KIT Receptor Research Articles

Recent advances in the treatment of gastrointestinal stromal tumors.

Constitutively activating mutations in the KIT and platelet-derived growth factor receptor α (PDGFRA) RTKs play a crucial role in the biology of gastrointestinal stromal tumors (GISTs), and this disease has served as an effective model for targeting gain-of-function kinase mutations in cancer. Imatinib has entered the clinical arena in the last decade and substantially improved the outcome in these formerly untreatable cancers. However, most advanced GISTs responding to imatinib progress within 2-3 years due to heterogeneous subclones harboring a range of imatinib-resistant secondary KIT mutations. Sunitinib, and more recently, regorafenib, have obtained US Food and Drug Administration approval for the treatment of GISTs after imatinib failure, and thus expanded the treatment options in resistant disease. Within this framework, we present an evaluation of current GIST management, emphasizing the most recent advances in the field together with a discussion on future steps to be taken in refractory disease.

Unbiased Compound Screening Identifies Unexpected Drug Sensitivities and Novel Treatment Options for Gastrointestinal Stromal Tumors

Most gastrointestinal stromal tumors (GIST) are caused by oncogenic KIT or platelet-derived growth factor receptor activation, and the small molecule kinase inhibitor imatinib mesylate is an effective first-line therapy for metastatic or unresectable GIST. However, complete remissions are rare and most patients ultimately develop resistance, mostly because of secondary mutations in the driver oncogenic kinase. Hence, there is a need for novel treatment options to delay failure of primary treatment and restore tumor control in patients who progress under therapy with targeted agents. Historic data suggest that GISTs do not respond to classical chemotherapy, but systematic unbiased screening has not been performed. In screening a compound library enriched for U.S. Food and Drug Administration (FDA)-approved chemotherapeutic agents (NCI Approved Oncology Drugs Set II), we discovered that GIST cells display high sensitivity to transcriptional inhibitors and topoisomerase II inhibitors. Mechanistically, these compounds exploited the cells' dependency on continuous KIT expression and/or intrinsic DNA damage response defects, explaining their activity in GIST. Mithramycin A, an indirect inhibitor of the SP1 transcription factor, and mitoxantrone, a topoisomerase II inhibitor, exerted significant antitumor effects in mouse xenograft models of human GIST. Moreover, these compounds were active in patient-derived imatinib-resistant primary GIST cells, achieving efficacy at clinically relevant concentrations. Taken together, our findings reveal that GIST cells have an unexpectedly high and specific sensitivity to certain types of FDA-approved chemotherapeutic agents, with immediate implications for encouraging their clinical exploration.

Current concepts on diagnosis and treatment of mastocytosis.

Mastocytosis is a heterogeneous group of disorders characterized by a clonal proliferation and accumulation of mast cells in one or more organ, primarily in the skin and bone marrow. The clinical spectrum of the disease varies from relatively benign forms with isolated skin lesions to very aggressive variants with extensive systemic involvement and poor prognosis. The growth and proliferation of clonal mast cells is caused by an activating mutation of the tyrosine kinase receptor Kit for Stem Cell Factor, the main growth factor for mast cells. Clinical symptoms are related to mast-cell mediator release, to the tissue mast cell infiltration or both. The degree of infiltration and cell activation determines the highly variable clinical and morphological features. Current treatment of mastocytosis includes symptomatic, antimediator drugs and cytoreductive targeted therapies.

Membrane Tyrosine Kinase Receptors Kit and FLT3 are an Important Targets for the Therapy of Acute Myeloid Leukemia

