Altered DNA methylation: a secondary mechanism involved in carcinogenesis

DNA methylation modifications induced by hexavalent chromium

Hexavalent chromium [Cr (VI)] contributes a significant health risk and causes a number of chronic diseases and cancers. While the genotoxic and carcinogenic effects of hexavalent chromium exposure are explicit and better-characterized, the exact mechanism underlying the carcinogenic process of Cr (VI) is still a matter of debate. In recent years, studies have shown that epigenetic modifications, especially DNA methylation, may play a significant role in Cr (VI)-induced carcinogenesis. The aim of this review is to summarize our understanding regarding the effects of Cr (VI) on global and gene-specific DNA methylation.

Goodbye to the bioassay

It is time to say goodbye to the standard two-year rodent bioassay. While a few, primarily genotoxic, compounds which are clearly associated with human cancer test positive in the bioassay, there is no science-based, sound foundation for presuming it provides either a valid broad (across different chemicals) capability for discerning potential human carcinogens or a valid starting point for making human risk assessment decisions. The two basic assumptions underlying the bioassay are: (1) rodent carcinogens are human carcinogens; and (2) results obtained at high doses are indicative of results that will occur at lower, environmentally relevant, doses. Both of these assumptions are not correct. Furthermore, a reevaluation of National Toxicology Program bioassay data has revealed that if the dose group size were increased from 50 to 200 rodents per group the number of bioassays deemed to be positive would increase from approximately 50% to very close to 100%. Thus, under the extreme conditions of the bioassay (e.g., high doses, lifetime exposure and, at times, a non-physiological route of administration) virtually all chemicals tested could be made into rodent carcinogens. In recent years there have been a number of proposals to move away from the standard bioassay. In particular, a recently formulated decision tree (Cohen, 2017), which places an emphasis on dose-response relationships and invites the use of MOA information, provides a sound basis for moving on from the bioassay and towards a rational approach to both identify chemicals which appear to have the potential to cause cancer in humans and take dose-response relationships into consideration in order to place the extent, if any, of the risk they might pose into proper perspective.

DNA methylation analysis in rat kidney epithelial cells exposed to 3-MCPD and glycidol

3-Monochloropropane-1,2-diol (3-MCPD) is a well-known food processing contaminant that has been regarded as a rat carcinogen, which is known to induce Leydig-cell and mammary gland tumors in males, as well as kidney tumors in both genders. 3-MCPD is highly suspected to be a non-genotoxic carcinogen. 2,3-Epoxy-1-propanol (glycidol) can be formed via dehalogenation from 3-MCPD. We aimed to investigate the cytotoxic effects of 3-MCPD and glycidol, then to demonstrate the possible epigenetic mechanisms with global and gene-specific DNA methylation in rat kidney epithelial cells (NRK-52E). IC₅₀ value of 3-MCPD was determined as 48 mM and 41.39 mM, whereas IC₅₀ value of glycidol was 1.67 mM and 1.13 mM by MTT and NRU test, respectively. Decreased global DNA methylation at the concentrations of 100 μM and 1000 μM for 3-MCPD and 100 μM and 500 μM for glycidol were observed after 48 h exposure by using 5-methylcytosine (5-mC) ELISA kit. Methylation changes were detected in promoter regions of c-myc and Rassf1a in 3-MCPD and glycidol treated NRK-52E cells by using methylation-specific PCR (MSP), whereas changes on gene expression of c-myc and Rassf1a were observed by using real-time PCR. However, e-cadherin, p16, VHL and p15 genes were unmethylated in their CpG promoter regions in response to treatment with 3-MCPD and glycidol. Alterations in DNA methylation might be key events in the toxicity of 3-MCPD and glycidol.

The effect of acute dichlorodiphenyltrichloroethane exposure on hypermethylation status and down-regulation of p53 and p16INK4a genes in rat liver

The aim of the study was to investigate the early effect of acute dichlorodiphenyltrichloroethane (DDT) exposure on the methylation status of the promoter region of two tumor suppressor genes: p53 and p16(INK4a) (p16) in rat liver. We analyzed their transcript and protein expression profiles concurrently with the examination of transcriptional and protein expression levels of DNA (cytosine-5)-methyltransferase 1 (Dnmt1). Male Wistar rats were treated with a single dose of DDT (57 mg kg(-1) of body weight) and the methylation status of p53 and p16 genes was examined after 24 h using methylation-sensitive restriction analysis-MSRA. The obtained results indicate that DDT induced alternations in methylation of the promoter region in both p53 and p16 genes. In all the tested samples, the promoter CpG islands of p53 (-261, -179, and -450) were methylated within 100% as compared to control samples (0%). The methylation status of the p16 promoter (-11 and +77) was also altered due to exposure to DDT. Methylated cytosines were detectable in 75% of the tested DNA samples. The Real-time PCR and western blot analyses showed a decrease in mRNA and protein levels of p53, respectively, which was related to the increase in DNA synthesis. These relationships were also observed for mRNA and protein expressions of p16, although to a slighter extent. We also showed that hypermethylation in the promoter region of both tumor suppressor genes was consistent with an increased Dnmt1 mRNA level, and this relationship was further confirmed at the protein level of DNMT1. Concluding, our data suggests that epigenetically mediated changes in gene expression may play an important role in the mechanism of DDT toxicity, including carcinogenic action.

Chemopreventive mechanism of polypeptides from Chlamy Farreri (PCF) against UVB-induced malignant transformation of HaCaT cells

To investigate polypeptide from Chlamy Farreri (PCF)'s protective effect against skin cancer, we used a cellular model of ultraviolet B (UVB)-induced malignant transformation. The human keratinocyte cell line HaCaT was repeatly exposed to UVB (10 mJ/cm(2), 20 times) and malignant transformation was confirmed by Gimesa staining, cell cycle analysis and various assays [anchorage independent growth, matrix metalloproteinase-9 (MMP9) activity, plating efficiency]. The malignant transformation was found to be effectively prevented by PCF pretreatment (2.84mM for 2h prior to each UVB exposure). We investigated the mechanism of PCF-mediated action by determining its effect on DNA methylation status of the tumour suppressor genes [P16 and ras association domain family 1 A (RASSF1A)] in the UVB-transformed cells. Both genes were found to be hypermethylated by chronic UVB exposure. The expression levels of P16, RASSF1A, DNA methyltransferases (DNMTs) and DNA damage inducible protein a (GADD45a) were measured by reverse transcriptase-polymerase chain reaction and western blotting. While chronic UVB exposure was found to suppress the expression of P16 and RASSF1A, it enhanced the expression of DNMT3b. In the early phase of UVB-induced malignant transformation, the GADD45a expression was increased, however, it declined with a continued irradiation of the cells. The UVB-induced DNA hypermethylation of P16 and RASSF1A and subsequent gene silencing was reversed by PCF treatment. The inhibition of DNMTs expression suggested that PCF blocked DNA methylation and thereby the silencing of tumour suppressor genes. Furthermore, the PCF-mediated substantial increase in GADD45a expression indicated that PCF promoted demethylation of tumour suppressor genes via GADD45a induction.

Cadmium as a possible cause of bladder cancer: a review of accumulated evidence

Bladder cancer is a significant disease, the rates of which have increased over the few last years. However, its etiology remains as yet undefined. Cadmium, a widespread environmental carcinogen that has received considerable interest, presents evidence as a possible cause of bladder cancer. A literature review was conducted from the years 1984-2013 to study the accumulated evidence for cadmium as a possible cause of bladder cancer, including routes of cadmium exposure, accumulation, toxicity, carcinogenicity, and evidence from epidemiological and experimental studies. Special reference is devoted to cadmium nephrotoxicity, which illustrates how cadmium exerts its effects on the transitional epithelium of the urinary tract. Mechanisms of carcinogenesis are discussed. The effects of cadmium on gene expression in urothelial cells exposed to cadmium are also addressed. Despite different methodologies, several epidemiologic and nephrotoxicity studies of cadmium indicate that occupational exposure to cadmium is associated with increased risk of bladder cancer and provide additional evidence that cadmium is a potential toxic element in urothelial cells. In vitro studies provide further evidence that cadmium is involved in urothelial carcinogenesis. Animal studies encounter several problems such as morphology differences between species. Among the complex mechanisms of cadmium carcinogenesis, gene expression deregulation is the subject of recent studies on bladder cadmium-induced carcinogenesis. Further research, however, will be required to promise a better understanding of the mechanisms underlying cadmium carcinogenesis and to establish the precise role of cadmium in this important malignancy.

