CALPHOSTIN-C SYNERGISTICALLY INDUCES APOPTOSIS WITH VP-16 IN LYMPHOMA-CELLS WHICH EXPRESS ABUNDANT PHOSPHORYLATED BCL-2 PROTEIN

A newly established human lymphoma cell line (OZ) has the t(14;18)(q32 ;q21) translocation and expresses large amounts of Bcl-2, compared to CCRF-CEM cells. VP-16 (40 mu g/mL), a promising agent against lymphoma , caused DNA fragmentation (26.9% of total DNA) typical for apoptosis at 6 h in CCRF-CEM cells, but no significant changes in OZ cells until 24 h after the addition of VP-16. However, coincubation with calphost in C (0.2 mu g/mL), a protein kinase C (PKC) inhibitor, induced DNA fr agmentation in VP-16-treated OZ cells (13.5% of total DNA) at 6 h afte r the treatment. Simultaneous immunoblot analysis revealed that this i nduction of apoptosis coincided with the downregulation of serine-phos phorylated Bcl-2 (13% of control cells). By contrast, apoptosis induce d by VP-16 in CCRF-CEM cells was attenuated by the addition of 0.5 mu M phorbol 12-myristate 13-acetate, a potent PKC stimulator. These obse rvations suggest that Bcl-2 function is partly regulated by phosphoryl ation/dephosphorylation mechanisms of the PKC system, and that phospho rylated Bcl-2 in lymphoma cells may play a role in the prevention of a poptosis.