CircNSD2 promotes metastasis and immune escape by increasing the USP10/SRSF6-mediated alternative splicing of TPM1 in TNBC

Triple-negative breast cancer (TNBC) is the most malignant BC subtype, and metastasis remains the major cause of poor prognosis. TGF-β plays crucial roles in TNBC metastasis, yet the underlying mechanisms remain unclear. Circular RNAs (circRNAs) are a novel type of regulatory RNA characterized by high evolutionary conservation and stability. This study aims to investigate the roles and mechanisms of circRNAs in TGF-β-induced TNBC metastasis. CircNSD2 was detected in TNBC cells through high-throughput RNA sequencing. Gain- and loss-of-function assays were performed to evaluate the role of circNSD2. Chromatin immunoprecipitation (ChIP) and luciferase assays verified the regulatory effects of TGF-β on circNSD2. RNA pulldown, proteomic analyses, and RNA immunoprecipitation were conducted to identify the downstream targets of circNSD2. CircNSD2 expression was upregulated in patients with metastatic TNBC and correlated with an unfavorable prognosis. In vitro and in vivo studies indicated that circNSD2 facilitates TGF-β-induced TNBC metastasis and EMT. Mechanistically, circNSD2 is activated by TGF-β and cyclized by KHSRP. Moreover, circNSD2 functions as a scaffold to enhance the interaction between the SRSF6 and USP10 proteins, thereby preventing K48-linked polyubiquitination of SRSF6 at lysine 16 and inhibiting its proteasomal degradation. Furthermore, stabilized SRSF6 reprogrammed TPM1 alternative splicing, which resulted in TNBC metastasis. In addition, circNSD2 promoted immune escape in TNBC by upregulating PD-L1 expression and suppressing the antitumor immunity of CD8+ T cells. Our study revealed that circNSD2 increased TNBC metastasis and immune escape by promoting the USP10/SRSF6/TPM1 axis and that circNSD2 could serve as a potential diagnostic biomarker and therapeutic target for TNBC patients.