Etk/BMX is a non-receptor protein tyrosine kinase that requires a functional phosphatidylinositol 3-kinase via the pleckstrin homology domain to be activated by cytokine. In the present study, a conditionally active form of Etk was constructed by fusing the hormone-binding domain of estrogen receptor (ER) to an amino terminus truncated form of Etk, PHA1-68Etk, to generate ΔEtk:ER. In stably transfected Pa-4AEtk:ER cells, the activity of ΔEtk:ER was stimulated within minutes by the treatment of ΔEtk:ER stimulant, estradiol, and sustained for greater than 24 h. A robust induction in the phosphorylation of signal transducers and activators of transcription (STAT) proteins, including STAT1, STAT3, and STAT5, was accompanied with ΔEtk:ER activation. Moreover, the conditionally activated Etk stimulated STAT1- and STAT5-dependent reporter activities by ~160- and ~15-fold, respectively, however, elicited only a modest STAT3-mediated reporter activation. Qualitatively comparable results were obtained in lung A549 cells, indicating that ΔEtk:ER inducible system could function in an analogous fashion in different epithelial cells. Furthermore, we demonstrated that Etk activation alone augmented cyclin D1 promoter/enhancer activity via its STAT5 response element in both Pa-4ΔEtK:ER and A549 cells. Altogether, these findings support the notion that the activation of Etk kinase is sufficient to transactivate STAT-mediated gene expression. Hence, our inducible ΔEtK:ER system represents a novel approach to investigate the biochemical events following Etk activation and to evaluate the contribution by kinase activation of Etk alone or in conjunction with other signaling pathway(s) to the ultimate biological responses.
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