Impaired expression of HIF‐2α induces compensatory expression of HIF‐1α for the recovery from anemia

Abstract

Erythropoiesis is strongly influenced by the interactions between stromal cells and erythroid progenitors, as well as by a key regulatory factor, erythropoietin (EPO). We previously generated mice with a knockdown mutation of Hif‐2α (referred to as kd/kd) and found that these kd/kd mice exhibited normocytic anemia, even though the EPO expression was not severely affected. However, the VCAM‐1 expression in spleen endothelial cells (EC), which is regulated by HIF‐2α, was impaired, resulting in defective erythroid maturation. A deficiency of HIF‐2α clearly led to pancytopenia. However, the critical level of HIF‐2α required for erythropoiesis has not yet been elucidated. In this study, we generated HIF‐2α knockdown/knockout heterozygous mice (kd/null). Strikingly, anemia was observed in the kd/null mice, but the red blood cell indices were significantly improved compared to those of kd/kd mice. In the spleens of kd/null mice, higher HIF‐1α activity and expansion of the red pulp area were observed compared to those of kd/kd mice. Importantly, EC isolated from kd/null spleens showed high expression of VEGF receptors, FLK‐1 and FLT‐1, which are regulated by HIF‐1α instead of HIF‐2α under hypoxic conditions. We also found higher expression of phosphorylated ERK and higher proliferative activity in the EC isolated from kd/null mice compared to those from kd/kd mice. While the HIF‐2α expression was diminished, HIF‐1α bound to the HRE region in the promoters of genes that are normally regulated by HIF‐2α. These results suggest that there is a compensatory pathway involving HIF‐1α that regulates the expression of some HIF‐2α target genes. J. Cell. Physiol. 230: 1534–1548, 2015. © 2015 Wiley Periodicals, Inc., A Wiley Company