Lipid abnormalities in the nephrotic syndrome: causes, consequences, and treatment

Glucose and free radicals impair the antioxidant properties of serum albumin

Epidemiological data consistently show that reduced levels of serum albumin, which is the most abundant protein in plasma, are associated with an increased mortality risk. Various biological properties evidenced by direct effects of the albumin molecule may explain its beneficial effects. The present work aimed to investigate in vitro whether glycation or free radicals or both factors would affect the antioxidant properties of bovine serum albumin (BSA). Glycation was performed by long-term incubations (60 days) of BSA with increasing concentrations of glucose (up to 500 mmol/l) at 37 degreesC. Minimally oxidized BSA was obtained after controlled incubations of dialyzed BSA samples with a water-soluble free radical generator [2,2' azo-bis(2-amidinopropane) HCl]. The glycation-mediated modifications and the free radical-induced conformational changes of BSA were monitored using intrinsic fluorescence measurements of the tryptophan residues and acrylamide as a quenching agent. Thiol groups, Amadori glycophore contents, and boronate binding were also measured. We found that the changes observed in the conformation of the BSA molecule were associated with modifications of its antioxidant properties. The latter were studied by the copper-mediated oxidation of human low density lipoproteins and the free radical-induced blood hemolysis test. Our data support the concept that oxidative-induced BSA modifications are important determinants in the antioxidant properties of BSA. Glycated BSA still behaved as an antioxidant but became pro-oxidant in the presence of copper, probably by generating oxygenated species. These data confirm the key role of metals ions in this process. Although these results warrant further in vivo investigations, we propose that, considering the poor glucose control found in diabetics as well as the key role of oxidative stress in vascular complications, glycation-mediated and free radical-induced impairment of the antioxidant properties of albumin might be important parameters in vascular complications encountered in diabetes.

Statins and the kidney

Experience to date suggests that 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors or statins can be used relatively safely and effectively to treat dyslipidaemia complicating renal disease. Recent studies suggest that in addition to lowering plasma lipid levels, these drugs modify other factors that contribute to vascular injury. Furthermore, statins could slow the progression of chronic renal failure and may augment the action of immunosuppressive therapy after renal transplantation. Such newly defined actions, some of which could be unrelated to lipid lowering, are likely to extend the applications of statins in nephrology.

Drugs causing dyslipoproteinemia

Diuretics and beta-blockers have a strong tendency to affect serum lipids adversely, whereas the peripherally acting alpha-blocking agents consistently result in beneficial effects. Most of the other antihypertensive agents (calcium channel blockers, ACE inhibitors, angiotensin II receptor antagonists, and drugs that act centrally) are lipid neutral. The effect of steroid hormones varies with the drug, dose, and route of administration. In general, androgens lower HDL-C and have a variable effect on LDL-C. The effects of progestins vary greatly depending on their androgenicity, and estrogens are beneficial except when hypertriglyceridemia occurs with oral estrogens. Glucocorticoids raise HDL-C and may also increase triglycerides and LDL-C. Retinoids increase triglycerides and LDL-C and also reduce HDL-C. Interferons can cause hypertriglyceridemia. Following organ transplantation, a dyslipidemia often ensues. This is caused in part by the medications used to prevent rejection (glucocorticoids, cyclosporine, and FK-506) and requires close attention and, in some patients, drug therapy to prevent coronary artery disease.

Lipoprotein kinetics in patients with analbuminemia. Evidence for the role of serum albumin in controlling lipoprotein metabolism

In vitro data suggested that albumin is a key factor controlling apolipoprotein (apo) synthesis by hepatocytes. Studies in analbuminemic rats have shown an increase in secretion of apoB-containing lipoprotein from the liver. We studied the kinetic aspects of apoB- and apoAI-containing lipoprotein metabolism in two sisters with analbuminemia using a constant 14-hour infusion of leucine labeled with stable isotopes. Compared with control subjects, total cholesterol was higher in the two patients (432 and 461 versus 155 +/- 14 mg/dL), as was apoB (257 and 230 versus 72 +/- 7 mg/dL). Triglycerides were slightly increased (134 and 105 versus 89 +/- 9 mg/dL), whereas apoAI was lower (109 and 105 versus 124 +/- 6 mg/dL). VLDL-apoB production was higher, as was the production of IDL-apoB and LDL-apoB (32.8 and 36.0 versus 24.8 +/- 5.9, 32.1 and 27.2 versus 16.4 +/- 2.3, and 14.1 and 17.6 versus 10.3 +/- 1.2 mg.kg-1.d-1, respectively). The fractional catabolic rate of all the apoB-containing lipoproteins was decreased (0.23 and 0.37 versus 0.48 +/- 0.05, 0.27 and 0.28 versus 0.62 +/- 0.08, and 0.012 and 0.009 versus 0.022 +/- 0.002.h-1, respectively). A similar mechanism could explain the dyslipidemia observed in other conditions associated with low albumin levels, such as nephrotic syndrome.

