Differential roles of Ras and Rap1 in growth factor-dependent activation of phospholipase Cε

Phospholipase Cepsilon is a phosphoinositide-specific phospholipase C that selectively associates with Ras and Rap small GTPases as a target. Here we explored the molecular basis of the Rap1- as well as Ras-mediated regulation of phospholipase Cepsilon upon platelet-derived growth factor stimulation by using a receptor mutant deficient in its ability to phosphorylate and activate phospholipase Cgamma. Following platelet-derived growth factor treatment, this receptor induces persistent activation of ectopically expressed PLCepsilon through activation of Ras and Rap1. The rapid and initial phase of the activation is mediated by Ras, whereas Rap1 is responsible for the prolonged activation. We further demonstrate that the CDC25 homology domain, which exhibits guanine nucleotide exchange factor activity toward Rap1, but not Ras, is critical for the prolonged activation of phospholipase Cepsilon. Platelet-derived growth factor prevented the hematopoietic BaF3 cells containing the mutant receptor from undergoing apoptosis, and enabled these cells to proliferate, only when phospholipase Cepsilon was expressed. Therefore, the phospholipase C signal is suggested to be critical for survival and growth of BaF3 cells.