Gut Microbiota-Derived Butyric Acid Alleviates Glucocorticoid-Associated Osteonecrosis of the Femoral Head via Modulating Inflammatory Cytokines in Bone Marrow Mesenchymal Stem Cells.

Background: The role of gut microbiota and its metabolites in regulating bone metabolism has been well established, with inflammatory immune responses potentially playing a critical role. Glucocorticoid-associated osteonecrosis of the femoral head (GA-ONFH), caused by high-dose glucocorticoid use for inflammatory or immune-related diseases, is a prevalent condition of bone metabolic imbalance. However, the regulatory role and mechanisms of gut microbiota and its metabolites in the development and progression of GA-ONFH remain unclear. This study aims to investigate the intervention effects of gut microbiota and its metabolite butyric acid on GA-ONFH through a series of multi-omics in vitro and in vivo experiments. Methods: Sprague Dawley rats were randomly divided into four groups. The gut microbial composition of the groups was analyzed through 16S rDNA sequencing. Targeted metabolomics was employed to assess differences in short-chain fatty acids (SCFAs) among the groups. Butyric acid, identified as a key differential metabolite, was then selected for further exploration of its effects on bone marrow mesenchymal stem cells (BMSCs) and GA-ONFH rat models through in vitro and in vivo experiments. Results: 16S rDNA sequencing revealed alterations in gut microbiota structure in GA-ONFH rats. Micro-CT and HE staining demonstrated that depletion of gut microbiota with broad-spectrum antibiotics prior to GA-ONFH modeling exacerbated the disease's development. In contrast, fecal microbiota transplantation (FMT) was shown to alleviate GA-ONFH progression. Targeted metabolomics indicated that FMT mitigated the reduction in butyric acid levels induced by dexamethasone (DXM). Subsequent in vitro cell experiments confirmed that butyric acid promotes BMSC proliferation, migration, and osteogenic differentiation. RNA sequencing revealed that butyric acid regulates T cell-mediated inflammatory cytokine genes in BMSCs, while Western blot and immunofluorescence assays confirmed that butyric acid modulates the expression of TNF-α and IL-2/IL-4 in BMSCs. Finally, in vivo experiments demonstrated that butyric acid supplementation attenuated the progression of GA-ONFH and improved the expression of inflammation-related cytokines in femoral head tissue. Conclusions: Our study demonstrates that gut microbiota depletion exacerbates GA-ONFH, while FMT restores butyric acid levels and alleviates disease severity. Butyric acid reduced the expression of TNF-α and IL-2 while increasing the level of IL-4 in vivo and in vitro, thereby improving the local inflammatory environment of the femoral head and alleviating the progression of GA-ONFH. These findings highlight that reduction in butyric acid levels due to gut microbiota dysbiosis is a crucial factor in the progression of GA-ONFH.