Allelic loss and mutations in a new ETRG-1 gene are early events in diethylstilbestrol-induced renal carcinogenesis in Syrian hamsters

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

Disruptive chemicals, senescence and immortality

Carcinogenesis is thought to be a multistep process, with clonal evolution playing a central role in the process. Clonal evolution involves the repeated 'selection and succession' of rare variant cells that acquire a growth advantage over the remaining cell population through the acquisition of 'driver mutations' enabling a selective advantage in a particular micro-environment. Clonal selection is the driving force behind tumorigenesis and possesses three basic requirements: (i) effective competitive proliferation of the variant clone when compared with its neighboring cells, (ii) acquisition of an indefinite capacity for self-renewal, and (iii) establishment of sufficiently high levels of genetic and epigenetic variability to permit the emergence of rare variants. However, several questions regarding the process of clonal evolution remain. Which cellular processes initiate carcinogenesis in the first place? To what extent are environmental carcinogens responsible for the initiation of clonal evolution? What are the roles of genotoxic and non-genotoxic carcinogens in carcinogenesis? What are the underlying mechanisms responsible for chemical carcinogen-induced cellular immortality? Here, we explore the possible mechanisms of cellular immortalization, the contribution of immortalization to tumorigenesis and the mechanisms by which chemical carcinogens may contribute to these processes.

Cadmium induced cell apoptosis, DNA damage, decreased DNA repair capacity, and genomic instability during malignant transformation of human bronchial epithelial cells

Cadmium and its compounds are well-known human carcinogens, but the mechanisms underlying the carcinogenesis are not entirely understood. Our study was designed to elucidate the mechanisms of DNA damage in cadmium-induced malignant transformation of human bronchial epithelial cells. We analyzed cell cycle, apoptosis, DNA damage, gene expression, genomic instability, and the sequence of exons in DNA repair genes in several kinds of cells. These cells consisted of untreated control cells, cells in the fifth, 15th, and 35th passage of cadmium-treated cells, and tumorigenic cells from nude mice using flow cytometry, Hoechst 33258 staining, comet assay, quantitative real-time polymerase chain reaction (PCR), Western blot analysis, random amplified polymorphic DNA (RAPD)-PCR, and sequence analysis. We observed a progressive increase in cell population of the G0/G1 phase of the cell cycle and the rate of apoptosis, DNA damage, and cadmium-induced apoptotic morphological changes in cerebral cortical neurons during malignant transformation. Gene expression analysis revealed increased expression of cell proliferation (PCNA), cell cycle (CyclinD1), pro-apoptotic activity (Bax), and DNA damage of the checkpoint genes ATM, ATR, Chk1, Chk2, Cdc25A. Decreased expression of the anti-apoptotic gene Bcl-2 and the DNA repair genes hMSH2, hMLH1, ERCC1, ERCC2, and hOGG1 was observed. RAPD-PCR revealed genomic instability in cadmium-exposed cells, and sequence analysis showed mutation of exons in hMSH2, ERCC1, XRCC1, and hOGG1 in tumorigenic cells. This study suggests that Cadmium can increase cell apoptosis and DNA damage, decrease DNA repair capacity, and cause mutations, and genomic instability leading to malignant transformation. This process could be a viable mechanism for cadmium-induced cancers.

Cadmium-induced DNA damage and mutations in Arabidopsis plantlet shoots identified by DNA fingerprinting

Random amplified polymorphic DNA (RAPD) test is a feasible method to evaluate the toxicity of environmental pollutants on vegetal organisms. Herein, Arabidopsis thaliana (Arabidopsis) plantlets following Cadmium (Cd) treatment for 26 d were screened for DNA genetic alterations by DNA fingerprinting. Four primers amplified 20-23 mutated RAPD fragments in 0.125-3.0 mg L(-1) Cd-treated Arabidopsis plantlets, respectively. Cloning and sequencing analysis of eight randomly selected mutated fragments revealed 99-100% homology with the genes of VARICOSE-Related, SLEEPY1 F-box, 40S ribosomal protein S3, phosphoglucomutase, and noncoding regions in Arabidopsis genome correspondingly. The results show the ability of RAPD analysis to detect significant genetic alterations in Cd-exposed seedlings. Although the exact functional importance of the other mutated bands is unknown, the presence of mutated loci in Cd-treated seedlings, prior to the onset of significant physiological effects, suggests that these altered loci are the early events in Cd-treated Arabidopsis seedlings and would greatly improve environmental risk assessment.

