Hypoxia-induced p21waf and p27 regulation in c-myc immortalized cells
21125 Background: Hypoxia is an adaptative response to tissue damage whose mechanisms have not been yet characterized. From a clinical viewpoint, tumours with more necrosis, an indicator of severe hypoxic stress, are often resistant to conventional treatment. Moreover, numerous tumours overexpressed c-myc which strongly enforces proliferation in oxygen deprived conditions. Evidences showed that c-myc blocked cyclin-dependent kinase inhibitors (CDKI) through a complex network of interactions inducing proliferation and G0-S transition. We examined whether Rat-1 immortalized fibroblasts underwent cell cycle arrest p21waf- and p27- mediated in response to O2-deprivation. Materials and Methods: c-myc null cells (-/-) were derived by gene targeting from an immortalized but otherwise non- transformed Rat-1 fibroblast cell line. Oxygen deprivation conditions (0.2% and 1% O2) were achieved in a humidified anaerobic workstation at 37 °C for 6 or 24 hours. Protein concentration was determined by the Lawry assay method. Immunoblots for c-myc and p21waf and p27 were performed. Results: c-myc (+/+) samples collected after 6 hours of hypoxia, which were supposed to be slowly proliferating, showed lower p21waf and p27 levels than c-myc (-/-). Over 24 hours of hypoxia, cell cultures were supposed to be oxygen-dependent and we found higher CDKI levels in c-myc (+/+) cells. Discussion: Our data could explain same resistance mechanisms of tumours cells. In fact, 6 hours O2-deprived-cells, which are unable to supply autonomous O2 support (neo-angiogenesis, in vivo), downregulated p21waf and p27 expression to overcome CDKI cell-cycle arrest. When the cells acquired O2-dependent growth mechanism, low O2 level induced c-myc mediated p21waf and p27 up-regulation to induce cell cycle arrest. Despite the up-regulation of CDKI, PS 341 (a proteasome inhibitors) has been demonstrated to induce apoptosis. Our data kindly encouraging trials on the use of p21waf and p27 stabilizers (like PS 341) in those tumours which overexpressed c-myc. No significant financial relationships to disclose.