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.

Differential use of BTK and PLC in FcεRI- and KIT-mediated mast cell activation: A marginal role of BTK upon KIT activation

In mast cells (MCs), the TEC family kinase (TFK) BTK constitutes a central regulator of antigen (Ag)-triggered, FcεRI-mediated PLCγ phosphorylation, Ca²⁺ mobilization, degranulation, and pro-inflammatory cytokine production. Less is known about the function of BTK in the context of stem cell factor (SCF)-induced KIT signaling. In bone marrow-derived MCs (BMMCs), Ag stimulation caused intense phosphorylation of BTK at Y551 in its active center and at Y223 in its SH3-domain, whereas in response to SCF only Y223 was significantly phosphorylated. Further data using the TFK inhibitor Ibrutinib indicated that BTK Y223 is phosphorylated by a non-BTK TFK upon SCF stimulation. In line, SCF-induced PLCγ1 phosphorylation was stronger attenuated by Ibrutinib than by BTK deficiency. Subsequent pharmacological analysis of PLCγ function revealed a total block of SCF-induced Ca²⁺ mobilization by PLC inhibition, whereas only the sustained phase of Ca²⁺ flux was curtailed in Ag-stimulated BMMCs. Despite this severe stimulus-dependent difference in inducing Ca²⁺ mobilization, PLCγ inhibition suppressed Ag- and SCF-induced degranulation and pro-inflammatory cytokine production to comparable extents, suggesting involvement of additional TFK(s) or PLCγ-dependent signaling components. In addition to PLCγ, the MAPKs p38 and JNK were activated by Ag in a BTK-dependent manner; this was not observed upon SCF stimulation. Hence, FcεRI and KIT employ different mechanisms for activating PLCγ, p38, and JNK, which might strengthen their cooperation regarding pro-inflammatory MC effector functions. Importantly, our data clearly demonstrate that analyzing BTK Y223 phosphorylation is not sufficient to prove BTK activation.

Cytokine Receptor Signal Transduction Mechanisms in Immuno-Hematopoietic Cells

Novel aspects of cytokine receptor signal transduction are discussed and cytokine receptors classified based on ligand-dependent signalling. An introductory section presents an overview of the role of cytokines in hematopoiesis. A brief explanation of basic concepts, such as redundancy, pleiotropy, synergism, etc., important for the understanding of cell response to cytokines, is also included. Three of five classes of receptors show the involvement of tyrosine kinase activity as the key step in signal transduction. The importance of tyrosine phosphorylation in cellular response to cytokines is pointed out.

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.

Novel somatic KIT exon 8 mutation with dramatic response to imatinib in a patient with mucosal melanoma: a case report

Primary mucosal melanomas represent ∼1.3% of all cases of melanoma diagnosed in the USA. The sinonasal location is the most common primary site. Mutations in the KIT gene occur in 10-22% of mucosal melanomas. Tumor response to imatinib mesylate has been reported in about half of the patients with tumors harboring KIT mutations. Responses are almost exclusively restricted to tumors with mutations in KIT exon 9 or 11. We report a case of a patient with a sinonasal mucosal melanoma with a novel exon 8 mutation (C443S) who had marked initial response to imatinib. Somatic exon 8 KIT mutations have not been previously reported in mucosal melanoma or in other human solid tumors; however, such mutations have been reported in canine and feline mast cell tumors. Protein transcripts from exon 8 play an important role in the structural and functional integrity of the extracellular domain of KIT. In preclinical studies, a mutation in exon 8 led to autophosphorylation, independent of KIT ligand, and constitutive activation of the tyrosine kinase. This biology may explain the successful application of imatinib in animals with tumors harboring exon 8 KIT mutations and in our patient with mucosal melanoma. This report expands the population of patients with melanoma who might benefit from imatinib to those with somatic exon 8 KIT mutations. Such mutations should be looked for in patients with mucosal melanoma.

Genetic variation in AKT1, PTEN and the 8q24 locus, and the risk of testicular germ cell tumor

STUDY QUESTION

Is there an association between testicular germ cell tumor (TGCT) and genetic polymorphisms in AKT1, PTEN and the 8q24 locus?

SUMMARY ANSWER

Our findings suggest that genetic variation in PTEN may influence the risk of TGCT.

WHAT IS KNOWN ALREADY

There is strong evidence that genetic variation influences the risk of TGCT. The oncogene, AKT1, the tumor suppressor gene, PTEN and the chromosome 8q24 locus play important roles in cancer development in general.

STUDY DESIGN, SIZE, DURATION

We have conducted a population-based Norwegian-Swedish case-parent study, based on cases diagnosed in 1990-2008, including 831 triads (TGCT case and both parents), 474 dyads (TGCT case and one parent) and 712 singletons (only the TGCT case). In addition we expanded the study to include 3922 unrelated male controls from the TwinGene project.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We genotyped 26 single nucleotide polymorphisms (SNPs) in AKT1, PTEN and the 8q24 locus. First, triads and dyads were included in a likelihood-based association test. To increase the statistical power, case singletons and controls from the TwinGene project were included in a single test for association. We examined if the allelic effect on TGCT risk differed by histological subgroup, country of origin or parent of origin. Odds ratios (ORs) and 95% confidence intervals (CI) were calculated with Bonferroni correction (P bonf) for multiple testing.

MAIN RESULTS AND THE ROLE OF CHANCE

In the case-parent analyses, none of the 26 SNPs were significantly associated with TGCT. Of the 23 SNPs investigated in the combined study, one SNP in PTEN (rs11202586) remained associated with TGCT risk after adjusting for multiple testing (OR = 1.16, 95% CI = 1.06-1.28, P bonf = 0.040). We found no difference in risk according to histological subgroup, parent of origin or between countries.

LIMITATIONS, REASONS FOR CAUTION

Our study is strengthened by the population-based design and large sample size, which gives high power to detect risk alleles. The reported association was not highly significant, and although it was based on an a priori hypothesis of this tumor suppressor gene being implicated in the etiology of TGCT, replication studies, as well as functional studies of this polymorphism, are warranted.

WIDER IMPLICATIONS OF THE FINDINGS

We report, to our knowledge, a novel association between TGCT and a marker in the tumor suppressor gene PTEN. Previous studies have linked PTEN to TGCT etiology, and there is also a link between PTEN and KITLG, which contains TGCT susceptibility loci revealed through recent genome-wide studies.

