Multi-omics integration reveals the CTSK-cholesterol metabolic axis and the cholesterol–RORA/LDLR paracrine axis in SPP1+ macrophages as mediators of 6PPD-quinone-induced idiopathic pulmonary fibrosis

Objective This study aimed to screen and identify potential biomarkers linking 6PPD-quinone (6PPD-Q) exposure to idiopathic pulmonary fibrosis (IPF), generating a mechanistic hypothesis for future experimental investigation and therapeutic development.Methods 6PPD-Q-related genes and IPF-associated genes were obtained from public databases. Candidate biomarkers were identified via differential expression analysis, machine learning, Mendelian randomization, Receiver operating characteristic (ROC) curve analysis, and gene expression profiling. Functional enrichment, immune infiltration, molecular docking, molecular dynamics (MD) simulation, single-cell RNA sequencing (scRNA-seq), and spatial transcriptomics were employed to explore underlying mechanisms.Results CTSK was identified as a candidate biomarker significantly upregulated in IPF samples and enriched in cholesterol metabolism pathways. Differentially infiltrated B cells showed strong correlation with CTSK. Molecular docking predicted a favorable interaction between 6PPD-Q and CTSK (-6.108 kcal/mol), while MD simulations supported the structural stability of the predicted complex. scRNA-seq and spatial transcriptomics suggested SPP1+ macrophages (Macro-SPP1) as a key cell population, in which high CTSK expression and cholesterol metabolic dysregulation may contribute to fibrosis.Conclusion CTSK-associated cholesterol metabolic dysregulation in Macro-SPP1 may represent a potential molecular link between 6PPD-Q exposure and IPF-related fibrotic processes. The cholesterol-associated RORA/LDLR signaling axis may further contribute to macrophage-fibroblast interactions and the development of a pro-fibrotic microenvironment. These findings suggest a potential 6PPD-Q-CTSK-cholesterol regulatory axis and provide candidate molecular targets for future experimental validation and therapeutic investigation.