Metabolism
Abstract
In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARγ dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARγ ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARγ target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARγ ligands regulate lipid metabolism and insulin sensitivity in adipocytes.
Authors
Patricia C. Chui, Hong-Ping Guan, Michael Lehrke, Mitchell A. Lazar
Abstract
In this study, a large number of naturally processed peptides was isolated and identified from the human diabetes-susceptible class II MHC molecules HLA-DQ8 (DQA1*0301,DQB1*0302) and from murine I-Ag7 species, both of which are expressed in genetically identical APC lines. The peptides presented during the processing of autologous proteins were highly selective in showing sequence specificity, mainly consisting of 1 or more acidic residues at their C terminus. Testing for binding to the MHC molecules revealed that the position 9 (P9) acidic residues of the peptides contributed decisively to binding. For HLA-DQ8, the P1 residue, which was also an acidic amino acid, influenced binding positively. Both HLA-DQ8 and I-Ag7 selected for common peptides that bound in the same register. There was no evidence for selection of peptides having nonspecific or promiscuous binding. Thus, diabetogenic class II MHC molecules are highly selective in terms of the peptides presented by their APCs, and this is governed by the features of their P9 anchor pocket. These results are in striking contrast to those from studies examining synthetic peptide or phage display libraries, in which many peptides were shown to bind.
Authors
Anish Suri, James J. Walters, Michael L. Gross, Emil R. Unanue
Abstract
Metabolic flexibility of skeletal muscle, that is, the preference for fat oxidation (FOx) during fasting and for carbohydrate oxidation in response to insulin, is decreased during insulin resistance. The aim of this study was to test the hypothesis that the capacity of myotubes to oxidize fat in vitro reflects the donor’s metabolic characteristics. Insulin sensitivity (IS) and metabolic flexibility of 16 healthy, young male subjects was determined by euglycemic hyperinsulinemic clamp. Muscle samples were obtained from vastus lateralis, cultured, and differentiated into myotubes. In human myotubes in vitro, we measured suppressibility (glucose suppression of FOx) and adaptability (an increase in FOx in the presence of high palmitate concentration). We termed these dynamic changes in FOx metabolic switching. In vivo, metabolic flexibility was positively correlated with IS and maximal oxygen uptake and inversely correlated with percent body fat. In vitro suppressibility was inversely correlated with IS and metabolic flexibility and positively correlated with body fat and fasting FFA levels. Adaptability was negatively associated with percent body fat and fasting insulin and positively correlated with IS and metabolic flexibility. The interindividual variability in metabolic phenotypes was preserved in human myotubes separated from their neuroendocrine environment, which supports the hypothesis that metabolic switching is an intrinsic property of skeletal muscle.
Authors
Barbara Ukropcova, Michele McNeil, Olga Sereda, Lilian de Jonge, Hui Xie, George A. Bray, Steven R. Smith
Abstract
Diabetes mellitus is a major health concern, affecting more than 5% of the population. Here we describe a potential novel therapeutic agent for this disease, FGF-21, which was discovered to be a potent regulator of glucose uptake in mouse 3T3-L1 and primary human adipocytes. FGF-21–transgenic mice were viable and resistant to diet-induced obesity. Therapeutic administration of FGF-21 reduced plasma glucose and triglycerides to near normal levels in both ob/ob and db/db mice. These effects persisted for at least 24 hours following the cessation of FGF-21 administration. Importantly, FGF-21 did not induce mitogenicity, hypoglycemia, or weight gain at any dose tested in diabetic or healthy animals or when overexpressed in transgenic mice. Thus, we conclude that FGF-21, which we have identified as a novel metabolic factor, exhibits the therapeutic characteristics necessary for an effective treatment of diabetes.
