ERO1α is a novel endogenous marker of hypoxia in human cancer cell lines

Background: Hypoxia is an important factor that contributes to tumour aggressiveness and correlates with poor prognosis and resistance to conventional therapy. Therefore, identifying hypoxic environments within tumours is extremely useful for understanding cancer biology and developing novel therapeutic strategies. Several studies have suggested that carbonic anhydrase 9 (CA9) is a reliable biomarker of hypoxia and a potential therapeutic target, while pimonidazole has been identified as an exogenous hypoxia marker. However, other studies have suggested that CA9 expression is not directly induced by hypoxia and it is not expressed in all types of tumours. Thus, in this study, we focused on endoplasmic reticulum disulphide oxidase 1α (ERO1α), a protein that localises in the endoplasmic reticulum and is involved in the formation of disulphide bonds in proteins, to determine whether it could serve as a potential tumour-hypoxia biomarker. Methods: Using quantitative real-time polymerase chain reaction, we analysed the mRNA expression of ERO1α and CA9 in different normal and cancer cell lines. We also determined the protein expression levels of ERO1α and CA9 in these cell lines by western blotting. We then investigated the hypoxia-inducible ERO1α and CA9 expression and localisation in HCT116 and HeLa cells, which express low (CA9-low) and high (CA9-high) levels of CA9, respectively. A comparative analysis was performed using pimonidazole, an exogenous hypoxic marker, as a positive control. The expression and localisation of ERO1α and CA9 in tumour spheres during hypoxia were analysed by a tumour sphere formation assay. Finally, we used a mouse model to investigate the localisation of ERO1α and CA9 in tumour xenografts using several cell lines. Results: We found that ERO1α expression increased under chronic hypoxia. Our results show that ERO1α was hypoxia-induced in all the tested cancer cell lines. Furthermore, in the comparative analysis using CA9 and pimonidazole, ERO1α had a similar localisation to pimonidazole in both CA9-low and CA9-high cell lines. Conclusion: ERO1α can serve as a novel endogenous chronic hypoxia marker that is more reliable than CA9 and can be used as a diagnostic biomarker and therapeutic target for cancer.