The role of p16INK4a in senescence induction is well documented though data from these Silmitasertib PKC inhibitor studies were produced in normoxic conditions, as well. We show here that p16INK4a protein expression is down regulated in HDFs under hypoxia, independent of HIF-1a and its target MIF. A, previous report showed that the expression of p16INK4a was down regulated under hypoxia/anoxia, which was dependent on constitutive activation of PI3K/Akt and phosphorylation of GSK3b in cancer cells. Consistent with these reports we propose here that other transcription factors normally activated in hypoxia may be also involved in hypoxia-dependent regulation of p16INK4a. In addition the severity of hypoxic condition or cell type may also affect the hypoxia dependent modulation of p16INK4a expression. Our knowledge of p16INK4a and its regulation under hypoxic environment is currently limited and further investigations are underway to elucidate the possible mechanisms. According to recent studies, cells cultured under hypoxic conditions may acquire ability to prevent senescence through HIF-1a’s central role and loss of HIF-1a in hypoxia or even in normoxia restores the cell’s ability to reinstate senescence. Interestingly, in HDFs knock down of HIF-1a did not reinstate HRasV12 induced senescence but instead induced cell death under hypoxic conditions. Previous reports indicate that regulation of cellular senescence is different between human and mouse cells, suggesting that the results obtained in a mouse model may not be necessarily valid for human cells. On the other hand oncogenic Ras may exert both proapoptotic and anti-apoptotic effects depending on the eminence of Ras effector pathway and the apoptotic machinery. In different studies, it has been reported that oncogenic Ras signaling via RAF pathway may generate apoptotic response mediated by p53. Thus, according to our data we suggest that the reinstatement of H-RasV12 induced senescence in human diploid fibroblasts under hypoxic environment might depend on restored expression of p16INK4a. Further, we cannot rule out the possibility that increased expression of Ras and p53, but lack of HIF-1a, which exerts antiapoptotic effects in hypoxia, may favor the induction of apoptosis instead of senescence in HDFs. Recent studies have shown the involvement of DNA damage signaling through ATM/ATR kinases as a crucial mediator of oncogene induced senescence. However, studies reporting preventive role for hypoxia on induction of senescence did not considerably elucidate regulation of DNA damage response under hypoxic conditions or whether it is involved in hypoxia dependent suppression of senescence. In a recent report, hypoxia did not decrease levels of DDR and cell cycle arrest caused by etoposide in immortalized human fibroblasts. On the other hand, extremely low levels of hypoxia have been found to induce DDR, involving both ATR- and ATM-mediated signaling. Consequently hypoxiainduced DDR triggers p53-dependent apoptosis. Our data suggest moderate down regulation of DDR under hypoxic conditions in H-RasV12 expressing HDFs, to best of our knowledge this is the first comprehensive data elucidating the effects of hypoxia on OIS and DDR together.