Characterization of the thermotolerant cell. I. Effects on protein synthesis activity and the regulation of heat-shock protein 70 expression

Effect of L-azetidine 2-carboxilic acid on the activity of the general amino-acid permease from Saccharomyces cerevisiae var. ellipsoideus

Addition of the L-proline analogue L-azetidine 2-carboxylic acid to growing cultures of Saccharomyces cerevisiae var. ellipsoideus promoted fast deactivation of the general aminoacid permease, measured as L-valine uptake, without an immediate decrease in the growth rate. Cells preincubated with the analogue for 3 h were unable to restore either growth ability or general aminoacid permease activity in analogue-free medium. Eadie-Hofstee plots of L-valine uptake in the presence of the analogue are consistent with a strong reduction in the number of active molecules of the general amino-acid permease located in the plasma membrane. Inhibitory effects on protein synthesis were seen after preincubations of the yeast with the analogue for 3 h although a 30 min preincubation had no effect.

Patterns of protein synthesis in various cells after extreme heat shock

The analysis of proteins synthesized in rat thymocytes and mouse teratocarcinoma PCC-4 Aza 1 and myeloma Sp2/0 cells after 1 h of treatment at 42 or 44 degrees C was carried out. Shock at 42 degrees C reduced the total synthetic rate of proteins in all three cell lines and induced "classical" heat-shock protein with a mass of 70 kDa (hsp 70). Heat shock at 44 degrees C resulted in almost complete inhibition of protein synthesis; only a small amount of hsp 70 was synthesized. Meanwhile a new 48-kDa polypeptide (pI = 7.5) was found in the cells exposed to severe heat shock. This protein was compared by peptide mapping with other known polypeptides of the same size: heat-shock protein from chicken embryo cells and mitogen-stimulated polypeptide from human lymphoid cells. The peptide maps were not identical. It was also shown that after a shock at 44 degrees C teratocarcinoma cells were able to accumulate anomalous amounts of hsp 70 despite hsp 70 synthesis inhibition. The data show that reaction of various cells to extreme heat shock depends heavily on cell type.

Regulation of heat shock protein synthesis in rat astrocytes

Rat forebrain astrocytes synthesize heat shock proteins with molecular weights 97, 89, 70, 68, and 30-34 kilodaltons. The stress inducible 68-kDa heat shock protein (HSP-68) was vigorously expressed by astrocytes in culture after a 45 degrees C, 20 min heat shock. HSP-68 synthesis was poorly inducible by a second heat shock given 16 hr after the initial heat shock. Decreased [35S]methionine incorporation into HSP-68 correlated with low levels of HSP-68 mRNA present after the second heat shock. The data suggest that control of HSP-68 mRNA levels by transcriptional/posttranscriptional mechanisms is a major site for regulation of HSP-68 synthesis.

Hypoxia induces a specific set of stress proteins in cultured endothelial cells

Vascular endothelial cells (EC) are the initial cells within the vascular wall exposed to decreases in blood ambient oxygen concentration. The mechanisms by which they tolerate low levels of oxygen are unknown, but may parallel the response to other cellular stresses, such as heat shock. After 4-8 h of hypoxia, we found a decrease in total protein synthesis in both cultured bovine aortic and pulmonary arterial EC. SDS-PAGE and autoradiographic analysis of [35S]methionine-labeled proteins demonstrated the concomitant induction of a specific set of proteins (Mr 34, 36, 47, and 56 kD) in both cell types. These hypoxia-associated proteins (HAPs) were cell-associated and up-regulated in a time- and oxygen concentration-dependent manner. Comparison of these proteins with heat shock proteins (HSPs) demonstrated that HAPs were distinct from HSPs. EC maintained chronically in 3% O2 continued to synthesize elevated levels of HAPs, yet further up-regulated these proteins when exposed to 0% O2. The presence of five times the normal media glucose concentration did not alter the appearance of HAPs. Hypoxia sensitive renal tubular epithelial cells up-regulated no proteins corresponding to HAPs and were irreversibly damaged within 8 h of exposure to 0% O2. In vitro translation experiments demonstrated that the steady-state level of several mRNAs was higher in the anoxic EC than in normoxic EC and encoded for proteins of Mr 32, 35, 37, 40, and 48 kD that were different from proteins encoded by HSP mRNAs. The induction of HAPs during acute hypoxia and their continued synthesis in chronic hypoxia suggest that HAPs may be important in the maintenance of endothelial cell integrity under conditions of decreased ambient oxygen.

Response of mammalian cells to metabolic stress; changes in cell physiology and structure/function of stress proteins

In response to adverse changes in their local environment, cells or tissues from all organisms increase the expression of a group of proteins referred to as heat shock or stress proteins. Collectively, the stress proteins are thought to provide the cell with some degree of protection during the environmental insult as well as facilitate the repair and recovery of metabolic pathways perturbed as a consequence of the stress event. Within the past few years it has become apparent that most all of the stress proteins are present in appreciable levels in the unstressed cell and are involved in a number of very basic and essential biochemical pathways. The present review has discussed pertinent changes in cell physiology in mammalian cells experiencing metabolic stress. In addition, considerable attention has been given to discussing the properties and possible functions of the individual stress proteins.

