Opposing actions of angiotensins on angiogenesis

Angiogenic growth factors and hypertension

Emerging evidence supports a novel view of hypertension as a disease of inadequate or aberrant responses to angiogenic growth factors (AGF). Patients with hypertension have reduced microvascular density, with some evidence supporting a primary role for rarefaction in causing hypertension. Two clinical models have demonstrated a link between inhibition of AGF activity and hypertension. A major side effect of bevacizumab, a monoclonal antibody to vascular endothelial growth factor (VEGF), is hypertension. Pre-eclampsia is accompanied by high circulating levels of soluble VEGF receptor-1, which forms inactive complexes with VEGF and placental growth factor (PlGF). Paradoxically, early studies have demonstrated high circulating levels of AGF in hypertension. Several mechanisms may account for this finding including increased vascular stretch, tissue ischemia, compensatory responses, decreased clearance or a combination of these mechanisms. High AGF in hypertension could contribute to clinical sequelae such as peripheral and pulmonary edema, microalbuminuria, and progression of atherosclerosis. However, a role for altered angiogenesis in the pathogenesis of hypertension or its sequelae has not been established. Novel studies to understand the roles of AGF in hypertensive patients are warranted.

Distribution of angiotensin-(1-7) and ACE2 in human placentas of normal and pathological pregnancies

This work was designed to study the expression of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] and its generating enzyme (ACE2) in the uteroplacental interface. Placentas were obtained from 11 early pregnancy failures (5 miscarriages and 6 ectopic pregnancies), 15 normotensive, and 10 preeclamptic gestations. In placental villi, the main sites of immunocytochemical expression of Ang-(1-7) and ACE2 were the syncytiotrophoblast, cytotrophoblast, endothelium and vascular smooth muscle of primary and secondary villi. Syncitial Ang-(1-7) expression in samples obtained from miscarriages and ectopic pregnancies was increased compared to normal term pregnancy [2.0 (2.0-2.25 for the 25 and 75% interquartile range) vs 1.3 (1.0-1.9), p<0.01]. In the maternal stroma, Ang-(1-7) and ACE2 were expressed in the invading and intravascular trophoblast and in decidual cells in all 3 groups. Ang-(1-7) and ACE2 staining was also found in arterial and venous endothelium and smooth muscle of the umbilical cord. The expression of Ang-(1-7) and ACE2 was similar in samples obtained from normal term or preeclamptic pregnancies, except for increased expression of ACE2 in umbilical arterial endothelium in preeclampsia [0.5 (0.5-0.8) vs 0.0 (0.0-0.0), p<0.01]. The uteroplacental location of Ang-(1-7) and ACE2 in pregnancy suggests an autocrine function of Ang-(1-7) in the vasoactive regulation that characterizes placentation and established pregnancy.

The biological "scrabble" of pulmonary arteriovenous malformations: considerations in the setting of cavopulmonary surgery

Pulmonary arteriovenous fistulas are vascular malformations, which, by virtue of producing abnormal vascular connections proximal to the units of gas exchange, result in intrapulmonary right-to-left shunting. These malformations or fistulas reflect at least in part disordered angiogenesis, and less commonly recruitment and dilation of pre-existing vascular channels. Pulmonary arteriovenous fistulas occur in a number of diverse clinical settings. Such fistulas are a well-established feature of the Weber-Osler-Rendu complex, or hereditary haemorrhagic telangiectasia, an autosomal dominant vascular dysplasia characterized by mucocutaneous telangiectasis, epistaxis, gastrointestinal haemorrhage, and arteriovenous malformations in the lung, brain, liver and elsewhere. They are also seen in the patient with acute or chronic liver disease, disease that is usually but not invariably severe, or those with non-cirrhotic portal hypertension. They may occur as congenital malformations, single or diffuse, large or small in isolation, and when large or extensive enough may result in hypoxaemia, clinical cyanosis, and heart failure. Cerebral vascular accidents are also a well-known complication of this disorder. An extensive literature has accumulated with regard to the pulmonary arteriovenous fistulas seen in the setting of the Weber-Osler-Rendu complex, and there is considerable information on the genetics, basic biology, clinical findings, complications and therapeutic interventions of these malformations in the setting of this syndrome. These issues, however, are not the primary considerations of this review, although some aspects of this fascinating disorder will be discussed later. Rather the focus will be on pulmonary arteriovenous malformations that develop in the setting of cavopulmonary surgery, and their relationship to the pulmonary arteriovenous fistulas occurring in the hepatopulmonary syndrome. The complex tapestry of these overlapping and intersecting clinical observations will be unfolded in the light of their chronology.

