Vasoactive intestinal peptide-induced expression of cytochrome P450 cholesterol side-chain cleavage and 17α-hydroxylase enzyme activity in hen granulosa cells

Abstract

Experiments were conducted to determine whether vasoactive intestinal peptide (VIP) can regulate expression of cytochrome P450 side-chain cleavage (P450(scc)) and P450 17α-hydroxylase (P450 17α-OH) mRNA levels and enzyme activity in granulosa cells from nonhierarchal (6-8-mm) follicles. Initial studies demonstrated that immunoreactive VIP is localized within the theca (but not granulosa) layer of both resting (< 0.5-mm follicles) and 6-8-mm follicles, thus providing a potential paracrine mechanism of action for VIP. While short-term (3 h) incubation of granulosa cells with VIP (0.001-1.0 μM) failed to stimulate progesterone production from 6-8-mm follicle granulosa cells, a 4-h culture period in the presence of VIP resulted in increased cyclic AMP (cAMP) accumulation, and a 24-h culture period resulted in progesterone synthesis and increased P450(scc) mRNA levels; control levels of each endpoint measurement were not altered within the period observed. By contrast, culture with the growth factor transforming growth factor α (TGFα) in the presence of VIP (1 μM) prevented increases in P450(scc) mRNA levels and progesterone production. Similar effects of VIP and TGFα in the presence of VIP were demonstrated for P450 17α-OH mRNA levels and enzyme activity. Finally, there was an additive effect of VIP (0.1 μM) plus recombinant human (rh) FSH (100 mIU) on the initiation of progesterone production in cultured 6-8-mm follicle granulosa cells compared to the addition of VIP or rhFSH alone. The ability of VIP to increase P450(scc) and P450 17α-OH mRNA levels and induce steroid production in nonhierarchal follicles is similar to previously published results following culture with rhFSH. Therefore, we suggest that VIP and FSH may act in concert to initiate steroid production at the time of follicle selection (proposed to occur at the 9-12-mm stage of development), possibly via the adenylyl cyclase/cAMP second messenger system. On the other hand, TGFα may represent a physiological mechanism to prevent premature expression of these enzymes.