Contribution of HIF-1 alpha or HIF-2 alpha to erythropoietin expression: in vivo evidence based on chromatin immunoprecipitation

Circulating erythropoietin (EPO) is mainly produced by the kidneys and mediates erythrogenesis in bone marrow and nonhematopoietic cell survival. EPO is also produced in other tissues where it functions as a paracrine. Moreover, the hypoxic induction of EPO is known to be mediated by HIF-1 alpha and HIF-2 alpha, but it remains obscure as to which of these two mediators mainly contributes to EPO expression. Thus, we designed in vivo experiments to evaluate the contributions made by HIF-1 alpha and HIF-2 alpha to EPO expression. In mice exposed to mild whole body hypoxia, HIF-1 alpha and HIF-2 alpha were both induced in all tissues examined. However, EPO mRNA was expressed in kidney and brain, but not in liver and lung. Likewise, chromatin immunoprecipitation (CHIP) analyses demonstrated that HIF-1 alpha or HIF-2 alpha binding to the EPO gene increased under hypoxic conditions only in kidney and brain. A comparison of CHIP data and EPO mRNA levels suggested that, during mild hypoxia, renal EPO transcription is induced equally by HIF-1 alpha and HIF-2 alpha, but that brain EPO is mainly induced during hypoxia by HIF-2 alpha. Thus, HIF-1 alpha and HIF-2 alpha appear to contribute to EPO expression tissue specifically.