Effects of arsenic on telomerase and telomeres in relation to cell proliferation and apoptosis in human keratinocytes and leukemia cells in vitro

Growth, bioluminescence and shoal behavior hormetic responses to inorganic and/or organic chemicals: a review

A biphasic dose response, termed hormesis, is characterized by beneficial effects of a chemical at a low dose and harmful effects at a high dose. This biphasic dose response phenomenon has the potential to strongly alter toxicology in a broad range. The present review focuses on the progress of research into hormetic responses in terms of growth (in plants, birds, algae and humans), bioluminescence, and shoal behavior as end points. The paper describes how both inorganic and organic chemicals at a low dose show stimulatory responses while at higher doses are inhibitory. The article highlights how factors such as symbiosis, density-dependent factors, time, and contrasting environmental factors (availability of nutrients, temperature, light, etc.) affect both the range and amplitude of hormetic responses. Furthermore, the possible underlying mechanisms are also discussed and we suggest that, for every end point, different hormetic mechanisms may exist. The occurrences of varying interacting receptor systems or receptor systems affecting the assessment of hormesis for each endpoint are discussed. The present review suggests that a hormetic model should be adopted for toxicological evaluations instead of the older threshold and linear non-threshold models.

Mitochondrial hormesis links low-dose arsenite exposure to lifespan extension

Arsenite is one of the most toxic chemical substances known and is assumed to exert detrimental effects on viability even at lowest concentrations. By contrast and unlike higher concentrations, we here find that exposure to low-dose arsenite promotes growth of cultured mammalian cells. In the nematode C. elegans, low-dose arsenite promotes resistance against thermal and chemical stressors and extends lifespan of this metazoan, whereas higher concentrations reduce longevity. While arsenite causes a transient increase in reactive oxygen species (ROS) levels in C. elegans, co-exposure to ROS scavengers prevents the lifespan-extending capabilities of arsenite, indicating that transiently increased ROS levels act as transducers of arsenite effects on lifespan, a process known as mitohormesis. This requires two transcription factors, namely DAF-16 and SKN-1, which employ the metallothionein MTL-2 as well as the mitochondrial transporter TIN-9.1 to extend lifespan. Taken together, low-dose arsenite extends lifespan, providing evidence for nonlinear dose-response characteristics of toxin-mediated stress resistance and longevity in a multicellular organism.

Role of genomic instability in arsenic-induced carcinogenicity. A review

Exposure to chronic arsenic toxicity is associated with cancer. Although unstable genome is a characteristic feature of cancer cells, the mechanisms leading to genomic instability in arsenic-induced carcinogenesis are poorly understood. While there are excellent reviews relating to genomic instability in general, there is no comprehensive review presenting the mechanisms involved in arsenic-induced genomic instability. This review was undertaken to present the current state of research in this area and to highlight the major mechanisms that may involved in arsenic-induced genomic instability leading to cancer. Genomic instability is broadly classified into chromosomal instability (CIN), primarily associated with mitotic errors; and microsatellite instability (MIN), associated with DNA level instability. Arsenic-induced genomic instability is essentially multi-factorial in nature and involves molecular cross-talk across several cellular pathways, and is modulated by a number of endogenous and exogenous factors. Arsenic and its metabolites generate oxidative stress, which in turn induces genomic instability through DNA damage, irreversible DNA repair, telomere dysfunction, mitotic arrest and apoptosis. In addition to genetic alteration; epigenetic regulation through promoter methylation and miRNA expression alters gene expression profiling leading to genome more vulnerable and unstable towards cancer risk. Moreover, mutations or silencing of pro-apoptotic genes can lead to genomic instability by allowing survival of damaged cells that would otherwise die. Although a large body of information is now generated regarding arsenic-induced carcinogenesis; further studies exploring genome-wide association, role of environment and diet are needed for a better understanding of the arsenic-induced genomic instability.

Systems biology approaches to evaluate arsenic toxicity and carcinogenicity: an overview

Long term exposure to arsenic, either through groundwater, food stuff or occupational sources, results in a plethora of dermatological and non-dermatological health effects including multi-organ cancer and early mortality. Several epidemiological studies, across the globe have reported arsenic-induced health effects and cancerous outcomes; but the prevalence of such diseases varies depending on environmental factors (geographical location, exposure level), and genetic makeup (and variants thereof); which is further modulated by several other factors like ethnicity, age-sex, smoking status, diet, etc. It is also interesting to note that, chronic arsenic exposure to a similar extent, even among the same family members, result in wide inter-individual variations. To understand the adverse effect of this toxic metabolite on biological system (cellular targets), and to unravel the underlying molecular basis (at the level of transcript, proteome, or metabolite), a holistic, systems biology approach was taken. Due to the paradoxical nature and unavailability of any suitable animal model system; the literature review is primarily based on cell line and population based studies. Thus, here we present a comprehensive review on the systems biology approaches to explore the underlying mechanism of arsenic-induced carcinogenicity, along with our own observations and an overview of mitigation strategies and their effectiveness till date.

