Toshio Hirano

Full Oncogenic Activities of v-Src Are Mediated by Multiple Signaling Pathways Ras AS AN ESSENTIAL MEDIATOR FOR CELL SURVIVAL

Tyrosine kinase oncoproteins cause simultaneous activation of multiple intracellular signaling pathways. However, the precise mechanisms by which individual pathways induce oncogenesis are not well understood. We have investigated the roles of individual signaling pathways in v-Src-dependent cell growth and survival by inhibiting one particular pathway. v-Src induced constitutive activation of signal transducers and activators of transcription 3 (STAT3), phosphatidylinositol 3-kinase, and Ras in murine Ba/F3 cells and led to factor-independent proliferation. Dominant-negative mutants of STAT3 (STAT3D) and phosphatidylinositol 3-kinase (Δp85) inhibited v-Src-dependent growth by ∼60 and ∼40%, respectively. Moreover, dominant-negative Ras (N17) induced severe apoptosis, which was accompanied by down-regulation of Bcl-2 and activation of caspase-3. Although cells overexpressing Bcl-2 or caspase-3 inhibitors remained viable even when N17 was expressed, the growth was reduced by ∼85%. During N17- and STAT3D-induced growth suppression, expression of cyclin D2,cyclin D3, c-myc, and c-fos was suppressed by N17, whereas that of cyclin D2,cyclin E, and c-myc was suppressed by STAT3D. Thus, v-Src-activated Ras and STAT3 are involved in distinct but partly overlapping transcriptional regulation of cell cycle regulatory molecules. These results suggest that the full oncogenic activity of v-Src requires simultaneous activation of multiple signalings, in which Ras is particularly required for survival.

Regulation of Human B Cell Differentiation: Molecular Structure and Immunological Functions of Human B Cell Differentiation Factor (BSF-2)

Regulation of human B cell differentiation into antibody-secreting cells was studied. By utilizing monoclonal antibodies, B cells can be isolated into three subpopulations with respect to their activation stage: resting B cells, activated B cells, and B cells at the final differentiation stage. Anti-IgM together with BSF-1 activate resting B cells into the activated stage and activated B cells secrete BCGF and become responsive to BCGF. Then, BCDF (BSF-2) induces Ig secretion in activated B cells. cDNA for BSF-2 has been cloned and recombinant BSF-2 induced Ig secretion in B lymphoblastoid cells as well as in normal cells. The amino acid sequence deduced from the cDNA sequence does not have any homology with other lymphokines. Patients with cardiac myxoma often show autoantibody production. The result showed that cardiac myxoma cells secrete large amounts of BSF-2, indicating that constitutive production of BSF-2 in vivo may be responsible for autoantibody production.