Type 2 angiotensin II receptor is downregulated in cardiomyocytes of patients with heart failure

Angiotensin AT2 receptors reduce inflammation and fibrosis in cardiovascular remodeling

The angiotensin AT2 receptor (AT2R), an important member of the "protective arm" of the renin-angiotensin system (RAS), has been recently defined as a therapeutic target in different pathological conditions. The AT2R activates complex signalling pathways linked to cellular proliferation, differentiation, anti-inflammation, antifibrosis, and induction or inhibition of apoptosis. The anti-inflammatory effect of AT2R activation is commonly associated with reduced fibrosis in different models. Current discoveries demonstrated a direct impact of AT2Rs on the regulation of cytokines, transforming growth factor beta1 (TGF-beta1), matrix metalloproteases (MMPs), and synthesis of the extracellular matrix components. This review article summarizes current knowledge on the AT2R in regard to immunity, inflammation and fibrosis in the heart and blood vessels. In particular, the differential influence of the AT2R on cardiovascular remodeling in preclinical models of myocardial infarction, heart failure and aneurysm formation are discussed. Overall, these studies demonstrate that AT2R stimulation represents a promising therapeutic approach to counteract myocardial and aortic damage in cardiovascular diseases.

Adaptive and maladaptive roles of different angiotensin receptors in the development of cardiac hypertrophy and heart failure

Angiotensin II (Ang II) is formed by the action of angiotensin-converting enzyme (ACE) in the renin-angiotensin system. This hormone is known to induce cardiac hypertrophy and heart failure and its actions are mediated by the interaction of both pro- and antihypertrophic Ang II receptors (AT1R and AT2R). Ang II is also metabolized by ACE 2 to Ang-(1-7), which elicits the activation of Mas receptors (MasR) for inducing antihypertrophic actions. Since heart failure under different pathophysiological situations is preceded by adaptive and maladaptive cardiac hypertrophy, we have reviewed the existing literature to gain some information regarding the roles of AT1R, AT2R, and MasR in both acute and chronic conditions of cardiac hypertrophy. It appears that the activation of AT1R may be involved in the development of adaptive and maladaptive cardiac hypertrophy as well as subsequent heart failure because both ACE inhibitors and AT1R antagonists exert beneficial effects. On the other hand, the activation of both AT2R and MasR may prevent the occurrence of maladaptive cardiac hypertrophy and delay the progression of heart failure, and thus therapy with different activators of these antihypertrophic receptors under chronic pathological stages may prove beneficial. Accordingly, it is suggested that a great deal of effort should be made to develop appropriate activators of both AT2R and MasR for the treatment of heart failure subjects.

Pathophysiology and pharmacology of G protein-coupled receptors in the heart

G protein-coupled receptors (GPCRs), comprising the largest superfamily of cell surface receptors, serve as fundamental modulators of cardiac health and disease owing to their key roles in the regulation of heart rate, contractile dynamics, and cardiac function. Accordingly, GPCRs are heavily pursued as drug targets for a wide variety of cardiovascular diseases ranging from heart failure, cardiomyopathy, and arrhythmia to hypertension and coronary artery disease. Recent advancements in understanding the signalling mechanisms, regulation, and pharmacological properties of GPCRs have provided valuable insights that will guide the development of novel therapeutics. Herein, we review the cellular signalling mechanisms, pathophysiological roles, and pharmacological developments of the major GPCRs in the heart, highlighting the β-adrenergic, muscarinic, and angiotensin receptors as exemplar subfamilies.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Effect of angiotensin-converting enzyme inhibition on cardiovascular adaptation to exercise training