KIT and FLT3 are class III trans membrane tyrosine kinases playing key roles in the control of hematopoietic stem cell survival and proliferation. KIT is mutated in about 2.5-3% of acute myeloid leukemia (AML) patients mainly by point mutations occurring at the level of tyrosine kinase domains. FLT3 is mutated in about 30% of AML patients, either by internal tandem duplication of the juxtamembrane domain or by point mutations occurring at the level of tyrosine kinase domains. All these types of mutations lead to the constitutive activation of KIT and FLT3 receptors, respectively. In some AML subsets, the occurrence of FLT3/ITD mutation is associated with a poor prognosis. These observations have represented the basis for the development of a large number of Tyrosine Kinase Inhibitors (TKIs) with activity against KIT and FLT3. Some of these inhibitors are still in the preclinical phase, but many have been tested in phase I/II/III clinical studies. Although these inhibitors when used as single agents have lead to a significant reduction of the number of leukemic blasts, they have produced only transient and limited clinical responses. This efficacy may be related to the limited inhibitory caefficacy of these inhibitors, occurrence of resistance and to the complexity of the genetic abnormalities occurring in AMLs. However, recently Quizartinib, a potent FLT3 inhibitor, showed an impressive rate of remission induction in patients with chemotherapy-refractory FLT3-ITD-positive AMLs, but most remissions do not meet classical criteria of complete remissions and instead represent clearance of bone marrow blasts with incomplete recovery of blood cell counts. However, the majority of these patients after an initial sustained response exhibit a leukemic relapse due to the rare pre-exisisting or new FLT3-ITD-TKD mutants surviving to the treatment with this drug and becaming driver leukemic clones. The combination of various types of FLT3 TKIs, including a type II TKI like Quizartinib with a type I FLT3 inhibitor efficiently targeting TKD mutants, such as Crenolanib, or the combination of FLT3 inhibitors with other molecularly targeted agents inhibiting other pathways activated in leukemic cells would determine a major progress in leukemia therapy. The efficacy of FLT3 TKIs in combination with standard anti-leukemic chemotherapeutics is also under evaluation in several combination clinical trials. It is expected that these trials could determine an improvement in the negative prognosis of AML patients with FLT3/ITD mutations. Keywords: Membrane tyrosine kinase receptors, leukemia, cytogenetic.

Phenotyping of Human Melanoma Cells Reveals a Unique Composition of Receptor Targets and a Subpopulation Co-Expressing ErbB4, EPO-R and NGF-R

Malignant melanoma is a life-threatening skin cancer increasingly diagnosed in the western world. In advanced disease the prognosis is grave. Growth and metastasis formation in melanomas are regulated by a network of cytokines, cytokine-receptors, and adhesion molecules. However, little is known about surface antigens and target expression profiles in human melanomas. We examined the cell surface antigen profile of human skin melanoma cells by multicolor flow cytometry, and compared their phenotype with 4 melanoma cell lines (A375, 607B, Mel-Juso, SK-Mel28). Melanoma cells were defined as CD45−/CD31− cells co-expressing one or more melanoma-related antigens (CD63, CD146, CD166). In most patients, melanoma cells exhibited ErbB3/Her3, CD44/Pgp-1, ICAM-1/CD54 and IGF-1-R/CD221, but did not express CD20, ErbB2/Her2, KIT/CD117, AC133/CD133 or MDR-1/CD243. Melanoma cell lines were found to display a similar phenotype. In most patients, a distinct subpopulation of melanoma cells (4–40%) expressed the erythropoietin receptor (EPO-R) and ErbB4 together with PD-1 and NGF-R/CD271. Both the EPO-R+ and EPO-R− subpopulations produced melanoma lesions in NOD/SCID IL-2Rgammanull (NSG) mice in first and secondary recipients. Normal skin melanocytes did not express ErbB4 or EPO-R, but expressed a functional KIT receptor (CD117) as well as NGF-R, ErbB3/Her3, IGF-1-R and CD44. In conclusion, melanoma cells display a unique composition of surface target antigens and cytokine receptors. Malignant transformation of melanomas is accompanied by loss of KIT and acquisition of EPO-R and ErbB4, both of which are co-expressed with NGF-R and PD-1 in distinct subfractions of melanoma cells. However, expression of EPO-R/ErbB4/PD-1 is not indicative of a selective melanoma-initiating potential.

Clonal expansion capacity defines two consecutive developmental stages of long-term hematopoietic stem cells

Long-term hematopoietic stem cells (HSCs [LT-HSCs]) are well known to display unpredictable differences in their clonal expansion capacities after transplantation. Here, by analyzing the cellular output after transplantation of stem cells differing in surface expression levels of the Kit receptor, we show that LT-HSCs can be systematically subdivided into two subtypes with distinct reconstitution behavior. LT-HSCs expressing intermediate levels of Kit receptor (Kit(int)) are quiescent in situ but proliferate extensively after transplantation and therefore repopulate large parts of the recipient's hematopoietic system. In contrast, metabolically active Kit(hi) LT-HSCs display more limited expansion capacities and show reduced but robust levels of repopulation after transfer. Transplantation into secondary and tertiary recipient mice show maintenance of efficient repopulation capacities of Kit(int) but not of Kit(hi) LT-HSCs. Initiation of differentiation is marked by the transit from Kit(int) to Kit(hi) HSCs, both of which precede any other known stem cell population.

