Mutations of ras protooncogenes and p53 tumor suppressor gene in cardiac hemangiosarcomas from B6C3F1 mice exposed to 1,3-butadiene for 2 years

Comparison of Endothelial Cell Proliferation in Normal Liver and Adipose Tissue in B6C3F1 Mice, F344 Rats, and Humans

Peroxisome proliferator-activated receptor gamma (PPARγ) and dual PPARα and γ agonists have been developed for use in the treatment of diabetes and hyperlipidemias. Vascular tumors were increased in mice treated with some PPAR agonists, but not in rats. Spontaneous hemangiosarcomas are common in several strains of mice, uncommon in rats, and rarely occur in humans. The objective of this study was to determine the endothelial cell proliferation rate in liver and adipose tissue of B6C3F1 mice, F344 rats, and humans to aid in investigations of the genesis and development of hemangiosarcoma formation, and to determine the relevance of the increased endothelial cell proliferation rate in drug-treated rodents in assessing the risk of these drugs in humans. We determined the endothelial cell labeling index (LI) in untreated mice, rats, and humans, in normal liver, brown fat (rats and mice only) and white fat by dual immunohistochemistry of CD31 and Ki-67. The LI, highest in mice and lowest in humans, was statistically significantly greater in the mouse compared to the human and rat. The increased rate of spontaneous or PPAR agonist-induced hemangiosarcoma formation in mice may be related to the higher background endothelial cell proliferation rate compared to rats and humans.

Comparative genomic hybridization in detection of DNA changes in canine lymphomas

In this study, chromosomal imbalances in tumor tissues (lymphomas) and nucleotide changes in tumor suppressor TP53 were studied in a Bernese Mountain dog bitch and a cross breed bitch. Using comparative genomic hybridization, numerous chromosomal rearrangements were detected, which indicated the heterogeneity in tumor growth: in the cross breed bitch, a deletion on the chromosome 9, and duplications on chromosomes 5, 8 and 17 have been found. In the Bernese Mountain Dog bitch, losses on chromosomes 1, 5, 8, 12, 18, 22, 27, 29 and gains on chromosomes 1, 2, 9, 11, 15, 16, 18, 20, 23, 24, 25, 28, 29, 30, 34, 36, 37 and 38 were identified. With the sequencing of the TP53 gene, one silent mutation, transition A/G at position 138 in exon 5 was detected, without changing the amino acid.

Hyaluronan/RHAMM interactions in mesenchymal tumor pathogenesis: role of growth factors

Fibrosarcoma belongs to the sarcoma cancer group, which are spindle cell malignancies of mesenchymal origin, and owe their name to the predominant cell line that is present within the tumor. The extracellular matrix (ECM) is a complicated structure that surrounds and supports cells within tissues. Its main components are proteoglycans, collagens, glycoproteins, hyaluronan (HA), and several matrix-degrading enzymes. During cancer progression, significant changes can be observed in the structural and mechanical properties of ECM components. The ECM provides a physical scaffold to which tumor cells attach and migrate. Thus, it is required for key cellular events such as cell motility, adhesion, proliferation, invasion, and metastasis. Importantly, fibrosarcomas were shown to have a high content and turnover of ECM components including HA, proteoglycans, collagens, fibronectin, and laminin. In this review, we will focus on the HA component of fibrosarcoma ECM and critically discuss its role and involved mechanisms during fibrosarcoma pathogenesis.

Molecular fingerprints of environmental carcinogens in human cancer

Identification of specific molecular changes (fingerprints) is important to identify cancer etiology. Exploitable biomarkers are related to DNA, epigenetics, and proteins. DNA adducts are the turning point between environmental exposures and biological damage. DNA mutational fingerprints are induced by carcinogens in tumor suppressor and oncogenes. In an epigenetic domain, methylation changes occurs in specific genes for arsenic, benzene, chromium, and cigarette smoke. Alteration of specific microRNA has been reported for environmental carcinogens. Benzo(a)pyrene, cadmium, coal, and wood dust hits specific heat-shock proteins and metalloproteases. The multiple analysis of these biomarkers provides information on the carcinogenic mechanisms activated by exposure to environmental carcinogens.

Modeling human endothelial cell transformation in vascular neoplasias

Endothelial cell (EC)-derived neoplasias range from benign hemangioma to aggressive metastatic angiosarcoma, which responds poorly to current treatments and has a very high mortality rate. The development of treatments that are more effective for these disorders will be expedited by insight into the processes that promote abnormal proliferation and malignant transformation of human ECs. The study of primary endothelial malignancy has been limited by the rarity of the disease; however, there is potential for carefully characterized EC lines and animal models to play a central role in the discovery, development and testing of molecular targeted therapies for vascular neoplasias. This review describes molecular alterations that have been identified in EC-derived neoplasias, as well as the processes that underpin the immortalization and tumorigenic conversion of ECs. Human EC lines, established through the introduction of defined genetic elements or by culture of primary tumor tissue, are catalogued and discussed in relation to their relevance as models of vascular neoplasia.

