J Mol Cell Cardiol 34, 1455ÿ1462 (2002)

doi:10.1006/jmcc.2002.2077, available online at http://www.idealibrary.com on1

Cardiac Renin±Angiotensin Aldosterone System

Role of the Local Renin±angiotensinSystem in Cardiac Damage: a MinireviewFocussing on Transgenic Animal ModelsMichael Bader

Max-DelbruÈck-Center for Molecular Medicine (MDC), D-13092 Berlin-Buch, Germany

(Received 30 May 2002, accepted for publication 3 June 2002)

M. BADER. Role of the Local Renin±angiotensin System in Cardiac Damage: a Minireview Focussing on TransgenicAnimal Models. Journal of Molecular and Cellular Cardiology (2002) 34, 1455±1462. The local generation of allcomponents of the renin-angiotensin system (RAS) in the heart has been the basis for the postulation of a tissueRAS in this organ. Since angiotensin II is involved in the induction of cardiac hypertrophy and ®brosis the localgeneration of this peptide may be of highest clinical importance. Several transgenic animal models have beengenerated to evaluate the functional importance of the cardiac RAS. We have established a new hypertensivemouse model lacking local angiotensinogen expression in the heart. In these animals, cardiac weight and collagensynthesis are increased compared to normotensive control mice but to a lesser extent than in mice with equallyenhanced blood pressure but intact cardiac angiotensinogen generation. Thus, we have shown that local synthesisof this protein is involved but not essential in the development of cardiac hypertrophy and ®brosis.

# 2002 Published by Elsevier Science Ltd

Key Words: Transgenic animals; Renin-angiotensin system; Renin; Angiotensinogen; Angiotensin-converting enzyme; Angiotensin receptors; Cardiac ®brosis; Cardiac hypertrophy.

Introduction

The renin±angiotensin system (RAS) was describedfor more than a century as a circulating hormonesystem regulating blood pressure and kidney func-tion. The advent of molecular biology has added anovel aspect to the picture of the RAS namely thelocal production and action of components of thesystem in several organs including brain, adrenalgland, vascular wall, kidney, and heart.1 In par-ticular the interest has focussed on the cardiacRAS,2,3 since genetic as well as pharmacologicstudies have suggested an important role of angio-tensin in the heart: Polymorphisms in the gene forangiotensin-converting enzyme (ACE) were linkedto the risk for cardiac diseases and inhibitors of thisenzyme as well as angiotensin (Ang) receptor

Please address all correspondence to: Michael Bader, Max-DelbruÈ ck-CTel: 49 30 9406-2193; Fax: 49 30 9406-2110; E-mail: mbader@md

0022±2828/02/111455�08 $35.00/0

antagonists turned out to be extremely effectivedrugs for the treatment of heart failure.4±11 Allcomponents of the RAS are synthesized in theheart and, therefore, local production of Ang IIoccurs in this organ. The interplay of this localsystem with the circulating RAS, mechanicalstretch, and the sympathetic nervous system hasrecently been addressed by the generation of trans-genic animal models with targeted alterations inthe expression of RAS components in the heart.The results are consistent with a pivotal functionof the local RAS in the regulation of cardiac hyper-trophy, remodelling, and ®brosis. However, thereare still open questions concerning the autonomyof the RAS in these effects or its dependence onadditional stimuli such as mechanical stretch orgrowth factors.

entrum for Molecular Medicine (MDC), D-13092 Berlin, Germany.c-berlin.de

# 2002 Published by Elsevier Science Ltd

1456 M. Bader

Components of RAS in the Heart

All components of the RAS are expressed in theheart. Angiotensinogen is synthesized in all partsof the heart and in cultured cardiac myocytes and®broblasts.12±16 ACE is also produced in cardiactissue and is mainly localized on ®broblasts and onthe endothelium.17,18 Human heart chymase alsoconverts Ang I to Ang II and may contribute tocardiac Ang-II generation.19

Renin mRNA is present in atria, ventricles, andisolated cardiomyocytes, however, in minuteamounts,14,20,21 and, therefore, the source of car-diac renin has been debated. Circulating renin andprorenin are ef®ciently taken up by the cardiac cir-culation and may be the major source of Ang-Igenerating activity (Fig. 1).22,23 This mechanismwas recently corroborated by a transgenic mousemodel. Human renin derived from the liver ef®cient-ly metabolized human angiotensinogen expressedin the heart of these mice.24 The uptake pathwayneeds mannose-6-phosphate residues on circulat-ing renin and a role of a renin-binding protein inthis process has been suggested.22,25 In addition torenin, other enzymes such as cathepsins or toninthat also liberate angiotensins from angiotensino-gen have been detected in the heart.26±29 However,the ®nding that bilateral nephrectomy, whichcompletely blunts circulating renin also abolishes

Figure 1 The cardiac renin-angiotensin system. Renin isgenerated locally but can also come from circulating sources.be generated by mast-cell derived heart chymase. Ang II®broblasts eliciting growth and ®brosis, respectively. Moreovsympathetic nerve endings and TGFb from cardiac cells thaincrease in local angiotensinogen and AT1 receptor expressio®broblasts. For more details see text. NA, norepinephrine; a,

cardiac Ang II levels argues in favour of kidney-derived renin as a major source of Ang I-generatingactivity at least in the rat heart.30,31

