Abstract 2233: The prostaglandin E receptor EP4 is upregulated on breast cancer stem-like cells and regulates sensitivity to Natural Killer cells
Authors: Fulton Amy M. , Kundu Namita , Ma Xinrong , Kochel Tyler , Reader Jocelyn 
Published: August 2015

Abstract
The cyclooxygenase-2 (COX-2) pathway is highly expressed in many breast tumors and contributes to poor outcomes. The COX-2 product prostaglandin E2 (PGE2) promotes tumor growth and metastasis by acting on the G-protein-coupled receptor EP4. Our goal is to understand the role that EP4 and COX-2 play in the survival of breast cancer cells with stem-like properties. We compared EP4 and COX-2 expression in mammosphere-forming and bulk populations derived from a panel of human and murine luminal and basal type tumor cells with different metastatic capacities. Expression of both EP4 and COX-2 were markedly increased in mammosphere-forming cells derived from basal and/or metastatic cells relative to the bulk population, but neither COX-2 nor EP4 levels were elevated in mammospheres derived from luminal or non-metastatic cells. Breast cancer stem-like cells are more sensitive than the bulk population to inhibition with small molecule EP4 antagonists in vitro. Consistent with these data, tumor-initiating capacity in vivo is markedly inhibited by EP4 antagonists. PGE2 potently suppresses Natural Killer (NK) cell activities through the EP4 receptor and the administration of EP4 antagonists to tumor-bearing mice restores NK activities to normal levels. We asked if NK cells can recognize and lyse breast cancer stem cells and if EP4 plays a role in this process. We compared the lytic sensitivity of murine mammary-forming tumor cells versus the bulk population to cytotoxicity mediated by normal murine NK cells. In metastatic tumor cell lines 66.1 and 410.4, the breast cancer stem cell enriched population was less sensitive to NK-mediated lysis than the bulk population. Gene silencing of EP4 modestly increased the sensitivity of tumor target cells to NK killing which corresponded to increased expression of the NK activating ligand H60. We are delineating the mechanism by which EP4 and COX-2 are upregulated in cells with stem-like properties. Like EP4 and COX-2, STAT3 is upregulated in breast cancer stem cells. Inhibition of STAT3 reduces mammosphere-forming capacity. Our studies support a mechanism whereby COX-2/EP4 signaling induces STAT3 to support breast cancer stem cell survival by a feed-forward mechanism and that these cells may be relatively resistant to NK-mediated control. These studies support the continued examination of EP4 as a therapeutic target. In preclinical studies, EP4 antagonists protect NK cells from tumor-mediated immune suppression and thereby prevent tumor metastasis and inhibit cancer stem cells.
Citation Format: Amy M. Fulton, Namita Kundu, Xinrong Ma, Tyler Kochel, Jocelyn Reader. The prostaglandin E receptor EP4 is upregulated on breast cancer stem-like cells and regulates sensitivity to Natural Killer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2233. doi:10.1158/1538-7445.AM2015-2233