Effects of treatment with clofibrate, bezafibrate, and ciprofibrate on the metabolism of cholesterol in rat liver microsomes

An insight into primary biliary cholangitis and its recent advances in treatment: semi-synthetic analogs to combat ursodeoxycholic-acid resistance

INTRODUCTION

Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease which on progression causes cirrhosis; various studies also suggested that several diseases can co-exist in patients. In existing depiction of disease PBC, apart from entire use of ursodeoxycholic acid (UDCA), several patients need to step forward to liver-transplantation or death due to resistance or non-responder with UDCA monotherapy.

AREAS COVERED

To overcome this non-respondent treatment, novel bile acid semi-synthetic analogs have been identified which shows their potency against for farnesoid X receptor and transmembrane G protein-coupled receptor-5 which are identified as target for many developing analogs which have desirable pharmacokinetic profiles.

EXPERT OPINION

A range of studies suggests that adding semisynthetic analogs in therapeutic regime improves liver biochemistries in patients with suboptimal response to UDCA. Thus, the aspire of this review is to abridge and compare therapeutic value and current markets affirm of various bile acids semi-synthetic analogs which certainly are having promising effects in PBC monotherapy or in pooled treatment with UDCA for PBC.

Editor's Highlight: Clofibrate Decreases Bile Acids in Livers of Male Mice by Increasing Biliary Bile Acid Excretion in a PPARα-Dependent Manner

Fibrates and their receptor, namely peroxisome proliferator-activated receptor α (PPARα), have been reported to regulate bile acid (BA) synthesis and transport. However, the effect of fibrate treatment and PPARα activation on BA homeostasis remains controversial. In this study, both wild-type (WT) and PPARα-null male mice were treated with clofibrate (CLOF) for 4 days to evaluate the effects of short-term PPARα activation on BA homeostasis. Although a decrease in total BAs (ΣBAs) was observed in livers of CLOF-treated WT mice, it was not observed in PPARα-null mice. CLOF-mediated decrease in ΣBAs in the liver was not likely due to the reduction in BA synthesis or BA uptake, as evidenced by an increase in the BA synthetic enzyme (Cyp7a1) and 2 BA uptake transporters (Na (+)-taurocholate cotransporting polypeptide [Ntcp] and organic anion transporting polypeptide [Oatp]1b2). Instead, the decrease in liver BAs by CLOF is largely a result of increased biliary excretion of BAs, which was associated with a significant induction of the canalicular efflux transporter (bile salt export pump [Bsep]) in the liver. The PPARα-mediated increase in Cyp7a1 in CLOF-treated WT mice was not due to farnesoid X receptor (Fxr)-small heterodimer partner (Shp) signaling in the liver, but due to suppression of Fxr- fibroblast growth factor15 signaling in the ileum. Additionally, CLOF also suppressed intestinal BA transporters (apical sodium-dependent bile acid transporter and organic solute transporterβ) and cholesterol efflux transporters (Abcg5 and Abcg8) in a PPARα-dependent manner. In summary, this study provides the first comprehensive analysis on the effect of a short-term CLOF treatment on BA homeostasis, and revealed an essential role of PPARα in regulating BA synthesis, transport and signaling.

Integrated physiology and systems biology of PPARα

The Peroxisome Proliferator Activated Receptor alpha (PPARα) is a transcription factor that plays a major role in metabolic regulation. This review addresses the functional role of PPARα in intermediary metabolism and provides a detailed overview of metabolic genes targeted by PPARα, with a focus on liver. A distinction is made between the impact of PPARα on metabolism upon physiological, pharmacological, and nutritional activation. Low and high throughput gene expression analyses have allowed the creation of a comprehensive map illustrating the role of PPARα as master regulator of lipid metabolism via regulation of numerous genes. The map puts PPARα at the center of a regulatory hub impacting fatty acid uptake, fatty acid activation, intracellular fatty acid binding, mitochondrial and peroxisomal fatty acid oxidation, ketogenesis, triglyceride turnover, lipid droplet biology, gluconeogenesis, and bile synthesis/secretion. In addition, PPARα governs the expression of several secreted proteins that exert local and endocrine functions.

Peroxisome proliferator-activated receptor alpha target genes

The peroxisome proliferator-activated receptor alpha (PPARα) is a ligand-activated transcription factor involved in the regulation of a variety of processes, ranging from inflammation and immunity to nutrient metabolism and energy homeostasis. PPARα serves as a molecular target for hypolipidemic fibrates drugs which bind the receptor with high affinity. Furthermore, PPARα binds and is activated by numerous fatty acids and fatty acid-derived compounds. PPARα governs biological processes by altering the expression of a large number of target genes. Accordingly, the specific role of PPARα is directly related to the biological function of its target genes. Here, we present an overview of the involvement of PPARα in lipid metabolism and other pathways through a detailed analysis of the different known or putative PPARα target genes. The emphasis is on gene regulation by PPARα in liver although many of the results likely apply to other organs and tissues as well.

Therapeutic effects of fibrates in postprandial lipemia

Hypertriglyceridemia is observed in many metabolic diseases such as the metabolic syndrome, diabetes mellitus, or mixed dyslipidemia frequently leading to premature coronary heart disease (CHD). Additionally, several studies have shown that postprandial hypertriglyceridemia is pronounced in patients with CHD, metabolic syndrome, hypertension, and other pathologic conditions. The triglyceride-rich lipoprotein remnants accumulating in the postprandial state seem to be involved in atherogenesis and in events leading to thrombosis. Since abnormal postprandial lipemia is associated with pathologic conditions, its treatment is of clinical importance.Fibrates are of significant help in managing hypertriglyceridemia. This review summarizes the effect of fibric acid derivatives on postprandial lipemia. Fibrates decrease the production of and enhance the catabolism of triglyceride-rich lipoproteins through the activation of peroxisome proliferator-activated receptor-alpha. Results of clinical studies with fibrates have confirmed their action in decreasing postprandial triglyceride levels by increasing lipoprotein lipase activity, decreasing apolipoprotein CIII production, and by increasing fatty acid oxidation in the liver.It is concluded that fibrates are effective agents in lowering the postprandial increase in remnant lipoprotein particles and retinyl palmitate. Furthermore, fibrates can also affect the postprandial lipid profile by increasing hepatic lipase levels and in some cases, by reducing cholesterol ester transfer protein activity. The main target of fibrate therapy is to improve fasting hypertriglyceridemia, which is an essential component associated with improving postprandial lipemia. Fibrates are well tolerated by patients and adverse effects have been reported rarely after their administration.

