# Synergistic Effects of PPARγ Ligands and Retinoids in Cancer Treatment

Authors: Masahito Shimizu; Hisataka Moriwaki
Journal: PPAR Research
 (2008)

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## Abstract

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily.  The activation of PPARs by their specific ligands is regarded as one of the promising strategies to inhibit cancer cell growth.  However, recent clinical trials targeting several common cancers showed no beneficial effect when PPAR ligands are used as a monotherapy.  Retinoid X receptors (RXRs), which play a critical role in normal cell proliferation as a master regulator for nuclear receptors, preferentially form heterodimers with PPARs.  A malfunction of RXRα due to phosphorylation by the Ras/MAPK signaling pathway is associated with the development of certain types of human malignancies.  The activation of PPARγ/RXR heterodimer by their respective ligands synergistically inhibits cell growth, while inducing apoptosis in human colon cancer cells when the phosphorylation of RXRα was inhibited.  We here in review the synergistic antitumor effects produced by the combination of the PPAR, especially PPARγ, ligands plus other agents, especially retinoids, in a variety of human cancers.  We also focus on the phosphorylation of  RXRα because the inhibition of RXRα phosphorylation and the restoration of its physiological function may activate PPAR/RXR heterodimer and, therefore, be a potentially effective and critical strategy for the inhibition of cancer cell growth.


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## Body

## 1. Introduction

Peroxisome proliferator-activated receptors (PPARs) are members of a
superfamily of nuclear hormone receptors comprising three isoforms, PPARα,
PPARβ/δ,
and PPARγ,
which act as ligand-activated transcription factors.  PPARs play key roles in energy homeostasis by
modulating glucose and lipid metabolism and transport.  Through these metabolic actions, PPARs can
regulate cell proliferation, differentiation and survival [1, 2].
PPARs also control immune and inflammatory responses [3].
Because these physiological activities of PPARs are closely associated
with normal cell homeostasis, the aberrant expression and function of PPARs have
been observed in a variety of human malignancies.  Moreover, these reports also suggest the
possibility that targeting PPARs might be a critical strategy for inhibiting
the development and growth of cancers.
Indeed, numerous in vivo and in vitro studies have demonstrated that
PPAR agonists, especially, PPARγ
ligands can inhibit cell growth, cause apoptosis, and thus exert antitumor
effects in various types of human malignancies [4–6].
Based on the antigrowth and prodifferentiation action of PPARs, several
clinical studies have been conducted using the PPAR ligands in human
cancers.  However, with the exception of
a small trial on liposarcomas, the clinical trials have so far indicated that the
PPAR agonists may not be useful as a monotherapy for advanced malignancies [7–10].On
the other hand, recent preclinical studies show absorbing evidence that the combined
treatment with PPAR ligands plus a variety of other agents can cause a synergistic
effect to inhibit growth in cancer cells.
For instance, we recently found that the activation of PPARγ/RXR
heterodimer by their respective ligands synergistically inhibited cell growth
and induced apoptosis in human colon cancer cells [11].
Therefore, the aim of this paper is to review the possibility that the combined
usage of the PPAR ligands with other agents may therefore be a critical strategy
for the treatment of certain types of human cancers.  We also review the significance of the
aberrant phosphorylation of retinoid X receptor (RXR), which is a heterodimeric
partner for PPARs, as described in the next section.


