Differential regulation of endoplasmic reticulum stress by protein tyrosine phosphatase 1B and T cell protein tyrosine phosphatase

Protein-tyrosine phosphatase 1B (PTP1B) and T cell protein-tyrosine phosphatase (TCPTP) are closely related intracellular phosphatases implicated in the control of glucose homeostasis. PTP1B and TCPTP can function coordinately to regulate protein tyrosine kinase signaling, and PTP1B has been implicated previously in the regulation of endoplasmic reticulum (ER) stress. In this study, we assessed the roles of PTP1B and TCPTP in regulating ER stress in the endocrine pancreas. PTP1B and TCPTP expression was determined in pancreases from chow and high fat fed mice and the impact of PTP1B and TCPTP over- or underexpression on palmitate- or tunicamycin-induced ER stress signaling assessed in MIN6 insulinoma β cells. PTP1B expression was increased, and TCPTP expression decreased in pancreases of mice fed a high fat diet, as well as in MIN6 cells treated with palmitate. PTP1B overexpression or TCPTP knockdown in MIN6 cells mitigated palmitate- or tunicamycin-induced PERK/eIF2α ER stress signaling, whereas PTP1B deficiency enhanced ER stress. Moreover, PTP1B deficiency increased ER stress-induced cell death, whereas TCPTP deficiency protected MIN6 cells from ER stress-induced death. ER stress coincided with the inhibition of Src family kinases (SFKs), which was exacerbated by PTP1B overexpression and largely prevented by TCPTP knockdown. Pharmacological inhibition of SFKs ameliorated the protective effect of TCPTP deficiency on ER stress-induced cell death. These results demonstrate that PTP1B and TCPTP play nonredundant roles in modulating ER stress in pancreatic β cells and suggest that changes in PTP1B and TCPTP expression may serve as an adaptive response for the mitigation of chronic ER stress.     

C-terminal Src kinase-homologous kinase (CHK), a unique inhibitor inactivating multiple active conformations of Src family tyrosine kinases

The Src family of protein kinases (SFKs) mediates mitogenic signal transduction, and constitutive SFK activation is associated with tumorigenesis. To prevent constitutive SFK activation, the catalytic activity of SFKs in normal mammalian cells is suppressed mainly by two inhibitors called C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK), which inactivate SFKs by phosphorylating a consensus tyrosine near the C terminus of SFKs (Y(T)). The phosphorylated Y(T) intramolecularly binds to the SH2 domain of SFKs. This interaction, known as pY(T)/SH2 interaction, together with binding between the SH2 kinase linker and the SH3 domain of SFKs (linker/SH3 interaction) stabilizes SFKs in a "closed" inactive conformation. We previously discovered an alternative mechanism CHK employs to inhibit SFKs. This mechanism, referred to as the non-catalytic inhibitory mechanism, involves tight binding of CHK to SFKs; the binding alone is sufficient to inhibit SFKs. Herein, we constructed multiple active conformations of an SFK member, Hck, by systematically disrupting the two inhibitory interactions. We found that CHK employs the non-catalytic mechanism to inactivate these active conformations of Hck. However, CHK does not bind Hck when it adopts the inactive conformation in which both inhibitory interactions are intact. These data indicate that binding of CHK to SFKs via the non-catalytic mechanism is governed by the conformations of SFKs. Although CSK is also an inhibitor of SFKs, it does not inhibit SFKs by a similar non-catalytic mechanism. Thus, the non-catalytic inhibitory mechanism is a unique property of CHK that allows it to down-regulate multiple active conformations of SFKs.     

The Interaction of Protein-tyrosine Phosphatase α (PTPα) and RACK1 Protein Enables Insulin-like Growth Factor 1 (IGF-1)-stimulated Abl-dependent and -independent Tyrosine Phosphorylation of PTPα

