Halofenate and clofibrate: mechanism of hypotriglyceridemic action in the rat

MBX-102/JNJ39659100, a novel non-TZD selective partial PPAR-γ agonist lowers triglyceride independently of PPAR-α activation

MBX-102/JNJ-39659100 (MBX-102) is a selective, partial PPAR-γ agonist that lowers glucose in the absence of some of the side effects, such as weight gain and edema, that are observed with the TZDs. Interestingly MBX-102 also displays pronounced triglyceride lowering in preclinical rodent models and in humans. Although in vitro reporter gene studies indicated that MBX-102 acid is a highly selective PPAR-γ agonist that lacks PPAR-α activity, we sought to determine if PPAR-α activation in vivo could possibly contribute to the triglyceride lowering abilities of MBX-102. In vivo studies using ZDF and ZF rats demonstrated that MBX-102 lowered plasma triglycerides. However in ZF rats, MBX-102 had no effect on liver weight or on hepatic expression levels of PPAR-α target genes. Further in vitro studies in primary human hepatocytes supported these findings. Finally, the ability of MBX-102 to lower triglycerides was maintained in PPAR-α knockout mice, unambiguously establishing that the triglyceride lowering effect of MBX-102 is PPAR-α independent. The in vivo lipid lowering abilities of MBX-102 are therefore mediated by an alternate mechanism which is yet to be determined.

Alterations in rat serum lipids and apolipoproteins following clofibrate treatment

The in vivo effects of the hypolipidemic drug clofibrate (0.5 mmol/kg body wt daily p.o. for 7 days) on serum lipids and apolipoproteins have been studied in male rats. Clofibrate caused an increase in liver weight without affecting body weight. Triglyceride, total and free cholesterol and HDL cholesterol were decreased in sera of clofibrate-treated rats. The relative abundance, and accordingly the absolute quantities, of the polyunsaturated fatty acids linoleic (18:2), linolenic (18:3) and docosahexenoic (22:6) in serum triglyceride decreased in response to clofibrate treatment. The concentrations of serum apolipoproteins A-I, B and C-III were reduced in clofibrate-treated rats. The apolipoprotein E level was not altered. The distribution of apolipoproteins A-I, B, C-III and E between heparin-Mn supernatant and precipitate were unaffected. The unchanged C-III distribution indicates unaltered intravascular VLDL catabolism. Concurrent reductions in serum HDL cholesterol and ApoA-I in clofibrate-treated rats suggest a diminished production of lipoprotein particles containing ApoA-I. Reductions in serum ApoB and in the mass ratio of serum triglyceride to ApoB indicate a decrease in the number and size, respectively, of circulating triglyceride-rich lipoprotein particles. These observations suggest that the hypolipidemic effect of clofibrate in the normolipemic rat is caused mainly by diminished hepatic secretion, rather than by enhanced catabolism, of triglyceride-rich lipoproteins.

Stimulation of fatty acid utilization by sodium clofibrate in rat and monkey hepatocytes

The acute effects of sodium clofibrate (NaCPIB) on the metabolism of [1-14C]palmitate, [1-14C]octanoate, [1-14C]butyrate, and [2-3H]glycerol by freshly isolated hepatocytes were tested to explore its mechanism of action. Labeled long-, medium-, and short-chain fatty acids were incorporated into all the major lipid classes and were oxidized to 14CO2 by the liver cells. The partitioning of labeled fatty acids from lipogenic towards oxidative pathways was inversely related to fatty acid chain length. [1-14C]Palmitate was incorporated mainly into cellular triglycerides and phospholipids; [1-14C]octanoate, mainly into triglycerides and free cholesterol; and [1-14C]butyrate, mainly into free cholesterol and phospholipids of the cells. NaCPIB (1-3 mM) rapidly stimulated the esterification of labeled palmitate or glycerol to triglycerides, but drug levels greater than 5 mM were inhibitory to esterification. NaCPIB (1 mM) increased the oxidation of [1-14C]palmitate to 14CO2 by either rat or monkey hepatocytes and enhanced the release of labeled lipids from [2-3H]glycerol-prelabeled cells into the extracellular medium. Accelerated [1-14C]octanoate incorporation into glycerolipids and sterols and increased [1-14C]octanoate conversion to 14CO2 were observed in rat liver cells incubated with 1 mM NaCPIB. In contrast, the same drug level stimulated the oxidation of [1-14C]butyrate to 14CO2 but greatly diminished its incorporation into hepatocellular sterols or glycerolipids. These results indicate that (a) NaCPIB acutely alters hepatic ultilization of fatty acids by actions at diverse loci; (b) these metabolic alterations vary with fatty acid chain length; and (c) these effects are probably due to rapid changes in biochemical regulatory mechanism and/or in substrate channelling within the cells. These data further suggest that the early hypolipidemic effect of the drug in rats and primates may be related to an enhanced hepatic oxidation of long-chain fatty acids, but cannot be attributed simply to a reduction in their esterification to complex lipids.

