Brain angiotensin receptor subtypes AT1, AT2, and AT4 and their functions

New Approaches Targeting the Renin-Angiotensin System: Inhibition of Brain Aminopeptidase A, ACE2 Ubiquitination, and Angiotensinogen

Despite the availability of various therapeutic classes of antihypertensive drugs, hypertension remains poorly controlled, in part because of poor adherence. Hence, there is a need for the development of antihypertensive drugs acting on new targets to improve control of blood pressure. This review discusses novel insights (including the data of recent clinical trials) with regard to interference with the renin-angiotensin system, focusing on the enzymes aminopeptidase A and angiotensin-converting enzyme 2 (ACE2) in the brain, as well as the substrate of renin- angiotensinogen-in the liver. It raises the possibility that centrally acting amino peptidase A inhibitors (eg, firibastat), preventing the conversion of angiotensin II to angiotensin III in the brain, might be particularly useful in African Americans and patients with obesity. Firibastat additionally upregulates brain ACE2, allowing the conversion of angiotensin II to its protective metabolite angiotensin-(1-7). Furthermore, antisense oligonucleotides or small interfering ribonucleic acids suppress hepatic angiotensinogen for weeks to months after 1 injection and thus could potentially overcome adherence issues. Finally, interference with ACE2 ubiquitination is emerging as a future option for the treatment of neurogenic hypertension, given that ubiquitination resistance might upregulate ACE2 activity.

Roles of Angiotensin III in the brain and periphery

Angiotensin (Ang) III, a biologically active peptide of the renin angiotensin system (RAS) is predominantly known for its central effects on blood pressure. Our understanding of the RAS has evolved from the simplified, classical RAS, a hormonal system regulating blood pressure to a complex system affecting numerous biological processes. Ang II, the main RAS peptide has been widely studied, and its deleterious effects when overexpressed is well-documented. However, other components of the RAS such as Ang III are not well studied. This review examines the molecular and biological actions of Ang III and provides insight into Ang III's potential role in metabolic diseases.

Firibastat: A Novel Brain Aminopeptidase Inhibitor - A New Era of Antihypertensive therapy

Global incidence and prevalence of hypertension continues to increase and remains a significant challenge. The ever-increasing number of cases are due to comorbid conditions such as obesity and diabetes, as well as lifestyle indiscretions such as excessive salt intake. Hypertension, congestive heart failure, and kidney disease are all conditions resulting from abnormal Renin-Angiotensin-Aldosterone activation and adverse remodeling. Firibastat, a novel Brain Aminopeptidase inhibitor, may be able to help achieve blood pressure control in those with resistant hypertension. In this review article, we will discuss the biochemical pathway of firibastat and various trials assessing drug efficacy in animals and humans. This drug has the potential to curb the risk of uncontrolled hypertension and help improve long term cardiovascular morbidity and mortality.

Brain ACE2 activation following brain aminopeptidase A blockade by firibastat in salt-dependent hypertension

In the brain, aminopeptidase A (APA), a membrane-bound zinc metalloprotease, generates angiotensin III from angiotensin II. Brain angiotensin III exerts a tonic stimulatory effect on the control of blood pressure (BP) in hypertensive rats and increases vasopressin release. Blocking brain angiotensin III formation by the APA inhibitor prodrug RB150/firibastat normalizes arterial BP in hypertensive deoxycorticosterone acetate (DOCA)-salt rats without inducing angiotensin II accumulation. We therefore hypothesized that another metabolic pathway of brain angiotensin II, such as the conversion of angiotensin II into angiotensin 1-7 (Ang 1-7) by angiotensin-converting enzyme 2 (ACE2) might be activated following brain APA inhibition. We found that the intracerebroventricular (icv) administration of RB150/firibastat in conscious DOCA-salt rats both inhibited brain APA activity and induced an increase in brain ACE2 activity. Then, we showed that the decreases in BP and vasopressin release resulting from brain APA inhibition with RB150/firibastat were reduced if ACE2 was concomitantly inhibited by MLN4760, a potent ACE2 inhibitor, or if the Mas receptor (MasR) was blocked by A779, a MasR antagonist. Our findings suggest that in the brain, the increase in ACE2 activity resulting from APA inhibition by RB150/firibastat treatment, subsequently increasing Ang 1-7 and activating the MasR while blocking angiotensin III formation, contributes to the antihypertensive effect and the decrease in vasopressin release induced by RB150/firibastat. RB150/firibastat treatment constitutes an interesting therapeutic approach to improve BP control in hypertensive patients by inducing in the brain renin-angiotensin system, hyperactivity of the beneficial ACE2/Ang 1-7/MasR axis while decreasing that of the deleterious APA/Ang II/Ang III/ATI receptor axis.

Firibastat, the first-in-class brain aminopeptidase a inhibitor, in the management of hypertension: A review of clinical trials

An unfortunate subset of hypertensive patients develops resistant hypertension in which optimal doses of three or more first-line antihypertensive drugs fail to sufficiently control blood pressure. Patients with resistant hypertension represent a high-risk and difficult-to-treat group, and such patients are at amplified jeopardies for substantial hypertension-related multi-organ failure, morbidity, and mortality. Thus, there is a pressing requirement to better improve blood pressure control through the pharmaceutical generation of novel classes of antihypertensive drugs that act on newer and alternative therapeutic targets. The hyperactivity of the brain renin-angiotensin system (RAS) has been shown to play a role in the pathogenesis of hypertension in various experimental and genetic hypertensive animal models. In the brain, angiotensin-II is metabolized to angiotensin-III by aminopeptidase A (APA), a membrane-bound zinc metalloprotease enzyme. A large body of evidence has previously established that angiotensin-III is one of the main effector peptides of the brain RAS. Angiotensin-III exerts central stimulatory regulation over blood pressure through several proposed mechanisms. Accumulating evidence from preclinical studies demonstrated that the centrally acting APA inhibitor prodrugs (firibastat and NI956) are very safe and effective at reducing blood pressure in various hypertensive animal models. The primary purpose of this study is to narratively review the published phase I-II literature on the safety and efficacy of APA inhibitors in the management of patients with hypertension. Moreover, a summary of ongoing clinical trials and future perspectives are presented.

