Alpha1-adrenergic receptors prevent a maladaptive cardiac response to pressure overload

The alpha1-adrenergic receptors in cardiac hypertrophy: signaling mechanisms and functional implications

Cardiac hypertrophy is a complex remodeling process of the heart induced by physiological or pathological stimuli resulting in increased cardiomyocyte size and myocardial mass. Whereas cardiac hypertrophy can be an adaptive mechanism to stressful conditions of the heart, prolonged hypertrophy can lead to heart failure which represents the primary cause of human morbidity and mortality. Among G protein-coupled receptors, the α1-adrenergic receptors (α1-ARs) play an important role in the development of cardiac hypertrophy as demonstrated by numerous studies in the past decades, both in primary cardiomyocyte cultures and genetically modified mice. The results of these studies have provided evidence of a large variety of α1-AR-induced signaling events contributing to the defining molecular and cellular features of cardiac hypertrophy. Recently, novel signaling mechanisms have been identified and new hypotheses have emerged concerning the functional role of the α1-adrenergic receptors in the heart. This review will summarize the main signaling pathways activated by the α1-AR in the heart and their functional implications in cardiac hypertrophy.

The α1A-adrenergic receptor subtype mediates increased contraction of failing right ventricular myocardium

Dysfunction of the right ventricle (RV) is closely related to prognosis for patients with RV failure. Therefore, strategies to improve failing RV function are significant. In a mouse RV failure model, we previously reported that α1-adrenergic receptor (α1-AR) inotropic responses are increased. The present study determined the roles of both predominant cardiac α1-AR subtypes (α1A and α1B) in upregulated inotropy in failing RV. We used the mouse model of bleomycin-induced pulmonary fibrosis, pulmonary hypertension, and RV failure. We assessed the myocardial contractile response in vitro to stimulation of the α1A-subtype (using α1A-subtype-selective agonist A61603) and α1B-subtype [using α1A-subtype knockout mice and nonsubtype selective α1-AR agonist phenylephrine (PE)]. In wild-type nonfailing RV, a negative inotropic effect of α1-AR stimulation with PE (force decreased ≈50%) was switched to a positive inotropic effect (PIE) with bleomycin-induced RV injury. Upregulated inotropy in failing RV occurred with α1A-subtype stimulation (force increased ≈200%), but not with α1B-subtype stimulation (force decreased ≈50%). Upregulated inotropy mediated by the α1A-subtype involved increased activator Ca(2+) transients and increased phosphorylation of myosin regulatory light chain (a mediator of increased myofilament Ca(2+) sensitivity). In failing RV, the PIE elicited by the α1A-subtype was appreciably less when the α1A-subtype was stimulated in combination with the α1B-subtype, suggesting functional antagonism between α1A- and α1B-subtypes. In conclusion, upregulation of α1-AR inotropy in failing RV myocardium requires the α1A-subtype and is opposed by the α1B-subtype. The α1A subtype might be a therapeutic target to improve the function of the failing RV.

The PTIP-Associated Histone Methyltransferase Complex Prevents Stress-Induced Maladaptive Cardiac Remodeling

Pressure overload induces stress-induced signaling pathways and a coordinated transcriptional response that begets concentric cardiac hypertrophy. Although concentric hypertrophy initially attenuates wall stress and maintains cardiac function, continued stress can result in maladaptive cardiac remodeling. Cardiac remodeling is orchestrated by transcription factors that act within the context of an epigenetic landscape. Since the epigenetic landscape serves as a molecular link between environmental factors (stress) and cellular phenotype (disease), defining the role of the epigenome in the development and progression of cardiac remodeling could lead to new therapeutic approaches. In this study, we hypothesized that the epigenetic landscape is important in the development of cardiac hypertrophy and the progression to maladaptive remodeling. To demonstrate the importance of the epigenome in HF, we targeted the PTIP-associated histone methyltransferase complex in adult cardiac myocytes. This complex imparts histone H3 lysine 4 (H3K4) methylation marks at actively expressed genes. We subjected PTIP null (PTIP-) mice to 2 weeks of transverse aortic constriction, a stress that induces concentric hypertrophy in control mice (PTIP+). PTIP- mice have a maladaptive response to 2wk of transverse aortic constriction (TAC)-induced pressure overload characterized by cardiac dilatation, decreased LV function, cardiac fibrosis, and increased cell death. PTIP deletion resulted in altered stress-induced gene expression profiles including blunted expression of ADRA1A, ADRA1B, JUN, ATP2A2, ATP1A2, SCN4B, and CACNA1G. These results suggest that H3K4 methylation patterns and the complexes that regulate them, specifically the PTIP-associated HMT, are necessary for the adaptive response to TAC.

Deubiquitinating enzyme CYLD mediates pressure overload-induced cardiac maladaptive remodeling and dysfunction via downregulating Nrf2

Ubiquitin proteasome system (UPS) consists of ubiquitin, ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin ligases (E3s), proteasomes, and deubiquitinating enzymes (DUBs). Ubiquitin, E1s, several E2s, E3s, and proteasomes play an important role in the regulation of cardiac homeostasis and dysfunction; however, less is known about the role of DUBs in the heart. Here, we uncovered a crucial role of cyclindromatosis (CYLD), a DUB, in mediating cardiac maladaptive remodeling and dysfunction. CYLD expression was dramatically upregulated in the cardiomyocytes of hypertrophic and failing human and murine hearts. Knockout of CYLD improved survival rate and alleviated cardiac hypertrophy, fibrosis, apoptosis, oxidative stress, and dysfunction in mice that were subjected to sustained pressure overload induced by transverse aortic constriction. Deep sequencing and gene array analyses revealed that the most dramatically changed genes are those involving in the free radical scavenging pathway and cardiovascular disease, including fos, jun, myc, and nuclear factor erythroid-2 related factor 2 (Nrf2) in the heart. Moreover, knockdown of CYLD enhanced mitogen-activated protein kinase (MAPK) ERK- and p38-mediated expression of c-jun, c-fos, and c-myc, which govern Nrf2 expression in cardiomyocytes. The CYLD deficiency-induced suppression of reactive oxygen species (ROS) formation, death and hypertrophy in cardiomyocytes was blocked by additional knockdown of Nrf2. Taken together, our findings demonstrate for the first time that CYLD mediates cardiac maladaptive remodeling and dysfunction, most likely via enhancing myocardial oxidative stress in response to pressure overload. At the molecular level, CYLD interrupts the ERK- and p38-/AP-1 and c-Myc pathways to suppress Nrf2-operated antioxidative capacity, thereby enhancing oxidative stress in the heart.

Isoproterenol acts as a biased agonist of the alpha-1A-adrenoceptor that selectively activates the MAPK/ERK pathway

The α_1A-AR is thought to couple predominantly to the Gα_q/PLC pathway and lead to phosphoinositide hydrolysis and calcium mobilization, although certain agonists acting at this receptor have been reported to trigger activation of arachidonic acid formation and MAPK pathways. For several G protein-coupled receptors (GPCRs) agonists can manifest a bias for activation of particular effector signaling output, i.e. not all agonists of a given GPCR generate responses through utilization of the same signaling cascade(s). Previous work with Gα_q coupling-defective variants of α_1A-AR, as well as a combination of Ca²⁺ channel blockers, uncovered cross-talk between α_1A-AR and β₂-AR that leads to potentiation of a Gα_q-independent signaling cascade in response to α_1A-AR activation. We hypothesized that molecules exist that act as biased agonists to selectively activate this pathway. In this report, isoproterenol (Iso), typically viewed as β-AR-selective agonist, was examined with respect to activation of α_1A-AR. α_1A-AR selective antagonists were used to specifically block Iso evoked signaling in different cellular backgrounds and confirm its action at α_1A-AR. Iso induced signaling at α_1A-AR was further interrogated by probing steps along the Gα_q /PLC, Gα_s and MAPK/ERK pathways. In HEK-293/EBNA cells transiently transduced with α_1A-AR, and CHO_α_1A-AR stable cells, Iso evoked low potency ERK activity as well as Ca²⁺ mobilization that could be blocked by α_1A-AR selective antagonists. The kinetics of Iso induced Ca²⁺ transients differed from typical Gα_q- mediated Ca²⁺ mobilization, lacking both the fast IP₃R mediated response and the sustained phase of Ca²⁺ re-entry. Moreover, no inositol phosphate (IP) accumulation could be detected in either cell line after stimulation with Iso, but activation was accompanied by receptor internalization. Data are presented that indicate that Iso represents a novel type of α_1A-AR partial agonist with signaling bias toward MAPK/ERK signaling cascade that is likely independent of coupling to Gα_q.

