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 has an important role in calcium signaling and cell survival in response to stress signals. In cancer cell lines and human tumors Sig1R is often up-regulated suggesting that it might play a relevant role in tumorigenesis, although detailed functional studies are missing.
In this study we examined whether Sig1R sustained proliferation, survival and tumorigenic properties of human cancer cells. Knockdown of Sig1R using small interfering RNA (siRNA) in human prostate and lung cancer cells had profound effect on proliferation, clonogenic capability and tumor sphere formation, indicating reversal of tumorigenicity and stem-like phenotype in absence of Sig1R.
Next, we tested a series of structurally diverse Sig1R ligands selected for high affinity and selectivity for the receptor in the attempt to discover pharmacological agents that could phenocopy the effects of the genetic knockdown on cancer cells. 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 low micromolar range (1-10 mM). 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. 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 demonstrate that the presence of functional Sig1R ensures survival and sustains tumorigenicity and stem-like properties of cancer cells by enhancing their metabolic flexibility. Highly selective Sig1R antagonists are promising leads for discovery of innovative therapeutic strategies and might represent effective candidates for pharmacological targeting of stem-like and tumor-initiating cells in human cancers.