Integrins Modulate Cellular Fibrogenesis at Multiple Levels: Regulation of TGF-β Signaling

Fibrosis could occur in virtually any organ or tissue. The fibrotic lesion indolently disrupts the structure of the healthy organ, thereby hampering its proper function, consequence of which is devastating. Among the myriad factors that modulate fibrogenesis, transforming growth factor β (TGF-β) is one of the most studied and its central role for fibrotic disorders has been strongly suggested. Due to the pleiotropic nature of this cytokine, TGF-β modulates multiple cellular responses throughout fibrogenesis. The complexity is supported by the TGF-β receptor-specific phosphorylation of both the canonical, Smad-, and non-canonical, “non-Smad,” pathways. The latter modulates Smad activity either independent of Smad or by phosphorylating the Smad linker region, distinct from those receptor-regulated. Despite the commodity of this mediator, the mechanism by which TGF-β signaling causes specific pathogenesis and disease varies depending on the nature of the organ and the cells that compose that organ. Cells express a specific series of integrins that act as cellular sensors for the extracellular environment, determining subsequent cellular signals in a cell-type specific manner. Integrins may change their expression pattern under pathological conditions and contribute to the regulation of fibrogenesis via modulating ambient TGF-β activity. This regulation includes release of active TGF-β from its latent form and modulation of various signals downstream of integrin-engagement, which participate in the non-canonical regulation of TGF-β signaling. TGF-β also induces expression of integrins, as well as their ligand extracellular matrix, generating an amplification loop. Furthermore, myriads of intracellular signaling molecules that associate with integrin engagement could noncanonically modulate TGF-β signals. The entire picture of this mutual regulation between integrin and the TGF-β pathways might be difficult to draw. Instead, this review intends to depict several critical aspects of this regulation, with examples from various types of fibrosis in different tissues to help understand the integrin-modulation of fibrogenesis, a critical clue for therapeutic approaches to fibrosis.