Sheila Higginbotham

Evidence that a burst of DNA depurination in SENCAR mouse skin induces error-prone repair and forms mutations in the H-ras gene.

Treatment of SENCAR mouse skin with dibenzo[a,l]pyrene results in abundant formation of abasic sites that undergo error-prone excision repair, forming oncogenic H-ras mutations in the early preneoplastic period. To examine whether the abundance of abasic sites causes repair infidelity, we treated SENCAR mouse skin with estradiol-3,4-quinone (E2-3,4-Q) and determined adduct levels 1 h after treatment, as well as mutation spectra in the H-ras gene between 6 h and 3 days after treatment. E2-3,4-Q formed predominantly (⩾99%) the rapidly-depurinating 4-hydroxy estradiol (4-OHE2)-1-N3Ade adduct and the slower-depurinating 4-OHE2-1-N7Gua adduct. Between 6 h and 3 days, E2-3,4-Q induced abundant A to G mutations in H-ras DNA, frequently in the context of a 3′-G residue. Using a T.G-DNA glycosylase (TDG)-PCR assay, we determined that the early A to G mutations (6 and 12 h) were in the form of G.T heteroduplexes, suggesting misrepair at A-specific depurination sites. Since G-specific mutations were infrequent in the spectra, it appears that the slow rate of depurination of the N7Gua adducts during active repair may not generate a threshold level of G-specific abasic sites to affect repair fidelity. These results also suggest that E2-3,4-Q, a suspected endogenous carcinogen, is a genotoxic compound and could cause mutations.

Detection of dibenzo[a,l]pyrene-induced H-ras codon 61 mutant genes in preneoplastic SENCAR mouse skin using a new PCR-RFLP method.

One of the key events in tumor initiation in mouse skin is mutational activation of the H-ras gene. Papillomas induced by the most carcinogenic environmental polycyclic aromatic hydrocarbon (PAH), dibenzo[a,l]pyrene (DB[a,l]P), in SENCAR mouse skin contain a specific H-ras codon 61 (CAA→CTA) mutation. We describe here detection of these mutations in preneoplastic skin by measuring the frequency of an induced XbaI RFLP, created by the mutation. Development of the PCR-XbaI RFLP method, sensitive enough to detect 1 codon 61 mutant allele among 10 000 wild-type genes, is described. The results indicate that codon 61 mutations are induced 1  day (0.1%) after DB[a,l]P treatment on mouse skin, reach a high value (5%) by day 3, rapidly decline between days 7–9 and increase again during the clonal expansion of pre-papillomas into tumors. The detection of codon 61 mutations 1 day after DB[a,l]P exposure suggests that mutations occurred by pre-replication misrepair.

Depurinating naphthalene-DNA adducts in mouse skin related to cancer initiation.

Naphthalene has been shown to be a weak carcinogen in rats. To investigate its mechanism of metabolic activation and cancer initiation, mice were topically treated with naphthalene or one of its metabolites, 1-naphthol, 1,2-dihydrodiolnaphthalene (1,2-DDN), 1,2-dihydroxynaphthalene (1,2-DHN), and 1,2-naphthoquinone (1,2-NQ). After 4 h, the mice were sacrificed, the treated skin was excised, and the depurinating and stable DNA adducts were analyzed. The depurinating adducts were identified and quantified by ultraperformance liquid chromatography/tandem mass spectrometry, whereas the stable adducts were quantified by (32)P-postlabeling. For comparison, the stable adducts formed when a mixture of the four deoxyribonucleoside monophosphates was treated with 1,2-NQ or enzyme-activated naphthalene were also analyzed. The depurinating adducts 1,2-DHN-1-N3Ade and 1,2-DHN-1-N7Gua arise from reaction of 1,2-NQ with DNA. Similarly, the major stable adducts appear to derive from the 1,2-NQ. The depurinating DNA adducts are, in general, the most abundant. Therefore, naphthalene undergoes metabolic activation to the electrophilic ortho-quinone, 1,2-NQ, which reacts with DNA to form depurinating adducts. This is the same mechanism as other weak carcinogens, such as the natural and synthetic estrogens, and benzene.

