Selective Loss of Fine Tuning of Gq/11 Signaling by RGS2 Protein Exacerbates Cardiomyocyte Hypertrophy

Role of nuclear unphosphorylated STAT3 in angiotensin II type 1 receptor-induced cardiac hypertrophy

AIMS

Cardiac hypertrophy is a risk factor independent of blood pressure; however, the mechanisms that distinguish pathological remodelling due to local cues from pressure overload are unresolved. This study was aimed at discovering a novel gene expression mechanism in heart failure.

METHODS AND RESULTS

In angiotensin II type 1 receptor (AT1R) transgenic mice (TG), we found a significant increase of mRNA and total STAT3 (T-STAT3) protein, but not STAT3 phosphorylated at residues Y705 and S727. A net increase in nuclear accumulation of this unphosphorylated form of STAT3 (U-STAT3) correlated with the development of cardiac hypertrophy and dysfunction, which are associated with abnormal expression of osteopontin and regulator of G protein signalling 2 genes. Nuclear accumulation of U-STAT3 is induced by angiotensin II treatment in neonatal cardiac myocytes, fibroblasts, and AT1R-expressing human embryonic kidney 293 (HEK-AT1R) cells. Chromatin immunoprecipitation demonstrated that U-STAT3 binds to the target gene promoter, and siRNA-mediated knockdown of STAT3 expression significantly altered the expression of target genes in HEK-AT1R cells. T-STAT3 in TG mouse hearts and the phosphorylation-deficient Y705F mutant STAT3 in HEK-AT1R cells physically interacted with transcription co-activator p300.

CONCLUSION

Chronic activation of AT1R induces unregulated expression of the Stat3 gene, leading to nuclear accumulation of U-STAT3, which significantly correlated with progression of cardiac hypertrophy.

Deregulation of RGS2 in cardiovascular diseases

Alteration of G protein-coupled receptor (GPCR) signaling is a salient feature of hypertension and the associated heart diseases. Recent studies have revealed a large family of Regulators of G-protein Signaling (RGS) proteins as important endogenous regulators of GPCR signaling. RGS2 selectively regulates Galphaq/11 signaling, an essential cause of hypertension and cardiac hypertrophy. Both clinical and animal studies have shown that deregulation of RGS2 leads to exacerbated Galphaq/11 signaling. There is an inverse correlation between RGS2 expression and blood pressure, as well as a selective down-regulation of RGS2 in various models of cardiac hypertrophy. The causal relationship has been established in animal studies. RGS2 knockout mice exhibit not only hypertension phenotype but also accelerated cardiac hypertrophy and heart failure in response to pressure-overload. Further in vitro studies have shown that RGS2 knockdown with RNA interference exacerbates, whilst RGS2 over-expression completely abolishes the Galphaq/11-induced hypertrophy. These findings indicate that deregulation of RGS2 plays a crucial role in the pathogenesis of cardiovascular diseases, marking RGS2 as a potential therapeutic target or biomarker of hypertension or hypertensive heart diseases.

Ischemia induces regulator of G protein signaling 2 (RGS2) protein upregulation and enhances apoptosis in astrocytes

Regulator of G protein signaling (RGS) family members, such as RGS2, interact with Galpha subunits of heterotrimeric G proteins, accelerating the rate of GTP hydrolysis and attenuating the intracellular signaling triggered by the G protein-coupled receptor-ligand interaction. They are also reported to regulate G protein-effector interactions and form multiprotein signaling complexes. Ischemic stress-induced changes in RGS2 expression have been described in astrocytes, and these changes are associated with intracellular signaling cascades, suggesting that RGS2 upregulation may be an important mechanism by which astrocytes may regulate RGS2 function in response to physiological stress. However, information on the functional roles of stress-induced modulation of RGS2 protein expression in astrocyte function is limited. We report the role of ischemic stress in RGS2 protein expression in rat C6 astrocytoma cells and primary mouse astrocytes. A marked increase in RGS2 occurred after ischemic stress induced by chemicals (sodium azide and 2-deoxyglucose) or oxygen-glucose deprivation (OGD, real ischemia). RGS2 mRNA expression was markedly enhanced by 1 h of exposure to chemical ischemia or 6 h of OGD followed by 2 or 6 h of recovery, respectively. This enhanced expression in primary astrocytes and C6 cells was restored to baseline levels after 12 h of recovery from chemically induced ischemic stress or 4-6 h of recovery from OGD. RGS2 protein was also significantly expressed at 12-24 h of recovery from ischemic insult. Ischemia-induced RGS2 upregulation was associated with enhanced apoptosis. It significantly increased annexin V-positive cells, cleaved caspase-3, and enhanced DNA ladder formation and cell cycle arrest. However, a small interfering RNA (siRNA)-mediated RGS2 knockdown reversed the apoptotic cell death associated with ischemia-induced RGS2 upregulation. Upregulated RGS2 was significantly inhibited by SB-203580, a p38 MAPK inhibitor. Rottlerin, a potent inhibitor of PKCdelta, completely abrogated the increased RGS2 expression. We also examine whether ischemia-induced RGS2-mediated apoptosis is affected by siRNA-targeted endogenous PKCdelta downregulation or its phosphorylation. Although RGS2 upregulation was not affected, siRNA transfection significantly suppressed endogenous PKCdelta mRNA and protein expressions. Ischemia-induced PKCdelta phosphorylation and caspase-3 cleavage were dose dependently inhibited by PKCdelta knockdown, and this endogenous PKCdelta suppression reversed ischemia-induced annexin V-positive cells. This study suggests that ischemic stress increases RGS2 expression and that this condition contributes to enhanced apoptosis in C6 cells and primary astrocytes. The signaling it follows may involve PKCdelta and p38 MAPK pathways.

