The Type III Transforming Growth Factor-β receptor (Betaglycan or BG) Modulates TGF-β Signaling and Wound Healing in Human Endometrial Cells and in Endometriosis

Abstract

Endometriosis is an estrogen-responsive disease defined as the presence of endometrial tissue at ectopic sites. TGF-β superfamily members are pleiotropic cytokines that exhibit cell-type-specific effects and regulate a plethora of diverse cellular responses. Perturbations in TGF-β superfamily components and signaling have been implicated in endometriosis. Betaglycan (BG) is a vital co-receptor and modulator of TGF-β superfamily. It functions as a tumor suppressor in numerous cancers. In the current investigation, we addressed the modulation of BG shedding by TGF-β, activin A and inhibin A and the molecular mechanisms involved using endometrial in vitro models. We also analyzed the influence of TGF-βs and BG on wound healing in endometriotic cells. Additionally, we investigated the utility of serum/endocervical mucus sBG and ADAM12 levels as potential non-invasive diagnostic biomarkers for endometriosis. The results demonstrated that TGF-β1/2 and activin A suppress shedding of BG via the ALK-4/5–SMAD3 axis in human endometriotic cells. We observed that inhibin A also suppresses BG shedding. Notably, shedding was modulated by MMPs. Furthermore, recombinant BG significantly reduced secretion of TGF-β1/2 and the rate of wound closure in endometriotic cells but had no influence on their viability. Interestingly, TGF-β1 significantly augmented secretion of MMP2/3 and enhanced the rate of wound closure in endometriotic cells. Conversely, TGF-β2 promoted secretion of MMP2 only and had no effect on the rate of wound closure. Activin A but not inhibin A significantly enhanced secretion of MMP2/3. Remarkably, endocervical mucus but not serum sBG levels were significantly reduced in endometriosis patients compared to controls. TGF-β1/2 significantly augmented secretion of ADAM12 in endometrial stromal cells in vitro. ADAM12 localizes mainly to the luminal and glandular epithelial cells as well as smooth muscle cells of eutopic and ectopic endometrium. No significant differences were observed in ADAM12 levels in endometrial staining intensity as well as in serum/endocervical mucus samples from endometriosis patients relative to controls. These findings provide new insights into the roles of BG in TGF-β signaling, wound healing, and in the pathophysiology of endometriosis. The strong reduction in BG shedding indicates that the TGF-β ligands require mBG for signaling, since it is particularly vital in establishing the potency of its ligands on their target cells. We speculate that the reduced endocervical mucus sBG levels in endometriosis patients may indicate enhanced TGF-β functions, which may in part be linked to the chronic inflammation observed in endometriosis. Although speculative, we propose that sBG may represent a potential therapeutic agent and an early diagnostic biomarker and hence, further studies to elucidate its function in endometriosis will undoubtedly provide indispensable insights.