Liver-Specific Bmal1 Depletion Reverses the Beneficial Effects of Nobiletin on Liver Cholesterol Homeostasis in Mice Fed with High-Fat Diet

Nobiletin (NOB), a naturally occurring small-molecule compound abundant in citrus peels, has displayed potential lipid-lowering and circadian-enhancing properties in preclinical studies. However, the requirement of specific clock genes for the beneficial effects of NOB is not well understood. In the current study, mice with a liver-specific deletion of the core clock component, Bmal1—Bmal1LKO—were fed a high-fat diet (HFD) ad libitum for eight weeks, while NOB (200 mg/kg) was administered by daily oral gavage from the fifth week and throughout the last four weeks. NOB decreased liver triglyceride (TG) alongside the decreasing mRNA levels of de novo lipogenesis (DNL) genes in both Bmal1(flox/flox) and Bmal1LKO mice. NOB increased serum very low-density lipoprotein (VLDL) levels in Bmal1LKO mice, which was consistent with higher liver Shp and lower Mttp mRNA expression levels, the key genes that facilitate VLDL assembly and secretion. NOB decreased liver and serum cholesterol levels in the Bmal1(flox/flox) mice, consistent with lower Hmgcr and higher Cyp7a1, Cyp8b1, Gata4 and Abcg5 mRNA levels in the liver. In contrast, in the Bmal1LKO mice, NOB increased Hmgcr mRNA levels and had no effect on the above-mentioned genes related to bile acid synthesis and cholesterol excretion, which might contribute to the elevation of liver and serum cholesterol levels in NOB-treated Bmal1LKO mice. NOB inhibited hepatic DNL and decreased liver TG levels in HFD-fed mice independently of liver Bmal1, whereas liver-specific Bmal1 depletion reversed the beneficial effects of NOB on liver cholesterol homeostasis. The complex interactions between NOB, the circadian clock and lipid metabolism in the liver warrant further research.