Molecular mechanisms of altered cholesterol metabolism in rats with spontaneous focal glomerulosclerosis

BACKGROUND: Imai rats exhibit spontaneous focal glomerulosclerosis (FGS), which is marked by heavy proteinuria, severe hyperlipidemia, and progressive renal insufficiency beginning at 8 to 10 weeks of age. In an earlier study, we reported severe skeletal muscle and adipose tissue lipoprotein lipase, and very low-density lipoprotein (VLDL) receptor deficiencies, which account for elevated plasma VLDL and triglycerides in Imai rats at 34 weeks of age. In this study, we investigated key factors involved in cholesterol metabolism. METHODS: Male Imai and Sprague-Dawley control rats were fed a regular rat chow and observed from age 8 through 34 weeks. Hepatic 3-hydroxy-3 methylglutaryl coenzyme A (HMG-CoA) reductase, cholesterol 7alpha-hydroxylase, low-density lipoprotein (LDL) receptor and acyl Co A:cholesterol acyltransferase (ACAT) were measured by Western blot and plasma lecithin:cholesterol acyltransferase (LCAT) protein was measured by enzyme-linked immunosorbent assay (ELISA). RESULTS: At 34 weeks of age, the Imai rats showed severe proteinuria, hypoalbuminemia, 60% reduction in glomerular filtration rate (GFR), elevated plasma total and LDL cholesterol and LDL/high-density lipoprotein (HDL) ratio. Imai rats showed a twofold elevation of hepatic HMG-CoA reductase, the rate-limiting step in cholesterol biosynthesis, but no significant change in cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in cholesterol catabolism to bile acids. This was accompanied by and largely due to a threefold down-regulation of hepatic LDL receptor, which limits hepatic uptake of LDL; and a threefold up-regulation of hepatic ACAT (P < 0.01), which augments esterification of hepatocyte free cholesterol, thus, limiting cholesterol-mediated feedback regulation of cholesterol synthesis and catabolism. Moreover, plasma LCAT concentration was severely depressed (by fourfold) in Imai rats. This abnormality can impair HDL-mediated cholesterol transport from extrahepatic tissues to the liver. CONCLUSION: The study revealed marked abnormalities of the key proteins involved in regulation of hepatic cholesterol metabolism. These abnormalities can account for severe dysregulation of cholesterol metabolism in Imai rats with spontaneous FGS, which closely resembles FGS in humans.     

Hepatic HDL receptor, SR-B1 and Apo A-I expression in chronic renal failure.

BACKGROUND: Chronic renal failure (CRF) is associated with hypertriglyceridaemia and depressed plasma high-density lipoprotein (HDL)-cholesterol and apolipoprotein A-I (Apo A-I) concentrations. Uraemic hypertriglyceridaemia is due, in part, to lipoprotein lipase and hepatic lipase deficiencies, which are causally linked to excess parathormone (PTH). This study was designed to test the hypothesis that depressed plasma concentration and abnormal composition of HDL in CRF may be due to dysregulation of hepatic expression of Apo A-I and/or the newly discovered HDL receptor. METHODS: Hepatic Apo A-I and HDL receptor mRNA abundance (Northern blot), and HDL receptor protein mass (Western blot) were determined in CRF rats (5/6 nephrectomy), parathyroidectomized CRF rats (CRF-PTx) and sham-operated controls. RESULTS: The CRF group exhibited normal hepatic HDL receptor mRNA and HDL receptor protein abundance coupled with reduced hepatic Apo A-I mRNA. Hepatic Apo A-I mRNA, HDL receptor mRNA and protein abundance were not affected by PTx. CONCLUSIONS: CRF results in the down-regulation of hepatic Apo A-I gene expression, which accounts for the known reduction in plasma Apo A-I concentration. However, CRF does not affect HDL receptor mRNA or protein expression in this model. Neither Apo A-I nor HDL receptor expression were modified by PTx in CRF rats.     

