Protein kinase C translocation and PKC-dependent protein phosphorylation during myocardial ischemia

Role of Connexin 43 phosphorylation on Serine-368 by PKC in cardiac function and disease

Intercellular communication mediated by gap junction channels and hemichannels composed of Connexin 43 (Cx43) is vital for the propagation of electrical impulses through cardiomyocytes. The carboxyl terminal tail of Cx43 undergoes various post-translational modifications including phosphorylation of its Serine-368 (S368) residue. Protein Kinase C isozymes directly phosphorylate S368 to alter Cx43 function and stability through inducing conformational changes affecting channel permeability or promoting internalization and degradation to reduce intercellular communication between cardiomyocytes. Recent studies have implicated this PKC/Cx43-pS368 circuit in several cardiac-associated diseases. In this review, we describe the molecular and cellular basis of PKC-mediated Cx43 phosphorylation and discuss the implications of Cx43 S368 phosphorylation in the context of various cardiac diseases, such as cardiomyopathy, as well as the therapeutic potential of targeting this pathway.

Alcohol exposure alters mouse lung inflammation in response to inhaled dust

Alcohol exposure is associated with increased lung infections and decreased mucociliary clearance. Occupational workers exposed to dusts from concentrated animal feeding operations (CAFOs) are at risk for developing chronic inflammatory lung diseases. Agricultural worker co-exposure to alcohol and organic dust has been established, although little research has been conducted on the combination effects of alcohol and organic dusts on the lung. Previously, we have shown in a mouse model that exposure to hog dust extract (HDE) collected from a CAFO results in the activation of protein kinase C (PKC), elevated lavage fluid cytokines/chemokines including interleukin-6 (IL-6), and the development of significant lung pathology. Because alcohol blocks airway epithelial cell release of IL-6 in vitro, we hypothesized that alcohol exposure would alter mouse lung inflammatory responses to HDE. To test this hypothesis, C57BL/6 mice were fed 20% alcohol or water ad libitum for 6 weeks and treated with 12.5% HDE by intranasal inhalation method daily during the final three weeks. Bronchoalveolar lavage fluid (BALF), tracheas and lungs were collected. HDE stimulated a 2–4 fold increase in lung and tracheal PKCε (epsilon) activity in mice, but no such increase in PKCε activity was observed in dust-exposed mice fed alcohol. Similarly, alcohol-fed mice demonstrated significantly less IL-6 in lung lavage in response to dust than that observed in control mice instilled with HDE. TNFα levels were also inhibited in the alcohol and HDE-exposed mouse lung tissue as compared to the HDE only exposed group. HDE-induced lung inflammatory aggregates clearly present in the tissue from HDE only exposed animals were not visually detectable in the HDE/alcohol co-exposure group. Statistically significant weight reductions and 20% mortality were also observed in the mice co-exposed to HDE and alcohol. These data suggest that alcohol exposure depresses the ability of the lung to activate PKCε-dependent inflammatory pathways to environmental dust exposure. These data also define alcohol as an important co-exposure agent to consider in the study of inhalation injury responses.

Role of calcium and PKC in salivary mucous cell exocrine secretion

Fluid and exocrine secretion of mucins by salivary mucous glands is regulated predominantly by parasympathetic activation of muscarinic receptors. A direct role for subsequent putative signaling steps, phospholipase C (PLC), increased intracellular calcium ([Ca(2+)](i)), and isoforms of protein kinase C (PKC) in mediating muscarinic exocrine secretion has not been elucidated, and these are potential therapeutic targets to enhance mucin secretion in hyposalivary patients. We found that muscarinic-induced mucin secretion by rat sublingual tubulo-acini was dependent upon PLC activation and the subsequent increase in [Ca(2+)](i), and further identified a transient PKC-independent component of secretion dependent upon Ca(2+) release from intracellular stores, whereas sustained secretion required entry of extracellular Ca(2+). Interactions among carbachol, PKC inhibitors, phorbol 12-myristate 13-acetate, and thapsigargin to modulate [Ca(2+)](i) implicated conventional PKC isoforms in mediating sustained secretion. With increasing times during carbachol perfusion of glands, in situ, PKC-α redistributed across glandular membrane compartments and underwent a rapid and persistent accumulation near the luminal borders of mucous cells. PKC-β1 displayed transient localization near luminal borders, whereas the novel PKCs, PKC-δ or PKC-ε, displayed little or no redistribution in mucous cells. Collective results implicate synergistic interactions between diacylglycerol (DAG) and increasing [Ca(2+)](i) levels to activate cPKCs in mediating sustained muscarinic-induced secretion.

Propylthiouracil attenuates monocrotaline-induced pulmonary arterial hypertension in rats

BACKGROUND

Propylthiouracil (PTU) enhances nitric oxide production and inhibits smooth muscle cell proliferation, suggesting a possible role in the prevention of pulmonary arterial hypertension (PAH).

METHODS AND RESULTS

The 30 male Sprague-Dawley rats were randomized to receive saline injection only (group 1), monocrotaline (MCT) (70 mg/kg) only (group 2) or MCT + 0.1% PTU in drinking water (group 3) given on day 5 after MCT administration. By day 35, western blot showed lower connexin43 (Cx43) and membranous protein kinase C-epsilon expressions in the right ventricle (RV) of group 2 animals than in the other groups (all P<0.05). Conversely, Cx43 expression in the lung was higher in group 2 than in other groups (all P<0.02). Additionally, mRNA expressions of matrix metalloproteinase-9, tissue necrotic factor-alpha, and caspase-3 were higher, whereas Bcl-2 and endothelial nitric oxide synthase were lower, in the lungs and RV of group 2 rats than in the other groups (all P<0.05). Moreover, the numbers of alveolar sacs and lung arterioles were also reduced in group 2 than in other groups (all P<0.05), and RV systolic pressure and RV weight were increased in group 2 compared with other groups (all P<0.001).

CONCLUSIONS

PTU effectively attenuates complications associated with MCT-induced PAH.