Alpha-1-adrenergic receptors in heart failure: the adaptive arm of the cardiac response to chronic catecholamine stimulation

Alpha-1-adrenergic receptors (ARs) are G protein-coupled receptors activated by catecholamines. The alpha-1A and alpha-1B subtypes are expressed in mouse and human myocardium, whereas the alpha-1D protein is found only in coronary arteries. There are far fewer alpha-1-ARs than beta-ARs in the nonfailing heart, but their abundance is maintained or increased in the setting of heart failure, which is characterized by pronounced chronic elevation of catecholamines and beta-AR dysfunction. Decades of evidence from gain and loss-of-function studies in isolated cardiac myocytes and numerous animal models demonstrate important adaptive functions for cardiac alpha-1-ARs to include physiological hypertrophy, positive inotropy, ischemic preconditioning, and protection from cell death. Clinical trial data indicate that blocking alpha-1-ARs is associated with incident heart failure in patients with hypertension. Collectively, these findings suggest that alpha-1-AR activation might mitigate the well-recognized toxic effects of beta-ARs in the hyperadrenergic setting of chronic heart failure. Thus, exogenous cardioselective activation of alpha-1-ARs might represent a novel and viable approach to the treatment of heart failure.

Nuclear localization of a1A-adrenergic receptors is required for signaling in cardiac myocytes: an “inside-out” a1-AR signaling pathway

Background

Recent studies indicate that α1‐adrenergic receptors (α1‐ARs) are cardioprotective by preventing cardiac myocyte death and augmenting contractility in heart failure. Although G‐protein‐coupled receptors are assumed to localize to and signal at the plasma membrane, we previously demonstrated that endogenous α1‐ARs localize to the nuclei in adult cardiac myocytes. However, the functional consequence of this nuclear localization remains unclear. Here, we attempted to reconcile nuclear localization of α1‐ARs with their physiologic function by examining α1‐AR‐induced contractility in adult cardiac myocytes.

Methods and Results

By measuring shortening in unloaded, cultured adult cardiac myocytes, we found that the α1A‐subtype regulated contractility through phosphorylation of cardiac troponin I (cTnI) at the protein kinase C (PKC) site, threonine 144. Reconstitution of an α1A‐subtype nuclear localization mutant in cardiac myocytes lacking α1‐ARs failed to rescue nuclear α1A‐mediated phosphorylation of cTnI and myocyte contractility. Leptomycin B, the nuclear export inhibitor, also blocked α1A‐mediated phosphorylation of cTnI. These data indicate that α1‐AR signaling originates in the nucleus. Consistent with these observations, we localized the α1A‐subtype to the inner nuclear membrane, identified PKCα, δ, and ε in the nucleus, and found that α1‐ARs activate PKCδ in nuclei isolated from adult cardiac myocytes. Finally, we found that a PKCδ nuclear localization mutant blunted α1‐induced phosphorylation of cTnI.

Conclusions

Together, our data identify a novel, “inside‐out” nuclear α1A‐subtype/PKCδ/cTnI‐signaling pathway that regulates contractile function in adult cardiac myocytes. Importantly, these data help resolve the discrepancy between nuclear localization of α1‐ARs and α1‐AR‐mediated physiologic function.

Alpha1a-adrenoceptor genetic variant induces cardiomyoblast-to-fibroblast-like cell transition via distinct signaling pathways

The role of naturally occurring human α1a-Adrenergic Receptor (α1aAR) genetic variants associated with cardiovascular disorders is poorly understood. Here, we present the novel findings that expression of human α1aAR-247R (247R) genetic variant in cardiomyoblasts leads to transition of cardiomyoblasts into a fibroblast-like phenotype, evidenced by morphology and distinct de novo expression of characteristic genes. These fibroblast-like cells exhibit constitutive, high proliferative capacity and agonist-induced hypertrophy compared with cells prior to transition. We demonstrate that constitutive, synergistic activation of EGFR, Src and ERK kinases is the potential molecular mechanism of this transition. We also demonstrate that 247R triggers two distinct EGFR transactivation-dependent signaling pathways: 1) constitutive Gq-independent β-arrestin-1/Src/MMP/EGFR/ERK-dependent hyperproliferation and 2) agonist-induced Gq- and EGFR/STAT-dependent hypertrophy. Interestingly, in cardiomyoblasts agonist-independent hyperproliferation is MMP-dependent, but in fibroblast-like cells it is MMP-independent, suggesting that expression of α1aAR genetic variant in cardiomyocytes may trigger extracellular matrix remodeling. Thus, these novel findings demonstrate that EGFR transactivation by α1aAR-247R leads to hyperproliferation, hypertrophy and alterations in cardiomyoblasts, suggesting that these unique genetically-mediated alterations in signaling pathways and cellular function may lead to myocardial fibrosis. Such extracellular matrix remodeling may contribute to the genesis of arrhythmias in certain types of heart failure.

Adrenergic signaling in heart failure: a balance of toxic and protective effects

Heart failure with reduced ejection fraction involves activation of the sympathetic nervous system and chronic hyperactivation of the sympatho-adrenergic receptors (ARs) β-ARs and α1-ARs, which are thought to be cardiotoxic and worsen pathological remodeling and function. Concurrently, the failing heart manifests significant decreases in sympathetic nerve terminal density, decreased cardiac norepinephrine levels, and marked downregulation of β-AR abundance and signaling. Thus, a state of both feast and famine coexist with respect to the adrenergic state in heart failure. For the failing heart, the hyperadrenergic state is toxic. However, the role of hypoadrenergic mechanisms in the pathophysiology of heart failure is less clear. Cardiotoxic effects are known to arise from the β1-AR subtype, and use of β-AR blockers is a cornerstone of current heart failure therapy. However, cardioprotective effects arise from the β2-AR subtype that counteract hyperactive β1-AR signaling, but unfortunately, β2-AR cardioprotective signaling in heart failure is inhibited by β-AR blocker therapy. In contrast to current dogma, recent research shows β1-AR signaling can also be cardioprotective. Moreover, for some forms of heart failure, β2-AR signaling is cardiotoxic. Thus for both β-AR subtypes, there is a balance between cardiotoxic versus cardioprotective effects. In heart failure, stimulation of α1-ARs is widely thought to be cardiotoxic. However, also contrary to current dogma, recent research shows that α1-AR signaling is cardioprotective. Taken together, recent research identifies cardioprotective signaling arising from β1-AR, β2-AR, and α1-ARs. A goal for future therapies will to harness the protective effects of AR signaling while minimizing cardiotoxic effects. The trajectory of heart failure therapy changed radically from the previous and intuitive use of sympathetic agonists, which unfortunately resulted in greater mortality, to the current use of β-AR blockers, which initially seemed counterintuitive. As a cautionary note, if the slow adoption of beta-blocker therapy in heart failure is any guide, then new treatment strategies, especially counterintuitive therapies involving stimulating β-AR and α1-AR signaling, may take considerable time to develop and gain acceptance.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

Role of AKAP79/150 protein in β1-adrenergic receptor trafficking and signaling in mammalian cells

Protein kinase A-anchoring proteins (AKAPs) participate in the formation of macromolecular signaling complexes that include protein kinases, ion channels, effector enzymes, and G-protein-coupled receptors. We examined the role of AKAP79/150 (AKAP5) in trafficking and signaling of the β1-adrenergic receptor (β1-AR). shRNA-mediated down-regulation of AKAP5 in HEK-293 cells inhibited the recycling of the β1-AR. Recycling of the β1-AR in AKAP5 knockdown cells was rescued by shRNA-resistant AKAP5. However, truncated mutants of AKAP5 with deletions in the domains involved in membrane targeting or in binding to calcineurin or PKA failed to restore the recycling of the β1-AR, indicating that full-length AKAP5 was required. Furthermore, recycling of the β1-AR in rat neonatal cardiac myocytes was dependent on targeting the AKAP5-PKA complex to the C-terminal tail of the β1-AR. To analyze the role of AKAP5 more directly, recycling of the β1-AR was determined in ventricular myocytes from AKAP5(-/-) mice. In AKAP5(-/-) myocytes, the agonist-internalized β1-AR did not recycle, except when full-length AKAP5 was reintroduced. These data indicate that AKAP5 exerted specific and profound effects on β1-AR recycling in mammalian cells. Biochemical or real time FRET-based imaging of cyclic AMP revealed that deletion of AKAP5 sensitized the cardiac β1-AR signaling pathway to isoproterenol. Moreover, isoproterenol-mediated increase in contraction rate, surface area, or expression of β-myosin heavy chains was significantly greater in AKAP5(-/-) myocytes than in AKAP5(+/+) myocytes. These results indicate a significant role for the AKAP5 scaffold in signaling and trafficking of the β1-AR in cardiac myocytes and mammalian cells.

