Molecular cloning and expression of chicken C-terminal Src kinase: lack of stable association with c-Src protein

A concise review on tyrosine kinase targeted cancer therapy

Abstract:

The tyrosine kinase (TK) family is considered one of the important family members of the kinase family due to its important role in various cellular processes like cell growth, cell differentiation, apoptosis, etc. Mutation, overexpression, and dysfunction of tyrosine kinase receptors lead to the development of malignancy; thus, they are considered as one of the important targets for the development of anti-cancer molecules. The tyrosine kinase family is majorly divided into two classes; receptor and non-receptor tyrosine kinase. Both of the classes have an important role in the development of tumour cells. Currently, there are more than 40 FDA-approved tyrosine kinase inhibitors, which are used in the treatment of various types of cancers. Tyrosine kinase inhibitors mainly block the phosphorylation of tyrosine residue of the corresponding kinase substrate and so activation of downstream signalling pathways can be inhibited. The promising results of tyrosine kinase inhibitors in solid tumours provide a revolution in oncology research. In this article, we had summarized the role of some important members of the tyrosine kinase family in the development and progression of tumour cells and the significance of tyrosine kinase inhibitors in the treatment of various types of cancer.

The Rho guanine nucleotide exchange factor PLEKHG1 is activated by interaction with and phosphorylation by Src family kinase member FYN

Rho family small GTPases (Rho) regulate various cell motility processes by spatiotemporally controlling the actin cytoskeleton. Some Rho-specific guanine nucleotide exchange factors (RhoGEFs) are regulated via tyrosine phosphorylation by Src family tyrosine kinase (SFK). We also previously reported that PLEKHG2, a RhoGEF for the GTPases Rac1 and Cdc42, is tyrosine-phosphorylated by SRC. However, the details of the mechanisms by which SFK regulates RhoGEFs are not well understood. In this study, we found for the first time that PLEKHG1, which has very high homology to the Dbl and pleckstrin homology domains of PLEKHG2, activates Cdc42 following activation by FYN, a member of the SFK family. We also show that this activation of PLEKHG1 by FYN requires interaction between these two proteins and FYN-induced tyrosine phosphorylation of PLEKHG1. We also found that the region containing the Src homology 3 and Src homology 2 domains of FYN is required for this interaction. Finally, we demonstrated that tyrosine phosphorylation of Tyr-720 and Tyr-801 in PLEKHG1 is important for the activation of PLEKHG1. These results suggest that FYN is a regulator of PLEKHG1 and may regulate cell morphology through Rho signaling via the interaction with and tyrosine phosphorylation of PLEKHG1.

Understanding the cellular roles of Fyn-related kinase (FRK): implications in cancer biology

The non-receptor tyrosine kinase Fyn-related kinase (FRK) is a member of the BRK family kinases (BFKs) and is distantly related to the Src family kinases (SFKs). FRK was first discovered in 1993, and studies pursued thereafter attributed a potential tumour-suppressive function to the enzyme. In recent years, however, further functional characterization of the tyrosine kinase in diverse cancer types suggests that FRK may potentially play an oncogenic role as well. Specifically, while ectopic expression of FRK suppresses cell proliferation and migration in breast and brain cancers, knockdown or catalytic inhibition of FRK suppresses these cellular processes in pancreatic and liver cancer. Such functional paradox is therefore evidently exhibited in a tissue-specific context. This review sheds light on the recent developments emerged from investigations on FRK which include: (a) a review of the expression pattern of the protein in mammalian cells/tissues, (b) underlying genomic perturbations and (c) a mechanistic function of the enzyme across different cellular environments. Given its functional heterogeneity observed across different cancers, we also discuss the therapeutic significance of FRK.

Non-receptor protein tyrosine kinases signaling pathways in normal and cancer cells

Protein tyrosine kinases (PTKs) are enzymes that transfer phosphate groups to tyrosine residues on protein substrates. Phosphorylation of proteins causes changes in their function and/or enzymatic activity resulting in specific biological responses. There are two classes of PTKs: the transmembrane receptor PTKs and the cytoplasmic non-receptor PTKs (NRTKs). NRTKs are involved in transduction of signals originating from extracellular clues, which often interact with transmembrane receptors. Thus, they are important components of signaling pathways which regulate fundamental cellular functions such as cell differentiation, apoptosis, survival, and proliferation. The activity of NRTKs is tightly regulated, and de-regulation and/or overexpression of NRTKs has been implicated in malignant transformation and carcinogenesis. Research on NRTKs has shed light on the mechanisms of a number of cellular processes including those involved in carcinogenesis. Not surprisingly, several tyrosine kinase inhibitors are in use as treatment for a number of malignancies, and more are under investigation. This review deals with the structure, function, and signaling pathways of nine main families of NRTKs in normal and cancer cells.

Chemopreventive activity of plant flavonoid isorhamnetin in colorectal cancer is mediated by oncogenic Src and β-catenin

Analysis of the Polyp Prevention Trial showed an association between an isorhamnetin-rich diet and a reduced risk of advanced adenoma recurrence; however, the mechanism behind the chemoprotective effects of isorhamnetin remains unclear. Here, we show that isorhamnetin prevents colorectal tumorigenesis of FVB/N mice treated with the chemical carcinogen azoxymethane and subsequently exposed to colonic irritant dextran sodium sulfate (DSS). Dietary isorhamnetin decreased mortality, tumor number, and tumor burden by 62%, 35%, and 59%, respectively. MRI, histopathology, and immunohistochemical analysis revealed that dietary isorhamnetin resolved the DSS-induced inflammatory response faster than the control diet. Isorhamnetin inhibited AOM/DSS-induced oncogenic c-Src activation and β-catenin nuclear translocation, while promoting the expression of C-terminal Src kinase (CSK), a negative regulator of Src family of tyrosine kinases. Similarly, in HT-29 colon cancer cells, isorhamnetin inhibited oncogenic Src activity and β-catenin nuclear translocation by inducing expression of csk, as verified by RNA interference knockdown of csk. Our observations suggest the chemoprotective effects of isorhamnetin in colon cancer are linked to its anti-inflammatory activities and its inhibition of oncogenic Src activity and consequential loss of nuclear β-catenin, activities that are dependent on CSK expression.

Fulvestrant-induced cell death and proteasomal degradation of estrogen receptor α protein in MCF-7 cells require the CSK c-Src tyrosine kinase

Fulvestrant is a representative pure antiestrogen and a Selective Estrogen Receptor Down-regulator (SERD). In contrast to the Selective Estrogen Receptor Modulators (SERMs) such as 4-hydroxytamoxifen that bind to estrogen receptor α (ERα) as antagonists or partial agonists, fulvestrant causes proteasomal degradation of ERα protein, shutting down the estrogen signaling to induce proliferation arrest and apoptosis of estrogen-dependent breast cancer cells. We performed genome-wide RNAi knockdown screenings for protein kinases required for fulvestrant-induced apoptosis of the MCF-7 estrogen-dependent human breast caner cells and identified the c-Src tyrosine kinase (CSK), a negative regulator of the oncoprotein c-Src and related protein tyrosine kinases, as one of the necessary molecules. Whereas RNAi knockdown of CSK in MCF-7 cells by shRNA-expressing lentiviruses strongly suppressed fulvestrant-induced cell death, CSK knockdown did not affect cytocidal actions of 4-hydroxytamoxifen or paclitaxel, a chemotherapeutic agent. In the absence of CSK, fulvestrant-induced proteasomal degradation of ERα protein was suppressed in both MCF-7 and T47D estrogen-dependent breast cancer cells whereas the TP53-mutated T47D cells were resistant to the cytocidal action of fulvestrant in the presence or absence of CSK. MCF-7 cell sensitivities to fulvestrant-induced cell death or ERα protein degradation was not affected by small-molecular-weight inhibitors of the tyrosine kinase activity of c-Src, suggesting possible involvement of other signaling molecules in CSK-dependent MCF-7 cell death induced by fulvestrant. Our observations suggest the importance of CSK in the determination of cellular sensitivity to the cytocidal action of fulvestrant.

