Kidney International, Vol. 67 (2005), pp. 21012113PERSPECTIVES IN RENAL MEDICINEPreeclampsia: A renal perspectiveS. ANANTH KARUMANCHI, SHARON E. MAYNARD, ISAAC E. STILLMAN, FRANKLIN H. EPSTEIN,and VIKAS P. SUKHATMERenal Division and Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston,Massachusetts; Department of Obstetrics & Gynecology, Beth Israel Deaconess Medical Center and Harvard Medical School,Boston, Massachusetts; and Department of Pathology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston,MassachusettsPreeclampsia: A renal perspective. Preeclampsia is a syndromethat affects 5% of all pregnancies, producing substantial mater-nal andperinatal morbidityandmortality. Theaimof this reviewis to summarize our current understanding of the pathogenesisof preeclampsia with special emphasis on the recent discoverythat circulating anti-angiogenic proteins of placental origin mayplay an important role in the pathogenesis of proteinuria andhypertension of preeclampsia.Preeclampsia, the syndrome of hypertension and pro-teinuria that heralds the seizures of eclampsia, remainsoneof thegreat mysteries intheeldof obstetrics.Although our understanding of the pathophysiologyof preeclampsiahasincreasedoverthepast 50years,it is quiteincomplete, andmanagement remains sup-portive: close observation, treatment with antihyperten-siveagentsandmagnesiumsulfate, andif progressivesigns and symptoms occur, urgent delivery of the fetus.Preeclampsia is still one of the leading causes of maternaland neonatal mortality in the world.Preeclampsiais characterizedbyaconstellationofsigns andsymptoms, includingthenewonset of hy-pertensionandproteinuriaduringthelasttrimesterofpregnancy, usuallyassociatedwithedemaandhyper-uricemia[1, 2]. It occursonlyinthepresenceof theplacenta, evenwhenthereisnofetus(asinhydatidi-form mole) and remits dramatically postpartum [3]. Theplacentainpreeclampsiaisusuallyabnormal, withev-idenceofhypoperfusionandischemia.Althoughtheseplacental changes are neither universal nor specic forpreeclampsia, there appears to be a correlation betweenKey words: pregnancy, HELLP syndrome, angiogenesis, pseudovascu-logenesis, VEGF, PlGF, soluble Flt-1, soluble VEGFR-1, proteinuria,edema.Received for publication February 9, 2004and in revised for April 23, 2004Updated on November 23, 2004Accepted on January 12, 2005C 2005 by the International Society of Nephrologythe severity of the disease and the extent of placental ab-normalities. The clinical ndings of severe preeclampsiaare unied by the presence of systemic endothelial dys-function and microangiopathy, in which the target organmay be the brain (seizures or eclampsia), the liver [thehemolysis, elevated liver function tests, and low plateletcount(HELLP)syndrome],orthekidney(glomerularendotheliosisandproteinuria). Severepreeclampsiaisalso associated with small for gestational age (SGA) fe-tuses. Because of the strong clinical and experimental ev-idence of early placental involvement and dysfunction ofthematernalendothelium, itiscurrentlybelievedthatpreeclampsia has its origin in disordered vascular devel-opment of the placenta, whichinturnleads towidespreadmaternal vascular endothelial effects [4, 5] (Fig. 1). Thisreviewwill discussmechanismsunderlyingtheclinicalmanifestations of preeclampsia. In addition, we will de-scribe evidence suggesting that placental secretion of anantiangiogenic protein may contribute to the endothelialdysfunction of preeclampsia.RISK FACTORS FOR THE DEVELOPMENT OFPREECLAMPSIAThe risk factors for the development of preeclampsiaare listedinTable 1 [1, 57]. Preeclampsia is morecommon not only in rst pregnancies, but also in multi-gravidas who have a new partner, suggesting that priorexposure to paternal antigens may be protective [8, 9].However, recent evidence from a large Norwegian birthregistry suggests that prolonged interpregnancy interval,rather than primipaternity, accounts for this increase inrisk[10], thoughwhythisoccursisunclear. Althoughpreeclampsia is traditionally not considered to be agenetic disease, it is clear that genetic factors contributeto the susceptibility to preeclampsia [6]. A family history(mother, sister, orboth)ofpreeclampsiaisassociatedwith a fourfold increased risk for preeclampsia [6]. Otherepidemiologic studies suggest that paternal geneticcontributions tothe zygotic genotype inadditionto21012102 Karumanchi et al: Preeclampsia and angiogenic factorsTable1. Risk factors for the development of preeclampsiaFamily history of preeclampsiaNulliparityMultiple gestationMolar pregnanciesOlder maternal ageObesityPreexisting hypertensionChronic renal diseaseDiabetes mellitusThrombotic vascular diseasematernal genes may contribute susceptibility topreeclampsia [11]. Polymorphisms of genes involved inthe regulation of blood pressure or coagulation, such asrenin, angiotensinogen(T235), endothelialnitricoxidesynthase(eNOS), prothrombin, factor VLeiden, andmethyltetrahydrofolate (MTHFR), though promising inearly studies [1215], have