Tissue renin angiotensin systems

The RAAS is a powerful regulator of vascular tone and intravascular volume and of tissue architecture and a variety of other functions. The recent appreciation of the immunoregulatory role of angiotensin II and its possible involvement in the genesis of atherosclerosis and in plaque rupture all speak to the wide-ranging physiologic and pathophysiologic activities of the peptide. So do its actions in fat cell differentiation and in neuromodulation. The system exists in the circulation, and RAASs, whole or partial, exist in many tissues. These systems are regulated at many levels ranging from the synthesis of renin to the dimerization of angiotensin receptors. Regulation occurs in multiple tissues and, as a result, tissue concentrations of angiotensin II and the concentration of other RAS components and their active metabolites can vary independently of the circulating system in these tissues. An RAS seems also to function within certain cells. Therapeutic interventions involving ACEIs and ARBs seem likely to provide benefit at least in part through the interruption of local systems. It is to be expected that with enhanced understanding of the biology of the multiple RASs, new suggestions for therapeutic interventions will be forthcoming.

Inhibition of angiogenesis: a new function for angiotensinogen and des(angiotensin I)angiotensinogen

Angiotensinogen (AGT) can be schematically considered to consist of a combination of an angiotensin I (Ang I) function, located at the N-terminal end, and the presence of a serpin (serine protease inhibitor) structure at the opposite end. des(Ang I)AGT, which accounts for more than 97% of the molecule, apparently has no function. Several serpins (antithrombin, maspin, pigment epithelial-derived factor, and kallistatin) have been recently shown to exert an antiangiogenic activity, suggesting a common mechanism of endothelial cell proliferation and migration. AGT and its renin-cleaved product, des(Ang I)AGT, are also angiogenesis inhibitors, both in vitro and in vivo at concentrations within the range of those observed in plasma. This property most likely results from the structure analogy of AGT with serpins. The pathologic relevance of this new function is still not known, but AGT produced by glial cells may play a role in the stabilization of the blood-brain barrier. These new data must be considered in light of the overall action of the renin-angiotensin system in angiogenesis.

Angiotensin Converting Enzyme-2 (ACE2) and its Possible Roles in Hypertension, Diabetes and Cardiac Function

Angiotensin converting enzyme-2 (ACE2) is a recently described homologue of the vasoactive peptidase, angiotensin converting enzyme (ACE). Like ACE, ACE2 is an integral (type I) membrane zinc metallopeptidase, which exists as an ectoenzyme. ACE2 is less widely distributed than ACE in the body, being expressed at highest concentrations in the heart, kidney and testis. ACE2 also differs from ACE in its substrate specificity, functioning exclusively as a carboxypeptidase rather than a peptidyl dipeptidase. A key role for ACE2 appears to be emerging in the conversion of angiotensin II to angiotensin (1-7), allowing it to act as a counter-balance to the actions of ACE. ACE2 has been localised to the endothelial and epithelial cells of the heart and kidney where it may have a role at the cell surface in hydrolysing bioactive peptides such as angiotensin II present in the circulation. A role for ACE2 in the metabolism of other biologically active peptides also needs to be considered. ACE2 also serendipitously appears to act as a receptor for the severe acute respiratory syndrome (SARS) coronavirus. Studies using ace2 -/- mice, and other emerging studies in vivo and in vitro, have revealed that ACE2 has important functions in cardiac regulation and diabetes. Together with its role as a SARS receptor, ACE2 is therefore likely to be an important therapeutic target in a diverse range of disease states.

