Protein Kinase C (PKC) is a family of serine/threonine-isozymes that are involved in many signaling events in normal and disease states. Previous studies from our lab have demonstrated that εPKC plays a pivotal role in neuroprotection induced by ischemic preconditioning. However, the role of εPKC during and after brain ischemia is not clearly defined. Therefore, in the present study, we tested the hypothesis that activation of εPKC during an ischemic event is neuroprotective. Furthermore, other studies have demonstrated that εPKC mediates cerebral ischemic tolerance in the rat brain by decreasing vascular tone. Thus, we also tested the effects of εPKC activation during ischemia on cerebral blood flow (CBF). We found that ψε-Receptors for activated C kinase (RACK), a εPKC-selective peptide activator, injected intravenously 30 minutes before induction of global cerebral ischemia conferred neuroprotection in the CA1 region of the rat hippocampus. Moreover, measurements of CBF before, during and after cerebral ischemia revealed a significant reduction in the reperfusion phase of rats pretreated with ψεRACK compared to Tat peptide (vehicle). Our results suggest that εPKC can protect the rat brain against ischemic damage by regulating CBF. Thus, εPKC may be one of the treatment modalities against ischemic injury.
Ischemia; epsilon Protein Kinase C; Cerebral Blood Flow; Neuroprotection
Delayed neuroprotection against ischemic challenges is conferred by both ischemic preconditioning (IPC) and preconditioning by activation of the ε-isoform of protein kinase C (εPKC-PC). In vivo, ischemic preconditioning enhances GABA release and ameliorates glutamate release during lethal cerebral ischemia. We tested the hypothesis that IPC and εPKC-PC confer neuroprotection by GABA synapses in rat organotypic hippocampal slices. Ischemic preconditioning or εPKC-PC was induced with 15 mins oxygen-glucose deprivation (OGD) or ψεRACK, a selective εPKC activator; and test ischemia consisted of 40 mins OGD. At the time of peak neuroprotection (48 h after preconditioning), we recorded GABAA receptor-mediated miniature postsynaptic currents (GABA mPSCs) in vulnerable CA1 pyramidal neurons using whole-cell voltage clamp techniques. The frequency and amplitude of GABA mPSCs significantly increased 48 h after IPC. In contrast, εPKC-PC enhanced only the amplitude of GABA mPSCs with no effect on frequency. We next asked if neuroprotection depended on these changes in GABA synapses. Weak antagonism of the GABAA receptor with bicuculline (100 nmol/L) decreased the amplitude of GABA mPSCs by 20.9 ± 6.1%. When applied during test ischemia, 100 nmol/L bicuculline abolished neuroprotection conferred by either IPC or εPKC-PC. We conclude that neuroprotection conferred by preconditioning depends on functional modifications of GABA synapses.
εPKC; inhibition; ischemia; ischemic tolerance; organotypic slice
We have previously shown that domains involved in binding of protein kinase C (PKC1) isozymes to their respective anchoring proteins (RACKs2) and short peptides derived from these domains are PKC isozyme-selective antagonists. We also identified PKC isozyme-selective agonists, named ψRACK3 peptides, derived from a sequence within each PKC with high homology to its respective RACK. We noted that all the ψRACK sequences within each PKC isozyme have at least one non-homologous amino acid difference from their corresponding RACK that constitutes a charge change. Based on this information, we have devised here a new approach to design an isozyme-selective PKC antagonist, derived from the ψRACK sequence. We focused on εPKC ψRACK peptide, where the pseudo-εRACK sequence (ψεRACK; HDAPIGYD; corresponding to εPKC85-92) is different in charge from the homologous RACK-derived sequence (NNVALGYD; corresponding to εRACK285-292) in the second amino acid. Here we show that changing the charge of the ψεRACK peptide through a substitution of only one amino acid (aspartate to asparagine) resulted in a peptide with an opposite activity on the same cell function and a substitution for aspartate with an alanine resulted in an inactive peptide. These data support our hypothesis regarding the mechanism by which pseudo-RACK peptide activates PKC in heart cells and suggest that this approach is applicable to other signaling proteins with inducible protein-protein interactions.