Impact of hyperglycemic control on left ventricular myocardium. A molecular and cellular basic study in a diabetic rat model

This experimental study investigated the impact of hyperglycemic control on left ventricular (LV) function using a model of diabetes mellitus (DM) (induced by streptozocin 60 mg/kg). Sixteen adult-Sprague Dawley rats were divided into group 1 (poor hyperglycemic control, n = 8) and group 2 (good hyperglycemic control, n = 8). Diabetic rats and 8 healthy rats serving as controls (group 3) were sacrificed on day 28 after DM induction. The results demonstrated that HbA(1C) on day 28 was higher in group 1 than in groups 2 and 3 (P < 0.0001). The mRNA expressions of MMP-9 and endothelin-1 were elevated in group 1 compared with that in groups 2 and 3 (P < 0.05), whereas PGC-1alpha and eNOS were lower in group 1 than in groups 2 and 3 (P < 0.05). The number of apoptotic nuclei was higher in group 1 than in groups 2 and 3 (P < 0.01). The integrated area (microm(2)) of connexin43 (Cx43), Cx43 protein expression, and LV function were lower in group 1 than in groups 2 and 3 (P < 0.05). Moreover, PKC-epsilon expression in the mitochondrial compartment was decreased in group 1 compared to that in groups 2 and 3 (P < 0.005).

Adenoprotection of the heart involves phospholipase C-induced activation and translocation of PKC-epsilon to RACK2 in adult rat and mouse

Adenosine protects the heart from adrenergic overstimulation. This adenoprotection includes the direct anti-adrenergic action via adenosine A(1) receptors (A(1)R) on the adrenergic signaling pathway. An indirect A(1)R-induced attenuation of adrenergic responsiveness involves the translocation of PKC-epsilon to t-tubules and Z-line of cardiomyocytes. We investigated with sarcomere imaging, immunocytochemistry imaging, and coimmunoprecipitation (co-IP) whether A(1)R activation of PKC-epsilon induces the kinase translocation to receptor for activated C kinase 2 (RACK2) in isolated rat and mouse hearts and whether phospholipase C (PLC) is involved. Rat cardiomyocytes were treated with the A(1)R agonist chlorocyclopentyladenosine (CCPA) and exposed to primary PKC-epsilon and RACK2 antibodies with secondaries conjugated to Cy3 and Cy5 (indodicarbocyanine), respectively. Scanning confocal microscopy showed that CCPA caused PKC-epsilon to reversibly colocalize with RACK2 within 3 min. Additionally, rat and mouse hearts were perfused and stimulated with CCPA or phenylisopropyladenosine to activate A(1)R, or with phorbol 12-myristate 13-acetate to activate PKC. RACK2 was immunoprecipitated from heart extracts and resolved with SDS-PAGE. Western blotting showed that CCPA, phenylisopropyladenosine, and phorbol 12-myristate 13-acetate in the rat heart increased the PKC-epsilon co-IP with RACK2 by 186, 49, and >1,000%, respectively. The A(1)R antagonist 8-cyclopentyl-1,3-dipropylxanthine prevented the CCPA-induced co-IP with RACK2. In mouse hearts, CCPA increased the co-IP of PKC-epsilon with RACK2 by 61%. With rat cardiomyocytes, the beta-adrenergic agonist isoproterenol increased sarcomere shortening by 177%. CCPA reduced this response by 47%, an action inhibited by the PLC inhibitor U-73122 and 8-cyclopentyl-1,3-dipropylxanthine. In conclusion, A(1)R stimulation of the heart is associated with PLC-initiated PKC-epsilon translocation and association with RACK2.

Differential effects of phosphorylation of regions of troponin I in modifying cooperative activation of cardiac thin filaments

Ischemia and heart failure are associated with protein kinase C (PKC) dependent phosphorylation of cardiac troponin I (cTnI). We investigated the effect of phosphorylation of cTnI PKC sites S43, S45 and T144 under normal (pH 7.0) and acidic (pH 6.5) conditions on tension in skinned fiber bundles from a mouse heart. To mimic the PKC phosphorylation, we exchanged troponin (cTn) in these fiber bundles with cTn complex containing either cTnI-(S43E/S45E) or cTnI-(T144E). We determined how pseudo-phosphorylation and acidic pH affect activation of thin filaments by strongly bound crossbridges by use of n-ethyl maleimide (NEM-S1) to mimic rigor. We hypothesized that PKC phosphorylation of cTnI amplifies the effect of ischemic/hypoxic conditions to depress myofilament force and Ca(2+)-responsiveness by reducing the ability of rigor crossbridge to activate force. Pseudo-phosphorylation of cTnI at S43/S45 exacerbated the effect of acidic pH to induce a rightward shift in the Ca(2+)-tension relation. Under acidic conditions, fibers regulated by cTnI-(S43E/S45E) demonstrated a significant reduction in the ability of NEM-S1 to recruit cycling crossbridges, when compared to controls regulated by cTnI. Similar effects of pseudo-phosphorylation of cTnI-(T144) occurred, but to a lesser extent that those of pseudo-phosphorylation of S43/S45. We conclude that under acidic conditions PKC phosphorylation of cTnI residues at S43/S45 and at T144 is likely to have differential, but significant effects in depressing the ability of both Ca(2+) and rigor crossbridges to activate force generation. Although these effects of PKC dependent phosphorylation may be maladaptive in heart failure, they may also spare ATP consumption and be cardio-protective in ischemia.

