{"paper_id":"1b9d9c61-983f-47b4-9782-bf349429bf59","body_text":"Discovered in 1990,\nPPARs are known for their biological role in inducing the proliferation of\nperoxisomes in rodents [ 1 ]. They are transcription factors\nbelonging to the ligand-activated nuclear hormone receptor superfamily [ 2 ] and have been identified in\ndifferent species such as the xenopus, mouse, rat, and humans. In all these\nspecies, PPARs present three isotypes encoded by distinct single-copy genes:\nPPAR α  (NR1C1), PPAR β / δ  (also called NUC1 or NR1C2), and PPAR γ  (NR1C3), located\non chromosomes 15, 17, 6 in the mouse and chromosomes 22, 6, 3\nin humans, respectively. The PPAR γ  gene alternative\npromoters give rise to three different isoforms named  γ 1,  γ 2, and  γ 3 which\ndiffer at their 5′ends (see  Figure 1(a) ) [ 3 ]. PPAR  α ,  β ,  γ 1/ γ 3,  γ 2 translation produces proteins of 468, 441, 475, and 505 amino acids,\nrespectively, with a molecular weight of\n49 to 56 kDa [ 4 ]. By performing multiple PPAR nucleotide/protein\nalignments of PPARs in different species, a strong interspecies identity\n(human, mouse, rat, bovine,  ≈ 90%) has been\nestablished, illustrating a strong evolutionary conservation among species by\nderivation from a common ancestor ( Table 1 ). PPAR γ  shows the highest\nconservation in terms of cDNA and proteins.\nLike several other\nmembers of the nuclear receptor superfamily, PPARs possess the typical\nstructure organised in six domains named A to F (see  Figure 1(b) ) [ 5 ]. Domain C (DBD: DNA binding domain)\ncontains two zinc fingers and allows promoter target gene interaction and\ndimerization with its preferential nuclear receptor: retinoid X receptor (RXR).\nThe PPAR/RXR heterodimer binds to the target gene promoter response element\nnamed peroxisome proliferator response element (PPRE) which is made up of two\nhalf site AGGTCA separated by one or two nucleotides (also called DR1 or DR2\nfor direct repeat 1 or 2) and a  5 ′  extension A (A/T) CT. Domain E/F allows\nligand binding and contains a ligand-dependent transactivation function called\nAF2 (activating function 2). It is involved in dimerization and interaction\nwith cofactors.\nAs with the other nuclear receptors, the binding of the ligand is a key step in\nthe control of PPAR transcriptional activity. In the absence of a ligand,\ncorepressors and histone deacetylases (HDAC) bind to PPARs and inhibit the transcription\nactivation of target genes. PPAR ligands have the ability to dissociate the\ncorepressor complexes from the PPAR/RXR heterodimer, allowing the binding of\nthe coactivators in order to initiate and activate transcription.\nThere are two kinds of\nligands for the PPARs: natural and synthetic. Among the natural ligands the\nmonounsaturated fatty acids (FA) (e.g., oleic acid) and the polyunsaturated\nfatty acids (PUFA) (e.g., linoleic acid, linolenic acid, and arachidonic acid) are\ndescribed as ligands for PPAR α , PPAR β , and PPAR γ . They act with concentrations\nconsistent with those found in human serum [ 6 ]. The different \nPUFA metabolites: 8(S)- and 15-hydroxyeicosatetraenoic acid (8(S)- and 15-HETE), leukotriene B4\n(LTB4), 9- and 13-hydroxyoctadedienoic acid (9-HODE and 13-HODE) and 15-deoxy-Δ 12,\n14 -prostaglandin J2 (PGJ2) are potent selective activators of PPAR α  and\nPPAR γ . Some oxidized low-density lipoproteins (LDLs), oxidized\nalkyphospholipids, nitrolinolenic acid, and prostaglandin metabolites can also\nactivate PPAR γ  [ 7 ]. Recently, it has been demonstrated\nthat P450 eicosanoids are potent PPAR α  and PPAR γ  ligands [ 8 ]. Indeed, \nNg et al. [ 8 ] have shown that P450 catalysed arachidonic acid metabolites like\n20-hydroxyeicosatetraenoic acid (20-HETE) or 11, 12-epoxyeicosatrienoic acid\n(11, 12-EET) can activate PPAR α  and PPAR γ . These ligands induce PPAR binding to\nPPRE and can modify the expression of PPAR α  responsive genes like apoA-I or\napoA-II in the same way than synthetic ligands. Thus the finely regulated\nconversion of PUFAs to eicosanoids through either the lipoxygenase, cyclooxygenase,\nor cytochrome P450 monooxygenase pathways may provide a mechanism for the\ndifferential regulation of PPAR α  and PPAR γ  and their respective target genes.