Xiao-Yao-San suppresses psychological distress-related colorectal cancer metastasis and enhances antitumor immune responses through regulating gut microbiota–bile acid axis

• XYS inhibits liver metastasis and alleviates depression in CRS-induced CRLM mice by modulating gut microbiota and bile acid profiles. • XYS restores BSH-expressing bacteria in the cecal microbiota and the free BA proportion in the serum pool. • XYS and its elevated hyocholic acid boost antitumor immune responses in liver metastatic foci of CRS-induced CRLM mice. Psychiatric disorders are recognized as risk factors for tumor progression and metastasis in colorectal cancer (CRC). Xiao-Yao-San (XYS), a traditional Chinese herbal formula with antidepressant properties, has been reported to inhibit CRC growth and modulate gut microbiota. However, its efficacy and underlying mechanisms in preventing metastasis remain unclear. The antimetastatic effect of XYS was assessed in a chronic restraint stress (CRS)-induced CRC model. XYS-associated gut microbiota was evaluated using 16S ribosomal RNA (rRNA) sequencing. Peripheral bile acid (BA) profiles were analyzed through targeted metabolomics. Fecal microbiota and commercial bacterial strains were anaerobically cultured to identify bile salt hydrolase ( bsh )-expressing bacteria. XYS-mediated antitumor immune responses were determined via flow cytometry, immunohistochemistry, and cytotoxic T lymphocytes (CD8⁺ T cells)–murine colon adenocarcinoma cell line (MC38) co-culture assays. Administration of dried XYS decoction powder reduced liver metastatic nodules and prevented body weight loss in CRS-induced CRC mice, while also alleviating depressive-like behaviors and lowering serum stress hormone levels. High-dose XYS (2 g/kg/day) produced antimetastatic and antidepressant effects comparable to those of sertraline, although these effects were largely abolished in mice treated with antibiotic cocktails. XYS significantly modulated gut microbiota composition and serum BA profiles, particularly by restoring bsh -expressing bacteria in the cecum and increasing the proportion of free BAs. XYS also reshaped the immune landscape within metastatic foci, where enhanced antitumor CD8⁺ T cell responses were strongly associated with the altered BA profile. Among these, hyocholic acid (HCA), a free BA elevated by XYS, suppressed liver metastasis in vivo , alleviated exhaustion and restored effector function of CD8⁺ T cells in metastatic tumor microenvironment (TME), together with dose-dependently enhanced CD8 + T cell cytotoxicity against tumor cells in vitro . XYS prevents psychiatric disorder-associated CRC metastasis and strengthens antitumor immune responses by regulating bsh -expressing gut bacteria and their derived BA profiles. These findings thus offer a novel mechanistic rationale for the potential use of XYS in treating CRC patients with comorbid psychiatric disorders.