# The Potential Applications of Peroxisome Proliferator-Activated Receptorδ Ligands in Assisted Reproductive Technology

Authors: Jaou-Chen Huang
Journal: PPAR Research
 (2008)

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## Abstract

Peroxisome proliferator-activated receptorδ (PPARδ, also known as PPARβ) has ubiquitous distribution and extensive biological functions. The reproductive function of PPARδ was first revealed in the uterus at the implantation site. Since then, PPARδ and its ligand have been discovered in all reproductive tissues, including the gametes and the preimplantation embryos. PPARδ in preimplantation embryos is normally activated by oviduct-derived PPARδ ligand. PPARδ activation is associated with an increase in embryonic cell proliferation and a decrease in programmed cell death (apoptosis). On the other hand, the role of PPARδ and its ligand in gamete formation and function is less well understood. This review will summarize the reproductive functions of PPARδ and project its potential applications in assisted reproductive technology.


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## Body

## 1. Introduction

Assisted  reproduction uses a spectrum of
technologies to enhance fertility. In vitro fertilization (IVF) is the most
advanced and most sophisticated assisted reproductive technology (ART). Since the
first “IVF baby”, Louise Brown, was born in 1978, IVF has gradually been
accepted by the general public. Nowadays, IVF is a routine procedure to treat the
infertile couples. Recent data shows more than 45 000 IVF babies were born in
the US
each year (http://www.cdc.gov/ART/ART2005/section1.htm) and there are more than three millions “IVF
babies” in the world (http://news.bbc.co.uk/1/hi/health/5101684.stm).Compared with embryos in natural pregnancies, IVF
embryos have low implantation potential (about 15–20% per embryo,http://apps.nccd.cdc.gov/ART2005/nation05.asp). In order to increase IVF success, it is common
to transfer two or more embryos to the uterus. This practice not only increases
the odds of pregnancy but also increases the chance of multiple pregnancy.
Compared with natural conception, IVF pregnancies are thirty times more likely
to be multiple (32% versus 1%, http://www.cdc.gov/ART/ART2005/sect2_fig5-15.htm#Figure 8). Furthermore, 13% of the IVF multiple pregnancies
are “high-order” multiple pregnancies, that is, triplets or more. These pregnancies
are prone to develop obstetrical complications and pose great risks to mothers
and infants. As a result, some infants suffer short-term complications and
life-long sequels. Therefore, one of the most urgent tasks in ART is to enhance
the implantation potential of IVF embryos so that transferring single embryo
yields an acceptable chance of success.It has long been observed that, compared with in vivo
embryos, embryos cultured in simple media develop at a slower pace [1, 2] and have more apoptosis [3]. It is generally accepted that, compared with in
vivo embryos, IVF embryos are in a less optimal environment—they are not in the supportive and protective
environment of the oviduct. As a result, IVF embryos do not develop to their
full potential and do not implant as well as their in vivo counterparts. It has
been proposed that modifying embryo culture conditions, making them similar to
those of the oviduct, may improve embryo development and enhance IVF success.
Recent reports show embryos express PPARδ and that PPARδ activation by oviduct-derived ligand enhances
embryo development and implantation (more below). Thus PPARδ is a novel pathway that could be exploited to
enhance ART outcome. This article will review the current literature regarding
PPARδ and reproduction and outline the potential applications of its ligands in ART. Because PPARδ interacts with PPARα
and -γ, relevant information regarding PPARα and -γ will also be provided.


