Oncogenic activation of p59fyn tyrosine protein kinase by mutation of its carboxyl-terminal site of tyrosine phosphorylation, tyrosine 528

Fyn Kinase in Alzheimer's Disease: Unraveling Molecular Mechanisms and Therapeutic Implications

Alzheimer's disease, characterized by the accumulation of abnormal protein aggregates and neuronal damage in the brain, leads to a gradual decline in cognitive function and memory. As a complex neurodegenerative disorder, it involves disruptions in various biochemical pathways and neurotransmitter systems, contributing to the progressive loss of neurons and synaptic connections. The complexity of Alzheimer's signaling pathways complicates treatment, presenting a formidable challenge in the quest for effective therapeutic interventions. A member of the Src family of kinases (SFKs), Fyn, is a type of non-receptor tyrosine kinase that has been linked to multiple essential CNS processes, such as myelination and synaptic transmission. Fyn is an appealing target for AD treatments because it is uniquely linked to the two major pathologies in AD by its interaction with tau, in addition to being activated by amyloid-beta (Aβ) through PrPC. Fyn mediates neurotoxicity and synaptic impairments caused by Aβ and is involved in regulating the process of Aβ synthesis.Additionally, the tau protein's tyrosine phosphorylation is induced by Fyn. Fyn is also a challenging target because of its widespread body expression and strong homology with other kinases of the Src family, which could cause unintentional off-target effects. This review emphasizes signaling pathways mediated by Fyn that govern neuronal development and plasticity while also summarizing the most noteworthy recent research relevant to Fyn kinase's function in the brain. Additionally, the therapeutic inhibition of Fyn kinase has been discussed, with a focus on the Fyn kinase inhibitors that are in clinical trials, which presents a fascinating opportunity for targeting Fyn kinase in the creation of possible therapeutic approaches for the management of Alzheimer's disease.

FYN: emerging biological roles and potential therapeutic targets in cancer

Src family protein kinases (SFKs) play a key role in cell adhesion, invasion, proliferation, survival, apoptosis, and angiogenesis during tumor development. In humans, SFKs consists of eight family members with similar structure and function. There is a high level of overexpression or hyperactivity of SFKs in tumor, and they play an important role in multiple signaling pathways involved in tumorigenesis. FYN is a member of the SFKs that regulate normal cellular processes. Additionally, FYN is highly expressed in many cancers and promotes cancer growth and metastasis through diverse biological functions such as cell growth, apoptosis, and motility migration, as well as the development of drug resistance in many tumors. Moreover, FYN is involved in the regulation of multiple cancer-related signaling pathways, including interactions with ERK, COX-2, STAT5, MET and AKT. FYN is therefore an attractive therapeutic target for various tumor types, and suppressing FYN can improve the prognosis and prolong the life of patients. The purpose of this review is to provide an overview of FYN's structure, expression, upstream regulators, downstream substrate molecules, and biological functions in tumors.

Hesperetin Inhibits TGF-β1-Induced Migration and Invasion of Triple Negative Breast Cancer MDA-MB-231 Cells via Suppressing Fyn/Paxillin/RhoA Pathway

Triple-negative breast cancer is an aggressive subtype of breast cancer with poor clinical outcomes and poor prognosis. Hesperetin is an active component extracted from Citrus fruits and Traditional Chinese Medicine has a wide range of pharmacological effects. Here, we assessed the anti-migration and anti-invasive effects and explored inhibitory mechanisms of hesperetin on metastasis of human triple negative breast cancer MDA-MB-231 cells. Cell viability experiments revealed that 200 μM hesperetin has a clear inhibitory effect on MDA-MB-231 cells. TGF-β1 treatment induces apparent tumor progression in MDA-MB-231 cells including aberrant wound-healing and invasion ability, which is effectively suppressed by hesperetin co-treatment. Additionally, hesperetin inhibited the TGF-β1-mediated actin stress fiber formation. Western blot results showed that hesperetin suppressed the TGF-β1-mediated (i) activation of Fyn, (ii) phosphorylation of paxillin at Y31, Y88, and Y118 sites, (iii) the increased expression of RhoA, and (iv) activation of Rho-kinase. We demonstrated the increased interaction of Fyn with paxillin and RhoA protein in the TGF-β1-induced metastasis of MDA-MB-231 cells. Small interfering RNA Fyn inhibited phosphorylation of paxillin (Y31) and activation of Rho-kinase induced by TGF-β1. In conclusion, hesperetin has a significant inhibitory effect on migration and invasion of MDA-MB-231 cells induced by TGF-β1, which might be attributed to inhibiting the Fyn/paxillin/RhoA pathway.