not been conrmed in largerstudies [1620]. Genome-wide scanning of Icelandicfamilies revealed a signicant locus on chromosome 2p13[logarithmof theodds(LOD)score4.70] [21]. Morerecently, the 2p locus was conrmed in a study of patientsfrom New Zealand and Australia [22]. Finally, a Dutchstudyreportedlinkageof HELLPsyndrome, but notpreeclampsia, with a locus on 12q, suggesting that geneticfactorsimportantinHELLP(hemolysis,elevatedliverfunctiontests,lowplatelets)syndromemaybedistinctfrom those in preeclampsia [23]. Women with trisomy 13fetuses have a higher incidence of preeclampsia, regard-less of parity, suggesting that a gene on chromosome 13maybeimportantinpreeclampsia[24]. Anactivatingmineralocorticoid receptor mutation was described in arare group of patients who have only pregnancy-inducedhypertension without proteinuria [25]. The incidence ofpreeclampsia is also higher in women who live at highaltitudes and in the third world, suggesting that hypoxiaand/or hithertounknownenvironmental factors mayalso contribute to the development of preeclampsia [26,27].CLINICAL MANIFESTATIONS AND THEIRPATHOPHYSIOLOGIC UNDERPINNINGSHypertensionWhile in normal pregnancy peripheral vascular resis-tance and blood pressure are decreased, in preeclamp-sia these changes are reversed. Increasedperipheralvascular resistance, rather than increased cardiac output,is the chief cause of hypertension [28]. Sympathetic ac-tivation is noted in preeclampsia as it is in other formsof hypertension, aconclusionsupportedbyelectricalrecordingsof sympatheticnerveimpulses[29] andbyreportsofincreasedconcentrationsofcirculatingcate-cholamines [30]. Sympathetic activation may be respon-sible for the increase in cardiac output noted by some inthe early stages of preeclampsia [31]. Preeclampsia is alsonotableforanexaggeratedresponsetoangiotensinII,catecholamines, andother hypertensive stimuli whencomparedtonormal pregnant controls [32, 33]. Onegroup has reported that this response may precede theonset of overt hypertensionbyweekstomonths[34](andingdisputedbyothers[35]), reminiscentoftheexaggeratedresponsetovasoconstrictorsdescribedinnormotensive relatives of patients with essential hyper-tension [36].Althoughtotalplasmavolumehasbeenreportedtobelowinpreeclampsia[37], theremaybeincreasedeffective circulating volume as evidenced by sup-pressed renin and aldosterone [38, 39] and elevated brainnatriuretichormone[40]relativetonormalpregnancy.These ndings are reminiscent of the fall in plasma vol-umeandriseinbloodpressureproducedbyinfusionof vascoconstrictors suggesting that peripheral vasocon-striction, especially of small venules, shifts blood to thearteries and central veins while elevating capillary pres-sure [41]. Blood pressure rises, and renin and aldosteronelevels fall as a secondary phenomenon.Generalized vascular constriction is universallypresent in preeclampsia, at least compared to the physi-ologicvasodilationofnormalpregnancy[28]. Thereissubstantial evidence that this may be due to endothelialdysfunction.Amyriadofmarkersforendothelialacti-vationanddysfunction, includingendothelin, cellularbronectin, plasminogenactivatorinhibitor-1(PAI-1),and von Willebrands factor, are altered in preeclampsia.Womenwithpreeclampsiahaveenhancedresponsive-ness to vasopressors as compared with normal pregnantwomen. Women with a history of preeclampsia exihibitevidence of impaired endothelial-dependent vasore-laxation as measured by brachial artery owmediateddilatationupto3yearsafterdelivery, implyingthesechanges in the maternal endothelium may be more thantransient [42, 43]. Alterations of endothelial functionhavebeennotedinpreeclampticvesselsexaminedinvitro, supportingthehypothesis that endothelial dys-function may underlie the hypertension of preeclampsia[4, 5]. The increased incidence of preeclampsia in womenwith chronic diseases such as diabetes and hypertensionalso suggests some factor in the maternal milieu may alsolendsusceptibilitytopreeclampsia. Inadditiontoin-creased vascular reactivity, the vasoconstriction appearstobemediatedat least inpart byalterationsinlocalconcentrations of several vasoactive molecules, includingthe vasoconstrictors norepinephrine, endothelin, andperhaps thromboxane, and the vasodilators prostacyclinand perhaps nitric oxide.Prostaglandin I2(PGI2) (prostacyclin), a circu-lating vasodilator produced primarily by the en-dothelial and smooth muscle layers of blood vessels, isKarumanchi et al: Preeclampsia and angiogenic factors 2103GeneticfactorsEnvironmentalfactorsOtherPlacentaldysfunctionCirculatingfactor(s)Vascular endothelial dysfunctionHypertension Glomerularendotheliosisproteinuria edemarenal insufficiencyHeadacheCerebral edemaSeizuresLFTsHELLP syndromeFig. 