PKC (protein kinase C); RACK (receptor for activated C-kinase); ψRACK (pseudo RACK); intramolecular interaction; carrier peptide
In the brain, ischemic preconditioning (IPC) diminishes mitochondrial dysfunction after ischemia and confers neuroprotection. Activation of ε protein kinase C (εPKC) has been proposed to be a key neuroprotective pathway during IPC. We tested the hypothesis that IPC increases the levels of εPKC in synaptosomes from rat hippocampus, resulting in improved synaptic mitochondrial respiration. Preconditioning significantly increased the level of hippocampal synaptosomal εPKC to 152% of sham-operated animals at 2 d of reperfusion, the time of peak neuroprotection. We tested the effect of εPKC activation on hippocampal synaptic mitochondrial respiration 2 d after preconditioning. Treatment with the specific εPKC activating peptide, tat-ψεRACK (tat-ψε-receptor for activated C kinase), increased the rate of oxygen consumption in the presence of substrates for complexes I, II, and IV to 157, 153, and 131% of control (tat peptide alone). In parallel, we found that εPKC activation in synaptosomes from preconditioned animals resulted in altered levels of phosphorylated mitochondrial respiratory chain proteins: increased serine and tyrosine phosphorylation of 18 kDa subunit of complex I, decreased serine phosphorylation of FeS protein in complex III, increased threonine phosphorylation of COX IV (cytochrome oxidase IV), increased mitochondrial membrane potential, and decreased H2O2 production. In brief, ischemic preconditioning promoted significant increases in the level of synaptosomal εPKC. Activation of εPKC increased synaptosomal mitochondrial respiration and phosphorylation of mitochondrial respiratory chain proteins. We propose that, at 48 h of reperfusion after ischemic preconditioning, εPKC is poised at synaptic mitochondria to respond to ischemia either by direct phosphorylation or activation of the εPKC signaling pathway.
cerebral ischemia; phosphorylation; electron transport chain; neuroprotection; cell death; hippocampus
In response to mild ischemic stress, the brain elicits endogenous survival mechanisms to protect cells against a subsequent lethal ischemic stress, referred to as ischemic tolerance. The molecular signals that mediate this protection are thought to involve the expression and activation of multiple kinases, including protein kinase C (PKC). Here we demonstrate that εPKC mediates cerebral ischemic tolerance in vivo. Systemic delivery of ψεRACK, an εPKC-selective peptide activator, confers neuroprotection against a subsequent cerebral ischemic event when delivered immediately prior to stroke. In addition, activation of εPKC by ψεRACK treatment decreases vascular tone in vivo, as demonstrated by a reduction in microvascular cerebral blood flow. Here we demonstrate the role of acute and transient εPKC in early cerebral tolerance in vivo and suggest that extra-parenchymal mechanisms, such as vasoconstriction, may contribute to the conferred protection.
Ischemia; preconditioning; protein kinase C; cerebral blood flow
Activation of ε protein kinase C (εPKC) protects hearts from ischemic injury. However, some of the mechanism(s) of εPKC mediated cardioprotection are still unclear. Identification of εPKC targets may aid to elucidate εPKC–mediated cardioprotective mechanisms. Previous studies, using a combination of εPKC transgenic mice and difference in gel electrophoresis (DIGE), identified a number of proteins involved in glucose metabolism, whose expression was modified by εPKC. These studies, were accompanied by metabolomic analysis, and suggested that increased glucose oxidation may be responsible for the cardioprotective effect of εPKC. However, whether these εPKC-mediated alterations were due to differences in protein expression or phosphorylation was not determined.
Methods and Results
Here, we used an εPKC-specific activator peptide, ψεRACK, in combination with phosphoproteomics to identify εPKC targets, and identified proteins whose phosphorylation was altered by selective activation of εPKC most of the identified proteins were mitochondrial proteins and analysis of the mitochondrial phosphoproteome, led to the identification of 55 spots, corresponding to 37 individual proteins, which were exclusively phosphorylated, in the presence of ψεRACK. The majority of the proteins identified were proteins involved in glucose and lipid metabolism, components of the respiratory chain as well as mitochondrial heat shock proteins.
In summary the protective effect of εPKC during ischemia involves phosphorylation of several mitochondrial proteins involved in glucose, lipid metabolism and oxidative phosphorylation. Regulation of these metabolic pathways by εPKC phosphorylation may lead to εPKC-mediated cardioprotection induced by ψεRACK.
εPKC; ischemia; phosphorylation; mitochondria
Neuroprotection against cerebral ischemia conferred by ischemic preconditioning (IPC) requires translocation of epsilon protein kinase C (εPKC). A major goal in our laboratory is to define the cellular targets by which εPKC confers protection. We tested the hypothesis that εPKC targets the mitochondrial
KATP+ channel (
mtKATP+) after IPC. Our results demonstrated a rapid translocation of εPKC to rat hippocampal mitochondria after IPC. Because in other tissues εPKC targets
mtKATP+ channels, but its presence in brain mitochondria is controversial, we determined the presence of the
KATP+ channel-specific subunits (Kir6.1 and Kir6.2) in mitochondria isolated from rat hippocampus. Next, we determined whether
mtKATP+ channels play a role in the IPC induction. In hippocampal organotypic slice cultures, IPC and lethal ischemia were induced by oxygen-glucose deprivation. Subsequent cell death in the CA1 region was quantified using propidium iodide staining. Treatment with the
KATP+ channel openers diazoxide or pinacidil 48 h prior to lethal ischemia protected hippocampal CA1 neurons, mimicking the induction of neuroprotection conferred by either IPC or εPKC agonist-induced preconditioning. Blockade of
mtKATP+ channels using 5-hydroxydecanoic acid abolished the neuroprotection due to either IPC or εPKC preconditioning. Both ischemic andεPKC agonist-mediated preconditioning resulted in phosphorylation of the
mtKATP+ channel subunit Kir6.2. After IPC, selective inhibition of εPKC activation prevented Kir6.2 phosphorylation, consistent with Kir6.2 as a phosphorylation target of εPKC or its downstream effectors. Our results support the hypothesis that the brain
mtKATP+ channel is an important target of IPC and the signal transduction pathways initiated by εPKC.
