Metformin is the first-line drug for the treatment of type 2 diabetes. Besides its well-characterized antihyperglycemic properties, metformin also lowers plasma VLDL triglyceride (TG). In this study, we investigated the underlying mechanisms in APOE*3-Leiden.CETP mice, a well-established model for human-like lipoprotein metabolism. We found that metformin markedly lowered plasma total cholesterol and TG levels, an effect mostly due to a decrease in VLDL-TG, whereas HDL was slightly increased. Strikingly, metformin did not affect hepatic VLDL-TG production, VLDL particle composition, and hepatic lipid composition but selectively enhanced clearance of glycerol tri[ 3 H]oleate-labeled VLDL-like emulsion particles into brown adipose tissue (BAT). BAT mass and lipid droplet content were reduced in metformin-treated mice, pointing to increased BAT activation. In addition, both AMP-activated protein kinase a1 (AMPKa1) expression and activity and HSL and mitochondrial content were increased in BAT.Furthermore, therapeutic concentrations of metformin increased AMPK and HSL activities and promoted lipolysis in T37i differentiated brown adipocytes. Collectively, our results identify BAT as an important player in the TG-lowering effect of metformin by enhancing VLDL-TG uptake, intracellular TG lipolysis, and subsequent mitochondrial fatty acid oxidation. Targeting BAT might therefore be considered as a future therapeutic strategy for the treatment of dyslipidemia.

Dyslipidemia is one of the classical risk factors for cardiovascular diseases (CVD) besides hypertension, type 2 diabetes, and smoking ( 1 ). Dyslipidemia is defi ned as an elevation of plasma LDL-cholesterol (LDL-C), triglycerides (TG), or both, with or without a lowering of HDLcholesterol (HDL-C) ( 2 ). Whereas elevated LDL-C is a well-established major predictor of CVD and has been the primary target for lipid-lowering strategies, evidence suggests that an elevated TG level is an independent risk factor for CVD development as well ( 3, 4 ). Plasma TG levels are considered elevated when they exceed 150 mg/dl, which is observed in 31% of the adult US population ( 5 ). Although hypertriglyceridemia can be caused by rare monogenic disorders, it is mostly caused by a complex interaction between environmental factors and subtle variations in genes involved in lipoprotein metabolism ( 5 ). Current treatments for hypertriglyceridemia are aimed at either increasing TG clearance (e.g., fi brates) ( 6 ) or at decreasing lipolysis in WAT (e.g., niacin) ( 7 ). In addition, reduction of VLDL-TG production lowers plasma TG levels (e.g., exendin-4) ( 8 ).In recent years, the autonomic nervous system, which consists of a sympathetic and a parasympathetic branch, emerged as an important regulator of metabolic homeostasis. Whereas the role of the sympathetic nervous system (SNS) in the regulation of glucose metabolism has been fi rmly established (for review, see Ref. 9 ), considerably fewer studies have focused on its role in TG metabolism. This review provides an update specifi cally on the role of Abstract Important players in triglyceride (TG) metabolism include the liver (production), white adipose tissue (WAT) (storage), heart and skeletal muscle (combustion to generate ATP), and brown adipose tissue (BAT) (combustion toward heat), the collective action of which determine plasma TG levels. Interestingly, recent evidence points to a prominent role of the hypothalamus in TG metabolism through innervating the liver, WAT, and BAT mainly via sympathetic branches of the autonomic nervous system. Here, we review the recent fi ndings in the area of sympathetic control of TG metabolism. Various neuronal populations, such as neuropeptide Y (NPY)-expressing neurons and melanocortin-expressing neurons, as well as peripherally produced hormones (i.e., GLP-1, leptin, and insulin), modulate sympathetic outfl ow from the hypothalamus toward target organs and thereby infl uence peripheral TG metabolism. We conclude that sympathetic stimulation in general increases lipolysis in WAT, enhances VLDL-TG production by the liver, and increases the activity of BAT with respect to lipolysis of TG, followed by combustion of fatty acids toward heat. Moreover, the increased knowledge about the involvement of the neuroendocrine system in TG metabolism presented in this review offers new therapeutic options to fi ght hypertriglyceridemia by specifi cally modulating sympathetic nervous system outfl ow toward liver, BAT, or WAT .-Geerling, J. J., M. R. B...