Dose- and time-dependent epigenetic changes in the livers of Fisher 344 rats exposed to furan

The presence of furan in common cooked foods along with evidence from experimental studies that lifetime exposure to furan causes liver tumors in rats and mice has caused concern to regulatory public health agencies worldwide; however, the mechanisms of the furan-induced hepatocarcinogenicity remain unclear. The goal of the present study was to investigate whether or not long-term exposure to furan causes epigenetic alterations in rat liver. Treating of male Fisher 344 rats by gavage 5 days per week with 0, 0.92, 2.0, or 4.4 mg furan/kg body weight (bw)/day resulted in dose- and time-dependent epigenetic changes consisting of alterations in DNA methylation and histone lysine methylation and acetylation, altered expression of chromatin modifying genes, and gene-specific methylation. Specifically, exposure to furan at doses 0.92, 2.0, or 4.4 mg furan/kg bw/day caused global DNA demethylation after 360 days of treatment. There was also a sustained decrease in the levels of histone H3 lysine 9 and H4 lysine 20 trimethylation after 180 and 360 days of furan exposure, and a marked reduction of histone H3 lysine 9 and H3 lysine 56 acetylation after 360 days at 4.4 mg/kg bw/day. These histone modification changes were accompanied by a reduced expression of Suv39h1, Prdm2, and Suv4-20h2 histone methyltransferases and Ep300 and Kat2a histone acetyltransferases. Additionally, furan at 2.0 and 4.4 mg/kg bw/day induced hypermethylation-dependent down-regulation of the Rassf1a gene in the livers after 180 and 360 days. These findings indicate possible involvement of dose- and time-dependent epigenetic modifications in the furan hepatotoxicity and carcinogenicity.

2,4-Dichlorophenol induces global DNA hypermethylation through the increase of S-adenosylmethionine and the upregulation of DNMTs mRNA in the liver of goldfish Carassius auratus

Altered DNA methylation is associated with changes in gene expression, signal transduction and stress response after exposure to a wide range of exogenous compounds, and abnormal methylation is a major toxic effect induced by chemicals such as benzene and phenols. 2,4-Dichlorophenol (2,4-DCP), a derivative of phenol, has been classified as a priority pollutant by the US EPA due to its toxic effects on aquatic organisms. However, the effect of 2,4-DCP on DNA methylation and its potential mechanism in fish are rarely understood. The present study aims to figure out whether 2,4-DCP could impact DNA methylation and explore its potential mechanisms by measuring the global DNA methylation levels, S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) contents, the mRNA expression of DNA methyltransferase1 (DNMT1) and DNA methyltransferase3 (DNMT3) in the liver of goldfish Carassius auratus. DNA methylation levels were analyzed using high performance liquid chromatography (HPLC) and MspI/HpaII ethidium bromide assay, SAM and SAH contents were determined by HPLC, the mRNA expression of DNMT1 and DNMT3 was measured by quantitative-PCR (qPCR). The results showed that 2,4-DCP caused global DNA hypermethylation, elevated the methylation levels of CpG islands, increased the SAM and SAH contents, decreased the SAM/SAH ratio, and upregulated the mRNA expression of DNMT1 and DNMT3, while depletion of SAM with Na2SeO3 and inhibition of DNMTs activity with 5-aza-2'-deoxycytidine (5AdC) impaired 2,4-DCP-induced global DNA hypermethylation, suggesting that the increase of SAM contents and upregulation of the mRNA expression of DNMT1 and DNMT3 may play important roles in 2,4-DCP-induced global DNA hypermethylation process. Our report is the first one to show that short-term 2,4-DCP exposure caused the global DNA hypermethylation via altered SAM level and DNMTs expression in fish.

Changes of c-Myc and DNMT1 mRNA and protein levels in the rat livers induced by dibutyl phthalate treatment

We investigated the relationship between dibutyl phthalate (DBP)-induced hypomethylation of the c-Myc promoter region (as evident in our early study) and the expression of c-Myc and DNMT1 genes (at messenger RNA (mRNA) and protein level) in the rat liver. Male Wistar rats received DBP in 1, 3, or 14 daily doses of 1800 mg kg−1 body weight. Levels of DNMT1, c-Myc mRNA, and proteins were detected using real-time polymerase chain reaction and Western blot analysis, respectively. Our findings indicate that DBP caused an increase in mRNA levels of c-Myc at all time points. The results showed that protein levels of c-Myc in rat liver also increased significantly by DBP treatment, which were more pronounced at last time point (after 14 doses). Furthermore, overexpression of DNMT1gene have been found after one dose of DBP, which was confirmed at the protein level by Western blot analysis. Reduced levels of DNMT1mRNA and proteins (3 and 14 doses) were coordinated with depletion DNA synthesis (reported previously). Based on our previous results and those presented here, the following conclusion could be drawn: (1) DBP exerted biological activity through epigenetic modulation of c-Myc gene expression; (2) it seems possible that DBP-induced active demethylation of c-Myc gene through mechanism(s) linked to generation of reactive oxygen species by activated c-Myc; and (3) control of DNA replication was not directly dependent on c-Myc transcriptional activity and we attribute this finding to DNMT1gene expression which was tightly coordinated with DNA synthesis.

Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are important nuclear receptors involved in the regulation of cellular responses from exposure to many xenobiotics and various physiological processes. Phenobarbital (PB) is a non-genotoxic indirect CAR activator, which induces cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes and is known to produce liver foci/tumors in mice and rats. From literature data, a mode of action (MOA) for PB-induced rodent liver tumor formation was developed. A MOA for PXR activators was not established owing to a lack of suitable data. The key events in the PB-induced liver tumor MOA comprise activation of CAR followed by altered gene expression specific to CAR activation, increased cell proliferation, formation of altered hepatic foci and ultimately the development of liver tumors. Associative events in the MOA include altered epigenetic changes, induction of hepatic CYP2B enzymes, liver hypertrophy and decreased apoptosis; with inhibition of gap junctional intercellular communication being an associative event or modulating factor. The MOA was evaluated using the modified Bradford Hill criteria for causality and other possible MOAs were excluded. While PB produces liver tumors in rodents, important species differences were identified including a lack of cell proliferation in cultured human hepatocytes. The MOA for PB-induced rodent liver tumor formation was considered to be qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies conducted in human populations chronically exposed to PB in which there is no clear evidence for increased liver tumor risk.

Baicalin modulates microRNA expression in UVB irradiated mouse skin

This study aimed to evaluate the effects of baicalin on ultraviolet radiation B (UVB)-mediated microRNA (miRNA) expression in mouse skin. We determined miRNA expression profiles in UVB irradiated mice, baicalin treated irradiated mice, and untreated mice, and conducted TargetScan and Gene Ontology analyses to predict miRNA targets. Three miRNAs (mmu-miR-125a-5p, mmu-miR-146a, and mmu-miR-141) were downregulated and another three (mmu-miR-188-5p, mmu-miR-223 and mmu-miR-22) were upregulated in UVB irradiated mice compared with untreated mice. Additionally, these miRNAs were predicted to be related to photocarcinogenesis, hypomethylation and apoptosis. Three miRNAs (mmu-miR-378, mmu-miR-199a-3p and mmu-miR-181b) were downregulated and one (mmu-miR-23a) was upregulated in baicalin treated mice compared with UVB irradiated mice, and they were predicted to be related to DNA repair signaling pathway. These deregulated miRNAs are potentially involved in the pathogenesis of photodamage, and may aid treatment and prevention of UVB-induced dermatoses.

Identification of Dlk1-Dio3 imprinted gene cluster noncoding RNAs as novel candidate biomarkers for liver tumor promotion

The molecular events during nongenotoxic carcinogenesis and their temporal order are poorly understood but thought to include long-lasting perturbations of gene expression. Here, we have investigated the temporal sequence of molecular and pathological perturbations at early stages of phenobarbital (PB) mediated liver tumor promotion in vivo. Molecular profiling (mRNA, microRNA [miRNA], DNA methylation, and proteins) of mouse liver during 13 weeks of PB treatment revealed progressive increases in hepatic expression of long noncoding RNAs and miRNAs originating from the Dlk1-Dio3 imprinted gene cluster, a locus that has recently been associated with stem cell pluripotency in mice and various neoplasms in humans. PB induction of the Dlk1-Dio3 cluster noncoding RNA (ncRNA) Meg3 was localized to glutamine synthetase-positive hypertrophic perivenous hepatocytes, suggesting a role for β-catenin signaling in the dysregulation of Dlk1-Dio3 ncRNAs. The carcinogenic relevance of Dlk1-Dio3 locus ncRNA induction was further supported by in vivo genetic dependence on constitutive androstane receptor and β-catenin pathways. Our data identify Dlk1-Dio3 ncRNAs as novel candidate early biomarkers for mouse liver tumor promotion and provide new opportunities for assessing the carcinogenic potential of novel compounds.