## 2. PPARs and Their Ligands

Peroxisomes are
organelles in eukaryotes that participate in fatty acid oxidation. In 1990, the
first PPAR (PPARα) was discovered in the mouse [4]. Two years later, PPARα
and two additional PPAR isotypes, PPARβ (also known as PPARδ) and PPARγ, were
discovered in the Xenopus [5]. Subsequently, all three isotypes were found in
mouse and many species including human. PPARδ was originally discovered in a
human osteosarcoma cell line [6] and later found to be the human homolog of PPARβ
in the Xenopus.PPARs are ligand-activated
transcription factors. They form heterodimers with another nuclear receptor,
retinoid X receptor (RXR), which also has three isotypes: RXRα, β, and γ [7]. The functions of
PPAR-RXR complexes are determined by PPAR isotypes. Although either PPAR or RXR
ligand can activate PPAR-RXR complexes, simultaneous PPAR and RXR binding yields
more potent activities [8]. Whereas PPARα and
PPARγ activate genes related to glucose and lipid metabolism, only a few genes
are reported to be directly regulated by PPARδ. PPARδ reportedly upregulates
PDK1, ILK [9] and 14-3-3ε [10], and downregulates PTEN [9] and 11 beta hydroxysteroid
dehydrogenase type 2 gene [11]. The functions of these genes, unlike those regulated by PPARα and
PPARγ, are not limited to energy homeostasis. They include carbohydrate
homeostasis (PDK1), cell migration, proliferation and adhesions
(ILK), signal transduction (ILK) and its modulation (14-3-3ε) and, finally, tumor suppression (PTEN).Unlike PPARα and PPARγ,
the outcome of PPARδ activation is not limited to the transcriptional
activities of genes directly regulated by PPARδ because PPARδ also modulates
the transcriptional activities of PPARα, PPARγ, other nuclear receptors (such
as estrogen receptor), and BCL-6 (a transcriptional repressor). A recent report
shows that binding of PPARδ by its ligand allows full transcriptional
activities of PPARα and PPARγ, which is normally inhibited by nonliganded
PPARδ [12]. In addition, binding of PPARδ by its ligand
releases a transcription repressor BCL-6 [13] which targets a group of genes with diverse
activities including transcription
regulation (n=18), protein binding (n=11), signal transduction (n=10),
catalysis (n=8), structural molecule activity (n=3), enzyme activity regulation
(n=3), protein transportation (n=2), cell movement (n=2), chaperone (n=1), and
unknown function (n=3) [14]. Thus
PPARδ interacts with an extensive array of intracellular proteins to regulate
cellular functions.A diverse group of chemicals including hypolipidemic drugs,
herbicides, and industrial plasticizers causes liver tumors in the rodents.
They induce peroxisome proliferation and led to the discovery of PPARα  [4]. Fatty acids, particularly the unsaturated fatty
acids, and certain eicosanoids bind to PPARα, -γ, and -δ with varying
affinities [8, 15]. Although all PPAR
isotypes bind to unsaturated fatty acids, PPARα has the highest affinity.
Eicosanoids from the lipoxygenase pathway (such as leukotrienes and
hydroxyeicosatetraenoic acids—HETEs) and the cyclooxygenase pathway (such as
prostaglandins—PGs) bind to PPARs: leukotriene B4 and 8(S)-HETE are PPARα ligand,
15-deoxy-Δ12,14-PGJ2 (a PGD2 derivative) is a PPARγ ligand, and PGI2 is a PPARδ
ligand [15]. In addition to the natural ligands, PPARs also
respond to synthetic ligands. Some of the synthetic PPAR ligands are currently used
to treat metabolic diseases: fibrates, which bind to PPARα, are hypolipidemic
agents; thiazolidinediones (TZDs), which bind to PPARγ, are insulin sensitizers.
A recent report shows that retinoic acid, depending on the ratio of cellular
retinoic acid binding protein 2 (CRABP-II)
and fatty acid binding protein 5 (FABP5), may function as a PPARδ ligand [16].


## 3. Biological Functions of PPARδ

The roles of PPARα and
PPARγ in energy homeostasis are relatively easy to understand because the
former is predominantly expressed in the brown adipose tissue and liver, and
the latter, the adipose tissue [8, 17]. While PPARα catabolizes lipid in the liver, PPARγ
facilitates fatty acid storage in adipose tissue by inducing the maturation of
preadipocyte to fat cells.The functions of PPARδ,
on the other hand, are not as easy to ascertain because PPARδ has a ubiquitous distribution (including high levels
of expression in the gut, kidney, and heart,  and a lower level of expression in
the liver) and interacts with extensive arrays of proteins in the cells (more in Section 2). Reported functions of PPARδ include the
formation of intestinal adenoma [18] and colon cancer [19], the healing of skin [20], the development of hair follicles [21], and the protection of cells against noxious
stimuli [10, 22]. The reproductive
function of PPARδ was revealed for the first time during the investigation of
cyclooxygenase-2 knockout mouse [23].


