Cholesterol is required for the secretion of the very-low-density lipoprotein: in vivo studies

Regulation of cholesterol metabolism during high fatty acid-induced lipid deposition in calf hepatocytes

Cholesterol in the circulation is partly driven by changes in feed intake, but aspects of cholesterol metabolism during development of fatty liver are not well known. The objective of this study was to investigate mechanisms of cholesterol metabolism in calf hepatocytes challenged with high concentrations of fatty acids (FA). To address mechanistic insights regarding cholesterol metabolism, liver samples were collected from healthy control dairy cows (n = 6; 7-13 d in milk) and cows with fatty liver (n = 6; 7-11 d in milk). In vitro, hepatocytes isolated from 3 healthy female calves (1 d old) were challenged with or without a mix of 1.2 mM FA to induce metabolic stress. In addition, hepatocytes were processed with 10 µmol/L of the cholesterol synthesis inhibitor simvastatin or 6 µmol/L of the cholesterol intracellular transport inhibitor U18666A with or without the 1.2 mM FA mix. To evaluate the role of cholesterol addition, hepatocytes were treated with 0.147 mg/mL methyl-β-cyclodextrin (MβCD + FA) or 0.147 mg/mL MβCD with or without 10 and 100 µmol/L cholesterol before incubation with FA (CHO10 + FA and CHO100 + FA). In vivo data from liver biopsies were analyzed by 2-tailed unpaired Student's t-test. Data from in vitro calf hepatocytes were analyzed by one-way ANOVA. Compared with healthy cows, blood plasma total cholesterol and plasma low-density lipoprotein cholesterol content in cows with fatty liver was markedly lower, whereas the hepatic total cholesterol content did not differ. In contrast, compared with healthy controls, the triacylglycerol content in the liver and the content of FA, β-hydroxybutyrate, and aspartate aminotransferase in the plasma of cows with fatty liver were greater. The results revealed that both fatty liver in vivo and challenge of calf hepatocytes with 1.2 mM FA in vitro led to greater mRNA and protein abundance of sterol regulatory element binding transcription factor 1 (SREBF1) and fatty acid synthase (FASN). In contrast, mRNA and protein abundance of sterol regulatory element binding transcription factor 2 (SREBF2), acyl coenzyme A-cholesterol acyltransferase, and ATP-binding cassette subfamily A member 1 (ABCA1) were lower. Compared with the FA group, the cholesterol synthesis inhibitor simvastatin led to greater protein abundance of microsomal triglyceride transfer protein and mRNA abundance of SREBF2, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), ACAT2, and lower ABCA1 and FASN protein abundance. In contrast, compared with the FA group, the cholesterol intracellular transport inhibitor U18666A + FA led to greater total cholesterol concentration and greater protein and mRNA abundance of FASN. Compared with the MβCD + FA group, the addition of 10 µmol/L cholesterol led to greater concentration of cholesteryl ester and excretion of apolipoprotein B100, and greater protein and mRNA abundance of ABCA1 and microsomal triglyceride transfer protein, and lower concentration of malondialdehyde. Overall, a reduction in cholesterol synthesis promoted FA metabolism in hepatocytes likely to relieve the oxidative stress caused by the high FA load. The data suggest that maintenance of normal cholesterol synthesis promotes very low-density lipoprotein excretion and can reduce lipid accumulation and oxidative stress in dairy cows that experience fatty liver.

Multiple effects of cold exposure on livers of male mice

Cold exposure of mice is a common method to stimulate brown adipose tissue (BAT) activity and induce browning of white adipose tissue (WAT) that has beneficial effects on whole-body lipid metabolism, including reduced plasma triglyceride (TG) concentrations. The liver is a key regulatory organ in lipid metabolism as it can take up as well as oxidize fatty acids. The liver can also synthesize, store and secrete TGs in VLDL particles. The effects of cold exposure on murine hepatic lipid metabolism have not been addressed. Here, we report the effects of 24-h exposure to 4°C on parameters of hepatic lipid metabolism of male C57BL/6J mice. Cold exposure increased hepatic TG concentrations by 2-fold (P < 0.05) but reduced hepatic lipogenic gene expression. Hepatic expression of genes encoding proteins involved in cholesterol synthesis and uptake such as the LDL receptor (LDLR) was significantly increased upon cold exposure. Hepatic expression of Cyp7a1 encoding the rate-limiting enzyme in the classical bile acid (BA) synthesis pathway was increased by 4.3-fold (P < 0.05). Hepatic BA concentrations and fecal BA excretion were increased by 2.8- and 1.3-fold, respectively (P < 0.05 for both). VLDL-TG secretion was reduced by approximately 50% after 24 h of cold exposure (P < 0.05). In conclusion, cold exposure has various, likely intertwined effects on the liver that should be taken into account when studying the effects of cold exposure on whole-body metabolism.

Atorvastatin and pitavastatin enhance lipoprotein lipase production in L6 skeletal muscle cells through activation of adenosine monophosphate-activated protein kinase

Pravastatin and atorvastatin increase the serum level of lipoprotein lipase (LPL) mass in vivo but do not increase LPL activity in 3T3-L1 preadipocytes in vitro. LPL is mainly produced by adipose tissue and skeletal muscle cells. Metformin enhances LPL in skeletal muscle through adenosine monophosphate-activated protein kinase (AMPK) activation but not in adipocytes. This study aimed to examine the effect of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) on LPL production and to investigate the mechanism by which statins enhance skeletal muscle cell LPL production. L6 skeletal muscle cells were incubated with pravastatin, simvastatin, atorvastatin or pitavastatin. LPL activity, protein levels and mRNA expression were measured. Atorvastatin and pitavastatin significantly increased LPL activity, protein levels and mRNA expression in L6 skeletal muscle cells at 1 μmol/L, but neither statin had an effect at 10 μmol/L. We measured AMPK to clarify the mechanism by which statins increase LPL production in skeletal muscle cells. At 1 μmol/L, both atorvastatin and pitavastatin enhanced AMPK activity, but this enhancement was abolished when AMPK signaling was blocked by compound C. The increased expressions of LPL protein and mRNA by atorvastatin and pitavastatin were reduced by compound C. In addition, mevalonic acid abolished atorvastatin- and pitavastatin-induced AMPK activation and LPL expression. These results suggest that atorvastatin and pitavastatin increase LPL activity, protein levels and LPL mRNA expression by activating AMPK in skeletal muscle cells.

Hepatic ABCA1 and VLDL triglyceride production

Elevated plasma triglyceride (TG) and reduced high density lipoprotein (HDL) concentrations are prominent features of metabolic syndrome (MS) and type 2 diabetes (T2D). Individuals with Tangier disease also have elevated plasma TG concentrations and a near absence of HDL, resulting from mutations in ATP binding cassette transporter A1 (ABCA1), which facilitates the efflux of cellular phospholipid and free cholesterol to assemble with apolipoprotein A-I (apoA-I), forming nascent HDL particles. In this review, we summarize studies focused on the regulation of hepatic very low density lipoprotein (VLDL) TG production, with particular attention on recent evidence connecting hepatic ABCA1 expression to VLDL, LDL, and HDL metabolism. Silencing ABCA1 in McArdle rat hepatoma cells results in diminished assembly of large (>10nm) nascent HDL particles, diminished PI3 kinase activation, and increased secretion of large, TG-enriched VLDL1 particles. Hepatocyte-specific ABCA1 knockout (HSKO) mice have a similar plasma lipid phenotype as Tangier disease subjects, with a two-fold elevation of plasma VLDL TG, 50% lower LDL, and 80% reduction in HDL concentrations. This lipid phenotype arises from increased hepatic secretion of VLDL1 particles, increased hepatic uptake of plasma LDL by the LDL receptor, elimination of nascent HDL particle assembly by the liver, and hypercatabolism of apoA-I by the kidney. These studies highlight a novel role for hepatic ABCA1 in the metabolism of all three major classes of plasma lipoproteins and provide a metabolic link between elevated TG and reduced HDL levels that are a common feature of Tangier disease, MS, and T2D. This article is part of a Special Issue entitled: Triglyceride Metabolism and Disease.