Tyrosine kinase Fyn promotes apoptosis after intracerebral hemorrhage in rats by activating Drp1 signaling

Tyrosine kinase Fyn is a member of the Src kinase family, which is involved in neuroinflammation, apoptosis, and oxidative stress. Its role in intracerebral hemorrhage (ICH) is not fully understood. In this study, we found that Fyn was significantly elevated in human brain tissue after ICH. Accordingly, we investigated the role of Fyn in a rat ICH model, which was constructed by injecting blood into the right basal ganglia. In this model, Fyn expression was significantly upregulated in brain tissue adjacent to the hematoma. SiRNA-induced Fyn knockdown was neuroprotective for secondary cerebral damage, as demonstrated by reduced brain edema, suppression of the modified neurological severity score, and mitigation of blood-brain barrier permeability and neuronal damage. Fyn downregulation reduced apoptosis following ICH, as indicated by downregulation of apoptosis-related proteins AIF, Cyt.c, caspase 3, and Bax; upregulation of anti-apoptosis-related protein Bcl-2; and decreased tunnel staining. Mdivi-1, a Drp1 inhibitor, reversed Fyn overexpression induced pro-apoptosis. However, Fyn did not significantly affect inflammation-related proteins NF-κB, TNF-α, caspase 1, MPO, IL-1β, or IL-18 after ICH. Fyn activated Drp1 signaling by phosphorylating Drp1 at serine 616, which increased apoptosis after ICH in rats. This study clarifies the relationship between Fyn, apoptosis, and inflammation following ICH and provides a new strategy for exploring the prevention and treatment of ICH. KEY MESSAGES: ICH induced an increase in Fyn expression in human and rat cerebral tissues. Knockdown of Fyn prevented cerebral damage following ICH. Inhibition of Fyn had no significant effects on inflammatory responses. However, the downregulation of Fyn exerted neuroprotective effects on apoptosis. Fyn perturbed ICH-induced cell apoptosis by interacting with and phosphorylating (Ser616) Drp1 in a rat ICH model.

Caffeoyl-Prolyl-Histidine Amide Inhibits Fyn and Alleviates Atopic Dermatitis-Like Phenotypes via Suppression of NF-κB Activation

Caffeic acid (CA) is produced from a variety of plants and has diverse biological functions, including anti-inflammation activity. It has been recently demonstrated that caffeoyl-prolyl-histidine amide (CA-PH), which is CA conjugated with proline-histidine dipeptide, relieves atopic dermatitis (AD)-like phenotypes in mouse. In this study, we investigated the molecular mechanism underlying CA-PH-mediated alleviation of AD-like phenotypes using cell line and AD mouse models. We confirmed that CA-PH suppresses AD-like phenotypes, such as increased epidermal thickening, infiltration of mast cells, and dysregulated gene expression of cytokines. CA-PH suppressed up-regulation of cytokine expression through inhibition of nuclear translocation of NF-κB. Using a CA-PH affinity pull-down assay, we found that CA-PH binds to Fyn. In silico molecular docking and enzyme kinetic studies revealed that CA-PH binds to the ATP binding site and inhibits Fyn competitively with ATP. CA-PH further suppressed spleen tyrosine kinase (SYK)/inhibitor of nuclear factor kappa B kinase (IKK)/inhibitor of nuclear factor kappa B (IκB) signaling, which is required for nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. In addition, chronic application of CA-PH, in contrast with that of glucocorticoids, did not induce up-regulation of regulated in development and DNA damage response 1 (REDD1), reduction of mammalian target of rapamycin (mTOR) signaling, or skin atrophy. Thus, our study suggests that CA-PH treatment may help to reduce skin inflammation via down-regulation of NF-κB activation, and Fyn may be a new therapeutic target of inflammatory skin diseases, such as AD.

TCR Triggering Induces the Formation of Lck-RACK1-Actinin-1 Multiprotein Network Affecting Lck Redistribution

The initiation of T-cell signaling is critically dependent on the function of the member of Src family tyrosine kinases, Lck. Upon T-cell antigen receptor (TCR) triggering, Lck kinase activity induces the nucleation of signal-transducing hubs that regulate the formation of complex signaling network and cytoskeletal rearrangement. In addition, the delivery of Lck function requires rapid and targeted membrane redistribution, but the mechanism underpinning this process is largely unknown. To gain insight into this process, we considered previously described proteins that could assist in this process via their capacity to interact with kinases and regulate their intracellular translocations. An adaptor protein, receptor for activated C kinase 1 (RACK1), was chosen as a viable option, and its capacity to bind Lck and aid the process of activation-induced redistribution of Lck was assessed. Our microscopic observation showed that T-cell activation induces a rapid, concomitant, and transient co-redistribution of Lck and RACK1 into the forming immunological synapse. Consistent with this observation, the formation of transient RACK1-Lck complexes were detectable in primary CD4⁺ T-cells with their maximum levels peaking 10 s after TCR-CD4 co-aggregation. Moreover, RACK1 preferentially binds to a pool of kinase active pY394^Lck, which co-purifies with high molecular weight cellular fractions. The formation of RACK1-Lck complexes depends on functional SH2 and SH3 domains of Lck and includes several other signaling and cytoskeletal elements that transiently bind the complex. Notably, the F-actin-crosslinking protein, α-actinin-1, binds to RACK1 only in the presence of kinase active Lck suggesting that the formation of RACK1-pY394^Lck-α-actinin-1 complex serves as a signal module coupling actin cytoskeleton bundling with productive TCR/CD4 triggering. In addition, the treatment of CD4⁺ T-cells with nocodazole, which disrupts the microtubular network, also blocked the formation of RACK1-Lck complexes. Importantly, activation-induced Lck redistribution was diminished in primary CD4⁺ T-cells by an adenoviral-mediated knockdown of RACK1. These results demonstrate that in T cells, RACK1, as an essential component of the multiprotein complex which upon TCR engagement, links the binding of kinase active Lck to elements of the cytoskeletal network and affects the subcellular redistribution of Lck.

