A Life or Death Choice

Abstract

Gab1, a docking protein that is tyrosine-phosphorylated in response to various growth factors, activates signaling pathways involved in cell growth and in protection from apoptosis. Holgado-Madruga and Wong investigated the role of Gab1 in the response to oxidative stress and uncovered evidence implicating Gab1 in mediating positive and negative survival signals through distinct signaling pathways. The authors used Western analysis of Gab1 immunoprecipitates to demonstrate that H 2 O 2 elicited tyrosine phosphorylation of Gab1 in several cell lines. Pharmacological analysis revealed that phosphorylation depended on activity of Src family tyrosine kinases. Experiments comparing wild-type mouse embryo fibroblasts (MEFs) to MEFs that lacked Gab1 (Gab1 −/− ) showed that Gab1 was necessary for H 2 O 2 -dependent activation of JNK (c-Jun N-terminal kinase) but not for JNK activation after other stimuli or for H 2 O 2 -dependent activation of other mitogen-activated protein kinases (MAPKs). Expression of Gab1 in Gab-deficient MEFs rescued H 2 O 2 -dependent activation of JNK; this depended on the presence of an intact SHP2 [Src homology 2 (SH2) domain-containing protein tyrosine phosphatase 2] binding domain. In cell viability assays, Gab1 −/− cells were more sensitive than wild-type MEF to H 2 O 2 -triggered apoptosis. Gab1 expression in Gab-deficient MEFs enhanced viability; however, cells expressing a GAB1 mutant that could not bind SHP2 showed improved survival over those expressing wild-type Gab1. Mutational analyses indicated that, after exposure to H 2 O 2 , Gab1 interactions with phosphatidylinositol 3-kinase promoted cell survival, whereas interactions with SHP2 promoted cell death. Thus, in addition to its roles in the response to growth factors, Gab1 appears to integrate positive and negative survival signals in response to oxidative stress. M. Holgado-Madruga, A. J. Wong, Gab1 is an integrator of cell death versus cell survival signals in oxidative stress. Mol. Cell. Biol . 23 , 4471-4484 (2003). [Abstract] [Full Text]