Indian Journal of Experimental Biology Vol. 42, August 2004, pp. 747-757

Review Article

Fetal growth and development: Roles of fatty acid transport proteins and nuclear transcription factors in human placenta

Asim K Duttaroy Department of Nuiriti on, Faculty of Medic ine, University of Oslo, P.O. Box. 1046, N-0316 Oslo Norway

In the feto-placental unit, preferential transport of maternal plasma arachidonic acid (20:4n-6) and docosahexaenoic acid (22:5n-3) across the pl acenta is of critical importance for fetal growth and development. More than 90 per cent of the fat deposition in the fetu s occurs in the last 10 weeks of pregnancy. All of the n -3 and n -6 fatty acid structures acquired by the fetus have to cross the pl acenta and fetal blood are enriched in long chain polyunsaturated fatty acids (LCPUFA) relative to the maternal supply. Fatty acids cross the placental microvillous and basal membranes by simple diffusion and via the action of membrane bound (FAT, FATP and p-FABPpm) and cytoplasmic fatty acid-binding proteins (FABPs). The direction and magnitude of fatty acid flux is mainly dictated by the relative abundance of available binding sites. The existence of a fatty-acid-transport system compri sing multiple binding proteins in human placenta may be essential to faci litate the preferential transport of maternal plasma fatty acids in order to meet the requirements of the growing fetus. The critical importance of long-chain fatty acids in cellular homeostas is demands an effici ent uptake system for these fatty acids and their metabolism in ti ssues. In fact , involvement of several nuclear transcription factors (PPARy, LXR, RXR , and SREBP-1) is critical in the expression of genes responsible for fatty acids uptake, placental trophoblast differentiation and hCG production. These indicate that these receptors are potential regulators of placental lipid transfer and homeostasis. This rev iew di scusses importance of nuclear receptors and fatty acid binding/transport proteins in placental fatty acid uptake, transport and metabolism.

Keywords: Fatty acid transport protein, Fetal growth, Human placenta, Nuclear transcription factor

Fatty acids serve as a metabolic energy source, as building blocks for membrane lipids, and as cellular signalling molecules such as eicosanoids 1-6• In fact, essential fatty acids (EFA) and their long chain polyunsaturated fatty acids (LCPUFA) derivatives are of critical importance in cell growth and development2

-6

. They are also increasingly being recognised as important intracellular mediators of gene expression7

. Because of the fundamental role of EFA and LCPUFA it was hypothesised that the EFAILCPUFA status of tissue or cells is an important determinant of health and disease2

-6

. The multiple roles of fatty acids suggest that careful regulation of all aspects of their di sposition, including cellular uptake and subsequent intracellular transport is critical for the maintenance of cellular integrity. However, the very property that makes long chain fatty acid well suited to be components of membranes, i.e. their acyl chain hydrophobicity, complicates for the organism the task

Corresponding author Phone: +47 22 85 15 47; Fax: + 47 22 85 13 41 Email: a.k.duttaroy@medisin .uio. no

of transporting them from sites of intestinal absorption, hepatic synthesis and lipolysis to sites of utili sation. The insolubility of fatty acids in aqueous environment requires that specific trafficking mechanisms to deliver fatty acids for cellular needs8

• The uptake of fatty acids was long considered to be a passive process, involving partitioning of the fatty acid molecule into lipid bilayer of the plasma membrane, however studies from various laboratories clearly demonstrated the presence of different fatty acid-binding proteins both in the cytosol as well as in the cell membranes, and their involvement in the uptake and intracellular transport of these water­insoluble molecules9

-12

• In this review, I will discuss the importance of fatty acid binding/transpott proteins and their relationships with nuclear transcription factors in LCPUF A uptake by the human placenta and the growth and development of the feto-placental unit.

