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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)-β is known to mediate renal fibrosis, we proposed a profibrotic role for cross talk between the TGF-β1 and HIF-1α signaling pathways in kidney cells. Hypoxic incubation increased HIF-1α protein expression in cultured human renal tubular epithelial cells and mouse embryonic fibroblasts. TGF-β1 treatment further increased HIF-1α expression in cells treated with hypoxia and also increased HIF-1α in normoxic conditions. TGF-β1 did not increase HIF-1α mRNA levels nor decrease the rate of protein degradation, suggesting that it enhances normoxic HIF-1α translation. TGF-β receptor (ALK5) kinase activity was required for increased HIF-1α expression in response to TGF-β1, but not to hypoxia. A dominant negative Smad3 decreased the TGF-β-stimulated reporter activity of a HIF-1α-sensitive hypoxia response element. Conversely, a dominant negative HIF-1α construct decreased Smad-binding element promoter activity in response to TGF-β. Finally, blocking HIF-1α transcription with a biochemical inhibitor, a dominant negative construct, or gene-specific knockdown decreased basal and TGF-β1-stimulated type I collagen expression, while HIF-1α overexpression increased both. Taken together, our data demonstrate cooperation in signaling between Smad3 and HIF-1α and suggest a new paradigm in which HIF-1α is necessary for normoxic, TGF-β1-stimulated renal cell fibrogenesis.

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