Stimulation of α1a adrenergic receptors induces cellular proliferation or antiproliferative hypertrophy dependent solely on agonist concentration

Stimulation of α_1aAdrenergic Receptors (ARs) is known to have anti-proliferative and hypertrophic effects; however, some studies also suggests this receptor can increase cell proliferation. Surprisingly, we find the α_1aAR expressed in rat-1 fibroblasts can produce either phenotype, depending exclusively on agonist concentration. Stimulation of the α_1aAR by high dose phenylephrine (>10⁻⁷ M) induces an antiproliferative, hypertrophic response accompanied by robust and extended p38 activation. Inhibition of p38 with SB203580 prevented the antiproliferative response, while inhibition of Erk or Jnk had no effect. In stark contrast, stimulation of the α_1aAR with low dose phenylephrine (∼10⁻⁸ M) induced an Erk-dependent increase in cellular proliferation. Agonist-induced Erk phosphorylation was preceded by rapid FGFR and EGFR transactivation; however, only EGFR inhibition blocked Erk activation and proliferation. The general matrix metalloprotease inhibitor, GM6001, blocked agonist induced Erk activation within seconds, strongly suggesting EGFR activation involved extracellular triple membrane pass signaling. Erk activation required little Ca²⁺ release and was blocked by PLCβ or PKC inhibition but not by intracellular Ca²⁺ chelation, suggesting Ca²⁺ independent activation of novel PKC isoforms. In contrast, Ca²⁺ release was essential for PI3K/Akt activation, which was acutely maximal at non-proliferative doses of agonist. Remarkably, our data suggests EGFR transactivation leading to Erk induced proliferation has the lowest activation threshold of any α_1aAR response. The ability of α_1aARs to induce proliferation are discussed in light of evidence suggesting antagonistic growth responses reflect native α_1aAR function.

Hormonal regulation of β-myosin heavy chain expression in the mouse left ventricle

We investigated the influence of sex hormones on the expression of α- and β-cardiac myosin heavy chain isoforms (α-MHC and β-MHC) in C57bl/6 mice of both sexes under physiological and pathological conditions. In the left ventricles (LVs) of fertile female mice, β-MHC expression was tenfold higher compared with the age-matched males, whereas no difference was found in α-MHC expression. These differences disappeared after ovariectomy or in immature mice. We also found a sex-related difference in expression of β-adrenoceptors (β1-AR), as mRNA levels of this gene were 40% lower in fertile females compared with males of the same age but did not differ in prepubertal or ovariectomized animals. Interestingly, the deletion of both β1- and β2-ARs abolished sex difference of β-MHC expression, as mRNA levels in the LVs of knockout males were increased and reached values comparable to those of knockout females. Moreover, the β1-AR antagonist metoprolol induced about a threefold increase in β-MHC expression in adult male mice. The capability of gender to regulate β-MHC expression was also evaluated in the presence of hemodynamic overload. Thoracic aortic coarctation (TAC) produced cardiac hypertrophy along with a 12-fold increase in β-MHC and a 50% decrease in β1-AR expression in males but not in females, thus abolishing the gender difference observed in sham animals for such genes. By contrast, TAC did not change β2-AR expression. In conclusion, our results show that the expression of β-MHC and β1-AR in the LVs undergo gender-related and correlated changes under both physiological and pathological conditions and suggest a role of β1-AR-mediated signaling.

α₁A-adrenergic receptors regulate cardiac hypertrophy in vivo through interleukin-6 secretion

The role of α₁-adrenergic receptors (ARs) in the regulation of cardiac hypertrophy is still unclear, because transgenic mice demonstrated hypertrophy or the lack of it despite high receptor overexpression. To further address the role of the α₁-ARs in cardiac hypertrophy, we analyzed unique transgenic mice that overexpress constitutively active mutation (CAM) α₁A-ARs or CAM α₁B-ARs under the regulation of large fragments of their native promoters. These constitutively active receptors are expressed in all tissues that endogenously express their wild-type counterparts as opposed to only myocyte-targeted transgenic mice. In this study, we discovered that CAM α₁A-AR mice in vivo have cardiac hypertrophy independent of changes in blood pressure, corroborating earlier studies, but in contrast to myocyte-targeted α₁A-AR mice. We also found cardiac hypertrophy in CAM α₁B-AR mice, in agreement with previous studies, but hypertrophy only developed in older mice. We also discovered unique α₁-AR-mediated hypertrophic signaling that was AR subtype-specific with CAM α₁A-AR mice secreting atrial naturietic factor and interleukin-6 (IL-6), whereas CAM α₁B-AR mice expressed activated nuclear factor-κB (NF-κB). These particular hypertrophic signals were blocked when the other AR subtype was coactivated. We also discovered that crossbreeding the two CAM models (double CAM α₁A/B-AR) inhibited the development of hypertrophy and was reversible with single receptor activation, suggesting that coactivation of the receptors can lead to novel antagonistic signal transduction. This was confirmed by demonstrating antagonistic signals that were even lower than normal controls in the double CAM α₁A/B-AR mice for p38, NF-κB, and the IL-6/glycoprotein 130/signal transducer and activator of transcription 3 pathway. Because α₁A/B double knockout mice fail to develop hypertrophy in response to IL-6, our results suggest that IL-6 is a major mediator of α₁A-AR cardiac hypertrophy.

Different expression of adrenoceptors and GRKs in the human myocardium depends on heart failure etiology and correlates to clinical variables

Downregulation of β(1)- adrenergic receptors (β(1)-ARs) and increased expression/function of G-protein-coupled receptor kinase 2 (GRK2) have been observed in human heart failure, but changes in expression of other ARs and GRKs have not been established. Another unresolved question is the incidence of these compensatory mechanisms depending on heart failure etiology and treatment. To analyze these questions, we quantified the mRNA/protein expressions of six ARs (α(1A), α(1B), α(1D), β(1), β(2), and β(3)) and three GRKs (GRK2, GRK3, and GRK5) in left (LV) and right ventricle (RV) from four donors, 10 patients with ischemic cardiomyopathy (IC), 14 patients with dilated cardiomyopathy (DC), and 10 patients with nonischemic, nondilated cardiopathies (NINDC). We correlated the changes in the expressions of ARs and GRKs with clinical variables such as left ventricular ejection fraction (LVEF) and left ventricular end-systolic and left ventricular end-diastolic diameter (LVESD and LVEDD, respectively). The main findings were 1) the expression of the α(1A)-AR in the LV positively correlates with LVEF; 2) the expression of GRK3 and GRK5 inversely correlates with LVESD and LVEDD, supporting previous observations about a protective role for both kinases in failing hearts; and 3) β(1)-AR expression is downregulated in the LV and RV of IC, in the LV of DC, and in the RV of NINDC. This difference, better than an increased expression of GRK2 (not observed in IC), determines the lower LVEF in IC and DC vs. NINDC.

Nuclear localization drives α1-adrenergic receptor oligomerization and signaling in cardiac myocytes

Conventional models of G-protein coupled receptor (GPCR) signaling describe cell surface receptors binding to external ligands, such as hormones or circulating peptides, to induce intracellular signaling and a physiologic response. However, recent studies identify new paradigms indicating that GPCRs localize to and signal at the nucleus and that GPCR oligomers can influence receptor function. Previously, we reported that endogenous α1-adrenergic receptors (α1-ARs) localize to and signal at the nuclei in adult cardiac myocytes. In this study, we examined the mechanisms behind α1-AR nuclear localization and how nuclear localization impacted receptor function. We verified that endogenous α1-ARs localized to the nuclear membrane of intact nuclei isolated from wild-type adult cardiac myocytes. Next, we identified and disrupted putative nuclear localization sequences in both the α1A- and α1B-adrenergic receptors, which led to mis-localization of α1-ARs in cultured adult cardiac myocytes. Using these mutants, we demonstrated that nuclear localization was required for α1-signaling in adult cardiac myocytes. We also found that the nuclear export inhibitor leptomycin B inhibited α1-AR signaling, indicating α1-AR signaling must arise in the nucleus in adult cardiac myocytes. Finally, we found that co-localization of the α1-subtypes at the nuclei in adult cardiac myocytes facilitated the formation of receptor oligomers that could affect receptor signaling. In summary, our data indicate that α1-AR nuclear localization can drive the formation of receptor oligomers and regulate signaling in adult cardiac myocytes.

