Doxorubicin induces prostate cancer drug resistance by upregulation of ABCG4 through GSH depletion and CREB activation: Relevance of statins in chemosensitization.

Multidrug resistance mediated by ATP-binding cassette (ABC) transporters remains a major impediment to cancer chemotherapy. In the present study, we documented that doxorubicin (Dox) or cisplatin-induced prostate cancer (PCa) chemoresistance is predominantly mediated by the induction of ABCG4 in androgen-independent PCa cells. Treatment of DU-145 or PC-3 cells with Dox significantly enhanced the expression of ABCG4 that resulted in the efflux of intracellular Dox. However, incubation of cells with ABCG4 short hairpin RNA resulted in a significant accumulation of Dox and sensitized cells to Dox-induced cytotoxicity. Interestingly, simvastatin synergistically potentiated Dox-induced cytotoxicity by inhibiting ABCG4 in DU-145 and DU-145 Dox^res cells. Mechanistically, ABCG4 expression was regulated redox-dependently by intracellular glutathione (GSH) levels. Treatment of cells with N-acetylcysteine or simvastatin restored Dox-induced depletion of GSH levels that in turn inhibited ABCG4 levels. In addition, a reduction in GSH levels by Dox caused a nuclear factor-κB dependent enhancement of c-Myc expression, which led to cAMP-regulatory element-binding protein (CREB) activation. Furthermore, chromatin immunoprecipitation experiments revealed that Dox-induced CREB activation transcriptionally upregulates ABCG4 expression. These results were further confirmed in an in vivo PCa xenograft mice model. Combination of simvastatin and Dox significantly regressed the tumor growth and size with no noticeable Dox-induced cardiotoxic side effects. Intriguingly, DU-145 cells with stably depleted ABCG4 levels not only significantly delayed the development of the tumor but also greatly sensitized the tumor to a low dose of Dox that resulted in complete tumor regression. Collectively, this data reinforces a novel function of ABCG4 in Dox-mediated chemoresistance, and as a potential therapeutic target in drug-induced PCa chemoresistance.

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