Angiotensin II type 2 receptor stimulation: a novel option of therapeutic interference with the renin-angiotensin system in myocardial infarction?

Stimulation of angiotensin AT2 receptors by the non-peptide agonist, Compound 21, evokes vasodepressor effects in conscious spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Angiotensin type 2 receptor (AT(2) receptor) stimulation evokes vasodilator effects in vitro and in vivo that oppose the vasoconstrictor effects of angiotensin type 1 receptors (AT(1) receptors). Recently, a novel non-peptide AT(2) receptor agonist, Compound 21, was described, which exhibited high AT(2) receptor selectivity.

EXPERIMENTAL APPROACH

Functional cardiovascular effects of the drug candidate Compound 21 were assessed, using mouse isolated aorta and rat mesenteric arteries in vitro and in conscious spontaneously hypertensive rats (SHR).

KEY RESULTS

Compound 21 evoked dose-dependent vasorelaxations in aortic and mesenteric vessels, abolished by the AT(2) receptor antagonist, PD123319. In vivo, Compound 21 administered alone, at doses ranging from 50 to 1000 ng.kg(-1).min(-1) over 4 h did not decrease blood pressure in conscious normotensive Wistar-Kyoto rats or SHR. However, when given in combination with the AT(1) receptor antagonist, candesartan, Compound 21 (300 ng.kg(-1).min(-1)) lowered blood pressure in SHR only. Further analysis in separate groups of conscious SHR revealed that, at a sixfold lower dose, Compound 21 (50 ng.kg(-1).min(-1)) still evoked a significant depressor response in adult SHR ( approximately 30 mmHg) when combined with different doses of candesartan (0.01 or 0.1 mg.kg(-1)). Moreover, the Compound 21-evoked depressor effect was abolished when co-infused (50 microg.kg(-1).min(-1) for 2 h) with the AT(2) receptor antagonist PD123319.

CONCLUSION AND IMPLICATIONS

Collectively, our results indicate that acute administration of Compound 21 evoked blood pressure reductions via AT(2) receptor stimulation. Thus Compound 21 can be considered an excellent drug candidate for further study of AT(2) receptor function in cardiovascular disease.

Stimulation of AT2 receptor exerts beneficial effects in stroke-prone rats: focus on renal damage

BACKGROUND AND AIM

Angiotensin II acts through two major receptors: AT1-R and AT2-R. It is known that the stimulation of AT1-R mediates vasoconstriction, cell proliferation and fibrosis, aldosterone release and inflammatory response but, although the stimulation of AT2-R is thought to promote vasodilation and anti-inflammatory effects, its real in-vivo functions are still unclear. The aim of this study was to investigate the effects of specific and selective AT2-R stimulation on the pathological events occurring in spontaneously hypertensive stroke-prone rats (SHRSPs).

METHODS AND RESULTS

SHRSPs who were fed a high-salt diet underwent long-term treatment with vehicle or compound 21 (C21), a nonpeptide selective AT2-R agonist, at doses of 0.75, 5 and 10 mg/kg per day. The vehicle-treated rats developed brain abnormalities detectable by magnetic resonance imaging after 42.5 +/- 7.5 days, and died 43 +/- 9.5 days after the start of the dietary treatment. The highest C21 dose delayed the occurrence of brain damage (P < 0.001 vs. vehicle-treated SHRSPs) and prolonged survival (P < 0.001) without affecting blood pressure. These beneficial effects of C21 were abolished by the administration of PD123319, an AT2-R antagonist. C21 treatment preserved renal structure by preventing inflammatory cell infiltration, collagen accumulation, and the neo-expression of vimentin; it also prevented the increased plasma renin activity and accumulation of urinary acute-phase proteins observed in the vehicle-treated rats.

CONCLUSION

Specific and selective AT2-R stimulation has beneficial effects on the pathological events occurring in SHRSPs. These data indicate a new avenue for the pharmacological treatment of diseases in which modulation of the renin-angiotensin system is required.

