Transcriptional regulation mechanisms of hypoxia-induced neuroglobin gene expression

Ngb (neuroglobin) has been identified as a novel endogenous neuroprotectant. However, little is known about the regulatory mechanisms of Ngb expression, especially under conditions of hypoxia. In the present study, we located the core proximal promoter of the mouse Ngb gene to a 554 bp segment, which harbours putative conserved NF-κB (nuclear factor κB)- and Egr1 (early growth-response factor 1) -binding sites. Overexpression and knockdown of transcription factors p65, p50, Egr1 or Sp1 (specificity protein 1) increased and decreased Ngb expression respectively. Experimental assessments with transfections of mutational Ngb gene promoter constructs, as well as EMSA (electrophoretic mobility-shift assay) and ChIP (chromatin immunoprecipitation) assays, demonstrated that NF-κB family members (p65, p50 and cRel), Egr1 and Sp1 bound in vitro and in vivo to the proximal promoter region of the Ngb gene. Moreover, a κB3 site was found as a pivotal cis-element responsible for hypoxia-induced Ngb promoter activity. NF-κB (p65) and Sp1 were also responsible for hypoxia-induced up-regulation of Ngb expression. Although there are no conserved HREs (hypoxia-response elements) in the promoter of the mouse Ngb gene, the results of the present study suggest that HIF-1α (hypoxia-inducible factor-1α) is also involved in hypoxia-induced Ngb up-regulation. In conclusion, we have identified that NF-κB, Egr1 and Sp1 played important roles in the regulation of basal Ngb expression via specific interactions with the mouse Ngb promoter. NF-κB, Sp1 and HIF-1α contributed to the up-regulation of mouse Ngb gene expression under hypoxic conditions.