In vitro promotion of SDF-1α on the migration and proliferation of C-kit+ cardiac stem cells in neonatal mice.

The aim of this study was to explore the In Vitro effects of stromal-derived factor-1α (SDF-1α) on the migration and proliferation of c-kit+ cardiac stem cells. The lentivirus containing SDF-1α (LV-SDF-1α) was constructed. Primary myocardial fibroblasts were transfected by LV-SDF-1α, followed by primary culture of cardiac tissue cells and separation of c-kit+ cardiac stem cells with a flow cytometer, in order to investigate the effects of SDF-1α on the migration and proliferation of c-kit+ cardiac stem cells using cell co-culture, immunofluorescence and EdU tracing technologies. The results showed that myocardial fibroblasts could secrete SDF-1α after the transfection with LV-SDF-1α. High-purity c-kit+ cardiac stem cells were obtained through flow cytometry sorting and the positive rate was about 40%. The c-kit+ cardiac stem cells cultured In Vitro could be differentiated into cTnT positive cardiomyocyte-like cells. After co-culture of myocardial fibroblasts and c-kit+ cardiac stem cells transfected with lentivirus, SDF-1α might increase the migration of c-kit+ cardiac stem cells, but SDF-1α did not promote the proliferation of c-kit+ cardiac stem cells. In conclusion, the myocardial fibroblasts transfected with lentivirus can highly express SDF-1α, c-kit+ cardiac stem cells can be differentiated into cTnT positive cardiomyocyte-like cells and SDF-1α can effectively enhance the migration of c-kit+ cardiac stem cells but fails to stimulate the proliferation.

4-(Imidazo[1,2-a]pyridin-3-yl): pyrimidine derivatives as anticancer agents.

A series of 4-(imidazo[1,2-a]pyridin-3-yl)-pyrimidine derivatives are claimed as inhibitors of c-KIT and as potential treatments for cancer. Their chemical preparation and biological evaluation against imatinib-resistant tumor cells have been described. Several claimed molecules have excellent IC50 values in the nanomolar range. Several molecules were also selective against a wide panel of kinases. Few specific inhibitors have been found to have promising oral bioavailability and acceptable to excellent values regarding the inhibition of hERG channel. This class represents a new platform for developing new anticancer treatment against a wide range of c-KIT mutations and secondary mutations that may arise in gastrointestinal stromal tumor patients.

Effects of Emodin on Protein Expression Related to Autophagy of Interstitial Cells of Cajal in Diabetic Rats.

This work aims to investigate the effects and mechanism of emodin in treating diabetic gastroenteropathy and colonic dysmotility in STZ + HS/HF diet induced diabetic gastroenteropathy rats. Diabetic colonic dysmotility model was established by high-fat/high-glucose (HS/HF) feeding combined with streptozotocin (STZ). Emodin was divided into high, medium and low dose groups. After eight weeks of intervention, fasting blood glucose (FBG) and body weight were measured. Gastrointestinal transmission time was evaluated. Serum vasoactive intestinal peptide (VIP) and substance P (SP) were detected. Colonic protein expression of selective autophagy adaptor proteins p62 and beclin1 were detected by immunohistochemistry. Colonic protein expression of beclin1, autophagy related gene 5 (Atg5), C-kit and p62 were detected by Western blot. After treating with emodin, gastrointestinal transmission rate was improved. The expression of serum SP was increased and serum VIP was decreased. Colonic c-kit and p62 were up-regulated. The expressions of beclin1 and Atg5 were down-regulated. Emodin can improve colonic dysmotility and promote the recovery of colonic motility and intestinal defecation in diabetic rats. Its mechanism may involved with up-regulating the expression of C-kit and P62, down-regulating the expression of Beclin1 and Atg5 in colon, which are associated with colon over-autophagy of Cajal interstitial cell (ICC).

Avapritinib for Systemic Mastocytosis.

