Current findings of renin-angiotensin-aldosterone system ( RAAS ) bring about the connexion with diabetes mellitus, and how of import angiotonin IV ( Ang IV ) in diabetic research and intervention. Ang IV physiologically antagonizes angiotenisn II ( Ang II ) , and gives benefits to human system and protracting action of peptides such as antidiuretic hormone, lys-bradykinin, and oxytocin. Therefore, Ang IV is believed to cut down exasperations in macrovascular and microvascular associated diabetes. However, Ang IV causes insulin-resistance in glucose transporter ( GLUT4 ) due to adhering on insulin-regulated aminopeptidase ( IRAP ) or angiotensin subtype IV receptor ( AT4 ) . Therefore, insulin signalling in GLUT4 translocation reduced and glucose could non be uptake into cell. Research has been done to increase sensitiveness, such as AL-11 development, which has more powerful and specific compared with the current drug, divalinal-Ang IV. In decision, Ang IV has good and bad belongingss, and farther research is needed to understand its actions in patients suffer from diabetes mellitus.
Angiotensin IV ( Ang IV ) , Renin-Angiotensin-Aldosterone-System ( RAAS ) , Diabetes Mellitus, Insulin-regulated Aminopeptidase ( IRAP ) , Angiotensin Subtype IV Receptor ( AT4 )
The renin-anigotensin-aldosterone system ( RAAS ) is the major regulative tract mediated by secernment of renin due to the high degrees of aldostrone and to certain extent, hydrocortisone. The system is responsible in H2O and Na keeping, every bit good as keeping blood force per unit area of the organic structure [ 1 ] . However, new finds led to the sensing of angiotonin IV ( Ang IV ) and its receptor known as angiotonin subtype IV ( AT4 ) receptor, which subsequently is besides known as insulin-regulated aminopeptidase ( IRAP ) enzyme that is found in glucose transporter known as GLUT4 [ 1, 2 ] . Several findings indicate that IRAP may hold relationship with glucose degrees in the organic structure via GLUT4, and therefore, may hold connexion with diabetes mellitus [ 3 ] . Apart from that, there are besides other findings which are related to Ang IV and linked with other systems [ 4 ] . Therefore, the study will cover the belongingss and relationship between RAAS, AT4, IRAP, and diabetes mellitus in general, every bit good as the importance of Ang IV in diabetic research and intervention [ 1, 3, 4 ] .
1. Renin-Angiotensin-Aldosterone System ( RAAS )
Figure 1: The RAAS System and Mechanism of Action of Angiotensins I, II, and IV, and Ang ( 1-7 ) on the Human System. Beginning: [ 1 ]
The classical renin-angiotensin-aldosterone system ( RAAS ) is involved in synthesis of angiotonin I ( Ang I ) and angiotensin II ( Ang II ) , when angiotensinogen, an inactive peptide which produced from liver is converted to Ang I by renin, and later, Ang II produced becomes the chief peptide responsible for the ordinance of RAAS in nephritic system. However, for the past 10 old ages, there are finds which led to the alterations in the classical construct of RAAS.
The newer finds include the presence of local or tissue RAAS such as in bosom, blood vass, pancreas, cardinal nervous system ( CNS ) , generative system, lymphatic system, and adipose tissue which act independently or interact together with adrenal secretory organs and kidney [ 1 ] . Not merely that, based on other research, womb, placenta, and, encephalon besides have a local RAAS [ 5 ] . These localised systems could impact intracrine, autocrine, paracrine, and endocrinal maps [ 1, 2 ] .
