p38 MAP-Kinases pathway regulation, function and role in human diseases
Ana Cuendaa,b,⁎, Simon Rousseaua
aMRC Protein Phosphorylation Unit, College of life Sciences, University of Dundee, Dundee DD1 5EH, Scotland, UK
bDepartamento de Inmunología y Oncología, Centro Nacional de Biotecnología-CSIC, Campus de Cantoblanco-UAM, 28049-Madrid, Spain
Received 22 August 2006; received in revised form 13 March 2007; accepted 19 March 2007
Available online 24 March 2007
Mammalian p38 mitogen-activated protein kinases (MAPKs) are activated by a wide range of cellular stresses as well as in response to
inflammatory cytokines. There are four members of the p38MAPK family (p38α, p38β, p38γ and p38δ) which are about 60% identical in their
amino acid sequence but differ in their expression patterns, substrate specificities and sensitivities to chemical inhibitors such as SB203580. A
large body of evidences indicates that p38MAPK activity is critical for normal immune and inflammatory response. The p38MAPK pathway is a
key regulator of pro-inflammatory cytokines biosynthesis at the transcriptional and translational levels, which makes different components of this
pathway potential targets for the treatment of autoimmune and inflammatory diseases. However, recent studies have shed light on the broad effect
of p38MAPK activation in the control of many other aspects of the physiology of the cell, such as control of cell cycle or cytoskeleton
remodelling. Here we focus on these emergent roles of p38MAPKs and their implication in different pathologies.
© 2007 Elsevier B.V. All rights reserved.
Keywords: p38 MAP kinase; Cellular stress; Pro-inflammatory cytokines; Cellular differentiation; Inflammatory disease; Cancer
Cells respond to changes in the physical and chemical
properties of the environment by altering many cellular
functions such as survival, proliferative potential, metabolism
rate, interaction with other cells, and numerous cellular
processes involved in the homeostasis and health of the
organism. These environmental changes include alterations in
the concentrations of nutrients, growth factors, cytokines and
cell damaging agents, but also physical stimulation mediated by
changes in the osmolarity of the medium. In response to those
changes, mammalian cells activate four well-characterised
subfamilies of mitogen-activated protein kinases (MAPKs):
ERK1/2, ERK5, JNKs and p38s. Integral to all MAPK path-
ways, are central three-tiered core signalling modules consisting
of the following protein kinase families: MAPK kinase kinases
(MKKKs), MAPK kinases (MKKs) and MAPKs. The MAPKs
are activated upon dual phosphorylation of tyrosine and
threonine residues in a conserved Thr–Xaa–Tyr motif (where
Xaa is any amino-acid) in the activation loop of kinase
subdomain VIII. MAPK phosphatases reverse the phosphoryla-
tion and return the MAPK to their inactive state. Phosphoryla-
tion of MAPKs is catalysed by the dual specificity kinases,
MKKs, whichare inturn activated upon phosphorylation ofSer/
Thr residues, and are highly selective in phosphorylating
specific MAPKs. The mechanism that accounts for the
specificity of MKKs to activate individual MAPK isoforms is
mediated, in part, by an interaction between an N-terminal
region located on the MKK and different docking sites present
on the MAPK, and also by the structure of the MAPK activation
loop that contains the Thr–Xaa–Tyr dual phosphorylation motif
[1–6]. The first component activated in the MAPK core
signalling module are MKKKs which phosphorylate specific
MKKs and have distinct motifs in their sequences that confers
selectivity to their activation in response to different stimuli.
2. Identification of mammalian p38 MAP-kinases
There are four p38 MAP kinases in mammals: α, β, γ and δ.
Among all p38 MAPK isoforms, p38α is the best characterised
Biochimica et Biophysica Acta 1773 (2007) 1358–1375
⁎Corresponding author. MRC Protein Phosphorylation Unit, College of life
Sciences, University of Dundee, Dundee DD1 5EH, Scotland, UK. Tel.: +44
1382 384241; fax: +44 1382 223778 or Tel.: +34 915855451; fax: +34 91
E-mail address: firstname.lastname@example.org (A. Cuenda).
0167-4889/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
and is expressed in most cell types. p38α MAPK was initially
identify as a 38 kDa polypeptide that underwent tyrosine
phosphorylation in response to endotoxin treatment and
hyperosmolarity shock . p38α was the mammalian MAPK
orthologue ofHog1,the osmosensingMAPKofSaccharomyces
cerevisiae . In parallel, two other groups independently
identified p38α as a kinase activated by stress (also called
Reactivating Kinase, RK)  and IL-1 (named p40)  that
could directly phosphorylate and activate the Ser/Thr protein
kinase MAPK-activated protein kinase 2 (MAPKAP-K2),
which, in turn, phosphorylates the small 27 kDa heat shock
protein, HSP27 [10–12]. Later on that year, another identifica-
a class of pyridinyl imidazole anti-inflammatory drugs, called
cytokine-suppressive anti-inflammatory drugs (CSAIDs), with
SB203580 being the most extensively studied compound .
A few years after the identification of p38α, three additional
isoforms were described encoded by different genes: p38β
, p38γ (also called SAPK3 and ERK6) [15,16] and p38δ
(also called SAPK4) [17,18]. Splicing of p38β has been also
reported [19,20]. Although all p38 isoforms are widely
expressed, p38γ is most significantly expressed in skeletal
muscle and p38δ is mainly found in testis, pancreas, kidney and
small intestine .
The p38 MAPK subfamily can further be divided into two
distinct subsets, on the one hand p38α and p38β and on the
other, p38γ and p38δ (see Fig. 1). This is evident firstly from
their amino-acid sequence identity: p38α and p38β are 75%
identical, whereas p38γ and p38δ are 62% and 61% identical to
p38α, respectively. Of note, p38γ and p38δ are more identical
(∼70%) to each other. Secondly, their susceptibilities to
inhibition at low concentrations by the compounds SB203580
and SB202190. In vitro and in vivo assays demonstrated that
only p38α and p38β are inhibited by these compounds, whereas
p38γ and p38δ were completely unaffected by the drugs
[17,21,22]. The basis of this inhibition was revealed in the
crystal structure of p38α complexed with SB203580. Thr106 in
the hinge of the p38α and p38β ATP binding pocket interacts
with a fluorine atom in the SB203580 structure. This orients the
drug to interact with His107 and Leu108 of the pocket
preventing ATP binding [23,24]. p38γ and p38δ possess Met,
a large side chain amino acid, at the Thr106 equivalent position
in the ATP binding pocket that prevents inhibitor binding.
Substitution of residue Thr106 in p38α, alone or in combination
with His107 or Leu108, with the corresponding more bulky
residue from p38γ or p38δ (Met and Pro or Phe, respectively, in
both cases) abolishes SB203580 binding. Conversely, if the
amino acid of p38γ, p38δ, or even JNK1 which corresponds to
p38α Thr106 is replaced with Thr, the resulting mutants display
at least partial sensitivity to SB203580 [23,24]. A third dif-
ference between these two subgroups of p38MAPKs is with
regard to substrate selectivity of these kinases. For example,
microtubule-associated protein Tau is a better in vitro substrate
for p38γ and p38δ than p38α and p38β [25,26], and this is also
true for the scaffold proteins α1-syntrophin, SAP90/PSD95 and
SAP97/hDlg [27–29]. Conversely, MAPKAP-K2, MAPKAP-
K3 and glycogen synthase are better phosphorylated by p38α
and p38β than p38γ and p38δ [17,21,30].
