Phosphorylation of Cbl following stimulation with interleukin-3 and its association with Grb2, Fyn, and phosphatidylinositol 3-kinase

We have demonstrated that a 120-kDa protein, identified as Cbl, becomes rapidly phosphorylated on tyrosine residues following stimulation of factor-dependent cells with interleukin-3 (IL-3). Little or no phosphorylation of Cbl was observed in the absence of IL-3 stimulation and phosphorylation is maximal by 20-30 min after IL-3 stimulation. Association of Cbl with Grb2 was noted in unstimulated cells, and the amount of Cbl associated with Grb2 increased following IL-3 stimulation. The p85 subunit of phosphatidylinositol 3-kinase was constitutively associated with Cbl. Approximately 10% of the PI kinase activity present in anti-phosphotyrosine immunoprecipitates was present in anti-Cbl immunoprecipitates of IL-3-stimulated cells. The constitutive association of Cbl with Fyn was also observed. Cbl was observed to bind to bacterial fusion proteins encoding the unique, SH3, and SH2 domains of Fyn, Hck, and Lyn. The SH2 domain of Fyn alone was able to bind Cbl to nearly the same extent as did the fusion protein encoding the unique, SH3, and SH2 domains. This was not the case for the SH2 domain of Hck, however, as binding of the Hck fusion protein to Cbl appeared to require multiple domains. The binding of the fusion proteins to Cbl occurred regardless of whether Cbl was tyrosine-phosphorylated or not, and the binding could not be disrupted by the addition of 30 mM free phosphotyrosine. These data suggest the unexpected conclusion that the Fyn SH2 domain may bind to Cbl in a phosphotyrosine-independent manner.

Hck is activated by opsonized zymosan and A23187 in distinct subcellular fractions of human granulocytes

Regulation of neutrophil responses is known to involve tyrosine phosphorylation. Hck, a major neutrophil protein-tyrosine kinase, becomes expressed during differentiation of human promyelocytic NB4 cells into neutrophil-like cells. Hck is mainly localized in a secretory granule-enriched cell fraction, but it is also present in a granule-free membrane fraction and the cytosol. Hck is rapidly and transiently activated upon stimulation of differentiated NB4 cells or human neutrophils with serum-opsonized zymosan or the calcium ionophore A23187, but not by phorbol 12-myristate 13-acetate. In NB4 cells, Hck is also weakly activated by fMet-Leu-Phe. Cell fractionation showed that opsonized zymosan and A23187 induce Hck activation in distinct subcellular fractions. Both stimuli activate Hck in the secretory granule-enriched fraction, but only A23187 activates the kinase in the granule-free membrane fraction. Our results suggest that Hck might regulate early signal transduction events induced by opsonized zymosan and A23187, and that the different subcellular fractions of Hck might serve discrete functions, one of which could be regulation of the degranulation response.

Critical role of Lyn kinase in inhibition of neutrophil apoptosis by granulocyte-macrophage colony-stimulating factor

The signal pathway for control of apoptosis in human neutrophils is currently unknown. In this study, we provide the first evidence that a Src family tyrosine kinase, Lyn, plays a key role in inhibition polymorphonuclear (PMN) cell death. Several nuclear proteins associated with apoptosis, i.e., p53, cdc2, and Rb, were absent from PMN. Bcl-2, known to inhibit apoptosis, was also not expressed. Programmed cell death that rapidly occurred in PMN could be arrested by granulocyte-macrophage CSF (GM-CSF), but this activation did not induce p53, cdc2, retinoblastoma, or Bcl-2 expression. Instead, GM-CSF produced a rapid activation of Lyn and Hck, but not Fgr, tyrosine phosphorylation within 1 min. Co-immunoprecipitation studies indicated that only Lyn, but not Hck, was physically coupled to GM-CSF receptor. By histologic assessment and evaluation of DNA fragmentation, only antisense Lyn, but not antisense Hck or antisense Fgr, could reverse the cell survival advantage provided by GM-CSF. Therefore, the physical coupling of Lyn to GM-CSF receptor and its early activation are required for inhibition or delay of apoptosis in PMN.

Expression of the activated (Y501-F501) hck tyrosine kinase in 32Dcl3 myeloid cells prolongs survival in the absence of IL-3 and blocks granulocytic differentiation in response to G-CSF

The hematopoietic cell kinase (hck), a member of the src family of intracellular, membrane-associated protein tyrosine kinases, is primarily expressed in mature granulocytes and monocyte/macrophages. Hck kinase activity plays an essential role in macrophage activation and may be an important signaling molecule in granulocytes. To examine the potential role of hck in hematopoietic differentiation pathways, retroviral vectors were used to express wild-type and mutant forms of p59hck in the hck-negative, interleukin-3 (IL-3) -dependent murine myeloid cell line, 32Dcl3. Constitutive expression of an activated form of hck markedly prolonged the viability of 32Dcl3 cells in the absence of IL-3 but failed to abrogate the requirement for IL-3 for proliferation. Moreover, enforced expression of the activated hck kinase (and less so, the wild-type kinase) blocked granulocytic differentiation of 32Dcl3 cells in response to granulocyte colony-stimulating factor. These findings indicate that up-regulation of hck expression is not required for (and may interfere with) granulocytic differentiation.

Reading between the lines: SH3 recognition of an intact protein

The structure of Nef-SH3 domain complex reveals how an SH3 domain can more effectively 'read' its linear proline-rich recognition element when it is presented within the context of a folded protein.

