Mitochondrial uncoupling protein-2 inhibition promotes gemcitabine response in cancer cells.

Among chemotherapy agents, the pyrimidine nucleoside analog gemcitabine (GEM) is one of the most safe and employed drugs against a variety of solid tumors. Various studies have shown that the mitochondrial uncoupling protein-2 (UCP-2) can mitigate oxidative stress by increasing entrance of protons into the mitochondrial matrix, thus rendering more efficient the electron flow through the respiratory complexes. Here, we show that low doses of the chemical uncoupler FCCP [carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone] or the retinoid TTNPB (5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl-1-propenyl benzoic acid), an activator of UCP-2 proton conductance, prevent mitochondrial superoxide induction by GEM and protect cancer cells from GEM-induced apoptosis. Inversely, the addition to GEM of genipin, a highly selective inhibitor of UCP-2, synergistically inhibits proliferation of cancer cell lines. This effect is mediated by a strong induction of mitochondrial superoxide and apoptosis, which was analyzed by annexinV binding, caspase-3 and -7 activity, and PARP cleavage. The radical scavenger N-acetyl-L-cysteine strongly prevents superoxide generation, apoptosis, and cell growth inhibition by GEM/genipin. Similar data were obtained by over-expressing manganese superoxide dismutase (MnSOD), indicating that mitochondrial oxidative stress plays a key role in the synergistic cell growth inhibition and apoptosis by GEM/genipin combination. In normal primary fibroblasts, the combined treatment induces a very low induction of superoxide ion, apoptosis, and cell growth inhibition, as compared to cancer cells, probably because of a low UCP-2 function, which is instead particularly high in cancer cells to maintain reactive oxygen species within tolerable limits. These data encourage us to develop in vivo studies aimed to enhance GEM response by UCP-2 inhibition.