(Circulation. 1999;99:22-25.)
© 1999 American Heart Association, Inc.
Brief Rapid Communications |
From the Cardiovascular Division, The Harvard Thorndike Laboratory, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Mass.
Correspondence to Beverly H. Lorell, MD, Cardiovascular Division, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215. E-mail blorell{at}bidmc.harvard.edu
| Abstract |
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Methods and ResultsFlaccid buffer-perfused adult normal and hypertrophied rat hearts were perfused with Ang II 10-8 mol/L plus prazosin 10-7 mol/L or Ang II plus the AT2 blocker PD 123319 5x10-7 mol/L. New protein synthesis was measured by the rate of [3H]phenylalanine incorporation into the LV proteins. In normal hearts, Ang II (n=8) increased the rate of [3H]phenylalanine incorporation by 74±27% (P<0.05 versus no drug). Treatment with PD123319 (n=8) did not increase protein synthesis compared with Ang II alone (32±11% versus Ang II alone, P=NS). In hypertrophied hearts, Ang II alone (n=6) increased the rate of [3H]phenylalanine incorporation only by 23±13% (P=NS versus no drug). In contrast, treatment with PD123319 (n=7) induced a 76±21% increase in new LV protein synthesis compared with Ang II alone (P<0.05). AT2 receptor blockade in Ang IIstimulated hypertrophied hearts was associated with enhanced membrane protein kinase C translocation and reduced LV cGMP content.
ConclusionsThese data support the hypothesis that in adult hypertrophied rat hearts, inhibition of cardiac AT2 receptors, which are upregulated in chronic LV hypertrophy, amplifies the immediate LV growth response to Ang II. This appears to be related to augmented Ang IIstimulated PKC activation and suppression of cGMP signaling.
Key Words: angiotensin hypertrophy signal transduction
| Introduction |
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from cytosol to membrane
fraction and is blocked by the specific Ang II type 1
(AT1) receptor antagonist
losartan.1 2 3 4 5 6 In contrast with neonatal myocytes,
early signals of the cardiac growth response to Ang II in adult rat
hearts do not involve the induction of proto-oncogenes, which occurs in
response to load.1 2 3 4 5 6 7 8 We have also shown that the acute
cardiac growth response of new cardiac protein synthesis and PKC
translocation in response to Ang II is blunted in hypertrophied hearts
with aortic stenosis relative to normal hearts.2 3
Recent observations suggest that this may be mediated by the Ang II
type 2 (AT2) receptor. We9 and
others10 have demonstrated an upregulation of LV
AT2 receptors in hypertrophied hearts. Similar
upregulation of AT2 receptors was reported in
human failing myocardium.11 12 Several studies
have proposed that downstream signaling related to the
AT2 receptor differs strikingly from signaling of
the AT1 receptor and involves kinin/cGMP
signaling rather than the PKC pathway.13 14 15 16 17 Transgenic
experiments suggest that the AT2 receptor
signaling cascade has distinct biological roles compared with
AT1 receptormediated
signaling.18 19 Consistent with this hypothesis,
recent studies in vascular smooth muscle cells20 and
endothelial cells21 demonstrated that the
AT2 receptor exerts antiproliferative effects
counteracting the growth-promoting effects of the
AT1 receptor. In contrast, the role of
AT2 receptormediated signaling in cardiac
growth of the intact adult heart is not known. Thus, in the present
study, we tested the hypothesis that in hypertrophied hearts, the
AT2 receptor mediates inhibitory
effects on the new cardiac protein synthesis and PKC activation in
response to acute Ang II stimulation. | Methods |
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To investigate the effects of AT2 blockade on Ang
IIinduced new cardiac protein synthesis, normal and hypertrophied
hearts were perfused with no drug (n=4), with Ang II
10-8 mol/L plus 10-7
mol/L prazosin (n=6 to 8), and with Ang II plus the selective
AT2 blocker PD 122319 (Parke-Davis,
5x10-7 mol/L, n=6 or 7). The
1-blocker prazosin was used to prevent any
indirect stimulation of protein synthesis via activation of the
postsynaptic sympathetic system.3 4 The dose of PD 122319
was chosen on the basis of previous studies9 13 14
demonstrating a selective AT2 receptor blockade
in response to Ang II. After 60 minutes of perfusion, hearts were
perfused for another 120 minutes with the same buffer, to which 0.5
mCi/L [3H]phenylalanine was
added.2 3 8 22 The net LV protein synthesis during the 120
minutes of perfusion was assumed to be linear2 3 8 22 and
was calculated as follows: phenylalanine incorporation (moles ·
g protein-1 · h-1)
= phenylalanine (dpm · g protein-1
· h-1)/perfusate phenylalanine
specific activity (dpm/mol).