Fatty acid uptake in feto-placental unit During fetal development the deposit of LCPUFA

in the fetus is rapid during growth, and it is suggested that a failure to accomplish a specific component of brain growth due to inadequacy of LCPUFA of

748 INDIAN J EXP BIOL, AUGUST 2004

critical membrane lipids may lead to irrevocable damage13

. Fetal brain and retina are very rich in arachidonic acid, 20:4n-6, (ARA) and docosahexaenoic acid, 22:6n-3(DHA), and a sufficient supply of these fatty acids during the last trimester of pregnancy and the neonatal period is of great importance 13

-15

• Many studies have shown the levels of LCPUFA are higher in the fetal than the maternal circulation 13

•16

·17

• At birth, linoleic acid,l8:2n-6 (LA) represents about 10% of the total fatty acids in cord plasma compared to 30% in maternal plasma, but surprisingly, ARA concentration in cord plasma is twice (-10%) that observed in the mother (-5%). Similarly, a-linolenic acid, 18:2n-3 (ALA) concentration in the new-born (0.6%) is half that in the mother (0.3%), whereas DHA concentration is double (3% versus 1.5%) 13

"16

"17

• This situation in which the relative plasma concentration of n-3 and n-6 LCPUFA (mainly DHA, and ARA) exceeds that of their precursors (ALA, and LA) is specific to the new born and is never observed in adult. It is obviously an extremely favourable situation for the development of the new-born, especially at a time when high quantities of ARA and particularly of DHA are needed by brain and retina. Placental transport of LCPUFA from the maternal plasma is in practice crucial for fetal growth and development because fetal synthesis of LCPUFA is thought to be very low 13

-14

.18

• Moreover, human placental tissue lacks both the delta 6 and delta 5 desaturase activities 13

•18

, therefore any LCPUFA in the fetal circulation must primarily be derived from the maternal plasma. In the materna-fetal unit, free fatty acids (FFA) are the main class of naturally occurring lipids transferred across the placenta, irrespective of species or of the maternal source from which they originate in the maternal circulation 13•

19• Maternal

lipoprotein lipase must be active to facilitate placental uptake of FFA from circulating triacylglycerol but not from chylomicrons 13

-14

• We have demonstrated that placental microvillous membranes (MVM) exhibited two distinct tiiacylglycerol hydrolase activities- a minor activity at pH 8.0; and second major activity at pH 6.020

·2 1

. Triacylglycerol hydrolase activity at pH 8.0 of MVM appears to be lipoprotein lipase whereas the activity at pH 6.0 is unique as it get almost abolished by serum but is not affected by high NaCI concentrations. Therefore, it is possible that the inhibitory effect of serum on triacylglycerol hydrolase activity at pH 6.0 in MVM may not allow it to act on maternal lipoproteins, but only on intracellular triacylglycerol stores in placenta. If it is true then this

membrane bound triacylglycerol hydrolase (optima pH 6.0) may play an important role in releasing FFA by hydrolysing the placental stores of tiiacylglycerol. This unique triacylglycerol hydrolase activity at pH 6.0 therefore, may be involved in the packaging and/or release of FFA from the placenta, whereas lipoprotein lipase which is mostly present in placental macrophages may be responsible for hydrolysis of maternal plasma lipoproteins as suggested by Bonet et

/22 a . Uptake of FFA was long considered to be a passive

process, involving partitioning of the fatty acids molecule into lipid bilayer of the plasma membrane. Evidence has been provided for the involvement of several membrane associated fatty acid­binding/transport proteins in the uptake of long chain FFA (LCFA) into a variety of mammalian ti ssues: hepatocytes, adipocytes, cardiomyocytes, and jejunal mucosal cells23

·24

. Although, long chain FFA move very rapidly across artificial protein-free lipid bilayers, fatty acid binding or transport proteins are required in biological membranes to promote cellular uptake of LCFA from a complex environment 2526

. Several membrane proteins have been suggested to mediate FFA uptake into cells, such as plasma membrane fatty acid binding protein (FABPpm• 40kDa) 24

·27

, fatty acid translocase (FAT, 88 kDa) or CD36 (28) and fatty acids transport protein (FATP, 63kDa). Most of the tissues such as intestine, adipose, liver etc. have more than one membrane fatty acid transporters, however physiological implications are yet to be understood clearly 8

·29

.

