Siah proteins induce the epidermal growth factor-dependent degradation of phospholipase Cε

Abstract

Phospholipase Cε (PLCε) is activated by various growth factors or G-protein-coupled receptor ligands via different activation mechanisms. The Ras association (RA) domain of PLCε is known to be important for its ability to bind with Ras-family GTPase upon growth factor stimulation. In the present study, we identified Siah1 and Siah2 as novel binding partners of the PLCε RAdomain. Both Siah1 and Siah2 interacted with the RA2 domain of PLCε, and the mutation of Lys-2186 of the PLCε RA2 domain abolished this association. Moreover, Siah induced the ubiquitination and degradation of PLCε upon epidermal growth factor (EGF) stimulation, and Siah proteins were phosphorylated on multiple tyrosine residues via an Src-dependent pathway upon EGF treatment. The Src inhibitor abolished the EGF-dependent ubiquitination of PLCε, and the Siah1 phosphorylation-deficient mutant could not increase the EGF-dependent ubiquitination and degradation of PLCε. The EGF-dependent degradation of PLCε was blocked in mouse embryonic fibroblast (MEF) cells derived from Siah1a/Siah2 double knockout mice, and the extrinsic expression of wild-type Siah1 restored the degradation of PLCε, whereas the phosphorylation- deficient mutant did not. Siah1 expression abolished PLCε-dependent potentiation of EGF-dependent cell growth. In addition, the expression of wild-type Siah1 in Siah1a/Siah2-double knockout MEF cells inhibited EGF-dependent cell growth, and this inhibition was abolished by PLCε knockdown. Our results suggest that the Siah-dependent degradation of PLCε plays a role in the regulation of growth factor-dependent cell growth. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.