Cytoglobin is up-regulated by tumour hypoxia and silenced by promoter hypermethylation in head and neck cancer

Abstract

Background Cytoglobin (Cygb) was first described in 2002 as an intracellular globin of unknown function. We have previously demonstrated down-regulation of cytoglobin as a key event in a familial cancer syndrome of the upper aerodigestive tract. Method Cygb expression and promoter methylation were investigated in sporadic head and neck squamous cell carcinoma (HNSCC) using a cross-section of clinical samples. Additionally, the putative mechanisms of Cygb expression in cancer were explored by subjecting HNSCC cell lines to hypoxic culture conditions and 5-aza-2-deoxycitidine treatment. Results In clinically derived HNSCC samples, CYGB mRNA expression showed a striking correlation with tumour hypoxia (measured by HIF1A mRNA expression P=0.013) and consistent associations with histopathological measures of tumour aggression. CYGB expression also showed a marked negative correlation with promoter methylation (P=0.018). In the HNSCC cell lines cultured under hypoxic conditions, a trend of increasing expression of both CYGB and HIF1A with progressive hypoxia was observed. Treatment with 5-aza-2-deoxycitidine dramatically increased CYGB expression in those cell lines with greater baseline promoter methylation. Conclusion We conclude that the CYGB gene is regulated by both promoter methylation and tumour hypoxia in HNSCC and that increased expression of this gene correlates with clinco-pathological measures of a tumour’s biological aggression.