# PPAR Action in Human Placental Development and Pregnancy and Its Complications

Authors: Fritz Wieser; Leslie Waite; Christophe Depoix; Robert N. Taylor
Journal: PPAR Research
 (2008)

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## Abstract

During pregnancy crucial anatomic, physiologic, and metabolic changes challenge the mother and the fetus.
The placenta is a remarkable organ that allows the mother and the fetus to adapt to the new metabolic, immunologic,
and angiogenic environment imposed by gestation. One of the physiologic systems that appears to have evolved to
sustain this metabolic regulation is mediated by peroxisome proliferator-activated receptors (PPARs).
In clinical pregnancy-specific disorders, including preeclampsia, gestational diabetes, and intrauterine growth restriction,
aberrant regulation of components of the PPAR system parallels dysregulation of metabolism, inflammation and
angiogenesis. This review summarizes current knowledge on the role of PPARs in regulating human trophoblast invasion,
early placental development, and also in the physiology of clinical pregnancy and its complications. As increasingly
indicated in the literature, pregnancy disorders, such as preeclampsia and gestational diabetes, represent potential
targets for treatment with PPAR ligands. With the advent of more specific PPAR agonists that exhibit efficacy in ameliorating
metabolic, inflammatory, and angiogenic disturbances, further studies of their application in pregnancy-related diseases
are warranted.


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## Body

## 1. Introduction

Peroxisome
proliferator-activated receptors (PPARs) are major regulators of lipid and
glucose metabolism, inflammation,
and angiogenesis [1–6] that allow adaptation of the mother to the nutritional
and perfusion requirements of the fetus [3, 7, 8]. PPARs, members of the nuclear hormone receptor
superfamily, are ligand-activated transcription factors. The PPAR amino acid
sequence can be divided into five modular domains: A/B, C, D, E, and F. Domain
E is the ligand binding domain (LBD) and contains a ligand-dependent transcriptional
activation function (AF-2). Domain C is the DNA binding domain, formed of two
typical zinc fingers. PPARs activate DNA direct repeat response elements
by binding as heterodimers with retinoic acid receptor (RXR) partners [9]. There are three PPAR isotypes, PPARα,
PPARγ,
and PPARβ/δ, that are highly conserved across species, with mouse, rat, and human sequences sharing >80% amino acid homology [6, 10]. The conserved expression of different PPAR and RXR isotypes
in both rat and human placentas [11] suggests that these receptors play
functional roles in placental lipid transfer and homeostasis. PPARα
has a wide distribution and is prominent in tissues with high metabolic rates
such as liver, heart, skeletal muscle, and kidney and in steroidogenic organs
such as the adrenals [12]. PPARγ has three isoforms (PPARγ1,
γ2,
and γ3)
and is expressed in brown and white adipose tissue, large intestine, to a
lesser extent in immune cells (monocytes, macrophages, Peyer’s patches of the
digestive tract), the mucosa of colon and cecum, and placental trophoblasts [13–16]. PPARβ/δ is distributed in all tissues tested
with particularly high expression in placenta and large intestine [8, 17, 18]. PPARα and PPARγ
are involved in adipocyte differentiation, lipid metabolism, insulin action,
and in the regulation of inflammatory responses [1, 5, 16], particularly involving the macrophage [19]. PPARβ/δ is known to be involved in lipid
metabolism and inflammation, as well as keratinocyte differentiation and wound
healing [5, 20, 21].The PPAR system is intimately involved
in cardiovascular disease, obesity, as well as pregnancy-specific diseases [6, 22]. Over the past decade studies have
shown that all three PPAR isotypes are expressed in human placental trophoblast
cells [11] and that they are involved in the
regulation of pregnancy physiology and its clinical complications. Physiological
and pathophysiological conditions that modulate the PPAR system [22–35] influence the risk and course of preeclampsia (PE),
gestational diabetes mellitus (GDM), or intrauterine growth restriction (IUGR) [36–53]. Some of these diseases and factors involving the PPAR
system are summarized in
Tables 1 and 2.Table 1
Effects of physiological and pathophysiological conditions on PPAR.
Influence on PPAR actionConditionsPPAR-actionModelReferenceDiabetesIncreases PPARγ in skeletal
muscleMurinePark et al. [22]AgeIncreases PPARγ in subcutaneous fat in older manHumanImbeault et al. [23]Decreases PPARα in heartMurineIemitsu et al.  [24]HypertensionIncreases PPARα and γ in aorta and mesenteric arteriesMurineDiep and Schiffrin [25]DietSoy extract increases PPARα and γ in macrophagesIn vitroMezei et al. [28]High-fat diet increases adipose tissue expression
of PPARγ and induces PPARγ2 mRNA expression in liver (obese mice)MurineVidal-Puig et al. [26]Hyperlipid diet reduces PPARγ in colonic epitheliumMurineDelage et al. [29]Low-calorie diet decreases PPARγ in subcutaneous fatHumanBastard et al. [27]ExerciseIncreases PPARγ DNA binding
activity in fat depotsMurinePetridou et al. [30]Increases
PPARα in heartMurineIemitsu et al. [24]Increases
PPARβ/δ in skeletal muscleHumanFritz et al. [34]ObesityIncreases
of PPARγ2 and PPARγ2/PPARγ1 ratio in adipose tissueHumanVidal-Puig et al. [31]Metabolic syndromeDominant-negative
mutation in PPARγ
induces
metabolic syndromeHumanSavage et al. [35]Insulin resistance (IR)Pioglitazone
ameliorates IRMurineDing et al. [33]PPARγ Ala allele protects against
hyperinsulinemiaHumanJaziri et al. [32]Vitamin AIncreases
PPARγ
in
colonic mucosaMurineDelage et al. [29]Table 2
Effects of metabolic conditions on pregnancy-specific diseases (GDM: gestational diabetes mellitus; PE: preeclampsia; IUGR: Intrauterine growth restriction;
−: reduced risk; +: increased risk).
Influence on pregnancy-specific diseasesConditionsGDMPEIUGRReferenceDiabetes—+—Ostlund et al. [36]Advanced
maternal age+++Delbaere et al.  [53]
Odibo et al. [37]Hypertension—++Sibai et al. [38]Optimal nutrition−−−Artal et al. [41]
Saftlas et al. [43]
Scholl et al. [39]Optimal exercise−−—Artal et al. [41]
Zhang et al. [44]
Sorensen et al. [42]
Saftlas et al. [43]Obesity+++Cedergren [48]
Saftlas et al. [43]
O’Brien et al. [47]
Ros et al. [45]
Sebire et al. [46]
Bodnar et al. [49]Metabolic syndrome+++Ray et al. [50]Insulin resistance—+—Wolf et al. [51]Periconceptional multivitamins—−—Bodnar et al. [52]In early pregnancy, immediately after embryonic
implantation, major maternal physiologic changes occur in the cardiovascular,
hepatic, and endocrine systems with resultant anatomical and metabolic
modifications that serve to promote maternal immune tolerance of the conceptus
and to provide the fetus with its increased nutritional needs [54, 55]. Metabolic changes (including increased availability of
glucose, low density lipoprotein, and fatty acids)  increased insulin resistance and altered amino acid
metabolism, immunologic, and hematologic changes (including an increase in
plasma volume). Establishment of a thrombophilic state and extensive placental
and decidual angiogenesis are observed in pregnancy, and these changes require
a complex activation of regulating mediators [56–58].Pregnancy complications result when
the mother and/or fetus fail to adapt to these new metabolic, angiogenic, and
thrombogenic challenges. Women with preexisting compromise to their vascular homeostasis,
such as underlying hypertension, diabetes mellitus, or metabolic syndrome, have
a significantly increased risk of developing pregnancy complications (see Table2). Placenta-associated complications also can lead to impaired growth or fetal
demise [59, 60]. These placental conditions share vasculopathological
mechanisms in common with atherosclerosis and represent early markers for
maternal risk of cardiovascular disease [61, 62] and hypertension [61, 63, 64]. Curiously, a prior history of preeclampsia appears to
confer protection against the future development of endometriosis and some
cancers [65, 66].PPARs
can be activated by natural ligands, like prostaglandins (PGs), fatty acids,
and their derivatives, as well as by synthetic ligands. PPAR medications have
been developedand discovered to be relatively safe drugs with
benefits in multiple disease states including diabetes and
cardiovascular disease [67]. Fibrate drugs used to treat
hyperlipidemia, and thiazolidinedione drugs used to treat type 2 diabetes are potent and
relatively specific ligand activators of PPARα and γ, respectively,
and are widely used clinically [68, 69]. A number of naturally-occurring PPAR ligands have been identified,
including long-chain fatty acids (C16 and greater), eicosanoids such as 8(S)-HETE
(PPARα)
and 9-and13-HODE (PPARγ), and PGs such as
PGA1,which binds to PPARα,
PPARβ/δ,
and 15-deoxy-delta12,14-prostaglandin
J2(15dPGJ2), which in turn binds to PPARγ
[70–72]. Both the expression of PPAR and the production of their
potential ligands are altered during pregnancy and its related diseases. We
postulate that pathologic diversion of fatty-acid metabolism away from the
production of eicosanoid ligands in preeclampsia and gestational diabetes might
be corrected using synthetic ligands.


