Chronic toxic and carcinogenic effects of cadmium chloride in male DBA/2NCr and NFS/NCr mice: strain-dependent association with tumors of the hematopoietic system, injection site, liver, and lung

Toxicity Tolerance in the Carcinogenesis of Environmental Cadmium

Cadmium (Cd) is an environmental toxicant of worldwide public health significance. Diet is the main non-workplace Cd exposure source other than passive and active smoking. The intestinal absorption of Cd involves transporters for essential metals, notably iron and zinc. These transporters determine the Cd body burden because only a minuscule amount of Cd can be excreted each day. The International Agency for Research on Cancer listed Cd as a human lung carcinogen, but the current evidence suggests that the effects of Cd on cancer risk extend beyond the lung. A two-year bioassay demonstrated that Cd caused neoplasms in multiple tissues of mice. Also, several non-tumorigenic human cells transformed to malignant cells when they were exposed to a sublethal dose of Cd for a prolonged time. Cd does not directly damage DNA, but it influences gene expression through interactions with essential metals and various proteins. The present review highlights the epidemiological studies that connect an enhanced risk of various neoplastic diseases to chronic exposure to environmental Cd. Special emphasis is given to the impact of body iron stores on the absorption of Cd, and its implications for breast cancer prevention in highly susceptible groups of women. Resistance to cell death and other cancer phenotypes acquired during Cd-induced cancer cell transformation, under in vitro conditions, are briefly discussed. The potential role for the ZnT1 efflux transporter in the cellular acquisition of tolerance to Cd cytotoxicity is highlighted.

Estimation of health risks associated with dietary cadmium exposure

In much of the world, currently employed upper limits of tolerable intake and acceptable excretion of cadmium (Cd) (ECd/Ecr) are 0.83 µg/kg body weight/day and 5.24 µg/g creatinine, respectively. These figures were derived from a risk assessment model that interpreted β2-microglobulin (β2MG) excretion > 300 μg/g creatinine as a "critical" endpoint. However, current evidence suggests that Cd accumulation reduces glomerular filtration rate at values of ECd/Ecr much lower than 5.24 µg/g creatinine. Low ECd/Ecr has also been associated with increased risks of kidney disease, type 2 diabetes, osteoporosis, cancer, and other disorders. These associations have cast considerable doubt on conventional guidelines. The goals of this paper are to evaluate whether these guidelines are low enough to minimize associated health risks reliably, and indeed whether permissible intake of a cumulative toxin like Cd is a valid concept. We highlight sources and levels of Cd in the human diet and review absorption, distribution, kidney accumulation, and excretion of the metal. We present evidence for the following propositions: excreted Cd emanates from injured tubular epithelial cells of the kidney; Cd excretion is a manifestation of current tissue injury; reduction of present and future exposure to environmental Cd cannot mitigate injury in progress; and Cd excretion is optimally expressed as a function of creatinine clearance rather than creatinine excretion. We comprehensively review the adverse health effects of Cd and urine and blood Cd levels at which adverse effects have been observed. The cumulative nature of Cd toxicity and the susceptibility of multiple organs to toxicity at low body burdens raise serious doubt that guidelines concerning permissible intake of Cd can be meaningful.

Gestational Cd Exposure in the CD-1 Mouse Induces Sex-Specific Hepatic Insulin Insensitivity, Obesity, and Metabolic Syndrome in Adult Female Offspring

There is compelling evidence that developmental exposure to toxic metals increases risk for obesity and obesity-related morbidity including cardiovascular disease and type 2 diabetes. To explore the hypothesis that developmental Cd exposure increases risk of obesity later in life, male, and female CD-1 mice were maternally exposed to 500 ppb CdCl2 in drinking water during a human gestational equivalent period (gestational day 0-postnatal day 10 [GD0-PND10]). Hallmark indicators of metabolic disruption, hepatic steatosis, and metabolic syndrome were evaluated prior to birth through adulthood. Maternal blood Cd levels were similar to those observed in human pregnancy cohorts, and Cd was undetected in adult offspring. There were no observed impacts of exposure on dams or pregnancy-related outcomes. Results of glucose and insulin tolerance testing revealed that Cd exposure impaired offspring glucose homeostasis on PND42. Exposure-related increases in circulating triglycerides and hepatic steatosis were apparent only in females. By PND120, Cd-exposed females were 30% heavier with 700% more perigonadal fat than unexposed control females. There was no evidence of dyslipidemia, steatosis, increased weight gain, nor increased adiposity in Cd-exposed male offspring. Hepatic transcriptome analysis on PND1, PND21, and PND42 revealed evidence for female-specific increases in oxidative stress and mitochondrial dysfunction with significant early disruption of retinoic acid signaling and altered insulin receptor signaling consistent with hepatic insulin sensitivity in adult females. The observed steatosis and metabolic syndrome-like phenotypes resulting from exposure to 500 ppb CdCl2 during the pre- and perinatal period of development equivalent to human gestation indicate that Cd acts developmentally as a sex-specific delayed obesogen.

Prolonged cadmium exposure alters benign uterine fibroid cell behavior, extracellular matrix components, and TGFB signaling

The heavy metal Cadmium (Cd), a widespread environmental contaminant, poses serious hazards to human health and is considered a metallohormone and carcinogen. In women with uterine fibroids, there is a significant association between blood Cd levels and increased fibroid tumor size. The aim of this study was to determine if benign human uterine leiomyoma (fibroid) cells could be malignantly transformed in vitro by continuous Cd exposure and, if so, explore a molecular mechanism by which this could occur. We found when fibroid cells were exposed to 10 µM CdCl₂ for 8 weeks, a robust and fast‐growing Cd‐Resistant Leiomyoma (CR‐LM) cell culture was established. The CR‐LM cells formed viable colonies in soft agar and had increased cytoplasmic glycogen aggregates, enhanced cell motility, a higher percentage of cells in G2/M phase, and increased expression of the proliferation marker Ki‐67. NanoString analysis showed downregulation of genes encoding for extracellular matrix (ECM) components, such as collagens, fibronectins, laminins, and SLRP family proteins, whereas genes involved in ECM degradation (MMP1, MMP3, and MMP10) were significantly upregulated. A volcano plot showed that the top differentially genes favored cancer progression. Functional analysis by ingenuity pathway analysis predicted a significant inhibition of TGFB1 signaling, leading to enhanced proliferation and attenuated fibrosis. Prolonged Cd exposure altered phenotypic characteristics and dysregulated genes in fibroid cells predicative of progression towards a cancer phenotype. Therefore, continuous Cd exposure alters the benign characteristics of fibroid cells in vitro, and Cd exposure could possibly pose a health hazard for women with uterine fibroids.