Depending on the species, Ang II receptors of bothsubtypes, AT1 and AT2, are about equally expres-sed in the heart and in isolated cardiomyo-cytes.32±37 Cardiac ®broblasts express only AT1receptors under normal conditions38,39 but canreactivate the AT2 receptor in heart failure.40±42

Thus, all the necessary components to generateAng II are present in the heart and Ang-II synthesiswas recently shown to occur in the cardiacinterstitium.23,43±45

Actions of Ang II in the Heart

Of prime importance are the hypertrophic and pro-®brotic actions of Ang II on cardiomyocytes. Acc-ordingly, models of left ventricular hypertrophy likeSHR, TGR(mREN2)27, or isoproterenol-infused ratsrespond readily to drugs interfering with the RASwith a reduction in left ventricular weight accom-panied by a decreased myocardial ®brosis.46±51 Insome models, these changes were not correlatedwith blood pressure reduction or plasma Ang IIlevels, but instead with a reduction in cardiacAng II concentrations.49±51

primarily taken up from the plasma. angiotensinogen isACE is available in abundance, although Ang II may alsointeracts with AT1 receptors on cardiac myocytes ander, Ang II stimulates the release of norepinephrine from

t further enhance its effects. Mechanical stretch elicits ann and thereby Ang-II generation on cardiac myocytes andb, a- and b-adrenergic receptor; TR, TGFb receptor.

1457Cardiac Renin±angiotensin System

How does Ang II elicit growth in cardiomyocytes?The AT1 receptor mediates most of the hypertro-phic effects of Ang II via several intracellularsignaling pathways (Fig. 1).52±54 After G-proteinactivation the activity of tyrosine kinases are stimu-lated including several members of the mitogen-activated protein (MAP) kinase family and theJAK/STAT pathway. Both pathways ®nally activatetranscription factors like AP1 and the STATs,respectively, which initiate the expression ofgrowth-related genes. Moreover, the phosphoryla-tion of the ribosomal protein S6 and thereby proteinsynthesis is increased.

Cofactors may be essential mediators of Ang IIaction on cardiomyocyte growth. Endothelin hasbeen shown to be released by stretch and Ang II inthe heart and in some models endothelin receptorantagonists block cardiac hypertrophy induced byAng II.55±57 Moreover, release of norepinephrine byAng II from sympathetic nerve endings in the heartmay trigger hypertrophy and RAS activation.58,59

In a transgenic rat model with de®cient angiotensi-nogen expression in the brain60 we have recentlyshown that a reduced central RAS activity is accom-panied by a blunted hypertrophic response of theheart to low-dose Ang-II infusion.61 Consequently,we suggest that enhanced circulating Ang II mayemploy the brain RAS to activate the sympatheticnervous system, which in turn may be pivotal forhypertrophy induction.

The interplay between mechanical stretch andlocal Ang II generation may also be highly import-ant for the development of cardiac hypertrophy.52

Stretch induces the release of Ang II in the myo-cardium as well as from cardiomyocytes in cul-ture.15,62 Ang II in turn increases the expressionof RAS components such as angiotensinogen, renin,ACE, AT1, and AT2 in a positive feed back loop.63±65

Left ventricular hypertrophy induced by aortic liga-tion results in a signi®cant upregulation of angio-tensinogen and renin mRNA in the left ventricle,whereas no correlation existed with plasma reninlevels which were only transiently increased afterthe constriction.66,67

Concomitantly to hypertrophy, most stimuli alsoinduce cardiac ®brosis namely the proliferation ofcardiac ®broblasts and the excessive deposition ofextracellular matrix in the cardiac interstitium.68

The resulting increase in stiffness causes ventriculardysfunction and ®nally heart failure mostly throughdiastolic dysfunction. Therefore, ®brosis is of majorpathophysiological relevance. Ang II is directlyinvolved in the development of cardiac ®brosis.68

Chronic Ang II infusion induces ®brosis and ACEinhibitors as well as AT1 receptor antagonists can

ameliorate ®brosis induced by pressure overload.Activation of the AT1 receptor in ®broblasts againactivates the MAP kinases and the JAK/STAT path-way which induce expression of angiotensinogenand ®brosis-related proteins such as collagens aswell as cell proliferation (Fig. 1).68,69

Transgenic Animal Models

Transgenic animal models have been generated tosolve the question whether mechanical stretch orwhether the cardiac RAS alone are able to inducecardiac hypertrophy independently. Mice lackingangiotensinogen (own unpublished results) orAT1 receptors70,71 develop cardiac hypertrophyafter volume or pressure overload, respectively.Cardiomyocytes isolated from angiotensinogen-knockout mice respond to mechanical stretch byactivating MAP kinases as do control cells. How-ever, in contrast to control cells this effect is notblocked by AT1 antagonists.72 These results indi-cate that there are redundant pathways of growthinduction by stretch in cardiomyocytes circumvent-ing the RAS. However, the default mechanisminvolves Ang II and the AT1 receptor.