Cell cycle control of germ cell differentiation

The germ cell lineage is our lifelong reservoir of reproductive stem cells and our mechanism for transmitting genes to future generations. These highly specialised cells are specified early during development and then migrate to the embryonic gonads where sex differentiation occurs. Germ cell sex differentiation is directed by the somatic gonadal environment and is characterised by two distinct cell cycle states that are maintained until after birth. In the mouse, XY germ cells in a testis cease mitotic proliferation and enter G(1)/G(0) arrest from 12.5 dpc, while XX germ cells in an ovary enter prophase I of meiosis from 13.5 dpc. This chapter discusses the factors known to control proliferation and survival of germ cells during their journey of specification to sex differentiation during development.

The interstitial cells of Cajal of the equine gastrointestinal tract: what we know so far

Gastrointestinal motility disorders are a serious problem in both veterinary and human medicine and may represent a dysfunction of the neural, muscular or pacemaker components (interstitial cells of Cajal) of bowel control. The interstitial cells of Cajal are considered to be the pacemakers and mediators of certain forms of neurotransmission in the gastrointestinal tract. These cells have been implicated, either primarily or secondarily, in the pathogenesis of gastrointestinal disease processes in which there is a prominent element of disturbance to intestinal motility. In the horse, their involvement has been implicated in large intestinal obstructive colic and grass sickness (equine dysautonomia). This review highlights the properties of the interstitial cells of Cajal and the role these cells play in orchestrating gastrointestinal motility patterns. In addition, it examines their role in intestinal motility disorders and summarises our current understanding of their importance in the equine gastrointestinal tract.

Targeted therapy for gastrointestinal stromal tumors: current status and future perspectives

Gastrointestinal stromal tumors (GISTs) present 80% of gastrointestinal tract mesenchymal tumors, with systemic chemotherapy and radiotherapy being unable to improve survival of patients with advanced disease. The identification of activating mutations in either KIT cell surface growth factor receptor or platelet-derived growth factor receptor alpha, which lead to ligand-independent signal transduction, paved the way for the development of novel agents that selectively inhibit key molecular events in disease pathogenesis. The development of imatinib mesylate in the treatment of metastatic GIST represents a therapeutic breakthrough in molecularly targeted strategies, which crucially improved patients' prognosis while its usefulness in adjuvant and neoadjuvant setting is under study. Sunitinib malate is available in the second-line setting, with ongoing studies evaluating its role in an earlier disease stage, while other targets are under intense investigation in order to enrich the therapeutical armamentarium for this disease. GIST phenotype seems to be an essential indicator of treatment response; thus, obtaining genotype information of each patient may be critical in order to tailor individualized treatment strategies and achieve maximal therapeutic results.

Homocysteine-induced oxidative stress upregulates chymase in mouse mastocytoma cells

Reactive oxygen species (ROS) such as hydrogen peroxide (H(2)O(2)), O(*-)(2) and OH(*) participate in the pathogenesis of ischemia/reperfusion injury, inflammation and atherosclerosis. Our previous studies have suggested that increased angiotensin II (Ang II)-forming chymase may be involved in the development of atherosclerosis. However, the regulatory mechanism of chymase expression has not yet been clarified. In this study, we tested whether oxidative stress upregulates mouse mast cell proteinase chymase, mouse mast cell proteinase (MMCP)-5 or MMCP-4. We also examined the expression and activity of these proteins after treatment. Cultured mouse mastocytoma cells (MMC) displaying chymase-dependent Ang II-forming activity were treated with H(2)O(2) and several aminothiols with or without anti-oxidants. The levels of MMCP-5 and MMCP-4 expression were determined by quantitative RT-PCR; the level of chymase-dependent Ang II-forming activity was measured by high performance liquid chromatography using Ang I as a substrate. Treatment of MMC with homocysteine (0.1-3 mmol l(-1)) significantly increased MMCP-5 and MMCP-4 expression, as well as Ang II-forming activity. These effects were significantly inhibited by the addition of catalase and further suppressed by the combination of catalase and superoxide dismutase. Incubation with hydrogen peroxide alone caused a significant increase in Ang II-forming activity, which was completely suppressed by co-treatment with catalase. Furthermore, MMCP-5 and MMCP-4 expression levels were drastically suppressed and chymase induction by homocysteine was diminished under the GATA-inhibited condition. Homocysteine increased mast cell chymase expression and activity through the mechanism of oxidative stress. Our results suggest that there is a biochemical link between oxidative stress and the local Ang II-forming system.

Expression of KIT Receptor in Feline Cutaneous Mast Cell Tumors

Twenty-seven feline cutaneous mast cell tumors (MCTs) were selected for this retrospective study. Samples were routinely processed and stained with hematoxylin and eosin (HE) and toluidine blue, and tumors were classified as well-differentiated (19/27), atypical or poorly granulate (7/27), and pleomorphic (1/27). Immunohistochemistry to detect KIT protein was performed on all samples. The immunoreactivity was recorded by distribution within the tumor, cellular location, and intensity. Well-differentiated MCTs were predominantly characterized by diffuse cytoplasmic (8/19) and membranous stain (7/19); a diffuse distribution of KIT positive cells was displayed in most of these tumors as well (15/19). Atypical MCTs showed diffuse distribution of labeled cells (4/7), and diffuse cytoplasm immunostaining was seen most (5/7). The pleomorphic MCT showed diffuse cytoplasmic KIT stain, with moderate labeling intensity, typically displaying focal distribution in deeper areas of the neoplasm. According to the results, there was no correlation between the type of MCTs and KIT expression, although the use of feline KIT immunohistochemistry could be useful to assess the mast cell origin.

Lnk adaptor protein down-regulates specific Kit-induced signaling pathways in primary mast cells

Stem cell factor (SCF) plays critical roles in proliferation, survival, migration, and function of hematopoietic progenitor and mast cells through binding to Kit receptor. Previous studies have implicated the adaptor protein Lnk as an important negative regulator of SCF signaling. However, the molecular mechanism underlying this regulation is unclear. Here, we showed that the Src homology 2 domain (SH2) of Lnk binds directly and preferentially to phosphorylated tyrosine 567 in Kit juxtamembrane domain. Using Lnk−/− bone marrow mast cells (BMMCs) transduced with different Lnk proteins, we demonstrated that Lnk down-regulates SCF-induced proliferation with attenuation of mitogen-activated protein kinase (MAPK) and c-jun N-terminal kinase signaling. Furthermore, we showed that Lnk−/− BMMCs displayed increased SCF-dependent migration compared with wild-type cells, revealing a novel Lnk-mediated inhibitory function. This correlated with enhanced Rac and p38 MAPK activation. Finally, we found that Lnk domains and carboxy-terminal tyrosine contribute differently to inhibition of in vitro expansion of hematopoietic progenitors. Altogether, our results demonstrate that Lnk, through its binding to Kit tyrosine 567, negatively modulates specific SCF-dependent signaling pathways involved in the proliferation and migration of primary hematopoietic cells.