Aminopeptidases in Cardiovascular and Renal Function. Role as Predictive Renal Injury Biomarkers

Aminopeptidases (APs) are metalloenzymes that hydrolyze peptides and polypeptides by scission of the N-terminus amino acid and that also participate in the intracellular final digestion of proteins. APs play an important role in protein maturation, signal transduction, and cell-cycle control, among other processes. These enzymes are especially relevant in the control of cardiovascular and renal functions. APs participate in the regulation of the systemic and local renin-angiotensin system and also modulate the activity of neuropeptides, kinins, immunomodulatory peptides, and cytokines, even contributing to cholesterol uptake and angiogenesis. This review focuses on the role of four key APs, aspartyl-, alanyl-, glutamyl-, and leucyl-cystinyl-aminopeptidases, in the control of blood pressure (BP) and renal function and on their association with different cardiovascular and renal diseases. In this context, the effects of AP inhibitors are analyzed as therapeutic tools for BP control and renal diseases. Their role as urinary biomarkers of renal injury is also explored. The enzymatic activities of urinary APs, which act as hydrolyzing peptides on the luminal surface of the renal tubule, have emerged as early predictive renal injury biomarkers in both acute and chronic renal nephropathies, including those induced by nephrotoxic agents, obesity, hypertension, or diabetes. Hence, the analysis of urinary AP appears to be a promising diagnostic and prognostic approach to renal disease in both research and clinical settings.

Protective effects of losartan on some type 2 diabetes mellitus-induced complications in Wistar and spontaneously hypertensive rats

Diabetes mellitus type 2 (T2DM) is characterized by resistance of insulin receptors and/or inadequate insulin secretion resulting in metabolic and structural complications including vascular diseases, arterial hypertension and different behavioral alterations. We aimed to study the effects of the antihypertensive angiotensin AT1 receptor antagonist losartan on the T2DM-induced changes of exploratory behavior, anxiety, nociception and short term memory in normotensive Wistar and spontaneously hypertensive rats (SHRs). The experimental model of T2DM induced by a combination of high fat diet and streptozotocin, decreased exploratory activity and increased the level of carbonylated proteins in selected brain structures in both strains; as well it increased corticosterone level, pain threshold, anxiety-like behavior, and decline short term memory only in SHRs. Losartan treatment alleviated some of the T2DM- induced metabolic complications, abolished the T2DM-induced hypo activity, and normalized the corticosterone level, carbonylated proteins in brain, nociception and memory. Losartan did not exert effect on the anxiety behavior in both strains. We showed that T2DM exerted more pronounced negative effects on the rats with comorbid hypertension as compared to normotensive rats. Overall effects on the studied behavioral parameters are related to decreased exploration of the new environment, increased anxiety-like behavior, and decline in short-term memory. The systemic sub-chronic treatment with an angiotensin AT1 receptor antagonist losartan ameliorated most of these complications.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Roles and functions of Atp6ap2 in the brain

The classical renin-angiotensin system (RAS) in the body has been studied intensively in the last decades, since it is known that this system is involved in the regulation of blood pressure. Since nearly all members of the classical RAS have also been identified within the brain in the last decades and due to the existence of the blood-brain barrier, a RAS within the brain (bRAS) that is largely independent from the peripheral RAS has been postulated. All members of the angiotensin family as e.g., angiotensin II, angiotensin IV and angiotensin II (1-7) along with the respective receptors (e.g., angiotensin II receptor type 1 (AT1), angiotensin II receptor type 2 (AT2), angiotensin IV receptor (AT4), angiotensin II (1-7) receptor (Mas)) have been identified within the brain. Moreover, a receptor capable of binding renin and the renin precursor prorenin with high affinity has also been detected within the brain. This protein functions as a membrane receptor for (pro)renin and also represents a V-ATPase subunit and is therefore termed (P)RR or Atp6ap2, respectively. In this review we shed light on the (known as well as putative) roles and functions of Atp6ap2 in the brain under physiological and pathophysiological conditions.

A PCR-RFLP method for detection of the LNPEP encoding human insulin-regulated aminopeptidase (IRAP) rs4869317 polymorphism

Background & objectives:

Insulin regulated aminopeptidase (IRAP) has been related to certain pathologies such as breast cancer, Alzheimer's disease and septic shock. IRAP is encoded by the leucyl/cystinyl aminopeptidase (LNPEP) gene. The genetic variation in the LNPEP gene has been analyzed in relation with the mortality and vasopressin clearance in septic shock. The LNPEP rs4869317 SNP (single nucleotide polymorphism) was the most significantly associated SNP with vasopressinase activity, being TT genotype associated with increased mortality. The objective of the present study was to develop a simple method to allow a quick and affordable genotyping for the rs4869317 SNP of LNPEP gene.

Methods:

Blood DNA samples were obtained from randomly selected healthy volunteers (n=28). A pair of primers was designed to amplify an 834 bp region of the LNPEP gene containing the rs4869317 SNP. The two alleles (T or A) were detected by digestion of the PCR products with the PacI restriction endonuclease. This enzyme only cuts the PCR products when the adenine is present in the SNP.

Results:

All individuals showed RFPL (restriction fragment length polymorphism) fragments for the expected genotypes (TT, TA or AA). The methodology was validated by sequencing of the amplified DNAs from several ‘T/T’ and ‘A/A’ homozygotes and ‘T/A’ heterozygotes. The results from both methods showed agreement.

Interpretation & conclusions:

The PCR-RFLP is a simple and reliable method that allows a quick genotyping for the rs4869317 SNP of LNPEP gene. The study of this polymorphism could be useful in future investigations to analyze the role of genetic variants of IRAP in several physiological/pathological conditions.

Novel adipocyte aminopeptidases are selectively upregulated by insulin in healthy and obese rats

The lack of a complete assembly of the sensitivity of subcellular aminopeptidase (AP) activities to insulin in different pathophysiological conditions has hampered the complete view of the adipocyte metabolic pathways and its implications in these conditions. Here we investigated the influence of insulin on basic AP (APB), neutral puromycin-sensitive AP (PSA), and neutral puromycin-insensitive AP (APM) in high and low density microsomal and plasma membrane fractions from adipocytes of healthy and obese rats. Catalytic activities of these enzymes were fluorometrically monitoring in these fractions with or without insulin stimulus. Canonical traffic such as insulin-regulated AP was not detected for these novel adipocyte APs in healthy and obese rats. However, insulin increased APM in low density microsomal and plasma membrane fractions from healthy rats, APB in high density microsomal fraction from obese rats and PSA in plasma membrane fraction from healthy rats. A new concept of intracellular compartment-dependent upregulation of AP enzyme activities by insulin emerges from these data. This relatively selective regulation has pathophysiological significance, since these enzymes are well known to act as catalysts and receptor of peptides directly related to energy metabolism. Overall, the regulation of each one of these enzyme activities reflects certain dysfunction in obese individuals.

