Loss of beta1-integrin enhances TGF-beta1-induced collagen expression in epithelial cells via increased alphavbeta3-integrin and Rac1 activity

Hayashida, T.; Jones, J. C.; Lee, C. K.; Schnaper, H. W.

J Biol Chem. 2010 Jul 24; 285(40):30741-51

Abstract

Transforming growth factor beta (TGF-beta) promotes tissue fibrosis via the receptor-specific Smad pathway and non-canonical pathways. We recently reported that TGF-beta1-stimulated collagen expression by cultured kidney cells requires integrin-dependent activation of focal adhesion kinase (FAK) and consequent ERK MAP kinase activity leading to Smad3 linker region phosphorylation. Here, we defined a role for alphavbeta3-integrin in this non-canonical pathway. A human kidney tubular cell line in which beta1-integrin was knocked down (beta1-k/d) demonstrated enhanced type I collagen mRNA expression and promoter activity. A second shRNA to either alphav-integrin or beta3-integrin, but not to another alphav-binding partner, beta6-integrin, abrogated the enhanced COL1A2 promoter activity in beta1-k/d cells. Although alphavbeta3-integrin surface expression levels were not different, alphavbeta3-integrins colocalized with sites of focal adhesion significantly more in beta1-k/d cells, and activated alphavbeta3-integrin was detected only in beta1-k/d cells. Further, the collagen response was decreased by a function-blocking antibody or a peptide inhibitor of alphavbeta3-integrin. In cells lacking alphavbeta3-integrin, the responses were attenuated, whereas the response was enhanced in alphavbeta3-overexpressing cells. Rac1 and ERK, previously defined mediators for this non-canonical pathway, showed increased activities in beta1-k/d cells. Finally, inhibition of alphavbeta3-integrin decreased Rac1 activity and COL1A2 promoter activity in beta1-k/d cells. Together, our results indicate that decreasing beta1 chain causes alphavbeta3-integrin to become functionally dominant and promotes renal cell fibrogenesis via Rac1-mediated ERK activity.

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