Amplifying Btk's Signal

LPS-induced vein endothelial cell injury and acute lung injury have Btk and Orai 1 to regulate SOC-mediated calcium influx

Patients with sepsis and sepsis-related complications have a high mortality. Endothelial cell dysfunction plays a central role in sepsis pathophysiological process. In sepsis patients, endothelial cell apoptosis is associated with intracellular calcium overload. Multiple functions in the apoptotic process have been found to be regulated by calcium signaling. Our previous work had proved that LPS-induced cell injury was associated with store-operated calcium (SOC) entry mediated by stromal interaction molecule-1 (STIM 1) in Human umbilical vein endothelial cells (HUVEC), but the underlying molecular mechanism has not been adequately defined. Here we report that the LPS-induced cell injury is related to the calcium overload in HUVEC. SOC entry mediated by calcium release-activated calcium modulator (Orai) 1 and transient receptor potential canonical (TRPC) 1 was associated with LPS-induced calcium overload and cell apoptosis. Bruton's tyrosine kinase (Btk)/Phospholipase C(PLC) γ/inositol 1,4,5-triphosphate receptor (IP3R) played a major role in regulating calcium overload in LPS-induced HUVEC. Knockdown of Btk markedly inhibited the expressions of Orai 1 and its downstream molecule IP3R but not that of TRPC1 in LPS-induced HUVEC. In mice, knockdown of Btk and Orai 1 inhibited LPS-induced calcium overload, pulmonary vascular endothelial cell (VEC) injury and acute lung injury. These findings demonstrated that Btk acts as a regulator of calcium-dependent signaling, especially in the Orai 1-mediated SOC entry of the LPS-induced VEC.

Noncatalytic Bruton's tyrosine kinase activates PLCγ2 variants mediating ibrutinib resistance in human chronic lymphocytic leukemia cells

Treatment of patients with chronic lymphocytic leukemia (CLL) with inhibitors of Bruton's tyrosine kinase (BTK), such as ibrutinib, is limited by primary or secondary resistance to this drug. Examinations of CLL patients with late relapses while on ibrutinib, which inhibits BTK's catalytic activity, revealed several mutations in BTK, most frequently resulting in the C481S substitution, and disclosed many mutations in PLCG2, encoding phospholipase C-γ₂ (PLCγ₂). The PLCγ₂ variants typically do not exhibit constitutive activity in cell-free systems, leading to the suggestion that in intact cells they are hypersensitive to Rac family small GTPases or to the upstream kinases spleen-associated tyrosine kinase (SYK) and Lck/Yes-related novel tyrosine kinase (LYN). The sensitivity of the PLCγ₂ variants to BTK itself has remained unknown. Here, using genetically-modified DT40 B lymphocytes, along with various biochemical assays, including analysis of PLCγ₂-mediated inositol phosphate formation, inositol phospholipid assessments, fluorescence recovery after photobleaching (FRAP) static laser microscopy, and determination of intracellular calcium ([Ca²⁺] _i ), we show that various CLL-specific PLCγ₂ variants such as PLCγ₂S707Y are hyper-responsive to activated BTK, even in the absence of BTK's catalytic activity and independently of enhanced PLCγ₂ phospholipid substrate supply. At high levels of B-cell receptor (BCR) activation, which may occur in individual CLL patients, catalytically-inactive BTK restored the ability of the BCR to mediate increases in [Ca²⁺] _i Because catalytically-inactive BTK is insensitive to active-site BTK inhibitors, the mechanism involving the noncatalytic BTK uncovered here may contribute to preexisting reduced sensitivity or even primary resistance of CLL to these drugs.

Particles from the Echinococcus granulosus Laminated Layer Inhibit CD40 Upregulation in Dendritic Cells by Interfering with Akt Activation

The larval stage of the cestode Echinococcus granulosus causes cystic echinococcosis in humans and livestock. This larva is protected by the millimeter-thick, mucin-based laminated layer (LL), from which materials have to be shed to allow parasite growth. We previously reported that dendritic cells (DCs) respond to microscopic pieces of the mucin gel of the LL (pLL) with unconventional maturation phenotypes, in the absence or presence of Toll-like receptor (TLR) agonists, including lipopolysaccharide (LPS).

