Peptide growth factors markedly decrease the ligand binding of angiotensin II type 2 receptor in rat cultured vascular smooth muscle cells

Angiotensin II activates matrix metalloproteinase type II and mimics age-associated carotid arterial remodeling in young rats

Increased angiotensin II (Ang II), matrix metalloproteinase type II (MMP2), and sympathetic activity accompany age-associated arterial remodeling. To analyze this relationship, we infused a low subpressor dose of Ang II into young (8 months old) rats. This increased carotid arterial MMP2 transcription, translation, and activation, as well as transforming growth factor-beta1 activity and collagen deposition. A higher Ang II concentration, which increased arterial pressure to that of old (30 months old) untreated rats, produced carotid media thickening and intima infiltration by vascular smooth muscle cells (VSMCs). Ex vivo, Ang II increased MMP2 activity in carotid rings from young rats to that of untreated old rats. Ang II also increased the ability of early passage VSMCs from young rats to invade a synthetic basement membrane, similar to that of untreated VSMCs from old rats. The MMP inhibitor GM6001 and the AT1 receptor antagonist Losartan inhibited these effects. The alpha-adrenoreceptor agonist phenylephrine increased arterial Ang II protein, causing MMP2 activation and intima and media thickening. Exposure of young VSMCs to phenylephrine in vitro increased Ang II protein and MMP2 activity to the levels of old VSMCs; Losartan abolished these effects. Thus, Ang II-induced effects on MMP2, transforming growth factor-beta1, collagen, and VSMCs are central to the arterial remodeling that accompanies advancing age.

Effect of angiotensin AT2 receptor stimulation on vascular cyclic GMP production in normotensive Wistar Kyoto rats

In the present study in normotensive Wistar Kyoto rats (WKY), we investigated whether any angiotensin II (ANG II) increases in vascular cyclic GMP production were via stimulation of AT(2) receptors. Adult WKY were infused for 4h with ANG II (30 ng/kg per min, i.v.) or vehicle (0.9% NaCl, i.v.) after pretreatment with (1) vehicle, (2) losartan (100 mg/kg p.o.), (3) PD 123319 (30 mg/kg i.v.), (4) losartan+PD 123319, (5) icatibant (500 microg/kg i.v.), (6) L-NAME (1 mg/kg i.v.), (7) minoxidil (3 mg/kg i.v.). Mean arterial blood pressure (MAP) was continuously monitored, and plasma ANG II and aortic cyclic GMP were measured at the end of the study. ANG II infusion over 4h raised MAP by a mean of 13 mmHg. This effect was completely prevented by AT(1) receptor blockade. PD 123319 slightly attenuated the pressor effect induced by ANG II alone (123.4+/-0.8 versus 130.6+/-0.6) but did not alter MAP in rats treated simultaneously with ANG II + losartan (113+/-0.6 versus 114.3+/-0.8). Plasma levels of ANG II were increased 2.2-3.7-fold by ANG II infusion alone or ANG II in combination with the various drugs. The increase in plasma ANG II levels was most pronounced after ANG II+losartan treatment but absent in rats treated with losartan alone. Aortic cyclic GMP levels were not significantly changed by either treatment. Our results demonstrate that the AT(2) receptor did not contribute to the cyclic GMP production in the vascular wall of normotensive WKY.

Differential ANG II-induced growth activation pathways in mesenteric artery smooth muscle cells from SHR

Angiotensin II-induced growth signaling mechanisms were investigated in vascular smooth muscle cells (VSMCs) from mesenteric arteries of spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY). In WKY, angiotensin II significantly increased protein synthesis ([(3)H]leucine incorporation) but not DNA synthesis ([(3)H]thymidine incorporation). In SHR, angiotensin II increased protein and DNA synthesis. VSMCs from both strains expressed angiotensin type 1 (AT(1)) and type 2 (AT(2)) receptors. Losartan (an AT(1) receptor antagonist) but not PD-123319 (an AT(2) receptor antagonist) attenuated angiotensin II-stimulated protein synthesis in WKY VSMCs. In SHR, losartan and PD-123319 partially inhibited angiotensin II-induced VSMC proliferation. The mitogen-activated protein kinase or extracellular signal-regulated protein kinase (ERK) kinase inhibitor PD-98059 blocked VSMC growth responses to angiotensin II in both strains. Angiotensin II increased ERK1/2 activation more in SHR than WKY, an effect inhibited by losartan but not PD-123319. LY-294002 [a phosphatidylinositol-3 (PI3) kinase inhibitor] blocked angiotensin II-stimulated ERK1/2 activation in SHR but not in WKY, whereas bisindolylmaleimide [a protein kinase C (PKC) inhibitor] was ineffective. In conclusion, angiotensin II stimulates VSMC proliferation via AT(1) and AT(2) receptors in SHR. In WKY, angiotensin II induces VSMC hypertrophy via AT(1) receptors. ERK1/2-dependent pathways regulated by intracellular Ca(2+) but not PKC mediate these effects. In SHR VSMCs, PI3 kinase plays a role in augmented angiotensin II-induced ERK1/2 phosphorylation. These angiotensin II-mediated signaling events could contribute to vascular remodeling in SHR.

Angiotensin and its AT2 receptor: new insights into an old system

The AT2 receptor represents a true receptor, but signals and functions in unexpected ways compared to the respective features of the 'classical' AT1 receptor. Moreover, some of the actions of the AT2 receptor are even directly opposed to those of the AT1 receptor, especially concerning the growth- and differentiation-modulating actions of ANG II. The regulation of the AT2 receptor itself by its agonist, as well as by growth factors during ontogenesis, and its acknowledged effects on the regulation of cell growth, differentiation and apoptosis, points towards a role of a program modulator in embryonic development and regeneration.

