n sporadic RCC. In mouse tissue specific VHL knockout was not found to induce tumors, but generated cysts either alone or in conjunction with a PTEN knockout. Nutlin3 web HIF-2a Induces Fibrosis & Cysts Human clear cell RCCs typically show global oxygen independent activation of HIF-1a and HIF-2a. Stabilization of HIFa subunits in early lesions of human RCC may be a decisive step in renal tumorigenesis. Experimental studies have shown that HIF-2a, and not HIF-1a, appears to be the decisive subunit mediating tumorigenic features. Mechanistically this may be due to a cellular proliferative effect of HIF2a, whereas HIF-1a may have opposite effects. Nevertheless, overexpression of HIF-1a in murine proximal tubuli has recently been shown to lead to RCC. Constant activation of HIF by genetic inactivation of VHL in tubular epithelia has further shown to induce renal fibrosis, which may indicate a common pathway of epithelial dedifferentiation. In summary, HIF effects play an important role in the kidney, which can be beneficial or deleterious, depending on the setting and the timing. The specific roles of the different HIFa isoforms in this context are not PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22182733 well defined. Furthermore, there is no knowledge of the differential expression patterns of HIF-1a and HIF-2a in the human kidney. Experimental data originates mostly from VHL knockout studies in the mouse, where HIFa stabilization is an inevitable consequence. However, other HIF independent effects with oncogenic potential are known to be released when VHL is inactivated. We therefore aimed to clearly define the physiological expression patterns of HIFa subunits in normal kidney epithelial cells, as well as in response to experimental VHL knockout and human VHL disease. Furthermore, we aimed to study the tumorigenic potential of tubular HIF-2a overexpression in a novel mouse model. Results Distinct expression patterns of HIF-1a and HIF-2a in hypoxic mouse and human kidneys Identically to the predescribed situation in the rat, we see distinct expression patterns for the two HIFa subunits in mouse and man. Hypoxic human kidneys were investigated from victims of carbon monoxide intoxication, where the tissues are severely hypoxic due to the reduced oxygen transport capacity of the haemoglobin and in kidney tissue adjacent to RCCs, where hypoxic microenvironment is often caused by expansive tumor growth. In all cases investigated, hypoxia leads to HIF-1a accumulation exclusively in tubular epithelial cells, whereas HIF-2a is stabilized in interstitial cells and in glomeruli. Hypoxia does not enable HIF-2a expression in renal tubular cells, in any species. Inducible VHL knockout in mouse renal tubular cells enables HIF-2a expression We next studied HIF expression in an inducible murine VHL knockout model, which was implied by the Pax8 promoter and confers an inducible Cre-driven knockout in the complete tubular system of the kidney. In these animals VHL expression is lost after doxycycline treatment. Control animals without doxycycline showed no expression of either HIFa subunit. In contrast to the physiological HIF expression, the knockout mice showed tubular expression of HIF2a in the tubules upon a 3-day treatment with 2 HIF-2a Induces Fibrosis & Cysts doxycyclin. Thus, it appears that VHL represses renal tubular HIF-2a specifically. Biallelic inactivation of VHL releases HIF-2a expression in 
distinct early lesions of the distal tubule in kidneys of the human VHL disease cadherin. In the light of the mou