## 2. RXRs and PPARs

RXRs and retinoic acid receptors (RARs), both of which are composed of
three subtypes (α,
β,
and γ),
are also members of the nuclear hormone
receptor superfamily.  The ligands for RXRs and RARs are the retinoids, a
group of structural and functional analogues of vitamin A, and the retinoids
have a profound effect on such cellular activities as growth, differentiation,
apoptosis, and morphogenesis primarily through binding to RXRs and/or
RARs.  A small portion of dietary
retinoids is converted to retinoic acid (RA), which is an active metabolite of the
retinoids.  RXR is specific for the 9-cis
RA, while RAR binds both 9-cis RA and all-trans RA (ATRA).  The nuclear retinoid receptors are
ligand-dependent transcription factors that bind to the retinoic acid receptor
responsive element (RARE) and retinoid X receptor responsive element (RXRE),
which are present in the promoter regions of retinoid responsive target genes,
thereby modulating the gene expression [12, 13].
Other nuclear receptors, including PPARs, also require RXR as a
heterodimeric partner in order to exert their function.  After ligand binding, PPARs can regulate
target gene expression by binding to the peroxisome proliferator responsive
element (PPRE) in target genes as a heterodimer with RXRs (see
Figure 1) [14, 15].
Therefore, RXRs play a fundamental role in controlling normal cell
proliferation and metabolism and act as a master regulator of nuclear
receptors.  Among the retinoid receptors,
RXRα
is thought to be one of the most important receptors with respect to the regulation
of the essential effects of cell activities.Figure 1
PPAR activation
pathway and transcriptional regulation of target genes.  After ligand binding, PPARs form heterodimers
with RXR in the nucleus. The PPAR/RXR
heterodimers interact with transcriptional coactivators (CoActs) and bind to
sequence specific PPRE located in the target genes that control glucose and
insulin homeostasis, lipid metabolism, inflammation, and cellular
differentiation. L: ligand.


## 3. Structure of RXRα and Significance of RXRα Phosphorylation

RXRs have a variable N-terminal domain (A/B domain;
AF-1), a highly conserved DNA-binding
domain (DBD),
a nonconserved hinge, and a moderately conserved C-terminus including the ligand-binding domain (LBD).Transcriptional activation is mediated by LBD,
which contains four more-or-less overlapping surfaces: a ligand-binding pocket
for the binding of small, lipophilic molecules, a transactivation domain (AF-2
or helix 12), a cofactor binding surface, and a dimerization surface
[16].  Recent studies revealed that phosphorylation
processes are critical for the transcriptional activity of RAR/RXR
heterodimers.  Bruck et al. [17] reported
that the activation of c-Jun N-terminal kinases (JNKs) induces phosphorylation
of both at three residues (serine 61, serine 75, and threonine 87) located in
the N-terminal AF-1 domain and one residue (serine 265) in the Omega loop in
LBD (AF-2 domain) of RXRα.  The RA-induced
phosphorylation of the same three residues in the AF-1 domain is required for
the cooperation of RXRα with RARγ for maximal transcriptional activity [18].  The phosphorylation of RXRα in its N-terminal domain plays a role to activate a
subset of RA-responsive genes and for the antiproliferative effect of RA [19].  These findings suggest that RXRα “positively” regulates the transactivation of target
genes through phosphorylation [20].On the other hand, there are some contrary reports which show the phosphorylation of RXRα to “negatively” modulate the function of its heterodimeric binding
partners.  Indeed, MAPK-mediated phosphorylation of the
RXRα LBD impairs the transcriptional activity of RXR/RAR [21, 22] and RXR/vitamin D3 receptor (VDR) [23].  These “negative” effects of RXRα via its phosphorylation might be associated with certain
types of human diseases, including cancer [20].
In the next section, we review
the specific roles of the aberrant phosphorylation of RXRα
in carcinogenesis, especially focusing on the development of hepatocellular
carcinoma (HCC).