Angiotensin II activates nuclear transcription factor kappaB through AT(1) and AT(2) in vascular smooth muscle cells: molecular mechanisms

Nuclear factor-kappaB (NF-kappaB) regulates many genes involved in vascular physiopathology. We have previously observed in vivo NF-kappaB activation in injured vessels that diminished by angiotensin-converting enzyme inhibition. In the present work, we investigated the effect of angiotensin II (Ang II) on NF-kappaB activity in rat vascular smooth muscle cells, evaluating the molecular mechanisms and the specific receptor subtype involved. Ang II increased NF-kappaB DNA binding (5-fold, 10(-)(9) mol/L at 1 hour; electrophoretic mobility shift assay), nuclear translocation of p50/p65 subunits, and cytosolic inhibitor kappaBalpha (IkappaBalpha) degradation. Ang II elicited NF-kappaB-mediated transcription (transfection of a reporter gene) and expression of NF-kappaB-related genes (monocyte chemoattractant protein-1 and angiotensinogen). AT(1) (DUP753) and AT(2) (PD123319 and CGP42112) receptor antagonists inhibited Ang II-induced NF-kappaB DNA binding in a dose-dependent manner ( approximately 85% for each one; 10(-)(5) mol/L at 1 hour). The AT(2) agonist p-aminophenylalanine(6)-Ang II augmented NF-kappaB binding (4.6-fold, 10(-)(9) mol/L at 1 hour), p65 nuclear levels, and transcription of an NF-kappaB reporter gene. AT(1) antagonist markedly inhibited NF-kappaB-mediated transcription and gene expression. Some differences between AT(1)/AT(2) intracellular signals were found. Antioxidants and ceramide inhibitors, but not protein kinase C inhibitors, diminished NF-kappaB activation elicited by both Ang II and the AT(2) agonist, while tyrosine kinase inhibitors only decreased Ang II-induced NF-kappaB activity. Our results demonstrate that Ang II activates NF-kappaB via AT(1) and AT(2), although NF-kappaB-mediated transcription occurred mainly through AT(1). Both receptors share some signaling pathways (oxygen radicals and ceramide); however, tyrosine kinases only participate in AT(1)/NF-kappaB responses. These data provide novel insights into Ang II actions, suggesting a potential implication of the AT(2) in the pathobiology of vascular cells.

Probing intracellular dynamics in living cells with near-field optics

Near-field optics (NFO) overcomes the diffraction limit of light microscopes and permits visualization of single molecules. However, despite numerous applications of NFO in the physical sciences, there is still a paucity of applications in the neurosciences. In this work, the authors have developed NFO probes to image intracellular dynamic processes in living cells. This is the first time a NFO probe has been inserted inside a living cell to deliver light to a spatially controlled region for optical measurements and to record cellular responses to external stimuli. Two different optical detection systems (CCD camera and avalanche photon detection) were developed to monitor cellular responses to drug administration in two different cell types. NG108-15 neuroblastoma cells and vascular smooth muscle cells (VSMC) were penetrated with NFO probes. Intracellular Ca2+ increases post drug stimulation were detected by NFO probes. The cells were loaded with either fura-2/AM or fluo-3/AM calcium dyes. VSMC were stimulated with angiotensin II, resulting in a precise area of intracellular Ca2+ increase. Different response profiles of Ca2+ increases were observed after ionomycin and bradykinin administration in NG108-15 cells. Responsive heterogeneities due to ionomycin among different cells of the same type were recorded. The results show that NFO probes make possible real-time visualization of intracellular events. With refinement, intracellular NFO probes offer the potential of probing cell function with fast temporal and excellent spatial resolutions.

Characterization of AT2 receptor expression in NIH 3T3 fibroblasts

1. A high expression of angiotensin II receptors and of angiotensin-converting enzyme (ACE) activity was detected in confluent NIH 3T3 fibroblasts. 2. Characterization with selective ligands, dithiothreitol, and GTP gamma S, indicated that only the AT2 subtype was expressed. 3. AT2 receptors and ACE expression were strictly dependent on the cell density and growth phase of the cells, with AT2 receptors being expressed earlier than ACE. In contrast, high expression of AT2 receptors irrespective of their growth state was observed in NIH 3T3 cells lacking contact inhibition upon neoplastic transformation with ras. 4. Our results imply a possible relation of AT2 receptors to cell growth and cell-cell contact.

Role of AT2 receptors in angiotensin II-stimulated contraction of small mesenteric arteries in young SHR

This study assesses the receptor subtype (AT1 and AT2) through which angiotensin II (Ang II) mediates contraction in small arteries of young and adult spontaneously hypertensive rats (SHR). Segments of third-order mesenteric arteries ( approximately 200 microm in lumen diameter) were mounted in a pressurized system. Systolic blood pressure and media:lumen ratio of small arteries were significantly greater (P<0.001) in young SHR and adult SHR than in age-matched Wistar-Kyoto rats (WKY). Ang II-induced contractile effects were significantly increased (P<0.05) in young SHR compared with age-matched WKY. AT1 blockade with losartan, and combined AT1 and AT2 blockade with losartan and PD123319, abolished Ang II-stimulated contraction in young and adult rats. AT2 blockade (PD123319) significantly reduced (P<0.01) Ang II-elicited contraction in young SHR but had no effect in WKY or adult SHR, indicating that AT2 receptors may contribute to Ang II-induced contraction in young SHR. To determine the Ang receptor status in rat mesenteric vessels, AT1 and AT2 receptor mRNA expression was determined by reverse transcription-polymerase chain reaction. AT1 and AT2 receptor protein expression were detected by Western blot analysis. AT1 receptor mRNA was equally expressed in age-matched rats, but expression was significantly lower in young rats compared with adult rats. AT2 receptor mRNA was weakly expressed in WKY and adult SHR. In vessels from young SHR, AT2 receptor mRNA expression was significantly increased compared with the other groups. AT1 receptor protein was equally expressed in adult rats of both strains but was undetectable in young rats. AT2 receptor protein was only detectable in young rats, with the magnitude of expression greater in SHR than WKY. In conclusion, Ang II-stimulated contractile responses are augmented in vessels from young SHR. These effects are reduced by selective AT2 blockade and abolished by AT1 blockade, indicating that both Ang receptor subtypes are involved in contraction in young SHR. In WKY and adult SHR, losartan, but not PD123319, inhibited Ang II-induced contraction, indicating the exclusive involvement of AT1 receptors. Thus, in SHR, in the phase of developing hypertension, enhanced Ang II-stimulated vascular contraction may be associated with changes in Ang II receptor status, as evidenced pharmacologically and by increased vascular AT2 receptor mRNA and protein expression.