c-Kit in early onset of diabetes: a morphological and functional analysis of pancreatic beta-cells in c-KitW-v mutant mice

c-Kit tyrosine receptor kinase, a well-established stem cell marker, is expressed in a variety of tissues including the pancreas. The involvement of c-Kit in fetal rat and human endocrine pancreatic development, survival, and function has been well characterized but primarily using in vitro experimental approaches. Therefore, the aim of the current study was to examine whether deficiency of a functional c-Kit receptor would have physiological and functional implications in vivo. We characterized the c-Kit mutant mouse, c-Kit(W-v/+), to evaluate the in vivo role of c-Kit in beta-cell growth and function. Here we report that male c-Kit(W-v/+) mice, at 8 wk of age, showed high fasting blood glucose levels and impaired glucose tolerance, which was associated with low levels of insulin secretion after glucose stimulation in vivo and in isolated islets. Morphometric analysis revealed that beta-cell mass was significantly reduced (50%) in male c-Kit(W-v/+) mice when compared with controls (c-Kit(+/+)) (P < 0.05). In parallel, a reduction in pancreatic duodenal homeobox-1 and insulin gene expression in whole pancreas as well as isolated islets of c-Kit(W-v/+) male mice was noted along with a decrease in pancreatic insulin content. Furthermore, the reduction in beta-cell mass in male c-Kit(W-v/+) mice was associated with a decrease in beta-cell proliferation. Interestingly, these changes were not observed in female c-Kit(W-v/+) mice until 40 wk of age. Our results clearly demonstrate that the c-Kit receptor is involved in the regulation of glucose metabolism, likely through an important role in beta-cell development and function.

The Scaffolding Adapter Gab2, via Shp-2, Regulates Kit-evoked Mast Cell Proliferation by Activating the Rac/JNK Pathway

The scaffolding adapter Gab2 mediates cell signaling and responses evoked by various extracellular stimuli including several growth factors. Kit, the receptor for stem cell factor (SCF), plays a critical role in the proliferation and differentiation of a variety of cell types, including mast cells. Kit, via Tyr(567) and Tyr(719), activates Src family kinases (SFK) and PI3K respectively, which converge on the activation of a Rac/JNK pathway required for mast cell proliferation. However, how Kit Tyr(567) signals to Rac/JNK is not well understood. By analyzing Gab2(-/-) mast cells, we find that Gab2 is required for SCF-evoked proliferation, activation of Rac/JNK, and Ras. Upon Kit activation in wild-type mast cells, Gab2 becomes tyrosyl-phosphorylated and associates with Kit and Shp-2. Tyr(567), an SFK binding site in Kit, and SFK activity were required for Gab2 tyrosyl phosphorylation and association with Shp-2. By re-expressing Gab2 or a Gab2 mutant that cannot bind Shp-2 in Gab2(-/-) mast cells or acutely by deleting Shp-2 in mast cells, we found that Gab2 requires Shp-2 for SCF-evoked Rac/JNK, Ras activation, and mast cell proliferation. Lastly, by analyzing mast cells from mice with compound Gab2 and Kit Y719F mutations (i.e., Gab2(-/-): KitY719F/Y719F mice), we find that Gab2, acting in a parallel pathway to PI3K from Kit Tyr(719), regulates mast cell proliferation and development in specific tissues. Our data show that Gab2 via Shp-2 is critical for transmitting signals from Kit Tyr(567) to activate the Rac/JNK pathway controlling mast cell proliferation, which likely contributes to mast cell development in specific tissues.

Stem cell factor expression, mast cells and inflammation in asthma

The Kit ligand SCF or stem cell factor (SCF) is a multipotent growth factor, acting as an important growth factor for human mast cells. SCF induces chemotaxis and survival of the mast cell, as well as proliferation and differentiation of immature mast cells from CD34(+) progenitors. Additionally, SCF enhances antigen-induced degranulation of human lung-derived mast cells, and induces a mast cell hyperplasia after subcutaneous administration. SCF expression increases in the airways of asthmatic patients, and this is reversed after treatment with glucocorticoids. A role for SCF may thus be hypothesized in diseases associated with a local increase in the number and/or activation of mast cells, as occurring in the airways in asthma. SCF will be reviewed as a potential therapeutic target in asthma, to control the regulation of mast cell number and activation. We here report the main pathways of SCF synthesis and signalling, and its potential role on airway function and asthma.

Stem cell factor and its receptor c-Kit as targets for inflammatory diseases

Stem cell factor (SCF), the ligand of the c-Kit receptor, is expressed by various structural and inflammatory cells in the airways. Binding of SCF to c-Kit leads to activation of multiple pathways, including phosphatidyl-inositol-3 (PI3)-kinase, phospholipase C (PLC)-gamma, Src kinase, Janus kinase (JAK)/Signal Transducers and Activators of Transcription (STAT) and mitogen activated protein (MAP) kinase pathways. SCF is an important growth factor for mast cells, promoting their generation from CD34+ progenitor cells. In vitro, SCF induces mast cells survival, adhesion to extracellular matrix and degranulation, leading to expression and release of histamine, pro-inflammatory cytokines and chemokines. SCF also induces eosinophil adhesion and activation. SCF is upregulated in inflammatory conditions both in vitro and in vivo, in human and mice. Inhibition of the SCF/c-Kit pathway leads to significant decrease of histamine levels, mast cells and eosinophil infiltration, interleukin (IL)-4 production and airway hyperresponsiveness in vivo. Taken together, these data suggest that SCF/c-Kit may be a potential therapeutic target for the control of mast cell and eosinophil number and activation in inflammatory diseases.

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.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

Myeloid leukaemias are frequently associated with translocations and mutations of tyrosine kinase genes. The products of these oncogenes, including BCR-ABL, TEL-PDGFR, Flt3 and c-Kit, have elevated tyrosine kinase activity and transform haematopoietic cells, mainly by augmentation of proliferation and enhanced viability. Activated ABL kinases are associated with chronic myeloid leukaemia. Mutations in platelet-derived growth factor receptor beta are associated with chronic myelomonocytic leukaemia. Flt3 or c-Kit cooperate with other types of oncogenes to create fully transformed acute leukaemias. Elevated activity of these tyrosine kinases is crucial for transformation, thus making the kinase domain an ideal target for therapeutic intervention. Tyrosine kinase inhibitors for various kinases are currently being evaluated in clinical trials and are potentially useful therapeutic agents in myeloid leukaemias. Here, the authors review the signalling activities, mechanism of transformation and therapeutic targeting of several tyrosine kinase oncogenes important in myeloid leukaemias.

