Human renin-dependent hypertension in rats transgenic for human angiotensinogen

Activation of the Human Angiotensin-(1-12)-Chymase Pathway in Rats With Human Angiotensinogen Gene Transcripts

Angiotensin-(1-12) [Ang-(1-12)], an alternate substrate for tissue angiotensin II (Ang II) formation, underscores the importance of alternative renin-independent pathway(s) for the generation of angiotensins. Since renin enzymatic activity is species-specific, a transgenic model of hypertension due to insertion of the human angiotensinogen (AGT) gene in Sprague Dawley rats allowed for characterizing the contribution of a non-renin dependent mechanism for Ang II actions in their blood and heart tissue. With this in mind, we investigated whether TGR(hAGT)L1623 transgenic rats express the human sequence of Ang-(1-12) before and following a 2-week oral therapy with the type I Ang II receptor (AT1-R) antagonist valsartan. Plasma and cardiac expression of angiotensins, plasma renin activity, cardiac angiotensinogen, and chymase protein and the enzymatic activities of chymase, angiotensin converting enzyme (ACE) and ACE2 were determined in TGR(hAGT)L1623 rats given vehicle or valsartan. The antihypertensive effect of valsartan after 14-day treatment was associated with reduced left ventricular wall thickness and augmented plasma concentrations of angiotensin I (Ang I) and Ang II; rat and human concentrations of angiotensinogen or Ang-(1-12) did not change. On the other hand, AT₁-R blockade produced a 55% rise in left ventricular content of human Ang-(1-12) concentration and no changes in rat cardiac Ang-(1-12) levels. Mass-Spectroscopy analysis of left ventricular Ang II content confirmed a >4-fold increase in cardiac Ang II content in transgenic rats given vehicle; a tendency for decreased cardiac Ang II content following valsartan treatment did not achieve statistical significance. Cardiac chymase and ACE2 activities, significantly higher than ACE activity in TGR(hAGT)L1623 rats, were not altered by blockade of AT₁-R. We conclude that this humanized model of angiotensinogen-dependent hypertension expresses the human sequence of Ang-(1-12) in plasma and cardiac tissue and responds to blockade of AT₁-R with further increases in the human form of cardiac Ang-(1-12). Since rat renin has no hydrolytic activity on human angiotensinogen, the study confirms and expands knowledge of the importance of renin-independent mechanisms as a source for Ang II pathological actions.

Cardiac angiotensin-(1-12) expression and systemic hypertension in rats expressing the human angiotensinogen gene

Angiotensin-(1-12) [ANG-(1-12)] is processed into ANG II by chymase in rodent and human heart tissue. Differences in the amino acid sequence of rat and human ANG-(1-12) render the human angiotensinogen (hAGT) protein refractory to cleavage by renin. We used transgenic rats harboring the hAGT gene [TGR(hAGT)L1623] to assess the non-renin-dependent effects of increased hAGT expression on heart function and arterial pressure. Compared with Sprague-Dawley (SD) control rats (n= 11), male homozygous TGR(hAGT)L1623 (n= 9) demonstrated sustained daytime and nighttime hypertension associated with no changes in heart rate but increased heart rate lability. Increased heart weight/tibial length ratio and echocardiographic indexes of cardiac hypertrophy were associated with modest reduction of systolic function in hAGT rats. Robust human ANG-(1-12) immunofluorescence within myocytes of TGR(hAGT)L1623 rats was associated with a fourfold increase in cardiac ANG II content. Chymase enzymatic activity, using the rat or human ANG-(1-12) as a substrate, was not different in the cardiac tissue of SD and hAGT rats. Since both cardiac angiotensin-converting enzyme (ACE) and ACE2 activities were not different among the two strains, the changes in cardiac structure and function, blood pressure, and left ventricular ANG II content might be a product of an increased cardiac expression of ANG II generated through a non-renin-dependent mechanism. The data also underscore the existence in the rat of alternate enzymes capable of acting on hAGT protein. Homozygous transgenic rats expressing the hAGT gene represent a novel tool to investigate the contribution of human relevant renin-independent cardiac ANG II formation and function.