INTRODUCTION: Systemic mastocytosis (SM) is a rare myeloid neoplasm driven in ≈95% of cases by activating KIT mutations, usually D816V. SM can be indolent (ISM), smoldering (SSM) and advanced (AdvSM), the latter characterized by organ damage resulting from infiltrating neoplastic mast cells. The vast majority of cases are indolent, with near-normal life expectancy, although symptoms can be severe. AdvSM, comprising aggressive SM, SM with an associated hematologic neoplasm and mast cell leukemia, however, carries a poor prognosis. Avapritinib is a highly potent and selective inhibitor of mutant KIT. AREAS COVERED: We provide an overview of SM, including the current therapeutic landscape, and discuss avapritinib in detail: its chemistry and discovery, pharmacodynamic and pharmacokinetic data, current approval status and safety and efficacy profiles in both advanced and non-advanced SM. EXPERT OPINION: With a response rate of 75% amongst evaluable patients with AdvSM and marked reductions observed in measures of mast cell and disease burden, avapritinib stands out as a highly effective targeted therapy for this mutant KIT-driven disease. Cognitive impairment may occur, and intracranial hemorrhage has been reported, particularly in association with severe thrombocytopenia. Early results in patients with ISM/SSM are encouraging. Avapritinib is now approved in the US for AdvSM.

FIH-1/c-kit signaling: a novel contributor to corneal epithelial glycogen metabolism.

PURPOSE: Corneal epithelial cells have large stores of glycogen, which serve as their primary energy source. Recently, we demonstrated that factor-inhibiting hypoxia-inducible factor 1 (FIH-1) diminished glycogen stores in vitro and in vivo, working through the Akt/Glycogen Synthase Kinase (GSK)-3ß pathway. In this study we investigated the relationship between FIH-1 and c-kit as it pertains to limbal and corneal epithelial glycogen stores. METHODS: Limbal and corneal epithelia from wild-type FIH-1(-/-) and Kit(W/Wv) mice were stained with periodic acid Schiff (PAS) to detect glycogen. RNA samples prepared from laser-capture microdissected populations of limbal epithelium were subjected to real-time quantitative PCR to determine c-kit ligand expression. Submerged cultures of primary human corneal epithelial keratinocytes (HCEKs) transduced with FIH-1 were treated with c-kit ligand to establish further a FIH-1/c-kit interaction via Western analysis. Akt phosphorylation was assessed by Western blotting. RESULTS: The limbal epithelial cells of FIH-1 null mice had an increase in glycogen levels as well as increased c-kit ligand mRNA compared with wild-type controls. Consistent with a FIH-1/c-kit association, the diminished Akt signaling observed in FIH-1-overexpressing HCEKs could be restored by the addition of c-kit ligand. Interestingly, Akt signaling and glycogen content of the corneal epithelium were significantly decreased in c-kit mutant mice. CONCLUSIONS: c-Kit signaling has been shown to affect glucose metabolism via the Akt/GSK-3ß pathway. An inverse relationship between FIH-1 and c-kit signaling pathways accounts, in part, for differences in glycogen content between corneal and limbal epithelial cells.

[Expression of phosphorylated STAT5 in bone marrow hematopoietic stem cells of patients with paroxysmal nocturnal hemoglobinuria before and after in vitro G-CSF or SCF stimulation].

OBJECTIVE: To study the expressions of phosphorylated STAT5 (P-STAT5) in CD34(+)CD59(-) and CD34(+)CD59(+) bone marrow cells of the patients with paroxysmal nocturnal hemoglobinuria (PNH) before and after in vitro G-CSF or SCF stimulation, then evaluate the functions of G-CSF and SCF receptors in PNH clone cells. METHODS: Bone marrow mononuclear cells (BMMNC) of 26 PNH patients and 14 normal controls were stimulated in vitro with G-CSF (100 ng/ml) or SCF (100 ng/ml) for 10 min. Before and after these stimulations, the mean fluorescence intensity (MFI) of P-STAT5 in CD34(+)CD59(+) BMMNC and CD34(+)CD59(-) BMMNC were measured by flow cytometry. RESULTS: (1) The P-STAT5 MFI was (24 ± 18) in unstimulated CD34(+)CD59(-) cells. And it was significantly lower than that in unstimulated CD34(+)CD59(+) cells of PNH patients (64 ± 49) and normal controls (61 ± 33) (both P < 0.01). No statistic difference existed between the latter two. (2) The P-STAT5 MFI was (36 ± 35) in G-CSF stimulated CD34(+)CD59(-) cells of PNH patients. And it was significantly lower than that in G-CSF stimulated CD34(+)CD59(+) cells of PNH patients (124 ± 84) and normal controls (116 ± 59) (both P < 0.01). There was no statistic difference between the latter two. (3) The P-STAT5 MFI was (34 ± 27) in SCF stimulated CD34(+)CD59(-) cells of PNH patients. And it was significantly lower than that in SCF stimulated CD34(+)CD59(+) cells of PNH patients (124 ± 97) and normal controls (128 ± 62) (both P < 0.01). And no statistic difference existed between the latter two. (4) The increased P-STAT5 MFI in G-CSF stimulated CD34(+)CD59(-) cells of PNH patients was significantly lower than that in G-CSF stimulated CD34(+)CD59(+) cells of PNH patients and normal controls (both P < 0.01). No statistic difference existed between the latter two. The increase of P-STAT5 MFI in SCF stimulated CD34(+)CD59(-) cells of PNH patients was significantly lower than that in SCF stimulated CD34(+)CD59(+) cells of PNH patients and normal controls (both P < 0.01). No statistic difference existed between the latter two. CONCLUSION: There is a lower expression of P-STAT5 expressed in G-CSF or SCF stimulated PNH clone cells compared to that in normal clone cells.