The roles of hyaluronan/RHAMM/CD44 and their respective interactions along the insidious pathways of fibrosarcoma progression

Fibrosarcomas are rare malignant mesenchymal tumors originating from fibroblasts. Importantly, fibrosarcoma cells were shown to have a high content and turnover of extracellular matrix (ECM) components including hyaluronan (HA), proteoglycans, collagens, fibronectin, and laminin. ECMs are complicated structures that surround and support cells within tissues. During cancer progression, significant changes can be observed in the structural and mechanical properties of the ECM components. Importantly, hyaluronan deposition is usually higher in malignant tumors as compared to benign tissues, predicting tumor progression in some tumor types. Furthermore, activated stromal cells are able to produce tissue structure rich in hyaluronan in order to promote tumor growth. Key biological roles of HA result from its interactions with its specific CD44 and RHAMM (receptor for HA-mediated motility) cell-surface receptors. HA-receptor downstream signaling pathways regulate in turn cellular processes implicated in tumorigenesis. Growth factors, including PDGF-BB, TGFβ2, and FGF-2, enhanced hyaluronan deposition to ECM and modulated HA-receptor expression in fibrosarcoma cells. Indeed, FGF-2 through upregulation of specific HAS isoforms and hyaluronan synthesis regulated secretion and net hyaluronan deposition to the fibrosarcoma pericellular matrix modulating these cells' migration capability. In this paper we discuss the involvement of hyaluronan/RHAMM/CD44 mediated signaling in the insidious pathways of fibrosarcoma progression.

Hemangiosarcoma in mice administered pregabalin: analysis of genotoxicity, tumor incidence, and tumor genetics

Pregabalin, (S)-3-(aminomethyl)-5-methylhexanoic acid, binds with high affinity to the α(2)δ subunit of voltage-gated calcium channels and exerts analgesic, anxiolytic, and antiseizure activities. Two-year carcinogenicity studies were completed in B6C3F1 and CD-1 mice and two separate studies in Wistar rats. Doses in mice were 200, 1000, and 5000 mg/kg/day, with systemic exposures (AUC(0-24 h)) up to 31 times the mean exposure in humans, given the maximum recommended clinical dose. In rats, doses were 50, 150, and 450 mg/kg/day in males and 100, 300, and 900 mg/kg/day in females; systemic exposures up to 24 times were achieved in clinical trials. In both strains of mice, pregabalin treatment was associated with an increased incidence of hemangiosarcoma primarily in liver, spleen, and bone marrow. The incidence of hemangiosarcoma was higher in B6C3F1 mice than in CD-1 mice, consistent with its spontaneous incidence. Pregabalin did not increase the incidence of any other tumor type in rats and was not genotoxic, based on an extensive battery of in vivo and in vitro tests in bacterial and mammalian systems. Thus, pregabalin is a single-species, single tumor-type, nongenotoxic mouse carcinogen. Hemangiosarcomas occurring in mice treated with pregabalin were genotypically distinct from hemangiosarcomas induced by genotoxic carcinogens in humans with respect to ras and p53 mutation patterns and were similar to spontaneous tumors. Furthermore, there was a strong association between pregabalin treatment and bone marrow changes in these studies in mice, suggesting a possible link between the effects observed in bone marrow and the increase in tumor incidence in pregabalin-treated mice.

MicroRNA-mediated regulation of the angiogenic switch

PURPOSE OF REVIEW

It has been known for decades that in order to grow, tumors need to activate quiescent endothelial cells to form a functional vascular network, a process termed 'angiogenesis'. However, the molecular determinants that reverse this endothelial quiescence to facilitate pathological angiogenesis are not yet completely understood. This review examines a critical regulatory switch at the level of Ras that activates this angiogenic switch process and the role that microRNAs play in this process.

RECENT FINDINGS

In the last few years, microRNAs, a new class of small RNA molecules, have emerged as key regulators of several cellular processes, including angiogenesis. MicroRNAs such as miR-126, miR-296, and miR-92a have been shown to play important roles in angiogenesis. We recently described how miR-132, an angiogenic growth factor inducible microRNA in the endothelium, facilitates pathological angiogenesis by downregulating p120RasGAP, a molecular brake for Ras. Importantly, targeting miR-132 with a complementary, synthetic antimicroRNA restored the brake and decreased angiogenesis and tumor burden in multiple tumor models. Taken together, emerging evidence suggests a central role for microRNAs downstream of multiple growth factors in regulating endothelial proliferation, migration, and vascular patterning.

SUMMARY

Further research into miR-132-p120RasGAP biology and more broadly, microRNA regulation of Ras pathways in the endothelium will not only advance our understanding of angiogenesis but also provide opportunities for therapeutic intervention.

