Secretion of nascent lipoproteins by isolated hepatocytes from hypothyroid and hypothyroid, hypercholesterolemic rats

Evaluation of cultured hamster hepatocytes as an experimental model for the study of very low density lipoprotein secretion

The secretion of triacylglycerol, cholesterol and cholesteryl ester in very low density lipoprotein (VLDL) by cultured hamster hepatocytes was studied, and the results compared with those obtained previously using cultured rat hepatocytes and the human hepatoma cell line HepG2. The hamster cells secreted apolipoprotein B and VLDL triacylglycerol, cholesterol and cholesteryl ester linearly during 24 h in culture, and this time period was used in all experiments. Addition of oleate (1 mM) to the culture medium resulted in increased secretion of triacylglycerol, but cholesterol ester output were unchanged. Triacylglycerol secretion was also increased in the presence of lipogenic substrates (10 mM lactate + 1 mM pyruvate) plus dexamethasone (1 microM), but not with either of these agents alone. Inhibition of cholesterol synthesis in the hamster cells by incubation with mevinolin (2 micrograms/ml) did not change VLDL lipid secretion, but stimulation using mevalonate lactone resulted in decreased triacylglycerol output. Manipulation of the rate of cholesterol esterification in the hepatocytes by inhibiting or stimulating the activity of acyl coenzyme A cholesterol:acyl transferase using the inhibitor Dup128 (25 microM) and 25-hydroxycholesterol (50 microM), respectively, had no effect on the secretion of VLDL lipid. In the presence of 1 mM oleate plus 25-hydroxycholesterol, however, a rise in the output of triacylglycerol and cholesteryl ester was observed. Hepatocytes prepared from hamsters fed 2% cholestyramine secreted significantly less triacylglycerol than those from animals given the control diet, but cholesterol and cholesteryl ester output were unchanged, despite a decrease of about 40% in the total cholesterol content of the cells. These results show that the secretion of lipid in VLDL in hamster hepatocytes differs from that in rat and human liver in its response to dietary cholestyramine, and from rat hepatocytes and HepG2 cells in its response to changes in the rate of lipogenesis and cholesterol synthesis and esterification. Overall, hamster hepatocytes appear to be less susceptible to modification the rate of hepatic VLDL secretion, and should provide a useful additional tool for the investigation of this process.

Secretion of apolipoprotein E by an astrocytoma cell line

Apolipoprotein (apo) E is a predominant protein in developing mammalian brain and in damaged peripheral nerve. Of particular interest is the observation that astrocytes in the central nervous system cease to produce apoE after nerve damage, whereas an increase in apoE production results after peripheral nerve injury. Differences in the response to injury with regard to the production of apoE may be related to dissimilarities in the abilities of the central and peripheral nervous systems to regenerate. As there are few data concerning the regulation of apoE gene expression in extrahepatic tissues, we employed a human astrocytoma cell line (CCF-STTG1) as a model to study apoE production in astrocytes. CCF-STTG1 cells secreted apoE constitutively in serum-free media. Cholesterol added to the media as cholesterol:phospholipid liposomes (2-100 micrograms/ml) or as human plasma LDL increased the amount of apoE secreted into the media, but had little or no effect on the relative abundance of apoE mRNA. By contrast, the commercially available triglyceride-phospholipid emulsion Intralipid added at dilutions of 1:50 to 1:500 caused a total inhibition of apoE secretion by the cells, but again, little change was noted in the relative abundance of apoE mRNA. Insulin (5 micrograms/ml) caused a 45-55% reduction in the amount of apoE secreted by the astrocytoma cells. Glucagon (5 micrograms/ml), on the other hand, did not increase apoE secretion, and apoE mRNA concentrations were not affected by either hormone treatment. ApoE was secreted from the astrocytoma cells associated with particles of plasma VLDL to IDL and HDL size. After feeding the cells with 20 micrograms/ml cholesterol as cholesterol:phospholipid liposomes, an increased proportion of apoE was secreted associated with the larger VLDL to IDL size particles, with a concomitant decrease in the proportion associated with the smaller HDL-size particles. When cells were incubated with 5 micrograms/ml insulin, most of the apoE was associated with the HDL-size particles. When cholesterol:phospholipid liposomes were added in the presence of insulin virtually all of the secreted apoE was found associated with the VLDL to IDL size particles. In summary, the regulation of apoE production in CCF-STTG1 cells in many respects resembles that of other cells, including hepatocytes. However, it is clear that there remain to be identified cell specific factors which regulate apoE production in astrocytes. The CCF-STTG1 cell line promises to provide a suitable model to investigate these questions.

