Reduction of endoplasmic reticulum Ca²⁺ levels favors plasma membrane surface exposure of calreticulin

Some chemotherapeutic agents can elicit apoptotic cancer cell death, thereby activating an anticancer immune response that influences therapeutic outcome. We previously reported that anthracyclins are particularly efficient in inducing immunogenic cell death, correlating with the pre-apoptotic exposure of calreticulin (CRT) on the plasma membrane surface of anthracyclin-treated tumor cells. Here, we investigated the role of cellular Ca²⁺ homeostasis on CRT exposure. A neuroblastoma cell line (SH-SY5Y) failed to expose CRT in response to anthracyclin treatment. This defect in CRT exposure could be overcome by the overexpression of Reticulon-1C, a manipulation that led to a decrease in the Ca²⁺ concentration within the endoplasmic reticulum lumen. The combination of Reticulon-1C expression and anthracyclin treatment yielded more pronounced endoplasmic reticulum Ca²⁺ depletion than either of the two manipulations alone. Chelation of intracellular (and endoplasmic reticulum) Ca²⁺, targeted expression of the ligand-binding domain of the IP₃ receptor and inhibition of the sarco-endoplasmic reticulum Ca²⁺-ATPase pump reduced endoplasmic reticulum Ca²⁺ load and promoted pre-apoptotic CRT exposure on the cell surface, in SH-SY5Y and HeLa cells. These results provide evidence that endoplasmic reticulum Ca²⁺ levels control the exposure of CRT.