Thiol reducing reagents inhibit the heat shock response. Involvement of a redox mechanism in the heat shock signal transduction pathway

Gambogic acid and gambogenic acid induce a thiol-dependent heat shock response and disrupt the interaction between HSP90 and HSF1 or HSF2

Cancer cells rely on heat shock proteins (HSPs) for growth and survival. Especially HSP90 has multiple client proteins and plays a critical role in malignant transformation, and therefore different types of HSP90 inhibitors are being developed. The bioactive natural compound gambogic acid (GB) is a prenylated xanthone with antitumor activity, and it has been proposed to function as an HSP90 inhibitor. However, there are contradicting reports whether GB induces a heat shock response (HSR), which is cytoprotective for cancer cells and therefore a potentially problematic feature for an anticancer drug. In this study, we show that GB and a structurally related compound, called gambogenic acid (GBA), induce a robust HSR, in a thiol-dependent manner. Using heat shock factor 1 (HSF1) or HSF2 knockout cells, we show that the GB or GBA-induced HSR is HSF1-dependent. Intriguingly, using closed form ATP-bound HSP90 mutants that can be co-precipitated with HSF1, a known facilitator of cancer, we show that also endogenous HSF2 co-precipitates with HSP90. GB and GBA treatment disrupt the interaction between HSP90 and HSF1 and HSP90 and HSF2. Our study implies that these compounds should be used cautiously if developed for cancer therapies, since GB and its derivative GBA are strong inducers of the HSR, in multiple cell types, by involving the dissociation of a HSP90-HSF1/HSF2 complex.

Visualizing cellular stress: A hypothesis-driven confocal laboratory exercise to identify compounds that activate heat shock factor binding at Hsp70 loci

Exposure of organisms to high temperatures and various chemical and physical stressors can cause protein misfolding and aggregation. In turn, this can disrupt the functions of proteins, threatening both development and homeostasis. To overcome this, cells can initiate the highly conserved heat shock (HS) stress response pathway. In eukaryotes, this is a coordinated cellular response, in which the master HS activator, heat shock factor (HSF), is rapidly recruited to the HS protein genes, and triggers the recruitment of additional coactivator proteins that facilitate gene expression. This results in the production of HS proteins that function as nuclear and cytosolic molecular chaperones, to promote refolding of proteins and prevent aggregation and increase protein degradation pathways. Here, we describe a laboratory exercise in which students visualize and quantify Green Fluorescent Protein (GFP)-tagged HSF binding to the HS protein genes in living Drosophila salivary gland nuclei as an output of chemically induced protein misfolding. Students are assigned an array of chemicals, and using the scientific literature, predict impacts of these chemicals on protein folding. Students then test the effects of their chemicals by measuring GFP-tagged HSF binding to the HS genes in salivary glands using confocal microscopy. Designed for junior and senior level students in a cell/molecular biology course, this is a two-part lab, in which student work closely with an instructor to help familiarize them with developing hypotheses supported by scientific literature and testing these hypotheses by quantitating the levels of GFP-HSF binding, using confocal microscopy of living Drosophila cells. © 2018 International Union of Biochemistry and Molecular Biology, 46(5):445-452, 2018.

Oridonin Triggers Chaperon-mediated Proteasomal Degradation of BCR-ABL in Leukemia

Inducing degradation of oncoproteins by small molecule compounds has the potential to avoid drug resistance and therefore deserves to be exploited for new therapies. Oridonin is a natural compound with promising antitumor efficacy that can trigger the degradation of oncoproteins; however, the direct cellular targets and underlying mechanisms remain unclear. Here we report that oridonin depletes BCR-ABL through chaperon-mediated proteasomal degradation in leukemia. Mechanistically, oridonin poses oxidative stress in cancer cells and directly binds to cysteines of HSF1, leading to the activation of this master regulator of the chaperone system. The resulting induction of HSP70 and ubiquitin proteins and the enhanced binding to CHIP E3 ligase hence target BCR-ABL for ubiquitin-proteasome degradation. Both wild-type and mutant forms of BCR-ABL can be efficiently degraded by oridonin, supporting its efficacy observed in cultured cells as well as mouse tumor xenograft assays with either imatinib-sensitive or -resistant cells. Collectively, our results identify a novel mechanism by which oridonin induces rapid degradation of BCR-ABL as well as a novel pharmaceutical activator of HSF1 that represents a promising treatment for leukemia.

SIRT1 knockdown promotes neural differentiation and attenuates the heat shock response

Neurons have a limited capacity for heat shock protein (HSP) induction and are vulnerable to the pathogenic consequence of protein misfolding and aggregation as seen in age-related neurodegenerative diseases. Sirtuin 1 (SIRT1), an NAD(+) -dependent lysine deacetylase with important biological functions, has been shown to sustain the DNA-binding state of HSF1 for HSP induction. Here we show that differentiation and maturation of embryonic cortical neurons and N2a neuroprogenitor cells is associated with decreases in SIRT1 expression and heat shock-dependent induction of HSP70 protein. Tests of a pharmacological activator and an inhibitor of SIRT1 affirm the regulatory role of SIRT1 in HSP70 induction. Protein cross-linking studies show that nuclear SIRT1 and HSF1 form a co-migrating high molecular weight complex upon stress. The use of retroviral vectors to manipulate SIRT1 expression in N2a cells show that shRNA-mediated knock down of SIRT1 causes spontaneous neurite outgrowth coincident with reduced growth rate and decreased induction of hsp70-reporter gene, whereas SIRT1 over-expression blocks the induced neural differentiation of N2a cells. Our results suggest that decreased SIRT1 expression is conducive to neuronal differentiation and this decrease contributes to the attenuated induction of HSPs in neurons.

Arabidopsis heat shock factor HsfA1a directly senses heat stress, pH changes, and hydrogen peroxide via the engagement of redox state

Arabidopsis heat shock factor HsfA1a is present in a latent, monomeric state under normal conditions; its activation involves heat stress-induced trimerization, binding to heat shock element in target promoters, and the acquisition of transcriptional competence. HsfA1a is an important regulator for heat stress-induced gene expression and thermotolerance. However, it is not clear whether HsfA1a is directly activated by stress and the mechanisms of the stress signaling are poorly understood. We analyzed HsfA1a activation by trimerization and DNA-binding assays in vitro and in vivo in response to heat stress, low/high pH, and hydrogen peroxide treatments. Our results show that purified recombinant HsfA1a was activated by these stress treatments in vitro. The same treatments also induced the binding to HSP18.2 and HSP70 promoters as examined by chromatin immunoprecipitation, and the HsfA1a DNA binding paralleled the mRNA expression of its target genes induced by different stresses. Stress-induced DNA-binding could be reversed, both in vitro and in vivo, by subsequent incubation with reducing agents (DTT, NADPH). These data suggest that HsfA1a can directly sense stress and become activated, and this process is dependent on the redox state. An N-terminal deletion of the amino acid residues from 48 to 74 negatively affected pH- and hydrogen peroxide-, but not heat-stress sensing.