## 4. PPARδ and Reproduction

In primates, including humans, mature eggs are picked
up by the fimbria and become fertilized in the ampulla. The zygotes remain in
the oviduct for 72 hours; develop to morula/early blastocyst stage embryos
before entering into the uterus. During this period, the oviduct produces
soluble factors to promote embryo development and protect the embryo.As mentioned earlier, the
link between PPARδ and reproduction was first revealed at the implantation site
of cyclooxygenase-2 knock out mice [23]. Since then, it was learned that embryos express
PPARδ and that oviducts and embryos produce PGI2. Recent studies also show that exogenous PPARδ
ligand promotes the development of embryos and enhances their implantation
potential (more in Section 4.4).


### 4.1. Female Reproductive Tract and Embryos Produce PPARδ Ligand

We analyzed the metabolites of arachidonic acid by human
[24] and mouse [25] oviducts and found substantial amount of PGI2.
Further analysis shows that PGI2  production by the oviducts varies
according to the estrus cycle. It peaks shortly after ovulation, coincides with
the presence of cleaving zygotes in the oviduct and the “window” of embryonic
responsiveness to PGI2  [25]. Oviducts posses PGI2 synthase and
cyclooxygenase-2; the latter is the rate limiting enzyme of PG synthesis. The
increased PGI2 production is due to upregulation of the cyclooxygenase-2
gene. Oviduct also produces retinoic acid [26], the effects of oviduct-derived retinoic acid on
embryo development is controversial (details below).Similar to oviducts, embryos also metabolize
arachidonic acid via cyclooxygenase and lipoxygenase pathways. PGI2  is the most abundant metabolites of arachidonic acid by mouse embryos [27]. Preimplantation embryos express PGI2 synthase, and cyclooxygenase-1 and -2; all are expressed in early stage and
throughout the preimplantation period. The preimplantation embryos also produce
retinoic acid [28] but its role in embryo development is yet to be
determined.The uterus is known to produce PGI2 [29] but its central role in assisting embryo
implantation was not revealed until twenty years later [23]. The uterus produces retinoic acid [30] but its biological role is unclear. Similarly, the
ovary produces retinoic acid [31] and PGI2  [32] but the extent to which they interact with PPARδ to
influence oocyte maturation is not clear.


### 4.2. Testes Express PPARδ

All three PPAR isotypes are present in Sertoli and
Leydig cells of the testes: PPARα and -δ transcript and protein are expressed in Leydig cells and Sertoli cells of
rat [33], PPARγ1 transcript
is detected in human testis [34], both PPARα and -γ transcripts and
proteins are expressed in mouse Sertoli
cells [35]. In addition, mouse spermatids and spermatocytes
express PPARδ
[36]. The functionality of PPARδ in the testes is
supported by the presence of Ssm, a novel PPARδ target gene in mouse testes [37]. Thus PPARδ may directly or indirectly (i.e., via
PPARα or -γ) affect spermatogenesis. Information
regarding PPARδ expression and action in mature sperms is limited. We
previously report that iloprost (a PGI2  analog) does not affect
sperm activity [38]. However, the response of mature sperms to
synthetic PPARδ ligand or retinoic acid has not been reported.


### 4.3. Ovaries Express PPARδ

Similar to testes, the
ovary expresses all three PPAR isotypes [39]. While PPARδ is expressed throughout the ovary,
PPARα is mainly expressed in the theca and the stroma, and PPARγ, in the granulosa
cells (of human, pig, rodents, and sheep)
and the oocytes (of cattle and zebrafish).
Of the three PPAR isotypes in the ovary, only PPARγ shows cyclic changes thus
implying its role in follicular genesis and/or oocyte maturation. Since PPARδ may
regulate the transcriptional activity of PPARγ, the growth of follicles or
oocytes may be indirectly modulated by PPARδ ligand. The expression of PPARδ and
the effect of its activation on the oocytes remain unclear at the moment.