Fyn deficiency attenuates renal fibrosis by inhibition of phospho-STAT3

The hallmark of renal tubulointerstitial fibrosis is the accumulation of myofibroblasts and extracellular matrix proteins. Fyn, a member of the Src family of kinases, has diverse biological functions including regulation of mitogenic signaling and proliferation and integrin-mediated interaction. Src family proteins promote pulmonary fibrosis by augmenting transforming growth factor-β signaling, but their role in renal fibrosis is less understood. We observed upregulation of Fyn in a renal fibrosis model induced by unilateral ureteral obstruction. Upon ureteral obstruction, Fyn-deficient mice exhibited attenuated renal fibrosis relative to wild-type mice. Furthermore, obstruction-induced renal expression of type I collagen, fibronectin, α-smooth muscle actin, and plasminogen activator inhibitor-1 was suppressed. Pharmacologic inhibition of Fyn blocked induction of extracellular matrix proteins in kidney cell lines. Importantly, the attenuation of renal fibrosis by Fyn deficiency was not accompanied by changes in the Smad pathway. Rather, the antifibrotic effect of Fyn deficiency was associated with downregulation of signal transducer and activator of transcription 3 (STAT3). Small, interfering RNA targeting STAT3 in Fyn-deficient cells further suppressed α-smooth muscle actin expression, whereas a STAT3 activator partially restored plasminogen activator inhibitor-1 expression, indicating that STAT3 signaling is critically involved in this process. Thus, Fyn plays an important role in renal fibrosis. Hence, Fyn kinase inhibitors may be therapeutically useful against renal fibrosis.

Neuroprotective Effects of Inhibiting Fyn S-Nitrosylation on Cerebral Ischemia/Reperfusion-Induced Damage to CA1 Hippocampal Neurons

Nitric oxide (NO) can regulate signaling pathways via S-nitrosylation. Fyn can be post-translationally modified in many biological processes. In the present study, using a rat four-vessel-occlusion ischemic model, we aimed to assess whether Fyn could be S-nitrosylated and to evaluate the effects of Fyn S-nitrosylation on brain damage. In vitro, Fyn could be S-nitrosylated by S-nitrosoglutathione (GSNO, an exogenous NO donor), and in vivo, endogenous NO synthesized by NO synthases (NOS) could enhance Fyn S-nitrosylation. Application of GSNO, 7-nitroindazole (7-NI, an inhibitor of neuronal NOS) and hydrogen maleate (MK-801, the N-methyl-d-aspartate receptor (NMDAR) antagonist) could decrease the S-nitrosylation and phosphorylation of Fyn induced by cerebral ischemia/reperfusion (I/R). Cresyl violet staining validated that these compounds exerted neuroprotective effects against the cerebral I/R-induced damage to hippocampal CA1 neurons. Taken together, in this study, we demonstrated that Fyn can be S-nitrosylated both in vitro and in vivo and that inhibiting S-nitrosylation can exert neuroprotective effects against cerebral I/R injury, potentially via NMDAR-mediated mechanisms. These findings may lead to a new field of inquiry to investigate the underlying pathogenesis of stroke and the development of novel treatment strategies.

The Fyn-ADAP Axis: Cytotoxicity Versus Cytokine Production in Killer Cells

Lymphocyte signaling cascades responsible for anti-tumor cytotoxicity and inflammatory cytokine production must be tightly regulated in order to control an immune response. Disruption of these cascades can cause immune suppression as seen in a tumor microenvironment, and loss of signaling integrity can lead to autoimmunity and other forms of host-tissue damage. Therefore, understanding the distinct signaling events that exclusively control specific effector functions of “killer” lymphocytes (T and NK cells) is critical for understanding disease progression and formulating successful immunotherapy. Elucidation of divergent signaling pathways involved in receptor-mediated activation has provided insights into the independent regulation of cytotoxicity and cytokine production in lymphocytes. Specifically, the Fyn signaling axis represents a branch point for killer cell effector functions and provides a model for how cytotoxicity and cytokine production are differentially regulated. While the Fyn–PI(3)K pathway controls multiple functions, including cytotoxicity, cell development, and cytokine production, the Fyn–ADAP pathway preferentially regulates cytokine production in NK and T cells. In this review, we discuss how the structure of Fyn controls its function in lymphocytes and the role this plays in mediating two facets of lymphocyte effector function, cytotoxicity and production of inflammatory cytokines. This offers a model for using mechanistic and structural approaches to understand clinically relevant lymphocyte signaling.