Clinicopathologic and immunohistochemical characteristics of adult primary cardiac angiosarcomas: analysis of 10 cases

Primary cardiac angiosarcoma is a rare but the most common malignant neoplasm of the heart in adults. The objective of this study is to analyze the clinicopathologic characteristics of primary cardiac angiosarcoma. Ten cases of primary cardiac angiosarcoma treated in a single institution were analyzed for their clinical, pathologic, and immunohistochemical features. There were 6 men and 4 women, with a mean age of 40 years (range, 20-61 years). The patients commonly presented with dyspnea and distant metastasis. All tumors were located in the right atrium, with a mean tumor size of 6.8 cm. Tumors were hemorrhagic, with variegated tan-brown solid areas. Histologically, they exhibited high-grade morphology with mixed solid growth and anatomizing channels. Frequent mitoses and tumor necrosis were common. The tumors were strongly positive for CD31, CD34, FLI-1, and WT-1 but negative for AE1/3, D2-40, human herpesvirus 8, and epidermal growth factor receptor. The tumor cells were focally reactive to p53, with a high rate of Ki-67 expression. A complete tumor resection was not possible in any of the patients because of the size or extensive local invasion of the tumor. Overall survival ranged from 1 to 81 months (mean, 26.6 months) after initial histologic diagnosis. Primary cardiac angiosarcomas are rare tumors that commonly arise in the right atrium. The mean age is much younger than that of soft tissue angiosarcoma. Regional tumor extension and distant metastasis are extremely common at the time of diagnosis. Surgical resection with adjuvant chemotherapy is currently the preferred treatment, and survival time appears to be inversely correlated with the tumor size and degree of regional tumor extension at the time of surgery.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

1,3-Butadiene: III. Assessing carcinogenic modes of action

1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with greater potency in the mouse than the rat, and is associated with an increase in leukemia mortality in highly exposed workers. Species differences in the formation of reactive metabolites underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action (MOAs) for human leukemia and rodent tumors are both likely related to mutagenic potencies of one or more of these metabolites. However, differences in the nature of genotoxic lesions associated with human leukemia and rodent tumors, along with their implications for risk assessment, require that they be discussed separately. The MOAs for BD are assessed in this review using the modified Hill criteria and human relevance framework. Key events in MOAs for human and rodent cancers are identified, along with important species differences and sources of nonlinearity for each event that can affect extrapolations made from high- to low-dose exposures. Because occupational exposures to BD have also included co-exposures to styrene and dimethyldithiocarbamide (DMDTC), potential interactions with BD carcinogenicity are also discussed. The MOAs for BD carcinogenesis will be used to guide key decisions made in the quantitative cancer dose-response assessment.

Transgenic model of cardiac rhabdomyosarcoma formation

OBJECTIVES

Cardiac rhabdomyosarcomas are rare, and the pathogenesis of this detrimental disease is widely unknown. Most data are obtained from case reports or small series, and models for systematic pathogenetic studies are lacking. We aimed to establish a transgenic mouse model of cardiac rhabdomyosarcoma formation.

METHODS

Standard techniques were used to construct a minigene comprised of the 5' region of the 1.4-kb SM22alpha gene (expressed in embryonic cardiac muscle) and the 2.7-kb SV40 T antigen early region. This T antigen fragment includes the coding sequences for the binding sites of p53 and the proteins of the pRb family. Genotyping of transgenic mice was performed by means of polymerase chain reaction, and phenotypic expression was evaluated by means of immunohistochemistry.

RESULTS

Transgenic mice were studied at the age of approximately 8 to 12 weeks. Cardiac tumors were found of variable size in the left or right sides of the heart and were associated with T antigen expression. Histologic analysis revealed a 3.1-fold enhanced cell density, enlarged cell nuclei, and a 3.4-fold enhanced DNA content. Phenotypic characterization of cardiac tumors resulted in positive staining for desmin, smooth muscle alpha-actin, troponin C, and Myo D1, which met the criteria for rhabdomyosarcomas.

CONCLUSIONS

To the best of our knowledge, the present study is the first description of a mouse model of cardiac rhabdomyosarcoma formation based on genetic modulation. Our model will be a valuable tool for illuminating the pathogenesis of cardiac rhabdomyosarcomas and will allow the testing of new therapeutic approaches to fight this dreadful disease.

Genetic alterations in K-ras and p53 cancer genes in lung neoplasms from B6C3F1 mice exposed to cumene

The incidences of alveolar/bronchiolar adenomas and carcinomas in cumene-treated B6C3F1 mice were significantly greater than those of the control animals. We evaluated these lung neoplasms for point mutations in the K-ras and p53 genes that are often mutated in humans. K-ras and p53 mutations were detected by cycle sequencing of PCR-amplified DNA isolated from paraffin-embedded neoplasms. K-ras mutations were detected in 87% of cumene-induced lung neoplasms, and the predominant mutations were exon 1 codon 12 G to T transversions and exon 2 codon 61 A to G transitions. P53 protein expression was detected by immunohistochemistry in 56% of cumene-induced neoplasms, and mutations were detected in 52% of neoplasms. The predominant mutations were exon 5, codon 155 G to A transitions, and codon 133 C to T transitions. No p53 mutations and one of seven (14%) K-ras mutations were detected in spontaneous neoplasms. Cumene-induced lung carcinomas showed loss of heterozygosity (LOH) on chromosome 4 near the p16 gene (13%) and on chromosome 6 near the K-ras gene (12%). No LOH was observed in spontaneous carcinomas or normal lung tissues examined. The pattern of mutations identified in the lung tumors suggests that DNA damage and genomic instability may be contributing factors to the mutation profile and development of lung cancer in mice exposed to cumene.

