Initiation of protein synthesis and heat-shock protein-72 expression in the rat brain following severe insulin-induced hypoglycemia

Short-term but not long-term hypoglycaemia enhances plasma levels and hepatic expression of HSP72 in insulin-treated rats: an effect associated with increased IL-6 levels but not with IL-10 or TNF-α

The inducible expression of the 70-kDa heat shock proteins (HSP70) is associated with homeostatically stressful situations. Stresses involving sympathetic nervous system (SNS) activation, including α1-adrenergic agonists and physical exercise, are capable of inducing HSP70 expression and release of the HSP70 inducible form, HSP72. However, whether hypoglycaemia is capable of influencing HSP70 status under a stressful situation such as insulin-induced hypoglycaemia (IIH), which also involves SNS activation, is unsettled. Hence, we decided to investigate whether the predominant signal for HSP70 expression and delivery into the blood comes from either low glucose, high insulin, or both during short-term IIH (STIIH) and long-term IIH (LTIIH). Our data indicated that low glucose level (up to 1.56 ± 0.14 mM), but not insulin, is the triggering factor responsible for a dramatic rise in HSP72 plasma concentrations (from 0.15 ± 0.01 in fed state to 0.77 ± 0.13 ng/mL during hypoglycaemic episodes). This was observed in parallel with up to 7-fold increases in interleukin-6 (IL-6) but not interleukin-10 (IL-10) or tumour necrosis factor-α (TNF-α) at STIIH. Together, the observations may suggest that HSP72 is released under hypoglycaemic conditions as a part of the homeostatic stress response, whereas at long-term, both hypoglycaemia and insulin may influence HSP72 expression in the liver, but not in kidneys. Secreted extracellular HSP72 (eHSP72) may be purely a danger signal to all the tissues of the body for the enhancement of immune and metabolic surveillance state or actively participates in glycaemic control under stressful situations.

Anesthetics, cerebral protection and preconditioning

BACKGROUND AND OBJECTIVES

Several studies demonstrate that cerebral preconditioning is a protective mechanism against a stressful situation. Preconditioning determinants are described, as well as the neuroprotection provided by anesthetic and non-anesthetics agents.

CONTENT

Review based on the main articles addressing the pathophysiology of ischemia-reperfusion and neuronal injury and pharmacological and non-pharmacological factors (inflammation, glycemia, and temperature) related to the change in response to ischemia-reperfusion, in addition to neuroprotection induced by anesthetic use.

CONCLUSIONS

The brain has the ability to protect itself against ischemia when stimulated. The elucidation of this mechanism enables the application of preconditioning inducing substances (some anesthetics), other drugs, and non-pharmacological measures, such as hypothermia, aimed at inducing tolerance to ischemic lesions.

Anesthetics, cerebral protection and preconditioning

BACKGROUND AND OBJECTIVES

Several studies demonstrate that cerebral preconditioning is a protective mechanism against a stressful situation. Preconditioning determinants are described, as well as the neuroprotection provided by anesthetic and non-anesthetics agents.

CONTENT

Review based on the main articles addressing the pathophysiology of ischemia-reperfusion and neuronal injury and pharmacological and non-pharmacological factors (inflammation, glycemia, and temperature) related to the change in response to ischemia-reperfusion, in addition to neuroprotection induced by anesthetic use.

CONCLUSIONS

The brain has the ability to protect itself against ischemia when stimulated. The elucidation of this mechanism enables the application of preconditioning inducing substances (some anesthetics), other drugs, and non-pharmacological measures, such as hypothermia, aimed at inducing tolerance to ischemic lesions.

