Ligation of CD40 rescues Ramos‐Burkitt lymphoma B cells from calcium ionophore‐ and antigen receptor‐triggered apoptosis by inhibiting activation of the cysteine protease CPP32/Yama and cleavage of its substrate PARP

Abstract

The new and growing family of interleukin‐1β‐converting enzyme (ICE) cysteine proteases are now recognised to be major effectors of cellular death by apoptosis. Like other members of this family, the CPP32/Yama proform is activated by processing to its active heterodimeric enzyme or apopain when it likely contributes to the process of apoptosis by cleaving poly(ADP‐ribose) polymerase (PARP) and thereby inhibiting much of its DNA repair activity. Apoptosis plays a fundamental role in the regulation of the immune system where it is involved in the selection of both T and B lymphocytes bearing antigen receptor (AgR) for non‐self. Cells of the Ramos Epstein‐Barr virus (EBV)‐genome‐negative Burkitt lymphoma (BL) B cell line (Ramos‐BL) can be triggered into growth arrest and apoptosis by treating with the calcium ionophore ionomycin or by crosslinking their surface AgR with antibodies directed against immunoglobulin (Ig)M (anti‐IgM). Ionomycin‐ and AgR‐triggered growth arrest and apoptosis are arrested by signals transduced through the surface CD40 of Ramos‐BL B cells. Both ionomycin and anti‐IgM trigger activation of CPP32 and cleavage of PARP prior to the onset of apoptosis; this process is abrogated by treatment with anti‐CD40 and is independent of Bcl‐2 expression. A tripeptide inhibitor of ICE family cysteine proteases, Z‐Val‐Ala‐Asp‐fluoromethylketone (zVAD‐fmk) inhibits ionomycin‐ and AgR‐triggered CPP32 activation, PARP cleavage and apoptosis, but not growth arrest, in Ramos‐BL B cells. Thus, in this report we demonstrate that in a physiological system, activation of endogenous members of the ICE family, including CPP32, and cleavage of the death substrate PARP act as major effectors of apoptotic death.