Protein kinase Calpha is involved in interferon regulatory factor 3 activation and type I interferon-beta synthesis

Cyclic AMP response element-binding protein (CREB) transcription factor in astrocytic synaptic communication

Astrocytes are known to actively participate in synaptic communication by forming structures called tripartite synapses. These synapses consist of presynaptic axon terminals, postsynaptic dendritic spines, and astrocytic processes where astrocytes release and receive transmitters. Although the transcription factor cyclic AMP response element (CRE)-binding protein (CREB) has been actively studied as an important factor for mediating synaptic activity-induced responses in neurons, its role in astrocytes is relatively unknown. Synaptic signals are known to activate various downstream pathways in astrocytes, which can activate the CREB transcription factor. Therefore, there is a need to summarize studies on astrocytic intracellular pathways that are induced by synaptic communication resulting in activation of the CREB pathway. In this review, we discuss the various neurotransmitter receptors and intracellular pathways that can induce CREB activation and CREB-induced gene regulation in astrocytes.

Desmosomes as Signaling Hubs in the Regulation of Cell Behavior

Desmosomes are intercellular junctions, which preserve tissue integrity during homeostatic and stress conditions. These functions rely on their unique structural properties, which enable them to respond to context-dependent signals and transmit them to change cell behavior. Desmosome composition and size vary depending on tissue specific expression and differentiation state. Their constituent proteins are highly regulated by posttranslational modifications that control their function in the desmosome itself and in addition regulate a multitude of desmosome-independent functions. This review will summarize our current knowledge how signaling pathways that control epithelial shape, polarity and function regulate desmosomes and how desmosomal proteins transduce these signals to modulate cell behavior.

Signal Transduction in Immune Cells and Protein Kinases

Immune response relies upon several intracellular signaling events. Among the protein kinases involved in these pathways, members of the protein kinase C (PKC) family are prominent molecules because they have the capacity to acutely and reversibly modulate effector protein functions, controlling both spatial distribution and dynamic properties of the signals. Different PKC isoforms are involved in distinct signaling pathways, with selective functions in a cell-specific manner.In innate system, Toll-like receptor signaling is the main molecular event triggering effector functions. Various isoforms of PKC can be common to different TLRs, while some of them are specific for a certain type of TLR. Protein kinases involvement in innate immune cells are presented within the chapter emphasizing their coordination in many aspects of immune cell function and, as important players in immune regulation.In adaptive immunity T-cell receptor and B-cell receptor signaling are the main intracellular pathways involved in seminal immune specific cellular events. Activation through TCR and BCR can have common intracellular pathways while others can be specific for the type of receptor involved or for the specific function triggered. Various PKC isoforms involvement in TCR and BCR Intracellular signaling will be presented as positive and negative regulators of the immune response events triggered in adaptive immunity.

Combating herpesvirus encephalitis by potentiating a TLR3-mTORC2 axis

TLR3 is a sensor of double-stranded RNA that is indispensable for defense against infection with herpes simplex virus type 1 (HSV-1) in the brain. We found here that TLR3 was required for innate immune responses to HSV-1 in neurons and astrocytes. During infection with HSV-1, TLR3 recruited the metabolic checkpoint kinase complex mTORC2, which led to the induction of chemokines and trafficking of TLR3 to the cell periphery. Such trafficking enabled the activation of molecules (including mTORC1) required for the induction of type I interferons. Intracranial infection of mice with HSV-1 was exacerbated by impairment of TLR3 responses with an inhibitor of mTOR and was significantly 'rescued' by potentiation of TLR3 responses with an agonistic antibody to TLR3. These results suggest that the TLR3-mTORC2 axis might be a therapeutic target through which to combat herpes simplex encephalitis.

HDAC6 regulates cellular viral RNA sensing by deacetylation of RIG-I

RIG-I is a key cytosolic sensor that detects RNA viruses through its C-terminal region and activates the production of antiviral interferons (IFNs) and proinflammatory cytokines. While posttranslational modification has been demonstrated to regulate RIG-I signaling activity, its significance for the sensing of viral RNAs remains unclear. Here, we first show that the RIG-I C-terminal region undergoes deacetylation to regulate its viral RNA-sensing activity and that the HDAC6-mediated deacetylation of RIG-I is critical for viral RNA detection. HDAC6 transiently bound to RIG-I and removed the lysine 909 acetylation in the presence of viral RNAs, promoting RIG-I sensing of viral RNAs. Depletion of HDAC6 expression led to impaired antiviral responses against RNA viruses, but not against DNA viruses. Consequently, HDAC6 knockout mice were highly susceptible to RNA virus infections compared to wild-type mice. These findings underscore the critical role of HDAC6 in the modulation of the RIG-I-mediated antiviral sensing pathway.

