Induction of mitochondrial chaperonin, hsp60, by cadmium in human hepatoma cells

Bio-indicators in cadmium toxicity: Role of HSP27 and HSP70

Heat shock proteins (HSPs) are a family of proteins that are expressed by cells in reply to stressors. The changes in concentration of HSPs could be utilized as a bio-indicator of oxidative stress caused by heavy metal. Exposure to the different heavy metals may induce or reduce the expression of different HSPs. The exposure to cadmium ion (Cd²⁺) could increase HSP70 and HSP27 over 2- to 10-fold or even more. The in vitro and in vivo models indicate that the HSP70 family is more sensitive to Cd intoxication than other HSPs. The analyses of other HSPs along with HSP70, especially HSP27, could also be useful to obtain more accurate results. In this regard, this review focuses on examining the literature to bold the futuristic uses of HSPs as bio-indicators in the initial assessment of Cd exposure risks in defined environments.

Effects of endocrine disrupting chemicals on estrogen receptor alpha and heat shock protein 60 gene expression in primary cultures of loggerhead sea turtle (Caretta caretta) erythrocytes

The loggerhead turtle (Caretta caretta) can be considered a good indicator species for studying the ecological impact of endocrine disrupting chemicals (EDCs) on wildlife. However, the effect of these environmental pollutants on nuclear steroid hormone signaling has not yet been addressed in sea turtles mainly due to the legal constraints of their endangered status. Here we describe the use of primary erythrocyte cell cultures as in vitro models for evaluating the effects of different EDCs on the expression of estrogen receptor α (ERα). In addition, we evaluated erythrocyte toxicity caused by EDCs using Alamar Blue assay and heat shock proteins 60 (HSP60) expression. Primary cultures of erythrocytes were exposed to increasing concentrations of 4-nonylphenol (4NP), Diisodecyl phthalate (DiDP), Tri-m-cresyl phosphate (TMCP) and Tributyltin (TBT) for 48h. Alamar Blue demonstrated that exposure of erythrocytes to each contaminant for up to 48h led to a significant impairment of cellular metabolic activity at 100μM, with the exception of TBT. Moreover, our data indicate that loggerhead erythrocytes constitutively express ERα and HSP60 at the transcript level and respond to EDCs by up-regulating their expression. In this regard, ERα was up-regulated in a dose-dependent manner after 48h exposure to both 4NP and TMCP. Interestingly, the dosage-dependent effects of DiDP on ERα expression were opposite in comparison to that obtained following exposure to the other tested compounds. This work provides the first indication regarding the potential of primary erythrocytes as study models for evaluating the effects of EDCs on sea turtles.

Identification of molecular candidates and interaction networks via integrative toxicogenomic analysis in a human cell line following low-dose exposure to the carcinogenic metals cadmium and nickel

Cadmium and nickel have been classified as carcinogenic to humans by the World Health Organization's International Agency for Research on Cancer. Given their prevalence in the environment, the fact that cadmium and nickel may cause diseases including cancer even at low doses is a cause for concern. However, the exact mechanisms underlying the toxicological effects induced by low-dose exposure to cadmium and nickel remain to be elucidated. Furthermore, it has recently been recognized that integrative analysis of DNA, mRNA and proteins is required to discover biomarkers and signaling networks relevant to human toxicant exposure. In the present study, we examined the deleterious effects of chronic low-dose exposure of either cadmium or nickel on global profiling of DNA copy number variation, mRNA and proteins. Array comparative genomic hybridization, gene expression microarray and functional proteomics were conducted, and a bioinformatics tool, which predicted signaling pathways, was applied to integrate data for each heavy metal separately and together. We found distinctive signaling networks associated with subchronic low-dose exposure to cadmium and nickel, and identified pathways common to both. ACTB, HSP90AA1, HSPA5 and HSPA8, which are key mediators of pathways related to apoptosis, proliferation and neoplastic processes, were key mediators of the same pathways in low-dose nickel and cadmium exposure in particular. CASP-associated signaling pathways involving CASP3, CASP7 and CASP9 were observed in cadmium-exposed cells. We found that HSP90AA1, one of the main modulators, interacted with HIF1A, AR and BCL2 in nickel-exposed cells. Interestingly, we found that HSP90AA1 was involved in the BCL2-associated apoptotic pathway in the nickel-only data, whereas this gene interacted with several genes functioning in CASP-associated apoptotic signaling in the cadmium-only data. Additionally, JUN and FASN were main modulators in nickel-responsive signaling pathways. Our results provide valuable biomarkers and distinctive signaling networks that responded to subchronic low-dose exposure to cadmium and nickel.