3. Regulation of the p38 MAPK signalling module
3.1. Dual phosphorylation by MKKs
The p38 MAPKs are strongly activated in vivo by
environmental stresses and inflammatory cytokines, and less
by serum and growth factors. Together with the JNK family,
p38 MAPKs are also known as Stress-Activated Protein
Kinases (SAPKs). The canonical activation of p38 MAPKs
occurs via dual phosphorylation of their Thr–Gly–Tyr motif, in
the activation loop, by MKK3 and MKK6 , . Upon
activation, the dually phosphorylated p38 MAPK goes through
characteristic global conformational changes that alters the
alignment of the two kinase halves (N-terminal and C-terminal
domains) of the folded protein and enhances access to substrate,
which together increases enzymatic activity [33,34].
MKK3 and MKK6 (also called SKK3) are highly selective
for p38 MAPKs and do not activate JNKs or ERK1/2 [21,35–
37]. The importance of these two kinases physiological function
comes from knockout studies, where mice lacking both MKK3
and MKK6 are not viable, dying in midgestation with defects in
the placenta and the development of the embryonic vasculature
. This observation indicates that MKK3 and MKK6 have
some redundant roles, because loss of either gene alone yields
healthy mice [39–41]. Using MKK targeted gene disruption and
siRNA approaches, it has been shown that in response to most
stimuli MKK3 and MKK6 are the main MKKs activating p38α,
although in some circumstances, such as ultraviolet radiation,
MKK4, an activator of JNK, may contribute to the p38α
activation . The major MKK required for p38 activation
may not only be affected by the stimuli, but also by cell type as
their level of expression varies. For instance, MKK3 has been
shown to be the major p38 activator in mesangial cells
stimulated by transforming growth factor , while MKK6
appears to be the predominant isoform in thymocytes .
Although, unlike p38α, the activation of p38β, γ and δ
isoforms has not been extensively examined in MKKs knock-
outs, it has been suggested that the pattern of downstream
p38 MAPK activation in the particular response may be
Fig. 1. p38 MAPKs signalling pathways.
1359A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
determined by the level of MKK6 activity, which activates all
p38 isoforms in vitro, triggered by a given stimulus .
MKK6 and MKK3 are in turn activated by phosphorylation
by a MAPK kinase kinase (MKKK). The MKKK responsible for
activating the p38 MAPK pathways appears to be cell type and
stimulus specific. Several MKKKs have been implicated in the
regulationofp38 MAPKsignalling,these include MLKs,ASK1,
TAK1 and some members of the MEKK family [32,44–47].
Contributing to p38 activation upstream of MKKK are the low
molecular weight GTP-binding proteins from the Rho subfamily
suchasRac1, Cd242, Rho and Rit [48–50] and heterotrimericG-
protein coupled receptors .
Recently, it has been shown that, despite p38 MAPK
pathway is normally regulated by MKKK–MKK pathway in
mammalian cells, two other possible mechanisms of activating
p38α have been proposed. In a yeast two-hybrid screen TAB1
(Transforming growth factor-β-activated protein 1 (TAK1)-
binding protein 1) was identified to bind to p38α MAPK ,
but not to interact with other p38 family member. During the
past 4 years, some studies have established that this adaptor
protein represent an example of MKK-independent activation of
p38 MAPK implicated in the ischemic heart and immunological
processes [53–55]. This MKK-independent activation is
achieved through the autophosphorylation and activation of
p38α after interaction with TAB1 . This mechanism appears
to be involved in the AMPK activation of p38α in ischemic
heart . Intriguingly, in cardiomyocytes it was observed that
although TAB1 lead to an increase in p38α activity, none of the
classic downstream activities of the enzyme (associated with the
activation of p38α by MKKs) were observed . The authors
of the study found that TAB1 sequestered p38α to the cytosol
and this could be a mean of preventing some of the MKK-
activated p38α functions. It remains to be demonstrated how
widespread this model of activation is and if it leads to any
cellular functions of p38α. However, this mechanism does not
contribute to p38 MAPK activation in fibroblasts or epithelial
cells under the same conditions [38,44].
Another MKK-independent mechanism of activation of
p38α has been observed in Tcells stimulated through the Tcell
antigen receptor (TCR). In this system, p38α is activated by an
alternative mechanism in which TCR-mediated stimulation
activates proximal tyrosine kinases that results in the
phosphorylation of p38α on a noncanonical activating residue,
Tyr323. This phosphorylation activates p38α, probably by
causing changes in its structural conformation, to phosphor-
ylate third party substrates as well as its own Thr–Gly–Tyr
3.3. p38 scaffold and binding proteins
The protein kinases that form the MAPK signalling modules
may interact via a series of sequential binary interactions to
create a protein kinase cascade. However these protein kinases
may be organized into signal complexes to create a functional
MAPK module. This organization may be mediated by the
interaction of the protein kinase with one member of the cascade
or, alternatively, by a scaffold protein that interacts with each of
been implicated in the regulation of different MAPK signalling
MAPK cascade. One example, is the protein Osmosensing
Scaffold for MEKK3 (OSM) which forms a complex with Rac,
MEKK3 and MKK3 in the activation of p38α in response to
hyperosmotic stress . OSM could be the mammalian
counterpart of STE50 in S. cerevisiae, which is required for
the regulation of Hog1 under the same stress . Similar roles
in the activation of the p38 MAPK pathway have also been
proposed for the members of the JNK-interacting protein (JIP)
family JIP2 and JIP4, which are scaffold proteins for the JNK
pathway .JIP2 canbindtoJNKand totheother components
of this pathway, MKK7 and MLK3 , but also binds to
MKK3, p38α and p38δ MAPKs [64–66]. On the other hand,
JIP4 also binds to JNK but not to MKK7 or MLK3, and its
function is different to other JIP proteins, it does not affect the
activation of JNK pathway. In contrast, JIP4 appears to be a new
component of the p38 MAPK pathway since it is an activator of
this cascade by a mechanism that requires MKK3 and MKK6.
JIP4 interacts with p38α and p38β, but not with p38γ or p38δ
TAB1, as mentioned previously, has been linked with a novel
way of activating p38α, but, in contrast, it was also found that
TAB1 complexed with TAK1 and TAB2 or TAB3 can mediate a
negative feedback loop following phosphorylation by p38α of
TAB1 in response to inflammatory stimuli .
3.4. Downregulation of p38MAPK pathway
The magnitude and duration of p38 MAPK signal transduc-
tion are critical determinants of its biological effects. Activation
of p38 MAPK occurs within minutes in response to most stimuli
and is transient. This suggests that p38 MAPK functions as a
biological switch that must be downregulated, both under basal
conditions and during adaptation. In mammalian cells several
protein phosphatases interact with and inactivate p38 MAPK
pathway, both PP2C (Ser/Thr phosphatase) and PTP (Tyr
phosphatase) have been shown to regulate p38 MAPK [68, 69].
Moreover, another family of dual-specificity phosphatases
plays a key role in the regulation of these MAPKs, since, for
example, the M3/6 and MKP-7 phosphatases have been shown
to regulate JNK and p38 MAPKs [70,71].
