Transforming Growth Factor-β-Dependent and -Independent Pathways of Induction of Tubulointerstitial Fibrosis in β6−/−Mice

Transforming growth factor-β1 (TGF-β1) and the renin-angiotensin-aldosterone system are key mediators in kidney fibrosis. Integrin αvβ6, a heterodimeric matrix receptor expressed in epithelia, binds and activates latent TGF-β1. We used β6 integrin-null mice (β6−/−) to determine the role of local TGF-β1 activation in renal fibrosis in the unilateral ureteral obstruction (UUO) model. Obstructed kidneys from β6−/−mice showed less injury than obstructed kidneys from wild-type (WT) mice, associated with lower collagen I, collagen III, plasminogen activator inhibitor (PAI-1), and TGF-β1 mRNA levels and lower collagen content. Infusion with either angiotensin II (Ang II) or aldosterone (Aldo) or combination in β6−/−UUO mice significantly increased collagen contents to levels comparable to those in identically treated WT. Active TGF-β protein expression in β6−/−mice was less in UUO kidneys with or without Ang II infusion compared to matched WT mice. Activated Smad 2 levels in β6−/−obstructed kidneys were lower than in WT UUO mice, and did not increase when fibrosis was induced in β6−/−UUO mice by Ang II infusion. Anti-TGF-β antibody only partially decreased this Ang II-stimulated fibrosis in β6−/−UUO kidneys. In situhybridization and immunostaining showed low expression of PAI-1 mRNA and protein in tubular epithelium in β6−/−UUO kidneys, with increased PAI-1 expression in response to Ang II, Aldo, or both. Our results indicate that interruption of αvβ6-mediated activation of TGF-β1 can protect against tubulointerstitial fibrosis. Further, the robust induction of tubulointerstitial fibrosis without increase in activated Smad 2 levels in obstructed β6−/−mice by Ang II suggests the existence of a TGF-β1-independent pathway of induction of fibrosis through angiotensin.