Expression of thermotolerance following microinjection of poly(A)RNA isolated from thermotolerant CHO cells

Poly(A)RNA was isolated from thermotolerant cells and microinjected into recipient non-tolerant Chinese hamster ovary (CHO) cells. The injected cells expressed thermotolerance to a subsequent test heat treatment both in terms of the end-points of colony formation (cell survival) and resumption of protein synthesis after test heating (translational labelling). The magnitude of thermotolerance expression was dependent on the experimental end-point (increase up to 3.8-fold for translational labelling and approximately 2-fold for survival) and on the time between microinjection and the test heat treatment. Control experiments showed that poly(A)RNA from non-tolerant cells did not alter the heat response of microinjected cells. Proteins corresponding to the poly(A)RNA from thermotolerant cells were analysed by in vitro translation and by labelling of microinjected cells, followed by SDS-PAGE. In vitro translations showed high levels of transcripts for classical heat-shock proteins (HSP 70/72, 89, 110) in poly(A)RNA from thermotolerant versus control cells. However, proteins synthesized in intact cells showed no detectable differences when cells were microinjected with poly(A)RNA from thermotolerant versus control cells, or not injected at all. In principle the data show that microinjection of specific poly(A)RNA fractions can be used for defining the contribution of individual gene products to the cellular heat response.

T lymphocyte stress response. II. Protection of translation and DNA replication against some forms of stress by prior hyperthermic stress

We have compared the effects of a mild heat shock and febrile temperatures on heat-shock protein (hsp) synthesis and development of stress tolerance in T lymphocytes. Our previous studies demonstrated that febrile temperatures (less than or equal to 41 degrees C) induced the synthesis of hsp110, hsp90, and the constitutive or cognate form of hsp70 (hscp70; a weak induction of the strongly stress-induced hsp70 was also observed. In the studies reported herein, we demonstrate that a mild heat shock (42.5 degrees C) reverses this ratio; that is, hsp70 and not hscp70 is the predominate member of this family synthesized at this temperature. Modest heat shock also enhanced the synthesis of hsp110 and hsp90. In order to assess the relationship between hsp synthesis and the acquisition of thermotolerance, purified T cells were first incubated at 42.5 degrees C (induction temperature) and then subsequently subjected to a severe heat-shock challenge (45 degrees C, 30 min). T cells first incubated at a mild heat-shock temperature were capable of total protein synthesis at a more rapid rate following a severe heat shock than control cells (induction temperature 37 degrees C). This phenomenon, which has been previously termed translational tolerance, did not develop in cells incubated at the febrile temperature (induction temperature 41 degrees C). Protection of translation also extended to immunologically relevant proteins such as interleukin-2 and the interleukin-2 receptor. Because clonal expansion is a critical event during an immune response, the effects of hyperthermic stress on DNA replication (mitogen-induced T cell proliferation) was also evaluated in thermotolerant T cells. DNA synthesis in control cells (induction temperature 37 degrees C) was severely inhibited following heat-shock challenge at 44 degrees C or 45 degrees C; in contrast, T cells preincubated at 42.5 degrees C rapidly recovered their DNA synthetic capacity. T cells preincubated at a febrile temperature were moderately protected against hyperthermic stress. The acquisition of thermotolerance was also associated with enhanced resistance to chemical (ethanol)-induced stress but not to heavy metal toxicity (cadmium) or dexamethasone-induced immunosuppression. These studies suggest that prior hsp synthesis may protect immune function against some forms of stress (e.g., febrile episode) but would be ineffective against others such as elevated glucocorticoid levels which normally occur during an immune response.

Expression of constitutive and inducible HSP70 and HSP47 is enhanced in cells persistently spread on OPN1 or collagen

Cells persistently spread on OPN or collagen survive heat shock better than cells transiently spread on fibronectin or tissue culture plates. Thus, a central question is whether constitutively or inducible stress proteins are enhanced in cells grown on adhesive proteins that maintain a persistent spread cell shape. Levels of Hsp 72,73, and colligin/Hsp47 were determined by Western blot analyses. The inducible Hsp 72 was prominently expressed following heat shock in cells grown on OPN or collagen, but not in cells plated on fibronectin coated substratum or on tissue culture plates. Colligin/Hsp 47 and Hsp 73 manifested a similar pattern of expression indicating that these adhesive attachment proteins accommodate cell function through organization of cell architecture.