Arsenic exposure through drinking water is associated with longer telomeres in peripheral blood

Inorganic arsenic is a strong carcinogen, possibly by interaction with the telomere length. The aim of the study was to evaluate how chronic arsenic exposure from drinking water as well as the arsenic metabolism efficiency affect the individual telomere length and the expression of telomere-related genes. Two hundred two women with a wide range in exposure to arsenic via drinking water (3.5-200 μg/L) were recruited. Concentrations of arsenic metabolites in urine [inorganic arsenic (iAs), methylarsonic acid (MMA), and dimethylarsinic acid (DMA)] were measured. The relative telomere length in blood was measured by quantitative real-time polymerase chain reaction. Genotyping (N = 172) for eight SNPs in AS3MT and gene expression of telomere-related genes (in blood; N = 90) were performed. Urinary arsenic (sum of metabolites) was positively associated with telomere length (β = 0.65 × 10(-4), 95% CI = 0.031 × 10(-4)-1.3 × 10(-4), adjusted for age and BMI). Individuals with above median fractions of iAs and MMA showed significantly longer telomeres by increasing urinary arsenic (β = 1.0 × 10(-4), 95% CI = 0.21 × 10(-4)-1.8 × 10(-4) at high % iAs; β = 0.88 × 10(-4) 95% CI = 0.12 × 10(-4)-1.6 × 10(-4) at high % MMA) than those below the median (p = 0.80 and 0.44, respectively). Similarly, carriers of the slow and more toxic metabolizing AS3MT haplotype showed stronger positive associations between arsenic exposure and telomere length, as compared to noncarriers (interaction urinary arsenic and haplotype p = 0.025). Urinary arsenic was positively correlated with the expression of telomerase reverse transcriptase (TERT, Spearman r = 0.22, p = 0.037), but no association was found between TERT expression and telomere length. Arsenic in drinking water influences the telomere length, and this may be a mechanism for its carcinogenicity. A faster and less toxic arsenic metabolism diminishes arsenic-related telomere elongation.

Antioxidant and metabolic impairment result in DNA damage in arsenic-exposed individuals with severe dermatological manifestations in Eastern India

Arsenic is an environmental toxicant, free-radical generator, carcinogenic agent, and aging promoter. Recently, blood samples were analyzed from individuals (control- male 12, female 13; arsenic-exposed- male 16, female 14; and exposed to ≥100 μg/L As, ≥10 y) with dermatological symptoms in few affected villages in Eastern India to unravel their hematopoietic, metabolic, and antioxidant profiles. White blood cells recovered from buffy coat were used for DNA fragmentation test. Present observation suggests that significant number of individuals developed pigmentation and palmoplantar hyperkeratosis with black-brownish patch on their body and many of those developed carcinomas. Hematopoietic data show a significant increase in eosinophil and decrease in monocyte count in either sex. Though insignificant, an increase in neutrophil in female and lymphocyte count in male arsenic-exposed individuals are supported by the earlier report on sex dimorphic immune sensitization. Significant increase in serum alanine transaminase in both sexes and bilirubin only in male suggests the eventuality of hepatic disintegration. Arsenic exposure significantly decreased serum amylase in female. A significant decrease in antioxidant components like catalase, soluble thiol, and recently recognized uric acid worsened the situation by generating free radicals as observed in significant rise in malondialdehyde level, which finally increased DNA fragmentation and arsenic-associated mutagenesis and carcinogenesis. This could attribute to lowering in immune competence and related necrotic and/or apoptotic manifestations.

PCB153 reduces telomerase activity and telomere length in immortalized human skin keratinocytes (HaCaT) but not in human foreskin keratinocytes (NFK)

Polychlorinated biphenyls (PCBs), ubiquitous environmental pollutants, are characterized by long term-persistence in the environment, bioaccumulation, and biomagnification in the food chain. Exposure to PCBs may cause various diseases, affecting many cellular processes. Deregulation of the telomerase and the telomere complex leads to several biological disorders. We investigated the hypothesis that PCB153 modulates telomerase activity, telomeres and reactive oxygen species resulting in the deregulation of cell growth. Exponentially growing immortal human skin keratinocytes (HaCaT) and normal human foreskin keratinocytes (NFK) were incubated with PCB153 for 48 and 24days, respectively, and telomerase activity, telomere length, superoxide level, cell growth, and cell cycle distribution were determined. In HaCaT cells exposure to PCB153 significantly reduced telomerase activity, telomere length, cell growth and increased intracellular superoxide levels from day 6 to day 48, suggesting that superoxide may be one of the factors regulating telomerase activity, telomere length and cell growth compared to untreated control cells. Results with NFK cells showed no shortening of telomere length but reduced cell growth and increased superoxide levels in PCB153-treated cells compared to untreated controls. As expected, basal levels of telomerase activity were almost undetectable, which made a quantitative comparison of treated and control groups impossible. The significant down regulation of telomerase activity and reduction of telomere length by PCB153 in HaCaT cells suggest that any cell type with significant telomerase activity, like stem cells, may be at risk of premature telomere shortening with potential adverse health effects for the affected organism.

