FGF21 decreases plasma triglycerides (TGs) in rodents and humans; however, the underlying mechanism or mechanisms are unclear. In the present study, we examined the role of FGF21 in production and disposal of TG-rich lipoproteins (TRLs) in mice. Treatment with pharmacological doses of FGF21 acutely reduced plasma non-esterified fatty acids (NEFAs), liver TG content, and VLDL-TG secretion. In addition, metabolic turnover studies revealed that FGF21 facilitated the catabolism of TRL in white adipose tissue (WAT) and brown adipose tissue (BAT). FGF21-dependent TRL processing was strongly attenuated in CD36-deficient mice and transgenic mice lacking lipoprotein lipase in adipose tissues. Insulin resistance in diet-induced obese and ob/ob mice shifted FGF21 responses from WAT toward energy-combusting BAT. In conclusion, FGF21 lowers plasma TGs through a dual mechanism: first, by reducing NEFA plasma levels and consequently hepatic VLDL lipidation and, second, by increasing CD36 and LPL-dependent TRL disposal in WAT and BAT.

This article is available online at http://www.jlr.org related chronic metabolic diseases. The natural function of BAT is to combust energy from high-caloric nutrients to defend the body against cold environments ( 4 ). The ability to burn energy-dense triglycerides as fuels for heat production could enable BAT to diminish hypertrophic white adipose tissue (WAT) depots, a prerequisite for the prevention of metabolic lifestyle diseases ( 5, 6 ). In humans, BAT activity, determined by positron emission tomographycomputed tomography (PET-CT), is positively correlated with BAT mass ( 1-3 ), BAT activation status ( 2 ), and environmental factors such as low temperatures ( 7 ). Repeated cold exposure leads to increased BAT activity ( 8, 9 ), a condition that is associated with a self-reported decrease in sensitivity to cold. The thermogenic process is dependent on the presence of uncoupling protein 1 (UCP1), a protein located in the inner membrane of mitochondria that is able to separate electron transport in the respiratory chain from the production of energy in the form of ATP. The heat generated by this exothermic reaction is transported via the blood circulation system to maintain body temperature ( 10 ). Low BAT activity in humans correlates with ageing and obesity ( 2, 11 ), suggesting a causal link between decreased BAT activity, weight gain, and the development of metabolic diseases. In this context, channeling fatty acids and triglycerides into BAT could attenuate deleterious effects that saturated fatty acids cause by ectopic lipid accumulation in the liver or heart . In fact, up to 90% of energy for heat production is derived from fatty acids that are delivered by triglyceriderich lipoproteins ( 4, 10, 12 ). These latter are hepatic VLDLs Abstract The endocannabinoids and their main receptor, cannabinoid type-1 (CB1), suppress intracellular cyclic AMP levels and have emerged as key players in the control of energy metabolism. CB1 agonists and blockers have been reported to infl uence the thermogenic function of white and brown adipose tissue (WAT and BAT), affecting body weight through the inhibition and stimulation of energy expenditure, respectively. The purpose of the current study was to investigate the regulation of the endocannabinoid system in WAT and BAT following exposure to either cold or specifi c agonism of ␤ 3-adrenoceptors using CL316,243 (CL), conditions known to cause BAT activation and WAT browning. To address this question, we performed quantitative PCRbased mRNA profi ling of genes important for endocannabinoid synthesis , degradation, and signaling, and determined endocannabinoid levels by LC-MS in WAT and BAT of control, cold-exposed, and CL-treated wild-type mice as well as primary brown adipocytes. Treatment with CL and exposure to cold caused an upregulation of endocannabinoid levels and biosynthetic enzymes in WAT. Acute ␤ 3-adrenoceptor activation increased endocannabinoids and a subset of genes of biosynthesis in BAT and primary brown adipocytes. We suggest that the cold-mediated ...

ObjectiveInsulin resistance is associated with impaired receptor dependent hepatic uptake of triglyceride-rich lipoproteins (TRL), promoting hypertriglyceridemia and atherosclerosis. Next to low-density lipoprotein (LDL) receptor (LDLR) and syndecan-1, the LDLR-related protein 1 (LRP1) stimulated by insulin action contributes to the rapid clearance of TRL in the postprandial state. Here, we investigated the hypothesis that the adaptor protein phosphotyrosine interacting domain-containing protein 1 (PID1) regulates LRP1 function, thereby controlling hepatic endocytosis of postprandial lipoproteins.MethodsLocalization and interaction of PID1 and LRP1 in cultured hepatocytes was studied by confocal microscopy of fluorescent tagged proteins, by indirect immunohistochemistry of endogenous proteins, by GST-based pull down and by immunoprecipitation experiments. The in vivo relevance of PID1 was assessed using whole body as well as liver-specific Pid1-deficient mice on a wild type or Ldlr-deficient (Ldlr−/−) background. Intravital microscopy was used to study LRP1 translocation in the liver. Lipoprotein metabolism was investigated by lipoprotein profiling, gene and protein expression as well as organ-specific uptake of radiolabelled TRL.ResultsPID1 co-localized in perinuclear endosomes and was found associated with LRP1 under fasting conditions. We identified the distal NPxY motif of the intracellular C-terminal domain (ICD) of LRP1 as the site critical for the interaction with PID1. Insulin-mediated NPxY-phosphorylation caused the dissociation of PID1 from the ICD, causing LRP1 translocation to the plasma membrane. PID1 deletion resulted in higher LRP1 abundance at the cell surface, higher LDLR protein levels and, paradoxically, reduced total LRP1. The latter can be explained by higher receptor shedding, which we observed in cultured Pid1-deficient hepatocytes. Consistently, PID1 deficiency alone led to increased LDLR-dependent endocytosis of postprandial lipoproteins and lower plasma triglycerides. In contrast, hepatic PID1 deletion on an Ldlr−/− background reduced lipoprotein uptake into liver and caused plasma TRL accumulation.ConclusionsBy acting as an insulin-dependent retention adaptor, PID1 serves as a regulator of LRP1 function controlling the disposal of postprandial lipoproteins. PID1 inhibition provides a novel approach to lower plasma levels of pro-atherogenic TRL remnants by stimulating endocytic function of both LRP1 and LDLR in the liver.

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