Re-oxygenation causes hypoxic tumor regression through restoration of p53 wild-type conformation and post-translational modifications.

Hypoxic tumors are resistant to conventional therapies through indirect mechanisms such as the selection of resistant phenotype under chronic hypoxia. Hyperbaric oxygen (HBO) therapy has been shown to increase oxygen level and induce apoptosis in hypoxic tumor. However, it could produce significant adverse effects including oxygen toxic seizures and severe radiation tissue injury due to high pressure. We have shown that repeated oxygenation at 30% O(2) (1 atmospheres absolute) results in significant regression of MCF-7 tumor xenografts without any adverse effect. In MCF-7 cells, re-oxygenation showed an eightfold increase in cellular apoptosis. Both in hypoxic tumor and in hypoxic cells, that exclusively favor p53 to exist in mutant conformation, re-oxygenation restores p53 wild-type conformation. The oxygen-mediated rescue of mutant p53 followed by its trans-activation is responsible for the induction of p53-downstream apoptotic, cell-cycle arrest and DNA-repair genes. Further, p53 trans-activation may thus be due to its post-translational modifications as a result of re-oxygenation. We have thus concluded that oxygen therapy without pressure, as opposed to HBO therapy, may be ideal for hypoxic tumor regression, which functions through oxygen-mediated rescue of mutant p53 followed by induction of apoptosis.