Preconditioning prevents ischemia-induced neuronal death through persistent Akt activation in the penumbra region of the rat brain

Activation of the Akt1-CREB pathway promotes RNF146 expression to inhibit PARP1-mediated neuronal death

Progressive degeneration of dopaminergic neurons characterizes Parkinson's disease (PD). This neuronal loss occurs through diverse mechanisms, including a form of programmed cell death dependent on poly(ADP-ribose) polymerase-1 (PARP1) called parthanatos. Deficient activity of the kinase Akt1 and aggregation of the protein α-synuclein are also implicated in disease pathogenesis. Here, we found that Akt1 suppressed parthanatos in dopaminergic neurons through a transcriptional mechanism. Overexpressing constitutively active Akt1 in SH-SY5Y cells or culturing cells with chlorogenic acid (a polyphenol found in coffee that activates Akt1) stimulated the CREB-dependent transcriptional activation of the gene encoding the E3 ubiquitin ligase RNF146. RNF146 inhibited PARP1 not through its E3 ligase function but rather by binding to and sequestering PAR, which enhanced the survival of cultured cells exposed to the dopaminergic neuronal toxin 6-OHDA or α-synuclein aggregation. In mice, intraperitoneal administration of chlorogenic acid activated the Akt1-CREB-RNF146 pathway in the brain and provided neuroprotection against both 6-OHDA and combinatorial α-synucleinopathy in an RNF146-dependent manner. Furthermore, dysregulation of the Akt1-CREB pathway was observed in postmortem brain samples from patients with PD. The findings suggest that therapeutic restoration of RNF146 expression, such as by activating the Akt1-CREB pathway, might halt neurodegeneration in PD.

Pterostilbene induces Nrf2/HO-1 and potentially regulates NF-κB and JNK-Akt/mTOR signaling in ischemic brain injury in neonatal rats

Hypoxic-ischemic (HI) brain injury has a high occurrence rate of 1-4 per 1000 live births and is the leading cause of neurological disabilities. Despite the improvement in neonatal care, the effectiveness of current therapeutic strategies is limited, and thus, additional therapies with better results are of much needed. Pterostilbene is a stilbenoid possessing numerous preventive and therapeutic properties. The current study aimed to assess whether pterostilbene exerted protective effects in neonatal rats against experimentally induced ischemic brain injury. Pterostilbene was administered via oral gavage from postnatal day 3 to day 8. Rat pups that were seven-day-old were exposed to hypoxic-ischemic insult via ligation of the common carotid artery and hypoxic environment exposure. Pterostilbene treatment reduced neuronal loss and infarct volume. Pterostilbene administration regulated the NF-κB pathway, and the levels of inflammatory mediators (Nitric oxide, TNF-α, IL-1β, and IL-6) were reduced. HI-induced oxidative stress was significantly reduced by pterostilbene, as presented by decreased production of malondialdehyde and reactive oxygen species. Levels of glutathione were enhanced by pterostilbene. Pterostilbene regulated Nrf2/HO-1 and JNK expression and activated the PI3K/Akt-mTOR signals. These findings suggest that pterostilbene is a candidate compound for the treatment of neonatal HI.

Refocusing the Brain: New Approaches in Neuroprotection Against Ischemic Injury

A new era for neuroprotective strategies is emerging in ischemia/reperfusion. This has forced to review the studies existing to date based in neuroprotection against oxidative stress, which have undoubtedly contributed to clarify the brain endogenous mechanisms, as well as to identify possible therapeutic targets or biomarkers in stroke and other neurological diseases. The efficacy of exogenous administration of neuroprotective compounds has been shown in different studies so far. However, something must be missing to get these treatments successfully applied in the clinical environment. Here, the mechanisms involved in neuronal protection against physiological level of ROS and the main neuroprotective signaling pathways induced by excitotoxic and ischemic stimuli are reviewed. Also, the endogenous ischemic tolerance in terms of brain self-protection mechanisms against subsequent cerebral ischemia is revisited to highlight how the preconditioning has emerged as a powerful tool to understand these phenomena. A better understanding of endogenous defense against exacerbated ROS and metabolism in nervous cells will therefore aid to design pharmacological antioxidants targeted specifically against oxidative damage induced by ischemic injury, but also might be very valuable for translational medicine.

Remote Ischemic Conditioning in Cerebral Diseases and Neurointerventional Procedures: Recent Research Progress

Cerebral ischemia and stroke are increasing in prevalence and are among the leading causes of morbidity and mortality in both developed and developing countries. Despite the progress in endovascular treatment, ischemia/reperfusion (IR) injury is an important contributor to post-surgical mortality and morbidity affecting a wide range of neurointerventional procedures. However, pharmacological recruitment of effective cerebral protective signaling has been largely disappointing to date. In remote ischemic conditioning (RIC), repetitive transient mechanical obstruction of vessels at a limb remote from the IR injury site protects vital organs from IR injury and confers infarction size reduction following prolonged arterial occlusion. Results of pharmacologic agents appear to be species specific, while RIC is based on the neuroprotective influences of phosphorylated protein kinase B, signaling proteins, nitric oxide, and transcriptional activators, the benefits of which have been confirmed in many species. Inducing RIC protection in patients undergoing cerebral vascular surgery or those who are at high risk of brain injury has been the subject of research and has been enacted in clinical settings. Its simplicity and non-invasive nature, as well as the flexibility of the timing of RIC stimulus, also makes it feasible to apply alongside neurointerventional procedures. Furthermore, despite nonuniform RIC protocols, emerging literature demonstrates improved clinical outcomes. The aims of this article are to summarize the potential mechanisms underlying different forms of conditioning, to explore the current translation of this paradigm from laboratory to neurovascular diseases, and to outline applications for patient care.

