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Topical Review |
1 Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
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Abstract |
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(Received 5 December 2006;
accepted after revision 9 January 2007;
first published online 11 January 2007)
Corresponding author P. S. Leung: Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. Email: psleung{at}cuhk.edu.hk
The renin–angiotensin system
The renin–angiotensin system (RAS) is classically known as a circulating or hormonal system regulating blood pressure, electrolyte and fluid homeostasis (Peach, 1977). Such an endocrine function is mediated largely by its potent effects on vascular smooth muscle and on the renal reabsorption of electrolytes as well as water via direct tubule actions, and via stimulation of aldosterone and vasopressin (Matsusaka & Ichikawa, 1997). This classical RAS consists of several key components: the hepatic derived precursor angiotensinogen, two critical enzymes, namely the renal synthesized renin and pulmonary-bound angiotensin-converting enzyme (ACE), and the physiologically active peptide, angiotensin II, as well as its receptors. The sequential actions of renin and ACE on plasma angiotensinogen produce the decapeptide angiotensin I and octapeptide angiotensin II, respectively. In addition, alternative enzymes to renin and ACE generate a number of bioactive peptides from angiotensin I and/or angiotensin II, such as angiotensin III, angiotensin IV and angiotensin (1–7). Such angiotensin-processing peptidases include, to name but a few, chymase, chymotrypsin, tonin, ACE2 (a homologue of ACE) and aminopeptidase A, as well as the aminopeptidase B/N. Angiotensin II, together with these bioactive peptides, mediates their specific functions via the respective cellular receptors of target tissue organs (Lavoie & Sigmund, 2003). The biological actions of the RAS are mediated by the angiotensin II type 1 and type 2 receptors, i.e. AT1 and AT2 receptors; most of the functions are, however, mediated by the AT1 receptor (De Gasparo et al. 2000).
The local RAS in organ systems
The presence of local tissue RASs has been increasingly recognized over the last 15 years, with mounting recognition of their clinical importance (Montgomery et al. 2003). These functional local RASs have been found in such diverse organ systems as the pancreas, heart, kidney, vasculature and adipose tissue as well as the nervous, reproductive and digestive systems (see review by Paul et al. 2006). Such local RASs operate in an autocrine, paracrine and/or intracrine manner and exhibit multiple physiological effects at the cellular level which adds to, and/or differs from, the circulating RAS. In addition to haemodynamic actions, the local RAS has multiple and novel functions including the regulation of cell growth, differentiation, proliferation and apoptosis, reactive oxygen species (ROS) generation, tissue inflammation and fibrosis, and hormonal secretion (Leung, 2004). Such a diversity of roles makes tissue RASs attractive therapeutic targets in diverse disease states. This topical review is undertaken to give a critical appraisal on the physiological roles of a local RAS in the pancreas and, additionally, to discuss its clinical relevance.
The existence of a local pancreatic RAS
The accumulated evidence supports the existence of a complete pancreatic RAS (where renin and ACE are involved in the biosynthetic pathway), although the data are somewhat conflicting. Thus, angiotensinogen and renin are expressed in rat pancreas (Leung et al. 1999), whilst neither angiotensin I nor renin activity have been identified in the dog pancreas (Chappell et al. 1991). On the other hand, binding sites for angiotensin II receptors have been characterized in the endocrine and exocrine cells of pancreas (Chappell et al. 1992, 1995; Ghiani & Masini, 1995). Indeed, AT1 and AT2 receptors and angiotensin II have been specifically localized to different cell types of the pancreas including endothelial, ductal, acinar and islet cells (Leung et al. 1997, 1998). Consistent with this finding, mRNA for the AT1 receptor subtypes (AT1a and AT1b) and the AT2 receptor has also been found in the rat pancreas (Leung et al. 1999). In the human pancreas, AT1 receptors and (pro)renin have been shown to be localized not only in exocrine cells but also the beta cells of the endocrine pancreas (Tahmasebi et al. 1999). The presence of a pancreatic RAS in the human pancreas is further substantiated by the expression and localization of angiotensinogen and AT1 receptors, notably in pancreatic islets and ducts (Lam & Leung, 2002).
