Very low density lipoprotein secretion by cultured rat hepatocytes. Inhibition by albumin and other macromolecules

Increased VLDL in nephrotic patients results from a decreased catabolism while increased LDL results from increased synthesis

Increased very low density lipoprotein (VLDL) in nephrotic patients results from a decreased catabolism while increased low density lipoprotein (LDL) results from increased synthesis. Hyperlipidemia is a hallmark of nephrotic syndrome that has been associated with increased risk for ischemic heart disease as well as a loss of renal function in these patients. The hyperlipidemia usually is characterized by increased cholesterol levels, although hypertriglyceridemia may be present as well. The factors that determine the phenotype of nephrotic dyslipidemia are not understood, nor has the primary stimulus for nephrotic hyperlipidemia been identified. One hypothesis is that nephrotic hyperlipidemia is the result of a coordinate increase in synthesis of proteins by the liver. To address these issues we simultaneously measured the in vivo rate of VLDL apolipoprotein B100 (apo B100) secretion, LDL apo B100 synthesis and albumin synthesis in patients with a nephrotic syndrome (N = 8) and compared them with a control group (N = 7) using a primed/continuous infusion of the stable isotope L-[1-13C] valine for six hours. Kinetic data were analyzed by multicompartmental analysis. Patients studied had combined hyperlipidemia as reflected by an significant increase in both VLDL and LDL apo B100 pool sizes. In contrast, the albumin pool size was significantly decreased. VLDL apo B100 levels were primarily increased as a consequence of a decrease in fractional catabolic rate (FCR) rather than from an increase in the absolute synthesis rate (ASR). Both VLDL apo B100 and triglycerides were inversely related to the fractional catabolism (FCR) of VLDL apo B100 (r2 = 0.708; P = 0.0088) while neither had any relationship to the ASR of VLDL apo B100. In contrast to VLDL, increased LDL apo B100 was not a consequence of decreased catabolism. The LDL apo B100 ASR was significantly increased (P = 0.001) in the nephrotic patients compared to controls. Low density lipoprotein apo B100 ASR was greater than that of VLDL apo B100 in some patients, suggesting that LDL in these patients was not only derived from VLDL delipidation, but also by an alternative secretory pathway. There was no clear relationship between the ASR of VLDL apo B100 and the ASR of albumin within the current study population. Our data indicate that increased VLDL in nephrotic patients results from a decreased catabolism, while increased LDL results from increased synthesis.

Role of lipoprotein-bound NEFAs in enhancing the specific activity of plasma CETP in the nephrotic syndrome

Plasma cholesteryl ester transfer protein (CETP) activity, evaluated by the transfer of radiolabeled cholesteryl esters from a tracer dose of tritiated HDL to the plasma apolipoprotein B-containing lipoproteins, was significantly higher in patients with untreated idiopathic nephrotic syndrome (n = 15) than in normolipidemic control subjects (n = 22) (81.5 +/- 8.4 versus 43.1 +/- 3.1 micrograms CE.mL-1.h-1, respectively; P < .001). The increased CETP activity in nephrotic plasma was explained by a significant rise in both the CETP mass concentration (3.2 +/- 0.2 versus 2.1 +/- 0.1 mg/L; P < .001), and the specific CETP activity, calculated as the ratio of CETP activity to CETP mass (25.3 +/- 1.7 versus 20.4 +/- 1.6 micrograms CE.mg-1.h-1; P < .05). Elevated CETP activity in nephrotic patients was shown to be associated with a significant decrease in the mean size of LDL (24.4 +/- 0.5 versus 26.3 +/- 0.5 nm; P < .0001) as well as in the relative abundance of HDL2a (29.6 +/- 1.6% versus 34.8 +/- 1.1%; P < .05). The nephrotic syndrome was characterized by a significant increase in the relative proportion of lipoprotein-bound nonesterified fatty acids (NEFAs) (35.4 +/- 7.7% versus 7.6 +/- 3.0% of total; P < .01), leading to a significant increase in the electronegative charge of LDL (-4.3 +/- 0.1 versus -3.9 +/- 0.1 mV; P < .05) and HDL (-11.5 +/- 0.1 versus -11.1 +/- 0.2 mV; P < .05). Compared with native, non-supplemented plasma, removal of lipoprotein-bound NEFAs by addition of fatty acid-poor albumin to total plasma from nephrotic patients or control subjects significantly decreased CETP activity and specific CETP activity. Specific CETP activity no longer differed between nephrotic and control groups after albumin supplementation (19.7 +/- 1.5 versus 17.7 +/- 1.5 micrograms CE.mg-1.h-1; NS). It is concluded that, in addition to elevated CETP mass concentration, lipoprotein-bound NEFAs, by increasing the negative electrostatic charge of nephrotic lipoproteins, can facilitate the CETP-mediated neutral-lipid transfer reaction in total plasma from nephrotic patients.