HMG-CoA reductase,cholesterol 7alpha-hydroxylase,LCAT, ACAT,LDL receptor,and SRB-1 in hereditary analbuminemia

BACKGROUND: Hereditary analbuminemia is associated with hypercholesterolemia, which has been shown to be primarily caused by increased extrahepatic production of cholesterol. Nagase rats with hereditary analbuminemia (NAR) have been used as a model to dissect the effect of primary hypoalbuminemia from that caused by proteinuria in nephrotic syndrome. The present study was undertaken to explore the effect of hereditary analbuminemia on protein expression of the key factors involved in cholesterol metabolism. METHODS: Hepatic tissue protein abundance of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, cholesterol 7alpha-hydroxylase (a rate-limiting enzyme in cholesterol catabolism), low density lipoprotein (LDL) receptor, high density lipoprotein (HDL) receptor (SRB-1), acyl-coA cholesterol acyltransferase-2 (ACAT-2), and plasma concentration of lecithin cholesterol acyltransferase (LCAT), as well as HMG-CoA reductase, ACAT, and LCAT activities were determined in fasting male NAR and Sprague-Dawley control rats. RESULTS: The NAR group exhibited significant up-regulation of HMG-CoA reductase protein abundance but normal HMG-CoA reductase enzymatic activity. This was coupled with a significant up-regulation of cholesterol 7alpha-hydroxylase and a mild up-regulation of ACAT protein abundance and activity. However, hepatic LDL receptor and HDL receptor and plasma LCAT protein concentration and activity were normal in NAR. CONCLUSION: Hypercholesterolemia in NAR is associated with elevated hepatic HMG-CoA reductase protein abundance, but normal HMG-CoA reductase activity. These findings point to post-translational regulation of this enzyme and favor an extrahepatic origin of hypercholesterolemia in NAR. The observed up-regulation of cholesterol 7alpha-hydroxylase represents a compensatory response to the associated hypercholesterolemia. Unlike nephrotic syndrome, which causes severe LDL receptor, HDL receptor, and LCAT deficiencies, hereditary analbuminemia does not affect these proteins.     

Cellular cholesterol efflux to plasma from proteinuric patients is elevated and remains unaffected by antiproteinuric treatment.

BACKGROUND: Lipid derangements are assumed to contribute to the elevated cardiovascular risk in proteinuric patients. The impact of proteinuria on reverse cholesterol transport (RCT) is unknown. The first step in RCT, cellular cholesterol efflux to plasma, may be altered in proteinuria, consequent to changes in pre-beta high-density lipoprotein (HDL) formation and plasma phospholipid transfer protein (PLTP) activity. METHODS: In six non-diabetic male patients with nephrotic-range proteinuria and 12 matched healthy men, plasma (apo)lipoproteins, pre-beta HDL formation, PLTP activity as well as the ability of plasma to promote cholesterol efflux out of cultured human skin fibroblasts were determined. These variables were also measured in response to antiproteinuric treatment, consisting of single and dual RAAS blockade by losartan and lisinopril. RESULTS: Plasma total cholesterol (P<0.05), triglycerides (P<0.05), apolipoprotein (apo) A-I (P<0.001), apo B (P<0.001), PLTP activity (P<0.005) and pre-beta HDL formation (P<0.001) were higher in proteinuric patients. Cellular cholesterol efflux to plasma from proteinuric patients was 41% higher than to plasma from healthy subjects (P<0.001). Reduction of proteinuria from 5.0 to 1.4 g/day by dual RAAS blockade was associated with a 23% reduction in plasma apo B levels (P<0.05). Pre-beta HDL formation and plasma PLTP activity did not change significantly. Combined antiproteinuric treatment did not reduce the elevated cellular cholesterol efflux. CONCLUSION: Cellular cholesterol efflux to plasma from patients with nephrotic-range proteinuria is enhanced, in conjunction with elevated pre-beta HDL formation and plasma PLTP activity. These changes may attenuate the cardiovascular risk associated with proteinuria-associated hyperlipidaemia. Antiproteinuric therapy lowers plasma apo B, but does not affect cell-derived cholesterol efflux, suggesting that this therapy beneficially affects cardiovascular risk in proteinuric patients.     

