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Review

An overview of mammalian p38 mitogen-activated protein kinases, central regulators of cell stress and receptor signaling

[version 1; peer review: 2 approved]
PUBLISHED 29 Jun 2020
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OPEN PEER REVIEW
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Abstract

The p38 family is a highly evolutionarily conserved group of mitogen-activated protein kinases (MAPKs) that is involved in and helps co-ordinate cellular responses to nearly all stressful stimuli. This review provides a succinct summary of multiple aspects of the biology, role, and substrates of the mammalian family of p38 kinases. Since p38 activity is implicated in inflammatory and other diseases, we also discuss the clinical implications and pharmaceutical approaches to inhibit p38.

Keywords

p38, MAPK, inflammation, signalling

p38 mitogen-activated protein kinases

p38α (originally named p38) was identified and cloned as a 38 kDa protein that was tyrosine-phosphorylated in response to LPS stimulation in mammalian cells1,2. Sequence comparison, on the day p38α was cloned, revealed that it belonged to the mitogen-activated protein kinase (MAPK) family and that a Saccharomyces cerevisiae osmotic response protein kinase HOG1 was a p38α homologue35. p38α was also named cytokine suppressive drug binding protein (CSBP) because it was identified as the target of a series of anti-inflammatory pyridinyl-imidazole compounds and as reactivating kinase (RK) because it phosphorylated and activated MK235. There are four members of the p38 group of MAPKs encoded by four different genes in mammals: p38α (MAPK14, chromosome 6p21.31 in humans), p38β (MAPK11, SAPK2b, Chr22q13.33)6, p38γ (MAPK12, ERK6, SAPK3, Chr22q13.33)7,8, and p38δ (MAPK13, SAPK4, Serk4, Chr6p21.31)9,10. As can be surmised from their chromosomal locations, MAPK14/p38α and MAPK13/p38δ are physically close and separated by just over 15 kb, as are MAPK12/p38β and MAPK11/p38γ, which are separated by less than 2 kb. All the p38s contain a conserved Thr–Gly–Tyr (TGY) dual phosphorylation motif within the kinase activation loop, and both Thr and Tyr phosphorylation are necessary to fully activate the kinase11. However, monophosphorylated p38α Thr180 has some kinase activity in vitro, but a different substrate specificity, when compared with dual-site phosphorylated p38α12. p38 group members are expressed ubiquitously, but p38γ and p38δ are enriched in certain cell types and tissues, such as p38γ in skeletal muscle and p38δ in the salivary, pituitary, and adrenal glands13. p38β shares more amino acid sequence identity with p38α (~70%), while p38γ and p38δ share ~60% identity with p38α. p38γ and p38δ also share high sequence homology with cyclin-dependent kinases (CDKs) and are sensitive to some CDK inhibitors14.

Activation and inactivation of p38

p38α is involved in the response to almost all stressful stimuli, including LPS, UV light, heat shock, osmotic shock, inflammatory cytokines, T cell receptor ligation, glucose starvation, and oncogene activation2,4,5,1520. Under certain circumstances, it is also activated upon growth factor stimulation. It should be noted that the activation of p38 in some cases is cell type specific, since an activating stimulus in one cell type may inhibit p38 in other cell types21. The study of p38 group members other than p38α has been less intensive; however, where it has been examined, the other p38s are frequently co-activated with p38α22.

Like other MAPK signaling pathways, the activation of all p38s is mediated by a kinase cascade: MAPKKK (MAP3K), which activates MAPKK (MAP2K), which in turn activates MAPK. The MAP2K kinases MKK3 and MKK6 are the major upstream kinases for p38 activation2325. Although MKK3 and MKK6 phosphorylate most p38 isoforms in vitro, selective activation and substrate specificity have been observed in vivo26. MKK4 has also been reported to phosphorylate p38α and p38δ in specific cell types9. A number of MAP3Ks have been reported to participate in p38 activation including TAK127, ASK128, DLK29, and MEKK429,30. Low-molecular-weight GTP-binding proteins in the Rho family, such as Rac1 and Cdc42, can activate p38 through binding to MEK1 or MLK1, which function as upstream activators of MAP3K31,32.

p38α can also be activated by MAP2K-independent mechanisms. TAB1 (TAK1-binding protein 1) directly interacts with p38α and can promote trans autophosphorylation on Thr180 and Tyr182 and thus full activation of p38α33. A subsequent study revealed that autophosphorylation of Thr180 and Tyr182 requires a conserved Thr185 residue34. TAB1-dependent p38α activation has been implicated in ischemic myocardial injury and T cell anergy35,36. TAB1 is also claimed to play a role in Sestrin-mediated p38α activation12. Another MAP2K-independent activation is mediated by ZAP70 after T cell receptor ligation. ZAP70 can directly phosphorylate p38α/β on Tyr32318, leading to autophosphorylation on Thr180, one of the dual phosphorylation sites. As discussed, mono-Thr180 phosphorylated p38 still has some kinase activity37, and loss of ZAP70-mediated p38 activation in p38αβY323F double knock-in mice reduces autoimmunity and inflammation in several autoimmune disease models3840. Interestingly, p38α also phosphorylates ZAP70, resulting in a decrease in the size and persistence of the T cell receptor signaling complex, and therefore acts as a feedback regulator of ZAP7041.

