Targeting breast cancer stem cells by restoring RBM38 function: mechanism and protein delivery therapy.

Breast cancer stemness drives tumor progression and therapy resistance, yet the underlying mechanisms remain poorly understood. In this study, we observed that RBM38 expression gradually decreases during breast cancer progression in MMTV-PyMT transgenic mice. Using a model with targeted RBM38 expression in breast tissue, we found it suppressed tumor initiation, decreased tumor size and metastasis, and reduced breast cancer stem cells. Multi-omics analyses, including single-cell and spatial transcriptomics, RIP-sequencing, and proteomics, were used to reveal the mechanisms. We identified HOXD10 mRNA as a critical downstream target of RBM38. Phosphorylation of RBM38 at Ser117, Thr120, and Thr132 by SLK and PGAM5 is required for RBM38-mediated stemness suppression, protein complex formation, and HOXD10 mRNA regulation. Based on these insights, we explored RBM38's therapeutic potential using an activatable supercharged polypeptide (ASCP) delivery platform. In patient-derived xenograft (PDX) models, ASCP-RBM38 achieved effective tumor localization, reduced stemness, and protected against doxorubicin (DOX)-induced cardiotoxicity. Clinical analysis of human samples validated this mechanism. Our study underscores the pivotal role of phosphorylation-mediated regulation and protein complex formation in RBM38-mediated control of breast cancer stemness, highlighting its therapeutic potential.