Function Of Src Family Kinases Research Articles

Development of FRET Biosensor to Characterize CSK Subcellular Regulation.

C-terminal Src kinase (CSK) is the major inhibitory kinase for Src family kinases (SFKs) through the phosphorylation of their C-tail tyrosine sites, and it regulates various types of cellular activity in association with SFK function. As a cytoplasmic protein, CSK needs be recruited to the plasma membrane to regulate SFKs' activity. The regulatory mechanism behind CSK activity and its subcellular localization remains largely unclear. In this work, we developed a genetically encoded biosensor based on fluorescence resonance energy transfer (FRET) to visualize the CSK activity in live cells. The biosensor, with an optimized substrate peptide, confirmed the crucial Arg107 site in the CSK SH2 domain and displayed sensitivity and specificity to CSK activity, while showing minor responses to co-transfected Src and Fyn. FRET measurements showed that CSK had a relatively mild level of kinase activity in comparison to Src and Fyn in rat airway smooth muscle cells. The biosensor tagged with different submembrane-targeting signals detected CSK activity at both non-lipid raft and lipid raft microregions, while it showed a higher FRET level at non-lipid ones. Co-transfected receptor-type protein tyrosine phosphatase alpha (PTPα) had an inhibitory effect on the CSK FRET response. The biosensor did not detect obvious changes in CSK activity between metastatic cancer cells and normal ones. In conclusion, a novel FRET biosensor was generated to monitor CSK activity and demonstrated CSK activity existing in both non-lipid and lipid raft membrane microregions, being more present at non-lipid ones.

Src family kinases involved in the differentiation of human preadipocytes

BackgroundObesity is characterized by the excess accumulation of white adipose tissue (WAT). Src family kinases (SFKs) are non-receptor tyrosine kinases consisting of eight members (SRC, FYN, YES1, HCK, LCK, LYN, FGR and BLK) that have been studied extensively in mammalian cells. Although individual members in murine cells provide some clues that are associated with the regulation of adipogenesis, the specific role of this family in adipocyte differentiation has rarely been assessed, especially in human adipocytes. MethodsHerein, we first explored the expression profiles of SFKs during human preadipocyte differentiation. Then, we used the pyrazolo-pyrimidinyl-amine compound PP1, a potent SFK inhibitor, to evaluate the function of SFKs during adipocyte differentiation. Furthermore, we adopted a loss-of-function strategy with siRNAs to determine the role of FGR in adipocyte differentiation. ResultsHere, we found that SRC, FYN, YES1, LYN and FGR were expressed in human preadipocytes and induced after the initiation of differentiation. Furthermore, the SFK inhibitor PP1 suppressed adipocyte differentiation. We also found that PP1 significantly suppressed the SFK activity in preadipocytes and decreased the expression of adipogenic genes in early and late differentiation. Given that FGR exhibited the most expression enhancement in mature adipocytes, we focused on FGR and found that its knockdown reduced lipid accumulation and adipogenic gene expression. ConclusionsCollectively, these findings suggest that SFKs, especially FGR, are involved in the differentiation of human preadipocytes. Our results lay a foundation for further understanding the role of SFKs in adipocyte differentiation and provide new clues for anti-obesity therapies.

The promoting role of lysosome-localized c-Src in autophagosome-lysosome fusion

Src-family kinases (SFKs), such as c-Src, Lyn and Fyn, belong to non-receptor-type tyrosine kinases and play key roles in cell proliferation, adhesion, and migration. SFKs are anchored to the plasma membrane, Golgi membranes and lysosomal membranes through lipid modifications. Although the functions of SFKs being localized to the plasma membrane are intensively studied, those of SFKs being localized to organelle membranes are poorly understood. Here, we show that, among SFKs, c-Src in particular is involved in a decrease in the amount of LC3-II. c-Src and non-palmitoylated Lyn [Lyn(C3S) (cysteine-3 → serine-3)], which are localized onto lysosomes, decrease the amount of LC3-II and treatment with SFK inhibitors increases the amount of LC3-II, suggesting the importance of SFKs' lysosomal localization for a change of autophagic flux in a kinase activity-dependent manner. Colocalization of LC3-II with the lysosome-associated membrane protein LAMP1 shows that lysosome-localized SFKs promote the fusion of autophagosomes with lysosomes. Lysosome-localized SFKs play a positive role in the maintenance of cell viability under starvation conditions, which is further supported by knockdown of c-Src. Therefore, our results suggest that autophagosome-lysosome fusion is promoted by lysosome-localized c-Src, leading to cell survival under starvation conditions.

