Physiological induction of a beta-adrenergic receptor kinase inhibitor transgene preserves ss-adrenergic responsiveness in pressure-overload cardiac hypertrophy

Single-nucleus transcriptomics reveal cardiac cell type-specific diversification in metabolic disease transgenic pigs

Cardiac damage is widely present in patients with metabolic diseases, but the exact pathophysiological mechanisms involved remain unclear. The porcine heart is an ideal material for cardiovascular research due to its similarities to the human heart. This study evaluated pathological features and performed single-nucleus RNA sequencing (snRNA-seq) on myocardial samples from both wild-type and metabolic disease-susceptible transgenic pigs (previously established). We found that transgenic pigs exhibited lipid metabolism disturbances and myocardial injury after a high-fat high-sucrose diet intervention. snRNA-seq reveals the cellular landscape of healthy and metabolically disturbed pig hearts and identifies the major cardiac cell populations affected by metabolic diseases. Within metabolic disorder hearts, metabolically active cardiomyocytes exhibited impaired function and reduced abundance. Moreover, massive numbers of reparative LYVE1+ macrophages were lost. Additionally, proinflammatory endothelial cells were activated with high expression of multiple proinflammatory cytokines. Our findings provide insights into the cellular mechanisms of metabolic disease-induced myocardial injury.

Cardiac-specific disruption of Bin1 in mice enables a model of stress- and age-associated dilated cardiomyopathy

Non-compensated dilated cardiomyopathy (DCM) leading to death from heart failure is rising rapidly in developed countries due to aging demographics, and there is a need for informative preclinical models to guide the development of effective therapeutic strategies to prevent or delay disease onset. In this study, we describe a novel model of heart failure based on cardiac-specific deletion of the prototypical mammalian BAR adapter-encoding gene Bin1, a modifier of age-associated disease. Bin1 deletion during embryonic development causes hypertrophic cardiomyopathy and neonatal lethality, but there is little information on how Bin1 affects cardiac function in adult animals. Here we report that cardiomyocyte-specific loss of Bin1 causes age-associated dilated cardiomyopathy (DCM) beginning by 8-10 months of age. Echocardiographic analysis showed that Bin1 loss caused a 45% reduction in ejection fraction during aging. Younger animals rapidly developed DCM if cardiac pressure overload was created by transverse aortic constriction. Heterozygotes exhibited an intermediate phenotype indicating Bin1 is haplo-insufficient to sustain normal heart function. Bin1 loss increased left ventricle (LV) volume and diameter during aging, but it did not alter LV volume or diameter in hearts from heterozygous mice nor did it affect LV mass. Bin1 loss increased interstitial fibrosis and mislocalization of the voltage-dependent calcium channel Cav 1.2, and the lipid raft scaffold protein caveolin-3, which normally complexes with Bin1 and Cav 1.2 in cardiomyocyte membranes. Our findings show how cardiac deficiency in Bin1 function causes age- and stress-associated heart failure, and they establish a new preclinical model of this terminal cardiac disease.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

A beta-blocker, propranolol, decreases the efficacy from enzyme replacement therapy in Pompe disease

Enzyme replacement therapy (ERT) with recombinant human acid α-glucosidase (rhGAA) fails to completely reverse muscle weakness in Pompe disease. β2-agonists enhanced ERT by increasing receptor-mediated uptake of rhGAA in skeletal muscles.

PURPOSE

To test the hypothesis that a β-blocker might reduce the efficacy of ERT, because the action of β-blockers opposes those of β2-agonists.

METHODS

Mice with Pompe disease were treated with propranolol (a β-blocker) or clenbuterol in combination with ERT, or with ERT alone.

RESULTS

Propranolol-treated mice had decreased weight gain (p<0.01), in comparison with clenbuterol-treated mice. Left ventricular mass was decreased (and comparable to wild-type) in ERT only and clenbuterol-treated groups of mice, and unchanged in propranolol-treated mice. GAA activity increased following either clenbuterol or propranolol in skeletal muscles. However, muscle glycogen was reduced only in clenbuterol-treated mice, not in propranolol-treated mice. Cell-based experiments confirmed that propranolol reduces uptake of rhGAA into Pompe fibroblasts and also demonstrated that the drug induces intracellular accumulation of glycoproteins at higher doses.