To investigate whether AT2 blockade modulates Ang
IIstimulated PKC translocation, separate groups of normal and
hypertrophied hearts were perfused for 15 minutes with no drug (n=4),
with Ang II 10-8 mol/L plus
10-7 mol/L prazosin (n=5 or 6), and with Ang II
plus the AT2 blocker
5x10-7 mol/L (n=6 or 7). This time period was
chosen on the basis of the time course of maximal activation of PKC by
Ang II.2 Cytosolic and membrane fractions from frozen LV
tissue were prepared as described previously.2 23 Samples
containing 50 µg protein were then separated on a 7.5% SDS-PAGE gel.
Detection of PKC was carried out by use of antiPKC-
antibody
(1:1000, Life Technologies) and an enzyme-linked
chemiluminescenceWestern blotting analysis system
(Amersham International).
To investigate whether AT2 blockade modifies LV cGMP content in Ang IIperfused hearts, additional groups of normal and hypertrophied hearts were subjected to Ang II 10-8 mol/L plus 10-7 mol/L prazosin (n=4 per group) and to Ang II plus the AT2 blocker 5x10-7 mol/L (n=5 per group). LV cGMP levels were determined by enzymatic assay (Amersham, Life Science) and expressed as mol/mg protein.
All data are presented as mean±SEM. Two-way ANOVA followed by Fisher's exact test for post hoc analyses was used for multiple comparisons. Significance was accepted at P<0.05.
| Results |
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Effects of AT2 Blockade on Ang IIInduced
Phenylalanine Incorporation
The rate of [3H]phenylalanine
incorporation into proteins in the absence of drug was similar in
normal and hypertrophied hearts (Figure 1
). Corroborating our previous
findings,2 Ang IIinduced phenylalanine incorporation
increased in normal hearts but did not in hypertrophied hearts. In
normal hearts stimulated with Ang II, the rate of phenylalanine
incorporation did not increase further during AT2
receptor blockade. In contrast, in hypertrophied hearts stimulated with
Ang II, phenylalanine incorporation was increased by
AT2 receptor blockade.
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Effects of AT2 Blockade on Ang IIInduced PKC-
Translocation in Adult Rat Hearts
In normal and hypertrophied hearts with no drug, the membrane
fraction of PKC composed 41.6±2.7% and 46.3±1.3% of total PKC
content (P=NS). Normal hearts stimulated with Ang II
demonstrated a significant increase in membrane fraction of PKC
(24.9±8.6% versus no drug, P<0.05) that was not further
modified by AT2 blockade (25.5±5.8% versus no
drug, Figure 2
, left). In hypertrophied
hearts, Ang II alone did not cause a significant increase in membrane
translocation of PKC (8.8±4.1% versus no drug, Figure 2
, right). In the presence of AT2 blockade, however,
Ang II significantly increased the membrane fraction of PKC compared
with hypertrophied hearts with no drug (15.4±4.7% versus no drug,
P<0.05).
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Effects of AT2 Blockade on LV cGMP Content
In both normal and hypertrophied hearts stimulated with Ang II,
selective AT2 blockade significantly depressed LV
cGMP levels (98.6±7.6 versus 64.7±6.1 fmol/mg protein,
P<0.05, and 75.5±11.9 versus 45.8±6.2 fmol/mg protein,
P<0.05, respectively).
| Discussion |
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Recent studies suggested that AT2 receptorrelated biological effects are due to activation of kinins and intracellular cGMP signaling.13 14 15 16 17 Our data corroborate these findings by demonstrating that AT2 blockade causes a decrease in LV cGMP content in intact hearts stimulated by Ang II. In addition, AT2 blockade amplified PKC translocation in hypertrophied hearts stimulated with Ang II. Of note, this effect was absent in normal hearts. The presence of cross talk between PKC translocation and selective AT2 blockade in hypertrophied hearts with upregulated AT2 receptors9 10 11 12 is consistent with a report of Yamada et al,24 who demonstrated that AT2 receptor activation inhibits mitogen-activated protein kinase. In addition, several authors proposed that AT2 receptor activation may enhance apoptotic cell death.17 24 These hypotheses merit future studies in the present aortic stenosis model of chronic LV hypertrophy and failure.
The present study has several limitations. First, the isolated-perfused heart model does not allow investigation of later components of the growth response, namely myocyte hypertrophy. Second, it does not distinguish whether changes in protein synthesis or PKC translocation are localized predominantly to cardiac myocytes or also to matrix cells that express Ang II receptors.
In summary, the present study shows that AT2 receptor blockade amplifies new cardiac protein synthesis in response to Ang II in hypertrophied hearts. This appears to be related to both augmented PKC activation by Ang II and suppression of cGMP signaling. Thus, these data suggest for the first time that the AT2 receptor upregulation in chronic LV hypertrophy may blunt the acute LV growth response to Ang II. The beneficial or detrimental effects of chronic AT2 receptor blockade on hypertrophic remodeling and the transition to failure in pressure-overload hypertrophy remain to be investigated.
| Acknowledgments |
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Received July 31, 1998; revision received November 3, 1998; accepted November 5, 1998.
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