The plasma kallikrein-kinin system(KKS), first recognized over 40 yearsago, was originally believed to con-tribute to physiologic hemostasis. Atthe time, factor XII (Hageman factor,FXII) and the related proteinsprekallikrein (PK) and high molecu-lar–weight kininogen (HK) were knownto be essential for efficient surface-acti-vated blood coagulation, as measuredin the activated partial thromboplastintime (APTT) test. Indeed, in this test,autoactivation of FXII in glass tubespromotes thrombin formation.

According to the then-current “con-tact activation” hypothesis, FXII acti-vation on a negatively charged surfacewas thought to initiate hemostasis ina similar manner by a cascade of pro-teolytic reactions that culminate inthrombin formation. This model wasundermined by the failure to identifysuch a physiologically relevant surface,coupled with evidence that individualsdeficient in FXII, PK, or HK are free ofbleeding disorders. In addition, the

recognition that factor XI, whose defi-ciency is associated with bleeding, canbe activated by thrombin provided abypass mechanism that obviated aneed for FXII to activate factor XI.

Only over the last 6 years have alterna-tive explanations for the physiologicrole of the KKS and its assembly andactivation begun to emerge. Rather thanassembling on a negatively charged sur-face such as that used in the APTT test,the proteins of the plasma KKS are nowknown to bind a multiprotein receptorcomplex in the intravascular compart-ment. As shown in Figure 1, HK, the

critical regulator of plasma KKS assem-bly and activation, binds an endothelialcell surface receptor complex containingcytokeratin 1 (CK1), urokinase plas-minogen activator receptor (uPAR), andgC1qR (1–6). Recent studies with cul-tured human umbilical vein endothelialcells indicate that FXII can also bind tothis receptor (7), but that this interac-tion is highly regulated. Plasma concen-trations of HK completely block FXIIbinding to the multiprotein receptorcomplex. Further, FXII binding requiresa 30-fold higher free Zn2+ concentrationthan does HK, which can only be

The Journal of Clinical Investigation | April 2002 | Volume 109 | Number 8 1007

The plasma kallikrein-kinin system counterbalances the renin-angiotensin system

Alvin H. SchmaierUniversity of Michigan, Departments of Internal Medicine and Pathology, 5301 Medical Science Research Building III, 1150 West Medical Center Drive, Ann Arbor, Michigan 48109-0640, USA. Phone: (734) 647-3124; Fax: (734) 647-5669; E-mail: aschmaie@umich.edu.

J. Clin. Invest. 109:1007–1009 (2002). DOI:10.1172/JCI200215490.

CommentarySee related article, pages 1057–1063.

Figure 1Assembly and activation of the plasma KKS on endothe-lial cells. Plasma PK circulates in complex with HK. TheHK•PK complex binds to a multiprotein receptor com-plex that consists of cytokeratin 1 (CK1), urokinase plas-minogen activator receptor (uPAR) and gC1qR. The pro-teins of the HK•PK receptor complex co-localize onendothelial cell membranes. When HK•PK binds toendothelial cells, PK is rapidly converted to kallikrein (K)by the enzyme prolylcarboxypeptidase (PRCP), which isconstitutively active on endothelial cell membranes. Theresulting kallikrein autodigests its receptor, HK, to liber-ate bradykinin (BK), which can liberate tissue plasmino-gen activator (tPA), nitric oxide (NO), and prostacyclin(PGI2) from endothelial cells. Kallikrein also activatesFXII, which binds to the same multiprotein receptorcomplex as HK in its absence. In this revised hypothesisfor assembly and activation of the proteins of the plas-ma KKS, FXII is activated by kallikrein after PK activation.ScuPA, single chain urokinase plasminogen activator.

achieved in a milieu of activatingplatelets or other cells (7). Thus, underphysiologic conditions, HK binds tothis endothelial cell complex but FXII isprevented from doing so.