CONCLUSION

Propranolol, a commonly prescribed β-blocker, reduced weight, increased left ventricular mass and decreased glycogen clearance in skeletal muscle following ERT. β-Blockers might therefore decrease the efficacy from ERT in patients with Pompe disease.

GRK2 in the heart: a GPCR kinase and beyond

SIGNIFICANCE

Heart failure (HF) is a common end point for many underlying cardiovascular diseases. Down-regulation and desensitization of β-adrenergic receptors (β-AR) caused by G-protein-coupled receptor (GPCR) kinase 2 (GRK2) are prominent features of HF. Recent Advances and Critical Issues: Significant progress has been made to understand the pathological role of GRK2 in the heart both as a GPCR kinase and as a molecule that can exert GPCR-independent effects. Inhibition of cardiac GRK2 has proved to be therapeutic in the failing heart and may offer synergistic and additional benefits to β-blocker therapy. However, the mechanisms of how GRK2 directly contributes to the pathogenesis of HF need further investigation, and additional verification of the mechanistic details are needed before GRK2 inhibition can be used for the treatment of HF.

FUTURE DIRECTIONS

The newly identified characteristics of GRK2, including the S-nitrosylation of GRK2 and the localization of GRK2 on mitochondria, merit further investigation. They may contribute to it being a pro-death kinase and result in HF under stressed conditions through regulation of intracellular signaling, including cardiac reduction-oxidation (redox) balance. A thorough understanding of the functions of GRK2 in the heart is necessary in order to finalize it as a candidate for drug development.

Role of β-adrenergic receptor signaling and desensitization in heart failure: new concepts and prospects for treatment

The use of beta-blockers to antagonize beta-adrenergic receptor signaling in the heart has become a standard method of treatment for heart failure, resulting in positive clinical outcomes alone and in conjunction with other modulators of cardiomyocyte contractility. However, an entire explanation for improved cardiac function in patients using beta-blockers is unknown, and in fact may be quite complicated, considering the numerous intracellular signaling pathways associated with beta-adrenergic receptors. Stimulation of beta-adrenergic receptors during both normal conditions and during heart failure activate several distinct signaling cascades, which influence cardiomyocyte contraction, hypertrophy and apoptosis. This review explores the signaling cascades induced by beta-adrenergic receptor activation in normal and desensitized states to provide new insight into the effective treatment of cardiac dysfunction.

Cardiomyocyte-restricted inhibition of G protein-coupled receptor kinase-3 attenuates cardiac dysfunction after chronic pressure overload

Transgenic mice with cardiac-specific expression of a peptide inhibitor of G protein-coupled receptor kinase (GRK)3 [transgenic COOH-terminal GRK3 (GRK3ct) mice] display myocardial hypercontractility without hypertrophy and enhanced α(1)-adrenergic receptor signaling. A role for GRK3 in the pathogenesis of heart failure (HF) has not been investigated, but inhibition of its isozyme, GRK2, has been beneficial in several HF models. Here, we tested whether inhibition of GRK3 modulated evolving cardiac hypertrophy and dysfunction after pressure overload. Weight-matched male GRK3ct transgenic and nontransgenic littermate control (NLC) mice subjected to chronic pressure overload by abdominal aortic banding (AB) were compared with sham-operated (SH) mice. At 6 wk after AB, a significant increase of cardiac mass consistent with induction of hypertrophy was found, but no differences between GRK3ct-AB and NLC-AB mice were discerned. Simultaneous left ventricular (LV) pressure-volume analysis of electrically paced, ex vivo perfused working hearts revealed substantially reduced systolic and diastolic function in NLC-AB mice (n = 7), which was completely preserved in GRK3ct-AB mice (n = 7). An additional cohort was subjected to in vivo cardiac catheterization and LV pressure-volume analysis at 12 wk after AB. NLC-AB mice (n = 11) displayed elevated end-diastolic pressure (8.5 ± 3.1 vs. 2.9 ± 1.2 mmHg, P < 0.05), reduced cardiac output (3,448 ± 323 vs. 4,488 ± 342 μl/min, P < 0.05), and reduced dP/dt(max) and dP/dt(min) (both P < 0.05) compared with GRK3ct-AB mice (n = 16), corroborating the preserved cardiac structure and function observed in GRK3ct-AB hearts assessed ex vivo. Increased cardiac mass and myocardial mRNA expression of β-myosin heavy chain confirmed the similar induction of cardiac hypertrophy in both AB groups, but only NLC-AB hearts displayed significantly elevated mRNA levels of brain natriuretic peptide and myocardial collagen contents as well as reduced β(1)-adrenergic receptor responsiveness to isoproterenol, indicating increased LV wall stress and the transition to HF. Inhibition of cardiac GRK3 in mice does not alter the hypertrophic response but attenuates cardiac dysfunction and HF after chronic pressure overload.