Membrane associated fatty acid-binding proteins in human placenta

The placenta is comprised highly specialised trophoblast cells, which arise from the embryo and differentiate to perform specialised functions . These functions include invasion of the uterine wall, nutrient and waste transport, metabolism, evasion of the maternal immune system, and cytokine and hormone production. While passive diffusion does occur for some nutrient transfer, the fetal requirement for these nutrients is so great that passive diffuison alone is not adequate. Therefore, specific nutrient carriers, or transport proteins are located in the placenta that act to facilitate transfer and meet the increased nut1ients demand of the fetus during gestation.

The mechanisms involving preferential uptake of maternal LCPUFA by the placenta was first examined

DUTTAROY: FETAL GROWTH AND DEVELOPMENT 749

in our lab by determining the fatty acid binding characteristics of human placental membranes30

. The binding of EFNLCPUFA to human placental membranes is highly reversible. In addition, oleic acid, 18:1 n-9 (OA) binding is inhibited strongly by LCPUFA (ARA, y-linolenic acid, 18:3n-6, and eicosapentaenoic acid, 20:5n-3, EPA), followed by the relevant parent EFA (LA and ALA). The lack of strong inhibition of binding of EFNLCPUFA to placental membranes by OA suggests the existence of stronger affinities for EFNLCPUFA compared with OA30

. EPA and its eicosanoid metabolites are growth inhibitory as they reduce the availability of ARA and its metabolites by competing at cyclooxygenase/lipoxygenase and desaturationlelongation pathways of EF A metabolism. However, EPA has little inhibitory effect on ARA binding to human placental membranes30

. In contrast to EPA, its parent fatty acid, ALA inhibited the binding of both ARA and LA strongly. The competition experiments also suggest that the binding sites in human placental membranes have heterogeneous binding affinities for fatty acids. Binding sites have a strong preference for LCPUFA: the order of competition is AA>>>LA>ALA>>>>OA30

.

Evidence for the involvement of membrane protein in fatty acid uptake comes from the trypsin treated placental membranes which show a decrease in specific [14C] fatty acid binding compared with that of untreated membranes30

. The presence of FABPpm both in sheep and human placental membranes has been demonstrated3 u 2

. Protein (40 kDa) which binds only LCFA, was isolated and purified from human placental membranes24

• The pi value and the amino acid composition of human placental membrane fatty acid binding protein are different from those of hepatic or gut FABPpm32

. In addition, unlike ubiquitous FABPpm24

, placental FABPpm (p-FABPpm) does not have glutamic-oxaloacetic-transaminase (GOT:L-aspartate:2-oxaloglutarate: aminotransferase, E.C2.6. 1.1) acti vit/3

. Therefore, despite similar size ( 40kDa) and their membrane location (both are peripherally membrane bound protein), p-FABPpm and ubiquitous FABPpm differ both in structure and function. Pre-incubation of placental membranes with polyclonal antiserum against p-FABPpm inhibited the binding of fatty acids but the degree of inhibition varied depending on the type of fatty acid30

. Antibody mediated inhibition is much more acute for LCPUFA binding than the OA binding, suggesting again the heterogeneity of fatty acid, binding activity of p-

FABPpm and that this may be preferentially involved in the placental LCPUFA uptake. Fatty acid binding activity, PAGE radiobinding assay, and Western blot analysis of microvillous and basal membranes of the human placenta has clearly demonstrated that the p­FABPpm is exclusively located in the microvillous membranes of the human placenta34

. p-FABPpm binds only 14.58 % of the total fatty acids when the protein is incubated with fatty acid mixture with the exact composition and concentration as the maternal plasma FFA pool during the last trimester of pregnancy. Almost 98% of ARA and 87% of DHA present in the mixture bind to the protein which is much higher than for OA and LA (21% and 13% respectively)35

·36

.

