RBM7 deficiency promotes breast cancer metastasis by coordinating MFGE8 splicing switch and NF-kB pathway

Abstract

Aberrant alternative splicing is well-known to be closely associated with tumorigenesis of various cancers. However, the intricate mechanisms underlying breast cancer metastasis driven by deregulated splicing events remain largely unexplored. Here, we unveiled RBM7 as a novel regulator of alternative splicing that plays a crucial role in counteracting the metastatic potential of breast cancer. Through bioinformatics analysis and IHC staining validation, we revealed that RBM7 is decreased in lymph node and distant organ metastases of breast cancer as compared to primary lesions. Furthermore, we found that low expression of RBM7 is correlated with the reduced disease-free survival of breast cancer patients. Breast cancer cells with RBM7 depletion exhibited an increased potential for lung metastasis compared to scramble control cells. The absence of RBM7 stimulated breast cancer cell migration, invasion, and angiogenesis. Mechanistically, RBM7 regulates the splicing of MFGE8 directly, favoring the production of the predominant MFGE8-L isoform. This results in the attenuation of STAT1 phosphorylation and alterations in cell adhesion molecules. The MFGE8-L isoform exerted an inhibitory effect on the migratory and invasive capability of breast cancer cells, while the MFGE8-S isoform had the opposite effect. Particularly, the ectopic expression of MFGE8-L significantly reversed the pro-invasion effect of RBM7 silencing, but did not contribute to the promotion of angiogenesis-related secreted proteins. In RBM7 depleted cells, a gene set enrichment analysis revealed a significant amplification of the NF-κB cascade. Concordantly, RBM7 negatively regulates p65 phosphorylation. Furthermore, an NF-κB inhibitor could obstruct the increase in HUVEC tube formation caused by RBM7 silencing. Clinically, we noticed a positive correlation between RBM7 expression, MFGE-8 exon7 inclusion, and p65 downstream targets. Therefore, our study not only offer mechanistic insights into how abnormal splicing contributes to the aggressiveness of breast cancer, but also provide a new approach for molecular-targeted therapy in combating breast cancer.