FYNagling divergent adhesive functions for Fyn in keratinocytes

Fyn, a member of the Src family kinases (SFKs), has been shown to play important yet contradictory roles in keratinocyte (KC) adhesion. During KC differentiation, physiological activation of Fyn results in the formation of adherens junctions, recruiting junctional components and inducing signaling pathways that control the differentiation program. However, in KC transformation and oncogenesis, increased Fyn activity has been implicated in the dissolution of adhesion structures and an increased migratory phenotype. Fyn activity is also associated with both the formation and dissolution of focal adhesions, and to a lesser extent hemidesmosomes and desmosomes. This viewpoint article aims to reconcile these disparate bodies of literature regarding Fyn's role in cell-cell and cell-matrix adhesion by proposing several alternative, testable hypotheses that unify Fyn's fractured functions.

Expression and activity of Fyn mediate proliferation and blastic features of chronic myelogenous leukemia

The BCR-ABL1 oncogene is a tyrosine kinase that activates many signaling pathways, resulting in the induction of chronic myeloid leukemia (CML). Kinase inhibitors, such as imatinib, have been developed for the treatment of CML; however, the terminal, blast crisis phase of the disease remains a clinical challenge. Blast crisis CML is difficult to treat due to resistance to tyrosine kinase inhibitors, increased genomic instability and acquired secondary mutations. Our recent studies uncovered a role for Fyn in promoting BCR-ABL1 mediated cell growth and sensitivity to imatinib. Here we demonstrate that Fyn contributes to BCR-ABL1 induced genomic instability, a feature of blast crisis CML. Bone marrow cells and mouse embryonic fibroblasts derived from Fyn knockout mice transduced with BCR-ABL1 display slowed growth and clonogenic potential as compared to Fyn wild-type BCR-ABL1 expressing counterparts. K562 cells overexpressing constitutively active Fyn kinase were larger in size and displayed an accumulation of genomic abnormalities such as chromosomal aberrations and polyploidy. Importantly, loss of Fyn protected mouse embryonic fibroblast cells from increased number of chromosomal aberrations and fragments induced by BCR-ABL1. Together, these results reveal a novel role for Fyn in regulating events required for genomic maintenance and suggest that Fyn kinase activity plays a role in the progression of CML to blast crisis.

Cyanidin-3-glucoside suppresses B[a]PDE-induced cyclooxygenase-2 expression by directly inhibiting Fyn kinase activity

Benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) is a well-known carcinogen that is associated with skin cancer. Abnormal expression of cyclooxygenase-2 (COX-2) is an important mediator in inflammation and tumor promotion. We investigated the inhibitory effect of cyanidin-3-glucoside (C3G), an anthocyanin present in fruits, on B[a]PDE-induced COX-2 expression in mouse epidermal JB6 P+ cells. Pretreatment with C3G resulted in the reduction of B[a]PDE-induced expression of COX-2 and COX-2 promoter activity. The activation of activator protein-1 (AP-1) and nuclear factor-κB (NF-κB) induced by B[a]PDE was also attenuated by C3G. C3G attenuated the B[a]PDE-induced phosphorylation of MEK, MKK4, Akt, and mitogen-activated protein kinases (MAPKs), but no effect on the phosphorylation of the upstream MAPK regulator Fyn. However, kinase assays demonstrated that C3G suppressed Fyn kinase activity and C3G directly binds Fyn kinase noncompetitively with ATP. By using PP2, a pharmacological inhibitor for SFKs, we showed that Fyn kinase regulates B[a]PDE-induced COX-2 expression by activating MAPKs, AP-1 and NF-κB. These results suggest that C3G suppresses B[a]PDE-induced COX-2 expression mainly by blocking the activation of the Fyn signaling pathway, which may contribute to its chemopreventive potential.

Fyn: a novel molecular target in cancer

Fyn is 59-kDa member of the Src family of kinases that is historically associated with T-cell and neuronal signaling in development and normal cellular physiology. Whereas Src has been heavily studied in cancer, less attention has been traditionally awarded to the other Src kinases such as Fyn. Our group has shown that Fyn is particularly upregulated in prostate cancer in contrast to the alternative members of the Src family. This suggests that it may mediate several important processes attributed to Src kinases in prostate cancer and other malignancies. These functions include not only cellular growth and proliferation but also morphogenesis and cellular motility. Together, these suggest a role for Fyn in both progression and metastasis. As several agents in clinical development affect Fyn activation, understanding the role that Fyn plays in cancer is of great importance in oncology. Cancer 2010. (c) 2010 American Cancer Society.

Alternative splicing in the differentiation of human embryonic stem cells into cardiac precursors

The role of alternative splicing in self-renewal, pluripotency and tissue lineage specification of human embryonic stem cells (hESCs) is largely unknown. To better define these regulatory cues, we modified the H9 hESC line to allow selection of pluripotent hESCs by neomycin resistance and cardiac progenitors by puromycin resistance. Exon-level microarray expression data from undifferentiated hESCs and cardiac and neural precursors were used to identify splice isoforms with cardiac-restricted or common cardiac/neural differentiation expression patterns. Splice events for these groups corresponded to the pathways of cytoskeletal remodeling, RNA splicing, muscle specification, and cell cycle checkpoint control as well as genes with serine/threonine kinase and helicase activity. Using a new program named AltAnalyze (http://www.AltAnalyze.org), we identified novel changes in protein domain and microRNA binding site architecture that were predicted to affect protein function and expression. These included an enrichment of splice isoforms that oppose cell-cycle arrest in hESCs and that promote calcium signaling and cardiac development in cardiac precursors. By combining genome-wide predictions of alternative splicing with new functional annotations, our data suggest potential mechanisms that may influence lineage commitment and hESC maintenance at the level of specific splice isoforms and microRNA regulation.