Protective Effects of Notoginsenoside R1 via Regulation of the PI3K-Akt-mTOR/JNK Pathway in Neonatal Cerebral Hypoxic-Ischemic Brain Injury

Notoginsenoside R1 (NGR1) is a predominant phytoestrogen extracted from Panax notoginseng that has recently been reported to play important roles in the treatment of cardiac dysfunction, diabetic kidney disease, and acute liver failure. Studies have suggested that NGR1 may be a viable treatment of hypoxic-ischemic brain damage (HIBD) in neonates by reducing endoplasmic reticulum stress via estrogen receptors (ERs). However, whether NGR1 has other neuroprotective mechanisms or long-term neuroprotective effects is unclear. In this study, oxygen-glucose deprivation/reoxygenation (OGD/R) in primary cortical neurons and unilateral ligation of the common carotid artery (CCL) in 7-day-old postnatal Sprague Dawley (SD) rats followed by exposure to a hypoxic environment were used to mimic an HIBD episode. We assessed the efficacy of NGR1 by measuring neuronal damage with MTT assay and assessed brain injury by TTC staining and brain water content detection 24–48 h after OGD/HIE. Simultaneously, we measured the long-term neurophysiological effects using the beam walking test (5 weeks after HI) and Morris water maze test 5–6 weeks after HI. Expression of PI3K-Akt-mTOR/JNK (24 h after HI or OGD/R) proteins was detected by Western blotting after stimulation with HI, NGR1, LY294002 (PI3K inhibitor), 740Y-P (PI3K agonist), or ICI 182780(estrogen receptors inhibitor). The results indicated that NGR1 exerted neuroprotective effects by inhibiting neuronal apoptosis and promoting cell survival via the PI3K-Akt-mTOR/JNK signaling pathways by targeting ER in neonatal hypoxic–ischemic injury.

Limb remote ischaemic postconditioning-induced elevation of fibulin-5 confers neuroprotection to rats with cerebral ischaemia/reperfusion injury: Activation of the AKT pathway

Limb remote ischaemic postconditioning (RIPostC) is an effective and well-acknowledged treatment for brain ischaemia injury. The present study aimed to evaluate the role of fibulin-5 in the neuroprotection of RIPostC against cerebral ischaemia/reperfusion (I/R) injury in rats. The middle cerebral artery occlusion (MCAO) model was established in rats and then RIPostC was carried out by three cycles of 10 minutes occlusion/10 minutes release of the bilateral femoral artery at the beginning of the reperfusion. To downregulate the fibulin-5 level, fibulin-5 siRNA was injected into the lateral ventricle 24 hours before MCAO. According to our present study, RIPostC attenuated cerebral I/R injury by decreasing infarct volume, improving neurobehavioral score and suppressing blood brain barrier (BBB) leakage. Moreover, the mRNA and protein levels of fibulin-5 were upregulated by RIPostC at 24 hours and 72 hours after reperfusion. Downregulation of fibulin-5 attenuated the neuroprotection of RIPostC. Finally, the result showed that fibulin-5 was upregulated by RIPostC via activation of the PI3K/AKT pathway. Taken together, these results provide evidence that upregulation of fibulin-5 is involved in the beneficial effect of RIPostC against cerebral I/R injury.

Role of the PI3K/AKT signalling pathway in apoptotic cell death in the cerebral cortex of streptozotocin-induced diabetic rats

Diabetes mellitus is associated with cognitive dysfunction. Numerous previous studies have shown that type 1 diabetes-induced hyperglycaemia causes structural brain damage, such as a decrease in whole-brain grey matter. The impact of diabetes mellitus on the cerebral cortex is poorly understood and requires further clarification. In the present study, diabetes was induced via an intraperitoneal injection of streptozotocin (50 mg/kg). Hematoxylin and eosin (H&E) staining was performed to detect the morphological changes in the cerebral cortex, terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) staining was used to detect neuronal apoptosis and western blotting was performed to determine protein expression levels. Nine weeks after the induction of diabetes, the body weight was significantly lower and the blood glucose levels were significantly higher in the diabetic rats than in the control rats (P<0.05). H&E staining revealed nuclear chromatin condensation and cytoplasmic shrinkage in the cerebral cortex of the diabetic rats and TUNEL staining further indicated apoptotic changes in the cerebral cortex of the diabetic rats. The ratio of B-cell lymphoma 2 (Bcl-2) -associated X protein/Bcl-2 and the expression of cytochrome c and activated caspase-3 (cleaved caspase-3) were significantly increased, whereas the ratio of phosphorylated AKT/AKT was significantly decreased in the diabetic rats compared with that in the control rats (P<0.05). Taken together, these results suggested that diabetes mellitus may induce neuronal apoptosis in the cerebral cortex by downregulating AKT phosphorylation.

Crosstalk Between MAPK/ERK and PI3K/AKT Signal Pathways During Brain Ischemia/Reperfusion

The epidermal growth factor receptor (EGFR) is linked to the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) and Raf/mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK_1/2) signaling pathways. During brain ischemia/reperfusion, EGFR could be transactivated, which stimulates these intracellular signaling cascades that either protect cells or potentiate cell injury. In the present study, we investigated the activation of EGFR, PI3K/AKT, and Raf/MAPK/ERK_1/2 during ischemia or reperfusion of the brain using the middle cerebral artery occlusion model. We found that EGFR was phosphorylated and transactivated during both ischemia and reperfusion periods. During ischemia, the activity of PI3K/AKT pathway was significantly increased, as judged from the strong phosphorylation of AKT; this activation was suppressed by the inhibitors of EGFR and Zn-dependent metalloproteinase. Ischemia, however, did not induce ERK_1/2 phosphorylation, which was dependent on reperfusion. Coimmunoprecipitation of Son of sevenless 1 (SOS1) with EGFR showed increased association between the receptor and SOS1 in ischemia, indicating the inhibitory node downstream of SOS1. The inhibitory phosphorylation site of Raf-1 at Ser259, but not its stimulatory phosphorylation site at Ser338, was phosphorylated during ischemia. Furthermore, ischemia prompted the interaction between Raf-1 and AKT, while both the inhibitors of PI3K and AKT not only abolished AKT phosphorylation but also restored ERK_1/2 phosphorylation. All these findings suggest that Raf/MAPK/ERK_1/2 signal pathway is inhibited by AKT via direct phosphorylation and inhibition at Raf-1 node during ischemia. During reperfusion, we observed a significant increase of ERK_1/2 phosphorylation but no change in AKT phosphorylation. Inhibitors of reactive oxygen species and phosphatase and tensin homolog restored AKT phosphorylation but abolished ERK_1/2 phosphorylation, suggesting that the reactive oxygen species-dependent increase in phosphatase and tensin homolog activity in reperfusion period relieves ERK_1/2 from inhibition of AKT.

Neuroprotective effects of electroacupuncture on hypoxic-ischemic encephalopathy in newborn rats are associated with increased expression of GDNF-RET and protein kinase B

OBJECTIVE

To explore the neuroprotective effects of electroacupuncture (EA) on hypoxic-ischemic encephalopathy (HIE) and to further investigate the role of glial cell line-derived neurotrophic factor (GDNF) family receptor member RET (rearranged during transfection) and its key downstream phosphatidylinositol 3 kinase (PI-3K)/protein kinase B (Akt) pathway in the process.

