Showing papers in "Carcinogenesis in 1986"

Nuclease P1-mediated enhancement of sensitivity of 32P-postlabeling test for structurally diverse DNA adducts.

Abstract: Exceedingly sensitive procedures are required to detect the presence of covalent DNA adducts in humans exposed to environmental genotoxicants because of low levels of such derivatives (1 adduct in 10(8)-10(10) DNA nucleotides). A 32P-postlabeling assay for detection and quantitation of carcinogen--DNA adducts with a sensitivity limit of 1 adduct in 10(7)-10(8) nucleotides has been described previously. In the standard procedure, DNA is enzymatically digested to 3'-phosphorylated normal and adducted mononucleotides, which are quantitatively 32P-labeled at their 5'-hydroxyl groups by T4 polynucleotide kinase-catalyzed [32P]phosphate transfer from [gamma-32P]ATP. 32P-labeled derivatives are resolved by t.l.c., detected by autoradiography and quantitated by counting. We now report that a minor modification of this procedure, entailing the postincubation of DNA digests with Penicillium citrinum nuclease P1 before 32P-labeling, enhanced the technique's sensitivity to 1 adduct in approximately 10(10) nucleotides for a 10-micrograms DNA sample. Nuclease P1 cleaves deoxyribonucleoside 3'-monophosphates of normal nucleotides to deoxyribonucleosides which do not serve as substrates for polynucleotide kinase, while most adducted nucleotides were found to be totally or partially resistant to the 3'-dephosphorylating action of nuclease P1. The additional enzymatic step enabled specific labeling of adducts in up to 20 micrograms of DNA with excess carrier-free [gamma-32P]ATP. The enzymatic digestion conditions were standardized to afford optimal adduct recovery. The new procedure was found to be simple, highly reproducible, and applicable to the detection and measurement of aromatic or bulky non-aromatic DNA adducts formed with such structurally diverse carcinogens as benzo[a]pyrene, 7,12-dimethyl-benz[a]anthracene, dibenzo[c,g]carbazole, 4-aminobiphenyl, safrole and mitomycin C; most adducts were recovered quantitatively with a 500- to 1000-fold increase in 32P-count rates as compared with the standard procedure.

Formation of 8-hydroxyguanine moiety in cellular DNA by agents producing oxygen radicals and evidence for its repair

Abstract: 8-Hydroxydeoxyguanosine (8-OH-dG) was detected in DNA isolated from HeLa cells after the cells in tissue culture had been irradiated with X-rays and from the liver of mice after the whole animals had been irradiated with gamma-rays. The amounts of 8-OH-dG in DNA after in vivo irradiation were three orders of magnitude lower than those after in vitro irradiation (0.008-0.032 8-OH-dG residue/10(5) dG/krad). The 8-OH-dG produced in liver DNA by irradiation of mice decreased with time, suggesting the presence of a repair enzyme(s) acting on 8-OH-dG in mouse liver. Treatment of Salmonella typhimurium cells with hydrogen peroxide also caused increase in the 8-OH-dG content. These results indicate that 8-OH-dG is formed in vivo in cellular DNA on treatment with various oxygen radical-producing agents and that it is repairable.

A glutathione transferase in human leukocytes as a marker for the susceptibility to lung cancer.

Abstract: Leukocyte isozyme(s) of glutathione transferase (GT-tSBO) has been shown to be dominantly inherited. The frequency of the phenotypes of this isozyme in bronchial carcinoma and control patients matched for age and smoking history is reported here. Control smokers showed an increased likelihood of having GT-tSBO (59%) compared with lung cancer patients (35%). The lack of GT-tSBO was related to the extent of smoking by the lung cancer patients but not to the pathology of the lung tumor. It was concluded that the gene expressing this isozyme(s) may be a host genetic determinant of susceptibility to lung cancer in smokers.

The isolation and identification of a new mutagen from fried ground beef: 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

Abstract: A new mutagenic compound has been isolated from ground beef which was fried at 300 degrees C for 5.5 min on each side. The new mutagen was purified using an aqueous acid extraction, XAD-2 adsorption-solvent elution, a series of preparative and analytical h.p.l.c. purification steps, and monitored with the Ames/Salmonella assay. This study reveals a new mutagen member of the amino-imidazoazaarene class of aromatic amines, having a mol. w of 224, and a formula of C13H12N4 as determined by high-resolution mass spectrometry. N.m.r. spectrometry supports the structure, 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP), for the new mutagen. The 1-methyl and 3-methyl synthesized isomers of PhIP were compared to the purified mutagen. The two isomers had identical mass spectra to the purified compound, but only the 1-methyl isomer showed similar u.v. and n.m.r. spectra. The two synthetic isomers were separable by h.p.l.c. and the beef derived component co-eluted with the 1-methyl-PhIP isomer. PhIP has a specific activity in the Ames/Salmonella assay of 1950 revertants/microgram. Although it is not as mutagenic as other compounds isolated from fried beef (e.g. MeIQx, 58 000 revertants/microgram) it is the most abundant mutagenic compound by mass in fried beef. PhIP is present at approximately 15 p.p.b. of the original weight of uncooked beef (accounting for 75% of the mass of genotoxic material) and contributes 18% of the total mutagenicity of the fried beef.

Comparison of O6-alkylguanine-DNA alkyltransferase activity based on cellular DNA content in human, rat and mouse tissues.