Cadmium-stimulated invasion of rat liver cells during malignant transformation: Evidence of the involvement of oxidative stress/TET1-sensitive machinery

Cadmium (Cd) is recognized as a highly toxic heavy metal for humans in part because it is a multi-organ carcinogen. To clarify the mechanism of Cd carcinogenicity, we have established an experimental system using rat liver TRL1215 cells exposed to 2.5 μM Cd for 10 weeks and then cultured in Cd-free medium for an additional 4 weeks (total 14 weeks). Recently, we demonstrated, by using this experimental system, that 1) Cd stimulates cell invasion by suppression of apolipoprotein E (ApoE) expression, and 2) Cd induces DNA hypermethylation of the regulatory region of the ApoE gene. However, the underlying mechanism(s) as well as other potential genetic participants in the Cd-stimulated invasion are undefined. In the present work, we found that concurrent with enhanced invasion, Cd induced oxidative stress, coupled with the production of oxidative stress-sensitive metallothionein 2A (MT2A), which lead to down-modulation of ten-eleven translocation methylcytosine dioxygenase 1 (TET1: DNA demethylation) in addition to ApoE, without impacting DNA methyltransferases (DNMTs: DNA methylation) levels. Furthermore, the expression of tissue inhibitor of metalloproteinase 2 and 3 (TIMP2 and TIMP3) that are positively regulated by TET1, were decreased by Cd. The genes (ApoE/TET1/TIMP2/TIMP3) suppressed by Cd were further suppressed by hydroquinone (HQ; a reactive oxygen species [ROS] producer), whereas N-acetyl-l-cysteine (NAC; a ROS scavenger) prevented the suppression of their expression by HQ. In addition, NAC reversed their expression suppressed by Cd. Cd-stimulated cell invasion was clearly dampened by NAC in a concentration-dependent manner. Overall these findings suggest that 1) altered TET1 expression and activity together with ApoE are likely involved in the enhanced invasiveness due to Cd exposure, and 2) Cd down-regulation of TET1 likely evokes a reduction in ApoE expression (possible by DNA hypermethylation), and 3) anti-oxidants are effective in abrogation of the enhanced invasiveness that occurs concurrently with Cd-induced malignant transformation.

Role of the PI3K/Akt pathway in cadmium induced malignant transformation of normal prostate epithelial cells

This study investigated the role of the PI3K/Akt pathway in cadmium (Cd) induced malignant transformation of normal prostate epithelial (PWR1E and RWPE1) cells. Both PWR1E and RWPE1 cells were exposed to 10 μM Cd for one year and designated as Cd-PWR1E and Cd-RWPE1. Cd-RWPE1 cells robustly formed tumors in athymic nude mice. Functionally, Cd-exposure induced tumorigenic attributes indicated by increased wound healing, migration and invasion capabilities in both cell lines. RT²-array analysis revealed many oncogenes including P110α, Akt, mTOR, NFKB1 and RAF were induced whereas tumor suppressor (TS) genes were attenuated in Cd-RWPE1. This was validated by individual quantitative-real-time-PCR at transcriptional and by immunoblot at translational levels. These results were consistent in Cd-PWR1E vs parental PWR1E cells. Gene Set Enrichment Analysis revealed that five prostate cancer (PCa) related pathways were enriched in Cd-exposed cells compared to their normal controls. These pathways include the KEGG- Pathways in cancer, Prostate Cancer Pathway, ERBB, Apoptosis and MAPK pathways. We selected up- and down-regulated genes randomly from the PI3K/Akt pathway array and profiled these in the TCGA/GDC prostate-adenocarcinoma (PRAD) patient cohort. An upregulation of oncogenes and downregulation of TS genes was observed in PCa compared to their normal controls. Taken together, our study reveals that the PI3K/Akt signaling is one of the main molecular pathways involved in Cd-driven transformation of normal prostate epithelial cells to malignant form. Understanding the molecular mechanisms involved in the Cd-driven malignant transformation of normal prostate cells will provide a significant insight to develop better therapeutic strategies for Cd-induced prostate cancer.

Epithelial-mesenchymal transition in breast epithelial cells treated with cadmium and the role of Snail

Epidemiological and experimental studies have implicated cadmium (Cd) with breast cancer. In breast epithelial MCF10A and MDA-MB-231 cells, Cd has been shown to promote cell growth. The present study examined whether Cd also promotes epithelial-mesenchymal transition (EMT), a hallmark of cancer progression. Human breast epithelial cells consisting of non-cancerous MCF10A, non-metastatic HCC 1937 and HCC 38, and metastatic MDA-MB-231 were treated with 1 or 3 μM Cd for 4 weeks. The MCF10A epithelial cells switched to a more mesenchymal-like morphology, which was accompanied by a decrease in the epithelial marker E-cadherin and an increase in the mesenchymal markers N-cadherin and vimentin. In both non-metastatic HCC 1937 and HCC 38 cells, treatment with Cd decreased the epithelial marker claudin-1. In addition, E-cadherin also decreased in the HCC 1937 cells. Even the mesenchymal-like MDA-MB-231 cells exhibited an increase in the mesenchymal marker vimentin. These changes indicated that prolonged treatment with Cd resulted in EMT in both normal and cancer-derived breast epithelial cells. Furthermore, both the MCF10A and MDA-MB-231 cells labeled with Zcad, a dual sensor for tracking EMT, demonstrated a decrease in the epithelial marker E-cadherin and an increase in the mesenchymal marker ZEB-1. Treatment of cells with Cd significantly increased the level of Snail, a transcription factor involved in the regulation of EMT. However, the Cd-induced Snail expression was completely abolished by actinomycin D. Luciferase reporter assay indicated that the expression of Snail was regulated by Cd at the promotor level. Snail was essential for Cd-induced promotion of EMT in the MDA-MB-231 cells, as knockdown of Snail expression blocked Cd-induced cell migration. Together, these results indicate that Cd promotes EMT in breast epithelial cells and does so by modulating the transcription of Snail.

