The potential involvement of FABP4 in rheumatoid arthritis-associated osteoporosis: a comprehensive bioinformatics analysis and experimental validation

<h4>Objective</h4>This study investigates the role and underlying mechanism of FABP4 in RA secondary OP by examining its correlation with the condition.<h4>Methods</h4>DEGs related to RA and OP were identified using GEO datasets (GSE17755 and GSE230665), focusing on bone metabolism genes. Candidate genes were identified using WGCNA, PPI networks, and four algorithms (Degree, BottleNeck, MCC, EPC), followed by enrichment analysis. Key genes were pinpointed using machine learning methods (SVM-RFE, LASSO, Boruta) and validated with ROC curves, all showing AUC > 0.7. MicroRNA prediction and genomic mapping were also done. For <i>in vivo</i> validation, CIA rats (n=5 per group) were analyzed at 2, 6, and 12 weeks against a control group using ELISA, micro-CT, H&E staining, IHC, and quantitative image analysis.<h4>Results</h4>Bioinformatics identified eight hub genes: PTEN, CDC42, MFN2, UBC, RAS5F1, MMP9, CCL17, and FABP4. In CIA rats, FABP4, TNF-α, IL-6, IL-1β, RANKL, and β-CTX levels rose with disease progression, showing slight increases at 2 weeks and significant rises at 6 and 12 weeks (P<0.05). At 12 weeks, these levels were higher than at 6 weeks (P<0.01). FABP4 levels positively correlated with TNF-α, IL-6, IL-1β, RANKL, and β-CTX (P<0.01). Micro-CT showed worsening ankle joint damage over time. At 2 weeks, CIA rats had no significant BS/BV difference from controls but showed decreased BV/TV and Tb.Th and increased Tb.Sp (P<0.05). At 6 and 12 weeks, CIA rats had significantly lower BV/TV, BS/BV, and Tb.Th and higher Tb.Sp compared to controls (P<0.01). IHC analysis showed high FABP4, TNF-α, and RANKL expression in CIA rats' synovium and cartilage at 2 weeks. TNF-α levels stabilized by 6 weeks and returned to normal by 12 weeks. FABP4 peaked at 12 weeks, while RANKL decreased but stayed above 2-week levels.<h4>Conclusion</h4>FABP4 is closely associated with abnormal bone metabolism in RA secondary OP, potentially through the RANKL/RANK/OPG system. It may represent a potential therapeutic target that warrants further investigation.