A phase I, open-label, multicenter, first-in-human study of the safety, tolerability, pharmacokinetics, and antitumor activity of TPX-0022, a novel MET/CSF1R/SRC inhibitor, in patients with advanced solid tumors harboring genetic alterations in MET.

TPS3663 Background: Alterations in the MET gene, including amplifications, chromosomal translocations, and activating mutations (kinase domain [KD] or exon 14 [Δex14]), occur across various tumors and may function as oncogenic drivers. SRC family kinases function as a key downstream node for MET signaling. CSF1R is a receptor tyrosine kinase associated with tumor progression and suppression of the immune response in the tumor microenvironment. TPX-0022 is a type I kinase inhibitor with a novel macrocyclic structure that potently inhibits MET, CSF1R and SRC to simultaneously target oncogenic MET signaling, its key downstream mediators, and the tumor microenvironment. Methods: This is a multicenter phase 1 first-in-human, open-label study to determine the safety, tolerability, PK, and preliminary efficacy of TPX-0022 in adults with advanced solid tumors harboring genetic alterations in MET. TPX-0022 will be administered orally in continuous 28-day cycles. The primary endpoint is the incidence of DLTs and determination of recommended phase 2 dose (RP2D). Secondary endpoints include ORR by blinded independent central review, intra-cranial response rate, PFS and OS (dose expansion only). In the dose escalation portion, ~30 subjects age ≥18 with solid tumors harboring MET gene amplifications, Δex14, fusions or KD mutations as determined by local tissue-based or liquid biopsy will be enrolled in a 3+3 design. Intrasubject dose escalation will also be allowed. Once the RP2D has been determined, a food effect sub-study will be conducted and ~80 subjects will be enrolled in a dose expansion portion of the study into the following cohorts: I: non-small cell lung cancer (NSCLC) Δex14 (MET therapy naïve), II: NSCLC Δex14 (MET therapy pre-treated), III: MET amplified NSCLC, gastric, or hepatocellular carcinoma, IV: solid tumors with MET KD mutations or fusions. Correlative studies will include analysis of circulating cell-free DNA to identify genomic alterations that may predict activity of TPX-0022 as well as circulating protein biomarkers such as s-MET, HGF, CSF1 and serum cytokines. The study is open and enrolling in the dose escalation portion at the time of submission. Clinical trial information: NCT03993873 .

WHIMP links the actin nucleation machinery to Src-family kinase signaling during protrusion and motility.

Cell motility is governed by cooperation between the Arp2/3 complex and nucleation-promoting factors from the Wiskott-Aldrich Syndrome Protein (WASP) family, which together assemble actin filament networks to drive membrane protrusion. Here we identify WHIMP (WAVE Homology In Membrane Protrusions) as a new member of the WASP family. The Whimp gene is encoded on the X chromosome of a subset of mammals, including mice. Murine WHIMP promotes Arp2/3-dependent actin assembly, but is less potent than other nucleation factors. Nevertheless, WHIMP-mediated Arp2/3 activation enhances both plasma membrane ruffling and wound healing migration, whereas WHIMP depletion impairs protrusion and slows motility. WHIMP expression also increases Src-family kinase activity, and WHIMP-induced ruffles contain the additional nucleation-promoting factors WAVE1, WAVE2, and N-WASP, but not JMY or WASH. Perturbing the function of Src-family kinases, WAVE proteins, or Arp2/3 complex inhibits WHIMP-driven ruffling. These results suggest that WHIMP-associated actin assembly plays a direct role in membrane protrusion, but also results in feedback control of tyrosine kinase signaling to modulate the activation of multiple WASP-family members.

