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

Core genome responses involved in acclimation to high temperature

Plants can acclimate rapidly to environmental conditions, including high temperatures. To identify molecular events important for acquired thermotolerance, we compared viability and transcript profiles of Arabidopsis thaliana treated to severe heat stress (45 degrees C) without acclimation or following two different acclimation treatments. Notably, a gradual increase to 45 degrees C (22 degrees C to 45 degrees C over 6 h) led to higher survival and to more and higher-fold transcript changes than a step-wise acclimation (90 min at 38 degrees C plus 120 min at 22 degrees C before 45 degrees C). There were significant differences in the total spectrum of transcript changes in the two treatments, but core components of heat acclimation were apparent in the overlap between treatments, emphasizing the importance of performing transcriptome analysis in the context of physiological response. In addition to documenting increases in transcripts of specific genes involved in processes predicted to be required for thermotolerance (i.e. protection of proteins and of translation, limiting oxidative stress), we also found decreases in transcripts (i.e. for programmed cell death, basic metabolism, and biotic stress responses), which are likely equally important for acclimation. Similar protective effects may also be achieved differently, such as prevention of proline accumulation, which is toxic at elevated temperatures and which was reduced by both acclimation treatments but was associated with transcript changes predicted to either reduce proline synthesis or increase degradation in the two acclimation treatments. Finally, phenotypic analysis of T-DNA insertion mutants of genes identified in this analysis defined eight new genes involved in heat acclimation, including cytosolic ascorbate peroxidase and the transcription factors HsfA7a (heat shock transcription factor A7a) and NF-X1.

Thermotolerance and cell death are distinct cellular responses to stress: dependence on heat shock proteins

We tested the hypothesis that heat shock protein (Hsp) induction and cell death are mutually exclusive responses to stress. Despite activation of heat shock transcription factor 1 at temperatures ranging from 40 to 46 degrees C, Hsp72 and Hsp27 were not induced above 42 degrees C. Moreover, cells underwent apoptosis at 44 degrees C and necrosis at 46 degrees C, with mitochondrial cytochrome c release at both temperatures. However, only apoptosis was associated with caspase activation. Treatment of cells with z-VAD-fmk prior to heat shock at 44 degrees C failed to restore Hsp induction despite inhibition of heat-induced apoptosis. Furthermore, accumulation of Hsps after incubation at 42 degrees C rendered the cells resistant to apoptosis. These results suggest that lack of Hsp induction is the cause rather than the consequence of cell death.

Tissue-specific changes in RNA synthesis in vivo during anoxia in crucian carp

The overall energy budget for protein synthesis (i.e., transcription plus translation) is thought to consist of fixed and variable components, with RNA synthesis accounting for the former and protein synthesis the latter. During anoxia, the downregulation of protein synthesis (i.e., the variable component), to reduce energetic demand, is an important aspect of survival in crucian carp. The present study examines RNA synthesis during anoxia by labeling with [(3)H]uridine. A novel synthesis rate calculation is presented, which allows for the tissue-specific salvage of uridine, with synthesis rates finally expressed relative to DNA. After 48 h anoxia, the decline (29%) in brain RNA synthesis and increases in the heart and liver (132 and 871%, respectively) support known RNA functions during hypoxic/anoxic survival. This study provides evidence that, in an anoxia-tolerant species, survival mechanisms involving RNA are able to operate because tissue-specific restructuring of the RNA synthesis process enables fixed synthesis costs to be maintained; this may be as vital to survival as exploiting the variable energetic demand of protein synthesis.

