Proteolytic cleavage of phospholipase C-γ1 during apoptosis in Molt-4 cells

Apoptosis is a cell suicide mechanism that requires the activation of cellular death proteases for its induction. We examined whether the progress of apoptosis involves cleavage of phospholipase C-γ1 (PLC-γ1), which plays a pivotal role in mitogenic signaling pathway. Pretreatment of T leukemic Molt-4 cells with PLC inhibitors such as U-73122 or ET-18-OCH₃ potentiated etoposide-induced apoptosis in these cells. PLC-γ1 was fragmented when Molt- 4 cells were treated with several apoptotic stimuli such as etoposide, ceramides, and tumor necrosis factor α. Cleavage of PLC-γ1 was blocked by overexpression of Bcl-2 and by specific inhibitors of caspases such as Z- DEVD-CH₂F and YVAD-cmk. Purified caspase-3 and caspase-7, group II caspases, cleaved PLC-γ1 in vitro and generated a cleavage product of the same size as that observed in vivo, suggesting that PLC-γ1 is cleaved by group II caspases in vivo. From point mutagenesis studies, Ala-Glu-Pro-Asp⁷⁷⁰ was identified to be a cleavage site within PLC-γ1. Epidermal growth factor receptor (EGFR) -induced tyrosine phosphorylation of PLC-γ1 resulted in resistance to cleavage by caspase-3 in vitro. Furthermore, cleaved PLC-γ1 could not be tyrosine-phosphorylated by EGFR in vitro. In addition, tyrosine- phosphorylated PLC-γ1 was not significantly cleaved during etoposide-induced apoptosis in Molt-4 cells. This suggests that the growth factor-induced tyrosine phosphorylation may suppress apoptosis-induced fragmentation of PLC- γ1. We provide evidence for the biochemical relationship between PLC-γ1- mediated signal pathway and apoptotic signal pathway, indicating that the defect of PLC-γ1-mediated signaling pathway can facilitate an apoptotic progression.