Cytotoxicity patterns of arsenic trioxide exposure on HaCaT keratinocytes

BACKGROUND

Arsenic is a ubiquitous environmental toxicant, and abnormalities of the skin are the most common outcomes of long-term, low-dose, chronic arsenic exposure. If the balance between keratinocyte proliferation, differentiation, and death is perturbed, pathologic changes of the epidermis may result, including psoriasis, atopic dermatitis, and certain forms of ichthyosis. Therefore, research investigations using in vitro human epidermal cells could help elucidate cellular and molecular processes in keratinocytes affected by arsenic. Data from such investigations could also provide the basis for developing cosmetic intervention for skin diseases caused by arsenic.

METHODS

The viability of HaCaT keratinocyte cultures with or without prior exposure to low-dose arsenic trioxide was compared for varying concentrations of arsenic trioxide over a time course of 14 days because in untreated control cultures, approximately 2 weeks is required to complete cell differentiation. Long-term cultures were established by culturing HaCaT cells on collagen IV, and cells were subsequently exposed to 0 parts per million (ppm), 1 ppm, 5 ppm, 7.5 ppm, 10 ppm, and 15 ppm of arsenic trioxide. The percentages of viable cells as well as DNA damage after exposure were determined on Day 2, Day 5, Day 8, and Day 14.

RESULTS

Using both statistical and visual analytics approaches for data analysis, we have observed a biphasic response at a 5 ppm dose with cell viability peaking on Day 8 in both chronic and acute exposures. Further, a low dose of 1 ppm arsenic trioxide enhanced HaCaT keratinocyte proliferation, whereas doses above 7.5 ppm inhibited growth.

CONCLUSION

The time course profiling of arsenic trioxide cytotoxicity using long-term HaCaT keratinocyte cultures presents an approach to modeling the human epidermal cellular responses to varying doses of arsenic trioxide treatment or exposure. A low dose of arsenic trioxide appears to aid cell growth but concomitantly disrupts the DNA transcription process.

Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms

Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here.

Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms

Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here.

Altered gene expression by low-dose arsenic exposure in humans and cultured cardiomyocytes: assessment by real-time PCR arrays

Chronic arsenic exposure results in higher risk of skin, lung, and bladder cancer, as well as cardiovascular disease and diabetes. The purpose of this study was to investigate the effects on expression of selected genes in the blood lymphocytes from 159 people exposed chronically to arsenic in their drinking water using a novel RT-PCR TaqMan low-density array (TLDA). We found that expression of tumor necrosis factor-α (TNF-α), which activates both inflammation and NF-κB-dependent survival pathways, was strongly associated with water and urinary arsenic levels. Expression of KCNA5, which encodes a potassium ion channel protein, was positively associated with water and toe nail arsenic levels. Expression of 2 and 11 genes were positively associated with nail and urinary arsenic, respectively. Because arsenic exposure has been reported to be associated with long QT intervals and vascular disease in humans, we also used this TLDA for analysis of gene expression in human cardiomyocytes exposed to arsenic in vitro. Expression of the ion-channel genes CACNA1, KCNH2, KCNQ1 and KCNE1 were down-regulated by 1-μM arsenic. Alteration of some common pathways, including those involved in oxidative stress, inflammatory signaling, and ion-channel function, may underlay the seemingly disparate array of arsenic-associated diseases, such as cancer, cardiovascular disease, and diabetes.

Airborne polychlorinated biphenyls (PCBs) reduce telomerase activity and shorten telomere length in immortal human skin keratinocytes (HaCat)

PCBs, a group of 209 individual congeners, are ubiquitous environmental pollutants and classified as probable human carcinogens. One major route of exposure is by inhalation of these industrial compounds, possibly daily from inner city air and/or indoor air in contaminated buildings. Hallmarks of aging and carcinogenesis are changes in telomere length and telomerase activity. We hypothesize that semi-volatile PCBs, like those found in inner city air, are capable of disrupting telomerase activity and altering telomere length. To explore this possibility, we exposed human skin keratinocytes to a synthetic Chicago Airborne Mixture (CAM) of PCBs, or the prominent airborne PCB congeners, PCB28 or PCB52 for up to 48 days and determined telomerase activity, telomere length, cell proliferation, and cell cycle distribution. PCBs 28, 52 and CAM significantly reduced telomerase activity from days 18-48. Telomere length was shortened by PCB 52 from day 18 and PCB 28 and CAM from days 30 on. All PCBs decreased cell proliferation from day 18; only PCB 52 produced a small increase of cells in G0/G1 of the cell cycle. This significant inhibition of telomerase activity and reduction of telomere length by PCB congeners suggest a potential mechanism by which these compounds could lead to accelerated aging and cancer.