A novel role for ABCA1-generated large pre-beta migrating nascent HDL in the regulation of hepatic VLDL triglyceride secretion

In Tangier disease, absence of ATP binding cassette transporter A1 (ABCA1) results in reduced plasma HDL and elevated triglyceride (TG) levels. We hypothesized that hepatocyte ABCA1 regulates VLDL TG secretion through nascent HDL production. Silencing of ABCA1 expression in oleate-stimulated rat hepatoma cells resulted in: 1) decreased large nascent HDL (>10 nm diameter) and increased small nascent HDL (<10 nm) formation, 2) increased large buoyant VLDL1 particle secretion, and 3) decreased phosphatidylinositol-3 (PI3) kinase activation. Nascent HDL-containing conditioned medium from rat hepatoma cells or HEK293 cells transfected with ABCA1 was effective in increasing PI3 kinase activation and reducing VLDL TG secretion in ABCA1-silenced hepatoma cells. Addition of isolated large nascent HDL particles to ABCA1-silenced hepatoma cells inhibited VLDL TG secretion to a greater extent than small nascent HDL. Similarly, addition of recombinant HDL, but not human plasma HDL, was effective in attenuating TG secretion and increasing PI3 kinase activation in ABCA1-silenced cells. Collectively, these data suggest that large nascent HDL particles, assembled by hepatic ABCA1, generate a PI3 kinase-mediated autocrine signal that attenuates VLDL maturation and TG secretion. This pathway may explain the elevated plasma TG concentration that occurs in most Tangier subjects and may also account, in part, for the inverse relationship between plasma HDL and TG concentrations in individuals with compromised ABCA1 function.

Knockdown of FABP5 mRNA decreases cellular cholesterol levels and results in decreased apoB100 secretion and triglyceride accumulation in ARPE-19 cells

To maintain normal retinal function, retinal pigment epithelial (RPE) cells engulf photoreceptor outer segments (ROS) enriched in free fatty acids (FFAs). We have previously demonstrated fatty acid-binding protein 5 (FABP5) downregulation in the RPE/choroidal complex in a mouse model of aging and early age-related macular degeneration. FABPs are involved in intracellular transport of FFAs and their targeting to specific metabolic pathways. To elucidate the role of FABP5 in lipid metabolism, the production of the FABP5 protein in a human RPE cell line was inhibited using RNA interference technology. As a result, the levels of cholesterol and cholesterol ester were decreased by about 40%, whereas FFAs and triglycerides were increased by 18 and 67% after siRNA treatment, respectively. Some species of phospholipids were decreased in siRNA-treated cells. Cellular lipid droplets were evident and apoB secretion was decreased by 76% in these cells. Additionally, we discovered that ARPE-19 cells could synthesize and secrete Apolipoprotein B100 (apoB100), which may serve as a backbone structure for the formation of lipoprotein particles in these cells. Our results indicate that FABP5 mRNA knockdown results in the accumulation of cellular triglycerides, decreased cholesterol levels, and reduced secretion of apoB100 protein and lipoprotein-like particles. These observations indicated that FABP5 plays a critical role in lipid metabolism in RPE cells, suggesting that FABP5 downregulation in the RPE/choroid complex in vivo might contribute to aging and early age-related macular degeneration.

Influence of simvastatin on apoB-100 secretion in non-obese subjects with mild hypercholesterolemia

Statins decrease apoB-100-containing lipoproteins by increasing their fractional catabolic rates through LDL receptor-mediated uptake. Their influence on hepatic secretion of these lipoproteins is controversial. The objective of the study was to examine the influence of simvastatin on the secretion of apoB-100-containing lipoproteins in fasting non-obese subjects. Turnover of apoB-100-containing lipoproteins was investigated using stable isotope-labeled tracers. Multicompartmental modeling was used to derive kinetic parameters. Eight male subjects (BMI 25 +/- 3 kg/m(2)) with mild hypercholesterolemia (LDL cholesterol 135 +/- 30 mg/dL) and normal triglycerides (111 +/- 44 mg/dL) were examined under no treatment (A), under chronic treatment with simvastatin 40 mg/day (B) and after an acute-on-chronic dosage of 80 mg simvastatin under chronic simvastatin treatment (C). Lipoprotein concentrations changed as expected under 40 mg/day simvastatin. Fractional catabolic rates increased in IDL and LDL but not in VLDL fractions versus control [VLDL +35% in B (n.s.) and +21% in C (n.s.); IDL +169% in B (P = 0.08) and +187% in C (P = 0.032); LDL +87% in B (P = 0.025) and +133% in C (P = 0.025)]. Chronic (B) and acute-on-chronic simvastatin treatment (C) did not affect lipoprotein production rates [VLDL -8 and -13%, IDL +47 and +38%, and LDL +19 and +30% in B and C, respectively (all comparisons n.s.)]. The data indicate that simvastatin does not influence the secretion of apoB-100-containing lipoproteins in non-obese subjects with near-normal LDL cholesterol concentrations.

Apolipoprotein B100 biogenesis: a complex array of intracellular mechanisms regulating folding, stability, and lipoprotein assemblyThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Apolipoprotein B100 (apoB) is a large amphipathic lipid-binding protein that is synthesized by hepatocytes and used to assemble and stabilize very low density lipoproteins (VLDL). It may have been derived through evolution from other lipid-associating proteins such as microsomal triglyceride transfer protein or vitellogenin. The correct folding of apoB requires assistance from chaperone proteins in co-translational lipidation, disulfide bond formation, and glycosylation. Any impairment in these processes results in co-translational targeting of the misfolded apoB molecule for proteasomal degradation. In fact, most of the regulation of apoB production is mediated by intracellular degradation. ApoB that misfolds post-translationally, perhaps as a result of oxidative stress, may be eliminated through autophagy. This review focuses on the proposed pentapartite domain structure of apoB, the role that each domain plays in the binding of lipid species and regulation of apoB synthesis, and the process of VLDL assembly. The factors involved in the recognition, ubiquitination, and proteasomal delivery of defective apoB molecules are also discussed.