Exercise training and experimental diabetes modulate heat shock protein response in brain

In diabetes, defense systems against cellular stress are impaired. Heat shock proteins (HSPs) function primarily as molecular chaperones. Factors that raise tissue HSP levels may slow progression of diabetes and improve diabetic complications that also affect brain tissue. This study tested the effect of an 8-week exercise training on brain HSP response in rats with or without streptozotocin-induced diabetes (SID). In untrained animals, the HSP levels were not different between SID and non-diabetic groups. Endurance training, however, increased HSP72 and HSP90 protein in non-diabetic rats, whereas SID significantly decreased the effect of training on these HSPs. At the mRNA level, HSP60, HSP90 and GRP75 were increased due to training, whereas HSP72 mRNA was only increased in exercise-trained diabetic animals. Training or diabetes had no effect on protein carbonyl content, a marker of oxidative damage. Altogether, our findings suggest that endurance training increases HSP expression in the brain, and that experimental diabetes is associated with an incomplete HSP response at the protein level.

Edward F. Adolph Distinguished Lecture: Muscle as an endocrine organ: IL-6 and other myokines

Skeletal muscle is an endocrine organ that produces and releases myokines in response to contraction. Some myokines are likely to work in a hormone-like fashion, exerting specific endocrine effects on other organs such as the liver, the brain, and the fat. Other myokines will work locally via paracrine mechanisms, exerting, e.g., angiogenetic effects, whereas yet other myokines work via autocrine mechanisms and influence signaling pathways involved in fat oxidation and glucose uptake. The finding that muscles produce and release myokines creates a paradigm shift and opens new scientific, technological, and scholarly horizons. This finding represents a breakthrough within integrative physiology and contributes to our understanding of why regular exercise protects against a wide range of chronic diseases. Thus the myokine field provides a conceptual basis for the molecular mechanisms underlying, e.g., muscle-fat, muscle-liver, muscle-pancreas, and muscle-brain cross talk.

Muscle as an endocrine organ: focus on muscle-derived interleukin-6

Skeletal muscle has recently been identified as an endocrine organ. It has, therefore, been suggested that cytokines and other peptides that are produced, expressed, and released by muscle fibers and exert paracrine, autocrine, or endocrine effects should be classified as "myokines." Recent research demonstrates that skeletal muscles can produce and express cytokines belonging to distinctly different families. However, the first identified and most studied myokine is the gp130 receptor cytokine interleukin-6 (IL-6). IL-6 was discovered as a myokine because of the observation that it increases up to 100-fold in the circulation during physical exercise. Identification of IL-6 production by skeletal muscle during physical activity generated renewed interest in the metabolic role of IL-6 because it created a paradox. On one hand, IL-6 is markedly produced and released in the postexercise period when insulin action is enhanced but, on the other hand, IL-6 has been associated with obesity and reduced insulin action. This review focuses on the myokine IL-6, its regulation by exercise, its signaling pathways in skeletal muscle, and its role in metabolism in both health and disease.

Developmental exposure to polychlorinated biphenyls influences stroke outcome in adult rats

BACKGROUND

The "developmental origins of adult disease" hypothesis was originally derived from evidence linking low birth weight to cardiovascular diseases including stroke. Subsequently, it has been expanded to include developmental exposures to environmental contaminants as risk factors for adult onset disease.

OBJECTIVE

Our goal in this study was to test the hypothesis that developmental exposure to poly-chlorinated biphenyls (PCBs) alters stroke outcome in adults.

METHODS

We exposed rats to the PCB mixture Aroclor 1254 (A1254) at 0.1 or 1 mg/kg/day in the maternal diet throughout gestation and lactation. Focal cerebral ischemia was induced at 6-8 weeks of age via middle cerebral artery occlusion, and infarct size was measured in the cerebral cortex and striatum at 22 hr of reperfusion. PCB congeners were quantified in brain tissue by gas chromatography with microelectron capture detection, and cortical and striatal expression of Bcl2 and Cyp2C11 were quantified by quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Developmental exposure to A1254 significantly decreased striatal infarct in females and males at 0.1 and 1 mg/kg/day, respectively. Predominantly ortho-substituted PCB congeners were detected above background levels in brains of adult females and males exposed to A1254 at 1 but not 0.1 mg/kg/day. Effects of developmental A1254 exposure on Bcl2 and Cyp2C11 expression did not correlate with effects on infarct volume.