Effects of HMG-CoA reductase inhibition on hepatic expression of key cholesterol-regulatory enzymes and receptors in nephrotic syndrome

BACKGROUND: Hypercholesterolemia is one of the major manifestations of nephrotic syndrome. We have previously shown that nephrotic hypercholesterolemia is associated with and, in part, due to dysregulation of hepatic HMG-CoA reductase, acyl-CoA:cholesterol acyltransferase (ACAT) and cholesterol 7alpha-hydroxylase, as well as lecithin:cholesterol acyltransferase (LCAT), low-density lipoprotein (LDL) receptor and high-density lipoprotein (HDL) receptor deficiencies. This study was carried out to discern the effect of inhibition of HMG-CoA reductase on expression of the key enzymes and receptors involved in cholesterol metabolism in the liver. METHODS: Rats with puromycin-induced nephrotic syndrome were treated with either a statin (rosuvastatin 20 mg/kg/day) or placebo for 2 weeks. Placebo-treated normal rats served as controls. Gene expression, protein abundance and/or activities of relevant receptors and enzymes were quantified. RESULTS: The untreated nephrotic rats showed heavy proteinuria, hypoalbuminemia, hypercholesterolemia, elevated total cholesterol:HDL cholesterol ratio and normal creatinine clearance. This was associated with severe reductions in hepatic LDL receptor, hepatic HDL receptor and plasma LCAT concentration, marked upregulation of hepatic ACAT, and unchanged cholesterol 7alpha-hydroxylase (rate-limiting step in cholesterol catabolism). Statin administration for 2 weeks ameliorated hepatic LDL receptor and HDL receptor deficiencies and significantly lowered plasma cholesterol, LDL cholesterol, total cholesterol:HDL cholesterol ratio and proteinuria. CONCLUSIONS: HMG-CoA reductase inhibition improved hepatic LDL and HDL receptor deficiencies, and ameliorated the associated hyperlipidemia in the nephrotic rats.     

HMG-CoA reductase, cholesterol 7alpha-hydroxylase, LDL receptor, SR-B1, and ACAT in diet-induced syndrome X

BACKGROUND: Long-term consumption of Western diets can lead to acquired syndrome X, which presents with obesity, insulin resistance, hypertension, hyperlipidemia, and risk of atherosclerotic cardiovascular disease. While plasma lipid abnormalities in syndrome X have been well characterized, their molecular basis remains unclear. This study explored potential mechanisms of hypercholesterolemia in diet-induced syndrome X. METHODS: Female Fischer rats were fed a high-fat, refined-carbohydrate (sucrose) diet (HFS) or standard rat chow (low-fat, complex carbohydrate, LFCC) for 20 months. Plasma lipids and hepatic tissue mRNA, protein, and/or activities of the key enzymes and receptors involved in cholesterol metabolism were determined. RESULTS: The HFS group exhibited hypertension, hyperlipidemia, insulin resistance, obesity, significant down-regulation of hepatic cholesterol 7alpha-hydroxylase (the rate-limiting step in cholesterol catabolism) and low-density lipoprotein (LDL) receptor (LDL-R, the primary pathway of LDL clearance). In contrast, hepatic tissue acyl-coenzyme A:cholesterol acyltransferase (ACAT-2, the primary enzyme involved in intracellular esterification of cholesterol) and scavenger-receptor class B, type 1 (SR-B1 or HDL receptor) were up-regulated. While 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA expression was increased, its protein abundance and activity were unchanged, and HMG-CoA reductase-to-cholesterol 7alpha-hydroxylase ratio was increased in HFS-fed animals. CONCLUSION: Hypercholesterolemia in diet-induced syndrome X is associated with depressed cholesterol 7alpha-hydroxylase, diminished LDL-R, elevated ACAT, and increased HMG-CoA reductase-to-cholesterol 7alpha-hydroxylase ratio. These findings point to impaired hepatic catabolism and uptake of cholesterol and inappropriate cholesterol production capacity as the underlying causes of hypercholesterolemia in rats with diet-induced syndrome X.     

Down-regulation of hepatic high-density lipoprotein receptor, SR-B1, in nephrotic syndrome.

