Novel protein-tyrosine kinase gene (hck) preferentially expressed in cells of hematopoietic origin

Characteristic expression of Hck in human B-cell precursors

OBJECTIVE

To identify molecules involved in signaling for early B-cell development, we investigated the expression of signal transduction-related proteins in B-cell progenitors.

MATERIALS AND METHODS

[corrected] Normal as well as leukemic B-cell progenitors were examined by immunoblotting and immunofluorescence study.

RESULTS

[corrected] In a survey of the expression of a broad range of signal transduction molecules, the Src-family protein tyrosine kinases were found to be differentially expressed in early B-cell differentiation. [corrected] Analysis of freshly prepared precursor-B acute lymphoblastic leukemia cells and B-lineage cell lines showed Hck and Lyn are major Src-family protein tyrosine kinases expressed in this type of leukemic blasts. [corrected] However, heterogeneity of Hck and Lyn expression was found in these cells, and precursor-B acute lymphoblastic leukemia cells subsequently were classified according to the expression pattern of Hck and Lyn as Hck/Lyn dual-negative, Hck-predominant, Hck/Lyn dual-positive, and Lyn-predominant. Further studies on normal B-lineage cells indicated that the Src-family protein tyrosine kinases are expressed sequentially in a differentiation-dependent fashion during B-cell ontogeny and that the predominant expression of Hck is a common feature in B-cell progenitors, whereas Lyn expression is more significant in mature B cells.

CONCLUSIONS

Although the biologic significance remains unknown, sequential expression of Src-family protein tyrosine kinases should play a role in regulation of early B-cell differentiation.

The HIV Nef protein alters Ca(2+) signaling in myelomonocytic cells through SH3-mediated protein-protein interactions

Human immunodeficiency virus Nef plays an important role in AIDS pathogenesis. In addition to the well known down-regulation of cell surface receptors (CD4, MHCI), Nef is able to alter cellular signaling. Of particular interest for this study is the ability of Nef to bind with a very high affinity to SH3 domains of myelomonocyte-specific protein-tyrosine kinases of the Src family (Src-like PTK). We have therefore investigated Ca(2+) signaling in HL60 cells retrovirally transduced with wild type Nef or with a Nef mutant deficient in the SH3-interacting proline-rich motif (Nef((PXXP)4(-))). In differentiated HL60 cells, Nef markedly altered cellular Ca(2+) signaling; the amount of intracellularly stored Ca(2+) was increased, and as a consequence, store-operated Ca(2+)-influx was decreased. This effect was not observed in undifferentiated HL60 cells or in CEM T-lymphocytes and correlated with the differentiation-induced up-regulation of Src-like PTK. The Nef effect on Ca(2+) signaling depended entirely on the integrity of its PXXP motif. The Src-like PTK p56/59(hck) co-immunoprecipitated with both Nef and with the inositol 1,4,5-trisphosphate receptor, providing a possible mechanistic link between the viral protein and intracellular Ca(2+) stores of the host cell. Collectively, our results demonstrate that the human immunodeficiency virus 1 Nef protein manipulates intracellular Ca(2+) stores through SH3-mediated interactions in myelomonocytic cells.

Multiple roles for Src in a PDGF-stimulated cell

The observation that platelet-derived growth factor (PDGF) increases the catalytic activity of Src family members (Src) suggests that they contribute to PDGF-dependent responses. The role of Src in PDGF-dependent cell cycle progression, phosphorylation of proteins, and chemotaxis has been tested by investigators using a variety of cell types and approaches, and it appears that the contribution of Src is highly variable. This idea is perhaps best illustrated by the finding that Src plays radically different roles downstream of the PDGF alpha- and beta-receptor subunits. Hence, Src is a versatile signal relay enzyme, whose contribution to a signaling cascade depends on variables such as the nature of the receptor via which the cell is activated, as well as the cell type itself.

SH2-kinase linker mutations release Hck tyrosine kinase and transforming activities in Rat-2 fibroblasts

Biochemical and structural studies of Src and related kinases demonstrate that two intramolecular interactions suppress kinase activity. These interactions involve binding of the SH2 domain to a phosphotyrosine residue in the C-terminal tail and association of the SH3 domain with a polyproline type II helix formed by amino acids linking the SH2 and kinase domains. Recent studies have shown that high affinity interaction of the SH3 domain of Hck with the human immunodeficiency virus type I Nef protein activates Hck tyrosine kinase and biological activities, suggesting a mechanism that involves disruption of the SH3-linker interaction. To test the role of this interaction in the regulation of Hck kinase activity in living cells, we substituted alanines for prolines 225 and 228 in the linker region and observed that the resulting mutant (Hck-2PA) demonstrated strong transforming activity in a Rat-2 fibroblast focus-forming assay. Hck-2PA also exhibited elevated tyrosine kinase activity in terms of autophosphorylation, endogenous substrate phosphorylation, and in an in vitro kinase assay. The transforming and kinase activities of Hck-2PA were remarkably similar to those observed with a Hck mutant activated by Phe substitution of the conserved tail Tyr residue and with wild-type Hck following co-expression with human immunodeficiency virus Nef. Introduction of the 2PA and tail mutations into a single Hck expression construct did not increase kinase or transforming activity relative to the individual mutations. These data provide new evidence that SH3-linker interaction may represent the dominant mechanism controlling Hck tyrosine kinase activity in vivo.