METHODS

A total of 220 seven-day-old SD rats (of either sex, from 22 broods) were randomly divided into two groups, one (30 rats) for sham-surgery group and the other (190 rats) for HIE model group. The HIE model was established using the left common carotid artery ligation method in combination with hypoxic treatment. The successfully established rats were randomly divided into five groups, including control model group, EA group, sham-EA group, antagonist group and antagonist plus electroacupuncture group, with 35 rats in each group. Baihui (GV 20), Dazhui (GV 14), Quchi (LI 11) and Yongquan (KI 1) acupoints were chosen for acupuncture. EA was performed at Baihui and Quchi for 10 min once a day for continuous 1, 3, 7 and 21 days, respectively. The rats were then killed after the operation and injured cerebral cortex was taken for the measurement of neurologic damage by hematoxylin-eosin (HE) staining and the degenerative changes of cortical ultrastructure by transmission electron microscopy. RET mRNA level and Akt protein level were detected by real-time reverse-transcription polymerase chain reaction (RT-PCR) and western blot analysis, respectively.

RESULTS

EA could ameliorate neurologic damage of the first somatic sensory area (S1Tr) and alleviate the degenerative changes of ultrastructure of cortical neurons in rats subjected to HIE. And the longer acupuncture treatment lasted, the better its therapeutic effect would be. This was accompanied by gradually increased expression of GDNF family receptor RET at the mRNA level and its downstream signaling Akt at the protein level in the ischemic cortex.

CONCLUSION

EA has neuroprotective effects on HIE and could be a potential therapeutic strategy for HIE in the neonate. Activation of RET/Akt signaling pathway might be involved in this process.

Ischemic conditioning-induced endogenous brain protection: Applications pre-, per- or post-stroke

In the area of brain injury and neurodegenerative diseases, a plethora of experimental and clinical evidence strongly indicates the promise of therapeutically exploiting the endogenous adaptive system at various levels like triggers, mediators and the end-effectors to stimulate and mobilize intrinsic protective capacities against brain injuries. It is believed that ischemic pre-conditioning and post-conditioning are actually the strongest known interventions to stimulate the innate neuroprotective mechanism to prevent or reverse neurodegenerative diseases including stroke and traumatic brain injury. Recently, studies showed the effectiveness of ischemic per-conditioning in some organs. Therefore the term ischemic conditioning, including all interventions applied pre-, per- and post-ischemia, which spans therapeutic windows in 3 time periods, has recently been broadly accepted by scientific communities. In addition, it is extensively acknowledged that ischemia-mediated protection not only affects the neurons but also all the components of the neurovascular network (consisting of neurons, glial cells, vascular endothelial cells, pericytes, smooth muscle cells, and venule/veins). The concept of cerebroprotection has been widely used in place of neuroprotection. Intensive studies on the cellular signaling pathways involved in ischemic conditioning have improved the mechanistic understanding of tolerance to cerebral ischemia. This has added impetus to exploration for potential pharmacologic mimetics, which could possibly induce and maximize inherent protective capacities. However, most of these studies were performed in rodents, and the efficacy of these mimetics remains to be evaluated in human patients. Several classical signaling pathways involving apoptosis, inflammation, or oxidation have been elaborated in the past decades. Newly characterized mechanisms are emerging with the advances in biotechnology and conceptual renewal. In this review we are going to focus on those recently reported methodological and mechanistic discoveries in the realm of ischemic conditioning. Due to the varied time differences of ischemic conditioning in different animal models and clinical trials, it is important to define optimal timing to achieve the best conditioning induced neuroprotection. This brings not only an opportunity in the treatment of stroke, but challenges as well, as data is just becoming available and the procedures are not yet optimized. The purpose of this review is to shed light on exploiting these ischemic conditioning modalities to protect the cerebrovascular system against diverse injuries and neurodegenerative disorders.

Ischemic tolerance modulates TRAIL expression and its receptors and generates a neuroprotected phenotype

TNF-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily released by microglia, appears to be involved in the induction of apoptosis following focal brain ischemia. Indeed, brain ischemia is associated with progressive enlargement of damaged areas and prominent inflammation. As ischemic preconditioning reduces inflammatory response to brain ischemia and ameliorates brain damage, the purpose of the present study was to evaluate the role of TRAIL and its receptors in stroke and ischemic preconditioning and to propose, by modulating TRAIL pathway, a new therapeutic strategy in stroke. In order to achieve this aim a rat model of harmful focal ischemia, obtained by subjecting animals to 100 min of transient occlusion of middle cerebral artery followed by 24 h of reperfusion and a rat model of ischemic preconditioning in which the harmful ischemia was preceded by 30 mins of tMCAO, which represents the preconditioning protective stimulus, were used. Results show that the neuroprotection elicited by ischemic preconditioning occurs through both upregulation of TRAIL decoy receptors and downregulation of TRAIL itself and of its death receptors. As a counterproof, immunoneutralization of TRAIL in tMCAO animals resulted in significant restraint of tissue damage and in a marked functional recovery. Our data shed new light on the mechanisms that propagate ongoing neuronal damage after ischemia in the adult mammalian brain and provide new molecular targets for therapeutic intervention. Strategies aimed to repress the death-inducing ligands TRAIL, to antagonize the death receptors, or to activate the decoy receptors open new perspectives for the treatment of stroke.