Pathophysiological role of angiotensin II type 2 receptor in cardiovascular and renal diseases

Since the discovery of nonpeptidic ligands, the receptors for angiotensin (Ang) II have been classified into 2 subtypes (Ang II type 1 receptor [AT1-R] and Ang II type 2 receptor [AT2-R]). AT1-R mediates most of the cardiovascular actions of Ang II. AT2-R is expressed at very high levels in the developing fetus. Its expression is very low in the cardiovascular system of the adult. The expression of AT2-R can be modulated by pathological states associated with tissue remodeling or inflammation. In failing hearts or neointima formation after vascular injury, AT2-R is reexpressed in cells proliferating in interstitial regions or neointima and exerts an inhibitory effect on Ang II-induced mitogen signals or synthesis of extracellular matrix proteins, resulting in attenuation of the tissue remodeling. An extreme form of cell growth inhibition ends in programmed cell death, and this process, which is initiated by the withdrawal of growth factors, is also enhanced by AT2-R. Cardiac myocyte- or vascular smooth muscle-specific mice that overexpress AT2-R display an inhibition of Ang II-induced chronotropic or pressor actions, suggesting the role of AT2-R on the activity of cardiac pacemaker cells and the maintenance of vascular resistance. AT2-R also activates the kinin/nitric oxide/cGMP system in the cardiovascular and renal systems, resulting in AT2-R-mediated cardioprotection, vasodilation, and pressure natriuresis. These effects, transmitted by AT2-R, are mainly exerted by stimulation of protein tyrosine or serine/threonine phosphatases in a Gi protein-dependent manner. The expression level of AT2-R is much higher in human hearts than in rodent hearts, and the AT2-R-mediated actions are likely enhanced, especially by clinical application of AT1-R antagonists. Thus, in this review, the regulation of AT2-R expression, its cellular localization, its pathological role in cardiovascular and kidney diseases, and pharmacotherapeutic effects of AT2-R stimulation are discussed.

Regulation of cell proliferation and angiotensin II type 2 receptors in R3T3 cells

The effects of insulin-like growth factor 1 (IGF-1), basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGFbeta1), fetal calf serum (FCS) and angiotensin II (AngII) on cell proliferation, (3H-thymidine incorporation and cell number) and AT2 receptor number and mRNA levels in R3T3 cells have been studied. All growth factors as well as FCS markedly increased cell proliferation, whereas AngII increased slightly 3H-thymidine incorporation, but not cell number. TGFbeta1, bFGF and FCS reduced by more than 80% both AT2 receptor number and mRNA, by inhibiting the transcription rate. In contrast, IGF-1 and AngII increased about 4-fold AT2 receptor number, but only IGF-1 increased AT2 mRNA. When added together the effects of IGF-1 and AngII were more than additive on AT2 receptor number, but not on mRNA level. None of the factors studied modified AT2 mRNA half-life. In conclusion, the present results demonstrated that: 1/ cell proliferation is not correlated with AT2 expression; 2/ growth factors regulate, positively or negatively, AT2 transcription rate, whereas AngII regulates the translation rate of AT2 mRNA; 3/ all the effects of AngII on R3T3 are mediated by AT2 receptors since they are mimicked by the AT2 agonist CGP42112 and blocked by the AT2 antagonist PD123177.

Angiotensin II type 1 receptor blockade with candesartan protects the porcine myocardium from reperfusion-induced injury

Angiotensin-converting enzyme (ACE) inhibitors reduce myocardial ischemia/reperfusion injury. It is unclear whether reduced formation of angiotensin II or attenuated degradation of bradykinin is responsible for the beneficial effects. We investigated the role of endogenous angiotensin II in ischemia/reperfusion injury by studying the effects of the angiotensin II type 1 receptor antagonist candesartan on myocardial function, infarct size, and perfusion after ischemia/reperfusion. Anesthetized pigs were subjected to 45 min of regional ischemia and 240 min of reperfusion. Starting 5 min before reperfusion, four groups of pigs (n = 6 in each) received coronary venous retroinfusion of candesartan (0.2, 2, or 20 microg/kg) or vehicle for 30 min. Myocardial regional blood flow was measured with radioactive microspheres in two separate groups (n = 6 in each) given 20 microg/kg candesartan or vehicle. Retroinfusion of 20 microg/kg of candesartan improved recovery of left ventricular systolic segment shortening measured by sonomicrometry in the ischemic area compared with 0.2 microg/kg of candesartan and vehicle. Infarct size, as a percentage of the area at risk, was smaller in the 2 and 20 microg/kg groups than in the vehicle group (39.1 +/- 11.6% and 34.8 +/- 10.2% vs. 78.3 +/- 8.9%, p < 0.01). There was no difference between candesartan and vehicle in their effects on regional myocardial blood flow. Angiotensin II type 1 receptor blockade supports myocardial functional recovery and reduces infarct size. This effect is not related to improved regional myocardial blood flow during reperfusion.

Effects of growth factors on cell proliferation and angiotensin II type 2 receptor number and mRNA in PC12W and R3T3 cells

Previous studies have suggested that the expression of angiotensin type 2 receptor was inversely related to cell proliferation. We examined the effects of insulin-like growth factor (IGF-1), basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGFbeta1) and fetal calf serum (FCS) on cell proliferation and AT2 binding sites and mRNA level in PC12W (rat pheochromocytoma cell line) and R3T3 (mouse fibroblast cell line) which express abundant AT2 receptors. In both cell lines, serum deprivation markedly increased both AT2 receptor number and mRNA. However, in the absence of serum cell proliferation continued in PC12W and R3T3 at late passages (R3T3 LP) but not at early passages (R3T3 EP). In PC12W, none of the three growth factors studied stimulated cell proliferation, but TGFbeta1 and more particularly bFGF markedly reduced AT2 expression. In R3T3 LP, IGF-1 and bFGF, but not TGFbeta1, slightly stimulated cell proliferation, but the three factors, specially bFGF, reduced AT2 expression. In contrast, in R3T3 EP, the three growth factors significantly increased cell proliferation, but whereas TGFbeta1 and bFGF markedly reduced AT2 binding sites and mRNA, IGF-1 caused the opposite effects. These results indicate that regulation of AT2 expression is not correlated with cell proliferation and appears to be more complex than initially suspected. In addition, they show that the same factor can have an opposite effect on AT2 expression in the same cell line depending upon the cell passage.