Translational control by RGS2

The regulator of G protein signaling (RGS) proteins are a family of guanosine triphosphatase (GTPase)-accelerating proteins. We have discovered a novel function for RGS2 in the control of protein synthesis. RGS2 was found to bind to eIF2Bepsilon (eukaryotic initiation factor 2B epsilon subunit) and inhibit the translation of messenger RNA (mRNA) into new protein. This effect was not observed for other RGS proteins tested. This novel function of RGS2 is distinct from its ability to regulate G protein-mediated signals and maps to a stretch of 37 amino acid residues within its conserved RGS domain. Moreover, RGS2 was capable of interfering with the eIF2-eIF2B GTPase cycle, which is a requisite step for the initiation of mRNA translation. Collectively, this study has identified a novel role for RGS2 in the control of protein synthesis that is independent of its established RGS domain function.

Regulator of G protein signaling 5: a new player in vascular remodeling

Regulators of G protein signaling (RGS) proteins are important modulators of G protein-coupled receptors and, therefore, critical for cardiovascular functions. One family member, RGS5, has recently been identified as a key regulator of vascular remodeling and pericyte maturation in tumors. Here, we discuss a potential role for RGS5 and its relatives, RGS2 and 4, within the cardiovascular system. Insights into RGS5 signaling are likely to be highly significant for vascular pathologies such as hypertension, atherosclerosis, and angiogenesis.

Adenosine regulation of microtubule dynamics in cardiac hypertrophy

There is evidence that endogenous extracellular adenosine reduces cardiac hypertrophy and heart failure in mice subjected to chronic pressure overload, but the mechanism by which adenosine exerts these protective effects is unknown. Here, we identified a novel role for adenosine in regulation of the cardiac microtubule cytoskeleton that may contribute to its beneficial effects in the overloaded heart. In neonatal cardiomyocytes, phenylephrine promoted hypertrophy and reorganization of the cytoskeleton, which included accumulation of sarcomeric proteins, microtubules, and desmin. Treatment with adenosine or the stable adenosine analog 2-chloroadenosine, which decreased hypertrophy, specifically reduced accumulation of microtubules. In hypertrophied cardiomyocytes, 2-chloroadenosine or adenosine treatment preferentially targeted stabilized microtubules (containing detyrosinated alpha-tubulin). Consistent with a role for endogenous adenosine in reducing microtubule stability, levels of detyrosinated microtubules were elevated in hearts of CD73 knockout mice (deficient in extracellular adenosine production) compared with wild-type mice (195%, P < 0.05). In response to aortic banding, microtubules increased in hearts of wild-type mice; this increase was exaggerated in CD73 knockout mice, with significantly greater amounts of tubulin partitioning into the cold-stable Triton-insoluble fractions. The levels of this stable cytoskeletal fraction of tubulin correlated strongly with the degree of heart failure. In agreement with a role for microtubule stabilization in promoting cardiac dysfunction, colchicine treatment of aortic-banded mice reduced hypertrophy and improved cardiac function compared with saline-treated controls. These results indicate that microtubules contribute to cardiac dysfunction and identify, for the first time, a role for adenosine in regulating cardiomyocyte microtubule dynamics.

RGS proteins: identifying new GAPs in the understanding of blood pressure regulation and cardiovascular function

Understanding the mechanisms that underlie BP (blood pressure) variation in humans and animal models may provide important clues for reducing the burden of uncontrolled hypertension in industrialized societies. High BP is often associated with increased signalling via G-protein-coupled receptors. Three members of the RGS (regulator of G-protein signalling) superfamily RGS2, RGS4 and RGS5 have been implicated in the attenuation of G-protein signalling pathways in vascular and cardiac myocytes, as well as cells of the kidney and autonomic nervous system. In the present review, we discuss the current state of knowledge regarding their differential expression and function in cardiovascular tissues, and the likelihood that one or more of these alleles are candidate hypertension genes. Together, findings from the studies described herein suggest that development of methods to modulate the expression and function of RGS proteins may be a possible strategy for the treatment and prevention of hypertension and cardiovascular disease.

