<i>Clostridium butyricum</i> alleviates multiple myeloma by remodeling the bone marrow microenvironment and inhibiting PI3K/AKT pathway through the gut‒bone axis

Emerging evidence reveals a strong connection between the gut microbiota and cancer. However, the exact role of gut microbiota dysbiosis in multiple myeloma (MM) is poorly understood, and the therapeutic potential of microbiota-targeted interventions represents a promising strategy that demands urgent mechanistic and translational investigation. First, we conducted a comprehensive microbiome-metabolite analysis between MM patients and healthy individuals. The result revealed a marked compositional difference characterized by reduced abundances of butyrate-producing bacteria and diminished butyrate levels in the MM cohort. Subsequent fecal microbiota transplantation demonstrated that the gut microbiota critically modulates MM progression, with healthy donor-derived microbiota reducing the tumor burden and concomitantly elevating serum butyrate. Furthermore, through function-based culturomics screening, <i>Clostridium butyricum</i> (<i>C. butyricum</i>) was identified as a key butyrate-producing specialist. <i>C. butyricum</i> or its metabolite butyrate significantly reduced the systemic tumor burden in 5TGM1 mice. Notably, <i>C. butyricum</i> and butyrate alleviated bone marrow inflammation and osteolytic lesions by suppressing Th17 cells and IL-17 levels in the bone marrow. Moreover, cellular assays and transcriptome sequencing further revealed that butyrate could induce MM cells' apoptosis via HDAC inhibition-mediated upregulation of PPARγ, leading to sequential suppression of the PI3K/AKT pathway and antiapoptotic BCL-2 expression. This apoptotic signaling cascade was reversed by PPARγ antagonism. The direct antitumor effect was further confirmed in M-NSG mice. Our research systematically verifies the specific role of the gut microbiota in MM and provides the first evidence of the immune and molecular mechanisms by which <i>C. butyricum</i> alleviates MM progression, offering preclinical support for probiotic-based therapies against MM.