Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases. Microbiol Mol Biol Rev. 2011;75:50-83. [PMID: 21372320 DOI: 10.1128/mmbr.00031-10]

P90 RSK arranges Chk1 in the nucleus for monitoring of genomic integrity during cell proliferation

The ataxia telangiectasia mutated- and rad3-related kinase (ATR)/Chk1 pathway is a sentinel of cell cycle progression. On the other hand, the Ras/mitogen-activated protein kinase/90-kDa ribosomal S6 kinase (p90 RSK) pathway is a central node in cell signaling downstream of growth factors. These pathways are closely correlated in cell proliferation, but their interaction is largely unknown. Here we show that Chk1 is phosphorylated predominantly at Ser-280 and translocated from cytoplasm to nucleus in response to serum stimulation. Nonphosphorylated Chk1-Ser-280 mutation attenuates nuclear Chk1 accumulation, whereas the phosphomimic mutation has a reverse effect on the localization. Treatment with p90 RSK inhibitor impairs Chk1 phosphorylation at Ser-280 and accumulation at the nucleus after serum stimulation, whereas these two phenomena are induced by the expression of the constitutively active mutant of p90 RSK in serum-starved cells. In vitro analyses indicate that p90 RSK stoichiometrically phosphorylates Ser-280 on Chk1. Together with Chk1 phosphorylation at Ser-345 by ATR and its autophosphorylation at Ser-296, which are critical for checkpoint signaling, Chk1-Ser-280 phosphorylation is elevated in a p90 RSK-dependent manner after UV irradiation. In addition, Chk1 phosphorylation at Ser-345 and Ser-296 after UV irradiation is also attenuated by the treatment with p90 RSK inhibitor or by Ser-280 mutation to Ala. These results suggest that p90 RSK facilitates nuclear Chk1 accumulation through Chk1-Ser-280 phosphorylation and that this pathway plays an important role in the preparation for monitoring genetic stability during cell proliferation.

Group VIA PLA2 (iPLA2β) is activated upstream of p38 mitogen-activated protein kinase (MAPK) in pancreatic islet β-cell signaling

Group VIA phospholipase A(2) (iPLA(2)β) in pancreatic islet β-cells participates in glucose-stimulated insulin secretion and sarco(endo)plasmic reticulum ATPase (SERCA) inhibitor-induced apoptosis, and both are attenuated by pharmacologic or genetic reductions in iPLA(2)β activity and amplified by iPLA(2)β overexpression. While exploring signaling events that occur downstream of iPLA(2)β activation, we found that p38 MAPK is activated by phosphorylation in INS-1 insulinoma cells and mouse pancreatic islets, that this increases with iPLA(2)β expression level, and that it is stimulated by the iPLA(2)β reaction product arachidonic acid. The insulin secretagogue D-glucose also stimulates β-cell p38 MAPK phosphorylation, and this is prevented by the iPLA(2)β inhibitor bromoenol lactone. Insulin secretion induced by d-glucose and forskolin is amplified by overexpressing iPLA(2)β in INS-1 cells and in mouse islets, and the p38 MAPK inhibitor PD169316 prevents both responses. The SERCA inhibitor thapsigargin also stimulates phosphorylation of both β-cell MAPK kinase isoforms and p38 MAPK, and bromoenol lactone prevents both events. Others have reported that iPLA(2)β products activate Rho family G-proteins that promote MAPK kinase activation via a mechanism inhibited by Clostridium difficile toxin B, which we find to inhibit thapsigargin-induced β-cell p38 MAPK phosphorylation. Thapsigargin-induced β-cell apoptosis and ceramide generation are also prevented by the p38 MAPK inhibitor PD169316. These observations indicate that p38 MAPK is activated downstream of iPLA(2)β in β-cells incubated with insulin secretagogues or thapsigargin, that this requires prior iPLA(2)β activation, and that p38 MAPK is involved in the β-cell functional responses of insulin secretion and apoptosis in which iPLA(2)β participates.