A spatially and temporally restricted mouse model of soft tissue sarcoma

Soft tissue sarcomas are mesenchymal tumors that are fatal in approximately one-third of patients. To explore mechanisms of sarcoma pathogenesis, we have generated a mouse model of soft tissue sarcoma. Intramuscular delivery of an adenovirus expressing Cre recombinase in mice with conditional mutations in Kras and Trp53 was sufficient to initiate high-grade sarcomas with myofibroblastic differentiation. Like human sarcomas, these tumors show a predilection for lung rather than lymph node metastasis. Using this model, we showed that a prototype handheld imaging device can identify residual tumor during intraoperative molecular imaging. Deletion of the Ink4a-Arf locus (Cdkn2a), but not Bak1 and Bax, could substitute for mutation of Trp53 in this model. Deletion of Bak1 and Bax, however, was able to substitute for mutation of Trp53 in the development of sinonasal adenocarcinoma. Therefore, the intrinsic pathway of apoptosis seems sufficient to mediate p53 tumor suppression in an epithelial cancer, but not in this model of soft tissue sarcoma.

Genetic alterations in K-ras and p53 cancer genes in lung neoplasms from Swiss (CD-1) male mice exposed transplacentally to AZT

A transplacental carcinogenicity study was conducted by exposing pregnant Swiss (CD-1) mice to 0, 50, 100, 200, or 300 mg of 3'-azido-3'-deoxythymidine (AZT)/kg bw/day, through a 18 to 19-day gestation [National Toxicology Program, NIH Pub. No. 04-4458, 2004]. The incidences of alveolar/bronchiolar adenomas and carcinomas, in the 200 and 300 mg/kg male treatment groups, were significantly greater than that of the controls. In the present study, we evaluated the benign and malignant lung neoplasms from this bioassay for point mutations, in the K-ras and p53 cancer genes that are often mutated in human lung tumors. K-ras and p53 mutations were detected by cycle sequencing of polymerase chain reaction-amplified DNA, isolated from formalin-fixed, paraffin-embedded neoplasms. K-ras mutations were detected in 25 of 38 (66%) of the AZT-induced lung tumors, and the predominant mutations were codon 12 G-->T transversions. p53 mutations were detected in 32 of 38 (84%) of the AZT-induced lung tumors, with the predominant mutations being exon 8, codon 285 A-->T transversions, and exon 6, codon 198 T-->A transversions. No K-ras or p53 mutations were detected in five tumors, examined from control mice. The patterns of mutations identified in the lung tumors suggest that incorporation of AZT or its metabolites into DNA, oxidative stress, and genomic instability may be the contributing factors to the mutation profile and development of lung cancer in these mice.

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

Evaluation of genetic alterations in cancer-related genes in lung and brain tumors from B6C3F1 mice exposed to 1,3-butadiene or chloroprene

1,3-Butadiene and chloroprene are multisite carcinogens in B6C3F1 mice with the strongest tumor response being the induction of lung neoplasms in females. Incidence of brain tumors in mice exposed to 1,3-butadiene was equivocal. This article reviews the efforts of our laboratory and others to uncover the mechanisms of butadiene and chloroprene induced lung and brain tumor responses in the B6C3F1 mouse. The formation of lung tumors by these chemicals involved mutations in the K-ras cancer gene and loss of heterozygosity in the region of K-ras on distal chromosome 6, while alterations in p53 and p16 were implicated in brain tumorigenesis.

Organ-specific susceptibility of p53 knockout mice to N-bis(2-hydroxypropyl)nitrosamine carcinogenesis

To elucidate which is the major determinant of susceptibility of p53 deficient mice, the carcinogen or the target organ, N-bis(2-hydroxypropyl)nitrosamine was administered to induce tumors in multi-organs. In a 15-week experiment, the incidences of both lung and hepatic vascular tumors were found to be significantly higher in p53 nullizygous (-/-) than in heterozygous (+/-) and wild-type (+/+) mice, indicating universal susceptibility of p53 (-/-) mice. In a 40-week experiment, p53 (+/-) mice showed increased susceptibility only with regard to vascular tumors, coinciding with significantly more frequent (60%) p53 gene mutations, in comparison with lung tumors with their low mutation rate (10.8%) (P<0.005). These results indicate that the target organ may be a more important factor than the carcinogen in determining susceptibility of p53 (+/-) mice.

