Bruton's tyrosine kinase (BTK) is a member of the SRC-related TEC family of protein tyrosine kinases (PTKs). DT-40 lymphoma B cells, rendered BTK-deficient through targeted disruption of the btk gene by homologous recombination knockout, did not undergo radiation-induced apoptosis, but cells with disrupted lyn or syk genes did. Introduction of the wild-type, or a SRC homology 2 domain or a plecstrin homology domain mutant (but not a kinase domain mutant), human btk gene into BTK-deficient cells restored the apoptotic response to radiation. Thus, BTK is the PTK responsible for triggering radiation-induced apoptosis of lymphoma B cells, and its kinase domain is indispensable for the apoptotic response.

Very little is known regarding the effects of ionizing radiation on cytoplasmic signal transduction pathways.Here, we show that ionizing radiation induces enhanced tyrosine phosphorylation of multiple substrates in human B-lymphocyte precursors. This response to ionizing radiation was also observed in cells pretreated with vanadate, a potent proteintyrosine-phosphatase (PTPase) inhibitor, and phosphotyrosyl [Vals]angiotensin II phosphatase assays showed no decreased PTPase activity in irradiated cells. Thus, enhanced tyrosine phosphorylation in irradiated B-lymphocyte precursors is not triggered by inhibition of total cellular PTPase activity. Immune-complex kinase assays using anti-phosphotyrosine antibodies demonstrated enhanced protein-tyrosine kinase (PTK) activity in the immunoprecipitates from irradiated cells, and the PTK inhibitors genistein and herbimycin effectively prevented radiation-induced tyrosine phosphorylation. Immune-complex kinase assays on irradiated and unirradiated B-lymphocyte precursors using antibodies prepared against unique amino acid sequences of pv9nf, p56/p53'Yn, p55""', and p561ck demonstrated that these Src-family tyrosine kinases were not the primary PTKs responsible for enhanced tyrosine kinase activity in the anti-phosphotyrosine antibody immunoprecipitates or for enhanced tyrosine phosphorylation of multiple substrates. Thus, our fmdings favor the hypothesis that ionizing radiation induces enhanced tyrosine phosphorylation in B-lymphocyte precursors by stimulation of as yet unidentified PTKs. Tyrosine phosphorylation appears to be an important proximal step in radiationinduced apoptosis and clonogenic cell death because inhibition of PTK prevents DNA fragmentation and loss of clonogenicity of irradiated B-lymphocyte precursors. Since PTKs play myriad roles in the regulation of cell function and proliferation, the activation of a PTK cascade, as detailed in this report, may explain some of the pleiotropic effects of ionizing radiation on cellular functions of B-lymphocytes and their precursors.The molecular mechanism by which ionizing radiation inhibits and destroys mammalian cells has been widely explored but not precisely deciphered (1, 2). Protein-tyrosine kinases (PTKs) participate and likely play pivotal roles in initiation of signal cascades that affect proliferation and survival of human B-lymphocyte precursors (3-7). The purpose of this study was to examine the effects of ionizing radiation on PTKs in human B-lymphocyte precursors at discrete developmental stages of B-cell ontogeny.MATERIALS AND METHODS Patient Material and Cell Lines. We used the fetal liver pro-B cell line FL112, the pre-pre-B cell line Reh, the pre-B cell line Nalm-6, and the early B/Burkitt lymphoma cell lines Daudi, Ramos, and Ramos-1 (a subclone of Ramos). The immunophenotypic and genotypic features and the radiation sensitivity of these human B-lymphocyte precursor cell lines were detailed in a previous report (8). We also used primary bone marrow blasts from a pre-pre-B acute lymphoblas...

Exposure of human B-cell precursors (BCP) to ionizing radiation results in cell cycle arrest at the G2-M checkpoint as a result of inhibitory tyrosine phosphorylation of p34cdc2 . Here, we show that ionizing radiation promotes physical interactions between p34cdc2 and the Src family protein-tyrosine kinase Lyn in the cytoplasm of human BCP leading to tyrosine phosphorylation of p34cdc2. Lyn kinase immunoprecipitated from lysates of irradiated BCP as well as a full-length glutathione S-transferase (GST)-Lyn fusion protein-phosphorylated recombinant human p34cdc2 on tyrosine 15. Furthermore, Lyn kinase physically associated with and tyrosine-phosphorylated p34cdc2 kinase in vivo when co-expressed in COS-7 cells. Binding experiments with truncated GST-Lyn fusion proteins suggested a functional role for the SH3 rather than the SH2 domain of Lyn in Lyn-p34cdc2 interactions in BCP. The first 27 residues of the unique amino-terminal domain of Lyn were also essential for the ability of GST-Lyn fusion proteins to bind to p34cdc2 from BCP lysates. Ionizing radiation failed to cause tyrosine phosphorylation of p34cdc2 or G2 arrest in Lyn kinase-deficient BCP, supporting an important role of Lyn kinase in radiation-induced G2 phase-specific cell cycle arrest. Our findings implicate Lyn as an important cytoplasmic suppressor of p34cdc2 function.

Extensive immunologic surface marker analyses and binding competition assays demonstrated that B43 monoclonal antibody (MoAb) is a new member of the CD19 cluster that recognizes the same surface epitope as several other anti-CD19 MoAbs. We used B43 MoAb to test for CD19 expression on neoplastic cells from 340 leukemia and 151 malignant lymphoma patients and on nonneoplastic cells in normal lymphohematopoietic and nonlymphohematopoietic tissues. Our study more than doubles the total number of cases with classified hematologic malignancies that have been examined for CD19 antigen expression. The data presented confirm that CD19 is the most reliable B lineage surface marker and support our view that this B lineage-restricted surface determinant may be an important functional receptor. Our findings provide unique and direct evidence that (a) CD19 is expressed on leukemic B lineage lymphoid progenitor cells freshly obtained from B lineage acute lymphoblastic leukemia patients but not on normal myeloid, erythroid, megakaryocytic, or multilineage bone marrow progenitor cells; (b) ligation of CD19 with B43 MoAb induces sustained increases in [Ca2+]i when crosslinked and inhibits high-molecular weight B cell growth factor (HMW-BCGF)-induced proliferation of activated B cells without affecting their low- molecular weight B cell growth factor (LMW-BCGF) response; therefore CD19 may be a unique signal receptor; (c) HMW-BCGF and LMW-BCGF augment expression of CD19, which suggests that CD19 and BCGF receptors may be under coordinate regulatory control; (d) approximately two million B43 MoAb molecules per cell can be bound to target B lineage lymphoma cells with a Ka of 1.9 x 10(8)/mol/L; (e) CD19 can undergo B43 MoAb-induced internalization; and (f) the opportunity is thus provided for using anti-CD19 MoAb to deliver toxins to B lineage neoplastic cells for more effective treatment of high-risk leukemia/lymphoma patients.

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