Steroidogenesis-inducing protein interacts with transforming growth factor-β to stimulate DNA synthesis in rat granulosa cells

Abstract

We have examined the effects of Steroidogenesis-inducing protein (SIP), previously isolated from human follicular fluid, on the synthesis of DNA by granulosa cells isolated from diethylstilbestrol-primed immature rats. SIP alone had no effect but in conjunction with transforming growth factor-β (TGF-β) there was an increase in [ 3H]thymidine incorporation into granulosa cell DNA. The increase in [ 3H]thymidine into DNA was due to an increase in the number of labeled granulosa cells as assessed by autoradiography. Epidermal growth factor (EGF) and transforming growth factor-α (TGF-α) interfered with the ability of SIP and TGF-β to promote DNA synthesis. Previously, we reported that the growth-promoting action of follicle-stimulating hormone (FSH) on rat granulosa cells in vitro was dependent on TGF-β, and EGF inhibited the actions of FSH plus TGF-β on [ 3H]thymidine incorporation into DNA. Since the dependency of SIP on its interactions with TGF-β and the ability of EGF to interfere with the process were similar to the properties reported for FSH, this raised the possibility that the actions of SIP were mediated through the accumulation of intracellular cAMP. However, when the hypothesis was tested, SIP had no effect on cAMP levels in the presence or absence of TGF-β, under conditions in which FSH stimulated cAMP accumulation. In conclusion, DNA synthesis in rat granulosa cells is dependent on the presence of TGF-β. In the presence of TGF-β, FSH or SIP, acting through cAMP-dependent and cAMP-independent mechanisms respectively, can recruit more cells to enter the cell cycle and initiate DNA synthesis. While FSH plays a major role in regulating granulosa cell proliferation, our results indicate that an intragonadal peptide, SIP, can promote growth as effectively as FSH.