Local renin-angiotensin system and the brain--a continuous quest for knowledge

The ancient renin-angiotensin system (RAS) was discovered more than a hundred years ago by identifying the rate-limiting enzyme of the system and its relevance to blood pressure regulation. Forty years ago, Detlev Ganten et al. postulated the existence of a tissue RAS. In these forty years, he kept developing the knowledge of these systems either directly or by training or attracting the interest of many researchers. Through the present review, we try to highlight recent advancements that originated from the postulation of local brain RAS. Although a large amount of knowledge accumulated, this system continues to intrigue and stimulate the interest and imagination of many researchers.

Intrarenal renin angiotensin system revisited: role of megalin-dependent endocytosis along the proximal nephron

The existence of a local renin angiotensin system (RAS) of the kidney has been established. Angiotensinogen (AGT), renin, angiotensin-converting enzyme (ACE), angiotensin receptors, and high concentrations of luminal angiotensin II have been found in the proximal tubule. Although functional data have documented the relevance of a local RAS, the dualism between biosynthesis and endocytotic uptake of its components and their cellular processing has been incompletely understood. To resolve this, we have selectively analyzed their distribution, endocytosis, transcytosis, and biosynthesis in the proximal tubule. The presence of immunoreactive AGT, restricted to the early proximal tubule, was due to its retrieval from the ultrafiltrate and storage in endosomal and lysosomal compartments. Cellular uptake was demonstrated by autoradiography of radiolabeled AGT and depended on intact endocytosis. AGT was identified as a ligand of the multiple ligand-binding repeats of megalin. AGT biosynthesis was restricted to the proximal straight tubule, revealing substantial AGT mRNA expression. Transgenic AGT overexpression under the control of an endogenous promoter was also restricted to the late proximal tubule. Proximal handling of renin largely followed the patterns of AGT, whereas its local biosynthesis was not significant. Transcytotic transport of AGT in a proximal cell line revealed a 5% recovery rate after 1 h. ACE was expressed along late proximal brush-border membrane, whereas ACE2 was present along the entire segment. Surface expression of ACE and ACE2 differed as a function of endocytosis. Our data on the localization and cellular processing of RAS components provide new aspects of the functional concept of a "self-contained" renal RAS.

Ablation of angiotensin (1-7) receptor Mas in C57Bl/6 mice causes endothelial dysfunction

The Mas gene codes for an angiotensin (1-7) receptor. There is accumulating evidence that Mas is involved in vascular homeostasis. We have recently backcrossed Mas-knockout mice to two different genetic backgrounds, C57Bl/6 and FVB/N. FVB/NMas-deficient mice exhibited elevation in blood pressure (BP) and impaired endothelial function. In the present study, we aimed to address the question whether this phenotype is strain-specific. Therefore, we evaluated endothelial function in C57Bl/6Mas-deficient mice. Similar to FVB/NMas-knockout animals, Mas-deficiency in C57Bl/6 mice leads to endothelial dysfunction evaluated by the acute BP effect of acetylcholine administration. Measurements of nitric oxide (NO) and reactive oxygen species (ROS) and the systems involved in their metabolism revealed an imbalance between these vasoactive factors in C57Bl/6Mas-knockout mice, which may explain the impairment of endothelial function in these animals. However, endothelial dysfunction was less prominent in Mas-deficient mice on a C57Bl/6 background compared to FVB/N. Moreover, C57Bl/6Mas-deficient mice remained normotensive while FVB/N-based animals exhibited elevated BP. The impairment of endothelium-dependent vasodilatory response to acetylcholine in two different mouse strains with Mas deficiency indicates a key role of Mas in endothelial function by its effects on the generation and metabolism of NO and ROS.

Renin increases mesangial cell transforming growth factor-beta1 and matrix proteins through receptor-mediated, angiotensin II-independent mechanisms

