Interactions of gender, growth hormone, and phenobarbital induction on murine Cyp2b expression

The interactions of gender, growth hormone, and phenobarbital induction on Cyp2b expression were examined in phenotypically normal (lit/+) and growth-hormone deficient "little" (lit/lit) mice. Using an immunocrossreactive monoclonal antibody designed to identify rat CYP2B1 and 2B2 proteins, we observed three hepatic Cyp2b proteins in control (lit/+) females, but only two proteins, one at trace levels, in control males. Phenobarbital administration to lit/+ mice increased the expression of the two Cyp2b isoforms in the males by 3- to 4-fold, but produced an approximately 75% reduction in the female-expressed proteins. Whereas growth hormone depletion (lit/lit) had no effect on the expression profile of Cyp2b proteins in females, it had a de-repressive effect in males, resulting in the expression of three proteins at concentrations now comparable to those observed in female liver. Generally, phenobarbital had no inductive effects in the lit/lit mice of both sexes. In all groups, transcript levels measured by a CYP2B1 probe were in agreement with the protein findings. In contrast, Cyp2b mRNA identified by an oligonucleotide probe for CYP2B2 were repressed completely by growth hormone in both sexes, and was expressed as a female-predominant transcript in the lit/lit mice. In spite of an apparent high degree of sequence homology between the rat CYP2B and murine Cyp2b gene families, the present findings highlight fundamental differences in their constitutive and gender-dependent expression, growth hormone regulation, and phenobarbital inducibility.