Stimulation of c-Src by prolactin is independent of Jak2

Interaction of prolactin (PRL) with its receptor (PRLR) leads to activation of Jak and Src family tyrosine kinases. The PRL/growth hormone/cytokine re ceptor family conserves a proline-rich sequence in the cytoplasmic juxtamem brane region (Box 1) required for association and subsequent activation of Jaks. In the present work, we studied the mechanisms underlying c-Src kinas e activation by PRL and the role that Jak2 plays in this process. PRL addit ion to chicken embryo fibroblasts (CEF) expressing the rat PRLR long form r esulted in activation of c-Src and Jak2 and in tyrosine phosphorylation of the receptor. Receptor phosphorylation was due to associated Jak2, since in cells expressing either a Box 1 mutated PRLR (PRLR4P-A), which is unable t o interact with Jak2, or a kinase-domain-deleted Jak2 (Jak2 Delta k), PRL d id not stimulate receptor phosphorylation. Interestingly, addition of PRL t o cells expressing PRLR4P-A resulted in an activation of c-Src equivalent t o that observed with the wild-type receptor. These findings indicate that P RL-mediated stimulation of c-Src was independent of Jak2 activation and of receptor phosphorylation. Our results suggest that PRL-activated Src could send signals to downstream cellular targets independently of Jak2.