4. Downstream targets and some physiological roles of the
4.1. Downstream targets for p38 MAPKs
The identification of physiological substrates for p38α and
p38β has been facilitated by the availability of relatively
specific pyridinyl imidazole inhibitors such as SB203580 and
SB202190 [11,72]. These compounds have been very important
tools for the delineation of pathways in which these MAPKs are
1360A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
involved. However, because they inhibit both kinases, p38α and
p38β, with similar IC50values, it is not possible to distinguish
between the effects of either MAPK or other kinases also
inhibited by these inhibitors with similar potency . This
problem can be partly solved by the use of mice-deficient for
each p38 MAPKs. Knockout mice for p38α have been
generated, but they die at midgestation [74–76], whereas
tissue-specific knockouts have implicated p38α in cardiomyo-
cyte proliferation and survival [77,78]. Recently p38β, p38γ
and p38δ and double p38γ/p38δ knockout mice have also been
generated, which are viable and fertile [28,79]. Moreover, the
diaryl urea compound BIRB796  is not only a potent
inhibitor of p38α and p38β, but also inhibits p38γ and p38δ at
higher concentrations in cell-based assays providing a new tool
for identifying physiological roles of these two p38 MAPK
isoforms by using varying concentrations of this new compound
in combination with the pyridinyl imidazoles .
Many p38 MAPK targets have been described, both in the
cytoplasm and in the nucleus, which indicates that multiple
cellular functions are under their control. p38α MAPK was
shown to be present in both the nucleus and the cytoplasm of
quiescent cells, but upon cell stimulation, some evidence
suggests that it translocates from the cytoplasm to the nucleus
. However, other data indicate that activated p38 MAPK
is also present in the cytoplasm of stimulated cells . p38
MAPK are proline-directed kinases. However, substrate
specificity is not only determined by the targeted amino acids
but also by specific docking domains present on the substrate
protein and by specific substrate binding motif in the
p38 MAPK. Thus, although the substrate specificity of all
p38 MAPK isoforms is known to overlap, there are some
differences between the two subgroups of p38 MAPKs with
regard to substrate selectivity of these kinases.
Some p38α and p38β physiological substrates are sum-
marised in Fig. 2, these have been shown to be transcription
factors, other protein kinases which in turn phosphorylate
transcription factors, cytoskeletal proteins, and translational
machinery components, and other proteins such as metabolic
enzyme, glycogen synthase or cytosolic phospholipase A2
p38γ and p38δ MAPK isoforms can phosphorylate typical
p38 MAPK substrates such as the transcription factors ATF2,
Elk-1 or SAP1. However, they cannot phosphorylate MAP-
KAP-K2 or MAPKAP-K3, which are good substrates for the
other two p38 MAPK isoforms [17,21]. A feature that makes
p38γ unique among the p38 MAPKs is its short C-terminal
sequence -KETXL, an amino acid sequence ideal for binding
PDZ domains in proteins. p38γ binds to a variety of these
proteins, such as α1-syntrophin, SAP90/PSD95 and SAP97/
hDlg, and under stress conditions is able to phosphorylate them
and modulate their activity [27–29]. These proteins are scaffold
proteins usually targeted to the plasma membrane cytoskeleton
Fig. 2. Substrates and function of the p38α/β MAPKs, and of their substrates. The list of substrates indicated in this figure is not complete but shows the many
important substrates and physiological roles described for these kinases to date. CHOP, CCAAT/enhancer-binding protein-homologous protein; MEF, myocyte
enhancing factor; PGC, peroxisome proliferators activated receptor γ coactivator; SAP, serum response factor accessory protein; HBP, high mobility group-box
transcription factor; NFAT, nuclear factor of activated T-cells; ATF, activating transcription factor; MAPKAP-K, mitogen activated protein kinase activated protein
kinase; MSK, mitogen and stress activated protein kinase; MNK, mitogen activated protein kinase-interacting protein; TAK, transforming growth factor-β-activated
1361A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
at specialised sites such as the neuromuscular junction and gap
junctions through protein–protein interactions. In the case of
SAP97/hDlg its phosphorylation by p38γ provided a mechan-
ism of dissociating SAP97/hDlg from the cytoskeleton .
On the other hand, p38δ possibly plays a role in cytoskeleton
regulation as it has been reported to phosphorylate the
cytoplasmic protein stathmin which has been liked to regulation
of microtubule dynamics . Microtubule-associated protein
tau is another protein substrate of p38δ [25,26]. Eukaryotic
elongation factor 2 (eEF2) kinase was identified in a screen for
substrates for p38δ and shown to be inhibited upon phosphor-
ylation on Ser359 [85,86]. Moreover, it has been suggested that
p38δ regulates involucrin gene expression through the tran-
scription factor CHOP [87,88].
4.2. Roles of p38 MAPKs in cellular differentiation
Evidences from a number of studies carried out during the
past few years establish a key role for the p38 MAPK pathway
in the conversion of myoblasts to differentiated myotubes
during myogenic progression [89–92]. Myogenic differentia-
tion proceeds through irreversible cell cycle arrest of myoblasts
followed by a gradual increase in expression of muscle-specific
genes. The persistent activation of p38 MAPK, induced at early
stage of this process, leads to upregulation of myogenic markers
and accelerates myotube formation in cell culture models.
Treatment with the p38α/β inhibitor SB203580 blocks fusion of
myoblasts to myotubes, as well as the induction of muscle-
specific genes, [89–92], whereas activation of p38 MAPK by
over-expression of a constitutively active mutant of its activator
MKK6 (MKK6-EE) is sufficient to stop cell proliferation and to
induce both the expression of differentiation markers and the
appearance of multinucleated myotubes [91,93]. Recent in vivo
studies demonstrated that p38 MAPK signalling is a crucial
determinant of myogenic differentiation during early embryonic
myotome development .
p38 MAPK controls progression of myoblasts differentiation
at multiple levels: transcription factor activity, chromatin
remodelling and turnover of mRNAs encoding certain regula-
tors of muscle differentiation. Different reports have shown that
p38α and p38β phosphorylate and enhance the transcriptional
activity of members of the myocyte enhancer factor-2 (MEF2)
family, MEF2A and MEF2C, but not MEF2D [95–97]. By
contrast, p38γ only weakly phosphorylates MEF2A, MEF2C
and MEF2D in vitro and barely stimulates their transcriptional
activities in vivo, whereas p38δ does not phosphorylate any of
them [51,91]. MEF2 factors cannot activate muscle gene on
their own, but they do potentiate the activity of basic helix–
loop–helix myogenic regulatory factors (MFRs), which control
the activation of muscle differentiation-specific genes, and their
transcriptional co-activators, including a chromatin remodelling
enzyme [98,99]. Moreover, the phosphorylation of the protein
E47 by p38 MAPK, which is a partner of MyoD (a MRF family
member), promotes functional MyoD-E47 hetero-dimerization,
and targets chromatin-remodelling enzymes SWI/SNF and
RNA polymerase II to muscle-specific loci [91,92,97,99–
101], thereby inducing transcription of muscle specific genes.
One recent study has reported evidences that p38 MAPK may
control expression of specific set of muscle-specific genes
acting not only at transcriptional level, but also on mRNA
turnover. Thus, activation of p38 MAPK in myoblasts causes
stabilization of some muscle-specific mRNAs by phosphorylat-
ing the AU-rich element (ARE)-binding protein KSRP, which
controls the turnover of several transcripts during the transition
from myoblasts to myotubes .