A high-cholesterol, n-3 polyunsaturated fatty acid diet causes different responses in rats and hamsters

This study was designed to investigate the response to a high-cholesterol, n-3 polyunsaturated fatty acid (PUFA) or n-6 PUFA diet in rats and hamsters. Animals were fed n-3 or n-6 PUFA with a cholesterol-free diet, or with a diet enriched with cholesterol (0.5%, w/w) for 2 weeks. In rats and hamsters fed a cholesterol-free diet, plasma cholesterol, triglycerides and very-low-density lipoprotein (VLDL)-triglyceride levels in n-3 PUFA group were significantly lower than those in n-6 PUFA group. In contrast, when diets were supplemented with 0.5% cholesterol, the plasma cholesterol- and triglyceride-lowering effect of dietary n-3 PUFA disappeared. In hamsters fed with the atherogenic diet (0.5% dietary cholesterol) for 2 weeks, n-3 PUFA induced hypercholesterolemia more than n-6 PUFA, the increase being in the VLDL and low-density lipoprotein (LDL) fractions. Our data thus indicate that elevation of VLDL- and LDL-cholesterol in hamsters by n-3 PUFA, compared with n-6 PUFA, is dependent on 0.5% dietary cholesterol supplementation. In rats, on the other hand, dietary n-3 PUFA did not induce hypercholesterolemia more than n-6 PUFA when 0.5% cholesterol was supplemented. Although the effects of n-3 PUFA on plasma cholesterol, triglycerides and VLDL-triglycerides were similar in hamsters and rats, the interactive effects of n-3 PUFA and cholesterol on plasma and lipoprotein cholesterol levels differed in the two species. It was also found that plasma triglycerides, cholesterol and lipoprotein cholesterol levels in hamsters are higher than in rats in the presence and absence of dietary cholesterol. In addition, cholesterol feeding induces hypertriglyceridemia and hypercholesterolemia only in hamsters. Moreover, liver triglyceride concentrations increased in rats fed a cholesterol-rich diet and hepatic triglyceride levels of the n-3 PUFA-fed rats were significantly lower than those in the n-6 PUFA-fed rats in the presence and absence of dietary cholesterol. However, triglycerides did not accumulate in the liver in hamsters fed a cholesterol-rich diet and hepatic triglyceride levels of the n-3 PUFA-fed hamsters were not significantly different from those in the n-6 PUFA-fed hamsters in the presence and absence of dietary cholesterol. Therefore, these studies confirm marked species differences in response to the interactive effects of dietary n-3 PUFA and cholesterol.

ApoA-I Lipidation in Primary Mouse Hepatocytes

The liver is the major site of both apolipoprotein A-I (apoA-I) synthesis and ATP-binding cassette transporter A1 (ABCA1) expression. Here, we compare the lipidation with cholesterol and phospholipid of newly synthesized human apoA-I (hapoA-I) using adenoviral vector-mediated endogenous expression or exogenously added hapoA-I in wild type and ABCA1-null hepatocytes. Hepatocytes were labeled with [3H]cholesterol (delivered with LDL or methyl-beta-cyclodextrin), [3H]mevalonate, or [3H]choline. ABCA1 deficiency decreased apoA-I phospholipidation by 80%, but acquisition of de novo synthesized and exogenous cholesterol only decreased by 40-60%. The transfer of de novo synthesized cholesterol to apoA-I was decreased at all time points, but that of exogenously delivered cholesterol was independent of ABCA1 activity at the early time points. Progesterone does not affect apoA-I synthesis or its lipidation but inhibited the early phase of apoA-I cholesterol lipidation in both wild type and ABCA1-null hepatocytes. Fast protein liquid chromatography analysis of medium lipoproteins confirmed that with ABCA1 deficiency, the proportion of secreted high density lipoprotein-associated apoA-I and cholesterol decreased by about 50%. The very low density lipoprotein (VLDL)/LDL size fraction also contained a significant level of cholesterol in ABCA1 deficiency, consistent with the result of immunoprecipitations showing the presence of lipoproteins with both apoA-I and murine apoB. ApoA-I lipidation with newly synthesized cholesterol in ABCA1-null hepatocytes was significantly decreased by brefeldin A and monensin. In conclusion, we demonstrate that: (i) whereas most hepatic phospholipidation of apoA-I is mediated by ABCA1, acquisition of cholesterol depends on active transfer from intracellular compartments by ABCA1-dependent and -independent pathways, both sensitive to progesterone and (ii) there is separate regulation of phospholipid and cholesterol lipidation of apoA-I in hepatocytes.

Synthesis and biological activity of a novel squalene epoxidase inhibitor, FR194738

The synthesis and biological properties of a novel squalene epoxidase inhibitor, FR194738, are described. This compound displayed potent in vitro inhibitory activities against squalene epoxidase and cholesterol synthesis, and lowered plasma cholesterol and triglyceride levels in dogs.

The P-407–Induced Murine Model of Dose-Controlled Hyperlipidemia and Atherosclerosis

We are continuing to both elucidate underlying mechanisms and identify clinical applications for a chemically induced murine model of dose-controlled hyperlipidemia and atherosclerosis. This murine model neither utilizes genetically modified mice nor a high-fat, cholate-containing diet, although simultaneous ingestion of a high-fat, cholate-enriched diet potentiates the hyperlipidemic response and the number and size of aortic atherosclerotic lesions formed in C57BL/6 mice. The chemical agent used to induce hyperlipidemia is poloxamer 407 (P-407), a nonionic surface-active-agent. To date, we have investigated the effect of P-407 on the biologic activity of a variety of key enzymes involved with lipid metabolism and transport. These enzymes include 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, lipoprotein lipase (LPL), cholesterol 7alpha-hydoxylase (C7alphaH), sterol 27-hydroxylase (S27H), lecithin cholesterol acyltransferase (LCAT), cholesteryl-ester-transfer-protein (CETP), hepatic lipase (HL), and endothelial lipase (EL). P-407 directly inhibits the heparin-releasable fraction of LPL and HL and indirectly increases the biologic activity of CETP and LCAT. Long-term (> 4 months) administration of P-407 to C57BL/6 mice appears to have no effect on the biologic activity of S27H and HMG-CoA reductase, but decreases the activity of C7alphaH. This would suggest that hypertriglyceridemia and hypercholesterolemia result from inhibition of LPL and C7alphaH, respectively, while the biologic activity of CETP and LCAT are indirectly increased to compensate for the increased cholesterol burden. The above model has proven useful for predicting the therapeutic efficacy of existing and possibly newer statin drugs, as well as evaluating the potential of one statin drug (atorvastatin calcium) to cause the regression of P-407-induced atherosclerotic lesions in mice. The P-407-induced murine model of atherogenesis represents an alternative to the use of either genetically modified or diet-induced models and may also prove beneficial for the evaluation of newer classes of antihyperlipidemic agents such as antioxidants, CETP inhibitors, and liver X receptor (LXR) agonists.

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

High levels of expression of the ATP binding cassette transporter A1 (ABCA1) in the liver and the need to over- or underexpress hepatic ABCA1 to impact plasma HDL levels in mice suggest a major role of the liver in HDL formation and in determining circulating HDL levels. Cultured murine hepatocytes were used to examine the role of hepatic ABCA1 in mediating the lipidation of apolipoprotein A-I (apoA-I) for HDL particle formation. Exogenous apoA-I stimulated cholesterol efflux to the medium from wild-type hepatocytes, but not from ABCA1-deficient (abca1(-/-)) hepatocytes. ApoA-I induced the formation of new HDL particles and enhanced the lipidation of endogenously secreted murine apoA-I in ABCA1-expressing but not abca1(-/-) hepatocytes. ABCA1-dependent cholesterol mobilization to apoA-I increased new cholesterol synthesis, indicating depletion of the regulatory pool of hepatocyte cholesterol during HDL formation. Secretion of triacylglycerol and apoB was decreased following apoA-I incubation with ABCA1-expressing but not abca1(-/-) hepatocytes. These results support a major role for hepatocyte ABCA1 in generating a critical pool of HDL precursor particles that enhance further HDL generation and passive cholesterol mobilization in the periphery. The results also suggest that diversion of hepatocyte cholesterol into the "reverse" cholesterol transport pathway diminishes cholesterol availability for apoB-containing lipoprotein secretion by the liver.