Genetic alterations in brain tumors following 1,3-butadiene exposure in B6C3F1 mice

The nervous system of the B6C3F1 mouse has rarely been a target for chemical carcinogenesis in the National Toxicology Program (NTP) bioassays. However, 6 malignant gliomas and 2 neuroblastomas were observed in B6C3F1 mice exposed to 625 ppm 1,3-butadiene (NTP technical reports 288 and 434). These mouse brain tumors were evaluated with regard to the profile of genetic alterations that are observed in human brain tumors. Alterations in the p53 tumor suppressor gene were common. Missense mutations were observed in 3/6 malignant gliomas and 2/2 neuroblastomas and were associated with loss of heterozygosity. Most of the mutations occurred in exons 5-8 of the p53 gene and were G-->A transitions, and did not involve CpG sites. Loss of heterozygosity at the Ink4a/Arf gene locus was observed in 5/5 malignant gliomas and 1/1 neuroblastoma, while the PTEN(phosphatase and tensin homologue) gene locus was unaffected by deletions. One of 2 neuroblastomas had a mutation in codon 61 of H-ras, while H-ras mutations were not observed in the malignant gliomas examined. Only 1 brain tumor has been reported from control mice of over 500 NTP studies. This malignant glioma showed no evidence of alterations in the p53 gene or K- and H-ras mutations. It is likely that the specific genetic alterations observed were induced or selected for by 1,3-butadiene treatment that contributed to the development of mouse brain tumors. The observed findings are similar in part to the genetic alterations reported in human brain tumors.

Analysis of p53 tumor suppressor gene, H-ras protooncogene and proliferating cell nuclear antigen (PCNA) in squamous cell carcinomas of HRA/Skh mice following exposure to 8-methoxypsoralen (8-MOP) and UVA radiation (PUVA therapy)

Treatment with 8-methoxypsoralen (8-MOP) and ultraviolet radiation (primarily UVA), called PUVA therapy, has been used to treat different chronic skin diseases but led to a significant increased risk for skin cancer. The National Toxicology Program (NTP) performed a study in mice treated with PUVA that showed a significant increase in squamous cell carcinomas of the skin. In the present study, we evaluated the protein expression of p53 and PCNA and DNA mutations of p53 and H-ras genes in both hyperplastic and neoplastic squamous cell lesions from the NTP study. By immunohistochemical staining, protein expression of both p53 and PCNA was detected in 3/16 (19%) of hyperplastic lesions and 14/17 (82%) of SCCs in groups treated with both 8-MOP and UVA. The mutation frequency of p53 in SCCs from mice administered 8-MOP plus UVA was 15/17 (88%) with a predominant distribution of mutations in exon 6 (14/15 - 93%). No H-ras mutations were detected in the hyperplastic lesions/tumors. The mutagenic effect of PUVA on the p53 tumor suppressor gene may lead to a conformational modification and inactivation of the p53 protein, which are considered critical steps in PUVA-induced skin carcinogenesis. The p53 mutational frequency and patterns from our study were different from those reported in human PUVA-type tumors.

Acute exposure of human lung cells to 1,3-butadiene diepoxide results in G1 and G2 cell cycle arrest

1,3-Butadiene (BD) causes genetic damage, including adduct formation, sister chomatid exchange, and point mutations. Previous studies have focused on the types of genetic damage and tumors found after long-term exposure of rodents to butadiene. This study examined the effect of the most active BD metabolite, butadiene diepoxide (BDO2), on cell cycle entry and progression in human lung fibroblasts (LU cells) with a normal diploid karyotype. Serum-arrested (G0) LU cells were exposed to BDO2 for 1 hr and stimulated to divide with medium containing 10% fetal bovine serum. The BDO2-treated LU cells were evaluated for cell cycle progression, nuclear localization of arrest mediators, mitotic index, and cellular proliferation. The BDO2-treated cells demonstrated a substantial inhibition of cell proliferation when treated with 100 microM BDO2 for 1 hr. No appreciable levels of apoptosis or mitotic figures were observed in the BDO2-treated cells through 96 hr posttreatment. Flow cytometric analysis revealed that the lack of proliferation in BDO2-treated LU cells was related to G1 arrest in about half of the cells and a delayed progression through S and G2 arrest in nearly all of the remaining cells. Both G1 and G2 arrest were prolonged and only a very small percentage of BDO2-treated cells were eventually able to replicate. Increased nuclear localization of both p53 and p21(cip1) was observed in BDO2-treated cells, suggesting that the cell cycle arrest was p21(cip1)-mediated. These results demonstrate that BDO2 induces cell cycle perturbation and arrest even with short-term exposure that does not produce other pathologic cellular effects.