Cardiomyocyte overexpression of the α1A-adrenergic receptor in the rat phenocopies second but not first window preconditioning

We examined α(1A)-adrenergic receptor (AR) mediation of preconditioning in a novel α(1A)-AR cardiac transgenic (TG) rat model (α(1A)-TG). Compared with nontransgenic littermates (NTLs), in conscious α(1A)-TG rats, heart rate was reduced, contractility [left ventricle (LV) +dP/dt, ejection fraction, end-systolic elastance] was significantly enhanced, and triple product (LV systolic wall stress × LV +dP/dt × heart rate) was unchanged. However, infarct size (IS)/area at risk (AAR) in response to ischemia-reperfusion (30 min coronary occlusion/3 h reperfusion) was reduced to 35 ± 4.6% in α(1A)-TGs vs. 52 ± 2.2% in NTLs (P < 0.05). Second window preconditioning reduced IS/AAR in NTLs to 29 ± 2.7% but did not afford further protection in α(1A)-TGs. In contrast, with first window preconditioning, IS/AAR was reduced to similar levels in both α(1A)-TGs (12 ± 1.4%) and NTLs (10 ± 1.1%). In untreated α(1A)-TGs, cardioprotection was associated with enhanced myocardial phosphorylated (p)-mitogen/extracellular signal-regulated kinase (MEK), p-extracellular signal-regulated kinase (ERK), and inducible nitric oxide synthase (iNOS) at the protein level, along with a 1.3-fold increase in total nitric oxide synthase activity like in second window preconditioning. Affymetrix microarrays revealed that few genes (4.6% of 3,172 upregulated; 8.8% of 3,498 downregulated) showed directionally similar changes in α(1A)-TGs vs. NTLs subjected to second window preconditioning. Thus, second, but not first, window cardioprotection is evident in α(1A)-TGs in the absence of ischemic preconditioning and is mediated by iNOS activation associated with MEK/ERK phosphorylation. Transcriptionally, however, second window preconditioning is considerably more complex than α(1A)-TG preconditioning, with the alteration of thousands of additional genes affording no further protection than that already available in α(1A)-TG rats.

cMyBP-C as a promiscuous substrate: phosphorylation by non-PKA kinases and its potential significance

It is now generally accepted that phosphorylation of cMyBP-C is critically important in maintaining normal cardiac function. Although much of the work to date on phospho-regulation of cMyBP-C has focused on the role of protein kinase A (PKA, also known as cAMP-dependent protein kinase), recent evidence suggests that a number of non-PKA serine/threonine kinases, such as Ca(2+)/calmodulin-dependent protein kinase II, protein kinase C, protein kinase D and the 90-kDa ribosomal S6 kinase are also capable of targeting this key regulatory sarcomeric protein. This article reviews such evidence and proposes a hypothetical role for some of the pertinent signalling pathways in phospho-regulation of cMyBP-C in the setting of heart failure.

β-myosin heavy chain is induced by pressure overload in a minor subpopulation of smaller mouse cardiac myocytes

RATIONALE

Induction of the fetal hypertrophic marker gene β-myosin heavy chain (β-MyHC) is a signature feature of pressure overload hypertrophy in rodents. β-MyHC is assumed present in all or most enlarged myocytes.

OBJECTIVE

To quantify the number and size of myocytes expressing endogenous β-MyHC by a flow cytometry approach.

METHODS AND RESULTS

Myocytes were isolated from the left ventricle of male C57BL/6J mice after transverse aortic constriction (TAC), and the fraction of cells expressing endogenous β-MyHC was quantified by flow cytometry on 10,000 to 20,000 myocytes with use of a validated β-MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size. β-MyHC-positive myocytes constituted 3 ± 1% of myocytes in control hearts (n=12), increasing to 25 ± 10% at 3 days to 6 weeks after TAC (n=24, P<0.01). β-MyHC-positive myocytes did not enlarge with TAC and were smaller at all times than myocytes without β-MyHC (≈70% as large, P<0.001). β-MyHC-positive myocytes arose by addition of β-MyHC to α-MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had α-MyHC only. Myocytes positive for β-MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium.

CONCLUSIONS

β-MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have α-MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller working ventricular myocytes with more myosin.

Cardiac pressure overload hypertrophy is differentially regulated by β-adrenergic receptor subtypes

In isolated myocytes, hypertrophy induced by norepinephrine is mediated via α(1)-adrenergic receptors (ARs) and not β-ARs. However, mice with deletions of both major cardiac α(1)-ARs still develop hypertrophy in response to pressure overload. Our purpose was to better define the role of β-AR subtypes in regulating cardiac hypertrophy in vivo, important given the widespread clinical use of β-AR antagonists and the likelihood that patients treated with these agents could develop conditions of further afterload stress. Mice with deletions of β(1), β(2), or both β(1)- and β(2)-ARs were subjected to transverse aortic constriction (TAC). After 3 wk, β(1)(-/-) showed a 21% increase in heart to body weight vs. sham controls, similar to wild type, whereas β(2)(-/-) developed exaggerated (49% increase) hypertrophy. Only when both β-ARs were ablated (β(1)β(2)(-/-)) was hypertrophy totally abolished. Cardiac function was preserved in all genotypes. Several known inhibitors of cardiac hypertrophy (FK506 binding protein 5, thioredoxin interacting protein, and S100A9) were upregulated in β(1)β(2)(-/-) compared with the other genotypes, whereas transforming growth factor-β(2), a positive mediator of hypertrophy was upregulated in all genotypes except the β(1)β(2)(-/-). In contrast to recent reports suggesting that angiogenesis plays a critical role in regulating cardiac hypertrophy-induced heart failure, we found no evidence that angiogenesis or its regulators (VEGF, Hif1α, and p53) play a role in compensated cardiac hypertrophy. Pressure overload hypertrophy in vivo is dependent on a coordination of signaling through both β(1)- and β(2)-ARs, mediated through several key cardiac remodeling pathways. Angiogenesis is not a prerequisite for compensated cardiac hypertrophy.

Cardiac and neuroprotection regulated by α(1)-adrenergic receptor subtypes

Sympathetic nervous system regulation by the α(1)-adrenergic receptor (AR) subtypes (α(1A), α(1B), α(1D)) is complex, whereby chronic activity can be either detrimental or protective for both heart and brain function. This review will summarize the evidence that this dual regulation can be mediated through the different α(1)-AR subtypes in the context of cardiac hypertrophy, heart failure, apoptosis, ischemic preconditioning, neurogenesis, locomotion, neurodegeneration, cognition, neuroplasticity, depression, anxiety, epilepsy, and mental illness.

Omega-3 fatty acids prevent pressure overload-induced cardiac fibrosis through activation of cyclic GMP/protein kinase G signaling in cardiac fibroblasts

BACKGROUND

Omega-3 polyunsaturated fatty acids (eicosapentaenoic acid and docosahexaenoic acid) from fish oil ameliorate cardiovascular diseases. However, little is known about the effects of ω-3 polyunsaturated fatty acids on cardiac fibrosis, a major cause of diastolic dysfunction and heart failure. The present study assessed the effects of ω-3 polyunsaturated fatty acids on cardiac fibrosis.

METHODS AND RESULTS

We assessed left ventricular fibrosis and pathology in mice subjected to transverse aortic constriction after the consumption of a fish oil or a control diet. In control mice, 4 weeks of transverse aortic constriction induced significant cardiac dysfunction, cardiac fibrosis, and cardiac fibroblast activation (proliferation and transformation into myofibroblasts). Dietary supplementation with fish oil prevented transverse aortic constriction-induced cardiac dysfunction and cardiac fibrosis and blocked cardiac fibroblast activation. In heart tissue, transverse aortic constriction increased active transforming growth factor-β1 levels and phosphorylation of Smad2. In isolated adult mouse cardiac fibroblasts, transforming growth factor-β1 induced cardiac fibroblast transformation, proliferation, and collagen synthesis. Eicosapentaenoic acid and docosahexaenoic acid increased cyclic GMP levels and blocked cardiac fibroblast transformation, proliferation, and collagen synthesis. Eicosapentaenoic acid and docosahexaenoic acid blocked phospho-Smad2/3 nuclear translocation. DT3, a protein kinase G inhibitor, blocked the antifibrotic effects of eicosapentaenoic acid and docosahexaenoic acid. Eicosapentaenoic acid and docosahexaenoic acid increased phosphorylated endothelial nitric oxide synthase and endothelial nitric oxide synthase protein levels and nitric oxide production.

