Identification of JAK protein tyrosine kinases as signaling molecules for prolactin. Functional analysis of prolactin receptor and prolactin‐erythropoietin receptor chimera expressed in lymphoid cells

The mechanism of action of prolactin (PRL) was studied in murine lymphoid BAF‐3 cells transfected with either the long form of the PRL receptor (PRL‐R), or a chimeric receptor consisting of the extracellular domain of the PRL‐R and the transmembrane and intracellular domain of the erythropoietin receptor (PRL/EPO‐R). PRL sustained normal and long‐term proliferation of BAF‐3 cells expressing either the PRL‐R or the hybrid PRL/EPO‐R. Upon [125I]PRL cross‐linking, both types of BAF‐3 transfectants were shown to express two [125I]PRL cross‐linked species differing in size by 20 kDa. These cross‐linked complexes, after denaturation, were recognized by antibody against the PRL‐R, indicating that they contain the transfected receptor. PRL induced rapid and transient tyrosine phosphorylation of both the PRL‐R and the PRL/EPO‐R in BAF‐3 transfectants. Furthermore, PRL induced rapid tyrosine phosphorylation of the Janus kinase 2 (JAK2) which was already physically associated with the PRL‐R or the PRL/EPO‐R in the absence of ligand. JAK1 was also associated with PRL‐R and PRL/EPO‐R in the absence of ligand. However, only in BAF‐3 cells expressing the PRL‐R does PRL induce rapid and transient tyrosine phosphorylation of JAK1. These results demonstrate that JAK protein tyrosine kinases couple PRL binding to tyrosine phosphorylation and proliferation.