Published online before print August 7, 2008, doi: 10.1161/CIRCRESAHA.108.175018
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© 2008 American Heart Association, Inc.
Integrative Physiology |
Phosphatidylinositol 3-Kinase 
Is a Critical Mediator of Myocardial Ischemic and Adenosine-Mediated Preconditioning
From the Departments of Medicine and Physiology (K.B., A.J.C., M.P., P.H.B., G.Y.O., R.G.T.), Heart and Stroke/Richard Lewar Centre of Excellence (P.H.B., G.Y.O., R.G.T.), and Division of Cardiology (P.H.B., G.Y.O., R.G.T.), University of Toronto, Ontario, Canada; Department of Biology (S.S., R.G.T.), York University, Toronto, Ontario, Canada; Novartis Institutes for Biomedical Research (S.I.); and Institute of Molecular Biotechnology (J.M.P.), Austrian Academy of Sciences. Present address for G.Y.O.: Division of Cardiology, Department of Medicine and Mazankowski Alberta Heart Institute, University of Alberta, Edmonton, Canada.
Correspondence to Dr Robert G. Tsushima, Department of Biology, York University, 4700 Keele St, FS 344, Toronto, ON, Canada M3J 1P3. E-mail tsushima{at}yorku.ca
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Abstract |
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Ischemic preconditioning (IPC) is a potent cellular protective mechanism whereby brief periods of sublethal ischemia protect the myocardium from prolonged ischemia-induced injury. We demonstrate the selective role of phosphatidylinositol 3-kinase (PI3K) isoforms in IPC. Hearts from PI3K
knockout mice (PI3K–/–) displayed poorer functional recovery and greater tissue injury following IPC compared to wild-type and PI3K+/– hearts. Examination of the cell-signaling pathways revealed restored phosphorylation levels of Akt and glycogen synthase kinase (GSK)3β in wild-type hearts, which were abolished in PI3K–/– hearts subjected to IPC. Inhibition of GSK3β by LiCl reversed the loss in protection in PI3K–/– hearts. In contrast, mice expressing a cardiac-specific kinase-deleted PI3K
(PI3KDN) were resistant to injury induced by 30 minutes of ischemia followed by 40 minutes of reperfusion. Furthermore, the resistance of PI3KDN hearts to ischemia/reperfusion correlated with the persistent expression of p110 and was blocked by the PI3K inhibitor wortmannin, suggesting the possible enhanced cell signaling through the PI3K pathway. These results demonstrate the importance of the PI3K-Akt-GSK3β signaling pathway in IPC. Selective activation of myocardial PI3K may be an attractive target for the treatment of ischemic heart disease.
Key Words: heart • ischemic preconditioning • PI3K • Akt • GSK3β, transgenic mice
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Introduction |
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Ischemic preconditioning (IPC) is a potent cellular protective mechanism that is initiated by brief periods of sublethal ischemic stress (reviewed elsewhere1). First identified by Murry et al,2 they observed a reduction in infarct size when canine hearts were subjected to 4 brief episodes of 5 minutes of ischemia and 5 minutes of reperfusion before a period of sustained ischemia. It has been concluded that the heart adapted within minutes to become resistant against ischemia-induced injury.1
The cellular mechanisms underlying the protection initiated by myocardial IPC have been scrutinized intensively,1,3 given the potential to identify novel targets for treating ischemic heart disease. Work has focused on identifying the triggers, mediators, and end effectors of IPC. Numerous signal transduction pathways have been demonstrated to be the mediators affording protection by IPC.3 Phosphatidylinositol 3-kinase (PI3K) is a family of conserved lipid and protein kinases that are ubiquitously expressed in many cells, including the heart (reviewed elsewhere4,5). These enzymes phosphorylate phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2 or PIP2] to form phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3 or PIP3]. Class IA and IB PI3Ks are heterodimeric enzymes that have a regulatory subunit coupled to a tightly bound catalytic subunit. Class IA PI3Ks are activated through their coupling with tyrosine kinase receptors, whereas class IB activation occurs through G protein–coupled receptors.6 The class IA family is comprised of PI3K, -β, and -
, with PI3K constituting class IB.4,5 PI3K, -β, and - are found in the heart, whereas PI3K is considered to be exclusively expressed in leukocytes.4,5
It has been recently established PI3K isoforms convey distinct roles in cardiac physiology and development; PI3K regulates cell growth and apoptosis, whereas PI3K acts to control cardiac contractility.7,8 In IPC, there is biphasic activation of PI3K signaling: first, during the IPC phase, where it leads to the opening of mitochondrial ATP-sensitive potassium channels; and subsequently at the time of reperfusion, where it is proposed to result in the downstream closure of the mitochondrial permeability transition pore (reviewed elsewhere9). Insight into the role of PI3K in mediating IPC protection has been highlighted by the use of the pharmacological PI3K inhibitors wortmannin and LY294002.10–13 However, these agents are somewhat limited because of their inability to distinguish between the different PI3K isoforms. Accordingly, we set out to investigate the distinct and selective role of PI3K (Class IA) and PI3K (Class IB) in myocardial IPC. In the present study, we demonstrate the preferential role of PI3K in mediating IPC protection through the downstream activation of the Akt–glycogen synthase kinase (GSK)3β signaling pathway. Surprisingly, suppression of PI3K in the heart induced resistance to ischemia/reperfusion injury through maintained signaling via PI3K.
