Hyperthermia stimulates energy-proteasome-dependent protein degradation in cultured myotubes

Deregulated JNK signaling enhances apoptosis during hyperthermia

PURPOSE

c-Jun N-terminal kinases (JNKs) comprise a subfamily of mitogen-activated protein kinases (MAPKs). The JNK group is known to be activated by a variety of stimuli. However, the molecular mechanism underlying heat-induced JNK activation is largely unknown. The aim of this study was to clarify how JNK activity is stimulated by heat.

METHODS AND MATERIALS

The expression levels of various MAPK members in HeLa cells, with or without hyperthermia treatment, were evaluated via western blotting. The kinase activity of MAPK members was assessed through in vitro kinase assays. Cell death was assessed in the absence or presence of siRNAs targeting MAPK-related members.

RESULTS

Hyperthermia decreased the levels of MAP3Ks, such as ASK1 and MLK3 which are JNK kinase kinase members, but not those of the downstream MAP2K/SEK1 and MAPK/JNK. Despite the reduced or transient phosphorylation of ASK1, MLK3, or SEK1, downstream JNK was phosphorylated in a temperature-dependent manner. In vitro kinase assays demonstrated that heat did not directly stimulate SEK1 or JNK. However, the expression levels of DUSP16, a JNK phosphatase, were decreased upon hyperthermia treatment. DUSP16 knockdown enhanced the heat-induced activation of ASK1-SEK1-JNK pathway and apoptosis.

CONCLUSION

JNK was activated in a temperature-dependent manner despite reduced or transient phosphorylation of the upstream MAP3K and MAP2K. Hyperthermia-induced degradation of DUSP16 may induce activation of the ASK1-SEK1-JNK pathway and subsequent apoptosis.

Potential role of passively increased muscle temperature on contractile function

Declines in muscle force, power, and contractile function can be observed in older adults, clinical populations, inactive individuals, and injured athletes. Passive heating exposure (e.g., hot baths, sauna, or heated garments) has been used for health purposes, including skeletal muscle treatment. An acute increase in muscle temperature by passive heating can increase the voluntary rate of force development and electrically evoked contraction properties (i.e., time to peak twitch torque, half-relation time, and electromechanical delay). The improvements in the rate of force development and evoked contraction assessments with increased muscle temperature after passive heating reveal peripheral mechanisms’ potential role in enhancing muscle contraction. This review aimed to summarise, discuss, and highlight the potential role of an acute passive heating stimulus on skeletal muscle cells to improve contractile function. These mechanisms include increased calcium kinetics (release/reuptake), calcium sensitivity, and increased intramuscular fluid.

Decrease in MAP3Ks expression enhances the cell death caused by hyperthermia

PURPOSE

Hyperthermia is a promising anticancer treatment modality. However, the molecular mechanism underlying the thermal sensitivity of tumor cells is largely unknown. The aim of this study was to clarify how biochemical changes triggered by heat stimulate antitumor activity.

METHODS AND MATERIALS

The expression levels of various MAPK members in HeLa cells with or without hyperthermia were evaluated by western blotting and RT-PCR. The intracellular Ca²⁺ concentration [Ca²⁺]_i was monitored by digital imaging using CaTM-2 AM. An in vitro cleavage assay was used to determine whether calcium-dependent protease calpain cleaves MAPK components. Cell proliferation and clonogenicity were assessed in the absence or presence of siRNAs targeting MAPK members.

RESULTS

Hyperthermia decreased the levels of MAP3K TAK1, RAF1 and MEKK2 but not of the downstream MAP2K and MAPK members. The hyperthermia-induced degradation of TAK1 and MEKK2 was rescued by either the proteasome inhibitor MG132 or the calpain inhibitor ALLN; however, RAF1 was not affected by the inhibitors. Heat induced down regulation of RAF1. Hyperthermia increased [Ca²⁺]_i and calpain I expression. The calcium ionophore A23187 decreased TAK1 and MEKK2 levels. An in vitro cleavage assay demonstrated that TAK1 and MEKK2 are calpain I substrates. Knockdown of TAK1, RAF1 and MEKK2 suppressed cell proliferation and clonogenicity.