ObjectiveIn addition to improve glucose intolerance, recent studies suggest that glucagon-like peptide-1 (GLP-1) receptor agonism also decreases triglyceride (TG) levels. The aim of this study was to evaluate the effect of GLP-1 receptor agonism on very-low-density lipoprotein (VLDL)-TG production and liver TG metabolism.Experimental ApproachThe GLP-1 peptide analogues CNTO3649 and exendin-4 were continuously administered subcutaneously to high fat diet-fed APOE*3-Leiden transgenic mice. After 4 weeks, hepatic VLDL production, lipid content, and expression profiles of selected genes involved in lipid metabolism were determined.ResultsCNTO3649 and exendin-4 reduced fasting plasma glucose (up to −30% and −28% respectively) and insulin (−43% and −65% respectively). In addition, these agents reduced VLDL-TG production (−36% and −54% respectively) and VLDL-apoB production (−36% and −43% respectively), indicating reduced production of VLDL particles rather than reduced lipidation of apoB. Moreover, they markedly decreased hepatic content of TG (−39% and −55% respectively), cholesterol (−30% and −55% respectively), and phospholipids (−23% and −36% respectively), accompanied by down-regulation of expression of genes involved in hepatic lipogenesis (Srebp-1c, Fasn, Dgat1) and apoB synthesis (Apob).ConclusionGLP-1 receptor agonism reduces VLDL production and hepatic steatosis in addition to an improvement of glycemic control. These data suggest that GLP-receptor agonists could reduce hepatic steatosis and ameliorate dyslipidemia in patients with type 2 diabetes mellitus.

Background and Purpose The aetiology of inflammation in the liver and vessel wall, leading to non‐alcoholic steatohepatitis (NASH) and atherosclerosis, respectively, shares common mechanisms including macrophage infiltration. To treat both disorders simultaneously, it is highly important to tackle the inflammatory status. Exendin‐4, a glucagon‐like peptide‐1 (GLP‐1) receptor agonist, reduces hepatic steatosis and has been suggested to reduce atherosclerosis; however, its effects on liver inflammation are underexplored. Here, we tested the hypothesis that exendin‐4 reduces inflammation in both the liver and vessel wall, and investigated the common underlying mechanism. Experimental Approach Female APOE*3‐Leiden.CETP mice, a model with human‐like lipoprotein metabolism, were fed a cholesterol‐containing Western‐type diet for 5 weeks to induce atherosclerosis and subsequently treated for 4 weeks with exendin‐4. Key Results Exendin‐4 modestly improved dyslipidaemia, but markedly decreased atherosclerotic lesion severity and area (−33%), accompanied by a reduction in monocyte adhesion to the vessel wall (−42%) and macrophage content in the plaque (−44%). Furthermore, exendin‐4 reduced hepatic lipid content and inflammation as well as hepatic CD68+ (−18%) and F4/80+ (−25%) macrophage content. This was accompanied by less monocyte recruitment from the circulation as the Mac‐1+ macrophage content was decreased (−36%). Finally, exendin‐4 reduced hepatic chemokine expression in vivo and suppressed oxidized low‐density lipoprotein accumulation in peritoneal macrophages in vitro, effects dependent on the GLP‐1 receptor. Conclusions and Implications Exendin‐4 reduces inflammation in both the liver and vessel wall by reducing macrophage recruitment and activation. These data suggest that exendin‐4 could be a valuable strategy to treat NASH and atherosclerosis simultaneously.

The central nervous system (CNS) is highly sensitive to insulin ( 1-6 ). Insulin in the brain is mostly derived from the circulation, and only a modest amount, if any, is produced locally ( 6, 7 ). Circulating insulin can cross the blood-brain barrier ( 8, 9 ) and exert metabolic effects in peripheral organs via the CNS. Intracerebroventricular (ICV) administration of insulin decreases food intake, resulting in reduced body weight ( 3, 10, 11 ). In addition, the central action of insulin plays a crucial role in the inhibitory effect of the hormone on hepatic glucose production ( 4, 12 ).Recently, a novel regulatory function for the effects of insulin through the CNS with regard to adipose tissue metabolism has been proposed, suggesting that intracellular lipolysis and lipogenesis in WAT is under the neuronal control of central insulin ( 13,14 ). Furthermore, it has been shown that central glucose lowers plasma triglyceride Abstract Insulin signaling in the central nervous system (CNS) is required for the inhibitory effect of insulin on glucose production. Our aim was to determine whether the CNS is also involved in the stimulatory effect of circulating insulin on the tissue-specifi c retention of fatty acid (FA) from plasma. In wild-type mice, hyperinsulinemic-euglycemic clamp conditions stimulated the retention of both plasma triglyceride-derived FA and plasma albumin-bound FA in the various white adipose tissues (WAT) but not in other tissues, including brown adipose tissue (BAT). Intracerebroventricular (ICV) administration of insulin induced a similar pattern of tissue-specifi c FA partitioning. This effect of ICV insulin administration was not associated with activation of the insulin signaling pathway in adipose tissue. ICV administration of tolbutamide, a K ATP channel blocker, considerably reduced (during hyperinsulinemic-euglycemic clamp conditions) and even completely blocked (during ICV administration of insulin) WAT-specifi c retention of FA from plasma. This central effect of insulin was absent in CD36-defi cient mice, indicating that CD36 is the predominant FA transporter in insulin-stimulated FA retention by WAT. In diet-induced insulin-resistant mice, these stimulating effects of insulin (circulating or ICV administered) on FA retention in WAT were lost. In conclusion, in insulinsensitive mice, circulating insulin stimulates tissue-specifi c partitioning of plasma-derived FA in WAT in part through activation of K ATP channels in the CNS. Apparently, circulating insulin stimulates fatty acid uptake in WAT but not in BAT, directly and indirectly through the CNS. -Coomans,