AT2 receptor stimulation increases aortic cyclic GMP in SHRSP by a kinin-dependent mechanism

In the present study we tested the hypothesis whether an angiotensin AT2 receptor-mediated stimulation of the bradykinin (BK)/nitric oxide (NO) system can account for the effects of AT1 receptor antagonism on aortic cGMP described previously in SHRSP. Adult SHRSP were treated for 4 hours with angiotensin II (ANG II) (30 ng/kg per min IV) or vehicle (0.9% NaCl I.V.). Animals were pretreated with vehicle, losartan (100 mg/kg P.O.), PD 123319 (30 mg/kg I.V.), losartan plus PD 123319, icatibant (500 microg/kg I.V.), N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/kg I.V.), or minoxidil (3 mg/kg I.V.). Mean arterial blood pressure (MAP) was continuously monitored over the 4-hour experimental period, and plasma ANG II and aortic cGMP were measured by RIA at the end of the study. ANG II infusion over 4 hours raised MAP by about 20 mm Hg. Losartan alone or losartan plus ANG II as well as minoxidil plus ANG II markedly reduced blood pressure when compared to vehicle-treated or ANG II-treated animals, respectively. Plasma levels of ANG II were increased 2-fold by ANG II infusion alone or by ANG II in combination with icatibant, L-NAME, or minoxidil. The increase in plasma ANG II levels was even more pronounced after losartan treatment. Aortic cGMP content was significantly increased by ANG II, losartan, losartan plus ANG II, and minoxidil plus ANG II by 60%, 45%, 68%, and 52%, respectively (P<.05). The effects of ANG II and of losartan plus ANG II on aortic cGMP content were both blocked by cotreatment with the AT2 receptor antagonist PD 123319. Icatibant and L-NAME abolished the effects of ANG II on aortic cGMP. Our results demonstrate the following: (1) ANG II increases aortic cGMP by an AT2 receptor-mediated action because the effect could be prevented by an AT2 receptor antagonist; (2) the effect of ANG II was not secondary to blood pressure increase because it remained under reduction of MAP with minoxidil; (3) losartan increased aortic cGMP most likely by increasing plasma ANG II levels with a subsequent stimulation of AT2 receptors; and (4) the effects of AT2 receptor stimulation are mediated by BK and, subsequently, NO because they were abolished by B2 receptor blockade as well as by NO synthase inhibition.

Modulation of angiotensin II type 2 receptor mRNA in rat hypothalamus and brainstem neuronal cultures by growth factors

This study investigates the regulatory effects of growth factors upon angiotensin II type 2 (AT2) mRNA levels in neurons co-cultured from newborn rat hypothalamus and brainstem. Incubation of cultured neurons with nerve growth factor (NGF; 5-50 ng/ml) caused time-dependent changes in the steady-state levels of AT2 receptor mRNA. Short-term (0.5-1.0 h) incubations with NGF resulted in significant increases in AT2 receptor mRNA, whereas longer-term incubations (4-24 h) caused significant decreases. Activation of NGF receptors is known to stimulate phospholipase C-gamma and subsequently activate protein kinase C (PKC). Incubation of cultures with the PKC activator, phorbol-12-myristate-13-acetate (PMA; 100 nM), caused temporal changes in AT2 receptor mRNA levels similar to those observed with NGF. By contrast, insulin (0.1-10 microg/ml) elicited only significant decreases in AT2 receptor mRNA levels. The observed abilities of NGF and insulin to regulate the expression of AT2 receptor mRNA are consistent with the fact that the AT2 receptor gene promoter region contains several cis DNA regulatory elements that respond to growth factor-stimulated transcription factors. These novel observations which show that NGF and insulin can regulate AT2 receptor mRNA in neurons derived from neonatal rat CNS lend support to the idea that AT2 receptors have a role in development and differentiation.

Interferon regulatory factor-1 up-regulates angiotensin II type 2 receptor and induces apoptosis

The expression of the angiotensin II type 2 (AT2) receptor is developmentally and growth regulated. In cultured R3T3 cells, expression of this receptor is markedly induced at the confluent state and with serum deprivation. In this study we demonstrated that the removal of serum from culture media resulted in the induction of apoptosis in these cells and the addition of angiotensin II further enhanced apoptosis. We have previously identified an interferon regulatory factor (IRF) binding motif in the mouse AT2 receptor gene promoter region. In this report, we observed that serum removal increased IRF-1 expression, with a rapid and transient decrease of IRF-2. To prove that the changes in IRFs after serum removal mediated apoptosis and up-regulated AT2 receptor, we transfected antisense oligonucleotides for IRF-1 or IRF-2 into R3T3 cells and observed that IRF-1 antisense oligonucleotide attenuated apoptosis and abolished the up-regulation of AT2 receptor. IRF-2 antisense oligonucleotide pretreatment did not affect the onset of apoptosis after serum removal; instead, it increased AT2 receptor binding and enhanced angiotensin II-mediated apoptosis. Taken together, these results suggest that increased IRF-1 after serum starvation contributes to the induction of apoptosis and that increased IRF-1 up-regulates the AT2 receptor expression after serum starvation, resulting in enhanced angiotensin II-mediated apoptosis.

Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms

The regulatory effects of angiotensin II (AngII) on its receptor subtypes, AT1 and AT2, were studied using cultured bovine adrenal cells (BAC), which express both receptor subtypes, and PC12W and R3T3 cells, which express only AT2 receptors. In BAC, AngII caused a decrease in AT1- and AT2-binding sites and their corresponding messenger RNAs (mRNAs), but with different kinetics. AT1-binding sites decreased by more than 50% within the first 3 h, whereas AT1 mRNA started to decline after a lag period of 3 h. Both AT2-binding sites and mRNA remained stable within the first 6 h of AngII treatment. Then, AT2 mRNA decreased rapidly with an apparent half-life of 2-3 h, whereas AT2-binding sites declined with an apparent half-life of about 16 h. Measurement of transcription rate and mRNA half-life by the [3H]uridine-thiouridine method revealed that AngII reduced by 90% the rate of AT1 transcription, but had no effect on AT1 mRNA half-life, whereas it slightly reduced AT2 transcription, but markedly reduced AT2 mRNA stability. All of the effects of AngII on both AT1 and AT2 receptors were blocked by losartan, indicating that they were mediated exclusively through the AT1 receptor. In PC12W cells, AngII was unable to modify AT2-binding sites or mRNA. Moreover, in BAC, [125I]AngII was internalized through the AT1 receptor, whereas occupancy of AT2 receptors in either BAC or PC12W did not produce internalization of the hormone. These results indicate that AngII, through the AT1 receptor, down-regulates both AT1 and AT2, but by different mechanisms; AT1 receptor is regulated through internalization-degradation of the occupied receptor and inhibition of transcription, whereas AT2 receptor is regulated mainly by decreasing the stability of its mRNA. Moreover, the phorbol ester phorbol 12-myristate 13-acetate mimicked most of the effects of AngII in BAC and decreased both AT2-binding sites and mRNA on PC12W cells, indicating that the hormonal regulation of both AT1 and AT2 receptors is mediated through protein kinase C activation.