Overexpression of TNNI3K, a cardiac-specific MAP kinase, promotes P19CL6-derived cardiac myogenesis and prevents myocardial infarction-induced injury

TNNI3K is a new cardiac-specific MAP kinase whose gene is localized to 1p31.1 and that belongs to a tyrosine kinase-like branch in the kinase tree of the human genome. In the present study we investigated the role of TNNI3K in the cardiac myogenesis process and in the repair of ischemic injury. Pluripotent P19CL6 cells with or without transfection by pcDNA6-TNNI3K plasmid were used to induce differentiation into beating cardiomyocytes. TNNI3K promoted the differentiation process, judging from the increasing beating mass and increased number of alpha-actinin-positive cells. TNNI3K improved cardiac function by enhancing beating frequency and increasing the contractile force and epinephrine response of spontaneous action potentials without an increase of the single-cell size. TNNI3K suppressed phosphorylation of cardiac troponin I, annexin-V(+) cells, Bax protein, and p38/JNK-mediated apoptosis. Intramyocardial administration of TNNI3K-overexpressing P19CL6 cells in mice with myocardial infarction improved cardiac performance and attenuated ventricular remodeling compared with injection of wild-type P19CL6 cells. In conclusion, our study clearly indicates that TNNI3K promotes cardiomyogenesis, enhances cardiac performance, and protects the myocardium from ischemic injury by suppressing p38/JNK-mediated apoptosis. Therefore, modulation of TNNI3K activity would be a useful therapeutic approach for ischemic cardiac disease.

Inhibition of protein kinase C alpha improves myocardial beta-adrenergic receptor signaling and ventricular function in a model of myocardial preservation

OBJECTIVE

The specific effect of protein kinase C alpha, the primary ventricular calcium-dependent protein kinase C isoform, on myocardial protection is unclear. The objective of this study was to determine the role of protein kinase C alpha in myocardial protection and recovery of function after cardioplegic arrest, cold preservation, and normothermic reperfusion, as relevant to cardiac transplantation.

METHODS

We used an ex vivo murine model, and hearts were arrested with cold crystalloid cardioplegia or saline as a control and maintained at 4 degrees C for 4 hours. This was followed by normothermic reperfusion for 90 minutes. Transgenic hearts with cardiac-specific activation or inhibition of protein kinase C alpha were then studied to specifically examine the effects of protein kinase C alpha on myocardial preservation in this model.

RESULTS

Cardioplegic arrest with University of Wisconsin solution led to significantly improved postreperfusion hemodynamics and inhibition of myocardial protein kinase C alpha activity relative to that seen in saline-treated control hearts. Beta-adrenergic receptor signaling was also preserved with University of Wisconsin solution. Transgenic hearts with enhanced protein kinase C alpha activity had poor postreperfusion hemodynamics, impaired beta-adrenergic receptor signaling, and increased G protein-coupled receptor kinase 2 activity compared with those seen in nontransgenic control hearts. In contrast, transgenic hearts with inhibited protein kinase C alpha activity had even better myocardial protection relative to control hearts and preserved beta-adrenergic receptor signaling.

CONCLUSIONS

Current techniques of myocardial preservation are associated with inhibition of protein kinase C alpha activity and maintenance of intact beta-adrenergic receptor signaling. Activation of protein kinase C alpha leads to enhanced beta-adrenergic receptor desensitization and impaired signaling and ventricular function as a result of increased G protein-coupled receptor kinase 2 activity. This is a novel in vivo mechanism of G protein-coupled receptor kinase 2 activation. Strategies to specifically inhibit these kinases might improve long-term myocardial protection.

Interruption of endothelin signaling modifies membrane type 1 matrix metalloproteinase activity during ischemia and reperfusion

The matrix metalloproteinases (MMPs), in particular, membrane type 1 MMP (MT1-MMP), are increased in the context of myocardial ischemia and reperfusion (I/R) and likely contribute to myocardial dysfunction. One potential upstream induction mechanism for MT1-MMP is endothelin (ET) release and subsequent protein kinase C (PKC) activation. Modulation of ET and PKC signaling with respect to MT1-MMP activity with I/R has yet to be explored. Accordingly, this study examined in vivo MT1-MMP activation during I/R following modification of ET signaling and PKC activation. With the use of a novel fluorogenic microdialysis system, myocardial interstitial MT1-MMP activity was measured in pigs (30 kg; n = 9) during I/R (90 min I/120 min R). Local ET(A) receptor antagonism (BQ-123, 1 microM) and PKC inhibition (chelerythrine, 1 microM) were performed in parallel microdialysis probes. MT1-MMP activity was increased during I/R by 122 +/- 10% (P < 0.05) and was unchanged from baseline with ET antagonism and/or PKC inhibition. Selective PKC isoform induction occurred such that PKC-betaII increased by 198 +/- 31% (P < 0.05). MT1-MMP phosphothreonine, a putative PKC phosphorylation site, was increased by 121 +/- 8% (P < 0.05) in the I/R region. These studies demonstrate for the first time that increased interstitial MT1-MMP activity during I/R is a result of the ET/PKC pathway and may be due to enhanced phosphorylation of MT1-MMP. These findings identify multiple potential targets for modulating a local proteolytic pathway operative during I/R.

Signaling Pathways Involved in the Cardioprotective Effects of Cannabinoids

The aim of the present article is to review the cardioprotective properties of cannabinoids, with an emphasis on the signaling pathways involved. Cannabinoids have been reported to protect against ischemia in rat isolated hearts, as well as in rats and mice in vivo. Although these effects have been observed mostly with a pre-treatment of a cannabinoid, we report that the selective CB(2)-receptor agonist JWH133 is able to reduce infarct size when administered either before ischemia, during the entire ischemic period, or just upon reperfusion. Little is known about the signaling pathways involved in these cardioprotective effects. Likely candidates include protein kinase C (PKC) and mitogen-activated protein kinases (MAPK) since they are activated during ischemia-reperfusion and contribute to the protective effect ischemic preconditioning. The use of pharmacological inhibitors suggests that PKC, p38 MAPK, and p42/p44 MAPK (ERK1/2) contribute to the protective effect of cannabinoids. In addition, perfusion with JWH133 in healthy hearts caused an increase in both p38 MAPK phosphorylation level and activity, whereas the CB(1)-receptor agonist ACEA was associated with an increase in the phosphorylation status of both ERK1 and ERK2 without any change in activity. During ischemia, both agonists doubled p38 MAPK activity, whereas ERK1/2 phosphorylation level and activity during reperfusion were enhanced only by the CB(1)-receptor agonist. Finally, although nitric oxide (NO) was shown to exert both pro and anti-apoptotic effects on cardiomyocytes, with an apparently controversial effect on myocardial survival, our data suggest that NO may contribute to the cardioprotective effect of some cannabinoids.