Systemic mastocytosis

Systemic mastocytosis is a fascinating disease with diverse clinical features. There have been numerous advances in understanding the basis of clinical manifestations of this disease and of its molecular pathogenesis in the last several decades. The development of methods to study mast cell biology using cell culture and murine models has proven invaluable in this regard. Clarification of the roles of mast cells in various biological processes has expanded our understanding of their importance in innate immunity, as well as allergy. New diagnostic methods have allowed the design of detailed criteria to assist in distinguishing reactive mast cell hyperplasia from systemic mastocytosis. Variants and subvariants of systemic mastocytosis have been defined to assist in determining prognosis and in management of the disease. Elucidation of the roles of the Kit receptor tyrosine kinase and signal transduction pathway activation has contributed to development of potential targeted therapeutic approaches that may prove useful in the future.

Kit-activating mutations cooperate with Spi-1/PU.1 overexpression to promote tumorigenic progression during erythroleukemia in mice

The erythroleukemia developed by spi-1/PU.1 transgenic mice is a multistage process characterized by an early arrest of the proerythroblast differentiation followed later on by malignant transformation. Herein, we report the presence of acquired mutations in the SCF receptor gene (Kit) in 86% of tumors isolated during the late stage of the disease. Kit mutations affect codon 814 or 818. Ectopic expression of Kit mutants in nonmalignant proerythroblasts confers erythropoietin independence and tumorigenicity to cells. Using PP1, PP2, and imatinib mesylate, we show that Kit mutants are responsible for the autonomous expansion of malignant cells via Erk1/2 and PI3K/Akt activations. These findings represent a proof of principle for oncogenic cooperativity between one proliferative and one differentiation blocking event for the development of an overt leukemia.

Signaling by Kit protein-tyrosine kinase--the stem cell factor receptor

Signaling by stem cell factor and Kit, its receptor, plays important roles in gametogenesis, hematopoiesis, mast cell development and function, and melanogenesis. Moreover, human and mouse embryonic stem cells express Kit transcripts. Stem cell factor exists as both a soluble and a membrane-bound glycoprotein while Kit is a receptor protein-tyrosine kinase. The complete absence of stem cell factor or Kit is lethal. Deficiencies of either produce defects in red and white blood cell production, hypopigmentation, and sterility. Gain-of-function mutations of Kit are associated with several human neoplasms including acute myelogenous leukemia, gastrointestinal stromal tumors, and mastocytomas. Kit consists of an extracellular domain, a transmembrane segment, a juxtamembrane segment, and a protein kinase domain that contains an insert of about 80 amino acid residues. Binding of stem cell factor to Kit results in receptor dimerization and activation of protein kinase activity. The activated receptor becomes autophosphorylated at tyrosine residues that serve as docking sites for signal transduction molecules containing SH2 domains. The adaptor protein APS, Src family kinases, and Shp2 tyrosyl phosphatase bind to phosphotyrosine 568. Shp1 tyrosyl phosphatase and the adaptor protein Shc bind to phosphotyrosine 570. C-terminal Src kinase homologous kinase and the adaptor Shc bind to both phosphotyrosines 568 and 570. These residues occur in the juxtamembrane segment of Kit. Three residues in the kinase insert domain are phosphorylated and attract the adaptor protein Grb2 (Tyr703), phosphatidylinositol 3-kinase (Tyr721), and phospholipase Cgamma (Tyr730). Phosphotyrosine 900 in the distal kinase domain binds phosphatidylinositol 3-kinase which in turn binds the adaptor protein Crk. Phosphotyrosine 936, also in the distal kinase domain, binds the adaptor proteins APS, Grb2, and Grb7. Kit has the potential to participate in multiple signal transduction pathways as a result of interaction with several enzymes and adaptor proteins.

KIT (c-kit oncogene product) pathway is constitutively activated in human testicular germ cell tumors

We investigated the expression of KIT (product of c-kit oncogene), gain-of-function mutations, and activation of its downstream signal transduction in human testicular cancers. KIT was expressed in 88% (22/25) of seminomas and in 44.4% (4/9) of non-seminomas compared to adjacent normal testicular tissue. Nine of the KIT-expressing seminomas had mutations (40.9%; 9/22) in the c-kit gene; two cases in exon 11 and 7 cases in exon 17. Two of these mutations in exon 17 were novel, and the other seven mutations were identical to the already known gain-of-function mutations which cause activation of KIT without ligand stem cell factor. All of the mutant KIT and 53.8% (7/13) of wild-type KIT were phosphorylated (activated) and associated with phosphorylated phosphatidylinositol 3-kinase (PI3K). Akt was also phosphorylated in these seminomas, suggesting that the KIT-PI3K-Akt pathway is activated in seminoma. These findings suggest that the KIT-PI3K-Akt pathway is constitutively activated in testicular germ cell tumors, due to overexpression of KIT protein and/or gain-of-function mutations in the c-kit gene.

Update on the biology and therapy of gastrointestinal stromal tumors

BACKGROUND

Gastrointestinal stromal tumors (GISTs), the most common mesenchymal tumors of the gastrointestinal tract, are an example of a disease with an effective, molecularly targeted therapy.

METHODS

Published articles and author experience were used to comprehensively define the clinical features, biology, and state-of-the-art therapy of GISTs.

RESULTS

GISTs are thought to originate from the neoplastic transformation of the interstitial cells of Cajal, the intestinal pacemaker cells. GISTs commonly have mutations in the kit gene, resulting in a gain-of-function mutation and ligand-independent constitutive activation of the KIT receptor tyrosine kinase. Successful tyrosine kinase inhibitors target the aberrant pathways that are critical for tumor cell viability. The development of imatinib mesylate (formerly STI 571) in the treatment of metastatic GISTs represents a therapeutic breakthrough.

CONCLUSIONS

Progress in the clinical diagnosis has led to an increased recognition of this disease as a distinct clinical entity. Treatment of metastatic GIST with imatinib has led to unprecedented improvements in progression-free and overall survival. The use of imatinib in the preoperative and postoperative treatment of GISTs is an area of intense investigation.

Observations on the Interstitial Cells of Cajal and Neurons in a Recovered Case of Equine Dysautonomia (Grass Sickness)

This paper describes the histopathological and immunohistochemical changes in the central, autonomic and enteric nervous systems in a well-documented case of equine dysautonomia (ED), after the animal had recovered without significant residual clinical signs. Evidence of neuronal degeneration, such as neuronal chromatolysis, glial scars or a decrease in density of neurons, was not observed in the midbrain, pons, medulla oblongata or spinal cord, including the nuclei of cranial nerves III, V, VII, X and XII. In addition, no evidence of muscle denervation or re-innervation, such as group atrophy of clusters of cells or grouping of cells of similar size in the triceps muscle, was observed. These findings support the hypothesis that the chromatolytic motor neurons may not progress to cell death in ED. The autonomic ganglia showed evidence of previous neuronal loss. In the myenteric and submucosal plexuses of the ileum there was significant loss of enteric neurons. However, in the ileum, a continuous network of interstitial cells of Cajal (ICC) was observed by means of immunohistochemical techniques. It is possible that recovery without significant residual clinical signs was associated with lack of damage to, or more likely recovery of, the ICC, which have a pacemaker function in the intestinal tract.

