Heat shock proteins in toxicology: How close and how far?

The response to stress triggers activation of the genes involved in cell survival and/or cell death. Stress response is a ubiquitous feature of cells that is induced under stress conditions. As a part of this response a set of genes called stress genes are induced to synthesize a group of proteins called heat shock proteins (Hsps). The Hsps play an essential role as molecular chaperones by assisting the correct folding of nascent and stress-accumulated misfolded proteins, and by preventing their aggregation. Because of their sensitivity to even minor assaults, Hsps are suitable as an early warning bio-indicator of cellular hazard. Despite having enormous use in toxicology, the current state of knowledge in defining a mechanism of action or accurately predicting toxicity based on stress gene expression warrants further investigation. The goal of this review is to summarize current developments in the application of stress genes and their products ‘Hsps’ in toxicology with a brief discussion of the caveats. While focusing on hsp70 because of its higher conservation across the taxa and since it is one of the first to be induced under stress conditions, we will also discuss other members of the stress gene family.     

The heat shock response: life on the verge of death

Organisms must survive a variety of stressful conditions, including sudden temperature increases that damage important cellular structures and interfere with essential functions. In response to heat stress, cells activate an ancient signaling pathway leading to the transient expression of heat shock or heat stress proteins (Hsps). Hsps exhibit sophisticated protection mechanisms, and the most conserved Hsps are molecular chaperones that prevent the formation of nonspecific protein aggregates and assist proteins in the acquisition of their native structures. In this Review, we summarize the concepts of the protective Hsp network.     

The evolution of heat shock genes and expression patterns of heat shock proteins in the species from temperature contrasting habitats

Heat shock genes are the most evolutionarily ancient among the systems responsible for adaptation of organisms to a harsh environment. The encoded proteins (heat shock proteins, Hsps) represent the most important factors of adaptation to adverse environmental conditions. They serve as molecular chaperones, providing protein folding and preventing aggregation of damaged cellular proteins. Structural analysis of the heat shock genes in individuals from both phylogenetically close and very distant taxa made it possible to reveal the basic trends of the heat shock gene organization in the context of adaptation to extreme conditions. Using different model objects and nonmodel species from natural populations, it was demonstrated that modulation of the Hsps expression during adaptation to different environmental conditions could be achieved by changing the number and structural organization of heat shock genes in the genome, as well as the structure of their promoters. It was demonstrated that thermotolerant species were usually characterized by elevated levels of Hsps under normal temperature or by the increase in the synthesis of these proteins in response to heat shock. Analysis of the heat shock genes in phylogenetically distant organisms is of great interest because, on one hand, it contributes to the understanding of the molecular mechanisms of evolution of adaptogenes and, on the other hand, sheds the light on the role of different Hsps families in the development of thermotolerance and the resistance to other stress factors.     

The evolutionary and ecological role of heat shock proteins

Most heat shock proteins (Hsp) function as molecular chaperones that help organisms to cope with stress of both an internal and external nature. Here, we review the recent evidence of the relationship between stress resistance and inducible Hsp expression, including a characterization of factors that induce the heat shock response and a discussion of the associated costs. We report on studies of stress resistance including mild stress, effects of high larval densities, inbreeding and age on Hsp expression, as well as on natural variation in the expression of Hsps. The relationship between Hsps and life history traits is discussed with special emphasis on the ecological and evolutionary relevance of Hsps. It is known that up‐regulation of the Hsps is a common cellular response to increased levels of non‐native proteins that facilitates correct protein folding/refolding or degradation of non‐functional proteins. However, we also suggest that the expression level of Hsp in each species and population is a balance between benefits and costs, i.e. a negative impact on growth, development rate and fertility as a result of overexpression of Hsps. To date, investigations have focused primarily on the Hsp70 family. There is evidence that representatives of this Hsp family and other molecular chaperones play significant roles in relation to stress resistance. Future studies including genomic and proteonomic analyses will increase our understanding of molecular chaperones in stress research.     

Geldanamycin restores a defective heat shock response in vivo

Induced expression of heat shock proteins (Hsps) plays a central role in promoting cellular survival after environmental and physiological stress. We have previously shown that scrapie-infected mouse neuroblastoma (ScN2a) cells fail to induce the expression of Hsp72 and Hsp28 after various stress conditions. Here we present evidence that this impaired stress response is due to an altered regulation of HSF1 activity. Upon stress in ScN2a cells, HSF1 was converted into hyperphosphorylated trimers but failed to acquire transactivation competence. A kinetic analysis of HSF1 activation revealed that in ScN2a cells trimer formation after stress was efficient, but disassembly of trimers proceeded much faster than in the uninfected cell line. Geldanamycin, a Hsp90-binding drug, significantly delayed disassembly of HSF1 trimers after a heat shock and restored stress-induced expression of Hsp72 in ScN2a cells. Heat-induced Hsp72 expression required geldanamycin to be present; following removal of the drug ScN2a cells again lost their ability to mount a stress response. Thus, our studies show that a defective stress response can be pharmacologically restored and suggest that the HSF1 deactivation pathway may play an important role in the regulation of Hsp expression.     

