The polycyclic aromatic compounds Beta-naphthoflavone (BetaNF), 3-methylcholanthrene (3MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are potent inducers of microsomal mono-oxygenase enzymes involved in the metabolism of foreign compounds (xenobiotics) in many tissues. This laboratory has studied the effects of administration of BetaNF, 3MC and TCDD to pregnant rats on the monooxygenase enzymes involved in steroid biosynthesis in the placenta. Androgen biosynthesis and progesterone metabolism in placental homogenates were significantly decreased following BetaNF and 3MC, but not TCDD treatment. Our data indicate that physiologically important oxidative and reductive enzymes in the placenta may be altered by polyaromatic compounds. The proposed research will first examine the effects of BetaNF, 3MC and TCDD on steroid hormone synthesis in intact placental tissue in order to establish whether the observed suppression of androgen biosynthesis in placental homogenates relates to altered endocrine function of intact tissue. Secondly we will examine the hypothesis that the suppressive effects of BetaNF and 3MC on steroid metabolism are related to the induction of placental xenobiotic metabolism. Experiments will establish whether BetaNF, 3MC and TCDD have similar inductive effects on the microsomal enzymes involved in the metabolism of the prototype xenobiotic benzo(a)pyrene (BP). The catabolism of [14C]BP to its dihydrodiol, quinone and phenolic metabolites will be analyzed by HPLC; the nature of the covalent binding of [14C]BP to DNA in vitro will be analyzed by Sephadex LH-20 chromatography. Subsequent experiments will assess whether the loss of steroid pathways following treatment with BetaNF and 3MC are cellularly and temporally related to the induction of BP metabolism. Should BetaNF, 3MC and TCDD have similar inductive effects on BP metabolism, these data will provide insight into whether the generation of cytotoxic intermediates of BetaNF and 3MC underlie the decrease in steroid metabolism. The third objective is to examine the hypothesis that the suppressive effects of BetaNF and 3MC on steroid pathways are related to alterations in placental receptors for epidermal growth factor (EGF). Experiments will study the effects of BetaNF, 3MC and TCDD on the binding of [125I] EGF and [125I] insulin to placental membranes. Alterations in the binding of these peptide hormones will be further studied in relationship to changes in both steroid and BP metabolism. In view of the reported sensitivity of the EGF receptor to cytotoxic actions of polyaromatic compounds, these data will help to distinguish between effects on the cytochrome P1-450 induction process compared to cytotoxic effects on placental endocrine function. This approach will further delineate mechanisms which may underlie the adverse effects of polyaromatic compounds on feto-placental growth.
