Early signaling pathways activated by c-Kit in hematopoietic cells

STAT3 activation is required for Asp(816) mutant c-Kit induced tumorigenicity

Activating mutations of c-kit at codon 816 (Asp(816)) have been identified in variety of malignancies, including acute myeloid leukemia (AML), mastocytosis and germ cell tumors. The mutant c-Kit receptor confers cytokine independence and induces tumorigenicity. However, the molecular mechanisms, particularly the changes in the signal transduction pathways, responsible for these biological effects induced by mutant c-Kit are largely undefined. Using the human embryonic kidney cell line, 293, we show in the current report that constitutive activation of STAT3 and STAT1 is associated with D816H mutant c-Kit. Transfection of dominant negative STAT3, but not STAT1 inhibits mutant c-Kit mediated anchorage-independent growth in vitro and tumor formation in vivo. Expression of constitutively activated STAT3 restores the mutant c-Kit receptor's transforming ability in 293 cells. These results demonstrate that activation of STAT3 by Asp(816) mutant c-Kit is required for the anchorage-independent growth and tumorigenicity induced by Asp(816) mutant c-Kit.

Stem cell factor protects c-kit+ human primary erythroid cells from apoptosis

OBJECTIVE

It has been reported that stem cell factor (SCF) promotes cell survival in primary cultured human erythroid colony-forming cells (ECFC). Given the heterogeneous nature of ECFC, which may affect interpretation of the data, we purified c-kit+ ECFC and investigated the specificity and mechanisms of the anti-apoptotic effects of SCF on these cells.

MATERIALS AND METHODS

Glycophorin A+ (GPA+) c-kit+ cells were purified from primary cultured ECFC derived from purified human CD34+ cells. The GPA+c-kit- and nonerythroid cells were generated from the same CD34+ cells. Apoptosis of ECFC was investigated in the absence or presence of SCF and erythropoietin (EPO) in serum-free medium. DNA fragmentation was measured with enzyme linked immunosorbent assay for oligonucleosome-sized DNA, gel electrophoresis, and annexin V labeling. Characterization of expanded cells and enriched cells was performed using multiparameter flow cytometry. For Akt assay, cells were lysed and the cleared lysates subjected to SDS-PAGE followed by Western blotting.

RESULTS

In GPA+c-kit+ cells, deprivation of cytokine caused rapid DNA fragmentation within 4 hours that reached a maximum at 6 hours. This was partially but clearly prevented by SCF or EPO. In contrast, no significant DNA fragmentation was seen in GPA+c-kit- and nonerythroid cells within 24 hours. PP2, a specific Src family kinase inhibitor, but not its inactive analogue PP3, reversed the anti-apoptotic effects of SCF. PP2 also inhibited SCF-induced phosphorylation of Akt.

CONCLUSION

These data indicate that SCF protects purified human GPA+c-kit+ cells from apoptosis and suggest that kit-mediated Src kinase activation is involved in Akt activation and cell survival.

Stem cell factor-induced migration of mast cells requires p38 mitogen-activated protein kinase activity

Stem cell factor (SCF) can be considered a cardinal cytokine in mast cell biology as it affects mast cell differentiation, survival, and migration. The objective of this study was to investigate the role of two mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase (ERK) and p38, in SCF-induced cell migration. This was examined in mouse mast cells by using PD 098059 and SB203580, which are specific inhibitors of mitogen-induced extracellular kinase (MEK) and p38 MAP kinase, respectively. SCF induced a rapid and transient activation of ERK and p38 in a dose-dependent manner. Inhibition of p38 activity by SB203580 was paralleled with a marked reduction of migration toward SCF, whereas the effect of the MEK inhibitor was less pronounced. This is the first report of a physiological function of SCF-dependent activation of p38. Whether p38-mediated mast cell migration is a possible target for suppression of mast cell hyperplasia remains to be determined.