Identification and characterization of a family of Caenorhabditis elegans genes that is homologous to the cadmium-responsive gene cdr-1

Six Caenorhabditis elegans genes that are homologous to the novel, cadmium-responsive gene cdr-1 have been identified and characterized. Nucleotide and amino acid sequence comparisons among the CDR family, which includes cdr-1, cdr-2, cdr-3, cdr-4, cdr-5, cdr-6, and cdr-7, reveals a high degree of identity among the seven members in this family. There are high levels of amino acid and nucleotide sequence similarity in the lengths of the open reading frames, predicted sizes, and protein characteristics. The seven proteins are predicted to be extremely hydrophobic, and are classified as integral membrane proteins. Structural analysis of the predicted proteins suggests that they may have similar biological functions. In response to cadmium exposure, cdr-1, cdr-2, cdr-3, and cdr-4 transcription significantly increases. In contrast, the levels of cdr-5, cdr-6, and cdr-7 transcription are not significantly affected or inhibited by cadmium exposure. Further, in non-exposed C. elegans, cdr-2, cdr-4, cdr-6, and cdr-7 are constitutively expressed. When CDR-1 expression was inhibited using RNAi, numerous fluid droplets were observed throughout the nematode body cavity. This phenotype became more pronounced in the presence of hypotonic stress. This suggests that CDR-1 may function in osmoregulation to maintain salt balance in C. elegans.

Molecular and cellular mechanisms of cadmium carcinogenesis

Cadmium is a heavy metal, which is widely used in industry, affecting human health through occupational and environmental exposure. In mammals, it exerts multiple toxic effects and has been classified as a human carcinogen by the International Agency for Research on Cancer. Cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities. Cd2+ does not catalyze Fenton-type reactions because it does not accept or donate electrons under physiological conditions, and it is only weakly genotoxic. Hence, indirect mechanisms are implicated in the carcinogenicity of cadmium. In this review multiple mechanisms are discussed, such as modulation of gene expression and signal transduction, interference with enzymes of the cellular antioxidant system and generation of reactive oxygen species (ROS), inhibition of DNA repair and DNA methylation, role in apoptosis and disruption of E-cadherin-mediated cell-cell adhesion. Cadmium affects both gene transcription and translation. The major mechanisms of gene induction by cadmium known so far are modulation of cellular signal transduction pathways by enhancement of protein phosphorylation and activation of transcription and translation factors. Cadmium interferes with antioxidant defense mechanisms and stimulates the production of reactive oxygen species, which may act as signaling molecules in the induction of gene expression and apoptosis. The inhibition of DNA repair processes by cadmium represents a mechanism by which cadmium enhances the genotoxicity of other agents and may contribute to the tumor initiation by this metal. The disruption of E-cadherin-mediated cell-cell adhesion by cadmium probably further stimulates the development of tumors. It becomes clear that there exist multiple mechanisms which contribute to the carcinogenicity of cadmium, although the relative weights of these contributions are difficult to estimate.