Identification of a novel splice variant of C3G which shows tissue-specific expression

C3G is a guanine nucleotide-releasing protein that binds to the Src homology 3 (SH3) domain of the adapter protein Crk. In this study, we isolated cDNAs coding for rat C3G. Northern blot analysis of RNA from various rat tissues and cell lines showed a major transcript of about 7 kb which was present at the highest level in testis. A comparison of the amino acid sequence (derived from the cDNA sequence) of rat C3G with the human form showed 87.3% sequence identity. The principal difference was the presence of an additional 51 amino acids in the rat C3G sequence after the fifth PXXP motif. This difference may be attributable to alternative splicing of the primary transcript. This interpretation was supported by reverse transcription-polymerase chain reaction (RT-PCR) assays, which resulted in two products differing by 153 bp. The RT-PCR analysis of RNA from various rat tissues showed that the relative expression levels of the two splice forms were variable. The form of C3G with the insertion of 51 amino acids (named C3G-2) was present in rat testis at a high level and, to a lesser extent, in brain, but it was not seen (or was present at a very low level) in other rat tissues and certain rat and mouse cell lines. This expression pattern of the C3G-2 form was confirmed by Northern blotting using the insert region as a probe. The C3G-1 form, without the insertion of 51 amino acids, was present in almost all rat tissues except testis and in cell lines of rat, mouse, or human origin. Thus, in rat cells, we have identified a novel splice variant of C3G. The expression pattern indicates that the form of C3G described here is likely to serve a tissue- or cell-specific physiological function.

Identification of receptor genes in renal cell carcinoma associated with angiogenesis by differential hybridization technique

To screen the receptor genes in renal cell carcinoma (RCC) associated with angiogenesis, we performed differential hybridization of the cDNA library of membrane-type protein tyrosine kinases (mPTKs). Three thousand plaques of a mPTKs-enriched cDNA library were screened with mPTKs mixture probes produced from hypervascular RCC tissues and RCC cell lines. Six different cDNA fragments of the PTK genes were isolated, and the sequence analysis showed that these represented cDNAs for TIE1, KDR, FMS, FGFR-4, JAK1 and HCK. Of these genes, the expression of TIE1, KDR, and FGFR-4 was studied in RCC tissue and cell lines by Northern blot analysis. We also investigated the expression of vascular endothelial growth factor (VEGF), placenta growth factor (PlGF) and their receptor FLT-1. In all the hypervascular RCC tissues, the amounts of mRNAs for KDR and FLT-1 were increased compared to adjacent normal tissues. The TIE1 and FGFR-4 genes were also overexpressed in most of the hypervascular RCC tissues, while no mRNA of KDR, FLT-1, or TIE1 could be detected in any of the four human RCC cell lines. The amounts of the VEGF and PlGF mRNAs were increased in hypervascular RCC tissues, while VEGF mRNA was detected in the four cell lines but PlGF mRNA was not. FGFR-4 mRNA was expressed in three of the four cell lines. These results suggest that KDR, FLT-1, PlGF and TIE1 mRNAs are present in the mesenchymal cells of RCC, while VEGF and FGFR-4 genes are expressed in RCC cells themselves in vivo.

The proto-oncogene p120(Cbl) is a downstream substrate of the Hck protein-tyrosine kinase

Hematopoietic cell kinase (Hck) is a member of the Src-family of protein tyrosine kinases. We have found that upon enzymatic activation of Hck by the heavy metal mercuric chloride, there was a rapid increase in the levels of tyrosine phosphorylation of several proteins including the proto-oncogene p120(Cbl). Fibroblasts that are transformed with an activated allele of Hck exhibit constitutive Cbl phosphorylation. Upon Fcgamma receptor activation, a more physiologically relevant extracellular signal, Cbl is tyrosine phosphorylated and the Src-family selective inhibitor, PP1, can prevent this phosphorylation on Cbl. Hck phosphorylates Cbl in vitro and the interaction between Cbl and Hck is direct, requiring Hck's unique, SH3 and SH2 domains for optimal binding. Using a novel estrogen-regulated chimera of Hck we have shown a hormone-dependent association between Hck and Cbl in murine fibroblasts. This work suggests that Cbl serves as a key mediator of Hck induced signalling in hematopoietic cells.

Intramolecular regulatory interactions in the Src family kinase Hck probed by mutagenesis of a conserved tryptophan residue

Intramolecular interactions between the Src homology domains (SH2 and SH3) and the catalytic domains of Src family kinases result in repression of catalytic activity. The crystal structure of the Src family kinase Hck, with its regulatory domains intact, has been solved. It predicts that a conserved residue, Trp260, at the end of the linker between the SH2 and the catalytic domains plays an important role in regulation by the SH3 and SH2 domains. We have mutated this residue and compared the activities of C-terminally phosphorylated wild type Hck and W260A Hck. The W260A mutant has a higher specific activity than wild type Hck. The W260A mutant requires autophosphorylation at Tyr416 for full activity, but it is not activated by ligand binding to the SH3 or SH2 domains. This mutation also changes the accessibility of the SH2 and SH3 domains to their cognate peptide ligands. Our results indicate that Trp260 plays a critical role in the coupling of the regulatory domains to the catalytic domain, as well as in positioning the ligand binding surfaces.

The transcription factor Sp1 regulates the myeloid-specific expression of the human hematopoietic cell kinase (HCK) gene through binding to two adjacent GC boxes within the HCK promoter-proximal region

The human hemopoietic cell kinase (HCK) is a member of the src family of protein tyrosine kinases specifically expressed in myeloid cells and to a minor extent in B-lymphoid cells. HCK expression is up-regulated at the transcriptional level during myeloid differentiation of hematopoietic cells. To elucidate the molecular basis of the differential HCK gene expression, the genomic region containing the HCK promoter was isolated and functionally characterized. A DNA fragment containing 101 base pairs of the 5'-flanking sequence showed strong promoter activity in the macrophage cell line RAW264 but was inactive in the non-monocytic cell lines HUT-78 and NIH-3T3. Site-directed mutagenesis of the proximal promoter region showed that two GC-rich sequence elements are essential for transcriptional activity in myeloid cells. Electrophoretic mobility shift analysis using nuclear extracts obtained from RAW264 cells and from the promonocytic cell line U-937 revealed the formation of at least three distinct protein-DNA complexes at each of these sites, one of which was found to contain the transcription factor Sp1. Expression of a reporter gene linked to the -101 HCK promoter region was up-regulated by Sp1, but not by other members of the Sp1 family of transcription factors, in Drosophila Schneider cells. A synergistic effect on HCK promoter activity was observed at high concentrations of Sp1. Our results show that Sp1 plays an essential role in the regulation of the differential gene expression of the HCK gene.

