Asparagine synthetase activity of mouse leukemias

Various transplanted leukemias and normal tissues of the mouse were tested for asparagine synthetase activity. Leukemias susceptible to suppression by asparaginase have little or no synthetase activity. In contrast, leukemias insensitive to asparaginase exhibit substantial and often very high asparagine synthetase activity. Asparaginase-resistant variants of sensitive leukemias also have considerable synthetase activity. Thus the requirement by certain malignant cells of exogenous asparagine, which entails sensitivity to asparaginase, may be ascribed to lack of asparagine synthetase. Development of asparaginase-resistant variants from asparaginase-sensitive lines is consistently associated with acquisition of asparagine synthetase activity.

Effect of dietary curcumin and dibenzoylmethane on formation of 7,12-dimethylbenz[a]anthracene-induced mammary tumors and lymphomas/leukemias in Sencar mice

Female Sencar mice (6 weeks old) were administered 1 mg of 7,12-dimethylbenz[a]anthracene (DMBA) by oral gavage once a week for 5 weeks. At 20 weeks after the first dose of DMBA, 68% of mice developed mammary tumors (the average 1.08 tumors per mouse) and 45% had lymphomas/leukemias. Feeding 1% dibenzoylmethane (DBM) in AIN 76A diet, starting at 2 weeks before the first dose of DMBA and continuing until the end of the experiment, inhibited both the multiplicity and incidence of DMBA-induced mammary tumor by 97%. The incidence of lymphomas/leukemias was completely inhibited by 1% DBM diet. In contrast, feeding 2% curcumin diet had little or no effect on the incidence of mammary tumors, and the incidence of lymphomas/leukemias was reduced by 53%.

Carcinogenicity in mice of a mutagenic compound, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) from cooked foods

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), which is a mutagenic compound present in fried beef and beef extracts, was given orally to CDF1 mice at a concentration of 0.06% in the diet for 84 weeks. Liver tumors were induced in 43% of males and 91% of females fed MeIQx. The incidences of liver tumors in mice of both sexes were significantly higher in groups fed MeIQx than in control groups. The incidences of lung tumors in females fed MeIQx and of lymphomas and leukemias in both sexes fed MeIQx were also significantly higher than in the respective controls.

Methyl-tertiary-butyl ether (MTBE)--a gasoline additive--causes testicular and lymphohaematopoietic cancers in rats

In the framework of a series of experiments conducted to evaluate the carcinogenic effects of oxygenated gasoline additives, MTBE was analyzed in an oral lifetime carcinogenicity study using 8-week-old male and female Sprague-Dawley rats. These experiments were part of a large research project on gasoline carcinogenicity performed at the Bentivoglio (BT) Castle Cancer Research Center of the Ramazzini Foundation and of the Bologna Institute of Oncology, MTBE, dissolved in oil, was administered by stomach tube at the doses of 1000, 250, or 0 mg/kg b.w., once daily, four days weekly, for 104 weeks. The animals were maintained until natural death. The last animal died 166 weeks after the start of the experiment, i.e., at 174 weeks of age. Under the tested experimental conditions, MTBE was shown to cause an increase in Leydig interstitial cell tumors of the testes and a dose-related increase in lymphomas and leukemias in female rats.

Benzene inhalation produces leukemia in mice

Female C57Bl/6 mice were exposed to 300 ppm benzene 6 hr/day, 5 days/week for 16 weeks and then held for lifetime observation. Sixty-four weeks after commencement of the study, 10 of 90 exposed mice had died as opposed to only 1 of 88 controls. Of the 10 exposed mice that died, 6 had thymic lymphomas, 2 had unspecified lymphomas, 1 was killed when moribund and found leukemia-free, and 1 was undiagnosed due to autolysis and partial cannibalization. The single dead control animal did not have lymphoma or leukemia. These data provide proof of the leukemogenicity of benzene in female C57Bl/6 mice.