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.

Angiotensin type 1a receptors on corticotropin-releasing factor neurons contribute to the expression of conditioned fear

Although generally associated with cardiovascular regulation, angiotensin II receptor type 1a (AT1a R) blockade in mouse models and humans has also been associated with enhanced fear extinction and decreased post-traumatic stress disorder (PTSD) symptom severity, respectively. The mechanisms mediating these effects remain unknown, but may involve alterations in the activities of corticotropin-releasing factor (CRF)-expressing cells, which are known to be involved in fear regulation. To test the hypothesis that AT1a R signaling in CRFergic neurons is involved in conditioned fear expression, we generated and characterized a conditional knockout mouse strain with a deletion of the AT1a R gene from its CRF-releasing cells (CRF-AT1a R((-/-)) ). These mice exhibit normal baseline heart rate, blood pressure, anxiety and locomotion, and freeze at normal levels during acquisition of auditory fear conditioning. However, CRF-AT1a R((-/-)) mice exhibit less freezing than wild-type mice during tests of conditioned fear expression-an effect that may be caused by a decrease in the consolidation of fear memory. These results suggest that central AT1a R activity in CRF-expressing cells plays a role in the expression of conditioned fear, and identify CRFergic cells as a population on which AT1 R antagonists may act to modulate fear extinction.

Autonomic and inflammatory consequences of posttraumatic stress disorder and the link to cardiovascular disease

Stress- and anxiety-related disorders are on the rise in both military and general populations. Over the next decade, it is predicted that treatment of these conditions, in particular, posttraumatic stress disorder (PTSD), along with its associated long-term comorbidities, will challenge the health care system. Multiple organ systems are adversely affected by PTSD, and PTSD is linked to cancer, arthritis, digestive disease, and cardiovascular disease. Evidence for a strong link between PTSD and cardiovascular disease is compelling, and this review describes current clinical data linking PTSD to cardiovascular disease, via inflammation, autonomic dysfunction, and the renin-angiotensin system. Recent clinical and preclinical evidence regarding the role of the renin-angiotensin system in the extinction of fear memory and relevance in PTSD-related immune and autonomic dysfunction is also addressed.

The renin-angiotensin system: a possible contributor to migraine pathogenesis and prophylaxis

The presence of a tissue-based renin-angiotensin system, independent of the systemic one, has been identified in several organs including the brain. Experimental models have suggested the involvement of the renin-angiotensin system in neurogenic inflammation, susceptibility to oxidative stress, endothelial dysfunction, and neuromodulation of nociceptive transmission, thus potentially contributing to the pathogenesis of migraine. Genetic factors that increase susceptibility to migraine may include angiotensin-converting enzyme polymorphism, although available data are controversial. Clinical studies have suggested that angiotensin-converting enzyme inhibitors and angiotensin receptor blockers may be effective in migraine prophylaxis. However, further research should clarify whether the postulated preventive effect is attributable to a pharmacological action over and above the antihypertensive effect and should test their tolerability in subjects with normal blood pressure values. In patients with contraindications or not responding to conventional prophylactic drugs and in patients with comorbid arterial hypertension, angiotensin-converting enzyme inhibitors and angiotensin receptor blockers may be used for migraine prophylaxis.

Angiotensin type 1 receptor inhibition enhances the extinction of fear memory

BACKGROUND

The current effective treatment options for posttraumatic stress disorder (PTSD) are limited, and therefore the need to explore new treatment strategies is critical. Pharmacological inhibition of the renin-angiotensin system is a common approach to treat hypertension, and emerging evidence highlights the importance of this pathway in stress and anxiety. A recent clinical study from our laboratory provides evidence supporting a role for the renin-angiotensin system in the regulation of the stress response in patients diagnosed with PTSD.

METHODS

With an animal model of PTSD and the selective angiotensin receptor type 1 (AT1) antagonist losartan, we investigated the acute and long-term effects of AT1 receptor inhibition on fear memory and baseline anxiety. After losartan treatment, we performed classical Pavlovian fear conditioning pairing auditory cues with footshocks and examined extinction behavior, gene expression changes in the brain, as well as neuroendocrine and cardiovascular responses.

RESULTS

After cued fear conditioning, both acute and 2-week administration of losartan enhanced the consolidation of extinction memory but had no effect on fear acquisition, baseline anxiety, blood pressure, and neuroendocrine stress measures. Gene expression changes in the brain were also altered in mice treated with losartan for 2 weeks, in particular reduced amygdala AT1 receptor and bed nucleus of the stria terminalis c-Fos messenger RNA levels.

CONCLUSIONS

These data suggest that AT1 receptor antagonism enhances the extinction of fear memory and therefore might be a beneficial therapy for PTSD patients who have impairments in extinction of aversive memories.

Effects of angiotensin (5-8) microinfusions into the ventrolateral periaqueductal gray on defensive behaviors in rats

Peptides of the renin-angiotensin system modulate blood pressure and hydro-electrolyte composition. Angiotensin (Ang) receptors are localized in brain areas related to the regulation of autonomic and endocrine control and involved in sensory perception, memory process and behavioral responses. Among these areas, the ventrolateral periaqueductal gray (vlPAG) is one of the most important structures of the neuronal circuitry controlling the autonomic and behavioral components of emotional states. Although Ang II metabolism in the vlPAG forms several Ang-peptides including Ang (5-8), the role of this tetrapeptide in the organization of defensive responses has not yet been described. To address this issue, the purpose of the present study was to determine the effects of intra-vlPAG injections of Ang (5-8) (0.2, 0.4 and 0.8 nmol/0.25 μL) in rats submitted to the elevated plus-maze (EPM) test. Additionally, it was evaluated the effects of intra-vlPAG Ang (5-8) on the expression of conditioned fear, assessed by the fear-potentiated startle and contextual conditioned freezing tests. The results showed that Ang (5-8) produced an intense, dose-related reduction in the entries into and time spent in the open arms of the EPM, decreased direct exploration and increased risk assessment behaviors. Moreover, intra-vlPAG injections of Ang (5-8) before the test session promoted pro-aversive effects in the FPS and enhanced contextual freezing. Taken together, these results point out to an important anxiogenic-like action for Ang (5-8) in the mediation of defensive behaviors organized in the vlPAG.

