Abstract 3: Cellular Contributions of the Circadian Clock in Atherogenesis

Abstract

Atherosclerosis is a leading cause of death despite the improvements in lipid and blood pressure control. The circadian clock, a molecular network of genes and proteins that controls 24-hour timing, has emerged to have a surprising role in the control of metabolic and vascular function. Herein we examined the impact of circadian rhythm dysfunction in atherogenesis by implementation of vascular transplant and PCSK9 based approaches to induce accelerated lesion development in mice. We find that atherogenesis is exacerbated in Bmal1-KO aortic grafts immersed in the hypercholesterolemic milieu of ApoE -/- mice. To assess if atherosclerosis was ‘circadian rhythm dependent’ we subjected wild-type mice to a shortened light cycle (4L/4D) and induced atherosclerosis by intravenous injection of a human PCSK-9 adeno associated virus. Atherosclerosis in the jet-lagged PCSK-9 mice was robustly increased relative to the atherosclerosis observed in WT mice on a normal light cycle (12L/12D), providing further evidence that circadian rhythm and the circadian clock contribute to atherosclerosis. However, atherosclerosis is a complex disease that is the net result of interplay between intrinsic (vascular cells) and extrinsic mechanisms (metabolism, blood pressure, and hormones) and the importance of clock function in individual cell types is poorly understood. We found that deletion or silencing of key circadian transcription factors resulted in an enhanced inflammatory and pro-oxidant phenotype with diminished NO production and greater lipid uptake in both macrophages and endothelial cells. Loss of circadian function in smooth muscle cells similarly resulted in enhanced production of reactive oxygen species and greater cell proliferation. Surprising, the silencing of Bmal2 in endothelial cells resulted in greater lipid uptake in oxLDL treated HAEC as well as increased expression of markers of autophagy, suggesting that Bmal2 may orchestrate numerous output functions in different cell types. In conclusion, we find that the circadian clock and circadian rhythm have a profound impact on atherosclerosis, to influence vascular cell inflammatory and lipid uptake responses, and identify an unexpectedly prominent role for the side-partner of Bmal1, Bmal2.