# Regulation of Cell Proliferation and Differentiation by PPARβ/δ

Authors: Rolf Müller; Markus Rieck; Sabine Müller-Brüsselbach
Journal: PPAR Research
 (2008)

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## Abstract

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a ligand-activated transcription factor with essential functions in the regulation of lipid catabolism, glucose homeostasis, and inflammation, which makes it a potentially relevant drug target for the treatment of major human diseases. In addition, there is strong evidence that PPARβ/δ modulates oncogenic signaling pathways and tumor growth. Consistent with these observations, numerous reports have clearly documented a role for PPARβ/δ in cell cycle control, differentiation, and apoptosis. However, the precise role of PPARβ/δ in tumorigenesis and cell proliferation remains controversial. This review summarizes our current knowledge and proposes a model corroborating the discrepant data in this area of research.


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## Body

## 1. Introduction

Peroxisome  proliferator-activated receptor-β/δ
(PPARβ/δ)
is a transcription factor that is activated by endogenous fatty acid ligands
and by synthetic agonists [1, 2]. Major functions of
PPARβ/δ are associated with the regulation of glucose, energy, and lipid
metabolism [3], and the control of
inflammatory responses [4, 5]. PPARβ/δ, therefore, represents a promising drug target for the treatment of
common diseases such as obesity, metabolic syndrome, chronic inflammation, and
arteriosclerosis, which has led to the development of synthetic drug agonists
with subtype selectivity and high-affinity binding [6]. Mice lacking PPARβ/δ show an aberrant development of the placenta and exhibit a defect in
wound healing associated with alterations in cell proliferation,
differentiation, and cellular survival [7–10]. Experimental
evidence obtained with cultured cells has provided additional strong evidence
for a role of PPARβ/δ in cell cycle regulation and differentiation in different cell types (see
Table 1). Consistent with these physiological functions, there is also clear
evidence for a role of PPARβ/δ in oncogenesis and tumor growth.
These findings might provide a basis for the development of novel strategies
for the treatment of proliferative diseases, but also demand some caution with
respect to the clinical use of PPARβ/δ-directed dugs. A detailed knowledge of the role of PPARβ/δ in cell proliferation and its effects on tumor growth are therefore of
paramount importance.Table 1
Effects of PPARβ/δ
on cell proliferation and differentiation.
Cell typeExp. approachRole of PPARβ/δ  inAffected pathwayReferencesProlif.Diff.Epithelial
cellsKeratinocyteAgonist, wt versus null↘↗AKT[11]KeratinocyteAgonist, wt versus null↘↗ERK[12–17]KeratinocyteAgonist,
RNAi, wt versus null↗[18, 19]AdipocyteAgonist,
wt versus null↗PPARγ[20–22]Trophoblastwt versus null↗AKT[10, 23]Paneth cells (in vivo)wt versus null↗Hedgehog[24]Hepatic stellate cellAgonist↗[25]OligodendrocyteAgonist↗[26]Mesenchymal
cellsFibroblastAgonist↘(↘*)G0S2**, PTEN[27]Fibroblastwt versus null, re-expression in null↘p57KIP2[28, 29]Vascular smooth muscle cellsAgonist↘PDGF[30]Tumor endotheliumwt versus null↘↗[29]Endothelial cellsAgonist↗↗[31]Human
tumor cell linesMCF-7 breast carcinoma; UACC903 melanomaAgonist↘[32]HT29, HCT116, LS-174T colon carcinoma;
HepG2, HuH7 hepatomaAgonist↘[33]HCT116 colon carcinomaRNAi↘[34]SH-SY5Y neuroblastomaAgonist↗[35]NSC lung carcinomaAgonist↗AKT, NFκB[36]A549 NSC lung ca.Agonist↘[37]*transdifferentiation
into myofibroblasts.**G0S2: G0/G1 switch gene 2 (cell cycle
inhibitor).


## 2. PPARβ/δ Affects Tumorigenesis

The role of PPARβ/δ in tumorigenesis has been explored predominantly in epithelial tumors
of the skin, lung, and intestine and in the tumor stroma. PPARβ/δ inhibits chemically induced skin carcinogenesis, since an enhancement
of chemically induced skin tumor growth is seen in mice with a global
disruption of Pparb [38]. However, no effect on skin
carcinogenesis is observed in mice lacking PPARβ/δ specifically in basal keratinocytes [39], suggesting that the
tumor suppressive effect of PPARβ/δ is due to a function in other cell types. A tumor suppressive role for
PPARβ/δ has also been described for a transgenic mouse model of Raf
oncogene-induced lung adenoma formation, but similar to skin carcinogenesis the
precise mechanisms and cell types involved are not known [40]. Effects of PPARβ/δ have also been reported in different mouse models of intestinal
carcinogenesis, that is, the Apc/Min mouse lacking functional APC protein and
chemically induced intestinal carcinogenesis, but these studies differ in their
conclusions [41]. Thus, PPARβ/δ has been reported to have either no effect on intestinal tumorigenesis [9] to attenuate tumor
growth by promoting terminal differentiation of colonocytes [33, 42–45] or to potentiate
tumorigenesis [46–48]. The reason for
these discrepancies remains unclear at present [49], but may be in part
related to a function of PPARβ/δ
in host cells recruited by the tumor, such as endothelial cells, fibroblasts,
and macrophages [50]. Indeed, recent work
showed that PPARβ/δ is indispensable for the formation of functional tumor microvessels [29, 51], suggesting that PPARβ/δ
may have different functions in the tumor stroma and in tumor cells with
opposing effects on tumor growth. The role of PPARβ/δ
in tumor stroma cells is  further discussed below.


