Cystatin C, Albuminuria, and Mortality Among Older Adults With Diabetes

OBJECTIVE

Albuminuria and impaired glomerular filtration rate (GFR) are each associated with poor health outcomes among individuals with diabetes. Joint associations of albuminuria and impaired GFR with mortality have not been comprehensively evaluated in this population.

RESEARCH DESIGN AND METHODS

This is a cohort study among Cardiovascular Health Study participants with diabetes, mean age 78 years. GFR was estimated using serum cystatin C and serum creatinine. Albumin-to-creatinine ratio (ACR) was measured in single-voided urine samples.

RESULTS

Of 691 participants, 378 died over 10 years of follow-up. Cystatin C–estimated GFR <60 ml/min per 1.73 m², creatinine-based estimated GFR <60 ml/min per 1.73 m², and urine ACR ≥30 mg/g were each associated with increased mortality risk with hazard ratios of 1.73 (95% CI 1.37–2.18), 1.54 (1.21–1.97), and 1.73 (1.39–2.17), respectively, adjusting for age, sex, race, diabetes duration, hypoglycemic medications, hypertension, BMI, smoking, cholesterol, lipid-lowering medications, prevalent cardiovascular disease (CVD), and prevalent heart failure. Cystatin C–estimated GFR and urine ACR were additive in terms of mortality risk. Cystatin C–estimated GFR predicted mortality more strongly than creatinine-based estimated GFR.

CONCLUSIONS

Albuminuria and impaired GFR were independent, additive risk factors for mortality among older adults with diabetes. These findings support current recommendations to regularly assess both albuminuria and GFR in the clinical care of patients with diabetes; a focus on interventions to prevent or treat CVD in the presence of albuminuria, impaired GFR, or both; and further consideration of cystatin C use in clinical care.Kidney disease is a major complication of diabetes that markedly increases risk of cardiovascular disease (CVD) and mortality. Albuminuria, which is believed to reflect hemodynamic disturbances within the glomerulus, has long been identified as a major prognostic indicator in individuals with diabetes (1,2). Impaired glomerular filtration rate (GFR), a complementary sign of kidney damage, is also associated with increased risk (2,3). As a result, screening for kidney disease using urinary albumin and serological markers of kidney function has become a cornerstone of diabetes care, facilitating targeted interventions to prevent CVD and kidney disease progression (4).Albuminuria and impaired GFR may provide additive prognostic information regarding health outcomes in diabetes. However, the joint association of albuminuria and impaired GFR with mortality has not been defined in this setting, because few studies have evaluated albuminuria and GFR simultaneously, studies that have reached different conclusions (3,5), and GFR has only been estimated using serum creatinine. GFR estimated from serum cystatin C may reflect kidney function more precisely than GFR estimated using traditional serum creatinine-based methods and is more strongly linked with adverse health outcomes in community-based, mostly nondiabetic populations (6,7).We examined associations of cystatin C–estimated GFR, creatinine-estimated GFR, change in estimated GFR over time, and albuminuria with mortality among older adults with diabetes. The relationship of kidney disease with mortality may be particularly important among older adults. In the general population, traditional cardiovascular risk factors predict CVD weakly in older compared with younger adults, and the prognostic value of kidney disease is increased (8). Our aim was to assess whether cystatin C–estimated GFR would provide additive prognostic information to urine albumin excretion and to compare the strength of the association with mortality with that of creatinine-estimated GFR.     

Comparison of Two Creatinine-Based Estimating Equations in Predicting All-Cause and Cardiovascular Mortality in Patients With Type 2 Diabetes

OBJECTIVE

To compare the performance of two glomerular filtration rate (GFR)-estimating equations in predicting the risk of all-cause and cardiovascular mortality in type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

We followed 2,823 type 2 diabetic outpatients for a period of 6 years for the occurrence of all-cause and cardiovascular mortality. GFR was estimated using the four-variable Modification of Diet in Renal Disease (MDRD) study equation and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation.

RESULTS

At baseline, an estimated GFR (eGFR) <60 mL/min/1.73 m² was present in 22.0 and 20.2% of patients using the MDRD study equation and the CKD-EPI equation, respectively. A total of 309 patients died during the follow-up (152 patients from cardiovascular causes). Both creatinine-based equations were associated with an increased risk of all-cause and cardiovascular mortality. However, the CKD-EPI equation provided a more accurate risk prediction of mortality than the MDRD study equation. Receiving operating characteristic curves showed that the areas under the curve (AUCs) for all-cause mortality (AUC 0.712 [95% CI 0.682–0.741]) and cardiovascular mortality (0.771 [0.734–0.808]) using eGFR_CKD-EPI were significantly greater (P < 0.0001 by the z statistic) than those obtained by using eGFR_MDRD (0.679 [0.647–0.711] for all-cause mortality and 0.739 [0.698–0.783] for cardiovascular mortality).

CONCLUSIONS

Our findings suggest that the estimation of GFR using the CKD-EPI equation more appropriately stratifies patients with type 2 diabetes according to the risk of all-cause and cardiovascular mortality compared with the MDRD study equation.Chronic kidney disease (CKD) is a major public health problem because its prevalence is rapidly increasing worldwide and it is strongly associated with increased risks of end-stage renal disease, death, cardiovascular disease (CVD), and hospitalization (1–5). Glomerular filtration rate (GFR) is the best overall measure of kidney function. Current diagnosis, evaluation, and management of CKD routinely rely on estimates of GFR (eGFRs) usually derived from creatinine-based equations such as the Modification of Diet in Renal Disease (MDRD) study equation, which incorporates information on serum creatinine concentration, age, sex, and race (1,6,7). This equation is the most commonly used method for estimating kidney function in routine clinical practice. Its prognostic value has been validated in several studies and populations (1,6,7). Decreased eGFR_MDRD has been shown to be an important risk factor for death, CVD events, and other adverse clinical outcomes, specifically in patients with a GFR level <60 mL/min/1.73 m² (1–5). However, despite its widespread use, it is known that the major limitations of the MDRD study equation are imprecision and systematic underestimation of measured GFR (bias) at higher values (1,6,7).The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) investigators recently developed and validated a new equation to improve the estimation of GFR (eGFR_CKD-EPI) by using a large database pooled from 10 studies (8). This equation, which uses the same four variables as the MDRD study equation (i.e., serum creatinine level, age, sex, and race), has been shown to be more precise and accurate than the MDRD study equation in estimating measured GFR, especially at higher GFR values (8). Improved accuracy of the CKD-EPI equation could have important implications for public health and clinical practice (8). In addition, some large population-based cohort studies have recently shown that the CKD-EPI equation also has superior accuracy in classifying individuals at risk for CVD events and death compared with the MDRD study equation (9–11).However, the CKD-EPI equation might not work equally well in people at high CVD risk, such as type 2 diabetic individuals. Whether the use of the CKD-EPI equation provides more accurate prognostic information than the MDRD study equation with respect to the risk of all-cause and CVD mortality in patients with type 2 diabetes is currently unknown. Thus, the aim of this prospective, longitudinal study was to compare the performance of the MDRD study equation and the CKD-EPI equation in predicting all-cause and CVD mortality in a large sample of type 2 diabetic individuals during a follow-up period of 6 years.     

Renal Hyperfiltration and the Development of Microalbuminuria in Type 1 Diabetes

OBJECTIVE

The purpose of this study was to examine prospectively whether renal hyperfiltration is associated with the development of microalbuminuria in patients with type 1 diabetes, after taking into account known risk factors.

RESEARCH DESIGN AND METHODS

The study group comprised 426 participants with normoalbuminuria from the First Joslin Kidney Study, followed for 15 years. Glomerular filtration rate was estimated by serum cystatin C, and hyperfiltration was defined as exceeding the 97.5th percentile of the sex-specific distribution of a similarly aged, nondiabetic population (134 and 149 ml/min per 1.73 m² for men and women, respectively). The outcome was time to microalbuminuria development (multiple albumin excretion rate >30 μg/min). Hazard ratios (HRs) for microalbuminuria were calculated at 5, 10, and 15 years.

RESULTS

Renal hyperfiltration was present in 24% of the study group and did not increase the risk of developing microalbuminuria. The unadjusted HR for microalbuminuria comparing those with and without hyperfiltration at baseline was 0.8 (95% CI 0.4–1.7) during the first 5 years, 1.0 (0.6–1.7) during the first 10 years, and 0.8 (0.5–1.4) during 15 years of follow-up. The model adjusted for baseline known risk factors including A1C, age at diagnosis of diabetes, diabetes duration, and cigarette smoking resulted in similar HRs. In addition, incorporating changes in hyperfiltration status during follow-up had minimal impact on the HRs for microalbuminuria.

