Melatonin Stimulates STAR Expression and Progesterone Production via Activation of the PI3K/AKT Pathway in Bovine Theca Cells

Melatonin is present in mammalian follicular fluid and plays an important role in regulating steroidogenesis in follicular cells. In this study, we report the effect of melatonin on steroidogenesis in the theca interna (TI) in small bovine follicles and theca cells (TCs) cultured in vitro. Treatment with melatonin significantly increased the expression of steroidogenic acute regulatory protein (STAR) and the production of progesterone in both TI and in TCs. Melatonin stimulated the phosphorylation of AKT but not ERK1/2, and the addition of luzindole (a nonspecific MT1 and MT2 inhibitor) or 4P-PDOT (specific MT2 inhibitor) reduced melatonin-induced STAR expression, progesterone secretion, and PI3K/AKT pathway activation. The effect of melatonin on the TI in follicles was more obvious than on the TCs in vitro. Results indicate that melatonin stimulates the steroidogenesis of TCs mainly via the activation of the PI3K/AKT pathway by MT1 and MT2.