## 4. RXRα Phosphorylation and Cancer

Abnormalities in the expression and function of retinoids and their
receptors play an important role in influencing the development of various
human malignancies and, therefore, might be critical targets for cancer chemoprevention
and/or chemotherapy [24].  Specifically,
we previously reported that hepatocarcinogenesis is accompanied by an
accumulation of the phosphorylated (i.e.,  inactivated) form of RXRα
and the inhibition of RXRα
phosphorylation may thus
be
an effective strategy for preventing the development of HCC.  Initially, we showed that the RXRα
protein is anomalously phosphorylated at a specific site of the
serine/threonine residues and is accumulated both in human HCC tissue as well
as in HCC cell lines [22].
Phosphorylation at serine 260 of RXRα,
a consensus site of mitogen-activated protein kinase (MAPK), is closely linked
to its retarded degradation, low transcriptional activity, and the promotion of
cancer cell growth, and the abrogation of phosphorylation by MAPK-specific
inhibitors restores the degradation of RXRα
in a ligand-dependent manner [22, 25].  In
addition, the phosphorylated form of RXRα
(p-RXRα)
is also resistant to ubiquitination and subsequent proteasome-mediated breakdown
in both human HCC tissues and a human HCC cell line, whereas RXRα
protein is unphosphorylated and highly ubiquitinated in the normal liver and in
nonproliferating hepatocyte cultures [26].  The
phosphorylation of RXRα
abolishes its ability to form heterodimers with RARβ,
thus resulting in the loss of cell growth control, resistance to retinoids, and
the acceleration of cancer development [27].
These findings suggest that the accumulation of p-RXRα
(i.e., nonfunctional RXRα),
which can escape from the proteasome-mediated degradation system, may interfere
with the function of normal RXRα
in a dominant-negative manner, thereby playing a critical role in the
development of HCC (see Figure 2) [28].Figure 2
A schematic
representation of RXRα phosphorylation in HCC cells.  In normal hepatocytes, when the ligand
(retinoid) binds to and activates RXRα, the receptor becomes able to heterodimerize with other
nuclear receptors, including RAR, and then activates the expression of the
target genes, which may regulate normal cell proliferation and
differentiation, by binding to
the specific responsive element.
In HCC cells, the Ras/MAPK pathway is highly activated and
phosphorylates RXRα at serine residues, thus impairing the
functions of the receptor.  Therefore,
the accumulated p-RXRα interferes with the remaining normal RXRα, presumably, in a dominant negative
manner, thereby playing a critical role in the development of HCC.
L: ligand. Ub: ubiquitin.In addition to HCC, a malfunction of RXRα
due to a posttranslational modification by phosphorylation is also associated
with the development of other types of human malignancies.  We recently reported that RXRα
protein is highly phosphorylated and also accumulated in human colon cancer
tissue samples as well as human colon cancer cell lines, while the levels of
expression of p-RXRα
do not increase in normal colonic epithelial cells; RXRα
protein is phosphorylated in 75% of colorectal cancer tissues when compared
with corresponding normal colon epithelial tissues [11].
Similar results have also been observed in human pancreatic cancer
(manuscript in preparation).  Moreover,
Kanemura et al. [29] reported the abnormal phosphorylation of
RXRα
protein to play a role in the enhancement of cell proliferation, while
producing an antiapoptotic effect, and also presumably acquiring RA-resistance
in HL-60R human leukemia cells.  In
addition to these malignancies, full-length RXRα
is anomalously phosphorylated and accumulated in leiomyoma when compared to
myometrial cells and this is associated with a resistance to ligand-mediated
ubiquitination and a delay in the receptor proteolytic degradation [30].What are the precise mechanisms by which phosphorylation of RXRα
loses its transcriptional activity?
Recent studies indicate that the phosphorylation of RXRα
can regulate the function of its heterodimeric binding partners.  For instance, Solomon et al. [23] reported
that phosphorylation of RXRα
at serine 260, which is located in the Omega loop of the LBD, results in the
attenuation of ligand-dependent transactivation by RXR/VDR complex in human
keratinocytes, thus resulting in the induction of malignant transformation.  The residues located in the AF-2 domain are also
phosphorylated in response to stress agents, including JNKs and MKK4/SEK1 [21, 31],
among which serine 265 located in the Omega loop [31],
and these phosphorylations are closely linked to inhibit the transcription of RA target genes.  The phosphorylation of RXRα
at serine 260 is also associated with retinoid resistance [22, 27].  Therefore, these findings indicate that RXRα
phosphorylation, which occurs at specific residues located in the Omega loop of
the LBD, is apparently associated with a malfunction in the retinoid-dependent
signaling pathway.  The Omega loop,
located between helices H1 and H3, is a very flexible and dynamic region that
moves substantially during the conformational rearrangement that accompanies
ligand binding to the LBD [32].  It has
therefore been proposed that phosphorylation of the residues in this loop might
alter the dynamics of this region and create conformational changes within the
LBD, thus disrupting the interactions with coactivators and therefore
inhibiting the activation of RA-responsive genes [17, 33].