Independent regulation of chylomicron lipolysis and particle removal rates: effects of insulin and thyroid hormones on the metabolism of artificial chylomicrons

The processes of chylomicron lipolysis and removal from plasma were investigated by the intra-arterial infusion of doubly labeled artificial chylomicrons in rats. The rate of lipolysis was measured as a delipidation index (DI), which is the glyceryl-tri-9,10(N)-3H oleate (3H-TO) fraction removed from the particle as fatty acids, whereas the cholesteryl(1-14C) oleate (14C-CO) plasma disappearance rate measures the splanchnic organ particle uptake. In the alloxan-diabetic rats, despite a normal DI, the 14C-CO plasma residence time (RT) was longer than in control animals and remained longer after stimulation of the lipoprotein lipase by heparin. DI and 14C-CO removal rate were not significantly altered by insulin administration to glucose-supplemented control rats. Lipolysis was remarkable in propylthiouracil (PTU)-induced hypothyroidism, and yet the 14C-CO removal rate was retarded. In hypothyroidism, heparin enhanced the 14C-CO removal more than in the control group; however, after heparin, the 14C-CO RT still remained higher in the hypothyroid animals as compared with the control group. Hyperthyroidism lowered the DI; nevertheless, the 14C-CO disappearance rate was faster than in controls. In summary, lack or excess of thyroid hormone influences both the chylomicron lipolysis and removal systems, whereas lack of insulin impairs mostly the particle removal from plasma, and excess of insulin has no effect on the chylomicron metabolism.

Effects of hypothyroidism and high-fat feeding on mRNA concentrations for the low-density-lipoprotein receptor and on acyl-CoA:cholesterol acyltransferase activities in rat liver

1. Induction of hypothyroidism in rats by feeding propylthiouracil (PTU) significantly increased serum cholesterol concentrations, and the effect was more pronounced for cholesterol in low-density lipoproteins (LDL) rather than high-density lipoproteins (HDL). The concentrations of serum triacylglycerol were decreased in hypothyroidism. These effects on serum lipids were also seen when the normal rats were pair-fed with the PTU-treated group. 2. Feeding a diet rich in saturated fat and cholesterol further increased cholesterol concentrations in LDL and also elevated that in very-low-density lipoprotein (VLDL) of hypothyroid rats. In euthyroid rats such a diet resulted in a relatively small increase in VLDL cholesterol, whereas LDL cholesterol was decreased. 3. Steady-state concentrations of mRNA for the hepatic LDL receptor were significantly decreased in the livers of hypothyroid rats, but were not significantly changed by high-fat feeding in euthyroid or hypothyroid rats. 4. The expression of the LDL receptor in hepatocytes cultured from hypothyroid rats was decreased relative to the euthyroid controls. 5. Whereas the esterification of cholesterol with oleate in hepatocytes cultured from hypothyroid rats was decreased, the activity of acyl-CoA:cholesterol acyltransferase (ACAT) in the livers of these animals was not changed. 6. High-fat feeding increased the hepatic ACAT activity in normal and hypothyroid rats. 7. Incubation of rat hepatocytes with 10 nM-tri-iodothyronine for 4 h increased the relative concentration of the mRNA for the LDL receptor by 25%. 8. It is therefore concluded that thyroid hormones stimulate the synthesis and expression of the hepatic LDL receptor. Elevated cholesterol concentrations in LDL in hypothyroidism probably result from a primary defect in the expression of the hepatic receptor, rather than indirectly via changes in ACAT activity.