BACKGROUND: Nephrotic syndrome (NS) is a prototype of acquired hypercholesterolemia. Hepatic synthesis and removal of cholesterol play major roles in the regulation of plasma concentration of this sterol. Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) particles are the primary vehicles for cholesterol transport to the liver. We have recently demonstrated that NS results in acquired hepatic LDL receptor deficiency in rats. This study was undertaken to determine the effect of NS on hepatic expression of the newly discovered, long-sought HDL receptor. METHODS: Hepatic HDL receptor and apolipoprotein A-I (apo A-I) expressions were studied in rats with puromycin-induced NS. The results were compared with those obtained in placebo-treated, normal controls. RESULTS: The NS group exhibited a marked reduction in hepatic tissue HDL receptor protein abundance when compared with the control group. In contrast, hepatic HDL receptor mRNA abundance in the NS group was similar to that of the control group. As expected, the NS group showed a marked increase in hepatic apo A-I mRNA abundance. CONCLUSIONS: The study explored the effect of experimental NS on hepatic HDL receptor expression, and the results revealed a marked down-regulation of HDL receptor in rats with NS. In contrast, hepatic expression of Apo A-I, the principal protein constituent of HDL, was markedly increased in NS rats. The HDL receptor deficiency shown here can potentially limit the efficiency of HDL as the primary vehicle for reverse cholesterol transport in NS.     

Impaired metabolism of high density lipoprotein in uremic patients.

We measured lipoproteins, apolipoproteins, lipoprotein lipase (LPL), hepatic triglyceride lipase (HTGL), lecithin: cholesterol acyltransferase (LCAT) and parameters of calcium metabolism to evaluate the roles of these enzymes and hypertriglyceridemia for impaired high-density lipoprotein (HDL) metabolism in chronic renal failure, and to examine the impact of altered calcium homeostasis on the lipoprotein-regulating enzymes. The subjects were 25 healthy volunteers and 66 uremic patients, 24 treated with hemodialysis (HD) and 42 with continuous ambulatory peritoneal dialysis (CAPD). Lipoprotein analysis revealed: (1) reduction in HDL cholesterol especially in HDL2 subfraction; (2) increase in HDL triglyceride; and (3) decreased ratio of HDL2 cholesterol to HDL3 cholesterol in both HD and CAPD patients. Simple regression analysis showed: (1) a positive correlation between VLDL triglyceride and triglyceride/cholesterol ratio of HDL; (2) positive correlations of LPL level in post-heparin plasma to cholesterol concentrations in HDL2, HDL3 and total HDL, and to apolipoproteins A-I and A-II; and (3) inverse correlations of HTGL to HDL2 cholesterol and to the ratio of HDL2 cholesterol/HDL3 cholesterol. Multiple regression analysis of HDL cholesterol indicated positive association with LPL and inverse correlation with VLDL triglyceride. Four variables including LPL, HTGL, LCAT and VLDL triglyceride explained 51.5% of the variation of HDL cholesterol. HDL2 cholesterol was associated positively with LPL and negatively with VLDL triglyceride in the model. HDL3 cholesterol was associated positively with LPL, HTGL and LCAT and inversely with VLDL triglyceride. Stepwise multiple regression analysis indicated that independent predictors of HTGL were gender, parathyroid hormone levels by a mid-portion assay, ionized calcium and age, and that those of LCAT were ionized calcium and age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Innovation in the Treatment of Uremia: Proceedings from the Cleveland Clinic Workshop: Causes of Dysregulation of Lipid Metabolism in Chronic Renal Failure

