DOMINANT-NEGATIVE VARIANTS OF THE SHP-2 TYROSINE PHOSPHATASE INHIBIT PROLACTIN ACTIVATION OF JAK2 (JANUS-KINASE-2) AND INDUCTION OF STAT5 (SIGNAL TRANSDUCER AND ACTIVATOR OF TRANSCRIPTION-5)-DEPENDENT TRANSCRIPTION

PRL plays a central role in the regulation of milk protein gene expres sion 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 t o receptor dimerization and activation of the tyrosine kinase Jak (jan us kinase) 2, associated with the membrane-proximal, intracellular dom ain of the receptor. Jak2 phosphorylates and activates Stat5, a member of the Stat (signal transducers and activators of transcription) fami ly. PRL receptor also activates SHP-2, a cytosolic tyrosine phosphatas e. We investigated the connection between these two signaling events a nd 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 SH P-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-Stat5a Delta 749, as well as the transactivation p otential 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 inhi bitory tyrosine phosphorylation event in Jak2 required for Jak2 activi ty, Stat5 phosphorylation, and transcriptional induction.