Inhibition of 12-O-tetradecanoylphorbol-13-acetate-enhanced transformation in vitro by radical scavengers

Reactive Oxygen Species and Metabolism in Leukemia: A Dangerous Liaison

Reactive oxygen species (ROS), previously considered toxic by-products of aerobic metabolism, are increasingly recognized as regulators of cellular signaling. Keeping ROS levels low is essential to safeguard the self-renewal capacity of hematopoietic stem cells (HSC). HSC reside in a hypoxic environment and have been shown to be highly dependent on the glycolytic pathway to meet their energy requirements. However, when the differentiation machinery is activated, there is an essential enhancement of ROS together with a metabolic shift toward oxidative metabolism. Initiating and sustaining leukemia depend on the activity of leukemic stem cells (LSC). LSC also show low ROS levels, but unlike HSC, LSC rely on oxygen to meet their metabolic energetic requirements through mitochondrial respiration. In contrast, leukemic blasts show high ROS levels and great metabolic plasticity, both of which seem to sustain their invasiveness. Oxidative stress and metabolism rewiring are recognized as hallmarks of cancer that are intimately intermingled. Here we present a detailed overview of these two features, sustained at different levels, that support a two-way relationship in leukemia. Modifying ROS levels and targeting metabolism are interesting therapeutic approaches. Therefore, we provide the most recent evidence on the modulation of oxidative stress and metabolism as a suitable anti-leukemic approach.

Suppression of depleted uranium-induced neoplastic transformation of human cells by the phenyl fatty acid, phenyl acetate: chemoprevention by targeting the p21RAS protein pathway

Depleted uranium is a dense heavy metal used primarily in military applications. Published data from our laboratory have demonstrated that exposure to depleted uranium in vitro can transform immortalized human osteoblast (HOS) cells to the tumorigenic phenotype (associated with aberrant RAS oncogene expression and tumor suppressor protein production). Since depleted uranium is used in military applications, it would therefore be beneficial to identify and test potential antitumor-promoting agents. Chemopreventive interventions that target deregulated signal transduction pathways may be effective strategies to prevent carcinogenesis. Since the RAS protein plays a key role in signal transduction, disruption of its signaling pathway may be particularly effective. The phenyl fatty acid, phenyl acetate, a differentiation inducer that affects post-translational processing of RAS, was tested for its ability to prevent depleted uranium-induced neoplastic transformation using HOS cells. After a 24-h exposure to insoluble depleted uranium-uranium dioxide (1 mg/ml), cells were incubated for 1 day to 6 weeks with 2.5 mM phenyl acetate. Treatment with depleted uranium resulted in transformation to the tumorigenic phenotype. In contrast, HOS cells exposed to depleted uranium and then treated with phenyl acetate did not exhibit transformation to the tumorigenic phenotype. These data suggest that depleted uranium-induced neoplastic transformation in vitro can be prevented by targeting the RAS protein.

Lipid peroxidation, free radical production and antioxidant status in breast cancer

Reactive oxygen metabolites (ROMs), including superoxide anion (O2*-), hydrogen peroxide (H2O2) and hydroxyl radical (*OH), play an important role in carcinogenesis. There are some primary antioxidants such as superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) which protect against cellular and molecular damage caused by the ROMs. We conducted the present study to determine the rate of O2*- and H2O2 production, and concentration of malondialdehyde (MDA), as an index of lipid peroxidation, along with the SOD, GPx and CAT activities in 54 breast cancer (BC) patients. Forty-two age- and sex-matched patients with minor surgical problems, who had no history of any neoplastic or breast disorders, were taken as controls. The rate of O2*- production was significantly higher (p < 0.001) in BC patients than controls, irrespective of clinical stages and menopausal status. Similarly, H2O2 production was significantly higher in BC patients, especially in stage III and postmenopausal groups, as compared to the respective controls. MDA concentration was also observed significantly elevated in stage II (p < 0.001), stage III (p < 0.01), postmenopausal (p < 0.005), and premenopausal (p < 0.02) group as compared to their corresponding controls. SOD and GPx activities were found significantly raised in all the groups (p < 0.001), except the GPx activity was found a smaller alteration in stage IV (p < 0.02). On the contrary, CAT activity was found significantly depressed in all the study groups. The maximum depression was observed in stage II (-61.8%). Lower CAT activity in our study may be the effect of higher production of ROMs, particularly O2*- and *OH. SOD and GPx, however, were less effected by these higher ROMs production. The results of our study have shown a higher ROMs production and decreased CAT activity, which support the oxidative stress hypothesis in carcinogenesis. The relatively higher SOD and GPx may be due to the response of increased ROMs production in the blood. However, the higher SOD and GPx activities may be inadequate to detoxify high levels of H2O2 into H2O leading to the formation of the most dangerous *OH radical followed by MDA. Therefore, administration of CAT may be helpful in the management of BC patients. However, further elaborate clinical studies are required to evaluate the role of such antioxidant enzymes in BC management.