Activation of the rat scavenger receptor class B type I gene by PPARalpha

Peroxisomal proliferator activated receptor alpha (PPARalpha) is activated by fibrate drugs which are known to protect against atherosclerosis. The present study examines the effects of PPARalpha on SR-BI expression. For this study, a rat SR-BI promoter-luciferase reporter gene construct was co-transfected into different cell lines with expression vectors that encode for PPARalpha+/-retinoic X receptor alpha (RXRalpha). PPARalpha/RXR increased the activity of the SR-BI promoter, an effect that was enhanced by clofibrate. Sequence analysis of the rat SR-BI promoter revealed the presence of a putative peroxisomal proliferator response element (PPRE) at bp -1,622. Electrophoretic mobility shift assays demonstrated that PPARalpha and RXRalpha are able to bind to the SR-BI PPRE motif. In addition, mutational analysis studies confirmed that this PPRE motif is responsible for the PPARalpha/RXRalpha-dependent activation of the rat SR-BI promoter in the cell lines examined.

Fenofibrate Induces a Novel Degradation Pathway for Scavenger Receptor B-I Independent of PDZK1

Fibrate drugs improve cardiovascular health by lowering plasma triglycerides, normalize low density lipoprotein levels, and raise high density lipoprotein (HDL) levels in patients with dyslipidemias. The HDL-raising effect of fibrates has been shown to be due in part to an increase in human apolipoprotein AI gene expression. However, it has recently been shown that fibrates can affect HDL metabolism in mouse by significantly decreasing hepatic levels of the HDL receptor scavenger receptor B-I (SR-BI) and the PDZ domain containing protein PDZK1. PDZK1 is essential for maintaining hepatic SR-BI levels. Therefore, decreased SR-BI might be secondary to decreased PDZK1, but the mechanism by which fibrates lower SR-BI has not been elucidated. Here we show that feeding PDZK1-deficient mice fenofibrate resulted in the near absence of SR-BI in liver, definitively demonstrating that the effect of fenofibrate on SR-BI is PDZK1-independent. Metabolic labeling experiments in primary hepatocytes from fenofibrate-fed mice demonstrated that fenofibrate enhanced the degradation of SR-BI in a post-endoplasmic reticulum compartment. Moreover, fenofibrate-induced degradation of SR-BI was independent of the proteasome, calpain protease, or the lysosome, and antioxidants did not inhibit fenofibrate-induced degradation of SR-BI. Using metabolic labeling coupled with cell surface biotinylation assays, fenofibrate did not inhibit SR-BI trafficking to the plasma membrane. Together, the data support a model in which fenofibrate enhances the degradation of SR-BI in a post-ER, post-plasma membrane compartment. The further elucidation of this novel degradation pathway may provide new insights into the physiological and pathophysiological regulation of hepatic SR-BI.

Effects of peroxisome proliferator-activated receptor α activation on pathways contributing to cholesterol homeostasis in rat hepatocytes

Peroxisome proliferator-activated receptor alpha (PPARalpha) activation by fibrates controls expression of several genes involved in hepatic cholesterol metabolism. Other genes could be indirectly controlled in response to changes in cellular cholesterol availability. To further understand how fibrates may affect cholesterol synthesis, we investigated in parallel the changes in the metabolic pathways contributing to cholesterol homeostasis in liver. Ciprofibrate increased HMG-CoA reductase and FPP synthase mRNA levels in rat hepatocytes, together with cholesterogenesis from [(14)C] acetate and [(3)H] mevalonate. The up-regulation observed in fenofibrate- and WY-14,643-treated mice was abolished in PPARalpha-null mice, showing an essential role of PPARalpha. Among the three sterol regulatory element-binding protein (SREBP) mRNA species, only SREBP-1c level was significantly increased. In ciprofibrate-treated hepatocytes, cholesterol efflux was decreased, in parallel with cholesteryl ester storage and bile acids synthesis. As expected, AOX expression was strongly induced, supporting evidence of the peroxisome proliferation. Taken together, these results show that fibrates can cause cholesterol depletion in hepatocytes, possibly in part as a consequence of an important requirement of cholesterol for peroxisome proliferation, and increase cholesterogenesis by a compensatory phenomenon afterwards. Such cholesterogenesis regulation could occur in vivo, in species responsive to the peroxisome proliferative effect of PPARalpha ligands.

Peroxisome proliferator-activated receptor alpha (PPARalpha) activators induce hepatic farnesyl diphosphate synthase gene expression in rodents

Fibrates are hypolipidemic drugs that exert multiple effects on lipid metabolism by activating peroxisome proliferator-activated receptor alpha (PPARalpha) and modulating the expression of many target genes. In order to investigate the link between PPARalpha and cholesterol synthesis, we analysed the effect of fibrates on expression of the farnesyl diphosphate synthase (FPP synthase) gene, known to be regulated by sterol regulatory element-binding proteins (SREBPs), in conjunction with HMG-CoA reductase. In wild-type mice, both fenofibrate and WY 14,643 induced FPP synthase gene expression, an effect impaired in PPARalpha-null mice. A three-fold induction was observed in ciprofibrate-treated rat hepatocytes, in primary culture. This effect was decreased in presence of 5,6-dichlorobenzimidazole riboside (DRB) and cycloheximide (CHX), transcription and translation inhibitors, respectively. Acyl-CoA oxidase (AOX), a bona fide PPARalpha target gene, was induced by ciprofibrate but slower and more strongly than FPP synthase. In addition, induction of FPP synthase gene expression was abolished in the presence of 25-hydroxycholesterol (25-OH Chol). Thus, activation of PPARalpha by fibrates induced FPP synthase gene expression in both hepatocytes in culture and in mouse liver. This effect is likely to be dependent on cellular sterol level, possibly through SREBP-mediated transcriptional activation.

Inhibitory effects of Polygonum cuspidatum water extract (PCWE) and its component rasveratrol on acyl-coenzyme A–cholesterol acyltransferase activity for cholesteryl ester synthesis in HepG2 cells

The pharmacological effects of Polygonum cuspidatum water extract (PCWE) on lipid biosynthesis were investigated in cultured human hepatocyte HepG2 cells. The addition of PCWE (5 and 20 microg/ml), which had no effect on cell proliferation and cellular protein content, caused a marked decrease in the cellular cholesterol content, particularly, the cholesteryl ester content following 24 h of incubation. The incorporation of (14)C-oleate into the cellular cholesteryl ester fraction was also reduced remarkably during incubation for 6 and 24 h. The effect of PCWE on acyl-coenzyme A-cholesterol acyltransferase (ACAT) activity were studied in vitro to explore the mechanism by which PCWE inhibits cholesterol ester formation. The data confirmed that PCWE, in a dose dependent manner, remarkably inhibits ACAT activity. Among the main active chemicals of P. cuspidatum, resveratrol, a kind of flavonoid, decreased ACAT activity in a dose-dependent manner from the level of 10(-3) M. Theses results strongly suggest that PCWE reduces the cholesteryl ester formation in human hepatocytes by inhibiting ACAT.

