HIF-1α is a key molecule for the progression of EMT-mediated renal fibrosis

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Epithelial-mesenchymal transition(EMT) plays a central role in mediating tubulointerstitial fibrosis. In human biopsy samples, tubular epithelial cells(TECs) surrounding the fibrotic area expressed fibroblast specific protein 1(FSP1), although TECs positive for α smooth muscle actin were very rare. Therefore, it is essential to examine FSP1 expression in TECs for the detection of EMT in renal fibrosis. Previous studies have implicated that chronic hypoxia is the main contributor to the development and progression of tubulointerstitial fibrosis. In this study, we examined whether hypoxia could be a factor mediating EMT in vitro and in vivo. We established the method to detect EMT in vitro, using primary culture of TECs genetically marked by Cre-loxP system. TECs started to elongate after hypoxic exposure and became fibroblast-like cells morphologically at 5 days after hypoxia. Immunocytochemistry revealed that more than 40% of TECs expressed FSP1 at 5 days after hypoxia. The chief mediator of hypoxic response is hypoxia-inducible factor 1(HIF-1), and its oxygen-sensitive component HIF-1α which is promptly degraded by a kind of ubiquitin ligase, the von Hippel-Lindau tumor suppressor (VHL) in normoxia. To study more directly whether HIF-1α functions in renal fibrosis in vivo, we generated a mouse line with epithelium-targeted expression of VHL gene (constitutively active HIF-1), using Cre-loxP-mediated recombination. Tubulointerstitial fibrosis was induced by unilateral ureteral obstruction(UUO). Both obstructed and unobstructed kidneys were harvested for immunohistochemistry 8 days after UUO. FSP1 positive cell numbers increased significantly in unobstructed kidneys of VHL mutant mice compared with wild type mice. These results strongly suggest that HIF-1α activates tissue fibroblasts in renal fibrosis via the induction of epithelial-mesenchymal transition(EMT).