Abstract: O6-Alkylguanine-DNA alkyltransferase (alkyltransferase) is the repair protein for O6-alkylguanine, a pre-mutagenic adduct formed by a variety of alkylating agents. Previous comparisons of the repair capacity of O6-alkylguanine in different tissues have expressed the alkyltransferase activity relative to total protein, and have asserted that tissues with low levels of activity were at greater risk for mutagenic damage than tissues with higher levels of activity. Because the alkyltransferase uses DNA as substrate, and because tissues vary greatly in protein content, comparisons of tissue alkyltransferase activity may be more appropriately based on cellular DNA content. We compared alkyltransferase activity relative to tissue DNA content with the activity related to protein content in human, rat and mouse tissues. In each species, liver containing the highest level of activity using either method. In agreement with the findings of others, low levels of alkyltransferase activity relative to protein were seen in human brain, rat brain and small intestine, and mouse kidney. However, based on alkyltransferase activity relative to DNA content, low levels of activity were seen in human bone marrow myeloid precursors, rat bone marrow, brain and intestine, and mouse spleen and bone marrow. The range of activity between tissues was 18-fold in human, 15-fold in rat and 8-fold in mouse. In general, the rank of alkyltransferase activity relative to DNA for each tissue was human greater than rat greater than mouse. These results suggest that the mouse is more susceptible to nitrosoureas than rat or human. In each species, the organs with low levels of alkyltransferase activity relative to tissue DNA content would appear to be targets for mutagenic damage following nitrosourea exposure.

Preneoplastic lesions as end points in carcinogenicity testing. I. Hepatic preneoplasia.

Abstract: The evaluation of the carcinogenic risk deriving from chemical compounds depends mainly on conventional histopathology up to the present. The accepted end point in carcinogenicity testing is the tumor as defined histologically. A great disadvantage of this approach is the long lag period in the development of tumors induced by chemicals. In order to overcome this drawback, many efforts have been made to detect early biological or morphological lesions which might be specific for carcinogens. A great number of short-term tests carried out in vitro provided valuable information about the reactions of cellular constituents and biological macromolecules to the chemicals tested, but they did not allow an unequivocal prediction of the carcinogenic potential of the respective compounds in whole animals, not to mention man. The introduction of modern micromorphological methods such as electron microscopy and cytochemistry in the evaluation of whole-animal studies also revealed many new aspects on carcinogen-induced cellular and subcellular alterations which appeared to be unreliable as indicators of the carcinogenic risk of chemicals. However, during the past two decades a number of characteristic cellular changes has been detected in various tissues, especially in the liver. These changes regularly precede the development of certain tumor types, and are regarded as preneoplastic lesions (1,2). The altered cell populations usually form well-defined foci. They appear prior to the development of tumors in the target tissue of the carcinogen, and should be duly considered in the evaluation of the carcinogenic risk from chemicals in bioassays.

Inhibition of experimental oral carcinogenesis by topical beta carotene

Abstract: beta-Carotene was found to significantly inhibit the formation of 7,12-dimethylbenz[a]anthracene (DMBA)-induced squamous cell carcinoma of hamster buccal pouch when applied topically on days alternate to the application of 0.25% DMBA in heavy mineral oil thrice weekly for 22 weeks. An initial experiment utilized 40 male young adult Syrian hamsters divided into four equal groups. Group 1 had DMBA applied to left buccal pouches thrice weekly. Group 2 had DMBA applied as in group 1 but also beta-carotene thrice weekly on days alternate to the DMBA application. Group 3 animals were painted with only beta-carotene and group 4 animals were untreated controls. In a second experiment with 80 animals, beta-carotene was found to inhibit oral carcinogenesis in an initiation--promotion hamster buccal pouch system using 0.1% DMBA as initiator and 40% benzoyl peroxide as promoter. beta-Carotene inhibited both initiation and promotion.

Tumorigenicity of nitrated derivatives of pyrene, benz[a]anthracene, chrysene and benzo[a]pyrene in the newborn mouse assay.

Abstract: Eight nitropolycyclic aromatic hydrocarbons (PAHs), including 1- and 4-nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene, 7-nitrobenz[a]anthracene, 6-nitrochrysene and 6-nitrobenzo-[a]pyrene and their parent PAHs were tested fro tumorigenicity in the newborn mouse model by i.p. administration at 1, 8, and 15 days after birth. Both pyrene and 1-nitropyrene induced similar incidences of hepatic tumors in males, yielding a 12-15% and a 21-28% tumor incidence at total doses of 700 and 2800 nmol per mouse, respectively. Liver tumors did not occur in females and the 3-10% lung tumor yield in both sexes was similar to that found in solvent-treated controls. The presumed proximate carcinogen, 1-nitrosopyrene, administered at 700 nmol per mouse, caused liver tumors in 45% of the males and in 9% of the females. 4-Nitropyrene was more tumorigenic than pyrene or 1-nitropyrene; at a dose of 2800 nmol, it induced liver tumors in 83% of the males and 7% of the females, with a lung tumor yield of 38 and 31%, respectively. Female mice treated with 200 nmol of 1,3-, 1,6- or 1,8-dinitropyrene did not develop liver tumors but the hepatic tumor incidence in males was 20, 32 and 16%, respectively, which was significantly greater than that found in mice treated with pyrene. In male mice administered 2800 nmol of benz[a]anthracene, the hepatic tumor incidence was 79%, while treatment with 7-nitrobenz[a]anthracene showed an incidence of only 28%. Similarly, 560 nmol of benzo[a]pyrene caused a 49% liver tumor yield in males while those given 6-nitrobenzo[a]pyrene had a 28% incidence. Treatment with benzo[a]pyrene also induced a 35 and 48% lung tumor incidence in males and females while the comparable values in 6-nitrobenzo[a]pyrene-treated mice were 14 and 2%. Chrysene administered at 2800 nmol per mouse induced hepatic and lung tumors in 41% and 21% of the males, respectively; at the 700-nmol dose, it induced only liver tumors in 29% of the males and in none of the females. In contrast, treatment with 6-nitrochrysene at 700 nmol per mouse resulted in a 76 and 23% hepatic tumor incidence in males and females, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Age- and tissue-related DNA modifications in untreated rats: detection by 32P-postlabeling assay and possible significance for spontaneous tumor induction and aging.