Triiodothyronine increases rat apolipoprotein A-I synthesis and alters high-density lipoprotein composition in vivo

The effects of altered serum 3,3',5-triiodothyronine levels on rat lipoprotein metabolism were examined. Daily injections of the hormone (50 micrograms/100 g body mass) over a period of six days led to an increase of 6.4-fold in the hepatic mRNA level for apolipoprotein(apo)A-I, and a 21% increase in serum apoA-I levels. 12h after a single injection of 3,3',5-triiodothyronine the rate of [14C]leucine incorporation into apoA-I increased 2.1 fold. Conversely, in hypothyroid rats there was a decrease in hepatic mRNA levels for apoA-I and a decreased rate of [14C]leucine incorporation into apoA-I. The increase in hepatic apoA-I mRNA levels following 3,3',5-triiodothyronine treatment occurred prior to significant changes in serum triacylglycerol levels. High-density lipoprotein (HDL) particles isolated from the serum of hyperthyroid rats were smaller and enriched in apoA-I compared to apoA-IV and apoE. Similar changes in HDL composition were observed following in vitro incubations of normal rat serum with purified rat apoA-I. The results suggest that during altered thyroid status, changes in serum HDL size and composition occur in association with significant changes in apoA-I gene expression.

Effect of age on serum lipids and lipoproteins of male and female JCR:LA-corpulent rats

The JCR:LA-cp rat is a strain incorporating the corpulent (cp) gene. When homozygous for the cp gene, the rats are hyperphagous, hyperinsulinemic, hyperlipidemic and obese. The corpulent male rats develop atherosclerotic and myocardial lesions from an early age, while corpulent female and lean rats do not develop lesions. The hyperlipidemia is due to elevated levels of VLDL resulting in moderately raised cholesterol levels and markedly elevated triacylglycerol levels. The VLDL concentrations are similar in corpulent male and female rats at an early age with both having much higher levels than lean rats. As the animals age, the VLDL hyperlipidemia in the corpulent male increases at 3 months and then decreases slowly and rises again at 12 months of age. The corpulent female rats show higher triacylglycerol and phospholipid concentrations than the males at 3 months age and reach values over 1000 mg/100 ml by 9 months of age, then decrease at 12 months of age. The cholesterol concentrations of the corpulent females are greater than those of the males from 9 months of age. Thus, in the period of life up to middle age, the cardiovascular disease incidence does not correlate with the degree of hyperlipidemia. The disease progression does correlate with the severity of insulin resistance and glucose intolerance, which is more severe in the corpulent male than female rats. The results suggest that the hyperlipidemia must be a necessary condition for development of atherosclerotic disease in this strain of rats, but it is not a sufficient condition.

Bile acid synthesis and intracellular and extracellular cholesterol concentrations in isolated rat hepatocytes: the effect of dietary cholesterol

Bile acid synthesis in isolated hepatocytes prepared from rats given 1% cholesterol in the diet and incubated for 1 h in suspension was not increased compared to that in cells from control rats. When the hepatocytes were maintained in monolayer culture for 24 h, however, increased production of bile acid (X2.5) was observed in the cholesterol-fed group. The amount of bile acid synthesised during incubation in suspension was significantly correlated with intracellular unesterified cholesterol levels, but showed no correlation with intracellular esterified or medium cholesterol concentrations after 1 h. Bile acid production in hepatocytes maintained in monolayer culture was also significantly correlated with the intracellular unesterified, but not esterified, cholesterol content. In addition, in this case, there was a significant correlation with the levels of both unesterified and esterified cholesterol found in the medium after 24 h. These results suggest that the amount of cholesterol available to liver cells from extracellular sources has a role in the regulation of bile acid synthesis in cholesterol-fed rats, while the concentrations of esterified cholesterol stored within the cells are not important in this process.