Regulation of the SRC family kinases by Csk

The non-receptor tyrosine kinase Csk serves as an indispensable negative regulator of the Src family tyrosine kinases (SFKs) by specifically phosphorylating the negative regulatory site of SFKs, thereby suppressing their oncogenic potential. Csk is primarily regulated through its SH2 domain, which is required for membrane translocation of Csk via binding to scaffold proteins such as Cbp/PAG1. The binding of scaffolds to the SH2 domain can also upregulate Csk kinase activity. These regulatory features have been elucidated by analyses of Csk structure at the atomic levels. Although Csk itself may not be mutated in human cancers, perturbation of the regulatory system consisting of Csk, Cbp/PAG1, or other scaffolds, and certain tyrosine phosphatases may explain the upregulation of SFKs frequently observed in human cancers. This review focuses on the molecular bases for the function, structure, and regulation of Csk as a unique regulatory tyrosine kinase for SFKs.

SRC family kinases are required for limb trajectory selection by spinal motor axons

Signal relay by guidance receptors at the axonal growth cone is a process essential for the assembly of a functional nervous system. We investigated the in vivo function of Src family kinases (SFKs) as growth cone guidance signaling intermediates in the context of spinal lateral motor column (LMC) motor axon projection toward the ventral or dorsal limb mesenchyme. Using in situ mRNA detection we determined that Src and Fyn are expressed in LMC motor neurons of chick and mouse embryos at the time of limb trajectory selection. Inhibition of SFK activity by C-terminal Src kinase (Csk) overexpression in chick LMC axons using in ovo electroporation resulted in LMC axons selecting the inappropriate dorsoventral trajectory within the limb mesenchyme, with medial LMC axon projecting into the dorsal and ventral limb nerve with apparently random incidence. We also detected LMC axon trajectory choice errors in Src mutant mice demonstrating a nonredundant role for Src in motor axon guidance in agreement with gain and loss of Src function in chick LMC neurons which led to the redirection of LMC axons. Finally, Csk-mediated SFK inhibition attenuated the retargeting of LMC axons caused by EphA or EphB over-expression, implying the participation of SFKs in Eph-mediated LMC motor axon guidance. In summary, our findings demonstrate that SFKs are essential for motor axon guidance and suggest that they play an important role in relaying ephrin:Eph signals that mediate the selection of motor axon trajectory in the limb.

Reactive landing of gas-phase ions as a tool for the fabrication of metal oxide surfaces for in situ phosphopeptide enrichment

Zirconium, titanium, and hafnium oxide-coated stainless steel surfaces are fabricated by reactive landing of gas-phase ions produced by electrospray ionization of group IVB metal alkoxides. The surfaces are used for in situ enrichment of phosphopeptides before analysis by matrix-assisted laser desorption ionization (MALDI) mass spectrometry. To evaluate this method we characterized ZrO(2) (zirconia) surfaces by (1) comparison with the other group IVB metal oxides of TiO(2) (titania) and HfO(2) (hafnia), (2) morphological characterization by SEM image analysis, and (3) dependence of phosphopeptide enrichment on the metal oxide layer thickness. Furthermore, we evaluated the necessity of the reactive landing process for the construction of useful metal oxide surfaces by preparing surfaces by electrospray deposition of Zr, Ti, and Hf alkoxides directly onto polished metal surfaces at atmospheric pressure. Although all three metal oxide surfaces evaluated were capable of phosphopeptide enrichment from complex peptide mixtures, zirconia performed better than hafnia or titania as a result of morphological characteristics illustrated by the SEM analysis. Metal oxide coatings that were fabricated by atmospheric pressure deposition were still capable of in situ phosphopeptide enrichment, although with inferior efficiency and surface durability. We show that zirconia surfaces prepared by reactive landing of gas-phase ions can be a useful tool for high throughput screening of novel phosphorylation sites and quantitation of phosphorylation kinetics.

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK)--endogenous negative regulators of Src-family protein kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors of the Src-family protein tyrosine kinases (SFKs). Since constitutive activation of SFKs contributes to cancer formation and progression, to prevent excessive activation of SFKs, their activity in normal cells is kept at the basal level by CSK and CHK. CSK and CHK inactivate SFKs by specifically phosphorylating a consensus tyrosine (called Y(T)) near their C-termini. Upon phosphorylation, the phospho-Y(T) engages in intramolecular interactions that lock the SFK molecule in an inactive conformation. SFKs are anchored to the plasma membrane, while CSK and CHK are localized predominantly in the cytosol. To inhibit SFKs, CSK and CHK need to translocate to the plasma membrane. Recruitment of CSK and CHK to the plasma membrane is mediated by the binding of their SH2, SH3 and/or kinase domains to specific transmembrane proteins, G-proteins and adaptor proteins located near the plasma membrane. For CSK, membrane recruitment often accompanies activation. CSK and CHK employ two types of direct interactions with SFKs to achieve efficient Y(T) phosphorylation: (i) short-range interactions involving binding of the active sites of CSK and CHK to specific residues near Y(T), (ii) long-range non-catalytic interactions involving binding of SFKs to motifs located distally from the active sites of CSK and CHK. The interactions between CSK and SFKs are transient in nature. Unlike CSK, CHK binds tightly to SFKs to form stable protein complexes. The binding is non-catalytic as it is independent of Y(T). More importantly, the tight binding alone is sufficient to completely inhibit SFKs. This non-catalytic inhibitory binding represents a novel mechanism employed by CHK to inhibit SFKs. Given that SFKs are implicated in cancer development, compounds mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics.

A novel non-catalytic mechanism employed by the C-terminal Src-homologous kinase to inhibit Src-family kinase activity

Although C-terminal Src kinase (CSK)-homologous kinase (CHK) is generally believed to inactivate Src-family tyrosine kinases (SFKs) by phosphorylating their consensus C-terminal regulatory tyrosine (Tyr(T)), exactly how CHK inactivates SFKs is not fully understood. Herein, we report that in addition to phosphorylating Tyr(T), CHK can inhibit SFKs by a novel non-catalytic mechanism. First, CHK directly binds to the SFK members Hck, Lyn, and Src to form stable protein complexes. The complex formation is mediated by a non-catalytic Tyr(T)-independent mechanism because it occurs even in the absence of ATP or when Tyr(T) of Hck is replaced by phenylalanine. Second, the non-catalytic CHK-SFK interaction alone is sufficient to inactivate SFKs by inhibiting the catalytic activity of autophosphorylated SFKs. Third, CHK and Src co-localize to specific plasma membrane microdomains of rat brain cells, suggesting that CHK is in close proximity to Src such that it can effectively inactivate Src in vivo. Fourth, native CHK.Src complex exists in rat brain, and recombinant CHK.Hck complex exists in transfected HEK293T cells, implying that CHK forms stable complexes with SFKs in vivo. Taken together, our findings suggest that CHK inactivates SFKs (i) by phosphorylating their Tyr(T) and (ii) by this novel Tyr(T)-independent mechanism involving direct binding of CHK to SFKs. It has been documented that autophosphorylated SFKs can still be active, in some cases even when their Tyr(T) is phosphorylated. Thus, the ability of the Tyr(T)-independent mechanism to suppress the activity of both non-phosphorylated and autophosphorylated SFKs represents a fail-safe measure employed by CHK to down-regulate SFK signaling under all circumstances.

