(Hypertension. 2008;52:715.)
© 2008 American Heart Association, Inc.
Original Articles |
From the Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston.
Correspondence to Julie Chao, PhD, Department of Biochemistry and Molecular Biology, Medical University of South Carolina, 173 Ashley Ave, Charleston, SC 29425. E-mail chaoj{at}musc.edu
| Abstract |
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-nitro-L-arginine methylester (an NO synthase inhibitor). One day after myocardial infarction, tissue kallikrein treatment significantly improved cardiac contractility and reduced myocardial infarct size and left ventricle end diastolic pressure in Brown Norway Katholiek rats. Kallikrein attenuated ischemia-induced apoptosis and monocyte/macrophage accumulation in the ischemic myocardium in conjunction with increased NO levels and reduced myeloperoxidase activity. Icatibant and N
-nitro-L-arginine methylester abolished kallikreins effects, indicating a kinin B2 receptor NO-mediated event. Moreover, inactive kallikrein had no beneficial effects in cardiac function, myocardial infarction, apoptosis, or inflammatory cell infiltration after myocardial infarction. In primary cardiomyocytes derived from Brown Norway Katholiek rats under serum-free conditions, active, but not inactive, kallikrein reduced hypoxia/reoxygenation-induced apoptosis and caspase-3 activity, and the effects were mediated by kinin B2 receptor/nitric oxide formation. This is the first study to demonstrate that tissue kallikrein directly activates kinin B2 receptor in the absence of kininogen to reduce infarct size, apoptosis, and inflammation and improve cardiac performance of infarcted hearts.
Key Words: apoptosis cardiac function infarct size kinin B2 receptor tissue kallikrein
| Introduction |
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Brown Norway Katholiek (BNK) rats, genetically deficient in kininogen secretion, retain kinin B2 receptor expression. Therefore, the kininogen-deficient BNK rat is an ideal model to investigate whether tissue kallikrein has a direct effect on kinin B2 receptor in triggering biological functions. In this study, we determined whether tissue kallikrein has a cardioprotective role by direct activation of the kinin B2 receptor in BNK rats after myocardial infarction (MI). Our results showed that tissue kallikrein through kinin B2 receptor activation and NO formation improved cardiac performance and reduced ischemia-induced infarction, cardiomyocyte apoptosis, and intramyocardial inflammation in BNK rats. This is the first study to identify a direct biological function of tissue kallikrein through kinin B2 receptor activation independent of kinin formation in vivo.
| Methods |
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-nitro-L-arginine-methyl-ester (L-NAME) (35 mg/kg). The sixth group received inactive tissue kallikrein in a similar manner. One day after coronary artery ligation, hemodynamic parameters were analyzed as previously described,14 animals were euthanized, and heart tissues were harvested for morphological and biochemical analyses.
Inactivation of Tissue Kallikrein by Aprotinin
Tissue kallikrein was inactivated by prior incubation with 5-fold molar excess of aprotinin at 37°C for 1 hour. Inactivation of tissue kallikrein was determined by enzymatic assay with S2266, a chromogenic substrate.15 Aprotinin-treated tissue kallikrein exhibited less than 5% of active kallikrein activity.
Myocardial Infarct Size Determination
The middle part of the heart (2 mm) was sectioned transversely and incubated with 1.5% 2,3,5-triphenyltetrazolium chloride (Sigma) for 5 minutes at 37°C. The ratio of infarcted area to the area at risk was then calculated. The infarcted area was distinguished by 2,3,5-triphenyltetrazolium chloride staining using computer-assisted planimetry (National Institutes of Health Image 1.57).
Histological and Immunohistological Analysis
For histological analyses, the left ventricle was fixed with 4% paraformaldehyde, dehydrated, embedded, and cut into 4-µm sections. Primary antibody against ED-1 (Chemicon, 1:200) was used for immunostaining of monocytes/macrophages. The number of ED-1-positive cells was counted in a double-blind fashion from 8 to 10 different fields of each section (n=10) at 400x magnification. Apoptosis was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling).16 The ratio of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling-positive cardiomyocytes to the total number of cardiomyocytes was calculated.
Hypoxia/Reoxygenation of Primary Cultured Cardiomyocytes
Cardiomyocytes were isolated from the hearts of 2- to 3-day-old BNK rats as previously described.17 Cardiomyocytes were grown in DMEM supplemented with 10% fetal bovine serum. Cardiomyocyte origin was confirmed immunocytochemically using antibody to sarcomeric
-actinin (Sigma). Subcultured cells were maintained in serum-free DMEM for 24 hours and then incubated with serum-free DMEM supplemented with active tissue kallikrein (0.2 µmol/L) or aprotinin-inactivated tissue kallikrein (0.2 µmol/L) under the condition of hypoxia for 12 hours (95% N2 and 5% CO2) followed by 24-hour reoxygenation (95% O2 and 5% CO2). Apoptotic cardiomyocytes were identified by Hoechst 33342 staining. Hoechst-positive apoptotic cells were determined by counting cardiomyocytes in 6 randomly chosen fields. Caspase-3 activity in cardiomyocyte lysates was determined using a fluorometric caspase-3 assay kit (Oncogene) according to the manufacturers instructions.
Nitrate/Nitrite and Myeloperoxidase Assays
Nitrate/nitrite levels, an indicator of NO production, were measured by a fluorometric assay as previously described.18 Myeloperoxidase activity in cardiac extracts was measured as previously described.19
Statistical Analysis
Data were compared among experimental groups using analysis of variance followed by Fishers partial least squares difference. Data are expressed as mean±SEM. Differences were considered statistically significant at a value of P<0.05.
| Results |
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Intramyocardial injection of tissue kallikrein, but not inactive kallikrein, significantly reduced infarct size in the left ventricle 1 day after MI compared with the MI control group, as determined by 2,3,5-triphenyltetrazolium chloride staining and quantitative analysis (Figure 1A and 1B). Coadministration of icatibant and L-NAME abrogated kallikreins effect. However, icatibant, L-NAME, or aprotinin alone had no effect on myocardial infarct size as compared with the MI control (data not shown).
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Kallikrein Reduces MI-Induced Cardiomyocyte Apoptosis and Intramyocardial Inflammation
Apoptotic cardiomyocytes were detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining in the infarcted myocardium 1 day after MI, and kallikrein treatment reduced the number of apoptotic cells (Figure 2A). Icatibant and L-NAME abrogated kallikreins effect, and inactive kallikrein had no effect. Quantitative analysis showed that active, but not inactive, kallikrein significantly reduced the ratio of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling-positive cardiomyocytes to total number of cardiomyocytes as compared with the control group. However, kallikreins protective effect was blocked by icatibant and L-NAME (Figure 2B). Furthermore, inflammatory cell accumulation in the infarcted region of the heart was identified by ED-1 immunostaining (Figure 3A). ED-1 positive cells were counted for quantification of monocyte/macrophage number (Figure 3B). Increased inflammatory cell infiltration was detected in the infarcted area of the heart after acute MI, but kallikrein injection significantly decreased monocytes/macrophages compared with the control. Icatibant and L-NAME blocked kallikreins effect, but aprotinin-inactivated kallikrein had no protective effect against the inflammatory response.
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Kallikrein Increases NO Levels and Reduces Myeloperoxidase Activity
Kallikrein treatment resulted in a significant increase in cardiac nitrate/nitrite production compared with the MI control group, and this effect was abrogated by icatibant and L-NAME (Figure 4A). Moreover, kallikrein prevented the increase in cardiac myeloperoxidase levels induced by MI damage, and the effect of kallikrein was blocked by icatibant and L-NAME (Figure 4B). Again, inactive kallikrein had no effect in increasing NO formation or inhibiting oxidative stress.
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Active Kallikrein Reduces Hypoxia/Reoxygenation-Induced Apoptosis and Caspase-3 Activity in Primary Cultured Cardiomyocytes
Representative Hoechst-positive staining and quantitative analysis showed that active tissue kallikrein, but not aprotinin-inactivated tissue kallikrein, significantly reduced hypoxia/reoxygenation-induced apoptosis in cultured primary cardiomyocytes derived from BNK rats (Figure 5A and 5B). Similarly, active kallikrein, but not inactive kallikrein, significantly reduced hypoxia/reoxygenation-induced caspase-3 activity (Figure 5C). Icatibant and L-NAME abrogated kallikreins effects on both apoptosis and caspase-3 activity.
