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Am J Physiol Heart Circ Physiol 282: H1970-H1977, 2002. First published February 14, 2002; doi:10.1152/ajpheart.01029.2001
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Vol. 282, Issue 6, H1970-H1977, June 2002

Cardioprotection mediated by sphingosine-1-phosphate and ganglioside GM-1 in wild-type and PKCepsilon knockout mouse hearts

Zhu-Qiu Jin1, Hui-Zhong Zhou1, Peili Zhu1, Norman Honbo1, Daria Mochly-Rosen2, Robert O. Messing3, Edward J. Goetzl4, Joel S. Karliner1, and Mary O. Gray1

1 Cardiology Section, Veterans Affairs Medical Center, San Francisco, 94121; 2 Department of Molecular Pharmacology, Stanford University, Stanford 94305; 3 Ernest Gallo Clinic and Research Center, University of California, San Francisco 94608; and 4 Immunology Division, Department of Medicine, University of California, San Francisco, San Francisco, California 94143


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Sphingosine-1-phosphate (S1P) protects neonatal rat cardiac myocytes from hypoxic damage through unknown signaling pathways. We tested the hypothesis that S1P-induced cardioprotection requires activation by the epsilon -isoform of protein kinase C (PKCepsilon ) by subjecting hearts isolated from PKCepsilon knockout mice and wild-type mice to 20 min of global ischemia and 30 min of reperfusion. Pretreatment with a 2-min infusion of 10 nM S1P improved recovery of left ventricular developed pressure (LVDP) in both wild-type and PKCepsilon knockout hearts and reduced the rise in LV end-diastolic pressure (LVEDP) and creatine kinase (CK) release. Pretreatment for 2 min with 10 nM of the ganglioside GM-1 also improved recovery of LVDP and suppressed CK release in wild-type hearts but not in PKCepsilon knockout hearts. Importantly, GM-1 but not S1P, increased the proportion of PKCepsilon localized to particulate fractions. Our results suggest that GM-1, which enhances endogenous S1P production, reduces cardiac injury through PKCepsilon -dependent intracellular pathways. In contrast, extracellular S1P induces equivalent cardioprotection through PKCepsilon -independent signaling pathways.

ischemia-reperfusion injury; epsilon -isoform of protein kinase C


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

SPHINGOSINE-1-PHOSPHATE (S1P) is a lysophospholipid growth factor and mediator of diverse cellular functions, which transmits signals to cells by autocrine and paracrine mechanisms (24). It is generated from sphingomyelin and other phospholipid precursors that are stored in membranes and is secreted by cardiac myocytes, platelets, macrophages, and epithelial cells to produce up to micromolar concentrations in normal serum (24). S1P binds to G protein-coupled cell surface receptors formerly termed endothelial differentiation gene (EDG) receptors (12), now called S1P receptors. The high level of expression of the EDG 1 (S1P1), EDG 3 (S1P3), and EDG 5 (S1P2) receptor mRNAs in mouse heart (31) and the essential role of the S1P1 receptor in heart development (17) raise the possibility that this lipid mediator could alter cardiac function under some physiological and/or pathological conditions, such as ischemia. S1P is well recognized as a survival factor in a number of native cell types and cell lines (7). Gangliosides are a class of sialic acid-containing glycosphingolipids associated with the plasma membrane, and the ganglioside GM-1, which stimulates generation of S1P, also acts as a survival factor (4). In a recent study, Karliner et al. (14) showed in cultured neonatal rat cardiac myocytes that pretreatment with S1P or GM-1 reduced hypoxic cell death. This cardioprotection was abolished by the putative nonselective protein kinase C (PKC) inhibitor chelerythyrine (14).

