Abstract 3307: Transcriptional silencing of c-Myc by promoter-directed siRNA efficiently target prostate cancer stem cells.
Authors: Civenni Gianluca , Malek Anastasia , Albino Domenico , Sarti Manuela , Pinton Sandra , Di Marco Stefano , Carbone Giuseppina M. , Catapano Carlo V. 
Published: August 2014

Abstract
Prostate cancer is the most frequent epithelial cancer in elderly men in Western countries. Depending on the stage, prostate cancer treatment requires surgery, hormone therapy, radio- and chemo-therapy. c-MYC is a transcription factor activated by mitogenic signaling pathways and playing a central function in stem cell biology. Over-activity of c-Myc plays a central role in tumorigenesis by affecting cell proliferation, metabolic adaptation and survival. Amplification of c-MYC is one of the most common genetic alterations occurring in cancer genomes. c-Myc expression is significantly elevated in invasive prostate adenocarcinomas compared to benign prostatic hyperplasia and normal prostate tissue. Particularly, c-Myc is highly expressed in prostate cancer cells having the CD44+/CD24- phenotype, which is described as a hallmark of cancer progenitor/stem cells (CSCs). Importantly, several studies link therapy resistance and disease progression to the CSC subpopulation within the highly heterogeneous cellular composition of the tumor. CSCs are likely the tumor initiating population that produces metastasis and disease recurrence. These lines of evidence suggest a central role of c-Myc in the maintenance of CSC compartment in human tumors and justify c-Myc as a therapeutic target.
Previously, we showed that the c-Myc gene could be silenced with small interfering RNAs (siRNA) targeting its promoter. Interestingly, single transfection with myc targeting siRNA (myc-siRNA) induced long lasting effects on cell proliferation and clonogenicity, indicative of a persistent loss of proliferative and clonogenic potential. In this study, we investigated the effects of transcriptional silencing of c-Myc on the CSC subpopulation in human prostate cancer cell lines in vitro and in vivo. We found that treatment with myc-siRNA significantly reduced the fraction of CSCs defined by expression of stem cell surface markers CD44 and CD24, in vitro sphere forming ability and self-renewal. Furthermore, combined analysis of senescence and cell surface markers showed that senescence occurred prevalently in the CD44+/CD24- cell subpopulation leading to its depletion, and reduced self renewal capability, in vivo tumorigenicity and metastatic potential. Moreover, repeated intraperitoneal administration of myc-siRNA over a 4-week period reduced tumor growth in a xenogeneic prostate cancer model. These results are consistent with the role of c-Myc in the maintenance of the CSC subpopulation in human prostate tumors and suggest that c-Myc downregulation induces an irreversible loss of clonogenic and tumor-initiating capability linked to the induction of cell senescence in CSCs. Our findings show also that an RNAi-based transcriptional therapy directed to genes directly involved in the maintenance of the cancer stem cells could be an efficient approach to block progression and relapse of prostate cancer.
Citation Format: Gianluca Civenni, Anastasia Malek, Domenico Albino, Manuela Sarti, Sandra Pinton, Stefano Di Marco, Giuseppina M. Carbone, Carlo V. Catapano. Transcriptional silencing of c-Myc by promoter-directed siRNA efficiently target prostate cancer stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3307. doi:10.1158/1538-7445.AM2013-3307