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Am J Physiol Heart Circ Physiol 291: H1754-H1760, 2006. First published May 12, 2006; doi:10.1152/ajpheart.01199.2005
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Competitive displacement of phosphoinositide 3-kinase from beta-adrenergic receptor kinase-1 improves postinfarction adverse myocardial remodeling

Antonio Curcio, Takahisa Noma, Sathyamangla V. Naga Prasad, Matthew J. Wolf, Anthony Lemaire, Cinzia Perrino, Lan Mao, and Howard A. Rockman

Department of Medicine, Cell Biology, and Molecular Genetics, Duke University Medical Center, Durham, North Carolina

Submitted 13 November 2005 ; accepted in final form 17 April 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Adverse remodeling after myocardial infarction (MI) determines the progression of heart failure. Failing hearts are characterized by downregulation of beta-adrenergic receptor (beta-AR) signaling in part because of increased beta-AR kinase 1 activity. Our previous studies have shown that overexpression of the phosphoinositide kinase (PIK) domain of phosphoinositide 3-kinase (PI3K), prevents beta-AR downregulation and enhances adrenergic agonist responsiveness by inhibiting the targeting of PI3K to the beta-AR complex. To investigate whether preventing beta-AR downregulation in the heart ameliorates cardiac function post-MI, transgenic mice with cardiac-specific overexpression of the PIK domain peptide (TgPIK) underwent left coronary artery ligation and were subsequently followed by serial echocardiography at 4, 8, 12, 16, and 20 wk. Despite having similar infarction sizes, TgPIK mice showed better systolic function, less cardiac dilatation, and improved hemodynamic response to dobutamine compared with littermate controls after MI. To test that displacement of PI3K from the beta-AR complex, but not the total loss of PI3K-{gamma}, is critical for amelioration of cardiac function, mice lacking the PI3K-{gamma} (PI3K-{gamma}-KO) underwent MI, and their cardiac function was assessed 20 wk post-MI. Serial echocardiographic measurements showed severe reduction in contractile performance in PI3K-{gamma}-KO compared with TgPIK mice. Furthermore, significant beta-AR downregulation and desensitization were only seen in infarcted wild-type and PI3K-{gamma}-KO mice and not in TgPIK mice. Together, these results demonstrate that adverse remodeling of the ventricle after MI can be attenuated by a strategy that prevents recruitment of PI3K to the plasma membrane and restores normal beta-AR function.

heart failure; ventricular remodeling; transgenic models

DESPITE IMPROVEMENTS IN the diagnosis and management of coronary atherosclerotic disease, heart failure resulting from myocardial infarction (MI) remains one of the leading causes of morbidity and mortality in the United States (6). Preservation of functional myocardium after MI is a critical strategy to prevent progressive increases in left ventricular (LV) size and induction of myocyte hypertrophy (23). These adverse processes, defined as LV remodeling, represent strong prognostic factors of increased morbidity and mortality, and accumulating evidence indicates that heightened sympathetic nervous system activity promotes adverse remodeling. Moreover, beta-adrenergic receptor (beta-AR) antagonists have been shown to reduce post-MI remodeling and heart failure development and to improve patients' outcome (2, 19).

Hyperactivation of the sympathetic nervous system in heart failure leads to the chronic increase in catecholamines and eventually to desensitization and downregulation of beta-ARs (26, 27). beta-AR downregulation is a process resulting from agonist-induced receptor internalization in endosomal compartments of the cell (22) that involves, as the initial step, phosphorylation of the agonist-occupied receptor by beta-AR kinase 1 (beta-ARK1, also known as GRK2; see Refs. 5, 26, 28). Subsequent to receptor phosphorylation, beta-AR internalization follows a sequence of translocation events that lead to the assembly of a multiprotein complex at the plasma membrane that contain arrestin proteins and phosphoinositide 3-kinase (PI3K) (13, 16). In cell culture systems, and in the heart, it has recently been shown that recruitment of beta-ARK1 to the receptor facilitates agonist-mediated translocation of PI3K to the plasma membrane (14) and that both protein kinase and lipid kinase activities of PI3K are required for beta-AR internalization (15). The region of PI3K that specifically associates beta-ARK1 was identified as the phosphoinositide kinase (PIK) domain of the enzyme (13). Our previous in vitro studies have shown that overexpression of the PIK domain competes with endogenous PI3K for binding with beta-ARK1, thereby displacing PI3K from the beta-ARK1/PI3K complex, resulting in inhibition of agonist-stimulated beta-AR internalization (13).