\nPPAR β  can be activated by different types of eicosanoids including prostaglandinA1\n(PGA1) and prostaglandin D2 (PGD2). Many synthetic ligands exist and have been\nused in PPAR work. These ligands include prostaglandin 12 analogs, pirinixic\nacid (Wy-14643) for PPAR α , hypolipidemic and hypoglycemic agents\n(nonthiazolidinedione) for PPAR β , and thiazolidinediones (e.g., rosiglitasone,\ntroglitazone) for PPAR γ  [ 2 ].\n\nThe adult PPAR expression patterns have been extensively established at the mRNA\nand protein levels in several species ( Table 2 ) [ 8 ,  9 ]. Several studies conducted during\nmammalian gestation have established the placenta as an important expression\nsite of the different PPARs isoforms. Our review will focus only on term\nplacental expression and on the amniotic/fetal membranes. The placental dynamic\nexpression of the 3 PPARs during early and midgestation (of mouse, rat, and\nhuman) is well described in Fournier et\nal., 2007 [ 4 ]. In rat placenta, all three PPAR\nisoforms are ubiquitously expressed from 11 days postcoitum (dpc) [ 10 ]. Both PPAR β / δ  and PPAR γ  are expressed after 8.5 dpc in mouse placenta. By immunohistochemistry\nand RT-PCR, the three PPAR isoforms are been shown to be expressed in the\nvillous trophoblastic cells and syncytiotrophoblasts of the human term placenta\n[ 4 ]. To extend the previously published\nresults [ 11 ] and to assess the potential\nimportance of PPAR proteins in fetal membranes, RT-PCR and immunohistochemistry\nexperiments were performed on human term placental samples. The three PPARs are\npresent in total placenta, amnion, chorion, and in amnion-derived WISH\nepithelial cell line at the mRNA (see  Figure 2(a) ) and protein levels (see  Figure\n2(b) ). The expression of PPAR α  and PPAR γ  seems to be weaker than that observed\nfor PPAR β / δ . In addition, a greater amplification of the PPAR γ  cDNA is obtained\nin chorion than in amnion, where PPAR γ  is almost undetectable.\n\nThe lipids of human amnion and chorion are enriched in the essential fatty acid\narachidonic acid, which is the precursor of all the prostaglandins of the 2 series\n[ 13 ]. Sixty-six percent of the\narachidonic acid of the human fetal membranes are available in\nthe glycerophospholipids of these tissues and can easily be converted into PGD 2  [ 14 ]. The placenta produces considerable\namounts of PGD2 [ 15 ]. The enzymes necessary to convert PGD 2  into prostaglandin J2 (PGJ2) are present and coexpressed with PPAR γ  in placenta. 15-Deoxy-Δ 12, 14 -PGJ2 (15dPGJ2) and its precursor PGD2 are present in amniotic fluid at concentrations that do\nnot exceed 3 nM [ 16 ]. However,\nthis amniotic fluid concentration cannot be an exact representation of the\nphysiological placental reality for PPARs ligands because the nuclear\nconcentration is not measured. The maternal blood may also be a source of PPAR\nligands for the human placenta and the fetal membranes. It has been established\nthat a heat-stable compound (not a protein, but rather a prostanoid or a fatty\nacid) is detected in maternal blood serum and is able to activate the PPAR γ  [ 17 ]. The presence of classical and new PPARs\nligands (e.g., P450 eicosanoids, PUFA metabolites) in placenta and fetal\nmembranes suggests that they could activate PPAR, induce PPAR binding to PPRE,\nand modify the expression of PPAR target genes; but this hypothesis has to be\nconfirmed by further analysis, based on PPARs activation in other organs. For\nexample, PUFAs, such as and eicosapentaenoic acid (EPA) and docosahexaenoic\nacid (DHA), increased PPAR γ  mRNA expression and binding to PPRE in renal\ntubular epithelial cell line\n(HK-2). Furthermore, they downregulate LPS-induced activation of NF- κ B via a\nPPAR γ -dependent pathway\nin HK-2 cells [ 18 ]. Another example showed that PGD2\nis among the most abundantly produced prostaglandins in synovial fluid by\nsynovial fibroblasts [ 19 ]. It can be converted into PGJ2. It has been demonstrated that PPAR γ \nligands (15dPGJ2) inhibit IL-1 β –induced\nproduction of nitric oxide (NO) and matrix metalloproteinase-13 (MMP-13) in\nchondrocytes. This inhibition was PPAR γ -dependent and occurred at the\ntranscriptional level, through repression of NF- κ B signalling [ 20 ]. These two examples support a role\nof PPAR ligands in fetal membranes.