Phosphorylation of NF-κB1/p105 by oncoprotein kinase Tpl2: Implications for a novel mechanism of Tpl2 regulation

A Dual Role for Cysteinyl Leukotriene Receptors in the Pathogenesis of Corneal Infection

Cysteinyl leukotrienes (CysLTs) have been defined as central mediators of inflammation. Despite our extensive understanding of these bioactive lipid mediators in the pathogenesis of diseases such as asthma, allergic rhinitis, and even neurological disorders, information regarding the eye is markedly lacking. As a result, this study examined the expression profiles of two major CysLT receptors, CysLT1 and CysLT2, in the cornea using experimental mouse models of Pseudomonas aeruginosa-induced keratitis with contrasting outcomes: susceptible C57BL/6 (B6) and resistant BALB/c. Postinfection, disparate levels of CysLT receptors were accompanied by distinct expression profiles for select proinflammatory and anti-inflammatory cell surface markers detected on macrophages and polymorphonuclear neutrophils between the two strains. Further, inhibition of either CysLT receptor converted the disease response of both strains, where corneal perforation was prevented in B6 mice, and BALB/c mice fared significantly worse. In addition, receptor antagonist studies revealed changes in inflammatory cell infiltrate phenotypes and an influence on downstream CysLT receptor signaling pathways. Although the B6 mouse model highlights the established proinflammatory activities related to CysLT receptor activation, results generated from BALB/c mice indicate a protective mechanism that may be essential to disease resolution. Further, basal expression levels of CysLT1 and CysLT2 were significantly higher in uninfected corneas of both mouse strains as opposed to during infection, suggestive of a novel role in homeostatic maintenance within the eye. In light of these findings, therapeutic targeting of CysLT receptors extends beyond inhibition of proinflammatory activities and may impact inflammation resolution, as well as corneal surface homeostasis.

Rigosertib inhibits MEK1-ERK pathway and alleviates lipopolysaccharide-induced sepsis

Background

Here, by using the lipopolysaccharide (LPS)‐induced mice sepsis model, we treated septic wild‐type (WT) mice or MEK1^DD mice with rigosertib to evaluate its prospective effects on sepsis.

Methods

We also generated macrophages derived from bone marrow from WT or MEK1^DD mice. These macrophages were pretreated with rigosertib and then induced with LPS or poly I:C.

Results

Rigosertib suppressed LPS or poly I:C‐induced expression of inflammatory cytokines (tumor necrosis factor‐alpha [TNF‐α] and interleukin‐6 [IL‐6], and IL‐23) in WT bone marrow–derived macrophages while failed to affect the upregulation of TNF‐α and IL‐6 in LPS‐treated bone marrow–derived macrophages from MEK1^DD mice. Rigosertib promoted survival rate, ameliorated lung injury, and reduced inflammatory cytokine levels in serum of WT septic mice.

Conclusion

In contrast, the effects of rigosertib on sepsis were abrogated in septic MEK1^DD mice, which had inducible constitutive activation of MEK1 signaling. Rigosertib alleviated LPS‐induced sepsis inhibits MEK1/ERK signaling pathway.

TPL2 enforces RAS-induced inflammatory signaling and is activated by point mutations

NF-κB transcription factors, driven by the IRAK/IKK cascade, confer treatment resistance in pancreatic ductal adenocarcinoma (PDAC), a cancer characterized by near-universal KRAS mutation. Through reverse-phase protein array and RNA sequencing we discovered that IRAK4 also contributes substantially to MAPK activation in KRAS-mutant PDAC. IRAK4 ablation completely blocked RAS-induced transformation of human and murine cells. Mechanistically, expression of mutant KRAS stimulated an inflammatory, autocrine IL-1β signaling loop that activated IRAK4 and the MAPK pathway. Downstream of IRAK4, we uncovered TPL2 (also known as MAP3K8 or COT) as the essential kinase that propels both MAPK and NF-κB cascades. Inhibition of TPL2 blocked both MAPK and NF-κB signaling, and suppressed KRAS-mutant cell growth. To counter chemotherapy-induced genotoxic stress, PDAC cells upregulated TLR9, which activated prosurvival IRAK4/TPL2 signaling. Accordingly, a TPL2 inhibitor synergized with chemotherapy to curb PDAC growth in vivo. Finally, from TCGA we characterized 2 MAP3K8 point mutations that hyperactivate MAPK and NF-κB cascades by impeding TPL2 protein degradation. Cancer cell lines naturally harboring these MAP3K8 mutations are strikingly sensitive to TPL2 inhibition, underscoring the need to identify these potentially targetable mutations in patients. Overall, our study establishes TPL2 as a promising therapeutic target in RAS- and MAP3K8-mutant cancers and strongly prompts development of TPL2 inhibitors for preclinical and clinical studies.