The role of the central renin-angiotensin system in Parkinson's disease

Since the discovery of a renin-angiotensin system (RAS) in the brain, several studies have linked this central RAS to neurological disorders such as ischaemia, Alzheimer's disease and depression. In the last decade, evidence has accumulated that the central RAS might also play a role in Parkinson's disease. Although the exact cause of this progressive neurodegenerative disorder of the basal ganglia remains unidentified, inflammation and oxidative stress have been suggested to be key factors in the pathogenesis and the progression of the disease. Since angiotensin II is a pro-inflammatory compound that can induce the production of reactive oxygen species due to activation of the NADPH-dependent oxidase complex, this peptide might contribute to dopaminergic cell death. In this review, three different strategies to interfere with the pathogenesis or the progression of Parkinson's disease are discussed. They include inhibition of the angiotensin-converting enzyme, blockade of the angiotensin II type 1 receptor and stimulation of the angiotensin II type 2 receptor.

The past, present and future of angiotensin II type 2 receptor stimulation

Studying the angiotensin type 2 receptor (AT2) has been problematic in the past because a pharmacological tool for direct, specific in vitro and in vivo stimulation of the receptor has been lacking. Consequently, current knowledge about AT2 receptor signalling and function had to be obtained by indirect approaches, like studying animals or cells with genetically altered AT2 receptor expression levels, inhibitory experiments using specific AT2 receptor antagonists, stimulation with angiotensin II under concomitant angiotensin II type 1 receptor blockade or stimulation with the peptide agonist CGP42112A, which has additional AT2 receptor antagonistic properties. The recently developed non-peptide AT2 receptor agonist Compound 21 now, for the first time, allows direct, selective and specific AT2 receptor stimulation in vitro and in vivo . This new tool will certainly revolutionise AT2 receptor research, enable many new insights into AT2 receptor function and may also have the potential to become a future medical drug. This article reviews milestone findings about AT2 receptor functional properties obtained by ‘conventional’ experimental approaches within the last 20 years. Moreover, it provides an overview of the first results obtained by direct AT2 receptor stimulation with Compound 21, comprising effects on alkaline secretion, neurite outgrowth, blood pressure and post-infarct cardiac function.

The angiotensin II type 2 receptor in renal disease

Suppression of angiotensin II formation by angiotensin-converting enzyme inhibitors or blockade of the angiotensin II receptor by angiotensin receptor blockers is a powerful therapeutic strategy to slow the progression of renal disease. However, angiotensin-converting enzyme inhibitors and angiotensin receptor blockers provide only imperfect protection against the progression of chronic kidney disease to end-stage renal failure. Hence, innovative approaches are needed to keep patients with chronic kidney disease off dialysis. Angiotensin II activates at least two receptors, namely the angiotensin II type 1 (AT( 1)) and angiotensin II type 2 (AT(2)) receptors. The majority of the effects of angiotensin II, such as vasoconstriction, inflammation, and matrix deposition, are mediated via the AT(1) receptor. It is thought that the AT(2) receptor counteracts these effects and plays a role in nephroprotection. However, recent data support the notion that the AT(2) receptor transduces pro-inflammatory effects and promotes fibrosis and hypertrophy. Therefore, the question of whether stimulation of the AT(2) receptor could represent a silver bullet for the treatment of chronic kidney disease or may, on the contrary, exert detrimental effects on renal physiology remains unresolved. Recent data from AT(2) receptor-knockout mice demonstrate that the loss of AT(2) receptor signalling is associated with increased renal injury and mortality in chronic kidney disease. This raises the expectation that pharmacological stimulation of the AT(2) receptor may positively influence renal pathologies. However, further research is needed to explore the question whether AT(2) receptor stimulation may represent a new therapeutic strategy for the treatment of chronic kidney disease.