Csk defines the ability of integrin-mediated cell adhesion and migration in human colon cancer cells: implication for a potential role in cancer metastasis

Progression of human colon cancer is often associated with elevated expression and activity of the Src family tyrosine kinase (SFK). SFK is ordinarily in equilibrium between inactive and primed states by a balance of negative regulatory kinase Csk and its counteracting tyrosine phosphatase(s), both of which act on the regulatory C-terminal tyrosine of SFK. To evaluate the contribution of the regulatory system of SFK in cancer progression, we here modulated the equilibrium status of SFK by introducing wild-type or dominant-negative Csk in human epithelial colon cancer cells, HCT15 and HT29. Overexpression of wild-type Csk induced decreased SFK activation, increased cell-cell contacts mediated by E-cadherin, decreased the number of focal contacts and decreased cell adhesion/migration and in vitro invasiveness. Conversely, expression of a dominant-negative Csk resulted in elevated SFK activation, enhanced phosphorylation of FAK and paxilllin, enhanced cell scattering, an increased number of focal contacts, dramatic rearrangement of actin cytoskeleton and increased cell adhesion/migration and in vitro invasiveness. In these scattered cells, however, localization, expression and phosphorylation of either E-cadherin or beta-catenin were not significantly affected, suggesting that the E-cadherin-mediated cell-cell contact is indirectly regulated by SFK. Furthermore, all these events occurred absolutely dependent on integrin-mediated cell adhesion. These findings demonstrate that Csk defines the ability of integrin-SFK-mediated cell adhesion signaling that influences the metastatic potential of cancer cells.

TRAF6 and C-SRC induce synergistic AP-1 activation via PI3-kinase-AKT-JNK pathway

Interleukin-1 (IL-1) induces multiple genes via activation of transcription factors that include NF-kappa B and activator protein-1 (AP-1). We found that IL-1-mediated c-Src activation was required for AP-1 activation, but not for NF-kappa B activation and also revealed that c-Src-induced AP-1 activation was enhanced synergistically by the coexpression of TNF receptor associated factor 6 (TRAF6). In addition, c-Src interacts with TRAF6 in response to IL-1 and this interaction is required for c-Src activity. However, neither dominant negative mutants of TRAF6 (TRAF6 DN) nor kinase-dead mutant of c-Src (c-Src KD) counteracted each-induced AP-1 activation, suggesting no hierarchy between these two molecules. During the TRAF6 and c-Src-induced AP-1 activation, phosphatidylinositol 3 (PI3)-kinase, its downstream signaling molecule, Akt and c-Jun N-terminal kinase (JNK) were significantly activated and inhibition of these kinase activities down-regulated AP-1 activation through the suppression of c-fos expression. Furthermore, TRAF6 and c-Src-induced JNK activation was significantly inhibited by PI3-kinase inhibitor or a dominant negative mutant of Akt (Akt DN). Taken together, our results demonstrate that c-Src and TRAF6 are key mediators of IL-1-induced AP-1 activation and provide evidence of cross talk between c-Src and TRAF6 molecules through PI3 kinase-Akt-JNK pathways.

Ca2+/calmodulin-dependent protein kinase II stimulates c-fos transcription and DNA synthesis by a Src-based mechanism in glomerular mesangial cells

Mesangial cell growth factors elevate intracellular free [Ca2+]i, but mechanisms linking [Ca2+]i to gene expression and DNA synthesis are unclear. This study investigated the hypothesis that Ca2+/calmodulin-dependent protein kinase II (CaMK II), which is activated by elevated [Ca2+]i, increases c-fos transcription and DNA synthesis via a Src-based mechanism. In cultured rat mesangial cells, dominant negative Src (SrcK-) blocked activation of the c-fos gene promoter by CaMK II 290, a constitutively active form of CaMK IIalpha. Activation of the c-fos promoter by CaMK II 290 was also blocked by COOH-terminal Src kinase, which phosphorylates and inactivates c-Src. A pharmacologic CaMK inhibitor, KN-93, did not block activation of the c-fos promoter by ectopically expressed v-Src. Stimulation of c-Src by endothelin-1 required CaMK II activity, further supporting the notion that CaMK II acts upstream of Src in a signaling cassette. Activation of the c-fos promoter by CaMKII290 and Src required the c-fos serum response element. Dominant negative SrcK- also blocked induction of DNA synthesis in mesangial cells by CaMK II 290. Collectively, these results suggest that in mesangial cells Src protein tyrosine kinases act downstream of CaMKII in a signaling pathway in which [Ca2+]i induces the c-fos promoter and increases DNA synthesis.

A novel, specific interaction involving the Csk SH3 domain and its natural ligand

C-terminal Src kinase (Csk) takes part in a highly specific, high affinity interaction via its Src homology 3 (SH3) domain with the proline-enriched tyrosine phosphatase PEP in hematopoietic cells. The solution structure of the Csk-SH3 domain in complex with a 25-residue peptide from the Pro/Glu/Ser/Thr-rich (PEST) domain of PEP reveals the basis for this specific peptide recognition motif involving an SH3 domain. Three residues, Ala 40, Thr 42 and Lys 43, in the SH3 domain of Csk specifically recognize two hydrophobic residues, Ile 625 and Val 626, in the proline-rich sequence of the PEST domain of PEP. These two residues are C-terminal to the conventional proline-rich SH3 domain recognition sequence of PEP. This interaction is required in addition to the classic polyproline helix (PPII) recognition by the Csk-SH3 domain for the association between Csk and PEP in vivo. NMR relaxation analysis suggests that Csk-SH3 has different dynamic properties in the various subsites important for peptide recognition.

Expression of protein tyrosine kinases and stem cell factor in chicken blastodermal cells

Chicken blastodermal cells (CBC) from Stage X embryos, which were isolated from newly laid, fertile, unincubated eggs, are pluripotent cells and can produce somatic and germline chimeras when injected into recipient stage X embryos. The CBC retain their pluripotential ability for up to 7 d in vitro. The molecular mechanisms that control proliferation and differentiation of CBC are largely unknown, although protein tyrosine kinases (PTK) are known to play important roles in these processes in similar cells. To understand better the molecular mechanisms of proliferation and differentiation in CBC, expression profiles of PTK and stem cell factor (SCF) were analyzed by reverse transcription polymerase chain reaction (RT-PCR) using gene-specific and degenerate oligonucleotide primers. Seventeen distinct PTK, including 14 receptor-type and 3 nonreceptor-type PTK and SCF were identified by RT-PCR. Expression of all of the genes was confirmed by northern blot analysis. The northern blot analysis showed that all probes hybridized with one or more transcripts at various expression levels. The expression of the 17 PTK and SCF genes in CBC suggests that they might play a role in signal transduction pathways that control the proliferation or differentiation in CBC.

Meltrin alpha cytoplasmic domain interacts with SH3 domains of Src and Grb2 and is phosphorylated by v-Src

Meltrin alpha/ADAM12 is a member of the ADAM/MDC family proteins characterized by the presence of metalloprotease and disintegrin domains. This protein also contains a single transmembrane domain and a relatively long cytoplasmic domain containing several proline-rich sequences. These sequences are compatible with the consensus sequences for binding the Src homology 3 (SH3) domains. To determine whether the proline-rich sequences interact with SH3 domains in several proteins, binding of recombinant SH3 domains to the meltrin alpha cytoplasmic domain was analysed by pull-down assays. The SH3 domains of Src and Yes bound strongly, but that of Abl or phosphatidylinositol 3-kinase p85 subunit did not. Full-length Grb2/Ash bound strongly, whereas its N-terminal SH3 domain alone did less strongly. Src and Grb2 in bovine brain extracts also bound to meltrin alpha cytoplasmic domain on affinity resin. Furthermore, immunoprecipitation with a monoclonal antibody to meltrin alpha resulted in coprecipitation of Src and Grb2 with meltrin alpha in cell extracts, suggesting that Src and Grb2 are associated in vivo with meltrin alpha cytoplasmic domain. This notion was also supported by the findings that exogenously expressed meltrin cytoplasmic domain coexisted with Src and Grb2 on the membrane ruffles. The C-terminal Tyr901 of meltrin alpha was phosphorylated both in vitro and in cultured cells by v-Src. These results may imply that meltrin alpha cytoplasmic domain is involved in a signal transduction for some biological function through the interaction with SH3-containing proteins.