Recent data demonstrates that the specific disruption of the beta-ARK1/PI3K interaction by overexpressing a catalytically inactive mutant of PI3K-{gamma} delays the development of heart failure after chronic pressure overload (17) and prolongs survival in a genetic model of heart failure (21). Moreover, overexpression of the PIK domain peptide restores the normal beta-AR responsiveness to agonist by preventing PI3K translocation at the membrane and inhibits adverse receptor redistribution in cardiomyocytes from failing pigs and in transgenic mice chronically stimulated with isoproterenol (22).

Because adverse remodeling occurs after MI, we tested whether cardiac-specific expression of the PIK domain would ameliorate adverse remodeling after coronary artery ligation. Moreover, as an extension of our previous investigation of the role of PI3K-{gamma} in beta-AR downregulation, we studied the relationship between the deterioration of cardiac function after MI and the extent of infarction size in control, transgenic (TgPIK), and PI3K-{gamma} knockout mice.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Experimental model of MI. Generation of genetically modified TgPIK mice (transgenic mice with cardiac-specific expression of PIK domain) and PI3K-{gamma}-KO mice (mice lacking PI3K-{gamma}) has been previously described (17, 22). Adult mice (C57BL/6), 8–10 wk old, weighing 19–23 g of either sex, were anesthetized with a mixture of ketamine and xylazine (100 and 2.5 mg/kg, respectively, ip injection). Under a dissecting microscope (Nissho Optical TZ-240; Labtek, Campbell, CA), animals were placed in the supine position on a heated operation board, and a midline cervical incision was made to expose the trachea. After successful endotracheal intubation, the cannula was connected to a volume-cycled rodent ventilator (model 683; Harvard Aapparatus, Holliston, MA) on room air with a stroke volume of 0.25 ml and respiratory rate of 100/min. The chest cavity was entered in the fourth intercostal space at the left sternal border through a small incision, and MI was produced by ligating the left anterior descending coronary artery with an 8–0 prolene suture at the site of the vessels' emergence past the tip of the left atrium. Animals were handled according to the approved protocols and animal welfare regulations of the Institutional Review Board at Duke University Medical Center.

Serial echocardiography. Transthoracic echocardiography was performed with a linear 30-MHz transducer (Vevo 660 High Resolution Imaging System; VisualSonics, Toronto, Ontario, Canada), as previously described (4, 5, 17, 21, 26, 28). LV end-diastolic and end- systolic dimensions (LVEDD and LVESD, respectively), fractional shortening (FS), interventricular septal wall thickness, posterior wall thickness, heart rate (HR), and velocity of circumferential fiber shortening corrected by HR (Vcfc) were acquired in conscious mice placed on a heated pad.

Hemodynamic studies. Cardiac catheterization in TgPIK and wild-type (WT) mice 20 wk after MI and 4 wk after sham operation was performed as previously described (29). Briefly, a MPCU-200 catheter (Millar Instruments, Houston, TX) was inserted retrogradely through the right carotid artery in the LV, and a polyethylene-50 catheter was placed in the left external jugular vein for dobutamine infusion (5 µg·kg–1·min–1). Steady-state hemodynamic parameters were recorded for 3 min, and, subsequently, the contractile state was increased with a dobutamine infusion; after a steady state was reached (~3 min), baseline recordings were performed. All data were recorded digitally at 1 kHz in a PowerLab DAQ System (ADInstruments, Colorado Springs, CO), and parameters of LV pressure and the first derivative of LV pressure were analyzed (Millar Instruments).

Morphological evaluation of the infarct size. Twenty weeks after coronary artery ligation, mice were terminated, and hearts were arrested in diastole with a 300-µl injection of saturated KCl solution in the right atrium. Each LV was cut at the level of the suture in a proximal segment that was flash-frozen in liquid nitrogen for biochemical analysis and a distal segment that was fixed in 10% buffered formalin for histological studies. For infarct size evaluation, tissues were embedded in paraffin, and two 5-µm sections from the beginning of the infarction area and two sections from the middle portion were cut and stained with Masson's trichrome (18). Total LV epicardial and endocardial circumferences and epicardial and endocardial borders of infarcted regions were measured and averaged using computer-assisted image analysis software (ImageJ NIH; see Ref. 18).

