Phenobarbital induction and tissue-specific expression of the rat CYP2B2 gene in transgenic mice

To investigate molecular events regulating the transcription of genes inducible by phenobarbital, transgenic mouse strains were developed incorporating rat cytochrome P450 2B2 (CYP2B2) genes. Expression in mouse tissues was analyzed for two series of rat CYP2B2 gene constructs, of 19 and 39 kilobase pairs total length, each containing the entire coding region, introns, and 3′-flanking sequences of CYP2B2, but differing in the respective lengths of 5′-flanking sequence. One group of mice, whose transgene included the complete 2B2 gene but only 800 base pairs of 5′-proximal sequence, were not phenobarbital-inducible in mouse liver or in any extrahepatic tissue; rather, these genes were expressed at very high levels constitutively and selectively in only kidney and liver. A second group of mice with an identical transgene, except for the presence of an additional 19 kilobase pairs of 5′-flanking sequence, expressed 2B2 only in the liver and at high levels only after phenobarbital treatment, analogous to the expression pattern observed for the endogenous CYP2B2 gene in the rat. These results demonstrate that, in vivo, phenobarbital induction and tissue-specific control requires interaction of regulatory elements far upstream of the core CYP2B2 promoter region and upstream of motifs indicated previously as determinants of phenobarbital responsiveness.