Mechanisms of enhanced beta-adrenergic reserve from cardiac resynchronization therapy

BACKGROUND

Cardiac resynchronization therapy (CRT) is the first clinical heart failure treatment that improves chamber systolic function in both the short-term and long-term yet also reduces mortality. The mechanical impact of CRT is immediate and well documented, yet its long-term influences on myocyte function and adrenergic modulation that may contribute to its sustained benefits are largely unknown.

METHODS AND RESULTS

We used a canine model of dyssynchronous heart failure (DHF; left bundle ablation, atrial tachypacing for 6 weeks) and CRT (DHF for 3 weeks, biventricular tachypacing for subsequent 3 weeks), contrasting both to nonfailing controls. CRT restored contractile synchrony and improved systolic function compared with DHF. Myocyte sarcomere shortening and calcium transients were markedly depressed at rest and after isoproterenol stimulation in DHF (both anterior and lateral walls), and CRT substantially improved both. In addition, beta(1) and beta(2) stimulation was enhanced, coupled to increased beta(1) receptor abundance but no change in binding affinity. CRT also augmented adenylate cyclase activity over DHF. Inhibitory G-protein (Galpha(i)) suppression of beta-adrenergic stimulation was greater in DHF and reversed by CRT. Galpha(i) expression itself was unaltered; however, expression of negative regulators of Galpha(i) signaling (particularly RGS3) rose uniquely with CRT over DHF and controls. CRT blunted elevated myocardial catecholamines in DHF, restoring levels toward control.

CONCLUSIONS

CRT improves rest and beta-adrenergic-stimulated myocyte function and calcium handling, upregulating beta(1) receptors and adenylate cyclase activity and suppressing G(i)-coupled signaling associated with novel RGS upregulation. The result is greater rest and sympathetic reserve despite reduced myocardial neurostimulation as components underlying its net benefit.

Sphingosine-1-phosphate regulates RGS2 and RGS16 mRNA expression in vascular smooth muscle cells

Regulator of G protein signalling (RGS) protein expression is altered under growth promoting conditions in vascular smooth muscle cells (VSMCs). Since sphingosine-1-phosphate (S1P) is an important growth stimulatory factor, we investigated whether stimulation of VSMCs with S1P results in alterations in mRNA expression levels of several RGS proteins and which signalling components are involved. VSMCs were stimulated with S1P and mRNA expression levels of RGS2, RGS3, RGS4, RGS5 and RGS16 were measured by real-time polymerase chain reaction. S1P caused a time-dependent up-regulation of RGS2 and RGS16 mRNA expression. FTY720-P, a S1P(1)/S1P(3-5) agonist, did not regulate RGS2 mRNA levels although it did up-regulate RGS16 mRNA expression. Pertussis toxin treatment revealed that the S1P-induced RGS16 expression was G(i/o)-dependent whereas up-regulation of RGS2 mRNA was not. Phosphatidylinositol 3-kinase, protein kinase C and mitogen-activated protein kinase kinase apparently were not involved in the S1P-induced up-regulation of both RGS proteins. The present study demonstrates that S1P induces RGS2 and RGS16 mRNA expression but uses distinct S1P receptor subtypes and signalling pathways to regulate expression of these RGS proteins.

Regulator of G protein signaling 2 mediates cardiac compensation to pressure overload and antihypertrophic effects of PDE5 inhibition in mice

The heart initially compensates for hypertension-mediated pressure overload by enhancing its contractile force and developing hypertrophy without dilation. Gq protein-coupled receptor pathways become activated and can depress function, leading to cardiac failure. Initial adaptation mechanisms to reduce cardiac damage during such stimulation remain largely unknown. Here we have shown that this initial adaptation requires regulator of G protein signaling 2 (RGS2). Mice lacking RGS2 had a normal basal cardiac phenotype, yet responded rapidly to pressure overload, with increased myocardial Gq signaling, marked cardiac hypertrophy and failure, and early mortality. Swimming exercise, which is not accompanied by Gq activation, induced a normal cardiac response, while Rgs2 deletion in Galphaq-overexpressing hearts exacerbated hypertrophy and dilation. In vascular smooth muscle, RGS2 is activated by cGMP-dependent protein kinase (PKG), suppressing Gq-stimulated vascular contraction. In normal mice, but not Rgs2-/- mice, PKG activation by the chronic inhibition of cGMP-selective phosphodiesterase 5 (PDE5) suppressed maladaptive cardiac hypertrophy, inhibiting Gq-coupled stimuli. Importantly, PKG was similarly activated by PDE5 inhibition in myocardium from both genotypes, but PKG plasma membrane translocation was more transient in Rgs2-/- myocytes than in controls and was unaffected by PDE5 inhibition. Thus, RGS2 is required for early myocardial compensation to pressure overload and mediates the initial antihypertrophic and cardioprotective effects of PDE5 inhibitors.