### 4.4. Preimplantation Embryos Express PPARδ

Compared with PPARγ [28, 40] or PPARα [28, 40], there is more
information regarding the expression of PPARδ and the outcome of its activation
on preimplantation embryos [28, 40]. Mouse embryos express
PPARδ at an early stage [40, 41] and throughout the
preimplantation period. Blastocyst stage embryos express PPARδ in the inner
cell mass and the trophectoderm [40]. PPARδ activation is associated with embryonic
cell proliferation and improved embryo development [40]. Supplementing L-165,041 (a synthetic PPARδ  ligand)
or iloprost (a PGI2 analog) to culture media enhances embryo hatching
[38, 40–42]. Pretreatment with iloprost also increases the
potentials of implantation and live birth of mouse embryos [40, 43].The reported effects of
retinoic acid on embryo development are inconclusive: some show it is
beneficial to embryo development [44, 45], others show it is
detrimental [46, 47]. A recent report shows
LG100268 (a synthetic RXR ligand) reduces trophectoderm cell proliferation in a
concentration-dependent manner, but enhances the development of bovine
blastocysts at 0.1 μM [48]. Since retinoic acid binds to retinoic acid
receptor (RAR), RXR, and (depending on the balance of intracellular CRABP-II and
FABP5) PPARδ, the effect of retinoic acid on embryo development is likely to
depend on its concentration as well as receptor availability. The latter is
likely to change according to the developmental stage of the embryo. More studies are needed to resolve this complex
issue.


## 4.1. Female Reproductive Tract and Embryos Produce PPARδ Ligand

We analyzed the metabolites of arachidonic acid by human
[24] and mouse [25] oviducts and found substantial amount of PGI2.
Further analysis shows that PGI2  production by the oviducts varies
according to the estrus cycle. It peaks shortly after ovulation, coincides with
the presence of cleaving zygotes in the oviduct and the “window” of embryonic
responsiveness to PGI2  [25]. Oviducts posses PGI2 synthase and
cyclooxygenase-2; the latter is the rate limiting enzyme of PG synthesis. The
increased PGI2 production is due to upregulation of the cyclooxygenase-2
gene. Oviduct also produces retinoic acid [26], the effects of oviduct-derived retinoic acid on
embryo development is controversial (details below).Similar to oviducts, embryos also metabolize
arachidonic acid via cyclooxygenase and lipoxygenase pathways. PGI2  is the most abundant metabolites of arachidonic acid by mouse embryos [27]. Preimplantation embryos express PGI2 synthase, and cyclooxygenase-1 and -2; all are expressed in early stage and
throughout the preimplantation period. The preimplantation embryos also produce
retinoic acid [28] but its role in embryo development is yet to be
determined.The uterus is known to produce PGI2 [29] but its central role in assisting embryo
implantation was not revealed until twenty years later [23]. The uterus produces retinoic acid [30] but its biological role is unclear. Similarly, the
ovary produces retinoic acid [31] and PGI2  [32] but the extent to which they interact with PPARδ to
influence oocyte maturation is not clear.


## 4.2. Testes Express PPARδ

All three PPAR isotypes are present in Sertoli and
Leydig cells of the testes: PPARα and -δ transcript and protein are expressed in Leydig cells and Sertoli cells of
rat [33], PPARγ1 transcript
is detected in human testis [34], both PPARα and -γ transcripts and
proteins are expressed in mouse Sertoli
cells [35]. In addition, mouse spermatids and spermatocytes
express PPARδ
[36]. The functionality of PPARδ in the testes is
supported by the presence of Ssm, a novel PPARδ target gene in mouse testes [37]. Thus PPARδ may directly or indirectly (i.e., via
PPARα or -γ) affect spermatogenesis. Information
regarding PPARδ expression and action in mature sperms is limited. We
previously report that iloprost (a PGI2  analog) does not affect
sperm activity [38]. However, the response of mature sperms to
synthetic PPARδ ligand or retinoic acid has not been reported.


## 4.3. Ovaries Express PPARδ

Similar to testes, the
ovary expresses all three PPAR isotypes [39]. While PPARδ is expressed throughout the ovary,
PPARα is mainly expressed in the theca and the stroma, and PPARγ, in the granulosa
cells (of human, pig, rodents, and sheep)
and the oocytes (of cattle and zebrafish).
Of the three PPAR isotypes in the ovary, only PPARγ shows cyclic changes thus
implying its role in follicular genesis and/or oocyte maturation. Since PPARδ may
regulate the transcriptional activity of PPARγ, the growth of follicles or
oocytes may be indirectly modulated by PPARδ ligand. The expression of PPARδ and
the effect of its activation on the oocytes remain unclear at the moment.