Piperine modulation of carcinogen induced oxidative stress in intestinal mucosa

Reactive oxygen species (ROS) and reactive metabolic intermediates generated from various chemical carcinogens are known to play an important role in cell damage and in the initiation and progression of carcinogenesis. Many radical scavengers, interestingly naturally occuring antioxidants have been found to be effective in inhibiting the induction of carcinogenesis by a wide variety of chemical carcinogens. Studies have also indicated that various spice principles form an important group as antioxidants. In the present study our goal was to investigate whether piperine an pungent principle of black and long peppers was able to inhibit or reduce the oxidative changes induced by chemical carcinogens in rat intestinal model. Carcinogenesis was initiated in intestinal lumen of male rats with 7,12,dimethyl benzanthracene, dimethyl amino-methyl azobenzene and 3-methyl cholenthrene. Oxidative alterations were assessed by determining thiobarbituric reactive substances, mainly malonaldehyde (as a measure of lipid peroxidation), thiol status and expression of gamma-GT and Na+-K+-ATPase activity in intestinal mucosa. Data indicated that carcinogens treatment induced GSH depletion with substantial increase in thiobarbituric reactive substances and enzyme activities. Piperine treatment with carcinogens resulted in inhibition of thiobarbituric reactive substances. It mediated a significant increase in the GSH levels and restoration in gamma-GT and Na+-K+-ATPase activity. The studies thus indicate a protective role of piperine against the oxidative alterations by carcinogens. It may be suggested that piperine modulates the oxidative changes by inhibiting lipid peroxidation and mediating enhanced synthesis or transport of GSH thereby replenishing thiol redox.

Inhibitory action of (-)-epigallocatechin gallate on radiation-induced mouse oncogenic transformation

The anticarcinogenic activity of a major component of green tea, (-) epigallocatechin gallate (EGCg) was examined by using the radiation-induced oncogenic transformation in C3H10T1/2 cells. EGCg substantially suppressed the radiation-induced transformation so that the transformation frequency with 15 microM of EGCg was reduced nearly to spontaneous levels. This effect of EGCg was in a dose-dependent manner and significant suppression of transformation was observed even in treatment of cells with 5 microM of EGCg concentration where the cytotoxicity was mild. The inhibitory effect of EGCg was maximal when it was present during the entire incubation period. However, neither treatment prior to nor concurrent with radiation was effective, suggesting that EGCg action is mainly involved in the promotional stage of C3H10T1/2 cell transformation.

Inhibitory effects of Rooibos tea, Aspalathus linealis, on X-ray-induced C3H10T1/2 cell transformation

Oncogenic transformation of mouse C3H10T1/2 cells induced by X-rays was suppressed in the presence of extract of Rooibos tea, Aspalathus linealis. Transformation was reduced with increased concentration of the extract, so that at an extract concentration of 10%, transformation incidence was similar to the spontaneous level. Suppression was also dependent on treatment time with the extract and was maximal when present during the entire incubation period. In contrast, green tea extract at an equitoxic concentration showed no detectable effect on transformation incidence.

Antipromoting effects of 5-lipoxygenase inhibitors, 3-nitro-2,4,6- trihydroxybenzamide derivatives, on TPA-promoted transformation in BALB 3T3 cells

In order to discover the role in the promotion process of 5-lipoxygenase (5-LO), a key enzyme in the arachidonate cascade, the antipromoting effects of the 5-LO inhibiting 3-nitro-2,4,6-trihydroxybenzamide (NTB) derivatives were studied in a two-stage transformation assay system using BALB 3T3 cells. All compounds inhibited TPA-promoted transformation in a dose-dependent manner. Most of them achieved a 70-80% inhibition. Good correlations were observed between the inhibition of TPA-promoted transformation and that of 5-LO. These results indicate that 5-LO plays an important role in the promotion stage of the transformation of BALB 3T3 cells.

Inhibitory effects of radical scavengers on diacylglycerol-promoted transformation in BALB/3T3 cells

To discover the relationship between the activation of protein kinase C and the generation of reactive oxygen in the tumor promotion process, we investigated the effects of radical scavengers on diacylglycerol-promoted transformation in BALB/3T3 cells. Superoxide dismutase (SOD) showed inhibitory effects on both 1-oleoyl-2-acetylglycerol (OAG)-promoted and diolein-promoted transformation. Catalase (CT) suppressed the promoting effects of diolein by up to 70%. Mannitol (MT), a hydroxyl radical (.OH) scavenger, inhibited diacylglycerol-promoted transformation dose-dependently. These results suggest that activation of protein kinase C alone is insufficient and that generation of reactive oxygen accompanied by activation of the enzyme is essential to the promotion process in BALB/3T3 cells.

Rapid in vitro transformation system for liver epithelial cells by iron chelate, Fe-NTA

We have previously found toxic effects of iron chelate, Fe-NTA on cultured normal rat liver epithelial cells (RL34). In the present study, when RL34 cells were exposed to 50 micrograms/ml iron of Fe-NTA for 15 days, besides the expected cytolytic effects in most cells, the appearance of resistant cells was observed. The resistant cells showed drastic morphological transformation, grew in soft agar, and induced hepatocellular carcinomas when transplanted into syngeneic newborn rats in a short period of time. Since DNA instability in the transformed cells was ascertained by differential AO staining, it is suggested that DNA damage by Fe-NTA is of a critical importance for extremely rapid neoplastic transformation of normal epithelial cells.

