Roles of p38α mitogen-activated protein kinase in mouse models of inflammatory diseases and cancer.

The p38α mitogen-activated protein kinase pathway not only regulates the production of inflammatory mediators, but also controls processes related to tissue homeostasis, such as cell proliferation, differentiation and survival, which are often disrupted during malignant transformation. The versatility of this signaling pathway allows for the regulation of many specific functions depending on the cell type and context. Here, we discuss mouse models that have been used to identify in vivo functions of p38α signaling in the pathogenesis of inflammatory diseases and cancer. Experiments using genetically modified mice and pharmacological inhibitors support that targeting the p38α pathway could be therapeutically useful for some inflammatory diseases and tumor types.

azoxymethane

activator protein‐1

mice with a point mutation at the Apc gene

apolipoprotein E

apoptosis signal‐regulating kinase

constitutively‐active form

collagen antibody‐induced arthritis

collagen‐induced arthritis

cecal ligation and puncture

chronic obstructive pulmonary disease

cyclooxygenase‐2

colorectal cancer

tamoxifen‐inducible Cre

cigarette smoke solution

Cre‐versible allele

Chemokine (C‐X‐C motif) ligand

dendritic cell

N‐diethylnitrosamine

7,12‐dimethylbenz[a]anthracene

dominant‐negative form

dextran sodium sulfate

experimental autoimmune encephalomyelitis

epidermal growth factor receptor

extracellular signal‐regulated kinase

flox/flox conditional allele

hepatocellular carcinoma

hepatocyte nuclear factor 3β

hepatocyte growth factor

inflammatory bowel disease

intercellular adhesion molecule

intestinal epithelial cell

interferon

interleukin

c‐Jun N‐terminal kinase

mice expressing both the T cell receptor transgene KRN and the major histocompatibility complex class II molecule A(g7)

knockout

low‐density lipoprotein receptor

lipopolysaccharide

mitogen‐activated protein kinase

MAPK kinase kinase

monocyte chemoattractant protein

macrophage inflammatory protein 1a

matrix metalloproteinase

myeloperoxidase

multiple sclerosis

mitogen‐ and stress‐activated protein kinase

nuclear factor kappa B

phenobarbital

phorbol 12‐myristate 13‐acetate

phosphatase and tensin homolog

polyoma middle T‐antigen

rheumatoid arthritis

reactive oxygen species

squamous cell carcinomas

signal transducer and activator of transcription

tumor necrosis factor

12‐O‐tetradecanoylphorbol‐13‐acetate

tobacco smoke

vascular cell adhesion molecule

wild‐type

Introduction

Mitogen‐activated protein kinases (MAPKs) are evolutionarily conserved kinases that control many cellular processes. Eukaryotic cells contain several MAPK pathways that function in parallel and are activated by different extracellular stimuli. The p38 MAPK family includes four members: p38α, p38β, p38δ and p38γ, which are approximately 60% identical in their amino acid sequences, are encoded by different genes, and have different tissue expression patterns. Most cell types express substantial levels of p38α, whereas the other p38 MAPKs have more tissue‐specific expression patterns. p38α was originally identified as a protein kinase implicated in stress and inflammatory responses 1, 2, 3, 4. Activation of p38α is usually triggered by the MAPK kinases MKK3 and MKK6, although sometimes by MKK4 or by autophosphorylation independently of MAPK kinases. More than 100 proteins can be directly phosphorylated by p38α, including other protein kinases and many transcription factors 5, 6. The number and variety of p38α substrates identified is consistent with the ability of this signaling pathway to regulate numerous cellular processes. Indeed, the use of pyridinyl imidazole inhibitors such as SB203580 and SB202190, which inhibit p38α and p38β, has allowed the identification of many functions potentially regulated by p38 MAPKs beyond the stress response 7, 8. In vivo physiological roles for p38 MAPK signaling have been determined by the generation of genetically modified mice. Of note, p38α knockout (KO) mice are embryonic lethal as a result of a defect in placenta morphogenesis 9, 10, whereas the KOs for other p38 MAPKs are viable 11, 12. The double KO for MKK3 and MKK6 is also embryonic lethal and shows a similar phenotype to the p38α KO 13. Moreover, there is evidence that p38α and p38β play overlapping in vivo functions during mouse development 14. Genetically modified mice have provided evidence showing that p38α signaling plays an important role controlling inflammation, as well as the proliferation, differentiation and survival of different cell types 15, 16. Here, we discuss the roles of p38α in mouse models of inflammatory diseases and cancer.

p38α MAPK in inflammatory diseases

There is in vivo and in vitro evidence linking p38α signaling to the production of inflammatory mediators and pro‐inflammatory cytokines in several cell types via transcriptional and post‐transcriptional mechanisms 7, 16.

Mice deficient for the p38α substrate MK2 provided the first in vivo evidence for the implication of this pathway in inflammation. The MK2 KO mice are more resistant to lipopolysaccharide (LPS)‐induced endotoxic shock as a result of the reduced production of tumor necrosis factor‐α (TNF‐α) 17. Additional studies show that the MK2‐related kinase MK3 contributes to regulating LPS‐induced TNF‐α production in vivo, although to a lesser extent than MK2 18. The use of mice deficient for p38α either in myeloid cells or in epithelial cells has further supported the implication of this pathway in cytokine production and inflammatory responses in vivo 19, 20, 21. The connection of p38α with the production of inflammatory mediators has prompted the use of mouse models to investigate the in vivo functions of this pathway in the pathogenesis of inflammatory diseases (Fig. 1 and Table 1). It should be noted that p38β appears to be required neither for the acute, nor chronic inflammatory responses 11, 22, whereas myeloid cells deficient in p38γ and p38δ are impaired in the LPS‐induced production of several cytokines, which correlates with reduced levels of the MAPK kinase kinase (MAP3K) TPL‐2 and extracellular signal‐regulated kinase (ERK)1/2 signaling 23. Interestingly, p38α activation does not appear to be affected by p38γ and p38δ downregulation 23, suggesting that they regulate the inflammatory response by distinct mechanisms.