PPARs: transcription factors controlling lipid and lipoprotein metabolism

Nuclear receptors are transcription factors that are activated by ligands and subsequently bind to regulatory regions in target genes, thereby modulating their expression. Nuclear receptors thus allow the organism to integrate signals coming from the environment and to adapt by modifying the expression levels of relevant genes. The peroxisome proliferator-activated receptors (PPARs) alpha, beta/delta, and gamma constitute a subfamily of nuclear receptors. PPARalpha has been shown to bind and to be activated by leukotriene B4 and the hypolipidemic drugs of the fibrate class; PPARbeta/delta ligands are polyunsaturated fatty acids and prostaglandins; while prostaglandin J2 derivatives and the antidiabetic glitazones are, respectively, natural and synthetic ligands for PPARgamma. Upon binding and activation by their ligands, they regulate the transcription of numerous genes involved in intracellular lipid metabolism, lipoprotein metabolism, and reverse cholesterol transport in a subtype- and tissue-specific manner. PPARs therefore constitute interesting targets for the development of therapeutic compounds useful in the treatment of disorders of lipid and lipoprotein metabolism.

Fibrates suppress bile acid synthesis via peroxisome proliferator-activated receptor-alpha-mediated downregulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase expression

Fibrates are hypolipidemic drugs that affect the expression of genes involved in lipid metabolism by activating peroxisome proliferator-activated receptors (PPARs). Fibrate treatment causes adverse changes in biliary lipid composition and decreases bile acid excretion, leading to an increased incidence of cholesterol gallstones. In this study, we investigated the effect of fibrates on bile acid synthesis. Ciprofibrate and the PPARalpha agonist Wy14,643 decreased bile acid synthesis in cultured rat hepatocytes and suppressed cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities, paralleled by a similar reduction of the respective mRNAs. Treatment of rats with 0.05% (wt/wt) ciprofibrate decreased cholesterol 7alpha-hydroxylase enzyme activity and mRNA. The functional involvement of PPARalpha in the suppression of both enzymes was proven with the use of PPARalpha-null mice. In wild-type mice, ciprofibrate reduced cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase enzyme activities and mRNA. The decrease in mRNA of both enzymes is regulated transcriptionally and posttranscriptionally, respectively, resulting in a decline in the output of fecal bile acids (-45%) and a 3-fold increase in fecal cholesterol secretion. These effects were completely abolished in PPARalpha-null mice. A decreased bile acid production by PPARalpha-mediated downregulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase may contribute to the increased risk of gallstone formation after fibrate treatment.

Peroxisome proliferator-activated receptors: from transcriptional control to clinical practice

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that control energy homeostasis through genomic actions. Over the past few years significant advances have been made in unravelling the pathways that are modulated by PPARs. Gene targeting experiments in mice and genetic studies in humans have demonstrated a physiological role for these receptors in adipocyte function, glucose homeostasis, and lipid and lipoprotein metabolism. Recent data indicate that PPARs enhance the reverse cholesterol transport pathway by regulating genes that control macrophage cholesterol efflux, cholesterol transport in plasma and bile acid synthesis. Clinical and experimental evidence suggest that PPAR activation decreases the incidence of cardiovascular disease not only by correcting metabolic disorders, but also through direct actions at the level of the vascular wall. Thus, dysregulation of PPAR activity modulates the onset and evolution of metabolic disorders such as dyslipidaemia, obesity and insulin resistance, predisposing to atherosclerosis.

The murine and human cholesterol 7alpha-hydroxylase gene promoters are differentially responsive to regulation by fatty acids mediated via peroxisome proliferator-activated receptor alpha

We determined if fatty acids can regulate the murine Cyp7a1 and human CYP7A1 gene promoters via peroxisome proliferator-activated receptor alpha (PPARalpha)/9-cis-retinoic acid receptor alpha (RXRalpha). In transfected cells, the murine Cyp7a1 gene promoter displayed markedly lower basal activity, but greater sensitivity to fatty acid- or WY 14,643-activated PPARalpha/RXRalpha when compared with the human CYP7A1 gene promoter. PPARalpha/RXRalpha can bind to a site (Site II) located within the region at nucleotides -158 to -132 of both promoters. Mutagenesis of the human CYP7A1 Site II element abolished the response to activated PPARalpha/RXRalpha. The murine Cyp7a1 gene promoter contains an additional PPARalpha/RXRalpha-binding site (Site I) located within nucleotides -72 to -57. Replacement of a single residue in human CYP7A1 Site I with that found in the murine Cyp7a1 Site I sequence enabled PPARalpha/RXRalpha binding, and this mutation resulted in reduced basal activity, but substantially improved the response to activated PPARalpha/RXRalpha in transfected cells. We conclude that fatty acids can regulate the cyp7a gene promoter via PPARalpha/RXRalpha. The differential response of the murine Cyp7a1 and human CYP7A1 gene promoters to PPARalpha activators is attributable to the additional PPARalpha/RXRalpha-binding site in the murine Cyp7a1 gene promoter.

Catabolites of cholesterol synthesis pathways and forskolin as activators of the farnesoid X-activated nuclear receptor

The nuclear receptors are a family of transcriptional mediators that, upon activation, bind DNA and regulate gene transcription. Among these receptors, the farnesoid X-activated receptor (FXR) has recently been identified as one activated by bile acids and farnesol. To investigate the potential of other sterols to activate FXR, as well as to examine relevant relationships among identified activators of FXR, the current study used a mammalian cell transcription assay to quantify and compare activation potential. In addition to the classical bile acids deoxycholate (DCA) and chenodeoxycholate (CDCA), FXR was shown to be transcriptionally active in the presence of the androgen catabolites 5alpha-androstan-3alpha-ol-17-one (androsterone) and 5beta-androstan-3alpha-ol-17-one (etiocholanolone), as well as the sterol bronchodilatory drug forskolin. Conversely, cholesterol and several other key precursors to the androgens and bile acids were either not active or only slightly active. Furthermore, it was observed that the bile acid ursodeoxycholate (UDCA) could inhibit DCA and CDCA activation of FXR in a manner parallel to its ability to antagonize DCA and CDCA induction of apoptosis. By far, the most efficacious activator of FXR was forskolin. Interestingly, although it is classically viewed as an initiator of the adenylate cyclase/protein kinase A (PKA) pathway, PKA inhibition did not inhibit forskolin's activation of FXR nor was cyclic AMP (cAMP) able to stimulate FXR-mediated transcription. These data would suggest that forskolin acts as a ligand for FXR rather than as a secondary activator of FXR and could have important implications with respect to its potential toxicity and pharmacological use.