## 4.4. Preimplantation Embryos Express PPARδ

Compared with PPARγ [28, 40] or PPARα [28, 40], there is more
information regarding the expression of PPARδ and the outcome of its activation
on preimplantation embryos [28, 40]. Mouse embryos express
PPARδ at an early stage [40, 41] and throughout the
preimplantation period. Blastocyst stage embryos express PPARδ in the inner
cell mass and the trophectoderm [40]. PPARδ activation is associated with embryonic
cell proliferation and improved embryo development [40]. Supplementing L-165,041 (a synthetic PPARδ  ligand)
or iloprost (a PGI2 analog) to culture media enhances embryo hatching
[38, 40–42]. Pretreatment with iloprost also increases the
potentials of implantation and live birth of mouse embryos [40, 43].The reported effects of
retinoic acid on embryo development are inconclusive: some show it is
beneficial to embryo development [44, 45], others show it is
detrimental [46, 47]. A recent report shows
LG100268 (a synthetic RXR ligand) reduces trophectoderm cell proliferation in a
concentration-dependent manner, but enhances the development of bovine
blastocysts at 0.1 μM [48]. Since retinoic acid binds to retinoic acid
receptor (RAR), RXR, and (depending on the balance of intracellular CRABP-II and
FABP5) PPARδ, the effect of retinoic acid on embryo development is likely to
depend on its concentration as well as receptor availability. The latter is
likely to change according to the developmental stage of the embryo. More studies are needed to resolve this complex
issue.


## 5. Clinical Applications of PPARδ Ligand in ART

PPARδ can be activated via one of the three methods: PGI2  (either stable analog or natural PGI2), synthetic PPARδ ligand, or
retinoic acid. The data presented above supports the notion that embryonic PPARδ activation
by iloprost or synthetic PPARδ ligand may enhance ART outcome. However, using PPARδ ligand in ART should be approached with
caution. A PPARδ ligand suitable for ART use should have passed extensive
reproductive toxicology studies involving laboratory animals as well as
domestic species to assure the health of progeny. The potential applications of
PPARδ ligand in ART are listed below.


### 5.1. Using PPARδ Ligand to Enhance Gamete Function

The potential of PPARδ ligand in enhancing male gamete
function or production is unknown because there is limited information on the
effects of PPARδ ligand on the spermatogenesis and sperm activities. PPARδ may have
no effect on sperm function because PGI2 analog does not seem to
affect the motility of human sperms [38]. However, synthetic PPARδ ligand or retinoic acid
was not tested in the previous report.It is not clear the extent to which PPARδ ligand affects
the oocytes either directly or indirectly (through granulosa cells). At the
molecular level, PPARδ ligand has the potential to influence follicular genesis
and/or oocyte maturation via PPARδ or PPARγ. This influence depends on the
species-specific expression of PPARγ and PPARδ in the oocytes and the granulosa
cells [39]. Recent
reports show PPARγ ligand enhances the meiotic resumption of mouse oocytes [49] and reverses the adverse effects of diet-induced obesity
on the oocytes [50]. More research is needed to understand the
potential targets of PPARδ ligand in the ovary, that is, oocyte versus granulosa cells and PPARδ versus
PPARγ.


