Renal Unit, Fundación Jiménez Díaz, Universidad Autónoma, Madrid, Spain
Introduction
The classical view of angiotensin II (Ang II) as a vasoactive agent that participates in local and systemic haemodynamic regulation has been recently enlarged to consider it as a true cytokine with an active role in renal and cardiovascular pathology (reviewed in [1,2]). In several models of kidney damage, the blockade of Ang II actions by angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor antagonists ameliorates proteinuria, inflammatory cell infiltration and fibrosis [36]. In addition, several authors, including ourselves, have demonstrated that Ang II is a renal growth factor that modulates cell growth and extracellular matrix production [1,2,,7,,8]. Moreover, Ang II participates in the inflammatory response in the kidney through the synthesis of chemotactic factors [9]. Finally, recent studies suggested that angiotensin peptides other than Ang II are bioactive agents with potential implication in renal pathology (reviewed in [10]). In this editorial, we comment on some new aspects of the reninangiotensin system (RAS) that provide a wider view of this complex system.
Action of Ang II on cell growth and matrix synthesis
One common feature of progressive renal diseases is the proliferation of resident renal cells and accumulation of extracellular matrix [11]. Several studies have demonstrated that local Ang II could contribute to these phenomena [18]. Ang II modulates cell growth, inducing hyperplasia/hypertrophy, depending on the cell type [1]. Ang II activates mesangial cells, tubular cells, and renal interstitial fibroblasts, increasing the expression and synthesis of extracellular matrix proteins [1,2,8]. Some of these effects seem to be mediated by the release of growth factors such as transforming growth factor-ß (TGF-ß) and platelet-derived growth factor (PDGF) [1,2,12]. In experimental models of renal damage elevated renal Ang II production as well as upregulation of renal TGF-ß expression was noted, and this was associated with increased expression of mRNA of extracellular matrix proteins and matrix deposition. These abnormalities were reduced by ACE inhibition and AT1 receptor blockade [5,6,12].
Ang II and mononuclear cell recruitment
A novel function of Ang II is its participation in inflammatory cell recruitment. The first evidence of a potential role of Ang II in immune responses was suggested by the presence of Ang II receptors on human monocytes [13], as well as by the accumulation of mononuclear cells in the kidney of rats after 714 days of systemic Ang II infusion [1416]. Ang II may be involved in different steps in the onset and progression of inflammation. This peptide is a chemotactic factor for mononuclear cells, which induces adhesion molecules expression in human endothelial cells and activates human monocytes, leading to increased adhesion to endothelial cells [1720].
Most kidney diseases are characterized by the infiltration of monocytes, and those cells seem to play a crucial role in the progression to irreversible structural renal changes [21]. Many studies have shown that in several models of renal injury, associated or not to hypertension, ACE inhibitors reduce the number of infiltrating cells in the glomeruli and interstitium [5,22,23]. We have recently demonstrated that this beneficial effect could be due to interference with chemokine production [9]. Among chemokines, monocyte chemoattractant protein-1 (MCP-1) has emerged as an important mediator of monocyte infiltration [24,25]. Increased renal MCP-1 expression has been reported in experimental and human nephritis [26]. Moreover, the administration of anti-MCP-1 antibodies to rats with nephrotoxic nephritis and with anti-thymocyte antibody-induced nephritis reduced the glomerular infiltration by monocytes/macrophages [27,28]. Recent studies have further expanded the potential link between Ang II and MCP-1. Thus, in a model of immune complex nephritis, characterized by increased local Ang II generation in the absence of systemic hypertension, we have observed an upregulation of renal MCP-1 (mRNA and protein), coincidentally with mononuclear cell infiltration. Both effects were markedly reduced by treatment with the ACE inhibitor quinapril [9]. MCP-1 upregulation was seen in glomerular and tubular epithelial cells, as well as in infiltrating mononuclear cells, suggesting that this chemokine is produced by both intrinsic and infiltrating cells in paracrine/autocrine fashion as hypothesized by Tang et al. [27]. In cultured glomerular mesangial cells and mononuclear cells, Ang II is a potent activator of MCP-1, to an extent comparable to that of other cytokines [9]. The role of Ang II in immune-mediated nephritis and in the regulation of MCP-1 expression has been recently confirmed in a model of antiglomerular basement membrane nephritis induced in AT-1a-deficient homozygous mice [29]. All these data strongly suggest that Ang II plays a central role in the regulation of inflammatory cell recruitment during renal injury via chemokine production.