Peroxisome proliferator-induced long chain acyl-CoA thioesterases comprise a highly conserved novel multi-gene family involved in lipid metabolism

Long chain acyl-CoA esters are important intermediates in degradation and synthesis of fatty acids, as well as having important functions in regulation of intermediary metabolism and gene expression. Although the physiological functions for most acyl-CoA thioesterases have not yet been elucidated, previous data suggest that these enzymes may be involved in lipid metabolism by modulation of cellular concentrations of acyl-CoAs and fatty acids. In line with this, we have cloned four highly homologous acyl-CoA thioesterase genes from mouse, showing multiple compartmental localizations. The nomenclature for these genes has tentatively been assigned as CTE-I (cytosolic), MTE-I (mitochondrial), and PTE-Ia and Ib (peroxisomal), based on the identification of putative targeting signals. Although the various isoenzymes show between 67% and 94% identity at amino acid level, each individual enzyme shows a specific tissue expression. Our data suggest that all four genes are located within a very narrow cluster on chromosome 12 in mouse, similar to a sequence cluster on human chromosome 14, which identified four genes homologous to the mouse thioesterase genes. Four related genes were also identified in Caenorhabditis elegans, all containing putative PTS1 targeting signals, suggesting that the ancestral type I thioesterase gene(s) is/are of peroxisomal origin. All four thioesterases are differentially expressed in tissues examined, but all are inducible at mRNA level by treatment with the peroxisome proliferator clofibrate, or during the physiological condition of fasting, both of which conditions cause a perturbation in overall lipid homeostasis. These results strongly support the existence of a novel multi-gene family cluster of mouse acyl-CoA thioesterases, each with a distinct function in lipid metabolism.

Lack of predictability of classical animal models for hypolipidemic activity: a good time for mice?

Hypolipidemic drugs that are efficacious in man are not always active in classical animal models of dyslipidemia. Inhibitors of HMG-CoA reductase (statins) do not lower plasma cholesterol in rats, but yet this species was alone in providing activity for fibrate-type drugs. Nicotinic acid possesses many desirable features with regard to clinical use, but most of these actions are lacking in rats and monkeys. The metabolism of low density lipoproteins in hamsters is widely thought to be similar to that in humans, yet neither statins or fibrates lower plasma lipids in these species. With the advent of mouse models expressing specific human genes (or disruption of genes) it is now possible to re-examine the effect of established drugs and to characterize new hypolipidemic compounds with respect to site and mechanism of action. Drug responses observed in humans are now being seen in such mouse models (e.g. HDL elevation with fenofibrate in mice with the human apo A-I gene). Moreover, mice are now being screened for compounds that lower plasma (human) Lp(a), or lower plasma cholesterol in the absence of LDL receptors. It is proposed that these new genetic mouse models may afford a more focused examination of drug action and provide, for new compounds, better prediction of the human response.

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.

Reduction of oxysterol levels up-regulates HMG-CoA reductase activity in rat liver

Cholesterol regulates hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity by feedback inhibition. It has been suggested that oxidized derivatives of cholesterol (oxysterols) play an important role, as an intracellular mediator, in the feedback inhibition of cholesterol biosynthesis. We, therefore, investigated the role of intracellular oxysterols in the regulation of HMG-CoA reductase activity. Rats were fed with food (control), cholesterol, clofibrate as a potentiator of the microsomal monooxygenase cytochrome P-450 enzyme system, ketoconazole as a strong inhibitor of the system, or butylated hydroxytoluene (BHT) as an antioxidant. We analyzed and compared hepatic microsomal oxysterol levels among the groups. The results of this study indicated that the oxysterol level, especially 7beta-hydroxycholesterol and 7-ketocholestrol, in the liver was lowered by the administration of ketoconazole and BHT, and HMG-CoA reductase activity was increased in response to these agents. However, there was no change in the HMG-CoA reductase activity, after the administration of clofibrate. We conclude that reduced levels of oxysterol may release the inhibitory effect on the HMG-CoA reductase enzyme and lead to up-regulation of the enzyme.

Opposite effects of bezafibrate and gemfibrozil in both normal and hypertriglyceridemic rats

Chow and sucrose-fed rats were used as animal models to study the dose-responses of bezafibrate and gemfibrozil in normolipidemic and hypertriglyceridemic states, respectively. Although both drugs lowered plasma triglycerides (TG) to about the same extent in chow-fed rats, gemfibrozil lowered liver TG as well as plasma total and LDL-cholesterol (LDL-C), but elevated HDL-cholesterol (HDL-C) and plasma apo E concentrations. Bezafibrate produced opposite effects, namely, decreased HDL-C, apo E and liver TG, and tended to increase LDL-C. TG lowering for both drugs in chow-fed rats was not due to changes in TG secretion (production) in normal rats but was associated with enhanced LPL activity. In hypertriglyceridemic rats both drugs modestly reduced TG secretion rates about 40% at a dose producing maximal TG lowering, but again, gemfibrozil elevated and bezafibrate lowered HDL-C and apo E. Unlike gemfibrozil, bezafibrate induced the appearance of LDL-C in hypertriglyceridemic rats which was not detected in control animals, and also tended to increase rather than decrease plasma apo B levels. Finally, changes in liver TG concentration (mg/g) in hypertriglyceridemic rats were opposite for these drugs, resulting in significant drug-related differences in liver TG content (mg/organ). From these data we postulate that, although similar with regard to TG lowering activity and mechanisms thereof, gemfibrozil and bezafibrate produce fundamentally different effects on LDL, HDL and apolipoprotein metabolism (apo B and apo E) in rats which may relate to potential differential effects on reverse cholesterol transport and atherogenesis.

Bezafibrate and clofibric acid are novel inhibitors of phosphatidylcholine synthesis via the methylation of phosphatidylethanolamine

The effects of bezafibrate and clofibric acid, fibrate hypolipidemic agents, on phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway were studied. In cultured rat hepatocytes, bezafibrate and clofibric acid added to the medium rapidly and markedly reduced the conversion of ethanolamine-labeled PE to PC (IC50 30 and 150 microM, respectively). Furthermore, the methylation of PE derived from serine was also blocked by bezafibrate, as was the secretion of PC derived from either serine or ethanolamine. The microsomal activity of PE N-methyltransferase was inhibited by these agents. Perfluorooctanoic acid but not DCQVA, though both are potent peroxisome proliferators comparable to fibrates, produced this inhibition. The inhibitory effects produced by these agents were diminished by dithiothreitol (DTT) added to the assay or alkaline pH assay condition. Inhibition by oleic acid was also attenuated under these conditions, suggesting a common mechanism of inhibition. However, unlike fatty acids, fibrates did not have rapid stimulatory effects on the CDP-choline pathway in hepatocytes. These results suggest that fibrates may mimic fatty acids in regulating PC synthesis from the PE methylation pathway but not the CDP-choline pathway.