Involvement of the Leishmania donovani virulence factor A2 in protection against heat and oxidative stress

Leishmania is an obligate intracellular protozoan parasite that infects cells of the reticulo-endothelial system. Host defences against Leishmania include fever and oxidant production, and the parasite has developed a number of defence mechanisms to neutralize the host response. The Leishmania donovani A2 family of proteins has been shown to be essential for survival in mammalian visceral organs. Here we provide evidence that A2 proteins protect the parasite against host defences, namely heat stress (fever) and oxidative stress. A2 is however unable to protect the cells from endoplasmic reticulum stress induced by dithiothreitol. To downregulate A2 protein expression, L. donovani was transfected with an A2 antisense RNA expressing-vector, resulting in significant reduction of A2 levels. The resulting A2-deficient cells were more sensitive to heat shock and this was associated with increased production of internal oxidants during heat shock. Moreover, axenic amastigotes with downregulated A2 expression had increased internal oxidants and decreased viability following treatment with hydrogen peroxide or a nitric oxide donor when compared to control cells. Overall, these results suggest that A2 protects L. donovani from a variety of stresses, thereby allowing it to survive in the internal organs of the mammalian host and to cause visceral disease.

Modulation of heat shock transcription factor 1 as a therapeutic target for small molecule intervention in neurodegenerative disease

A yeast-based small molecule screen identifies a novel activator of human HSF1 and protein chaperone expression and which appears to alleviate the toxicity of protein misfolding diseases.

Neurodegenerative diseases such as Huntington disease are devastating disorders with no therapeutic approaches to ameliorate the underlying protein misfolding defect inherent to poly-glutamine (polyQ) proteins. Given the mounting evidence that elevated levels of protein chaperones suppress polyQ protein misfolding, the master regulator of protein chaperone gene transcription, HSF1, is an attractive target for small molecule intervention. We describe a humanized yeast-based high-throughput screen to identify small molecule activators of human HSF1. This screen is insensitive to previously characterized activators of the heat shock response that have undesirable proteotoxic activity or that inhibit Hsp90, the central chaperone for cellular signaling and proliferation. A molecule identified in this screen, HSF1A, is structurally distinct from other characterized small molecule human HSF1 activators, activates HSF1 in mammalian and fly cells, elevates protein chaperone expression, ameliorates protein misfolding and cell death in polyQ-expressing neuronal precursor cells and protects against cytotoxicity in a fly model of polyQ-mediated neurodegeneration. In addition, we show that HSF1A interacts with components of the TRiC/CCT complex, suggesting a potentially novel regulatory role for this complex in modulating HSF1 activity. These studies describe a novel approach for the identification of new classes of pharmacological interventions for protein misfolding that underlies devastating neurodegenerative disease.

The misfolding of proteins into a toxic state contributes to a variety of neurodegenerative diseases such as Huntington, Alzheimer, and Parkinson disease. Although no known cure exists for these afflictions, many studies have shown that increasing the levels of protein chaperones, proteins that assist in the correct folding of other proteins, can suppress the neurotoxicity of the misfolded proteins. As such, increasing the cellular concentration of protein chaperones might serve as a powerful therapeutic approach in treating protein misfolding diseases. Because the levels of protein chaperones in the cell are primarily controlled by the heat shock transcription factor 1 [HSF1], we have designed and implemented a pharmacological screen to identify small molecules that can promote human HSF1 activation and increase the expression of protein chaperones. Through these studies, we have identified HSF1A, a molecule capable of activating human HSF1, increasing the levels of protein chaperones and alleviating the toxicity of misfolded proteins in both cell culture as well as fruit fly models of neurodegenerative disease.

Aberrant regulation and modification of heat shock factor 1 in senescent human diploid fibroblasts

Induction of the heat shock response (HSR), determined by hsp70-luciferase reporter and HSP70 protein expression, is attenuated as a function of age of the IMR-90 human diploid fibroblasts. To better understand the underlying mechanism, we evaluated changes in the regulation and function of the HSF1 transcription factor. We show that the activation of HSF1 both in vivo and in vitro was decreased as a function of age, and this was attributable to a change in the regulation of HSF1 as the abundance of HSF1 protein and mRNA was unaffected. HSF1 was primarily cytosolic in young cells maintained at 37 degrees C, and heat shock promoted its quantitative nuclear translocation and trimerization. In old cells, some HSF1 was nuclear sequestered at 37 degrees C, and heat shock failed to promote the quantitative trimerization of HSF1. These changes in HSF1 could be reproduced by treating young cells with H2O2 to stunt them into premature senescence. Flow cytometry measurement of peroxide content showed higher levels in old cells and H2O2-induced premature senescent cells as compared to young cells. Experiments using isoelectric focusing and Western blot showed age-dependent changes in the mobility of HSF1 in a pattern consistent with its S-glutathiolation and S-nitrosylation; these changes could be mimicked by treating young cells with H2O2. Our results demonstrated dynamic age-dependent changes in the regulation but not the amount of HSF1. These changes are likely mediated by oxidative events that promote reversible and irreversible modification of HSF1 including S-glutathiolation and S-nitrosylation.

Riluzole increases the amount of latent HSF1 for an amplified heat shock response and cytoprotection

Background

Induction of the heat shock response (HSR) and increased expression of the heat shock proteins (HSPs) provide mechanisms to ensure proper protein folding, trafficking, and disposition. The importance of HSPs is underscored by the understanding that protein mis-folding and aggregation contribute centrally to the pathogenesis of neurodegenerative diseases.

Methodology/Principal Findings

We used a cell-based hsp70-luciferease reporter gene assay system to identify agents that modulate the HSR and show here that clinically relevant concentrations of the FDA-approved ALS drug riluzole significantly increased the heat shock induction of hsp70-luciferse reporter gene. Immuno-Western and -cytochemical analysis of HSF1 show that riluzole increased the amount of cytosolic HSF1 to afford a greater activation of HSF1 upon heat shock. The increased HSF1 contributed centrally to the cytoprotective activity of riluzole as hsf1 gene knockout negated the synergistic activity of riluzole and conditioning heat shock to confer cell survival under oxidative stress. Evidence of a post-transcriptional mechanism for the increase in HSF1 include: quantitation of mRNA^hsf1 by RT-PCR showed no effect of either heat shock or riluzole treatment; riluzole also increased the expression of HSF1 from a CMV-promoter; analysis of the turnover of HSF1 by pulse chase and immunoprecipitation show that riluzole slowed the decay of [³⁵S]labeled-HSF1. The effect of riluzole on HSF1 was qualitatively different from that of MG132 and chloroquine, inhibitors of the proteasome and lysosome, respectively, and appeared to involve the chaperone-mediated autophagy pathway as RNAi-mediated knockdown of CMA negated its effect.

Conclusion/Significance

We show that riluzole increased the amount of HSF1 to amplify the HSR for cytoprotection. Our study provides novel insight into the mechanism that regulates HSF1 turnover, and identifies the degradation of HSF1 as a target for therapeutics intervention.