CONCLUSIONS

Hyperthermia decreased the levels of MAP3K TAK1, RAF1 and MEKK2, without reduction of the downstream components in the MAP3K-MAP2K-MAPK cascade, by a calpain-dependent degradation pathway or transcriptional regulation. TAK1, RAF1 and/or MEKK2 play crucial roles in cell proliferation and clonogenicity and are potential molecular targets for hyperthermia.

Proteasome Biology: Chemistry and Bioengineering Insights

Proteasomal degradation provides the crucial machinery for maintaining cellular proteostasis. The biological origins of modulation or impairment of the function of proteasomal complexes may include changes in gene expression of their subunits, ubiquitin mutation, or indirect mechanisms arising from the overall impairment of proteostasis. However, changes in the physico-chemical characteristics of the cellular environment might also meaningfully contribute to altered performance. This review summarizes the effects of physicochemical factors in the cell, such as pH, temperature fluctuations, and reactions with the products of oxidative metabolism, on the function of the proteasome. Furthermore, evidence of the direct interaction of proteasomal complexes with protein aggregates is compared against the knowledge obtained from immobilization biotechnologies. In this regard, factors such as the structures of the natural polymeric scaffolds in the cells, their content of reactive groups or the sequestration of metal ions, and processes at the interface, are discussed here with regard to their influences on proteasomal function.

Identification of the acclimation genes in transcriptomic responses to heat stress of White Pekin duck

White Pekin duck is an important meat resource in the livestock industries. However, the temperature increase due to global warming has become a serious environmental factor in duck production, because of hyperthermia. Therefore, identifying the gene regulations and understanding the molecular mechanism for adaptation to the warmer environment will provide insightful information on the acclimation system of ducks. This study examined transcriptomic responses to heat stress treatments (3 and 6 h at 35 °C) and control (C, 25 °C) using RNA-sequencing analysis of genes from the breast muscle tissue. Based on three distinct differentially expressed gene (DEG) sets (3H/C, 6H/C, and 6H/3H), the expression patterns of significant DEGs (absolute log2 > 1.0 and false discovery rate < 0.05) were clustered into three responsive gene groups divided into upregulated and downregulated genes. Next, we analyzed the clusters that showed relatively higher expression levels in 3H/C and lower levels in 6H/C with much lower or opposite levels in 6H/3H; we referred to these clusters as the adaptable responsive gene group. These genes were significantly enriched in the ErbB signaling pathway, neuroactive ligand-receptor interaction and type II diabetes mellitus in the KEGG pathways (P < 0.01). From the functional enrichment analysis and significantly regulated genes observed in the enriched pathways, we think that the adaptable responsive genes are responsible for the acclimation mechanism of ducks and suggest that the regulation of phosphoinositide 3-kinase genes including PIK3R6, PIK3R5, and PIK3C2B has an important relationship with the mechanisms of adaptation to heat stress in ducks.

Effect of heat stress-induced production of mitochondrial reactive oxygen species on NADPH oxidase and heme oxygenase-1 mRNA levels in avian muscle cells

Heat stress is a major factor inducing oxidative disturbance in cells. In the present study, we investigated the mechanism of overproduction of reactive oxygen species (ROS) in cultured avian muscle cells in response to heat stress, and also focused attention on the interaction of mitochondrial superoxide anions with altered NADPH oxidase (NOX), superoxide dismutase (SOD) and heme oxygenase-1 (HO-1) mRNA levels in heat-stressed cells. Exposure of cells to heat stress conditions (41°C, 6h) resulted in increased mitochondrial superoxide and intracellular ROS levels, and increased carbonyl protein content as compared with that of normal cells (37°C). The mitochondrial uncoupler 2,4-dinitrophenol lowered intracellular ROS levels in heat-stressed cells. Heat stress increased NOX4 mRNA and decreased HO-1 mRNA levels, while SOD1 and SOD2 mRNA levels remained relatively stable in heat-stressed cells. Addition of the superoxide scavenger 4-hydroxy TEMPO to the culture medium of heat-stressed cells restored mitochondrial superoxide and intracellular ROS levels as well as NOX4 and HO-1 mRNA levels to near-normal values. We suggest that mitochondrial superoxide production could play an influential role in augmenting oxidative damage to avian muscle cells, possibly via the up-regulation of NOX4 and down-regulation of HO-1 in heat-stressed avian muscle cells.