Abstract: When liver, kidney, lung and heart DNA preparations of untreated Sprague-Dawley rats of different ages (3 days-10 months) were analyzed for the possible presence of covalent modifications by a 32P-postlabeling assay, characteristic tissue-specific patterns of 32P-labeled spots (termed I-spots) were observed on thin-layer chromatograms. Amounts of these DNA derivatives (termed I-compounds), which were not detected in newborn rat DNA, markedly increased with age. This novel type of DNA modification could be due to environmental (e.g. dietary) factors or to endogenous DNA-reactive metabolites and may conceivably play a role in the initiation of spontaneous cancers or other adverse health effects related to aging.

Genetic control of hepatocarcinogenesis in C57BL/6J and C3H/HeJ inbred mice.

Abstract: Treatment of newborn male C3H/HeJ mice with N,N-diethyl-nitrosamine (DEN) or N-ethyl-N-nitrosourea (ENU) resulted in the induction of hepatocellular adenomas and carcinomas with a mean number of tumors per animal that was approximately 20- to 50-fold higher than that for similarly treated C57BL/6J male mice. We used two methods to study the genetic basis for this difference in susceptibility to liver tumor induction. Analysis of DEN-induced liver tumor multiplicities as a quantitative genetic trait in segregating crosses between C3H/HeJ and C57BL/6J mice indicated that allelic differences for at least two loci contributed to the higher sensitivity to hepatocarcinogenesis of C3H/HeJ mice relative to C57BL/6J mice. However, a single locus, which we have denoted Hcs (hepatocarcinogen sensitivity), was responsible for approximately 85% of the difference in susceptibility. The C57BL/6J and C3H/HeJ alleles at this locus were semi-dominant. This result was confirmed by analysis of hepatocarcinogenesis by ENU in BXH (C57BL/6J X C3H/HeJ) recombinant inbred mice. Four of the nine recombinant inbred strains studied were highly susceptible to the induction of liver tumors by ENU, three of the strains exhibited the resistant phenotype of the C57BL/6J parent, and two of the strains were of intermediate sensitivity to hepatocarcinogenesis. Newborn male C3H/HeJ and C57BL/6J mice did not significantly differ in the extent of ethylation of hepatic DNA, or in the relative levels of N-7-ethylguanine or O6-ethylguanine after treatment with [1-14C]DEN.

Inhibitory effects of 5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione (Oltipraz) on carcinogenesis induced by benzo[a]pyrene, diethylnitrosamine and uracil mustard.

Abstract: 5-(2-Pyrazinyl)-4-methyl-1,2-dithiol-3-thione (Oltipraz) was studied for its capacity to inhibit carcinogen-induced neoplasia in female ICR/Ha mice. When administered by oral intubation 48 h prior to benzo[a]pyrene (BP), also given by oral intubation, Oltipraz inhibited the occurrence of pulmonary adenomas and tumors of the forestomach. The ratio of the number of tumors occurring in the mice receiving Oltipraz to that of the corresponding controls was: lung, 0.36 and forestomach 0.38. Inhibition also occurred when Oltipraz was given p.o. 24 h prior to BP. In other experiments, oral administration of Oltipraz 48 h prior to p.o. administration of diethylnitrosamine or uracil mustard inhibited pulmonary adenoma formation but to a lesser extent than with BP as the carcinogen. The low toxicity of Oltipraz found previously, coupled with evidence of protective effects against chemically diverse carcinogens, suggests that this compound should be studied further for its possible use as an agent for the chemoprevention of neoplasia.

sn-1,2-Diacylglycerols mimic the effects of 12-O-tetradecanoylphorbol-13-acetate in vivo by inducing biochemical changes associated with tumor promotion in mouse epidermis.