A redox-linked novel pathway for arsenic-mediated RET tyrosine kinase activation

We examined the biochemical effects of arsenic on the activities of RET proto-oncogene (c-RET protein tyrosine kinases) and RET oncogene (RET-MEN2A and RET-PTC1 protein tyrosine kinases) products. Arsenic activated c-RET kinase with promotion of disulfide bond-mediated dimerization of c-RET protein. Arsenic further activated RET-MEN2A kinase, which was already 3- to 10-fold augmented by genetic mutation compared with c-RET kinase activity, with promotion of disulfide bond-mediated dimerization of RET-MEN2A protein (superactivation). Arsenic also increased extracellular domain-deleted RET-PTC1 kinase activity with promotion of disulfide bond-mediated dimerization of RET-PTC1 protein. Arsenic increased RET-PTC1 kinase activity with cysteine 365 (C365) replaced by alanine with promotion of dimer formation but not with cysteine 376 (C376) replaced by alanine. Our results suggest that arsenic-mediated regulation of RET kinase activity is dependent on conformational change of RET protein through modulation of a special cysteine sited at the intracellular domain in RET protein (relevant cysteine of C376 in RET-PTC1 protein). Moreover, arsenic enhanced the activity of immunoprecipitated RET protein with increase in thiol-dependent dimer formation. As arsenic (14.2 microM) was detected in the cells cultured with arsenic (100 microM), direct association between arsenic and RET in the cells might modulate dimer formation. Thus, we demonstrated a novel redox-linked mechanism of activation of arsenic-mediated RET proto-oncogene and oncogene products.

Oncogene expression profiles in K6/ODC mouse skin and papillomas following a chronic exposure to monomethylarsonous acid

We have previously observed that a chronic drinking water exposure to monomethylarsonous acid [MMA(III)], a cellular metabolite of inorganic arsenic, increases tumor frequency in the skin of keratin VI/ornithine decarboxylase (K6/ODC) transgenic mice. To characterize gene expression profiles predictive of MMA(III) exposure and mode of action of carcinogenesis, skin and papilloma RNA was isolated from K6/ODC mice administered 0, 10, 50, and 100 ppm MMA(III) in their drinking water for 26 weeks. Following RNA processing, the resulting cRNA samples were hybridized to Affymetrix Mouse Genome 430A 2.0 GeneChips(R). Micoarray data were normalized using MAS 5.0 software, and statistically significant genes were determined using a regularized t-test. Significant changes in bZIP transcription factors, MAP kinase signaling, chromatin remodeling, and lipid metabolism gene transcripts were observed following MMA(III) exposure as determined using the Database for Annotation, Visualization and Integrated Discovery 2.1 (DAVID) (Dennis et al., Genome Biol 2003;4(5):P3). MMA(III) also caused dose-dependent changes in multiple Rho guanine nucleotide triphosphatase (GTPase) and cell cycle related genes as determined by linear regression analyses. Observed increases in transcript abundance of Fosl1, Myc, and Rac1 oncogenes in mouse skin support previous reports on the inducibility of these oncogenes in response to arsenic and support the relevance of these genomic changes in skin tumor induction in the K6/ODC mouse model.

Analysis of genomic dose-response information on arsenic to inform key events in a mode of action for carcinogenicity

A comprehensive literature search was conducted to identify information on gene expression changes following exposures to inorganic arsenic compounds. This information was organized by compound, exposure, dose/concentration, species, tissue, and cell type. A concentration-related hierarchy of responses was observed, beginning with changes in gene/protein expression associated with adaptive responses (e.g., preinflammatory responses, delay of apoptosis). Between 0.1 and 10 microM, additional gene/protein expression changes related to oxidative stress, proteotoxicity, inflammation, and proliferative signaling occur along with those related to DNA repair, cell cycle G2/M checkpoint control, and induction of apoptosis. At higher concentrations (10-100 microM), changes in apoptotic genes dominate. Comparisons of primary cell results with those obtained from immortalized or tumor-derived cell lines were also evaluated to determine the extent to which similar responses are observed across cell lines. Although immortalized cells appear to respond similarly to primary cells, caution must be exercised in using gene expression data from tumor-derived cell lines, where inactivation or overexpression of key genes (e.g., p53, Bcl-2) may lead to altered genomic responses. Data from acute in vivo exposures are of limited value for evaluating the dose-response for gene expression, because of the transient, variable, and uncertain nature of tissue exposure in these studies. The available in vitro gene expression data, together with information on the metabolism and protein binding of arsenic compounds, provide evidence of a mode of action for inorganic arsenic carcinogenicity involving interactions with critical proteins, such as those involved in DNA repair, overlaid against a background of chemical stress, including proteotoxicity and depletion of nonprotein sulfhydryls. The inhibition of DNA repair under conditions of toxicity and proliferative pressure may compromise the ability of cells to maintain the integrity of their DNA.

Down-regulation of wt1 expression in leukemia cell lines as part of apoptotic effect in arsenic treatment using two compounds

Arsenic trioxide (As2O3) induces remission in patients with acute promyelocytic leukemia (APL). To better understand molecular mechanisms of arsenic actions, this study investigated the effect of two different arsenic compounds on gene expression of apoptosis and cellular proliferation related genes. The Wilms' tumor gene (wt1) is up-regulated in acute myeloid leukemia (AML) and a variety of leukemia cell lines. The expression of wt1 in these cells is proposed to have an anti-apoptotic effect. HL-60 and K562 were treated with arsenic trioxide (As2O3) and sodium arsenite (NaAsO2) at concentrations between 0 - 10 microM for up to 48 h. The induction of apoptosis was accompanied by down-regulation of hTERT and wt1 mRNA and protein expression but up-regulation of par-4. Low concentrations of 0.1 microM arsenic induced expression of the anti-apoptotic bcl-2 gene in both cell lines HL-60 and K562. There were no major differences encountered between compounds. After arsenic treatment of the leukemia cell lines HL-60 and K562 the up-regulation of par-4 may contribute to the induction of apoptosis rather than down-regulation of bcl-2. The therapeutic effect of arsenic is the induction of apoptosis by modulating the gene expression profile of pro- and anti-apoptotic genes including the wt1 gene.