Recent evidence suggesting a strong interplay between components of the renin-angiotensin system and key mediators of fibrosis led us to hypothesize that renin, independent of its enzymatic action to enhance angiotensin (Ang) II synthesis, directly increases production of the fibrogenic cytokine transforming growth factor (TGF)-beta. Human or rat mesangial cells (MCs) were treated with human recombinant renin (HrRenin) or rat recombinant renin (RrRenin) and the effects on TGF-beta1, plasminogen activator inhibitor-type 1 (PAI-1), fibronectin (FN) and collagen 1 mRNA and protein were investigated. Blockade of the rat MC renin receptor was achieved using siRNA. HrRenin or RrRenin, at doses shown to be physiologically relevant, induced marked dose- and time-dependent increases in TGF-beta1. These effects were not altered by adding an inhibitor of renin's enzymatic action (RO 42-5892), the Ang II receptor antagonist losartan or the Ang-converting enzyme inhibitor enalapril. RrRenin also induced PAI-1, FN and collagen 1 mRNA and PAI-1 and FN protein in a dose-dependent manner. Neutralizing antibodies to TGF-beta partially blocked these effects. Supernatant and cell lysate Ang I and Ang II levels were extremely low. MC angiotensinogen mRNA was undetectable both with and without added renin. Targeting of the rat renin receptor mRNA with siRNA blocked induction of TGF-beta1. We conclude that renin upregulates MC TGF-beta1 through a receptor-mediated mechanism, independent of Ang II generation or action. Renin-induced increases in TGF-beta1 in turn stimulate increases in PAI-1, FN and collagen I. Thus, renin may contribute to renal fibrotic disease, particularly when therapeutic Ang II blockade elevates plasma renin.

Myocardial adrenergic dysfunction in rats with transgenic, human renin-dependent hypertension

OBJECTIVES

We investigated cardiac function in rats transgenic for the human renin and angiotensinogen genes (TGR) to test the hypothesis that elevated local angiotensin II precipitates adrenergic dysfunction and abnormal contractile function.

METHODS

Hearts from TGR and Sprague-Dawley control rats, aged 6 weeks, were studied using the Langendorff model and papillary muscle preparations (n = 6-10 per group). Incremental isoproterenol (1 - 1000 nmol/l) and external Ca2+-concentrations (0.75-6.0 mmol/l) were tested. Cardiac protein and mRNA expression levels were determined by Western blot and RNAase protection assay.

RESULTS

TGR rats showed left ventricular hypertrophy (54%), higher blood pressures (76 mmHg), and elevated plasma renin activity (seven-fold) compared to controls (P < 0.01). The effect of isoproterenol on TGR rat systolic and diastolic left ventricular performance was decreased in both in-vitro models compared to controls (two- to threefold, P < 0.01). TGR rat papillary muscles showed impaired force generation with abnormal basal and Ca2+-dependent relaxation. Gialpha2 and Gialpha3 protein levels were increased (20-30%) and SERCA2a and adenylyl cyclase protein levels were decreased (23 and 37%, respectively) in TGR hearts compared to controls, while Gsalpha or beta1 and beta2-receptor levels were unchanged. Cardiac angiotensin converting enzyme and atrial natriuretic peptide mRNA levels were increased more than four-fold in TGR with no differences for the angiotensin type1 receptor, beta1-receptor, SERCA2a, phospholamban, adenylyl cyclase V and angiotensinogen genes.

CONCLUSIONS

TGR rat hearts develop severe adrenergic dysfunction with decreased adenylyl cyclase and abnormal intracellular Ca2+-homeostasis. Our findings emphasize angiotensin II as a major risk factor promoting early functional decline in cardiac hypertrophy. The data may have implications for patients with activating polymorphisms of the renin-angiotensin system and support the need for an early therapeutic intervention.

Genetic polymorphisms in the renin-angiotensin system: relevance for susceptibility to cardiovascular disease

The renin-angiotensin system plays an important role in the pathogenesis of cardiovascular disease. Cloning of the human genes coding for the angiotensin-converting enzyme, angiotensinogen, and angiotensin II type 1 receptor has led to the discovery of several polymorphisms, which may be implicated in the pathogenesis of cardiovascular disease. The deletion/insertion (D/I) polymorphism of the angiotensin-converting enzyme gene is associated with hypertension in men, left ventricular hypertrophy in untreated hypertensive patients, various atherosclerotic cardiovascular complications, and microvascular disorders. The M235T polymorphism of the angiotensinogen gene may be associated with a higher risk of hypertension. The A1166C polymorphism of the angiotensin II type 1 receptor gene is probably correlated with hypertension and through an epistatic interaction with the D/I polymorphism of the angiotensin-converting enzyme gene possibly also with coronary heart disease. Several other gene polymorphisms, in particular those in the promoter area of the angiotensinogen gene, have been studied in relation to cardiovascular disease. Based on the insights gained from the reports summarized in this review article, population-based genetic studies of nuclear families are currently being conducted in Belgium and in the People's Republic of China with blood pressure and hypertension as the main outcome variables.