miR-24, miR-30b, and miR-142-3p regulate phagocytosis in myeloid inflammatory cells

Micro-RNAs (miRNAs) are small noncoding RNAs that regulate various biological pathways. As their role in phagocytosis remains poorly understood, we investigated their impact on phagocytosis in myeloid inflammatory cells. Seven miRNAs (miR-24, -30b, -101, 142-3p, -652-3p, -652-5p, and -1275) that were differentially expressed during monocyte to macrophage (Mφ) and monocyte to dendritic cell (DC) differentiation were screened for their potential role in phagocytosis. Among these, overexpression of miR-24, miR-30b, and miR-142-3p in human monocyte-derived Mφ, DC, monocytes, and PBMCs significantly attenuate phagocytosis of Escherichia coli and Staphylococcus aureus, as well as the secretion of inflammatory mediators, including TNF-α, IL-6, and IL-12p40. miRNA-mediated changes in cytokine profiles were observed at transcriptional and/or posttranscriptional levels and importantly exhibit miRNA-specific impact. To examine the underlying mechanism, we monitored the expression of phagocytosis pathway-associated genes and identified several genes that were altered in Mφ and DC transfected with miR-24, miR-30b, and miR-142-3p mimics. Some of these genes with altered expression also harbor putative miRNA binding sites. We show that miR-142-3p directly regulates protein kinase Cα (PKCα), a key gene involved in phagocytosis. Interestingly, miR-142-3p and PKCα exhibit antagonistic expression during Mφ and DC differentiation. Short interfering RNA-mediated knockdown of PKCα in Mφ leads to reduced bacterial uptake, further highlighting the role of the gene in phagocytosis. Overall, these results demonstrate that miR-24, miR-30b, and miR-142-3p regulate phagocytosis and associated cytokine production in myeloid inflammatory cells through modulation of various genes involved in the pathway.

Adenosine triphosphate hydrolysis reduces neutrophil infiltration and necrosis in partial-thickness scald burns in mice

Extracellular adenosine triphosphate (ATP), present in thermally injured tissue, modulates the inflammatory response and causes significant tissue damage. The authors hypothesize that neutrophil infiltration and ensuing tissue necrosis would be mitigated by removing ATP-dependent signaling at the burn site. Mice were subjected to 30% TBSA partial-thickness scald burn by dorsal skin immersion in a water bath at 60 or 20°C (nonburn controls). In the treatment arm, an ATP hydrolyzing enzyme, apyrase, was applied directly to the site immediately after injury. Skin was harvested after 24 hours and 5 days for hematoxylin and eosin stain, elastase, and Ki-67 staining. Tumor necrosis factor (TNF)-α and interferon (IFN)-β expression were measured through quantitative real-time polymerase chain reaction. At 24 hours, the amount of neutrophil infiltration was different between the burn and burn + apyrase groups (P < .001). Necrosis was less extensive in the apyrase group when compared with the burn group at 24 hours and 5 days. TNF-α and IFN-β expression at 24 hours in the apyrase group was lower than in the burn group (P < .05). However, Ki-67 signaling was not significantly different among the groups. The results of this study support the role of extracellular ATP in neutrophil activity. The authors demonstrate that ATP hydrolysis at the burn site allays the neutrophil response to thermal injury and reduces tissue necrosis. This decrease in inflammation and tissue necrosis is at least partially because of TNF-α and IFN-β signaling. Apyrase could be used as topical inflammatory regulators to quell the injury caused by inflammation.

Bruton's tyrosine kinase and protein kinase C µ are required for TLR7/9-induced IKKα and IRF-1 activation and interferon-β production in conventional dendritic cells

Stimulation of TLR7/9 by their respective ligands leads to the activation of IκB kinase α (IKKα) and Interferon Regulatory Factor 1 (IRF-1) and results in interferon (IFN)-β production in conventional dendritic cells (cDC). However, which other signaling molecules are involved in IKKα and IRF-1 activation during TLR7/9 signaling pathway are not known. We and others have shown that Bruton's Tyrosine Kinase (BTK) played a part in TLR9-mediated cytokine production in B cells and macrophages. However, it is unclear if BTK participates in TLR7/9-induced IFN-β production in cDC. In this study, we show that BTK is required for IFN-β synthesis in cDC upon TLR7/9 stimulation and that stimulated BTK-deficient cDC are defective in the induction of IKKα/β phosphorylation and IRF-1 activation. In addition, we demonstrate that Protein Kinase C µ (PKCµ) is also required for TLR7/9-induced IRF-1 activation and IFN-β upregulation in cDC and acts downstream of BTK. Taken together, we have uncovered two new molecules, BTK and PKCµ, that are involved in TLR7/9-triggered IFN-β production in cDC.