Membrane fractionation, beta-AR radioligand binding, and cAMP assay. Membranes were prepared as described previously (4, 31). beta-AR densities were evaluated by incubating 20 µg of membranes with a saturating concentration of 125I-labeled cyanopindolol (200 pmol/l) and 100 µmol alprenolol. Determination of total cAMP levels in the hearts was performed by using the cAMP (3H) assay system (Amersham Biosciences, Piscataway, NJ). Briefly, samples were extracted by homogenization in buffer containing 4 mM EDTA (to prevent enzymatic degradation of cAMP), followed by heating for several minutes in a boiling water bath to coagulate protein. After centrifugation, the cAMP levels in the supernatants were assayed according to the manufacturer's protocol.

Statistical analysis. Data are expressed as means ± SE. Two-way repeated-measures ANOVA was used to evaluate echocardiographic measurements in TgPIK, WT, and PI3K-{gamma}-KO mice at 4, 8, 12, 16, and 20 wk post-MI. Post hoc analysis was performed with Neuman-Keuls correction. One-way ANOVA with Bonferroni correction for multiple comparisons was used to evaluate data from the infarct size vs. FS and Vcfc. Unpaired t-tests were used to analyze beta-AR density and total cAMP generation between sham and MI treatment. For all analysis, a value of P < 0.05 was considered significant.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Cardiac expression of PIK attenuated LV dilatation and prevented decay in cardiac function after MI. To test the hypothesis that cardiac selective displacement of PI3K from beta-ARK1/PI3K has a beneficial role in reducing adverse remodeling after MI, we assessed the development of progressive cardiac dysfunction by serial echocardiography at 4, 8, 12, 16, and 20 wk after coronary artery ligation. The range of infarction size was similar in all three groups, as shown in Table 1. The overall mean infarct size for WT mice was 40.2 ± 3.3%, whereas it was 38.7 ± 4.3% for TgPIK mice and 30.1 ± 5.3% for PI3K-{gamma}-KO mice [P = not significant (NS), PI3K-{gamma}-KO vs. TgPIK and WT]. At any given infarct size, TgPIK showed less LV enlargement compared with WT mice, as assessed by echocardiographic evaluation of LVEDD and LVESD, thus yielding improved cardiac function as measured by a greater %FS in the TgPIK mice compared with WT mice (Table 1 and Fig. 1, A–C).


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  		Table 1. Morphometric and echocardiographic parameters from WT, TgPIK, and PI3K-{gamma}-KO mice before and 20 wk after MI

 

Figure 1
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  		Fig. 1. Cardiac overexpression of phosphoinositide kinase (PIK) domain peptide attenuates left ventricular dilatation and ameliorates systolic function after myocardial infarction. Absolute values (left) and %changes (right) in wild-type (WT, n = 20, {blacksquare}), transgenic PIK (TgPIK, n = 14, {circ}), and phosphoinositide 3-kinase-{gamma} knockout mice (PI3K-{gamma}-KO, n = 8, {square}) from before (Pre) surgery to 20 wk (w) after infarction of left ventricular end-diastolic dimension (LVEDD; A), left ventricular end-systolic dimension (LVESD; B), and fractional shortening (FS; C). *P < 0.003 TgPIK vs. WT and {dagger}P < 0.005 TgPIK vs. PI3K-{gamma}-KO by 2-way repeated-measures ANOVA.

 
Because there was a global beneficial effect on cardiac function upon PIK expression, we investigated whether variability in infarct size, related to small differences in the anatomy of the left anterior descending coronary artery (11), could be correlated to the extent of cardiac dysfunction post-MI. We examined infarct size vs. FS and Vcfc, two well-established indexes of cardiac function status (1). The beneficial effects observed in TgPIK mice were present over the range of infarct size as demonstrated by dichotomizing into small-moderate infarctions (10–35%) and large-extensive infarctions (35–70%; Fig. 2, A and B).


Figure 2
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  		Fig. 2. PIK domain cardiac expression improves cardiac function over all ranges of infarctions. FS (A) and velocity of circumferential fiber shortening corrected by heart rate (Vcfc; B) at 20 wk are plotted against infarct size. For each group (WT, n = 20, {blacksquare}; TgPIK, n = 14, {circ}; PI3K-{gamma}-KO, n = 8, {square}), a cutoff of 35% infarction size was established, and the animals were grouped in small-to-moderate infarction groups and large-to-extensive infarction groups (left and right, respectively, in A and B). No significant difference was found among subgroups of the same range of infarctions. {dagger}P < 0.05 TgPIK vs. PI3K-{gamma}-KO and *P < 0.05 TgPIK vs. WT by 1-way ANOVA with Bonferroni correction.