The effect of individual amino acids on ApoB100 and Lp(a) secretion by HepG2 cells

The rate at which HepG2 cells secrete apoB100 lipoproteins is inversely related to the concentration of amino acids in the medium (Zhang, Z., Sniderman, A. D., Kalant, D., Vu, H., Monge, J. C., Tao, Y., and Cianflone, K. (1993) J. Biol. Chem. 268, 26920-26926). The purpose of the present study was to determine the effect of individual amino acids on apoB100 and lipoprotein secretion. Asparagine was associated with modestly increased secretion. The branched chain amino acids (leucine, isoleucine, and valine) and lysine had minor inhibitory effects. The other amino acids, by contrast, decreased apoB secretion, although the magnitude of the effect varied considerably, the most potent being tyrosine, cysteine, phenylalanine, tryptophan, methionine, and glutamine. Although the effect on Lp(a) generally paralleled that on apoB100, it was usually much less pronounced. No amino acid caused a marked decrease in albumin, apoAI, or total protein secreted from the HepG2 cells. The amino acid effect on apoB was paralleled by similar decreases in secreted cholesterol ester (CE) primarily in the low density lipoprotein density range (d < 1.006-1.063 g/ml), although there was no significant change in intracellular CE. Neither intracellular nor secreted triglycerides (TG) or free cholesterol changed, resulting in a slightly larger TG-enriched particle being secreted. The effect was confirmed in cultured primary hamster hepatocytes, where a mixture of amino acids also caused a decrease in apoB secretion (up to 40%). ApoAI appeared to increase as with the HepG2 cells. Secreted CE paralleled apoB . There was no change in intracellular or secreted TG or free cholesterol, resulting in a substantially larger TG-rich particle being secreted. mRNA for apoB100 increased with asparagine, decreased moderately with branched chain amino acids, and decreased further with glutamine, as shown by dot blot and Northern blotting. Pulse-chase studies indicated that there was no change in apoB secretion efficiency under any condition. These results extend our previous observations by demonstrating specificity of the amino acid effect on apoB100 secretion. Although an effect on transcription is the likely mechanism, the exact basis for this remains to be determined.

Dyslipidemia in diabetes mellitus

Patients with diabetes mellitus have a higher rate of mortality than the general population. This higher mortality may be attributed mainly to cardiovascular disease. A high prevalence of dyslipidemia in diabetics can be one of the reasons for this. The most commonly recognized lipid abnormality in non-insulin-dependent diabetics (NIDDM) is hypertriglyceridemia, which is known to be an independent risk factor for coronary heart disease in diabetics. Hypertriglyceridemia can be produced by two mechanisms, increased synthesis of very-low-density lipoprotein (VLDL) triglyceride and removal defect of plasma triglyceride. It has been a matter of debate whether insulin always stimulates hepatic VLDL secretion but it is generally accepted that insulin deficiency results in an impairment of plasma triglyceride clearance. Considerable attention has recently been focused on the atherogenecity of postprandial hyperlipidemia, remnant lipoproteins, small, dense LDL, lipoprotein (a) [Lp(a)] and isolated hypo-alphalipoproteinemia in NIDDM subjects. Several reports suggested that these atherogenic lipoprotein abnormalities are present in NIDDMs even if they are apparently normolipidemic. Association of visceral fat obesity, insulin resistance and nephropathy may aggravate the atherogenic lipoprotein profile. Therefore, we propose here that plasma lipid levels of diabetic subjects must be more strictly controlled than for the non-diabetic population in order to avoid an increased risk for coronary heart disease. If they are obese or associated with insulin resistance or nephropathy, these conditions should be carefully controlled.

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.

Metabolism of low density lipoproteins in nephrotic dyslipidemia: comparison of hypercholesterolemia alone and combined hyperlipidemia

High levels of low-density lipoprotein cholesterol (LDL) (hypercholesterolemia) are commonly present in the nephrotic syndrome. Another pattern of dyslipidemia in nephrotic patients is an elevation of both cholesterol and triglyceride levels (combined hyperlipidemia). It has been postulated that the underlying cause of nephrotic dyslipidemia is an hepatic overproduction of apolipoprotein B (apo B)-containing lipoproteins. To examine this hypothesis, the metabolism of LDL-apo B was compared between nephrotic patients with hypercholesterolemia and with combined hyperlipidemia. Thirteen patients (7 with hypercholesterolemia, and 6 with combined hyperlipidemia) underwent measurements of turnover rates of autologous LDL apo B. The results were compared to normolipidemic controls and to patients with primary combined hyperlipidemia previously studied in our laboratory. Nephrotic patients with hypercholesterolemia generally had: (a) lower fractional catabolic rates of LDL apo B than normolipidemic healthy individuals; (b) LDL particles enriched in cholesterol; but (c) no overproduction of LDL apo B. In contrast, patients with combined hyperlipidemia were found to have: (a) high fractional catabolic rates for LDL apo B compared to normolipidemic controls; (b) cholesterol-poor LDL particles; and (c) markedly elevated production rates for LDL. Also, for the group as a whole, there was a positive correlation between plasma triglyceride levels and fractional catabolic rates. These data indicate that the metabolism of LDL is strikingly different between the two forms of nephrotic dyslipidemia. Although there may be common mechanisms contributing to LDL levels in nephrotic patients, there also appears to be a divergence of mechanisms depending on whether hypertriglyceridemia is associated with hypercholesterolemia.

