Endogenous interaction profiling identifies DDX5 as an oncogenic coactivator of transcription factor Fra-1.

Fra-1, a member of the activator protein 1 (AP-1) family, is overexpressed in triple-negative breast cancer (TNBC) and plays crucial roles in tumor growth. Here we report the identification of 118 proteins interacting with endogenous chromatin-bound Fra-1 in TNBC cells, highlighting DDX5 as the most enriched Fra-1-interacting protein. DDX5, a previously unrecognized protein in the Fra-1 transcriptional network, shows extensive overlap with Fra-1 cistrome and transcriptome that are highly associated with the TNBC cell growth. We provide evidence that DDX5 expression enhances Fra-1 transcriptional activity and potentiates Fra-1-driven cell proliferation. Furthermore, we show that the DDX5 target gene signature predicts poor clinical outcome in breast cancer patients. DDX5 protein level was higher in triple-negative basal-like tumors than in non-basal-like tumors, including luminal A, luminal B, and HER2-enriched subtypes. Collectively, by combining proteomic and genomic approaches we reveal a role for DDX5 as a regulatory protein of Fra-1 signaling and suggest DDX5 as a potential therapeutic target for TNBC.