The reprogramming of pluripotent stem cells from adult cells is a crucial step toward producing patient-specific cells for transplant therapy. Critical to this goal is the ability to reproducibly drive the differentiation of these cells to specific fates, such as cardiac and neural cells. While gene expression is important in tissue specific differentiation, the impact of alternative splicing on the biology of differentiating cells has not been fully realized. To identify specific splicing events that may determine cell-type-specific differentiation, we compared splicing profiles of human embryonic stem cells (ESCs) and derived cardiac and neural precursors using Affymetrix exon tiling arrays. Segregation of splicing profiles into cardiac-restricted and common cardiac/neural differentiation pattern groups revealed unique groups of genes with clear implications for the biology of cardiomyocyte function and the maintenance of pluripotent ESCs. Alternative splicing of many of these genes, notably regulators of cell death and proliferation, were often predicted to impact protein domain or microRNA binding site inclusion, suggesting that the function or expression of these proteins is altered during differentiation. These results provide further evidence that alternative splicing is important in shaping the functional repertoire of ESCs and differentiated cells.

Caffeic acid, a phenolic phytochemical in coffee, directly inhibits Fyn kinase activity and UVB-induced COX-2 expression

Caffeic acid (3,4-dihydroxycinnamic acid) is a well-known phenolic phytochemical present in many foods, including coffee. Recent studies suggested that caffeic acid exerts anticarcinogenic effects, but little is known about the underlying molecular mechanisms and specific target proteins. In this study, we found that Fyn, one of the members of the non-receptor protein tyrosine kinase family, was required for ultraviolet (UV) B-induced cyclooxygenase-2 (COX-2) expression, and caffeic acid suppressed UVB-induced skin carcinogenesis by directly inhibiting Fyn kinase activity. Caffeic acid more effectively suppressed UVB-induced COX-2 expression and subsequent prostaglandin E(2) production in JB6 P+ mouse skin epidermal (JB6 P+) cells compared with chlorogenic acid (5-O-caffeoylquinic acid), an ester of caffeic acid with quinic acid. Data also revealed that caffeic acid more effectively induced the downregulation of COX-2 expression at the transcriptional level mediated through the inhibition of activator protein-1 (AP-1) and nuclear factor-kappaB transcription activity compared with chlorogenic acid. Fyn kinase activity was suppressed more effectively by caffeic acid than by chlorogenic acid, and downstream mitogen-activated protein kinases (MAPKs) were subsequently blocked. Pharmacological Fyn kinase inhibitor (3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine and leflunomide) data also revealed that Fyn is involved in UVB-induced COX-2 expression mediated through the phosphorylation of MAPKs in JB6 P+ cells. Pull-down assays revealed that caffeic acid directly bound with Fyn and non-competitively with adenosine triphosphate. In vivo data from mouse skin also supported the idea that caffeic acid suppressed UVB-induced COX-2 expression by blocking Fyn kinase activity. These results suggested that this compound could act as a potent chemopreventive agent against skin cancer.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

Srcasm corrects Fyn-induced epidermal hyperplasia by kinase down-regulation

Src family tyrosine kinases (SFKs) are important regulators of epithelial cell growth and differentiation. Characterization of cellular mechanisms that regulate SFK activity will provide insights into the pathogenesis of diseases associated with increased SFK activity. Keratin 14-Fyn (K14) transgenic mice were derived to characterize the effect of Fyn on epidermal growth and differentiation in vivo. The epidermis of K14-Fyn mice is thickened, manifests prominent scale, and exhibits features consistent with hyperproliferation. Increased epidermal Fyn levels correlate with activation of p44/42 MAP kinases, STAT-3, and PDK-1, key signaling molecules that promote epithelial cell growth. The Src-activating and signaling molecule (Srcasm) is a substrate of SFKs that becomes tyrosine-phosphorylated downstream of the EGF receptor. In vitro, increased Srcasm levels promote activation of endogenous Fyn and keratinocyte differentiation. To study the in vivo effect of Srcasm upon Fyn, double transgenic lines were derived. K14-Fyn/Srcasm transgenic mice did not manifest the hyperproliferative phenotype. In contrast, K14-Fyn/Srcasm-P transgenic mice, which express a nonphosphorylatable Srcasm mutant, maintained the hyperproliferative phenotype. Resolution of the hyperproliferative phenotype correlated with reduced Fyn levels in vivo in three experimental systems: transgenic mice, primary keratinocytes, and cell lines. Biochemical studies revealed that Srcasm-dependent Fyn down-regulation requires Fyn kinase activity, phosphorylation of Srcasm, and the Srcasm GAT domain. Therefore, Srcasm is a novel regulator of Fyn promoting kinase down-regulation in a phosphorylation-dependent manner. Srcasm may act as a molecular "rheostat" for activated SFKs, and cellular levels of Srcasm may be important for regulating epithelial hyperproliferation associated with increased SFK activity.

