Effects of fish oil feeding and fasting on LXRα/RXRα binding to LXRE in the SREBP-1c promoter in mouse liver

High-fish oil feeding and fasting down-regulate sterol regulatory element-binding protein-1c (SREBP-1c) mRNA level and suppress lipogenesis in mouse liver. Previous promoter analysis revealed that liver X receptor α (LXRα)/retinoid X receptor α (RXRα) complex was required for SREBP-1c gene expression in cell culture. In in vitro studies, polyunsaturated fatty acids (PUFAs, n-6, n-3) inhibited binding of LXRα/RXRα heterodimer to LXR responsive elements (LXREs) in the SREBP-1c promoter. To examine whether fish oil feeding and fasting would also inhibit its binding to LXREs in mouse liver, active liver nuclear extracts were prepared by percoll gradient centrifugation, and gel mobility shift assay was conducted. Although 1- to 5-day fish oil feeding and 2-day fasting decreased SREBP-1c mRNA by 45-68% and 65%, respectively, fish oil feeding decreased binding of LXR/RXR heterodimer to LXREs by 0-26%, while 2-day fasting decreased their binding by 40-56%. Luciferase assay using mutation of LXREs in mouse primary hepatocytes revealed that the LXR ligand, T0901317, induced increased transcription of SREBP-1c mRNA was mediated by LXREs, but it is unknown whether fish oil/eicosapentaenoic acid (EPA)-induced down-regulation of SREBP-1c mRNA was mediated by LXREs. These data indicate that high-fish oil feeding might decrease SREBP-1c mRNA partly by decreased transcription of SREBP-1c, but if so, the binding inhibition of LXRα to LXREs might not be a major cause, while fasting decreased SREBP-1c mRNA, mainly by its binding inhibition of LXRα to LXREs in the SREBP-1c promoter.