REVIEW ARTICLE
Roles of AP-2 transcription factors in the regulation of
cartilage and skeletal development
Ann-Kathrin Wenke and Anja K. Bosserhoff
Institute of Pathology, University of Regensburg, Germany
The AP-2 family
AP-2awas first identified by its ability to bind to enhan-
cer regions of SV40 and human metallothionein IIA [1].
The AP-2 family of transcription factors is composed of
five members: AP-2a, AP-2b, AP-2c, AP-2d, and AP-2e
[2–7], described for humans and mice. Orthologs of
some AP-2s have also been found in frogs and fish, and
homologs occur in invertebrates. All AP-2s have a
highly conserved basic helix–span–helix DNA-binding
and dimerization domain at their C-terminus, and a less
conserved proline-rich and glutamine-rich transactiva-
tion domain at their N-terminus [8–10]. Most isoforms
also have a PY-motif (XPPXY) in the N-terminal trans-
activation domain that is important for their role as
transcriptional activators [9]. The AP-2 factors form
homodimers and heterodimers for their transcriptional
activity. A multiple alignment of all five human AP-2s,
illustrating their domain structure, is shown in Fig. 1.
A detailed and extensive overview of the AP-2 family
is given in the review of Eckert et al., [11] which also
contains a schematic illustration of the AP-2 structure.
Expression patterns of AP-2 molecules
and functional implications
The expression and function of AP-2 isoforms have
been systematically analyzed during murine embryo-
genesis and in studies of the corresponding knockout
mice.
AP-2a, AP-2band AP-2cshow partially overlap-
ping expression patterns in neural crest cells (NCCs),
the peripheral nervous system, the facial mesenchyme,
the limbs, various epithelia of the developing embryo,
Keywords
AP-2; cartilage; chondrogenesis; limb;
transcriptional regulation
Correspondence
A.-K. Bosserhoff, Institute of Pathology,
University of Regensburg,
Franz-Josef-Strauss-Allee 11, D-93053
Regensburg, Germany
Fax: +49 941 944 6602
Tel: +49 941 944 6705
E-mail: anja.bosserhoff@klinik.uni-regens
burg.de
(Received 12 October 2009, revised 13
November 2009, accepted 20 November
2009)
doi:10.1111/j.1742-4658.2009.07509.x
During embryogenesis, most of the mammalian skeletal system is preformed
as cartilaginous structures that ossify later. The different stages of cartilage
and skeletal development are well described, and several molecular factors
are known to influence the events of this enchondral ossification, especially
transcription factors. Members of the AP-2 family of transcription factors
play important roles in several cellular processes, such as apoptosis, migra-
tion and differentiation. Studies with knockout mice demonstrate that a main
function of AP-2s is the suppression of terminal differentiation during
embryonic development. Additionally, the specific role of these molecules as
regulators during chondrogenesis has been characterized. This review gives
an overview of AP-2s, and discusses the recent findings on the AP-2 family,
in particular AP-2a, AP-2b, and AP-2e, as regulators of cartilage and skeletal
development.
Abbreviations
NCC, neural crest cell; RA, retinoic acid; ZPA, zone of polarizing activity.
894 FEBS Journal 277 (2010) 894–902 ª2009 The Authors Journal compilation ª2009 FEBS
and the extraembryonic trophectoderm [4,12,13]. In
contrast to the other AP-2s, AP-2dis specifically
expressed in the central nervous system, retina, and
developing heart [6]. AP-2eexpression has been
detected in the developing olfactory bulb, neural tis-
sue, especially the midbrain and hindbrain [7,14], and
hypertrophic chondrocytes during chondrogenesis [15].
