Antioxidant defense mechanisms of human mesothelioma and lung adenocarcinoma cells

Abstract

The development of drug resistance of tumors is multifactorial and still poorly understood. Some cytotoxic drugs generate free radicals, and, therefore, antioxidant enzymes may contribute to drug resistance. We investigated the levels of manganese superoxide dismutase (Mn SOD), its inducibility, and its protective role against tumor necrosis factor-α and cytotoxic drugs (cisplatin, epirubicin, methotrexate, and vindesin) in human pleural mesothelioma (M14K) and pulmonary adenocarcinoma (A549) cells. We also studied other major antioxidant mechanisms in relation to oxidant and drug resistance of these cells. A549 cells were more resistant than M14K cells toward both oxidants (hydrogen peroxide and menadione) and all the cytotoxic drugs tested. M14K cells contained higher basal Mn SOD activity than A549 cells (28.3 ± 3.4 vs. 1.8 ± 0.3 U/mg protein), and Mn SOD activity was significantly induced by tumor necrosis factor-α only in A549 cells (+524%), but the induction did not offer any protection during subsequent oxidant or drug exposure. Mn SOD was not induced significantly in either of these cell lines by any of the cytotoxic drugs (0.007–2 μM, 48 h) tested when assessed by Northern blotting, Western blotting, or specific activity. A549 cells contained higher catalase activity than M14K cells (7.6 ± 1.3 vs. 3.6 ± 0.5 nmol O2⋅ min−1⋅ mg protein−1). They also contained twofold higher levels of glutathione and higher immunoreactivity of the heavy subunit of γ-glutamylcysteine synthetase than M14K cells. Experiments with inhibitors of γ-glutamylcysteine synthetase and catalase supported our conclusion that mechanisms associated with glutathione contribute to the drug resistance of these cells.