Our long-term goal is to understand the molecular mechanisms pertaining to diurnal regulation of plasma lipid levels and to determine how perturbations in this regulation contribute to dyslipidemia and atherosclerosis. We observed that plasma triglyceride and cholesterol mainly associated with non-HDL apoB-lipoproteins exhibit diurnal rhythms. We showed that plasma lipid diurnal rhythms are altered when animals are subjected to food entrainment and are not seen in Clock mutant and Bmal1-/- mice. Our studies have revealed that both Clock and Bmal1 (1) control diurnal regulation of MTP, an essential chaperone for the assembly of apoB-containing triglyceride-rich lipoproteins, involving SHP; (2) regulate expression of ABCA1 via USF2 in macrophages and control cholesterol efflux and atherosclerosis; and (3) regulate hepatic cholesterol excretion to bile by modulating ABCG5/G8 expression via GATA4. We also observed that mice deficient in Clock or Bmal1 exhibit sustained hyperlipidemia and are more susceptible to atherosclerosis. Hence, our overall postulate is that circadian mechanisms protect against hyperlipidemia and atherosclerosis. This proposal pertains to circadian regulation of fat mobilization that involves uptake of fatty acids, re-synthesis of triacylglycerols and lipoprotein assembly by enterocytes and hepatocytes. Our first aim is to elucidate the role of Bmal1 in fatty acid uptake. We will test the hypothesis that Bmal1 regulates CD36 expression via Rev-erbα to modulate fatty acid uptake. Deregulation of this pathway increases fatty acid uptake by enterocytes and hepatocytes, thereby contributing to steatosis and hyperlipidemias. Fatty acids taken up by enterocytes and hepatocytes are packaged into lipoproteins involving two steps. We previously showed that the first step of primordial lipoprotein assembly is regulated by Clock:Bmal1 by controlling MTP. Here, we hypothesize that Bmal1 also regulates the second step lipoprotein core expansion by regulating the expression of Crebh and apoAIV. Therefore, the second aim is to explain mechanisms by which Bmal1 regulates Crebh to modulate the assembly of large lipoproteins. We postulate that Bmal1 regulates Crebh involving two mechanisms. First, Bmal1 upregulates Crebh and Rev- erbα, when Rev-erbα levels increase it represses Crebh expression. At the end of the study, we expect to demonstrate that Bmal1 controls lipid uptake and lipoprotein assembly and explain the molecular mechanisms for this regulation. These studies will (1) elucidate key physiological, biochemical, and molecular mechanisms regulated by Bmal1 that control fatty acid uptake and lipoprotein assembly; (2) show that lipid metabolism and circadian regulatory mechanisms are intertwined; and (3) demonstrate that deregulation of circadian pathways enhances hyperlipidemia, thereby increasing risk for atherosclerosis. These regulatory mechanisms might have evolved to optimize fat absorption. The outcomes from these studies will impact two fields of biology; lipid metabolism and circadian regulation. Novel understanding of the circadian regulatory mechanisms of fatty acid uptake and triglyceride mobilization will be garnered.

Circadian rhythms control physiology and behavior. Disruptions in these regulatory mechanisms predispose individuals to various metabolic diseases, such as atherosclerosis and hepatosteatosis. Hence, understanding the circadian mechanisms that control lipid metabolism at molecular level might be useful for the prevention and treatment of these diseases. The studies proposed here will explain how Bmal1 controls circadian regulation of fatty acid uptake, assembly of large lipoproteins, and plasma and tissue lipid homeostasis.