Nuclear compartmentalization of FAK and FRNK in cardiac myocytes

Focal adhesion kinase (FAK) and FAK-related non-kinase (FRNK) accumulate in the nucleus of cardiac myocytes during hypertensive hypertrophy. Nuclear FAK and FRNK are phosphorylated on different serines and form distinct bright spots. The subnuclear distribution of serine-phosphorylated FAK and FRNK was examined in this study by double labeling with fibrillarin, a component of nucleoli, and Sam68, a constituent of Sam68 nuclear bodies. We also investigated the role of protein kinase C (PKC)-mediated phosphorylation of FAK and FRNK on nuclear translocation. PKC activation by 12-O-tetradecanoylphorbol 13-acetate treatment increased serine phosphorylation of FAK and FRNK. Specifically, FAK was phosphorylated on serine 722 but not serine 910. On the other hand, FRNK was phosphorylated on serine 217, the equivalent site of FAK serine 910, but not serine 30, the homologous site of FAK serine 722. Serine-phosphorylated FAK and FRNK redistributed into the nucleus and formed distinct patterns. FAK with phosphorylation on serine 722 colocalized with Sam68 but not fibrillarin. On the contrary, FRNK phosphorylated on 217 coexisted with fibrillarin but not Sam68. Immunoprecipitation also confirmed that FAK associated with Sam68 and FRNK interacted with fibrillarin, respectively. These results suggest that FAK and FRNK target different nuclear subdomains by their association with distinct nuclear proteins.

Involvement of FrzA/sFRP-1 and the Wnt/frizzled pathway in ischemic preconditioning

Phosphorylation and subsequent inactivation of glycogen synthase kinase (GSK)-3beta via the Akt/PI3-Kinase pathway during ischemic preconditioning (PC) has been shown to be cardioprotective. As FrzA/sFRP-1, a secreted antagonist of the Wnt/Frizzled pathway, is expressed in the heart and is able to decrease the phosphorylation of GSK-3beta in vitro on vascular cells, we examined its effect during PC using transgenic mouse overexpressing FrzA in cardiomyocytes (alpha-MHC promoter) under a conditional transgene expression approach (tet-off system). Overexpression of FrzA inhibited the increase in GSK-3beta phosphorylation as well as protein kinase C (PKC) epsilon activation in transgenic mice after PC as compared with littermates. Phospho-Akt (P-Akt), phospho-JNK, or the cytoplasmic beta-catenin levels were not modified, phospho-p38 (P-p38) was slightly increased in transgenic mice after PC as compared with littermates. FrzA transgenic mice displayed a larger infarct size and a greater worsening of cardiac function compared with littermates. All these differences were reversed by the addition of doxycycline. This study demonstrates for the first time that disruption of a beta-catenin independent Wnt/Frizzled pathway induces the activation of GSK-3beta and reverses the benefit of preconditioning.

Arginine-glycine-aspartic acid (RGD)-containing peptides inhibit the force production of mouse papillary muscle bundles via alpha 5 beta 1 integrin

Integrins are considered to be an important mechanosensor in cardiac myocytes. To test whether integrins can influence cardiac contractile function, the force-frequency relationships of mouse papillary muscle bundles were measured in the presence or absence of a synthetic integrin-binding peptide, GRGDNP (gly-arg-gly-asp-asn-pro). Results demonstrate that in the presence of an arginine-glycine-aspartic acid (RGD)-containing synthetic peptide, contractile force was depressed significantly by, 28% at 4 Hz, 37.7% at 5 Hz and 20% at 10 Hz (n = 6, P < 0.01). Treatment of myofibres with either protease-generated fragments of denatured collagen (Type I) or denatured collagen that contain the RGD motif, also reduced force production significantly. An integrin-activating antibody for beta(1) integrin inhibited the force similar to synthetic RGD peptide. Function-blocking integrin antibodies for alpha(5) and beta(1) integrins reversed the effect of the RGD-containing peptide, and alpha(5) integrin also reversed the effect of proteolytic fragments of denatured collagen on contractile force, whereas experiments with function-blocking antibody for beta(3) integrin did not reverse the effect of RGD peptide. Force-[Ca(2)(+)](i) measurements showed that the depressed rate of force generation observed in the presence of the RGD-containing peptide was associated with reduced [Ca(2)(+)](i). Data analyses further demonstrated that force per unit of Ca(2)(+) was reduced, suggesting that the myofilament activation process was altered. In addition, inhibition of PKC enzyme using the selective, cell-permeable inhibitor Ro-32-0432, reversed the activity of RGD peptide on papillary muscle bundles. In conclusion, these data indicate that RGD peptide, acting via alpha(5)beta(1) integrin, depresses the force production from papillary muscle bundles, partly associated with changes in [Ca(2)(+)](i) and the myofilament activation processes, that is modulated by PKCepsilon.