Protein tyrosine phosphatase α (PTPα) promotes integrin-stimulated cell migration in part through the role of Src-phosphorylated PTPα-Tyr(P)-789 in recruiting and localizing p130Cas to focal adhesions. The growth factor IGF-1 also stimulates PTPα-Tyr-789 phosphorylation to positively regulate cell movement. This is in contrast to integrin-induced PTPα phosphorylation, that induced by IGF-1 can occur in cells lacking Src family kinases (SFKs), indicating that an unknown kinase distinct from SFKs can target PTPα. We show that this IGF-1-stimulated tyrosine kinase is Abl. We found that PTPα binds to the scaffold protein RACK1 and that RACK1 coordinates the IGF-1 receptor, PTPα, and Abl in a complex to enable IGF-1-stimulated and Abl-dependent PTPα-Tyr-789 phosphorylation. In cells expressing SFKs, IGF-1-stimulated phosphorylation of PTPα is mediated by RACK1 but is Abl-independent. Furthermore, expressing the SFKs Src and Fyn in SFK-deficient cells switches IGF-1-induced PTPα phosphorylation to occur in an Abl-independent manner, suggesting that SFK activity dominantly regulates IGF-1/IGF-1 receptor signaling to PTPα. RACK1 is a molecular scaffold that integrates growth factor and integrin signaling, and our identification of PTPα as a RACK1 binding protein suggests that RACK1 may coordinate PTPα-Tyr-789 phosphorylation in these signaling networks to promote cell migration.     

c-Src but not Fyn promotes proper spindle orientation in early prometaphase

Src family tyrosine kinases (SFKs) participate in mitotic signal transduction events, including mitotic entry, cleavage furrow ingression, and cytokinesis abscission. Although SFKs have been shown to associate with the mitotic spindle, the role of SFKs in mitotic spindle formation remains unclear. Here, we show that c-Src promotes proper spindle orientation in early prometaphase. Src localizes close to spindle poles in a manner independent of Src kinase activity. Three-dimensional analyses showed that Src inhibition induced spindle misorientation, exhibiting a tilting spindle in early prometaphase. Spindle misorientation is frequently seen in SYF cells, which harbor triple knock-out mutations of c-Src, c-Yes, and Fyn, and reintroduction of c-Src but not Fyn into SYF cells rescued spindle misorientation. Spindle misorientation was also observed upon Src inhibition under conditions in which Aurora B was inhibited. Inducible expression of c-Src promoted a properly oriented bipolar spindle, which was suppressed by Src inhibition. Aster formation was severely inhibited in SYF cells upon Aurora B inhibition, which was rescued by reintroduction of c-Src into SYF cells. Furthermore, reintroduction of c-Src facilitated microtubule regrowth from cold-induced depolymerization and accelerated M phase progression. These results suggest that c-Src is involved in spindle orientation through centrosome-mediated aster formation.     

Inhibition of protein-tyrosine phosphatase 1B (PTP1B) mediates ubiquitination and degradation of Bcr-Abl protein

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder characterized at the molecular level by the expression of Bcr-Abl, a chimeric protein with deregulated tyrosine kinase activity. The protein-tyrosine phosphatase 1B (PTP1B) is up-regulated in Bcr-Abl-expressing cells, suggesting a regulatory link between the two proteins. To investigate the interplay between these two proteins, we inhibited the activity of PTP1B in Bcr-Abl-expressing TonB.210 cells by either pharmacological or siRNA means and examined the effects of such inhibition on Bcr-Abl expression and function. Herein we describe a novel mechanism by which the phosphatase activity of PTP1B is required for Bcr-Abl protein stability. Inhibition of PTP1B elicits tyrosine phosphorylation of Bcr-Abl that triggers the degradation of Bcr-Abl through ubiquitination via the lysosomal pathway. The degradation of Bcr-Abl consequently inhibits tyrosine phosphorylation of Bcr-Abl substrates and the downstream production of intracellular reactive oxygen species. Furthermore, PTP1B inhibition reduces cell viability and the IC(50) of the Bcr-Abl inhibitor imatinib mesylate. Degradation of Bcr-Abl via PTP1B inhibition is also observed in human CML cell lines K562 and LAMA-84. These results suggest that inhibition of PTP1B may be a useful strategy to explore in the development of novel therapeutic agents for the treatment of CML, particularly because host drugs currently used in CML such as imatinib focus on inhibiting the kinase activity of Bcr-Abl.     