Other of import findings for the newer RAAS construct are such as word picture of angiotonin II ( 1-7 ) or Ang II- ( 1-7 ) or Ang ( 1-7 ) , angiotensin- ( 3-7 ) or Ang ( 3-7 ) , and angiotensin A ( or Ang A ) as a biological active metabolite of RAAS and degraded by aminopeptidase or carboxypeptidase, the connexion and grounds that angiotonin subtype IV receptor ( AT4 ) is the insulin-regulated aminopeptidase ( IRAP ) enzyme [ 1, 2 ] , the find of renin/prorenin receptor, findings of angiotensin-converting enzyme II ( ACE2 ) as an angiotonin peptide processing enzyme and homologue of angiotensin-converting enzyme ( ACE ) , and presence of Ang ( 1-7 ) and Ang ( 3-7 ) as substrates for both G-coupled receptor Mas and AT4 receptor severally. These finds lead to the apprehension of the RAAS as cascade that has multiple go-betweens and receptors, every bit good as multi-functional enzymes [ 1 ] .
Normally, RAAS is associated with local regulative mechanisms which involved in homeostatic tracts. Cellular growing, extracellular matrix formation, vascular proliferation, endothelium map, and besides programmed cell death can be induced via the action of Ang II ( and Ang III which act chiefly in encephalon ) [ 1 ] . Other engagements of RAAS include the repression of the system without medicine through antibody suppression or cistron therapy for long clip every bit good as engagement of ribozyme or by interfering courier ribonucleic acid ( messenger RNA ) . The illustrations can been seen in rat theoretical accounts whereby vectors showing antisense messenger RNA of angiotonin subtype I ( AT1 ) receptor or angiotensinogen antisense messenger RNA stabilizes blood force per unit area in hypertensive rat theoretical accounts. Not merely that, rats actively immune with Ang I analogues or anti-angiotensinogen antibody reduced the represser ‘s response to exogenic Ang I and therefore, caused a autumn in blood force per unit area. However, in worlds, hypotensive consequence can merely be seen at higher antibody titers. This shows that the RAAS is a localized system that able to bring forth multiple effects dependant on their substrates and tracts [ 2 ] .
2. Angiotensin IV
2.1 Biochemical Properties of Angiotensin IV and its Related Ligands
Ang IV or Ang ( 3-8 ) is a hexapeptide which consist of the 3rd until 8th peptide of the original angiotensinogen peptide [ 1, 6, 7 ] . It is formed in two ways. One of it is from the Ang II via aminopeptidase A and aminopeptidases B, M, and N and therefore, aspartic acid and arginine is removed from the first and 2nd place of the peptide [ 8 ] . It can besides be converted straight from Ang II via D-aminopeptidase [ 1, 2 ] .
Figure 2: The angiotonins ‘ production tract. Beginning: [ 1 ]
The pharmacological belongingss of Ang IV is chiefly induced onto its receptor known as AT4 receptor or IRAP and sometimes, AT1 and angiotonin subtype II ( AT2 ) receptors at higher concentrations [ 1, 2, 4, 7, 9 ] . The findings are based on affinity and specificity of its radioligands to the receptor found in assorted variety meats [ 4 ] . Other ligands which can adhere on AT4 receptor is LVV-hemorphin 7 ( LVV-H7 ) which is biochemically unrelated with Ang IV, divalinal-Ang IV ( which can besides bring forth counter effects in certain occasions ) , and norleucine1-Ang IV ( Nle1-Ang IV ) . Meanwhile, lipid-lowering medicine drugs such as amastatin are powerful inhibitor of aminopeptidases A and N which is responsible in production of Ang IV but has no direct consequence on Ang IV towards endothelial cells [ 1, 4, 6, 8, 9, 10 ] . Ang IV could non be cleaved by both ACE and ACE II which cleaved most angiotensin peptides and other go-betweens such as bradykinins [ 7, 8, 11 ] .
Figure 3: The ACE and ACE2 cleaveage sites of angiotonin peptides, apelin, bradykinins, dynorphin A, and I?-casomorphin. Beginning: [ 11 ]
2.2 Cellular Mechanism of Angiotensins IV
Due to the similarity of action with insulin, Ang IV can trip certain protein kinases pathway particularly phosphoinositol III kinase ( PI3 K ) and PI-dependent kinase-1 ( PDK-1 ) activities in the endothelial cells where it can heighten the action of insulin, promote action of bradykinin and azotic oxide, and causes vasodilatation [ 1, 4, 10 ] . It besides caused intracellular Ca release and activates phospholipase C via the addition of inositol phosphate, doing musculus contraction [ 4, 10 ] .