Cadmium-induced malignant transformation of rat liver cells: Potential key role and regulatory mechanism of altered apolipoprotein E expression in enhanced invasiveness

Cadmium is a transition metal that is classified as human carcinogen by the International Agency for Research on Cancer (IARC) with multiple target sites. Many studies using various model systems provide evidence of cadmium-induced malignancy formation in vivo or malignant cell transformation in vitro. Nonetheless, further studies are needed to completely understand the mechanisms of cadmium carcinogenicity. Our prior studies have utilized a rat liver epithelial cell line (TRL 1215) as a model for cadmium-induced malignant transformation. In the present study, we focused on the molecular mechanisms of this malignant transformation, especially with regard to hyper-invasiveness stimulated by cadmium transformation. By performing a series of biochemical analyses on cadmium transformed cells, it was determined that cadmium had significantly down-regulated the expression of apolipoprotein E (ApoE). ApoE was recently established as a suppressor of cell invasion. A key factor in the suppression of ApoE by cadmium appeared to be that the metal evoked a 5-aza-2'-deoxycytidine-sensitive hypermethylation of the regulatory region of ApoE, coupled with interference of the action of liver X receptor α (LXRα), a transcriptional regulator for ApoE. Furthermore, the expression of LXRα itself was suppressed by cadmium-mediated epigenetic modification. Re-expression of ApoE clearly abrogated the cell invasion stimulated by cadmium-induced malignant transformation. Together, the current results suggest that the cadmium-mediated enhanced cell invasion is linked to down-regulation of ApoE during malignant transformation these liver cells.

Non‑infective occupational risk factors for hepatocellular carcinoma: A review (Review)

Liver cancer is the second leading worldwide cause of cancer‑associated mortalities. Hepatocellular carcinoma, which accounts for the majority of liver tumors, ranks fifth among types of human cancer. Well‑established risk factors for liver cancer include the hepatitis B and C viruses, aflatoxins, alcohol consumption, and oral contraceptives. Tobacco smoking, androgenic steroids, and diabetes mellitus are suspected risk factors. Current knowledge regarding non‑infective occupational risk factors for liver cancer is inconclusive. The relevance of liver disorders to occupational medicine lies in the fact that the majority of chemicals are metabolized in the liver, and toxic metabolites generated via metabolism are the predominant cause of liver damage. However, their non‑specific clinical manifestations that are similar in a number of liver diseases make diagnosis difficult. Furthermore, concomitant conditions, such as viral hepatitis and alcohol or drug abuse, may mask liver disorders that result from occupational hepatotoxic agents and block the demonstration of an occupational cause. The identification of environmental agents that result in human cancer is a long and often difficult process. The purpose of the present review is to summarize current knowledge regarding the association of non‑infective occupational risk exposure and HCC, to encourage further research and draw attention to this global occupational public health problem.

Cadmium exposure and cancer mortality in a prospective cohort: the strong heart study

BACKGROUND

Cadmium (Cd) is a toxic metal classified as a human carcinogen by the International Agency for Research on Cancer.

OBJECTIVE

We evaluated the association of long-term Cd exposure, as measured in urine, with cancer mortality in American Indians from Arizona, Oklahoma, and North and South Dakota who participated in the Strong Heart Study during 1989-1991.

METHODS

The Strong Heart Study was a prospective cohort study of 3,792 men and women 45-74 years of age who were followed for up to 20 years. Baseline urinary Cd (U-Cd) was measured using inductively coupled plasma mass spectrometry. We assessed cancer events by annual mortality surveillance.

RESULTS

The median (interquintile range) U-Cd concentration was 0.93 (0.55, 1.63) μg/g creatinine. After adjusting for sex, age, smoking status, cigarette pack-years, and body mass index, the adjusted hazard ratios (HRs) comparing the 80th versus the 20th percentiles of U-Cd were 1.30 (95% CI: 1.09, 1.55) for total cancer, 2.27 (95% CI: 1.58, 3.27) for lung cancer, and 2.40 (95% CI: 1.39, 4.17) for pancreatic cancer mortality. For all smoking-related cancers combined, the corresponding HR was 1.56 (95% CI: 1.24, 1.96). Cd was not significantly associated with liver, esophagus and stomach, colon and rectum, breast, prostate, kidney, or lymphatic and hematopoietic cancer mortality. On the basis of mediation analysis, we estimated that the percentage of lung cancer deaths due to tobacco smoking that could be attributed to Cd exposure was 9.0% (95% CI: 2.8, 21.8).

CONCLUSIONS

Low-to-moderate Cd exposure was prospectively associated with total cancer mortality and with mortality from cancers of the lung and pancreas. The implementation of population-based preventive measures to decrease Cd exposure could contribute to reducing the burden of cancer.