Differential effects of fibrates on the acyl composition of microsomal phospholipids in rats

1. The time-course and comparative effects of treatment with clofibrate (CFB), bezafibrate (BFB), and gemfibrozil (GFB) on the acyl composition of the main microsomal phospholipids, i.e. phosphatidylcholine and phosphatidylethanolamine, have been studied in male Sprague-Dawley rats. 2. The administration of the three fibrates caused a strong peroxisomal induction and a hypolipidaemic effect. Concerning the changes in acyl composition, CFB and BFB behaved in a similar way, with differences which could be attributed to their different potency as peroxisome inducers, whereas GFB showed a somewhat distinct profile. 3. The three drugs increased the relative content of palmitic, palmitoleic and oleic acids, whereas the levels of stearic acid and also those of long chain, highly unsaturated fatty acids docosatetraenoic, docosapentaenoic and docosahexaenoic acids were reduced. In general, these effects appeared from the first day of treatment and were highly correlated with peroxisomal proliferation. In addition, they were more evident in the phosphatidylcholine than in the phosphatidylethanolamine fraction. 4. Fibrates increased total monounsaturated fatty acids, whereas a decrease in total polyunsaturated fatty acids in the phosphatidylcholine fraction was observed in CFB- and BFB-, but not in GFB-treated rats. Clear differences appeared between CFB and BFB on the one hand, and GFB on the other when the influence of fibrate treatment on the molar percentages of linoleic, eicosatrienoic, arachidonic and mead acids was analyzed. 5. GFB increased linoleic acid content in phosphatidylethanolamine, whereas CFB and BFB decreased its level in both phospholipid fractions. In contrast, CFB and BFB enhanced eicosatrienoic and mead acids in both fractions and arachidonic acid in phosphatidylethanolamine, whereas GFB had practically no effect.

Effect of etofibrate on bile production in the normolipidemic rat

1. The effect of etofibrate, the ethandiol-1,2 diester of nicotinic and clofibric acids on bile production was studied in male rats that received a daily dose of 300 mg of etofibrate/kg body weight by stomach tube for 10 days and were compared with control rats receiving the medium. 2. The bile duct was cannulated, animals were intravenously given 1 microCi (4-14C)-cholesterol/100 b.w. and bile was collected at different intervals for a total of 4 hr. 3. Etofibrate treatment decreased plasma cholesterol and triglyceride concentrations and increased the bile flow. The cummulative amount of both bile volume and total bile radioactivity secreted increased linearly in all the animals; the respective slopes being higher in etofibrate treated rats than in controls. 4. The main labelled component found in the bile was always bile acids rather than cholesterol and the proportion of each of these compounds was similar in both groups. Neither was any difference between the groups found in the concentration of bile acids, cholesterol and phospholipids nor in the cholesterol/(bile+phospholipid) ratio. 5. Besides other factors, the present results indicate that an increase in bile flow and biliary cholesterol excretion in its free form and after its conversion into bile acids should contribute to the hypocholesterolemic effect of etofibrate.

Effect of gemfibrozil treatment in hypercholesterolemia on low density lipoprotein (LDL) subclass distribution and LDL-cell interaction

Gemfibrozil, a widely used fibric acid derivative, corrects hypercholesterolemia in a non-negligible fraction of patients. To investigate the mechanism of the cholesterol-lowering activity of fibric acids, a study was performed in 12 type IIa hyperlipidemic patients treated with gemfibrozil for 12 weeks. Changes in low density lipoprotein (LDL) structure and composition, agonist capacity of LDL against the LDL-receptor in human skin fibroblasts, LDL-receptor activity in mononuclear cells, lecithin:cholesterol acyltransferase (LCAT) and cholesterol ester transfer protein (CETP) activity, were evaluated. Plasma total and LDL cholesterol levels decreased by 17% and 20% after 12 weeks of treatment, the reduction being directly correlated with the baseline levels (r = 0.75 and 0.78, respectively). The mean LDL diameter increased significantly, from 25.5 to 26.1 nm, while the relative content of small LDL particles (< 25.1 nm) increased from 23.4% to 32.8% of total LDL. Neither the apolipoprotein (apo) B secondary structure nor the affinity of LDL for the LDL-receptor of fibroblasts were affected. The LDL-receptor activity in patients' mononuclear cells increased 3-fold, the rise being unrelated to the plasma cholesterol reduction. LCAT activity did not change, while CETP activity was reduced by 25% (P = 0.13) after treatment. These findings indicate that gemfibrozil causes significant changes in LDL structure that do not, however, affect the LDL interaction with peripheral cells.

Effects of pregnenolone-16 alpha-carbonitrile on the metabolism of cholesterol in rat liver microsomes

The effects of pregnenolone-16 alpha-carbonitrile (PCN) on hepatic metabolism of cholesterol were studied in rat liver microsomes in order to clarify the underlying mechanisms of the PCN-induced biliary hypersecretion of cholesterol. Male Sprague-Dawley rats were fed a diet supplemented with 0.05% of PCN for one week. The microsomal activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, regulating cholesterol biosynthesis, decreased from 577 +/- 46 (SEM) to 367 +/- 38 pmol/min/mg protein compared to the controls. Cholesterol 7 alpha-hydroxylase activity, governing bile acid synthesis, was 9.0 +/- 1.1 pmol/min/mg protein in the treated group and 34.8 +/- 7.4 pmol/min/mg protein in the controls, a reduction of 74% (P < 0.01). The acyl CoA:cholesterol acyltransferase (ACAT) activity, catalyzing the esterification of cholesterol, remained unchanged, as did the levels of total and free cholesterol in liver homogenates and microsomes. The results of this study provide evidence that the increase in biliary cholesterol secretion during PCN treatment is not caused by a change in ACAT activity, but can be explained by a decreased catabolism of cholesterol to bile acids.