ischemic tolerance; diazoxide; protein kinase C; organotypic slice culture; cell death; signal transduction
Protein kinase C-ε (εPKC) induces neurite outgrowth in neuroblastoma cells but molecular mechanism of the εPKC-induced neurite outgrowth is not fully understood. Therefore, we investigated the ability of phosphatidylinositol 4,5-bisphosphate (PIP2) binding of εPKC and its correlation with the neurite extension. We found that full length εPKC bound to PIP2 in a 12-ο-tetradecanoylphorbol-13-acetate dependent manner, while the regulatory domain of εPKC (εRD) bound to PIP2 without any stimulation. To identify the PIP2 binding region, we made mutants lacking several regions from εRD, and examined their PIP2 binding activity. The mutants lacking variable region 1 (V1) bound to PIP2 stronger than intact εRD, while the mutants lacking pseudo-substrate or common region 1 (C1) lost the binding. The PIP2 binding ability of the V3-deleted mutant was weakened. Those PIP2 bindings of εPKC, εRD and the mutants well correlated to their neurite induction ability. In addition, a chimera of pleckstrin homology domain of phospholipase Cδ and the V3 region of εPKC revealed that PIP2 binding domain and the V3 region are sufficient for the neurite induction, and a first 16 amino acids in the V3 region was important for neurite extension. In conclusion, εPKC directly binds to PIP2 mainly through pseudo-substrate and common region 1, contributing to the neurite induction activity.
actin; neurite outgrowth; neuroblastoma; phosphatidylinositol 4,5-bisphosphate; protein kinase C
The balance between endothelial nitric oxide synthase (eNOS)-derived nitric oxide (NO) and reactive oxygen species (ROS) production determines endothelial-mediated vascular homeostasis. Activation of protein kinase C (PKC) has been linked to imbalance of the eNOS/ROS system, which leads to endothelial dysfunction. We previously found that selective inhibition of delta PKC (δPKC) or selective activation of epsilon PKC (εPKC) reduces oxidative damage in the heart following myocardial infarction. In this study we determined the effect of these PKC isozymes in the survival of coronary endothelial cells (CVEC). We demonstrate here that serum deprivation of CVEC increased eNOS-mediated ROS levels, activated caspase-3, reduced Akt phosphorylation and cell number. Treatment with either the δPKC inhibitor, δV1-1, or the εPKC activator, ψεRACK, inhibited these effects, restoring cell survival through inhibition of eNOS activity. The decrease in eNOS activity coincided with specific de-phosphorylation of eNOS at Ser1179, and eNOS phosphorylation at Thr497 and Ser116. Furthermore, δV1-1 or ψεRACK induced physical association of eNOS with caveolin-1, an additional marker of eNOS inhibition, and restored Akt activation by inhibiting its nitration. Together our data demonstrate that 1) in endothelial dysfunction, ROS and reactive nitrogen species (RNS) formation result from uncontrolled eNOS activity mediated by activation of δPKC or inhibition of εPKC 2) inhibition of δPKC or activation of εePKC correct the perturbed phosphorylation state of eNOS, thus increasing cell survival. Since endothelial health ensures better tissue perfusion and oxygenation, treatment with a δPKC inhibitor and/or an εPKC activator in diseases of endothelial dysfunction should be considered.
The response of the myocardium to an ischaemic insult is regulated by two highly homologous protein kinase C (PKC) isozymes, δ and εPKC. Here, we determined the spatial and temporal relationships between these two isozymes in the context of ischaemia/reperfusion (I/R) and ischaemic preconditioning (IPC) to better understand their roles in cardioprotection.
Methods and results
Using an ex vivo rat model of myocardial infarction, we found that short bouts of ischaemia and reperfusion prior to the prolonged ischaemic event (IPC) diminished δPKC translocation by 3.8-fold and increased εPKC accumulation at mitochondria by 16-fold during reperfusion. In addition, total cellular levels of δPKC decreased by 60 ± 2.7% in response to IPC, whereas the levels of εPKC did not significantly change. Prolonged ischaemia induced a 48 ± 11% decline in the ATP-dependent proteasomal activity and increased the accumulation of misfolded proteins during reperfusion by 192 ± 32%; both of these events were completely prevented by IPC. Pharmacological inhibition of the proteasome or selective inhibition of εPKC during IPC restored δPKC levels at the mitochondria while decreasing εPKC levels, resulting in a loss of IPC-induced protection from I/R. Importantly, increased myocardial injury was the result, in part, of restoring a δPKC-mediated I/R pro-apoptotic phenotype by decreasing pro-survival signalling and increasing cytochrome c release into the cytosol.
