Dyslipoproteinemia in an inbred rat strain with spontaneous chronic progressive nephrotic syndrome

Age-related glomerulosclerosis and interstitial fibrosis in Milan normotensive rats: a podocyte disease

In Milan normotensive (MNS) rats glomerulosclerosis and interstitial fibrosis develop spontaneously in the absence of hypertension. Renal changes were sequentially assessed in these rats between 2 and 10 months of age. At 10 months, rats were characterized by heavy proteinuria, increased serum creatinine, focal or global glomerulosclerosis in 51 +/- 12% of the glomeruli as well as tubulointerstitial injury involving > 25% of the section area. Cell injury in podocytes (evidenced as increased expression of desmin and by electron microscopy) and interstitial fibroblasts (increased expression of alpha-smooth muscle actin) and mild glomerular hypertrophy were witnessed as early as three to four months of age and preceded glomerulosclerosis and interstitial fibrosis. Only minor evidence of mesangial cell activation (as assessed by glomerular (de novo alpha-smooth muscle actin or type I collagen expression or increased cell proliferation) was noted throughout the observation period. Later stages of the disease were characterized by glomerular and/or tubulointerstitial macrophage influx and osteopontin expression (a chemoattractant), mild accumulation of lymphocytes, platelets, fibrinogen, as well as by a progressive accumulation of various matrix proteins. Progressive renal disease in MNS rats is thus noteworthy for the relative lack of mesangial cell activation. Rather, early podocyte damage, induced by yet unknown mechanisms, may underlie the development of glomerulosclerosis and subsequent interstitial fibrosis.

The serum lathosterol to cholesterol ratio, an index of cholesterol synthesis, is not elevated in patients with glomerular proteinuria and is not associated with improvement of hyperlipidemia in response to antiproteinuric treatment

The hypothesis that increased cholesterol synthesis provides a mechanism that contributes to nephrotic syndrome-associated hyperlipidemia is mainly based on experimental evidence. The serum level of the cholesterol precursor, lathosterol (expressed per millimole cholesterol), is a reliable marker of whole-body cholesterol synthesis in normocholesterolemia and primary hypercholesterolemia. Serum lathosterol and lipoprotein levels were measured in 11 moderately hyperlipidemic patients with nephrotic-range proteinuria and 22 matched controls. The proteinuric patients were evaluated before and during three antiproteinuric treatment periods with angiotensin-converting enzyme (ACE) inhibition therapy (n = 6) or a low-protein diet (n = 5) alone, in combination, and again as a single treatment. In untreated patients, serum total cholesterol, very-low-density (VLDL) and low-density (LDL) lipoprotein cholesterol, apolipoprotein B (apo B), and lipoprotein (a) [Lp(a)] levels were higher than in controls (P < .01 to P < .001), but the lathosterol to cholesterol ratio tended to be lower in patients (0.99 +/- 0.43 micromol/mmol) as compared with controls (1.29 +/- 0.41 micromol/mmol, P < .10). During combined antiproteinuric treatment, total and VLDL + LDL cholesterol, apo B, and Lp(a) decreased (P < .02 to P < .01), but remained higher than levels in controls. Yet the serum lathosterol to cholesterol ratio changed little and was even lower (P < .05) in treated patients than in controls. Serum total cholesterol (r = -.82, P < .01) and apo B (r = -.84, P < .01) were inversely correlated with serum albumin in untreated patients, whereas the serum lathosterol to cholesterol ratio was not (r = -.01, NS). In the patient group, multiple regression analysis showed that changes in the lathosterol to cholesterol ratio during the study were only related to changes in the dietary polyunsaturated to saturated fatty acids ratio (P:S) coinciding with the low-protein diet (P < .01). In contrast, the decrease of VLDL + LDL cholesterol, apo B, and Lp(a) was independently related to reduction of proteinuria (P < .02 to P < .001), but not to changes in the lathosterol to cholesterol ratio. In conclusion, the present data, based on the serum lathosterol to cholesterol ratio, do not support the concept that increased cholesterol synthesis plays an important role in the maintenance of human nephrotic syndrome-associated hypercholesterolemia. Moreover, it appears unlikely that the decrease of apo B-containing lipoproteins with antiproteinuric treatment is attributable to inhibition of cholesterogenesis. These findings warrant further documentation of cholesterol synthesis in human nephrotic syndrome by direct methods.