Neural differentiation and the attenuated heat shock response

Differentiation of neural progenitor cells of neuroblastoma, pheochromocytoma, and surrogate stem cell lineages from a state resembling stem cells to a state resembling neurons is accompanied by a marked attenuation in induction of the heat shock protein 70 promoter driven-luciferase reporter gene, and induction of the reporter gene in primary embryonic neurons from hippocampus, cortex, and spinal cord is lower still when compared to the differentiated cells. Neural specificity of this phenotype is demonstrated by a negative correlation of hsp70-reporter gene expression and neurite extension under various experimental conditions. Analysis of biochemical events involved in induction of the heat shock response (HSR) reveal a blunted activation of HSF1 DNA-binding activity, and decreased induction of the mRNA(hsp70) and the 72 kDa HSP70 protein. Immunocytochemical staining for HSP70 demonstrates a cytoplasmic staining pattern; heat shock greatly increased the HSP70 staining intensity in the undifferentiated cells and less so in the differentiated cells. Vulnerability of the differentiated cells towards the oxidizer, arsenite, and the excitotoxic glutamate/glycine is demonstrated by the dose-dependent cytotoxic effects of these agents on cell viability and activation of caspase 3/7. Importantly, conditioning heat shock as well as increased expression of HSP70 by gene transfer conferred protection against such cytotoxicity. Together, our results show that neural differentiation is associated with a decreased induction of the heat shock response and an increased vulnerability to stress induced pathologies and death.

Changes in the regulation of heat shock gene expression in neuronal cell differentiation

Neuronal differentiation of the NG108-15 neuroblastoma-glioma hybrid cells is accompanied by a marked attenuation in the heat shock induction of the Hsp70-firefly luciferase reporter gene activity. Analysis of the amount and activation of heat shock factor 1, induction of mRNA(hsp), and the synthesis and accumulation of heat shock proteins (HSPs) in the undifferentiated and differentiated cells suggest a transcriptional mechanism for this attenuation. Concomitant with a decreased induction of the 72-kDa Hsp70 protein in the differentiated cells, there is an increased abundance of the constitutive 73-kDa Hsc70, a protein known to function in vesicle trafficking. Assessment of sensitivity of the undifferentiated and differentiated cells against stress-induced cell death reveals a significantly greater vulnerability of the differentiated cells toward the cytotoxic effects of arsenite and glutamate/glycine. This study shows that changes in regulation of the HSP and HSC proteins are components of the neuronal cell differentiation program and that the attenuated induction of HSPs likely contributes to neuronal vulnerability whereas the increased expression of Hsc70 likely has a role in neural-specific functions.

Adaptive responses of mouse skeletal muscle to contractile activity: The effect of age

This study has characterised the time course of two major transcriptional adaptive responses to exercise (changes in antioxidant defence enzyme activity and heat shock protein (HSP) content) in muscles of adult and old male mice following isometric contractions and has examined the mechanisms involved in the age-related reduction in transcription factor activation. Muscles of B6XSJL mice were subjected to isometric contractions and analysed for antioxidant defence enzyme activities, heat shock protein content and transcription factor DNA binding activity. Data demonstrated a significant increase in superoxide dismutase (SOD) and catalase activity and HSP content of muscles of adult mice following contractile activity which was associated with increased activation of the transcription factors, nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1) and heat shock factor (HSF) following contractions. Significant increases in SOD and catalase activity and heat shock cognate (HSC70) content were seen in quiescent muscles of old mice. The increase in antioxidant defence enzyme activity following contractile activity seen in muscles of adult mice was not seen in muscles of old mice and this was associated with a failure to fully activate NF-kappaB and AP-1 following contractions. In contrast, although the production of HSPs was also reduced in muscles of old mice following contractile activity compared with muscles of adult mice following contractions, this was not due to a gross reduction in the DNA binding activity of HSF.

Global effects of homocysteine on transcription in Escherichia coli: induction of the gene for the major cold-shock protein, CspA

Homocysteine (Hcy) is a thiol-containing amino acid that is considered to be medically important because it is linked to the development of several life-threatening diseases in humans, including cardiovascular disease and stroke. It inhibits the growth of Escherichia coli when supplied in the growth medium. Growth inhibition is believed to arise as a result of partial starvation for isoleucine, which occurs because Hcy perturbs the biosynthesis of this amino acid. This study attempted to further elucidate the inhibitory mode of action of Hcy by examining the impact of exogenously supplied Hcy on the transcriptome. Using gene macroarrays the transcript levels corresponding to 68 genes were found to be reproducibly altered in the presence of 0.5 mM Hcy. Of these genes, the biggest functional groups affected were those involved in translation (25 genes) and in amino acid metabolism (19 genes). Genes involved in protection against oxidative stress were repressed in Hcy-treated cells and this correlated with a decrease in catalase activity. The gene showing the strongest induction by Hcy was cspA, which encodes the major cold-shock protein CspA. RT-PCR and reporter fusion experiments confirmed that cspA was induced by Hcy. Induction of cspA by Hcy was not caused by nutritional upshift, a stimulus known to induce CspA expression, nor was it dependent on the presence of a functional CspA protein. The induction of cspA by Hcy was suppressed when isoleucine was included in the growth medium. These data suggest that the induction of CspA expression in the presence of Hcy occurs because of a limitation for isoleucine. The possibility that Hcy-induced cspA expression is triggered by translational stalling that occurs when the cells are limited for isoleucine is discussed.

Specificity in reactive oxidant signaling: think globally, act locally

Although reactive oxidants have long been stigmatized as unwanted metabolic byproducts, the expression of oxidases specifically functioning to produce these same molecules in a regulated fashion is surprisingly pervasive throughout metazoan and plant evolution. Although the involvement of oxidants in many signaling pathways is well documented, the cellular strategies for conferring pathway specificity to such reactive molecules have remained more recondite. Recent studies now suggest that cells may spatially restrict oxidant production to allow microdomain-specific signaling.

Redox regulatory mechanisms in cellular stress responses

BACKGROUND

Reactive oxygen species are produced in a highly localized and specific pattern in biological stress responses. The present review examines the redox regulatory aspects of a number of molecular stress response mechanisms in both prokaryotes and eukaryotes.

SCOPE

The present review provides examples representing both the cytoplasmic stress response, often studied as the heat shock response, as well as the stress response of the endoplasmic reticulum, known as the unfolded protein response. The examples have been selected to illustrate the variety of ways that redox signals mediate and affect stress responses.

CONCLUSIONS

Redox regulatory mechanisms are intricately embedded in both the cytoplasmic and endoplasmic reticulum stress responses at multiple levels. Many different stimuli, both internal and external, activate endogenous production of reactive oxygen species as a necessary part of the intracellular communication system that activates stress responses.

