SOCS (suppressor of cytokine signaling) proteins have been shown to be negative regulators of cytokine receptor signaling via the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway. We have cloned a member of this family (hSOCS-2) by utilizing the insulin-like growth factor I receptor (IGF-IR) cytoplasmic domain as bait in a yeast two-hybrid screen of a human fetal brain library. The hSOCS-2 protein interacted strongly with the activated IGF-IR and not with a kinase negative mutant receptor in the two-hybrid assay. Mutation of receptor tyrosines 950, 1250, 1251, and 1316 to phenylalanine or deletion of the COOH-terminal 93 amino acids did not result in decreased interaction of the receptor with hSOCS-2 protein. hSOCS-1 protein also interacted strongly with IGF-IR in the two-hybrid assay. Glutathione S-transferase-hSOCS-2 associated with activated IGF-IR in lysates of mouse fibroblasts overexpressing IGF-IR. Human embryonic kidney cells (293) were transiently transfected with vectors containing IGF-IR and FLAG epitope-tagged hSOCS-2. After IGF-I stimulation, activated IGF-IR was found in anti-FLAG immunoprecipitates and, conversely, FLAG-hSOCS-2 was found in anti IGF-IR immunoprecipitates. Thus, hSOCS-2 interacted with IGF-IR both in vitro and in vivo. HSOCS-2 mRNA was expressed in many human fetal and adult tissues with particularly high abundance in fetal kidney and adult heart, skeletal muscle, pancreas, and liver. These results raise the possibility that SOCS proteins may also play a regulatory role in IGF-I receptor signaling.
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