The role of angiotensin receptors in cardiovascular diseases

As an antihypertensive regimen, angiotensin I-converting enzyme (ACE) inhibition appears to have an antiproliferative cardiovascular effect that is not caused by blood pressure reduction alone. On the other hand, ACE inhibition has been shown to induce neocapillarization in hypertrophied myocardium. The possible mechanisms behind these beneficial cardiovascular effects of ACE inhibition are the suppression of angiotensin II formation and the potentiation of bradykinin. Angiotensin II receptor antagonism appears to have a similar antiproliferative effect on myocardium and vascular smooth muscle as ACE inhibition. This suggests that the antiproliferative action of both regimens is due only to the reduction of the pressor and growth effects of angiotensin II, or that both regimens have an additional, similarly effective antiproliferative action. Recently, knowledge about angiotensin II receptors has almost exponentially expanded. The two main classes of angiotensin II receptors, type 1 and 2 (AT1 and AT2), have been shown to belong to the same receptor family. However, their signal transduction and function seem to differ totally. The function and signal transduction of AT1 are to a large extent known. All the well-known physiological and pathophysiological effects of angiotensin II have been attributed to AT1. On the other hand, AT2 has quite recently been shown to mediate antiproliferation and differentiation at least in some tissues and cells, e.g. in vascular endothelial cells and some cells of neuronal origin. This review highlights the recent findings on angiotensin II receptors, and discusses the mechanisms behind the beneficial cardiovascular effects of interfering with the renin-angiotensin system.

In vitro and in vivo effects of UP 269-6, a new potent orally active nonpeptide angiotensin II receptor antagonist, on vascular smooth muscle cell proliferation

1. The present studies were designed to measure the affinity of UP 269-6, a newly developed angiotensin AT1 receptor antagonist, for vascular AT1 receptors from normotensive and hypertensive rats and to investigate in vitro, its effects on angiotensin II (AII)-induced hyperplasia and hypertrophy of vascular smooth muscle cells (VSMC). In addition the in vivo effects of UP 269-6 on neointimal proliferation in a carotid artery balloon injury in normotensive rats were also investigated. 2. UP 269-6 selectively inhibited [125I]-Sar1-Ile8-AII binding to vascular AT1 receptors present on VSMC derived from normotensive Wistar rat and from SHR (Ki = 16.6 +/- 3.6 nM and 7.5 +/- 2.0 nM, respectively). In comparison, losartan and its metabolite, EXP 3174, inhibited [125I]-Sar1-Ile8-AII binding to vascular AT1 receptors derived from both cell models with Ki values slightly lower (losartan) and higher (EXP 3174), respectively, than that of UP 269-6. 3. AII (1 microM) induced a weak and variable hyperplastic response (4 to 32% increase in cell number) in Wistar rat VSMC after 96 h. 4. AII (1 microM) induced a time-dependent increase in cell number in VSMC from SHR. UP 269-6 inhibited concentration-dependently this effect with an IC50 value of 159 +/- 58 nM. Losartan was clearly less potent and EXP 3174 showed nearly the same inhibitory potency, compared to UP 269-6. UP 269-6 (1 microM) inhibited nearly completely the action of AII. 5. AII (500 nM) caused maximal stimulation of protein synthesis in Wistar rat VSMC (117 +/- 36%). UP 269-6, losartan and EXP 3174 totally inhibited this stimulation with IC50 values of 28 +/- 6 nM, 3504 +/- 892 nM and 21 +/- 3 nM, respectively. 6. AII (50 nM) induced maximal stimulation of protein synthesis in SHR VSMC (237 +/- 67%). UP 269-6, losartan and EXP 3174 totally inhibited this stimulation with IC50 values of 16 +/- 3 nM, 282 +/- 122 nM and 3.3 +/- 1.0 nM, respectively. 7. UP 269-6 (75 mg kg-1 day-1) administered orally in the diet for 20 days induced a 38% reduction in neointimal area and a 36% reduction in neointima/media ratio associated with the intimal thickening induced by carotid artery balloon injury. 8. In conclusion, UP 269-6 was shown to be a potent antiproliferative agent both in vitro on AII-induced hyperplasia and hypertrophy of VSMC derived from normotensive and hypertensive rats, and in vivo upon intimal thickening induced by carotid artery balloon injury in the rat.

The renin-angiotensin system and vascular hypertrophy

In addition to its vasoconstrictor and aldosterone-stimulating action, angiotensin II also drives cell growth and replication in the cardiovascular system, which may result in myocardial hypertrophy and hypertrophy or hyperplasia of conduit and resistance vessels in certain subjects. These actions are mediated through angiotensin II receptors (subtype AT1), which activate the G protein, phospholipase C, diacylglycerol and inositol trisphosphate pathway, to increase the expression of certain protooncogenes (c-fos, c-myc and c-jun) and growth factors (platelet-derived growth factor-A-chain, transforming growth factor-beta 1 and basic fibroblast growth factor). The cellular responses to angiotensin II in vascular smooth muscle have been shown in different hypertensive vessels to be either hypertrophy alone, hypertrophy and DNA synthesis without cell division (polyploidy) or DNA synthesis with cell division (hyperplasia). In genetic hypertension, the altered structure of small arteries is due to either cellular hyperplasia or remodeling, whereas in renovascular hypertension there is hypertrophy of vascular smooth muscle cells. Angiotensin II also increases synthesis of some matrix components, activates blood monocytes and is thrombogenic. Angiotensin-converting enzyme (ACE) inhibitors prevent or reverse vascular hypertrophy in animal models of hypertension; this seems to be a class effect, shared to some extent with calcium channel blocking agents. In human hypertension, ACE inhibitors reduce the increased media/lumen ratio of large and small arteries in hypertension and increase arterial compliance. These properties are also shared by losartan, the first of the new class of angiotensin II receptor (AT1) antagonists. The clinical implications of these findings need to be tested through rigorous and prospective clinical trials.

