The placenta is the primary source of estrogens and progesterone during pregnancy. Because pregnant diabetic women are reported to have lower serum estrogen and higher progesterone levels than nondiabetic pregnant women, and placental insulin-like growth factor II (IGF-II) production may be elevated during diabetic pregnancy, the role of IGF-II in the regulation of human cytotrophoblastic aromatase, 3 beta-hydroxysteroid dehydrogenase (3 beta HSD), and P450 cholesterol side-chain cleavage (P450scc) enzyme activities was studied. Incubation of cytotrophoblasts with IGF-II for 24 h significantly diminished the ability of these cells to convert androstenedione to estrogens by 92.3 +/- 6.6 (SE)%. IGF-II could suppress aromatase activity at a concentration as low as 2.0 ng/ml. Preincubation of cells with either insulin, IGF-I, or a monoclonal anti-IGF-I receptor antibody did not alter IGF-II's potent inhibitory effect. Treatment with mannose 6-phosphate alone also resulted in significant suppression of aromatase activity, and concurrent treatment with both mannose 6-phosphate and IGF-II resulted in greater inhibition than with either agent alone. These observations suggest that IGF-II suppresses aromatase activity by activation of its own specific receptor. In contrast, incubation of cytotrophoblasts with IGF-II for 24 h significantly increased the 3 beta HSD activity (as determined by the conversion of pregnenolone to progesterone) and P450scc activity (as determined by the conversion of 25-hydroxycholesterol to progesterone) of these cells. IGF-II's ability to stimulate these enzymatic processes was found to be comparable in magnitude to that of IGF-I. IGF-II-stimulated 3 beta HSD activity was completely inhibited by concurrent treatment with either actinomycin D or cycloheximide, suggesting that new mRNA and protein synthesis are required for IGF-II to exert its stimulatory effect. These studies indicate that IGF-II is a potent inhibitor of human cytotrophoblastic aromatase activity in vitro. In addition, IGF-II can stimulate cytotrophoblastic 3 beta HSD and P450scc activities. Since placental IGF-II production in pregnant diabetic women may be augmented, these observations may help explain the lower serum estrogen and higher progesterone levels associated with pregnancy in the diabetic patient.