Tumor Necrosis Factor Receptor-Associated Factor Regulation of Nuclear Factor κB and Mitogen-Activated Protein Kinase Pathways

Tumor necrosis factor receptor (TNFR)-associated factors (TRAFs) are a family of structurally related proteins that transduces signals from members of TNFR superfamily and various other immune receptors. Major downstream signaling events mediated by the TRAF molecules include activation of the transcription factor nuclear factor κB (NF-κB) and the mitogen-activated protein kinases (MAPKs). In addition, some TRAF family members, particularly TRAF2 and TRAF3, serve as negative regulators of specific signaling pathways, such as the noncanonical NF-κB and proinflammatory toll-like receptor pathways. Thus, TRAFs possess important and complex signaling functions in the immune system and play an important role in regulating immune and inflammatory responses. This review will focus on the role of TRAF proteins in the regulation of NF-κB and MAPK signaling pathways.

Regulation of NF-κB by the p105-ABIN2-TPL2 complex and RelAp43 during rabies virus infection

At the crossroad between the NF-κB and the MAPK pathways, the ternary complex composed of p105, ABIN2 and TPL2 is essential for the host cell response to pathogens. The matrix protein (M) of field isolates of rabies virus was previously shown to disturb the signaling induced by RelAp43, a NF-κB protein close to RelA/p65. Here, we investigated how the M protein disturbs the NF-κB pathway in a RelAp43-dependant manner and the potential involvement of the ternary complex in this mechanism. Using a tandem affinity purification coupled with mass spectrometry approach, we show that RelAp43 interacts with the p105-ABIN2-TPL2 complex and we observe a strong perturbation of this complex in presence of M protein. M protein interaction with RelAp43 is associated with a wide disturbance of NF-κB signaling, involving a modulation of IκBα-, IκBβ-, and IκBε-RelAp43 interaction and a favored interaction of RelAp43 with the non-canonical pathway (RelB and p100/p52). Monitoring the interactions between host and viral proteins using protein-fragment complementation assay and bioluminescent resonance energy transfer, we further show that RelAp43 is associated to the p105-ABIN2-TPL2 complex as RelAp43-p105 interaction stabilizes the formation of a complex with ABIN2 and TPL2. Interestingly, the M protein interacts not only with RelAp43 but also with TPL2 and ABIN2. Upon interaction with this complex, M protein promotes the release of ABIN2, which ultimately favors the production of RelAp43-p50 NF-κB dimers. The use of recombinant rabies viruses further indicates that this mechanism leads to the control of IFNβ, TNF and CXCL2 expression during the infection and a high pathogenicity profile in rabies virus infected mice. All together, our results demonstrate the important role of RelAp43 and M protein in the regulation of NF-κB signaling.

Rabies virus is a recurring zoonosis responsible for about 60,000 deaths per year. A key feature of rabies virus is its stealth, allowing it to spread within the host and escape the immune response. To do so, rabies virus developed several mechanisms, including a thorough interference with cell signaling pathways. Here, we focused our attention on the molecular aspects of rabies virus escape to the NF-κB pathway through the interaction between the M protein and the NF-κB protein RelAp43. Monitoring close range interactions, we found that RelAp43 plays an important role in the stabilization of the p105-ABIN2-TPL2 complex, which is essential in the regulation of both NF-κB and MAPK pathways, and we brought a new insight on the dynamics within the host protein complex. These results were confirmed in living cells and in mice. Overall, our data suggest that rabies virus interference with the p105-ABIN2-TPL2 complex is a cornerstone of its stealth strategy to escape the immune response.