In addition to the promyogenic role of p38 MAPK at early
myogenic stages, an unexpected repressive p38 MAPK
function, which operates at late stages of muscle differentiation,
has also been described using different approaches [93,103].
Most of the work that demonstrates the requirement for p38
MAPK in myogenesis is based on the use of the compound
SB203580, which only inhibits p38α and p38β. The contribu-
tion of the different p38 MAPK family members to the
differentiation process has been recently examined using mice
lacking individually one of the four p38 MAPKs . This
study shows that p38α plays a central role in myogenesis, since
myoblasts from mice lacking this kinase, but not those lacking
p38β or p38δ, do not differentiate to multinucleated myotubes
. Given that p38γ expression is exceptionally high in
skeletal muscle in comparison to other tissues and that, endo-
genous p38γ protein levels increase as myoblast differentiate
into myotubes [89,105], it is not surprising that it may play a
cardinal role in skeletal muscle differentiation. Indeed, Lechner
et al.  initially showed that over-expression of p38γ in
skeletal muscle cells leads to differentiation from myoblast to
myotubes, and that a dominant-negative mutant of p38γ
prevented this differentiation process. In addition, p38γ-
deficient myoblasts show attenuated fusion in vitro although
no major alteration was detected on neonatal or adult muscle
a possible compensatory mechanism due to the redundancy of
functions among the p38 isoforms.
Some reports have suggested a role for p38δ in keratinocyte
differentiation by regulating the expression of involucrin, which
is a protein expressed during keratinocyte differentiation .
Keratinocyte differentiation is a multistage process that is
initiated in the proliferative basal layer of the epidermis and
proceeds through the metabolically active spinous and granular
layers, until the cell is released from the cell surface at the
cornified envelope . It has been shown that activation of
exogenously expressed p38δ by differentiation-inducing agents
such as a bioactive green tea polyphenol (EGCG), okadaic acid
(OA) or the phorbol ester TPA, correlated with increased
involucrin promoter activity in keratinocytes via increased
activity at AP1, Sp1 and C/EBP sites [107,108]. Of note, this
occurred in an SB203580-independent manner and what is
more, p38γ is not expressed in keratinocytes , although the
mechanisms by which p38δ may regulates keratinocyte
differentiation is still unknown. However, the regulation of
keratinocyte differentiation does not seem to be exclusive for
p38δ, thus it has been shown that treatment of keratinocytes
with agents to deplete cholesterol, induces the upregulation of
involucrin mRNA in a p38α-dependent manner, but not by
p38δ . Moreover, in the presence of exogenous constitu-
1362A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
tively active MKK6 or MKK7, a role of p38α has also been
On the other hand, it has been also claimed that p38δ may
have a dual role in keratinocytes contributing not only to the
differentiation process, but also to their apoptosis in a PKCδ
dependent manner, and in response to OA or H2O2[108, 110]. It
is important to notice that most of the evidences involving p38δ
in regulating keratinocyte differentiation or apoptosis are based
in overexpression experiments, and require verification using
other tools to both, inhibit the activity or the expression of
different p38 MAPKs.
Other differentiation processes, in which p38 MAPKs have
been shown to be implicated in either positive or inhibitory roles
are: early stages of osteoclastogenesis from bone marrow
cells differentiation [115,116] and neuronal plasticity [117,118].
Interestingly, the number of studies highlighting the role of
p38 MAPK pathway in stem cells differentiation is greatly
unknown. Thus, it has been reported that p38α/β MAPK
function as a molecular switch to activate the quiescent satellite
cells, which are muscle stem cells . In addition, there are
also evidences showing that in two different stem cell lines, the
control of p38 MAPK activity also constitutes an early switch,
committing stem cells into either neurogenesis or cardiomyo-
genesis . Moreover, the differentiation of pancreatic
progenitors to β-cells induced by the agent conophylline occurs
through a p38 MAPK-dependent mechanism . In hemato-
poietic stem cells (HSC), self-renewal is crucial for hemato-
poietic homeostasis. In this case it seems that activation of p38
detrimental effect to their self-renewal capacity . Thus
inhibition of p38 MAPK may provide beneficial therapeutic
target in some human diseases, as in aplastic anemia .
4.3. p38 MAPK pathways and cell migration
Another role for p38α that was first elucidated by using
SB203580 is its involvement in chemotactic cell migration. It
was first reported that SB203580 inhibited endothelial cell
migration stimulated by vascular endothelial growth factor
(VEGF) . Subsequently, p38α was shown to relay
chemotactic signals in numerous systems; N-formyl-L-leucyl-
L-phenylalanine (fMLP) and C5a induced neutrophil migration
[124,125], migration of vascular smooth muscle cells in
response to PDGF, TGFβ and IL-1β [126,127], mast cells
treated with stem cell factors , Gas6-Ark stimulated
Gonadotropin-releasing hormone neuronal cells , epithe-
lial cells stimulated with CXCL12/SDF1α, EGF, HGF, PDGF
and TGFβ [130,131] and invasion of human breast epithelial
cells by H-Ras . The role of the four p38 isoforms in
transducing chemotactic signals was evaluated using cells
derived from mice lacking the different isoforms and the results
demonstrated that only p38α but none of the other p38 isoforms
were involved in relaying these signals . Partial loss of
pro-angiogenic (for example VEGF and HGF) signalling
leading to increase cell migration could in part explain the
embryonic lethality due to defect in labyrinth invasion by
allantoic mesenchyme seen in the p38α-deficient mice [74,75].
It is worth noting that HGF-deficient mice also show a loss of
proper organization in the labyrinth region. This can delineate a
signalling pathway leading from tyrosine kinase receptors (c-
Met and VEGFR2) to p21-activated protein kinase 1/2  to
p38α that regulates angiogenesis. Interestingly it is known that
p38α is activated by hypoxia, a condition that leads to VEGF
expression, which could also contribute to defective angiogenic
signalling seen in p38α-deficient mice. The VEGF mRNA
contains an ARE, and as mentioned previously, these sequences
can be the targets of p38α-dependent regulation .
Most investigations on the role of p38α in cell migration
have focused on links with the cytoskeleton rather than changes
in gene expression. Initial studies have revealed that inhibition
of p38α activity and the subsequent phosphorylation of HSP27
by MAPKAP-K2 could prevent actin cytoskeleton reorganiza-
tion necessary for cell migration [123,126,134]. The role of the
protein kinase MAPKAP-K2 in this pathway was confirmed in
mice lacking this enzyme where vascular smooth muscle cells,
fibroblasts and macrophages migration was all found to be
impaired [127,131]. Interestingly, mice lacking MSK1 and
MSK2, two other protein kinases that can be activated by p38α
were found not to be involved in chemotactic cell migration
. Initial study on the role of the p38α-MAPKAP-K2 axis
focused on HSP27. Unphosphorylated HSP27 has been shown
to block actin polymerization and act as an actin cap-binding
protein in vitro, which is modulated by phosphorylation [135–
137]. Interestingly, at least two other substrates of MAPKAP-
K2 could affect actin polymerization. CapZ-interactin protein
(CAPZIP) as its name implies interacts with the actin cap-
binding protein CapZ and is phosphorylated by MAPKAP-K2
. Although the role of this protein in a model of cell
migration has not been proven, it would possible to envisage
that phosphorylation of CAPZIP could regulate the function of
CapZ on actin polymerization. Recently, LIMK1 was shown to
be phosphorylated and activated by MAPKAP-K2 in response
to VEGF in endothelial cells . LIMK1 induces actin
remodelling by phosphorylating and inactivating cofilin, an
actin-depolymerizing factor. Thus LIMK1 forms a crucial and
integral part of the migratory response to VEGF in endothelial
Caldesmon is an actin- and myosin-binding protein that is
also involved in the assembly of actin filaments. It was found to
be phosphorylated downstream of p38α in uPa-stimulated
smooth muscle cells  and important for endothelial cell
migration where it is abundantly found . Paxillin is a
phosphoprotein found at focal adhesions, crucial links between
the extracellular matrix and the cell cytoskeleton. It was
reported to be phosphorylated at Ser83 by p38α in NGF-
stimulated PC12 cells . Although it remains to be
demonstrated that phosphorylation of Ser83 of paxillin is
essential for cell migration, this provides another possible
mechanism for p38α-dependent cell migration.