Role of endogenous RGS proteins on endothelial ERK 1/2 activation

Endothelial cells are maintaining atherosclerotic signaling mediated by Extracellular Regulated Kinases 1 and 2 (ERK). Signaling gets activated upon stimulation of G protein-coupled receptors mediated by G(q) and G(i/o) proteins subjected to regulation by RGS proteins. The goal of the study was to delineate the specificity of RGS proteins modulating induced ERK phosphorylation. We used stimulated HUVEC, silenced specifically RGS proteins and compared assessed ERK 1/2 activation with immunohistochemical stainings on atherosclerotic plaques. Increased ERK phosphorylation was detected upon stimulation with Phenylephrine (2.6+/-0.1 times over basal), Endothelin-1 (1.8+/-0.2), Dopamine (5.1+/-0.2), TNF (9.8+/-0.7) or IL-4 (3.1+/-0.3). RGS silencing increased activation of ERK 1/2: Phen (RGS3, 5), ET-1 (RGS3, 4), Dopa (RGS3), TNF (RGS2, 3, 4) or IL-4 (RGS2, 3, 4). Immunohistochemically, increased ERK activation was detected on atherosclerotic plaques. This data supports the role of RGS proteins on ERK activation in human atherosclerosis which identifies RGS proteins as new therapeutical targets.

AMP activated protein kinase-alpha2 deficiency exacerbates pressure-overload-induced left ventricular hypertrophy and dysfunction in mice

AMP activated protein kinase (AMPK) plays an important role in regulating myocardial metabolism and protein synthesis. Activation of AMPK attenuates hypertrophy in cultured cardiac myocytes, but the role of AMPK in regulating the development of myocardial hypertrophy in response to chronic pressure overload is not known. To test the hypothesis that AMPKalpha2 protects the heart against systolic overload-induced ventricular hypertrophy and dysfunction, we studied the response of AMPKalpha2 gene deficient (knockout [KO]) mice and wild-type mice subjected to 3 weeks of transverse aortic constriction (TAC). Although AMPKalpha2 KO had no effect on ventricular structure or function under control conditions, AMPKalpha2 KO significantly increased TAC-induced ventricular hypertrophy (ventricular mass increased 46% in wild-type mice compared with 65% in KO mice) while decreased left ventricular ejection fraction (ejection fraction decreased 14% in wild-type mice compared with a 43% decrease in KO mice). AMPKalpha2 KO also significantly exacerbated the TAC-induced increases of atrial natriuretic peptide, myocardial fibrosis, and cardiac myocyte size. AMPKalpha2 KO had no effect on total S6 ribosomal protein (S6), p70 S6 kinase, eukaryotic initiation factor 4E, and 4E binding protein-1 or their phosphorylation under basal conditions but significantly augmented the TAC-induced increases of p-p70 S6 kinase(Thr389), p-S6(Ser235), and p-eukaryotic initiation factor 4E(Ser209). AMPKalpha2 KO also enhanced the TAC-induced increase of p-4E binding protein-1(Thr46) to a small degree and augmented the TAC-induced increase of p-Akt(Ser473). These data indicate that AMPKalpha2 exerts a cardiac protective effect against pressure-overload-induced ventricular hypertrophy and dysfunction.

Adenosine A3 receptor deficiency exerts unanticipated protective effects on the pressure-overloaded left ventricle

BACKGROUND

Endogenous adenosine can protect the overloaded heart against the development of hypertrophy and heart failure, but the contribution of A(1) receptors (A(1)R) and A(3) receptors (A(3)R) is not known.

METHODS AND RESULTS

To test the hypothesis that A(1)R and A(3)R can protect the heart against systolic overload, we exposed A(3)R gene-deficient (A(3)R knockout [KO]) mice and A(1)R KO mice to transverse aortic constriction (TAC). Contrary to our hypothesis, A(3)R KO attenuated 5-week TAC-induced left ventricular hypertrophy (ratio of ventricular mass/body weight increased to 7.6+/-0.3 mg/g in wild-type mice compared with 6.3+/-0.4 mg/g in KO mice), fibrosis, and dysfunction (left ventricular ejection fraction decreased to 43+/-2.5% and 55+/-4.2% in wild-type and KO mice, respectively). A(3)R KO also attenuated the TAC-induced increases of myocardial atrial natriuretic peptide and the oxidative stress markers 3'-nitrotyrosine and 4-hydroxynonenal. In contrast, A(1)R KO increased TAC-induced mortality but did not alter ventricular hypertrophy or dysfunction compared with wild-type mice. In mice in which extracellular adenosine production was impaired by CD73 KO, TAC caused greater hypertrophy and dysfunction and increased myocardial 3'-nitrotyrosine. In neonatal rat cardiomyocytes induced to hypertrophy with phenylephrine, the adenosine analogue 2-chloroadenosine reduced cell area, protein synthesis, atrial natriuretic peptide, and 3'-nitrotyrosine. Antagonism of A(3)R significantly potentiated the antihypertrophic effects of 2-chloroadenosine.

CONCLUSIONS

Adenosine exerts protective effects on the overloaded heart, but the A(3)R acts counter to the protective effect of adenosine. The data suggest that selective attenuation of A(3)R activity might be a novel approach to treat pressure overload-induced left ventricular hypertrophy and dysfunction.