## 5. Phosphorylated RXRα is a Critical Target for Cancer Treatment

The above findings support the possibility that the inhibition of RXRα
phosphorylation and the restoration of its physiological function as a master
regulator of nuclear receptors must be an effective strategy for controlling cell
growth in various types of human cancers.
It has been shown that the new synthetic retinoid, acyclic retinoid
(ACR, NIK-333: Kowa Pharmaceutical Company Ltd., Tokyo, Japan), which was originally developed as an agonist
for both RXR and RAR [34, 35], can restore the function of RXRα
by inactivating the Ras-Erk signaling system and thereby inhibiting RXRα
phosphorylation [25].
Practically, this agent has demonstrated several beneficial effects in
experimental studies both in vivo and in vitro.
For instance, ACR inhibited chemically induced
hepatocarcinogenesis in rats as well as spontaneously occurring
hepatoma in mice [36].
This agent also inhibited growth and induced apoptosis in human HCCderived cells [37–42].  Similar growth inhibitory effects are also
observed in other types of human cancer cells, such as squamous cell carcinoma
or leukemia cells [43, 44].In addition, we also confirmed the chemopreventive effect
of ACR on recurrent and second primary HCCs in patients who received anticancer treatment for an initial
HCC in a double-blind and placebocontrolled clinical study.  Namely, the oral administration of ACR for 12
months significantly reduced the incidence of posttherapeutic
recurrence of HCC and improved the survival rate in patients who underwent
potentially curative treatments, without causing any severe adverse effects [45–47].  These
findings suggest that ACR is a promising agent for the chemoprevention of HCC
and that p-RXRα
may be a critical target for the chemoprevention and/or treatment of some types
of human cancers, including HCC, which show the accumulation of p-RXRα
protein.


## 6. Synergy between PPARγ Ligands and Retinoids in Cancer

Since RXR forms a permissive heterodimeric complex with PPAR, and the activation
of PPARγ
exerts antigrowth effects in cancer cells [4–6], it seems to be reasonable that the
combination of RXR and PPARγ
agonists may offer new therapeutic strategies for various types of human
malignancies.  Firstly, Tontonoz
et al. [15] reported
that the combined use of PPARγ and RXRα specific ligands is able to trigger
terminal differentiation of primary human liposarcoma cells in vitro. This result suggests that the combination of these ligands may be a
useful therapy for the treatment of liposarcoma [15].
Beneficial effects for the combined treatment with PPAR ligands plus
retinoids are extensively reported in preclinical studies of the hematologic
malignancies [48–51]. Therefore, the combination of PPARγ
ligand with RXR agonist or RAR agonist can enhance the differentiating and
growth-inhibitory effects in human leukemia cells [48].  The
combination of PPARγ
ligand, ciglitazone, and ATRA synergistically reduces the cell growth rates and
cell cycle arrest at the G1 phase in HL-60 human leukemia cells, and this is
associated with synergistic upregulation of PTEN expression [49]. The
combination of 9-cis RA and PPARγ
ligand shows significant synergistic effects for the induction of apoptosis in
multiple myeloma cells [50]. These reports suggest that the combination of PPARγ
ligands plus retinoids holds promise as a novel therapy for some types of
hematologic malignancies by activating the transcription of target genes that
control apoptosis and differentiation in these malignant cells.In
addition to the hematologic malignancies, a number of preclinical studies
indicate the preferable effects by the combination of PPAR ligands plus retinoids
on the inhibition of cell growth in solid malignancies, especially in breast
caner [52–55]. For
instance, Rubin et al. [55] showed that a combination of ligands
for PPARγ and RXR inhibits breast aromatase
expression induced by tumor-derived factors.
Because aromatase activates estrogen biosynthesis, the combination of
these ligands may be able to find utility in thetreatment of
estrogen-dependent carcinogenesis, such as breast cancer and endometrial cancer [56]. The
combination of RXR ligand with ciglitazone also cooperatively inhibits the
growth of breast cancer and lung cancer cells by activating the RARE promoter
activity and inducing RARβ,
which plays a critical role in mediating the growth-inhibitory effects of
retinoids in various cancer cells [57].  In
addition, the synergistic or cooperative effect of RXR and PPARγ
agonists for growth inhibition and apoptosis induction is found in colon cancer
cells [11, 58]. The
detailed effects of PPARγ
ligands plus retinoids to inhibit growth in colorectal cancer cells are
discussed in the next section.What
are the molecular mechanisms by which the combination of PPARγ
ligands and retinoids synergistically induce anticancer effects?  Yang et al. [59] reported that the PPARγ
and RXR ligands have been shown to differentially recruit subsets of
transcriptional coactivators (i.e., p160 by RXR and DRIP205 by PPARγ)
to the receptor complex, thus leading to an enhanced transcriptional activation
and cellular effects.  The
transcriptional activity of PPRE is additively induced by treatment with a PPARγ
activator plus 9-cis RA, and RXRα
accumulation, by inhibiting its degradation due to the proteasome system,
therefore contributes to the enhancement of PPARγ/RXR
activation [60].  The
transactivation of the PPRE by PPARγ/RXR
heterodimer enhances the expression of the glutathione S-transferase gene, which is responsible for the cellular metabolism as well as the
detoxification of several xenobiotics and carcinogenic compounds [61].  The
findings of these reports suggest that the accumulation of the unphosphorylated
form (i.e., functional form) of RXRα
activates the transcriptional activity of PPRE and thereby enhances the
expression of important target genes.  The
significance of the restoration of RXRα
by inhibiting its aberrant phosphorylation is reported in the studies using the
cell lines of HCC [22, 28], leukemia [29], and colon cancer [11], as discussed below.