Essential requirement for c-kit and common gamma chain in thymocyte development cannot be overruled by enforced expression of Bcl-2

The thymus in mice lacking both the receptor tyrosine kinase c-kit and the common cytokine receptor gamma chain (gamma(c)) is alymphoid because these receptors provide essential signals at the earliest stages of thymocyte development. The signals transduced by these receptors potentially regulate proliferation, survival, or differentiation, but the contribution of each receptor to distinct intracellular signaling cascades is only poorly defined. Here, we have examined whether enforced expression of Bcl-2 can rescue thymocyte development in c-kit and gamma(c) single or double mutant mice. A bcl-2 transgene (E(mu)-bcl-2-25; expressed in the T cell lineage) was introduced into (a) c-kit and gamma(c) wild-type (c-kit+gamma(c)+bcl+), (b) c-kit-deficient (c-kit(-)gamma(c)+bcl+), (c) gamma(c)-deficient (c-kit+gamma(c)-bcl+), or (d) c-kit and gamma(c) double-deficient mice (c-kit-gamma(c)-bcl+). The bcl-2 transgene was functionally active in wild-type and c-kit or gamma(c) single mutants, as it promoted survival of ex vivo isolated thymocytes, including pro-T cells. In vivo, however, transgenic Bcl-2 did not release T cell precursors from their phenotypic block and failed to increase progenitor or total thymocyte cellularity in c-kit or gamma(c) single or double mutants. These data argue strongly against a role for Bcl-2 as a key mediator in signaling pathways linked to cytokine and growth factor receptors driving early thymocyte development.

Signal transducer and activator of transcription 3 activation is required for Asp816 mutant c-Kit–mediated cytokine-independent survival and proliferation in human leukemia cells

Activating mutations of c-kit at codon 816 (Asp816) have been implicated in a variety of malignancies, including acute myeloid leukemia (AML). The mutant c-Kit receptor confers cytokine-independent survival of leukemia cells and induces tumorigenicity. Changes in the signal transduction pathways responsible for Asp816 mutant c-Kit–mediated biologic effects are largely undefined. The results of this study show that Asp816 mutant c-Kit induces constitutive activation of signal transducer and activator of transcription 3 (STAT3) and STAT1, and up-regulates STAT3 downstream targets, Bcl-xL and c-myc. The phosphatidylinositol-3-kinase (PI-3K)/Akt pathway, but not the Ras-mediated mitogen-activated protein (MAP) kinase pathway, is also constitutively activated by Asp816 mutant c-Kit. Suppression of STAT3 activation by a dominant negative molecule in MO7e leukemia cells transduced with mutant c-kit inhibits stem cell factor (SCF)-independent survival and proliferation, accompanied by the down-regulation of Bcl-xL and c-myc. However, activated STAT3 does not appear to be the sole mediator that is responsible for the phenotypic changes induced by Asp816 mutant c-Kit, because expression of constitutively activated STAT3 in MO7e cells does not completely reconstitute cytokine independence. Activation of other signaling components by mutant c-Kit, such as those in the PI-3K/Akt pathway, is demonstrated and may also be needed for the mutant c-Kit–mediated biologic effects. The investigation of altered signal transduction pathways and the resulting functional consequences mediated by Asp816 mutant c-Kit should provide important information for the characterization of subsets of leukemia and potential molecular pathways for therapeutic targeting.

Induction of Telomerase Activity During Development of Human Mast Cells from Peripheral Blood CD34+Cells: Comparisons with Tumor Mast-Cell Lines

To further characterize the development of mast cells from human hemopoietic pluripotent cells we have investigated the expression of telomerase activity in cultured human peripheral blood CD34+ cells, and CD34+ /CD117+ /CD13+ progenitor mast cells selected therefrom, with the idea that induction of telomerase is associated with clonal expansion of CD34+ /CD117+ /CD13+ cells. A rapid increase in telomerase activity preceded proliferation of both populations of cells in the presence of stem cell factor and either IL-3 or IL-6. The induction was transient, and telomerase activity declined to basal levels well before the appearance of mature mast cells. Studies with pharmacologic inhibitors suggested that this induction was initially dependent on the p38 mitogen-activated protein kinase and phosphatidylinositol 3'-kinase, but once cell replication was underway telomerase activity, but not cell replication, became resistant to the effects of inhibitors. Tumor mast cell lines, in contrast, expressed persistently high telomerase activity throughout the cell cycle, and this expression was unaffected by inhibitors of all known signaling pathways in mast cells even when cell proliferation was blocked for extended periods. These results suggest that the transient induction of telomerase activity in human progenitor mast cells was initially dependent on growth factor-mediated signals, whereas maintenance of high activity in tumor mast cell lines was not dependent on intracellular signals or cell replication.