Regulation of hepatic angiotensinogen gene expression in nephrotic rats

Plasma angiotensinogen (Ao) concentration (PAC), urinary Ao excretion (UAE), hepatic levels of Ao mRNA and plasma renin concentration (PRC) were studied in control and nephrotic rats subjected to the following treatments: dexamethasone (DEX), ethinyl-estradiol (EE), tri-iodothyronine (T3), bilateral nephrectomy (NX), captopril (CAP) and adrenalectomy (ADX). In nephrotic rats PAC diminished, UAE and PRC augmented and Ao mRNA levels were not altered. In control rats, DEX, EE, T3 and NX increased PAC and Ao mRNA levels whereas CAP diminished PAC but not affected Ao mRNA. ADX diminished PAC and Ao mRNA levels. In nephrotic rats, these treatments produced the same effect than in control rats except in ADX which did not affect PAC. These data suggest that the decreased PAC is not related to alterations in hepatic Ao gene expression but to elevated PRC and UAE.

Pathogenesis of dyslipoproteinemia in renal insufficiency: the role of lipoprotein lipase and hepatic lipase

The lipoprotein pattern, observed in patients with renal failure, suggests impaired catabolism of triglyceride-rich lipoproteins. This is supported by the findings of numerous studies addressing the pathogenesis of the dyslipoproteinemia of uremia. Aberrant lipoprotein composition, resulting in disturbed substrate characteristics for lipoprotein lipase and unfavourable receptor ligand function, probably constitutes the primary pathology. The structural details of the lipoproteins that are responsible for this dysfunction are not yet established. In this regard, abnormal apolipoprotein pattern and, possibly more important, biological modifications must be taken into consideration. Low activity of lipoprotein lipase does not seem to be a primary pathogenetic factor. However, there is little doubt that it plays a contributory part. The role of hepatic lipase is controversial.

Proteinuria and albumin homeostasis in the nephrotic syndrome: effect of dietary protein intake

Nephrotic syndrome is analyzed in the light of interventions designed to decrease proteinuria and renal injury. The effect of dietary protein intake on urinary protein losses and albumin homeostasis are discussed on the basis of the pathophysiologic mechanisms known to account for changes in renal function of nephrotic patients. In addition, the effect of angiotensin-converting enzyme inhibitors for reduction of proteinuria is discussed in terms of the modulation of glomerular permselectivity and hemodynamics.

Fibrinogen, coagulation factor VII, tissue plasminogen activator, plasminogen activator inhibitor-1, and lipid as cardiovascular risk factors in chronic hemodialysis and continuous ambulatory peritoneal dialysis patients

Mortality rates associated with cardiovascular disease (CVD) are high in long-term dialysis patients. Increased levels of plasma fibrinogen (FBG), coagulation factor VII (FVII), tissue plasminogen activator (t-PA), and plasminogen activator inhibitor-1 (PAI-1) as well as hyperlipidemia are regarded as important risk factors for CVD. To investigate whether there are differences in the risk of CVD between chronic hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD) patients, serum lipid levels and plasma FBG, FVII, t-PA, and PAI-1 levels were measured in 17 patients on HD and 17 patients on CAPD. FBG was measured by the thrombin time method, FVII activity (FVIIc) by the chromogenic prothrombin time method, and t-PA and PAI-1 activity by the chromogenic substrate assay. No difference was found in body mass index (BMI) between HD and CAPD patients. Total cholesterol (TC), TC/high-density lipoprotein (HDL)-C ratio, low-density lipoprotein (LDL)-C, and triglycerides (TG) were significantly increased, and HDL-C was significantly decreased in CAPD patients compared with HD patients. FBG and FVIIc were significantly elevated in CAPD patients compared with controls or HD patients. T-PA activities were significantly higher in HD and CAPD patients than in controls. CAPD patients showed significantly higher PAI-1 activities than controls or HD patients. Significant positive correlations were found between FBG or FVIIc and TC, between FBG and LDL-C or TG, and between FVIIc and LDL-C in these patients. T-PA showed significant negative correlations with FBG, PAI-1, TC, LDL-C, and TG. There was a significant positive correlation between PAI-1 and TG and a significant negative correlation between PAI-1 and HDL-C. We conclude that CAPD patients may have a greater risk of CVD than do HD patients, and that coagulation and fibrinolytic activity are correlated with lipid disorders in these patients.

Factors influencing plasma lipid profiles including lipoprotein (a) concentrations in renal transplant recipients

Fasting plasma cholesterol, triglycerides, high-density lipoprotein (HDL) and apoprotein (apo) B were elevated in 214 nondiabetic renal transplant recipients when compared to a reference group. Apo (a) was slightly but not significantly lower in transplant recipients (median 118 mg/dl, range 16-1680 vs 130 mg/dl, 10-1176) and this difference could be predicted from Lp (a) isoform analysis. Cholesterol, triglyceride, apo B and apo (a) concentrations correlated negatively with creatinine clearance but none of these parameters showed a significant association with proteinuria. Patients treated with steroids had higher plasma HDL concentrations than those receiving cyclosporin monotherapy (P < 0.01). The use of diuretics was associated with raised triglycerides (P < 0.001) and cholesterol (P < 0.01) and with reduced HDL (P < 0.01) whilst patients receiving beta-blockers had significantly higher triglycerides (P < 0.01) and lower HDL levels (P < 0.02). In multiple regression analysis, age (P < 0.01), creatinine clearance (P < 0.05) and diuretic therapy (P < 0.005) were independent risk factors for increased cholesterol whilst apo (a) levels correlated negatively with creatinine clearance (P < 0.005). These results suggest that impaired renal function, steroids and non-immunosuppressive drugs contribute to lipid abnormalites in renal transplant recipients.