Ischemic brain extract increases SDF-1 expression in astrocytes through the CXCR2/miR-223/miR-27b pathway

Ischemic cerebral stroke is one of the leading global causes of mortality and morbidity. Ischemic preconditioning (IPC) refers to a sublethal ischemia and resulting in tolerance to subsequent severe ischemic injury. Although several pathways are reportedly involved in IPC-mediated neuroprotection, the functional role of astrocytes is not fully understood. Stromal cell-derived factor-1 (SDF-1), a CXC chemokine produced mainly in astrocytes, is a ligand for chemokine receptor CXCR4. SDF-1 is reported to play a critical role in neuroprotection after stroke by mediating the migration of neuronal progenitor cells. We hypothesized that stimuli derived from ischemic brain were involved in the protective effects of IPC. To investigate this hypothesis, the mechanism in which ischemic brain extract (IBE) induced SDF-1 expression was investigated in C6 astrocytoma cells. IBE treatment of C6 cells increased SDF-1 expression compared to that in untreated or normal brain extract (NBE)-treated cells by downregulating SDF-1 targeting miRNA, miR-27b. MiR-223 was inversely upregulated in IBE-treated cells; overexpression of miR-223 decreased the expression of miR-27b by suppressing IKKα expression. Analysis of cytokine array data revealed an IBE associated enhanced expression of CINC-1 (CXCL1) and LIX1 (CXCL5). Knockdown or inhibition of their receptor, CXCR2, abolished IBE-mediated increased expression of SDF-1. These results were confirmed in primary cultured astrocytes. Taken together, the data demonstrate that IBE-elicited signals increase SDF-1 expression through the CXCR2/miR-223/miR-27b pathway in C6 astrocytoma cells and primary astrocytes, supporting the view that increased expression of SDF-1 by ischemic insults is a possible mechanism underlying therapeutic application of IPC.

Effects of pharmacological inhibition of AMP-activated protein kinase on GLUT3 expression and the development of ischemic tolerance in astrocytes

Ischemic tolerance resulting from preconditioning ischemia is a neuroprotective mechanism. In cultured astrocytes, its development depends on regulation of the expression of glucose transporter 3 (GLUT3) by the stress sensor/effector AMP-activated protein kinase (AMPK). Here we demonstrate that GLUT3 is upregulated during preconditioning and then downregulated during recovery. We also found that, although AMPK inhibition during preconditioning initially suppressed the upregulation of GLUT3 as shown previously, this was followed by a period of GLUT3 upregulation, enhanced glycogen accumulation, and enhanced tolerance to a subsequent ischemic challenge. These results reveal that AMPK has a complex influence on ischemic tolerance.

Impact of acellular hemoglobin-based oxygen carriers on brain apoptosis in rats

BACKGROUND

Extracellular hemoglobin (Hb)-based oxygen carriers (HBOCs) are under extensive consideration as oxygen therapeutics. Their effects on cellular mechanisms related to apoptosis are of particular interest, because the onset of proapoptotic pathways may give rise to tissue damage.

STUDY DESIGN AND METHODS

The objective was to assess whether the properties of the Hb that replaces blood during an isovolemic hemodilution would modulate apoptotic-response mechanisms in rat brain and whether such signaling favors cytoprotection or damage. We exposed rats to exchange transfusion (ET; 50% blood volume and isovolemic replacement with Hextend [negative colloid control], MP4OX [PEGylated HBOC with high oxygen affinity], and ααHb [αα-cross-linked HBOC with low oxygen affinity; n=4-6/group]). Sham rats acted as control. Animals were euthanized at 2, 6, and 12 hours after ET; brain tissue was harvested and processed for analysis.

RESULTS

In MP4OX animals, the number of neurons that overexpressed the hypoxia-inducible factor (HIF)-1α was higher than in ααHb, particularly at the early time points. In addition, MP4OX was associated with greater phosphorylation of protein kinase B (Akt), a well-known cytoprotective factor. Indeed, the degree of apoptosis, measured as terminal deoxynucleotidyl transferase-positive neurons and caspase-3 cleavage, ranked in order of MP4OX < Hextend < ααHb.

CONCLUSION

Even though both HBOCs showed increased levels of HIF-1α compared to shams or Hextend-treated animals, differences in signaling events resulted in very different outcomes for the two HBOCs. ααHb-treated brain tissue showed significant neuronal damage, measured as apoptosis. This was in stark contrast to the protection seen with MP4OX, apparently due to recruitment of Akt and neuronal specific HIF-1α pathways.

Temporal and regional patterns of Smad activation in the rat hippocampus following global ischemia

In this study, we examined the temporal and regional patterns of Smad activation in the rat hippocampus following global ischemia. We also examined the association between Smad activation and ischemia-induced pathology in the hippocampus. We found that 1) Smad1, -2, -3, and -5 proteins were detected in the rat hippocampus by means of western blot and immunohistochemistry; 2) after 5 min of ischemia, Smad2 and Smad3 proteins accumulated in the nuclei of pyramidal cells in the CA1 region, which is vulnerable to ischemia; 3) after 3 min of ischemia, which was non-lethal, there was no such nuclear accumulation of Smad2 and Smad3 in the CA1 region; 4) following injection of activin A, nuclear accumulation of Smad2 and Smad3 was induced not only in pyramidal cells of the CA1 region, but also in pyramidal cells of the CA3 region as well as in granule cells of the DG region; 5) activin A-induced nuclear accumulation of Smad2 and Smad3 neither caused degeneration of hippocampal neurons nor prevented degeneration induced by ischemia. These results suggest that in the hippocampus, ischemia-induced activation of Smad2 and Smad3 is associated with the response to stress but is not related to neuronal survival or death.

From rapid to delayed and remote postconditioning: the evolving concept of ischemic postconditioning in brain ischemia

Ischemic postconditioning is a concept originally defined to contrast with that of ischemic preconditioning. While both preconditioning and postconditioning confer a neuroprotective effect on brain ischemia, preconditioning is a sublethal insult performed in advance of brain ischemia, and postconditioning, which conventionally refers to a series of brief occlusions and reperfusions of the blood vessels, is conducted after ischemia/reperfusion. In this article, we first briefly review the history of preconditioning, including the experimentation that initially uncovered its neuroprotective effects and later revealed its underlying mechanisms-of-action. We then discuss how preconditioning research evolved into that of postconditioning--a concept that now represents a broad range of stimuli or triggers, including delayed postconditioning, pharmacological postconditioning, remote postconditioning--and its underlying protective mechanisms involving the Akt, MAPK, PKC and K(ATP) channel cell-signaling pathways. Because the concept of postconditioning is so closely associated with that of preconditioning, and both share some common protective mechanisms, we also discuss whether a combination of preconditioning and postconditioning offers greater protection than preconditioning or postconditioning alone.

Repeated Intrathecal Administration of Ropivacaine Causes Neurotoxicity in Rats

Previous studies have shown that ropivacaine is the least neurotoxic local anaesthetic. Most of the data derive from short-term ropivacaine injection into the subarachnoid space. Intrathecal administration for a prolonged period, and the histological changes and behavioural effects of repeated intrathecal administration, have not previously been investigated. We studied the possible neurotoxicity of intrathecal injection of ropivacaine in a rat model.