End‐stage renal disease (ESRD) is associated with accelerated atherosclerosis and premature death from cardiovascular disease. These events are driven by oxidative stress inflammation and lipid disorders. ESRD‐induced lipid abnormalities primarily stem from dysregulation of high‐density lipoprotein (HDL), triglyceride‐rich lipoprotein metabolism, and oxidative modification of lipoproteins. In this context, production and plasma concentration of Apo‐I and Apo‐II are reduced, HDL maturation is impaired, HDL composition is altered, HDL antioxidant and anti‐inflammatory functions are depressed, clearance of triglyceride‐rich lipoproteins and their atherogenic remnants is impaired, their composition is altered, and their plasma concentration is elevated in ESRD. The associated defect in HDL maturation is largely caused by acquired lecithin‐cholesterol acyltransferase deficiency while its triglyceride enrichment is due to hepatic lipase deficiency. Hypertriglyceridemia, abnormal composition, and impaired clearance of triglyceride‐rich lipoproteins and their remnants are mediated by down‐regulation of lipoprotein lipase, hepatic lipase, very low‐density lipoprotein (VLDL) receptor, and LDL receptor‐related protein, relative reduction in ApoC‐II/ApoC‐III ratio, up‐regulation of acyl‐CoA cholesterol acyltransferase, and elevated plasma level of cholesterol ester‐poor prebeta HDL. Impaired clearance and accumulation of oxidation‐prone VLDL and chylomicron remnants and abnormal LDL composition in the face of oxidative stress and inflammation favors their uptake by macrophages and resident cells in the artery wall. The effect of heightened influx of lipids is compounded by impaired HDL‐mediated reverse cholesterol transport leading to foam cell formation which is the central event in atherosclerosis plaque formation and subsequent plaque rupture, thrombosis, and tissue damage.     

Down-regulation of lipoprotein lipase and VLDL receptor in rats with focal glomerulosclerosis.

BACKGROUND: Patients and animals with nephrotic syndrome and those with chronic renal failure (CRF) often exhibit hypertriglyceridemia and impaired very low-density lipoprotein (VLDL) clearance. Imai rats that were originally derived from Sprague-Dawley rats develop spontaneous proteinuria, hyperlipidemia, progressive renal insufficiency and histologic changes of focal glomerulosclerosis (FGS), closely resembling human FGS. This study was undertaken to test the hypothesis that elevation of plasma triglyceride and VLDL concentrations in the Imai rats is associated with deficiency of lipoprotein lipase (LPL) and VLDL receptor which are the main pathways of triglyceride-rich lipoprotein clearance. METHODS: Male Imai and Sprague-Dawley control rats were fed regular rat chow and studied at 10 and 34 weeks of age. Tissue LPL and VLDL-r protein abundance (Western analysis) and post-heparin lipolytic activity were determined. RESULTS: At 10 weeks of age, Imai rats showed mild proteinuria, moderate hyperlipidemia, normal creatinine clearance and blood pressure. By 34 weeks of age, the study animal exhibited severe proteinuria, marked hyperlipidemia, significant renal insufficiency and hypertension. This was associated with a severe progressive reduction in skeletal muscle and adipose tissue LPL and VLDL-r protein abundance and depressed plasma post heparin, lipolytic activity. CONCLUSION: Progressive hyperlipidemia in the Imai rats with spontaneous FGS is accompanied by severe combined LPL and VLDL-r deficiencies that can, in part, account for the associated hypertriglyceridemia and elevated plasma VLDL concentrations.     

Decreased plasma lecithin: cholesterol acyltransfer and associated changes in plasma and red cell lipids in uraemia

Plasma lipids, lecithin : cholesterol acyltransferase (LCAT)activity and erythrocyte lipid composition were compared fora group of newly diagnosed uraemic patients and a group of healthysubjects. Plasma triacylglycerol was increased and both totaland high-density lipoprotein (HDL) cholesterol were decreased.A lower percentage of total cholesterol in patients' plasmawas in the esterified form and plasma values of the phospholipid,lysolecithin, were also lower. The plasma LCAT activity of uraemicpatients, whether expressed as nmol or percentage of cholesterolesterified per hour, was significantly lower than for normals.Both LCAT activity and lysolecithin in uraemic plasma were inverselycorrelated with the concentration of urea. The lipid compositionof erythrocytes from patients was also abnormal, with both freecholesterol and lecithin being increased. These results areconsistent with the occurrence of an acquired deficiency ofLCAT in uraemia, comparable to that previously described inhepatic disease. The LCAT enzyme is secreted by the liver, andthe inverse correlation noted in this study between LCAT activityand urea suggests that the increased urea in renal disease mayinhibit the synthesis and secretion of the enzyme by the liver.The resulting reduction in LCAT activity may lead to the accumulationof cholesterol and lecithin in cell membranes and contributeto the overall pathophysiology of renal disease.     

