Differential Stimulation of Testicular Steroidogenesis by Orthosteric and Allosteric Agonists of Luteinizing Hormone Receptor

Luteinizing hormone (LH) and human chorionic gonadotropin (hCG), due to binding to the LH/hCG receptor, activate the adenylyl cyclase (AC) system which regulates testosterone (T) production. Long-term administration of LH and hCG causes desensitization of this system and attenuates the steroidogenic response, thereby necessitating the search for new agonists of the LH/hCG receptor. The aim of the work was to explore, in comparison with hCG, the stimulatory effects of the previously developed thieno[2,3-d]pyrimidines, TP03 and TP04, and a new thieno[2,3-d]pyrimidine derivative, 5-amino-N-(tert-butyl)-4-(3-(4-aminopyrimidine-5-carboxamido)-phenyl)-2-(methylthio)thieno[2,3-d]pyrimidine-6-carboxamide (TP37), on AC activity in rat testicular membranes, as well as on T production and gene expression of the LH/hCG receptor and the key testicular steroidogenic proteins under conditions of a single and 3-day administration to male rats. hCG increased AC activity in testicular membranes more efficiently compared to thieno[2,3-d]pyrimidines, and after a single injection (50 and 100 IU/rat) was superior to TP03 and TP04 (15-50 mg/kg) in its steroidogenic effect. After a 3-day administration, the steroidogenic effect of hCG was attenuated compared to that for TP03 and TP04. After 3 days of treatment with gonadotropin, testicular expression of genes encoding the StAR protein and cytochrome P450scc was considerably increased, but expression of the Lhr and Cyp17a1 genes encoding LH/hCG receptor and cytochrome P450-17 alpha was suppressed. TP03 and TP04 slightly increased StAR gene expression but did not affect expression of other genes. TP37, which was active in vitro, after a short stimulation of T production, suppressed the steroidogenic function at a dose of 50 mg/kg, probably due to its degradation and the ability to suppress Cyp17a1 gene expression. Our data indicate significant differences in the mechanisms underlying the effect of gonadotropins and thieno[2,3-d]pyrimidines with an activity of LH/hCG receptor agonists on testicular steroidogenesis. We also demonstrate that long-term administration of thieno[2,3-d]pyrimidines to stimulate T production does not attenuate steroidogenesis and induces no LH resistance.