Regulator of G-protein signaling subtype 4 mediates antihypertrophic effect of locally secreted natriuretic peptides in the heart

BACKGROUND

Mice lacking guanylyl cyclase-A (GC-A), a natriuretic peptide receptor, have pressure-independent cardiac hypertrophy. However, the mechanism underlying GC-A-mediated inhibition of cardiac hypertrophy remains to be elucidated. In the present report, we examined the role of regulator of G-protein signaling subtype 4 (RGS4), a GTPase activating protein for G(q) and G(i), in the antihypertrophic effects of GC-A.

METHODS AND RESULTS

In cultured cardiac myocytes, treatment of atrial natriuretic peptide stimulated the binding of guanosine 3',5'-cyclic monophosphate-dependent protein kinase (PKG) I-alpha to RGS4, PKG-dependent phosphorylation of RGS4, and association of RGS4 and Galpha(q). In contrast, blockade of GC-A by an antagonist, HS-142-1, attenuated the phosphorylation of RGS4 and association of RGS4 and Galpha(q). Moreover, overexpressing a dominant negative form of RGS4 diminished the inhibitory effects of atrial natriuretic peptide on endothelin-1-stimulated inositol 1,4,5-triphosphate production, [(3)H]leucine incorporation, and atrial natriuretic peptide gene expression. Furthermore, expression and phosphorylation of RGS4 were significantly reduced in the hearts of GC-A knockout (GC-A-KO) mice compared with wild-type mice. For further investigation, we constructed cardiomyocyte-specific RGS4 transgenic mice and crossbred them with GC-A-KO mice. The cardiac RGS4 overexpression in GC-A-KO mice significantly reduced the ratio of heart to body weight (P<0.001), cardiomyocyte size (P<0.01), and ventricular calcineurin activity (P<0.05) to 80%, 76%, and 67% of nontransgenic GC-A-KO mice, respectively. It also significantly suppressed the augmented cardiac expression of hypertrophy-related genes in GC-A-KO mice.

CONCLUSIONS

These results provide evidence that GC-A activates cardiac RGS4, which attenuates Galpha(q) and its downstream hypertrophic signaling, and that RGS4 plays important roles in GC-A-mediated inhibition of cardiac hypertrophy.

Ecto-5'-nucleotidase deficiency exacerbates pressure-overload-induced left ventricular hypertrophy and dysfunction

This study examined whether endogenous extracellular adenosine acts to facilitate the adaptive response of the heart to chronic systolic overload. To examine whether endogenous extracellular adenosine can protect the heart against pressure-overload-induced heart failure, transverse aortic constriction was performed on mice deficient in extracellular adenosine production as the result of genetic deletion of CD73. Although there was no difference in left ventricular size or function between CD73-deficient mice (knockout [KO] mice) and wild-type mice under unstressed conditions, aortic constriction for 2 or 4 weeks induced significantly more myocardial hypertrophy, left ventricular dilation, and left ventricular dysfunction in KO mice compared with wild-type mice. Thus, after 2 weeks of transverse aortic constriction, left ventricular fractional shortening decreased to 27.4+/-2.5% and 21.9+/-1.7% in wild-type and KO mice, respectively (P<0.05). Consistent with a role of adenosine in reducing tissue remodeling, KO mice displayed increased myocardial fibrosis and myocyte hypertrophy compared with wild-type mice. Furthermore, adenosine treatment reduced phenylephrine-induced cardiac myocyte hypertrophy and collagen production in cultured neonatal rat cardiac myocytes and cardiac fibroblasts, respectively. Consistent with a role for adenosine in modulating cardiomyocyte hypertrophy, KO mice demonstrated increased activation of mammalian target of rapamycin signaling, accompanied by higher expression of the hypertrophy marker atrial natriuretic peptide. Conversely, the adenosine analogue 2-chloro-adenosine significantly reduced cell size, mammalian target of rapamycin/p70 ribosomal S6 kinase activation, and atrial natriuretic peptide expression in cultured neonatal cardiomyocytes. These data demonstrate that CD73 helps to preserve cardiac function during chronic systolic overload by preventing maladaptive tissue remodeling.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

A role for RGS10 in beta-adrenergic modulation of G-protein-activated K+ (GIRK) channel current in rat atrial myocytes