A comet assay of DNA damage and repair in K562 cells after photodynamic therapy using haematoporphyrin derivative, methylene blue and meso-tetrahydroxyphenylchlorin

Single-cell electrophoresis (comet assay) has been used to evaluate DNA damage and repair in the human myeloid leukaemia cell line K562 after low-dose (predominantly sub-lethal) treatments of hyperthermia and photodynamic therapy (PDT). Three different photosensitizers were examined: haematoporphyrin derivative (HpD), methylene blue (MB) and meso-tetrahydroxyphenylchlorin (mTHPC). None of the drugs in the absence of light, nor in light alone, resulted in detectable DNA damage. However, a significant amount of DNA damage was detected immediately after treatment with haematoporphyrin derivative or methylene blue PDT compared with drug-only or light-only treatments; no residual level of DNA damage was evident for either drug following a 4-h post-treatment incubation at 37 degrees C. No significant DNA damage was detected after meso-tetrahydroxyphenylchlorin PDT or hyperthermia either immediately or 4 h after treatment. We conclude that the alkaline comet assay can be applied as an effective screening assay for DNA damage induced by a range of laser therapies.

Heat shock proteins, thermotolerance, and their relevance to clinical hyperthermia

Mammalian cells, when exposed to a non-lethal heat shock, have the ability to acquire a transient resistance to subsequent exposures at elevated temperatures, a phenomenon termed thermotolerance. The mechanism(s) for the development of thermotolerance is not well understood, but earlier experimental evidence suggests that protein synthesis may play a role in its manifestation. On the molecular level, heat shock activates a specific set of genes, so-called heat shock genes, and results in the preferential synthesis of heat shock proteins. The heat shock response, specifically the regulation, expression and functions of heat shock proteins, has been extensively studied in the past decades and has attracted the attention of a wide spectrum of investigators ranging from molecular and cell biologists to radiation and hyperthermia oncologists. There is much data supporting the hypothesis that heat shock proteins play important roles in modulating cellular responses to heat shock, and are involved in the development of thermotolerance. This review summarizes some current knowledge on thermotolerance and the functions of heat shock proteins, especially hsp70. The relationship between thermotolerance development and hsp70 synthesis in tumours and in normal tissues is examined. The possibility of using hsp70 as an indicator for thermotolerance is discussed.

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.

Differential heat stress stability of epidermal growth factor receptor and erbB-2 receptor tyrosine kinase activities

The epidermal growth factor (EGF) and erbB-2 receptors are structurally related membrane-bound tyrosine kinases. While these proteins exhibit close sequence homology, 50% overall and 80% in the tyrosine kinase domains, they respond very differently to heat stress. In NIH-3T3 or NR6 cells transfected with wild-type EGF-R and incubated at 37 degrees C or heat shocked at 46 degrees C, EGF binds to its receptor and stimulates receptor autophosphorylation to equivalent extents. At 46 degrees C, however, the basal tyrosine kinase activity of the wild-type erbB-2 receptor is rapidly lost. When cells containing chimeric receptors composed of the EGF-R extracellular domain and intracellular domain of erbB-2 were heat stressed, 125I-EGF bound to the receptors, but did not stimulate receptor autophosphorylation. The decline in EGF-stimulated chimeric erbB-2 receptor autophosphorylation is dependent on the length of heat shock, with nearly 100% of the kinase activity lost after 60 min at 46 degrees C. The loss of chimeric receptor erbB-2 kinase activity is not due to degradation of receptor protein, nor is it attributable to a specific transmembrane domain from either the EGF or erbB-2 receptors. Sensitivity of erbB-2 to heat stress is also not a result of denaturation of this receptor's carboxy-terminal domain. Insertion of the erbB-2 tyrosine kinase domain into the EGF-R confers heat stress sensitivity to the resultant chimeric receptor. Thus, although the EGF-R and erbB-2 kinase domains show a high degree of homology, the secondary/tertiary structures of these domains would seem to be stabilized in distinct manners.