Taken together, our findings indicate that IPC prevents I/R injury at reperfusion by protecting ATP-dependent 26S proteasomal function. This decreases the accumulation of the pro-apoptotic kinase, δPKC, at cardiac mitochondria, resulting in the accumulation of the pro-survival kinase, εPKC.
Cardioprotection; Ischaemia/reperfusion; Apoptosis; Proteasome; PKC; Ischaemic preconditioning
The cardioprotective effects of moderate alcohol consumption have been well documented in animal models and in humans. Protection afforded against ischemia and reperfusion injury (I/R) proceeds through an ischemic preconditioning-like mechanism involving the activation of epsilon protein kinase C (εPKC) and is dependent on the time and duration of ethanol treatment. However, the substrates of εPKC and the molecular mechanisms by which the enzyme protects the heart from oxidative damage induced by I/R are not fully described. Using an open-chest model of acute myocardial infarction in vivo, we find that intraperitoneal injection of ethanol (0.5 g/kg) 60 minutes prior to (but not 15 minutes prior to) a 30-minute transient ligation of the left anterior descending coronary artery reduced I/R-mediated injury by 57% (measured as a decrease of creatine phosphokinase release into the blood). Only under cardioprotective conditions, ethanol treatment resulted in the translocation of εPKC to cardiac mitochondria, where the enzyme bound aldehyde dehydrogenase-2 (ALDH2). ALDH2 is an intra-mitochondrial enzyme involved in the detoxification of toxic aldehydes such as 4-hydroxy-2-nonenal (4-HNE) and 4-HNE mediates oxidative damage, at least in part, by covalently modifying and inactivating proteins (by forming 4-HNE adducts). In hearts subjected to I/R after ethanol treatment, the levels of 4-HNE protein adducts were lower and JNK1/2 and ERK1/2 activities were diminished relative to the hearts from rats subjected to I/R in the absence of ethanol. Together, this work provides an insight into the mitochondrial-dependent basis of ethanol-induced and εPKC-mediated protection from cardiac ischemia, in vivo.
We have previously identified the PKC (protein kinase C)-anchoring protein RACK1 (receptor for activated C-kinase 1), as a specific binding partner for the cAMP-specific phosphodiesterase PDE4D5, suggesting a potential site for cross-talk between the PKC and cAMP signalling pathways. In the present study we found that elevation of intracellular cAMP, with the β2-adrenoceptor agonist isoproterenol (isoprenaline), led to activation of PDE4 enzymes in the particulate and soluble fractions of HEK (human embryonic kidney)-293 cells. In contrast activation of PDE4D5, with isoproterenol and the PKC activator PMA, was restricted to the particulate fraction, where it interacts with RACK1; however, RACK1 is dispensable for anchoring PDE4D5 to the particulate fraction. Kinetic studies demonstrated that RACK1 alters the conformation of particulate-associated PDE4D5 so that it more readily interacts with its substrate cAMP and with rolipram, a PDE4 inhibitor that specifically targets the active site of the enzyme. Interaction with RACK1 was also essential for PKC-dependent and ERK (extracellular-signal-regulated kinase)-independent phosphorylation (on Ser126), and activation of PDE4D5 in response to PMA and isoproterenol, both of which trigger the recruitment of PKCα to RACK1. Together these results reveal novel signalling cross-talk, whereby RACK1 mediates PKC-dependent activation of PDE4D5 in the particulate fraction of HEK-293 cells in response to elevations in intracellular cAMP.
cAMP; intracellular targeting; phosphodiesterase (PDE); protein kinase C (PKC); receptor for activated C-kinase 1 (RACK1); β2-AR, β2-adrenoreceptor; CREB, cAMP-response-element-binding protein; DMEM, Dulbecco's modified Eagle's medium; DOTAP, dioleoyltrimethylammonium propane; ECL, enhanced chemiluminescence; ERK, extracellular-signal-regulated kinase; GFX, GF109203X; GST, glutathione transferase; HEK, human embryonic kidney; HRP, horseradish peroxidase; IGF, insulin-like growth factor; MEK, MAPK (mitogen-activated protein kinase)/ERK kinase; PDE, phosphodiesterase; PKA, protein kinase A; PKC, protein kinase C; PP2A, protein phosphatase 2A; RACK1, receptor for activated C-kinase 1; TBST, Tris-buffered saline containing 0.1% Tween 20; VSV, vesicular stomatitis virus
Pervious biochemical and hemodymanic studies have highlighted the important role of εPKC in cardioprotection during ischemic preconditioning. However, little is known about the electrophysiological consequences of εPKC modulation in ischemic hearts. Membrane permeable peptide εPKC selective activator and inhibitor were used to investigate the role of εPKC modulation in reperfusion arrhythmias.