DNA Microarray Analysis of Altered Gene Expression in Cadmium‐exposed Human Cells

Cadmium (Cd) is a heavy metal known to be toxic and carcinogenic, but its mechanism of action remains to be elucidated. Development of the DNA microarray technology has recently made the comprehensive analysis of gene expression possible, and it could be a powerful tool also in toxicological studies. With microarray slides containing 7,000-9,000 genes, we have been studying the gene expression profiles of a human cell line exposed to Cd. By exposure to a non-lethal concentration of Cd, 46 upregulated and 10 downregulated genes whose expression levels changed twofold or greater were observed. The expression of genes related to cellular protection and damage control mechanisms such as those encoding metallothioneins, anti-oxidant proteins and heat shock proteins was simultaneously induced. In addition, altered expression of many genes involved in signaling, metabolism and so on was newly observed. As a whole, a number of genes appear to be coordinately regulated toward survival from Cd toxicity. When cells were exposed to a higher concentration of Cd, more remarkable effects were observed both in the number of affected genes and in the extent of altered expression. These findings will contribute to the understanding of the complicated biological effects of Cd.

Upregulation of stress response mRNAs in COS-7 cells exposed to cadmium

Exposure of cells to cadmium (Cd) is known to stimulate the expression of various types of genes. These changes in gene expression are presumed to be related to the cellular response to Cd toxicity. To better understand the mechanisms related to Cd toxicity, suppression subtractive hybridization was carried out on COS-7 cells (African green monkey kidney cells) and the gene expression induced by Cd exposure was investigated. Heat shock protein (hsp) 10, 40, 60, 70, 89alpha and metallothionein II (MTII) mRNAs were found to be induced by Cd. This is the first report to describe the Cd-inducibility of hsp10, 40 and 89alpha mRNAs. Semi-quantitative reverse-transcription polymerase chain reaction showed the diverse expression patterns of these genes, depending on Cd concentration and exposure time. A marked elevation of hsp70 mRNA and induction of mRNA for the co-chaperone, hsp40, were detected. A relatively low level of hsp10 and hsp60 mRNAs was induced, with only a 2-fold increase within 24 h. Hsp89alpha mRNA was induced shortly after Cd exposure. These various induction patterns suggest that hsps play different roles in the cell against Cd toxicity.

Acute cadmium exposure induces stress-related gene expression in wild-type and metallothionein-I/II-null mice

This study examined the effect of acute cadmium on stress-related gene expression and free radical production in wild-type and metallothionein-I/II-null (MT-null) mice. Atlas Toxicology arrays showed that acute cadmium (40 micromol/kg as CdCl(2), ip for 3 h) markedly increased the expression of genes encoding heat-shock proteins, heme oxygenase-1, and genes in response to DNA damage/repair. The expression of genes encoding cytochrome P450 enzymes, UDP-glucuronosyltransferases, Mn-superoxide dismutase, and catalase was suppressed by cadmium. MT-null mice were more sensitive than wild-type mice to cadmium-induced, stress-related gene expression, in accord with greater activation of transcription factor AP-1 and phosphorylated JNK and ERK. To evaluate free radical production, mice were simultaneously given the spin trap agent, N-tert-butyl-alpha-phenylnitrone (PBN, 250 mg in DMSO/kg, ip) with cadmium, and livers were removed 30 min later for PBN-trapped radical extraction with chloroform:methanol (2:1), and detected with electron spin resonance (ESR). Cadmium treatment caused detectable ESR signals for PBN adducts as well as lipid peroxidation in the liver similarly in both wild-type and MT-null mice. Thus, the mechanism of acute cadmium toxicity involves multiple facets including oxidative damage and aberrant gene expression, and absence of MT exacerbates Cd-induced aberrant gene expression.