Downregulation of telomerase activity in human promyelocytic cell line using RNA interference

Telomerase is a ribonucleoprotein complex. It consists of two main components, human telomerase reverse transcriptase (hTERT) and human telomerase RNA. High telomerase activity is present in most malignant cells, but it is barely detectable in majority of somatic cells. The direct correlation between telomerase reactivation and carcinogens has made hTERT a key target for anticancer therapeutic studies. In this study, for the first time, we evaluated the ability of the new generation of short interfering RNA (siRNA) to regulate telomerase activity in the human promyelocytic leukemia cell line (HL-60). Transient transfection cell line by hTERT siRNAs resulted in statistically significant suppression of hTERT messenger RNAs which were detected by quantitative real-time polymerase chain reaction, while the expressed hTERT protein levels were measured by flow cytometry. The results of telomeric repeat amplification protocol showed that telomerase activity was significantly reduced upon transfection of the HL-60 cell line with hTERT siRNAs. The results of this study showed that telomerase activity and cell proliferation were efficiently inhibited in the hTERT siRNA-treated leukemic cell line.

Elevated human telomerase reverse transcriptase gene expression in blood cells associated with chronic arsenic exposure in Inner Mongolia, China

BACKGROUND

Arsenic exposure is associated with human cancer. Telomerase-containing human telomerase reverse transcriptase (hTERT) can extend telomeres of chromosomes, delay senescence, and promote cell proliferation leading to tumorigenesis.

OBJECTIVE

The goal of this study was to investigate the effects of As on hTERT mRNA expression in humans and in vitro.

METHOD

A total of 324 Inner Mongolia residents who have been exposed to As via drinking water participated in this study. Water and toenail samples were collected and analyzed for As. Blood samples were quantified for hTERT mRNA expression using real-time polymerase chain reaction. The hTERT mRNA levels were linked to water and nail As concentrations and skin hyperkeratosis. Human epidermal keratinocytes were treated with arsenite to assess effects on cell viability and hTERT expression in vitro.

RESULTS

hTERT mRNA expression levels were significantly associated with As concentrations of water (p<0.0001) and nails (p=0.002) and also associated with severity of skin hyperkeratosis (p<0.05), adjusting for age, sex, smoking, and pesticide use. Females showed a higher slope than males (females: 0.126, p=0.0005; males: 0.079, p=0.017). In addition to water and nail As concentrations, age (p<0.0001) and pesticide use (p=0.025) also showed significant associations with hTERT expression. The hTERT expression levels decreased with age. Tobacco smoking did not affect hTERT expression (p=0.13). hTERT expression was significantly correlated with OGG1 and ERCC1 expression. The in vitro results also showed a dose-response relationship between arsenite concentrations and hTERT expression and reached the peak at 1 microM.

CONCLUSIONS

hTERT expression was associated with As exposure in vivo and in vitro. The increased hTERT expression may be a cellular response to genomic insults by As and may also indicate that As may function as a tumor promoter in carcinogenesis in humans.

Intracellular redox status and oxidative stress: implications for cell proliferation, apoptosis, and carcinogenesis

Oxidative stress can be defined as the imbalance between cellular oxidant species production and antioxidant capability. Reactive oxygen species (ROS) are involved in a variety of different cellular processes ranging from apoptosis and necrosis to cell proliferation and carcinogenesis. In fact, molecular events, such as induction of cell proliferation, decreased apoptosis, and oxidative DNA damage have been proposed to be critically involved in carcinogenesis. Carcinogenicity and aging are characterized by a set of complex endpoints, which appear as a series of molecular reactions. ROS can modify many intracellular signaling pathways including protein phosphatases, protein kinases, and transcription factors, suggesting that the majority of the effects of ROS are through their actions on signaling pathways rather than via non-specific damage of macromolecules; however, exact mechanisms by which redox status induces cells to proliferate or to die, and how oxidative stress can lead to processes evoking tumor formation are still under investigation.

2-(4'-CHLOROPHENYL)-1,4-BENZOQUINONE INCREASES THE FREQUENCY OF MICRONUCLEI AND SHORTENS TELOMERES

The toxicity of polychlorinated biphenyls (PCBs) has been attributed widely to receptor-mediated effects, buttressed by the popularity of the Toxic Equivalency Factor. We propose that a crucial toxic mechanism of lower-chlorinated PCBs is their enzymatic biotransformation to electrophiles, including quinoid metabolites, that bind intracellular sulfhydryl groups, such as those found in microtubulin and enzymes like telomerase. To test this hypothesis, we have examined micronuclei induction, cell cycle, and telomere shortening in cells in culture. Our findings show a large increase in micronuclei frequency and cell cycle perturbation in V79 cells, and a marked decrease in telomere length in HaCaT cells exposed to 2-(4'-chlorophenyl)-1,4-benzoquinone (PCB3pQ).

