Endoglin promotes TGF-β/Smad1 signaling in scleroderma fibroblasts

Abstract

TGF-β is the primary inducer of extracellular matrix (ECM) proteins in scleroderma (systemic sclerosis, SSc). Previous studies indicate that in a subset of SSc fibroblasts TGF-β signaling is activated via elevated levels of activin receptor-like kinase (ALK) 1 and phosphorylated Smad1 (pSmad1). The goal of this study was to determine the role of endoglin/ALK1 in TGF-β/Smad1 signaling in SSc fibroblasts. In SSc fibroblasts, increased levels of endoglin correlated with high levels of pSmad1, collagen, and connective tissue growth factor (CCN2). Endoglin depletion via siRNA in SSc fibroblasts inhibited pSmad1 but did not affect pSmad2/3. Following endoglin depletion mRNA and protein levels of collagen and CCN2 were significantly decreased in SSc fibroblasts but remained unchanged in normal fibroblasts. ALK1 was expressed at similar levels in SSc and normal fibroblasts. Depletion of ALK1 resulted in inhibition of pSmad1 and a moderate but significant reduction of mRNA and protein levels of collagen and CCN2 in SSc fibroblasts. Furthermore, constitutively high levels of endoglin were found in complexes with ALK1 in SSc fibroblasts. Overexpression of constitutively active ALK1 (caALK1) in normal and SSc fibroblasts led to a moderate increase of collagen and CCN2. However, caALK1 potently induced endothelin 1 (ET-1) mRNA and protein levels in SSc fibroblasts. Additional experiments demonstrated that endoglin and ALK1 mediate TGF-β induction of ET-1 in SSc and normal fibroblasts. In conclusion, this study has revealed an important profibrotic role of endoglin in SSc fibroblasts. The endoglin/ALK1/Smad1 pathway could be a therapeutic target in patients with SSc if appropriately blocked.

Overexpression of the short endoglin isoform reduces renal fibrosis and inflammation after unilateral ureteral obstruction

“…In agreement with these reports, our present data in the UUO model of tubulo-interstitial fibrosis shows the increase in both Eng mRNA and protein expression in the O kidney ( Figure 1). Several authors demonstrated that Eng promotes fibrosis in different kidney experimental models [15,16,[33][34][35][36][37] and other experimental models of tissue fibrosis in organs such as heart [13,38], liver [39] or skin [40]. Two different Eng isoforms that differs exclusively in the intracellular tail have been reported in both humans [18] and mice [19].…”

Glucocorticoids Recruit Tgfbr3 and Smad1 to Shift Transforming Growth Factor-β Signaling from the Tgfbr1/Smad2/3 Axis to the Acvrl1/Smad1 Axis in Lung Fibroblasts

“…2B), suggesting a role for Smad1, which is the key mediator of the Acvrl1/Smad1 axis, in the effects of dexamethasone on TGF-␤ signaling. The Acvrl1/Smad1 axis, which is traditionally considered to be active primarily in the endothelium (29, 48 -50), was demonstrated to be active in NIH/3T3 cells (results not shown) and in fibroblasts (51,52). To examine the functional contribution of Smad1 to the effects of dexamethasone on TGF-␤ signaling, the expression of smad1 was ablated by transfection of NIH/3T3 cells with siRNA directed against smad1, with scrambled siRNA serving as a negative control.…”

The cytokines of pulmonary fibrosis: Much learned, much more to learn

“…Both canonical and noncanonical TGF-b signaling pathways are inhibited by Smad6 and/ or Smad7 through a negative feedback loop [119]. Non-canonical signaling occurs through several other intracellular signaling molecules, including but not limited to endoglin/Smad1 [114,115,[120][121][122][123]; TRAF6 [119,[124][125][126]; ERK1/2 [115,121,[127][128][129][130], p38 [131,132] and JunB [133] MAPK; c-Abl [134]; mTOR [134]; PI3K/ AKT [115,128,132,135]; and NF-jB [136]. Much attention has additionally been directed recently to the Wnt/b-catenin pathway, which appears to promote proliferation and migration of lung fibroblasts and profibrotic gene expression [137,138], control TGF-b production and profibrotic activity [139,140], and facilitate EMT through direct interaction between Smad3 and b-catenin [141].…”