Status of therapeutic gene transfer to treat cardiovascular disease in dogs and cats

Gene therapy is a procedure resulting in the transfer of a gene(s) into an individual's cells to treat a disease, which is designed to produce a protein or functional RNA (the gene product). Although most current gene therapy clinical trials focus on cancer and inherited diseases, multiple studies have evaluated the efficacy of gene therapy to abrogate various forms of heart disease. Indeed, human clinical trials are currently underway. One goal of gene transfer may be to express a functional gene when the endogenous gene is inactive. Alternatively, complex diseases such as end stage heart failure are characterized by a number of abnormalities at the cellular level, many of which can be targeted using gene delivery to alter myocardial protein levels. This review will discuss issues related to gene vector systems, gene delivery strategies and two cardiovascular diseases in dogs successfully treated with therapeutic gene delivery.

G protein-coupled receptor kinases in normal and failing myocardium

Heart failure (HF) is the end stage of many underlying cardiovascular diseases and is among the leading causes of morbidity and mortality in industrialized countries. One of the striking characteristics of HF is the desensitization of G protein-coupled receptor (GPCR) signaling, particularly the beta-adrenergic receptor (betaAR) system. GPCR desensitization is initiated by phosphorylation by GPCR kinases (GRKs), followed by downregulation and functional uncoupling from their G proteins. In the heart, the major GRK isoforms, GRK2 and GRK5, undergo upregulation due to the heightened sympathetic nervous system activity that is characteristic of HF as catecholamine levels increase in an effort to drive the failing pump. This desensitization leads to the distinctive loss of inotropic reserve and functional capacity of the failing heart. Moreover, GRK2 and GRK5 have an increasing non-GPCR interactome, which may play critical roles in cardiac physiology. In the current review, the canonical GPCR kinase function of GRKs and the novel non-GPCR kinase activity of GRKs, their contribution to the pathogenesis of cardiac hypertrophy and HF, and the possibility of GRKs serving as future drug targets will be discussed.

Impaired β-adrenergic responsiveness accentuates dysfunctional excitation-contraction coupling in an ovine model of tachypacing-induced heart failure

Reduced inotropic responsiveness is characteristic of heart failure (HF). This study determined the cellular Ca2+ homeostatic and molecular mechanisms causing the blunted β-adrenergic (β-AR) response in HF.We induced HF by tachypacing in sheep; intracellular Ca2+ concentration was measured in voltage-clamped ventricular myocytes. In HF, Ca2+ transient amplitude and peak L-type Ca2+ current (ICa-L) were reduced (to 70 ± 11% and 50 ± 3.7% of control, respectively, P <0.05) whereas sarcoplasmic reticulum (SR) Ca2+ content was unchanged. β-AR stimulation with isoprenaline (ISO) increased Ca2+ transient amplitude, ICa-L and SRCa2+ content in both cell types; however, the response of HF cells was markedly diminished (P <0.05).Western blotting revealed an increase in protein phosphatase levels (PP1, 158 ± 17% and PP2A, 188 ± 34% of control, P <0.05) and reduced phosphorylation of phospholamban in HF (Ser16, 30 ± 10% and Thr17, 41 ± 15% of control, P <0.05). The β-AR receptor kinase GRK-2 was also increased in HF (173 ± 38% of control, P <0.05). In HF, activation of adenylyl cyclase with forskolin rescued the Ca2+ transient, SR Ca2+ content and SR Ca2+ uptake rate to the same levels as control cells in ISO. In conclusion, the reduced responsiveness of the myocardium to β-AR agonists in HF probably arises as a consequence of impaired phosphorylation of key intracellular proteins responsible for regulating the SR Ca2+ content and therefore failure of the systolic Ca2+ transient to increase appropriately during β-AR stimulation.