Figure 1

Implication of p38α MAPK in mouse models of inflammatory diseases. For details, see Table 1.

Table 1

p38α MAPK signaling in mouse models of inflammatory diseases and cancer. Specificity of mouse lines: Alb, hepatocytes; CD4, T cells; CD11c, dendritic cells; K14, ectoderm and derivatives; Lck, T cells and thymocytes; LysM, myeloid cells; MMTV, breast epithelial cells; More, embryos; Mx‐Cre, liver and lymphocytes; RERTn, ubiquitously expressed; Rosa26, ubiquitously expressed; SP‐C, type II alveolar epithelial cells; Tie, endothelial cells; Villin, intestinal epithelial cells.

Mouse lines	Models	Phenotypes	Molecules/processes involved	References
p38α T106M (knock‐in)	CAIA; p38 MAPK inhibitor	No effect	Not applicable	22	Arthritis
MK2−/−	CIA	Reduced arthritis severity and incidence	Reduced TNF‐α, IL‐6	28
MKK3−/−	K/BxN passive arthritis	Reduced arthritis severity	Reduced P‐p38, IL‐1β, CXCL‐1, IL‐6, MMP3	29
MKK6−/−	K/BxN passive arthritis	Reduced arthritis, cartilage destruction and bone erosion	Reduced P‐p38, P‐MK2, P‐MSK1, IL‐6, MMP3	30
ASK1−/−	K/BxN passive arthritis	Reduced arthritis, cartilage destruction and bone erosion	Reduced IL‐1β, IL‐6, CXCL‐1, TNF‐α, CCL2	31
WT	K/BxN passive arthritis; p38 MAPK inhibitor
p38α (F/F) LysM‐Cre	K/BxN passive arthritis	Enhanced arthritis severity	Enhanced IL‐6, IL‐1β, P‐Stat3	32
	Antigen‐induced arthritis
WT	EAE; p38 MAPK inhibitors	Reduced EAE severity	Reduced IL‐17	39, 40	EAE
ASK1−/−	EAE	Reduced EAE severity	Reduced MCP‐1, RANTES, MIP‐1α	41
p38α+/−	EAE	Reduced EAE severity	Reduced IL‐17	39
Lck‐p38α DN (transgenic)	EAE	Reduced EAE severity	Reduced IL‐17 and P‐p38 in T cells	40
MKK3−/− MKK6+/−
Lck‐MKK6 CA (transgenic)		Enhanced EAE susceptibility	Enhanced IL‐17
p38α/p38β Y323F (knock‐in)	EAE and CIA	Reduced EAE and CIA severity	Reduced IFN‐γ, TNF‐α and T‐bet expression but enhanced IL‐10	44
p38α (F/F) Rosa26‐Cre‐ERT2	EAE	Reduced EAE severity	Not determined	42
p38α (F/F) CD4‐Cre		No effect	Not applicable
p38α (F/F) LysM‐Cre		No effect	Not applicable
p38α (F/F) CD11c‐Cre		Reduced EAE	Reduced IL‐6 and Th17 differentiation
MK2−/−	EAE	Delayed EAE onset and prolonged activity	Reduced TNF‐α, FasR, enhanced leukocyte infiltration and reduced apoptosis	43
MK2−/−	Ldlr−/−	Reduced severity to Atherosclerosis	Reduced VCAM‐1, ICAM‐1, MCP‐1	45	Atherosclerosis
ApoE−/−	Virus‐induced acceleration in ApoE−/− model; p38 MAPK inhibitor	Reduced viral load and pro‐atherogenic molecules	Reduced E‐selectin, VCAM‐1, ;ICAM‐1, MCP‐1	46
p38α (F/F) LysM‐Cre	ApoE−/−	No effect on disease initiation	Not applicable	47
		enhanced apoptosis and Advanced plaque progression	Reduced AKT activity
p38α (F/F) LysM‐Cre	ApoE−/−	No effect	Not applicable	49
p38α (F/F) Tie‐Cre‐ERT2
WT	TS‐induced COPD; p38 MAPK inhibitor	Reduced lung inflammation	Reduced COX‐2, IL‐6	53	COPD and asthma
SP‐C‐MKK6 CA (transgenic)	CSS/LPS‐ induced COPD	Enhanced disease severity	Increased IL‐16, CXCL‐1, MMP‐12, TCA‐3, Leptin	54
WT	LPS‐induced lung inflammation; p38 MAPK inhibitor	Reduced lung inflammation	Reduced TNF‐α, IL‐1β and neutrophil accumulation	55
WT	Ova‐induced asthma; p38α antisense oligonucleotide	Reduced disease symptoms	Reduced IL‐4, IL‐5, IL‐13 and eosinophil recruitment	56
WT	Ova/ozone‐induced asthma; p38 MAPK inhibitor and dexamethasone	Reduced disease symptoms	Reduced TNF‐α, IL‐13, CXCL‐1, GM‐CSF and MKP‐1	57
p38α(F/F) LysM‐Cre	SDS‐ and UVB‐induced skin injury	Reduced inflammatory response	Enhanced P‐JNK, P‐ERK and Reduced CXCL‐1, CXCL‐2, IL‐10	20	Sepsis and skin inflammation
p38α (F/F) K14‐Cre
MSK1−/− MSK2−/−	LPS‐induced sepsis	Reduced resistance to sepsis	Enhanced TNF‐α, IL‐6, IL‐12, Reduced IL‐10	33
	CLP‐induced sepsis	Increased resistance to sepsis	Reduced IL‐10 but no differences in IL‐6, IL‐12, TNF‐α
	PMA‐induced eczema	Increased inflammation	Enhanced MPO activity and infiltration