Deficiency of Src family kinases p59/61hck and p58c-fgr results in defective adhesion-dependent neutrophil functions

Cross-linking of the neutrophil-beta 2- or beta 3-related leukocyte response integrins by extracellular matrix (ECM) proteins or monoclonal antibodies (mAb) stimulates cytoskeletal rearrangement leading to cell spreading and respiratory burst. Tyrosin phosphorylation of a variety of proteins and activation of the Src family kinases within polymorphonuclear leukocytes (PMN) have recently been implicated in the intracellular signaling pathways generated by leukocyte integrins (Yan, S.R., L. Fumagalli, and G Berton. 1995. J. Inflammation. 45:217-311.) To directly test whether these functional responses are dependent on the Src family kinases p59/61hck and p58c-fgr, we examined adhesion-dependent respiratory burst in PMNs isolated from hck -/-, fgr -/-, and hck -/- fgr -/- knockout mice. Purified bone marrow PMNS from wild-type mice released significant amounts of O2- when adherent to fibrinogen-, fibronectin-, or collagen-coated surfaces, in the presence of activating agents such as tumor necrosis factor (TNF) or formyl-methionyl-leucyl-phenylalanine, as described for human PMNs. PMNs from hck-/-fgr-/- double-mutant mic, however, failed to respond. This defect was specific for integrin signaling, since respiratory burst was normal in hck-/-fgr-/-PMNs stimulated by immune complexes or PMA. Stimulation of respiratory burst was observed in TNF-primed wild-type PMN plated on surfaces coated with murine intracellular adhesion molecule-1 (ICAM-1), while hck-/-fgr-/- PMNs, failed to respond. Direct cross-linking of the subunits of beta 2 and beta 2 integrins by surface-bound mAbs was elicited O2- production by wild-type PMNs, while the double-mutant hck-/-fgr-/- cells failed to respond. Photomicroscopy and cell adhesion assays revealed that the impaired functional responses of hck-/-fgr-/- PMNs were caused by defective spreading and tight adhesion on either ECM protein- or mAb-coated surfaces. In contrast, hck-/-or fgr-/-single mutant cells produced O2- at levels equivalent to wild-type cells on ECM protein, murine ICAM-1, and antiintegrin mAb-coated surfaces. Hence, either p59/61 hck and p 58c-fgr is required for signaling through leukocyte beta 2 and beta 3 integrins leading to PMN spreading and respiratory burst. This is the first direct genetic evidence of the importance of Src family kinases in integrin signaling within leukocytes, and it is also the best example of overlapping function between members of this gene family within a defined signal transduction pathway.

Improved 3D gd-HCACO and gd-(H)CACO-TOCSY experiments for isotopically enriched proteins dissolved in H2O

Pulsed field gradients were incorporated into the HCACO experiment for acquiring spectra on isotopically enriched protein samples dissolved in H2O. Excellent water suppression and spectral quality were achieved using the modified pulse sequence (gd-HCACO), as demonstrated for a 13C-/15N-labeled sample of the SH2 domain from the hematopoietic cellular kinase dissolved in 90% H2O/10% D2O. Strong correlations for all residues were observed in the gd-HCACO spectrum, even for residues having alpha-protons resonating exactly at the H2O frequency. The HCACO-TOCSY experiment was modified to correlate intraresidue 13C alpha (rather than 1H alpha), carbonyl (13C'), and aliphatic side-chain protons [(H)CACO-TOCSY]. Pulsed field gradients were also incorporated into the (H)CACO-TOCSY experiment for water suppression.

Proteolytic cleavage of the urokinase receptor substitutes for the agonist-induced chemotactic effect

Physiological concentrations of urokinase plasminogen activator (uPA) stimulated a chemotactic response in human monocytic THP-1 through binding to the urokinase receptor (uPAR). The effect did not require the protease moiety of uPA, as stimulation was achieved also with the N-terminal fragment (ATF), while the 33 kDa low molecular weight uPA was ineffective. Co-immunoprecipitation experiments showed association of uPAR with intracellular kinase(s), as demonstrated by in vitro kinase assays. Use of specific antibodies identified p56/p59hck as a kinase associated with uPAR in THP-1 cell extracts. Upon addition of ATF, p56/p59hck activity was stimulated within 2 min and returned to normal after 30 min. Since uPAR lacks an intracellular domain capable of interacting with intracellular kinase, activation of p56/p59hck must require a transmembrane adaptor. Evidence for this was strongly supported by the finding that a soluble form of uPAR (suPAR) was capable of inducing chemotaxis not only in THP-1 cells but also in cells lacking endogenous uPAR (IC50, 5 pM). However, activity of suPAR require chymotrypsin cleavage between the N-terminal domain D1 and D2 + D3. Chymotrypsin-cleaved suPAR also induced activation of p56/p59hck in THP-1 cells, with a time course comparable with ATF. Our data show that uPA-induced signal transduction takes place via uPAR, involves activation of intracellular tyrosine kinase(s) and requires an as yet undefined adaptor capable of connecting the extracellular ligand binding uPAR to intracellular transducer(s).

Late administration of a lipophilic tyrosine kinase inhibitor prevents lipopolysaccharide and Escherichia coli-induced lethal toxicity

Septic shock induced by gram-negative bacteria results primarily from excessive stimulation by lipopolysaccharide (LPS) of macrophages to produce tumor necrosis factor (TNF)-alpha and interleukin (IL)-1. The cellular effects of LPS, TNF-alpha, and IL-1 are mediated via tyrosine phosphorylation pathways. A recent report indicated that selective inhibitors of tyrosine kinases, tyrphostins of the AG126 family, protect mice against LPS-induced lethal toxicity in mice. Protection was most effective when the tyrphostin was injected before the LPS. In the present study, tyrphostin AG556, which is more lipophilic than those of the AG126 family, was effective in preventing LPS-induced lethal toxicity when administered 2 h after LPS. AG556 also prevented viable Escherichia coli-induced lethal toxicity when given 2 h before and, to a lesser extent, 2 h after the bacterial inoculation. AG556 may block a critical step downstream of the signaling pathway induced by LPS after TNF-alpha production.

