SUMMARY The proposed work will define the role of the lipid droplet-associated protein CIDEC (also known as FSP27) on the metabolic fate of lipids in liver and intestine, circulating lipoproteins, and oxidative lipid metabolism in muscle and brown adipose tissue (BAT). Dysregulation of lipid metabolism is the basis of some of the most common medical disorders in Western populations, such as cardiovascular disease, hyperlipidemia, fatty liver diseases, obesity, and insulin resistance. The long-term goal in our laboratory is to elucidate molecular and cellular mechanisms governing whole-body lipid homeostasis, both under physiological and pathological conditions. CIDEC/FSP27 encodes 2 isoforms, a and b, with strict tissue distribution. Here we will exploit Fsp27bKO mice to reveal the role of FSP27b (the sole FSP27 expressed in liver and intestine, and the major isoform in BAT) on the partitioning of lipids for storage/oxidation/secretion. We will test the new ideas that FSP27b is a critical regulator of hepatic APOB lipidation and secretion, dietary lipid absorption and postprandial lipemia, and fatty acid utilization in peripheral tissues. The proposed studies will be transformative for our understanding of: i) mechanisms governing the biogenesis and secretion of pro-atherogenic lipoproteins; ii) lipid biology in major oxidative tissues; and iii) metabolic cross-talk between liver and other tissues, an area that has been largely neglected in the past. To achieve the goal of defining the role of FSP27b on lipid metabolism, we plan three specific aims. Aim 1 will test the hypothesis that FSP27b promotes the lipidation and secretion of APOB-containing lipoproteins in liver and intestine. Aim 2 will test the hypothesis that hepatic FSP27b limits the availability of PPAR agonists for peripheral tissues. Aim 3 will test the hypothesis that FSP27b reduces energy expenditure in brown adipose tissue. Importantly, some of the proposed research will correct the published scientific record on FSP27, which is based on a flawed “Fsp27-floxed” mice. The translational relevance of our studies is highlighted by reports showing that polymorphisms in FSP27 are associated with elevated fasting triglyceridemia in humans, and that loss-of- function mutations result in familial partial lipodystrophy, type 5 (FPLD5). Overall, success of the proposed studies will fill a large gap of knowledge in whole-body lipid (patho)physiology, by defining molecular mechanisms of lipid droplet-mediated control of triglyceride metabolism in liver, intestine, muscle, and brown adipose tissue, with particular attention to multi-organ metabolic cross-talk via circulating lipids. These studies may also establish FSP27 as a valid pharmacological target to manage fatty liver diseases, diet-induced obesity, dyslipidemias, and cardiovascular risk in patients.