Hormesis [biological effects of low level exposures (BELLE)] and dermatology

Hormesis, or biological effects of low level exposures (BELLE), is characterized by nonmonotonic dose response which is biphasic, displaying opposite effects at low and high dose. Its occurrence has been documented across a broad range of biological models and diverse type of exposure. Since hormesis appears to be a relatively common phenomenon in many areas, the objective of this review is to explore its occurrence related to dermatology and its public health and risk assessment implication. Hormesis appears to be a common phenomenon in in-vitro skin biology. However, in vivo data are lacking and the clinical relevance of hormesis has yet to be determined. Better understanding of this phenomenon will likely lead to different strategies for risk assessment process employed in the fields of dermatologic toxicology and pharmacology. We believe that hormesis is a common phenomenon and should be given detailed consideration to its concept and its risk assessment implications, and how these may be incorporated into the experimental and regulatory processes in dermatology. The skin, with its unique characteristics, its accessibility, and the availability of non-invasive bioengineering and DNA microarray technology, will be a good candidate to extend the biology of hormesis.

Modulation of DNA polymerase beta-dependent base excision repair in cultured human cells after low dose exposure to arsenite

Base excision repair (BER) is crucial for development and for the repair of endogenous DNA damage. However, unlike nucleotide excision repair, the regulation of BER is not well understood. Arsenic, a well-established human carcinogen, is known to produce oxidative DNA damage, which is repaired primarily by BER, whilst high doses of arsenic can also inhibit DNA repair. However, the mechanism of repair inhibition by arsenic and the steps inhibited are not well defined. To address this question we have investigated the regulation of DNA polymerase beta (Pol beta) and AP endonuclease (APE1), in response to low, physiologically relevant doses of arsenic. GM847 lung fibroblasts and HaCaT keratinocytes were exposed to sodium arsenite, As(III), and mRNA, protein levels and BER activity were assessed. Both Pol beta and APE1 mRNA exhibited significant dose-dependant down regulation at doses of As(III) above 1 microM. However, at lower doses Pol beta mRNA and protein levels, and consequently, BER activity were significantly increased. In contrast, APE1 protein levels were only marginally increased by low doses of As(III) and there was no correlation between APE1 and overall BER activity. Enzyme supplementation of nuclear extracts confirmed that Pol beta was rate limiting. These changes in BER correlated with overall protection against sunlight UV-induced toxicity at low doses of As(III) and produced synergistic toxicity at high doses. The results provide evidence that changes in BER due to low doses of arsenic could contribute to a non-linear, threshold dose response for arsenic carcinogenesis.

Biphasic effect of arsenite on cell proliferation and apoptosis is associated with the activation of JNK and ERK1/2 in human embryo lung fibroblast cells

Biphasic dose-response relationship induced by environmental agents is often characterized with the effect of low-dose stimulation and high-dose inhibition. Some studies showed that arsenite may induce cell proliferation and apoptosis via biphasic dose-response relationship in human cells; however, mechanisms underlying this phenomenon are not well understood. In the present study, we aimed at investigating the relationship between biphasic effect of arsenite on cell proliferation and apoptosis and activation of JNK and ERK1/2 in human embryo lung fibroblast (HELF) cells. Our results demonstrated that cell proliferation may be stimulated at lower concentrations (0.1 and 0.5 microM) arsenite but inhibited at higher concentrations (5 and 10 microM). When cell apoptosis was used as the endpoint, the concentration-response curves were changed to U-shapes. During stimulation phospho-JNK levels were significantly increased at 3, 6, and 12 h after 0.1 or 0.5 microM arsenite exposure. Phospho-ERK1/2 levels were increased with different concentrations (0.1-10 microM) of arsenite at 6, 12, and 24 h. Blocking of JNK pathway with 20 microM SP600125 or ERK1/2 by 100 microM PD98059 significantly inhibited biphasic effect of arsenite in cells. Data in the present study suggest that activation of JNK and ERK1/2 may be involved in biphasic effect of arsenite when measuring cell proliferation and apoptosis in HELF cells. JNK activation seems to play a more critical role than ERK1/2 activation in the biphasic process.