p38α (F/F) LysM‐Cre	LPS‐ and CLP‐induced sepsis	Increased resistance to sepsis	Reduced TNF‐α, AP‐1, C/EBP‐β and CREB activity	19
MK2−/−	LPS‐ induced sepsis	Increased resistance to sepsis	Reduced TNF‐α, IFN‐γ, IL‐6, NO	17
MK2−/− MK3−/−	LPS‐ induced sepsis	Not determined	Further reduced TNF‐α and TTP compared to MK2−/−	18
p38α (F/F) Villin‐Cre	DSS‐induced colitis	Increased colitis	Enhanced apoptosis, increased Bak, IL‐6, COX‐2 and JNK activation	21, 91, 92	Colitis
p38α (F/F) LysM‐Cre		Reduced colitis	Reduced AP‐1, NFkB activity, IL‐6, COX‐2	21
p38α (F/F) Alb‐Cre	LPS/TNF‐induced liver damage	No effect	Enhanced JNK activation	86	Liver damage
p38α (F/F) IKK2 (F/F) Alb‐Cre		Enhanced liver toxicity	Enhanced hepatocyte apoptosis, Reduced c‐FLIP(L) levels
Wip1−/−	MMTV‐ErbB2 and MMTV‐Hras	Reduced breast tumorigenesis	Enhanced P‐p38, reduced proliferation and increased apoptosis	68	Breast cancer
MMTV‐Wip1 (transgenic)	MMTV‐ErbB2	Enhanced breast tumorigenesis	Increased proliferation	69
MMTV‐MKK6 (transgenic)		No effect	Increased Wip1
MMTV‐Wip1 MMTV‐MKK6 (transgenic)		Reduced tumorigenesis compared to MMTV‐Wip1	Reduced proliferation
GADD45α−/−	MMTV‐Ras	Enhanced breast Tumorigenesis	Reduced P‐p38 and Ras‐induced senescence	70
WT	MMTV‐PyMT; p38 MAPK inhibitor and cisplatin treatment	Reduced tumor growth and malignancy	Enhanced apoptosis and JNK activity	71
p38α (F/F) RERTn‐Cre‐ERT2	Kras LSL‐G12V	Enhanced lung tumorigenesis	Reduced C/EBPα, HNF3β,Increased AKT/EGFR signaling	67	Lung cancer
MK2 (CV/CV) p53 (F/F)	Kras LSL‐G12D + Adeno‐Cre (intratracheal)	No effect on tumor initiation	Not applicable	80
		MK2/p53 double KO tumors grow faster (tumor progression)	Increased proliferation; increased apoptosis in response to cisplatin
p38α (F/F) Alb‐Cre	DEN/Pb	Increased liver cancer	Enhanced proliferation and JNK‐c‐Jun signaling	66	Liver cancer
p38α (F/F) Mx‐Cre
p38α (F/F) Alb‐Cre	DEN	Increased liver cancer	Increased ROS, hepatocype death, IL‐α secretion and hepatocyte Compensatory proliferation	84
p38α (F/F) Mx‐Cre		No effect	Reduced IL‐6, IL‐1β, HGF
p38α (F/F) Alb‐Cre	Thioacetamide	Increased liver cancer	Enhanced SOX‐2, c‐Jun	85
p38α (F/F) Villin‐Cre	AOM/DSS	Increased colon cancer	Altered colon homeostasis and barrier function	92, 94	Colon cancer
p38α (F/F) Villin‐Cre‐ERT2
p38α (F/F) Villin‐Cre‐ERT2	p38α deletion in AOM/DSS‐induced colon tumors	Reduced colon cancer	Reduced proliferation, P‐Stat3, IL‐6, Mcl‐1, increased apoptosis and P‐JNK	92
ASK1−/−	AOM/DSS	Increased colon cancer	Increased coltitis, macrophage apoptosis and enhanced TNF‐α, IL‐6, COX2, IL‐1β	95
APCmin	AOM/APCmin; p38 MAPK inhibitor	Reduced colon cancer	Reduced proliferation, enhanced p21, PTEN, nuclear FoxO3A	96
WT	AOM/DSS; p38 MAPK inhibitor and MEK1 inhibitor	Reduced colon cancer	enhanced apoptosis, reduced proliferation	97
PRAK (MK5) −/−	DMBA	Increased skin cancer	Impaired Ras‐induced senescence, enhanced Ki67, reduced DcR2, p16	107	Skin cancer
PRAK (MK5) −/−	DMBA/TPA	Reduced skin cancer progression	Impaired angiogenesis, enhanced apoptosis	108
ASK1−/−	DMBA/TPA	Dual function; ASK1 alone‐ tumor promoting role. Reduced inflammation	Reduced P‐p38, P‐JNK, TNF‐α, IL‐6	110
ASK2−/−		ASK2 in cooperation with ASK1‐ tumor suppressive role	Reduced P‐p38, P‐JNK, reduced apoptosis
GADD45α−/−	UV	Increased skin cancer	Reduced apoptosis, P‐p38, P‐JNK, p53	111
p53−/− SKH‐1	UV; p38 MAPK inhibitor	Increased skin cancer	Increased P‐c‐Jun, cyclin D1, NOX‐2	112
MK2−/−	DMBA/TPA	Reduced skin cancer	Increased apoptosis and p53, reduced IL‐1β, IL‐6, TNF‐α	113
K14‐p38α DN (transgenic)	UVB	Reduced skin cancer	Reduced AP‐1 activity, reduced COX‐2	115
K14‐p38α DN (transgenic)	Solar UV	Reduced skin cancer	Reduced edema, inflammation and proliferation	116
MSK1−/− MSK2−/−	DMBA/TPA	Reduced skin cancer	Enhanced IL‐1β, TNF‐α, increased MPO activity	114

