The Steel/W transduction pathway: kit autophosphorylation and its association with a unique subset of cytoplasmic signaling proteins is induced by the Steel factor

Cellular Energetics of Mast Cell Development and Activation

Mast cells are essential first responder granulocytes in the innate immune system that are well known for their role in type 1 immune hypersensitivity reactions. Although mostly recognized for their role in allergies, mast cells have a range of influences on other systems throughout the body and can respond to a wide range of agonists to properly prime an appropriate immune response. Mast cells have a dynamic energy metabolism to allow rapid responsiveness to their energetic demands. However, our understanding of mast cell metabolism and its impact on mast cell activation and development is still in its infancy. Mast cell metabolism during stimulation and development shifts between both arms of metabolism: catabolic metabolism-such as glycolysis and oxidative phosphorylation-and anabolic metabolism-such as the pentose phosphate pathway. The potential for metabolic pathway shifts to precede and perhaps even control activation and differentiation provides an exciting opportunity to explore energy metabolism for clues in deciphering mast cell function. In this review, we discuss literature pertaining to metabolic environments and fluctuations during different sources of activation, especially IgE mediated vs. non-IgE mediated, and mast cell development, including progenitor cell types leading to the well-known resident mast cell.

Loss of PI3 kinase association improves the sensitivity of secondary mutation of KIT to Imatinib

Background

KIT mutations are the predominant driver mutations in gastrointestinal stromal tumors (GISTs), and targeted therapy against KIT has improved treatment outcome dramatically. However, gaining secondary mutation of KIT confers drug resistance of GISTs leading to treatment failure.

Results

In this study, we found that secondary mutation of KIT dramatically increases the ligand-independent activation of the receptor and their resistance to the often used KIT inhibitor Imatinib in the treatment of GISTs. PI3 kinase plays essential roles in the cell transformation mediated by the primary mutation of KIT. We found that loss of PI3 kinase association, but not the inhibition of the lipid kinase activity of PI3 kinase, inhibits the ligand-independent activation of secondary mutations of KIT, and increases their sensitivity to Imatinib, and loss of PI3 kinase association inhibits secondary mutations of KIT mediated cell survival and proliferation in vitro. The in vivo assay further showed that the growth of tumors carrying secondary mutations of KIT is more sensitive to Imatinib when PI3 kinase association is blocked while inhibition of the lipid kinase activity of PI3 kinase cannot inhibit tumor growth, indicating that PI3 kinase is important for the drug resistance of secondary mutation of KIT independent of the lipid kinase activity of PI3 kinase.

Conclusions

Our results suggested that PI3 kinase is necessary for the ligand-independent activation of secondary mutations of KIT, and loss of PI3 kinase association improves the sensitivity of secondary mutations to the targeted therapy independent of the lipid kinase activity of PI3 kinase.

C-Kit Promotes Growth and Migration of Human Cardiac Progenitor Cells via the PI3K-AKT and MEK-ERK Pathways

A recent phase I clinical trial (SCIPIO) has shown that autologous c-kit+ cardiac progenitor cells (CPCs) improve cardiac function and quality of life when transplanted into patients with ischemic heart disease. Although c-kit is widely used as a marker of resident CPCs, its role in the regulation of the cellular characteristics of CPCs remains unknown. We hypothesized that c-kit plays a role in the survival, growth, and migration of CPCs. To test this hypothesis, human CPCs were grown under stress conditions in the presence or absence of SCF, and the effects of SCF-mediated activation of c-kit on CPC survival/growth and migration were measured. SCF treatment led to a significant increase in cell survival and a reduction in cell death under serum depletion conditions. In addition, SCF significantly promoted CPC migration in vitro. Furthermore, the pro-survival and pro-migratory effects of SCF were augmented by c-kit overexpression and abrogated by c-kit inhibition with imatinib. Mechanistically, c-kit activation in CPCs led to activation of the PI3K and the MAPK pathways. With the use of specific inhibitors, we confirmed that the SCF/c-kit-dependent survival and chemotaxis of CPCs are dependent on both pathways. Taken together, our findings suggest that c-kit promotes the survival/growth and migration of human CPCs cultured ex vivo via the activation of PI3K and MAPK pathways. These results imply that the efficiency of CPC homing to the injury site as well as their survival after transplantation may be improved by modulating the activity of c-kit.

Gastrointestinal stromal tumor and its targeted therapeutics

Over the past 60 years, investigators of basic science, pathology, and clinical medicine have studied gastrointestinal stromal tumor (GIST) and made minor advances in patient care. Recent discoveries have led to an understanding of the biological role of KIT and platelet-derived growth factor receptor-α in GIST and the development of the tyrosine kinase inhibitor imatinib mesylate (Gleevec, formerly STI-571), one of the most exciting examples of targeted therapy to date. The success of targeted therapy in GIST has lead to new developments in our understanding of the medical and surgical management of the disease. Intense study of GIST may lead to new paradigms in the management of cancer.

Imatinib upregulates compensatory integrin signaling in a mouse model of gastrointestinal stromal tumor and is more effective when combined with dasatinib

Activating mutations in the Kit receptor tyrosine kinase are associated with gastrointestinal stromal tumor (GIST). Imatinib inhibits Kit and is front-line therapy for GIST. However, imatinib most often elicits a partial response or stable disease, and most GIST patients who initially respond to imatinib eventually acquire resistance. Thus, improved treatment strategies for GIST are needed. We investigated the role of Src family kinases (SFK) in tumorigenesis in a mouse model of human GIST. The SFKs Src and Lyn were active in GIST, and surprisingly, imatinib treatment stimulated their phosphorylation/activation. We show that integrin signaling activates focal adhesion kinase and, consequently, SFKs in GIST and that imatinib enhances integrin signaling, implying a role for the extracellular matrix and integrin signaling in tumor maintenance and imatinib resistance. Dasatinib, an inhibitor of SFKs and Kit, inhibited SFK and focal adhesion kinase activation in GIST but also inhibited Kit and Kit-dependent downstream signaling pathways including phosphoinositide 3-kinase and mitogen-activated protein kinase, but not signal transducer and activator of transcription (STAT) signaling. Whereas dasatinib and imatinib alone both produced a minimal histopathologic response, combination therapy improved their efficacy, leading to increased necrosis in GIST. These results highlight the importance of SFK and STAT signaling in GIST and suggest that the clinical efficacy of imatinib may be limited by the stimulation of integrin signaling.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

SHIP deficiency enhances HSC proliferation and survival but compromises homing and repopulation

The SH2 domain-containing inositol 5'-phosphatase-1 (SHIP) has the potential to modulate multiple signaling pathways downstream of receptors that impact hematopoietic stem cell (HSC) biology. Therefore, we postulated that SHIP might play an important role in HSC homeostasis and function. Consistent with this hypothesis, HSC proliferation and numbers are increased in SHIP(-/-) mice. Despite expansion of the compartment, SHIP(-/-) HSCs exhibit reduced capacity for long-term repopulation. Interestingly, we observe that SHIP(-/-) stem/progenitor cells home inefficiently to bone marrow (BM), and consistent with this finding, have reduced surface levels of both CXCR4 and vascular cell adhesion marker-1 (VCAM-1). These studies demonstrate that SHIP is critical for normal HSC function, homeostasis, and homing.

Imatinib inhibits proliferation of Ewing tumor cells mediated by the stem cell factor/KIT receptor pathway, and sensitizes cells to vincristine and doxorubicin-induced apoptosis

PURPOSE AND EXPERIMENTAL DESIGN

The stem cell factor/KIT receptor loop may represent a novel target for molecular-based therapies of Ewing tumor. We analyzed the in vitro impact of KIT blockade by imatinib in Ewing tumor cell lines.