Foe turned friend: multiple functional roles attributable to hyper-activating stem cell factor receptor mutant in regeneration of the haematopoietic cell compartment.

OBJECTIVES: Stem cell factor receptor, c-kit, is considered to be the master signalling molecule of haematopoietic stem cells. It develops the orchestral pattern of haematopoietic cell lineages, seen by its varying degree of omnipresence in progenitors, lineage committed and mature cells. We have investigated the effect of over-expressing c-kit on early recovery of the haematopoietic compartment, in irradiated hosts. MATERIALS AND METHODS: Normal bone marrow cells (BMCs) were transfected with Kit(wt) (wild-type c-kit) or its variant Kit(mu) (asp814tyr) by electroporation. Lethally irradiated mice were transplanted with normal or transfected congeneic BMCs. The effect of ectopic expression of c-kit on haematopoietic cell recovery was determined by analysing donor-derived cells. Furthermore, effects of both types of c-kit over-expression on progenitor and lineage-committed cells were examined by flow cytometric analysis of Sca-1 and lineage-committed (Lin(+)) cells respectively. RESULTS: Hyper-activating Kit(mu) significantly improved recovery of the haematopoietic system in irradiated hosts. In vivo results showed that the donor-derived c-kit(+) cell population was increased to more than 3-fold in the case of Kit(mu)-transfected cells compared to normal and Kit(wt) over-expressing BMCs. In general, survival of progenitor and committed cell was improved in the Kit(mu) over-expressing system compared to the other two cohorts. CONCLUSION: These results suggest that recruitment of the hyper-activating variant of c-kit (Kit(mu)) lead to early recovery of the bone marrow of lethally irradiated mice.

c-Kit receptor expression in normal human Schwann cells and Schwann cell lines derived from neurofibromatosis type 1 tumors.

The growth factor receptor c-Kit has several well-characterized functions during the development of numerous cell types, including red blood cells, mast cells, and melanocytes. Its role in Schwann cells has been described in transformed cells derived from malignant peripheral nerve sheath tumors from patients with neurofibromatosis type 1 (NF1 MPNST; Badache et al. [1998] Oncogene 17:795-800). However, c-Kit functions have not been investigated in normal Schwann cells. We report here that neonatal rat Schwann cells express low c-Kit levels, whereas expression levels for c-Kit are high for Schwann cells derived from MPNST of NF1 patients. In addition, c-Kit expression is not detectable in normal adult human Schwann cells. Although the c-Kit ligand stem cell factor (SCF) induces the phosphorylation of protein kinase B (or Akt) and prevents apoptosis in Schwann cells, SCF has no effect on the proliferation or differentiation of Schwann cells.

The multi-targeted kinase inhibitor SU5416 inhibits small cell lung cancer growth and angiogenesis, in part by blocking Kit-mediated VEGF expression.

