View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Current Biology, Vol. 12, R745–R752, October 29, 2002, ©2002 Elsevier Science Ltd. All rights reserved. PII S0960-9822(02)01255-1 Just The Beginning: Novel Functions Review for Angiotensin-Converting Enzymes 1 1 Urs Eriksson , Ursula Danilczyk and Josef M. results in the generation of the main effector molecule 2 Penninger angiotensin II, which acts both as a systemic and a locally generated paracrine or autocrine effector peptide [1–3] (Figure 1). The protease renin, which is Cardiovascular disease is predicted to be the secreted by juxtaglomerular cells at the renal afferent commonest cause of death worldwide by the year arterioles, cleaves the liver-derived precursor peptide 2020. Diabetes, smoking and hypertension are the angiotensinogen into the decameric peptide angio- main risk factors. The renin-angiotensin system plays tensin I. Angiotensin I is further hydrolyzed into the a key role in regulating blood pressure and fluid and vasoconstrictor octapeptide angiotensin II by the pro- electrolyte homeostasis in mammals. The discovery teolytic dipeptidase ACE, located on the luminal side of specific drugs that block either the key enzyme of of the vascular endothelium, and other non-ACE the renin-angiotensin system, angiotensin-convert- systems, such as tissue chymase at the tissue level [8]. ing enzyme (ACE), or the receptor for its main effec- ACE also inactivates the vasodilator peptides tor angiotensin II, was a major step forward in the bradykinin and kallidin. Thus, activation of the renin- treatment of hypertension and heart failure. In recent angiotensin system mainly results in a systemic vaso- years, however, the renin-angiotensin system has pressor response by blocking the hypotensive been shown to be a far more complex system than kinin-bradykinin pathway and by generation of the initially thought. It has become clear that additional vasoconstrictor angiotensin II [4]. peptide mediators are involved. Furthermore, a new Angiotensin II is not just a potent vasopressor which ACE, angiotensin-converting enzyme 2 (ACE2), has raises systemic blood pressure; it also mediates a been discovered which appears to negatively regu- broad array of physiological and pathophysiological late the renin-angiotensin system. In the heart, ACE2 effects by binding to specific cell membrane receptors deficiency results in severe impairment of cardiac [9,10]. So far, two distinct types of G-protein-coupled contractility and upregulation of hypoxia-induced receptors for angiotensin II have been cloned and char- genes. We shall discuss the interplay of the various acterised in humans and rodents. The angiotensin II effector peptides generated by angiotensin-convert- type 1 (AT1) and type 2 (AT2) receptors are heteroge- ing enzymes ACE and ACE2, highlighting the role of neously distributed in peripheral tissues and the brain ACE2 as a negative regulator of the renin-angiotensin [9,10]. The AT1 receptor is predominantly expressed in system. the kidneys, adrenal glands, vascular smooth muscle cells and the heart, and it is to this receptor that the regulatory actions of angiotensin II on blood pressure and salt/water balance have been attributed [9,11,12]. Introduction The AT2 receptor is present at high density during fetal The renin-angiotensin system is a complex regulatory development; in the adult, significant AT2 receptor system that plays a key role in maintaining blood expression occurs only in the adrenal medulla, uterus, pressure homeostasis, as well as fluid and salt ovary, vascular endothelium, adrenal glands and certain balance in mammals [1–4]. The angiotensin convert- areas in the brain [4,9]. The AT2 receptor is thought to ing-enzyme (ACE) is an important regulator of the counterbalance effects mediated by the AT1 receptor: it renin-angiotensin system. Recently, a homologue of appears to induce vasodilation and may be involved in human ACE, angiotensin-converting enzyme 2 (ACE2), the control of cell proliferation, differentiation and has been discovered [5,6]. Characterisation of its angiogenesis [4,10,11]. function and substrate specificity [5,6], together with data from ace2 mutant mice [7], suggests that ACE2 A Broad Array of Biologically Active Peptides negatively regulates the activated renin-angiotensin In vivo, angiotensin II is degraded into several meta- system. In this review we shall discuss the functions bolites by different enzymes (Figure 2). So far, it has of ACE and ACE2 and their substrates and products in been shown that at least five of these products are the regulation of the renin-angiotensin system. biologically active [4]. Interestingly, these metabolites have different biological effects. Angiotensin