Abstract: 12-O-Tetradecanoylphorbol-13 acetate (TPA) or various acylglycerols were applied topically to CD-1 mice, and biochemical changes associated with tumor promotion in the epidermis were examined. The topical application of 5 mumol of sn-1,2-didecanoylglycerol caused a 40-fold increase in ornithine decarboxylase activity which was similar to that found after the topical application of 2 nmol of TPA. The time course for the induction of ornithine decarboxylase activity by TPA and the time course for its induction by sn-1,2-didecanoylglycerol were similar; both compounds produced rapid increases in ornithine decarboxylase activity with peak induction occurring 4-6 h after application of the inducing chemical. sn-1,2-Dioctanoylglycerol and sn-1-oleoyl-2-acetylglycerol also increased ornithine decarboxylase activity in mouse epidermis, but sn-1,2-dioleoylglycerol, 1,3-didecanoylglycerol and rac-1-monodecanoylglycerol were inactive at the dose tested. trans-Retinoic acid, a potent inhibitor of tumor promotion, markedly inhibited the epidermal induction of ornithine decarboxylase activity that resulted from the topical administration of sn-1,2-didecanoylglycerol or TPA. The effects of TPA and the acylglycerols on epidermal DNA synthesis in vivo were determined by measuring the incorporation of [3H]thymidine into epidermal DNA. The application of sn-1,2-didecanoylglycerol or TPA to mouse skin stimulated epidermal DNA synthesis. The maximum increase occurred 18 h after administration of the inducing chemical, and the increase in DNA synthesis was proportional to the dose of sn-1,2-didecanoylglycerol. Although sn-1,2-didecanoylglycerol, sn-1,2-dioctanoylglycerol and sn-1,2-dioleoylglycerol stimulated epidermal DNA synthesis, sn-1-oleoyl-2-acetylglycerol, 1,3-didecanoylglycerol and rac-1-monodecanoylglycerol had little or no effect. The increase in epidermal DNA synthesis induced by sn-1,2-didecanoylglycerol or TPA was inhibited by the simultaneous application of fluocinolone acetonide, a potent inhibitor of tumor promotion. The results indicate that several sn-1,2-diacylglycerols mimic TPA in vivo with respect to their effects on certain biochemical parameters associated with tumor promotion in mouse skin.

Tissue doses of ethylene oxide in cigarette smokers determined from adduct levels in hemoglobin.

Abstract: Determination of adducts to hemoglobin (Hb) is a useful approach for monitoring tissue doses of ultimate carcinogens. This approach provides a basis for both risk estimation and for the identification of a priori unknown environmental carcinogens. This paper describes the application of a new method for the analyses of Hb adducts to cigarette smokers and non-smokers. The results demonstrate a raised level of hydroxyethylation of N-terminal valine of Hb of smokers that is quantitatively compatible with ethene in the smoke being the source. The magnitude of the tissue doses of ethylene oxide originating from inhaled ethene suggests that this factor is a major contributor to smoking-associated cancer risk.

Anti-mutagenesis and anti-promotion by apigenin, robinetin and indole-3-carbinol.

Abstract: We assessed the anti-mutagenic and anti-promotion properties of two flavones, apigenin and robinetin, and of indole-3-carbinol, because these compounds have been reported in vegetables, the consumption of which has been associated with reduced rates of cancer. However, the active components of these foods and their effects on carcinogenesis have not been established. Anti-mutagenicity was determined in the Salmonella typhimurium assay by measuring the effects of the test compounds on bacterial mutagenesis induced by methyl-nitrosourea (MNU), methyl-n-nitro-N-nitrosoguanidine (MNNG), benzo[a]pyrene (BaP) or 2-aminoanthracene (2-AA). Inclusion of apigenin resulted in a 62% and a 43% inhibition of mutagenicity with 13 nmol of 2-AA and 30 nmol BaP respectively. Robinetin caused an 87% inhibition of mutagenicity by 2-AA, but indole-3-carbinol had little or no effect on the mutagenicity of any of the compounds. None of the three compounds inhibited mutagenesis by MNU or MNNG and none were mutagenic or toxic when tested in the absence of mutagenic compounds at doses up to 20 micrograms/plate. Anti-promotion properties were assessed by measuring the effects of apigenin, robinetin and indole-3-carbinol on induction of ornithine decarboxylase activity (ODC) in mouse epidermis by 17 nmol 12-O-tetradecanoyl phorbol-13-acetate (TPA). Pretreatment of the skin half an hour before TPA with apigenin, robinetin, butylated hydroxyanisole, 13-cis-retinoic acid (all at 50 mumol) or di-fluoromethylornithine (1.6 mumol) inhibited ODC induction at 6 h after TPA by 67-80%. Pretreatment with 50 mumol indole-3-carbinol caused a 78% elevation in the TPA induction at this time. Dose response measurements were conducted with apigenin, indole-3-carbinol and robinetin. Inhibition by 30-90% of TPA-induced ODC was observed at 6 h after TPA in mice pretreated with 12.5-100 mumol apigenin. Pretreatment with 37.5 or 50 mumol indole-3-carbinol or 0.5, 12.5 or 25 mumol robinetin resulted in elevated induction of epidermal ODC by TPA at 6 h after TPA. However, treatment with 50 or 100 mumol robinetin diminished ODC induction at 6 h after TPA. Treatment with 100 mumol apigenin or 50 or 100 mumol indole-3-carbinol in non-TPA-treated mouse skin caused elevations in epidermal ODC. In comparing the time course of ODC induction, indole-3-carbinol (50 mumol) pretreatment shifted the induction of epidermal ODC to earlier times, in addition to elevating ODC induction by TPA.(ABSTRACT TRUNCATED AT 400 WORDS)

Aflatoxin B1 binding to plasma albumin and liver DNA upon chronic administration to rats.