Effects of F-strain Mycoplasma gallisepticum inoculation on serum very low density lipoprotein diameter and fractionation of cholesterol among lipoproteins in commercial egg-laying hens

Experimental inoculation with the F-strain of Mycoplasma gallisepticum (FMG) at 12 wk of age has been shown to affect the performance, liver, reproductive organs, and yolk lipid characteristics of commercial layers. Therefore, this study was conducted to determine the serum lipoprotein characteristics of commercial egg-laying hens at 16 wk of age and throughout lay after inoculation with FMG at 12 wk of age. Mean diameters of very low density lipoproteins (VLDL) were determined for the 10th, 50th, and 90th percentiles of serum total VLDL of each hen. Percentages of total serum cholesterol recovered in VLDL and low and high density lipoprotein particle classes were also determined. Inoculation of birds with FMG at 12 wk did not change the physical properties or relative concentrations of their circulating lipoproteins. However, the age of the bird had significant differential effects on all the parameters examined. These data demonstrate that FMG-inoculation at 12 wk of age does not affect the lipoproteins of laying hens, but because these birds were housed in biological isolation units, these results do not preclude the possibility that these yolk precursors may be affected in FMG-infected birds that are housed in facilities in which there are increased levels of environmental stress. These data further suggest that alterations in liver, reproductive organs, and yolk lipid characteristics in response to FMG, as noted in previous reports on commercial layers, are not mediated through changes in circulating VLDL diameters.

Differential effects of fenofibrate or simvastatin treatment of rats on hepatic microsomal overt and latent diacylglycerol acyltransferase activities

Hepatic triacylglycerol secretion is elevated in insulin-resistant states. Microsomal diacylglycerol acyltransferase (DGAT) catalyzes the final reaction in the synthesis of triacylglycerol (TAG). We have previously described two DGAT activities in rat liver microsomes, one overt (cytosol-facing) and one latent (endoplasmic reticulum lumen-facing) (Owen MR, Corstorphine CG, Zammit VA: Overt and latent activities of diacylglycerol acytransferase in rat liver microsomes: possible roles in very-low-density lipoprotein triacylglycerol secretion. Biochem J 323:17-21, 1977). It was suggested that they are involved in the synthesis of TAG for the cytosolic droplet and VLDL lipidation, respectively. In the present study, we measured the overt and latent DGAT activities in rats fed diets containing one of two hypolipidemic drugs: fenofibrate (a peroxisome proliferator-activated receptor alpha [PPARalpha] agonist) and simvastatin (a 3-hydroxy-3-methylglutaryl [HMG]-CoA reductase inhibitor). We found that the activities of the two DGATs could be varied independently by these treatments. Fenofibrate raised overt DGAT activity but lowered that of latent DGAT. In contrast, simvastatin markedly lowered overt DGAT activity without affecting that of latent DGAT. The increase in overt DGAT activity induced by fenofibrate could not be mimicked by feeding a diet enriched in n-3 polyunsaturated fatty acids (PUFA), which lowered overt DGAT activity but did not affect latent DGAT, suggesting that n-3 PUFA act through a mechanism independent of PPARalpha activation. The fibrate-induced increase in overt DGAT activity and the inhibition of latent DGAT may provide a mechanism through which acyl moieties are retained within the liver for oxidation through the pathways concomitantly upregulated by PPARalpha activation.

Dipyridamole prevents the coconut oil-induced hypercholesterolemia. A study on lipid plasma and lipoprotein composition

For a better understanding of the hypolipidemic function of dipyridamole, we have studied the comparative effects of diet supplementation with 10% coconut oil with and without dipyridamole on the lipid plasma and lipoprotein composition in chicks. This study was performed under postprandial and food-deprivation (12h) conditions. Coconut oil induced a clear hypercholesterolemia under both feeding conditions. Simultaneous administration of dipyridamole maintained total and esterified cholesterol at levels similar to those observed in control animals sacrificed under postprandial conditions. Under these conditions, our results also show that dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. Nevertheless, it should be emphasised that the levels of total cholesterol found in intermediate- and very-low-density lipoproteins were lower than in control. All chemical components of these fractions were significantly decreased by dipyridamole. The effects were not significant in chicks deprived of food. In conclusion, our results show that the hypercholesterolemia induced by coconut oil was prevented by dipyridamole. To our knowledge, this is one of the first reports on the antihypercholesterolemic effects of dipyridamole.

Dipyridamole reduces cholesterol and triglyceride levels in plasma and lipoproteins from young chicks fed a saturated fat-enriched diet

We have studied the value of dipyridamole for the prevention of hypercholesterolemia caused by saturated fat. This study shows that supplementation of 10-20% coconut oil to the diet induced a significant hypercholesterolemia under postprandial conditions. Simultaneous administration of dipyridamole and 10% coconut oil for 2 weeks maintained plasma cholesterol and triglycerides at levels similar to control, counteracting the increase induced by coconut oil. Dipyridamole significantly reduced cholesterol levels in all the chick plasma lipoproteins that were increased by coconut oil administration. However, it should be emphasized that cholesterol and other chemical components were more drastically reduced by dipyridamole in intermediate- and very-low-density lipoproteins than in other lipoprotein fractions.

Effect of atorvastatin treatment on lipoprotein lipase mass in the pre-heparin plasma in Japanese hyperlipidemic subjects

BACKGROUND

Atorvastatin is a recently introduced statin that lowers LDL-cholesterol (LDL-C) and triglycerides more than some of the older statins.

METHODS

Twenty-one Japanese hyperlipidemic subjects were treated with atorvastatin (10 mg/day) for 6 weeks. Plasma lipid concentrations and pre-heparin plasma LPL mass before and after oral administration were evaluated using an open crossover trial format. LPL mass in the pre-heparin plasma was measured by a sandwich enzyme immunoassay.

RESULTS

Atorvastatin decreased plasma triglyceride (TG) concentration (-21%, p<0.05), as well as plasma total and LDL-cholesterol concentrations. LPL mass in the pre-heparin plasma did not change significantly by this treatment during this period. Both apolipoprotein (apo) B and E decreased considerably (-33%, p<0.001 for apo B; -29% p<0.001 for apo E), while apo A-I concentration did not change. Other clinical parameters such as body mass index, blood pressure, and fasting plasma glucose concentration of these subjects did not change during this treatment.

CONCLUSIONS

Atorvastatin is effective in reducing plasma TG, which did not appear to be associated with an increased LPL mass.

