Follicle-Stimulating Hormone and Luteinizing Hormone Mediate the Androgenic Pathway in Leydig Cells of an Evolutionary Advanced Teleost

The endocrine pathways controlling vertebrate spermatogenesis are well established in mammals where the pituitary gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) exclusively activate the FSH receptor (FSHR) in Sertoli cells and the LH/choriogonadotropin receptor (LHCGR) in Leydig cells, respectively. In some teleosts, however, it has been shown that Lh can cross-activate the Fshra ortholog, and that Leydig cells coexpress the Lhcgrba and Fshra paralogs, thus mediating the androgenic function of Fsh in the testis. Here, we investigated whether these proposed mechanisms are conserved in an evolutionary advanced pleuronectiform teleost, the Senegalese sole (Solea senegalensis). Transactivation assays using sole Fshra- and Lhcgrba-expressing cells and homologous single-chain recombinant gonadotropins (rFsh and rLh) showed that rFsh exclusively activated Fshra, whereas rLh stimulated both Lhcgrba and Fshra. The latter cross-activation of Fshra by rLh occurred with an EC50 4-fold higher than for rFsh. Both recombinant gonadotropins elicited a significant androgen release response in vitro and in vivo, which was blocked by protein kinase A (PKA) and 3beta-hydroxysteroid dehydrogenase inhibitors, suggesting that activation of steroidogenesis through the cAMP/PKA pathway is the major route for both Lh- and Fsh-stimulated androgen secretion. Combined in situ hybridization and immunocytochemistry using cell-specific molecular markers and antibodies specifically raised against sole Fshra and Lhcgrba demonstrated that both receptors are expressed in Leydig cells, whereas Sertoli cells only express Fshra. These data suggest that Fsh-mediated androgen production through the activation of cognate receptors in Leydig cells is a conserved pathway in Senegalese sole.