Regulation of steroid production in cultured porcine thecal cells by transforming growth factor-β

Abstract

Evidence that transforming growth factor-β (TGFβ) 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 TGFβ 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 μg/ml), progesterone production increases over a 3-day culture period. TGFβ (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). TGFβ 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 TGFβ (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 TGFβ appeared to be distal to cAMP formation, the effect of TGFβ 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 TGFβ appears to be exerted distal to cAMP formation, neither cholesterol sidechain cleavage nor the 3β-hydroxysteroid dehydrogenase: Δ 5− Δ 4 isomerase reactions are primary targets of this factor. Together with evidence of thecal production of TGFβ, the results of this study indicate that this peptide may be an autocrine regulator of thecal steroidogenesis.