Metallothionein blocks oxidative DNA damage in vitro

The role of metallothionein (MT) in mitigation of oxidative DNA damage (ODD) induced by either cadmium (Cd) or the direct oxidant hydrogen peroxide (H(2)O(2)) was systematically examined using MT-I/II double knockout (MT-null) or MT-competent wild-type (WT) cells. Both toxicants were much more lethal to MT-null cells (Cd LC(50) = 6.6 μM; H(2)O(2) LC(50) = 550 μM) than to WT cells (Cd LC(50) = 16.5 μM; H(2)O(2) LC(50) = 930 μM). Cd induced concentration-related MT increases in WT cells, while the basal levels were undetectable and not increased by Cd in MT-null cells. ODD, measured by the immuno-spin trapping method, was minimally induced by sub-toxic Cd levels (1 or 5 μM; 24 h) in WT cells, but markedly increased in MT-null cells (>430 %). Similarly, ODD was induced to higher levels by lower concentrations of H(2)O(2) in MT-null cells than WT cells. Transfection of MT-I into MT-null cells reduced both Cd- and H(2)O(2)-induced cytolethality and ODD. Cd increased the expression of the oxidant defense genes, HO-1, and GSTa2 to a much greater extent in MT-null cells than in WT. Cd or H(2)O(2) exposure increased the expression of key transport genes, Mrp1 and Mrp2, in WT cells but not in MT-null cells. MT protects against Cd- and H(2)O(2)-induced ODD in MT-competent cells possibly by multiple mechanisms, potentially including direct metal ion sequestration and sequestration of oxidant radicals by MT. MT-deficient cells appear to adapt to Cd primarily by turning on oxidant response systems, while MT-competent cells activate MT and transport systems.

Emerging roles of cadmium and heme oxygenase in type-2 diabetes and cancer susceptibility

Many decades after an outbreak of severe cadmium poisoning, known as Itai-itai disease, cadmium continues to pose a significant threat to human health worldwide. This review provides an update on the effects of this environmental toxicant cadmium, observed in numerous populations despite modest exposure levels. In addition, it describes the current knowledge on the link between heme catabolism and glycolysis. It examines novel functions of heme oxygenase-2 (HO-2) that protect against type 2-diabetes and obesity, which have emerged from diabetic/obese phenotypes of the HO-2 knockout mouse model. Increased cancer susceptibility in type-2 diabetes has been noted in several large cohorts. This is a cause for concern, given the high prevalence of type-2 diabetes worldwide. A lifetime exposure to cadmium is associated with pre-diabetes, diabetes, and overall cancer mortality with sex-related differences in specific types of cancer. Liver and kidney are target organs for the toxic effects of cadmium. These two organs are central to the maintenance of blood glucose levels. Further, inhibition of gluconeogenesis is a known effect of heme, while cadmium has the propensity to alter heme catabolism. This raises the possibility that cadmium may mimic certain HO-2 deficiency conditions, resulting in diabetic symptoms. Intriguingly, evidence has emerged from a recent study to suggest the potential interaction and co-regulation of HO-2 with the key regulator of glycolysis: 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4). HO-2 could thus be critical to a metabolic switch to cancer-prone cells because the enzyme PFKFB and glycolysis are metabolic requirements for cell proliferation and resistance to apoptosis.

Cancer mortality in a Chinese population surrounding a multi-metal sulphide mine in Guangdong province: an ecologic study

BACKGROUND

The Dabaoshan mine in the southeast of Guangdong Province, China, is at high risk of multi-metal pollutant discharge into a local river (Hengshihe) and the surrounding area. Following approximately 30 years of exposure to these metals, little is known regarding the subsequent health effects and risks for the local residents. In our present study, we have estimated the relationships between long-term environmental exposure to multiple heavy metals and the risk of cancer mortality in a Chinese population in the vicinity of Dabaoshan.

METHODS

An ecologic study was performed. Between 2000-2007, a total population of 194,131 lived in the nine agricultural villages that surround the Hengshihe area. Heavy metals concentrations were determined in local environmental samples (water and crops) and whole blood taken from 1152 local residents of both a high-exposure area (HEA) and a low-exposure area (LEA). We calculated the rate ratio and standardized mortality ratios based on age- and gender-specific cancer mortality rates for the different reference populations (based on district, county and province). Simple, multiple linear and ridge regression models were used to evaluate the associations between exposure to multiple heavy metals and cancer mortality in the nine villages, after adjustment for age and sex.

RESULTS

The geometric mean blood levels of cadmium and lead were measured at 24.10 μg/L and 38.91 μg/dL for subjects (n = 563) in the HEA and 1.87 μg/L and 4.46 μg/dL for subjects (n = 589) from the LEA, respectively (P < 0.001). The rate of mortality from all cancers in the HEA was substantially elevated in comparison with the corresponding mortality rate in the LEA for men (rate ratio = 2.13; 95% confidence intervals = 1.63 - 2.77) and women (2.83; 1.91 - 4.19); rates were also significantly elevated compared with the rate when compared to the entire Wengyuan County area, or the provincial reference population. In addition, mortality rates were significantly increased for stomach, lung and esophageal cancer in the HEA in comparison with the corresponding rates in the LEA, in Wengyuan County and the provincial reference population for men, women and both combined. Further analysis showed that there were significantly positive correlations between exposure to cadmium and lead and the risk of all-cancers and stomach cancer mortality among women and both sexes, whilst zinc exposure showed no association with the risk of site-specific cancer mortality in the nine villages evaluated.

CONCLUSIONS

The findings of this study reveal probable associations between long-term environmental exposure to both cadmium and lead and an increased risk of mortality from all cancer, as well as from stomach, esophageal and lung-cancers.

Quantitative classification of DNA damages induced by submicromolar cadmium using oligonucleotide chip coupled with lesion-specific endonuclease digestion

Implementation of proper analytical tool for systematic investigation and quantitative determination of different classes of cadmium ion-induced DNA damages, especially at low metal ion concentrations, is still lacking. Using lesion-specific enzymes that cleave DNA at specific classes of damage and a fluorometric approach developed for quantifying fluorophore-labeled oligonucleotides bound to chip surfaces, we determined the frequencies of different lesions (strand breaks, oxidized purines, oxidized pyrimidines, or abasic sites) induced by submicromolar Cd(2+). Cd(2+)-treated oligonucleotide chips were digested with various endonucleases (Fpg protein, endonuclease III, endonuclease IV), producing a de novo single strand break (SSB) at their substrate modifications. The frequency of SSB and double strand break (DSB) was computed from the difference of pre- and post-Cd(2+)-treatment oligonucleotide coverage on the chip. While the frequency of SSBs and oxidized bases were successfully quantified even at 0.5 μM of Cd(2+), DSB frequency could be easily quantitated at 8.7 μM [Cd(2+)]. The numbers of abasic sites were below the oligonucleotide detection limit (2.4 amole; equivalent to 0.24 fM for a reaction volume of 100 μL). SSBs were found to constitute about 85-90% of single strand damages, while oxidized bases comprise only 4-7% of the total at 0.9 to 8.7 μM [Cd(2+)].