Global methods to monitor the thiol-disulfide state of proteins in vivo

Cysteines play an important role in protein biochemistry. The unique chemical property and high reactivity of the free thiol group makes reduced cysteine a versatile component of catalytic centers and metal binding sites in many cytosolic proteins and oxidized cystine a stabilizing component in many secreted proteins. Moreover, cysteines readily react with reactive oxygen and nitrogen species to form reversible oxidative thiol modifications. As a result, these reversible thiol modifications have found a use as regulatory nano-switches in an increasing number of redox sensitive proteins. These redox-regulated proteins are able to adjust their activity quickly in response to changes in their redox environment. Over the past few years, a number of techniques have been developed that give insight into the global thiol-disulfide state of proteins in the cell. They have been successfully used to find substrates of thiol-disulfide oxidoreductases and to discover novel redoxregulated proteins. This review will provide an overview of the current techniques, focus on approaches to quantitatively describe the extent of thiol modification in vivo, and summarize their applications.

Dynamic regulation and involvement of the heat shock transcriptional response in arsenic carcinogenesis

The objective of this study is to better define induction of the heat shock response by arsenite, and to evaluation if induction of heat shock proteins (HSPs) contributes to the carcinogenic activity of arsenite. We show here that arsenite is a ubiquitous inducer of the heat shock response in mammalian cells: that it activated heat shock transcription factor 1 (HSF1) DNA-binding activity, enhanced hsp 70 promoter, and induced hsp70mRNA and synthesis of HSP chaperones. Using a high throughput hsp70 promoter-luciferase reporter assay, we observed a hormetic dose response where low concentrations of arsenite stimulated and high concentrations inhibited. Further, the response was time-dependent such that with longer times of incubation, the dose response shifted to the left. The effect of arsenite in inducing the hsp 70-luciferase reporter absolutely required a functional HSF1 as it was not observed in HSF1 minus cells but re-instated by expression of HSF1. Consistent with the suggestion that arsenic targets vicinal cysteine-SH, we showed that dithiothreitol blocked the effect of arsenite. Assays of cell viability and caspase showed that arsenite caused a dose-dependent increase in cell death by activation of caspase 3/7 and pre-induction of HSPs blunted these effects. Using anchorage independent cell growth as a late stage tumor promotion assay, we showed that low concentrations of arsenite had a growth promoting effect, which was enhanced by moderate heat shock. Our study provides evidence that induction of the heat shock response is a sensitive biomarker of arsenic exposure and that induction of HSPs likely contributes to the tumor promotion effect of arsenic.

Hypoxia upregulates osteopontin expression in NIH-3T3 cells via a Ras-activated enhancer

Osteopontin (OPN) is a secreted phosphoglycoprotein that has been linked to tumor progression and survival in several solid tumors, including head and neck cancers. Previous studies showed that OPN expression is induced by tumor hypoxia, and its plasma levels can serve as a surrogate marker for tumor hypoxia and treatment outcome in head and neck cancer patients. In this study, we investigate the transcriptional mechanism by which hypoxia enhances OPN expression. We found that OPN is induced in head and neck squamous cell carcinoma (HNSCC) cell lines and in NIH3T3 cells by hypoxia at both mRNA and protein levels in a time-dependent manner. Actinomycin D chase experiments showed that hypoxic induction of OPN was not due to increased mRNA stability. Deletion analyses of the mouse OPN promoter regions indicated that a ras-activated enhancer (RAE) located at -731 to -712 relative to the transcription start site was essential for hypoxia-enhanced OPN transcription. Using electrophoretic mobility shift assays with the RAE DNA sequence, we found that hypoxia induced sequence-specific DNA-binding complexes. Furthermore, hypoxia and ras exposure resulted in an additive induction of OPN protein and mRNA levels that appeared to be mediated by the RAE. Induction of OPN through the RAE element by hypoxia is mediated by an Akt-kinase signaled pathway as decreasing Akt levels with dominant negative constructs resulted in inhibition of OPN induction by hypoxia. Taken together, these results have identified a new hypoxia responsive transcriptional enhancer that is regulated by Akt signaling.

Potential roles of hepatic heat shock protein 25 and 70i in protection of mice against acetaminophen-induced liver injury

The aim of the present study was to assess the contribution of the level of expression of heat shock protein 25 (HSP25), 60 (HSP60), 70 (HSC70) and 70i (HSP70i) in mouse livers after a lethal dose of acetaminophen (APAP) to their survival. We examined changes in survival ratio, plasma APAP level and alanine aminotransferase (ALT) activity, and hepatic reduced glutathione (GSH), HSP25, HSP60, HSC70 and HSP70i levels following treatment of mice with APAP (500 mg/kg, p.o.). The plasma APAP level increased rapidly, and reached a maximum 0.5 h after APAP treatment. Hepatic GSH decreased rapidly, and was almost completely depleted 1 h after APAP treatment. Plasma ALT activity, an index of liver injury, significantly increased from 3 h onwards after APAP treatment. The survival ratios 9 h, 24 h and 48 h after APAP treatment were 96%, 38% and 36%, respectively. We found a remarkable difference in the patterns of hepatic HSP25 and HSP70i induction in mice that survived after APAP treatment. HSP70i levels increased from 1 h onwards after APAP treatment in a time-dependent manner, and reached a maximum at 9 h. In contrast, HSP25 could be detected just 24 h after APAP treatment, and maximal accumulation was observed at 48 h. Other HSPs examined were unchanged. Notably, the survival ratio dropped by only 2% after HSP25 expression. Recently, a novel role for HSP25 as an anti-inflammatory factor was suggested. We have already shown that 48-h treatment with APAP induces severe centrilobular necrosis with inflammatory cell infiltration in mouse livers. Taken together, the level of expression of hepatic HSP25 may be a crucial determinant of the fate of mice exposed to APAP insult.

Hepatoprotective mechanism of schisandrin B: role of mitochondrial glutathione antioxidant status and heat shock proteins

In this study, the time course of schisandrin B- (Sch B-) induced changes in hepatic mitochondrial glutathione antioxidant status (mtGAS) and heat shock protein (HSP) 25/70 induction was examined to study their differential roles in the hepatoprotection afforded by Sch B pretreatment against carbon tetrachloride (CCl(4)) toxicity in mice. Dimethyl diphenyl bicarboxylate (DDB), a nonhepatoprotective analog of Sch B, was also included for comparison. The results indicate that Sch B treatment (2 mmol/kg) produced maximum enhancement in hepatic mtGAS and increases in both hepatic HSP 25 and HSP 70 levels at 24 h after dosing. While the extent of hepatoprotection afforded by Sch B pretreatment against CCl(4) was found to correlate inversely with the elapsed time postdosing, the protective effect was associated with the ability to sustain mtGAS and/or HSP 70 levels in a CCl(4)-intoxicated condition. On the other hand, DDB (2 mmol/kg) treatment, which did not sustain mtGAS and HSP 70 level, could not protect against CCl(4) toxicity. Abolition of the Sch B-mediated enhancement of mtGAS by buthionine sulfoximine/phorone did not completely abrogate the hepatoprotective action of Sch B. The results indicate that Sch B pretreatment independently enhances mtGAS and induces HSP 25/70 production, particularly under conditions of oxidative stress, thereby protecting against CCl(4) hepatotoxicity.

