Alpha-adrenergic preservation of myocardial pH during ischemia is PKC isoform dependent

Current Developments on the Role of α1-Adrenergic Receptors in Cognition, Cardioprotection, and Metabolism

The α₁-adrenergic receptors (ARs) are G-protein coupled receptors that bind the endogenous catecholamines, norepinephrine, and epinephrine. They play a key role in the regulation of the sympathetic nervous system along with β and α₂-AR family members. While all of the adrenergic receptors bind with similar affinity to the catecholamines, they can regulate different physiologies and pathophysiologies in the body because they couple to different G-proteins and signal transduction pathways, commonly in opposition to one another. While α₁-AR subtypes (α_1A, α_1B, α_1C) have long been known to be primary regulators of vascular smooth muscle contraction, blood pressure, and cardiac hypertrophy, their role in neurotransmission, improving cognition, protecting the heart during ischemia and failure, and regulating whole body and organ metabolism are not well known and are more recent developments. These advancements have been made possible through the development of transgenic and knockout mouse models and more selective ligands to advance their research. Here, we will review the recent literature to provide new insights into these physiological functions and possible use as a therapeutic target.

Activation of α1B-adrenoceptors contributes to intermittent hypobaric hypoxia-improved postischemic myocardial performance via inhibiting MMP-2 activation

Inhibition of matrix metalloproteinases-2 (MMP-2) activation renders cardioprotection from ischemia/reperfusion (I/R) injury; however, the signaling pathways involved have not been fully understood. Intermittent hypobaric hypoxia (IHH) has been shown to enhance myocardial tolerance to I/R injury via triggering intrinsic adaptive responses. Here we investigated whether IHH protects the heart against I/R injury via the regulation of MMP-2 and how the MMP-2 is regulated. IHH (Po2 = 84 mmHg, 4-h/day, 4 wk) improved postischemic myocardial contractile performance, lactate dehydrogenase (LDH) release, and infarct size in isolated perfused rat hearts. Moreover, IHH reversed I/R-induced MMP-2 activation and release, disorders in the levels of MMP-2 regulators, peroxynitrite (ONOO(-)) and tissue inhibitor of metalloproteinase-4 (TIMP-4), and loss of the MMP-2 targets α-actinin and troponin I. This protection was mimicked, but not augmented, by a MMP inhibitor doxycycline and lost by the α1-adrenoceptor (AR) antagonist prazosin. Furthermore, IHH increased myocardial α1A-AR and α1B-AR density but not α1D-AR after I/R. Concomitantly, IHH further enhanced the translocation of PKC epsilon (PKCε) and decreased the release of mitochondrial cytochrome c due to I/R via the activation of α1B-AR but not α1A-AR or α1D-AR. IHH-conferred cardioprotection in the postischemic contractile function, LDH release, MMP-2 activation, and nitrotyrosine as well as TIMP-4 contents were mimicked but not additive by α1-AR stimulation with phenylephrine and were abolished by an α1B-AR antagonist chloroethylclonidine and a PKCε inhibitor PKCε V1-2. These findings demonstrate that IHH exerts cardioprotection through attenuating excess ONOO(-) biosynthesis and TIMP-4 loss and sequential MMP-2 activation via the activation of α1B-AR/PKCε pathway.

Sevoflurane-induced cardioprotection depends on PKC-alpha activation via production of reactive oxygen species

BACKGROUND

We previously demonstrated the involvement of the Ca2+-independent protein kinase C-delta (PKC-delta) isoform in sevoflurane-induced cardioprotection against ischaemia and reperfusion (I/R) injury. Since sevoflurane is known to modulate myocardial Ca2+-handling directly, in this study we investigated the role of the Ca2+-dependent PKC-alpha isoform in sevoflurane-induced cardioprotective signalling in relation to reactive oxygen species (ROS), adenosine triphosphate-sensitive mitochondrial K+ (mitoK+(ATP)) channels, and PKC-delta.

METHODS

Preconditioned (15 min 3.8 vol% sevoflurane) isolated rat right ventricular trabeculae were subjected to I/R, consisting of 40 min superfusion with hypoxic, glucose-free buffer, followed by normoxic glucose-containing buffer for 60 min. After reperfusion, contractile recovery was expressed as percentage of force development before I/R. The role of PKC-alpha, ROS, mitoK+(ATP) channels, and PKC-delta was established using the following pharmacological inhibitors: Go6976 (GO; 50 nM), n-(2-mercaptopropionyl)-glycine (MPG; 300 microM), 5-hydroxydecanoic acid sodium (5HD; 100 microM), and rottlerin (ROT; 1 microM).