CONCLUSIONS

Renal hyperfiltration does not have an impact on the development of microalbuminuria in type 1 diabetes during 5, 10, or 15 years of follow-up.The glomerular filtration rate (GFR), the volume of water filtered out of the plasma per unit of time, is indicative of overall kidney function. However, measuring GFR with the gold standard technique is an intensive process and difficult for both the operator and the participant. Thus, it has not been practical to determine GFR in large epidemiological studies. Instead, serum creatinine has been widely used to estimate low levels of GFR when loss of kidney function has already occurred. However, serum creatinine is not sensitive enough to detect changes when renal function is normal or abnormally elevated (1). A laboratory test to estimate GFR based on serum cystatin C levels has been developed recently. Cystatin C assays are easy to perform and have been shown to yield accurate estimates even in the normal or elevated ranges of filtration (2,3). This development has created a new opportunity for studying early diabetic renal function abnormalities in large epidemiological studies.Hyperfiltration has been suggested as a risk factor for the development of microalbuminuria (4). The increase in pressure and flow may lead to functional and structural changes in the kidney (5,6). In several small studies, hyperfiltration was associated with the development of microalbuminuria in type 1 diabetes, but results have been inconsistent. Some studies were conducted in children beginning at diagnosis or early in the course of diabetes, and usually a few events of microalbuminuria were observed (7–11). Yip et al. (12) found no association between hyperfiltration and microalbuminuria in a 10-year prospective case control study of 25 adult pairs who had diabetes duration between 1 and 19 years. None of these studies adequately addressed confounders. Little subsequent research in large cohorts has been conducted on the role of hyperfiltration, primarily due to difficulties in determining GFR.Scott et al. (13) studied microalbuminuria onset in the First Joslin Study on the Natural History of Microalbuminuria (First Joslin Kidney Study) during the first 4 years of follow-up (13). They found that younger age at diabetes diagnosis, longer diabetes duration, poorer glycemic control, and cigarette smoking were associated with the development of microalbuminuria. Serum cystatin C measurements (to estimate GFR) were not available at the time of that work. The current project builds upon this prior study by examining whether hyperfiltration, as measured by cystatin C, is associated with the development of microalbuminuria during 15 years of follow-up, after taking into account known risk factors.     

Plasma Growth Differentiation Factor-15 Independently Predicts All-Cause and Cardiovascular Mortality As Well As Deterioration of Kidney Function in Type 1 Diabetic Patients With Nephropathy

OBJECTIVE

Growth deferentiation factor-15 (GDF-15) is involved in inflammation and apoptosis. Expression is induced in the heart in response to ischemia and in atherosclerotic plaques. The aim of this study was to investigate GDF-15 levels in relation to all-cause mortality, cardiovascular mortality and morbidity, decline in glomerular filtration rate (GFR), and progression toward end-stage renal disease (ESRD).

RESEARCH DESIGN AND METHODS

The study was a prospective observational follow-up study including 451 type 1 diabetic patients with diabetic nephropathy (274 men, aged 42.1 ± 0.5 years [means ± SD], diabetes duration 28.3 ± 8.9 years, GFR 76 ± 33 ml/min/1.73 m²) and a control group of 440 patients with longstanding type 1 diabetes and persistent normoalbuminuria (232 men, aged 45.4 ± 11.5 years, duration of diabetes 27.7 ± 10.1 years). The patients were followed for 8.1 (0.0–12.9) years (median [range]).

RESULTS

Among normoalbuminuric patients, GDF-15 above the median predicted an adjusted (age, systolic blood pressure [sBP], and estimated GFR) increased risk of all-cause mortality (hazard ratio [HR] 3.6 [95% CI 1.3–10.3]; P = 0.014). Among patients with diabetic nephropathy, higher (fourth quartile) versus lower (first quartile) GDF-15 levels predict all-cause mortality (covariate-adjusted [sex, age, smoking, blood pressure, A1C, cholesterol, GFR, N-terminal prohormone B-type natriuretic peptide, antihypertensive treatment, and previous cardiovascular events]; HR 4.86 [95% CI 1.37–17.30]) as well as fatal and nonfatal cardiovascular events (adjusted HR 5.59 [1.23–25.43] and 3.55 [1.08–11.64], respectively). In addition, higher GDF-15 levels predict faster decline in GFR (P < 0.001) but not development of ESRD.

CONCLUSIONS

Higher levels of GDF-15 are a predictor of all-cause and cardiovascular mortality and morbidity in patients with diabetic nephropathy. Furthermore, higher levels of GDF-15 are associated with faster deterioration of kidney function.Diabetes is associated with accelerated atherosclerosis and an increased risk of cardiovascular disease (CVD), which has become the major cause of morbidity and mortality among patients with diabetic nephropathy (1). Left ventricular hypertrophy, hypertension, and diabetes are leading predictors for the development of heart failure and sudden death (2,3). In general, the hypertrophic growth of the myocardium is regulated by a number of pro- and antigrowth factors, e.g., angiotensin-II and B-type natriuretic peptide (BNP) related to the transforming growth factor-β superfamily (4–6).Recently, growth differentiation factor-15 (GDF-15) has been identified as a novel anti-hypertrophic regulatory factor (7). GDF-15 is generated as a 40-kDa propeptide from which the NH₂-terminus is cleaved and a 30-kDa protein secreted as the active form (8).GDF-15 is induced in the hypertrophic and dilated cardiomyopathy following hypertension/volume overload, ischemia, and heart failure, possibly via proinflammatory cytokine and oxidative stress-dependent signaling pathways (9,10). GDF-15 is highly expressed in the infarcted myocardium in predominantly nondiabetic patients suffering an acute myocardial infarction (MI) (9) and in atherosclerotic plaques obtained from carotid artery surgery (11). In a nested case-control study, GDF-15 was shown to be associated with adverse cardiovascular outcomes in women (12). Furthermore, GDF-15 has been shown to predict mortality in patients with both ST-elevation MI (STEMI) and non–STEMI, independent of known biomarkers such as N-terminal prohormone B-type natriuretic peptide (NT-proBNP) (13,14).Therefore, we investigated the predictive value of circulating GDF-15 levels on all-cause mortality, fatal and nonfatal CVD, decline in GFR, as well as progression to end-stage renal disease (ESRD) in a well-characterized population of type 1 diabetic patients with or without diabetic nephropathy.     

High-Normal Serum Uric Acid Increases Risk of Early Progressive Renal Function Loss in Type 1 Diabetes: Results of a 6-year follow-up

OBJECTIVE

We previously described a cross-sectional association between serum uric acid and reduced glomerular filtration rate (GFR) in nonproteinuric patients with type 1 diabetes. Here, we prospectively investigated whether baseline uric acid impacts the risk of early progressive renal function loss (early GFR loss) in these patients.

RESEARCH DESIGN AND METHODS

Patients with elevated urinary albumin excretion (n = 355) were followed for 4–6 years for changes in urinary albumin excretion and GFR. The changes were estimated by multiple determinations of albumin-to-creatinine ratios (ACRs) and serum cystatin C (GFRcystatin).

RESULTS

At baseline, the medians (25th–75th percentiles) for uric acid, ACR, and GFRcystatin values were 4.6 mg/dl (3.8–5.4), 26.2 mg/g (15.1–56.0), and 129 ml/min per 1.73 m² (111–145), respectively. During the 6-year follow-up, significant association (P < 0.0002) was observed between serum uric acid and development of early GFR loss, defined as GFRcystatin decline exceeding 3.3% per year. In baseline uric acid concentration categories (in mg/dl: <3.0, 3.0–3.9, 4.0–4.9, 5.0–5.9, and ≥6), the risk of early GFR loss increased linearly (9, 13, 20, 29, and 36%, respectively). This linear increase corresponds to odds ratio 1.4 (95% CI 1.1–1.8) per 1 mg/dl increase of uric acid. The progression and regression of urinary albumin excretion were not associated with uric acid.

CONCLUSIONS

We found a clear dose-response relation between serum uric acid and risk of early GFR loss in patients with type 1 diabetes. Clinical trials are warranted to determine whether uric acid–lowering drugs can halt renal function decline before it becomes clinically significant.The paradigm of early diabetic nephropathy in type 1 diabetes has changed during the past few years. Previously, it focused on urinary leakage of small amounts of albumin, designated microalbuminuria, which was believed to predict progression to overt proteinuria and ultimately end-stage renal disease (ESRD) (1). However, recent studies (2,3) documented that rather than progression, regression to normoalbuminuria is the fate of microalbuminuria in the majority of patients. Progression to proteinuria is the fate in a minority only. At the same time, the onset of progressive renal function loss was observed in a subset of these patients well before the onset of proteinuria (4). Because this decline occurs while renal function is still normal, we label it “early” progressive renal function loss (early GFR loss). Whereas microalbuminuria may regress, remain stable, or progress during the course of diabetes, early GFR loss progresses to chronic kidney disease (CKD) and ESRD (4–6).Because early GFR loss is detectable while renal function is normal or elevated, an opportunity may exist for effective intervention many years before the occurrence of meaningful loss of renal function. However, our knowledge of early GFR loss determinants is limited (4). Discovery of these determinants, especially modifiable ones, may enable the design of more effective programs for the prevention of CKD and ESRD.A body of evidence suggests that elevated serum uric acid may cause kidney injury in animal models (7). Also, mounting evidence points to a role of elevated serum uric acid in the development of kidney disease in humans. In cohort studies of subjects without diabetes, hyperuricemia predicted the development of CKD stage 3 (8) and ESRD (9,10), and in community-based cohorts moderately elevated serum uric acid concentrations were associated with the development of CKD stage 3 (11,12). Recently, in a type 1 diabetic cohort in Denmark, moderately elevated uric acid concentrations were found to be associated with risk of proteinuria during an 18-year follow-up (13). However, changes in renal function were not examined.The Second Joslin Study on the Natural History of Microalbuminuria in Type 1 Diabetes (Second Joslin Study) is a follow-up study designed to determine the natural history of early diabetic nephropathy in nonproteinuric patients with type 1 diabetes. In baseline data, high-normal serum uric acid concentration was strongly associated with reduced glomerular filtration rate (GFR), as estimated by serum cystatin C (GFRcystatin) (14). Whether elevated serum uric acid preceded or followed reduced GFR could not be determined in that cross-sectional data. Therefore, we prospectively measured GFRcystatin during 4–6 years of follow-up and investigated the association between baseline serum uric acid concentration and changes in renal function. A secondary aim was to explore the role of serum uric acid in the progression and regression of urinary albumin excretion.     