Authors
Alexei Kharitonenkov, Tatiyana L. Shiyanova, Anja Koester, Amy M. Ford, Radmila Micanovic, Elizabeth J. Galbreath, George E. Sandusky, Lisa J. Hammond, Julie S. Moyers, Rebecca A. Owens, Jesper Gromada, Joseph T. Brozinick, Eric D. Hawkins, Victor J. Wroblewski, De-Shan Li, Farrokh Mehrbod, S. Richard Jaskunas, Armen B. Shanafelt
Abstract
CD36 mediates the transfer of fatty acids (FAs) across the plasma membranes of muscle and adipose cells, thus playing an important role in regulating peripheral FA metabolism in vivo. In the proximal intestine, CD36 is localized in abundant quantities on the apical surface of epithelial cells, a pattern similar to that of other proteins implicated in the uptake of dietary FAs. To define the role of CD36 in the intestine, we examined FA utilization and lipoprotein secretion by WT and CD36-null mice in response to acute and chronic fat feeding. CD36-null mice given a fat bolus by gavage or fed a high-fat diet accumulated neutral lipid in the proximal intestine, which indicated abnormal lipid processing. Using a model in which mice were equipped with lymph fistulae, we obtained evidence of defective lipoprotein secretion by directly measuring lipid output. The secretion defect appeared to reflect an impaired ability of CD36-null enterocytes to efficiently synthesize triacylglycerols from dietary FAs in the endoplasmic reticulum. In the plasma of intact mice, the reduced intestinal lipid secretion was masked by slow clearance of intestine-derived lipoproteins. The impaired clearance occurred despite normal lipoprotein lipase activity and likely reflected feedback inhibition of the lipase by FAs due to their defective removal from the plasma. We conclude that CD36 is important for both secretion and clearance of intestinal lipoproteins. CD36 deficiency results in hypertriglyceridemia both in the postprandial and fasting states and in humans may constitute a risk factor for diet-induced type 2 diabetes and cardiovascular disease.
Authors
Victor A. Drover, Mohammad Ajmal, Fatiha Nassir, Nicholas O. Davidson, Andromeda M. Nauli, Daisy Sahoo, Patrick Tso, Nada A. Abumrad
Abstract
Endogenous cannabinoids acting at CB1 receptors stimulate appetite, and CB1 antagonists show promise in the treatment of obesity. CB1–/– mice are resistant to diet-induced obesity even though their caloric intake is similar to that of wild-type mice, suggesting that endocannabinoids also regulate fat metabolism. Here, we investigated the possible role of endocannabinoids in the regulation of hepatic lipogenesis. Activation of CB1 in mice increases the hepatic gene expression of the lipogenic transcription factor SREBP-1c and its targets acetyl-CoA carboxylase-1 and fatty acid synthase (FAS). Treatment with a CB1 agonist also increases de novo fatty acid synthesis in the liver or in isolated hepatocytes, which express CB1. High-fat diet increases hepatic levels of the endocannabinoid anandamide (arachidonoyl ethanolamide), CB1 density, and basal rates of fatty acid synthesis, and the latter is reduced by CB1 blockade. In the hypothalamus, where FAS inhibitors elicit anorexia, SREBP-1c and FAS expression are similarly affected by CB1 ligands. We conclude that anandamide acting at hepatic CB1 contributes to diet-induced obesity and that the FAS pathway may be a common molecular target for central appetitive and peripheral metabolic regulation.
Authors
Douglas Osei-Hyiaman, Michael DePetrillo, Pál Pacher, Jie Liu, Svetlana Radaeva, Sándor Bátkai, Judith Harvey-White, Ken Mackie, László Offertáler, Lei Wang, George Kunos
Severe impairment in liver insulin signaling fails to alter hepatic insulin action in conscious mice
Abstract
Insulin exerts its potent effects on hepatic glucose fluxes via direct and indirect mechanisms. Whereas a liver-specific insulin receptor (IR) knockout (LIRKO) mouse exhibits glucose intolerance as well as insulin resistance, it is unclear whether a more acute decrease in the expression of hepatic IR would be sufficient to induce hepatic insulin resistance. Here we report that the downregulation of hepatic IR expression by up to 95% does not modify hepatic insulin action. The i.p. administration (2 injections over 1 week) of an antisense oligodeoxynucleotide (ASO) directed to reduce insulin expression downregulated hepatic IR expression in C57BL6J mice. A high dose of IR-ASO decreased IR protein approximately 95%, while a control-ASO failed to modify IR expression. At this dose, the IR-ASO also decreased IR expression in adipose tissue but did not significantly decrease IR expression in hypothalamus or skeletal muscle. Insulin action was assessed with insulin clamp studies in conscious mice. The rate of glucose infusion during the clamp studies was comparable in control-ASO– and IR-ASO–treated mice. Importantly, the depletion of liver IR protein markedly impaired downstream insulin signaling in the liver, but it failed to modify the rate of glucose production. Thus, near ablation of liver IR does not alter insulin action on glucose production.