Stimulation of nAchRα7 Receptor Inhibits TNF Synthesis and Secretion in Response to LPS Treatment of Mast Cells by Targeting ERK1/2 and TACE Activation

The cholinergic anti-inflammatory pathway is recognized as one of the main mechanisms of neuromodulation of the immune system. Activation of the α7 nicotinic acetylcholine receptor (nAchRα7) suppresses cytokine synthesis in distinct immune cells but the molecular mechanisms behind this effect remain to be fully described. Mast cells (MCs) are essential players of allergic reactions and innate immunity responses related to chronic inflammation. Activation of TLR4 receptor in MCs leads to the rapid secretion of pre-synthesized TNF from intracellular pools and to the activation of NFκB, necessary for de novo synthesis of TNF and other cytokines. Here we report that the nAchRα7 receptor specific agonist GTS-21 inhibits TLR4-induced secretion of preformed TNF from MCs in vivo and in vitro. Utilizing bone marrow-derived mast cells (BMMCs) it was found that GTS-21 also diminished secretion of de novo synthesized TNF, TNF mRNA accumulation and IKK-dependent p65-NFκB phosphorylation in response to LPS. nAchRα7 triggering prevented TLR4-induced ERK1/2 phosphorylation, which resulted an essential step for TNF secretion due to the phosphorylation of the metallopeptidase responsible for TNF maturation (TACE). Main inhibitory actions of GTS-21 were prevented by AG490, an inhibitor of JAK-2 kinase. Our results show for the first time, that besides the prevention of NFκB-dependent transcription, inhibitory actions of nAchRα7 triggering include the blockade of pathways leading to exocytosis of granule-stored cytokines in MCs.

NFKB1: a suppressor of inflammation, ageing and cancer

The pleiotropic consequences of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) pathway activation result from the combinatorial effects of the five subunits that form the homo- and heterodimeric NF-κB complexes. Although biochemical and gene knockout studies have demonstrated overlapping and distinct functions for these proteins, much is still not known about the mechanisms determining context-dependent functions, the formation of different dimer complexes and transcriptional control in response to diverse stimuli. Here we discuss recent results that reveal that the nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (NFKB1) (p105/p50) subunit is an important regulator of NF-κB activity in vivo. These effects are not restricted to being a dimer partner for other NF-κB subunits. Rather p50 homodimers have a critical role as suppressors of the NF-κB response, while the p105 precursor has a variety of NF-κB-independent functions. The importance of Nfkb1 function can be seen in mouse models, where Nfkb1(-/-) mice display increased inflammation and susceptibility to certain forms of DNA damage, leading to cancer, and a rapid ageing phenotype. In humans, low expression of Kip1 ubiquitination-promoting complex 1 (KPC1), a ubiquitin ligase required for p105 to p50 processing, was shown to correlate with a reduction in p50 and glioblastoma incidence. Therefore, while the majority of research in this field has focused on the upstream signalling pathways leading to NF-κB activation or the function of other NF-κB subunits, such as RelA (p65), these data demonstrate a critical role for NFKB1, potentially revealing new strategies for targeting this pathway in inflammatory diseases and cancer.

Lipopolysaccharide directly stimulates Th17 differentiation in vitro modulating phosphorylation of RelB and NF-κB1

Toll-like receptors (TLRs) recognize a wide range of pathogen-associated molecular patterns (PAMP) and are preferentially expressed in innate immune cells. TLR-mediated activation of these cells activates the adaptive immune system. However, it has become clear that TLRs are not only expressed but also functionally active in CD4 T cells. The intestines are continuously exposed to TLR ligands, including lipopolysaccharide (LPS), a TLR4 ligand, and TLR4 is expressed higher in Th17 cells than Th1 and Th2 cells. In addition, development of Th17 cells in the gut mucosa is more dependent on gut microbiota than Th1, Th2, and Treg. Thus, we examined whether LPS directly regulates Th17 differentiation. LPS directly stimulated Th17 differentiation in vitro. In Th17 cells, LPS increased phosphorylation of NF-κB1, resulting in an increase of p50, the processed form of NF-κB1, whereas it decreased phosphorylation of RelB, leading to the up-regulation of RelB. Subcutaneous injection of LPS increased the frequency of IL-17 producing cells in inguinal lymph nodes, worsening experimental autoimmune encephalomyelitis (EAE). Additionally, expression of TLR1, TLR2, TLR4, and TLR5 was reduced upon T cell activation and LPS showed modest effect on TLR4 expression. These findings provide the first evidence that TLR4 activation directly regulate Th17 differentiation.