Mobile phones, heat shock proteins and cancer

There are several reports which indicate that electromagnetic radiation (such as from mobile phones) at non-thermal levels may elicit a biological effect in target cells or tissues. Whether or not these biological effects lead to adverse health effects, including cancer, is unclear. To date there is limited scientific evidence of health issues, and no mechanism by which mobile phone radiation could influence cancer development. In this paper, we develop a theoretical mechanism by which radiofrequency radiation from mobile phones could induce cancer, via the chronic activation of the heat shock response. Upregulation of heat shock proteins (Hsps) is a normal defence response to a cellular stress. However, chronic expression of Hsps is known to induce or promote oncogenesis, metastasis and/or resistance to anticancer drugs. We propose that repeated exposure to mobile phone radiation acts as a repetitive stress leading to continuous expression of Hsps in exposed cells and tissues, which in turn affects their normal regulation, and cancer results. This hypothesis provides the possibility of a direct association between mobile phone use and cancer, and thus provides an important focus for future experimentation.     

The stress response is repressed during fermentation in brewery strains of yeast

Yeast cells encounter a variety of environmental stresses during brewing and must respond to ensure cell survival. Cells can respond to stress by inducing a Heat Shock Response in which heat shock proteins (Hsps) are synthesized. In laboratory strains of Saccharomyces cerevisiae, the heat shock protein, Hsp104, plays a major role in the acquisition of tolerance to a variety of stresses such as heat, ethanol and sodium arsenite, and as such acts as an excellent stress indicator. The induction of Hsp104 in bottom-and top-fermenting brewery strains was examined when grown under laboratory and industrial fermentation conditions, and it was found that each brewing strain exhibits its own unique pattern of Hsp104 expression. During industrial fermentations, brewery strains are capable of mounting a stress response at the early stages of fermentation. However, as the fermentation proceeds, the response is repressed. The results suggest that conditions experienced in industrial brewing prevent the activation of the stress response. This study increases our understanding of alterations in gene expression patterns during the brewing process, and yields information that will aid in the definition of best practice in yeast management.     

Quantitative evidence that both Hsc70 and Hsp70 contribute to thermal adaptation in hybrids of the livebearing fishes Poeciliopsis

The 70-kilodalton heat shock protein family is composed of both environmentally inducible (Hsp) and constitutively expressed (Hsc) family members. While the role of the constitutively expressed stress proteins in thermotolerance is largely unknown, de novo expression stress proteins in response to elevated temperatures has been associated with increased thermotolerance in many cell lines, developing embryos and adult organisms. Distinct, hemiclonal hybrids between the livebearing fish species Poeciliopsis monacha and P. lucida varied in their abilities to survive temperature stress, with survival being greatest when rates of temperature increase to 40°C were slowest and when P. monacha genomes were combined with a sympatric P. lucida genome. Quantification of Hsp70 under heat shock conditions and Hsc70 under normal physiological conditions indicated that variation in survival among hemiclones was best explained by the combined effects of these two proteins. Similar complex interactions between maternal and paternal genomes and rate of temperature increase were found to underline patterns of survival, Hsp70 accumulation and Hsc70 abundance. These data suggest that the relationship between Hsps and thermotolerance is more intricate than previously thought and that Hsps contribute to thermal adaptation in these fishes through genetic interactions specific to particular environments.     

Exogenous Hsc70, but not thermal preconditioning, confers protection to motoneurons subjected to oxidative stress

Proper sensing of stress and the initiation of the stress response are critical to maintaining cell viability in response to noxious stimuli. Induction of the stress response prior to the exposure of a lethal stress (preconditioning) can be protective. Heat shock proteins (Hsps), the main products of the stress response, are considered to be responsible for this protective effect. Most cells readily initiate a stress response, but some neuronal phenotypes, including motoneurons (MNs), have a diminished capacity to do so. We have found that, given a proper stimulus, MNs can execute a heat stress response; but, it does not protect them from death caused by hydrogen peroxide (H(2)O(2)) induced oxidative stress, despite inhibiting H(2)O(2)-induced caspase activation. Conversely, we demonstrate that incubation with the heat shock cognate 70 (Hsc70) protein prior to oxidative insult can protect MNs from oxidative stress. This survival promoting effect may be mediated through the substrate binding domain (SBD) of Hsc70. Our data suggest that stress preconditioning may not be beneficial to MNs, but that pharmacological interventions and alternative means of acquiring components of the stress response are an effective means of ameliorating lethal stress in MNs and may be potentially useful therapeutically in preventing pathological MN loss.