 
To test whether the complete absence of PI3K would be equivalent to displacing PI3K itself from the receptor complex, PI3K-{gamma}-KO mice underwent ligation of the left anterior descending coronary artery and were followed for 20 wk. Strikingly, we found that the PI3K-{gamma}-KO mice displayed a worse cardiac function compared with TgPIK mice at each time point (Fig. 1C) and in an independent manner from the infarction size (Fig. 2A). Moreover, Vcfc was reduced significantly compared with TgPIK mice (Table 1) at 20 wk post-MI, and a significant worsening of cardiac function was observed for both small-moderate and large-extensive infarctions (Fig. 2B). These findings are consistent with previous studies showing that global absence of PI3K is deleterious and leads to progressive decay in contractile performance upon stress (20).

Mortality data. The overall mortality was 43% in the WT group, 38% in TgPIK group, and 40% in PI3K-{gamma}-KO group. We did not observe any significant difference in mortality between the three groups. After excluding intraoperative deaths, mortality rate was 22% in the TgPIK group, 21% in the PI3K-{gamma}-KO group, and 28% in the WT group.

PIK domain prevents beta-AR downregulation and cAMP generation deregulation in transgenic mice. Because previous studies have demonstrated that inhibition of receptor-targeted PI3K results in preserved beta-AR function, we examined whether inhibition of membrane-targeted PI3K would prevent beta-AR downregulation in failing hearts after MI. beta-AR density was significantly reduced by 24% in infarcted WT mice and by 22% in infarcted PI3K-{gamma}-KO mice compared with their respective shams. In contrast, infarcted TgPIK mice showed no significant reduction in beta-AR levels compared with transgenic sham mice (Fig. 3A). These data show that, after MI, cardiac expression of PIK domain peptide prevents beta-AR downregulation by preserving the membrane localization of receptors (22).


Figure 3
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  		Fig. 3. PIK-overexpressing mice do not develop beta-adrenergic receptor (beta-AR) dysfunction in long-term heart failure after infarction. A: beta-AR downregulation is prevented in TgPIK mice compared with WT and PI3K-{gamma}-KO. beta-AR densities 20 wk after infarction (fmol/mg protein): 49.9 ± 3.7 in TgPIK mice (n = 7); 39.0 ± 1.5 in WT mice (n = 6); 38.0 ± 2.8 in PI3K-{gamma}-KO mice (n = 6). MI, myocardial infarction. B: hearts from infarcted TgPIK mice show reduced total levels of cAMP compared with infarcted WT and PI3K-{gamma}-KO mice. Total cAMP 20 wk after infarction (pmol/mg protein): 0.37 ± 0.03 in WT mice (n = 5); 0.25 ± 0.02 in TgPIK mice (n = 5); 0.39 ± 0.04 in PI3K-{gamma}-KO mice (n = 7). P values from unpaired Student's t-tests are shown.

 
We next examined beta-AR downstream signaling as assessed by total cAMP accumulation in the heart. WT and PI3K-{gamma}-KO mice showed increased total cAMP levels compared with TgPIK hearts 20 wk post-MI (Fig. 3B). These results indicate that cardiac-specific disruption of beta-ARK1/PI3K complex by PIK prevents beta-AR downregulation but does not promote the adverse accumulation of cardiac cAMP concentrations.

PIK cardiac expression preserves beta-AR function. To test in vivo whether competitive displacement of PI3K from beta-ARK1 results in preservation of receptor responsiveness, two subgroups of animals displaying similar infarction sizes (TgPIK mice, n = 6, infarct size = 37.8% vs. WT mice, n = 8, infarct size = 38.6%) were challenged with the beta-AR agonist dobutamine, and the response was measured by hemodynamic monitoring. The increase in contractile function with agonist was assessed by measuring the maximal and minimal first derivative of the LV pressure over time [LV(dP/dtmax) and LV(dP/dtmin), respectively], before and after a 3-min dobutamine infusion at 5 µg·kg–1·min–1 (Table 2). After dobutamine challenge, infarcted TgPIK mice showed a significantly greater LV(dP/dtmax) compared with infarcted WT mice (13,945 ± 1,037 vs. 9,822 ± 1,040 mmHg/s, P < 0.005; Fig. 4A and Table 2). We further derived the change in LV(dP/dtmax) [LV(dP/dtmax) after dobutamine – basal LV(dP/dtmax)] as an index of beta-AR responsiveness in failing hearts. beta-AR responsiveness was significantly greater in the TgPIK mice after MI compared with WT mice (8,657 ± 470 vs. 5,387 ± 718 mmHg/s, P < 0.05; Table 2). Finally, when the change in LV(dP/dtmax) is plotted against infarction size, we found that the infarcted TgPIK mice had significant preservation of beta-AR responsiveness compared with infarcted WT mice over the complete range of infarct sizes (Fig. 4B). These data demonstrate that overexpression of the PIK peptide preserves cardiac beta-AR function even after 20 wk from MI.


