In vitro BPDE-induced DNA adducts in peripheral lymphocytes as a risk factor for squamous cell carcinoma of the head and neck

The level of DNA adducts under the same conditions of carcinogen exposure and cell proliferation reflects an integrated measure of carcinogen metabolism and DNA repair. Therefore, such DNA adduct levels have the potential to be a biomarker for susceptibility to chemical carcinogenesis. In a pilot study of 91 patients with squamous cell carcinomas of the head and neck and 115 controls who were frequency matched by age, sex, ethnicity, and smoking status, we applied a newly developed in vitro assay of benzo[a]pyrene diol epoxide (BPDE)-induced DNA adducts in short-term peripheral lymphocytes cultures. Levels of BPDE-DNA adducts were found to be significantly higher in cases than in controls (mean +/- SD, 76.8 +/- 77.4/10(7) and 47.1 +/- 48.0/10(7) nucleotides, respectively; p < 0.001). Using the median level of control values (35/10(7)) as the cut-off point, about 66% of cases were distributed above this level. Logistic regression analysis revealed that the level of BPDE-induced DNA adducts was an independent risk factor (odds ratio = 2.22; 95% confidence interval = 1.22--4.04) after adjustment for age, sex and smoking status. Further stratified analyses showed that levels of the induced adducts between cases and controls were significantly higher in both age groups, that is, younger or older than 60, as well as in both men and women. Smoking had a positive effect on the induced adducts. The highest level of induced adducts was seen in current smokers, then former smokers and non-smokers. There was a statistically significant dose--response relationship between the quartile levels of BPDE-induced DNA adducts and the risk of head and neck cancer (trend test, p = 0.003). Despite the relatively small sample size, the association of BPDE-induced DNA adducts and cancer risk suggests that this assay has the potential to complement with other biomarkers in identifying individuals at increased risk of developing tobacco-related cancers.

Sensitivity to DNA damage induced by benzo(a)pyrene diol epoxide and risk of lung cancer: a case-control analysis

Levels of DNA adducts vary greatly in vivo, attributable to individual differences in enzymatic bioactivation of benzo(a)pyrene. We developed an assay to measure the levels of DNA adducts induced in vitro by benzo(a)pyrene diol epoxide (BPDE), a bioactivated form of benzo(a)pyrene. In this large molecular epidemiological study of lung cancer, we tested the hypothesis that the level of in vitro BPDE-induced adducts is associated with risk of lung cancer. This hospital-based case-control study included 221 newly diagnosed lung cancer cases and 229 healthy controls frequency matched on age, sex, ethnicity, and smoking status. Short-term cultured peripheral blood lymphocytes from each subject were exposed in vitro to BPDE (4 microm) for 5 h, and the 32P-postlabeling method was then used to measure BPDE-induced DNA adducts in the host cells. Overall, the patients had significantly higher levels of BPDE-DNA adducts than did the controls (mean +/- SD per 107 nucleotides, 93.2+/-89.3 for cases versus 63.7+/-61.1 for controls; P = 0.001). Univariate and multivariate logistic regression analyses were performed to calculate the crude and adjusted odds ratios and their 95% confidence intervals. When the median adduct level of controls (46/10(7) nucleotides) was used as the cutoff point, 64% of cases had higher levels (odds ratio, 2.15; 95% confidence interval, 1.39-3.33, adjusted for age, sex, ethnicity, body mass index, recent weight loss, pack-years smoked, smoking in the last 24 h, and family history of cancer). Stratified analyses showed consistently higher levels of BPDE-induced adducts in cases than in controls, regardless of subgroup of age, sex, ethnicity, body mass index, recent weight loss, pack-years smoked, smoking in the last 24 h, and family history of cancer. A significant dose-response relationship between the quartile levels of BPDE-induced DNA adducts and the risk of lung cancer was observed (trend test, P < 0.001). The significant association between the level of in vitro BPDE-induced DNA adducts and risk for lung cancer suggests that subjects very sensitive to BPDE-induced DNA damage may have a suboptimal ability to remove the BPDE-DNA adducts and so are susceptible to tobacco carcinogen exposure and, therefore, may be at increased risk of lung cancer.