\nAs a determining result, the knockout of the PPAR γ  in mice [ 21 ] yielded the first findings indicating\nthe importance of this factor in early embryonic and perinatal development.\nThese results are concomitant with those obtained by the generation of RXR α  or\n β  null mice (PPAR γ  partner in the functional heterodimer), also showing an\nembryonic lethality explained by the lack of generation of a functional\nlabyrinthine zone [ 22 ]. Furthermore, complementary studies\nconducted by the inactivation of PPAR γ  coactivators or coregulators, such as peroxisome\nproliferators activator receptor-binding protein (PBP) and peroxisome proliferator-activated receptor-interacting protein (PRIP),\nalso lead to severe placental dysfunction, such as inadequate vascularisation\nof the structure [ 23 – 25 ]. Recently, Barak et al. also\ndemonstrated that the inactivation of PPAR β / δ  led to the formation of abnormal\ngaps and a thinner but fully differentiated vascular structure in the placentodecidual\ninterface [ 26 ]. These results establish the nonredundant\nroles of PPAR γ  and PPAR β / δ  in early mouse placental\ndevelopment. By contrast, the inactivation of PPAR α  has no\neffect on placental formation or on the developing foetus and by the way theirs\npossible roles during pregnancy had to be clarified [ 2 ]. In humans, the studies are almost\nexclusively focused on the PPAR γ  roles during early placentation. It\nhas been clearly established that all three PPARs can stimulate or inhibit the\ndifferentiation and/or proliferation of the villous cytotrophoblasts into\nsyncytiotrophoblasts and the synthesis of chorionic gonadotrophic hormone, and\nmay hamper extravillous trophoblastic cell invasion (for more details, see\nFournier et al., 2007 [ 4 ]).\nAs one of the first\nfunctions described for PPAR γ  in other tissues, trophoblastic lipid uptake and\naccumulation are also regulated in part by this factor [ 27 ]. The PPAR γ  ligands seem to increase the uptake and accumulation of the fatty acids\nin human placenta [ 28 ]. This regulation is associated with\nan enhanced expression of adipophilin (fat droplet-associated protein) and fatty acid transport\nproteins (1 and 4) in human trophoblasts [ 28 – 30 ]. These results were confirmed\nrecently by the in vivo activation of PPAR γ  by its agonist rosiglitazone in\nmice, which also leads to the enhancement of the previous described genes plus\ntwo new ones involved in the lipid transport: S3-12 (plasma associated protein)\nand myocardial lipid droplet protein/MLDP [ 27 ]. Taken together, these results\nconfirm the results obtained on PPAR γ -null mutants: the absence of the\nlipid droplets normally present around the fetal vessels in the wild-type\nplacenta [ 21 ].\nAt this stage of our knowledge of PPARs, the most interesting results have been\nobtained with the study of their involvement in the inflammation process,\nwhich may be linked to labor at term and also to the premature rupture of fetal\nmembranes (see  Figure 3 ). Term labor is associated with an increase in proinflammatory\nproteins and cytokines such as IL1 β , IL6, IL8, IL10, and TNF- α . This increase in\nproinflammatory proteins and cytokines induces uterine contractions. PPAR γ  ligands have been demonstrated to inhibit the secretion of IL6, IL8,\nand TNF- α  in amnion and chorion [ 31 ], highlighting the role of PPARs in\nthe regulation of the inflammatory response in human gestational tissues and\ncells [ 32 – 35 ]. The parathyroid hormone-related\nprotein (presenting a cytokine-like action) is involved in many processes\nduring normal and pathological pregnancies, and is decreased by PPAR γ  stimulation [ 36 ], which also blocks proinflammatory\ncytokine release by adiponectin and leptin [ 37 ]. The production of