Appearance of interstitial cells of Cajal in the human midgut

Several subtypes of the interstitial cells of Cajal (ICC) form networks that play a role in gastrointestinal motor control. ICC express c-kit and depend on signaling via Kit receptors for development and phenotype maintenance. At 7-8 weeks of development, c-kit-immunoreactive (c-kit-IR) cells are present in the human oesophagus, stomach and proximal duodenum wall. In the remaining small and large bowel, c-kit-IR cells appear later. The object of the present study is to determine the timing of the appearance of c-kit-IR ICC in the parts of the digestive tube originating from the midgut (distal duodenum, jejunum, ileum and proximal colon). Specimens were obtained from eight human embryos and 11 fetuses at 7-12 weeks of gestational age. The specimens were exposed to anti-c-kit antibodies to investigate ICC differentiation. The differentiation of enteric neurons and smooth muscle cells was immunohistochemically examined by using anti-PGP9,5 and anti-desmin antibodies, respectively. In the distal duodenum, jejunum and ileum, c-kit-IR cells emerged at week 9 at the level of the myenteric plexus in the form of a thin row of cells encircling the inception of the ganglia. These cells were multipolar or spindle-shaped with two long processes and corresponded to the ICC of the myenteric plexus. In the proximal colon, c-kit-IR cells emerged at week 9-10 in the form of two parallel belts of cells extending at the submucosal plexus and the myenteric plexus levels. We conclude that ICC develop following two different patterns in the human midgut.

Allosteric pathway identification through network analysis: from molecular dynamics simulations to interactive 2D and 3D graphs

Allostery is a universal phenomenon that couples the information induced by a local perturbation (effector) in a protein to spatially distant regulated sites. Such an event can be described in terms of a large scale transmission of information (communication) through a dynamic coupling between structurally rigid (minimally frustrated) and plastic (locally frustrated) clusters of residues. To elaborate a rational description of allosteric coupling, we propose an original approach - MOdular NETwork Analysis (MONETA) - based on the analysis of inter-residue dynamical correlations to localize the propagation of both structural and dynamical effects of a perturbation throughout a protein structure. MONETA uses inter-residue cross-correlations and commute times computed from molecular dynamics simulations and a topological description of a protein to build a modular network representation composed of clusters of residues (dynamic segments) linked together by chains of residues (communication pathways). MONETA provides a brand new direct and simple visualization of protein allosteric communication. A GEPHI module implemented in the MONETA package allows the generation of 2D graphs of the communication network. An interactive PyMOL plugin permits drawing of the communication pathways between chosen protein fragments or residues on a 3D representation. MONETA is a powerful tool for on-the-fly display of communication networks in proteins. We applied MONETA for the analysis of communication pathways (i) between the main regulatory fragments of receptors tyrosine kinases (RTKs), KIT and CSF-1R, in the native and mutated states and (ii) in proteins STAT5 (STAT5a and STAT5b) in the phosphorylated and the unphosphorylated forms. The description of the physical support for allosteric coupling by MONETA allowed a comparison of the mechanisms of (a) constitutive activation induced by equivalent mutations in two RTKs and (b) allosteric regulation in the activated and non-activated STAT5 proteins. Our theoretical prediction based on results obtained with MONETA was validated for KIT by in vitro experiments. MONETA is a versatile analytical and visualization tool entirely devoted to the understanding of the functioning/malfunctioning of allosteric regulation in proteins - a crucial basis to guide the discovery of next-generation allosteric drugs.