The larval stage of the cestode Echinococcus granulosus causes cystic echinococcosis in humans and livestock. This larva is protected by the millimeter-thick, mucin-based laminated layer (LL), from which materials have to be shed to allow parasite growth. We previously reported that dendritic cells (DCs) respond to microscopic pieces of the mucin gel of the LL (pLL) with unconventional maturation phenotypes, in the absence or presence of Toll-like receptor (TLR) agonists, including lipopolysaccharide (LPS). We also reported that the presence of pLL inhibited the activating phosphorylation of the phosphatidylinositol 3-kinase (PI3K) effector Akt induced by granulocyte-macrophage colony-stimulating factor or interleukin-4. We now show that the inhibitory effect of pLL extends to LPS as a PI3K activator, and results in diminished phosphorylation of GSK3 downstream from Akt. Functionally, the inhibition of Akt and GSK3 phosphorylation are linked to the blunted upregulation of CD40, a major feature of the unconventional maturation phenotype. Paradoxically, all aspects of unconventional maturation induced by pLL depend on PI3K class I. Additional components of the phagocytic machinery are needed, but phagocytosis of pLL particles is not required. These observations hint at a DC response mechanism related to receptor-independent mechanisms proposed for certain crystalline and synthetic polymer-based particles; this would fit the previously reported lack of detection of molecular-level motifs necessary of the effects of pLL on DCs. Finally, we report that DCs exposed to pLL are able to condition DCs not exposed to the material so that these cannot upregulate CD40 in full in response to LPS.

Itk-mediated integration of T cell receptor and cytokine signaling regulates the balance between Th17 and regulatory T cells

Loss of the Tec family kinase Itk results in a bias to FoxP3⁺ T_reg cell differentiation and reduced TCR-induced phosphorylation of mTOR targets.

A proper balance between Th17 and T regulatory cells (T_reg cells) is critical for generating protective immune responses while minimizing autoimmunity. We show that the Tec family kinase Itk (IL2-inducible T cell kinase), a component of T cell receptor (TCR) signaling pathways, influences this balance by regulating cross talk between TCR and cytokine signaling. Under both Th17 and T_reg cell differentiation conditions, Itk^−/− CD4⁺ T cells develop higher percentages of functional FoxP3⁺ cells, associated with increased sensitivity to IL-2. Itk^−/− CD4⁺ T cells also preferentially develop into T_reg cells in vivo. We find that Itk-deficient T cells exhibit reduced TCR-induced phosphorylation of mammalian target of rapamycin (mTOR) targets, accompanied by downstream metabolic alterations. Surprisingly, Itk^−/− cells also exhibit reduced IL-2–induced mTOR activation, despite increased STAT5 phosphorylation. We demonstrate that in wild-type CD4⁺ T cells, TCR stimulation leads to a dose-dependent repression of Pten. However, at low TCR stimulation or in the absence of Itk, Pten is not effectively repressed, thereby uncoupling STAT5 phosphorylation and phosphoinositide-3-kinase (PI3K) pathways. Moreover, Itk-deficient CD4⁺ T cells show impaired TCR-mediated induction of Myc and miR-19b, known repressors of Pten. Our results demonstrate that Itk helps orchestrate positive feedback loops integrating multiple T cell signaling pathways, suggesting Itk as a potential target for altering the balance between Th17 and T_reg cells.

