Epigenetic silencing of tumor suppressor genes commonly occurs in human cancers via increasing DNA methylation and repressive histone modifications at gene promoters. However, little is known about how pathogenic environmental factors contribute to cancer development by affecting epigenetic regulatory mechanisms. Previously, we reported that both hypoxia and nickel (an environmental carcinogen) increased global histone H3 lysine 9 methylation in cells through inhibiting a novel class of iron- and α-ketoglutarate-dependent histone demethylases. Here, we investigated whether inhibition of histone demethylase JMJD1A by hypoxia and nickel could lead to repression/silencing of JMJD1A-targeted gene(s). By using Affymetrix GeneChip and ChIP-on-chip technologies, we identified Spry2 gene, a key regulator of receptor tyrosine kinase/extracellular signal-regulated kinase (ERK) signaling, as one of the JMJD1A-targeted genes in human bronchial epithelial BEAS-2B cells. Both hypoxia and nickel exposure increased the level of H3K9me2 at the Spry2 promoter by inhibiting JMJD1A, which probably led to a decreased expression of Spry2 in BEAS-2B cells. Repression of Spry2 potentiated the nickel-induced ERK phosphorylation, and forced expression of Spry2 in BEAS-2B cells decreased the nickel-induced ERK phosphorylation and significantly suppressed nickel-induced anchorage-independent growth. Taken together, our results suggest that histone demethylases could be targets of environmental carcinogens and their inhibition may lead to altered gene expression and eventually carcinogenesis.

Iron-and 2-oxoglutarate-dependent dioxygenases are a diverse family of non-heme iron enzymes that catalyze various important oxidations in cells. A key structural motif of these dioxygenases is a facial triad of 2-histidines-1-carboxylate that coordinates the Fe(II) at the catalytic site. Using histone demethylase JMJD1A and DNA repair enzyme ABH2 as examples, we show that this family of dioxygenases is highly sensitive to inhibition by carcinogenic nickel ions. We find that, with iron, the 50% inhibitory concentrations of nickel (IC 50 Nickel compounds are human respiratory carcinogens (1), causing a very high incidence of lung and nasal cancers in nickel refinery workers (2). Over 20 years ago, our group reported that cells phagocytosed particulate nickel compounds, and the dissolution of these particles inside of the cells generated high concentrations of free nickel ions in the cytoplasm and nucleus (3). Using a dye that fluoresces when intracellular nickel ion binds to it, we showed that both soluble and insoluble nickel compounds were able to elevate the levels of nickel ions in the cytoplasmic and nuclear compartments (4). A strong correlation was found between the uptake of particulate nickel compounds by cells and subsequent cell transformation (5), suggesting that intracellular nickel ion concentration is a major determinant of toxicity and carcinogenicity of nickel compounds. Identifying the intracellular targets of nickel ions is therefore crucial to understand the underlying mechanism for the carcinogenic effects of nickel compounds.Silencing of tumor suppressor gene(s) by epigenetic mechanisms represents one of the potential mechanisms of nickel carcinogenesis. Epigenetic events, which include DNA methylation and histone modifications, are ubiquitously involved in the regulation of gene expression. By using a transgenic cell model with the target gene placed near heterochromatin, we were the first to demonstrate that nickel exposure caused a very high frequency of transgene silencing by increasing DNA methylation and repressive histone marks at the promoter of the silenced transgene (6 -8). In animal experiments, injection of particulate nickel compounds (nickel sulfide or nickel subsulfide) into mice induced formation of malignant fibrous histiocytomas and sarcomas, with the p16 and Fhit genes often found to be epigenetically silenced in these cancers (9,10). Additional studies have demonstrated that nickel exposure caused truncation of histone H2B and H2A as well as global alterations of a variety of histone modifications, such as histone acetylation, methylation, phosphorylation, and ubiquitination (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). However, the underlying mechanisms responsible for these nickel-induced epigenetic alterations are poorly understood. In our recent study, we reported that nickel increases the global levels of mono-and di-methylated histone H3 lysine 9 (H3K9me1 and H3K9me2) not by affecting histone methyltransferases but rather by inhibiting a group of unidentified iron-and...

