The Brown Strain of Flammulina velutipes Singer Attenuates 5-Fluorouracil-Induced Intestinal Injury by Suppressing Inflammation, Oxidative Stress, and Barrier Disruption via Modulation of Epithelial–Mesenchymal Transition and Tight Junction Integrity

5-Fluorouracil (5-FU) remains a cornerstone chemotherapeutic for colorectal cancer, exerting its antitumor effects primarily through disruption of DNA and RNA synthesis and subsequent induction of apoptosis. Nonetheless, its clinical efficacy is often compromised by prominent adverse effects, particularly mucositis. This study examines the potential of brown-strain Flammulina velutipes Singer (FVB) to alleviate 5-FU-associated intestinal damage in a mouse model, offering insights into its possible role in mitigating chemotherapy-induced toxicity. 5-FU treatment significantly exacerbated gastrointestinal toxicity, as evidenced by severe diarrhea, shortened colon length, villus atrophy, and architectural disorganization of the intestine. It also inhibited crypt cell proliferation and induced body weight loss. Mechanistically, 5-FU activated pro-inflammatory, apoptotic, oxidative stress, and EMT pathways and disrupted mucosal tight junctions. Notably, FVB administration mitigated these pathological changes, indicating its protective role against 5-FU-induced intestinal injury. In summary, this investigation presents novel evidence for the protective role of FVB in mitigating 5-FU-induced intestinal mucositis. The results highlight the therapeutic potential of FVB as an adjunct to chemotherapy, potentially reducing treatment-related toxicity and enhancing the clinical care and quality of life of individuals undergoing colorectal cancer therapy.