RESULTS

KIT expression was detected in 4 of 4 Ewing tumor cell lines and in 49 of 110 patient samples (44.5%) by immunohistochemistry and/or Western blot analysis. KIT expression was stronger in Ewing tumors showing EWS-FLI1 nontype 1 fusions. Despite absence of c-kit mutations, constitutive and ligand-inducible phosphorylation of KIT was found in all tumor cell lines, indicating an active receptor. Treatment with KIT tyrosine kinase inhibitor imatinib (0.5-20 micro M) induced down-regulation of KIT phosphorylation and dose response inhibition of cell proliferation (IC(50), 12-15 micro M). However, imatinib administered alone at doses close to IC(50) for growth inhibition (10 micro M) did not induce a significant increase in apoptosis. We then analyzed if blockade of KIT loop through imatinib (10 micro M) was able to increase the antitumor in vitro effect of doxorubicin (DXR) and vincristine (VCR), drugs usually used in Ewing tumor treatment. Addition of imatinib decreased in 15-20 and 15-36% of the proliferative rate of Ewing tumor cells exposed to DXR and VCR, respectively, and increased in 15 and 30% of the apoptotic rate of Ewing tumor cells exposed to the same drugs.

CONCLUSIONS

Inhibition of Ewing tumor cell proliferation by imatinib is mediated through blockade of KIT receptor signaling. Inhibition of KIT increases sensitivity of these cells to DXR and VCR. This study supports a potential role for imatinib in the treatment of Ewing tumor.

Signal transduction by several KIT juxtamembrane domain mutations

Mutations of KIT receptor tyrosine kinase are found in the majority of patients with mastocytosis and in most gastrointestinal stromal tumors. Oncogenic KIT mutations in GISTs are located in the KIT juxtamembrane domain (JMD), while codon 816 in the KIT kinase domain is mutated in systemic mastocytosis. We describe and characterize a mutation in the KIT-JMD named Kdelta27. We show that Kdelta27 mutant is constitutively dimerized and phosphorylated. Kdelta27 ectopic expression renders both the Ba/F3 cell line and primary cultures of bone marrow mast cells independent of cytokines for proliferation and cell survival. The classical signaling pathways activated by wild-type KIT upon ligand stimulation are constitutively activated by Kdelta27 and other JMD mutations. However, a side-to-side comparison revealed differences between the wild-type and JMD mutations. First, in vitro kinase assays reveal a change in peptide substrate specificity. Second, STAT proteins are preferentially phosphorylated by KIT mutants. Third, inhibitors of KIT kinase are more efficient on JMD mutations than on WT KIT. We conclude that Kdelta27 is a new oncogenic KIT mutation showing constitutive activation of downstream signaling pathways, and suggest that specific pathways are activated by oncogenic KIT.

Cytokine signaling and hematopoietic homeostasis are disrupted in Lnk-deficient mice

The adaptor protein Lnk, and the closely related proteins APS and SH2B, form a subfamily of SH2 domain-containing proteins implicated in growth factor, cytokine, and immunoreceptor signaling. To elucidate the physiological function of Lnk, we derived Lnk-deficient mice. Lnk(-/-) mice are viable, but display marked changes in the hematopoietic compartment, including splenomegaly and abnormal lymphoid and myeloid homeostasis. The in vitro proliferative capacity and absolute numbers of hematopoietic progenitors from Lnk(-/-) mice are greatly increased, in part due to hypersensitivity to several cytokines. Moreover, an increased synergy between stem cell factor and either interleukin (IL)-3 or IL-7 was observed in Lnk(-/-) cells. Furthermore, Lnk inactivation causes abnormal modulation of IL-3 and stem cell factor-mediated signaling pathways. Consistent with these results, we also show that Lnk is highly expressed in multipotent cells and committed precursors in the erythroid, megakaryocyte, and myeloid lineages. These data implicate Lnk as playing an important role in hematopoiesis and in the regulation of growth factor and cytokine receptor-mediated signaling.

The complexity of KIT gene mutations and chromosome rearrangements and their clinical correlation in gastrointestinal stromal (pacemaker cell) tumors

Gastrointestinal stromal (pacemaker cell) tumors (GIST/GIPACTs) are frequently associated with activating KIT mutations, primarily of exon 11 and rarely of exons 9 and 13, as well as certain chromosome rearrangements. Reports regarding the frequency and prognostic significance of KIT mutations are conflicting and few cases have been completely sequenced. Furthermore, there are few detailed analyses of chromosome alterations in GIST/GIPACTs. In a detailed analysis of 14 GIST/GIPACTs from 12 patients, we found a wider spectrum of KIT mutations than previously reported, including 11 different in-frame mutations involving exons 11, 14, and 15. No mutations were detected in four malignant tumors. The shorter (GNNK-) KIT isoform was preferentially expressed. Cytogenetic and spectral karyotype analyses of 10 tumors revealed clonal abnormalities in eight tumors; the most common were terminal 1p deletions and losses of chromosomes 14 and/or 22. Neither KIT mutation status nor chromosome aberrations correlated with tumor phenotype or clinical behavior in our series. Collectively, these findings indicate that the role of KIT mutations and chromosomal rearrangements in the pathogenesis of GIST/GIPACTs are more complex than previously recognized.

Kit signaling and negative regulation of daunorubicin-induced apoptosis: role of phospholipase Cgamma

Previous studies have demonstrated that activation of Kit by stem cell factor (SCF), its natural ligand, or by gain-of-function point mutation in its intracellular domain, confers significant protection against apoptosis induced by growth factor deprivation or radiation. However, the effects of Kit activation on the cellular response to anti-tumor agents have not been so extensively documented. This study shows that daunorubicin-induced apoptosis and cytotoxicity were reduced in the murine Ba/F3 cells transfected with Kit (Ba/F3-Kit) in the presence of SCF and in Ba/F3 cells transfected with a constitutively active Kit variant (Ba/F3-KitDelta27), compared to either parental Ba/F3 (Ba/F3-wt) or unstimulated Ba/F3-Kit cells. In Ba/F3-wt and in Ba/F3-Kit cells, daunorubicin stimulated within 8-15 min neutral sphingomyelinase and ceramide production but not in SCF-stimulated Ba/F3-Kit or in Ba/F3-KitDelta27 whereas all Ba/F3 cells were equally sensitive to exogenous cell-permeant C6-ceramide. In Ba/F3-Kit, SCF-induced Kit activation resulted in a rapid phospholipase Cgamma (PLCgamma) tyrosine phosphorylation followed by diacylglycerol release and protein kinase C (PKC) stimulation. U-73122, a PLCgamma inhibitor, not only blocked diacylglycerol production and PKC stimulation but also restored daunorubicin-induced sphingomyelinase stimulation, ceramide production, and apoptosis. These results suggest that Kit activation results in PLCgamma-mediated PKC-dependent sphingomyelinase inhibition which contributes to drug resistance in Kit-related malignancies.