Effects of hydrogen peroxide on bovine leukemia virus expression

Several activators of bovine leukemia virus (BLV) expression, including lipopolysaccharides, phorbol esters and calcium ionophores, are known to generate reactive oxygen species (ROS). Therefore the influence of H2O2 on BLV expression in two BLV producing cell lines was investigated. The effect of H2O2 on BLV expression is apparently dose-dependent. Incubation of FLK/BLV cells with low concentrations of H2O2 (2.5 to 10 microM) induced a marked enhancement of BLV p24 synthesis and an activation of the long terminal repeat (LTR). Higher concentrations resulted in a decrease of proliferation, induction of apoptosis and in a decrease of BLV synthesis. Furthermore, in both cell lines H2O2 treatment led to the activation of NF-kappaB. Pretreatment of cells with antioxidants abrogated the H2O2-induced BLV expression. Taken together, our findings suggest that oxidative stress stimulates BLV expression via activation of NF-kappaB, raising the possibility that biological sources of H2O2, such as stimulated phagocytes, may influence BLV expression.

Redox regulation of mammalian heat shock factor 1 is essential for Hsp gene activation and protection from stress

The activation of eukaryotic heat shock protein (Hsp) gene expression occurs in response to a wide variety of cellular stresses including heat shock, hydrogen peroxide, uncoupled oxidative phosphorylation, infection, and inflammation. Biochemical and genetic studies have clearly demonstrated critical roles for mammalian heat shock factor 1 (HSF1) in stress-inducible Hsp gene expression, resistance to stress-induced programmed cell death, extra-embryonic development, and other biological functions. Activation of mammalian Hsp gene expression involves the stress-inducible conversion of HSF1 from the inactive monomer to the DNA-binding competent homotrimer. Although Hsp activation is a central conserved process in biology, the precise mechanisms for stress sensing and signaling to activate HSF1, and the mechanisms by which many distinct stresses activate HSF1, are poorly understood. In this report we demonstrate that recombinant mammalian HSF1 directly senses both heat and hydrogen peroxide to assemble into a homotrimer in a reversible and redox-regulated manner. The sensing of both stresses requires two cysteine residues within the HSF1 DNA-binding domain that are engaged in redox-sensitive disulfide bonds. HSF1 derivatives in which either or both cysteines were mutated are defective in stress-inducible trimerization and DNA binding, stress-inducible nuclear translocation and Hsp gene trans-activation, and in the protection of mouse cells from stress-induced apoptosis. This redox-dependent activation of HSF1 by heat and hydrogen peroxide establishes a common mechanism in the stress activation of Hsp gene expression by mammalian HSF1.

Molecular chaperones, stress proteins and redox homeostasis

Protection against oxidative stress is highly interrelated with the function of the most ancient cellular defense system, the network of molecular chaperones, heat shock, or stress-proteins. These ubiquitous, conserved proteins help other proteins and macromolecules to fold or re-fold and reach their final, native conformation. Redox regulation of protein folding becomes especially important during the preparation of extracellular proteins to the outside oxidative milieu, which should take place in a gradual and step-by-step controlled manner in the endoplasmic reticulum or in the periplasm. Several chaperones, such as members of the Hsp33 family in yeast and the plethora of small heat shock proteins as well as one of the major chaperones, Hsp70 are able to act against cytoplasmic oxidative damage. Abrupt changes of cellular redox status lead to chaperone induction. The function of several chaperones is tightly regulated by the surrounding redox conditions. Moreover, our recent data suggest that chaperones may act as a central switchboard for the transmission of redox changes in the life of the cell.

Differential acquisition of antigenic peptides by Hsp70 and Hsc70 under oxidative conditions

Hsp70 and Hsc70 are two chaperones of high homology expressed under contrasting situations. Hsc70 is constitutively expressed and poorly stress-inducible, whereas Hsp70 is unabundant in normal physiological situations and strongly induced under oxidative stress. In the present study we show that the chaperoning activity of purified Hsp70 and Hsc70 is minimal under reducing conditions and increases in environments that mimic oxidative stress. Association with peptides is more pronounced for Hsp70 than for Hsc70 in every condition tested and is accompanied with a gradual change in secondary structure during oxidation. The binding of peptides to Hsp70 and Hsc70 under oxidative conditions is not reversible by treatment with a reducing agent, confirming that other chaperone-associated factors are required for substrate release. These findings support the idea that formation of HSP70-peptide complexes and possibly their immunogenicity is enhanced in conditions of stress.

Redox signaling of cardiac HSF1 DNA binding

Experiments involving chemical induction of the heat shock response in simple biological systems have generated the hypothesis that protein denaturation and consequential binding of heat shock transcription factor 1 (HSF1) to proximal heat shock elements (HSEs) on heat shock protein (hsp) genes are the result of oxidation and/or depletion of intracellular thiols. The purpose of the present investigation was to determine the role of redox signaling of HSF1 in the intact animal in response to physiological and pharmacological perturbations. Heat shock and exercise induced HSF1-HSE DNA binding in the rat myocardium (P < 0.001) in the absence of changes in reduced glutathione (GSH), the major nonprotein thiol in the cell. Ischemia-reperfusion, which decreased GSH content (P < 0.05), resulted in nonsignificant HSF1-HSE formation. This dissociation between physiological induction of HSF1 and changes in GSH was not gender dependent. Pharmacological ablation of GSH with L-buthionine-[S,R]-sulfoximine (BSO) treatment increased myocardial HSF1-HSE DNA binding in estrogen-naive animals (P = 0.007). Thus, although physiological induction of HSF1-HSE DNA binding is likely regulated by mediators of protein denaturation other than cellular redox status, the proposed signaling pathway may predominate with pharmacological oxidation and may represent a plausible and accessible strategy in the development of HSP-based therapies.

Complex effects of sulfhydryl reagents on ligand interactions with nucleoside transporters: evidence for multiple populations of ENT1 transporters with differential sensitivities to N-ethylmaleimide

Functional studies have implicated cysteines in the interaction of ligands with the ENT1 nucleoside transporter. To better define these interactions, N-ethylmaleimide (NEM) and p-chloromercuribenzylsulfonate (pCMBS) were tested for their effects on ligand interactions with the [(3)H] nitrobenzylthioinosine (NBMPR) binding site of the ENT1 transporters of mouse Ehrlich ascites cells and human erythrocytes. NEM had biphasic, concentration-dependent effects on NBMPR binding to intact Ehrlich cells, plasma membranes, and detergent-solubilized membranes, with about 35% of the binding activity being relatively insensitive to NEM inhibition. NBMPR binding to human erythrocyte membranes also displayed heterogeneity in that about 33% of the NBMPR binding sites remained, albeit with lower affinity for NBMPR, even after treatment with NEM at concentrations in excess of 1 mM. However, unlike that seen for Ehrlich cells, no "reversal" in NBMPR binding to human erythrocyte membranes was observed at the higher concentrations of NEM. pCMBS inhibited 100% of the NBMPR binding to both Ehrlich cell and human erythrocyte membranes, but had no effect on the binding of NBMPR to intact cells. The effects of NEM on NBMPR binding could be prevented by coincubation of membranes with nonradiolabeled NBMPR, adenosine, or uridine. Treatment with NEM and pCMBS also decreased the affinity of other nucleoside transport inhibitors for the NBMPR binding site, but enhanced the affinities of nucleoside substrates. These data support the existence of at least two populations of ENT1 in both erythrocyte and Ehrlich cell membranes with differential sensitivities to NEM. The interaction of NEM with the mouse ENT1 protein may also involve additional sulphydryl groups not present in the human ENT1.