Abstract: The hepatocarcinogen aflatoxin B1 (AFB1) was administered to male Wistar rats by oral intubation in either single or repeated doses and the binding to plasma protein and liver DNA determined. Twenty-four hours after a single dose (3.5-200 micrograms/kg AFB1) a constant ratio was found between levels of aflatoxin bound to plasma protein and that bound to liver DNA. In total 0.98-2.15% of the administered dose was bound to the plasma protein at this time point. In the chronic study rats received two doses of 0.5 microgram AFB1/day and groups of animals were killed on days 2, 3, 7, 14, 21 and 24. Binding of aflatoxin to plasma protein accumulated to a level 3-fold higher than that seen after a single dose. Levels of binding reached a plateau between days 7 and 14 of treatment and then remained stable until the end of the experiment. Binding to DNA also accumulated, 2.5-fold and in parallel to plasma protein, binding reached a plateau between days 7 and 14 of treatment. In both the chronic and acute studies fractionation of the plasma proteins by Sephadex G-200 chromatography showed that all detectable bound aflatoxin was associated with a single peak corresponding to albumin. Thus, a constant ratio was observed, after chronic or single exposure, between the concentration of plasma albumin-bound aflatoxin and that bound to DNA of the liver, the target organ for carcinogenesis by AFB1. In order to investigate the proposed role of AFB1 in the aetiology of primary hepatocellular carcinoma in man it would be of great value to have a method for assessing long-term human exposure at an individual level. The relevance of the observations presented in this paper are discussed in the light of such a requirement.

DNA strand breaks in human leukocytes induced by superoxide anion, hydrogen peroxide and tumor promoters are repaired slowly compared to breaks induced by ionizing radiation.

Abstract: The repair kinetics and sensitivity to inhibitors of DNA strand breaks caused by superoxide anion, hydrogen peroxide, benzoyl peroxide and anthralin have been studied and compared with strand breaks caused by well-studied agents such as ionizing radiation and bleomycin. The latter two agents are generally believed to produce breaks indirectly by producing hydroxyl radicals, a very potent oxidizing species, which attack the phosphodiester backbone of DNA. As expected from earlier results, breaks induced by radiation and bleomycin rapidly disappear during post-treatment incubation as a consequence of the action of cellular DNA repair enzymes. Thus, strand breaks produced by hydroxyl radicals appear to be readily repaired in human leukocytes. By contrast, breaks caused by extracellular superoxide anion appeared not to be readily repaired, implying that some mechanism other than the generation of hydroxyl radicals in the vicinity of the DNA was involved. Inhibitors such as 3-aminobenzamide, cytosine arabinoside and adenine arabinoside affected the apparent rate of repair of radiation and methylmethane sulfonate-induced breaks but there was no indication that they affected superoxide anion-induced breaks. They partially inhibited repair of hydrogen peroxide-induced breaks. Breaks caused by benzoyl peroxide and anthralin were also apparently not repaired. We conclude that hydroxyl radicals are not likely the ultimate DNA strand-breaking species in cells exposed to extracellular superoxide anion and that the observed slow repair of DNA strand breaks may be significant in the mechanism of action of tumor-promoting agents.

Oncogenes and human cancer: cause or consequence?

Abstract: During the last five years we have witnessed a series of scientific developments that are making possible for the first time the unveiling of molecular events involved in the onset of human neoplasia. Identification and subsequent characterization of oncogenes in human tumors occupies the most prominent place among these scientific developments. The concept of oncogene emanated from work started almost 20 years ago with acute transforming retroviruses (see 1, 2 for reviews). Genetic and biochemical evidence unquestionably defined the existence of a small region in the viral genome whose expression was sufficient to trigger carcinogenesis. At least 20 retroviral oncogenes have been identified and characterized so far, particularly since the advent of recombinant DNA technology (2). However, many investigators have remained skeptical about the significance of oncogene research in regard to our understanding of human cancer. After all, the concept of oncogene as defined by retroviruses is in serious conflict with the multi-stage nature of human malignancies. The realization that retroviral oncogenes are of cellular origin (3,4) aJerted scientists to the possibility that related dominant oncogenes may also exist in non-virally induced neoplasms. It was reasoned that cellular genes (proto-oncogenes) may become activated by somatic mutations that mimic the changes imposed upon these loci during retroviral transduction. Testing this hypothesis required technology capable of transferring single copy genes from one mammalian cell to another. In 1973, Graham and van der Eb developed a calcium precipitation technique which allowed them to introduce subgenomic fragments of adenovirus DNA into rodent cells (5). Four years later, this technique was successfully utilized to introduce single copy genes into mammalian cells utilizing either metaphase chromosomes or deproteinized genomic DNA (6,7).

Dose-related inhibition of aflatoxin B1 induced hepatocarcinogenesis by the phenolic antioxidants, butylated hydroxyanisole and butylated hydroxytoluene

Abstract: The effect of butylated hydroxyanisole (BHA) or butylated hydroxytoluene (BHT) on the carcinogenicity in rats of aflatoxin B1 (AFB1) was investigated. AFB1 was administered by gastric intubation to male F344 rats at 25 micrograms/kg body wt three times a week such that a total dose of 1.5 mg/kg (0.48 mmol/kg) body wt was given over a period of 20 weeks and diets containing either 1000 or 6000 p.p.m. BHA or BHT were fed starting one more week before carcinogen, during administration and for one week after cessation. Animals were killed during exposure and at intervals up to 24 weeks after cessation. Liver altered foci and neoplasms were quantified using the exclusion of cellular iron after iron-loading and gamma-glutamyl transpeptidase reaction, as well as conventional staining for identification. Exposure to AFB1 alone induced substantial numbers of altered foci after 20 weeks, and at 24 weeks after cessation of exposure, the incidence of hepatocellular neoplasms was 63%. In the groups receiving BHA or BHT together with AFB1, the numbers of altered foci were decreased at all time points and at termination, the final incidence of liver cell neoplasms and number of neoplasms per animal were also reduced in a dose-related manner. Neoplasms in other organs were rare and were not affected by antioxidant treatment, except for a possible reduction of colon cancer. Thus, BHA and BHT inhibited the hepatocarcinogenesis of concurrently administered AFB1 without shifting the organotropism.