Point mutation in kit receptor tyrosine kinase reveals essential roles for kit signaling in spermatogenesis and oogenesis without affecting other kit responses

The Kit receptor tyrosine kinase functions in hemato- poiesis, melanogenesis and gametogenesis. Kit receptor-mediated cellular responses include proliferation, survival, adhesion, secretion and differentiation. In mast cells, Kit-mediated recruitment and activation of phosphatidylinositol 3'-kinase (PI 3-kinase) produces phosphatidylinositol 3'-phosphates, plays a critical role in mediating cell adhesion and secretion and has contributory roles in mediating cell survival and proliferation. To investigate the consequences in vivo of blocking Kit-mediated PI 3-kinase activation we have mutated the binding site for the p85 subunit of PI 3-kinase in the Kit gene, using a knock-in strategy. Mutant mice have no pigment deficiency or impairment of steady-state hematopoiesis. However, gametogenesis is affected in several ways and tissue mast cell numbers are affected differentially. While primordial germ cells during embryonic development are not affected, Kit(Y719F)/Kit(Y719F) males are sterile due to a block at the premeiotic stages in spermatogenesis. Furthermore, adult males develop Leydig cell hyperplasia. The Leydig cell hyperplasia implies a role for Kit in Leydig cell differentiation and/or steroidogenesis. In mutant females follicle development is impaired at the cuboidal stages resulting in reduced fertility. Also, adult mutant females develop ovarian cysts and ovarian tubular hyperplasia. Therefore, a block in Kit receptor-mediated PI 3-kinase signaling may be compensated for in hematopoiesis, melanogenesis and primordial germ cell development, but is critical in spermatogenesis and oogenesis.

Isoforms of c-KIT differ in activation of signalling pathways and transformation of NIH3T3 fibroblasts

Alternate splicing of mRNA encoding c-KIT results in isoforms which differ in the presence or absence of four amino acids (GNNK) in the juxtamembrane region of the extracellular domain of the receptor. In this study we show that these isoforms of human c-KIT, expressed at similar levels in NIH3T3 cells, display differential effects on various attributes of transformation. The GNNK- isoform strongly promoted anchorage independent growth (colony formation in semi-solid medium), loss of contact inhibition (focus formation), and led to tumorigenicity in nude mice. In contrast, the GNNK+ isoform elicited colony formation but relatively poor focus formation and no tumorigenicity. Saturation binding analysis indicated that the isoforms do not differ significantly in their affinity for the KIT ligand, Steel Factor (SLF). Negligible ligand-independent receptor phosphorylation was observed in either case but, after ligand stimulation, the GNNK- isoform displayed more rapid and extensive tyrosine autophosphorylation and faster internalization. Both isoforms recruited the p85 subunit of phosphatidylinositol 3-kinase and led to similar phosphorylation of its downstream effector c-Akt, but the GNNK- isoform gave rise to more MAP kinase phosphorylation. Thus the c-KIT isoforms display different signalling characteristics and have different transforming activity in NIH3T3 cells.

Role of PI3-kinase in the development of interstitial cells and pacemaking in murine gastrointestinal smooth muscle

1. Development of the pacemaker system in the small intestine depends upon signalling via tyrosine kinase (Kit) receptors. The downstream pathways initiated by Kit in interstitial cells of Cajal (ICC) have not been investigated. Wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), inhibitors of phosphatidylinositol 3'-kinase (PI3-kinase), were used to test the involvement of this pathway in the development and maintenance of ICC and electrical rhythmicity in the murine small intestine. 2. ICC and electrical slow waves were present in the murine jejunum at birth. ICC and electrical rhythmicity continued to develop in neonates such that adult activity was recorded after 1 week. Development of ICC and rhythmicity were maintained in organ culture. 3. Wortmannin or LY 294002 inhibited the development of slow waves and blocked rhythmicity within 2-4 days. Loss of slow waves was preceded by disappearance of Kit-positive cells from the myenteric (IC-MY) and deep muscular plexus (IC-DMP) regions. Wortmannin or LY 294002 had no acute effect on slow waves. 4. Muscles from older animals (day 10-day 30) developed resistance to wortmannin treatment, but when the exposure to wortmannin was increased to 35 days, damage to ICC networks and electrical dysrhythmias were observed. 5. PI3-kinase appears to be a critical downstream signalling element linking Kit receptors to ICC development and maintenance of phenotype. ICC are more sensitive to Kit or PI3-kinase blockade at birth, but the importance of the PI3-kinase signalling in the maintenance of ICC persists into adulthood. Interference with PI3-kinase signalling in immature or adult animals could result in disruption of ICC and gastrointestinal dysrhythmias.

Negative Regulation of c-kit-Mediated Cell Proliferation by FcγRIIB

FcγRIIB are single-chain low-affinity receptors for IgG that bear an immunoreceptor tyrosine-based inhibition motif in their intracytoplasmic domain and that negatively regulate immunoreceptor tyrosine-based activation motif-dependent cell activation. They are widely expressed by cells of hematopoietic origin. We investigated here whether FcγRIIB could also negatively regulate protein tyrosine kinase receptor (RTK)-dependent cell proliferation. As an experimental model, we used growth factor-dependent mast cells that constitutively express FcγRIIB and c-kit, an RTK prototype. We found that anti-c-kit Abs mimicked the effect of stem cell factor and induced thymidine incorporation in FcγRIIB−/−, but not in wild-type (wt) mast cells unless FcγRIIB were blocked or anti-c-kit F(ab′)2 were used. When coaggregated with c-kit by intact Abs in wt mast cells, FcγRIIB inhibited thymidine incorporation, as well as cell proliferation, and inhibition was correlated with an arrest of cells in G1 during the cell cycle. The coaggregation of c-kit with FcγRIIB did not affect ligand-induced c-kit phosphorylation and induced the tyrosyl-phosphorylation of FcγRIIB, which selectively recruited the Src homology 2 domain-bearing inositol 5-phosphatase SHIP. Our results indicate that IgG Abs to growth factors or growth factor receptors may control RTK-dependent proliferation of a variety of cells that express FcγRIIB.

Affinity Modulation of Very Late Antigen-5 Through Phosphatidylinositol 3-Kinase in Mast Cells

Adhesiveness of integrins is up-regulated rapidly by a number of molecules, including growth factors, cytokines, chemokines, and other cell surface receptors, through a mechanism termed inside-out signaling. The inside-out signaling pathways are thought to alter integrin affinity for ligand, or cell surface distribution of integrin by diffusion/clustering. However, it remains to be clarified whether any physiologically relevant agonists induce a rapid change in the affinity of β1 integrins and how ligand-binding affinity is modulated upon stimulation. In this study, we reported that affinity of β1 integrin very late Ag-5 (VLA-5) for fibronectin was rapidly increased in bone marrow-derived mast cells by Ag cross-linking of FcεRI. Ligand-binding affinity of VLA-5 was also augmented by receptor tyrosine kinases when the phospholipase Cγ-1/protein kinase C pathway was inhibited. Wortmannin suppressed induction of the high affinity state VLA-5 in either case. Conversely, introduction of a constitutively active p110 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) increased the binding affinity for fibronectin. Failure of a constitutively active Akt to stimulate adhesion suggested that the affinity modulation mechanisms mediated by PI 3-kinase are distinct from the mechanisms to control growth and apoptosis by PI 3-kinase. Taken together, our findings demonstrated that the increase of affinity of VLA-5 was induced by physiologically relevant stimuli and PI 3-kinase was a critical affinity modulator of VLA-5.

Socs1 binds to multiple signalling proteins and suppresses steel factor-dependent proliferation

We have identified Socs1 as a downstream component of the Kit receptor tyrosine kinase signalling pathway. We show that the expression of Socs1 mRNA is rapidly increased in primary bone marrow-derived mast cells following exposure to Steel factor, and Socs1 inducibly binds to the Kit receptor tyrosine kinase via its Src homology 2 (SH2) domain. Previous studies have shown that Socs1 suppresses cytokine-mediated differentiation in M1 cells inhibiting Janus family kinases. In contrast, constitutive expression of Socs1 suppresses the mitogenic potential of Kit while maintaining Steel factor-dependent cell survival signals. Unlike Janus kinases, Socs1 does not inhibit the catalytic activity of the Kit tyrosine kinase. In order to define the mechanism by which Socs1-mediated suppression of Kit-dependent mitogenesis occurs, we demonstrate that Socs1 binds to the signalling proteins Grb-2 and the Rho-family guanine nucleotide exchange factors Vav. We show that Grb2 binds Socs1 via its SH3 domains to putative diproline determinants located in the N-terminus of Socs1, and Socs1 binds to the N-terminal regulatory region of Vav. These data suggest that Socs1 is an inducible switch which modulates proliferative signals in favour of cell survival signals and functions as an adaptor protein in receptor tyrosine kinase signalling pathways.