Another way, by which the p38α pathway can regulate
angiogenesis and cell motility or invasion, is through the
regulation of matrix metalloproteases called MMPs. Inhibition
1363 A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
of p38α MAPK activity with SB203580 was shown to block
MMP-9 expression in phorbol myristate acetate (PMA)-treated
human squamous cell carcinoma . This was found to be
also dependent on the transcription factor AP-1 . In cells
central to angiogenesis, like endothelial cells or macrophages, a
similar role for the p38α MAPK has been proposed. In LPS-
stimulated RAW macrophages, the same p38α MAPK-AP-1
pathway was shown to regulate MMP-9 expression . This
was also found to be the case in TNFα-stimulated monocytes
. In endothelial cells, sheer-stress induced MMP-9
expression was shown to be dependent on both the p38α
MAPK and the ERK1/2 pathways . Interestingly, MMP-9
expression correlates with severity of atherosclerosis ,
increasing the clinical interest of this MAPK pathway. In
contrast to MMP-9, the regulation of MMP-2 in endothelial
cells seems to be more dependent on the ERK1/2 MAPK than
the p38α MAPK [149,150].
The importance of p38α in relaying chemotactic signals will
be important for normal physiological functions like neutrophil
migration and angiogenesis. However, this also raises the
possibility of targeting p38α and its substrates involved in cell
migration in situations where aberrant cell motility leads to
disease development, like tumour growth (through angiogen-
esis), invasion and metastasis.
5. Pathological implication of the p38MAPKs
5.1. Role of p38MAPK pathways in inflammation
As mentioned above, p38α MAPK was first recognized for
its role in inflammation in regulating the biosynthesis of pro-
inflammatory cytokines, namely IL-1 and TNFα, in endotoxin-
stimulated monocytes . Subsequently it was found to be
involved in regulating the production of IL-8 in response to IL-1
or osmotic shock  and the production of IL-6 in response to
TNFα . COX-2, another inflammatory mediator, was
shown to be regulated at least in part by p38α [153–155]. p38α
production as mice lacking p38β, the most closely related
isoform, showed no defect in cytokine production or in immune
function in an initial study . Moreover, recently it has been
shown that, in p38α deficient Th1 cells, the IFN-γ secretion
stimulated by IL12/IL18 is defective compared with the
secretion induced by TCR. Suggesting that the activity of
p38α in Th1 cells may be restricted to one of the two pathways
involved in IFN-γ production, . No functions in inflam-
mation for the other two isoforms of p38 MAPKs have been
reported yet. Although regulation of these inflammatory
mediators can arise at different levels, a lot of focus has been
on the role of the p38α MAPK pathway in post-transcriptional
regulation. These genes all share an ARE found in the 3′
untranslated region (3′UTR) of their mRNA. The presence of
this element is known to shorten the half-life of the mRNA
containing them and in some cases (as for TNFα) block their
translation. The importance of this ARE found in the 3′UTR of
TNFα mRNA in mediating the p38α regulation of translation
has been elegantly demonstrated in a study of mice lacking the
ARE of TNFα, where they became irresponsive to LPS-
stimulated p38α-mediated TNFα translation . In another
mice model, this time lacking the protein kinase MAPKAP-K2,
the downstream target of p38α, there is marked decrease in
production of TNFα and IL-6 in response to LPS , which
has also been linked to post-transcriptional regulation via the 3′
COX-2 . Thus it seems that a common mechanism for the
gene regulation by the p38α MAPK pathway is the post-
transcriptional control via the ARE. The exact mechanisms by
which p38α MAPK controls post-transcriptional regulation is
still unknown and will most likely involve the phosphorylation
and/or activation of numerous proteins. So far a number of
substrates for MAPKAP-K2 could potentially be involved in
post-transcriptional regulation; these include the ARE binding
proteins TTP  and hnRNP A0  and the poly (A)-
binding protein PABP1 . Moreover, hnRNP A1, an ARE
MNK1, another protein kinase found downstream of p38α and
to be important for TNFα production in T cells .
Furthermore, another ARE binding protein, KSRP has been
shown to be directly phosphorylated at Thr692 by p38α. This
phosphorylation blocks KSRP binding to the mRNA and
therefore its destabilizing function .
Pro-inflammatory cytokine (PIC) production plays a key role
in the pathogenesis of many chronic inflammatory and
rheumatic diseases. In particular, TNFα, IL-1β and IL-6 are
key players in rheumatoid arthritis, Crohn's disease (a subset of
inflammatory bowel diseases), psoriasis, ankylosing spondylitis
and chronic asthma [164–166]. Not only they are a cause of
inflammatory diseases, but there is evidence that they play an
important role in other diseases, including heart failure
[167,168], ischaemic retinopathies  and the development
of insulin resistance in diabetes . Consequently, blocking
the action of these PICs is an attractive therapeutic strategy. In
view of this a number of pharmaceutical companies have
developed drugs that target p38α MAPK in order to block PICs
production. Some ofthese drugs have advance to human clinical
trials. They include AMG 548 (Amgen), BIRB 796 (Boeringer
Ingelheim), SCIO 469 and SCIO 323 (Scios, Johnson and
Johnson) and VX-702 (Vertex). Although AMG 548 showed
greater than 85% inhibition of ex-vivo LPS-induced TNFα and
IL-1β in healthy males, its production was suspended due to
random liver enzyme elevations that were not dose or exposure
dependent . Similarly BIRB 796 has shown some liver
enzymes elevations that were above the upper limit of normal
and there was no evidence for clinical efficacy of BIRB 796 in
phase II clinical trials for pain, multiple myeloma and
rheumatoid arthritis and SCIO 323 is in phase I clinical trial
for myelodysplastic syndrome, multiple myeloma, rheumatoid
arthritis, cerebral ischemia and diabetes mellitus whereas VX-
702 is a second generation inhibitor developed against inflam-
matory disorders that was designed to replace VX-745 shown to
cross the blood–brain barrier . However, in view of the
many roles of p38α and the requirement of long-term treatment
for chronic diseases, it is possible that downstream effectors or
1364 A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
specific upstreamactivators of p38α involved in PIC production
may make better drug targets. This could explain in part why
even with more than a hundred compound developed to inhibit
this kinase none have found their way yet as a bona fide drug.
5.2. p38 MAPK pathways and cancer
During the last few years, members of the p38 MAPKs
subfamily have joined the group of canonical signalling
pathways involved in the transformation process. It has been
shown that p38 MAPK pathway could be involved in some of
the alterations observed in the physiology of transformed cells:
self-sufficiency in growth signals, unlimited replication poten-
tial, protection against apoptotic cell death, de novo angiogen-
esis, and tissue invasion and metastasis .