Therapeutic implications of targeting the PI3Kinase/AKT/mTOR signaling module in melanoma therapy

The PI3Kinase/AKT/mTOR signaling module is implicated in various cellular functions including cell survival, growth and proliferation, glucose metabolism, apoptosis, migration, and angiogenesis. Increased expression of AKT and its up- and downstream regulators is linked to several types of cancer. Aberrant expression of AKT is observed in nearly 60% of melanomas culminating in apoptosis resistance via deactivation of apoptotic molecules Bad and Cas-pase-9. Through cross-talk with NF-κB, ERK1/2, JNK and p38MAPK signaling pathways, AKT induces a plethora of cellular effects often leading to tumor development and progression. Due to frequently observed resistance to other common cancer treatments such as chemotherapy, immunotherapy, and radiation, and the detrimental consequences of constitutive activation of the PI3Kinase/AKT/mTOR signaling module, targeted inhibition of the effectors and substrates involved in this module has become a viable and attractive option for molecular targeted therapy in melanoma. Pharmacological inhibitors of various components of this module, either alone or in combination with other agents, have shown significant decrease in proliferation, tumorigenesis, cell growth and survival of various tumors in phases I and II clinical trials. Some inhibitors have even received their Food and Drug Administration (FDA) approval. This review summarizes the current knowledge on this module, its cross-talk with other major cell survival pathways and its targeted inhibition for therapeutic purposes in melanoma.

Posttranslational modification of vesicular stomatitis virus glycoprotein, but not JNK inhibition, is the antiviral mechanism of SP600125

Vesicular stomatitis virus (VSV), a negative-sense single-stranded-RNA rhabdovirus, is an extremely promising oncolytic agent for cancer treatment. Since oncolytic virotherapy is moving closer to clinical application, potentially synergistic combinations of oncolytic viruses and molecularly targeted antitumor agents are becoming a meaningful strategy for cancer treatment. Mitogen-activated protein kinase (MAPK) inhibitors have been shown to impair liver cell proliferation and tumor development, suggesting their potential use as therapeutic agents for hepatocellular carcinoma (HCC). In this work, we show that the impairment of MAPK in vitro did not interfere with the oncolytic properties of VSV in HCC cell lines. Moreover, the administration of MAPK inhibitors did not restore the responsiveness of HCC cells to alpha/beta interferon (IFN-α/β). In contrast to previous reports, we show that JNK inhibition by the inhibitor SP600125 is not responsible for VSV attenuation in HCC cells and that this compound acts by causing a posttranslational modification of the viral glycoprotein.

Regulation and function of the RSK family of protein kinases

The RSK (90 kDa ribosomal S6 kinase) family comprises a group of highly related serine/threonine kinases that regulate diverse cellular processes, including cell growth, proliferation, survival and motility. This family includes four vertebrate isoforms (RSK1, RSK2, RSK3 and RSK4), and single family member orthologues are also present in Drosophila and Caenorhabditis elegans. The RSK isoforms are downstream effectors of the Ras/ERK (extracellular-signal-regulated kinase) signalling pathway. Significant advances in the field of RSK signalling have occurred in the past few years, including several new functions ascribed to the RSK isoforms, the discovery of novel protein substrates and the implication of different RSK isoforms in cancer. Collectively, these new findings increase the diversity of biological functions regulated by RSK, and highlight potential new directions of research. In the present paper, we review the structure, expression and activation mechanisms of the RSK isoforms, and discuss their physiological roles on the basis of established substrates and recent discoveries.

The macrophage response towards LPS and its control through the p38(MAPK)-STAT3 axis