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.

Inhibition of brain angiotensin III attenuates sympathetic hyperactivity and cardiac dysfunction in rats post-myocardial infarction

AIMS

In rats post-myocardial infarction (MI), activation of angiotensinergic pathways in the brain contributes to sympathetic hyperactivity and progressive left ventricle (LV) dysfunction. The present study examined whether angiotensin III (Ang III) is one of the main effector peptides of the brain renin-angiotensin system controlling these effects.

METHODS AND RESULTS

After coronary artery ligation, Wistar rats were infused intracerebroventricularly for 4 weeks via minipumps with vehicle, the aminopeptidase A (APA) inhibitor RB150 (0.3 mg/day), which blocks the formation of brain Ang III, or losartan (0.25 mg/day). Blood pressure (BP), heart rate, and renal sympathetic nerve activity in response to air stress and acute changes in BP were measured, and LV function was evaluated by echocardiography and Millar catheter. At 4 weeks post-MI, brain APA activity was increased, sympatho-excitatory and pressor responses to air stress enhanced, and arterial baroreflex function impaired. LV end-diastolic pressure (LVEDP) was increased and ejection fraction (EF) and maximal first derivative of change in pressure over time (dP/dt(max)) were decreased. Central infusion of RB150 during 4 weeks post-MI normalized brain APA activity and responses to stress and baroreflex function, and improved LVEDP, EF, and dP/dt(max). Central infusion of losartan had similar effects but was somewhat less effective, and had no effect on brain APA activity.

CONCLUSION

These results indicate that brain APA and Ang III appear to play a pivotal role in the sympathetic hyperactivity and LV dysfunction in rats post-MI. RB150 may be a potential candidate for central nervous system-targeted therapy post-MI.

Angiotensin (5-8) modulates nociception at the rat periaqueductal gray via the NO-sGC pathway and an endogenous opioid

Angiotensins (Angs) modulate blood pressure, hydro-electrolyte composition, and antinociception. Although Ang (5-8) has generally been considered to be inactive, we show here that Ang (5-8) was the smallest Ang to elicit dose-dependent responses and receptor-mediated antinociception in the rat ventrolateral periaqueductal gray matter (vlPAG). Ang (5-8) antinociception seems to be selective, because it did not alter blood pressure or act on vascular or intestinal smooth muscle cells. The non-selective Ang-receptor (Ang-R) antagonist saralasin blocked Ang (5-8) antinociception, but selective antagonists of Ang-R types I, II, IV, and Mas did not, suggesting that Ang (5-8) may act via an unknown receptor. Endopeptidase EP 24.11 and amastatin-sensitive aminopeptidase from the vlPAG catalyzed the synthesis (from Ang II or Ang III) and inactivation of Ang (5-8), respectively. Selective inhibitors of neuronal-nitric oxide (NO) synthase, soluble guanylyl cyclase (sGC) and a non-selective opioid receptor (opioid-R) inhibitor blocked Ang (5-8)-induced antinociception. In conclusion, Ang (5-8) is a new member of the Ang family that selectively and strongly modulates antinociception via NO-sGC and endogenous opioid in the vlPAG.

Enzymatic activity of DPIV and renin-angiotensin system (RAS) proteases in patients with left ventricular dysfunction and primary prevention implantable cardioverter/defibrillator (ICD)

BACKGROUND

Patients (pts) with severely decreased left ventricular ejection fraction (LV-EF ≤ 35%) are at high risk for sudden cardiac death (SCD). We sought to investigate, if pts with primary prevention ICD hold alterations in enzyme-activities of the dipeptidyl-aminopeptidase IV (DPIV) and the renin-angiotensin system (RAS) before VT/VF occurrence.

METHODS

57 Pts (53 male, mean age 64.9 [42-84] years, mean LV-EF 26 ± 5%) with ischemic (n=49) or non-ischemic cardiomyopathy (n=8) who had received an ICD/CRT-D for primary prevention, were included. Pts were assessed for appropriate ICD intervention for VT/VF during a mean follow-up of 365 ± 90 days. Serum levels of dipeptidyl-aminopeptidase IV (DPIV), aminopeptidase N (APN), aminopeptidase B (APB), insulin-regulated aminopeptidase (IRAP), and angiotensin-converting enzyme 2 (ACE2) were determined.

RESULTS

Pts with appropriate ICD intervention (n=16) had higher serum activities of IRAP (mean difference=12.681 pkat/mL; p=0.007), and DPIV (mean difference=117.557 pkat/mL; p=0.032) than pts without appropriate ICD intervention. Furthermore, ACE2 activity was significantly higher (median: 223.7 RFU/smL vs. 169.10 RFU/smL; p=0.037). A Cox regression analysis indicated DPIV activity >50th centile to have a hazard ratio (HR) of 5.955 (CI 95%: 1.670-21.241; p=0.006) for prediction of appropriate ICD intervention. In a multivariate Cox regression model, DPIV and IRAP >50th centile remained predictive for appropriate ICD intervention.

CONCLUSION

Our prospective study shows that pts with primary prevention ICD, who receive appropriate ICD intervention during follow-up, can be identified by elevated activities of DPIV and several RAS proteases. Hence, theses biomarkers seem to be of prognostic relevance in a primary prevention collective. Our data has to be proven in larger cohorts.

Development of oxytocin- and vasopressin-network in the supraoptic and paraventricular nuclei of fetal sheep

The hypothalamic supraoptic and paraventricular nuclei consist of oxytocin and arginine vasopressin synthesizing neurons that send projections to the neurohypophysis. A growing body of evidence in adult animals and young animals at near term confirmed the structure and function in the vasopressinergic and oxytocinergic network. However, whether those distinctive neural networks are formed before near term is largely unknown. This study determined the special patterns in location and distribution of oxytocin- and vasopressin-neurons in the paraventricular and supraoptic nuclei from preterm to term in the ovine fetuses. The results showed that oxytocin- and vasopressin-neurons were present in both nuclei at the three gestational time periods (preterm, near term, and term). In the paraventricular nuclei, vasopressin-cells concentrated mainly in the core of the middle magnocellular paraventricular nuclei, and oxytocin-cells were scattered surrounding the core. In the supraoptic nuclei, vasopressin-cells mostly located in the ventral part, and oxytocin-cells in the dorsal part. The data demonstrated that the special distributed patterns of vasopressin- and oxytocin-neuron network have formed in those two nuclei at least from preterm. Intracerebroventricular injection of angiotensin II significantly increased fetal plasma oxytocin and vasopressin levels at preterm, which was associated with an increase of oxytocin- and vasopressin-neuron activity marked with c-fos expression. The data provided new evidence for the structural and functional development of the oxytocin- and vasopressin-network before birth.

