Cancer cells are commonly more resistant to cell death activated by the membranolytic protein complex C5b-9. Several surface-expressed and intracellular proteins that protect cells from complement-dependent cytotoxicity (CDC) have been identified. In this study, we investigated the function of heat shock protein 90 (Hsp90), an essential and ubiquitously expressed chaperone, overexpressed in cancer cells, in C5b-9-induced cell death. As shown, inhibition of Hsp90 with geldanamycin or radicicol is enhancing sensitivity of K562 erythroleukemia cells to CDC. Similarly, Hsp90 inhibition confers in Ramos B cell lymphoma cells elevated sensitivity to treatment with rituximab and complement. C5b-9 deposition is elevated on geldanamycin-treated cells. Purified Hsp90 binds directly to C9 and inhibits zinc-induced C9 polymerization, indicating that Hsp90 may act directly on the C5b-9 complex. Mortalin, also known as stress protein 70 or GRP75, is a mitochondrial chaperone that confers resistance to CDC. The postulated cooperation between Hsp90 and mortalin in protection from CDC was tested. Geldanamycin failed to sensitize toward CDC cells with knocked down mortalin. Direct binding of Hsp90 to mortalin was shown by co-immunoprecipitation in cell extracts after triggering with complement as well as by using purified recombinant proteins. These results provide an insight into the protective mechanisms utilized by cancer cells to evade CDC. They suggest that Hsp90 protects cells from CDC by inhibiting, together with mortalin, C5b-9 assembly and/or stability at the plasma membrane.

Cancer cells have developed numerous strategies to maintain their proliferative capacity and to withstand different kinds of stress. The mitochondrial stress-70 protein named glucose regulated protein 75 (GRP75), also known as mortalin, is an intriguing cancer pro-survival factor. It is constitutively expressed in normal tissues but is upregulated in many tumors, and was shown to be a cancer prognostic biomarker. Mortalin is an inhibitor of complement-dependent cytotoxicity (CDC) and may therefore protect cells from antibody-based immunotherapy. To target mortalin for cancer therapy, our laboratory designed several mortalin mimetic peptides with sequences predicted to be involved in mortalin binding to its client proteins. The peptides were synthesized with a C-terminal transactivator of transcription sequence. By using cell death methodologies, the mechanism of action of the mortalin mimetic peptides on cancer cells was studied. Two peptides in particular, Mot-P2 and Mot-P7, were found to be highly toxic to lymphoma and ovarian, breast and prostate carcinoma cells. The analysis of their mode of action revealed that they may induce, within minutes, plasma membrane perturbations and mitochondrial stress. Furthermore, Mot-P2 and Mot-P7 activated necrotic cell death, leading to plasma membrane perforation, mitochondrial inner membrane depolarization and decrease in ATP level. In addition, Mot-P7, but not Mot-P2, required extracellular calcium ions to fully mediate cell death and was partially inhibited by plasma membrane cholesterol. At sub-toxic concentrations, the two peptides moderately inhibited cancer cell proliferation and blocked cell cycle at G2/M. Both peptides may bind intracellularly to mortalin and/or a mortalin-binding protein, hence knocking down mortalin expression reduced cell death. Combining treatment with Mot-P2 or Mot-P7 and CDC resulted in increased cell death. This study identified highly cytotoxic mortalin mimetic peptides that may be used as monotherapy or combined with complement-activating antibody therapy to target mortalin for precision cancer therapy.