Regulation of intestinal and hepatic apoprotein synthesis after chronic fat and cholesterol feeding

Although diet influences levels of lipoproteins and their corresponding apoproteins, its effects on the molecular regulation of apoprotein synthesis are relatively unknown. Male Sprague-Dawley rats were fed an atherogenic diet containing cholesterol and propylthiouracil (PTU). Intestinal apo AI and AIV mRNA concentrations were decreased by the atherogenic diet, but apo AI and AIV synthesis was increased in vitro (organ explants) and in vivo (polysome runoff), consistent with regulation at the translational level. In contrast, hepatic apo E mRNA concentration and synthesis were increased after the atherogenic diet, consistent with pretranslational regulation. The response to cholesterol feeding for hepatic apo AI and E showed a third pattern of regulation, in which synthesis increased and mRNA content remained stable or fell, again suggesting translational control, but polysome runoff synthesis was unchanged. The apparent importance of translational regulation in the intestine is consistent with the necessity for the tissue to respond rapidly to changes in intraluminal content.

Atherogenesis in two strains of obese rats. The fatty Zucker and LA/N-corpulent

Two strains of obese rats, the fatty Zucker and the LA/N-corpulent have been compared at 6 months age for the presence of vascular and myocardial disease. Both strains, when obese, exhibit a VLDL hyperlipidemia with elevated triglycerides and moderate elevations of plasma cholesterol concentrations compared to the lean rats of the same strain. The hyperlipidemia is more modest in the fatty Zucker than the corpulent LA/N, and the serum lipid concentrations of the lean Zucker are lower than those of the lean LA/N. Apolipoprotein concentrations were similar and elevated in the two obese genotypes compared to the lean genotypes which were also similar to each other. Male and female obese animals of both strains exhibited hyperinsulinemia under fasting conditions and after oral glucose, with obese male LA/N rats exhibiting the most severe hyperinsulinemia. Glucose tolerance was impaired in obese LA/N animals but was normal in lean rats of both strains and fatty Zucker rats of both sexes. The glucose intolerance observed in obese LA/N animals was more severe in the male than in the female rats. Unlike the corpulent rat, which develops atherosclerotic lesions, the fatty Zucker shows no evidence of advanced vascular lesions on scanning electron microscopy. The fatty Zucker also does not develop the myocardial lesions that are frequent in the male corpulent LA/N rat. It is suggested that the initiation of the atherogenic process is dependent upon elevated insulin levels or transient hyperglycemia. Development of the advanced lesions appears to require the presence of hyperlipidemia.

Serum lipids and lipoproteins in the atherosclerosis prone LA/N corpulent rat

The LA/N rat is one of two congenic strains bred from the original obese, hyperphagic and hypertensive rats of Koletsky. With the exception of hypertension the LA/N strain, when homozygous for the corpulent gene, is phenotypically similar to the parent Koletsky strain and prone to the development of vascular and myocardial lesions. Here we report a detailed analysis of the serum lipids, lipoproteins and apolipoproteins B, E and A-I levels in young adult homozygous corpulent (cp/cp) rats of both sexes and in lean males of the same age which were demonstrable non-carriers (+/+) of the cp gene. Both male and female cp/cp rats were hypertriglyceridemic (282-512 mg/100 ml) and moderately hypercholesterolemic (74-84 mg/100 ml). Elevations in these lipids reflected the presence of large (622 A), triacylglycerol-rich and apoprotein-poor VLDL containing both apolipoproteins Bh and B1 and increased phospholipid-rich HDL. Similar, but less pronounced, elevations in serum apolipoproteins B and E in the cp/cp rats when compared to the +/+ animals were also noted. Apolipoproteins A-I levels were 2.7-3-fold higher in cp/cp rats. The levels of VLDL were significantly higher in female cp/cp rats; however, the levels of IDL (intermediate-density lipoproteins), LDL and HDL were significantly lower than in the more atherosclerosis prone male cp/cp rats. Similarly, apolipoprotein A-I was higher and apolipoprotein B lower in the male cp/cp than in the female cp/cp rats. The LDL (d = 1.030-1.063 g/ml) in cp/cp rats, like that in normal animals, was heterogeneous and contained apolipoproteins Bh, E, A-I and C. This fraction was significantly elevated in male cp/cp rats when compared to females but still represented less than 13% of the total serum cholesterol and less than 6% of the total serum lipids in 3-month-old cp/cp animals. The ratio of cholesterol to phospholipids was significantly lower for all lipoproteins in cp/cp rats when compared to +/+ males and these ratios for female cp/cp rats were in all cases lower than those of male cp/cp animals.

