POSTTRANSCRIPTIONAL REGULATION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR BY HYPOXIA

The major control point for the hypoxic induction of the vascular endo thelial growth factor (VEGF) gene is the regulation of the steady-stat e level of the mRNA. We previously demonstrated a discrepancy between the transcription rate and the steady-state mRNA level induced by hypo xia. This led us to examine the post-transcriptional regulation of VEG F expression. Actinomycin D experiments revealed that hypoxia increase d VEGF mRNA half-life from 43 +/- 6 min to 106 +/- 9 min. Using an in vitro mRNA degradation assay, the half-life of VEGF mRNA 3'-untranslat ed region (UTR) transcripts were also found to be increased when incub ated with hypoxic versus normoxic extracts. Both cis-regulatory elemen ts involved in VEGF mRNA degradation under normoxic conditions and in increased stabilization under hypoxic conditions were mapped using thi s degradation assay. A hypoxia-induced protein(s) was found that bound to the sequences in the VEGF 3'-UTR which mediated increased stabilit y in the degradation assay. Furthermore, genistein, a tyrosine kinase inhibitor, blocked the hypoxia-induced stabilization of VEGF 3'-UTR tr anscripts and inhibited hypoxia-induced protein binding to the VEGF 3' -UTR. These findings demonstrate a significant post-transcriptional co mponent to the regulation of VEGF.