Role of the angiotensin converting enzyme 1/angiotensin II/angiotensin receptor 1 axis in interstitial collagenase expression in human carotid atheroma

BACKGROUND AND AIM

Angiotensin II (AII) receptor 1 (ATR1) and angiotensin converting enzyme 1 (ACE1) blockers have been shown to reduce acute cardiovascular events in patients, improve plaque stability and modify matrix metalloproteinase (MMP) expression. However, the role of the ACE1/AII/ATR1 axis in interstitial collagenase regulation has not been fully explored. In this study, we investigated the effect of ATR1 and ACE1 blockade on the expression and activity of MMP-1, -8 and -13 in human carotid atheroma.

METHODS

Atheroma samples (n = 24) were obtained from patients undergoing carotid endarterectomy. The effects of ATR1 (irbesartan), ACE1 (quinapril), ACE2 (DX600) and MMP (GM6001) blockade on the expression of AII, the interstitial collagenases and soluble elastin fragments were investigated in explant culture supernatants. Paired atheroma samples were incubated with intervention or media control for 4 days. Protein levels (AII, MMP-1, -8, -13 and soluble elastin) were determined by ELISA.

RESULTS

ATR1, but not ACE1, blockade significantly reduced MMP-1 and -8 concentrations in atheroma supernatants. ACE2 blockade significantly increased MMP-1 and -8 concentrations in atheroma supernatants. AII concentration in atheroma supernatants significantly increased after ATR1, ACE1 and ACE2 blockade. Release of soluble elastin fragments increased after ATR1 and ACE1 blockade, but was not changed by an MMP inhibitor.

CONCLUSIONS

Our findings suggest that ATR1 blockade alters AII, MMP-1, MMP-8 expression and a marker of elastin degradation in human atheroma, but that the elastin degradation response is not MMP driven. This data contributes to the recognised ability of ATR1 blockade to modify plaque stability.

Angiotensin-(1-7) dose-dependently inhibits atherosclerotic lesion formation and enhances plaque stability by targeting vascular cells

OBJECTIVE

To test the hypothesis that chronic infusion of angiotensin-(1-7) [Ang-(1-7)] may dose-dependently inhibit atherosclerotic lesion formation by targeting vascular smooth muscle cells and a large dose of Ang-(1-7) may stabilize mature plaque by targeting macrophages.

APPROACH AND RESULTS

In vivo, the effects of Ang-(1-7) on atherogenesis and plaque stability were observed in ApoE(-/-) mice fed a high-fat diet and chronic angiotensin II infusion. In vitro, the effects of Ang-(1-7) on vascular smooth muscle cells' proliferation and migration, and macrophage inflammatory cytokines were examined. Ang-(1-7) dose-dependently attenuated early atherosclerotic lesions and inhibited vascular smooth muscle cells' proliferation and migration via suppressing extracellular regulated protein kinase/P38 mitogen-activated protein kinase and janus kinase/signal transducers and activators of transcription activities and enhancing smooth muscle 22α and angiotensin II type 2 receptor expression. Ang-(1-7) treatment resulted in high contents of collagen and vascular smooth muscle cells, and low contents of macrophages and lipids in carotid mature plaques. Ang-(1-7) lowered the expression levels of proinflammatory cytokines and activities of matrix metalloproteinases in mature plaques.

CONCLUSIONS

Ang-(1-7) treatment inhibits early atherosclerotic lesions and increases plaque stability in ApoE(-/-) mice, thus providing a novel and promising approach to the treatment of atherosclerosis.

Suppressing inflammation by inhibiting the NF-κB pathway contributes to the neuroprotective effect of angiotensin-(1-7) in rats with permanent cerebral ischaemia

BACKGROUND AND PURPOSE

Angiotensin-(1-7) [Ang-(1-7)] has anti-inflammatory effects in peripheral organs, but its effects in ischaemic stroke are unclear as yet. We investigated whether its anti-inflammatory effect contributes to the neuroprotection induced by Ang-(1-7) in a rat model of permanent middle cerebral artery occlusion (pMCAO).