There is abundant evidence that activation of the epsilon -isoform of PKC (PKCepsilon ) is required for cardioprotection induced by brief bouts of ischemia or hypoxia or by pharmacological agents. PKCepsilon is activated by peconditioning (23, 25), and a selective peptide inhibitor of PKCepsilon blocks this cardioprotection (11). Furthermore, expression of a selective peptide activator of PKCepsilon in the heart of transgenic mice increases cardioprotection (8). Because Karliner et al. (14) found that S1P provides protection from hypoxic damage in culture, we set out to determine whether such protection can be induced in the isolated beating heart. We also wanted to learn whether PKCepsilon mediates these cardioprotective effects, and to this end we used PKCepsilon knockout mice.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals. Mutant mice lacking PKCepsilon were originally derived using homologous recombination in embryonic stem cells with a neomycin gene insert to disrupt the first coding exon of the PKCepsilon gene (15). The resulting chimeric mice were crossed with C57B16/J mice to test for germ line transmission of the mutation. Chimeras were then crossed with 129SvJae mice to establish an inbred 129SvJae line harboring the null mutation. Male PKCepsilon (+/-) heterozygous 129SvJae mice were crossed with C57BL/6J female mice to yield F1 generation hybrid C57B1/6J x129SvJae heterozygous progeny, which were intercrossed to generate F2 generation wild-type and PKCepsilon null male mice for the study. Homozygous male knockout mice bred from these animals are of normal body weight and lifespan compared with wild-type mice and cannot be distinguished from littermates in the normal cage environment. Genotyping using PCR to confirm the absence of PKCepsilon DNA was routinely performed on tail samples. Only male wild-type mice and PKCepsilon knockout mice were used in the present study.

Mice were fed standard rodent chow and water ad libitum. They were housed in a quiet quarantine room for at least 3 days before each experiment. All studies were performed in accordance with the guidelines of the Animal Care Subcommittee of the San Francisco Department of Veterans Affairs Medical Center and with the Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23, Revised 1996).

Langendorff isolated perfused heart preparation. Mice were heparinized (500 U/kg ip) and anesthetized with pentobarbital sodium (60 mg/kg ip). Hearts were rapidly excised, washed in ice-cold arresting solution (120 mmol/l NaCl, 30 mmol/l KCl), and cannulated via the aorta on a 20-gauge stainless steel blunt needle. Hearts were perfused at 70 mmHg on a modified Langendorff apparatus using Krebs-Henseleit solution containing (in mmol/l) 118 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2 MgSO4, 1.2 KH2PO4, 24 NaHCO3, 5.5 glucose, 5.0 Na pyruvate, 0.5 EDTA, and bubbled with 95% O2-5% CO2 at 37°C. Platinum electrodes connected to a Grass Instrument stimulus generator were used to pace hearts at 360 beats/min.

Ischemia-reperfusion experimental protocol. Hearts were perfused continuously with oxygenated Krebs-Henseleit solution for 70 min to establish the stability of the preparation. Subsequently, separate ischemia-reperfusion studies were carried out as follows: baseline left ventricular (LV) developed pressure (LVDP), LV end-diastolic pressure (LVEDP), and coronary flow (CF) were measured after a 20-min equilibration period. All hearts were then treated with vehicle or agonist for 2 min, which was then washed out for 5 min. The hearts were then subjected to 20 min of global ischemia, which was achieved by turning a stopcock just proximal to the aortic cannula to completely halt coronary perfusion. During global ischemia, pacing was temporarily stopped, and cardiac temperature was maintained at 37°C in a humidified chamber. All hearts were then reperfused with Krebs-Henseleit solution for 30 min. Hemodynamics were recorded continuously and measured every 5 min during reperfusion.

Measurement of LV performance. LVDP (LVDP = LV systolic pressure - LVEDP) was measured using a 1.4-Fr micromanometer (Millar Instruments) passed into a polyvinylchloride film balloon filled with water to set the LVEDP at <10 mmHg. The balloon was inserted through the left atrium into the left ventricle, and pressures were recorded continuously on a Gould TA 240 chart recorder. CF was measured by collecting effluent from the coronary sinus.

Measurement of creatine kinase release. Coronary effluent was collected every 5 min during the reperfusion period. Creatine kinase (CK) release was measured using a commercially available kit (Sigma). Values were corrected for CF rate and wet heart weight.