In macrophages detection of gram-negative bacteria particularly involves binding of the outer-wall component lipopolysaccharide (LPS) to its cognate receptor complex, comprising Toll like receptor 4 (TLR4), CD14 and MD2. LPS-induced formation of the LPS receptor complex elicits a signaling network, including intra-cellular signal-transduction directly activated by the TLR4 receptor complex as well as successional induction of indirect autocrine and paracrine signaling events. All these different pathways are integrated into the macrophage response towards an inflammatory stimulus by a highly complex cross-talk of the pathways engaged. This also includes a tight control by several intra- and inter-cellular feedback loops warranting an inflammatory response sufficient to battle invading pathogens and to avoid non-essential tissue damage caused by an overwhelming inflammatory response. Several evidences indicate that the reciprocal cross-talk between the p38(MAPK)-pathway and signal transducer and activator of transcription (STAT)3-mediated signal-transduction forms a critical axis successively activated by LPS. The balanced activation of this axis is essential for both induction and propagation of the inflammatory macrophage response as well as for the control of the resolution phase, which is largely driven by IL-10 and sustained STAT3 activation. In this context regulation of suppressor of cytokine signaling (SOCS)3 expression and the recently described divergent regulatory roles of the two p38(MAPK)-activated protein kinases MK2 and MK3 for the regulation of LPS-induced NF-κB- and IRF3-mediated signal-transduction and gene expression, which includes the regulation of IFNβ, IL-10 and DUSP1, appears to play an important role.

The yeast mitogen-activated protein kinase Slt2 is involved in the cellular response to genotoxic stress

BACKGROUND

The maintenance of genomic integrity is essential for cell viability. Complex signalling pathways (DNA integrity checkpoints) mediate the response to genotoxic stresses. Identifying new functions involved in the cellular response to DNA-damage is crucial. The Saccharomyces cerevisiae SLT2 gene encodes a member of the mitogen-activated protein kinase (MAPK) cascade whose main function is the maintenance of the cell wall integrity. However, different observations suggest that SLT2 may also have a role related to DNA metabolism.

RESULTS

This work consisted in a comprehensive study to connect the Slt2 protein to genome integrity maintenance in response to genotoxic stresses. The slt2 mutant strain was hypersensitive to a variety of genotoxic treatments, including incubation with hydroxyurea (HU), methylmetanosulfonate (MMS), phleomycin or UV irradiation. Furthermore, Slt2 was activated by all these treatments, which suggests that Slt2 plays a central role in the cellular response to genotoxic stresses. Activation of Slt2 was not dependent on the DNA integrity checkpoint. For MMS and UV, Slt2 activation required progression through the cell cycle. In contrast, HU also activated Slt2 in nocodazol-arrested cells, which suggests that Slt2 may respond to dNTP pools alterations. However, neither the protein level of the distinct ribonucleotide reductase subunits nor the dNTP pools were affected in a slt2 mutant strain. An analysis of the checkpoint function revealed that Slt2 was not required for either cell cycle arrest or the activation of the Rad53 checkpoint kinase in response to DNA damage. However, slt2 mutant cells showed an elongated bud and partially impaired Swe1 degradation after replicative stress, indicating that Slt2 could contribute, in parallel with Rad53, to bud morphogenesis control after genotoxic stresses.

CONCLUSIONS

Slt2 is activated by several genotoxic treatments and is required to properly cope with DNA damage. Slt2 function is important for bud morphogenesis and optimal Swe1 degradation under replicative stress. The MAPK Slt2 appears as a new player in the cellular response to genotoxic stresses.

OxLDL up-regulates Niemann-Pick type C1 expression through ERK1/2/COX-2/PPARα-signaling pathway in macrophages

The Niemann-Pick type C1 (NPC1) is located mainly in the membranes of the late endosome/lysosome and controls the intracellular cholesterol trafficking from the late endosome/lysosome to the plasma membrane. It has been reported that oxidized low-density lipoprotein (oxLDL) can up-regulate NPC1 expression. However, the detailed mechanisms are not fully understood. In this study, we investigated the effect of oxLDL stimulation on NPC1 expression in THP-1 macrophages. Our results showed that oxLDL up-regulated NPC1 expression at both mRNA and protein levels in a dose-dependent and time-dependent manner. In addition, oxLDL also induced the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2). Treatment with oxLDL significantly increased cyclooxygenase-2 (COX-2) mRNA and protein expression in the macrophages, and these increases were suppressed by the ERK1/2 inhibitor PD98059 or ERK1/2 small interfering RNA (siRNA) treatment. OxLDL up-regulated the expression of peroxisome proliferator-activated receptor α (PPARα) at the mRNA and protein levels, which could be abolished by COX-2 siRNA or COX-2 inhibitor NS398 treatment in these macrophages. OxLDL dramatically elevated cellular cholesterol efflux, which was abrogated by inhibiting ERK1/2 and/or COX-2. In addition, oxLDL-induced NPC1 expression and cellular cholesterol efflux were reversed by PPARα siRNA or GW6471, an antagonist of PPARα. Taken together, these results provide the evidence that oxLDL can up-regulate the expression of the NPC1 through ERK1/2/COX-2/PPARα-signaling pathway in macrophages.