Toll-like receptor 3 activation promotes desensitization of histamine response in human gingival fibroblasts: Poly (I:C) induces histamine receptor desensitization in human gingival fibroblasts

Viruses are associated with the development of periodontal disease, particularly during periods of suppressed cellular immunity. For this reason, we evaluated the hypothesis that viral components regulate the actions of histamine, an important mediator of immune responses. We assessed the effect of Poly (I:C) on histamine-mediated intracellular calcium mobilization in human gingival fibroblasts. Our results show that histamine induces an increase in intracellular calcium concentrations in a dose-dependent manner. This response was blocked when cells were incubated in the presence of Poly (I:C). In addition, phorbol esters, a diacylglycerol analog, mimics the inhibitory actions of Poly (I:C) in response to histamine. The effect of Poly (I:C) was reversed by Stuarosporine (1 μM), GÖ6983 (7 μM), Bisindolylmaleimide (1 μM) [a protein inhibitor (PKC)], and SB 203580 (3 μM) (a p38-MAPK inhibitor). These findings suggest that Poly (I:C) regulates histamine-induced calcium mobilization through activation of PKC and p38.

Polyinosinic:polycytidylic acid induces protein kinase D-dependent disassembly of apical junctions and barrier dysfunction in airway epithelial cells

BACKGROUND

Disruption of the epithelial barrier might be a risk factor for allergen sensitization and asthma. Viral respiratory tract infections are strongly associated with asthma exacerbation, but the effects of respiratory viruses on airway epithelial barrier function are not well understood. Many viruses generate double-stranded RNA, which can lead to airway inflammation and initiate an antiviral immune response.

OBJECTIVES

We investigated the effects of the synthetic double-stranded RNA polyinosinic:polycytidylic acid (polyI:C) on the structure and function of the airway epithelial barrier in vitro.

METHODS

16HBE14o- human bronchial epithelial cells and primary airway epithelial cells at an air-liquid interface were grown to confluence on Transwell inserts and exposed to polyI:C. We studied epithelial barrier function by measuring transepithelial electrical resistance and paracellular flux of fluorescent markers and structure of epithelial apical junctions by means of immunofluorescence microscopy.

RESULTS

PolyI:C induced a profound decrease in transepithelial electrical resistance and increase in paracellular permeability. Immunofluorescence microscopy revealed markedly reduced junctional localization of zonula occludens-1, occludin, E-cadherin, β-catenin, and disorganization of junction-associated actin filaments. PolyI:C induced protein kinase D (PKD) phosphorylation, and a PKD antagonist attenuated polyI:C-induced disassembly of apical junctions and barrier dysfunction.

CONCLUSIONS

PolyI:C has a powerful and previously unsuspected disruptive effect on the airway epithelial barrier. PolyI:C-dependent barrier disruption is mediated by disassembly of epithelial apical junctions, which is dependent on PKD signaling. These findings suggest a new mechanism potentially underlying the associations between viral respiratory tract infections, airway inflammation, and allergen sensitization.

PKC alpha regulates Sendai virus-mediated interferon induction through HDAC6 and β-catenin

Recognition of viral RNA by cytoplasmic retinoic acid inducible gene I (RIG-I)-like receptors initiates signals leading to the induction of type I interferon (IFN) transcription via transcription factors such as interferon regulatory factor 3 (IRF3) and nuclear factor κB (NF-κB). Here, we describe a new signalling pathway that involves protein kinase C alpha (PKCα), histone deacetylase 6 (HDAC6) and beta-catenin (β-catenin), which is essential for IFN gene induction following virus infection. Knockdown of PKCα in various human cells, including primary cells, inhibited Sendai virus (SeV)-mediated IFN induction and enhanced virus replication. In the absence of this pathway IRF3 becomes activated, but does not bind to its promoter and is thus unable to support transcription. Mechanistically, SeV infection induced the activation of PKCα, which promoted its interaction with HDAC6 and enhanced its deacetylation activity in a phosphorylation-dependent manner. Further downstream, HDAC6 caused deacetylation of β-catenin and enhanced its nuclear translocation and promoter binding. In the nucleus, β-catenin acted as a co-activator for IRF3-mediated transcription. Our findings suggest an important role of a novel signalling pathway mediated by PKCα-HDAC6-β-catenin in controlling IRF3-mediated transcription.