Potency of select statin drugs in a new mouse model of hyperlipidemia and atherosclerosis

Poloxamer-407 (P-407) is a nonionic surfactant that induces atheroma formation in the aortas of C57BL/6 mice with long-term (14 weeks) administration. The objectives of the present study were to determine the mechanism(s) responsible for the induction of hypercholesterolemia as well as to determine whether this animal model may be of potential use in rank ordering the efficacy (lipid lowering) of various statin drugs. The effect of long-term (16 weeks) administration of P-407 on the catalytic activities of rate-limiting enzymes of cholesterol biosynthesis [HMG-CoA reductase (HMGR)] and catabolism [microsomal cholesterol 7alpha-hydroxylase (C7alphaH) and mitochondrial sterol 27 hydroxylase (S27H)] was assessed in C57BL/6 mice. Effects of P-407 on these enzymes were compared in mice fed an atheroma-inducing diet (high-cholesterol, supplemented with cholic acid) and animals maintained on a basal diet and injected with saline (controls) after 16 weeks. The mean value for the activities of C7alphaH in P-407-injected mice was 24.3+/-3.8 pmol min(-1) mg(-1) and was significantly (P<0.05) less than the mean value determined for sham-injected control animals (37.0+/-14.3 pmol min(-1) mg(-1)). In contrast, the mean values for the catalytic activities of S27H and HMGR did not change with P-407 administration. Neither C7alphaH nor S27H activity in mice fed the high-cholesterol diet differed from values for control animals, whereas the mean HMGR activity was drastically reduced (-94%, P<0.05). The hypercholesterolemic effect of P-407 is not due to altered cholesterol biosynthesis, but is mediated by reduced cholesterol catabolism due to decreased activity of the rate limiting enzyme (C7alphaH) in the classic bile acid synthetic pathway. Plasma triglyceride lowering resulting from the oral administration of equal doses of various statin drugs appeared, in general, to be positively correlated with their relative aqueous solubility and paralleled the efficacy of these agents to lower low-density-lipoprotein-associated cholesterol (LDL-C) in humans. The plasma triglyceride lowering effect of the five statin drugs tested produced the following rank order; pravastatin sodium (-44%)>atorvastatin calcium (-36%)>simvastatin (-33%)>lovastatin (-25%)>fluvastatin sodium (-19%). While reductions in plasma total cholesterol following administration of the statin drugs was not as profound as that observed with triglycerides, the relative rank order or trend was preserved. The percent reduction in plasma triglycerides in the present model appears to be a useful parameter with which to predict the relative reduction in plasma LDL-C expected for these agents in humans.

Dietary plant stanol esters reduce VLDL cholesterol secretion and bile saturation in apolipoprotein E*3-Leiden transgenic mice

Dietary plant stanols lower serum cholesterol levels in humans and in hyperlipidemic rodents, mainly by inhibition of the intestinal cholesterol absorption. We used female apolipoprotein E*3-Leiden transgenic mice to investigate the consequences of this effect on serum lipid levels and hepatic lipid metabolism. Five groups of 6 or 7 mice received for 9 weeks a diet containing 0.25% cholesterol and 0.0%, 0.25%, 0.5%, 0.75%, or 1.0% (wt/wt) plant stanols (sitostanol 88% [wt/wt], campestanol 10% [wt/wt]) esterified to fatty acids. Compared with the control diet, plant stanol ester treatment dose-dependently reduced serum cholesterol levels by 10% to 33% (P<0.05), mainly in very low density lipoproteins (VLDLs), intermediate density lipoproteins, and low density lipoproteins. Furthermore, 1.0% of the dietary plant stanols significantly decreased the liver contents of cholesteryl esters (-62%), free cholesterol (-31%), and triglycerides (-38%) but did not change the hepatic VLDL-triglyceride and VLDL-apolipoprotein B production rates. However, plant stanol ester feeding significantly decreased the amounts of cholesteryl esters and free cholesterol incorporated in nascent VLDLs by 72% and 30%, respectively, resulting in a net 2-fold decreased VLDL cholesterol output. Liver mRNA levels of low density lipoprotein receptors, 3-hydroxy-3-methylglutaryl coenzyme A synthase, cholesterol 7alpha-hydroxylase, and sterol 27-hydroxylase were not changed by plant stanol ester feeding. Nevertheless, the serum lathosterol-to-cholesterol ratio was significantly increased by 23%, indicating that dietary plant stanol esters increased whole-body cholesterol synthesis. Plant stanol esters also significantly decreased the cholesterol saturation index in bile by 55%. In conclusion, in apolipoprotein E*3-Leiden transgenic mice, plant stanol ester feeding dose-dependently lowered serum cholesterol levels as a result of a reduced secretion of VLDL cholesterol. This was caused by a decreased hepatic cholesterol content that also resulted in a lowered biliary cholesterol output, indicative of a reduced lithogenicity of bile in these mice.

Elevated baseline triglyceride levels modulate effects of HMGCoA reductase inhibitors on plasma lipoproteins

BACKGROUND

The response in levels of very-low-density (VLDL) and low-density (LDL) lipoproteins varies substantially among hyperlipidemic patients during treatment with HMGCoA reductase inhibitors. Apolipoprotein E genotype and gender are known to contribute to the regulation of steady state levels of plasma lipoproteins. This study explores the effect of these and other potential determinants of the response of VLDL and LDL to treatment with reductase inhibitors.

METHODS

Using mixed linear statistical models, the response of lipoprotein lipid values was studied in 142 hyperlipidemic individuals who were treated with reductase inhibitors. Patients received one or more of the following drugs individually for a total of 623 treatment observations: lovastatin, pravastatin, simvastatin, or atorvastatin. For evaluation of the effects of treatment in the aggregate, actual doses were expressed as equivalent doses of atorvastatin, using factors based on random assignment comparisons in 16 reported studies. The analysis factors considered were apolipoprotein E genotype, baseline average triglycerides >170 mg/dL (vs less), and gender.

RESULTS

Presence of an apo epsilon4 allele was associated with a trend toward greater reduction of triglyceride levels and a diminished ability of the reductase inhibitors to reduce LDL cholesterol levels. Gender had only minimal effect on the response of either LDL cholesterol or triglycerides. However, the effect of elevated baseline triglycerides on the response of both triglycerides and LDL cholesterol was striking and was exerted in opposite directions. The triglyceride-lowering effect of reductase inhibitors was greater in patients with initial triglyceride levels above 170 mg/dL (P=0.0001). The effect was even greater in patients with initial triglyceride levels over 250 mg/dL (P=0.015). Conversely, for LDL cholesterol levels, elevated baseline triglycerides were associated with a significantly decreased response to the drugs (P=0.0015).

CONCLUSIONS

These findings indicate that baseline triglyceride levels are an important predictor of response of plasma lipoproteins to HMGCoA reductase inhibitors, perhaps reflecting fundamental differences in mechanism underlying the hyperlipidemic phenotype.

Fish oil reduces cholesterol and arachidonic acid levels in plasma and lipoproteins from hypercholesterolemic chicks

The value of fish oil for prevention and/or treatment of human atherosclerosis has not been fully established. This study shows that replacement of saturated fat in young chick diet with menhaden oil produced a significant reversion of the hypercholesterolemia previously induced by coconut oil feeding. Fish oil also produced a clear decrease of plasma triacylglycerol levels. Coconut oil increased the percentages of 12:0 and 14:0 fatty acids, while menhaden oil increased those of 20:5 n-3 and 22:6 n-3. Percentages of 20:4 n-6, 18:2 n-6 and 18:1 n-9 significantly decreased by fish oil addition to the diet. Total cholesterol, phospholipid and protein contents of high and low density lipoproteins increased by coconut oil feeding. When coconut oil was replaced by menhaden oil, total cholesterol was significantly reduced in high, low and very low density lipoproteins. All chemical components of VLDL were decreased by menhaden oil feeding. Our results show a strong hypocholesterolemic effect of menhaden oil when this fat was supplemented to hypercholesterolemic chicks. The clear decrease found in arachidonic acid content of chick plasma and lipoproteins may contribute to the beneficial effects of fish oil consumption by lowering the production of its derived eicosanoids.

