Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5 B were compared with their subcellular localization determined using indire ct immunofluorescence microscopy. Following prolactin activation, both STAT 5A and STAT5B were rapidly translocated into the nucleus and displayed a de tergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA bind ing of both STAT5A and STAT5B. However, nuclear translocation of only STAT5 B 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 preve nted prolactin-induced tyrosine phosphorylation and nuclear translocation o f STAT5A and STAT5B but did not block src kinase activation and nuclear tra nslocation of STAT5B. In co-transfection assays, prolactin-mediated activat ion but not src kinase-mediated activation of STAT5B resulted in the induct ion of a beta-casein promoter-driven reporter construct. These results sugg est that STATE activation by src may occur by a mechanism distinct from tha t employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.