On the mechanism of induction of microsomal cytochrome P450IVA1 and peroxisome proliferation in rat liver by clofibrate.

The time course of induction of microsomal and peroxisomal lipid-metabolizing enzymes in male Wistar rat liver has been investigated following a single i.p. dose of clofibrate (250 mg/kg). The microsomal enzyme, cytochrome P450IVA1, demonstrated a biphasic response to sodium clofibrate administration, the biphasic response consisting of an initial small response, peaking at approximately 30 min post-dose and returning to near baseline values after 2 hr. A second major induction of cytochrome P450IVA1 occurred between 18 and 24 hr post-dose. This biphasic phenomenon for cytochrome P450IVA1 was observed for the enzyme activity (lauric acid hydroxylase), immunodetectable protein (using a specific ELISA method) and at the mRNA level (using a 2.1 kilobase cytochrome P450IVA1 cDNA probe). In contrast, peroxisomal fatty acid beta-oxidation enzymes responded in a monophasic manner to clofibrate administration, peaking approximately 24 hr post-dose. Accordingly, microsomal cytochrome P450IVA1 was induced before the peroxisomal enzymes of fatty acid beta-oxidation. The effect of cycloheximide on the induction of peroxisome proliferation by clofibrate was additionally investigated. The prior administration of cycloheximide to Wistar rats ablated the clofibrate-dependent induction of both cytochrome P450IVA1 and peroxisomal-dependent lipid metabolism and also blocked the corresponding synthesis of enzyme proteins. Cycloheximide additionally inhibited the clofibrate-dependent increase in peroxisomal acyl-CoA oxidase mRNA, but was without effect on the induced cytochrome P450IVA1 mRNA levels, indicating a protein or enzyme dependency for the phenomenon of peroxisome proliferation. Taken collectively, our data strongly argues that the regulation of microsomal cytochrome P450IVA1 and peroxisomal fatty acid beta-oxidation enzymes are closely related, possibly through the initial, clofibrate-dependent regulation of cytochrome P450IVA1.