A Dimerization Function in the Intrinsically Disordered N-Terminal Region of Src

SUMMARYThe mode of regulation of Src kinases has been elucidated by crystallographic studies identifying conserved structured protein modules involved in an orderly set of intramolecular associations and ligand interactions. Despite these detailed insights, much of the complex behavior and diversity in the Src family remains unexplained. A key missing piece is the function of the unstructured N-terminal region. We report here the function of the N-terminal region in binding within a hydrophobic pocket in the kinase domain of a dimerization partner. Dimerization substantially enhances autophosphorylation and phosphorylation of selected substrates, and interfering with dimerization is disruptive to these functions. Dimerization and Y419 phosphorylation are codependent events creating a bistable switch. Given the versatility inherent in this intrinsically disordered region, its multisite phosphorylations, and its divergence within the family, the unique domain likely functions as a central signaling hub overseeing much of the activities and unique functions of Src family kinases.

Structure, Dynamics, and Function of the Membrane Associated SRC Family Kinase HCK

The Src family kinases (SFKs) are a group of nine non-receptor tyrosine kinases known for their involvement in critical signal transduction pathways. Structurally, SFKs share a common multi-domain architecture, consisting of SH3, SH2, and catalytic (SH1) domains attached to a membrane-anchoring SH4 domain through an intrinsically disordered ∼80 residue region of low sequence conservation called the Unique (U) domain. Despite the wealth of information on SFK structure and function, little is known about the nature of these enzymes in the membrane-associated form. Interestingly, SH4-U domains in different species share common sequence features, despite differences between SFK members, arguing for their importance in differentiating the functions of SFKs and raising questions about their ability to participate in SFK signaling.Hematopoetic cell kinase (Hck), a phagocyte specific proto-oncogene member of the SFKs, is a potential drug target for HIV infections and Bcr/Abl-chronic myeloid leukemia. To achieve structural characterization of the membrane-associated Hck via its SH4-U domains, data from solution and solid-state NMR, neutron reflection, and fluorescence resonance energy transfer were computationally integrated using a novel maximum-entropy multi-resolution “restrained-ensemble molecular dynamics” (reMD) simulation scheme. This framework has allowed us to detail features of the membrane-protein complex at atomic resolution and identify possible new constraints on Hck's ability to phosphorylate downstream targets. In vivo assays are being developed to test the relevance of these findings.

Structural recognition mechanisms between human Src homology domain 3 (SH3) and ALG-2-interacting protein X (Alix)

The functions of Src family kinases are tightly regulated through Src homology (SH) domain-mediated protein–protein interactions. We previously reported the biophysical characteristics of the apoptosis-linked gene 2-interacting protein X (Alix) in complex with the haemopoietic cell kinase (Hck) SH3 domain. In the current study, we have combined ITC, NMR, SAXS and molecular modeling to determine a 3D model of the complex. We demonstrate that Hck SH3 recognizes an extended linear proline-rich region of Alix. This particular binding mode enables Hck SH3 to sense a specific non-canonical residue situated in the SH3 RT-loop of the kinase. The resulting model helps clarify the mechanistic insights of Alix–Hck interaction. Structured summary of protein interactionsHckphysically interacts with SAM68 by two hybrid (View interaction)FynR96Iphysically interacts with Alix by two hybrid (View interaction)Hckbinds to Alix by pull down (View interaction)Fynphysically interacts with SAM68 by two hybrid (View interaction)Hck and Alixbind by nuclear magnetic resonance (View interaction)FynR96I and Alixbind by isothermal titration calorimetry (View Interaction: 1, 2)FynR96I and Alixbind by nuclear magnetic resonance (View interaction)FynR96Ibinds to Alix by pull down (View interaction)Hckphysically interacts with Alix by two hybrid (View interaction)FynR96I and Alixbind by x ray scattering (View interaction)Hckphysically interacts with NEF by two hybrid (View interaction)FynR96Iphysically interacts with NEF by two hybrid (View interaction)

Src family kinase potentiates the activity of nicotinic acetylcholine receptor in rat autonomic ganglion innervating urinary bladder