Constitutive and growth factor-regulated phosphorylation of caveolin-1 occurs at the same site (Tyr-14) in vivo: identification of a c-Src/Cav-1/Grb7 signaling cassette

Caveolin-1 was first identified as a phosphoprotein in Rous sarcoma virus (RSV)-transformed chicken embryo fibroblasts. Tyrosine 14 is now thought to be the principal site for recognition by c-Src kinase; however, little is known about this phosphorylation event. Here, we generated a monoclonal antibody (mAb) probe that recognizes only tyrosine 14-phosphorylated caveolin-1. Using this approach, we show that caveolin-1 (Y14) is a specific tyrosine kinase substrate that is constitutively phosphorylated in Src- and Abl-transformed cells and transiently phosphorylated in a regulated fashion during growth factor signaling. We also provide evidence that tyrosine-phosphorylated caveolin-1 is localized at the major sites of tyrosine-kinase signaling, i.e. focal adhesions. By analogy with other signaling events, we hypothesized that caveolin-1 could serve as a docking site for pTyr-binding molecules. In support of this hypothesis, we show that phosphorylation of caveolin-1 on tyrosine 14 confers binding to Grb7 (an SH2-domain containing protein) both in vitro and in vivo. Furthermore, we demonstrate that binding of Grb7 to tyrosine 14-phosphorylated caveolin-1 functionally augments anchorage-independent growth and epidermal growth factor (EGF)-stimulated cell migration. We discuss the possible implications of our findings in the context of signal transduction.

Intracellular pathways linking hypoxia to activation of c-fos and AP-1

Organisms respond to hypoxia through detection of blood oxygen levels by sensors at peripheral chemoreceptors and by receptors in certain key cells of the body. The pathways over which peripheral chemoreceptor signals are transmitted to respiratory muscles are well established. However, the intracellular pathways that transmit hypoxic stimulus to gene activation are just being identified. Using anti-sense c-fos strategy, we have shown that c-fos is essential for the activation of activator protein-1 transcription factor complex (AP-1) and subsequent stimulation of downstream genes such as tyrosine hydroxylase (TH; Mishra et al. 1998). The purpose of the present study was to identify intracellular pathways that link hypoxia to activation of c-fos. The results of the present study show that hypoxia causes Ca2+ influx through L-type voltage gated Ca2+ channels and that hypoxia-induced c-fos gene expression is Ca2+/calmodulin dependent. We also demonstrate that hypoxia activates the extracellular-regulated kinase (ERK) and p38, but not JNK. Further, phosphorylation of ERK is essential for c-fos activation via SRE cis-element. Further characterization of nuclear signalling pathways provides evidence for the involvement of Src, a non receptor protein tyrosine kinase, and Ras, a small G protein, in the hypoxia-induced c-fos gene expression. These results suggest a possible role for non-receptor protein tyrosine kinases in propagating signals from G-protein coupled receptors to the activation of immediate early genes such as c-fos during hypoxia.

Overexpression of C-terminal Src kinase blocks 14, 15-epoxyeicosatrienoic acid-induced tyrosine phosphorylation and mitogenesis

We have previously reported that 14,15-epoxyeicosatrienoic acid (14, 15-EET) is a potent mitogen for the renal epithelial cell line, LLCPKcl4. This mitogenic effect is dependent upon activation of a protein-tyrosine kinase cascade that results in activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase. Because of suggestive evidence that 14,15-EET also activated Src in these cells, we stably transfected LLCPKcl4 with an expression construct of the C-terminal Src kinase (CSK), which inhibits Src family kinase activity. In vitro Src kinase activity assays confirmed that in empty vector-transfected cells (Vector cells), 14, 15-EET increased Src kinase activity, while in clones overexpressing CSK mRNA and immunoreactive protein (CSK cells), 14,15-EET-induced activation of Src was almost completely blocked (94% inhibition). Of interest, epidermal growth factor (EGF) and fetal bovine serum (FBS) also increased Src activity in Vector cells, but not in CSK cells, further confirming the ability of CSK overexpression to prevent Src activation. CSK cells failed to increase [(3)H]thymidine incorporation in response to exogenous 14,15-EET. In contrast, both EGF and FBS significantly increased [(3)H]thymidine incorporation in CSK cells. Immunoprecipitation with anti-phosphotyrosine antibodies and immunoblotting with an antibody against extracellular signal-regulated kinase (ERK) indicated that in CSK cells, 14,15-EET failed to activate ERK1 and ERK2; however, EGF- and FBS-induced activation of ERKs was not different from that seen in Vector cells. In Vector cells, the 14,15-EET-stimulated tyrosine phosphorylation of ERKs was blocked by pretreatment with 1 microm PP2, a selective inhibitor of Src kinases. The present study demonstrates that 14, 15-EET exerts its mitogenic effects predominantly through a Src kinase-mediated pathway, which is the most upstream signaling step determined to date in the 14,15-EET-activated tyrosine kinase cascade in renal epithelial cells.

The fibroblast growth factor receptor FGFR-4 acts as a ligand dependent modulator of erythroid cell proliferation

Receptor and non-receptor tyrosine kinases constitute a large family of proteins that play a pivotal role in hematopoiesis. Here we conducted a comprehensive survey of tyrosine kinase gene expression in primary erythroid progenitor cells from bone marrow by employing a PCR-based strategy that targets the conserved kinase encoding region. We demonstrate that erythroid progenitor cells express several receptor and non-receptor tyrosine kinases, like c-kit, Jak1, Ryk, FAK, Syk, Arg, Csk and members of the insulin receptor family. Specific changes in the expression profile of tyrosine kinases were observed following differentiation induction. We also report on the identification of a new ligand dependent modulator of erythropoiesis, fibroblast growth factor receptor-4 (FGFR-4). FGFR-4 is effectively expressed in erythroid progenitors and downregulated when cells differentiate. Furthermore, the FGFR-4 ligand, basic fibroblast growth factor (bFGF), enhanced erythroid cell proliferation induced by SCF or insulin, and thus modulated both erythroid proliferation and differentiation in vitro.

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.

Epithelial cell adhesion to extracellular matrix proteins induces tyrosine phosphorylation of the Epstein-Barr virus latent membrane protein 2: a role for C-terminal Src kinase

The Epstein-Barr virus (EBV) latent membrane protein 2 (LMP2) is expressed in latently EBV-infected B cells, where it forms patches in the plasma membrane and interferes with B-cell receptor signal transduction through dominant-negative effects on protein kinases. LMP2 transcripts are detected in nasopharyngeal carcinoma, an epithelial-cell malignancy. In this study the function of LMP2A in epithelial cells was investigated. LMP2A was found to coprecipitate with protein kinase activities and to become phosphorylated in in vitro kinase assays. Analysis of LMP2A deletion mutants demonstrated that tyrosines implicated in interacting with Src family kinase SH2 domains and the SH2 domain of Csk, as well as the LMP2A immunoreceptor tyrosine-based activation motif, are important for its phosphorylation in epithelial cells. LMP2A tyrosine phosphorylation was triggered by cell adhesion to extracellular-matrix (ECM) proteins. Src family kinases, whose involvement in cell-ECM signaling and LMP2A phosphorylation in B lymphocytes has been well established, were found not to be responsible for LMP2A phosphorylation in epithelial cells. Instead, coexpression of Csk, a negative Src regulator, and LMP2A led to an increase in LMP2A phosphorylation both in nonadherent cells and upon cell adhesion. Csk also phosphorylated LMP2A in vitro. These results suggest that LMP2A has a different role in epithelial cells, where it interacts with cell adhesion-initiated signaling pathways. Although tyrosine phosphorylation of LMP2A occurs in both cell types, different protein kinases seem to be used: Src family kinases in B lymphocytes and Csk in epithelial cells.