Effect of endotoxin-induced cell injury on 70-kD heat shock proteins in bovine lung endothelial cells

Heat shock proteins (HSPs) have been remarkably conserved throughout evolution. It has been assumed that induction of HSPs remains a stereotypic response to injury, important for survival of eukaryotic cells during euthermic injury. However, there are few studies of this phenomenon in endothelial cells, and none in pulmonary endothelial cells. We studied the induction of synthesis of 70-kD proteins in bovine pulmonary artery endothelial cells (BPAECs) in response to heat shock and to euthermic injury induced by bacterial endotoxin. First, in response to heat, BPAECs showed rapid and reversible heat-induced synthesis of 70-kD proteins, readily detectable by one-dimensional SDS-PAGE of [35S]methionine-labeled BPAECs. Heat shock at 42 degrees C for 3 h or 43 degrees C for 2 h suppressed total protein synthesis by 30% (P less than 0.001) but an increased rate of synthesis of 70-kD protein continued, representing an increasing fraction of total protein synthesis. Heat-induced synthesis of 70-kD protein returned to baseline levels 8 h after heat shock. Northern analysis showed that mRNA for a protein homologous to a conserved amino acid sequence in the family of species-homologous 70-kD heat shock proteins (HSP 70) was induced by a 15-min incubation at 42 degrees C and remained detectably increased for 6 h. We next assessed whether euthermic injury by bacterial endotoxin (LPS) generated a similar response. LPS was cytotoxic by BPAECs as assessed morphologically, by release of 51Cr from prelabeled cells, and by a significant suppression of total protein synthesis (range, 35 to 70%; P less than 0.001). Despite cytotoxicity, LPS did not induce 70-kD protein at a level that could be detected by SDS-PAGE, and no increase in mRNA for HSP 70 was detected by Northern analysis. LPS-injured BPAECs remained "competent" to induce both 70-kD proteins and mRNA for HSP 70 in response to heat shock. We conclude that at least quantitatively, induction of HSP 70 by BPAECs is not a stereotypic response to injury but rather is at least relatively injury-specific. However, competence to induce HSP 70 appears to be extremely resilient: it is retained in dysfunctional BPAECs in the face of profound inhibition of global protein synthesis, suggesting an important homeostatic role.

Organization and disorganization of actin filaments in human epidermal keratinocytes: heat-shock treatment and recovery process

We investigated alterations of actin organization due to heat shock and recovery from the collapse in human epidermal keratinocytes. Exposure of keratinocytes to elevated temperature caused the rapid disintegration of actin filaments. With a heat-shock treatment at 45 degrees C for 10 min, actin filaments disappeared and granular actin was distributed diffusely in the cytoplasm. After return to 37 degrees C, recovery of actin organization was observed. Completely disintegrated granular actin assembled to form actin dots, which tended to group. The grouping actin dots often took a circular, semicircular or arched form. Filamentous actin then extended out from the actin dots. Fine short bundles of actin filaments had a rippled appearance or were polygonal in structure, with actin filaments converged at many points. On the seventh day after heat-shock treatment, actin organization had almost returned to the pre-heat-shock condition, with diffusely distributed actin filaments. In previous studies, we observed such aberrant structures in human malignant keratinocytes and human epidermal keratinocytes treated with 12-O-tetradecanoylphorbol-13-acetate. The observations presented here indicate that those structures are not specific to malignancy or to the process of malignant transformation, but represent less mature and aberrant organizations of actin.

Effect of in vitro heat shock upon the synthesis and secretion of prostaglandins and protein by uterine and placental tissues of the sheep

The objectives of this study were to determine the secretion patterns of prostaglandins (PG) and protein during mid- (Day 100) and late- (Day 140) pregnancy in the ewe and to ascertain whether that pattern is altered by in vitro heat shock. Explant cultures were prepared from intercaruncular endometrium, caruncular endometrium, fetal cotyledon and interplacentomal placenta. Cultures were incubated at 39 or 42 degrees C for 18 h in the presence of arachidonic acid or L-[4,5(3)H]leucine. There were no effects of day of gestation or consistent effects of temperature upon de novo synthesis of tissue and secretory protein. Elevated temperature generally depressed PGE(2) secretion by maternal tissues and PGF secretion by caruncular endometrium but had little effect on PGE(2) release by fetal tissues or on PGF release by intercaruncular endometrium or fetal tissues. Day of gestation by tissue type interactions were found for PGF and PGE(2) release. At Day 100, maternal tissues secreted more PGF and PGE(2) than fetal tissues; at Day 140, PG secretion from fetal tissues was greater than at Day 100, and fetal PGE(2) release exceeded release from maternal tissues. Tissue proteins resolved by SDS-PAGE revealed the appearance in heat-shocked tissue of 93 and 72 kDa heat-shock proteins. In conclusion, elevated temperature depressed PGE(2) release, particularly from maternal tissues. Changes in PGE(2) suggest that the increase in utero-placental PGE(2) with increasing gestational age is due to changes in secretion of the fetal placenta.