DNA methylation and hepatocarcinogenesis in rats fed a choline-devoid diet

Abstract: Groups of male Fischer-344 rats were fed either a choline-supplemented or a choline-devoid (CD) diet, for up to 14 months. In rats fed the CD diet, hepatic lesions developed and progressed through two distinct stages, the first characterized by severe steatosis and an increase in cell turnover and the second by gradual clearance of the deposited fat, fibrosis and parenchymal nodularity. Large hepatocellular carcinomas were found in rats killed at 14 months. DNA was purified from the livers of all groups of rats and from the tumors, and its level of methylation was analyzed using the restriction endonucleases HpaII and MspI. DNA undermethylation was detected only in the livers of rats fed the CD diet for 14 months, whether bearing tumors or not, and in three of four hepatocellular carcinomas. Undermethylation of liver total DNA is therefore a late effect of dietary choline deficiency in the rat.

Glutathione transferases in rat lung: the presence of transferase 7-7, highly efficient in the conjugation of glutathione with the carcinogenic (+)-7β,8α-dihydroxy-9α,10α-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene

Abstract: The enzyme-catalysed conjugation of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha, 10 alpha-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+/-)-anti-BPDE] with glutathione (GSH) by cytosolic GSH transferases isolated primarily from rat lung has been studied. GSH transferase 4-4 was active in the GSH conjugation of anti-BPDE, whereas transferases 2-2 and 3-3 showed little activity. GSH transferase 1-1 did not contribute to the activity since significant amounts were not detected in the rat lung. Activity was also obtained with several acidic pulmonary GSH transferases and with a newly described form, transferase 7-7, also isolated from rat kidney and from hyperplastic liver nodules. The catalytic efficiency (kcat/Km) of transferase 7-7 was seven times that of transferase 4-4, the most active rat transferase previously identified. When the GSH concentration was varied at constant (+/-)-anti-BPDE concentration in the presence of transferases 4-4, 7-7 or the major acidic transferase, non-linear Lineweaver-Burk plots were obtained. Resolution of the GSH conjugates of the two enantiomers of (+/-)-anti-BPDE by h.p.l.c. showed that all isoenzymes with notable activity were selective (greater than or equal to 97%) for the (+)-enantiomer of anti-BPDE, which is generally considered to be the most carcinogenic form of BPDE. The possibility that one enantiomer inhibits the conjugation of the other enantiomer with GSH cannot be excluded and may quantitatively affect the results obtained.

Response of experimental animals to human carcinogens: an analysis based upon the IARC Monographs programme

Abstract: Only the results of epidemiological studies can be used to establish a causal relationship between an exposure to an agent and human cancer; however, such studies often cannot be carried out due to limitations of population or latent period or to the presence of mixed exposures. It is essential, therefore, that the validity be established of extrapolating to humans the results obtained from long-term carcinogenicity tests in animals. The responses of experimental animals to known and suspected human carcinogens, as evaluated in the IARC Monographs series, were analysed as an indication of the sensitivity of animal tests for predicting human carcinogens. Although the response was high - 84% - it would have been even higher had all the compounds been adequately tested experimentally. An additional finding was that for many exposures causally related to human cancer, there is a target organ in common between humans and at least one animal species, despite many inherent physiological differences. These findings show clearly the importance of experimental carcinogenicity studies in the primary prevention of cancer.

The wavelength dependence of u.v.-induced pyrimidine dimer formation, cell killing and mutation induction in human diploid skin fibroblasts.

Abstract: In this study, we determined the wavelength dependence of u.v.-induced pyrimidine dimer formation, cell killing and mutation induction in human diploid skin fibroblasts. Pyrimidine dimers were quantified using the T4 endonuclease V assay, cell killing was measured as loss of colony forming ability and mutation induction was detected at the HPRT locus. U.v. irradiation was performed with monochromatic light of four different wavelengths (254, 297, 302 and 365 nm) and with polychromatic light of a Philips TL-01 lamp (predominantly 312 nm). The relative wavelength dependence for cell killing and mutation induction did not correlate with that for dimer formation. Toxicity and mutagenicity per equivalent initial dimer load increase with increasing wavelength. The relative wavelength dependence for cell killing and mutation induction is essentially the same, except at 365 nm.

Aromatic DNA adducts in human bone marrow and peripheral blood leukocytes.

Abstract: DNA from normal human bone marrow mononuclear and non-mononuclear cells was analysed by 32P-post-labelling for the presence of aromatic adducts. Ten out of 10 individuals showed the presence of adducts at levels of 1-9 adducts per 10(8) nucleotides that were not detected in four samples of human foetal bone marrow. Inter-individual variations in the patterns of these presumed aromatic adducts were observed. Similar adducts were also present in the DNA of peripheral white blood cells of both smokers and non-smokers, although at lower levels than in bone marrow. The data suggest that the adducts result from environmental exposure to an as-yet-unidentified genotoxic agent or agents.

Palytoxin is a non-12-O-tetradecanoylphorbol-13-acetate type tumor promoter in two-stage mouse skin carcinogenesis.