JNK phosphorylates the HSF1 transcriptional activation domain: role of JNK in the regulation of the heat shock response

The role of c-Jun NH2-terminal kinase (JNK) signaling cascade in the stress-inducible phosphorylation of heat shock factor 1 (HSF1) was investigated using known agonists and antagonists of JNK. We showed that treatment of HeLa cells with MG132, a proteasome inhibitor and known INK activator, caused the transcriptional activation domain of HSF1 to be targeted and phosphorylated by JNK2 in vivo. Dose-response and time course studies of the effects of heat shock and anisomycin treatment showed a close correlation of the activation of JNK and hyperphosphorylation of HSF1. SB203580 inhibited INK at the 100 microM concentration and significantly reduced the amount of hyperphosphorylated HSF1 upon heat shock or anisomycin treatment. SB203580 and dominant-negative JNK suppress hsp70 promoter-driven reporter gene expression selectively at 45 degrees C but not at 42 degrees C heat stress, suggesting that JNK would be preferentially associated with the protective heat shock response against severe heat stress. The possibility that JNK-mediated phosphorylation of HSF1 may selectively stabilize the HSF1 protein and confers protection to cells under conditions of severe stress is discussed.

Resolution, detection, and characterization of redox conformers of human HSF1

We describe here an experimental protocol for the resolution, detection, and quantitation of the reduced and oxidized conformers of human heat shock factor 1 (hHSF1) and report on the effects in vitro and in vivo of redox-active agents on the redox status, structure, and function of hHSF1. We showed that diamide, a reagent that promotes disulfide bond formation, caused a loss of immunorecognition of the monomeric hHSF1 protein in a standard Western blot detection procedure. Modification of the Western blot procedure to include dithiothreitol in the equilibration and transfer buffers after gel electrophoresis allowed for the detection of a compact, intramolecularly disulfide cross-linked oxidized hHSF1 (ox-hHSF1) in the diamide-treated sample. The effect of diamide was blocked by pretreatment with N-ethylmaleimide and was reversed by dithiothreitol added to the sample prior to gel electrophoresis. Incubation with nitrosoglutathione at 42 degrees C also promoted the conversion of HSF1 to ox-HSF1; at 25 degrees C, however, nitrosoglutathione was by itself without effect but blocked the formation of ox-hHSF1 in the presence of diamide. The disulfide cross-linked ox-hHSF1 was monomeric and resistant to the in vitro heat-induced trimerization and activation. The possibility that ox-HSF1 may occur in oxidatively stressed cells was evaluated. Treatment of HeLa cells with 2 mm l-buthionine sulfoximine promoted the formation of ox-HSF1 and blocked the heat-induced activation of HSF DNA binding activity. Our result suggests that hHSF1 may have integrated redox chemistry of cysteine sulfhydryl into its functional responses.

Activation of heat shock factor 1 by pyrrolidine dithiocarbamate is mediated by its activities as pro-oxidant and thiol modulator

Pyrrolidine dithiocarbamate (PDTC) is known to inhibit NF-kappa B, which plays a critical role(s) as an immediate early mediator of immune and inflammatory responses. Here we show that PDTC induces heat shock factor 1 (HSF1) activation and heat shock protein expression, while other antioxidants such as butylated hydroxytoluene (BHT), n-propylgallate (PG), ascorbic acid (AA), and N-acetyl-L-cysteine (NAC) do not. Since PDTC exerts other functions than antioxidant, e.g., a pro-oxidant, metal chelator, and thiol group modulator, we examined which of these activities is responsible for the PDTC-induced HSF1 activation. PDTC-induced HSF1 activation was not prevented by metal chelators, EDTAs, indicating that the metal chelating effect of PDTC is not linked to the HSF1 activation. PDTC increased intracellular GSSG level. In addition, PDTC-induced activation of HSF1 was significantly inhibited by NAC and a thiol-reducing agent dithiothreitol (DTT), while it was partially prevented by other antioxidants, AA, BHT, and PG. These results suggest that the activation of HSF1 by PDTC may be due to its activities as pro-oxidant and thiol group modulator rather than anti-oxidant.

Heat shock and oxidative stress-induced exposure of hydrophobic protein domains as common signal in the induction of hsp68

The hypothesis of a common signal for heat shock (HS) and oxidative stress (OS) was analyzed in C6 cells with regard to the induction of heat shock proteins (Hsps). The synthesis rate and level of the strictly inducible Hsp68 was significantly higher after HS (44 degrees C) compared with OS (2 mm H2O2). This difference corresponded to higher and lower activation of the heat shock factor (HSF) by HS and OS, respectively. OS, on the other hand, showed stronger cytotoxicity compared with HS as indicated by drastic lipid peroxidation and inhibition of protein synthesis as well as of mitochondrial and endocytotic activity. Lactic dehydrogenase also revealed stronger inhibition of enzyme activity by OS than by HS as shown in cells and in vitro experiments. Conformational analysis of lactic dehydrogenase by the fluorophore 1-anilinonaphtalene-8-sulfonic acid, however, showed stronger exposure of hydrophobic domains after HS than after OS which correlates positively with the Hsp68 response. Treatment of cells with deoxyspergualin, which exhibits high affinity to Hsps, the putative inhibitors of HSF, strongly increased only OS-induced hsp68 expression. In conclusion, the results suggest that exposure of hydrophobic domains of cytosolic proteins represents the common first signal in the multistep activation pathway of HSF.

Pervanadate induces the hyperphosphorylation but not the activation of human heat shock factor 1

In this study, we evaluated the effects of pervanadate, a tyrosine phosphatase inhibitor, on the regulation and function of heat-shock factor 1 (HSF1) in HeLa cells. We showed that 50-100 microM pervanadate induced the hyperphosphorylation of the latent HSF1, as demonstrated by a retarded mobility of the HSF1 protein in SDS-polyacrylamide gel electrophoresis and as supported by the reversal of this mobility shift upon treatment of the cell extract with acid phosphatase. Pervanadate by itself had no effect on the monomeric stoichiometry and DNA-binding activity of HSF1. Upon heat shock, the pervanadate-induced hyperphosphorylated HSF1 formed DNA-binding trimers and translocated into the nuclear compartment. At high concentration (approximately 500 microM), pervanadate also induced the tyrosine phosphorylation of many cellular proteins and blunted the heat-induced transcription of hsp 70. N-acetyl cysteine inhibited these effects of pervanadate, suggesting a redox-based mechanism for its activity. Analysis of the activation of mitogen-activated protein kinases (MAPKs) using antibodies specific for the phospho-form (activated) of the kinases in Western blot showed that pervanadate activated extracellular signal-regulated kinase (ERK1/2), c-Jun-N-terminal kinase 1/2 (JNK1/2), and p-38 kinase. Pharmacological inhibitors of the ERK1/2 kinase pathway or the p38 kinase had little or no effect on the pervanadate-induced hyperphosphorylation of HSF1. Our results show that hyperphosphorylation of hHSF1 can occur prior to and independent of other events involved in the activation of hHSF1. The possibility that activation of the MAPK signaling cascade, notably JNK, may contribute to the hyperphosphorylation of human HSF1 (hHSF1) is discussed.