## 7. Synergy between PPARγ Ligands and Retinoids in Colorectal Cancer

Among the PPAR targeting therapies, the activation of PPARγ
by its ligand is regarded as a potentially useful strategy for the
chemoprevention and/or treatment of colorectal cancer because many in vivo and
in vitro preclinical studies have demonstrated that PPARγ
ligands can inhibit cell growth, cause apoptosis, and thus exert antitumor
effects in this malignancy [5, 62–65].  As
a result, there has been considerable interest in utilizing the combination of
ligands for PPARγ
and RXR for the prevention and treatment of colorectal cancer.  In fact, it has been reported that in human
colon cancer cells the combination of the RXR and PPARγ
agonists produces greater efficacy in growth inhibition than either single
agent alone, and this is associated with a cooperative reduction in the levels
of cyclooxygenase-2 (COX-2) expression and prostaglandin E2 (PGE2)
synthesis [58].  The
simultaneous exposure of HT-29 human colon cancer cells to ciglitazone and
9-cis RA results in an increased apoptotic effect and greater inhibition of
COX-2 expression, in comparison to cells treated with either ciglitazone or
9-cis RA alone [66].  We
recently reported that the combination of 9-cis RA and ciglitazone causes a
synergistic inhibition in the growth of human colon cancer Caco2 cells, which
express high levels of p-RXRα
protein, and this is associated with the induction of apoptosis and inhibition
in the expression of both COX-2 and c-Jun proteins and mRNAs.  The combination of these agents has a
synergistic effect in increasing the PPRE activity and decreasing the AP-1
activity.  However, we should emphasize
that these preferable effects are observed when the phosphorylation of RXRα
protein is inhibited [11].
Therefore, the inhibition of the phosphorylation of this
protein appears to play a critical role in inducing the synergistic growth
inhibitory effect in colon cancer cells.The above findings indicate that the activation of the RXR/PPARγ
heterodimer by their specific ligands can decrease the expression of COX-2, which
is one of the main mediators in the inflammatory signaling pathway.  This seems to be significant because COX-2
plays a critical role in the development of colorectal cancer and might, therefore,
be an important molecular target for colorectal cancer prevention and treatment
[67].  Recent
studies have revealed 2, 4, 6-trinitrobenzene sulfonic acid-induced colitis to be
significantly reduced after the administration of both PPARγ and RXR agonists, and this beneficial effect is reflected by the
reduction in the NF-κB DNA binding activity in the colon [68].  The inhibition of the β-catenin mediated pathway, which
promotes the development of colon cancer and is stimulated by COX-2 as well as PGE2 [69, 70], by nonsteroidal anti-inflammatory drugs,
requires a high-level expression of RXRα
and PPARγ
[71].
Therefore, the activation of the RXR/PPARγ heterodimer by the coadministration of their ligands is
clinically useful for the prevention
and/or treatment of colon cancer as well as colonic inflammation [72], due to their
synergistic effects on the COX-2/PGE2 axis.