Development of an in vitro model of acquired resistance to toceranib phosphate (Palladia®) in canine mast cell tumor

BackgroundMast cell tumors (MCTs) are the most common skin tumors in dogs and exhibit variable biologic behavior. Mutations in the c-kit proto-oncogene are associated with the tumorigenesis of MCTs, resulting in growth factor-independent and constitutive phosphorylation of the KIT receptor tyrosine kinase (RTK). Toceranib (TOC) phosphate (Palladia®) is a KIT RTK inhibitor that has biological activity against MCTs. Despite these benefits, patients ultimately develop resistance to TOC. Therefore, there is a need to identify distinguishing clinical and molecular features of resistance in this population.ResultsThe canine C2 mastocytoma cell line contains an activating mutation in c-kit. Three TOC-resistant C2 sublines (TR1, TR2, TR3) were established over seven months by growing cells in increasing concentrations of TOC. TOC inhibited KIT phosphorylation and cell proliferation in a dose-dependent manner in the treatment-naïve, parental C2 line (IC50 < 10 nM). In contrast, the three sublines were resistant to growth inhibition by TOC (IC50 > 1,000 nM) and phosphorylation of the KIT receptor was less inhibited compared to the TOC-sensitive C2 cells. Interestingly, sensitivity to three structurally distinct KIT RTK inhibitors was variable among the sublines, and all 3 sublines retained sensitivity to the cytotoxic agents vinblastine and lomustine. Sequencing of c-kit revealed secondary mutations in the juxtamembrane and tyrosine kinase domains of the resistant sublines. These included point mutations in TR1 (Q574R, M835T), TR2 (K724R), and TR3 (K580R, R584G, A620S). Additionally, chronic TOC exposure resulted in c-kit mRNA and KIT protein overexpression in the TOC-resistant sublines compared to the parental line. C2, TR1, TR2, and TR3 cells demonstrated minimal P-glycoprotein (P-gp) activity and no functional P-gp.ConclusionsThis study demonstrates the development of an in vitro model of acquired resistance to targeted therapy in canine MCTs harboring a c-kit-activating mutation. This model may be used to investigate the molecular basis of and strategies to overcome TOC resistance.

Increased Risk of Malignant Melanoma in Patients with Systemic Mastocytosis?

Mastocytosis, a group of rare disorders that occur in both children and adults, is characterised by abnormal growth and pathological accumulation of mast cells in one or more organs, most commonly the skin (1). Urticaria pigmentosa (UP) is the most common cutaneous variant. In cases of extracutaneous involvement, systemic mastocytosis (SM) can be diagnosed on the basis of the criteria formulated by the WHO. The course of SM in most patients (90%) is indolent, with more aggressive presentation in only a few.The incidence of cutaneous melanoma is increasing and although this malignancy and mastocytosis originate from 2 different types of cells (melanocytes from the neural crest and mast cells from haematopoetic stem cells, respectively) they share certain similarities, including expression of the transcription factors MITF and STAT3, and dependence of the growth factor receptor KIT and its ligand stem cell factor for their growth and development (2, 3). We have found 5 published case reports that suggest a relationship between these 2 pathologies. In the first, published in 1979, a patient with nodular mastocytosis de -veloped both melanocytoma and mastocytoma (4). In the second, UP and SM preceded a metastatic melanoma (5) and the third involved combined mastocytoma-junctional naevus (6). In the fourth case, malignant melanoma was diagnosed prior to SM (7). And finally, a patient with telangiectasia macularis eruptive perstans (TEMP), a rare form of cutaneous mastocytosis, was found to have a malignant melanoma (8).Here, we describe our 4 additional cases and discuss possible associations between these 2 diseases.MATERIAL AND METHODS

Combined KIT and FGFR2b Signaling Regulates Epithelial Progenitor Expansion during Organogenesis

SummaryOrgan formation and regeneration require epithelial progenitor expansion to engineer, maintain, and repair the branched tissue architecture. Identifying the mechanisms that control progenitor expansion will inform therapeutic organ (re)generation. Here, we discover that combined KIT and fibroblast growth factor receptor 2b (FGFR2b) signaling specifically increases distal progenitor expansion during salivary gland organogenesis. FGFR2b signaling upregulates the epithelial KIT pathway so that combined KIT/FGFR2b signaling, via separate AKT and mitogen-activated protein kinase (MAPK) pathways, amplifies FGFR2b-dependent transcription. Combined KIT/FGFR2b signaling selectively expands the number of KIT+K14+SOX10+ distal progenitors, and a genetic loss of KIT signaling depletes the distal progenitors but also unexpectedly depletes the K5+ proximal progenitors. This occurs because the distal progenitors produce neurotrophic factors that support gland innervation, which maintains the proximal progenitors. Furthermore, a rare population of KIT+FGFR2b+ cells is present in adult glands, in which KIT signaling also regulates epithelial-neuronal communication during homeostasis. Our findings provide a framework to direct regeneration of branched epithelial organs.

