C-terminal Src kinase controls development and maintenance of mouse squamous epithelia

Carboxy-terminal Src kinase (Csk) is a negative regulator of Src family kinases, which play pivotal roles in controlling cell adhesion, migration, and cancer progression. To elucidate the in vivo role of Csk in epithelial tissues, we conditionally inactivated Csk in squamous epithelia using the keratin-5 promoter/Cre-loxP system in mice. The mutant mice developed apparent defects in the skin, esophagus, and forestomach, with concomitant hyperplasia and chronic inflammation. Histology of the mutant epidermis revealed impaired cell-cell adhesion in basal cell layers. Analysis of primary keratinocytes showed that the defective cell-cell adhesion was caused by cytoskeletal remodeling via activation of the Rac1 pathway. Mutant keratinocytes also showed elevated expression of mesenchymal proteins, matrix metalloproteinases (MMPs), and the proinflammatory cytokine TNF-alpha. Inhibition of the expression of TNF-alpha and MMP9 by the anti-inflammatory reagent FK506 could cure the epidermal hyperplasia, suggesting a causal link between inflammation and epidermal hyperplasia. These observations demonstrate that the Src/Csk circuit plays crucial roles in development and maintenance of epithelia by controlling cytoskeletal organization as well as phenotypic conversion linked to inflammatory events.

SFK-STAT pathway: an alternative and important way to malignancies

Signal transducers and activators of transcription (STAT) proteins play a crucial role in mediating signals from a diverse spectrum of cytokine receptors. STAT is thought to be activated by JAK family kinases (JFK) in many cytokine receptor signal pathways; however, recent studies have demonstrated an alternative pathway to activate STAT by Src family kinases (SFK) in growth factor receptor signal. We also observed STAT5 phosphorylation by Lyn, a member of SFK, in our two recent studies. We introduce these studies and review the literature of STAT activation by SFK and aberrant activation of STAT by oncogenic signals.

An Integrated Genomic-Based Approach to Individualized Treatment of Patients With Advanced-Stage Ovarian Cancer

Purpose

The purpose of this study was to develop an integrated genomic-based approach to personalized treatment of patients with advanced-stage ovarian cancer. We have used gene expression profiles to identify patients likely to be resistant to primary platinum-based chemotherapy and also to identify alternate targeted therapeutic options for patients with de novo platinum-resistant disease.

Patients and Methods

A gene expression model that predicts response to platinum-based therapy was developed using a training set of 83 advanced-stage serous ovarian cancers and tested on a 36-sample external validation set. In parallel, expression signatures that define the status of oncogenic signaling pathways were evaluated in 119 primary ovarian cancers and 12 ovarian cancer cell lines. In an effort to increase chemotherapy sensitivity, pathways shown to be activated in platinum-resistant cancers were subject to targeted therapy in ovarian cancer cell lines.

Results

Gene expression profiles identified patients with ovarian cancer likely to be resistant to primary platinum-based chemotherapy with greater than 80% accuracy. In patients with platinum-resistant disease, we identified expression signatures consistent with activation of Src and Rb/E2F pathways, components of which were successfully targeted to increase response in ovarian cancer cell lines.

Conclusion

We have defined a strategy for treatment of patients with advanced-stage ovarian cancer that uses therapeutic stratification based on predictions of response to chemotherapy, coupled with prediction of oncogenic pathway deregulation, as a method to direct the use of targeted agents.

Sphingosine-1-Phosphate Stimulates the Functional Capacity of Progenitor Cells by Activation of the CXCR 4 -Dependent Signaling Pathway via the S1P 3 Receptor

Objective— Sphingosine-1-phosphate (S1P) is a bioactive lipid, which influences migration and proliferation of endothelial cells through activation of S1P receptors and has been shown to support SDF-1 induced migration and bone marrow homing of CD34 + progenitors.