1. Placental dysfunctionandendothelialdysfunction in the pathogenesis of preeclamp-sia. Placental dysfunction, triggered by poorlyunderstoodmechanisms, whichmayincludegenetic, immunologic, and environmental fac-tors, playsanearlyandprimaryroleinthedevelopmentofpreeclampsia. Thediseasedplacenta in turn secretes a factor(s) intothe maternal circulation, causing systemicendothelial cell dysfunction. Most of themanifestations of preeclampsia, includinghypertension, proteinuria (glomerular en-dotheliosis), seizures (cerebral edema and/orvasospasm), and the hemolysis, elevatedliver functiontests, andlowplatelet count(HELLP) syndrome can be attributed to vas-cular and endothelial effects.Fig. 2. Glomerular endotheliosis. (A) Nor-mal human glomerulus (hematoxylin andeosin stain). (B) Human preeclampticglomerulus (hematoxylin and eosin stain). A33-year-oldwomanwithtwingestationandseverepreeclampsiaat 26weeks gestationwithurineprotein/creatinineratioof 26atthe time of biopsy. (C) Electron microscopyof glomerulus of the above patient describedin (B). Note occlusion of capillary lumen cy-toplasm and expansion of the subendothelialspace with some electron-dense material.Podocyte cytoplasms show protein resorptiondropletsandrelativelyintactfootprocesses(original magnication 1500). (D) Controlrat glomerulus (hematoxylin and eosinstain). Note normal cellularity and opencapillary loops. (E) Soluble Flt-1treated rat(hematoxylin and eosin stain). Note occlusionof capillary loops by swollen cytoplasm withminimal increase in cellularity. (F) Electronmicroscopy of sFlt-1treated rat. Note occlu-sion of capillary loops by swollen cytoplasmwithrelativepreservationof podocytefootprocesses (original magnication 2500). Alllight micrographs taken at identical originalmagnication of 40).2104 Karumanchi et al: Preeclampsia and angiogenic factorsincreasedinnormal pregnancy[44]. Inpreeclampsia,prostacyclinproductionislowerthaninnormal preg-nancy well before the onset of hypertension and protein-uria [45, 46]. Endothelial cells incubated with serumfromwomen with preeclampsia (vs. serum from normotensivepregnant women) produce less prostacyclin in vitro, sug-gesting the presence of a circulating factor suppressingprostacyclinsynthesis[47]. Prostacyclinconcentrationsarenormalinpregnantwomenwithchronichyperten-sionbut without preeclampsia[48], providingfurthercircumstantial evidence that decreased prostacyclin is acontributing cause of preeclampsia rather than a conse-quence of hypertension.ThromboxaneA2(TXA2)isapotent vasoconstric-tor synthesized by endothelial cells, activated platelets,macrophages, andother organs. UrinaryexcretionofTXA2 metabolites has been reported to be increased inpreeclampsia by some [49] but not all investigators [46,50]. TXA2 production appears to parallel the severity ofpreeclampsia, being higher in patients with coagulopathyand pronounced platelet activation [50]. Some attemptsto prevent preeclampsia in high risk patients by inhibitingplateletthromboxanesynthesisusingaspirinweresuc-cessful but others found no benet, either in low risk orhigh risk populations [51, 52].Thereissomeevidencethatnitricoxide, avascularsmooth muscle relaxant synthesized by vascular endothe-lium, may also mediate the generalized vasodilation ofnormal pregnancy[53, 54]. Inrodent experiments, ni-tricoxideinhibitionduringpregnancyinduces hyper-tension[5557] andproteinuria[58] andreversesthenorepinephrine and angiotensin II resistance character-istic of pregnancy [59, 60]. Thus, damage to vascular en-dothelium with decreased nitric oxide production mightcontribute to vascular constriction in preeclampsia. Al-thoughsomehumanstudies havereporteddecreasednitric oxide production in preeclampsia [6163], othersreport nitric oxide production to be unchanged [64, 65]or even increased [66, 67]. Unfortunately, studies of ni-tricoxideproductionaredifcultduetoitsextremelyshort circulating half-life andother challenges toitsmeasurement.Endothelin-1, a potent vasoconstrictor that can be re-leasedbyvascularendothelial cellsinresponsetoin-jury [68], has also been implicated in the hypertension ofpreeclampsia. Reports of plasma endothelin concentra-tions in preeclampsia have been mixed, with most [6971]but not all [50] investigators reporting a modest increasein circulating concentrations. The release of endothelinbyculturedendothelial cellsisenhancedbyexposureto plasma from preeclamptic patients, as compared withplasma from women with normal pregnancies [72]. Lim-itedanimaldatasuggestthathypertensioninducedbyendothelin-1 administration during pregnancy producesproteinuria [73].Renal dysfunction, proteinuria, and renal pathologyIn normal pregnancy, glomerular ltration rate (GFR)as measured by inulin clearance and renal plasma ow(para-aminohippurateclearance)increasesby40%to60%during the rst trimester [74, 75], resulting in a fall inserum markers of renal clearance, including blood ureanitrogen (BUN), creatinine, and uric acid. In preeclamp-sia, both GFR and renal plasma ow decrease by 30%to40%comparedwithnormalpregnancyofthesameduration [76, 77]. Rarely, prolonged renal hypoperfusionwithresultingacutetubular necrosiscanoccur inseverepreeclampsia. It is important to note that in preeclamp-sia, BUNand creatinine often remain in the normal