Binding of palmitic and a-linolenic acid was nil or insignificant. In contrast, under identical experimental conditions, human serum albumin (HSA) binds 36% of the total fatty acids, but does not show any preference for particular fatty acids35

.36

. Radiolabelled fatty acid binding reveals that p-FABPpm has higher affinities (Kd) and binding capacities (B max) for LCPUFA compared with other fatty acids. The apparent Bmax values for oleic acid, linoleic acid, arachidonic acid and docosahexaenoic acid were 2.0 ± 0.14, 2.1 ± 0.17, 3.5 ± 0.11, 4.0 ± 0.10 mole/mole of p-FABPpm. whereas the apparent Kct values are 1±.0.07, 0.73 ± 0.04, 0.45 ± 0.03 and 0.4 ± 0.02 11M, respectively 37

. In the case of HSA, Kd and Bmax values for all fatty acids are around 111M and 5 mole/mole of protein, respectively. This data provides direct evidence for the role of p-FABPpm in preferential sequestration of maternal LCPUFA by the placenta for transport to the fetus 36

.

Human placental choriocarcinoma (BeWo) cells display placental differentiation markers including the production of placental specific proteins, and has been used extensively to study lipoprotein metaboli sm. Fatty acid uptake by BeWo cells is temperature­dependent and exhibits saturable kinetics. OA is taken up least and DHA most by these cells33

• Western blot analyses demonstrate the presence of p-FABPpm in BeWo cells which is absent from HepG2 cells, confirming our previous observation that the placental FABPpm is different from the hepatic protein33

.

Furthermore, pre-treatment of BeWo cells with anti-p­FABPpm antibodies inhibits most of the uptake of DHA (64%) and AA (68%), whereas OA uptake is inhibited only 32% compared with the controls treated with pre­immune serum33

. The order of fatty acid uptake inhibition by the antibodies is

750 INDIAN J EXP BIOL, AUGUST 2004

DHA>AA>>>ALA>LA>>>OA, indicating that p­FABPrm is involved in preferential uptake of LCPUFA by these cells similar to what was observed with human placental membranes30

. These results clearly demonstrate that p-FABPpm may be involved in the preferential uptake of LCPUFA by these cells. Studies on distribution of radiolabelled fatty acids in the cellular lipids of BeWo cells show that DHA was incorporated mai nly in tri acy lglycerol fraction , followed by phospholipid fraction, whereas for AA the reverse is true38

. Almost 60% of the total DH A taken up by the cells gets esterified into triacylglycerol, whereas 37% is in phospholipids fractions . One of the mai n functions of the placenta is to deliver DH A into the fetal circulation, therefore, the triacylglycerol form may favour the transport of DH A to the fetus circulation.

In addi tion to p-FABPrm• the presence of other two fatty acid transporters, FA T/CD36, and FA TP, has been demonstrated in human placenta using pure trophoblast cells and placental mi crovi llous and basal membrane preparations39

. FAT and FA TP are present in both the placental membranes, microvillous and basal membranes whereas p-FABPrm is only present in microvillous membranes39

. Although , the presence of multiple membrane fatty acid transporters in different tissues have been reported40 their complex interactions in the uptake of FFA are yet to be understood. Studies in other ti ssues suggest that these membrane proteins alone or in tandem may be iJ1volved in effective uptake of FFA 40. 11_

Unlike p-FABPrm. other fatty acid transporters such as FAT/CD36 and FATP do not have any preference for LCPUFA38

. Therefore, location of FAT and FATP on both sides of the bipolar placental cells may a llow bi-directional flow of all types of FFA (non-essential , EFA, and LCPUFA) across the placenta, whereas the exclusive location of p-FABPpm on the maternal side may favour the unidirectional flow of maternal plasma LCPUFA to the fetus by virtue of its preference for these fatty acids.