Protein transduction domain from HIV- TAT was used as a carrier for peptide delivery into intact Langendorff perfused guinea pig hearts. Action potentials were imaged and mapped (124 sites) using optical techniques and surface ECG was continuously recorded. Hearts were exposed to 30 min stabilization period, 15 min of no-flow ischemia, followed by 20 min reperfusion. Peptides (0.5 μM) were infused as follows: a) control (vehicle-TAT peptide; TAT-scrambled ψεRACK peptide); b) εPKC agonist (TAT-ψεRACK); c) εPKC antagonist (TAT-εV1).
Hearts treated with εPKC agonist ψεRACK had reduced incidence of ventricular tachycardia (VT, 64%) and fibrillation (VF, 50%) compared to control (VT, 80%, p<0.05) and (VF, 70%, P<0.05). However, the highest incidence of VT (100%, P<0.05) and VF (80%) occurred in hearts treated with εPKC antagonist peptide εV1 compared to control and to εPKC agonist ψεRACK. Interestingly, at 20 min reperfusion, 100% of hearts treated with εPKC agonist ψεRACK exhibited complete recovery of action potentials compared to 40% (p<0.05) of hearts treated with εPKC antagonist peptide, εV1 and 65% (P<0.5) of hearts in control. At 20 min reperfusion, maps of action potential duration from εPKC agonist ψεRACK showed minimal dispersion (48.2±9 ms) compared to exacerbated dispersion (115.4±42 ms, P<0.05) in εPKC antagonist and control (67±20 ms, P<0.05). VT/VF and dispersion from hearts treated with scrambled agonist or antagonist peptides were similar to control.
the results demonstrate that εPKC activation by ψεRACK peptide protects intact hearts from reperfusion arrhythmias and affords better recovery. On the other hand, inhibition of εPKC increased the incidence of arrhythmias and worsened recovery compared to controls. The results carry significant therapeutic implications for the treatment of acute ischemic heart disease by preconditioning-mimicking agents.
cardiac electrophysiology; Protein Kinase C; reperfusion arrhythmia; optical mapping
Nuclear factor-kappaB (NF-κB) activation occurs following ischemic preconditioning (IPC) in brain. However, the upstream signaling messengers and down-stream targets of NF-κB required for induction of IPC remain undefined. In a previous study, we demonstrated that epsilon protein kinase c (εPKC) was a key mediator of IPC in brain. Activation of εPKC induced cyclooygenase-2 (COX-2) expression and conferred ischemic tolerance in the neuronal and hippocampal slice models. Here, we hypothesized that IPC-mediated COX-2 expression was mediated by NF-κB. We tested this hypothesis in mixed cortical neuron/astrocyte cell cultures. To simulate IPC or ischemia, cell cultures were exposed to 1 or 4 h of oxygen–glucose deprivation, respectively. Our results demonstrated translocation of p65 and p50 subunits of NF-κB into nucleus following IPC or εPKC activation. NF-κB inhibition with pyrrolidine dithiocarbamate (10 μM) abolished IPC or εPKC activator-mediated neuroprotection indicating that NF-κB activation was involved in ischemic tolerance. In parallel studies, inhibition of either εPKC or the extracellular signal-regulated kinase (ERK 1/2) pathway reduced IPC-induced NF-κB activation. Finally, inhibition of NF-κB blocked IPC-induced COX-2 expression. In conclusion, we demonstrated that IPC-signaling cascade comprises εPKC activation→ERK1/2 activation→NF-κB translocation to nucleus→COX-2 expression resulting in neuroprotection in mixed neuronal culture.
Cerebral ischemia; Ischemic tolerance; Epsilon protein kinase C; Extracellular signal-regulated kinase (ERK1/2); Neuroprotection; Mixed cortical neuron/astrocyte cell cultures
Dopamine cellular signaling, via the D1 receptor (D1R), involves both protein kinase A (PKA) and protein kinase C (PKC), but the PKC isoform involved has not been determined. Therefore, we tested the hypothesis that the D1R-mediated inhibition of NADPH oxidase activity involves cross-talk between PKA and specific PKC isoform(s). In HEK-293 cells heterologously expressing human D1R (HEK-hD1), fenoldopam, a D1R agonist, and phorbol-12-myristate-13-acetate (PMA), a PKC activator, inhibited oxidase activity in a time- and concentration-dependent manner. The D1R-mediated inhibition of oxidase activity (68.1±3.6%) was attenuated by two different PKA inhibitors, H89 (10 µmol/L) (88±8.1%) and Rp-cAMP (10 µmol/L) (97.7±6.7%), and two different PKC inhibitors, bisindolylmaleimide I (1 µmol/L) (94±6%) and staurosporine (10 nmol/L) (93±8%), which by themselves, had no effect (n=4–8/group). The inhibitory effect of PMA (1 µmol/L) on oxidase activity (73±3.2%) was blocked by H89 (100±7.8%) (n=5–6/group). The PMA-mediated inhibition of NADPH oxidase activity was accompanied by an increase in PKCθS676, an effect that was also blocked by H89. Fenoldopam (1 µmol/L) also increased PKCθS676 in HEK-hD1 and human renal proximal tubule (RPT) cells. Knockdown of PKCθ with siRNA in RPT cells prevented the inhibitory effect of fenoldopam on NADPH oxidase activity. Our studies demonstrate for the first time that cross-talk between PKA and PKCθ plays an important role in the D1R-mediated negative regulation of NADPH oxidase activity in human kidney cells.