Direct stimulation of angiotensin II type 2 receptor enhances spatial memory

We examined the possibility that direct stimulation of the angiotensin II type 2 (AT(2)) receptor by a newly generated direct AT(2) receptor agonist, Compound 21 (C21), enhances cognitive function. Treatment with C21 intraperitoneal injection for 2 weeks significantly enhanced cognitive function evaluated by the Morris water maze test in C57BL6 mice, but this effect was not observed in AT(2) receptor-deficient mice. However, C21-induced cognitive enhancement in C57BL6 mice was attenuated by coadministration of icatibant, a bradykinin B(2) receptor antagonist. Administration of C21 dose dependently increased cerebral blood flow assessed by laser speckle flowmetry and hippocampal field-excitatory postsynaptic potential (f-EPSP) determined by electrophysiological techniques in C57BL6 mice. Furthermore, activation of the AT(2) receptor by C21 promoted neurite outgrowth of cultured hippocampal neurons prepared from fetal transgenic mice expressing green fluorescent protein. Finally, we investigated the pathologic relevance of C21 for spatial learning using an Alzheimer's disease mouse model with intracerebroventricular injection of amyloid-β (1 to 40). We observed that treatment with C21 prevented cognitive decline in this model. These results suggest that a direct AT(2) receptor agonist, C21, enhances cognitive function at least owing to an increase in CBF, enhancement of f-EPSP, and neurite outgrowth in hippocampal neurons.

Role of angiotensin II receptor subtype activation in cognitive function and ischaemic brain damage

Recent clinical studies have demonstrated that angiotensin II type 1 (AT(1) ) receptor blockers (ARBs) reduce the onset of stroke, stroke severity and the incidence and progression of Alzheimer's disease and dementia. We can expect that ARBs exert these effects by both AT(1) receptor blockade and angiotensin II type 2 (AT(2) ) receptor stimulation. Moreover, recent experimental results support the notion that AT(2) receptor stimulation with AT(1) receptor blockade could contribute to protection against ischaemic brain damage at least partly due to an increase in cerebral blood flow and decrease in oxidative stress, and prevent cognitive decline. Cellular therapy has been focused on as a new therapeutic approach to restore injured neurons. In this context, it has been reported that AT(2) receptor stimulation enhances neurite outgrowth and decreases neural damage, thereby enhancing neurogenesis. Moreover, additional beneficial effects of ARBs with an AT(1) receptor blocking action with a partial peroxisome proliferator-activated receptor (PPAR)-γ agonistic effect have been reported, and interaction of AT(2) receptor activation and PPAR-γ might be involved in these ARBs' effects. This article reviews the effects of regulation of activation of angiotensin II receptor subtypes on ischaemic brain damage and cognitive function, focusing on the effects of AT(2) receptor stimulation.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Correlations between angiotensinase activity asymmetries in the brain and paw preference in rats

The function of angiotensin peptides is dependent upon the action of several aminopeptidases (APs) termed angiotensinases. Soluble (SOL) and membrane (MEM)-bound alanyl-AP (AlaAP) and cystinyl-AP (CysAP) are involved in the metabolism of angiotensins and related to the modulation of behavior and memory. To study the interactions between angiotensinase activity in the hippocampus and behavioral lateralization, Wistar rats were selected on the basis of their performance in the paw preference test (left-handed, ambidextrous and right-handed) and the activities of SOL-AlaAP/CysAP and MEM-AlaAP/CysAP were measured in the both hippocampuses. We observed that: (1) the left hippocampus had higher activities of SOL-AlaAP/CysAP and MEM-AlaAP/CysAP than the right hippocampus; (2) rats showed significant differences in the activities of SOL-AlaAP/CysAP and MEM-AlaAP/CysAP in the hippocampus depending on the behavioral lateralization detecting by paw preference; (3) in three groups of rats, hemispheric dominance - %R/T [%R/T=right hemisphere/(right hemisphere+left hemisphere)x100] activities of MEM-AlaAP, SOL-CysAP and MEM-CysAP was significantly different whereby %RT was lower in left-handed, higher in ambidextrous and intermediate in right-handed rats; (4) individual %R/T activities of SOL-CysAP and MEM-CysAP in the hippocampus were positively correlated with paw preference scores. Finally, we used the passive avoidance behavior test to demonstrate the differences of long-term memory among the three groups. These results suggested that the asymmetric activity of angiotensinase in the rat hippocampus may be associated with both the direction and the intensity of behavioral lateralization as expressed by paw preference.

Regulation of central angiotensin type 1 receptors and sympathetic outflow in heart failure

Angiotensin type 1 receptors (AT(1)Rs) play a critical role in a variety of physiological functions and pathophysiological states. They have been strongly implicated in the modulation of sympathetic outflow in the brain. An understanding of the mechanisms by which AT(1)Rs are regulated in a variety of disease states that are characterized by sympathoexcitation is pivotal in development of new strategies for the treatment of these disorders. This review concentrates on several aspects of AT(1)R regulation in the setting of chronic heart failure (CHF). There is now good evidence that AT(1)R expression in neurons is mediated by activation of the transcription factor activator protein 1 (AP-1). This transcription factor and its component proteins are upregulated in the rostral ventrolateral medulla of animals with CHF. Because the increase in AT(1)R expression and transcription factor activation can be blocked by the AT(1)R antagonist losartan, a positive feedback mechanism of AT(1)R expression in CHF is suggested. Oxidative stress has also been implicated in the regulation of receptor expression. Recent data suggest that the newly discovered catabolic enzyme angiotensin-converting enzyme 2 (ACE2) may play a role in the modulation of AT(1)R expression by altering the balance between the octapeptide ANG II and ANG- (1-7). Finally, exercise training reduces both central oxidative stress and AT(1)R expression in animals with CHF. These data strongly suggest that multiple central and peripheral influences dynamically alter AT(1)R expression in CHF.