CONCLUSION

Our data provide proof of principle that developmental exposures to environmental contaminants influence the response of the adult brain to ischemic injury and thus represent potentially important determinants of stroke susceptibility.

Mechanisms of Brain Injury after Global Cerebral Ischemia

Cerebral ischemia results in a rapid depletion of energy stores that triggers a complex cascade of cellular events such as cellular depolarization and Ca2+ influx, resulting in excitotoxic cell death. The critical determinant of severity of brain injury is the duration and severity of the ischemic insult and early restoration of CBF. Induced therapeutic hypothermia following CA is the only strategy that has demonstrated improvement in outcomes in prospective, randomized clinical trials. Although pharmacologic neuro-protection has been disappointing thus far in a variety of experimental animal models, further research efforts are directed at using some agents that demonstrate marginal or moderate efficacy in combination with hypothermia. Although the signal transduction pathways and intracellular molecular events during cerebral ischemia and reperfusion are complex, potential therapeutic neuroprotective strategies hold promise for the future.

Vitamin E isoform-specific inhibition of the exercise-induced heat shock protein 72 expression in humans

Increased levels of reactive oxygen and nitrogen species, as seen in response to exercise, challenge the cellular integrity. Important protective adaptive changes include induction of heat shock proteins (HSPs). We hypothesized that supplementation with antioxidant vitamins C (ascorbic acid) and E (tocopherol) would attenuate the exercise-induced increase of HSP72 in the skeletal muscle and in the circulation. Using randomization, we allocated 21 young men into three groups receiving one of the following oral supplementations: RRR-alpha-tocopherol 400 IU/day + ascorbic acid (AA) 500 mg/day (CEalpha), RRR-alpha-tocopherol 290 IU/day + RRR-gamma-tocopherol 130 IU/day + AA 500 mg/day (CEalphagamma), or placebo (Control). After 28 days of supplementation, the subjects performed 3 h of knee extensor exercise at 50% of the maximal power output. HSP72 mRNA and protein content was determined in muscle biopsies obtained from vastus lateralis at rest (0 h), postexercise (3 h), and after a 3-h recovery (6 h). In addition, blood was sampled for measurements of HSP72, alpha-tocopherol, gamma-tocopherol, AA, and 8-iso-prostaglandin-F2alpha (8-PGF2alpha). Postsupplementation, the groups differed with respect to plasma vitamin levels. The marker of lipid peroxidation, 8-iso-PGF2alpha, increased from 0 h to 3 h in all groups, however, markedly less (P < 0.05) in CEalpha. In Control, skeletal muscle HSP72 mRNA content increased 2.5-fold (P < 0.05) and serum HSP72 protein increased 4-fold (P < 0.05) in response to exercise, whereas a significant increase of skeletal muscle HSP72 protein content was not observed (P = 0.07). In CEalpha, skeletal muscle HSP72 mRNA, HSP72 protein, and serum HSP72 were not different from Control in response to exercise. In contrast, the effect of exercise on skeletal muscle HSP72 mRNA and protein, as well as circulating HSP72, was completely blunted in CEalphagamma. The results indicate that gamma-tocopherol comprises a potent inhibitor of the exercise-induced increase of HSP72 in skeletal muscle as well as in the circulation.

Hypoglycemia enhances the expression of prion protein and heat-shock protein 70 in a mouse neuroblastoma cell line

Cellular prion protein (PrP(C)) expression can be regulated by heat-shock stress, and we designed the present study to determine whether hypoglycemia could affect PrP(C) expression. RT-PCR and Western blotting were used to measure the expression of PrP(C) and heat-shock protein (Hsp70) in mouse neuroblastoma (N18) cells cultured 3 hr to 3 days in media deprived of 97.5% (L) or 75% (M) of its glucose. Hypoglycemia caused a concomitant time-dependent and glucose dose-dependent increase in PrP(C) and Hsp70. In addition, hypoglycemia also increased phosphorylated c-Jun N-terminal kinase (JNK) protein levels in a time-dependent manner. The upregulation of PrP(C) and Hsp70 under hypoglycemic conditions was disrupted by the specific JNK inhibitor SP600125. It was also found from in vitro studies that hypoglycemic conditions induced higher levels of PrP(C) promoter activity in PrP(C) promoters containing a heat-shock element (HSE) than in PrP(C) promoters lacking HSE. We propose that hypoglycemia-increased PrP(C) expression might be due to JNK phosphorylation of a heat-shock transcriptional factor, which then interacts with HSE in the promoter of PrP(C).

