Single-cell and bulk transcriptomic sequencing reveals tryptophan metabolism-linked biomarkers of obesity

Obesity remains a major public health challenge, and the contribution of tryptophan metabolism to obesity-associated inflammation remains incompletely understood. Here, we integrated bulk transcriptomic and single-cell RNA sequencing datasets from the Gene Expression Omnibus (GSE25401, GSE217007, GSE166047, and GSE176171) with a curated set of tryptophan metabolism-related genes. Weighted gene co-expression network analysis and differential expression analysis identified 95 differentially expressed tryptophan metabolism-related genes. Protein-protein interaction analysis, univariate logistic regression, and machine-learning models including random forest, support vector machine, and generalized linear model prioritized five biomarkers: SPI1, ITGB2, CD86, CYBB, and TLR8. All five genes were upregulated in obesity and were enriched in immune-related pathways. Single-cell analysis identified major adipose tissue cell populations and showed that SPI1, CD86, CYBB, and TLR8 were predominantly expressed in monocytes, whereas ITGB2 was enriched in natural killer cells. Immune infiltration analysis further supported close associations between these biomarkers and obesity-related immune dysregulation. RT-qPCR validation confirmed increased expression of ITGB2, CD86, and SPI1 in adipose tissue from obese individuals, whereas CYBB and TLR8 were not significantly different between groups. Together, these findings identify tryptophan metabolism-linked biomarkers associated with obesity and suggest that their main translational relevance may lie in stratifying or targeting obesity-associated secondary inflammation.