Benzo[a]pyrene-induced murine skin tumors exhibit frequent and characteristic G to T mutations in the p53 gene

Cellular signaling crosstalk between Wnt signaling and gap junctions inbenzo[a]pyrene toxicity

Gap junctional intercellular communication (GJIC) is considered a key biological mechanism to maintain homeostasis in cell differentiation and growth. In addition, as another major signaling pathway associated with cell proliferation and differentiation, Wnt/β-catenin signaling appears to trigger several cellular responses against injury. The purpose of the present study was to investigate the effects of a known toxic agent, benzo[a]pyrene (BaP), on the regulation and interaction between GJIC and Wnt/β-catenin signaling. BaP treatment resulted in GJIC inhibition and decreases the major GJIC protein connexin 43 (Cx43) in WB-F344 rat liver epithelial cells. We also found BaP-mediated downregulation of Wnt/β-catenin signaling related to the PI3K-Akt pathway. To identify the relationship between GJIC and Wnt/β-catenin signaling, we treated WB-F344 cells with the Wnt agonist CHIR99021 and found that it inhibited GJIC while causing a significant reduction in Cx43 expression at both the mRNA and protein levels, through the repression of promoter activity. This Wnt agonist-mediated GJIC inhibition was confirmed using a small interfering RNA directed against the Wnt antagonist Dact2, indicating that Wnt/β-catenin signaling negatively regulates GJIC. Despite the inverse correlation between Wnt/β-catenin signaling and Cx43 promoter activation as indicated by downregulation of β-catenin nuclear translocation and upregulation of Cx43 promoter activation involving HNF3β, BaP treatment decreased the Cx43 protein expression, which was associated with protein degradation, possibly through protein kinase C activation. In conclusion, our results revealed the mechanism of BaP-induced inhibition of GJIC and Wnt/β-catenin signaling. More importantly, linking Wnt/β-catenin signaling to Cx protein expression will have profound implications in understanding the relationships among different major signaling pathways associated with cell proliferation and differentiation in toxicity.

Dietary phytochemicals as the potential protectors against carcinogenesis and their role in cancer chemoprevention

Health-threatening consequences of carcinogen exposure are mediated via occurrence of electrophiles or reactive oxygen species. As a result, the accumulation of biomolecular damage leads to the cancer initiation, promotion or progression. Accordingly, there is an association between lifestyle factors including inappropriate diet or carcinogen formation during food processing, mainstream, second or third-hand tobacco smoke and other environmental or occupational carcinogens and malignant transformation. Nevertheless, increasing evidence supports the protective effects of naturally occurring phytochemicals against carcinogen exposure as well as carcinogenesis in general. Isolated phytochemicals or their mixtures present in the whole plant food demonstrate efficacy against malignancy induced by carcinogens widely spread in our environment. Phytochemicals also minimize the generation of carcinogenic substances during the processing of meat and meat products. Based on numerous data, selected phytochemicals or plant foods should be highly recommended to become a stable and regular part of the diet as the protectors against carcinogenesis.

Cigarette smoke enhances oncogene addiction to c-MET and desensitizes EGFR-expressing non-small cell lung cancer to EGFR TKIs

Cigarette smoking is one of the leading risks for lung cancer and is associated with the insensitivity of non‐small cell lung cancer (NSCLC) to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). However, it remains undetermined whether and how cigarette smoke affects the therapeutic efficacy of EGFR TKIs. In this study, our data showed that chronic exposure to cigarette smoke extract (CSE) or tobacco smoke‐derived carcinogen benzo[α]pyrene, B[α]P, but not nicotine‐derived nitrosamine ketone (NNK), reduced the sensitivity of wild‐type EGFR‐expressing NSCLC cells to EGFR TKIs. Treatment with TKIs almost abolished EGFR tyrosine kinase activity but did not show an inhibitory effect on downstream Akt and ERK pathways in B[α]P‐treated NSCLC cells. CSE and B[α]P transcriptionally upregulate c‐MET and activate its downstream Akt pathway, which is not inhibited by EGFR TKIs. Silencing of c‐MET reduces B[α]P‐induced Akt activation. The CSE‐treated NSCLC cells are sensitive to the c‐MET inhibitor crizotinib. These findings suggest that cigarette smoke augments oncogene addiction to c‐MET in NSCLC cells and that MET inhibitors may show clinical benefits for lung cancer patients with a smoking history.

MUC16 overexpression induced by gene mutations promotes lung cancer cell growth and invasion

Air pollution is one of the leading causes of lung cancer. Air pollution-related lung cancer is a deteriorating public health problem, particularly in developing countries. The MUC16 gene is one of the most frequently mutated genes in air pollution-related lung cancer. In the present study, MUC16 mRNA expression was increased in ∼50% of air pollution-related lung cancer samples obtained from patients residing in air-polluted regions (Xuanwei and Fuyuan, Yunnan, China), and MUC16 mRNA levels were correlated with the degree of air pollution. Furthermore, sequencing of the captured MUC16 gene identified 561 mutation sites within the MUC16 gene in the air pollution-related lung cancer tissues. Interestingly, some mutations at specific sites and one region were associated with MUC16 mRNA up-regulation. Therefore, we further investigated the impacts of gene mutation on MUC16 expressions and cell behaviors in cultured cells by inducing certain mutations within the MUC16 gene using CRISPER/Cas9 genome editing technology. Certain mutations within the MUC16 gene induced MUC16 overexpression at both the mRNA and the protein level in the cultured cells. Additionally, MUC16 overexpression induced by gene mutations had functional effects on the behavior of lung cancer cells, including increasing their resistance to cisplatin, promoting their growth, and enhancing their migration and invasion capabilities. Based on the data, we suggest that MUC16 mutations potentially associated with air pollution may participate in the development and progression of air pollution-related lung cancer. In addition to ovarian cancer, MUC16 may be a candidate biomarker for lung cancer.

Mechanism of error-free replication across benzo[a]pyrene stereoisomers by Rev1 DNA polymerase

Benzo[a]pyrene (BP) is a carcinogen in cigarette smoke which, after metabolic activation, can react with the exocyclic N² amino group of guanine to generate four stereoisomeric BP-N²-dG adducts. Rev1 is unique among translesion synthesis DNA polymerases in employing a protein-template-directed mechanism of DNA synthesis opposite undamaged and damaged guanine. Here we report high-resolution structures of yeast Rev1 with three BP-N²-dG adducts, namely the 10S (+)-trans-BP-N²-dG, 10R (+)-cis-BP-N²-dG, and 10S ( − )-cis-BP-N²-dG. Surprisingly, in all three structures, the bulky and hydrophobic BP pyrenyl residue is entirely solvent-exposed in the major groove of the DNA. This is very different from the adduct alignments hitherto observed in free or protein-bound DNA. All complexes are well poised for dCTP insertion. Our structures provide a view of cis-BP-N²-dG adducts in a DNA polymerase active site, and offer a basis for understanding error-free replication of the BP-derived stereoisomeric guanine adducts.

Benzo[a]pyrene (BP) is a carcinogen in cigarette smoke that upon metabolic activation reacts with guanine. Here, the authors present the structures of the translesion DNA synthesis polymerase Rev1 in complex with three of the four possible stereoisomeric BP-N² -dG adducts, which gives insights how Rev1 achieves error-free replication.