Rats received 0.12 ml/kg body weight of ropivacaine at concentrations of 0.5 or 1%, or normal saline only, via an implanted intrathecal catheter at 90-minute intervals for 12 hours. On days 1, 3, 5, 7, 14 and 28, the spinal cord was examined by light and electron microscopy at the L3 level. We assessed sensory thresholds to noxious stimulation, behavioural change and protein kinase B immunoreactivity for possible neuronal injury within the spinal cord.

Ropivacaine 1% induced thermal hyperalgesia and mechanical allodynia, neuronal injury characterised by tissue oedema, proliferation of glial cells, neuronal morphology changes and degeneration and protein kinase B expression. There were no significant differences in motor function as a result of different concentrations of ropivacaine.

Repeated intrathecal injection of ropivacaine 1% can induce neurotoxicity in rats. Our data suggests that expression of protein kinase B might be involved in this neurotoxicity.

Signal transducers and activators of transcription: STATs-mediated mitochondrial neuroprotection

Cerebral ischemia is defined as little or no blood flow in cerebral circulation, characterized by low tissue oxygen and glucose levels, which promotes neuronal mitochondria dysfunction leading to cell death. A strategy to counteract cerebral ischemia-induced neuronal cell death is ischemic preconditioning (IPC). IPC results in neuroprotection, which is conferred by a mild ischemic challenge prior to a normally lethal ischemic insult. Although many IPC-induced mechanisms have been described, many cellular and subcellular mechanisms remain undefined. Some reports have suggested key signal transduction pathways of IPC, such as activation of protein kinase C epsilon, mitogen-activated protein kinase, and hypoxia-inducible factors, that are likely involved in IPC-induced mitochondria mediated-neuroprotection. Moreover, recent findings suggest that signal transducers and activators of transcription (STATs), a family of transcription factors involved in many cellular activities, may be intimately involved in IPC-induced ischemic tolerance. In this review, we explore current signal transduction pathways involved in IPC-induced mitochondria mediated-neuroprotection, STAT activation in the mitochondria as it relates to IPC, and functional significance of STATs in cerebral ischemia.

Mechanisms and prospects of ischemic tolerance induced by cerebral preconditioning

In the brain, brief episodes of ischemia induce tolerance against a subsequent severe episode of ischemia. This phenomenon of endogenous neuroprotection is known as preconditioning-induced ischemic tolerance. The purpose of this review is to summarize the current state of knowledge about mechanisms and potential applications of cerebral preconditioning and ischemic tolerance. Articles related to the terms ischemic preconditioning and ischemic tolerance were systematically searched via MEDLINE/PubMed, and articles published in English related to the nervous system were selected and analyzed. The past two decades have provided interesting insights into the molecular mechanisms of this neuroprotective phenomenon. Although both rapid and delayed types of tolerance have been documented in experimental settings, the delayed type has been found to be more prominent in the case of neuronal ischemic tolerance. Many intracellular signaling pathways have been implicated regarding ischemic preconditioning. Most of these are associated with membrane receptors, kinase cascades, and transcription factors. Moreover, ischemic tolerance can be induced by exposing animals or cells to diverse types of endogenous and exogenous stimuli that are not necessarily hypoxic or ischemic in nature. These cross-tolerances raise the hope that, in the future, it will be possible to pharmacologically activate or mimic ischemic tolerance in the human brain. Another promising approach is remote preconditioning in which preconditioning of one organ or system leads to the protection of a different (remote) organ that is difficult to target, such as the brain. The preconditioning strategy and related interventions can confer neuroprotection in experimental ischemia, and, thus, have promise for practical applications in cases of vascular neurosurgery and endo-vascular therapy.

Preconditioning-induced ischemic tolerance: a window into endogenous gearing for cerebroprotection

Ischemic tolerance defines transient resistance to lethal ischemia gained by a prior sublethal noxious stimulus (i.e., preconditioning). This adaptive response is thought to be an evolutionarily conserved defense mechanism, observed in a wide variety of species. Preconditioning confers ischemic tolerance if not in all, in most organ systems, including the heart, kidney, liver, and small intestine. Since the first landmark experimental demonstration of ischemic tolerance in the gerbil brain in early 1990's, basic scientific knowledge on the mechanisms of cerebral ischemic tolerance increased substantially. Various noxious stimuli can precondition the brain, presumably through a common mechanism, genomic reprogramming. Ischemic tolerance occurs in two temporally distinct windows. Early tolerance can be achieved within minutes, but wanes also rapidly, within hours. Delayed tolerance develops in hours and lasts for days. The main mechanism involved in early tolerance is adaptation of membrane receptors, whereas gene activation with subsequent de novo protein synthesis dominates delayed tolerance. Ischemic preconditioning is associated with robust cerebroprotection in animals. In humans, transient ischemic attacks may be the clinical correlate of preconditioning leading to ischemic tolerance. Mimicking the mechanisms of this unique endogenous protection process is therefore a potential strategy for stroke prevention. Perhaps new remedies for stroke are very close, right in our cells.

The neuroprotective mechanism of brain ischemic preconditioning

Brain ischemia is one of the most common causes of death and the leading cause of adult disability in the world. Brain ischemic preconditioning (BIP) refers to a transient, sublethal ischemia which results in tolerance to later, otherwise lethal, cerebral ischemia. Many attempts have been made to understand the molecular and cellular mechanisms underlying the neuroprotection offered by ischemic preconditioning. Many studies have shown that neuroprotective mechanisms may involve a series of molecular regulatory pathways including activation of the N-methyl-D-aspartate (NMDA) and adenosine receptors; activation of intracellular signaling pathways such as mitogen activated protein kinases (MAPK) and other protein kinases; upregulation of Bcl-2 and heat shock proteins (HSPs); and activation of the ubiquitin-proteasome pathway and the autophagic-lysosomal pathway. A better understanding of the processes that lead to cell death after stroke as well as of the endogenous neuroprotective mechanisms by which BIP protects against brain ischemic insults could help to develop new therapeutic strategies for this devastating neurological disease. The purpose of the present review is to summarize the neuroprotective mechanisms of BIP and to discuss the possibility of mimicking ischemic preconditioning as a new strategy for preventive treatment of ischemia.