The angiotensin II type 2 receptor in the brain

Recent clinical studies indicate that blockade of the renin-angiotensin system is important to prevent stroke, and accumulating results of basic research also indicate the possible involvement of the central renin-angiotensin system in ischaemic brain damage and cognition. When the angiotensin II type 1 receptor is blocked by an angiotensin type 1 receptor blocker, unbound angiotensin II acts preferentially on the angiotensin II type 2 (AT(2)) receptor. These results suggest the pathophysiological importance of the AT(2) receptor in the clinical use of angiotensin type 1 receptor blockers, which are widely used in patients with hypertension with the expectation of a decrease in the onset of cardiovascular and cerebrovascular disease. We review here the possible roles of AT(2) receptor activation in the brain, focusing on ischaemic stroke, cognitive function and neurogenesis, and potential effects of specific AT(2) receptor agonists.

Angiotensin III modulates the nociceptive control mediated by the periaqueductal gray matter

Endogenous angiotensin (Ang) II and/or an Ang II-derived peptide, acting on Ang type 1 (AT(1)) and Ang type 2 (AT(2)) receptors, can carry out part of the nociceptive control modulated by periaqueductal gray matter (PAG). However, neither the identity of this putative Ang-peptide, nor its relationship to Ang II antinociceptive activity was clarified. Therefore, we have used tail-flick and incision allodynia models combined with an HPLC time course of Ang metabolism, to study the Ang III antinociceptive effect in the rat ventrolateral (vl) PAG using peptidase inhibitors and receptor antagonists. Ang III injection into the vlPAG increased tail-flick latency, which was fully blocked by Losartan and CGP 42,112A, but not by divalinal-Ang IV, indicating that Ang III effect was mediated by AT(1) and AT(2) receptors, but not by the AT(4) receptor. Ang III injected into the vlPAG reduced incision allodynia. Incubation of Ang II with punches of vlPAG homogenate formed Ang III, Ang (1-7) and Ang IV. Amastatin (AM) inhibited the formation of Ang III from Ang II by homogenate, and blocked the antinociceptive activity of Ang II injection into vlPAG, suggesting that aminopeptidase A (APA) formed Ang III from Ang II. Ang III can also be formed from Ang I by a vlPAG alternative pathway. Therefore, the present work shows, for the first time, that: (i) Ang III, acting on AT(1) and AT(2) receptors, can elicit vlPAG-mediated antinociception, (ii) the conversion of Ang II to Ang III in the vlPAG is required to elicit antinociception, and (iii) the antinociceptive activity of endogenous Ang II in vlPAG can be ascribed preponderantly to Ang III.

The angiotensin AT4 receptor subtype as a target for the treatment of memory dysfunction associated with Alzheimer's disease

Over recent years antihypertensive drugs, particularly angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), have been reported to have beneficial effects upon cognitive impairment. Such findings suggest that pharmacological manipulation of angiotensin ligands may be of clinical importance in slowing or halting the cognitive deterioration seen in vascular dementia and Alzheimer's disease. The mechanism(s) underlying these improvements in cognitive function remains unclear; however, important leads are emerging. The angiotensin AT4 receptor subtype, discovered by our laboratory in 1992, influences several important behaviours and physiologies, including learning and memory, and may play a role in this cognitive improvement. This review initially describes the therapeutic drugs approved by the Federal Drug Administration and new approaches presently being developed to treat Alzheimer's disease-induced cognitive impairment. Next, the biologically-active angiotensin ligands and their respective receptor subtypes are discussed, followed by the roles of angiotensin II, angiotensin IV, ACE inhibitors and ARBs in cognitive function. We conclude with a working hypothesis concerning the importance of the AT4 receptor subtype as a new potential drug target for the treatment of Alzheimer's disease-associated memory loss.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Development of fetal brain renin-angiotensin system and hypertension programmed in fetal origins

Since the concept of fetal origins of adult diseases was introduced in 1980s, the development of the renin-angiotensin system (RAS) in normal and abnormal patterns has attracted attention. Recent studies have shown the importance of the fetal RAS in both prenatal and postnatal development. This review focuses on the functional development of the fetal brain RAS, and ontogeny of local brain RAS components in utero. The central RAS plays an important role in the control of fetal cardiovascular responses, body fluid balance, and neuroendocrine regulation. Recent progress has been made in demonstrating that altered fetal RAS development as a consequence of environmental insults may impact on "programming" of hypertension later in life. Given that the central RAS is of equal importance to the peripheral RAS in cardiovascular regulation, studies on the fetal brain RAS development in normal and abnormal patterns could shed light on "programming" mechanisms of adult cardiovascular diseases in fetal origins.

Human brain contains a novel non-AT1, non-AT2 binding site for active angiotensin peptides

AIMS

To determine whether the novel non-AT1, non-AT2 binding site for angiotensins recently discovered in rodent brains occurs in the human brain.

MAIN METHODS

Radioligand binding assays of (125)I-sarcosine(1), isoleucine(8) angiotensin II binding were carried out in homogenates of the rostral pole of the temporal cortex of human brains containing 0.3 mM parachloromercuribenzoate (PCMB), 10 microM losartan to saturate AT1 receptors, 10 microM PD123319 to saturate AT2 receptors, with or without 10 microM angiotensin II to define specific binding. Competition binding assays employed a variety of angiotensin peptides, specific angiotensin receptor antagonists, several neuropeptides and an endopeptidase inhibitor to determine pharmacological specificity for this binding site.

KEY FINDINGS

The novel non-AT1, non-AT2 binding site was present in similar amounts in female and male brains: Bmax 1.77+/-0.16 and 1.52+/-0.17 fmol/mg initial wet weight in female and male brains, respectively. The K(D) values, 1.79+/-0.09 nM for females, and 1.53+/-0.06 nM for males were also similar. The binding site shows pharmacological specificity similar to that in rodent brains: sarcosine(1), isoleucine(8) angiotensin II>angiotensin III>angiotensin II>angiotensin I'angiotensin IV>angiotensin 1-7. Shorter angiotensin fragments and non-angiotensin peptides showed low affinity for this binding site.

SIGNIFICANCE

The presence in human brain of this novel non-AT1, non-AT2 binding site supports the concept that this binding site is an important component of the brain angiotensin system. The functional significance of this binding site, either as a novel angiotensin receptor or a highly specific angiotensinase remains to be determined.