Tissue sample preparation for Western blot. After 20 min of stabilization, isolated mouse hearts were perfused with 10 nM GM-1 or S1P for 2 min, followed by a 5-min washout period. These pretreated hearts were immediately put into liquid nitrogen and stored at -80°C. Frozen myocardial ventricular tissue samples were minced. Total cellular proteins were prepared by homogenization of the minced tissue (Brinkman Polytron PT 3000) in sample buffer containing 10 mM Tris · HCl (pH 7.4), 1 mM EDTA, 1 mM EGTA, 0.25 M sucrose, 25 µg/ml aprotinin, 25 µg/ml leupeptin, 20 µg/ml soybean trypsin inhibitor, and 17 µg/ml phenylmethylsulfonyl fluoride. The cytosolic and particulate portions of total cellular proteins were separated by a 30-min centrifugation at 47,000 g (25). After dilution, protein concentration was determined by bovine serum albumin as a standard according to the method of Bradford (Bio-Rad Laboratories; Hercules, CA). The yields of total cellular proteins, cytosolic proteins, and particulate proteins were recorded for each tissue sample tested.

PKC Western blot. Assessment of PKC isozymes was conducted using standard SDS-PAGE Western blot techniques. Briefly, 50 µg of protein derived from the cytosolic fraction or the particulate fraction of the homogenate was electrophoresed on a 7.5% denaturing gel at 30 mA per lane for 1-2 h. Proteins were electrotransferred onto a nitrocellulose membrane (Bio-Rad) at 200 mA for 2 h. Adequate background blocking was accomplished by incubating the nitrocellulose membrane with 5% nonfat dry milk in phosphate buffer solution (pH 7.4). Antibodies against PKC isozymes alpha , epsilon , and delta  (Transduction Laboratories; Lexington, KY) were used to measure the expression of individual PKC isozymes. PKC immunoreactive bands were quantitated by densitometric analysis of digitized autoradiograms with NIH Image 1.61 software.

Infarct size determination with triphenyltetrazolium chloride staining. After 20 min of global ischemia and 30 min of reperfusion, a subset of hearts in each group was infused with 15 ml of 1% triphenyltetrazolium chloride (Sigma) in phosphate-buffered saline at a rate of 1.5 ml/min. Hearts were then removed from the cannula, weighed, and fixed overnight in 10% formalin, after which they were removed from formalin and stored frozen at -20°C until sectioning for analysis of LV infarct size. Hearts were sliced into 2-mm transverse sections from apex to base and digitally photographed on each side (Camedia E-10, Olympus Camera). Computerized area analysis was performed with National Institutes of Health Image software. The infarct size of each section was expressed as a fraction of the area at risk defined as the total area of the left ventricle in this global ischemia model.

Statistical analysis. Results are reported as means ± SE. Comparisons between groups were made using repeated- measures or one-way analysis of variance, followed by post hoc testing (Newman-Keuls). P < 0.05 was considered statistically significant.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Baseline functional measurements in wild-type hearts. There were no significant baseline differences in hemodynamic measurements among hearts from wild-type or PKCepsilon knockout mice (Table 1). After pretreatment before ischemia-reperfusion with either exogenous S1P or GM-1, which elicits generation of endogenous S1P (4), these values remained unchanged among groups and in comparison with baseline values (Table 2).

                              
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Table 1.   Baseline functional parameters before pretreatment with S1P and ganglioside GM-1


                              
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Table 2.   Cardiac functional parameters change after pretreatment with S1P and ganglioside GM-1

S1P and GM-1 reduce myocardial reperfusion injury. Hearts were suspended in the Langendorff apparatus and treated with a 2-min infusion of either 10 nmol/l S1P or 10 nmol/l GM-1 at the end of a 20-min equilibration period as described under methods. Hearts were then subjected to 20 min of global ischemia and 30 min of reperfusion. As can be seen in Fig. 1, both S1P and GM-1 markedly improved LV systolic function throughout reperfusion as measured by LVDP and maximal rate of LV pressure development over time (+dP/dtmax). Cardiac diastolic function (LVDP and -dP/dtmax) were also improved after S1P and GM-1 pretreatment (Fig. 2). CF exceeded control values in both the S1P group and the GM-1 group (Fig. 3). These interventions also reduced myocardial injury as measured by decreases in CK release (Fig. 4). As shown in Fig. 5, infarct size was also reduced in both the S1P- and GM-1-pretreated groups compared with control.


