Bone Morphogenetic Proteins/Growth Differentiation Factors and Smad Activation in Ovarian Granulosa Cells and Carcinoma

Abstract

The ligands of the transforming growth factor β (TGF-β) superfamily are important paracrine/autocrine regulators of the ovarian follicular development. This group includes TGF-β. activins, bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs), and others. Nine BMPs and GDFs are expressed in the ovary, with a well-conserved pattern of expression among mammalian species. Oocytes express BMP6, BMP15, and GDF9: theca cells express BMP3, BMP3b, BMP4, and BMP7; and granulosa cells express BMP2 and BMP5. In vivo studies have revealed the importance of GDF9 and BMP15 in the control of early follicular growth and ovulation. In vitro, most of the BMPs and GDFs induce granulosa cell proliferation but reduce follicle stimulating hormone (FSH)-induced progesterone production, thus delaying the differentiation process.The general mechanism of the signaling pathway for TGF-β family members has been well described. The ligands interact with type II and type I serine/threonine kinase receptors leading to the activation of two sets of downstream Smad-transcriptional factors. Owing to their common evolutionary origin, more than thirty related members of the TGF-β superfamily likely interact with seven type I (ALK) and five type II receptors, thereby activating Smad proteins. Instead of purifying the GDF9 receptors from granulosa cells for their identification, we hypothesized that GDF9 activates the known serine/threonine kinase receptors. We demonstrated the ability of the BMP receptor type II (BMPRII) ectodomain to block GDF9 stimulation of rat granulosa cell proliferation. In a GDF9-nonresponsive cell line, overexpression of the TGF-β type I receptor, ALK5, but not any other six type I receptors, conferred GDF9 responsiveness. The predicted role of BMPRII and ALK5 as type II and type I receptors, respectively, for GDF9 was validated in granulosa cells following the suppression of endogenous BMPRII and ALK5 expression using gene “knockdown” approaches. In parallel, Moore et al. have shown that BMP15, the GDF9 paralog, is likely to signal in granulosa cells by binding to BMPRII and ALK6. Less is known about the receptors involved in the signaling of the other ovarian BMPs and GDFs in granulosa cells. Ligands of the TGF-β superfamily occupy a central position in the signaling networks that control the growth and differentiation of cells. However, in the ovary, data are limited on the involvement and the disruption of BMPs and GDFs pathways in granulosa and epithelium cell tumor formation.Identification of GDF9 receptors in granulosa cells based on the coevolution of ligands and receptors has proved that evolutionary tracing of polypeptide ligands, receptors, and downstream signaling molecules in their respective subgenomes verifies a new paradigm for hormone research. These studies suggested that more than thirty ligands of the TGF-β family interact with a limited number of receptors in a combinatorial manner to activate two downstream Smad pathways. In granulosa cells and ovarian cancer cells, identification of the components of the Smad-dependent signaling pathway for the nine ovarian BMPs and GDFs and its multilevel regulations should be pursued. These studies will be essential for identification of potential alterations in the BMPs and GDFs signaling leading to carcinogenesis.