Abstract

Erythropoietin (EPO) is the primary regulator of red blood cell development. Although hypoxic regulation of EPO has been extensively studied, the mechanism(s) for basal regulation of EPO are not well understood. In vivo studies in healthy human volunteers and animal models indicated that angiotensin II (Ang II) and angiotensin converting enzyme inhibitors regulated blood EPO levels. In the current study, we found that Ang II induced EPO expression in situ in murine kidney slices and in 786-O kidney cells in culture as determined by reverse transcription polymerase chain reaction. We further investigated the signaling mechanism of Ang II regulation of EPO in 786-O cells. Pharmacological inhibitors of Ang II type 1 receptor (AT₁R) and extracellular signal-regulated kinase 1/2 (ERK1/2) suppressed Ang II transcriptional activation of EPO. Inhibitors of AT₂R or Src homology 2 domain–containing tyrosine phosphatase had no effect. Coimmunoprecipiation experiments demonstrated that p21Ras was constitutively bound to the AT₁R; this association was increased by Ang II but was reduced by the AT₁R inhibitor telmisartan. Transmembrane domain (TM) 2 of AT₁R is important for G protein–dependent ERK1/2 activation, and mutant D74E in TM2 blocked Ang II activation of ERK1/2. Ang II signaling induced the nuclear translocation of the Egr-1 transcription factor, and overexpression of dominant-negative Egr-1 blocked EPO promoter activation by Ang II. These data identify a novel pathway for basal regulation of EPO via AT₁R-mediated Egr-1 activation by p21Ras-mitogen-activated protein kinase/ERK kinase-ERK1/2. Our current data suggest that Ang II, in addition to regulating blood volume and pressure, may be a master regulator of erythropoiesis.