Heat shock

Abstract: Our cells and tissues are challenged constantly by exposure to extreme conditions that cause acute and chronic stress. Consequently, survival has necessitated the evolution of stress response networks to detect, monitor, and respond to environmental changes (Morimoto et al. 1990, 1994a; Baeuerle 1995; Baeuerle and Baltimore 1996; Feige et al. 1996; Morimoto and Santoro 1998). Prolonged exposure to stress interferes with efficient operations of the cell, with negative consequences on the biochemical properties of proteins that, under ideal conditions, exist in thermodynamically stable states. In stressed environments, proteins can unfold, misfold, or aggregate. Therefore, the changing demands on the quality control of protein biogenesis, challenges protein homeostasis, for which the heat shock response, through the elevated synthesis of molecular chaperones and proteases, repairs protein damage and assists in the recovery of the cell. The inducible transcription of heat shock genes is the response to a plethora of stress signals (Lis and Wu 1993; Morimoto 1993; Wu 1995) (Fig. 1), including (1) environmental stresses, (2) nonstress conditions, and (3) pathophysiology and disease states. Although changes in heat shock protein (HSP) expression are associated with certain diseases (Morimoto et al. 1990), these observations leave open the question of whether this is an adaptation to the particular pathophysiological state, a reflection of the suboptimal cellular environment associated with the disease, or serves to warn other cells and tissues of imminent danger. The protective role of HSPs is a measure of their capacity to assist in the repair of protein damage. Whether in prokaryotes, plants, or animals, overexpression of one or more HSPs is often sufficient to protect cells and tissues against otherwise lethal exposures to diverse environmental stresses including hydrogen peroxide and other oxidants, toxic chemicals, extreme temperatures, and ethanol-induced toxicity (Parsell and Lindquist 1994). In vertebrate tissue culture cells and animal models, elevating HSPs level, either by modulation of the heat shock response or by constitutive overexpression of specific heat shock proteins, restricts or substantially reduces the level of pathology and cell death (Mizzen and Welch 1988; Huot et al. 1991; Jaattela et al. 1992; Parsell and Lindquist 1994; Mestril et al. 1994; Plumier et al. 1995; Marber et al. 1995; Mehlen et al. 1995; Mosser et al. 1997). This has led to the recognition that HSPs, via their chaperoning effects on proteins, protect cells from many forms of stress-induced cell damage and could influence the course of disease.

Abstract: The enhanced synthesis of a few proteins immediately after subjecting cells to a stress such as heat shock was first reported for drosophila cells in 1974 (l), and the universality of the response from bacteria to human was recognized shortly thereafter (reviewed in Ref. 2). In the ensuing 16 years, a vast literature has accumulated that describes a wide variety of events in a cell’s response to a heat shock. The scope of the data ranges from x-ray crystallographic measurements and physical chemical studies on specific heat shock proteins to the effects of heat shock gene expression on an organism’s ecological niche. Recently, the emphasis in this field has focused on the unction of various heat shock proteins and their possible role as “molecular chaperones”’ (3). In the subject matter that follows, I review recent data and other aspects of the heat shock phenomenon. A more thorough and comprehensive discussion of this topic can be found in a number of recent reviews and monographs (4-10). Primary references to much of the material described here are in these reviews. Much of the initial molecular biology and biochemistry of heat shock consisted of cloning the genes, determining primary sequences of the proteins, and probing the regulatory factors responsible for their induction. From these latter studies, we learned that the DNA sequence responsible for regulating heat shock gene expression in the eukaryotic cell was invariant from yeast to human (reviewed in Ref. 11). The most recent analysis of this element suggests it is an inverted repeat of the 5-nucleotide base pair, nGAAn (12). The presence of this element located about 80-150 base pairs upstream of the start site of RNA transcription is the most definitive evidence that the gene encodes a heat shock protein. However, most of these genes have other regulatory signals that activate the gene when the appropriate protein factors are present. For example, there are at least four sequence motifs upstream of the human hsp702 gene that are responsive individually to serum factors, heavy metals, and the ElA protein of adenovirus (13). The obvious interpretation of these results is that the hsp70 protein is synthesized for reasons other than heat shock and, in fact, this gene is activated at a specific stage (early S) in the cell’s mitotic cycle, during mitogenesis and upon other stress conditions (see Table I). Most of the heat shock proteins are induced by other stress agents (a listing is in Ref. 14) and during normal development of an organism (reviewed in Ref. 15). The gene encoding the protein that binds to the heat shockresponsive DNA sequence has been cloned from yeast (16,17) and shown to be essential for viability of this organism. This

Abstract: 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.

Abstract: Cells from virtually all organisms respond to a variety of stresses by the rapid synthesis of a highly conserved set of polypeptides termed heat shock proteins (HSPs). The precise functions of HSPs are unknown, but there is considerable evidence that these stress proteins are essential for survival at both normal and elevated temperatures. HSPs also appear to play a critical role in the development of thermotolerance and protection from cellular damage associated with stresses such as ischemia, cytokines, and energy depletion. These observations suggest that HSPs play an important role in both normal cellular homeostasis and the stress response. This mini-review examines recent evidence and hypotheses suggesting that the HSPs may be important modifying factors in cellular responses to a variety of physiologically relevant conditions such as hyperthermia, exercise, oxidative stress, metabolic challenge, and aging.