Angiotensin-converting enzyme (ACE) activity may be one determinant of adaptability to exercise training, but well-controlled studies in humans without confounding conditions are lacking. Thus, the purpose of the present study was to investigate whether ACE inhibition affects cardiovascular adaptations to exercise training in healthy humans. Healthy participants of both genders (40 ± 7 years) completed a randomized, double-blind, placebo-controlled trial. Eight weeks of exercise training combined with placebo (PLA, n = 25) or ACE inhibitor (ACEi, n = 23) treatment was carried out. Before and after the intervention, cardiovascular characteristics were investigated. Mean arterial blood pressure was reduced (p < 0.001) by -5.5 [-8.4; -2.6] mmHg in ACEi , whereas the 0.7 [-2.0; 3.5] mmHg fluctuation in PLA was non-significant. Maximal oxygen uptake increased (p < 0.001) irrespective of ACE inhibitor treatment by 13 [8; 17] % in ACEi and 13 [9; 17] % in PLA. In addition, skeletal muscle endurance increased (p < 0.001) to a similar extent in both groups, with magnitudes of 82 [55; 113] % in ACEi and 74 [48; 105] % in PLA. In contrast, left atrial volume decreased (p < 0.05) by -9 [-16; -2] % in ACEi , but increased (p < 0.01) by 14 [5; 23] % in PLA. Total hemoglobin mass was reduced (p < 0.01) by -3 [-6; -1] % in ACEi , while a non-significant numeric increase of 2 [-0.4; 4] % existed in PLA. The lean mass remained constant in ACEi but increased (p < 0.001) by 3 [2; 4] % in PLA. In healthy middle-aged adults, 8 weeks of high-intensity exercise training increases maximal oxygen uptake and skeletal muscle endurance irrespective of ACE inhibitor treatment. However, ACE inhibitor treatment counteracts exercise training-induced increases in lean mass and left atrial volume. ACE inhibitor treatment compromises total hemoglobin mass.

Alterations of the renin angiotensin system in human end-stage heart failure before and after mechanical cardiac unloading by LVAD support

Heart transplantation is often an unrealizable therapeutic option for end-stage heart failure, which is why mechanical left ventricular assist devices (LVADs) become an increasingly important therapeutic alternative. Currently, there is a lack of information about molecular mechanisms which are influenced by LVADs, particularly regarding the pathophysiologically critical renin angiotensin system (RAS). We, therefore, determined regulation patterns of key components of the RAS and the β-arrestin signaling pathways in left ventricular (LV) tissue specimens from 8 patients with end-stage ischemic cardiomyopathy (ICM) and 12 patients with terminal dilated cardiomyopathy (DCM) before and after LVAD implantation and compared them with non-failing (NF) left ventricular tissue samples: AT1R, AT2R, ACE, ACE2, MasR, and ADAM17 were analyzed by polymerase chain reaction. ERK, phosphorylated ERK, p38, phosphorylated p38, JNK, phosphorylated JNK, GRK2, β-arrestin 2, PI3K, Akt, and phosphorylated Akt were determined by Western blot analysis. Angiotensin I and Angiotensin II were quantified by mass spectrometry. Patients were predominantly middle-aged (53 ± 10 years) men with severely impaired LV function (LVEF 19 ± 8%), when receiving LVAD therapy for a mean duration of 331 ± 317 days. Baseline characteristics did not differ significantly between ICM and DCM patients. By comparing failing with non-failing left ventricles, i.e., before LVAD implantation, a downregulation of AT1R, AT2R, and MasR and an upregulation of ACE, ACE2, GRK, β-arrestin, ERK, PI3K, and Akt were seen. Following LVAD support, then angiotensin I, ACE2, GRK, and β-arrestin were downregulated and AT2R, JNK, and p38 were upregulated. ACE, angiotensin II, AT1R, ADAM17, MasR, ERK, PI3K, and Akt remained unchanged. Some regulation patterns were influenced by the underlying etiology of heart failure, the severity of LV dysfunction at baseline, and the duration of LVAD therapy. Key components of the RAS and β-arrestin signaling pathways were divergently altered in failing left ventricles both before and after LVAD implantation, whereas a remarkable fraction remained unchanged. This indicates a rather incomplete molecular reverse remodeling, whose functional relevance has to be further evaluated.

