Abstract
BackgroundGrowth differentiation factor-11 (GDF-11) belongs to the transforming growth factor-β (TGF-β) superfamily. To date, the expression of GDF-11 in the ovary and its role in regulating ovarian function are completely unknown. Ovarian granulosa cell-mediated steroidogenesis plays a pivotal role in maintaining normal female reproductive function. GDF-11 and GDF-8 share high sequence similarity and exhibit many similar features and functions. Steroidogenic acute regulatory protein (StAR) regulates the rate-limiting step in steroidogenesis and its expression can be downregulated by GDF-8. Polycystic ovary syndrome (PCOS) is the most common cause of female infertility. The expression levels of GDF-8 are upregulated in the human follicular fluid and granulosa-lutein (hGL) cells of PCOS patients. However, whether similar results can be observed for the GDF-11 needs to be determined.MethodsThe effect of GDF-11 on StAR expression and the underlying molecular mechanisms were explored by a series of in vitro experiments in a primary culture of hGL cells obtained from patients undergoing in vitro fertilization (IVF) treatment. Human follicular fluid samples were obtained from 36 non-PCOS patients and 36 PCOS patients. GDF-11 levels in follicular fluid were measured by ELISA.ResultsGDF-11 downregulates StAR expression, whereas the expression levels of the P450 side-chain cleavage enzyme (P450scc) and 3β-hydroxysteroid dehydrogenase (3β-HSD) are not affected by GDF-11 in hGL cells. Using pharmacological inhibitors and a siRNA-mediated approach, we reveal that ALK5 but not ALK4 mediates the suppressive effect of GDF-11 on StAR expression. Although GDF-11 activates both SMAD2 and SMAD3 signaling pathways, only SMAD3 is involved in the GDF-11-induced downregulation of StAR expression. In addition, we show that SMAD1/5/8, ERK1/2, and PI3K/AKT signaling pathways are not activated by GDF-11 in hGL cells. RT-qPCR and ELISA detect GDF-11 mRNA expression in hGL cells and GDF-11 protein expression in human follicular fluid, respectively. Interestingly, unlike GDF-8, the expression levels of GDF-11 are not varied in hGL cells and follicular fluid between non-PCOS and PCOS patients.ConclusionsThis study increases the understanding of the biological function of GDF-11 and provides important insights into the regulation of ovarian steroidogenesis.
Highlights
Growth differentiation factor-11 (GDF-11) belongs to the transforming growth factor-β (TGF-β) superfamily
The suppressive effect of GDF‐11 on steroidogenic acute regulatory protein (StAR) expression is mediated by ALK5 but not activin receptor-like kinase 4 (ALK4) Similar to GDF-8, TGF-β type I receptors, ALK4 and ALK5, are putative receptors for GDF-11 that mediate its biological functions [13]
The inhibitory effects of GDF-11 on StAR mRNA and protein levels were abolished by the knockdown of ALK5 (Figs. 2D and 2E). These results indicate that the suppressive effect of GDF-11 on StAR expression is mediated by ALK5 but not ALK4 in human follicular fluid and granulosa-lutein (hGL) cells
Summary
Growth differentiation factor-11 (GDF-11) belongs to the transforming growth factor-β (TGF-β) superfamily. The expression of GDF-11 in the ovary and its role in regulating ovarian function are completely unknown. Ovarian granulosa cell-mediated steroidogenesis plays a pivotal role in maintaining normal female repro‐ ductive function. Steroidogenic acute regulatory protein (StAR) regulates the rate-limiting step in steroidogenesis and its expression can be downregulated by GDF-8. The key regulatory step involved in ovarian steroidogenesis is the regulation of the transport of cholesterol from the outer to the inner membrane of the mitochondria, where the cytochrome P450 side-chain cleavage enzyme (P450scc), the enzyme catalyzes the first step of steroidogenesis, is located [1]. It has been well characterized that cholesterol transfer from the outer to the inner membrane of the mitochondria is mediated by the steroidogenic acute regulatory protein (StAR) [1, 2]. Ovarian StAR expression is regulated by gonadotropins through the cAMP-PKA signaling pathway [3]
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