Effect of UVB irradiation on microRNA expression in mouse epidermis

The aim of this study was to assess the effects of UVB irradiation on miRNA expression in the mouse epidermis. We determined miRNA expression profiles in the epidermis of UVB irradiated mice and untreated mice, and conducted TargetScan and Gene Ontology analyses to predict miRNA targets. Three miRNAs were downregulated and three were upregulated in the epidermis of UVB irradiated mice compared with untreated mice, and were predicted to be associated with photocarcinogenesis, hypomethylation and apoptosis. miRNAs are potentially involved in the pathogenesis of photodamage, and may aid in the treatment and prevention of UVB-induced dermatoses.

Epigenetic alterations in ultraviolet radiation-induced skin carcinogenesis: interaction of bioactive dietary components on epigenetic targets

The importance of epigenetic alterations in the development of various diseases including the cancers has been realized. As epigenetic changes are reversible heritable changes, these can be utilized as an effective strategy for the prevention of cancers. DNA methylation is the most characterized epigenetic mechanism that can be inherited without changing the DNA sequence. Although limited available data suggest that silencing of tumor suppressor genes in ultraviolet (UV) radiation-exposed epidermis leads to photocarcinogenesis and is associated with a network of epigenetic modifications including alterations in DNA methylation, DNA methyltransferases and histone acetylations. Various bioactive dietary components have been shown to protect skin from UV radiation-induced skin tumors in animal models. The role of bioactive dietary components, such as, (-)-epicatechins from green tea and proanthocyanidins from grape seeds has been assessed in chemoprevention of UV-induced skin carcinogenesis and underlying epigenetic mechanism in vitro and in vivo animal models. These bioactive components have the ability to block UV-induced DNA hypermethylation and histone modifications in the skin required for the silencing of tumor suppressor genes (e.g. Cip1/p21, p16(INK4a) ). This information is of importance for understanding the role of epigenetic modulation in UV-induced skin tumor and the chemopreventive mechanism of bioactive dietary components.

Epigenetic inactivation of PCDH10 in human prostate cancer cell lines

PCDH10 (protocadherin-10), a novel tumour suppressor gene, is down-regulated in several human cancers due to hypermethylation of promoter CGIs (CpG islands). Here, we investigated the expression of PCDH10 in different normal adult tissues and in a panel of prostate cancer cell lines. PCDH10 was widely expressed in normal tissues with higher levels in the prostate. The expression of PCDH10 was markedly reduced or silenced in prostate cancer cell lines compared with normal adult prostate tissue. Decreased PCDH10 expression was correlated with the methylation status of the PCDH10 promoter. Furthermore, the DNA demethylating agent 5'-azacytidin restored PCDH10 expression by suppressing PCDH10 promoter methylation in prostate cancer cell lines. Treatment with Trichostatin A alone had no significant effect on the expression of PCDH10 but enhanced the effect of 5'-azacytidin. In conclusion, we found that the decreased PCDH10 expression in prostate cancer cells was associated with the aberrant methylation of PCDH10 promoter CGI. Our results may contribute to the understanding of the role of PCDH10 inactivation in the progression of prostate cancers.

Aberrant DNA hypermethylation patterns lead to transcriptional silencing of tumor suppressor genes in UVB-exposed skin and UVB-induced skin tumors of mice

Overexposure of the human skin to solar ultraviolet (UV) radiation is the major etiologic factor for development of skin cancers. Here, we report the results of epigenetic modifications in UV-exposed skin and skin tumors in a systematic manner. The skin and tumor samples were collected after chronic exposure of the skin of SKH-1 hairless mice to UVB radiation using a well-established photocarcinogenesis protocol. We found a distinct DNA hypermethylation pattern in the UVB-exposed epidermal skin and UVB-induced skin tumors that was associated with the elevated expression and activity of the DNA methyltransferases (Dnmt) 1, Dnmt3a and Dnmt3b. To explore the role of hypermethylation in skin photocarcinogenesis, we focused on the p16(INK4a) and RASSF1A tumor suppressor genes, which are transcriptionally silenced on methylation. We established that the silencing of these genes in UVB-exposed epidermis and UVB-induced skin tumors is associated with a network of epigenetic modifications, including hypoacetylation of histone H3 and H4 and increased histone deacetylation, as well as recruitment of methyl-binding proteins, including MeCP2 and MBD1, to the methylated CpGs. Higher levels of DNA methylation and DNMT activity in human squamous cell carcinoma specimens than in normal human skin suggest that the data are relevant clinically. Our data indicate for the first time that UVB-induced DNA hypermethylation, enhanced Dnmt activity and histone modifications occur in UVB-exposed skin and UVB-induced skin tumors and suggest that these events are involved in the silencing of tumor suppressor genes and in skin tumor development.

Enforced SOCS1 and SOCS3 expression attenuates Lck-mediated cellular transformation

Lck is an Src family protein tyrosine kinase with predominant T cell expression. Aberrant expression or activation of Lck kinase has been reported in both lymphoid and non-lymphoid malignancies. We showed previously that the signal transduction pathway involving Janus kinase (JAK) and signal transducer and activator of transcription (STAT) is constitutively activated and contributes to Lck-mediated oncogenesis. Under normal physiological conditions, active STAT proteins induce the expression of suppressor of cytokine signaling (SOCS) family proteins to inhibit further JAK/STAT signaling. It is not fully understood whether and how SOCS-mediated negative feedback control is dysregulated in Lck-transformed cells. Here we report that two SOCS family members, SOCS1 and SOCS3, are not expressed in Lck-transformed LSTRA leukemia. While SOCS1 gene is silenced by DNA hypermethylation, loss of SOCS3 expression is through a mechanism independent of epigenetic silencing by DNA methylation. Furthermore, ectopic expression of SOCS1 or SOCS3 leads to reduced cell proliferation and increased apoptosis in Lck-transformed cells. This is consistent with the attenuation of Lck kinase activity by exogenous SOCS1 or SOCS3 expression. Downstream STAT5 activity is also inhibited as shown by reduced STAT5 tyrosine phosphorylation and in vitro DNA binding. All together, our data highlight the importance of silencing multiple SOCS genes in tumorigenesis and support the roles of SOCS1 and SOCS3 as tumor suppressors toward oncogenic Lck kinase.

Clinical significance of melanoma antigen-encoding gene-1 (MAGE-1) expression and its correlation with poor prognosis in differentiated advanced gastric cancer

BACKGROUND

Melanoma antigen-encoding gene-1 (MAGE-1), a cancer/testis antigen, has been reported to be expressed in various types of cancer. We investigated the clinicopathological features and prognostic significance of MAGE-1 expression in advanced gastric cancer (AGC).

METHODS

Immunohistochemical staining for MAGE-1 was performed on surgical specimens obtained from 135 patients with AGC.

RESULTS

Positive expression of MAGE-1 detected in cytoplasm was observed in 44 of 135 cases (32.6%) in primary tumors and 26 of 96 (27.1%) in lymph node metastases. In noncancerous gastric tissues, apparent MAGE-1 expression was not detected. MAGE-1 in primary tumor was correlated with advanced age (P < 0.001), macroscopic infiltrated type (P = 0.035), and presence of vascular invasion (P = 0.027). The 5-year cancer-specific survival rates of AGC patients with positive MAGE-1 expression were significantly lower than those of patients with negative MAGE-1 (positive: 31.6%, negative: 57.6%, P = 0.038). On multivariate analysis, MAGE-1 expression was not an independent prognostic predictor of AGC (P = 0.064). In differentiated AGC patients, MAGE-1 expression was correlated with advanced age (P = 0.003), macroscopic infiltrated type (P = 0.009), and presence of lymph node metastasis (P = 0.033). The cancer-specific survival rates of differentiated AGC patients with positive MAGE-1 were significantly lower than those of patients with negative MAGE-1 (P = 0.003). Positive MAGE-1 expression was an independent prognostic factor of differentiated AGC patients on multivariate analysis (P = 0.031).

