Thermotolerance in mammalian cells. Protein denaturation and aggregation, and stress proteins

Role of the mitochondrial DnaJ homologue, Mdj1p, in the prevention of heat-induced protein aggregation

The role of the mitochondrial Hsp70 system in the prevention of heat-induced protein aggregation was studied in isolated mitochondria from Saccharomyces cerevisiae. Firefly luciferase was employed as a thermolabile tester protein. After shift to 40 degrees Celsius transient increase of mt-Hsp70/luciferase complex was observed, which required functional Mdj1p and Mge1p, the mitochondrial homologues of DnaJ and GrpE. The kinetics of luciferase aggregation, however, were not influenced by mutations in either mt-Hsp70 or Mge1p. Only the absence of Mdj1p led to enhanced protein aggregation. Thus, a central role in the transient protection against heat stress is attributed to this mitochondrial DnaJ homologue.

Thermotolerance and heat shock proteins: possible involvement of Ku autoantigen in regulating Hsp70 expression

Here we characterize and compare the phenomenon of thermotolerance and permanent heat resistance in mammalian cells. The biochemical and molecular mechanisms underlying the induction of thermotolerance, and the role that heat shock proteins play in its development and decay are discussed. Finally, we describe a novel constitutive HSE-binding factor (CHBF/Ku) that appears to be involved in the regulation of the heat shock response.

Heat-shock induced protein modifications and modulation of enzyme activities

Upon heat stress, the cell physiology is profoundly altered. The extent of the alterations depends on the severity of the stress and may lead to cell death. The heat shock response is an array of metabolic changes characterized by the impairment of major cellular functions and by an adaptative reprogramming of the cell metabolism. The enhanced synthesis of the HSPs is a spectacular manifestation of this reprogramming. Numerous post translational modifications of proteins occur in response to heat stress and can be related to altered cellular functions. Some proteins are heat-denatured and temporarily inactivated. Heat-denaturation is reversible, chaperones may contribute to the repair. The extent of heat-denaturation depends on the cell metabolism: (a) it is attenuated in thermotolerant cells or in cells overexpressing the appropriate chaperones (b) it is enhanced in energy-deprived cells. Covalent modifications may also rapidly alter protein function. Changes in protein glycosylation, methylation, acetylation, farnesylation, ubiquitination have been found to occur during stress. But protein phosphorylation is the most studied modification. Several protein kinase cascades are activated, among which the various mitogen activated protein kinase (MAP kinase) cascades which are also triggered by a wide range of stimuli. As a possible consequence, stress modifies the phosphorylation status and the activity of components from the transcriptional and translational apparatuses. The same kinases also target key enzymes of the cellular metabolism. Protein denaturation results in constitutive hsp titration, this titration is a signal to trigger the heat-shock gene transcription and to activate some of the protein kinase cascades.

Thermostability of a nuclear-targeted luciferase expressed in mammalian cells. Destabilizing influence of the intranuclear microenvironment

Protein denaturation and aggregation are most likely the cause for the noxious effects of heat shock. There are some indications that the nucleus is one of the most sensitive cellular compartments. To test the possibility that the intranuclear microenvironment might be detrimental to the heat stability of proteins, we compared the in situ thermal stability of a reporter protein localized in the nucleus or in the cytoplasm. A recombinant firefly (Photynus pyralis) luciferase carrying a point mutation in the C-terminal domain remains in the cytoplasm (cyt-luciferase). A nuclear localization sequence was fused to the N-terminal domain of cyt-luciferase; the resulting nuc-luciferase was efficiently targeted to the cell nucleus. In both cases, decreased luciferase activity and solubility were found in lysates from heat-shocked cells. These characteristics were taken as an indication of thermal denaturation in situ. The heat-inactivated luciferases were partially reactivated during recovery after stress, indicating the capacity of both the cytoplasmic and nuclear compartments to reassemble proteins from an aggregated state. Although both the nuc- and the cyt-luciferases were heat inactivated at similar rates in vitro, nuc-luciferase was more susceptible to thermal denaturation in situ compared to cyt-luciferase. This observation suggests that the microenvironment of an intracellular compartment may modulate the thermal stability of proteins. The local concentration might be one element of this microenvironment affecting the heat-stability of proteins. In cells made thermotolerant by a priming shock, the thermal inactivation of the recombinant luciferases occurred at a slower rate during a second challenging stress. However, this decreased thermal sensitivity was less pronounced for the nuc-luciferase (threefold) than for the cyt-luciferase (sevenfold). The nuclear luciferase might become a useful tool to investigate the action of molecular chaperones in the nucleus.

Thermotolerance and nuclear protein aggregation: protection against initial damage or better recovery?

