[CANCER RESEARCH 47, 5249-5255, October 15, 1987) Perspectivesin CancerResearch Heat Shock Proteins in Thermotolerance and Other Cellular Processes1 Stephen W. Carper,2 John J. Duffy, and Eugene W. Gerner Department of Radiation Oncology and Cancer Center, The University of Arizona Health Sciences Center, Tucson, Arizona S5724 Heat shock proteins are a unique set of highly conserved erentially synthesized (for recent reviews, see Refs. 7-13). The proteins induced by heat and other stresses. Their role in expression of hsps and the amino acid sequence of several hsps cellular responses to stress is currently unclear, due in part to are highly conserved throughout evolution, implying that they a variety of experimental findings that are apparently contra may have some universally important function(s). It is com dictory. The purpose of this review is to evaluate the evidence monly accepted that the function of the hsps is either to protect regarding the role of lisps-' in cellular responses to stress and to cells against subsequent heat stress or to enhance the ability of speculate on other possible physiological functions of these cells to recover from the toxic effects of heat or other stresses. unique proteins. This Perspectives article evaluates current evidence regarding Over the past 10 years, hyperthermia has gained increasing this possible role of hsps and considers alternate effects of hsps acceptance as a mode of cancer treatment, especially when used on cellular physiological responses. in combination with radiotherapy (1). In 1985, the United States Food and Drug Administration formally recognized hy perthermia therapy, generally meaning the elevation of tumor Operational Definitions of hsps and Thermotolerance temperatures to 40-45°C, as an effective form of treatment for certain types of cancer. A variety of preclinical laboratory Heat stress causes a variety of alterations in cellular physi studies have documented a wide range of biological effects in ology, including increases in the synthesis of certain proteins both normal and tumor cells and tissues (2-5) and have con and enzymes (2-13). Increased synthesis of proteins can occur tributed to the development of this therapy. One such effect is as a result of changes ranging from elevated transcription rates the development of thermotolerance (6). to enhanced translation of existing message. Further, activities Thermotolerance is the term generally used to refer to the of enzymes can be increased due to posttranslational modifi transient, nonheritable state of resistance to the cytotoxic ef cations which might be induced by heat shock. In order to more fects of subsequent hyperthermic treatments, which can be precisely define a heat shock protein, we will adopt the defini induced by a short exposure to a nonlethal heat treatment. The tion used by Schlesinger (12), which restricts the designation degree of resistance can be large. For example, mammalian heat shock protein to those proteins with encoding genes that cells expressing thermotolerance can exhibit survival levels contain one or more heat shock consensus sequences and the several hundred times those of control cells when both are synthesis of which is strongly stimulated by an environmental stress. The HSE sequence for eukaryotes is (CT-GAA—TTC- subjected to the same lethal hyperthermic temperature. If this phenomenon could be manipulated, such that normal tissues AG), and it is sometimes referred to as the Pelham box after expressed thermotolerance while tumor cells did not, a tumor- Hugh Pelham (14) who was one of the first to describe this selective cytotoxicity might be exploited for therapeutic gain. conserved unit. It is located on the coding strand of DNA This possibility has provided the impetus for the study of upstream from the TATA box and is essential for the heat mechanism(s) responsible for the development of thermotoler shock inducibility of transcription from these genes. This defi nition has several advantages at a time when many new so- ance in normal and neoplastic cells. Although many models called "hsps" are being discovered. Proteins with mRNA that have been proposed, no single mechanism for the development of thermotolerance has been shown to apply generally. Several is increased due to enhanced transcription but with encoding DNA that does not contain a HSE, such as the recently reported agents are known that can increase or decrease cellular ability chicken "hsp" 108 (15), would be differentiated from hsps. This to develop thermotolerance, but the molecular mechanisms responsible for these changes are not known (7). Until the distinction (the presence of a HSE) may be desirable in eukar detailed mechanism for the development of thermotolerance is yotic