Signaling through the extracellular calcium-sensing receptor (CaSR)

The extracellular calcium ([Formula: see text])-sensing receptor (CaSR) was the first GPCR identified whose principal physiological ligand is an ion, namely extracellular Ca(2+). It maintains the near constancy of [Formula: see text] that complex organisms require to ensure normal cellular function. A wealth of information has accumulated over the past two decades about the CaSR's structure and function, its role in diseases and CaSR-based therapeutics. This review briefly describes the CaSR and key features of its structure and function, then discusses the extracellular signals modulating its activity, provides an overview of the intracellular signaling pathways that it controls, and, finally, briefly describes CaSR signaling both in tissues participating in [Formula: see text] homeostasis as well as those that do not. Factors controlling CaSR signaling include various factors affecting the expression of the CaSR gene as well as modulation of its trafficking to and from the cell surface. The dimeric cell surface CaSR, in turn, links to various heterotrimeric and small molecular weight G proteins to regulate intracellular second messengers, lipid kinases, various protein kinases, and transcription factors that are part of the machinery enabling the receptor to modulate the functions of the wide variety of cells in which it is expressed. CaSR signaling is impacted by its interactions with several binding partners in addition to signaling elements per se (i.e., G proteins), including filamin-A and caveolin-1. These latter two proteins act as scaffolds that bind signaling components and other key cellular elements (e.g., the cytoskeleton). Thus CaSR signaling likely does not take place randomly throughout the cell, but is compartmentalized and organized so as to facilitate the interaction of the receptor with its various signaling pathways.

Arabidopsis MKKK20 is involved in osmotic stress response via regulation of MPK6 activity

Plants have developed various regulatory pathways to adapt to environmental stresses. In this study, we identified Arabidopsis MKKK20 as a regulator in the response to osmotic stress. mkkk20 mutants were found to be sensitive to high concentration of salt and showed higher water loss rates than wild-type (WT) plants under dehydration conditions. In addition, mkkk20 mutants showed higher accumulation of superoxide, a reactive oxygen species (ROS), compared to WT plants under high salt condition. In contrast, transgenic plants overexpressing MKKK20 displayed tolerance to salt stress. MKKK20 transcripts were increased by the treatments with NaCl, mannitol, MV, sorbitol, and cold, suggesting that MKKK20 is involved in the response to osmotic, ROS, and cold stresses. In-gel kinase assay showed that MKKK20 regulates the activity of MPK6 under NaCl, cold, and H(2)O(2) treatments. Taken together, our results suggest that MKKK20 might be involved in the response to various abiotic stresses, especially osmotic stress, through its regulation of MPK6 activity.

Characterization of p38 MAPKs from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

p38 mitogen-activated protein kinases (MAPKs) are broadly expressed signaling molecules that involves in the regulation of cellular responsible for various extracellular stimuli. In this study, three p38 MAPK genes (Ec-p38a, p38b and p38β) were cloned from grouper, Epinephelus coioides and their characteristics were investigated in vitro. Although Ec-p38a, p38b and p38β showed high homologies to other fish p38a MPAK, p38b MAPK and p38β MAPK, respectively, they all contained the conserved structures of Thr-Gly-Tyr (TGY) motif and substrate binding site Ala-Thr-Arg-Trp (ATRW). Phylogenetic analysis indicated that Ec-p38a, p38b and p38β are more closely related to those from fish than mammals. The tissue distribution patterns of Ec-p38a, p38b and p38β were different, and Ec-p38β was up-regulated most obviously in head kidney after Singapore grouper iridovirus (SGIV) infection. Overexpression of Ec-p38β in FHM cells delayed the occurrence of CPE induced by SGIV infection. Further analysis indicated that overexpression of Ec-p38β inhibited viral gene transcription and protein synthesis, as well as SGIV induced typical apoptosis in fish cells. Taken together, our data indicated that Ec-p38β played a crucial role in regulating apoptosis and virus replication during iridovirus infection.