CONCLUSION

Omega-3 fatty acids prevent cardiac fibrosis and cardiac dysfunction by blocking transforming growth factor-β1-induced phospho-Smad2/3 nuclear translocation through activation of the cyclic GMP/protein kinase G pathway in cardiac fibroblasts.

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Functional alpha-1B adrenergic receptors on human epicardial coronary artery endothelial cells

Alpha-1-adrenergic receptors (α1-ARs) regulate coronary arterial blood flow by binding catecholamines, norepinephrine (NE), and epinephrine (EPI), causing vasoconstriction when the endothelium is disrupted. Among the three α1-AR subtypes (α1A, α1B, and α1D), the α1D subtype predominates in human epicardial coronary arteries and is functional in human coronary smooth muscle cells (SMCs). However, the presence or function of α1-ARs on human coronary endothelial cells (ECs) is unknown. Here we tested the hypothesis that human epicardial coronary ECs express functional α1-ARs. Cultured human epicardial coronary artery ECs were studied using quantitative real-time reverse transcription polymerase chain reaction, radioligand binding, immunoblot, and (3)H-thymidine incorporation. The α1B-subtype messenger ribonucleic acid (mRNA) was predominant in cultured human epicardial coronary ECs (90-95% of total α1-AR mRNA), and total α1-AR binding density in ECs was twice that in coronary SMCs. Functionally, NE and EPI through the α1B subtype activated extracellular signal-regulated kinase (ERK) in ECs, stimulated phosphorylation of EC endothelial nitric oxide synthase (eNOS), and increased deoxyribonucleic acid (DNA) synthesis. These results are the first to demonstrate α1-ARs on human coronary ECs and indicate that the α1B subtype is predominant. Our findings provide another potential mechanism for adverse cardiac effects of drug antagonists that nonselectively inhibit all three α1-AR subtypes.

Adrenergic signaling polymorphisms and their impact on cardiovascular disease

This review examines the impact of recent discoveries defining personal genetics of adrenergic signaling polymorphisms on scientific discovery and medical practice related to cardiovascular diseases. The adrenergic system is the major regulator of minute-by-minute cardiovascular function. Inhibition of adrenergic signaling with pharmacological beta-adrenergic receptor antagonists (beta-blockers) is first-line therapy for heart failure and hypertension. Advances in pharmacology, molecular biology, and genetics of adrenergic signaling pathways have brought us to the point where personal genetic differences in adrenergic signaling factors are being assessed as determinants of risk or progression of cardiovascular disease. For a few polymorphisms, functional data generated in cell-based systems, genetic mouse models, and pharmacological provocation of human subjects are concordant with population studies that suggest altered risk of cardiovascular disease or therapeutic response to beta-blockers. For the majority of adrenergic pathway polymorphisms however, published data conflict, and the clinical relevance of individual genotyping remains uncertain. Here, the current state of laboratory and clinical evidence that adrenergic pathway polymorphisms can affect cardiovascular pathophysiology is comprehensively reviewed and compared, with a goal of placing these data in the broad context of potential clinical applicability.

Post-transcriptional control of human ether-a-go-go-related gene potassium channel protein by alpha-adrenergic receptor stimulation

Stimulation of alpha1-adrenoreceptors (alpha1-AR) acutely alters ion channel behavior via several signaling pathways [calcium and protein kinase C (PKC)]. Little is known about sustained alpha1-adrenergic/PKC signaling and channel regulation as may occur during cardiovascular disease states. Here we describe the effects of prolonged alpha1A-AR and PKC activity on human ether-a-go-go-related gene (HERG) K(+) channels (Kv11.1) expressed in a heterologous expression system. Stimulation of alpha1A-AR with phenylephrine or direct activation of PKC with phorbol ester increased HERG channel protein abundance and K(+) current density in a time- and dose-dependent manner. Channel augmentation reached a steady-state plateau within 24 h with a 2- to 6-fold induction. Phorbol ester and moderate alpha1A-AR stimulation enhanced HERG abundance in a PKC-dependent fashion but with stronger alpha1A-adrenergic stimulation; protein kinase A (PKA)-dependent activity also contributed. Comparable channel induction of other cardiac K(+) channels was not seen in this system. Comparison of wild-type HERG and channels with either mutated PKC phosphorylation sites (HERGDeltaPKC) or mutated PKA phosphorylation sites (HERGDeltaPKA) suggested that the mechanisms of augmentation of HERG by the two kinases were partially overlapping. The PKC-dependent effect was largely due to enhanced synthetic rates. Stimulation of alpha1-AR in cultured rat neonatal cardiac myocytes also enhanced the abundance of ERG channels. These findings show that alpha1A-AR stimulation is capable of influencing the balance of HERG channel synthesis and degradation via multiple signaling pathways, a process that may have relevance in cardiac diseases and treatment.

Molecular distinction between physiological and pathological cardiac hypertrophy: experimental findings and therapeutic strategies

Cardiac hypertrophy can be defined as an increase in heart mass. Pathological cardiac hypertrophy (heart growth that occurs in settings of disease, e.g. hypertension) is a key risk factor for heart failure. Pathological hypertrophy is associated with increased interstitial fibrosis, cell death and cardiac dysfunction. In contrast, physiological cardiac hypertrophy (heart growth that occurs in response to chronic exercise training, i.e. the 'athlete's heart') is reversible and is characterized by normal cardiac morphology (i.e. no fibrosis or apoptosis) and normal or enhanced cardiac function. Given that there are clear functional, structural, metabolic and molecular differences between pathological and physiological hypertrophy, a key question in cardiovascular medicine is whether mechanisms responsible for enhancing function of the athlete's heart can be exploited to benefit patients with pathological hypertrophy and heart failure. This review summarizes key experimental findings that have contributed to our understanding of pathological and physiological heart growth. In particular, we focus on signaling pathways that play a causal role in the development of pathological and physiological hypertrophy. We discuss molecular mechanisms associated with features of cardiac hypertrophy, including protein synthesis, sarcomeric organization, fibrosis, cell death and energy metabolism and provide a summary of profiling studies that have examined genes, microRNAs and proteins that are differentially expressed in models of pathological and physiological hypertrophy. How gender and sex hormones affect cardiac hypertrophy is also discussed. Finally, we explore how knowledge of molecular mechanisms underlying pathological and physiological hypertrophy may influence therapeutic strategies for the treatment of cardiovascular disease and heart failure.

Cardiac signaling genes exhibit unexpected sequence diversity in sporadic cardiomyopathy, revealing HSPB7 polymorphisms associated with disease

Sporadic heart failure is thought to have a genetic component, but the contributing genetic events are poorly defined. Here, we used ultra-high-throughput resequencing of pooled DNAs to identify SNPs in 4 biologically relevant cardiac signaling genes, and then examined the association between allelic variants and incidence of sporadic heart failure in 2 large Caucasian populations. Resequencing of DNA pools, each containing DNA from approximately 100 individuals, was rapid, accurate, and highly sensitive for identifying common and rare SNPs; it also had striking advantages in time and cost efficiencies over individual resequencing using conventional Sanger methods. In 2,606 individuals examined, we identified a total of 129 separate SNPs in the 4 cardiac signaling genes, including 23 nonsynonymous SNPs that we believe to be novel. Comparison of allele frequencies between 625 Caucasian nonaffected controls and 1,117 Caucasian individuals with systolic heart failure revealed 12 SNPs in the cardiovascular heat shock protein gene HSPB7 with greater proportional representation in the systolic heart failure group; all 12 SNPs were confirmed in an independent replication study. These SNPs were found to be in tight linkage disequilibrium, likely reflecting a single genetic event, but none altered amino acid sequence. These results establish the power and applicability of pooled resequencing for comparative SNP association analysis of target subgenomes in large populations and identify an association between multiple HSPB7 polymorphisms and heart failure.