Effects of angiotensin II and its metabolites in the rat coronary vascular bed: is angiotensin III the preferred ligand of the angiotensin AT2 receptor?

Aminopeptidases metabolize angiotensin II to angiotensin-(2-8) (=angiotensin III) and angiotensin-(3-8) (=angiotensin IV), and carboxypeptidases generate angiotensin-(1-7) from angiotensin I and II. Angiotensin-converting enzyme (ACE) inhibitors and/or angiotensin II type 1 (AT1) receptor blockers affect the concentrations of these metabolites, and they may thus contribute to the beneficial effects of these drugs, possibly through stimulation of non-classical angiotensin AT receptors. Here, we investigated the effects of angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) in the rat coronary vascular bed, with or without angiotensin AT1 - or angiotensin II type 2 (AT2) receptor blockade. Results were compared to those in rat iliac arteries and abdominal aortas. Angiotensin II, angiotensin III and angiotensin IV constricted coronary arteries via angiotensin AT1 receptor stimulation, angiotensin III and angiotensin IV being approximately 20- and approximately 8000-fold less potent than angiotensin II. The angiotensin AT2 receptor antagonist PD123319 greatly enhanced the constrictor effects of angiotensin III, starting at angiotensin III concentrations in the low nanomolar range. PD123319 enhanced the angiotensin II-induced constriction at submicromolar angiotensin II concentrations only. Angiotensin-(1-7) exerted no effects in the coronary circulation, although, at micromolar concentrations, it blocked angiotensin AT1 receptor-induced constriction. Angiotensin AT2 receptor-mediated relaxation did not occur in iliac arteries and abdominal aortas, and the constrictor effects of the angiotensin metabolites in these vessels were identical to those in the coronary vascular bed. In conclusion, angiotensin AT2 receptor activation in the rat coronary vascular bed results in vasodilation, and angiotensin III rather than angiotensin II is the preferred endogenous agonist of these receptors. Angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) do not exert effects through non-classical angiotensin AT receptors in the rat coronary vascular bed, iliac artery or aorta.

Role of central and peripheral aminopeptidase activities in the control of blood pressure: a working hypothesis

Although there is a large body of knowledge on protein synthesis, the available data on protein catabolism, although quite substantial, are still inadequate. This is due to the marked differences in the activity of proteolytic enzymes, compounded by different substrate specificities and multiple environmental factors. Understanding enzyme behavior under physiological and pathological conditions requires the identification of specific proteolytic activities, such as aminopeptidases, as able to degrade certain peptidergic hormones or neuropeptides. Another requirement is the isolation, purification and characterization of the enzymes involved. In addition, systematic studies are needed to determine each enzyme's subcellular location, tissue distribution, and the influence of environmental factors such as diurnal rhythm, age, gender, diet, cholesterol, or steroids. Central and peripheral aminopeptidases may play a role in the control of blood pressure by coordinating the effect of the different peptides of the renin-angiotensin system cascade, acting through the AT(1), AT(2), and AT(4) receptors. Our review of the available data suggests the hypothesis that cholesterol or steroids, particularly testosterone, significantly influence aminopeptidase activities, their substrate availability and consequently their functions. These observations may have relevant clinical implications for a better understanding of the pathophysiology of cardiovascular diseases, and thus for their treatment with aminopeptidase inhibitors.

Orally active aminopeptidase A inhibitors reduce blood pressure: a new strategy for treating hypertension

Overactivity of the brain renin-angiotensin system has been implicated in the development and maintenance of hypertension. We reported previously that angiotensin II is converted to angiotensin III by aminopeptidase A in the mouse brain. We then used specific and selective aminopeptidase A inhibitors to show that angiotensin III is one of the main effector peptides of the brain renin-angiotensin system, exerting tonic stimulatory control over blood pressure in hypertensive rats. Aminopeptidase A, the enzyme generating brain angiotensin III, thus represents a potential candidate central nervous system target for the treatment of hypertension. Given this possible clinical use of aminopeptidase A inhibitors, it was, therefore, important to investigate their pharmacological activity after oral administration. We investigated RB150, a dimer of the selective aminopeptidase A inhibitor, EC33, generated by creating a disulfide bond. This chemical modification allows prodrug to cross the blood-brain barrier when administered by systemic route. Oral administration of RB150 in conscious DOCA-salt rats inhibited brain aminopeptidase A activity, resulting in values similar to those obtained with the brains of normotensive rats, demonstrating the central bioavailability of RB150. Oral RB150 treatment resulted in a marked dose-dependent reduction in blood pressure in DOCA-salt but not in normotensive rats, with an ED(50) in the 1-mg/kg range, achieved in <2 hours and lasting for several hours. This treatment also significantly decreased plasma arginine-vasopressin levels and increased diuresis, which may participate to the blood pressure decrease by reducing the size of fluid compartment. Thus, RB150 may be the prototype of a new class of centrally active antihypertensive agents.

Type 1 angiotensin receptor (AT1-R)-mediated decrease in type 2 angiotensin receptor mRNA level is dependent on Gq and extracellular signal-regulated kinase 1//2 in AT1-R-transfected PC12 cells

Angiotensin II (Ang II) functions through two major Ang II receptor subtypes, type 1 (AT1-R) and type 2 (AT2-R), both of which are classified to be G protein-coupled receptors. AT2-R is highly expressed at the fetal stage, and in heart remodelling and brain ischaemia; therefore, it is important to clarify the regulatory mechanisms of AT2-R expression. Although AT1-R is generally believed to modulate AT2-R expression in some tissues or cells, the exact mechanism remains to be clarified. In the present study, we examined the effect of AT1-R stimulation on expression of endogenous rat AT2-R (rAT2-R) in AT1-R-transfected PC12 cells. rAT2-R mRNA and protein expression were decreased by Ang II in PC12 cells transfected with rAT1A-R in a time-dependent manner, mediated through a decline in mRNA stability. The C-terminus of G protein-coupled receptor (GPCR) is important for GPCR-mediated signal transduction. Therefore, we used C-terminus-deleted human AT1-R (hAT1-327STOP), which is thought to be a nondesensitised mutant of hAT1-R. As a result, Ang II decreased rAT2-R mRNA expression to a greater extent in cells transfected with hAT1-327STOP than with wild-type hAT1-R. The decrease was completely reversed by AT1-R antagonist candesartan, G(q) inhibitor YM254980, and mitogen-activated protein kinase (MAPK) kinase 1/2 inhibitor U0126, but not by pertussis toxin, which uncouples the receptor with G(i), or p38 MAPK inhibitor SB203580. We suggest, possibly for the first time, that the hAT1-R/G(q)/extracellular signal-regulated kinase 1/2 pathway is involved in the down-regulation of AT2-R using PC12 cells transfected with AT1-R.