Applications and pitfalls of stress-proteins in biomonitoring

The vast number of potentially hazardous chemicals and the complex interactions that can occur between them in environmental mixtures, call for inexpensive, early and sensitive endpoints that reflect their biological effect. The existing validated bioassays, mostly based on lethality or reproduction, have been shown to be inadequate in respect of their sensitivity, the duration and expense of the test. In contrast, changes at biochemical level are usually the first detectable responses to environmental perturbation. Because these alterations underlie all effects at higher organisational level, they have often been shown to be very sensitive indicators of pollution. Stress-proteins (also referred to as heat-shock proteins or hsp) have recently been recognised as being one of the primary defence mechanisms that are activated by the occurrence of denatured proteins in the cell. Four major stress-protein families of 90,70,60 and 16-24 kDa are the most prominent and are frequently referred to as hsp90, hsp70, hsp60 and low molecular weight (LMW) stress-proteins. Three aspects of stress-proteins have been characterised that are essential if they want to be used as biomarkers of pollution: (1) they are part of the cellular protective response; (2) their synthesis is likely to be induced by a large number of chemicals; and (3) they are highly conserved in all organisms from bacteria to plants and man. Also, field studies have shown (be it for a limited number of stressors) that the stress response can occur even at the minute concentrations of pollutants that are usually found in the environment. However, increasing knowledge on the kinetics and persistence of the stress response to complex environmental mixtures, on the influence of both physiological and environmental parameters (pH, eutrophication, ellipsis), on the constitutive levels of stress-proteins and on the acquisition of tolerance, is required before one could safely apply stress-proteins to assess on-site pollution. Still, included in a test battery of complementary bioassays, stress protein may be very valuable as tier I biomarkers, i.e. broad response biomarkers that are used for preliminary screening of the environment.

Cadmium-induced mRNA expression of Hsp32 is augmented in metallothionein-I and -II knock-out mice

Cadmium is toxic and carcinogenic to humans and animals. The testis and lung are the target organs for cadmium carcinogenesis. Heat shock proteins (HSPs) as well as metallothionein (MT) and glutathione (GSH) play an important role in protection against its toxicity. HSP32, also known as heme oxygenase-1, is a 32-kDa protein induced by heme, heavy metals, oxidative stresses, and heat. We investigated expression of the Hsp32 gene of various organs (the liver, lung, heart, stomach, kidney, and testis) in transgenic mice deficient in the MT-I and -II genes (MT-KO) and in control mice (MT-W) after an injection of cadmium chloride (CdCl2). Survival of MT-W mice after a subcutaneously injection of CdCl2 was higher than that of MT-KO mice, while no significant difference was observed in the level of GSH in each organ between MT-W and MT-KO mice. Northern blot analysis showed that the MT-I mRNA was more extensively induced in the liver, kidney, and heart than other organs 6 h after an injection of CdCl2 (30 micromol/kg body wt, sc). There was little increase of the MT-I mRNA in the testis when induced by CdCl2. Expression of the Hsp32 gene in the liver and kidney in response to CdCl2 was more extensively augmented in MT-KO mice than in MT-W mice. In the lung and testis, there was little induction and no augmentation in expression of the Hsp32 gene induced by CdCl2 in both MT-W and MT-KO mice. In the stomach, there was little induction of the Hsp32 mRNA in MT-W mice, but was increased in MT-KO mice. Immunohistochemical staining revealed that the HSP32 protein was strongly expressed in the kidney and liver of MT-W mice 24 h after an injection of CdCl2 (20 micromol/kg body wt, sc), while the expression of HSP32 protein was not increased in the testis. In metabolically active organs such as the liver and kidney, expression of the Hsp32 gene as well as the MT-I gene was extensively induced by cadmium in MT-W mice, and more eminently induced in MT-KO mice. We suggest that organs of low stress response to cadmium such as the testis and lung may be vulnerable target sites for cadmium toxicity and carcinogenesis.

Attenuation by glutathione of hsp72 gene expression induced by cadmium in cisplatin-resistant human ovarian cancer cells

Intracellular GSH has some effects on protecting cells against cadmium and is involved in the development of resistance to cisplatin (CDDP). To determine the effects of intracellular GSH on expression of the heat shock genes (hsp) induced by cadmium in CDDP-resistant cancer cells, we used two human ovarian cancer cell lines: CDDP-sensitive A2780 and its CDDP-resistant derivative A2780CP. The concentration of intracellular GSH was significantly higher in A2780CP than in A2780 cells. A2780CP cells were more resistant to CdCl2 exposure than A2780 cells. The treatment of the two cell lines with 50 microM CdCl2 induced hsp72, hsp32 and metallothionein (MT-II) mRNAs, and the induction level of each mRNA did not differ in the two cell lines. However, the treatment with 20 microM CdCl2 induced the hsp72 and hsp32 mRNAs in A2780CP cells less than in A2780 cells, while the MT-II mRNA was induced to similar levels in the two cell lines. The DNA binding activity of the heat shock factor (HSF) in response to 20 microM CdCl2 exposure was also significantly lower in A2780CP cells. The treatment of A2780 cells with N-acetyl-L-cysteine increased the intracellular GSH concentration, and profoundly suppressed hsp72 mRNA induction and HSF activation by CdCl2. These results indicate that the regulation of the hsp72 gene expression induced by CdCl2 was more suppressive in A2780CP than in A2780 cells. Our findings suggest that increased GSH biosynthesis in CDDP-resistant cancer cells may be involved in the attenuation of HSF activation by CdCl2.