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  		Table 2. Hemodynamic parameters in WT and TgPIK mice 20 wk post-MI

 

Figure 4
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  		Fig. 4. beta-AR response to agonist is preserved in TgPIK mice 20 wk after MI. A: both TgPIK (n = 6) and WT (n = 8) groups show significant dobutamine (Dob) response (*P < 0.001, ANOVA), but the response to dobutamine is significantly greater in the TgPIK group compared with WT ({dagger}P < 0.005, ANOVA). B: TgPIK mice display better cardiac function assessed as change in maximal first derivative of the left ventricular pressure over time [LV(dP/dtmax)] compared with WT mice at any given infarct size.

 
Sham-operated WT mice (n = 8) and TgPIK mice (n = 9) underwent cardiac catheterization and hemodynamic evaluation 4 wk after surgery (Table 3). Both groups displayed a significant increase in each parameter after dobutamine infusion. Interestingly, TgPIK mice showed a small but significant greater response to dobutamine for LV(dP/dtmax), LV(dP/dtmin), and LV systolic pressure (Table 3). beta-AR responsiveness as measured by the change in LV(dP/dtmax) was not different between sham-operated TgPIK mice and sham WT mice [change in LV(dP/dtmax), 7,966 ± 715 vs. 9,219 ± 1,060 mmHg/s, P = NS].


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  		Table 3. Hemodynamic parameters in sham-operated WT and TgPIK mice

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Long-term prognosis after MI depends chiefly on the postinfarction LV function. Although current therapeutic strategies are targeted to attenuate adverse remodeling (32), the mortality rate for patients with chronic heart failure remains high and indicates a need for novel strategies that function synergistically with current treatments (6). Our current study demonstrates that the disruption of beta-ARK1/PI3K interaction diminished the development of adverse ventricular remodeling in the mouse after MI. Importantly, the beneficial effects of the competitive displacement of PI3K from beta-ARK1/PI3K cytosolic complex through overexpression of PIK domain peptide results in preserved beta-AR function as evaluated by different contractile parameters over a wide range of infarction. Data from our laboratory and others (3, 4, 27) have consistently shown that chronic beta-AR dysfunction is a universal finding in heart failure; however, whether abnormal beta-AR function is pathogenic in heart failure is unknown. In our current study, we show how preserved beta-AR function post-MI is associated with improved cardiac function and supports the concept that persistent beta-AR dysfunction is deleterious to the heart under conditions of chronic stress (17, 21, 22).

It has been shown that there is a linear relationship between the number of dysfunctional LV segments and remodeling, which is ultimately related to the extension of the infarcted area (24). Importantly, we show that, for infarctions in the large-extensive range (35–70%), TgPIK mice displayed better cardiac function and were associated with reduced LV enlargement. In contrast, we demonstrate that the post-MI decline in FS and Vcfc was greater for the PI3K-{gamma}-KO mice compared with both TgPIK mice and WT mice. These data support our hypothesis that it is the competitive displacement of PI3K that inhibits targeting of active PI3K to the beta-AR complex that is the critical factor in preserving cardiac function. This is consistent with our previous studies wherein cardiac-specific overexpression of inactive PI3K preserves beta-AR function after chronic administration of isoproterenol and pressure overload, which is associated with the amelioration of cardiac dysfunction (17). In contrast, chronic administration of isoproterenol in the PI3K-{gamma}-KO mice results in beta-AR dysfunction resulting from the interaction of PI3K-{alpha} with beta-ARK1 in the absence of PI3K-{gamma} (17). Therefore, overexpression of PIK domain (conserved among the members of the PI3K family) would lead to displacement of all isoforms of PI3K from the beta-ARK1/PI3K complex resulting in reduced beta-ARK1-associated PI3K activity at the receptor complex.