Conversely, de-phosphorylation of both threonine and tyrosine residues in the activation loop inactivates MAPKs, and this is mainly carried out by dual-specificity phosphatases of the MAPK phosphatase (MKP)/dual specificity phosphatase (DUSP) family42. Although several MKPs have been reported to dephosphorylate p38α, MKP1/DUSP1, MKP5/DUSP10, MKP8/DUSP26, and DUSP8 are more potent inhibitors of p38α and JNK than ERK43. A recent report showed that DUSP12 is also a p38α phosphatase44. While there are a number of p38α DUSPs, no DUSP for p38γ or p38δ has been reported, and these two p38s are resistant to several known p38α MKPs such as MKP1, 3, 5, and 745. p38α-dependent upregulation of MKP1 was reported and is believed to be part of a negative feedback loop of p38α activation46. Other types of phosphatases have also been reported to target p38 MAPKs, such as CacyBP/SIP47, Wip148, and PP2C49,50. The substrate specificity between p38 and phosphatases and the related physiological functions in vivo still need further investigation. p38γ has also been reported to be degraded by a p38/JNK/ubiquitin-proteasome-dependent pathway, which represents an additional mechanism by which p38 kinases may cross regulate each other51. Yet other ways of regulating p38 are suggested from studies in Caenorhabditis elegans, where a genetic screen for resistance against bacterial infection identified RIOK-1, an atypical serine kinase and human RIO kinase homolog, as a suppressor of the p38 pathway52. As RIOK-1 is a transcriptional target of the p38 pathway in C. elegans, this suggests that RIOK-1 is part of a negative feedback loop. A brief summary of the p38 pathway is shown in Figure 1.

2a3116c1-c38e-4e20-ab4b-f97903684b74_figure1.gif

Figure 1. A diagram of the p38 pathway.

MKP, mitogen-activated protein kinase phosphatase; TAB1, TAK1-binding protein 1; Tyr, tyrosine.

Downstream substrates of p38

Protein kinases

The p38 MAPK cascade does not end at p38. Members of the MAPK-activated protein kinase (MAPKAPK) family such as MK2, MK3, and MK5 (PRAK) are all p38 substrates3,4,5355. The MKs have a broad range of substrates that extend the range of functions regulated by p38 kinases. Mitogen- and stress-activated protein kinase-1/2 (MSK1/2), which are important for CREB activation and chromosome remodeling, have also been identified as substrates of p38α56. MNK1/2, kinases that phosphorylate the eukaryotic initiation factor-4e (eIF-4E), are phosphorylated by p38α57,58. p38α has also been reported to inactivate murine GSK3β by phosphorylating Ser389, and since GSK3β is required for the continuous degradation of β-catenin in the Wnt signaling pathway, this can lead to an accumulation of β-catenin59,60. It was also reported that p38δ negatively regulates insulin secretion by catalyzing an inhibitory phosphorylation of PKD161. A number of p38 protein kinase substrates are summarized in Table 1.

Table 1. Substrates of p38 group members – kinases.

SubstrateKinaseFunctionReferences
MAPKAPK2
(MK2)
p38α, p38β, p38γ,
p38δ
Activates the kinase substrateFreshney NW et al., Cell, 19944
Rouse J et al., Cell, 19943
MAPKAPK3
(MK3)
p38α, p38β, p38γ,
p38δ
Activates the kinase substrateMcLaughlin MM et al., J Biol Chem,
199654
MNK1/2p38αActivates the kinase substrateFukunaga R et al., EMBO J, 199758
Waskiewicz AJ et al., EMBO J,
199757
MSK1/2p38αActivates the kinase substrateDeak M et al., EMBO J, 199856
Pierrat B et al., J Biol Chem, 199877
PAK6p38αActivates the kinase substrateKaur R et al., J Biol Chem, 200578
PIP4Kbp38αInactivates the kinase substrateJones DR et al., Mol Cell, 200679
RPAK
(MK5)
p38α, p38βActivates the kinase substrateNew L et al., EMBO J, 199855
PKCεp38α, p38βCompletes cytokinesisSaurin AT et al., Nat Cell Biol, 200880
GSK3βp38αInactivates the kinase
substrate, activates Wnt
pathway.
Bikkavilli RK et al., J Cell Sci, 200860
Thornton TM et al., Science, 200859

GSK3β, glycogen synthase kinase 3 beta; MAPKAPK, mitogen-activated protein kinase activated protein kinase; MSK1/2, mitogen- and stress-activated protein kinase; PAK6, p21-activated kinase 6; PIP4Kb, phosphatidylinositol 5 phosphate 4-kinase; PKCε, protein kinase C epsilon type.

Transcription factors

p38 targets a large number of transcription factors, including myocyte-specific enhancer factor 2 (MEF2) family members, cyclic AMP-dependent transcription factor 1, 2, and 6 (ATF-1/2/6), CHOP (growth arrest and DNA damage inducible gene 153, or GADD153), p53, C/EBPβ, MITF1, DDIT3, ELK1/4, NFAT, and STAT1/4. p38 phosphorylation of transcription factors predominantly leads to enhanced transcriptional activity. However, in some cases, it represses transcription, and this is summarized in Table 2. Transcription factor phosphorylation by p38 is often stimulus and cell type dependent and plays a role in the cellular response to inflammation, DNA damage, metabolic stress, and many other stresses6276. The effects of p38 on transcription seem to constitute the major part of p38’s responses to stress stimuli.

Table 2. Substrates of p38 group members – transcription factors.

