S100A4 facilitates radiation-induced tumor repopulation by driving polyploid giant cancer cells budding.

Radiotherapy, a pivotal treatment for colorectal cancer, is compromised by tumor repopulation, which is characterized by accelerated growth and increased treatment resistance. Although radiation-induced DNA breaks eliminate most cells, a subset of polyploid giant cancer cells (PGCCs) evade death through massive genomic amplification, subsequently undergoing depolyploidization via a viral budding-like process to generate proliferative progeny. Critically, these PGCCs drive tumor repopulation and underpin therapeutic failure. However, the molecular mechanisms that regulate PGCCs budding and the mechanisms by which they bypass the intrinsic safeguards against abnormal proliferation remain unclear. In this study, we dissected the life cycle of PGCCs in colorectal cancer post-radiation. Specifically, we used a combination of single-cell transcriptomics, functional genomics, and longitudinal models to map the dynamic transition from polyploid persistence to PGCCs budding. We reveal that S100A4-a key mediator of PGCCs repopulation-is significantly upregulated in post-radiotherapy PGCCs. S100A4 depletion significantly suppresses budding capacity by blunting viral budding-like proliferation, mechanistically achieved through suppression of IRF3-mediated interferon-I signaling, thereby alleviating ISG15/BST2-mediated suppression of budding. Further mechanistic studies revealed that S100A4 binds the RAGE receptor to facilitate this process. Pharmacological inhibition of RAGE phenocopied S100A4 depletion, impairing PGCCs budding and restoring radiation sensitivity. These results nominate the S100A4-ISG15 axis as a central regulator of radiotherapy resistance and suggest that stratifying patients by S100A4-ISG15 expression or the "viral budding score" could predict radiotherapy resistance. Additionally, targeting this axis may offer an opportunity to overcome adaptive persistence in colorectal cancer and possibly other cancer types.