One new role for the p38 MAPK pathway that has been
elucidated in recent years is the regulation of checkpoint controls
on the cell type, p38 MAPK can either induce progression or
inhibition at G1/S transition by differential regulation of specific
retinoma protein (pRb), which is a hallmark of G1/S progression
[38,174],andby phosphorylation ofthe p53 tumor suppressor on
two activating sites in the N-terminal region (Ser33 and Ser46)
[174–176]. On the other hand, activation of p38 MAPK in
mammalian cells in response to various environmental insults,
including ultraviolet (UV) light, disruption of the microtubule
cytoskeleton, hyperosmotic stress and inhibition of histone
deacetylase, initiates G2/M checkpoint [177–182], and G1 arrest
following UV-induced DNA damage [183,184].
DNA damage checkpoints function as surveillance mechan-
isms during cell division to ensure that each step is completed
properly, thus maintaining genetic integrity. In mammalian
cells, the G2/M checkpoint response is complex and mediated
by a number of signalling pathways, including the ataxia-
telangiectasia mutated (ATM) and ATM-related (ATR) path-
ways  and, more recently, the p38 MAPK pathway .
Activation of the p38 MAPK signalling pathway, in response to
stimuli that impose cell cycle arrest and/or cell death, results in
the activation of its physiological substrate, MAPKAP-K2,
which phosphorylates Cdc25B on Ser309 or Ser323 and
Cdc25C on Ser216 . This mediates the subsequent binding
of 14-3-3 proteins and the shuttling of Cdc25B into the
cytoplasm, which together induce G2 delay [177,179,183]. A
specific inhibitor of p38 MAPK was shown to significantly
reduce phosphorylation of Cdc25B on Ser309, which in turn
significantly reduced Cdc25B-14-3-3 interactions and initiated
the G2/M checkpoint response . Cdc25s are protein
phosphatases that activate cyclin dependent protein kinase
activity (which is the major regulator of G2/M transition) .
More recently, p38 MAPK has been reported to block entry in S
phase by phosphorylating CdC25A on Ser76 and Ser124 and of
causing the degradation of Cdc25A protein in response to
hyperosmotic stress and cytokine withdrawal in interleukin-7
(IL-7) and IL-3-dependent mouse lymphocytes [186,187].
Most of the work published on cell cycle regulation by p38
MAPK pathway has been focussed on studying the role of the
isoforms p38α and β. In the case of p38γ, one report indicates
its activation after ionizing radiation could be dependent on
ATM . However, all p38 MAPKs show robust induction
by stresses such as UV radiation, whereas their induction by
ionizing radiation is highly infrequent  [Y. Kuma, A.
Cuenda unpublished results]. However, it has been shown that,
Xenopus p38γ promotes meiotic G2/M transition in Xenopus
oocytes treated with progesterone and activates XCdc25C by
phosphorylating it at Ser205, whereas p38α or p38β have no
effect . Fully grown Xenopus oocytes are arrested in G2/
prophase of meiosis I and are induced to proceed through
meiosis by progesterone stimulation .
On the other hand, it has been shown that p38 MAPK is
involved in the growth-inhibitory signalling cascade of contact
inhibition in fibroblasts. This novel physiological function of
p38α in cell cycle control provides further mechanism support
for the idea that p38α may act as suppressor of tumorigenesis.
Proliferation of non-transformed cells is regulated by cell-cell
contact, which is referred to as contact-inhibition . There is
a sustained activation of p38α in response to cell–cell contact.
Contact inhibition is impaired by p38α/β inhibitors as well as in
p38α−/− fibroblasts. Moreover, p38α−/− fibroblasts show a
higher saturation density compare to wild-type fibroblasts,
which is reversed by reconstituted expression of p38α .
All these findings suggest that defects in p38α MAPK
function may contribute to cell cycle defects and tumorigenesis.
A clear example is the role of p38α in inducing terminal
differentiation and inhibiting proliferation of rhabdomyosar-
coma-derived cells, one of the more common solid tumors of
childhood, as a consequence of defects in differentiation of
muscle precursor cells. This defect is attributed to deficiency in
p38 MAPK activity . Another case of inhibition of
tumorigenesis through p38 MAPK was reported in mice
expressing the oncogenes ErbB2 or Ha-Ras, but disrupted for
the gene Wip1 (alsoPPM1D), which encodes a phosphatase that
inhibits p38 MAPK, manifested impaired mammary carcino-
genesis [193,194]. Similarly, decreased p38 MAPK activity
caused in cells by deletion of both upstream activators, MKK3
and MKK6, enhances proliferation of fibroblasts on low serum
and increases tumorigenesis when immortalised MKK3/6 (−/−)
fibroblasts (compared to wild-type cells) were injected
subcutaneously in athymic nude mice . Another phospha-
tase, DUSP26 (also known as MAPK-phosphatase 8) was
shown to dephosphorylate p38 MAPK and promotes survival of
anaplastic thyroid cancer cells .
Taken together these results clearly point to a role of the p38
MAPK activity in restraining uncontrolled cell proliferation.
This was clearly established in a recent report demonstrating
that a key role for the p38α MAPK in preventing tumorigenesis
is to promote growth arrest and apoptosis specifically in
response to reactive oxygen species (ROS) . Dolado et al.
showed that p38α-deficient cells are resistant to ROS-induced
apoptosis and oncogenes that generate high levels of ROS leads
to transformation of p38α-deficient MEFs. ROS can activate
the MKKK ASK1, an upstream activator of the p38α MAPK,
by dissociating it from the glutathione-S-transferase (GST) mu
. Alternatively, ROS could also activate p38α MAPK
1365A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
through the inactivation of phosphatases by oxidation of the
active site cysteine residue, consistent with the tumorigenic
roles proposed for some p38α MAPK phosphatases previously
mentioned [193–195]. Dolado et al. also showed that cancer
cells have found ways to uncouple the activation of p38α
MAPK by ROS by increasing levels of GST proteins. Thus,
drugs that could restore this activation, by targeting GST
proteins for example, could prove to be useful anti-cancer
agents for certain tumours.
In contrast to the number of publications discussing the role
of p38 MAPK pathway as a tumor suppressor, only a few
publications have provided evidence for an oncogenic potential
of this pathway . This could involve supporting tumour
invasion. Growth of tumors beyond a certain size results in
hypoxia and requires the formation of new blood vessels for
further growth, which is controlled through the production and
secretion of angiogenic factors. p38α MAPK is activated by
hypoxic conditions and is involved in the production of VEGF
. Interestingly, p38 MAPK also plays a role in the down-
stream signalling of VEGF leading to angiogenesis [199,200].
As mentioned previously, the p38α MAPK pathway was
suggested to play a role in MMP-9 regulation, an observation
initially made in a carcinoma cell line . This suggests a
potential role for the p38α MAPK pathway in promoting cell
invasion and metastasis not only by mediating chemotactic
signalling but also by controlling ECM remodelling. Interest-
ingly, increase levels of angiogenic factors like VEGF and
MMP-9 (that are both regulated by the p38α MAPK) correlate
with unfavourable prognosis in many tumours.
There are now numerous reports supporting such hypothesis.
In B-cell chronic lymphatic leukaemia cells constitutive expres-
sion of MMP-9 was dependent on p38α MAPK .