SubstrateKinaseFunctionReferences
ATF2p38α, p38β,
p38γ, p38δ
Enhances transcriptional activityCuenda A et al., EMBO J, 199781
Jiang Y et al., J Biol Chem, 19979
C/EBPαp38αEnhances transcriptional activityQiao L et al., J Biol Chem, 200682
C/EBPβp38αEnhances transcriptional activityEngelman JA et al., J Biol Chem, 199883
C/EBPεp38αEnhances transcriptional activityWilliamson EA et al., Blood, 200584
CHOPp38α, p38βEnhances transcriptional activityWang XZ et al., Science, 199668
E2F4p38αEnhances transcriptional activityMorillo SM et al., Mol Cell Biol, 201285
Elk-1p38αEnhances transcriptional activity
in specific cell types
Janknecht R et al., EMBO J, 199767
Whitmarsh AJ et al., Mol Cell Biol,
199766
ERαp38αEnhances nuclear localization
and transcriptional activity
Lee H et al., Mol Cell Biol, 200286
Fosp38α, p38β,
p38γ, p38δ
Enhances transcriptional activityTanos T et al., J Biol Chem, 200587
FOXO3ap38αEnhances nuclear relocalizationHo KK et al., J Biol Chem, 201288
GRp38αEnhances transcriptional activityMiller AL et al., Mol Endocrinol,
200589
IUF1p38α, p38βEnhances transcriptional activityMacfarlane WM et al., J Biol Chem, 199790
JDP2p38αN/DKatz S et al., Biochem J, 200291
c-JUNp38α, p38β,
p38γ
Enhances transcriptional activityHumar M et al., Int J Biochem Cell
Biol, 200792
MafAp38α, p38β,
p38γ, p38δ
Enhances transcriptional activitySii-Felice K et al., FEBS Lett, 200593
MEF2Ap38α, p38β,
p38δ
Enhances transcriptional activityZhao M et al., Mol Cell Biol, 199994
MEF2Cp38α, p38β
p38γ, p38δ
Enhances transcriptional activityHan J et al., Nature, 199762
MEF2Dp38αEnhances recruitment of Ash2L
to muscle-specific promoters
Zhao M et al., Mol Cell Biol, 199994
Rampalli S et al., Nat Struct Mol Biol,
200773
MITFp38αEnhances transcriptional activityMansky KC et al., J Biol Chem,
200295
MRF4p38αRepresses transcriptional activitySuelves M et al., EMBO J, 200496
NFATc1p38αEnhances transcriptional activity
and interaction with PU.1
Matsumoto M et al., J Biol Chem,
200497
NFATc4p38α, p38β
p38γ
Represses nuclear localization
and transcriptional activity
Yang TT et al., Mol Cell Biol, 200298
NR4Ap38αEnhances transcriptional activitySekine Y et al., J Cell Sci, 201199
Nur77p38αDisrupts interaction with p65 and
represses transcriptional activity
Li L et al., Nat Chem Biol, 2015100
Osterixp38αEnhances recruitment of
coactivators
Ortuño MJ et al., J Biol Chem,
2010101
p53p38αIncreases protein stability and
apoptosis
Bulavin DV et al., EMBO J, 199969
Pax6p38αEnhances transcriptional activityMikkola I et al., J Biol Chem, 1999102
PPARαp38αEnhances transcriptional activityBarger PM et al., J Biol Chem, 2001103
SAP1p38α, p38β
p38γ, p38δ
Enhances transcriptional activityJanknecht R et al., EMBO J, 199767
Smad3p38αEnhances nuclear translocationHayes SA et al., Oncogene, 2003104
Snailp38αIncreases protein stability and
transcriptional activity
Ryu KJ et al., Cancer Res, 2019105
STAT1p38α, p38βEnhances transcriptional activityKovarik P et al., Proc Natl Acad Sci
U S A, 1999106
STAT4p38αEnhances transcriptional activityVisconti R et al., Blood, 2000107
TEAD4p38αEnhances cytoplasmic
translocation and suppresses
transcriptional activity
Lin KC et al., Nat Cell Biol, 201776
Twist1p38αIncreases protein stability and
transcriptional activity
Hong J et al., Cancer Res, 2011108
USF1p38αEnhances transcriptional activityGalibert MD et al., EMBO J, 200171
Xbp1sp38αEnhances nuclear translocation
and transcriptional activity
Lee J et al., Nat Med, 201175

ATF2, activating transcription factor 2; C/EBP, CCAAT/enhancer binding protein; CHOP, CCAAT/enhancer-binding protein homologous protein; ER, estrogen receptor; GR, glucocorticoid receptor; IUF1, insulin upstream factor 1; JDP2, Jun dimerization protein 2; MEF, myocyte-specific enhancer factor; MITF, microphthalmia transcription factor; MRF, muscle regulatory factor; NFAT, nuclear factor of activated T cells; Pax6, paired box 6; PPARα, peroxisome proliferator-activated receptor alpha; TEAD4, TEA domain family transcription factor 4; USF1, upstream transcription factor 1; Xbp1s, spliced form of X-box binding protein 1.

Transcriptional regulators

A large number of transcriptional regulators, including epigenetic enzymes, are substrates of p38, and these are summarized in Table 3. The SWI–SNF complex subunit BAF60 is phosphorylated and inactivated by p38 during skeletal myogenesis109,110, and EZH2, the catalytic component of the Polycomb Repressive Complex 2 (PRC2), was also found to be phosphorylated by p38, particularly in ER-negative breast cancer samples111. Besides its transcriptional function, dATF-2 is also involved in heterochromatin formation, and stress-induced phosphorylation of dATF-2 by p38 disrupts heterochromatin in Drosophila112.

