A critical role for STAT3 Thr714 phosphorylation in NPM-ALK-driven tumorigenesis.

The oncogenic fusion protein NPM-ALK drives anaplastic large cell lymphoma (ALCL) by activating the transcription factor STAT3. While STAT3 phosphorylation at Y705 and S727 is well characterized, the present study defines a mechanistic role for phosphorylation at T714 in supporting full STAT3 functionality. In NPM-ALK-positive ALCL cells, STAT3 is phosphorylated at Y705, S727, and T714, and this is suppressed by ALK inhibition. Enforced NPM-ALK expression in Ba/F3 cells induces phosphorylation at all three sites in a kinase-dependent manner. To investigate the role of T714, wild-type STAT3 or a T714A mutant was reconstituted into STAT3-knockdown Ba/F3 cells expressing NPM-ALK. Wild-type STAT3 underwent Y705 and S727 phosphorylation and nuclear translocation, whereas the T714A mutant was phosphorylated at S727 only and failed to translocate. The reduced expression of STAT3 target genes (Cyclin D1, Pim1, Pim2, and Socs3) with STAT3 knockdown was restored by wild-type STAT3, but not by the T714A mutant. In vivo, STAT3 knockdown suppressed tumor formation and hepatosplenomegaly in mice inoculated with Ba/F3 cells expressing NPM-ALK, and these phenotypes were rescued by wild-type STAT3, but not by the T714A mutant. These findings indicate that STAT3 phosphorylation at T714 is required for subsequent Y705 phosphorylation, nuclear translocation, and transcriptional activation specifically within the context of NPM-ALK-mediated signaling.