PPARA and nicotine dependence: Subsequently, KEGG enrichment analysis revealed multiple pathways associated with COPD, including p53 signaling pathway, MAPK signaling pathway, PI3K-Akt signaling pathway, Hippo signaling pathway, Nicotine addiction, PPAR signaling pathway, Cholesterol metabolism, etc., which was consistent with previous studies (33–38), and could initially confirm the reliability of this study.