Table 3. Substrates of p38 group members – transcriptional regulators.

SubstrateKinaseFunctionReferences
Chromatin
remodeling
regulators
BAF60cp38α, p38βActivates transcription of MyoD-
target genes
Simone C et al., Nat Genet,
2004109
Forcales SV et al., EMBO J,
2012110
RNF2p38αModulates gene expression and
histone 2B acetylation
Rao PS et al., Proteomics,
2009124
EZH2p38αPromotes cytoplasmic localizationAnwar T et al., Nat Commun,
2018111
dAFF2p38α, p38βDisrupts heterochromatin
formation
Seong K-H et al., Cell, 2011112
Other
regulators
CRTC2p38αEnhances nucleocytoplasmic
transport and represses
transcription activity
Ma H et al., Mol Cell Biol, 2019125
E47p38α, p38βEnhances the formation of MyoD/
E47 heterodimers
Page JL et al., J Biol Chem, .
2004126
Lluís F et al., EMBO J, 2005127
HBP1p38αIncreases protein stability and
represses transcription
Xiu M et al., Mol Cell Biol, 2003128
p18(Hamlet)p38α, p38βIncreases protein stability and
enhances transcription
Cuadrado A et al., EMBO J,
2007129
PGC-1αp38α, p38βIncreases protein stability and
enhances transcription
Puigserver P et al., Mol Cell,
2001130
Rb1p38α, p38γInduces Rb degradation and cell
death; suppresses Rb activity and
promotes the G0-to-G1 transition
Delston RB et al., Oncogene,
2011131
Tomás-Loba A et al., Nature,
201914
SRC-3p38αInduces SRC-3 degradation and
suppresses RARα-dependent
transcription
Giannì M et al., EMBO J, 2006132

CRTC2, CREB-regulated transcription coactivator 2; HBP1, HMG-box transcription factor 1; PGC-1α, peroxisome proliferator-activated receptor gamma co-activator 1 alpha; RAR, retinoic acid receptor; RNF2, ring finger protein 2.

Other substrates

Given the wide range of responses that p38 is involved in, it is not surprising that many p38 substrates cannot be so easily categorized into groups, and these miscellaneous substrates are summarized in Table 4. Some of them are involved in metabolism such as Raptor phosphorylation by p38β, which enhances mTORC1 activity in response to arsenite-stress113, and DEPTOR (mTOR-inhibitory protein) phosphorylation by p38γ and p38δ, leading to its degradation and mTOR hyperactivation114. p38α phosphorylation of Tip60 at Thr158 promotes senescence and DNA-damage-induced apoptosis115,116. Some p38 substrates are cell death regulators. In the ER stress response, p38α locates to the lysosome and phosphorylates the chaperone-mediated autophagy (CMA) receptor LAMP2A, leading to activation of CMA and thus protecting cells from ER stress-induced death117.

Table 4. Substrates of p38 group members – others.

