Role of oxygen radicals in tumor promotion

Oxidative stress in chemical toxicity

The toxic effects of compounds which undergo redox cycling via enzymatic one-electron reduction are reviewed. First of all, the enzymatic reduction of these compounds leads to reactive intermediates, mainly radicals which react with oxygen, whereby superoxide anion radicals are formed. Further oxygen metabolites are hydrogen peroxide, singlet oxygen and hydroxyl radicals. The role of these oxygen metabolites in toxicity is discussed. The occurrence of lipid peroxidation during redox cycling of quinonoide compounds, e.g., adriamycin, and the possible relationship to their toxicity is critically evaluated. It is shown that iron ions play a crucial role in lipid peroxidation induced by redox cycling compounds. DNA damage by metal chelates, e.g., bleomycin, is discussed on the basis of findings that enzymatic redox cycling of a bleomycin-iron complex has been observed. The involvement of hydroxyl radicals in bleomycin-induced DNA damage occurring during redox cycling in cell nuclei is claimed. Redox cycling of other substances, e.g., aromatic amines, is discussed in relation to carcinogenesis. Other chemical groups, e.g., nitroaromatic compounds, hydroxylamines and azo compounds are included. Other targets for oxygen radical attack, e.g., proteins, are also dealt with. It is concluded that oxygen radical formation by redox cycling may be a critical event in toxic effects of several compounds if the protective mechanisms of cells are overwhelmed.

Chloramine-induced sister-chromatid exchanges

Nitrogen-chlorine (N-Cl) derivatives are a class of long-lived oxidants produced by stimulated phagocytes which may be important mediators of the inflammatory response. Because other phagocyte-generated oxidants cause genetic damage in cultured mammalian cells, we studied the ability of the synthetic N-Cl compound, chloramine T (Cl-T), to produce sister-chromatid exchanges (SCEs) in cultured Chinese hamster ovary (CHO) cells. CHO cells were incubated for 30 h with Cl-T (10(-8) M-10(-5) M) and genetic damage was analyzed utilizing the SCE assay. A significant (p less than 0.0005) dose-dependent increase in SCEs was observed. This effect was diminished when cells were treated concomitantly with methionine (10(-5) M), a thioether which reduces N-Cl back to the parent amine. Extracellularly-generated oxidants must traverse long distances before interacting with nuclear target molecules. Therefore, long-lived N-Cl derivatives may represent an important class of oxidants which mediate the process of carcinogenesis associated with chronic inflammatory states in vivo.

Stimulation of superoxide anion formation by the non-TPA type tumor promoters palytoxin and thapsigargin in porcine and human neutrophils

The non-12-O-tetradecanoylphorbol-13-acetate (TPA) type tumor promoters palytoxin and thapsigargin provoked a respiratory burst in porcine and human neutrophils. The amounts of oxygen consumed and superoxide anion (O2-) produced were found to be stoichiometric. Concentrations of 6.5 X 10(-8) M palytoxin and 1.2 X 10(-6) M thapsigargin were required for half-maximal stimulation to 3 X 10(6) porcine cells/ml in Hanks' solution. Combinations of palytoxin and thapsigargin, and of one non-TPA type and one TPA-type tumor promoter, had synergistic effects in stimulating O2- formation in neutrophils, suggesting that these compounds activate the cells by different signal transduction mechanisms.

Modulation of growth, differentiation and carcinogenesis by dehydroepiandrosterone