Rats transgenic for human renin and human angiotensinogen as a model for gestational hypertension

Animal models of gestational hypertension are problematic. A novel mouse model was described earlier. The dams in that study were transgenic for human angiotensinogen and the sires for human renin; human renin was expressed in and produced by the placenta. This model was adapted to the rat, which has greater utility in terms of chronic instrumentation and physiologic measurements. Female rats transgenic for human angiotensinogen were mated with rats transgenic for human renin. Telemetry BP increased on day 5 of pregnancy from 110/80 mmHg to as high as 180/140 mmHg, while heart rate increased slightly. The renin transgene was expressed in the placenta, which resulted in increased human plasma renin concentration from 0 to 937 +/- 800 ng angiotensin I ml/h; the values returned to 0 after delivery. Female rats transgenic for human renin that were mated with male rats transgenic for human angiotensinogen in contrast exhibited a decrease in BP. In these rats, human angiotensinogen in plasma remained undetectable. Double transgenic offspring of these transgenic rats developed hypertension and end-organ damage, regardless of the source of the transgenes. The conclusion is that transgenic rats that bear human renin and angiotensinogen genes make an attractive model for gestational hypertension. The rat model will have greater utility than the mouse model.

The molecular basis of hypertension

More than 50 million Americans display blood pressures outside the safe physiological range. Unfortunately for most individuals, the molecular basis of hypertension is unknown, in part because pathological elevations of blood pressure are the result of abnormal expression of multiple genes. This review identifies a number of important blood pressure regulatory genes including their loci in the human, mouse, and rat genome. Phenotypes of gene deletions and overexpression in mice are summarized. More detailed discussion of selected gene products follows, beginning with proteins involved in ion transport, specifically the epithelial sodium channel and sodium proton exchangers. Next, proteins involved in vasodilation/natriuresis are discussed with emphasis on natriuretic peptides, guanylin/uroguanylin, and nitric oxide. The renin angiotensin aldosterone system has an important role antagonizing the vasodilatory cyclic GMP system.

Angiotensinogen concentrations and renin clearance : implications for blood pressure regulation

Renin (REN) requires seconds to convert angiotensinogen (AGT) to angiotensin I. We tested the hypothesis that this long catalytic cycle might indicate an influence of AGT concentrations on REN clearance. We studied 2 transgenic rat (TGR) strains for human (h) AGT; one strain has hAGT values approximately 7-fold higher than the other (68+/-18 versus 10+/-4 microg angiotensin I/mL). hREN (30 000 pg) was bolus-infused into both lines and into nontransgenic controls. The terminal half-life (T1/2beta) was increased (130 versus 82 minutes) and the metabolic clearance rate (MCR) was decreased (0.83+/-0.29 versus 2.2+/-0.66 microL. min(-1). g(-1)) in the high hAGT strain compared with the low hAGT strain. The difference was not related to volume of distribution at steady state. Infused hREN blocked with remikiren resulted in T1/2beta and MCR values that were not different from control values. Infused unblocked and blocked radiolabeled hREN was distributed similarly in the hAGT TGR strains. Infused mouse REN, which cannot convert hAGT, had similar T1/2beta and MCR values in hAGT TGR. Measuring REN with direct radioimmunoassay or by enzyme kinetic assay gave similar results. We next crossed homozygous hAGT TGR from both strains with homozygous hREN TGR. Heterozygous offspring from the low hAGT TGR strain had plasma REN activity, hREN concentration, and rat AGT values that were no different from those of their parents. However, TGR offspring with high hAGT values had massively elevated plasma REN activity and hREN concentration as well as elevated blood pressure, even though both the hREN and rREN genes are downregulated. We conclude that increased AGT concentrations decrease REN MCR and increase REN T1/2beta. The REN-AGT complex may stabilize plasma REN concentration and regulate plasma REN activity independent of renal REN secretion and angiotensin II-mediated feedback. These effects could augment angiotensin I generation and influence blood pressure. The notion that AGT is merely a passive substrate reservoir for REN should be revised.