Authors
Christoph Buettner, Rima Patel, Evan D. Muse, Sanjay Bhanot, Brett P. Monia, Rob McKay, Silvana Obici, Luciano Rossetti
Abstract
Partial restoration of insulin receptor Insr expression in brain, liver, and pancreatic β cells is sufficient for rescuing Insr knockout mice from neonatal death, preventing diabetes ketoacidosis, and normalizing life span and reproductive function. However, the transgenically rescued mice (referred to as L1) have marked hyperinsulinemia, and approximately 30% develop late-onset type 2 diabetes. Analyses of protein expression indicated that L1 mice had modestly reduced Insr content but normal insulin-stimulated Akt phosphorylation in the liver. Conversely, L1 mice had a near complete ablation of Insr protein product in the arcuate and paraventricular nuclei of the hypothalamus, which was associated with a failure to undergo insulin-dependent Akt phosphorylation in the hypothalamus. To test whether reconstitution of insulin signaling in the liver is sufficient for restoring in vivo hepatic insulin action, we performed euglycemic hyperinsulinemic clamp studies in conscious L1 and WT mice. During the clamp, L1 mice required an approximately 50% lower rate of glucose infusion than did WT controls, while the rate of glucose disappearance was not significantly altered. Conversely, the rate of glucose production was increased approximately 2-fold in L1 mice. Thus, restoration of hepatic insulin signaling in Insr knockout mice fails to normalize the in vivo response to insulin.
Authors
Haruka Okamoto, Silvana Obici, Domenico Accili, Luciano Rossetti
Abstract
Elevated plasma levels of VLDL triglycerides (TGs) are characteristic of patients with type 2 diabetes mellitus (T2DM) and are associated with increased production rates (PRs) of VLDL TGs and apoB. Lipoprotein lipase–mediated (LPL-mediated) lipolysis of VLDL TGs may also be reduced in T2DM if the level of LPL is decreased and/or the level of plasma apoC-III, an inhibitor of LPL-mediated lipolysis, is increased. We studied the effects of pioglitazone (Pio), a PPARγ agonist that improves insulin sensitivity, on lipoprotein metabolism in patients with T2DM. Pio treatment reduced TG levels by increasing the fractional clearance rate (FCR) of VLDL TGs from the circulation, without changing direct removal of VLDL particles. This indicated increased lipolysis of VLDL TGs during Pio treatment, a mechanism supported by our finding of increased plasma LPL mass and decreased levels of plasma apoC-III. Lower apoC-III levels were due to reduced apoC-III PRs. We saw no effects of Pio on the PR of either VLDL TG or VLDL apoB. Thus, Pio, a PPARγ agonist, reduced VLDL TG levels by increasing LPL mass and inhibiting apoC-III PR. These 2 changes were associated with an increased FCR of VLDL TGs, almost certainly due to increased LPL-mediated lipolysis.
Authors
Kazunori Nagashima, Carlos Lopez, Daniel Donovan, Colleen Ngai, Nelson Fontanez, André Bensadoun, Jamila Fruchart-Najib, Steve Holleran, Jeffrey S. Cohn, Rajasekhar Ramakrishnan, Henry N. Ginsberg
Abstract
Authors
Cullen M. Taniguchi, Kohjiro Ueki, C. Ronald Kahn
Copyright © 2026 American Society for Clinical Investigation
ISSN: 0021-9738 (print), 1558-8238 (online)