Critical role of Gα12 and Gα13 proteins in TGF-β-induced myofibroblast differentiation.

Myofibroblast differentiation, characterized by accumulation of cytoskeletal and extracellular matrix proteins by fibroblasts, is a key process in wound healing and pathogenesis of tissue fibrosis. Transforming growth factor-{beta} (TGF-{beta}) is the most powerful known driver of myofibroblast differentiation. TGF-{beta} signals through transmembrane receptor serine/threonine kinases that phosphorylate Smad transcription factors (Smad2/3) leading to activation of transcription of target genes. Heterotrimeric G proteins mediate a distinct signaling from seven-transmembrane G protein coupled receptors, not commonly linked to Smad activation. We asked if G protein signaling plays any role in TGF-{beta}-induced myofibroblast differentiation, using primary cultured human lung fibroblasts. Activation of Gs by cholera toxin blocked TGF-{beta}-induced myofibroblast differentiation without affecting Smad2/3 phosphorylation. Inhibition of Gi by pertussis toxin, or siRNA-mediated combined knockdown of Gq and G11 had no significant effect on TGF-{beta}-induced myofibroblast differentiation. A combined knockdown of G12 and G13 resulted in a drastic inhibition of TGF-{beta}-stimulated expression of myofibroblast marker proteins (collagen-1, fibronectin, smooth-muscle -actin), with siG12 being significantly more potent than siG13. Mechanistically, a combined knockdown of G12 and G13 resulted in a substantially reduced phosphorylation of Smad2 and Smad3 in response to TGF-{beta}, which was accompanied by a significant decrease in the expression of TGF{beta} receptors (TGFBR1, TGFBR2) and of Smad3 under siG12/13 conditions. In conclusion, our study uncovers a novel role of G12/13 proteins in the control of TGF-{beta} signaling and myofibroblast differentiation.