Effects of albumin on lipid synthesis, apo B-100 secretion, and LDL catabolism in HepG2 cells

This study examines the effects of extracellular albumin on hepatic apo B-100 metabolism. To do so, a transformed human liver cell line, HepG2, was used as a hepatocyte model and the concentration of albumin in the medium was varied between 0 and 5 g%. Apo B-100 and apo A1 concentrations in the medium were determined by specific enzyme-linked immunoassay (ELISA) and intracellular synthesis of cholesterol ester and triglyceride were determined by addition of appropriate radiolabels to the medium. The data demonstrate that the reduction of extracellular albumin concentration resulted in increased apo B-100 concentration in the medium. Apo A1 secretion, however, was unaffected. While the differences in apo B-100 concentration in the medium were statistically significant (33% +/- 7%, P < 0.0025, 0 g% albumin compared to 5 g% albumin in the medium), the absolute magnitude of the effect under these conditions was relatively modest. Nevertheless, the changes were consistent and evident over incubation periods as long as 8 days. Of interest, although triglyceride synthesis was unaffected, cholesterol ester synthesis changed such that as albumin concentration decreased, synthesis of cholesterol ester increased paralleling the changes in apo B-100 (170% +/- 9%, P < 0.005). These findings were extended by studying interventions which altered cholesterol ester synthesis. Addition of the compound 58-035 (5 micrograms/ml, a specific inhibitor of acylcholesterol acyltransferase activity) resulted in substantial inhibition of cholesterol ester synthesis (39% to 66%, P < 0.025 and P < 0.005, respectively) and apo B-100 concentrations in the medium which decreased by 20% to 28%, P < 0.025. Triglyceride synthesis, in contrast, increased significantly by 32% P < 0.025. Therefore, addition of 58-035 confirmed the previous findings of a parallel relation between cholesterol ester synthesis and apo B-100 concentration in the medium. Nonetheless, albumin still had an additional inhibitory effect on cholesterol ester and apo B-100 secretion. Of interest, when chylomicron remnants (25 micrograms/ml cholesterol), which cause apo B-100 secretion to increase by more than threefold, were added to the medium, albumin now had a more pronounced absolute effect on apo B-100 secretion with a 48% inhibition observed as albumin was increased from 0 to 5 g% in the medium (P < 0.0125). The effect of extracellular albumin on the low density lipoprotein (LDL) pathway was also examined. No differences in non-specific cell association component were detected.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Downregulation of hepatic albumin mRNA in response to induced hypercholesterolemia in rabbits

Albumin gene expression was studied in rabbits fed on a cholesterol-rich diet for up to 16 weeks. Livers from experimental animals showed extensive lipid deposition. Hepatic albumin mRNA abundance decreased along the treatment to very low levels after 16 weeks. An 8-fold decrease in the rate of transcription of this gene was also detected. This downregulation of albumin gene expression cannot be attributed to a general impairment of RNA synthesis, as expression of other liver-specific and housekeeping genes did not vary significantly. There was a decrease in ascitic fluid albumin levels after 10 weeks, although serum albumin levels remained unchanged throughout the treatment. Our results are discussed in view of the relationship of albumin levels with hypercholesterolemia.

Lipoprotein secretion by the human hepatoma cell line Hep G2: differential rates of accumulation of apolipoprotein B and lipoprotein lipids in tissue culture media in response to albumin, glucose and oleate

Serum low-density lipoprotein (LDL) concentration is a major determinant of susceptibility to the development of atherosclerosis. A major component of the protein moiety of LDL and its precursor very-low-density lipoprotein is apolipoprotein B (apo B). The human hepatoma cell line, Hep G2, was used as a model for the investigation of mechanisms which control hepatic secretion of the apo B and lipid components of lipoproteins. Using a sensitive immunoradiometric assay for apo B developed in this laboratory, we showed that bovine serum albumin inhibited and glucose, and fatty acids enhanced the rate of accumulation of apo B in the culture medium of Hep G2 cells. However, these substances did not necessarily affect LDL lipids in the same way as apo B. This finding appeared to be due to Hep G2 cells expressing lipase activities which led to triacylglycerol and phospholipid hydrolysis and lipid reuptake. Reuptake of apo B also occurred, but its rate of accumulation in the culture medium suggested it was a closer reflection of its true secretory rate.

Hep-G2 cells and primary rat hepatocytes differ in their response to inhibitors of HMG-CoA reductase

CI-981, a novel synthetic inhibitor of HMG-CoA reductase, was previously reported to be highly liver-selective using an ex vivo approach. In order to determine liver-selectivity at the cellular level, CI-981 was evaluated in cell culture and compared to lovastatin, pravastatin, fluvastatin and BMY-21950. Using human cell lines, none of the compounds tested showed liver-selectivity, i.e. strong inhibition of cholesterol synthesis in Hep-G2 cells (liver model) but weak inhibition in human fibroblasts (peripheral cell model). In contrast, all drugs tested produced equal and potent inhibition of sterol synthesis in primary cultures of rat hepatocytes, and CI-981, pravastatin and BMY-21950 were more than 100-fold more potent in rat hepatocytes compared to human fibroblasts. Since all compounds were also equally potent at inhibiting sterol synthesis in a rat subcellular system and in vivo, the data suggest that the use of Hep-G2 cells may not be the cell system of choice in which to study inhibition of hepatic cholesterogenesis or to demonstrate liver selectivity of inhibitors of HMG-CoA reductase.

