Negative regulation of growth hormone receptor/JAK2 signaling by signal regulatory protein α

Abstract

Signal regulatory proteins (SIRPs) are receptor-like transmembrane proteins, the majority of which contain a cytoplasmic proline-rich region and four cytoplasmic tyrosines that, when phosphorylated, bind SH2 domain-containing protein tyrosine phosphatases (SHP). We demonstrated previously that growth hormone (GH) induces tyrosyl phosphorylation of SIRPα and association of SIRPα with SHP-2. The GH-activated tyrosine kinase JAK2 associates with and tyrosyl-phosphorylates SIRPα1. Here we show that JAK2-SIRPα1 association does not require phosphotyrosines in SIRPα1 or JAK2 or the proline-rich region of SIRPα1. However, when the C-terminal 30 amino acids of SIRPα1 containing the proline-rich region and tyrosine 495 are deleted, tyrosyl phosphorylation of SIRPα1 by JAK2 and association of SHP-2 with SIRPα1 are reduced. GH-dependent tyrosyl phosphorylation of JAK2 is reduced when wild-type SIRPα1 compared with SIRPα1 lacking the four cytoplasmic tyrosines (SIRP 4YF) is expressed in cells, suggesting that SIRPα1 negatively regulates GHR/JAK2 signaling. Consistent with reduced JAK2 activity, overexpression of wild-type SIRPα1 but not SIRP 4YF reduces GH-induced phosphorylation of ERKs 1 and 2, STAT3, and STAT5B. These results suggest that SIRPα1 is a negative regulator of GH signaling and that the ability of SIRPα1 mutants to negatively regulate GHR-JAK2 signaling correlates with their ability to bind SHP-2.