Cytoplasmic fatty acid-binding proteins (F ABP) in human placenta

Once taken up in the cell the intracellular transport of FFA is also though t to be mediated by FABP within the cytosol. Human placenta contains both liver type (L-) and heart type (H-) FABP39

. As there are many differences in L-FABP and H-FABP41

, the express ion of these two proteins (L-FABP and H­FABP) in the placental trophoblasts may be related to

differential fatty acids transport and metaboli sm to meet demands for feto-placental growth and development33

. The stoichiometry of binding is 1 mole fatty acid per mole protein for all FABP types except for the liver FABP which binds 1-2 mole FFA42

. H­FABP only binds FFA, whereas L-FABP binds heterogeneous ligands9

'42

. L-FABP has been implicated in cell growth and regulation by virtue of its binding to vmious growth stimulatory and inhibitory eicosanoids as well as selenium9

'14

'26

.42

• Prostaglandin , (PG)E 1 and haem bind L-FABP with higher affi nity than fatty acids, as do the lipoxygenase metabolites of arachidonic acid43

. The cyclooxygenase metabolites PGE2, TxB2 and LTB4 , do not bind L-FABP, whereas it binds cyclopentenone prostaglandins (PGA~. PGA2,

PGJ2, and ~ 1 2-PG]z) more av idly than oleic acid to L­

FABP. PGD2, PGE2 and PGF:w are poor competitors and PGE 1 is intermediate. L-FABP binds these cyclopentenone prostglandins avidly, and probably responsible for transport them to the nuclear membranes. Recently, it has been demonstrated that L­FABP transports ligands to peroxysome proliferator activated receptor (PPAR) through protein-protei n interaction, and thus may play an important ro le in gene expression44

.45

. However, no such role for H­FABP is known yet. Differential effects of FABPs on fatty acid uptake and esterification have been demonstrated using transfected mouse L-cell fibroblasts. L-FABP increases fatty ac id uptake and targets this fatty acid for esterification into phospholipids, while 1-FABP does not increase fatty ac id uptake but preferentially stimulates esterification into triacylglycero l46

. It is poss ible that these proteins have different roles in terms of intracellular traffickjng of fatty acids in syncytiotrophoblast. T he L-FABP and f-FABP genes exhibit distinct patterns of tissue specific and developmental regulation47

. In developing heatt, H-FABP mRNA has been detected at the earliest time point in rat ( 19 day gestation pe1iod)47

. However, very little is known on the regulation of expression of L­FABP and H-FABP in human placenta syncytiotrophoblasts . The presence of two different FABPs in placenta may be involved in channelling of specific fatty acids to ~-oxidation , in the synthesis of structural and storage lipid in the placenta (Fig. I ), in the transfer to fetal circulation or in placental growth and regulation, and PPAR activation.

Nuclear transcription factors and gene expression Nuclear receptors compri se a large family of ligand

regulated transcription factors48• They have been

DUTIAROY: FETAL GROWTH AND DEVELOPMENT 751

categorised into two groups---class I nuclear receptors, comprising steroid hormone receptors, which usually are sequestered in . the cytoplasm by heat shock proteins and upon ligand binding they homodimerize and migrate into the nucleus where they regulate transcription of key target genes. Whereas, class II nuclear receptors, including a large number of receptors that are localised in the nucleus also when not activated by their ligand, and depending on the receptor, can bind the DNA either as heterodimers, homodimers and, in some cases, as monomers. Several members of class II nuclear receptors are still named 'orphans' because their ligands have not been identified yet. The modular structure of nuclear receptors is generally conserved in almost all the members of this super-family, and includes AlB domain, contammg a ligand­independent transcription activation function (AF-1) at the N-terminus; the C domain containing a DNA­binding domain (DBD), characterised by two typical zinc-fingers that are involved in the recognition of specific DNA consensus sequences (Fig. 2); the D domain, also known as hinge region, which confers flexibility to the receptor for proper dimerisation and interaction to the DNA consensus sequences located on target genes and that can also interact with co­repressor proteins4950

