15-Deoxy-\#916;12,14-prostaglandin J2 (15d-PGJ2), a representative J-series cyclopentenone prostaglandin (CyPG), is one of the terminal products of the cyclooxygenase-2 pathway, and may act as a lynchpin in inflammation-associated cancer. This CyPG has been reported to exert Janus-faced (pro- and anti-apoptotic) effects. In the present study, we have found that human breast cancer (MCF-7) cells pretreated with 15d-PGJ2 are less prone to undergo doxorubicin-induced apoptosis. This prompted us to further investigate the molecular mechanism underlying 15d-PGJ2-induced resistance to doxorubicin-induced MCF-7 cell death. An elevated level of glutathione (GSH) in cancer cells has been associated with resistance to certain chemotherapeutics. Treatment of MCF-7 cells with 15d-PGJ2 (10 \#956;M) caused upregulation of glutamate cystein ligase catalytic (GCLC) subunit, the rate-limiting enzyme in the GSH synthesis. Nrf2 is a redox-sensitive transcription factor involved in the transactivation of many genes encoding phase 2 detoxifying/antioxidant enzymes including GCLC. 15d-PGJ2 treatment resulted in nuclear translocation, subsequent binding to antioxidant response element (ARE) and transactivation of Nrf2. siRNA knockdown of Nrf2 gene in MCF-7 cells abrogated 15d-PGJ2-induced GCLC expression at 24 h. Following 15d-PGJ2 treatment, the intracellular GSH level was initially diminished, but eventually enhanced. 15d-PGJ2 contains an electrophilic \#945;,\#946;-unsaturated carbonyl moiety and readily forms adducts with various thiols including GSH. It is likely that the formation of a 15d-PGJ2-GSH conjugate can result in a transient decrease in the GSH level, which triggers the intracellular accumulation of ROS, provoking Nrf2 activation. This, in turn, induces expression of GCLC and subsequent enhancement of GSH levels in MCF-7 cells. Multidrug-resistance protein 1 (MRP1) is considered to be involved in GSH homeostasis via efflux of GSH conjugated with many electrophiles and chemotherapeutics out of the cell. In addition to GCLC, 15d-PGJ2 up-regulated the expression of MRP1 in a Nrf2-dependent manner. As doxorubicin is also a substrate for MRP1, the up-regulation of MRP1 may account for 15d-PGJ2-induced doxorubicin resistance. Interestingly, 15d-PGJ2-induced GCLC expression was attenuated by the MRP1 antagonist MK571 (100 \#956;M) and also by siRNA knockdown of MRP1 gene, suggesting that MRP1 contributes to 15d-PGJ2-mediated up-regulation of GCLC by pumping out the 15d-PGJ2-GSH conjugate. Effluxed 15d-PGJ2-GSH conjugate may be broken down to regenerate lipophilic free 15d-PGJ2, which can readily re-enter the cell. In conclusion, the coordinated up-regulation of GCLC and MRP1 induced by 15d-PGJ2 via the Nrf2-ARE signaling may confer resistance to doxorubicin-induced MCF-7 cell death.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 916.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO