(Circulation. 2002;105:293.)
© 2002 American Heart Association, Inc.
Brief Rapid Communications |
From the Department of Cardiology, Guys, Kings and St Thomas School of Medicine, Kings College London, London, UK.
Correspondence to Prof A.M. Shah, Dept of Cardiology, GKT School of Medicine, Bessemer Road, London, SE5 9PJ, UK. E-mail ajay.shah{at}kcl.ac.uk
| Abstract |
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Methods and Results Mice with targeted disruption of the NADPH oxidase subunit gp91phox (gp91phox-/-) and matched wild-type mice were subjected to subcutaneous angiotensin II infusion at a subpressor dose (0.3 mg/kg/day) for 2 weeks. Systolic blood pressure was unaltered by angiotensin II in either group. Angiotensin II significantly increased heart/body weight ratio, atrial natriuretic factor and ß-myosin heavy chain mRNA expression, myocyte area, and cardiac collagen content in wild-type but not gp91phox-/- mice. Angiotensin II treatment increased myocardial NADPH oxidase activity in wild-type but not gp91phox-/- mice.
Conclusions A gp91phox-containing NADPH oxidase plays an important role in the development of angiotensin II-induced cardiac hypertrophy, independent of changes in blood pressure.
Key Words: hypertrophy angiotensin free radicals myocardium
| Introduction |
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Phagocyte-type NADPH oxidases, a major source of ROS in cardiovascular cells, are implicated in Ang II-induced VSM hypertrophy4 and hypertension.6 These oxidases are expressed in endothelium,7,8 VSM,4 adventitial fibroblasts,9 and cardiomyocytes.10 In endothelium and fibroblasts, gp91phox is the major subunit responsible for enzyme activity, whereas in VSM, homologues such as nox1 may be more important.11 To date, the potential role of a gp91phox-containing NADPH oxidase in the development of cardiac hypertrophy has not been studied.
| Methods |
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gp91phox mRNA expression was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in left ventricle (LV) homogenate and isolated cardiomyocytes, using specific primers targeted to exon 3, which is disrupted in gp91phox-/- mice.12
NADPH Oxidase Activity
Isolated gp91phox-/- and control hearts (n
4/group) were perfused with Ang II (0.1 to 1 µmol/L) for 10 minutes and then snap-frozen. NADPH-dependent superoxide production was measured in LV homogenates using lucigenin (5 µmol/L)-enhanced chemiluminescence (NADPH 300 µmol/L; 100 µg protein; 37°C).4,7 Some experiments were performed in the presence of a cell-permeable O2- scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron, 20 mmol/L), the flavoprotein inhibitor diphenyleneiodonium (DPI, 10 µmol/L), or a nitric oxide synthase inhibitor N
-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 µmol/L).
Animal Model
Wild-type and gp91phox-/- mice (42±2 days old) were anesthetized by inhalation of 2% isoflurane, 98% oxygen. Osmotic minipumps (Alzet Model 1002; Alza Corp) containing either Ang II (infusion rate 0.3 mg/kg/day) or vehicle were implanted in the midscapular region. Blood pressure was monitored by tail cuff plethysmography (World Precision Instruments, UK) in conscious mice (
9/group) following
3 training periods.
Assessment of Hypertrophy
Mice (n
6/group) were euthanized and body and heart weights recorded. Additional hearts (n
3/group) were fixed, sectioned (5 µm), and labeled with anti-laminin B2 antibody and counterstained with hematoxylin. Myocyte areas (>50 cells/section) were measured from transverse sections, by a blinded observer, using a digital image analyzer (Openlab 3.3.3, Improvision, UK). Collagen content was assessed by quantifying the blue pixel content from LV cryosections (6 µm) (n
3/group) stained with Masons trichrome.
Atrial natriuretic factor (ANF) and ß-myosin heavy chain (ß-MHC) mRNA expression was measured by semiquantitative RT-PCR, with normalization to GAPDH expression.
Statistics
Data are presented as mean±SEM. Comparisons between groups were made by 1-way ANOVA, followed by Fischers least significance post hoc test or Students unpaired t test. A value of P<0.05 was considered significant.
| Results |
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NADPH-Dependent Superoxide Production
Ang II treatment induced significant dose-dependent increases in NADPH oxidase activity in wild-type hearts but had no effect in gp91phox-/- hearts (Figure 1B). NADPH-dependent superoxide production was abolished by DPI and Tiron in all groups, but was unaffected by L-NAME (data not shown).
Blood Pressure
Basal systolic blood pressure was significantly lower in gp91phox-/- compared with wild-type mice (127±2.2 mm Hg versus 139±2.3 mm Hg). Ang II infusion (0.3 mg/kg/day) did not increase systolic blood pressure in either group (Figure 1C).
Cardiac Hypertrophic Response
Ang II infusion caused a significant increase in heart/body weight ratio in wild-type mice. This effect was substantially blunted and nonsignificant in gp91phox-/- mice (
8% increase versus 20% in wild-type) (Figure 1D). Expression of ANF and ß-MHC mRNA, 2 molecular markers of cardiac hypertrophy, was significantly increased after Ang II infusion in wild-type but not gp91phox-/- mice (Figures 2A and 2B). LV myocyte area was significantly increased in myocardial sections of wild-type mice treated with Ang II but not in corresponding gp91phox-/- animals (Figure 2C). Interstitial cardiac fibrosis was also significantly increased by Ang II infusion in wild-type mice, whereas no increase was observed in gp91phox-/- animals (Figure 2D).
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| Discussion |
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In VSM, Ang II increases NADPH oxidase-dependent ROS production, which is thought to activate signaling pathways involved in the hypertrophic response.4 In the present study, we found that NADPH oxidase activity was increased by Ang II in wild-type hearts but that Ang II had no effect in gp91phox-/- animals. This finding is consistent with the hypothesis that ROS produced by a gp91phox-containing NADPH oxidase are involved in Ang II-mediated cardiac hypertrophy. The downstream pathways modulated by Ang II-stimulated ROS production in cardiac myocytes will require further study. In preliminary studies, we found no differences in the activation of extracellular signal regulated kinase (ERK1/2), c-Jun N-terminal kinase, or p38 MAPK between wild-type and gp91phox-/- mice treated with Ang II (data not shown).
An interesting observation in this study was that baseline systolic blood pressure was significantly lower in conscious gp91phox-/- mice compared with wild-type controls. Whereas gp91phox homologues, such as nox1, are thought to be more important than gp91phox itself for VSM function,11 the present data suggest that gp91phox is also involved in the regulation of VSM tone. Very recently, Wang et al15 also reported a reduced systolic blood pressure in gp91phox-/- mice. These authors also found that infusion of pressor doses of Ang II did increase blood pressure in gp91phox-/- mice. In the present study, it is unlikely that the lower blood pressure in gp91phox-/- mice accounted for the attenuated cardiac hypertrophic response to Ang II because we studied subpressor doses. Furthermore, in pilot studies, infusion of a pressor dose of Ang II failed to induce hypertrophy in these animals. The gp91phox-/- mice tended to have very slightly higher baseline heart/body weight ratios than wild-types (P=NS); however, this is unlikely to have limited hypertrophy because maximal hypertrophy in wild-type animals can exceed 100% (eg, after aortic banding; data not shown).
The present results add significantly to the growing body of evidence in support of an important role for NADPH oxidase in cardiovascular physiology and pathophysiology.16 Previous studies using the same mice with targeted disruption of gp91phox demonstrated an involvement of gp91phox in Ang II-induced vascular hypertrophy in vivo15 and in the regulation of vascular tone7 and basal systemic blood pressure.15 The present study is the first to suggest a direct functional role for a gp91phox-containing NADPH oxidase in the pathogenesis of Ang II-induced cardiac hypertrophy.
| Acknowledgments |
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Received October 31, 2001; revision received December 4, 2001; accepted December 4, 2001.
| References |
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