DNA demethylation by 5-aza-2-deoxycytidine treatment abrogates 17 beta-estradiol-induced cell growth and restores expression of DNA repair genes in human breast cancer cells

Prolonged exposure to elevated levels of estrogen is a risk factor for breast cancer. Though increased cell growth and loss of DNA repair capacity is one of the proposed mechanisms for estrogen-induced cancers, the mechanism through which estrogen induces cell growth and decreases DNA repair capacity is not clear. DNA hypermethylation is known to inactivate DNA repair genes and apoptotic response in cancer cells. Therefore, the objective of this study was to determine the role of DNA hypermethylation in estrogen-induced cell growth and regulation of DNA repair genes expression in breast cancer cells. To achieve this objective, the estrogen-responsive MCF-7 cells either pretreated with 5-aza-2-deoxycytidine (5-aza-dC) or untreated (as control) were exposed to 17 beta-estradiol (E2), and its effect on cell growth and expression of DNA repair genes were measured. The result revealed that 5-aza-dC abrogates the E2-induced growth in MCF-7 cells. An increased expression of OGG1, MSH4, and MLH1 by 5-aza-dC treatment alone, suggest the DNA hypermethylation as a potential cause for decreased expression of these genes in MCF-7 cells. The decreased expression of ERCC1, XPC, OGG1, and MLH1 by E2 alone and its restoration by co-treatment with 5-aza-dC further suggest that E2 reduces the expression of these DNA repair genes potentially through promoter hypermethylation. Reactivation of mismatch repair (MMR) gene MLH1 and abrogation of E2-induced cell growth by 5-aza-dC treatment suggest that estrogen causes increased growth in breast cancer cells potentially through the inhibition of MMR-mediated apoptotic response. In summary, this study suggests that estrogen increases cell growth and decreases the DNA repair capacity in breast cancer cells, at least in part, through epigenetic mechanism.

Simulated microgravity-induced epigenetic changes in human lymphocytes

Real space flight and modeled microgravity conditions result in changes in the expression of genes that control important cellular functions. However, the mechanisms for microgravity-induced gene expression changes are not clear. The epigenetic changes of DNA methylation and chromatin histones modifications are known to regulate gene expression. The objectives of this study were to investigate whether simulated microgravity alters (a) the DNA methylation and histone acetylation, and (b) the expression of DNMT1, DNMT3a, DNMT3b, and HDAC1 genes that regulate epigenetic events. To achieve these objectives, human T-lymphocyte cells were grown in a rotary cell culture system (RCCS) that simulates microgravity, and in parallel under normal gravitational conditions as control. The microgravity-induced DNA methylation changes were detected by methylation sensitive-random amplified polymorphic DNA (MS-RAPD) analysis of genomic DNA. The gene expression was measured by Quantitative Real-time PCR. The expression of DNMT1, DNMT3a, and DNMT3b was found to be increased at 72 h, and decreased at 7 days in microgravity exposed cells. The MS-RAPD analysis revealed that simulated microgravity exposure results in DNA hypomethylation and mutational changes. Gene expression analysis revealed microgravity exposure time-dependent decreased expression of HDAC1. Decreased expression of HDAC1 should result in increased level of acetylated histone H3, however a decreased level of acetylated H3 was observed in microgravity condition, indicating thereby that other HDACs may be involved in regulation of H3 deacetylation. The findings of this study suggest that epigenetic events could be one of the mechanistic bases for microgravity-induced gene expression changes and associated adverse health effects.

Long duration exposure to cadmium leads to increased cell survival, decreased DNA repair capacity, and genomic instability in mouse testicular Leydig cells

Epidemiological and experimental studies have shown that cadmium is carcinogenic to human and experimental animals, however, the mechanism of cadmium-induced carcinogenesis is not clear. The aberrant expression of cell cycle and DNA repair genes resulting in increased cell proliferation and genomic instability are the characteristic features of cancer cells. The purpose of this study was to determine if exposure to cadmium can perturb cell proliferation/survival and causes genomic instability in TM3 cells, a mouse testicular Leydig cell line. The results of this study revealed that short-duration exposure to lower doses of cadmium significantly increase the growth of TM3 cells, whereas, higher doses are toxic and cause cell death. The long duration exposure to higher doses of cadmium, however, results in increased cell survival and acquisition of apoptotic resistance. Gene expression analysis by real-time PCR revealed increased expression of the anti-apoptotic gene Bcl-2, whereas decreased expression of pro-apoptotic gene Bax. Decreased expression of genes for maintenance of DNA methylation, DNMT1, and DNA repair, OGG1 and MYH, was also observed in cells exposed to cadmium for 24h. The random amplified polymorphic DNA (RAPD) assay revealed genomic instability in cells with chronic exposure to cadmium. The findings of this study indicate that mouse testicular Leydig cells adapt to chronic cadmium exposure by increasing cell survival through increased expression of Bcl-2, and decreased expression of Bax. The increased proliferation of cells with genomic instability may result in malignant transformation, and therefore, could be a viable mechanism for cadmium-induced cancers.