REGULATION OF RAT HEPATIC CYTOCHROME-P450 EXPRESSION BY STEROL BIOSYNTHESIS INHIBITION - INHIBITORS OF SQUALENE SYNTHASE ARE POTENT INDUCERS OF CYP2B EXPRESSION IN PRIMARY CULTURED RAT HEPATOCYTES AND RAT-LIVER

The effects of treatment with squalestatin 1, a potent inhibitor of sq ualene synthase, the first committed enzyme of sterol biosynthesis, we re examined on cytochrome P450 expression in primary cultured rat hepa tocytes and rat liver. Incubation of cultured hepatocytes with squales tatin 1 caused marked accumulations (maximal elevations that were simi lar to 25-100% of phenobarbital-elicited increases) of CYP2B mRNA and immunoreactive protein but not of CYP1A, CYP3A, or CYP4A. Squalestatin 1 treatment increased CYP2B and 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA content in hepatocyte cultures with comparable potenci es (ED50 = 5.0 and 18 nM, respectively), and significantly induced CYP 2B (mRNA, immunoreactive protein, and pentoxyresorufin O-dealkylase ac tivity) in the livers of treated rats, producing maximal increases at a dose of 25 mg/kg/day that were similar to 32-87% of phenobarbital-in duced increases. Squalestatin 1 treatment induced both CYP2B1 and CYP2 B2 and activated reporter gene expression in cultured hepatocytes tran siently transfected with a plasmid containing similar to 2.4 kb of CYP 2B1 gene 5'-flanking region or containing a previously described pheno barbital-responsive region. Coincubation of cultured hepatocytes with 25-hydroxycholesterol suppressed squalestatin 1-mediated CYP2B and 3-h ydroxy-3-methylglutaryl coenzyme A mRNA induction with approximately t he same potency. Treatment of cultures with SQ-34919, a structurally d istinct squalene synthase inhibitor, produced the same selective CYP2B mRNA induction as did squalestatin 1. These results suggest that inhi bition of hepatic sterol synthesis activates processes that culminate in increased CYP2B gene transcription.