Relationship of hypoxia-inducible factor (HIF)-1 alpha and HIF-2 alpha expression to vascular endothelial growth factor induction and hypoxia survival in human breast cancer cell lines

Hypoxia-inducible factors (HLF-1 and HIF-2) are two closely related protein complexes that activate transcription of target genes in response to hypox ia. Expression of HIF-1 alpha and HIF-2 alpha and their effects on survival under hypoxia were studied in six human breast cancer cell lines. me also evaluated the basal and inducible expression of two hypoxically regulated g enes, vascular endothelial growth factor (VEGF) and lactate dehydrogenase-A (LDH-A), All of the cell lines studied expressed HIF-1 alpha at various le vels, but HIF-2 alpha was low or absent from the more aggressive cell lines , There was an inverse correlation between HIF-1 alpha and HIF-2 alpha indu ction and clonogenic survival under hypoxia. Thus, cell lines with reduced induction of HIF-1 alpha or HIF-2 alpha showed high basal levels of VEGF an d improved survival under hypoxia, A reduction in HIF expression was also a ssociated with a more aggressive phenotype in vivo, To confirm these result s, we carried out stable transfection of the MDA 435 cell line with human H IF-2 alpha cDNA, There was no change in the growth rate in monolayer cultur e. However, in vitro growth as colonies and irt vivo tumor growth of the HI F-2 alpha overexpressing cells were significantly impaired Compared with th e control transfectants. Thus, despite the fact that HIF proteins are neces sary for optimal tumor growth and angiogenesis in vivo, overexpression of t hese molecules seems detrimental to tumor growth. A balance between the ang iogenic and tumor-inhibiting levels of HIF proteins may, therefore, be nece ssary for optimal tumor growth.