Dehydroepiandrosterone (3 beta-hydroxy-5-androsten-17-one; DHEA) and its conjugates are abundant circulating steroids that originate largely from the adrenal cortex. Their levels decline profoundly with age in human beings of both sexes, as the incidence of most cancers rises. Low levels of these steroids have been associated with the presence and risk of development of cancer. Administration of DHEA to rodents produces protection against spontaneous tumors and chemical carcinogenesis, suppresses weight gain without significantly affecting food intake, ameliorates the severity of diabetes in genetically diabetic mice, and restrains autoimmune processes. DHEA and related steroids also depress the mitogenic effects of carcinogens, tumor promoters and plant lectins, and block viral and carcinogen-induced cell transformations. DHEA and certain congeners are also potent and quite specific inhibitors of mammalian glucose-6-phosphate dehydrogenases. We have observed that the conversion of 3T3-L1 and 3T3-F442A preadipocyte clones to the adipocyte phenotype, in response to appropriate differentiation stimuli (fetal calf serum, insulin, dexamethasone, and 1-methyl-3-isobutylxanthine), is blocked by DHEA and other steroidal inhibitors of glucose-6-phosphate dehydrogenase. The structural requirements for blocking adipocyte differentiation and for inhibiting glucose-6-phosphate dehydrogenase are closely correlated. Evidence is reviewed suggesting that the inhibition of glucose-6-phosphate dehydrogenase is central to the anticarcinogenic and differentiation-blocking actions of DHEA and related steroids. The 3T3 preadipocyte clones provide a valuable system for the analysis of the mechanisms of the effects of DHEA on growth, differentiation and carcinogenesis.

Antioxidant prevention of tumor promoter induced inhibition of mouse hepatocyte intercellular communication

The liver tumor promoters, phenobarbital (20-500 micrograms/ml), lindane (1,2,3,4,5,6-hexachlorocyclohexane, gamma-isomer; 0.1-5.0 micrograms/ml), and DDT (1,1-bis[4-chlorophenyl]-2,2,2-trichloroethane; 0.5-10.0 micrograms/ml), and the hydrogen peroxide-generating enzyme, glucose oxidase (0.01-0.10 units/ml) inhibited gap junctional intercellular communication between B6C3F1 mouse hepatocytes in primary culture. Addition of the antioxidants, superoxide dismutase (100 units/ml), DPPD (N,N'-diphenyl-1,4-phenylenediamine; 25 microM), and vitamin E (DL-alpha-tocopherol acetate; 100 microM), to tumor promoter-treated cultures prevented the inhibition of hepatocyte intercellular communication. DPPD and vitamin E, prevented the inhibition of hepatocyte intercellular communication by glucose oxidase. Superoxide dismutase had no effect on the inhibition of intercellular communication caused by glucose oxidase. These results suggest that activated oxygen species are produced during liver tumor promoter treatment of cultured mouse hepatocytes and are responsible for the inhibition of mouse hepatocyte intercellular communication by the promoters.

Effects of hydrogen peroxide on oral carcinogenesis in hamsters

The effects of twice weekly topical applications of hydrogen peroxide on the buccal epithelium of Syrian hamsters were studied. Animals were treated either with hydrogen peroxide alone, with hydrogen peroxide and the carcinogen 9, 10-dimethyl-1,2-benzanthracene (DMBA), or with DMBA alone. In animals treated with 30% H2O2 alone, histopathologic examination after 22 weeks revealed hyperkeratosis and hyperplasia in all animals with hyperchromatic cells and mild dysplasia in four of nine: no tumors were seen. In animals treated with DMBA alone, three of seven (43%) developed epidermoid carcinoma, while six of 11 (55%) of animals treated with DMBA plus 3% hydrogen peroxide and five of five (100%) of animals treated with DMBA plus 30% hydrogen peroxide (P = 0.054) developed carcinoma. Thus, hydrogen peroxide can, by itself, induce pathologic changes frequently associated with preneoplastic lesions; it may also augment carcinogenesis associated with DMBA.

Protective effects of 2(3)-tert-butyl-4-hydroxyanisole on chemical carcinogenesis

2(3)-tert-Butyl-4-hydroxyanisole (2(3)-BHA) inhibits the action of a broad spectrum of carcinogens, mutagens and tumour promoters. This review discusses available information on four mechanisms by which 2(3)-BHA brings about these inhibitory effects. The first mechanism is induction of increased activities of enzyme systems having the capacity to detoxify carcinogenic compounds. The second is an alteration of the cytochrome P-450 system, resulting in metabolism of carcinogens to less carcinogenic metabolites. The third is the antioxidant effect of 2(3)-BHA. The fourth is poorly defined and relates to experiments in which 2(3)-BHA inhibits carcinogenesis when administered subsequent to the carcinogen.

