Clustered CD20 induced apoptosis: src-family kinase, the proximal regulator of tyrosine phosphorylation, calcium influx, and caspase 3-dependent apoptosis.
Anti-CD20 antibodies may reduce or eliminate non-Hodgkin's lymphoma B cells in patients, although the mechanism of action is not clear. To explore mechanism(s), we examined the induction of signal transduction events using anti-CD20 monoclonal antibodies (mAb) in the human non-Hodgkin's lymphoma Ramos B cell line. We found that while Rituximab (a human-mouse hybrid mAb) alone induced apoptotic cell death, other murine anti-CD20 mAbs induced apoptosis of Ramos B cells only upon clustering with a secondary antibody. CD20 clustering was accompanied by activation of tyrosine protein kinase activity, PLCgamma2 phosphorylation, influx of Ca(2+), and activation of caspase 3. All signaling events, as well as the subsequent apoptosis, were blocked by PP2, a selective inhibitor of Src-family kinases. Treatment of Ramos with EGTA and BAPTA to block changes in cytoplasmic Ca(2+) likewise prevented CD20-induced apoptosis. Our findings support a model in which CD20 clustering activates members of the Src family of protein tyrosine kinases, leading to phosphorylation of PLCgamma2 and increased cytoplasmic Ca(2+). These early signal transduction events activate caspase 3 to promote apoptotic cell death of NHL B cells.
Hofmeister
J K
JK
Ohio State University Comprehensive Cancer Center, 300 West 12th Avenue, Columbus, Ohio 43210, USA.
Cooney
D
D
Coggeshall
K M
KM
D000911:Antibodies, Monoclonal / Q000276:immunology; D018951:Antigens, CD20 / Q000276:immunology*; D017209:Apoptosis / Q000276:immunology*; D002118:Calcium / Q000276:immunology; D053148:Caspase 3; D020169:Caspases / Q000276:immunology*; D004789:Enzyme Activation; D006801:Humans; D016393:Lymphoma, B-Cell / Q000276:immunology* / Q000378:metabolism / Q000473:pathology*; D010766:Phosphorylation; D015398:Signal Transduction / Q000276:immunology*; D014407:Tumor Cells, Cultured; D014443:Tyrosine; D019061:src-Family Kinases / Q000276:immunology*
D000911:Antibodies, Monoclonal; D018951:Antigens, CD20; D014443:Tyrosine; D019061:src-Family Kinases; C505407:CASP3 protein, human; C505409:Casp3 protein, mouse; D053148:Caspase 3; D020169:Caspases; D002118:Calcium