rangefor nonpregnant women despite a signicant decrease inGFR from the high level of normal pregnancy.Proteinuria may rarely precede hypertension but usu-allyaccompaniesorfollowsit. Afterpregnancyister-minated, proteinuria commonly disappears within 3 to 8weeks, but occasionally persists for months. The quantityof protein excreted in the urine varies widely from lessthan a gram to 8 to 10 g per day. The urinary sediment isusually bland; red blood cells and cellular casts are rare.Preeclampsia is the leading cause of nephrotic syndromeduring pregnancy. Recent data suggest that a loss of bothsize and charge selectivity of the glomerular barrier con-tribute to the development of albuminuria [77].Preeclampsia is associated with a characteristicglomerular lesion, glomerular capillary endothelio-sis(Fig. 2AtoC) [7880]. Bylight microscopy, theglomeruli are enlarged and the glomerular capillary lu-men is bloodless due to endothelial and mesangial cellswelling and hypertrophy. While these changes may befocally present in other conditions (such as abruptio pla-centae), only in preeclampsia is endotheliosis so promi-nent and widespread. Glomerular cellularity is at mostslightly increased. Mesangial interposition may occur inseverecasesorinthehealingstages. Glomerularvis-ceral epithelial cells (podocytes) usually appear swollenwith periodic acid-Schiff (PAS)-positive hyaline droplets.Immunouorescence may reveal deposition of brinorbrinogenderivativesparticularlyinbiopsiesdonewithin2weekspostpartum[81]. Electronmicroscopyis important toconrmendotheliosis andtheloss ofendothelial fenenstrae. Remarkably, despite heavy pro-teinuria the podocyte foot processes are relatively pre-served[82]. Glomerularsubendothelialandoccasionalmesangial electron-densedepositscanbeseen. Theselikely relate tobrinor relatedbreakdownproducts. Mildglomerular endotheliosis has been noted in up to 50%ofpatientswithpregnancy-inducedhypertensionwith-out proteinuria [83], suggesting that pregnancy-inducedhypertensionmayinsomecases reect anearlier ormilder form of the same syndrome. Glomerular enlarge-ment and endothelial swelling usually disappear within8weeksof delivery, coincidingwithresolutionof theKarumanchi et al: Preeclampsia and angiogenic factors 2105hypertension and proteinuria. Focal segmental glomeru-losclerosis (FSGS) can accompany the generalizedglomerular endotheliosis of preeclampsia in50%or moreof cases [84, 85].EdemaAlthough the presence of edema is not necessary forthediagnosis, suddenweight gain, withedemaof thefeet, hands, andface, is acommonpresentingsymp-tom in preeclampsia [32]. Salt loads are excreted moreslowly than in normal pregnancy [86, 87]. The edema ofpreeclampsiaisunlikethatincongestiveheartfailure,hepaticcirrhosis, andnephroticsyndrome, whereloweffective circulating volume leads to high plasma reninand aldosterone concentrations and secondary renalsodiumretention (underll edema) [88]. Instead, reninand aldosterone concentrations are suppressed relativetonormalpregnancy[39]. Theedemaofpreeclampsiathusresemblestheoverlledemaofacuteglomeru-lonephritisorofacuteischemicrenal failureinwhichGFR is decreased out of proportion to the drop in renalplasma ow and in which glomerular-tubular imbalancehas been invoked as a cause of salt retention.Decreased GFR, increased capillary permeability, andhypoalbuminemia may contribute to the edema for-mation. Albumin-bound Evans blue dye disappearsmore quickly from the intravascular space in preeclamp-tic women compared with normal pregnant women,suggesting endothelial permeability is increased[89].However, this phenomenon is also seen in non-pregnantpatients with heart failure and the nephrotic syndrome[90, 91]. Hypoalbuminemiais commonandmaycon-tribute to edema. However, correction of hypoalbumine-mia withalbumininfusions inpreeclampsia patientsusually does not produce a diuresis, suggesting that thehypoalbuminemia alone is not responsible for the edema[92].HyperuricemiaTheassociationbetweenpreeclampsiaandelevatedserumuric acidwas rst notedmore than80 yearsago [93]. Serum uric acid levels are usually elevated inpreeclampsia, and the degree of uric acid elevation hasbeencorrelatedwiththeseverityof proteinuria[94], renalpathologic changes [83], maternal morbidity [95], and fe-tal demise [96]. Often, the elevation of uric acid precedesthe onset of proteinuria and fall in GFR [97]. Althoughit has been proposed that uric acid generation may be in-creased in preeclampsia as a result of tissue ischemia [98],most evidence suggests that decreased renal clearance isthe more important mechanism[99]. Similar decreases inuric acid clearance have been noted by infusions of vaso-constrictorsinhumans[100]. Ithasrecentlybeensug-gested that hyperuricemia may also directly contributeto vascular damage and hypertension [101, 102].Coagulopathy and HELLP syndromePreeclampsia is sometimes complicated by consump-tive coagulopathy and thrombotic microangiopathy cul-minatingintheHELLPsyndrome. HELLPsyndromedevelops in