Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2011
Publication Date: 8/1/2011
Citation: Kuang, Y., Paulson, K., Lichtenstein, A., Matthan, N., Lamon-Fava, S. 2011. Docosahexaenoic acid suppresses apolipoprotein A-I gene expression through hepatocyte nuclear factor-3beta. American Journal of Clinical Nutrition. 94:1-8. Interpretive Summary: Dietary fish-oil supplementation has been shown in human kinetic studies to lower the production rate of apolipoprotein (apo) A-I, the major protein component of high density lipoprotein (HDL). The underlying mechanism responsible for this effect is not fully understood. We investigated the effect and the mechanism of action of omega-3 containing polyunsaturated fatty acid, docosahexaenoic acid (DHA), relative to the saturated fatty acid (palmitic acid), on the hepatic expression of apo A-I in HepG2 cells. Findings from this study suggest that, in human hepatoma cells (HepG2), DHA can repress the production of (apo) A-I. This information is useful for developing means to stimulate HDL production in humans.
Technical Abstract: BACKGROUND: Dietary fish-oil supplementation has been shown in human kinetic studies to lower the production rate of apolipoprotein (apo) A-I, the major protein component of HDL. The underlying mechanism responsible for this effect is not fully understood. OBJECTIVE: We investigated the effect and the mechanism of action of the very-long-chain n-3 (omega-3) polyunsaturated fatty acid docosahexaenoic acid (DHA), relative to the saturated fatty acid palmitic acid (PA), on the hepatic expression of apo A-I in HepG2 cells. DESIGN: HepG2 cells were treated with different doses of DHA and PA (0-200 umol/L). mRNA expression levels of apo A-I were assessed by real-time polymerase chain reaction, and apo A-I protein concentrations were measured by immunoassay. DHA dose-dependently suppressed apo A-I mRNA levels and also lowered apo A-I protein concentrations in the media, with maximum effects at 200 umol/L. This concentration of fatty acids was used in all subsequent experiments. RESULTS: To elucidate the mechanism mediating the reduction in apo A-I expression by DHA, transfection experiments were conducted with plasmid constructs containing serial deletions of the apo A-I promoter. The DHA-responsive region was mapped to the -185 to -148 nucleotide region of the apo A-I promoter, which binds the hepatocyte nuclear factor (HNF)-3beta. Nuclear extracts from cells treated with DHA or PA had a similar nuclear abundance of HNF-3beta. However, electrophoresis mobility shift assays showed less binding of HNF-3beta to the -180 to -140 sequence of the apo A-I promoter than did PA-treated cells. As shown by chromatin immunoprecipitation analysis, less HNF-3beta was recruited to the apo A-I promoter in DHA-treated cells than in PA-treated cells, which supports the concept of an interference of DHA with the binding of HNF-3beta to the apo A-I promoter. CONCLUSION: These findings suggest that, in human hepatoma HepG2 cells, DHA inhibits the binding of HNF-3beta to the apo A-I promoter, resulting in the repression of apo A-I promoter transactivity and thus a reduction in apo A-I expression.