Interestingly, in contrast to what was observed in endothelial
cells (see cell migration section) increase MMP-2 expression
seem to be dependent on the p38α MAPK in many transformed
cell lines. The TGFβ-mediated increase production of MMP-2
by prostate cancer cells involves the p38α MAPK–MAPKAP-
K2–HSP27 axis [202,203]. Similar results were obtained in pre-
neoplastic human breast epithelial cells whereas the TGFβ-
stimulated MMP-2 expression was regulated by phosphorylation
of ATF2 through the p38α MAPK signalling . Similarly, in
H-ras but not N-ras transformation of the same cell line, the
production of MMP-2 was dependent on p38α MAPK but not
the ERK1/2 MAPK [132,205]. Additionally, ras-dependent
cellular invasion requires the p38α MAPK regulation of the
serine protease urokinase plasminogen activator (u-PA) . In
human melanoma cells, the tetraspanin CD9 induces the
expression of MMP-2 through the activation of both the p38α
MAPK and the JNK pathway . Thus, there seem to be an
important role of the p38α MAPK pathway in up-regulating the
expression of MMPs that correlates with an increased invasive
phenotypeofcancer cells.Now itwill beimportant tounderstand
the exact mechanisms by which the p38α MAPK pathway
regulates these proteases. For example, does this regulation only
occur at the transcriptional level via the ATF1 or ATF2
transcription factors or is the p38α MAPK pathway also
involved in the translation and maturation of these proteases.
Thisknowledgewillhopefullyincrease the arrayoftoolsthatcan
be used to disrupt the invasive phenotype of tumours.
Not only p38α but also other isoforms could mediate
tumorigenesis. Recent data suggest that the oncogene Ras
positively regulate expression of p38γ isoform and this may be
involved in Ras-transformation in rat intestinal epithelial cells
and in Ras-increased invasion in breast cancer cells [208,209].
5.3. p38 MAPKs in cardiovascular dysfunction
Cardiovascular mortality is an important health problem in
human populations. Two leading causes of cardiac morbidity
are pressure overload cardiac hypertrophy resulting from
hypertension and cardiomyocyte apoptosis and necrosis
following ischemic injuries. Shear stress from pressure overload
can activate stress-activated protein kinases pathways ,
whereas both ischemia and reperfusion of isolated rat hearts
lead to activation of p38MAPK and MAPKAP-K2 [211,212].
p38α was found to be the dominant p38 MAPK isoform found
in the heart, with p38β levels undetectable and low levels of
p38γ and p38δ . Studies have proposed both a protective
and damaging roles of p38MAPK in the stressed myocardium.
This seemed to be dependent in part in the system studied. For
example, in ischemia reperfusion, there is a difference in
myocardial responsiveness between a mouse and a pig model;
in that study p38 activation in the mouse contributes to acute
cellular injury and death, while the same activation in pig has no
causative effect .
The role of p38 MAPKs in cardiac hypertrophy has been
suggested by studies using over-expressed active forms of their
upstream activators MKK3 and MMK6 in cardiomyocytes. In
these studies it was found that the active mutants elicited
characteristic hypertrophic responses [210,215]. Moreover,
over-expression of MKK3 in cardiomyocytes leads to an
increase in apoptosis . Interestingly, the differences
between MKK3 and MKK6 can points to distinct roles of p38
isoforms, whereas the more restricted activation spectrum of
promoting apoptosis. In a transgenic mice model using the same
MKK3 and MKK6 constructs to activate p38 MAPK, it was
found that p38 MAP kinase signalling can contribute to the loss
of contractility and myocardium stiffness and promotes specific
remodelling process in heart failure but there was minimal
change in ventricular mass, suggesting that this pathway in vivo
is not sufficient to induce hypertrophy, contrary to the report in
cardiomyocytes . Similarly, in a study using transgenic
expression of dominant negative forms of MKK3, MKK6 and
p38α in the heart, it was found that the p38 pathway can have an
anti-hyperthropic effect and could function to restrain calci-
neurin-mediated hypertrophy through NFAT transcription
factors . However, a transgene expressing an active form
of TAK1 in mice lead to increase p38α phosphorylation and
cardiac hypertrophy, interstitial fibrosis, severe myocardial
dysfunction, fetal gene induction, apoptosis and early lethality
. But over-expression of TAK1 not only activates p38α but
JNK and NFκB , two pathways, which have themselves
1366A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
been link to cardiac hypertrophy [210,220]. In studies of human
hearts with either compensated hypertrophy or advanced heart
failure, only in the latter case was there any detectable p38
activity . It is conceivable that activation of the stress-
activated pathway in the failing heart is an attempt to respond to
shear stress but due to the weakness of the failing heart to adapt
this activation might result in a more detrimental phenotype.
MAPK and this protein kinase has been more extensively
studied in this context. During ischemia in perfused heart,
inhibition of p38MAPKs activity protects against hypoxic
induced apoptosis and necrosis. Hypoxia was also found to
lead to intracellular acidosis, which augments p38α activation
and leads to apoptotic cell death . Reactive oxygen species
generated from the mitochondria during ischemia and reperfu-
sion activates p38α MAPK and inhibition of p38α significantly
prevented celled death arising from ischemia reperfusion .
activators of AMPK) treated cells, SB203580 blocked Bax
translocation to the mitochondria, a hallmark of apoptosis,
linking p38α activation by ischemia to increase cell death .
Thus the p38α MAPK has been linked with induction of pro-
apoptotic signals. Similarly, MAPKAP-K2-deficient mice were
found resistant to myocardial ischemic reperfusion injury,
pro-apoptotic signals . However, a number of studies have
proposed a role in cardioprotection for the p38 MAPK pathway.
Protection due to ischemic pre-conditioning, the protection of
myocardium conferred by cycles of brief ischemia–reperfusion,
correlates with the activation and phosphorylation of p38α at
Tyr182 in the rabbit heart . It had been previously
established that p38α was activated by cellular stresses like
heat shock, oxidative stress or osmotic shock and that cellular
resistance to these stresses were increased through enhanced
actin cytoskeleton reorganization via the p38–MAPKAP-K2–
HSP27 pathway [226,227]. In cardiac myocytes, activation of
the p38α MAPK–MAPKAP-K2 and the subsequent phosphor-
protein family, provide protection against stress-induced
apoptosis or PDGF-BB improved cardiac function following
myocardial infarction [228,229]. A protective role for this
pathway has been further supported by a study that showed that
over-expression of MKK6 lead to an increased αB-crystallin
levels and could explain the cardioprotective effect of MKK6
transgene over-expression . An interesting explanation
between these different results was suggested by a study where
they found that p38 MAPK-mediated F-actin reorganization
may stimulate apoptotic cell death but conversely can protect
against osmotic-derived necrosis in cardiomyocyte .
Another study also found a similarly dual role of the p38α
pathway where treatment with SB203580 aggravated myocyte
necrosis but also revealed a cardioprotective role for the
inhibition of p38α activity as it blocks contractility during
reperfusion . This effect on contractility was diminished as
well in hearts of mice lacking MKK3 or MAPKAP-K2 when
stimulated with TNFα . Thus, although in various models
inhibition of p38α leads to some degree of cardioprotection
still unclear how this would apply to human clinical conditions
andthere isa needforgreater research into appropriatetargetsof
the p38 MAPKs. One attractive possibility would be to
differentiate between targets of the p38α pathway that lead to
apoptosis and those that protects against necrosis in order to
develop drugs that only target the deteriorating branch of the
p38α activity while maintaining the protection it confers.
