Orally administered novokinin, an angiotensin AT2 receptor agonist, suppresses food intake via prostaglandin E2-dependent mechanism in mice

Update on Angiotensin II Subtype 2 Receptor: Focus on Peptide and Nonpeptide Agonists

Angiotensin II (Ang II) is the most dominant effector component of the renin-angiotensin system (RAS) that generally acts through binding to two main classes of G protein-coupled receptors, namely Ang II subtype 1 receptor (AT1R) and angiotensin II subtype 2 receptor (AT2R). Despite some controversial reports, the activation of AT2R generally antagonizes the effects of Ang II binding on AT1R. Studying AT2R signaling, function, and its specific ligands in cell culture or animal studies has confirmed its beneficial effects throughout the body. These characteristics classify AT2R as part of the protective arm of the RAS that, along with functions of Ang (1-7) through Mas receptor signaling, modulates the harmful effects of Ang II on AT1R in the activated classic arm of the RAS. Although Ang II is the primary ligand for AT2R, we have summarized other natural or synthetic peptide and nonpeptide agonists with critical evaluation of their structure, mechanism of action, and biologic activity. SIGNIFICANCE STATEMENT: AT2R is one of the main components of the RAS and has a significant prospective for mediating the beneficial action of the RAS through its protective arm on the body's homeostasis. Targeting AT2R offers substantial clinical application possibilities for modulating various pathological conditions. This review provided concise information regarding the AT2R peptide and nonpeptide agonists and their potential clinical applications for various diseases.

Intracerebroventricular infusion of angiotensin AT2 receptor agonist novokinin aggravates some diabetes-mellitus-induced alterations in Wistar rats

Cumulative data suggest the significant role of the renin–angiotensin system in the development of the pathological consequences of diabetes mellitus (DM). Newly synthesized AT2 receptor agonists gained importance as a target for creating new antihypertensives. The aim of the present work was to study the effects of peptide AT2 agonist novokinin, infused intracerebroventricularly, on the consequences of the streptozotocin-induced type 1 DM (T1DM) in Wistar rats. Food and water consumption, body mass, urine excretion (metabolic cages), motor activity (open-field test), anxiety (elevated plus maze), nociception (paw pressure analgesimeter test), spatial memory (T-maze alternation test), and plasma levels of glucose and corticosterone (ELISA) were assessed 2 weeks after the T1DM induction. Novokinin increased water and food consumption, as well as urine output, and reduced mass gain in the control rats. Diabetic rats demonstrated hyperalgesia, increased level of plasma corticosterone, decreased motor and exploratory activity, and impaired spatial memory. Novokinin infusion increased water intake, diuresis, and mortality rate, decreased food intake, exacerbated diabetes-induced hyperalgesia, and provoked anxiety-like behavior but improved spatial memory in diabetic rats. These initial data suggest that angiotensin AT2 receptors participate in the pathogenesis of T1DM-induced complications in the function of the nervous system.

AT2 receptor agonist Compound 21: A silver lining for diabetic nephropathy

The currently available therapies for diabetic nephropathy, one of the leading causes of renal failure globally are based on inhibition of renin angiotensin system. However, recently, the focus has shifted towards activation of its protective arm rather than the inhibition of deteriorative axis, using specific agonists. Compound 21 (C21), a novel non-peptide Angiotensin II type 2 receptor (AT₂) agonist, recently granted orphan drug status for the treatment of a rare disease, idiopathic pulmonary fibrosis has also shown a potent anti-inflammatory, anti-fibrotic, antioxidant and anti-apoptotic potential in various diseases including heart failure, myocardial infarction, chronic inflammatory diseases, and neurological diseases such as ischemic stroke. A pool of evidences suggest that C21, either alone or in combination with angiotensin receptor blockers could be extremely beneficial in the treatment of diabetic nephropathy, a chronic inflammatory condition sharing its pathogenesis with aforementioned diseases. The review analyses the new therapeutic tool, C21, its mechanisms of action for renoprotection in diabetic nephropathy, and its future perspectives and thereby provides an insight into the potential application of C21 as a novel therapeutic tool in the eradication of diabetic nephropathy.