Mechanisms of KKS activationThis interaction of HK with its endothe-lial cell–surface receptor is key to theregulated activation of PK, which circu-lates in the plasma in a complex withHK. Work with cultured endothelialcells suggests a novel mechanism bywhich HK-bound PK is rapidly convert-ed to kallikrein in a process that is inde-pendent of FXII (8, 9). On culturedendothelial cells and cell matrices, FXIIactivation occurs subsequent to PK acti-vation (9, 10). Activated forms of FXIIthus do not initiate PK activation,although they can feed back to increasethe rate and extent of its activation.Activation of PK — unlike that of astructurally related zymogen, factor XI— can therefore proceed even in theabsence of factor XIIa (11). As shown in

Figure 1, endothelial cell–associatedactive kallikrein then cleaves HK to lib-erate bradykinin. The assembly of theHK•PK complex on endothelial cells isthus predicted to lead to constitutiveproduction of bradykinin, which canthen activate the bradykinin B2 receptor.Activation of this receptor regulates vas-cular tone by stimulating NO forma-tion in endothelial cells (12).

This mechanism for bradykinin pro-duction depends on the activity of anendothelial cell–borne PK activator,whose identity was unknown until quiterecently. We have now reported that theserine protease prolylcarboxypeptidase(PRCP, lysosomal carboxypeptidase,angiotensinase C) represents one suchenzyme (13). Its Km for PK (7 nM) is con-sistent with a physiologic role as a spe-cific PK activator (13), and it is found onendothelial cell membranes and in theendothelial cell–endomembrane systemthat communicates between the cell’slysosomal and membrane compart-ments. Previous investigations by Erdös

and colleagues identified angiotensin II(Km = 0.2 mM) and bradykinin (Km = 1mM) as substrates of the same process-ing enzyme (14). PRCP can activate thebiologically inert angiotensin I or thevasoconstrictor angiotensin II to formangiotensin II1–7, a biologically activepeptide that induces vasodilation bystimulating NO formation (15, 16) (Fig-ure 2). The finding that PRCP also acti-vates PK indicates that it can producetwo biologically active peptides, brady-kinin and angiotensin II1–7, each ofwhich can reduce blood pressure, coun-terbalancing the vasoconstrictive effectsof angiotensin II.

The KKS in thrombosisThe known ability of angiotensin II toinduce plasminogen activator inhibitor1 secretion implicates the renin-angiotensin system (RAS) in promotingthrombosis (17). The recent findingsthat PRCP activates PK (13) and inacti-vates angiotensin II (14) indicate animportant and previously unappreciat-ed interaction between the plasma KKSand the RAS (Figure 2), suggesting thatthese pathways jointly not only regulateblood pressure, but may also influencethrombosis. Two other known interac-tions between the KKS and the RAShave already been documented: plasmakallikrein can activate prorenin to renin(18), while angiotensin-convertingenzyme (kininase II) can convert brady-kinin into the thrombin-inhibitorypeptide bradykinin1–5 and can also con-vert angiotensin I to angiotensin II (19,20). The plasma KKS is therefore pre-dicted to be anticoagulant and profib-rinolytic (8, 16). Indeed, bradykinin is apotent stimulator of NO formation,prostacyclin liberation, and tissue plas-minogen activator release as well as aninhibitor of thrombin (12, 20–22).Moreover, kallikrein is a kineticallyfavorable activator of single-chainurokinase (8). Considering the estab-lished role of bradykinin as a hypoten-sive peptide, it appears that the plasmaKKS serves as a physiologic counterbal-ance to the hypertensive, prothrombot-ic RAS (Figure 2).

Evidence for the physiologicactivities of the KKSTo date there has been a paucity of ani-mal models in which the role of theplasma KKS can be studied. One suchmodel is the bradykinin B2 receptorknockout mouse (BKB2R–/–). Mice with