RESULTS

Preconditioning of trabeculae with sevoflurane improved contractile recovery after I/R [65 (3)% (I/R + SEVO) vs 47 (3)% (I/R); n = 8; P < 0.05]. This cardioprotective effect was attenuated in trabeculae treated with GO [42 (4)% (I/R + SEVO + GO); P > 0.05 vs (I/R)]. In sevoflurane-treated trabeculae, PKC-alpha translocated towards mitochondria, as shown by immunofluorescent co-localization analysis. GO and MPG, but not 5HD or ROT, abolished this translocation.

CONCLUSIONS

Sevoflurane improves post-ischaemic contractile recovery via activation of PKC-alpha. ROS production, but not opening of mitoK+(ATP) channels, precedes PKC-alpha translocation towards mitochondria. This study shows the involvement of Ca2+-dependent PKC-alpha in addition to the well-established role of Ca2+-independent PKC isoforms in sevoflurane-induced cardioprotection.

Protein kinase C-epsilon coimmunoprecipitates with cytochrome oxidase subunit IV and is associated with improved cytochrome-c oxidase activity and cardioprotection

We have utilized an in situ rat coronary ligation model to establish a PKC-epsilon cytochrome oxidase subunit IV (COIV) coimmunoprecipitation in myocardium exposed to ischemic preconditioning (PC). Ischemia-reperfusion (I/R) damage and PC protection were confirmed using tetrazolium-based staining methods and serum levels of cardiac troponin I. Homogenates prepared from the regions at risk (RAR) and not at risk (RNAR) for I/R injury were fractionated into cell-soluble (S), 600 g low-speed centrifugation (L), Percoll/Optiprep density gradient-purified mitochondrial (M), and 100,000 g particulate (P) fractions. COIV immunoreactivity and cytochrome-c oxidase activity measurements estimated the percentages of cellular mitochondria in S, L, M, and P fractions to be 0, 55, 29, and 16%, respectively. We observed 18, 3, and 3% of PKC-delta, -epsilon, and -zeta isozymes in the M fraction under basal conditions. Following PC, we observed a 61% increase in PKC-epsilon levels in the RAR M fraction compared with the RNAR M fraction. In RAR mitochondria, we also observed a 2.8-fold increase in PKC-epsilon serine 729 phosphoimmunoreactivity (autophosphorylation), indicating the presence of activated PKC-epsilon in mitochondria following PC. PC administered before prolonged I/R induced a 1.9-fold increase in the coimmunoprecipitation of COIV, with anti-PKC-epsilon antisera and a twofold enhancement of cytochrome-c oxidase activity. Our results suggest that PKC-epsilon may interact with COIV as a component of the cardioprotection in PC. Induction of this interaction may provide a novel therapeutic target for protecting the heart from I/R damage.

Increased isoform-specific membrane translocation of conventional and novel protein kinase C in human neuroblastoma SH-SY5Y cells following prolonged hypoxia

Several studies have suggested that protein kinase C (PKC) plays a key role in the mechanism of cerebral ischemic/hypoxic preconditioning (I/HPC). However, detailed information regarding PKC isoforms in response to brain ischemia/hypoxia and their potential role in neuroprotection is unclear. Previous studies in our laboratory have demonstrated that the levels in membrane translocation of conventional PKC (cPKC) betaII, gamma, and novel PKCepsilon (nPKC), but not cPKCalpha, betaI, nPKCdelta, eta, mu, theta, and atypical PKC (aPKC) zeta and iota/lambda, were increased significantly in the hippocampus and cortex of intact mice with hypoxic preconditioning. To further detect cPKC and nPKC isoforms activation following prolonged hypoxia in vitro, we tested the membrane translocation (an indicator of PKC activation) of cPKCalpha, betaI, betaII, and gamma, and nPKCdelta, epsilon, eta, mu, and theta in a human neuroblastoma SH-SY5Y cell line following sustained hypoxic exposure (1% O(2)/5% CO(2)/94% N(2)). Using Western blot and immunocytochemistry methods, we found that the levels of cPKCalpha, betaI, betaII, and nPKCepsilon, but not nPKCdelta, eta, mu, and theta, membrane translocation were increased significantly (P < 0.05, n = 8) in a time-dependent manner (from 0.5 to 24 h) following sustained hypoxic exposure. Similarly, the immunostaining experiment also showed a noticeable translocation of cPKCalpha, betaI, betaII, and nPKCepsilon from the cytosol to the perinuclear or membrane-related areas after 6 h posthypoxic exposure. In addition, no cPKCgamma was detected in this cell line under either a normoxic or hypoxic condition. These results suggested that prolonged hypoxia may induce the activation of cPKCalpha, betaI, betaII, and nPKCepsilon by triggering their membrane translocation in SH-SY5Y cells.

