Regulation of the mouse liver cytochrome P450 2B subfamily by sex hormones and phenobarbital

Abstract

The sex-dependent expression and inducibility of the cytochrome P450 2B subfamily was studicd in DBA/2 and Balb/c mice and their F1 recombinants at the mRNA protein and activity levels. Analysis of poly(A)+ RNA with specific oligonucleotide probes directed to known mRNAs within the mouse 2B subfamily revealed that the levels of P450 2b-10 and 2b-9 mRNAs were co-regulated with two proteins (56 and 53 kDa) detected by a 2B-specific polyclonal antibody. Other mRNAs related to the 2B subfamily were barely or not at all detectable and did not coincide with protein expression suggesting that P450s 2b-9 and 2b-10 arc the major 2B isoenzymes present in mouse liver. Specifically. castration of males increased the expression of 2b-9 and 2b-10 mRNAs and protein up to female levels, and testosterone administration to castrated mice reversed these changes. Ovariectomy of females appears to increase the expression of these P450s slightly. 2b-10, but not 2b-9 mRNA and protein were induced by phenobarbital. Based on immunoinhibition studies and the levels of these isoenzymes P4502b-10 appears to be the major catalyst of 7-pentoxyresorufin O-dealkylation. Both P4502b-9 and P4502b-10 contribute up to 30 % of the testosterone 16α-hydroxylation the balance being catalysed by P450s within the 2D subfamily. These experiments show that the female-predominant expression of the two mouse liver isoenzymes P4502b-9 and P4502b-10 is dependent on sex hormones. The fact that P4502b-9 does not respond to phenobarbital while P4502b-10 is regulated by both phenobarbital and sex hormones demonstrates the complexity of P450 expression even within one subfamily.