Suppressor of cytokine signaling (SOCS)-3 protein interacts with the insulin-like growth factor-I receptor

SOCS proteins are a class of proteins that are negative regulators of cytok ine receptor signaling via the Janus kinase (JAK)/signal transducer and act ivator of transcription (STAT) pathway. In a yeast two-hybrid screen of a h uman fetal brain library, we have previously identified SOCS-2 as a binding partner of the activated IGF-I receptor (IGFIR). To test whether or not SO CS-3 also binds to the IGFIR, we cloned human SOCS-3 by reverse transcripti on-polymerase chain reaction from human skeletal muscle mRNA. SOCS-3 mRNA w as expressed in many human fetal and adult tissues and in some human cancer cell lines (Hela, A549 pulmonary adenocarcinoma and G361 human melanoma). We found that human SOCS-3 protein interacts directly with the cytoplasmic domains of the activated IGFIR and the insulin receptor (IR) in the yeast t wo-hybrid assay. In GST-SOCS-3 pull-down experiments using IGFIR from mamma lian cells and in immunoprecipitation experiments in which IGFIR and FLAG-S OCS-3 were transiently expressed in human embryonic kidney 293 cells, we fo und that SOCS-3 interacts constitutively with IGFIR in vitro and in intact cells. Unlike SOCS-2, hSOCS-3 was phosphorylated on tyrosines in response t o IGF-I addition to 293 cells. We conclude that SOCS-3 binds to the IGFIR a nd may be a direct substrate for the receptor tyrosine kinase. (C) 2000 Aca demic Press.