Diepoxybutane induces caspase and p53-mediated apoptosis in human lymphoblasts

Diepoxybutane (DEB) is the most potent metabolite of the environmental chemical 1,3-butadiene (BD), which is prevalent in petrochemical industrial areas. BD is a known mutagen and human carcinogen, and possesses multiorgan systems toxicity that includes bone marrow depletion, spleen, and thymus atrophy. Toxic effects of BD are mediated through its epoxy metabolites. In working towards elucidating the cellular and molecular mechanisms of BD toxicity, we investigated the ability of DEB to induce apoptosis in human lymphoblasts. DEB induced a concentration and exposure time-dependent apoptosis, which accounted for the DEB-induced loss of cell viability observed in TK6 lymphoblasts. The DEB-induced apoptosis was inhibited by inhibitors of caspases 3 and 9. The role of p53 in mediating the DEB-induced apoptosis was also investigated. DEB induced elevated p53 levels in direct correlation to the extent of DEB-induced apoptosis, as the concentration of DEB increased up to 5 microM. The extent of DEB-induced apoptosis was dramatically higher in TK6 lymphoblasts as compared to the genetically paired p53-deficient NH32 lymphoblasts under the same experimental conditions. Our results confirm and extend observations on the occurrence of apoptosis in DEB exposed cells, and demonstrate for the first time the elevation of p53 levels in human lymphoblasts in response to DEB exposure. In addition, our results demonstrate for the first time that DEB-induced apoptosis is mediated by caspases 3 and 9, as well as the p53 protein. It is possible that DEB-induced apoptosis may explain BD-induced bone marrow depletion, spleen and thymus atrophy in BD-exposed animals.

Activated Ras induces a proangiogenic phenotype in primary endothelial cells

Angiogenic factors alter endothelial cell phenotype to promote the formation of new blood vessels, a process critical for a number of normal and pathological conditions. Ras is required for the induction of the angiogenic phenotype in response to vascular endothelial growth factor (VEGF). However, VEGF generates many signals, several of which are not dependent upon Ras activation. Our current study investigates the sufficiency of Ras activation for driving angiogenic responses. An activated Ras(V12) mutant induces prominent membrane ruffling, branching morphogenesis on three-dimensional collagen, DNA synthesis, and cell migration in primary endothelial cells. An upregulation of PI3K/AKT, Erk, and Jnk signaling pathways accompany these phenotypic changes. The inhibition of Erk blocked cell proliferation, but only partially attenuated migration. Blocking PI3K had no effect on DNA synthesis, but caused a modest reduction in cell migration. Lastly, Jnk played a significant role in both the proliferation and migration response. These effects of Ras(V12) are not the result of increased autocrine secretion of VEGF. These data suggest that the acquisition of activating Ras mutations can lead to a proangiogenic conversion in the phenotype of primary endothelial cells. Furthermore, these data raise the possibility that chronic Ras activation in endothelial cells may be sufficient to promote angiogenesis and the development of vascular anomalies.

Chemical-specific alterations in ras, p53, and beta-catenin genes in hemangiosarcomas from B6C3F1 mice exposed to o-nitrotoluene or riddelliine for 2 years

The most prominent neoplastic lesions in mice in the 2-year studies of o-nitrotoluene and riddelliine were hemangiosarcomas. Fifteen o-nitrotoluene-induced hemangiosarcomas of the skeletal muscle, subcutaneous tissue, and mesentery; 12 riddelliine-induced hemangiosarcomas of the liver; and 15 spontaneous subcutaneous hemangiosarcomas were examined for genetic alterations in ras, p53, and beta-catenin genes. Mutations in at least one of these genes were identified in 13 of 15 (87%) of the o-nitrotoluene-induced hemangiosarcomas with missense mutations in p53 exons 5-8 detected in 11 of 15 (73%) of these neoplasms. Seven of 15 (47%) hemangiosarcomas from mice exposed to o-nitrotoluene had deletions at exon 2 splice sites or smaller deletions in the beta-catenin gene. K-ras mutation was detected in only 1 of the 15 (7%) o-nitrotoluene-induced hemangiosarcomas. In contrast to the o-nitrotoluene study, 7/12 (58%) riddelliine-induced hemangiosarcomas had K-ras codon 12 GTT mutations and, when screened by immunohistochemistry, 9/12 (75%) had strong staining for the p53 protein in malignant endothelial cells, the cells of origin of hemangiosarcomas. Riddelliine-induced hemangiosarcomas were negative for the beta-catenin protein. Spontaneous hemangiosarcomas from control mice lacked both p53 and beta-catenin protein expression and ras mutations. Our data indicated that p53 and beta-catenin mutations in the o-nitrotoluene-induced hemangiosarcomas and K-ras mutations and p53 protein expression in riddelliine-induced hemangiosarcomas most likely occurred as a result of the genotoxic effects of these chemicals. It also suggests that these mutations play a role in the pathogenesis of the respective hemangiosarcomas in B6C3F1(1) mice.

Carcinogenicity and mechanistic insights on the behavior of epoxides and epoxide-forming chemicals

Many epoxides and their precursors are high production volume chemicals that have major uses in the polymer industry and as intermediates in the manufacture of other chemicals. Several of these chemicals were demonstrated to be carcinogenic in laboratory animal studies conducted by the Ramazzini Foundation (e.g., vinyl chloride, acrylonitrile, styrene, styrene oxide, and benzene) and by the National Toxicology Program (e.g., ethylene oxide, 1,3-butadiene, isoprene, chloroprene, acrylonitrile, glycidol, and benzene). The most common sites of tumor induction were lung, liver, harderian gland, and circulatory system in mice; Zymbal's gland and brain in rats; and mammary gland and forestomach in both species. Differences in cancer outcome among studies of epoxide chemicals may be related to differences in study design (e.g., dose, duration, and route of exposure; observation period; animal strains), as well as biological factors affecting target organ dosimetry of the DNA-reactive epoxide (toxicokinetics) and tissue response (toxicodynamics). N7-Alkylguanine, N1-alkyladenine, and cyclic etheno adducts, as well as K-ras and p53 mutations, have been detected in animals and/or workers exposed to several of these chemicals. The classifications of these chemical carcinogens by IARC and NTP are based on animal and human data and results of mechanistic studies. Reducing occupational and environmental exposures to these chemicals will certainly reduce human cancer risks.

