DNA damage is a prerequisite for p53-mediated proteasomal degradation of HIF-1α in hypoxic cells and downregulation of the hypoxia marker carbonic anhydrase IX

We investigated the relationship between the tumor suppressor p53 and the hypoxia-inducible factor-1 (HIF-1)-dependent expression of the hypoxia marker, carbonic anhydrase IX (CAIX). MCF-7 (wt p53) and Saos-2 (p53-null) cells displayed similar induction of CAIX expression and CA9 promoter activity under hypoxic conditions. Activation of p53 by the DNA damaging agent mitomycin C (MC) was accompanied by a potent repression of CAIX expression and the CA9 promoter in MCF-7 but not in Saos-2 cells. The activated p53 mediated increased proteasomal degradation of HIF-1alpha protein, resulting in considerably lower steady-state levels of HIF-1alpha protein in hypoxic MCF-7 cells but not in Saos-2 cells. Overexpression of HIF-1alpha relieved the MC-induced repression in MCF-7 cells, confirming regulation at the HIF-1alpha level. Similarly, CA9 promoter activity was downregulated by MC in HCT 116 p53(+/+) but not the isogenic p53(-/-) cells. Activated p53 decreased HIF-1alpha protein levels by accelerated proteasome-dependent degradation without affecting significantly HIF-1a transcription. In summary, our results demonstrate that the presence of wtp53 under hypoxic conditions has an insignificant effect on the stabilization of HIF-1alpha protein and HIF-1-dependent expression of CAIX. However, upon activation by DNA damage, wt p53 mediates an accelerated degradation of HIF-1alpha protein, resulting in reduced activation of CA9 transcription and, correspondingly, decreased levels of CAIX protein. A model outlining the quantitative relationship between p53, HIF-1alpha, and CAIX is presented.