Anti-cancer agent OK432 induces manganese superoxide dismutase in human granulocytes

Synergistic effects of apigenin and paclitaxel on apoptosis of cancer cells

Background

It was well known that the clinical use of chemotherapeutic drugs is restricted by severe adverse reactions and drug resistances. Thus it is necessary to figure out a strategy to increase the specific anti-tumor efficiency of chemotherapeutic drugs. Apigenin, a kind of flavonoids, has been reported to possess anticancer activities with very low cytotoxicity to normal tissue.

Methodology/Principal Findings

Our results from cell viability assay, western-blots and TdT-mediated dUTP-biotin nick end labeling (TUNEL) assay demonstrated the synergistic pro-apoptotic effects of a low dose of apigenin and paclitaxel in human cancer cell lines. To analyze the underlying mechanism, we examined reactive oxygen species (ROS) staining after cells were treated with a combination of apigenin and paclitaxel, or each of them alone. Data from flow-cytometry showed that superoxides but not reduction of peroxides accumulated in HeLa cells treated with apigenin or a combination of apigenin and paclitaxel. Apigenin and paclitaxel-induced HeLa cell apoptosis was related to the level of ROS in cells. We further evaluated activity and protein level of superoxide dismutase (SOD). Apigenin significantly inhibited SOD activity but did not alter the SOD protein level suggesting that apigenin promoted ROS accumulation through suppressing enzyme activity of SOD. Addition of Zn²⁺, Cu²⁺ and Mn²⁺ to cell lysates inhibited apigenin's effects on SOD activity. At the same time, data from caspase-2 over-expression and knocked-down experiments demonstrated that caspase-2 participated in apigenin and paclitaxel-induced HeLa cell apoptosis.

Conclusions/Significance

Taken together, our study demonstrated that apigenin can sensitize cancer cells to paclitaxel induced apoptosis through suppressing SOD activity, which then led to accumulation of ROS and cleavage of caspase-2, suggesting that the combined use of apigenin and paclitaxel was an effective way to decrease the dose of paclitaxel taken.

Superoxide dismutases in malignant cells and human tumors

Reactive oxygen metabolites have multifactorial effects on the regulation of cell growth and the capacity of malignant cells to invade. Overexpression of the superoxide dismutases (SODs) in vitro increases cell differentiation, decreases cell growth and proliferation, and can reverse a malignant phenotype to a nonmalignant one. The situation in vivo is more complex due to multiple interactions of tumor cells with their environment. Numerous in vivo studies show that the superoxide dismutases can be highly expressed in aggressive human solid tumors. Furthermore, high SOD has occasionally been associated with a poor prognosis and with resistance to cytotoxic drugs and radiation. Most of the apparent conflicts between the above in vitro and in vivo observations can be reconciled by considering the net redox status of tumor cells in different environments. Administering high concentrations of SOD to cells in vitro is usually associated with a non- or less malignant phenotype, whereas secondary induction of SOD in tumors in vivo can be associated with an aggressive malignant transformation probably due to the altered (oxidative) redox state in the malignant cells. This concept suggests that for many types of tumors antioxidants could be used to diminish the invasive capability of malignant cells.

Superoxide dismutases in the lung and human lung diseases

The lungs are directly exposed to higher oxygen concentrations than most other tissues. Increased oxidative stress is a significant part of the pathogenesis of obstructive lung diseases such as asthma and chronic obstructive pulmonary disease, parenchymal lung diseases (e.g., idiopathic pulmonary fibrosis and lung granulomatous diseases), and lung malignancies. Lung tissue is protected against these oxidants by a variety of antioxidant mechanisms among which the superoxide dismutases (SODs) are the only ones converting superoxide radicals to hydrogen peroxide. There are three SODs: cytosolic copper-zinc, mitochondrial manganese, and extracellular SODs. These enzymes have specific distributions and functions. Their importance in protecting lung tissue has been confirmed in transgenic and knockout animal studies. Relatively few studies have been conducted on these enzymes in the normal human lung or in human lung diseases. Most human studies suggest that there is induction of manganese SOD and, possibly, extracellular SOD during inflammatory, but not fibrotic, phases of parenchymal lung diseases and that both copper-zinc SOD and manganese SOD may be downregulated in asthmatic airways. Many previous antioxidant therapies have been disappointing, but newly characterized SOD mimetics are being shown to protect against oxidant-related lung disorders in animal models.

MnSOD expression is less frequent in tumour cells of invasive breast carcinomas than in in situ carcinomas or non-neoplastic breast epithelial cells

Manganese superoxide dismutase (MnSOD) is an antioxidant enzyme capable of neutralizing superoxide anion molecules. In previous studies it has been suggested to suppress both tumour proliferation and apoptosis. This study investigated 65 invasive, 50 in situ and 19 benign hyperplastic breast lesions for its immunohistochemical expression. MnSOD expression was also tested with in situ hybridization. To study cell proliferation, apoptosis and their association with MnSOD expression the neoplastic breast lesions were immunostained with a monoclonal antibody to Ki-67 and the extent of apoptosis in them was determined by the TUNEL method. 32/65 (49%) of the invasive ductal carcinomas, 41/50 (82%) of the in situ and 15/19 (79%) of the benign hyperplasias expressed the MnSOD protein. There were significantly more MnSOD positive cases in in situ carcinoma and in benign hyperplasia than in invasive carcinoma (p=0.00016 and p=0.022, respectively). Positivity was also more frequently found in non-neoplastic ductal and acinar epithelial cells than in invasive carcinoma. On the other hand, neoplastic epithelial cells of invasive and in situ carcinoma showed strong positivity more often than the epithelial cells of benign hyperplasia or non-neoplastic epithelium. In breast lesions, MnSOD positivity did not associate with proliferation or apoptosis. The lower frequency of MnSOD positive cases in invasive breast carcinoma suggests that the lack of its expression might contribute to the development of an invasive breast carcinoma phenotype and that it could in this way operate as a tumour suppressor gene, as previously suggested.

Antioxidant defense mechanisms of human mesothelioma and lung adenocarcinoma cells

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-alpha 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-alpha 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 microM, 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 O(2). min(-1). mg protein(-1)). They also contained twofold higher levels of glutathione and higher immunoreactivity of the heavy subunit of gamma-glutamylcysteine synthetase than M14K cells. Experiments with inhibitors of gamma-glutamylcysteine synthetase and catalase supported our conclusion that mechanisms associated with glutathione contribute to the drug resistance of these cells.