Implication of HIF-1α but Not HIF-2α in the Hypoxic Response of Human Hematopoietic Progenitors.

Maturing hematopoietic cells are exposed to hypoxia as they develop and migrate within the bone marrow microenvironment. Previous studies using non hematopoietic cell lines and monocytes showed that CXCR4 is strongly induced by hypoxia but little is known on the regulation of CXCR4 by hypoxia in the other hematopoietic cells and during hematopoietic development. We analyzed the expression and regulation of hypoxia-inducible transcription factor-1a (HIF1a) and 2a (HIF2a), the master regulators of metabolic adaptation to hypoxia, during hematopoiesis. Real time quantitative RT-PCR showed that HIF-1a mRNA was present on all the non hematopoietic and hematopoietic cells lines including HL-60, HEL, TF1, K562, KG1, U937, Jurkat and Mo7e. In contrast, HIF-2a mRNA expression was variable among the cell lines and was detected only at very low level in some cells such as KG1, Jurkat and HEL. Hypoxia exposure rapidly induced VEGF mRNA expression in the cells that expressed HIF-1a mRNA and exhibited HIF-1a protein accumulation. Interestingly, CXCR4 induction was observed only in the cells that exhibit significant expression of HIF-2a mRNA and HIF-2a protein accumulation. A strong correlation between HIF-2a mRNA levels and the induction of CXCR4 mRNA expression by hypoxia was found. Human CD34+ cells also expressed high levels of HIF-1a mRNA, whereas HIF-2a mRNA was barely detected. Interestingly, as observed for several myeloid cell lines, CD34+ cells exhibited a strong induction of VEGF expression in response to hypoxia and hypoxia mimetic agents cobalt chloride and desferrrioxamine whereas CXCR4 receptor expression was not induced suggesting that CXCR4 mRNA induction is related to the expression of HIF-2a. Altogether these data indicated that the hypoxic responses of human hematopoietic progenitors are independent of HIF-2a. Moreover, they establish that CXCR4 regulation by hypoxia is linked to HIF-2a protein expression.