approximately 10% to 20% of women withsevere preeclampsia. The tendency of blood to coagulateis increased in normal pregnancy at least in part becauseof changes in the circulating concentrations of coagulantfactors. In the indolent microangiopathy of preeclamp-sia, itismainlythefactorsthataresynthesizedinthevascular endothelium that are abnormal. These includeprostacyclins(PGI2), vonWillebrandfactor, thrombo-modulin, cellular bronectin, and PAI-1 [103106]. Eveninthe absence of overt coagulopathy, serumandurine b-rin degradation products are increased, implicating sub-clinical activation of the coagulation cascade.Thesechangesarenot simplyepiphenomenainre-sponse to hypertension. Plasma concentrations of cellu-lar bronectin may be increased weeks before the onsetof hypertension [107]. Exposure of cultured endothelialcellstoserumfromwomenwithpreeclampsiatriggersincreasedcellularlbronectin[108, 109]andthrombo-modulin[110]releasecomparedwithserumfromnor-motensive pregnant women, again suggesting that afactor in preeclamptic serumis directly activatingendothelial cells. It is interesting tonote that otherendothelial disorders, including thrombotic thrombocy-topenic purpura/hemolytic uremic syndrome, vasculitis,and disseminated intravascular coagulation can producesimilaralterationsinmarkersofendothelialactivation[111113]. There is also evidence that platelet activationis present in preeclampsia as a consequence of endothe-lial damage. Markers of platelet activation such as betathromboglobulin have been found to be elevated prior tothe onset of clinical disease [114]. Furthermore, adhesionmolecules such as soluble P-selectin, soluble E-selectin,andsoluble vascular cell adhesionmolecule-1 (VCAM-1)that reect endothelial, platelet, and leukocyte activationhave also been reported to be elevated in preeclampsia[115]. The severe thrombocytopenia that is occasionallyseenislikelyduetoconsumptionduringintravascularcoagulation.Neurologic abnormalitiesPreeclampsia is occasionally complicated by the devel-opment of seizures (eclampsia). Other neurologic symp-tomsincludeheadache, blurredvision, andtemporaryloss of vision. The neurologic changes in eclampsia andpreeclampsia have beenattributedtocerebral edema andvasoconstriction. Brainedemabymagneticresonance2106 Karumanchi et al: Preeclampsia and angiogenic factors(MR) imaging and abnormalities in circulating endothe-lial markers correlatecloselywiththeoccurrenceofeclampsia, suggesting that endothelial damage and dis-ruption of the blood-brain barrier may be an underlyingmechanism [116]. The cerebral edema of eclampsia in-volves predominantly the posterior portions of the whitematter, andhas beenreferredtoas reversibleposte-riorleukoencephalopathysyndrome(RPLS)[117]. In-terestingly,patientswiththromboticthrombocytopenicpurpura (TTP), another disorder with microangiopathyalsohavefeaturesofRPLSonMRimagingexamina-tions[118], asdosomepatientswithhypertensiveen-cephalopathy.ABNORMAL PLACENTATION INPREECLAMPSIAIt was 1939, when E.W. Page rst proposed thatplacentaplaysacentral roleinpreeclampsia[3]. Pla-centas from severe preeclamptic pregnancies classicallyhavenumerous infarcts, sclerotic narrowingof arter-ies and arterioles, and brin deposition and thrombosis.Physiologicstudieshaveconrmedthatuteroplacentalbloodowis diminishedanduterine vascular resistance isincreased in preeclamptic women. In addition, placentalischemia induced by mechanical constriction of the uter-ine arteries or aorta produces hypertension, proteinuria,and, variably, glomerularendotheliosis, inavarietyofspecies [119121]. Placental ischemia may be contributedto by a decrease in placental production of nitric oxide[122]. However, placental ischemia alone, as in intrauter-ine growth restriction, does not appear to be sufcientto produce preeclampsia. Thus, although uteroplacentalischemia is an important trigger of preeclampsia, the re-sponsetothisischemiaeitherattheplacentalorthematernal systemic levelmust be variable.Evidence suggests that placental ischemia inpreeclampsia occurs as a result of defective placen-tal vascular remodeling. Early in normal placentaldevelopment, cytotrophoblast cells, fetal in origin, attachtotheuterinedeciduasviaanchoringvilli(seeFig.3).Floating villi, containing fetal vessels which participate innutrient exchange, are bathed in maternal blood whichlls the intravillous space via uterine spiral arteries.Duringplacentalvasculogenesis,asmallpercentageofcytotrophoblasts intheanchoringvilli breakthroughthe syncytiumand migrate into the endometrium.These extravillous trophoblasts invade the uterinespiral arteries of the myometrium, a process that peaksat about 12weeks gestation. By18to20weeks, theendometrial and supercial myometrial segments ofthespiralarteriesarelinedbycellsofcytotrophoblastorigin,withtransformationofthesevesselsfromsmallresistance vessels to accid, high-caliber capacitancevessels[123, 124]. Thisdramatictransformationallowstheincreaseinplacental bloodowneededtosustainthe fetus