Abstract 11421: High Fructose Diet Downregulates Acyl-coA Synthetase-3 Expression and Activity in Liver of Hamsters via Impairing LXR/RXR Signaling Pathway

Abstract

The liver plays a central role in whole-body lipid metabolism by regulating the uptake, synthesis, oxidation and export of lipids. Dysfunction of lipid metabolism in liver underlies the development of cardiovascular diseases and diabetes. Long-chain acyl-CoA synthetases (ACSL1, ACSL3, ACSL4, ACSL5, ACSL6) have been shown to play key roles in fatty acids metabolism in liver. Because each isoform of ACSL family has a distinct function in directing acyl-CoAs to one or more specific downstream pathways, the level of expression/activity of individual ACSL isozymes could directly influence the fatty acid metabolic fates in liver tissue. Currently, the cellular mechanisms that regulate the hepatic expression of ACSLs under changing pathophysiological conditions remain largely unknown. In this study, we examined the impact of dyslipidemic high fructose diet on the expression of ACSL isozymes in liver of hamsters. We show that fructose diet (FD) specifically downregulated ACSL3 expression in liver of hamsters (n=9). This was companied by reductions of ACSL activities to synthesize arachidonyl-CoA and oleoyl-CoA in liver extracts of FD-fed hamsters as compared to hamsters (n=6) fed a normal diet (ND). Examinations of nuclear receptors revealed that mRNA and protein levels of three key components of the LXR signaling pathway including LXRα, LXRβ and RXRβ in liver were downregulated by the fructose diet. We further isolated the 5’proximal region of hamster ACSL3 gene containing a putative LXRE motif and demonstrated the response of this promoter region to LXR ligand induction of promoter activity by reporter assays conducted in HepG2 cells. Electrophoretic mobility shift assays using ³²P-labeled hamster ACSL3-LXRE probe attested the reduced binding of LXRs to the ACSL3-LXRE sequence with nuclear extracts of FD-fed hamsters, relative to the ND-fed hamsters. Finally, we demonstrate that siRNA-mediated depletion of LXRα/β abolished the LXR ligand-stimulated transcription of ACSL3 in HepG2 cells. In conclusion, our studies provide the first in vivo demonstration for the important role of LXR signaling pathway in control of hepatic ACSL3 expression and the negative effects of fructose diet on LXR signaling pathway and the expression of LXR downstream target genes.