Hypolipidemic activity of select fibrates correlates to changes in hepatic apolipoprotein C-III expression: a potential physiologic basis for their mode of action.

For the last 30 years fibrates have been widely prescribed to treat human dyslipidemia. However, the primary mechanism by which they lower plasma lipid levels is still unknown. Studies with transgenic mice have suggested that changes in apoC-111 expression levels have a dramatic influence on plasma triglyceride levels. These results suggested that fibrates could reduce lipid levels by lowering apoC-I11 gene expression. In the current studies, we sought to determine whether the selected fibrates, bezafibrate, clofibrate, fenofibrate, and gemfibrozil, could reduce hepatic apoC-I11 mRNA and plasma apoC-I11 levels. Chow-fed rats were orally gavaged daily with a dosing vehicle alone or with 100 mg/kg of each of the fibrates for 1 week and in addition with gemfibrozil for 2 weeks. Bezafibrate and fenofibrate lowered plasma triglyceride by approximately half and dramatically reduced hepatic apoC-I11 mRNA and plasma apoCI11 levels. In contrast, clofibrate did not reduce plasma triglyceride levels and only partially reduced apoC-111 mRNA and plasma protein levels. Gemfibrozil strongly reduced plasma triglyceride levels and had an intermediate but significant effect on apoC-I11 mRNA and plasma apoC-111 levels. Some of the fibrates, especially gemfibrozil also reduced plasma apoC-I1 levels, an effect that could contribute to the observed triglyceride-lowering effect. In addition, the ratio of plasma apoE to plasma apoC-I1 plus apoC-111 was strongly and inversely correlated with plasma triglyceride levels. As plasma apoE levels were not reduced in gemfibrozil-treated animals, this could also have contributed to the triglyceridelowering effect of this fibrate. Fibrate-mediated triglyceride lowering was not the result of a decreased apoB or VLDL production and, therefore, suggested an enhanced VLDL remnant catabo1ism.l Our results suggest that the mechanism by which fibrates lower plasma triglycerides is by reducing the level of hepatic apoC-I11 expression.-Haubendner, S., A. D. Essenburg, B. C. Barnett, M. E. Pape, R B. DeMattos, B. R. Krause, L. L. Minton, B. J. Auerbach, R. S. Newton, T. Leff, and C. L. Bisgaier. Hypolipidemic activity of select fibrates correlates to changes in hepatic apolipoprotein C-I11 expression: a potential physiologic basis for their mode of action. J. Lipid Res. 1995. 36 2541-2551. Supplementary key words gemfibrozil bezafibrate fenofibrate clofibrate hepatic lipase coronary heart disease hypertriglyceridemia * apolipoprotein C-I1 * apolipoprotein E * lipoprotein lipase Hypertriglyceridemia and low levels of HDL may each be independent risk factors for cerebrovascular and coronary heart disease (CHD) (1-3). Conventional therapy for hypertriglyceridemia includes dietary weight loss, and drug intervention if appropriate, which might include fibrate therapy, and gemfibrozil when HDL levels are also diminished, nicotinic acid, fish oils, and under certain conditions, reductase inhibitors and bile acid sequestrants (4-7). A primary prevention clinical trial has shown that gemfibrozil lowers the incidence of CHD in large part by elevating HDL (8). Surprisingly, however, little is known of the mechanisms by which fibrates lower triglycerides and in the case of gemfibrozil, the mechanism that leads to HDL elevation. Although some results have shown an effect of fibrates on hepatic (HL) and lipoprotein lipase (LPL) activity and/or expression, no consensus has emerged from these studies. Similarly, both in vitro and in vivo studies on apolipoprotein levels and expression have suggested that fibrates have differential effects on Abbreviations: apo, apolipoprotein; CHD, coronary heart disease; HL, hepatic lipase; HPGC, high performance gel chromatography; LPL, lipoprotein lipase; PPAR, peroxisome proliferator activated receptor; PPRE, peroxisome proliferator activated receptor response element; HDL, high density lipoprotein; LDL, low density lipoprotein; VLDL, very low density lipoprotein. 'To whom correspondence should be addressed. Haubenwallner et al. Hypolipidemic effect of fibrates and apoGIII expression 2541 apolipoprotein synthesis. For example, clofibrate, fenofibrate, and gemfibrozil in rats do not affect hepatic apoE mRNA levels but these compounds either reduce (clofibrate and fenofibrate) or elevate (gemfibrozil) plasma apoE levels (9). Similarly, in the cholesterol-fed rat, only gemfibrozil, but neither ciprofibrate, fenofibrate, clofibrate, nor bezafibrate elevated plasma apoE (10). Studies of Staels et al. (9) have shown rat hepatic apoA-I mRNA levels diminish after clofibrate, fenofibrate, or gemfibrozil treatment, which is consistent with diminished plasma levels observed with clofibrate and fenofibrate, but not with gemfibrozil where plasma apoA-I levels were unchanged. In the cholesterol-fed rat, gemfibrozil actually elevates plasma apoA-I (10). Thus, causal relationships between apolipoprotein production and plasma levels may be inconclusive and can simply reflect a complex relationship between apolipoprotein synthesis, intracellular degradation, secretion, redistribution between plasma lipoproteins, and clearance. Previous observations have suggested that reduced plasma apoC-I11 levels are associated with reduced triglycerides (1 1). ApoC-I11 inhibits hepatic clearance of triglyceride-rich remnants (12) and lipoprotein lipase activity in vitro (13). Genetic studies have demonstrated a strong association of a polymorphism in the 3’ untranslated region of the apoC-I11 gene with hypertriglyceridemia in several distinct populations (14). Similarly, transgenic mice overexpressing apoC-I11 are hypertriglyceridemic (1517), while mice genetically deficient in apoC-I11 are hypotriglyceridemic (18). In the current study we report that apoC-111 expression is reduced during fibrate therapy, suggesting that a possible mechanism for the hypolipidemic action of these compounds is the reduction of apoC-I11 gene expression. MATERIALS AND METHODS Animal diets and treatments Male Sprague-Dawley rats (100-200 g) were obtained from Charles River Laboratories and housed 4-6 rats per cage. All animals were allowed normal rat chow (Ralston-Purina) and water ad libitum in temperaturecontrolled rooms, under a 12-h light, 12-h dark cycle beginning with lights on at 6 AM. Rats were dosed daily between 6 and 9 AM by oral gavage using a suspension vehicle (0.2% Tween-20 plus 1.5% carboxymethylcellulose). Control animals received vehicle alone. Vehicle volume represented 0.25% of body weight. Fibrates were administered at 100 mg/kg per day for 1 or 2 weeks, as indicated for the specific experiments. For all but the production rate studies, non-fasted rats were anesthetized by ether inhalation 1012 h post-dosing, One Week Two Weeks 100