The Csk homologous kinase, Chk, binds tyrosine phosphorylated paxillin in human blastic T cells

In determining the role of Chk in T cell signaling, we have focused on its protein-protein interactions. We detected a tyrosine phosphoprotein that coimmunoprecipitated with Chk from pervanadate stimulated human blastic T cells. Subsequent Western blot analysis identified this tyrosine phosphoprotein as paxillin. Paxillin, a cytoskeletal protein involved in focal adhesions, was first identified as a v-Src substrate in transformed fibroblasts. Interestingly, Chk specifically bound tyrosine phosphorylated paxillin. Consistent with our in vivo data, Chk and paxillin were observed to localize in similar cellular regions prior to and following stimulation. Using GST fusion proteins, we determined that the Chk SH2 domain, not the SH3 domain, bound tyrosine phosphorylated paxillin. Specifically, paxillin bound to the FLVRES motif of the Chk SH2 domain. Using Far Western analysis, we revealed that the Chk SH2 domain directly associates with tyrosine phosphorylated paxillin. Finally, p52(Chk) expression in Csk-deficient mouse embryo fibroblasts decreased total phosphotyrosine levels of paxillin, implying a physiological role for Chk. These studies provide important insight into the role of Chk in tyrosine mediated signaling, as well as T cell physiology.

C-terminal Src kinase associates with ligand-stimulated insulin-like growth factor-I receptor

Increased expression of the insulin-like growth factor-I receptor (IGF-IR) protein-tyrosine kinase occurs in several kinds of cancer and induces neoplastic transformation in fibroblast cell lines. The transformed phenotype can be reversed by interfering with the function of the IGF-IR. The IGF-IR is required for transformation by a number of viral and cellular oncoproteins, including SV40 large T antigen, Ras, Raf, and Src. The IGF-IR is a substrate for Src in vitro and is phosphorylated in v-Src-transformed cells. We observed that the IGF-IR and IR associated with the C-terminal Src kinase (CSK) following ligand stimulation. We found that the SH2 domain of CSK binds to the tyrosine-phosphorylated form of IGF-IR and IR. We determined the tyrosine residues in the IGF-IR and in the IR responsible for this interaction. We also observed that fibroblasts stimulated with IGF-I or insulin showed a rapid and transient decrease in c-Src tyrosine kinase activity. The results suggest that c-Src and CSK are involved in IGF-IR and IR signaling and that the interaction of CSK with the IGF-IR may play a role in the decrease in c-Src activity following IGF-I stimulation.

Oncogenic src, raf, and ras stimulate a hypertrophic pattern of gene expression and increase cell size in neonatal rat ventricular myocytes

In response to hormones and growth factors, cultured neonatal ventricular myocytes increase in profile, exhibit myofibrillogenesis, and re-express genes whose expression is normally restricted to the fetal stage of ventricular development. These include atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), and skeletal muscle (SkM)-alpha-actin. By using luciferase reporter plasmids, we examined whether oncogenes that activate the extracellular signal-regulated kinase cascade (srcF527, Ha-rasV12, and v-raf) increased expression of "fetal" genes. Transfection of myocytes with srcF527 stimulated expression of ANF, SkM-alpha-actin, and beta-MHC by 62-, 6.7-, and 50-fold, respectively, but did not induce DNA synthesis. Stimulation of ANF expression by srcF527 was greater than by Ha-rasV12, which in turn was greater than by v-raf. General gene expression was also increased but to a lesser extent. The response to srcF527 was inhibited by dominant-negative Ha-rasN17. Myocyte area was increased by srcF527, Ha-rasV12, and v-raf, and although it altered myocyte morphology by causing a pseudopodial appearance, srcF527 did not detectably increase myofibrillogenesis either alone or in combination with Ha-rasV12. A kinase-dead src mutant increased myocyte size to a much lesser extent than srcF527 and also did not inhibit ANF-luciferase expression in response to phenylephrine. We conclude that members of the Src family of tyrosine kinases may be important in mediating the transcriptional changes occurring during cardiac myocyte hypertrophy and that Ras and Raf may be downstream effectors.

Kinetic properties of the C-terminal Src kinase, p50csk

Csk is an important regulator of tyrosine kinases of the Src family. In this paper, we have characterised the kinetics and catalytic properties of a highly active and stable enzyme obtained in milligram amounts by expressing the enzyme as a fusion protein with glutathione-S-transferase (GST) in Escherichia coli. Using the synthetic polyamino acid poly(Glu, Tyr) as substrate, phosphotransferase activity was linear for 7-8 min with Mg2+ and 5 min with Mn2+. With Mg2+ and Mn2+, respectively, K(m) (ATP) was 56.9 +/- 6.2 and 5.4 +/- 0.6 microM and Vmax was 293 +/- 52 and 217 +/- 38 pmol phosphate transferred (microgram Csk)-1 min-1. Optimal concentrations of Mg2+ and Mn2+ were 4-10 mM and 2-3 mM, respectively, and higher concentrations of both cations were inhibitory. The Csk activity was highly sensitive to monovalent (Na+, K+) and divalent (Ca2+) cations, the sensitivity being 2-5-fold higher with Mg2+ than Mn2+. Physiological concentrations of Ca2+ (less than 10 microM) were without effect. Autophosphorylation of Csk was demonstrated in vitro, but did not influence the catalytic activity. Addition of inorganic phosphate above 100 microM strongly inhibited Csk catalytic activity towards poly(Glu, Tyr) in the presence of Mn2+, but not in the presence of Mg2+. Phosphorylation of a physiological substrate (Lck) and autophosphorylation of Csk was not inhibited by phosphate, indicating that the phosphate-dependent inhibition of Csk activity was substrate specific.

Overexpression of C-terminal Src kinase homologous kinase suppresses activation of Lyn tyrosine kinase required for VLA5-mediated Dami cell spreading

The Csk homologous kinase (Chk), which is co-expressed with C-terminal Src kinase (Csk) in hematopoietic cells, negatively regulates Src family kinases in vitro with selectivity toward Lyn but not c-Src in platelets. To explore the role of Src family kinases in hematopoietic cell adhesion, we overexpressed Chk in the megakaryocytic cell line Dami and established clones exhibiting a 10-fold increase in the amount of Chk. Overexpression of Chk was found to suppress VLA5 integrin-mediated cell spreading, but not cell attachment, throughout fibronectin (FN) stimulation. Deletion and point mutagenesis analyses of Chk showed that this suppression was dependent upon both the SH3 domain, which is responsible for membrane anchoring, and kinase activity. FN-induced cell spreading accompanied a sustained increase in Lyn activity with coincidental kinetics and the activation of Lyn was also suppressed by overexpression of Chk but not a Chk mutant lacking the SH3 domain. Expression of a truncated Lyn mutant lacking the kinase domain inhibited both cell spreading and Lyn activation upon stimulation with FN. These results suggest that sustained activation of Lyn, which is regulated by membrane-anchored Chk, plays a crucial role in VLA5-mediated cell spreading but not cell attachment to a FN substrate.