Winger et al. [16] analyzed the expression of all five
mouse AP-2 family members in the unfertilized
oocyte and from zygote formation to the blastocyst
Transactivation domain
with PY-motif
Dimerization domain
DNA-binding domain
Alpha -----------MLWKLTDNIKYEDC-EDRHDGTSNGTARLPQLGTVGQSPYTSAPPLSHT
Beta MHSPPRDQAAIMLWKLVENVKYEDIYEDRHDGVPSHSSRLSQLGSVSQGPYSSAPPLSHT
Gamma -----------MLWKITDNVKYEEDCEDRHDGSSNGNPRVPHLSSAGQHLYSPAPPLSHT
Epsilon -----------MLVHTYSAME-------RPDGLG-AAAGGARLSSLPQAAYGPAPPLCHT
Delta -----------MSTTFPGLVHDAEIRHDGSNSYRLMQLGCLESVANSTVAYSSSSPLTYS
* :. :. . : * .:.** ::
Alpha PNA----DFQPP-YFPPPY--QPI-YPQSQDP---YSHVN-DPYS--LNPLHAQPQP--Q
Beta PSS----DFQPP-YFPPPY--QPLPYHQSQDP---YSHVN-DPYS--LNPLHQ-PQ---Q
Gamma GVA----EYQPPPYFPPPY--QQLAYSQSADP---YSHLG-EAYAAAINPLHQPAPTGSQ
Epsilon PAATAAAEFQPP-YFPPPYPQPPLPYGQAPDAAAAFPHLAGDPYGG-LAPLAQPQPP---
Delta TTG---TEFASP-YFSTNHQYTPL-HHQSFHYEFQHSHPAVTPDAYSLNSLHHSQQYYQQ
. :: .* **.. : : : *: . ..* . . : .*
Alpha HPGWPGQRQ---SQESGLLHTHRGLPHQLSG-LDP-----RRDY---RRHEDLLHGP-HA
Beta HPWGQRQRQEVGSEAGSLLPQPRAALPQLSG-LDP-----RRDYHSVRRPDVLLHSAHHG
Gamma QQAWPGRQSQEGAGLPSHHGRPAGLLPHLSG-LEAGAVSARRDAY--RRSDLLLPHAHAL
Epsilon QAAWAAPRAAARAHEE--PPGLLAPPARALG-LDP-----RRDYA--TAVPRLLHGLADG
Delta IHHGEPTDFINLHNARALKSSCLDEQRRELGCLDAYR---RHDLS--LMSHGSQYGMHPD
: * *:. *:*
Alpha LSSGLGD-LSIHSLPH--AIEEVPHVEDP---GINIPDQT-VIKKGPVSLSKSNSNAVSA
Beta LDAGMGDSLSLHGLGHP-GMEDVQSVEDANNSGMNLLDQS-VIKKVPVPP-----KSVTS
Gamma DAAGLAENLGLHDMPH--QMDEVQNVDDQ---HLLLHDQT-VIRKGPISMT----KNPLN
Epsilon AHGLADAPLGLPGLAAAPGLEDLQAMDEP---GMSLLDQS-VIKKVPIPSK---ASSLSA
Delta QRLLPGPSLGLAAAGA----DDLQGSVEAQ-CGLVLNGQGGVIRRG--------------
*.: ::: : : : .* **::
Alpha IPINKDNLFGGV-VNPNEVFCSVPGRLSLLSSTSKYKVTVAEVQRRLSPPECLNASLLGG
Beta LMMNKDGFLGGMSVNTGEVFCSVPGRLSLLSSTSKYKVTVGEVQRRLSPPECLNASLLGG
Gamma LPCQKE--LVGAVMNPTEVFCSVPGRLSLLSSTSKYKVTVAEVQRRLSPPECLNASLLGG
Epsilon LSLAKDS-LVGGITNPGEVFCSVPGRLSLLSSTSKYKVTVGEVQRRLSPPECLNASLLGG
Delta ---------GTCVVNPTDLFCSVPGRLSLLSSTSKYKVTIAEVKRRLSPPECLNASLLGG
*. ::********************:.**:****************
Alpha VLRRAKSKNGGRSLREKLDKIGLNLPAGRRKAANVTLLTSLVEGEAVHLARDFGYVCETE
Beta VLRRAKSKNGGRSLRERLEKIGLNLPAGRRKAANVTLLTSLVEGEAVHLARDFGYICETE
Gamma VLRRAKSKNGGRSLREKLDKIGLNLPAGRRKAAHVTLLTSLVEGEAVHLARDFAYVCEAE
Epsilon VLRRAKSKNGGRCLRERLEKIGLNLPAGRRKAANVTLLTSLVEGEAVHLARDFGYVCETE
Delta ILRRAKSKNGGRCLREKLDRLGLNLPAGRRKAANVTLLTSLVEGEALHLARDFGYTCETE
:***********.***:*:::************:************:******.* **:*
Alpha FPAKAVAEFLNRQHSD-PNEQVTRKNMLLATKQICKEFTDLLAQDRSPLGNSRPNPILEP
Beta FPAKAVSEYLNRQHTD-PSDLHSRKNMLLATKQLCKEFTDLLAQDRTPIGNSRPSPILEP
Gamma FPSKPVAEYLTRPHLGGRNEMAARKNMLLAAQQLCKEFTELLSQDRTPHGTSRLAPVLET
Epsilon FPAKAAAEYLCRQHAD-PGELHSRKSMLLAAKQICKEFADLMAQDRSPLGNSRPALILEP
Delta FPAKAVGEHLARQHME-QKEQTARKKMILATKQICKEFQDLLSQDRSPLGSSRPTPILDL
**:*...*.* * * : :**.*:**::*:**** :*::***:* *.** :*:
Alpha GIQSCLTHFNLISHGFGSPAVCAAVTALQNYLTEALKAMDKMYLS-----NNP-NSHTDN
