Inhibition of fatty acid biosynthesis by bezafibrate in different rat cells

Bezafibrate decreases growth stimulatory action of the sphingomyelin signaling pathway in regenerating rat liver

Liver regeneration after partial hepatectomy (PH) is achieved through proliferation of hepatocytes and non-parenchymal cells. The nuclear peroxisome proliferator-activated receptor alpha (PPARalpha) is involved in regulation of lipid metabolism and proliferation of hepatic cells. The sphingomyelin signal transduction pathway is involved in the regulation of the cell cycle in eukaryotic organisms. Sphingosine-1-phosphate (S1P) and ceramide (CER)-- the intermediates of the pathway--are known to stimulate and to inhibit cellular proliferation. The aim of the present study was to investigate the effect of PPARalpha activation by bezafibrate on the sphingomyelin signaling pathway during the first 24h of liver regeneration after PH in the rat. The content of sphingomyelin, ceramide, sphingosine, sphinganine, sphingosine-1-phosphate and the activity of sphingomyelinases and ceramidases were determined at various time points after PH. It has been found that the activity of neutral Mg(2+)-dependent sphingomyelinase (nSMase) increased, whereas the activity of acidic sphingomyelinase (aSMase) decreased in the regenerating liver. Activation of PPARalpha by bezafibrate lower the activity of nSMase and increased the activity of aSMase in the regenerating rat liver. The content of ceramide was higher in bezafibrate-treated rats, whereas the content of sphingosine-1-phosphate was markedly lower as compared to the untreated rats. Therefore, it is concluded that activation of PPARalpha by bezafibrate decreases the growth-stimulatory activity of the sphingomyelin pathway in regenerating rat liver.

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.