Impaired Expression Of Hif-2 Alpha Induces Compensatory Expression Of Hif-1 Alpha For The Recovery From Anemia

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-2 (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-2, was impaired, resulting in defective erythroid maturation. A deficiency of HIF-2 clearly led to pancytopenia. However, the critical level of HIF-2 required for erythropoiesis has not yet been elucidated. In this study, we generated HIF-2 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-1 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-1 instead of HIF-2 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-2 expression was diminished, HIF-1 bound to the HRE region in the promoters of genes that are normally regulated by HIF-2. These results suggest that there is a compensatory pathway involving HIF-1 that regulates the expression of some HIF-2 target genes. J. Cell. Physiol. 230: 1534-1548, 2015. (c) 2015 Wiley Periodicals, Inc., A Wiley Company