SubstrateKinaseFunctionReferences
Cell-cycle
regulators
Cdc25Ap38αIncreases protein stabilityGoloudina A et al., Cell Cycle, 2003133
Cdc25Bp38αIncreases protein stabilityLemaire M et al., Cell Cycle, 2006134
Cyclin D1p38αCauses ubiquitination and degradation
of cyclin D1
Casanovas O et al., J Biol Chem, 2000135
Cyclin D3p38α, p38β
p38γ, p38δ
Causes ubiquitination and degradation
of cyclin D3
Casanovas O et al., Oncogene, 2004136
p57kip2p38αEnhances interaction with CDKs and
inhibits CDKs
Joaquin M et al., EMBO J, 2012137
Cell-death
regulators
Baxp38αPrevents Bcl-2–Bax heterodimer
formation, enhances apoptosis
Min H et al., Mol Carcinog, 2012138
BimELp38αEnhances apoptosisCai B et al., J Biol Chem, 2006139
Caspase-3p38αInhibits caspase-3 activity and
apoptosis
Alvarado-Kristensson M et al., J Exp Med,
2004140
Caspase-8p38αInhibits caspase-8 activity and
apoptosis
Alvarado-Kristensson M et al., J Exp Med,
2004140
Caspase-9p38αInhibits caspase-9 activity and
apoptosis
Seifert A et al., Cell Signal, 2009141
DNA/RNA
binding proteins
Cdt1p38α, p38βIncreases protein stabilityChandrasekaran S et al., Mol Cell Biol,
2011142
Droshap38αEnhances nuclear export and
degradation
Yang Q et al., Mol Cell, 2015143
FBP2p38αPromotes prothrombin mRNA 3' end
processing
Danckwardt S et al., Mol Cell, 2011144
FBP3p38αPromotes prothrombin mRNA 3' end
processing
Danckwardt S et al., Mol Cell, 2011144
H2AXp38α, p38βPromotes serum starvation-induced
apoptosis
Lu C et al., FEBS Lett, 2008145
H3p38αN/DZhong SP et al., J Biol Chem, 2000146
HuRp38α, p38βEnhances cytoplasmic accumulation
and increases mRNA stability
Lafarga V et al., Mol Cell Biol, 2009147
KSRPp38α, p38βPrevents KSRP-mediated ARE-directed
mRNA decay
Briata P et al., Mol Cell, 2005148
Rps27p38αN/DKnight JD et al., Skelet Muscle, 2012149
SPF45p38αInhibits Fas alternative splicing (exon 6
exclusion)
Al-Ayoubi AM et al., Mol Cell Biol, 2012150
Endocytosis
regulators
EEA1p38αPromotes recruitment to endocytic
membranes and enhances MOR
endocytosis
Macé G et al., EMBO J, 2005151
Rabenosyn-5p38αPromotes recruitment to endocytic
membranes and enhances MOR
endocytosis
Macé G et al., EMBO J, 2005151
GDI-2p38αEnhances GDI:Rab5 complex formation
and modulates endocytosis
Cavalli V et al., Mol Cell, 2001152
MAPK pathway
regulator
JIP4p38αEnhances p38 activityKelkar N et al., Mol Cell Biol, 2005153
Tip60p38αEnhances the pro-senescent function
of Tip60
Zheng H et al., Mol Cell, 2013115
TAB1p38αInhibits TAK1 activityCheung PC et al., EMBO J, 2003154
TAB3p38αInhibits TAK1 activityMendoza H et al., Biochem J, 2008155
FRS2p38αDownregulates FGF1-induced signalingZakrzewska M et al., Int J Mol Sci, 2019156
Membrane
proteins
EGFRp38αInduces EGFR internalizationWinograd-Katz SE et al., Oncogene, 2006157
FGFR1p38αRegulates translocation of exogenous
FGF1 into the cytosol/nucleus
Sørensen V et al., Mol Cell Biol, 2008158
Nav1.6p38αPromotes interaction with NEDD-4 and
protein degradation
Gasser A et al., J Biol Chem, 2010159
NHE1p38αInduces intracellular alkalinizationKhaled AR et al., Mol Cell Biol, 2001160
PLA2p38αN/DBörsch-Haubold AG et al., J Biol Chem, 1998161
TACEp38α, p38βIncreases TACE-mediated ectodomain
shedding and TGF-alpha family ligand
release
Xu P et al., Mol Cell, 2010162
ZAP70p38αPhosphorylation of ZAP70 increases
stability of T cell receptor
Giardino Torchia ML et al., Proc Natl Acad Sci
U S A, 201841
Structure
proteins
Caldesmonp38αN/DHedges JC et al., Am J Physiol, 1998163
Hsp27p38αN/DKnight JD et al., Skelet Muscle, 2012149
Keratin 8p38αRegulates cellular keratin filament
reorganization
Ku NO et al., J Biol Chem, 2002164
Lamin B1p38αEnhances lamin B1 accumulationBarascu A et al., EMBO J, 2012165
Paxillinp38αRequired for NGF-induced neurite
extension of PC-12 cells
Huang C et al., J Cell Biol, 2004166
Stathminp38δN/DParker CG et al., Biochem Biophys Res
Commun, 1998167
SAP97p38γModulating the association of this
protein with other cytoskeleton proteins
Sabio G et al., EMBO J, 2005168
Taup38α, p38γ, p38δEnhances formation of paired helical
filaments
Inhibits amyloid-β toxicity in Alzheimer's
mice
Reynolds CH et al., J Neurochem,1997169
Ittner A et al., Science, 2016170
Tensin1p38αRegulates the binding specificity of
tensin1 to different proteins
Hall EH et al., Mol Cell Proteomics, 2010171
OthersDEPTORp38γ, p38δEnhances degradation and mTOR
hyperactivation
González-Terán B et al., Nat Commun, 2016114
GSp38βRequired for subsequent
phosphorylation to inhibit enzyme activity
Kuma Y et al., Biochem J, 2004172
LAMP2Ap38αActivates chaperone-mediated
autophagy
Li W et al., Nat Commun, 2017117
Parkinp38αDecreases its interaction with PINK1
and suppresses mitophagy
Chen J et al., Cell Death Dis, 2018173
p47phoxp38αPromotes NADPH oxidase activation
and superoxide production
Makni-Maalej K et al., J Immunol, 2012174
p62p38γ, p38δEnhances mTORC1 activityLinares JF et al., Cell Rep, 2015175
Koh A et al., Cell, 2018176
Raptorp38βEnhances mTORC1 activity in response
to arsenite stress
Wu X-N et al., J Biol Chem, 2011113
Rpn2p38αInhibits proteasome activityLee SH et al., J Biol Chem, 2010177
Siah2p38αIncreases Siah2-mediated degradation
of PHD3
Khurana A et al., J Biol Chem, 2006178

CDK, cyclin-dependent kinase; EGFR, epidermal growth factor receptor; FBP1, far upstream binding protein; FGF1, fibroblast growth factor 1; FGFR1, fibroblast growth factor receptor 1; FRS2, fibroblast growth factor receptor substrate 2; GDI, GDP dissociation inhibitor; KSRP, hnRNPK-homology type splicing regulatory protein; MAPK, mitogen-activated protein kinase; mTORC1, mammalian target of rapamycin complex 1; NADPH, nicotinamide adenine dinucleotide phosphate; NGF, nerve growth factor; NHE1, Na+/H+ exchanger isoform 1; PHD3, prolyl hydroxylase 3; PLA2, phospholipase A2; SAP97, synapse-associated protein 97; TAB, transforming growth factor-β-activated protein kinase-1-binding protein; TACE, tumor necrosis factor-alpha-converting enzyme; TAK1, transforming growth factor β-activated kinase 1; TGF, transforming growth factor.

