Gata4 and Sp1 regulate expression of the erythropoietin receptor in cardiomyocytes

Experimental studies indicate significant cardioprotective effects of recombinant erythropoietin (Epo) by binding to the Epo receptor (EpoR) and by inducing various molecular mechanisms, including activation of Gata4, a transcription factor that induces anti-apoptotic genes. However, specific molecular mechanisms of EpoR regulation in cardiomyocytes are unknown. We identified a 774 bp regulatory domain in the EpoR 5' flanking region by reporter gene assays in murine HL-1 cardiomyocytes. The binding sites for Gata and Sp transcription factors both significantly contributed to EpoR promoter activity. DNA-binding studies (EMSA and ChIP assays) identified Gata4 and Sp1 as EpoR promoter-binding proteins in HL1 cardiomyocytes. Although Sp1 alone stimulates EpoR only slightly, forced expression of Gata4 significantly induced EpoR mRNA expression. In addition, knockdown of Gata4 (but also of Sp1) resulted in a significant decrease of EpoR transcript levels in HL-1 cardiomyocytes. Cumulative in vitro data suggest that function of the Sp1 site is essential for the Gata4-mediated transcription. In vivo, analysis of transgenic mice expressing an inducible small-hairpin RNA against Gata4 confirmed suppression of EpoR expression in the heart. Treating mice with high-dose doxorubicin not only resulted in Gata4 protein depletion, but also down-regulated EpoR, followed by up-regulation of EpoR transcripts when Gata4 levels recovered. In conclusion, we identified Gata4 as novel regulator of EpoR transcription in cardiomyocytes. In models of cardiac injury, down-regulation of Gata4 or Sp1 may limit the accessibility of the EpoR for binding of erythropoiesis-stimulating agents (ESA). Thereby our data underline the essential role of Gata4 in mediating cardioprotective effects.