Evolution of renal hyperfiltration and arterial stiffness from adolescence into early adulthood in type 1 diabetes

OBJECTIVE

To determine, in a small but carefully physiologically characterized cohort of subjects with uncomplicated type 1 diabetes, the changes in renal hemodynamic function and arterial stiffness that occur over time as the participants transitioned from adolescence into early adulthood. The classical paradigm for type 1 diabetes suggests that glomerular filtration rate (GFR) declines in patients with renal hyperfiltration, but the natural history of peripheral vascular function abnormalities in uncomplicated type 1 diabetes is less well understood, particularly as patients transition from adolescence to early adulthood.

RESEARCH DESIGN AND METHODS

Renal hemodynamic function (inulin and p-aminohippuric acid clearance), blood pressure, arterial stiffness (radial augmentation index), albumin excretion, and circulating renin-angiotensin system measures were obtained during clamped euglycemia at baseline and at follow-up 6.8 ± 2.5 years later in 10 patients with hyperfiltration (GFR ≥135 mL/min/1.73 m²) and in 8 with normofiltration.

RESULTS

Compared with baseline values, GFR (171 ± 20 to 120 ± 15 mL/min/1.73 m²) and filtration fraction (FF, 0.24 ± 0.06 to 0.18 ± 0.03) declined in hyperfilterers (ANOVA P ≤ 0.033), and renal vascular resistance increased (0.0678 ± 0.0135 to 0.0783 ± 0.0121 mmHg/L/min, P = 0.017). GFR and FF did not change in normofiltering subjects. In contrast, the radial augmentation index decreased in hyperfiltering (1.2 ± 11.7 to −11.0 ± 7.8%) and normofiltering (14.3 ± 14.0 to 2.5 ± 14.6%) subjects (within-group changes, ANOVA P ≤ 0.030). The decline in circulating aldosterone levels was similar in both groups.

CONCLUSIONS

During the transition from adolescence to early adulthood, hyperfiltration is not sustained in subjects with type 1 diabetes, whereas GFR remains stable in normofiltering subjects. Our findings suggest early normofiltration may predict stable renal function. In contrast, arterial stiffness decreased in all patients regardless of filtration status, suggesting that age-related increases in arterial stiffness occur at older ages.Hyperfiltration is associated with the development of diabetic nephropathy, possibly because of high intraglomerular pressure that results in glomerular injury (1). A variety of hormonal factors influence hyperfiltration, including the renin-angiotensin system (RAS), cyclooxygenase 2, protein kinase C-β, and changes in hormones related to puberty (2–5). Blockade of these hormonal pathways partially reduces the glomerular filtration rate (GFR) in hyperfiltering subjects but has no effect in normofilterers, suggesting that individuals with hyperfiltration are physiologically distinct (2–4). More recently, hyperfiltration has been associated with peripheral vascular alterations, including low arterial stiffness and endothelial dysfunction (6,7). It has therefore been suggested that the hyperfiltration state reflects generalized microvascular and macrovascular functional changes (6–8).Although it is generally accepted that hyperfiltration represents a renal risk factor in diabetes, the natural history for changes in renal function in normofiltering subjects remains poorly defined. For example, normofilterers may represent a group of former hyperfiltering individuals who have had a decline in kidney function and were simply studied at a time when GFR was within the normal range. Alternatively, normofiltration may represent a subgroup that is protected against renal and vascular injury.Peripheral vascular function testing has suggested that normofiltration is associated with preserved endothelial function, which is important for two reasons (6). First, this observation suggests that normofiltration represents a “protective” vascular phenotype. Second, measures of peripheral vascular function, such as arterial stiffness, may offer additional, noninvasive insight into renal and vascular risk, before the onset of clinical end points such as declining renal function, hypertension, or microalbuminuria (9).Although renal hyperfiltration and changes in macrovascular function, such as low arterial stiffness, appear to be linked in cross-sectional studies of early type 1 diabetes, the interaction between these preclinical abnormalities over time, in the same individuals, is not known (7). This association is important, because hyperfiltration is associated with declining renal function (1). For example, if declining GFR in hyperfiltering subjects is also associated with a deleterious rise in arterial stiffness, this could yield important pathophysiologic insights into mechanisms of disease progression and clinical prognostic information (1,7). A more complete understanding of preclinical factors that may increase future renal risk is of particular importance during the transition from adolescence to early adulthood, which may represent a crucial period when renal injury is initiated (10).We initially measured GFR and arterial stiffness in a well-characterized adolescent cohort with uncomplicated type 1 diabetes (3,4). The same measures were repeated 6.8 ± 2.5 years later in a subgroup during the transition from adolescence to early adulthood. The goals of the present analysis were to 1) describe the relationship between changes in GFR and arterial compliance and 2) elucidate the natural history of arterial stiffness in patients with early type 1 diabetes renal hyperfiltration or normofiltration. Our hypotheses were that 1) renal hyperfiltration would decrease, in association with renal vasoconstriction and a concordant rise in arterial stiffness, as study participants transitioned from adolescence into early adulthood, and 2) normofiltration represents a state of preserved kidney function and stable macrovascular function that does not change significantly in early type 1 diabetes.     

Early Diabetic Nephropathy: A complication of reduced insulin sensitivity in type 1 diabetes

OBJECTIVE

Diabetic nephropathy (DN) is a major cause of mortality in type 1 diabetes. Reduced insulin sensitivity is a well-documented component of type 1 diabetes. We hypothesized that baseline insulin sensitivity would predict development of DN over 6 years.

RESEARCH DESIGN AND METHODS

We assessed the relationship between insulin sensitivity at baseline and development of early phenotypes of DN—microalbuminuria (albumin-creatinine ratio [ACR] ≥30 mg/g) and rapid renal function decline (glomerular filtration rate [GFR] loss >3 mL/min/1.73 m² per year)—with three Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations over 6 years. Subjects with diabetes (n = 449) and without diabetes (n = 565) in the Coronary Artery Calcification in Type 1 Diabetes study had an estimated insulin sensitivity index (ISI) at baseline and 6-year follow-up.

RESULTS

The ISI was lower in subjects with diabetes than in those without diabetes (P < 0.0001). A higher ISI at baseline predicted a lower odds of developing an ACR ≥30 mg/g (odds ratio 0.65 [95% CI 0.49–0.85], P = 0.003) univariately and after adjusting for HbA_1c (0.69 [0.51–0.93], P = 0.01). A higher ISI at baseline conferred protection from a rapid decline of GFR as assessed by CKD-EPI cystatin C (0.77 [0.64–0.92], P = 0.004) and remained significant after adjusting for HbA_1c and age (0.80 [0.67–0.97], P = 0.02). We found no relation between ISI and rapid GFR decline estimated by CKD-EPI creatinine (P = 0.38) or CKD-EPI combined cystatin C and creatinine (P = 0.50).

