Sigma-2 receptors are highly overexpressed in cancer cells compared to normal cells, as well as in proliferative versus quiescent cells. Sigma-2 receptors have been of interest as an antineoplastic target, as sigma-2 agonists cause cell death via a variety of pathways. Recently, a new class of sigma-2 agonists has been discovered, with ligands such as CM764, that do not induce cell death but that affect cellular metabolism, as shown by increased MTT reduction, increased ATP level, and HIF-1α stabilization. Previously, our lab has shown that treatment of breast tumor cells with sigma-2 receptor agonists CB-184 and BD737 caused dose dependent increases in ceramide and simultaneous decreases in sphingomyelin, suggesting that sigma-2 receptors modulate the sphingolipid pathway. The ceramide pathway has been shown to play both prosurvival and pro-apoptotic roles in cancer cells. Here, we examine the role of ceramide in sigma-2 receptor mediated pathways in human SK-N-SH neuroblastoma cells. A 24 h treatment of SK-N-SH neuroblastoma cells with 20 μM exogenous C6-ceramide caused less than 20% cell death. Interestingly, this compared to over 50% cell death in MCF-7 breast adenocarcinoma cells and PANC-1 pancreatic cancer cells, indicating a differential role of ceramide in cell death pathways across cell lines. SK-N-SH cells were treated with 10 μM of either CM572, a classic sigma-2 agonist that causes cell death, or CM764, a sigma-2 agonist that stimulates metabolism. Fumonisin B1 is a potent inhibitor of ceramide synthase. Treatment of cells with fumonisin B1 (10 μM) did not significantly alter CM572-induced cytotoxicity. Conversely, treatment with fumonisin B1 did significantly inhibit the MTT reduction caused by CM764. Taken together, these results suggest that ceramide formation may play more of a role in the metabolic stimulative effect of sigma-2 receptors, as opposed to the cytotoxic effect. Decreased levels of ceramide have been shown to increase cellular resistance to apoptosis. Therefore, it has been of interest to increase intracellular ceramide levels in drug resistant cancer cells in order to sensitize the resistant cells to chemotherapeutics. Multidrug resistance (MDR) in cancer cells is often attributed to the overexpression of particular ABC transporters that effectively pump drugs out of the cells. One such protein is the multidrug resistance-associated protein 1 (MRP1). It has been shown that increased expression of drug efflux pumps also decreases cell sensitivity to ceramide. Treatment with CM572 caused a dose-dependent decrease in MRP1 protein levels, while treatment with CM764 had little effect on MRP1. We will examine how this modulation of MRP1 affects sensitivity to C6-ceramide. We will also examine whether combination of C6-ceramide with sigma-2 receptor ligands augments the cytotoxic and/or metabolic stimulative effects of sigma-2 receptors.
Citation Format: Cheri Z. Liu, Ellen Sukharevsky, Wayne D. Bowen. The role of ceramide in the dual cytotoxic and metabolic stimulative effects of sigma-2 receptors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2017. doi:10.1158/1538-7445.AM2017-2017