Biochemical and molecular studies using human autopsy brain tissue

The use of human brain tissue obtained at autopsy for neurochemical, pharmacological and physiological analyses is reviewed. RNA and protein samples have been found suitable for expression profiling by techniques that include RT-PCR, cDNA microarrays, western blotting, immunohistochemistry and proteomics. The rapid development of molecular biological techniques has increased the impetus for this work to be applied to studies of brain disease. It has been shown that most nucleic acids and proteins are reasonably stable post-mortem. However, their abundance and integrity can exhibit marked intra- and intercase variability, making comparisons between case-groups difficult. Variability can reveal important functional and biochemical information. The correct interpretation of neurochemical data must take into account such factors as age, gender, ethnicity, medicative history, immediate ante-mortem status, agonal state and post-mortem and post-autopsy intervals. Here we consider issues associated with the sampling of DNA, RNA and proteins using human autopsy brain tissue in relation to various ante- and post-mortem factors. We conclude that valid and practical measures of a variety of parameters may be made in human brain tissue, provided that specific factors are controlled.

Ischemic tolerance

A brief period of cerebral ischemia confers transient tolerance to a subsequent ischemic challenge in the brain. This phenomenon of ischemic tolerance has been confirmed in various animal models of forebrain ischemia and focal cerebral ischemia. Since the ischemic tolerance afforded by preceding ischemia can bring about robust protection of the brain, the mechanism of tolerance induction has been extensively studied. It has been elucidated that ischemic tolerance protects neurons, and at the same time, it preserves brain function. Further experiments have shown that metabolic and physical stresses can also induce cross-tolerance to cerebral ischemia, but the protection by cross-tolerance is relatively modest. The underlying mechanism of ischemic tolerance still is not fully understood. Potential mechanisms may be divided into two categories: (1) A cellular defense function against ischemia may be enhanced by the mechanisms inherent to neurons. They may arise by posttranslational modification of proteins or by expression of new proteins via a signal transduction system to the nucleus. These cascades of events may strengthen the influence of survival factors or may inhibit apoptosis. (2) A cellular stress response and synthesis of stress proteins may lead to an increased capacity for health maintenance inside the cell. These proteins work as cellular "chaperones" by unfolding misfolded cellular proteins and helping the cell to dispose of unneeded denatured proteins. Recent experimental data have demonstrated the importance of the processing of unfolded proteins for cell survival and cell death. The brain may be protected from ischemia by using multiple mechanisms that are available for cellular survival. If tolerance induction can be manipulated and accelerated by a drug treatment that is safe and effective enough, it could greatly improve the treatment of stroke.

Possible mechanisms involved in the down-regulation of translation during transient global ischaemia in the rat brain