Novokinin inhibits gastric acid secretion and protects against alcohol-induced gastric injury in rats

Novokinin (Arg-Pro-Leu-Lys-Pro-Trp), a potent vasorelaxing and hypotensive peptide modified from ovokinin, exhibits highly selective affinity for the AT₂ receptor. However, its role in gastrointestinal functions is still not fully understood. In this study, we found that novokinin inhibited basal gastric acid secretion and protected gastric mucosa from alcohol-induced injury in a dose-related manner in rats after intracerebroventricular (i.c.v.) administration. Novokinin significantly decreased basal gastric acid output at the dose of 50 and 100 nmol/rat. The effect of novokinin on gastric acid secretion was reversed by central injection of PD 123319 (10 nmol/rat), an AT₂ receptor antagonist, and peripheral injection of indomethacin (10 mg/kg), an inhibitor of prostaglandin synthesis. Meanwhile, pre-treatment with novokinin at doses of 10, 50, and 100 nmol/rat significantly reduced the alcohol-induced gastric mucosal injury compared to the ulcer-control group, which was inhibited by indomethacin (10 mg/kg). The result showed a remarkable increase in the level of prostaglandin E2 (PGE₂), glutathione (GSH), and a decrease in malondialdehyde (MDA) after i.c.v. administration of novokinin. These findings suggest that the inhibitory effect of novokinin on gastric acid secretion is probably mediated via an AT₂ receptor-prostaglandins (PGs) pathway. The gastroprotective effect of novokinin might be attributed to the inhibition of acid secretion, the cytoprotection of PGs, and the antioxidant property.

Bioactive peptides derived from natural proteins with respect to diversity of their receptors and physiological effects

We have found various bioactive peptides derived from animal and plant proteins, which interact with receptors for endogenous bioactive peptides such as opioids, neurotensin, complements C3a and C5a, oxytocin, and formyl peptides etc. Among them, rubiscolin, a δ opioid peptide derived from plant RuBisCO, showed memory-consolidating, anxiolytic-like, and food intake-modulating effects. Soymorphin, a μ opioid peptide derived from β-conglycinin showed anxiolytic-like, anorexigenic, hypoglycemic, and hypotriglyceridemic effects. β-Lactotensin derived from β-lactoglobulin, the first natural ligand for the NTS2 receptor, showed memory-consolidating, anxiolytic-like, and hypocholesterolemic effects. Weak agonist peptides for the complements C3a and C5a receptors were released from many proteins and exerted various central effects. Peptides showing anxiolytic-like antihypertensive and anti-alopecia effects via different types of receptors such as OT, FPR and AT2 were also obtained. Based on these study, new functions and post-receptor mechanisms of receptor commom to endogenous and exogenous bioactive peptides have been clarified.

Reporter mouse strain provides a novel look at angiotensin type-2 receptor distribution in the central nervous system

Angiotensin-II acts at its type-1 receptor (AT1R) in the brain to regulate body fluid homeostasis, sympathetic outflow and blood pressure. However, the role of the angiotensin type-2 receptor (AT2R) in the neural control of these processes has received far less attention, largely because of limited ability to effectively localize these receptors at a cellular level in the brain. The present studies combine the use of a bacterial artificial chromosome transgenic AT2R-enhanced green fluorescent protein (eGFP) reporter mouse with recent advances in in situ hybridization (ISH) to circumvent this obstacle. Dual immunohistochemistry (IHC)/ISH studies conducted in AT2R-eGFP reporter mice found that eGFP and AT2R mRNA were highly co-localized within the brain. Qualitative analysis of eGFP immunoreactivity in the brain then revealed localization to neurons within nuclei that regulate blood pressure, metabolism, and fluid balance (e.g., NTS and median preoptic nucleus [MnPO]), as well as limbic and cortical areas known to impact stress responding and mood. Subsequently, dual IHC/ISH studies uncovered the phenotype of specific populations of AT2R-eGFP cells. For example, within the NTS, AT2R-eGFP neurons primarily express glutamic acid decarboxylase-1 (80.3 ± 2.8 %), while a smaller subset express vesicular glutamate transporter-2 (18.2 ± 2.9 %) or AT1R (8.7 ± 1.0 %). No co-localization was observed with tyrosine hydroxylase in the NTS. Although AT2R-eGFP neurons were not observed within the paraventricular nucleus (PVN) of the hypothalamus, eGFP immunoreactivity is localized to efferents terminating in the PVN and within GABAergic neurons surrounding this nucleus. These studies demonstrate that central AT2R are positioned to regulate blood pressure, metabolism, and stress responses.

Effect of egg white and its hydrolysate on stearoyl-CoA desaturase index and fat accumulation in rat tissues

We investigated the dietary effects of egg white (EW) and its hydrolysate (EWH) on fat metabolism in rats. Wistar rats were divided into casein, EW and EWH dietary groups, and fed their respective diet for 8 weeks. Dietary EW and EWH decreased food intake, body weight gain and fat accumulation in the carcass, liver, muscles and adipose tissues, but muscle weight was increased. In addition, dietary EW and EWH decreased stearoyl-CoA desaturase (SCD) indices and glucose-6-phosphate dehydrogenase activity of the liver and gastrocnemius muscle. Dietary EW also increased the fecal excretion of triacylglycerol, cholesterol and total bile acids, and decreased the serum levels of triacylglycerol and leptin. The suppressive effects of dietary EW on food intake and body fat accumulation were weakened by dietary EWH. These findings indicate that EW and EWH, especially EW, are effective in reducing body fat accumulation by regulating hepatic and muscular SCD indices.