Localization and retention of p90 ribosomal S6 kinase 1 in the nucleus: implications for its function

Ribosomal S6 kinase 1 (RSK1), which plays a critical role in cell survival and proliferation, contains a bipartite nuclear localization sequence that permits its entry into the nucleus. RSK1 is retained in the nucleus via its indirect interactions with AKAP95. Interference with its nuclear entry or retention decreases DNA synthesis.

Ribosomal S6 kinase 1 (RSK1) belongs to a family of proteins with two kinase domains. Following activation in the cytoplasm by extracellular signal-regulated kinases (ERK1/2), it mediates the cell-proliferative, cell-growth, and survival-promoting actions of a number of growth factors and other agonists. These diverse biological actions of RSK1 involve regulation of both cytoplasmic and nuclear events. However, the mechanisms that permit nuclear accumulation of RSK1 remain unknown. Here, we show that phosphorylation of RSK1 on S221 is important for its dissociation from the type Iα regulatory subunit of protein kinase A (PKA) in the cytoplasm and that RSK1 contains a bipartite nuclear localization sequence that is necessary for its nuclear entry. Once inside, the active RSK1 is retained in the nucleus via its interactions with PKA catalytic subunit and AKAP95. Mutations of RSK1 that do not affect its activity but disrupt its entry into the nucleus or expression of AKAP95 forms that do not enter the nucleus inhibit the ability of active RSK1 to stimulate DNA synthesis. Our findings identify novel mechanisms by which active RSK1 accumulates in the nucleus and also provide new insights into how AKAP95 orchestrates cell cycle progression.

RON is associated with tumor progression via the inhibition of apoptosis and cell cycle arrest in human gastric cancer

The recepteur d'origine nantais (RON) receptor tyrosine kinase is overexpressed in epithelial cancers, including gastric cancer. The aims of the present study were to evaluate whether RON affects tumor cell behaviors and oncogenic signaling pathways, and to document the relationship of its expression with various clinicopathological parameters in gastric cancer. The biological role of RON in tumor cell behaviors and oncogenic signaling pathways was investigated by using small interfering RNA in gastric cancer cell lines including AGS and MKN28. The expression of RON in gastric cancer tissues was investigated by using reverse transcription polymerase chain reaction and immunohistochemistry. Knockdown of RON suppressed tumor cell migration and invasion in AGS and MKN28, induced apoptosis through modulation of anti-apoptotic and pre-apoptotic genes and induced cell cycle arrest by decreasing cyclin D1, cyclin D3 and CDK4, and by inducing p21 and p27 expression. Signaling cascades, including Akt and mitogen-activated protein kinase (MAPK), were significantly blocked by knockdown of RON. Expression of RON was significantly associated with tumor size, depth of invasion, lymph node metastasis, tumor stage and poor survival. These results indicate that RON is associated with tumor progression via the inhibition of apoptosis and cell cycle arrest in human gastric cancer.

The mitogen-activated protein kinome from Anopheles gambiae: identification, phylogeny and functional characterization of the ERK, JNK and p38 MAP kinases

BACKGROUND

Anopheles gambiae is the primary mosquito vector of human malaria parasites in sub-Saharan Africa. To date, three innate immune signaling pathways, including the nuclear factor (NF)-kappaB-dependent Toll and immune deficient (IMD) pathways and the Janus kinase/signal transducers and activators of transcription (Jak-STAT) pathway, have been extensively characterized in An. gambiae. However, in addition to NF-kappaB-dependent signaling, three mitogen-activated protein kinase (MAPK) pathways regulated by JNK, ERK and p38 MAPK are critical mediators of innate immunity in other invertebrates and in mammals. Our understanding of the roles of the MAPK signaling cascades in anopheline innate immunity is limited, so identification of the encoded complement of these proteins, their upstream activators, and phosphorylation profiles in response to relevant immune signals was warranted.

