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Angiogenesis plays an integral role in follicular and luteal development and is positively regulated by several intra-ovarian factors including vascular endothelial growth factor A (VEGFA) and fibroblast growth factor 2 (FGF2). Various transforming growth factor-β (TGF-β) superfamily members function as intra-ovarian regulators of follicle and luteal function, but their potential roles in modulating ovarian angiogenesis have received little attention. In this study, we used a bovine theca interna culture model (exhibiting characteristics of luteinization) to examine the effects of TGF-β1 and bone morphogenetic protein 6 (BMP6) on angiogenesis and steroidogenesis. VEGFA/FGF2 treatment promoted endothelial cell network formation but had little or no effect on progesterone and androstenedione secretion or expression of key steroidogenesis-related genes. TGF-β1 suppressed basal and VEGFA/FGF2-induced endothelial cell network formation and progesterone secretion, effects that were reversed by an activin receptor-like kinase 5 (ALK5) inhibitor (SB-431542). The ALK5 inhibitor alone raised androstenedione secretion and expression of several transcripts including  CYP17A1 . BMP6 also suppressed endothelial cell network formation under VEGFA/FGF2-stimulated conditions and inhibited progesterone secretion and expression of several steroidogenesis-related genes under basal and VEGFA/FGF2-stimulated conditions. These effects were reversed by an ALK1/2 inhibitor (K02288). Moreover, the ALK1/2 inhibitor alone augmented endothelial network formation, progesterone secretion, androstenedione secretion and expression of several steroidogenesis-related genes. The results indicate dual suppressive actions of both TGF-β1 and BMP6 on follicular angiogenesis and steroidogenesis. Further experiments are needed to unravel the complex interactions between TGF-β superfamily signalling and other regulatory factors controlling ovarian angiogenesis and steroidogenesis.

Modulatory effects of TGF-β1 and BMP6 on thecal angiogenesis and steroidogenesis in the bovine ovary