Obligatory role for cooperative signaling by pre-TCR and Notch during thymocyte differentiation

The first checkpoint during T cell development, known as beta selection, requires the successful rearrangement of the TCR-beta gene locus. Notch signaling has been implicated in various stages during T lymphopoiesis. However, it is unclear whether Notch receptor-ligand interactions are necessary during beta selection. Here, we show that pre-TCR signaling concurrent with Notch receptor and Delta-like-1 ligand interactions are required for the survival, proliferation, and differentiation of mouse CD4(-)CD8(-) thymocytes to the CD4(+)CD8(+) stage. Furthermore, we address the minimal signaling requirements underlying beta selection and show a hierarchical positioning of key proximal signaling molecules. Collectively, our results demonstrate an essential role for Notch receptor-ligand interactions in enabling the autonomous signaling capacity of the pre-TCR complex.

The exon 7-spliced Lck isoform in T lymphocytes: a potential regulator of p56lck signaling pathways

The protein-tyrosine kinase p56lck is the product of the lck gene. It plays a pivotal role in T-lymphocyte activation and thymocyte development, as indicated by the defective immune responses of lck-/- mice. We have demonstrated that an exon 7-deleted lck mRNA is produced by alternative splicing in all human cells expressing the lck gene. We have now looked for the protein encoded by this spliced lck mRNA and attempted to determine the function of the deleted Lck protein. This paper shows that the LckDelta7 protein is present in JCaM1.6 T-cells and we inferred that this isoform accounts for 15% of the total Lck proteins in the parental Jurkat T-cell line. We report that deletion of the first 51 amino-acids (exon 7) of the Lck catalytic domain greatly reduces the kinase activity of the recombinant protein. The residual activity can, nevertheless, be enhanced by adding Mn(2+), whereas this cation has no effect on the activity of the p56lck mutated in its active site (K273E). The enforced production of LckDelta7 protein in transfected Jurkat cells results in slower cell proliferation than does p56lck. These findings suggest that the LckDelta7 protein is a p56lck cell-signaling regulator. This mechanism could be common to both humans and mice, in which we also found the exon 7-spliced lck transcript.

Cutting edge: recruitment of the ancestral fyn gene during emergence of the adaptive immune system

The adaptive immune system (AIS) is characterized by the MHC molecules and the rearranging Ag receptors, and was established in a common ancestor of jawed vertebrates. Fyn, a Src-family tyrosine kinases, is important for normal development and function of T lymphocytes and neuronal cells. Indeed, as the result of an alternative splicing of a distinct exon 7, fyn encodes for two isoforms, FynT in T lymphocytes and FynB in the brain. How this alternative splicing of fyn transcripts has emerged and evolved in relation to the setting of the AIS remains to be established. In this study, we show that exon capture in a vertebrate ancestor by the fynT-like gene has yielded a novel fyn-encoded isoform, fynB. Unexpectedly, the newly established AIS recruited the ancestral Fyn isoform, FynT, whereas the CNS expresses the most recent one, FynB. These results shed new light on the emergence of the AIS.

A novel adaptor-like protein which is a substrate for the non-receptor tyrosine kinase, BRK

The brk gene encodes a non-receptor tyrosine kinase that has been found to be overexpressed in approximately two thirds of breast tumours. Using a yeast two-hybrid based screen, we have cloned cDNAs encoding a novel protein, BKS, that is a substrate for the kinase activity of BRK and has the characteristics of an adaptor protein. BKS possesses an N-terminal PH-like domain followed by an SH2-like domain. In co-transfection experiments, high levels of phosphotyrosine were observed on BKS and BRK was found to be associated with BKS, both of which were dependent on the catalytic activity of BRK. The phosphorylation of and association with BKS by BRK was also dependent on the SH2-like domain present within BKS. In addition, BKS recruited an unidentified 100 kDa protein that was also phosphorylated on tyrosine residues in the presence of BRK. We have determined that the BKS protein is expressed in most adult human tissues. Oncogene (2000) 19, 4273 - 4282

Lck domains differentially contribute to pre-T cell receptor (TCR)- and TCR-alpha/beta-regulated developmental transitions

Maturational changes at the CD4(-)CD8(-) double negative (DN) to CD4(+)CD8(+) double positive (DP) transition are dependent on signals generated via the pre-T cell receptor (TCR) and the nonreceptor protein tyrosine kinase p56(lck) (Lck). How Lck activities are stimulated or relayed after pre-TCR formation remains obscure. Our structure-function mapping of Lck thymopoietic properties reveals that the noncatalytic domains of Lck are specialized to signal efficient cellular expansion at DN to DP transition. Moreover, although substitution of the Lck catalytic domain with FynT sequences minimally impacts DP development, single positive thymocytes are most efficiently produced in the presence of kinases containing both the NH(2)-terminal and catalytic regions of Lck. These findings demonstrate that the Lck structure is uniquely adapted to mediate signals at both major transitions in thymopoiesis.