; the E domain comprising the ligand-binding domain (LBO), an hydrophobic pocket that can accommodate the selective ligand for the receptor, and the ligand-dependent transcription activation function (AF-2); the F domain, which is present in some nuclear receptors and seems to

FA

Fig. !-Involvement of membrane fatty acid transport proteins and cytoplamsic fatty acid binding proteins in fatty acid uptake. [Fatty acids are taken up cell membrane fatty acid transporters, and subsequently their intracellular transport carried out by FABPs for fatty acid metabolism (oxidation, storage) or-gene expression via nuclear transcription factors].

negatively modulate the transcription activity of the receptor51

'52

.

Nuclear receptors control gene transcription through a series of complex events. Upon binding of the ligand to the hydrophobic pocket, the receptor undergoes a conformational change that exposes a helix 12, the surface for interaction with transcription co-activators, which has been identified as the AF-2 of the receptor53

.

At the same time, the structural modifications elicited by the ligand decrease the affinity to co-repressors, which are released from the receptors495054

• However, nuclear receptors can also be regulated in a ligand­independent fashion through the AF-1. Post­translational modifications such as phosphorylation55

and acetylation56 have been reported to modulate receptor activity. Co-activators and co-repressors interacting with nuclear receptors regulate the rate of transcription initiation not only by interacting with proteins of the pre-initiation complex and engaging RNA polymerase II57

, but also by recruiting histone acetyl transferases (HATs) or histone deacetylases (HDACs). Some of these co-regulators are also part of the HAT or HDAC complex. HATs and HDACs have opposite effects on the nucleosomal structure and local chromatin assembly, and consequently they also play a key role in activation and repression of gene transcription respectively, by affecting the accessibility of transcription factors to DNA and the epigenetic marking system constituting the so-called ' histone code' 58

. Because there is a large number of co­regulators that can be recruited by nuclear receptors, it