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| Discussion |
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It has been shown that only active tissue kallikrein, but not active site-inhibited kallikrein, can stimulate the kinin B2 receptor in cultured cells, suggesting that cleavage of a peptide bond in the receptor is necessary for its activation by kallikrein.8 To determine whether tissue kallikrein has a direct proteolytic action on the kinin B2 receptor, we evaluated the effect of purified active and inactive forms of tissue kallikrein on hypoxia/reoxygenation-induced programmed cell death in cultured cardiomyocytes. Our results showed that active kallikrein, but not inactive kallikrein, inhibited hypoxia-induced apoptosis and caspase-3 activity in cultured cardiomyocytes derived from Sprague-Dawley (data not shown) and kininogen-deficient rats. These results indeed indicate that cleavage of a peptide bond in the kinin B2 receptor is necessary for direct activation of kinin B2 receptor by tissue kallikrein.
Kinin is capable of generating NO by causing an increase in the phosphorylation of endothelial NO synthase.22 Similarly, tissue kallikrein gene transfer has been shown to promote muscular neovascularization by endothelial NO synthase upregulation and Akt activation.23 Moreover, endothelial NO synthase gene delivery protected against cardiac remodeling through reduction of oxidative stress after MI.24 NO is a potent antioxidant25 and is capable of inhibiting neutrophil superoxide anion production through a direct action on the membrane components of NADPH oxidase and the assembly of NADH/NADPH oxidase subunits.26,27 Our present finding showed that icatibant and L-NAME abolished kallikreins effects in promoting nitrate/nitrite (an indicator of NO) levels as well as suppressing superoxide production in infarcted hearts. These combined results indicate that kallikrein/kinin is capable of improving cardiac function through increased NO formation.
A recent study showed that the kinin B1 and B2 receptors may serve a protective role in cardiac dysfunction.28 However, we previously demonstrated that myocardial hypertrophy induced by aortic occlusion is mediated by B2 receptor, but not by B1 receptor, using kinin receptor knockout mice.29 In addition, intact bradykinin, but not des-Arg9-bradykinin (a kinin B1 receptor agonist), prevented cardiomyocyte apoptosis and ventricular remodeling after acute ischemia/reperfusion, supporting a role of kinin B2 receptor, but not B1 receptor, in cardiac protection.3 The potential role of tissue kallikrein in protection against other organ damage such as the kidney, blood vessels, and brain through direct kinin B2 receptor activation awaits further investigation.
It has been recently reported that the kallikrein–kinin system is involved in protease-activated receptor-mediated inflammation in rodents.30 In this regard, we observed that tissue kallikrein independent of kinin formation promotes the migration of cultured human keratinocytes through direct activation of protease-activated receptor-1 (Gao L, Chao L, Chao J, unpublished results). Whether tissue kallikrein can act on protease-activated receptors to prevent ischemic heart disease, independent of its interaction with kinin receptors, is a new avenue for future studies.
Perspectives
This is the first study to demonstrate that tissue kallikrein improves cardiac function by inhibiting apoptosis and inflammation through direct activation of the kinin B2 receptor without kinin formation in the infarcted myocardium. Comparison of active and inactive tissue kallikrein indicates that cleavage of a peptide bond is required for B2 receptor stimulation by kallikrein in vitro and in vivo. This is an innovative finding because it is well characterized that tissue kallikrein exerts biological functions through generation of kinin peptides from kininogen substrate. Because ACE inhibition can potentiate kallikreins effect on kinin B2 receptor activation, tissue kallikrein, in addition to kinin formation, could provide advanced therapeutic benefits of ACE inhibition in protection against cardiovascular and renal diseases.
| Acknowledgments |
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This work was supported by National Institutes of Health grants HL29397, DK066350, and C06 RR015455 from the Extramural Research Facilities Program of the National Center for Research Resources.
Disclosures
None.
Received April 9, 2008; first decision May 6, 2008; accepted August 1, 2008.
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