Abstract

PRL plays a central role in the regulation of milk protein gene expression in mammary epithelial cells and in the growth and differentiation of lymphocytes. It confers its activity through binding to a specific transmembrane, class I hematopoietic receptor. Ligand binding leads to receptor dimerization and activation of the tyrosine kinase Jak (janus kinase) 2, associated with the membrane-proximal, intracellular domain of the receptor. Jak2 phosphorylates and activates Stat5, a member of the Stat (signal transducers and activators of transcription) family. PRL receptor also activates SHP-2, a cytosolic tyrosine phosphatase. We investigated the connection between these two signaling events and derived a dominant negative mutant of SHP-2 comprising the two SH2 domains [SHP-2(SH2)2]. An analogous variant of the SHP-1 phosphatase [SHP-1(SH2)2] was used as a control. The dominant negative mutant of SHP-2 was found to inhibit the induction of tyrosine phosphorylation and DNA-binding activity of m-Stat5a, m-Stat5b, and the carboxyl-terminal deletion variant m-Stat5adelta749, as well as the transactivation potential of m-Stat5a and m-Stat5b. The dominant negative mutant SHP-1(SH2)2 had no effect. The kinase activity of Jak2 is also dependent on a functional SHP-2 phosphatase. We propose that SHP-2 relieves an inhibitory tyrosine phosphorylation event in Jak2 required for Jak2 activity, Stat5 phosphorylation, and transcriptional induction.