EXPERIMENTAL APPROACH

We infused Ang-(1-7), Mas receptor antagonist A-779, angiotensin II type 2 receptor antagonist PD123319 or artificial CSF into the right lateral ventricle of male Sprague-Dawley rats from 48 h before onset of pMCAO until the rats were killed. Twenty-four hours after pMCAO, the neuroprotective effect of Ang-(1-7) was analysed by evaluating infarct volume and neurological deficits. The levels of oxidative stress were detected by spectrophotometric assay. The activation of NF-κB was assessed by Western blot and immunohistochemistry analysis. The level of COX-2 was tested by Western blot analysis and concentrations of pro-inflammatory cytokines were measured by elisa.

KEY RESULTS

Infusion of Ang-(1-7), i.c.v., significantly reduced infarct volume and improved neurological deficits. It decreased the levels of oxidative stress and suppressed NF-κB activity, which was accompanied by a reduction of pro-inflammatory cytokines and COX-2 in the peri-infarct regions. These effects of Ang-(1-7) were reversed by A-779 but not by PD123319. Additionally, infusion of A-779 alone increased oxidative stress levels and enhanced NF-κB activity, which was accompanied by an up-regulation of pro-inflammatory cytokines and COX-2.

CONCLUSION AND IMPLICATIONS

Our findings indicate that suppressing NF-κB dependent pathway via Mas receptor may represent one mechanism that contributes to the anti-inflammatory effects of Ang-(1-7) in rats with pMCAO.

Angiotensin AT2 receptor activates the cyclic-AMP signaling pathway in eel

A unique angiotensin type 2 receptor (AT2) that induces a cAMP signaling pathway was cloned and characterized for the first time in fish, Anguilla japonica. Phylogeny and synteny results showed that the AT2s among fishes and tetrapods share the same origin despite a sub-cluster formation among eel, salmon, and zebrafish. The eel AT2 was expressed abundantly in the spleen and localized at straight arterioles and ellipsoid regions prior to the sinusoid, suggesting a role in the regulation of microcirculation and/or immune response. Various angiotensin (Ang) peptides, including Ang II, Ang III, and Ang IV, were detected in the spleen by a radioimmunoassay coupled with HPLC separation, and these endogenous peptides stimulated a cAMP signaling, which has no crosstalk with cGMP pathway. The common and contrasting features of AT2 between fishes and mammals imply some ancestral characters of AT2, which are important information for receptor binding and evolutionary studies.

Association of angiotensin-converting enzyme gene insertion/deletion polymorphism with decreased risk for basal cell carcinoma

The incidence of basal cell carcinoma (BCC) is significantly reduced in individuals treated with inhibitors of angiotensin I-converting enzyme (ACE) that produces angiotensin II. The objective of this study was to investigate the possible association of a functional polymorphism in the ACE gene, which affects its transcription, with risk for BCC. In DNA samples of 92 patients with BCC and 103 healthy controls of Greek origin and comparable age and gender, we studied the ACE gene insertion/deletion (I/D) polymorphism. Fisher's exact test was used for comparison of allele and genotype frequencies between the control and patients' groups. The detected low expression I allele frequency in the group of BCC patients was significantly decreased compared to controls (15.8 vs. 31.1 %, respectively; P = 0.001). ID heterozygotes exhibited 3.06 times lower BCC risk, compared with DD homozygotes (P = 0.001; OR = 0.327, 95 % CI = 0.174-0.615). The protective role of I allele was particularly prominent in women (P = 0.007, OR = 0.299, 95 % CI = 0.125-0.716), while for men it exhibited a marginal level (P = 0.041). These findings indicate that the low expression ACE I allele carriers have a decreased risk for BCC. The protective effect of the ID genotype against BCC may be explained by a possible underlying mechanism involving the effect of produced angiotensin II levels on its receptors due to putatively different binding affinity.

Angiotensin II and III metabolism and effects on steroid production in the HAC15 human adrenocortical cell line