Skeletal muscle interleukin-6 regulation in hyperthermia

We previously reported that IL-6 production is acutely elevated in skeletal muscles exposed to ≥41°C, but the regulatory pathways are poorly understood. The present study characterizes the heat-induced transcriptional control of IL-6 in C2C12 muscle fibers. Hyperthermia exposure (42°C for 1 h) induced transcription from an IL-6 promoter-luciferase reporter plasmid. Heat shock factor-1 (HSF-1), a principal mediator of the heat shock response, was then tested for its role in IL-6 regulation. Overexpression of a constitutively active HSF-1 construct increased basal (37°C) promoter activity, whereas overexpression of a dominant negative HSF-1 reduced IL-6 promoter activity during basal and hyperthermia conditions. Since hyperthermia also induces stress-activated protein kinase (SAPK) signaling, we tested whether mutation of a transcription site downstream of SAPK, (i.e., activator protein-1, AP-1) influences IL-6 transcription in hyperthermia. The mutation had no effect on baseline reporter activity but completely inhibited heat-induced activity. We then tested whether pharmacologically induced states of protein stress, characteristic of cellular responses to hyperthermia and known to induce SAPKs and HSF-1, would induce IL-6 production in the absence of heat. The proteasome was inhibited with MG-132 in one set of experiments, and the unfolded protein response was stimulated with dithiothreitol, thapsigargin, tunicamycin, or castanospermine in other experiments. All treatments stimulated IL-6 protein secretion in the absence of hyperthermia. These studies demonstrate that IL-6 regulation in hyperthermia is directly controlled by HSF-1 and AP-1 signaling and that the IL-6 response in C2C12 myotubes is sensitive to categories of protein stress that reflect accumulation of damaged or unfolded proteins.

β2-Adrenergic agonists and the treatment of skeletal muscle wasting disorders

β2-Agonists are traditionally used for the treatment of bronchospasm associated with asthma and the treatment of symptomatic patients with COPD. However, β2-agonists are also powerful anabolic agents that trigger skeletal muscle hypertrophy. Investigating the effects of β2-agonists in skeletal muscle over the past 30 years in different animal models has led to the identification of potential therapeutic applications in several muscle wasting disorders, including neuromuscular diseases, cancer cachexia, sepsis or thermal injury. In these conditions, numerous studies indicate that β2-agonists can attenuate and/or reverse the decrease in skeletal muscle mass and associated weakness in animal models of muscle wasting but also in human patients. The purpose of this review is to present the biological and clinical significance of β2-agonists for the treatment of skeletal muscle wasting. After the description of the molecular mechanisms involved in the hypertrophy and anti-atrophy effect of β2-agonists, we will review the anti-atrophy effects of β2-agonist administration in several animal models and human pathologies associated with or leading to skeletal muscle wasting. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Temperature and neuromuscular function

This review focuses on the effects of different environmental temperatures on the neuromuscular system. During short duration exercise, performance improves from 2% to 5% with a 1 °C increase in muscle temperature. However, if central temperature increases (i.e., hyperthermia), this positive relation ceases and performance becomes impaired. Performance impairments in both cold and hot environment are related to a modification in neural drive due to protective adaptations, central and peripheral failures. This review highlights, to some extent, the different effects of hot and cold environments on the supraspinal, spinal and peripheral components of the neural drive involved in the up- and down-regulation of neuromuscular function and shows that temperature also affects the neural drive transmission to the muscle and the excitation-contraction coupling.

