Role for mitochondrial chaperones in cancer cells resistance to antibody-based immunotherapy

Introduction: Antibodies are widely used in cancer therapy since they can selectively target specific molecules expressed on cancer cells. Complement-dependent cytotoxicity (CDC) of the cancer cells activated by antibodies is induced by the complement C5b-9 membrane attack complex, which activates a programmed necrotic cell death. Mitochondria damaged during CDC lose their main activities, including energy production, regulation of cellular metabolism, calcium homeostasis, and detoxification of reactive oxygen radicals. As we have shown, to counteract the CDC, cancer cells utilize their mitochondrial chaperone Mortalin. Mortalin, the mitochondrial heat shock protein (Hsp) 70 plays a crucial role in protein folding, mitochondrial function, stress response, and regulation of cell death pathways. TRAP1 is an Hsp90-related mitochondrial chaperone that contributes to protein folding and stabilization, regulation of mitochondrial functions, and protection from oxidative stress. Both mortalin and TRAP1 are overexpressed in cancer, and support cancer growth and progression, and resistance to CDC.

Material and methods: Human lymphoma and carcinoma cell lines are subjected to CDC and extent of cell death is determined by FACS after labeling with propidium iodide. C3 and C5b-9 deposition is quantified by FACS and by Western blotting.

Results and Discussion: We have also established a role for TRAP1 and for DNAJ/B6, a chaperone that belongs to the DNAJ/Hsp40 family, in conferring resistance of cancer cells to CDC. Our results show that knocking down mortalin, TRAP1, and DNAJB6 in cancer cells enhances cell death, whereas their overexpression reduces cell death. The mode of action and cooperation of these 3 chaperones in the protection of cancer cells from CDC is investigated. According to our results, they reduce the quantity of complement C5b-9 complexes deposited on the cancer cells during CDC. Most interestingly, they appear to collaborate with each other in their action against CDC.

Conclusion: Our ongoing research aims to define the unified molecular cascade of events that enable cancer cells to resist antibody and complement attack. Furthermore, we are developing reagents that will sensitize cancer cells to antibody-based immunotherapy.