Macrophage migration inhibitory factor protects from nonmelanoma epidermal tumors by regulating the number of antigen-presenting cells in skin

The response of the skin to harmful environmental agents is shaped decisively by the status of the immune system. Keratinocytes constitutively express and secrete the chemokine-like mediator, macrophage migration inhibitory factor (MIF), more strongly than dermal fibroblasts, thereby creating a MIF gradient in skin. By using global and epidermis-restricted Mif-knockout (Mif^-/- and K14-Cre^+/tg; Mif^fl/fl) mice, we found that MIF both recruits and maintains antigen-presenting cells in the dermis/epidermis. The reduced presence of antigen-presenting cells in the absence of MIF was associated with accelerated and increased formation of nonmelanoma skin tumors during chemical carcinogenesis. Our results demonstrate that MIF is essential for maintaining innate immunity in skin. Loss of keratinocyte-derived MIF leads to a loss of control of epithelial skin tumor formation in chemical skin carcinogenesis, which highlights an unexpected tumor-suppressive activity of MIF in murine skin.-Brocks, T., Fedorchenko, O., Schliermann, N., Stein, A., Moll, U. M., Seegobin, S., Dewor, M., Hallek, M., Marquardt, Y., Fietkau, K., Heise, R., Huth, S., Pfister, H., Bernhagen, J., Bucala, R., Baron, J. M., Fingerle-Rowson, G. Macrophage migration inhibitory factor protects from nonmelanoma epidermal tumors by regulating the number of antigen-presenting cells in skin.

Exposure of Human Lung Cells to Tobacco Smoke Condensate Inhibits the Nucleotide Excision Repair Pathway

Exposure to tobacco smoke is the number one risk factor for lung cancer. Although the DNA damaging properties of tobacco smoke have been well documented, relatively few studies have examined its effect on DNA repair pathways. This is especially true for the nucleotide excision repair (NER) pathway which recognizes and removes many structurally diverse DNA lesions, including those introduced by chemical carcinogens present in tobacco smoke. The aim of the present study was to investigate the effect of tobacco smoke on NER in human lung cells. We studied the effect of cigarette smoke condensate (CSC), a surrogate for tobacco smoke, on the NER pathway in two different human lung cell lines; IMR-90 lung fibroblasts and BEAS-2B bronchial epithelial cells. To measure NER, we employed a slot-blot assay to quantify the introduction and removal of UV light-induced 6–4 photoproducts and cyclobutane pyrimidine dimers. We find a dose-dependent inhibition of 6–4 photoproduct repair in both cell lines treated with CSC. Additionally, the impact of CSC on the abundance of various NER proteins and their respective RNAs was investigated. The abundance of XPC protein, which is required for functional NER, is significantly reduced by treatment with CSC while the abundance of XPA protein, also required for NER, is unaffected. Both XPC and XPA RNA levels are modestly reduced by CSC treatment. Finally, treatment of cells with MG-132 abrogates the reduction in the abundance of XPC protein produced by treatment with CSC, suggesting that CSC enhances proteasome-dependent turnover of the protein that is mediated by ubiquitination. Together, these findings indicate that tobacco smoke can inhibit the same DNA repair pathway that is also essential for the removal of some of the carcinogenic DNA damage introduced by smoke itself, increasing the DNA damage burden of cells exposed to tobacco smoke.

Skin Carcinogenesis Studies Using Mouse Models with Altered Polyamines

Nonmelanoma skin cancer (NMSC) is a major health concern worldwide. With increasing numbers in high-risk groups such as organ transplant recipients and patients taking photosensitizing medications, the incidence of NMSC continues to rise. Mouse models of NMSC allow us to better understand the molecular signaling cascades involved in skin tumor development in order to identify novel therapeutic strategies. Here we review the models designed to determine the role of the polyamines in NMSC development and maintenance. Elevated polyamines are absolutely required for tumor growth, and dysregulation of their biosynthetic and catabolic enzymes has been observed in NMSC. Studies using mice with genetic alterations in epidermal polyamines suggest that they play key roles in tumor promotion and epithelial cell survival pathways, and recent clinical trials indicate that pharmacological inhibitors of polyamine metabolism show promise in individuals at high risk for NMSC.

Characterization of Somatic Mutations in Air Pollution-Related Lung Cancer

Air pollution has been classified as Group 1 carcinogenic to humans, but the underlying tumorigenesis remains unclear. In Xuanwei City of Yunnan Province, the lung cancer incidence is among the highest in China attributed to severe air pollution generated by combustion of smoky coal, providing a unique opportunity to dissect lung carcinogenesis of air pollution. Here we analyzed the somatic mutations of 164 non-small cell lung cancers (NSCLCs) from Xuanwei and control regions (CR) where smoky coal was not used. Whole genome sequencing revealed a mean of 289 somatic exonic mutations per tumor and the frequent C:G → A:T nucleotide substitutions in Xuanwei NSCLCs. Exome sequencing of 2010 genes showed that Xuanwei and CR NSCLCs had a mean of 68 and 22 mutated genes per tumor, respectively (p < 0.0001). We found 167 genes (including TP53, RYR2, KRAS, CACNA1E) which had significantly higher mutation frequencies in Xuanwei than CR patients, and mutations in most genes in Xuanwei NSCLCs differed from those in CR cases. The mutation rates of 70 genes (e.g., RYR2, MYH3, GPR144, CACNA1E) were associated with patients' lifetime benzo(a)pyrene exposure. This study uncovers the mutation spectrum of air pollution-related lung cancers, and provides evidence for pollution exposure-genomic mutation relationship at a large scale.

TP53 mutations induced by BPDE in Xpa-WT and Xpa-Null human TP53 knock-in (Hupki) mouse embryo fibroblasts

Somatic mutations in the tumour suppressor gene TP53 occur in more than 50% of human tumours; in some instances exposure to environmental carcinogens can be linked to characteristic mutational signatures. The Hupki (human TP53 knock-in) mouse embryo fibroblast (HUF) immortalization assay (HIMA) is a useful model for studying the impact of environmental carcinogens on TP53 mutagenesis. In an effort to increase the frequency of TP53-mutated clones achievable in the HIMA, we generated nucleotide excision repair (NER)-deficient HUFs by crossing the Hupki mouse with an Xpa-knockout (Xpa-Null) mouse. We hypothesized that carcinogen-induced DNA adducts would persist in the TP53 sequence of Xpa-Null HUFs leading to an increased propensity for mismatched base pairing and mutation during replication of adducted DNA. We found that Xpa-Null Hupki mice, and HUFs derived from them, were more sensitive to the environmental carcinogen benzo[a]pyrene (BaP) than their wild-type (Xpa-WT) counterparts. Following treatment with the reactive metabolite of BaP, benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), Xpa-WT and Xpa-Null HUF cultures were subjected to the HIMA. A significant increase in TP53 mutations on the transcribed strand was detected in Xpa-Null HUFs compared to Xpa-WT HUFs, but the TP53-mutant frequency overall was not significantly different between the two genotypes. BPDE induced mutations primarily at G:C base pairs, with approximately half occurring at CpG sites, and the predominant mutation type was G:C>T:A in both Xpa-WT and Xpa-Null cells. Further, several of the TP53 mutation hotspots identified in smokers' lung cancer were mutated by BPDE in HUFs (codons 157, 158, 245, 248, 249, 273). Therefore, the pattern and spectrum of BPDE-induced TP53 mutations in the HIMA are consistent with TP53 mutations detected in lung tumours of smokers. While Xpa-Null HUFs exhibited increased sensitivity to BPDE-induced damage on the transcribed strand, NER-deficiency did not enhance TP53 mutagenesis resulting from damage on the non-transcribed strand in this model.