Aldosterone is synthesized in the zona glomerulosa of the adrenal cortex under primary regulation by the renin-angiotensin system. Angiotensin II (A-II) acts through the angiotensin types 1 and 2 receptors (AT1R and AT2R). A-II is metabolized in different tissues by various enzymes to generate two heptapeptides A-III and angiotensin 1-7, which can then be catabolized into smaller peptides. A-II was more potent than A-III in stimulating aldosterone secretion in the adrenocortical cell line HAC15, and A-II, but not A-III, stimulated cortisol secretion. A-II stimulated mRNA expression of steroidogenic acute regulatory protein, 3β-hydroxysteroid dehydrogenase, CYP11B1, and CYP11B2, whereas A-III stimulated 3β-hydroxysteroid dehydrogenase, CYP11B1, and CYP11B2 but decreased the expression of CYP17A1 required for cortisol synthesis. The stimulation of aldosterone secretion by A-II and A-III was blocked by the AT1R receptor blocker, losartan, but not by an AT2R blocker. A-II was rapidly metabolized by the HAC15 cells to mainly to angiotensin 1-7, but not to A-III, and disappeared from the supernatant within 6 h. A-III was metabolized rapidly and disappeared within 1 h. In conclusion, A-II was not converted to A-III in the HAC15 cell and is the more potent stimulator of aldosterone secretion and cortisol of the two. A-III stimulated aldosterone secretion but not cortisol secretion.

Similarities and differences between effects of angiotensin III and angiotensin II on human prostate cancer cell migration and proliferation

Proliferation plays a critical role in tumor growth when cell migration is essential to invasion. The effect of Ang III and Ang II was evaluated on these important processes. Changes in the migration potential of prostate cancer cells were investigated using Wound Healing Test and a Transwell Migration Chamber with a 3 μm pore size. Cell proliferation was measured with a BrdU Assay and Countess Automated Cell Counter, thus determining the influence of angiotensins on hormone-dependent (LNCaP) and hormone-independent (DU-145) human prostate cancer lines. The influence of Ang III and Ang II on classic receptors may be inhibited by Losartan or PD123319. Test peptide modulation of the AT1 and AT2 receptors was examined by Western Blot and fluorescent immunocytochemistry. The results indicate that Ang III promotes the migration of both LNCaP and DU-145 lines, whereas Ang II stimulates this process only in androgen-independent cells. Both angiotensin peptides can induce prostate cancer cell proliferation in a time- and dose-dependent manner. The obtained results show that Ang III and Ang II can modify the expression of classic receptors, particularly AT2. These results suggest that the investigated peptide can modulate cell migration and proliferation in prostate cancer cells. Angiotensins probably have a greater influence on proliferation in the early-stage prostate cancer model than hormone-independent cell lines. Assume also that Ang II can enhance the migration tendency aggressive prostate cancer cells, while Ang III does so more effective in non-metastatic cells.

Sex differences in the renal vascular response to angiotensin II involves the Mas receptor

AIM

The renin-angiotensin system (RAS) depressor arm, particularly renal angiotensin type 2 receptor (AT(2) R) and Mas receptor (masR) expression, is enhanced in females, which may contribute to renal and cardiovascular protection. We examined the hypotheses that masR activation increases renal blood flow (RBF) at rest and attenuates the reduction in RBF in response to angiotensin II (AngII) infusion in female rats. Furthermore, we postulated that combined activation of the AT(2) R and masR would produce a greater response than masR activation alone.

METHODS

In anaesthetized male and female Wistar rats, mean arterial pressure (MAP) and RBF responses during graded AngII infusion (30-1000 ng kg(-1)  min(-1) i.v.) were assessed following pre-treatment with vehicle, the masR antagonist A779, or A779 plus the AT(2) R antagonist PD123319.

RESULTS

Basal MAP was not altered by any pre-treatment. Basal RBF decreased approx. 20% in female (P < 0.05), but not male rats in response to A779. However, basal RBF was not altered by A779 + PD123319. AngII infusion reduced RBF in a dose-related fashion (P(dose)  < 0.0001) and masR blockade did not alter the RBF response to AngII infusion in male or female rats. However, A779 + PD123319 attenuated the reduction in RBF response to AngII in females (P(group)  < 0.005), but not males.

CONCLUSION

The impact of the masR on renal haemodynamics appears to be sexually dimorphic, with greater effects in female than male rats. However, the paradoxical effects of dual AT(2) R and masR blockade suggest that a greater understanding of the complex interactions between RAS components is required before the therapeutic opportunities of AT(2) R and/or masR stimulation can be advanced.

