Abstract

We hypothesized that the drug efflux protein P-glycoprotein (Pgp), the product of the multidrug resistance gene MDR1, might influence hepatic expression of CYP3A or other cytochromes P-450 (P-450s) because Pgp can transport endogenous regulators of these cytochromes. We began with variants of a CF-1 mouse strain containing a defective mdr1a gene that is inherited in a Mendelian fashion. The amount of CYP3A protein in liver was inversely related to the gene dose of the normal mdr1a allele in these mice.mdr1a knockout mice of either mixed (FVB × 129/Ola) or pure FVB genetic background and housed in Amsterdam display an increased expression of CYP2B and CYP3A proteins. However, becausemdr1a ablation causes a compensatory increase in hepaticmdr1b (which can efflux intracellular glucocorticoids), we reasoned that mdr1b might mask the overall effect ofmdr1a absence on P-450 gene expression. Targeted inactivation of the mdr1b gene increased P-450 expression, but the effect was modest compared withmdr1a ablation. Mice nullizygous for bothmdr1a and mdr1b-type Pgps and kept in Amsterdam had dramatically increased levels of CYP3A protein as well as other P-450s examined and of the electron donor P-450 reductase. Consistent with the protein results, CYP3A catalytic activity measured as midazolam 1′- and 4-hydroxylation in liver microsomes from these knockout mice revealed a rank order of activities withmdr1a/1b > mdr1a >mdr1b > (+/+) mice. In contrast to results in mice housed in Amsterdam, in the genetically identical mdr1aor mdr1a/1b (−/−) male mice housed in the United States, hepatic P-450 expression was unaffected by mdr1genotype or actually showed a slight decrease in mdr1a(−/−) mice. These results provide a revealing picture ofmdr1-type Pgp as an upstream regulator of hepatic P-450 expression, and demonstrate that these pharmacologically relevant phenotypes in knockout mice depend not only on the genetic make-up of the mice but also on the environment.