Oleic acid stimulation of apolipoprotein B secretion from HepG2 and Caco-2 cells occurs post-transcriptionally

HepG2 and Caco-2 cells were studied to compare the regulation of liver and intestinal apolipoprotein (apo) biosynthesis and secretion by fatty acids. Incubation with fatty acid consistently stimulated apoB production by both HepG2 and Caco-2 cells. Media concentrations of apoB, determined by radioimmunoassay, were approx. 3-fold greater for cells incubated for 24 h in serum-free medium containing oleic acid bound to albumin than for cells incubated with albumin alone. Oleic acid also resulted in a 2-3-fold accumulation of cellular triacylglycerol for HepG2 cells and Caco-2 cells. Cellular apoB and media and cellular apoA-I concentrations were not affected by oleic acid. Immunoblotting with a monoclonal antibody against human apoB confirmed a greater mass of apoB in media from HepG2 and Caco-2 cells incubated with oleic acid. Radiolabeled apoB-100 was also increased in media from HepG2 and Caco-2 cells incubated with [35S]methionine for 24 h in the presence of oleic acid, suggesting enhanced apoB synthesis. However, apoB mRNA concentrations were unchanged in response to oleic acid. Gel filtration of media by fast protein liquid chromatography (FPLC) revealed a redistribution of apoB from LDL-sized particles to VLDL or chylomicrons in media from Caco-2 cells incubated with oleic acid, whereas apoB remained in LDL for HepG2 cells. ApoA-I in media from HepG2 and Caco-2 cells eluted as free or lipid-poor apoA-I, and the apoA-I distribution was unaltered by incubation with oleic acid. These data demonstrate that HepG2 and Caco-2 cells maintained in supplemented serum-free medium respond to oleic acid by a similar post-transcriptional increase in apoB synthesis, but different packaging of apoB into triacylglycerol-rich lipoproteins.

Low density lipoprotein metabolism in rats with puromycin aminonucleoside-induced nephrotic syndrome

With puromycin aminonucleoside-induced nephrotic syndrome (NS) in rats, twofold elevated levels of lipoproteins were observed. These levels were not related to proteinuria or to plasma albumin levels. Ultrastructural lesions induced in the kidneys by puromycin aminonucleoside were consistent with NS, while there was little or no hepatic involvement. Apolipoprotein B (apo B) kinetic measurements using homologous 125I-labeled low density lipoproteins (LDL) demonstrated a higher synthetic rate in nephrotic rats relative to controls (6.18 +/- 1.86 micrograms x g-1 x d-1 v 3.94 +/- 0.66 micrograms x g-1 x d-1 respectively, P less than .005), while the fractional catabolic rate was only marginally reduced (1.64 +/- 0.28 pools x day-1 in NS v 1.83 +/- 0.37 pools x day-1 in controls, P less than 0.4). These results indicate that in rats with experimentally induced NS, the expanded apo B-LDL pool results from increased synthesis of this apoprotein while no significant role can be ascribed to alterations in its catabolism. These data are consistent with our preliminary findings in NS in humans.

Modulation of the synthesis of apolipoproteins in rat hepatoma cells

The present study was designed to investigate whether plasma lipoproteins and albumin can affect the basal synthetic rate of apolipoproteins in differentiated rat hepatoma cells (Fao) incubated in serum-free medium. The synthesis of apolipoproteins was measured by the incorporation of [35S]methionine into medium lipoproteins isolated by density gradient ultracentrifugation. Under all the experimental conditions used, Fao cells synthesized almost exclusively apolipoprotein E. When cells were incubated in the presence of 5-10% rat plasma the synthesis of apolipoprotein E increased 2-3-fold; lipoprotein-deficient serum had a negligible effect. Fatty acid-poor bovine serum albumin (BSA), which had been found to reduce very-low-density lipoprotein secretion in isolated rat hepatocytes, did not modify the synthesis of apolipoprotein E. When Fao cells were incubated in medium containing rat plasma lipoprotein fractions, the synthesis of apolipoprotein E increased. The d less than 1.090 g/ml plasma lipoprotein fraction had the major stimulatory effect. Increased apolipoprotein E synthesis was observed when cells were incubated in the presence of lipids extracted from rat plasma lipoproteins. These results suggest that the intracellular accumulation of lipoprotein-lipids plays an important role in regulating apolipoprotein E synthesis in Fao cells.