Effect of NK-104, a new synthetic HMG-CoA reductase inhibitor, on triglyceride secretion and fatty acid oxidation in rat liver

For the investigation of the mechanism responsible for the hypotriglyceridemic effect of NK-104, a new synthetic inhibitor of HMG-CoA reductase, the rate-limiting enzyme for cholesterol synthesis, isolated rat liver was perfused with or without NK-104 in the presence of exogenous [1-(14)C]oleic acid substrate. Addition of NK-104 tended to increase the ketone body production while it caused a significant decrease in the secretion rate of triglyceride by the perfused liver without affecting uptake of exogenous [1-(14)C]oleic acid. The inhibitor also significantly decreased hepatic triglyceride concentration. The altered triglyceride secretion was accompanied by a concomitant decreased incorporation of exogenous [1-(14)C]oleate into triglyceride. The conversion of exogenous [1-(14)C]oleic acid substrate indicated an inverse relationship between the pathways of oxidation and esterification. No effect of NK-104 on hepatic secretion of cholesterol was observed. These results suggest that NK-104 exerts its hypotriglyceridemic action, primarily by diverting the exogenous free fatty acid to the pathways of oxidation at the expense of esterification.

Dietary fish oil reduces cholesterol and arachidonic acid levels in chick plasma and very low density lipoprotein

The mechanisms involved in the hypolipidemic effects of fish oil have not been clearly established. This study shows that supplementation of 10% menhaden oil to the chick diet for 7 days produced a significant hypocholesterolemia and hypotriglyceridemia. Fatty acid composition of chick plasma drastically changed by the same dietary manipulation. Percentages of 20:5 and 22:6 n-3 fatty acids strongly increased, while percentages of 20:4 n-6, 18:2 n-6, and 18:1 n-9 significantly decreased. Changes observed in the relative percentages were parallel to those obtained in the amount of each fatty acid. Ratio of n-3/n-6 clearly decreased in plasma by fish oil feeding. Total cholesterol and triacylglycerol contents decreased in high density lipoprotein (HDL) but did not change in low density lipoprotein (LDL). All chemical constituents of very low density lipoprotein (VLDL) significantly decreased after the first week of menhaden oil supplementation to the diet. Similar modifications in fatty acid composition of the three lipoprotein fractions were also found. Our results suggest that the hypocholesterolemic effects of fish oil may be mediated by the depletion in VLDL synthesis and secretion into the chick plasma. On the other hand, the strong decrease found in the arachidonic acid (AA) content of chick plasma and lipoproteins may contribute to the beneficial effects of fish oil consumption by lowering the production of its derived eicosanoids.

Manipulation of cholesterol and cholesteryl ester synthesis has multiple effects on the metabolism of apolipoprotein B and the secretion of very-low-density lipoprotein by primary hepatocyte cultures

Inhibition of esterified and non-esterified cholesterol synthesis by lovastatin in primary rat hepatocytes suppressed the net synthesis and very-low-density lipoprotein (VLDL) secretion of apolipoprotein B (apoB)-48 and apoB-100. Lovastatin did not alter the rates of apoB-48 and apoB-100 post-translational degradation. 25-Hydroxycholesterol, which inhibited non-esterified cholesterol synthesis but increased the synthesis of cholesteryl ester, showed differential effects on the metabolism of apoB-48 and apoB-100. Whereas the secretion of apoB-48 VLDL was suppressed there was no effect on the secretion of apoB-100 VLDL. The post-translational degradation of apoB-48, but not of apoB-100, was enhanced by 25-hydroxycholesterol. The net synthesis rates of apoB-48 and apoB-100 were unaffected by 25-hydroxycholesterol. The inhibitory effect of lovastatin alone on the net synthesis of apoB-48 and apoB-100 was reversed by the simultaneous presence of 25-hydroxycholesterol, suggesting a role for newly synthesised cholesteryl ester. Prevention of the reversal effect by the acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor YM 17E supported this interpretation. In the presence of lovastatin, restoration of the net synthesis of apoB by 25-hydroxycholesterol was not accompanied by an increased VLDL output of apoB-48 and apoB-100. However, under these conditions there was an increased post-translational degradation of apoB-48 and apoB-100. These results suggest that interference with intracellular cholesterol and cholesteryl ester metabolism interrupts VLDL assembly at sites of both apoB net synthesis and post-translational degradation.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Different effect of simvastatin and atorvastatin on key enzymes involved in VLDL synthesis and catabolism in high fat/cholesterol fed rabbits

The effects of atorvastatin (3 mg kg(-1)) and simvastatin (3 mg kg(-1)) on hepatic enzyme activities involved in very low density lipoprotein metabolism were studied in coconut oil/cholesterol fed rabbits. Plasma cholesterol and triglyceride levels increased 19 and 4 fold, respectively, after 7 weeks of feeding. Treatment with statins during the last 4 weeks of feeding abolished the progression of hypercholesterolaemia and reduced plasma triglyceride levels. 3-Hydroxy-3-methyl-glutaryl Coenzyme A reductase, acylcoenzyme A:cholesterol acyltransferase, phosphatidate phosphohydrolase and diacylglycerol acyltransferase activities were not affected by drug treatment. Accordingly, hepatic free cholesterol, cholesteryl ester and triglyceride content were not modified. Simvastatin treatment caused an increase (72%) in lipoprotein lipase activity without affecting hepatic lipase activity. Atorvastatin caused a reduction in hepatic phospholipid content and a compensatory increase in CTP:phosphocholine cytidylyl transferase activity. The results presented in this study suggest that, besides the inhibitory effect on 3-hydroxy-3-methyl-glutaryl Coenzyme A reductase, simvastatin and atorvastatin may have additional effects that contribute to their triglyceride-lowering ability.

Enhancement of fatty acid and cholesterol synthesis accompanied by enhanced biliary but not very-low-density lipoprotein lipid secretion following sustained pravastatin blockade of hydroxymethyl glutaryl coenzyme A reductase in rat liver

A 3-week treatment of rats with pravastatin (PV) augmented biliary cholesterol and phospholipid output 3.6- and 2.2-fold over controls, while bile acid (BA) output and kinetics were unchanged. No major changes were detected in hepatic and serum cholesterol concentrations despite the PV inhibitory property on hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase. To evaluate the mechanisms of this adaptive phenomenon, several parameters of hepatic lipid homeostasis were assessed. Biliary cholesterol changes could not be attributed to an increased influx of lipoprotein cholesterol to the liver and bile. Hepatic low-density lipoprotein (LDL) receptor content, as inferred from Western blot analysis, was unchanged, as was the biliary excretion of labeled cholesterol derived from chylomicron remnants. In vivo 3H2O-incorporation studies showed an 80% increase in hepatic cholesterol synthesis, evidence for bypass of the PV block. Remarkably, fatty acid synthesis was also stimulated twofold, providing substrate for hepatic triglycerides, which were slightly enhanced. However, serum triglycerides decreased 52% associated with a 22% decrease in hepatic very-low-density lipoprotein (VLDL) secretion. Thus, the biochemical adaptation following PV treatment produces complex alterations in hepatic lipid metabolism. An enhanced supply of newly synthesized cholesterol and fatty acids in association with a limited VLDL secretion rate augments the biliary lipid secretion pathway in this experimental model.