Angiotensinogen variants and human hypertension

The research on molecular genetics of human hypertension aims to identify the loci involved in the regulation of blood pressure, detect gene variants within the identified loci, associate them with intermediate phenotypes, and ultimately estimate their quantitative effects on blood pressure level and their interaction with main environmental factors. So far, the angiotensinogen (AGT) gene is one of the few candidate genes that has been investigated using these multiple statistical, clinical, and biochemical strategies. A highly polymorphic dinucleotide GT repeat (80% heterozygosity) has been used in several linkage studies. Other diallelic polymorphisms, located in the 5' regulatory region of the gene in intronic and exonic sequences, have been described, which were then used in association studies in different clinical settings. Positive associations between the M235T and the G-6A polymorphisms and plasma angiotensinogen levels indicates a pathway by which the AGT locus could be involved in essential hypertension.

M235T angiotensinogen gene polymorphism and cardiovascular renal risk

OBJECTIVE

In this meta-analysis, we attempted to derive pooled estimates for the putative associations between various cardiovascular-renal disorders and the M235T polymorphism of the angiotensinogen gene.

METHODS

Case-control studies were combined, using the Mantel and Haenszel approach. Joint P values for continuous variables were calculated by Stouffer's method. Continuous measurements reported in different units were expressed on a percentage scale using the intrastudy mean of the MM genotype as denominator.

RESULTS

The computerized database used for this analysis included 69 reports with an overall sample size of 27,906 subjects. Overall, possession of the T allele was associated with an increased risk of hypertension. In comparison with the MM reference group (number of studies, n = 32), the excess risk was 31% (P = 0.001) in TT homozygotes and 11% (P = 0.03) in TM heterozygotes. The sensitivity analysis showed that this association was present only in whites (T allelic frequency, f = 42.2%), but not in blacks (f = 77.0%) or Asians (f = 78.0%). Atherosclerotic complications (n = 12), renal microvascular disorders (n = 13), cardiomyopathy (n = 2) or diabetic retinopathy (n = 3) were not correlated with the M235T polymorphism. Publication bias was observed for hypertension, but not for coronary heart disease, including myocardial infarction, and for microvascular nephropathy. Furthermore, in comparison with the MM control group, the circulating angiotensinogen levels (n = 8) were raised by 11 and 7% (P = 0.01) in TT and TM subjects, respectively. In contrast, plasma levels of the angiotensin I converting enzyme (n = 4) and body mass index (n = 15) were not associated with the T allele.

CONCLUSION

The T allele encoding angiotensinogen is not associated with atherosclerotic or microvascular complications, but in Caucasians behaves as a marker for hypertension. This association, which may have been inflated by publication bias, does not necessarily imply causality.

Transcription of the prorenin gene in normal and diseased breast

The angiotensin II type 1 (AT1) receptor is present in a wide variety of human and animal tissues, and is particularly abundant in epithelial cells. Because of this, and because it is known that tissue renin angiotensin systems (RASs) exist that have specific local functions, we investigated the expression and localisation of components of the RAS in normal and diseased breast tissue. Using a monoclonal antibody to the AT1 receptor, immunocytochemistry confirmed that the AT1 receptor was characteristically distributed in ductal epithelial cells in both normal and malignant tissue, and in most, although not all, cells in invasive tumours. Transcription of prorenin mRNA was studied by in situ hybridisation, using a DIG-ddUTP tail-labelled probe specific for the human prorenin gene. In normal tissue, and in cases of ductal carcinoma in situ, prorenin mRNA was distributed in myoepithelial cells and in a band of connective tissue cells completely surrounding the AT1-containing ductal epithelial cells. This prorenin transcribing tissue was disrupted and attenuated in invasive tumours, and in some of these, prorenin mRNA transcription could not be detected at all. Functions ascribed to the tissue RASs include regulation of mitosis and tissue modelling, as well as fluid and electrolyte transport. The results presented here strongly suggest the possibility that a tissue RAS may also be present in the breast, closely coupled to the provision of angiotensin II to the AT1 receptors in ductal epithelial cells. This mechanism is disrupted in cancer.