Differential role of hypoxia-inducible factor 1 alpha in toll-like receptor-mediated and allergic inflammatory reactions

Hypoxia-inducible factor 1 (HIF-1) is a transcription complex that plays a pivotal role in cellular adaptation to hypoxic conditions. The role of this factor in inflammatory reactions associated with infections and allergies has recently become evident. In this review we summarize our current knowledge concerning the accumulation and role of HIF-1 in Toll-like receptor-mediated and allergic inflammation. The differential molecular mechanisms used to stabilize this protein in various settings and its ability to support both proinflammatory and angiogenic responses suggest important functional roles in both innate immune responses and allergies. Importantly, the HIF-1 transcription complex is activated in human basophils during IgE-mediated inflammatory responses. It is involved in VEGF expression and subsequent promotion of angiogenesis and in controlling energy metabolism.

Btk regulation in human and mouse B cells via protein kinase C phosphorylation of IBtkγ

The inhibitor of Bruton tyrosine kinase γ (IBtkγ) is a negative regulator of the Bruton tyrosine kinase (Btk), which plays a major role in B-cell differentiation; however, the mechanisms of IBtkγ-mediated regulation of Btk are unknown. Here we report that B-cell receptor (BCR) triggering caused serine-phosphorylation of IBtkγ at protein kinase C consensus sites and dissociation from Btk. By liquid chromatography and mass-mass spectrometry and functional analysis, we identified IBtkγ-S87 and -S90 as the critical amino acid residues that regulate the IBtkγ binding affinity to Btk. Consistently, the mutants IBtkγ carrying S87A and S90A mutations bound constitutively to Btk and down-regulated Ca(2+) fluxes and NF-κB activation on BCR triggering. Accordingly, spleen B cells from Ibtkγ(-/-) mice showed an increased activation of Btk, as evaluated by Y551-phosphorylation and sustained Ca(2+) mobilization on BCR engagement. These findings identify a novel pathway of Btk regulation via protein kinase C phosphorylation of IBtkγ.

Hypoxia-inducible factor 1 as one of the "signaling drivers" of Toll-like receptor-dependent and allergic inflammation

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription complex which plays a crucial role in cellular adaptation to low oxygen availability. In the last years there has been increasing evidence about the role of this factor in inflammatory/innate immune reactions. It has also been found to contribute to different types of allergic inflammation. In this review the current knowledge about the accumulation and role of HIF-1 in Toll-like receptor-mediated and allergic inflammation is summarized. Differential biochemical mechanisms employed to stabilize the protein in different cases are discussed.

The involvement of hypoxia-inducible factor 1 alpha in Toll-like receptor 7/8-mediated inflammatory response

Toll-like receptors (TLRs) 7 and 8 are crucial in host defence against single-stranded RNA (ssRNA) viruses. Such viruses cause severe illnesses, which remain a serious medical burden in both industrialised and developing countries. TLR7/8 downstream signaling leads to a dramatic cellular stress associated with energy consumption. However, the molecular mechanisms of cell survival and adaptation to TLR7/8-induced stress, which give the cells an opportunity to initiate proper inflammatory reactions, are not clear at all. Here we report for the first time that ligand-induced activation of TLR7/8 leads to the accumulation of hypoxia-inducible factor 1 alpha (HIF-1alpha) protein in THP-1 human myeloid macrophages via redox- and reactive nitrogen species-dependent mechanisms. MAP kinases and phosphoinositol-3K are not involved in TLR7/8-mediated HIF-1alpha accumulation. Experiments with HIF-1alpha knockdown THP-1 cells have clearly demonstrated that HIF-1alpha is important for the protection of these cells against TLR7/8-induced depletion of ATP. Thus, HIF-1alpha might support both cell survival and the production of pro-inflammatory cytokines upon TLR7/8 activation.

LPS-induced Toll-like receptor 4 signalling triggers cross-talk of apoptosis signal-regulating kinase 1 (ASK1) and HIF-1alpha protein

Toll-like receptor 4 (TLR4) is required for recognition of lipopolysaccharide (LPS) of Gram-negative bacteria and induction of the innate immune response to them. Nevertheless, the involvement of some crucial pathways in TLR4 signalling is poorly understood. Here, we report that LPS-induced TLR4 signalling triggers cross talk of HIF-1alpha and ASK1 in THP-1 human myeloid monocytic leukaemia cells. Both pathways are activated via redox-dependent mechanism associated with tyrosine kinase/phospholipase C-1gamma-mediated activation of protein kinase C alpha/beta, which are known to activate NADPH oxidase and the production of reactive oxygen species that activate both HIF-1alpha and ASK1. ASK1 contributes to the stabilisation of HIF-1alpha, most likely via activation of p38 MAP kinase.