## 2. PPARs in Trophoblast Invasion and Placental Development

In first trimester, human placental
bed biopsies, PPAR-γ is expressed predominantly in
invasive trophoblasts, whereas in the second-trimester PPARγ
is expressed in the columns of anchoring villi and cytotrophoblasts [73, 74]. In the third trimester, PPARγ
principally localizes to extravillous cytotrophoblasts (EVCT) and villous syncytiotrophoblasts
[75], where it appears to regulate placental hormone
production and secretion. Although the focus of this review is to summarize findings
on PPAR/RXR heterodimers in human placentation, much of the direct evidence for
a role of these receptors in trophoblast invasion and placental development has
emerged from studies in knockout mouse models. This topic is reviewed
comprehensively in Schaiff et al. [3], and is summarized briefly here and
in Table 3 [76–81].Table 3
PPAR knock out models and placental pathology (PRIP: peroxisome proliferator-activated
receptor-(PPAR) interacting protein; RAP 250: nuclear receptor-activating protein
250).
PPAR knockout modelPlacental pathologyLethalityReferencePPARαNo
significant effect on placentation20%Yessoufou et al. [76]PPARβ/δPoor
placentation>90%Barak et al. [77]PPARγPoorly
developed labyrinth100%Barak et al. [15]
Kubota et al. [82]PPARγ coactivator PRIPReduced
spongiotrophoblast layer100%Zhu et al. [79]PPARγ coactivator RAP250Reduced
spongiotrophoblast layer100%Antonson et al. [80]RXRα
or βLack
of labyrinth zone100%Sapin et al. [81]PPARγ/RXRα heterodimers play a key regulatory
role in murine placental development. PPARγ deficiency was shown to interfere
with terminal trophoblast differentiation and placental vascularization [78]; embryos without this gene
show massive placental defects that can be rescued by restoration of
the trophoblast PPARγ gene via tetraploid chimeras [15]. Deletion
of RXRα and RXRβ also leads to embryo lethality [15, 81, 83]. Both PPAR-interacting protein (PRIP) and nuclear
receptor-activating protein 250 (RAP250) encode nuclear receptor coactivators
that associate with PPARs, RXRs, and other nuclear receptor proteins. Genetic
disruption of PRIP or RAP250 in mouse models results in embryonic lethality at
postconception days 11.5 and 13.5, respectively [79, 80]. Placentas of PRIP (−/−) and RAP250 (−/−) embryos
exhibited dramatically reduced spongiotrophoblast and labyrinth layers as well
as failure of blood vessel maturation in the region bordering the
spongiotrophoblast [79, 80].In addition to placentation per se, PPARγ
appears to play an important role in the uterine preparation for embryonic
implantation. Peeters et al. demonstrated that PPARγ
ligands reduced the production of the endometrial angiogenic factor VEGF, and postulated
that this pathway might influence early embryonic vascularization [84]. By contrast, PPARγ
agonists induce angiogenesis in cardiac myofibroblasts, smooth muscle cells,
and macrophages [85–87]. Recent preliminary data by our lab and others suggest
that the PPARγ
system also stimulates VEGF expression in trophoblast (JEG-3) cells (Depoix et
al., unpublished).The functional role of PPARγ activity
is well studied in trophoblast physiology (Table 4). PPARγ
agonists inhibit invasion of cultured EVCT isolated from human first-trimester
placenta, whereas PPARγ antagonists promoted EVCT invasion
and repressed the PPARγ agonist-mediated effects [78]. PPARγ controls mucin (MUC)-1 transcription
and regulates maternal-fetal transport in mouse models [88]. Moreover, PPARγ and RXRα play a role in human chorionic
gonadotropin (hCG) expression, trophoblast differentiation, and regulation of
fatty acid transport and storage in human placental trophoblasts [89, 90]. PPARγ diminishes leptin-induced
inflammatory responses in the human placenta [91] and inhibits PAPP-A expression [92].Table 4
PPAR action in trophoblast development and placental function (MUC-1: mucin-1; EVCT: extravillous cytotrophoblast; hCG: human chorionic gonadotropin; Th2 T-helper 2 cell).
PPAR action in trophoblast development and placentationPPARPPAR actionModelReferencePPARγInhibits EVCT
invasionIn vitroFournier et al.
[78]Promotes
trophoblast differentiation hCG secretionIn vitroTarrade et al. [89]Induces
hCG productionIn vitroSchild et al. [93]AntiinflammatoryIn vitroLappas et al. [91]Regulates
fatty acid transportIn vitroSchaiff et al. [90]Increases
VEGF expressionIn vitroDepoix,
unpublishedTerminal
differentiation, placental vascularizationMurineBarak et al. [15]Controls
MUC-1 expressionMurineShalom-Barak et al. [88]Stimulates
trophoblast maturationMurineAsami-Miyagishi et al. [94]Modulates
placental lipid metabolismMurineCapobianco et al. [95]PPARβ/δPromotes
placental developmentMurineNadra et al. [8]PPARαRegulates
placental lipid transferMurine/HumanWang et al. [74]PPAR
action in pregnancyPPARγAntiinflammatoryIn vitroLappas et al. [96]Involved
in inflammatory control and remodeling in the placentaIn
vitroMarvin et al. [97]Increased
circulating PPARγ
activators
in normal pregnancyIn vitro/humanWaite et al. [73]Decreases in fetal membrane with laborHumanDunn-Albanese et al. [98]PPARβ/δIncreases
in amnion with laborHumanBerry et al. [99]PPARαStimulates
Th2 cytokine pattern during pregnancyMurineYessoufou et al. [76]Declines
in choriodecidua with laborHumanBerry et al. [99]Regulation of PPARγ
in human placental tissues is thought to occur through natural ligands (e.g.,
15dPGJ2, 9-HODE, 13-HODE, and 15-HETE) through direct binding to the receptor’s
ligand binding pocket [11, 100]. These ligands are likely to be synthesized locally within
the placenta. Furthermore, crosstalk between the mitogen-activated protein
kinase (MAPK) p38 and PPARγ occurs within cultured trophoblast
cells [101]. PPARγ decreases IGFII secretion and is thought
to inhibit trophoblast invasion via the PAPP-A cascade [92].In
young PPARα knock out mice, no major phenotypic differences
of gross pathology of internal organs were described [76, 102]. However, disturbance of the Th1/Th2 T-lymphocyte ratio,
rather than placental malformation, is thought to be responsible for an increased
abortion rate (20%) in PPARα null mice. During normal pregnancy Th1
cytokines are downregulated and Th2 cytokines are upregulated [103].The third distinct PPAR, PPARβ/δ
also is essential for placentation as demonstrated in PPARβ/δ
knockout mice (Table 3) [77], and is involved in the regulation of implantation in other
animal models  [17, 104, 105]. The implantation of cultured embryos is enhanced by
PPARβ/δ
activation and this receptor even has been postulated as a novel therapeutic
target to improve clinical IVF outcomes [104]. PPARβ/δ is induced during decidualization of
the implantation site and requires close contact with the blastocyst. PPARβ/δ
null mice die between 9.5 to 10.5 embryonic days due to abnormal cell-cell
communication at the placental-decidual interface [8].Together these data suggest that PPARs
are required not only for trophoblast invasion and differentiation but also for
establishment of the placental maternal-fetal transport.