NADPH oxidase; protein kinase A; protein kinase C; D1 dopamin receptor
AC2 (adenylate cyclase 2) is stimulated by activation of Gq-coupled muscarinic receptors through PKC (protein kinase C) to generate localized cAMP in HEK (human embryonic kidney)-293 cells. In the present study, we utilized a sensitive live-cell imaging technique to unravel the proteins that play essential roles in a Gq-coupled muscarinic receptor-mediated cAMP signalling complex. We reveal that, upon agonist binding to the Gq-coupled muscarinic receptor, AKAP79 (A-kinase-anchoring protein 79) recruits PKC to activate AC2 to produce cAMP. The cAMP formed is degraded by PDE4 (phosphodiesterase 4) activated by an AKAP-anchored PKA (protein kinase A). Calcineurin, a phosphatase bound to AKAP79, is not involved in this regulation. Overall, a transient cAMP increase is generated from AC2 by Gq-coupled muscarinic receptor activation, subject to sophisticated regulation through AKAP79, PKC, PDE4 and PKA, which significantly enhances acetylcholine-mediated signalling.
adenylate cyclase 2 (AC2); A-kinase-anchoring protein 79 (AKAP79); muscarinic receptor; phosphodiesterase 4 (PDE4); protein kinase A (PKA); protein kinase C (PKC)
Protein kinase C (PKC) is implicated in the potentiation of Cav 2.3 currents by acetyl-β-methylcholine (MCh), a muscarinic M1 receptor agonist or phorbol-12-myristate, 13-acetate (PMA). The PKC isozymes responsible for the action of MCh and PMA were investigated using translocation as a measure of activation and with isozyme-selective antagonists and siRNA. Cav channels were expressed with α1 2.3, β1b and α2δ subunits and muscarinic M1 receptors in the Xenopus oocytes and the expressed currents (IBa) were studied using Ba2+ as the charge carrier. Translocation of PKC isozymes to the membrane studied by Western blot revealed that all eleven known PKC isozymes are present in the Xenopus oocytes. Exposure of the oocytes to MCh led to the translocation of PKC α whereas PMA activated PKC βII and ε isozymes. The action of MCh was inhibited by Go 6976, an inhibitor of cPKC isozymes or PKC α siRNA. PMA-induced potentiation of Cav 2.3 currents was inhibited by CG533 53, a PKC βII antagonist, βIIV5.3, a peptide translocation inhibitor of PKC βII or PKC βII siRNA. Similarly, εV1.2, a peptide translocation inhibitor of PKC ε or PKC ε siRNA inhibited PMA action. The inhibitors of PKC increased the basal IBa slightly. It is possible that some PKC isozymes have negative control over the IBa. Our results implicate PKC α in the potentiation of Cav 2.3 currents by MCh and PKC βII and ε in the potentiation of Cav 2.3 currents by PMA.
Section: 3. Neurophysiology, Neruopharmacology and other forms of Interceullular communication; PKC; Voltage-gated Calcium channels; Xenopus oocytes; Phosphorylation; Muscarinic receptors; Phrobol ester; MCh
The pineal hormone, melatonin (5-methoxy N-acetyltryptamine) induces a rapid aggregation of melanin-containing pigment granules in isolated melanophores of Xenopus laevis. Treatment of melanophores with activators of protein kinase C (PKC), including phorbol esters, mezerein and a synthetic diacylglycerol, did not affect pigment granule distribution but did prevent and reverse melatonin-induced pigment aggregation. This effect was blocked by an inhibitor of PKC, Ro 31- 8220. The inhibitory effect was not a direct effect on melatonin receptors, per se, as the slow aggregation induced by a high concentration of an inhibitor of cyclic AMP-dependent protein kinase (PKA), adenosine 3',5'-cyclic monophosphothioate, Rp-diastereomer (Rp- cAMPS), was also reversed by PKC activation. Presumably activation of PKC, like PKA activation, stimulates the intracellular machinery involved in the centrifugal translocation of pigment granules along microtubules. alpha-Melanocyte stimulating hormone (alpha-MSH), like PKC activators, overcame melatonin-induced aggregation but this response was not blocked by the PKC inhibitor, Ro 31-8220. This data indicates that centrifugal translocation (dispersion) of pigment granules in Xenopus melanophores can be triggered by activation of either PKA, as occurs after alpha-MSH treatment, or PKC. The very slow aggregation in response to inhibition of PKA with high concentrations of Rp-cAMPS, suggests that the rapid aggregation in response to melatonin may involve multiple intracellular signals in addition to the documented Gi-mediated inhibition of adenylate cyclase.