Non-Structural protein 1 (NS1) gene of Canine Parvovirus-2 regresses chemically induced skin tumors in Wistar rats

The Non-Structural protein 1 of Canine Parvovirus-2 (CPV2.NS1) plays a major role in viral cytotoxicity and pathogenicity. CPV2.NS1 has been proven to cause apoptosis in HeLa cells in vitro in our laboratory. Here we report that CPV2.NS1 has no toxic side effects on healthy cells but regresses skin tumors in Wistar rats. Histopathological examination of tumor tissue from CPV2.NS1 treated group revealed infiltration of mononuclear and polymorphonuclear cells with increased extra cellular matrix, indicating signs of regression. Tumor regression was also evidenced by significant decrease in mitotic index, AgNOR count and PCNA index, and increase in TUNEL positive apoptotic cells in CPV2.NS1 treated group. Further, CPV2.NS1 induced anti-tumor immune response through significant increase in CD8(+) and NK cell population in CPV2.NS1 treated group. These findings suggest that CPV2.NS1 can be a possible therapeutic candidate as an alternative to chemotherapy for the treatment of cancer.

Enhanced aggressiveness of benzopyrene-induced squamous carcinomas in transgenic mice overexpressing the proprotein convertase PACE4 (PCSK6)

PACE4 (PCSK6) is a proprotein convertase (PC) capable of processing numerous substrates involved in tumor growth, invasion, and metastasis. Because of the human relevancy of the tobacco-associated carcinogen benzo[a]pyrene (B(a)P) we investigated whether transgenic mice in which this PC is targeted to the epidermis (K5-PACE4) may be more susceptible to B(a)P complete carcinogenesis than wild type (WT) mice. In an in vitro experiment, using cell lines derived from skin tumors obtained after B(a)P treatment, we observed that PACE4 overexpression and activity accounts for an increased proliferation rate, exaggerated sensitivity to the PC inhibitor CMK, and interference with IGF-1R autophosphorylation. Squamous cell carcinomas, obtained from K5-PACE4 mice subjected to complete chemical carcinogenesis, were characterized by a 50% increase in cell proliferation, when compared with similar tumors from WT mice. In addition, tumors from K5-PACE4 mice showed deeper invasion into the underlying dermis. Thus, mice overexpressing PACE4 exhibited tumors of increased growth rate and invasive potential when exposed to the human carcinogen B(a)P, further supporting the significance of PCs in tumor growth and progression.

Mechanisms of oral carcinogenesis induced by dibenzo[a,l]pyrene: an environmental pollutant and a tobacco smoke constituent

We previously reported that dibenzo[a,l]pyrene (DB[a,l]P), the most potent known environmental carcinogen among polycyclic aromatic hydrocarbons (PAH) congeners, is carcinogenic in the oral tissues of mice. We have now developed a new mouse model which employs the oral application of the fjord region diol epoxide, (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DB[a,l]PDE), a metabolite of the tobacco smoke constituent DB[a,l]P, and we show its specific induction of oral squamous cell carcinoma (OSCC) in both tongue and other oral tissues. Groups of B6C3F1 mice (20/group) received 6 or 3 nmol of (±)-anti-DB[a,l]PDE administered into the oral cavity; 3 times per week for 38 weeks. Additional groups received the vehicle alone or were left untreated. Mice were sacrificed 42 weeks after the first carcinogen administration. The high dose induced 74 and 100% OSCC in the tongue and other oral tissues, respectively; the corresponding values at the lower dose were 45 and 89%. Using immunohistochemistry, we showed that DB[a,l]PDE resulted in overexpression of p53 and COX-2 proteins in malignant tissues when compared to normal oral tissues and tongues. Consistent with the carcinogenicity, we demonstrated powerful mutagenicity in cII gene in B6C3F1 (Big Blue) mouse tongue. The mutational profile in lacI reporter gene is similar to those detected in human head and neck cancer, and p53 mutations were observed in mouse oral tumor tissues. Taken together, we conclude that the formation of diol epoxides plays a major role among the mechanisms by which DB[a,l]P exerts its oral mutagenicity and tumorigenicity.

Non-melanoma skin cancer in mouse and man

As a frontier organ, skin is exposed to different environmental and/or occupational chemicals which cause cutaneous cancers in experimental animals. In mice, 7,12-dimethylbenz[a]anthrancene (DMBA) and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) are frequently used as skin model tumor initiator and promoter, respectively. The sequential administration of DMBA and TPA leads to the appearance of a large number of benign papillomas, of which some convert later into invasive squamous cell carcinomas (SCC). At the molecular level, initiation of carcinogenesis in mouse skin consists in the mutational activation of the Ha-ras oncoprotein. HA-RAS mutations are rare in human SCC, but HA-RAS-mutated tumors appear in melanoma patients treated with B-raf inhibitors, indicating that initiated, HA-RAS-mutated stem cells also reside in human skin. Similarly, UV-induced human SCC show footprint mutations in the tumor suppressor gene TP53 which are also observed in UV-induced mouse SCC. Strong species differences exist with respect to phorbol ester-mediated tumor promotion. While certain mouse strains are very susceptible, other rodent species are much less sensitive. Likewise, humans appear to be much more resistant to phorbol ester-mediated skin toxicity. Papilloma formation as a result of a chemical insult is uncommon in men, questioning the relevance of this preneoplastic lesion for humans. However, skin tumorigenesis in the experimental situation and in humans appears to follow common molecular mechanisms, even though there are species differences in the morphological correlates to the preneoplastic state. Therefore, we recommend not simply labeling them as irrelevant for human risk assessment.

Evaluation of chemicals requiring metabolic activation in the EpiDerm™ 3D human reconstructed skin micronucleus (RSMN) assay

The in vitro human reconstructed skin micronucleus (RSMN) assay in EpiDerm™ is a promising new assay for evaluating genotoxicity of dermally applied chemicals. A global pre-validation project sponsored by the European Cosmetics Association (Cosmetics Europe - formerly known as COLIPA), and the European Center for Validation of Alternative Methods (ECVAM), is underway. Results to date demonstrate international inter-laboratory and inter-experimental reproducibility of the assay for chemicals that do not require metabolism [Aardema et al., Mutat. Res. 701 (2010) 123-131]. We have expanded these studies to investigate chemicals that do require metabolic activation: 4-nitroquinoline-N-oxide (4NQO), cyclophosphamide (CP), dimethylbenzanthracene (DMBA), dimethylnitrosamine (DMN), dibenzanthracene (DBA) and benzo(a)pyrene (BaP). In this study, the standard protocol of two applications over 48h was compared with an extended protocol involving three applications over 72h. Extending the treatment period to 72h changed the result significantly only for 4NQO, which was negative in the standard 48h dosing regimen, but positive with the 72h treatment. DMBA and CP were positive in the standard 48h assay (CP induced a more reproducible response with the 72h treatment) and BaP gave mixed results; DBA and DMN were negative in both the 48h and the 72h dosing regimens. While further work with chemicals that require metabolism is needed, it appears that the RMSN assay detects some chemicals that require metabolic activation (4 out of 6 chemicals were positive in one or both protocols). At this point in time, for general testing, the use of a longer treatment period in situations where the standard 48h treatment is negative or questionable is recommended.