Beta GIQSCLTHFSLITHGFGAPAICAALTALQNYLTEALKGMDKMFLN-----NTTTNRHTSG
Gamma NIQNCLSHFSLITHGFGSQAICAAVSALQNYIKEALIVIDKSYMN-----PGD-QSPADS
Epsilon GVQSCLTHFSLITHGFGGPAICAALTAFQNYLLESLKGLDKMFLS-----SVG-SGHGET
Delta DIQRHLTHFSLITHGFGTPAICAALSTFQTVLSEMLNYLEKHTTHKNGGAADSGQGHANS
.:* *:**.**:**** *:***::::*. : * * ::* . .
Alpha N----AKSSDKEEKHRK-----
Beta EGP-GSKTGDKEEKHRK-----
Gamma N-----KTLEKMEKHRK-----
Epsilon K------ASEKDAKHRK-----
Delta EKAPLRKTSEAAVKEGKTEKTD
: : : *. *
Fig. 1. Multiple alignment of AP-2a, AP-2b, AP-2c, AP-2d, and AP-2e. The proline-rich and glutamine-rich N-terminus, which is important for
transactivation, is shown in yellow, and contains the PY-motif (green). The helix–span–helix domain at the C-terminus shown in blue medi-
ates dimerization and, together with the basic domain, (red) DNA-binding. ‘*’, amino acids that are identical in all sequences in the align-
ment; ‘:’, conserved substitutions have been observed; ‘.’, semiconserved substitutions.
A.-K. Wenke and A.-K. Bosserhoff AP-2 proteins in cartilage differentiation
FEBS Journal 277 (2010) 894–902 ª2009 The Authors Journal compilation ª2009 FEBS 895
stage of development. They found that AP-2a,
AP-2b, AP-2cand AP-2eare differentially expressed
during the preimplantation period, and, with the
exception of AP-2a, also in unfertilized oocytes.
Furthermore, they determined that functional redun-
dancy occurs between these proteins during at least
the preimplantation period [16].
However, gene knockout experiments indicate that
the AP-2s perform individual and nonredundant
functions during mouse development. Analyses of
AP-2a-null mice have demonstrated that AP-2ais a
fundamental regulator of mammalian craniofacial
development. AP-2aknockout mice die perinatally
with craniofacial defects, thoracoabdominoschisis, and
severe skeletal defects in the head and trunk region
[17,18]. Studies of earlier embryonic stages of these
mice indicate a failure of cranial neural tube closure
and defects in cranial ganglia development. Another
role of AP-2apreviously masked in the knockout mice
became apparent in chimeric mice composed of both
wild-type and AP-2a-null cells [19]. These chimeras
reveal the major influence of AP-2aon eye forma-
tion and limb pattern formation typified by limb
duplications.
In contrast to these defects, the lack of AP-2b
leads to enhanced apoptotic cell death of renal epi-
thelial cells. AP-2bknockout mice die shortly after
birth because of polycystic kidney disease and termi-
nal renal failure [20,21]. The targeted deletion of
AP-2calso has severe consequences. The loss of
AP-2cis already lethal in early embryogenic develop-
ment directly after implantation during gastrulation,
because AP-2ccontrols proliferation and differentia-
tion of extraembryonic trophectodermal cells [22,23].