IκB kinase-induced interaction of TPL-2 kinase with 14-3-3 is essential for Toll-like receptor activation of ERK-1 and -2 MAP kinases

The MEK-1/2 kinase TPL-2 is critical for Toll-like receptor activation of the ERK-1/2 MAP kinase pathway during inflammatory responses, but it can transform cells following C-terminal truncation. IκB kinase (IKK) complex phosphorylation of the TPL-2 C terminus regulates full-length TPL-2 activation of ERK-1/2 by a mechanism that has remained obscure. Here, we show that TPL-2 Ser-400 phosphorylation by IKK and TPL-2 Ser-443 autophosphorylation cooperated to trigger TPL-2 association with 14-3-3. Recruitment of 14-3-3 to the phosphorylated C terminus stimulated TPL-2 MEK-1 kinase activity, which was essential for TPL-2 activation of ERK-1/2. The binding of 14-3-3 to TPL-2 was also indispensible for lipopolysaccharide-induced production of tumor necrosis factor by macrophages, which is regulated by TPL-2 independently of ERK-1/2 activation. Our data identify a key step in the activation of TPL-2 signaling and provide a mechanistic insight into how C-terminal deletion triggers the oncogenic potential of TPL-2 by rendering its kinase activity independent of 14-3-3 binding.

FlowMax: A Computational Tool for Maximum Likelihood Deconvolution of CFSE Time Courses

The immune response is a concerted dynamic multi-cellular process. Upon infection, the dynamics of lymphocyte populations are an aggregate of molecular processes that determine the activation, division, and longevity of individual cells. The timing of these single-cell processes is remarkably widely distributed with some cells undergoing their third division while others undergo their first. High cell-to-cell variability and technical noise pose challenges for interpreting popular dye-dilution experiments objectively. It remains an unresolved challenge to avoid under- or over-interpretation of such data when phenotyping gene-targeted mouse models or patient samples. Here we develop and characterize a computational methodology to parameterize a cell population model in the context of noisy dye-dilution data. To enable objective interpretation of model fits, our method estimates fit sensitivity and redundancy by stochastically sampling the solution landscape, calculating parameter sensitivities, and clustering to determine the maximum-likelihood solution ranges. Our methodology accounts for both technical and biological variability by using a cell fluorescence model as an adaptor during population model fitting, resulting in improved fit accuracy without the need for ad hoc objective functions. We have incorporated our methodology into an integrated phenotyping tool, FlowMax, and used it to analyze B cells from two NFκB knockout mice with distinct phenotypes; we not only confirm previously published findings at a fraction of the expended effort and cost, but reveal a novel phenotype of nfkb1/p105/50 in limiting the proliferative capacity of B cells following B-cell receptor stimulation. In addition to complementing experimental work, FlowMax is suitable for high throughput analysis of dye dilution studies within clinical and pharmacological screens with objective and quantitative conclusions.

Macrophages in multiple myeloma: emerging concepts and therapeutic implications

Multiple myeloma, a clonal plasma cell malignancy, has long provided a prototypic model to study regulatory interactions between malignant cells and their microenvironment. Myeloma-associated macrophages have historically received limited scrutiny, but recent work points to central and non-redundant roles in myeloma niche homeostasis. The evidence supports a paradigm of complex, dynamic and often mutable interactions between macrophages and other cellular constituents of the niche. We and others have shown that macrophages support myeloma cell growth, viability and drug resistance through both contact-mediated and non-contact-mediated mechanisms. These tumor-beneficial roles have evolved in opposition to, or in parallel with, intrinsic pro-inflammatory and tumoricidal properties. Thus, simple blockade of protective "don't eat me" signals on the surface of myeloma cells leads to macrophage-mediated myeloma cell killing. Macrophages also enhance the tumor-supportive role of mesenchymal stem/stromal cells (MSCs) in the niche: importantly, this interaction is bidirectional, producing a distinct state of macrophage polarization that we termed "MSC-educated macrophages." The intriguing pattern of cross-talk between macrophages, MSCs and tumor cells highlights the myeloma niche as a dynamic multi-cellular structure. Targeted reprogramming of these interactions harbors significant untapped therapeutic potential, particularly in the setting of minimal residual disease, the main obstacle toward a cure.