The solution structure of HIV-1 Nef reveals an unexpected fold and permits delineation of the binding surface for the SH3 domain of Hck tyrosine protein kinase

The solution structure of HIV-1 Nef has been solved by multidimensional heteronuclear NMR spectroscopy. The construct employed to circumvent problems associated with aggregation was a double-deletion mutant (delta2-39, delta159-173) in which conformationally disordered regions of the protein at the N terminus and in a long solvent-exposed flexible loop were removed, without affecting the properties or structural integrity of the remainder of the protein. Despite the absence of any sequence similarity, the overall fold of Nef is reminiscent of that of the family of winged helix-turn-helix DNA binding proteins. The binding surface of Nef for the SH3 domain of Hck tyrosine protein kinase has been mapped and reveals a non-contiguous (in terms of amino-acid sequence) interaction surface. This unique feature may suggest possible avenues for drug design aimed at inhibiting the interaction between Nef and SH3 domains.

Polyomavirus middle-T antigen associates with the kinase domain of Src-related tyrosine kinases

Middle-T antigen of mouse polyomavirus, an oncogenic DNA virus, associates with and activates the cellular tyrosine kinases c-Src, c-Yes, and Fyn. This interaction is essential for polyomavirus-mediated transformation of cells in culture and tumor formation in animals. To determine the domain of c-Src directing association with middle-T, mutant c-Src proteins lacking the amino-terminal unique domain and the myristylation signal, the SH2 domain, the SH3 domain, or all three of these domains were coexpressed with middle-T in NIH 3T3 cells. All mutants were found to associate with middle-T, demonstrating that the kinase domain of c-Src, including the carboxy-terminal regulatory tail, is sufficient for association with middle-T. Moreover, we found that Hck, another member of the Src kinase family, does not bind middle-T, while chimeric kinases consisting of the amino-terminal domains of c-Src fused to the kinase domain of Hck or the amino-terminal domains of Hck fused to the kinase domain of c-Src associated with middle-T. Hck mutated at its carboxy-terminal regulatory residue, tyrosine 501, was also found to associate with middle-T. These results suggest that in Hck, the postulated intramolecular interaction between the carboxy-terminal regulatory tyrosine and the SH2 domain prevents association with middle-T. This intramolecular interaction apparently also limits the ability of c-Src to associate with middle-T, since removal of the SH2 or SH3 domain increases the efficiency with which middle-T binds c-Src.

Deficiency of the Hck and Src tyrosine kinases results in extreme levels of extramedullary hematopoiesis

Expression of the Src-family kinases--Src, Hck, and Fgr--increases dramatically during myeloid cell development. Src-deficient mice exhibit functional abnormalities in only one myeloid cell type, the osteoclast, resulting in impaired bone remodeling and osteopetrosis, while hck-/- or fgr-/- mice have few and subtle myeloid cell deficiencies. To determine whether these limited phenotypes are due to the coexpression of multiple Src-family kinases with overlapping functions, we have intercrossed src-/- mice to hck-/- and fgr-/- mutants to produce double mutants. Two thirds of hck-/- src-/- double mutants die at birth; surviving animals develop a severe form of osteopetrosis, resulting in extreme levels of splenic extramedullary hematopoiesis, anemia, and leukopenia. These hematopoietic defects are caused by abnormalities in the bone marrow environment because hck-/- src-/- mutant stem cells reconstitute a normal hematopoietic system in irradiated wild-type mice. In contrast, fgr-/- src-/- double mutants have no defects beyond those observed in src-/- animals. Cultured normal murine osteoclasts express abundant amounts of Src, Hck, and Fgr and Hck levels are increased in src-/- osteoclasts. These observations suggest that Hck and Src serve partially overlapping functions in osteoclasts and that the expression of Hck in src-/- osteoclasts ameliorates their functional defects.

CD66 family members are associated with tyrosine kinase activity in human neutrophils

The granulocyte activation Ags, CD66a, CD66b, CD66c, and CD66d, are expressed at low levels on resting blood granulocytes, but their surface expression is up-regulated following stimulation. CD66a, in contrast to CD66b and CD66c which are anchored to the membrane via a glycosyl-phosphatidylinositol linkage, is a transmembrane protein with a cytoplasmic domain. We have previously reported that CD66a is phosphorylated in human neutrophils, largely on tyrosine, with a lower level of phosphoserine. We have now found that CD66a undergoes a rapid increase in phosphorylation following stimulation with FMLP, platelet-activating factor, and 12-O-tetradecanoyl-phorbol-13-acetate. This increase in phosphorylation was transient, with maximal phosphorylation observed by 1 min and a return to base line by 5 min following stimulation. Protein kinase activity was detected in neutrophils associated with CD66a, CD66b, and CD66c. Most of the protein kinase activity associated with these Ags was tyrosine kinase activity, with a lesser amount of threonine and serine kinase activities. Lyn and Hck accounted for much of the associated tyrosine kinase activity. The data suggest that phosphorylation of CD66a on tyrosine by an associated tyrosine kinase may play a role in the function of CD66a. In addition, associated tyrosine kinase activity may play a role in signal transduction from CD66a, CD66b, and CD66c to regulate other cell functions.