胆固醇逆向转运紊乱在兔胆红素钙结石成石过程中的作用

对免胆道不全梗阻与感染致胆红素钙结石成石过程中脂蛋白，尤其是高密度脂蛋白（HDL）代谢全过程作了测定。结果：实验组与对照组相比，血清游离胆固醇及HDL3-胆固醇明显升高，而HDL2-胆固醇及HDL-胆固醇明显减少；血浆卵磷酯-胆固醇酸基转移酶（LCAT）活性及肝细胞膜HDL受体最大结合力明显下降；胆汁中甘氨胆酸、甘氨脱氧胆酸及总胆酸明显减少；而肝组织中总胆固醇、甘油三酯及磷脂则均无显著性差异。本实验结果提示：在胆道不全梗阻及感染致胆红素钙结石成石过程中，存在因LCAT活性不足所致的HDL合成减少及成熟障碍，并有肝细胞膜HDL受体结合力下降，由于胆固醇逆向转运过程紊乱，在其它致石因素共同作用下导致色素性胆石的发生。     

Up-regulation of acyl-coenzyme A:cholesterol acyltransferase (ACAT) in nephrotic syndrome

BACKGROUND: We have previously demonstrated that hypercholesterolemia in rats with puromycin-induced nephrotic syndrome (NS) is associated with up-regulation of hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and relative down-regulation of cholesterol 7alpha-hydroxylase (Ch-7alpha), which represent the rate-limiting steps in cholesterol biosynthesis and catabolism. Expression of HMG-CoA reductase is inhibited and Ch-7alpha is augmented by intracellular free cholesterol, which is avidly esterified by acyl-CoA:cholesterol acyltransferase (ACAT). Therefore, we hypothesized that NS may result in up-regulation of hepatic ACAT. METHODS: Hepatic tissue ACAT mRNA (Northern blot), protein (Western blot) and enzymatic activity were determined in rats with puromycin-induced NS, placebo-treated control rats and Nagase hypoalbuminemic (NAG) rats. RESULTS: The NS group exhibited heavy proteinuria, hypoalbuminemia, normal creatinine clearance, severe hypercholesterolemia and hypertriglyceridemia. Despite severe hypoalbuminemia, NAG rats with inherited hypoalbuminemia exhibited only a mild elevation of plasma cholesterol and triglycerides. Severe hypercholesterolemia in the NS group was coupled with depressed liver tissue free cholesterol concentration and marked increases in hepatic ACAT mRNA, protein and enzymatic activity. In contrast, ACAT mRNA and protein contents of the liver were normal and ACAT activity was mildly elevated in the NAG group. CONCLUSIONS: NS results in marked up-regulation of hepatic ACAT, which is primarily due to proteinuria and not hypoalbuminemia, since the latter alone, as seen in NAG rats, does not significantly impact ACAT expression. Elevated ACAT in NS can contribute to dysregulation of cholesterol biosynthesis and catabolism by limiting the normal cholesterol signaling involved in regulation of these processes.     

肾移植患者的血脂改变

目的 研究肾移植患者的血脂变化。方法 对上海市8家医院174例肾移植的资料进行调查分析。结果 与正常对照组相比，肾移植患者术前血胆固醇（Tch)、低密度脂蛋白胆固醇（LDL－ch)显著增高，血甘油三酯（TG）、高密度脂蛋白胆固醇（HDL－ch)、极低密度脂蛋白胆固醇（VLDL－ch)与正常对照组的差异无显著性；载脂蛋白Al(Apo A1)及卵磷脂胆固醇酰基转移酶（LCAT）均低于正常对照组，脂蛋白（a)[Lp(a)]的水平显著高于正常对照组；肾移植患者手术前后上述各指标的变化不显著。结论 肾移植患者脂质紊乱相当常见，应高度重视。     

Downregulation of high-density lipoprotein receptor in human fibroblasts by insulin and IGF-I

