Prolactin activates tyrosyl phosphorylation of insulin receptor substrate 1 and phosphatidylinositol-3-OH kinase.

Prolactin (PRL) has been demonstrated to induce tyrosine phosphorylation and activation of the cytoplasmic tyrosine kinase JAK2. The present study represents an initial effort to identify the phosphorylation repertoire of the PRL receptor (PRLR). For this purpose we have modified the rat PRLR cDNA to encode an additional N-terminal epitope specifically designed to allow the rapid purification of the PRLR and associated proteins from transfected cells. The Flag-tagged PRLR was stably expressed in the human 293 cell line. PRL induced tyrosine phosphorylation of proteins of 85, 95, and 185 kDa from 10 to 30 min after PRL stimulation. Immunoblot analysis of immunoprecipitation indicates that p85 corresponds to the 85-kDa regulatory subunit of phosphatidylinositol (PI)-3' kinase, p95 to PRLR, and p185 to insulin receptor substrate 1 (IRS-1). Both PI-3' kinase and IRS-1 appear to associate with PRLR in a PRL-dependent manner. These results thus indicate that kinases other than JAK2, namely PI-3' kinase, are activated by PRL.