Protein kinase Cepsilon interacts with cytochrome c oxidase subunit IV and enhances cytochrome c oxidase activity in neonatal cardiac myocyte preconditioning

We have previously identified a phorbol ester-induced PKCepsilon (protein kinase Cepsilon) interaction with the ( approximately 18 kDa) COIV [CO (cytochrome c oxidase) subunit IV] in NCMs (neonatal cardiac myocytes). Since PKCepsilon has been implicated as a key mediator of cardiac PC (preconditioning), we examined whether hypoxic PC could induce PKCepsilon-COIV interactions. Similar to our recent study with phorbol esters [Ogbi, Chew, Pohl, Stuchlik, Ogbi and Johnson (2004) Biochem. J. 382, 923-932], we observed a time-dependent increase in the in vitro phosphorylation of an approx. 18 kDa protein in particulate cell fractions isolated from NCMs subjected to 1-60 min of hypoxia. Introduction of a PKCepsilon-selective translocation inhibitor into cells attenuated this in vitro phosphorylation. Furthermore, when mitochondria isolated from NCMs exposed to 30 min of hypoxia were subjected to immunoprecipitation analyses using PKCepsilon-selective antisera, we observed an 11.1-fold increase in PKCepsilon-COIV co-precipitation. In addition, we observed up to 4-fold increases in CO activity after brief NCM hypoxia exposures that were also attenuated by introducing a PKCepsilon-selective translocation inhibitor into the cells. Finally, in Western-blot analyses, we observed a >2-fold PC-induced protection of COIV levels after 9 h index hypoxia. Our studies suggest that a PKCepsilon-COIV interaction and an enhancement of CO activity occur in NCM hypoxic PC. We therefore propose novel mechanisms of PKCepsilon-mediated PC involving enhanced energetics, decreased mitochondrial reactive oxygen species production and the preservation of COIV levels.

Preconditioning: Gender Effects1

Preconditioning is injury induced protection from subsequent injury. During preconditioning protective cellular responses to injury are up regulated resulting in acute and delayed defense against further damage. Several studies indicate that females experience a protective advantage after acute insult compared to males. Despite evidence of gender differences in acute injury, relatively few studies have evaluated whether there are sex differences in preconditioning. Variations in patients' pre-morbid preconditioning status may explain outcome variations that are not apparent in small animal studies. This review discusses the differences in response to acute injury experienced by males and females, the basic mechanisms of preconditioning, and the sex differences in the mechanisms of preconditioning.

Identification of protein kinase C isoforms involved in cerebral hypoxic preconditioning of mice

Recently, accumulated studies have suggested that protein kinases C (PKC) play a central role in the development of ischemic-hypoxic preconditioning (I/HPC) in the brain. However, which types of PKC isoforms might be responsible for neuroprotection is still not clear, especially when the systematic investigation of PKC isoform-specific changes in brain regions was rare in animals with ischemic-hypoxic preconditioning. By using Western blot, we have demonstrated that the levels of cPKC betaII and gamma membrane translocation were increased in the early phase of cerebral hypoxic preconditioning. In this study, we combined the Western blot and immunostaining methods to investigate the effects of repetitive hypoxic exposure (H1-H4, n = 6 for each group) on membrane translocation and protein expression of several types of PKC isoforms, both in the cortex and hippocampus of mice. We found that the increased membrane translocation of nPKCepsilon (P < 0.05, versus normoxic H0) but not its protein expression levels in both the cortex and hippocampus during development of cerebral HPC in mice. However, there were no significant changes in both membrane translocation and protein expression levels of nPKCdelta, theta, eta, mu, and aPKC iota/lambda, zeta in these brain areas after hypoxic preconditioning. Similarly, an extensive subcellular redistribution of cPKCbetaII, gamma, and nPKCepsilon was observed by immunostaining in the cortex after three series of hypoxic exposures (H3). These results indicate that activation of cPKCbetaII, gamma, and nPKCepsilon might be involved in the development of cerebral hypoxic preconditioning of mice.

