| As tumors grow and outstrip the ability of the existing vasculature to supply nutrients and oxygen, they become hypoxic. For a tumor to grow beyond a microscopic mass, the cells must adapt to the hypoxic environment. HIF1 α is the primary transcription factor that is induced by hypoxia and induces genes that allow the tumor to adapt to the hypoxic environment. Hypoxia also inhibits global translation, which not only conserves energy but also alters protein expression in the cell. Work in the Johannes lab has shown that certain mRNAs such as HIF1α, NDRG1 and HIF2α continue to associate with polysomes and be translated when global translation is inhibited by prolonged hypoxic exposure. The data presented here shows that both glucose and hypoxia are required for translational repression and that this inhibition is mediated primarily by eIF2α phosphorylation. Data from dicistronic vector assays suggest that the 5' UTRs of HIF1α, NDRG1 and HIF2α contain IRES elements that allow them to continue to translate under hypoxic conditions. However, stringent RNA analysis using siRNA, dicistronic vectors and direct RNA transfections conclusively shows that alternate monocistronic mRNAs are derived from the dicistronic vectors. These aberrant transcripts are responsible for the IRES activity measured in the dicistronic assays. Together this data strongly indicates that IRES mediated translation is not the mechanism by HIF1α, HIF2α and NDRG1 continue to be translated under conditions of hypoxic stress. To further investigate the mechanism involved in the translational inhibition under conditions of glucose and oxygen depletion, the phosphorylation of eIF4E was examined. The results show that eIF4E is phosphorylated by MNK2 under hypoxic conditions. We believe that this is the first report that shows a unique function of MNK2. The role of eIF4E phosphorylation on the regulation of translation under hypoxia will be the subject of future investigations. |