Dibenzo[a,I]pyrene: The Most Potent Carcinogenic Aromatic Hydrocarbon

Abstract Dibenzo[a,l]pyrene (DB[a,l]P) is the most potent carcinogen among dibenzo[a]pyrenes. DB[a,l]P is more tumorigenic than 7,12−dimethylbenz[a]anthracene, DB[a,l]P 11,12−dihydrodiol, and 100–200 times more tumorigenic than benzo[a]pyrene. DB[a,l]P is also extremely toxic. Dose-response studies were conducted in the skin of female SENCAR mice by initiation-promotion to compare the tumorigenicity of DB[a,l]P to that of (±)−trans−DB[a,l]P−11, 12−dihydrodiol, (±)−anti−DB[a,l]P diol epoxide (anti−DB[a,l]PDE) and (±)−syn−DB[a,l]PDE. Mice were initiated with 12, 4 or 1.33 nmol of compound and promoted with tetradecanoyl phorbol acetate. DB[a,l]P at 12, 4 or 1.33 nmol induced 9.3, 7.1 or 5.2 tumors per mouse (t/m), respectively. DB[a,l]P−11,12−dihydrodiol induced 4.6, 4.3 or 2.8 t/m. Anti−DB[a,l]PDE resulted in 2.0, 0.7 or 0.7 t/m vs 1.8, 1.5 or 1.8 with syn−DB[a,l]PDE. The experiment confirms that DB[a,l]P is more potent than DB[a,l]P−11,12−dihydrodiol and shows that the two diol epoxides are less tumorigen...

Tumor-initiating activity in mouse skin and carcinogenicity in rat mammary gland of fluorinated derivatives of benzo[a]pyrene and 3-methylcholanthrene

SummaryComparative studies of tumor-initiating activity in mouse skin and carcinogenicity in rat mammary gland were conducted with benzo[a]pyrene (BP) and 3-methylcholanthrene (MC) derivatives. SENCAR mice were initiated with BP, 6-fluorobenzo[a]pyrene (6-FBP), 6-methylBP, 7-FBP, 8-FBP, 9-FBP, 10-FBP, or 10-azaBP and promoted with tetradecanoyl phorbol acetate. The same compounds plus BP 7,8-dihydrodiol were tested by intramammillary injection in female Sprague-Dawley rats. Tumor-initiating activity in mice and/or carcinogenicity in rats were observed for BP, 6-methylBP, 6-, 7-, 8-, and 10-FBP, whereas 9-FBP was inactive in both experiments and 10-azaBP was only marginally active in the mammary gland. BP 7,8-dihydrodiol was carcinogenic in rat mammary gland, although it was less potent than BP. MC, 8-FMC, 10-FMC, and 3-methylcholanthrylene were also tested in Sprague-Dawley rats by intramammillary injection. All compounds were carcinogenic, with MC displaying the most potent activity. The less potent carcinogenic activity of BP 7,8-dihydrodiol in the mammary gland, compared with BP, and the moderate-to-weak tumor-initiating and/or carcinogenic activity of 7-, 8-, and 10-FBP suggest that the bay-region diol-epoxide pathway does not play a significant role in the activation of BP in these two target tissues. Similarly, the carcinogenic activity of 8-FMC and 10-FMC, in which the bay-region diol-epoxide pathway is blocked, suggests that this mechanism of activation is not important in the carcinogenicity of MC in rat mammary gland.

Inflammatory response of mouse skin exposed to the very potent carcinogen dibenzo[a,l]pyrene: a model for tumor promotion.

The potent carcinogenicity of dibenzo[a,l]pyrene in mouse skin is associated with an inflammation unique among polycyclic aromatic hydrocarbons and expressed as erythema. The time course of erythema and the associated histological events in the skin of female SENCAR mice were determined after a single application of 6.25-200 nmol dibenzo[a,l]pyrene or selected metabolites. Dibenzo[a,l]pyrene and dibenzo[a,l]pyrene-11,12-dihydrodiol, precursor to the bay-region diol epoxide, induced an erythema first present 5-6 days after treatment. Dibenzo[a,l]pyrene-8,9-dihydrodiol and other dibenzo[a, l]pyrene metabolites, however, did not induce erythema. These findings suggest a central role for the bay-region diol epoxide in the induction of the observed inflammation. The intensity and duration of erythema were dose-dependent, whereas the delayed appearance of erythema was constant and dose-independent. These results suggest induction of an immune hypersensitivity by dibenzo[a, l]pyrene and its 11,12-dihydrodiol. Histological changes in the skin were consistent with a contact hypersensitivity reaction and included, in association with erythema, epidermal hyperplasia and the presence of mononuclear leukocytes in the dermis. Animals were tested for dibenzo[a,l]pyrene-induced contact hypersensitivity. Female SENCAR mice were treated with a single dermal application of dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene. Five days later, the animals were challenged with a single application of dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene to the ear pinna. Ear swelling exhibited features of a contact hypersensitivity reaction, including (1) delayed appearance after challenge, (2) noninducibility in animals not previously exposed to chemical sensitizer, and (3) chemical specificity. The results suggest that dibenzo[a,l]pyrene induces, via its bay-region diol epoxide, a contact hypersensitivity reaction that may promote tumor development and thereby enhance carcinogenic potency.