Protein kinase C and toll-like receptor signaling

Protein kinase C (PKC) is a family of kinases that are implicated in a plethora of diseases, including cancer and cardiovascular disease. PKC isoforms can have different, and sometimes opposing, effects in these disease states. Toll-like receptors (TLRs) are a family of pattern recognition receptors that bind pathogens and stimulate the secretion of cytokines. It has long been known that PKC inhibitors reduce LPS-stimulated cytokine secretion by macrophages, linking PKC activation to TLR signaling. Recent studies have shown that PKC-α, -δ, -ε, and -ζ are directly involved in multiple steps in TLR pathways. They associate with the TLR or proximal components of the receptor complex. These isoforms are also involved in the downstream activation of MAPK, RhoA, TAK1, and NF-κB. Thus, PKC activation is intimately involved in TLR signaling and the innate immune response.

A conventional protein kinase C inhibitor targeting IRF-3-dependent genes differentially regulates IL-12 family members

Protein kinase C (PKC) isoforms play a critical role in the regulation of innate immune responses. We have previously demonstrated that conventional PKC (cPKC) α is involved in interferon regulatory factor 3 (IRF-3) activation and IFN-β synthesis. Herein, we investigated the role of cPKCs in the regulation of IL-12 family members expression mediated by the Toll-like receptor 3 (TLR3) and TLR4. First, inhibition of cPKCs activity in human DCs by a cPKC-specific inhibitor, Gö6976 downregulated the expression of IL-12p70 and IL-27p28 but not IL-12/IL-23p40, IL-23, IL-27EBI3 induced by LPS or poly(I:C). Furthermore, reporter gene assays in RAW 264.7 macrophages showed that cPKCs regulate IL-12p35 and IL-27p28 promoter activities since Gö6976 repressed LPS and poly(I:C)-mediated transcriptional activities of IL-12p35 and IL-27p28. In contrast, no effect was observed with IL-12/IL-23p40 and IL-23p19 reporter constructs. These results prompted us to study the role of IRF-3 on IL-23 expression. Bone marrow-derived DC (BMDCs) from IRF-3(-/-) mice produced comparable levels of IL-23 induced by both LPS and poly(I:C) as compared to wild type BMDCs, indicating that IRF-3 is not involved in IL-23 production. Finally, BMDCs from PKCα(-/-) mice displayed a reduced synthesis of IL-27 induced by poly(I:C). Collectively, these data identify cPKCs as critical components that control IRF-3-dependent IL-12p35 and IL-27p28 gene expression downstream of TLR3 and TLR4.

CKD712, (S)-1-(α-naphthylmethyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, inhibits the lipopolysaccharide-stimulated secretion of HMGB1 by inhibiting PI3K and classical protein kinase C

CKD712, (S)-1-(α-naphthylmethyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, was considered as a new effective drug candidate to sepsis, based on its anti-inflammatory activity. It was reported that CKD712 inhibited various signal pathways which play a key role in production of proinflammatory cytokines. Here, we examined the effect of CKD712 on the secretion of high mobility group box 1 (HMGB1), which is one of the proinflammatory cytokines. CKD712 can reduce Gram-negative lipopolysaccharide (LPS)- and Gram-positive lipoteichoic acid (LTA)-stimulated HMGB1 secretion in RAW264.7 and human peripheral blood monocytes (PBMo), and also reduce LPS-induced nucleocytoplasmic translocation of HMGB1 1h before or after LPS treatment. CKD712 could dose-dependently inhibit the activation of PI3K and PI3K-dependent kinase 1 (PDK1), which are involved in HMGB1 secretion signaling pathway. In addition, CKD712 inhibited classical protein kinase C (cPKC), the effective kinase for phosphorylation of HMGB1 for secretion, however, had no effect on histone acetyl-transferase activity, which is another mechanism known for HMGB1 secretion. Thus, we suggest that CKD712 could inhibit LPS- and LTA-stimulated HMGB1 secretion through the inhibition of HMGB1 phosphorylation by inhibiting PI3K-PKC signaling pathway.