Critical role of angiotensin II type 2 receptors in the control of mitochondrial and cardiac function in angiotensin II-preconditioned rat hearts

Angiotensin II preconditioning (APC) involves an angiotensin II type 1 receptor (AT1-R)-dependent translocation of PKCε and survival kinases to the mitochondria leading to cardioprotection after ischemia-reperfusion (IR). However, the role that mitochondrial AT1-Rs and angiotensin II type 2 receptors (AT2-Rs) play in APC is unknown. We investigated whether pretreatment of Langendorff-perfused rat hearts with losartan (L, AT1-R blocker), PD 123,319 (PD, AT2-R blocker), or their combination (L + PD) affects mitochondrial AT1-R, AT2-R, PKCε, PKCδ, Akt, PKG-1, MAPKs (ERK1/2, JNK, p38), mitochondrial respiration, cardiac function, and infarct size (IS). The results indicate that expression of mitochondrial AT1-Rs and AT2-Rs were enhanced by APC 1.91-fold and 2.32-fold, respectively. Expression of AT2-R was abolished by PD but not by L, whereas the AT1-R levels were abrogated by both blockers. The AT1-R response profile to L and PD was also shared by PKCε, Akt, MAPKs, and PKG-1, but not by PKCδ. A marked increase in state 3 (1.84-fold) and respiratory control index (1.86-fold) of mitochondria was observed with PD regardless of L treatment. PD also enhanced the post-ischemic recovery of rate pressure product (RPP) by 74% (p < 0.05) compared with APC alone. Losartan, however, inhibited the (RPP) by 44% (p < 0.05) before IR and reduced the APC-induced increase of post-ischemic cardiac recovery by 73% (p < 0.05). Finally, L enhanced the reduction of IS by APC through a PD-sensitive mechanism. These findings suggest that APC upregulates angiotensin II receptors in mitochondria and that AT2-Rs are cardioprotective through their permissive action on AT1-R signaling and the suppression of cardiac function.

AT2 receptors in cardiovascular and renal diseases

The renin-angiotensin system (RAS) plays an important role in the initiation and progression of cardiovascular and renal diseases. These actions mediated by AT1 receptor (AT1R) are well established and led to development of selective AT1R blockers (ARBs). In contrast, there is scientific evidence that AT2 receptor (AT2R) mediates effects different from and often opposing those of the AT1R. Meagrely expressed in healthy tissue the AT2R is upregulated in injuries providing an endogenous protection to inflammatory, oxidative and apoptotic processes. Interestingly the beneficial effects mediated by AT2R can be further enhanced by pharmacological intervention using the recently developed AT2R agonists. This review article summarizes our current knowledge about regulation, signalling and effects mediated by AT2R in health and disease, with emphasis on cardiac and renal systems. At the end a novel concept of natural protective systems will be introduced and discussed as an attractive target in drug development.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Non-canonical signalling and roles of the vasoactive peptides angiotensins and kinins

GPCRs (G-protein-coupled receptors) are among the most important targets for drug discovery due to their ubiquitous expression and participation in cellular events under both healthy and disease conditions. These receptors can be activated by a plethora of ligands, such as ions, odorants, small ligands and peptides, including angiotensins and kinins, which are vasoactive peptides that are classically involved in the pathophysiology of cardiovascular events. These peptides and their corresponding GPCRs have been reported to play roles in other systems and under pathophysiological conditions, such as cancer, central nervous system disorders, metabolic dysfunction and bone resorption. More recently, new mechanisms have been described for the functional regulation of GPCRs, including the transactivation of other signal transduction receptors and the activation of G-protein-independent pathways. The existence of such alternative mechanisms for signal transduction and the discovery of agonists that can preferentially trigger one signalling pathway over other pathways (called biased agonists) have opened new perspectives for the discovery and development of drugs with a higher specificity of action and, therefore, fewer side effects. The present review summarizes the current knowledge on the non-canonical signalling and roles of angiotensins and kinins.