### 5.2. Using PPARδ Ligand to Enhance Embryo Development

Three independent laboratories using different
strains of mice show iloprost (a PGI2 analog) enhances embryo development
[10, 38, 41]. A recent report, based on 60 cryopreserved human embryos donated by
patients for research, confirmed the enhancing effects of iloprost on human
embryos [51]. Mouse embryos previously exposed to iloprost produce
more implantation sites and yield more live pups [43]. These positive findings support the use of PPARδ
ligand to enhance IVF outcome.Whereas no synthetic PPARδ ligand has been approved
by the FDA for clinical use, iloprost (which activates PGI2 receptor
and PPARδ) is approved by the FDA to treat pulmonary hypertension and
peripheral vascular disease. Iloprost has undergone rigorous toxicology test
and shows no teratogenicity [52]. Various animals (including rats, rabbit, and
monkeys) exposed to iloprost during the peri-implantation period and throughout
the pregnancy produce normal offspring. Based on four positive results from
independent sources (three involving mouse embryos and one involving human
embryo described above) and reassuring reproductive toxicology profile, a small
scale, phase I/II clinical trial using iloprost may be considered.
Supplementing iloprost to IVF embryos mimics the environment of the oviduct
which provides a PGI2  rich environment surrounding the embryos [24, 25].There is a long way before retinoic acid is ready
for use in ART. Given that PPARδ activation by retinoic acid is dependent on
the ratio of CRABP-II and FABP5, the extent to which retinoic acid functions as
a PPARδ ligand in the embryos is likely to vary based on the developmental
stage of the embryos. More research is needed to ascertain the stage-dependent
response of embryos to retinoic acid.


### 5.3. Using PPARδ Ligand to Augment PPARδ System at the Implantation Site

The uterus, being the site of implantation, is as
important as the embryos in ensuring a good ART outcome. Therefore, although
uterus is not an area served by ART, a discussion about PPARδ ligand and ART is
not complete without a brief discussion of the uterus.It is PPARδ at the implantation site that
establishes the first link between PPAR and reproduction [23]. Recent evidence suggests that a host of signaling
pathways involved in decidualization, implantation, and placentation converge
on PPARδ: maternal PPARδ is important for decidualization and implantation;
embryonic PPARδ is important for placentation [53]. Thus enhancing the PPARδ system of the uterus may
be a novel method to ensure ART success. Activating PPARδ system at the
implantation site is different from activating PPARδ system in the
preimplantation embryos. The former involves administering PPARδ ligand to
potential mothers during the peri-implantation period; the latter involves
exposing IVF embryos to PPARδ ligand prior to embryo transfer. The former may require
days of treatment; the latter only takes 18–24 hours (i.e., during
the eight cell to morular stage transition). In addition, more information regarding
genes and pathways activated by PPARδ ligand in the uterus and their impacts on
the progeny needs to be obtained before it can be used to target the uterus.


### 5.4. Using PPARδ Ligand to Improve Oocyte Cryopreservation Outcome

One important aspect of ART is the cryopreservation
of oocytes. Oocyte cryopreservation is considered as a solution to ovarian
aging in women who wish to defer raising their family; it is also viewed as one
of the methods to preserve fertility in young women who are about to receive
chemotherapy or irradiation. The outcome of oocytes cryopreservation is,
however, far from satisfactory [54]; its success is hampered by freezing injury [55]. Lipid in the egg membrane and inside the
cytoplasm is believed to be one of the contributing factors. Modifying membrane
lipid [56] or removing excess cytoplasmic lipid [57] reportedly enhances oocyte survival after
cryopreservation. Since PPARδ regulates lipid metabolism, it may mobilize lipid
and augment the viability and the developmental competence of cryopreserved
oocytes. The suitability of the above method in human ART requires more
research because lipid content in the oocytes varies among species [58].


## 5.1. Using PPARδ Ligand to Enhance Gamete Function

The potential of PPARδ ligand in enhancing male gamete
function or production is unknown because there is limited information on the
effects of PPARδ ligand on the spermatogenesis and sperm activities. PPARδ may have
no effect on sperm function because PGI2 analog does not seem to
affect the motility of human sperms [38]. However, synthetic PPARδ ligand or retinoic acid
was not tested in the previous report.It is not clear the extent to which PPARδ ligand affects
the oocytes either directly or indirectly (through granulosa cells). At the
molecular level, PPARδ ligand has the potential to influence follicular genesis
and/or oocyte maturation via PPARδ or PPARγ. This influence depends on the
species-specific expression of PPARγ and PPARδ in the oocytes and the granulosa
cells [39]. Recent
reports show PPARγ ligand enhances the meiotic resumption of mouse oocytes [49] and reverses the adverse effects of diet-induced obesity
on the oocytes [50]. More research is needed to understand the
potential targets of PPARδ ligand in the ovary, that is, oocyte versus granulosa cells and PPARδ versus
PPARγ.