So far, nothing is known about chondrogenic defects
mediated by knocking out AP-2bor AP-2c.
However, all these types of grave damage after
deletion of AP-2 transcription factors demonstrate
the importance of the AP-2s for several functions
during embryonic development. To date, knockout
studies concerning AP-2dor AP-2ehave not been
published.
Regulation of AP-2 and AP-2 target
genes
The expression of the AP-2atranscription factor is
induced by different signal-transducing agents, such as
retinoic acid (RA), cAMP, phorbol ester, UV light, and
singlet oxygen [2,24–26]. RA plays an important role in
the process of chondrocyte differentiation [27]. AP-2
mediates transcriptional activation in response to two
different signal transduction pathways, the phorbol
ester-activated protein kinase C pathway, or the cAMP-
dependent protein kinase A pathway [28]. Here, cAMP
may modulate AP-2 activity by protein kinase A-induced
phosphorylation of the transcription factor [29].
So far, interactions with AP-2 have been described
for many proteins. For example, CBP ⁄p300-interacting
transactivator with ED-rich tail 2 interacts with and co-
activates all isoforms of AP-2, and the interaction with
AP-2ais suggested to be necessary for normal neural
tube and cardiac development [30,31]. The Kru
¨ppel-
related zinc finger protein AP-2rep (Klf12) has been
characterized as a repressor of AP-2a. Repression of
AP-2atranscription by AP-2rep is dependent on an
N-terminal PVDLS motif that interacts specifically with
the corepressor CtBP1 [32,33]. Recently, it was shown
that the broad-complex, tramtrack and bric-a-brac
domain containing protein KCTD1 directly binds to
AP-2aand acts as a negative regulator for AP-2atrans-
activation [34]. It was also demonstrated in other studies
that the nuclear protein poly(ADP-ribose) polymerase-1
interacts with the C-terminus of AP-2aand enhances its
transcriptional activity in normal circumstances,
whereas its enzymatic activity is used as a temporary
shut-off mechanism during unfavorable conditions
[35,36]. Little is known about the interaction of AP-2
and its binding partners in cartilage. However, at least
CBP ⁄p300-interacting transactivator with ED-rich
tail 2 and protein poly(ADP-ribose) polymerase-1 are
expressed in this tissue [37–40]. It would be interesting
to further analyze their interactions with AP-2 and the
functional role of these in chondrocytes.
Furthermore, it is speculated that in melanoma,
where AP-2aacts as a tumor suppressor, the loss of
AP-2ais caused by a failure in post-transcriptional
processing of the protein [41]. Additionally, it is evi-
dent that AP-2 transcription factors can indirectly
modulate genes by functional interactions with other
transcription factors, e.g. c-myc, rBP, and p53 [42–44].
The formation of AP-2 homodimers and heterodimers
could also be important for their regulatory activity,
but no studies have been published so far.
For the regulation of target gene expression, the
AP-2 transcription factors bind to the palindromic
recognition sequence 5¢-GCCN
3
GGC-3¢or variations
of this GC-rich sequence within multiple gene promot-
ers [45]. AP-2s play a dual role as transcriptional acti-
vators and repressors. By regulating target genes with
AP-2-binding sites within their promoter sequences,
the AP-2 transcription factors play important roles in
cellular processes, such as morphogenesis, in particular
proliferation, differentiation, cell cycle regulation, and
apoptosis [11,45,46]. Through suppression of genes
inducing terminal differentiation, apoptosis, and
AP-2 proteins in cartilage differentiation A.-K. Wenke and A.-K. Bosserhoff
896 FEBS Journal 277 (2010) 894–902 ª2009 The Authors Journal compilation ª2009 FEBS
growth retardation, AP-2s play vital roles in cell prolif-
eration. Besides the functions of AP-2s in physiological
processes, they have also crucial roles in pathological
processes such as tumorigenesis and genetic diseases
[47].
Most analyses of the regulation of AP-2 and the
interactions of the transcription factor with binding
partners, as well as of the regulation of target gene
expression, have been performed for AP-2a.Upto
now, there have been no similar studies for the other
AP-2 isoforms.
