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)

Association between Citrus Consumption and Melanoma Risk in the NIH-AARP Diet and Health Study

It has been hypothesized that consumption of citrus, a group of foods particularly rich in a class of photoactive compounds known as furocoumarins, may increase the risk of malignant melanoma. However, this hypothesis has not been rigorously studied in a general sample of US men and women. This study examined the relationship between citrus intake and melanoma risk in subjects of the NIH-AARP Diet and Health Study. Among 388,467 adults, 3,894 melanoma cases were identified during a median follow-up of 15.5 years. After adjustment for relevant potential confounders, total citrus consumption was not significantly associated with melanoma risk in this cohort. Among those with higher estimated exposure to ultraviolet radiation, and among those aged 60+ years at baseline, there were significant trends toward increased melanoma risk associated with whole citrus fruit consumption (P trends = 0.01 and 0.02, respectively), but the hazard ratios of the top consumers (2+ cups per week) vs. nonconsumers were nonsignificant. Further research is needed to explore associations of citrus with melanoma risk among older adults and those with high sun exposure.

Frequent p53 and H-ras Mutations in Benzene- and Ethylene Oxide-Induced Mammary Gland Carcinomas from B6C3F1 Mice

Benzene and ethylene oxide are multisite carcinogens in rodents and classified as human carcinogens by the National Toxicology Program. In 2-year mouse studies, both chemicals induced mammary carcinomas. We examined spontaneous, benzene-, and ethylene oxide-induced mouse mammary carcinomas for p53 protein expression, using immunohistochemistry, and p53 (exons 5–8) and H -ras (codon 61) mutations using cycle sequencing techniques. p53 protein expression was detected in 42% (8/19) of spontaneous, 43% (6/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. However, semiquantitative evaluation of p53 protein expression revealed that benzene- and ethylene oxide-induced carcinomas exhibited expression levels five- to six-fold higher than spontaneous carcinomas. p53 mutations were found in 58% (7/12) of spontaneous, 57% (8/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. H -ras mutations were identified in 26% (5/19) of spontaneous, 50% (7/14) of benzene-, and 33% (4/12) of ethylene oxide-induced carcinomas. When H- ras mutations were present, concurrent p53 mutations were identified in 40% (2/5) of spontaneous, 71% (5/7) of benzene-, and 75% (3/4) of ethylene oxide-induced carcinomas. Our results demonstrate that p53 and H -ras mutations are relatively common in control and chemically induced mouse mammary carcinomas although both chemicals can alter the mutational spectra and more commonly induce concurrent mutations.

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.

Induction of cytochrome P450 1 genes and stress response genes in developing zebrafish exposed to ultraviolet radiation

Ultraviolet (UV) radiation damages cell molecules, and has been suggested to up-regulate mammalian cytochrome P4501 (CYP1) genes through an aryl hydrocarbon receptor (AHR) mediated mechanism. In this study, embryos and larvae of zebrafish (Danio rerio) were exposed to UV to determine the effects on expression of CYP1 and stress response genes in vivo in these fish. Zebrafish embryos were exposed for varying times to UV on two consecutive days, with exposure beginning at 24 and 48h post-fertilization (hpf). Embryos exposed for 2, 4 or 6h twice over 2 days to UVB (0.62 W/m(2); 8.9-26.7 kJ/m(2)) plus UVA (2.05 W/m(2); 29.5-144.6 kJ/m(2)) had moderately (2.4+/-0.8-fold) but significantly up-regulated levels of CYP1A. UVA alone had no effect on CYP1A expression. Proliferating cellular nuclear antigen (PCNA) and Cu-Zn superoxide dismutase (SOD1) transcript levels were induced (2.1+/-0.2 and 2.3+/-0.5-fold, respectively) in embryos exposed to two 6-h pulses of 0.62 W/m(2) UVB (26.8 kJ/m(2)). CYP1A was induced also in embryos exposed to higher intensity UVB (0.93 W/m(2)) for two 3-h or two 4-h pulses (20.1 or 26.8 kJ/m(2)). CYP1B1, SOD1 and PCNA expression was induced by the two 3-h pulses of the higher intensity UVB, but not after two 4-h pulses of the higher intensity UVB, possibly due to impaired condition of surviving embryos, reflected in a mortality of 34% at that UVB dose. A single 8-h long exposure of zebrafish larvae (8dpf) to UVB at 0.93 W/m(2) (26.8 kJ/m(2)) significantly induced CYP1A and CYP1B1 expression, but other CYP1 genes (CYP1C1, CYP1C2 and CYP1D1) showed no significant increase. The results show that UVB can induce expression of CYP1 genes as well stress response genes in developing zebrafish, and that UVB intensity and duration influence the responses.