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Materials and Methods |
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Experimental Animals
Mice used in this study were C57BL/6 (wild-type [WT]), PI3K
+/–, PI3K–/–, and cardiac-specific kinase-deleted PI3K (PI3KDN) mice. All mice were outbred into a C57BL/6 background for at least 8 generations. Experiments were performed in accordance with the Canadian Council of Animal Care. Generation of PI3K–/–, PI3K +/–, and PI3KDN mice were as described previously.7,8 As reported previously, both PI3K–/– and PI3KDN have normal body weight, whereas PI3KDN display smaller heart weights attributable to altered myocardial growth (Table I in the online data supplement, available at http://circres.ahajournals.org).7,8
Isolated Mouse Heart Perfusion
Measurement of left ventricular developed pressure (LVDP) from isolated perfused mouse hearts subjected to ischemia/reperfusion alone or IPC was performed as described in the expanded Materials and Methods section in the online data supplement.
Determination of Tissue Viability
Measurement and comparison of cell viability from the WT and PI3K transgenic hearts during I-R or IPC were assessed by lactate dehydrogenase (LDH) release measured from the coronary venous effluent. All samples were stored at –20°C until analysis. LDH concentration was determined by an enzymatic assay kit (Sigma, St Louis, Mo).
Immunoblotting
Changes in total and phosphorylated protein levels were determined by Western blot analysis (see the online data supplement). Protein concentration was determined using the Bradford method.
Drugs
Adenosine (50 µmol/L) and LiCl (3 mmol/L) (Sigma) were infused for 15 minutes and 30 minutes, respectively, in WT and PI3K–/– hearts subjected to 30 minutes of ischemia and 40 minutes of reperfusion. Wortmannin (200 nmol/L; Sigma) was infused for 30 minutes in PI3KDN hearts before 30 minutes of sustained ischemia. All drug stock solutions were made initially in water and then dissolved in Krebs–Henseleit solution to the desired final concentration.
Statistical Analysis
Statistical significance was determined using an unpaired Student t test or an ANOVA, followed by a post hoc Student–Newman–Keuls test. Probability values of <0.05 were considered statistically significant. Data are shown as means±SEM.
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Results |
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PI3K
Mediates IPCDeveloped pressure was measured in isolated perfused WT and age-matched heterozygous PI3K
+/– and homozygous PI3K–/– hearts subjected to either 30 minutes of ischemia followed by 40 minutes of reperfusion (I-R), or the well-established IPC protocol of 4 cycles of 5 minutes of I-R before the prolonged 30 minutes of ischemia and 40 minutes of reperfusion.11–13 Representative LVDP recordings from WT and PI3K–/– hearts during I-R and IPC are shown in Figure 1. In WT hearts, I-R was associated with elevated diastolic pressure concomitant with compromised systolic pressure, which was prevented by IPC (Figure 1A). Similar findings were observed in PI3K+/– hearts (data not shown). In contrast, IPC afforded no protection in PI3K–/– hearts. Moreover, we observed no further decrease in functional recovery in PI3K–/– hearts following I-R compared to WT hearts (Figure 1B).