prostaglandins\nby the endometrium, the myometrium, and the fetal membranes induces the\ncontraction of the myometrium during labor. This generation of uterotonic\nprostaglandins correlates with the increased prostaglandin-endoperoxide\nsynthase type 2/cyclooxygenase type 2 (COX-2) activity and the increased secretory phospholipase A2-IIA (sPLA2) mRNA, proteins and activities. By inhibiting the production of the COX-2 and\nsPLA2 in fetal membranes, PPAR γ  promotes the quiescence of the\nuterus during gestation [ 34 ]. The molecular action of 15dPGJ2\nseems to involve interactions of the NF-Kappa B signaling pathway, inducing\nreduction of PGF2 α , PGE2, and MMP9 release in the placental\nenvironment [ 31 ]. This suppressive action of PPAR γ \non inflammation is apparently time-dependent during pregnancy. The PPAR γ  level\nof expression remains stable throughout gestation, except for the period just\nbefore labor, when its expression in fetal membranes declines. This reduction\nis coincidental with a relative increase in COX-2 expression [ 38 ]. Further work has shown this simple\nscheme to be more complex. While the expression of PPAR α  does not change at\nterm in amnion, it decreases in chorion. An increase was also demonstrated for\nPPAR β / δ  in chorionic and amniotic zones [ 11 ]. These last two findings raise the\nquestion of the involvement of the  α  and  β  isoforms in this process. The\nabsence of a real link between COX-2 and PPAR γ  is presented by Lindstrom and Bennett [ 39 ]. Finally, the PPAR action seems to\nbe concentration-dependent. A small amount of 15dPGJ2 ( < 0.1  μ M) acts through\nthe PPAR γ  signaling pathway, where at high\nconcentration (1  μ M) its actions are most probably mediated through\nother pathways: PPAR β / δ  and/or an inhibition of NF- κ B\nindependent of PPARs [ 35 ]. Furthermore, 15dPGJ2 and troglitazone were also demonstrated to have some antiinflammatory or apoptosis-induction\nspecific effects by PPAR γ -independent pathways. This was suggested by the work of Lappas et\nal. on human gestational tissues, demonstrating that this effect could\npassed by antagonist effect of 15dPGJ2 on the NF- κ B pathways and\nantioxidant effects of the troglitazone, a synthetic ligand of PPAR γ  [ 31 ].\nIn contrast to the\ndifferent roles described for PPARs during human placentation, only a few\nstudies on PPARs and placental pathologies have been conducted. In choriocarcinoma\nand hydatiform moles, a downregulation of the PPAR γ  expression is observed but\nthis real influence needs to be elucidated [ 40 ]. The potential involvement of PPAR γ  on preeclampsia is suggested by the fact that this pathology is\nassociated with an increased peroxidation in trophoblasts [ 41 ,  42 ]. An overproduction of 15-HETE has\nalso been noted, suggesting a deregulation of PPAR γ  [ 43 ]. This can cause a strong\ntransactivation of PPAR γ  during early pregnancy, resulting\nin a reduction of extravillous trophoblastic invasion, one cellular explanation\noften cited in the physiopathology of preeclampsia [ 44 ,  45 ]. Other abnormal transactivation of\nPPARs may be hypothesized to explain placental pathologies. The 15dPGJ2 has\nbeen shown to induce apoptosis of the placental (JEG-3) and amniotic (WISH) established cell\nline, [ 46 ,  47 ]. An excess of 15dPGJ2 production can\nbe a source of placental dysfunction linked to an increase in trophoblastic\ndeath. It is also established that deletion of PPAR γ , PPAR β / δ , and some of their coactivators (PBP, PRIP, and\nRAP250) induce abnormal placental phenotypes (abruption,\nreduction of fetomaternal exchanges, and alterations of trophoblastic\ndifferentiation) in null mutants [ 21 ,  23 ,  24 ,  26 ,  48 ,  49 ]. Chromosomal and/or genetic alterations\n(point mutation or deletion) may occur for these genes, inducing human\nplacental alterations. The placental 11 β \n    hydroxysteroid dehydrogenase type\n2 is a target gene of PPARs [ 50 ]. This enzyme plays a key role in\nfetal development by controlling fetal exposure to maternal glucocorticoids. An\nabnormal regulation by PPARs may result in an absence of fetal protection. In\nthe rat placental HRP-1 established cell line, the phthalate and derivatives\ntransactivate PPARs ( α  and  γ ) induced an increase in uptake rates of fetal essential fatty acid and\nthe transport of arachidonic and docosahexaenoic acid [ 51 ]. If such a mechanism can be induced\nby the phthalates during human placentation, this may strongly affect the fetal\nessential fatty acid content during growth.