Distinct role for c-kit receptor tyrosine kinase and SgIGSF adhesion molecule in attachment of mast cells to fibroblasts

Binding of stem cell factor (SCF) to c-kit receptor tyrosine kinase (KIT) transduces signals essential for mast cell development via several pathways including activation of phosphatidylinositol 3-kinase (PI3-K). When cultured mast cells (CMCs) are cocultured with fibroblasts expressing membrane-bound SCF, CMCs with normal KIT adhere to fibroblasts and proliferate, whereas CMCs lacking cell surface expression of KIT do neither. Spermatogenic immunoglobulin superfamily (SgIGSF) was identified as another molecule that participates in mast cell adhesion to fibroblasts. Since the IC-2 mast cell line expressed neither KIT nor SgIGSF, the effect of ectopic expression of KIT or SgIGSF on the adhesion of IC-2 cells was examined. Three forms of KIT with the normal ectodomain were used: wild-type (KIT-WT) and two mutant types with a phenylalanine substitution at the tyrosine residue 719 (KIT-Y719F) or 821 (KIT-Y821F). KIT-Y719F does not activate PI3-K, whereas KIT-Y821F does. Firstly, KIT or SgIGSF was expressed singly in IC-2 cells. All three forms of KIT increased the adhesion level of IC-2 cells, whereas SgIGSF did not. Secondly, SgIGSF was coexpressed with one of the three forms of KIT. Coexpression of SgIGSF with KIT-WT or KIT-Y821F increased the adhesion level more markedly than was achieved by KIT-WT or KIT-Y821F alone. The effect was abolished by an antibody that blocks SCF-KIT interaction. In contrast, coexpression of SgIGSF with KIT-Y719F did not increase the adhesion level induced by KIT-Y719F alone. In adhesion of mast cells to fibroblasts, KIT appeared to behave as an adhesion molecule and as an activator of other adhesion molecules through phosphorylating PI3-K.

Targeting c-Kit mutations: basic science to novel therapies

The Kit receptor tyrosine kinase is a transmembrane receptor that is expressed in a variety of different tissues and mediates pleiotropic biological effects through its ligand stem cell factor (SCF). Sporadic mutations of Kit as well as autocrine/paracrine activation mechanisms of the SCF/Kit pathway have been implicated in a variety of malignancies, where its primary contribution to metastases is in enhancing tumor growth and reducing apoptosis. For example, Kit is frequently mutated and activated in gastrointestinal stromal tumors (GISTs) and there is ligand-mediated activation of Kit in some lung cancers. Kit is a convenient target in Kit-induced tumors and inhibition of this receptor with the small molecule drug Gleevec (imatinib mesylate, STI571) in GIST has shown dramatic efficacy. Unfortunately, past experience has demonstrated that chemotherapy of cancers with a single drug often leads to resistance of the cancer. Further understanding of the molecular mechanisms underlying Kit-mediated transformation is therefore important and may lead to the identification of further novel drug targets. These Kit-specific signaling pathways may then be targeted to overcome potential drug resistance. This review will focus on our understanding of the molecular mechanisms involved in transformation by Kit. The potential mechanisms by which Kit induces cellular transformation are described. We will also discuss the role and expression of Kit in various malignancies. Ultimately, the understanding of c-Kit biology, biochemistry, and mutational analysis will lead to better therapeutics.

Expression of protooncogene c-kit receptor in rat testis and uniqueness of extracellular domain across the species with potential in molecular phylogeny

We studied expression of protooncogene c-kit receptor in Brown Norway rat Rattus norvegicus testis during different stages of postnatal development. Several regions from within the c-kit gene encompassing different domains were amplified employing reverse transcriptase polymerase chain reaction, and the resultant amplicons were cloned and characterized. Maximum expression of c-kit was observed in the testes during the days 10 to 30, suggesting its involvement in transition of primary spermatocytes towards formation of mature spermatozoa. Multiple novel transcripts originating from the extracellular domain were also identified, though their functions remained unknown. The evolutionary divergence of c-kit cDNA of 10 other vertebrates was studied using their sequences from the GenBank. Analyses of c-kit cDNA and its protein sequences in rat and related genomes showed organizational uniqueness across the species. Construction of phylogenetic tree, based on c-kit cDNA and protein sequences delineated all the species successfully and was found to be in accordance with the established positioning of these animals. The organizational uniqueness of c-kit cDNA sequences from the extracellular domain may be exploited as a useful tool in delineating phylogenetic relationship of different species.

Activation of the MAP kinase pathway by c-Kit is PI-3 kinase dependent in hematopoietic progenitor/stem cell lines

The Steel factor (SF) and its receptor c-Kit play a critical role for various cell types at different levels in the hematopoietic hierarchy. Whether similar or distinct signaling pathways are used upon c-Kit activation in different cell types within the hematopoietic hierarchy is not known. To study c-Kit signaling pathways in the hematopoietic system we have compared c-Kit downstream signaling events in SF-dependent hematopoietic stem cell (HSC)-like cell lines to those of mast cells. Both Erk and protein kinase B (PKB)/Akt are activated by ligand-induced activation of the c-Kit receptor in the HSC-like cell lines. Surprisingly, phosphoinositide-3 (PI-3) kinase inhibitors block not only PKB/Akt activation but also activation of Raf and Erk. SF-induced activation of Ras is not affected by inhibition of PI-3 kinase. In mast cells and other more committed hematopoietic precursors, the activation of Erk by SF is not PI-3 kinase dependent. Our results suggest that a molecular signaling switch occurs during differentiation in the hematopoietic system whereby immature hematopoietic progenitor/stem cells use a PI-3 kinase-sensitive pathway in the activation of both Erk and PKB/Akt, which is then switched upon differentiation to the more commonly described PI-3 kinase-independent mitogen-activated protein (MAP) kinase pathway.

