Summary Lipid droplet (LD) lipolysis in brown adipose tissue (BAT) is generally considered to be required for cold-induced nonshivering thermogenesis. Here we show that mice lacking BAT Comparative Gene Identification-58 (CGI-58), a lipolytic activator essential for the stimulated LD lipolysis, have normal thermogenic capacity and are not cold sensitive. Relative to littermate controls, these animals had higher body temperatures when they were provided food during cold exposure. The increase in body temperature in the fed, cold-exposed knockout mice was associated with increased energy expenditure and increased sympathetic innervation and browning of white adipose tissue (WAT). Mice lacking CGI-58 in both BAT and WAT were cold sensitive, but only in the fasted state. Thus, LD lipolysis in BAT is not essential for cold-induced nonshivering thermogenesis in vivo. Rather CGI-58-dependent LD lipolysis in BAT regulates WAT thermogenesis, and our data uncover an essential role of WAT lipolysis in fueling thermogenesis during fasting.

BackgroundColorectal cancer is strongly associated with lipid metabolism. NPC1L1, a sterol transporter, plays a key role in modulating lipid homeostasis in vivo. Its inhibitor, ezetimibe, began to be used clinically to lower cholesterol and this caused the great debate on its role in causing carcinogenesis. Here we explored the role of NPC1L1 in colorectal tumorigenesis.MethodsWild-type mice and NPC1L1−/− (NPC1L1 knockout) mice were treated with azoxymethane (AOM)-dextran sodium sulfate (DSS) to induce colitis-associated colorectal tumorigenesis. Mice were sacrificed 10, 15, 18 or 20 weeks after AOM treatment, respectively. Colorectal tumors were counted and analyzed. Plasma lipid concentrations were measured using enzymatic reagent kit. Protein expression level was assayed by western blot.ResultsNPC1L1−/− mice significantly had fewer tumors than wild-type. The ratio of malignant/tumor in NPC1L1−/− mice was significantly lower than in wild-type 20 weeks after AOM-DSS treatment. NPC1L1 was highly expressed in the small intestine of wild-type mice but its expression was undetectable in colorectal mucous membranes or tumors in either group. NPC1L1 knockout decreased plasma total cholesterol and phospholipid. NPC1L1−/− mice had significant lower intestinal inflammation scores and expressed inflammatory markers p-c-Jun, p-ERK and Caspase-1 p20 lower than wild-type. NPC1L1 knockout also reduced lymphadenectasis what may be caused by inflammation. NPC1L1 knockout in mice decreased β-catenin in tumors and regulated TGF-β and p-gp in adjacent colons or tumors. There was not detectable change of p53 by NPC1L1 knockout.ConclusionsOur results provide the first evidence that NPC1L1 knockout protects against colitis-associated tumorigenesis. NPC1L1 knockout decreasing plasma lipid, especially cholesterol, to reduce inflammation and decreasing β-catenin, p-c-Jun and p-ERK may be involved in the mechanism.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-015-1230-0) contains supplementary material, which is available to authorized users.

Milk lipid is secreted by a unique process, during which triacylglycerol droplets bud from mammary cells coated with an outer bilayer of apical membrane. In all current schemes, the integral protein butyrophilin 1A1 (BTN) is postulated to serve as a transmembrane scaffold, which interacts, either with itself, or with the peripheral proteins, xanthine oxidoreductase (XOR) and possibly perilipin-2 (PLIN2), to form an immobile bridging complex between the droplet and apical surface. In one such scheme, BTN on the surface of cytoplasmic lipid droplets interacts directly with BTN in the apical membrane without binding to either XOR or PLIN2. We tested these models using both biochemical and morphological approaches. BTN was concentrated in the apical membrane in all species examined and contained mature N-linked glycans. We found no evidence for the association of unprocessed BTN with intracellular lipid droplets. BTN-enhanced-green-fluorescent-protein was highly mobile in areas of mouse milk-lipid droplets that had not undergone post-secretion changes, and endogenous mouse BTN comprised only 0.5–0.7%, (w/w) of the total protein, i.e., over fifty-fold less than in the milk-lipid droplets of cow and other species. These data are incompatible with models of milk-lipid secretion in which BTN is the major component of an immobile global adhesive complex and suggest that interactions between BTN and other proteins at the time of secretion are more transient than previously predicted. The high mobility of BTN in lipid droplets, mark it as a potential mobile signaling molecule in milk.

Cold exposure activates the sympathetic nervous system. It is generally thought that this sympathetic activation induces heat production by stimulating lipolysis of cytosolic lipid droplets (LDs) in brown adipocytes. However, this concept was not examined in vivo due to lack of appropriate animal models. Recently, we and others have demonstrated that LD lipolysis in brown adipocytes is not required for cold-induced nonshivering thermogenesis. Our studies uncovered an essential role of white adipose tissue (WAT) lipolysis in fueling thermogenesis during fasting. In addition, we showed that lipolysis deficiency in brown adipose tissue (BAT) induces WAT browning. This commentary further discusses the significance of our findings and how whole body may be heated up without BAT lipolysis.

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The heart primarily uses fatty acids as energy substrates. Adipose lipolysis is a major source of fatty acids, particularly under stress conditions. In this study, we showed that mice with selective inactivation of the lipolytic coactivator comparative gene identification-58 (CGI-58) in adipose tissue (FAT-KO mice), relative to their littermate controls, had lower circulating FA levels in the fed and fasted states due to impaired adipose lipolysis. They preferentially utilized carbohydrates as energy fuels and were more insulin sensitive and glucose tolerant. Under cold stress, FAT-KO versus control mice had >10-fold increases in glucose uptake in the hearts but no increases in other tissues examined. Plasma concentrations of atrial natriuretic peptide and cardiac mRNAs for atrial and brain-type natriuretic peptides, two sensitive markers of cardiac remodeling, were also elevated. After one week of cold exposure, FAT-KO mice showed reduced cardiac expression of several mitochondrial oxidative phosphorylation proteins. After one month of cold exposure, hearts of these animals showed depressed functions, reduced SERCA2 protein, and increased proteins for MHC-β, collagen I proteins, Glut1, Glut4 and phospho-AMPK. Thus, CGI-58-dependent adipose lipolysis critically regulates cardiac metabolism and function, especially during cold adaptation. The adipose-heart axis may be targeted for the management of cardiac dysfunction.