A review of the molecular mechanisms of chemically induced neoplasia in rat and mouse models in National Toxicology Program bioassays and their relevance to human cancer

Tumor response in the B6C3F1 mouse, F344 rat, and other animal models following exposure to various compounds provides evidence that people exposed to these or similar compounds may be at risk for developing cancer. Although tumors in rodents and humans are often morphologically similar, underlying mechanisms of tumorigenesis are often unknown and may be different between the species. Therefore, the relevance of an animal tumor response to human health would be better determined if the molecular pathogenesis were understood. The underlying molecular mechanisms leading to carcinogenesis are complex and involve multiple genetic and epigenetic events and other factors. To address the molecular pathogenesis of environmental carcinogens, the authors examine rodent tumors (e.g., lung, colon, mammary gland, skin, brain, mesothelioma) for alterations in cancer genes and epigenetic events that are associated with human cancer. National Toxicology Program (NTP) studies have identified several genetic alterations in chemically induced rodent neoplasms that are important in human cancer. Identification of such alterations in rodent models of chemical carcinogenesis caused by exposure to environmental contaminants, occupational chemicals, and other compounds lends further support that they are of potential human health risk. These studies also emphasize the importance of molecular evaluation of chemically induced rodent tumors for providing greater public health significance for NTP evaluated compounds.

Optimization of the comet assay for the sensitive detection of PUVA-induced DNA interstrand cross-links

Psoralen plus ultraviolet A (PUVA), commonly used for the treatment of hyperproliferative skin disorders, has been found to be associated with an increased risk of squamous cell cancer. Interstrand cross-link (ICL) formation by PUVA treatment is considered the major factor contributing to the carcinogenesis. However, it remains unclear how PUVA causes, or promotes cancers, in humans. As an initial step in understanding the mechanisms of mutagenesis and carcinogenesis of PUVA photochemotherapy, we have optimized and subsequently utilized a modified alkaline comet assay involving a post-lysis gamma-irradiation at 9 Gy to sensitively measure the formation and repair of PUVA-induced ICLs in the immortalized human keratinocyte cell line HaCaT. A clear dose-dependent response of HaCaT cells to PUVA exposure was observed with a combination of a fixed UVA dose at 0.05 J/cm(2) and a dose of 8-methoxypsoralen ranging from 10 to 100 microM. Results also indicated that the ICL repair was concentration dependent. We have also demonstrated that PUVA-induced monoadduct formation, at an estimated ratio of 3:1 to ICLs in the present experimental conditions, does not interfere with the detection of the ICLs in the modified alkaline comet assay. Furthermore, comparison of the amount of ICL formation between the single-dose UVA treatment and a split-dose protocol was performed. The split-dose protocol was believed to generate more ICLs than the single-dose treatment, thus more effective in PUVA photochemotherapy. Our results demonstrate that comparable amounts of ICLs were formed in HaCaT cells for each dose of UVA, using either the split-dose or single-dose protocols.

Gene Expression Studies Demonstrate that the K-ras/Erk MAP Kinase Signal Transduction Pathway and Other Novel Pathways Contribute to the Pathogenesis of Cumene-induced Lung Tumors

National Toxicology Program (NTP) inhalation studies demonstrated that cumene significantly increased the incidence of alveolar/bronchiolar adenomas and carcinomas in B6C3F1 mice. Cumene or isopropylbenzene is a component of crude oil used primarily in the production of phenol and acetone. The authors performed global gene expression analysis to distinguish patterns of gene regulation between cumene-induced tumors and normal lung tissue and to look for patterns based on the presence or absence of K- ras and p53 mutations in the tumors. Principal component analysis segregated the carcinomas into groups with and without K- ras mutations, but failed to separate the tumors based on p53 mutation status. Expression of genes associated with the Erk MAP kinase signaling pathway was significantly altered in carcinomas with K- ras mutations compared to tumors without K- ras mutations or normal lung. Gene expression analysis also suggested that cumene-induced carcinomas with K- ras mutations have greater malignant potential than those without mutations. In addition, significance analysis of function and expression (SAFE) demonstrated expression changes of genes regulated by histone modification in carcinomas with K- ras mutations. The gene expression analysis suggested the formation of alveolar/bronchiolar carcinomas in cumene-exposed mice typically involves mutation of K- ras, which results in increased Erk MAP kinase signaling and modification of histones.

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.