Kit/stem cell factor receptor-induced phosphatidylinositol 3'-kinase signalling is not required for normal development and function of interstitial cells of Cajal in mouse gastrointestinal tract

Signalling mediated by the receptor tyrosine kinase c-Kit is required for normal development of interstitial cells of Cajal (ICC). c-Kit activates several signalling pathways, including the phosphatidylinositol 3'-kinase (PI3'-kinase) pathway. The signals required for ICC development and maintenance are not well understood. Studies indicate a role for PI3'-kinase. We studied ICC function and morphology in mice homozygous for the tyrosine 719 to phenylalanine c-Kit mutation, which disrupts all PI3'-kinase binding to c-Kit. Functionally, the electrical slow waves in the jejunum and inhibitory junction potentials were normal in adult mutants. Morphologically, the distribution of ICC was not altered in mutants. There was no difference in the density of ICC in the jejunum of adults or newborns from quantitative analysis of c-Kit immunoreactivity. The number of ICC obtained in culture was the same using mutants or wild-type littermates. The density and organization of nerves in the jejunum of mutants was not affected. Deletion of c-Kit-induced PI3'-kinase signalling does not affect the function or development of ICC in the mouse. This is an important and counterintuitive result, given the role of PI3'-kinase signalling downstream of c-Kit and the role of both c-Kit and PI3'-kinase individually in ICC development.

The phospholipase C gamma 1-dependent pathway of Fc epsilon RI-mediated mast cell activation is regulated independently of phosphatidylinositol 3-kinase

Mast cell degranulation following Fc epsilon RI aggregation is generally believed to be dependent on phosphatidylinositide 3-kinase (PI 3-kinase)-mediated phospholipase C (PLC)gamma activation. Here we report evidence that the PLC gamma 1-dependent pathway of Fc epsilon RI-mediated activation of mast cells is independent of PI 3-kinase activation. In primary cultures of human mast cells, Fc epsilon RI aggregation induced a rapid translocation and phosphorylation of PLC gamma 1, and subsequent inositol trisphosphate (IP3) production, which preceded PI 3-kinase-related signals. In addition, although PI 3-kinase-mediated responses were completely inhibited by wortmannin, even at high concentrations, this PI 3-kinase inhibitor had no effect on parameters of Fc epsilon RI-mediated PLC gamma activation, and had little effect on the initial increase in intracellular calcium levels that correlated with PLC gamma activation. Wortmannin, however, did produce a partial (approximately 50%) concentration-dependent inhibition of Fc epsilon RI-mediated degranulation in human mast cells and a partial inhibition of the later calcium response at higher concentrations. Further studies, conducted in mast cells derived from the bone marrow of mice deficient in the p85 alpha and p85 beta subunits of PI 3-kinase, also revealed no defects in Fc epsilon RI-mediated PLC gamma 1 activation. These data are consistent with the conclusion that the PLC gamma-dependent component of Fc epsilon RI-mediated calcium flux leading to degranulation of mast cells is independent of PI 3-kinase. However, PI 3-kinase may contribute to the later phase of Fc epsilon RI-mediated degranulation in human mast cells.

Clonogenic culture of normal spermatogonia: in vitro regulation of postnatal germ cell proliferation

The stem cell properties of neonatal germ cells have recently been demonstrated by in vivo transplantation. Regulation of proliferation of these cells, however, is not yet understood, and an in vitro system is needed for directly testing the action of differentiation and proliferation-related factors for germ cells. We developed an in vitro model involving micromanipulation and a single-cell clonogenic assay in which results from independent experiments on spermatogonia and gonocytes have been analyzed and compared. Neonatal germ cells can be distinguished by their large size both in vivo and in vitro in a single-cell suspension. These cells are picked up singly using a micropipette and deposited into a 96-well plate precoated with an extracellular matrix component, e.g., collagen IV. The effect of growth factors or cocultured somatic cells was assayed by counting the percentage of wells containing a colony and comparing this percentage with that of control cultures. Addition of platelet-derived growth factor significantly shifted the modal colony size for gonocytes from >16-64 to >64-128 cells/colony (P < 0.001, chi2) but had no effect on spermatogonia-derived colony size and number. For testis somatic cell underlays, there was a profound inhibition of all colony types, and immunohistochemical staining of testis cell underlays showed inhibin/activinbetaA subunit expression. This finding suggests that negative regulation of germ cell proliferation is mediated by inhibin. Addition of activin A to these cultures resulted in significant recovery (P = 0.046) of gonocyte-derived colony numbers but not spermatogonia-derived colonies, which may reflect the functional regulation by these factors observed in vivo. This proliferation assay also highlights many similarities in the regulation of gonocyte and spermatogonia proliferation in vitro, suggesting that proliferation potential is not noticeably affected by the transition of gonocytes to spermatogonia. For example, the average colony cloning efficiency was 80% for gonocytes and 76% for spermatogonia. This technology forms a basis for optimizing growth of neonatal germ cells for applications such as introduction of genetic material into the germ line to produce transgenic mice and to explore gene therapy.

Necessity of tyrosine 719 and phosphatidylinositol 3'-kinase-mediated signal pathway in constitutive activation and oncogenic potential of c-kit receptor tyrosine kinase with the Asp814Val mutation

Substitution of valine (Val) for aspartic acid (Asp) at codon 814 constitutively activates murine c-kit receptor tyrosine kinase (KIT), and Asp816Val mutation, corresponding to murine Asp814Val mutation, is found in patients with mastocytosis and acute myelocytic leukemia. However, the signal transduction pathways responsible for oncogenesis by the Asp814Val mutant (KIT(Val814)) are not fully understood. To examine the oncogenic signal transduction of KIT(Val814), we converted 20 tyrosine (Tyr) residues to phenylalanine (Phe) in the cytoplasmic domain of KIT(Val814) or deleted the C-terminal region containing 2 other tyrosine residues (Del). Among various KIT(Val814)- derived mutants, KIT(Val814-Tyr719Phe) and KIT(Val814-Del) severely impaired receptor tyrosine phosphorylation and association with the p85 subunit of phosphatidylinositol 3'-kinase (p85 (PI3-K)). Moreover, KIT(Val814-Tyr719Phe) and KIT(Val814-Del) failed to induce ligand-independent growth in Ba/F3 cells, indicating that Tyr719, the binding site for p85(PI3-K), and the C-terminal region are indispensable for factor-independent growth by KIT(Val814). Although the C-terminal region was also required for ligand-dependent growth by wild-type KIT (KIT(WT)), the Tyr719Phe substitution had negligible effects on ligand-dependent growth by KIT(WT). Furthermore, dominant-negative PI3-K significantly inhibited ligand-independent growth by KIT(Val814). These results demonstrate that Tyr719 is crucial for constitutive activation of KIT(Val814), but not for the ligand-induced activation of KIT(WT), and that the downstream signaling of PI3-K plays an important role in ligand-independent growth and tumorigenicity by KIT(Val814), thereby suggesting that KIT(Val814) is a unique activating mutation that leads to a distinguishable function from the effects of KIT(WT).