Putative determinants of the cellular response to hyperthermia

Recently, it has been demonstrated that two different thermal resistant states found in Chinese hamster cells, one transient, associated with thermotolerance, and the other permanent, associated with the increased expression of the cognate member of the hsp 70 family, are characterized by faster recovery from heat-induced perturbations in several cellular processes (Laszlo 1992b). These processes include total cellular protein and RNA synthesis, the localization of hsp70, the organization of vimentin, and the protein composition of the nucleus. In the present study, the recovery from heat-induced perturbations in cellular physiology was extended further to two more types of Chinese hamster cells: permanently heat resistant cells in which thermoresistance is associated with the overexpression of hsp27 and heat-sensitive cell lines. When the heat-resistant hsp27 transfected cell lines were compared with the control wild-type cell line, the recovery of protein synthesis from heat-induced inhibition was similar in the normal and hsp27 transfected cells, while the recovery from heat-induced inhibition of total RNA synthesis and the recovery from heat-induced increased association of hsp70 with nuclei were both more rapid in the hsp27 transfected cell lines. In the permanently heat-sensitive cell lines, the kinetics of recovery from heat-induced inhibition of protein synthesis did not correlate with the heat sensitive state. However, delays in the recovery from heat-induced alterations in total cellular RNA synthesis and from heat-induced excess nuclear association of hsp70 were associated with the heat-sensitive state. Overall, these results suggest that the kinetics of recovery from heat-induced alterations in total cellular RNA synthesis and the localization of hsp 70 are putative candidates for being determinants of the cellular response to hyperthermia, and thus have the potential to form the basis of predictive assays for use in conjunction with clinical hyperthermia.

Effect of amino acid analogs on the development of thermotolerance and on thermotolerant cells

Exposure of HA-1 Chinese hamster fibroblasts to amino acid analogs has been shown to have a heat-sensitizing effect as well as inducing the heat shock response (Li and Laszlo, 1985a). In this study, we have examined the effect of amino acid analogs on the development of thermotolerance after a brief heat shock or exposure to sodium arsenite and the effect of amino acid analogs on cells that are already thermotolerant. Exposure of HA-1 cells to amino acid analogs inhibited the development of thermotolerance following a mild heat shock or treatment with sodium arsenite. However, cells that were already thermotolerant were resistant to the sensitizing action of amino acid analogs. The refractoriness of thermotolerant cells to amino acid analog treatment developed in parallel with thermotolerance. The uptake of the arginine analog, canavanine, and its incorporation into proteins was not altered in the thermotolerant cells. Furthermore, another biological consequence of exposure to amino acid analogs, sensitization to ionizing radiation, also was not altered in the thermotolerant cells. The inhibition of the development of thermotolerance by amino acid analogs and the refractoriness of thermotolerant cells to the heat-sensitizing action of amino acid analogs lend further support the role of heat-shock proteins in the phenomenon of thermotolerance.

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

Cells that have been pre-exposed to thermal stress can acquire a transient resistance against the killing effect of a subsequent thermal stress. The cause for this phenomenon, called thermotolerance, seems to be an enhanced resistance of proteins against thermal denaturation and aggregation. This resistance can be expressed as an attenuation of damage formation (less initial damage) or as a better repair of the protein damage (facilitated recovery). Heat Shock (or better, Stress) Proteins (HSPs) may play a role in and even be required for thermal resistance. However, rather than stress-induced enhanced synthesis and elevated total levels of HSPs per se, the concentration of, both constitutive and inducible, HSPs at and/or (re)distributed to specific subcellular sites may be the most important factor for the acquisition of thermotolerance. Specific HSPs may be involved either in damage protection or in damage repair.

The thermoresistant state: protection from initial damage or better repair?

The induction of and recovery from heat-induced perturbations in several cellular parameters were examined in normal, transiently thermotolerant, and permanently heat-resistant HA-1 Chinese hamster fibroblasts. The initial heat-induced perturbations in total cellular protein synthesis, RNA synthesis, vimentin-containing intermediate filaments, and nuclear protein mass were similar in the three different cell types which display various levels of thermal resistance as determined by clonogenic survival. The posthyperthermia recovery from the heat-induced perturbations in all of the cellular parameters was more rapid in both the permanently heat-resistant cells and in the transiently thermotolerant cells. This response was observed in cells in which transient thermotolerance was induced by either a mild heat shock or exposure to sodium arsenite. The development and decay of the capacity for more rapid recovery from the initial heat-induced perturbations in total cellular protein and RNA synthesis paralleled the development and decay of clonogenic thermotolerance. Overall, these results support the notion that more rapid recovery from similar levels of heat-induced perturbations in various cellular parameters are a salient feature of both the transiently and permanently heat-resistant state.