Differential subcellular targeting of PKC-epsilon in response to pharmacological or ischemic stimuli in intestinal epithelia

Ischemia is the central pathogenic factor underlying a spectrum of intestinal disorders. The study of the cellular signaling responses to ischemic stress in nonepithelial cells has progressed substantially in the previous several years, but little is known about the response in epithelial cells. Unique features of the epithelial response to ischemic stress suggest differential regulation with regards to signaling. The PKC family of proteins has been implicated in ischemic stress in nonepithelial systems. The role of PKC isoforms in chemical ischemia in intestinal epithelial cells is evaluated in this study. Additionally, the phosphorylation of the F-actin cross-linking protein myristoylated alanine-rich C kinase substrate (MARCKS) is also studied. Chemical ischemia resulted in the transient activation of only the isoform PKC-epsilon as detected by translocation employing the subcellular fractionation technique. The pharmacological agonists phorbol 12-myristate 13-acetate and carbachol also led to the translocation of PKC-epsilon. By immunofluoresence, MARCKS is noted to be located at the lateral membrane under control conditions. In response to carbachol, MARCKS translocates to the cytosol, indicating its phosphorylation, which is additionally confirmed biochemically. Consistent with this observation, carbachol induces the translocation of PKC-epsilon to proximity with MARCKS at the lateral membrane. In response to chemical ischemia, MARCKS fails to translocate and phosphorylation does not increase. Additionally, the translocation of PKC-epsilon is not to the lateral membrane but rather basally. The data suggest that the differential translocation of PKC-epsilon in response to pharmacological agonists versus ischemic stress may lead to different effects on downstream targets.

Increased expression and altered subcellular distribution of PKC-delta in chronically hypoxic rat myocardium: involvement in cardioprotection

We examined the role of protein kinase C (PKC) in the cardioprotective mechanism induced by long-term adaptation to chronic intermittent hypoxia. Adult male Wistar rats were exposed to hypobaric hypoxia of 7,000 m for 8 h/day, 5 days/wk; the total number of exposures was 24-32. A control group was kept under normoxic conditions. Western blot analysis of PKC isoforms-delta and -epsilon was performed in the cytosol and three particulate fractions of left ventricular myocardium. Infarct size was determined in open-chest animals subjected to 20-min coronary artery occlusion and 3-h reperfusion. The PKC inhibitors chelerythrine (1 or 5 mg/kg) or rottlerin (selective for PKC-delta isoform; 0.3 mg/kg) were administered intravenously as a single bolus 15 min before ischemia. Chronic hypoxia had no effect on the expression and distribution of PKC-epsilon. The relative amount of PKC-delta increased in the cytosol and nuclear-cytoskeletal, mitochondrial, and microsomal fractions of chronically hypoxic myocardium by 100%, 212%, 237%, and 146%, respectively, compared with corresponding normoxic values. Chronic hypoxia decreased the size of myocardial infarction (normalized to the area at risk) by about one-third on the average (P < 0.05). Both doses of chelerythrine tended to reduce infarction in controls, and only the high dose completely abolished the improvement of ischemic tolerance in hypoxic hearts (P < 0.05). Rottlerin attenuated the infarct size-limiting effect of chronic hypoxia (P < 0.05), and it had no effect in controls. These results suggest that chronic intermittent hypoxia-induced cardioprotection in rats is partially mediated by PKC-delta; the contribution of other isoforms remains to be determined.

Sphingosine kinase activation mediates ischemic preconditioning in murine heart

BACKGROUND

Phosphorylation of sphingosine by sphingosine kinase (SK) is the rate-limiting step in the cellular synthesis of sphingosine 1-phosphate (S1P). The monoganglioside GM1, which stimulates SK, is cardioprotective in part through increased generation of S1P that protects myocytes by diverse mechanisms. Because protein kinase C (PKC)epsilon activation is necessary for myocardial ischemic preconditioning (IPC) and PKC activators increase SK activity, we tested the hypothesis that SK may be a central mediator of IPC.

METHODS AND RESULTS

In adult murine hearts, IPC sufficient to reduce infarct size significantly increased cardiac SK activity, induced translocation of SK protein from the cytosol to membranes, and enhanced cardiac myocyte survival. IPC did not increase SK activity in PKCepsilon-null mice. The SK antagonist N,N-dimethylsphingosine inhibited PKCepsilon activation and directly abolished the protective effects of IPC and the enhanced SK activity induced by IPC.

CONCLUSIONS

These findings demonstrate that PKCepsilon is thus recruited by IPC and induces activation of SK that then mediates IPC-induced cardioprotection in murine heart.

Inhibition of PKC phosphorylation of cTnI improves cardiac performance in vivo

Protein kinase C (PKC) modulates cardiomyocyte function by phosphorylation of intracellular targets including myofilament proteins. Data generated from studies on in vitro heart preparations indicate that PKC phosphorylation of troponin I (TnI), primarily via PKC-epsilon, may slow the rates of cardiac contraction and relaxation (+dP/dt and -dP/dt). To explore this issue in vivo, we employed transgenic mice [mutant TnI (mTnI) mice] in which the major PKC phosphorylation sites on cardiac TnI were mutated by alanine substitutions for Ser(43) and Ser(45) and studied in situ hemodynamics at baseline and increased inotropy. Hearts from mTnI mice exhibited increased contractility, as shown by a 30% greater +dP/dt and 18% greater -dP/dt than FVB hearts, and had a negligible response to isoproterenol compared with FVB mice, in which +dP/dt increased by 33% and -dP/dt increased by 26%. Treatment with phenylephrine and propranolol gave a similar result; FVB mouse hearts demonstrated a 20% increase in developed pressure, whereas mTnI mice showed no response. Back phosphorylation of TnI from mTnI hearts demonstrated that the mutation of the PKC sites was associated with an enhanced PKA-dependent phosphorylation independent of a change in basal cAMP levels. Our results demonstrate the important role that PKC-dependent phosphorylation of TnI has on the modulation of cardiac function under basal as well as augmented states and indicate interdependence of the phosphorylation sites of TnI in hearts beating in situ.

