Modulation of TRAIL-induced tumor cell apoptosis in a hypoxic environment

Hypoxia induces Hif-1 alpha and selects for loss of wild-type p53 function, both of which can promote tumor cell survival. We evaluated the ability of TRAIL to induce apoptosis of human tumor cell lines exposed to hypoxia. H460 lung cancer cells express low levels of Hif-1 alpha, stabilize wild-type p53 during hypoxia, and undergo TRAIL-induced apoptosis. In U2OS osteosarcoma or PA1 ovarian teratocarcinoma cells, high levels of Hif-1 alpha and low levels of stable p53 are detected during hypoxia, and cells undergo low levels of TRAIL-induced apoptosis as compared to H460 cells. H460 cells are sensitized to TRAIL- induced apoptosis, whereas U2OS are protected, and little apoptosis is observed in relatively TRAIL-resistant PA1 during hypoxia. Forced expression of Hif-1 alpha is also surprisingly a potent inducer of apoptosis in wild-type p53 expressing H460 cells and further promotes TRAIL-induced apoptosis. TRAIL-sensitive wild-type p53-expressing HCT116 colon carcinoma cells modestly elevate Hif-1 alpha levels and are equally or slightly more sensitive to TRAIL during hypoxia. In contrast, p53-null HCT116 have higher levels of Hif-1 alpha during normoxia and are extremely sensitive to TRAIL, but are protected from TRAIL-induced apoptosis during hypoxia. We hypothesize that a hypoxic tumor microenvironment may alter sensitivity to TRAIL, which may be impacted by Hif-1 alpha levels and p53 status. These findings suggest that particular attention to hypoxic regions of tumors and sensitizers to hypoxia-induced cell death may be required to optimize therapeutic combinations using TRAIL.