Lovastatin therapy in nephrotic hyperlipidemia: effects on lipoprotein metabolism

The nephrotic syndrome is characterized by proteinuria, hypoalbuminemia, and hypercholesterolemia. Hypertriglyceridemia often is present as well. In this study, the kinetics of plasma lipoproteins were investigated in four patients with nephrotic hyperlipidemia, and repeat studies were carried out in three of these patients during therapy with lovastatin. Before lovastatin therapy, the patients had an extremely delayed catabolism of very low density lipoproteins (VLDL) without evidence of overproduction of lipoproteins in this fraction. Three of four patients had elevated levels of low density lipoprotein (LDL) that were due mainly to increased production rates for LDL. In the three patients treated with lovastatin, the drug therapy lowered plasma concentrations of total cholesterol, triglycerides, VLDL-cholesterol, and LDL-cholesterol, and raised high density lipoprotein (HDL)-cholesterol. Lovastatin therapy decreased VLDL-triglycerides primarily by enhancing their catabolism, and lowered LDL-cholesterol levels mainly by reducing input rates for LDL. Overall, lovastatin appears to be an effective drug for the treatment of hyperlipidemia in the nephrotic syndrome.

Comparative effects of dietary fish oil and carbohydrate on plasma lipids and hepatic activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase and neutral lipase activities in the rat

In rats fed a fish oil-enriched diet, plasma triacylglycerols were lowered 51%. At the same time there was a mean 45% reduction in Mg2+-dependent phosphatidate phosphohydrolase activity in liver microsomes and a mean 20% decrease in microsomal triacylglycerol (neutral) and diacylglycerol hydrolase activities, but not of diacylglycerol acyltransferase. These observations support the hypothesis that decreases in the activities of phosphatidate phosphohydrolase and of both lipases are involved in the expression of the inhibitory effects of fish oil feeding on hepatic lipoprotein triacylglycerol secretion. Conversely, the feeding of a sucrose-enriched diet resulted in a mean 39% rise in plasma triacylglycerols, a 19% increase in triacylglycerol hydrolase and a mean 45% increase in Mg2+-dependent microsomal phosphohydrolase activity. The effects of the two nutritional interventions on phosphatidate phosphohydrolase activity confirm a key function for this enzyme in triacylglycerol formation.

Changes in apolipoprotein A-I mRNA level in the liver of rats with experimental nephrotic syndrome

In previous studies we had shown that: one of the most specific feature of hyperlipoproteinemia found in rats with experimental nephrotic syndrome is the accumulation of apolipoprotein A-I-rich HDL in plasma and this disorder is associated with an overproduction of apolipoprotein A-I by the liver. The present study was designed to investigate whether the increased hepatic synthesis of apolipoprotein A-I was due to an accumulation of functionally active apolipoprotein A-I mRNA in liver of nephrotic rats. Hepatic mRNA was translated in vitro by rabbit reticulocyte lysate in the presence of [35S]methionine and in vitro synthesized apolipoprotein A-I, albumin and apolipoprotein E were immunoprecipitated by specific rabbit IgG. In nephrotic rats the amount of in vitro synthesized apolipoprotein A-I was almost twice that found in the controls, suggesting that functionally active apolipoprotein A-I mRNA was increased in liver of nephrotic rats. To confirm that this difference in apolipoprotein A-I mRNA activity was due to an actual increase of hepatic apolipoprotein A-I mRNA sequences, we performed nucleic acid hybridization experiments (northern blot) using several cloned cDNA probes (rat and human apolipoprotein A-I, rat apolipoprotein E and apolipoprotein A-II). The results indicate that in nephrotic rats the amount of hybridizable apolipoprotein A-I mRNA sequences was about 3-fold higher than that in controls. In contrast, there was no difference in the amount of hybridizable apolipoprotein A-II and apolipoprotein E mRNA sequences, indicating that the change in apolipoprotein A-I mRNA induced by the nephrotic state was specific for this mRNA.

Serum and urinary high density lipoproteins in glomerular disease with proteinuria

The serum lipoprotein concentrations, including high-density lipoprotein (HDL) subfractions and apolipoproteins Al and B were measured in 21 patients (14 male and seven female) with nephrotic range proteinuria (greater than 3g/24hr), well maintained renal function (creatinine clearance greater than 35 mliter/min/1.73m2) and biopsy-proven primary glomerular disease. In these, and in a further five patients (creatinine clearance greater than 15 mliter/min/1.73m2), urinary apolipoprotein Al output was determined. Total HDL cholesterol was similar in patients and controls, but in male patients, HDL2 was low (0.54 +/- 0.10 mmole/liter, mean +/- SEM) compared to controls (0.75 +/- 0.04 mmole/liter, P less than HDL3 was high (0.81 +/- 0.07 in patients and 0.63 +/- 0.02 mmole/liter in controls, P less than 0.01). In women, there was a similar tendency for HDL2 to be lower in patients (0.68 +/- 0.18 mmole/liter) than in controls (0.85 +/- 0.10 mmole/liter). Multiple regression analysis revealed that major determinants of the urinary apolipoprotein Al output were the urinary protein output and selectivity index (multiple r = 0.85). Furthermore, some patients lost apolipoprotein Al into their urine at rates indicating increased production of apolipoprotein Al in the nephrotic syndrome. The serum HDL subfraction concentrations in the nephrotic syndrome could be explained by a combination of increased HDL production and increased urinary loss of low molecular wt HDL.

