Abstract
Loss of the VHL tumor suppressor is regarded as an initiating event in the development of clear-cell renal carcinoma. Surprisingly, loss of VHL induces senescence in mouse fibroblasts in vitro, a response that would restrict development of renal carcinoma in vivo. Typical in vitro cell culture levels of oxygen, however, are significantly higher than physiological levels of oxygen, which have been shown to abrogate senescence induced by many stimuli. Therefore, we investigated the oxygen dependence of VHL loss-induced senescence. Using mouse fibroblasts and primary renal epithelial cells in vitro, we found that VHL loss leads to senescence under atmospheric conditions (21% O(2)), partly through increasing p27 levels, but not under physiological oxygenation (2% to 5% O(2)), despite maintaining increased p27 expression. This suggests that VHL inactivation sensitizes cells to oxidative stress. In support of this concept, senescence following VHL loss depends on p53 activity, which decreases under the less stressful conditions of mild hypoxia. We confirmed these observations in vivo by treating kidney-specific VHL knockout animals with the potent oxidizer paraquat and observed a robust induction of cellular senescence. Together, these data demonstrate that in vivo oxygenation promotes tolerance of VHL loss in renal epithelia, which may promote the development of renal carcinoma.