CONCLUSIONS

These findings suggest that MAGE-1 protein expression can serve as a predictive marker of poor prognosis in differentiated AGC patients.

Simultaneous determination of 8-hydroxy-2'-deoxyguanosine and 5-methyl-2'-deoxycytidine in DNA sample by high performance liquid chromatography/positive electrospray ionization tandem mass spectrometry

8-Hydroxy-2'-deoxyguanosine (8-OHdG) and 5-methyl-2'-deoxycytidine (5-mdC) are utilized as useful biomarkers not only for early diagnosis but also for the detection and assessment of high-risk individuals. In the present study, a sensitive and specific method was developed for simultaneous determination of 8-OHdG and 5-mdC in DNA by high performance liquid chromatography/positive electrospray ionization tandem mass spectrometry. The limits of quantification for 8-OHdG and 5-mdC were 80 and 40pg/ml, respectively. The calibration curves of 8-OHdG and 5-mdC were linear over the concentration range of 0.02-100ng/ml and the correlation coefficients were higher than 0.9990. The intra-day and inter-day relative standard derivative values were in the range of 0.70-7.47% for 8-OHdG and 1.07-7.06% for 5-mdC, respectively. The recoveries were 93.4-108.5% for 8-OHdG and 87.4-104.9% for 5-mdC, respectively. This method was validated by determination of the background levels of 8-OHdG and 5-mdC in calf thymus DNA, and satisfactory results were obtained.

Effect of cadmium on cortisol production and 11beta-hydroxysteroid dehydrogenase 2 expression by cultured human choriocarcinoma cells (JEG-3)

Cadmium is a toxicant with known carcinogenic and endocrine disruptor effects. We have previously reported that prenatal exposure to cadmium may induce low birth weight which is associated to increased foetal exposure to glucocorticoids; both signals constitute "hallmarks" of developmental programming. Since the effect of cadmium on the glucocorticoid system of placental carcinogenic cells is unknown, in the present work, we studied the effect of acute low dose of cadmium on cortisol production and 11beta-HSD2 expression and activity by cultured human choriocarcinoma cells (JEG-3). In addition, it was also evaluated whether those effects were related to the methylation index of the HSD11B2 gene, which can be regulated by epigenetic mechanisms. Cells were incubated with low cadmium dose (0.5 and 1 microM) for 24h and viability (MTT), cortisol production (EIA), 11beta-HSD2 expression (qRT-PCR) and activity (radioassay), and methylation index of the HSD11B2 gene were determined. Results show lower cortisol concentrations in the incubation media of exposed cells, which were associated to increased 11beta-HSD2 expression and activity and to a lower methylation index of the gene. These results suggest that cadmium-induced endocrine disruptor effects on JEG-3 cells could be mediated by changes in the methylation status of some target genes.

Methylation status of CDH1 gene in samples of gastric mucous from brazilian patients with chronic gastritis infected by Helicobacter pylori

CONTEXT: Gastric cancer is one of the top list of cancer types that most leads to death in Brazil and worldwide. Helicobacter pylori(H. pylori) is a class I carcinogen and infect almost 90% of chronic gastritis patients. Some genotypes confer different virulent potential to H. pylori and can increase the risk of gastritis development. Methylation of CpG islands can inactivate tumor suppressor genes and therefore, it can be involved in the tumorigenic process. CDH1 is a tumor suppressor gene that encodes the E-cadherin protein, which is important in maintaining cell-cell contacts. The inactivation of this gene can increase the chance of metastasis. Promoter methylation of CDH1 at early steps of gastric carcinogenesis is not yet completely understood. OBJECTIVE: In this study, we investigated the methylation status of CDH1 in chronic gastritis samples and correlated it with the presence of H. pylori. METHODS: Sixty gastric mucosal biopsies were used in this study. The detection of H. pylori was performed with the PCR primers specific to urease C gene. H. pylori genotyping was performed by PCR to cagA and vacA (s and m region). The methylation status of these gene CDH1 was analyzed using methylation-specific polymerase chain reaction and direct sequencing of the PCR products was performed using primers methylated and unmethylated in both forward and reverse directions. RESULTS: H. pylori was detected in 90% of chronic gastritis samples; among these 33% were cagA positive and 100% vacA s1. The genotype vacA s2/m1 was not detected in any sample analyzed. Methylation of CDH1 was detected in 63.3% of chronic gastritis samples and 95% of them were also H. pylori-positive. CONCLUSION: This work suggests that CDH1 gene methylation and H. pylori infection are frequent events in samples from Brazilian patients with chronic gastritis and reinforces the correlation between H. pylori infection and CDH1 inactivation in early steps of gastric tumorigenesis.

Effects of arsenic exposure on DNA methylation and epigenetic gene regulation

Arsenic is a nonmutagenic human carcinogen that induces tumors through unknown mechanisms. A growing body of evidence suggests that its carcinogenicity results from epigenetic changes, particularly in DNA methylation. Changes in gene methylation status, mediated by arsenic, have been proposed to activate oncogene expression or silence tumor suppressor genes, leading to long-term changes in the activity of genes controlling cell transformation. Mostly descriptive, and often contradictory, studies have demonstrated that arsenic exposure is associated with both hypo- and hyper-methylation at various genetic loci in vivo or in vitro. This ambiguity has made it difficult to assess whether the changes induced by arsenic are causally involved in the transformation process or are simply a reflection of the altered physiology of rapidly dividing cancer cells. Here, we discuss the evidence supporting changes in DNA methylation as a cause of arsenic carcinogenesis and highlight the strengths and limitations of these studies, as well as areas where consistencies and inconsistencies exist.

DNA hypomethylation induced by tributyltin, triphenyltin, and a mixture of these in Sebastiscus marmoratus liver

Tributyltin (TBT) and triphenyltin (TPT) coexist in freshwater and marine environments. However, the effects of TBT, TPT, and a mixture of the two on DNA methylation in marine fish livers and the mechanism involved remain to be elucidated. Previous study have proved that abnormal methylation patterns are induced by the balance of transmethylation reaction including the tissue level of S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH) or the activity of DNA (cytosine-5) methyltransferase 1 (DNMT1). Therefore, in the present study, we assessed their ability to cause hepatic DNA hypomethylation in Sebastiscus marmoratus liver and the related mechanism. The results showed that TBT, TPT, and a mixture of the two significantly induced DNA hypomethylation in the fish livers in a dose-dependent manner. Using Pearson correlation coefficient analysis, we identified strong linear correlations between S-adenosylhomocysteine, S-adenosylmethionine, or the SAM to SAH ratio and the hepatic genome-wide 5-methylcytosine content of the DNA, but no correlation between the latter and the DNMT1 expression level. It is therefore proposed that the organotins hypomethylation induced in the marine fish livers was due to altering the balance of the substrate and the product in transmethylation reactions.

A Data-Based Assessment of Alternative Strategies for Identification of Potential Human Cancer Hazards

The two-year cancer bioassay in rodents remains the primary testing strategy for in-life screening of compounds that might pose a potential cancer hazard. Yet experimental evidence shows that cancer is often secondary to a biological precursor effect, the mode of action is sometimes not relevant to humans, and key events leading to cancer in rodents from nongenotoxic agents usually occur well before tumorigenesis and at the same or lower doses than those producing tumors. The International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) hypothesized that the signals of importance for human cancer hazard identification can be detected in shorter-term studies. Using the National Toxicology Program (NTP) database, a retrospective analysis was conducted on sixteen chemicals with liver, lung, or kidney tumors in two-year rodent cancer bioassays, and for which short-term data were also available. For nongenotoxic compounds, results showed that cellular changes indicative of a tumorigenic endpoint can be identified for many, but not all, of the chemicals producing tumors in two-year studies after thirteen weeks utilizing conventional endpoints. Additional endpoints are needed to identify some signals not detected with routine evaluation. This effort defined critical questions that should be explored to improve the predictivity of human carcinogenic risk.