Is albumin synthesis regulated by the colloid osmotic pressure? Effect of albumin and dextran on albumin and total protein synthesis in isolated rat hepatocytes

In order to study the influence of the colloid osmotic pressure on albumin and total liver protein synthesis, rat hepatocytes were isolated by collagenase perfusion and incubated in Krebs-Ringer-buffer for 4 h. The colloid osmotic pressure produced by different bovine serum albumin (BSA) or dextran 60 concentrations varied from 3 to 80 mm Hg. A physiological colloid osmotic pressure of 20 mm Hg was obtained with 5.7 g BSA or 3.7 g dextran 60 per 100 ml of buffer. Albumin synthesis was measured by Laurell rocket immunoelectrophoresis. Total liver protein and total secretory protein synthesis were determined by the measurement of 1-14C-leucine incorporation. Albumin synthesis was not primarily regulated by the colloid osmotic pressure as was demonstrated by a lack of inhibition after addition of BSA. There was no significant influence of the oncotic pressure on the incorporation of 14C-leucine into total liver proteins. The incorporation into total secretory proteins was inhibited by an increasing colloid osmotic pressure, mediated either by BSA or dextran, suggesting an inhibition of the secretion of plasma proteins other than albumin.

Release of hepatic lipase and very low density lipoprotein by cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study the release of hepatic lipase and very low density lipoprotein (VLDL). The presence of hepatic lipase activity was proved by salt-resistance, affinity chromatography and inactivation by a hepatic lipase antibody. Cellular rate of hepatic lipase release increased by prolonged time in culture, whereas VLDL secretion decreased. Oleic acid and dextran-70 had no effect on release of hepatic lipase, whereas VLDL secretion was increased and decreased, respectively. Calcium antagonists (cobalt and verapamil), monensin and cycloheximide inhibited both the release of hepatic lipase and VLDL. Colchicine and chloroquine, which decreased VLDL secretion, had no effect on release of hepatic lipase. The present results suggest that release of hepatic lipase and secretion of VLDL are not coordinated and exhibit different sensitivity towards certain compounds altering secretory functions.

Stimulation by oleic acid of incorporation of L-[4,5-3H]leucine into very low density lipoprotein apoprotein by the isolated perfused rat liver

Livers from normal fed or fasted (24h) rats were perfused in vitro to determine whether fatty acid affects the biosynthesis of very low density lipoprotein (VLDL) apoprotein. Oleate stimulated VLDL triacylglycerol output and increased incorporation of L-[4,5-3H]leucine into VLDL apoprotein in both the fed and fasted groups. The increased incorporation of [3H]leucine was mainly into VLDL-apoprotein E. The total mass of VLDL apoprotein secreted was also stimulated by oleate proportionately. These data suggest that fatty acids may stimulate hepatic synthesis and/or secretion of the VLDL apoproteins and that apo E, may be required for the formation and secretion of triacyl-glycerol in the VLDL.

Characterisation of rat hepatocyte monolayers for investigation of the metabolism of bile salts

Rat hepatocyte monolayers were maintained for periods up to 24 h during which time their viability was greater than 85%. Using specific radioimmunoassays, the hepatocyte monolayers were shown to synthesise conjugated cholic, chenodeoxycholic and beta-muricholic acids. Feeding the bile salt sequestrant, cholestyramine, to donor animals increased synthesis of the major bile salt conjugates by the cells. Incubation of hepatocyte monolayers with bovine serum albumin decreased total synthesis of the three bile acids measured, but increased the amount of conjugated chenodeoxycholic acid detected. In order to test whether the effect of bovine serum albumin on bile salt synthesis was due to binding of bile salts, hepatocyte monolayers were incubated with antiserum to conjugated chenodeoxycholic acid. This treatment increased conjugated chenodeoxycholic acid production but had no effect on the other bile salt conjugates. It is concluded that the increase in conjugated chenodeoxycholic acid synthesis seen with bovine serum albumin and antiserum to conjugated chenodeoxycholic acid is caused by binding of the bile salt in the medium.

The hyperlipidemia of the nephrotic syndrome. Relation to plasma albumin concentration, oncotic pressure, and viscosity