Human 60-kDa Heat-Shock Protein: A Danger Signal to the Innate Immune System

Mammalian 60-kDa heat-shock protein (hsp60) is a key target of T cell and Ab responses in chronic inflammation or atherosclerosis. We show in this study that human hsp60 is also an Ag recognized by cells of the innate immune system, such as macrophages. Both mouse and human macrophages respond to contact with exogenous human hsp60 with rapid release of TNF-α; mouse macrophages in addition produce nitric oxide. The proinflammatory macrophage response is hsp60 dose dependent and similar in kinetics and extent to LPS stimulation. Human hsp60 was found to synergize with IFN-γ in its proinflammatory activity. Finally, human hsp60 induces gene expression of the Th1-promoting cytokines IL-12 and IL-15. These findings identify autologous hsp60 as a danger signal for the innate immune system, with important implications for a role of local hsp60 expression/release in chronic Th1-dependent tissue inflammation.

Cadmium-regulated genes from the nematode Caenorhabditis elegans. Identification and cloning of new cadmium-responsive genes by differential display

The transition metal cadmium is a pervasive and persistent environmental contaminant that has been shown to be both a human toxicant and carcinogen. To inhibit cadmium-induced damage, cells respond by increasing the expression of genes encoding stress-response proteins. In most cases, the mechanism by which cadmium affects the expression of these genes remains unknown. It has been demonstrated in several instances that cadmium activates gene transcription through signal transduction pathways, mediated by protein kinase C, cAMP-dependent protein kinase, or calmodulin. A codicil is that cadmium should influence the expression of numerous genes. To investigate the ability of cadmium to affect gene transcription, the differential display technique was used to analyze gene expression in the nematode Caenorhabditis elegans. Forty-nine cDNAs whose steady-state levels of expression change 2-6-fold in response to cadmium exposure were identified. The nucleotide sequences of the majority of the differentially expressed cDNAs are identical to those of C. elegans cosmids, yeast artificial chromosomes, expressed sequence tags, or predicted genes. The translated amino acid sequences of several clones are identical to C. elegans metallothionein-1, HSP70, collagens, and rRNAs. In addition, C. elegans homologues of pyruvate carboxylase, DNA gyrase, beta-adrenergic receptor kinase, and human hypothetical protein KIAA0174 were identified. The translated amino acid sequences of the remaining differentially expressed cDNAs encode novel proteins.

Mechanisms of signal transduction in the stress response of hepatocytes

Adaptation of animals to stress is a unique property of life which allows the survival of the species. The stress response of hepatocytes is a very complex phenomenon, sometimes involving a cascade of events. The general stress signals are elucidated by mobilization of carbohydrate stores and akin to the insulin mediators. Oxidative signals are generated by pesticides, heavy metals, drugs, and alcohol which may or may not be under the purview of peroxisomes. Peroxisomal responses are well-defined involving specific receptors, whereas nonperoxisomal responses may be signaled by calcium, the Ah receptor, or built-in antioxidant systems. The intoxication signals are generally thought to be membrane defects induced by xenobiotics which then lead to highly nonspecific responses of hepatocytes. Detoxication signals, on the other hand, are specific responses of hepatocytes triggering de novo syntheses of detoxifier proteins or enzymes. Evidence reveals the existence of two distinct mechanisms of signal transduction in stressed hepatocytes--one involving the peroxisome and the other the plasma membrane.