Biological functions of the p38 pathway

Embryo development

p38α is required for embryo development, since the mouse Mapk14–/– embryo dies between embryonic days (E) 10.5 and 12.5118121. Mutant mice with a single Thr180 to Ala mutation or with the double T180A Y182F mutation are also embryonic lethal122,123. Surprisingly, given the importance of the dual phosphorylation for complete p38 activation, substitution of Tyr182 with Phe results in mice that have reduced p38 signaling but are nevertheless viable123, although this is consistent with previous studies showing that the p38 phosphorylated on Thr180 alone retains some activity in vitro37. Histological analysis demonstrates that p38α is required for placental angiogenesis, but not embryonic cardiovascular development, and tetraploid rescue of the placental defect in Mapk14–/– embryos confirmed that p38α is essential for extraembryonic development120,121. Given the important role that p38 and MK2 plays in regulating TNF-induced cell death179182, it is intriguing that the Mapk14–/– embryonic lethal phenotype is very similar to that observed in other mice with defects in the TNF death pathway. Caspase-8, FADD, and cFLIP knock-out mice also die at E10.5, and this is due to TNF-dependent endothelial cell death and disruption of the vasculature in the yolk sac183,184. Other p38 isoforms are not necessary for embryo development, but p38α and p38β have overlapping functions, as Mapk14loxp/loxpMapk11–/–Sox2-Cre embryos die before E16.5 with spina bifida that correlates with neural hyperproliferation and increased apoptosis in the liver, which was not observed in Mapk14∆/∆Sox2-Cre embryos185. Remarkably, p38α appears to have a very specific function during embryogenesis because when p38α was replaced by p38β in the Mapk14 chromosomal locus, which thereby placed p38β under the control of the endogenous p38α promoter, it was unable to rescue the embryonic lethality induced by loss of p38α185.

Immune responses

p38 is activated by many inflammatory stimuli, and its activity is important for inflammatory responses. Macrophage-specific deletion of Mapk14 inhibits inflammatory cytokine production and protects mice from CLP-induced sepsis186. p38α controls the production of inflammatory cytokines, such as TNF and IL-6, at many levels. It directly phosphorylates transcription factors, such as MEF2C62,186, and regulators of mRNA stability, such as hnRNPK-homology (KH) type splicing regulatory protein (KSRP)187. MEF2C appears to play an anti-inflammatory role in endothelial cells in vivo188. Via MK2/MK3, p38 also upregulates cytokine mRNA transcription by the serum response transcription factor (SRF)189, and similarly, via MK2/MK3, p38 regulates mRNA stability by phosphorylating and inactivating TTP/Zfp36, a protein that promotes rapid turnover of AU-rich mRNAs, many of which are cytokine mRNAs187,190. p38 activation also induces the expression of inflammatory mediators such as COX-2, MMP9, iNOS, and VCAM-1, which are involved in tissue remodeling and oxidation regulation191194. The p38 pathway also regulates adaptive immunity. p38α participates in antigen processing in CD8+ cDCs195, and ZAP70-mediated p38α/β activation is important for T cell homeostasis and function18. In B cells, p38α is important for CD40-induced gene expression and proliferation of B cells196, and the p38α–MEF2c axis is believed to be necessary for germinal center B (GCB) cell proliferation and survival197,198. Excessive activation of p38α has been observed in many inflammatory diseases, such as inflammatory bowel disease (IBD), asthma, rheumatoid arthritis, and steatohepatitis199201. The other members of the p38 family also play roles in immune responses. For example, p38γ and p38δ are required for neutrophil migration to damaged liver in non-alcoholic fatty liver disease202 and inhibition of eukaryotic elongation factor 2 in LPS-induced liver damage203. p38δ is required for neutrophil accumulation in acute lung injury204. These observations, and the role that p38s play in TNF production, led to enormous pharmaceutical efforts to develop p38 inhibitors to treat chronic inflammatory diseases. However, unfortunately, these drugs were not efficacious in these diseases205.

Cell cycle

p38 has been implicated in G1 and G2/M phases of the cell cycle in several studies. The addition of activated recombinant p38α caused mitotic arrest in vitro, and an inhibitor of p38α/β suppressed activation of the checkpoint by nocodazole in NIH3T3 cells206. G1 arrest caused by Cdc42 overexpression is also dependent on p38α in NIH3T3 cells207. Besides, p38γ is specially required for gamma-irradiation-induced G2 arrest208. The link between p38 and cell cycle control has been proposed through the regulation of several p38 substrates. Both p38α and p38γ regulate cell cycle progression via Rb but in opposite directions14,209. HBP1 represses the expression of cell cycle regulatory genes during cell cycle arrest in a p38-dependent manner210; p53 and p21 activation by p38α prevented G1 progression through blockade of CDK activity211,212. The p38 pathway is also involved in cell cycle progress, as it is essential for self-renewal of mouse male germline stem cells213 and its regulation of G1-length plays a role in cell size uniformity214.

Cell differentiation

Participation of p38 in cell differentiation has been reported in certain cell types. p38α activity is essential for neuronal differentiation in PC-12 cells and EPO-induced differentiation in SKT6 cells20,215. Treatment of 3T3-L1 fibroblasts with specific p38α/β inhibitors prevents their differentiation into adipocytes by reducing C/EBPβ phosphorylation83, and p38α-dependent phosphorylation of MEF2C and BAF60 is critical for myogenic differentiation110,216. Intestinal epithelial cell-specific deletion of p38α also influences goblet cell differentiation in a Notch-dependent manner200.

