Interdependence of HIF-1α and TGF-β/Smad3 signaling in normoxic and hypoxic renal epithelial cell collagen expression

Basu RK, Hubchak S, Hayashida T, Runyan CE, Schumacker PT, Schnaper HW. Interdependence of HIF-1 alpha and TGF-beta/Smad3 signaling in normoxic and hypoxic renal epithelial cell collagen expression. Am J Physiol Renal Physiol 300: F898-F905, 2011. First published January 5, 2011; doi:10.1152/ajprenal.00335.2010.-Increasing evidence suggests that chronic kidney disease may develop following acute kidney injury and that this may be due, in part, to hypoxia-related phenomena. Hypoxia-inducible factor (HIF) is stabilized in hypoxic conditions and regulates multiple signaling pathways that could contribute to renal fibrosis. As transforming growth factor (TGF)-beta is known to mediate renal fibrosis, we proposed a profibrotic role for cross talk between the TGF-beta 1 and HIF-1 alpha signaling pathways in kidney cells. Hypoxic incubation increased HIF-1 alpha protein expression in cultured human renal tubular epithelial cells and mouse embryonic fibroblasts. TGF-beta 1 treatment further increased HIF-1 alpha expression in cells treated with hypoxia and also increased HIF-1 alpha in normoxic conditions. TGF-beta 1 did not increase HIF-1 alpha mRNA levels nor decrease the rate of protein degradation, suggesting that it enhances normoxic HIF-1 alpha translation. TGF-beta receptor (ALK5) kinase activity was required for increased HIF-1 alpha expression in response to TGF-beta 1, but not to hypoxia. A dominant negative Smad3 decreased the TGF-beta-stimulated reporter activity of a HIF-1 alpha-sensitive hypoxia response element. Conversely, a dominant negative HIF-1 alpha construct decreased Smad-binding element promoter activity in response to TGF-beta. Finally, blocking HIF-1 alpha transcription with a biochemical inhibitor, a dominant negative construct, or gene-specific knockdown decreased basal and TGF beta 1-stimulated type I collagen expression, while HIF-1 alpha overexpression increased both. Taken together, our data demonstrate cooperation in signaling between Smad3 and HIF-1 alpha and suggest a new paradigm in which HIF-1 alpha is necessary for normoxic, TGF-beta 1-stimulated renal cell fibrogenesis.