Heat shock inhibits caspase-1 activity while also preventing its inflammasome-mediated activation by anthrax lethal toxin

Anthrax lethal toxin (LT) rapidly kills macrophages from certain mouse strains in a mechanism dependent on the breakdown of unknown protein(s) by the proteasome, formation of the Nalp1b (NLRP1b) inflammasome and subsequent activation of caspase-1. We report that heat-shocking LT-sensitive macrophages rapidly protects them against cytolysis by inhibiting caspase-1 activation without upstream effects on LT endocytosis or cleavage of the toxin's known cytosolic substrates (mitogen-activated protein kinases). Heat shock protection against LT occurred through a mechanism independent of de novo protein synthesis, HSP90 activity, p38 activation or proteasome inhibition and was downstream of mitogen-activated protein kinase cleavage and degradation of an unknown substrate by the proteasome. The heat shock inhibition of LT-mediated caspase-1 activation was not specific to the Nalp1b (NLRP1b) inflammasome, as heat shock also inhibited Nalp3 (NLRP3) inflammasome-mediated caspase-1 activation in macrophages. We found that heat shock induced pro-caspase-1 association with a large cellular complex that could prevent its activation. Additionally, while heat-shocking recombinant caspase-1 did not affect its activity in vitro, lysates from heat-shocked cells completely inhibited recombinant active caspase-1 activity. Our results suggest that heat shock inhibition of active caspase-1 can occur independently of an inflammasome platform, through a titratable factor present within intact, functioning heat-shocked cells.

Regulation of ubiquitin-proteasome system mediated degradation by cytosolic stress

ER-associated, ubiquitin-proteasome system (UPS)-mediated degradation of the wild-type (WT) gap junction protein connexin32 (Cx32) is inhibited by mild forms of cytosolic stress at a step before its dislocation into the cytosol. We show that the same conditions (a 30-min, 42 degrees C heat shock or oxidative stress induced by arsenite) also reduce the endoplasmic reticulum (ER)-associated turnover of disease-causing mutants of Cx32 and the cystic fibrosis transmembrane conductance regulator (CFTR), as well as that of WT CFTR and unassembled Ig light chain. Stress-stabilized WT Cx32 and CFTR, but not the mutant/unassembled proteins examined, could traverse the secretory pathway. Heat shock also slowed the otherwise rapid UPS-mediated turnover of the cytosolic proteins myoD and GFPu, but not the degradation of an ubiquitination-independent construct (GFP-ODC) closely related to the latter. Analysis of mutant Cx32 from cells exposed to proteasome inhibitors and/or cytosolic stress indicated that stress reduces degradation at the level of substrate polyubiquitination. These findings reveal a new link between the cytosolic stress-induced heat shock response, ER-associated degradation, and polyubiquitination. Stress-denatured proteins may titer a limiting component of the ubiquitination machinery away from pre-existing UPS substrates, thereby sparing the latter from degradation.

Proteomic analysis of altered protein expression in skeletal muscle of rats in a hypermetabolic state induced by burn sepsis

mRNA profiling has been extensively used to study muscle wasting. mRNA level changes may not reflect that of proteins, especially in catabolic muscle where there is decreased synthesis and increased degradation. As sepsis is often associated with burn injury, and burn superimposed by sepsis has been shown to result in significant loss of lean tissues, we characterized changes in the skeletal-muscle proteome of rats subjected to a cutaneous burn covering 20% of the total body surface area, followed 2 days later by sepsis induced by CLP (caecal ligation and puncture). EDL (extensor digitorum longus) muscles were dissected from Burn-CLP animals (n=4) and controls (sham-burned and sham-CLP-treated, n=4). Burn-CLP injury resulted in a rapid loss of EDL weight, increased ubiquitin-conjugated proteins and increased protein carbonyl groups. EDL protein profiles were obtained by two-dimensional gel electrophoresis using two immobilized pH gradient strips with overlapping pH range covering a pH 3-8 range. Seventeen spots were significantly altered in the Burn-CLP compared with the control group, representing 15 different proteins identified by peptide mass fingerprinting. The identities of three proteins including transferrin were further confirmed by liquid chromatography-tandem MS. The significant changes in transferrin and HSP27 (heat-shock protein 27) were verified by Western-blot analysis. HSP60, HSP27 and HSPbeta6 were down-regulated, along with HSP70, as detected by Western blotting. Six metabolic enzymes related to energy production were also down-regulated. A simultaneous decrease in chaperone proteins and metabolic enzymes could decrease protein synthesis. Furthermore, decreased HSPs could increase oxidative damage, thus accelerating protein degradation. Using cultured C2C12 myotubes, we showed that H2O2-induced protein degradation in vitro could be partially attenuated by prior heat-shock treatment, consistent with a protective role of HSP70 and/or other HSPs against proteolysis.

