MicroRNA-21 Controls Circadian Regulation of Apoptosis in Atherosclerotic Lesions

Background: The necrotic core partly formed by ineffective efferocytosis increases the risk of an atherosclerotic plaque rupture. Microribonucleic acids contribute to necrotic core formation by regulating efferocytosis and macrophage apoptosis. Atherosclerotic plaque rupture occurs at increased frequency in the early morning, indicating diurnal changes in plaque vulnerability. Although circadian rhythms play a role in atherosclerosis, the molecular clock output pathways that control plaque composition and rupture susceptibility are unclear.

Methods: Circadian gene expression, necrotic core size, apoptosis, and efferocytosis in aortic lesions were investigated at different times of the day in mice and mice after consumption of a high-fat diet for 12 weeks. Genome-wide gene expression and lesion formation were analyzed in bone marrow-transplanted mice. Diurnal changes in apoptosis and clock gene expression were determined in human atherosclerotic lesions.

Results: The expression of molecular clock genes, lesional apoptosis, and necrotic core size were diurnally regulated in mice. Efferocytosis did not match the diurnal increase in apoptosis at the beginning of the active phase. However, in parallel with apoptosis, expression levels of oscillating strands decreased in the mouse atherosclerotic aorta. knockout abolished circadian regulation of apoptosis and reduced necrotic core size but did not affect core clock gene expression. Further, knockout upregulated expression of proapoptotic Xaf1 (XIAP-associated factor 1) in the atherosclerotic aorta, which abolished circadian expression of Xaf1. The antiapoptotic effect of was mediated by noncanonical targeting of through both strands. knockout in bone marrow cells also reduced atherosclerosis and necrotic core size. Circadian regulation of clock gene expression was confirmed in human atherosclerotic lesions. Apoptosis oscillated diurnally in phase with expression, demonstrating an early morning peak antiphase to that of the strands.

Conclusions: Our findings suggest that the molecular clock in atherosclerotic lesions induces a diurnal rhythm of apoptosis regulated by circadian expression in macrophages that is not matched by efferocytosis, thus increasing the size of the necrotic core.