Codon 12 and codon 13 mutations at the K-ras gene induce different soft tissue sarcoma types in nude mice

K-ras codon 12 mutation is more oncogenic in in vitro and in vivo experimental systems than K-ras codon 13 mutation. Moreover, human colorectal tumors bearing a codon 12 mutation are more aggressive, invasive, and metastatic than the same tumor types carrying a codon 13 mutation. However, despite the association between specific sarcoma types and codon 12 or codon 13 mutations, the relationship between the position of the mutated codon at ras genes and tumor aggressiveness has not been studied in this tumor type. Here, we used a nude mice model to evaluate the tumorogenic capacity of stable transfectants of NIH3T3 fibroblasts, expressing K-ras mutated at codon 12 (K12) or 13 (K13), and morphologically, functionally, and molecularly compared these tumors. We found histopathological differences between them, K12-derived tumors showing fibrosarcoma-like features, whereas K13-derived tumors resembled malignant fibrous histiocytomas. Moreover, K12 tumors showed shorter latency of appearance, lower apoptotic and mitotic rates, and higher expression of markers for sarcoma aggressiveness (Ki67, p53 and c-myc) than K13 tumors. They also showed differences in the expression or activation of Ras, Ras downstream pathways [c-Jun N-terminal kinase (JNK), MAPK and AKT], and apoptotic [AKT, Bcl-2, Focal adhesion kinase (FAK)] and mitotic (cyclin B1) regulators, which could explain their functional differences. Most remarkably, the significantly diminished apoptotic rate observed in K12-derived tumors was associated with enhanced antiapoptotic signaling through the AKT pathway. These morphological, functional, and molecular differences demonstrate that codon 12 and codon 13 mutations in the K-ras oncogene can induce two different soft tissue sarcoma types in our in vivo model.

Novel and nondetected human signaling protein polymorphisms

The frequency of single nucleotide polymorphisms (SNPs) in downstream signaling proteins was determined by combination heteroduplex HPLC and double-stranded sequencing of genomic DNA from 96-144 congestive heart failure (CHF) patients. Analysis of 56 coding exons in 9 signaling genes revealed 17 novel and 8 previously reported synonymous (no change in amino acid) SNPs, as well as one novel nonsynonymous SNP in the Rad small G protein. Because this initial analysis failed to detect numerous SNPs reported in the NCBI and Celera databases, double-strand sequencing of relevant exons from 74-91 CHF patients was used to confirm the absence of 10 previously reported nonsynonymous SNPs. Our results show that synonymous SNPs are frequent in signaling protein genes, whereas nonsynonymous SNPs are rare, suggesting a high degree of evolutionary conservation among these downstream signaling molecules. Comparisons of our results to the NCBI and Celera databases indicates that 56% of their SNP entries are not detected in our cohort. Importantly, while 31% of database SNPs were verified, 69% of SNPs detected in our cohort are not included in these databases. These findings indicate that caution may be warranted in relying exclusively on SNP databases as catalogs for polymorphic signaling protein genes.

Mutational spectral analysis at the HPRT locus in healthy children

There is growing evidence linking somatic mutational events during fetal development and childhood to an increasing number of multifactorial human diseases. Despite this, little is known about the relationship between endogenous and environmentally induced exogenous mutations during human development. Here we describe a comparative spectral analysis of somatic mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) reporter gene locus in healthy children. We observed an age-specific decrease in the proportion of large alterations and a corresponding increase in the proportion of small alterations with increasing age following birth (P<0.001). The age specific decrease in the proportion of large alterations (67-30%) was mainly due to a decrease in the proportion of aberrant variable (V), diversity (D) and joining (J) (V(D)J) recombinase mediated HPRT deletions (P<0.001). The increase in the proportion of small alterations with age (28-64%) was associated with an increase in transversions from 8% in children at the late stages of fetal development to 31% in children 12-16 years old (P=0.003). Transitions decreased with age, especially at CpG dinucleotides (P=0.010), as transversions increased (P=0.009). These patterns of mutations provide insight into important spontaneous, genotoxic, and site-specific recombinational somatic mutational events associated with the age-specific development of human disease in children as well as adults.

