Effect of CD37 small modular immuno-pharmaceutical (SMIP) on direct apoptosis in chronic lymphocytic leukemia cells via transcriptional up-regulation of the BH3 family member BIM

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3035 Background: CD37 is a tetraspan transmembrane family protein selectively expressed on normal and transformed B-cells. A novel CD37SMIP was previously demonstrated to mediate superior direct apoptosis and NK-cell mediated killing of chronic lymphocytic leukemia (CLL) and other B-cell malignancies. Methods: Given the superior in vitro apoptosis observed with CD37SMIP treatment and early clinical activity observed in highly refractory CLL patients, we hypothesized that a unique mechanism of cell killing was utilized by CD37SMIP. This was pursued in preclinical studies outlined below. Results: Unlike many other agents utilized to treat CLL, death mediated by CD37SMIP does not depend upon caspase activation. Nonetheless, CD37SMIP treatment of CLL cells promotes time-dependent induction of mitochondrial membrane depolarization, mitochondrial translocation of Bax, and up-regulation of Bim protein. CD37SMIP Bim protein induction occurred concomitantly with an increase in BIM mRNA levels. Electrophoretic mobility shift assay using oligonucleotides of the BIM promoter demonstrated increased protein binding activity in nuclear extracts derived from CD37SMIP treated cells and the physical interaction of FoxO3a transcription factor with the FoxO3a responsive element in the BIM promoter was demonstrated using a “protein pull down” assay and confirmed by chromatin immunoprecipitation assays. Furthermore, CD37SMIP treatment significantly increased BIM promoter regulated luciferase reporter expression in B-CLL cells. Consistent with a primary role of Bim up-regulation in mitochondrial membrane destabilization and apoptosis, transfection of CLL cells with BIM siRNA resulted in inhibition of CD37SMIP-induced mitochondrial membrane depolarization and apoptosis. Conclusions: These studies demonstrate CD37SMIP mediated apoptosis in CLL cells occurs via FoxO3a-dependent transcriptional up-regulation of BIM protein. This distinct mechanism of apoptosis utilized by CD37SMIP contrasts it with other agents used for CLL treatment. Additionally, it provides a mechanism for the promising clinical activity of TRU-016 (humanized CD37SMIP) observed to date in refractory CLL patients. [Table: see text]