Increasing Melanoma Cell Death Using Inhibitors of Protein Disulphide Isomerases to Abrogate Survival Responses to Endoplasmic Reticulum Stress

Exploiting vulnerabilities in the intracellular signalling pathways of tumor cells is a key strategy for the development of new drugs. Agents that disrupt normal signalling pathways may induce homeostatic responses to restore normal function. The endoplasmic reticulum (ER) is responsible for regulation of intracellular calcium (Ca2+) and the synthesis of cell-surface or secretory proteins. Disruption of ER function induces a stress response characterised by the up-regulation of ER chaperones and a cascade of transcriptional regulation allowing the cell to adapt and focus resources for damage repair. The unfolded protein response (UPR) is an important ER stress response which rescues the cell by removing unfolded or misfolded proteins. However, ER stress will induce apoptotic death if homeostatic mechanisms are insufficient to protect or repair the cell. The ability of ER stress to drive apoptosis could be harnessed to increase the effectiveness of cancer treatment if homeostatic responses can be attenuated with appropriate drugs. Recent studies in which elements of the ER stress response have been down-regulated or blocked have shown that this can shift the balance towards apoptosis in cells treated with ER stress-inducing agents. Cancer cell types differ in their susceptibility to chemotherapy and malignant melanoma, one of the most difficult cancers to treat, is largely unresponsive to conventional chemotherapy, resulting in low 5-year survival rates. Melanoma cells have extensive repertoires of molecular defences against immunological and cytotoxic attack resulting in defective apoptotic signalling. The induction of ER stress chaperones can be an early event in tumour initiation and targeting ER stress responses of melanoma cells will represent a novel therapeutic approach. Many ER stress-response chaperones have protein disulfide isomerase (PDI) activity or PDI-like domains and blocking this activity may be a way to attenuate ER stress responses and tip the balance towards apoptosis in stressed cells. The aim of the present study was to test the hypothesis that apoptosis in response to ER stress can be increased using PDI inhibitors to attenuate homeostatic mechanisms