Role of the enterohepatic circulation of bile salts in lipoprotein metabolism

The enterohepatic circulation of bile salts and cholesterol plays a central role in maintaining whole body cholesterol homeostasis. Hepatic lipoprotein metabolism is reviewed and the role of disturbances in bile salt metabolism in the pathogenesis of dyslipidemias is discussed. Further, the manipulation of bile salt metabolism to treat dyslipidemia is reviewed.

Dietary pectin lowers sphingomyelin concentration in VLDL and raises hepatic sphingomyelinase activity in rats

There is evidence that cholesterol and sphingomyelin metabolism are interrelated, and thus the hypothesis tested was that dietary pectin, because it can alter hepatic cholesterol metabolism, would also alter hepatic sphingomyelin metabolism. For that purpose, 4-wk-old female Wistar rats were fed a diet without or with pectin (20 g/100 g) up to 21 d. In accordance with previous work, pectin consumption caused a significant (P < 0.001) reduction in hepatic (65%), whole plasma (37%), and VLDL (80%) cholesterol levels. Pectin also significantly reduced VLDL sphingomyelin concentrations (57%), but raised the amount of sphingomyelin in the high density lipoproteins (HDL)-2 fractions (58%), so that the level of sphingomyelin in whole plasma remained unaffected. Pectin did not affect the sphingomyelin concentration in the liver. Pectin consumption did not affect the hepatic sphingomyelin synthesizing enzymes, serine palmitoyltransferase, phosphatidylcholine:ceramide phosphocholine transferase, or phosphatidylethanolamine:ceramide phosphoethanolamine transferase. In contrast, dietary pectin activated both lysosomal (28%) and plasma membrane (26%) sphingomyelinase and thus may have enhanced sphingomyelin degradation. An attempt was made to describe the effects of dietary pectin on sphingomyelin metabolism in terms of altered fluxes through liver and plasma, with whole liver and whole plasma concentrations of sphingomyelin remaining unaffected.

The role of ATP citrate-lyase in the metabolic regulation of plasma lipids. Hypolipidaemic effects of SB-204990, a lactone prodrug of the potent ATP citrate-lyase inhibitor SB-201076

ATP citrate (pro-S)-lyase (EC 4.1.3.8), a cytosolic enzyme that generates acetyl-CoA for cholesterol and fatty acid synthesis de novo, is a potential target for hypolipidaemic intervention. Here we describe the biological effects of the inhibition of ATP citrate-lyase on lipid metabolism in Hep G2 cells, and plasma lipids in rats and dogs, by using SB-204990, the cell-penetrant gamma-lactone prodrug of the potent ATP citrate-lyase inhibitor SB-201076 (Ki=1 microM). Consistent with an important role of ATP citrate-lyase in the supply of acetyl-CoA units for lipid synthesis de novo, SB-204990 inhibited cholesterol synthesis and fatty acid synthesis in Hep G2 cells (dose-related inhibition of up to 91% and 82% respectively) and rats (76% and 39% respectively). SB-204990, when administered orally to rats, was absorbed into the systemic circulation; pharmacologically relevant concentrations of SB-201076 were recovered in the liver. When administered in the diet (0.05-0. 25%, w/w) for 1 week, SB-204990 caused a dose-related decrease in plasma cholesterol (by up to 46%) and triglyceride levels (by up to 80%) in rats. This hypolipidaemic effect could be explained, at least in part, by a decrease (up to 48%) in hepatic very-low-density lipoprotein (VLDL) production as measured by the accumulation of VLDL in plasma after injection of Triton WR-1339. SB-204990 (25 mg/kg per day) also decreased plasma cholesterol levels (by up to 23%) and triglyceride levels (by up to 38%) in the dog, preferentially decreasing low-density lipoprotein compared with high-density lipoprotein cholesterol levels. Overall these results are consistent with the concept that ATP citrate-lyase is an important enzyme in controlling substrate supply for lipid synthesis de novo and a potential enzyme target for hypolipidaemic intervention.

Effect of hypolipidemic drugs on key enzyme activities related to lipid metabolism in normolipidemic rabbits

The effect of atorvastatin (3 mg kg(-1) day(-1)), simvastatin (3 mg kg(-1) day(-1)) and bezafibrate (100 mg kg(-1) day(-1)) administered for 4 weeks to male New Zealand white rabbits on enzyme activities related to lipid metabolism has been studied. Only the statins reduced plasma cholesterol values, while none of the drugs modified plasma triglyceride or high density lipoprotein (HDL)-cholesterol concentrations, nor the activity of enzymes such as hepatic diacylglycerol acyltransferase, lipoprotein lipase or hepatic lipase, directly involved in triglyceride metabolism. Both statins elicited similar increases in the hepatic microsomal 3-hydroxy-3-methyl-glutaryl Coenzyme A (CoA) reductase activity (147 and 109% induction for simvastatin and atorvastatin, respectively), and none of the drugs assayed modified hepatic acyl-coenzyme A:cholesterol acyltransferase activity significantly. Only bezafibrate induced a significant 57% reduction in the activity of hepatic microsomal cholesterol 7alpha-hydroxylase. Regarding the rate limiting enzyme of phosphatidylcholine biosynthesis, CTP:phosphocholine cytidylyl transferase, atorvastatin and bezafibrate behaved similarly, decreasing the enzyme activity in the liver by 45% and 54%, respectively; simvastatin induced no modification of this activity. The reduction of CTP:phosphocholine cytidylyl transferase activity is not caused by a direct inhibition of the enzyme by bezafibrate and atorvastatin. Further, the inhibitory effect of atorvastatin appears to be unrelated to the inhibition of 3-hydroxy-3-methyl-glutaryl CoA reductase elicited in vivo.

Mapping a gene for combined hyperlipidaemia in a mutant mouse strain

Familial combined hyperlipidaemia (FCHL) is a common, multifactorial disorder associated with elevated levels of plasma triglyceride, cholesterol, or both. A characteristic feature is increased secretion of very low density lipoproteins (VLDL) and apolipoprotein B (apoB). Although FCHL is the most common cause of premature coronary artery disease (CAD), accounting for over 10% of cases, its aetiology remains largely unknown. One powerful approach to the dissection of complex genetic traits involves the use of animal models. We have identified a mouse strain, HcB-19/Dem (HcB-19), which exhibits hypertriglyceridaemia, hypercholesterolaemia and elevated levels of plasma apoB. Like FCHL patients, HcB-19 mice also exhibit increased secretion of triglyceride-rich lipoproteins, and their hyperlipidaemia becomes progressively more severe with age. It is likely that the hyperlipidaemia results from a mutation of a novel gene that arose during development of strain HcB-19. We mapped the hyperlipidaemia gene (Hyplip1) to the distal portion of mouse chromosome 3. This region is syntenic to human chromosome 1q21-q23, which has recently been shown to harbour a gene associated with FCHL in families from a Finnish isolate.