Rac1 regulates stress-induced, redox-dependent heat shock factor activation

The signaling pathway by which environmental stresses activate heat shock factors (HSFs) is not completely understood. We show that the small GTPase rac1, and Rac1-regulated reactive oxygen species (ROS) play an important role in stress-stimulated heat shock response. A dominant-negative allele of Rac1 (Rac1N17) inhibits the hypoxia/reoxygenation and sodium arsenite-induced transcriptional activity of HSF-1 and the transcription of heat shock protein 70. Rac1N17 also suppresses the production of intracellular ROS induced by hypoxia/reoxygenation or sodium arsenite. Moreover, direct suppression of intracellular ROS levels by antioxidants decreases stress-stimulated HSF activity. However, expression of a constitutively active mutant of Rac1 (Rac1V12) in the absence of extracellular stresses does not increase intracellular ROS levels or induce the heat shock response. These results show that Rac1 is a necessary but insufficient component of the stress-induced signaling pathway that leads to ROS production, activation of HSFs, and transcription of heat shock proteins.

Phenylarsine oxide inhibits heat shock protein 70 induction in cultured guinea pig gastric mucosal cells

Phenylarsine oxide (PAO) forms a stable ring complex with vicinal dithiols that can be reversed with 2,3-dimercaptopropanol (DMP) but not by dithiothreitol (DTT) or 2-mercaptoethanol (2-ME). PAO at 2 microM or higher inhibited heat shock protein 70 (HSP70) induction within minutes in cultured guinea pig gastric mucosal cells exposed to heat (43 degrees C) for 30 min. PAO did not affect the nuclear translocation and phosphorylation of heat shock factor 1 (HSF1) induced by heat stress, but it completely blocked the binding activity of HSF1 to the heat shock element (HSE), leading to the block of expression of HSP70 mRNA and accumulation of HSP70 in the cells. These inhibitions were completely reversed with 2 microM DMP but not with 0.1 mM DTT or 1 mM 2-ME, suggesting specific interactions between PAO and vicinal dithiol-containing molecules. Thioredoxin (Trx) reversed the inhibition of the binding activity of HSF1 in whole cell extracts prepared from PAO-treated, heat-stressed cells. Our results suggest that PAO may react with vicinal-containing molecules including Trx and specifically block the interaction between HSF1 and HSE.

Exposure of developing oligodendrocytes to cadmium causes HSP72 induction, free radical generation, reduction in glutathione levels, and cell death

Primary cultures of oligodendrocytes were used to study the toxic effects of cadmium chloride. Cell viability was evaluated by the mitochondrial dehydrogenase activity and confirmed by propidium iodide (PI) fluorescence staining. The expression of the 72 kDa stress protein, HSP72, was assayed by Western blot analysis. The results showed that Cd(2+)-induced toxicity was dependent on the time and dose of exposure, as well as on the developmental stage of the cultures. Oligodendrocyte progenitors were more vulnerable to Cd(2+) toxicity than were mature oligodendrocytes. Mature oligodendrocytes accumulated relatively higher levels of Cd(2+) than did progenitors, as determined by (109)CdCl(2) uptake; treatment with the metal ion caused a more pronounced reduction in intracellular glutathione levels and significantly higher free radical accumulation in progenitors. The latter could explain the observed differences in Cd(2+) susceptibility. HSP72 protein expression was increased both in progenitors and in mature cells exposed to Cd(2+). Pretreatment with N-acetylcysteine, a thiocompound with antioxidant activity and a precursor of glutathione, prevented Cd(2+)-induced (i) reduction in glutathione levels and (ii) induction of HSP72 and diminished (i) Cd(2+) uptake and (ii) Cd(2+)-evoked cell death. In contrast, buthionine sulfoximine, an inhibitor of gamma-glutamyl-cysteine synthetase, depleted glutathione, and potentiated the toxic effect of Cd(2+). These results strongly suggest that Cd(2+)-induced cytotoxicity in oligodendrocytes is mediated by reactive oxygen species and is modulated by glutathione levels.

Oxidants, nitrosants, and the lung

The respiratory tract is subjected to a variety of environmental stresses, including oxidizing gases, particulates, and airborne microorganisms, that together, may injure structural and functional lung components and thereby jeopardize the primary lung function of gas exchange. To cope with such various environmental threats, the lung has developed elaborate defense mechanisms that include inflammatory-immune pathways as well as several antioxidant systems. These defense systems operate largely in extracellular spaces, thus protecting underlying bronchial and alveolar epithelial cells from injury, although these cells themselves are also active participants in such (inflammatory) defense mechanisms. Although potentially harmful, oxidants are increasingly recognized as pathophysiologic mediators produced primarily by inflammatory-immune cells as a host defense mechanism, but also by various other cell types as an intracellular mediator in various cell responses, thus affecting inflammatory-immune processes or inducing resistance. The molecular mechanisms and signaling pathways involved in such processes are the focus of much current investigation. Nitric oxide, a messenger molecule produced by many lung cell types, also modulates oxidant-mediated processes, thereby giving rise to a new family of reactive nitrogen species ("nitrosants") with potentially unique signaling properties. The complex role of oxidants and nitrosants in various pathophysiologic processes in the lung have confounded the design of therapeutic approaches with antioxidant substrates. This review discusses current knowledge regarding extracellular antioxidant defenses in the lung, and oxidant/nitrosant mechanisms operating under inflammatory-immune conditions and their potential contribution to common lung diseases. Finally, some recent developments in antioxidant therapeutic strategies are discussed.

The influence of hyperthermic stress on the redox state of glucocorticoid receptor

Glucocorticoid receptor (GR) is hormone-dependent transcription factor which participates in intracellular signal transduction. The reduced state of the receptor sulfhydryl groups is considered a necessary prerequisite for its normal functioning under the homeostatic conditions. The aim of the work presented in this paper was to examine the influence of non-homeostatic conditions - whole body hyperthermic stresses at 41 degrees C and 42 degrees C, on GR redox state. Non-reducing SDS-PAGE and immunoblot analysis were used to trace alterations of the receptor's redox state. The steroid binding assay was performed in order to examine direct influence of the whole body heat stresses on the receptor thiols. The results obtained show that the 41 degrees C stress leads to formation of intermolecular disulfide bonds between apo-GR and associated heat shock proteins (Hsp90, Hsp70). Apart from intermolecular GR-Hsp90 and GR-Hsp70 disulfide linkages, 42 degrees C hyperthermic stress also caused creation of intramolecular ones within GR. The results imply malfunctioning of intracellular redox control mechanisms under the hyperthermic conditions.