A perspective on benzene leukemogenesis

Although benzene is best known as a compound that causes bone marrow depression leading to aplastic anemia in animals and humans, it also induces acute myelogenous leukemia in humans. The epidemiological evidence for leukemogenesis in humans is contrasted with the results of animal bioassays. This review focuses on several of the problems that face those investigators attempting to unravel the mechanism of benzene-induced leukemogenesis. Benzene metabolism is reviewed with the aim of suggesting metabolites that may play a role in the etiology of the disease. The data relating to the formation of DNA adducts and their potential significance are analyzed. The clastogenic activity of benzene is discussed both in terms of biomarkers of exposure and as a potential indication of leukemogenesis. In addition to chromosome aberrations, sister chromatid exchange, and micronucleus formation, the significance of chromosomal translocations is discussed. The mutagenic activity of benzene metabolites is reviewed and benzene is placed in perspective as a leukemogen with other carcinogens and the lack of leukemogenic activity by compounds of related structure is noted. Finally, a pathway from exposure to benzene to eventual leukemia is discussed in terms of biochemical mechanisms, the role of cytokines and related factors, latency, and expression of leukemia.

Potential role of free radicals in benzene-induced myelotoxicity and leukemia

Occupational exposure to benzene, a major industrial chemical, has been associated with various blood dyscrasias and increased incidence of acute myelogenous leukemia in humans. It is established that benzene requires metabolism to induce its effects. Benzene exposure in humans and animals has also been shown to result in structural and numerical chromosomal aberrations in lymphocytes and bone marrow cells, indicating that benzene is genotoxic. In this review we have attempted to compile the available evidence on the role of increased free radical activity in benzene-induced myelotoxic and leukemogenic effects. Benzene administration to rodents has been associated with increased lipid peroxidation in liver, plasma, and bone marrow, as shown by an increase in the formation of thiobarbituric-acid reactive products that absorb at 535 nm. Benzene administration to rodents also results in increased prostaglandin levels indicating increased arachidonic acid peroxidation. Other evidence includes the fact that bone marrow cells and their microsomal fractions isolated from rodents following benzene-treatment have a higher capacity to form oxygen free radicals. The bone marrow contains several peroxidases, the most prevalent of which is myeloperoxidase. The peroxidatic metabolism of the benzene metabolites, phenol and hydroquinone, results in arachidonic acid peroxidation and oxygen activation to superoxide radicals, respectively. These metabolites, upon co-administration also produce a myelotoxicity similar to that observed with benzene. Recently, we have found that exposure of human promyelocytic leukemia (HL-60) cells (a cell line rich in myeloperoxidase), to the benzene metabolites, hydroquinone and 1,2,4-benzenetriol results in increased steady-state levels of 8-hydroxydeoxyguanosine a marker of oxidative DNA damage. Peroxidatic metabolism of benzene's phenolic metabolites may therefore be responsible for the increased free radical activity and toxicity produced by benzene in bone marrow. We thus hypothesize that free radicals contribute, at least in part, to the toxic and leukemogenic effects of benzene.

The tumorigenicity of 5-azacytidine in the male Fischer rat

5-Azacytidine was administered to young adult male Fischer rats. Tumors were found in 31 out of 70 rats that had received 5-azacytidine and survived 18 months from the start of the experiment. Several rats had multiple primary tumors. In the rats that were tested for complete carcinogenicity a variety of tumor types was found. These included acute leukemia and malignant reticuloendotheliosis, and tumors of the testis, skin, and bronchus. No hepatic tumors were found in the group that was tested for hepatic tumor initiation. Hepatocellular carcinomas were found only in the group that was examined for hepatic tumor promotion by receiving a prior initiating dose of diethylnitrosamine. No tumors were found in the age controls. Thus, in these initial experiments, 5-azacytidine appeared to be a complete carcinogen, inducing tumors in several organs, and a tumor promoter but not a complete carcinogen for the liver.

Chromosome changes (trisomy 15) in murine T-cell leukemia induced by 1,12-dimethylbenz(a)anthracene (DMBA)

The banding pattern of DMBA-induced leukemias in C57BL/6 mice revealed a very constant chromosome pattern: the presence of trisomy 15 in almost all leukemic cells. This finding strongly suggests that chromosome 15 trisomy is the first detectable specific chromosome change associated with the development of DMBA-induced T-cell lymphomas. A similar association was previously shown with regard to development of radiation-leukemia-virus-induced T-cell lymphoma. It is conceivable that in tumors of diverse etiologies common cytogenetic changes may appear in the same common target-cell precursor, by a process of the "convergent microevolution" type.