However, it besides triggers mitogenic activated protein kinase ( MAPK ) tract which leads to small vasoconstriction action, apart from triping mitogenesis and tropic activity of cell [ 1, 7 ] . It besides increases look of plasminogen activator inhibitor ( PAI-1 ) to forestall myocardial infarction but may increase hazard of thrombosis, in which is due to the incresase of Ang IV via addition in Ang II and plasma renin activity [ 1, 8 ] .
Other tracts that are activated via Ang IV action are such as atomic factor kappa-B ( NF-?B ) and the plasminogen activator inhibitor I ( PAI-1 ) pathways [ 8, 10 ] .
2.3 Pharmacological Actions and Effectss of Angiotensins IV and its Related Ligands on Human System
Due to its ability to do many biological actions, scientists had focused on Ang IV although ab initio considered being biologically inactive. The general actions of Ang IV are fundamentally the antonym with Ang II [ 4, 6 ] .
Ang IV has a distinguishable intracerebroventricular consequence and increased in memory and acquisition by moving on CNS [ 2, 4, 7 ] . This triggers neurotransmitter release in hippocampus and Ponss via initiation of K ( K+ ) [ 2 ] . There are three hypotheses proposed refering memory-potentiating effects of Ang IV and LVV-hemorphin 7 in which they are powerful inhibitors of IRAP, prolong action of endogenous peptides, and besides modulate glucose uptake by modulating GLUT4 trafficking [ 1 ] .
As for the vascular system, it has prevailing vasodilatation consequence. For illustration, some surveies shows vasodilatation of pneumonic arteria provided the endothelium is integral although it is pre-constricted. Besides that, it besides causes the addition of endothelial azotic oxide ( eNO ) activity and cyclic guanyl monophosphate ( cGMP ) content via bradykinin-nitric oxide-cGMP vasodilative tract which had manifested in porcine pneumonic arterial endothelial cells [ 1, 5, 7 ] . It besides showed vasoprotective consequence, reduces superoxide formation, and better aortal endothelial map in Apolipoprotein E ( Apo-E ) deficient mice [ 12 ] . However, high blood pressure can happen due to action on AT1 receptor at high concentrations but it can be reversed via AT1 receptor blockers [ 2, 4, 7 ] .
For nephritic, Ang IV could increase nephritic cortical blood flow and decreases Na ( Na+ ) conveyance in nephritic proximal tubule without alteration in systemic blood force per unit area. Apart from vasodilative consequence, surveies besides show decrease sum and regional nephritic blood flow in rats. [ 1 ]
In cardiovascular system, there is decrease in pressure-development and expulsion capablenesss, and increase sensitiveness of left ventricular bosom wall during systole and speeded relaxation [ 1 ] . However, due to look of PAI-1, it may increase the hazard of coronary artery disease and impact fibrinolytic system based on surveies affecting ACE inhibitors and angiotonin receptor blockers ( ARB ) , and comparing PAI-1 look with other angiotonin peptides [ 8, 13 ] .
Meanwhile, Ang IV is besides involved in metamorphosis and may extent half life of biologically active neuropeptides such as neuromedin B and neurokinin A through competitory suppression and prevent cleavage by AT4 receptor [ 4, 8 ] .
As for other Ang IV ligands such as divalinal-Ang IV, it can rarefy the consequence of Ang IV in biological systems. Therefore, there is engagement of other mechanism which perchance due to antagonism to the activation of 2nd courier system [ 4, 8 ] . As for LVV-H7, the action is much similar to Ang IV except in peripheral actions. However, it do non do addition in intracellular activity, and Ang IV could move as partial adversary against LVV-H7 [ 1, 6 ] .