A single amino acid in the SH3 domain of Hck determines its high affinity and specificity in binding to HIV-1 Nef protein

We have examined the differential binding of Hck and Fyn to HIV-1 Nef to elucidate the structural basis of SH3 binding affinity and specificity. Full-length Nef bound to Hck SH3 with the highest affinity reported for an SH3-mediated interaction (KD 250 nM). In contrast to Hck, affinity of the highly homologous Fyn SH3 for Nef was too weak (KD > 20 microM) to be accurately determined. We show that this distinct specificity lies in a variable loop, the 'RT loop', positioned close to conserved SH3 residues implicated in the binding of proline-rich (PxxP) motifs. A mutant Fyn SH3 with a single amino acid substitution (R96I) in its RT loop had an affinity (KD 380 nM) for Nef comparable with that of Hck SH3. Based on additional mutagenesis studies we propose that the selective recognition of Nef by Hck SH3 is determined by hydrophobic interactions involving an isoleucine residue in its RT loop. Although Nef contains a PxxP motif which is necessary for the interaction with Hck SH3, high affinity binding was only observed for intact Nef protein. The binding of a peptide containing the Nef PxxP motif showed > 300-fold weaker affinity for Hck SH3 than full-length Nef.

Activation of src-related tyrosine kinases by IL-3

The activation of src-related tyrosine kinases following IL-3 stimulation was examined in 32Dcl3 cells. Three src-related tyrosine kinases were activated following IL-3 stimulation: fyn, hck, and lyn. 32Dcl3 cells were transfected with retroviral vectors expressing each of these kinases and independent clones overexpressing each kinase were isolated. In cells overexpressing either fyn or hck, IL-3 stimulated a rapid increase in catalytic activity, which remained elevated longer compared with the kinetics observed in parental 32Dcl3 cells. An increase in the number of tyrosine-phosphorylated proteins in the presence and absence of IL-3 stimulation was observed in cells overexpressing fyn or hck. Transfection of 32Dcl3 cells with a retroviral vector encoding lyn also resulted in an elevated level of kinase activity, although the increase was not as dramatic as that observed with fyn or hck. Consistent with observations in parental 32Dcl3 cells, a high basal level of lyn kinase activity was observed in unstimulated lyn-transfected cells and IL-3 stimulation resulted in an approximate threefold increase in kinase activity. Overexpression of c-src in 32Dcl3 did not result in IL-3-stimulated activation of c-src, indicating specificity for fyn, hck, and lyn. While the overexpression of fyn, hck, or lyn in 32Dcl3 cells resulted in increased kinase activity and IL-3 stimulated tyrosine phosphorylation, it did not render the cells more sensitive to IL-3. These results suggests that in addition to the JAK2 tyrosine kinase, src-related kinases may play a significant role in signal transduction by cytokine receptors.

The src-family protein-tyrosine kinase p59hck is located on the secretory granules in human neutrophils and translocates towards the phagosome during cell activation

The src-family protein-tyrosine kinase p59hck is mainly expressed in neutrophils; however, its functional role in these cells is unknown. Several other src-family members are localized on secretory vesicles and have been proposed to regulate intracellular traffic. We have established here the subcellular localization of p59hck in human neutrophils. Immunoblotting of subcellular fractions showed that approx. 60% of the p59hck per cell is localized on the secretory granules; the other 40% is distributed equally between non-granular membranes and the cytosol. Immunofluorescence of neutrophils and HL60 cells suggests that the p59hck-positive granules are azurophil granules. Granular p59hck is highly susceptible to degradation by an azurophil-granule proteinase. Different forms of p59hck occur in the three subcellular compartments: a 61 kDa form is mainly found in the granules, a 59 kDa form is predominant in the non-granular membranes, whereas cytosolic p59hck migrates as a doublet at 63 kDa. During the process of phagocytosis-linked degranulation, induced by serum-opsonized zymosan in neutrophils or HL60 cells, granular p59hck translocates towards the phagosome. The subcellular localization of p59hck suggests that the enzyme could be involved in the regulation of the degranulation process.

Myristoylation and differential palmitoylation of the HCK protein-tyrosine kinases govern their attachment to membranes and association with caveolae

The human proto-oncogene HCK encodes two versions of a protein-tyrosine kinase, with molecular weights of 59,000 (p59hck) and 61,000 (p61hck). The two proteins arise from a single mRNA by alternative initiations of translation. In this study, we explored the functions of these proteins by determining their locations within cells and by characterizing lipid modifications required for the proteins to reach those locations. We found that p59hck is entirely associated with cellular membranes, including the organelles known as caveolae; in contrast, only a portion of p61hck is situated on membranes, and none is detectable in preparations of caveolae. These distinctions can be attributed to differential modification of the two HCK proteins with fatty acids. Both proteins are at least in part myristoylated, p59hck more so than p61hck. In addition, however, p59hck is palmitoylated on cysteine 3 in the protein. Palmitoylation of the protein requires prior myristoylation and, in turn, is required for targeting to caveolae. These findings are in accord with recent reports for other members of the SRC family of protein-tyrosine kinases. Taken together, the results suggest that HCK and several of its relatives may participate in the functions of caveolae, which apparently include the transduction of signals across the plasma membrane to the interior of the cell.