Influence of bezafibrate on hepatic cholesterol metabolism in gallstone patients: reduced activity of cholesterol 7 alpha-hydroxylase

Bezafibrate is a hypolipidemic fibric acid derivative known to induce cholesterol supersaturation of bile. To characterize its effects on hepatic cholesterol metabolism, 31 normolipidemic, normal-weight patients with gallstones undergoing cholecystectomy were studied. Eleven patients (5 men) were randomized to treatment with bezafibrate, 200 mg three times daily for 4 weeks before operation; the remaining 20 patients (5 men) served as nontreatment controls. At operation, a liver biopsy specimen was obtained under standardized conditions and several important parameters of cholesterol metabolism were assayed. Bezafibrate treatment lowered total plasma cholesterol and triglycerides 30% and 37%, respectively. The hepatic cholesterol 7 alpha-hydroxylase activity was reduced by approximately 60% in the bezafibrate treated patients compared with the controls, whereas the acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was similar in the two groups. The total 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity was increased twofold in the treated patients, whereas the active enzyme remained about the same as in the controls. The low-density lipoprotein (LDL) receptor binding activity was unaffected by the treatment. Bezafibrate treatment significantly reduces cholesterol 7 alpha-hydroxylase activity, and it is suggested that this may play an important role for the development of supersaturated bile during such therapy.

The effects of probucol and clofibrate alone and in combination on hepatic cholesterol metabolism in the male rat

Male rats were fed for 10 days on a diet supplemented with either probucol or clofibrate, alone or in combination, and the effects of the drugs on hepatic cholesterol metabolism studied. Plasma triacylglycerols were significantly lowered (15.6%, P < 0.05) by the drugs in combination but not individually whereas plasma cholesterol levels were reduced by probucol alone (22.4%, P < 0.05) and the combined treatment effected a further decrease leading to a total reduction of 50.6% (P < 0.001). Probucol reduced hepatic cellular triacylglycerols (20.0%, P < 0.05) and cholesterol (15.3%, P < 0.05) but cholesteryl esters were unaffected. In combination with clofibrate, probucol accentuated the reductions in both cellular cholesterol and cholesteryl esters produced by clofibrate alone and lowered their levels by 22.8%, P < 0.01 and 38.5%, P < 0.001, respectively. Although probucol, on its own, did not affect the activity of acyl-coenzyme A:cholesterol acyltransferase (ACAT), its combination with clofibrate caused less inhibition (43.5%, P < 0.01) of this enzyme activity than clofibrate alone (65.7%, P < 0.001). Probucol had a similarly moderating effect on the clofibrate-induced reductions in microsomal cholesterol and cholesteryl esters. Neither the microsomal nor the cytosolic neutral cholesteryl ester hydrolase was affected by probucol alone although both enzymes were dramatically increased (between 350% and 550%) by clofibrate and the combined treatment. The activity of the hepatic cytosolic inhibitor of cholesteryl ester hydrolase was unaffected by clofibrate or probucol individually but the two drugs in combination increased the total activity of the inhibitor by 52.1%, P < 0.01. When allowance was made for this increased inhibitor activity, it was clear that probucol accentuated the stimulatory effect of clofibrate on the cytosolic nCEH.

The effects of clofibrate and bezafibrate on cholesterol metabolism in the liver of the male rat

Fibric acid derivatives are used to treat hyperlipidemia and have wide ranging effects on lipid metabolism. The action of these compounds on cholesterol esterification, catalyzed by acyl-coenzyme A:cholesterol acyltransferase (ACAT), has been quite widely studied, but their effect on cholesteryl ester hydrolysis and the enzyme neutral cholesteryl ester hydrolase (nCEH) has been largely ignored. Male rats were therefore fed for 10 d on a standard chow diet supplemented with either clofibrate or bezafibrate, to study their effects on plasma lipid levels and hepatic cholesterol metabolism. Plasma triacylglycerols were not significantly altered by these diets, but bezafibrate significantly lowered plasma cholesterol levels (29.7%, P < 0.01). When expressed per unit weight of DNA, both fibrates reduced the hepatic content of triacylglycerol, cholesterol and cholesteryl esters (40, 18.7, 16.5 and 66.7, 28.6, 34.2% for clofibrate and bezafibrate, respectively). ACAT activity was significantly reduced by both drugs, but clofibrate (65% inhibition) was more effective than bezafibrate (35% inhibition). The most dramatic effect of the diets was a marked increase in the activity of both the microsomal and the cytosolic nCEH. When expressed on a whole liver basis, the effect of bezafibrate on the cytosolic enzyme (13.6-fold increase in activity) was much greater than that of clofibrate (4.8-fold increase). Increases in the activity of a cytosolic protein that inhibits the activity of nCEH were also noted, but these changes were relatively small. The results suggest that the activation of nCEH, in combination with the inhibition in ACAT activity, contributes to a decrease in the cholesteryl ester content of the liver which may influence the secretion of very low density lipoprotein.

Relationship between plasma lipids and palmitoyl-CoA hydrolase and synthetase activities with peroxisomal proliferation in rats treated with fibrates

1. The time-course of the effect of clofibrate (CFB), bezafibrate (BFB) and gemfibrozil (GFB) on lipid plasma levels and palmitoyl-CoA hydrolase and synthetase activities, as well as the correlations with the peroxisomal proliferation phenomenon have been studied in male Sprague-Dawley rats. 2. The administration of the three drugs caused a significant reduction in body weight gain, accompanied with a paradoxical increase in food intake in groups treated with BFB and GFB. 3. Drug treatment produced gross hepatomegaly and increase in peroxisomal beta-oxidation, and these parameters were strongly correlated. The order of potency was BFB > CFB > or = GFB. 4. Both plasma cholesterol (BFB approximately CFB > GFB) and triglyceride (BFB approximately GFB > CFB) levels were reduced in treated animals. There was an inverse correlation between these parameters and peroxisomal beta-oxidation, although the peroxisomal proliferation seemed to explain only a small part of the hypolipidemic effect observed. 5. Cytosolic and microsomal (but not mitochondrial) palmitoyl-CoA hydrolase activities were increased by the three drugs (BFB > CFB > GFB), probably by inducing the hydrolase I isoform, which is insensitive to inhibition by fibrates in vitro. The increased hydrolase activities were directly and strongly correlated with peroxisomal beta-oxidation. 6. Palmitoyl-CoA synthetase activity was also increased by the treatment with fibrates (BFB > CFB > GFB), probably as a consequence of the enhancement of hydrolase activities. 7. Some of the effects of fibrate treatment can be explained, at least in part, in terms of peroxisomal induction and caution should be exercised in the extrapolation of these results to species, such as man,that are insensitive to peroxisomal proliferation.