Cadmium-induced cancers in animals and in humans

Discovered in the early 1800s, the use of cadmium and various cadmium salts started to become industrially important near the close of the 19th century, rapidly thereafter began to flourish, yet has diminished more recently. Most cadmium used in the United States is a byproduct from the smelting of zinc, lead, or copper ores, and is used to manufacture batteries. Carcinogenic activity of cadmium was discovered first in animals and only subsequently in humans. Cadmium and cadmium compounds have been classified as known human carcinogens by the International Agency for Research on Cancer and the National Toxicology Program based on epidemiologic studies showing a causal association with lung cancer, and possibly prostate cancer, and studies in experimental animals, demonstrating that cadmium causes tumors at multiple tissue sites, by various routes of exposure, and in several species and strains. Epidemiologic studies published since these evaluations suggest that cadmium is also associated with cancers of the breast, kidney, pancreas, and urinary bladder. The basic metal cationic portion of cadmium is responsible for both toxic and carcinogenic activity, and the mechanism of carcinogenicity appears to be multifactorial. Available information about the carcinogenicity of cadmium and cadmium compounds is reviewed, evaluated, and discussed.

Integrating time-course microarray gene expression profiles with cytotoxicity for identification of biomarkers in primary rat hepatocytes exposed to cadmium

MOTIVATION

DNA microarrays can provide information about the expression levels of thousands of genes simultaneously at the transcriptomic level, while conventional cell viability and cytotoxicity measurement methods provide information about the biological functions at the cellular level. Integrating these data at different levels provides a promising approach for evaluating or predicting how cells respond to chemical exposure. It is important to investigate the multi-scale biological system in a systematic way to better understand the gene regulation networks and signal transduction pathways involved in the cellular responses to environmental factors.

RESULTS

Primary rat hepatocytes were exposed to cadmium acetate at 0, 1.25 and 2 microM. mRNA expression profiles at 0, 3, 6, 12 and 24 h were measured using the Affymetrix RatTox U34 GeneChip arrays. Simultaneously, cytotoxicity was assessed by lactase dehydrogenase leakage assay. Gene expression profiles at different time points were used to evaluate cytotoxicity at subsequent time points using partial least squares, and it was found that gene expression profiles at 0 h had the best prediction accuracy for the cytotoxicity observed at 12 h. Some biomarkers whose expression profiles showed strong relationship with cytotoxicity were identified and the underlying pathways were reconstructed to illustrate how hepatocytes respond to cadmium exposure. Permutation studies were also applied to assess the reliability of the predictive models.

AVAILABILITY

Matlab source code is available upon request and DNA microarray data are available at GEO (http://www.ncbi.nlm.nih.gov/geo).

Cadmium-induced malignant transformation in rat liver cells: Role of aberrant oncogene expression and minimal role of oxidative stress

Our study examined the role of oxidative stress and aberrant gene expression in malignant transformation induced by chronic, low-level cadmium exposure in non-tumorigenic rat liver epithelial cell line, TRL 1215. Cells were cultured in 1.0 microM cadmium (as CdCl(2)) for up to 28 weeks and compared to passage-matched control cells. The level of cadmium used for transformation produced no evidence of increased superoxide (O(2) (-*.)) or hydrogen peroxide (H(2)O(2)) levels in the early stages of exposure (</=24 hr). The chronic cadmium exposed liver epithelial cells (CCE-LE) were hyperproliferative with a growth rate about 3-fold higher than control cells. CCE-LE cells produced highly aggressive tumors upon inoculation into mice confirming malignant transformation. Analysis of cellular reactive oxygen species (ROS) showed that CCE-LE cells possessed markedly lower basal levels of intracellular O(2) (-*.)and H(2)O(2) and were very tolerant to high-dose (50 microM) cadmium-induced ROS. Time course studies showed the production of ROS by high-dose cadmium was abolished well in advance of malignant transformation. In contrast, marked overexpression of the oncogenes c-myc and c-jun occurred in transformed CCE-LE cells as evidenced by up to 10-fold increases in both transcript and protein. A significant increase in DNA-binding activity of the transcription factors AP-1 and NF-kappaB occurred in CCE-LE cells. Increases in oncogene expression and transcription factor activity occurred concurrently with malignant transformation. Thus, cadmium-induced ROS occurs as an early, high-dose event but is abolished well in advance of malignant transformation. Low-level chronic cadmium triggers oncogene overexpression possibly by altering critical transcription factor activity. Such changes in cellular gene expression likely culminate in the loss of growth control and cadmium-induced neoplastic transformation in CCE-LE cells, whereas generation of ROS by cadmium seemed to play a minimal role in this transformation.

Effects of cadmium on DNA-(Cytosine-5) methyltransferase activity and DNA methylation status during cadmium-induced cellular transformation

Cadmium is a human carcinogen that likely acts via epigenetic mechanisms. Since DNA methylation alterations represent an important epigenetic event linked to cancer, the effect of cadmium on DNA methyltransferase (MeTase) activity was examined using in vitro (TRL1215 rat liver cells) and ex vivo (M.SssI DNA MeTase) systems. Cadmium effectively inhibited DNA MeTases in a manner that was noncompetitive with respect to substrate (DNA), indicating an interaction with the DNA binding domain rather than the active site. Based on these results, the effects of prolonged cadmium exposure on DNA MeTase and genomic DNA methylation in TRL1215 cells were studied. After 1 week of exposure to 0-2.5 microM cadmium, DNA MeTase activity was reduced (up to 40%) in a concentration-dependent fashion, while genomic DNA methylation showed slight but significant reductions at the two highest concentrations. After 10 weeks of exposure, the cells exhibited indications of transformation, including hyperproliferation, increased invasiveness, and decreased serum dependence. Unexpectedly, these cadmium-transformed cells exhibited significant increases in DNA methylation and DNA MeTase activity. These results indicate that, while cadmium is an effective inhibitor of DNA MeTase and initially induces DNA hypomethylation, prolonged exposure results in DNA hypermethylation and enhanced DNA MeTase activity.