## 3. PPARs and Pregnancy

Based on its regulatory functions and
known eicosanoid ligands, PPARγ has emerged as an excellent candidate
to play a role in the regulation of maternal metabolism, maintenance of uterine
quiescence, and onset of labor by regulating proinflammatory cytokines and
prostaglandins (Table 4). Normal pregnancy is accompanied by changes in lipid
and glucose metabolism, but further dysregulation of these pathways can lead to
pregnancy complications such as PE or GDM. Hence, PPAR regulators of these
metabolic pathways might be expected to be important in human pregnancy.Some of our initial studies in this
field were designed to screen for potential activators of PPARγ
in the circulation of pregnant women. Human choriocarcinoma JEG-3 cells were
transfected with peroxisome-proliferator responsive reporter plasmids; and pooled
sera from pregnant and nonpregnant women were added to the cell culture medium [73]. Peroxisome proliferator responsive
element (PPRE) luciferase reporter activation was dramatically increased by
sera from pregnant women compared to nonpregnant women (Figures 1 and 2). We
showed that PPARγ (and to some extent PPARα)
activity is increased from the earliest stages of pregnancy (Figure 2). The
findings suggested that circulating PPARγ-activating factors, presumably
eicosanoids, were present throughout the course of gestation. We hypothesized
that activation of PPARγ by sera of pregnant women is a
regulatory adaptation of the maternal organism to increased lipid and glucose
loading in pregnancy [73].Figure 1
JEG-3 cells were transfected with PPRE-luciferase reporter vectors
and treated with pooled sera (10%) from non-pregnant (NP), pregnant
(P) and preeclamptic (PE) women. Luciferase acitivity, relative to cells
treated with 10% dextran charcoal-shipped fetal calf serum (DCSS), is
reported on the ordinate.Figure 2
PPARγ activation is present throughout the course of normal
pregnancy. All serum samples were collected from the same subject
and PPRE-luciferase reporter experiments were performed using
10% serum as described in Figure 1. Luciferase activity was normalized
to DCSS to determine relative activation. Black horizontal bar
represents the level of signaling seen with 10% serum from the same
woman six weeks after delivery.It also has been hypothesized that
PPARγ
activation regulates uterine quiescence by influencing Nuclear Factor-Kappa B
(NFκB)
and cyclooxygenase (COX-2) expression [96, 97, 106]. Reciprocal expression of PPARγ
and (COX)-2 in human term placenta suggests a role of the PPAR system in the
initiation of labor [98]. Under conditions of high PPARγ
expression, antiinflammatory actions dominate; however, with onset of labor
PPARγ
levels drop and COX-2 concomitantly increases in the fetal membranes [98]. Elevated COX-2 activity in the human amnion is observed
in the settings of term and idiopathic preterm labor, contributing to the
generation of uterotonic prostaglandins (PGs), which are known to participate
in parturition [107]. PPARγ ligands have been shown to antagonize
NF-κB
activation and reduce inflammatory cytokine gene expression (IL-1β,
IL-6, IL-10 and TNF-α) and COX-2 [108]. Both natural (e.g., 15dPGJ2) and synthetic ligands
(e.g., troglitazone) were shown to have anti-inflammatory effects in human
gestational tissues, significantly decreasing basal and LPS-stimulated PGE2 and PGF2α release from placenta and amnion [108]. PGF2α
, also a marker of oxidative stress, is
increased in women with preeclampsia [109]. Given the inflammatory changes observed
in pregnancy-specific diseases, a potential role of PPAR agonist treatment has
been entertained for the treatment of PE, GDM, and other pregnancy-specific
diseases such as the prevention of preterm labor [96].PPARα and β/δ also play a role in maintaining
pregnancy and parturition. PPARα and β/δ are expressed in the amnion,
choriodecidua, and villous placental tissues. Data from PPARα knockout
mice suggest that PPARα maintains pregnancy by stimulating a
Th2 cytokine response [76]. In normal pregnancy, expression of PPARα
declines in the choriodecidua with the onset of labor [99]. By contrast, PPARβ/δ
expression, which is temporally upregulated between the first and third
trimester of pregnancy [99], increases further in the amnion coincidental
with the onset of labor [99].Few studies have elucidated substantial
risk of PPAR agonists during pregnancy in animal models, but these drugs carry a
“C” classification from the FDA. For example, rosiglitazone
did not damage blastocyst development in vitro or harm mouse fetuses when given
during murine pregnancy
[110]. While the use of
rosiglitazone during pregnancy is generally considered to be safe [110]; more data need
to be acquired before these drugs can be recommended.


## 4. PPARs and Pregnancy-Specific Diseases

Failure of metabolic adaptation to
pregnancy can result in pregnancy-specific complications such as PE and GDM. We
and others have postulated that angiogenic factors and cytokines that lead to
pathological gestational changes are likely to be regulated by the PPAR system (Table5).Table 5
PPAR in pregnancy-specific diseases.
PPARPPAR-actionDiseaseModelReferencePPARγReduced circulating PPARγ activators in serum from women with PEPEIn vitroWaite et al. [111]Placental 15dPGJ2level are
decreased in diabetesGDMMurineCapobianco et al. [95]Association
of PPAR-γ2 Pro12Ala with
weight gainGDMHumanTok et al. [112]Placental 15dPGJ2levels are
decreasedGDMHumanJaverbaum et al. [113]DecreasedHydatidiform moleHumanCapparuccia et al. [114]DecreasedChoriocarcinomaHumanCapparuccia et al. [114]Placental
PPAR expression is not involvedIUGRHumanRodie et al. [115]Association of PPAR-γ2 Pro12Ala polymorphismPreterm
birthHumanMeirhaeghe et al. [116]PPARαLack of PPAR-α upregulates Th1 cytokinesAbortion/neonatal mortalityMurineYessoufou et al. [76]


