Prolactin Receptor Regulates Stat5 Tyrosine Phosphorylation and Nuclear Translocation by Two Separate Pathways

Abstract

The SH2 domain containing signal transducers and activators of transcription (Stat proteins) are effector molecules downstream of cytokine receptors. Ligand/ receptor engagement triggers Stat proteins tyrosine phosphorylation, dimerization, and translocation to the nucleus where they regulate gene transcription. Stat5, originally identified as a mammary gland growth factor, is an essential mediator of prolactin (PRL)-induced milk protein gene activation. Prolactin receptor (PRLR) is a member of the cytokine/growth hormone/PRL receptor superfamily. The mechanism through which PRLR modulates Stat5 tyrosine phosphorylation, nuclear translocation, and DNA binding was analyzed in HC11 cells, a mammary epithelial cell line, and 293-LA cells, a human kidney cell line stably overexpressing Jak2 kinase. We have found that in HC11 cells, Stat5 is specifically activated by PRL treatment, demonstrating that Stat5 is a physiological substrate downstream of PRLR. Furthermore, using different forms natural forms of the PRLR as well as receptor tyrosine to phenylalanine mutant forms, we determined that tyrosine phosphorylation of Stat5 is independent of PRLR phosphotyrosines. We established, however, that the C-terminal tyrosine of the PRLR Nb2 form, Tyr 382 , plays an essential positive role in PRLR-dependent Stat5 nuclear translocation and subsequently DNA binding. All together, our data propose a new model for activation of Stat5 through the PRLR, suggesting that Stat5 tyrosine phosphorylation and nuclear translocation are two separately regulated events. Prolactin (PRL)1 is a pituitary polypeptide hormone as well as a local growth factor that regulates several physiological functions such as reproduction, promotion of the growth and differentiation of the mammary gland, and immune function (1). PRL interacts with specific cell surface receptors expressed on different target tissues (reviewed in Ref. 2).The receptor for PRL (PRLR) belongs to a large group of receptors known as the cytokine/GH/PRL receptor superfamily, which includes the receptors for GH, EPO, granulocyte-macrophage colony-stimulating factor, and several interleukins (2, 3). These receptors share common extracellular structural motifs such as two disulfide loops and intracellular such as the proline-rich Box1 homology domain. These receptors do not possess intrinsic kinase activity but signal through cytoplasmic protein tyrosine kinases of the Janus kinase family (Jak/Tyk kinases) and the Src-kinase family. Ligand binding to the cytokine/GH/PRL receptor family induces receptor dimerization and activation of the associated kinases. This leads in turn to tyrosine phosphorylation of multiple cellular proteins including the receptors themselves (4).The downstream signaling molecules activated by this receptor family have not been completely elucidated. However, it has been shown that several of the SH2 domain containing molecules interact with cytokine receptors, e.g. phospholipid metabolizing enzymes, phospholipase C-␥, and the regulatory unit of phosphatidylino...

Role of the STAT1-SH2 Domain and STAT2 in the Activation and Nuclear Translocation of STAT1

“…It thus appears that the priming of cells with a subthreshold concentration of h/v facilitates an increase in the activation of STAT1-SH2mut after IFN␣/␤ while preserving the IFN-dependent nature of the stimulation. Earlier studies showed that in some cases the autophosphorylation sites of receptors are dispensable for STAT activation, leaving the possibility that the STAT-SH2 domain binds to a receptor-associated, tyrosine phosphorylated protein (24,45). In contrast, our findings suggest for Whole cell extracts (15 g) were subjected to immunoprecipitation with antiserum directed against the C terminus of STAT1, and isolated proteins were resolved on a 7.5% SDS-polyacrylamide gel.…”

The Coactivator of Transcription CREB-binding Protein Interacts Preferentially with the Glycosylated Form of Stat5

“…This domain of the protein is known to be important for protein interactions and tetramer formation of Stat5 dimers on target gene promoters (55). Glycosylated Stat5a was found to be localized mainly in the nucleus after cytokine stimulation of mammary epithelial cells (56,57). This implies that Stat5a is glycosylated by the OGlcNAc transferase either in the nucleus or after induction on the way to the nucleus.…”

Recruitment of the Protein-tyrosine Phosphatase SHP-2 to the C-terminal Tyrosine of the Prolactin Receptor and to the Adaptor Protein Gab2

“…The C-terminal Tyrosine of the PRLR Regulates the State of SHP-2 Tyrosine Phosphorylation-We have previously reported that the C-terminal tyrosine of the PRLR is critical for the induction of the prolactin responsive ␤-casein gene (29) and the activation of Stat5 (32). In addition, a positive role for SHP-2 in the activation of Stat5 and the induction of early genes has also been reported (7,33).…”

“…This receptor form has a 27-amino acid internal deletion and lacks tyrosine 309. Tyrosine to phenylalanine point mutants of the membrane proximal (LY237F, NY237F, and ⌬Y237F) and the C-terminal tyrosines (LY580F, NY382F, and ⌬Y382F) in the long form, Nb2 form, and the ⌬296 -322 mutant form were described previously (29,32). Finally, the PRLR Nb2 mutant form ⌬243-268 is also used in our studies as a negative control (29).…”