Negative Regulator Of Src Family Kinases Research Articles

Src family kinases suppress differentiation of brown adipocytes and browning of white adipocytes.

Brown adipocytes and beige adipocytes can expend energy, generate heat, and increase whole-body energy expenditure. The detailed mechanisms of adipogenesis and thermogenesis of these cells are still obscure. Here, we show that Src family kinases (SFKs) regulate both brown adipogenesis and browning of white adipocytes. To identify factors involved in brown adipogenesis, we first examined the effect of several chemical inhibitors on the differentiation of brown preadipocytes isolated from mouse brown adipose tissue (BAT) and found that treatment with PP2, the specific inhibitor of SFKs, promoted the differentiation. Another inhibitor of SFKs, PP1, also promoted the brown adipogenesis, whereas an inactive analogue of PP2, PP3, did not. Moreover, over-expression of C-terminal Src kinase (CSK), the negative regulator of SFKs, also promoted brown adipogenesis. Next, we examined the effect of inhibition of SFKs on the differentiation of white preadipocytes isolated from white adipose tissue (WAT). Our results showed that either PP2 treatment or CSK-over-expression generated Ucp1-positive beige adipocytes, thus inducing browning of white adipocytes. Finally, our analysis showed that the expression levels and activity of SFKs in WAT were much higher than in BAT. These results taken together suggest that SFKs regulate differentiation and browning of fat cells in vivo.

Small molecule inhibition of Csk alters affinity recognition by T cells.

The C-terminal Src kinase (Csk), the primary negative regulator of Src-family kinases (SFK), plays a crucial role in controlling basal and inducible receptor signaling. To investigate how Csk activity regulates T cell antigen receptor (TCR) signaling, we utilized a mouse expressing mutated Csk (Csk(AS)) whose catalytic activity is specifically and rapidly inhibited by a small molecule. Inhibition of Csk(AS) during TCR stimulation led to stronger and more prolonged TCR signaling and to increased proliferation. Inhibition of Csk(AS) enhanced activation by weak but strictly cognate agonists. Titration of Csk inhibition revealed that a very small increase in SFK activity was sufficient to potentiate T cell responses to weak agonists. Csk plays an important role, not only in basal signaling, but also in setting the TCR signaling threshold and affinity recognition.

Inhibition of the kinase Csk in thymocytes reveals a requirement for actin remodeling in the initiation of full TCR signaling.

T cell receptor (TCR) signaling is initiated by Src-family kinases (SFKs). To understand how C-terminal Src kinase (Csk), the negative regulator of SFKs, controls the basal state and the initiation of TCR signaling, we generated mice expressing a PP1-analog inhibitor-sensitive Csk variant (CskAS). Inhibition of CskAS in thymocytes, without TCR engagement, induced potent SFK activation and proximal TCR signaling up to phospholipase C-γ1 (PLC-γ1). Surprisingly, increases in inositol phosphates (InsP), intracellular calcium and Erk phosphorylation were impaired. Altering the actin cytoskeleton pharmacologically or providing CD28 costimulation rescued these defects. Thus, Csk plays a critical role in preventing TCR signaling. However, our studies also revealed a requirement for actin remodeling, initiated by costimulation, for full TCR signaling.

Mammalian Pragmin regulates Src family kinases via the Glu-Pro-Ile-Tyr-Ala (EPIYA) motif that is exploited by bacterial effectors

Several pathogenic bacteria have adopted effector proteins that, upon delivery into mammalian cells, undergo tyrosine phosphorylation at the Glu-Pro-Ile-Tyr-Ala (EPIYA) or EPIYA-like sequence motif by host kinases such as Src family kinases (SFKs). This EPIYA phosphorylation triggers complex formation of bacterial effectors with SH2 domain-containing proteins that results in perturbation of host cell signaling and subsequent pathogenesis. Although the presence of such an anomalous protein interaction suggests the existence of a mammalian EPIYA-containing protein whose function is mimicked or subverted by bacterial EPIYA effectors, no molecule that uses the EPIYA motif for biological function has so far been reported in mammals. Here we show that mammalian Pragmin/SgK223 undergoes tyrosine phosphorylation at the EPIYA motif by SFKs and thereby acquires the ability to interact with the SH2 domain of the C-terminal Src kinase (Csk), a negative regulator of SFKs. The Pragmin-Csk interaction prevents translocalization of Csk from the cytoplasm to the membrane and subsequent inactivation of membrane-associated SFKs. As a result, SFK activity is sustained in cells where Pragmin is phosphorylated at the EPIYA motif. Because EPIYA phosphorylation of Pragmin is mediated by SFKs, cytoplasmic sequestration of Csk by Pragmin establishes a positive feedback regulation of SFK activation. Remarkably, the Helicobacter pylori EPIYA effector CagA binds to the Csk SH2 domain in place of Pragmin and enforces membrane recruitment of Csk and subsequent inhibition of SFKs. This work identifies Pragmin as a mammalian EPIYA effector and suggests that bacterial EPIYA effectors target Pragmin to subvert SFKs for successful infection.