Tec kinases, actin, and cell adhesion

The Tec family non-receptor tyrosine kinases have been recognized for their roles in the regulation of phospholipase C-gamma and Ca(2+) mobilization downstream from antigen receptors on lymphocytes. Recent data, however, show that the Tec family kinase interleukin-2-inducible T-cell kinase (Itk) also participates in pathways regulating the actin cytoskeleton and 'inside-out' signaling to integrins downstream from the T-cell antigen receptor. Data suggest that Itk may function in a kinase-independent fashion to regulate proper recruitment of the Vav1 guanine nucleotide exchange factor. By enhancing actin cytoskeleton reorganization, recruitment of signaling molecules to the immune synapse, and integrin clustering in response to both antigen and chemokine receptors, the Tec kinases serve as modulators or amplifiers that can increase the duration of T-cell signaling and regulate T-cell functional responses.

LIME acts as a transmembrane adapter mediating BCR-dependent B-cell activation

Assembly of a signaling complex around the transmembrane adapter LAT is essential for the transmission of T-cell receptor (TCR)-mediated signaling. However, a LAT-like molecule responsible for the initial activation events in B-cell receptor (BCR) signaling has not yet been identified. Here, we show that LIME is a transmembrane adaptor required for BCR-mediated B-cell activation. LIME was found to be expressed in mouse splenic B cells. Upon BCR cross-linking, LIME was tyrosine phosphorylated by Lyn and associated with Lyn, Grb2, PLC-gamma2, and PI3K. Reduction of LIME expression by the introduction of siRNA resulted in the disruption of BCR-mediated activation of MAPK, calcium flux, NF-AT, PI3K, and NF-kappaB. Taken together, these results establish that LIME is an essential transmembrane adaptor linking BCR ligation to the downstream signaling events that lead to B-cell activation.

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

Phosphoinositide 3-kinase and its targets in B-cell and T-cell signaling

Phosphoinositide 3-kinase (PI3K) activation is essential for lymphocyte proliferation driven by receptors for antigen, costimulatory ligands and cytokines. The lipid products of PI3K contribute to the assembly of membrane-associated signaling complexes by promoting recruitment of selected proteins from the cytoplasm. Many proteins possess domains that are able to bind selectively to PI3K products. Different 'PI3K effector' proteins are coupled to distinct biological responses, depending on cell type and on the receptor that is engaged. In B cells and T cells, Tec-family tyrosine kinases and Akt serine/threonine kinases are emerging as crucial mediators of proliferation and survival signals downstream of PI3K. Of particular interest is recent evidence that PI3K signaling controls increases in lymphocyte size and metabolic activity that accompany cell cycle progression.

Regulation of phospholipase C-gamma2 networks in B lymphocytes

The modulation of inositol-1,4,5-trisphosphate (IP3), a product of phospholipase C (PLC) activity, is one of a common signaling mechanism used in many biological systems. B lymphocytes also rely on IP3 and subsequent calcium signaling to ensure appropriate developmental outcomes, as well as antigen-specific responses. In establishing the optimal intensity and duration of the PLC-gamma activity, an important role has emerged for adaptor molecules, which direct the appropriate subcellular localization of PLC-gamma and induce its conformational changes. Generated IP3 binds to IP3 receptors located on the endoplasmic reticulum (ER), which in turn is essential for triggering calcium release from the ER and subsequent entry of extracellular calcium by so-called Ca2+ entry channels. Recent data has begun to shed new light on the connection between the calcium release and the influx of extracellular calcium, and the molecular identity of the Ca2+ entry channels.