through the pregnancy.The characteristic pathologic placental lesion in severepreeclampsia is diminished endovascular invasion by cy-totrophoblasts and failure of uterine spiral arteriolar re-modeling [123, 125]. Cytotrophoblast invasion is limitedto the proximal decidua, and the myometrial segmentsof the spiral arteries remain narrow and undilated [126],resulting in uterine hypoperfusion. Zhou et al [127, 128]have shownthat invasive cytotrophoblasts down-regulatetheexpressionofadhesionmoleculescharacteristicoftheirepithelial cell originandadoptacell-surfacead-hesion phenotype typical of endothelial cells, a processreferred to as pseudovasculogenesis. They hypothesizedand conrmed that in preeclampsia, cytotrophoblast cellsfail toundergothisswitchingof cell-surfaceintegrinsandadhesionmolecules[129]. Thisworksuggeststhatcytotrophoblast differentiationis abnormal inseverepreeclampsia, and may be an early defect that eventuallyleads to placental ischemia. Others have demonstratedthat hypoxia-inducible factor-1 (HIF-1) is up-regulatedinpreeclampsia and have suggested that HIF-1 target genessuch as transforming growth factor-beta 3 (TGF-b3) mayblock the cytotrophoblast invasion [130132]. More re-cently, heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) has been demonstrated tobe decreased in preeclamptic cytotrophoblasts; however,its role in cytotrophoblast differentiation and invasion isunclear [133]. Interestingly, uteroplacental ischemia pro-duced in monkeys by aortic constriction late in gestationdoes not produce the defective placental cytotrophoblastinvasionseeninpreeclampsia, thoughit is associatedwithproteinuria and hypertension [134].THE PLACENTAL FACTOR IN PREECLAMPSIAAll of the clinical manifestations of preeclampsia canbe attributed to endothelial cell dysfunction leading toend-organdamageandhypoperfusion. Basedonthisobservation, it hasbeensuggestedthat theremaybea circulating factor, likely placental inorigin, whichaffects systemic endothelial endothelial cell functionandleadstotheclinicalsyndromeofpreeclampsia[4,135]. Intenseinvestigationhas focusedonthesearchfor a placental factor which might induce the maternalsyndrome.Althoughmanycandidatefactors,includingtumor necrosis factor-a (TNF-a), interleukin (IL)-6,IL-1a, IL-1b, Fas ligand, neurokinin B, and asymmetricdimethy L-arginine (ADMA) have been suggested, noneso far has been proven to be etiologic [1, 2, 5, 136, 137].Recent data suggest that increased syncytiotrophoblastsheddingasaconsequenceofplacentalapoptosismaycontributetoendothelial dysfunction[138140]; how-ever, there is no in vivo evidence so far that circulatingKarumanchi et al: Preeclampsia and angiogenic factors 2107Placenta Anchoring villuscytotrophoblastcolumnDecidua MyometriumCytotrophoblaststem cellsCytotrophoblastCytotrophoblast EndovascularcytotrophoblastMaternalendothelialcellsSyncytiotrophoblastMaternal bloodFloating villusBlood flow SpiralarteryTunica mediasmooth musclePlacenta Anchoring villuscytotrophobastcolumnDecidua MyometriumCytotrophoblaststem cellsCytotrophoblastCytotrophoblastMaternalendothelial cellsSyncytiotrophoblastMaternal bloodFloating villusFetal bloodvesselSpiral arteryTunica mediavascular smoothmuscle layerLessbloodflowFig. 3. Abnormal placentation in preeclamp-sia. Exchange of oxygen, nutrients, and wasteproductsbetweenthefetusandthemotherdepends on adequate placental perfusion bymaternal vessels. In normal placental devel-opment, invasive cytotrophoblasts of fetalorigin invade the maternal spiral arteries,transformingthemfromsmall-caliberresis-tance vessels to high-caliber capacitance ves-sels capable of providing placental perfusionadequate to sustain the growing fetus. Duringthe process of vascular invasion, the cytotro-phoblasts differentiate fromanepithelial phe-notype to an endothelial phenotype, a processreferred to as pseudovasculogenesis (upperpanel). In preeclampsia, cytotrophoblasts failto adopt aninvasive endothelial phenotype.Instead, invasion of the spiral arteries is shal-low and they remain small caliber, resistancevessels (lower panel). This may result in theplacental ischemia.VEGFPIGFFLT-1sFLT-1HealthyplacentaPreeclampsiaplacentaBlood vesselBlood vesselAnticoagulant andvasodilatory factorsProcoagulant andvasoconstricting factorsHealthy endothelial cellDysfunctional endothelial cellFig. 