Abstract B119: Blocking metabolic stress response with genetic knockdown and selective ligands of sigma-1 receptor in cancer cells

There is increasing interest in the biological pathways that determine the ability of cancer cells to adapt and survive to metabolic and micro-environmental stress and sustain tumor-initiating and metastatic properties. A better understanding of these pathways may lead to the identification of key nodes and targetable elements for development novel therapeutic strategies for cancer. The sigma-1 receptor (Sig1R) is a ubiquitously expressed membrane-bound protein that acts as ligand-activated molecular chaperone. Sig1R is localized preferentially at the endoplasmic reticulum (ER) and the mitochondria-associated ER membrane (MAM) domains and controls calcium signaling between ER and mitochondria in response to stress signals. Sig1R is often up-regulated in cancer cell lines and human tumors suggesting that it might have a role in tumorigenesis, although detailed functional studies are missing. In this study we examined whether Sig1R sustains proliferation, survival and tumorigenic properties of human cancer cells. Knockdown of Sig1R using small interfering RNA (siRNA) in human prostate and lung cancer cell lines had profound effect on proliferation, clonogenic capability and tumor-sphere formation, indicating reversal of the tumorigenic and stem-like phenotype in absence of Sig1R. Next, in the attempt to discover pharmacological agents that could phenocopy the effects of the genetic knockdown in cancer cells we tested a series of structurally diverse Sig1R ligands selected for high affinity and selectivity for the receptor. We identified various Sig1R ligands that behaved as antagonists of the receptor functions in cancer cells inhibiting clonogenicity and tumor-sphere formation. Effective concentrations of the ligands were in the micromolar range (1-10 μM). At these doses, Sig1R antagonistic ligands almost completely suppressed clonogenic and tumor sphere forming capability of cancer cells. Furthermore, whereas cell proliferation and viability under standard culture conditions were minimally affected by Sig1R ligands, their effects were more pronounced under glucose starvation, a condition that causes metabolic stress in cancer cells. Notably, both Sig1R knockdown and pharmacological antagonists led to impaired mitochondrial function, which was more evident under glucose starvation. Thus, the absence of functional Sig1R reduced mitochondrial activity and adaptability of cancer cells to metabolic stress. These results indicate that the presence of functional Sig1R sustains tumorigenic and stem-like properties by enhancing the metabolic flexibility of cancer cells through the control of ER-mitochondria functions. Highly selective Sig1R antagonists are promising leads for discovering innovative therapeutic strategies and represent effective candidates for pharmacological targeting of stem-like and tumor-initiating cells in human cancers. Citation Format: Gianluca Civenni, Celeste De Monte, Federica Sereni, Sara Allegrini, Roberto Bosotti, Erik Laurini, Bernhard Wunsch, Sabrina Pricl, Giuseppina M. Carbone, Carlo V. Catapano. Blocking metabolic stress response with genetic knockdown and selective ligands of sigma-1 receptor in cancer cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B119.