Table 1 summarizes the accumulated data on the evidence for the existence of a pancreatic RAS from different animal species and isolated cells as well as cell lines of the pancreas.
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It is intriguing that the pancreatic RAS components are responsive to various physiological and pathophysiological stimuli, including hypoxia, pancreatitis, islet transplantation, hyperglycaemia, type 2 diabetes mellitus (T2DM), and pancreatic cancer (Leung, 2004).
In chronic hypoxia, the expression of several major components of the pancreatic RAS is increased (Chan et al. 2000). Of interest in this context is the reversibility and adaptability of the RAS activation by chronic hypoxia, further indicating its physiological relevance to the pancreas (Ip et al. 2003b). Hypoxia causes a decrease of blood flow (and the presence of ischaemia) in several tissues including the pancreas, and leads to enhanced tissue inflammation and injury (Kuwahira et al. 1993). The up-regulation of RAS by hypoxia could contribute to ischaemia via vasoconstriction of the pancreatic microcirculation (Carlsson et al. 1998). Acute pancreatitis is associated with increased expression of major components of the pancreatic RAS (Leung et al. 2000). On the other hand, expression of local RAS components are up-regulated in human pancreatic endocrine tumours (Lam & Leung, 2002) and ACE inhibition may also modulate mitosis and gene expression in pancreatic cancer cells (Reddy et al. 1995).
The pancreatic RAS is also subjected to regulation by such conditions as islet transplantation, diabetes mellitus, and hyperglycaemia. Of note, AT1 receptor and angiotensinogen expression are markedly up-regulated in islets retrieved from 4-week-old islet transplants (Lau et al. 2004) and in islets or pancreas from animal models of T2DM (Tikellis et al. 2004; Chu et al. 2006) as well as in islets or pancreatic stellate cells exposed to hyperglycaemia (Lupi et al. 2006; Ko et al. 2006). Taken together, the regulation of the pancreatic RAS by (patho)physiological conditions suggests that a functional RAS exists in the pancreas and that interference with such a local RAS may be promising in the prevention and treatment of pancreatic inflammation and disease.
Table 2 summarizes the regulated expression of RAS components by physiological and pathophysiological stimuli from various cells in the pancreas.
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Recent studies have suggested novel roles for the pancreatic RAS in the regulation of pancreatic duct cell and acinar cell secretion in the exocrine pancreas. In pancreatic duct cells, angiotensin II influences anion secretion via the mediation of AT1 receptors. By using a short-circuit current (ISC) technique, angiotensin II was found to dose-dependently increase the ISC in a cystic fibrosis pancreatic cell line, which effect is completely abolished by losartan, an AT1 receptor blocker (Chan et al. 1997). This AT1 receptor-mediated anion secretion is dependent on Ca2+ and cAMP as its signal transduction events (Cheng et al. 1999). In isolated dog pancreatic epithelial cells and cystic fibrosis pancreatic cell cultures, AT1 receptor activation of calcium channels is involved in bicarbonate secretion (Fink et al. 2002). Interestingly, captopril reduces secretin-induced bicarbonate output in conscious dogs (Howard-McNatt & Fink, 2002).
In pancreatic acinar cells, AT1 receptors have been identified in rat pancreatic AR42J cells and angiotensin II mediates a dose-dependent increase in amylase secretion and inositol 1,3,4-triphosphate production (Chappell et al. 1995; Cheung et al. 1999). This effect can be inhibited by losartan (an AT1 receptor antagonist), but not by CGP42112 (an AT2 receptor antagonist). More recently, several key RAS components (AT1 and AT2 receptors and angiotensinogen) have been found to be expressed in isolated rat pancreatic acinar cells (Tsang et al. 2004a). Exogenous addition of angiotensin II stimulates a dose-dependent release of digestive enzyme secretion (amylase and lipase) from these isolated pancreatic acini. Administration of losartan significantly inhibits acinar digestion enzyme secretion while the specific AT2 receptor blocker PD123319 did not exhibit any such suppressive effect (Tsang et al. 2004a).