Growth, body weight, survival, and tumor trends in F344/N rats during an eleven-year period

Time trends for growth, body weight, survival and tumor prevalences in 144 diet control groups with a total of 5,184 male F344/N rats and 146 diet control groups with a total of 5,289 female rats of NCI-NTP 2-yr chemical carcinogenicity studies started during an 11-yr period (1971 to 1981) in 11 toxicology testing laboratories were evaluated. Male and female rats in more recent studies grew faster and attained a higher body weight than rats from earlier studies. Survival of males showed a significantly decreasing trend over time, which may have been related to diseases associated with increasing body weight, prevalence of leukemia and changes in criteria for euthanasia of moribund animals. The time trend for survival of females was not significant. There were highly significant (p less than 0.001) positive time trends for prevalences of leukemia, anterior pituitary tumors and thyroid C-cell tumors in both sexes, adrenal pheochromocytomas in males and mammary tumors and endometrial stromal polyps in females. The prevalence of mammary tumors in females and pituitary tumors in males had a highly significant (p less than 0.01) positive association with body weight. Histological reevaluation of tumor prevalences in approximately 250 rats of each sex at each of 4 different time periods indicated that changes in diagnostic criteria may have contributed to but could not totally explain the increased prevalence of leukemia. Changes in diagnostic criteria and the amount of tissue examined may have contributed to the increased prevalence of anterior pituitary tumors in both sexes and adrenal pheochromocytomas in males. Interlaboratory variability and changes in diet may also have contributed to the time-related trends.

Benzene: a multipotential carcinogen. Results of long-term bioassays performed at the Bologna Institute of Oncology

Until recently, the incidence of benzene carcinogenicity was based only on the association between benzene occupational exposure and human leukemia, with many limited case reports and scanty epidemiological data. Available experimental studies up to 1976 on animals were rare, fragmentary, and inadequate, and had failed to prove the carcinogenic effects of benzenes. However, an integrated project of long-term carcinogenicity bioassays, begun in our laboratory in 1976 and still continuing, has shown that benzene produces a variety of tumors in rats including Zymbal gland carcinomas, carcinomas of the oral cavity, hepatocarcinomas, and possibly mammary carcinomas, lymphoreticular neoplasias, and other malignancies. Some of the tumors caused by benzene are uncommon or unusual in the breed of rats studied. Therefore benzene must be considered, under the studied experimental conditions, a strong multipotential carcinogen. The need for more experimental research is emphasized, particularly to assess the carcinogenic effects of low doses. Also recommended are more comprehensive epidemiological investigations, extended to all types of malignancies, and the application of adequate measures for primary prevention.

Influence of cyclophosphamide and other immunosuppressive drugs on immune disorders and neoplasia in NZB/NZW mice

NZB/NZW mice were treated with various immunosuppressive drugs used in human SLE. Cyclophosphamide (5 mg/kg 6 days out of 7), alone or with prednisolone, was better than azathioprine, prednisolone, or azathioprine-plus-prednisolone, in prolonging survival and/or reducing proteinuria, Coomb's antibodies, antinuclear antibodies, and glomerular deposits of gamma-globulin. Intermittent bolus therapy with cyclophosphamide (59 mg/kg/10 days) was as effective as daily therapy. However, 61% of the mice receiving any cyclophosphamide regimen developed malignant tumors compared to none in the other groups.