Rheumatoid arthritis (RA)

RA is an autoimmune and chronic inflammatory disorder that affects the joints of hands and feet. Both p38 MAPK and the activators MKK3 and MKK6 are phosphorylated in their activation residues in synovial tissue from RA patients 24, 25, suggesting the implication of p38 MAPK signaling in RA. In a collagen‐induced model of experimental arthritis (CIA), the p38 MAPK inhibitor SD‐282 attenuates disease progression and reverses cartilage and bone destruction 26, 27. There is evidence that only p38α but not p38β is involved in collagen‐antibody or TNF‐α‐driven arthritis 22. Deficiency of MK2 protects against CIA by reducing the serum levels of interleukin (IL)‐6 and TNF‐α 28, suggesting that MK2 plays a critical role downstream of p38α signaling in arthritis. Disruption of the p38 MAPK activators MKK3 and MKK6 or the MAP3K apoptosis signal‐regulating kinase 1 (ASK1) also protects against experimental arthritis 29, 30, 31. These studies suggest that inhibition of p38 MAPK signaling may have therapeutic potential in arthritis patients. However, a recent study shows that p38α downregulation in myeloid cells exacerbates the severity of arthritis symptoms 32. This p38α effect could be mediated through the substrates mitogen‐ and stress‐activated protein kinase (MSK)‐1 and ‐2, which control transcriptional activation of the anti‐inflammatory cytokine IL‐10 33. Collectively, p38α signaling appears to have both pro‐ and anti‐inflammatory roles, which could explain the modest effect of p38 MAPK inhibitors in RA patients 34, 35. Targeting of the activators MKK3 and MKK6 or the substrate MK2 has been proposed as alternative therapeutic strategy in RA aiming to avoid the anti‐inflammatory effects of p38α 32, 36. In support of this idea, treatment of rats with the MK2 inhibitor PF‐3644022 reduces both LPS‐induced TNF‐α production and chronic inflammation in the streptococcal cell wall‐induced arthritis model 37.

Multiple sclerosis (MS)

MS is an inflammatory disease of the central nervous system that affects young adults. MS can be modeled in mice by immunization with myelin antigen combined with adjuvant, termed experimental autoimmune (or allergic) encephalomyelitis (EAE). EAE development requires elevated cytokine expression levels, which are also detected in MS patients. Interestingly, p38α is upregulated in MS lesions and the levels of phosphorylated p38 MAPK are enhanced in EAE rat models 38, suggesting the implication of p38 MAPK in EAE. In agreement with this idea, p38 MAPK inhibitors markedly suppress EAE in mouse models, correlating with decreased IL‐17 levels 39, 40. Furthermore, downregulation of p38α or its activator ASK1 ameliorates the severity of EAE 39, 41, 42. By contrast to the above results, mice deficient in the p38α substrate MK2 show a delayed onset of EAE but prolonged disease activity, which is probably the result of a lack of TNF‐α and an altered immune response in the central nervous system 43. These results suggest a predominant role for p38α substrates other than MK2 in the regulation of EAE development.

Recent studies show that p38α autophosphorylation is required for the production of IL‐17 by T cells and impairment of this alternative activation pathway reduces EAE severity in mice 44. Expression in T cells of a nonphosphorylatable p38α mutant, which is considered to work in a dominant‐negative manner, or the deletion of the activators MKK3 and MKK6, greatly reduces phosphorylation of p38 MAPK, production of IL‐17 and EAE symptoms in mice 40. Conversely, forced activation of p38 MAPK in mouse T cells by expression of constitutively active MKK6 results in enhanced IL‐17 production and an increased susceptibility to EAE 40. However, specific deletion of p38α in T cells does not affect EAE development 42. This might be explained by insufficient downregulation of p38α in T cells or by compensation via other p38 MAPK family members because p38β has a partial redundant role in T cell function 44. Deletion of p38α in macrophages does not affect EAE development, whereas deletion of p38α in CD11c+ dendritic cells (DCs) reduces EAE symptoms, as well as the expression of IL‐6 and differentiation of T cells producing IL‐17 (TH17) 42. These results suggest that p38α‐dependent expression of IL‐6 in DCs is required for TH17 differentiation and EAE development 42. Altogether, p38α activation in DCs and T cells appears to be important for the pathogenesis of EAE, suggesting that targeting this pathway might have therapeutic value in MS.

Atherosclerosis

Atherosclerosis is a chronic inflammatory cardiovascular disease and a leading cause of both mortality and morbidity worldwide. The atherogenic process has been widely studied using mice deficient either for apolipoprotein E (ApoE), which develop spontaneous atherosclerosis, or for low‐density lipoprotein receptor (Ldlr), which need a cholesterol diet to develop hypercholesterolemia and atherosclerosis.