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The temporal and quantitative analysis of the LVDP of both WT and PI3K+/– hearts shows a precipitous reduction in functional recovery during reperfusion, which was mitigated by IPC (Figure 2A and 2B). PI3K–/– hearts remained severely depressed in both I-R and IPC (Figure 2C), confirming a lack of IPC protection. Similar results were seen when adjusting for percent recovery of LVDP at 40 minutes of reperfusion (Figure 2D). To explore this genotype-specific difference in IPC, we measured LDH release as a marker of cell injury. It has been demonstrated that there is a strong positive correlation with LDH release and the degree of infarct size in isolated perfused mouse hearts in response to global ischemia and reperfusion.14 There was a significant reduction in LDH release during the 40-minute reperfusion period in WT hearts subjected to IPC (234±68 U/mL per minute per gram of heart weight, n=6) compared to hearts exposed to I-R alone (1165±134 U/mL per minute per gram of heart weight; n=6) (P<0.05) (Figure 2E). A similar trend was observed in PI3K+/– hearts. In contrast, PI3K–/– displayed elevated LDH release in both I-R (918±89 U/mL per minute per gram of heart weight, n=6; P<0.05 compared to WT I-R group) and IPC groups (1486±137 U/mL per minute per gram of heart weight, n=6; P<0.05 compared to PI3K–/– I-R group). As such, the enhanced tissue viability closely mimicked the improved functional recovery afforded by IPC. The lack of difference in IPC between the WT and heterozygous PI3K+/– hearts confirms that a complete loss of PI3K signaling is required to compromise myocardial IPC protection. These results provide convincing evidence that PI3K is a critical mediator of myocardial IPC.
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Cell-Signaling Pathways in IPC
The parallel survival kinase pathways, PI3K-Akt and mitogen-activated protein kinase–extracellular signal-regulated kinase 1/2 (MAPK-ERK1/2), have been proposed to mediate cell protection following lethal ischemia.3,15 We hypothesized that the PI3K-Akt pathway would be selectively diminished in the PI3K–/– hearts, leaving the MAPK pathways intact. Therefore, we examined the downstream signaling molecules of the PI3K and MAPK pathways to determine their differential activation in response to IPC in WT and PI3K–/– hearts using Western blot analysis. Immunoblotting was performed on hearts collected at the end of the reperfusion phase. PDK1 (3'-phosphoinositide-dependent kinase-1) is activated downstream of PI3K and is the upstream regulator of Akt16,17 and p70 ribosomal S6 kinase (p70S6K).18 Phosphorylated levels of PDK1 (Ser241) were significantly depressed following I-R in WT hearts, while being moderately but significantly restored when hearts were subjected first to IPC before I-R (Figure 3A). Akt possesses 2 key phosphorylated residues (Thr308 and Ser473), which are required to induce maximal activity.16 Consistent with our hypothesis denoting the importance of downstream Akt, we observed restoration phosphorylation of Akt at Ser473 and Thr308 in response to IPC relative to I-R in WT hearts (Figure 3B and 3C). In PI3K–/– hearts, phospho-PDK1 levels were diminished following I-R and remained reduced in the IPC group (Figure 3A). A marked effect was the dramatic reduction in phosphorylated Akt (Ser473) levels, but not Akt (Thr308), in the normoxic PI3K–/– hearts, which remained low following both I-R and IPC (Figure 3B).
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P<0.05 compared to the I-R group.
Akt can phosphorylate the downstream target GSK3β19 a key player in IPC protection,12,20 leading to its inhibition. Similar to our observations with Akt, IPC maintained the phosphorylation level of GSK3β (Ser9) compared with WT hearts subjected to 2 hours of normoxic perfusion, whereas I-R decreased the phosphorylation level of this kinase (Figure 3D). Remarkably, there was a dramatic reduction in the phosphorylated levels of GSK3β in PI3K–/– hearts following both I-R and IPC. Ribosomal p70S6K is another downstream target of PDK121,22 and Akt.23 We observed differential phosphorylation of p70S6K at Thr389 and Ser421/Ser424 in WT after I-R and IPC but not in PI3K–/– hearts (Figure 3E and 3F).
Consistent with a selective loss of the PI3K-Akt-GSK3β signaling pathways, there was a uniform elevation of the phosphorylation levels of the MAPK proteins ERK1/2 and p38 in both WT and PI3K–/– hearts exposed to I-R and IPC (supplemental Figure I). Together, these results suggest that the loss of IPC protection in PI3K–/– hearts is attributable to a relative lack of Akt and GSK3β phosphorylation, and therefore alterations in their activities.
