Proper control of hepatic glucose production is central to whole body glucose homeostasis and its disruption plays a major role in diabetes. Here we demonstrate that, although established as an intracellular lipid chaperone, aP2 is in fact actively secreted from adipocytes to regulate liver glucose metabolism. Fasting and lipolysisrelated signals regulate secretion of aP2 from adipocytes, and circulating aP2 levels are markedly elevated in mouse and human obesity. Recombinant aP2 stimulates glucose production and gluconeogenic activity in primary hepatocytes in vitro and in lean mice in vivo. In contrast, neutralization of secreted aP2 reduces glucose production and corrects the diabetic phenotype of obese mice. Hyperinsulinemiceuglycemic and pancreatic clamp studies demonstrated actions of aP2 in liver upon aP2 administration or neutralization. We conclude that aP2 is a novel adipokine linking adipocytes to hepatic glucose production and that neutralizing secreted aP2 may represent an effective therapeutic strategy for diabetes.

The term "lipid" is used to identify a large set of hydrophobic and amphiphilic molecules such as free fatty acids, sterols, fatty acid esters, and phospholipids. These molecules are involved in forming fundamental structures in cells and tissues, providing energy for the metabolic needs of organisms, and regulating several homeostatic processes within and outside of cells, including organelle homeostasis, immune function, inter-organ communication, energy metabolism, and cell survival. However, when the balance in their metabolism and composition is altered by environmental or metabolic stress, lifestyle, and genetic or epigenetic factors, lipids can also become critical components of pathophysiological cascades that are detrimental to healthy cell and tissue function. Hence, although lipids play fundamental physiological roles, in excess or in improper composition, they can be highly damaging, leading to organelle dysfunction, cell death, chronic inflammation, and disturbances in energy and substrate metabolism and survival responses. In this review, we primarily focus on the roles of lipid classes that regulate immune responses and signaling mechanisms, which perpetuate a vicious cycle of metabolic and inflammatory disturbances leading to disease. , arginase 2-deficient; ATGL, adipose triglyceride lipase; BAT, brown adipose tissue; DAG, diacylglycerol; ER, endoplasmic reticulum; FAHFA, fatty acid hydroxy fatty acid; FXR, farnesoid X receptor; GPR120, G protein-coupled receptor 120; iNKT, invariant natural killer T; iNOS, induced nitric oxide synthase (iNOS); LD, lipid droplet; LXR, liver X receptor; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; NKT, natural killer T; PKC, protein kinase C; PKR, protein kinase R; ROS, reactive oxygen species; snoRNA, small nucleolar RNA; TLR4, tolllike receptor 4; TUDCA, tauroursodeoxycholic acid. Abstract Lipids encompass a wide variety of molecules

Journal of Lipid Research Volume 56, 2015 423Adipose tissue is an endocrine organ whose products orchestrate the metabolic functions of various tissues, including brain, pancreas, and liver, to maintain systemic homeostasis. Adipocytes respond to metabolic and immune cues by mobilizing their fat stores through lipolysis and by secreting a variety of hormones and cytokines ( 1, 2 ). Such signals converge on target tissues, for example on liver to regulate glucose production, and on ␤ cells to modulate insulin production. A critical molecule for the integration of adipocyte biology with systemic metabolic regulation is aP2 [fatty acid binding protein (FABP) 4], a lipid binding protein that is upregulated during differentiation of adipocytes and upon macrophage activation ( 3, 4 ). Since its identifi cation, aP2 has been studied primarily for its intracellular functions in lipid metabolism and infl ammation ( 3,4 ). Genetic deletion models demonstrated that this FABP plays a critical role in the pathogenesis of several chronic metabolic diseases, including diabetes, atherosclerosis, and fatty liver. Mice defi cient in aP2 or aP2 and the related protein FABP5/mal1 together have improved adipose and liver function, increased insulin sensitivity, and reduced fatty liver and cardiovascular disease in the context of high-fat diet and genetic mouse models of obesity and atherosclerosis ( 5-12 ). The link between aP2 and metabolic disease is also supported by genetic association studies in multiple populations demonstrating metabolic and cardiovascular benefi ts in individuals carrying a rare haploinsuffi ciency mutation in the aP2 locus, validating the relevance of this pathway in human disease ( 13,14 ).

FAHFAs are a class of bioactive lipids which show great promise for treating diabetes and inflammatory diseases. Deciphering the metabolic pathways that regulate endogenous FAHFA levels is critical for developing diagnostic and therapeutic strategies. However, it remains unclear how FAHFAs are metabolized in cells or tissues. Here, we investigate whether FAHFAs can be incorporated into other lipid classes and identify a novel class of endogenous lipids, FAHFAcontaining triacylglycerols (FAHFA-TGs), which contain a FAHFA group esterified to the glycerol backbone. Isotope-labeled FAHFAs are incorporated into FAHFA-TGs when added to differentiated adipocytes which implies the existence of enzymes and metabolic pathways capable of synthesizing these lipids. Induction of lipolysis (i.e., triacylglycerol hydrolysis) in adipocytes is associated with marked increases in nonesterified FAHFA levels demonstrating that FAHFA-TGs breakdown is a regulator of cellular FAHFA levels. To quantify FAHFA levels in FAHFA-TGs and determine their regioisomeric distributions, we developed a mild alkaline hydrolysis method that liberates FAHFAs from triacylglycerols for easier detection. FAHFA-TG concentrations are greater than 1oo-fold than that of nonesterified FAHFAs, indicating that FAHFA-TGs are a major reservoir of FAHFAs in cells and tissues. The discovery of FAHFA-TGs reveals a new branch of TG and FAHFA metabolism with potential roles in metabolic health and regulation of inflammation.

Fatty acid esters of hydroxy fatty acids (FAHFAs) are a recently discovered class of endogenous lipids with antidiabetic and anti-inflammatory activities. Interest in these lipids is due to their unique biological activites and the observation that insulin-resistant people have lower palmitic acid esters of hydroxystearic acid (PAHSA) levels, suggesting that a FAHFA deficiency may contribute to metabolic disease. Rigorous testing of this hypothesis will require the measurement of many clinical samples; however, current analytical workflows are too slow to enable samples to be analyzed quickly. Here we describe the development of a significantly faster workflow to measure FAHFAs that optimizes the fractionation and chromatography of these lipids. We can measure FAHFAs in 30 min with this new protocol versus 90 min using the older protocol with comparable performance in regioisomer detection and quantitation. We also discovered through this optimization that oleic acid esters of hydroxystearic acids (OAHSAs), another family of FAHFAs, have a much lower background signal than PAHSAs, which makes them easier to measure. Our faster workflow was able to quantify changes in PAHSAs and OAHSAs in mouse tissues and human plasma, highlighting the potential of this protocol for basic and clinical applications.