### 4.1. PPARs and Preeclampsia

PE is a multifactorial,
pregnancy-related disorder that is defined by new-onset hypertension and
proteinuria after 20 weeks of gestation [117]. PE is a common cause of maternal and
infant morbidity and mortality worldwide, and is responsible for about 20% of
pregnancy-related maternal deaths in the US [118]. Women with PE have increased insulin
resistance as well as hypertriglyceridemia relative to normal pregnant women [119]. To date, no effective treatment has been found that either
prevents or reverses the development of the disease. Modern concepts of PE
pathophysiology invoke a two-stage process. The first stage is believed to be
initiated by impaired trophoblast invasion and abnormal uterine vessel
remodeling. The second stage is postulated to result from circulating factors
claimed to be derived from the ischemic placenta that stimulate an inflammatory
activation of maternal vascular endothelial cells. PE presents clinically in
the second or third trimester, however, fundamental inflammatory and angiogenic
biomarkers in the serum are detectable as early as the first trimester in women
with PE. Elevated concentrations of IL-2, TNFα, and sVEGFR-1 and reduced concentrations
of PlGF, IGFBP-1, and HLA-G in the maternal serum precede the clinical
manifestations of PE [119–123].While the cause of PE remains unknown,
several environmental and genetic risk factors have been identified (Table2).
Relevant to this review are hypertension, diabetes, and high (>29) body mass
index (BMI) [47, 124, 125]. Black race also appears to be a risk factor for PE [126] although this may be confounded by increased rates of
the above risk factors. Key inflammatory and angiogenic pathways involved in
the pathogenesis of PE are regulated by the PPAR system, which itself is
influenced by environmental and genetic factors. We believe that exogenous and endogenous
lipid regulators of PPAR play a role in maternal metabolism and
immune functionin normal and pathological pregnancies. For example,
dietary factors and physical activity that modulate the PPAR system have been
shown to reduce the risk and course of PE (Table 2).Similarly, genetic variations in the
PPARγ
gene have been proposed to modify the risk of PE. For example, the Pro467Leu
mutation of PPARγ [127–129] is a dominant negative mutant resulting from a C-to-T
transition in exon 6. A report of two individuals (one woman, one man) with
this mutation showed that they developed type 2 diabetes at young ages (26 and
27 years at diagnosis), as well as early hypertension (37 and 27 years at
diagnosis). Intriguingly, the woman had two pregnancies, both of which were
complicated by severe PE. The Pro12Ala polymorphism occurs in PPARγ2
[130], a second isoform of PPARγ that is expressed mainly in adipose
tissue. This mutation is the result of a C-to-G transversion in exon B. This is
by far the most studied allelic variation in any PPAR, and occurs at a rate of
about 12% in the Caucasian US population. While the resulting
phenotype is highly diverse and even apparently contradictory, it appears that
the penetrance of this mutation is influenced by other genetic, environmental,
ethnic, and gender differences. The studies generally agree that the presence
of the Ala
allele is associated with increased BMI, an independent risk factor for PE.
Thus, this polymorphism is a candidate affecting pregnancy outcome. Preliminary data of a study on the PPAR
gene variations (in PPAR gene) showed no association with PE or severity of PE in
a Finnish population [131]. Further studies on the association
of PPAR α, β,  and γ gene variations of mothers and offspring and
pregnancy-specific diseases need to be performed in different ethnic
populations.PE is marked by hyperlipidemia, and is
characterized by a state of oxidative stress. Circulating lipids in PE women are
more highly oxidized, and oxidized low-density lipoproteins (oxLDLs), in
particular, are highly elevated [132]. Given the circulating plasma lipid disturbances in PE,
our group performed experiments comparing sera from normal and PE
patients. We found that serum from women with severe PE had reduced
levels of PPAR activating lipids compared with serum of parity and
gestational age-matched women and also diminished the expression of PPARγ
in trophoblast cells (Figures 1 and 3) [111]. The reduction of transcriptional
activity observed in preeclamptic women’s sera was shown for PPARγ
and PPARα,
however not for PPARβ/δ or RXR. The reduction in potential
circulating PPAR activatorswas observed weeks and sometimes months
before the onset of maternal symptoms and clinicaldiagnosis of PE
[133]. Our results are consistent with other clinical evidence that antiinflammatory
regulation is challenged and further compromised in the maternal syndrome of
PE. Normal pregnancy manifests as a physiologic inflammatory state postulated
to be tolerated to serve the nutritional needs of the fetus, whereas, in PE
regulatory inflammatory mechanisms are excessively amplified, leading to
vascular damage in the mother [133]. In this “hyperinflammatory” state
of PE [134], the cytokines TNFα
and IL-1β
which are typically controlled by the NF-κB pathway in a negative-feedback
loop with PPAR, are elevated [26, 60, 119]. Elevated inflammatory parameters in PE accompany
altered levels of PG metabolites and circulating fatty acids. As noted,  PG metabolites as well as fatty acids are
important ligands of the PPAR system [135]. PG metabolism is altered
during normal pregnancy with levels of vasorelaxants suchas prostacyclin
increasing, whereas vasoconstrictive prostaglandin levelstend to be
