MYC-Driven Activation of USP39 Enhances SRSF1 Stability and Promotes PDAC Progression

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy primarily driven by oncogenic KRAS signaling. The splicing factor SRSF1 plays a key oncogenic role in PDAC, where its tightly regulated expression constrains KRAS-driven signaling under normal conditions, while its upregulation promotes tumorigenesis. SRSF1 expression is regulated in part by proteostasis. However, the precise mechanisms remain unclear. Here, we identify USP39 as a deubiquitinase that interacts with SRSF1 in an RNA-independent manner and stabilizes it by reducing ubiquitination. USP39 expression is elevated in PDAC tumors and precancerous lesions, and correlates with poor patient survival. USP39 knockdown suppresses PDAC cell proliferation and migration, effects that are partially rescued by SRSF1 overexpression. Mechanistically, we show that MYC directly activates USP39 transcription via E-box motifs within its exon 1b promoter, linking MYC-driven transcriptional regulation to post- translational stabilization of SRSF1. Together, these findings define a MYC-USP39-SRSF1 regulatory axis that integrates transcriptional and post-translational mechanisms in PDAC and highlight USP39 as a potential therapeutic target.