Cytoglobin (CYGB) is ubiquitously expressed in all tissues and has been characterized as a respiratory protein in connective tissues. CYGB is up-regulated during hypoxia, implicating its function in maintaining the homeostasis redox of the cell. Here, we study the underlying molecular mechanisms by which hypoxia regulates human CYGB gene expression. When cells were subjected to hypoxia, the expression of endogenous CYGB was up-regulated similar to 1.7-fold in BEAS-213 cells (p <= 0.05) and similar to 1.6-fold in HeLa cells (P <= 0.05). Dual-luciferase assays and site directed mutagenesis showed the presence of hypoxia responsive elements (HREs) at positions -141, -144 and -448 that were essential for activation of CYGB expression under hypoxic conditions. The binding of hypoxia inducible factor (HIF-1) protein to the HREs was confirmed by gel shift and chromatin immunoprecipitation (ChIP) assays. These results indicate that HRE motifs are directly involved in the activation of the CYGB transcription under hypoxia. (C) 2007 Elsevier Inc. All rights reserved.