Abstract
ATP-binding cassette, subfamily G, member 2 (ABCG2) is expressed in both normal and cancer cells and plays a crucial role in side population (SP) formation and efflux of xenobiotics and drugs. Nrf2, a redox-sensing transcription factor, on constitutive activation in non-small-cell lung cancer cells upregulates a wide spectrum of genes involved in redox balance, glutathione metabolism, and drug detoxification, which contribute to chemoresistance and tumorigenicity. This study examined the mechanism underlying Nrf2-dependent expression of ABCG2 and its role in the multidrug resistance phenotype. In silico analysis of the 5'-promoter flanking region of ABCG2 identified an antioxidant response element (ARE) at -431 to -420 bp. A detailed promoter analysis using luciferase reporter assays showed that ARE at -431 to -420 bp is critical for the Nrf2-mediated expression in lung cancer cells. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays revealed that Nrf2 interacts with the ABCG2 ARE element at -431 to -420 bp in vitro and in vivo. Disruption of Nrf2 expression in lung and prostate cancer cells, by short hairpin RNA, attenuated the expression of ABCG2 transcript and protein, and dramatically reduced the SP fraction in Nrf2-depleted cancer cells. Moreover, depleted levels of ABCG2 in these Nrf2 knockdown cells sensitized them to mitoxantrone and topotecan, two chemotherapy drugs detoxified mainly by ABCG2. As expected, overexpression of Nrf2 cDNA in lung epithelial cells led to an increase in ABCG2 expression and a 2-fold higher SP fraction. Thus, Nrf2-mediated regulation of ABCG2 expression maintains the SP fraction and confers chemoresistance.