AT2 receptors: beneficial counter-regulatory role in cardiovascular and renal function

The renin-angiotensin system (RAS) is a coordinated hormonal cascade intimately involved in cardiovascular and renal control and blood pressure regulation. Angiotensin II (Ang II), the major RAS effector peptide, binds two distinct receptors, the angiotensin type-1 receptor (AT(1)R) and the angiotensin type-2 (AT(2)R) receptor. The vast majority of the physiological actions of Ang II, almost all of them detrimental, are mediated by AT(1)Rs. In contrast, AT(2)Rs negatively modulate the actions of AT(1)Rs under the majority of circumstances and generally possess beneficial effects. AT(2)Rs induce vasodilation in both resistance and capacitance vessels, mediating natriuresis directly and via interactions with dopamine D1 receptors in the renal proximal tubule. AT(2)Rs inhibit renin biosynthesis and secretion and protect the kidneys from inflammation and ischemic injury. Our understanding of the exact role of AT(2)Rs in physiology and pathophysiology continues to expand; the purpose of this review is to provide an up-to-date summary of the functional role of AT(2)Rs at the organ, tissue, cellular, and subcellular levels with emphasis on the vascular and renal actions that bear on blood pressure regulation and hypertension.

Compound 21 Induces Vasorelaxation via an Endothelium- and Angiotensin II Type 2 Receptor-Independent Mechanism

Angiotensin II type 2 (AT 2 ) receptor stimulation has been linked to vasodilation. Yet, AT 2 receptor-independent hypertension and hypotension (or no effect on blood pressure) have been observed in vivo after application of the AT 2 receptor agonist compound 21 (C21). We, therefore, studied its effects in vitro, using preparations known to display AT 2 receptor-mediated responses. Hearts of Wistar rats, spontaneously hypertensive rats (SHRs), C57Bl/6 mice, and AT 2 receptor knockout mice were perfused according to Langendorff. Mesenteric and iliac arteries of these animals, as well as coronary microarteries from human donor hearts, were mounted in Mulvany myographs. In the coronary vascular bed of Wistar rats, C57Bl/6 mice, and AT 2 receptor knockout mice, C21 induced constriction followed by dilation. SHR hearts displayed enhanced constriction and no dilation. Irbesartan (angiotensin II type 1 receptor blocker) abolished the constriction and enhanced or (in SHRs) reintroduced dilation, and PD123319 (AT 2 receptor blocker) did not block the latter. C21 relaxed preconstricted vessels of all species, and this did not depend on angiotensin II receptors, the endothelium, or the NO-guanylyl cyclase-cGMP pathway. C21 constricted SHR iliac arteries but none of the other vessels, and irbesartan prevented this. C21 shifted the concentration-response curves to U46619 (thromboxane A 2 analog) and phenylephrine (α-adrenoceptor agonist) but not ionomycine (calcium ionophore) to the right. In conclusion, C21 did not cause AT 2 receptor-mediated vasodilation. Yet, it did induce vasodilation by blocking calcium transport into the cell and constriction via angiotensin II type 1 receptor stimulation. The latter effect is enhanced in SHRs. These data may explain the varying effects of C21 on blood pressure in vivo.

Changes in plasma angiotensin subtypes in Japanese eel acclimated to various salinities from deionized water to double-strength seawater

Our knowledge of complexity of the renin-angiotensin system (RAS) has grown in recent years and various angiotensin peptides including Ang II, Ang III, Ang IV, and Ang (1-7) were found to have specific functions. Using a combination of HPLC and radioimmunoassay (RIA), we established a high resolution method to quantify various angiotensin subtypes in the plasma of eel acclimated to deionized water (dW), freshwater (FW), seawater (SW), and double-strength seawater (DSW). [Asn(1), Val(5)]-Ang II, [Asp(1), Val(5)]-Ang II, [Val(4)]-Ang III, and [Val(3)]-Ang IV are all present in the circulation and both Ang II subtypes were significantly higher in DSW eel. When the eel was transferred from FW to SW, plasma immunoreactive (ir) Ang II concentration increased and its levels were highly correlated to plasma osmolality, suggesting that the elevated plasma osmolality is the major stimulus for activating the RAS during high salinity transfer. To examine the conversion of [Asn(1)] to [Asp(1)] residue in vivo and in vitro, synthetic [Asn(1), Val(5)]-Ang II was injected into the circulation or incubated with plasma, but the production of [Asp(1), Val(5)]-Ang II was insignificant, which implies that the conversion may occur at the angiotensinogen level. An asparaginase assay was further developed for measuring asparaginase activity and the highest activity was in liver in both FW and SW eel. This new method of analysis can be extended to study the endogenous angiotensin ligands in the local RAS. The potential significance of [Asn(1)] to [Asp(1)] conversion on Ang II metabolism and function is discussed.