Differentiation-induced skin cancer suppression by FOS, p53, and TACE/ADAM17

Squamous cell carcinomas (SCCs) are heterogeneous and aggressive skin tumors for which innovative, targeted therapies are needed. Here, we identify a p53/TACE pathway that is negatively regulated by FOS and show that the FOS/p53/TACE axis suppresses SCC by inducing differentiation. We found that epidermal Fos deletion in mouse tumor models or pharmacological FOS/AP-1 inhibition in human SCC cell lines induced p53 expression. Epidermal cell differentiation and skin tumor suppression were caused by a p53-dependent transcriptional activation of the metalloprotease TACE/ADAM17 (TNF-α-converting enzyme), a previously unknown p53 target gene that was required for NOTCH1 activation. Although half of cutaneous human SCCs display p53-inactivating mutations, restoring p53/TACE activity in mouse and human skin SCCs induced tumor cell differentiation independently of the p53 status. We propose FOS/AP-1 inhibition or p53/TACE reactivating strategies as differentiation-inducing therapies for SCCs.

Different sets of translesion synthesis DNA polymerases protect from genome instability induced by distinct food-derived genotoxins

DNA lesions, induced by genotoxic compounds, block the processive replication fork but can be bypassed by specialized translesion synthesis (TLS) DNA polymerases (Pols). TLS safeguards the completion of replication, albeit at the expense of nucleotide substitution mutations. We studied the in vivo role of individual TLS Pols in cellular responses to benzo[a]pyrene diolepoxide (BPDE), a polycyclic aromatic hydrocarbon, and 4-hydroxynonenal (4-HNE), a product of lipid peroxidation. To this aim, we used mouse embryonic fibroblasts with targeted disruptions in the TLS-associated Pols η, ι, κ, and Rev1 as well as in Rev3, the catalytic subunit of TLS Polζ. After exposure, cellular survival, replication fork progression, DNA damage responses (DDR), and the induction of micronuclei were investigated. The results demonstrate that Rev1, Rev3, and, to a lesser extent, Polη are involved in TLS and the prevention of DDR and of DNA breaks, in response to both agents. Conversely, Polκ and the N-terminal BRCT domain of Rev1 are specifically involved in TLS of BPDE-induced DNA damage. We furthermore describe a novel role of Polι in TLS of 4-HNE-induced DNA damage in vivo. We hypothesize that different sets of TLS polymerases act on structurally different genotoxic DNA lesions in vivo, thereby suppressing genomic instability associated with cancer. Our experimental approach may provide a significant contribution in delineating the molecular bases of the genotoxicity in vivo of different classes of DNA-damaging agents.

Benzo[a]pyrene, aflatoxine B₁ and acetaldehyde mutational patterns in TP53 gene using a functional assay: relevance to human cancer aetiology

Mutations in the TP53 gene are the most common alterations in human tumours. TP53 mutational patterns have sometimes been linked to carcinogen exposure. In hepatocellular carcinoma, a specific G>T transversion on codon 249 is classically described as a fingerprint of aflatoxin B₁ exposure. Likewise G>T transversions in codons 157 and 158 have been related to tobacco exposure in human lung cancers. However, controversies remain about the interpretation of TP53 mutational pattern in tumours as the fingerprint of genotoxin exposure. By using a functional assay, the Functional Analysis of Separated Alleles in Yeast (FASAY), the present study depicts the mutational pattern of TP53 in normal human fibroblasts after in vitro exposure to well-known carcinogens: benzo[a]pyrene, aflatoxin B₁ and acetaldehyde. These in vitro patterns of mutations were then compared to those found in human tumours by using the IARC database of TP53 mutations. The results show that the TP53 mutational patterns found in human tumours can be only partly ascribed to genotoxin exposure. A complex interplay between the functional impact of the mutations on p53 phenotype and the cancer natural history may affect these patterns. However, our results strongly support that genotoxins exposure plays a major role in the aetiology of the considered cancers.

Environmental exposure of the mouse germ line: DNA adducts in spermatozoa and formation of de novo mutations during spermatogenesis

BACKGROUND

Spermatozoal DNA damage is associated with poor sperm quality, disturbed embryonic development and early embryonic loss, and some genetic diseases originate from paternal de novo mutations. We previously reported poor repair of bulky DNA-lesions in rodent testicular cells.

METHODOLOGY/PRINCIPAL FINDINGS

We studied the fate of DNA lesions in the male germ line. B[a]PDE-N(2)-dG adducts were determined by liquid chromatography-tandem mass spectrometry, and de novo mutations were measured in the cII-transgene, in Big Blue mice exposed to benzo[a]pyrene (B[a]P; 3 x 50 mg/kg bw, i.p.). Spermatozoa were harvested at various time-points following exposure, to study the consequences of exposure during the different stages of spermatogenesis. B[a]PDE-N(2)-dG adducts induced by exposure of spermatocytes or later stages of spermatogenesis persisted at high levels in the resulting spermatozoa. Spermatozoa originating from exposed spermatogonia did not contain DNA adducts; however de novo mutations had been induced (p = 0.029), specifically GC-TA transversions, characteristic of B[a]P mutagenesis. Moreover, a specific spectrum of spontaneous mutations was consistently observed in spermatozoa.

CONCLUSIONS/SIGNIFICANCE

A temporal pattern of genotoxic consequences following exposure was identified, with an initial increase in DNA adduct levels in spermatozoa, believed to influence fertility, followed by induction of germ line de novo mutations with possible consequences for the offspring.

VILIP-1 expression in vivo results in decreased mouse skin keratinocyte proliferation and tumor development

VILIP-1, a member of the neuronal Ca²⁺ sensor protein family, is able to act as a tumor suppressor in carcinoma cells by inhibiting cell proliferation and migration. In order to study the role of VILIP-1 in skin carcinogenesis we generated transgenic mice overexpressing VILIP-1 in epidermis under the control of the bovine keratin K5 promoter (K5-VILIP-1). We studied the susceptibility of FVB wild type and VILIP-1 transgenic mice to chemically mediated carcinogenesis. After 30 weeks of treatment with a two-stage carcinogenesis protocol, all animals showed numerous skin tumors. Nevertheless, K5-VILIP-1 mice showed decreased squamous cell carcinoma (SCC) multiplicity of ∼49% (p<0.02) with respect to the corresponding SCC multiplicity observed in wild type (WT) mice. In addition, the relative percentage of low-grade cutaneous SCCs grade I (defined by the differentiation pattern according to the Broders grading scale) increased approximately 50% in the K5-VILIP1 mice when compared with SCCs in WT mice. Similar tendency was observed using a complete carcinogenesis protocol for skin carcinogenesis using benzo(a)pyrene (B(a)P). Further studies of tumors and primary epidermal keratinocyte cultures showed that matrix metalloproteinase 9 (MMP-9) levels and cell proliferation decreased in K5-VILIP-1 mice when compared with their wild counterparts. In addition tissue inhibitor of metalloproteinase 1 (TIMP-1) expression was higher in K5-VILIP-1 keratinocytes. These results show that VILIP-1 overexpression decreases the susceptibility to skin carcinogenesis in experimental mouse cancer models, thus supporting its role as a tumor suppressor gene.