Overall, Ang IV is involved in interfering of the cleavage of other bio-active peptides from AT4 or IRAP, suppressing its allosteric mechanism [ 4, 10 ] . Ang IV can be utile to explicate the function of IRAP in CNS and insulin antiphonal tissues due to its high-affinity to AT4 receptors every bit good as importance in knowledge, cardiovascular, and nephritic metamorphosis every bit good as pathophysiological conditions such as diabetes, coronary artery disease, and high blood pressure [ 4 ] . It could besides lend to inflammatory events in cardiovascular diseases via NF-kI? and Jak/Tak kinase tracts mediated by tyrosine phosphorylation and engagement in ordinance of proinflammatory cistrons [ 1, 10 ] .
2.4 Binding Affinity and Potency of Angiotensin IV on AT4 Receptor
The AT4 receptor is known as insulin-regulated membrane aminopeptidase ( IRAP ) . IRAP is antecedently known as placental leucine aminopeptidase ( PLAP ) and oxytocinase ( OTase ) based on their mechanism of action to split Pitocin and are homologues with 87 % similarity in the amino acerb sequence. It can be farther cogent evidence by similar regional distribution of messenger RNA, immunoreactivity, and binding of [ 125I ] -Nle1-Ang IV in thin subdivisions of mouse encephalon [ 1, 3, 6, 10, 14 ] . It belongs to Type II built-in membrane and M1 household of aminopeptidases, peculiarly Zn metallopeptidases [ 1, 14, 15 ] . It has a Zn binding motive and an exopeptidase motive where the catalytic site is extracellular [ 14 ] .
It was foremost cloned in 1995 and originally identified in adipocytes and skeletal musculus cells which is associated with insulin-responsive cells as a major protein in cysts incorporating the insulin regulated glucose transporter ( GLUT4 ) [ 1, 3, 4, 6 ] . There is similarity between sub-cellular distributions of IRAP with GLUT4 under basal and insulin-stimulated conditions [ 3 ] .
Ang IV binds at high affinity to AT4 receptor in pharmacologically-distinct adhering site such as encephalon compared to Ang III which has ten times lower affinity every bit good as Ang ( 3-7 ) [ 1, 4 ] . Meanwhile, Ang II with its parallel [ Sar1-Ile8 ] Ang II has clearly lower affinity, and other drugs such as losartan, PD 123177, and CGP 42112A show no affinity to it [ 4 ] . The authority between Ang IV with its parallel is in order of Ang IV & A ; gt ; Nle1-Ang IV & A ; gt ; LVV-H7 & A ; gt ; divalinal-Ang IV ( D1318 ) [ 14 ] .
The substrates of AT4 receptor or IRAP are such as antidiuretic hormone ( an anti-diuretic endocrine ) , oxytocin, met-enkephalin ( met-encephalon ) , lys-bradykinin, Ang III and IV, dynorphin A, neurokinin A, and neuromedin B [ 3, 14, 15 ] . Some surveies shows the importance of soluble IRAP in oxytocin ordinance degrees during gestation whereas in other surveies, Ang IV, and its ligands such as LVV-H7 could suppress catalytic activity of IRAP and the ligands resist proteolytic cleavage, and therefore, identified as enzyme-inhibitor interaction [ 4, 14 ] .
Vasopressin, Pitocin, and met-enkephalin which quickly cleaved by IRAP can be interrupted by the Ang IV and its ligands, which protracting the effects of these peptides [ 14 ] . Meanwhile, insulin, calcitonin, and endothelin are non IRAP substrates while antidiuretic hormone is a alone substrate which can be cleave N-terminal cysteine in vitro apart from oxytocin [ 14, 15 ] . It is believed that Ang IV may hold actions connected with diabetes mellitus or other metabolic syndromes [ 1, 16 ] .