CONCLUSIONS

Over 6 years, a higher ISI independently predicts a lower odds of developing microalbuminuria and rapid GFR decline as estimated with cystatin C, suggesting a relationship between insulin sensitivity and early phenotypes of DN.Diabetic nephropathy (DN) is a common and serious complication of diabetes. Its incidence is rising rapidly (1), and it is the most common cause of end-stage renal disease in the U.S. and Europe (2). The 2011 U.S. Renal Data System showed that DN accounted for 44.5% of all cases of end-stage renal disease in 2009 (3). Despite improvements in the outlook of this complication in past decades, it continues to be one of the major causes of morbidity and mortality in type 1 diabetes (4,5). DN is an important risk factor for coronary artery disease (6–8) and overall mortality (6,9). These findings highlight the need for improved methods of identifying persons at high risk for DN (10).The role of insulin sensitivity in the development and progression of macro- (7,11,12) and microvascular complications (12,13) in type 1 diabetes is increasingly recognized. Reduced insulin sensitivity also is a plausible mechanism linking renal disease with excess mortality in type 1 diabetes. Historically, when glycemic control is poor, reduced insulin sensitivity was believed to be directly related to body weight and HbA_1c (14,15), but more recent data suggest that reduced insulin sensitivity cannot simply be explained by weight or poor glycemic control. In fact, reduced insulin sensitivity has been documented in type 1 diabetic subjects with normal BMI and HbA_1c compared with nondiabetic individuals (16). The Coronary Artery Calcification in Type 1 Diabetes (CACTI) longitudinal cohort study of adults with type 1 diabetes investigated the determinants of early and accelerated atherosclerosis and found that insulin sensitivity independently predicted coronary artery calcification (17,18). Reduced insulin sensitivity has also been shown to predict diabetic retinopathy, neuropathy, and nephropathy in subjects with type 1 diabetes (13).Despite advances in the estimation of insulin sensitivity (insulin sensitivity index [ISI]) (19) and glomerular filtration rate (GFR) (20), research in the association of insulin sensitivity with DN has been limited since the Pittsburgh Epidemiology of Diabetes Complications (EDC) cohort showed more than a decade ago that the estimated glucose disposal rate (eGDR) predicts overt nephropathy (13). To readdress this relationship with contemporary data and estimating equations, we hypothesized that higher insulin sensitivity measured by ISI at baseline would be associated with decreased odds of developing two early phenotypes of DN—microalbuminuria (albumin-creatinine ratio [ACR] ≥30 mg/g) and rapid renal function decline (GFR loss >3 mL/min/1.73 m² per year) (21–23)—calculated by the three Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations (20) over 6 years in the CACTI study.     

Clinical utility of creatinine- and cystatin C-based definition of renal function for risk prediction of primary cardiovascular events in patients with diabetes

OBJECTIVE

To assess the cardiovascular risk of diabetic subjects with chronic kidney disease (CKD) based on different estimated glomerular filtration rate (eGFR) equations and to evaluate which definition of CKD best improves cardiovascular risk prediction of the Framingham Cardiovascular Risk Score (Framingham-CV-RS).

RESEARCH DESIGN AND METHODS

CKD was defined as eGFR <60 mL/min/1.73 m², estimated by the creatinine-based Modification of Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations and a cystatin C–based equation (CKD-CysC). Cox regression was used to estimate hazard ratios (HRs) of subjects with CKD for incident cardiovascular events in a cohort of 1,153 individuals with diabetes (baseline age 50–74 years). Furthermore, the CKD definitions were added individually to a reference model comprising the Framingham-CV-RS variables and HbA_1c, and measures of model discrimination and reclassification were assessed.

RESULTS

During 5 years of follow-up, 95 individuals had a primary cardiovascular event. Crude HRs were increased for all CKD definitions. However, after adjusting for established cardiovascular risk factors, HRs for both creatinine-based CKD definitions were attenuated to point estimates of 1.03, whereas the HRs for the cystatin C–based CKD definition remained significantly increased (HR 1.75 [95% CI 1.07–2.87]). Extension of the reference model by the different CKD definitions resulted in an increase in the c statistic only when adding CKD-CysC (from 0.638 to 0.644) along with a net reclassification improvement of 8.9%.

CONCLUSIONS

Only the cystatin C–based CKD definition was an independent risk predictor for cardiovascular events in our diabetic study cohort and indicated a potentially better clinical utility for cardiovascular risk prediction than creatinine-based equations.Chronic kidney disease (CKD) is a frequent disease in the elderly, especially among older adults with diabetes (1,2). However, epidemiologic data about the prevalence of CKD in patients with diabetes remain sparse and the accuracy of the different estimating equations to assess renal function in clinical routine is still debated (1,3,4).CKD can be classified with an estimated glomerular filtration rate (eGFR) of <60 mL/min/1.73 m² (CKD stages 3–5) (5). The most commonly used equation to estimate glomerular filtration rate (GFR) is the serum creatinine–based abbreviated Modification of Diet in Renal Disease (MDRD) equation (6), although it is well known that it underestimates GFR in the normal and high-normal range (7). Recently, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation has been introduced as a better means of estimate eGFR in observational research (8). However, data from patients with diabetes comparing the CKD-EPI and MDRD equations are still limited (8). Performance of creatinine-based eGFR in patients with diabetes and nephropathy lacks accuracy to monitor kidney function (9), especially in the early phases of renal impairment, and it can take years until other signs of a glomerulopathy such as albuminuria appear (10). Therefore, cystatin C–based estimating equations are suggested to show better clinical utility compared with creatinine-based equations (11,12).Which formula is best to be used to classify CKD in subjects with diabetes is an important question, especially because effective interventions exist to reduce the risk for cardiovascular disease and progression to end-stage renal disease (13). However, no study thus far has compared the MDRD and CKD-EPI formulas with a cystatin C–based equation in patients with diabetes (14). An eligible end point to shed further light on this question is an estimated predictive value of each equation for cardiovascular disease because CKD is clearly associated with cardiovascular end points, independent of established cardiovascular risk factors (15,16).Therefore, the objective of this analysis is to estimate the prognostic utility of serum creatinine– and cystatin C–based CKD definitions for incident cardiovascular events in subjects with diabetes.     

Measuring glomerular filtration rate in acute kidney injury: Yes,but not yet

Acute kidney injury has become a major focus for nephrologists and critical care physicians. The development of structural biomarkers is proceeding, but the results to date have been disappointing. The use of a shortened creatinine clearance as a functional acute kidney injury biomarker is not new but has not been compared with that of other diagnostic approaches. A rapid, repeatable, and accurate measured glomerular filtration rate would be the gold standard for a functional biomarker and is not far off.Acute kidney injury (AKI) remains a vexing clinical problem resulting in unacceptably high patient mortality, development of chronic kidney disease, and enhanced progression to end-stage kidney disease [1]. Although clinical risk factors for developing AKI have been identified, there is no reasonable surveillance technique (''biomarker'') to either definitively and rapidly diagnose the injury or determine the extent of its severity. Since patient outcomes correlate with the extent of injury and effective therapy requires early intervention, the ability to rapidly diagnose and stratify patients by their level of kidney injury is of paramount importance for therapeutic progress. Therefore, a search for a biomarker of kidney injury has intensified and is now considered by many experts to be the highest priority in the field of AKI [2]. It is likely that a combination of structural and functional markers of AKI will provide the highest clinical utility.Glomerular filtration rate (GFR), which measures the amount of plasma filtered through glomeruli within a given period of time, is clinically the most widely used indicator of kidney function. Yet a rapid quantitative technique with clinical utility has not been developed. Reduction in the GFR, secondary to kidney injury, is the hallmark of AKI and results in increased levels of blood urea nitrogen (BUN) and serum creatinine. Unfortunately, the rates of increase in BUN and serum creatinine do not parallel the fall in GFR in a time frame that is clinically useful. In addition, since both creatinine production from muscle and GFR determine the serum creatinine level, using serum creatinine as an indicator of GFR is highly patient-specific and often problematic or even misleading. These issues have been described elsewhere [3].Achieving the ability to rapidly and accurately measure GFR in an early stage of AKI would be beneficial for many reasons. It would rapidly identify and determine the extent of injury, allowing early pharmacologic or dialytic treatment (or both), enrollment and stratification for clinical studies, and prognostic information. It could also be used to determine the effect of a clinical maneuver on GFR, such as volume resuscitation and the use of pressors to support blood pressure, and enable appropriate drug dosing for agents cleared by the kidney or nephrotoxins.On this well-known background, Pickering and colleagues [1] set out to determine the clinical utility of a 4-hour creatinine clearance (CrCl), compared with plasma creatinine, for diagnosing AKI. The authors found that CrCl increased the likelihood of diagnosing AKI; a decreasing CrCl correlated with increased kidney injury severity, death, or dialysis; and the CrCl was most helpful when patients began with a serum creatinine in the normal range. Although the receiver operating characteristics were not impressive, the study was a small pilot study.This is not the first time a shortened CrCl has been used to estimate GFR in clinical settings. Herget-Rosenthal and colleagues [4] used a 2-hour CrCl in stable patients and found an acceptable and repeatable correlation with the 24-hour CrCl. However, the accuracy and utility of a shortened collection in unstable patients were questioned by two studies conducted in patients with AKI [5,6]. This limitation may relate to reduced production of creatinine in sepsis [7], increased production of creatinine with trauma, increased metabolism including the use of glucocorticoids, or the changing of GFRs during the collection periods. Pickering and colleagues did not attempt to validate their 4-hour CrCl rates with 6-hour iohexol or iothalamate infustion studies, currently the gold standard for clinical studies. This validation could have been done nearly simultaneously with the 4-hour CrCl urine collection. Until such studies are conducted, confidence for using a CrCl may be limited.A clinically useful technique to measure GFR in AKI has been a long-sought-after goal [8]. The cumbersome, time-consuming, and expensive techniques currently available have not met this important clinical need. The development and use of estimating formulas based on serum creatinine or cystatin C have been disappointing for several well-explained reasons [3]. Therefore, the development of a rapid, accurate, safe, easy, and in-expensive technique has high clinical importance both in and out of the hospital. An approach being developed commercially, used primarily in preclinical studies to date, is the use of bedside techniques to measure GFR. The use of inexpensive, nontoxic, easily sized fluorescent molecules has emerged as a likely candidate, and several groups are now working with this approach in preclinical models [9,10]. It is likely that several of these approaches will enter clinical phase studies, and improvements in design and clinical utility are continually being developed. Test rapidity, convenience, cost, safety, accuracy, and repeatability are all critical characteristics. Hopefully, one or more of these approaches will emerge and alleviate the need for multiple-hour urine collection and analysis.     

