Rituximab antiproliferative effect in B-lymphoma cells is associated with acid-sphingomyelinase activation in raft microdomains

Rituximab is a chimeric human immunoglobulin G1 (IgG1) anti-CD20 monoclonal antibody with significant activity against CD20(+) malignant B cells. Rituximab is currently used with success in the treatment of B-cell-clerived lymphoid neoplasias either alone or in combination with chemotherapy. However, the predominant mechanism by which rituximab exerts its antitumor properties in vivo remains unknown. In the present study, we demonstrate that in Daudi and RL B-lymphoma cells, rituximab (without cross-linking) used at the saturating dose of 10 mug/mL induced moderate accumulation in G, phase, growth inhibition, and significant loss in clonogenic potential. However, in these cells, rituximab induced no apoptosis. Furthermore, we observed that treatment with rituximab resulted in a rapid and transient increase in acid-sphingomyelinase (A-SMase) activity and concomitant cellular ceramide (CER) generation in raft microdomains. We also observed that rituximab-treated cells externalized both A-SMase and CER that colocalized with the CD20 receptor. Finally, we present evidence that ritux-imab-induced growth inhibition may be mediated through a CER-triggered signaling pathway, leading to the induction of cell cycle-dependent kinase inhibitors such as p27(Kip1) through a mitogen-activated protein kinase (MAPK)-dependent mechanism. (C) 2004 by The American Society of Hematology.