Chondrogenesis and skeletal
development
Most elements of the vertebrate skeleton are built
through enchondral ossification. This is a complex pro-
cess beginning with the migration of undifferentiated
mesenchymal cells to regions determined to differenti-
ate into bone, followed by aggregation and the forma-
tion of mesenchymal condensation [48,49]. These
resting and proliferating chondrocytes produce an
extracellular matrix mainly consisting of aggrecan and
type II collagen. As skeletogenesis proceeds, proliferat-
ing chondrocytes exit the cell cycle, become hypertro-
phic, express type X collagen, and reduce the
expression of type II collagen [50]. Hypertrophic chon-
drocytes undergo terminal differentiation before they
finally become apoptotic. Through the invasion of
blood vessels from the perichondrium, the cartilage
becomes vascularized. Additionally, osteoblasts invade
the cartilage and start to replace it with mineralized
bone [48].
Many molecules and signaling cascades are neces-
sary to regulate these molecular processes of chondro-
genic and skeletal development, including transcription
factors. Essential transcription factors in chondrocyte
differentiation are Sox9 and Runx2. Sox9 plays a key
role in chondrogenesis, as an inactivating mutation in
the gene encoding Sox9 leads to severe cartilage abnor-
malities called campomelic dysplasia [51,52]. The effect
of a complete loss of Sox9 during chondrogenesis was
analyzed using a model of mice chimeras injected with
homozygous embryonic Sox9
)⁄)
stem cells into wild-
type blastocysts, because Sox9 knockout mice are not
viable [53]. The Sox9
)⁄)
cells were excluded from mes-
enchymal condensation and had no expression of the
chondrocytic markers type II collagen, type IX colla-
gen, type X collagen, and aggrecan. Besides type II
collagen and aggrecan, Sox9 also regulates the expres-
sion of the cartilage-derived retinoic acid-sensitive pro-
tein [54,55]. Sox5 and Sox6, members of the Sox
family, are also important for chondrocyte differentia-
tion, as embryos lacking Sox5 and Sox6 die at embry-
onic day 16.5 and display a failure of chondrocyte
progenitor cells to differentiate into hypertrophic chon-
drocytes [56].
Two members of the Runx family of transcription
factors, Runx2 and Runx3, are positive regulators of
chondrocyte hypertrophy. Runx2 is transiently
expressed in prehypertrophic chondrocytes, and
enforced expression of Runx2 in these cells in trans-
genic mice leads to ectopic chondrocyte hypertrophy
[57]. Mice lacking both Runx2 and Runx3 do not have
hypertrophic chondrocytes or type X collagen-express-
ing cells, showing that both Runx2 and Runx3 are
important regulators for hypertrophic development of
chondrocytes [58]. Alongside the important function
for chondrogenesis, Runx2 is also a key regulator for
osteoblast differentiation. In particular, Runx2 is
expressed in cells prefiguring the vertebrate skeleton as
early as embryonic day 10.5 [59]. Runx2 regulates
many genes that determine the osteoblast phenotype,
as the forced expression of Runx2 in nonosteoblast
cells is sufficient to induce the osteoblast-specific gene
osteocalcin [60]. The inactivation of both Runx2 alleles
in mice results in a lack of osteoblasts throughout the
skeleton [61,62]. It has also been shown that deletions
resulting in the heterozygous loss of runx2 cause cleid-
ocranial dysplasia [63].
Role of AP-2a,AP-2band AP-2ein
chondrogenesis and skeletal development
In addition to Sox and Runx transcription factors,
members of the AP-2 family also have important func-
tions in chondrogenesis and development of the verte-
brate skeleton during embryogenesis. Especially for
AP-2a, but also for AP-2band AP-2e, a role as a reg-
ulator of cartilage differentiation has been shown
[64–69]. The functional and important roles of AP-2
transcription factors during chondrogenesis are illus-
trated in Fig. 2.
AP-2ais expressed in the growth plate and in articu-
lar cartilage, and has been described as a negative
regulator of chondrocyte differentiation [64]. The
expression of cartilage-derived retinoic acid-sensitive
protein and type II collagen is negatively correlated
with AP-2aexpression, and AP-2athus acts as a sup-
pressor of these two cartilage matrix genes during car-
tilage differentiation [64–66] (Fig. 2). High expression
levels of AP-2ain chondroprogenitor cells maintain
these cells in an early differentiation phenotype and
inhibit the transition to differentiated chondrocytes.