Conditional targeting of C-terminal Src kinase (Csk) in NK cells (135.2)

Abstract Csk is a key negative regulator of Src-family kinases and has been shown to be a strong inhibitor of immunoreceptor signalling in T cells and granulocytes. Our hypothesis is that an increase in Src kinase activity might take place in NK cells following the elimination of Csk, resulting in an enhanced cellular response even at low levels of stimulation. We aim to establish the role of Csk in NK cells using conditional knockout mice. A transgenic mouse has been developed where the Csk gene is flanked by LoxP (floxed) sites. Crossing Csk floxed transgenic mice with transgenic mice expressing Cre recombinase under the control of a human CD2 gene (hCD2-iCre) will enable the deletion of the Csk gene in a tissue-specific manner. The ROSA26 reporter gene system suggests that Cre deletes the floxed gene in approximately 30% of splenic NK cells and, as reported before, in the vast majority of T cells and B cells. Intracellular staining for Csk demonstrates a correlation between Cre targeting of the ROSA26 locus and the target locus. There is a reduction in the number of lymphocytes in the Csk knockout (KO) mouse. More specifically, there is a decrease in B cells, suggesting a role for Csk in B cell development. However, no obvious phenotype is detected in NK cells. Ongoing studies of NK cell functions will be presented.

Cell cycle-dependent regulation of SFK, JAK1 and STAT3 signaling by the protein tyrosine phosphatase TCPTP

Janus-activated kinases (JAKs) and Src family kinases (SFKs) and their common substrate signal transducer and activator of transcription (STAT)-3 are frequently hyperactivated in human cancer contributing to the proliferative drive by promoting G1/S and G2/M progression. Previous studies have established that the protein tyrosine phosphatase TCPTP can dephosphorylate and inactivate the SFK and JAK protein tyrosine kinases (PTKs) to attenuate cytokine signalling in vivo. In this study we determined whether TCPTP regulates SFK and JAK signalling during the cell cycle. We used primary mouse embryonic fibroblasts (MEFs) isolated from TCPTP-/- versus +/+ mice, immortalised TCPTP-/- MEFs versus those reconstituted with physiological levels of TCPTP and HeLa cells in which TCPTP protein levels had been suppressed by RNA interference, to establish TCPTP as a negative regulator of SFK, JAK1 and STAT3 signalling during the cell cycle. We found that the progression of TCPTP-deficient MEFs after the G1 restriction point into S-phase was enhanced. We used RNA interference and pharmacological inhibitors to demonstrate that elevated SFK and downstream phosphatidylinositol 3-kinase signalling but not JAK1 or STAT3 signalling were required for the enhanced G1/S transition. These results identify TCPTP as a negative regulator of the cell cycle.

Ablation of Csk in neural crest lineages causes corneal anomaly by deregulating collagen fibril organization and cell motility

Src family kinases (SFKs) have been implicated in the regulation of cell motility. To verify their in vivo roles during development, we generated mutant mice in which Csk, a negative regulator of SFKs, was inactivated in neural crest lineages using the Protein zero promoter in a Cre–loxP system. Inactivation of Csk caused deformities in various tissues of neural crest origins, including facial dysplasia and corneal opacity. In the cornea, the stromal collagen fibril was disorganized and there was an overproduction of collagen 1a1 and several metalloproteases. The corneal endothelium failed to overlie the central region of the eye and the peripheral endothelium displayed a disorganized cytoskeleton. Corneal mesenchymal cells cultured from mutant mice showed attenuated cell motility. In these cells, p130 Crk-associated substrate (Cas) was hyperphosphorylated and markedly downregulated. The expression of a dominant negative Cas (CasΔSD) could suppress the cell motility defects. Fluorescence resonance energy transfer analysis revealed that activation of Rac1 and Cdc42 was depolarized in Csk-inactivated cells, which was restored by the expression of either Csk or CasΔSD. These results demonstrate that the SFKs/Csk circuit plays crucial roles in corneal development by controlling stromal organization and by ensuring cell motility via the Cas-Rac/Cdc42 pathways.