Ischemia-induced phosphorylation of phospholemman directly activates rat cardiac Na/K-ATPase

Regulation of the Na/K ATPase by protein kinases is model-specific. We have observed a profound activation of the sarcolemmal Na/K ATPase during cardiac ischemia, which is masked by an inhibitor of the enzyme in the cytosol. The aim of these studies was to characterize the pathways involved in this activation in the Langendorff-perfused rat heart. Na/K ATPase activity was determined by measuring ouabain-sensitive phosphate generation by cardiac homogenates at 37 degrees C. In isolated sarcolemma, ischemia (30 min) caused a substantial activation of the Na/K ATPase compared with aerobic controls, which was abolished by perfusing the heart with staurosporine or H89. However, the alpha1 subunit of the Na/K ATPase was not phosphorylated during ischemia. The sarcolemmal protein phospholemman (PLM) was found associated with the Na/K ATPase alpha1 and beta1 but not alpha2 subunits, and PLM increased its association with the catalytic subunit of PKA following ischemia. In vitro 14-3-3 binding assays indicated that PLM was phosphorylated following ischemia. These results indicate that the ischemia-induced activation of the Na/K ATPase is indirect, through phosphorylation of PLM, which is an integral part of the Na/K ATPase enzyme complex in the heart. The role of PLM is analogous to phospholamban in regulating the sarcoplasmic reticulum calcium ATPase.

Regulation of membrane-associated iPLA2 activity by a novel PKC isoform in ventricular myocytes

Thrombin stimulation of rabbit ventricular myocytes increases membrane-associated, Ca2+-independent phospholipase A2 (iPLA2) activity, resulting in accelerated hydrolysis of membrane plasmalogen phospholipids and increased production of arachidonic acid and lysoplasmenylcholine. This study was designed to investigate the signal transduction pathways involved in activation of membrane-associated iPLA2. Incubation of isolated membrane fractions suspended in Ca2+-free buffer with thrombin or phorbol 12-myristate 13-acetate resulted in a two- to threefold increase in iPLA2 activity. Prior treatment with the PKC inhibitor GF-109203X blocked iPLA2 activation by thrombin. These data suggest that a novel PKC isoform present in the membrane fraction modulates iPLA2 activity. Immunoblot analysis revealed a significant portion of PKC-epsilon present in the membrane fraction, but no other membrane-associated novel PKC isoform was detected by this method. These data indicate that activation of membrane-associated iPLA2 is mediated by a membrane-associated novel PKC isoform in thrombin-stimulated rabbit ventricular myocytes.

Myocyte redistribution of GRK2 and GRK5 in hypertensive, heart-failure-prone rats

Gprotein-coupled receptor kinases (GRKs) are known to be involved in the development of cardiac hypertrophy. Their exact role and subcellular distribution during cardiac hypertrophy and failure remain to be elucidated. We examined expression and subcellular distribution of GRK2 and GRK5 in the left ventricle of female spontaneously hypertensive heart failure (SHHF) rats at 6 months of age using Western blots and fluorescent confocal microscopy. GRK2 was expressed mainly in the Triton X-100 soluble fraction in the left ventricle with similar expression levels between SHHF and age-matched Wistar-Kyoto (WKY) rats. GRK2 had a striated pattern which colocalized with sarcomeric alpha-actinin and G protein in both SHHF and WKY rat myocytes and specifically accumulated in the intercalated disks of myocytes from SHHF but not WKY rats. GRK5 was expressed in both the Triton X-100 soluble fraction and Triton X-100 insoluble fraction in the left ventricle with similar expression levels between SHHF and WKY rats. GRK5 distributed diffusely in the cytoplasm in both SHHF and WKY rat myocytes and specifically accumulated in the nucleus of myocytes from SHHF but not WKY rats. GRK5 colocalized with coilin, the major component of the nuclear substructure involved in RNA synthesis and processing. The results suggest different roles for GRK2 and GRK5 in G-protein signaling and RNA biogenesis. Subcellular redistribution of GRK2 and GRK5 may be involved in cardiac hypertrophy resulting from chronic hypertension.

Protection of myocytes from hypoxia-reoxygenation injury by nitric oxide is mediated by modulation of transforming growth factor-beta1

BACKGROUND

Reoxygenation injury is a result of several complex events, including release of reactive oxygen species, protein kinase C (PKC) activation, and altered expression of transforming growth factor-beta1 (TGF-beta(1)). Nitric oxide (NO) generally protects tissues from reperfusion injury.

METHODS AND RESULTS

We examined the modulation of TGF-beta1 expression and activity and PKC activation in cultured rat heart myocytes exposed to a brief period of hypoxia-reoxygenation (H-R) by NO donor 3-morpholino-sydnonimine (SIN-1). H-R resulted in an increased expression of total TGF-beta1 (mRNA and protein) but a decrease in the release of active TGF-beta1. Myocyte PKC-alpha protein level was not altered by H-R, but its phosphorylation was augmented. Pretreatment of myocytes with SIN-1 diminished myocyte injury quantified as lactate dehydrogenase release. Simultaneously, release of active TGF-beta1 increased and total TGF-beta1 expression decreased (all P<0.05 versus H-R alone). PKC-alpha phosphorylation increased further in cells treated with SIN-1. The effects of SIN-1 were blocked by the NO scavenger phenyl-tetramethyl-imidazoline-oxyl-oxide as well as by the PKC inhibitor staurosporine. To examine if another NO donor would have a similar effect, cardiomyocytes were treated with nitroglycerin before H-R. With nitroglycerin treatment, similar to SIN-1 treatment, myocyte injury was diminished, TGF-beta1 release increased, and total TGF-beta1 expression decreased.

CONCLUSIONS

These observations suggest modulation of TGF-beta1 expression as a novel mechanism of salutary effect of NO donors. PKC-alpha activation may play an important role in the protective effect of NO against H-R injury.

Post-ischemic PKC inhibition impairs myocardial calcium handling and increases contractile protein calcium sensitivity

OBJECTIVE

Protein kinase C (PKC) activation impairs contractility in the normal heart but is protective during myocardial ischemia. We hypothesized that PKC remains activated post-ischemia and modulates myocardial excitation-contraction coupling during early reperfusion.

METHODS

Langendorff-perfused rabbit hearts where subjected to 25 min unmodified ischemia and 30 min reperfusion. Total PKC activity was measured, and the intracellular translocation pattern of PKC-alpha, -delta, -epsilon, and -eta assessed by immunohistochemistry and fractionated Western immunoblotting. The PKC-inhibitors chelerythrine and GF109203X were added during reperfusion and also given to non-ischemic hearts. Measurements included left ventricular function, intracellular calcium handling measured by Rhod-2 spectrofluorometry, myofibrillar calcium responsiveness in beating and tetanized hearts, and metabolic parameters.

