Prolactin influences many different systems within the body, however it is most well known for its effect on the mammary gland where it is required for development of the gland and the induction of lactation. The receptor for prolactin is a member of the cytokine receptor superfamily. The binding of ligands to cytokine receptors results in the activation of one or more member of the Janus family and one or more members of the src-family of tyrosine kinases. In the case of prolactin, JAK2 is rapidly activated and leads to the phosphorylation of the receptor and a DNA binding protein known as STAT5. These two molecules appear to be the primary substrates for JAK2. Additional signaling molecules are also activated following stimulation of cells with prolactin, including ras, raf, mitogen-activated protein kinase, and phosphatidylinositol 3-kinase. Our hypothesis is that activation of src-related tyrosine kinase is required for mitogenic signaling by the prolactin receptor. We propose that activation of src-related kinases is meditated by the binding of their SH2 and/or SH3 domains to specific sequences in the cytoplasmic tail of the prolactin receptor. This hypothesis will be tested by examining the ability of the prolactin receptor to stimulate proliferation of cells lacking specific tyrosine kinases, and by testing the ability of dominant negative mutants of src-like kinases to block mitogenesis. The ability of src- like kinases to bind to and become activated by specific sequences in the cytoplasmic tail of the receptor will also be examined. Signaling molecules phosphorylated and/or activated in a src-kinase dependent manner will be identified. In addition, we will identify the critical signaling pathways utilized by a constitutively activated mutant of the prolactin receptor that is able to induce growth factor-independent proliferation.