The effect of beta-adrenergic stimulation on endogenous G-protein-activated K(+) (GIRK) current has been investigated in atrial myocytes from hearts of adult rats. Beta-adrenergic stimulation (10 microm isoprenaline, Iso) had no effect on activation kinetics, peak current or steady-state current but resulted in slowing of deactivation upon washout of acetylcholine (ACh), the time constant (tau(d)) being increased by a factor of about 2.5. The effect of Iso could be mimicked by inclusion of cAMP (500 microm) in the filling solution of the patch clamp pipette. The Iso-induced increase in tau(d) was blocked by the selective beta(1) receptor antagonist CGP-20112A (2 microm) and by the PKA inhibitor H9 (100 microm included in the pipette solution). A candidate for mediating these effects is RGS10, one of the regulators of G-protein signalling (RGS) species expressed in cardiac myocytes. Overexpression of RGS10 by adenoviral gene transfer resulted in a reduction in tau(d) of 60%. Sensitivity of tau(d) to Iso remained in cells overexpressing RGS10. Overexpression of RGS4 caused a comparable reduction in tau(d), which became insensitive to Iso. Expression of an RGS10 carrying a mutation (RGS10-S168A), which deletes a PKA phosphorylation site, caused a decrease in tau(d) comparable to overexpression of wild-type RGS10. Sensitivity of tau(d) to Iso was lost in RGS10-S168A-expressing myocytes. Silencing of RGS10 by means of adenovirus-mediated transcription of a short hairpin RNA did not affect basal tau(d) but removed sensitivity to Iso. These data suggest that endogenous RGS10 has GTPase-activating protein (GAP) activity on the G-protein species that mediates activation of atrial GIRK channels. Moreover, RGS10, via PKA-dependent phosphorylation, enables a crosstalk between beta-adrenergic and muscarinic cholinergic signalling.

RGS protein specificity towards Gq- and Gi/o-mediated ERK 1/2 and Akt activation, in vitro

Extracellular Regulated Kinases (ERK) and Protein Kinase B (Akt) are intermediaries in relaying extracellular growth signals to intracellular targets. Each pathway can become activated upon stimulation of G protein-coupled receptors mediated by G(q) and G(i/o) proteins subjected to regulation by RGS proteins. The goal of the study was to delineate the specificity in which cardiac RGS proteins modulate G(q)and G(i/o)-induced ERK and Akt phosphorylation. To isolate G(q)- and G(i/o)-mediated effects, we exclusively expressed muscarinic M(2) or M(3) receptors in COS-7 cells. Western blot analyses demonstrated increase of phosphorylation of ERK 1.7-/3.3-fold and Akt 2.4-/6-fold in M(2)-/M(3)- expressing cells through carbachol stimulation. In co-expressions, M(3)/G(q)-induced activation of Akt was exclusively blunted through RGS3s/RGS3, whereas activation of ERK was inhibited additionally through RGS2/RGS5. M(2)/G(i/o) induced Akt activation was inhibited by all RGS proteins tested. RGS2 had no effect on M(2)/G(i/o)-induced ERK activation. The high degree of specificity in RGS proteins-depending modulation of G(q)- and G(i/o)-mediated ERK and Akt activation in the muscarinic network cannot merely be attributed exclusively to RGS protein selectivity towards G(q) or G(i/o) proteins. Counter-regulatory mechanisms and inter-signaling cross-talk may alter the sensitivity of GPCR-induced ERK and Akt activation to RGS protein regulation.

Phosphodiesterase type 5: expanding roles in cardiovascular regulation

Phosphodiesterase type 5A (PDE5A) selectively hydrolyzes cyclic GMP. Inhibitors of PDE5A such as sildenafil are widely used to treat erectile dysfunction, but growing evidence supports important roles for the enzyme in both the vasculature and heart. In disorders such as cardiac failure, PDE5A upregulation may contribute to a decline in cGMP and protein kinase G signaling, exacerbating dysfunction. PDE5A plays an important role in the pulmonary vasculature where its inhibition benefits patients with pulmonary hypertension. In the heart, PDE5A signaling appears compartmentalized, and its inhibition is cardioprotective against ischemia-reperfusion and antracycline toxicity, blunts acute adrenergic contractile stimulation, and can suppress chronic hypertrophy and dysfunction attributable to pressure-overload. In this review, we discuss the molecular biology, pharmacology, and physiology of PDE5A, mechanisms of vascular and cardiac regulation, and recent evidence supporting the utility of selective PDE5A inhibition for the treatment of cardiovascular disorders.

Sphingosine-1-phosphate and endothelin-1 induce the expression of rgs16 protein in cardiac myocytes by transcriptional activation of the rgs16 gene

The expression of the negative Regulator of G protein signaling 16 (RGS16) is rapidly induced in cardiomyocytes by various stimuli. To identify the promoter of the mouse RGS16 gene, a 1.8-kb deoxyribonucleic acid fragment 5' of the RGS16-coding region was subcloned into a firefly-luciferase reporter vector and four overlapping fragments were analyzed. The luciferase production was quantified in neonatal rat cardiac myocytes (NRCM). A 0.6-kb fragment that induced a tenfold increase in luciferase activity contained the minimal promoter sequence. Its activity was twofold stimulated by fetal calf serum, endothelin-1 (ET-1), and sphingosine 1-phosphate (S1P), which stimuli also elevated the level of RGS16 protein. Stimulation of NRCM with ET-1 induced activation of the monomeric GTPases RhoA and Rac1, whereas S1P and the selective S1P1 receptor agonist SEW2871 only induced a pronounced activation of Rac1. In accordance, the treatment with the Rho-, Rac-, and Cdc42-inactivating Clostridium difficile Toxin B (TcdB) 10463 inhibited ET-1 and S1P-induced transcriptional activation. The ET-1-induced activation was insensitive to pertussis toxin but selectively suppressed by the RhoA-C-specific C2I-C3 ADP-ribosyl transferase and the ET(B) receptor antagonist BQ788. The S1P-induced activation was specifically inhibited by pertussis toxin and the Rac-inactivating TcdB 1470. All stimulated transcriptional activity was abolished by the negative transcription factor Yin Yang 1 (YY1), which binds to a consensus sequence within the minimal promoter. Taken together, our data show that most likely ET(B)- and S1P1-receptors induce RGS16 protein expression in cardiac myocytes by increasing the transcriptional activity of the rgs16 gene. This activation is mediated by heterotrimeric G proteins, Rho GTPases, and is under negative control of the transcription factor YY1.