Src family kinases (SFKs), one of the tyrosine kinase groups, are primary regulators of signal transductions that control cellular functions such as cell proliferation, differentiation, survival, metabolism, and other important roles of the cell. One of the crucial functions of SFKs is to regulate the activities of various neuronal channels. In this study, we investigated the modulatory action of SFK on nicotinic acetylcholine receptors (nAChRs) expressed in rat major pelvic ganglion (MPG) neurons innervating the urinary bladder. PP1 and PP2 (5 μM), selective Src-kinase inhibitors, attenuated ACh-induced ionic currents and [Ca 2+] i transients in MPG neurons, whereas PP3, an inactive analogue, had no effect. Blocking the tyrosine kinase activity of Src kinase by pp60 c-src inhibitory peptide also reduced the ACh-induced currents. Conversely, sodium orthovanadate (200 μM), a tyrosine phosphatase inhibitor, significantly augmented the ACh-induced currents. In the kinase assay, the activities of SFKs in MPG neurons were also inhibited by PP2, but not by PP3. These data suggests that SFKs may have a facilitative role on the synaptic transmission in rat pelvic autonomic ganglion.

Functions of Fyn kinase in the completion of meiosis in mouse oocytes

Oocyte maturation invokes complex signaling pathways to achieve cytoplasmic and nuclear competencies for fertilization and development. The Src-family kinases FYN, YES and SRC are expressed in mammalian oocytes but their function during oocyte maturation remains an open question. Using chemical inhibitor, siRNA knockdown, and gene deletion strategies the function of Src-family kinases was evaluated in mouse oocytes during maturation under in vivo and in vitro conditions. Suppression of Src-family as a group with SKI606 greatly reduced meiotic cell cycle progression to metaphase-II. Knockdown of FYN kinase expression after injection of FYN siRNA resulted in an approximately 50% reduction in progression to metaphase-II similar to what was observed in oocytes isolated from FYN (−/−) mice matured in vitro. Meiotic cell cycle impairment due to a Fyn kinase deficiency was also evident during oocyte maturation in vivo since ovulated cumulus oocyte complexes collected from FYN (−/−) mice included immature metaphase-I oocytes (18%). Commonalities in meiotic spindle and chromosome alignment defects under these experimental conditions demonstrate a significant role for Fyn kinase activity in meiotic maturation.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Tyrosine phosphatases such as SHP-2 act in a balance with Src-family kinases in stabilization of postsynaptic clusters of acetylcholine receptors

BackgroundDevelopment of neural networks requires that synapses are formed, eliminated and stabilized. At the neuromuscular junction (NMJ), agrin/MuSK signaling, by triggering downstream pathways, causes clustering and phosphorylation of postsynaptic acetylcholine receptors (AChRs). Postnatally, AChR aggregates are stabilized by molecular pathways that are poorly characterized. Gain or loss of function of Src-family kinases (SFKs) disassembles AChR clusters at adult NMJs in vivo, whereas AChR aggregates disperse rapidly upon withdrawal of agrin from cultured src-/-;fyn-/- myotubes. This suggests that a balance between protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) such as those of the Src-family may be essential in stabilizing clusters of AChRs.ResultsWe have analyzed the role of PTPs in maintenance of AChR aggregates, by adding and then withdrawing agrin from cultured myotubes in the presence of PTP or PTK inhibitors and quantitating remaining AChR clusters. In wild-type myotubes, blocking PTPs with pervanadate caused enhanced disassembly of AChR clusters after agrin withdrawal. When added at the time of agrin withdrawal, SFK inhibitors destabilized AChR aggregates but concomitant addition of pervanadate rescued cluster stability. Likewise in src-/-;fyn-/- myotubes, in which agrin-induced AChR clusters form normally but rapidly disintegrate after agrin withdrawal, pervanadate addition stabilized AChR clusters. The PTP SHP-2, known to be enriched at the NMJ, associated and colocalized with MuSK, and agrin increased this interaction. Specific SHP-2 knockdown by RNA interference reduced the stability of AChR clusters in wild-type myotubes. Similarly, knockdown of SHP-2 in adult mouse soleus muscle by electroporation of RNA interference constructs caused disassembly of pretzel-shaped AChR-rich areas in vivo. Finally, we found that src-/-;fyn-/- myotubes contained elevated levels of SHP-2 protein.ConclusionOur data are the first to show that the fine balance between PTPs and SFKs is a key aspect in stabilization of postsynaptic AChR clusters. One phosphatase that acts in this equilibrium is SHP-2. Thus, PTPs such as SHP-2 stabilize AChR clusters under normal circumstances, but when these PTPs are not balanced by SFKs, they render clusters unstable.

