Cell cycle dysregulation is a defining feature of breast cancer. Here, 1âmethylânicotinamide (1âMNA), metabolite of nicotinamide Nâmethyltransferase(NNMT) is identified, as a novel driver of cellâcycle progression in breast cancer. NNMT, highly expressed in breast cancer tissues, positively correlates with tumor grade, TNM stage, Kiâ67 index, and tumor size. Ablation of NNMT expression dramatically suppresses cell proliferation and causes cellâcycle arrest in G0/G1 phase. This phenomenon predominantly stems from the targeted action of 1âMNA, resulting in a specific downâregulation of p27 protein expression. Mechanistically, 1âMNA expedites the degradation of p27 proteins by enhancing cullinâ1 neddylation, crucial for the activation of Cullinâ1âRING E3 ubiquitin ligase(CRL1)âan E3 ubiquitin ligase targeting p27 proteins. Â NNMT/1âMNA specifically upâregulates the expression of UBC12, an E2 NEDD8âconjugating enzyme required for cullinâ1 neddylation. 1âMNA showes high binding affinity to UBC12, extending the halfâlife of UBC12 proteins via preventing their localization to lysosome for degradation. Therefore, 1âMNAÂ is a bioactive metabolite that promotes breast cancer progression by reinforcing neddylation pathwayâmediated p27 degradation. The study unveils the link between NNMT enzymatic activity with cellâcycle progression, indicating that 1âMNA may be involved in the remodeling of tumor microenvironment.