Erythropoietin (EPO) expression is controlled by tissue specific activation of the transcription factor hypoxia inducible factor (HIF) in liver / kidney and neuronal cells. HIF is a heterodimer with an O2-labile a-subunit and a constitutive nuclear ß-subunit. Under hypoxia HIF-a accumulates and translocates into the nucleus. After dimerisation with the HIF-ß subunit further tissue-specific cofactors enhance EPO expression.

Here, we studied the effect of Retinoic acid (RA) on hypoxia-inducible EPO gene expression in the human neuroblastoma cell line SH-SY5Y and Kelly. We found that RA significantly enhanced EPO gene expression under hypoxia in a cell specific manner. Immunoblot analysis revealed that RA increased HIF-1a and HIF-2a protein levels in SH-SY5Y, but not in Kelly cells. HIF-1 DNA binding (EMSA) and transcriptional activity of HIF in the luciferase assays was increased in response to RA stimulation in SH-SY5Y. Knock down of HIF-1a or HIF-2a prevented RA induced EPO expression under hypoxia. Mutational analysis of the 3' EPO enhancer demonstrated that RA induced amplification of EPO gene expression in hypoxia required both an intact HIF binding site (HBS) and DR2 site.