Prolactin induces rapid phosphorylation and activation of prolactin receptor-associated RAF-1 kinase in a T-cell line.

The receptor for prolactin (PRL) is a member of the hematopoietic receptor family that also includes the receptors for interleukins 2-7. PRL is synthesized and is secreted by human T lymphocytes and acts as a lymphokine necessary, but not sufficient, for T lymphocyte progression through the G1 phase of the cell cycle. Although data now indicate that PRL serves an immunomodulatory role in vitro and in vivo, the mechanisms of PRL receptor signal transduction in T cells have not been defined. We demonstrate here that PRL induced the phosphorylation of the p72-74 serine/threonine kinase c-Raf-1 in the PRL-dependent rat T-cell line Nb2. Associated with this inducible phosphorylation of Raf-1 was a concentration- and time-dependent activation of in vitro Raf-1 autokinase and substrate kinase activities, which correlated with the PRL-induced proliferation of Nb2 cells. Co-immunoprecipitation studies revealed association of Raf-1 with PRL receptors in Nb2 cells. These results revealed that all isoforms of the PRL receptor (short, intermediate, and long) are expressed in Nb2 cells and associate with Raf-1. In contrast to the PRL-dependent Nb2 cells, phosphorylation and activation of Raf-1 were constitutive in the Nb2-derived, PRL-independent, T-cell line Sp. These studies demonstrate for the first time an association between the PRL receptor and a serine/threonine kinase affiliated with signal transduction.