RESULTS

In this study, we present the orthologs and phylogeny of 17 An. gambiae MAPKs, two of which were previously unknown and two others that were incompletely annotated. We also provide detailed temporal activation profiles for ERK, JNK, and p38 MAPK in An. gambiae cells in vitro to immune signals that are relevant to malaria parasite infection (human insulin, human transforming growth factor-beta1, hydrogen peroxide) and to bacterial lipopolysaccharide. These activation profiles and possible upstream regulatory pathways are interpreted in light of known MAPK signaling cascades.

CONCLUSIONS

The establishment of a MAPK "road map" based on the most advanced mosquito genome annotation can accelerate our understanding of host-pathogen interactions and broader physiology of An. gambiae and other mosquito species. Further, future efforts to develop predictive models of anopheline cell signaling responses, based on iterative construction and refinement of data-based and literature-based knowledge of the MAP kinase cascades and other networked pathways will facilitate identification of the "master signaling regulators" in biomedically important mosquito species.

The mechanism of MAP kinase activation under acidic condition in feline esophageal smooth muscle cells

Reflux esophagitis results from repeated exposure of the esophagus to acidic gastric juice or bile-containing duodenal contents. In Barrett's adenocarcinoma, acid increases proliferation via ERK and p38 MAPK activation. This study was focused on determination of the mechanism(s) underlying MAPKs (ERK 1/2, p38 MAPK, and JNK) activation induced by acidic medium at pH 4 in normal feline primary cultured esophageal smooth muscle cells (FESMCs). We detected ERK 1/2 and p38 MAPK phosphorylation after exposure to pH 4 or neutral media in the presence or absence of several inhibitors and quantified the MAPK levels using western blotting analysis and densitometry. Acidic medium markedly increased the phosphorylation of ERK 1/2 and p38 MAPK within 10 min. Acid-induced ERK 1/2 and p38 MAPK activation was inhibited by pertussis toxin (PTX-sensitive G(i/o) protein inhibitor), DEDA (phospholipase (PL) A(2) inhibitor), ρCMB (PLD inhibitor), GF109203X (protein kinase C (PKC) inhibitor) and D609 (phosphatidylcholine-specific PLC inhibitor). But, genistein (tyrosine kinase inhibitor), forskolin (adenylate cyclase activator) and U73122 (phosphatidylinositol-specific PLC inhibitor) had no effect on acid-induced ERK1/2 and p38 MAPK activation. These findings indicate that the activation of ERK 1/2 and p38 MAPK pathways by acidic conditions, at least in part, may be mediated by activation of the G(i/o) protein coupled receptors, PC-PLC, PLD, PLA(2), and PKC in FESMCs.

The dysregulation of the monocyte/macrophage effector function induced by isopropanol is mediated by the defective activation of distinct members of the AP-1 family of transcription factors

Isopropanol is the second most common cause of short-chain alcohol acute intoxication. Nonethanolic short-chain alcohols mediate their immunomodulatory effect by interfering with nuclear factor of activated T cells (NFAT) activation with or without additional activator protein-1 (AP-1) involvement. In the present study, we examined the immunomodulation induced by isopropanol in conditions that are not reliant on NFAT: the inflammatory cytokine response of lipopolysaccharide (LPS)-stimulated monocytes. Our hypothesis was that isopropanol acute exposure would have an attenuated effect or no consequence in this setting. To our surprise, the impairment of AP-1 activation was sufficient to mediate a severe and dose-dependent phenotype in human monocytes in vitro at alcohol concentrations as low as 0.16% (or 26 mM). There were three outcomes: interleukin (IL)-1β/IL-8 were unaltered; IL-6 was upregulated; and tumor necrosis factor alpha (TNF-α)/CCL2 were downregulated. The effector function of human monocyte-derived macrophages was also compromised. Our results showed that Toll-like receptor 4 early signaling was preserved, as isopropanol did not change the kinase activity of the IL-1 receptor-associated kinase 1 in LPS-stimulated cells. The nuclear factor-κB signaling cascade and the p38/c-Jun N-terminal kinase modules of the mitogen-activated protein kinase pathway were alcohol insensitive. Conversely, the activation of extracellular signal-regulated protein kinase and, ultimately, of c-Fos and JunB were impaired. The alcohol-induced cytokine dysregulation was confirmed in a mouse model of isopropanol intoxication in which the production of TNF-α in response to LPS challenge was virtually abolished. The magnitude of this alcohol effect was sufficiently high to rescue animals from LPS-induced toxic shock. Our data contribute to the dismal body of information on the immunotoxicology of isopropanol, one of the most ubiquitous chemicals to which the general population is significantly exposed.