Characterization of benzo[a]pyrene quinone-induced toxicity to primary cultured bone marrow stromal cells from DBA/2 mice: potential role of mitochondrial dysfunction

Oral exposure of DBA/2 mice to benzo[a]pyrene (BP) has been shown to result in hematotoxicity which is manifested as aplastic anemia and leukemia. Since normal hematopoiesis is regulated by bone marrow stromal cells, in this study we have characterized the bone marrow stromal toxicity induced by BP and BP-derived metabolites, particularly quinones. Incubation of stromal cells with various concentrations of BP-1,6-, 3,6-, 6,12-, or 7,8-quinone for 24 hr resulted in a significant decrease of cell survival in a concentration-dependent manner, while cells treated with BP or BP-7,8-dihydrodiol did not exhibit any significant loss of cell survival. Among the BP quinones examined, BP-1,6-quinone was the most cytotoxic to stromal cells. The cytotoxicity induced by BP-1,6-quinone also exhibited a time-dependent relationship. Pretreatment of stromal cells with 1,2-dithiole-3-thione (D3T) resulted in a significant induction of both cellular reduced glutathione (GSH) content and quinone reductase (QR) activity in a concentration-dependent manner. However, D3T pretreatment did not offer any protection against BP-1,6-quinone-induced toxicity. Furthermore, dicumarol, a potent inhibitor of QR, or buthionine sulfoximine, a specific inhibitor of GSH biosynthesis, did not potentiate BP-1,6-quinone-induced cytotoxicity was not altered. However, incubation of stromal cells with BP-1,6-quinone resulted in a significant depletion of cellular ATP content and mitochondrial morphological changes, which preceded the loss of cell survival. In addition to BP-1,6-quinone, other cytotoxic BP quinones also exhibited a capacity to deplete cellular ATP level in stromal cells, while BP, which was not cytotoxic to stromal cells, did not elicit any significant decrease in cellular ATP level. These observations suggest that mitochondria may be a potential target of BP quinones. Overall, the above results indicate that neither cellular GSH and QR nor reactive oxygen species appear to be involved in BP quinone-induced stromal cell injury and that BP quinones may elicit cytotoxicity to stromal cells through directly disrupting mitochondrial energy metabolism.

Cancer risk assessment for health care workers occupationally exposed to cyclophosphamide

In the present study a cancer risk assessment of occupational exposure to cyclophosphamide (CP), a genotoxic carcinogenic antineoplastic agent, was carried out following two approaches based on (1) data from an animal study and (2) data on primary and secondary tumors in CP-treated patients. Data on the urinary excretion of CP in health care workers were used to estimate the uptake of CP, which ranged from 3.6 to 18 micrograms/day. Based on data from an animal study, cancer risks were calculated for a health care worker with a body weight of 70 kg and a working period of 40 years, 200 days a year (linear extrapolation). The life-time risks (70 years) of urinary bladder cancer in men and leukemias in men and women were found to be nearly the same and ranged from 95 to 600 per million. Based on the patient studies, cancer risks were calculated by multiplication of the 10-year cumulative incidence per gram of CP in patients by the estimated mean total uptake in health care workers over 10 years, 200 days a year. The risk of leukemias in women over 10 years ranged from 17 to 100 per million using the secondary tumor data (linear extrapolation). Comparable results were obtained for the risk of urinary bladder tumors and leukemias in men and women when primary tumor data were used. Thus, on an annual basis, cancer risks obtained from both the animal and the patient study were nearly the same and ranged from about 1.4 to 10 per million. In The Netherlands it is proposed that, for workers, a cancer risk per compound of one extra cancer case per million a year should be striven for ("target risk") and that no risk higher than 100 per million a year ("prohibitory risk") should be tolerated. From the animal and the patient study it appears that the target risk is exceeded but that the risk is still below the prohibitory risk.

Carcinogenicity of oral cadmium in the male Wistar (WF/NCr) rat: effect of chronic dietary zinc deficiency