Associations between polycyclic aromatic hydrocarbon-related exposures and p53 mutations in breast tumors

BACKGROUND

Previous studies have suggested that polycyclic aromatic hydrocarbons (PAHs) may be associated with breast cancer. However, the carcinogenicity of PAHs on the human breast remains unclear. Certain carcinogens may be associated with specific mutation patterns in the p53 tumor suppressor gene, thereby contributing information about disease etiology.

OBJECTIVES

We hypothesized that associations of PAH-related exposures with breast cancer would differ according to tumor p53 mutation status, effect, type, and number.

METHODS

We examined this possibility in a population-based case-control study using polytomous logistic regression. As previously reported, 151 p53 mutations among 859 tumors were identified using Surveyor nuclease and confirmed by sequencing.

RESULTS

We found that participants with p53 mutations were less likely to be exposed to PAHs (assessed by smoking status in 859 cases and 1,556 controls, grilled/smoked meat intake in 822 cases and 1,475 controls, and PAH-DNA adducts in peripheral mononuclear cells in 487 cases and 941 controls) than participants without p53 mutations. For example, active and passive smoking was associated with p53 mutation-negative [odds ratio (OR) = 1.55; 95% confidence interval (CI), 1.11-2.15] but not p53 mutation-positive (OR = 0.77; 95% CI, 0.43-1.38) cancer (ratio of the ORs = 0.50, p < 0.05). However, frameshift mutations, mutation number, G:C-->A:T transitions at CpG sites, and insertions/deletions were consistently elevated among exposed subjects.

CONCLUSIONS

These findings suggest that PAHs may be associated with specific breast tumor p53 mutation subgroups rather than with overall p53 mutations and may also be related to breast cancer through mechanisms other than p53 mutation.

Mutational spectra of human cancer

The purpose of this review is to summarize the evidence that can be used to reconstruct the etiology of human cancers from mutations found in tumors. Mutational spectra of the tumor suppressor gene p53 (TP53) are tumor specific. In several cases, these mutational spectra can be linked to exogenous carcinogens, most notably for sunlight-associated skin cancers, tobacco-associated lung cancers, and aristolochic acid-related urothelial tumors. In the TP53 gene, methylated CpG dinucleotides are sequences selectively targeted by endogenous and exogenous mutagenic processes. Recent high-throughput sequencing efforts analyzing a large number of genes in cancer genomes have so far, for the most part, produced mutational spectra similar to those in TP53 but have unveiled a previously unrecognized common G to C transversion mutation signature at GpA dinucleotides in breast cancers and several other cancers. Unraveling the origin of these G to C mutations will be of importance for understanding cancer etiology.

Different patterns of cyclin D1/CDK4-E2F-1/4 pathways in human embryo lung fibroblasts treated by benzo[a]pyrene at different doses

OBJECTIVE

To investigate the roles of the cyclin D1/CDK4 and E2F-1/4 pathways and compare their work patterns in cell cycle changes induced by different doses of B[a]P.

METHODS

Human embryo lung fibroblasts (HELFs) were treated with 2 micromol/L or 100 micromol/L B[a]P which were provided with some characteristics of transformed cells (T-HELFs). Cyclin D1, CDK4 and E2F-1/4 expressions were determined by Western blotting. Flow cytometry was used to detect the distribution of cell cycle.

RESULTS

After B[a]P treatment, the proportion of the first gap (G1) phase cells decreased. CDK4 and E2F-4 expression did not change significantly. In 2 micromol/L treated cells, a marked overexpression of cyclin D1 and E2F-1 was observed. However, in T-HELFs overexpression was limited to cyclin D1 only, and no overexpression of E2F-1 was observed. The decreases of G1 phase in response to B[a]P treatment were blocked in antisense cyclin D1 and antisense CDK4 transfected HELFs (A-D1 and A-K4) and T-HELFs (T-A-D1 and T-A-K4). After 2 micromol/L B[a]P treatment, overexpression of E2F-1 was attenuated in A-D1, and E2F-4 expression was decreased significantly in A-K4. In T-A-D1 and T-A-K4, E2F-4 expression was increased significantly, compared with T-HELFs. The E2F-1 expression remained unchanged in T-A-D1 and T-A-K4.

CONCLUSIONS

Cyclin D1/CDK4-E2F-1/4 pathways work in different patterns in response to low dose and high dose B[a]P treatment. In HELFs treated with 2 micromol/L B[a]P, cyclin D1 positively regulates the E2F-1 expression while CDK4 negatively regulates the E2F-4 expression; however, in HELFs treated with 100 micromol/L B[a]P, both cyclin D1 and CDK4 negatively regulate the E2F-4 expression.

Chemopreventive effects of sarcotriol on ultraviolet B-induced skin tumor development in SKH-1 hairless mice

Sarcotriol (ST) has been shown to be chemopreventive on 7,12-dimethyl-benz(a)anthracene (DMBA) initiated and 12-O-tetradecanoylphorbol-13-acetate (TPA)-promoted skin tumor development in CD-1 mice in recent studies from our laboratory. The objective of this study was to determine the chemopreventive effects of ST on ultraviolet B (UVB)-induced skin tumor development in female SKH-1 hairless mice, an experimental model relevant to human skin cancer development, and its possible mechanisms of action. Female SKH-1 mice were divided into two groups: Control and ST treated. Control was topically treated with 100 microliter acetone and ST treated group administered with 30 microgram ST in 100 microliter acetone one hour before UVB exposure. For UVB-induced tumorigenesis, carcinogenesis was initiated and promoted by UVB (180 mJ/cm(2)). Group weights and tumor counts were taken once every week. After 30 weeks, mice were sacrificed and dorsal skin samples were collected. The proteins from the skin sample were further used for SDS-PAGE and Western blotting using specific antibodies against caspase-3, caspase-8, caspase-9 and p53. Tumor multiplicity was found 19.6, 5.2 in the control and ST treated groups respectively. Caspase-3, -8, -9 and p53 were significantly (P < 0.05) upregulated in ST treated group compared to Control group. Together, this study for the first time identifies the chemopreventive effects of ST in UVB-induced carcinogenesis possibly by inducing apoptosis and upregulating p53.

Preoptic nitric oxide attenuates endotoxic fever in guinea pigs by inhibiting the POA release of norepinephrine

Lipopolysaccharide (LPS) administration induces hypothalamic nitric oxide (NO); NO is antipyretic in the preoptic area (POA), but its mechanism of action is uncertain. LPS also stimulates the release of preoptic norepinephrine (NE), which mediates fever onset. Because NE upregulates NO synthases and NO induces cyclooxygenase (COX)-2-dependent PGE2, we investigated whether NO mediates the production of this central fever mediator. Conscious guinea pigs with intra-POA microdialysis probes received LPS intravenously (2 μg/kg) and, thereafter, an NO donor (SIN-1) or scavenger (carboxy-PTIO) intra-POA (20 μg/μl each, 2 μl/min, 6 h). Core temperature (Tc) was monitored constantly; dialysate NE and PGE2 were analyzed in 30-min collections. To verify the reported involvement of α2-adrenoceptors (AR) in PGE2 production, clonidine (α2-AR agonist, 2 μg/μl) was microdialyzed with and without SIN-1 or carboxy-PTIO. To assess the possible involvement of oxidative NE and/or NO products in the demonstrated initially COX-2-independent POA PGE2 increase, (+)-catechin (an antioxidant, 3 μg/μl) was microdialyzed, and POA PGE2, and Tc were determined. SIN-1 and carboxy-PTIO reduced and enhanced, respectively, the rises in NE, PGE2, and Tc produced by intravenous LPS. Similarly, they prevented and increased, respectively, the delayed elevations of PGE2 and Tc induced by intra-POA clonidine. (+)-Catechin prevented the LPS-induced elevation of PGE2, but not of Tc. We conclude that the antipyretic activity of NO derives from its inhibitory modulation of the LPS-induced release of POA NE. These data also implicate free radicals in POA PGE2 production and raise questions about its role as a central LPS fever mediator.