Mice deficient for the p38α substrate MK2, which are impaired in pro‐inflammatory cytokine production 17, are resistant to atherosclerosis by reducing vascular lipid deposition and macrophages in hypercholesterolemic Ldlr−/− mice. MK2 also regulates aortic expression of the vascular cell adhesion molecule (VCAM)‐1 and the chemokine monocyte chemoattractant protein (MCP)‐1, which are key for the recruitment of monocytes/macrophages to the vascular wall 45. p38 MAPK has also been suggested to regulate the pro‐atherogenic molecules VCAM‐1 and MCP‐1 in ApoE−/− mice 46. These studies support the implication of p38 MAPK signaling in the development of atherosclerosis, although the cell type responsible remains unclear. Phosphorylated MK2 is detected in the endothelium and macrophage‐rich plaque areas within aortas of hypercholesterolemic Ldlr−/− mice, suggesting that atherosclerosis development might involve p38α activation in these cells. Surprisingly, p38α downregulation in macrophages does not affect the formation of atherosclerotic plaques or macrophage recruitment in ApoE−/− mice but, instead, leads to macrophage apoptosis and other markers of advanced plaque progression, which were not checked in the MK2 KO mice, by suppressing AKT activation 47. It was subsequently shown that the inhibition of p38α or both MK2 and MK3 impairs the LPS‐induced activation of AKT in bone‐marrow derived macrophages, although KO of either MK2 or MK3 alone has little effect on AKT phosphorylation 48. It is therefore possible that p38α plays a pro‐survival role in the macrophages of advanced atherosclerotic plaques, whereas a deficiency of MK2 alone does not affect the AKT survival pathway. Thus, the phenotype of MK2 KO mice might be a result of the role of MK2 in other cells, such as endothelial cells. By contrast to this possibility, a recent study reports that endothelial or macrophage specific‐downregulation of p38α affects neither the development, nor the characteristics of atherosclerotic plaques in ApoE−/− mice 49. The controversial findings could be explained by the different genetic backgrounds of the mice used in the studies, which can influence the extent of atherosclerosis in ApoE−/− mice 50. More studies are warranted to define the role of p38α signaling in atherosclerosis, especially regarding the analysis of other cell types that could be involved, such as smooth muscle cells.

Chronic obstructive pulmonary disease (COPD)

The p38 MAPK pathway has been linked to lung inflammatory diseases such as COPD and asthma. Phosphorylated p38 MAPK has been detected in both alveolar macrophages and the alveolar walls of COPD patients 51. Increased activation of p38 MAPK has been also reported in alveolar macrophages of patients with severe asthma 52. Activation of p38 MAPK in alveolar macrophages may induce the secretion of pro‐inflammatory cytokines and chemokines required for the pathogenesis of COPD. Inhibition of p38 MAPK using SD‐282 reduces inflammation in a model of tobacco smoke‐induced airway inflammation with decreased expression of cyclooxygenase‐2 (COX‐2) and IL‐6 mRNAs 53. Studies using pharmacological inhibitors have also implicated p38 MAPK in mouse models of COPD, asthma and acute lung inflammation 54, 55, 56, 57, suggesting that p38 MAPK inhibition could have therapeutic effects in lung inflammatory diseases.

p38α MAPK in cancer

Cell proliferation, differentiation and survival are tightly regulated under physiological conditions to maintain tissue homeostasis and dysregulation of these processes is a hallmark of cancer. Immune and inflammatory responses are also important for cancer initiation and progression. During tumorigenesis, cells of both the innate (macrophages, neutrophils, DCs) and adaptive (T and B lymphocytes) immune systems infiltrate the tumor microenvironment and regulate tumor cell fate either directly or via the production of extracellular factors. Immune cells rely on the p38α pathway to regulate multiple functions and to produce cytokines and chemokines 58, 59, 60, 61, which may either promote or suppress tumor growth. For example, COX‐2, IL‐6 and IL‐17 can be regulated by p38α and have important effects on tumorigenesis 62, 63, 64. However, the precise contribution of p38α‐mediated immune responses to tumor initiation and progression is still poorly characterized. This review will focus on the role of p38α in tumor cells. There is evidence implicating p38α in the regulation of cell proliferation, differentiation, survival, migration and invasion in various cancer cell lines 7, 8, 15, 16, 65. Initial experiments using mouse models of cancer indicated that p38α can suppress lung and liver tumor formation in vivo 66, 67. However, additional studies show that p38α may play tumor suppressor or tumor promoter roles depending on the tissue and the tumorigenesis stage (Fig. 2 and Table 1).

Figure 2

Regulation of tumorigenesis by p38α MAPK in mouse models of cancer. Key molecules and processes are indicated. For further details, see Table 1. Normal cells are indicated in beige and tumor cells are indicated in blue. Breast and lung tumor cells treated with cisplatin rely on p38α for survival. The link between PRAK and angiogenesis has been reported in endothelial cells.

Breast cancer

Mouse models have provided in vivo evidence for the implication of p38 MAPK signaling in breast cancer. Studies using mice deficient in Wip1, a phosphatase that can target p38α, show significantly reduced breast tumorigenesis upon expression of Erbb2 or H‐Ras, which correlates with higher p38 MAPK activation 68. The p38α and p38β inhibitor SB203580 restores the Erbb2 driven tumorigenesis in Wip1 KO mice, suggesting that p38 MAPK hyperactivation contributes to the reduced breast tumorigenesis observed in the absence of Wip1. Conversely, mice overexpressing Wip1 in the breast epithelium are more susceptible to breast tumor development induced by ErbB2, a phenotype that was attenuated upon co‐expression of constitutively active MKK6 to activate the p38 MAPK pathway 69. Mice deficient in Gadd45α, an activator of the c‐Jun N‐terminal kinase (JNK) and p38 MAPK pathways, also show accelerated breast tumorigenesis induced by Ras, which correlates with reduced activation of p38 MAPK and reduced levels of Ras‐induced senescence 70.

By contrast to the above tumor suppressive role in breast tumor initiation, recent reports suggest that p38 MAPK signaling may also play pro‐tumorigenic roles. For example, the p38α and p38β inhibitor PH797804 impairs the growth of breast tumors induced by polyoma middle T (PyMT), which correlates with increased apoptosis and decreased proliferation of tumor cells 71. Interestingly, p38 MAPK inhibition potentiates the chemotherapeutic drug cisplatin, reducing the size and malignancy of PyMT‐induced breast tumors. At the molecular level, inhibition of p38 MAPK results in reactive oxygen species (ROS)‐dependent upregulation of the JNK pathway, which in turn mediates cisplatin‐induced apoptosis 71. The inhibitor LY2228820 also reduces tumor growth in a xenograft model based on the MDA‐MB‐468 breast cancer cell line 72. These results indicate that p38 MAPK signaling contributes to breast tumor progression in mouse models.