Concentration-dependent differential effects of N-acetyl-L-cysteine on the expression of HSP70 and metallothionein genes induced by cadmium in human amniotic cells

Cadmium induces the expression of the 70 kDa heat shock protein (HSP70) and metallothionein (MT), both of which are considered to be associated with intracellular glutathione (GSH) metabolism in the cellular protection mechanism against cadmium-induced cellular injury. We determined the effects of N-acetyl-L-cysteine (NAC), which increases the intracellular GSH levels, on the induction of HSP70 and MT gene expression in a cultured cell line of human amniotic cells (WISH) exposed to CdCl2. The mRNA level of MT-II, a major isoform of MT genes, was more prominently increased than that of HSP70 when WISH cells were exposed to CdCl2 (5-15 microM, for 6 h). The treatment of WISH cells with 1.5 and 30 mM NAC for 2 h increased the intracellular GSH levels by 1.4- and 3.1-fold, respectively. Pretreatment of cells with 30 mM NAC significantly reduced both HSP70 and MT-II mRNA levels in the cells exposed to 50 microM CdCl2. This concentration of NAC also efficiently suppressed the cadmium-induced lethality. On the contrary, pretreatment with 1.5 mM NAC suppressed only the induction of HSP70 gene expression in the 50 microM CdCl2-treated cells, and did not inhibit the metal toxicity. However, this low concentration of NAC efficiently suppressed lipid peroxidation which was increased by 50 microM CdCl2. Furthermore, this low concentration of NAC also decreased the CdCl2-induced gene expression of HSP32 which represents a general response to oxidative stress. Taken together, NAC seems to have at least two concentration-dependent functions in WISH cells exposed to CdCl2; the low concentration of NAC can suppress the induction of HSP70 gene expression as well as the increase of lipid peroxidation via an antioxidant pathway, while the high concentration of NAC can suppress the induction of MT-II mRNA as well as cadmium-induced cell death. Our present data suggest that changes in intracellular redox status, as reflected by GSH concentration, have more important effects on the induction of HSP70 mRNA rather than that of MT-II mRNA in human amniotic cells exposed to cadmium.

Dose-dependent induction of heat shock protein 70 synthesis in Raphidocelis subcapitata following exposure to different classes of environmental pollutants

In this study an enzyme-linked immunosorbent assay (ELISA) was developed to detect the stress protein Hsp70 in the green alga Raphidocelis subcapitata. Using this ELISA, the response to a variety of pollutants, including ZnCl2, SeO2 (heavy metals), lindane (organochlorine pesticide), pentachlorophenol (PCP, chlorinated hydrocarbon insecticide and fungicide), carbaryl (carbamate pesticide) and sodium dodecyl sulphate (SDS; surfactant) was tested. Our results show that Hsp 70 is produced in a dose-dependent way in response to most chemicals investigated (except PCP) and at concentrations below the range of classical cytotoxicity testing (i.e. growth inhibition, lethality). Still, the potential to induce Hsp70 varied among the pollutants tested, the heavy metals ZnCl2 and SeO2 being the strongest inducers of Hsp70. Combined with the existing literature, these results indicate that Hsp70 in R. subcapitata is a sensitive biomarker for a wide range of environmental pollutants.

Expression of stress proteins and mitochondrial chaperonins in chronically stimulated skeletal muscle