Cell metabolism

p38 group members participate in many cellular events related to metabolism. The p38β–PRAK axis specifically phosphorylates Rheb and suppresses mTORC1 activity under energy depletion conditions22. DEPTOR, an inhibitor of mTORC, can be phosphorylated by p38γ and p38δ, leading to its degradation123. Meanwhile, p38δ directly phosphorylated p62 to enhance mTORC1 activity in response to amino acids175. In brown adipocytes, p38α functions as a central mediator in β-adrenergic-induced UCP1 expression217,218, while in white adipocytes, p38α inactivation leads to elevated white-to-beige adipocyte reprogramming and resistance to diet-induced obesity219,220. In hepatocytes, p38α controls lipolysis and protects against nutritional steatohepatitis. Thus, mice with hepatocyte-specific loss of p38α developed more severe steatohepatitis than wild type mice when fed high-fat or -cholesterol diets. Intriguingly, macrophage specific deletion of p38 had the opposite effect in the same high-fat diets and resulted in less steatohepatitis than in wild type mice, which probably reflects the inflammatory role of p38 in macrophages199. p38α also directly phosphorylates Xbp1s to enhance its nuclear migration for maintaining glucose homeostasis in obesity75. However, p38α also functions as a negative regulator of AMPK signaling in maintaining gluconeogenesis, and hepatic p38α could be a drug target for hyperglycemia221. It was also reported that p38γ directly phosphorylated p62 under imidazole propionate stimulation to promote mTORC1 activity in hepatocytes176. Interestingly, AMPK also triggers the recruitment of p38α to scaffold protein TAB1 for p38α autoactivation in human T cells222.

Cell senescence

p38α appears to play a pivotal role in senescence. Constitutive activation of the p38 pathway by active MKK3 or MKK6 induces senescence in several cell types223,224, and p38α activity is responsible for senescence induced by multiple stimuli, such as telomere shortening225,226, H2O2 exposure227,228, and chronic oncogene activation19,223,229. p38α/β-specific inhibitors have been successfully used to prevent cellular senescence in cultivated human corneal endothelial cells230. Since cellular senescence is considered a defense strategy against oncogene activation, the p38 pathway plays important roles in tumorigenesis231. Meanwhile, p38α activity is important for senescence-associated secretory phenotype (SASP), and its inhibition markedly reduces the secretion of most SASP factors, suggesting multiple roles for the p38 pathway in senescence232235.

Cell survival and death

The role of the p38 pathway in cell fate is cell type and stimulus dependent. For example, p38α becomes activated upon NGF withdrawal in PC-12 cells, and p38α activated by overexpression of MKK3 induced apoptosis in NGF differentiated PC-12 cells211. Similarly, inhibition of p38 with PD169316 blocked NGF withdrawal-induced apoptosis in PC-12 cells236,237. The interplay between the p38 pathway and caspases, the central regulators/executors of apoptosis, is complicated because p38α activity can be elevated in a caspase-dependent manner in death stimulus treated cells238,239, and caspase activity can also be elevated in MKK6E (dominant active form) overexpressed cells239,240. In contrast, inhibition of caspase-8 and caspase-3 by p38α-mediated phosphorylation in neutrophils was also reported140. Recent studies show that p38-activated MK2 directly phosphorylates RIPK1 in TNF-treated cells or pathogen-infected cells, limiting TNF-induced cell death180182. This represents an interesting link between cytokine production induced by TNF and cell death because TNF-induced MK2/MK3 phosphorylation of tristetraprolin/Zfp36 inactivates it and leads to increased stability of cytokine mRNAs190. Aberrant p38α activity is observed in many tumor cells, and inhibition of p38α/β enhances cell death in these cells241,242.

Perspectives

p38 is one of the most researched of all proteins, let alone kinases, and a search in PubMed for p38 MAPK or p38 kinase returns more than 36,000 publications, which is a higher number than some proteins listed in a review of the "top 10" most studied genes243. By contrast, searches for the kinases Raf and Src return about 17,000 and 25,000 hits, respectively. In 2018, there were more than 2,000 publications that mention p38, and it is clearly impractical to summarize such a vast amount of literature. As might be surmised from the preceding commentary, the studies are on a wide range of topics; however, the publications are more concentrated in some areas than others. The role of the p38 pathway in cancers (>10,000)244246, inflammation (>8,000)247249, and infections (>3,600)250,251 was intensively studied. About 1,600 publications include the specific term "p38 inhibitor". This reflects the previously mentioned enormous interest of the pharmaceutical industry in developing p38 inhibitors to treat chronic inflammatory diseases, such as rheumatoid arthritis. Yet other publications report natural products that can activate or inhibit p38, with the ultimate aim of using them clinically252258. In 2011, the European Commission approved Esbriet (pirfenidone), which was described as a p38γ inhibitor, for the treatment of idiopathic pulmonary fibrosis259. However, when this drug was approved by the FDA in 2014 for treating the same disease, it was described as a compound that acts on multiple pathways. In 2008, there were 27 clinical trials listed testing the use of p38 inhibitors in inflammatory disease settings205, while a search today for p38 inhibitors in clinicaltrials.gov returns 44 studies for conditions as diverse as pain, asthma, cognitive impairment, rheumatoid arthritis, cancer, myelodysplastic syndrome, and depression (Table 5). This indicates that there remains clinical interest in targeting the pathway and that there is therefore a need for more specific inhibitors of each of the p38 group members and more basic research to fully understand how the pathway, especially how each member of the p38 family, is utilized and regulated.