Angiotensin converting enzyme-2 (ACE2) and its possible roles in hypertension, diabetes and cardiac function

Angiotensin converting enzyme-2 (ACE2) is a recently described homologue of the vasoactive peptidase, angiotensin converting enzyme (ACE). Like ACE, ACE2 is an integral (type I) membrane zinc metallopeptidase, which exists as an ectoenzyme. ACE2 is less widely distributed than ACE in the body, being expressed at highest concentrations in the heart, kidney and testis. ACE2 also differs from ACE in its substrate specificity, functioning exclusively as a carboxypeptidase rather than a peptidyl dipeptidase. A key role for ACE2 appears to be emerging in the conversion of angiotensin II to angiotensin (1-7), allowing it to act as a counter-balance to the actions of ACE. ACE2 has been localised to the endothelial and epithelial cells of the heart and kidney where it may have a role at the cell surface in hydrolysing bioactive peptides such as angiotensin II present in the circulation. A role for ACE2 in the metabolism of other biologically active peptides also needs to be considered. ACE2 also serendipitously appears to act as a receptor for the severe acute respiratory syndrome (SARS) coronavirus. Studies usingace2 -/- mice, and other emerging studiesin vivo andin vitro, have revealed that ACE2 has important functions in cardiac regulation and diabetes. Together with its role as a SARS receptor, ACE2 is therefore likely to be an important therapeutic target in a diverse range of disease states.

Old antihypertensives as novel antineoplastics: angiotensin-I-converting enzyme inhibitors and angiotensin II type 1 receptor antagonists

Angiogenesis, cellular growth and invasion of a cancer cell are attractive targets for new treatment strategies of malignancies in recent years. The evidences are accumulating that ACE inhibitors and angiotensin II type 1 antagonists could be novel anti-angiogenic, anti-invasive, and even anti-growth agents against neoplastic tissues: The renin-angiotensin system promotes angiogenesis directly or indirectly and growth of neoplastic cell. Some tumors carry angiotensin II type 1 receptors. Angiotensin II antagonists and angiotensin-I-converting enzyme inhibitors have shown some anti-neoplastic actions. Angiotensin II receptor blocker losartan antagonises platelets, which are thought to modulate via vascular endothelial growth factor. They may even protect the patient from the major toxicity of chemotherapy and/or radiotherapy, myelotoxicity, enabling us to give higher doses and end up with higher success rate. We believe that these agents can be useful on clinical grounds and suggest their incorporation into clinical studies.

Significance of vascular endothelial cell growth factor up-regulation mediated via a chymase-angiotensin-dependent pathway during angiogenesis in hamster sponge granulomas

Chymase is a serine protease responsible for local production of angiotensin (Ang) II from its precursor Ang I in several species, including humans, dogs, and hamsters. We have previously reported that chymase facilitates angiogenesis in sponge granulation tissues via local production of Ang II. Herein, we report the significance of vascular endothelial growth factor (VEGF) up-regulation mediated by Ang II during angiogenesis in hamster sponge granulomas. Treatment of granulation tissues with an anti-VEGF neutralizing antibody or antisense oligomers against VEGF mRNA significantly reduced Ang II-induced angiogenesis, supporting a significant role for VEGF during angiogenesis. In cultured fibroblasts prepared from granulation tissues, VEGF mRNA was up-regulated in response to Ang II within 2 h and this enhanced expression was abolished in the presence of an Ang II type 1 receptor-selective antagonist, an inhibitor of nuclear factor-kappaB activation, or an activator protein-1 inhibitor. To study the significance of local production of Ang II by chymase, we examined the effects of chymostatin on in vivo angiogenesis. We found that chymostatin markedly inhibited both up-regulation of VEGF mRNA and angiogenesis in granulation tissues treated by compound 48/80 or basic fibroblast growth factor. Our results suggest that Ang II directly acts on fibroblasts in granulation tissue to up-regulate VEGF mRNA and thereby induce angiogenesis. Furthermore, a chymase-Ang II-VEGF pathway may operate in granulation tissue as the primary mediator of angiogenesis.

Differential effects of angiotensin II and angiotensin-(1-7) at the nucleus tractus solitarii of transgenic rats with low brain angiotensinogen

OBJECTIVES

In this study, we investigated the effects of angiotensin II and angiotensin-(1-7) at the nucleus tractus solitarii (nTS) in transgenic rats with a severe deficit in brain angiotensinogen production, TGR(ASrAOGEN) (TGR).