Evaluation of cadmium-induced transcriptome alterations by three color cDNA labeling microarray analysis on a T-cell line

Beside heavy metals, cadmium (Cd(2+)) is a ubiquitous toxic metal with a well established apoptotic and genotoxic effect, chronic exposure of which has been involved in a variety of pathological conditions. In the present study, we investigated by 1455 genes cDNA microarrays the toxic and apoptotic effect of Cd(2+), on the T-cell line CCRF-CEM, applying a three laser differential analysis, on the same microarray slide. The cells were cultured for 6 and 24 h in the absence (control) or presence of Cd(2+) (10 or 20 microM), RNAs were extracted and the produced cDNAs were labeled with rhodamine derivatives fluorescent dyes. A microarray slide was simultaneously hybridized by the labeled cDNAs and analyzed. We found that, in relation to control, treatment of the cells for 6 h with 10 and 20 microM Cd(2+), induces up-regulation in 20 and 34 genes, respectively. Treatment for 24 h with 10 and 20 microM Cd(2+) induces up-regulation in 22 and 84 genes, respectively. Twenty-eight genes were found down-regulated only after treatment for 24 h with Cd(2+) 10 microM. These data suggest that Cd(2+) produces a time- and dose-dependent molecular cascade, induces disturbances in different subcellular compartments, influencing thereafter the normal cellular functions, the differentiation process, the malignant transformation and the cell death.

Studies on the interaction between Cd(2+) ions and DNA

Cadmium is a potent carcinogen in rodents and has recently been accepted by the International Agency for Research on Cancer as a category 1 (human) carcinogen, but the molecular mechanism of its action remains largely unclear. It has however been suggested that cadmium-induced carcinogenesis may involve either direct or indirect interaction of Cd(2+) with DNA. Cd(2+) is believed to bind covalently with N7 centres of adenine and guanine. At low concentrations (< or =50 mM), Cd(2+) is found to react with plasmid DNA to produce a mixture of Form I and Form II bands whereas at higher concentrations (> or =100 mM), Cd(2+) causes extensive damage to DNA at a pH 5.8 solution of cadmium nitrate. Within the range 0-100 mM (when pH is adjusted to 7.4 by adding NaOH) an increase in concentration of Cd(2+) is found to cause a decrease in the gel mobility rate of plasmid and an increase in the intensity of the Form II band. When plasmid DNA is digested with BamH1, only the Form III band is observed both in the presence and absence of Cd(2+). However, the mobility of the band is found to decrease with the increase in the concentration of Cd(2+). When the enzyme Ssp1 which cuts plasmid DNA at the AT sites is used instead of BamH1, two bands are observed in the presence of cadmium as against one band in the absence of cadmium. These results suggest that Cd(2+) binds covalently with DNA (possibly at G, A and T centres) and can form intrastrand bifunctional AT adducts but not the GG adducts. It may also be that neither GG nor AT adducts are formed and yet Ssp1 digestion is prevented because of a structural modification introduced in adenine by its interaction with Cd(2+). In the presence of antioxidants such as cysteine, glutathione and ascorbate (especially cysteine and ascorbate), DNA damage is found to be greater than expected for the combined effects of the antioxidant and Cd(2+). The increased DNA damage is believed to be due to the formation of reactive oxygen species (ROS).

Studies on the interaction between Cd(2+) ions and nucleobases and nucleotides

Cadmium is a potent carcinogen in rodents and has recently been accepted by the International Agency for Research on Cancer as a category 1 (human) carcinogen, but the molecular mechanism of its action remains largely unclear. It has however been suggested that cadmium-induced carcinogenesis may involve either direct or indirect interaction of Cd(2+) with DNA. In this study it is found that when Cd(2+) is allowed to interact with adenine and guanine, there is a marked change in the high performance liquid chromatography (HPLC) retention time for adenine but not for guanine. Since Cd(2+) is believed to bind covalently to adenine and guanine, the changes in retention time but absence of any cadmium in the peak fraction point to the following: (i) lability of cadmium-nucleobase adducts, and (ii) introduction of some kind of chemical modification in adenine but not in guanine as a result of covalent binding. This result is different from that for Ni(2+) in which case a change in retention time was observed for guanine but not for adenine.

Alteration of cadmium-induced mutational spectrum by catalase depletion in Chinese hamster ovary-K1 cells

Previously, we have demonstrated that cadmium acetate significantly induces hprt mutation frequency in Chinese hamster ovary (CHO)-K1 and that 3-amino-1,2,4-triazole (3AT), a catalase inhibitor, potentiates the mutagenicity of cadmium [Chem. Res. Toxicol. 9 (1996) 1360-1367]. In this study, we investigate the role of intracellular peroxide in the molecular nature of mutations induced by cadmium. Using 2',7'-dichlorofluorescin diacetate and fluorescence spectrophotometry, we have shown that cadmium dose-dependently increased the amounts of intracellular peroxide and the levels were significantly enhanced by 3AT. Furthermore, we have characterized and compared the hprt mutation spectra in 6-thioguanine-resistant mutants derived from CHO-K1 cells exposed to 4 microM of cadmium acetate for 4h in the absence and presence of 3AT. The mutation frequency induced by cadmium and cadmium plus 3AT was 11- and 16-fold higher than that observed in untreated populations (2.2 x 10(-6)), respectively. A total of 40 and 51 independent hprt mutants were isolated from cadmium and cadmium plus 3AT treatments for mRNA-polymerase chain reaction (PCR), genomic DNA-PCR and DNA sequencing analyses. 3AT co-administration significantly enhanced the frequency of deletions induced by cadmium. Cadmium induced more transversions than transitions. In contrast, 3AT co-administration increased the frequency of GC-->AT transitions and decreased the frequencies of TA-->AT and TA-->GC transversions. Together, the results suggest that intracellular catalase is important to prevent the formation of oxidative DNA damage as well as deletions and GC-->AT transitions upon cadmium exposure.