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Fig. 1.   Effect of ganglioside GM-1 (GM-1) or sphingosine-1-phosphate (S1P) pretreatment on cardiac systolic function of isolated wild-type mouse hearts during reperfusion after 20 min of ischemia. A: left ventricular developed pressure (LVDP). B: maximal positive rate of pressure development (+dP/dtmax). *P < 0.05, compared with control group. n = 5-6 hearts for each group.



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Fig. 2.   Effect of GM-1 or S1P pretreatment on cardiac diastolic function of isolated wild-type mouse hearts during reperfusion after 20-min of ischemia. A: LV end-diastolic pressure (LVEDP). B: maximum negative rate of pressure development (-dP/dtmax). *P < 0.05, compared with control group. n = 5-6 hearts for each group.



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Fig. 3.   Effect of GM-1 or S1P pretreatment on coronary flow (CF) in wild-type hearts at the end of 20 min ischemia and 30 min reperfusion. *P < 0.05, compared with control group. n = 8-9 hearts for each group.



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Fig. 4.   Effect of GM-1 or S1P pretreatment on release of CK from isolated wild-type mouse hearts subjected to 20 min ischemia and 30 min reperfusion. *P < 0.05, compared with control group. n = 8-9 hearts each group.



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Fig. 5.   Infarct size determination in GM-1 or S1P wild-type pretreatment groups. open circle , Individual mice; , means ± SE for respective group. *P < 0.05, compared with control group. n = 4 hearts for each group.

Effects of S1P and GM-1 on PKCepsilon translocation in wild-type mouse hearts. Because the nonselective PKC inhibitor chelerythrine abolished S1P and GM-1 cardioprotection in cultured neonatal rat cardiac myocytes (14), we wondered whether the ability to induce cardioprotection was isozyme specific. Hearts from wild-type mice were treated with 10 nmol/l S1P or 10 nmol/l GM-1 or vehicle for 2 min in the Langendorff apparatus and then homogenized. Western blot analysis was then performed as described in METHODS. As can be seen in Fig. 6, GM-1, but not S1P, caused translocation of PKCepsilon , but not PKCalpha or PKCdelta , in wild-type mouse hearts. This observation suggested that GM-1 requires PKCepsilon translocation for cardioprotection, but that S1P might not act through PKCepsilon . To test this hypothesis, we examined the cardioprotective effect of S1P and GM-1 in mice that lack PKCepsilon .


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Fig. 6.   Translocation of protein kinase C (PKC) isozymes in wild-type hearts with GM-1 or S1P pretreatment. A: PKCepsilon ; B: PKCalpha ; C: PKCdelta . Top: representative immunoblots of particulate (P) and cytosolic (C) fractions. Bar graphs show the proportion of the particulate fraction compared with the total amount of isozyme. STD, standard. *P < 0.05, compared with control group. n = 3-4 hearts for each group.

Do S1P and GM-1 reduce myocardial ischemic injury in PKCepsilon knockout mice? At baseline and immediately after infusion of S1P and GM-1, there were no differences in hemodynamic measurements and CF between PKCepsilon knockout mice and wild-type mice (Tables 1 and 2). We reasoned that pretreatment with GM-1 would not be cardioprotective in the PKCepsilon knockout mouse if translocation of PKCepsilon is an absolute requirement for reducing ischemia-reperfusion injury. As shown in Fig. 7, GM-1 did not protect ischemic myocardium in PKCepsilon knockout mice as evidenced by the absence of improvement in LVDP and CK release during reperfusion. In contrast, S1P, which did not induce PKCepsilon translocation (Fig. 6), reduced ischemia-reperfusion injury. LVDP was improved and CK release was reduced in the S1P group (Figs. 7 and 8), consistent with an PKCepsilon -independent mechanism of action.