FUNCTIONAL DECIPHERING OF THE CORE SIGNALING MECHANISM FOR SPERMATOGONIAL STEM CELL SELF-RENEWAL AND SURVIVAL

The biological fate decisions of self-renewal and differentiation by spermatogonial stem cells (SSCs) provide the foundation for continual spermatogenesis in the mammalian testis. Since SSCs are rare in the testis, it is extremely difficult to study the mechanisms that regulate their biological functions. The SSC self-renewal fate decision has been shown to be dependent on glial cell line-derived neurotrophic factor (GDNF) influence. However, the intracellular mechanisms utilized by GDNF in SSCs are unknown. Recently we identified GDNF-regulated genes in SSCs, which likely encode for molecules that regulate self-renewal. From these data significant up-regulated expression of the transcription factors Bcl6b, Erm, and Lhx1 was identified and Bcl6b was subsequently shown to be important for SSC self-renewal in vitro and in vivo. The objective of this study was to begin deciphering the intracellular signaling mechanisms utilized by GDNF to regulate SSC self-renewal and survival. We utilized a mouse GDNF-dependent culture system that maintains an undifferentiated germ cell population enriched for self-renewing SSCs. Continuous in vitro culture and transplantation analysis are critical elements to define the presence of SSCs in any population of cells subject to examination for factors regulating SSC function. Using siRNA to reduce expression in cultured SSCs and transplantation to quantify stem cell numbers, Bcl6b, Erm, and Lhx1 were all shown to be important for SSC self-renewal in vitro. Next, GDNF was shown to activate AKT in SSCs and this activation could be blocked by inhibition of both PI3K and Src Family Kinases (SFKs). Culture of SSCs with inhibitors to AKT (AKT inhibitor IV), PI3K (LY294002), or SFKs (SU6656) followed by transplantation analysis showed significant impairment of SSC maintenance in vitro compared to control cultures. Apoptosis analysis revealed a significant increase in the percentage of apoptotic cells when both AKT and PI3K inhibitors, but not SFK inhibitor, was added to culture media indicating that multiple signaling pathways are responsible for SSC selfrenewal and survival. Up-regulation of Bcl6b, Erm, and Lhx1 gene expression by GDNF in self-renewing SSCs could be blocked by inhibition of AKT. However, expression of both Plzf and Oct4, which are expressed in SSCs but not GDNF-regulated, were also significantly reduced when AKT was inhibited. GDNF was further shown to phosphorylate SFKs in cultured SSCs and inhibition of SFK function could specifically block GDNF-regulated expression of Bcl6b, Erm, and Lhx1 without effecting either Plzf or Oct4 expression. These data demonstrate that GDNFactivated AKT signaling is essential for SSC survival, but not a specific pathway used for GDNF-regulated gene expression. Importantly, these data also demonstrate that SFK stimulation is an essential intracellular signaling mechanism utilized by GDNF to specifically regulate SSC selfrenewal. (platform)

Multiple Effects of Ephrin-A5 on Cortical Neurons Are Mediated by Src Family Kinases

The Eph receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are involved in a variety of developmental processes such as axonal guidance, cell migration, cell adhesion, proliferation, and differentiation. In addition to repulsive effects, ephrins can also induce attractive responses. Up to now, little was known about the underlying signaling mechanisms that regulate attractive versus repulsive effects. In this study, we show that ephrin-A5 enhances the motility of cortical neurons that is dependent on the activity of Src-family kinases (SFKs). Ephrin-A5 further changes the adhesive properties of neurons by inducing the formation of cell aggregates. Using the stripe assay, we found that the motogenic effect of ephrin-A5 is the result of repulsive ephrin-A interactions. Blocking SFK function leads to a conversion of repulsion into adhesion, suggesting that SFKs can act as a biological switch for the response of EphA receptors. Finally, we discovered a ligand-induced release of membrane particles containing EphA receptors, suggesting membrane ripping as a novel mechanism to overcome the "ephrin paradox" of repulsion after high-affinity receptor-ligand binding.