Status of therapeutic gene transfer to treat canine dilated cardiomyopathy in dogs

Therapeutic gene transfer holds promise as a way to treat dilated cardiomyopathy from any underlying cause because the approach attempts to address metabolic disturbances that occur at the molecular level of the failing heart. Calcium-handling abnormalities and increased rates of apoptosis are abnormalities that occur in many types of heart disease, and gene therapies that target these metabolic defects have proven to be beneficial in numerous rodent models of heart disease. The authors are currently evaluating this approach to treat canine idiopathic dilated cardiomyopathy.

G-protein-coupled receptor kinases in cardiovascular conditions: focus on G-protein-coupled receptor kinase 2, a gain in translational medicine

With increasing knowledge of the regulatory mechanisms of G-protein-coupled receptor signaling in heart physiology, many studies have focused on the role of this system in cardiovascular disease. In recent years, scientists have moved their attention from the receptors to their regulatory proteins: the G-protein-coupled receptor kinases. This class of protein is indispensable for terminating signaling of G-protein-coupled receptors through receptor desensitization and downregulation. This article attempts to assemble the currently available information regarding G-protein-coupled receptor kinases and their role in cardiovascular disease and, in particular, the potential employment of G-protein-coupled receptor kinase 2 as biomarker of cardiac dysfunction.

Molecular aspects of ischemic heart disease: ischemia/reperfusion-induced genetic changes and potential applications of gene and RNA interference therapy

Molecular biologic techniques have a variety of applications in the study of ischemic heart disease, including roles in elucidating cardiac genetic changes resulting from ischemia as well as in developing therapeutic interventions to treat ischemic heart disease. This review describes recent studies documenting genetic changes associated with myocardial ischemia and infarction as well as those investigating the safety and effectiveness of gene therapy for stimulating angiogenesis, protecting the heart against reperfusion injury, and treating heart failure. Also discussed are future research directions, including the potential use of RNA interference and combined stem cell therapy and gene therapy for the treatment of cardiovascular disease.

Efficient myocyte gene delivery with complete cardiac surgical isolation in situ

BACKGROUND

Previously, we used cardiopulmonary bypass with incomplete cardiac isolation and antegrade administration of vector for global cardiac gene delivery. Here we present a translatable cardiac surgical procedure that allows for complete surgical isolation of the heart in situ with retrograde (through the coronary venous circulation) administration of both vector and endothelial permeabilizing agents to increase myocyte transduction efficiency.

METHODS

In 6 adult dogs the heart was completely isolated with tourniquets placed around both vena cavae and cannulas and all pulmonary veins. On cardiopulmonary bypass, the aorta and pulmonary artery were crossclamped, and the heart was isolated. Crystalloid cardioplegia at 4 degrees C containing 10(13) particles of adenovirus encoding LacZ and 15 microg of vascular endothelial growth factor was infused retrograde into the coronary sinus and recirculated for a total of 30 minutes. The dogs were then weaned from cardiopulmonary bypass and allowed to recover. With a catheter, 3 control dogs underwent retrograde infusion of the same cocktail without cardiac isolation or cardiopulmonary bypass.

RESULTS

Beta-galactosidase activities in the cardiopulmonary bypass group were several orders of magnitude higher in both the right and left ventricles when compared with those in the control group (P < .05). X-gal staining from the cardiopulmonary bypass group showed unequivocal evidence of myocyte gene expression globally in a significant proportion of cardiac myocytes. No myocyte gene expression was observed in the control group.

CONCLUSION

A novel cardiac surgical technique has been developed. This approach with cardiac isolation and retrograde delivery of vector through the coronary sinus results in efficient myocyte transduction in an adult large animal in vivo.

Alterations in cAMP-mediated signaling and their role in the pathophysiology of dilated cardiomyopathy

Dilated cardiomyopathy is a disease characterized by enlargement of the chambers of the heart and a decrease in contractility of the heart muscle. The process involves several alterations in proteins involved in cyclic adenosine monophosphate (cAMP) generation that result in a decrease in intracellular cAMP content per unit of adrenergic stimulation in cardiac myocytes. A fundamental question is whether these changes constitute a pathologic mechanism that contributes to chamber enlargement and hypocontractility or a compensatory adaptation that protects the heart from the adverse effects of increased catecholamine stimulation. Clinical studies in humans suggest that the latter effect may be more important. Studies in animal models, however, make the picture more complex: changes in cAMP-mediated signaling can have different effects depending on the specific protein whose expression or function is altered and the setting in which the alteration occurs. It may be that dilated cardiomyopathy represents a collection of different diseases in which alterations in cAMP-mediated signaling have different roles in the pathophysiology of the disease, and, furthermore, that changes in the phosphorylation of individual substrates of cAMP-dependent protein kinase may be either beneficial or harmful. Identifying differences among patients with dilated cardiomyopathy with respect to the role of altered cAMP-mediated signaling in their pathology, and identifying the "good" and "bad" substrates of cAMP-dependent protein kinase, are important areas for further research.

