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 RA domain. 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.