Angiotensin receptors and inflammation
Ang II elicits cellular responses through its binding to two specific receptor subtypes, AT1 and AT2 [30]. Most of the known actions of Ang II are mediated by AT1, such as vasoconstriction and deposition of matrix [1,2]. The receptor subtype involved in promoting inflammatory processes in the kidney is not yet elucidated; however, in the model of unilateral ureteral obstruction, monocyte/macrophage infiltration was only reduced by ACE inhibition, but not by AT1 receptor blockade [23]. In the models of mesangial proliferative nephritis and ureteral obstruction treatment with AT1 receptor antagonists caused a significant reduction, but not complete abolition, of renal MCP-1 expression [31,32]. A similar reduction of MCP-1 was observed in AT1 knockout mice [29]. On the other hand, in cultured glomerular endothelial cells, Ang II induced the expression of another chemokine, RANTES, through the AT2 receptor [16]. Moreover in vivo treatment with an AT2 antagonist diminished Ang II-induced glomerular monocyte infiltration [16]. The effect of AT2 inhibition on MCP-1 expression is at present unknown. Although future studies are necessary to clarify this point, these data suggest that ACE inhibitors or a combination of AT1 and AT2 blockers could be more effective than an AT1 receptor antagonist alone in the control of inflammatory cell infiltration. Finally, an effect of Ang II on MCP-1 production is not only demonstrable in renal cells: Ang II also increases MCP-1 expression in vascular smooth muscle [33], as well as in various target organs of hypertensive rats [35]. On the whole, such experimental evidence shows that Ang II must be viewed as a true cytokine that is involved in the regulation of inflammatory responses.
Ang II and nuclear factor-B
The molecular signalling pathways elicited by Ang II have been investigated extensively. Ang II activates several second-messenger systems, including calcium mobilization and activation of protein kinases, such as the protein kinase C and mitogen activated proteins (MAP) cascades [30,35]. One important event in the cellular response to stimulation is the transduction of the signals to the nucleus. Some studies have shown that Ang II activates some nuclear transcription factors, including STAT family, AP-1 and CREB [3538]. These proteins can translocate into the nucleus where they bind to specific DNA sequences, increasing the transcription of related genes. We have demonstrated that Ang II activates nuclear factor-B (NF-
B) in vascular smooth muscle and mesangial cells [9,33]. NF-
B plays an important role in the regulation of the expression of proinflammatory genes, cell adhesion proteins, nitric oxide synthase and angiotensinogen, and other gene products involved in inflammation, immune response, renal damage, and cell proliferation [39,40]. In a normotensive model of immune glomerulonephritis, we found elevated tissue NF-
B activity that was well correlated with mononuclear cell infiltration, and renal MCP-1 expression that was diminished by ACE inhibition [9]. Moreover, in the model of ureteral obstruction which is characterized by an increase in the expression of MCP-1 and adhesion molecules (ICAM-1 and VCAM-1), two different studies have shown that NF-
B activity was elevated in the kidney cortex [41,42]. We have recently observed that after systemic infusion of Ang II renal NF-
B activity increases both in resident renal cells and in infiltrating monocytes [43]. Moreover, in other pathological settings associated with local Ang II production, such as experimental models of atherosclerosis and the model of wounded aortic endothelium, elevated tissue NF-
B activity was correlated with expression of MCP-1 and leukocyte adhesion molecule VCAM-1 respectively [33,44]. All these data strongly suggest that NF-
B is involved in the pathogenesis of renal and cardiovascular disease. In addition, the anti-inflammatory effect of ACE inhibitors may be due to the decreased activation of NF-
B.