Ischemic preconditioning blocks BAD translocation, Bcl-xL cleavage, and large channel activity in mitochondria of postischemic hippocampal neurons

Transient forebrain or global ischemia induces delayed neuronal death in vulnerable CA1 pyramidal cells with many features of apoptosis. A brief period of ischemia, i.e., ischemic preconditioning, affords robust protection of CA1 neurons against a subsequent more prolonged ischemic challenge. Here we show that preconditioning acts via PI3K/Akt signaling to block the ischemia-induced cascade involving mitochondrial translocation of Bad, assembly of Bad with Bcl-x(L), cleavage of Bcl-x(L) to form its prodeath fragment, DeltaN-Bcl-x(L), activation of large-conductance channels in the mitochondrial outer membrane, mitochondrial release of cytochrome c and Smac/DIABLO (second mitochondria-derived activator of caspases/direct IAP-binding protein with low pI), caspase activation, and neuronal death. These findings show how preconditioning acts to prevent the release of cytochrome c and Smac/DIABLO from mitochondria and to preserve the integrity of the mitochondrial membrane. The specific PI3K inhibitor LY294002 administered in vivo 1 h before or immediately after ischemia or up to 120 h later significantly reverses preconditioning-induced protection, indicating a requirement for sustained PI3K signaling in ischemic tolerance. These findings implicate PI3K/Akt signaling in maintenance of the integrity of the mitochondrial outer membrane.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

In vivo and in vitro characterization of a novel neuroprotective strategy for stroke: ischemic postconditioning

As clinical trials of pharmacological neuroprotective strategies in stroke have been disappointing, attention has turned to the brain's own endogenous strategies for neuroprotection. Recently, a hypothesis has been offered that modified reperfusion subsequent to a prolonged ischemic episode may also confer ischemic neuroprotection, a phenomenon termed 'postconditioning'. Here we characterize both in vivo and in vitro models of postconditioning in the brain and offer data suggesting a biological mechanism for protection. Postconditioning treatment reduced infarct volume by up to 50% in vivo and by approximately 30% in vitro. A duration of 10 mins of postconditioning ischemia after 10 mins of reperfusion produced the most effective postconditioning condition both in vivo and in vitro. The degree of neuroprotection after postconditioning was equivalent to that observed in models of ischemic preconditioning. However, subjecting the brain to both preconditioning as well as postconditioning did not cause greater protection than each treatment alone. The prosurvival protein kinases extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and Akt show prolonged phosphorylation in the cortex of postconditioned rats. Neuroprotection after postconditioning was inhibited only in the presence of LY294002, which blocks Akt activation, but not U0126 or SB203580, which block ERK and P38 MAP kinase activity. In contrast, preconditioning-induced protection was blocked by LY294002, U0126, and SB203580. Our data suggest that postconditioning may represent a novel neuroprotective approach for focal ischemia/reperfusion, and one that is mediated, at least in part, by the activation of the protein kinase Akt.

Inhalational anesthetics as preconditioning agents in ischemic brain

While many pharmacological agents have been shown to protect the brain from cerebral ischemia in animal models, none have translated successfully to human patients. One potential clinical neuroprotective strategy in humans may involve increasing the brain's tolerance to ischemia by preischemic conditioning (preconditioning). There are many methods to induce tolerance via preconditioning such as ischemia itself, pharmacological, hypoxia, endotoxin, and others. Inhalational anesthetic agents have also been shown to result in brain preconditioning. Mechanisms responsible for brain preconditioning are many, complex, and unclear and may involve Akt activation, ATP-sensitive potassium channels, and nitric oxide, amongst many others. Anesthetics, however, may play an important and unique role as preconditioning agents, particularly during the perioperative period.

Gender-specific response to isoflurane preconditioning in focal cerebral ischemia

Inhalation anesthetics are effective chemical preconditioning agents in experimental cerebral ischemia. However, previous work has been performed exclusively in male animals. We determined if there is a gender difference in ischemic outcome after isoflurane preconditioning (IsoPC), and if this sex-specific response is linked to differences in Akt phosphorylation or expression of neuronal inducible cell-death putative kinase (NIPK), a negative modulator of Akt activation. Young and middle-aged male and female mice were preconditioned for 4 h with air (sham PC) or 1.0% IsoPC and recovered for 24 h. Cortices were subdissected from preconditioned young male and female mice for measurement of Akt phosphorylation (Western blot) and NIPK mRNA (quantitative polymerase chain reaction). Additional cohorts underwent 2 h of reversible middle cerebral artery occlusion. Lastly, male and female Akt1(+/+) and Akt1(-/-) mice were studied to determine if gender differences in ischemic outcome after IsoPC is Akt1-dependent. Infarction volume was determined at 22 h reperfusion (2,3,5-triphenyltetrazolium chloride). As expected, IsoPC decreased ischemic damage as compared with sham PC in young and middle-aged male mice. In contrast, IsoPC markedly increased infarction in young female mice and had no effect in middle-aged female mice. Cortical phospho-Akt was increased by IsoPC versus sham PC only in male mice. No increase was observed in IsoPC female mice. NIPK mRNA was higher in female mice than in male mice regardless of preconditioning status. Male IsoPC neuroprotection was lost in Akt1-deficient male mice. We conclude that IsoPC is beneficial only in ischemic male brain and that sex differences in IsoPC are mediated through Akt activation and basal NIPK expression.

Ischaemic preconditioning of the brain, mechanisms and applications

BACKGROUND

The concept of ischaemic preconditioning was introduced in the late 1980s. The concept emerged that a brief subcritical ischaemic challenge could mobilize intrinsic protective mechanisms that increased tolerance against subsequent critical ischaemia. Tissues with a high sensitivity against ischaemia, i.e. myocardium and central nervous system, present the most promising targets for therapeutic application of ischaemic preconditioning. During the last years the mechanisms of neuronal preconditioning were systematically studied and a number of molecular regulation pathways were discovered to participate in preconditioning. The purpose of the present review is to survey the actual knowledge on cerebral preconditioning, and to define the practical impact for neurosurgery.

METHODS

A systematic medline search for the terms preconditioning and postconditioning was filed. Publications related to the nervous system were selected and analysed.