Characterization of the major DNA adducts in the liver of rats chronically exposed to tamoxifen for 18 months

Our previous study has shown that chronic exposure to tamoxifen (TAM) induced formation of high levels of DNA adducts in the liver, the target tissue of TAM-induced carcinogenesis in rats. One of the major DNA adducts (spot 1), as detected by 32P-postlabeling, accounted for 53% of the total adducts. To characterize this major adduct, the current study has compared spot 1 with two previously identified TAM-DNA adducts, i.e. alpha-TAM-N2-deoxyguanine (alpha-TAM-N2-dG) and alpha-N-desmethyl TAM-N2-deoxyguanine (alpha-N-dmTAM-N2-dG) by various rechromatography methods. It was found that spot 1 was further resolved into two fractions during rechromatography analysis, one fraction co-migrated with the alpha-TAM-N2-dG and the other fraction co-migrated with the alpha-N-dmTAM-N2-dG. These findings have demonstrated that chronic exposure to tamoxifen induced the same major DNA adducts, i.e. alpha-TAM-N2-dG and alpha-N-dmTAM-N2-dG as those detected in acutely exposed rats.

Action of curcumin on the cytochrome P450-system catalyzing the activation of aflatoxin B1

Curcumin, in a dose-dependent manner, inhibited the formation of covalent adduct between aflatoxin B1 and DNA, as catalyzed by microsomes or a reconstituted microsomal monooxygenase system. Its effect on the cytochrome P450-system was investigated in the latter system. The inhibition (50%) of aflatoxin B1-DNA adduct formation by curcumin in this system could be reversed by increasing the amount of cytochrome P450 but not by that of NADPH-cytochrome P450 reductase. Curcumin inhibited the reductase activity when measured by the reduction of cytochrome C but not when measured by the reduction of dichlorophenolindophenol, an artificial electron acceptor. These results, as well as the reversal of curcumin-induced inhibition of P450 reductase activity by higher amounts of cytochrome C, indicated a strong affinity of curcumin towards cytochromes. This was further substantiated from the observation that curcumin-pretreated cytochrome P450 had reduced ability to catalyze aflatoxin B1-DNA adduct formation in the reconstituted system. Curcumin, thus, may inhibit chemical carcinogenesis by modulating cytochrome P450 function.

Effects of natural polyphenols on aflatoxin B1 activation in a reconstituted microsomal monooxygenase system

Covalent adduct formation between aflatoxin B1 and DNA, as catalyzed by a reconstituted microsomal monooxygenase system containing purified cytochrome P450 and NADPH-cytochrome P450 reductase, was observed to be inhibited by certain polyphenolic compounds of natural origin. Polyhydroxylated flavonoids were found to be more effective than phenolic acids and displayed dose-dependent inhibition. The inhibition (50%) could be reversed by increasing the amount of cytochrome P450 but not by increasing the amount of reductase. Each polyphenol inhibited NADPH-cytochrome P450 reductase activity as measured by reduction of cytochrome C. This inhibition could be reversed with higher amounts of cytochrome C. This inhibition, however, could not be reversed if an artificial electron acceptor, dichlorophenolindophenol, was used in place of cytochrome C. These results suggest a strong affinity of polyphenols toward cytochromes. This conclusion was further supported from the observation that pretreatment of cytochrome P450 with each polyphenol reduced its ability to catalyze aflatoxin B1-DNA adduct formation in the reconstituted system. Natural polyphenols, thus, may have the ability to modulate chemical carcinogenesis by modulating cytochrome P450 function.

Modulation by some natural carotenoids of DNA adduct formation by aflatoxin B1 in vitro

Nine natural carotenoids have been tested for their ability to modulate formation of DNA adduct by the carcinogen aflatoxin B1 (AFB1) in an in vitro reaction catalyzed by rat liver microsomes. Certain apo-carotenoids, which are precursors of vitamin A, were found to be very efficient in inhibiting the adduct formation. Some other carotenoids, which although have less pro-vitamin A activity, also showed similar inhibitory effect. The DNA adduct formation appeared to be modulated through the action of the carotenoids on microsomal enzymes, since an effective inhibition in each case was observed on the formation of the activated AFB1-metabolite. It is envisaged that such inhibition could form a basis by which natural carotenoids could counteract the carcinogenic action of AFB1.