Effects of activators and inhibitors of protein kinase C on two-stage transformation in BALB/3T3 cells

To determine the relationship between protein kinase C and the promotion of carcinogenesis, we investigated the effects of activators and inhibitors of protein kinase C on two-stage transformation in BALB/3T3 cells. Diacylglycerols, which are activators, and specific inhibitors, such as palmitoyl-DL-carnitine chloride (PC), 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), and staurosporine (ST) were used. Treatment with diacylglycerols enhanced focus formation in 3-methylcholanthrene (3-MC)-initiated cells, but not as much as 12-O-tetradecanoylphorbol-13-acetate (TPA). PC and H-7 inhibited TPA-enhanced transformation by 76 and 79%, respectively. ST, the most potent inhibitor of protein kinase C, had a low inhibitory effect on transformation at non-toxic doses (33% inhibition). The results suggest that protein kinase C may play an important role in the process by which transformation is promoted in BALB/3T3 cells.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-enhanced transformation in vitro by inhibitors of phospholipid metabolism

The effects of some inhibitors of phospholipid metabolism were studied using a two-stage transformation assay system in vitro with 3-methylcholanthrene (3-MC) initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion in BALB/3T3 cells. Inhibitors of phospholipid metabolism such as hydrocortisone (HC), p-bromophenacyl bromide (BPB), nordihydroguaiaretic acid (NDGA), quercetin and indomethacin (IM) were used. They were demonstrated to inhibit strongly TPA-promoted transformation. In particular, HC at 1 microgram/ml inhibited the in vitro transformation completely. All the inhibitors used inhibited transformation in a dose-dependent manner. The results suggest that phospholipid metabolism is important for the promotion stage of transformation in BALB/3T3 cells.

Free radicals, antioxidant enzymes, and carcinogenesis

Free radicals are found to be involved in both initiation and promotion of multistage carcinogenesis. These highly reactive compounds can act as initiators and/or promoters, cause DNA damage, activate procarcinogens, and alter the cellular antioxidant defense system. Antioxidants, the free radical scavengers, however, are shown to be anticarcinogens. They function as the inhibitors at both initiation and promotion/transformation stage of carcinogenesis and protect cells against oxidative damage. Altered antioxidant enzymes were observed during carcinogenesis or in tumors. When compared to their appropriate normal cell counterparts, tumor cells are always low in manganese superoxide dismutase activity, usually low in copper and zinc superoxide dismutase activity and almost always low in catalase activity. Glutathione peroxidase and glutathione reductase activities are highly variable. In contrast, glutathione S-transferase 7-7 is increased in many tumor cells and in chemically induced preneoplastic rat hepatocyte nodules. Increased glucose-6-phosphate dehydrogenase activity is also found in many tumors. Comprehensive data on free radicals, antioxidant enzymes, and carcinogenesis are reviewed. The role of antioxidant enzymes in carcinogenesis is discussed.

Antioxidants and multistage carcinogenesis in mouse skin

The two-step initiation-promotion protocol for the induction of skin tumors in mice is a convenient model to elucidate what molecular events are involved in the multistage process of carcinogenesis and how they can be modulated. The current theories concerning the mechanisms of skin tumor initiation, stages 1 and 2 of tumor promotion, and tumor progression are reviewed. Because chemical carcinogens and tumor promoters may, directly or indirectly, generate reactive oxygen species (ROS) and because various antioxidants inhibit effectively some of the biochemical and biological events linked to tumor initiation, promotion and/or progression, it is conceivable that different sequences and levels of free radical-induced macromolecule damage may contribute to the evolution of the epidermal target cells from the preneoplastic stage to the malignant stage.

Direct evidence of intercellular sharing of glutathione via metabolic cooperation

Intracellular glutathione (GSH) content and cell density are known to be two important determinants of cell sensitivity to free radicals and radiation. We have investigated intercellular sharing of GSH via metabolic cooperation (MC) by measuring the GSH content of Chinese hamster V79 cells under conditions that varied MC among cells. GSH was measured by flow cytometry with monochlorobimane, which becomes fluorescent after conjugation to GSH by GSH-S-transferase. High-performance liquid chromatography was used to confirm the accuracy of GSH measurements by flow cytometry. Several lines of evidence indicate sharing of GSH or its precursor gamma-glutamylcysteine via MC. These include a cell density-dependent heterogeneity in GSH content, reconstitution of GSH in GSH-depleted cells by coculture with nondepleted cells (except when the depleted cells were MC deficient), and decreased equilibration of GSH among GSH-depleted cells and nondepleted cells when an inhibitor of MC (phorbol myristate acetate) was present. The equilibration of GSH among GSH-depleted cells and nondepleted cells in coculture was not inhibitable by acivicin, suggesting that this form of intercellular sharing of GSH does not rely on gamma-glutamyltransferase-mediated extracellular transport of GSH.