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.

Specific and common activities of the FLT3 and KIT tyrosine kinase receptors revealed by the use of cultured mast cells

Development of the hematopoietic lineages is partially under the control of hematopoietic receptors with tyrosine kinase activity (RTK). To compare the cellular functions of two of the class III RTK, FLT3 and KIT, a murine chimeric FMS/FLT3 (FF3) receptor was expressed ectopically using retroviral infection, in normal IL3-derived cultured mast cells. Stimulation of the chimeric receptor produced a full mitogenic signal and led to mast cell maturation, as occurs upon activation of the endogenous KIT receptor. When introduced into mast cells derived from KIT-deficient White spotting (W) mutant mice, the FF3 receptor bypassed their mitogenic defect. KIT activation induced a synergistic mitogenic activity in mast cells upon IL3 stimulation, whereas FF3 appeared to down-modulate the IL3 response. Adhesion to fibronectin was specifically associated with KIT signaling.

Phosphatidylinositol-3' kinase is not required for mitogenesis or internalization of the Flt3/Flk2 receptor tyrosine kinase

Flt3/Flk2 is a receptor tyrosine kinase that is expressed on early hematopoietic progenitor cells. Flt3/Flk2 belongs to a family of receptors, including Kit and colony-stimulating factor-1R, which support growth and differentiation within the hematopoietic system. The Flt3/Flk2 ligand, in combination with other growth factors, stimulates the proliferation of hematopoietic progenitors of both lymphoid and myeloid lineages in vitro. We report that phosphatidylinositol 3'-kinase (PI3K) binds to a unique site in the carboxy tail of murine Flt3/Flk2. In distinction to Kit and colony-stimulating factor-1R, mutant receptors unable to couple to PI3K and expressed in rodent fibroblasts or in the interleukin 3-dependent cell line Ba/F3 provide a mitogenic signal comparable to wild-type receptors. Flt3/Flk2 receptors that do not bind to PI3K also normally down-regulate, a function ascribed to PI3K in the context of other receptor systems. These data point to the existence of other unidentified pathways that, alone or in combination with PI3K, transduce these cellular responses following the activation of Flt3/Flk2.

Substrate specificities and identification of a putative binding site for PI3K in the carboxy tail of the murine Flt3 receptor tyrosine kinase

Flt3 is a receptor tyrosine kinase (RTK) structurally related to the CSF-1R encoded by the c-fms locus, Kit and the PDGFR which is restricted in its expression to hematopoietic precursor populations and several distinct cell types within the central nervous system. Although the ligand for Flt3 has recently been identified, the developmental function of Flt3 within these tissues has not yet been described. In order to examine the signalling properties of this receptor, we previously constructed a chimeric molecule containing the extracellular domain of CSF-1R fused to the transmembrane and cytoplasmic domain of mouse Flt3 (FF3). The ability of the FF3 to directly associate with or tyrosine phosphorylate specific cytoplasmic signalling molecules in vivo was examined. GAP, Vav, Shc, and to a lesser extent PLC gamma become tyrosine-phosphorylated but no in vivo association with the receptor was detectable. FF3 associates with PI3K activity and the SH2 domains of p85 and Grb-2. Phosphopeptide competition experiments suggest that the PI3K binding site is located outside of the kinase insert in the carboxy tail of the receptor.

Analysis of the mitogenic pathway of the FLT3 receptor and characterization in its C terminal region of a specific binding site for the PI3' kinase

The FLT3 receptor tyrosine kinase (RTK) belongs to the class III subfamily which includes PDGF, CSF1 and SLF receptors. The recent cloning of the FLT3 ligand suggesting its important role in the differentiation and proliferation of the hematopoietic stem cells, has confirmed the initial expression analysis showing restricted pattern of receptor expression within the primitive hematopoietic population. To better understand the function of the FLT3 receptor and its relationship with the other hematopoietic RTKs, we analyzed the mitogenic pathway and substrate specificity of this receptor. The construction of a chimeric receptor called FF3, between the extracellular region of the CSF1 receptor fused with the transmembrane and the cytoplasmic regions of FLT3, has allowed an analysis in the absence of FLT3 ligand. We have shown in previous studies that FF3 is able to transduce the signal induced by CSF1, to induce tyrosine phosphorylation and/or association of several cytoplasmic proteins. We show here that this new receptor is fully functional in Ba/F3 hematopoietic cells, inducing a CSF1 dependence when expressed at the surface of this IL3 dependent cell line. The PI3' Kinase interacts with the FF3 receptor through SH2 domains and its binding site is localized on the tyrosine residue 958 in the C terminal part of the receptor.

An alternative method for T-cell receptor repertoire analysis: clustering of human V-beta subfamilies selected in responses to staphylococcal enterotoxins B and E

We have designed a convenient procedure for the analysis of V beta repertoire expression in polyclonal T-cell populations. In this procedure T-cell RNA is converted to cDNA, polydC-tailed with terminal deoxynucleotidyl transferase and submitted to one-side specificity PCR amplification with a constant region oligonucleotide primer. The amplified material is then analysed by reverse spot-test hybridization: after 32P-labelling, the amplification product is put to hybridize on a membrane where specially designed V beta subfamily-specific probes are immobilized. The radioactivity fixed on each probe can then be easily quantified and the signal obtained is directly proportional to the initial amount of homologous RNA. We applied this technique to the study of V beta gene selection following T-cell stimulation by staphylococcal enterotoxins B and E. We show that with these toxins two almost non-overlapping sets of T-cells are recruited and that this selection is likely to be dependent on specific amino acid residues shaping the fourth complementarity determining region of the TCR-beta chain. These residues constitute two tandemly-conserved tripeptide sequences (Asp39Pro40Gly41)-(Val69Ser70Arg71) and (Arg66Phe67Ser68)-(Asp88Ser89Ala90) in the SEB- and the SEE-responsive V beta gene clusters respectively.