Early ®ndings employing AT2 antagonistsshowed antigrowth effects of this receptor.37 Never-theless, recent studies using AT2-knockout micehave shown that this receptor is essential for hyper-trophy induction by pressure overload or Ang-IIinfusion.73,74 This effect may be mediated by areduced phosphorylation of S6. A transgenicmouse overexpressing the AT2 receptor in cardio-myocytes was less susceptible to AT1-mediatedhypertensive and chronotropic actions, comparedto controls.75 However, these mice developed thesame degree of hypertrophy after Ang-IIinfusion.76 Taken together, cardiomyocyte AT2receptors do not cause hypertrophy by their ownbut AT2 receptors in cardiac myocytes or ®broblastsor even in other tissues may be essential for hyper-trophy induction via the AT1 receptor.

Transgenic experiments designed to solve theopposite question, whether or not Ang II alone isable to induce cardiac hypertrophy withoutmechanical stretch gave controversial results.Transgenic rats overexpressing ACE or AT1 pre-dominantly in the heart have been produced.77,78

Despite very high cardiac levels of cardiac transgeneexpression, there were no morphological alterationsunless the heart was pressure overloaded by aorticbanding. This treatment resulted in a signi®cantlyhigher hypertrophic response in both transgenic ratstrains than in control animals. The results support

1458 M. Bader

the important role of Ang II in stretch-inducedhypertrophy but speak against an autonomouseffect. In contrast, mice expressing angiotensino-gen79 or AT180,81 exclusively in the heart remainednormotensive but nevertheless developed cardiachypertrophy. These ®ndings indicate that the localformation of Ang II induces cardiac damage inde-pendent of blood pressure elevation. Nevertheless,transgenic mice expressing directly Ang II82

released from an arti®cial protein in the heart onlydeveloped hypertrophy when the spillover of thepeptide into the circulation induced hypertension.In transgenic lines, which just exhibit increasedcardiac Ang II levels without haemodynamic altera-tions, only cardiac ®brosis is observed. Thus, theissue whether or not Ang II alone can induce car-diac hypertrophy remains controversial.

This transgenic experiment, however, underlinedthe importance of Ang II for the induction of ®brosis.Chimeric mice carrying cardiac cells without AT1receptors surrounded by normal tissue revealed thatthe activation of ®broblasts by Ang II depends onthe interaction of the peptide with neighbouringcardiomyocytes.83 This observation indicates thatcardiomyocytes release a paracrine mitogenic

Figure 2 Cardiac phenotype of mice lacking local angiotensgen expression; (B) Mean arterial pressure (MAP); (C) Relativeratio]. (D) Immunohistochemistry for collagen I to visualize ®ns, not signi®cant; n�4±7. (Modi®ed from 88).

factor, possibly TGF-b84 after Ang-II stimulation.On the other hand, growth-promoting actions ofAng II on pure cardiac ®broblasts in culture havebeen repeatedly demonstrated68 and these cellswhich secrete TGF-b by themselves after Ang-IIstimulation may also play a major role in thetrophic actions of Ang II on cardiomyocytes.85

We were also interested in the role of local angio-tensinogen generation for the induction of hyper-trophy and ®brosis in the heart. Therefore, we havegenerated a new mouse model by breeding angio-tensinogen-de®cient mice (TLM86) with transgenicanimals expressing the rat angiotensinogen geneonly in brain and liver (TGM12387) [Fig. 2(A)].The resulting animals (TLM12388) were hyperten-sive and reached blood pressure levels indistin-guishable from TGM123 [Fig. 2(B)]. In contrast tonormal mice, however, crossbred animals lackeddetectable angiotensinogen expression in the heart.As a consequence, the extent of cardiac hyper-trophy observed in TLM123 was signi®cantlylower than the one in TGM123 with preservedlocal angiotensinogen expression [Fig. 2(C)]. More-over, perivascular and interstitial ®brosis was lesspronounced in TLM123 than in TGM123

inogen synthesis. (A) Breeding schedule and angiotensino-heart weight [heart weight (HW, mg)/body weight (BW, g)brosis. * P , 0.05 vs WT; ** P , 0.01 vs WT; ## P , 0.01;

1459Cardiac Renin±angiotensin System

[Fig. 2(D)]. These animals show that local angioten-sinogen synthesis is important for cardiac damage.However, since the animals lacking cardiac angio-tensinogen expression still develop hypertrophy and®brosis, import of this protein or angiotensin pep-tides from the circulation or other RAS-independentmechanisms are also involved in these processes.

Conclusion

A cardiac RAS exists consisting of locally synthe-sized renin, angiotensinogen, ACE, and AT1 andAT2 receptors. In addition, circulating renin andangiotensinogen and locally synthesized heart chy-mase may contribute to the local Ang-II generatingactivity. The cardiac RAS is involved in the induc-tion of hypertrophy and ®brosis and is therefore ofgreat clinical relevance for hypertension and myo-cardial infarction.

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