Methods and Results— Here, we show that incubation of patient-derived endothelial progenitor cells (EPCs) with S1P or its synthetic analog FTY720 improved blood flow recovery in ischemic hind limbs. Likewise, recovery of blood flow was dramatically reduced after induction of hindlimb ischemia in mice deficient for the S1P receptor 3 (S1P 3 ). S1P 3 −/− bone marrow–derived mononuclear cells (BMCs) failed to augment neovascularization after hind limb ischemia. Of note, treatment of BMCs derived from S1P 3 −/− mice with S1P did not rescue blood flow recovery. Mechanistically, S1P and FTY720 induced phosphorylation of CXCR 4 , activated the Src kinase, and stimulated phosphorylation of JAK2. The contribution of CXCR 4 for S1P-mediated effects was further supported by the findings that S1P preincubation failed to stimulate invasion capacity and in vivo blood flow recovery of BMCs from CXCR 4 +/− mice. The activation of CXCR 4 was dependent on the Src kinase family as demonstrated by preincubation with the Src inhibitor PP2. The activation of the CXCR 4 signaling by S1P is mediated via the S1P 3 receptor, since S1P-induced Src phosphorylation was abrogated in EPC from S1P 3 −/− mice.

Conclusions— S1P agonists might serve as sensitizers of CXCR 4 -mediated signaling and may be applied in clinical progenitor cell therapy to improve EPC or BMC function in patients with coronary artery disease.

Microenvironmental Regulation of Membrane Type 1 Matrix Metalloproteinase Activity in Ovarian Carcinoma Cells via Collagen-induced EGR1 Expression

Late stage ovarian cancer is characterized by disseminated intraperitoneal metastasis as secondary lesions anchor in the type I and III collagen-rich submesothelial matrix. Ovarian carcinoma cells preferentially adhere to interstitial collagen, and collagen-induced integrin clustering up-regulates the expression of the transmembrane collagenase membrane type 1 matrix metalloproteinase (MT1-MMP). Collagenolytic activity is important in intraperitoneal metastasis, potentiating invasion through the mesothelial cell layer and colonization of the submesothelial collagen-rich matrix. The objective of this study was to elucidate a potential mechanistic link between collagen adhesion and MT1-MMP expression. Our results indicate that culturing cells on three-dimensional collagen gels, but not thin layer collagen or synthetic three-dimensional hydrogels, results in rapid induction of the transcription factor EGR1. Integrin signaling through a SRC kinase-dependent pathway is necessary for EGR1 induction. Silencing of EGR1 expression using small interfering RNA abrogated collagen-induced MT1-MMP expression and inhibited cellular invasion of three-dimensional collagen gels. These data support a model for intraperitoneal metastasis wherein collagen adhesion and clustering of collagen binding integrins activates integrin-mediated signaling via SRC kinases to induce expression of EGR1, resulting in transcriptional activation of the MT1-MMP promoter and subsequent MT1-MMP-catalyzed collagen invasion. This model highlights the role of unique interactions between ovarian carcinoma cells and interstitial collagens in the ovarian tumor microenvironment in inducing gene expression changes that potentiate intraperitoneal metastatic progression.

Spontaneous class switching and B cell hyperactivity increase autoimmunity against intracellular self antigen in Lyn-deficient mice

IgG autoantibodies cause pathology due to their ability to bind self antigens. However, the extent to which the initial B cell activation and isotype switching is antigen-driven is unclear and it has been widely proposed that intrinsic B cell hyperactivity may be a contributing factor. To explore this issue we generated mice with B cell hyperactivity secondary to deficiency in the src kinase Lyn that also expressed a gene-targeted anti-hen egg lysozyme Ig construct (VDJkappa) capable of class switching to all isotypes. The B cell hyperactivity caused spontaneous hypersecretion of antibodies and class switching to IgM, IgA, IgG1 and IgG3 isotypes in the absence of self antigen, and this persisted as an autoimmune phenomenon in the presence of intracellularly expressed hen egg lysozyme. Exaggerated class switching was also unaffected by antigen in vitro. These findings show that systemic high-avidity intracellular self antigens do not induce self tolerance in the face of B cell hyperactivity. Under these circumstances, spontaneous activation of hyperactive B cells leads to isotype switching and the development of high titres of IgG autoantibodies against intracellular proteins.