FINDINGS

Preconditioning can be subdivided into early and late mechanisms, depending on whether the effect appears immediately after the nonlethal stress or with a delay of some hours or days. In general early effects can be linked to adaptation of membrane receptors whereas late effects are the result of gene up- or downregulation. Not only subcritical ischaemia can trigger preconditioning but also hypoxia, hyperthermia, isoflurane and other chemical substances. Although a vast amount of knowledge has been accumulated regarding neural preconditioning, it is unknown whether the effects can be potentiated by pharmacological or hypothermic neuroprotection during the critical ischaemia. Furthermore, although the practical importance of these findings is obvious, the resulting protective manipulations have so far not been transferred into clinical neurosurgery. Postconditioning and remote ischaemic preconditioning are additional emerging concepts. Postconditioning with a series of mechanical interruptions of reperfusion can apparently reduce ischaemic damage. Remote ischaemic preconditioning refers to the concept that transient ischaemia for example of a limb can lead to protection of the myocardium and possibly the brain.

CONCLUSION

Possible cumulative neuroprotection by preconditioning and pharmacological protection during critical ischaemia should be studied systematically. Easy to apply methods of preconditioning, such as the application of volatile anaesthetics or erythropoietin some hours or days prior to planned temporary ischaemia, should be introduced into the practice of operative neurosurgery.

Regulation of brain cell death and survival after cardiopulmonary bypass

BACKGROUND

This study investigated the effect of low flow cardiopulmonary bypass, circulatory arrest, and selective cerebral perfusion on expression and phosphorylation of selected regulators of cell death and survival in striatum of newborn piglets.

METHODS

Animals were assigned to sham operation and three experimental groups. The experimental groups were placed on bypass, cooled to 18 degrees C, and subjected to 90 minutes of deep hypothermic circulatory arrest (DHCA), low-flow cardiopulmonary bypass (LFCPB) at mL/(kg x min), or selective cerebral perfusion (SCP) at 20 mL/(kg x min), followed by rewarming and 2 hours of recovery. The oxygen pressure in the microcirculation of the cortex was measured by quenching of phosphorescence. Levels of phosphorylated and total protein were determined by Western blot analysis.

RESULTS

Control oxygen pressure was 55 +/- 9 mm Hg and decreased during DHCA, LFCPB, and SCP to 1.1 +/- 0.6 mm Hg, 9.8 +/- 2.3 mm Hg, and 9.3 +/- 1.9 mm Hg, respectively (p < 0.001). After DHCA, N-terminal of Bcl-2-associated X protein (N-Bax) levels increased (295% +/- 15%, p < 0.01), B-cell leukemia protein (Bcl-2) levels decreased (31% +/- 9%, p < 0.01), and phosphorylation level of protein kinase B (pAkt) and extracellular signal-regulated kinase 1/2 (pERK1/2) did not change. After LFCPB and SCP, N-Bax and Bcl-2 levels were unchanged, pAkt levels increased (367% +/- 122%, p < 0.05 and 337% +/- 47%, p < 0.01, respectively), pERK1 (484% +/- 70% and 501% +/- 255%, respectively; p < 0.01) and pERK2 (569% +/- 128%; p < 0.001 and 494% +/- 162%; p < 0.05, respectively) levels increased, and total ERK2 levels also increased (279% +/- 90% and 153% +/- 44%, respectively, p < 0.05).

CONCLUSIONS

Stable levels of Bcl-2 and Bax and the increases in pAkt and pERK1/2 after LFCPB and SCP are likely indicators of improved chances for cell survival.

Moderate cerebral venous congestion induces rapid cerebral protection via adenosine A1 receptor activation

Stroke is a devastating complication in cardiovascular surgery, and neuronal damage is worsened by intracranial pressure elevation caused by cerebral venous circulatory disturbances (CVCD). However, we have previously reported that CVCD before cerebral ischemia decreases the infarct area. In the present study, focal cerebral ischemia was induced in spontaneously hypertensive rats by filament insertion through the carotid artery. Rats were divided into the following four groups: sham-operated, mild or severe venous congestion (VC), and DPCPX. The DPCPX group received the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) prior to mild VC. Behavior, infarct volume, edema and S-100 protein were evaluated among the four groups. The infarct volume rates in mild VC and severe VC groups were significantly less than that in sham-operated and DPCPX groups. However, the mortality of the severe VC group worsened in a time-dependent manner. We observed a significant decrease in edema in the mild VC group compared to the DPCPX group. Behavioral scores also indicated that the mild VC group had fewer neurological deficits than the other three groups, including the DPCPX group. We were able to induce rapid cerebral protection via adenosine A1 receptor activation by administering an appropriate degree of VC prior to cerebral ischemia produced by middle cerebral artery occlusion. Our work suggests possible mechanisms by which such effective VC may lead to cerebral protection and adenosine A1 receptor activation.

Improved regional cerebral blood flow is important for the protection seen in a mouse model of late phase ischemic preconditioning

INTRODUCTION

Ischemic preconditioning (IPC) induces protection to cerebral ischemia. However, it was previously unclear whether this protection resulted from altered susceptibility to ischemia. The current study examines the effects of late phase ischemic preconditioning in a mouse model of middle cerebral artery occlusion (MCAO). Specific examination of the regional cerebral blood flow (rCBF) was conducted.

EXPERIMENTAL PROCEDURE

Intra-abdominal radiofrequency probes were implanted in animals and core temperature was regulated. Mice were subjected to MCAO: (1) brief 15 min duration (preconditioning ischemia) and (2) 45 min MCAO (injurious ischemia). Naive (i.e. not preconditioned) animals were compared with preconditioned animals (preconditioning ischemia plus injurious ischemia at 72 h reperfusion). rCBF was measured using laser Doppler flowmetry (LDF) and magnetic resonance cerebral perfusion (MRP) arterial spin labeling. Percentage of brain infarcted was compared between groups.

RESULTS

rCBF was significantly improved in the preconditioned cohorts of mice. Naive animals showed flow reductions to 16+/-3.59% (MCAO_45; injurious, unpreconditioned) and 17.1+/-8.6% (MCAO_15; preconditioning ischemia alone) of baseline, while preconditioned animals had flows 33.9+/-13.2% (IPC_45; preconditioned animals with injurious ischemia at 72 h reperfusion) of baseline (p=0.001). Percentage of brain infarcted was 17.2+/-6.2% in naive animals, while it was 5.1+/-4.6% in the preconditioned animals (p=0.003). MRP of the perfusion to the ischemic hemisphere, in a striatal coronal slice of the brain was 26.7+/-5.8% of the contralateral hemisphere in naive animals while preconditioned mice had flows of 38.7+/-6.8% of contralateral (p=0.04).

CONCLUSIONS

Improved rCBF is an important factor in the protection of IPC, during injurious MCAO in the mouse. Stringent monitoring of rCBF is required in future studies of IPC.

