Signaling Cross-Talk between Transforming Growth Factor-Beta (TGF-β) and Bone Morphogenic Protein (BMP) pathways in Human Endometrial and Endometriotic Cells

Transforming growth factor-beta (TGF-β) and bone morphogenic protein (BMP) signaling pathways are involved in the vast majority of cellular processes and are fundamentally important during the entire life of all metazoans. TGF-βs and BMPs transduce their signals via canonical Smad-dependent pathways which involve TGF-β/BMP ligands, receptors and Smad molecules. Also, non-canonical Smad-independent signalling pathways are involved in TGF-β/BMP signal transduction. Here we investigated signalling cross-talk between the pathways downstream of TGF-β and BMP signalling in endometrial and endometriotic cells. Treatment of endometrial and endometriotic stromal and epithelial cells with TGF-β1 or TGF-β2 increased secretion of plasminogen activator inhibitor 1 (PAI-1) dramatically in all cell lines. Of note, higher PAI-1 secretion was observed in endometriotic cells compared to endometrial cells. Both a transforming growth factor-beta receptor-1 (TβRI) and a Bone morphogenic protein receptor-1(BMPR1) inhibitors completely blocked the TGF-β-induced PAI-1 secretion in all cell lines while a specific inhibitor of Smad3 (SiS3) had a partial effect on PAI-1 secretion in all cell lines. Additionally, we showed that Activin Receptor-Like kinase (ALK-2) is the main BMP receptor that is responsible for complete blockage of TGF-β-induced PAI-1 secretion whereas, ALK-3 and ALK-6 exhibited partial effects on PAI-1 secretion. In summary, the complete inhibition of TGF-β-induced PAI-1 secretion by a general BMPR1 and ALK-2 inhibitors suggest a possible cross-talk between TGF-β and BMP pathways. Since only ALK-2 had a complete decrease of TGF-β-induced PAI-1 secretion compared to ALK-3 and ALK-6 in all cells, then our results demonstrate the importance of ALK-2 as a point of cross-talk of TGF-β and BMP pathways. Furthermore, we showed TGF-β-induced phosphorylation of Smad1 in all cells upon TGF-beta treatment. Our result further confirms that the Smads are the most important intracellular transducers of TGFβ and BMP signals