Src and focal adhesion kinase mediate mechanical strain-induced proliferation and ERK1/2 phosphorylation in human H441 pulmonary epithelial cells

Pulmonary epithelial cells are exposed to repetitive deformation during physiological breathing and mechanical ventilation. Such deformation may influence pulmonary growth, development, and barotrauma. Although deformation stimulates proliferation and activates extracellular signal-regulated kinases (ERK1/2) in human pulmonary epithelial H441 cells, the upstream mechanosensors that induce ERK activation are poorly understood. We investigated whether c-Src or focal adhesion kinase (FAK) mediates cyclic mechanical strain-induced ERK1/2 activation and proliferation in human pulmonary epithelial (NCI-H441) cells. The H441 and A549 cells were grown on collagen I-precoated membranes and were subjected to an average 10% cyclic mechanical strain at 20 cycles/min. Cyclic strain activated Src within 2 min by increasing phosphorylation at Tyr(418), followed by rapid phosphorylation of FAK at Tyr(397) and Tyr(576) and ERK1/2 at Thr(202)/Tyr(204) (n = 5, P < 0.05). Twenty-four (A549 cells) and 24-72 h (H441 cells) of cyclic mechanical strain increased cell numbers compared with static culture. Twenty-four hours of cyclic strain also increased H441 FAK, Src, and ERK phosphorylation without affecting total FAK, Src, or ERK protein. The mitogenic effect was blocked by Src (10 micromol/l PP2 or short interfering RNA targeted to Src) or MEK (50 micromol/l PD-98059) inhibition. PP2 also blocked strain-induced phosphorylation of FAK-Tyr(576) and ERK-Thr(202)/Tyr(204) but not FAK-Tyr(397). Reducing FAK by FAK-targeted short interfering RNA blocked mechanical strain-induced mitogenicity and significantly attenuated strain-induced ERK activation but not strain-induced Src phosphorylation. Together, these results suggest that repetitive mechanical deformation induced by ventilation supports pulmonary epithelial proliferation by a pathway involving Src, FAK, and then ERK signaling.

Sorting of Fas ligand to secretory lysosomes is regulated by mono-ubiquitylation and phosphorylation

Fas ligand (FasL), a potent mediator of apoptosis expressed by CTL and NK cells, is sorted into the inner vesicles of secretory lysosomes for release via exosome-like vesicles. Previous studies identified a proline-rich domain in the cytoplasmic tail required for sorting FasL to secretory lysosomes, but the mechanisms by which this occurs have not been identified. Here we demonstrate that the PRD of FasL binds Fgr, Fyn and Lyn tyrosine kinases, leading to phosphorylation of FasL. Loss of phosphorylation reduces internalisation of FasL into multivesicular bodies. FasL is also directly mono-ubiquitylated at lysines flanking the PRD and mutation of these lysines reduces MVB localisation of FasL. Phosphorylation is not required for ubiquitylation because FasL lacking all tyrosines undergoes mono-ubiquitylation. These studies show that phosphorylation and ubiquitin signals regulate the sorting of FasL to secretory lysosomes by controlling entry into multivesicular bodies.

PRL3 promotes cell invasion and proliferation by down-regulation of Csk leading to Src activation

Phosphatase of regenerating liver 3 (PRL3) is overexpressed in a variety of tumors, and high levels of PRL3 expression are associated with tumorigenesis and metastasis. Consistent with an oncogenic role for PRL3, we show that ectopic PRL3 expression promotes cell proliferation and invasion. However, little is known about the molecular basis for PRL3 function. Obtaining this knowledge is vital for understanding PRL3-mediated disease processes and for the development of novel anticancer therapies targeted to PRL3. Here we report that up-regulation of PRL3 activates the Src kinase, which initiates a number of signal pathways culminating in the phosphorylation of ERK1/2, STAT3, and p130(Cas). The activation of these pathways likely contributes to the increased cell growth and motility of PRL3 cells. We provide evidence that PRL3 induces Src activation through down-regulation of Csk, a negative regulator of Src. Importantly, Src activation and Csk down-regulation are also observed in colon cancer cells expressing a higher level of PRL3. Thus, we have revealed a biochemical mechanism for the PRL3-mediated cell invasion and proliferation in which elevated PRL3 expression causes a reduction in Csk level, leading to Src activation.