Proteomic profiling reveals comprehensive insights into adrenergic receptor-mediated hypertrophy in neonatal rat cardiomyocytes

Myocardial adrenergic receptors (ARs) play important roles in cardiac hypertrophy. However, the detailed molecular mechanism of AR-mediated cardiac hypertrophy remains elusive to date. To gain full insight into how ARs are involved in the regulation of cardiac hypertrophy, protein expression profiling was performed with comparative proteomics approach on neonatal rat cardiomyocytes. Forty-six proteins were identified as differentially expressed in hypertrophic cardiomyocytes induced by AR stimulation. To better understand the biological significance of the obtained proteomic data, we utilized the ingenuity pathway analysis tool to construct biological networks and analyze function and pathways that might associate with AR-mediated cardiac hypertrophy. Pathway analysis strongly suggested that ROS may be involved in the development of AR-mediated cardiac hypertrophy, which was then confirmed by further experimentation. The results showed that a marked increase in ROS production was detected in AR-mediated cardiac hypertrophy and blocking of ROS production significantly inhibited AR-mediated cardiac hypertrophy. We further proved that the ROS production was through NADPH oxidase or the mitochondrial electron transport chain and this ROS accumulation resulted in activation of extracellular signal-regulated kinase 1/2 leading to AR-mediated cardiac hypertrophy. These experimental results support the hypothesis, from the ingenuity pathway analysis, that AR-mediated cardiac hypertrophy is associated with the dysregulation of a complicated oxidative stress-regulatory network. In conclusion, our results provide a basis for understanding the detailed molecular mechanisms of AR-mediated cardiac hypertrophy.

The alpha-1D Is the predominant alpha-1-adrenergic receptor subtype in human epicardial coronary arteries

OBJECTIVES

The goal was to identify alpha-1-adrenergic receptor (AR) subtypes in human coronary arteries.

BACKGROUND

The alpha1-ARs regulate human coronary blood flow. The alpha1-ARs exist as 3 molecular subtypes, alpha1A, alpha1B, and alpha1D, and the alpha1D subtype mediates coronary vasoconstriction in the mouse. However, the alpha1A is thought to be the only subtype in human coronary arteries.

METHODS

We obtained human epicardial coronary arteries and left ventricular (LV) myocardium from 19 transplant recipients and 6 unused donors (age 19 to 70 years; 68% male; 32% with coronary artery disease). We cultured coronary rings and human coronary smooth muscle cells. We assayed alpha1- and beta-AR subtype messenger ribonucleic acid (mRNA) by quantitative real-time reverse transcription polymerase chain reaction and subtype proteins by radioligand binding and extracellular signal-regulated kinase (ERK) activation.

RESULTS

The alpha1D subtype was 85% of total coronary alpha1-AR mRNA and 75% of total alpha1-AR protein, and alpha1D stimulation activated ERK. In contrast, the alpha1D was low in LV myocardium. Total coronary alpha1-AR levels were one-third of beta-ARs, which were 99% the beta2 subtype.

CONCLUSIONS

The alpha1D subtype is predominant and functional in human epicardial coronary arteries, whereas the alpha1A and alpha1B are present at very low levels. This distribution is similar to the mouse, where myocardial alpha1A- and alpha1B-ARs mediate beneficial functional responses and coronary alpha1Ds mediate vasoconstriction. Thus, alpha1D-selective antagonists might mediate coronary vasodilation, without the negative cardiac effects of nonselective alpha1-AR antagonists in current use. Furthermore, it could be possible to selectively activate beneficial myocardial alpha1A- and/or alpha1B-AR signaling without causing coronary vasoconstriction.

Sex-dependent gene regulatory networks of the heart rhythm

Expression level, control, and intercoordination of 66 selected heart rhythm determinant (HRD) genes were compared in atria and ventricles of four male and four female adult mice. We found that genes encoding various adrenergic receptors, ankyrins, ion channels and transporters, connexins, cadherins, plakophilins, and other components of the intercalated discs form a complex network that is chamber dependent and differs between the two sexes. In addition, most HRD genes in atria had higher expression in males than in females, while in ventricles, expression levels were mostly higher in females than in males. Moreover, significant chamber differences were observed between the sexes, with higher expression in atria than ventricles for males and higher expression in ventricles than atria for females. We have ranked the selected genes according to their prominence (new concept) within the HRD gene web defined as extent of expression coordination with the other web genes and stability of expression. Interestingly, the prominence hierarchy was substantially different between the two sexes. Taken together, these findings indicate that the organizational principles of the heart rhythm transcriptome are sex dependent, with the newly introduced prominence analysis allowing identification of genes that are pivotal for the sexual dichotomy.

{alpha}1-Adrenergic receptor subtypes in nonfailing and failing human myocardium

BACKGROUND

alpha1-adrenergic receptors (alpha1-ARs) play adaptive roles in the heart and protect against the development of heart failure. The 3 alpha1-AR subtypes, alpha1A, alpha1B, and alpha1D, have distinct physiological roles in mouse heart, but very little is known about alpha1 subtypes in human heart. Here, we test the hypothesis that the alpha1A and alpha1B subtypes are present in human myocardium, similar to the mouse, and are not downregulated in heart failure.

METHODS AND RESULTS

Hearts from transplant recipients and unused donors were failing (n=12; mean ejection fraction, 24%) or nonfailing (n=9; mean ejection fraction, 59%) and similar in age ( approximately 44 years) and sex ( approximately 70% male). We measured the alpha1-AR subtypes in multiple regions of both ventricles by quantitative real-time reverse-transcription polymerase chain reaction and radioligand binding. All 3 alpha1-AR subtype mRNAs were present, and alpha1A mRNA was most abundant ( approximately 65% of total alpha1-AR mRNA). However, only alpha1A and alpha1B binding were present, and the alpha1B was most abundant (60% of total). In failing hearts, alpha1A and alpha1B binding was not downregulated, in contrast with beta1-ARs.

CONCLUSIONS

Our data show for the first time that the alpha1A and alpha1B subtypes are both present in human myocardium, but alpha1D binding is not, and the alpha1 subtypes are not downregulated in heart failure. Because alpha1 subtypes in the human heart are similar to those in the mouse, where adaptive and protective effects of alpha1 subtypes are most convincing, it might become feasible to treat heart failure with a drug targeting the alpha1A and/or alpha1B.

Impact of alpha 1-adrenergic antagonist use for benign prostatic hypertrophy on outcomes in patients with heart failure

Previous clinical trials have shown that alpha(1)-adrenergic antagonists are not effective in subjects with heart failure (HF) and might increase HF rates when used for hypertension. However, alpha(1)-adrenergic antagonists may be prescribed to subjects with HF who have symptomatic benign prostatic hyperplasia. We sought to determine any association between alpha(1)-adrenergic antagonist use, commonly prescribed for benign prostatic hyperplasia, and the clinical outcomes of subjects with HF receiving contemporary therapy. An existing database of 388 subjects with decompensated HF admissions from 2002 to 2004 at the Veterans Affairs Hospital was analyzed according to the use of alpha(1)-adrenergic antagonists at discharge. Covariate-adjusted Cox proportional hazard models were used to examine any association with future admissions for decompensated HF and total mortality. Alpha-1-adrenergic antagonist therapy was prescribed in 25% of our HF population, predominantly for benign prostatic hyperplasia, and was not associated with significant increases in the combined risk of all-cause mortality and rehospitalization for HF (hazard ratio 1.24, 95% confidence interval 0.93 to 1.65, p = 0.14), HF hospitalization (hazard ratio 1.20, 95% confidence interval 0.85 to 1.70, p = 0.31), or all-cause mortality (hazard ratio 1.10, 95% confidence interval 0.78 to 1.56, p = 0.57). In patients not receiving beta-blocker therapy, alpha(1)-adrenergic antagonist therapy was significantly associated with increased HF hospitalizations (hazard ratio 1.94, 95% confidence interval 1.14 to 3.32, p = 0.015). In conclusion, in patients with chronic HF, the use of alpha(1)-adrenergic antagonists was significantly associated with more HF hospitalizations when prescribed without concomitant beta blockade. Thus, background beta-blocker therapy appears to be protective against the potential harmful effects of alpha(1)-adrenergic antagonist therapy in patients with HF.

Nrf2 protects against maladaptive cardiac responses to hemodynamic stress

BACKGROUND

Reactive oxygen species (ROS) play an important role in the maintenance of cardiovascular homeostasis. The present study sought to determine whether nuclear factor erythroid-2 related factor 2 (Nrf2), a master gene of the endogenous antioxidant defense system, is a critical regulator of the cardiac hypertrophic response to pathological stress.