Enhanced synthesis of heat shock proteins and augmented thermotolerance after induction of differentiation in HL-60 human leukemia cells

The effects of the induction of differentiation were investigated on the expression of heat shock proteins (hsps) and thermotolerance. The synthesis of the major hsps in response to heat stress was markedly enhanced in HL-60 human leukemia cells after differentiation. An increased amount of mRNA transcripts for hsp 70 was also noted. In addition, induction of differentiation resulted in acquisition of greater resistance to heat, which may be advantageous since cells in the peripheral blood must survive many stresses.

Novel ATP-binding heat-inducible protein of Mr = 37,000 that is sensitive to transformation in BALB/3T3 cells

Using affinity chromatography on ATP-agarose, we have identified a major ATP-binding protein in Nonidet P-40 extracts of avian and mammalian cells labeled with [35S]methionine. After washing ATP-agarose beads with high-ionic-strength buffer (0.4 M NaCl), the 37-kD protein was shown to be one of the major ATP-binding proteins while p72 and grp78, which are members of the hsp70 family, also bound to ATP-agarose. This protein consisted of several spots on two-dimensional gel electrophoresis. The isoelectric point of the most basic spot was approximately 9.2 in chick embryo fibroblasts, whereas it was about 8.8 in mouse 3T3 cells. The identities of these proteins in mouse and chick cells were confirmed by peptide mapping. After heat-shock treatment of BALB/3T3 cells, the major heat-shock protein, hsp70, was shown to be induced very rapidly after heat shock and was recovered in the ATP-binding fraction. Besides hsp70, a 37-kD protein was also found to be induced by heat shock. This protein was drastically induced by treating the cells with alpha,alpha'-dipyridyl, an iron chelating reagent, but not with sodium arsenite, calcium ionophore, or tunicamycin. The synthesis and the total amount of this ATP-binding protein increased in mouse 3T3 cells transformed by simian virus 40, methylcholanthrene, or activated c-Ha-ras oncogene compared to their normal counterparts. The incorporation of [32P]orthophosphate was not detected in either normal or transformed cells. These studies established that a major ATP-binding protein of Mr = 37,000 is a heat-inducible protein and that the synthesis of this protein is regulated by malignant transformation.

Induction of cellular hsp70 expression by human cytomegalovirus

Expression of the cellular heat shock protein 70 gene (hsp70) is transiently induced by human cytomegalovirus (HCMV) infection of permissive human diploid fibroblasts. Induction of the cellular heat shock response during critical times of infection had previously been reported to alter the growth of HCMV in vitro. Thus, a potential interaction between heat shock proteins and HCMV expression was indicated. HCMV dramatically increased expression of hsp70 RNA within 8 h of infection. hsp70 RNA remained elevated at 24 and 48 h postinfection and decreased to low levels of 72 h postinfection. Induction of HSP70 protein occurred more slowly; inducible HSP70 protein encoded by this RNA increased within 16 h postinfection and continued to increase throughout infection until 72 h postinfection, when the highest abundance of inducible HSP70 protein was observed. Cells that received both heat (43 degrees C for 70 min) treatment and HCMV infection expressed hsp70 RNA to levels above the sum of levels present in cells given either treatment alone. Furthermore, hsp70 RNA induction occurred earlier and remained elevated longer than in cells infected with HCMV alone or in cells treated with heat alone, respectively. Nevertheless, the pattern of HCMV immediate-early transcript expression at 2, 4, and 6 h postinfection appeared to be unchanged by this prior heat treatment. Our results suggest that heat shock treatment and HCMV infection can act additively in stimulating hsp70 RNA expression. The previously reported stimulation of HCMV growth in vitro following the heat shock response apparently does not result from alterations in the steady-state expression of HCMV immediate-early transcripts.

Hypoxia-ischemia induces heat shock protein-like (HSP72) immunoreactivity in neonatal rat brain

The expression of heat shock protein immunoreactivity in rat brain was evaluated in a model of neonatal hypoxia-ischemia. One-week-old rats were subjected to left carotid artery coagulation and exposure to 8% O2/92% N2 for 2 h (moderate injury) or 3.5 h (severe injury). Animals were sacrificed 1, 12 and 24 h after the hypoxic insult. Cells immunoreactive for the 72 kDa heat shock protein (HSP72) were observed in ipsilateral cortex as early as 1 h after the termination of the hypoxia. After 12 h, neurons in the ipsilateral hippocampus and cortex stained intensely for HSP72 immunoreactivity in the moderately injured group. In the severely injured brains, bilateral staining was observed in neurons and vessels of the hippocampus and cortex. Therefore, cells containing HSP72 immunoreactivity may serve as an early marker for neuronal injury from hypoxia-ischemia in the neonatal rat brain and more importantly may illustrate previously unrecognized areas of central nervous system vulnerability.