Inhibitory effects of sesquiterpenes from bay leaf on nitric oxide production in lipopolysaccharide-activated macrophages: structure requirement and role of heat shock protein induction

The methanolic extract from the leaves of Laurus nobilis (bay leaf, laurel) was found to inhibit nitric oxide (NO) production in lipopolysaccharide (LPS)-activated mouse peritoneal macrophages. Through bioassay-guided separation, fourteen known sesquiterpenes were isolated from the active fraction and were examined for ability to inhibit the NO production. Seven sesquiterpene lactones (costunolide, dehydrocostus lactone, eremanthine, zaluzanin C, magnolialide, santamarine and spirafolide) potently inhibited LPS-induced NO production (IC50 = 1.2 approximately 3.8 microM). Other sesquiterpene constituents also showed the inhibitory activity (IC50 > or = 21 microM), but their inhibitory activities were less than those of sesquiterpene lactones. Alpha-methylene-gamma-butyrolactone also showed inhibitory activity (IC50 = 9.6 microM), while mokko lactone and watsonol A etc., reductants of the alpha-methylene-gamma-butyrolactone moiety by NaBH4 or DIBAL, and a 2-mercaptoethanol adduct of dehydrocostus lactone showed little activity (IC50 > or = 18 microM). These results indicated that the alpha-methylene-gamma-butyrolactone moiety is important for the activity. Furthermore, costunolide and dehydrocostus lactone inhibited inducible nitric oxide synthase (iNOS) induction in accordance with induction of heat shock protein 72 (HSP 72). These results suggested that, as one of their mechanisms of action, sesquiterpene lactones induce HSP 72 thereby preventing nuclear factor-kappaB activation followed by iNOS induction.

Linking gene expression to mechanisms of toxicity

Activation of gene expression is one of the earliest cellular responses to toxicity. However, our understanding of the biological and biochemical signals that activate these toxicant-responsive genes as well as the consequences of gene activation to survival of the organism remains sketchy. In this article, strategies that can be used to link changes in gene expression to biochemical mechanisms of toxicity are addressed using the hsp70 and grp78 genes as examples. The data indicate that activation of hsp70 is linked to changes in thiol-disulfide redox perturbations while grp78 activation may be caused by loss of calcium from the endoplasmic reticulum. Each gene is part of a discrete feedback regulated signaling pathway designed to protect cells against the toxic signals that activate gene expression.

Calmodulin and cyclic ADP-ribose interaction in Ca2+ signaling related to cardiac sarcoplasmic reticulum: superoxide anion radical-triggered Ca2+ release

Reactive oxygen species (ROS) are often shown to damage cellular functions. The targets of oxidative damage depend on the nature of ROS produced and the site of generation. In contrast, ROS can also regulate signal transduction. In this case, ROS may either induce or enhance events, which lead to forward directions of cellular signaling. The consequences of regulation of signal transduction can be observed in physiological processes such as muscle contraction. Here, we discuss the concentration-dependent effects of superoxide anion radical (*O2-) on Ca2+ release from the cardiac sarcoplasmic reticulum (SR). Recent studies suggest that the ADP-ribosyl cyclase pathway, through its production of cyclic adenosine 5'-diphosphoribose (cADPR), may control Ca2+ mobilization in cardiac muscle cells. *O2- has dual effects that are concentration dependent. At low concentrations (nearly nanomolar levels), *O2- induces Ca2+ release by stimulating synthesis of cADPR, which requires calmodulin for sensitization of ryanodine-sensitive Ca2+-release channels (RyRC). At these low concentrations, *O2- is responsible for regulation of cellular signal transduction. At higher concentrations (micromolar levels), *O2- produces a loss in the function of calmodulin that is to inhibit RyRC. This results in an increase in Ca2+ release, which is linked to cell injury. The difference in the functions of low and high concentrations of *O2- may result in two distinct physiological roles in cardiac muscle Ca2+ signaling.

Salicylic acid influences Hsp70/Hsc70 expression in Lycopersicon esculentum: dose- and time-dependent induction or potentiation

Overexpression of heat-shock (HS) proteins (HSP) is often sufficient to protect against lethal environmental stresses. Anti-inflammatory salicylates potentiate the induction of the 70 kDa HSP (Hsp70) in mammals in response to HS and enhance thermotolerance. In plants, salicylic acid (SA) is a natural signalling molecule, mediating resistance in response to avirulent pathogens. The influence of SA on the HS response in plants is, however, unknown. We investigated the effect of SA, alone or with HS, on Hsp70/Hsc70 expression in tomato cells using biometabolic labelling and Western blotting. A dose- and time-dependent influence on Hsp70/Hsc70 accumulation was observed: SA at 1.0 mM (3 h) potentiated heat-induced accumulation, while 1.0 mM (5 h) and 0.5 mM (8 h) induced expression, the latter preceded by increased membrane permeability. These results suggest that in plants, as in mammals, low SA concentrations do not induce Hsp70/Hsc70 expression but potentiate HS induction and confer membrane protection, while cytotoxic levels induce Hsp70/Hsc70.

Gene expression and the thiol redox state

The intracellular redox status is a tightly regulated parameter which provides the cell with an optimal ability to counteract the highly oxidizing extracellular environment. Intracellular redox homeostasis is regulated by thiol-containing molecules, such as glutathione and thioredoxin. Essential cellular functions, such as gene expression, are influenced by the balance between pro- and antioxidant conditions. The mechanism by which the transcription of specific eukaryotic genes is redox regulated is complex, however, recent findings suggest that redox-sensitive transcription factors play an essential role in this process. This review is focused on the recent knowledge concerning some eukaryotic transcription factors, whose activation and DNA binding is controlled by the thiol redox status of the cell.

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.

Roles of reactive oxygen species: signaling and regulation of cellular functions

Reactive oxygen species (ROS) are the side products (H2O2, O2.-, and OH.) of general metabolism and are also produced specifically by the NADPH oxidase system in most cell types. Cells have a very efficient antioxidant defense to counteract the toxic effect of ROS. The physiological significance of ROS is that ROS at low concentrations are able to mediate cellular functions through the same steps of intracellular signaling, which are activated by natural stimuli. Moreover, a variety of natural stimuli act through the intracellular formation of ROS that change the intracellular redox state (oxidation-reduction). Thus, the redox state is a part of intracellular signaling. As such, ROS are now considered signal molecules at nontoxic concentrations. Progress has been achieved in studying the oxidative activation of gene transcription in animal cells and bacteria. Changes in the redox state of intracellular thiols are considered to be an important mechanism that regulates cell functions.