The effect of chronic dietary zinc deficiency on the carcinogenic potential of dietary cadmium was assessed in male Wistar (WF/NCr) rats. Groups (n = 28) of rats were fed diets adequate (60 ppm) or marginally deficient (7 ppm) in zinc and containing cadmium at various levels (0, 25, 50, 100, or 200 ppm). Lesions were assessed over the following 77 weeks. Zinc deficiency alone had no effect on survival, growth, or food consumption. Cadmium treatment did not reduce survival or food consumption and only at the highest doses of cadmium (100 and 200 ppm) was body weight reduced (maximum 17%). The incidence of prostatic proliferative lesions, both hyperplasias and adenomas, was increased over that seen in controls (1.8%) in both zinc-adequate (20%) and zinc-deficient rats (14%) fed 50 ppm cadmium. The overall incidence for prostatic lesions for all cadmium treatment groups was, however, much lower in zinc-deficient rats, possibly because of a marked increase in prostatic atrophy that was associated with reduced zinc intake. Cadmium treatment resulted in an elevated leukemia incidence (maximum 4.8-fold over control) in both zinc-adequate and zinc-deficient groups, although zinc deficiency reduced the potency of cadmium in this respect. Testicular tumors were significantly elevated only in rats receiving 200 ppm cadmium and diets adequate in zinc. Both zinc-deficient and zinc-adequate groups showed significant positive trends for development of testicular neoplasia with increasing cadmium dosage. Thus, oral cadmium exposure is clearly associated with tumors of the prostate, testes, and hematopoietic system in rats, while dietary zinc deficiency has complex, apparently inhibitory, effects on cadmium carcinogenesis by this route.

Chronic toxicity and carcinogenic evaluation of diisononyl phthalate in rats

Groups of 110 Fischer 344 rats/sex were fed diisononyl phthalate (DINP) at dietary levels of 0, 0.03, 0.3, and 0.6 wt% for periods up to 2 years. Interim sacrifices of 10 predesignated rats/sex/dose were at 6, 12, and 18 months with surviving animals sacrificed at 24 months. At study termination, survival was in excess of 60% for every group. At the mid or high dose, the following biological effects were noted: slight decreases in food consumption and body weight; slight increase in mortality; a dose-related increase in relative organ weights of liver and kidney; and some slight effects on urinalysis, hematologic, and clinical chemistry parameters. No peroxisome induction was observed in livers of treated rats compared with controls. No clear treatment-related nonneoplastic or neoplastic lesions were found. However, mononuclear cell leukemia (MNCL) and changes known to be associated with an increased incidence of MNCL were seen in the mid-dose and high-dose groups. A literature review suggests that MNCL is a common finding in aging F344 rats and that this increased incidence in rats treated with DINP is not relevant to man. A clear no-observed-effect level was demonstrated for all biological end points at a dietary level of 0. 03 wt% or approximately 17 mg/kg/day of DINP.

Formaldehyde: an experimental multipotential carcinogen

Male and female Sprague-Dawley rats of different ages at the start of the experiments (12 day embryos, and 7 and 25 weeks old) were administered fromaldehyde in drinking water at different doses (2,500 or 1,500, 1,000, 500, 100, 50, 10, 0 ppm). An increased incidence of leukemias and of gastro-intestinal tumors was observed in formaldehyde treated rats. Gastro-intestinal tumors are exceptionally rare in the rats of the colony used. These results, together with the ones obtained by other Authors on rats exposed by inhalation to formaldehyde, indicate that this compound is an experimental multipotential carcinogen. The experimental results presented in this report give scientific support to the epidemiological observation of a higher incidence of leukemias and of gastro-intestinal cancers among the people occupationally exposed.

Carcinogenesis bioassay of technical-grade piperonyl butoxide in F344 rats

The insecticide synergist piperonyl butoxide--alpha-[2-(2-butoxyethoxy)ethoxy]-4,5-methylenedioxy-2-propyltoluene--was tested for carcinogenicity in inbred F344 rats in a 2-year study employing doses of 10,000 and 5,000 ppm of the compound administered continuously in the feed. Although a statistically significant dose-related increase in the incidence of lymphoreticular neoplasia was associated with administration of the compound to females, the incidence of that class of neoplasm was higher in control males than in treated males. The finding of statistical significance in one sex is not considered by itself to constitute sufficient evidence of a biologic effect to justify an indictment of carcinogenic action. However, inasmuch as the chief use of this substance is to alter the in vivo metabolism of other chemicals, its possible role as a cocarcinogen should be carefully considered in any risk-benefit evaluation aimed at setting policies regarding its uses.