Adapter proteins and promoter regulation of the angiotensin AT2 receptor — implications for cardiac pathophysiology

The angiotensin AT 2 receptor (AT2R) represents an important component of the renin-angiotensin system since it is involved in the (patho) physiology of different cardiovascular and neuronal diseases. Furthermore, AT2 receptors can partly mediate beneficial effects of angiotensin AT 1 receptor (AT1R) blockers, and direct pharmacological AT 2 receptor agonism emerges as a novel therapeutic strategy. This review discusses the constitutive and ligand-mediated activity as well as the signal transduction of the AT2 receptor, focusing on adapter proteins which directly bind to this receptor. Direct protein-protein interaction partners of the AT2 receptor described so far include the transcription factor promyelocytic zinc finger protein, AT2 receptor binding protein and the AT1 receptor. In addition, the putative crosstalk of the AT2 receptor with the renin/ prorenin receptor (RER) via the promyelocytic zinc finger protein (PLZF) and the role of oestrogens on the regulation of the AT2 receptor are presented. Conceiving the coupling of the AT2 receptor to different adapter proteins with distinct and partly opposing cellular effects and the implications of its constitutive activity might help to overcome the current controversies on the (patho)physiological role of the AT2 receptor.

Small but powerful: short peptide hormones and their role in autoimmune inflammation

In the recent years, it has become increasingly clear that the immune response is also influenced by mediators which were first discovered as regulators in the nervous or also cardiovascular system. Here, small peptide hormones may play an important role. Kinins like bradykinins act on the endothelium and play a role for trafficking of lymphocytes over the blood-brain barrier. Neuropeptides like vasoactive intestinal peptide or neuropeptide Y also directly act on T cells and favour the differentiation of Th2 cells or regulatory T cell populations. Recently, the renin-angiotensin system (RAS) came into the focus of interest. Inhibition of the RAS at different levels may influence autoimmune responses and involve T cells as well as antigen-presenting cells, probably via different signalling pathways. Inhibitors of angiotensin converting enzyme and antagonists of the angiotensin 1 receptors are used in the treatment of hypertension, kidney disease or stroke by millions of people worldwide. These inexpensive and safe pharmaceuticals may also represent an interesting and innovative approach for the (combination) treatment of autoimmune diseases like multiple sclerosis.

Pharmacological treatment for heart failure: a view from the brain

Systolic heart failure is a feed-forward phenomenon with devastating consequences. Impaired cardiac function is the initiating event, but central nervous system mechanisms activated by persistent altered neural and humoral signals from the periphery play an important sustaining role. Animals with experimentally induced heart failure have neurochemical abnormalities in the brain that, when manipulated, profoundly affect sympathetic drive, volume regulation, and cardiac remodeling--critical determinants of outcome. This brief review explores recent studies that provide a strong rationale for the development of pharmaceutical agents that target central nervous system abnormalities in heart failure.

Devil and angel in the renin-angiotensin system: ACE-angiotensin II-AT1 receptor axis vs. ACE2-angiotensin-(1-7)-Mas receptor axis

Recent studies have established a new regulatory axis in the renin–angiotensin system (RAS). In this axis, angiotensin (Ang)-(1–7) is finally produced from Ang I or Ang II by the catalytic activity of angiotensin-converting enzyme 2 (ACE2). Ang-(1–7) shows actions different from those of AT₁ receptor stimulation, such as vasodilatation, natriuresis, anti-proliferation and an increase in the bradykinin–NO (nitric oxide) system. As the catalytic efficiency of ACE2 is approximately 400-fold higher with Ang II as a substrate than with Ang I, this axis is possibly acting as a counter-regulatory system against the ACE/Ang II/AT₁ receptor axis. The signaling pathway of the ACE2–Ang-(1–7) axis has not yet been totally and clearly understood. However, a recent report suggests that the Mas oncogene acts as a receptor for Ang-(1–7). Intracellular signaling through Mas is not clear yet. Several factors such as Akt phosphorylation, protein kinase C activation and mitogen-activated protein (MAP) kinase inhibition seem to be involved in this signaling pathway. Further investigations are needed to clarify the regulation and mechanism of action of ACE2 and Ang-(1–7). However, this second axis through ACE2 and Ang-(1–7) in RAS can be an important target for the therapy of cardiovascular and metabolic disorders.