Scaffold Protein JLP Is Critical for CD40 Signaling in B Lymphocytes

CD40 expression on the surface of B lymphocytes is essential for their biological function and fate decision. The engagement of CD40 with its cognate ligand, CD154, leads to a sequence of cellular events in B lymphocytes, including CD40 cytoplasmic translocation, a temporal and spatial organization of effector molecules, and a cascade of CD40-induced signal transduction. The JLP scaffold protein was expressed in murine B lymphocytes. Using B lymphocytes from jlp-deficient mice, we observed that JLP deficiency resulted in defective CD40 internalization upon CD154/CD40 engagement. Examination of interactions and co-localization among CD40, JLP, dynein, and Rab5 in B lymphocytes suggested that CD40 internalization is a process of JLP-mediated vesicle transportation that depends on Rab5 and dynein. JLP deficiency also diminished CD40-dependent activation of MAPK and JNK, but not NF-κB. Inhibiting vesicle transportation from the direction of cell periphery to the cell center by a dynein inhibitor (ciliobrevin D) impaired both CD154-induced CD40 internalization and CD40-dependent MAPK activities in B lymphocytes. Collectively, our data demonstrate a novel role of the JLP scaffold protein in the bridging of CD154-triggered CD40 internalization and CD40-dependent signaling in splenic B lymphocytes.     

Requirement for CD45 in fine-tuning mast cell responses mediated by different ligand–receptor systems

The receptor-like protein tyrosine phosphatase CD45, the most abundant cell surface phosphatase on all nucleated hemopoietic cells, is a critical regulator of the activation status of Src family kinases (SFKs). To study the impact of CD45 on mast cell function, we compare bone marrow-derived mast cells (BMMCs) from CD45-deficient mice and from mice expressing an activating point mutation (E613R) in the juxtamembrane wedge of CD45. In response to Ag-triggered FcεR1-mediated activation, CD45-deficient BMMCs exhibit increased inhibitory Lyn phosphorylation and drastically reduced effector functions (degranulation and cytokine secretion). In contrast, CD45 E613R BMMCs show stronger effector functions after Ag-triggering than wild-type (WT) BMMCs. Despite these dichotomous phenotypes, phosphorylation of the inhibitory tyrosine in the SFK Lyn of CD45 E613R BMMCs is comparable to CD45-deficient BMMCs. This unexpected phenotype most likely is due to attenuated interaction between CD45 E613R and Lyn and a hyper-activation of the Fyn-regulated phosphatidylinositol-3-kinase pathway. Interestingly, depending on the receptor system addressed, CD45-deficient and CD45 E613R BMMCs show uniform phenotypes as well. Proliferation of both cell types in response to IL-3 and/or SF is enhanced compared to WT BMMCs. Together, the data indicate that CD45 plays a complex and essential role in fine-tuning mast cell responses mediated by different ligand–receptor systems.     

A CSF-1 receptor phosphotyrosine 559 signaling pathway regulates receptor ubiquitination and tyrosine phosphorylation

Receptor tyrosine kinase (RTK) activation involves ligand-induced receptor dimerization and transphosphorylation on tyrosine residues. Colony-stimulating factor-1 (CSF-1)-induced CSF-1 receptor (CSF-1R) tyrosine phosphorylation and ubiquitination were studied in mouse macrophages. Phosphorylation of CSF-1R Tyr-559, required for the binding of Src family kinases (SFKs), was both necessary and sufficient for these responses and for c-Cbl tyrosine phosphorylation and all three responses were inhibited by SFK inhibitors. In c-Cbl-deficient macrophages, CSF-1R ubiquitination and tyrosine phosphorylation were substantially inhibited. Reconstitution with wild-type, but not ubiquitin ligase-defective C381A c-Cbl rescued these responses, while expression of C381A c-Cbl in wild-type macrophages suppressed them. Analysis of site-directed mutations in the CSF-1R further suggests that activated c-Cbl-mediated CSF-1R ubiquitination is required for a conformational change in the major kinase domain that allows amplification of receptor tyrosine phosphorylation and full receptor activation. Thus the results indicate that CSF-1-mediated receptor dimerization leads to a Tyr-559/SFK/c-Cbl pathway resulting in receptor ubiquitination that permits full receptor tyrosine phosphorylation of this class III RTK in macrophages.     

