Implication of both progesterone and glucocorticoid receptors in the induction of cytochrome p4504a4 in rabbit lung

We have previously localized cytochrome P4504A4 to the microvascular endothelial cells (MEC) of maternal rabbit lung in late gestation, thereby implicating CYP4A4 via its metabolism of eicosanoids, in hemodynamic regulation in the lung. CYP4A4 co-hydroxylates various substrates, including arachidpnic acid, prostaglandin E; (PGEj), and PGF:a, but not laurate. Here, we examine the induction of CYP4A4, which appears to be positively regulated through progesterone (P) and/or the glucccorticoid (G) receptors. Interestingly, P and G levels progressively increase throughout pregnancy and appear to coincide with the gradual rise in CYP4A4 levels. In this study, the induction of CYP4A4 after treatments with various steroid hormones has been examined. CYP4A4 induction was monitored in rabbit lung microsomes by the CYP4A4-specific PG& o-hydroxylation reaction. In parallel, CYP4A4 (as well as CYP4A5, CYP4A6, and CYP4A7) mRNA levels were also monitored by RNase protection assays. Injection with either P or a synthetic G, dexamethasone (D) resulted in a significant increase in PGEi co-hydroxylase activity, whereas estradiol, aktosterone, dehvdroepiandrosterone (DHEA), and DHEA-sulfate did not significantly increase PGEi <ohydroxylase activity. In separate experiments, pulmonary CYP4A4 PGEi (o-hydroxylase activity was examined at various times following a single injection of various doses of P and D; resulting in peak induction at 24 hours for both agents at the times tested. The results suggest that D was a more potent inducer of CYP4A4 than P, i.e., with 1 mg D/kg body weight, PGEi ro-hydroxylase activity was 250 pmd/min/mg, while P at up to 8 mg/kg body weight reached a maximum activity of only about 28 prnol/min/mg Simultaneous injection of D (0.5 mg/kg) and the G/P antagonists (RU38486, RU40555, or RU43044) resulted in at least a 50% decrease in PGEi w-hydroxylase activity and mRNA levels. These findings suggest involvement of one or both of these steroid receptors in the induction of CYP4A4 in rabbit lung. Continuing experiments to delineate the induction mechanism are in progress and include culture of lung MEC and induction with P/D in rabbits. [Supported by NIH GM31296 to BSSM).