Organ-dependent susceptibility of p53 knockout mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)

p53 knockout mice are now being frequently used to identify the carcinogenic potential of chemicals, thus it is important to precisely assess the susceptibility of the animals to various test chemicals. In the present study the susceptibility of p53 nullizygous((-/-)), heterozygous((+/-)), and wild-type((+/+)) mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was investigated. Mice of all three genotypes were first fed a diet containing 100 or 300 p.p.m. IQ for 15 weeks in a short-term experiment. p53((+/-)) and ((+/+)) mice were then treated with IQ for 40 weeks and maintained without further treatment for an additional 12 weeks in the long-term experiment. In the forestomach, the incidence of squamous cell hyperplasia was significantly higher in p53((-/-)) than in ((+/-)) and ((+/+)) mice at 15 weeks and higher in ((+/-)) mice than ((+/+)) mice with long-term IQ treatment, indicating an elevated susceptibility of p53 knockout mice. In contrast, in the liver, various hepatocellular lesions developed mainly in female mice with long-term IQ exposure but no significant differences were evident between p53 knockout and wild-type mice, indicating a lack of elevated susceptibility in the knockout animals. Furthermore, polymerase chain reaction-single strand conformation polymorphism and sequencing analysis revealed relatively high (13/30) and low (1/15) incidences of p53 mutations (exons 5-8) in squamous cell hyperplasia and hepatocellular tumors, respectively. These results clearly indicate that the susceptibility of p53 knockout mice is organ-dependent, coinciding to some extent with the likelihood of p53 gene alteration in the induced tumors.

UV fingerprints predominate in the PTCH mutation spectra of basal cell carcinomas independent of clinical phenotype

Basal cell carcinoma (BCC) shows a wide interpatient variation in lesion accrual. To determine whether certain tumorigenic fingerprints and potentially predisposing patched (PTCH) tumor suppressor single-nucleotide polymorphisms (SNPs) are distributed differently among sporadic BCC patients, we compared the PTCH mutation spectra in early-onset BCC (first lesion at age < 35 years), regular BCC (first lesion at age > or = 35 years and < 10 lesions), and multiple BCC (> or = 10 lesions). The PTCH gene was mutated in 29 of 60 cases (48%). Most of the PTCH mutations bore the UV fingerprint (i.e., C --> T or tandem CC --> TT transitions at dipyrimidine sites). However, neither the proportion nor the spectra of exonic PTCH mutations differed significantly among the three groups. A large number of SNPs (IVS10+99C/T, IVS11-51G/C, 1665T/C, 1686C/T, IVS15+9G/C, IVS16-80G/C, IVS17+21G/A, and 3944C/T or its combinations) were also detected, but again their incidence did not differ significantly among the groups. Interestingly, expression of the IVS16-80G/C and the IVS17+21G/A genotype did not achieve the Hardy-Weinberg equilibrium in patients with regular and/or early-onset BCC. These data suggest that a (UV-) mutated PTCH gene is important for sporadic BCC formation independent of clinical phenotype and that the IVS16-80G/C and/or IVS17+21G/A SNP site might be important for tumorigenesis in certain BCC patients.

Immunohistochemical analysis for Mdm2 and p53 proteins in methylcholanthrene-induced mouse rhabdomyosarcomas

3-methylcholanthrene (MC)-induced mouse 10 embryonal (ERSs) and 24 pleomorphic rhabdomyosarcomas (PRSs) of the dermis were examined immunohistochemically for nuclear reactivity of Mdm2, p53, and proliferating cell nuclear antigen (PCNA). ERSs were microscopically present in the rhabdium layer of the dermis from 10 to 13 weeks post injection (PI), and PRSs developed from 13 weeks PI. Moderate to marked Mdm2 reactivity was observed in each of the 10 ERSs, and 23 of the 24 PRSs. Moderate to marked p53 reactivity was observed in 5 of the 10 ERSs, and 19 of the 24 PRSs. p53 reactivity increased in PRSs compared with ERSs. The level of Mdm2 expression was significantly higher compared with p53 expression. Discordant Mdm2 overexpression was observed in 5 ERSs and 5 PRSs, and discordant p53 overexpression was observed in 1 PRSs, although co-overexpression of Mdm2 and p53 was observed in 5 ERSs and 18 PRSs. PCNA reactivity significantly increased in PRSs compared with ERSs. These results suggest that Mdm2 overexpression is an important pathogenic event in MC-induced mouse rhabdomyosarcomas, and its expression may be induced by p53-independent pathway.

Mutagenesis at methylated CpG sequences

5-Methylcytosine in DNA is genetically unstable. Methylated CpG (mCpG) sequences frequently undergo mutation resulting in a general depletion of this dinucleotide sequence in mammalian genomes. In human genetic disease- and cancer-relevant genes, mCpG sequences are mutational hotspots. It is an almost universally accepted dogma that these mutations are caused by random deamination of 5-methylcytosines. However, it is plausible that mCpG transitions are not caused simply by spontaneous deamination of 5-methylcytosine in double-stranded DNA but by other processes including, for example, mCpG-specific base modification by endogenous or exogenous mutagens or, alternatively, by secondary factors operating at mCpG sequences and promoting deamination. We also discuss that mCpG sequences are favored targets for specific exogenous mutagens and carcinogens. When adjacent to another pyrimidine, 5-methylcytosine preferentially undergoes sunlight-induced pyrimidine dimer formation. Certain polycyclic aromatic hydrocarbons form guanine adducts and induce G to T transversion mutations with high selectivity at mCpG sequences.

Induction of Invasive Mouse Skin Carcinomas in Transgenic Mice with Mutations in Both H-ras and p53