Effects of hypolipidaemics cetaben and clofibrate on mitochondrial and peroxisomal enzymes of rat liver

Clofibrate or cetaben was administered to male rats for 10 days. Peroxisomal and mitochondrial enzymes were assayed in liver subcellular fractions. Clofibrate affected the specific activities of both mitochondrial enzymes (glycerol-3-phosphate dehydrogenase and nicotinamide-linked isocitrate dehydrogenase) and peroxisomal enzymes (fatty acyl-CoA oxidase, glycerone phosphate acyltransferase, urate oxidase, and D-amino-acid oxidase). In contrast, cetaben raised only the peroxisomal enzymes, acyl-CoA oxidase, glycerone-phosphate acyltransferase, D-amino-acid oxidase, catalase, and urate oxidase. Thus, the hypolipidaemic activity of these drugs may be exclusively related to stimulated peroxisomal functioning, while mitochondria play only a minor role.

Studies with etofibrate in the rat. Part II: A comparison of the effects of prolonged and acute administration on plasma lipids, liver enzymes and adipose tissue lipolysis

To contribute to the understanding of the hypolipidemic action of etofibrate, which is the 1,2-ethandiol ester of clofibric acid and nicotinic acid, 300 mg of this drug/kg body weight or of the medium were administered daily by a stomach tube to normolipidemic rats. Some animals were decapitated at the 10th day of daily treatment (prolonged treatment), whereas others were studied at different times after one single administration (acute treatment). In animals on prolonged treatment etofibrate decreased plasma levels of cholesterol, triacylglycerols, free fatty acids (FFA) and glycerol, as well as the total and unesterified cholesterol concentrations, in liver microsomes. In these rats, etofibrate increased the activity of liver cytosolic glycerol-3-P dehydrogenase, whereas it decreased the activity of both microsomal HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and did not affect acyl-CoA: cholesterol acyltransferase (ACAT). At 3, 5 and 7 h after acute treatment, etofibrate decreased plasma levels of triacylglycerols, glycerol and FFA, and this effect disappeared at 24 h, whereas plasma cholesterol did not change 3 h after etofibrate but decreased at 5 and 7 h and remained low after 24 h, and a similar change was found in the liver microsomes free cholesterol concentration. However, with the exception of a significant reduction in cytosolic glycerol-3-P dehydrogenase at 7 h and in ACAT at 5 h, acute etofibrate treatment did not affect the activity of the liver enzymes studied. At low concentrations (10(-5) M) in the incubation medium, etofibrate decreased the release of both FFA and glycerol by epididymal fat pad pieces incubated in vitro. These findings together with those previously reported by us in rats using a similar etofibrate treatment protocol [6] indicate that etofibrate decreases the availability of lipolytic products in the liver by acting on their release from adipose tissue and on their intrinsic hepatic metabolism. Consequently, this drug would decrease liver VLDL triacylglycerol synthesis and secretion, which together with facilitating the clearance of circulating triacylglycerols causes its hypotriglyceridemic effect. The hypocholesterolemic effect of etofibrate after acute treatment may be a secondary consequence of the reduced liver VLDL production caused by decreased adipose tissue lipolysis, but after prolonged treatment, this effect also seems to be influenced by the inhibition of HMG-CoA reductase activity which would reduce cholesterol synthesis.

Mechanisms of action in the liver of crilvastatin, a new hydroxymethylglutaryl-coenzyme A reductase inhibitor

Crilvastatin is a drug from the pyrrolidone family that had been shown to induce non-competitive inhibition of rat hydroxymethylglutaryl-coenzyme A reductase activity in vitro. The aim of this study was to evaluate the activity of crilvastatin on the hepatic metabolism of cholesterol in rats. Crilvastatin increased low density lipoprotein (LDL)-cholesterol uptake by the liver more than high density lipoprotein (HDL) uptake, thus increasing by up 30% the clearance of excess plasma cholesterol. In normolipidemic rats, crilvastatin significantly enhanced acyl coenzyme A:cholesterol acyl transferase and cholesterol 7 alpha-hydroxylase activity. In rats with a previous high cholesterolemia, crilvastatin also enhanced cholesterol 7 alpha-hydroxylase activity and did not increase liver acyl coenzyme A:cholesterol acyl transferase activity. These findings suggest that a drug such as crilvastatin could have a hypocholesterolemic effect by a mechanism other than the sole inhibition of cholesterol synthesis, possibly by stimulating cholesterol and bile salt secretion via the biliary tract in previously hypercholesterolemic rats.

Mode of action of fibrates

Fibrates are a class of hypolipidaemic drugs that effectively reduce plasma triglyceride and cholesterol levels, but also raise HDL cholesterol. In recent years the attention of pharmacologists and clinicians to fibrates has been renewed also in the light of a multifaceted action on plasma lipids as well as on factors modulating the thrombotic homeostasis in blood. The mechanisms of actions underlying these effects are discussed in this short review.

Fibrates influence the expression of genes involved in lipoprotein metabolism in a tissue-selective manner in the rat

The influence of different fibrates on apolipoprotein metabolism was investigated. Administration of fenofibrate provoked a dose-dependent decrease in plasma cholesterol concentration that was already evident after 1 day. Intestinal apolipoprotein (apo) A-I and apo A-IV mRNA levels remained fairly constant. In contrast, liver apo A-I, apo A-II, and apo A-IV mRNA levels decreased in a dose-dependent fashion, which was associated with a lower transcription rate of the apo A-I but not the apo A-II gene. The decline in hepatic apo A-I, apo A-II, and apo A-IV mRNA had already started after 1 day and was associated with a drop in plasma apo A-I and apo A-IV concentrations. Plasma apo E had already decreased after 1 day of fenofibrate, whereas apo B initially remained constant and increased only after 14 days of fenofibrate at the highest dose. Hepatic and intestinal apo B mRNA contents and liver, heart, kidney, and testis apo E mRNA contents were only marginally affected after treatment with fenofibrate. Liver low density lipoprotein receptor mRNA levels rose slightly after a 3-day administration of the highest dose of fenofibrate. Both clofibrate and gemfibrozil had effects comparable to those of fenofibrate on liver and intestinal apolipoprotein mRNA levels except for liver apo A-II mRNA, which decreased only marginally. Compared with fenofibrate, clofibrate caused similar changes in plasma cholesterol, apo A-I, apo A-IV, and apo E concentrations, whereas gemfibrozil increased plasma cholesterol and apo E without changing apo A-I and apo A-IV concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of lipoprotein production in Hep G2 cells by fenofibrate and clofibrate

