Disease relevance of Csk

• Functional analysis of Csk and CHK kinases in breast cancer cells [1].
• The recombinant adenovirus vector carrying csk cDNA induced Csk expression in mouse osteoclast-like cells formed in vitro and clearly reduced c-Src kinase activity in a dose-dependent manner [2].
• Src/Csk transfectants developed normally in the initial stages of neural differentiation, but exhibited an apparent defect in cell-to-cell interaction, i.e. neurite fasciculation and aggregation of cell bodies, in the latter stages [3].
• Of these candidates, Csk is the strongest for NC dermatitis, since its map position was most tightly linked to the derm1 locus [4].
• Importantly, Csk overexpression in NL-17 cells resulted in significant suppression of in vivo metastasis, without affecting its tumorgenicity [5].

High impact information on Csk

• These results demonstrate that Src family kinase activity is critically dependent on phosphorylation by Csk and suggest that the regulation of kinase activity may be essential during embryogenesis [6].
• To analyze the significance of this regulation during development, we have generated mice deficient in Csk, a kinase that phosphorylates this tyrosine, by gene targeting in embryonic stem cells [6].
• This provides evidence for the involvement of Csk in the regulation of T-cell activation [7].
• Hyperphosphorylation of key signaling proteins such as Syk and Paxillin in mutant granulocytes further supported breakdown of the activation threshold set by Csk [8].
• Csk-deficient granulocytes showed enhanced spontaneous and ligand-induced degranulation with hyperinduction of integrins [8].

Chemical compound and disease context of Csk

• Overexpression of Csk decreased Src tyrosine kinase activity in NL-17 cells, the highly metastatic clone of mouse colon adenocarcinoma 26 [5].
• First, we could show that chemical inhibition of c-src kinase could produce practically the same phenomenon of reduced toxicity of TCDD in wild-type mice cotreated with geldanamycin, a specific chemical inhibitor known to suppress c-src kinase [9].
• The gender-specific mechanisms of TCDD-induced toxicity appear to involve c-Src kinase, nPKC epsilon, and pan-Ras, as well as overlap in the cytosolic signal transduction pathways of TCDD and sex steroids [10].

Biological context of Csk

• The function of Csk as a repressor of Lck and Fyn activity suggests activation of these PTKs is solely responsible for the phenotype observed in csk-deficient T lineage cells [11].
• These findings suggest that Cbp could serve as a sensor of SFK activity in early stages of cell adhesion signaling, and that Csk-mediated down-regulation of SFK is essential to allow dynamic cellular events involved in the regulation of cell spreading and migration [12].
• Our studies showed that PAG tyrosine phosphorylation and association with Csk are suppressed in response to activation of normal mouse T cells [13].
• In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the Fc gamma R-mediated signaling that leads to phagocytosis in a kinase-dependent manner [14].
• These findings imply that Csk is involved in the regulation of Src family kinases that play roles in cell-to-cell interaction mediated by cell adhesion molecules [3].

Anatomical context of Csk

• It remains unclear whether activation of Lck and Fyn in the absence of Csk uncouples alpha/beta T cell development entirely from engagement of surface-expressed receptors [11].
• Cbp, a C-terminal Src kinase (Csk)-binding protein, is a transmembrane phosphoprotein that has been implicated in the regulation of the Src family kinase (SFK) through recruiting Csk, a negative regulator of SFK, to a membrane microdomain of lipid rafts [12].
• They also show a role for Csk in P. aeruginosa invasion, while providing further evidence that actin cytoskeleton disruption contributes to ExsA-regulated P. aeruginosa cytotoxicity and invasion inhibition [15].
• Both p130Cas and c-Cbl have been reported to play critical roles in osteoclast function as downstream targets of c-Src kinase [16].
• We measured c-Src kinase activity in cell lysates from proliferating, growth-arrested and differentiating 3T3-L1 cells [17].