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Fig. 7.   Effect of GM-1 or S1P pretreatment on cardiac systolic function and diastolic function of isolated PKCepsilon knockout (KO) mouse hearts during reperfusion after 20 min of ischemia. A: LVDP. B: LVEDP. *P < 0.05, compared with control group. n = 4-6 hearts for each group.



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Fig. 8.   Effect of GM-1 or S1P pretreatment CK release after ischemia-reperfusion injury in isolated PKCepsilon KO mouse hearts. CK was assayed at the end of the 30-min reperfusion period. *P < 0.05, compared with control group. n = 4-6 hearts for each group.

It should be noted that the absolute values of LVEDP were not significantly different in the hearts treated with GM-1 compared with the hearts treated with S1P (Fig. 7, bottom). However, when the relative change is taken into account, the values are in fact significantly different, because as shown in Table 2, the hearts treated with GM-1 had an initial mean LVEDP of 4.5 ± 0.9 mmHg, whereas the hearts treated with S1P had a higher value of 7.2 ± 0.4 mmHg. Although these values were not significantly different at baseline before ischemia-reperfusion, the GM-1 hearts had values at the end of reperfusion of 16 ± 3.2 mmHg compared with S1P-treated hearts, which averaged 8.5 ± 1.0 mmHg. This represents a 4.1 ± 1.0-fold increase for the GM-1 hearts compared with only a 1.1 ± 0.2-fold increase for the S1P-treated hearts, a difference that is significant (P < 0.05).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The major finding of this study is that both S1P and GM-1 protect the isolated buffer-perfused mouse heart against ischemia-reperfusion injury. These observations are the first to show in any whole organ that S1P reduces damage due to ischemia-reperfusion. Using PKCepsilon knockout mice, we also found that S1P given exogenously does not require PKCepsilon for its cardioprotective action. In contrast, the ganglioside GM-1, which increases intracellular S1P levels by stimulating sphingosine kinase activity (4), has an absolute requirement for PKCepsilon . This PKCepsilon dependence may be for S1P production or intracellular signaling, or both. These findings are consistent with experiments showing that GM-1, but not S1P, translocates PKCepsilon but not PKCalpha or PKCdelta , which indicates PKCepsilon engagement in intracellular S1P generation and/or signaling.

S1P and protection from cell death. All prior studies of S1P as a survival factor have relied on in vitro data in cell lines and have focused on apoptosis as the sole model of cell death. From such studies, Spiegel and colleagues developed a model describing an intracellular ceramide-S1P "rheostat" (7, 16). This model is founded on observations that increases in the concentration of the proapoptotic molecule ceramide can be countered by increases in the intracellular levels of S1P (7). Thus inhibition of sphingosine kinase (the final step in S1P synthesis) induces cell death, whereas activation of sphingosine kinase by PKC increases intracellular S1P and enhances cell survival (7, 16). Overexpression of sphingosine kinase in cell lines also protects against apoptosis induced by serum deprivation or ceramide elevation (22). In addition, the ganglioside GM-1 stimulates synthesis of S1P by activating sphingosine kinase and protects cultured rat heart fibroblasts from ceramide-induced apoptosis (4). In cultured neonatal rat ventricular myocytes, Karliner et al. (14) have previously shown that both S1P and GM-1 reduce hypoxic cell death and prevent cell death due to inhibition of sphingosine kinase under normoxic conditions.

There is also evidence that S1P acts extracellularly to enhance cell survival via activation of EDG (S1P) receptors, particularly S1P3 and S1P2 (1). Goetzl and colleagues (9) demonstrated that exogenous S1P inhibits ceramide-induced apoptosis via the S1P3 receptor in human lymphoblastoma cells by suppressing cellular levels of the apoptosis-promoting protein Bax without alteration in levels of Bcl-xL or Bcl-2. Transfection of plasmids encoding transcripts antisense to the S1P3 receptor inhibited S1P-induced reduction of Bax expression and protection against apoptosis. Recently, An et al. (1) reported that S1P3 and S1P2 receptors mediate protection against apoptosis induced by serum starvation in HTC4 hepatoma cells.