All the convergent data indicate that the pancreatic RAS plays a physiological role in regulating pancreatic ductal bicarbonate and acinar digestive enzyme secretion. What, then, are its roles in diseases of the exocrine pancreas, and in pancreatitis? To address this issue, the differential effects of RAS blockade on pancreatitis-induced inflammation and injury have been studied. Intriguingly, saralasin, a non-specific antagonist of AT1/AT2 receptors, is effective in improving pancreatitis-induced injury in the pancreas (Tsang et al. 2003), perhaps through an inhibition of RAS activation of ROS (Ip et al. 2003a). Prophylactic and therapeutic treatment with losartan, but not PD123319, also reduces the pancreatitis-induced oxidative stress, presumably by improving pancreatic microcirculation and by diminishing AT1 receptor-mediated NADPH oxidase-dependent production of ROS (Tsang et al. 2004b). Furthermore, AT1 receptor antagonism has been shown to be protective against acute pancreatitis and its associated pulmonary injury (Chan & Leung, 2006).
Using a rat model of obstructive pancreatitis, the involvement of angiotensin II might be due to AT1 receptor-mediated NADPH oxidase-dependent NF
B-dependent activation, and thus angiotensin II ultimately promotes pro-inflammatory actions during the development of pancreatitis (Y. C. Chan & P. S. Leung, unpublished data). On the other hand, a recent study has shown that ACE inhibition attenuates chronic pancreatitis-induced injury and pancreatic fibrosis, possibly via the prevention of pancreatic stellate cell activation (Kuno et al. 2003). In this regard, losartan also prevents apoptosis of pancreatic acinar cell by AT1 receptor-mediated pancreatic cell fibrosis (Wang et al. 2004). All the available data support the potential clinical value of RAS blockade in protecting and treating pancreatitis and fibrosis. Figure 1 summarizes the physiological roles of pancreatic RAS in the exocrine pancreas and the clinical implications.
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In the endocrine pancreas, islet RASs play a novel role in regulating islet glucose-stimulated insulin secretion and thus glucose homeostasis. In this context, pancreatic blood flow and preferentially islet blood flow is significantly impaired by locally generated angiotensin II, as demonstrated by perfused rat pancreas; this inhibitory effect was rescued by RAS antagonists (Carlsson et al. 1998). Captopril and irbesartan (an AT1 receptor antagonist) selectively enhance pancreatic islet blood flow, insulin secretion and glucose tolerance (Huang et al. 2006, 2007). Interestingly, angiotensin II dose-dependently inhibits insulin release from isolated mouse islets in response to a high glucose concentration (Lau et al. 2004). This inhibitory action is due, at least in part, to a decreased (pro)insulin biosynthesis which is preventable by losartan treatment. These data from isolated islets rule out the possibility that the AT1 receptor-mediated effect on glucose-stimulated insulin secretion is exclusively attributable to changes in pancreatic islet blood flow (Carlsson et al. 1998; Lau et al. 2004).
Although the AT2 receptor has been expressed in isolated mouse islets, angiotensin II does not affect glucose-stimulated insulin secretion via this receptor subtype, as shown by PD123319 treatment (Lau et al. 2004). In contrast, AT2 receptor has also been localized to the outer region of islets and colocalizes with somatostatin-secreting cells in the endocrine pancreas and in the immortalized rat pancreatic cell lines RIN-m and RIN-14B (Wong et al. 2004). In RIN-14B cells, angiotensin II stimulates somatostatin secretion in a dose-dependent manner. This action seems to be mediated by AT2 receptors, as CGP42112 treatment abolishes the response to angiotensin II (Wong et al. 2004). These data prompt us to conclude that the pancreatic islet RAS has a functional role in regulating pancreatic islet insulin and somatostatin secretion and thus physiologically involved in glucose homeostasis.