Requirement of NOX2 and reactive oxygen species for efficient RIG-I-mediated antiviral response through regulation of MAVS expression

The innate immune response is essential to the host defense against viruses, through restriction of virus replication and coordination of the adaptive immune response. Induction of antiviral genes is a tightly regulated process initiated mainly through sensing of invading virus nucleic acids in the cytoplasm by RIG-I like helicases, RIG-I or Mda5, which transmit the signal through a common mitochondria-associated adaptor, MAVS. Although major breakthroughs have recently been made, much remains unknown about the mechanisms that translate virus recognition into antiviral genes expression. Beside the reputed detrimental role, reactive oxygen species (ROS) act as modulators of cellular signaling and gene regulation. NADPH oxidase (NOX) enzymes are a main source of deliberate cellular ROS production. Here, we found that NOX2 and ROS are required for the host cell to trigger an efficient RIG-I-mediated IRF-3 activation and downstream antiviral IFNbeta and IFIT1 gene expression. Additionally, we provide evidence that NOX2 is critical for the expression of the central mitochondria-associated adaptor MAVS. Taken together these data reveal a new facet to the regulation of the innate host defense against viruses through the identification of an unrecognized role of NOX2 and ROS.

Genetic polymorphisms, their allele combinations and IFN-beta treatment response in Irish multiple sclerosis patients

INTRODUCTION

IFN-beta is widely used as first-line immunomodulatory treatment for multiple sclerosis. Response to treatment is variable (30-50% of patients are nonresponders) and requires a long treatment duration for accurate assessment to be possible. Information about genetic variations that predict responsiveness would allow appropriate treatment selection early after diagnosis, improve patient care, with time saving consequences and more efficient use of resources.

MATERIALS & METHODS

We analyzed 61 SNPs in 34 candidate genes as possible determinants of IFN-beta response in Irish multiple sclerosis patients. Particular emphasis was placed on the exploration of combinations of allelic variants associated with response to therapy by means of a Markov chain Monte Carlo-based approach (APSampler).

RESULTS

The most significant allelic combinations, which differed in frequency between responders and nonresponders, included JAK2-IL10RB-GBP1-PIAS1 (permutation p-value was p(perm) = 0.0008), followed by JAK2-IL10-CASP3 (p(perm) = 0.001).

DISCUSSION

The genetic mechanism of response to IFN-beta is complex and as yet poorly understood. Data mining algorithms may help in uncovering hidden allele combinations involved in drug response versus nonresponse.

The TAK1-JNK cascade is required for IRF3 function in the innate immune response

Interferon regulatory factor (IRF)3 is critical for the transcriptional induction of chemokines and cytokines during viral or bacterial invasion. The kinases Tank binding kinase (TBK)1 and Ikappa B kinase (IKK)epsilon can phosphorylate the C-terminal part of IRF3 and play important roles in IRF3 activation. In this study, we show that another kinase, c-Jun-NH(2)-terminal kinase (JNK), phosphorylates IRF3 on its N-terminal serine 173 residue, and TAK1 can stimulate IRF3 phosphorylation via JNK. JNK specific inhibitor SP600125 inhibits the N-terminal phosphorylation without affecting the C-terminal phosphorylation. In addition, IRF3-mediated gene expressions on lipopolysaccharide (LPS) or polyinosinic-cytidylic acid (polyI:C) treatment are severely impaired by SP600125, as well as for reporter gene assay of IRF3 activation. Knockdown of TAK1 further confirmed these observations. Interestingly, constitutive active IRF3(5D) can be inhibited by SP600125; JNK1 can synergize the action of IRF3(5D), but not the S173A-IRF3(5D) mutant. More importantly, polyI:C failed to induce the phosphorylation of mutant S173A and SP600125 dramatically abrogated IRF3 phosphorylation and dimerization that was stimulated by polyI:C. Thus, this study demonstrates that the TAK1-JNK cascade is required for IRF3 function, in addition to TBK1/IKKvarepsilon, uncovering a new mechanism for mitogen-activated protein (MAP) kinase to regulate the innate immunity.