Coordinate regulation of TPL-2 and NF-κB signaling in macrophages by NF-κB1 p105

The role of IκB kinase (IKK)-induced proteolysis of NF-κB1 p105 in innate immune signaling was investigated using macrophages from Nfkb1(SSAA/SSAA) mice, in which the IKK target serines on p105 are mutated to alanines. We found that the IKK/p105 signaling pathway was essential for TPL-2 kinase activation of extracellular signal-regulated kinase (ERK) mitogen-activate protein (MAP) kinase and modulated the activation of NF-κB. The Nfkb1(SSAA) mutation prevented the agonist-induced release of TPL-2 from its inhibitor p105, which blocked activation of ERK by lipopolysaccharide (LPS), tumor necrosis factor (TNF), CpG, tripalmitoyl-Cys-Ser-Lys (Pam(3)CSK), poly(I · C), flagellin, and R848. The Nfkb1(SSAA) mutation also prevented LPS-induced processing of p105 to p50 and reduced p50 levels, in addition to decreasing the nuclear translocation of RelA and cRel. Reduced p50 in Nfkb1(SSAA/SSAA) macrophages significantly decreased LPS induction of the IκBζ-regulated Il6 and Csf2 genes. LPS upregulation of Il12a and Il12b mRNAs was also impaired although specific blockade of TPL-2 signaling increased expression of these genes at late time points. Activation of TPL-2/ERK signaling by IKK-induced p105 proteolysis, therefore, induced a negative feedback loop to downregulate NF-κB-dependent expression of the proinflammatory cytokine interleukin-12 (IL-12). Unexpectedly, TPL-2 promoted soluble TNF production independently of IKK-induced p105 phosphorylation and its ability to activate ERK, which has important implications for the development of anti-inflammatory drugs targeting TPL-2.

Tpl2 kinase signal transduction in inflammation and cancer

The activation of mitogen-activated protein kinases (MAPKs) is critically involved in inflammatory and oncogenic events. Tumor progression locus 2 (Tpl2), also known as COT and MAP3 kinase 8 (MAP3K8), is a serine-threonine kinase with an important physiological role in tumor necrosis factor, interleukin-1, CD40, Toll-like receptor and G protein-coupled receptor-mediated ERK MAPK signaling. Whilst the full characterization of the biochemical events that lead to the activation of Tpl2 still represent a major challenge, genetic and molecular evidence has highlighted interesting interactions with the NF-κB network. Here, we provide an overview of the multifaceted functions of Tpl2 and the molecular mechanisms that govern its regulation.

Regulation and function of TPL-2, an IκB kinase-regulated MAP kinase kinase kinase

The IκB kinase (IKK) complex plays a well-documented role in innate and adaptive immunity. This function has been widely attributed to its role as the central activator of the NF-κB family of transcription factors. However, another important consequence of IKK activation is the regulation of TPL-2, a MEK kinase that is required for activation of ERK-1/2 MAP kinases in myeloid cells following Toll-like receptor and TNF receptor stimulation. In unstimulated cells, TPL-2 is stoichiometrically complexed with the NF-κB inhibitory protein NF-κB1 p105, which blocks TPL-2 access to its substrate MEK, and the ubiquitin-binding protein ABIN-2 (A20-binding inhibitor of NF-κB 2), both of which are required to maintain TPL-2 protein stability. Following agonist stimulation, the IKK complex phosphorylates p105, triggering its K48-linked ubiquitination and degradation by the proteasome. This releases TPL-2 from p105-mediated inhibition, facilitating activation of MEK, in addition to modulating NF-κB activation by liberating associated Rel subunits for translocation into the nucleus. IKK-induced proteolysis of p105, therefore, can directly regulate both NF-κB and ERK MAP kinase activation via NF-κB1 p105. TPL-2 is critical for production of the proinflammatory cytokine TNF during inflammatory responses. Consequently, there has been considerable interest in the pharmaceutical industry to develop selective TPL-2 inhibitors as drugs for the treatment of TNF-dependent inflammatory diseases, such as rheumatoid arthritis and inflammatory bowel disease. This review summarizes our current understanding of the regulation of TPL-2 signaling function, and also the complex positive and negative roles of TPL-2 in immune and inflammatory responses.