High-density lipoprotein (HDL3) particles bind to a cell surface receptor, thereby promoting the efflux of cholesterol from extrahepatic nonsteroidogenic cells. This receptor appears to be upregulated by increased cell cholesterol content and also may be responsive to the growth state of cells. Because insulin can be mitogenic, the effect of insulin on HDL-receptor function was tested. HDL-receptor activity of cholesterol-loaded fibroblasts was inhibited by insulin treatment. Insulin decreased HDL binding in a log-dose fashion (-25% at 67 nM insulin) in association with increases in [3H]thymidine incorporation into DNA. HDL-mediated cholesterol efflux from cholesterol-loaded cells was diminished by insulin treatment of cells in parallel with decreased HDL binding. Insulin induced reciprocal changes in HDL- and low-density lipoprotein (LDL)-receptor activity. In cells in which these receptors were upregulated by varying cell cholesterol content, insulin increased LDL binding (+88%) and decreased HDL binding (-24%). Insulin-like growth factor I (IGF-I, 100 ng/ml) also significantly decreased HDL binding and HDL-mediated cholesterol efflux to a comparable degree. Pooled human serum similarly induced a reduction in HDL binding to its receptor. These results are consistent with the hypothesis that growth factors in general, and insulin and IGF-I in particular, decrease HDL-receptor activity, possibly to promote retention of cholesterol needed for new membrane synthesis during cell proliferation. Such a mechanism could be partly responsible for accumulation of cholesteryl esters in arterial wall cells during atherogenesis in diabetes mellitus.     

Biomarkers associated with high-density lipoproteins in atherosclerotic kidney disease

High-density lipoproteins (HDL) originate as discoidal particles that are rapidly converted by lecithin:cholesterol acyltransferase (LCAT) into the spherical particles that predominate in normal human plasma. Spherical HDL consist of multiple populations of particles that vary widely in size, composition and function. Human population studies have established that high plasma HDL cholesterol levels are associated with a reduced incidence of cardiovascular disease. The mechanistic basis of this relationship is not well understood, but most likely involves a number of the cardioprotective functions of HDL. These include the ability of apolipoprotein (apo) A-I, the main apolipoprotein constituent of HDL, to remove cholesterol from macrophages in the artery wall. HDL also have antioxidant and anti-inflammatory properties that are potentially cardioprotective. Evidence that some of these beneficial properties are compromised in people with diabetes and renal disease is emerging. Persistently elevated plasma glucose levels in people with diabetes and poor glycemic control can lead to irreversible, non-enzymatic glycation of plasma proteins, including apoA-I. Non-enzymatically glycated proteins are also prevalent in people with diabetes and end-stage renal disease who are at high cardiovascular risk. Evidence that non-enzymatically glycated apoA-I inhibits the LCAT reaction and impairs some of the cardioprotective properties of HDL is also emerging. This review is concerned with how non-enzymatic glycation of apoA-I affects the ability of LCAT to convert discoidal HDL into spherical HDL, how it affects cholesterol efflux from macrophages and how it affects the anti-inflammatory and antioxidant properties of HDL.     

Plasma lipoproteins and lecithin:cholesterol acyltransferase distribution in patients on dialysis

Plasma lipoproteins and LCAT activity were studied using a single spin density gradient separation and an exogenous substrate enzyme assay in 41 patients on chronic hemodialysis and in 11 normal subjects. The plasma HDL cholesterol was markedly decreased (33 vs. 63 mg/dl, P less than 0.001) while total and LDL-cholesterol were unchanged in the patients. Plasma LCAT activity was significantly lower in the patient group (42 vs. 59 nmoles/4 hr/ml, P less than 0.001), but the distribution of activity (studied in 13 dialysis patients and 12 control subjects) was not different between the two groups: 90% being associated with HDL and VHDL lipoprotein fractions. To examine the possible genetic influence on the development of hypertriglyceridemia in the patient group, we examined the ratio of apolipoproteins E3/E2 and CII/CIII in ten of the patients and another group of 13 control subjects. The frequency of heterozygotes for E3 deficiency was not different between the patient (one of ten) and the control (two of 13) groups. While the patient group had lower CII/CIII ratio, the figures did not reach statistical significance. The low LCAT activity in the face of higher plasma triglycerides and low HDL may contribute to impaired lipolysis previously documented in uremic patients. A follow-up study performed 1 year after the initial study confirmed the decreased HDL (51 vs. 71 mg/dl, P less than 0.01) and LCAT activity (50 vs. 59 nmoles/hr/ml, P less than 0.02) in an exogenous substrate system (N = 20). LCAT measured using the endogenous substrate was not significantly different from the control group (49 vs. 55 nmoles/hr/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Lecithin: cholesterol acyltransfer, dyslipoproteinaemia and membrane lipids in uraemia.