Smooth muscle DNA replication in response to angiotensin II is regulated differently in the neointima and media at different times after balloon injury in the rat carotid artery. Role of AT1 receptor expression

We have reported that angiotensin II (Ang II) infusion to rats during the third and fourth weeks after vascular injury stimulates DNA replication in a larger proportion of smooth muscle cells (SMCs) in the arterial neointima than in the underlying media or the normal arterial media. Whether this increased responsiveness to Ang II is a transient or stable property of neointimal cells after vascular injury remained unclear. The present study examined smooth muscle DNA replication in response to Ang II infusion (250 ng.kg-1.min-1 for 2 weeks) at 3 to 4, 9 to 10, or 27 to 28 weeks after balloon injury to the rat carotid artery. Control rats received Ringer's lactate. BrdU (0.8 mg.kg-1.d-1) was coinfused to label replicating DNA. The increased replicative response to Ang II in the neointima versus the normal arterial media did not persist beyond the period of rapid lesion growth shortly after injury, even in neointimal areas without endothelial regeneration. By 9 to 10 weeks after injury, replication frequencies were comparable in the neointima and the normal arterial wall. In the presence of a regenerated endothelium, neointimal DNA replication was lowered but not abolished. After the early period, however, the most marked difference may be the loss of ability of medial SMCs to respond mitogenically to systemic Ang II. As a consequence, Ang II-induced DNA replication in injured arteries was greater in the neointima than in the underlying media at all times studied after injury. DNA replication levels correlated with AT1 receptor levels in the injured artery neointima but not media, as shown by receptor binding in vascular sections at 3 and 10 weeks after injury. The growth response to systemic Ang II is differentially regulated in adjacent smooth muscle layers in the injured arterial wall in vivo via mechanisms that include, but are not restricted to, the regulation of AT1 receptor expression in SMCs.

Insulin and insulin-like growth factors induce expression of angiotensin type-2 receptor in vascular-smooth-muscle cells

Angiotensin type-2 receptor (AT2) is abundant in fetal tissues, including aorta, and its expression level declines after birth. In the present study, the regulation of its expression was studied in cultured vascular-smooth-muscle cells (VSMC). The maximum number of binding sites of AT2 increased in VSMC after they were cultured without serum in the presence of insulin, which was essential for its expression. AT2 expression was inhibited by treatment with phorbol ester. Northern blot analyses revealed that insulin-dependent expression is due to elevation of mRNA level of AT2. Similar induction was observed when insulin-like growth factor (IGF)-I or IGF-II was used instead of insulin. The study on the dose dependencies of these factors revealed that the induction of AT2 expression was mediated through the activation of IGF-I receptors. The insulin-induced expression of AT2 was detected in the aorta of genetically obese (fa/fa) Zucker rats, which reportedly have approximately tenfold-higher plasma concentrations of insulin than their lean littermates. The insulin-dependence seems characteristic of VSMC, because it was not observed for pheochromocytoma cells or adrenal glands. These results suggest that the expression of AT2 is regulated by at least two mechanisms, that is, IGF-I receptor dependent and IGF-I receptor independent, and that the former may play an important role in the expression of AT2 in VSMC.

Angiotensin II-elicited signal transduction via AT1 receptors in endothelial cells

1. Angiotensin II (AII) actions are mediated by two distinct types of receptors: AT1, which includes two subtypes, AT1A and AT1B, and AT2. AII produces vasoconstriction on the vascular wall acting directly on smooth muscle cells via AT1 receptors. AII receptors have recently been demonstrated on endothelial cells. But the pharmacological characteristics of these receptors and the intracellular signal pathways coupled to them remain unclear. 2. The aim of this work was to characterize the AII receptor subtypes in rat aortic endothelial cells (RAEC) in primary culture and to evaluate the signal pathways coupled to these receptors by measuring the activation of phospholipase C (PLC) and phospholipase A2 (PLA2). 3. Labelled AII bound to RAEC in a specific, saturable manner. Scatchard analysis showed a Kd of 1.87 +/- 0.49 nM and a Bmax of 50.2 +/- 10.9 x 10(3) sites per cell. AII was displaced by the AT1-specific antagonist, DuP753 with a Ki of 17.37 +/- 1.49 nM, but not by the AT2 receptor analogues CGP42771B or PD123177. These data were confirmed by the finding of AT1 mRNA in endothelial cells. Analysis of RNA expression by RT-PCR showed the presence of both subtypes, AT1A and AT1B in endothelial cells, whereas smooth muscle cells express only AT1A. 4. The activation of PLC and PLA2 in response to AII was evaluated by measuring inositol phosphate production and arachidonic acid release, respectively. Both were enhanced by AII in a dose-dependent manner, and inhibited by DuP753, but not by PD123177. 5. We conclude that AT1 receptors are expressed by endothelial cells in primary culture and that phospholipase C and phospholipase A2 activated via this receptor.

Localization and quantitation of angiotensin AT1 and AT2 receptors in the pregnant and non-pregnant sheep uterus

The two angiotensin II receptor subtypes, AT1 and AT2, have been reported to be differentially expressed in the myometrial membrane preparations of nulliparous and pregnant sheep, however, their distribution in the sheep reproductive tract has not been reported. The aim of this study is to map the distribution of AT1 and AT2 receptors in the anoestrus reproductive tract of the sheep by quantitative in vitro autoradiography and to investigate if the density and distribution of the receptors change during pregnancy. The AT2 receptor is abundant in a discrete layer in the myometrium of the anoestrus sheep uterus, whilst the AT1 receptor is expressed at lower levels, predominantly in the endometrium. Near-term pregnant uteri, show a marked change in the expression of angiotensin II receptors: the myometrium no longer expresses detectable AT2 receptors but rather, expresses low levels of AT1 receptors. Angiotensin converting enzyme is found in high concentrations in the blood vessels of the pregnant and non-pregnant sheep reproductive system and on the epithelial cells of the fallopian tubes of the non-pregnant sheep. These studies reveal marked reciprocal changes of angiotensin II receptors, with myometrial AT1 receptors increasing during pregnancy, whilst AT2 receptors fall markedly. These changes suggest that angiotensin II may be involved in regulating changes of uterine structure and function during pregnancy by interaction with multiple receptor subtypes.