SU5416 is a multi-targeted kinase inhibitor that potentially has the ability to directly block tumor growth by inhibiting Kit signaling, as well as blocking angiogenesis by inhibiting vascular endothelial growth factor receptor (VEGFR) signaling. Previous work has demonstrated that SU5416 efficiently blocks Kit-mediated growth of small cell lung cancer (SCLC) in vitro. To determine the drug's effect on in vivo growth of SCLC, we studied its activity, alone and in combination with carboplatin, in chemotherapy-resistant H526, and chemotherapy-sensitive H209 murine xenograft models. SU5416 efficiently inhibited Kit activity in vivo when administered on a twice-weekly schedule. When administered over a 3-week period to animals bearing established tumors, it inhibited growth by at least 70%. It was at least as effective as carboplatin in suppressing growth of H526 xenografts. However, the combination with carboplatin was not superior to the most active single agent in either xenograft model at the doses and schedule utilized. SU5416 clearly inhibited growth in part by inhibiting angiogenesis, with microvessel density dropping by approximately 50% in treated xenografts. In addition to the recognized mechanism of inhibition of VEGFR, we uncovered a novel mechanism of angiogenesis suppression by demonstrating reduced VEGF expression in SU5416-treated xenografts. In vitro, stem cell factor treatment of the H526 cell line enhanced expression of VEGF, which was efficiently blocked with SU5416. Thus, we have demonstrated that SU5416 can inhibit SCLC growth by directly inhibiting tumor cell proliferation and by inhibiting angiogenesis, in part by inhibiting Kit-mediated VEGF expression. These data suggest that kinase inhibitors that target both Kit and VEGFR could play an important role in the treatment of SCLC, as well as other malignancies that express Kit.

Biology of gastrointestinal stromal tumors: KIT mutations and beyond.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the digestive tract. Aspects of the morphology and immunophenotype in GISTs resemble those in the interstitial cells of Cajal (ICC), which are a specialized cell type responsible for coordinating peristaltic activity throughout the gastrointestinal tract. Therefore, it is possible that GISTs result from transformation of nonneoplastic progenitor cells that would normally differentiate towards an ICC endpoint. Activation of the KIT receptor tyrosine kinase is required for differentiation and proliferation of nonneoplastic ICC, and oncogenic KIT mutations are a crucial event in the development of most GISTs. These mutations can involve either the extracellular or intracellular domains of the KIT receptor, giving rise to conformational changes that enable constitutive, ligand-independent, activation of the KIT protein. Oncogenic KIT activation leads to phosphorylation of various substrate proteins and, in turn, to activation of signal transduction cascades regulating cell proliferation, apoptosis, chemotaxis, and adhesion. Recently, a small molecule tyrosine kinase inhibitor (STI571, imatinib mesylate, Gleevec) directed against the enzymatic (kinase) domain of the KIT protein was found to produce dramatic clinical responses as monotherapy for metastatic GISTs. This review focuses on the biological and molecular genetic principles of GISTs, and particularly the role of mutant KIT as a therapeutic target.

Soluble c-kit receptor mobilizes hematopoietic stem cells to peripheral blood in mice.

OBJECTIVE: The mechanisms of mobilization of hematopoietic stem cells (HSC) from bone marrow to peripheral blood (PB) by cytokines are poorly understood. One hypothesis is that cytokines disrupt cytoadhesive interactions of stem cells with bone marrow stroma. The soluble portion of c-kit (s-kit) binds stem cell factor (SCF) and can specifically block the ability of SCF to bind HSC. MATERIALS AND METHODS: To examine stem cell mobilization by s-kit, we prepared PB mononuclear cells from s-kit- or granulocyte colony-stimulating factor (G-CSF)-treated mice and assayed their colony-forming abilities and their long-term reconstituting abilities by transplantation into lethally irradiated Ly-5.2 congenic mice. RESULTS: We confirmed the published findings that human recombinant s-kit can block SCF-stimulated hematopoietic colony growing. We then found that s-kit could mobilize colony-forming cells from bone marrow to PB, and we found long-term reconstitution cells in the PB from s-kit-treated mice. The majority of s-kit-mobilized stem cells were in the CD34(+) cell population. We also tested the additive effect between G-CSF and s-kit. The mean percentages of donor cells in the mice transplanted with Lin(-) cells from the G-CSF-treated mice and the G-CSF/s-kit-treated mice were 44.6% and 64.8%, respectively (p=0.028). CONCLUSIONS: These findings demonstrate that stem cells with long-term engraftment capabilities can be mobilized by s-kit, and that s-kit combined with G-CSF treatment leads to significant enhancement of engraftment efficiency, suggesting mobilization via disruption between c-kit and SCF as the mechanism.

Tr-kit promotes the formation of a multimolecular complex composed by Fyn, PLCgamma1 and Sam68.

