Differential Effects of Prolactin andsrc/abl Kinases on the Nuclear Translocation of STAT5B and STAT5A

Abstract

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergentresistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinasemediated activation of STAT5B resulted in the induction of a ␤-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.Cytokines influence a variety of cellular functions including proliferation, growth arrest, and differentiation. The neuroendocrine hormone prolactin (Prl) 1 plays a central role in the development and differentiation of the mammary gland. Binding of Prl to its cell surface receptor, a member of the cytokine receptor superfamily, regulates the transcription of several milk protein genes, including the whey acidic protein (1), ␤-lactoglobulin (2), and ␤-casein (3, 4) genes. The Prl receptor (PrlR) transmits signals in part via activation of the JAK/signal transducers and activators of transcription (STAT) pathway. Interaction of Prl with PrlR induces receptor dimerization, activation of the JAK2 protein-tyrosine kinase (3, 5-7), and tyrosine phosphorylation of transcription factors that belong to the STAT family.Among the seven mammalian STAT proteins that have been discovered (8), STAT1, STAT3, and STAT5 are capable of activation by the PrlR (9). STAT5, however, plays a key role in Prl-induced milk protein gene expression and mammary gland differentiation (10, 11). Tyrosine 700 of rat STAT5 is the site of phosphorylation by JAK2 and the primary regulator of STAT5 DNA binding (3). After tyrosine phosphorylation, STAT5 dimerizes and translocates into the nucleus, where it binds to specific DNA elements and activates transcription of target genes, such as the milk protein genes.Two different STAT5 genes encoding STAT5A and STAT5B have been identified that share 93% identity at the amino acid level with the primary differences occurring at their carboxyl termini (12-14). STAT5A was discovered as a mediator of Prl response in mammary epithelial cells and was originally designated as mammary gland-specific factor, or MGF (4, 15). ...

IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled

“…A second line of evidence comes from the study of Kazansky et al (1999) who studied the DNA-binding and tyrosine phosphorylation of STAT-5A and STAT-5B following prolactin activation or Src activation. Their results show that following prolactin activation, both STAT5A and STAT5B were rapidly phosphorylated and translocated to the nucleus.…”

EGF receptor‐mediated, c‐Src‐dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes

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Hypoxia activates the cyclin D1 promoter via the Jak2/STAT5b pathway in breast cancer cells

“…However, surprisingly, we observed differences in transcriptional activities of STAT5a and -5b induced by hypoxia. It is known that ligand-dependent and -independent signal transduction pathways may differentially regulate STAT5a and -5b nuclear translocation and, therefore, have the potential to differentially regulate STAT5-dependent gene expression (Kazansky et al, 1999).…”