Zinc supplementation is required for the cytotoxic and immunogenic effects of chemotherapy in chemoresistant p53-functionally deficient cells

Optimal tumor eradication often results from the death of malignant cells, as induced by chemotherapeutic agents, coupled to the induction of antitumor immune responses. However, cancer cells frequently become resistant to the cytotoxic activity of chemotherapy. The aim of the present study was to evaluate whether zinc dichloride (ZnCl2), which was known to re-establish the chemosensitivity of cancer cells by reactivating p53, promotes immunogenic instances of cell death. We found that ZnCl2, in combination with chemotherapeutic agents such as cisplatin and adriamycin (ADR), favors the apoptotic demise of chemoresistant cells, while cisplatin and ADR alone fail to do so. The co-culture of immature dendritic cells (DCs) with cancer cells succumbing to the co-administration of chemotherapy and ZnCl2 led to DC activation, as indicated by the upregulation of the activation markers CD83 and CD86. In part, such process depended on cell death, as it was limited (but not abrogated) by the pan-caspase inhibitor Z-VAD-fmk. Moreover, DC activation relied on the ZnCl2-induced exposure of calreticulin (CRT) on the surface of cancer cells, correlating with the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2 alpha), a marker of endoplasmic reticulum stress. The siRNA-mediated knockdown of CRT as well as the inhibition of CRT exposure with brefeldin A strongly impaired DC maturation, indicating CRT translocation as induced by that ZnCl2 is a key event in this setting. Altogether, these results suggest that ZnCl2, has the potential to enhance the therapeutic effects of antineoplastic agents not only by improving their cytotoxic activity but also by promoting CRT exposure.