We constructed mutant protein kinase C (PKC) cDNAs which expressed PKC activity in vivo in the absence of phorbol ester activation. A hybrid PKC gene, PKAC, was constructed by substituting the coding region for the N-terminal 253 amino acids of PKC alpha with the N-terminal 17 amino acids of the cyclic AMP-dependent protein kinase catalytic subunit (PKA). A truncated PKC gene, delta PKC beta, lacking the coding region for amino acid positions 6 to 159 of PKC beta was also constructed. These mutant kinase genes expressed under the control of the SR alpha promoter activated the c-fos gene enhancer in Jurkat cells and initiated maturation of Xenopus laevis oocytes. Phorbol ester binding activity was absent in both constructs but was preserved in another hybrid gene, PKCA, which was composed of the coding region for 1 to 253 amino acids of PKC alpha at the N-terminal side and the coding region for 18 to 350 amino acids of PKA at the C-terminal side. These results indicate that elimination of the regulatory domain of PKC produces constitutively active PKC that can bypass activation by the phorbol ester. delta PKC beta, in synergy with a calcium ionophore, was capable of activating the interleukin 2 promoter, indicating that cooperation of PKC-dependent and calcium-dependent pathways is necessary for activation of the interleukin 2 gene.
We tested the hypothesis that protein kinase A (PKA) inhibits K2P currents activated by protein kinase C (PKC) in freshly isolated aortic myocytes. PDBu, the PKC agonist, applied extracellularly, increased the amplitude of the K2P currents in the presence of the “cocktail” of K+ channel blockers. Gö 6976 significantly reduced the increase of the K2P currents by PDBu suggesting the involvement of either α or β isoenzymes of PKC. We found that forskolin, or membrane permeable cAMP, did not inhibit K2P currents activated by the PKC. However, when PKA agonists were added prior to PDBu, they produced a strong decrease in the K2P current amplitudes activated by PKC. Inhibition of PDBu-elicited K2P currents by cAMP agonists was not prevented by the treatment of vascular smooth muscle cells with PKA antagonists (H-89 and Rp-cAMPs). Zn2+ and Hg2+ inhibited K2P currents in one population of cells, produced biphasic responses in another population, and increased the amplitude of the PDBu-elicited K+ currents in a third population of myocytes, suggesting expression of several K2P channel types. We found that cAMP agonists inhibited biphasic responses and increase of amplitude of the PDBu-elicited K2P currents produced by Zn2+ and Hg2. 6-Bnz-cAMp produced a significantly altered pH sensitivity of PDBu-elicited K2P-currents, suggesting the inhibition of alkaline-activated K2P-currents. These results indicate that 6-Bnz-cAMP and other cAMP analogs may inhibit K2P currents through a PKA-independent mechanism. cAMP analogs may interact with unidentified proteins involved in K2P channel regulation. This novel cellular mechanism could provide insights into the interplay between PKC and PKA pathways that regulate vascular tone.
The members of the atypical subfamily of protein kinase C (PKC) show dramatic structural and functional differences from other PKC isotypes. Thus, in contrast to the classical or novel PKCs, they are not activated by diacylglycerol or phorbol esters. However, the atypical PKCs are the target of important lipid second messengers such as ceramide, phosphatidic acid, and 3'-phosphoinositides. The catalytic and pseudosubstrate sequences in the two atypical PKCs (lambda/iota PKC and zeta PKC) are identical but are significantly different from those of conventional or novel PKCs. It has been shown that microinjection of a peptide with the sequence of the pseudosubstrate of the atypical PKC isotypes but not of alpha PKC or epsilon PKC dramatically inhibited maturation and NF-kappa B activation in Xenopus oocytes, as well as reinitiation of DNA synthesis in quiescent mouse fibroblasts. This indicates that either or both atypical isoforms are important in cell signalling. Besides the pseudosubstrate, the major differences in the sequence between lambda/iota PKC and zeta PKC are located in the regulatory domain. Therefore, any functional divergence between the two types of atypical PKCs will presumably reside in that region. We report here the molecular characterization of lambda-interacting protein (LIP), a novel protein that specifically interacts with the zinc finger of lambda/iota PKC but not zeta PKC. We show in this paper that this interaction is detected not only in vitro but also in vivo, that LIP activates lambda/iota PKC but not zeta PKC in vitro and in vivo, and that this interaction is functionally relevant. Thus, expression of LIP leads to the transactivation of a kappa B-dependent promoter in a manner that is dependent on lambda/iota PKC. To our knowledge, this is the first report on the cloning and characterization of a protein activator of a PKC that binds to the zinc finger domain, which has so far been considered a site for binding of lipid modulators. The fact that LIP binds to lambda/iota PKC but not to the highly related zeta PKC isoform suggests that the specificity of the activation of the members of the different PKC subfamilies will most probably be accounted for by proteins like LIP rather than by lipid activators.
