부산대학교 도서관

The receptor-like phosphotyrosine phosphatase η (PTPη) is an important intracellular effector of the cytostatic action of SST. Here we characterize, in Chinese hamster ovary-k1 cells, the intracellular pathway that from somatostatin receptor 1 (SSTR1), leads to the activation of PTPη and that involves, in a multimeric complex and sequential activation, the tyrosine kinases Janus kinase (JAK) 2 and Src, and the cytosolic phosphotyrosine phosphatase SHP-2. We show that inhibitors of JAK2 and Src and dominant-negative mutants of SHP-2 and Src abolished the SSTR1-mediated PTPη activation, suggesting that all these effectors participate in the activation of PTPη. In basal conditions, JAK2 forms a multimeric complex with SHP-2, Src and PTPη. In response to SST, JAK2 is activated in a G protein-dependent manner, dissociates from and phosphorylates SHP-2, increasing its activity. Subsequently, SHP-2 dissociates from Src, dephosphorylates the Src inhibitory tyrosine-529, and causes an autocatalytical increase of the phosphorylation of Src tyrosine 418, located inside its kinase activation loop. Active Src, in turn, controls the activity of PTPη, via a direct interaction and phosphorylation of the phosphatase. These data for the first time depict an intracellular pathway involving a precise sequence of interactions and cross-activation among tyrosine phosphatases and kinases acting upstream of PTPη. In particular the sequential activation of JAK2, SHP-2, and Src conveys the molecular signaling from SSTR1 to the activation of this phosphatase that is responsible for the final biological effects of SST.