Cooperative inhibition of T-cell antigen receptor signaling by a complex between a kinase and a phosphatase

Antigen receptor-triggered T-cell activation is mediated by the sequential action of the Src and Syk/Zap-70 families of protein tyrosine kinases (PTKs). Previously, we reported that another PTK termed p50(csk) was a potent negative regulator of T-cell receptor (TCR) signaling because of its ability to inactivate Src-related kinases. This inhibitory effect required the catalytic activity of Csk, as well as its Src homology (SH)3 and SH2 domains. Subsequent studies uncovered that, via its SH3 domain, p50(csk) was associated with PEP, a proline-enriched protein tyrosine phosphatase (PTP) of unknown function expressed in hemopoietic cells. Herein, we have attempted to identify the role of the Csk-PEP complex in T lymphocytes. The results of our experiments showed that, like Csk, PEP was a strong repressor of TCR signaling. This property was dependent on the phosphatase activity of PEP, as well as on the sequence mediating its binding to p50(csk). Through reconstitution experiments in Cos-1 cells, evidence was obtained that Csk and PEP act synergistically to inhibit protein tyrosine phosphorylation by Src-related kinases, and that this effect requires their association. Finally, experiments with a substrate-trapping mutant of PEP suggested that PEP functions by dephosphorylating and inactivating the PTKs responsible for T-cell activation. In addition to giving novel insights into the mechanisms involved in the negative regulation of T-cell activation, these findings indicate that the association of an inhibitory PTK with a PTP constitutes a more efficient means of inhibiting signal transduction by Src family kinases in vivo.

Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins

Abeta1-42 is a self-associating peptide whose neurotoxic derivatives are thought to play a role in Alzheimer's pathogenesis. Neurotoxicity of amyloid beta protein (Abeta) has been attributed to its fibrillar forms, but experiments presented here characterize neurotoxins that assemble when fibril formation is inhibited. These neurotoxins comprise small diffusible Abeta oligomers (referred to as ADDLs, for Abeta-derived diffusible ligands), which were found to kill mature neurons in organotypic central nervous system cultures at nanomolar concentrations. At cell surfaces, ADDLs bound to trypsin-sensitive sites and surface-derived tryptic peptides blocked binding and afforded neuroprotection. Germ-line knockout of Fyn, a protein tyrosine kinase linked to apoptosis and elevated in Alzheimer's disease, also was neuroprotective. Remarkably, neurological dysfunction evoked by ADDLs occurred well in advance of cellular degeneration. Without lag, and despite retention of evoked action potentials, ADDLs inhibited hippocampal long-term potentiation, indicating an immediate impact on signal transduction. We hypothesize that impaired synaptic plasticity and associated memory dysfunction during early stage Alzheimer's disease and severe cellular degeneration and dementia during end stage could be caused by the biphasic impact of Abeta-derived diffusible ligands acting upon particular neural signal transduction pathways.

Physical and functional interactions between receptor-like protein-tyrosine phosphatase alpha and p59fyn

We have examined the in vivo activity of receptor-like protein-tyrosine phosphatase alpha (PTPalpha) toward p59(fyn), a widely expressed Src family kinase. In a coexpression system, PTPalpha effected a dose-dependent tyrosine dephosphorylation and activation of p59(fyn), where maximal dephosphorylation correlated with a 5-fold increase in kinase activity. PTPalpha expression resulted in increased accessibility of the p59(fyn) SH2 domain, consistent with a PTPalpha-mediated dephosphorylation of the regulatory C-terminal tyrosine residue of p59(fyn). No p59(fyn) dephosphorylation was observed with an enzymatically inactive mutant form of PTPalpha or with another receptor-like PTP, CD45. Many enzyme-linked receptors are complexed with their substrates, and we examined whether PTPalpha and p59(fyn) underwent association. Reciprocal immunoprecipitations and assays detected p59(fyn) and an appropriate kinase activity in PTPalpha immunoprecipitates and PTPalpha and PTP activity in p59(fyn) immunoprecipitates. No association between CD45 and p59(fyn) was detected in similar experiments. The PTPalpha-mediated activation of p59(fyn) is not prerequisite for association since wild-type and inactive mutant PTPalpha bound equally well to p59(fyn). Endogenous PTPalpha and p59(fyn) were also found in association in mouse brain. Together, these results demonstrate a physical and functional interaction of PTPalpha and p59(fyn) that may be of importance in PTPalpha-initiated signaling events.

Rapid impact of beta-amyloid on paxillin in a neural cell line

Beta-amyloid1-42 (Abeta) is a naturally occuring peptide whose accumulation in the brain is putatively coupled to Alzheimer's disease pathogenesis. Deleterious effects of Abeta on neurons have been linked to the inappropriate activation of signaling pathways within the cell (reviewed in Yankner, 1996), including tyrosine phosphorylation of focal adhesion kinase (FAK) (Zhang et al., 1994, 1996a,b). Here we have investigated the effects of Abeta on paxillin in a neural cell line. Paxillin, a substrate for FAK, is thought to act as a signal "integrator," functioning to link other proteins into multi-molecular signaling complexes (reviewed in Turner, 1994). Treatment of the rat central nervous system B103 cell line with aggregates of Abeta was found to induce the tyrosine phosphorylation of paxillin within 30 min, nearly 24 hr prior to significant cell death. Particularly striking was a subsequent "mobilization" of paxillin to the cytoskeleton in Abeta-treated cells. The amount of paxillin associated with the cytoskeleton in Abeta-treated cells was increased 10-fold over controls. The Abeta-induced paxillin accumulation could be visualized immunocytochemically, with an increase in number and size of paxillin-labeled focal contacts upon treatment with Abeta. This effect was specific, in that vinculin, another focal contact protein, was unaffected by Abeta. Disruption of f-actin, which inhibits both Abeta-induced neurotoxicity (Furukawa and Mattson, 1995) and focal contact signaling in B103 cells (Zhang et al., 1996b) was found to block the cytoskeletal paxillin accumulation. The rapid tyrosine phosphorylation and cytoskeletal mobilization of paxillin links Abeta to the activation of focal contact signaling events that may influence neuronal cytoskeletal architecture, gene expression, synaptic plasticity and cell viability.