Multiple growth factors modulate mRNA expression of angiotensin II type-2 receptor in R3T3 cells

Previous studies showed that angiotensin II type-2 receptor (AT2) sites were increased when R3T3 cells were growth arrested and decreased when they were stimulated with fibroblast growth factor or serum. We examined the effects of several other growth factors on the expression of AT2 mRNA to clarify the relation between the AT2 receptor and growth factors. R3T3 cells were cultured in the medium containing 10% FCS until they were confluent and then serum was removed. AT2 mRNA was increased after serum was depleted, and the expression level reached a plateau after 2 days of serum depletion. The presence of serum (10%), fibroblast growth factor (10 ng/mL), or lysophosphatidic acid (1 mumol/L) reduced the AT2 mRNA expression. Phorbol ester (1 to 100 nmol/L) also suppressed the AT2 mRNA expression in a dose-dependent manner. Interleukin-1 beta (1 ng/mL) enhanced the AT2 mRNA expression 1.6-fold and the AT2 receptor number 1.4-fold. Insulin (100 nmol/L) enhanced AT2 mRNA expression 1.4-fold and the AT2 receptor number 1.6-fold. These results suggest that AT2 mRNA expression is modulated by multiple growth factors in both positive and negative directions. The presence of potential cis DNA elements that respond to interleukin-1 beta (CCAAT enhancer binding protein site), insulin [insulin response sequence of phospho(enol)pyruvate carboxykinase gene], and phorbol ester (AP-1 site) in the promoter region of the mouse AT2 gene suggests that the effects of these growth factors and phorbol ester may be mediated via these cis DNA elements.

Angiotensin II-induced responses in vascular smooth muscle cells: inhibition by non-peptide receptor antagonists

The present study investigates the effect of angiotensin II and LR-B/081 (-methyl 2-[[4-butyl-2-methyl-6-oxo-5-[[2'-(1H-tetra-zol-5-yl) [1,1'-biphenyl]-4-yl] methyl]-1(6H)-pyrimidinyl] methyl]-3-thiophenecarboxylate), a novel non-peptide angiotensin II receptor antagonist, on both early and late responses in rat vascular smooth muscle cells. Angiotensin II induced a rapid and transient elevation of inositol trisphosphate intracellular levels, triggered the release of both prostaglandin E2 and prostaglandin I2 (EC50 = 21 +/- 3 and 16 +/- 2 nM, respectively), and, in long-term studies, increased leucine and thymidine incorporation. All angiotensin II effects were antagonized by LR-B/081 and losartan, the reference non-peptide angiotensin AT1-selective receptor antagonist, whereas they were unaffected by PD123177 (1-(4-amino-3-methylphenyl)methyl-5-diphenylacetyl-4,5,6,7-tetr ahy dro-1H- imidazo[4,5-c]pyridine carboxylic acid), a non-peptide angiotensin AT2-selective receptor antagonist. LR-B/081 displayed a much higher potency than losartan in inhibiting angiotensin II-induced prostaglandin E2 (IC50 = 0.15 +/- 0.02 and 39 +/- 9 nM, respectively) and prostaglandin I2 release (IC50 = 0.18 +/- 0.04 and 134 +/- 40 nM, respectively) and was also more potent in blocking the increase in protein synthesis (IC50 = 242 +/- 119 nM and 1221 +/- 687 nM, respectively). Moreover, LR-B/081 and losartan blocked the response to angiotensin III but failed to inhibit the prostaglandin release stimulated by vasopressin or the mitogenic effect of serum. LR-B/081 and losartan were devoid of intrinsic properties in the experimental conditions employed.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression, genomic organization, and transcription of the mouse angiotensin II type 2 receptor gene

Although the rat angiotensin II type 2 receptor (AT2) was cloned and shown to be a member of the seven transmembrane domain-type receptor family, its signaling mechanism and biological roles have not been established. To acquire additional information on the structure and functions of AT2 genomic DNA, we cloned the mouse AT2 gene and examined its expression, transcription, and genomic organization. The amino acid sequence of the mouse AT2 cDNA showed a 98.5% sequence identity with the rat AT2. In mouse fetus, mRNA of the AT2 was highly expressed in the eviscerated carcass and brain. This expression decreased rapidly after birth. In 10-week-old mice, mRNA of the AT2 could be detected in the brain by Northern blot analysis. However, reverse transcription-polymerase chain reaction showed that mRNA of the AT2 was expressed in all organs examined, indicating that the AT2 is expressed at a low level in other organs. Southern blot analysis of the genomic DNA of the mouse liver digested with BamHI, EcoRI, and HindIII resulted in single bands, indicating that the AT2 gene probably exists at a single locus in the mouse genome. The nucleotide sequence of the AT2 gene (4.5 kb of the EcoRI fragment) revealed the presence of three exons. An entire coding sequence was included in the third exon. Primer extension experiments showed the presence of two transcription initiation sites in the mouse AT2 gene. A DNA segment of about 1.5 kb of the promoter region (-1497 to +56 bp) of the mouse AT2 gene was fused to a luciferase reporter gene.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro pharmacological characterization of UP 269-6, a novel nonpeptide angiotensin II receptor antagonist

f1p4in vitro pharmacology of UP 269-6, a novel nonpeptide angiotensin II antagonist, was examined in radioligand binding and functional isolated tissue assays. UP 269-6 bound selectively to AT1 receptors as evidenced by the inhibition of specific [125I] Sar1, Ile8-AII binding in rat adrenal membranes (IC50 = 35.8 nM) and in cultured vascular smooth muscle cells (IC50 = 23.8 nM). UP 269-6 displayed a very high selectivity for the AT1 compared to the AT2 receptor subtype (IC50 > 10,000 nM). UP 269-6 inhibited the AII-induced contraction of isolated rabbit aortic strips. The pattern of AII antagonism suggested competitive antagonism at low concentrations (10(-10), 3 x 10(-10), 10(-9) M) of UP 269-6 and insurmountable antagonism at higher concentrations (3 x 10(-9), 10(-8), 3 x 10(-8) M). Based on the calculated pA2 values, UP 269-6 (9.86 +/- 0.25) was an angiotensin II receptor antagonist as potent as L-158,809 (9.82 +/- 0.37) and much more potent than losartan (7.96 +/- 0.38). UP 269-6 was devoid of affinity (IC50 > 10,000 nM) for many other receptors, ion channels and uptake sites, demonstrating its high specificity for AII receptors. Furthermore, this compound did not affect the contractile response to KCl or phenylephrine in rabbit aorta and exhibited no effect on angiotensin converting enzyme activity. These data demonstrate that UP 269-6 is a highly potent, selective and specific AT1 receptor antagonist.