The Ras GTPase-activating protein (GAP) is an SH3 domain-binding protein and substrate for the Src-related tyrosine kinase, Hck

The Ras GTPase-activating protein (GAP) is a target for protein tyrosine kinases of both the receptor and cytoplasmic classes and may serve to integrate tyrosine kinase and Ras signaling pathways. In this report, we provide evidence that GAP is an SH3 domain-binding protein and substrate for the Src-related tyrosine kinase Hck, which has been implicated in the regulation of myeloid cell growth, differentiation, and function. Wild-type (WT) or kinase-inactive (K269E) mutant Hck proteins were co-expressed with bovine GAP using the baculovirus/Sf-9 cell system. GAP was readily phosphorylated on tyrosine by WT but not K269E Hck. GAP was present in WT Hck immunoprecipitates from the co-infected cells, indicative of Hck.GAP complex formation. Unexpectedly, GAP also associated with the kinase-inactive mutant of Hck, suggesting that tyrosine autophosphorylation of Hck is not required for complex formation. The WT and K269E forms of Hck also associated with GAP mutants lacking either the C-terminal catalytic domain (delta CAT) or the Src homology region (delta SH), indicating that these GAP domains are dispensable for complex formation. Recombinant GST fusion proteins containing the Hck, Src, Fyn, or Lck SH3 domains associated with full-length GAP, delta CAT, and delta SH, all of which share an N-terminal proline-rich region resembling an SH3-binding motif (PPLPPPPPQLP). Deletion of the highly conserved YXY sequence from the Hck SH3 domain abolished binding. GAP-SH3 interaction was also inhibited by the proline-rich peptide GFPPLPPPPPQLPTLG, which corresponds to N-terminal amino acids 129-144 of bovine GAP. An N-terminal deletion mutant of GAP lacking this proline-rich region did not bind to the Hck SH3 domain. These data implicate the Hck SH3 domain in GAP interaction, and suggest a general function for the SH3 domains of Src family kinases in recognition of GAP via its proline-rich N-terminal domain.

The Fc gamma RI receptor signals through the activation of hck and MAP kinase

U937 cells differentiated with IFN-gamma (termed U937IF cells) were used to study Fc gamma RI signaling. IFN induces a functional Fc gamma RI receptor signaling pathway in U937 cells, leading to the activation of the respiratory burst. IFN induces the expression of the nonreceptor protein tyrosine kinase, hck, and cross-linking the Fc gamma RI receptor in U937IF cells results in the activation of hck kinase as evidenced by the three- to fivefold increased tyrosine phosphorylation of hck. In vitro kinase assays demonstrate that the specific kinase activity of hck is increased 10-fold after Fc gamma RI stimulation. hck is observed to associate with two prominent tyrosine-phosphorylated proteins, p72 and p95, after Fc gamma RI-activation. Fc gamma RI cross-linking also results in mobility shift in MAP kinase in U937IF cells, suggesting that the Fc gamma RI receptor signals through the activation of MAP kinase. The data suggest that hck, p72, p95, and MAP kinase are involved in signal transduction through the Fc gamma RI receptor.

Urokinase plasminogen activator receptor, beta 2-integrins, and Src-kinases within a single receptor complex of human monocytes

The glycosylphosphatidylinositol (GPI)-anchored membrane protein urokinase plasminogen activator-receptor (uPA-R; CD87) is one of the key molecules involved in migration of leukocytes and tumor cells. uPA bound to uPA-R provides the cell proteolytic potential used for degradation of extracellular matrix. uPA-R is also involved in induction of cell adhesion and chemotaxis. Here, we provide a molecular explanation for these uPA-R-related cellular events. By size fractionation of monocyte lysate and affinity isolation on its natural ligand uPA, we demonstrate uPA-R as a component of a receptor complex of relatively large size. Reprecipitation and immunoblotting techniques allowed us to detect the protein tyrosine kinases (PTKs) p60fyn, p53/56lyn, p58/64hck, and p59fgr as components of this "uPA-R complex". Activation of monocytes even with enzymatically inactivated uPA resulted in induction of tyrosine phosphorylation, suggesting modulation of uPA-R-associated PTKs upon ligand binding. In spite of their presence in large complexes, we did not find the GPI-linked proteins CD14, CD58, and CD59 in the uPA-R complex, which indicates the presence of different receptor domains containing GPI-linked proteins in monocytes. However, we identified the leukocyte integrins LFA-1 and CR3 as components of the uPA-R complex as indicated by coisolation of these molecules, as well as by cocapping and comodulation of uPA-R and leukocyte integrins on the monocyte surface. The assemblage of uPA-R, PTKs and membrane spanning beta 2-integrins in one receptor complex indicates functional cooperation. In regard to the involvement of these molecules in pericellular proteolysis, signal transduction, as well as adhesion and chemotactic movement, we suggest uPA-R complex as a potential cellular device for cell migration.

Proline-rich (PxxP) motifs in HIV-1 Nef bind to SH3 domains of a subset of Src kinases and are required for the enhanced growth of Nef+ viruses but not for down-regulation of CD4

Human immunodeficiency virus (HIV) and simian immunodeficiency virus Nef proteins contain a conserved motif with the minimal consensus (PxxP) site for Src homology region 3 (SH3)-mediated protein-protein interactions. Nef PxxP motifs show specific binding to biotinylated SH3 domains of Hck and Lyn, but not to those of other tested Src family kinases or less related proteins. A unique cooperative role of a distant proline is also observed. Endogenous Hck of monocytic U937 cells can be specifically precipitated by matrix-bound HIV-1 Nef, but not by mutant protein lacking PxxP. Intact Nef PxxP motifs are dispensable for Nef-induced CD4 down-regulation, but are required for the higher in vitro replicative potential of Nef+ viruses. Thus, CD4 down-regulation and promotion of viral growth are two distinct functions of Nef, and the latter is mediated via SH3 binding.