Rapid activation of sodium-proton exchange and extracellular signal-regulated protein kinase in fibroblasts by G protein-coupled 5-HT1A receptor involves distinct signalling cascades

These experiments tested the hypothesis that signalling elements involved in the activation of the extracellular signal-regulated protein kinase (ERK) mediate rapid activation of sodium-proton exchange (NHE) in fibroblasts when both signals are initiated by a single G protein-coupled receptor, the 5-HT1A receptor. Similarities between the two processes were comparable concentration-response curves and time-courses, and overlapping sensitivity to some pharmacological inhibitors of tyrosine kinases (staurosporine and genistein), and phosphoinositide 3'-kinase (wortmannin and LY204002). Activation of NHE was much more sensitive to the phosphatidylcholine-specific phospholipase inhibitor (D609) than was ERK. Neither pathway was sensitive to manoeuvres designed to block PKC. In contrast, Src or related kinases appear to be required to activate ERK, but not NHE. Transfection of cDNA constructs encoding inactive mutant phosphoinositide 3'-kinase, Grb2, Sos, Ras, and Raf molecules were successful in attenuating ERK, but had essentially no effect upon NHE activation. Finally, PD98059, an inhibitor of mitogen activated/extracellular signal regulated kinase kinase, blocked ERK but not NHE activation. Thus, in CHO fibroblast cells, activation by the 5-HT1A receptor of ERK and NHE share a number of overlapping features. However, our studies do not support a major role for ERK, when activated by the 5-HT1A receptor, as a short-term upstream regulator of NHE activity.

Characterization of the novel focal adhesion kinase RAFTK in hematopoietic cells

Protein tyrosine kinases (PTKs) mediate signals that respond to many pivotal cellular functions. Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and PTKs, is a critical control mechanism for various physiological processes, including cell growth, differentiation, metabolism, cell cycle regulation and cytoskeleton function. The focal adhesion kinase (FAK) is a widely expressed non-receptor tyrosine kinase that is implicated in integrin-mediated signaling and plays a role in signal transduction pathways mediating cell adhesion, motility and anchorage-independent growth. Recently, we and others have identified a novel protein tyrosine kinase termed RAFTK, (also known as Pyk2 or Cak-beta), which is related to FAK. This review describes the role of RAFTK in various signaling cascades mainly in reference to hematopoietic cell lineages.

Translocation of the Csk homologous kinase (Chk/Hyl) controls activity of CD36-anchored Lyn tyrosine kinase in thrombin-stimulated platelets

Chk/Hyl is a recently isolated non-receptor tyrosine kinase with greatest homology to a ubiquitous negative regulator of Src family kinases, Csk. To understand the significance of co-expression of Chk and Csk in platelets, we examined the subcellular localization of each protein. Chk, but not Csk, was completely translocated from the Triton X-100-soluble to the Triton X-100-insoluble cytoskeletal fraction within 10 s of thrombin stimulation. Chk and Lyn, but not Csk and c-Src, co-fractionated in the higher density lysate fractions of resting platelets, with Chk being found to localize close to CD36 (membrane glycoprotein IV)-anchored Lyn. The kinase activity of co-fractionated Lyn was suppressed 3-fold. In vitro phosphorylation assays showed that Chk suppressed Lyn activity by phosphorylating its C-terminal negative regulatory tyrosine. Upon stimulation of platelets with thrombin, the rapid and complete translocation of Chk away from Lyn caused concomitant activation of Lyn. This activation was accompanied by dephosphorylation of Lyn at its C-terminal negative regulatory tyrosine in cooperation with a protein tyrosine phosphatase. These results suggest that Chk, but not Csk, may function as a translocation-controlled negative regulator of CD36-anchored Lyn in thrombin-induced platelet activation.

Identification of csk tyrosine phosphorylation sites and a tyrosine residue important for kinase domain structure

The lack of a conserved tyrosine autophosphorylation site is a unique feature of the C-terminal Src-kinase, Csk, although this protein tyrosine kinase can be autophosphorylated on tyrosine residues in vitro and in bacteria. Here we show that human Csk is tyrosine phosphorylated in HeLa cells treated with sodium pervanadate. Phosphorylation in vivo occurs mainly at Tyr-184 and in vitro mainly at Tyr-304. A Y304F mutation strongly decreased Csk phosphorylation in vitro, and a Y184F mutation abolished tyrosine phosphorylation in vivo. A catalytically inactive form of Csk was also phosphorylated on Tyr-184 in vivo, suggesting that this is not a site of autophosphorylation. The kinase activity of the Y184F protein was not changed, while the Y304F protein showed one-third of wild-type activity. Three-dimensional modelling of the Csk kinase domain indicated that the Y304F mutation abolishes one of two conserved hydrogen bonds between the upper and the lower lobes in the open conformation of the kinase domain. Phosphopeptide binding studies suggested that phosphorylation of Tyr-184 creates a binding site for low-molecular-mass proteins. Cellular Csk was associated with several phosphoproteins, some of which were interacting with the Csk SH2 domain. Taken together these results indicate that Csk can be phosphorylated in vivo at Tyr-184 by an as yet unknown tyrosine kinase, and that autophosphorylation of Tyr-304 occurs only at abnormally high Csk concentrations in vitro. Furthermore, Tyr-304 is required for the maintenance of the structure of the Csk kinase domain.

Induction of Hypoxia-Inducible Factor-1, Erythropoietin, Vascular Endothelial Growth Factor, and Glucose Transporter-1 by Hypoxia: Evidence Against a Regulatory Role for Src Kinase

The induction by hypoxia of genes such as erythropoietin, vascular endothelial growth factor (VEGF ), and glucose transporter-1 (Glut-1) is mediated in part by a transcriptional complex termed hypoxia-inducible factor-1 (HIF-1). Several lines of evidence have implicated protein phosphorylation in the mechanism of activation of HIF-1 by hypoxia. Recent reports have described the activation of the tyrosine kinase src by severe hypoxia, and a role in the induction of VEGF by severe hypoxia has been proposed. This led us to examine whether src and related kinases operated more widely in the hypoxic induction of HIF-1 and HIF-1–dependent genes regulated by hypoxia. Measurements of src kinase activity in cells exposed to varying severities of hypoxia showed activation by severe hypoxia (0.1% oxygen or catalyst induced anoxia), but not 1% oxygen. This contrasted with the marked induction of HIF-1 by exposure to 1% oxygen. Manipulations of src activity were produced by transient and stable transfection of Hep3B cells. Despite substantial changes in src activity, no alteration was seen in the normoxic or hypoxic expression of erythropoietin, VEGF, or Glut-1, or in the regulation of HIF-1–dependent reporter genes inducible by hypoxia. Similarly, we found that the expression of these genes in src- or c-src kinase-deficient cells did not differ from wild-type cells at either 1% oxygen or more severe hypoxia. These results indicate that src is not critical for the hypoxic induction of HIF-1, erythropoietin, VEGF, or Glut-1.

Exclusion of CD45 inhibits activity of p56lck associated with glycolipid-enriched membrane domains

p56lck (Lck) is a lymphoid-specific Src family tyrosine kinase that is critical for T-cell development and activation. Lck is also a membrane protein, and approximately half of the membrane-associated Lck is associated with a glycolipid-enriched membrane (GEM) fraction that is resistant to solubilization by Triton X-100 (TX-100). To compare the membrane-associated Lck present in the GEM and TX-100-soluble fractions of Jurkat cells, Lck from each fraction was immunoblotted with antibody to phosphotyrosine. Lck in the GEM fraction was found to be hyperphosphorylated on tyrosine, and this correlated with a lower kinase specific activity relative to the TX-100-soluble Lck. Peptide mapping and phosphatase diagests showed that the hyperphosphorylation and lower kinase activity of GEM-associated Lck was due to phosphorylation of the regulatory COOH-terminal Tyr505. In addition, we determined that the membrane-bound tyrosine phosphatase CD45 was absent from the GEM fraction. Cells lacking CD45 showed identical phosphorylation of Lck in GEM and TX-100-soluble membranes. We propose that the GEM fraction represents a specific membrane domain present in T-cells, and that the hyperphosphorylation of tyrosine and lower kinase activity of GEM-associated Lck is due to exclusion of CD45 from these domains. Lck associated with the GEM domains may therefore consitute a reservoir of enzyme that can be readily activated.