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Previous studies have demonstrated that exposure to polycyclic aromatic hydrocarbons (PAHs) and its derivatives is associated with an increased risk of skin cancers, and the carcinogenic effect of PAHs is thought to involve both tumor initiation and promotion. Whereas PAH tumor initiation is well characterized, the mechanisms involved in the tumor promotion of PAHs remain elusive. In the present study, we investigated the effects of PAHs on vascular endothelial growth factor 6 -8). 5-MCDE has also been found to be a strong complete carcinogen (9 -11). Therefore, PHAs are important compounds in the etiology of human cancers. Carcinogenesis is a multistage process that consists of initiation, promotion, and progression (13). Initiation is a rapid and irreversible course, whereas promotion is a long term process requiring chronic exposure to a certain compound with tumor promotion activities (14). It is very likely that PAHs contribute to carcinogenesis through involvement at these multiple stages (15, 16). The mutagenic effects of PAHs, which are responsible for tumor initiation, have been extensively documented (5, 9 -12). However, their tumor promotion effects, which are mainly mediated by activation of transcription factors, are not well understood. It is thought that the rate-limiting steps in multistage carcinogenesis occur during tumor promotion and tumor progression. Thus, the identification of signaling pathways involved in PAH tumor promotion is not only essential for understanding the tumorigenesis of PAH compounds, but also in providing useful information for tumor chemoprevention. So it is practically important to compare among the parental PAHs, their derivatives as well as their analogs, the potential effects on various signaling pathways and downstream target genes. Because vascular endothelial growth factor (VEGF) is one of the key growth factors involved in carcinogenesis, we here investigate the effects of subgroups of PAHs, including B[a]P and its derivative B[a]PDE, 5-MCDE, and its analog CDE, on VEGF induction, as well as the signaling pathways leading to this induction. * This work was supported in part by Grants CA112557, CA103180, and CA094964 from the NCI, National Institutes of Health and Grants ES012451, ES000260, and ES010344 from the NIEHS, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. -7,8-diol-9,10-epoxide; CDE, chrysene-1,2-diol-3,4-epoxide; 5-MCDE, (Ϯ)-anti-5-methylchrysene-1,2-diol-3,4-epoxide; FBS, fetal bovine serum; MEM, Eagle's minimal essential medium; PI-3K, phosphatidylinositol 3-kinase; EMSA, electrophoretic mobility shift assay; PBS, phosphate-buffered saline; RT, reverse transcriptase.

Our laboratory has shown that arsenite markedly increased the cancer rate caused by solar-simulation ultraviolet radiation (UVR) in the hairless mouse skin model. In the present study, we investigated how arsenite affected DNA photodamage repair and apoptosis after solar-simulation UVR in the mouse keratinocyte cell line 291.03C. The keratinocytes were treated with different concentrations of sodium arsenite (0.0, 2.5, 5.0 μM) for 24 hr and then were immediately irradiated with a single dose of 0.30 kJ/m 2 UVR. At 24 hr after UVR, DNA photoproducts [cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts (6-4PPs)] and apoptosis were measured using the enzyme-linked immunosorbent assay and the two-color TUNEL (terminal deoxynucleotide transferase dUTP nick end labeling) assay, respectively. The results showed that arsenite reduced the repair rate of 6-4PPs by about a factor of 2 at 5.0 μM and had no effect at 2.5 μM. UVR-induced apoptosis at 24 hr was decreased by 22.64% at 2.5 μM arsenite and by 61.90% at 5.0 μM arsenite. Arsenite decreased the UVR-induced caspase-3/7 activity in parallel with the inhibition of apoptosis. Colony survival assays of the 291.03C cells demonstrate a median lethal concentration (LC 50 ) of arsenite of 0.9 μM and a median lethal dose (LD 50 ) of UVR of 0.05 kJ/m 2 . If the present results are applicable in vivo , inhibition of UVR-induced apoptosis may contribute to arsenite’s enhancement of UVR-induced skin carcinogenesis.