Hypolipidemic effects of clofibrate and selected chroman analogs in fasted rats: I. Chow-fed animals

The hypolipidemic properties of ethyl 6-chlorochroman-2-carboxylate (II), ethyl 6-phenylchroman-2-carboxylate (III) and ethyl 6-cyclohexylchroman-2-carboxylate (IV) were compared to clofibrate (I) in fasted normolipidemic rats. The chroman analog II, like its parent compound, clofibrate, reduced serum and alpha-lipoprotein cholesterol concentrations. Although analog III had no effect on serum cholesterol, it caused a slight elevation of alpha-lipoprotein cholesterol concentration. Serum free cholesterol was increased and LCAT activity was reduced in clofibrate-treated rats. The hypolipidemic agents had no consistent effect on liver lipid concentrations and liver microsomal HMG-CoA reductase activity. In addition, we have shown that drug efficacies varied directly with seasonal variations in serum lipid concentrations.

[Effect of lipid normalizers (clofibrate and procetofene) on chylomicron metabolism in rats]

Clofibrate (250 mg/kg/day) and Procetofene (100 mg/kg/day) feedings determine a significant increase of the rat liver weight. After intravenous injection in treated and control rats of doubly labelled chylomicrons ([14C] glycerol and [3H] oleic acid) an enhancement of radioactive triglyceride hydrolysis was observed in the treated groups. The oxidation of exogenous oleic acid was increased by both drug feeding. Alterations of liver and blood lipids (especially large amounts of labelled diglycerides in the blood) could be probably related to the inhibition of the diglyceride acyltransferase in the liver.

Mode of action of the lipid-lowering agents, clofibrate and BM 15075, on cholesterol biosynthesis in rat liver

When rats were fed a diet containing 0.3% clofibrate or a derivative of this drug, BM 15075, serum cholesterol was lowered within 3-7 days by 26-38%. Both drugs diminished the activity of hydroxymethylglutaryl CoA reductase, the regulatory enzyme of hepatic cholesterol biosynthesis, in rat liver microsomes by about 60% under the same conditions. The decrease in the activity of the enzyme obviously is due to changes in the amount of enzyme protein. Under in vitro conditions microsomal hydroxymethylglutaryl CoA reductase was inhibited competitively by (1.35 mM) clofibric acid (sodium salt) and by BM 15075 (1 mM) with respect to its substrate. These results give evidence that these drugs can affect both, the rate of synthesis and the substrate affinity of hydroxymethylglutaryl CoA reductase.

Effect of clofibrate on in vivo triglyceride production and clearance in genetically hyperlipemic rats

Seventeen hyperlipemic and 17 normolipemic littermate Zucker rats were treated with clofibrate or normal saline to determine the effect of this drug upon hepatic triglyceride (TG) production and peripheral TG disposal. Peripheral Intralipid clearance was not different in hyperlipemic rats relative to control animals. Hyperlipemic animals demonstrated abnormally elevated TG production in the fasted state which was not corrected with clofibrate administration. Following treatment, peripheral Intralipid clearance was increased 100% in hyperlipemic rats, but unchanged in normolipemic animals. These observations provide further evidence that the predominant lipid reducing action of clofibrate is manifested only in the hyperlipemic state, and predominantly upon peripheral lipid disposal.

Studies on the mechanism of action of halofenate

This paper reviews most of the clinical studies on the mode of action of halofenate, an established hypolipidemichypouricemic agent in man. In yeast cutlures and in isolated rat adipocytes, halofenate was found to inhibit the conversion of pyruvate to acetyl CoA. While pyruvate dehydrogenase was inhibited in vitro, halofenate also inhibited the activety of various other isolated enzymes. In rats maintained on halofenate in the diet (0.02-0.10%) for 2-14 days, there were 20-40% decreases in plasma cholesterol, trigly cerides, phospholipids, and free fatty acids. Inhibition of liver HMG-CoTA reductase does not appear to account for the hypocholesterolemic effect, and activation of mitochondrial alpha-glycerophosphate dehydrogenase does not explain the hypotriglyceridemic action. Kinetic measurements of the serum appearance and disappearance of triglycerides in drug-treated rats suggest that the hypotriglyceridemic activity is due to a net inhibition of hepatic triglyceride synthesis. Reduction of very low density lipoprotein (VLDL) and high density lipoprotein (HDL) levels in rats with sucrose-induced hyperlipidemia and normalization of the altered apolipoprotein profiles are in accord with the effects of halofenate on plasma triglyceride and cholesterol levels. The reduced insulin-to-glucagon ratio observed in Zucker obese hyperlipemic rats is also consistent with halofenat's hypotriglyceridemic activity. Preliminary experiments in rats on the mechanism of its hypoglycemic activity, observed in some diabetic hyperlipidemic patients, indicate that halofenate acts differently than conventional oral hypoglycemic agents. Some, but not all, of the effects of halofenate were observed with clofibrate at two to ten times higher levels.