\nGestational diabetes is\nlinked to impaired lipids metabolism [ 52 ]. Decreased 15dPGJ2 in blood of\ndiabetic mothers is also linked to a decrease in\nplacental PPAR γ  expression. The inhibition of PPAR γ  results in an induction of a placental proinflammatory environment associated with an\nincrease in nitrogen monoxide production and release, which can impair fetoplacental\ndevelopment [ 53 ,  54 ].\nThe PPAR regulation of\ninflammation may be very important in another obstetrical pathology of the\namniotic membranes: the chorioamnionitis. This pathology, usually due to an\nascendant colonization of pathogenic microorganisms from the vagina to the\nuterus, is closely associated with preterm labor and premature rupture of\nmembranes (chorion and amnion). These\nruptures of membranes seem to arise from deregulated proinflammatory factor\nsynthesis. It has already been reported in this pathology that IL1 β , IL6, IL8,\nTNF- α , and prostaglandinE(2) show\ninadequate concentrations in placental membrane and in amniotic fluid [ 55 – 58 ]. As PPARs may be involved in the occurrence\nand control of this inflammatory response, further studies are needed to assess\ntheir importance in this process and to find new possible therapeutic strategies\nto prevent this damaging pathology.\nMore generally, the use of natural and\nsynthetic PPAR ligands looks to be a promising way in preventing placental\npathologies such as endometriosis or preeclampsia. An interesting study also\ndemonstrates that the reduction of LPS induction of cytokines is reduced by\nPPAR γ  ligands in fetal membranes. Nevertheless, the few studies already\nconducted were done practically only on animal (rodent) models and looks to\nhave positive effects on the pathologies (for review see Toth et al. [ 59 ]). Till now, the major problem using,\nfor example, TZD (thiazolidinedionzes) linking to the PPAR γ  pathways still the numerous adverse effects of this kind of treatment (e.g.,\nweight gain, anemia, leukopenia, etc.). These facts and the potential placental\nimpacts raised also the question of the use of these medical drugs to treat the\ngestational diabetes. Perhaps, at the level of clinician actual knowledge, PPAR γ  and its ligands could be used in\na first time, only as good early marker candidates for the diagnosis of\npregnancy pathologies like, for example, preeclampsia.\n\nSince the discovery of the PPARs, there\nhas been a marked increase in available data on their involvement in mammalian\ndevelopment. Concerning the placenta, all PPARs, but particularly PPAR γ , are\nessential for multiple physiological functions of the trophoblastic and\namniotic parts, leading to major involvement of PPARs in the pathophysiology of\ngestational diseases. However, special care must be taken when this particular PPAR\nsignaling cascade is involved, because part of the regulation may involve PPAR\nligand signalling (by the natural 15dPGJ2 ligand or the troglitazone\nsynthetic ligand) but may be transduced by independent nuclear receptor\npathways (as, e.g., by antagonizing effects on NF- κ B pathway for 15dPGJ2 and by acting as an antioxidant for troglitazone). This last point\nintroduces a new level of complexity in PPAR biology. It does not close preclusion of the\neventual use of PPARs for therapeutic treatment during pregnancy, but future\nmedical applications seem still to be a long way off. We can reasonably expect\nto see some obstetrical use of PPARs in diagnosis (detection of PPARs mutations\nin intrauterine growth retardation, predisposition of preeclampsia) and\ntherapeutics (tocolysis or treatment of chorioamniotis).","source_license":"CC-BY-4.0","license_restricted":false}