Impairment in a negative regulatory system for TCR signaling in CD4 + T cells from old mice

To examine the involvement of lipid rafts in an age-associated decline in T cell function, we analyzed the effect of aging on the constituents of lipid rafts in resting mouse CD4 + T cells. We found a pronounced, age-dependent reduction in PAG/Cbp, which is involved in the regulation of Src family kinases (SFKs) by recruiting Csk (a negative regulator of SFKs) to lipid rafts. This reduction is specific for T cells and is attributed, at least in part, to the reduction in its mRNA level. The reduction of PAG accompanies marked impairment in recruiting Csk to lipid rafts and a concomitant decrease in the inactive forms of SFKs. These findings indicate that old mouse CD4 + T cells have a defect in a negative SFK regulatory system.

Expression pattern of adaptor protein PAG: Correlation between secondary lymphatic follicle and histogenetically related malignant lymphomas

Transmembrane adaptor protein PAG, also known as Csk-binding protein (Cbp), which binds and activates the cytoplasmic tyrosine kinase Csk, the major negative regulator of Src-family kinases, was found to be expressed in germinal centers of lymphoid follicles as well as in follicular, but not mantle cell lymphomas. Expression of PAG may reflect its role in regulation of proliferation and differentiation of germinal center B-cells. From the routine histopathology point of view, PAG might be a new positive marker of follicular lymphoma and a negative marker of mantle cell lymphoma.

CD45 regulated signaling pathways.

CD45 is expressed on all nucleated haematopoietic cells and was originally identified as the first and prototypic transmembrane protein tyrosine phosphatase (PTPase). CD45 has been extensively studied for over two decades as a PTPase that functions in antigen receptor signaling by dephosphorylation of Src-kinases. CD45 can operate as a positive as well negative regulator of Src-family kinases. In CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient human SCID patients, CD45 is required for signal transduction through antigen receptors. Our group has recently shown that CD45 can also function as a Janus kinase (JAK) tyrosine phosphatase that negatively regulates cytokine receptor signaling involved in the differentiation, proliferation, and antiviral immunity of haematopoietic cells. Moreover, a point mutation in CD45, implicated in affecting CD45 dimerization, and a genetic polymorphism that affects alternative CD45 splicing have been implicated in autoimmunity in mice and humans. CD45 is expressed in multiple isoforms and modulation of specific CD45 splice variants with antibodies can prevent transplant rejections. Moreover, loss of CD45 can affect microglia activation in a mouse model for Alzheimer's disease. Modulation of CD45 splice variants and CD45 activity might provide a unique opportunity to design drugs that turn off or turn-on antigen and cytokine receptor signaling in cancer, allergy, transplantation, or autoimmunity.

New insights into the transmembrane protein tyrosine phosphatase CD45

CD45 is expressed on all nucleated haematopoietic cells and was originally identified as the first and prototypic transmembrane protein tyrosine phosphatase. In CD45 mutant cell lines, CD45-deficient mice and CD45-deficient human SCID patients, CD45 is required for signal transduction through antigen receptors. CD45 can operate as a positive as well as a negative regulator of Src-family kinases. Moreover, CD45 was identified as the elusive JAK tyrosine phosphatase that negatively regulates cytokine receptor activation involved in the differentiation, proliferation and antiviral immunity of haematopoietic cells. Modulation of CD45 splice variants provides a unique opportunity to design drugs that turn off or turn on antigen and cytokine receptor signaling in cancer, transplantation or autoimmunity

Regulation of the renal Na-HCO(3) cotransporter. XI. Signal transduction underlying CO(2) stimulation.

We have previously shown that CO(2) stimulation of the renal Na-HCO(3) cotransporter (NBC) activity is abrogated by general inhibitors of protein tyrosine kinases. The more selective inhibitor herbimycin also blocked this effect at concentrations known to preferentially inhibit Src family kinases (SFKs). We therefore examined a role for SFKs in CO(2)-stimulated NBC activity. To this end, we engineered OK cells to express the COOH-terminal Src kinase (Csk), a negative regulator of SFKs. CO(2) stimulated NBC activity normally in beta-galactosidase-expressing and untransfected control cells. In contrast, Csk-expressing cells had normal baseline NBC activity that was not stimulated by CO(2). CO(2) stimulation increased both total SFK activity and specific tyrosine phosphorylation of Src. The specific MEK1/2 inhibitor PD-98059 completely inhibited the CO(2) stimulation of NBC activity as well as the accompanying phosphorylation and activation of ERK1/2. Our data suggest the involvement of both SFKs, probably Src, and the "classic" MAPK pathway in mediating CO(2)-stimulated NBC activity in renal epithelial cells.