In prior work, Goetzl et al. (10) have shown that cultured cardiac myocytes express S1P3 and S1P2 receptors. From the cardioprotective action of S1P and GM-1 in cultured neonatal rat cardiac myocytes, we wondered whether such protection would occur in an isolated organ system. Our findings indicate that a short (2 min) exposure to a low concentration (10 nM) of either S1P or GM-1 reduced ischemia-reperfusion injury as evidenced by increased LVDP, decreased LVEDP, reduced CK release and infarct size. In cultured neonatal rat ventricular myocytes, much higher concentrations (10 µM) of these agents for a much longer time (2 h) were used to achieve cardioprotection (14). Although we did not directly explore the mechanism of cell death in the present study, the relatively short time of ischemia-reperfusion (total of 50 min), which leads to extensive CK release, as well as to massive necrosis in the control state, would seem to exclude apoptosis as an important mode of cell death. In this connection, it should be noted that a large infarct area resulted from a relatively short ischemic time of 20 min and reperfusion time of 30 min. In an identical model using a different a strain of mouse, Tekin et al. (28) recently reported that after 20 min of ischemia and 30 min of reperfusion, infarct size ranged between 30 and 38%. Our infarct size results in the control group are virtually identical to those of Tekin et al.

Infusion of 10 nM S1P or 10 nM GM-1 for 2 min via the aortic cannula in the isolated Langendorff buffer-perfused mouse heart preparation resulted in no significant changes in coronary flow, LVDP, or LVEDP. In preliminary studies, we found that 100 nM S1P but not 100 nM GM-1 caused apparent coronary vasoconstriction and decreased LVDP (data not shown). Others have reported related hemodynamic changes after administration of S1P. In a canine isolated, blood-perfused sinoatrial node and papillary muscle preparation, Sugiyama et al. (27) observed that 10 µg of S1P given over 4 s directly into a nutrient coronary artery caused sinus tachycardia and coronary vasoconstriction followed by a weak negative inotropic effect. In contrast, the same group noted in an in vivo rat model that intravenous injection of S1P decreased heart rate, ventricular contraction, and blood pressure (26). Bischoff et al. (2) reported that intravenous injection of S1P rapidly (within 30 s), transiently and dose dependently reduced rat renal blood flow and mesenteric blood flow without affecting mean arterial pressure or heart rate. Thus species difference, mode of administration, and drug concentration will be important considerations in understanding the role of S1P or GM-1 in future studies of myocardial protection and preservation.

PKC, PKCepsilon knockout mice, and sphingosine kinase. PKC is a family of isozymes that translocate between subcellular compartments upon activation (20). It is likely that following activation, these isozymes are bound to selective anchoring proteins or internal receptors for activated C-kinase (RACKs). There is abundant evidence that activated PKCepsilon translocates to its RACK during acute ischemia and pharmacological preconditioning (18). Isozyme-selective agonists and antagonists of PKC translocation and function have been developed (5, 8, 11, 13). Previously, we employed an PKCepsilon peptide antagonist to demonstrate that PKCepsilon mediates hypoxic preconditioning of cultured neonatal rat cardiomyocytes (11) and pharmacological preconditioning of neonatal mouse cardiac myocytes (13). Activation of particular PKC isozymes, such as PKCdelta or PKCepsilon , appears to have opposing actions on protection from ischemia-induced damage, with activation of PKCepsilon being cardioprotective, whereas activation of PKCdelta increases damage induced by ischemia in vitro and in vivo (5).

PKC activation may be an important factor in GM-1-mediated myocardial protection, because it stimulates sphingosine kinase, the terminal step in S1P synthesis (3, 7, 16), and thus may be critical for the generation of intracellular S1P. Cavallini et al. (4) have already shown in cardiac fibroblasts that GM-1 augments sphingosine kinase activity by more than twofold. Dimethylsphingosine, a potent competitive inhibitor of sphingosine kinase but not of protein kinase C, causes cell death in both cardiac myocytes (14) and cardiac fibroblasts (4). This cell death is markedly reduced by pretreatment with GM-1 (4, 14). Thus these observations indicate that GM-1 directly activates sphingosine kinase, and that inhibition of this enzyme leads to cell death.

