Tadashige Nozaki

Requirement of the caspase-3/CPP32 protease cascade for apoptotic death following cytokine deprivation in hematopoietic cells.

Hematopoietic cytokines transduce cell survival signals, which are distinct from the signals necessary for the stimulation of DNA synthesis. Recently, the Ras and phosphatidylinositol 3-kinase pathways have been shown to play important roles in preventing apoptosis in various cell types,e.g. hematopoietic cells and neuronal cells. Withdrawal of cytokine(s), in turn, results in rapid inactivation of these survival pathways and eventually leads to cell death accompanied by the hallmarks of apoptosis. However, the mechanism of cell death caused by cytokine deprivation has not been fully elucidated. In this study, we demonstrate that caspase-3/CPP32, a member of the caspase/interleukin-1β-converting enzyme family, is activated upon interleukin (IL)-3 deprivation in IL-3-dependent cells as well as IL-2 deprivation in IL-2-dependent cells. In addition, poly(ADP-ribose) polymerase, a cellular substrate for the caspase family proteases, was degraded into apoptotic fragments in both cell lines after cytokine removal. Furthermore, inhibition of a caspase family protease by synthetic peptides suppressed apoptotic death. These results indicate that the activation of a caspase-like protease(s) is required for the progression of apoptosis following cytokine deprivation. However, readdition of IL-3 did not restore the proliferative potential of the cells that survived in the presence of the peptide inhibitor after IL-3 depletion. Therefore, cellular commitment to apoptosis appears to precede the activation of a caspase-like protease(s).

Role of poly(ADP-ribose) polymerase in cell-cycle checkpoint mechanisms following γ-irradiation

Abstract A nuclear poly(ADP-ribose) polymerase (PARP) is activated by γ-irradiation and consequently synthesizes poly(ADP-ribose) by binding to DNA strand-breaks. This property suggests that PARP is a DNA strand-break-signal generator. Meanwhile, the cell-cycle arrest occurs in G1 and G2 phases following γ-irradiation. We found that PARP inhibitors including 3-aminobenzamide (3-AB) suppressed G1 arrest and enhanced G2 arrest following γ-irradiation. These observations suggested that PARP is critical for the induction of G1 arrest and is also involved in the regulation of G2 arrest. Furthermore, the effects of 3-AB on the G1-arrest signal-transduction pathway were also studied. We found that p53 stabilization following γ-irradiation was not inhibited but the p53-responsive transient increases of WAFI/CIPI/p21 and MDM-2 mRNA were suppressed by 3-AB. Therefore, it is suggested that PARP participates in G1-arrest signal-transduction pathway through the modulation of WAFI/CIPI/p21 and MDM-2 mRNA expression.