The signal transducers and activators of transcription (STATs) were found to be essential for cardioprotection. However, their role in preconditioning (PC) neuroprotection remains undefined. Previously, our studies showed that PC mediated a signaling cascade that involves activation of epsilon protein kinase C (εPKC), extracellular signal-regulated kinase (ERK1/2), and cyclooxygenase-2 (COX-2) pathways. However, the intermediate pathway by which ERK1/2 activates COX-2 was not defined. In this study, we investigated whether the PC-induced signaling pathway requires phosphorylation of STAT isoforms for COX-2 expression. To mimic PC or lethal ischemia, mixed cortical neuron/astrocyte cell cultures were subjected to 1 and/or 4 h of oxygen–glucose deprivation (OGD), respectively. The results indicated serine phosphorylation of STAT3 after PC or εPKC activation. Inhibition of either εPKC or ERK1/2 activation abolished PC-induced serine phosphorylation of STAT3. Additionally, inhibition of STAT3 prevented PC-induced COX-2 expression and neuroprotection against OGD. Therefore, our findings suggest that PC signaling cascade involves STAT3 activation after εPKC and ERK1/2 activation. Finally, we show that STAT3 activation mediates COX-2 expression and ischemic tolerance.
cerebral ischemia; extracellular signal-regulated kinase (ERK1/2); ischemic tolerance; neuroprotection; phosphorylation; protein kinase C
RACK1 (Receptor for Activated C-Kinase 1), an anchoring protein that shuttles activated PKC to cellular membranes, plays an important role in PKC-mediated signal transduction pathways. A significant loss of RACK1 has been found in the brain of aging animals and Alzheimer’s disease (AD) patients, which implicates the potential involvement of RACK1 in altered PKC activation associated with dementia. Our previous studies have demonstrated that GABAergic inhibition in prefrontal cortex, which is important for cognitive processes like “working memory”, is regulated by muscarinic receptors via a PKC-dependent mechanism, and this effect is impaired by β-amyloid peptide (Aβ). In this study, we found that Aβ oligomers decreased RACK1 distribution in the membrane fraction of cortical neurons. Moreover, overexpression of RACK1 rescued the effect of muscarinic receptors on GABAergic transmission in Aβ-treated cortical cultures in vitro and Aβ-injected cortical neurons in vivo. These results suggest that the Aβ-induced loss of RACK1 distribution in the cell membrane may underlie the Aβ impairment of muscarinic regulation of PKC and GABAergic transmission. Thus, RACK1 provides a potential therapeutic target that can restore some of the impaired cellular processes by Aβ.
Aβ; RACK1; PKC; muscarinic acetylcholine receptors; IPSC; Sindbis virus; stereotaxic injection
Pediatric bipolar disorder (PBD) is a major public health concern, however, its neurobiology is poorly understood. We therefore studied the role of protein kinase C (PKC) in the pathophysiology of bipolar illness.
We determined PKC activity and immunolabeling of various PKC isozymes (i.e., PKC α, PKC βI, PKC βII, and PKC δ) in the cytosol and membrane fractions of platelets obtained from PBD patients and normal control subjects. PKC activity and PKC isozymes were also determined after 8 weeks of pharmacotherapy of PBD patients (n = 16) with mood stabilizers.
PKC activity and the protein expression of PKC βI and βII, but not PKC α or PKC δ, were significantly decreased in both membrane as well as cytosol fractions of platelets obtained from medication-free PBD patients compared with normal control subjects. Eight weeks of pharmacotherapy resulted in significantly increased PKC activity but no significant changes in any of the PKC isozymes in PBD patients.
These results indicate that decreases of specific PKC isozymes and decreased PKC activity may be associated with the pathophysiology of PBD and that pharmacotherapy with mood stabilizing drugs results in an increase and normalization of PKC activity along with improvement in clinical symptoms.
Pediatric bipolar disorder; platelets; PKC isozymes; PKC activity; lithium; mood stabilizing drugs
During the pre-hibernation season, arctic ground squirrels (AGS) can tolerate 8 minutes of asphyxial cardiac arrest (CA) without detectable brain pathology. Better understanding of the mechanisms regulating innate ischemia tolerance in AGS has the potential to facilitate the development of novel, prophylactic agents to induce ischemic tolerance in patients at risk of stroke or cardiac arrest. We hypothesized that neuroprotection in AGS involves robust maintenance of ion homeostasis similar to anoxia-tolerant turtles. Ion homeostasis was assessed by monitoring ischemic depolarization (ID) in cerebral cortex during CA in vivo and during oxygen glucose deprivation in vitro in acutely prepared hippocampal slices. In both models, the onset of ID was significantly delayed in AGS compared to rats. The epsilon protein kinase C (εPKC) is a key mediator of neuroprotection and inhibits both Na+/K+-ATPase and voltage-gated sodium channels, primary mediators of the collapse of ion homeostasis during ischemia. The selective peptide inhibitor of εPKC (εV1–2) shortened the time to ID in brain slices from AGS but not in rats despite evidence that εV1–2 decreased activation of εPKC in brain slices from both rats and AGS. These results support the hypothesis that εPKC activation delays the collapse of ion homeostasis during ischemia in AGS.
brain ischemia; heart arrest; tolerance; neuroprotection