Inhibition of caspase activity delays apoptosis in a transfected NS/0 myeloma cell line

The productivity of NS/0 myeloma batch cultures is often compromised by the premature induction of apoptosis, now established to be the predominant method of cell death during culture decline. Caspase proteases have recently been shown to play a major role in the transmission of signals for apoptotic cell death. Using a specific inhibitor that targets a range of caspases (Z- VAD-fmk) we assessed whether inhibition of caspase activity could prolong the viability of NS/0 cells under conditions that cause apoptotic cell death in batch cultures. Z-VAD-fmk was found to significantly reduce apoptotic cell death (by ~50%) induced by cytotoxins and to preserve membrane integrity to a similar extent. In conditions of low serum, Z-VAD-fmk reduced certain features of apoptosis (e.g., DNA fragmentation), but only marginally improved viability. In medium-depleted batch cultures, Z-VAD-fmk afforded a delay of between 24 and 48 h in both the induction of apoptosis and loss of viability. Despite an apparent increase in viability in Z-VAD-fmk-treated NS/0 cultures, no improvement in productivity could be demonstrated, suggesting that at least some normal pathways for protein production are shut down upstream of caspase activation. An examination of mitochondrial membrane potential (ΔΨm) in Z-VAD-fmk-treated and untreated NS/0 cells revealed only a small initial difference (5%) in the levels of ΔΨm depolarization. Similar levels of mitochondrial dysfunction, despite caspase inactivity, may therefore be responsible for the comparable productivity in untreated and Z-VAD-fmk- treated cultures. Thus, this study suggests that, while a delay in cell death due to caspase inhibition may reduce problems associated with cellular disintegration, it does not permit productivity improvements in this type of culture.