Left-right asymmetric ventricular expression of CARP in the piglet heart: regional response to experimental heart failure

BACKGROUND AND AIM

Cardiac ankyrin repeat protein (CARP), whose expression is down-regulated in response to doxorubicin (Dox) in vitro, has been proposed to be a marker of experimentally-induced cardiac hypertrophy in rodent models. In piglets, the rapid hypertrophy rate of the left ventricle (LV) as compared to that of the right ventricle (RV) represents a natural model of asymmetric ventricular enlargement. We tested whether CARP expression correlates with postnatal ventricular hypertrophy and to what extent CARP can be sensitive to Dox treatment in vivo.

METHODS

CARP mRNA and protein levels were quantified (by Northern blot hybridization, semi-quantitative RT-PCR and Western blot) in the piglet heart, both during early postnatal development and upon Dox-induced cardiomyopathy (Dox-CM).

RESULTS

The study revealed: (1) significantly augmented CARP mRNA and protein levels in the LV compared to the RV resulting in left vs. right asymmetry in ventricular CARP expression throughout early postnatal development; (2) dose- and chamber-dependent CARP mRNA and protein enrichment in ventricular myocardium in response to Dox; and (3) abolishment of asymmetric patterns of ventricular CARP expression at heart failure resulting from Dox-CM.

CONCLUSIONS

(1) CARP is differentially regulated in the LV and RV during both postnatal development and disease; and (2) monitoring of ventricular CARP expression patterns can be used for further analysis of transition from compensated to overt heart failure.

Candidate mechanical stimuli for hypertrophy during volume overload

A myocyte system that senses and responds to mechanical inputs might be activated by any number of features of the time-varying length or force signals experienced by the myocytes. We therefore characterized left ventricular volume and wall stress signals during early volume overload with high spatial and temporal resolution. Left ventricular pressure and volume were measured in open-chest isoflurane-anesthetized male Sprague-Dawley rats 4 and 7 days after surgical creation of an infrarenal arteriovenous fistula or sham operation. Mean wall stresses were calculated by using a simple thick-walled ellipsoidal model. Consistent with previous reports, this surgical model produced a 66% increase in cardiac output and a 10% increase in left ventricular mass by day 7. A number of features of the time-varying volume signal (maximum, mean, amplitude, rates of rise and fall) were significantly altered during early volume overload, whereas many other proposed hypertrophic stimuli, including peak systolic wall stress and diastolic strain, were not. Treating hemodynamic variables more generally as time-varying signals allowed us to identify a wider range of candidate mechanical stimuli for hypertrophy (including some not previously proposed in the literature) than focusing on standard time points in the cardiac cycle. We conclude that features of the time-varying ventricular volume signal and related local deformations may drive hypertrophy during volume overload and propose that those features of the volume signal that also change during pressure overload might be the most interesting candidates for further exploration.

Molecular mechanisms of reduced beta-adrenergic signaling in the aged heart as revealed by genomic profiling

Myocardial aging leads to a reduction of beta-adrenergic receptor-induced metabolic and contractile responsiveness. We hypothesize that a change in the patterns of gene expression is important in these age-related events. To test this, hearts were harvested from young and aged male rats (3-4 and 20-22 mo, respectively). Total mRNA was extracted and prepared for hybridization to Affymetrix U34A GeneChips. Filtering criteria, involving fold change and a statistical significance cutoff were employed, yielding 263 probe pairs exhibiting differential signals. Of the 163 annotated genes, at least 56 (34%) were classified as signaling/cell communication. Of these 56, approximately half were directly involved in G protein-coupled receptor signaling pathways. We next determined which of these changes might be involved in anti-adrenergic activity and identified 19 potentially important gene products. Importantly, we observed a decrease in beta1-adrenergic receptor and adenylyl cyclase mRNAs, whereas the mRNA encoding beta-arrestin increased. Furthermore, the results demonstrate an increase in mRNAs encoding the adenosine A1 receptor and phospholipase D, which could increase anti-adrenergic effects. Moreover, the mRNAs encoding the muscarinic M3 receptor, nicotinic acetylcholine receptor beta3, and nicotinic acetylcholine receptor-related protein were increased as was the mRNA encoding guanylate kinase-associated protein. Interestingly, we also observed eight mRNAs whose abundance changed three- to sixfold with aging that could be considered as being compensatory. Although these results do not prove causality, they demonstrate that cardiac aging is associated with changes in the profiles of gene expression and that many of these changes may contribute to reduced adrenergic signaling.