Serum Levels of the Adipokine Chemerin in Relation to Renal Function

OBJECTIVE

To investigate serum levels of the adipokine chemerin in patients on chronic hemodialysis (CD) as compared with control patients with a glomerular filtration rate (GFR) >50 ml/min.

RESEARCH DESIGN AND METHODS

Chemerin was quantified by ELISA in control patients (n = 60) and CD patients (n = 60) and correlated with clinical and biochemical measures of renal function, glucose, and lipid metabolism, as well as inflammation, in both groups.

RESULTS

Median serum chemerin levels were more than twofold higher in CD patients (542.2 μg/l) compared with subjects with a GFR >50 ml/min (254.3 μg/l) (P < 0.001). Furthermore, GFR, as assessed by the original Modification of Diet in Renal Disease formula, independently predicted circulating chemerin concentrations in multiple regression analyses in both control patients (P < 0.05) and CD patients (P < 0.01).

CONCLUSIONS

We demonstrate that markers of renal function are independently related to circulating chemerin levels.Recently, chemerin has been identified as a novel adipocyte-secreted factor playing a crucial role in adipocyte differentiation and insulin signaling (1–4). Several studies have quantified circulating chemerin in humans. Thus, two reports found an independent association between chemerin and markers of inflammation (5,6). Furthermore, correlations between circulating chemerin and metabolic syndrome–related parameters have been described (6–8). In contrast to other adipokines (9–12), no data have been published so far about the relation of chemerin to renal function.     

Insulin resistance, cystatin C, and mortality among older adults

OBJECTIVE

Insulin resistance is a risk factor for cardiovascular and noncardiovascular diseases. Impaired kidney function is linked with insulin resistance and may affect relationships of insulin resistance with health outcomes.

RESEARCH DESIGN AND METHODS

We performed a cohort study of 3,138 Cardiovascular Health Study participants (age ≥65 years) without diabetes. Insulin sensitivity index (ISI) was calculated from fasting and 2-h postload insulin and glucose concentrations. Associations of ISI and fasting insulin concentration with all-cause mortality were tested using Cox proportional hazards models, adjusting for demographic variables, prevalent cardiovascular disease, lifestyle variables, waist circumference, and LDL cholesterol. Subsequent models were additionally adjusted for or stratified by glomerular filtration rate estimated using serum cystatin C (eGFR).

RESULTS

A total of 1,810 participants died during the 14.7-year median follow-up. Compared with the highest quartile of ISI, the lowest quartile (most insulin resistant) was associated with 21% (95% CI 6–41) and 11% (−3 to 29) higher risks of death without and with adjustment for eGFR, respectively. Compared with the lowest quartile of fasting insulin concentration, the highest quartile was associated with 22% (4–43) and 4% (−12 to 22) higher risks of death without and with adjustment for eGFR, respectively. Similar attenuation by eGFR was observed when blood pressure, triglycerides, HDL cholesterol, and C-reactive protein were included in models.

CONCLUSIONS

Insulin resistance measured as ISI or fasting insulin concentration is associated with increased risk of death among older adults, adjusting for conventional confounding characteristics. Impaired kidney function may mediate or confound this relationship.Insulin resistance is an established risk factor for cardiovascular and noncardiovascular diseases. Insulin resistance is associated with increased risk of cardiovascular disease events in diverse community-based populations, whether it is measured directly (1,2), estimated using fasting insulin concentration (e.g., homeostasis model assessment [HOMA]) (3–9), or calculated using dynamic testing (e.g., oral glucose tolerance test [OGTT]) (9,10). Insulin resistance is also associated with increased risk of noncardiovascular diseases, including cancer (2,11). Insulin resistance promotes endothelial dysfunction, oxidative stress, and inflammation and is closely linked with other cardiovascular risk factors (obesity, hypertension, and dyslipidemia) as part of metabolic syndrome (12). Through these mechanisms, insulin resistance may be causally related to adverse clinical outcomes.Kidney function may play an important role in the relationship of insulin resistance with adverse health outcomes. Impaired kidney function is known to be linked with insulin resistance (13). The causal nature of this relationship is not well defined: impaired kidney function may promote insulin resistance through retained uremic toxins, acidosis, and active vitamin D deficiency; insulin resistance may contribute to the development of impaired kidney function by damaging glomerular endothelial and epithelial cells; and/or shared risk factors (e.g., obesity or genetic predisposition) may underlie both insulin resistance and impaired kidney function (14–18). Lower glomerular filtration rate (GFR), even within the normal range (≥60 mL/min/1.73 m²), is strongly associated with increased risks of cardiovascular disease and death, particularly among older adults (19,20). It is therefore possible that impaired kidney function confounds or mediates known associations of insulin resistance with cardiovascular and noncardiovascular diseases.We explored whether impaired kidney function confounds or mediates the relationship of insulin resistance with mortality. We chose all-cause mortality as our primary outcome to reflect the pleiotropic effects of insulin resistance. We studied this relationship in the Cardiovascular Health Study (CHS), a community-based population of older adults, because insulin resistance and impaired kidney function are each known strong risk factors for adverse health outcomes among older people (1,19,20). In addition, CHS obtained baseline data ascertaining insulin resistance in both fasting and dynamic states; measured baseline serum cystatin C, which may better discriminate differences in kidney function and its associated health risks in the normal range (20–23); and followed participants for >15 years. These data allow a comprehensive evaluation of the relationships of interest.     

Development and Progression of Renal Insufficiency With and Without Albuminuria in Adults With Type 1 Diabetes in the Diabetes Control and Complications Trial and the Epidemiology of Diabetes Interventions and Complications Study

OBJECTIVE

This multicenter study examined the impact of albumin excretion rate (AER) on the course of estimated glomerular filtration rate (eGFR) and the incidence of sustained eGFR <60 ml/min/1.73 m² in type 1 diabetes up to year 14 of the Epidemiology of Diabetes Interventions and Complications (EDIC) study (mean duration of 19 years in the Diabetes Control and Complications Trial [DCCT]/EDIC).

RESEARCH DESIGN AND METHODS

Urinary albumin measurements from 4-h urine collections were obtained from participants annually during the DCCT and every other year during the EDIC study, and serum creatinine was measured annually in both the DCCT and EDIC study. GFR was estimated from serum creatinine using the abbreviated Modification of Diet in Renal Disease equation.

RESULTS

A total of 89 of 1,439 subjects developed an eGFR <60 ml/min/1.73 m² (stage 3 chronic kidney disease on two or more successive occasions (sustained) during the DCCT/EDIC study (cumulative incidence 11.4%). Of these, 20 (24%) had AER <30 mg/24 h at all prior evaluations, 14 (16%) had developed microalbuminuria (AER 30–300 mg/24 h) before they reached stage 3 chronic kidney disease, and 54 (61%) had macroalbuminuria (AER >300 mg/24 h) before they reached stage 3 chronic kidney disease. Macroalbuminuria is associated with a markedly increased rate of fall in eGFR (5.7%/year vs. 1.2%/year with AER <30 mg/24 h, P < 0.0001) and risk of eGFR <60 ml/min/1.73 m² (adjusted hazard ratio 15.3, P < 0.0001), whereas microalbuminuria had weaker and less consistent effects on eGFR.

CONCLUSIONS

Macroalbuminuria was a strong predictor of eGFR loss and risk of developing sustained eGFR <60 ml/min/1.73 m². However, screening with AER alone would have missed 24% of cases of sustained impaired eGFR.It has generally been thought that increases in urine albumin excretion rate (AER) precede a fall in glomerular filtration rate (GFR) in patients developing diabetic chronic kidney disease (1). Some large studies in patients with type 2 diabetes (2–4) and a few smaller studies in individuals with type 1 diabetes (5–9), however, have demonstrated that a substantial proportion of diabetic individuals with decreased GFR levels do not have increased AER.In this article, we examine the effects of prior and current levels of AER on the rate of decline in estimated GFR (eGFR) and on the risk of decreased levels of eGFR (<60 ml/min/1.73 m²) in subjects with type 1 diabetes in the Diabetes Control and Complications Trial (DCCT) and/or the follow-up Epidemiology of Diabetes Interventions and Complications (EDIC) study.     

Deficient mitochondrial biogenesis in critical illness: cause,effect, or epiphenomenon?