The Src-like tyrosine kinase Hck is activated by granulocyte colony-stimulating factor (G-CSF) and docks to the activated G-CSF receptor

Activation of the granulocyte colony-stimulating factor receptor (G-CSF-R) leads to tyrosine-phosphorylation of multiple cytoplasmic components. To date, the kinases Jak1, Jak2, Tyk2, Lyn, and Syk have been implicated in this process. However, it is unknown if other kinases might be involved in the diverse responses from the G-CSF-R, which include mitogenesis, survival, differentiation, and functional activation of responsive cells. The hematopoietic cell kinase (Hck) is a member of the Src-family of kinases known to be expressed in cells of the granulocytic lineage. It also interacts with the gp130 subunit of the LIF/IL-6 receptors, which is closely related to the G-CSF-R, and so represents a good candidate for mediating at least some of the downstream signaling from the G-CSF-R. Therefore, we investigated the activation of Hck by the G-CSF-R in intact cells as well as in vitro. These studies revealed recruitment of Hck to activated G-CSF-R, mediated by direct binding via its SH2 domain to multiple phosphotyrosines of the receptor. In addition, we show that Hck becomes activated upon G-CSF treatment and is, in turn, able to phosphorylate the G-CSF-R, indicating a clear functional and physical involvement in G-CSF signaling.

Expression Profile of Active Genes in Granulocytes

A number of genes active in granulocytes have been intensively studied as to the function of their products and their expression controls. However, the intensities and relative order of these gene activities have not been studied. This report describes an expression profile of 748 different species of active genes in human peripheral granulocytes obtained by analyzing a 3′-directed cDNA library that faithfully represents the mRNA population in the source cells. A significant fraction (20.3% of the total) of the expressed genes in granulocytes consisted of nuclear proteins such as DNA binding proteins, of secretory proteins such as cytokines, and of membrane proteins such as major histocompatibility complex (MHC) proteins and receptors. By comparing this expression profile with 11 profiles similarly obtained with unrelated human cells/tissues, we discovered 10 novel genes that are likely to act specifically in granulocytes. Comparison of this expression profile with that obtained with granulocytoids widely used as a granulocyte model by inducing a cultured promyelocytic leukemia cell line HL60 showed similarities and dissimilarities of gene expressions.

© 1998 by The American Society of Hematology.

Expression Profile of Active Genes in Granulocytes

A number of genes active in granulocytes have been intensively studied as to the function of their products and their expression controls. However, the intensities and relative order of these gene activities have not been studied. This report describes an expression profile of 748 different species of active genes in human peripheral granulocytes obtained by analyzing a 3′-directed cDNA library that faithfully represents the mRNA population in the source cells. A significant fraction (20.3% of the total) of the expressed genes in granulocytes consisted of nuclear proteins such as DNA binding proteins, of secretory proteins such as cytokines, and of membrane proteins such as major histocompatibility complex (MHC) proteins and receptors. By comparing this expression profile with 11 profiles similarly obtained with unrelated human cells/tissues, we discovered 10 novel genes that are likely to act specifically in granulocytes. Comparison of this expression profile with that obtained with granulocytoids widely used as a granulocyte model by inducing a cultured promyelocytic leukemia cell line HL60 showed similarities and dissimilarities of gene expressions.

© 1998 by The American Society of Hematology.

Parallel hybridization analysis of multiple protein kinase genes: identification of gene expression patterns characteristic of human hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the major causes of human cancer deaths worldwide. To identify alterations of the genetic program associated with human HCC, we designed a new protocol based on the high-density replica method to analyze protein kinase gene expression in normal liver, HCC, and HCC-derived cell lines. RNA was prepared for reverse transcription and cDNA was used for PCR amplification of the conserved catalytic domain of protein kinase genes. Initially, from a pair of HCC and the adjacent noncancerous tissues, we sequenced 228 samples and identified 26 genes that represent different tyrosine kinase subfamilies. High-density grid filters were then prepared to assist the identification, by hybridization, of genes that are differentially expressed in normal vs HCC samples. Eleven tyrosine kinase genes were tested, and positive signals were reliably scored by doubly offset duplicates and by two independent gene-specific probes. Of the 11 genes tested, PDGF receptor-beta, MEKK-3, axl, and FGFR-4 are preferentially expressed in tumor samples. Additionally, we analyzed protein kinase gene expression in five HCC cell lines and identified distinct kinase gene expression patterns in different cell lines. Our results suggest that multiple kinases are activated in different tumors and confirm that there is molecular heterogeneity in the mechanisms sustaining autonomous cell growth in liver tumor formation.

Nitrocellulose particles adsorbed to immunoglobulins are a new and effective approach to induce cell activation dependent on receptor aggregation

Nitrocellulose (NC) has proved to be a versatile tool for the isolation and characterization of various biomolecules. In this report we extend its scope by using antibody-coated NC particles to cross-link molecules on the surface of living cells. Ligation of receptors in Jurkat cells with NC-bound specific antibodies induced protein tyrosine phosphorylation patterns of cellular proteins comparable to conventional antibody cross-linking. In addition, the present study shows that application of NC particles coated with human IgA significantly activated monocytic cells via the Fc alpha receptor (Fc alphaR), whereas cross-linking of receptor-ligand complexes with isotype-specific antibody was less efficient. Subsequent immunoprecipitation and immunoblot analysis of aggregated Fc receptors (FcRs) complexed to Ig-adsorbed particles permits fast identification of molecules involved in the transmission of signals. Therefore, ligand-coated NC particles can be used to examine receptor-mediated cell activation events dependent upon extensive receptor aggregation.