Regulation of angiotensin-(1-7) and angiotensin II type 1 receptor by telmisartan and losartan in adriamycin-induced rat heart failure

AIM

To investigate the possible effects of telmisartan and losartan on cardiac function in adriamycin (ADR)-induced heart failure in rats, and to explore the changes in plasma level of angiotensin-(1-7)[Ang-(1-7)] and myocardial expression of angiotensin II type 1/2 receptors (AT(1)R / AT(2)R) and Mas receptor caused by the two drugs.

METHODS

Male Sprague-Dawley rats were randomly divided into 4 groups: the control group, ADR-treated heart failure group (ADR-HF), telmisartan plus ADR-treated group (Tel+ADR) and losartan plus ADR-treated group (Los+ADR). ADR was administrated (2.5 mg/kg, ip, 6 times in 2 weeks). The rats in the Tel+ADR and Los+ADR groups were treated orally with telmisartan (10 mg/kg daily po) and losartan (30 mg/kg daily), respectively, for 6 weeks. The plasma level of Ang-(1-7) was determined using ELISA. The mRNA and protein expression of myocardial Mas receptor, AT(1)R and AT(2)R were measured using RT-PCR and Western blotting, respectively.

RESULTS

ADR significantly reduced the plasma level of Ang-(1-7) and the expression of myocardial Mas receptor and myocardial AT(2)R, while significantly increased the expression of myocardial AT(1)R. Treatment with telmisartan and losartan effectively increased the plasma level of Ang-(1-7) and suppressed myocardial AT(1)R expression, but did not influence the expression of Mas receptor and AT(2)R.

CONCLUSION

The protective effects of telmisartan and losartan in ADR-induced heart failure may be partially due to regulation of circulating Ang-(1-7) and myocardial AT(1)R expression.

Identification of noncytotoxic and IL-10-producing CD8+AT2R+ T cell population in response to ischemic heart injury

Emerging evidence suggests a cardioprotective role of the angiotensin AT2R, albeit the underlying cellular mechanisms are not well understood. We aimed in this article to elucidate a potential role of cardiac angiotensin AT2R in regulating cellular immune response to ischemic heart injury. Seven days after myocardial infarction in rats, double-immunofluorescence staining showed that AT2R was detected in a fraction of CD8(+) T cells infiltrating in the peri-infarct myocardium. We developed a method that allowed the isolation of myocardial infiltrating CD8(+)AT2R(+) T cells using modified MACS, and further characterization and purification with flow cytometry. Although the CD8(+)AT2R(-) T cells exhibited potent cytotoxicity to both adult and fetal cardiomyocytes (CMs), the CD8(+)AT2R(+) T cells were noncytotoxic to these CMs. The CD8(+)AT2R(+) T cells were characterized by upregulated IL-10 and downregulated IL-2 and INF-γ expression when compared with CD8(+)AT2R(-) T cells. We further showed that IL-10 gene expression was enhanced in CD8(+) T cells on in vitro AT2R stimulation. Importantly, in vivo AT2R activation engendered an increment of CD8(+)AT2R(+) T cells and IL-10 production in the ischemic myocardium. In addition, intramyocardial transplantation of CD8(+)AT2R(+) T cells (versus CD8(+)AT2R(-)) led to reduced ischemic heart injury. Moreover, the CD8(+)AT2R(+) T cell population was also demonstrated in human peripheral blood. Thus, we have defined the cardioprotective CD8(+)AT2R(+) T cell population, which increases during ischemic heart injury and contributes to maintaining CM viability and providing IL-10, hence revealing an AT2R-mediated cellular mechanism in modulating adaptive immune response in the heart.