Mitogen-Activated Protein Kinase Activation Through Fcε Receptor I and Stem Cell Factor Receptor Is Differentially Regulated by Phosphatidylinositol 3-Kinase and Calcineurin in Mouse Bone Marrow-Derived Mast Cells

Aggregation of high affinity FcR for IgE (FcεRI) on mast cells activates intracellular signal transduction pathways, including the activation of protein tyrosine kinases, phosphatidylinositol 3-kinase (PI3-kinase), and protein kinase C. Binding of stem cell factor (SCF) to its receptor (SCFR, c-Kit) on mast cells also induces increases in intrinsic tyrosine kinase activity and activation of PI3-kinase. Although ligation of both receptors induces Ras and Raf-1 activation, the downstream consequences of these early activation events are not well defined, except for the activation of extracellular signal-regulated kinases (ERK). Addition of Ag (OVA) to mouse bone marrow-derived mast cells (BMMC) sensitized with anti-OVA IgE triggers the activation of three members of the mitogen-activated protein (MAP) kinase family, c-Jun amino-terminal kinase (JNK), p38 MAP kinase (p38), and extracellular signal-regulated kinases. SCF similarly activates all three MAP kinases. Wortmannin, an inhibitor of PI3-kinase, inhibited both FcεRI- and SCFR-mediated JNK activation and partially inhibited FcεRI, but not SCFR-mediated p38 activation. Cyclosporin A inhibited FcεRI-mediated JNK and p38 activation, but did not affect the activation of these kinases when stimulated through the SCFR. Wortmannin and cyclosporin A inhibited FcεRI-mediated production of TNF-α and IL-4 in addition to serotonin release in BMMC. These results indicate that both PI3-kinase and calcineurin may contribute to the regulation of cytokine gene transcription and the degranulation response by modulating JNK activity in BMMC.

Kit signaling through PI 3-kinase and Src kinase pathways: an essential role for Rac1 and JNK activation in mast cell proliferation

The receptor tyrosine kinase Kit plays critical roles in hematopoiesis, gametogenesis and melanogenesis. In mast cells, Kit receptor activation mediates several cellular responses including cell proliferation and suppression of apoptosis induced by growth factor deprivation and gamma-irradiation. Kit receptor functions are mediated by kinase activation, receptor autophosphorylation and association with various signaling molecules. We have investigated the role of phosphatidylinositol 3'-kinase (PI 3-kinase) and Src kinases in Kit-mediated cell proliferation and suppression of apoptosis induced both by factor deprivation and irradiation in bone marrow-derived mast cells (BMMC). Analysis of Kit-/- BMMC expressing mutant Kit receptors and the use of pharmacological inhibitors revealed that both signaling pathways contribute to these Kit-mediated responses and that elimination of both pathways abolishes them. We demonstrate that the PI 3-kinase and Src kinase signaling pathways converge to activate Rac1 and JNK. Analysis of BMMC expressing wild-type and dominant-negative mutant forms of Rac1 and JNK revealed that the Rac1/JNK pathway is critical for Kit ligand (KL)-induced proliferation of mast cells but not for suppression of apoptosis. In addition, KL was shown to inhibit sustained activation of JNK induced by gamma-irradiation and concomitant irradiation-induced apoptosis.

Involvement of phospholipase Cgamma1 in mouse egg activation induced by a truncated form of the C-kit tyrosine kinase present in spermatozoa

Microinjection of a truncated form of the c-kit tyrosine kinase present in mouse spermatozoa (tr-kit) activates mouse eggs parthenogenetically, and tr-kit- induced egg activation is inhibited by preincubation with an inhibitor of phospholipase C (PLC) (Sette, C., A. Bevilacqua, A. Bianchini, F. Mangia, R. Geremia, and P. Rossi. 1997. Development [Camb.]. 124:2267-2274). Co-injection of glutathione-S-transferase (GST) fusion proteins containing the src-homology (SH) domains of the gamma1 isoform of PLC (PLCgamma1) competitively inhibits tr-kit- induced egg activation. A GST fusion protein containing the SH3 domain of PLCgamma1 inhibits egg activation as efficiently as the whole SH region, while a GST fusion protein containing the two SH2 domains is much less effective. A GST fusion protein containing the SH3 domain of the Grb2 adaptor protein does not inhibit tr-kit-induced egg activation, showing that the effect of the SH3 domain of PLCgamma1 is specific. Tr-kit-induced egg activation is also suppressed by co-injection of antibodies raised against the PLCgamma1 SH domains, but not against the PLCgamma1 COOH-terminal region. In transfected COS cells, coexpression of PLCgamma1 and tr-kit increases diacylglycerol and inositol phosphate production, and the phosphotyrosine content of PLCgamma1 with respect to cells expressing PLCgamma1 alone. These data indicate that tr-kit activates PLCgamma1, and that the SH3 domain of PLCgamma1 is essential for tr-kit-induced egg activation.

Nf1 regulates hematopoietic progenitor cell growth and ras signaling in response to multiple cytokines

Neurofibromin, the protein encoded by the NF1 tumor-suppressor gene, negatively regulates the output of p21(ras) (Ras) proteins by accelerating the hydrolysis of active Ras-guanosine triphosphate to inactive Ras-guanosine diphosphate. Children with neurofibromatosis type 1 (NF1) are predisposed to juvenile chronic myelogenous leukemia (JCML) and other malignant myeloid disorders, and heterozygous Nf1 knockout mice spontaneously develop a myeloid disorder that resembles JCML. Both human and murine leukemias show loss of the normal allele. JCML cells and Nf1-/- hematopoietic cells isolated from fetal livers selectively form abnormally high numbers of colonies derived from granulocyte-macrophage progenitors in cultures supplemented with low concentrations of granulocyte-macrophage colony stimulating factor (GM-CSF). Taken together, these data suggest that neurofibromin is required to downregulate Ras activation in myeloid cells exposed to GM-CSF. We have investigated the growth and proliferation of purified populations of hematopoietic progenitor cells isolated from Nf1 knockout mice in response to the cytokines interleukin (IL)-3 and stem cell factor (SCF), as well as to GM-CSF. We found abnormal proliferation of both immature and lineage-restricted progenitor populations, and we observed increased synergy between SCF and either IL-3 or GM-CSF in Nf1-/- progenitors. Nf1-/- fetal livers also showed an absolute increase in the numbers of immature progenitors. We further demonstrate constitutive activation of the Ras-Raf-MAP (mitogen-activated protein) kinase signaling pathway in primary c-kit+ Nf1-/- progenitors and hyperactivation of MAP kinase after growth factor stimulation. The results of these experiments in primary hematopoietic cells implicate Nf1 as playing a central role in regulating the proliferation and survival of primitive and lineage-restricted myeloid progenitors in response to multiple cytokines by modulating Ras output.