Nevertheless, we were surprised to find that whereas S1P and GM-1 are both cardioprotective in this model, only GM-1 caused translocation of PKCepsilon . This led us to ask whether it would be possible to confirm the role of PKCepsilon in this model of cardioprotection by using the PKCepsilon knockout mouse. This mouse has been previously used for neurological studies that demonstrate altered nocioception (15) and supersensitivity to the sedative effects of ethanol, barbiturates, and benzodiazepines (21). The homozygous male mice used in our studies are of normal body weight and lifespan compared with their wild-type littermates and are indistinguishable from the latter in the normal cage environment. Their LV function and CF rates in the control state are identical to wild-type mice.

Results of the experiments in the PKCepsilon knockout mice were consistent with observations of PKCepsilon translocation using Western blotting. GM-1, which translocated PKCepsilon in wild-type mouse hearts, was ineffective in protecting PKCepsilon null mouse hearts from ischemia-reperfusion damage. Conversely, S1P, which did not translocate PKCepsilon in wild-type hearts, was still cardioprotective in PKCepsilon null mouse hearts. These observations are consistent with a requirement of PKCepsilon for the generation of intracellular S1P via activation of sphingosine kinase (3, 7, 16). These data also suggest that in the heart exogenously administered S1P acts through cognate receptors via yet to be defined PKC-independent pathways that are currently under study in our laboratory.

Previously we found that a putative PKC inhibitor chelerylthrine blocked the protective effect of S1P in neonatal rat cardiac myocytes (14). Recently, Yamamoto et al. (29) reported that chelerythrine induces apoptosis through generation of reactive oxygen species in neonatal rat cardiac myocytes. As pointed out by Clerk (6), PKC inhibitors may act independent of any effect on PKC. Thus, chelerythrine, which has been widely used as a PKC inhibitor, may act through another mechanism, and this may explain the apparent discrepancy between our current study and the prior in vitro result (14) with respect to the relation between PKCepsilon and S1P.

In summary, we have shown that both exogenous S1P and GM-1, a compound that activates sphingosine kinase and is known to increase endogenous S1P levels (4), protect the ex vivo heart from ischemia-reperfusion injury. GM-1 requires PKCepsilon for its cardioprotective action. As S1P is released by cardiac myocytes, as well as by platelets and macrophages (30), its enhancement, whether intracellular or extracellular, represents a novel model of cardioprotection that deserves further exploration.


    ACKNOWLEDGEMENTS

This study was supported by a Merit Review Grant from the Research Service of the Department of Veterans Affairs (to J. S. Karliner), by RO1 HL-31809 (to E. J. Goetzl), and RO1 HL-52141 (to D. Mochly-Rosen) from the National Institutes of Health. M. O. Gray is the recipient of a Research Career Development Award from the Department of Veterans Affairs Research Service.


    FOOTNOTES

Address for reprint requests and other correspondence: J. S. Karliner, Cardiology Section (111C), 4150 Clement St., San Francisco, CA 94121 (E-mail: Joel.Karliner{at}med.va.gov).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

First published February 14, 2002;10.1152/ajpheart.01029.2001

Received 27 November 2001; accepted in final form 12 February 2002.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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5.   Chen, L, Hahn H, Wu G, Chen CH, Liron T, Schechtman D, Cavallaro G, Banci L, Guo Y, Bolli R, Dorn GW, II, and Mochly-Rosen D. Opposing cardioprotective actions and parallel hypertrophic effects of delta PKC and epsilon PKC. Proc Natl Acad Sci USA 98: 11114-11119, 2001[Abstract/Free Full Text].

6.   Clerk, A. Death by protein kinase C inhibitor: a stressful event. J Mol Cell Cardiol 33: 1773-1776, 2001[ISI][Medline].

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