Ligand Binding dimerization

I

N--t AlB I c I D I E

~~~ DNA Binding region

I F rc y AF-2

Fig. 2--5chematic representation of a nuclear receptor. [A typical nuclear receptor is composed of several functional domains. The variable NH2-terrninal region (AlB) contains the ligand-independent AF-1 transactivation domain. The conserved DNA-binding domain (DBD), is responsible for the recognition of specific DNA sequences. A variable linker region D connects the DBD to the conserved ElF region that contains the ligand-binding domain (LBD) as well as the dimerisation surface. The ligand-independent transcriptional activation domain is contained within the AlB region, and the ligand-dependent AF-2 core transactivation domain within the COOH-terrninal portion of the LBD].

752 INDIAN J EXP BIOL, AUGUST 2004

is conceivable that gene activation or repression may be the result of their sequential or combinatorial assembly on specific promoters that recognizes chromatin templates and recruits basal transcription machinery and RNA polymerase II .

Peroxysome proliferator activated receptor (PPAR) in human placenta

PPARs are members of the nuclear receptor superfamily of transcritption factors that includes steroid receptors. They bind to fatty acids and fatty acids metabolites and so have been identified as regulators of lipid and glucose homeostasis. In addition, these receptors are involved in a wide variety of physiological processes ranging from cell differentiation, infl ammatory response, regulation of vascular biology, as well as in pathological conditions such as type II diabetes, atherosclerosis and tumors. Three distinct PPAR subtypes have been identified and characterised so far: the a (NR 1 C 1 ), ~/8 (NRIC3) subtype. Hypolipidemic drugs and fatty acids induce expression of L-FABP via PPARa in liver59. Recent evidence has shown that membrane fatty acid transport proteins (FAT, FA TP) are also modulated in a tissue specific manner by PPARs 60

·6I,

indicating that these fatty acid binding/transport proteins are the target genes for PPARs.

PPARy has been associated primarily with lipolytic effects and is known to regulate enzymes invol ved in adipose tissue differentiation and lipid storage and metabolism62. PPARy is the most intensively studied PPAR isoform. Studies have shown that this receptor participates in biological pathways of intense basic and clinical interest, such as differentiation, insulin sensitiVIty, type-2 diabetes, atherosclerosis, and cancer. PPARy exists in two protein isoforms that are created by alternative promoter usage and alternative splicing at the 5' end of the gene; PPARy2 contains 30 additional amino ac ids at theN terminus compared

with PPARYI· Whereas, many tissues express a low level of PPARYI· PPARy2 is fat-selective and is expressed at very high levels in that tissue.

Both PPARy and RXRa are expressed in human placenta, and that these two nuclear receptors cooperate to induce the synthesis of hCG, a hormone essential for human pregnancl3· 64

. The critical role of PPARy has been earlier demonstrated in mice deficient in PPARy exhibit abnormal placental development and trophoblastic differentiation65. PPARy and RXRa ligands increase hCGI3 transcript level, the hCGI3 promoter has been suggested to contain binding sites for PPARy/RXRa hetero­dimers63. In view of the general changes in placen tal trophic hormone production, this heterodimer may thus favour placental differentiation and maintenance. Therefore, stimulation of differentiation may occur through direct induction of placental hormone production triggered by the ligand-i nduced activation of PPARIRXR heterodimers, rather than through an increase in the levels of these receptors. Placental hormones would then induce and sustain mature placental function via a feed-forward amplification loop. Figure 3 shows the interaction of a generic PPAR with PPRE as a heterodimer with RXR, leading to expression of genes .

The tissue specific regulation of FAT/CD36 and FATP expression by PPARs has been demonstrated, with hepatic expression of FAT, and FATP under the control of PPARa whereas, in adipose tissue these proteins are in control of PPAR:y62·66

. In contrast , it appears that FABPpm expression is not under control of PPARa and PPARy 62·66

• However, no such information is available with regard to the human placental fatty acid transport/binding proteins. Several naturally occurring ligands for PPARy have been identified, including fatty acids, oxidised LDL derivatives, and prostaglandin metabolites 15-deoxy-t.-

Fig. 3-PPAR and molecular mechanism of action. [A generic PPAR is shown binding to a PPRE as a heterodimer with RXR in the presence of their specific ligands].·

DUTIAROY: FETAL GROWTH AND DEVELOPMENT 753

12,14 prostaglandin J2 (15~PGJ2)62 . The potential natural ligands involved in activating PPAR/RXR heterodimers in the placenta are not known yet, however it is possible that these ligands may activate expression of fatty acid binding/transport proteins in human placenta. Expression of p-FABPpm permits placenta to sequestrate ARA and DHA30 which may also act as ligands for PPAR and RXR. The presence of FAT, FATP, L-FABP, and H-FABP in placental trophoblasts30

.36 may help to deliver the ligands (fatty