Solution structure of the human Hck SH3 domain and identification of its ligand binding site

SH3 domains are protein binding domains that occur widely among signal transduction proteins. Here, we present the NMR-determined solution structure of the SH3 domain from the cytoplasmic protein tyrosine kinase, Hck. Hck is involved in a number of cell signal transduction pathways, frequently in pathways associated with immune response. SH3 domains bind proteins via a left-handed polyproline type II helix on the target protein. We have assessed the structural impact of binding to a ligand through addition of a peptide corresponding to a proline-rich region of a Hck target, the GTPase activating protein of the Ras pathway. Ligand binding effects small structural changes and stabilizes the SH3 domain structure. Also, we have compared the solution structure of the Hck SH3 domain to the crystal structure of Hck, in which the SH3 domain exhibits an intramolecular binding to an interdomain linker region. These structures are interpreted as the apo- and holo- forms of the Hck SH3 domain.

Physical and Functional Association of FcαR With Protein Tyrosine Kinase Lyn

In this report, we show that the Src family nonreceptor protein tyrosine kinase (PTK) Lyn associates with aggregated IgA Fc receptor (FcαR) in the monocytic cell line THP-1. Receptor aggregation and subsequent immunoprecipitation of receptor complexes with huIgA adsorbed to nitrocellulose particles shows that Lyn associates with FcαR by a mechanism sensitive to short treatment with the Src family-selective inhibitor PP1. However, interaction of Lyn with IgG Fc receptor (FcγR) in THP-1 cells was unaffected by short treatment with the PTK inhibitor. Cross-linking of FcαR induced tyrosine phosphorylation of several cellular proteins, including p72Syk, which appears to be a major target of early PTK activity. Unexpectedly, in vitro kinase assays showed that FcαR aggregation-induced tyrosine phosphorylation of Syk did not result in upregulation of Syk activity. Despite the lack of enhanced Syk kinase activity, downstream signaling after FcαR cross-linking was functional and induced the release of significant amounts of interleukin-1 receptor antagonist and interleukin-8. The induction of cytokine release was completely blocked by PP1, thus confirming the biological significance of the association of Lyn with aggregated FcαR. Our data show that early signal transduction after FcαR cross-linking as well as FcαR-mediated activation of cellular effector functions depends on Src family kinase activity. The Src-family PTK involved in FcαR-mediated tyrosine phosphorylation appears to be Lyn, which coprecipitated with aggregated FcαR complexes.

Physical and Functional Association of FcαR With Protein Tyrosine Kinase Lyn

In this report, we show that the Src family nonreceptor protein tyrosine kinase (PTK) Lyn associates with aggregated IgA Fc receptor (FcαR) in the monocytic cell line THP-1. Receptor aggregation and subsequent immunoprecipitation of receptor complexes with huIgA adsorbed to nitrocellulose particles shows that Lyn associates with FcαR by a mechanism sensitive to short treatment with the Src family-selective inhibitor PP1. However, interaction of Lyn with IgG Fc receptor (FcγR) in THP-1 cells was unaffected by short treatment with the PTK inhibitor. Cross-linking of FcαR induced tyrosine phosphorylation of several cellular proteins, including p72Syk, which appears to be a major target of early PTK activity. Unexpectedly, in vitro kinase assays showed that FcαR aggregation-induced tyrosine phosphorylation of Syk did not result in upregulation of Syk activity. Despite the lack of enhanced Syk kinase activity, downstream signaling after FcαR cross-linking was functional and induced the release of significant amounts of interleukin-1 receptor antagonist and interleukin-8. The induction of cytokine release was completely blocked by PP1, thus confirming the biological significance of the association of Lyn with aggregated FcαR. Our data show that early signal transduction after FcαR cross-linking as well as FcαR-mediated activation of cellular effector functions depends on Src family kinase activity. The Src-family PTK involved in FcαR-mediated tyrosine phosphorylation appears to be Lyn, which coprecipitated with aggregated FcαR complexes.

Three-dimensional structure of the Hck SH2 domain in solution

The hematopoietic cellular kinase (Hck) is a member of the Src family of non-receptor protein-tyrosine kinases that is expressed predominantly in granulocytes, monocytes and macrophages. Recent observations suggest that Hck may be activated in HIV-infected macrophages and in chronic myelogenous leukemia cells that express Bcr-Abl. In order to increase our understanding of the structural basis for regulation of Hck activity under normal and pathological conditions, we have solved the solution structure of the uncomplexed Hck SH2 domain using NMR spectroscopy. A novel procedure that uses intraresidue HN-H alpha distances as references for converting NOE intensities into distance restraints has been described. A total of 1757 significant experimental restraints were derived from NMR spectroscopic data including 238 medium-range and 487 long-range distance restraints and 177 torsion angle restraints. These restraints were used in a simulated annealing procedure to generate 20 structures with the program DYANA. Superimposition of residues 5-104 upon the mean coordinate set yielded an average atomic rmsd values of 0.42 +/- 0.08 A for the N,C alpha,C' atoms and 0.81 +/- 0.08 A for all heavy atoms. Rmsd values for those residues in the regions of ordered secondary structure were 0.27 +/- 0.04 A for the N,C alpha,C' atoms and 0.73 +/- 0.06 A for all heavy atoms.

SH3-mediated Hck tyrosine kinase activation and fibroblast transformation by the Nef protein of HIV-1

Tyrosine kinases of the Src family are regulated via their Src homology 2 (SH2) and SH3 domains. The Nef protein of human immunodeficiency virus-1 (HIV-1) has previously been shown to bind with high affinity and specificity in vitro to the SH3 domain of Hck, a Src family member expressed primarily in myeloid cells. However, the effect of Nef on Hck activity in living cells is unknown. Here we show that Rat-2 fibroblasts co-expressing Hck and Nef rapidly developed transformed foci, whereas control cells expressing either protein alone did not. Nef formed a stable complex with Hck and stimulated its tyrosine kinase activity in vivo. Mutagenesis of the Nef proline-rich motif essential for SH3 binding completely blocked complex formation, kinase activation, and transformation, indicating that the Nef SH3-binding function is required for its effects on Hck. These results provide direct evidence that SH3 engagement is sufficient to activate a Src family kinase in vivo and suggest that Hck may be activated by Nef in HIV-infected macrophages.

