Uric acid and incident chronic kidney disease in a large health check-up population in Taiwan

Aim: Uric acid (UA) is strongly associated with the confirmed chronic kidney disease (CKD) risk factors, such as hypertension, diabetes and metabolic syndrome (MS); however, whether higher UA is independently associated with CKD is still debatable. Other studies found that low UA level may reflect inadequate protection against oxidant‐mediated stress; it is also unknown whether hypouricemia may have a harmful effect on the kidney. No studies have examined whether there is a J‐shaped relationship between UA and incident CKD. Methods: The association between UA and incident kidney disease (Glomerular filtration rate <60 mL/min per 1.73 m²) was examined among 94 422 Taiwanese participants, aged ≥20 years with a mean 3.5 years follow‐up in a retrospective cohort. The association between UA and CKD was evaluated using Cox models with adjustment for confounders. Results: The adjusted hazard ratio (HR) for incident CKD was 1.03 (95% confidence interval (CI), 1.01 to 1.06) for baseline UA level (increase by 1 mg/dL). Compared with serum UA in the first quintile (2.0 to 4.5 mg/dL), the multivariate‐adjusted HR for CKD of the fifth (≥7.3 mg/dL), fourth (6.3 to 7.2 mg/dL), third (5.5 to 6.2 mg/dL), second (4.6 to 5.4 mg/dL) and hyopuricemia (<2.0 mg/dL) were 1.15 (95%CI, 1.01–1.30), 0.98 (95%CI, 0.87–1.10), 1.06 (95%CI, 0.94–1.19), 1.02 (95%CI, 0.91–1.14) and 1.65(95%CI, 0.53–5.15), respectively. The tests for the non‐linear association were all not significant for both male and female. Gender‐specific model revealed only the UA above 7.3 mg/dL with the increased risk of new‐onset CKD in males. Conclusion: Hyperuricemia is a risk factor for CKD in Taiwan, future studies are still necessary to determine whether hypouricemia increases the risk of CKD.     

Uric acid and the kidney

Uric acid, the end product of purine metabolism, is excreted predominantly by the proximal tubules. Abnormal serum levels of uric acid are due to alterations in production or excretion. Fractional excretion of uric acid is helpful in determining the underlying etiology of hypouricemia or hyperuricemia in children. Abnormalities in the molecular mechanisms that control renal uric acid tubular transport are implicated in various disorders associated with abnormal uric acid levels. Gout is rare in children; yet its presence necessitates evaluation for enzymatic defects in purine metabolism. Well-known effects of uric acid on the kidney include nephrolithiasis and acute kidney injury (AKI) in the setting of tumor lysis. However, recent data suggest that uric acid may be an important factor in the pathogenesis of AKI in general, as well as of chronic kidney disease (CKD) and hypertension. Hence, uric acid may not only be a marker but also a potential therapeutic target in kidney disease. Nonetheless, because of confounders, more studies are needed to clarify the association between uric acid and multifactorial disorders of the kidney.     

High-normal albuminuria and incident chronic kidney disease in a male nondiabetic population

Background

High-normal albuminuria is an important risk factor for incident chronic kidney disease in diabetic populations, in contrast to an uncertain association in nondiabetic populations. This study aimed to reveal the relationship between high-normal albuminuria and incident chronic kidney disease in a Japanese nondiabetic population.

Methods

A 10-year follow-up retrospective cohort study was performed involving 1378 Japanese men (mean age 44?±?5.3 years) without chronic kidney disease and diabetes mellitus. Chronic kidney disease was diagnosed as either estimated glomerular filtration rate <?60 mL/min/1.73 m² or a urine albumin-to-creatinine ratio ≥?30 mg/g.

Results

At baseline, age, estimated glomerular filtration rate, and the presence of hematuria, hypertension, and dyslipidemia were independently associated with the albumin-to-creatinine ratio. Among the 1378 participants, 185 (13.4%) fulfilled diagnostic criteria for chronic kidney disease over the 10-year follow-up period. Median annual estimated glomerular filtration rate decline showed a deterioration with increasing quartiles of baseline albumin-to-creatinine ratio (P?=?0.004). Participants who had a baseline albumin-to-creatinine ratio in the highest quartile (5.9–28.9 mg/g) were more likely to develop micro- or macroalbuminuria (odds ratio: 16.23, 95% confidence interval 6.56–54.03), chronic kidney disease (odds ratio: 2.48, 95% confidence interval 1.64–3.82), and hypertension (odds ratio 2.06, 95% confidence interval 1.30–3.31), but not diabetes mellitus compared with those who had an albumin-to-creatinine ratio in the lowest quartile (1.3–3.6 mg/g) after adjustment for potential confounders.

Conclusions

High-normal albuminuria was associated with incident chronic kidney disease in this Japanese nondiabetic male population.

     

Uric acid transporter ABCG2 is increased in the intestine of the 5/6 nephrectomy rat model of chronic kidney disease

Background

Uric acid (UA) remains a risk factor of chronic kidney disease (CKD). Therefore, it is important to clarify the mechanism of UA excretion in CKD. The specific mechanisms of extrarenal excretion from the intestine are unknown. We evaluated the expression of the UA transporter in the intestinal tract—the ATP-binding cassette transporter G2 (ABCG2)—in a 5/6 nephrectomy rat model of CKD.