NFX1 plays a role in human papillomavirus type 16 E6 activation of NFkappaB activity

High-risk human papillomavirus (HR HPV) requires differentiating epithelial cells to continue to divide in order to replicate the viral DNA. To achieve this, HPV perturbs several regulatory pathways, including cellular apoptosis and senescence signals. HPV E6 has been identified as a regulator of the NFκB signaling pathway, a pathway important in many cellular processes, as well as regulation of virus-host cell interactions. We report here that NFX1-91, an endogenously expressed transcriptional regulator of human telomerase reverse transcriptase (hTERT) that is targeted by HPV type 16 (HPV16) E6/E6-associated protein (E6AP) for degradation, is also critical for regulation of the NFκB pathway by HPV16 E6. Microarray analysis revealed induction of NFκB-responsive genes and reduction of NFκB inhibitors with knockdown of NFX1-91. Knockdown of NFX1-91 induced downregulation of p105, an NFκB inhibitor in both primary human foreskin keratinocytes (HFKs) and HCT116 cells. Chromatin immunoprecipitation assays further confirmed that NFX1-91 bound to the p105 promoter and upregulated its expression. Similarly, in HPV16 E6-positive cells, reduction of p105 expression was observed, paralleling knockdown of NFX1-91 expression. Overall, our data suggest a mechanism for HPV16 E6 activation of the NFκB pathway through NFX1-91. Also, it provides evidence that NFX1-91 can function as a dual regulator, not only a transcriptional repressor, but also a transcriptional activator, when bound to DNA.

TPL-2-mediated activation of MAPK downstream of TLR4 signaling is coupled to arginine availability

The innate immune response is influenced by the nutrient status of the host. Mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinase 1 (ERK1) and ERK2, are activated after the stimulation of macrophages with bacterial lipopolysaccharide (LPS) and are necessary for the optimal production of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha). We uncovered a role for the extracellular nutrient arginine in the activation of ERK1/2 in LPS-stimulated macrophages. Arginine facilitated the activation of MAPKs by preventing the dephosphorylation and inactivation of the MAPK kinase kinase tumor-promoting locus 2 (TPL-2). Starvation of mice decreased the concentration of arginine in the plasma and impaired the activation of ERK1/2 by LPS. Supplementation of starved mice with arginine promoted the subsequent activation of ERK1/2 and the production of TNF-alpha in response to LPS. Thus, arginine is critical for two aspects of the innate immune response in macrophages: It is the precursor used in the generation of the antimicrobial mediator nitric oxide, and it facilitates MAPK activation and consequently cytokine production.

Deregulation of Tpl2 and NF-kappaB signaling and induction of macrophage apoptosis by the anti-depressant drug lithium

Lithium is an anti-depressant drug that also possesses immunomodulatory functions. The anti-inflammatory effect of lithium is thought to involve activation of the transcription factor CREB, although the underlying mechanism is incompletely understood. We show here that in macrophages lithium stimulates Tpl2, a MAP kinase kinase kinase (MAP3K) known to mediate activation of extracellular signal regulated kinase (ERK) and the downstream target CREB. Lithium activates Tpl2 by inducing degradation of p105, an NF-kappaB precursor protein that functions as a physiological inhibitor of Tpl2. This novel function of lithium does not involve inhibition of a well-characterized lithium target, GSK3beta, since other known GSK3beta inhibitors do not induce p105 degradation or Tpl2 activation. Lithium also promotes the activation of Tpl2 and ERK by the TLR4 ligand LPS. On the other hand, prolonged incubation of macrophages with lithium results in dramatic loss of p105 and inhibition of LPS-stimulated NF-kappaB activation. Consequently, lithium both attenuates LPS-mediated pro-inflammatory gene induction and induces apoptosis in macrophages. These results provide novel insight into the anti-inflammatory function of lithium.

Extracellular signal-regulated kinase regulation of tumor necrosis factor-alpha mRNA nucleocytoplasmic transport requires TAP-NxT1 binding and the AU-rich element