Opposite roles of ERK and p38 mitogen-activated protein kinases in cadmium-induced genotoxicity and mitotic arrest

The roles of extracellular signal-regulated kinase (ERK) and p38 mitogen-activation protein kinase (MAPK) in guarding genome stability and regulating cell cycle progression were explored in CL3 human lung adenocarcinoma cells treated with cadmium (Cd), a human carcinogen. Exposing asynchronous cells to CdCl(2) for 2 h (45% viability) caused irreversible mitotic arrest. Exposing early-G(2) cells to Cd markedly delayed mitotic exit and subsequently induced sub-G(1) populations; however, this did not alter the levels of Cdc2 and cyclin B1. These results suggest that Cd elicits mitotic arrest without affecting the progression of G(2) to mitosis. Using counterflow centrifugal elutriation and flow cytometry analysis, CL3 cells synchronized at G(1)-, S-, and G(2)/M-phases were collected and treated with CdCl(2). G(2)/M was the most sensitive cell cycle phase to Cd for the induction of ERK and p38 MAPK activities, cytotoxicity, apoptosis, micronucleus, and intracellular peroxide; despite that similar Cd accumulation was observed in G(1)-, S-, and G(2)/M-cells. Co-treatment early-G(2) cells with Cd and SB202190, an inhibitor of p38 MAPK, significantly decreased the induction of micronucleus, mitotic arrest, and apoptosis. Conversely, PD98059, an inhibitor of the ERK upstream activators MKK1/2, enhanced micronucleus and apoptosis in Cd-treated early-G(2) cells. Together, the results suggest that intracellular peroxide may participate in the activation of ERK and p38 MAPK by Cd; also, the activated-p38 MAPK may contribute to mitotic arrest and genome instability, whereas the activated-ERK may help to maintain genome integrity and survival.

Cadmium carcinogenesis in review

Cadmium is an inorganic toxicant of great environmental and occupational concern which was classified as a human carcinogen in 1993. Occupational cadmium exposure is associated with lung cancer in humans. Cadmium exposure has also, on occasion, been linked to human prostate cancer. The epidemiological data linking cadmium and pulmonary cancer are much stronger than for prostatic cancer. Other target sites for cadmium carcinogenesis in humans (liver, kidney, stomach) are considered equivocal. In rodents, cadmium causes tumors at several sites and by various routes. Cadmium inhalation in rats results in pulmonary adenocarcinomas, supporting a role in human lung cancer. Prostate tumors and preneoplastic proliferative lesions can be induced in rats after cadmium ingestion or injection. Prostatic carcinogenesis in rats occurs only at cadmium doses below those that induce chronic degeneration and dysfunction of the testes, a well-known effect of cadmium, confirming the androgen dependency of prostate tumors. Other targets of cadmium in rodents include the testes, adrenals, injection sites, and hematopoietic system. Various treatments can modify cadmium carcinogenesis including supplemental zinc, which prevents cadmium-induced injection site and testicular tumors while facilitating prostatic tumors. Cadmium is poorly mutagenic and probably acts through indirect mechanisms, although the precise mechanisms remain unknown.

Inhibition of antimutagenic enzymes, 8-oxo-dGTPases, by carcinogenic metals. Recent developments

Nickel, cadmium, cobalt, and copper are carcinogenic to humans and/or animals, but the underlying mechanisms are poorly understood. Our studies have been focused on one such mechanism involving mediation by the metals of promutagenic oxidative damage to DNA bases. The damage may be inflicted directly in DNA or in the deoxynucleotide pool, from which the damaged bases are incorporated into DNA. Such incorporation is prevented in cells by 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphatases (8-oxo-dGTPases). Thus, inhibition of these enzymes should enhance carcinogenesis. We have studied effects of Cd(II), Cu(II), Co(II), and Ni(II) on the activity of isolated bacterial and human 8-oxo-dGTPases. Cd(II) and Cu(II) were strongly inhibitory, while Ni(II) and Co(II) were much less suppressive. After developing an assay for 8-oxo-dGTPase activity, we confirmed the inhibition by Cd(II) in cultured cells and in the rat testis, the target organ for cadmium carcinogenesis. 8-Oxo-dGTPase inhibition was accompanied by an increase in the 8-oxo-dG level in testicular DNA.

Expression of metallothionein protein in the lungs of Wistar rats and C57 and DBA mice exposed to cadmium oxide fumes

Chronic exposure to inhaled cadmium (Cd) has been shown to induce lung tumors in rats (Wistar strain) but not in mice (NMRI strain). The protein metallothionein (MT) plays an important role in Cd detoxification, and it has been suggested that differential inducibility of pulmonary MT may lead to interspecies susceptibility differences to inhaled Cd. Interstrain differences in the pulmonary response of the MT gene to Cd stimuli have not been examined in rats or mice. We compared pulmonary MT expression in Wistar Furth (WF) rats with that in DBA and C57 mice, following a single 3-h exposure to CdO fumes containing 1 mg Cd/m3. Induction of the MT gene was assessed by the levels of MT-I and MT-II transcripts, MT-protein content, and number of MT-labeled alveolar and bronchiolar epithelial cells immediately after Cd exposure and 1, 3, and 5 days later. Control animals were exposed to air/argon furnace gases. We observed differential intra- and interspecies inducibility of the MT gene in the lung following Cd inhalation. DBA mice exhibited greater levels of MT-mRNA, mainly for the MT-I isoform, MT-protein content, and number of MT positive cells relative to C57 mice. WF rats showed lower transcription and translation responses of the MT gene upon Cd stimuli than C57 mice. The present results, in concert with our previous findings of higher lung cell proliferation in Cd-exposed C57 relative to DBA mice, predict greater susceptibility of C57 to the carcinogenic effects of inhaled Cd. Furthermore, the low transcriptional and translation responses of the MT gene to Cd stimuli in WF rats might explain the higher susceptibility of this rat strain to develop malignant lung tumors after chronic exposure to Cd via inhalation. Parallel to our findings in mice, differences in the responsiveness of lung MT gene may exist across rat strains. Thus intraspecies genetic variability in pulmonary MT may influence the susceptibility of rats or mice to lung carcinogenesis induced by inhalation of Cd compounds.

