DHA supplement rescues high-fat diet-induced circadian rhythm disturbance of lipid metabolism
Disruptions to circadian rhythm have been associated with an increased risk of Nonalcoholic fatty liver disease (NAFLD). DHA has been found to affect both circadian rhythm and lipid metabolism. In this study, the relationship between DHA supplementation and its potential improvement of lipid metabolism and circadian clock regulation was investigated. Male C57BL/6 mice were fed a control, high fat or DHA supplemented diet for 12 weeks. Biochemical analyses and H&E staining showed that high-fat diet (HFD) could induce NAFLD, and DHA supplementation (AOH) could attenuate NAFLD. The qPCR results showed that the expression levels of core clock genes, CLOCK and BMAL1, were significantly higher at zeitgeber (ZT) 0 (7:00 am) than that at ZT12 (7:00 pm) in the control group, while this difference in day and night disappeared in the HFD group, but not in the AOH group. Western blot results indicated that the expressions of rhythm output molecules (RORα and REV-ERBα) and their downstream protein INSIG2 all exhibited corresponding circadian changes. The SREBPs-regulated proteins were significantly increased in the HFD group at both ZT0 and ZT12, but were decreased in the AOH, accompanied by the corresponding changes in the protein expression of HMGCR, LXR, CYP7A1 and CYP27A1. Taken together, our results indicate that HFD can decrease or disruptcircadian rhythm fluctuation by up-regulating the expression of core circadian rhythm genes, CLOCK and BMAL1, at ZT12, and by inducing metabolic abnormalities through the INSIG2-SREBPs pathway regulated by RORα and REV-ERBα. DHA supplementation, however, seemed to restore circadian rhythm to a normal "night-high, day-low" characteristic through regulating metabolic pathway via rhythmic nuclear receptors, thereby improving the lipid metabolism rhythm and reducing liver fat accumulation.