Effect of a ligand selective for peripheral benzodiazepine receptors on the expression of rat hepatic P-450 cytochromes: assessment of the effect in vivo and in a hepatocyte culture system.

The peripheral benzodiazepine receptor plays a role in the translocation of cholesterol into mitochondria where steroidogenesis occurs. Sterols have been suggested to be involved in the regulation of the cytochrome P-450 (CYP)2B subfamily as the endogenous suppressor of this CYP. To investigate the role of cholesterol metabolites on the expression of CYPs, the effect of PK11195, a specific ligand of the peripheral benzodiazepine receptor and a stimulator of cholesterol transportation, on CYP expression was examined in rats in vivo and in cultured hepatocytes. As judged by the change in testosterone metabolic activity catalyzed by liver microsomes, i.p. injection of PK11195 into rats increased the CYP2B subfamily significantly. A trend in the induction of the CYP2A1, 2C11, and 3A isozymes was also observed. When PK11195 was given to rats together with phenobarbital, an additive effect of these compounds on testosterone metabolic activity was observed. In cultured hepatocytes, PK11195 exhibited the same effect on CYP expression as seen in vivo, but the magnitude of the effect was much greater than that observed in vivo. The inductive effect of PK11195 toward the CYP2B and 3A subfamilies was 2.3- and 6.5-fold greater, respectively, than that with phenobarbital. The inductive effect of PK11195 was confirmed by immunoblotting with antibodies against CYP2A, 2B, 2C, and 3A proteins. These results indicate that PK11195 has an inductive effect on several subfamilies of CYPs by directly acting on liver cells and has no ability to suppress the expression of these CYPs. This observation suggests that, if certain sterols play a role in the suppressive control of the CYP2B subfamily, they are produced in organelles other than the mitochondria.