Alpha(1)-adrenergic activation of the cardiac ankyrin repeat protein gene in cardiac myocytes

Cardiac ankyrin repeat protein (CARP) is a nuclear transcription cofactor that is activated by multiple signaling pathways in hypertrophic cardiac myocytes. Since CARP has been reported to be a transcriptional co-repressor, its activation during hypertrophy might contribute to the deregulation of gene expression leading to heart failure. Here, we found that alpha(1)-adrenergic signaling activates CARP mRNA expression in rat cardiac myocytes. To examine how alpha(1)-adrenergic signaling activates the CARP gene, a 660 bp fragment of the mouse CARP promoter was cloned. Previous reports suggested that the mouse CARP promoter was dependent on the GATA4 transcription factor whereas the human CARP promoter was dependent on transcriptional enhancer factor-1 (TEF-1). TEF-1 and GATA4 transcription factors, known mediators of alpha(1)-adrenergic signaling, bound to the mouse CARP promoter at several sites as determined by gel mobility shift assays. These sites are highly conserved between the mouse and human promoters, suggesting that they are functionally important in both. Mutation analysis showed that binding of TEF-1 factors is required for basal activity of the CARP promoter in cardiac myocytes. However, over-expression of TEF-1 factors could not potentiate the response of the CARP promoter to alpha(1)-adrenergic stimulation. On the other hand, the alpha(1)-adrenergic response was potentiated by GATA4 over-expression. Taken together, our results demonstrate that alpha(1)-adrenergic signaling regulates CARP expression in cardiac myocytes, in part through the transcription factor GATA4.

Beta-adrenergic receptor-mediated DNA synthesis in cardiac fibroblasts is dependent on transactivation of the epidermal growth factor receptor and subsequent activation of extracellular signal-regulated kinases

Cardiac hypertrophy often leads to heart failure and is associated with abnormal myocardial adrenergic signaling. This enlargement of myocardial mass can involve not only an increase in cardiomyocyte size, but increased proliferation of cardiac fibroblasts. A potential key player in the cardiac hypertrophic response is the ERK family of MAPKs. To gain mechanistic insight into adrenergic regulation of myocardial mitogenic signaling, we examined beta-adrenergic receptor (beta-AR) stimulation of ERK activation and DNA synthesis in cultured adult rat cardiac fibroblasts, including the involvement of tyrosine kinases in this signaling pathway. Addition of the beta-AR agonist isoproterenol (ISO) to serum-starved cells induced DNA synthesis in a dose-dependent manner, and this was inhibited by selective inhibitors of the epidermal growth factor receptor (EGFR). Importantly and in agreement with the involvement of MAPKs and the EGFR in this response in cardiac fibroblasts, the EGFR inhibitor AG1478 attenuated ISO-induced ERK phosphorylation. Moreover, pretreatment with PP2, a selective inhibitor of the Src tyrosine kinase, attenuated both ISO-mediated EGFR phosphorylation and ERK activation. Furthermore, studies in these cardiac fibroblasts showed that phosphatidylinositol 3-kinase contributed to beta-AR-mediated ERK activation, but not to EGFR activation. Finally, studies using selective inhibitors of matrix metalloproteases indicated that they and heparin-bound EGF shedding were involved in beta-AR-induced ERK activation and subsequent DNA synthesis in cardiac fibroblasts. Because these cells primarily express the beta(2)-AR subtype, our findings indicate that beta(2)-AR-mediated EGFR transactivation of intracellular tyrosine kinase signaling pathways is the major signaling pathway responsible for the adrenergic stimulation of mitogenesis of cardiac fibroblasts.