Short dysfunctional telomeres impair the repair of arsenite-induced oxidative damage in mouse cells

Telomeres and telomerase appear to participate in the repair of broken DNA ends produced by oxidative damage. Arsenite is an environmental contaminant and a potent human carcinogen, which induces oxidative stress on cells via the generation of reactive oxygen species affecting cell viability and chromosome stability. It promotes telomere attrition and reduces cell survival by apoptosis. In this study, we used mouse embryonic fibroblasts (MEFs) from mice lacking telomerase RNA component (mTERC(-/-) mice) with long (early passage or EP) and short (late passage or LP) telomeres to investigate the extent of oxidative damage by comparing the differences in DNA damage, chromosome instability, and cell survival at 24 and 48 h of exposure to sodium arsenite (As3+; NaAsO2). There was significantly high level of DNA damage in mTERC(-/-) cells with short telomeres as determined by alkaline comet assay. Consistent with elevated DNA damage, increased micronuclei (MN) induction reflecting gross genomic instability was also observed. Fluorescence in situ hybridization (FISH) analysis revealed that increasing doses of arsenite augmented the chromosome aberrations, which contributes to genomic instability leading to possibly apoptotic cell death and cell cycle arrest. Microarray analysis has revealed that As3+ treatment altered the expression of 456 genes of which 20% of them have known functions in cell cycle and DNA damage signaling and response, cell growth, and/or maintenance. Results from our studies imply that short dysfunctional telomeres impair the repair of oxidative damage caused by arsenite. The results will have implications in risk estimation as well as cancer chemotherapy.

Chronic arsenic exposure and oxidative stress: OGG1 expression and arsenic exposure, nail selenium, and skin hyperkeratosis in Inner Mongolia

Arsenic, a human carcinogen, is known to induce oxidative damage to DNA. In this study we investigated oxidative stress and As exposure by determining gene expression of OGG1, which codes for an enzyme, 8-oxoguanine DNA glycosylase, involved in removing 8-oxoguanine in As-exposed individuals. Bayingnormen (Ba Men) residents in Inner Mongolia are chronically exposed to As via drinking water. Water, toenail, and blood samples were collected from 299 Ba Men residents exposed to 0.34-826 microg/L As. RNA was isolated from blood, and mRNA levels of OGG1 were determined using real-time polymerase chain reaction. OGG1 expression levels were linked to As concentrations in drinking water and nails, selenium concentrations in nails, and skin hyperkeratosis. OGG1 expression was strongly associated with water As concentrations (p < 0.0001). Addition of the quadratic term significantly improved the fit compared with the linear model (p = 0.05) . The maximal OGG1 response was at the water As concentration of 149 microg/L. OGG1 expression was also significantly associated with toenail As concentrations (p = 0.015) but inversely associated with nail Se concentrations (p = 0.0095) . We found no significant differences in the As-induced OGG1 expression due to sex, smoking, or age even though the oldest group showed the strongest OGG1 response (p = 0.0001) . OGG1 expression showed a dose-dependent increased risk of skin hyperkeratosis in males (trend analysis, p = 0.02) , but the trend was not statistically significant in females. The results from this study provide a linkage between oxidative stress and As exposure in humans. OGG1 expression may be useful as a biomarker for assessing oxidative stress from As exposure.

Arsenic in the aetiology of cancer

Arsenic, one of the most significant hazards in the environment affecting millions of people around the world, is associated with several diseases including cancers of skin, lung, urinary bladder, kidney and liver. Groundwater contamination by arsenic is the main route of exposure. Inhalation of airborne arsenic or arsenic-contaminated dust is a common health problem in many ore mines. This review deals with the questions raised in the epidemiological studies such as the dose-response relationship, putative confounders and synergistic effects, and methods evaluating arsenic exposure. Furthermore, it describes the metabolic pathways of arsenic, and its biological modes of action. The role of arsenic in the development of cancer is elucidated in the context of combined epidemiological and biological studies. However, further analyses by means of molecular epidemiology are needed to improve the understanding of cancer aetiology induced by arsenic.

Lack of poly(ADP-ribose) polymerase-1 gene product enhances cellular sensitivity to arsenite

Arsenite (As3+) has long been known to induce cancer and other degenerative diseases. Arsenite exerts its toxicity in part by generating reactive oxygen species. Identification of genetic factors that contribute to arsenic mutagenicity and carcinogenicity is critical for the treatment and prevention of arsenic exposure in human population. As poly(ADP-ribose) polymerase (PARP) is critical for genomic DNA stability, role of PARP-1 was evaluated in arsenic-induced cytotoxic and genotoxic effects. Our study revealed that telomere attrition, probably owing to arsenite-induced oxidative stress, was much more pronounced in PARP-1-/- mouse embryonic fibroblasts (MEF; 40%) compared with PARP-1+/+ MEFs (10-20%). Correlation observed between telomere reduction and apoptotic death in PARP-1 null cells strongly indicates that the telomere attrition might be a trigger for enhanced apoptotic death after arsenite treatment. Elevated DNA damage detected by alkaline comet assay points to an impaired repair ability of arsenite-induced DNA lesions in PARP-1-/- MEFs. Consistent with elevated DNA damage, increased micronuclei induction reflecting gross genomic instability was also observed in arsenite-treated PARP-1-/- MEFs. Microarray analysis has revealed that arsenite treatment altered the expression of about 311 genes majority of which have known functions in cellular responses to stress/external stimulus and cell growth and/or maintenance. Our results suggest an important role for PARP-1 gene product in the maintenance of chromosome-genome stability in response to arsenite-induced DNA damage.

