TRANSLATION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR MESSENGER-RNA BY INTERNAL RIBOSOME ENTRY - IMPLICATIONS FOR TRANSLATION UNDER HYPOXIA

Vascular endothelial growth factor (VEGF) is a hypoxia inducible angio genic growth factor that promotes compensatory angiogenesis in circums tances of oxygen shortage. The requirement for translational regulatio n of VEGF is imposed by the cumbersome structure of the 5' untranslate d region (5'UTR), which is incompatible with efficient translation by ribosomal scanning, and by the physiologic requirement for maximal VEG F production under conditions of hypoxia, where overall protein synthe sis is compromised. Using bicistronic reporter gene constructs, we sho w that the 1,014-bp 5'UTR of VEGF contains a functional internal ribos ome entry site (IRES), Efficient cap-independent translation is mainta ined under hypoxia, thereby securing efficient production of VEGF even under unfavorable stress conditions. To identify sequences within the 5'UTR required for maximal IRES activity, deletion mutants were analy zed. Elimination of the majority (851 nucleotides) of internal 5'UTR s equences not only maintained full IRES activity but also generated a s ignificantly more potent IRES, Activity of the 163-bp long ''improved' ' IRES element was abrogated, however, following substitution of a few bases near the 5' terminus as well as substitutions close to the tran slation start codon, Both the full-length 5'UTR and its truncated vers ion function as translational enhancers in the context of a monocistro nic mRNA.