Changes in cPKC isoform-specific membrane translocation and protein expression in the brain of hypoxic preconditioned mice

Previous studies have shown that the level of total conventional protein kinase C (cPKC) membrane translocation (activation) was increased in the brain of hypoxic preconditioned mice. In order to find out which isoform of cPKC may participate in the development of cerebral hypoxic preconditioning (HPC), we used Western bolt and immunohistochemistry to observe the effects of repetitive hypoxic exposure (H1-H6, n = 6 for each group) on the level of cPKC isoform-specific protein expression and its membrane translocation in the cortex and hippocampus of mice. We found that the levels of cPKC betaII and gamma membrane translocation were increased significantly (p < 0.05 versus normoxic H0 group, n = 6) in response to repetitive hypoxic exposure (H1-H4) at an early phase of hypoxic preconditioning, but no significant changes of cPKC alpha and betaI membrane translocation were found during cPKC alpha, betaI, betaII and gamma protein expression both in hippocampus and cortex. In addition, an extensive subcellular redistribution of cPKC betaII and gamma was detected by immunohistochemistry staining in the cortex after repetitive hypoxic exposures (H3). However, a significant decrease in the expression of cPKC gamma protein (p < 0.05 versus H0 group) was found only in the cortex of delayed hypoxic preconditioned mice (H5-H6). These results suggest that the activation of cPKC betaII and gamma may be involved in the early phase of cerebral hypoxic preconditioning and the changes in cPKC gamma protein expression may participate in the development of the late phase of cerebral hypoxic preconditioning as well as selective vulnerability to hypoxia both in cortex and hippocampus.

Preconditioning: evolution of basic mechanisms to potential therapeutic strategies

Preconditioning describes the phenomenon by which a traumatic or stressful stimulus confers protection against subsequent injury. Originally recognized in dog heart subjected to ischemic challenges, preconditioning has been demonstrated in multiple species, can be induced by various stimuli, and is applicable in different organ systems. Tremendous progress has been made elucidating the signal transduction cascade of preconditioning. Preconditioning represents a potent tissue-protective condition, and mechanistic understanding may allow safe clinical application. This review recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; summarizes the current mechanistic understanding of acute preconditioning; outlines the signal transduction cascade leading to the development of delayed preconditioning; discusses preconditioning in noncardiac tissue; and explores the potential of using preconditioning clinically.

Direct evidence that protein kinase C plays an essential role in the development of late preconditioning against myocardial stunning in conscious rabbits and that epsilon is the isoform involved

Brief ischemic episodes confer marked protection against myocardial stunning 1-3 d later (late preconditioning [PC] against stunning). The mechanism of this powerful protective effect is poorly understood. Although protein kinase C (PKC) has been implicated in PC against infarction, it is unknown whether it triggers late PC against stunning. In addition, the entire PKC hypothesis of ischemic PC remains controversial, possibly because the effects of PKC inhibitors on PC protection have not been correlated with their effects on PKC activity and/or translocation in vivo. Thus, conscious rabbits underwent a sequence of six 4-min coronary occlusion (O)/4-min reperfusion (R) cycles for three consecutive days (days 1, 2, and 3). In the control group (group I, n = 7), the recovery of systolic wall thickening after the six O/R cycles was markedly improved on days 2 and 3 compared with day 1, indicating the development of late PC against stunning. Administration of the PKC inhibitor chelerythrine at a dose of 5 mg/kg before the first O on day 1 (group II, n = 10) abrogated the late PC effect against stunning, whereas a 10-fold lower dose (0.5 mg/kg; group III, n = 7) did not. Administration of 5 mg/kg of chelerythrine 10 min after the sixth reperfusion on day 1 (group IV, n = 6) failed to block late PC against stunning. When rabbits were given 5 mg/kg of chelerythrine in the absence of O/R (group V, n = 5), the severity of myocardial stunning 24 h later was not modified. Pretreatment with phorbol 12-myristate 13-acetate (4 microg/kg) on day 1 without ischemia (group VI, n = 11) induced late PC against stunning on day 2 and the magnitude of this effect was equivalent to that observed after ischemic PC. In vehicle-treated rabbits (group VIII, n = 5), the six O/R cycles caused translocation of PKC isoforms epsilon and eta from the cytosolic to the particulate fraction without significant changes in total PKC activity, in the subcellular distribution of total PKC activity, or in the subcellular distribution of the alpha, beta1, beta2, gamma, delta, zeta, iota, lambda, and mu isoforms. The higher dose of chelerythrine (5 mg/kg; group X, n = 5) prevented the translocation of both PKC epsilon and eta induced by ischemic PC, whereas the lower dose (0.5 mg/kg; group XI, n = 5) prevented the translocation of PKC eta but not that of epsilon, indicating that the activation of epsilon is necessary for late PC to occur whereas that of eta is not. To our knowledge, this is the first demonstration that a PKC inhibitor actually prevents the translocation of PKC induced by ischemic PC in vivo, and that this inhibition of PKC translocation results in loss of PC protection. Taken together, the results demonstrate that the mechanism of late PC against myocardial stunning in conscious rabbits involves a PKC-mediated signaling pathway, and implicate epsilon as the specific PKC isoform responsible for the development of this cardioprotective phenomenon.