δ-Opioid receptor activation stimulates normal diet intake but conversely suppresses high-fat diet intake in mice

The central opioid system is involved in a broadly distributed neural network that regulates food intake. Here, we show that activation of central δ-opioid receptor not only stimulated normal diet intake but conversely suppressed high-fat diet intake as well. [D-Pen(2,5)]-enkephalin (DPDPE), an agonist selective for the δ-receptor, increased normal diet intake after central administration to nonfasted male mice. The orexigenic activity of DPDPE was inhibited by blockade of cyclooxygenase (COX)-2, lipocalin-type prostaglandin D synthase (L-PGDS), D-type prostanoid receptor 1 (DP(1)), and neuropeptide Y (NPY) receptor type 1 (Y1) for PGD(2) and NPY, respectively, suggesting that this was mediated by the PGD(2)-NPY system. In contrast, DPDPE decreased high-fat diet intake in mice fed a high-fat diet. DPDPE-induced suppression of high-fat diet intake was blocked by antagonists of melanocortin 4 (MC(4)) and corticotropin-releasing factor (CRF) receptors but not by knockout of the L-PGDS gene. These results suggest that central δ-opioid receptor activation suppresses high-fat diet intake via the MC-CRF system, independent of the orexigenic PGD(2) system. Furthermore, orally administered rubiscolin-6, an opioid peptide derived from spinach Rubisco, suppressed high-fat diet intake. This suppression was also blocked by centrally administered naltrindole, an antagonist for the δ-receptor, suggesting that rubiscolin-6 suppressed high-fat diet intake via activation of central δ-opioid receptor.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Angiotensinogen gene polymorphisms and food-intake behavior in young, normal female subjects in Japan

OBJECTIVE

We examined whether angiotensinogen (AGT) gene polymorphisms are associated with food preferences in young, normal female subjects.

METHODS

Fifty-two young, normal female subjects (21-22 y old) were recruited. After a 12-h fast, blood samples were obtained to examine the AGT gene polymorphisms (rs699 and rs7079), angiotensin-converting enzyme (ACE) insertion (I)/deletion (D), and adrenergic β3 receptor (ADRB3) gene polymorphisms (rs4994). A trained dietitian interviewed the participants to determine the portion size and frequency of food eaten for 1 wk by using the established questionnaire FFQg 3.0.

RESULTS

The genotypes of the AGT Met235Thr polymorphisms were TT:TC:CC = 2:19:31 (T:C = 0.22:0.78). The genotypes of AGT rs7079 were CC:CA:AA = 26:21:5 (C:A = 0.70:0.30), and those of ACE were DD:DI:II = 5:28:19 (D/I = 0.37:0.63). The genotypes of ADRB3 Trp64Arg were TT:TC:CC = 38:11:3 (T:C = 0.84:0.16). The total caloric intake was greater for those with the MM/MT genotype of AGT Met235Thr than for those with the TT genotype (1993 versus 1698 kcal/d, P < 0.05). The consumption of total lipids, cholesterol, and unsaturated free fatty acids was also higher in those with the MM/MT genotype of AGT Met235Thr than in those with the TT genotype. However, the AGT polymorphism (rs7079) and the ACE I/D were not associated with food preferences. In contrast, the subjects with ADRB3 Trp64 tended to show a high energy intake and preferences for proteins and lipids including fatty acids and cholesterol. They ate more fish and meat. Multiple regression analysis showed that the energy intake in subjects with the MM/MT genotype was independently determined by total lipids (B = 11.7, P < 0.0001) and carbohydrates (B = 4.6, P < 0.0001).

CONCLUSIONS

The AGT Met235Thr polymorphism was significantly associated with a higher caloric intake owing to total fat and carbohydrate consumption.