systems in that it classifies protein products (e.g., hsps, understood, the design of treatment protocols for tumor-selec steroid-regulated proteins, glucose-regulated proteins, etc.) into tive cytotoxicity will be severely hindered. groups based on common features in the encoding DNAs. In parallel with the preclinical studies referred to above, Various stresses such as heat shock and oxidative stress (16) investigators interested in developmental aspects of specific and heat shock and DNA-damaging agents (17) can induce the gene regulation discovered that hyperthermia, or heat shock, synthesis of common proteins in both prokaryotic and eukar caused certain genes to be activated. Like thermotolerance yotic organisms. However, not all proteins induced by one stress expression, this unique gene activation is apparently a universal are necessarily induced by the other stress, such that cellular response of all cells. When either prokaryotic, archaebacterial, responses to the various agents can be quite different and or eukaryotic cells are subjected to an increase in their normal complex (16, 17). This will most certainly also be the case in growth temperature, a specific set of proteins (hsps) are pref- mammalian cells. Table 1 lists a number of heat-inducible proteins, indicates Received 5/20/87; revised 7/22/87; accepted 7/23/87. whether the genes coding for these proteins contain an HSE, 1Supported by grants from the IS I'I IS National Cancer Institute (CA-17343, CA-30052, and CA-09213) and by an award from the Arizona Disease Research and also enumerates some of the known functions of these Commission. proteins in various species. These proteins, only some being 2To whom requests for reprints should be addressed. 3The abbreviations used are: hsps, heat shock proteins; hsp XX, a specific hsps by our definition, are involved in a variety of normal heat shock protein with a molecular mass of XX in kilodaltons; CHM, cyclohex- cellular functions, ranging from DNA and protein synthesis to imide; HSE, heat shock consensus element. glycolysis and changes in macromolecular structure. The most 5249 Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 1987 American Association for Cancer Research. HEAT SHOCK PROTEINS AND THERMOTOLERANCE Table 1 Functions of some heat-inducible proteins Protein Source Reported function Ref. HSEDnak /. Proteins with genes that contain a PCgròor grò coliE. synthesisNecessaryfor X-phage replication and RNA and DNA ESLysyl-tRNAEL, grò coliE. weakATPasefor morphogenesis of X-phage; gròEL displays activityCharging IIProtease synthetase Form coliE. tRNAATPof Lahsp coliYeastDrosophila proteaseInvolveddependent 70 (YG100)hsp temperaturesAssociateswith growth at high 32hsp28, 26, 23, cellsDrosophilaDrosophilaDrosophilaChickenKc cytoskeletonRecoverywith the 70hsp shocks.S. of nucleolar morphology after heat protein?1Involvednucleic acid-binding 68hsp 83Ubiquitinhsp fibroblastsChickenHeLaNecessaryembryodegradation791313131318192021222223,242526//.in targeting proteins for 70hsp 70E. HSEEnolase Proteins with genes that may contain a 1hsp inglycolysisCollagen-binding 47hsp fibroblastsDrosophila,embryo proteinAutodegradationmembrane 70hsp hamsterovary,Chinese endActin-binding via a serine-type protease in the COOH L-929ChineseMouse 90hsp ovaryMousehamster regulatedActin-bindingprotein calmodulin 90, hsp100hsp lineRatHeLaInvolvedlymphoma cell proteinFatty 71hsp proteinPolyadenylate-bindingacid-binding 73YeastChicken protein27282930313233///. HSEhsp Proteins with genes that probably do not contain a 70hps71hsp brainRatChickenATPase pitsFatty for clathrin-coated proteinBindsacid-binding 108Bovine to steroid receptor343215 abundant hsp in most species, hsp 70, has been shown to contain continuous heating protocols (6). Each method of heating re an ATPase activity in a highly conserved ammo-terminal region sults in a change in the shape of the single cell survival response (8, 12). hsp 70 has been reported to have autoproteolytic curve. The question of whether these different regimens of activity, which is of interest because two other hsps, the Ion A inducing thermotolerance actually represent different types of gene product (protease La) in Escherichia colt and ubiquitin in thermotolerance or a similar phenomenon remains unanswered. chickens, are known to either possess protease activity (protease Immediately after a heat shock there is a drastic reduction in La) or participate in specific protein degradation processes the rate of protein synthesis which can be measured by incor (ubiquitin). poration of radioactively labeled amino acids (usually [35S]- The definition of hsps used here is not meant to deemphasize methionine or (3H]leucine) into proteins. There is a decrease in the potential importance