RESULTS

Total PKC activity was increased at end-ischemia and remained elevated after 30 min of reperfusion. The translocation pattern indicated PKC-epsilon as the main active isoform during reperfusion. Post-ischemic PKC inhibition affected mainly diastolic relaxation, with lesser effect on contractility. Both PKC inhibitors increased the Ca(2+) responsiveness of the myofilaments as indicated by a leftward shift of the calcium-to-force relationship and increased maximum calcium activated tetanic pressure. Diastolic Ca(2+) removal was delayed and the post-ischemic [Ca(2+)](i) overload further exacerbated. Depressed systolic function was associated with a lower amplitude of [Ca(2+)](i) transients.

CONCLUSION

PKC is activated during ischemia and remains activated during early reperfusion. Inhibition of PKC activity post-ischemia impairs functional recovery, delays diastolic [Ca(2+)](i) removal, and increases Ca(2+) sensitivity of the contractile apparatus, resulting in impaired diastolic relaxation. Thus, post-ischemic PKC activity may serve to restore post-ischemic Ca(2+) homeostasis and attenuate contractile protein calcium sensitivity during the period of post-ischemic [Ca(2+)](i) overload.

Calcium-independent phospholipase A(2) mediates CREB phosphorylation and c-fos expression during ischemia

In isolated, perfused adult rat hearts, global ischemia increased the phosphorylation of cAMP response element-binding protein (CREB) relative to control levels, and this phosphorylation was reversed with reperfusion. CREB phosphorylation elicited by 5 min of global ischemia was sensitive to treatments with the calcium-independent phospholipase A(2) (iPLA(2)) inhibitor bromoenol lactone (BEL) and occurred in the absence of increases in myocardial cAMP content. In contrast, CREB phosphorylation elicited by 15 min of global ischemia was likely mediated by elevated cAMP levels. The expression of c-fos, in response to brief myocardial ischemia, was also sensitive to BEL treatment. The induction of iPLA(2)-mediated CREB phosphorylation was further substantiated by the observations that lysoplasmenylcholine increased both the phosphorylation of CREB and the induction of c-fos expression in the absence and presence of BEL. CREB phosphorylation in both ischemic hearts and lysoplasmenylcholine-perfused hearts was inhibited by pretreatment of hearts with the specific cAMP-dependent protein kinase (PKA) inhibitor H-89. Taken together, these data demonstrate that iPLA(2) mediates CREB phosphorylation through a PKA-dependent pathway during brief periods of myocardial ischemia, possibly through the formation of lysophospholipids.

Localization and kinetics of protein kinase C-epsilon anchoring in cardiac myocytes

Protein kinase C-epsilon (PKC-epsilon) plays a central role in cardiac cell signaling, but mechanisms of translocation and anchoring upon activation are poorly understood. Conventional PKC isoforms rely on a rapid Ca2+-mediated recruitment to cell membranes, but this mechanism cannot be employed by PKC-epsilon or other PKC isoforms lacking a Ca2+-binding domain. In this study, we used recombinant green fluorescent protein (GFP) fusion constructs and confocal microscopy to examine the localization, kinetics, and reversibility of PKC-epsilon anchoring in permeabilized rat cardiac myocytes. PKC-epsilon-GFP bound with a striated pattern that co-localized with alpha-actinin, a marker of the Z-line of the sarcomere. Binding required activation of PKC and occurred slowly but reversibly with apparent rate constants of k(on) = 4.6 +/- 1.2 x 10(3) M(-1) x s(-1) and k(off) = 1.4 +/- 0.5 x 10(-3) s(-1) (t1/2 = 8 min) as determined by fluorescence recovery after photobleaching and by perfusion experiments. A truncated construct composed of the N-terminal 144-amino-acid variable region of PKC-epsilon (epsilonV1-GFP), but not an analogous N-terminal domain of PKC-delta, mimicked the Z-line decoration and slow binding rate of the full-length enzyme. These findings suggest that the epsilonV1 domain is important in determining PKC-epsilon localization and translocation kinetics in cardiac muscle. Moreover, PKC-epsilon translocation is not a diffusion-controlled binding process but instead may be limited by intramolecular conformational changes within the V1 domain. The k(off) for epsilonV1-GFP was two- to threefold faster than for full-length enzyme, indicating that other domains in PKC-epsilon contribute to anchoring by prolonging the bound state.

Essential activation of PKC-delta in opioid-initiated cardioprotection

Stimulation of the delta(1)-opioid receptor confers cardioprotection to the ischemic myocardium. We examined the role of protein kinase C (PKC) after delta-opioid receptor stimulation with TAN-67 or D-Ala(2)-D-Leu(5)-enkephalin (DADLE) in a rat model of myocardial infarction induced by a 30-min coronary artery occlusion and 2-h reperfusion. Infarct size (IS) was determined by tetrazolium staining and expressed as a percentage of the area at risk (IS/AAR). Control animals, subjected to ischemia and reperfusion, had an IS/AAR of 59.9 +/- 1.8. DADLE and TAN-67 administered before ischemia significantly reduced IS/AAR (36.9 +/- 3.9 and 36.7 +/- 4.7, respectively). The delta(1)-selective opioid antagonist 7-benzylidenenaltrexone (BNTX) abolished TAN-67-induced cardioprotection (54.4 +/- 1.3). Treatment with the PKC antagonist chelerythrine completely abolished DADLE- (61.8 +/- 3.2) and TAN-67-induced cardioprotection (55.4 +/- 4.0). Similarly, the PKC antagonist GF 109203X completely abolished TAN-67-induced cardioprotection (54.6 +/- 6.6). Immunofluorescent staining with antibodies directed against specific PKC isoforms was performed in myocardial biopsies obtained after 15 min of treatment with saline, chelerythrine, BNTX, or TAN-67 and chelerythrine or BNTX in the presence of TAN-67. TAN-67 induced the translocation of PKC-alpha to the sarcolemma, PKC-beta(1) to the nucleus, PKC-delta to the mitochondria, and PKC-epsilon to the intercalated disk and mitochondria. PKC translocation was abolished by chelerythrine and BNTX in TAN-67-treated rats. To more closely examine the role of these isoforms in cardioprotection, we utilized the PKC-delta selective antagonist rottlerin. Rottlerin abolished opioid-induced cardioprotection (48.9 +/- 4.8) and PKC-delta translocation without affecting the translocation of PKC-alpha, -beta(1), or -epsilon. These results suggest that PKC-delta is a key second messenger in the cardioprotective effects of delta(1)-opioid receptor stimulation in rats.