The proto-oncogene c-fgr is expressed in normal mantle zone B lymphocytes and is developmentally regulated during myelomonocytic differentiation in vivo

The proto-oncogene c-fgr is a member of the c-src gene family of cytoplasmic tyrosine kinases. Previous studies have suggested that it is normally expressed in neutrophils, monocytes, macrophages, and natural killer cells. c-fgr is also expressed in the B cells of certain lymphoproliferative disorders, namely, Epstein-Barr virus-associated lymphoproliferative disease, and in chronic lymphocytic leukemia, but it has not previously been detected in normal or reactive human lymphoid tissue. In this study we have determined the pattern of p55c-fgr protein expression in normal human hematopoietic and lymphoid tissues at the single-cell level using immunohistochemical and immunofluorescent techniques. We show that p55c-fgr expression is developmentally regulated with high-level expression first evident at the myelocyte stage of myeloid differentiation. In addition, we show that p55c-fgr is expressed in circulating B lymphocytes isolated from chronic lymphocytic leukemia patients but is not expressed in normal circulating B lymphocytes. Surprisingly, p55c-fgr is also expressed in a subpopulation of normal B lymphocytes, the mantle zone B lymphocytes. This demonstration that p55c-fgr is expressed in a normal B-lymphocyte subpopulation suggests that its expression in certain B-cell lymphoproliferative disorders may be an indirect consequence of, rather than a primary cause of, the neoplastic transformation process.

Physical and functional association of the high affinity immunoglobulin G receptor (Fc gamma RI) with the kinases Hck and Lyn

The high affinity immunoglobulin G (IgG) receptor Fc gamma RI (CD64) is expressed constitutively on monocytes and macrophages, and is inducible on neutrophils. Fc gamma RI has recently been shown to be associated with the signal transducing gamma subunit of the high-affinity IgE receptor (Fc epsilon RI gamma). Induction of cytoplasmic protein tyrosine phosphorylation by Fc gamma RI cross-linking is known to be important in mediating Fc gamma RI-coupled effector functions. Recently, syk has been implicated in this role. We now report that the src-type kinases hck and lyn are physically and functionally associated with Fc gamma RI. Hck and lyn coimmunoprecipitated with Fc gamma RI from detergent lysates of normal human monocytes and of the monocytic line THP-1. Hck and lyn showed rapidly increased phosphorylation and increased exogenous substrate kinase activity after cross-linking of Fc gamma RI. These results demonstrate both physical and functional association of the Fc gamma RI/Fc epsilon RI gamma receptor complex with hck and lyn, and suggest a potential signal transducing role for these kinases in monocyte/macrophage activation.

Binding of Bruton's tyrosine kinase to Fyn, Lyn, or Hck through a Src homology 3 domain-mediated interaction

Bruton's tyrosine kinase (Btk) is a recently described B-cell-specific tyrosine kinase. Mutations in this gene lead to human X chromosome-linked agammaglobulinemia and murine X-linked immunodeficiency. Although genetic evidence strongly suggests that Btk plays a crucial role in B-lymphocyte differentiation and activation, its precise mechanism of action remains unknown, primarily because the proteins that it interacts with have not yet been identified. Here, we show that Btk interacts with Src homology 3 domains of Fyn, Lyn, and Hck, protein-tyrosine kinases that get activated upon stimulation of B- and T-cell receptors. These interactions are mediated by two 10-aa motifs in Btk. An analogous site with the same specificity is also present in Itk, the T-cell-specific homologue of Btk. Our data extend the range of interactions mediated by Src homology 3 domains and provide an indication of a link between Btk and established signaling pathways in B lymphocytes.

Hck expression correlates with granulocyte-macrophage colony-stimulating factor-induced proliferation in HL-60 cells

The human myeloid cell line HL-60 expresses approximately 300 high-affinity granulocyte-macrophage colony-stimulating factor receptors (GM-CSFRs), yet treatment of these cells with GM-CSF does not result in enhanced cellular proliferation or increases in protein tyrosine phosphorylation. In contrast, GM-CSF induces rapid increases in protein tyrosine phosphorylation and proliferative responses in HL-60 cells pretreated for 3 days in dimethyl sulfoxide (DMSO). Similarly, HL-60 cells pretreated with retinoic acid or 1,25 dihydroxyvitamin D3 were also capable of responding to GM-CSF. Interestingly, each of these treatments resulted in increased expression of the src-like tyrosine kinase hck. Stimulation with GM-CSF increased hck autophosphorylation in DMSO-treated HL-60 cells, suggesting that hck is a component of the GM-CSF signal transduction pathway. To determine if hck has a role in the DMSO-induced recoupling of the GM-CSFR, we overexpressed hck in HL-60 cells. The resulting cell line (HL-60/hck) expresses hck mRNA and protein at levels comparable with DMSO-treated HL-60 cells. Stimulation of HL-60/hck cells with GM-CSF results in activation of hck, increases in protein tyrosine phosphorylation, and increased proliferation. These results show that cytokine receptors can exist in an uncoupled form and suggest that in HL-60 cells, appropriate levels of the src-like tyrosine kinase hck are critical for functional coupling of the GM-CSFR to biologic responses.

Functional and biochemical association of Hck with the LIF/IL-6 receptor signal transducing subunit gp130 in embryonic stem cells

The role played by the Src-related tyrosine kinase, Hck, in embryonic stem (ES) cell differentiation was investigated by replacing a conserved C-terminally located tyrosine with phenylalanine by gene targeting. Targeted ES cells display a 7- to 9-fold elevation in constitutive Hck kinase activity and require approximately 15 times less leukaemia inhibitory factor (LIF) than parental ES cells to maintain their stem cell character in vitro. We also demonstrate a rapid and transient increase in Hck tyrosine kinase activity in parental ES cells stimulated by LIF and, finally, show that Hck is physically associated with gp130, an affinity converter and signal transducing component of the LIF receptor. Thus, these results provide biological and biochemical evidence that Hck participates in signal transduction from the LIF receptor.

