Schisandrin B Exerts Radiosensitizing Effects on Breast Cancer via Dual Mechanisms of Cell Cycle/DNA Repair and Gut Microbiota-Immune Axis Modulation.

Background/Objectives: Schisandrin B (Sch B), a bioactive lignan of Schisandra chinensis has been commonly investigated for its antitumor activities, yet its radiosensitizing effect and mechanism remain unclear. This study was conducted to investigate the radiosensitizing effects of Sch B in breast cancer (BC) and elucidate its molecular mechanisms, with a specific focus on the gut microbiota-immune axis. Methods: In vitro, CCK-8, colony formation, and 3D spheroid assays were used to evaluate the effects of Sch B on proliferation inhibition and radiosensitization, flow cytometry and immunofluorescence were used to elucidate the mechanisms involved. In vivo, 4T1 tumor-bearing mice were treated with Sch B, and 16S rDNA sequencing and LC-MS/MS were used to analyze the gut microbiota and short-chain fatty acid (SCFA) metabolism. IHC and qPCR detected antitumor immune responses. Results: Sch B inhibited the proliferation of BC cells in a time- and dose-dependent manner with negligible toxicity to the mammary epithelial cell line MCF-10A. Furthermore, Sch B enhanced the radiosensitivity (sensitization enhancement ratio: 1.20~1.77) of BC by inducing G1 phase cell cycle arrest and delaying radiation-induced DNA double-strand break repair. In vivo, Sch B suppressed BC growth in BALB/c mice without causing obvious systemic toxicity. Sch B reversed tumor-induced gut microbiota dysbiosis (restoring species abundance and the Firmicutes/Bacteroidetes ratio, enriching beneficial genera such as Lactobacillus and Butyrobacter) and normalized SCFA profiles (correlative evidence). Furthermore, Sch B modulated systemic immune responses by increasing the expression of Ifng, Cxcl10, Ddx58 and promoting CD3+ and CD8+ T-cell infiltration in tumors. Conclusions: Sch B exerts BC radiosensitization through dual mechanisms, direct regulation of the cell cycle and DNA repair, and indirect modulation of the gut microbiota-immune axis (correlative evidence), highlighting it as a safe and effective candidate for improving the efficacy of BC radiotherapy.