Acquired thermotolerance and heat shock in the extremely thermophilic archaebacterium Sulfolobus sp. strain B12

The extreme thermophile Sulfolobus sp. strain B12 exhibits an acquired thermotolerance response. Thus, survival of cells from a 70 degrees C culture at the lethal temperature of 92 degrees C was enhanced by as much as 6 orders of magnitude over a 2-h period if the culture was preheated to 88 degrees C for 60 min or longer before being exposed to the lethal temperature. In eubacteria and eucaryotes, acquired thermotolerance correlates with the induced synthesis of a dozen or so proteins known as heat shock proteins. In this Sulfolobus species, it correlates with the preferential synthesis of primarily one major protein (55 kilodaltons) and, to a much lesser extent, two minor proteins (28 and 35 kilodaltons). Since the synthesis of all other proteins was radically reduced and these proteins were apparently not degraded or exported, their relative abundance within the cell increased during the time the cells were becoming thermotolerant. They could not yet be related to known heat shock proteins. In immunoassays, they were not cross-reactive with antibodies against heat shock proteins from Escherichia coli (DnaK and GroE), which are highly conserved between eubacteria and eucaryotes. However, it appears that if acquired thermotolerance depends on the synthesis of protective proteins, then in this extremely thermophilic archaebacterium it depends primarily on one protein.

Heat shock causes the collapse of the intermediate filament cytoskeleton in Drosophila embryos

Heat shock has a dramatic effect on the organization of the cytoplasm, causing the intermediate filament cytoskeleton to aggregate at the nucleus. This has previously been shown in cultured Drosophila and mammalian cells. In this paper we analyze the heat lability of the intermediate filament cytoskeleton in early Drosophila embryos by indirect immunofluorescence. At all stages of embryogenesis tested, the intermediate filament cytoskeleton, which is maternally provided, is severely disturbed by 30 min heat shock at 37 degrees C. After the nuclei have migrated to the subcortical cytoplasm, it collapses around them. Nuclei in all heat-shocked embryos are considerably enlarged and become displaced. Embryos before cellular blastoderm stage, in which heat shock protein synthesis is not inducible, are irreversibly arrested in development by heat shock. Embryos at or after cellular blastoderm, which do synthesize heat shock proteins in response to stress, are also immediately arrested in development but continue development when returned to 25 degrees C. We discuss the possibility that cytoplasmic events such as the intermediate filament cytoskeleton rearrangement may be involved in heat shock-mediated phenocopy induction.

Biochemical analysis of heat-resistant mouse tumor cell strains: a new member of the HSP70 family

A series of heat-resistant mutants selected from a murine tumor cell line, RIF-1, display a markedly increased and stable resistance to heat shock. The mutant cell lines were analyzed for differences that may explain their increased resistance. Membrane lipid analysis showed no change in cholesterol content but an increase in the proportion of saturated fatty acids in the phospholipid fraction. Two-dimensional gel analysis revealed a generally increased constitutive synthesis of several major heat shock proteins (HSP), including HSP90, 68, 60, and 28. In addition, a new protein in the 70-kilodalton region is present in the resistant lines. The new protein has a lower isoelectric point than the constitutive HSP70 does, is only weakly induced by heat shock, and is immunologically cross-reactive with other members of the HSP70 family. After heat shock, the mutants display increases in HSP similar to those seen in the wild-type cells and they develop further transient tolerance to heat. Analysis of these mutants may help in understanding the function of HSP, both in normal growth and after heat shock.

Heat shock stimulates the release of arachidonic acid and the synthesis of prostaglandins and leukotriene B4 in mammalian cells

Heat shock has a profound influence on the metabolism and behavior of eukaryotic cells. We have examined the effects of heat shock on the release from cells of arachidonic acid and its bioactive eicosanoid metabolites, the prostaglandins and leukotrienes. Heat shock (42-45 degrees) increased the rate of arachidonic acid release from human, rat, murine, and hamster cells. Arachidonate accumulation appeared to be due, at least partially, to stimulation of a phospholipase A2 activity by heat shock and was accompanied by the accumulation of lysophosphatidyl-inositol and lysophosphatidylcholine in membranes. Induction of arachidonate release by heat did not appear to be mediated by an increase in cell Ca++. Stimulation of arachidonate release by heat shock in hamster fibroblasts was quantitatively similar to the receptor-mediated effects of alpha thrombin and bradykinin. The effects of heat shock and alpha thrombin on arachidonate release were inhibited by glucocorticoids. Increased arachidonate release in heat-shocked cells was accompanied by the accelerated accumulation of cyclooxygenase products prostaglandin E2 and prostaglandin F2 alpha and by 5-lipoxygenase metabolite leukotriene B4. Elevated concentrations of arachidonic acid and metabolites may be involved in the cytotoxic effects of hyperthermia, in homeostatic responses to heat shock, and in vascular and inflammatory reactions to stress.