Association of inhibitory tyrosine protein kinase p50csk with protein tyrosine phosphatase PEP in T cells and other hemopoietic cells

p50csk is a tyrosine protein kinase (TPK) that represses the activity of Src family TPKs. We previously showed that Csk is a potent negative regulator of antigen receptor signaling in T lymphocytes and that its Src homology (SH) 3 and SH2 domains are required to inhibit these signals. To test the idea that the Csk SH3 and SH2 domains mediate interactions with other cellular proteins, we attempted to identify Csk-associated polypeptides using the yeast two-hybrid system. The results of our experiments demonstrated that Csk physically associates with PEP, a protein tyrosine phosphatase (PTP) expressed in hemopoietic cells. 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. The association between Csk and PEP was documented in transiently transfected Cos-1 cells and in a variety of cells of hemopoietic lineages, including T cells. Additional analyses demonstrated that the association between Csk and PEP is highly specific. Together, these data indicated that PEP may be an effector and/or a regulator of p50csk in T cells and other hemopoietic cells. Moreover, they allowed the identification of PEP as the first known ligand for the Csk SH3 domain.

Csk enhances insulin-stimulated dephosphorylation of focal adhesion proteins

Insulin has pleiotropic effects on the regulation of cell physiology through binding to its receptor. The wide variety of tyrosine phosphorylation motifs of insulin receptor substrate 1 (IRS-1), a substrate for the activated insulin receptor tyrosine kinase, may account for the multiple functions of insulin. Recent studies have shown that activation of the insulin receptor leads to the regulation of focal adhesion proteins, such as a dephosphorylation of focal adhesion kinase (pp125FAK). We show here that C-terminal Src kinase (Csk), which phosphorylates C-terminal tyrosine residues of Src family protein tyrosine kinases and suppresses their kinase activities, is involved in this insulin-stimulated dephosphorylation of focal adhesion proteins. We demonstrated that the overexpression of Csk enhanced and prolonged the insulin-induced dephosphorylation of pp125FAK. Another focal adhesion protein, paxillin, was also dephosphorylated upon insulin stimulation, and a kinase-negative mutant of Csk was able to inhibit the insulin-induced dephosphorylation of pp125FAK and paxillin. Although we have shown that the Csk Src homology 2 domain can bind to several tyrosine-phosphorylated proteins, including pp125FAK and paxillin, a majority of protein which bound to Csk was IRS-1 when cells were stimulated by insulin. Our data also indicated that tyrosine phosphorylation levels of IRS-1 appear to be paralleled by the dephosphorylation of the focal adhesion proteins. We therefore propose that the kinase activity of Csk, through the insulin-induced complex formation of Csk with IRS-1, is involved in insulin's regulation of the phosphorylation levels of the focal adhesion proteins, possibly through inactivation of the kinase activity of c-Src family kinases.

FGF-1-dependent proliferative and migratory responses are impaired in senescent human umbilical vein endothelial cells and correlate with the inability to signal tyrosine phosphorylation of fibroblast growth factor receptor-1 substrates

Senescent cells do not proliferate in response to exogenous growth factors, yet the number and affinity of growth factor receptors on the cell surface appear to be similar to presenescent cell populations. To determine whether a defect in receptor signaling exists, we analyzed human umbilical vein endothelial cells (HUVEC) since HUVEC growth is absolutely dependent upon the presence of FGF. We report that in both presenescent and senescent HUVEC populations, FGF-1 induces the expression of cell cycle-specific genes, suggesting that functional FGF receptor (FGFR) may exist on the surface of these cells. However, the tyrosine phosphorylation of FGFR-1 substrates, Src and cortactin, is impaired in senescent HUVEC, and only the presenescent cell populations exhibit a FGF-1-dependent Src tyrosine kinase activity. Moreover, we demonstrate that senescent HUVEC are unable to migrate in response to FGF-1, and these data correlate with an altered organization of focal adhesion sites. These data suggest that the induction of gene expression is insufficient to promote a proliferative or migratory phenotype in senescent HUVEC and that the attenuation of the FGFR-1 signal transduction pathway may be involved in the inability of senescent HUVEC to proliferate and/or migrate.

Phosphorylation of caveolin by src tyrosine kinases. The alpha-isoform of caveolin is selectively phosphorylated by v-Src in vivo

Caveolae are flask-shaped plasma membrane specializations that are thought to exist in most cell types. A 22-kDa protein, caveolin, is an integral membrane component of caveolae membranes in vivo. Previous studies have demonstrated that caveolin is phosphorylated on tyrosine by oncogenic viral Src (v-Src) and that caveolin is physically associated as a hetero-oligomeric complex with normal cellular Src (c-Src) and other Src family tyrosine kinases. Caveolin contains eight conserved tyrosine residues that may serve as potential substrates for Src. Here, we have begun to study the phosphorylation of caveolin by Src family tyrosine kinases both in vitro and in vivo. Using purified recombinant components, we first reconstituted the phosphorylation of caveolin by Src kinase in vitro. Microsequencing of Src-phosphorylated caveolin revealed that phosphorylation occurs within the extreme N-terminal region of full-length caveolin between residues 6 and 26. This region contains three tyrosine residues at positions 6, 14, and 25. Deletion mutagenesis demonstrates that caveolin residues 1-21 are sufficient to support this phosphorylation event, implicating tyrosine 6 and/or 14. In vitro phosphorylation of caveolin-derived synthetic peptides and site-directed mutagenesis directly show that tyrosine 14 is the principal substrate for Src kinase. In support of these observations, tyrosine 14 is the only tyrosine residue within caveolin that bears any resemblance to the known recognition motifs for Src family tyrosine kinases. In order to confirm or refute the relevance of these in vitro studies, we next analyzed the tyrosine phosphorylation of endogenous caveolin in v-Src transformed NIH 3T3 cells. In vivo, two isoforms of caveolin are known to exist: alpha-caveolin contains residues 1-178 and beta-caveolin contains residues 32-178. Only alpha-caveolin underwent tyrosine phosphorylation in v-Src transformed NIH 3T3 cells, although beta-caveolin is well expressed in these cells. As beta-caveolin lacks residues 1-31 (and therefore tyrosine 14), these in vivo studies directly demonstrate the validity of our in vitro studies. Because alpha- and beta-caveolin are known to assume a distinct but overlapping subcellular distribution within a single cell, v-Src phosphorylation of alpha-caveolin may only affect a subpopulation of caveolae that contain alpha-caveolin.

Csk is constitutively associated with a 60-kDa tyrosine-phosphorylated protein in human T cells

The protein-tyrosine kinase Csk is one of the main down-regulators of the Src family of kinases. Csk may be involved in the down-regulation of T cell receptor (TCR) signaling by C-terminal tyrosine phosphorylation of Lck and Fyn; however, it is not known how Csk activity is regulated or how it targets these Src family members. We used Jurkat T cells and normal human T cells to examine proteins that bind to the SH2 domain of Csk. In both Jurkat and normal T cells, the Src homology 2 (SH2) domain of Csk bound constitutively to a tyrosine-phosphorylated protein of 60 kDa (p60). The 60-kDa protein was detected in Csk immunoprecipitates from both unstimulated and CD3-stimulated cells. In addition to p60, a protein of 190 kDa coprecipitated with Csk, and both proteins were phosphorylated on tyrosine residues by the immunocomplex. Small amounts of GTPase-activating protein (GAP) were detected in anti-Csk immunoprecipitates, suggesting that p60 may be a GAP-associated protein. Our data demonstrate that the SH2 domain of Csk specifically associates with at least two tyrosine-phosphorylated proteins in normal human T cells, that this association is independent of TCR/CD3 activation, and that Csk may be a part of a multiprotein complex containing GAP.