Recent studies indicate that mitochondrial dysfunction plays a role in the pathogenesis of a number of disease states. The importance of these organelles in shock and multiple organ dysfunction is of particular interest to those caring for the critically ill. Mitochondria have their own unique DNA (mtDNA) that encodes 13 essential subunits of electron transport chain enzymes, two ribosomal RNAs and 22 transfer RNAs. Importantly, mtDNA is especially susceptible to deletions, rearrangements and mutations because it is not bound by histones and lacks the extensive repair machinery present in the nucleus. The study by Côté et al. in this issue of Critical Care examines changes in mtDNA in critically ill patients. The results support further investigation into the role of mtDNA in the critically ill.The role of mitochondria in systemic disease has been under-appreciated, and in this issue of Critical Care, Côté et al. [1] examine changes in mitochondrial DNA (mtDNA) in critically ill patients. However, recent evidence has demonstrated impaired oxidative phosphorylation and defective mitochondrial homeostasis in a number of disorders [2,3]. Although the concept of mitochondrial dysfunction and bioenergetic failure during sepsis and shock is not new, recent experimental approaches have yielded novel and interesting findings [4-6]. These have led us and others to propose intriguing hypotheses regarding the pathogenesis of acquired mitochondrial dysfunction in a variety of disease states.In this issue, Côté et al. examine changes in mtDNA in critically ill patients. Their data demonstrate a 30% reduction in the ratio of mtDNA to nuclear DNA (nDNA) in circulating cells of 28 critically ill patients when compared to healthy controls [1]. More importantly, this ratio increased by almost 30% at four days in survivors while non-survivors experienced a further reduction in the mtDNA/nDNA ratio. One might conclude that loss or failed synthesis of mtDNA is a unifying cause of sepsis-induced mitochondrial dysfunction and that clinicians could use mtDNA copy number to predict mortality during critical illness. This requires a more detailed examination of mtDNA heterogeneity and mitochondrial regeneration.Each mitochondrion has 2–10 copies of its own circular genome. These encode for 13 essential subunits of electron transport chain enzymes, two ribosomal RNAs and 22 transfer RNAs [7]. The structural subunits of the electron transport complexes and other mitochondrial proteins arise from nuclear genes [8]. Thus, expression of the genes encoding mitochondrial enzyme complexes is under dual control. mtDNA is particularly prone to deletions, rearrangements and mutations caused by oxidative stress because it is unbound by histones and because these organelles lack the extensive repair systems seen in the nucleus [9]. Therefore, reactive oxygen species produced during oxidative phosphorylation in a variety of disease states can damage mtDNA and mitochondrial proteins. This would lead to decreased ATP production and enhanced programmed cell death [7].Heteroplasmy describes the coexistence of both mutant mtDNA and wild-type, non-mutant mtDNA within the same cell [8]. If the mitochondrial genome drift results in a significant amount of mutant mtDNA, cells exhibit reduced energy capacity and organs become dysfunctional [7]. The threshold for these processes is lower in highly oxidative tissue such as brain, heart, skeletal muscle, retina, kidney and endocrine organs [8]. This threshold effect explains tissue-related variability in the clinical presentation of both inherited and acquired mitochondrial diseases [8].Impaired mitochondrial biogenesis represents an additional manner in which mitochondria may contribute to acquired disorders. Biogenesis includes all of the processes needed for mitochondrial homeostasis and division. It requires precise coordination between both mitochondrial and nuclear-encoded gene products as well as maintenance and replication of mtDNA [10,11]. Recent investigation demonstrates that experimental murine sepsis caused mitochondrial oxidative stress, a loss of mtDNA copy number and depressed basal metabolism in the septic liver [12]. In the recovery phase, mitochondrial biogenesis restored mtDNA copy number and oxidative metabolism.Our understanding of bioenergetic failure in sepsis and shock has been largely limited by interpretation of early investigations. These studies assumed that preservation of cellular ATP indicated intact electron transport [13,14]. However, more recent data make it clear that cells can adapt and maintain viability by down-regulating oxygen consumption, energy requirements and ATP demand [15,16]. In the heart this response is called myocardial hibernation and results in cardiomyocyte hypocontractility with preserved cellular ATP [15]. Hibernating cells maintain ATP levels in the setting of defective oxidative phosphorylation by ceasing nonessential cellular functions to limit ATP utilization [15,16]. At the organ level, this down-regulated metabolic state may manifest as "organ dysfunction" or "organ failure". During hypoxia, ischemia and in early or non-fatal sepsis, such a response appears to be adaptive and often reversible as cells at risk maintain viability and recover after reoxygenation and reperfusion. Our data, however, indicate that during lethal sepsis a similar hibernation response, while initially adaptive, may become problematic as cells remain persistently down-regulated, enzyme complex content and activity decrease and organ failure becomes irreversible [3,4]. This may result from an acquired defect in gene expression and/or functional activity of any of the electron transport enzymes [17]. Our data suggest that persistently impaired mitochondrial gene expression may represent the irreversible defect that leads to organ failure and death.The hypothesis that therapeutically enhancing mitochondrial biogenesis could improve survival is fascinating, especially if defects in mitochondrial replication and mtDNA synthesis also occur in cells of solid organs. Based on recent reports, it is conceivable that stem cells or fibroblasts may be able to restore defective mitochondria in neighboring cells with wild-type mtDNA [18]. Thus, future investigation should focus on increasing and restoring wild-type mtDNA to restore cellular oxidative capacity and organ function in sepsis and shock.What is most exciting is that we are still gaining insight into this billion year old, complex organelle. However, it remains unclear if mitochondrial impairment causes organ dysfunction, is protective against impending organ injury or is an epiphenomenon. The data presented to date have not directly addressed this issue. These questions demand a more exhaustive investigation of the fascinating processes of mitochondrial biogenesis and homeostasis during both health and disease.     

TRAF-interacting Protein (TRIP): A Novel Component of the Tumor Necrosis Factor Receptor (TNFR)- and CD30-TRAF Signaling Complexes That Inhibits TRAF2-mediated NF-κB Activation

Through their interaction with the TNF receptor–associated factor (TRAF) family, members of the tumor necrosis factor receptor (TNFR) superfamily elicit a wide range of biological effects including differentiation, proliferation, activation, or cell death. We have identified and characterized a novel component of the receptor–TRAF signaling complex, designated TRIP (TRAF-interacting protein), which contains a RING finger motif and an extended coiled-coil domain. TRIP associates with the TNFR2 or CD30 signaling complex through its interaction with TRAF proteins. When associated, TRIP inhibits the TRAF2-mediated NF-κB activation that is required for cell activation and also for protection against apoptosis. Thus, TRIP acts as a receptor–proximal regulator that may influence signals responsible for cell activation/proliferation and cell death induced by members of the TNFR superfamily.Members of the TNF receptor (TNFR)¹ superfamily play important roles in the induction of diverse signals leading to cell growth, activation, and apoptosis (1). Whether the signals induced by a given receptor leads to cell activation or death is, however, highly cell-type specific and tightly regulated during differentiation of cells. For example, the TNFRs can exert costimulatory signals for proliferation of naive lymphocytes but also induce death signals required for deletion of activated T lymphocytes (1). The cytoplasmic domains of these receptors lack intrinsic catalytic activity and also exhibit no significant homology to each other or to other known proteins. Exceptions to this include Fas(CD95) and TNFR1 that share a significant homology within an 80–amino acid region of their cytoplasmic tails (called the “death domain”; 2, 3). Therefore, it is suggested that the TNFR family members can initiate different signal transduction pathways by recruiting different types of intracellular signal transducers to the receptor complex (1).Indeed, several types of intracellular signal transducers have been identified that initiate distinct signal transduction pathways when recruited to the members of TNFR superfamily (4–19). Recent biochemical and molecular studies showed that a class of signal-transducing molecules are recruited to Fas(CD95) or TNFR1 via interaction of the death domains (2, 3, 6, 12, 17, 20). For example, Fas(CD95) and TNFR1 recruit FADD(MORT1)/RIP or TRADD/FADD (MORT1)/RIP through the interactions of their respective death domains (2, 3, 6, 12, 17, 20, 21). Clustering of these signal transducers leads to the recruitment of FLICE/ MACH, and subsequently, to cell death (13, 14).The TNFR family members can also recruit a second class of signal transducers called TRAFs (TNFR-associated factor), some of which are responsible for the activation of NF-κB or JNK (9, 20, 22). TRAF proteins were identified by their biochemical ability to interact with TNFR2, CD40, CD30, or LT-βR (4, 5, 10, 11, 15, 23–27). These receptors interact directly with TRAFs via a short stretch of amino acids within their cytoplasmic tails, but do not interact with the death domain containing proteins (4, 5, 15, 24–27). To date, five members of the TRAF family have been identified as signaling components of the TNFR family members. All TRAF members contain a conserved TRAF domain, ∼230 amino acids in length, that is used for either homo- or heterooligomerization among the TRAF family, for interactions with the cytoplasmic regions of the TNFR superfamily, or for interactions with downstream signal transducers (4, 5, 8, 10, 11, 19, 23–25, 28). In addition to the TRAF domain, most of the TRAF family members contain an NH₂-terminal RING finger and several zinc finger structures, which appear to be important for their effector functions (4, 5, 10, 11, 23–25).Several effector functions of TRAFs were revealed by recent experiments based on a transfection system. TRAF2, first identified by its interaction with TNFR2 (4), was subsequently shown to mediate NF-κB activation induced by two TNF receptors, CD40 and CD30 (9, 28–30). TRAF5 was also implicated in NF-κB activation mediated by LTβR (10), whereas TRAF3 (also known as CRAF1, CD40bp, or LAP1; references 5, 11, 24, and 25) was shown to be involved in the regulation of CD40-mediated CD23 upregulation in B cells (5). The role of other TRAF members in the TNFR family–mediated signal transduction is not clear. They may possess some effector functions as yet to be revealed, or work as adapter proteins to recruit different downstream signal transducers to the receptor complex. For example, TRAF1 is required for the recruitment of members of the cellular inhibitor of apoptosis protein (c-IAP) family to the TNFR2-signaling complex (7). In addition to the signal transduction by the TNFR family members, TRAFs may regulate other receptor-mediated signaling pathways. For example, TRAF6 is a component of IL-1 receptor (IL1R)–signaling complex, in which it mediates the activation of NF-κB by IL-1R (31). Since TRAFs form homo- or heterooligomers, it is suggested that the repertoire of TRAF members in a given cell type may differentially affect the intracellular signals triggered by these receptors. This may be accomplished by the selective interaction of TRAFs with a specific set of downstream signal transducers. Although many aspects of TRAF-mediated effector functions leading to cellular activation have been defined, it needs to be determined whether TRAF proteins will also mediate the apoptotic signals induced by the “death-domain-less” members of the TNFR superfamily (1, 27, 32–36).Here we report the isolation and characterization of a novel component of the TNFR superfamily/TRAFs signaling complex, named TRIP (TRAF-interacting protein). TRIP associates with the receptor/TRAF signaling complex, and inhibits the TRAF2-mediated NF-κB activation. Biochemical studies indicate that TRIP associates with the TNFR2 or CD30 receptor complex via its interaction with TRAF proteins, suggesting a model which can explain why the ligation of these receptors can promote different cell fates: proliferation or death.     