Protein synthesis and protein phosphorylation during heat stress, recovery, and adaptation

Incubating cells at elevated temperatures causes an inhibition of protein synthesis. Mild heat stress at 41-42 degrees C inhibits the fraction of active, polysomal ribosomes from greater than 60% (preheating) to less than 30%. A return to 37 degrees C leads to an increase in protein synthesis, termed "recovery." Continuous incubation at 41-42 degrees C also leads to a gradual restoration of protein synthesis (greater than 70% of ribosomes reactivated by 2-4 h), termed "adaptation". Protein synthesis inhibition and reactivation is prestressed, recovered cells that contain elevated levels of the heat stress proteins occur to the same extent and at the same rate as in "naive" cells. The adaptation response requires transcription of new RNA whereas recovery does not. A large number of phosphorylation changes are induced by severe heat stress and occur with kinetics similar to the inhibition of protein synthesis. These include phosphorylation of eukaryotic protein synthesis initiation factor (eIF)-2 alpha and dephosphorylation of eIF-4B and eIF-4Fp25 (eIF-4E). However, the extent to which the modification occurs is proportional to the severity of the stress, and, under mild (41-42 degrees C) heat stress conditions, these initiation factor phosphorylation changes do not occur. Similarly, under conditions of severe heat stress eIF-2 alpha and eIF-4B frequently recover to their prestress phosphorylation state before the recovery of protein synthesis. eIF-4E dephosphorylation likewise does not occur under mild heat stress conditions. Therefore, these changes in phosphorylation states, which are thought to be sufficient cause, are not necessary for the inhibition of protein synthesis observed.

Regulation of the inducible heat shock 71 genes in early neural development of cultured rat embryos

Activation of the inducible heat shock 71 genes and their role in the heat shock response was studied in vitro in 9.5 day-old rat embryos at neural tube closure. The transcriptional response of a 71 kilodalton (kD) heat shock gene induced after various regimes of heat shock and acquired thermotolerance was investigated. Expression and accumulation of the heat shock (hs) 71 mRNA in the neuroectoderm was studied by Northern and dot blot analysis. Specific expression in various cell types and regions of the neuroectoderm were examined by in situ hybridization. Exposure of embryos to a heat shock at 43 degrees C for 7.5 min caused high levels of hs mRNA 71 accumulation in the neuroectoderm, pronounced protein synthesis inhibition, and regulated recovery. Specific neuroectoderm cell death followed, resulting in major developmental defects of the eye and forebrain region. A mild heat shock of 42 degrees C for 10 min induced the heat shock response, hsp synthesis, and cell recovery, but produced no cell death or deformities. Preheating the embryo at 42 degrees C resulted in acquired thermotolerance to an otherwise teratogenic 43 degrees C heat shock. Thermotolerance was associated with a rapid recovery of protein synthesis associated with hs 71 mRNA expression. Dot blot analysis showed that after a 42 degrees C heat shock, 71 mRNA was rapidly transcribed and transported into the cytoplasm where it was degraded within 2 hr of the initial response. The results suggest that the heat shock protein (hsp) 71 gene may have a protective rather than a rescuing function.

Association of a cellular heat shock protein with simian virus 40 large T antigen in transformed cells

The viral oncoprotein of simian virus 40, large T antigen (T-ag), is essential for viral replication and cellular transformation. To understand the mechanisms by which T-ag mediates its multifunctional properties, it is important to identify the cellular targets with which it interacts. A cellular protein of 73 kilodaltons (p73) which specifically associates with T-ag in simian virus 40-transformed BALB/c 3T3E cells has been identified. The binding of p73 to T-ag was demonstrated by coimmunoprecipitation analyses using polyclonal and monoclonal antibodies specific for T-ag. The interaction of p73 with T-ag was independent of T-ag complex formation with the cellular protein p53. Partial V8 protease cleavage maps for p73 and the cellular heat shock protein hsp70 were identical. Immunoblot analyses indicated that p73 complexed to T-ag was antigenically related to hsp70. T-ag deletion mutants were constructed that remove internal, amino-terminal, and carboxy-terminal sequences. These mutants mapped the p73 binding domain to the amino terminus of T-ag. The specific dissociation of p73 from the p73/T-ag complex was mediated by ATP; GTP, CTP, and UTP were also utilized as substrates. These characteristics suggest that p73 may be a member of the hsp70 family of heat shock proteins. The biologic significance of p73/T-ag complex formation has yet to be determined.

HSP70-related proteins in bovine skeletal muscle

Constitutive expression of HSP70-related proteins was detected in a variety of bovine tissues using a specific antibody. All tissues contained a 73 kilodalton protein. A lower molecular weight form (72 kilodaltons) that co-migrated on two-dimensional gels with the stressed-induced HSP70 was present in high levels in bovine skeletal muscle, but absent from rat skeletal muscle. Two-dimensional gel analysis revealed several isoforms for both the 73 and 72 kilodalton forms. Purification of HSP70-related proteins from bovine skeletal muscle, thymus gland and rat skeletal muscle demonstrated that the antibody recognized all the forms present in the tissue homogenates. The two proteins are similar but distinct as detected by one-dimensional peptide mapping. The lower molecular form was not present in fetal tissue but was detectable in newborn animals, suggesting that the levels are regulated during development.