PI3K in Pharmacological Preconditioning
The central role of PI3K in mediating G protein–coupled receptor (GPCR) signaling4,5 and the inability of the PI3K–/– hearts to undergo IPC protection suggest GPCR agonist signaling may be compromised. To examine this, we perfused WT and PI3K–/– hearts with adenosine (50 µmol/L), a well-established GPCR-mediated trigger of pharmacological preconditioning (PPC),1 before ischemia. Adenosine markedly increased the levels of phosphorylated GSK3β in WT hearts but not PI3K–/– hearts (Figure 4A), confirming that upstream PI3K is important in GSK3β signaling. LVDP recovery of WT hearts perfused with adenosine was significantly higher than the control group at 40 minutes of reperfusion (adenosine infusion: 74±4 mm Hg, n=6; control: 28±2 mm Hg, n=11; P<0.05) (Figure 4B). This protection by adenosine was completely abolished in the PI3K–/– hearts (adenosine: 39±3%, n=6; control: 36±4 mm Hg, n=12) (Figure 4B).
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Recent evidence shows that GSK3β is a key regulator of myocardial IPC, and GSK3β inhibition reduces I-R injury.12,20 We hypothesized that the well-known GSK3β inhibitor LiCl12,20 will enhance GSK3β phosphorylation (ie, inhibit GSK3β) and improve myocardial protection against I-R in PI3K–/– hearts. Lithium induces the N-terminal autophosphorylation of GSK3β.24 Indeed, LiCl (3 mmol/L) treatment before ischemia induced a significantly higher phosphorylation level of GSK3β in WT and PI3K–/– hearts in the absence of changes to the phosphorylation levels of Akt (Ser473) (Figure 5A and 5B). This was associated with greater LVDP recovery in both WT and PI3K–/– hearts following I-R (Figure 5C). These results suggest that the lack of phosphorylation of GSK3β was responsible for enhanced susceptibility of PI3K hearts to I-R injury.
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Resistance of PI3DN Hearts to I-R Injury
Based on our above results and the important role of GPCR in IPC protection, we hypothesized that the tyrosine kinase receptor–PI3K signaling pathway would play less of a role in IPC protection compared to PI3K. In PI3KDN hearts subjected to I-R, we surprisingly observed significantly greater functional recovery (Figure 6A and 6B) and markedly lower LDH release (improved cell viability; 268±20 U/mL per minute per gram of heart weight, I-R; 243±54 U/mL per minute per gram of heart weight, IPC; n=6) compared to WT hearts (Figure 6C). This benefit was not dependent on the duration of ischemia because extending the length of ischemia to 60 minutes still led to better protection in PI3KDN compared to WT hearts (Figure 6D). To investigate this phenomenon further, we examined the cell-signaling pathways known to be well-established mediators of IPC protection, as illustrated in Figure 3. We observed maintained phosphorylation levels of Akt (Ser473) (Figure 7A) and GSK3β (Ser9) (Figure 7B) following I-R in PI3KDN hearts compared to WT hearts. In PI3KDN hearts, we did not observe any corresponding maintenance of phosphorylated Akt (Thr308), p70S6K, ERK1/2, or p38 MAPK following I-R compared to WT hearts (data not shown). Moreover, the PI3KDN hearts did not show added improved functional recovery or higher phosphorylation levels of Akt (Ser473) and GSK3β (Ser9) compared to WT hearts following IPC (Figures 6B and 7
). Indeed, phosphorylation levels of these 2 kinases were comparable in PI3KDN hearts subjected to I-R and IPC. This suggests that PI3KDN hearts are in a "preconditioned" protected state.