SHP-1 binds and negatively modulates the c-Kit receptor by interaction with tyrosine 569 in the c-Kit juxtamembrane domain

The SH2 domain-containing SHP-1 tyrosine phosphatase has been shown to negatively regulate a broad spectrum of growth factor- and cytokine-driven mitogenic signaling pathways. Included among these is the cascade of intracellular events evoked by stem cell factor binding to c-Kit, a tyrosine kinase receptor which associates with and is dephosphorylated by SHP-1. Using a series of glutathione S-transferase (GST) fusion proteins containing either tyrosine-phosphorylated segments of the c-Kit cytosolic region or the SH2 domains of SHP-1, we have shown that SHP-1 interacts with c-Kit by binding selectively to the phosphorylated c-Kit juxtamembrane region and that the association of c-Kit with the larger of the two SHP-1 isoforms may be mediated through either the N-terminal or C-terminal SHP-1 SH2 domain. The results of binding assays with mutagenized GST-Kit juxtamembrane fusion proteins and competitive inhibition assays with phosphopeptides encompassing each c-Kit juxtamembrane region identified the tyrosine residue at position 569 as the major site for binding of SHP-1 to c-Kit and suggested that tyrosine 567 contributes to, but is not required for, this interaction. By analysis of Ba/F3 cells retrovirally transduced to express c-Kit receptors, phenylalanine substitution of c-Kit tyrosine residue 569 was shown to be associated with disruption of c-Kit-SHP-1 binding and induction of hyperproliferative responses to stem cell factor. Although phenylalanine substitution of c-Kit tyrosine residue 567 in the Ba/F3-c-Kit cells did not alter SHP-1 binding to c-Kit, the capacity of a second c-Kit-binding tyrosine phosphatase, SHP-2, to associate with c-Kit was markedly reduced, and the cells again showed hyperproliferative responses to stem cell factor. These data therefore identify SHP-1 binding to tyrosine 569 on c-Kit as an interaction pivotal to SHP-1 inhibitory effects on c-Kit signaling, but they indicate as well that cytosolic protein tyrosine phosphatases other than SHP-1 may also negatively regulate the coupling of c-Kit engagement to proliferation.

Structural determinants of SHP-2 function and specificity in Xenopus mesoderm induction

SHP-2 is a positive component of many receptor tyrosine kinase signaling pathways. The related protein-tyrosine phosphatase (PTP) SHP-1 usually acts as a negative regulator. The precise domains utilized by SHP-2 to transmit positive signals in vivo and the basis for specificity between SHP-1 and SHP-2 are not clear. In Xenopus, SHP-2 is required for mesoderm induction and completion of gastrulation. We investigated the effects of SHP-2 mutants and SHP-2/SHP-1 chimeras on basic fibroblast growth factor-induced mesoderm induction. Both SH2 domains and the PTP domain are required for normal SHP-2 function in this pathway. The N-terminal SH2 domain is absolutely required, whereas the C-terminal SH2 contributes to wild-type function. The C-terminal tyrosyl phosphorylation sites and proline-rich region are dispensable, arguing against adapter models of SHP-2 function. Although the SH2 domains contribute to SHP-2 specificity, studies of SHP chimeras reveal that substantial specificity resides in the PTP domain. Thus, PTP domains exhibit biologically relevant specificity in vivo, and noncatalytic and catalytic domains of PTPs contribute to specificity in a combinatorial fashion.

The road less travelled: c-kit and stem cell factor

Autocrine stimulation of growth factor receptors by autonomously produced ligands regulates different aspects of cellular transformation and progression. In several tumors, including gliomas, multiple autocrine systems are activated and may exert different functions in the malignant transformation process. The c-kit proto-oncogene is widely expressed in human gliomas, and it may be activated by its co-expressed ligand, stem cell factor (SCF). Studies in glioma cell lines as well as different tumor types suggest the possibility of intracellular interactions of c-kit with SCF. Although c-kit and SCF may not play a primary and causal role in the initiation and progression of glial tumors they may still be contributing factors in glioma biology. It can be hypothesized that the parallel activation of several autocrine systems including some of which have found less attention in gliomas, such as c-kit/SCF, could compromise the efficacy of therapies targeting different autocrine loops. A better understanding of the multiplicity and mechanisms of autocrine stimulation has implications for the development of new therapies interfering with autocrine tumor cell growth.

c-kit receptor signaling through its phosphatidylinositide-3'-kinase-binding site and protein kinase C: role in mast cell enhancement of degranulation, adhesion, and membrane ruffling

In bone marrow-derived mast cells (BMMCs), the Kit receptor tyrosine kinase mediates diverse responses including proliferation, survival, chemotaxis, migration, differentiation, and adhesion to extracellular matrix. In connective tissue mast cells, a role for Kit in the secretion of inflammatory mediators has been demonstrated as well. We recently demonstrated a role for phosphatidylinositide-3' (PI 3)-kinase in Kit-ligand (KL)-induced adhesion of BMMCs to fibronectin. Herein, we investigated the mechanism by which Kit mediates enhancement of Fc epsilon RI-mediated degranulation, cytoskeletal rearrangements, and adhesion in BMMCs. Wsh/Wsh BMMCs lacking endogenous Kit expression, were transduced to express normal and mutant Kit receptors containing Tyr-->Phe substitution at residues 719 and 821. Although the normal Kit receptor fully restored KL-induced responses in Wsh/Wsh BMMCs, Kit gamma 719F, which fails to bind and activate PI 3-kinase, failed to potentiate degranulation and is impaired in mediating membrane ruffling and actin assembly. Inhibition of PI 3-kinase with wortmannin or LY294002 also inhibited secretory enhancement and cytoskeletal rearrangements mediated by Kit. In contrast, secretory enhancement and adhesion stimulated directly through protein kinase C (PKC) do not require PI 3-kinase. Calphostin C, an inhibitor of PKC, blocked Kit-mediated adhesion to fibronectin, secretory enhancement, membrane ruffling, and filamentous actin assembly. Although cytochalasin D inhibited Kit-mediated filamentous actin assembly and membrane ruffling, secretory enhancement and adhesion to fibronectin were not affected by this drug. Therefore, Kit-mediated cytoskeletal rearrangements that are dependent on actin polymerization can be uncoupled from the Kit-mediated secretory and adhesive responses. Our results implicate receptor-proximal PI 3-kinase activation and activation of a PKC isoform in Kit-mediated secretory enhancement, adhesion, and cytoskeletal reorganization.

Characterization of a mast cell line that lacks the extracellular domain of membrane c-kit

Expression of the c-kit proto-oncogene receptor on mast cells is essential for their normal proliferation and maturation as well as for several biological responses such as chemotaxis and attachment. In the present study we report that the interleukin-3 (IL-3)-dependent mast cell line CFTL-15 lacks the extracellular domain of the c-kit receptor. This observation was made after noting that the c-kit ligand stem cell factor (SCF) could not prevent IL-3 deprivation-induced mast cell apoptosis and that CFTL-15 cells did not proliferate in response to SCF. Flow cytometric analysis employing monoclonal anti-c-kit antibodies, and immunogold labelling with analysis by electron microscopy, subsequently showed a diminished expression of c-kit on CFTL-15 cells. There was no identifiable message for the extracellular domain of c-kit in these cells, as determined by reverse transcriptase-polymerase chain reaction (RT-PCR). These previously unrecognized properties of the CFTL-15 mast cell line allowed the examination of other biological consequences of the lack of c-kit on mast cells. Analysing the ability of these cells to adhere to surface-bound fibronectin, it was found that addition of SCF did not increase their adhesion to this substrate, in opposition to what is reported with other mast cells. Similarly, CFTL-15 mast cells did not adhere to fibroblasts, which is known to require c-kit expression. Also, there was no protein tyrosine phosphorylation in these cells in response to SCF. CFTL-15 cells underwent apoptosis on removal of IL-3 coincident with a decrease in endogenous Bcl-2 mRNA. Overexpression of Bcl-2 cDNA prolonged survival of Bcl-2-transfected CFTL-15 cells upon withdrawal of IL-3. Thus, the CFTL-15 cell line that lacks surface c-kit is not able to proliferate in response to SCF, undergoes apoptosis in the presence of SCF, and does not adhere to fibroblasts. These results confirm earlier studies on the functional consequences of c-kit and provide a novel experimental model for further investigation.