Abstract: Palytoxon, which is a toxin with a molecular weight of 2681 daltons isolated from a marine coelenterate, is a potent skin irritant. However, it did not induce ornithine decarboxylase in mouse skin, or adhesion of human promyelocytic leukemia cells (HL-60). Moreover, it did not inhibit the specific binding of [3H]12-O-tetradecanoylphorbol-13-acetate (TPA) to a mouse skin particulate fraction or activate protein kinase C isolated from mouse brain in vitro. Since palytoxin showed strong irritation on mouse ear in one short-term screening test for a promoter, it was examined in a two-stage carcinogenesis experiment. The incidence of tumors in a group of mice treated with 7,12-dimethylbenz[a]anthracene plus palytoxin was 62.5% in week 25. These tumors were identified histologically as seven papillomas and one carcinoma. This paper reports the potent tumor-promoting activity of palytoxin, which is classified as a non-TPA-type tumor promoter.

Monitoring human exposure to ethylene oxide by the determination of haemoglobin adducts using gas chromatography-mass spectrometry.

Abstract: Globin samples from ethylene oxide-exposed workers and non-exposed referrents were analysed by two methods: (i) gas chromatography-mass spectrometry determination of Nt-(2-hydroxyethyl)histidine as its methyl ester heptafluorobutyryl derivative, after hydrolysis of the protein and isolation of the alkylated amino acid by ion exchange chromatography. The internal standard, Nt-(2-hydroxy-d4-ethyl)histidine, was added to the protein before hydrolysis. (ii) Determination of N-(2-hydroxyethyl)valine after derivatization of the protein by a modified Edman procedure, extraction and g.c.-m.s. determination of alkylated N-terminal valine in the form of its pentafluorophenylthiohydantoin derivative. The internal standard used was in this case a globin with a known content of hydroxy-d4-ethylated amino acids. The two methods gave consistent results, especially at high levels of alkylated products. The average content of hydroxyethylhistidine was 0.6 nmol/g higher than the content of hydroxyethylvaline. Higher levels of background alkylation (of unknown origin) were recorded with the histidine method as compared with the valine method, suggesting that the latter assay should show greater sensitivity for low level ethylene oxide exposure monitoring.

Interstrain differences in susceptibility to liver carcinogenesis initiated by N-nitrosodiethylamine and its promotion by phenobarbital in C57BL/6NCr, C3H/HeNCrMTV- and DBA/2NCr mice.

Abstract: Selected inbred strains of mice were compared with respect to their susceptibility to two-stage liver carcinogenesis. Five-week-old male mice of strains C57BL/6NCr (C57), C3H/HeNCrMTV- (C3H) and DBA/2NCr (DBA) were given a single i.p. injection of N-nitrosodiethylamine (DEN, 90 mg/kg body weight) or the solvent tricaprylin (10 ml/kg). Beginning 2 weeks later, half of the DEN-treated and half of the control mice were given drinking water containing 0.05% phenobarbital (PB). Ten mice from each treatment group were killed at 12, 24, 36 and 52 weeks of age (5, 17, 29 and 45 weeks exposure to PB). PB significantly increased both the number of hepatocellular foci/cm2 and the incidence of hepatocellular tumors after 17 weeks of treatment in 24-week-old DEN-initiated mice of strains C3H (0.11 +/- 0.07 versus 2.9 +/- 0.3 foci/cm2 and 20 versus 70% incidence of hepatocellular tumors) and DBA (0.09 +/- 0.09 versus 3.72 +/- 0.6 foci/cm2 and 0 versus 90% incidence of hepatocellular tumors) but was ineffective in C57 mice (0.04 +/- 0.04 versus 0.07 +/- 0.07 foci/cm2). At 36 weeks of age the incidence of liver cell tumors in mice given DEN but not PB was 10 (DBA), 10 (C57) and 50% (C3H); the incidence was increased by PB to 90% in DBA and 100% in C3H mice, but there was no increase in C57 mice. Even at 52 weeks, the low incidence of hepatocellular tumors in C57 mice given DEN only (20%) was not significantly increased by subsequent exposure to PB. Serum PB levels observed at 12, 24 and 36 weeks of age were significantly higher in DBA mice than in C57 or C3H mice. Similar results were observed in a separate study in which PB was administered in drinking water to 7-week-old male mice of these three strains for 20 days, during which period serum PB levels were measured at shorter intervals. DBA mice thus appear to be unable to metabolize PB, which itself rather than its metabolites is probably responsible for tumor-promoting effects. DBA mice were especially sensitive, while C57 mice were refractory to promotion of hepatocarcinogenesis by PB. These two strains, which differ with respect to other significant parameters for chemical carcinogenesis including inducibility for aryl hydrocarbon hydroxylase and susceptibility to promotion of hydrocarbon-initiated skin tumors by 12-O-tetradecanoylphorbol-13-acetate, thus also provide a means for analysis of the pharmacogenetics of susceptibility to hepatocellular tumor promotion.