Tr-kit is a truncated form of the tyrosine kinase receptor c-kit expressed in the haploid phase of spermatogenesis. Upon microinjection, tr-kit triggers metaphase-to-anaphase transition in mouse eggs by the sequential activation of Fyn and PLCgamma1. Here, we show that tr-kit promotes the interaction of several tyrosine-phosphorylated proteins with the SH3 domain of PLCgamma1. Western blot analysis indicates that one of these proteins is Sam68, an RNA-binding protein that is known to interact with and be phosphorylated by Src-like kinases in mitosis. tr-kit promotes the association of Sam68 with PLCgamma1 and Fyn in a multimolecular complex, as demonstrated by co-immunoprecipitation of the phosphorylated forms of these proteins using antibodies directed to anyone of the partners of the complex. Expression of tr-kit potentiates the interaction of endogenous Sam68 also with the SH3 domain of Fyn. Furthermore, the subcellular localization of Sam68 is affected by tr-kit through activation of Fyn in live cells. Lastly, we show that interaction with the SH3 domain of Fyn triggers the release of Sam68 from bound RNA. Thus, our data suggest that tr-kit promotes the formation of a multimolecular complex composed of Fyn, PLCgamma1 and Sam68, which allows phosphorylation of PLCgamma1 by Fyn, and may modulate RNA metabolism.

Imatinib mesylate causes hypopigmentation in the skin.

BACKGROUND: Imatinib mesylate is a tyrosine kinase inhibitor that targets the BCR-ABL protein in CML, c-kit (KIT) and platelet-derived growth factor receptors. In clinical trials with imatinib mesylate, common side effects of nausea, emesis, diarrhea, periorbital edema, fluid retention, and myelosuppression have been documented. METHODS: In this case series, the authors describe unique clinical findings of skin hypopigmentation in six patients with CML who were treated with imatinib mesylate. RESULTS: Most patients developed onset of skin hypopigmentation within the first month of treatment and all of the patients experienced additional drug toxicity. Despite patient susceptibility to toxicity, the presence of hypopigmentation did not appear to predict leukemic cell response or clinical outcome. All six patients established a hematologic response but only two patients had a complete cytogenetic response. Imatinib mesylate induced hypopigmentation also appeared to be reversible and potentially dose related. CONCLUSION: Skin hypopigmentation is a benign side effect from imatinib mesylate treatment that appears to be reversible upon discontinuation or dose reduction. Several lines of evidence have previously reported that KIT and its ligand stem cell factor (SCF) have a regulatory role in melanocyte development and survival, suggesting a rational mechanism of action for imatinib mesylate in the pathogenesis of hypopigmentation. The signal transduction mechanism currently is believed to involve SCF ligand binding of KIT and downstream activation of MAP kinase (Erk-2). Microphthalmia (Mi), a basic helix-loop-helix leucine zipper (bHLHZip) transcription factor, is phosphorylated by MAP kinase at a serine residue (S73). Once phosphorylated, Mi transactivates the tyrosine pigmentation gene promoter and affects pigment production.

[Treatment of systemic mastocytosis]. / Traitement des mastocytoses systémiques.

BACKGROUND: Systemic mastocytosis is a rare disease, characterized by mast cells proliferation in various organs. Two types of clinical manifestations can be distinguished: those related to mast cells mediators release and those related to tumoral proliferation involving different organs, these later defining aggressive systemic mastocytosis. Until recently, treatment was mainly symptomatic, without anti tumoral effect. RECENT FACTS: These last years, advances have been made in the understanding of the disease with the discovery of the c-kit oncogene mutation and the approach of the disease as a myeloproliferative disorder. PERSPECTIVES: Based on experiences acquired in the treatment of this kind of disorders, evaluation of new therapeutics, such as cladribine or combination of interferon-alpha and cytarabine is in progress. At least, tyrosine kinase inhibitors, a new family of molecules, are able of inhibiting some types of the mutated c-kit protein and one of them, imatinib mesylate, has shown a great efficacy in the treatment of gastro intestinal stromal tumors (GIST) which also involves the c-kit mutation. By analogy, treatment of patients with c-kit susceptible mutation might be treated with this molecule.

Activation of human mast cells through stem cell factor receptor (KIT) is associated with expression of bcl-2.

