Hypoxia Signaling Cascade for Erythropoietin Production in Hepatocytes

Erythropoietin (Epo) is produced in the kidney and liver in a hypoxia-inducible manner via the activation of hypoxia-inducible transcription factors (HIFs) to maintain oxygen homeostasis. Accelerating Epo production in hepatocytes is one plausible therapeutic strategy for treating anemia caused by kidney diseases. To elucidate the regulatory mechanisms of hepatic Epo production, we analyzed mouse lines harboring liver-specific deletions of genes encoding HIF-prolyl-hydroxylase isoforms (PHD1, PHD2, and PHD3) that mediate the inactivation of HIF1 alpha and HIF2 alpha under normal oxygen conditions. The loss of all PHD isoforms results in both polycythemia, which is caused by Epo overproduction, and fatty livers. We found that deleting any combination of two PHD isoforms induces polycythemia without steatosis complications, whereas the deletion of a single isoform induces no apparent phenotype. Polycythemia is prevented by the loss of either HIF2 alpha or the hepatocyte-specific Epo gene enhancer (EpoHE). Chromatin analyses show that the histones around EpoHE dissociate from the nucleosome structure after HIF2 alpha activation. HIF2 alpha also induces the expression of HIF3 alpha, which is involved in the attenuation of Epo production. These results demonstrate that the total amount of PHD activity is more important than the specific function of each isoform for hepatic Epo expression regulated by a PHD-HIF2 alpha-EpoHE cascade in vivo.