Molecular chaperones and cytosolic stress proteins are actively involved in the stabilization, import and refolding of precursor proteins into mitochondria. The purpose of the present study was to evaluate the relationship between mitochondrial content under steady-state conditions, and during the induction of organelle biogenesis, with the expression of stress proteins and mitochondrial chaperonins. A comparison of steady-state levels of mitochondrial enzyme activity [cytochrome c oxidase (CYTOX)] with chaperonin levels [the heat-shock protein HSP60, the glucose-regulated protein GRP75 (mtHSP70)] in striated muscles possessing a wide range of oxidative capacities revealed a proportional expression between the two. This relationship was disrupted by chronic contractile activity brought about by 10 days of 10 Hz stimulation of the tibialis anterior (TA) muscle, which induced 2.4-fold increases in CYTOX activity, but 3.2- and 9.3-fold increases in HSP60 and GRP75 respectively. The inducible stress protein HSP70i was detected at low levels in control TA muscle, and was increased 9.6-fold by chronic contractile activity, to values comparable with those found in the unstressed soleus muscle. This increase occurred in the absence of changes in type I MHC levels, indicating independent regulation of these genes. Despite the increases in HSP60 and HSP70i proteins, contractile activity did not alter their respective mRNA levels, illustrating post-transcriptional mechanisms of gene regulation during contractile activity. In contrast, the mRNA levels encoding the co-chaperonin CPN10 were increased 3.3-fold by contractile activity. Thus, the expression of individual mitochondrial chaperonins is independently regulated and uncoordinated. The extent of the induction of these stress proteins and chaperonins by contractile activity exceeded that of membrane enzymes (e.g. CYTOX). It remains to be determined whether this marked induction of proteins comprising part of the protein import machinery is beneficial for the translocation of enzyme precursors into the mitochondria during conditions of accelerated biogenesis.

Stressor-specific induction of heat shock proteins in rat hepatoma cells

In order to determine whether induction of specific stress proteins is dependent on a given stressor and whether induction of these proteins is linked to survival, Reuber H35 rat hepatoma cells were exposed to five different environmental stressors (heat shock, arsenite, cadmium, dinitrophenol and ethanol). The effect of these stressors was studied on cell survival as well as on inhibition and recovery of protein synthesis and on induction of heat shock proteins (hsps). In this article, we present evidence that several well-known hsp-inducers fail to stimulate specific hsps in a degree that is comparable to the induction of these hsps by heat shock. Most evidently, hsp60 is not induced by cadmium-treatment, whereas hsp100 is hardly induced by sodium arsenite. Treatment with DNP only slightly induces hsp68 and hsp84, whereas no detectable induction of hsps is observed after treatment with ethanol. In contrast, treatment with cadmium raises the amount of hsp28 to a higher level as compared to heat shock. A comparison of the stressor-specific induction of major hsps was also made under conditions of similar impact on cellular physiology: (a) stressor conditions up to the critical point that cell death starts to occur, and (b) conditions of iso-survival (50%). We conclude that hsps cannot be simply used as a general risk-assessment tool, and that the validation of stressor-specific risk-assessment warrants further research with larger groups of proteins.

Induction of heat shock 70 mRNA by cadmium is mediated by glutathione suppressive and non-suppressive triggers

The induction mechanism of heat shock 70 (Hsp70) gene by cadmium was investigated. In human amniotic WISH cells, Hsp 70 was induced by cadmium in a dose- and time-dependent manner. Cadmium-induced Hsp70 mRNA levels were enhanced 3- to 4-fold after depletion of intracellular glutathione (GSH) by either diethylmaleate or buthionine sulfoximine. Under these conditions, hydrogen peroxide might increase in the absence of substrate for glutathione peroxidase. We found that exogenous hydrogen peroxide alone induced Hsp70 which was further enhanced significantly after GSH-depletion by diethylmaleate. On the other hand, treatment of cells by diethyldithiocarbamate, an inhibitor of superoxide dismutase, induced Hsp70 2-fold over the level of control. This induction was further stimulated by cadmium even in the presence of GSH. Furthermore, a 4-fold increase of intracellular GSH by the treatment of cells with glutathione isopropyl ester did not diminish the cadmium-induced Hsp70. Gel mobility shift assays of nuclear extracts, from these differently treated cells, with oligonucleotide containing a promoter region of Hsp70 gene revealed that the levels of Hsp70 mRNA observed in the present study corresponded to the changes of transcription. These results imply that the induction of Hsp70 mRNA by cadmium is mediated at least partly via reactive oxygen species and attenuated by cellular GSH and that some part of cadmium-induced Hsp70 can not be eliminated by GSH, suggesting that multiple signals are functioning for this induction.