acid and eicosanoids) to PPARy for gene expression. Therefore, it is conceivable that natural ligands for PPAR are either synthesized or taken up by the trophoblast, enabling PPAR/RXR heterodimer activation and subsequent hCG production and placental invasion.

Sterol regulatory element binding proteins and liver X receptors in human placenta

In addition to the PPAR family, several other transcription factors have been identified as targets for fatty acids regulation, including HNFa, SREBP-1c, LXRa, and ~. RXRa, and NFKB 11 . In addition to PPARy, the placenta has also sterol regulatory element binding proteins (SREBP), transcription factors that regulate cholesterol, and in response to sterols, has provided important insights into the regulation of lipid synthesis67·68

. SREBP-1 and -2 are separate gene products of approximately 125 kDa located in the membranes of the endoplasmic reticulum and nuclear envelope. The gene for SREBP-1 has two different transcription start sites that generate two mRNAs and proteins, SREBP-1 a and -1 c. · SREBP-1 and -2, are both detected in placenta67·68

. Recent studies have demonstrated that SREBPs also regulate the transcription of ACC and FAS, key enzymes for fatty acid synthesis69

"71 . Insulin

is also known to act via SREBPlc via augmenting the nuclear content of SREBP-lc, while LCPUFA suppresses the nuclear content through a post­transcriptional mechanism67·68

. The simultaneous regulation of enzymes in both cholesterol and fatty acid synthesis pathways may allow for coordinate regulation of the synthesis of both lipid species in the placenta. Liver X receptor (LXR) is the newest twist to this story and we are at an early stage in understanding this pathway. LXRs play a major role in lipogenesis through. the regulation of the gene encoding SREBP-lc72

·73 . LXRa and ~ are also

expressed in human placental trophoblasts involved in lipid synthesis (unpublished observation from our Jab). However, in hepatic cell lines, it appears that n3-LCPUFA can interfere with LXR action74·75 . This network controls lipogenesis and triglyceride synthesis. While it appears that n3-PUFA oppose insulin action, it must be remembered that insulin induces SREBP-1 c gene transcription, while n3-PUFA controls SREBP-lc by enhancing the turnover of the mRNA. N3-PUFA regulate hepatic glucose/lipid homeostasis by controlling the activity and/or abundance of separate transcription factors. Together, these effects may determine the shift in metabolism from fatty acid synthesis and storage to fatty acid oxidation in human placenta.

Conclusions

Placental delivery of EFAs and their LCPUFAs derivatives is very important for the development of the fetus. Effective co-ordination between fatty acid transport/binding proteins are required for efficient transfer of these fatty acids from the maternal source across the placenta for fetal growth and development. Figure 4 summarises role of several fatty acid binding/transport proteins in fatty acid uptake by the human placental trophoblasts. Critical regulators in cellular lipid homeostasis in placenta are lipid-sensing nuclear transcnpt1on factors (PPARy, LXR, RXR, SREBP) which control expression of several lipid metabolic genes such as fatty acid transporter/binding proteins, lipase, acyl­CoA synthetase. In addition , LCPUFAs and their derivatives which may serve as potential natural ligands for these nuclear transcription factors 11 can also affect placental function and thus their own transport. Maternal lipids may regulate feto-placental lipid metabolism via nuclear transcription factors such as PPARy, SREBP, LXR and fatty acid transporters. Further understating of the placental fatty acid transport and metabolism system is required in order to better treat pathological hyperlipidemia in pregnancy which results in phenotypes associated with the development of pathologies such as atherosclerosis disease in the general population. In fact, fatty streak formation , the precursor of atherosclerosis, and oxidised LDL are present in human fetal aortas and associated this finding with maternal hypercholesterolemia. The consequences of the adverse lipid and lipoprotein

754 INDIAN 1 EXP BIOL, AUGUST 2004

FAT

Maternal Plasma

Fetal Plssms

Albumin ~

FFA '=' -lstoprotein

Fig. 4--Schematic diagram of the putative roles of fatty acid binding/transporter proteins in human placental f<>tty acid uptake and metabolism. [The presence of FAT and FATP on both sides of bipolar placental cell membranes allow transport of a ll sorts of fatty acids in both directions (from the mother to the fetus and vice versa). However, by virtue of its exclusive location on the maternal-facing plac.!ntal membrane, p-FABPpm sequesters maternal plasma LCPUFAs to the placenta for fetal supply. Cytoplasmic FABPs may be responsible for transcytoplasmic movement of FFAs to their sites of esterification, 13-oxidation, or induce gene expression by interacting with PPARs. (FAT, fatty acid translocase; FATP, fatty acid transport protein; L-, liver-type; H-, heart-type; PG, prostaglandins; LT, leukotrienes; TG, triacylglycerols, PL, phospholipids; CE, cholesteryl esters; HEPTE, hydroperoxyeicosatraenoic acid)] .

changes in the pregnant women on fetal development (such as macrosomic or thin fat babies etc) are yet to be explored.

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