Methods

Male Wistar rats (6 weeks old) were randomly assigned to the 5/6 nephrectomized (Nx) group or the sham-operated control group. Urine and blood samples were collected every 4 weeks. All the rats were killed at 8 weeks to obtain liver, duodenum, jejunum, ileum, and transverse colon tissues. Uricase activity was measured in the liver. Expression of ABCG2 in intestinal mucosa was measured with real time polymerase chain reaction (PCR).

Results

The Nx group showed significantly decreased urine UA excretion/body weight and UA clearance compared to the control group at 4 and 8 weeks after nephrectomy. In contrast, serum UA and uricase activity were not significant. The expression of ABCG2 in the ileum of the Nx group showed significantly increased upregulation, while no changes were seen in the intestines of the control group.

Conclusions

The Nx rats exhibited lower excretion of urine UA and over-expression of ABCG2 in the ileum. The fact that serum UA did not increase despite the decrease in UA excretion suggests that an excretory pathway other than the kidney, probably the intestine, may operate in a complementary role that corroborates the increase in ABCG2 expression in the ileum.     

Uric acid accumulated in isolated kidney grafts: An assessment of agonal ischaemia

Hypoxanthine-xanthine washed out from kidney tissue during preservation has previously been found to be a reliable measure of thein vitro assessment of the deleterious effect of ischaemia on the functional regenerative ability of the graft. We have now studied the question: can uric acid accumulation in the isolated kidney graft be employed as a retrospective measure of the agonal ischaemia? We have found that uric acid alone accumulates in renal tissue during the agonal phase, in an amount that remains unchanged during the subsequentin vitro ischaemia. The determination of hypoxanthine-xanthine and uric acid in kidney perfusate samples during preservation must thus be presumed to be an optimal clinical-chemical method of assessing the graft ischaemia.     

Uric acid and transplantation

Hyperuricemia is a common complication in organ transplant recipients, with a higher incidence in kidney and heart recipients. Risk factors for post-transplant hyperuricemia include reduced glomerular filtration rate, diuretic use, cyclosporine therapy, increasing age at transplant, obesity, and metabolic syndrome, as well as the presence of pretransplant hyperuricemia. The impact of hyperuricemia in patient and graft survival is unclear because uric acid only recently has been considered a risk factor for cardiovascular disease and graft survival. The effect of uric acid on graft function remains controversial, with studies suggesting that uric acid is an independent risk factor for chronic allograft dysfunction, contrasting with other studies suggesting that hyperuricemia is only a marker of reduced glomerular filtration rate. Strategies to reduce uric acid levels include reduction or avoidance of cyclosporine treatment, adequacy of antihypertension treatment, avoidance of diuretics, nutritional management, and use of uric acid–lowering agents. In this article, we review the incidence and risk factors for the development of post-transplant hyperuricemia, the effect of different immunosuppressive regimens in uric acid handling, and recent results from studies comparing uric acid levels and renal function in organ transplant recipients that try to identify which comes first: hyperuricemia or chronic allograft dysfunction?     

Uric acid and transplantation

Hyperuricemia is a common complication in organ transplant recipients, and frequently is associated with chronic cyclosporine immunosuppressive therapy. Kidney and heart transplant recipients are prone to develop posttransplant hyperuricemia. Risk factors for hyperuricemia include decreased glomerular filtration rate (GFR), diuretic use, and preexistent history of hyperuricemia. The influence of hyperuricemia in patient and graft survival is unclear because uric acid is not usually considered a common risk factor for cardiovascular disease that affects graft and patient survival. However, there have been small studies that have suggested that control of uric acid levels contributes to recovery of renal function (in heart and liver transplant recipients) and in an improvement in GFR in renal transplant recipients. Despite controversies in the need for hyperuricemia treatment in transplant patients, strategies to decrease uric acid levels includes a decrease or avoidance of cyclosporine treatment, adequacy of antihypertension treatment, avoidance of diuretics, nutritional management, and use of uric acid-decreasing agents. In this article we review the incidence and risk factors for the development of posttransplant hyperuricemia, discuss the influence of different immunosuppressive agents on uric acid metabolism, and suggest some alternative treatments for posttransplant hyperuricemia. We also consider that uric acid should be considered as a potential risk factor for renal allograft nephropathy or for renal dysfunction in nonrenal transplant recipients, as well as a comorbid factor for a decrease in patient and graft survival.     

Uric acid and hypertension

A link between serum uric acid and the development of hypertension was first hypothesized in the 1870s. Although numerous epidemiologic studies in the 1980s and 1990s suggested an association, relatively little attention was paid to it until recently. Animal models have suggested a two-step pathogenesis by which uric acid initially activates the renin angiotensin system and suppresses nitric oxide, leading to uric acid–dependent increase in systemic vascular resistance, followed by a uric acid–mediated vasculopathy, involving renal afferent arterioles, resulting in a late sodium-sensitive hypertension. Initial clinical trials in young patients have supported these mechanisms in young patients but do not yet support pharmacologic reduction of serum uric acid as first-line therapy for hypertension.     

Uric acid and preeclampsia

Increased uric acid level is a key clinical feature of preeclampsia; higher levels correlate with significant maternal and fetal morbidity and mortality. The cause of hyperuricemia and its specific role in the pathogenesis of preeclampsia, however, remain unclear. Although uric acid has been shown to roughly parallel the severity of the maternal syndrome, it has not been useful in predicting the development of preeclampsia. Nevertheless, there have been recent data supporting a pathogenic role potentially in the hypertension and endothelial cell dysfunction of preeclampsia. This article reviews our current understanding of hyperuricemia in the setting of preeclampsia, and highlights the hypothesis that hyperuricemia may contribute to vascular damage in preeclampsia.