Effect of plant flavonoids on microsome catalyzed reactions of aflatoxin B1 leading to activation and DNA adduct formation

Twenty-six plant phenolic flavonoids were tested for their ability to modulate microsome mediated activation of aflatoxin B1 (AFB1) and covalent adduct formation of activated metabolites with DNA. Some of the flavonoids were observed to inhibit both the reactions very significantly. A few of these showed inhibition of these reactions at varying degrees while others were inactive. Structurally, polyhydroxy flavonols such as robinetin, quercetin, fisetin and morin were found to be very active. Methylation and glycosylation usually rendered the flavonoid less active or inactive. With active flavonoids DNA adduct formation appeared to be modulated through their action on microsomal enzymes, and this could form a basis to counteract the carcinogenic action of aflatoxiin B1.

Action of vitamin A on DNA adduct formation by aflatoxin B1 in a microsome catalyzed reaction

The effect of vitamin A and some derivatives has been studied on the formation of DNA adduct by aflatoxin B1 (AFB1) in an in vitro reaction catalyzed by rat liver microsomes. Retinol, retinal, all trans retinoic acid and two retinyl esters were found to inhibit the adduct formation in a dose-dependent manner. The inhibition by retinol showed that it was prompt and could be reversed by increasing microsome concentration. Retinol also inhibited the formation of AFB1 adduct with microsomal protein. The inhibition of adduct formation with both DNA and protein was still apparent when repurified microsomes were used after pretreatment with retinol. Retinol inhibited adduct formation by virtue of its ability to interact with microsomal enzyme component(s), thereby interfering with the bioactivation of AFB1. The data are suggestive of a potential anticarcinogenic role for vitamin A against AFB1.

Modulation by certain factors of metabolic activation of aflatoxin B1 as detected in vitro in a simple fluorimetric assay

Forty compounds belonging to various chemical groups have been tested for their ability to suppress metabolic activation of aflatoxin B1 (AFB1) mediated by rat liver microsome. Microsomal activation has been carried out in an in vitro system containing Tris-buffer at pH 7.2. Production of the reactive metabolite, AFB1 8,9-epoxide, has been measured by separation and detection of its hydrolysis product AFB1 8,9-dihydrodiol as the Tris-diol complex. The complex is separated using simple procedures of biphasic extraction and deproteinization, and detected by its characteristic fluorescence. Quantitation is made by direct comparison of its fluorescence with that of an authentic Tris-diol prepared synthetically. The method is rapid and proved to be highly sensitive and reproducible. A large number of compounds have been observed to modulate at varying degrees the activation of AFB1 in this in vitro system. Many compounds have been tested at several concentration ranges and inhibition curve is constructed in each case from which ID50 values, i.e., the dose needed to bring about 50% inhibition can be obtained. These values expressed as nmol afford a direct and realistic comparison of the inhibitory potential of various modulators. Factors having great inhibitory potential have been identified as retinoids (retinol, retinal, retinoic acid, retinyl acetate), beta-carotene, riboflavin, ascorbic acid, copper, zinc, linoleic acid, p-hydroxy benzoic acid, butylated hydroxyanisole, butylated hydroxytoluene, disulfiram, and phenothiazine. Several other compounds have shown moderate inhibitory potential. The strong inhibition on Tris-diol formation by several vitamins, antioxidants and trace metals shows similarity with their effect on AFB1-DNA adduct formation. It is suggested that these agents may have potential anticarcinogenic activity against AFB1.

Factors modulating the formation of DNA adduct by aflatoxin B1 in vitro

Forty-two compounds belonging to various chemical groups have been tested for their ability to suppress formation of aflatoxin B1--DNA adduct mediated by microsome in vitro. While many of these compounds have either marginal or no modulating effect, some have been identified as effective inhibitors. The strong inhibition of DNA adduct formation by retinoids (retinol, retinal, retinoic acid and retinyl acetate), riboflavin, riboflavin 5'-phosphate, flavin adenine dinucleotide, Cu2+, 7,8-benzoflavone, disulfiram, butylated hydroxyanisole, butylated hydroxytoluene and phenothiazine suggests that these agents may have potential anticarcinogenic activity against aflatoxin B1.