Cardiac c-kit+AT2+ cell population is increased in response to ischemic injury and supports cardiomyocyte performance

The expression pattern of angiotensin AT2 receptors with predominance during fetal life and upregulation under pathological conditions during tissue injury/repair process suggests that AT2 receptors may exert an important action in injury/repair adaptive mechanisms. Less is known about AT2 receptors in acute ischemia-induced cardiac injury. We aimed here to elucidate the role of AT2 receptors after acute myocardial infarction. Double immunofluorescence staining showed that cardiac AT2 receptors were mainly detected in clusters of small c-kit+ cells accumulating in peri-infarct zone and c-kit+AT2+ cells increased in response to acute cardiac injury. Further, we isolated cardiac c-kit+AT2+ cell population by modified magnetic activated cell sorting and fluorescence activated cell sorting. These cardiac c-kit+AT2+ cells, represented approximately 0.19% of total cardiac cells in infarcted heart, were characterized by upregulated transcription factors implicated in cardiogenic differentiation (Gata-4, Notch-2, Nkx-2.5) and genes required for self-renewal (Tbx-3, c-Myc, Akt). When adult cardiomyocytes and cardiac c-kit+AT2+ cells isolated from infarcted rat hearts were cocultured, AT2 receptor stimulation in vitro inhibited apoptosis of these cocultured cardiomyocytes. Moreover, in vivo AT2 receptor stimulation led to an increased c-kit+AT2+ cell population in the infarcted myocardium and reduced apoptosis of cardiomyocytes in rats with acute myocardial infarction. These data suggest that cardiac c-kit+AT2+ cell population exists and increases after acute ischemic injury. AT2 receptor activation supports performance of cardiomyocytes, thus contributing to cardioprotection via cardiac c-kit+AT2+ cell population.

Angiotensin II type 2 receptor-mediated vasodilation. Focus on bradykinin, NO and endothelium-derived hyperpolarizing factor(s)

Angiotensin (Ang) II type 1 (AT(1)) receptors account for the majority of the cardiovascular effects Ang II, including vasoconstriction and growth stimulation. Recent evidence, mainly obtained in animals, suggests that Ang II type 2 (AT(2)) receptors counteract some or all of these effects. This review summarizes the current knowledge on the vasodilator effects induced by AT(2) receptors in humans and animals, focussing not only on the mediators of this effect, but also on the modulatory role of age, gender, and endothelial function. It is concluded that AT(2) receptor-mediated vasodilation most likely depends on the bradykinin-bradykinin type 2 (B(2)) receptor-NO-cGMP pathway, although evidence for a direct link between AT(2) and B(2) receptors is currently lacking. If indeed B(2) receptors are involved, this would imply that, in addition to NO, also the wide range of non-NO 'endothelium-derived hyperpolarizing factors' (EDHFs) that is released following B(2) receptor activation (e.g., K(+), cytochrome P450 products from arachidonic acid, H(2)O(2) and S-nitrososothiols), could contribute to AT(2) receptor-induced vasodilation.

Taurine May Prevent Diabetic Rats from Developing Cardiomyopathy also by Downregulating Angiotensin II Type2 Receptor Expression

OBJECTIVE

In diabetes, intracellular accumulation of sorbitol resulting from the high extracellular levels of glucose leads to depletion of intracellular compounds including taurine. This is associated with the development of late diabetic complications such as cardiomyopathy. The development of myocyte hypertrophy has been largely attributed to angiotensin II, whose growth properties are antagonized by taurine. However, the interaction between taurine, angiotensin II type2 receptor (AT2) and cardiomyopathy related to angiotensin II is still unknown. This study investigates the roles of taurine and AT2 in rats with streptozotocin (STZ)-induced diabetic cardiomyopathy.