III, which Classical Model of the Renin-Angiotensin System is cleaved from angiotensin II by an aminopeptidase, In the classical pathway of the renin-angiotensin binds to both AT1 and AT2 receptors and acts similar system, activation of a cascade of enzymatic reactions to angiotensin II [13]. In contrast, angiotensin(1–7), a product of angiotensin II as well as of the angiotensin 1 IMBA, Institute for Molecular Biotechnology of the Austrian I metabolite angiotensin(1–9), appears to have vasodi- Academy of Sciences, C/o Dr. Bohr Gasse 7, A-1030 Vienna, latatory [14–16] and antiproliferative effects mediated 2 Austria. University Health Network, Departments of Medical by binding a novel, not yet defined, angiotensin(1–7) Biophysics and Immunology, University of Toronto, Toronto, receptor [17]. Ontario, Canada. 620 University Avenue, Toronto, Ontario, Angiotensin IV is generated from angiotensin III by an Canada M5G 2C1. E-mail: [email protected] as yet unknown pathway. In the kidneys, angiotensin IV Review R746 Figure 1. The renin-angiotensin system: Kininogen Angiotensinogen the classical picture. Renin Vasodilation B1 Kinin Angiotensin I ACE Non-ACE enzymes Inactive product Angiotensin II AT 1 receptor AT 2 recpetor Vasoconstriction Vasodilation Proliferation Apoptosis Hypertrophy Growth arrest Current Biology enhances natriuresis and increases renal blood flow. of ACE remains unclear, but the fact that fertility is There is evidence that, in the central nervous system, impaired in ace-deficient mice [22,23], but not in AT1A angiotensin IV facilitates memory retention and retrieval receptor knock-out mice [24], suggests that testicular [4,12]. The effects of angiotensin IV involve a novel AT4 ACE does not mediate its effects via angiotensin receptor [18,19]. The AT4 receptor has also been iden- II–AT1A interactions. Both ACE isoforms are mem- tified as an insulin-regulated aminopeptidase [19], sug- brane-bound protein and, at the cell surface, they gesting an indirect mechanism whereby angiotensin IV function as ectoenzymes which hydrolyze circulating might extend the half-life of biologically active neu- peptides. ACE may be cleaved from the cell surface ropeptides by competitive inhibition of AT4’s enzyme and act as a soluble enzyme, but the biological signif- activity [4,19]. icance of soluble ACE remains unclear. The diversity of the renin-angiotensin system is not ACE is currently thought to have two major activities: only demonstrated by the different effects of the it acts as a peptidyl dipeptidase which removes the various degradation products of angiotensin I and carboxy-terminal dipeptide from its substrate, and as angiotensin II. Characterisation of receptor specificities endopeptidase on substrates that are amidated at the and enzymes involved in the degradation of angiotensin carboxyl terminus (Figure 3). The emerging picture of metabolites has revealed far more complex mediator ACE function is that it is more than just a key enzyme systems than previously recognised. It appears that the that catalyses cleavage of angiotensin I to the potent complexity of the renin-angiotensin system guarantees vasoconstrictor peptide angiotensin II [3,4,20]. ACE also well-tuned adaptation of the blood supply of the body hydrolyzes the inactive angiotensin(1-9) peptide into the compartments and organs to all possible physiologic vasodilator metabolite angiotensin(1–7) [25], and it is conditions. In this complex system, a key role has been additionally thought to inactivate the vasodilator pep- attributed to the rate-limiting enzymes, ACE and ACE2, tides bradykinin and kallidin [25,26]. Furthermore, it acts via their action in generating or degrading the various as endopeptidase on the multi-functional neuropep- active mediators. tides substance P and cholecystokinin, and may degrade the luteinizing hormone releasing hormone Angiotensin Converting-Enzyme (ACE) [4,27]. In this context, however, its biological signifi- ACE was termed a ‘hypertensin-converting enzyme’ cance is not known, nor has the possibility of functional when it was initially isolated in 1956 [20]. The human crosstalk between the systems been excluded. ace gene, located on chromosome 17, encodes a The finding that ACE catalyzes cleavage of angio- 180 kDa protein with two homologous domains. Each tensin I into angiotensin II, together with the eluci- domain has an active zinc-binding motif, His-Glu-X-X- dation of ACE’s three-dimensional structure, led to the His, which is found in many peptidases [3,21] (Figure design of specific ACE inhibitors. Inhibition of ACE 3). ACE is anchored to the plasma membrane through results in lowering of blood pressure by impaired for- a single carboxy-terminal transmembrane