4. Soluble Flt-1 (sFlt-1) causes endothelial dysfunction by antagonizing vascular endothelial growth factor (VEGF) and placental growthfactor (PlGF). There is mounting evidence that VEGF, and possibly PlGF, are required to maintain endothelial health in several tissues includingthe kidney and perhaps the placenta. In normal pregnancy, the placenta produces modest concentrations of VEGF, PlGF, and soluble Flt-1. Inpreeclampsia, excess placental soluble Flt-1 binds circulating VEGF and PlGF and prevents their interaction with endothelial cell-surface receptors.This results in endothelial cell dysfunction, including decreased prostacyclin, nitric oxide production and release of procoagulant proteins such asvon Willebrand factor, endothelin, cellular bronectin, and thrombomodulin.2108 Karumanchi et al: Preeclampsia and angiogenic factorsplacental debris induces preeclampsia-like features. Re-cently, preeclampsia-like features were notedinp57kip2(acell-cycle inhibitor) knockout mice, however, the mech-anisms responsible for this phenotype are unclear [141].Some [142], but not all [143], investigators have foundmarkers of oxidativestress tobeelevatedinwomenwith preeclampsia, suggesting that generation of reactiveoxygen-free radicals may contribute to endothelial dys-function [2]. Decreased intake of the antioxidant vitaminCandlowcirculatingascorbicacidconcentrationsareassociated with an increased risk of preeclampsia [144].A small randomized controlled trial found that antiox-idant therapy decreased the incidence of preeclampsia,suggestingthatoxidativestressmaybeacause,ratherthan a consequence, of the syndrome [145]. Additionalstudies are needed to test this hypothesis.As noted earlier, increased sensitivity to the vasopres-soreffectsof angiotensinisawell-establishedfeatureof preeclampsia. Two recent advances have illuminatedpossible mechanisms for this phenomenon. AbdAllaet al [146] reported increased angiotensin-1/bradykininB2 receptor heterodimers in women with preeclampsia,andshowedthat suchheterodimers canresult inen-hanced angiotensin II responsiveness [146]. In a similarvein, Wallukat et al [147] noted increased concentrationsof agonistic antibodies to the angiotensin II type 1 (AT-1) receptor in women with preeclampsia. These autoan-tibodies may induce the production of reactive oxygenspecies and block cytotrophoblast invasion in vitro [148,149], thus accounting for several of the clinical features ofpreeclampsia. Further work is needed to establish if thesealterations are etiologic or epiphenomenona that mightbe encountered in other examples of microangiopathy.Recently, gene expression proling has been used tosearch for candidate factors produced by the placenta inpreeclampsia. Using this approach, we found that placen-tal sFlt-1 mRNA (soluble fms-like tyrosine kinase-1) isup-regulated in preeclampsia [150]. sFlt-1 is a splice vari-ant of the vascular endothelial growth factor (VEGF))receptor Flt-1, lacking the transmembrane andcyto-plasmicdomains.sFltismadeinlargeamountsbytheplacentaandisreleasedintothematernal circulation[151153]. sFlt-1acts as apotent VEGFandplacen-tal growthfactor (PlGF) antagonist bybindingthesemolecules in the circulation [154]. Circulating sFlt-1 con-centrations are increasedinwomenwithestablishedpreeclampsia [150, 155157]. Consistent with the antag-onisticeffect of sFlt-1, free(orunbound)VEGFandfree (or unbound) PlGF concentrations are decreased inpreeclampticwomenatdiseasepresentationandevenbefore the onset of clinical symptoms [150, 158, 159] (seeFig. 4). When administered to pregnant and nonpregnantrats, sFlt-1producesasyndromeofhypertension, pro-teinuria, andglomerularendotheliosisthatmimicsthehuman syndrome of preeclampsia (Fig. 2D to F) [150].Recently, our laboratory as well as others have observedthat there is a marked rise in circulating sFlt-1 concen-tration beginning about 5 to 6 weeks before the onset ofclinical preeclampsia, accompanied by decreases in thecirculating free PlGF and VEGF [160, 161]. Moreover,alterations in these circulating angiogenic proteins corre-lated with disease severity, earlier onset of preeclampsia,andthe birthof aninfant small for gestational age. Finally,wehavealsorecentlyreportedthatdecreasedurinaryconcentrationsof freePlGFduringmidgestationpre-dict the subsequent development of clinical preeclampsia[162].Thesedatalendfurthersupporttothehypothe-sis that circulating sFlt-1 may have a pathogenic role inpreeclampsia.ThepossibilitythatantagonismofVEGFandPlGFmight play a role in preeclampsia has sound physiologicunderpinnings (Fig. 4). Inadditiontobeingapotentpromoter of angiogenesis, VEGFis knowntoinducenitricoxideandvasodilatoryprostacyclinsinendothe-lial cells, decreasingvasculartoneandbloodpressure[163]. There is evidence from animal models that VEGFis important in maintaining glomerular endothelial cellhealthandhealing [164, 165], andits absence induces pro-teinuria and glomerular endotheliosis [166, 167]. In an-tiangiogenic oncology trials, antagonism of VEGF usingneutralizingantibodiesandVEGFreceptorinhibitorscanproduceheadaches,hypertension,proteinuria,andcoagulopathy in human subjects [168170]. Furthermore,exogenous VEGF and PlGF can reverse the antiangio-genic properties of preeclamptic serum, as assessed by invitro angiogenesis assays [150]. Thus, the antiangiogeniceffects of sFlt-1 may account for many of the manifes-tations of preeclampsia, including the unique glomeru-lareffects. Sometissuestargetedinpreeclampsia(i.e.,renal glomeruli, hepatic sinusoids) have fenestrated en-dothelia, which are necessary for physiologic diffusion ofwater and solutes. It has been shown that VEGF, whichis expressed