The Fibroblast Integrin α11β1 Is Induced in a Mechanosensitive Manner Involving Activin A and Regulates Myofibroblast Differentiation

Fibrotic tissue is characterized by an overabundance of myofibroblasts. Thus, understanding the factors that induce myofibroblast differentiation is paramount to preventing fibrotic healing. Previous studies have shown that mechanical stress derived from the integrin-mediated interaction between extracellular matrix and the cytoskeleton promotes myofibroblast differentiation. Integrin alpha 11 beta 1 is a collagen receptor on fibroblasts. To determine whether alpha 11 beta 1 can act as a mechanosensor to promote the myofibroblast phenotype, mouse embryonic fibroblasts and human corneal fibroblasts were utilized. We found that alpha 11 mRNA and protein levels were up-regulated in mouse embryonic fibroblasts grown in attached three-dimensional collagen gels and conversely down-regulated in cells grown in floating gels. alpha 11 up-regulation could be prevented by manually detaching the collagen gels or by cytochalasin-D-treatment. Furthermore, SB-431542, an inhibitor of signaling via ALK4, ALK5, and ALK7, prevented the up-regulation of alpha 11 and the concomitant phosphorylation of Smad3 under attached conditions. In attached gels, TGF-beta 1 was secreted in its inactive form but surprisingly not further activated, thus not influencing alpha 11 regulation. However, inhibition of activin A attenuated the up-regulation of alpha 11. To determine the role of alpha 11 in myofibroblast differentiation, human corneal fibroblasts were transfected with small interfering RNA to alpha 11, which decreased alpha-smooth muscle actin expression and myofibroblast differentiation. Our data suggest that alpha 11 beta 1 is regulated by cell/matrix stress involving activin A and Smad3 and that alpha 11 beta 1 regulates myofibroblast differentiation.