Mutual Regulation of Src Family Kinases and the Neurotrophin Receptor TrkB

The neurotrophin receptor tyrosine kinase TrkB is critical to diverse biological processes. We investigated the interplay of Src family kinases (SFKs) and TrkB to better understand mechanisms of TrkB signaling in physiological and pathological conditions. We compared and contrasted the role of SFKs in TrkB signaling following activation of TrkB by two mechanisms, its transactivation by zinc, and its activation by its prototypic neurotrophin ligand, brain-derived neurotrophic factor (BDNF). Using biochemical, pharmacological, and chemical genetic studies of cultured rodent neurons, we found that zinc promotes preferential phosphorylation of Tyr-705/Tyr-706 of TrkB by a SFK-dependent but TrkB kinase-independent mechanism, a signaling event critical for transactivation of TrkB by zinc. By contrast, SFK activity is not essential for BDNF-mediated activation of TrkB, yet SFK activity is increased as a consequence of TrkB activation by BDNF. Moreover, BDNF-induced phosphorylation of Tyr-705/Tyr-706 of TrkB was inhibited by SFK inhibitors, implicating a regulatory role of SFKs in TrkB activation by BDNF. In sum, SFKs are activated by TrkB and, in turn, SFKs can promote TrkB activation. We propose models depicting the mutual regulation of SFKs and TrkB following activation of TrkB by zinc and BDNF.     

Paxillin family members function as Csk-binding proteins that regulate Lyn activity in human and murine platelets

SFKs (Src family kinases) contribute importantly to platelet function in haemostasis. SFK activity is controlled by Csk (C-terminal Src kinase), which phosphorylates a C-terminal tyrosine residue on SFKs, resulting in inhibition of SFK activity. Csk is recruited to sites of SFK activity by tyrosine-phosphorylated Csk-binding proteins. Paxillin, a multidomain adaptor protein, has been shown to act as a Csk-binding protein and to inhibit Src activity during growth factor signalling. Human platelets express Hic-5, a member of the paxillin family; however, its ability to act as a Csk-binding protein has not been characterized. We sought to identify and characterize the ability of paxillin family members to act as Csk-binding proteins during platelet activation. We found that murine and human platelets differ in the complement of paxillin family members expressed. Human platelets express Hic-5, whereas murine platelets express paxillin and leupaxin in addition to Hic-5. In aggregating human platelets, Hic-5 was tyrosine phosphorylated and recruited Csk via its SH2 domains. In aggregating murine platelets, however, Csk bound preferentially to paxillin, even though both paxillin and Hic-5 were abundantly present and became tyrosine phosphorylated. The SFK Lyn, but not Src or Fyn, was associated with paxillin family members in resting and aggregated human and murine platelets. Lyn, however, was phosphorylated on its C-terminal inhibitory tyrosine residue only following platelet aggregation, which was coincident with recruitment of Csk to paxillin and/or Hic-5 in a manner dependent on prior alpha(IIb)beta3 engagement. These observations support the notion that Hic-5 and paxillin function as negative feedback regulators of SFKs in aggregated platelets and that, when both are present, paxillin is preferentially used.     

Shp2 regulates SRC family kinase activity and Ras/Erk activation by controlling Csk recruitment

The protein-tyrosine phosphatase Shp2 plays an essential role in growth factor and integrin signaling, and Shp2 mutations cause developmental defects and/or malignancy. Previous work has placed Shp2 upstream of Ras. However, the mechanism of Shp2 action and its substrate(s) are poorly defined. Additional Shp2 functions downstream of, or parallel to, Ras/Erk activation also are proposed. Here, we show that Shp2 promotes Src family kinase (SFK) activation by regulating the phosphorylation of the Csk regulator PAG/Cbp, thereby controlling Csk access to SFKs. In Shp2-deficient cells, SFK inhibitory C-terminal tyrosines are hyperphosphorylated, and the tyrosyl phosphorylation of multiple SFK substrates, including Plcgamma1, is decreased. Decreased Plcgamma1 phosphorylation leads to defective Ras activation on endomembranes, and may help account for impaired Erk activation in Shp2-deficient cells. Decreased phosphorylation/activation of other SFK substrates may explain additional consequences of Shp2 deficiency, including altered cell spreading, stress fibers, focal adhesions, and motility.     