METHODS

Angiotensin II (10 pmol), angiotensin-(1-7) (10 pmol) or NaCl (0.9%/50 nl) were microinjected into the nTS of urethane-anaesthetized TGR (n = 36) and Sprague Dawley (SD) (n = 34) rats. Mean arterial pressure (MAP) and heart rate were measured via a femoral artery catheter and the baroreflex control of heart rate was evaluated after increases in MAP induced by phenylephrine (baroreflex bradycardia).

RESULTS

Angiotensin II microinjections into the nTS of the TGR induced a higher decrease in MAP and heart rate (-37 +/- 5 mmHg and -69 +/- 12 b.p.m., respectively) in comparison to SD rats (-18 +/- 1 mmHg and -43 +/- 5 b.p.m., respectively). In contrast, changes after angiotensin-(1-7) microinjections into the nTS of TGR (-6 +/- 1 mmHg and -13 +/- 4 b.p.m.) were significantly smaller than that induced in SD (-11 +/- 2 mmHg and -24 +/- 6 b.p.m.). The baseline baroreflex sensitivity to phenylephrine of TGR was accentuated in comparison to SD rats (0.70 +/- 0.06 versus 0.44 +/- 0.03 ms/mmHg). Angiotensin II microinjection into the nTS produced similar attenuation in the baroreflex bradycardia in both SD (0.28 +/- 0.07 versus 0.5 +/- 0.07 ms/mmHg, before injection) and TGR (0.44 +/- 0.1 versus 0.82 +/- 0.1 ms/mmHg, before injection). In contrast, angiotensin-(1-7) microinjection elicited a facilitation of the baroreflex bradycardia in SD (0.68 +/- 0.12 versus 0.41 +/- 0.03 ms/mmHg, before injection), while in TGR, angiotensin-(1-7) induced an attenuation of baroreflex bradycardia (0.34 +/- 0.07 ms/mmHg versus 0.55 +/- 0.05 ms/mmHg, before injection).

CONCLUSIONS

These results indicate that a permanent inhibition of angiotensinogen synthesis in the brain can lead to an increase in the sensitivity of the baroreflex control of heart rate (baroreflex bradycardia) and an increase in angiotensin II responsiveness at the nTS. However, the nTS effect of angiotensin-(1-7) was significantly attenuated in these transgenic rats. These data further indicate that the decrease in brain angiotensins in the transgenic rats may be functionally relevant and support the concept of differential regulatory mechanisms for the effects of the two angiotensin peptides.

Biological properties of the angiotensin peptides other than angiotensin II: implications for hypertension and cardiovascular diseases

Several peptides of the RAS other than angiotensin (1-8) have been identified. They are generally referred as 'angiotensin fragments': Ang (2-8), Ang (3-8) and Ang (1-7) and have been detected in human tissues. There is evidence that they may play a functional role in humans by acting in concert with angiotensin (1-8) and aldosterone. Available knowledge on the pathways leading to synthesis and degradation of angiotensin fragments, as well as on their interactions with receptors and on their possible role in cardiovascular homeostasis and disease are reviewed.

Microvascular angiogenesis and the renin-angiotensin system

The renin-angiotensin system is one of the major regulatory mechanisms essential for maintaining cardiovascular homeostasis. Angiotensin II is a multifunctional hormone that plays a key role in regulating this system. The importance of the renin-angiotensin system in controlling sodium homeostasis and vascular resistance is well established, however, in the past decade, much attention has been focused on the importance of angiotensin II as a regulator of microvessel density, acting through the AT1 and AT2 receptors. In this review, we discuss the connections between the renin-angiotensin system and other growth factor pathways known to be involved in pathologic and physiologic angiogenesis and rarefaction.