Heat-induced nuclear protein aggregation and subsequent disaggregation were measured in nonpreheated and preheated (thermotolerant) HeLa S3 cells. The effect of thermotolerance on the formation of and recovery from heat-induced nuclear protein aggregates was related to the cellular levels of hsp27, hsp60, hsp70, hsc70, and hsp90. Cells heated at different time points after the thermotolerance trigger showed various levels of protection against heat-induced nuclear protein aggregation. This protection, however, did not parallel the development and decay of thermotolerance on cell survival. The protection was maximal when the thermotolerance level already had started to decay. The level of protection against nuclear protein aggregation did however parallel the cellular level of hsp70 indicating that hsp70 may be involved in this process. At all stages during the development and decay, thermotolerant cells showed a more rapid recovery (disaggregation) from the heat-induced nuclear protein aggregates than non-thermotolerant cells. The rates of disaggregation during development and decay of thermotolerance paralleled the cellular levels of hsp27 suggesting that hsp27 is somehow involved in this recovery process from heat-induced nuclear protein aggregates. The total cellular levels of none of the individual hsp's completely correlate with development and decay of thermotolerance, indicating that overexpression of any of these hsp's alone does not determine the level of thermotolerance. Clonogenic cell survival paralleled the rates of disaggregation, leading to the notion that recovery processes are the most important determinant for the thermotolerant state of HeLa S3 cells. The best correlation with clonogenic survival was found when both initial aggregation and subsequent disaggregation were taken into account, suggesting that the combined action of various hsp's in these two processes have to be included in thermotolerance development and decay.

Modification of intracellular pH and thermotolerance development by amiloride

The intracellular pH (pHi) of cells heated at 45.0 degrees C in the presence or absence of amiloride and in choline chloride substituted sodium-free medium was measured with flow cytometry using the pH sensitive dye, carboxy-seminaphthorhodafluor (SNARF-1). Chinese hamster ovary (CHO) cells at pH 7.3 and low-pH-resistant (PHV2) cells at pH 6.6 were studied. Bimodal population distributions of pHi were obtained for both CHO and PHV2 cells following a treatment in which cells were heated 10 min at 45.0 degrees C, incubated 4 to 10 h at 37 degrees C, then reheated at 45 min at 45.0 degrees C. Amiloride or sodium-free medium modified the changes in pHi, but did not eliminate them entirely. Cells were sorted from the higher pHi and lower pHi subpopulations and plated for cell survival. The survival after both heat treatments was three to five-fold higher for cells sorted from the higher pHi subpopulation than cells sorted from the low pHi subpopulation. The development of thermotolerance was delayed in CHO cells but not in PHV2 cells when amiloride was present throughout the treatment regimen. Combining low pH with amiloride caused an even greater delay in thermotolerance development in CHO cells. However, the final fraction of thermotolerant cells after 14 h incubation was nearly identical, regardless of medium pH or the presence of amiloride.

Selective c-JUN expression in CA1 neurons of the gerbil hippocampus during and after acquisition of an ischemia-tolerant state

The selective delayed neuronal death of CA1 pyramidal cells after transient global ischemia in the gerbil brain can be prevented by preconditioning with a short sublethal period of ischemia 1-7 days prior to a subsequent, usually lethal ischemia of 5 min duration. Since changes of neuronal gene expression may play a crucial role in this tolerance induction, we investigated the postischemic expression profile of the fos, jun and Krox transcription factor families. We have previously reported that a single 5 min period of cerebral ischemia does not cause a de novo synthesis of immediate early gene (IEG) encoded proteins in CA1 neurons. In the present study, two experimental groups of Mongolian gerbils were investigated: one group was subjected to a single tolerance-inducing 2.5 min period of ischemia by bilateral occlusion of the common carotid artery. The second (combined ischemia) group was subjected to 2.5 min of ischemia, followed by 5 min of ischemia 4 days later. Post-ischemic expression of c-FOS, FOS B, c-JUN, JUN B, JUN D and KROX-24 was investigated by in situ hybridization and immunocytochemistry up to 48 h of recirculation. In contrast to a single 5 min period of ischemia, 2.5 min caused a postischemic expression of c-JUN protein, but no other IEGs, in CA1 neurons (peak at 6 h). Similarly, a selective but delayed c-JUN expression (peak at 18 h) was observed in animals subjected to combined ischemia. These results indicate that the induction of an endogenous neuroprotective state in CA1 neurons is associated with the activation of a genetic program which involves the expression of specific transcription factors.

Thermotolerant cells possess an enhanced capacity to repair heat-induced alterations to centrosome structure and function

To study the mechanisms of thermotolerance, the adaptive response by which cells become transiently resistant to killing by heat shock, we have focused on the centrosome, an organelle whose disorganization is closely correlated with thermal killing in Chinese hamster ovary (CHO) cells. Centrosome structure was studied by use of antisera directed against pericentrin, a 220 Kd protein of the pericentriolar material (PCM). Centrosome function was measured in intact cells by performing microtubule regrowth following exposure to the drug nocodazole. Immediately following heating at 45 degrees C for 4-18 min, centrosomal staining by antipericentrin decreased. Thereafter, staining gradually recovered, although abnormal configurations of staining appeared in heated cultures 10-20 h later. In contrast, abnormal patterns of staining rarely developed in thermotolerant cultures. Centriole number was not perturbed by heat, indicating that the heat effect was specific for the PCM. Heat also caused an immediate reduction in the number of microtubules nucleated by the PCM. As for staining by antipericentrin, microtubule nucleation recovered during 3-20 h at 37 degrees C after heating. The immediate, heat-induced decrease in antipericentrin staining or microtubule nucleation was similar in thermotolerant and nontolerant cells. In contrast, the inhibition for both endpoints recovered to control levels much more quickly in thermotolerant cells than in nontolerant cells. Furthermore, new protein synthesis was not required for the recovery of microtubule nucleation. These data show that thermotolerant cells have an enhanced capacity to repair thermal damage to centrosome structure and function, and suggest that a faster rate of recovery prevents disorganization of the PCM that is observed in nontolerant cells several hours after heating.