Oncogenic mutation in the Kit receptor tyrosine kinase alters substrate specificity and induces degradation of the protein tyrosine phosphatase SHP-1

Activating mutations in the Kit receptor tyrosine kinase have been identified in both rodent and human mast cell leukemia. One activating Kit mutation substitutes a valine for aspartic acid at codon 816 (D816V) and is frequently observed in human mastocytosis. Mutation at the equivalent position in the murine c-kit gene, involving a substitution of tyrosine for aspartic acid (D814Y), has been described in the mouse mastocytoma cell line P815. We have investigated the mechanism of oncogenic activation by this mutation. Expression of this mutant Kit receptor tyrosine kinase in a mast cell line led to the selective tyrosine phosphorylation of a 130-kDa protein and the degradation, through the ubiquitin-dependent proteolytic pathway, of a 65-kDa phosphoprotein. The 65-kDa protein was identified as the src homology domain 2 (SH2)-containing protein tyrosine phosphatase SHP-1, a negative regulator of signaling by Kit and other hematopoietic receptors, and the protein product of the murine motheaten locus. This mutation also altered the sites of receptor autophosphorylation and peptide substrate selectivity. Thus, this mutation activates the oncogenic potential of Kit by a novel mechanism involving an alteration in Kit substrate recognition and the degradation of SHP-1, an attenuator of the Kit signaling pathway.

Hepatocyte growth factor induces proliferation and differentiation of multipotent and erythroid hemopoietic progenitors

Hepatocyte growth factor (HGF) is a mesenchymal derived growth factor known to induce proliferation and "scattering" of epithelial and endothelial cells. Its receptor is the tyrosine kinase encoded by the c-MET protooncogene. Here we show that highly purified recombinant HGF stimulates hemopoietic progenitors to form colonies in vitro. In the presence of erythropoietin, picomolar concentrations of HGF induced the formation of erythroid burst-forming unit colonies from CD34-positive cells purified from human bone marrow, peripheral blood, or umbilical cord blood. The growth stimulatory activity was restricted to the erythroid lineage. HGF also stimulated the formation of multipotent CFU-GEMM colonies. This effect is synergized by stem cell factor, the ligand of the tyrosine kinase receptor encoded by the c-KIT protooncogene, which is active on early hemopoietic progenitors. By flow cytometry analysis, the receptor for HGF was found to be expressed on the cell surface in a fraction of CD34+ progenitors. Moreover, in situ hybridization experiments showed that HGF receptor mRNA is highly expressed in embryonic erythroid cells (megaloblasts). HGF mRNA was also found to be produced in the embryonal liver. These data show that HGF plays a direct role in the control of proliferation and differentiation of erythroid progenitors, and they suggest that it may be one of the long-sought mediators of paracrine interactions between stromal and hemopoietic cells within the hemopoietic microenvironment.

Cytokine-mediated protein kinase C activation is a signal for lineage determination in bipotential granulocyte macrophage colony-forming cells

Granulocyte macrophage colony-forming cells (GM-CFC) have the potential to develop into either macrophages and/or neutrophils. With a highly enriched population of these cells we have found that although GM-CFC are equally responsive to macrophage colony stimulating factor (M-CSF) and stem cell factor (SCF) in terms of DNA synthesis, M-CSF stimulated the development of colonies containing macrophages in soft gel assays, while SCF promoted neutrophilic colony formation. When SCF and M-CSF were combined, mainly macrophage development was stimulated both in soft agar colony-forming assays and liquid cultures. An analysis of some potential signaling mechanisms associated with cytokine-mediated developmental decisions in GM-CFC revealed that M-CSF, but not SCF, was able to chronically stimulate phosphatidylcholine breakdown and diacylglycerol production, indicating that protein kinase C (PKC) may be involved in the action of M-CSF. Furthermore, M-CSF, but not SCF, can increase the levels of PKC alpha (PKC alpha) expression and stimulate the translocation of PKC alpha to the nucleus. When the PKC inhibitor, calphostin C, was added to GM-CFC cultured in M-CSF then predominantly neutrophils were produced, conversely PKC activators added with SCF stimulated macrophage development. The data indicate a role for PKC in M-CSF-stimulated macrophage development from GM-CFC.

Direct association of p110 beta phosphatidylinositol 3-kinase with p85 is mediated by an N-terminal fragment of p110 beta

Phosphatidylinositol (PI) 3-kinase is a heterodimeric enzyme of 85-kDa (p85) and 110-kDa (p110) subunits implicated in mitogenic signal transduction by virtue of its activation in cells transformed by diverse viral oncoproteins and treated with various growth factors. We have identified a domain in p110 that mediates association with p85 in vitro and in intact cells. A glutathione S-transferase fusion protein containing the N-terminal 171 amino-acids of p110 beta bound to free p85 in cell lysates. This fusion protein also bound directly to p85 immobilized on nitrocellulose filters. An epitope-tagged fragment containing amino acids 31 to 150 of p110 beta associated with p85 upon expression in intact cells. Expression of either an N-terminal fragment of p110 beta or the p85 inter-SH2 domain, which mediates association with p110, reduced the association of endogenous PI 3-kinase activity with the activated platelet-derived growth factor receptor in intact cells. Hence, these defined regions of p85 and p110 mediate the interaction between the two subunits of PI 3-kinase.

Direct association of p110 beta phosphatidylinositol 3-kinase with p85 is mediated by an N-terminal fragment of p110 beta

Phosphatidylinositol (PI) 3-kinase is a heterodimeric enzyme of 85-kDa (p85) and 110-kDa (p110) subunits implicated in mitogenic signal transduction by virtue of its activation in cells transformed by diverse viral oncoproteins and treated with various growth factors. We have identified a domain in p110 that mediates association with p85 in vitro and in intact cells. A glutathione S-transferase fusion protein containing the N-terminal 171 amino-acids of p110 beta bound to free p85 in cell lysates. This fusion protein also bound directly to p85 immobilized on nitrocellulose filters. An epitope-tagged fragment containing amino acids 31 to 150 of p110 beta associated with p85 upon expression in intact cells. Expression of either an N-terminal fragment of p110 beta or the p85 inter-SH2 domain, which mediates association with p110, reduced the association of endogenous PI 3-kinase activity with the activated platelet-derived growth factor receptor in intact cells. Hence, these defined regions of p85 and p110 mediate the interaction between the two subunits of PI 3-kinase.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

Identification of mutations in the coding sequence of the proto-oncogene c-kit in a human mast cell leukemia cell line causing ligand-independent activation of c-kit product

The c-kit proto-oncogene encodes a receptor tyrosine kinase. Binding of c-kit ligand, stem cell factor (SCF) to c-kit receptor (c-kitR) is known to activate c-kitR tyrosine kinase, thereby leading to autophosphorylation of c-kitR on tyrosine and to association of c-kitR with substrates such as phosphatidylinositol 3-kinase (PI3K). In a human mast cell leukemia cell line HMC-1, c-kitR was found to be constitutively phosphorylated on tyrosine, activated, and associated with PI3K without the addition of SCF. The expression of SCF mRNA transcript in HMC-1 cells was not detectable by means of PCR after reverse transcription (RT-PCR) analysis, suggesting that the constitutive activation of c-kitR was ligand independent. Sequencing of whole coding region of c-kit cDNA revealed that c-kit genes of HMC-1 cells were composed of a normal, wild-type allele and a mutant allele with two point mutations resulting in intracellular amino acid substitutions of Gly-560 for Val and Val-816 for Asp. Amino acid sequences in the regions of the two mutations are completely conserved in all of mouse, rat, and human c-kit. In order to determine the causal role of these mutations in the constitutive activation, murine c-kit mutants encoding Gly-559 and/or Val-814, corresponding to human Gly-560 and/or Val-816, were constructed by site-directed mutagenesis and expressed in a human embryonic kidney cell line, 293T cells. In the transfected cells, both c-kitR (Gly-559, Val-814) and c-kitR (Val-814) were abundantly phosphorylated on tyrosine and activated in immune complex kinase reaction in the absence of SCF, whereas tyrosine phosphorylation and activation of c-kitR (Gly-559) or wild-type c-kitR was modest or little, respectively. These results suggest that conversion of Asp-816 to Val in human c-kitR may be an activating mutation and responsible for the constitutive activation of c-kitR in HMC-1 cells.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