Cloning, expression and regulation of angiotensin II receptors

Complementary DNAs for angiotensin II type 1 receptor isoforms AT1A and AT1B were cloned by expression cloning from bovine adrenal and rat vascular smooth muscles. Human AT1 receptor was also cloned. Seven transmembrane structures emerged. The AT1 type receptor interacted with more than one type of G-proteins. The ligand binding site of AT1 involving Arg167, Lys199, and Asp263 has been identified by site directed mutagenesis. The regulation of the receptors occur at many stages. The isoform, AT2, was also expression cloned from rat pheochromocytoma cells. Although its ligand binding is not affected by stable GTP analogs, it is a seven transmembrane domain receptor. It mediates the modulations of phosphotyrosine phosphatase by angiotensin II and AT2 specific CGP42112A. The modulation was abolished by pertussis toxin. Thus, AT2 belongs to a new class of angiotensin receptors with unique signalling and regulatory mechanisms. AT1 mediates cellular growth. Interestingly, AT2 expression is inversely related to the mitogenic activity of cells.

The angiotensin type 1 and type 2 receptor families. Siblings or cousins?

The diverse actions of angiotensin II (AngII) are mediated by cell surface receptors. Molecular cloning techniques have identified two distinct subtypes of AngII receptors referred to as AT1 and AT2. It is now well accepted that multiple forms of the AT1 receptor exist, but similar diversity of the AT2 subtype has not been conclusively demonstrated. Nonetheless, several converging lines of evidence do suggest that multiple AT2 receptors may be present in brain and cultured neuron-like cells lines. For instance, some AT2 receptors are regulated by guanine nucleotides and sulfhydryl-reducing agents, whereas others are insensitive. AT2 receptor populations also exhibit differing pharmacological profiles particularly with respect to their affinity for peptidic and non-peptidic ligands. Moreover, a recently developed anti-AT2 polyclonal antisera reveals a unique pattern of immunohistochemical staining in brain and it does not immunoreact with the recently cloned AT2 receptor. Collectively, these results support the hypothesis of multiple AT2 receptors at least within the CNS. Future studies should reveal whether these putative AT2 receptor subtypes result from unique genes or cell-specific post-translational modifications of a single gene product.

Differential gene expression and regulation of angiotensin II receptor subtypes in rat cardiac fibroblasts and cardiomyocytes in culture

Although both rat cardiac nonmyocytes (mostly fibroblasts) and cardiomyocytes have a functional angiotensin II (AngII) receptor, the regulation mechanism of its subtype expression in the rat heart remains unknown. In this study, by using a binding assay and a competitive reverse-transcriptase polymerase chain reaction, we examined the regulation of AngII types 1a and 1b (AT1a-R and AT1b-R) and type 2 receptor (AT2-R) expression in embryonal day 19 (E19) and neonatal (1-d) rat cardiac fibroblasts and cardiomyocytes. The number of AT2-R in E19 fibroblasts was dramatically decreased (from 305 to 41 fmol/mg protein) in 1-d fibroblasts, whereas that of AT1-R and the mRNA levels remained unchanged. The ratio of AT1a-R to AT1b-R mRNA in both E19 and 1-d fibroblasts was 9:1. The number of AT2-R in E19 cardiomyocytes was also significantly decreased (from 178 to 87 fmol/mg protein) in 1-d cardiomyocytes, whereas the magnitude was less prominent compared with that in fibroblasts. AT1-R expression remained unaltered in E19 and 1-d cardiomyocytes. In E19 and 1-d cardiomyocytes, the AT1b-R mRNA level was 1.5-fold higher than that of AT1a-R mRNA. Dexamethasone induced significant increases in AT1a-R mRNA (2.1-fold) and numbers (1.8-fold) without changing the affinity, whereas neither AT1b-R mRNA nor the number of AT2-R was affected by dexamethasone. The AT1a-R gene transcription rate, determined by means of a nuclear run-off assay, was increased (2-fold) by dexamethasone. The half-life of AT1a-R mRNA (18 h) was unchanged by dexamethasone. These data indicate that AngII receptor subtype expression in the rat heart is regulated in a cell- and subtype-specific manner.

Characterization of a new angiotensin antagonist selective for angiotensin-(1-7): evidence that the actions of angiotensin-(1-7) are mediated by specific angiotensin receptors

In this study we describe a new angiotensin antagonist [Asp1-Arg2-Val3-Tyr4-Ile5-His6-D-Ala7, (A-779)] selective for the heptapeptide angiotensin-(1-7) [Ang-(1-7)]. A-779 blocked the antidiuretic effect of Ang-(1-7) in water-loaded rats and the changes in blood pressure produced by Ang-(1-7) microinjection into the dorsal-medial and ventrolateral medulla. In contrast, A-779 did not change the dipsogenic, pressor, or myotropic effects of angiotensin II (Ang II). Also, A-779 did not affect the antidiuretic effect of vasopressin or the contractile effects of angiotensin III, bradykinin, or substance P on the rat ileum. In the rostral ventrolateral medulla, the pressor effect produced by Ang-(1-7) microinjection was completely blocked by A-779 but not by AT1 or AT2 receptor antagonists (DUP 753 and CGP 42112A, respectively). Conversely, the pressor effect produced by Ang II was not changed by A-779 but was completely blocked by DUP 753. Binding studies substantiated these observations: A-779 did not compete significantly for 125I-Ang II binding to adrenocortical membranes at up to a 1 microM concentration. Low affinity binding was also observed in adrenomedullary membranes with an IC50 greater than 10 microM. Our results show that A-779 is a potent and selective antagonist for Ang-(1-7). More importantly, our data indicate that specific angiotensin receptors mediate the central and peripheral actions of Ang-(1-7).