The clinical implication of tissue renin angiotensin systems

The presence, and in many cases the regulated synthesis, of components of the renin-angiotensin system have been demonstrated in multiple tissues, indicating the existence of tissue angiotensin-generating systems. These vary with respect to which renin-angiotensin system components are synthesized locally and which are taken up from plasma. Enzymes unrelated to the classical renin-angiotensin system may also contribute to tissue angiotensin synthesis. However, based on the available data, the prevailing opinion that kidney-derived renin is in all cases the only physiologically relevant renin in tissues must be revised. Also there is evidence indicating a role for tissue angiotensin systems in the pathogenesis of cardiovascular disease and in cardiovascular structural remodeling. The angiotensin-regulated synthesis of aldosterone in cardiac tissue has been described, suggesting the possibility that a renin-angiotensin-aldosterone system exists in the heart. In addition, intracellular (intracrine) sites of angiotensin action have been reported. Some of these findings have implications for therapeutics and, in particular, for the use of angiotensin converting-enzyme inhibitors and angiotensin receptor blockers. Finally, tissue angiotensin systems outside the cardiovascular system also appear to be physiologically relevant.

Protection against ischemia: a physiological function of the renin-angiotensin system

The renin-angiotensin system (RAS) is involved in a complex mechanism that serves to preserve the blood supply to organs so that they can maintain cellular function. Angiotensin II exerts this effect, independently of the blood pressure generated, through two time-related events: a fast opening of the reserve collateral circulation and a much slower response of new vessel formation or angiogenesis. This effect is observed in rats with ligation of the abdominal aorta and in gerbils with abrupt or progressive unilateral carotid artery ligation. Inhibition of the angiotensin-converting enzyme (ACE) or the angiotensin II receptor represses this effect, and it appears that it is mediated through a non-AT1 receptor site of angiotensin II. Many tumors, both benign and malignant, express renin and angiotensin. It seems that the stimulating action of angiotensin II on angiogenesis could also be involved in preserving the blood supply to tumor cells. Administration of converting enzyme inhibitors increases survival and decreases tumor size in tumor-bearing rats. These observations support the hypothesis that the RAS, directly or indirectly, is involved in situations in which the restoration of blood supply is critical for the viability of cells and that it is present not only in normal but also in pathological conditions such as tumors. In view of the ubiquitous presence of renins and angiotensins, it is also likely to be involved in other conditions, such as inflammation, arthritis, diabetic retinopathy, and retrolental fibroplasia, among others in which angiogenesis is prominent. In addition, angiotensin II could be involved, through the counterbalance of the AT1 and AT2 receptors, in the rarefaction of blood vessels as an etiologic component of essential hypertension.

Mechanisms of angiotensin-(1-7)-induced inhibition of angiogenesis

Angiotensin-(1-7) [ANG-(1-7)], an endogenous bioactive peptide constituent of the renin-angiotensin system, acts as an inhibitory growth factor in vitro and in vivo. In this study, we evaluated whether the antiangiogenic effect of ANG-(1-7) in the mouse sponge model of angiogenesis might be receptor mediated and involved in the release of nitric oxide (NO). The hemoglobin content (microg/mg wet tissue) of 7-day-old sponge implants was used as an index of the vascularization and showed that daily injections of ANG-(1-7) (20 ng) inhibited significantly the angiogenesis in the implants relative to the saline-treated group. The specific receptor antagonist D-Ala(7)-ANG-(1-7); A-779 prevented ANG-(1-7)-induced inhibition of angiogenesis. The antiangiogenic effect was also abolished by pretreatment with NO synthase inhibitors aminoguanidine (1 mg/ml) or N(G)-nitro-L-arginine methyl ester (0.3 mg/ml). Selective AT1 and AT2 angiotensin-receptor antagonists and an angiotensin-converting enzyme inhibitor, in combination with ANG-(1-7) or alone, did not alter angiogenesis in the implants. These results establish that the regulation of the vascular tissue growth by ANG-(1-7) is associated with NO release by activation of an angiotensin receptor distinct from AT1 and AT2.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

Angiotensin-(1-7): an update

The renin-angiotensin system is a major physiological regulator of arterial pressure and hydro-electrolyte balance. Evidence has now been accumulated that in addition to angiotensin (Ang) II other Ang peptides [Ang III, Ang IV and Ang-(1-7)], formed in the limited proteolysis processing of angiotensinogen, are importantly involved in mediating several actions of the RAS. In this article we will review our knowledge of the biological actions of Ang-(1-7) with focus on the puzzling aspects of the mediation of its effects and the interaction Ang-(1-7)-kinins. In addition, we will attempt to summarize the evidence that Ang-(1-7) takes an important part of the mechanisms aimed to counteract the vasoconstrictor and proliferative effects of Ang II.