CTLA4-Ig treatment improves RBP4-induced adipose tissue inflammation and insulin resistance triggered by MyD88, JNK, ERK and p38 pathways (IRC8P.443)

Adipose tissue (AT) inflammation and impaired insulin action is a major cause of type 2 diabetes. RBP4 is an adipocyte- and liver-derived protein that has an important role in insulin resistance, metabolic syndrome and AT inflammation. RBP4 elevation causes AT inflammation by activating innate immunity that elicits an adaptive immune response. Our aims are to determine the signaling pathways involved in RBP4-induced macrophage activation and the resulting antigen presentation and Th1 polarization and whether the blockade of antigen presentation improves AT inflammation and insulin resistance. RBP4-overexpressing mice (RBP4-Ox) are insulin resistant and glucose intolerant and have increased AT macrophage and Th1 cell infiltration. In RBP4-Ox, AT macrophages display enhanced JNK, ERK and p38 phosphorylation, and in vitro inhibition of these pathways reduces macrophage activation and macrophage-induced CD4 T cell proliferation and Th1 polarization. Moreover, macrophages obtained from MyD88 knockout mice and activated with RBP4 do not secrete TNF, IL12 and IL-1b and fail to induce CD4 T cell proliferation and Th1 polarization. Treatment of RBP4-Ox mice with CLTA4-Ig reduces AT inflammation and improves insulin resistance. Thus, RBP4 causes insulin resistance, at least partly, through MyD88 pathway and downstream by activating JNK, ERK and p38 pathways. These pathways induce macrophage activation and Th1 polarization, which can be blocked by inhibiting antigen presentation.

Expression of the adiponectin receptors AdipoR1 and AdipoR2 in lean rats and in obese Zucker rats

The adiponectin receptors, AdipoR1 and AdipoR2, are thought to transmit the insulin-sensitizing effects of adiponectin, an adipokine secreted by adipocytes. Modifications of their expression in insulin-sensitive tissues (skeletal muscle, liver, and adipose tissue) could therefore play a role in the control of insulin sensitivity and the development of insulin resistance. Recent data in mice supported this possibility. We examined whether the expression of adiponectin receptors (messenger RNA [mRNA] concentrations) is controlled in vivo in rats (Wistar) by nutritional factors (high-fat [HF] vs high-carbohydrate diet, fasting vs fed state) and whether this expression is decreased in an experimental model of insulin resistance, the obese Zucker rat. In Wistar rats, neither an HF diet nor fasting modified the mRNA concentrations of AdipoR1 in muscle, liver, or adipose tissue; the only modification observed was a decrease (P < .05) in AdipoR2 mRNA level in the liver of rats fed with an HF diet. In obese Zucker rats compared with their lean controls, neither AdipoR1 nor AdipoR2 expression was modified in muscle. AdipoR2 expression was slightly decreased in adipose tissue, whereas the expression of both AdipoR1 and AdipoR2 was increased (P < .05) in the liver of obese Zucker rats. In conclusion, contrary to what was reported in mice, the expression of adiponectin receptors in rats is poorly responsive to changes in nutritional conditions and is not decreased in a model of insulin resistance. These results do not support an important role for the expression of AdipoR1 and AdipoR2 in the modulation of sensitivity to insulin.

In vivo expression of carbohydrate responsive element binding protein in lean and obese rats

ChREBP (Carbohydrate response element binding protein) is considered to mediate the stimulatory effect of glucose on the expression of lipogenic genes. Its activity is stimulated by glucose. Less is known on the control of its expression. This expression could be controlled by nutritional (glucose, fatty acids) and hormonal (insulin) factors. We examined the in vivo nutritional control of ChREBP expression in liver and adipose tissue of Wistar rats. Compared respectively to the fed state and to a high carbohydrate diet, ChREBP mRNA concentrations were not modified by fasting or a high fat diet in rat liver and adipose tissue. FAS and ACC1 mRNA concentrations were on the contrary decreased as expected by fasting and high fat diets and these variations of FAS and ACC1 mRNA were positively related to those of SREBP-1c mRNA and protein, but not of ChREBP mRNA. Therefore i) ChREBP expression appears poorly responsive to modifications of nutritional condition, ii) modifications of the expression of ChREBP do not seem implicated in the physiological control of lipogenesis. To investigate the possible role of ChREBP in pathological situations we measured its mRNA concentrations in the liver and adipose tissue of obese Zucker rats. ChREBP expression was increased in the liver but not the adipose tissue of obese rats compared to their lean littermates. These results support a role of ChREBP in the development of hepatic steatosis and hypertriglyceridemia but not of obesity in this experimental model.