suppressed [136]. Failure of these alterations have
been suggested to lead to pregnancy complications (e.g., PE) [137]. For example, PGF2α,
which itself is stimulated by factorsin the plasma of women with PE
[138], can inhibit PPARγ effects [135]. Levelsof circulating
free fatty acids are in the normal range duringmost of pregnancy,
but rise dramatically during the final weeks of pregnancy and drop
precipitously at term [136]. In PE these levels are increased from
20 weeks’ gestation [133, 139]. We postulate that altered PG metabolism in this setting
[138] results in decreased PPARγ ligation and subsequent cytokine
activation. If this proposal is supported by more data, the use of PPAR ligands
might be proposed to ameliorate symptoms such as hypertension and inflammation.
Unfortunately, at present, the mechanism and site of this salutary of PPAR
ligand effect remain unknown in pregnancy, confounded by PPAR expression in
many cell types, including endothelial cells.Figure 3
Immunoblot of JEG-3 cells treated with pooled sera (10%) from nonpregnant
(NP), pregnant (P), and preeclamptic (PE) women. Cell lysates were
analyzed using a specific mouse anti-human PPARγ monoclonal antibody. Equal
amounts of protein (50 μg) were loaded into each lane. Factors in pregnant
serum up-regulate JEG-3 PPARγ expression. A decrease in PPARγ protein was
observed in cells exposed to PE sera (PE) compared to sera from normal
pregnant women (P).


### 4.2. PPARs and Gestational Diabetes

During normal
pregnancy, maternal lipid, and glucose metabolism is profoundly altered
[140]. The developing fetus uses
glucose as its predominant energy source, which puts a continuous
demand on the mother to provide this substrate [141]. This constant need for glucose
results in frequent hypoglycemia and postprandial hyperglycemia during normal
pregnancy [141].  Problems with energy metabolism such as GDM
are not uncommon and are often observed in susceptible women at this time. GDM
is defined as any degreeof glucose intolerance with onset or first
recognition during pregnancy. In women with GDM, defective β-cells
function cannot adequately compensate for free fatty acid-mediated insulin
resistance [142]. As elsewhere in our society, the
incidence of obesity, diabetes, and gestational diabetes mellitus are
increasing in the pregnant population [143]. In the United
States, the incidence of obesity among pregnant women ranges from
18.5% to 38.3% [144]; obesity comprises a major risk factor for GDM [145]. Morphological changes have been
identified in the syncytiotrophoblast, cytotrophoblast, trophoblastic basement
membrane, and fetal vessels within the placentae of these cases [146]. GDM is associated with several severe
neonatal complications (such as macrosomia, brachial plexus palsy, premature
delivery, IUGR, and intrauterine death) and maternal birth injuries also are
common [125, 147]. Furthermore, GDM has emerged as a risk factor for the
development of diabetes mellitus type 2 (DM2) and cardiovascular disease in
later life and shares a number of epidemiologic, pathophysiologic, and genetic
characteristics with DM2 [148]. GDM also has detrimental effects on the postnatal
infants [149].The PPAR system regulates the
metabolic and pathways involved in the establishment of GDM. PPAR-agonists
have antidiabetogenic, antiinflammatory, and antioxidant effects, which are all
potentially beneficial in the treatment of GDM [5].Environmental factors, such as diet
and exercise and genetic factors influence PPARα, γ activity [130, 150] as well as the risk for insulin resistance and GDM
(Table 2). Exercise activity initiated prepregnancy was shown to reduce the
risk of GDM and its complications [40, 41, 44, 151, 152]. Nutritional counseling, moderate physical exercise,
weight loss, and diet are successful therapies in some women with GDM,
improving glycemic control, reducing the incidence of LGA infants, and decreasing
the need for cesarean deliveries for cephalopelvic disproportion [41, 153].Candidate genes for GDM risk include
TNFα,
β3
adrenoreceptor (ADRB3), and PPARα and γ. The PPARγ
Pro12Ala polymorphism was not associated with increased insulin resistance in
Turkish women with GDM, however it was associated with weight gain [112]. The PPARγ
coactivator-1alpha (PGC-1) polymorphism also failed to be associated with the development
of GDM [154]. More studies on the association of various
genetic PPARα and γ variants and GDM in different ethnic
populations will be of interest.15dPGJ2 is a potent antiinflammatory
agent that represses the expression of a number of inflammatory
genes and regulating factors including the transcription factor NF-κB
[33, 108]. The concentration of 15dPGJ2 was reduced in
placentae from diabetic rats (Table 5) [95]. Placental 15dPGJ2 was noted to be diminished
in women with gestational and pregestational diabetes when compared to
controls, whereas levels of nitric oxide (a stimulator of placental
invasiveness, differentiation, and proliferation) were higher in term placental
explants from diabetic patients when compared to controls [113]. As PPARγ can
prevent nitric oxide overproduction in placenta from pregestational diabetic
women [113], it may have the potential to improve
fetal outcome in this condition.Sulfonylurea
agents including gliumepiride and glibenclamide exhibit PPARγ activity [155]. A randomized controlled trial to test
the effectiveness and safety of the sulfonylurea agent glyburide in the
management of women with GDM showed similar efficacy to insulin treatment [156]. Both the insulin- and
glyburide-treated women were able to achieve satisfactory glucose control and
had similar perinatal outcome [156].