Estrogen induces glomerulosclerosis in analbuminemic rats

Progression of chronic renal disease is usually more rapid in males, both in humans and in experimental animals. Estrogen-replacement studies indicate that this may be related to the beneficial effects of estrogen on the lipoprotein profile. However, in hyperlipidemic analbuminemic rats (NAR), females are more prone to develop renal injury than males, and ovariectomy tends to decrease triglyceride levels and prevent renal disease. Therefore, we studied the effects of estrogen administration on lipoproteins, and the induction of renal injury in uninephrectomized female and male NAR. Ovariectomized and orchidectomized uninephrectomized NAR were treated with estradiol implants for 24 weeks. In an additional group of ovariectomized rats, the implant was removed after 12 weeks. Both in ovariectomized and orchidectomized NAR, estradiol caused severe hypercholesterolemia (9 to 12 mmol/liter) and hypertriglyceridemia (6 to 8 mmol/liter) after six weeks. Subsequently, these rats developed severe proteinuria, reaching 209 +/- 25 and 95 +/- 43 mg/day, respectively, after 24 weeks. At this point there was severe glomerular sclerosis, with a respective score of 107 +/- 21 and 61 +/- 33. In terminal blood samples the most pronounced increase in lipid levels were observed in very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and low density lipoproteins (LDL). In contrast, ovariectomized NAR and orchidectomized NAR without exogenous estrogen had much lower lipid levels (cholesterol 5 to 7 mmol/liter and triglycerides 1 to 2 mmol/liter) after six weeks. These rats, or ovariectomized NAR where the estrogen treatment had been withdrawn, had practically no proteinuria (4 +/- 1, 19 +/- 11, and 13 +/- 4 mg/day, respectively) or renal damage (glomerulosclerosis score 1 +/- 0.4, 5 +/- 3 and 3 +/- 1, respectively) after 24 weeks. Thus, in hypertriglyceridemic analbuminemic rats, estrogen-treatment causes further increases in both triglycerides and cholesterol. Most probably these changes contribute to the development of renal injury by estrogen in this model. This effect of estrogen, which has also been observed in the Zucker rat, is unique for the hypertriglyceridemic state and deserves further study.

Direct association between the hepatic secretion of very-low-density lipoprotein apolipoprotein B-100 and plasma mevalonic acid and lathosterol concentrations in man

Apolipoprotein B-100 (apo B) is the principal structural and functional protein of the pro-atherogenic lipoproteins, but its homeostasis in man has not been clearly established. The hepatic availability of cholesterol substrate may be a determining factor. We examined whether there was a direct correlation between plasma concentrations of mevalonic acid (MVA) and lathosterol (indices of in vivo cholesterol synthesis) and hepatic secretion of very-low-density lipoprotein (VLDL) apo B in 13 normolipidaemic, healthy male subjects. The secretion of VLDL apo B was measured using a primed constant intravenous infusion of 1-[13C]-leucine (1 mg/kg per h) over 8 h. Gas-chromatography mass spectrometry (GCMS) was used to derive isotopic enrichment of apo B and fractional turnover rate was calculated using a monoexponential function. There was a highly significant positive correlation between the absolute secretion rate (ASR) of VLDL apo B and the plasma concentrations of mevalonic acid (r = 0.72, P = 0.005) and lathosterol (r = 0.81, P = 0.001) and the lathosterol:cholesterol ratio (r = 0.79, P = 0.001). In multiple regression analysis, these correlations remained significant after adjusting for waist circumference, age, apolipoprotein E genotype and dietary fat intake. The data further support the notion that the availability of cholesterol substrate regulates the hepatic secretion rate of apo B.

Genetic determinants of plasma lipoprotein levels and their dietary response

Hamsters were fed diets containing fat (5-20%, w/w) consisting of triolein (TO) alone or 50% triolein plus 50% trimyristin (TM), tripalmitin (TP) or tristearin (TS) for 28 days. Each fat had unique effects on lipoprotein concentrations which were related to changes in the expression of hepatic genes. Tripalmitin was the most hypercholesterolaemic of the saturated fats, causing dose-dependent increases in LDL and HDL cholesterol which correlated with decreases in the expression of HMGCoA reductase and LDL receptor genes. Tripalmitin also increased the expression of the apoB gene. It seems likely that fatty acids may regulate genes which are involved both in the synthesis and clearance of plasma lipoproteins. Inclusion of increasing amounts of cholesterol in diets containing 20% fat (50% TO plus 50% TP or TS) caused down-regulation of HMGCoAR and LDLR genes and up-regulation of the microsomal triglyceride transfer protein (MTP) gene for both fats. This was accompanied by increased LDL-cholesterol in the TP but not the TS group. In all experiments the concentration of VLDL-cholesterol correlated with the hepatic cholesterylester and MTP mRNA concentrations.

In vitro reconstitution of assembly of apolipoprotein B48-containing lipoproteins

Human apolipoprotein B48 (apoB48) and apoB15 (the NH2-terminal 48 and 15% of apoB100, respectively) were translated in vitro from their respective mRNAs using a rabbit reticulocyte lysate and microsomes derived from rat liver or dog pancreas. Synthesis of phosphatidylcholine and triacylglycerols was reconstituted in freshly isolated microsomes by the addition of precursors of these glycerolipids (acylcoenzyme A, glycerol 3-phosphate, and CDP-choline) before, during, or after translation. Assembly of apoB15 and apoB48 with newly synthesized phospholipids and triacylglycerols was favored by active, co-translational lipid synthesis. Moreover, translocation of apoB48 but not B15 into the microsomal lumen was increased in the presence of co-translational lipid synthesis. When apoB48 was translated in vitro, approximately 50% of apoB48 was buoyant at a density of <1.10 g/ml in the lumen of liver microsomes only when lipid synthesis was reconstituted during translation. Microsomal triacylglycerol transfer protein has been proposed to be essential for lipidation and/or translocation of apoB48. However, apoB48 was translocated into the lumen of dog pancreas microsomes in which the activity of the microsomal triacylglycerol transfer protein was not detectable. These data indicate that (i) apoB15 and apoB48 bind newly synthesized phosphatidylcholine during translocation; (ii) apoB48 but not apoB15 associates co-translationally with triacylglycerols; (iii) translocation of apoB48 but not apoB15 is stimulated by lipid synthesis; (iv) assembly of buoyant apoB48-containing lipoproteins can be reconstituted in vitro in the presence of active lipid synthesis; and (v) even in microsomes lacking microsomal triacylglycerol transfer protein activity, apoB48 is translocated into the lumen.

Intracellular degradation in the regulation of secretion of apolipoprotein B-100 by rabbit hepatocytes

Isolated rabbit hepatocytes were incubated with [35S]methionine to label intracellular pools of apolipoprotein B (apo-B). The cells were then reincubated with an excess of unlabelled methionine in the presence of oleate or protease inhibitors and the intracellular sites of accumulation of radiolabelled apo-B and the mass of apo-B were determined by isolation and analysis of subcellular fractions. Oleate or inhibitors of metalloproteases (o-phenanthroline), serine proteases (aprotinin), serine/cysteine proteases (leupeptin) or cysteins proteases (calpain inhibitor I; ALLN) but not aspartate proteases (pepstatin) resulted in inhibition of the cellular degradation of apo-B. The effect of o-phenanthroline was reversed by the addition of zinc ions. Oleate, o-phenanthroline and leupeptin also stimulated secretion of radiolabelled apo-B; the effects of the inhibitors and oleate were additive, suggesting that they could act via different mechanisms. o-Phenanthroline caused accumulation of apo-B in the rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) membranes; leupeptin caused accumulation of apo-B in the SER and cis-Golgi membranes, and ALLN and aprotinin caused accumulation of apo-B in the trans-Golgi membranes. These results suggest that intracellular degradation of apo-B occurs in the endoplasmic reticulum and in the trans-Golgi membranes and involves different proteases. Apo-B that accumulates in the ER membrane can be diverted into the lumen for secretion; however, apo-B that accumulates in the trans-Golgi membrane is irretrievably diverted from secretion.