constitutively in these organs, is necessaryto induce and maintain the health of the fenestrated en-dothelium [167, 171]. Thus, it is intuitive that tissues withfenestratedendothelium,especiallytherenalglomeru-lus, might be particularly susceptible to damage by sFlt-1mediated VEGF blockade.How placental dysfunction is related to placental sFlt-1production,andwhyplacentalperfusionisderangedinpreeclampsiaremainsunknown. Recent datausinginvitroprimarycytotrophoblast cultures suggest thatplacental hypoxiamayplayanimportant roleinup-regulating sFlt-1 production [172]. It remains to be shownif sFlt-1 can induce other features of preeclampsia suchas the HELLP syndrome or if additional synergistic fac-tors are needed. Additionally, the Flt-1 locus at 13q12 hasnot been localized within the genomic region identiedby genetic linkage studies for preeclampsia. However, iftheFlt-1locuscontributedtopreeclampsia, onecouldKarumanchi et al: Preeclampsia and angiogenic factors 2109hypothesizethatpatientswhocarrytrisomy13fetusesshould have a 50% increased risk of preeclampsia. Twocase-controlstudiesdoneseveralyearsagodidinfactdemonstrate higher incidence of preeclampsia in moth-ers who carry trisomy 13 fetuses as compared to other tri-somies or control pregnant patients [24, 173]. AlthoughsFlt-1 levels were not measured in the patients with tri-somy13, thesedataareconsistent withthehypothesis thatelevated production of sFlt-1 may lead to preeclampsia.IMPACT OF PREECLAMPSIA ON LONG-TERMMATERNAL HEALTHWomen with hypertensive disorders of pregnancy haveanincreasedlong-termincidenceof dyslipidemias, insulinresistance, and cardiovascular diseases [174176]. Sibai,el-Nazer, andGonzalez-Ruiz [177] reportedan18-foldin-crease in the incidence of chronic salt-sensitive hyperten-sion in women with a history of preeclampsia. It has beenspeculated that this late-onset hypertension may play arole in the eightfold increase in cardiovascular mortalitythat is also observed in these patients [176]. However, it isdifcult to determine causality, because many of the riskfactors for preeclampsia (suchas diabetes mellitus, hyper-tension, and renal insufciency) predispose to hyperten-sion and cardiovascular disease as well. It has also beensuggested that subtle renal injury caused by preeclampsiacan eventually lead to the development of chronic hyper-tension[178, 179]. It is temptingtospeculatethat thelong-termcardiovascular complications notedinsome patientswho have had preeclampsia may be due to a chronic an-tiangiogenic state resulting frompolymorphisms in genessuch as sFlt-1. Additionally, patients with preeclampsiaare saidtohave a decreasedlong-termincidence of malig-nancy [180], a provocative observation disputed by some[181], that may suggest that the antiangiogenic state ofpreeclampsia may reect a permanent state of the mater-nal milieu.CONCLUSIONPreeclampsia is a systemic disorder characterized bywidespread endothelial dysfunction. Substantial animaland human experimental evidence fullling Kochs pos-tulates [182] supports the hypothesis that placental sFlt1,an antiangiogenic protein, may be etiologic. Several im-portantquestionsremaintobeexplored,andmanyofthesecannowbeapproachedexperimentally. Whydocertain conditions (see Table 1) predispose to preeclamp-sia? Are there additional synergistic factors which mightinduce/predispose to HELLP syndrome? What is the re-lationshipofsFlt-1withotherpostulatedmediatorsofthe maternal syndrome such as hyperuricemia, ADMA,or antibodies toangiotensinII AT-1receptor?Whatgoverns regulationof sFlt1transcriptionandsplicing,and why is it abnormal in preeclampsia? Is placental is-chemiaacauseoraconsequenceofexcessiveplacen-tal sFlt1? Large genetic trials such as GOPEC (Geneticsof Preeclampsia) [6], ongoing proteomic/genomic stud-ies of tissueandbloodspecimens frompatients withpreeclampsia, and further characterization of the sFlt-1induced animal model of preeclampsia may help to an-swer these questions.Theimplications of theseadvances onour clinicalmanagement of preeclampsia may be profound. Large,prospective studies are needed to describe in detailthechanges insFlt1, VEGF, andPlGFthat precedepreeclampsiaonset; measurement of thesefactors inpregnant women may prove to be useful diagnosti-cally. Pharmacologic intervention aimed at restoringsFlt1/PlGF/VEGFbalancemaypreventormodifythecourse of preeclampsia. Safely prolonging pregnancybyonlyafewweeks couldsubstantiallyreducema-ternal andneonatal morbidityandmortalityresultingfrompreeclampsia, the worlds most common glomerulardisease.ACKNOWLEDGMENTSWewouldliketothankBenSachsandKee-HakLimoftheDe-partment of Obstetrics and Gynecology at the Beth Israel DeaconessMedical Center(BIDMC)andMarshall LindheimeroftheUniver-sity of Chicago for helpful discussions and strong support. This workwassupportedbyNIHgrants(DK02825, DK64255, andDK65997),the American Society of Nephrology Carl W. Gottschalk award, andBIDMC seed funds to S.A.K. 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