Tumor necrosis factor-alpha (TNF-alpha) production is regulated by transcriptional and posttranscriptional mechanisms. Lipopolysaccharide activates the NFkappaB pathway increasing TNF-alpha transcription. Lipopolysaccharide also activates the mitogen-activated protein kinase pathways, resulting in stabilization and enhanced translation of the TNF-alpha message. In addition, nuclear export of the TNF-alpha mRNA is a posttranscriptionally regulated process involving the Tpl2-ERK pathway and requiring the presence of the TNF-alpha AU-rich element (ARE). We demonstrate that nuclear export of the TNF-alpha message requires not only the TNF-alpha ARE but also the interaction of the proteins TAP and NxT1, both of which are involved in nucleocytoplasmic transport of mRNA. Through the use of dominant negative ERK1 and ERK2, we establish that control of TNF-alpha mRNA nuclear export operates specifically through ERK1. Finally, we examined the role of two established TNF-alpha ARE-binding proteins, HuR and tristetraprolin, that shuttle between the nucleus and cytoplasm. These data demonstrate that neither tristetraprolin nor HuR is required for TNF-alpha mRNA export. It is unclear at this time if ARE-binding protein(s) directly interact with the TAP-NxT1 complex, if each complex is independently targeted by ERK1, or if only one complex is targeted.

Phosphorylation of TPL-2 on serine 400 is essential for lipopolysaccharide activation of extracellular signal-regulated kinase in macrophages

Tumor progression locus 2 (TPL-2) kinase is essential for Toll-like receptor 4 activation of the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) and for upregulation of the inflammatory cytokine tumor necrosis factor (TNF) in lipopolysaccharide (LPS)-stimulated macrophages. LPS activation of ERK requires TPL-2 release from associated NF-kappaB1 p105, which blocks TPL-2 access to its substrate, the ERK kinase MEK. Here we demonstrate that TPL-2 activity is also regulated independently of p105, since LPS stimulation was still needed for TPL-2-dependent activation of ERK in Nfkb1(-/-) macrophages. In wild-type macrophages, LPS induced the rapid phosphorylation of serine (S) 400 in the TPL-2 C-terminal tail. Mutation of this conserved residue to alanine (A) blocked the ability of retrovirally expressed TPL-2 to induce the activation of ERK in LPS-stimulated Nfkb1(-/-) macrophages. TPL-2(S400A) expression also failed to reconstitute LPS activation of ERK and induction of TNF in Map3k8(-/-) macrophages, which lack endogenous TPL-2. Consistently, the S400A mutation was found to block LPS stimulation of TPL-2 MEK kinase activity. Thus, induction of TPL-2 MEK kinase activity by LPS stimulation of macrophages requires TPL-2 phosphorylation on S400, in addition to its release from NF-kappaB1 p105. Oncogenic C-terminal truncations of TPL-2 that remove S400 could promote its transforming potential by eliminating this critical control step.

Coordinating TLR‐activated signaling pathways in cells of the immune system

Toll-like receptor (TLR) signaling leads to the activation of mitogen-activated protein kinase and nuclear factor-kappaB signaling pathways. While the upstream signaling events initiated at the level of adaptors and the activation of the downstream signaling pathways have received a lot of attention, our understanding of how these signaling pathways are coordinated to regulate gene expression is poorly understood. This review gives a selective overview on our current understanding of signaling downstream of TLRs, with an emphasis on how the upstream kinases like the mitogen-activated protein kinase kinase kinases (TAK1 and Tpl2) and inhibitor of kappa-B kinase (IKK) coordinate the signaling events that steer the course of an immune response.

Differential regulation and properties of MAPKs

Mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs including embryogenesis, proliferation, differentiation and apoptosis based on cues derived from the cell surface and the metabolic state and environment of the cell. In mammals, there are more than a dozen MAPK genes. The best known are the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK(1-3)) and p38(alpha, beta, gamma and delta) families. ERK3, ERK5 and ERK7 are other MAPKs that have distinct regulation and functions. MAPK cascades consist of a core of three protein kinases. Despite the apparently simple architecture of this pathway, these enzymes are capable of responding to a bewildering number of stimuli to produce exquisitely specific cellular outcomes. These responses depend on the kinetics of their activation and inactivation, the subcellular localization of the kinases, the complexes in which they act, and the availability of substrates. Fine-tuning of cascade activity can occur through modulatory inputs to cascade component from the primary kinases to the scaffolding accessory proteins. Here, we describe some of the properties of the three major MAPK pathways and discuss how these properties govern pathway regulation and activity.