Molecular mechanisms utilized by alternative c-kit gene products in the control of spermatogonial proliferation and sperm-mediated egg activation

The c-kit proto-oncogene plays a dual role in the control of male fertility in mice through two alternative gene products: (1). c-kit [the transmembrane tyrosine kinase receptor for stem cell factor (SCF)], which is expressed and functional in differentiating spermatogonia of the postnatal testis, in which c-kit is essential for pre-meiotic proliferation; and (2). tr-kit, an intracellular protein which is specifically accumulated during spermiogenesis through the use of an alternative intronic promoter, and which is able to trigger mouse egg activation when microinjected into the cytoplasm of metaphase II arrested oocytes. Here, we summarize the most recent findings about the molecular pathways through which c-kit regulates cell cycle progression in mitotic germ cells, and those through which sperm-derived tr-kit triggers parthenogenetic completion of meiosis II and pronuclear formation in microinjected mouse eggs.

Evaluation of an intron deletion in the c-kit gene of canine mast cell tumors

OBJECTIVE

To evaluate molecular abnormalities in the c-kit gene of canine mast cell tumors (MCT) with different grades of cellular differentiation.

SAMPLE POPULATION

31 normal tissue specimens from dogs and 45 canine MCT classified according to grade of cell differentiation.

PROCEDURES

Genomic DNA extractions were made from canine MCT and normal tissues. Parts of exon 11, intron 11, and exon 12 of the c-kit gene were amplified by use of polymerase chain reaction. These regions were cloned, sequenced, and compared with GenBank sequences of the National Center for Biotechnology International. A statistical analysis was used to compare sequences from canine MCT and normal tissues.

RESULTS

A significantly higher percentage of homozygous intron 11 deletion was found in canine MCT (49%) than in normal tissues (13%). This percentage was also higher in moderately and poorly differentiated MCT, compared with well-differentiated MCT Although no mutations were detected in any of the specimens, a polymorphism at amino acid position 606 of the canine c-kit sequence was found in all the studied sequences.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated a relationship between intron 11 deletion and MCT and the grade of MCT differentiation. We suggest that intron 11 deletion may be implicated in the pathogenesis of MCT and could be used as a marker for diagnosis and prognosis of canine MCT.

Negative regulation of mast cell proliferation by FcgammaRIIB

FcgammaRIIB are single-chain low-affinity receptors for the Fc portion of IgG antibodies that are widely expressed by hematopoietic cells including mast cells. We previously demonstrated that FcgammaRIIB negatively regulate cell activation triggered by receptors that possess Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) including high-affinity IgE receptors (FcepsilonRI). FcgammaRIIB possess an Immunoreceptor Tyrosine-based Inhibition Motif (ITAM) whose deletion or mutation abolishes inhibition. When coaggregated with FcepsilonRI, the FcgammaRIIB ITIM is tyrosyl-phosphorylated by the src family protein tyrosine kinase lyn, and recruits the SH2 domain-containing inositol 5-phosphatase SHIP that accounts for inhibition of cell activation. We found recently that, when coaggregated with Kit, FcgammaRIIB can also inhibit mast cell proliferation: thymidine incorporation is inhibited, cells do not enter the G1 phase of the cell cycle, the induction of cyclins D2, D3 and A is inhibited, the activation of the MAP kinases Erk1/2, JNK and p38 is decreased, Akt phosphorylation is inhibited, and SHIP coprecipitates with FcgammaRIIB. Although inhibition of Akt phosphorylation and Erk activation was abrogated in SHIP(-/-) cells, inhibition of thymidine incorporation was only partially reduced. FcgammaRIIB-dependent inhibition of Kit-mediated mast cell proliferation was however mimicked by FcgammaRIIB whose intracytoplasmic domain was replaced by the catalytic domain of SHIP. We also found that FcgammaRIIB can inhibit the proliferation of cells whose proliferation was rendered growth factor-independent because they express a mutated form of Kit that renders this RTK constitutively activated. Based on these results we developed models aiming at using FcgammaRIIB as targets for new therapeutic approaches of disease associated with mast cell activation such as allergies and diseases associated with mast cell proliferation such as mastocytosis, mastocytomas or mast cell leukemias.

Kit signaling inhibits the sphingomyelin-ceramide pathway through PLCγ1: implication in stem cell factor radioprotective effect

Previous studies demonstrated that Kit activation confers radioprotection. However, the mechanism by which Kit signaling interferes with cellular response to ionizing radiation (IR) has not been firmly established. Based on the role of the sphingomyelin (SM) cycle apoptotic pathway in IR-induced apoptosis, we hypothesized that one of the Kit signaling components might inhibit IR-induced ceramide production or ceramide-induced apoptosis. Results show that, in both Ba/F3 and 32D murine cell lines transfected with wild-type c-kit, stem cell factor (SCF) stimulation resulted in a significant reduction of IR-induced apoptosis and cytotoxicity, whereas DNA repair remained unaffected. Moreover, SCF stimulation inhibited IR-induced neutral sphingomyelinase (N-SMase) stimulation and ceramide production. The SCF inhibitory effect on SM cycle was not influenced by wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor. The SCF protective effect was maintained in 32D-KitYF719 cells in which the PI3K/Akt signaling pathway is abolished due to mutation in Kit docking site for PI3K. In contrast, phospholipase C γ (PLCγ) inhibition by U73122 totally restored IR-induced N-SMase stimulation, ceramide production, and apoptosis in Kit-activated cells. Moreover, SCF did not protect 32D-KitYF728 cells (lacking a functional docking site for PLCγ1), from IR-induced SM cycle. Finally, SCF-induced radioprotection of human CD34+ bone marrow cells was also inhibited by U73122. Altogether, these results suggest that SCF radioprotection is due to PLCγ1-dependent negative regulation of IR-induced N-SMase stimulation. Beyond the scope of Kit-expressing cells, it suggests that PLCγ1 status could greatly influence the post-DNA damage cellular response to IR, and perhaps, to other genotoxic agents.

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.