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Both class IA (PI3K) and class IB (PI3K) PI3Ks are known to instigate Akt and GSK3β phosphorylation, as well as other downstream signaling molecules.4,5 Given the maintained phosphorylation of Akt (Ser473) and GSKβ (Ser9) in PI3KDN hearts, we hypothesized that there is enhanced signaling through the PI3K pathway in hearts in response to I-R. To test this hypothesis, we examined the protein levels of the catalytic subunit of PI3K, p110, following 2 hours of normoxic perfusion, I-R, and IPC. Although there were equivalent levels of p110 under normoxic conditions in WT and PI3KDN hearts (absent in PI3K–/– hearts), there was a drastic reduction in p110 only in WT hearts following I-R (Figure 8A). In contrast, with IPC there was less reduction of p110 protein in WT or PI3KDN hearts, suggesting that a fundamental mechanism of myocardial I-R injury is the selective degradation of p110 and loss of its protective downstream signaling mechanism. No changes in protein levels of p110, p110β, or the class IA regulatory subunit p85 were observed following I-R or IPC (data not shown). To test whether the maintained p110 expression is responsible for the resistance of PI3KDN hearts to I-R, we pretreated PI3KDN hearts with the PI3K inhibitor wortmannin (100 nmol/L) before ischemia. Remarkably, pharmacological inhibition of the residual PI3K activity resulted in a complete loss of protection in PI3KDN hearts observed following I-R (Figure 8B). Indeed, these hearts became phenotypically identical to WT hearts undergoing I-R injury. These results suggest that selective signaling through PI3K is responsible for the relative protection of the PI3KDN hearts to I-R injury, although the contribution to the other PI3K isoforms cannot be discounted (see below). Collectively, enhanced signaling through PI3K-Akt-GSK3β is a critical mediator of IPC and adenosine-mediated protection following I-R.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionSummaryReferences |
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Since the first discovery of myocardial IPC by Murry et al in 1986,2 this novel protective mechanism against ischemia/reperfusion injury has resulted in considerable interest. Fundamental questions still remain regarding the factors that are responsible for stimulating myocardial survival following lethal ischemia, and the molecular and cellular mechanisms for this protection. In addition, the direct clinical relevance has been highlighted.1,25 A common experimental approach has been to study the myriad of cell-signaling pathways that are associated with this cell survival mechanism.3,15 We demonstrate in the present study the critical role of PI3K
and the downstream signaling enzymes Akt and GSK3β in mediating IPC protection. More surprisingly is our observation that suppression of PI3K activity in the heart confers enhanced resistance to I-R injury because of augmented signaling through the PI3K-Akt-GSK3β pathway. Our results collectively provide definitive insight into the important role of PI3K in IPC.
Protective Effects of IPC Is Dependent on PI3K
The essential role of PI3K activation in IPC and cardioprotection is becoming more apparent. IPC protection is abolished by the PI3K inhibitors wortmannin and LY294002.10,11,13 Activation of PI3K during IPC results in phosphorylation of Akt and GSK3β, which are abolished indirectly by inhibiting upstream PI3K.10,12,13 The wealth of biochemical, physiological, and pathological data on the role of PI3K in IPC has been primarily derived using these pharmacological PI3K inhibitors. Both wortmannin and LY294002 are highly selectively for PI3K26; however, neither agent is isoform-selective. Moreover, LY294002 has effects independent of PI3K activity, specifically involving the direct blockade of voltage-gated K+ channels in the heart and pancreatic β cells.27,28 Despite the central and distinct roles of the PI3K isoforms in initiating receptor–mediated signaling,4,5 the precise physiological and pathological roles of the distinct PI3K isoforms IA (PI3KDN) and class IB (PI3K–/–) PI3K provides a unique tool to explore the role PI3K in myocardial I-R and IPC.
In the present study, we have reported on the distinct roles of PI3K and PI3K isoforms in I-R and IPC. The beneficial effects of IPC were completely abolished in PI3K–/– hearts, which have maintained p110, p110β, and p85 expression commensurate to WT levels.8 We suggest that these effects are attributable to the marked decrease in Akt and GSK3β signaling. Interestingly, we observed differences in the phosphorylation of Akt at Ser473 but not Thr308. PDK1 phosphorylates Akt at Thr308,16,17 but the exact kinase responsible for phosphorylating Ser473 remains controversial. Phosphorylation of both sites is deemed necessary for full Akt activity.16 It is not clear why the phosphorylation status at Ser473 was markedly affected in the PI3K–/– and PI3KDN hearts, but this suggests that the underlying mechanism responsible for phosphorylating this site on Akt may be differentially regulated during I-R. Our experimental results are consistent with previous findings by Murphy and colleagues, who demonstrated a loss of IPC protection in transgenic mouse hearts overexpressing kinase-dead PI3K lacking the ATP binding site.29 We suggest that PI3K confers IPC protection through the activation of Akt and subsequent inhibition of GSK3β activity. Acute overexpression of constitutively active Akt reduces myocyte apoptosis and improves contractile function in vitro30 and in vivo31 following hypoxia or ischemia. Our proposed mechanism is supported further by the PI3K hearts, which appear to be in a preconditioned state because the phosphorylated levels of both Akt and GSK3β remained similar following I-R compared to normoxia and IPC. We suggest this latter effect is attributable to the persistent levels of PI3K protein (and activity) in PI3KDN hearts subjected to I-R (Figure 8A). However, we cannot rule out the alternative possibility that the class IA PI3Kβ is involved in attenuating myocardial injury in the PI3KDN hearts following I-R. Interestingly, recent work has shown PI3Kβ signaling is coupled to GPCR and not tyrosine kinase receptors as is PI3K and .32,33 In macrophages and fibroblasts, PI3Kβ signaling is redundantly coupled to the same GPCR as PI3K. It is not yet known whether similar GPCR coupling to PI3Kβ occurs in the heart. However, our observed the lack of IPC protection in PI3K–/– hearts and the absence of any functional benefit following adenosine pretreatment suggests there may be a limited role for GPCR-PI3Kβ signaling in the heart during I-R or IPC. However, the role of PI3Kβ in IPC requires further investigation. Lastly, the mechanism by which suppressing PI3K activity in the heart can maintain the protein levels of PI3K following ischemic injury is unknown and remains to be determined.
