Metabolism of lipids such as cholesterol and triglycerides has daily variations and is controlled by a circadian clock. Sesamin isomers, a mixture of sesamin and episesamin (SE), are types of lignans in sesame seed that have shown the improvement of lipid metabolism with various diseases in an animal model. We therefore tested whether the effects of SE on lipid metabolism are influenced by timing of administration. High-fat diet (HFD)-loaded rat was administered SE in the ZT13 or 14 (at the beginning of the active phase) or ZT23 or 22 (at the end of the active phase) every day for 7 or 28 days, and the effects on lipid metabolism were evaluated. The effects of SE were enhanced by duration-dependency: 28-day administration of SE strongly affected some parameters related to lipid metabolism, particularly cholesterol metabolism, as compared to 7-day administration. In particular, in 28-day administration, the analysis of serum and liver cholesterol levels revealed that SE administration decreases more effectively at the beginning of the active phase when compared to at the end of that. Furthermore, quantitative real-time polymerase chain reaction (QRT-PCR) and functional analysis indicated that suppression of cholesterol synthesis in the liver and promotion of cholesterol excretion from the liver, as well as inhibition of the functional activity and gene expression of sterol response element-binding protein 2 (Srebp2), which is a transcriptional factor and controls the gene expression involved in cholesterol-metabolism enzymes, contribute to enhancement of SE’s effects at this administration timing. No significant differences were observed in triglyceride metabolism with regard to timing of SE administration. After 28-day administration of SE, administration at the beginning of the active phase only affected the expression of clock genes in the liver with phase-advance. In the pharmacokinetic study, administration time had no effect on the level of sesamin, episesamin or their metabolites in the liver after administration of SE for 28 days. The present results suggest that continuous long administration of SE at the beginning of the active phase is preferable for obtaining beneficial effects on lipid metabolism.
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