## 8. Synergy between PPARs Ligand and the Other Drugs except for Retinoids in Cancer

In addition to the retinoids, the synergistic
effects of PPARγ ligands with other agents have also been
reported by many investigators.  Girnun et
al. [73] found that agonist activation of PPARγ synergistically increases the growth-inhibitory
effect of the platinum-based drugs cisplatin and carboplatin in several
different types of cancers in both in vivo and in vitro studies.  This synergy is associated with the reduction
of multiple members of the metallothionein gene family expression, which
play a role in the resistance of certain cancers to platinum-based drugs [74] by PPARγ [73].  The synergistic
or enhancing effects induced by the combination of PPAR ligands plus other
conventional chemotherapeutic agents to inhibit cell growth are also reported
in several types of cancer cells [75–77].  In addition, it is also reported that the histone deacetylase (HDAC) inhibitors have a
synergistic effect with the thiazolidinediones in the activation of PPARγ
target genes [78].  In studies
using cancer cells, the combination
treatment using the PPARγ
agonist pioglitazone and the HDAC inhibitor valproic acid has been reported to
be more efficient at inhibiting prostate tumor growth than each individual
therapy alone [79].  An enhanced growth inhibition is observed
when neuroblastoma cells are treated with a PPARγ
ligand and a HDAC inhibitor, thus suggesting that a combination therapy to
treat neuroblastoma might prove more effective than using either agent alone [80].
These findings suggest that a combination therapy using PPARγ
agonists and HDAC inhibitors might therefore be potentially effective for the
treatment of some types of human malignancies.Recently, molecular-targeted therapy is
attractive as a new effective strategy to inhibit the growth of cancer cells, and
therefore, the combination therapy using such specific molecular-targeting
agents plus PPAR ligands may become an important regimen in near future.  For instance, the proteasome inhibitor bortezomib, which can inhibit the NF-κB activity,
augments the antiproliferative effects of the PPARγ
agonist rosiglitazone in human melanoma cells [81].  The
dual ligand specific for PPARα/γ
synergistically enhanced the antiproliferative and proapoptotic effect of
imatinib, a specific inhibitor of BCR-ABL tyrosine kinase, in Philadelphia
chromosome-positive lymphocytic leukemia and chronic myelogenous leukemia blast crisis cell lines [82, 83].  The
growth inhibitory effects by gefitinib, an EGFR tyrosine kinase inhibitor, on
the human lung cancer cell line are potentiated by the treatment with PPARγ
ligand, and this is associated with an increase in the expression of PTEN, but
a reduction in the expression of p-Akt [84].  It is
interesting to note that the
activation of PPARγ
by its ligand causes a dramatic inhibition of the tyrosine phosphorylation of
HER2 and HER3 receptors, the other member for the EGFR family of receptor
tyrosine kinases (RTKs), in human breast cancer cells [85].  The
PPARγ
ligand also blocks phosphorylation of other member of RTKs, such as IGF-1R,
thereby suppressing the proliferation of breast cancer cell lines [85].
These findings may explain the mechanisms in regard to precisely how the
PPARγ ligands can enhance the effects of specific
RTK inhibitors, although some other molecules may also play a role.


