The roles of two different pathways in hypoxia: p53/HDM2 and PERK/GCN2/eIF2alpha

Hypoxia always occurs in solid tumors. These hypoxic tumor cells are not sensitive to chemotherapic agents because of poor drug delivery and slow proliferation. Hypoxia activates two adaptive signaling pathways. On one hand, adaptation to hypoxia can be regulated by hypoxia-inducible factor 1 (HIF-1) and its downstream genes. On the other hand, hypoxia reduces protein synthesis and inhibits cell growth to adapt this stress. HIF-1alpha plays a crucial role in tumor hypoxia and therapeutic resistance and its protein level is under tight control. Previous studies show that p53 suppresses HIF-1alpha protein through HDM2-mediated ubiquitination and proteasomal degradation. However, the forced expression of HDM2 or growth factor-induced HDM2 can increase HIF-1alpha protein level, making it difficult to decipher how p53 and HDM2 regulate HIF-1alpha in hypoxia. In the first part of this study, we found that the increased p53 in hypoxia contributed to the downregulation of HIF-1alpha mRNA to suppress HIF-1alpha protein level. In addition, HIF-1alpha protein level was also inhibited by decreasing HDM2 protein level in hypoxia. Furthermore, p53 and HDM2 knockout MEF cells were employed to determine the biological functions of p53 and HDM2 in hypoxia through modulating HIF-1alpha protein level. We showed that the presence of p53 inhibited hypoxia-induced cell growth arrest and cell cycle arrest through the suppression of HIF-1alpha and its downstream target, p21; loss of HDM2 exhibited similar effects through the same mechanism; p53 strengthens chemotherapeutic-induced apoptosis in hypoxia via downregulating HIF-1alpha, loss of HDM2 displayed similar effects via the same mechanism. Recent studies suggest that activation of PERK and phosphorylation of alpha subunit of eIF2 (eIF2alpha) confer cell adaptation to hypoxic stress. However, eIF2a is still phosphorylated at a lowered level in PERK knockout cells under hypoxic conditions. The mechanism for eIF2alpha kinase(s)-increased cell survival is not clear. In the second part of this study, we investigated the roles of GCN2-mediated eIF2alpha phosphorylation in hypoxia. Here we provided evidence that another eIF2alpha kinase, GCN2, was also involved in hypoxia-induced eIF2alpha phosphorylation. We demonstrated that both GCN2 and PERK mediated the adaptation to hypoxic stress. High levels of eIF2alpha phosphorylation led to G1 arrest and protected cells from hypoxia-induced apoptosis. Reduced phosphorylation of eIF2alpha by knocking out either PERK or GCN2 suppressed hypoxia-induced G1 arrest and promoted apoptosis via activation of p53 signal cascade. However, totally abolishing phosphorylation of eIF2alpha inhibited G1 arrest without promoting apoptosis. In addition, reduced, but not abolished, phosphorylation of eIF2alpha sensitized cells to chemotherapeutics, but not to gamma-radiation in hypoxia. Based on our results, we propose that the levels of eIF2alpha phosphorylation serve as a "switch" in regulation of G1 arrest or apoptosis under hypoxic conditions.