Macrophage-mediated inhibition of mitochondrial respiration in EMT-6 murine mammary
adenocarcinoma cells can be mimicked in vitro by treatment of the cells with
interferon-gamma (IFN-gamma) in combination with
tumor necrosis factor,
interleukin-1, or
lipopolysaccharide. Conditioned supernatants obtained from activated macrophages appear to contain
interferon-gamma, suggesting that inhibition of mitochondrial respiration in
tumor cells was caused by synergy of IFN-gamma with other
cytokines. To further characterize
monokines that cause inhibition of mitochondrial respiration in
tumor cells, EA13.5 macrophage-like cells were isolated and selected for inhibition of mitochondrial respiration in EMT-6
tumor cells. After stimulation with IFN-gamma and
lipopolysaccharide, the EA13.5 cells released into conditioned supernatants a cytotoxic mediator that induced
nitric oxide synthesis and caused lesions in the electron transport chain of EMT-6 cells similar to the lesions caused by activated peritoneal macrophages.
Enzyme-linked
immunosorbent assay demonstrated that the conditioned supernatants produced by EA13.5 macrophage cells did not contain IFN-gamma. Treatment of the EA13.5 cell-conditioned supernatants with
neutralizing antibody against IFN-gamma did not abrogate the inhibition of mitochondrial respiration in EMT-6 cells caused by these conditioned supernatants. This study demonstrated that unidentified macrophage cytotoxic mediators distinct from IFN-gamma are involved in the induction of
nitric oxide synthesis and inhibition of mitochondrial respiration in
tumor cells.