Effects of captopril and angiotensin II receptor blockers (AT1, AT2) on myocardial ischemia-reperfusion induced infarct size

The renin-angiotensin system (RAS) plays a major role in regulating the cardiovascular system, and disorders of the RAS contribute largely to the cardiac pathophysiology, including myocardial ischemia-reperfusion (MI/R) injury. Two subtypes of angiotensin II (Ang II) receptors have been defined on the basis of their differential pharmacological properties. The current study was undertaken to address the question as to whether the inhibition of the angiotensin converting enzyme (ACE) by captopril and the AT1 and AT2 receptor blockers losartan and PD123319 modulate MI/R-induced infarct size in an in vivo rat model. To produce necrosis, a branch of the descending left coronary artery was occluded for 30 min followed by two hours of reperfusion. ECG changes, blood pressure, and heart rate were measured during the experiment. Captopril (3 mg/kg), losartan (2 mg/kg), and PD123319 (20 μg/kg/min) were given in an IV 10 min before ischemia and were continued during the ischemic period. The infarcted area was measured by TTC staining. The volume of infarct and the risk zone was determined by planimetry. Compared to the control group (55.62±4.00%) both captopril and losartan significantly reduced the myocardial infarct size (30.50±3.26% and 37.75±4.44%), whereas neither PD123319 nor PD123319+losartan affected the infarct size volume (46.50±3.72% and 54.62±2.43%). Our data indicates that captopril and losartan exert cardioprotective activity after an MI/R injury. Also, infarct size reduction by losartan was halted by a blockade of the AT2 receptor. Therefore, the activation of AT2 receptors may be potentially protective and appear to oppose the effects mediated by the AT1 receptors.

Orally administered soymorphins, soy-derived opioid peptides, suppress feeding and intestinal transit via gut mu(1)-receptor coupled to 5-HT(1A), D(2), and GABA(B) systems

We previously reported that soymorphins, mu-opioid agonist peptides derived from soy beta-conglycinin beta-subunit, have anxiolytic-like activity. The aim of this study was to investigate the effects of soymorphins on food intake and gut motility, along with their mechanism. We found that soymorphins decreases food intake after oral administration in fasted mice. Orally administered soymorphins suppressed small intestinal transit at lower dose than that of anorexigenic activity. Suppression of food intake and small intestinal transit after oral administration of soymorphins was inhibited by naloxone or naloxonazine, antagonists of mu- or mu(1)-opioid receptor, respectively, after oral but not intraperitoneal administration. The inhibitory activities of small intestinal transit by soymorphins were also inhibited by WAY100135, raclopride, or saclofen, antagonists for serotonin 5-HT(1A), dopamine D(2), or GABA(B) receptor, respectively. We then examined the order of activation of 5-HT(1A), D(2), and GABA(B) receptors, using their agonists and antagonists. The inhibitory effect of 8-hydroxy-2-dipropylaminotetralin hydrobromide, a 5-HT(1A) agonist, after oral administration on small intestinal transit was blocked by raclopride or saclofen. Bromocriptine, a D(2) agonist-induced small intestinal transit suppression, was inhibited by saclofen, but not by WAY100135. Baclofen, a GABA(B) agonist-induced small intestinal transit suppression, was not blocked by WAY100135 or raclopride. These results suggest that 5-HT(1A) activation elicits D(2) followed by GABA(B) activations in small intestinal motility. We conclude that orally administered soymorphins suppress food intake and small intestinal transit via mu(1)-opioid receptor coupled to 5-HT(1A), D(2), and GABA(B) systems.

beta-Lactotensin derived from bovine beta-lactoglobulin suppresses food intake via the CRF system followed by the CGRP system in mice

We found that beta-lactotensin (His-Ile-Arg-Leu), which has been isolated as an ileum-contracting peptide from chymotrypsin digest of bovine beta-lactoglobulin, dose-dependently suppresses food intake after intracerebroventricular (i.c.v.) or intraperitoneal administration at a dose of 40 nmol/mouse or 100mg/kg, respectively, in fasted mice. Orally administered beta-lactotensin also suppressed food intake at 500 mg/kg. We previously reported that beta-lactotensin acts as an agonist for neurotensin receptors; however, the anorexigenic activity of beta-lactotensin was not inhibited by i.c.v. co-administration with SR48692 or levocabastine, an antagonist for neurotensin NT(1) or NT(2) receptor, respectively. On the other hand, the anorexigenic effect of beta-lactotensin was blocked by i.c.v. co-administration with astressin or calcitonin gene-related peptide (CGRP)(8-37), an antagonist for corticotropin releasing factor (CRF) or CGRP, respectively. beta-Lactotensin had affinity for neither CRF nor CGRP receptor. In addition, CRF-induced anorexigenic activity after i.c.v. administration was completely blocked by CGRP(8-37), while CGRP-induced anorexigenic activity was not inhibited by astressin. These results suggest that the CGRP system is activated downstream of the CRF system in food intake regulation. Taken together, beta-lactotensin may suppress food intake by activating the CRF system followed by the CGRP system, independently of the neurotensin system.