The pro‐tumorigenic role of p38 MAPK is also supported by experiments showing that inhibition of this pathway impairs the proliferation of p53 mutant and estrogen receptor‐negative breast cancer cell lines in vitro 73. Of note, both MDA‐MB‐468 cells and PyMT breast tumors are estrogen receptor‐negative. Moreover, high levels of active p38 MAPK have been correlated with invasive and poor prognostic breast cancers, lymph node metastasis and tamoxifen resistance in patients 74, 75, 76, 77. Activation of p38 MAPK signaling downstream of the ubiquitin‐conjugating enzyme Ubc13 has been shown to contribute to metastasis and lung colonization by human and mouse breast cancer cells 78. Taken together, it appears that p38 MAPK inhibitors, either alone or in combination with chemotherapeutic drugs, could help to reduce breast tumor growth and metastasis.

Lung cancer

Studies using p38α conditional KO mice have provided in vivo evidence for the involvement of p38 MAPK in lung homeostasis 66, 67. Embryo‐specific deletion of p38α results in perinatal death as a result of distorted alveolar structures and massive infiltration of hematopoietic cells in the lungs 66. Postnatal deletion of p38α results in increased proliferation and defective differentiation of the lung stem and progenitor cells, which can be accounted for by the upregulation of epidermal growth factor receptor (EGFR) and lower expression of the transcription factor C/EBPα 67. Moreover, p38α signaling in lung stem cells induces the expression of CXCL‐12 that activates the stromal fibroblasts, whereas the endogenous p38α in lung fibroblasts is required for the induction of cytokines, which in turn trigger the recruitment of endothelial cells 79. These results support a key role for p38α in maintaining a functional lung microenvironment, suggesting that disruption of this signaling pathway may lead to lung diseases. In line with this idea, the altered lung homeostasis observed upon p38α downregulation facilitates lung tumorigenesis induced by oncogenic K‐rasG12V 67. The p38α‐deficient lung tumors exhibit poor differentiation and higher mitotic indices, which correlate with reduced levels of the differentiation markers C/EBPα and hepatocyte nuclear factor 3β (HNF3β), and with increased activation of AKT and EGFR signaling 67. The phenotype observed in p38α‐deficient lungs mimics the early stages of K‐rasG12V induced transformation, suggesting that the enhanced tumorigenesis might be related to changes in the lung cellular microenvironment rather than to the negative regulation of oncogenic signaling by p38α in tumor cells.

Deletion of the p38α substrate MK2 has no effect on the initiation of lung tumorigenesis in a similar mouse model, irrespective of the p53 status. However, MK2 restrains the progression of lung tumors in the absence of p53 but has no effect when p53 is expressed. MK2 disruption not only makes p53‐deficient lung tumors grow faster, but also sensitizes to DNA damage‐inducing drugs such as cisplatin 80.

Increased p38 MAPK phosphorylation has been reported in human lung tumors compared to normal tissue 81, suggesting that p38 MAPK might contribute to lung tumor progression. The regulation of lung inflammation by p38 MAPK signaling may also impinge on tumorigenesis, although it is unclear whether lung inflammatory diseases such as COPD and asthma are linked to increased risk of lung cancer 82. Further work is required to elucidate whether p38 MAPK inhibition might help lung cancer patients.

Liver cancer

p38α negatively regulates hepatocyte proliferation in adult mice during liver regeneration after partial hepatectomy or N‐nitrosodiethylamine (DEN)‐induced liver injury. Inactivation of c‐Jun in p38α deficient livers results in normal hepatocyte proliferation, suggesting that activation of the JNK‐c‐Jun pathway is responsible for the enhanced proliferation of p38α deficient hepatocytes 66.

Uncontrolled hepatocyte proliferation is considered to be important for liver cancer development. Hepatocellular carcinoma (HCC) is one of the most common forms of primary liver cancer in humans, with 70–90% of HCC cases occurring in patients with chronic liver diseases and cirrhosis, mainly as a result of hepatitis B virus infection and alcoholic liver disease 83. To study HCC in mice, DEN is used as an initiator and phenobarbital (Pb) as a promoting agent. Hepatocyte‐specific deletion of p38α facilitates DEN/Pb‐induced HCC, in which upregulation of the JNK‐c‐Jun pathway plays an important role by enhancing proliferation of p38α‐deficient tumor cells 66. p38α has been proposed to suppress ROS accumulation by modulating Hsp27 expression and cell death in DEN‐treated hepatocytes. Dying hepatocytes release IL‐1α, which stimulates DEN‐induced hepatocyte proliferation, facilitating HCC development 84. Similar results have been observed in a model of HCC related to liver cirrhosis, in which p38α deficiency in hepatocytes leads to ROS accumulation and enhanced thioacetamide‐induced liver damage and fibrosis 85. Another study using a model of LPS/TNF‐induced liver damage has shown that hyperactivation of the JNK pathway in p38α‐deficient hepatocytes is not sufficient to mediate TNF‐induced liver toxicity 86. However, the combined downregulation of p38α and IKK2 in hepatocytes results in liver failure upon LPS injection, suggesting that p38α collaborates with the nuclear factor kappa B (NF‐kB) pathway to protect the liver from cytokine‐induced damage by antagonizing JNK activation 86. These studies indicate that p38α can suppress HCC by regulating different molecular mechanisms depending on the stimuli.

In agreement with the observation that p38α suppresses HCC development in mouse models, reduced p38 MAPK and MKK6 activities have been reported in human HCC compared to nontumoral tissue 87. Moreover, phosphorylation of the p38α pathway target Hsp27 has been inversely correlated with tumor size, invasion and tumor stages of human HCCs 88. By contrast, another study positively correlated p38 MAPK phosphorylation with HCC tumor size and poor survival, although nontumoral areas were not analyzed 89. A larger cohort of human HCC samples should be analyzed to obtain conclusive data on the role of p38 MAPK signaling in human HCC progression.