5.4. Roles of p38 MAPK pathways in Alzheimer’s disease
The pathological hallmarks of Alzheimer's disease (AD) are
the accumulation of extracellular plaques and intracellular
neurofibrillary tangles that are composed of filaments polymers
of β-amyloid and the neuronal microtubule-associated protein
Tau, respectively. It has been proposed that elevated levels of
β-amyloid in AD brain induces microglial activation and
consequent release of pro-inflammatory cytokines induced by
the p38 MAPK pathway , which may contribute to the
development of this pathology together with other disorders
such as neuronal injury, trauma, ischemia and accumulation of
oxidants with brain aging.
Another major hallmarks of AD, and other neurodegenera-
tive disorders known as ‘tauopathies’, is the accumulation of
neurofilaments made by the protein Tau . The protein Tau
belongs to the family of microtubule-associated proteins. Tau
binds to β-tubulin and promotes microtubule assembly 
playing major regulatory roles in the organization and integrity
of the cytoskeleton network under normal physiological
conditions. Tau is functionally modulated by phosphorylation,
since the ability of Tau to bind and stabilize microtubules
correlates inversely with its phosphorylation which may
facilitates its self-assembly. Thus, when Tau is hyperphos-
phorylated (PHF-Tau), it dissociates from the cytoskeleton and
aggregates itself. This PHF-Tau is the major component of the
paired helical filaments (PHFs), which make up the neurofi-
brillary tangles that along with senile plaques, are the aberrant
structures found in the brains of patients with AD [240–242].
This hyperphosphorylation could result from an increased
activity of Tau kinases or the decreased activity of Tau
phosphatases. In AD at least thirty serine/threonine residues
are phosphorylated. Whereas numerous protein kinases have
been shown to phosphorylate Tau and regulate its function in
vitro, identification of the specific enzymes that regulate
phosphorylation of Tau in vivo has proved difficult .
Tau are serine and threonine residues followed by proline, it is
conceivable that members of the MAPK family play an
important role in phosphorylating Tau. Since aberrantly activated
JNK and p38s have been reported to be associated with cells that
contain filamentous Tau in some neurodegenerative diseases
[243–245], these kinases may contribute to the hyperphos-
phorylation of Tau protein. Moreover, the p38MAPK activator,
MKK6, has also been found to be active in neurodegenerative
diseases . Recently, all residues phosphorylated in Tau by
each p38 isoform have been identified by radioactive ATP-
labelling . Using phosphospecific antibodies, it has been
1367A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
demonstrated that these kinases phosphorylate Tau in vitro and
that in vivo they are implicated in Tau phosphorylation in
response to cellular stress [25,246,247]. In addition, p38s
phosphorylate Tau on residues that are phosphorylated in PHF-
Tau observed in AD brain [25,26], indicating that these kinases
may contribute to the hyperphosphorylation of Tau protein in
neurodegenerative diseases. In the last years it has been shown
that Tau is a good in vitro substrate for the p38 isoforms p38δ
and p38γ, and its phosphorylation by these two enzymes results
in a reduction in its ability to promote microtubule assembly
[25,26]. Moreover, overexpression of p38γ in neuroblastoma,
induces Tau phosphorylation which correlates with a decrease in
Tau associated to the cytoskeleton and an increase of soluble Tau
. It has been reported as well that p38δ is the major Tau
kinase in neuroblastoma in response to osmotic shock . All
these evidences indicate that p38MAPKs can regulate Tau
hyperphosphorylation in neurodegenerative disease and could be
potentially good therapeutic targets for those diseases.
6. Concluding remarks
Most of the studies to date have focused on the role of the
p38α isoform, which is widely referred as p38 in the literature.
However, there are three other p38 isoforms (p38β, p38γ and
p38δ) whose roles among the cellular functions and the
implication in some of the pathological conditions described in
question that remains to be answered is whether these p38
MAPK isoforms are differentially activated by certain stimuli to
mediate specific signals. Although, not many evidences exist so
far, it is possible that for example, in the implication of p38
MAPK in cancer, some isoforms may play a pro-oncogenic role
whereas other p38 isoforms act as tumor suppressors.
It is important to notice that the cell culture studies outlined
here require confirmation using in vivo models like knockouts.
Knockout mice for each p38 MAPK isoform have been
generated, and except p38α knockout, which is lethal, the
other knockouts lack of apparent phenotype. This may be due
to the functional redundancy caused by the existence of highly
related family members. This issue is demonstrated by Sabio
et al.  using cells from mouse knockouts lacking multiple
p38 family members in combination with the use of specific
inhibitors for the different kinases. Such redundancy may
account for the failure on finding a phenotype in the different
p38 knockout mice and point to the need to generate knock-in
mice expressing inactive p38MAPK and mice with tissue-
specific inactivation of the individual p38MAPK family
members. These mice, in combination with the use of specific
new kinase inhibitors, should provide powerful biological
models to address the specific roles of each p38MAPK isoform.
Studiescarriedout during thelast few years haveled tomajor
functions in vivo (Fig. 3). In particular, the use of kinase
inhibitors together with cells from genetically modified mice
lacking different components of this pathway have provided
important information regarding the physiological implication
of p38α MAPK in the immune response by regulating synthesis
of pro-inflammatory cytokines. This finding initiated a huge
Fig. 3. Physiological roles and pathological implications of p38 MAPKs pathways. p38 MAPKs play a central role in the regulation of many biological functions,
which contribute to physiological processes. Deregulation of p38 MAPKs pathways lead to the development of several pathological conditions.
1368 A. Cuenda, S. Rousseau / Biochimica et Biophysica Acta 1773 (2007) 1358–1375
effort by many companies to generate p38MAPK inhibitors as
potential targets for inflammatory diseases. Recently described
roles for p38 MAPK in cancer, heart and neurodegenerative
diseases make this pathway highly attractive for the develop-
ment of new therapeutics strategies to treat these pathologies as
well. On the other hand, the role of p38MAPK in cellular
differentiation, in particular its implication in the differentiation
of skeletal muscle and stem cells, suggests this pathway as a
potential key modulator of both tissue regeneration or cell
renewal processes triggered in response to tissue lost or
can have both protective and detrimental effects even in very
similar systems. Thus although the p38 MAPK pathway as
whole is an interesting therapeutical target, the p38 MAPK itself
may not be the ideal candidate. Identification and characteriza-
tion of the various substrates of these kinases involved in cell
differentiation/cell proliferation, heart failure and neurodegen-
eration, could provide much better targets enabling the ablation
ofthe deleteriouseffectswhile maintaining protectivefunctions.
The work in the authors' laboratories is supported by grants
from the Medical Research Council UK, pharmaceutical
companies that support the Division of Signal Transduction
Therapy (Astra-Zeneka, Boehringer-Ingelheim, GlaxoSmithK-
line, Merck and Co. Inc, Merck KgaA and Pfizer), Ministerio de
Educación y Ciencia (MEC) Spain (SAF2004-1933).
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kinases through docking-site-mediated interactions, Biochem. J. 372
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structures of MAP kinase p38 complexed to the docking sites on its
nuclear substrate MEF2A and activator MKK3b, Mol. Cell 9 (2002)
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