New developments in management of gastrointestinal stromal tumors: regorafenib, the new player in the team

New developments in management of gastrointestinal stromal tumors: regorafenib, the new player in the team Sergei Boichuk,1,2 Jessica L Rausch,1 Anette Duensing1,31Cancer Virology Program, University of Pittsburgh Cancer Institute, Hillman Cancer Center, Pittsburgh, PA, USA; 2Department of Pathology, Kazan State Medical University, Kazan, Russia; 3Department of Pathology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USAAbstract: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract and the most frequent single type of sarcoma, at least in some geographical regions. They arise from the interstitial cells of Cajal (or a common progenitor cell). The vast majority of GISTs are characterized by oncogenically activating mutations in the KIT or platelet-derived growth factor receptor alpha (PDGFRA) receptor tyrosine kinase genes. This molecular feature has been successfully exploited for therapeutic purposes, and as of a decade ago, GISTs have become the prototype of a solid tumor that can be targeted with small molecule kinase inhibitors. Imatinib mesylate (Gleevec&reg;/Glivec&reg;) benefits more than 85% of patients with unresectable and/or metastatic GIST. Unfortunately, the majority of patients develop resistance to imatinib within the first 2 years of treatment and new therapeutic options are needed. Although the broad-range kinase inhibitor sunitinib malate (Sutent&reg;) has been the second-line therapy approved by the US Food and Drug Administration since 2006, it was not until recently (February 2013) that regorafenib (Stivarga&reg;) was approved as a third-line therapeutic agent for GIST. This review summarizes the development process of regorafenib for GIST and highlights its biochemical, pharmacologic, and clinical properties.Keywords: gastrointestinal stromal tumors, GIST, regorafenib

Wnt/β-Catenin and Kit Signaling Sequentially Regulate Melanocyte Stem Cell Differentiation in UVB-Induced Epidermal Pigmentation

UV radiation is a well-known inducer of epidermal pigmentation that is utilized in therapy for vitiligo, one of the skin depigmentation disorders. Although it has been reported that melanocyte stem cells (McSCs) play essential roles in hair pigmentation, the relationship between McSCs and epidermal pigmentation remains unclear. Repetitive UVB irradiation on the dorsal skin of F1 mice of HR-1 × HR/De caused apparent epidermal pigmentation, and it was characterized by increase in the number of melanocytes. Interestingly, differentiation of McSCs into melanoblasts in hair follicles was followed by induction of epidermal melanocyte differentiation. Administration of a neutralizing antibody for Kit receptor that depletes resident melanoblasts could not suppress increased number of melanocytes. UVB irradiation also induced robust expression of Wnt7a as well as Kitl in epidermis, and β-catenin translocation into nucleus in McSCs. Intradermal injection of IWR-1 (inhibitor of Wnt response 1), a chemical inhibitor of β-catenin activation, and small interfering RNA (siRNA) against Wnt7a suppressed increase in the number of epidermal melanocytes. Taken altogether, it was demonstrated that Wnt7a triggered McSCs differentiation through β-catenin activation, and Kitl might induce following migration of melanoblasts to epidermis. These findings will help in developing therapeutic technologies for vitiligo and other pigmentary disorders.

Differential Regulation of Kit Ligand A Expression in the Ovary by IGF-I via Different Pathways