Biochemical analysis of heat-resistant mouse tumor cell strains: a new member of the HSP70 family

A series of heat-resistant mutants selected from a murine tumor cell line, RIF-1, display a markedly increased and stable resistance to heat shock. The mutant cell lines were analyzed for differences that may explain their increased resistance. Membrane lipid analysis showed no change in cholesterol content but an increase in the proportion of saturated fatty acids in the phospholipid fraction. Two-dimensional gel analysis revealed a generally increased constitutive synthesis of several major heat shock proteins (HSP), including HSP90, 68, 60, and 28. In addition, a new protein in the 70-kilodalton region is present in the resistant lines. The new protein has a lower isoelectric point than the constitutive HSP70 does, is only weakly induced by heat shock, and is immunologically cross-reactive with other members of the HSP70 family. After heat shock, the mutants display increases in HSP similar to those seen in the wild-type cells and they develop further transient tolerance to heat. Analysis of these mutants may help in understanding the function of HSP, both in normal growth and after heat shock.

Dynamic changes in the structure and intracellular locale of the mammalian low-molecular-weight heat shock protein

Mammalian cells grown at 37 degrees C contain a single low-molecular-weight heat shock (or stress) protein with an apparent mass of 28 kilodaltons (kDa) whose synthesis increases in cells after exposure to elevated temperatures or other forms of physiologic stress. Herein we present data demonstrating that heat shock protein 28 exists in a number of dynamic states depending upon the physiologic state of the cell. Biochemical fractionation of 37 degrees C cells in the absence of nonionic detergent revealed that the 28-kDa protein partitioned approximately equally between the soluble and insoluble fractions. The addition of detergent in the fractionation procedure resulted in all of the protein distributed within the soluble phase. In contrast, in cells first heat shocked and then fractionated in the presence of detergent, most of the 28-kDa protein was found within the insoluble fraction. These biochemical results appeared entirely consistent with indirect immunofluorescence experiments, demonstrating that the 28-kDa protein resided within the perinuclear region of 37 degrees C cells in close proximity to the Golgi complex. After heat shock treatment, the 28-kDa protein relocalized within the nucleus and resisted detergent extraction. The extent of 28-kDa protein redistribution into the nucleus and its detergent insolubility increased as a function of the severity of the heat shock treatment. With time of recovery from the heat treatment there occurred a gradual return of the 28-kDa protein into the detergent-soluble phase. Concomitant with these changes in 28-kDa protein solubility was a corresponding change in the apparent size of the protein as determined by gel filtration. While at 37 degrees C cells the protein exhibited a mass of 200 to 800 kDa; after heat shock the protein assumed sizes of 2 MDa or greater. Using immunoelectron microscopy, we show an accumulation of these aggregates of 28-kDa protein within the nucleus. Finally, we show that the heat-dependent redistribution of the 28-kDa protein from the cytoplasm into the nucleus was greatly diminished when the cells were first rendered thermotolerant, and we suggest that this simple assay (i.e., 28-kDa protein detergent solubility) may prove useful in evaluating the thermotolerant status of a cell or tissue.

Involvement of rRNA synthesis in the enhanced survival and recovery of protein synthesis seen in thermotolerance

Although acquired thermotolerance has been linked to the induction of heat shock proteins, the molecular mechanism(s) by which cells become resistant to heat is unknown. The present study shows a strong correlation between the survival of cells following heat shock and the rate of recovery of protein, total RNA, and rRNA synthesis. Increasing exposure of CHO cells to 45 degrees C was found to decrease survival and cause a lengthening delay in these synthetic processes. The same reciprocal correlation was seen in thermotolerant cells. As thermotolerance develops, more cells survive a heat challenge and the delay in synthesis decreases. These data argue that enhanced recovery of protein and RNA synthesis is one factor which plays a key role in thermotolerance. The involvement of rRNA synthesis was further investigated by using actinomycin D at 0.1 microgram m1(-1), a concentration at which rRNA synthesis is selectively inhibited. When the drug was present during the recovery from a challenge heat treatment, the survival of thermotolerant cells was approximately 3-fold lower than expected from the mild toxicity of the drug. As this could not be accounted for by an interaction of the drug with the response of cells to single heat treatments, it is concluded that the drug inhibits the expression of thermotolerance in cells which would otherwise express a full degree of thermotolerance. The time and concentration dependence of this effect indicates that the drug acts though inhibition of rRNA synthesis. Therefore, enhanced recovery of RNA synthesis, presumably rRNA synthesis, is identified as one of the mechanisms responsible for enhanced survival of thermotolerant cells following heat shock.

