Hypoxia-inducible factor-1alpha mRNA contains an internal ribosome entry site that allows efficient translation during normoxia and hypoxia

HIF-1alpha is the regulated subunit of the HIF-1 transcription factor, which induces transcription of a number of genes involved in the cellular response to hypoxia. The HIF-1alpha protein is rapidly degraded in cells supplied with adequate oxygen but is stabilized in hypoxic cells. Using polysome profile analysis, we found that translation of HIF-1alpha mRNA in NIH3T3 cells is spared the general reduction in translation rate that occurs during hypoxia. To assess whether the 5'UTR of the HIF-1alpha mRNA contains an internal ribosome entry site (IRES), we constructed a dicistronic reporter with the HIF-1alpha 5'UTR inserted between two reporter coding regions. We found that the HIF-1alpha 5'UTR promoted translation of the downstream reporter, indicating the presence of an IRES. The IRES had activity comparable to that of the well-characterized c-myc IRES. IRES activity was not affected by hypoxic conditions that caused a reduction in cap-dependent translation, and IRES activity was less affected by serum-starvation than was cap-dependent translation. These data indicate that the presence of an IRES in the HIF-1alpha 5'UTR allows translation to be maintained under conditions that are inhibitory to cap-dependent translation.