Adaptive and maladaptive mechanisms of cellular priming

OBJECTIVE

The mechanisms of cellular priming resulting in both adaptive and maladaptive responses to subsequent injury and strategies for manipulating this priming to constructive therapeutic advantage are explored.

BACKGROUND DATA

A cell is prepared or educated by an initial insult (priming stimulus). Investigations in both laboratory animals and humans indicate that cells, organs, and perhaps even whole patients respond differently to a proximal second insult ("second hit") by virtue of this prior environmental history. The opportunity to achieve the primed state appears to be conserved across almost all cell types. The initial stimulus transmits a message to the cellular machinery that influences the cell's response to a subsequent challenge. This response may result in an exaggerated inflammatory response in the case of the neutrophil (an often maladaptive process) or an improved tolerance to injury by the myocyte (adaptive response). Our global hypothesis is that cellular priming is a conserved, receptor-dependent process that invokes common intracellular targets across multiple cell types. We further postulate that these targets create a language based on the transient phosphorylation and dephosphorylation of intracellular enzymes that is therapeutically accessible.

CONCLUSIONS

Priming is a conserved, receptor-dependent process transduced by means of intracellular targets across multiple cell types. The potential therapeutic strategies outlined involve the receptor-mediated manipulation of cellular events. These events are transmitted through an intracellular language that instructs the cell regarding its behavior in response to subsequent stimulation. Understanding these intracellular events represents a realistic goal of priming and preconditioning biology and will likely lead to clinical control of the primed state.

Mechanisms of cardiac preconditioning: ten years after the discovery of ischemic preconditioning

Cardiac preconditioning describes the phenomenon by which transient ischemia induces myocardial protection against subsequent ischemia and reperfusion injury. Ten years have passed since the original description of this potent cardiac protective strategy and within this period tremendous progress has been made elucidating the mechanisms of preconditioning. Mechanistic understanding may allow safe clinical application. This review (1) recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; (2) summarizes the current mechanistic understanding of early preconditioning; (3) compares and contrasts the mechanisms of early versus delayed preconditioning; (4) suggests potential anti-inflammatory aspects of preconditioning; (5) examines limitations in laboratory models of preconditioning; and (6) explores the potential of using preconditioning clinically.

Protein kinase C isoform diversity in preconditioning

Protein kinase C (PKC) appears to be a common intracellular effector and signal collector during cardiac preconditioning; however, it remains unknown whether agonists that activate different PKC isoforms are also linked to select aspects of myocardial protection. Using agonists that are known to activate unique combinations of PKC isoforms, we interrogated the relationship between isoform activation and the different aspects (pH, function, and viability) of endogenous myocardial protection. To study this, isolated rat hearts were subjected to ischemia-reperfusion (I/R) (20 min/40 min), without (control = Ctrl) or with receptor-dependent [phenylephrine (PE), 50 microM; adenosine (ADO), 125 microM] or -independent [phorbol myristate acetate (PMA), 100 nM] activation of PKC. Function, pH, and viability were assessed by rate pressure product (%RPP) and coronary flow (CF; ml/min), by 31P NMR, and by CF creatine kinase (CK; U/liter) leak, respectively. PMA, which activates PKC delta but not eta, resulted in intracellular pH (pHi) and viability protection, but did not protect against postischemic myocardial stunning. ADO, which activates PKC eta but not delta, protects against stunning, but not acidosis or necrosis. PE, which activates PKC delta and eta, provided global myocardial protection against necrosis, acidosis, and stunning. Different PKC isoforms may be linked to distinct aspects of myocardial protection. Targeted activation of PKC isoforms may allow precise mechanistic application of preconditioning-like myocardial protection.