Effect of diabetes on severity and hospital mortality in patients with acute pancreatitis: a national population-based study

OBJECTIVE

Diabetes may increase the risk of acute pancreatitis (AP). We aimed to further investigate whether diabetes may also adversely affect outcomes of patients with AP.

RESEARCH DESIGN AND METHODS

In this retrospective cohort study, we compared 18,990 first-attack AP with diabetes to 37,980 matched control subjects from Taiwan’s National Health Insurance Research Database between 2000 and 2009. Primary outcomes were development of severe AP, defined by a modified Atlanta classification scheme, and hospital mortality. Analyses were performed using univariable and multivariable logistic regression model with generalized estimating equations accounting for hospital clustering effect.

RESULTS

After baseline characteristics were adjusted, AP patients with diabetes had a higher risk of a severe attack than their nondiabetic counterparts (adjusted odds ratio [OR] 1.21, 95% CI 1.16–1.26). When severity criteria were analyzed individually, diabetic AP patients had a 58% higher risk of intensive care unit admission and a 30% higher risk of local complications, but a 16% lower risk of gastrointestinal bleeding, than AP patients without diabetes. The risk of organ failure at least one system) was similar between the two groups. Conversely, AP patients with diabetes were associated with a lower risk of hospital mortality (adjusted OR 0.77, 95% CI 0.65–0.91).

CONCLUSIONS

Although diabetes may adversely affect the disease process of AP, it seems to protect patients from AP-related mortality.Acute pancreatitis (AP) is an acute inflammatory disease of the pancreas. The local inflammation is usually self-limited within a few days, but it can be destructive and cause a severe local complication and/or systemic reaction leading to organ failures and death. Although the case-fatality rate has been decreasing over the decades (1,2), severe cases still carry a high mortality (20–50%) and consume nearly half of the resources and costs incurred by all patients with AP (3). Accordingly, many efforts have been made to identify correlates of severity and predictors for mortality in patients with AP (4–6).In addition to older people (7), patients with certain comorbidities, such as obesity (8), hypertriglyceridemia (9), chronic renal failure (10), and systemic lupus erythematosus (11), are shown to be associated with greater risk of not only the incidence but also the severity and mortality of AP. Among various comorbidities, diabetes mellitus is relatively common in patients with AP; the prevalence was 11% in Japan (12), 17.7% in California (U.S.), (13) and 19.3% in Taiwan (3). These figures are expected to continuously increase in the future because diabetic patients not only are at risk for developing AP (14–16) but also are growing in prevalence worldwide (17). Nonetheless, the effect of diabetes on outcomes of patients with AP has not been adequately studied, and the results of available reports are inconsistent (13,18). For example, Frey and colleagues examined the effect of comorbidities on patients with AP and found that diabetes was not associated with early mortality (13), whereas Graham and coworkers assessed the effect of diabetes on critically ill patients and showed a reduced risk of hospital mortality in a subgroup patients with AP (18). In both studies, however, the effect of diabetes was not specifically examined and detailed analyses were not performed (13,18).In a recent national population-based study on Taiwanese patients with first-attack AP, we found that the prevalence of diabetes increased from 15.6% in 2000 to 2001 to 19.7% in 2008 to 2009 (1). In this study, we used the same cohort (1) to further investigate the effect of diabetes on outcomes of these patients. Because diabetic patients are likely to have a higher comorbid burden and hence a poorer reserve for acute illnesses, we hypothesized that diabetes is associated with a higher risk of severe attacks and hospital mortality in adult patients with first-attack AP.     

Anemia,red blood cell transfusion,and outcomes after severe traumatic brain injury

In the previous issue of Critical Care, Sekhon and colleagues report that mean 7-day hemoglobin concentration <90 g/l was associated with increased mortality among patients with severe traumatic brain injury (TBI). The adverse relationship between reduced hemoglobin concentrations and outcomes among those with TBI has been an inconsistent finding across available studies. However, as anemia is common among adults with severe TBI, and clinical equipoise may exist between specialists as to when to transfuse allogeneic red blood cells, randomized controlled trials of liberal versus restricted transfusion thresholds are indicated.In the previous issue of Critical Care, Sekhon and colleagues conducted a single-center retrospective cohort study to determine whether hemoglobin concentration was associated with outcomes among 273 critically ill adults with severe traumatic brain injury (TBI) [1]. After adjusting for age, Glasgow Coma Scale scores, external ventricular drain insertion, and allogeneic red blood cell (RBC) transfusion, the authors report that the estimated odds of in-hospital mortality among patients with a mean 7-day hemoglobin concentration <90 g/l was 3.1 (95% confidence interval, 1.5 to 6.3) times the estimated odds of in-hospital mortality among those with a mean 7-day hemoglobin concentration ≥90 g/l.Anemia is common among ICU patients [2]. The etiology of ICU anemia is multifactorial and includes the negative effects of the systemic inflammatory response on hematopoiesis, frequent phlebotomy, and hemodilution from intravenous fluid resuscitation [2]. Among ICU patients with TBI, the prevalence of reduced hemoglobin concentration ranges from 22 to 69%, depending on the presence or absence of extracranial hemorrhage and the timing of hemoglobin measurements [3].Although a hemoglobin transfusion threshold >70 g/l was adopted for ICU patients following publication of the Transfusion Requirements in Critical Care trial [4], this target may be poorly tolerated by those with severe TBI [2]. Anemia-induced compensatory mechanisms result in cerebral arteriolar dilatation and increased brain blood flow [2], which could be detrimental for those with cerebral edema or intracranial hypertension. Moreover, as brain tissue oxygen tension is dependent on systemic hemoglobin, reduced hemoglobin concentrations among those with TBI could decrease cerebral oxygen delivery and contribute to brain hypoxia [2].Although the findings of the study by Sekhon and colleagues provide support for the above physiologic concerns regarding reduced hemoglobin concentrations following brain injury [1], the adverse relationship between anemia and clinical outcomes is an inconsistent finding among available clinical studies [1,5-16]. Of the one randomized controlled trial [10] and the now 14 available cohort studies of which we are aware (two of which were based on post-hoc analyses of similar datasets derived from randomized controlled trials) [1,5-9,11-18], eight reported an association between anemia and an increased risk of poor neurological outcomes or mortality [1,5,9,11,13,14,17,18], while the remaining seven observed no such association. Moreover, in a recent systematic review of comparative studies, insufficient evidence was found to support a difference in outcomes between higher and lower hemoglobin levels among mostly TBI patients [19].Possible explanations for the inconsistency in results across studies include differences in TBI severity among study patients and inadequate consideration of the effects of anemia during critical time periods [20]. Although a set hemoglobin threshold may exist under which harm may occur among those with TBI, adverse outcomes may be more likely to occur during times of low cerebral blood flow, brain hypoxia, and/or ineffective autoregulation [16,20]. Some support for this argument was afforded by the findings of a recent retrospective cohort study, which reported that although anemia alone did not appear to be detrimental among patients with severe TBI, the simultaneous combination of anemia and brain hypoxia was linked with an increased risk of unfavorable outcomes [16].Another significant limitation of the existing literature on this topic has been the absence of a defined disease-exposure relationship among patients with TBI. Although it is plausible that development of reduced hemoglobin concentrations may be most important during the first 7 days following severe TBI [1], the use of the mean as a summary measure of exposure has the potential to result in exposure misclassification. Moreover, as the effects of anemia on outcomes following TBI are likely to be small, and a tremendous amount of brain hypoxia due to anemia would probably be needed to increase mortality, a sensitive measure of neurological performance or outcome is probably a more important outcome variable [20].Possibly the most important limitation of the available literature relating anemia to outcomes among those with TBI, however, is the inadequate consideration of the effects of RBC transfusion [19]. Although RBC transfusion often results in a small incremental increase in brain tissue oxygen tension in this patient population, transfused blood has important differences from the patients'' own blood and does not always improve cerebral metabolism [2]. Moreover, at least five retrospective cohort studies have reported that RBC transfusion increases the risk of death or worsened neurological outcome among those with TBI [9,13,14,21,22]. Admittedly, however, these observations could have been related to selection bias and an unbalanced distribution of outcome determinants between treatment groups [20]. Moreover, as anemia and RBC transfusion are probably highly correlated, those studies that used interaction terms for anemia and RBC transfusion in their regression models probably introduced multicollinearity, and therefore their estimated coefficients and odds ratios may be invalid [20].In summary, although preclinical experiments suggest several potential adverse effects of anemia among patients with TBI, the results of the available clinical studies are conflicting, and it remains unclear whether RBC transfusion may further increase risk of adverse outcomes. However, because anemia is common among adults with severe TBI, and a recent survey reported that clinical equipoise may exist among specialists as to when to transfuse allogeneic RBCs [23], randomized controlled trials of liberal versus restricted transfusion thresholds are required among adults with severe TBI. These trials will probably require use of multimodal monitoring to understand whether improved outcomes are only witnessed among those with simultaneous signs of brain hypoxia or cerebral ischemia.     