Quantification of apolipoproteins in rat serum and in cultured rat hepatocytes by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) has been developed to measure apolipoproteins in rat serum. Nondelipidated whole serum was heat-treated at 52 degrees C for 3 h in phosphate-buffered saline containing 0.1% Tween-20 before assay. Monospecific rabbit anti-rat apolipoprotein antibodies were added to 96-well polystyrene microtiter plates which had been coated with purified rat serum apolipoproteins or unknown samples. After incubation and washing, goat anti-rabbit serum antibodies conjugated with horseradish peroxidase were added to the plates and incubated. The bound peroxidase activity was assayed after further washing. Serum apolipoprotein concentrations were calculated by comparison against purified standards that were assayed simultaneously with the unknown samples. The intraassay coefficients of variation for apolipoprotein AI, E, and AIV (Apo AI, E, and AIV) were 2.3, 4.4, and 5.3%, and interassay coefficients of variation were 6.1, 5.5, and 7.9%, respectively. The ELISA assay is sensitive to nanogram quantities of rat serum apolipoproteins and the results agree well with those measured by densitometry. The serum concentrations of Apo AI, E, and AIV of a normal fed rat were found to be 504 +/- 8, 413 +/- 20, and 262 +/- 20 micrograms/ml, respectively. When cultured as monolayers in Waymouth's medium for 1 day, rat hepatocytes secreted Apo AI, E, and AIV at rates of 2.51, 61.8, and 48.9 ng protein/mg cell protein/h.

Post-secretory acquisition of apolipoprotein E by nascent rat hepatic very-low-density lipoproteins in the absence of cholesteryl ester transfer

Previous work has shown that nascent hepatic very-low-density lipoproteins (VLDL) in the rat are biosynthesized without the obligatory co-factor (apolipoprotein C-II) for lipoprotein lipase-mediated hydrolysis of their core triacylglycerols. Upon secretion, apolipoproteins C-II and C-III are rapidly transferred to the particles from high-density lipoprotein (HDL) within the space of Disse and upon the entry into the plasma. Here we extend those studies to include observations on the apolipoprotein E content and lipid composition of nascent hepatic VLDL before and after exposure to plasma components. We have elected to use hepatic secretory vesicle VLDL rather than liver perfusate VLDL as truly representative of the nascent lipoproteins. Nascent VLDL from fed rats has an apolipoprotein B/E ratio of 6.6 +/- 0.5, whereas that from fasted animals is 13.9 +/- 2.3. Incubation of nascent VLDL from fed and fasted rats with d greater than 1.063 g/ml rat serum, HDL or the d greater than 1.21 g/ml fraction resulted in a mass transfer of apolipoprotein E to the VLDL such that the apolipoprotein B/E ratio decreased to at least that of serum VLDL (3.4 +/- 0.3). The d greater than 1.21 g/ml fraction appeared to contain a species of apolipoprotein E which most actively transferred to VLDL. The acquisition of apolipoprotein E by nascent secretory vesicle VLDL was attended by a loss of phospholipids, particularly the C40 (stearoylarachidonyl) molecular species, and an increase in the cholesterol-to-phospholipid ratio from 0.11 +/- 0.01 to 0.18 +/- 0.03. No evidence was obtained to suggest a simultaneous acquisition of cholesteryl esters upon incubation of nascent VLDL with VLDL-free serum. We conclude that nascent hepatic VLDL is modified after secretion by acquisition of apolipoproteins C-II, C-III and E with a concomitant loss of phospholipids.