Endogenous and synthetic inhibitors of the Src-family protein tyrosine kinases

Src-family kinases (SFKs) are protooncogenic enzymes controlling mammalian cell growth and proliferation. The activity of SFKs is primarily regulated by two tyrosine phosphorylation sites: autophosphorylation of a conserved tyrosine (Y(A)) in the kinase domain results in activation while phosphorylation of the regulatory tyrosine (Y(T)) near the C-terminus leads to inactivation. The phosphorylated Y(T) (pY(T)) engages in intramolecular interactions that stabilise the inactive conformation of SFKs. These inhibitory intramolecular interactions include the binding of pY(T) to the SH2 domain and the binding of the SH2-kinase linker to the SH3 domain. Thus, SFKs are active upon (i) disruption of the inhibitory intramolecular interactions, (ii) autophosphorylation of Y(A) and/or (iii) dephosphorylation of pY(T). Since aberrant activation of SFKs contributes to cancer, SFKs in normal cells are kept inactive by multiple endogenous inhibitors classified as catalytic and non-catalytic inhibitors. The catalytic inhibitors include C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) that phosphorylate Y(T) of SFKs, as well as the protein tyrosine phosphatases that dephosphorylate pY(A) of the activated SFKs. The non-catalytic inhibitors inactivate SFKs by direct binding. CHK is unique among these inhibitors because it employs both catalytic and non-catalytic mechanisms to inhibit SFKs. Other known non-catalytic inhibitors include WASP, caveolin and RACK1, which function to down-regulate SFKs in specific subcellular locations. This review discusses how the various endogenous SFK inhibitors cooperate to regulate SFKs in normal cells. As chemical compounds that can selectively inhibit SFKs in vivo are potential anti-cancer therapeutics, this review also discusses how investigation into the inhibitory mechanisms of the endogenous inhibitors will benefit the design and screening of these compounds.     

Adaptor Protein GRB2 Promotes Src Tyrosine Kinase Activation and Podosomal Organization by Protein-tyrosine Phosphatase ? in Osteoclasts

The non-receptor isoform of protein-tyrosine phosphatase ϵ (cyt-PTPe) supports adhesion of bone-resorbing osteoclasts by activating Src downstream of integrins. Loss of cyt-PTPe reduces Src activity in osteoclasts, reduces resorption of mineralized matrix both in vivo and in cell culture, and induces mild osteopetrosis in young female PTPe KO mice. Activation of Src by cyt-PTPe is dependent upon this phosphatase undergoing phosphorylation at its C-terminal Tyr-638 by partially active Src. To understand how cyt-PTPe activates Src, we screened 73 Src homology 2 (SH2) domains for binding to Tyr(P)-638 of cyt-PTPe. The SH2 domain of GRB2 bound Tyr(P)-638 of cyt-PTPe most prominently, whereas the Src SH2 domain did not bind at all, suggesting that GRB2 may link PTPe with downstream molecules. Further studies indicated that GRB2 is required for activation of Src by cyt-PTPe in osteoclast-like cells (OCLs) in culture. Overexpression of GRB2 in OCLs increased activating phosphorylation of Src at Tyr-416 and of cyt-PTPe at Tyr-638; opposite results were obtained when GRB2 expression was reduced by shRNA or by gene inactivation. Phosphorylation of cyt-PTPe at Tyr-683 and its association with GRB2 are integrin-driven processes in OCLs, and cyt-PTPe undergoes autodephosphorylation at Tyr-683, thus limiting Src activation by integrins. Reduced GRB2 expression also reduced the ability of bone marrow precursors to differentiate into OCLs and reduced the fraction of OCLs in which podosomal adhesion structures assume organization typical of active, resorbing cells. We conclude that GRB2 physically links cyt-PTPe with Src and enables cyt-PTPe to activate Src downstream of activated integrins in OCLs.     

Receptor-type protein tyrosine phosphatase beta (RPTP-beta) directly dephosphorylates and regulates hepatocyte growth factor receptor (HGFR/Met) function