Physical and functional association of Src-related protein tyrosine kinases with Fc gamma RII in monocytic THP-1 cells

Aggregation of Fc gamma RII (CD 32), a low affinity receptor for immunoglobulin G (IgG), on the monocytic cell line THP-1 induces protein tyrosine kinase (PTK) activity. Several distinct cellular proteins, including Fc gamma RII itself, are phosphorylated on tyrosine following cross-linking of the receptor. Fc gamma RII lacks intrinsic PTK activity. In this report we demonstrate that a kinase activity was coprecipitated with Fc gamma RII in THP-1 cells. The kinetics of the receptor-associated kinase activity paralleled the appearance of tyrosine phosphorylation events observed following Fc gamma RII activation of THP-1 cells. Several proteins were associated with the receptor. Reimmunoprecipitation analysis demonstrated that lyn gene products were among the proteins coprecipitated with Fc gamma RII. p59hck (Hck) and p56lyn (Lyn) were the most abundant Src-related PTKs (Src-PTKs) in THP-1 cells. Enzymatic activity of both kinases, as measured by an in vitro kinase assay, was increased following specific cross-linking of Fc gamma RII. Furthermore, Fc gamma RII was specifically associated with both enzymes following its engagement and served as a substrate for both of these kinases. The association of Fc gamma RII with Src-PTK was specific for Fc gamma RII activation of THP-1 cells, since activation of cells via the high affinity Fc gamma receptor, Fc gamma RI (CD 64), did not result in association of Fc gamma RII with Hck or Lyn. Our data demonstrate a functional and physical association of Fc gamma RII with Hck and Lyn consistent with the involvement of Src-PTK in Fc gamma RII-mediated signal transduction.

Engagement of MHC class II molecules by staphylococcal superantigens activates src-type protein tyrosine kinases

Staphylococcal exotoxins (SE) are superantigens that bind to monomorphic determinants on major histocompatibility complex (MHC) class II molecules and stimulate human peripheral blood T lymphocytes in a V beta-specific manner. SE also deliver activation signals via MHC class II molecules that initiate cell adhesion and cytokine gene transcription. These events are preceded by tyrosine phosphorylation and are antagonized by inhibitors of tyrosine kinases, indicating an essential role for these kinases in signaling via class II molecules. We report that stimulation of human peripheral blood monocytes with SE induced rapid and selective activation of the src-related protein tyrosine kinases (PTK) fgr and hck. SE also induced the activation of fgr and lyn in B cells. PTK activation by SE required MHC class II expression, and was greatly potentiated in the presence of T cells bearing toxin-specific V beta chains. These results indicate that in addition to their antigen and superantigen-presenting function, MHC class II molecules act as signal-transducing receptors that are coupled to src-type PTK.

Hck tyrosine kinase activity modulates tumor necrosis factor production by murine macrophages

The hematopoietic cell kinase (hck) is a member of the src family of tyrosine kinases, and is primarily expressed in myeloid cells. Hck expression increases with terminal differentiation in both monocyte/macrophages and granulocytes and is further augmented during macrophage activation. Recent evidence has implicated src-related tyrosine kinases in critical signaling pathways in other hematopoietic lineages. Herein we demonstrate that manipulation of the level of hck expression in the murine macrophage cell line BAC1.2F5 alters the responsiveness of these cells to activation by bacterial lipopolysaccharide (LPS) but does not affect survival or proliferation. Overexpression of an activated mutant of hck in BAC1.2F5 cells augments tumor necrosis factor (TNF) production in response to LPS, whereas inhibition of endogenous hck expression, by antisense oligonucleotides, interferes with LPS-mediated TNF synthesis. Together, these observations suggest that hck is an important component of the signal transduction pathways in activated macrophages.

In vitro tyrosine phosphorylation of PLC-gamma 1 and PLC-gamma 2 by src-family protein tyrosine kinases

The phosphorylation of purified phospholipase C-gamma 1 (PLC-gamma 1) and PLC-gamma 2 by src-family-protein tyrosine kinases (PTKs) P56lck, p53/56lyn, p59hck, p59fyn, and p60src was studied in vitro. All five PTKs phosphorylated PLC-gamma 1 and PLC-gamma 2, suggesting that both PLC-gamma isozymes can be phosphorylated in cells by any of the src-family PTKs in response to the activation of cell surface receptors. Comparison of the in vitro phosphorylation rates revealed no distinct specificity between PLC-gamma 1 and PLC-gamma 2, or between the five PTKs.

Lipopolysaccharide- and interferon-gamma-induced expression of hck and lyn tyrosine kinases in murine bone marrow-derived macrophages

We have examined the role of tyrosine phosphorylation during the course of macrophage activation. Initial experiments indicated that vanadate, a known phosphotyrosine phosphatase inhibitor, enhanced the phorbol 12-myristate 13-acetate (PMA)-triggered respiratory burst and potentiated the priming effects of bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma), suggesting that tyrosine phosphorylation may be important in these end cell functions. As src-related kinases have been implicated in the activation of cells of other haemopoietic lineages, we examined the relationship between the activity of two such kinases, hck and lyn, and priming of the respiratory burst. We found that the level of hck and lyn is increased following exposure of bone marrow-derived macrophages (BMM) to LPS or IFN-gamma. The induction of both of these kinases follows similar kinetics with maximal activity occurring at 24-48 h. Interestingly, the kinetics of induction of hck and lyn kinase activity in BMM demonstrated a close temporal relationship with the priming effects of LPS and IFN-gamma on the macrophage respiratory burst. Collectively, these observations raise the possibility that modulation of expression of hck and lyn is involved in the regulation of the respiratory burst.