Regulation of biliary cholesterol secretion in the rat. Role of hepatic cholesterol esterification

Although the significance of the enterohepatic circulation of bile salts in the solubilization and biliary excretion of cholesterol is well established, little is known about the intrahepatic determinants of biliary cholesterol output. Studies were undertaken to elucidate some of these determinants in the rat. Feeding 1% diosgenin for 1 wk increased biliary cholesterol output and saturation by 400%. Bile flow, biliary bile salt, phospholipid and protein outputs remained in the normal range. When ethynyl estradiol (EE) was injected into these animals, biliary cholesterol output decreased to almost normal levels under circumstances of minor changes in the rates of biliary bile salt and phospholipid outputs. Similarly, when chylomicron cholesterol was intravenously injected into diosgenin-fed animals, biliary cholesterol output significantly decreased as a function of the dose of chylomicron cholesterol administered. Relative rates of hepatic cholesterol synthesis and esterification were measured in isolated hepatocytes. Although hepatic cholesterogenesis increased 300% in diosgenin-fed animals, the contribution of newly synthesized cholesterol to total biliary cholesterol output was only 19 +/- 9%, compared with 12 +/- 6% in control and 15 +/- 5% in diosgenin-fed and EE-injected rats. The rate of oleate incorporation into hepatocytic cholesterol esters was 30% inhibited in diosgenin-fed rats. When EE was injected into these animals, the rate of cholesterol esterification increased to almost 300%. To investigate further the interrelationship between hepatic cholesterol esterification and biliary cholesterol output, we studied 21 diosgenin-fed rats. Six of them received in addition EE and 10 received chylomicron cholesterol. The relationships between biliary cholesterol output as a function of both microsomal acyl-CoA:cholesterol acyltransferase (ACAT) activity and hepatic cholesterol ester concentration were significantly correlated in a reciprocal manner. From these results it is concluded that the size of the biliary cholesterol precursor pool can be rapidly modified through changes in the activity of the hepatic ACAT.

Determination of plasma total lipid profiles by capillary gas-liquid chromatography

We have obtained improved qualitative and quantitative information about plasma lipid composition by adopting capillary gas-liquid chromatography to the determination of plasma total lipid profiles. The new procedure takes advantage of the good recoveries of high molecular weight solutes provided by short capillary columns as well as of the absence of sample fractionation during on-column injection. The major improvements in the gas-liquid chromatographic determination of plasma lipid profiles are the greatly increased carbon number resolution of the solutes and the minimal sample size requirements on the capillary columns. In many instances there is a partial resolution of the saturated and unsaturated molecules within a carbon number, which provides clues to the fatty acid composition of the molecular species. In other respects the new plasma total lipid profiles are similar to those obtained on packed columns. The plasma lipids are dephosphorylated by phospholipase C and trimethylsilylated before analysis.

Effect of dexamethasone on mannolipid synthesis by hepatocytes prepared from control and inflamed rats

Hepatocytes were prepared from control and inflamed rats. Mannose incorporation into dolichol monophosphate mannose in homogenate and microsomal fraction of the hepatocytes was increased 2-fold over the controls 24 h after induction of inflammation by turpentine injection. Incubation of hepatocytes from both control and inflamed rats with 0.1-10 microM-dexamethasone produced a 1.5-fold increase of dolichol phosphate mannose formation, whereas, 100 microM-dexamethasone decreased its formation. The increase in the ratio of dolichol phosphate mannose formation in inflamed over controls was virtually eliminated when the cell homogenate assay mixtures included 30 nmol of exogenous dolichol phosphate. This supports the earlier suggestion that the increase in the enzyme activity in inflammation could be due to higher concentrations of endogenous dolichol phosphate [ Coolbear & Mookerjea (1981) J. Biol. Chem. 256, 4529-4535]. In contrast, the increase in the ratio of dolichol phosphate mannose formation between dexamethasone-treated and untreated hepatocytes remained unchanged when increasing concentrations of exogenous dolichol phosphate were added to the assays. This suggests that the increase in glycosylation of dolichol phosphate in dexamethasone-treated hepatocytes is probably due to the increased mannosyltransferase activity, rather than due to higher concentrations of endogenous dolichol phosphate in these cells.