The Nrf2 transcription factor contributes to resistance to cisplatin in bladder cancer

Cisplatin is the key systemic chemotherapeutic agent used for bladder cancer, but chemoresistance is a major clinical problem. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates various critical cellular processes, including cellular antioxidant response, cellular detoxification, and drug uptake/efflux. These processes, and the expression of multiple Nrf2 target genes, have been found to be associated with bladder cancer prognosis and chemotherapy resistance. We, therefore, investigated whether Nrf2 might regulate cisplatin resistance in bladder cancer.

We first used bladder cancer cell lines, including a cisplatin-resistant RT112 subline (RT112-CP), to investigate Nrf2 expression and activation and its association with cisplatin response. We then undertook immunohistochemical analysis of a tissue microarray of archival bladder cancer radical cystectomy specimens to test the relevance of clinical Nrf2 expression to outcomes following either neoadjuvant chemotherapy and cystectomy or cystectomy alone.

Bladder cancer cell lines showed variable Nrf2 expression. Nrf2 expression was greater in RT112-CP cisplatin-resistant cells compared with that in parental RT112 cells. Nrf2 overexpression was functional in this model as it was associated with increased antioxidant response element reporter construct activity, Nrf2 target gene expression (metallothionein and glutathione reductase), and basal glutathione levels. Cisplatin resistance was associated with Nrf2 expression, and in RT112-CP cells, its depletion partially restored cisplatin sensitivity. We demonstrated increased cytoplasmic or nuclear Nrf2 expression or both in 32% of clinical bladder cancer samples compared with that in normal tissue samples. Expression of Nrf2 in bladder cancer following radical cystectomy was associated with unfavorable overall (median = 0.65 vs. 2.11 y, P = 0.045), bladder cancer-specific, and recurrence-free survival in those patients who also received neoadjuvant cisplatin-based chemotherapy but not in those treated with cystectomy alone.

Nrf2 overexpression in bladder cancer is associated with clinically relevant cisplatin resistance that is reversible in experimental models and should now be tested in prospective studies.