METHODS AND RESULTS

Cardiac hypertrophy and dysfunction were established in mice by transverse aortic constriction (TAC). Nrf2 expression was transiently increased and then declined to the basal level while impairment of cardiac function proceeded. The knockout of Nrf2 (Nrf2(-/-)) did not cause any apparent structural and functional abnormalities in the unstressed heart. However, Nrf2(-/-) mice after TAC developed pathological cardiac hypertrophy, significant myocardial fibrosis and apoptosis, overt heart failure, and increased mortality, which were associated with elevated myocardial levels of 4-hydroxy-2-nonenal and 8-hydroxydeoxyguanosine and a complete blockade of the myocardial expression of several antioxidant genes. Overexpression of Nrf2 dramatically inhibited hypertrophic factor-induced ROS production and growth in both cardiomyocytes and cardiac fibroblasts, whereas knockdown of Nrf2 exerted opposite effects in both cells.

CONCLUSIONS

These findings demonstrate that activation of Nrf2 provides a novel mechanism to protect the murine heart against pathological cardiac hypertrophy and heart failure via suppressing oxidative stress.

Cardiac hypertrophy in neonatal nephrectomized rats: the role of the sympathetic nervous system

Cardiac hypertrophy is frequently encountered in patients with renal failure and represents an independent risk factor for cardiovascular morbidity and mortality. The pathogenesis of cardiac hypertrophy is related to multiple factors, including excess adrenergic activity. This study investigated how renal injury in the early stages of life affects the adrenergic system and thereby potentially influences cardiac growth. Biomarkers of cardiac hypertrophy were used to assess adrenergic function. Newborn male Sprague-Dawley rats were allocated to three groups of five rats each: 5/6 nephrectomy (Nx), pair-fed controls (PF), and sham-operated (SH). Nx animals had significantly higher plasma urea nitrogen, serum creatinine, and mean arterial blood pressure. The heart-weight/body-weight ratio of the Nx cohort was higher than SH and PF (p < 0.001) groups. Plasma norepinephrine (NE) of Nx animals was almost twofold higher than SH and PF (p < 0.01) animals. Compared with SH and PF, Nx animals had higher alpha1A-receptor protein expression, lower cardiac beta1- and beta2-receptor protein expression (p < 0.05), but higher G-protein-coupled receptor kinase-2 (GRK2) expression (p < 0.05). Norepinephrine transporter protein (NET) and renalase protein expression in cardiac tissue from Nx pups were significantly lower than SH and PF. Our data suggest that early age Nx animals have increased circulating catecholamines due to decreased NE metabolism. Enhancement of cardiac GRK2 and NE can contribute to cardiac hypertrophy seen in Nx animals. Furthermore, AKT (activated via alpha1A receptors), as well as increased alpha1A receptors and their agonist NE, might contribute to the observed hypertrophy.

How reliable are G-protein-coupled receptor antibodies?

A cluster of manuscripts in this issue of the Journal highlights a lack of selectivity of 49 antibodies against 19 subtypes of alpha(1)- and beta-adrenoceptors, muscarinic, dopamine and galanin receptors as well as vanilloid (TRPV1) receptors. Taken together these data demonstrate that lack of selectivity appears to be the rule rather than the exception for antibodies against G-protein-coupled and perhaps also other receptors. Thus, the previously often applied validation of such antibodies by the disappearance of staining in the presence of blocking peptide, i.e. the antigen against which the antibody was raised, alone is insufficient to demonstrate specificity. We propose that receptor antibodies should be validated by at least one of the following techniques: a) disappearance of staining in knock-out animals of the target receptor, b) reduction of staining upon knock-down approaches such as siRNA treatment, c) selectivity of staining in immunoblots or immunocytochemistry for the target receptor vs. related subtypes when expressed in the same cell line and/or d) antibodies raised against multiple distinct epitopes of a receptor yielding very similar staining patterns. Other issues of consideration to obtain reliable results based on receptor antibodies in applications such as immunohistochemistry or immunoblotting are also being discussed.

From the Cover: Whole-genome association study identifies STK39 as a hypertension susceptibility gene

Hypertension places a major burden on individual and public health, but the genetic basis of this complex disorder is poorly understood. We conducted a genome-wide association study of systolic and diastolic blood pressure (SBP and DBP) in Amish subjects and found strong association signals with common variants in a serine/threonine kinase gene, STK39. We confirmed this association in an independent Amish and 4 non-Amish Caucasian samples including the Diabetes Genetics Initiative, Framingham Heart Study, GenNet, and Hutterites (meta-analysis combining all studies: n = 7,125, P < 10(-6)). The higher BP-associated alleles have frequencies > 0.09 and were associated with increases of 3.3/1.3 mm Hg in SBP/DBP, respectively, in the Amish subjects and with smaller but consistent effects across the non-Amish studies. Cell-based functional studies showed that STK39 interacts with WNK kinases and cation-chloride cotransporters, mutations in which cause monogenic forms of BP dysregulation. We demonstrate that in vivo, STK39 is expressed in the distal nephron, where it may interact with these proteins. Although none of the associated SNPs alter protein structure, we identified and experimentally confirmed a highly conserved intronic element with allele-specific in vitro transcription activity as a functional candidate for this association. Thus, variants in STK39 may influence BP by increasing STK39 expression and consequently altering renal Na(+) excretion, thus unifying rare and common BP-regulating alleles in the same physiological pathway.

Facilitatory interplay in alpha 1a and beta 2 adrenoceptor function reveals a non-Gq signaling mode: implications for diversification of intracellular signal transduction

Agonist occupied alpha(1)-adrenoceptors (alpha(1)-ARs) engage several signaling pathways, including phosphatidylinositol hydrolysis, calcium mobilization, arachidonic acid release, mitogen-activated protein (MAP) kinase activation, and cAMP accumulation. The natural agonist norepinephrine (NE) activates with variable affinity and intrinsic efficacy all adrenoceptors, and in cells that coexpress alpha(1)- and beta-AR subtypes, such as cardiomyocytes, this leads to coactivation of multiple downstream pathways. This may result in pathway cross-talk with significant consequences to heart physiology and pathologic state. To dissect signaling components involved specifically in alpha(1A)- and beta(2)-AR signal interplay, we have developed a recombinant model system that mimics the levels of receptor expression observed in native cells. We followed intracellular Ca(2+) mobilization to monitor in real time the activation of both G(q) and G(s) pathways. We found that coactivation of alpha(1A)- and beta(2)-AR by the nonselective agonist NE or via a combination of the highly selective alpha(1A)-AR agonist A61603 and the beta-selective agonist isoproterenol led to increases in Ca(2+) influx from the extracellular compartment relative to stimulation with A61603 alone, with no effect on the associated transient release of Ca(2+) from intracellular stores. This effect became more evident upon examination of an alpha(1A)-AR variant exhibiting a partial defect in coupling to G(q), and we attribute it to potentiation of a non G(q)-pathway, uncovered by application of a combination of xestospongin C, an endoplasmic reticulum inositol 1,4,5-triphosphate receptor blocker, and 2-aminoethoxydiphenyl borate, a nonselective storeoperated Ca(2+) entry channel blocker. We also found that stimulation with A61603 of a second alpha(1A)-AR variant entirely unable to signal induced no Ca(2+) unless beta(2)-AR was concomitantly activated. These results may be accounted for by the presence of alpha(1A)/beta(2)-AR heterodimers or alternatively by specific adrenoceptor signal cross-talk resulting in distinct pharmacological behavior. Finally, our findings provide a new conceptual framework to rationalize outcomes from clinical studies targeting alpha- and beta-adrenoceptors.

Potential relevance of alpha(1)-adrenergic receptor autoantibodies in refractory hypertension

Background

Agonistic autoantibodies directed at the α₁-adrenergic receptor (α₁-AAB) have been described in patients with hypertension. We implied earlier that α₁-AAB might have a mechanistic role and could represent a therapeutic target.

Methodology/Principal Findings

To pursue the issue, we performed clinical and basic studies. We observed that 41 of 81 patients with refractory hypertension had α₁-AAB; after immunoadsorption blood pressure was significantly reduced in these patients. Rabbits were immunized to generate α₁-adrenergic receptor antibodies (α₁-AB). Patient α₁-AAB and rabbit α₁-AB were purified using affinity chromatography and characterized both by epitope mapping and surface plasmon resonance measurements. Neonatal rat cardiomyocytes, rat vascular smooth muscle cells (VSMC), and Chinese hamster ovary cells transfected with the human α_1A-adrenergic receptor were incubated with patient α₁-AAB and rabbit α₁-AB and the activation of signal transduction pathways was investigated by Western blot, confocal laser scanning microscopy, and gene expression. We found that phospholipase A2 group IIA (PLA2-IIA) and L-type calcium channel (Cacna1c) genes were upregulated in cardiomyocytes and VSMC after stimulation with both purified antibodies. We showed that patient α₁-AAB and rabbit α₁-AB result in protein kinase C alpha activation and transient extracellular-related kinase (EKR1/2) phosphorylation. Finally, we showed that the antibodies exert acute effects on intracellular Ca²⁺ in cardiomyocytes and induce mesentery artery segment contraction.