Sequential assignment and secondary structure determination for the Src homology 2 domain of hematopoietic cellular kinase

The hematopoietic cellular kinase (Hck) is a member of the Src family of non-receptor protein-tyrosine kinases and participates in signal transduction events regulating the growth, differentiation and function of phagocytes. The secondary structure of the SH2 domain for Hck was determined for a 13C/15N-enriched sample using multi-dimensional NMR spectroscopy. The secondary structure for the domain was determined from chemical shift indices [1H alpha, 13C alpha and 13C'], sequential NOEs [d(alphaN)(i, i+1) and d(NN)(i, i+1)], and 3J(alphaN) scalar coupling constants. The Hck SH2 domain consists of two alpha-helices and seven beta-strands. Complementary strands of beta-sheets were identified from long-range NOEs using a novel 3D, 13C/15N-edited HMQC-NOESY-(HCACO)NH experiment that correlated 1H alpha resonances between beta-strands. The secondary structure for Hck SH2 is similar to that predicted from the sequence alignment of the Src-family protein tyrosine kinases.

Activation of the Src-family tyrosine kinase Hck by SH3 domain displacement

The protein Hck is a member of the Src family of non-receptor tyrosine kinases which is preferentially expressed in haematopoietic cells of the myeloid and B-lymphoid lineages. Src kinases are inhibited by tyrosine-phosphorylation at a carboxy-terminal site. The SH2 domains of these enzymes play an essential role in this regulation by binding to the tyrosine-phosphorylated tail. The crystal structure of the downregulated form of Hck has been determined and reveals that the SH2 domain regulates enzymatic activity indirectly; intramolecular interactions between the SH3 and catalytic domains appear to stabilize an inactive form of the kinase. Here we compare the roles of the SH2 and SH3 domains in modulating the activity of Hck in an investigation of the C-terminally phosphorylated form of the enzyme. We show that addition of the HIV-1 Nef protein, which is a high-affinity ligand for the Hck SH3 domain, to either the downregulated or activated form of Hck causes a large increase in Hck catalytic activity. The intact SH3-binding motif in Nef is crucial for Hck activation. Our results indicate that binding of the Hck SH3 domain by Nef causes a more marked activation of the enzyme than does binding of the SH2 domain, suggesting a new mechanism for regulation of the activity of tyrosine kinases.

Crystal structure of the Src family tyrosine kinase Hck

The crystal structure of the haematopoietic cell kinase Hck has been determined at 2.6/2.9 A resolution. Inhibition of enzymatic activity is a consequence of intramolecular interactions of the enzyme's Src-homology domains SH2 and SH3, with concomitant displacement of elements of the catalytic domain. The conformation of the active site has similarities with that of inactive cyclin-dependent protein kinases.

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.

Protein tyrosine kinases in granulocyte colony stimulating factor receptor signal transduction, myeloid cell proliferation, and neutrophil activation

Granulocyte colony-stimulating factor (G-CSF) plays an important role in the growth and maturation of granulocytic precursor cells. Although the interaction between G-CSF and its receptor (G-CSF-R) is an obligatory event during proliferation and differentiation of myeloid cells, the signal transduction mechanisms leading to these effects are not completely known. We investigated the kinetics of protein tyrosine kinase (PTK) activation in G-CSF-R signal transduction in myeloid leukemic cell lines and peripheral blood neutrophils. G-CSF treatment of myeloid cell-lines (HL-60, KG-1, NFS-60) and neutrophils resulted in a rapid increase in PTK activity. This induction was inhibited by an anti-G-CSF monoclonal antibody and various PTK-specific inhibitors. PTK activity was important for proliferation of myeloid cells; its inhibition resulted in decreased proliferation and clonogenicity of these cells. PTK-induction was not involved in G-CSF-R expression, internalization or recycling, but was partially responsible for up-regulation of CD11b expression on neutrophils. In contrast to neutrophilic cell-lines, the myelo-monocytic cell lines (U-937, WEHI-3B) showed no change in PTK levels in response to G-CSF. The results indicate that the G-CSF-R-mediated PTK up-regulation may be a neutrophil-lineage-restricted signal, and that PTK may play an important role in the proliferation of neutrophil-precursors and functional activation of mature neutrophils.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

The role of Src family kinases in the normal and neoplastic gastrointestinal tract

Src family kinases are a group of non-receptor tyrosine kinases that mediate signal transduction pathways involved in the growth and differentiation of normal tissues. Considerable evidence exists for a role of these proteins in neoplastic progression in various organ systems including the nervous, hematopoietic and skeletal systems. In addition, the role of the Src kinase family has been characterized for colon cancer, but only limited progress has been made in delineating the role of Src kinases in the normal gastrointestinal (GI) tract and extracolonic GI cancers. In this review, we provide an up-to-date assessment of the Src family kinases in the normal and neoplastic GI tract.

Activation of Fyn tyrosine kinase upon secretagogue stimulation of bovine chromaffin cells

Adrenal medullary chromaffin cells derive from the neural crest during embryogenesis and differentiate into dedicated secretory cells that release catecholamines in response to acetylcholine in vivo or nicotinic agonists in vitro. Previous studies have indicated that tyrosine kinases participate in early secretagogue-induced events in these cells and are required for exocytosis. Abundant levels of the cytoplasmic tyrosine kinases, c-Src and c-Yes, have been detected in chromaffin cells, thereby implicating them as kinases relevant to these events. However, c-Src has been found to undergo a decrease in activity following secretagogue-stimulation, and c-Yes appears to exist in a constituitively low activity state, suggesting that other tyrosine kinases are involved. Furthermore, other members of the Src family of tyrosine kinases have been implicated as playing roles in secretion in a variety of cell types. Therefore, we sought to determine if other Src family members were present in chromaffin cells, and if so, to examine them for subcellular localization and changes in activity following treatment with nicotinic agonists. To this end, antibodies for Fyn, Lck, Lyn, and Fgr were assembled and used in immunoprecipitation, in vitro autokinase, and Western immunoblotting assays. Of these four kinases, only Fyn was found to be expressed at detectable levels. Differential centrifugation studies revealed that Fyn resides predominantly (> 95%) in the crude plasma membrane fraction and undergoes nicotinic-and carbachol-induced activation. This activation is reduced by the nicotinic antagonist, mecamylamine, is not elicited by muscarine, and is dependent upon the presence of extracellular Ca2+. These results suggest that Fyn is involved in signalling through the nicotinic receptor and may be one of the relevant kinases responsible for at least some of the tyrosine phosphorylations detected after stimulation.