Inhibition of glutathione S-transferase P type-positive foci development by linolic acid hydroperoxides and their secondary oxidative products in a rat in vivo mid-term test for liver carcinogens

The effects of linolic acid hydroperoxides (product A) and secondary oxidative products of product A (product B), were examined in an in vivo mid-term test for hepatocarcinogens or hepatopromoters in rats. The number of placental type of glutathione S-transferase (GST-P)-positive foci of the liver was significantly reduced in rats given diethylnitrosamine (DEN) followed by products A (4.64 +/- 1.09, P less than 0.05) or B (3.62 +/- 1.65, P less than 0.01) as compared to the controls given carcinogen alone (6.31 +/- 2.82). The area of GST-P positive foci was also significantly reduced in rats given DEN followed by product B (0.30 +/- 0.21, P less than 0.05) as compared to the controls (0.47 +/- 0.23). These results suggest that linolic acid hydroperoxides or their secondary oxidative products are not hepatocarcinogens and rather may possess inhibitory potential for liver carcinogenesis.

Peroxisome proliferators show tumor-promoting but no direct transforming activity in vitro

The chemically unrelated hypolipidemic drugs, tiadenol, niadenate, and clofibrate have been tested for carcinogenic and tumor-promoting potential in the C3H/10TI/2 C18 cell test system. None of these chemicals were carcinogenic, while both niadenate and clofibrate were active tumor promoters at micromolar concentrations. All 3 drugs induced the differentiation of C3H/10T1/2 C18 cells to adipocytes. This latter finding confirms previously observed effects of the tumor promoter TPA.

Copper ions and hydrogen peroxide form hypochlorite from NaCl thereby mimicking myeloperoxidase

Sea urchins have elaborated multiple defenses to assure monospermic fertilization. In this work, we have concentrated on a study of the mechanism(s) by which hydrogen peroxide (H2O2) prevents polyspermy in Arbacia punctulata. We found that it is not H2O2 but probably hypochlorous acid/hypochlorite (HOCl/OCl-) derived from H2O2 that is toxic to the supernumerary sperm. The spermicidal activity of H2O2 is potentiated by at least one order of magnitude by cupric ions (Cu2+). This increased toxicity is not due to the formation of hydroxyl radicals (.OH) because .OH scavengers did not counteract the activity of Cu2+. Moreover, substitution of Cu2+ by ferrous ions (Fe2+), which are known to cause formation of .OH from H2O2, had no effect on fertilization even at 10(2)-10(3) times higher concentrations. In contrast, 3-amino-1,2,4-triazole (AT), and HOCl/OCl- scavenger, totally reversed the toxic effects of Cu2+. Furthermore, we found that HOCl/OCl- is generated in solutions of H2O2 and Cu2+ in the presence of 0.5 M NaCl and that its accumulation is abolished by AT. Thus it is possible that the antifertility properties of copper are due to its ability to mediate formation of HOCl/OCl-. HOCl/OCl- generated by Cu2+ from H2O2 and Cl-, a low concentration of exogenously added HOCl/OCl-, or increased concentrations of H2O2 has similar inhibitory effects on the fertilization process in sea urchins. Therefore, we suggest that polyspermy is prevented by the action of a myeloperoxidase that affects the formation of HOCl/OCl- from the Cl- present in sea water through reaction with H2O2 generated by the newly fertilized egg.

Cancer and free radicals

It is now clear that free radical intermediates often are involved in the activation of many types of procarcinogens and promutagens to their active forms as well as in the binding of these activated species to DNA. In this chapter, a general introduction to free radical chemistry is presented, with some discussion of radical lifetimes and reactivities. Potential biological targets of radical attack include lipids, proteins, and nucleic acids, and the reactions of all three of these target molecules with radicals are discussed. Finally, the evidence linking free radical reactions with chemical carcinogenesis is reviewed. A mechanistic scheme that divides the mechanisms for activating procarcinogens into 5 types is suggested; of these, 3 types of mechanisms involve free radicals, either in the activation of the carcinogen or in its binding to DNA or both. It also is suggested that a "reverse binding" can occur in which radicals produced on the DNA backbone attack and bond to unactivated substrates, rather than activated substrates (such as radicals) attacking unactivated DNA. It is known that systems that produce superoxide can lead to the production of hydroxyl radicals and that these HO. radicals form radical sites on DNA; thus, reverse binding could occur when any species that can add to a free radical is in the vicinity of the radical-damaged DNA.