A New Mechanism of NK Cell Cytotoxicity Activation: The CD40–CD40 Ligand Interaction

NK recognition is regulated by a delicate balance between positive signals initiating their effector functions, and inhibitory signals preventing them from proceeding to cytolysis. Knowledge of the molecules responsible for positive signaling in NK cells is currently limited. We demonstrate that IL-2–activated human NK cells can express CD40 ligand (CD40L) and that recognition of CD40 on target cells can provide an activation pathway for such human NK cells. CD40-transfected P815 cells were killed by NK cell lines expressing CD40L, clones and PBLderived NK cells cultured for 18 h in the presence of IL-2, but not by CD40L-negative fresh NK cells. Cross-linking of CD40L on IL-2–activated NK cells induced redirected cytolysis of CD40-negative but Fc receptor-expressing P815 cells. The sensitivity of human TAP-deficient T2 cells could be blocked by anti-CD40 antibodies as well as by reconstitution of TAP/MHC class I expression, indicating that the CD40-dependent pathway for NK activation can be downregulated, at least in part, by MHC class I molecules on the target cells. NK cell recognition of CD40 may be important in immunoregulation as well as in immune responses against B cell malignancies.NK cells represent a distinct lineage of lymphocytes that are able to kill a variety of tumor (1), virus-infected (2), bone marrow transplanted (3), and allogeneic target cells (4). NK cells do not express T cell receptors or immunoglobulins and are apparently normal in mice with defects in the recombinase machinery (5, 6).Our knowledge about NK cell specificity has increased considerably in the last years. NK cells can probably interact with target cells by a variety of different cell surface molecules, some involved in cell adhesion, some activating the NK cytolytic program (7, 8), and other ones able to inhibit this activation by negative signaling (as reviewed in reference 9).A common feature of several inhibitory NK receptors is the capability to bind MHC class I molecules (10, 11), as predicted by the effector inhibition model within the missing self hypothesis of recognition by NK cells (12–14). Interestingly, the MHC class I receptors identified so far belong to different gene families in mouse and man; these are the p58/p70/NKAT or killer cell inhibitory receptors (KIR)¹ of the immunoglobulin superfamily in man and the Ly49 receptors of the C-type lectin family in the mouse. There is also evidence that MHC class I molecules can be recognized as triggering signals in NK cells of humans, rats as well as mice (13). The inhibitory receptors allow NK cells to kill tumor or normal cell targets with deficient MHC class I expression (12, 14). This does not exclude that other activating pathways can override inhibition by MHC class I molecules (15) and, even in their absence, there must be some activating target molecules that initiate the cytolytic program. Several surface molecules are able to mediate positive signals in NK cells. Some of these structures, like NKRP1 (16), CD69 (17), and NKG2 (18) map to the NK complex region (NKC) of chromosome 6 in mice and of chromosome 12 in humans (13). CD2 (19) and CD16 (20) molecules can also play a role in the activation pathway.NK cells resemble T cells in many respects, both may arise from an immediate common progenitor (21, 22), and share the expression of several surface molecules (23). NK cells produce cytokines resembling those secreted by some helper T cell subsets (24) and contain CD3 components in the cytoplasm (21). The expression of some surface structures, involved in TCR-dependent T cell costimulation, like CD28 in human (25), has been described on NK cells, but the functional relevance of these molecules for NK activation processes has not been fully established.Another T cell molecule of interest is CD40L, which interacts with CD40, a 50-kD membrane glycoprotein expressed on B cells (26), dendritic cells (27), and monocytes (28). CD40 is a member of the tumor necrosis factor/nerve growth factor receptor family (29) which includes CD27 (30), CD30 (31), and FAS antigen (32). Murine and human forms of CD40L had been cloned and found to be membrane glycoproteins with a molecular mass of ∼39 kD induced on T cells after activation (33). Also mast cells (34), eosinophils (35), and B cells (36) can be induced to express a functional CD40L. The CD40L–CD40 interaction has been demonstrated to be necessary for T cell–dependent B cell activation (33, 37). Mutations in the CD40L molecule cause a hyper-IgM immunodeficiency condition in man (38, 39, 40). On the other hand, CD40–CD40L interactions also orchestrate the response of regulatory T cells during both their development (41, 42) and their encounter with antigen (43, 44).NK cells have also been suggested to play a role in B cell differentiation and immunoglobulin production (45). Therefore, it was of interest to investigate whether NK cells could use a CD40-dependent pathway in their interactions with other cells. Therefore, we have investigated the ability of target cells expressing CD40 to induce activation of NK cytotoxicity.     

Renal Hyperfiltration Is a Determinant of Endothelial Function Responses to Cyclooxygenase 2 Inhibition in Type 1 Diabetes

OBJECTIVE

Our aim was to examine the effect of cyclooxygenase 2 (COX2) inhibition on endothelial function in subjects with type 1 diabetes analyzed on the basis of renal filtration status.

RESEARCH DESIGN AND METHODS

Flow-mediated dilation (FMD) was determined in type 1 diabetic subjects and hyperfiltration (glomerular filtration rate ≥135 ml/min/1.73 m², n = 13) or normofiltration (glomerular filtration rate ≥135 ml/min/1.73 m², n = 11). Studies were performed before and after celecoxib (200 mg daily for 14 days) during euglycemia and hyperglycemia.

RESULTS

Baseline parameters were similar in the two groups. Pretreatment, FMD was augmented in normofiltering versus hyperfiltering subjects during clamped euglycemia (10.2 ± 5.3% vs. 5.9 ± 2.3%, P = 0.003). COX2 inhibition suppressed FMD in normofiltering (10.2 ± 5.3% to 5.8 ± 3.4%, P = 0.006) versus hyperfiltering subjects (ANOVA interaction, P = 0.003).

CONCLUSIONS

Systemic hemodynamic function, including the response to COX2 inhibition, is related to filtration status in diabetic subjects and may reflect general endothelial dysfunction.Renal hyperfiltration is associated with an increased risk of progression to diabetic nephropathy in many, but not all, studies (1). Diabetic hyperfiltration may in part be due to cyclooxygenase 2 (COX2) upregulation (2,3). We have previously identified a cohort of subjects with uncomplicated type 1 diabetes who exhibit hyperfiltration (glomerular filtration rate [GFR] ≥135 ml/min/1.73 m²) or normofiltration (GFR <135 ml/min/1.73 m²) during clamped euglycemia (4). In hyperfiltering subjects, COX2 inhibition reduces GFR, whereas in subjects with normofiltration, COX2 inhibition is associated with an opposite GFR rise and an exaggerated suppression of vasodilatory prostaglandins (4). Together with previous observations (4–6), these findings suggest that hyperfiltering and normofiltering individuals are physiologically distinct.Previous studies have suggested that early type 1 diabetes is characterized by a state of generalized vasodilation due to nitric oxide upregulation (7). The role of COX2 in the systemic vasculature in humans with early type 1 diabetes is, however, incompletely understood (8–11). Accordingly, our goal was to study the effect of COX2 inhibition on endothelial function in diabetic subjects with hyperfiltration or normofiltration. Our hypothesis was that renal hemodynamic differences would also be reflected in the systemic circulation.