## 3. Attenuation of Tumor Stroma Cell Proliferation by PPARβ/δ

The inhibition of syngeneic tumor growth in
mice lacking PPARβ/δ
strongly correlates with a lower density of functional tumor microvessels [29, 51], which is  associated with a striking increase
in the proliferation of tumor endothelial cells and an inhibition of their
maturation [29]. The immature microvascular structures are also frequently surrounded
by perivascular cells expressing vast amounts of α-smooth
muscle actin, giving rise to an overall picture characteristic of tumor
endothelial hyperplasia. In vivo
microarray analysis led to the identification of PPARβ/δ
target genes with known inhibitory functions in angiogenesis, including Cd36 and Cdkn1c [29]. A crucial function of CD36 is to serve as a
receptor for thrombospondins which are known to attenuate the proliferation of
endothelial cells [52], and Cdkn1c codes for the cyclin-dependent kinase inhibitor p57KIP2 [53]. Consistent with the existence of a PPARβ/δ−p57KIP2 pathway in stroma cell
types, it was shown that the forced expression of PPARβ/δ
in Pparb null fibroblasts results in
a Cdkn1c-dependend inhibition of cell
proliferation [29]. Other PPARβ/δ
target genes with potential functions in cell proliferation and differentiation
were identified in the same study, suggesting that PPARβ/δ
regulates multiple genes with functions in cell proliferation in the context of
tumor stroma development and tumor angiogenesis.An antiproliferative effect of PPARβ/δ
agonists in fibroblasts and vascular smooth muscle cells has also been observed
in two other studies [27, 30], while opposite effects have been described
for endothelial cells [31]. At present, it is difficult to explain these
apparent discrepancies, since they cannot be narrowed down to a single
parameter, such as experimental strategy, cell type, expression level of PPARβ/δ,
or state of the cell (e.g., metabolic activity, proliferative status, stage of
differentiation, exogenous factors). This issue is discussed further in the
Conclusions section below.


## 4. Role of PPARβ/δ in Wound Healing and Keratinocyte Proliferation

Pparb null
mice exhibit a defect in wound healing by inhibiting apoptosis in keratinocytes
[8]. This survival function of PPARβ/δ has been explained by an induction of AKT/protein kinase B (PKB)
activity by PPARβ/δ resulting from an upregulation of the Pdk1 and Ilk genes and a
downregulation of Pten [11]. Increased AKT signaling is generally
associated with enhanced proliferation, yet others have reported that PPARβ/δ inhibits cell proliferation [7, 15]. In this case, however, AKT activity was not
affected by PPARβ/δ activation. Instead, a downregulation of protein kinase C and MAP
kinase signaling was observed [14]. The reason for these discrepancies is not
clear at present, however, in light of the relatively small effects of PPARβ/δ on the signaling pathways discussed above it is possible that subtle
differences in the experimental settings account for the apparent lack of
consistency.


## 5. Role of PPARβ/δ in Differentiation

Mice lacking PPARβ/δ show a very high degree of embryonic lethality due to an aberrant
development and malfunction of the placenta [7, 9, 10]. Consistent with this finding, the
differentiation and metabolic functions of trophoblast giant cells in vitro are dependent on PPARβ/δ [10]. In the same model, stimulatory effect of PPARβ/δ on AKT signaling was observed. Another tissue where PPARβ/δ plays a role in differentiation is the digestive tract, where PPARβ/δ promotes the differentiation of Paneth cells in the intestinal crypts
by down-regulating the hedgehog signaling pathway [24]. A differentiation promoting effect of PPARβ/δ has also been described for keratinocytes, adipocytes, endothelial
cells, and oligodendrocytes (see Table 1 for details).


## 6. Conclusions

Studies addressing the role of PPARβ/δ in differentiation have yielded a consistent picture and point to a
differentiation promoting in a wide spectrum of different cell types. Numerous reports have also clearly documented a
role for PPARβ/δ
in cell proliferation and tumorigenesis, yet
different studies have produced controversial results, even though the majority
of studies describe antiproliferative effects by PPARβ/δ (see Table 1).One reason for the apparently discrepant data may be associated
with the use of different experimental strategies. Since the precise mechanisms
of PPARβ/δ-mediated gene regulation are often not known, the results from
gain-of-function and loss-of-function are not always easy to interpret. Thus, ligand activation and genetic
inactivation of PPARβ/δ may
have opposite effects, as in the case of classical PPRE-driven genes, but may
also give similar results in other regulatory settings. The latter has been
described, for instance, for PPARβ/δ-mediated gene repression through direct interaction with the transcriptional
repressor BCL-6 in macrophages [54]. This aspect has not been thoroughly analyzed to date so that it is
difficult to judge its contribution to the deviant results published in
different studies.To help explain the discrepant published
data, we would therefore like to put forward another hypothesis. This model
postulates that PPARβ/δ is not a bona fide cell cycle
regulator with a defined function but rather affects the expression of both
inducers and inhibitors of cell proliferation (e.g., regulators of the AKT
pathway and PDGF versus the cell cycle inhibitors p57KIP2 and G0S2; see Table 1). This is conceivable both in view of the
large number of potential PPAR target genes, estimated at several thousand for
the human genome [55]. Depending on the particular cell
type, the metabolic or proliferative state of the cell or other experimental
conditions, positive or negative regulators of the cell cycle may prevail
resulting in opposite effects. This suggests that the precise effects of PPARβ/δ on cell proliferation are highly context-dependent and not predictable
on the basis of our current knowledge. Clearly, a better and detailed understanding of the effects of PPARβ/δ
on cell cycle regulation and differentiation will be a prerequisite for the
development of PPARβ/δ
directed drugs and their clinical application.



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