RNAi screens reveal novel metabolic regulators: RIP140, MAP4k4 and the lipid droplet associated fat specific protein (FSP) 27

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Abstract
Adipose tissue modulates whole body metabolism and insulin sensitivity by controlling circulating lipid levels and producing molecules that can regulate fatty acid metabolism in such tissues as muscle and liver. We have developed RNA interference (RNAi) screens to identify genes in cultured adipocytes that regulate insulin signalling and key metabolic pathways. These short interfering RNA (siRNA)‐based screens identified the transcriptional corepressor receptor interacting protein 140 (RIP140) (J Clin Invest116: 125, 2006) and the mitogen‐activated protein kinase (MAP4k4) (Proc Natl Acad Sci USA103: 2087, 2006) as negative regulators of insulin‐responsive hexose uptake and oxidative metabolism. Gene expression profiling revealed that RIP140 depletion upregulates the expression of clusters of genes in the pathways of glucose uptake, glycolysis, tricarboxylic acid cycle, fatty acid oxidation, mitochondrial biogenesis and oxidative phosphorylation. RIP140‐null mice resist weight gain on a high‐fat diet and display enhanced glucose tolerance. MAP4k4 depletion in adipocytes increases many of the RIP140‐sensitive genes, increases adipogenesis and mediates some actions of tumour necrosis factor‐α (TNF‐α). Remarkably, another hit in our RNAi screens was fat specific protein 27 (FSP27), a highly expressed isoform of Cidea. We discovered that FSP27 unexpectedly associates specifically with lipid droplets and regulates fat storage. We conclude that RIP140, MAP4k4 and the novel lipid droplet protein FSP27 are powerful regulators of adipose tissue metabolism and are potential therapeutic targets for controlling metabolic disease. The discovery of these novel proteins validates the power of RNAi screening for discovery of new therapeutic approaches to type 2 diabetes and obesity.