Evolution of the src-related protein tyrosine kinases

A phylogenetic analysis of src-related protein tyrosine kinases (PTKs) showed that one group of these genes is quite ancient in the animals, its divergence predating the divergence of the diploblast and triploblast phyla. Three other major groupings of genes were found to predate the divergence of protostome and deuterostome phyla. Most known src-related PTKs of mammals were found to belong to five well-differentiated families: srcA, srcB, abl, csk, and tec. One srcA gene (fyn) has an alternatively spliced seventh exon which shows a different pattern of relationship from the remainder of the gene; this suggests that this exon may have been derived by a recombinational event with another gene, perhaps one related to fgr. The recently published claim that mammalian members of this family expressed in the nervous system evolve more slowly at nonsynonymous nucleotide sites than do those expressed in the immune system was not supported by an analysis of 13 pairs of human and mouse orthologues. Rather, T-cell-specific src-related PTKs were found to have higher rates of nonsynonymous substitution than were those having broader expression. This effect was particularly marked in the peptide binding site of the SH2 domain. While the SH2 binding site was highly conserved among paralogous mammalian members of the srcA and srcB subfamilies, no such effect was seen in the comparison of paralogous members of the csk and tec subfamilies. This suggests that, while the peptide binding function of SH2 is conserved within both srcA and srcB subfamilies, paralogous members of the csk and tec subfamilies have diverged functionally with respect to peptide recognition by SH2.

Endogenous reactive oxygen intermediates activate tyrosine kinases in human neutrophils

In response to invading microorganisms, neutrophils produce large amounts of superoxide and other reactive oxygen intermediates (ROI) by assembly and activation of a multicomponent enzyme complex, the NADPH oxidase. While fulfilling a microbicidal role, ROI have also been postulated to serve as signaling molecules, because activation of the NADPH oxidase was found to be associated with increased tyrosine phosphorylation (Fialkow, L., Chan, C. K., Grinstein, S., and Downey, G.P. (1993) J. Biol. Chem. 268, 17131-17137). The mechanism whereby ROI induces phosphotyrosine accumulation was investigated using electroporated neutrophils stimulated with guanosine 5'-O-3-thiotriphosphate in order to bypass membrane receptors. In vitro immune complex assays and immunoblotting were used to identify five tyrosine kinases present in human neutrophils. Of these, p56/59hck, p72syk, and p77btk were activated during production of ROI. Interestingly, the in vitro autophosphorylation activities of p53/56lyn and p59fgr were found to decline with ROI production. The mode of regulation of p56/59hck was explored in detail. Oxidizing agents were unable to activate p56/59hck in vitro and, once activated in situ, reducing agents failed to inactivate it, suggesting that the effects of ROI are indirect. Tyrosine phosphorylation of p56/59hck paralleled its activation, and dephosphorylation in vitro reversed the stimulation. We therefore conclude that tyrosine phosphorylation is central to the regulation of p56/59hck and likely also of p72syk, which is similarly phosphorylated upon activation of the oxidase. Because ROI have been shown to reduce the activity of tyrosine phosphatases, we suggest that this inhibition allows constitutively active kinases to auto/transphosphorylate on stimulatory tyrosine residues, leading to an increase in their catalytic activity. Enhanced phosphotyrosine accumulation would then result from the combined effects of increased phosphorylation with decreased dephosphorylation.

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.

Cloning of BSK, a murine FRK homologue with a specific pattern of tissue distribution

A novel protein tyrosine kinase (PTK) was previously identified by us from the rat insulin-producing cell line, RINm5F, by polymerase chain reaction (PCR). By using this PCR fragment to screen a cDNA library from the mouse insulin-producing cell line beta TC-1, a cDNA clone of about 2.0 kb was obtained which encodes the entire amino acid (aa) sequence of the corresponding PTK. The deduced aa sequence reveals strong homology with the members of the SRC family of intracellular PTKs. We have designated the gene as BSK (beta-cell Src-homology tyrosine kinase). Southern blot analysis after PCR with primers specific for BSK confirmed its expression in fetal and adult islets of Langerhans, in RINm5F cells and in mouse kidney. Northern blots using poly(A)+RNA from non-beta-cell tissues showed that the BSK cDNA hybridized to three mRNA transcripts (2.9, 3.1 and 5.0 kb) present in kidney, liver and lung. Extensive homology of BSK with the recently identified human gene FRK was observed. It is concluded that Bsk is a murine Frk homologue with a specific pattern of tissue expression.

Serine/threonine kinase activation in human neutrophils: relationship to tyrosine phosphorylation

Tyrosine phosphorylation is among the earliest responses of neutrophils to chemotactic peptides. Tyrosine phosphorylated proteins comigrate with serine/threonine kinases of 65 and 72 kDa (PK65 and PK72), which are activated concomitantly by the chemoattractants. Studies were designed to test whether tyrosine phosphorylation is required for activation of PK65 and PK72. Pretreatment of cells with the tyrosine kinase inhibitors erbstatin or genistein prevented both phosphotyrosine accumulation and activation of PK65 and PK72. In nondenaturing lysates, PK65 and PK72 became spontaneously inactivated in parallel with rapid endogenous tyrosine dephosphorylation. Spontaneous dephosphorylation and inactivation of PK65 and PK72 were prevented in denatured lysates. Under these conditions, dephosphorylation could be induced by exogenous phosphotyrosine phosphatase 1B. PK65 and PK72 activation persisted despite virtually complete tyrosine dephosphorylation. Moreover, immunoprecipitation experiments indicated that PK65 and PK72 are not themselves tyrosine phosphorylated. We concluded that tyrosine phosphorylation is a necessary upstream event in the activation of the serine/threonine kinases. However, once the posttranslational modification that renders PK65 and PK72 active has occurred, tyrosine phosphorylation is no longer required for maintenance of their kinase activity.