Neuroprotective effect of an angiotensin receptor type 2 agonist following cerebral ischemia in vitro and in vivo

Background

Intracerebral administration of the angiotensin II type 2 receptor (AT₂R) agonist, CGP42112, is neuroprotective in a rat model of ischemic stroke. To explore further its possible cellular target(s) and therapeutic utility, we firstly examined whether CGP42112 may exert direct protective effects on primary neurons following glucose deprivation in vitro. Secondly, we tested whether CGP42112 is effective when administered systemically in a mouse model of cerebral ischemia.

Methods

Primary cortical neurons were cultured from E17 C57Bl6 mouse embryos for 9 d, exposed to glucose deprivation for 24 h alone or with drug treatments, and percent cell survival assessed using trypan blue exclusion. Ischemic stroke was induced in adult male C57Bl6 mice by middle cerebral artery occlusion for 30 min, followed by reperfusion for 23.5 h. Neurological assessment was performed and then mice were euthanized and infarct and edema volume were analysed.

Results

During glucose deprivation, CGP42112 (1x10^-8 M and 1x10^-7 M) reduced cell death by ~30%, an effect that was prevented by the AT₂R antagonist, PD123319 (1x10^-6 M). Neuroprotection by CGP42112 was lost at a higher concentration (1x10^-6 M) but was unmasked by co-application with the AT₁R antagonist, candesartan (1x10^-7 M). By contrast, Compound 21 (1x10^-8 M to 1x10^-6 M), a second AT₂R agonist, had no effect on neuronal survival. Mice treated with CGP42112 (1 mg/kg i.p.) after cerebral ischemia had improved functional outcomes over vehicle-treated mice as well as reduced total and cortical infarct volumes.

Conclusions

These results indicate that CGP42112 can directly protect neurons from ischemia-like injury in vitro via activation of AT₂Rs, an effect opposed by AT₁R activation at high concentrations. Furthermore, systemic administration of CGP42112 can reduce functional deficits and infarct volume following cerebral ischemia in vivo.

Angiotensin-(1-7) in kidney disease: a review of the controversies

Ang-(1-7) [angiotensin-(1-7)] is a biologically active heptapeptide component of the RAS (renin-angiotensin system), and is generated in the kidney at relatively high levels, via enzymatic pathways that include ACE2 (angiotensin-converting enzyme 2). The biological effects of Ang-(1-7) in the kidney are primarily mediated by interaction with the G-protein-coupled receptor Mas. However, other complex effects have been described that may involve receptor-receptor interactions with AT(1) (angiotensin II type 1) or AT(2) (angiotensin II type 2) receptors, as well as nuclear receptor binding. In the renal vasculature, Ang-(1-7) has vasodilatory properties and it opposes growth-stimulatory signalling in tubular epithelial cells. In several kidney diseases, including hypertensive and diabetic nephropathy, glomerulonephritis, tubulointerstitial fibrosis, pre-eclampsia and acute kidney injury, a growing body of evidence supports a role for endogenous or exogenous Ang-(1-7) as an antagonist of signalling mediated by AT(1) receptors and thereby as a protector against nephron injury. In certain experimental conditions, Ang-(1-7) appears to paradoxically exacerbate renal injury, suggesting that dose or route of administration, state of activation of the local RAS, cell-specific signalling or non-Mas receptor-mediated pathways may contribute to the deleterious responses. Although Ang-(1-7) has promise as a potential therapeutic agent in humans with kidney disease, further studies are required to delineate its signalling mechanisms in the kidney under physiological and pathophysiological conditions.

