Abstract / Summary
Abstract Resistance of tumor cells to complement-dependent cytotoxicity (CDC) is a major obstacle to the antitumor activity of therapeutic antibodies. The molecular network conferring resistance to complement attack is incompletely understood. Mitochondrial chaperones coordinate cellular adaptation to stress, but their contribution to complement resistance is only beginning to emerge. The mitochondrial chaperone TNF receptor-associated protein 1 (TRAP1) protects cells against oxidative and metabolic stress. As shown here, treatment of TRAP1 with the mitochondria-targeted inhibitor Gamitrinib-TTP, as well as TRAP1 knockdown or knockout, markedly sensitized cancer cells to CDC, whereas TRAP1 overexpression conferred resistance to CDC. TRAP1 deficiency increased C3b deposition and membrane attack complex (MAC; C5b-9) assembly without affecting antibody binding, C1q or C4b deposition, or the expression of the membrane complement regulators CD46, CD55 and CD59. Interestingly, in the absence of TRAP1 recruitment of the soluble complement regulatory protein Factor H to the cell surface was reduced. This may account for the increase in C3b binding. In addition, TRAP1 is shown to bind directly to C9 and inhibit its polymerization, disclosing a second inhibitory mechanism against CDC. Following complement attack, TRAP1 rapidly redistributed from mitochondria to the cell surface and the extracellular compartment. Neutralization of extracellular TRAP1 by specific antibodies enhanced MAC deposition and CDC, whereas extracellular recombinant TRAP1 reduced CDC. Beyond its effects on complement activation, TRAP1 is shown to protect mitochondria from complement-induced cell injury by limiting mitochondrial oxidative stress, preserving mitochondrial membrane potential and maintaining ATP production. Collectively, these findings identify TRAP1 as a multifunctional regulator that carries out extracellular complement regulation and intracellular mitochondrial stress adaptation, thereby determining tumor cell susceptibility to complement-mediated cell death. This work expands the emerging role of mitochondrial chaperones, like mortalin, in cell resistance to a complement stress and identifies TRAP1 as a promising therapeutic target for enhancing antibody-based cancer immunotherapy.