Protein tyrosine phosphorylation is a ubiquitous, fundamental biochemical mechanism that regulates essential eukaryotic cellular functions. The level of tyrosine phosphorylation of specific proteins is finely tuned by the dynamic balance between protein tyrosine kinase and protein tyrosine phosphatase activities. Hepatocyte growth factor receptor (also known as Met), a receptor protein tyrosine kinase, is a major regulator of proliferation, migration, and survival for many epithelial cell types. We report here that receptor-type protein tyrosine phosphatase β (RPTP-β) specifically dephosphorylates Met and thereby regulates its function. Expression of RPTP-β, but not other RPTP family members or catalytically inactive forms of RPTP-β, reduces hepatocyte growth factor (HGF)-stimulated Met tyrosine phosphorylation in HEK293 cells. Expression of RPTP-β in primary human keratinocytes reduces both basal and HGF-induced Met phosphorylation at tyrosine 1356 and inhibits downstream MEK1/2 and Erk activation. Furthermore, shRNA-mediated knockdown of endogenous RPTP-β increases basal and HGF-stimulated Met phosphorylation at tyrosine 1356 in primary human keratinocytes. Purified RPTP-β intracellular domain preferentially dephosphorylates purified Met at tyrosine 1356 in vitro. In addition, the substrate-trapping mutant of RPTP-β specifically interacts with Met in intact cells. Expression of RPTP-β in human primary keratinocytes reduces HGF induction of VEGF expression, proliferation, and motility. Taken together, the above data indicate that RPTP-β is a key regulator of Met function.     

A new pathway of CD5 glycoprotein-mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase

Triggering of the T cell receptor initiates a signaling cascade resulting in the activation of the T cell. These signals are integrated alongside those resulting from the triggering of other receptors whose function is to modulate the overall response. CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators such as SHP-1, RasGAP, or Cbl. We now propose that the inhibitory effects of CD5 are also mediated by a parallel pathway that functions at the level of inhibition of Fyn, a kinase generally associated with T cell receptor-mediated activation. After CD5 ligation, phosphorylation of the negative regulatory tyrosine (Tyr(531)) of Fyn increases, and this correlates with a substantial reduction in the kinase activity of Fyn and a profound inhibition of ZAP-70 activation. The effect requires the last 23 amino acids of the cytoplasmic domain of the receptor, strongly implying the involvement of a new CD5-interacting signaling or adaptor protein. Furthermore, we show that upon CD5 ligation there is a profound shift in its distribution from the bulk fluid phase to the lipid raft environment, where it associates with Fyn, Lck, and PAG. We suggest that the relocation of CD5, which we also show is capable of forming homodimers, to the proximity of raft-resident molecules enables CD5 to inhibit membrane proximal signaling by controlling the phosphorylation and activity of Fyn, possibly by interfering with the disassembly of C-terminal Src kinase (Csk)-PAG-Fyn complexes during T cell activation.     

Cellular settings mediating Src Substrate switching between focal adhesion kinase tyrosine 861 and CUB-domain-containing protein 1 (CDCP1) tyrosine 734

Reciprocal interactions between Src family kinases (SFKs) and focal adhesion kinase (FAK) are critical during changes in cell attachment. Recently it has been recognized that another SFK substrate, CUB-domain-containing protein 1 (CDCP1), is differentially phosphorylated during these events. However, the molecular processes underlying SFK-mediated phosphorylation of CDCP1 are poorly understood. Here we identify a novel mechanism in which FAK tyrosine 861 and CDCP1-Tyr-734 compete as SFK substrates and demonstrate cellular settings in which SFKs switch between these sites. Our results show that stable CDCP1 expression induces robust SFK-mediated phosphorylation of CDCP1-Tyr-734 with concomitant loss of p-FAK-Tyr-861 in adherent HeLa cells. SFK substrate switching in these cells is dependent on the level of expression of CDCP1 and is also dependent on CDCP1-Tyr-734 but is independent of CDCP1-Tyr-743 and -Tyr-762. In HeLa CDCP1 cells, engagement of SFKs with CDCP1 is accompanied by an increase in phosphorylation of Src-Tyr-416 and a change in cell morphology to a fibroblastic appearance dependent on CDCP1-Tyr-734. SFK switching between FAK-Tyr-861 and CDCP1-Tyr-734 also occurs during changes in adhesion of colorectal cancer cell lines endogenously expressing these two proteins. Consistently, increased p-FAK-Tyr-861 levels and a more epithelial morphology are seen in colon cancer SW480 cells silenced for CDCP1. Unlike protein kinase Cδ, FAK does not appear to form a trimeric complex with Src and CDCP1. These data demonstrate novel aspects of the dynamics of SFK-mediated cell signaling that may be relevant during cancer progression.     

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.