BACKGROUND: Mast cells (MCs) are multifunctional effector cells of the immune system. These cells originate from pluripotent hemopoietic progenitors. In contrast to basophils and other leukocytes, MCs exhibit a remarkably long life span (years) in vivo. Although a role for stem cell factor (SCF) and SCF receptor (KIT) in long-term survival of MCs has been proposed, the underlying biochemical mechanisms remain unknown. MATERIALS AND METHODS: We have examined expression of 'survival-related' molecules of the bcl-2 family including bcl-2 and bcl-x(L), in primary human MCs and the human MC line HMC-1. Primary MCs were isolated from dispersed lung tissue by cell sorting using an antibody against KIT. mRNA expression was analyzed by RT-PCR and Northern blotting. RESULTS: As assessed by RT-PCR, purified unstimulated lung MCs (>98% pure) exhibited KIT- and bcl-x(L) mRNA, but did not express bcl-2 mRNA. However, exposure of lung MCS to SCF (100 ng/ml) for 8 h resulted in expression of bcl-2 mRNA. Corresponding results were obtained by immunocytochemistry. In fact, exposure of MC to SCF resulted in expression of the bcl-2 protein whereas unstimulated MCs displayed only the bcl-x(L) protein without expressing the bcl-2 protein. The human MC leukemia cell line HMC-1, which contains a mutated and intrinsically activated SCF receptor, showed constitutive expression of both bcl-2 and bcl-x(L) at the mRNA and protein level. CONCLUSION: Our data show that human MCs can express members of the bcl-2 family. It is hypothesized that bcl-x(L) plays a role in KIT-independent growth of MCs, whereas bcl-2 may be involved in KIT-dependent functions of MCs.

Stem cell factor: biology and relevance to clinical practice.

The type III tyrosine kinase receptor c-KIT and its ligand stem cell factor (SCF; also known as KIT ligand, mast cell growth factor and steel factor) are closely involved in the regulation of a wide range of tissues at different stages of life. This review provides an outline of the discovery, structure and expression of SCF and c-KIT but concentrates on their respective roles in the regulation of human haemopoiesis and how this knowledge might be exploited in the clinical setting.

Activating mutations of c-kit at codon 816 confer drug resistance in human leukemia cells.

An improved understanding of how leukemia cells grow and become resistant to treatment remains critical for developing more effective therapies. We have identified activating mutations of c-kit at codon 816 (Asp(816) ) from a revertant of the cytokine-dependent acute myeloid leukemia (AML) cell line, MO7e (D816H), and de novo childhood AML (D816N). Following transduction of the mutant c-kit cDNAs, MO7e cells acquire a growth advantage and resistance to apoptosis in response to chemotherapeutic drugs and ionizing radiation, in addition to cytokine-independent survival. Although stimulation of mutant c-kit-bearing MO7e cells with stem cell factor (SCF), a ligand for c-Kit, does not have a significant effect on cell proliferation, SCF further inhibits apoptosis induced by cytotoxic agents. These results suggest a potentially important role of Asp(816) mutations of c-kit in both malignant cell proliferation and resistance to therapy.

Soluble Kit receptor blocks stem cell factor bioactivity in vitro.

Stem cell factor (SCF) is a growth factor that promotes the survival, proliferation, and differentiation of hematopoietic cells. SCF and its receptor, Kit, are normally present in both cell surface and soluble forms. Both forms of Kit can bind SCF. However, the function of soluble Kit is unknown. In order to determine if soluble Kit can modulate SCF activity, we produced a fusion protein, Kit-Fc, comprised of the extracellular domain of murine Kit and the Fc portion of human IgG(1) and investigated its ability to bind 125I-SCF and to inhibit SCF-stimulated hematopoietic colony growth in vitro. Stable cell lines expressing Kit-Fc were generated and Kit-Fc was purified to greater than 95% purity. Scatchard analysis demonstrated that Kit-Fc binds iodinated SCF with high affinity (Kd 570 pM). Kit-Fc also bound to transmembrane SCF displayed on the surface of fibroblasts. The murine mast cell line IC2 was engineered to express murine Kit on the cell surface and was demonstrated to proliferate in the presence of SCF. Kit-Fc completely blocked SCF-stimulated proliferation of IC2-Kit cells, but not IL-3-stimulated growth of IC2-Kit cells, demonstrating the specificity of Kit-Fc. We investigated the ability of Kit-Fc to block SCF-stimulated murine hematopoietic colony growth. Kit-Fc blocked SCF-stimulated erythroid colony growth as effectively as a neutralizing anti-Kit monoclonal antibody, ACK2, but did not block erythropoietin-stimulated erythroid colony growth. Likewise, Kit-Fc blocked SCF-stimulated myeloid colony growth as effectively as ACK2 antibody, but did not block IL-3- or GM-CSF-stimulated myeloid colony growth. These results indicate that a form of soluble Kit binds SCF with high affinity, and can specifically block the ability of SCF to stimulate hematopoietic colony growth, suggesting that one function of soluble Kit may be to modulate SCF bioactivity.