Fenofibrate and other fibrate derivatives are commonly used to treat hyperlipidemia. It is not yet clear how they exert their modulatory effects on plasma lipoproteins. To investigate whether these drugs act on the liver to primarily inhibit very low density lipoprotein production, we utilized the highly differentiated human hepatoma cell line, Hep G2. At concentrations greater than 15 micrograms/mL, fenofibrate caused a 30% decrease in secreted apolipoprotein B (apo B) after 4 days of treatment. Pulse-chase studies demonstrated that this was not due to inhibition of apo B synthesis. Triglyceride synthesis by fenofibrate-treated Hep G2 cells was decreased by 30%, and the amount secreted into the medium was reduced by 50%. At a low concentration of drug (5 micrograms/mL), triglyceride secretion was reduced markedly while apo B secretion remained unchanged. Thus, apo B secretion is less sensitive to fenofibrate than the synthesis and secretion of triglyceride, and may be secondary to changes in the latter. Fenofibrate has also been shown to raise plasma high density lipoprotein concentrations. We found that low concentrations of fenofibrate caused a 20-101% increase in secreted apolipoprotein AI (apo AI), and pulse-chase immunoprecipitation studies showed that this was due to an increase in apo AI synthesis. Fenofibrate was compared to clofibrate to investigate whether their relative effects on lipoprotein production in Hep G2 cells were comparable to their relative effects on plasma lipoproteins. Both fibrates decreased the secretion of apo B to the same extent, but only fenofibrate increased apo AI secretion. Fenofibrate was more effective than clofibrate in inhibiting the secretion of lipids by these cells. Thus, the known effects of fenofibrate on plasma lipoproteins can be attributed to its direct modulation of lipoprotein synthesis in the liver cell. Hep G2 cells may thus be useful in testing the relative efficacy of fibric acid derivatives in vitro.

Down-regulation of hepatic lipase gene expression and activity by fenofibrate

The influence of the hypolipidemic drug, fenofibrate, on hepatic lipase (HL) gene expression and activity was investigated in the rat. Fenofibrate treatment provoked a dose-dependent decrease in HL mRNA levels. At a dose of 0.5% (w/w), HL mRNA levels were reduced to nearly 50% the levels in untreated controls. This decrease was parallelled by a comparable reduction in liver HL activity. The decrease in HL mRNA levels was already observed after 1 day of fenofibrate treatment. Whole liver perfusion experiments showed that the heparin-releasable HL activity in fenofibrate-treated livers dropped to 10% the activity in control livers. In conclusion, treatment with fenofibrate decreases HL gene expression, leading to a lowered activity of endothelium bound HL in fenofibrate-treated livers.

Effects of fibric acid derivatives on accumulation of actin in myocardiocytes

We used sodium dodecylsulphate polyacrylamide gel electrophoresis and immunoblotting to analyze the effects of the fibric acid derivatives bezafibrate, fenofibrate and gemfibrozil on the accumulation of actin in the cytoplasmic and cytoskeletal fraction of cultured myocardiocytes. All three drugs tested modified cellular and subcellular actin in different ways, and the findings are thought to be related with the secondary effect of arrhythmia known to be caused by these drugs. Bezafibrate and gemfibrozil more markedly affected accumulation of actin by myocytes, while fenofibrate interfered less notably with the accumulation of this protein.

Age-related changes in the metabolism of cholesterol in rat liver microsomes

The effects of aging on the hepatic metabolism of cholesterol were studied in 1-, 6- and 24-month-old male Sprague-Dawley rats. Microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity, which regulates cholesterol biosynthesis, decreased from 835 +/- 144 (SEM) pmol/min/mg protein in the youngest group to 219 +/- 34 and 205 +/- 53 pmol/min/mg protein (p less than 0.001) in the 6- and 24-month-old groups, respectively. Cholesterol 7 alpha-hydroxylase activity, which governs bile acid synthesis, was gradually reduced from 70 +/- 14 pmol/min/mg protein in the 1-month-old group to 32 +/- 7 and 16 +/- 3 pmol/min/mg protein (p less than 0.05) in the 6- and 24-month-old groups, respectively. Acyl coenzyme A:cholesterol acyltransferase activity, which catalyzes the esterification of cholesterol, averaged 431 +/- 47 and 452 +/- 48 pmol/min/mg protein in the 1- and 6-month-old groups, respectively, and was increased to 585 +/- 55 pmol/min/mg protein (p less than 0.05) in the 24-month-old group. The level of total cholesterol showed an age-related increase from 1.56 +/- 0.16 mg/g liver in the 1-month-old group to 1.70 +/- 0.15 and 2.20 +/- 0.19 mg/g liver (p less than 0.05) in the 6- and 24-month-old groups, respectively. The increase was mainly caused by an accumulation of esterified cholesterol. We conclude that a marked decrease in HMG-CoA reductase occurs between 1 and 6 months of age; thereafter the enzyme activity stays unchanged. The activity of cholesterol 7 alpha-hydroxylase decreases progressively and drastically with age, whereas the capacity for esterifying cholesterol increases slightly.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bezafibrate on hepatic cholesterol metabolism

The influence of bezafibrate treatment on hepatic cholesterol metabolism was studied in rats and in humans. The activities of the three key enzymes involved in cholesterol metabolism [3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, cholesterol 7 alpha-hydroxylase, and acyl-coenzyme A: cholesterol acyltransferase (ACAT)] were suppressed by bezafibrate treatment in rats, but only the ACAT activity was significantly decreased when the activity was related to total liver weight. In humans, HMG-CoA reductase activity was increased about twice in the treated normolipidemic gallstone patients. In contrast, the concentration of lathosterol in serum decreased, indicating depression of the cholesterol synthesis. The increase in HMG-CoA reductase activity may be a compensatory effect of an inhibition of some other enzymes in the synthesis of cholesterol, as in vitro study on liver microsomes excluded a direct inhibitory effect of bezafibrate on HMG-CoA reductase. The ACAT activity was not significantly changed, and the cholesterol 7 alpha-hydroxylase activity was decreased by 55-60% compared with controls. The LDL-receptor-binding activity was unaffected by bezafibrate treatment.

Influence of fibric acid derivatives on intermediate filament proteins in myocardiocyte cultures

We analyzed desmin and vimentin accumulation in chick myocardiocyte cultures treated with the fibric acid derivatives bezafibrate, fenofibrate and gemfibrozil. The most noteworthy finding was the 50% decrease in the cytoplasmic desmin fraction in cells treated with gemfibrozil in comparison to control cultures, and the 19% increase in the cytoskeletal fraction in cultures treated with gemfibrozil and with bezafibrate. Vimentin accumulation by cells treated with bezafibrate was similar to that in control cultures, however the cytoskeletal vimentin fraction rose by 26% after treatment with gemfibrozil, and fell 13% after treatment with fenofibrate. No vimentin was found in the cytoplasmic fraction of cell treated with bezafibrate. Given the role of intermediate filaments in heart muscle contraction, fibric acid derivative- induced changes in the cytoplasmic and cytoskeletal concentrations of intermediate filament proteins may be related with the secondary effects of these drugs on heart rate.