The constitutive active/androstane receptor facilitates unique phenobarbital-induced expression changes of genes involved in key pathways in precancerous liver and liver tumors

Our overall goal is to elucidate progressive changes, in expression and methylation status, of genes which play key roles in phenobarbital (PB)-induced liver tumorigenesis, with an emphasis on their potential to affect signaling through critical pathways involved in the regulation of cell growth and differentiation. PB-elicited unique expression changes of genes, including some of those identified previously as exhibiting regions of altered DNA methylation, were discerned in precancerous liver tissue and/or individual liver tumors from susceptible constitutive active/androstane receptor (CAR) wild-type (WT) compared with resistant CAR knockout (KO) mice. Many of these function in crucial cancer-related processes, for example, angiogenesis, apoptosis, cell cycle, DNA methylation, Hedgehog signaling, invasion/metastasis, Notch signaling, and Wnt signaling. Furthermore, a subset of the uniquely altered genes contained CAR response elements (CAREs). This included Gadd45b, a coactivator of CAR and inhibitor of apoptosis, and two DNA methyltransferases (Dnmt1, Dnmt3a). The presence of CAREs in Dnmts suggests a potential direct link between PB and altered DNA methylation. The current data are juxtaposed with the effects of PB on DNA methylation and gene expression which occurred uniquely in liver tumor-prone B6C3F1 mice, as compared with the resistant C57BL/6, following 2 or 4 weeks of treatment. Collectively, these data reveal a comprehensive view of PB-elicited molecular alterations (i.e., changes in gene expression and DNA methylation) that can facilitate hepatocarcinogenesis. Notably, candidate genes for initial "fingerprints" of early and late stages of PB-induced tumorigenesis are proposed.

Phenobarbital elicits unique, early changes in the expression of hepatic genes that affect critical pathways in tumor-prone B6C3F1 mice

At 2 and 4 weeks following treatment with phenobarbital (PB), the classical nongenotoxic rodent liver carcinogen, we elucidated unique gene expression changes (both induction and repression) in liver tumor-susceptible B6C3F1 mice, as compared with the relatively resistant C57BL/6. Based on their cancer-related roles, we believe that altered expression of at least some of these genes might underlie PB-induced liver tumorigenesis. Putative constitutive active/androstane (CAR) response elements (CAREs), a subset of PB response elements, were present within multiple genes whose expression was uniquely altered in the B6C3F1 mice, suggesting a role for CAR in their regulation. Additionally, three DNA methyltransferase genes (Dnmt1, Dnmt3a, and Dnmt3b) were repressed uniquely in the tumor-prone B6C3F1 mice, and all possess putative CAREs, providing a potential direct link between PB and expression of key genes that regulate DNA methylation status. Previously, we demonstrated that PB-elicited unique regions of altered methylation (RAMs) in B6C3F1 mice, as compared with the relatively resistant C57BL/6, at 2 and 4 weeks, and annotation of the regions harboring these changes revealed 51 genes. This is extended by the current study, which employed RNA isolated from the same liver tissue used in the earlier investigations. Genes elucidated from both the methylation and expression analyses are involved in identical processes/pathways (e.g., cell cycle, apoptosis, angiogenesis, epithelial-mesenchymal cell transition, invasion/metastasis, and mitogen-activated protein kinase, transforming growth factor-beta, and Wnt signaling). Therefore, these changes might represent very early events that directly contribute to PB-induced tumorigenesis. It is instructive to consider the possibility that, in a hypothesis-driven fashion, these genes are initial candidates that could be utilized to develop a biomarker "fingerprint" of early exposure to PB and PB-like compounds.

Multiple genes exhibit phenobarbital-induced constitutive active/androstane receptor-mediated DNA methylation changes during liver tumorigenesis and in liver tumors

The constitutive active/androstane receptor (CAR) mediates responses to the nongenotoxic rodent liver tumor promoter phenobarbital (PB), including certain gene expression changes, hepatomegaly, and tumor formation. Aberrant DNA methylation represents epigenetic events that can play multiple roles in tumorigenesis. Previously, 146 unique PB-induced regions of altered DNA methylation (RAMs) were observed in liver tumor-susceptible CAR wild-type (WT) mice (in 23 weeks, precancerous tissue, and 32 weeks, tumor tissue), as compared to the resistant knockout (KO). We believe that at least some of these might be key for tumorigenesis. In the current study, cloning and annotation of a subset (82%) of the unique RAMs revealed 47 genes exhibiting altered methylation; 17 are already implicated in cancer or related processes and, thus, we have identified 30 "new" candidate genes that might be involved in carcinogenesis due to an epigenetic alteration. These may contribute to tumor development through their involvement in angiogenesis, apoptosis, epithelial-mesenchymal cell transition, growth/survival, and invasion/migration/metastasis. We have also, previously, discerned unique PB-elicited RAMs in liver tumor-prone B6C3F1 mice, as compared to the relatively resistant C57BL/6 strain, at 2 or 4 weeks, and identified 51 genes exhibiting altered methylation. Importantly, 11 of these genes were identified from identical, unique RAMs discerned in both the sensitive B6C3F1 and CAR WT mice, thus representing an initial, potential candidate "fingerprint" which might serve as a biomarker for PB-induced tumorigenesis. These two studies reveal "new" genes whose epigenetic statuses changed uniquely in liver tumor-susceptible mice (B6C3F1 and CAR WT), as compared to their resistant counterparts (C57BL/6 and CAR KO, respectively), within a continuum of PB-induced tumorigenesis.

Inhalation of cigarette smoke induces regions of altered DNA methylation (RAMs) in SENCAR mouse lung

The development of early biomarkers, both of exposure and effect, would substantially improve science-based risk assessment with regard to cigarette smoke (CS)-associated toxicity. Altered DNA methylation, an epigenetic mechanism, is linked to CS-induced lung tumorigenesis. We have taken an unbiased approach (i.e. genomic regions are not pre-selected) to assess early methylation changes within lung DNA from female SENCAR mice treated with a single dose of 7,12-dimethylbenz[a]anthracene (DMBA), and then exposed to air alone, or CS for 4 or 8 weeks. Regions of altered DNA methylation (RAMs) were detected in mice treated with DMBA alone, or DMBA+0.16, 0.32 or 0.48 mg wet total particulate matter per liter (WTPM/L) CS, using methylation-sensitive restriction digestion, arbitrarily primed PCR and capillary electrophoresis. Comparison of the RAMs that formed in different treatment groups revealed: (1) RAMs which "carried forward" across time (i.e. occurred at both 4 and 8 weeks) in a particular dose group, in addition to unique RAMs observed only at 8 weeks, and (2) RAMs which "carried forward" across dose (i.e. occurred in at least 2 dose groups at a particular time point), in addition to unique RAMs observed only in 1 dose group. Furthermore, a subset of RAMs was observed, at both 4 and 8 weeks, in DMBA-treated and DMBA+CS-exposed groups; the presence of unique RAMs in the latter suggest that combined DMBA+CS treatment more than just "magnifies" a subset of cell populations bearing the methylation changes induced by DMBA alone. Importantly, only minimal histopathological changes were observed in the lungs of CS-treated mice. This study is the first to demonstrate changes in lung DNA methylation at early times following exposure to CS, e.g., prior to overt histopathology. Thus, altered methylation might serve as a biomarker of CS exposure, and, in light of the fact that methylation changes are linked to CS-induced lung tumorigenesis, might also be useful as biomarkers of effect.

Biological Basis of Differential Susceptibility to Hepatocarcinogenesis among Mouse Strains

There is a vast amount of literature related to mouse liver tumorigenesis generated over the past 60 years, not all of which has been captured here. The studies reported in this literature have generally been state of the art at the time they were carried out. A PubMed search on the topic "mouse liver tumors" covering the past 10 years yields over 7000 scientific papers. This review address several important topics related to the unresolved controversy regarding the relevance of mouse liver tumor responses observed in cancer bioassays. The inherent mouse strain differential sensitivities to hepatocarcinogenesis largely parallel the strain susceptibility to chemically induced liver neoplasia. The effects of phenobarbital and halogenated hydrocarbons in mouse hepatocarcinogenesis have been summarized because of recurring interest and numerous publications on these topics. No single simple paradigm fully explains differential mouse strain responses, which can vary more than 50-fold among inbred strains. In addition to inherent genetics, modifying factors including cell cycle balance, enzyme induction, DNA methylation, oncogenes and suppressor genes, diet, and intercellular communication influence susceptibility to spontaneous and induced mouse hepatocarcinogenesis. Comments are offered on the evaluation, interpretation, and relevance of mouse liver tumor responses in the context of cancer bioassays.

Methylation of protocadherin 10, a novel tumor suppressor, is associated with poor prognosis in patients with gastric cancer

BACKGROUND & AIMS

By using methylation-sensitive representational difference analysis, we identified protocadherin 10 (PCDH10), a gene that encodes a protocadherin and is silenced in a tumor-specific manner. We analyzed its epigenetic inactivation, biological effects, and prognostic significance in gastric cancer.