Kit ligand (KITL) plays indispensable roles both in primordial follicle activation and in the maintenance of meiotic arrest of the oocyte. The regulation of KITL expression in the ovary, however, remains largely unknown. In the zebrafish, there are 2 paralogues of KITL, kitlga and kitlgb, and 2 Kit receptors, kita and kitb. Consistent with the situation in mammals, kitlga is only expressed in the ovarian follicle cells, and its cognate receptor kita is expressed in the oocyte. In the present study, we demonstrated that the expression of kitlga was promoted by IGF-I through its receptor IGF-IR. The stimulation involved transcription but not translation, suggesting that the kitlga gene is likely a direct downstream target of IGF-I signaling. Further experiments showed that the stimulatory effect of IGF-I was mediated by phosphatidyl inositol 3-kinase (PI3K)-Akt pathway. IGF-I also activated MEK-ERK pathway; however, this pathway suppressed kitlga expression. The regulation of kitlga expression by IGF-I appeared to depend on the stage of follicle development with a greater induction at early stage than late stage. This may be related to changes in IGF-I signaling pathways and/or local paracrine environment. In support of this were the differential expression of IGF-I receptors (igf1ra and igf1rb) and responsiveness of IGF-I signaling pathways, especially the PI3K-Akt pathway. Furthermore, the IGF-I-induced kitlga expression was inhibited by epidermal growth factor, an oocyte-derived paracrine factor in the zebrafish follicle. This study provides evidence for a controlling mechanism underlying the regulation of KITL expression in the ovary.

Single-cell analysis of K562 cells: An imatinib-resistant subpopulation is adherent and has upregulated expression of BCR-ABL mRNA and protein

In chronic myeloid leukemia (CML) cells from different stages of maturation may have differential expression of BCR-ABL at both messenger RNA (mRNA) and protein level. However, the significance of such differential expression to clinical disease behavior is unknown. Using the CML-derived, BCR-ABL expressing cell line, K562, distinct plastic-adherent (K562/Adh) and nonadherent (K562/NonAdh) subpopulations were established and then analyzed both as single cells and as bulk cell populations. BCR-ABL mRNA was upregulated in K562/Adh compared with K562/NonAdh cells in both single cell and bulk population analyses (p < 0.0001). Similarly, phosphorylation of BCR protein was upregulated in K562/Adh, compared with K562/NonAdh cells (63.42% vs. 23.1%; p = 0.007), and these two K562 subpopulations were found to express significantly different microRNA species. Furthermore, treatment with the BCR-ABL tyrosine kinase inhibitor, imatinib, reduced cell viability more rapidly in K562/NonAdh compared with K562/Adh cells (p < 0.005) both at single and bulk cell levels. This discovery of an adherent subpopulation of K562 cells with increased BCR-ABL mRNA, increased phosphorylated BCR protein expression, differential microRNA expression, and increased imatinib resistance suggests that a similar subpopulation of cells can also mediate clinical resistance to imatinib during treatment of patients with CML.

Phenotyping Of Leukemic Stem Cells In Ph+ ALL and Ph- ALL Reveals Unique Profiles Of Markers and Targets In Distinct Disease Variants