Intrarenal angiotensin III is the predominant agonist for proximal tubule angiotensin type 2 receptors

In angiotensin type 1 receptor-blocked rats, renal interstitial (RI) administration of des-aspartyl(1)-angiotensin II (Ang III) but not angiotensin II induces natriuresis via activation of angiotensin type 2 receptors. In the present study, renal function was documented during systemic angiotensin type 1 receptor blockade with candesartan in Sprague-Dawley rats receiving unilateral RI infusion of Ang III. Ang III increased urine sodium excretion, fractional sodium, and lithium excretion. RI coinfusion of specific angiotensin type 2 receptor antagonist PD-123319 abolished Ang III-induced natriuresis. The natriuretic response observed with RI Ang III was not reproducible with RI angiotensin (1-7) alone or together with angiotensin-converting enzyme inhibition. Similarly, neither RI angiotensin II alone or in the presence of aminopeptidase A inhibitor increased urine sodium excretion. In the absence of systemic angiotensin type 1 receptor blockade, Ang III alone did not increase urine sodium excretion, but natriuresis was enabled by the coinfusion of aminopeptidase N inhibitor and subsequently blocked by PD-123319. In angiotensin type 1 receptor-blocked rats, RI administration of aminopeptidase N inhibitor alone also induced natriuresis that was abolished by PD-123319. Ang III-induced natriuresis was accompanied by increased RI cGMP levels and was abolished by inhibition of soluble guanylyl cyclase. RI and renal tissue Ang III levels increased in response to Ang III infusion and were augmented by aminopeptidase N inhibition. These data demonstrate that endogenous intrarenal Ang III but not angiotensin II or angiotensin (1-7) induces natriuresis via activation of angiotensin type 2 receptors in the proximal tubule via a cGMP-dependent mechanism and suggest aminopeptidase N inhibition as a potential therapeutic target in hypertension.

Dopamine and angiotensin type 2 receptors cooperatively inhibit sodium transport in human renal proximal tubule cells

Little is known regarding how the kidney shifts from a sodium and water reclaiming state (antinatriuresis) to a state where sodium and water are eliminated (natriuresis). In human renal proximal tubule cells, sodium reabsorption is decreased by the dopamine D(1)-like receptors (D(1)R/D(5)R) and the angiotensin type 2 receptor (AT(2)R), whereas the angiotensin type 1 receptor increases sodium reabsorption. Aberrant control of these opposing systems is thought to lead to sodium retention and, subsequently, hypertension. We show that D(1)R/D(5)R stimulation increased plasma membrane AT(2)R 4-fold via a D(1)R-mediated, cAMP-coupled, and protein phosphatase 2A-dependent specific signaling pathway. D(1)R/D(5)R stimulation also reduced the ability of angiotensin II to stimulate phospho-extracellular signal-regulated kinase, an effect that was partially reversed by an AT(2)R antagonist. Fenoldopam did not increase AT(2)R recruitment in renal proximal tubule cells with D(1)Rs uncoupled from adenylyl cyclase, suggesting a role of cAMP in mediating these events. D(1)Rs and AT(2)Rs heterodimerized and cooperatively increased cAMP and cGMP production, protein phosphatase 2A activation, sodium-potassium-ATPase internalization, and sodium transport inhibition. These studies shed new light on the regulation of renal sodium transport by the dopaminergic and angiotensin systems and potential new therapeutic targets for selectively treating hypertension.

Regulation of angiotensin II receptors beyond the classical pathway

The RAS (renin–angiotensin system) plays a role not only in the cardiovascular system, including blood pressure regulation, but also in the central nervous system. AngII (angiotensin II) binds two major receptors: the AT1 receptor (AngII type 1 receptor) and AT2 receptor (AngII type 2 receptor). It has been recognized that AT2 receptor activation not only opposes AT1 receptor actions, but also has unique effects beyond inhibitory cross-talk with AT1 receptor signalling. Novel pathways beyond the classical actions of RAS, the ACE (angiotensin-converting enzyme)/AngII/AT1 receptor axis, have been highlighted: the ACE2/Ang-(1–7) [angiotensin-(1–7)]/Mas receptor axis as a new opposing axis against the ACE/AngII/AT1 receptor axis, novel AngII-receptor-interacting proteins and various AngII-receptor-activation mechanisms including dimer formation. ATRAP (AT1-receptor-associated protein) and ATIP (AT2-receptor-interacting protein) are well-characterized AngII-receptor-associated proteins. These proteins could regulate the functions of AngII receptors and thereby influence various pathophysiological states. Moreover, the possible cross-talk between PPAR (peroxisome-proliferator-activated receptor)-γ and AngII receptor subtypes is an intriguing issue to be addressed in order to understand the roles of RAS in the metabolic syndrome, and interestingly some ARBs (AT1-receptor blockers) have been reported to have an AT1-receptor-blocking action with a partial PPAR-γ agonistic effect. These emerging concepts concerning the regulation of AngII receptors are discussed in the present review.

