|Davis, J. Wade|
Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2008
Publication Date: 6/1/2008
Citation: Morin, B., Nichols, L.A., Zalasky, K.M., Davis, J., Manthey, J.A., Holland, L.J. 2008. The citrus flavonoids hesperetin and nobiletin differentially regulate low density lipoprotein receptor gene transcription in HepG2 liver cells. Journal of Nutrition. 138:1274-1281. Interpretive Summary: Citrus flavonoids, particularly those termed the "polymethoxylated flavones", have been shown to lower LDL cholesterol and serum triglycerides in animal studies. Detail biochemical studies have been done to uncover the mechanisms by which these beneficial effects occur. The mechanisms occur through the activation of specific and powerful activators for human liver cells.
Technical Abstract: The reduction of plasma cholesterol by citrus flavonoids is associated with effects on specific liver functions related to lipid handling. In previous in vivo studies, polymethoxylated flavones reduced plasma cholesterol levels at lower doses than required for flavanones. To delineate hepatic mechanisms that underlie this differential potency, we used HepG2 liver cells to quantitate effects on expression of the LDL receptor (LDLR) gene. A dose-response analysis showed that 200 µmol/L hesperetin, a flavanone that is present as a disaccharide in oranges, stimulated LDLR mRNA levels 3.6-4.7 fold. In contrast, nobiletin, a polymethoxylated flavone found at highest concentrations in oranges and tangerines, achieved maximal stimulation of 1.5-1.6 fold at only 5 µmol/L. Transcriptional mechanisms of action on the LDLR gene by citrus flavonoids have been implicated but, to our knowledge, not directly demonstrated previously. Here, using transfection vector constructs containing either 1480 or 142 bases of the upstream region of the LDLR gene, we show differences in both potency and efficacy in the induction of transcription, with maximal stimulation of 7.5-10 fold at 160 µmol/L hesperetin and 1.8-3.0 fold at 10-20 µmol/L nobiletin. The sterol regulatory element in the LDLR gene upstream region plays a crucial role, since mutation of this site eliminated induction in response to hesperetin or nobiletin. Thus, the citrus flavonoids are likely to act through the sterol regulatory element binding proteins, with polymethoxylated flavones activating these mechanisms at considerably lower concentrations than the flavanones.