The striking correlation between neuronal vulnerability and down-regulation of translation suggests that this cellular process plays a critical part in the cascade of pathogenetic events leading to ischaemic cell death. There is compelling evidence supporting the idea that inhibition of translation is exerted at the polypeptide chain initiation step, and the present study explores the possible mechanism/s implicated. Incomplete forebrain ischaemia (30 min) was induced in rats by using the four-vessel occlusion model. Eukaryotic initiation factor (eIF)2, eIF4E and eIF4E-binding protein (4E-BP1) phosphorylation levels, eIF4F complex formation, as well as eIF2B and ribosomal protein S6 kinase (p70(S6K)) activities, were determined in different subcellular fractions from the cortex and the hippocampus [the CA1-subfield and the remaining hippocampus (RH)], at several post-ischaemic times. Increased phosphorylation of the alpha subunit of eIF2 (eIF2 alpha) and eIF2B inhibition paralleled the inhibition of translation in the hippocampus, but they normalized to control values, including the CA1-subfield, after 4--6 h of reperfusion. eIF4E and 4E-BP1 were significantly dephosphorylated during ischaemia and total eIF4E levels decreased during reperfusion both in the cortex and hippocampus, with values normalizing after 4 h of reperfusion only in the cortex. Conversely, p70(S6K) activity, which was inhibited in both regions during ischaemia, recovered to control values earlier in the hippocampus than in the cortex. eIF4F complex formation diminished both in the cortex and the hippocampus during ischaemia and reperfusion, and it was lower in the CA1-subfield than in the RH, roughly paralleling the observed decrease in eIF4E and eIF4G levels. Our findings are consistent with a potential role for eIF4E, 4E-BP1 and eIF4G in the down-regulation of translation during ischaemia. eIF2 alpha, eIF2B, eIF4G and p70(S6K) are positively implicated in the translational inhibition induced at early reperfusion, whereas eIF4F complex formation is likely to contribute to the persistent inhibition of translation observed at longer reperfusion times.

Heat shock protein 60 in corpora amylacea

Heat shock protein 60 representation in the corpora amylacea of the brain was investigated in five different neurological diseases. In the cases with cerebral infarct, amyotrophic lateral sclerosis, multiple sclerosis, acute disseminated encephalomyelitis and primary tumors of the nervous system the corpora amylacea showed similar appearance with strong HSP-60 positivity in all investigated disorders at the predilection sites. In the inflammatory diseases, besides corpora amylacea, several cellular elements exhibited HSP-60 immunostaining too. In these cases, the widespread HSP-60 immunoreactivity associated with relative moderate corpora amylacea production as compared to other diseases. From this contradiction we concluded the corpora amylacea participate in the cellular stress reaction but stress protein synthesis certainly is not the primary event in corpora amylacea formation. In the development of the corpora amylacea the incipient process is most probably degenerative in nature, which later on is accompanied by stress protein synthesis and slow growing of these round structures designated for a protective role in the brain. However, the role of the stress protein synthesis in the corpora amylacea formation and growth was not unequivocally answered in this study. It is necessary to perform further comparative investigations of the stress protein representation and corpora amylacea formation in different diseases which may help in discovering useful pathogenetic data and the biological role of this degenerative structure.

Time course of brain neuronal degeneration and heat shock protein (72) expression following neck tourniquet-induced cerebral ischemia in the rat

1. The present study was designed to examine the regional expression of HSP72/73 protein after a 7.5-min period of cerebral ischemia and to compare the distribution of HSP neurons with the localization of irreversible neuronal degeneration as analyzed by silver impregnation technique. 2. During 6-24 hr after cerebral ischemia clear-cut neuronal argyrophilia developed in several brain regions including the hippocampal hilus, nucleus reticularis thalami, and colliculi inferiores. With the exception of the hippocampal hilus, the structures which showed silver impregnability were HSP72 negative at 6-24 hr. 3. Despite the clear HSP72 expression seen in hippocampal CA1 neurons, a significant loss of these neurons was seen at 7 days after ischemia. 4. These data show that in some structures the presence of HSP72 is indicative of higher resistance of these neurons to ischemia-induced degeneration, however, the process of delayed neuronal degeneration appears to be independent of the accelerated synthesis of HSP72 seen during the early period of reflow.

Long-term follow up of persistent hyperinsulinaemic hypoglycaemia of infancy

Twenty six children with hypoglycaemia were diagnosed and followed between 1975 and 1995. Diagnosis was confirmed by a high insulin:glucose ratio, and low free fatty acid and 3-hydroxybutyrate on fasting. All patients were treated with diazoxide at a maximum dose of 20 mg/kg/day. Requirement of a higher dose was considered as a failure of medical treatment and an indication for surgery. Sixteen children Responded to diazoxide; 10 failed to respond and underwent pancreatic resection. Six of the latter group started with symptoms in the neonatal period. Eleven of the 26 children have neurological sequelae. Head growth and neurological outcome correlated well. Additionally, non-specific electroencephalogram abnormalities (slow waves) appear to be indicative of subclinical hypoglycaemia during follow up.

