Phorbol ester stimulates the nonhypoxic induction of a novel hypoxia-inducible factor 1alpha isoform: implications for tumor promotion.

Hypoxia-inducible factor-1 (HIF-1), which is present at higher levels in human tumors, plays important roles in tumor promotion. It is composed of HIF-1alpha and HIF-1beta subunits and its activity depends on the amount of HIF-1alpha, which is tightly controlled by cellular oxygen tension. In addition to hypoxia, various nonhypoxic stimuli can stabilize HIF-1alpha in tumor cells, implying that both hypoxic and nonhypoxic stimuli contribute to the overexpression of HIF-1alpha in tumors. On the other hand, phorbol esters such as phorbol-12-myristate-13-acetate (PMA) are known to be potent tumor promoters. Here, we identified a novel HIF-1alpha isoform, which is regulated primarily by PMA. The variant mRNA lacks exon 11 and produces a 785-amino acid isoform (HIF-1alpha(785)) without altering the reading frame and therefore the COOH-terminal transcriptional activity. HIF-1alpha(785) is induced markedly by PMA and heat shock, the latter of which is also known to induce HIF-1alpha. HIF-1alpha(785) escapes from lysine acetylation because of the loss of Lys(532) and was stabilized under normoxic conditions. Its expression was blocked by reducing agents and by a mitogen-activated protein/extracellular signal-regulated kinase-1 inhibitor and enhanced by hydrogen peroxide. In addition, HIF-1alpha(785) overexpression strikingly enhanced tumor growth in vivo. These results suggest that HIF-1alpha(785) is induced by PMA under normoxic conditions via a redox-dependent mitogen-activated protein/extracellular signal-regulated kinase-1 pathway and that it plays an important role in tumor promotion.