METHODS

Methylation status was evaluated by combined bisulfite restriction analysis and bisulfite sequencing. The effects of PCDH10 re-expression were determined in growth, apoptosis, proliferation, and invasion assays. PCDH10 target genes were identified by complementary DNA microarray analysis.

RESULTS

PCDH10 was silenced or down-regulated in 94% (16 of 17) of gastric cancer cell lines; expression levels were restored by exposure to demethylating agents. Re-expression of PCDH10 in MKN45 gastric cancer cells reduced colony formation in vitro and tumor growth in mice; it also inhibited cell proliferation (P < .01), induced cell apoptosis (P < .001), and repressed cell invasion (P < .05), up-regulating the pro-apoptosis genes Fas, Caspase 8, Jun, and CDKN1A; the antiproliferation gene FGFR; and the anti-invasion gene HTATIP2. PCDH10 methylation was detected in 82% (85 of 104) of gastric tumors compared with 37% (38 of 104) of paired nontumor tissues (P < .0001). In the latter, PCDH10 methylation was higher in precancerous lesions (27 of 45; 60%) than in chronic gastritis samples (11 of 59; 19%) (P < .0001). After a median follow-up period of 16.8 months, multivariate analysis revealed that patients with PCDH10 methylation in adjacent nontumor areas had a significant decrease in overall survival. Kaplan-Meier survival curves showed that PCDH10 methylation was associated significantly with shortened survival in stage I-III gastric cancer patients.

CONCLUSIONS

PCDH10 is a gastric tumor suppressor; its methylation at early stages of gastric carcinogenesis is an independent prognostic factor.

Mapping the epigenome--impact for toxicology

Recent advances in technological approaches for mapping and characterizing the epigenome are generating a wealth of new opportunities for exploring the relationship between epigenetic modifications, human disease and the therapeutic potential of pharmaceutical drugs. While the best examples for xenobiotic-induced epigenetic perturbations come from the field of non-genotoxic carcinogenesis, there is growing evidence for the relevance of epigenetic mechanisms associated with a wide range of disease areas and drug targets. The application of epigenomic profiling technologies to drug safety sciences has great potential for providing novel insights into the molecular basis of long-lasting cellular perturbations including increased susceptibility to disease and/or toxicity, memory of prior immune stimulation and/or drug exposure, and transgenerational effects.

Mechanisms of peroxisome proliferator-induced DNA hypomethylation in rat liver

Genomic hypomethylation is a consistent finding in both human and animal tumors and mounting experimental evidence suggests a key role for epigenetic events in tumorigenesis. Furthermore, it has been suggested that early changes in DNA methylation and histone modifications may serve as sensitive predictive markers in animal testing for carcinogenic potency of environmental agents. Alterations in metabolism of methyl donors, disturbances in activity and/or expression of DNA methyltransferases, and presence of DNA single-strand breaks could contribute to the loss of cytosine methylation during carcinogenesis; however, the precise mechanisms of genomic hypomethylation induced by chemical carcinogens remain largely unknown. This study examined the mechanism of DNA hypomethylation during hepatocarcinogenesis induced by peroxisome proliferators WY-14,643 (4-chloro-6-(2,3-xylidino)-pyrimidynylthioacetic acid) and DEHP (di-(2-ethylhexyl)phthalate), agents acting through non-genotoxic mode of action. In the liver of male Fisher 344 rats exposed to WY-14,643 (0.1% (w/w), 5 months), the level of genomic hypomethylation increased by approximately 2-fold, as compared to age-matched controls, while in the DEHP group (1.2% (w/w), 5 months) DNA methylation did not change. Global DNA hypomethylation in livers from WY-14,643 group was accompanied by the accumulation of DNA single-strand breaks, increased cell proliferation, and diminished expression of DNA methyltransferase 1, while the metabolism of methyl donors was not affected. In contrast, none of these parameters changed significantly in rats fed DEHP. Since WY-14,643 is much more potent carcinogen than DEHP, we conclude that the extent of loss of DNA methylation may be related to the carcinogenic potential of the chemical agent, and that accumulation of DNA single-strand breaks coupled to the increase in cell proliferation and altered DNA methyltransferase expression may explain genomic hypomethylation during peroxisome proliferator-induced carcinogenesis.

Global DNA hypomethylation, rather than reactive oxygen species (ROS), a potential facilitator of cadmium-stimulated K562 cell proliferation

Cell proliferation plays a critical role in the process of cadmium (Cd) carcinogenesis. Although both induction of reactive oxygen species (ROS) and alteration of DNA methylation are involved in Cd-stimulated cell proliferation, the detailed mechanism of Cd-stimulated cell proliferation remains poorly understood. In this study, K562 cells pre-treated with N-acetylcysteine (NAC) or methionine (Meth) were exposed to Cd to investigate the potential contribution of ROS and global DNA methylation pathways in Cd-induced cell proliferation. The results showed that Cd-stimulated cell proliferation, increased ROS and DNA damage levels, and induced global DNA hypomethylation. The increases of ROS and DNA damage levels were attenuated by pre-treatment with NAC. Cd-stimulated cell proliferation did not appear to be suppressed through eliminating ROS by NAC. However, methionine was shown to prevent Cd-induced global DNA hypomethylation and Cd-stimulated cell proliferation. Our results suggest that global DNA hypomethylation, rather than ROS, is a potential facilitator of Cd-stimulated K562 cell proliferation.

Identification of genes that may play critical roles in phenobarbital (PB)-induced liver tumorigenesis due to altered DNA methylation

Aberrant DNA methylation plays important roles in tumorigenesis, and the nongenotoxic rodent tumor promoter phenobarbital (PB) alters methylation patterns to a greater extent in liver tumor susceptible as compared to resistant mice (Watson and Goodman, 2002). Unique hepatic regions of altered DNA methylation (RAMs) were identified in sensitive B6C3F1, as compared to resistant C57BL/6, mice at 2 or 4 weeks of PB treatment using a novel approach involving methylation-sensitive restriction digestion, arbitrarily primed PCR, and capillary electrophoresis (Bachman et al., 2006b). PCR products representing 90 of 170 (53%) total unique B6C3F1 RAMs at 2 or 4 weeks were cloned and subjected to BLAST-like alignment tool searches that resulted in 51 gene matches; some of these have documented oncogenic or tumor suppressor roles. Importantly, uniquely hypomethylated genes play roles in angiogenesis (e.g., chymase 1, tyrosine kinase nonreceptor 2, and possibly ephrin B2 and triple functional domain, PTPRF interacting) and invasion and metastasis, including those involved in the epithelial-mesenchymal transition (transcription factor 4, transforming growth factor beta receptor II, and ral guanine nucleotide dissociation stimulator). Common cellular targets and regulators of the genes representing unique B6C3F1 RAMs were uncovered, indicating that they might act in concert to more efficiently promote tumorigenesis. Genes not previously associated with mouse liver tumorigenesis exhibited altered methylation at these very early times following PB treatment. We hypothesize that at least some of the unique PB-induced B6C3F1 RAMs represent key events facilitating transformation, which is consistent with a causative role of altered DNA methylation during early stages of tumorigenesis.

Effects of long-term low-dose cadmium exposure on genomic DNA methylation in human embryo lung fibroblast cells

Cadmium is a toxic transition metal of continuing occupational and environmental concern. As a well-recognized human carcinogen, its carcinogenic mechanisms are still poorly understood. Cadmium has long been considered a non-genotoxic carcinogen and thought to act through epigenetic mechanisms. In the present study, we tested the effects of long-term low-dose cadmium exposure on DNA methylation in human embryo lung fibroblast (HLF) cells. After 2 months of exposure to 0-1.5 micromol/L cadmium, both the level of genomic DNA methylation and the enzyme activity of DNA methyltransferases (DNMTs) were increased in a concentration-related manner. Moreover, our results showed that cadmium exposure up-regulated the mRNA levels of DNMT1, DNMT3a and DNMT3b at higher concentrations. We further tested the growth dynamics of HLF cells, and observed significantly elevated growth rates, decreased cell population of G0/G1-phase and increased cell population of S-phase at 0.9, 1.2, and 1.5 micromol/L concentrations. Our study indicated that long-term low-dose cadmium exposure could disrupt DNA methylation, which may be one of the possible underlying carcinogenic mechanisms of cadmium.