### 4.3. PPARs and Other Pregnancy-Specific Diseases

Trophoblast research has emphasized
the similarities between the proliferative, migratory, andinvasive
properties of placental cells and those of cancer cells [157]. PPARγ, PPARβ/δ, and RXR appear to be linked to gestational
trophoblastic neoplasms, conditions associated with malignant trophoblast
behavior [114]. PPARγ agonists inhibit invasion of normal
extravillous cytotrophoblast isolated from human first-trimester placenta, and PPAR
activity has been shown to be downregulated in trophoblastic diseases including
hydatidiform mole and choriocarcinoma [114].PPARγ
has an effect on fetal and placental size influencing intrauterine growth. In an
intrauterine growth restriction (IUGR) model, glucocorticoids inhibited fetal
and placental growth partly by suppression of PPARγ
in the labyrinth zone of the placenta [158]. Activation of PPARγ
in the labyrinth trophoblasts is hypothesized to induce angiogenic factors and stimulate
the growth of fetal blood vessels, thereby promoting placental growth. However, treatment of pregnant mice with
rosiglitazone led to reduced thickness of the spongiotrophoblast layer and the
surface area of labyrinthine vasculature, and it altered expression of proteins
implicated in placental development [159].In vitro
and in vivo experiments
as well as animal models studies suggest a link between the PPAR system and
gestational duration, preterm labor, and birth weight
[116]. Variations in the PPAR genes influence other pregnancy-related mechanisms
including birth weight and gestational duration. In an Irish population, the
PPARγ Ala12
allele was associated with shorter gestational duration [116].PPAR ligands regulate apoptotic
mechanisms involved in rupture of the fetal membranes and may play a role in
preterm delivery, a condition associated with increased risk of neonatal sepsis
and newborn trauma [160]. 15d-PGJ2induced morphological
characteristics of apoptosis within 2 hours in an amniotic cell line [160]. In addition, ciglitizone also induced apoptosis,
whereas rosiglitazone had no effect on cell viability [160]. Prevention of apoptosis may have therapeutic potential
in preterm labor and premature rupture of the membranes and necessitates further
investigations.Interestingly, PPARα deficiency
is associated with miscarriage, neonatal mortality, and a shift from Th2 to a
Th1 cytokine phenotype [76]. Th1 predominant immunity is closely associated with inflammation,
endothelial dysfunction, and pregnancy complications. For example, interferonγ
is significantly reduced in the spleens of PPARα null mice [76]. Twenty percent of PPARα knockout mice aborted, and offspring
of PPAR-α null
mice exhibited increased neonatal mortality (13.3%). However the mechanism
whereby PPARα induces
a Th2 phenotype shift remains to be determined. PPARγ
ligands also were shown to decrease production of inflammatory ligands in
activated macrophages and T cells and to induce a shift from Th1 to Th2 cytokine
phenotype [161, 162].