Acute lymphoblastic leukemia (ALL) is a life-threatening hematopoietic neoplasm characterized by abnormal growth and accumulation of lymphatic blast cells in various hematopoietic tissues. In a substantial number of patients, the Philadelphia (Ph) chromosome and the related oncoprotein BCR/ABL, are detectable. Despite recent advances in the management and therapy of patients with ALL, including the use of BCR/ABL1 tyrosine kinase inhibitors (TKI), the prognosis is still poor. Therefore, several attempts have been made to improve targeted treatment approaches in ALL. One strategy is to identify markers and targets expressed on leukemic stem cells (LSC) in these patients and to apply targeted drugs in order to eliminate LSC. In patients with Ph+ ALL, the leukemia-initiating cell-population is considered to reside within a CD34+/CD38- fraction of the clone. In the present study, we examined the expression of various stem cell markers and target antigens in CD34+/CD38- stem cells and in more mature CD34+/CD38+ progenitor cells in patients with Ph+ ALL (n=12), Ph- ALL (n=13), Ph+ CML (n=20), and in control bone marrow (BM) samples (unexplained cytopenia, n=10). Surface expression of target antigens was analyzed by multicolor flow cytometry, and mRNA expression levels by qPCR. As assessed by flow cytometry, CD34+/CD38- cells were found to co-express CD19, the stem cell-homing receptor CD44, the Campath-1 antigen (CD52), AC133 (CD133), FLT3 (CD135), and CXCR4 (CD184) in all ALL patients examined. In a majority of the ALL patients tested (14/25), LSC also expressed Siglec-3 (CD33). In CML, LSC were found to express a similar profile of antigens, including CD33, CD44, CD52, CD133, CD135, and CXCR4, but these cells did not express CD19. In control BM samples, CD34+/CD38- cells expressed a similar phenotype, but the levels of CD33 and CD52 were lower compared to LSC in ALL and CML. The IL-1RAP was found to be expressed on LSC in patients with Ph+ CML and Ph+ ALL, but not on LSC in Ph- ALL or in normal BM stem cells. By contrast, the SCF receptor KIT (CD117) was found to be expressed on LSC in Ph+ CML but was hardly detectable on LSC in patients with Ph+ ALL or Ph- ALL. The IL-2RA (CD25) and the SDF-1-degrading surface enzyme dipeptidyl-peptidase IV (DPPIV=CD26) were expressed on LSC in patients with CML and in all patients with Ph+ ALL exhibiting BCR/ABL-p210, whereas in Ph+ ALL with BCR/ABL-p190, LSC variably expressed CD25, and did not express CD26. In patients with Ph- ALL and in the normal BM, CD34+/CD38- cells did not express CD25 or CD26. The target receptor CD20 was detectable on ALL LSC in 7/18 patients examined. All target receptors tested were also detectable on more mature CD34+/CD38+ progenitor cells in patients with Ph+ ALL and Ph- ALL. In consecutive studies, expression of target antigens was confirmed at the mRNA level by qPCR analyses of highly enriched ALL LSC. Finally, we were able to show that the CD52-targeting drug alemtuzumab induces rapid lysis of CD34+/CD38- ALL LSC in all patients examined (Figure). In summary, our data show that LSC in Ph+ ALL and Ph- ALL express a unique phenotype, including clinically relevant cytokine receptors and cell surface target antigens, including the Campath-1 antigen, CD52. In Ph+ ALL with BCR/ABL-p210, the phenotype of ALL LSC largely resembles the phenotype of LSC in Ph+ CML, confirming the close relationship and similar pathogenesis of these two types of leukemias. [Display omitted] Ficoll-isolated MNC of 4 patients with Ph+ ALL were incubated in control medium (Co) or in various concentrations of alemtuzumab (10-300 µg/ml) in RPMI 1640 medium in the presence of 30% human serum at 37°C for 1 hour. After washing, cells were stained with fluorochrome-conjugated mAb against CD34, CD38 and CD45 for 15 minutes. DAPI-staining was used to evaluate the percentage of viable cells. Cells were analysed using a FACSCanto II and FlowJo software. Results show the numbers of viable CD34+/CD38- cells and are expressed as percent of control (Co). Values represent the mean±S.D. of four independent experiments. Asterisk (*): p<0.05 compared to control. Disclosures:Valent:Novartis: Consultancy, Honoraria, Research Funding.

Structural basis for KIT receptor tyrosine kinase inhibition by antibodies targeting the D4 membrane-proximal region

Somatic oncogenic mutations in the receptor tyrosine kinase KIT function as major drivers of gastrointestinal stromal tumors and a subset of acute myeloid leukemia, melanoma, and other cancers. Although treatment of these cancers with tyrosine kinase inhibitors shows dramatic responses and durable disease control, drug resistance followed by clinical progression of disease eventually occurs in virtually all patients. In this report, we describe inhibitory KIT antibodies that bind to the membrane-proximal Ig-like D4 of KIT with significant overlap with an epitope in D4 that mediates homotypic interactions essential for KIT activation. Crystal structures of the anti-KIT antibody in complex with KIT D4 and D5 allowed design of affinity-matured libraries that were used to isolate variants with increased affinity and efficacy. Isolated antibodies showed KIT inhibition together with suppression of cell proliferation driven by ligand-stimulated WT or constitutively activated oncogenic KIT mutant. These antibodies represent a unique therapeutic approach and a step toward the development of "naked" or toxin-conjugated KIT antibodies for the treatment of KIT-driven cancers.