C-kit associated with the transmembrane 4 superfamily proteins constitutes a functionally distinct subunit in human hematopoietic progenitors

The transmembrane 4 superfamily (TM4SF) has come into prominence for its association with a wide range of cell surface molecules, especially integrins. We report that TM4SF molecules CD9, CD63, and CD81 are physically associated with c-kit receptor tyrosine kinase in the human factor-dependent myeloid cell line, MO7e. We characterized this complex using coimmunoprecipitation and colocalization methods. The c-kit coimmunoprecipitated with anti-TM4SF antibodies showed several distinct phenotypes compared to the total c-kit immunoprecipitated with anti-c-kit antibody. These included: (1) higher basal level of tyrosine phosphorylation without elevated kinase activity in the absence of Steel factor (SLF), (2) deficient enhancement of tyrosine phosphorylation and kinase activity in response to SLF, (3) elevated binding rate of SLF shown in chemical cross-linking studies, and (4) little internalization and degradation after SLF treatment. Cocapping studies in living cells showed that c-kit colocalized with TM4SF molecules after SLF stimulation, suggesting confirmation of the biochemical data obtained by the coimmunoprecipitation studies. Colocalization of c-kit with CD81 by SLF was also observed in cord blood CD34(+) cells, suggesting the existence of functional units of c-kit in TM4SF complexes in primary hematopoietic cells. This suggests that some TM4SF members may negatively modulate function of c-kit receptor tyrosine kinase and thus regulate receptor sensitivity to SLF in hematopoietic progenitors.

Physiology and pathophysiology of the interstitial cells of Cajal: from bench to bedside. IV. Genetic and animal models of GI motility disorders caused by loss of interstitial cells of Cajal

Several human motility disorders have been shown to be associated with loss or defects in interstitial cells of Cajal (ICC) networks. Because tissue samples for these studies were taken from patients with well-advanced motility problems, it is difficult to determine whether the loss of ICC is a cause or a consequence of the disease process. To establish the cause-and-effect relationship of ICC loss in motility disorders, it may be feasible to use animal models in which ICC are lost as motility dysfunction develops. Several models with defects in ICC networks have been developed, and these include animals with defects in the Kit signaling pathway (e.g., white-spotting mutants that have defects in Kit receptors; steel mutants that have mutations in stem cell factor, the ligand for Kit; and animals that are chronically treated with reagents that block Kit or downstream signaling proteins). ICC do not die when Kit signaling is blocked, rather, they redifferentiate into a smooth muscle-like phenotype. Diabetic animals (NOD/LtJ mice), animals with chronic bowel obstruction, and inflammatory bowel models also have defects in ICC networks that have been associated with motility disorders. By studying these models with molecular and genomic techniques it may be possible to determine the signals that cause loss of ICC and find ways of restoring ICC to dysfunctional tissues. This article discusses recent progress in the utilization of animal models to study the consequences of losing ICC on the development of motility disorders.

Requirement of Gab2 for mast cell development and KitL/c-Kit signaling

Mast cells are thought to participate in a variety of immune responses, such as parasite resistance and the allergic reaction. Mast cell development depends on stem cell factor (Kit ligand) and its receptor, c-Kit. Gab2 is an adaptor molecule containing a pleckstrin homology domain and potential binding sites for SH2 and SH3 domains. Gab2 is phosphorylated on tyrosine after stimulation with cytokines and growth factors, including KitL. Gab2-deficient mice were created to define the physiological requirement for Gab2 in KitL/c-Kit signaling and mast cell development. In Gab2-deficient mice, the number of mast cells was reduced markedly in the stomach and less severely in the skin. Bone marrow-derived mast cells (BMMCs) from the Gab2-deficient mice grew poorly in response to KitL. KitL-induced ERK MAP kinase and Akt activation were impaired in Gab2-deficient BMMCs. These data indicate that Gab2 is required for mast cell development and KitL/c-Kit signaling.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

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.

Phosphatidylinositol 3 kinase contributes to the transformation of hematopoietic cells by the D816V c-Kit mutant

Stem cell factor (SCF) binds the receptor tyrosine kinase c-Kit and is critical for normal hematopoiesis. Substitution of valine for aspartic acid 816 (D816V) constitutively actives human c-Kit, and this mutation is found in patients with mastocytosis, leukemia, and germ cell tumors. Immortalized murine progenitor cells (MIHCs) transduced with wild-type c-Kit proliferate in response to SCF, whereas cells expressing D816V c-Kit (MIHC-D816V) are factor-independent and tumorigenic. However, the mechanisms mediating transformation by D816V c-Kit are unknown. The objective of this study was to identify signaling components that contribute to D816V c-Kit-mediated transformation. SCF stimulates association of p85PI3K with phosphorylated tyrosine 721 of wild-type c-Kit. Phosphatidylinositol 3 kinase (PI3K) subsequently contributes to the activation of Akt and Jnks. In contrast, these studies demonstrated that the D816V c-Kit mutant was constitutively associated with phosphorylated p85PI3K, and, downstream of PI3K, Jnk 1 and Jnk 2 were activated but Akt was not. Interestingly, Erks 1 and 2 were not constitutively activated by D816V c-Kit. Thus, D816V c-Kit maintains the activity of PI3K but not of all signaling pathways activated by wild-type c-Kit. Further, all pathways downstream of PI3K are not constitutively active in MIHC-D816V cells. Studies with a PI3K inhibitor and D816V/Y721F c-Kit, a mutant incapable of recruiting PI3K, indicate that constitutive activation of PI3K through direct recruitment by D816V c-Kit plays a role in factor-independent growth of MIHC and is critical for tumorigenicity.

Src Homology 2 Domain-containing Inositol 5-Phosphatase 1 Mediates Cell Cycle Arrest by FcγRIIB

We previously found that low affinity receptors for the Fc portion of IgG, FcgammaRIIB, which are widely expressed by hematopoietic cells, can negatively regulate receptor tyrosine kinase-dependent cell proliferation. We investigated here the mechanisms of this inhibition. We used as experimental models wild-type mast cells, which constitutively express the stem cell factor receptor Kit and FcgammaRIIB, FcgammaRIIB-deficient mast cells reconstituted with wild-type or mutated FcgammaRIIB, and Src homology 2 domain-containing inositol polyphosphate 5-phosphatase 1 (SHIP1)-deficient mast cells. We found that, upon coaggregation with Kit, FcgammaRIIB are tyrosyl-phosphorylated, recruit SHIP1, but not SHIP2, SH2 domain-containing protein tyrosine phosphatase-1 or -2, abrogate Akt phosphorylation, shorten the duration of the activation of mitogen-activated protein kinases of the Ras and Rac pathways, abrogate cyclin induction, prevent cells from entering the cell cycle, and block thymidine incorporation. FcgammaRIIB-mediated inhibition of Kit-dependent cell proliferation was reduced in SHIP1-deficient mast cells, whereas inhibition of IgE-induced responses was abrogated. Cell proliferation was, however, inhibited by coaggregating Kit with FcgammaRIIB whose intracytoplasmic domain was replaced with the catalytic domain of SHIP1. These results demonstrate that FcgammaRIIB use SHIP1 to inhibit pathways shared by receptor tyrosine kinases and immunoreceptors to trigger cell proliferation and cell activation, respectively, but that, in the absence of SHIP1, FcgammaRIIB can use other effectors that specifically inhibit cell proliferation.