## 9. Is PPAR Phosphorylation Associated with Cancer?

As mentioned above, a malfunction of RXRα
due to phosphorylation is associated with cancer cell growth and retinoid
resistance [11, 22, 27, 29].  However,
a question which arises here is whether the phosphorylation of PPARs also plays
a role in carcinogenesis and/or resistance to the PPAR ligands.  Recent studies have shown that PPARs are
phosphoproteins, and their transcriptional activity is affected by several
kinases, including ERK/MAPK, both in a ligand-dependent and/or -independent
manner [86].
Although the significance of the PPARs phosphorylation in cancer has not
been clarified, at least in PPARγ,
the transcriptional activity of this receptor is inhibited by phosphorylation [87–89].
Extracellular signals that activate intracellular phosphorylation
pathways can influence the degradation process of PPARγ
[89, 90].
These reports may suggest that as well as RXRs [22, 27], the phosphorylation-mediated inhibition of
transcriptional activity of PPARs is associated with cancer [20].
Hedvat et al. [91] conducted an interesting study, reporting
that the activation of PPARγ
is sustained by the presence of HER-kinase inhibitor, suggesting that
HER-kinase and its downstream ERK/MAPK pathway phosphorylate PPARγ
and, therefore, abrogate the effects of PPARγ
activity through degradation of this nuclear receptor.  In this study, the inhibition of HER-kinase
activity was sufficient to inhibit PPARγ
protein degradation [91].
These findings suggest that, in future studies, the combination of PPARγ/RXR
ligands plus a specific agent which targets the RTKs and/or Ras/MAPK signaling
pathway may therefore become a promising strategy to inhibit the growth of
cancer cells by inhibiting the phosphorylation of PPARs/RXRs.


## 10. Conclusion

The combined use of two or more agents is often advantageous since it
may permit to lower the clinical dosages, thereby decreasing the overall
toxicity, and thus providing the potential for synergistic effects between
agents.  In this review, we made an attempt
to show the synergism between PPARγ
agonists and other agents (see Figure 3).
Among such preferable candidates, retinoids seem to be the best partner
of PPARγ
ligands in order to exert synergistic antitumor effects.  However, phosphorylation of RXRα
represses the PPARγ/RXR-dependent
anticancer effects.  In some cases, the inhibition
of PPAR phosphorylation may also support the antitumor function of these
nuclear receptors.  In addition, we
should keep in mind the fact that the encouraging results obtained from the
combined use of PPARγ
agonists plus other agents have been exclusively reported in preclinical
studies, and PPAR agonist monotherapy did not achieve a significant result for
advanced malignancies in clinical trials, except for a small trial on
liposarcomas [7–10].
Therefore, it might be expected that some combination with other agents may
lead to breakthrough in the clinical application of PPARγ
agonists for chemoprevention and/or treatment of malignancies.Figure 3
A hypothetical schematic representation of
the synergistic anticancer effects of the combination of PPAR ligands plus
other agents.  When PPARs are activated
by ligand binding, they are able to heterodimerize with RXR and activate the target
gene expression by binding to the PPRE element.
Therefore, the retinoids which bind to RXR may be the most preferable
partner for the PPAR agonists (A).
However, in some types of cancers, the MAPK pathway phosphorylates RXRα,
and the accumulated nonfunctional p-RXRα
interferes with the function of the remaining normal RXRα,
thereby promoting the growth of cancer cells.  The activation of RTKs by their specific
ligands (growth factors) can play a critical role in the stimulation of the
MAPK pathway.  Therefore, the agents
which target the activation of RTKs (B) and/or the MAPK pathway
(C)
restore the function of RXRα
as a master regulator of nuclear receptors in cancer cells and this will
support the synergistic growth inhibition by PPAR and
RXR ligands in cancer cells.  The HDACs
enforce a tight chromatin structure and thereby repress the transcription of
target genes controlled by PPAR/RXR.
Therefore, the combination of a PPAR
agonist plus an HDAC inhibitor is more efficient to inhibit the growth of
cancer cells (D).  Finally, the
conventional chemotherapeutic agents also cause synergistic or enhancing effects to inhibit
cancer cell growth by the combination of PPAR ligands (E). L: ligand.In conclusion, the combination treatment using the PPARγ
agonists and other agents might be an effective and promising strategy for
chemoprevention and/or treatment of various types of cancers.  Future studies will be necessary to improve
the anticancer efficacy of PPARγ
agonist plus retinoids by combining with appropriate specific kinase
inhibitors.



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*Source: 181047-2008-05-28.xml*