Abstract:
TGF-βs transduce their signals mainly through activation of Smad2 and Smad3. They can also strongly but only transiently induce phosphorylation and activation of Smad1, Smad5 and Smad8 (BMP-responsive Smads) in endothelial cells and epithelial cells, fibroblasts as well as epithelium derived cancer cells. These observations have raised questions on how activation of Smads1/5/8 by TGF-βs affect BMP responses. Previous studies have proposed a possible crosstalk of TGF-βs/BMP pathways which involve ALK-5 and formation of pSmad3-pSmad1/5 complexes. However, the location of cross-talk remains to be elucidated. This study sought to investigate location of cross-talk of the TGF-β and BMP pathways and understand their possible roles in pathophysiology of endometriosis. 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 cell numbers counted. The TGF-βs/BMP interaction was investigated by quantification of Plasminogen Activator Inhibitor 1 (PAI-1) secretion by the cells. Results showed that TGF-β1 or TGF-β2, respectively, increased PAI-1 secretion in all cell lines studied. Both BPM inhibitor and ALK-2 inhibitor demonstrated 100% inhibition of TGF-β1 or TGF-β2 induced-PAI-1 secretion in all cells lines tested while ALK-2 and ALK-6 inhibitors demonstrated partial effects at 40% and 25% to inhibition respectively while IgG1 (control) had no effect on TGF-β1 or TGF-β2 induced-PAI-1 secretion. The observation that BMP and ALK-2 inhibition completely blocked the TGF-β induced PAI-1 secretion while ALK-3 and ALK-6 inhibitors only showed partial inhibition strongly suggests that ALK-2 is the point of cross talk between BMP and TGF-βs pathways. This finding might provide new insights into the role of TGF-βs in pathophysiology of endometriosis. However, more studies are needed on BMP and other pathways in TGF-β signaling to elucidate the connection between BMP and TGF-βs since our study only gave a first glimpse into involvement of TGF-β signaling in endometriosis.