The {alpha}- and {gamma}-isoforms are the major PI3K expressed in the heart, and, while both share similar functional structures including lipid and protein kinase domains, each responds distinctly to external stimuli (15, 30). PI3K-{alpha} is mainly activated by growth hormone receptors, including insulin-like growth hormone receptor, to induce stimulation of Akt (30). PI3K-{gamma} is known to interact with beta-ARK1 and be recruited to free Gbeta{gamma} subunits upon G protein-coupled receptor (GPCR) activation to induce their internalization (13). Importantly, overexpression of PIK domain does not affect endogenous PI3K activity, and the TgPIK mice are characterized by intact PI3K signaling (22), a finding distinctly different from the PI3K-{gamma}-KO animals. Because PIK overexpression does not inhibit the activation of cellular PI3K signaling by either growth factor receptor or GPCR stimulation (16), activation of downstream anti-apoptotic pathways in the early phases of infarction should be similar in both TgPIK and WT hearts. Our data do, however, clearly document that reducing beta-ARK1-associated PI3K activity prevents beta-AR downregulation and restores beta-AR responsiveness post-MI. Consistent with these data, we postulate that the beneficial action of PIK is through modulation of beta-AR function.

Excessive beta-AR stimulation during heart failure is associated with adverse effects on myocardial structure and function. Increasing cAMP signaling in general has proven to be an unsuccessful strategy by which to treat heart failure and may not be selective enough to affect only the signaling pathways associated with decreased cardiac function (12). It has been demonstrated that elevated cAMP levels are deleterious on the heart, in part through persistent activation of protein kinase A (PKA) with subsequent phosphorylation of phospholamban, L-type Ca2+ channels, and possibly ryanodine receptors (10), resulting in increased cytosolic Ca2+ levels with toxic effect (7, 9). Indeed, loss of ryanodine receptor-associated phosphodiesterase (PDE)4D3 expression in the mouse leads to increased cAMP-mediated hyperphosphorylation of ryanodine receptor by PKA and resultant channel "leakiness" and development of cardiomyopathy (8). Our data show that beta-AR downregulation after infarction is prevented in TgPIK mice and inherently seems to lead to lower levels of total myocardial cAMP.

The mechanism for normalizing cAMP levels in TgPIK mice postinfarction is not clear. We postulate that normalizing beta-AR function through PIK overexpression prevents adverse remodeling of the heart and results in a lower sympathetic drive, thereby reducing second-messenger accumulation. Interestingly, an association between PI3K-{gamma} and a member of the cAMP-hydrolyzing PDE family, PDE3B, was recently identified (20). PI3K-{gamma} was shown to enhance PDE3B activity in a kinase-independent manner, which in turn increases cAMP hydrolysis, thereby reducing PKA activation (20). Therefore, PI3K-{gamma} not only regulates the internalization of beta-AR to desensitize cardiomyocytes to further beta-AR signaling but also influences cAMP levels generated by beta-AR stimulation. Through its interaction with both PDE3B and beta-ARK1, PI3K-{gamma} may regulate the PKA- and beta-ARK1-mediated phosphorylation of beta-ARs, in part determining the coupling of these receptors to downstream signaling cascades (25). Whether overexpression of PIK results in an increase in PDE3B or PDE4B levels to account for the lower cAMP levels in TgPIK hearts is not known and will require further investigation.

In conclusion, we show here that the progression of LV adverse remodeling after MI is associated with beta-AR downregulation and deregulation of cAMP signaling. Reversal of these maladaptive processes, through a competitive displacement of PI3K from agonist-stimulated beta-ARs, represents an innovative therapeutic strategy for ameliorating the development of heart failure after MI.


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
The study was supported by National Heart, Lung, and Blood Institute Grant P01 HL-75443 (to H. A. Rockman).


   ACKNOWLEDGMENTS
 
We thank Kristine Porter and Barbara Williams for excellent technical assistance.


   FOOTNOTES
 

Address for reprint requests and other correspondence: H. A. Rockman, Dept. of Medicine, Cell Biology, and Molecular Genetics, Duke Univ. Medical Center, DUMC 3104, Durham, NC 27710 (e-mail: h.rockman{at}duke.edu)

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.


   REFERENCES
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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