Although hyperlipidemia is a common feature of the nephrotic syndrome, the distribution of cholesterol among the plasma lipoproteins and the mechanism of the enhanced hepatic synthesis of lipoprotein lipids are not well understood. We studied the distribution of cholesterol among the plasma lipoproteins, as well as the relation between total cholesterol and plasma albumin concentration, oncotic pressure, and viscosity in 20 consecutive adult patients with uncomplicated nephrotic syndrome. The total plasma cholesterol (mean +/- S.D., 302 +/- 100 mg per deciliter [7.8 +/- 2.6 mmol per liter]) and low-density-lipoprotein cholesterol concentrations (215 +/- 89 mg per deciliter [5.6 +/- 2.3 mmol per liter]) were elevated in most patients, but the high-density-lipoprotein cholesterol level was normal or low (46 +/- 18 mg per deciliter [1.2 +/- 0.5 mmol per liter]) in 95 per cent of the patients. Thus, many hypercholesterolemic patients with unremitting nephrotic syndrome may be at increased risk for atherosclerotic heart disease. A significant inverse correlation was found between the total plasma cholesterol concentration and both the plasma albumin concentration (r = -0.528) and the plasma oncotic pressure (r = -0.674), but not the plasma viscosity (r = +0.319). Enhanced hepatic synthesis of lipoprotein lipids may be stimulated by a decreased plasma albumin concentration or oncotic pressure but does not appear to be due to changes in plasma viscosity.

Regulation of plasma lipid levels by plasma viscosity in nephrotic rats

The viscosity of the extracellular medium of cultured hepatocytes has been shown to be a regulator of the secretion and synthesis of very low-density lipoproteins (Yedgar et al., J. Biol. Chem. 257: 2188-2192, 1982). At present, the role of plasma viscosity in regulation of plasma lipoprotein levels was examined in vivo using nephrotic hyperlipidemic rats. Plasma viscosity was increased by injection of macromolecules: simultaneously with induction of nephrosis by aminonucleoside; and after the lipid level had reached its maximum. In experiment 1 the elevation of plasma viscosity (which persisted for at least 2 days) delayed the development of the hyperlipidemia by at least 2 days. In experiment 2 increasing the plasma viscosity reduced plasma triglyceride and cholesterol levels by 70 and 40%, respectively, within 2 days. The hyperlipidemia was accompanied by increased plasma viscosity. The contribution of lipoproteins to plasma viscosity was 27% in the nephrotic-hyperlipidemic rats, compared with 4% in normal rats. It is suggested that plasma viscosity regulates lipoprotein levels in vivo concordant with the observation in cultured hepatocytes.

Lipoprotein synthesis and secretion by cultured rat hepatocytes. Parallel inhibition of secretion of VLDL, HDL and albumin by monensin

The biosynthesis and secretion of very-low-density lipoproteins (VLDL) and high-density lipoproteins (HDL) by cultured normal rat hepatocytes was investigated with particular emphasis on its modification by monensin. This acidic ionophore coordinately inhibited the rates of secretion of the several VLDL apolipoproteins and the VLDL lipids, suggesting an effect late in the process of biosynthesis and secretion, probably at the stage of exiting from the Golgi apparatus. The secretion of immunoreactive albumin into the medium was comparably inhibited, implying that the pathway and mechanisms involved in albumin secretion may be closely similar to those for VLDL synthesis and secretion. Secretion of phospholipids and of apolipoproteins E and A-I in the HDL fraction increased progressively with time over 18 h in control incubations but was strongly inhibited by monensin. During extended incubation with monensin at high concentrations (10 microM), there was a net release to the medium of a number of hepatocyte proteins, including some that comigrated with apolipoprotein A-I and apolipoprotein C, making it appear that monensin increased the secretion of these apolipoproteins. However, using labeled amino acids, it was shown by autoradiography and by immunoprecipitation that secretion of newly-synthesized, radioactive apolipoprotein A-I and apolipoprotein C was actually inhibited by monensin. These results are compatible with the conclusion that HDL synthesis and secretion may occur by mechanisms closely related to those for synthesis and secretion of albumin and VLDL.

Secretion of the newly synthesized cholesterol by rat hepatocytes in primary culture

We used monolayer cultured rat hepatocytes as an experimental model to study the secretion of the newly synthesized cholesterol by the liver. Cellular cholesterol was labeled by exposing cultured hepatocytes to [14C]acetate prior to the study of secretion. Secretion of the newly synthesized cholesterol was measured by extracting cholesterol in the culture medium and assaying for the radioactivity of [14C]cholesterol. We found that: (a) cultured hepatocytes could secrete newly synthesized cholesterol in serum-free medium; (b) secreted [14C]cholesterol was bound to macromolecule(s) and the secretion rate was not affected by cycloheximide for up to 5 h; (c) serum added to the culture medium greatly enhanced hepatic cholesterol secretion; (d) serum high-density lipoproteins were most effective, lipoprotein-deficient serum (d greater than 1.21) less effective in stimulating cholesterol secretion, whereas low-density and very-low-density lipoproteins had little effect; (e) when the serum-free culture medium was fractionated by ultracentrifugation, a major portion of the secreted [14C]cholesterol was found in the high-density lipoprotein fraction; (f) part of the medium [14C]cholesterol also turned up in the high-density lipoprotein fraction when lipoprotein-deficient serum was added as the acceptor; (g) secreted [14C]cholesterol was found only in free form, although some of the cellular [14C]cholesterol was found as esters.