3. Diabetes Mellitus
Diabetess mellitus is a multisystem upset characterised by extra glucose degree in blood circulation. It is chiefly due to the deficiency of insulin production or opposition to insulin sensitiveness while, other factors such as polymorphism, inflammatory go-betweens, cytokines, and oxidative emphasis could besides lend. These can be made worse with patient ‘s unawareness and non-compliance [ 5 ] . Diabetes mellitus frequently consequences in hyperglycemia, high blood pressure, polydypsia, polyuria, and polyphagia, increased hypersensitivity and redness, and besides hypercholestrolaemia [ 1 ] . There are microvascular and macrovascular associated diabetes such as diabetic kidney disease, neuropathy, and retinopathy, and besides cardiovascular associated diseases [ 16 ] .
3.1 Relationship between RAAS, Angiotensin IV, AT4 Receptor, Insulin, GLUT4, and Diabetes Mellitus
The engagement of nephritic RAAS system could lend in the association both vascular and nephritic associated diabetes mellitus [ 1, 3, 5 ] . Equally far as we know, Ang II is straight involved in the exasperations of symptoms in microvascular and macrovascular diseases via vasoconstriction, decrease in eNO content, increased thrombin and fibrin formation, increased Na and H2O keeping, decreased in blood flow, bradykinin debasement, and in diabetic patients. These actions are physiologically antagonized by Ang IV and therefore, could cut down exasperation of symptoms in diabetic patients [ 1, 16 ] . Therefore, there may be possible that Ang IV is connected with Ang II, AT4 receptor or IRAP, and besides its substrates, insulin action, every bit good as diabetes mellitus [ 1 ] . Another minor association is the ACE2-Ang- ( 1-7 ) -Mas axis which show betterment in diabetic patients through RAAS encirclement during clinical tests and, could besides play a function in diabetes by countering the vasoconstriction consequence of ACE2-Ang II-AT1 axis [ 1, 5 ] . Furthermore, there is indirect connexion whereby there are actions of Ang II on both AT1 and AT2 receptors to trip the bradykinin-nitric oxide-cGMP vasodilative tract [ 5 ] .
The consequence of hyperglycaemic conditions in diabetic patients farther triggers RAAS and do farther vasoconstriction and increase blood force per unit area in diabetic patients [ 1 ] . At the same clip, the RAAS activation in diabetic patients associated with myocardial infarction causes syrupy rhythm, where the cardiac end product continues at a lower rate, doing uninterrupted action of the angiotonin peptides [ 17 ] . Furthermore, arthrosclerosis which consequence in free extremist formation in hyperglycaemic conditions could trip macrovascular complications [ 16 ] . Therefore, diabetic mellitus are normally associated with high blood pressure and myocardial infarction due to the engagement of RAAS and the suppression of IRAP is involved in the ordinance of GLUT4 trafficking by cut downing glucose uptake and doing hyperglycemia [ 15, 17 ] .
Figure 4: Connection between Ang II, Ang IV, and diabetes mellitus. Beginning: [ 1, 16 ]
Since there is connexion between RAAS, Ang II, and Ang IV, there is besides relationship between diabetes mellitus and Ang IV with insulin, IRAP, and GLUT4 whereby the presence of IRAP is needed to keep normal GLUT4 degrees irrespective of tissue type, sex, and age [ 3 ] . Ang IV antagonises insulin response on IRAP and may develop insulin opposition, cut downing response of insulin to IRAP, and at the same clip protracting actions of IRAP substrates. Therefore, diminishing Ang IV may diminish insulin-resistance but may besides down-regulate vasodilatation consequence, cognitive efficiency, eNO activity, mitogenic and tropic, every bit good as the cellular mechanism tracts [ 1, 2, 3, 4, 17 ] .
A survey by Keller ( 2004 ) explained the cause of GLUT4 lessening which is due to the alterations of the action of IRAP substrates. Prior to the deficiency of GLUT4 or increase in GLUT4 debasement, bosom size may increase and it may do hyperglycemia or diminished glucose disposal in fat or musculus tissue due to reduced translocation of GLUT4 onto the surface. This survey besides showed that GLUT4 damage is similar with those who are holding diabetes mellitus type 2 or insulin opposition and since GLUT4 is diminished, it is expected that IRAP action at the cell membrane are besides worsening in these patients. Therefore, the impaired IRAP map may play a function in development of complications in insulin-resistant persons and may be linked to the lessening of peptide substrate cleavage and alteration in endocrine action [ 2, 3, 17 ] .