Histidine increases the frequency of chromosomal aberrations induced by the xanthine oxidase-hypoxanthine system in V79 cells

The combined effects of the xanthine oxidase (XO)-hypoxanthine (HX) system and the various kinds of amino acids in Eagle's minimum essential medium on chromosomal aberrations were studied in Chinese hamster V79 cells. Among 13 amino acids tested, only histidine significantly increased the number of aberrant chromosomes and cytotoxicity in combination with the XO-HX system. This enhancing effect of histidine on chromosomal aberrations was dose-dependent at 0.063% - 0.25%; it was not affected by superoxide dismutase, but was strongly inhibited by catalase.

The production of DNA strand breaks in human leukocytes by superoxide anion may involve a metabolic process

H2O2 is known to be capable of inducing strand-break damage in intracellular DNA, but whether O2- also can do so in the absence of H2O2 is uncertain. The difficulty in distinguishing the effects of the two is that, under physiological conditions, dismutation of O2- to H2O2 can readily occur. When human leukocytes are stimulated with phorbol 12-myristate 13-acetate (PMA), they release O2-, and within a few minutes strand breakage in intracellular DNA can be observed. We have attempted to determine whether the O2- produced is itself capable of causing DNA damage or whether H2O2 alone, or in combination with O2-, is responsible for the observed damage. Addition of catalase (up to 250 micrograms/ml) to remove H2O2 prevented no more than about 50% of the DNA damage. The majority of the remaining damage could be blocked, in a dose-dependent manner, by superoxide dismutase (SOD) or a SOD-mimetic copper complex, identifying a fraction of damage to intracellular DNA dependent upon extracellular O2-. We studied this O2(-)-specific fraction through the use of three metabolic poisons (fluoride, 2-deoxyglucose, and A23187). These agents largely blocked DNA damage, while affecting extracellular O2- levels only slightly. For comparison, H2O2-induced DNA damage was studied with glucose oxidase to generate a flux of H2O2. The first two metabolic poisons had little effect, whereas A23187 did inhibit H2O2-induced DNA damage. We conclude that O2(-)-induced damage occurs through a mechanism that differs, at least in part, from the H2O2 damage pathway and that the former may involve one or more metabolic steps.

Oxidant-dependent metabolic activation of polycyclic aromatic hydrocarbons by phorbol ester-stimulated human polymorphonuclear leukocytes: possible link between inflammation and cancer

Oxidants, such as those generated by metabolically activated phagocytes in inflammation, have been implicated in the metabolic activation of carcinogens, and in this study we demonstrate that the interaction of (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP 7,8-dihydrodiol) with phorbol ester-stimulated polymorphonuclear leukocytes (PMNs) results in the generation of both a chemiluminescent intermediate and one that covalently binds to DNA. Cu(II)(3,5-diisopropylsalicylic acid)2 (CuDIPS), a biomimetic superoxide dismutase, and azide, a myeloperoxidase inhibitor, inhibited both of these reactions, indicating a dependency on oxygen-derived oxidants in these hydrocarbon-activation processes. Concordant with the formation of a carcinogen-DNA adduct, the admixture of BP 7,8-dihydrodiol and phorbol ester-stimulated PMNs elicited mutagenesis in Salmonella typhimurium strain TA100. 7,8-Dihydro-BP and BP cis-7,8-dihydrodiol were also mutagenic, whereas derivatives lacking a double bond at the 9,10 position were not. These results demonstrate that oxidants generated by metabolically stimulated PMNs can activate penultimate polycyclic aromatic hydrocarbons to a genotoxic metabolite and further defines a role for inflammation in carcinogenesis.

