B cell receptor-stimulated mitochondrial phospholipase A(2) activation andresultant disruption of mitochondrial membrane potential correlate with the induction of apoptosis in WEHI-231 B cells

Cross-linking of the Ag receptors on the immature B cell lymphoma, WEHI-231 , leads to growth arrest and apoptosis, We now show that although commitmen t to such B cell receptor (BCR)-mediated apoptosis correlates with mitochon drial phospholipase A(2) activation, disruption of mitochondrial function, and ATP depletion, it is executed independently of caspase activation. Firs t, we demonstrate a pivotal role for mitochondrial function in determining B cell fate by showing up-regulation of cytosolic phospholipase A(2) expres sion, induction of mitochondrial phospholipase A, activity, arachidonic aci d-mediated collapse of mitochondrial transmembrane inner potential (Delta p si (m)), and depletion of cellular ATP under conditions of apoptotic, but n ot proliferative, signaling via the BCR. Importantly, disruption of Delta p si (m), ATP depletion, and apoptosis can be prevented by rescue signals via CD40 or by Delta psi (m) stabilizers such as antimycin or oligomycin. Seco nd, we show that commitment and postmitochondrial execution of BCR-mediated apoptosis are not dependent on caspase activation by demonstrating that su ch apoptotic signaling does not induce release of cytochrome c from the mit ochondria or activation of effector caspases, as evidenced by poly(ADP-ribo se) polymerase or Bcl-x(L) cleavage. Indeed, apoptotic signaling via the BC R in WEHI-231 B cells does not stimulate the activation of caspase-3 and, c onsistent with this, BCR-mediated disruption of Delta psi (m) and commitmen t to apoptosis take place in the presence of caspase inhibitors. In contras t, BCR signaling induces the postmitochondrial activation of cathepsin B, a nd resultant apoptosis is blocked by the cathepsin B inhibitor, (23,35)tran s-epoxysuccinyl-L-leucylamindo-3-methylbutane ethyl ester (EST) suggesting a key role for this executioner protease in Ag receptor-driven apoptosis of WEHI-231 immature B cells.