Benzene hematotoxicity and leukemogenesis

Eight-to-twelve-week-old male and female C57B1/6 BNL mice were exposed to air or benzene vapor in air at a concentration of 10, 25, 100, 300, or 400 ppm. Benzene at concentrations of 100 ppm or higher for 10 exposures of 6 hours per day 5 days a week produced a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow. The fraction of stem cells in DNA synthesis was also increased. Exposure to 300 ppm 6 hours a day 5 days a week for 2, 4, 8, and 16 weeks produced a diminution in the stem cell levels in bone marrow which returned to those of controls 2 weeks after benzene exposure for 2 and 4 weeks, 16 weeks after exposure for 8 weeks, and to 92% of controls 25 weeks after 16 weeks of exposure. There was a more rapid return of blood lymphocytes to the control level. Mice exposed to 300 ppm for 6 hours/day, 5 days per week for 16 weeks began dying at 330 days of age, whereas no deaths were observed in sham-exposed mice until 440 days of age. The benzene-exposed mice died in two waves: the first was from 330-390 days of age, with a second wave commencing at 570 days of age. The first wave of mortality was due primarily to thymic lymphomata. The second wave was due to a mixture of nonthymic lymphomata and solid tumors.

The genetic basis of susceptibility to leukemia induction in mice by 3-methylcholanthrene applied percutaneously

Susceptibility to leukemia induction in mice by skin painting with 3-methylcholanthrene (MCA) is strain-specific, occurring only in strains relatively resistant to MCA-induced skin tumors. The Ah locus, which has a dominant allele (Ahb) for inducibility of the aryl hydrocarbon hydroxylase (AHH) enzyme system and a recessive allele (Ahd) for noninducibility, appears to be the major determinant of this trait. MCA-painted mice of strains and crosses carrying the Ahb allele usually show a high incidence of skin tumors (papillomas which may evolve into malignant tumors) and little or no leukemia, whereas in mice homozygous for the Ahd allele the treatment usually induces a high incidence of leukemia and few or no skin tumors. Among mice of a segregating backcross generation including both Ahb/Ahd heterozygotes and Ahd homozygotes, the occurrence of skin tumors was correlated directly with AHH inducibility and inversely with the leukemic response. Mice of Ahb strains with a high level of endogenous murine leukemia (MuLV) expression (C58, PL) show a much weaker skin tumor response than expected but no increase in leukemia incidence, and this observation tends to confirm the previous finding that MuLV infection of mice of low-MuLV strains results in reduced susceptibility to MCA tumorigenesis.

Patterns of tumor incidence in two-year cancer bioassay feeding studies in Fischer 344 rats

Patterns of tumor incidence in 25 of the National Toxicology Program's (NTP's) two-year cancer bioassay feeding studies in Fischer 344 rats were investigated. It was found that the overall frequencies of statistically significant (P less than 0.01) increases and decreases in organ-specific tumor incidence in treated groups relative to controls were approximately the same. The decreases were due primarily to mammary gland fibroadenomas in females (which were clearly associated with decreased weight gain in the treated groups) and leukemia/lymphoma in both sexes (which were frequently associated with increased liver tumor incidences in the treated groups). A clear explanation for this latter association was not apparent. The increased tumor incidences in the treated groups relative to controls were due primarily to liver neoplastic nodules. Treated animals of either sex had slightly (but significantly) improved overall survival relative to controls. The intra- and inter-laboratory variability in control tumor incidence frequently exceeded what one would expect to find by chance alone. This investigation helps explain the decreased tumor incidences observed in many NTP feeding studies. It also indicates that attention should be directed toward the determination of underlying mechanisms responsible for the negative correlation between the incidences of liver tumors and leukemia/lymphoma.

Chromosome abnormality in rat leukemia induced by 7,12-dimethylbenz[a]anthracene

A high percentage of consistent chromosome abnormality, trisomy of the longest telocentric chromosome, was found in leukemias induced in rats of the Long-Evans strain by pulse doses of 7,12-dimethylbenz[a]anthracene. Cells with this abnormality were large, immature, and mononuclear and tended toward erythroblastic maturation.