MK591 (Quiflapon) Downregulates c-Myc Oncogenic Signaling and Induces Apoptosis in Enzalutamide-Resistant Prostate Cancer Cells

Background: Enzalutamide is an FDA-approved drug commonly prescribed for advanced prostate cancer. Enzalutamide slows down prostate tumor growth but resistant disease invariably develops which is incurable primarily because currently available therapies cannot effectively kill enzalutamide-resistant prostate cancer cells. Mechanism(s) behind development of enzalutamide-resistance is not properly understood, though overactivation of c-Myc has been found to be a common event which plays an important role in the maintenance and progression of ERPC phenotype. However, direct-targeting of c-Myc poses special problem because of its non- enzymatic nature and certain amount of c-Myc activity is needed by non-cancer cells as well. Thus, c-Myc has emerged as an elusive target which needs to be managed by novel agents and strategies in a cancer-specific way for better control of ERPC. Methods: We addressed this problem by treating ERPC cells with a variety of cell survival and apoptosis- regulating agents followed by measurement of cell viability, and by analysis of the mRNA and protein levels of c- Myc. Gene expression was analyzed by Illumina Hi-Seq whole genome gene-expression array. Expression of c- Myc was confirmed by RT-PCR and Western blot. Apoptosis was measured by annexin-V binding, PARP- cleavage, and by detecting degradation of chromatin-DNA to nucleosomes. Transcriptional activity of c-Myc was analyzed by nuclear accumulation, DNA-binding, luciferase-reporter assays and expression of c-Myc-target genes. Results: We found that MK591, a leukotriene biosynthesis inhibitor, dramatically downregulates c-Myc in ERPC cells as revealed by RT-PCR, and Western blot as well as by the reduction in nuclear-accumulation and DNA- binding activities of c-Myc. Treatment with MK591 decreased the Myc-driven E-box-luciferase reporter activity, and substantially reduced the expression of c-Myc target genes (Cyclin D1, CDK4, survivin, Aurora kinase). Moreover, MK591 effectively blocked in vitro invasion and soft-agar colony-formation by ERPC cells. Interestingly, while MK591 strongly inhibits c-Myc function and kills ERPC cells via caspase-mediated apoptosis, it does not inhibit the basal c-Myc function or the viability of non-cancer cells, such as human foreskin fibroblasts (HFF). Conclusion: Our findings indicate that the expression and oncogenic-function of c-Myc in ERPC cells are severely downregulated by MK591, and suggest that MK591 may turn out to be a suitable new agent to treat advanced, aggressive prostate cancers which are resistant to enzalutamide therapy