Membrane-To-Nucleus Signaling Links Insulin-Like Growth Factor-1- and Stem Cell Factor-Activated Pathways

Stem cell factor (mouse: Kitl, human: KITLG) and insulin-like growth factor-1 (IGF1), acting via KIT and IGF1 receptor (IGF1R), respectively, are critical for the development and integrity of several tissues. Autocrine/paracrine KITLG-KIT and IGF1-IGF1R signaling are also activated in several cancers including gastrointestinal stromal tumors (GIST), the most common sarcoma. In murine gastric muscles, IGF1 promotes Kitl-dependent development of interstitial cells of Cajal (ICC), the non-neoplastic counterpart of GIST, suggesting cooperation between these pathways. Here, we report a novel mechanism linking IGF1-IGF1R and KITLG-KIT signaling in both normal and neoplastic cells. In murine gastric muscles, the microenvironment for ICC and GIST, human hepatic stellate cells (LX-2), a model for cancer niches, and GIST cells, IGF1 stimulated Kitl/KITLG protein and mRNA expression and promoter activity by activating several signaling pathways including AKT-mediated glycogen synthase kinase-3β inhibition (GSK3i). GSK3i alone also stimulated Kitl/KITLG expression without activating mitogenic pathways. Both IGF1 and GSK3i induced chromatin-level changes favoring transcriptional activation at the Kitl promoter including increased histone H3/H4 acetylation and H3 lysine (K) 4 methylation, reduced H3K9 and H3K27 methylation and reduced occupancy by the H3K27 methyltransferase EZH2. By pharmacological or RNA interference-mediated inhibition of chromatin modifiers we demonstrated that these changes have the predicted impact on KITLG expression. KITLG knock-down and immunoneutralization inhibited the proliferation of GIST cells expressing wild-type KIT, signifying oncogenic autocrine/paracrine KITLG-KIT signaling. We conclude that membrane-to-nucleus signaling involving GSK3i establishes a previously unrecognized link between the IGF1-IGF1R and KITLG-KIT pathways, which is active in both physiologic and oncogenic contexts and can be exploited for therapeutic purposes.

Pak and Rac GTPases promote oncogenic KIT–induced neoplasms

An acquired somatic mutation at codon 816 in the KIT receptor tyrosine kinase is associated with poor prognosis in patients with systemic mastocytosis and acute myeloid leukemia (AML). Treatment of leukemic cells bearing this mutation with an allosteric inhibitor of p21-activated kinase (Pak) or its genetic inactivation results in growth repression due to enhanced apoptosis. Inhibition of the upstream effector Rac abrogates the oncogene-induced growth and activity of Pak. Although both Rac1 and Rac2 are constitutively activated via the guanine nucleotide exchange factor (GEF) Vav1, loss of Rac1 or Rac2 alone moderately corrected the growth of KIT-bearing leukemic cells, whereas the combined loss resulted in 75% growth repression. In vivo, the inhibition of Vav or Rac or Pak delayed the onset of myeloproliferative neoplasms (MPNs) and corrected the associated pathology in mice. To assess the role of Rac GEFs in oncogene-induced transformation, we used an inhibitor of Rac, EHop-016, which specifically targets Vav1 and found that EHop-016 was a potent inhibitor of human and murine leukemic cell growth. These studies identify Pak and Rac GTPases, including Vav1, as potential therapeutic targets in MPN and AML involving an oncogenic form of KIT.

Histological specificity of alterations and expression of KIT and KITLG in non‐small cell lung carcinoma

Characterization of molecules within important oncogenetic pathways may have future implications for development of therapies and biomarkers in lung cancer. One such target is the tyrosine kinase receptor KIT (c-KIT). We evaluated alterations and expression of KIT and its ligand, KITLG (also known as SCF), in 72 clinical lung tumor specimens of different histologies. Gene copy number, mRNA expression levels, and protein expression were assayed using array-based comparative genomic hybridization, real-time quantitative reverse transcription PCR and immunohistochemistry, respectively. For validation, we investigated copy number alterations and mRNA expression in external microarray data sets of 1,600 and 555 primary lung tumors, respectively. Positivity for KIT staining was most common in large cell neuroendocrine carcinoma (LCNEC) which also showed the highest KIT mRNA expression levels whereas expression was lowest in squamous cell carcinoma (SqCC). KIT mRNA expression levels were higher in KIT immunopositive samples, but expression was not affected by KIT copy numbers. Copy number gains of KIT were significantly more frequent in SqCC compared with adenocarcinoma in our own series and in the 1,600-sample data set. Immunopositivity for both KIT and KITLG in the same tumor was rare except in LCNEC. Our results highlight an increased KIT mRNA expression and frequent KIT immunopositivity in LCNEC but point out a poor correlation between KIT copy numbers and expression in SqCC, perhaps reflecting the existence of a protective mechanism against KIT alterations in this subgroup.