## 4.1. PPARs and Preeclampsia

PE is a multifactorial,
pregnancy-related disorder that is defined by new-onset hypertension and
proteinuria after 20 weeks of gestation [117]. PE is a common cause of maternal and
infant morbidity and mortality worldwide, and is responsible for about 20% of
pregnancy-related maternal deaths in the US [118]. Women with PE have increased insulin
resistance as well as hypertriglyceridemia relative to normal pregnant women [119]. To date, no effective treatment has been found that either
prevents or reverses the development of the disease. Modern concepts of PE
pathophysiology invoke a two-stage process. The first stage is believed to be
initiated by impaired trophoblast invasion and abnormal uterine vessel
remodeling. The second stage is postulated to result from circulating factors
claimed to be derived from the ischemic placenta that stimulate an inflammatory
activation of maternal vascular endothelial cells. PE presents clinically in
the second or third trimester, however, fundamental inflammatory and angiogenic
biomarkers in the serum are detectable as early as the first trimester in women
with PE. Elevated concentrations of IL-2, TNFα, and sVEGFR-1 and reduced concentrations
of PlGF, IGFBP-1, and HLA-G in the maternal serum precede the clinical
manifestations of PE [119–123].While the cause of PE remains unknown,
several environmental and genetic risk factors have been identified (Table2).
Relevant to this review are hypertension, diabetes, and high (>29) body mass
index (BMI) [47, 124, 125]. Black race also appears to be a risk factor for PE [126] although this may be confounded by increased rates of
the above risk factors. Key inflammatory and angiogenic pathways involved in
the pathogenesis of PE are regulated by the PPAR system, which itself is
influenced by environmental and genetic factors. We believe that exogenous and endogenous
lipid regulators of PPAR play a role in maternal metabolism and
immune functionin normal and pathological pregnancies. For example,
dietary factors and physical activity that modulate the PPAR system have been
shown to reduce the risk and course of PE (Table 2).Similarly, genetic variations in the
PPARγ
gene have been proposed to modify the risk of PE. For example, the Pro467Leu
mutation of PPARγ [127–129] is a dominant negative mutant resulting from a C-to-T
transition in exon 6. A report of two individuals (one woman, one man) with
this mutation showed that they developed type 2 diabetes at young ages (26 and
27 years at diagnosis), as well as early hypertension (37 and 27 years at
diagnosis). Intriguingly, the woman had two pregnancies, both of which were
complicated by severe PE. The Pro12Ala polymorphism occurs in PPARγ2
[130], a second isoform of PPARγ that is expressed mainly in adipose
tissue. This mutation is the result of a C-to-G transversion in exon B. This is
by far the most studied allelic variation in any PPAR, and occurs at a rate of
about 12% in the Caucasian US population. While the resulting
phenotype is highly diverse and even apparently contradictory, it appears that
the penetrance of this mutation is influenced by other genetic, environmental,
ethnic, and gender differences. The studies generally agree that the presence
of the Ala
allele is associated with increased BMI, an independent risk factor for PE.
Thus, this polymorphism is a candidate affecting pregnancy outcome. Preliminary data of a study on the PPAR
gene variations (in PPAR gene) showed no association with PE or severity of PE in
a Finnish population [131]. Further studies on the association
of PPAR α, β,  and γ gene variations of mothers and offspring and
pregnancy-specific diseases need to be performed in different ethnic
populations.PE is marked by hyperlipidemia, and is
characterized by a state of oxidative stress. Circulating lipids in PE women are
more highly oxidized, and oxidized low-density lipoproteins (oxLDLs), in
particular, are highly elevated [132]. Given the circulating plasma lipid disturbances in PE,
our group performed experiments comparing sera from normal and PE
patients. We found that serum from women with severe PE had reduced
levels of PPAR activating lipids compared with serum of parity and
gestational age-matched women and also diminished the expression of PPARγ
in trophoblast cells (Figures 1 and 3) [111]. The reduction of transcriptional
activity observed in preeclamptic women’s sera was shown for PPARγ
and PPARα,
however not for PPARβ/δ or RXR. The reduction in potential
circulating PPAR activatorswas observed weeks and sometimes months
before the onset of maternal symptoms and clinicaldiagnosis of PE
[133]. Our results are consistent with other clinical evidence that antiinflammatory
regulation is challenged and further compromised in the maternal syndrome of
PE. Normal pregnancy manifests as a physiologic inflammatory state postulated
to be tolerated to serve the nutritional needs of the fetus, whereas, in PE
regulatory inflammatory mechanisms are excessively amplified, leading to
vascular damage in the mother [133]. In this “hyperinflammatory” state
of PE [134], the cytokines TNFα
and IL-1β
which are typically controlled by the NF-κB pathway in a negative-feedback
loop with PPAR, are elevated [26, 60, 119]. Elevated inflammatory parameters in PE accompany
altered levels of PG metabolites and circulating fatty acids. As noted,  PG metabolites as well as fatty acids are
important ligands of the PPAR system [135]. PG metabolism is altered
during normal pregnancy with levels of vasorelaxants suchas prostacyclin
increasing, whereas vasoconstrictive prostaglandin levelstend to be
suppressed [136]. Failure of these alterations have
been suggested to lead to pregnancy complications (e.g., PE) [137]. For example, PGF2α,
which itself is stimulated by factorsin the plasma of women with PE
[138], can inhibit PPARγ effects [135]. Levelsof circulating
free fatty acids are in the normal range duringmost of pregnancy,
but rise dramatically during the final weeks of pregnancy and drop
precipitously at term [136]. In PE these levels are increased from
20 weeks’ gestation [133, 139]. We postulate that altered PG metabolism in this setting
[138] results in decreased PPARγ ligation and subsequent cytokine
activation. If this proposal is supported by more data, the use of PPAR ligands
might be proposed to ameliorate symptoms such as hypertension and inflammation.
Unfortunately, at present, the mechanism and site of this salutary of PPAR
ligand effect remain unknown in pregnancy, confounded by PPAR expression in
many cell types, including endothelial cells.Figure 3
Immunoblot of JEG-3 cells treated with pooled sera (10%) from nonpregnant
(NP), pregnant (P), and preeclamptic (PE) women. Cell lysates were
analyzed using a specific mouse anti-human PPARγ monoclonal antibody. Equal
amounts of protein (50 μg) were loaded into each lane. Factors in pregnant
serum up-regulate JEG-3 PPARγ expression. A decrease in PPARγ protein was
observed in cells exposed to PE sera (PE) compared to sera from normal
pregnant women (P).