Synergistic interaction between H-ras and p53 were systematically examined during skin tumorigenesis. Concurrent expression of an activated H-ras gene and a mutant p53 gene was accomplished by crossing p53(Val135/wt) mice with TG.AC mice. Topical application to wild-type mice with benzo(a)pyrene (BaP) alone produced approximately 26% skin tumor incidence, whereas BaP treatment of p53(wt/wt)Hras(TG.AC/wt), p53(Val135/wt)Hras(wt/wt), and p53(Val135/wt)Hras(TG.AC/wt) mice produced a 75%, 77%, and 100% incidence of skin tumors, respectively. An average of 0.33 tumor per mouse was observed in wild-type (p53(wt/wt)Hras(wt/wt)) mice, whereas approximately 1.54, 1.96, and 3.08 tumors per mouse were seen in BaP-treated p53(wt/wt)Hras(TG.AC/wt), p53(Val135/wt)Hras(wt/wt), and p53(Val135/wt)Hras(TG.AC/wt) mice, respectively. The effects on total tumor volume were even more striking with 7-, 48-, and 588-fold increases in tumor volume compared with wild-type (p53(wt/wt)Hras(wt/wt)) in p53(wt/wt)Hras(TG.AC/wt), p53(Val135/wt)Hras(wt/wt), and p53(Val135/wt)Hras(TG.AC/wt) mice, respectively. Histopathologically, all tumors from p53(wt/wt)Hras(wt/wt) mice were either papillomas or well-differentiated squamous cell carcinomas, whereas the tumors in p53(wt/wt)Hras(TG.AC/wt), p53(Val135/wt)Hras(wt/wt), and p53(Val135/wt)Hras(TG.AC/wt) mice were principally squamous cell carcinomas with varying degree of invasiveness. Particularly, tumors in p53(Val135/wt)Hras(TG.AC/wt) mice exhibited the most rapid growth and the extreme form of tumor invasion. Microarray analysis revealed that dominant-negative p53 (Val135) and activated H-ras affected several cellular processes involved in tumorigenesis possibly through its effects on apoptosis, cell cycle arrest, and Ras-mitogen-activated protein kinase pathways. The present study provides the first in vivo evidence that a germ line p53 mutation and activated H-ras act synergistically to profoundly enhance tumor progression.

The combination of benzo[a]pyrene and ultraviolet A causes an in vivo time-related accumulation of DNA damage in mouse skin

Polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP), are ubiquitous environmental carcinogens. BaP is metabolized in vivo to reactive intermediates that become covalently bound to DNA and form BaP-DNA adducts, an initial event in carcinogenesis. Ultraviolet A (UVA) synergizes with BaP to significantly enhance genetic damage and accelerate carcinogenic processes. This study was initiated to investigate in vivo cellular changes related to carcinogenesis induced by repeated exposures to BaP plus UVA. Simulated chronic exposure to an environmental carcinogen and sunlight was conducted through biweekly topical application of BaP followed 2 h later by UVA exposure over a 10 week period. BaP diol epoxide (BPDE)-DNA adducts were measured in vivo by immunohistochemistry using an anti-BPDE-DNA monoclonal antibody. Oxidative DNA damage was measured by the detection of 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation using high-performance liquid chromatography. Alterations in the cell cycle that were relevant to carcinogenesis were revealed by changes in p53, as identified in vivo using a polyclonal anti-p53 antibody. We found that cells containing BPDE-DNA adducts and nuclear p53 expression significantly increased between 2 and 10 weeks of BaP-UVA treatment, whereas neither BPDE-DNA adducts nor significant changes in p53 were observed in untreated skin. Using regression analysis, oxidative 8-OHdG damage also showed a parallel increase over 2-10 weeks (r = 0.80). These results indicate that genetic damage caused by exposures to BaP plus UVA accumulates with time and increases the potential for inductive events leading to carcinogenesis and tumor formation.

On the origin of p53 G:C --> T:A transversions in lung cancers

The high frequency of G-->T transversions in the p53 gene is a distinctive feature of lung cancer patients with a smoking history and is commonly believed to reflect the direct mutagenic signature of polycyclic aromatic hydrocarbon (PAH) adducts along the gene. Using the April 2000 update of the p53 mutation database of the International Agency for Research on Cancer together with the primary literature, we confirm that the frequency of p53 G-->T transversions in lung cancer of smokers is about three times higher than their frequency in lung cancer of nonsmokers and in most other smoke-unrelated cancers. In contrast, the frequency of C-->A transversions, the DNA-strand mirror counterpart of G-->T transversions, appears to be similar in virtually all human cancers. Along with other data, this strand bias leads us to suggest that smoking may inhibit repair of G-->T primary lesions on the non-transcribed strand. As to the origin of G-->T primary lesions in the p53 gene, we unexpectedly found that cell lines derived from lung cancers, but not from other cancers, demonstrate significant additional excess of G-->T transversions when compared to p53 mutations in parent primary tumors. A detailed codon-by-codon comparison provides evidence in favor of the in vitro origin of this culture-associated G-->T augmentation. Since in culture lung cancer cell lines are not exposed to the carcinogens from smoke, one would rather ascribe these new G-->T transversions to some other mutagens such as, for example, reactive oxygen and nitrogen species. These results are consistent with our previous report [Proc. Natl. Acad. Sci. U.S.A. 97 (2000) 12244], and suggest that other factors, in addition to the direct mutagenic action of PAH-like carcinogens, contribute to p53 mutation-associated lung malignancy.

Selective clonal expansion and microenvironmental permissiveness in tobacco carcinogenesis

Historically our knowledge about the direct carcinogenic activity of cigarette smoke and its constituents grew from painting experiments on the skin of mice to produce papillomas and carcinomas. The neutral fraction of cigarette smoke condensate had most of the carcinogenic activity in this test and was rich in carcinogenic polycyclic aromatic hydrocarbons (PAHs), the most abundant by far being BP. However, the concentration of BP in the condensate was only about 2% the amount of pure BP required to cause skin tumors. In other fractions there were non-carcinogenic constituents that promoted tumor formation when applied repeatedly to mouse skin that had been initiated by a single subcarcinogenic application of BP. There were also constituents of cigarette smoke that acted as co-carcinogens when applied simultaneously with repeated applications of BP. BP was effective as an initiator at lower concentrations than as a complete carcinogen, and some non-carcinogenic PAHs in the condensate were also active initiators. It was concluded from these studies that cigarette smoke condensate is primarily a tumor-promoting and co-carcinogenic agent with weak activity as a complete carcinogen. A major effect of promoters, and possibly of co-carcinogens, is a diffuse hyperplasia which includes selective expansion of clones carrying endogenous mutations and/or mutations induced by PAHs and other carcinogens such as NNK. The induced mutations as well as damaged cells would occur throughout the exposed region and, along with the hyperplasia, increase the permissiveness of the cellular microenvironment for neoplastic expression of any potential tumor cell in its midst. Since neither the promoters nor co-carcinogens in tobacco smoke are known to interact directly with DNA, their effects can be considered epigenetic processes that act upon genetically altered cells. Examples are cited from studies of experimental skin carcinogenesis, smoking-induced histopathological changes in human lung and spontaneous transformation in cell culture to illustrate the genetic and epigenetic interactions of neoplastic development in general and their significance for smoking-induced lung cancer in particular. Certain dietary modifications that appear to be effective in moderating the promotional phase of animal and human carcinogenesis are suggested for trial in managing lung cancer.

Tobacco smoke carcinogens, DNA damage and p53 mutations in smoking-associated cancers

It is estimated that cigarette smoking kills over 1 000 000 people each year by causing lung cancer as well as many other neoplasmas. p53 mutations are frequent in tobacco-related cancers and the mutation load is often higher in cancers from smokers than from nonsmokers. In lung cancers, the p53 mutational patterns are different between smokers and nonsmokers with an excess of G to T transversions in smoking-associated cancers. The prevalence of G to T transversions is 30% in smokers' lung cancer but only 12% in lung cancers of nonsmokers. A similar trend exists, albeit less marked, in laryngeal cancers and in head and neck cancers. This type of mutation is infrequent in most other tumors aside from hepatocellular carcinoma. At several p53 mutational hotspots common to all cancers, such as codons 248 and 273, a large fraction of the mutations are G to T events in lung cancers but are almost exclusively G to A transitions in non-tobacco-related cancers. Two important classes of tobacco smoke carcinogens are the polycyclic aromatic hydrocarbons (PAH) and the nicotine-derived nitrosamines. Recent studies have indicated that there is a strong coincidence of G to T transversion hotspots in lung cancers and sites of preferential formation of PAH adducts along the p53 gene. Endogenously methylated CpG dinucleotides are the preferred sites for G to T transversions, accounting for more than 50% of such mutations in lung tumors. The same dinucleotide, when present within CpG-methylated mutational reporter genes, is the target of G to T transversion hotspots in cells exposed to the model PAH compound benzo[a]pyrene-7,8-diol-9,10-epoxide. As summarized here, a number of other tobacco smoke carcinogens also can cause G to T transversion mutations. The available data suggest that p53 mutations in lung cancers can be attributed to direct DNA damage from cigarette smoke carcinogens rather than to selection of pre-existing endogenous mutations.