The v-Src SH3 domain binds phosphatidylinositol 3'-kinase

Fibroblasts transformed by v-src or by related oncogenes encoding activated tyrosine kinases contain elevated levels of polyphosphoinositides with phosphate at the D-3 position of the inositol ring, as a result of the activation of phosphatidylinositol (PI) 3'-kinase. v-src-transformed cells also contain increased levels of PI 3'-kinase activity immunoprecipitable with anti-phosphotyrosine antibodies; furthermore, PI 3'-kinase can be detected in association with the v-Src tyrosine kinase. To identify regions of v-Src that can interact with PI 3'-kinase, the v-Src SH2 and SH3 domains were expressed in bacteria and incubated with lysates of normal chicken embryo fibroblasts. In vitro, the v-Src SH3 domain, but not the SH2 domain, bound PI 3'-kinase in lysates of uninfected chicken embryo fibroblasts. Substitutions of two highly conserved SH3 residues implicated in ligand binding abolished the ability of the v-Src SH3 domain to associate with PI 3'-kinase. Furthermore, the v-Src SH3 domain bound in vitro to the amino-terminal region of the p85 alpha subunit of PI 3'-kinase. These results suggest that the v-Src SH3 domain may mediate an interaction between the v-Src tyrosine kinase and PI 3'-kinase, by direct binding to p85.

The v-Src SH3 domain binds phosphatidylinositol 3'-kinase

Fibroblasts transformed by v-src or by related oncogenes encoding activated tyrosine kinases contain elevated levels of polyphosphoinositides with phosphate at the D-3 position of the inositol ring, as a result of the activation of phosphatidylinositol (PI) 3'-kinase. v-src-transformed cells also contain increased levels of PI 3'-kinase activity immunoprecipitable with anti-phosphotyrosine antibodies; furthermore, PI 3'-kinase can be detected in association with the v-Src tyrosine kinase. To identify regions of v-Src that can interact with PI 3'-kinase, the v-Src SH2 and SH3 domains were expressed in bacteria and incubated with lysates of normal chicken embryo fibroblasts. In vitro, the v-Src SH3 domain, but not the SH2 domain, bound PI 3'-kinase in lysates of uninfected chicken embryo fibroblasts. Substitutions of two highly conserved SH3 residues implicated in ligand binding abolished the ability of the v-Src SH3 domain to associate with PI 3'-kinase. Furthermore, the v-Src SH3 domain bound in vitro to the amino-terminal region of the p85 alpha subunit of PI 3'-kinase. These results suggest that the v-Src SH3 domain may mediate an interaction between the v-Src tyrosine kinase and PI 3'-kinase, by direct binding to p85.

Association of hematopoietic cell phosphatase with c-Kit after stimulation with c-Kit ligand

Protein tyrosine phosphorylation and dephosphorylation have been implicated in the growth and functional responses of hematopoietic cells. Recent studies have identified a novel protein tyrosine phosphatase, termed hematopoietic cell phosphatase (HCP) or PTP1C, that is predominantly expressed in hematopoietic cells. HCP encodes a cytoplasmic phosphatase that contains two src homology 2 (SH2) domains. Since SH2 domains have been shown to target the association of signal-transducing molecules with activated growth factor receptors containing intrinsic protein kinase activity, we assessed the association of HCP with two hematopoietic growth factor receptors, c-Kit and c-Fms. The results demonstrate that HCP transiently associates with ligand-activated c-Kit but not c-Fms and that this association occurs through the SH2 domains. In both colony-stimulating factor 1- and stem cell factor-stimulated cells, there is a marginal increase in tyrosine phosphorylation of HCP. Lastly, HCP can dephosphorylate autophosphorylated c-Kit and c-Fms in in vitro reactions. The potential role of HCP in stem cell factor signal transduction is discussed.

Association of hematopoietic cell phosphatase with c-Kit after stimulation with c-Kit ligand

Protein tyrosine phosphorylation and dephosphorylation have been implicated in the growth and functional responses of hematopoietic cells. Recent studies have identified a novel protein tyrosine phosphatase, termed hematopoietic cell phosphatase (HCP) or PTP1C, that is predominantly expressed in hematopoietic cells. HCP encodes a cytoplasmic phosphatase that contains two src homology 2 (SH2) domains. Since SH2 domains have been shown to target the association of signal-transducing molecules with activated growth factor receptors containing intrinsic protein kinase activity, we assessed the association of HCP with two hematopoietic growth factor receptors, c-Kit and c-Fms. The results demonstrate that HCP transiently associates with ligand-activated c-Kit but not c-Fms and that this association occurs through the SH2 domains. In both colony-stimulating factor 1- and stem cell factor-stimulated cells, there is a marginal increase in tyrosine phosphorylation of HCP. Lastly, HCP can dephosphorylate autophosphorylated c-Kit and c-Fms in in vitro reactions. The potential role of HCP in stem cell factor signal transduction is discussed.

Reversible expansion of primate mast cell populations in vivo by stem cell factor

Mast cell development in mice is critically regulated by stem cell factor (SCF), the term used here to designate a product of fibroblasts and other cell types that is a ligand for the tyrosine kinase receptor protein encoded by the proto-oncogene c-kit. However, the factors which regulate the size of mast cell populations in primates are poorly understood. Here we report that the subcutaneous administration of recombinant human SCF (rhSCF) to baboons (Papio cynocephalus) or cynomolgus monkeys (Macaca fascicularis) produced a striking expansion of mast cell populations in many anatomical sites, with numbers of mast cells in some organs of rhSCF-treated monkeys exceeding the corresponding values in control monkeys by more than 100-fold. Animals treated with rhSCF did not exhibit clinical evidence of mast cell activation, and discontinuation of treatment with rhSCF resulted in a rapid decline of mast cell numbers nearly to baseline levels. These findings are the first to demonstrate that a specific cytokine can regulate mast cell development in primates in vivo. They also provide the first evidence, in any mammalian species, to indicate that the cytokine-dependent expansion of tissue mast cell populations can be reversed when administration of the cytokine is discontinued.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

c-Kit-kinase induces a cascade of protein tyrosine phosphorylation in normal human melanocytes in response to mast cell growth factor and stimulates mitogen-activated protein kinase but is down-regulated in melanomas

The proto-oncogene c-Kit, a transmembrane receptor tyrosine kinase, is an important regulator of cell growth whose constitutively active oncogenic counterpart, v-kit, induces sarcomas in cats. Mutations in murine c-kit that reduce the receptor tyrosine kinase activity cause deficiencies in the migration and proliferation of melanoblasts, hematopoietic stem cells, and primordial germ cells. We therefore investigated whether c-Kit regulates normal human melanocyte proliferation and plays a role in melanomas. We show that normal human melanocytes respond to mast cell growth factor (MGF), the Kit-ligand that stimulates phosphorylation of tyrosyl residues in c-Kit and induces sequential phosphorylation of tyrosyl residues in several other proteins. One of the phosphorylated intermediates in the signal transduction pathway was identified as an early response kinase (mitogen-activated protein [MAP] kinase). Dephosphorylation of a prominent 180-kDa protein suggests that MGF also activates a phosphotyrosine phosphatase. In contrast, MGF did not induce proliferation, the cascade of protein phosphorylations, or MAP kinase activation in the majority of cells cultured from primary nodular and metastatic melanomas that grow independently of exogenous factors. In the five out of eight human melanoma lines expressing c-kit mRNAs, c-Kit was not constitutively activated. Therefore, although c-Kit-kinase is a potent growth regulator of normal human melanocytes, its activity is not positively associated with malignant transformation.

