Purpose of review: A diet that provides 2-5% of energy as highly unsaturated 20- and 22-carbon omega-6 or omega-3 fatty acids is associated with an inhibition of hepatic lipogenesis, a stimulation of hepatic fatty acid oxidation, and consequently a lowering of blood triglyceride levels. The purpose of this review is to demonstrate that highly unsaturated fatty acids regulate lipid metabolism by modulating protein expression at many levels including gene transcription, messenger RNA processing, mRNA decay, and post-translational protein modifications. Although the intracellular signaling mechanisms employed by highly unsaturated fatty acids are unknown, this review presents a summary of the emerging knowledge regarding highly unsaturated fatty acids as kinase cascade activators.
Recent findings: Highly unsaturated fatty acids suppress lipogenic gene transcription by reducing the DNA binding activity of several transcription factors, notably sterol regulatory-element binding protein 1 and nuclear factor Y. Highly unsaturated fatty acids inhibit the proteolytic release of sterol regulatory-element binding protein 1 from its membrane-anchored precursor through a ceramide-dependent signal, and impart a post-translational modification to nuclear factor Y. Highly unsaturated fatty acids accelerate sterol regulatory-element binding protein 1 mRNA decay and may function as antagonistic ligands for liver receptor X, thereby interfering with the liver receptor X stimulation of sterol regulatory-element binding protein 1 gene transcription. Highly unsaturated fatty acid activation of peroxisome proliferator-activated receptor alpha combined with their displacement of the oxysterol from liver receptor X may 'trap' liver receptor X as transcriptionally inactive peroxisome proliferator-activated receptor alpha/liver receptor X heterodimer. The gene expression consequences of liver receptor X 'trapping' may explain how dietary highly unsaturated fatty acids lead to a repartitioning of fatty acids away from storage and towards oxidation.
Summary: The liver appears to use the highly unsaturated fatty acid status as a nutrient sensor to determine whether fatty acids are to be stored or oxidized. In this way highly unsaturated fatty acids may function as nutritional factors that reduce the risk of developing hepatic lipotoxicity and insulin resistance.