Targeting cell migration and the endoplasmic reticulum stress response with calmodulin antagonists: a clinically tested small molecule phenocopy of SEC62 gene silencing in human tumor cells

Tumor cells benefit from their ability to avoid apoptosis and invade other tissues. The endoplasmic reticulum (ER) membrane protein Sec62 is a key player in these processes. Sec62 is essential for cell migration and protects tumor cells against thapsigargin-induced ER stress, which are both linked to cytosolic Ca²⁺. SEC62 silencing leads to elevated cytosolic Ca²⁺ and increased ER Ca²⁺ leakage after thapsigargin treatment. Sec62 protein levels are significantly increased in different tumors, including prostate, lung and thyroid cancer. In lung cancer, the influence of Sec62 protein levels on patient survival was analyzed using the Kaplan-Meier method and log-rank test. To elucidate the underlying pathophysiological functions of Sec62, Ca²⁺ imaging techniques, real-time cell analysis and cell migration assays were performed. The effects of treatment with the calmodulin antagonists, trifluoperazine (TFP) and ophiobolin A, on cellular Ca²⁺ homeostasis, cell growth and cell migration were compared with the effects of siRNA-mediated Sec62 depletion or the expression of a mutated SEC62 variant in vitro. Using Biacore analysis we examined the Ca²⁺-sensitive interaction of Sec62 with the Sec61 complex. Sec62 overproduction significantly correlated with reduced patient survival. Therefore, Sec62 is not only a predictive marker for this type of tumor, but also an interesting therapeutic target. The present study suggests a regulatory function for Sec62 in the major Ca²⁺ leakage channel in the ER, Sec61, by a direct and Ca²⁺-sensitive interaction. A Ca²⁺-binding motif in Sec62 is essential for its molecular function. Treatment of cells with calmodulin antagonists mimicked Sec62 depletion by inhibiting cell migration and rendering the cells sensitive to thapsigargin treatment. Targeting tumors that overproduce Sec62 with calmodulin antagonists in combination with targeted thapsigargin analogues may offer novel personalized therapeutic options.