Dysregulation of Src Family Kinases in Mast Cells from Epilepsy-Resistant ASK versus Epilepsy-Prone EL Mice

EL mice have been used as a model of epilepsy, whereas ASK mice are an epilepsy-resistant variant originating from a colony of EL mice. Mast cell-dependent anaphylaxis is easily inducible by stimulation with IgE and Ag in ASK mice, whereas EL mice are resistant to such stimuli. In this study we have characterized mast cells derived from these two strains. ASK mast cells proliferated more vigorously than EL cells in response to IL-3 and stem cell factor. Although ASK mast cells degranulated less vigorously than EL mast cells upon stimulation with IgE and Ag, ASK cells produced and secreted several-fold more TNF-alpha and IL-2 than EL cells. Consistent with the similarities of these ASK and EL mast cell responses with phenotypes of lyn(-/-) and wild-type mast cells, respectively, Lyn activity was reduced in ASK cells. In addition to the impaired Lyn activity, ASK cells just like lyn(-/-) cells exhibited reduced Syk activity, prolonged activation of ERK and JNK, and enhanced activation of Akt. Furthermore, the lipid raft-resident transmembrane adaptor protein Cbp/PAG that associates with Lyn was hypophosphorylated in ASK cells. Importantly, similar to lyn(-/-) cells, Fyn was hyperactivated in ASK cells. Therefore, these results are consistent with the notion that Lyn-dependent phosphorylation of Cbp/PAG negatively regulates Src family kinases. This study also suggests that reduced activity of Lyn, a negative regulator of mast cell activation, underlies the susceptibility of ASK mice to anaphylaxis and implies that dysregulation of Lyn and other Src family kinases contributes to epileptogenesis.

FAK and Src kinases are required for netrin-induced tyrosine phosphorylation of UNC5

During neuronal development, netrin and its receptors UNC5 and DCC (deleted in colorectal cancer) guide axonal growth cones in navigating to their targets. Netrin also plays important roles in the regulation of cell migration, tissue morphogenesis and tumor growth. Here, we show that netrin induces UNC5 tyrosine phosphorylation and that this effect of netrin is dependent on its co-receptor DCC. UNC5 tyrosine phosphorylation is known to be important for netrin to induce cell migration and axonal repulsion. Src tyrosine kinase activity is required for netrin to stimulate UNC5 tyrosine phosphorylation in neurons and transfected cells. The SH2 domain of Src kinase directly interacts with the cytosolic domain of UNC5 in a tyrosine-phosphorylation-dependent manner. Furthermore, the tyrosine kinase focal adhesion kinase (FAK) is also involved in netrin-induced UNC5 tyrosine phosphorylation. Both Src and FAK can phosphorylate UNC5. Our data suggest a model in which netrin stimulates UNC5 tyrosine phosphorylation and signaling in a manner dependent on the co-receptor DCC, through the recruitment of Src and FAK kinases.

Engagement of the B-cell antigen receptor activates STAT through Lyn in a Jak-independent pathway

Engagement of the B-cell antigen receptor (BCR) initiated by the Src kinase Lyn triggers rapid signaling cascades, leading to proliferation, differentiation or growth arrest of B cells. The Janus kinase (JAK)-STAT (signal transducer and activator of transcription) pathway, activated through cytokine receptors, mediates similar responses. Hypothesizing that Src and JAK pathways engage in crosstalk in B-cell signaling, we studied wild-type and Lyn-null B-cell lines, which express BCR. We found that activated BCR results in tyrosine phosphorylation of JAK-STAT, which required Lyn. To confirm that STAT activation is not due to JAK, we cloned the chicken homologs of JAK1 and JAK2 and made their antisense constructs. In cells expressing antisense JAK1 and JAK2, tyrosine phosphorylation of STAT was not inhibited following BCR stimulation. Using activation loop-specific phosphotyrosine antibodies, we did not detect phospho-JAK1 and phospho-JAK2 after BCR stimulation. The JAK inhibitor AG490 did not inhibit the tyrosine phosphorylation of Lyn or STAT after BCR simulation. An in vitro phosphorylation assay showed that Lyn directly phosphorylates STAT3. In an electrophoretic mobility shift assay, BCR stimulation led to enhanced DNA binding of the STAT3 in DT40, but not in the Lyn-null cells. We conclude that BCR engagement activates the STAT pathway via Lyn, independent of JAK.

CHK negatively regulates Lyn kinase and suppresses pancreatic cancer cell invasion

Among the most important signaling pathways operating in pancreatic cancer cells are those resulting from mutations in the Ras oncogene or from overexpression of ErbB-2 and associated Src-family kinases. In this study, we aimed to characterize CHK expression and function in pancreatic cancer. Our data demonstrated CHK expression in human pancreatic cancer tissues, and also showed that CHK associated with ErbB-2 via its SH2 domain in human PANC-1 pancreatic cancer cells. PANC-1 cells were found to express both Src kinase and Lyn kinase, although the expression of Lyn kinase was more abundant. Furthermore, CHK downregulated Lyn kinase activity and significantly inhibited the in vitro growth and invasion of PANC-1 cells upon EGF stimulation. These results indicate that CHK is a negative regulator of ErbB-2 and Lyn kinase signaling in pancreatic cancer cells.