Rapid interferon-gamma-stimulated tyrosine phosphorylation of cytosolic and membranal proteins in HL-60 promyelocytic cells

The involvement of tyrosine phosphorylation in the early stages of interferon-gamma (IFN gamma)-induced monocytic differentiation of HL-60 cells was studied. Immunoblotting analysis demonstrated that IFN gamma induced rapid changes in the tyrosine phosphorylation of several endogenous cytosolic and membranal proteins. The most prominent of these polypeptides was a 84 kDa protein. In membranes, the IFN gamma-induced phosphorylation of this protein was detectable in 5 min, remained elevated for 3 h and declined thereafter, while a gradual decrease in the phosphotyrosine content was observed in cytosols. In parallel, a 40% increase in the phosphotyrosine phosphatase activity was detected in the later stages of IFN gamma treatment. Rapid changes in tyrosine phosphorylation were detected also in a 64 kDa protein. In contrast, 2-day exposure to IFN gamma was needed to potentiate significantly the tyrosine phosphorylation of a 36 kDa membranal polypeptide. These data support the involvement of tyrosine phosphorylation in the early stages of IFN gamma-induced monocytic differentiation of HL-60 cells.

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.

pp60c-src enhances the acetylcholine receptor-dependent catecholamine release in vaccinia virus-infected bovine adrenal chromaffin cells

Secretion of catecholamines by adrenal chromaffin cells is a highly regulated process that involves serine/threonine and tyrosine phosphorylations. The nonreceptor tyrosine kinase pp60c-src is expressed at high levels and localized to plasma membranes and secretory vesicle membranes in these cells, suggesting an interaction of this enzyme with components of the secretory process. To test the hypothesis that pp60c-src is involved in exocytosis, we transiently expressed exogenous c-src cDNA using a vaccinia virus vector in primary cultures of bovine adrenomedullary chromaffin cells. Chromaffin cells infected with a c-src recombinant virus restored the diminished secretory activity accompanying infection by wild type virus alone or a control recombinant virus. The level of enhanced catecholamine release correlated directly with the time and level of exogenous c-src expression. These results could not be attributed to differences in cytopathic effects of wild type versus recombinant viruses as assessed by cell viability assays, nor to differences in norepinephrine uptake or basal release, suggesting that pp60c-src is involved in stimulus-secretion coupling in infected cells. Surprisingly, exogenous expression of an enzymatically inactive mutant c-src also restored catecholamine release, indicating that regions of the introduced c-src protein other than the kinase domain may affect catecholamine release. Secretory activity was elevated by both forms of c-src in response to either nicotine or carbachol (which activate the nicotinic and the nicotinic/muscarinic receptors, respectively). In contrast, release of catecholamines upon membrane depolarization (as elicited by 55 mM K+) or by treatment with the calcium ionophore A23187 was unaffected by either vaccinia infection or increased levels of pp60c-src. These results suggest that pp60c-src affects secretory processes in vaccinia-infected cells that are activated through ligand-gated, but not voltage-gated, ion channels.

Cloning of FRK, a novel human intracellular SRC-like tyrosine kinase-encoding gene

We report the cloning of a novel tyrosine kinase (TyK)-encoding gene (TYK) from the human hepatoma cell line Hep3B. Using the polymerase chain reaction (PCR) and oligodeoxyribonucleotide primers based on conserved TYK motifs, a 180-bp fragment was cloned and used to obtain full-length cDNA clones of 2.9 kb, with an open reading frame of 505 amino acids (aa). Restricted expression was detected by Northern blotting or reverse-transcribed PCR in a broad range of cell lines. The predicted aa sequence contains characteristic TyK motifs without a transmembrane region, suggesting an intracellular localization. There was 49% aa sequence identity with human FYN product and 47% with human SRC product; however, several structural differences distinguish this clone from other SRC subfamily members. This clone, FYN-related kinase or FRK, is a novel member of the intracellular TYK gene family.

Regulation of megakaryocytopoiesis

Megakaryocytopoiesis is the cellular developmental process prior to the release of platelets into the circulation. Regulation of megakaryocytopoiesis is a complex phenomenon that begins with commitment of hematopoietic stem cells to the replication and maturation of progenitor cells through endomitosis and megakaryocyte differentiation [1-4]. Platelet production is determined by the number and size of megakaryocytes in the marrow and may be regulated at two levels: at early stages of cell proliferation resulting in increased megakaryocyte numbers, and at later stages by endoreplication which increases DNA content and the size of megakaryocytes [5]. The mature megakaryocyte is a large polyploid cell with a highly defined invaginated membrane (demarcation membrane) and contains the membrane molecules necessary for platelet function [6-9]. Platelet shedding appears to occur by fragmentation of the cytoplasm of the megakaryocyte. Platelet release is thought to occur via transendothelial processes projecting into the vascular compartment [10, 11], although several studies indicate that megakaryocytes lodged in the lungs are capable of platelet formation [12-17]. The factors stimulating megakaryocytopoiesis in the lung have not been well characterized. In the past, the study of megakaryocyte development in vivo and in vitro was hampered by the rarity of megakaryocytes in the bone marrow, the poorly defined cell populations, and inadequate assays. These prior studies of megakaryocyte development have been discussed in the recent past by R. Hoffman [1], N. Williams [3], and M. W. Long [2]. An attempt will be made in this review to highlight and synthesize various new concepts of regulation of megakaryocytopoiesis.