METHODS

Of 60 female 4-week-old Wistar rats, 8 were treated with common diet and the other 52 with high sugar/fat diet (during the whole experiment) to induce insulin resistance. At the 4th week, of the 52 rats, 7 treated with sodium citrate buffer (pH = 4.5) were grouped into control group1 (con1) and the other 45 were treated by intraperitoneal injection (I.P) with STZ to develop type 2 diabetes. At the 28th week, the maximal velocity decrease of pressure per second in left ventricle within the period of isovolumic relaxation (-dp/dt(max)) was detected by a cannula through right carotid artery. After the cannula operation, of the 45 rats, all the living 24 with -dp/dt(max)< or = 5250 mmHg/s, who had developed diabetic cardiomyopathy, were grouped as follows: 7 treated with double distilled H2O (I.P) were grouped into control group2 (con2). 8 treated with AT2 agonist (CGP42112A) (I.P) were grouped into experimental group1 (exp1). Another 9 treated with taurine (I.P) were grouped into experimental group2 (exp2). All injections lasted 4 weeks (Q.D) and the heart weight (HW) was recorded. To examine cardiomyocyte apoptosis index (CAI), mRNA and protein of AT2 and Bcl-2 in cardiomyocytes, methods of terminal-deoxynucltidyl transferase mediated nick end labeling (TUNEL), reversal transcription polymerase chain reaction (RT-PCR) and immunoblot (Western Blot) were used, respectively.

RESULTS

Values of -dp/dt(max) in exp1, exp2 or con2 were much less than those in con1, respectively (p < 0.01). CAI (= stained cell number/total cell number x 100%) and AT2 values both in mRNA and protein levels in con1 were less than those in the other three groups, respectively (p < 0.01). The three parameters above were more in exp1 but less in exp2 than those in con2, respectively (p < 0.01). The three parameters and HW in exp1 were much higher than those in exp2, respectively (p < 0.01). Changes of Bcl-2 were opposed to those of AT2.

CONCLUSIONS

A high expression of AT2 may accelerate the apoptosis of cardiomyocytes in diabetic rats and play a role in precipitating diabetic cardiomyopathy; taurine may protect diabetic rats from developing cardiomyopathy also by downregulating AT2 receptors.

Astragalus prevents diabetic rats from developing cardiomyopathy by downregulating angiotensin II type2 receptors' expression

This study examined the potential roles of astragalus and angiotensin II type 2 receptor (AT2) in rats with streptozotocin (STZ)-induced diabetic cardiomyopathy. Of 52 female 4-week-old Wistar rats treated with high glucose and lipid diet to induce insulin resistance, 7 treated with sodium citrate buffer (pH=4.5) served as controls (con1) and the other 45 were treated by intraperitoneal injection (ip) of STZ to induce type 2 diabetes. After 20 weeks, the maximal velocity decrease of pressure per second in left ventricle within the period of isovolumic relaxation (-dp/dtmax) was detected by inserting cannula through right carotid artery. Of the 45 rats, 24 with -dp/dtmax < or = 700 mmHg/s (1 mmHg=0.133 kPa) developing diabetic cardiomyopathy were grouped as follows: 7 treated with double distilled H2O (ip) were included in control group 2 (con2); other 8 treated with AT2 agonist (CGP42112A, ip) were included in experimental group1 (exp); 9 treated with astragalus (po) constituted experimental group 2 (exp2). All injections lasted 4 weeks (qd) and the heart weight (HW) was recorded. Cardiomyocyte apoptosis index (CAI), mRNA of AT2 and Bcl-2 as well as AT2 and Bcl-2 protein values in cardiomyocytes were also measured. Our results showed that -dp/dtmax in exp1, exp2 and con2 were much lower than those in con1 (P<0.01). CAI and AT2 in both mRNA and protein in con1 were lower than those in the other three groups (P<0.01). The three parameters above were higher in exp1 but less in exp2 than those in con2, respectively (P<0.01). The three parameters and HW in exp1 were much higher than those in exp2 (P<0.01). Changes of Bcl-2 were opposite to those of AT2. Our results suggested that high expression of AT2 might accelerate the apoptosis of cardiomyocytes in diabetic rats and play an important role in precipitating diabetic cardiomyopathy and astragalus protects diabetic rats from developing cardiomyopathy by downregulating AT2.

Angiotensin AT2 receptors: cardiovascular hope or hype?