Ischemic dysfunction in transgenic mice expressing troponin I lacking protein kinase C phosphorylation sites

To determine the in vivo functional significance of troponin I (TnI) protein kinase C (PKC) phosphorylation sites, we created a transgenic mouse expressing mutant TnI, in which PKC phosphorylation sites at serines-43 and -45 were replaced by alanine. When we used high-perfusate calcium as a PKC activator, developed pressures in transgenic (TG) perfused hearts were similar to wild-type (WT) hearts (P = not significant, NS), though there was a 35% and 32% decrease in peak-systolic intracellular calcium (P < 0.01) and diastolic calcium (P < 0.005), respectively. The calcium transient duration was prolonged in the TG mice also (12-27%, ANOVA, P < 0.01). During global ischemia, TG hearts developed ischemic contracture to a greater extent than WT hearts (41 +/- 18 vs. 69 +/- 10 mmHg, perfusate calcium 3.5 mM, P < 0.01). In conclusion, expression of mutant TnI lacking PKC phosphorylation sites results in a marked alteration in the calcium-pressure relationship, and thus susceptibility to ischemic contracture. The reduced intracellular calcium and prolonged calcium transients suggests that a potent feedback mechanism exists between the myofilament and the processes controlling calcium homeostasis.

Role for PKC in the adenosine-induced decrease in shortening velocity of rat ventricular myocytes

We previously demonstrated that both adenosine receptor activation and direct activation of protein kinase C (PKC) decrease unloaded shortening velocity (V(max)) of rat ventricular myocytes. The goal of this study was to further investigate a possible link among adenosine receptors, phosphoinositide-PKC signaling, and V(max) in rat ventricular myocytes. We determined that the adenosine receptor agonist R-phenylisopropyladenosine (R-PIA, 100 microM) and the alpha-adrenergic receptor agonist phenylephrine (Phe, 10 microM) increased turnover of inositol phosphates. PKC translocation from the cytosol to the sarcolemma was used as an indicator of PKC activation. Western blot analysis demonstrated an increased PKC-epsilon translocation after exposure to R-PIA, Phe, and the PKC activators dioctanoylglycerol (50 microM) and phorbol myristate acetate (1 microM). PKC-alpha, PKC-delta, and PKC-zeta did not translocate to the membrane after R-PIA exposure. Finally, PKC inhibitors blocked R-PIA-induced decreases in V(max) as well as Ca(2+)-dependent actomyosin ATPase in rat ventricular myocytes. These results support the conclusions that adenosine receptors activate phosphoinositide-PKC signaling and that adenosine receptor-induced PKC activation mediates a decrease in V(max) in ventricular myocytes.

Role and mechanism of PKC in ischemic preconditioning of pig skeletal muscle against infarction

Protein kinase C (PKC) inhibitors, chelerythrine (Chel, 0.6 mg) and polymyxin B (Poly B, 1.0 mg), and PKC activators, phorbol 12-myristate 13-acetate (PMA, 0.05 mg) and 1-oleoyl-2-acetyl glycerol (OAG, 0.1 mg), were used as probes to investigate the role of PKC in mediation of ischemic preconditioning (IPC) of noncontracting pig latissimus dorsi (LD) muscles against infarction in vivo. These drugs were delivered to each LD muscle flap (8 x 12 cm) by 10 min of local intra-arterial infusion. It was observed that LD muscle flaps sustained 43 +/- 5% infarction when subjected to 4 h of global ischemia and 24 h of reperfusion. IPC with three cycles of 10 min ischemia-reperfusion reduced muscle infarction to 25 +/- 3% (P < 0.05). This anti-infarction effect of IPC was blocked by Chel (42 +/- 7%) and Poly B (37 +/- 2%) and mimicked by PMA (19 +/- 10%) and OAG (14 +/- 5%) treatments (P < 0.05), given 10 min before 4 h of ischemia. In addition, the ATP-sensitive K(+) (K(ATP)) channel antagonist sodium 5-hydroxydecanoate attenuated (P < 0.05) the anti-infarction effect of IPC (37 +/- 2%), PMA (44 +/- 17%), and OAG (46 +/- 9%). IPC, OAG, and Chel treatment alone did not affect mean arterial blood pressure or muscle blood flow assessed by 15-microm radioactive microspheres. Western blot analysis of muscle biopsies obtained before (baseline) and after IPC demonstrated seven cytosol-associated isoforms, with nPKCepsilon alone demonstrating progressive cytosol-to-membrane translocation within 10 min after the final ischemia period of IPC. Using differential fractionation, it was observed that nPKCepsilon translocated to a membrane compartment other than the sarcolemma and/or sarcoplasmic reticulum. Furthermore, IPC and preischemic OAG but not postischemic OAG treatment reduced (P < 0.05) muscle myeloperoxidase activity compared with time-matched ischemic controls during 16 h of reperfusion after 4 h of ischemia. Taken together, these observations indicate that PKC plays a central role in the anti-infarction effect of IPC in pig LD muscles, most likely through a PKC-K(ATP) channel-linked signal-transduction pathway.