Age-related variations in plasma and liver lipids of Yoshida rats: a comparison with Wistar rats

Lipoprotein and liver lipids of spontaneously hyperlipidemic Yoshida rats were compared with those of normolipidemic Wistar animals for studying their age- and strain-related differences. Both strains showed an age-related increase in the total plasma cholesterol concentration. However, the Yoshida strain had a higher content of cholesteryl esters and triglycerides than the Wistar strain in both young and adult animals (2- and 8-month-old animals, respectively). The free cholesterol content was also higher, but only in the 8-month-old animals. Both strains showed an age-related increase in the proportion of HDL1 and a symmetrical decrease in both the HDL2 and HDL3 subfractions, but the variations were more evident in the Yoshida strain. The study of strain-related differences suggested that the spontaneous hypertriglyceridemia of the Yoshida strain was not only related to the higher amount and proportion of the VLDL fraction, but also to the higher content of triglycerides in the LDL fraction. The livers of Yoshida rats accumulated more triglycerides (with an age-related progression) than those of Wistar rats. The major lipid classes in the liver of Yoshida rats contained a significantly higher proportion of monounsaturated fatty acyls. Furthermore, this proportion showed an age-related increase in all the lipid classes, but in cholesteryl esters. This suggested that liver desaturases had a relevant role in the development of hyperlipidemia, and of its age-related variations, in the Yoshida strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular apoprotein B degradation is suppressed by decreased albumin concentration in Hep G2 cells

It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.

The effect of a thromboxane A2 synthase inhibitor on the dyslipoproteinemia of an inbred rat strain with spontaneous age-related nephrotic syndrome

We have previously shown that the administration of a thromboxane A2 (TXA2) synthase inhibitor (FCE 22178) reduced the progression of glomerular lesions and proteinuria in MNS rats, an inbred strain which develops an age-related nephrotic syndrome. In the present study we investigated the effect of FCE 22178 on the plasma lipoproteins of MNS rats at 28 weeks of age (with mild proteinuria and moderate dyslipoproteinemia) and at 48 weeks of age (with heavy proteinuria and severe dyslipoproteinemia). Drug treatment reduced proteinuria (by 70% and 36% at 28 and 48 weeks of age, respectively) plasma cholesterol (by 36% and 27% at 28 and 48 weeks of age, respectively) and prevented the decrease of plasma albumin observed in untreated rats (C-MNS) 48 weeks old. In treated rats (T-MNS), the decrease of proteinuria was positively correlated with that of plasma cholesterol. FCE 22178 reduced the elevation in plasma HDL1 (by 17.4%) and HDL2 levels (by 30%), a key feature of nephrotic dyslipoproteinemia in the rat. From 28 to 48 weeks of age plasma apo A-I and apo E increased 217% and 128%, respectively, in C-MNS rats and 191% and 121%, respectively, in T-MNS rats. A significant increase of apo A-I/apo E ratio was found in C-MNS rats from 28 (2.28 +/- 0.36) to 48 weeks of age (3.84 +/- 0.9) but not in T-MNS rats. FCE 22178 altered the lipid composition of VLDL and HDL2 by reducing the content of cholesteryl esters and increasing that of free cholesterol and phospholipids. These findings suggest that the beneficial effect of FCE 22178 on the dyslipoproteinemia of nephrotic MNS rats is secondary to the amelioration in kidney function and to the reduction of proteinuria produced by this drug.

Age-related changes in blood and liver lipids of male wistar rats

The results of this study indicate that the age-dependent plasma cholesterol increase observed in male Wistar rats is correlated with changes in both the distribution of high-density lipoprotein fractions and the storage of hepatic cholesterol. Specifically, the lipoprotein distribution showed a significant increase in the proportion of HDL(1) and a symmetrical decrease in both the HDL(2) and HDL(3) fractions during the 3 month to 18 month age period. There were no significant changes in the very-low density and low-density lipoprotein fractions. The chemical composition of lipoproteins showed many age-related variations, especially in the proportion of cholesteryl ester and in the distribution of HDL subfractions. A study of fatty acyl composition of the major lipid classes showed that, within cholesteryl ester found in liver, there was an increase in the proportion of saturated fatty acids. Polyunsaturated fatty acids increased in the cholesteryl esters found in high-density lipoproteins of older rats. These observations suggest that the age-dependent accumulation of body cholesterol occurs by a reduced catabolism of HDL(1) fraction, and modifications in plasma and liver lipids.

Lipid synthesis and apolipoprotein gene expression in hepatocytes in primary culture from (puromycin-induced) nephrotic rats

Primary culture of hepatocytes from puromycin aminonucleoside-induced nephrotic rats were used to discriminate between the hepatic and extra-hepatic contribution to the hyperlipidemia occurring in the nephrotic syndrome. De novo lipogenesis and utilization of exogenous fatty acids were not modified in nephrotic hepatocytes as compared to controls. In contrast 2.2 and 5.3-fold more triacylglycerol and phospholipids were secreted respectively by nephrotic hepatocytes than by controls. Triacylglycerol overproduction was not associated with an increase either in apo B mRNA level or in apo B synthesis or secretion measured by [35S]-methionine incorporation and immunoprecipitation. We also observed a significant increase in apo AI and apo E synthesis and secretion by nephrotic hepatocytes. This increase was correlated with a greater amount of apo AI and apo E mRNA than in controls. The overproduction of apo AI and apo E by nephrotic hepatocytes might intervene in the clearance of plasma lipoproteins and the redistribution of plasma cholesterol.