British Journal of Pharmacology (2003) 140, 809–824. doi:10.1038/sj.bjp.0705448

Ventricular-specific expression of angiotensin II type 2 receptors causes dilated cardiomyopathy and heart failure in transgenic mice

The angiotensin II type 2 (AT2) receptor is upregulated in the left ventricle in heart failure, but its pathophysiological roles in vivo are not understood. In the present study, AT2 receptors were expressed in transgenic (TG) mice using the ventricular-specific myosin light-chain (MLC-2v) promoter. In TG compared with nontransgenic (NTG) mice, in vivo left ventricular (LV) systolic pressure and peak +dP/dt were depressed while LV diastolic pressure was elevated (P < 0.05). Echocardiography showed severely depressed LV fractional shortening, increased systolic and diastolic dimensions, and wall thinning (P < 0.05). Confocal and electron microscopy studies revealed an increase in the size of myocytes and interstitial spaces as well as an increase in interstitial collagen, disruption of the Z-band, and changes in cytochrome c localization. The changes were most prominent in the highest-expressing TG line, which implies a dose-response relationship. AT2 overexpression was also directly associated with the increase of phosphorylated protein levels of PKC-alpha, PKC-beta, and p70S6 kinase. These data demonstrate that ventricular myocyte-specific expression of AT2 receptors promotes the development of dilated cardiomyopathy and heart failure in vivo.

Alterations in G protein and MAP kinase signaling pathways during cardiac remodeling in hypertension and heart failure

The present study was undertaken to elucidate the G-protein and mitogen-activated kinase (MAP kinase) coupled signaling profile in a genetic model of hypertension and congestive heart failure (CHF) that mimics similar disease in humans. At the receptor level, Ang II type 1 receptor (AT1R) increased in left ventricular hypertrophy (LVH) and reverted to normal in CHF, whereas there was a downregulation of the Ang II type 2 receptor (AT2R) in CHF. At the transducer level, Galphaq and Galpha12 protein levels were unchanged during LVH but decreased significantly in CHF. In contrast, Gbeta and Galpha13 protein content were markedly upregulated in CHF. Furthermore, using phospho-specific antibodies in Western blots and in vitro kinase assays, we found at the effector level an upregulation of the small G-protein Rac1 activity during LVH but a decrease during CHF. In parallel, small G-protein Rho activity was significantly increased during LVH but was unchanged in failure. We found at the downstream level that MAP kinase isoforms extracellular signal regulated-kinase (ERK1/2), big mitogen-activated kinase (BMK1/ERK5), C-jun N-terminal-activated kinase (JNKs/SAPKs), and stress-activated kinase (p38) bioactivities were increased during LVH. During CHF, ERK1/2 and JNK1/2 kinase activities were decreased, whereas BMK1/ERK5 kinase activity reverted to normal values. In conclusion, this study demonstrates, for the first time, multistep alterations of G-protein and MAP kinase signaling pathways in LVH and progression to failure in a genetic model of hypertension and failure.

Exogenous angiotensin II does not facilitate norepinephrine release in the heart

Studies on the effect of angiotensin II on norepinephrine release from sympathetic nerve terminals through stimulation of presynaptic angiotensin II type 1 receptors are equivocal. Furthermore, evidence that angiotensin II activates the cardiac sympathetic nervous system in vivo is scarce or indirect. In the intact porcine heart, we investigated whether angiotensin II increases norepinephrine concentrations in the myocardial interstitial fluid (NE(MIF)) under basal conditions and during sympathetic activation and whether it enhances exocytotic and nonexocytotic ischemia-induced norepinephrine release. In 27 anesthetized pigs, NE(MIF) was measured in the left ventricular myocardium using the microdialysis technique. Local infusion of angiotensin II into the left anterior descending coronary artery (LAD) at consecutive rates of 0.05, 0.5, and 5 ng/kg per minute did not affect NE(MIF), LAD flow, left ventricular dP/dt(max), and arterial pressure despite large increments in coronary arterial and venous angiotensin II concentrations. In the presence of neuronal reuptake inhibition and alpha-adrenergic receptor blockade, left stellate ganglion stimulation increased NE(MIF) from 2.7+/-0.3 to 7.3+/-1.2 before, and from 2.3+/-0.4 to 6.9+/-1.3 nmol/L during, infusion of 0.5 ng/kg per minute angiotensin II. Sixty minutes of 70% LAD flow reduction caused a progressive increase in NE(MIF) from 0.9+/-0.1 to 16+/-6 nmol/L, which was not enhanced by concomitant infusion of 0.5 ng/kg per minute angiotensin II. In conclusion, we did not observe any facilitation of cardiac norepinephrine release by angiotensin II under basal conditions and during either physiological (ganglion stimulation) or pathophysiological (acute ischemia) sympathetic activation. Hence, angiotensin II is not a local mediator of cardiac sympathetic activity in the in vivo porcine heart.

