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Leydig Cells Express Follicle-Stimulating Hormone Receptors in African Catfish

Division of Endocrinology and Metabolism (A.G.-L., J.B., J.C.M.G., W.v.D., R.W.S.), Department of Biology, Faculty of Sciences, Utrecht University, 3584 CH Utrecht, The Netherlands; Center of Marine Biotechnology (J.M.T.), University of Maryland Biotechnology Institute, Baltimore, Maryland 21202; and Department of Physiology of Growth and Reproduction in Fish (G.L.T., R.W.S.), Institute of Marine Research, 5817 Bergen, Norway

Address all correspondence and requests for reprints to: Dr. Rüdiger W. Schulz, Utrecht University, Faculty of Sciences, Department of Biology, Division Endocrinology and Metabolism, Kruyt Building, Room W-604, Padualaan 8, 3584 CH Utrecht, The Netherlands. E-mail: r.w.schulz{at}uu.nl.

This report aimed to establish, using African catfish, Clarias gariepinus, as model species, a basis for understanding a well-known, although not yet clarified, feature of male fish reproductive physiology: the strong steroidogenic activity of FSHs. Assays with gonadotropin receptor-expressing cell lines showed that FSH activated its cognate receptor (FSHR) with an at least 1000-fold lower EC₅₀ than when challenging the LH receptor (LHR), whereas LH stimulated both receptors with similar EC₅₀s. In androgen release bioassays, FSH elicited a significant response at lower concentrations than those required to cross-activate of the LHR, indicating that FSH stimulated steroid release via FSHR-dependent mechanisms. LHR/FSHR-mediated stimulation of androgen release was completely abolished by H-89, a specific protein kinase A inhibitor, pointing to the cAMP/protein kinase A pathway as the main route for both LH- and FSH-stimulated steroid release. Localization studies showed that intratubular Sertoli cells express FSHR mRNA, whereas, as reported for the first time in a vertebrate, catfish Leydig cells express both LHR and FSHR mRNA. Testicular FSHR and LHR mRNA expression increased gradually during pubertal development. FSHR, but not LHR, transcript levels continued to rise between completion of the first wave of spermatogenesis at about 7 months and full maturity at about 12 months of age, which was associated with a previously recorded approximately 3-fold increase in the steroid production capacity per unit testis weight. Taken together, our data strongly suggest that the steroidogenic potency of FSH can be explained by its direct trophic action on FSHR-expressing Leydig cells.