The critical role of the phosphorylation of STAT3 at Y705 in ALCL-associated NPM-ALK-induced transforming activity

The fusion protein nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) drives oncogenesis in anaplastic large cell lymphoma (ALCL) by activating signal transducer and activator of transcription 3 (STAT3). NPM-ALK requires its kinase activity to induce STAT3 phosphorylation at tyrosine 705 (Y705) and promotes serine 727 (S727) phosphorylation via JNK activation. However, the role of these modifications in NPM-ALK-driven cellular transformation remains unclear. We herein utilized murine Ba/F3 cells expressing NPM-ALK to investigate the impact of these modifications. STAT3 knockdown, followed by reconstitution with wild-type STAT3 or its mutants (Y705F and S727A) revealed that Y705 phosphorylation was essential for NPM-ALK-mediated transformation. STAT3 knockdown suppressed the expression of NPM-ALK-induced STAT3 target genes (c-Myc, Pim, IL-6, and SOCS3) as well as cell proliferation, tumor formation, and spleen, liver, and lymph node enlargement. These effects were restored upon reconstitution with wild-type STAT3 or the S727A mutant, but not the Y705F mutant, confirming the essential role of Y705 phosphorylation in these biological processes. Additionally, wild-type and mutant STAT3 proteins exhibited differential stability, with Y705F being less stable and S727A being more stable. Lysosomal inhibition by bafilomycin A1 increased the expression of wild-type STAT3 and the Y705F mutant, but had no effect on the S727A mutant. Cycloheximide chase assays further confirmed that S727 phosphorylation regulated STAT3 degradation via the lysosomal pathway. These results identify a novel NPM-ALK-induced oncogenic mechanism mediated by STAT3 phosphorylation, highlighting potential therapeutic targets for ALCL.