Impaired Cholesterol Metabolism and Enhanced Atherosclerosis in Clock Mutant Mice
Background—Clock is a key transcription factor that positively controls circadian regulation. However, its role in plasma cholesterol homeostasis and atherosclerosis has not been studied.
Methods and Results—We show for the first time that dominant-negative Clock mutant protein (ClockΔ19/Δ19) enhances plasma cholesterol and atherosclerosis in three different mouse models. Detail analyses revealed that Clk∆19/∆19Apoe-/- mice display hypercholesterolemia due to the accumulation of apoB48-containing cholesteryl ester-rich lipoproteins. Physiologic studies showed that enhanced cholesterol absorption by the intestine contributes to hypercholesterolemia. Molecular studies indicated that the expression of NPC1L1, ACAT2 and MTP in the intestines of Clk∆19/∆19Apoe-/- mice was high and enterocytes assembled and secreted more chylomicrons. Further, we identified macrophage dysfunction as another potential cause of increased atherosclerosis in Clk∆19/∆19Apoe-/- mice. Macrophages from Clk∆19/∆19Apoe-/- mice expressed higher levels of scavenger receptors and took up more modified lipoproteins compared to Apoe-/- mice; but they expressed low levels of ABCA1 and were defective in cholesterol efflux. Molecular studies revealed that Clock regulates ABCA1 expression in macrophages by modulating USF2 expression.
Conclusions—ClockΔ19/Δ19 protein enhances atherosclerosis by increasing intestinal cholesterol absorption, augmenting uptake of modified lipoproteins by macrophages, and reducing cholesterol efflux from macrophages. These studies establish that circadian Clock activity is crucial in maintaining low plasma cholesterol levels and in reducing atherogenesis in mice.
- Received March 27, 2013.
- Revision received August 12, 2013.
- Accepted August 22, 2013.