Chk, a Csk family tyrosine protein kinase, exhibits Csk-like activity in fibroblasts, but not in an antigen-specific T-cell line

The Csk family of tyrosine protein kinases comprises two members named Csk and Chk. These enzymes phosphorylate the carboxyl-terminal tyrosine of Src-related kinases in vitro, thereby repressing their activity. Csk has been found to be necessary for normal embryonic development, and to be a potent negative regulator of antigen receptor signaling in T-lymphocytes. As the functions of Chk in mammalian cells are not known, we examined its ability to carry out Csk-like functions in vivo. Like p50csk, Chk reduced the elevated phosphotyrosine levels and the augmented activity of Src family kinases in Csk-deficient fibroblasts. Contrary to Csk, however, Chk was inefficient at repressing antigen receptor-induced signals in a T-cell line (BI-141). We also noted that Chk, but not Csk, failed to stably associate with cellular membranes following addition of a membrane targeting signal to its amino terminus. This observation suggested that Chk may contain dominant targeting sequences disallowing its recruitment to cellular membranes. Hence, these data demonstrate that Chk can mediate some, but not all, Csk-related functions in vivo. Moreover, they suggest that the "restricted" function of Chk may relate at least in part to its inability to be recruited to certain cellular locales.

Brk, a breast tumor-derived non-receptor protein-tyrosine kinase, sensitizes mammary epithelial cells to epidermal growth factor

brk (breast tumor kinase) shows homology to the src family of non-receptor protein-tyrosine kinases and is expressed in breast carcinomas. In order to investigate the role of brk in breast tumor development, we have examined the growth and transformation properties of human mammary epithelial cells engineered to overexpress Brk. Interestingly, like c-Src, overexpression of Brk leads to sensitization to EGF, and also results in a partially transformed phenotype. Further investigation of the latter activity was attempted by mutational analysis, targeting key residues known to affect tyrosine kinase activity in Src-like kinases. Mutation of amino acid residue Lys-219 to Met, by analogy to Src, abolished both kinase activity and transformation capacity. Mutation of amino acid residue Tyr-447 to Phe, however, resulted in a decrease in transforming potential without affecting kinase activity. These results suggest that while Src and Brk share some functional properties, they act differently during transformation. These differences are discussed in the context of the mechanisms underlying breast cancer development.

A beta peptide enhances focal adhesion kinase/Fyn association in a rat CNS nerve cell line

Neurotoxicity of the amyloid beta protein (A beta) is known to correlate with a selective change in protein tyrosine phosphorylation (Tyr(P)) of focal adhesion kinase (FAK) (Zhang et al., J. Biol. Chem., 269 (1994) 25247-25250). The current work has found that exposure of neuronal cells to A beta upregulates the stable association of FAK with Fyn, a neuronally-enriched protein tyrosine kinase of the Src-family. In cells incubated with aged A beta 1-42, the amount of immunoprecipitable FAK-Fyn complex increased approximately 280%. Equivalent results were obtained whether anti-FAK or anti-Fyn was used to precipitate the complex. Cells incubated with non-toxic A beta 17-42, which makes aggregates and attaches to cells but does not upregulate FAK Tyr(P), exhibited no increase in FAK-Fyn complex. Aberrant Fyn activity due to the A beta evoked association with FAK could play a role in neuronal degeneration and also cause anomalies in synaptic plasticity. These possibilities are of particular significance because of the reported increase in Fyn immunoreactivity in Alzheimer's-afflicted neurons (Shirazi and Wood, NeuroReport, 4 (1993) 435-437).

Production and characterization of monoclonal antibodies against the recombinant p50csk protein tyrosine kinase: a tool for signal transduction research

p50csk is a protein tyrosine kinase (PTK) that has been reported to regulate the activity of other PTKs belonging to the src gene family. Several hybridoma clones that produce monoclonal antibodies (MAbs) directed against recombinant p50csk were established. Five of the clones were analyzed for their ability to recognize native and denatured p50csk protein after undergoing native and denaturing polyacrylamide gel electrophoresis followed by western blotting. In addition, the clones were tested for their ability to immunoprecipitate p50csk and yet maintain tyrosine kinase activity of antibody-bound p50csk. None of the clones cross-reacted with pp60c-src, a PTK that shares with p50csk the homologous SH1 catalytic domain and SH2 and SH3 regulatory domains. These MAbs can be used to study p50csk directly, and its role in regulating members of the src family.