Type I keratin 17 protein is phosphorylated on serine 44 by p90 ribosomal protein S6 kinase 1 (RSK1) in a growth- and stress-dependent fashion

Keratin 17 (K17) is a type I intermediate filament protein that is constitutively expressed in ectoderm-derived epithelial appendages and robustly induced in epidermis following injury, during inflammation, and in chronic diseases such as psoriasis and cancer. Mutations within K17 are responsible for two rare diseases related to ectodermal dysplasias. Studies in K17-null mice uncovered several roles for K17, including structural support, resistance to TNFα-induced apoptosis, regulation of protein synthesis, and modulation of cytokine expression. Yet, little is known about the regulation of K17 protein via post-translational modification. Here, we report that serine 44 in the N-terminal head domain of K17 (K17-Ser(44)) is phosphorylated in response to extracellular stimuli (serum, EGF, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate) that alter skin keratinocyte growth, and to cellular stresses (sorbitol-induced hyperosmotic shock, UV irradiation, and hydrogen peroxide-induced oxidative stress). It also occurs in basaloid skin tumors in situ. Upon its stimulation in skin keratinocytes, K17-Ser(44) phosphorylation is induced rapidly but stays on transiently. The majority of the phosphorylated K17-Ser(44) pool is polymer-bound and is not obviously related to a change in filament organization. The amino acid sequence surrounding K17-Ser(44) matches the consensus for the AGC family of basophilic kinases. We show that p90 RSK1, an AGC kinase involved in the regulation of cell survival and proliferation, phosphorylates K17-Ser(44) in skin keratinocytes. These findings confirm and expand the tight link that has emerged between K17 up-regulation and growth and stress responses in the skin epithelium.

Simultaneous inhibition of mTOR-containing complex 1 (mTORC1) and MNK induces apoptosis of cutaneous T-cell lymphoma (CTCL) cells

Background

mTOR kinase forms the mTORC1 complex by associating with raptor and other proteins and affects a number of key cell functions. mTORC1 activates p70S6kinase 1 (p70S6K1) and inhibits 4E-binding protein 1 (4E-BP1). In turn, p70S6K1 phosphorylates a S6 protein of the 40S ribosomal subunit (S6rp) and 4E-BP1, with the latter negatively regulating eukaryotic initiation factor 4E (eIF-4E). MNK1 and MNK2 kinases phosphorylate and augment activity of eIF4E. Rapamycin and its analogs are highly specific, potent, and relatively non-toxic inhibitors of mTORC1. Although mTORC1 activation is present in many types of malignancies, rapamycin-type inhibitors shows relatively limited clinical efficacy as single agents. Initially usually indolent, CTCL displays a tendency to progress to the aggressive forms with limited response to therapy and poor prognosis. Our previous study (M. Marzec et al. 2008) has demonstrated that CTCL cells display mTORC1 activation and short-term treatment of CTCL-derived cells with rapamycin suppressed their proliferation and had little effect on the cell survival.

Methods

Cells derived from CTCL were treated with mTORC1 inhibitor rapamycin and MNK inhibitor and evaluated for inhibition of the mTORC1 signaling pathway and cell growth and survival.

Results

Whereas the treatment with rapamycin persistently inhibited mTORC1 signaling, it suppressed only partially the cell growth. MNK kinase mediated the eIF4E phosphorylation and inhibition or depletion of MNK markedly suppressed proliferation of the CTCL cells when combined with the rapamycin-mediated inhibition of mTORC1. While MNK inhibition alone mildly suppressed the CTCL cell growth, the combined MNK and mTORC1 inhibition totally abrogated the growth. Similarly, MNK inhibitor alone displayed a minimal pro-apoptotic effect; in combination with rapamycin it triggered profound cell apoptosis.