Nutritional effects on the lifespan of Syrian hamsters

Syrian hamsters were fed one of nine semipurified diets composed of three casein levels (9, 18, and 36 g/385 Kcal), with each of three corn oil levels (4.5, 9.0, and 18.0 g/385 Kcal). These diets were given either for five weeks and were followed by control diet (18 g casein and 9 g corn oil/385 Kcal) or control diet was fed for the first five weeks and was followed by the nine diets. Calorie consumption, maximum body weight and length of growth period and of life are reported. Calorie consumption was directly related to dietary fat levels. Maximum body weights increased with increasing dietary fat and protein when the various diets were fed during weeks 1–5. This result was not due to a conditioning of the animals fed high-fat or high-protein levels during weeks 1–5 to consume more calories after week 5, since after this time consumption was the same in all groups fed the control diet. When diets were fed from week 6 body weight increased in both sexes with increased dietary fat; however, higher dietary protein increased female and decreased male maximum body weight. Males took longer to reach these maximum weights than females, and were not affected by receiving the various diets during weeks 1–5. However, when diets were fed from week 6 until death, the growth period increased with higher dietary fat or protein. Male hamsters survived longer than females with each experimental treatment. Animals fed low-fat, low-protein diet or high-fat, high-protein diet during the first 5 weeks of the study survived longest. When diets were fed from 6 weeks until death, survival increased as dietary fat rose for both sexes. In contrast, survival improved as dietary protein rose for females or decreased for males. These studies establish a basis for further investigations on the link between nutrition and longevity in the Syrian hamster.

Tumorigenicity of 6-halogenated derivatives of benzo[a]pyrene in mouse skin and rat mammary gland

SummaryStudies of the tumorigenicity of 6-halogenated derivatives of benzo[a]pyrene (BP) can provide evidence about the role of the 6 position in the carcinogenic activation of BP. Female Swiss and A-strain mice were treated on the skin with BP, 6-fluorobenzo[a]pyrene (6-FBP), 6-chlorobenzo[a]pyrene (6-C1BP), 6-bromobenzo[a]pyrene (6-BrBP) and 6-iodobenzo[a]pyrene (6-IBP) by repeated application, and in some cases by initiation-promotion. While BP was more potent than 6-FBP, only these two compounds exhibites tumor-initiating and carcinogenic activity in mouse skin. Female Sprague-Dawley rats were treated with BP, 6-FBP, 6-C1BP, and 6-BrBP by intramammillary injection. BP and 6-FBP induced high levels of mammary epithelial tumors and fibrosarcomas. 6-C1BP elicited only a high percentage of fibrosarcomas, whereas 6-BrBP induced a few adenocarcinomas. These results indicate that chloro or bromo substitution at C-6 in BP reduces or eliminates carcinogenic activity. Conversely, 6-FBP, from which the fluoro substituent has been chemically and metabolically removed by one-electron oxidation, displays a moderate carcinogenic acitivity which is consistent with activation by either one-electron oxidation or monooxygenation.

Covalent Binding of Benzo[a]pyrene to Free and Membrane-Bound DNA in Isolated Liver Nuclei from 3-Methylcholanthrene-Induced Rats

Abstract Benzo[a]pyrene (BP) is bound to DNA by two major pathways: one-electron oxidation and monooxygenation. One-electron oxidation requires that the highly reactive radical cation be formed in close proximity to the DNA. Thus, in intact cell nuclei, binding of BP radical cation would be expected to occur predominantly in DNA associated with the membrane. To examine this, nuclei from the livers of 3-methylcholanthrene-induced male MRC Wistar rats were incubated with [3H]BP and NADPH. After the incubation, the “free” DNA was separated from “membrane-bound” DNA by extraction in low-Mg buffer. Binding of BP to membrane-bound DNA was 9.5 ± 2.0 μmol/mol DNA-P, whereas binding to free DNA was 2.2 ± 0.7 μmol/mol DNA-P. BP adducts formed by one-electron oxidation and lost from the nuclear DNA by depurination were also examined. The three depurination adducts previously found with rat liver microsomes and in mouse skin were also obtained from nuclei. These results suggest that binding of BP to DNA in intact nuc...