Tumor progression locus-2 is a critical regulator of pancreatic and lung inflammation during acute pancreatitis

Pancreatic and lung inflammation during acute pancreatitis is a poorly understood, but clinically important, phenomenon. The proto-oncogene Tpl2 (tumor progression locus-2) has recently been shown to have important immunomodulatory effects on some inflammatory processes, but its importance to pancreatitis has not been previously examined. Our studies were designed to (a) define the effects of Tpl2 on pancreatic and lung inflammation during pancreatitis and (b) identify mechanisms and cell types responsible for those effects. We examined pancreatitis-associated Tpl2 effects in wild type and Tpl2(-/-) mice subjected to either secretagogue-induced or bile salt-induced pancreatitis. To determine the myeloid or non-myeloid lineage of cells responsible for the Tpl2 effects, we used Tpl2(-/-) chimeric mice generated by lethal irradiation followed by bone marrow transplantation. Mechanisms responsible for the effects of Tpl2 ablation on caerulein-induced proinflammatory events were evaluated under in vivo and in vitro conditions using the techniques of electrophoretic mobility shift assay, immunoblot analysis, and quantitative reverse transcription-PCR. We found that Tpl2 ablation markedly reduced pancreatic and lung inflammation in these two dissimilar models of pancreatitis, but it did not alter pancreatic injury/necrosis in either model. The reduction in caerulein-induced pancreatic inflammation is dependent upon Tpl2 ablation in non-myeloid cells and is associated with both in vivo and in vitro inhibition of MEK, JNK, and AP-1 activation and the expression of MCP-1, MIP-2, and interleukin-6. Non-myeloid cell expression of Tpl2 regulates pancreatic inflammation during pancreatitis by mediating proinflammatory signals and the generation of neutrophil chemoattracting factors.

Post-translational modifications regulating the activity and function of the nuclear factor kappa B pathway

The diverse cellular and biological functions of the nuclear factor kappa B (NF-kappaB) pathway, together with the catastrophic consequences of its aberrant regulation, demand specific and highly regulated control of its activity. As described in this review, regulation of the NF-kappaB pathway is brought about through multiple post-translational modifications that control the activity of the core components of NF-kappaB signaling: the IkappaB kinase (IKK) complex, the IkappaB proteins and the NF-kappaB subunits themselves. These regulatory modifications, which include phosphorylation, ubiquitination, acetylation, sumoylation and nitrosylation, can vary, depending on the nature of the NF-kappaB-inducing stimulus. Moreover, they frequently have distinct, sometimes antagonistic, functional consequences and the same modification can have different effects depending on the context. Given the important role of NF-kappaB in human health and disease, understanding these pathways will not only provide valuable insights into mechanism and function, but could also lead to new drug targets and the development of diagnostic and prognostic biomarkers for many pathological conditions.

Arrestin-2 and G protein-coupled receptor kinase 5 interact with NFkappaB1 p105 and negatively regulate lipopolysaccharide-stimulated ERK1/2 activation in macrophages

Toll-like receptors (TLRs) are a recently described receptor class involved in the regulation of innate and adaptive immunity. Here, we demonstrate that arrestin-2 and GRK5 (G protein-coupled receptor kinase 5), proteins that regulate G protein-coupled receptor signaling, play a negative role in TLR4 signaling in Raw264.7 macrophages. We find that lipopolysaccharide (LPS)-induced ERK1/2 phosphorylation is significantly enhanced in arrestin-2 and GRK5 knockdown cells. To elucidate the mechanisms involved, we tested the effect of arrestin-2 and GRK5 knockdown on LPS-stimulated signaling components that are upstream of ERK phosphorylation. Upon LPS stimulation, IkappaB kinase promotes phosphorylation and degradation of NFkappaB1 p105 (p105), which releases TPL2 (a MAP3K), which phosphorylates MEK1/2, which in turn phosphorylates ERK1/2. We demonstrate that knockdown of arrestin-2 leads to enhanced LPS-induced phosphorylation and degradation of p105, enhanced TPL2 release, and enhanced MEK1/2 phosphorylation. GRK5 knockdown also results in enhanced IkappaB kinase-mediated p105 phosphorylation and degradation, whereas GRK2 and GRK6 knockdown have no effect on this pathway. In vitro analysis demonstrates that arrestin-2 directly binds to the COOH-terminal domain of p105, whereas GRK5 binds to and phosphorylates p105. Taken together, these results suggest that p105 phosphorylation by GRK5 and binding of arrestin-2 negatively regulates LPS-stimulated ERK activation. These results reveal that arrestin-2 and GRK5 are important negative regulatory components in TLR4 signaling.