## 5.2. Using PPARδ Ligand to Enhance Embryo Development

Three independent laboratories using different
strains of mice show iloprost (a PGI2 analog) enhances embryo development
[10, 38, 41]. A recent report, based on 60 cryopreserved human embryos donated by
patients for research, confirmed the enhancing effects of iloprost on human
embryos [51]. Mouse embryos previously exposed to iloprost produce
more implantation sites and yield more live pups [43]. These positive findings support the use of PPARδ
ligand to enhance IVF outcome.Whereas no synthetic PPARδ ligand has been approved
by the FDA for clinical use, iloprost (which activates PGI2 receptor
and PPARδ) is approved by the FDA to treat pulmonary hypertension and
peripheral vascular disease. Iloprost has undergone rigorous toxicology test
and shows no teratogenicity [52]. Various animals (including rats, rabbit, and
monkeys) exposed to iloprost during the peri-implantation period and throughout
the pregnancy produce normal offspring. Based on four positive results from
independent sources (three involving mouse embryos and one involving human
embryo described above) and reassuring reproductive toxicology profile, a small
scale, phase I/II clinical trial using iloprost may be considered.
Supplementing iloprost to IVF embryos mimics the environment of the oviduct
which provides a PGI2  rich environment surrounding the embryos [24, 25].There is a long way before retinoic acid is ready
for use in ART. Given that PPARδ activation by retinoic acid is dependent on
the ratio of CRABP-II and FABP5, the extent to which retinoic acid functions as
a PPARδ ligand in the embryos is likely to vary based on the developmental
stage of the embryos. More research is needed to ascertain the stage-dependent
response of embryos to retinoic acid.


## 5.3. Using PPARδ Ligand to Augment PPARδ System at the Implantation Site

The uterus, being the site of implantation, is as
important as the embryos in ensuring a good ART outcome. Therefore, although
uterus is not an area served by ART, a discussion about PPARδ ligand and ART is
not complete without a brief discussion of the uterus.It is PPARδ at the implantation site that
establishes the first link between PPAR and reproduction [23]. Recent evidence suggests that a host of signaling
pathways involved in decidualization, implantation, and placentation converge
on PPARδ: maternal PPARδ is important for decidualization and implantation;
embryonic PPARδ is important for placentation [53]. Thus enhancing the PPARδ system of the uterus may
be a novel method to ensure ART success. Activating PPARδ system at the
implantation site is different from activating PPARδ system in the
preimplantation embryos. The former involves administering PPARδ ligand to
potential mothers during the peri-implantation period; the latter involves
exposing IVF embryos to PPARδ ligand prior to embryo transfer. The former may require
days of treatment; the latter only takes 18–24 hours (i.e., during
the eight cell to morular stage transition). In addition, more information regarding
genes and pathways activated by PPARδ ligand in the uterus and their impacts on
the progeny needs to be obtained before it can be used to target the uterus.


## 5.4. Using PPARδ Ligand to Improve Oocyte Cryopreservation Outcome

One important aspect of ART is the cryopreservation
of oocytes. Oocyte cryopreservation is considered as a solution to ovarian
aging in women who wish to defer raising their family; it is also viewed as one
of the methods to preserve fertility in young women who are about to receive
chemotherapy or irradiation. The outcome of oocytes cryopreservation is,
however, far from satisfactory [54]; its success is hampered by freezing injury [55]. Lipid in the egg membrane and inside the
cytoplasm is believed to be one of the contributing factors. Modifying membrane
lipid [56] or removing excess cytoplasmic lipid [57] reportedly enhances oocyte survival after
cryopreservation. Since PPARδ regulates lipid metabolism, it may mobilize lipid
and augment the viability and the developmental competence of cryopreserved
oocytes. The suitability of the above method in human ART requires more
research because lipid content in the oocytes varies among species [58].


## 6. Summary

Competent gametes, quality embryos, and a receptive
endometrium are essential elements for a viable pregnancy. The outcome of ART
may thus be enhanced by improving any or all of the above elements. While it is
high time to consider using PPARδ ligand such as iloprost to enhance embryo
development, the application of PPARδ ligand in other areas of ART requires
more research. Continuing research on the reproductive functions of PPARδ and
the safety of its ligand will ensure a smooth translation of basic science to
clinical medicine.



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*Source: 794814-2008-12-16.xml*