REGULATION OF STEROID-PRODUCTION IN CULTURED PORCINE THECAL CELLS BY TRANSFORMING GROWTH-FACTOR-BETA

Evidence that transforming growth factor-beta (TGF-beta) is produced by porcine thecal cells and acts upon porcine granulosa cells suggests that this peptide may be a local regulator of follicular function in this species. The objective of the present study was to investigate the effects of TGF-beta on steroidogenesis in thecal cells from 4-6 mm follicles of prepubertal gilts. In this culture system, cells undergo functional luteinization such that production of androstenedione, the major steroid product in 24 h incubations, declines, and in the presence of luteinizing hormone (LH) (250 ng/ml) and insulin (1-mu-g/ml), progesterone production increases over a 3-day culture period. TGF-beta (0.1-10 ng/ml) had no effect on production of androstenedione from endogenous precursors in the presence or absence of LH, although there was a slight inhibition of androstenedione production in the presence of exogenous progesterone (up to 23%). As the cells luteinized in culture, the increase in progesterone production in response to LH increased (day 1, 4.4-fold; day 3, 13-fold). TGF-beta at concentrations as low as 0.1 ng/ml caused marked (up to 90%) inhibition of LH-stimulated progesterone production in day 3 cultures. In the presence of TGF-beta (10 ng/ml), the response to LH was completely abolished, and the response to dibutyryl cAMP was considerably attenuated (25% of controls). Since the primary site of action of TGF-beta appeared to be distal to cAMP formation, the effect of TGF-beta on conversion of exogenous 22-hydroxy-cholesterol and pregnenolone to progesterone was determined in day 3 cultures. 22-Hydroxycholesterol and pregnenolone restored progesterone production to at least 80% and 89% of controls, respectively. While the primary inhibitory action of TGF-beta appears to be exerted distal to cAMP formation, neither cholesterol sidechain cleavage nor the 3-beta-hydroxysteroid dehydrogenase: DELTA-5-DELTA-4 isomerase reactions are