Associations of Csk with chemical compounds

• Further study revealed that the Src kinase was activated by HgCl2 at least in part independent of the known Csk kinase-linked or Y527-phosphorylation/dephosphorylation-mediated control [18].
• Since SH2 domains recognize phosphotyrosine, it is possible that these two non-catalytic domains associate, and thereby repress c-Src kinase activity [19].
• Further analyses revealed that this interaction was mediated by the Csk SH3 domain and by a proline-rich region (PPPLPERTP) in the non-catalytic C-terminal portion of PEP [20].
• This phosphorylation induces the binding of Dok-3 to at least two inhibitory molecules, the 5' inositol phosphatase SHIP and the protein tyrosine kinase Csk [21].
• In Y567F, Lyn activation on SCF stimulation decreased and C-terminal Src kinase (Csk) suppressed KIT-mediated Ca(2+) influx and cell migration, suggesting that Y567-mediated Src family kinase (SFK) activation leads to Ca(2+) influx and migration [22].

Physical interactions of Csk

• CD45-D2 bound similarly to Src family kinases but bound Csk to a lesser extent and did not bind significantly to the less related kinase, Erk1 [23].
• In addition, tyrosine-phosphorylated PSD-93 was able to bind to Csk, a negative regulator of Src family kinases, in vitro as well as in a brain lysate [24].
• In this report, we have tested the possibility that Csk also interacts with PTP-PEST, a ubiquitous PTP sharing structural homology with PEP [25].

Enzymatic interactions of Csk

• Firm evidence that Ntk is a Csk-related enzyme was provided by the observation that it phosphorylated a Src-related polypeptide on the inhibitory carboxyl-terminal tyrosine [26].
• Paxillin was also tyrosine-phosphorylated in Csk-overexpressing cells, indicating that it can serve as a substrate of Csk [27].
• Csk binds specifically to the phosphorylated caveolin-1 and attenuates its stress-induced phosphorylation [28].

Regulatory relationships of Csk

• We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope [14].
• Csk recruited to rafts would then be activated and inhibit the kinase activity of Lck to keep resting T cells in a quiescent state [29].
• Recently, we have shown that these Neu-induced mammary tumors possess elevated c-Src tyrosine kinase activity [30].
• Cellular stress induces the tyrosine phosphorylation of caveolin-1 (Tyr(14)) via activation of p38 mitogen-activated protein kinase and c-Src kinase. Evidence for caveolae, the actin cytoskeleton, and focal adhesions as mechanical sensors of osmotic stress [31].
• Previously, we showed that RACK1 inhibits c-Src kinase activity and NIH3T3 cell growth [32].

Other interactions of Csk

• These data suggest a differential involvement of the Csk-mediated regulation of Src family PTKs in positive and negative selection of developing thymocytes [11].
• In resting T cells, Csk is constitutively localized in lipid rafts by virtue of interaction with a phosphorylated adaptor protein, Csk-binding protein (Cbp)/phosphoprotein associated with glycolipid-enriched microdomains, and sets an activation threshold in TCR signaling [29].
• Interestingly, there was a concomitant increase in the phosphorylation of Fyn on its Csk (C-terminal Src kinase) site, indicating feedback inhibition [33].
• Upon cell adhesion onto fibronectin, Cbp becomes transiently phosphorylated (consistent with SFK activation) and recruits Csk to lipid rafts [12].
• Csk expression and kinase activity were abundant in normal human breast tissues, breast carcinomas, and breast cancer cell lines, whereas CHK expression was negative in normal breast tissues and low in some breast tumors and in the MCF-7 breast cancer cell line [1].

Analytical, diagnostic and therapeutic context of Csk

• We used the polymerase chain reaction with degenerate oligonucleotide primers to search for Csk-related kinases [34].
• Semiquantitative RT-PCR analysis revealed that Csk mRNA increased during neonatal development and peaked at 2 weeks of age [35].
• In Western blot, immature testes showed higher expression of Csk protein than the pubertal or adult testes [35].
• Peptide mapping of in vivo labeled c-Src kinase excluded the involvement of phospho-Tyr-527 dephosphorylation in the SNAP-mediated activation mechanism [36].

References