Bmal1 Regulates Vascular Calcification via Noncanonical Circadian Pathway

BACKGROUND: Vascular calcification is a major contributor to cardiovascular morbidity and mortality in diabetes and is driven in part by osteogenic reprogramming of vascular smooth muscle cells (SMCs). Diabetes is also associated with circadian rhythm disruption; however, how circadian regulators contribute to vascular calcification remains poorly understood. We investigated the role of the core circadian protein Bmal1 (basic helix-loop-helix ARNT-like protein 1) in diabetes-associated vascular calcification. METHODS: Bmal1 expression was examined in diabetic mouse aortas, human diabetic arterial tissues, and vascular SMCs exposed to high glucose. SMC-specific Bmal1 knockout mice were generated and subjected to low-dose streptozotocin-induced diabetes. Vascular calcification and stiffness were assessed by calcium staining and quantification, and pulse wave velocity. RNA sequencing, chromatin immunoprecipitation, luciferase reporter assay, and clustered regularly interspaced short palindromic repeats-clustered regularly interspaced short palindromic repeat–associated 9–mediated genome editing were used to define Bmal1-regulated molecular mechanisms. RESULTS: Bmal1, but not other circadian regulators, was selectively upregulated in diabetic arteries and vascular SMCs under hyperglycemic conditions. SMC-specific deletion of Bmal1 markedly attenuated diabetes-induced vascular calcification, reduced aortic stiffness, and suppressed osteogenic gene expression in vivo and in vitro. RNA-sequencing analysis revealed enrichment of Bmal1-dependent genes involved in osteogenic differentiation and extracellular matrix organization. Mechanistically, Bmal1 directly binds to an E-box motif in the Runx2 (Runt-related transcription factor 2) promoter, enhancing Runx2 transcription independently of canonical circadian pathways. Disruption of Bmal1 binding to the Runx2 promoter abolished Bmal1-induced Runx2 expression. CONCLUSIONS: Our findings identify Bmal1 as a key mediator of diabetes-induced vascular calcification through direct transcriptional regulation of Runx2 via a noncanonical circadian mechanism. These results reveal a previously unrecognized pathogenic role for Bmal1 in vascular disease and suggest that targeting dysregulated circadian factors may represent a novel therapeutic strategy for preventing vascular calcification in diabetes.