Comparison of inflammatory lung responses in Wistar rats and C57 and DBA mice following acute exposure to cadmium oxide fumes

Inhalation of cadmium oxide (CdO) is a significant form of human exposure to cadmium (Cd). Furthermore, there is epidemiological and experimental data relating Cd inhalation with lung cancer. Animal studies indicate that rats are more susceptible to Cd-induced lung cancer than mice, but interstrain sensitivity differences to Cd-induced pulmonary inflammation or carcinogenesis have not been addressed in either species. We compared pulmonary inflammatory processes in Wistar Furth (WF) rats with those in C57 and DBA mice exposed to freshly generated CdO fumes in nose-only inhalation chambers. Animals were exposed to 1 mg Cd/m3 for 3 hr and terminated immediately or 1, 3, and 5 days after exposure. Control animals were exposed to air/argon furnace gases. Cd-induced lung injury was assessed by bronchoalveolar lavage fluid (BALF) analyses, histopathology, and immunohistochemical detection of cell proliferation. Inhalation of CdO resulted in pulmonary inflammatory processes that varied widely across species and strains. C57 mice responded with faster and greater influx of neutrophils and proliferation of alveolar macrophages, type II epithelial cells, and bronchiolar epithelial cells compared to DBA mice or WF rats. DBA mice retained a greater percentage of inhaled Cd in the lungs and presented higher levels of BALF protein than C57 mice or rats. In comparison to mice, WF rats responded with a more transient inflammatory response in BALF parameters and higher degree of acute inflammation in lung tissue. The more pronounced proliferation of alveolar and bronchiolar epithelial cells observed in C57 mice might indicate higher susceptibility of this mice strain to Cd-induced lung carcinogenesis compared to DBA mice or WF rats. Furthermore, the present results of fewer inflammatory cells and lower proliferation of epithelial cells in DBA mice in association with our previous observation of higher Cd-induced metallothionein protein in this strain suggest that DBA might be less susceptible to the pulmonary carcinogenic effects of inhaled Cd than C57 mice or WF rats. We conclude that mice might not necessarily be more resistant than rats to the carcinogenic effects of inhaled Cd, since intraspecies susceptibility differences are strongly suggested by the present data. An extrapolation of this conclusion is that genetic variations in the human population may determine individual sensitivity differences to inhaled Cd.

Testosterone pretreatment mitigates cadmium toxicity in male C57 mice but not in C3H mice

Previous work has indicated that testosterone pretreatment protects against cadmium-induced toxicity in male rats while other data indicate that pretreatment of mice with testosterone offers no such protection against cadmium. Since cadmium toxicity may vary widely with species and strain, we examined the effect of testosterone pretreatment on cadmium toxicity in two strains of mice, one that is sensitive (C3H) and one that is resistant (C57) to cadmium toxicity. A single sc injection of 20 micromol CdCl2/kg to C3H mice or 45 micromol CdCl2/kg to C57 mice proved very toxic, causing 50%, and 44% mortalities, respectively. However, when C57 mice were pretreated with testosterone (5 mg/kg, s.c., at - 48, - 24, and 0 h) prior to cadmium (45 micromol/kg), complete resistance to cadmium-induced lethality developed. Testosterone had no effect on cadmium-induced lethality in C3H mice. Testosterone prevented extensive hepatocellular damage caused by cadmium in C57 mice and also significantly reduced cadmium-induced elevations in serum lactate dehydrogenase (LDH) activity and blood urea nitrogen (BUN), which are indicators of hepatic and renal function, respectively. The toxicokinetics of cadmium were apparently not affected by testosterone pretreatment, as the distribution of cadmium to liver in either strain was unchanged by the steroid. Cadmium-induced metallothionein (MT) levels in liver and kidney of C57 mice were increased in testosterone-pretreated mice given the higher doses of metal but no such enhancement of MT synthesis occurred in C3H mice. This increase in MT may provide some level of protection against cadmium toxicity in the C57 mice. These results indicate that testosterone pretreatment prevents toxicity of cadmium in male C57 mice, possibly through enhancement of MT synthesis, but has no effect in male C3H mice.

Reactive oxygen species may participate in the mutagenicity and mutational spectrum of cadmium in Chinese hamster ovary-K1 cells

The molecular nature of mutations induced by Cd was investigated in this study to elucidate the role of Cd in the initiation of carcinogenesis. Exposing Chinese hamster ovary (CHO)-K1 cells to cadmium acetate markedly decreased the colony-forming ability of cells and induced mutation frequency in the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene. The mutation frequency induced by Cd at LD30-LD20 doses was approximately 20 times that of untreated cells. D-Mannitol, a scavenger of reactive oxygen species (ROS), significantly protects cells against Cd cytotoxicity and mutagenicity. Furthermore, non-cytotoxic doses of 3-amino-1,2,4-triazole, a catalase inhibitor, potentiates Cd cytotoxicity and mutagenicity. The cellular Cd uptake ability was not altered by the combined treatment with either D-mannitol or 3-amino-1,2,4-triazole. The GSH level and the activities of GSH peroxidase, GSSG reductase, and catalase in cells treated with Cd (4 microM, 4 h) decreased to 78%, 47%, 40%, and 22% of the untreated cells, respectively. Those enzymatic activities recovered to normal levels 8 h after removing Cd. Polymerase chain reaction and DNA sequencing analysis of 54 independent Cd mutants revealed Cd-induced base substitutions, splice mutations, and large genomic deletions. All six types of base substitutions were observed; however, base transversions (22/27; 81%) occurred more frequently than transitions (5/27; 19%). The frequencies of mutations occurring at T.A or G.C base pairs were roughly equal. Results in this study strongly suggest that Cd mutagenicity in CHO-K1 cells is ROS-dependent. Moreover, the unique mutational spectrum induced by Cd implies that specific DNA adducts generated through the interaction of Cd-DNA and ROS may play a role in the mutational specificity.