Induction of thermotolerance in T cells protects nuclear DNA topoisomerase I from heat stress

In this study, we have demonstrated that topoisomerase I DNA relaxing activity is protected against a severe heat shock in T cells made thermotolerant by a prior modest heat treatment. However, following a severe heat-shock challenge and incubation at 37 degrees C, topoisomerase activity in the control population eventually returned to levels similar to those detected in thermotolerant cells. This recovery of topoisomerase activity appears to result from the renaturation of heat-inactivated enzyme rather than from synthesis of new protein because the rate of recovery of catalytic activity was not inhibited by the presence of the protein synthesis inhibitor, cycloheximide.

Acquisition of thermotolerance induced by heat and arsenite in HeLa S3 cells: Multiple pathways to induce tolerance?

Recent data indicate that cells may acquire thermotolerance via more than one route. In this study, we observed differences in thermotolerance development in HeLa S3 cells induced by prior heating (15 minutes at 44 degrees C) or pretreatment with sodium-arsenite (1 hour at 37 degrees C, 100 microM). Inhibition of overall protein and heat shock protein (HSP) synthesis (greater than 95%) by cycloheximide (25 micrograms/ml) during tolerance development nearly completely abolished thermotolerance induced by arsenite, while significant levels of heat-induced thermotolerance were still apparent. The same dependence of protein synthesis was found for resistance against sodium-arsenite toxicity. Toxic heat, but not toxic arsenite treatments caused heat damage in the cell nucleus, measured as an increase in the protein mass of nuclei isolated from treated cells (intranuclear protein aggregation). Recovery from this intranuclear protein aggregation was observed during post-heat incubations of the cells at 37 degrees C. The rate of recovery was faster in heat-induced tolerant cells than in nontolerant cells. Arsenite-induced tolerant cells did not show an enhanced rate of recovery from the heat-induced intranuclear protein aggregation. In parallel, hyperthermic inhibition of RNA synthesis was the same in tolerant and nontolerant cells, whereas post-heat recovery was enhanced in heat-induced, but not arsenite-induced thermotolerant cells. The more rapid recovery from heat damage in the nucleus (protein aggregation and RNA synthesis) in cells made tolerant by a prior heat treatment seemed related to the ability of heat (but not arsenite) to induce HSP translocations to the nucleus.

Cell inactivation and cell cycle inhibition as induced by extreme hypoxia: the possible role of cell cycle arrest as a protection against hypoxia-induced lethal damage

Cycling mammalian cells that are rendered extremely hypoxic (less than 4 ppm O2) tend to accumulate in a pre-DNA-synthesis stage. It is not clear whether or not this is the result of an active regulation by the cells. In the present study we have rendered cells, synchronized by mitotic selection, extremely hypoxic over a relatively long period of time (up to 48 h). We have recorded cell cycle progression during hypoxia as well as cell inactivation depending on where in the cell cycle the cells were located when the hypoxic treatment was started. Three main conclusions are drawn: 1 the cell cycle arrest in late-G1 is complete even during a long-lasting (24 h) hypoxic treatment: 2 while cells in early- and mid-S are completely arrested and quickly inactivated under hypoxic conditions, cells in late-S, G2 and mitosis are able to continue cell cycle progression and divide; 3 whether the cells are located in G2, mitosis or early-G1 at the onset of hypoxia, they were able to survive relatively long-lasting hypoxic treatment. The present results are in favour of the view that the pre-DNA-synthetic arrest induced by extreme hypoxia may function to rescue the cells from severely damaging effects that would appear if the cells were able to initiate DNA synthesis.

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.

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.