We have established the importance of PI3K-dependent signaling in adenosine-mediated PPC. Adenosine enhanced postischemic recovery in WT and elevated phospho-GSK3β levels, effects that were absent in PI3K–/– hearts. Our results are consistent with previous work by Downey and colleagues showing adenosine preconditions the heart by activating PI3K34 and Akt.35
Downstream Effectors of the PI3K-Akt-GSK3β Signaling Cascade
Both Akt activation and GSK3β inhibition have been reported to enhance cell survival via PI3K activation.12,20 It has yet to be established how enhanced signaling through the Akt-GSK3β pathway confers protection to the ischemic heart. Juhaszova et al have hypothesized that IPC and PPC protection are mediated by the convergence of multiple signaling pathways onto GSK3β.20 Activation of the PI3K-Akt, mTOR-p70S6K, MAPK, phospholipase C-PKC, and PKA pathways all converge to GSK3β leading to its phosphorylation, and the subsequent inhibition of the end-effector, the mitochondrial permeability transition pore (MPTP). Pharmacological inhibition of GSK3β with LiCl, SB 216763, or SB 415286 mimicked the cellular protection produced by IPC or PPC. These effects were observed when only GSK3β, but not GSK3, expression was suppressed.
The MPTP is a large conductance nonspecific pore found on the inner mitochondrial membrane and can allow for the flux of molecules up to 1.5 kDa.36,37 Cyclosporin A and sanglifehrin A reduce ischemic injury by preventing MPTP activation.36 In contrast, atractyloside, an MPTP opener, abolishes IPC protection.38 Recently, phosphorylated GSK3β has been shown to bind directly to the adenine nucleotide translocase, suggested to be a component of the MPTP, resulting in a decrease in MPTP opening.39 Therefore, elevated levels of phospho-GSK3β may impair MPTP opening and account for the improved recovery observed in WT hearts subjected to IPC and PI3KDN hearts following I-R and for the lack of IPC protection in PI3K–/– hearts, which have significantly lower phospho-GSK3β levels.
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Summary |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionSummaryReferences |
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Our present results on the isoform specificity of PI3K in IPC strongly suggest the PI3K
isoform is critical for IPC-induced heart protection because genetic ablation of this enzyme in the heart eliminates IPC and adenosine-mediated preconditioning. In support of a pivotal role for PI3K in IPC, genetic suppression of PI3K activity induces resistance against prolonged ischemia through maintained signaling via the PI3K pathway. Finally, activation of the Akt-GSK3β signaling cascade by PI3K is important in mediating the cardioprotection of IPC and adenosine. The development of selective PI3K inhibitors have shown benefit in the treatment of inflammatory disease,40,41 and, as such, the use of these novel PI3K inhibitors may abrogate the benefits of IPC or some forms of PPC protection if PI3K activity is drastically reduced in the heart.
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Acknowledgments |
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Sources of Funding
This work was supported by grants from the Heart and Stroke Foundation of Ontario and a Premier’s Research Excellence Award (to R.G.T.). A.B. was supported by a John D. Schulz Science Summer Scholarship from the Heart and Stroke Foundation of Ontario. J.M.P. is supported by European Union grant EuGeneHeart. P.H.B. is Career Investigator with the Heart and Stroke Foundation of Ontario.
Disclosures
None.
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Footnotes |
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Original received March 2, 2008; revision received July 24, 2008; accepted July 24, 2008.
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References |
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