Cleavage of Bcl-2 is an early event in chemotherapy-induced apoptosis of human myeloid leukemia cells

The proto-oncogene product Bcl-2 protects a wide variety of cell types from apoptosis via a hitherto unknown mechanism. Bcl-2 has been shown to functi on upstream of the death proteases (caspases) in some, but not all, occurre nces of apoptotic cell death. Using the myeloid leukemic cell line P39 we r eport the chemotherapy-induced caspase-dependent cleavage of endogenous Bcl -2. Etoposide treatment of these cells triggered a time-dependent activatio n of type II and type III caspases and cleavage of Bcl-2 yielding a 23 kDa cleavage fragment. The emergence of this cleavage product was blocked by th e general caspase inhibitor zVAD-fmk, as well as the type ill caspase inhib itor IETD-fmk and the caspase-9-selective inhibitor LEHD-fmk, while the typ e II caspase inhibitor DEVD-fmk proved considerably less efficient. Bcl-2 c leavage preceded cleavage of the known caspase-3 substrate, poly(ADP-ribose ) polymerase (PARP), as well as that of the caspase-6 substrate, iamin B, i ndicating that Bcl-2 cleavage is a relatively early event in the apoptosis cascade in this experimental model. While evidence for cleavage of Bcl-2 in several subcellular compartments of etoposide-treated cells was obtained, this cleavage was detected predominantly in the mitochondrial fraction, thu s providing further support for the central role of mitochondria in apoptos is. Caspase-mediated cleavage following etoposide treatment of these myeloi d leukemic cells may represent a means for the attenuation of Bcl-2 functio n upon apoptosis induction.