BACKGROUND: Dyslipoproteinaemia is the most important complication linked to the increased morbidity and mortality of uraemic patients from cardiovascular disease. Many factors contribute to the dyslipoproteinaemia, including increased production of very low density lipoproteins (VLDL), decreased lipolysis and impaired low density lipoprotein (LDL) receptor activity. In this study, the role of decreased lecithin:cholesterol acyltransferase (LCAT) activity in relation to plasma and membrane lipid changes is examined. METHODS: Fasted blood samples were taken from 65 uraemic patients, including roughly equal numbers of haemodialysis, peritoneal dialysis and undialysed subjects, and from 29 apparently healthy individuals. Plasma total and free cholesterol, cholesteryl esters (CE), total and individual phospholipids, high density lipoprotein (HDL)-, LDL- and VLDL-cholesterol were all measured, as were erythrocyte and lymphocyte free cholesterol and phospholipids. RESULTS: More than half of all patients, including those both on haemodialysis and peritoneal dialysis, as well as untreated individuals, had relative plasma concentrations of CE below the normal mean - 2SD. These patients had significantly decreased LDL- (2.62 +/- 1.04 compared to 3.61 +/- 0.97 mmol/L; p < 0.001) and HDL-cholesterol (0.71 +/- 0.30 compared to 0.94 +/- 0.27 mmol/L; p < 0.01) and increased VLDL-cholesterol (0.60 +/- 0.50 compared to 0.47 +/- 0.26 mmol/L; p < 0.05) as well as significant increases in membrane cholesterol and cholesterol/phospholipid molar ratio in erythrocytes (3.30 +/- 0.49 and 0.87 +/- 0.08 compared to 2.95 +/- 0.18 mmol/g wet weight and 0.76 +/- 0.04 mol/mol respectively, both p < 0.001) and cholesterol/phospholipid molar ratio of lymphocytes (0.58 +/- 0.14 compared to 0.45 +/- 0.04 mol/mol; p < 0.001). They were markedly deficient in LCAT activity (56.1 +/- 20.4 compared to 105.5 +/- 17.5 nmol/ml/h; p < 0.001). The LCAT activity in plasma of patients with high CE was higher than for those with low CE, but it was also significantly less than normal and this group showed smaller changes in other lipid parameters. CONCLUSIONS: LCAT deficiency is common in uraemia and is associated with changes not just in plasma lipids, but also in membrane lipids which may be relevant to the progression of the disease.     

Disorders in high-density metabolism with insulin resistance and chronic kidney disease.

Cardiovascular risk increases with each decrement in renal function. Low-density lipoprotein (LDL) cholesterol levels are not associated with increased mortality, but high-density lipoprotein (HDL) levels are inversely associated with cardiovascular risk. Lipoprotein composition with increased abundance of small dense LDL and HDL and reduced levels of more buoyant isoforms is similar to what is found in states of insulin resistance and in the metabolic syndrome (MS). In both cases, high triglyceride levels are associated with reduced HDL levels. Chronic kidney disease (CKD) is itself associated with increasing insulin resistance as renal function fails. In both instances, decreased levels of apo A-I and apo A-II are a consequence of increased fractional catabolic rate (FCR), resulting from a predominance of small HDL particles. HDL maturation is impaired in CKD through decreased activity of lecithin:cholesterol acyltransferase (LCAT), and increased cholesterol ester transfer protein (CETP) activity in MS shuttles triglycerides back into HDL, thereby destabilizing it. Whether insulin resistance is entirely responsible for disorders of HDL metabolism in CKD, or whether the process is a result of unrelated pathophysiology, is currently unknown.