SH2 domains of the p85 alpha subunit of phosphatidylinositol 3-kinase regulate binding to growth factor receptors

The binding of cytoplasmic signaling proteins such as phospholipase C-gamma 1 and Ras GTPase-activating protein to autophosphorylated growth factor receptors is directed by their noncatalytic Src homology region 2 (SH2) domains. The p85 alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase, which associates with several receptor protein-tyrosine kinases, also contains two SH2 domains. Both p85 alpha SH2 domains, when expressed individually as fusion proteins in bacteria, bound stably to the activated beta receptor for platelet-derived growth factor (PDGF). Complex formation required PDGF stimulation and was dependent on receptor tyrosine kinase activity. The bacterial p85 alpha SH2 domains recognized activated beta PDGF receptor which had been immobilized on a filter, indicating that SH2 domains contact autophosphorylated receptors directly. Several receptor tyrosine kinases within the PDGF receptor subfamily, including the colony-stimulating factor 1 receptor and the Steel factor receptor (Kit), also associate with PI 3-kinase in vivo. Bacterially expressed SH2 domains derived from the p85 alpha subunit of PI 3-kinase bound in vitro to the activated colony-stimulating factor 1 receptor and to Kit. We infer that the SH2 domains of p85 alpha bind to high-affinity sites on these receptors, whose creation is dependent on receptor autophosphorylation. The SH2 domains of p85 are therefore primarily responsible for the binding of PI 3-kinase to activated growth factor receptors.

SH2 domains of the p85 alpha subunit of phosphatidylinositol 3-kinase regulate binding to growth factor receptors

The binding of cytoplasmic signaling proteins such as phospholipase C-gamma 1 and Ras GTPase-activating protein to autophosphorylated growth factor receptors is directed by their noncatalytic Src homology region 2 (SH2) domains. The p85 alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase, which associates with several receptor protein-tyrosine kinases, also contains two SH2 domains. Both p85 alpha SH2 domains, when expressed individually as fusion proteins in bacteria, bound stably to the activated beta receptor for platelet-derived growth factor (PDGF). Complex formation required PDGF stimulation and was dependent on receptor tyrosine kinase activity. The bacterial p85 alpha SH2 domains recognized activated beta PDGF receptor which had been immobilized on a filter, indicating that SH2 domains contact autophosphorylated receptors directly. Several receptor tyrosine kinases within the PDGF receptor subfamily, including the colony-stimulating factor 1 receptor and the Steel factor receptor (Kit), also associate with PI 3-kinase in vivo. Bacterially expressed SH2 domains derived from the p85 alpha subunit of PI 3-kinase bound in vitro to the activated colony-stimulating factor 1 receptor and to Kit. We infer that the SH2 domains of p85 alpha bind to high-affinity sites on these receptors, whose creation is dependent on receptor autophosphorylation. The SH2 domains of p85 are therefore primarily responsible for the binding of PI 3-kinase to activated growth factor receptors.

p21ras activation via hemopoietin receptors and c-kit requires tyrosine kinase activity but not tyrosine phosphorylation of p21ras GTPase-activating protein

Products of the ras gene family, termed p21ras, are GTP-binding proteins that have been implicated in signal transduction via receptors encoding tyrosine kinase domains. Recent findings have defined a superfamily of hemopoietin receptors that includes receptors for a number of interleukins and colony-stimulating factors. The intracellular portions of these receptors show only restricted homologies, have no tyrosine kinase domain, and provide no clues to the mode of signal transduction. However, in most cases the factors stimulate tyrosine phosphorylation. We demonstrate here that ligand-induced activation of the interleukin (IL)-2, IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor receptors resulted in activation of p21ras in various hemopoietic cell lines. The only cytokine tested that binds to a hemopoietin receptor and that did not activate p21ras was IL-4. Activation of p21ras was also observed in response to Steel factor, which stimulates the endogenous tyrosine kinase activity of the c-kit receptor, as well as with phorbol esters, which activate protein kinase C. Experiments with protein kinase inhibitors implicated tyrosine kinase activity, but not protein kinase C activity, as the upstream signal in p21ras activation via these growth factor receptors. Attempts to demonstrate tyrosine phosphorylation of the p21ras GTPase-activating protein (GAP) were negative, suggesting that phosphorylation of GAP may not be the major mechanism for regulation of p21ras activity by tyrosine kinases.

Interkinase domain of kit contains the binding site for phosphatidylinositol 3' kinase

Our previous analysis of the signal transduction pathway used by the c-kit-encoded receptor for the stem cell factor (SCF) indicated efficient coupling to the type I phosphatidylinositol 3' kinase (PI3K). In an attempt to localize the receptor's site of interaction with PI3K, we separately deleted either the noncatalytic 68-amino-acid-long interkinase domain or the carboxyl-terminal portion distal to the catalytic sequences. Loss of ligand-induced association of PI3K with the former deletion mutant and retention of the PI3K association by the carboxyl-terminally deleted receptor implied interactions of PI3K with the kinase insert. This was further supported by partial inhibition of the association by an anti-peptide antibody directed against the kinase insert and lack of effect of an antibody directed to the carboxyl tail of the SCF receptor. A bacterially expressed kinase insert domain was used as a fusion protein to directly test its presumed function as a PI3K association site. This protein bound PI3K from cell lysate as demonstrated by PI3K activity and by an associated phosphoprotein of 85 kDa. The association was dependent on phosphorylation of the tyrosine residues on the expressed kinase insert. On the basis of these observations, we conclude that the kinase insert domain of the SCF receptor selectively interacts with the p85 regulatory subunit of PI3K and that this association requires phosphorylation of tyrosine residues in the kinase insert region, with apparently no involvement of the bulk cytoplasmic structure or tyrosine kinase function of the receptor.

The rat c-kit ligand, stem cell factor, induces c-kit receptor-dependent mouse mast cell activation in vivo. Evidence that signaling through the c-kit receptor can induce expression of cellular function

Interactions between products of the mouse W locus, which encodes the c- kit tyrosine kinase receptor, and the Sl locus, which encodes a ligand for c-kit receptor, which we have designated stem cell factor (SCF), have a critical role in the development of mast cells. Mice homozygous for mutations at either locus exhibit several phenotypic abnormalities including a virtual absence of mast cells. Moreover, the c-kit ligand SCF can induce the proliferation and maturation of normal mast cells in vitro or in vivo, and also can result in repair of the mast cell deficiency of Sl/Sld mice in vivo. We now report that administration of SCF intradermally in vivo results in dermal mast cell activation and a mast cell-dependent acute inflammatory response. This effect is c-kit receptor dependent, in that it is not observed when SCF is administered to mice containing dermal mast cells expressing functionally inactive c- kit receptors, is observed with both glycosylated and nonglycosylated forms of SCF, and occurs at doses of SCF at least 10-fold lower on a molar basis than the minimally effective dose of the classical dermal mast cell-activating agent substance P. These findings represent the first demonstration in vivo that a c-kit ligand can result in the functional activation of any cellular lineage expressing the c-kit receptor, and suggest that interactions between the c-kit receptor and its ligand may influence mast cell biology through complex effects on proliferation, maturation, and function.