## 4.2. PPARs and Gestational Diabetes

During normal
pregnancy, maternal lipid, and glucose metabolism is profoundly altered
[140]. The developing fetus uses
glucose as its predominant energy source, which puts a continuous
demand on the mother to provide this substrate [141]. This constant need for glucose
results in frequent hypoglycemia and postprandial hyperglycemia during normal
pregnancy [141].  Problems with energy metabolism such as GDM
are not uncommon and are often observed in susceptible women at this time. GDM
is defined as any degreeof glucose intolerance with onset or first
recognition during pregnancy. In women with GDM, defective β-cells
function cannot adequately compensate for free fatty acid-mediated insulin
resistance [142]. As elsewhere in our society, the
incidence of obesity, diabetes, and gestational diabetes mellitus are
increasing in the pregnant population [143]. In the United
States, the incidence of obesity among pregnant women ranges from
18.5% to 38.3% [144]; obesity comprises a major risk factor for GDM [145]. Morphological changes have been
identified in the syncytiotrophoblast, cytotrophoblast, trophoblastic basement
membrane, and fetal vessels within the placentae of these cases [146]. GDM is associated with several severe
neonatal complications (such as macrosomia, brachial plexus palsy, premature
delivery, IUGR, and intrauterine death) and maternal birth injuries also are
common [125, 147]. Furthermore, GDM has emerged as a risk factor for the
development of diabetes mellitus type 2 (DM2) and cardiovascular disease in
later life and shares a number of epidemiologic, pathophysiologic, and genetic
characteristics with DM2 [148]. GDM also has detrimental effects on the postnatal
infants [149].The PPAR system regulates the
metabolic and pathways involved in the establishment of GDM. PPAR-agonists
have antidiabetogenic, antiinflammatory, and antioxidant effects, which are all
potentially beneficial in the treatment of GDM [5].Environmental factors, such as diet
and exercise and genetic factors influence PPARα, γ activity [130, 150] as well as the risk for insulin resistance and GDM
(Table 2). Exercise activity initiated prepregnancy was shown to reduce the
risk of GDM and its complications [40, 41, 44, 151, 152]. Nutritional counseling, moderate physical exercise,
weight loss, and diet are successful therapies in some women with GDM,
improving glycemic control, reducing the incidence of LGA infants, and decreasing
the need for cesarean deliveries for cephalopelvic disproportion [41, 153].Candidate genes for GDM risk include
TNFα,
β3
adrenoreceptor (ADRB3), and PPARα and γ. The PPARγ
Pro12Ala polymorphism was not associated with increased insulin resistance in
Turkish women with GDM, however it was associated with weight gain [112]. The PPARγ
coactivator-1alpha (PGC-1) polymorphism also failed to be associated with the development
of GDM [154]. More studies on the association of various
genetic PPARα and γ variants and GDM in different ethnic
populations will be of interest.15dPGJ2 is a potent antiinflammatory
agent that represses the expression of a number of inflammatory
genes and regulating factors including the transcription factor NF-κB
[33, 108]. The concentration of 15dPGJ2 was reduced in
placentae from diabetic rats (Table 5) [95]. Placental 15dPGJ2 was noted to be diminished
in women with gestational and pregestational diabetes when compared to
controls, whereas levels of nitric oxide (a stimulator of placental
invasiveness, differentiation, and proliferation) were higher in term placental
explants from diabetic patients when compared to controls [113]. As PPARγ can
prevent nitric oxide overproduction in placenta from pregestational diabetic
women [113], it may have the potential to improve
fetal outcome in this condition.Sulfonylurea
agents including gliumepiride and glibenclamide exhibit PPARγ activity [155]. A randomized controlled trial to test
the effectiveness and safety of the sulfonylurea agent glyburide in the
management of women with GDM showed similar efficacy to insulin treatment [156]. Both the insulin- and
glyburide-treated women were able to achieve satisfactory glucose control and
had similar perinatal outcome [156].


## 4.3. PPARs and Other Pregnancy-Specific Diseases

Trophoblast research has emphasized
the similarities between the proliferative, migratory, andinvasive
properties of placental cells and those of cancer cells [157]. PPARγ, PPARβ/δ, and RXR appear to be linked to gestational
trophoblastic neoplasms, conditions associated with malignant trophoblast
behavior [114]. PPARγ agonists inhibit invasion of normal
extravillous cytotrophoblast isolated from human first-trimester placenta, and PPAR
activity has been shown to be downregulated in trophoblastic diseases including
hydatidiform mole and choriocarcinoma [114].PPARγ
has an effect on fetal and placental size influencing intrauterine growth. In an
intrauterine growth restriction (IUGR) model, glucocorticoids inhibited fetal
and placental growth partly by suppression of PPARγ
in the labyrinth zone of the placenta [158]. Activation of PPARγ
in the labyrinth trophoblasts is hypothesized to induce angiogenic factors and stimulate
the growth of fetal blood vessels, thereby promoting placental growth. However, treatment of pregnant mice with
rosiglitazone led to reduced thickness of the spongiotrophoblast layer and the
surface area of labyrinthine vasculature, and it altered expression of proteins
implicated in placental development [159].In vitro
and in vivo experiments
as well as animal models studies suggest a link between the PPAR system and
gestational duration, preterm labor, and birth weight
[116]. Variations in the PPAR genes influence other pregnancy-related mechanisms
including birth weight and gestational duration. In an Irish population, the
PPARγ Ala12
allele was associated with shorter gestational duration [116].PPAR ligands regulate apoptotic
mechanisms involved in rupture of the fetal membranes and may play a role in
preterm delivery, a condition associated with increased risk of neonatal sepsis
and newborn trauma [160]. 15d-PGJ2induced morphological
characteristics of apoptosis within 2 hours in an amniotic cell line [160]. In addition, ciglitizone also induced apoptosis,
whereas rosiglitazone had no effect on cell viability [160]. Prevention of apoptosis may have therapeutic potential
in preterm labor and premature rupture of the membranes and necessitates further
investigations.Interestingly, PPARα deficiency
is associated with miscarriage, neonatal mortality, and a shift from Th2 to a
Th1 cytokine phenotype [76]. Th1 predominant immunity is closely associated with inflammation,
endothelial dysfunction, and pregnancy complications. For example, interferonγ
is significantly reduced in the spleens of PPARα null mice [76]. Twenty percent of PPARα knockout mice aborted, and offspring
of PPAR-α null
mice exhibited increased neonatal mortality (13.3%). However the mechanism
whereby PPARα induces
a Th2 phenotype shift remains to be determined. PPARγ
ligands also were shown to decrease production of inflammatory ligands in
activated macrophages and T cells and to induce a shift from Th1 to Th2 cytokine
phenotype [161, 162].


## 5. Conclusions

PPARs are involved in trophoblast
invasion, placental development, parturition, and pregnancy-specific diseases,
particularly PE and GDM. The role of the PPAR system in pregnancy under
physiologic and pathologic conditions has remained partly unclear due to lack
of knowledge about endogenous PPAR ligands. Pharmacological ligand research is
ahead of the identification of physiologic ligands. Partially characterized
inflammatory, angiogenic, and metabolic disturbances in pregnancy-related
diseases suggest that these synthetic PPAR agonists may be of potential use in
these conditions. Ongoing basic studies have elucidated the metabolic, antiinflammatory,
and angiogenic benefits of PPARα/β/δ and PPARγ/β/δ dual agonists and PPAR pan agonists
for treatment purposes. However, some experimental and clinical data have
uncovered unfortunate side effects of PPAR ligands, including cancer
progression and increased cardiac event rates. New generations of PPAR modulators
are under development and these promise to be more receptor-specific, and
hopefully will activate only a specific subset of target genes and metabolic
pathways to reduce untoward side effects. The potential role of PPARs in
regulation of inflammation and angiogenesis is intriguing and warrants further
studies. We submit that PPAR agonists may become beneficial drugs for
pregnancy-specific diseases, once their risks have been fully evaluated.



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*Source: 527048-2007-12-30.xml*