R4 RGS proteins: regulation of G-protein signaling and beyond

The regulators of G-protein signaling (RGS) proteins were initially characterized as inhibitors of signal transduction cascades initiated by G-protein-coupled receptors (GPCR) because of their ability to increase the intrinsic GTPase activity of heterotrimeric G proteins. This GTPase accelerating protein (GAP) activity enhances G protein deactivation and promotes desensitization. However, in addition to this signature trait, emerging data have revealed an expanding network of proteins, lipids, and ions that interact with RGS proteins and confer additional regulatory functions. This review highlights recent advances in our understanding of the physiological functions of one subfamily of RGS proteins with a high degree of homology (B/R4) gleaned from recent studies of knockout mice or cells with reduced RGS expression. We also discuss some of the newly appreciated interactions of RGS proteins with cellular factors that suggest RGS control of several components of G-protein-mediated pathways, as well as a diverse array of non-GPCR-mediated biological responses.

Small-molecule therapies for cardiac hypertrophy: moving beneath the cell surface

Pathological stress from cardiovascular disease stimulates hypertrophy of heart cells, which increases the risk of cardiac morbidity and mortality. Recent evidence has indicated that inhibiting such hypertrophy could be beneficial, encouraging drug discovery and development efforts for agents that could achieve this goal. Most existing therapies that have antihypertrophic effects target outside-in signalling in cardiac cells, but their effectiveness seems limited, and so attention has recently turned to the potential of targeting intracellular signalling pathways. Here, we focus on new developments with small-molecule inhibitors of cardiac hypertrophy, summarizing both agents that have been in or are poised for clinical testing, and pathways that offer further promising potential therapeutic targets.

Phospholipid-mediated signaling systems as novel targets for treatment of heart disease

The phospholipases associated with the cardiac sarcolemmal (SL) membrane hydrolyze specific membrane phospholipids to generate important lipid signaling molecules, which are known to influence normal cardiac function. However, impairment of the phospholipases and their related signaling events may be contributory factors in altering cardiac function of the diseased myocardium. The identification of the changes in such signaling systems as well as understanding the contribution of phospholipid-signaling pathways to the pathophysiology of heart disease are rapidly emerging areas of research in this field. In this paper, I provide an overview of the role of phospholipid-mediated signal transduction processes in cardiac hypertrophy and congestive heart failure, diabetic cardiomyopathy, as well as in ischemia-reperfusion. From the cumulative evidence presented, it is suggested that phospholipid-mediated signal transduction processes could serve as novel targets for the treatment of the different types of heart disease.

Cardiac hypertrophy: mechanisms and therapeutic opportunities

Cardiac hypertrophy and heart failure are major causes of morbidity and mortality in Western societies. Many factors have been implicated in cardiac remodeling, including alterations in gene expression in myocytes, cardiomyocytes apoptosis, cytokines and growth factors that influence cardiac dynamics, and deficits in energy metabolism as well as alterations in cardiac extracellular matrix composition. Many therapeutic means have been shown to prevent or reverse cardiac hypertrophy. New concepts for characterizing the pathophysiology of cardiac hypertrophy have been drawn from various aspects, including medical therapy and gene therapy, or use of stem cells for tissue regeneration. In this review, we focus on various types of cardiac hypertrophy, defining the causes of hypertrophy, describing available animal models of hypertrophy, discussing the mechanisms for development of hypertrophy and its transition to heart failure, and presenting the potential use of novel promising therapeutic strategies derived from new advances in basic scientific research.

N-terminal residues control proteasomal degradation of RGS2, RGS4, and RGS5 in human embryonic kidney 293 cells

