HER2 (neu) signaling increases the rate of hypoxia-inducible factor 1 alpha (HIF-1 alpha) synthesis: Novel mechanism for HIF-1-mediated vascular endothelial growth factor expression

Hypoxia inducible factor 1 (HIF-1) is a transcriptional activator composed of HIF-1 alpha and HIF-1 beta subunits. Several dozen HIF-1 targets are kno wn, including the gene encoding vascular endothelial growth factor (VEGF). Under hypoxic conditions, HIF-1 alpha. expression increases as a result of decreased ubiquitination and degradation. The tumor suppressors VHL (von Hi ppel-Lindau protein) and p53 target HIF-1 alpha for ubiquitination such tha t their inactivation in tumor cells increases the half-life of HIF-1 alpha. Increased phosphatidylinositol 3-kinase (PI3K) and AKT or decreased PTEN a ctivity in prostate cancer cells also increases HIF-1 alpha expression by a n undefined mechanism. In breast cancer, increased activity of the HER2 (al so known as neu) receptor tyrosine kinase is associated with increased tumo r grade, chemotherapy resistance, and decreased patient survival. HER2 has also been implicated as an inducer of VEGF expression. Here we demonstrate that HER2 signaling induced by overexpression in mouse 3T3 cells or heregul in stimulation of human MCF-7 breast cancer cells results in increased HIF- 1 alpha protein and VEGF mRNA expression that is dependent upon activity of PI3K, AKT (also known as protein kinase B), and the downstream kinase FRAP (FKBP-rapamycin-associated protein). In contrast to other inducers of HIF- 1 expression, heregulin stimulation does not affect the half-life of HIF-1 alpha. but instead stimulates HIF-1 alpha synthesis in a rapamycin-dependen t manner. The 5'-untranslated region of HIF-1 alpha mRNA directs heregulin- inducible expression of a heterologous protein. These data provide a molecu lar basis for VEGF induction and tumor angiogenesis by heregulin-HER2 signa ling and establish a novel mechanism for the regulation of HIF-1 alpha expr ession.