Epigenetic aspects of genotoxic and non-genotoxic hepatocarcinogenesis: studies in rodents

Hepatocellular carcinoma, which is one of the most prevalent life-threatening human cancers, is showing an increased incidence worldwide. Recent evidence indicates that the development of hepatocellular carcinoma is associated with not only genetic alterations, but also with profound epigenetic changes. This review summarizes the current knowledge about epigenetic alterations during rodent hepatocarcinogenesis, considers the similarities and differences in epigenetic effects of genotoxic and non-genotoxic rodent liver carcinogens, and discusses the possible role of these effects in the causality of liver tumor development.

Hypomethylation of hepatocyte nuclear factor-1beta (HNF-1beta) CpG island in clear cell carcinoma of the ovary

Expression of hepatocyte nuclear factor-1beta (HNF-1beta) is significantly up-regulated in ovarian clear cell carcinoma (CCC). The mechanism of up-regulation, however, remains unclear. It has been recognized that hypomethylation of specific gene promoters is involved in aberrant gene expression in carcinogenesis. In the present study, ovarian CCCs were examined whether there was a correlation between the methylation and expression status of HNF-1beta, using combined bisulfite restriction analysis (COBRA), bisulfite-sequencing and immunocyto/histochemistry. In 2 CCC cell lines, hypomethylation of HNF-1beta CpG island strongly correlated with HNF-1beta expression, at both the mRNA and protein levels. In archival surgical specimens, 20 of 20 CCCs were immunohistochemically positive for HNF-1beta, whereas none of 20 serous adenocarcinomas (SAs) or 12 normal ovaries were positive for it. By COBRA, methylation of HNF-1beta CpG island was less frequently detected in CCCs (8 of 20, 40%) than SAs (18 of 20, 90%) or normal ovaries (12 of 12, 100%) (p < 0.01), which was confirmed by bisulfite-sequencing. In addition, HNF-1beta hypomethylation correlated with a high HNF-1beta immunostaining score in CCCs. These results strongly suggest that hypomethylation of the HNF-1beta CpG island participates in the HNF-1beta up-regulation in ovarian CCC.

The effect of phenobarbital on the methylation level of the p16 promoter region in rat liver

It has been suggested that non-genotoxic carcinogens (NGCs) may cause modification of the DNA methylation status. We studied the effects of phenobarbital (PB) -- a non-genotoxic rodent liver carcinogen -- on the methylation level of the promoter region of the p16 suppressor gene, as well as on hepatomegaly, DNA synthesis, and DNA-methyltransferase (DNMTs) activity in the rat liver. Male Wistar rats received PB in 1, 3 or 14 daily oral doses (at 24-h intervals), each equivalent to 1/10 of the LD(50) value. The study showed that PB has caused persistent elevation in relative liver weight (RLW) as well as a transient increase in DNA synthesis. This suggests that the PB-induced increase in RLW was due to a combination of both hyperplasia and hypertrophy of liver cells. The effect of PB on DNA synthesis corresponded to an increase in the methylation pattern of the p16 promoter sequence. Methylation of cytosine in the analyzed CpG sites of the p16 gene was found after short exposure of the animals to PB. Treatment of rats with PB for 1 and 3 days also produced an increase in nuclear DNMTs activity. After prolonged administration (14 days), DNA synthesis declined, returning to the control level. No changes in methylation of the p16 gene nor in DNMTs activity were observed. The reversibility of early induced changes in target tissues is a mark characteristic of tumor promoters. Thus, transient changes in methylation of the p16 gene, although their direct role in the mechanisms of PB toxicity, including its carcinogenic action, remains doubtful, may therefore be a significant element of such processes.

Aberrant DNA methylation and gene expression in livers of newborn mice transplacentally exposed to a hepatocarcinogenic dose of inorganic arsenic

Our prior work showed that brief exposure of pregnant C3H mice to inorganic arsenic-induced hepatocellular carcinoma (HCC) formation in adult male offspring. The current study examined the early hepatic events associated with this oncogenic transformation. Pregnant mice were exposed to a known carcinogenic dose of arsenic (85 ppm) in the drinking water from gestation days 8 to 18. The dams were allowed to give birth and liver samples from newborn males were analyzed for arsenic content, global DNA methylation and aberrant expression of genes relevant to the carcinogenic process. Arsenic content in newborn liver reached 57 ng/g wet weight, indicating arsenic had crossed the placenta, reached the fetal liver and that significant amounts remained after birth. Global methylation status of hepatic DNA was not altered by arsenic in the newborn. However, a significant reduction in methylation occurred globally in GC-rich regions. Microarray and real-time RT-PCR analysis showed that arsenic exposure enhanced expression of genes encoding for glutathione production and caused aberrant expression of genes related to insulin growth factor signaling pathways and cytochrome P450 enzymes. Other expression alterations observed in the arsenic-treated male mouse newborn liver included the overexpression of cdk-inhibitors and stress response genes including increased expression of metallothionein-1 and decreased expression of betaine-homocysteine methyltransferase and thioether S-methyltransferase. Thus, transplacental exposure to arsenic at a hepatocarcinogenic dose induces alterations in DNA methylation and a complex set of aberrant gene expressions in the newborn liver, a target of arsenic carcinogenesis.

Epigenetic effects of the continuous exposure to peroxisome proliferator WY-14,643 in mouse liver are dependent upon peroxisome proliferator activated receptor alpha

Peroxisome proliferators are potent rodent liver carcinogens that act via a non-genotoxic mechanism. The mode of action of these agents in rodent liver includes increased cell proliferation, decreased apoptosis, secondary oxidative stress and other events; however, it is not well understood how peroxisome proliferators are triggering the plethora of the molecular signals leading to cancer. Epigenetic changes have been implicated in the mechanism of liver carcinogenesis by a number of environmental agents. Short-term treatment with peroxisome proliferators and other non-genotoxic carcinogens leads to global and locus-specific DNA hypomethylation in mouse liver, events that were suggested to correlate with a burst of cell proliferation. In the current study, we investigated the effects of long-term exposure to a model peroxisome proliferator WY-14,643 on DNA and histone methylation. Male SV129mice were fed a control or WY-14,643-containing (1000ppm) diet for one week, five weeks or five months. Treatment with WY-14,643 led to progressive global hypomethylation of liver DNA as determined by an HpaII-based cytosine extension assay with the maximum effect reaching over 200% at five months. Likewise, trimethylation of histone H4 lysine 20 and H3 lysine 9 was significantly decreased at all time points. The majority of cytosine methylation in mammals resides in repetitive DNA sequences. In view of this, we measured the effect of WY-14,643 on the methylation status of major and minor satellites, as well as in IAP, LINE1 and LINE2 elements in liver DNA. Exposure to WY-14,643 resulted in a gradual loss of cytosine methylation in major and minor satellites, IAP, LINE1 and LINE2 elements. The epigenetic changes correlated with the temporal effects of WY-14,643 on cell proliferation rates in liver, but no sustained effect on c-Myc promoter methylation was observed. Finally, WY-14,643 had no effect on DNA and histone methylation status in Pparalpha-null mice at any of the time points considered in this study. These data indicate the importance of epigenetic alterations in the mechanism of action of peroxisome proliferators and the key role of Pparalpha.

Assessment of the genotoxicity in toad Bufo raddei exposed to petrochemical contaminants in Lanzhou Region, China

Single cell gel electrophoresis or comet assay, micronucleus (MN) test and global DNA methylation detection were used to assess the genotoxicity in toad Bufo raddei exposed to the petrochemical (mainly oil and phenol) polluted area in Lanzhou Region (LZR) comparing with a relatively unpolluted area in Liujiaxia Region (LJXR). The results from the present study indicated that DNA damage and MN frequency in toad from LZR were significantly higher than those from LJXR at the same sampling month, whereas the degree of global DNA methylation was lower, which implies that the petrochemical contaminants at environmental level in LZR were genotoxic to B. raddei. The degree of genotoxic damage was obviously related with the extent of pollution among the three sampling months in LZR. The significantly positive correlations between DNA damage and concentrations of oil and/or phenol existed in liver cells but erythrocytes, implying that liver is more suitable as a sentinel tissue for the assessment of genotoxic impact of low-level contamination. The results from both comet assay and global DNA methylation detection on liver cells showed that the genotoxicity varied significantly with oil and/or phenol concentrations, suggesting that these two methods are relatively sensitive and suitable for monitoring the genotoxicity of petrochemical pollutants on amphibians.