Src family kinase activity regulates adhesion, spreading and migration of pancreatic endocrine tumour cells

Pancreatic endocrine tumours (PETs) are rare and 'indolent' neoplasms that usually develop metastatic lesions and exhibit poor response to standard medical treatments. Few studies have investigated pathways responsible for PET cell growth and invasion and no alternative therapeutic strategies have been proposed. In a recent microarray analysis for genes up-regulated in PETs, we have described the up-regulation of soluble Src family tyrosine kinases in this neoplasia, which may represent potentially promising candidates for therapy. Herein, we have investigated the expression and function of Src family kinases in PETS and PET cell lines. Western blot analysis indicated that Src is highly abundant in the PET cell lines CM and QGP-1. Immunohistochemistry and Western blot analyses showed that Src is up-regulated also in human PET lesions. Pharmacological inhibition of Src family kinases by the specific inhibitor PP2 strongly interfered with adhesion, spreading and migration of PET cell lines. Accordingly, the actin cytoskeleton was profoundly altered after inhibition of Src kinases, whereas even prolonged incubation with PP2 exerted no effect on cell cycle progression and/or apoptosis of PET cells. A transient increase in tyrosine phosphorylation of a subset of proteins was observed in QGP-1 cells adhering to the plate, with a peak at 75 min after seeding, when approximately 80% of cells were attached. Inhibition of Src kinases caused a dramatic reduction in the phosphorylation of proteins with different molecular weight that were isolated from the cell extracts by anti-phosphotyrosine immunoprecipitation or pull-down with the SH2 domain of Src. Among them, the docking protein p130Cas interacted with Src and is a major substrate of the Src kinases in QGP-1 cells undergoing adhesion. Our results suggest that Src kinases play a specific role during adhesion, spreading and migration of PET cells and may indicate therapeutical approaches directed to limiting the metastatic potential of these cells.

Src Family Kinases as Regulators of Angiogenesis: Therapeutic Implications

Despite its discovery nearly a century ago, the functions of the Src family of protein tyrosine kinases (SFKs) remain incompletely understood. While much has been learned regarding the functions of Src family kinases in the last few years, new roles for Src, particularly in promoting the progression of cancer towards the metastatic phenotype, continue to emerge. SFKs, through their functions as kinases and adapter proteins in signaling complexes, regulate such diverse cellular events as proliferation, migration, cell cycle control, and apoptosis. In tumor cells, the kinase activity of Src is frequently activated, with greater increases during progressive stages. Likewise, resistance to chemotherapy also corresponds with Src kinase activity. Thus, Src activation is predictive of poor prognosis in several tumors. Recently, selective SFK inhibitors are showing promise in clinical trials in imatinib mesylate (Gleevec, Novartis) resistant chronic myelogenous leukemia. However, in vitro studies have suggested that Src inhibitors may hold promise in the treatment of solid tumors such as colon and pancreatic cancer in which new therapeutic inhibitors are desperately needed. This review will summarize briefly the structure and function of Src and the evidence for Src in promoting tumor progression and metastasis. As recent work in this laboratory and others has demonstrated that Src is a regulator of expression of diverse pro-angiogenic factors produced by tumor cells, and a regulator of the endothelial cells that respond to these factors, this review will focus on the role of Src in angiogenesis and potential roles of Src inhibitors as antiangiogenic agents. Keywords: src kinases, angiogenesis, targeted therapy, chemotherapy, metastasis

Src family kinases, key regulators of signal transduction.

The Src family of protein tyrosine kinases (SFKs) plays key roles in regulating signal transduction by a diverse set of cell surface receptors in the context of a variety of cellular environments. SFKs have evolved many ingenious molecular strategies to couple receptors with the cytoplasmic signaling machinery. The contributions to this issue of ONCOGENE describe how this machinery regulates fundamental cellular processes, including cell growth, differentiation, cell shape, migration and survival, and specialized cell signals. The pleiotropic functions of Src and Src family members underscore the importance of these kinases and explain why many of the members of this family have been identified as cellular oncogenes. In this volume, we have attempted to provide the reader with an overview of the current understanding of the function of Src family kinases in the regulation of selected cellular signaling pathways.

