Involvement of ICE (Caspase) Family in γ‐Radiation‐Induced Apoptosis of Normal B Lymphocytes

Normal lymphocytes are highly sensitive to the damaging effects of ionizing radiation, and undergo cell death by apoptosis. We have investigated the possible involvement of the Interleukin-1β-converting enzyme (ICE) (Caspase) protease family, which appears to play an important role as intracellular mediator of apoptosis. Resting B lymphocytes isolated from human peripheral blood were irradiated (6 Gy) and cultured for 24 h, resulting in 25 ± 5.1% apoptotic cells, as measured by the TUNEL assay (mean ± SD, n = 6). Addition of the ICE family inhibitor Z-VAD.fmk (50 μM) completely inhibited apoptosis (2.0 ± 1.5% at 24 h). By using fluorogenic substrates containing the peptide recognition sequences DEVD and YVAD, the type of ICE family protease involved was examined more closely. A marked transient increase in DEVD-, and absent YVAD-cleavage activity indicated the involvement of a CPP32-like protease, not an ICE-like protease. Western blot analysis demonstrated that untreated B lymphocytes expressed the proform of the ICE family members CPP32 and ICH1L, but no detectable ICE. The induction of cell death by radiation was accompanied by the activation of CPP32 as shown by the cleavage of the proform to the active subunit p17, and the cleavage of poly(ADP-ribose) polymerase (PARP), one of the known substrates of CPP32. In contrast, no activation of ICH1L could be detected. These results indicate the involvement of CPP32 and possibly other CPP32-like proteases in radiation-induced apoptosis of resting B lymphocytes.