Requirements for the kit receptor tyrosine kinase during regeneration of zebrafish fin melanocytes

Embryonic neural crest-derived melanocytes and their precursors express the kit receptor tyrosine kinase and require its function for their migration and survival. However, mutations in kit also cause deficits in melanocytes that make up adult pigment patterns, including melanocytes that re-establish the zebrafish fin stripes during regeneration. As adult melanocytes in mice and zebrafish are generated and maintained by stem cell populations that are presumably established during embryonic development, it has been proposed that adult phenotypes in kit mutants result from embryonic requirements for kit. We have used a temperature-sensitive zebrafish kit mutation to show that kit is required during adult fin regeneration to promote melanocyte differentiation, rather than during embryonic stages to establish their stem cell precursors. We also demonstrate a transient role for kit in promoting the survival of newly differentiated regeneration melanocytes.

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.

Transcriptional and ERK1/2-dependent synergistic upregulation of p21(cip1/waf1) associated with steel factor synergy in MO7e

Steel factor (SLF) plus GM-CSF induces proliferative synergy in myeloid progenitors and factor-dependent cell line MO7e. We previously reported that the protein level of cyclin-dependent kinase inhibitor p21(cip1/waf1) (p21) increased synergistically when MO7e cells were stimulated with SLF plus GM-CSF and that p21 induction was required for SLF synergistic responses. Here we show that this p21 induction is regulated at the transcriptional level. Based on use of a multiprobe RNase protection assay, the synergistic increase of p21 mRNA was unique among many cell cycle regulators. While STAT5A and 5B were activated after stimulation with GM-CSF alone or SLF plus GM-CSF, there was no difference in activation between the groups. p44/42 MAP kinase (ERK1/2) was synergistically activated by SLF plus GM-CSF, but SAPK/JNK and p38 MAP kinase were not. Synergistic induction of p21 was significantly decreased with a MEK1 inhibitor, suggesting that the ERK1/2 pathway is involved in the synergistic increase of p21 after GM-CSF plus SLF stimulation.

Hematopoietic stem cells need two signals to prevent apoptosis; BCL-2 can provide one of these, Kitl/c-Kit signaling the other

Growth factors can cause cells to proliferate, differentiate, survive, or die. Distinguishing between these responses is difficult in multicellular, multiparameter systems. Yet this is essential to understand the impact on cells like hematopoietic stem cells (HSCs), which have strict and still poorly understood growth factor requirements. Single cell plating in serum-free medium allows direct assessment of growth factor responses. The range of tested factors can be expanded if the cells are protected from growth factor deprivation-induced apoptosis. BCL-2 is overexpressed in HSCs of H2K-BCL-2 transgenic mice, protecting them from many apoptotic stimuli. The response of single wild-type and transgenic HSCs to stimulations with individual factors was tested. Surprisingly, we find that high level BCL-2 expression does not prevent rapid death under serum-free conditions, even though it does in the presence of serum. We also find that transgenic, but not wild-type cells, survive and proliferate rapidly in response to steel factor (Kit ligand). These studies show that two separate signals are necessary to prevent apoptosis in HSCs, and that Kit ligand by itself provides a strong proliferative stimulus to HSCs. However, the proliferative response does not result in self-renewal, but in differentiation to all known hematopoietic oligolineage progenitors.

Role of Sertoli cells in injury-associated testicular germ cell apoptosis

This review examines experimental models of Sertoli cell injury resulting in germ cell apoptosis. Since germ cells exist in an environment created by Sertoli cells, paracrine signaling between these intimately associated cells must regulate the process of germ cell death. Germ cell apoptosis may be signaled by a decrease in Sertoli cell pro-survival factors, an increase in Sertoli cell pro-apoptotic factors, or both. The different models of Sertoli cell injury indicate that spermatogenesis is susceptible to disruption, and that targeting critical Sertoli cell functions can lead to rapid and massive germ cell death.