Bacterial Expression and Characterization of Catalytic Loop Mutants of Src Protein Tyrosine Kinase†

Protein tyrosine kinase Src is a key enzyme in mammalian signal transduction and an important target for anticancer drug discovery. Although recombinant expression in bacterial cells offers a convenient and rapid way for producing several other protein tyrosine kinases, active Src is difficult to produce in bacterial systems. However, a kinase-defective Src mutant (due to a single point mutation, Lys295Met) is expressed strongly in bacteria. We hypothesize that the difficulty with expressing active Src in bacteria is due to toxicity caused by Src kinase activity. To test this hypothesis, we generated a series of Src mutants by altering certain residues, especially His384, in the catalytic loop and examined their expression in the bacteria and their kinase activity. The results demonstrate that Src mutants with kinase activity above a certain threshold could not be purified from a bacterial expression system, while a variety of mutants with a kinase activity below this threshold could indeed be expressed and purified. These observations support the conclusion that Src activity is toxic to the bacteria, which prevents high-level expression of fully active Src. We further demonstrated that His384, a universally conserved residue among protein tyrosine kinases, is not essential for Src catalysis or its inactivation by C-terminal tail Tyr phosphorylation. Interestingly, His384 mutants undergo autophosphorylation on Tyr416 like wild-type Src but are not activated by autophosphorylation. The potential role of His384 in Src activation by autophosphorylation is discussed in the context of Src structure.

Update on HIV-associated nephropathy

PURPOSE OF REVIEW

HIV-associated nephropathy is characterized by a constellation of pathologic findings including a collapsing glomerulopathy, tubular dilatation, and interstitial infiltration with leukocytes. This review summarizes some of the recent advances in our understanding of the gene products and signaling pathways that contribute to the pathogenesis of HIV-associated nephropathy.

RECENT FINDINGS

Podocytes infected with HIV-associated nephropathy exhibit podocyte proliferation and de-differentiation. Restriction of HIV-1 transgene expression to the podocyte in a murine model supports the belief that podocyte infection is pivotal to the development of the disease. Recent studies have provided compelling in-vitro and in-vivo evidence that expression of the HIV-1 accessory gene nef is critical in altering the phenotype of mature podocytes and causing injury to these cells. An in-vitro study suggests that nef's effects in the podocyte appear to be mediated through Src kinase-dependent activation of the signal transducer and activator of transcription 3 and mitogen-activated protein kinase 1,2 signaling pathways.

SUMMARY

Recent evidence demonstrates that the viral protein nef plays a critical role in the development of HIV-associated nephropathy and provides a foundation for developing new therapeutic strategies for patients afflicted with this disease.

Transient anchorage of cross-linked glycosyl-phosphatidylinositol-anchored proteins depends on cholesterol, Src family kinases, caveolin, and phosphoinositides

How outer leaflet plasma membrane components, including glycosyl-phosphatidylinositol-anchored proteins (GPIAPs), transmit signals to the cell interior is an open question in membrane biology. By deliberately cross-linking several GPIAPs under antibody-conjugated 40-nm gold particles, transient anchorage of the gold particle-induced clusters of both Thy-1 and CD73, a 5' exonucleotidase, occurred for periods ranging from 300 ms to 10 s in fibroblasts. Transient anchorage was abolished by cholesterol depletion, addition of the Src family kinase (SFK) inhibitor PP2, or in Src-Yes-Fyn knockout cells. Caveolin-1 knockout cells exhibited a reduced transient anchorage time, suggesting the partial participation of caveolin-1. In contrast, a transmembrane protein, the cystic fibrosis transmembrane conductance regulator, exhibited transient anchorage that occurred without deliberately enhanced cross-linking; moreover, it was only slightly inhibited by cholesterol depletion or SFK inhibition and depended completely on the interaction of its PDZ-binding domain with the cytoskeletal adaptor EBP50. We propose that cross-linked GPIAPs become transiently anchored via a cholesterol-dependent SFK-regulatable linkage between a transmembrane cluster sensor and the cytoskeleton.