A Juxtamembrane Tyrosine in the Colony Stimulating Factor-1 Receptor Regulates Ligand-induced Src Association, Receptor Kinase Function, and Down-regulation

Recent literature implicates a regulatory function of the juxtamembrane domain (JMD) in receptor tyrosine kinases. Mutations in the JMD of c-Kit and Flt3 are associated with gastrointestinal stromal tumors and acute myeloid leukemias, respectively. Additionally, autophosphorylated Tyr559 in the JMD of the colony stimulating factor-1 (CSF-1) receptor (CSF-1R) binds to Src family kinases (SFKs). To investigate SFK function in CSF-1 signaling we established stable 32D myeloid cell lines expressing CSF-1Rs with mutated SFK binding sites (Tyr559-TFI). Whereas binding to I562S was not significantly perturbed, Y559F and Y559D exhibited markedly decreased CSF-1-dependent SFK association. All JMD mutants retained intrinsic kinase activity, but Y559F, and less so Y559D, showed dramatically reduced CSF-1-induced autophosphorylation. CSF-1-mediated wild-type (WT)-CSF-1R phosphorylation was not markedly affected by SFK inhibition, indicating that lack of SFK binding is not responsible for diminished Y559F phosphorylation. Unexpectedly, cells expressing Y559F were hyperproliferative in response to CSF-1. Hyperproliferation correlated with prolonged activation of Akt, ERK, and Stat5 in the Y559F mutant. Consistent with a defect in receptor negative regulation, c-Cbl tyrosine phosphorylation and CSF-1R/c-Cbl co-association were almost undetectable in the Y559F mutant. Furthermore, Y559F underwent reduced multiubiquitination and delayed receptor internalization and degradation. In conclusion, we propose that Tyr559 is a switch residue that functions in kinase regulation, signal transduction and, indirectly, receptor down-regulation. These findings may have implications for the oncogenic conversion of c-Kit and Flt3 with JMD mutations.

Src Family Kinases Are Involved in EphA Receptor-Mediated Retinal Axon Guidance

EphA receptor tyrosine kinases and their ephrin ligands play important roles in wiring of the developing nervous system. We have investigated here the function of Src family kinases (SFKs) in the retinotectal projection to dissect the signaling pathways by which EphA receptors control actin/microtubule rearrangements that underlie growth cone guidance and collapse. Both EphAs and SFKs are expressed broadly in retinal growth cones, and SFKs are recruited to EphA receptors after ephrinA stimulation. In the stripe and growth cone collapse assays we observe an abolition of EphA-mediated repulsion after inhibiting SFKs, either pharmacologically or enzymatically via electroporation-mediated overexpression of the SFK inhibitor Csk. In addition, we identify cortactin and the RhoGEF ephexin, which interact with EphA receptors in retinal axons, as targets of SFK-dependent tyrosine phosphorylation. In sum, our data suggest an important role of SFKs as downstream signaling molecules in EphA receptor-mediated repulsive axon guidance.

Drosophila Src-family kinases function with Csk to regulate cell proliferation and apoptosis.

Elevated Src protein levels and activity are associated with the development and progression of a variety of cancers. The consequences of deregulated Src activity have been studied extensively in cell culture; however, the effects of this deregulation in vivo, as well as the mechanisms of Src-induced tumorigenesis, remain poorly understood. In this study, the effect of expressing wild-type and constitutively active Drosophila Src-family kinases (SFKs) in the developing eye was examined. Overexpression of either wild-type Drosophila SFK (Src64 and Src42) is sufficient to induce ectopic proliferation in G1/G0-arrested, uncommitted cells in eye imaginal discs. In addition, both kinases trigger apoptosis in vivo, in a dosage-dependent manner. Constitutively active mutants are hypermorphic as they trigger proliferation and death more potently than their wild-type counterparts. Moreover, SFK-induced proliferation and apoptosis are largely independent events, as blocking ectopic proliferation does not block cell death. Further, DCsk (the Drosophila homolog of the C-terminal Src kinase) phosphorylates and interacts genetically with the wild-type SFKs, but not with the constitutively active mutants in which a conserved C-terminal tyrosine was mutated to phenylalanine, providing the first in vivo evidence that Csk regulates SFKs during development through phosphorylation of their C-terminal tyrosine.