PROLACTIN-INDUCED PROLIFERATION OF NB2 CELLS INVOLVES TYROSINE PHOSPHORYLATION OF THE PROLACTIN RECEPTOR AND ITS ASSOCIATED TYROSINE KINASEJAK2

The interaction of prolactin with its receptor in the Nb2 cell line ha s been shown to induce the phosphorylation of cell associated proteins and mitogenesis. It has been reported previously that one of these pr oteins, phosphorylated upon prolactin stimulation, was a tyrosine kina se. We have identified this kinase as JAK2, and demonstrate its associ ation with the prolactin receptor. In addition, we show that the prola ctin receptor itself becomes tyrosine phosphorylated upon ligand stimu lation in Nb2 cells. These actions are time-dependent and occur rapidl y after prolactin stimulation, with first the kinase being activated w ithin 5 min and then the receptor being phosphorylated maximally at 20 min. Moreover, phosphorylation of both JAK2 and the receptor as well as Nb2 cell proliferation are dependent on the concentration of lactog enic hormone, resulting in a bell-shaped response curve similar to tha t observed in the two site model of hGH action. This indicates that ea rly events in signal transduction as well as later events like mitogen esis and proliferation involve prolactin receptor dimerization. Togeth er these data indicate that the prolactin receptor in Nb2 cells is ass ociated to JAK2 and that upon ligand stimulation, and receptor dimeriz ation, the kinase and the receptor are tyrosine-phosphorylated, which represents the first event in the process of prolactin receptor signal transduction in Nb2 cells.