Table 5. Clinical trials of p38 inhibitors.

DrugTargetCondition or diseaseStatusNCT number
ARRY-371797p38Ankylosing spondylitisPhase 2NCT00811499
ARRY-371797p38Dental painPhase 2NCT00542035
NCT00663767
ARRY-371797p38HealthyPhase 1NCT00790049
ARRY-371797p38LMNA-related dilated cardiomyopathyPhase 2NCT02351856
NCT02057341
ARRY-371797p38Osteoarthritis of the kneePhase 2NCT01366014
ARRY-371798p38Rheumatoid arthritisPhase 1NCT00729209
ARRY-614p38 and
Tie2
Myelodysplastic syndromesPhase 1NCT01496495
NCT00916227
AZD7624p38Corticosteroid-resistant asthmaPhase 2NCT02753764
BIRB 796 BS p38HealthyPhase 1NCT02211170
BMS-582949p38αRheumatoid arthritisPhase 2NCT00605735
BMS-582949p38αVascular diseases (atherosclerosis)Phase 2NCT00570752
CHF6297p38αChronic obstructive pulmonary
disease
Phase 1/2NCT02815488
Losmapimod
(GS856553)
p38α/βAcute coronary syndromePhase
1/2/3
NCT01756495
NCT02145468
NCT00910962
Losmapimod
(GS856553)
p38α/βChronic obstructive pulmonary
disease
Phase 2NCT00642148
NCT01541852
Losmapimod
(GS856553)
p38α/βDepressive disorder, majorPhase 2NCT00976560
NCT00569062
Losmapimod
(GS856553)
p38α/βGlomerulosclerosis, focal segmentalPhase 2NCT02000440
Losmapimod
(GS856553)
p38α/βPain, neuropathicPhase 2NCT01110057
NCT00969059
LY3007113p38Metastatic cancerPhase 1NCT01463631
Neflamapimod
(VX-745)
p38αAlzheimer’s diseasePhase 2NCT03402659
NCT02423200
NCT02423122
Neflamapimod
(VX-745)
p38αDementia with Lewy bodiesRecruitingNCT04001517
P38 inhibitor
(4)
p38Rheumatoid arthritisPhase 2NCT00303563
NCT00316771
PF-03715455p38αAsthmaPhase 2NCT02219048
PF-03715455p38αChronic obstructive pulmonary
disease
Phase 2NCT02366637
PF-03715455p38αHealthyPhase 1NCT01226693
PH-797804p38α/βRheumatoid arthritis Phase 2NCT00383188
NCT00620685
Ralimetinib
(LY2228820)
p38α/βAdult glioblastomaPhase 1/2NCT02364206
Ralimetinib
(LY2228820)
p38α/βAdvanced cancerPhase 1NCT01393990
Ralimetinib
(LY2228820)
p38α/βEpithelial ovarian cancer Fallopian
tube cancer Primary peritoneal cancer
Phase 1/2NCT01663857
Ralimetinib
(LY2228820)
p38α/βPostmenopausal metastatic breast
cancer
Phase 2NCT02322853
SB-681323p38Acute lung injuryPhase 2NCT00996840
SB-681323p38Coronary heart diseasePhase 2NCT00291902
SB-681323p38Chronic obstructive pulmonary
disease
Phase 1/2NCT00564746
NCT00144859
SB-681323p38Pain, neuropathicPhase 2NCT00390845
SB-681323p38Rheumatoid arthritis InflammationPhase 1/2NCT00419809
NCT00439881
NCT00134693
Talmapimod
(SCIO-469)
p38αBone marrow diseases
Myelodysplastic syndromes
Hematologic diseases Bone marrow
neoplasms
Phase 2NCT00113893
Talmapimod
(SCIO-469)
p38αMultiple myelomaPhase 2NCT00095680
NCT00087867
Talmapimod
(SCIO-469)
p38αRheumatoid arthritisPhase 2NCT00043732
NCT00089921
VX-702p38αRheumatoid arthritisPhase 2NCT00395577
NCT00205478

One consequence of the massive pharmaceutical effort over the last 20 years is a large number of very specific, well-tolerated, and readily bioavailable drugs that can enable such basic research. For example, one study using a boutique panel of kinase inhibitors was able to demonstrate that 11 potent and specific p38 inhibitors synergized with Smac-mimetic drugs to kill a subset of AML leukemias, providing the strongest evidence implicating p38 in Smac-mimetic-induced killing179. Since several of these p38 inhibitors had already been clinically trialed, this presents an opportunity to fast-track such combinations into the clinic. In our opinion, it is likely that this is where the future of p38 research and p38 inhibitors lies, in revealing the intricate web of inter-connections and inter-dependencies of this core and central regulator of cell stress. We also believe that greater efforts to genetically assess the role of p38 and p38 isoforms in the pathophysiology of inflammatory and other diseases need to be made in order to push forward the clinical application of our burgeoning knowledge.

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Han J, Wu J and Silke J. An overview of mammalian p38 mitogen-activated protein kinases, central regulators of cell stress and receptor signaling [version 1; peer review: 2 approved]. F1000Research 2020, 9(F1000 Faculty Rev):653 (https://doi.org/10.12688/f1000research.22092.1)
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Jonathan D. Ashwell, Laboratory of Immune Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD, 20892, USA 
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