Induction of nitroblue tetrazolium reduction in mouse peritoneal macrophages by tumour promoters and inhibition of the induced nitroblue tetrazolium reduction by some inhibitors

Two polyacetates, aplysiatoxin and debromoaplysiatoxin, as well as 12-O-tetradecanoylphorbol-13-acetate (TPA), mezerein and teleocidin enhance nitroblue tetrazolium (NBT) reduction in mouse peritoneal macrophages in vitro. The ED50 values for NBT reduction of these 5 TPA-type tumor promoters were 4.2 ng/ml for TPA, 36 ng/ml for mezerein, 0.53 ng/ml for teleocidin, 1.5 ng/ml for aplysiatoxin and 108 ng/ml for debromoaplysiatoxin. The NBT reduction induced by the 5 tumor promoters is inhibited by 2 inhibitors of tumor promotion, retinoic acid and dibromoacetophenone. The possibility that tumor promotion by TPA-type tumor promoters involves similar mechanisms such as superoxide anion radicals release in cell membranes is discussed.

Are free radicals involved in tumor promotion?

Previously it was shown that lipophilic analogs of a free-radical scavenger, 2(3)-tert-butyl-4-hydroxyanisole (BHA), inhibit ornithine decarboxylase (ODC) activity which is induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis. With regard to this antitumor-promoting effect, eight analogs of BHA (2- and 3-BHA, 2-t-butyl-1, 4-dimethoxybenzene methyl-BHA), t-butylhydroquinone (t-BHQ), p-hydroquinone (HQ), 4-hydroxyanisole, phenol and 2-t-butylphenol) are evaluated herein for their antioxidant capacities for scavenging superoxide anions (O-2), of inhibiting lipid peroxidation and of inhibiting chemiluminescence (CL) in TPA-activated polymorphonuclear leukocytes (PMNs), an event associated with oxy-radical production. None of the analogs reacted with O-2, while 2- and 3-BHA suppressed the formation of O-2 by TPA-activated PMNs. T-BHQ underwent autoxidation in aqueous solution, reducing molecular oxygen and increasing the levels of O-2 that were formed chemically, enzymatically and cellularly. However, all of the phenolic antioxidant analogs of BHA inhibited TPA-stimulated CL in PMNs and ascorbate-initiated lipid peroxidation, while methyl-BHA (a non-antioxidant analog) was inactive. The inhibitory activities of these analogs for lipid peroxidation were related to both their lipophilic and antioxidant properties and corresponded favorably with their inhibitory activities for TPA-induced ODC activities in mouse epidermis. On the other hand, inhibition of the CL response by these antioxidants was independent of their lipophilicity and compared less favorably with their capacities to antagonize phorbol ester-induced ODC activity. These results imply that lipophilic BHA analogs inhibit TPA-induced ODC activity by scavenging free radicals other than O-2. Furthermore, the fact that t-BHQ was the most potent inhibitor of CL, lipid peroxidation and ODC activity and simultaneously reduced molecular oxygen, suggests the possibility that O-2 may act as a precursor to the formation of free radicals which are reactive with t-BHQ and more directly involved in the process of tumor promotion.

Superoxide anion is generated from cellular metabolites by solar radiation and its components

Several endogenous cellular constituents were tested for their ability to produce superoxide anion (O2-) from ground-state molecular oxygen upon irradiation by solar radiation. The pyridine cofactors NADPH and NADH, riboflavin, and the nucleosides 2-thiouracil and 4-thiouridine were found to sensitize the transmission of photon energy from solar radiation and monochromatic radiation (290, 334, 365, and 405 nm) to oxygen, resulting in O2- formation, as detected by superoxide dismutase-inhibitable cytochrome c reduction. Quantum yields for the production of O2- indicate that NADPH is the most efficient and riboflavin the least efficient of the compounds tested. These data indicate that endogenous compounds may participate in the production of O2- by solar radiation and imply that O2- may play a role in sunlight-induced erythema and dermal carcinogenesis.