Hypoxia-mediated selection of cells with diminished apoptotic potential in solid tumours

APOPTOSIS is a genetically encoded programme of cell death that can be activated under physiological conditions1,2 and may be an important safeguard against tumour development3–6. Regions of low oxygen (hypoxia) and necrosis are common features of solid tumours7,8.Here we report that hypoxia induces apoptosis in oncogenically transformed cells and that further genetic alterations, such as loss of the p53 tumour-suppressor gene or over-expression of the apoptosis-inhibitor protein Bcl-2, substantially reduce hypoxia-induced cell death. Hypoxia also selects for cells with defects in apoptosis, because small numbers of transformed cells lacking p53 overtake similar cells expressing wild-type p53 when treated with hypoxia. Furthermore, highly apoptotic regions strongly correlate with hypoxic regions in transplanted tumours expressing wild-type p53, whereas little apoptosis occurs in hypoxic regions of p53-deficient tumours. We propose that hypoxia provides a physiological selective pressure in tumours for the expansion of variants that have lost their apoptotic potential, and in particular for cells acquiring p53mutations.

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