Modulation by budesonide of a CpG endonuclease in mouse lung tumors

CpG endonuclease activity was identified in nuclear extracts obtained from mouse lung tumors. Enzyme activity was determined using a 333 bp polymerase chain reaction product of the estrogen receptor-alpha gene that contained either radiolabeled cytosine or tritium-labeled methyl groups at CpG sites. Activity was measured as the release of radioactivity from the substrate. The product of the nuclease activity was identified by high pressure liquid chromatography (HPLC) as either 5-methyl-2'-deoxycytidine when the CpG sites in the substrate were methylated or 2'-deoxycytidine when the CpG sites were not methylated. The CpG endonuclease activity was dependent on nuclear protein and temperature, had a proclivity for double-stranded over single-stranded DNA and was inhibited by ethylenediaminetetraacetic acid or 2-mercaptoethanol. Strain A/J mouse lung tumors induced by vinyl carbamate had a greater level of CpG endonuclease activity than non-involved lung tissue. Budesonide, a potent chemopreventive agent in mouse lung, not only prevented an increase in CpG endonuclease activity in lung tumors but, when administered to mice with established tumors, also decreased the level of endonuclease activity in the tumors. The effect of budesonide on CpG endonuclease activity in lung tumors was inversely related to its published effect on DNA methylation in mouse lung tumors, i.e. the drug decreased CpG endonuclease activity and increased the methylation of DNA. The increased CpG endonuclease activity in mouse lung tumors and its inhibition by budesonide would suggest this endonuclease as a potential molecular target for chemoprevention.

Altered methylation in gene-specific and GC-rich regions of DNA is progressive and nonrandom during promotion of skin tumorigenesis

Altered DNA methylation, an epigenetic mechanism, likely contributes to tumorigenesis, with an inverse relationship existing between methylation in a promoter region and transcription. Using the SENCAR two-stage mouse skin tumorigenesis model, altered methylation was characterized in precancerous tissue and in tumor tissue. Mouse skin was initiated with 7,12-dimethylbenz[a]anthracene and promoted three times a week with 3, 9, 18, or 27 mg cigarette smoke condensate (CSC) for 4, 8, or 29 weeks; tumors were collected at 29 weeks. In addition, reversibility of changes in methylation was assessed following cessation of the promoting stimulus. DNA was isolated, and GC-rich methylation was assessed quantitatively via methylation-sensitive restriction digestion, arbitrarily primed PCR, and electrophoretic separation of PCR products. Analysis focused on regions of altered methylation (RAMs), which persisted from 4 to 8 weeks and from 8 weeks to tumor tissue. Persistent RAMs (i.e., seen in precancerous tissue and carried forward to tumors) are likely to play a key role in tumorigenesis. Twenty-two CpG sites in the upstream region of the Ha-ras promoter were unmethylated in control skin, 27 mg CSC, and tumor tissue. At two CpG sites closer to the transcriptional start site the incidence of hypomethylation increased with the dose of CSC. Hypomethylation was detected in all tumor samples. Expression of Ha-ras increased with 18 and 27 mg CSC promotion and more so in tumor tissue. These data support our hypothesis that tumor promotion involves instability of the epigenome, providing an environment where changes in the methylation status of specific regions of the genome accumulate progressively and contribute to the clonal expansion of initiated cells that leads to tumor formation.

Loss of imprinting of the insulin-like growth factor 2 and the H19 gene in testicular seminomas detected by real-time PCR approach

IGF2 and H19 are imprinted genes in normal human tissue, but many studies have observed a loss of imprinting (LOI) of these genes in tumors as an epigenetic alteration of the DNA, that leads to a biallelic expression predisposing cells to carcinogenesis and tumor growth. The aim of this study was to test the reliability of LightCycler-assisted Real-time PCR in detecting LOI of IGF2 and H19 in 39 patients with testicular germ cell tumors by comparing these results with the analysis generated by the golden standard restriction fragment length polymorphism (RFLP). With LightCycler-assisted Real-time PCR for IGF2 44% and for H19 49% of the patients were found to be heterozygous. This was consistent with the results obtained by RFLP, but surprisingly RFLP failed in more than 7% of the patients. In detecting LOI (for IGF2 in 41% and for H19 in 68% of the informative patients) the approach by RFLP was superior, since the results derived from LightCycler-assisted Real-time PCR showed reliable results in 76 and 10% of the samples concerning IGF2 and H19, respectively. Again, no discrepancy between the results obtained by the two methods occurred. In sum, LightCycler-assisted Real-time PCR is a sufficiently working approach for the rapid and reliable detection of heterozygosity of IGF2 or H19 gene and identification of LOI of IGF2 and thus may be helpful in conducting large epidemiological studies. However, for the identification of LOI of the H19 gene in this cohort it possesses only restrictive use.

Phenobarbital induces progressive patterns of GC-rich and gene-specific altered DNA methylation in the liver of tumor-prone B6C3F1 mice

Altered DNA methylation contributes to tumorigenesis by affecting gene expression in a heritable fashion. Phenobarbital (PB) is a nongenotoxic rodent carcinogen which induces global hypomethylation and regions of hypermethylation in mouse liver. Liver tumor-sensitive (B6C3F1) and -resistant (C57BL/6) male mice were administered 0.05% (wt/wt) PB in drinking water for 2 or 4 weeks, and a 2-week recovery was included following each dosing period. DNA was isolated from liver (target) and kidney (nontarget) tissues. The methylation status of GC-rich regions of DNA was assessed via methylation-sensitive restriction digestion, arbitrarily primedpolymerase chain reaction, and capillary electrophoretic separation of products. PB-induced regions of altered methylation (RAMs) which carry forward from an early to a later time point are more likely to be mechanistically relevant as compared to those that do not. Twelve of 69 RAMs (17%) present in B6C3F1 liver at 2 weeks were also seen at 4 weeks, while only 1 of the 123 RAMs (< 1%) present in C57BL/6 liver was seen at 4 weeks. In the B6C3F1 mice, 57 unique (as compared to the C57BL/6) regions of altered hepatic methylation (RAMs), predominantly hypomethylation, were observed at 2 weeks, increasing to 86 at 4 weeks. Changes in methylation were largely reversible. Altered methylation in liver was highly dissimilar to that of kidney. Following 4 weeks PB, bisulfite sequencing revealed hypomethylation of Ha-ras in B6C3F1, but not C57BL/6, which correlated with increased gene expression. These data indicate that (1) progressive, nonrandom changes in methylation provide an epigenetic mechanism underlying the ability of PB to cause mouse liver tumorigenesis and (2) susceptibility to tumorigenesis is related inversely to the capacity to maintain normal patterns of methylation.

Diethanolamine and Phenobarbital Produce an Altered Pattern of Methylation in GC-Rich Regions of DNA in B6C3F1 Mouse Hepatocytes Similar to That Resulting from Choline Deficiency

DNA methylation is an epigenetic mechanism regulating transcription, which when disrupted, can alter gene expression and contribute to carcinogenesis. Diethanolamine (DEA), a non-genotoxic alkanolamine, produces liver tumors in mice. Studies suggest DEA inhibits choline uptake and causes biochemical changes consistent with choline deficiency (CD). Rodents fed methyl-deficient diets exhibit altered methylation of hepatic DNA and an increase in liver tumors, e.g., CD causes liver tumors in B6C3F1 mice. We hypothesize that DEA-induced CD leads to altered methylation patterns which facilitates tumorigenesis. B6C3F1 hepatocytes in primary culture were grown in the presence of either 4.5 mM DEA, 3 mM Phenobarbital (PB), or CD media for 48 h. These concentrations induced comparable increases in DNA synthesis. PB, a nongenotoxic rodent liver carcinogen known to alter methylation in mouse liver, was included as a positive control. Global, average, DNA methylation status was not affected. The methylation status of GC-rich regions of DNA, which are often associated with promoter regions, were assessed via methylation-sensitive restriction digestion and arbitrarily primed PCR with capillary electrophoretic separation and detection of PCR products. DEA, PB, and CD treatments resulted in 54, 63, and 54 regions of altered methylation (RAMs), respectively, and the majority were hypomethylations. A high proportion of RAMs (72%) were identical when DEA was compared to CD. Similarly, 70% were identical between PB and CD. Altered patterns of methylation in GC-rich regions induced by DEA and PB resemble that of CD and indicate that altered DNA methylation is an epigenetic mechanism involved in the facilitation of mouse liver tumorigenesis.