Induction of protein kinase PKR-dependent activation of interferon regulatory factor 3 by vaccinia virus occurs through adapter IPS-1 signaling

Interferon regulatory factor 3 (IRF-3) undergoes phosphorylation-induced activation in virus-infected cells and plays an important role in the antiviral innate immune response. The E3L protein encoded by vaccinia virus is known to impair phosphorylation and activation of IRF-3. Kinases in addition to I kappaB kinase-related kinases are implicated in the IRF-3-dependent antiviral response. To test in human cells the role of the protein kinase regulated by RNA (PKR) in IRF-3 activation, HeLa cells made stably deficient in PKR using an RNA interference strategy were compared with PKR-sufficient cells. Rapid phosphorylation and nuclear accumulation of IRF-3 were detected in PKR-sufficient cells following infection with E3L deletion mutant (DeltaE3L) virus. By contrast, the full IRF-3 activation response was largely abolished in PKR-deficient cells. The DeltaE3L virus-induced IRF-3 activation seen in PKR-sufficient cells was diminished by treatment with cytosine beta-D-arabinofuranoside. Furthermore, the vaccinia mutant ts23, which displays increased viral double-stranded RNA production at 39 degrees C, induced PKR-dependent IRF-3 phosphorylation at 39 degrees C but not at 31 degrees C. Both IRF-3 phosphorylation and cell apoptosis induced by infection with DeltaE3L virus were dependent upon RIG-I-like receptor signal transduction components, including the adapter IPS-1. These data suggest that PKR facilitates the host innate immune response and apoptosis in virus-infected cells by mediating IRF-3 activation through the mitochondrial IPS-1 signal transduction pathway.

Toll-like receptor 3 triggers apoptosis of human prostate cancer cells through a PKC-alpha-dependent mechanism

Toll-like receptors (TLRs) are known to play a key role in the innate immune system particularly in inflammatory response against invading pathogens. Recent reports strongly indicate that they play important roles in cancer cells. Prostate cancer represents one of the most common cancer for which no cure is available once metastatic and androgen refractory. Since TLR3 has been recently suggested as a possible therapeutic target in some cancer cell lines, we studied TLR3 expression and functionality in two human prostate cancer cell lines, LNCaP and PC3. We report that both cell lines express TLR3 and that the TLR3 agonist poly (I:C) activates mitogen-activated protein kinases and induces inhibition of proliferation as well as caspase-dependent apoptosis. By using pharmacological and genetic approaches, we demonstrate the involvement of TLR3 in poly (I:C)-induced effects. We also show that a novel interferon-independent pathway involving protein kinase C (PKC)-alpha activation, upstream of p38 and c-jun N-terminal kinase, is responsible for poly (I:C) pro-apoptotic effects on LNCaP cells. To our knowledge, this is the first report describing a role of PKC-alpha in poly (I:C)-mediated apoptosis. The comprehension of the mechanisms underlying TLR3-mediated apoptosis can contribute tools to develop new agonists useful for the treatment of prostate cancer.

The scaffold MyD88 acts to couple protein kinase Cepsilon to Toll-like receptors

Mice lacking protein kinase Cepsilon (PKCepsilon) are hypersensitive to both Gram-positive and Gram-negative bacterial infections; however, the mechanism of PKCepsilon coupling to the Toll-like receptors (TLRs), responsible for pathogen detection, is poorly understood. Here we sought to investigate the mechanism of PKCepsilon involvement in TLR signaling and found that PKCepsilon is recruited to TLR4 and phosphorylated on two recently identified sites in response to lipopolysaccharide (LPS) stimulation. Phosphorylation at both of these sites (Ser-346 and Ser-368) resulted in PKCepsilon binding to 14-3-3beta. LPS-induced PKCepsilon phosphorylation, 14-3-3beta binding, and recruitment to TLR4 were all dependent on expression of the scaffold protein MyD88. In mouse embryo fibroblasts and activated macrophages from MyD88 knock-out mice, LPS-stimulated PKCepsilon phosphorylation was reduced compared with wild type cells. Acute knockdown of MyD88 in LPS-responsive 293 cells also resulted in complete loss of Ser-346 phosphorylation and TLR4/PKCepsilon association. By contrast, MyD88 overexpression in 293 cells resulted in constitutive phosphorylation of PKCepsilon. A general role for MyD88 was evidenced by the finding that phosphorylation of PKCepsilon was induced by the activation of all TLRs tested that signal through MyD88 (i.e. all except TLR3) both in RAW cells and in primary human macrophages. Functionally, it is established that phosphorylation of PKCepsilon at these two sites is required for TLR4- and TLR2-induced NFkappaB reporter activation and IkappaB degradation in reconstituted PKCepsilon(-/-) cells. This study therefore identifies the scaffold protein MyD88 as the link coupling TLRs to PKCepsilon recruitment, phosphorylation, and downstream signaling.