Renal Tubular HIF-2α Expression Requires VHL Inactivation and Causes Fibrosis and Cysts

The Hypoxia-inducible transcription Factor (HIF) represents an important adaptive mechanism under hypoxia, whereas sustained activation may also have deleterious effects. HIF activity is determined by the oxygen regulated alpha-subunits HIF-1 alpha or HIF-2 alpha. Both are regulated by oxygen dependent degradation, which is controlled by the tumor suppressor "von Hippel-Lindau" (VHL), the gatekeeper of renal tubular growth control. HIF appears to play a particular role for the kidney, where renal EPO production, organ preservation from ischemia-reperfusion injury and renal tumorigenesis are prominent examples. Whereas HIF-1 alpha is inducible in physiological renal mouse, rat and human tubular epithelia, HIF-2 alpha is never detected in these cells, in any species. In contrast, distinct early lesions of biallelic VHL inactivation in kidneys of the hereditary VHL syndrome show strong HIF-2 alpha expression. Furthermore, knockout of VHL in the mouse tubular apparatus enables HIF-2 alpha expression. Continuous transgenic expression of HIF-2 alpha by the Ksp-Cadherin promotor leads to renal fibrosis and insufficiency, next to multiple renal cysts. In conclusion, VHL appears to specifically repress HIF-2 alpha in renal epithelia. Unphysiological expression of HIF-2 alpha in tubular epithelia has deleterious effects. Our data are compatible with dedifferentiation of renal epithelial cells by sustained HIF-2 alpha expression. However, HIF-2 alpha overexpression alone is insufficient to induce tumors. Thus, our data bear implications for renal tumorigenesis, epithelial differentiation and renal repair mechanisms.