Mouse and rat plasma renin concentration and gene expression in (mRen2)27 transgenic rats

The (mRen2)27 transgenic rat [TGR(mRen2)27] is said to have low plasma levels of active renin. We used a direct radioimmunoassay (RIA) for mouse submaxillary renin, as well as an indirect enzyme-kinetic assay based on the generation of angiotensin I with modification of the pH optimum, to measure rat and mouse plasma renin activity (PRA), plasma renin concentration (PRC), and plasma prorenin in TGR before and after lisinopril. The relationship between rat PRC and % rat kidney extract was steepest at pH 6.0 and flat at pH 8.5, whereas the relationship between mouse PRC and purified mouse renin was steepest at pH 8.5 and flat at pH 6.0. Mouse PRC was highly correlated with direct RIA measurements (r = 0.93). PRA before lisinopril was little influenced by pH, whereas the increase with lisinopril was greatest at pH 6.5. PRC before lisinopril was fourfold higher at pH 8.5 compared with that at pH 6.0. Lisinopril increased both PRC values but reversed the pH dependency. Prorenin was fourfold higher at pH 8.5 compared with that at pH 6.0 and decreased slightly with lisinopril. Renal renin concentration was higher at pH 6.0 than at pH 8.5. With lisinopril, renal renin concentration increased at both pH values. Mouse PRC was not changed by lisinopril. Ribonuclease protection assay showed both rat and mouse renin gene expression in the kidney, which increased with lisinopril. Thus TGR have circulating active rat and mouse renin and prorenin. The notion that TGR are a "low renin" model should be revised.

Angiotensin-converting enzyme and angiotensinogen gene polymorphisms and heart rate variability in twins

Decreased heart rate variability (HRV) is associated with congestive heart failure, post-myocardial infarction, ventricular arrhythmias, sudden cardiac death, and advancing age. A deletion/insertion polymorphism in the angiotensin-converting enzyme (ACE) gene and a substitution (M235T) in the angiotensinogen gene have been associated with risk for heart disease. The aim of this study was to determine the heritability of HRV and related parameters in monozygotic and dizygotic twins and to assess the influence of ACE and angiotensinogen polymorphisms. We studied 95 MZ pairs and 46 DZ pairs. We measured HRV and related parameters, ACE and angiotensinogen levels, plasma norepinephrine, ACE, and angiotensinogen genotypes. We found that HRV and related parameters were significantly influenced by genetic variability, although nonshared genetic effects were also important. Angiotensinogen and plasma norepinephrine were generally correlated with decreased HRV, whereas ACE was correlated with perturbances of normal rhythmic HRV. Nevertheless, the DD ACE genotype was associated with increased HRV (p <0.05), whereas angiotensinogen polymorphisms had no effect. We conclude that HRV and related parameters are in part heritable. Interestingly, the DD ACE genotype is associated with increased HRV.

Dose effects of human renin in rats transgenic for human angiotensinogen

We examined the effect of chronic human renin infusion and human renin inhibition on blood pressure in a unique transgenic rat model. We infused incremental doses of human renin (1 to 500 ng/h) with minipumps for 10 days into rats harboring the human angiotensinogen gene [TGR (hAOGEN)1623]. We measured blood pressure and heart rate continuously by telemetry. We found that human renin at 5 ng/h was necessary to increase blood pressure, whereas 10 ng/h caused systolic blood pressure to increase to 215 +/- 13 mm Hg. Heart rate decreased initially but then increased by 100 beats per minute compared with basal values. Drinking behavior also increased. Doses as high as 500 ng/h did not increase blood pressure further. A linear relationship was found between the log of plasma renin activity and systolic blood pressure that increased in slope from days 2 to 9. Rat angiotensinogen levels were low and not influenced by human renin infusion. Human angiotensinogen levels remained stable until 500 ng/h human renin was infused, at which time they decreased by 50% at 9 days. Rat renin gene expression (RNase protection assay) was decreased by human renin infusion, whereas rat and human angiotensinogen gene expressions in liver and kidney as well as angiotensin-converting enzyme gene expression in kidney were not affected. The human renin inhibitor Ro 42-5892 was given by gavage repeatedly to rats receiving human renin at 40 ng/h. Ro 42-5892 lowered blood pressure promptly to basal values. High human renin hypertension in this model is dose dependent, features a steeper relationship between blood pressure and plasma renin activity over time, and is associated with tachycardia and increased drinking. We conclude that the human angiotensinogen transgenic rat offers new perspectives in the study of human renin-induced hypertension.