Frequent mutations of the Trp53, Hras1 and beta-catenin (Catnb) genes in 1,3-butadiene-induced mammary adenocarcinomas in B6C3F1 mice

DNAs from 1,3-butadiene-induced mammary adenocarcinomas of B6C3F1 mice were examined for mutations in the Trp53 gene, the ras gene family and several components of the Wnt signaling pathway, including beta-catenin (Catnb), Apc and Axin. Trp53 mutations were detected in 41% (7 out of 17) of tumors. Each tumor with a Trp53 mutation also exhibited loss of the wild-type Trp53 allele, supporting the importance of Trp53 inactivation during development of these tumors. Analyses of the Hras1, Kras2 and Nras proto-oncogenes revealed Hras1 mutations in 53% (9 out of 17) of tumors. Seven of these mutations were a G-->C transversion in Hras1 codon 13, consistent with a 1,3-butadiene-specific Kras2 mutation previously reported in several other tumor types. Mutation screens in Catnb exon 2, the Apc mutation cluster region and the Catnb-binding domain of the Axin gene identified Catnb missense mutations in 3 out of 17 (18%) tumors. In total, mutations of the Trp53, Hras1 and/or Catnb genes were identified in 15 out of 17 1,3-butadiene-induced mammary adenocarcinomas. These results indicate that multiple genetic pathways are disrupted in chemically induced mammary tumors, and that studies in mouse models may help to understand the etiology of human breast cancers.

Mouse models for generating P53 gene mutation spectra

The p53 tumor suppressor gene lends itself to mutation spectra analysis, because the frequency of point mutations in human tumors is high, the locations of inactivating tumor mutations are numerous and dispersed, and all possible base substitutions are observed in human cancer. P53 tumor mutations induced experimentally in mice exposed to carcinogens have been described, but have not yet contributed significantly to our understanding of mutagenic mechanisms or of the origins of mutations in human cancers. Recently, gene-targeting technology has allowed development of a new mouse model, which explores experimentally the endogenous and environmental factors that may contribute to neoplastic disease in humans.

Mutational spectrum of 1,3-butadiene and metabolites 1,2-epoxybutene and 1,2,3,4-diepoxybutane to assess mutagenic mechanisms

1,3-Butadiene (BD) is a multisite carcinogen and is mutagenic in multiple tissues of B6C3F1 mice. BD is bioactivated to at least three directly mutagenic metabolites: 1,2-epoxybutene (EB), 1,2-epoxy-3,4-butanediol (EBD), and 1,2,3,4-diepoxybutane (DEB). However, the contribution of these individual metabolites to the carcinogenicity and in vivo mutatidnal spectrum of BD is uncertain. To assess the role of two BD metabolites EB and DEB in the in vivo mutagenicity of the parent compound BD, we examined the in vitro mutational spectra of EB and DEB in human and rodent cells. We also examined the in vivo mutagenicity and mutational spectrum of inhaled EB in the lung. In the bone marrow and spleen of B6C3F1 laci transgenic mice, BD-induced an increased frequency of the identical class of point mutations at A:T base pairs: AT-->GC transitions and AT-->TA transversions. BD exposure also induced an increased frequency of GC-->AT transitions in the spleen that was not observed in bone marrow, demonstrating tissue-specific differences in mutation spectrum. Exposure of Rat2 laci transgenic cells and human TK6 lymphoblasts to EB-induced an increased frequency of AT-->TA transversions. DEB exposure induced an increased frequency of AT-->TA transversions and partial deletions at hprt in human cells. In Rat laci transgenic cells, DEB was not mutagenic at laci but induced an increased frequency of micronuclei. In contrast to inhaled BD, inhaled DEB and EB were not mutagenic in the bone marrow or spleen. However, EB was mutagenic in the lungs. In the lung of mice, EB-induced specific increases in GC-->AT transitions, AT-->TA transversions, and deletion events. AT-->TA transversions are the most consistent mutation observed across biological systems following in vivo exposure to BD or in vitro exposures to EB and DEB. Although, BD exposure in mice induces chromosomal alterations and single base substitutions, the specific BD metabolite that induces the genetic events leading to tumors is uncertain. At present, it appears that only DEB can effectively induce this range of mutagenic events at levels of this metabolite that occur in the blood of mice exposed to BD. Detailed investigations to identify relevant biomarkers of BD exposure and response, particularly DNA adducts or lesions, that can be biologically linked to the range of genotoxic events known to occur in mice exposed to BD are needed.

New models for assessing carcinogenesis: an ongoing process

Traditionally, the use of rodent models in assessing the carcinogenic potential of chemicals has been expensive and lengthy, and the relevance of the carcinogenic effect to humans is often not fully understood. Today, however, with the rapid advances in molecular biology, genetically altered mice containing genes relevant to humans (e.g. oncogenes, tumor suppressor genes) and reporter genes (e.g. lacI) provide powerful tools for examining specific chemical-gene interactions thereby allowing a better understanding of the mechanisms of carcinogenesis in a shorter period of time. This paper will cover an overview of ongoing validation efforts, followed by examples of studies using several genetically engineered models including the p53def mouse model and the Big Blue transgenic mouse model. Specifically, examples where transgenic models were integrated into the testing program based on specific hypotheses dealing with genetic alterations in cancer genes and reporter genes will be discussed. The examples will highlight possible ways genetically altered mice may be integrated into a comprehensive research and testing strategy and thereby provide an improved estimation of human health risks.