Sex-dependent Expression and Clofibrate Inducibility of Cytochrome

The induction of liver cytochrome P450 4A-catalyzed fatty acid a-hydroxylase activity by clofibrate and other peroxisome proliferators has been proposed to be causally linked to the ensuing proliferation of peroxisomes in rat liver. Since female rats are less responsive than males to peroxisome proliferation induced by clofibrate, the influence of gender and hormonal status on the basal and clofibrate-inducible expression of the 4A P450s was examined. Northern blot analysis using gene-specific oligonucleotide probes revealed that in the liver, P450 4A1 and 4A3 mRNAs are induced to a much greater extent in male as compared to female rats following clofibrate treatment, whereas P450 4A2 mRNA is altogether absent from female rat liver. Male-specific expression of P450 4A2 mRNA was also observed in kidney. Western blot analysis indicated that a similar sex dependence characterizes both the basal expression and the clofibrate inducibility of the corresponding P450 4A proteins. This suggests that the lower responsiveness of female rats to clofibrate-induced peroxisome proliferation may reflect the lower inducibility of the P450 4A fatty acid hydroxylase enzymes in this sex. Investigation of the contribution of pituitary-dependent hormones to the male-specific expression of 4A2 revealed that this P450 mRNA is fully suppressed in liver following exposure to the continuous plasma growth hormone profile that characterizes adult female rats; in this and other regards liver P450 4A2 is regulated in a manner that is similar, but not identical to, P450 3A2, a malespecific testosterone GB-hydroxylase. In contrast, kidney 4A2 expression, although also male-specific, was not suppressed by continuous growth hormone treatment, but was regulated by pathways that, in part, involve testosterone as a positive regulator. The malespecific expression of liver and kidney P450 4A2 is thus under the control of distinct pituitary-dependent hormones acting in a tissue-specific manner.