Deficient expression of a B cell cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia

We describe a novel cytoplasmic tyrosine kinase, termed BPK (B cell progenitor kinase), which is expressed in all stages of the B lineage and in myeloid cells. BPK has classic SH1, SH2, and SH3 domains, but lacks myristylation signals and a regulatory phosphorylation site corresponding to tyrosine 527 of c-src. BPK has a long, basic amino-terminal region upstream of the SH3 domain. BPK was evaluated as a candidate for human X-linked agammaglobulinemia (XLA), an inherited immunodeficiency characterized by a severe deficit of B and plasma cells and profound hypogammaglobulinemia. BPK mapped to within 100 kb of a probe defining the polymorphism most closely linked to XLA at DXS178. Reduction in or the absence of BPK mRNA, protein expression, and kinase activity was observed in XLA pre-B and B cell lines. BPK is likely the XLA gene and functions in pathways critical to B cell expansion.

A novel tyrosine kinase, hyk, expressed in murine embryonic stem cells

To identify tyrosine kinases which play roles in mammalian early development, the 3' rapid amplification of cDNA ends (RACE) was performed on mouse embryonic stem (ES) cells. Among eight tyrosine kinases thus identified, we report here a novel tyrosine kinase, hyk (adhesion structures linked tyrosine kinase). The sequences of the 4.7 kb cDNA indicated the presence of RGD motif and three epidermal growth factor-like domains put between two immunoglobulin-like domains and three fibronectin type III domains in its extracellular region. It is strongly expressed in ES cells and later stages of embryos, but at low levels in midgestation embryos. It is also expressed at a low level in neural precursor cells from 10-day embryos, but at high levels in embryonic day 15 and neonatal brains. In adult tissues it is expressed ubiquitously.

New pathways of phagocyte activation: the coupling of receptor-linked phospholipase D and the role of tyrosine kinase in primed neutrophils

Protein kinase C (PKC) appears to have a central role in the O2- response of neutrophils following stimulation of membrane receptors. The second messenger, diacylglycerol (DG), that activates PKC is derived from membrane phospholipids via activation of phosphatidylinositol 4,5-bisphosphate (PIP2)-phospholipase C (PLC) and phospholipase D (PLD), with the latter pathway being more prominent in primed cells. In resting cells receptor coupling to PLD is through a G-protein. Priming brings a cytoplasmic tyrosine kinase into the transducer sequence which, through protein phosphorylation, increases the efficiency of coupling between membrane receptors and PLD. Phosphatidic acid (PA), the initial product of the PLD pathway, also appears to act as a second messenger by directly activating the NADPH oxidase responsible for generating O2-. Interconversion of PA and DG by phosphatidate phosphohydrolase and DG kinase determines which of these second messengers has the dominant role.

ZAP-70: a 70 kd protein-tyrosine kinase that associates with the TCR zeta chain

Protein-tyrosine kinases (PTKs) play an integral role in T cell activation. Stimulation of the T cell antigen receptor (TCR) results in tyrosine phosphorylation of a number of cellular substrates. One of these is the TCR zeta chain, which can mediate the transduction of extracellular stimuli into cellular effector functions. We have recently identified a 70 kd tyrosine phosphoprotein (ZAP-70) that associates with zeta and undergoes tyrosine phosphorylation following TCR stimulation. Here we report the isolation of a cDNA clone encoding ZAP-70. ZAP-70 represents a novel PTK and is expressed in T and natural killer cells. Moreover, tyrosine phosphorylation and association of ZAP-70 with zeta require the presence of src family PTKs and provide a potential mechanism by which the src family PTKs and ZAP-70 may interact to mediate TCR signal transduction.

Tumor-associated karyotypic lesions coselected with in vitro macrophage differentiation

Several cytogenetic lesions in chromosomes 2, 5, 12, and 16 have been repeatedly coselected with in vitro macrophage differentiation in a clonal murine thymic tumor cell line. Parental-type subclones, which show an extremely immature hemopoietic phenotype, do not carry the aberrations. The frequency of the stable differentiated variants is elevated by 5-azacytidine and bromodeoxyuridine, consistent with chromosome breakage being responsible for the phenotype. The frequency is also raised by dexamethasone. Since variants are 300-3,000-fold more resistant to dexamethasone than parental clones, we interpret this to be largely due to selection. Three of the lesions, on chromosome 2, match those previously described as associated specifically with in vivo-generated murine myeloid tumors, induced by X irradiation and corticosteroid treatment. Several implications follow from these observations. (1) In vitro differentiation in clonal tumor cell lines can be used to select for tumor-associated lesions. This should allow genetic and molecular analysis of the chromosome 2 lesions and of others that may pinpoint genes critical to macrophage differentiation and transformation. (2) Myeloid and lymphoid tumors that occur in response to X irradiation may diverge from a common initiating tumor. (3) The hemopoietic lineage switch phenomenon, previously described by several authors, may be caused by similar or identical chromosome aberrations.

A 16 amino acid synthetic peptide derived from human C3d triggers proliferation and specific tyrosine phosphorylation of transformed CR2-positive human lymphocytes and of normal resting B lymphocytes

We demonstrate herein that p16, a 16 amino acid synthetic peptide derived from human C3d, which carried LYNVEA sequence of C3d reacting with CR2 and C3d present in trypsin-cleaved C3, triggered "in vitro" and "in vivo" phosphorylations and "in vitro" proliferation of human B lymphocytes, depending on the stage of cell differentiation. Indeed, p16 and C3dT induced "in vivo" tyrosine phosphorylation of pp105 and "in vitro" proliferation only of CR2-positive and not of CR2-negative cell lines. In addition, p16 and C3dT also induced "in vivo" tyrosine phosphorylation of pp100 and "in vitro" proliferation of only small dense resting B lymphocytes and not other B lymphocyte subpopulations nor T lymphocytes. These data suggest that induction of pp100 and pp105 phosphorylation by p16 and C3dT could represent an early event associated with expression of CR2 in the regulation of human B lymphocyte proliferation.