Heat shock proteins and molecular chaperones: implications for adaptive responses in the skin

Recent advances in the biology of heat-shock proteins (hsps) are reviewed. These abundant and evolutionarily highly conserved proteins (also called stress proteins) act as molecular escorts. Hsps bind to other cellular proteins, help them to fold into their correct secondary structures, and prevent misfolding and aggregation during stress. Cytoplasmic hsp70 and hsp60 participate in complicated protein-folding pathways during the synthesis of new polypeptides. Close relatives of hsp70 and hsp60 assist in the transport and assembly of proteins inside intracellular organelles. Hsp90 may have a unique role, binding to the glucocorticoid receptor in a manner essential for proper steroid hormone action. Hsps may also be essential for thermotolerance and for prevention and repair of damage caused by ultraviolet B light. A unique class of T lymphocytes, the gamma delta T cells, exhibits a restricted specificity against hsps. These T cells may constitute a general, nonspecific immune mechanism directed against the hsps within invading organisms or against very similar hsps within invading organisms or against very similar hsps expressed by infected (stressed) keratinocytes. Immunologic cross-reactivity between hsps of foreign organisms and of the host may play a role in some autoimmune diseases. Although hsps are expressed in the skin, many questions remain about their role during injury, infection, and other types of cutaneous pathophysiology.

On the role of hsp72 in heat-induced intranuclear protein aggregation

Heat treatment of cells results in an increased protein content of nuclei and nuclear matrices when isolated after the heat treatment. This increase of TX-100 insoluble protein is interpreted as being the result of protein denaturation and subsequent aggregation. After the heat treatment cells can (partly) recover from these aggregates. Recent data suggest that heat shock proteins (hsps) might be involved in the recovery (disaggregation) from these heat-induced insoluble protein complexes. In this report, the role of hsp72 in the process of aggregation and disaggregation was investigated using: non-tolerant rat-1 cells, thermotolerant rat-1 cells (rat-1 TT), and transfected rat-1 cells constitutively expressing the human inducible hsp72 gene (HR-24 cells). After heating the various cells, it was observed that the expression of the human hsp72 confers heat resistance (43-45 degrees C). Heat-induced intranuclear protein aggregation was less in HR and rat-1 TT cells as compared to nontolerant rat-1 cells. After heat treatments leading to the same initial intranuclear protein aggregation, rat-1 TT cells recovered more rapidly from these aggregates, while HR cells recovered at the same rate as nontolerant rat-1 cells. Our data suggest that increased levels of hsp72 can confer heat resistance at the level of initial (nuclear) heat damage. Elevated levels of hsp72 alone, however, do not enable cells to recover more rapidly from heat-induced intranuclear protein aggregates.

Increased thermal aggregation of proteins in ATP-depleted mammalian cells

In an attempt to understand the influence of the intracellular environment on protein stability, the thermal denaturation of various reporter proteins was examined within cultured mammalian cells. Loss of solubility and of enzymatic activities were taken as indicators of thermal denaturation. Photinus pyralis luciferase, Escherichia coli beta-galactosidase, the 70-kDa constitutive heat-shock proteins and the 68-kDa dsRNA-dependent protein kinase are found mostly in the supernatant fractions of centrifuged lysates from control unshocked mammalian cells. However, when cells are lysed after heat shock, a proportion of the reporter molecules is found to be aggregated to the nuclear pellets. This insolubilization does not affect all cellular proteins; many of them remain unaffected by heat shock. The heat-induced insolubilization of all four reporter proteins is markedly enhanced when the intracellular ATP concentration is drastically decreased after inhibition of both oxidative phosphorylation and glycolysis. Although ATP molecules bind to luciferase and protect it from thermal inactivation in vitro, the consequences of strong ATP depletion on luciferase thermal stability within the cells are found to be much greater than expected from in vitro data. The 70-kDa constitutive heat-shock proteins and the 68-kDa protein kinase are ATP-binding proteins but ATP depletion also considerably increases the aggregation of beta-galactosidase to the nuclear pellets, although this enzyme is not known to be an ATP-binding molecule. Insolubilization of all four reporter proteins occurs in ATP-depleted cells even at normal growing temperatures (37 degrees C). Protein denaturation may be enhanced either by the aggregation and disappearance of the intracellular 'free' chaperones or by the trapping of unfolded protein molecules on chaperones; the chaperone/unfolded protein complexes could not dissociate in the absence of ATP. Enhanced protein denaturation due to ATP depletion is proposed to account for the greater heat sensitivity of ATP-depleted cells and for the ability of mitochondrial uncouplers to trigger a heat-shock response in some cells.