Src family kinases mediate neutrophil adhesion to adherent platelets

Polymorphonuclear leukocyte (PMN)-platelet interactions at sites of vascular damage contribute to local and systemic inflammation. We sought to determine the role of "outside-in" signaling by Src-family tyrosine kinases (SFKs) in the regulation of alphaMbeta2-integrin-dependent PMN recruitment by activated platelets under (patho)physiologic conditions. Activation-dependent epitopes in beta2 integrin were exposed at the contact sites between PMNs and platelets and were abolished by SFK inhibitors. PMNs from alphaMbeta2(-/-), hck(-/-)fgr(-/-), and hck(-/-)fgr(-/-)lyn(-/-) mice had an impaired capacity to adhere with activated platelets in suspension. Phosphorylation of Pyk2 accompanied PMN adhesion to platelets and was blocked by inhibition as well as by genetic deletion of alphaMbeta2 integrin and SFKs. A Pyk2 inhibitor reduced platelet-PMN adhesion, indicating that Pyk2 may be a downstream effector of SFKs. Analysis of PMN-platelet interactions under flow revealed that SFK signaling was required for alphaMbeta2-mediated shear-resistant adhesion of PMNs to adherent platelets, but was dispensable for P-selectin-PSGL-1-mediated recruitment and rolling. Finally, SFK activity was required to support PMN accumulation along adherent platelets at the site of vascular injury, in vivo. These results definitely establish a role for SFKs in PMN recruitment by activated platelets and suggest novel targets to disrupt the pathophysiologic consequences of platelet-leukocyte interactions in vascular disease.

A common signaling cascade may underlie "addiction" to the Src, BCR-ABL, and EGF receptor oncogenes

"Oncogene addiction" describes an unexplained dependency of cancer cells on a particular cellular pathway for survival or proliferation. We report that differential attenuation rates of prosurvival and proapoptotic signals in oncogene-dependent cells contribute to cell death following oncogene inactivation. Src-, BCR-ABL-, and EGF receptor-dependent cells exhibit a similar profile of signal attenuation following oncogene inactivation characterized by rapid diminution of phospho-ERK, -Akt, and -STAT3/5, and a delayed accumulation of the proapoptotic effector phospho-p38 MAPK. These findings implicate a transient imbalance in survival and apoptotic oncogenic outputs in the apoptotic response to oncogene inactivation. Moreover, these observations implicate a common profile of signal attenuation for multiple oncogenes and suggest that "addiction" associated with apoptosis reflects an active rather than a passive process.

Targeting multiple kinase pathways in leukemic progenitors and stem cells is essential for improved treatment of Ph+ leukemia in mice

It is generally believed that shutting down the kinase activity of BCR-ABL by imatinib will completely inhibit its functions, leading to inactivation of its downstream signaling pathways and cure of the disease. Imatinib is highly effective at treating human Philadelphia chromosome-positive (Ph(+)) chronic myeloid leukemia (CML) in chronic phase but not Ph(+) B cell acute lymphoblastic leukemia (B-ALL) and CML blast crisis. We find that SRC kinases activated by BCR-ABL remain fully active in imatinib-treated mouse leukemic cells, suggesting that imatinib does not inactivate all BCR-ABL-activated signaling pathways. This SRC pathway is essential for leukemic cells to survive imatinib treatment and for CML transition to lymphoid blast crisis. Inhibition of both SRC and BCR-ABL kinase activities by dasatinib affords complete B-ALL remission. However, curing B-ALL and CML mice requires killing leukemic stem cells insensitive to both imatinib and dasatinib. Besides BCR-ABL and SRC kinases, stem cell pathways must be targeted for curative therapy of Ph(+) leukemia.