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.

Detecting rare mutations associated with cancer risk

For more than a decade, investigators have been searching for a means of determining the risk of individuals developing cancer by detecting rare oncogenic mutations. The accumulation of mutations and the clonal evolvement of tumors provide opportunities for monitoring disease development and intervening prior to the presentation of clinical symptoms, or determining the risk of disease relapse during remission. A number of techniques, mostly polymerase chain reaction (PCR)-based, have been developed that enable the detection of rare oncogenic mutations within the range of 10(-2) to 10(-4) wild-type cells. Only a handful of procedures enable the detection of intragenic single base mutations at one mutant in 10-6 or better. These ultra-sensitive mutation detection techniques have produced some interesting results regarding single base mutation spectra and frequencies in p53, Harvey-ras, N-ras, and other reporter genes and DNA sequences in human tissues. Although there is evidence that some individuals may harbor cells or clones expressing genomic instability, the connection with the processes of carcinogenesis is still tenuous. There remains a need for rigorous epidemiological studies employing these ultra-sensitive mutation detection procedures. Since genomic instability is considered key to tumor development, the relevance of the detection of hypermutable clones in individuals is discussed in the context of cancer risk.

Frequent expression of new cancer/testis gene D40/AF15q14 in lung cancers of smokers

We found a significant correlation between lung cancer in smokers and the expression of a human gene, D40, predominantly expressed in testis and cancers. In an attempt to clone a novel human gene, we screened a cDNA library derived from a human B cell line and obtained a cDNA clone that we refer to as D40. A search for public databases for sequence homologies showed that the D40 gene is identical to AF15q14. D40 mRNA is predominantly expressed in normal testis tissue. However, this gene is also expressed in various human tumour cell lines and primary tumours derived from various organs and tissues, such as lung cancer. We examined the relationship between D40 expression and clinico-pathological characteristics of tumours in primary lung cancer. D40 expression did not significantly correlate with either histological type or pathological tumour stage. However, D40 expression was observed more frequently in poorly differentiated tumours than in well or moderately differentiated ones. Furthermore, the incidence of D40 expression was significantly higher in tumours from patients who smoke than in those from non-smokers. D40/AF15q14 is the first gene in the cancer/testis family for which expression is related to the smoking habits of cancer patients.

The influence of chronic pancreatitis on carcinogenesis: an experimental study in rats

BACKGROUND

The literature contains many controversial or unclearly defined opinions about the risk of development of carcinoma of the exocrine part of the pancreas in patients with chronic pancreatitis. This and our own clinical observations based on analysis of patients with chronic pancreatitis treated surgically (anastomotic and resectional procedures) formed the background to an experimental study to define the risk of carcinogenesis in the course of chronic pancreatitis in rats.

EXPERIMENTAL FINDINGS

In Wistar rats with chronic pancreatitis induced by etionine and then exposed to carcinogenic action of azaserine, proliferation, adenomas and acinic cell carcinomas of the exocrine part of the pancreas were diagnosed; the carcinomas were transplantable. In rats treated with azaserine only, benign proliferative lesions and adenomas were found. The presence of the p53 mutation protein was observed in carcinomatous pancreatic cells in malignant lesions of the pancreas in primary and transplantable cancers, but was not detected in benign proliferative lesions and adenomas. Chronic pancreatitis in Wistar rats predisposes the exocrine part of pancreas to malignant transformation. Growth of cancers of the exocrine part of the pancreas in male rats, but not in female rats, suggests hormonal determination of experimental pancreatic cancer.

CONCLUSIONS

Results demonstrate that chronic pancreatitis in rats predisposes to malignant proliferative lesions, including acinic cell carcinoma. Expression of the protein product of p53 gene mutations correlated with neoplastic transformation of pancreas preceded by chronic pancreatitis, and was also detected in transplantable tumours.

p53 gene expression in relation to indoor exposure to unvented coal smoke in Xuan Wei, China

Lung cancer mortality rates in Xuan Wei County, which are among the highest in China, have previously been associated with exposure to indoor emissions from burning smoky coal. To determine if this association is stronger among lung cancer patients with abnormal expression of p53, we performed a population-based case-control study. Ninety-seven newly diagnosed lung cancer patients and 97 controls, individually matched by age, sex, and home fuel type, were enrolled. We used immunocytochemical methods to assess p53 protein accumulation in exfoliated tumor cells isolated from sputum samples. As expected, the amount of lifetime smoky coal use was associated with an overall increase in lung cancer risk. Compared with subjects who used less than 130 tons of smoky coal during their lifetime, the odds ratios (OR) for lung cancer were 1.48 (95% confidence interval [CI], 0.73 to 3.02) for subjects exposed to 130 to 240 tons, and 3.21 (95% CI, 1.23 to 9.03) for subjects who used more than 240 tons of smoky coal (P for trend 0.01). The effect was due almost exclusively to the pattern in women, almost all of whom were nonsmokers. Further, among highly exposed women, the association was substantially larger and achieved statistical significance only among patients with sputum samples that were positive for p53 overexpression (OR, 18.72; 95% CI, 1.77 to 383.38 vs OR, 4.80; 95% CI, 0.66 to 43.87 for p53-negative cases). This study suggests that exposure to the combustion products of smoky coal in Xuan Wei is more strongly associated with women who have lung cancer accompanied by p53 protein overexpression in exfoliated tumor cells.

Valproate, but not lamotrigine, induces ovarian morphological changes in Wistar rats

Valproate (VPA) medication is associated with development of polycystic ovaries, menstrual disorders and hormonal changes in women with epilepsy. We sought to determine if changes in the ovaries also occurred in an animal model without epilepsy, and whether this effect could be related to a carcinogenic effect expressed by overexpression of p53. A potentially alternative antiepileptic drug, lamotrigine (LTG), was evaluated simultaneously. To this end, female Wistar rats were fed perorally with VPA 400 mg/kg/day (n = 15), VPA 600 mg/kg/day (n = 20), LTG 10 mg/kg/day (n = 15) or control solution (n = 15) for 90-95 days. There was a significant, dose-dependent increase in the number of follicular cysts, reduction in the number of corpora lutea and reduction of ovarian weight in the VPA group. No ovarian pathology was observed in the LTG group. In neither of the groups were morphological changes seen in other organs, nor was there any overexpression of the tumor suppressor gene p53 found. An alternative antiepileptic drug, LTG, showed no ovarian pathology, and there were no light microscopic changes in other organs, or evidence of pathologic p53 overexpression in the LTG-treated animals.