Cell cycle-dependent association of HSP70 with specific cellular proteins

In asynchronous populations of HeLa cells maintained at control or heat shock temperatures, HSP70 levels and its subcellular distribution exhibit substantial heterogeneity as demonstrated by indirect immunofluorescence with HSP70-specific monoclonal antibodies. Of particular interest is a subpopulation of cells in which the characteristic nuclear accumulation and nucleolar association of HSP70 is not detected after heat shock treatment. This apparent variation in the heat shock response is not observed when synchronized cells are examined. In this study, we demonstrate that three monoclonal antibodies to HSP70, in particular, do not detect nucleolar-localized HSP70 in heat-shocked G2 cells. This is not due to an inability of G2 cells to respond to heat shock as measured by increased HSP70 mRNA and protein synthesis, or due to a lack of accumulation of HSP70 after heat shock in G2. Rather the epitopes recognized by the various antibodies appear to be inaccessible, perhaps due to the association of HSP70 with other proteins. Non-denaturing immunoprecipitations with these HSP70-specific antibodies suggest that HSP70 may interact with other cellular proteins in a cell cycle-dependent manner.

Dynamic changes in the structure and intracellular locale of the mammalian low-molecular-weight heat shock protein

Mammalian cells grown at 37 degrees C contain a single low-molecular-weight heat shock (or stress) protein with an apparent mass of 28 kilodaltons (kDa) whose synthesis increases in cells after exposure to elevated temperatures or other forms of physiologic stress. Herein we present data demonstrating that heat shock protein 28 exists in a number of dynamic states depending upon the physiologic state of the cell. Biochemical fractionation of 37 degrees C cells in the absence of nonionic detergent revealed that the 28-kDa protein partitioned approximately equally between the soluble and insoluble fractions. The addition of detergent in the fractionation procedure resulted in all of the protein distributed within the soluble phase. In contrast, in cells first heat shocked and then fractionated in the presence of detergent, most of the 28-kDa protein was found within the insoluble fraction. These biochemical results appeared entirely consistent with indirect immunofluorescence experiments, demonstrating that the 28-kDa protein resided within the perinuclear region of 37 degrees C cells in close proximity to the Golgi complex. After heat shock treatment, the 28-kDa protein relocalized within the nucleus and resisted detergent extraction. The extent of 28-kDa protein redistribution into the nucleus and its detergent insolubility increased as a function of the severity of the heat shock treatment. With time of recovery from the heat treatment there occurred a gradual return of the 28-kDa protein into the detergent-soluble phase. Concomitant with these changes in 28-kDa protein solubility was a corresponding change in the apparent size of the protein as determined by gel filtration. While at 37 degrees C cells the protein exhibited a mass of 200 to 800 kDa; after heat shock the protein assumed sizes of 2 MDa or greater. Using immunoelectron microscopy, we show an accumulation of these aggregates of 28-kDa protein within the nucleus. Finally, we show that the heat-dependent redistribution of the 28-kDa protein from the cytoplasm into the nucleus was greatly diminished when the cells were first rendered thermotolerant, and we suggest that this simple assay (i.e., 28-kDa protein detergent solubility) may prove useful in evaluating the thermotolerant status of a cell or tissue.

Heat shock but not other stress inducers leads to the disruption of a sub-set of snRNPs and inhibition of in vitro splicing in HeLa cells

Splicing of pre-mRNA in HeLa cells exposed to various stress response inducers has been investigated. In vivo, intron-containing transcripts of the hsp27 gene accumulate in cells stressed by heat or sodium arsenite. In vitro analysis, however, reveals a differential effect of stress on splicing: nuclear extracts from cells exposed to a severe heat shock are incapable of splicing an exogenously supplied substrate while splicing is not perturbed in extracts treated with sodium arsenite, the amino acid analog canavinine or ethanol. Pretreatment of cells with a mild heat shock prior to a severe heat shock protects the splicing apparatus and allows splicing to proceed unimpeded. Analyses of the splicing defect in extracts from heat-shocked cells show that the inhibition of splicing cannot be accounted for by changes in the major RNA and protein components of small nuclear ribonucleoprotein particles (snRNPs) or in a previously described heat-labile factor that is essential for in vitro splicing. Fractionation of small nuclear ribonucleoprotein particles from heat-shock extracts by native polyacrylamide gel electrophoresis reveals dramatic changes in certain particles, most noticeably in a U4/U5/U6 snRNP complex and the U2 snRNP. Alterations in these particles are accompanied by the assembly of labeled pre-mRNA transcript into aberrant splicing complexes that differ from those formed in normal extracts.

Heat shock is lethal to fibroblasts microinjected with antibodies against hsp70

Synthesis of a small group of highly conserved proteins in response to elevated temperature and other agents that induce stress is a universal feature of prokaryotic and eukaryotic cells. Although correlative evidence suggests that these proteins play a role in enhancing survival during and after stress, there is no direct evidence to support this in mammalian cells. To assess the role of the most highly conserved heat shock protein (hsp) family during heat shock, affinity-purified monoclonal antibodies to hsp70 were introduced into fibroblasts by needle microinjection. In addition to impairing the heat-induced translocation of hsp70 proteins into the nucleus after mild heat shock treatment, injected cells were unable to survive a brief incubation at 45 degrees C. Cells injected with control antibodies survived a similar heat shock. These results indicate that functional hsp70 is required for survival of these cells during and after thermal stress.