Influence of Angiotensin II Subtype 2 Receptor (AT(2)R) Antagonist, PD123319, on Cardiovascular Remodelling of Aged Spontaneously Hypertensive Rats during Chronic Angiotensin II Subtype 1 Receptor (AT(1)R) Blockade

Cardiac AT₂R expression is upregulated in the normal process of aging. In this study we determined the contribution of AT₂R to chronic antihypertensive and remodelling effects of AT₁R blockade in aged hypertensive rats. Adult (20 weeks) and senescent (20 months) spontaneously hypertensive rats (SHRs) were treated with either the AT₁R antagonist, candesartan cilexetil (2 mg/kg/day), the AT₂R antagonist, PD123319 (10 mg/kg/day), or a combination of the 2 compounds. Mean arterial pressure (MAP) and left ventricular volume were markedly decreased by candesartan cilexetil, however, simultaneous treatment with PD123319 had no additional effect on either parameter. Perivascular fibrosis was significantly reduced by candesartan cilexetil in aged animals only, and this effect was reversed by concomitant PD123319 administration. Vascular hypertrophy was reduced by candesartan cilexetil, and these effects were reversed by simultaneous PD123319. These results suggest that AT₂R stimulation does not significantly influence the antihypertensive effect of chronic AT₁R blockade, but plays a role in the regulation of vascular structure. The severe degree of cardiac perivascular fibrosis in senescent animals was regressed by AT₁R blockade and this effect was reversed by simultaneous AT₂R inhibition, demonstrating an antifibrotic role of AT₂R stimulation in the aging hypertensive heart.

Angiotensin II type 2 receptor agonists: where should they be applied?

INTRODUCTION

Angiotensin II, the active endproduct of the renin-angiotensin system (RAS), exerts its effects via angiotensin II type 1 and type 2 (AT(1), AT(2)) receptors. AT(1) receptors mediate all well-known effects of angiotensin II, ranging from vasoconstriction to tissue remodeling. Thus, to treat cardiovascular disease, RAS blockade aims at preventing angiotensin II-AT(1) receptor interaction. Yet RAS blockade is often accompanied by rises in angiotensin II, which may exert beneficial effects via AT(2) receptors.

AREAS COVERED

This review summarizes our current knowledge on AT(2) receptors, describing their location, function(s), endogenous agonist(s) and intracellular signaling cascades. It discusses the beneficial effects obtained with C21, a recently developed AT(2) receptor agonist. Important questions that are addressed are do these receptors truly antagonize AT(1) receptor-mediated effects? What about their role in the diseased state and their heterodimerization with other receptors?

EXPERT OPINION

The general view that AT(2) receptors exclusively exert beneficial effects has been challenged, and in pathological models, their function sometimes mimics that of AT(1) receptors, for example, inducing vasoconstriction and cardiac hypertrophy. Yet given its upregulation in various pathological conditions, the AT(2) receptor remains a promising target for treatment, allowing effects beyond blood pressure-lowering, for example, in stroke, aneurysm formation, inflammation and myocardial fibrosis.

Angiotensin IV protects against angiotensin II-induced cardiac injury via AT4 receptor

Angiotensin II (Ang II) is an important regulator of cardiac function and injury in hypertension. The novel Ang IV peptide/AT4 receptor system has been implicated in several physiological functions and has some effects opposite to those of Ang II. However, little is known about the role of this system in Ang II-induced cardiac injury. Here we studied the effect of Ang IV on Ang II-induced cardiac dysfunction and injury using isolated rat hearts, neonatal cardiomyocytes and cardiac fibroblasts. We found that Ang IV significantly improved Ang II-induced cardiac dysfunction and injury in the isolated heart in response to ischemia/reperfusion (I/R). Moreover, Ang IV inhibited Ang II-induced cardiac cell apoptosis, cardiomyocyte hypertrophy, and proliferation and collagen synthesis of cardiac fibroblasts; these effects were mediated through the AT4 receptor as confirmed by siRNA knockdown. These findings suggest that Ang IV may have a protective effect on Ang II-induced cardiac injury and dysfunction and may be a novel therapeutic target for hypertensive heart disease.