Conclusions

These findings indicate that the combined inhibition of mTORC1 and MNK may prove beneficial in the treatment of CTCL and other malignancies.

Inhibition of SAPK2/p38 enhances sensitivity to mTORC1 inhibition by blocking IRES-mediated translation initiation in glioblastoma

A variety of mechanisms confer hypersensitivity of tumor cells to the macrolide rapamycin, the prototypic mTORC1 inhibitor. Several studies have shown that the status of the AKT kinase plays a critical role in determining hypersensitivity. Cancer cells in which AKT activity is elevated are exquisitely sensitive to mTORC1 inhibitors while cells in which the kinase is quiescent are relatively resistant. Our previous work has shown that a transcript-specific protein synthesis salvage pathway is operative in cells with quiescent AKT levels, maintaining the translation of crucial mRNAs involved in cell-cycle progression in the face of global eIF-4E-mediated translation inhibition. The activation of this salvage pathway is dependent on SAPK2/p38-mediated activation of IRES-dependent initiation of the cyclin D1 and c-MYC mRNAs, resulting in the maintenance of their protein expression levels. Here, we show that both genetic and pharmacologic inhibition of SAPK2/p38 in glioblastoma multiforme cells significantly reduces rapamycin-induced IRES-mediated translation initiation of cyclin D1 and c-MYC, resulting in increased G(1) arrest in vitro and inhibition of tumor growth in xenografts. Moreover, we observed that the AKT-dependent signaling alterations seen in vitro are also displayed in engrafted tumors cells and were able to show that combined inhibitor treatments markedly reduced the mRNA translational state of cyclin D1 and c-MYC transcripts in tumors isolated from mice. These data support the combined use of SAPK2/p38 and mTORC1 inhibitors to achieve a synergistic antitumor therapeutic response, particularly in rapamycin-resistant quiescent AKT-containing cells.

Physiological roles of mitogen-activated-protein-kinase-activated p38-regulated/activated protein kinase

Mitogen-activated protein kinases (MAPKs) are a family of proteins that constitute signaling pathways involved in processes that control gene expression, cell division, cell survival, apoptosis, metabolism, differentiation and motility. The MAPK pathways can be divided into conventional and atypical MAPK pathways. The first group converts a signal into a cellular response through a relay of three consecutive phosphorylation events exerted by MAPK kinase kinases, MAPK kinase, and MAPK. Atypical MAPK pathways are not organized into this three-tiered cascade. MAPK that belongs to both conventional and atypical MAPK pathways can phosphorylate both non-protein kinase substrates and other protein kinases. The latter are referred to as MAPK-activated protein kinases. This review focuses on one such MAPK-activated protein kinase, MAPK-activated protein kinase 5 (MK5) or p38-regulated/activated protein kinase (PRAK). This protein is highly conserved throughout the animal kingdom and seems to be the target of both conventional and atypical MAPK pathways. Recent findings on the regulation of the activity and subcellular localization, bona fide interaction partners and physiological roles of MK5/PRAK are discussed.

Physiological roles of mitogen-activated-protein-kinase-activated p38-regulated/activated protein kinase

Mitogen-activated protein kinases (MAPKs) are a family of proteins that constitute signaling pathways involved in processes that control gene expression, cell division, cell survival, apoptosis, metabolism, differentiation and motility. The MAPK pathways can be divided into conventional and atypical MAPK pathways. The first group converts a signal into a cellular response through a relay of three consecutive phosphorylation events exerted by MAPK kinase kinases, MAPK kinase, and MAPK. Atypical MAPK pathways are not organized into this three-tiered cascade. MAPK that belongs to both conventional and atypical MAPK pathways can phosphorylate both non-protein kinase substrates and other protein kinases. The latter are referred to as MAPK-activated protein kinases. This review focuses on one such MAPK-activated protein kinase, MAPK-activated protein kinase 5 (MK5) or p38-regulated/activated protein kinase (PRAK). This protein is highly conserved throughout the animal kingdom and seems to be the target of both conventional and atypical MAPK pathways. Recent findings on the regulation of the activity and subcellular localization, bona fide interaction partners and physiological roles of MK5/PRAK are discussed.