Inhibition by chloroquine of the secretion of very low density lipoproteins by cultured rat hepatocytes

Cultured rat hepatocytes were incubated in medium containing 1.0 mM oleic acid. The incorporation of [3H]glycerol into cell-associated and medium triacylglycerols was measured after 2 h incubation. More than 95% of the secreted [3H]triacylglycerols were recovered in the very low density lipoprotein (VLDL) fraction (d less than 1.006). Chloroquine and other lysosomotropic amines promoted a marked decrease in [3H]triacylglycerol secretion from the hepatocytes while the synthesis was unaffected. At 50-200 microM final concentration, chloroquine inhibited secretion of triacylglycerols by 70-90% of the control. Similar results were obtained when the mass of secreted triacylglycerols was measured. Chloroquine caused decreased secretion of [3H]triacylglycerols after 15-30 min incubation and the inhibitory effect was completely reversible within 1-2 h after washout of chloroquine. The reduced triacylglycerol secretion was not due to increased reuptake of secreted lipoproteins or decreased protein synthesis caused by chloroquine. Electron microscopy of chloroquine-treated cells showed that the inhibition of VLDL secretion occurs at or prior to the level of the Golgi apparatus. These results suggest that chloroquine interferes with crucial steps in the secretory process and/or that lysosomal function could be essential for secretion of VLDL.

Isolation and analysis of lipoproteins secreted by rat liver hepatocytes

A procedure has been developed for the small-scale isolation and characterization of lipoproteins secreted by cultured rat liver hepatocytes. The lipoproteins in the culture medium were separated into VLDL, LDL, HDL and a fraction with d greater than 1.21 on single-spin density-gradients. The lipoproteins were removed from the gradients by adsorption onto Cab-O-Sil, a hydrated colloidal silica. The lipid components were extracted from the silica with CHCl3/CH3OH and the apoproteins solubilized in a buffer that contained 2% sodium dodecyl sulfate and 6 M urea. The proteins were analyzed on 3-20% acrylamide electrophoresis gels that contained 1% sodium dodecyl sulfate. The two major rat-plasma lipoproteins, VLDL and HDL, were well separated by the gradients. The Cab-O-Sil was shown to bind 90-95% of the HDL and VLDL in the fractions from the gradient. The recovery of the lipid components was essentially quantitative. The recovery of the apolipoproteins was only about 60% but with very good precision. Over a 20 h period, the lipid phosphorus associated with secreted lipoproteins increased linearly. The secretion of apolipoprotein A1 and apolipoprotein E associated with HDL and apolipoprotein B associated with VLDL also increased as a nearly linear function with time. The secretion of apolipoprotein E associated with VLDL was linear only up to approx. 6 h. The availability of this procedure should greatly facilitate further studies on the characterization of lipoproteins secreted by hepatocytes and mechanisms that regulate lipoprotein synthesis and secretion.

Regulation and interaction of cholesterol, bile salt and lipoprotein synthesis in liver

The liver is the junction of several inter-organ metabolic cycles which are essential for the homeostasis of mammalian metabolism. Two of these are described in greater detail and their role in control of lipid metabolism will be presented. The fatty acid-triglyceride cycle is of particular importance for our understanding of the mechanisms governing serum lipid levels. This is due to the fact that the lipoprotein secreted by the liver in the course of this metabolic cycle - very low density lipoprotein - has a relatively long half-life in the plasma compartment. Data have been collected from the literature to show that different nutritional and pharmacological stimuli affecting serum lipid levels do so by interfering with the rate of very low density lipoprotein input into the plasma compartment. The enterohepatic circulation of steroids is another cycle which contributes to control of lipid metabolism. Data are presented which show that bile acids, the major steroids circulating in this cycle, exert direct feedback control of hepatic cholesterol synthesis. This characteristic of bile acids may explain why certain bile acids, when given orally, reduce serum cholesterol levels. Several clinical and experimental observations suggest a close relation between bile acid and triglyceride metabolism. It is characterized by an inverse relation between bile acid pool size and serum triglyceride levels. Moreover, a reduction of the bile acid pool size is accompanied by an enhanced hepatic fatty acid and triglyceride synthesis and secretion into blood. The molecular basis and physiological significance of these observations have still to be explored.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of cultured hepatocytes from fat- and cholesterol-fed rats

Hepatocytes were enzymatically dissociated from the livers of rats maintained for eight weeks on diets of (1) standard rat chow, (2) chow plus 20% olive oil, and (3) chow plus 20% olive oil and 2% cholesterol. Cells were cultured for up to one week and characterized by phase contrast light microscopy and transmission electron microscopy. Cultured hepatocytes derived from the cholesterol-fed rats (3) were distinguishable by phase contrast light microscopy from those derived from chow-or chow plus olive oil-fed (1 and 2) cells, since they contained numerous refractile cytoplasmic structures. Transmission electron microscopy demonstrated that these structures were lipid droplets. Hepatocytes were observed to accumulate non-membrane bound cytoplasmic lipid droplets in the order control-fed less than olive oil-fed less than cholesterol-fed. In addition to varying quantitatively among the feeding groups, lipid droplets also varied in size and osmiophilia as a function of the feeding regimen. Other than the differential accumulation of lipid droplets, the ultrastructure of the cultured hepatocytes was similar among cells derived from the various feeding groups.