Intracellular Signaling in the Gonads*

Abstract

The regulation of steroidogenesis in both the ovary and testis involves a complex interaction of a diversity of hormones and intracellular signaling pathways. The recent cloning of LH and FSH receptors has paved the way for an increased understanding of the mechanisms of receptor conformation, ligand-receptor interaction, and facilitation of post-receptor activity. The dominant role played by LH in the regulation of steroid production appears to be mediated by more than one intracellular signaling pathway. In addition to the stimulation of the adenylate cyclase-cAMP pathway, also known to be stimulated by FSH, the actions of LH may be additionally mediated by other intracellular messengers, such as those derived from the PLC pathway. Steroidogenesis in the gonads appears to be modulated by a variety of factors in addition to the gonadotropins. In this review, those factors of intracellular signaling mechanisms of which we have some understanding have been discussed. These include GnRH, PGF2 alpha, Ang II, VIP, GHRH, TNF alpha, CRF, EGF, and TGF alpha. Many of these factors have been shown to be locally synthesized, and specific receptors have been identified in the gonads. Many gonadal factors have the capacity to exert effects on steroidogenesis independent of the gonadotropins. Alternately, they have been demonstrated to alter the gonadal response to the gonadotropins via autocrine, paracrine, and intracrine mechanisms. As yet, our understanding of the intracellular signaling mechanisms used by novel gonadal regulators is limited. The involvement of the PLC, PLA2, and PLD pathways in this regard has been reviewed. It is becoming apparent that multiple signaling pathways may be stimulated by a single hormone, as in the case of GnRH, PGF2 alpha, and LH. The complexity of intracellular signal transduction in the gonads is enhanced by the potential cross-talk at numerous steps in the signaling cascades.