Figure 5: Connection between insulin signalling and Ang IV. Beginning: [ 3 ]
As we know, the current mechanism of glucose consumption is believed to be triggered by insulin whereby it activates to do translocation of GLUT4-IRAP composite to the cell surface which enables IRAP to degrade its substrates such as antidiuretic hormone. Therefore, insulin could advance inactivation of IRAP substrates and controls its actions and at the same clip, advance insulin-dependent glucose consumption into the cell [ 10 ] . Meanwhile, as have been discussed before, Ang IV has counter consequence on IRAP and allows protracting the action of its substrates such somatostatin and substance P [ 1, 4, 15 ] . With the current apprehension of its mechanism, there are surveies related to diabetic intervention via the use of AT4 receptor blocker such as AL-11 [ 18 ] .
4. Current Research Related to the Importance of Angiotensin IV in Diabetic Research and Treatment
Based on the consequence of both Ang IV and insulin on IRAP and GLUT4 translocation, Beckie et. Al. ( 2009 ) have came out with one solution to handle diabetic patients by increasing the insulin sensitiveness [ 18 ] . AL-11 is a man-made compound which have similar action with divalinal-Ang IV which antagonizes the binding of Ang IV, leting insuilin to move on IRAP and promote translocation of IRAP [ 1, 18 ] . The difference between these two compounds is that, AL-11 has higher authority and more specific in its action compared with divalinal-Ang IV. The prototype drug is patented in the United States, and the squad has besides suggested a chief whereby Ang IV up-regulates angiotensinogen look and AT1 receptors at high doses but down-regulates the consumption of glucose and IRAP look in endothelial blood vass [ 18 ] .
It has been tested in human smooth musculus cell civilizations and found out that MAPK activation could do suppression of p85 AKT tract which is responsible of GLUT4 translocation and glucose uptake [ 18 ] . Therefore, it is suggested that Ang IV promote insulin opposition at certain concentrations but could be prevented if by cut downing its maximal consequence [ 1, 4, 15, 18 ] . Furthermore, findings besides suggested that Ang IV has the opposite consequence of Ang II. Therefore, AL-11 can be used to handle diabetes mellitus Type II, insulin opposition, or cardiovascular hazard related with metabolic syndrome [ 18 ] .
Summary and Conclusion
Vasodilatation ( via increased cGMP and eNO content, bradykinin action, and decreased in Na and H2O keeping )
May addition coronary artery disease ( via PAI-1 )
Improve memory knowledge
May addition insulin opposition
Reduce fibrin and thrombin formation
Increased mitogenic and tropic action ( via MAPK tract )
Reduces oxidative emphasis ( due to increased eNO content and decreased free extremist formation )
Table 1: Summary of Ang IV Action in Human System. Beginning: [ 1, 2, 4 ]
The pharmacological actions of Ang IV in vascular system, every bit good as nephritic and cardiovascular systems are the antonym of Ang II and the binding of Ang IV towards IRAP are extremely specific compared with other angiotonin peptides [ 1, 4, 14 ] . This causes assorted effects which can be either good or bad to the human system and these effects are important as it involves the exasperations in diabetic patients. [ 1, 2, 4 ] Therefore, farther research is needed in order to to the full understand the mechanism of Ang IV action in patients enduring from diabetes mellitus [ 1, 5 ] .
Particular thanks to Dr. Dharmani Devi and all the staffs and lectors from Pharmacology Department, Faculty of Medicine, Universiti of Malaya for support and counsel. Not to bury, coursemates, staffs, and lectors of the Biomedical Science Programme, Faculty of Medicine, Univerisity Malaya.