1008 The Journal of Clinical Investigation | April 2002 | Volume 109 | Number 8

Figure 2The interaction between the plasma KKS and RAS. Plasma kallikrein converts prorenin to renin, andrenin has the ability to convert angiotensinogen to angiotensin I. Angiotensin-converting enzyme(ACE) converts inactive angiotensin I to the vasoconstrictor angiotensin II. Angiotensin II stimulatesplasminogen activator inhibitor 1 (PAI1) release from endothelial cells. At the same time ACEdegrades bradykinin into bradykinin(1–7) (not shown) or bradykinin(1–5), a peptide with thrombininhibitory activity. PRCP is the enzyme that degrades angiotensin II or angiotensin I to the vasodi-lating peptide, angiotensin II(1–7). Angiotensin II(1–7) stimulates NO and PGI2 formation, which poten-tiates the effects of bradykinin. PRCP also has the ability to convert PK to kallikrein. Formed kallikreindigests kininogens to liberate bradykinin, leaving a kinin-free kininogen (HKa) that has anti-prolif-erative and anti-angiogenic properties. Thus, PRCP, the same enzyme that degrades the vasocon-strictor angiotensin II, leads to the increased formation of the vasodilators bradykinin andangiotensin II(1–7). Finally, the resulting bradykinin stimulates tPA, NO, and PGI2 formation, thuscounterbalancing the prothrombotic effect of angiotensin II.

this defect have cardiac hypertrophy,chamber dilatation, and elevated leftventricular end-diastolic pressure, andthey show exaggerated vasopressorresponses to angiotensin II (23). In thepresence of angiotensin II infusion,these animals have increased bloodpressure and reduced renal blood flow(24). They also experience a dimin-ished cardioprotective benefit fromangiotensin I receptor antagonists andangiotensin-converting enzyme in-hibitors, relative to the response ofwild-type mice after ischemia-inducedheart failure (25). This animal modellinks bradykinin with angiotensin II inthe development of cardiac disease,but leaves open the question of theirinterrelationship in the area of throm-bosis, where the expected phenotype isby no means obvious. While it is possi-ble that the BKB2R–/– mouse will proveto be prothrombotic due to impairedtPA, NO, and prostacyclin liberation,the animal could equally be protectedfrom thrombosis as a result of reducedmetabolism of bradykinin.

In this issue of the JCI, Han et al. pres-ent another animal model thataddresses a different prediction of themodel proposed above, namely theidea that bradykinin is continuouslyformed in the intravascular compart-ment. These authors have studied micedeficient in the C1 inhibitor (C1 INH)(26), an inborn error that, in humans,is associated with hereditary angioede-ma (HAE). C1 INH, a member of theserpin family of serine proteinaseinhibitors, is a major inhibitor of C1r,C1s, plasma kallikrein, and factor XIIa.C1INH–/– mice show reduced plasmaC4 levels and low plasma total comple-ment levels from chronic complementactivation as a result of their C1 INHdeficiency. It is of interest that thesemice survive gestation, since homozy-gous humans deficient in C1 INH havenever been described. Both homozy-gous and heterozygous mutant miceexhibit increased vascular permeabilityafter injection of Evans blue dye. Thisphenotype can be corrected by provid-ing exogenous C1 INH.

Crucially, mice doubly deficient inboth C1 INH and the bradykinin B2

receptor are protected from this effect,indicating that the increase in vascularpermeability is mediated by bradykinin(22). These data indicate that brady-kinin is the key mediator of the edemain these animals, and presumably in

HAE patients as well. Since these ani-mals show constitutively increased per-meability, due to liberated bradykinin,plasma kallikrein formation also occurscontinuously in the intravascular com-partment. This interpretation is consis-tent with the mechanism of PK activa-tion shown in Figure 1. The C1INH–/–

mice provide in vivo support for thismodel of intravascular kallikrein for-mation, which was developed based oncell culture studies. There is presently noevidence that C1 INH inhibits endothe-lial surface-bound kallikrein as it does insolution, but it will be important to testthis possibility.

In sum, recent investigations indicatea physiologic basis for assembly andactivation of the plasma KKS. Thesemechanisms for assembly and activa-tion of the plasma KKS suggest that it isthe physiologic counterbalance to theRAS. The interaction of these two sys-tems represents a direct link betweenblood pressure regulation and throm-bosis. The details of this emerging para-digm are likely to be further elucidatedby additional animal models such as theone described here by Han et al. (26).

AcknowledgmentsI appreciate the critical review of thisCommentary by Lilli Petruzzelli andZia Shariat-Madar. This work is sup-ported by NIH grants HL52779,HL57346, and HL65194.

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