Detection of a physical and functional interaction between Csk and Lck which involves the SH2 domain of Csk and is mediated by autophosphorylation of Lck on tyrosine 394

The COOH-terminal Src kinase (Csk) is responsible for the phosphorylation of the conserved, negative regulatory, carboxyl-terminal tyrosine of most of the Src family protein tyrosine kinases. Up to now, no stable binding of Csk to Src kinases has been detected. We therefore decided to analyze this interaction using two systems which allow detection of transient interaction. We produced and purified recombinant proteins in the glutathione S-transferase prokaryotic expression system. First, using real-time biospecific interaction analysis (BIAcore(TM)), we detected in vitro a specific interaction between Csk and one of its substrates Lck, a lymphocyte-specific member of the Src family. This interaction requires the autophosphorylation of Lck on tyrosine 394 (the phosphorylation of which is correlated with an increase of the kinase activity) and involves a functional Csk SH2 domain. Second, using the yeast two-hybrid system, we confirmed in vivo the physical interaction between Csk and Lck. Furthermore, in vitro we showed that autophosphorylation of Lck on tyrosine 394 enhances the phosphorylation of Lck by Csk on the negative regulatory site, tyrosine 505, suggesting that activated Lck serves preferentially as substrate for Csk. These findings might explain the mechanism(s) by which Csk interacts with most of Src kinases to down-regulate their kinase activity.

Nuclear signaling by endothelin-1 requires Src protein-tyrosine kinases

In response to changes in vascular homeostasis, endothelial cells secrete endothelin-1 (ET-1), which in turn regulates gene expression and phenotype in underlying vascular cells. We characterized a nuclear signaling cascade in which Src protein-tyrosine kinases link the ET-1 receptor to induction of c-fos transcription. A dominant negative SrcK- kinase mutant blocked ET-1-stimulated c-fos transcription. Expression of the COOH-terminal Src kinase (Csk), which represses Src kinases, also blocked induction of c-fos transcription by ET-1. Activation of the c-fos promoter by ET-1 required both the CArG DNA sequence of the c-fos serum response element and the Ca2+/cAMP response element. In contrast, Src-induced c-fos transcription required only the CArG cis-element, demonstrating a divergence in signals regulating c-fos transcription. Thus, Src kinases contribute to a nuclear signaling cascade linking an ET-1 receptor to the CArG element of the c-fos serum response element. A Src-based pathway might play a more general role to propagate ET-1 nuclear signals that regulate cell growth and development. In addition, these results point to a widening role for nonreceptor protein-tyrosine kinases in propagating signals from G protein-coupled receptors.

Possible v-Crk-induced transformation through activation of Src kinases

p47gag-crk (v-Crk) encoded by avian sarcoma virus CT10, causes an elevation of tyrosine phosphorylation of several cellular proteins. The lack of a protein-tyrosine kinase domain in v-Crk suggests its co-operation with cellular protein-tyrosine kinase activity. We have shown that suppression of a certain fraction of c-Src activity by Csk may require the binding of Csk to tyrosine-phosphorylated paxillin. In this study, we detected co-immunoprecipitation of tyrosine-phosphorylated paxillin with v-Crk in CT10-transformed chicken embryo fibroblasts (CEF), and demonstrated that v-Crk binding to paxillin can inhibit Csk binding to paxillin. A phosphotyrosine peptide, which can inhibit v-Crk binding to paxillin, did not inhibit Csk binding to paxillin, suggesting that v-Crk and Csk bind to different tyrosine-phosphorylated sites in paxillin. We also found that the kinase activity of the endogenous c-Src in CEF is elevated severalfold after CT10-transformation. We therefore suggest that the competitive binding of overexpressed v-Crk affects an efficient interaction of Csk with tyrosine-phosphorylated paxillin in CT10-transformed CEF. This would result in a failure in the suppression of the kinase activities of a population of c-Src and other Src family protein-tyrosine kinases as well, and these kinases may then contribute to the phosphorylation of cellular proteins in CT10-transformed CEF.

Characterization of two activated mutants of human pp60c-src that escape c-Src kinase regulation by distinct mechanisms

Two activated transforming mutants of human pp60c-src were found to possess single point mutations within the regulatory carboxyl terminus (E527K in CY CST201) and the kinase domain (E381G in WO CST1), respectively, that do not directly interfere with either the regulatory c-Src kinase (CSK) phosphorylation site (Tyr530) or the SH2/3 domains. In vivo, both mutant proteins are hypophosphorylated on their carboxyl-terminal regulatory tyrosines and are hyperactive. In an in vitro Src kinase inactivation assay, both mutant Src proteins exhibited resistance to inactivation by CSK relative to wild-type Src. Under these in vitro conditions, E381G c-Src was found to be phosphorylated by CSK to wild-type levels, while E527K c-Src was not detectably phosphorylated. The ability of CSK to phosphorylate a carboxyl-terminal peptide modelled against E527K c-Src was also impaired, suggesting that CSK is unable to recognize E527K c-Src as an efficient substrate. In the case of E381G c-Src, examination of whether its SH2/3 domains were accessible to the carboxyl-terminal regulatory phosphotyrosine revealed a highly reduced ability of autophosphorylated E381G c-Src to bind to a synthetic phosphopeptide modelled from the SH2-binding region of polyoma middle-T antigen which binds to Src SH2 with high affinity. This suggests that the E381G c-Src mutation results in an altered or reduced accessibility of the SH2 domain of the autophosphorylated form of E381G c-Src and may represent a previously undescribed mode of Src activation. Further study of these and other Src mutants may offer additional new insights into the regulation of "Src family" kinases.

Role of tyrosine kinases in lymphocyte activation: targets for drug intervention

Recent developments in our understanding of lymphocyte receptor-associated signalling events have offered many new potential targets for modifying antigen and cytokine receptor signalling events in immune-related diseases such as allergy, autoimmunity and transplant rejection. As discussed below, these targets are largely tissue-restricted and are functionally confined to a limited set of receptors. Therefore, it is anticipated that selective inhibitors of these signalling events would offer safe and effective therapies for immunologically-based diseases. First, we review T and B cell antigen receptor signalling as targets for inhibiting lymphocyte responses. Second, targets in lymphocyte cytokine receptor signalling pathways are discussed. Finally, we review strategies for inhibition of receptor signalling.

The nonreceptor protein-tyrosine kinase CSK complexes directly with the GTPase-activating protein-associated p62 protein in cells expressing v-Src or activated c-Src

CSK is a predominantly cytosolic protein-tyrosine kinase (PTK) that negatively regulates Src family PTKs by phosphorylation of a conserved tyrosine near their C termini. Little is known about how CSK itself is regulated. On the basis of immunofluorescence studies, a model has been proposed that when c-Src is activated, it is redistributed to podosomes, in which substrates become phosphorylated, creating binding sites for CSK. CSK is recruited to these sites of c-Src activation via its SH2 and SH3 domains and is then in a position to downregulate c-Src activity (B. W. Howell and J. A. Cooper, Mol. Cell. Biol. 14:5402-5411, 1994). To identify phosphotyrosine (P.Tyr)-containing proteins that may mediate translocation of CSK due to c-Src activation, we have examined the whole spectrum of P.Tyr-containing proteins that associate with CSK in v-Src NIH 3T3 cells by anti-P.Tyr immunoblotting. Nine P.Tyr-containing proteins coimmunoprecipitated with CSK from v-Src NIH 3T3 cells. One of these, an approximately 62-kDa protein, also associated with CSK in NIH 3T3 cells treated with vanadate prior to lysis and in NIH 3T3 cells expressing an activated c-Src mutant. This 62-kDa protein was shown to be identical to the GTPase-activating protein (GAP)-associated p62 (GAP-A.p62) protein. The interaction between CSK and GAP-A.p62 could be reconstituted in vitro with glutathione S-transferase fusion proteins containing full-length CSK or the CSK SH2 domain. Furthermore, our data show that CSK interacts directly with GAP.A-p62 and that the complex between the two proteins is localized in subcellular membrane or cytoskeletal fractions. Our results suggest that GAP-A.p62 may function as a docking protein and may mediate translocation of proteins, including GAP and CSK, to membrane or cytoskeletal regions upon c-Src activation.