Colon cancer

The colon is part of the lower gastrointestinal tract. The intestinal epithelia serve as a barrier and play an important role in protecting the intestinal tract against luminal invading pathogens and ingested toxin, which can promote inflammatory responses. Colon inflammatory diseases such as inflammatory bowel disease (IBD) are associated with higher risk of colorectal cancer (CRC) development 90.

In vivo roles of p38α in colon homeostasis and tumor development have been studied using mice with p38α downregulation in intestinal epithelial cells (IECs) 21, 91, 92. These mice appear healthy but show changes in intestinal homeostasis, including increased IEC proliferation, which is associated with increased ERK1/2 and EGFR signaling 91, as well as reduced numbers of mucus producing goblet cells 21, 91, 92. Moreover, p38α regulates the assembly of intestinal epithelial tight junctions, probably by controlling the expression of ZO‐1 and other tight junction molecules 92.

Mice with IEC‐specific p38α downregulation are more susceptible to dextran sodium sulfate (DSS)‐induced colitis 21, 91, 92. DSS is toxic and induces epithelial cell apoptosis, which initiates intestinal inflammation and colitis in mice, and there is evidence that p38α plays a critical role protecting from epithelial apoptosis, thus preventing DSS‐induced colitis 21, 91, 92. Increased apoptosis correlates with increased JNK activation and accumulation of the pro‐apoptotic protein Bak in p38α‐deficient IEC 91, 92. Importantly, the enhanced colitis observed in these mice can be rescued by the administration of probiotics, which restore the altered epithelial permeability, supporting that regulation of the epithelial barrier function by p38α is critical for protection against DSS‐induced colitis 92. By contrast to the role of p38α in IEC, downregulation of p38α in myeloid cells reduces inflammatory responses and colon epithelial damage during DSS‐induced colitis 21. This correlates with reduced activity of NF‐kB and reduced expression of the inflammatory mediators COX‐2 and IL‐6 in the DSS‐treated mice 21. Thus, p38α signaling in different cell types appears to affect colitis progression differently. Of note, p38α in IEC not only regulates colon epithelial homeostasis, but also controls the expression of chemokines, which are essential for the recruitment of immune cells such as CD4+  T cells and subsequent clearance of Citrobacter rodentium infection 93. These studies indicate that p38α signaling in IEC is critical for the protection against DSS‐induced colitis and mucosal infections.

Chronic infection and inflammation can lead to colon tumor development. The initial stages of inflammation‐associated colon tumorigenesis are suppressed by p38α 92, 94. This is probably a result of the ability of p38α to regulate colon homeostasis and the epithelial barrier function 92. Similarly, mice deficient in the p38α activator ASK1 show enhanced DSS‐induced epithelial injury and inflammation and are more susceptible to inflammation‐associated colon tumorigenesis 95.

By contrast to the negative role of p38α in colon tumor initiation, p38 MAPK signaling can also perform pro‐tumorigenic functions in established colon tumors. Mice xenografted with colon cancer cell lines or expressing APCmin that are treated with the inhibitor SB202190 show reduced tumor growth, which correlates with a switch from HIF1α‐ to FoxO‐dependent transcription that affects glycolytic metabolism 96. Importantly, downregulation of p38α in colon tumor cells or pharmacological inhibition using PH797804 reduces tumor burden in mice, which correlates with activation of the JNK pathway, reduced expression of the anti‐apoptotic protein Mcl‐1 and downregulation of IL‐6/STAT3 signaling 92. Colon tumor growth is also reduced in azoxymethane‐treated APCmin mice by the combined inhibition of p38 MAPK using SB202190 and ERK1/2 signaling using PD0325901 97. These studies suggest a dual role for epithelial p38α signaling, suppressing inflammation‐associated colon tumor initiation but supporting colon tumor progression.

There is also evidence implicating p38α in colon cancer metastasis. In particular, reduced levels of p38 MAPK activity in colon cancer cells facilitate lung colonization from established liver metastasis by enhancing production of the cytokine PTHLH, which in turn induces endothelial cell death, enabling tumor cell extravasation to the lung 98. Taken together, p38α signaling appears to control colon tumor cell survival, proliferation and metastasis through distinct mechanisms.

As noted above, IBD patients have higher risk of developing CRC 90. The activating phosphorylation of p38 MAPK in IBD patients has been evaluated in several studies that yield contradictory results 99, 100, 101. Accordingly, the use of p38 MAPK inhibitors in clinical trials has not shown promising results. In patients with Crohn's disease, the p38 MAPK and JNK inhibitor CNI‐1493 showed some clinical improvement 102, whereas the p38 MAPK inhibitor BIRB796 showed no improvement 103. Mouse studies indicate that, during DSS‐induced colitis, p38α contributes to different functions in various cell types, which could explain the controversial effects reported using p38 MAPK inhibitors for therapy. In human CRC, enhanced levels of phosphorylated p38 MAPK have been reported both in tumor cells and in stromal cells 97, 104, 105, 106, suggesting a pro‐tumorigenic role of p38 MAPK. Moreover, high levels of phosphorylated p38 MAPK have been correlated with resistance to the chemotherapeutic drug irinotecan, as well as with poor overall survival in colon cancer patients 104, 106. Collectively, p38 MAPK inhibition in colon cancer patients appears as an attractive therapeutic possibility, although caution is warranted because p38 MAPK inhibition can result in adverse effects.