Involvement of Akt in preconditioning-induced tolerance to ischemia in PC12 cells

The serine-threonine protein kinase Akt has been identified as an important mediator of cell survival able to counteract apoptotic stimuli. However, hibernation, a model of natural tolerance to cerebral ischemia, is associated with downregulation of Akt. We previously established a model of ischemic tolerance in a PC12 cell line and using this model we now addressed the question whether ischemic tolerance also downregulates Akt in PC12 cells. Kinetic studies showed decreased Akt phosphorylation in tolerized cells. Similarly, phosphorylated levels of three major targets of Akt and well-known proapoptotic factors, the glycogen synthase kinase 3 (GSK-3), a Forkhead family member, FoxO4, and the protein murine double minute 2 (MDM2), all inactivated upon phosphorylation by Akt, were decreased in preconditioned cells. In addition, pharmacological blockade of the phosphoinositide 3-kinase (PI3K)/Akt pathway reduced cell death induced by oxygen and glucose deprivation (OGD) and increased the protective effect of preconditioning (PC). Furthermore, decreasing availability of P-Akt by transfecting PC12 cells with constructs of inactive Akt also resulted in protection against OGD and potentiation of the protective effect of PC. Depending on the environment, GSK-3, FOXO-4, and MDM2 can trigger apoptotic responses or cell cycle arrest, and thus, in a situation of reduced energy, driving the cells into a state of quiescence might be neuroprotective. This work suggests that in the context of tolerance downregulation of Akt is beneficial.

Effect of sublethal 6-hydroxydopamine on the response to subsequent oxidative stress in dopaminergic cells: evidence for preconditioning

Exposure to sublethal stress can trigger endogenous protection against subsequent, higher levels of stress. We tested for this preconditioning phenomenon in a model of Parkinson's disease by applying 6-hydroxydopamine to the dopaminergic MN9D cell line. Exposure to sublethal concentrations of 6-hydroxydopamine (5-10 microM) protected against the toxic effects of a subsequent exposure to a higher concentration (50 microM), as measured by the Hoechst assay for nuclear viability. This was accompanied by little or no protection against 6-hydroxydopamine-induced lactate dehydrogenase release, decline in ATP, or reduction in (3)H-dopamine uptake. The antioxidant, N-acetyl cysteine (20 mM), when applied during preconditioning, abolished protection, as did the protein synthesis inhibitor, cycloheximide (0.2 microM). Preconditioning did not affect superoxide dismutase or glutathione peroxidase enzymes, or levels of heat shock protein-72. However, Bcl-2 protein levels rose with preconditioning. Preconditioning rapidly increased phosphorylation of kinases ERK1/2, Akt and JNK, and was abolished by pharmacological inhibitors of their activity. Finally, sublethal 6-hydroxydopamine preconditioned against the toxicity of proteasome inhibitor, MG-132 (1 microM). Thus, exposure of a dopaminergic cell line to sublethal oxidative stress can protect against additional oxidative stress due to translational and post-translational modifications, as well as confer 'cross-tolerance' against a different insult, proteasome inhibition.

Protein kinase B/Akt modulates nephrotoxicant-induced necrosis in renal cells

Protein kinase B (Akt) activation is well known for its protective effects against apoptosis. However, the role of Akt in regulation of necrosis is unknown. This study was designed to test whether Akt activation protects against nephrotoxicant-induced injury and death in renal proximal tubular cells (RPTC). Exposure of primary cultures of RPTC to the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), resulted in 9% apoptosis and 30% necrosis at 24 h following the exposure. Akt was activated during 8 h but not at 24 h following toxicant exposure. No RPTC necrosis was observed during Akt activation. Blocking Akt activation using a phosphatidylinositol 3-kinase inhibitor, LY294002 (20 muM), or expressing dominant negative (inactive) Akt increased DCVC-induced RPTC necrosis to 42%. In contrast, Akt activation by expression of constitutively active Akt diminished necrosis to 15%. Modulation of Akt activity had no effect on DCVC-induced apoptosis. DCVC-induced RPTC injury was accompanied by decreases in respiration (51% of controls) and ATP levels (57% of controls). Akt inhibition exacerbated decreases in RPTC respiration and intracellular ATP content (both to 30% of controls). In contrast, Akt activation reduced DCVC-induced decreases in respiration (80% of controls) and prevented decline in ATP content. These data show that in RPTC, Akt activation reduces 1) toxicant-induced mitochondrial dysfunction, 2) decreases in ATP levels, and 3) necrosis. We conclude that Akt activation plays a protective role against necrosis caused by nephrotoxic insult in RPTC. Furthermore, we identified mitochondria as a subcellular target of protective actions of Akt against necrosis.

Dual inhibition of protein phosphatase-1/2A and calpain rescues nerve growth factor-differentiated PC12 cells from oxygen-glucose deprivation-induced cell death

In the present study, we examined how the cell survival signaling via cyclic AMP-responsive element binding protein (CREB) and Akt, and the cell death signaling via cystein proteases, calpain and caspase-3, are involved in oxygen-glucose deprivation (OGD) followed by reoxygenation (OGD/reoxygenation)-induced cell death in nerve growth factor (NGF)-differentiated PC12 cells. OGD/reoxygenation-induced cell death was evaluated by LDH release into the culture medium. The level of LDH release was low (9.0% +/- 4.1%) immediately after 4 hr of OGD (0 hr of reoxygenation), was significantly increased to 28.6% +/- 6.6% at 3 hr of reoxygenation, and remained at similar levels at 6 and 20 hr of reoxygenation, suggesting that reoxygenation at least for 3 hr resulted in the loss of cell membrane integrity. After 4 hr of OGD followed by 3 hr of reoxygenation, dephosphorylation of phosphorylated CREB (pCREB), but not phosphorylated Akt (pAkt), was induced. Under these conditions, calpain- but not caspase-3-mediated alpha-spectrin breakdown product was increased, indicating that OGD/reoxygenation also induced an increase in calpain activity. The restoration of pCREB by protein phosphatase (PP)-1/2A inhibitors or the inhibition of excessive activation of calpain by calpain inhibitor did not reduce OGD/reoxygenation-induced LDH release. Cotreatment with PP-1/2A and calpain inhibitors reduced OGD/reoxygenation-induced LDH release. The present study suggests that a balance in the phosphorylation and proteolytic signaling is involved in the survival of NGF-differentiated PC12 cells.