Such new data suggest a novel mechanism of how Ang II affects the inflammatory process, i.e. through NF-B activation (see Figure 1
]. In mesangial cells, Ang II caused a rapid and dose-related activation of NF-
B, with a potency similar to that of inflammatory cytokines such as TNF-
>, as indicated by mobility shift assays [9]. Several Ang II-induced genes are regulated by NF-
B, including chemokines (MCP-1 and RANTES), cytokines (IL-6) and angiotensinogen [9,16,45,46]. We have observed that Ang II-induced NF-
B activation preceded upregulation of MCP-1 gene expression [9]. The rapid rise of MCP-1 mRNA levels induced by Ang II, the kinetics of NF-
B activation, and the effect of NF-
B inhibitors on Ang II-induced MCP-1 expression, are consistent with a role for NF-
B in transcriptional activation of MCP-1 gene, as shown for inflammatory cytokines as IL-1ß> [47]. Another point under investigation is the receptor subtype involved in this process. Ang II increased the synthesis of IL-6 and angiotensinogen through the AT1 receptor [45,46] and expression of RANTES through the AT2 receptor [16]. In vascular smooth muscle cells, using specific Ang receptor antagonists, we have observed that the effect of Ang II on NF-
B activation and I
B
degradation was mediated by AT1 and AT2 receptors (unpublished data). Moreover, in vivo Ang II infusion increased renal NF-
B activity, and this was diminished at least in part, by treatment with AT1 and AT2 receptor antagonists [43]. In rats with unilateral ureteral obstruction both AT1 and AT2 antagonists decreased NF-
>B activation in the obstructed kidney [48]. Thus, this transcription factor apparently regulates some effects which are ascribed to the AT2 receptor, such as cell differentiation, apoptosis, and mononuclear cell recruitment [49,50,16], as well as other processes mediated by the AT1 receptor, such as cell proliferation and expression of cytokine and angiontesinogen mRNA [1,2,45,46]. NF-
B also regulates other proteins, including TNF-
, IL-1ß>, inducible nitric oxide synthase, and cycloxygenase-2. These proteins could potentially also be modulated by Ang II in renal cells. The above data document the complex mechanisms through only Ang II-induced tissue injury. They also illustrate that the field to be investigated is wide.
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New aspects of the RAS. The Ang II degradation products
Although Ang II has been considered the effector peptide of the reninangiotensin system, other angiotensin peptides also posses biological activities (review in [10]). Ang III presents some physiological functions similar to Ang II in the cardiovascular and central nervous systems [10]. Some effects of Ang (17) are opposite to those of Ang II. Ang (17) acts as a vasodilator and inhibits growth of vascular smooth-muscle cells [10]. Some of the actions of Ang II apparently are in reality due to these degradation peptides. Thus, the release of vasopressin requires the conversion of Ang II to Ang III [51], and the expression of the plasminogen activator inhibitor-1 induced by Ang II is mediated by Ang IV through the AT4 receptor [52]. The role of these Ang degradation products in the genesis of renal injury is under investigation. Renal infusion of Ang III into rats causes proteinuria [53]. In renal interstitial fibroblasts and mesangial cells, Ang III induced c-fos gene expression, increased TGF-ß> mRNA expression, and fibronectin production [54], suggesting that this peptide could participate in the control of cell proliferation and matrix accumulation observed during renal damage. In addition, we have observed that Ang III also upregulated MCP-1 gene expression, as occurs with Ang II (unpublished data). These data support the hypothesis that Ang II is not the one and only effector peptide of the RAS. As more and more information accumulates, we are forced to modify our classical view of this system.
Acknowledgments
The papers cited in this review have been supported by grants from Ministerio de Educación y Ciencia (PM 94/211, PM 95/93, PM97/85, SAF 97/55), EU Concerted Action Grant, BMH4-CT98-3631 (DG12-SSMI) and Instituto de Investigaciones Nefrológicas Reina Sofia.
Notes
Correspondence and offprint requests to: Jesus Egido MD, Renal and Vascular Research Laboratory, Fundación Jiménez Diaz, Avda. Reyes Católicos, 2, E-28040 Madrid, Spain.
References