Role of Bone Morphogenetic Proteins (BMPs) and TGF-β Receptors in the Pathogenesis of Endometriosis

Abstract

Endometriosis is a disease characterized by presence of endometrial tissue outside the uterus and affects approximately 2-10% women of the reproductive age and about 20 to 50% of women suffering from infertility. TGF-βs have been found to be highly elevated in blood and tissues of endometriotic patients. TGF-βs transduce their signals mainly through activation of Smad2 and Smad3 proteins. They can also strongly but only transiently induce phosphorylation and activation of Smad1, Smad5 and Smad8 (BMP-responsive Smads) in endothelial cells, epithelial cells, fibroblasts and epithelium derived cancer cells. These observations have raised several questions on activation of Smads1/5/8 by TGF-βs affect BMP responses. Previous studies have shown involvement of ALK-5 which is a BMP specific receptor and formation of pSmad3- pSmad1/5 complexes hence the need to investigate the exact role of TGF-β and BMP in the pathogenesis of endometriosis. This study sought to investigate the possible role of TGF-β and BMP pathways in the pathophysiology of endometriosis. Commercial human endometriotic and endometrial cell lines were used in this study. Immortalized human endometrial stromal (T- HESC), epithelial (HES), endometriotic stromal (22B) and epithelial (12ZVK) cell lines were treated with or without BMP inhibitors, with or without TGF-β1 or TGF-β2, respectively, and the cell numbers were counted. Quantification of TGF-βs and BMP interaction was investigated by measuring levels of Plasminogen Activator Inhibitor 1 (PAI-1) secretion by the cells. Results showed that, TGF-β1 and TGF-β2, respectively, increased PAI-1 secretion in all cell lines studied. Treatment of cell lines with the BMP inhibitor demonstrated a complete decrease (100%) of TGF- β1 or TGF-β2 induced-PAI-1 secretion in all cell lines studied. In addition, treatment of cell lines with ALK-2 inhibitor demonstrated 100% decrease (100%) of TGF-β1 or TGF-β2 induced-PAI-1 secretion in all cell lines, whereas the ALK-3 and ALK-6 inhibitors demonstrated only 40% and 25% decrease, respectively. Treatment of cells with the IgG1 (control) had no effect on TGF-β1 or TGF-β2 induced-PAI-1 secretion. Results of this study demonstrated that both BMP and TGF-β receptors are involved in signaling pathways in endometrial and endometriotic cells both BMP as well as an ALK-2 inhibitor completely blocked the TGF-β-induced PAI-1 secretion whereas ALK-3 and ALK-6 inhibitors only partly blocked it. These findings strongly suggest that ALK-2 is the point of cross talk between the BMP and TGF-βs pathways and that both pathways might be involved in pathogenesis of endometriosis. This finding might provide new insights into the roles of TGF-βs in the pathophysiology of endometriosis.