Regulation of aldo–keto reductase AKR1B10 gene expression: Involvement of transcription factor Nrf2

Aldo–keto reductase 1B10 (AKR1B10) is an aldose reductase-like oxidoreductase of human origin. The expression of AKR1B10 is highly induced in the cells of various cancers such as lung non-small-cell carcinoma and hepatocellular carcinoma. Since the enzyme exhibits broad substrate specificities toward various xenobiotics such as anti-tumor drugs or various endogenous compounds such as retinaldehyde, AKR1B10 may play an important role in tumor progression or drug resistance. However, very little is known about its gene regulation. In this study, we investigated the regulation of AKR1B10 expression. A −3282 bp of the 5′-flanking fragment of AKR1B10 gene was isolated from A549 lung carcinoma cells. This region contains several putative regulatory motifs such as AP-1, NF-κB and antioxidant response element. In addition, a complex polymorphic microsatellite with repetitive sequences enriched with C and T was found. However, luciferase reporter assay revealed that the microsatellite polymorphism did not influence the basal promoter activity. We found that an antioxidant ethoxyquin induced the AKR1B10 expression based on RT-PCR analysis and luciferase reporter assay. Since ethoxyquin is known to activate the gene expression mediated through transcription factor Nrf2, the involvement of Nrf2 was examined. Forced expression of dominant-negative Nrf2 mutant suppressed the ethoxyquin-induced AKR1B10 expression, and co-introduction of Nrf2 expression plasmid into the cells significantly augmented the luciferase reporter activity. Deletion analysis revealed that Nrf2-regulating cis-element(s) lay within −539 bp of the 5′-flanking region. These results suggest that Nrf2 is one of the major factors involved in the AKR1B10 gene regulation.