A large body of clinical studies has provided evidence indicating that pharmacological RAS blockade reduces the incidence of new-onset T2DM cases in high-risk patients with cardiovascular disease. In this respect, a recent meta-analysis of clinical trials of ACE inhibitors and AT1 receptor blockers has been conducted in order to evaluate these protective effects (Abuissa et al. 2005). This meta-analysis concluded that the mean risk for developing T2DM was reduced by 27% with ACE inhibitor treatments, 23% with AT1 receptor treatments, and 25% overall in a pooled analysis of these two RAS blockers. Nevertheless, the exact mechanism(s) for the protective effect of RAS blockade in T2DM remain ambiguous. Of note, recent identification of an islet RAS in the pancreas and its emerging role in islet function might provide a novel and alternative explanation for the reduced incidence of T2DM observed in these clinical trials (Lau et al. 2004).
In order to delineate this issue, several animal models of T2DM have recently been developed for this topical research. For the long-term effect (24 weeks) of ACE inhibitor on pancreatic islets, ramipril is administered to Otsuka Long-Evans Tokushima fatty (OLETF) rats; ramipril treatment can prevent islet destruction by fibrosis in these diabetic rats, as shown by the expression profile of TGF-
and its downstream signalling molecules (Ko et al. 2004). Using another rat model of T2DM, Zucker diabetic fatty rats, chronic treatment (10 weeks) of either an ACE inhibitor (perindopril) or AT1 receptor blocker (irbesartan) attenuates islet fibrosis as well as apoptosis and oxidative stress (Tikellis et al. 2004). These data suggest that islet RAS activation may be involved in oxidative stress-induced islet apoptosis and fibrosis, and thus islet dysfunction is observed in T2DM. Interestingly, hyperglycaemia as observed in T2DM has also recently been shown to activate the RAS in pancreatic islet and stellate cells; RAS blockade ameliorates the angiotensin II-induced pancreatic inflammation and fibrosis aggravated by chronic exposure to high glucose levels (Lupi et al. 2006; Ko et al. 2006).
Notwithstanding the presentation of this persuasive data, however, the precise mechanism(s) by which a local pancreatic RAS is involved in islet dysfunction has yet to be elucidated. More recently, a mouse model of T2DM, i.e. an obesity-induced db/db mouse, has been employed to test the hypothesis that there is a change in AT1 receptor expression in T2DM which enables endogenous levels of angiotensin II to impair islet function (Chu et al. 2006). These data clearly conclude that the pancreatic islet AT1 receptors, though not having any obvious effects on normal islet function, become up-regulated during T2DM; this up-regulation thus has negative effects on islet glucose-stimulated insulin secretion and (pro)insulin biosynthesis, and islet blood flow (Chu et al. 2006). These findings provide a novel and, at least, partial explanation for the reduced incidence of T2DM that has been observed in a number of clinical trials applying AT1 receptor blockade to individuals at high risk for this disease.
Using the db/db mouse model, it has been further shown that AT1 receptor antagonism (for 8 weeks) attenuates NADPH oxidase-induced oxidative stress; this, in turn, results in a down-regulation of uncoupling protein 2 (UCP2) expression, which is associated with improved beta-cell insulin secretion and reduces apoptosis-induced beta-cell mass loss (Chu & Leung, 2007). In keeping with these findings, chronic AT1 receptor antagonism improves islet cell function and structure, an effect that is apparently mediated by NADPH oxidase-induced oxidative stress in animal models of T2DM, i.e. OLETF rats and db/db mice (Nakayama et al. 2005; Shao et al. 2006). Taking all these data together, it is plausible to propose that AT1 receptor activation by T2DM mediates UCP2-driven oxidative stress and thus leads to pancreatic islet beta-cell dysfunction. Figure 2 summarizes the physiological roles of pancreatic RAS in endocrine pancreas and the clinical implications.
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Local RASs exist in major cell types of the endocrine and exocrine pancreas, the activities of which are subjected to regulation by physiological and pathophysiological stimuli. These functional RASs have important roles in the regulation of islet, acinar and duct cell secretion and their activation is involved in AT1 receptor-mediated oxidative stress-induced cell inflammation, apoptosis and fibrosis. Inhibition of the pancreatic RAS activation may shed new light on the prevention as well as treatment of pancreatic inflammation and diabetes mellitus.
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