Regulator of G protein signaling (RGS) proteins modulate G protein-coupled receptor (GPCR) signaling. The N termini of some RGS4-family proteins provide receptor specificity and also contain an N-end rule determinant that results in ubiquitylation and decreased protein expression. The relevance of these mechanisms to other RGS proteins is not fully understood. Thus we examined function, receptor specificity, and expression of R4 subfamily RGS proteins (RGS2, -3, -4, -5, and -8). Although the N terminus plays a key role in protein stability in human embryonic kidney (HEK) 293 cells, we were unable to demonstrate specificity of RGS2, -3, -4, -5, or -8 for muscarinic receptors (M(1), M(3), and M(5)). However, cellular RGS activity (8 = 3 > 2) was strongly correlated with expression; RGS4 and -5 had minimal expression and activity. Stabilizing mutations of RGS4 and -5 (C2S) enhanced expression and function with a greater influence on RGS4 than on RGS5. We were surprised to find that a predicted destabilizing mutation in RGS8 (A2C) did not markedly affect expression and had no effect on function. In contrast, a destabilizing mutation in RGS2 (RGS2-Q2L) recently identified as a rare N-terminal genetic variant in a Japanese hypertensive cohort (J Hypertens 23:1497-1505, 2005) showed significantly reduced expression and inhibition of angiotensin II (AT(1)) receptor-stimulated accumulation of inositol phosphates. We were surprised to find that RGS2-Q2R, also predicted to be destabilizing, showed nearly normal expression and function. Thus, proteasomal regulation of RGS expression in HEK293 cells strongly controls RGS function and a novel RGS2 mutation with decreased protein expression could be relevant to the pathophysiology of hypertension in humans.

How regulators of G protein signaling achieve selective regulation

The regulators of G protein signaling (RGS) are a family of cellular proteins that play an essential regulatory role in G protein-mediated signal transduction. There are multiple RGS subfamilies consisting of over 20 different RGS proteins. They are basically the guanosine triphosphatase (GTPase)-accelerating proteins that specifically interact with G protein alpha subunits. RGS proteins display remarkable selectivity and specificity in their regulation of receptors, ion channels, and other G protein-mediated physiological events. The molecular and cellular mechanisms underlying such selectivity are complex and cooperate at many different levels. Recent research data have provided strong evidence that the spatiotemporal-specific expression of RGS proteins and their target components, as well as the specific protein-protein recognition and interaction through their characteristic structural domains and functional motifs, are determinants for RGS selectivity and specificity. Other molecular mechanisms, such as alternative splicing and scaffold proteins, also significantly contribute to RGS selectivity. To pursue a thorough understanding of the mechanisms of RGS selective regulation will be of great significance for the advancement of our knowledge of molecular and cellular signal transduction.

Calcium signaling complexes in microdomains of polarized secretory cells

The highly polarized nature of epithelial cells in exocrine glands necessitates targeting, assembly into complexes and confinement of the molecules comprising the Ca(2+) signaling apparatus, to cellular microdomains. Such high degree of polarized localization has been shown for all Ca(2+) signaling molecules tested, including G protein coupled receptors and their associated proteins, Ca(2+) pumps, Ca(2+) influx channels at the plasma membrane and Ca(2+) release channels in the endoplasmic reticulum. Although the physiological significance of polarized Ca(2+) signaling is clear, little is known about the mechanism of targeting, assembly and retention of Ca(2+) signaling complexes in cellular microdomains. The present review attempts to summarize the evidence in favor of polarized expression of Ca(2+) signaling proteins at the apical pole of secretory cells with emphasis on the role of scaffolding proteins in the assembly and function of the Ca(2+) signaling complexes. The consequence of polarized enrichment of Ca(2+) signaling complexes at the apical pole is generation of an apical to basal pole gradient of cell responsiveness that, at low physiological agonist concentrations, limits Ca(2+) spikes to the apical pole, and when a Ca(2+) wave occurs, it always propagates from the apical to the basal pole. Our understanding of Ca(2+) signaling in microdomains is likely to increase rapidly with the application of techniques to controllably and selectively disrupt components of the complexes and apply high resolution recording techniques, such as TIRF microscopy to this problem.

Cellular mechanisms that determine selective RGS protein regulation of G protein-coupled receptor signaling

Regulators of G protein signaling (RGS proteins) bind directly to activated Galpha subunits to inhibit their signaling. However, recent findings show that RGS proteins selectively regulate signaling by certain G protein-coupled receptors (GPCRs) in cells, irrespective of the coupled G protein. New studies support an emerging model that suggests RGS proteins utilize both direct and indirect mechanisms to form stable functional pairs with preferred GPCRs to selectively modulate the signaling functions of those receptors and linked G proteins. Here, we discuss these findings and their implications for established models of GPCR signaling.

Urocortin inhibits Beclin1-mediated autophagic cell death in cardiac myocytes exposed to ischaemia/reperfusion injury

Autophagy is known to be a feature of cardiomyopathies and chronic ischaemia. Here we demonstrate that autophagy is also induced by a single cycle of ischaemia/reperfusion (I/R in neonatal and adult rat cardiac myocytes). Consistent with the critical role for Beclin1 in autophagocytosis, reduction of Beclin1 expression in cardiac myocytes by RNAi reduces I/R-induced autophagy and this is associated with enhanced cell survival. Autophagy is also reduced by urocortin, an endogenous cardiac peptide which we have previously shown to reduce other forms of myocyte cell death induced by I/R. The inhibition of autophagy by urocortin is mediated in part by inhibition of Beclin1 expression, an effect which is mediated by activation of the PI3 kinase/Akt pathway but which does not involve activation of p42/p44 MAPK.