Up-regulation of telomere-binding TRF1, TRF2 related to reactive oxygen species induced by As(2)O(3) in MGC-803 cells

In this work, our study focused on As(2)O(3) action in view point of telomere. Results showed that treatment of human gastric cancer MGC-803 cells with arsenic trioxide could cause up-regulation of telomeric repeat binding factor TRF1 and TRF2 mRNA and protein levels, and induced G2/M phase arrest and cell apoptosis. At the same time, telomere length shortening and telomerase inhibitory were not obvious. Flow cytometry measurements indicated that the increase of TRF1 and TRF2 proteins is related to oxidative stress by arsenic trioxide. Results also indicate that after arsenic trioxide treatment, p53 protein levels increased significantly and also could bind directly at the telomere t-loop junction. These findings demonstrate arsenic trioxide-induced cell cycle arrest and apoptosis might involve a novel pathway related to TRF1, TRF2 protein.

Retinoid/arsenic combination therapy of promyelocytic leukemia: induction of telomerase-dependent cell death

Acute promyelocytic leukemia (APL) is efficiently treated with a cell differentiation inducer, all-trans retinoic acid (ATRA). However, a significant percentage of patients still develop resistance to this treatment. Recently, arsenic trioxide (As2O3), alone or in combination with ATRA, has been identified as an alternative therapy in patients with both ATRA-sensitive and ATRA-resistant APL. Previous investigations restricted the mechanism of this synergism to the modulation and/or degradation of PML-RARalpha oncoprotein through distinct pathways. In this study, using several ATRA maturation-resistant APL cell lines, we demonstrate in vitro that the success of ATRA/As2O3 treatment in APL pathology can be explained, at least in part, by a synergistic effect of these two drugs in triggering downregulation of telomerase efficient enough to cause telomere shortening and subsequent cell death. Such long-term low-dose combinatorial therapy strategies, developed also to avoid acute side effects, reinforce the notion that the antitelomerase strategy, based on a combination of active agents, should now be considered and evaluated not only in APL but also in other malignancies.

Telomeres, telomerase, and apoptosis

Telomeres are specialized high-order chromatin structures that cap the ends of eukaryotic chromosomes. In vertebrates, telomeric DNA is composed of repetitions of the TTAGGG hexanucleotide, is bound to a set of specific proteins, and is elongated by the reverse transcriptase enzyme telomerase. Telomerase activity is promptly detected in cells with an indefinite replicative potential, such as cancer cells, while is almost undetectable in normal cells, which are characterized by a limited life span. Mounting evidence indicates that the maintenance of telomere integrity and telomerase protect cells from apoptosis. Disruption of the telomere capping function and (or) telomerase inhibition elicit an apoptotic response in cancer cells, while restoration of telomerase activity in somatic cells confers resistance to apoptosis. The possible mechanisms linking telomeres, telomerase and apoptosis are discussed in this review, together with the impact of this field in anticancer research.

Oxidative stress and apoptosis in metal ion-induced carcinogenesis

Epidemiological evidence suggests that exposure to certain metals causes carcinogenesis. The mechanisms of metal-induced carcinogenesis have been pursued in chemical, biochemical, cellular, and animal models. Significant evidence has accumulated that oxidative stress may be a common pathway in cellular responses to exposure to different metals. For example, in the last few years evidence in support of a correlation between the generation of reactive oxygen species, DNA damage, tumor promotion, and arsenic exposure has strengthened. This article summarizes the current literature on metal-mediated oxidative stress, apoptosis, and their relation to metal-mediated carcinogenesis, concentrating on arsenic and chromium.

Chromosome instability in T-cells cultured in the presence of pancuronium or fentanyl

BACKGROUND

Genomic instability is recognized as a cause of cellular apoptosis and certain drugs that exhibit a proapoptotic effect are also able to induce chromosome damage. Since we found in recent experiments that drugs such as pancuronium and fentanyl exerted an apoptogenic effect on T cells, we studied the capacity of those agents to promote chromosome instability, i.e. chromosome aberrations (CA) and telomeric associations (tas) in peripheral blood lymphocytes.

METHODS

Lymphocytes from healthy donors were cultured with pancuronium or fentanyl, using two different concentrations for each drug: 20 and 200 ng/ml for pancuronium and 10 and 30 ng/ml for fentanyl, respectively. Cells were exposed to each concentration of these drugs either for 24 or 48 h. The higher concentration chosen was the same at which we detected the proapoptotic effect in our previous works. Cytogenetic analysis was performed by means of a standard technique and chromosome aberrations or telomeric associations were blindly evaluated by two independent observers.

RESULTS

The chromosome aberrations we observed in treated cells were not significantly different from control lymphocytes. However, an unusual rate of telomeric associations (P < 0.001) was detected in cells exposed to both pancuronium and fentanyl, at each concentration tested and at each exposure time of the study.

CONCLUSIONS

Fentanyl and pancuronium do not have a direct clastogenic effect on T cultures, but at the same concentrations at which we demonstrated their apoptogenic power, these drugs are able to increase genomic instability through inducing an elevated rate of telomeric associations. Such a capacity could exploit in peripheral T cells the same mitochondrion-mediated signal pathway of apoptosis death.