Skin cancer

Mice deficient for the p38 MAPK substrate PRAK (also known as MK5) show enhanced 7,12‐dimethylbenz[a]anthracene (DMBA)‐induced skin carcinogenesis, which correlates with compromised senescence induction. In primary cells, inactivation of PRAK prevents senescence and promotes oncogenic transformation. The direct phosphorylation of p53 by PRAK has been proposed to mediate these effects 107. Interestingly, PRAK can also promote the growth of skin tumors induced by DMBA and 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) via regulation of tumor angiogenesis. However, this effect is mediated by PRAK signaling in endothelial cells rather than in keratinocytes. Thus, in endothelial cells, tumor‐secreted pro‐angiogenic factors activate vascular endothelial growth factor receptor 2, which in turn activates PRAK inducing migration of the endothelial cells and their incorporation into tumor vasculature. This PRAK function may be mediated by the phosphorylation of the focal adhesion kinase FAK and cytoskeletal reorganization 108. Of note, PRAK has been reported to also be a substrate of the MAPKs ERK3 and ERK4 109, and further studies are required to validate the contribution of p38α to PRAK activation in skin carcinogenesis. The p38α activator ASK1 has been also proposed to have a dual role in DMBA/TPA‐induced skin carcinogenesis, facilitating tumor promotion via regulation of the inflammatory response at the same time as playing a tumor suppressive role by cooperating with ASK2 in keratinocytes 110.

Mice deficient in Gadd45α show increased UV‐induced skin carcinogenesis, which correlates with reduced levels of phosphorylated JNK and p38 MAPK, as well as reduced p53 levels and apoptosis 111. Additionally, inhibition of p38 MAPK signaling has been associated with impaired capacity to repair UV‐induced DNA damage, a primary risk factor for human skin cancers. The levels of p38α are decreased in human cutaneous squamous cell carcinomas (SCC) and UV irradiation of p53‐deficient A431 keratinocytes (derived from SCC) decreases p38α expression. Consistently, treatment of p53−/− SKH‐1 mice with the p38 MAPK inhibitor SB203580 accelerates UV‐induced SCC carcinogenesis and increases the expression of the NAPDH oxidase Nox2. These findings support a tumor‐suppressive role for p38α in SCC pathogenesis, which is associated with the regulation of Nox2 112.

By contrast to the above observations, another study has implicated the p38α substrate MK2 in skin tumor development. MK2 deficient mice show reduced skin carcinogenesis after treatment with DMBA/TPA, which has been explained by the implication of MK2 in the production of pro‐inflammatory cytokines and in the regulation of p53‐dependent apoptosis 113. Similarly, combined deficiency of the p38α substrates MSK1 and MSK2 results in reduced skin carcinogenesis 114. However, the expression of IL‐1β and TNF‐α is upregulated in MSK1/2 double KO mice, probably as a result of weakened negative feedback loops that limit the inflammatory response 33. Therefore, a defective inflammatory response is unlikely to account for the reduced skin tumorigenesis observed in MSK1/2 double KO mice, which might be a result of impaired p38 MAPK‐triggered keratinocyte proliferation. Indeed, p38 MAPK signaling in keratinocytes has been reported to contribute to skin carcinogenesis by inducing activation of the transcription factor activator protein‐1 (AP‐1) and expression of COX‐2, which stimulate the proliferation of UVB‐irradiated epidermal keratinocytes 115. Mice expressing a p38α mutant protein, which may work in a dominant‐negative manner, also show reduced skin tumorigenesis in response to solar UV irradiation, suggesting that p38 MAPK activation by solar UV contributes to skin carcinogenesis 116.

Conclusions

There is good evidence implicating p38α signaling in inflammatory diseases, as well as during tumor initiation and progression (Table 1). The in vivo experiments using genetically modified mice and the use of pharmacological inhibitors suggest that targeting p38α signaling could be useful for the treatment of some inflammatory diseases. Several p38 MAPK inhibitors have been tested in clinical trials but have failed mainly as a result of side effects, such as skin rashes and liver toxicity. However, it is not clear whether these side effects are a result of the systemic inhibition of p38 MAPK signaling or the off‐target effects of the inhibitors. Nevertheless, promising results have been obtained in some cases 117, although systemic inhibition of the p38 MAPK pathway may not be beneficial in all the cases. Based on the studies using mouse models, p38α appears to have distinct roles in different cell types even within the same tissue. For example, inhibition of p38α in myeloid cells ameliorates the effects of colitis, whereas inhibition of p38α in IEC can have deleterious effects in the same model. This dual effect could explain the failure of p38 MAPK inhibitors in IBD patients.

Given the contribution of inflammation to tumorigenesis, inhibition of p38α signaling would be expected to benefit inflammation‐associated cancers. However, mouse studies indicate that the role of p38α signaling in cancer initiation and progression is cell type‐ and tumor type‐dependent. Because p38α may suppress some type of tumors at the same time as working as a tumor promoter in other cancers, the inhibitors should be used with caution. Thus, new strategies to target p38α signaling in cell type or tissue specific manners should be devised. Moreover, considering the cross‐talk among signaling pathways, it might be beneficial to use combination therapies for simultaneously targeting p38α and other signaling molecules. It should be noted that genetic analysis in mice have also implicated p38γ and p38δ in the in vivo regulation of colitis‐associated colon tumorigenesis 118 and skin cancer 119. It remains to be established whether different p38 MAPK family members might interplay during tumor development.

Recent studies have improved our understanding of the in vivo roles of p38α signaling in inflammation and cancer. Tumor‐suppressing and tumor‐promoting functions of the p38α pathway can be temporally and spatially separated during tumor development, depending on the tissue type and the tumor stage. More mechanistic studies are required to define the functions of p38α, as well as its key regulators and targets in mouse models of cancer. Future studies should also focus on the development of new models to regulate the p38 MAPK pathway in a time‐ and cell type‐dependent manner. These new models should provide valuable information on the role of p38α signaling at various stages of the disease and in different cell types, which in turn should be useful for developing improved therapeutic strategies.

Author contributions

JG and ARN wrote the paper.