Temporal ordering of caspase activation and substrate cleavage during antigen receptor-triggered apoptosis in Ramos-Burkitt lymphoma B cells

Abstract

We demonstrate here that selective activation of endogenous members of the caspase family and cleavage of substrates responsible for the maintenance of nuclear functional and structural integrity are major effectors of antigen receptor (AgR)- and ionomycin-triggered apoptosis in Ramos-Burkitt lymphoma (Ramos-BL) B cells. Ramos-BL B cells express significant proenzyme levels of caspase-2, -3, -7 and -8, low levels of caspase-6 and are caspase-1-negative. However, while anti-IgM and ionomycin trigger for significant activation of caspase-3, -7 and -8 at 12-16 h and at 4 h post-stimulation respectively, both anti-IgM and ionomycin fail to activate caspase-2 indicating that AgR- and ionomycin-triggered Ramos-BL B cell apoptosis is mediated by the selective activation of, at least, caspase-3, -7 and -8. Anti-IgM triggers for cleavage of the resident nuclear proteins poly(ADP-ribose) polymerase (PARP) at 8 h, lamins B1 and B2 from 12 to 16 h; likewise, ionomycin triggers for degradation of PARP at 2 h, lamins B1 and B2 at 4 h. Signal transduction through CD40 rescues Ramos-BL B cells from AgR- and ionomycin-triggered apoptosis at a very early stage of the apoptotic process by inhibiting both the early cleavage of PARP as well as the activation of caspase-3, -7 and -8 and cleavage of lamin B1; CD40-mediated rescue occurs upstream of CD40-induced expression of Bcl-2 and increased expression of Bcl-xL. In such cellular populations subject to regulation through apoptosis, dysregulation of the apoptotic mechanisms can have devastating consequences by contributing to the pathogenesis of malignancy as well as to lymphoproliferative and autoantibody disorders. An understanding of the role played by caspases in the execution of apoptosis may provide insight into the pathogenesis of these disease states and thereby provide targets for novel therapeutic strategy.