Conclusions/Significance

Patient α₁-AAB and rabbit α₁-AB can induce signaling pathways important for hypertension and cardiac remodeling. Our data provide evidence for a potential clinical relevance for α₁-AAB in hypertensive patients, and the notion of immunity as a possible cause of hypertension.

Ten commercial antibodies for alpha-1-adrenergic receptor subtypes are nonspecific

Commercial antibodies are used widely to quantify and localize the alpha1-adrenergic receptor (AR) subtypes, alpha1A, alpha1B, and alpha1D. We tested ten antibodies, from abcam and Santa Cruz, using western blot with heart and brain tissue from wild-type (WT) mice and mice with systemic knockout (KO) of one or all three subtypes. We found that none of the antibodies detected a band in WT that was absent in the appropriate KO or in the KO that was null for all alpha1-ARs (ABDKO). We conclude that the antibodies we tested are not specific for alpha1-ARs. These results raise caution with prior studies using these reagents. For now, competition radioligand binding is the only reliable approach to quantify the alpha1-AR subtype proteins. Receptor protein localization remains a challenge.

cAMP-mediated beta-adrenergic signaling negatively regulates Gq-coupled receptor-mediated fetal gene response in cardiomyocytes

The treatment with beta-blockers causes an enhancement of the norepinephrine-induced fetal gene response in cultured cardiomyocytes. Here, we tested whether the activation of cAMP-mediated beta-adrenergic signaling antagonizes alpha(1)-adrenergic receptor (AR)-mediated fetal gene response. To address this question, the fetal gene program, of which atrial natriuretic peptide (ANP) and the beta-isoform of myosin heavy chain are classical members, was induced by phenylephrine (PE), an alpha(1)-AR agonist. In cultured neonatal rat cardiomyocytes, we found that stimulation of beta-ARs with isoproterenol, a beta-AR agonist, inhibited the fetal gene expression induced by PE. Similar results were also observed when cardiomyocytes were treated with forskolin (FSK), a direct activator of adenylyl cyclase, or 8-CPT-6-Phe-cAMP, a selective activator of protein kinase A (PKA). Conversely, the PE-induced fetal gene expression was further upregulated by H89, a selective PKA inhibitor. To evaluate whether these results could be generalized to Gq-mediated signaling and not specifically to alpha(1)-ARs, cardiomyocytes were treated with prostaglandin F(2)alpha, another Gq-coupled receptor agonist, which is able to promote fetal gene expression. This treatment caused an increase of both ANP mRNA and protein levels, which was almost completely abolished by FSK treatment. The capability of beta-adrenergic signaling to regulate the fetal gene expression was also evaluated in vivo conditions by using beta1- and beta2-AR double knockout mice, in which the predominant cardiac beta-AR subtypes are lacking, or by administering isoproterenol (ISO), a beta-AR agonist, at a subpressor dose. A significant increase of the fetal gene expression was found in beta(1)- and beta(2)-AR gene deficient mice. Conversely, we found that ANP, beta-MHC and skACT mRNA levels were significantly decreased in ISO-treated hearts. Collectively, these data indicate that cAMP-mediated beta-adrenergic signaling negatively regulates Gq cascade activation-induced fetal gene expression in cultured cardiomyocytes and that this inhibitory regulation is already operative in the mouse heart under physiological conditions.

Nuclear alpha1-adrenergic receptors signal activated ERK localization to caveolae in adult cardiac myocytes

We previously identified an alpha1-AR-ERK (alpha1A-adrenergic receptor-extracellular signal-regulated kinase) survival signaling pathway in adult cardiac myocytes. Here, we investigated localization of alpha1-AR subtypes (alpha1A and alpha1B) and how their localization influences alpha1-AR signaling in cardiac myocytes. Using binding assays on myocyte subcellular fractions or a fluorescent alpha1-AR antagonist, we localized endogenous alpha1-ARs to the nucleus in wild-type adult cardiac myocytes. To clarify alpha1 subtype localization, we reconstituted alpha1 signaling in cultured alpha1A- and alpha1B-AR double knockout cardiac myocytes using alpha1-AR-green fluorescent protein (GFP) fusion proteins. Similar to endogenous alpha1-ARs and alpha1A- and alpha1B-GFP colocalized with LAP2 at the nuclear membrane. alpha1-AR nuclear localization was confirmed in vivo using alpha1-AR-GFP transgenic mice. The alpha1-signaling partners Galphaq and phospholipase Cbeta1 also colocalized with alpha1-ARs only at the nuclear membrane. Furthermore, we observed rapid catecholamine uptake mediated by norepinephrine-uptake-2 and found that alpha1-mediated activation of ERK was not inhibited by a membrane impermeant alpha1-blocker, suggesting alpha1 signaling is initiated at the nucleus. Contrary to prior studies, we did not observe alpha1-AR localization to caveolae, but we found that alpha1-AR signaling initiated at the nucleus led to activated ERK localized to caveolae. In summary, our results show that nuclear alpha1-ARs transduce signals to caveolae at the plasma membrane in cardiac myocytes.

GATA4 is a survival factor in adult cardiac myocytes but is not required for alpha1A-adrenergic receptor survival signaling

Recently, we defined an alpha1A-adrenergic receptor-ERK (alpha1A-AR-ERK) survival signaling pathway in adult cardiac myocytes. Previous studies in neonatal cardiac myocytes indicated that the cardiac-specific transcription factor GATA4 is a downstream mediator of alpha1-ERK signaling and that phosphorylation of GATA4 by ERK increases DNA binding and transcriptional activity. Therefore, we examined GATA4 as a potential downstream effector of alpha1A-ERK survival signaling in adult cardiac myocytes. We measured norepinephrine (NE)-induced cell death in cultured cardiac myocytes lacking alpha1-ARs (cultured from alpha1A/B-AR double-knockout mice, alpha1ABKO mice) that are susceptible to cell death induced by several proapoptotic stimuli, including NE. Our results show that overexpression of GATA4 is sufficient to protect alpha1ABKO cardiac myocytes from NE-induced cell death. However, we found that the alpha1A-subtype did not induce phosphorylation or increase the activity of GATA4 in adult mouse cardiac myocytes in culture or in vivo. Furthermore, we examined the effect of siRNA-mediated knockdown of GATA4 on alpha1A-survival signaling. In alpha1B-knockout cardiac myocytes, which express only the alpha1A-subtype and are protected from NE-induced cell death, GATA4 knockdown did not reverse alpha1A-survival signaling in response to NE. In summary, we found that GATA4 acted as a survival factor by preventing cell death in alpha1ABKO cardiac myocytes, but GATA4 was not activated by alpha1-AR stimulation and was not required for alpha1A-survival signaling in adult cardiac myocytes. This also identifies an important mechanistic difference in alpha1-signaling between adult and neonatal cardiac myocytes.

Characterization of the GNAQ promoter and association of increased Gq expression with cardiac hypertrophy in humans

AIMS

Transgenic mice with cardiac overexpression of Gq develop cardiac hypertrophy, apoptosis, and heart failure. Similar mechanisms may contribute to human left ventricular hypertrophy (LVH). However, mechanisms regulating transcription of the human GNAQ gene encoding the Gq protein are unknown and single-nucleotide polymorphisms have not been reported.

METHODS AND RESULTS

We delineated essential elements for transcription in the human GNAQ promoter using reporter assays and showed promoter induction by serum and angiotensin II. Sequencing of the whole promoter revealed a common (minor allele frequency 0.48) dinucleotide polymorphism at position -694/-695, resulting in an exchange of two adjacent nucleotides (TT > GC). The GC allele had increased transcription factor binding and was associated with enhanced transcriptional activation by serum or angiotensin II, resulting in enhanced Gq expression and intracellular signalling. Genotyping a population-based survey (n = 1204) revealed a higher prevalence of LVH in individuals with the GC/GC genotype [odds ratio (OR) 4.07; 95% CI 1.63-10.16; P = 0.003], this effect being more pronounced in women (OR 5.52; P = 0.005).

CONCLUSION

A novel polymorphism in the Gq promoter region is associated with enhanced promoter activity, Gq expression, intracellular signal transduction, and increased prevalence of LVH, particularly in women.