Hypertension, RAS, and gender: what is the role of aminopeptidases?

Hypertension is the major risk factor for coronary heart disease, stroke, and renal disease. Also, it is probably the most important risk factor for peripheral vascular disease and vascular dementia. Although hypertension occurs in both men and women, gender differences have been observed. However, whether sex hormones are responsible for the observed gender-associated differences in arterial blood pressure, and which is their mechanism of action, remains unclear. Local and circulating renin-angiotensin systems (RAS) are examples of systems that may be involved in the pathogenesis of hypertension. Classically, angiotensin II (Ang II) has been considered as the effector peptide of the RAS, but Ang II is not the only active peptide. Several of its degradation products, including angiotensin III (Ang III) and angiotensin IV (Ang IV) also possess biological functions. These peptides are formed via the activity of several aminopeptidases. This review will briefly summarize what is known about gender differences in RAS-regulating aminopeptidase activities, their relationship with sex hormones, and their potential role in controlling blood pressure acting through local and circulating RAS.

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT(1) receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named "The Angiotensin III Hypothesis" states that Ang II must be converted to Ang III in order to activate brain AT(1) receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT(1) receptor-mediated effects.

Therapeutic targeting of insulin-regulated aminopeptidase: heads and tails?

Insulin-regulated aminopeptidase, IRAP, is an abundant protein that was initially cloned from a rat epididymal fat pad cDNA library as a marker protein for specialized vesicles containing the insulin-responsive glucose transporter GLUT4, wherein it is thought to participate in the tethering and trafficking of GLUT4 vesicles. The same protein was independently cloned from human placental cDNA library as oxytocinase and is proposed to have a primary role in the regulation of circulating oxytocin (OXY) during the later stages of pregnancy. More recently, IRAP was identified as the specific binding site for angiotensin IV, and we propose that it mediates the memory-enhancing effects of the peptide. This protein appears to have multiple physiological roles that are tissue- and domain-specific; thus the protein can be specifically targeted for treating different clinical conditions.

Angiotensin, bradykinin and the endothelium

Angiotensins and kinins are endogenous peptides with diverse biological actions; as such, they represent current and future targets of therapeutic intervention. The field of angiotensin biology has changed significantly over the last 50 years. Our original understanding of the crucial role of angiotensin II in the regulation of vascular tone and electrolyte homeostasis has been expanded to include the discovery of new angiotensins, their important role in cardiovascular inflammation and the development of clinically useful synthesis inhibitors and receptor antagonists. While less applied progress has been achieved in the kinin field, there are continuous discoveries in bradykinin physiology and in the complexity of kinin interactions with other proteins. The present review focuses on mechanisms and interactions of angiotensins and kinins that deal specifically with vascular endothelium.

Central pressor actions of aminopeptidase-resistant angiotensin II analogs: challenging the angiotensin III hypothesis

Intracerebroventricular administration of angiotensins causes pronounced pressor and dipsogenic responses. The suggestion that angiotensin III rather than angiotensin II is the active peptide in the brain spawned what we call The Angiotensin III.

HYPOTHESIS

To test this hypothesis, 5 angiotensin II analogs containing zero or one position substitutions conferring resistance to aminopeptidases were administered intracerebroventricularly to determine their pressor and dipsogenic efficacies. Two aminopeptidase-resistant analogs caused significantly greater pressor responses than angiotensin II, whereas 3 analogs caused pressor responses similar to angiotensin II. Latency to cause a pressor response for 4 of the 5 aminopeptidase-resistant angiotensin II analogs was the same as for angiotensin II. There was no detectable formation of (125)I-angiotensin III from 1 of the intracerebroventricularly administered analogs, (125)I- N-Methyl-l-Asp(1)-angiotensin II, indicating its aminopeptidase resistance. Latency to drink also did not differ between the angiotensins. After the initial dipsogenic response, water was removed until 25 minutes after angiotensin administration to avoid interfering with the pressor response. The dipsogenic stimulus was sustained 25 minutes after intracerebroventricular injection of angiotensin II and its aminopeptidase-resistant analogs. Comparison of angiotensin III and angiotensin II showed equivalent pressor responses with similar latencies and durations. The latency to drink was similar for angiotensin III and angiotensin II. However, there was no dipsogenic response to angiotensin III 25 minutes after intracerebroventricular injection. These data do not support The Angiotensin III Hypothesis and suggest that conversion of exogenously applied angiotensin II to angiotensin III is not necessary to cause brain-mediated pressor or dipsogenic responses.

Angiotensin-(3-7) pressor effect at the rostral ventrolateral medulla

Ang-(3-7) is a fragment of the renin-angiotensin system that can be derived both from Ang II or Ang-(1-7). In the present study we determined the cardiovascular effects produced by angiotensin-(3-7) [Ang-(3-7)] microinjection into the rostral ventrolateral medulla (RVLM), a key region for the control of sympathetic drive to the periphery. RVLM microinjection of Ang-(3-7) (20, 40 or 80 ng) in male Wistar rats anesthetized with urethane produced significant increases in MAP (19+/-3.8 mm Hg, n=5; 16+/-1.6 mm Hg, n=15 and 11+/-1.2 mm Hg, n=4, respectively) as compared to saline (4+/-0.7 mm Hg, n=6). These alterations were similar to that induced by Ang-(1-7) (14+/-1.3 mm Hg, 40 ng; n=12) and Ang II (17+/-2.3 mm Hg, 40 ng; n=7). Microinjection of losartan (AT(1) receptor antagonist, 100 pmol) or A779 (selective Mas receptor antagonist, 100 pmol) did not alter the pressor effect caused by Ang-(3-7). Microinjection of an Ang-(3-7) analogue, d-Ala(7)-Ang-(3-7) (100 pmol), completely abolished the pressor effect caused by Ang-(3-7). These results suggest that Ang-(3-7) may be an additional peptide of the RAS to act as neuromodulator, at least at the RVLM. Further, the Ang-(3-7) pressor effect is not mediated by the interaction with AT(1) or the Ang-(1-7), Mas, receptors.