Immunohistochemical localization of angiotensin II receptor and local renin-angiotensin system in human colonic mucosa

Angiotensin II (Ang II) regulates water and sodium transport in renal tubules and gastrointestinal tract. Two types of Ang II receptors have been cloned, but their distributions have not been determined in human colon. In addition, tissue renin-angiotensin systems (RAS) are believed to exist and to regulate local actions in human colon. We studied by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC) the presence and localization of Ang II receptors Type 1 (AT(1)), Type 2 (AT(2)), and RAS components [angiotensinogen, renin, and angiotensin-converting enzyme (ACE)] in normal human colon. AT(1) receptors were localized in vessel walls, myofibroblasts, macrophages, and surface epithelium. AT(2) receptors were found in mesenchymal cells and weakly in parts of surface epithelium. Renin and ACE were distributed in vessel walls, mesenchymal cells, and in parts of surface epithelium. Angiotensinogen was also detected by RT-PCR. These findings demonstrated that Ang II receptors and RAS components were present in human colon, suggesting the possibility of its local regulation.

Enhanced angiotensin II activity in heart failure: reevaluation of the counterregulatory hypothesis of receptor subtypes

There are strong data favoring the pathogenic role of angiotensin II type 1 receptor (AT(1)) activation with subsequent promotion of myocyte growth and cardiac fibrosis in the development of cardiac hypertrophy and heart failure. An emerging hypothesis suggests that the activity of the angiotensin II type 2 receptor (AT(2)) may counterregulate AT(1) receptor effects during cardiac development and during the evolution of cardiac hypertrophy and heart failure. In this review, we examine the potential role of AT(2) activity in the context of this hypothesis. In contrast to the counterregulatory hypothesis, studies in mice with an overabundance of, or a deficiency in, the AT(2) receptor do not suggest that AT(2) signaling is essential for cardiac development. Moreover, the proposed antigrowth effects of AT(2) receptor signaling in pathological cardiac hypertrophy could not be shown in two mice models both deficient in AT(2) receptors. The role of AT(2) receptor signaling in cardiac fibrosis is, however, still debatable because of conflicting data in the same two studies. In angiotensin II-evoked apoptosis in cardiomyocytes, the proposed proapoptotic role of AT(2) activity could not be confirmed. Furthermore, in the progression from the bench to bedside, the results of two large clinical trials in heart failure, namely ELITE II and Val-HeFT, can be explained without ascribing a major protective role to the unopposed activity of the AT(2) receptor in the failing myocardium. In this review, we conclude that the collective evidence does not strongly support a net beneficial effect of AT(2) stimulation in the diseased myocardium.

Angiotensin II type 2 receptors: signalling and pathophysiological role

The signalling mechanisms and biological significance of the angiotensin II type 2 receptor have long been unknown. In recent years, studies, first in cell culture models but now increasingly also in vivo, have shed some light on the molecular events occurring after a stimulation of the receptor with its ligand as well as on its physiological effects and its significance for pathophysiological processes. There is increasing evidence that the angiotensin II type 2 receptor is involved in different pathophysiological processes, such as myocardial infarction, heart and kidney failure, and stroke.