Dominant male sterility in mice caused by insertion of a transgene

While examining a series of transgenic mouse lines carrying the HCK protooncogene, we encountered one line in which males hemizygous for the transgene were sterile. The sterile males mated normally but failed to impregnate females. Light and electron microscopy revealed that spermatogenesis proceeds normally until nuclear condensation, which occurs but gives rise to a variety of abnormally shaped nuclei. Expression of the transgene was not detectable. Thus, the insertion itself probably caused the abnormal phenotype by disrupting a gene (or genes) important in spermatogenesis. The mutation is genetically dominant, causing an abnormal phenotype even though the sterile mice carry an ostensibly normal counterpart of the disrupted locus. The mutant phenotype is completely penetrant only in some genetic backgrounds, suggesting a modifying influence from a second locus. Junctions between the inserted transgene and adjoining cellular DNA were cloned, allowing us to confirm the heterozygous nature of the genetic disruption and to detect and associated deletion. We have designated the mutation Lvs (lacking vigorous sperm) and presume that it may define a previously undescribed locus important in spermatogenesis.

Two isoforms of murine hck, generated by utilization of alternative translational initiation codons, exhibit different patterns of subcellular localization

Mammalian hck, a member of the src family of tyrosine kinases, is expressed predominantly in cells of the myeloid and B-lymphoid lineages. Using mutational analysis, we have investigated the molecular basis of two immunoreactive forms of murine hck of 56 and 59 kDa found in numerous hemopoietic cell types. Our results indicate that translation of murine p59hck initiates from a CTG codon located 21 codons 5' of an ATG that is utilized to generate p56hck. We provide evidence that two human hck isoforms are generated by the same mechanism. Subcellular fractionation studies reveal that while p59hck and p56hck are associated with membranes of various murine B-lymphoid and myeloid cell lines, p59hck alone is also located in the cytosol. In contrast to membrane-associated p59hck, which is metabolically labeled with [3H]myristic acid and exhibits amphiphilic properties in Triton X-114 detergent, cytosolic p59hck is hydrophilic, suggesting that it is not acylated. Possible mechanisms are proposed to account for these observations.

Expression of lineage-restricted protein tyrosine kinase genes in human natural killer cells

The hematopoietic lineage derivation, recognition structures and associated signal transduction pathways of CD3- natural killer (NK) cells have not been identified. Protein tyrosine kinases (PTK) structurally related to the product of the c-src protooncogene are differentially expressed in distinct hematopoietic differentiation lineages and may participate in specific signal transduction pathways. The present study was aimed at characterizing the expression of src-related PTK genes in normal human NK cells and in cells from patients with CD3- granular lymphocyte proliferative disease. CD3- normal NK cells had high levels of transcripts of the lck gene, which is highly expressed in T cells. CD8+ and CD8- NK cells expressed similarly high levels of lck mRNA. In contrast, NK cells expressed very low levels (25-80 times less than monocytes) of mRNA encoding the myelomonocytic PTK hck. NK cells also expressed fyn transcripts (p59fyn reportedly associates with the T cell receptor in T cells) and fgr transcripts, the latter observation confirming a previous report. The pattern of expression of the lineage-restricted PTKs lck and hck in NK cells is consistent with the hypothesis of an ontogenic relationship of this population with the lymphocytic rather than myelocytic differentiation pathway. PTK expressed in NK cells may participate in signal transduction pathways in this cell population.

Expression of the c-fgr and hck protein-tyrosine kinases in acute myeloid leukemic blasts is associated with early commitment and differentiation events in the monocytic and granulocytic lineages

Two members of the src proto-oncogene family of intracellular tyrosine kinases, c-fgr and hck, are selectively expressed in differentiated myeloid cells. To study the expression of these genes in acute myeloid leukemia (AML) and to determine the specific myeloid lineages and stages of myeloid differentiation at which the expression of these genes is acquired, we used a series of 79 cases of de novo AML as a differentiation model. The levels of c-fgr, hck, and c-fms (encoding the colony-stimulating factor-1 receptor) mRNA transcripts were correlated with the presence of specific cell surface antigens and the morphologic and cytochemical features in these AML blasts. Relatively undifferentiated leukemic myeloblasts with an HLA-DR, CD34, CD33, CD13+/- cell surface immunophenotype (French-American-British [FAB] M1 or M2) were characterized by a lack of c-fms and c-fgr expression, while low levels of c-fms and c-fgr could be detected in undifferentiated myeloblasts (FAB M1 or M2), which also expressed CD14 at low antigen density. The hck transcripts were either undetectable in these cells or were expressed at low levels. In contrast, only hck mRNA transcripts could be identified in blasts with progranulocytic morphology (FAB M3), while c-fms, c-fgr, and hck were all expressed at high levels in blasts with differentiated myelomonocytic or monocytic features (FAB M4 and M5). No c-fms, c-fgr, or hck transcripts were evident in leukemic cells of the erythroid lineage (FAB M6). When undifferentiated leukemic myeloblasts (HLA-DR, CD34, and CD33) were induced to differentiate in vitro to cells with monocytic characteristics, the expression of c-fms, c-fgr, and the CD14 cell surface antigen were induced to high levels, accompanied by the acquisition of hck and CD13 expression. In contrast, when HLA-DR, CD34, and CD33 blasts were induced to differentiate in vitro to cells with granulocytic characteristics, only hck and CD13 expression were induced. Our data suggest that the acquisition of c-fgr and/or hck expression is associated with early commitment and differentiation events in distinct myeloid lineages. Assessment of the expression of these kinases may provide a molecular tool to assign lineage in AML in conjunction with morphology, cytochemistry, and cell surface antigen expression.