Maximal beta3-adrenergic regulation of lipolysis involves Src and epidermal growth factor receptor-dependent ERK1/2 activation

Catecholamine-stimulated lipolysis is primarily a beta-adrenergic and cAMP-dependent event. In previous studies we established that the beta(3)-adrenergic receptor (beta(3)AR) in adipocytes utilizes a unique mechanism to stimulate extracellular signal-regulated kinases 1 and 2 (ERK) by direct recruitment and activation of Src kinase. Therefore, we investigated the role of the ERK pathway in adipocyte metabolism and found that the beta(3)AR agonist CL316,243 regulates lipolysis through both cAMP-dependent protein kinase (PKA) and ERK. Inhibition of PKA activity completely eliminated lipolysis at low (subnanomolar) CL316,243 concentrations and by 75-80% at higher nanomolar concentrations. The remaining 20-25% of PKA-independent lipolysis, as well as ERK activation, was abolished by inhibiting the activity of either Src (PP2 or small interfering RNA), epidermal growth factor receptor (EGFR with AG1478 or small interfering RNA), or mitogen-activated protein kinase kinase 1 or 2 (MKK1/2 with PD098059). PD098059 inhibited lipolysis by 53% in mice as well. Finally, the effect of estradiol, a reported acute activator of ERK and lipolysis, was also totally prevented by PP2, AG1478, and PD098059. These results suggest that ERK activation by beta(3)AR depends upon Src and epidermal growth factor receptor kinase activities and is responsible for the PKA-independent portion of the lipolytic response. Together these results illustrate the distinct and complementary roles for PKA and ERK in catecholamine-stimulated lipolysis.

SRC tail phosphorylation is limited by structural changes in the regulatory tyrosine kinase Csk

Src family tyrosine kinases are down-regulated through phosphorylation of a single C-terminal tyrosine by the nonreceptor tyrosine kinase Csk. Despite the fundamental role of Csk in controlling cell growth and differentiation, it is unclear what limits this key signaling reaction and controls the production of catalytically repressed Src. To investigate this issue, stopped-flow fluorescence experiments were performed to determine which steps modulate catalysis. Both Src binding and phosphorylation can be monitored by changes in intrinsic tryptophan fluorescence. Association kinetics are biphasic with the initial phase corresponding to the bimolecular interaction of both proteins and the second phase representing a slow conformational change that coincides with the rate of maximum turnover. The kinetic transients for the phosphorylation reaction are also biphasic with the initial phase corresponding to the rapid phosphorylation and the release of phospho-Src. These data, along with equilibrium sedimentation and product inhibition experiments, suggest that steps involving Src association, phosphorylation, and product release are fast and that a structural change in Csk participates in limiting the catalytic cycle.

PSD-95 is a negative regulator of the tyrosine kinase Src in the NMDA receptor complex

The tyrosine kinase Src upregulates the activity of the N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) and tyrosine phosphorylation of this receptor is critical for induction of NMDAR-dependent plasticity of synaptic transmission. A binding partner for Src within the NMDAR complex is the protein PSD-95. Here we demonstrate an interaction of PSD-95 with Src that does not require the well-characterized domains of PSD-95. Rather, we show binding to Src through a 12-amino-acid sequence in the N-terminal region of PSD-95, a region not previously known to participate in protein-protein interactions. This region interacts directly with the Src SH2 domain. Contrary to typical SH2 domain binding, the PSD-95-Src SH2 domain interaction is phosphotyrosine-independent. Binding of the Src-interacting region of PSD-95 inhibits Src kinase activity and reduces NMDAR phosphorylation. Intracellularly administering a peptide matching the Src SH2 domain-interacting region of PSD-95 depresses NMDAR currents in cultured neurons and inhibits induction of long-term potentiation in hippocampus. Thus, the PSD-95-Src SH2 domain interaction suppresses Src-mediated NMDAR upregulation, a finding that may be of broad importance for synaptic transmission and plasticity.

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

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

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

B cell receptor signaling in human systemic lupus erythematosus

PURPOSE OF REVIEW

The purpose of this review is to inform the scientific community of the most recent findings surrounding B cell receptor signaling function in human systemic lupus erythematosus and how altered B cell signaling may explain the characteristic hyperactivity of B cells in active disease and contribute to its pathogenesis.

RECENT FINDINGS

B cell receptor signaling is abnormal in patients with active systemic lupus erythematosus as demonstrated by increased calcium flux and global B cell hyperactivity. Altered signaling has been explained by a variety of factors such as defective FcgammaRIIB signaling, decreased expression of the protein tyrosine kinase Lyn, and increased serum levels of B lymphocyte stimulator.

SUMMARY

The studies reviewed suggest that B cells from systemic lupus erythematosus patients display molecular signaling defects that most likely contribute to pathogenesis of the disease and explain the characteristic hyperactivity of B cells in active disease.