Influence of mitochondrial protein synthesis inhibition on deafferentation-induced ultrastructural changes in nucleus magnocellularis of developing chicks

Following cochlea removal in developing chicks, about 30% of the neurons in the ipsilateral second-order auditory nucleus, nucleus magnocellularis, undergo cell death. Administration of chloramphenicol, a mitochondrial protein synthesis inhibitor, results in a pronounced increase in deafferentation-induced cell death. In this study, we examined whether the chloramphenicol enhancement of deafferentation-induced cell death reveals the same ultrastructural characteristics that are seen in degenerating nucleus magnocellularis neurons after cochlea removal alone. Unilateral cochlea removal was performed on anaesthetized posthatch chicks. One group of animals was simultaneously treated with chloramphenicol. Six, twelve, or twenty-four hours following cochlea removal, n. magnocellularis neurons were studied by routine transmission electron microscopy. Particular attention was paid to the integrity of the polyribosomes and rough endoplasmic reticulum. Two ultrastructurally different types of neuronal degeneration were observed in the deafferented nucleus magnocellularis neurons: an early onset electron-lucent type that always involved ribosomal dissociation and a late-onset electron-dense type displaying nuclear pyknosis and severely damaged mitochondria. The percentage of nucleus magnocellularis neurons displaying ribosomal disintegration following cochlea removal was found to be markedly increased after chloramphenicol treatment. This finding suggests that mitochondrial function is important for the maintenance of a functional protein synthesis apparatus following deafferentation.

Astrocyte survival and HSP70 heat shock protein induction following heat shock and acidosis

Although severe acidosis is an important mediator of brain infarction, recent evidence suggests that mild acidosis may protect ischemic cells. The HSP70 heat shock protein is induced by acidosis in cultured cells and in ischemic brain and protects cells against many types of injury. Therefore, this study determined whether induction of heat shock proteins protects cultured astrocytes against acidosis. Brief exposure of cultured cortical astrocytes to acid (pH 5.2 for 40 min) or heat shock (45 degrees C for 40 min) markedly induced hsp70 mRNA and HSP70 protein. HSP70 protein was detected with the C92 monoclonal antibody (Welch and Suhan: J Cell Biol 103:2035, 1986), which has been shown to recognize the protein product of the full-length rat hsp70 cDNA (Longo et al: J Neurosci Res 36:325, 1993). Heat shock of the cultured cortical astrocytes completely protected the astrocytes from an otherwise lethal heat exposure 24 h later (45 degrees C for 4 h). In contrast, heat pretreatment sensitized the astrocytes to injury from acidosis 24 h later. Acid pretreatment, which markedly induced the HSP70 protein without producing astrocytic cell death, similarly sensitized the cells to injury from acidosis 24 h later (60% survival following pH 5.2 for 3 h versus 90% survival in controls; P < 0.0001). Surprisingly, heat shock pretreatment protected astrocytes against exposure to acid 48 h later (P < 0.05, 1.5-3 h), whereas acid pretreatment had no effect on astrocyte survival 48 h later. Since heat shock did not protect against acidosis at 24 h when HSP70 induction was maximal but did protect at 48 h when HSP70 was markedly diminished, the protective effect of heat shock at 48 h may be related to stress proteins present at 48 h. It is concluded that induction of HSP70 and other heat shock proteins by heat shock protects astrocytes against subsequent lethal heat shock. However, heat shock and acid treatment increase the vulnerability of astrocytes to acidosis 24 h later in spite of the induction of HSP70 heat shock proteins. The finding that heat shock protected astrocytes against acidosis 2 days later may suggest that delayed induction of stress proteins partially protects the astrocytes against damage produced by high concentrations of hydrogen ions.