Identification and mutagenicity of metabolites of aristolochic acid formed by rat liver

Abstract: The rat liver 9000 g supernatant mediated metabolism of the carcinogenic aristolochic acid, which consists of aristolochic acid I (AAI) and aristolochic acid II (AAII), was investigated. Under anaerobic conditions the major metabolites were the corresponding aristolactams for both AAI and AAII. In contrast under aerobic conditions AAII was not detectably metabolized and the only metabolite found for AAI was the O-demethylated derivative aristolochic acid Ia (AAIa). The metabolites were identified by their u.v., mass and n.m.r. spectra and by comparison with reference standards. The mutagenic activities of the three metabolites were determined in Salmonella typhimurium strains TA1537 and TA 100. The aristolactams were mutagenic in both strains when a metabolizing system was present. These results indicate that AAI or AAII and their aristolactams exert their effect via a common reactive intermediate, probably the corresponding hydroxylamine. AAIa was only very weakly mutagenic and this metabolite may therefore not be regarded as a major mutagenic metabolite of AAI. These findings suggest that the acids are preferentially metabolized by two totally different pathways in vitro, namely an oxidative pathway for AAI and a reductive pathway for AAII.

Changes in nuclear protein acetylation in u.v.-damaged human cells.

Abstract: We have investigated the levels of nuclear protein acetylation in u.v.-irradiated human fibroblasts. Initially, we measured the levels of acetylation in total acid-soluble nuclear proteins and observed two distinct differences between the irradiated and unirradiated (control) cells. Immediately after irradiation, there is a 'wave' of protein hyperacetylation (i.e. a total acetylation level greater than that of unirradiated cells) that lasts for 2-6 h depending on the experimental conditions. This hyperacetylation phase is then followed by a hypoacetylation phase, lasting for many hours, and the total level of acetylation does not return to that of control cells until 24-72 h after u.v. damage. Both the magnitude and duration of each phase is dependent on the dose of u.v. light used. The wave of hyperacetylation is more pronounced at low u.v. doses (i.e. less than 5 J/m2), while the wave of hypoacetylation is more pronounced at higher u.v. doses (greater than or equal to 8 J/m2). Furthermore, the duration of each phase is prolonged when cells are exposed to 2 mM hydroxyurea, an agent which retards the rate of excision repair at u.v.-damaged sites. Examination of the acetylation levels of the individual nuclear proteins indicated that acetylation of the core histones follows the same pattern observed for the total acid-soluble protein fractions. Furthermore, these were the only major proteins in the total acid-soluble fraction observed to undergo the early, rapid hyperacetylation immediately following u.v. damage. Acetylation of histone H1 was negligible in both damaged and control cells, while three prominent non-histone proteins were acetylated only after long labeling times (greater than 4 h) in each case, gradually becoming hyperacetylated in the u.v.-damaged cells. These results raise the possibility that a causal relationship exists between nuclear protein acetylation and nucleotide excision repair of DNA in human cells.

Carcinogenicity and mutagenicity of chromium compounds: the association between bronchial metaplasia and neoplasia

Abstract: Over the last 40 years, experiments with animals and epidemiology of exposed human populations have established that certain hexavalent chromium-containing compounds are capable of causing cancer whilst trivalent materials are not. More recently, a variety of short-term genotoxicity tests (predictive of carcinogenicity) have clearly demonstrated that Cr[VI] is genotoxic per se, and that all Cr[VI]-containing materials which have been tested are genotoxic. What experiments have failed to demonstrate clearly, however, is which of the myriad of industrially available hexavalent materials are carcinogenic and which are not. In this long-term study we have looked at the incidence of squamous metaplasia in the bronchial epithelium of rats exposed to a range of chromium-containing materials by intrabronchial implantation. Squamous metaplasia is generally considered to be a transformed state from which squamous carcinoma may arise. We have shown that its incidence is increased in all groups exposed to Cr[VI]-materials, and in rats exposed to the reference carcinogen 20-methylcholanthrene. Squamous metaplasia was not increased in rats exposed to Cr[III] materials. Of rats exposed to Cr[VI], only those receiving materials of sparing aqueous solubility developed bronchial squamous carcinoma at statistically significant levels. These results strongly support the hypothesis that although Cr[VI] per se is biologically active, producing genotoxic effects and pathological changes which may predispose to the development of cancer, only Cr[VI] materials of sparing aqueous solubility seem to be capable of evoking a carcinogenic response.

Protection of crocin dyes on the acute hepatic damage induced by aflatoxin B1 and dimethylnitrosamine in rats.

Abstract: Chemopreventive agents are compounds that inhibit carcinogenesis when administered prior or subsequent to a course of carcinogen administration. The effects of dietary administration of crocin dyes on the hepatic damage induced by aflatoxin B1 (AFB1) and dimethylnitrosamine (DMN) in rats were investigated. Female Sprague-Dawley rats were treated with different dosages of AFB1 (0.9 or 4.5 mg/kg) or DMN (8 or 20 mg/kg) by i.p. administration, and the different degrees of hepatic damage were revealed by the elevations of levels of serum marker enzymes such as aspartate amino-transferase, alanine aminotransferase, alkaline phosphatase, gamma-glutamyl transpeptidase and lactic dehydrogenase. Pre-treatment of the animals with crocin dyes 50 mg/kg daily for three consecutive days, the enzyme elevations were significantly suppressed. This suggested that the crocin dyes possessed chemopreventive effects on the early acute hepatic damage induced by AFB1 or DMN. Feeding experiments demonstrated that crocin dyes at 0.1% in the diet could suppress partially the chronic hepatic damage induced by multiple dosages of AFB1 or DMN, but at a higher concentration of 1% crocin dye failed to do so because of their host toxicity. Crocin dyes are extracted from the fruits of Gardenia jasminoides and consist of carotenoids and geniposides as active principles. The protective mechanisms of crocin dyes may be attributed to their carotenoids which are converted metabolically to retinoids in rats.