The role of the ubiquitin/proteasome system in cellular responses to radiation

In the last few years, the ubiquitin(Ub)/proteasome system has become increasingly recognized as a controller of numerous physiological processes, including signal transduction, DNA repair, chromosome maintenance, transcriptional activation, cell cycle progression, cell survival, and certain immune cell functions. This is in addition to its more established roles in the removal of misfolded, damaged, and effete proteins. This review examines the role of the Ub/proteasome system in processes underlying the classical effects of irradiation on cells, such as radiation-induced gene expression, DNA repair and chromosome instability, oxidative damage, cell cycle arrest, and cell death. Furthermore, recent evidence suggests that the proteasome is a redox-sensitive target for ionizing radiation and other oxidative stress signals. In other words, the Ub/proteasome system may not simply be a passive player in radiation-induced responses, but may modulate them. The extent of the modulation will be influenced by the functional and structural diversity that is expressed by the system. Cell types vary in the Ub/proteasome structures they possess and the level at which they function, and this changes as they go from the normal to the cancerous condition. Cancer-related functional changes within the Ub/proteasome system may therefore present unique targets for cancer therapy, especially when targeting agents are used in combination with radio- or chemotherapy. The peptide boronic acid compound PS-341, which was designed to inhibit proteasome chymotryptic activity, is in clinical trials for the treatment of solid and hematogenous tumors. It has shown some efficacy on its own and in combination with chemotherapy. Preclinical studies have shown that PS-341 will also potentiate the cytotoxic effects of radiation therapy. In addition, other drugs in common clinical use have been shown to affect proteasome function, and their activities may be valuably reconsidered from this perspective.

Heat shock protects L6 myotubes from catabolic effects of dexamethasone and prevents downregulation of NF-kappaB

Glucocorticoids are the most important mediator of muscle cachexia in various catabolic conditions. Recent studies suggest that the transcription factor NF-kappaB acts as a suppressor of genes in the ubiquitin-proteasome proteolytic pathway and that glucocorticoids increase muscle proteolysis by downregulating NF-kappaB activity. The heat shock (stress) response, characterized by the induction of heat shock proteins, confers a protective effect against a variety of harmful stimuli. In the present study, we tested the hypothesis that the heat shock response protects muscle cells from the catabolic effects of dexamethasone and prevents downregulation of NF-kappaB. Cultured L6 myotubes were subjected to heat shock (43 degrees C for 1 h) followed by recovery at 37 degrees C for 1 h. Thereafter, cells were treated for 6 h with 1 microM dexamethasone, during which period protein degradation was measured as release of TCA-soluble radioactivity from proteins that had been prelabeled with [(3)H]tyrosine. Heat shock resulted in increased protein and mRNA levels for heat shock protein 70. The increase in protein degradation induced by dexamethasone was prevented in cells expressing the heat shock response. In the same cells, dexamethasone-induced downregulation of NF-kappaB DNA binding activity was blocked. The present results suggest that the heat shock response may protect muscle cells from the catabolic effects of dexamethasone and that this effect of heat shock may be related to inhibited downregulation of NF-kappaB activity.