The induction of Sox5 and Sox6 as well as that of
chondrocytic matrix genes such as type II collagen,
A.-K. Wenke and A.-K. Bosserhoff AP-2 proteins in cartilage differentiation
FEBS Journal 277 (2010) 894–902 ª2009 The Authors Journal compilation ª2009 FEBS 897
aggrecan and type X collagen are also delayed by
AP-2a[64,67].
Reports on AP-2aknockout mice clearly indicate
the importance of this transcription factor in regulat-
ing bone and cartilage development during embryogen-
esis, because of the severe skeletal defects in growth
and the development of face and limbs [17–19]. Don-
ner et al. tried to link the expression of AP-2ain these
tissues to upstream signaling pathways. They assessed
the organization of a cis-regulatory region within the
fifth intron specific for directing AP-2aexpression to
the developing frontal nasal process and limb bud mes-
enchyme, which they had previously identified in trans-
genic mice [70,71]. The results demonstrate that a
STAT binding site is required for robust AP-2aexpres-
sion in the face and limbs. In a follow-up study, they
found that this conserved cis-acting sequence serves to
maintain a level of AP-2aexpression that limits the
size of the hand plate and the associated number of
digit primordia [72].
AP-2 function was also analyzed in other species.
A similar role for AP-2aas a regulator for face and
limb bud development was described in chickens. AP-2
expression is completely downregulated after treatment
of the chick face with RA, and this is accompanied by
an increase in apoptosis [73]. The authors of this study
ascribe the regulation of outgrowth of limb buds and
patterning of the digits to the chicken AP-2.
The role of AP-2awas further studied in zebrafish.
It was confirmed that AP-2ais an essential regulator
of the development of neural crest derivates, including
embryonic cartilage and neurons, as well as pigmented
cells [74–76]. Knight et al. [77] demonstrated essential
functions for zebrafish AP-2a(tfap2a) and also AP-2b
(tfap2b) in the development of the facial ectoderm, and
for signals from this epithelium that induce skeletogen-
esis in NCCs. Zebrafish embryos lacking both tfap2a
and tfap2b have defects in epidermal cell survival and
deficient NCC-derived cartilage. The authors propose
that AP-2s have two distinct functions in cranial
NCCs: they play an early cell-autonomous role in cell
specification and survival, and a later nonautonomous
role as regulators of ectodermal signals that induce
skeletogenesis [77].
Luo et al. [78] characterized Inca (induced in the
neural crest by AP-2) as a target gene upregulated by
AP-2ain Xenopus embryos. Knockdown experiments
for Inca in frog and fish revealed essential functions in
a subset of NCCs that form craniofacial cartilage.
Cells deficient for Inca show normal migration but fail
to condense into skeletal primordia. This is an interest-
ing aspect, as, for murine embryonic development,
AP-2ais described as a suppressor of cartilage differ-
entiation, maintaining cells in an early differentiated
phenotype.
For AP-2b, expression in murine limbs has also been
demonstrated. AP-2bis expressed in the zone of polar-
izing activity (ZPA), the signaling center of the devel-
oping vertebrate limb [68]. A microarray approach
comparing gene expression in the ZPA with that in the
Hypertrophic zone Proliferative zone Resting zone
Sox9
AP-2
ε
Undifferentiated
mesenchymal cells
Differentiated
chondrocytes
Hypertrophic
chondrocytes
Condensed
mesenchymal cells
Sox9
Sox9
Sox5
Sox6
AP-2
α
Runx2
Runx3
Runx2
Runx2
Fig. 2. Functional role of AP-2aand AP-2e
in chondrogenesis. Overview of the differen-
tiation stages during chondrogenesis and
the involvement of transcription factors
(henatoxylin and eosin-stained section of an
embryonic cartilaginous limb).
AP-2 proteins in cartilage differentiation A.-K. Wenke and A.-K. Bosserhoff
898 FEBS Journal 277 (2010) 894–902 ª2009 The Authors Journal compilation ª2009 FEBS