Adipose triglyceride lipase: function, regulation by insulin, and comparison with adiponutrin

Adipose triglyceride lipase (ATGL) is a recently described adipose-enriched protein with triglyceride-specific lipase activity. ATGL shares the greatest sequence homology with adiponutrin, a nutritionally regulated protein of unclear biological function. Here we present a functional analysis of ATGL and adiponutrin and describe their regulation by insulin. Retroviral-mediated overexpression of ATGL in 3T3-L1 adipocytes increased basal and isoproterenol-stimulated glycerol and nonesterified fatty acid (NEFA) release, whereas siRNA-mediated knockdown of ATGL had the opposite effect. In contrast, siRNA-mediated knockdown of adiponutrin in 3T3-L1 adipocytes had no effect on glycerol or NEFA release. In mice, both ATGL and adiponutrin are nutritionally regulated in adipose tissue, with ATGL being upregulated and adiponutrin being downregulated by fasting. In 3T3-L1 adipocytes, insulin decreased ATGL and increased adiponutrin expression in a dose- and time-dependent manner, suggesting that insulin directly mediates this nutritional regulation. In addition, adipose expression of ATGL was increased by insulin deficiency and decreased by insulin replacement in streptozotocin-induced diabetic mice and was increased in fat-specific insulin receptor knockout mice, whereas adiponutrin showed the opposite pattern. These data suggest that murine ATGL but not adiponutrin contributes to net adipocyte lipolysis and that ATGL and adiponutrin are oppositely regulated by insulin both in vitro and in vivo.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

Uncoupling protein-2 negatively regulates insulin secretion and is a major link between obesity, beta cell dysfunction, and type 2 diabetes

beta cells sense glucose through its metabolism and the resulting increase in ATP, which subsequently stimulates insulin secretion. Uncoupling protein-2 (UCP2) mediates mitochondrial proton leak, decreasing ATP production. In the present study, we assessed UCP2's role in regulating insulin secretion. UCP2-deficient mice had higher islet ATP levels and increased glucose-stimulated insulin secretion, establishing that UCP2 negatively regulates insulin secretion. Of pathophysiologic significance, UCP2 was markedly upregulated in islets of ob/ob mice, a model of obesity-induced diabetes. Importantly, ob/ob mice lacking UCP2 had restored first-phase insulin secretion, increased serum insulin levels, and greatly decreased levels of glycemia. These results establish UCP2 as a key component of beta cell glucose sensing, and as a critical link between obesity, beta cell dysfunction, and type 2 diabetes.

Effects of isoenergetic high-carbohydrate compared with high-fat diets on human cholesterol synthesis and expression of key regulatory genes of cholesterol metabolism

BACKGROUND

High-carbohydrate diets improve plasma cholesterol concentrations but increase triacylglycerol concentrations; the latter effect increases the risk of cardiovascular disease (CVD). Triacylglycerol concentrations increase only during very-high-carbohydrate diets consisting mainly of simple sugars.

OBJECTIVE

We compared the CVD risk profile, cholesterol metabolism, and glucose tolerance of 7 healthy subjects during 2 isoenergetic diets: a high-fat, low-carbohydrate diet (HF diet) and a moderately high-carbohydrate, low-fat diet (HC diet).

DESIGN

In a randomized crossover study, we measured the effects of the HF diet [40% carbohydrate and 45% fat (15% saturated, 15% monounsaturated, and 15% polyunsaturated)] and HC diet [55% carbohydrate (mainly complex) and 30% fat (10% saturated, 10% monounsaturated, and 10% polyunsaturated)] (3 wk each) on plasma lipid concentrations, oral glucose tolerance, cholesterol synthesis rate, and the messenger RNA (mRNA) concentrations of beta-hydroxy-beta-methylglutaryl coenzyme A (HMG-CoA) reductase, the LDL receptor, and the LDL-receptor-related protein (LRP).

RESULTS

Compared with the HF diet, the HC diet lowered total, LDL, and HDL cholesterol (P < 0.05 for all) without modifying the ratio of LDL to HDL cholesterol; triacylglycerol concentrations were unchanged. Lower cholesterol concentrations occurred despite a higher cholesterol synthesis rate (P < 0.05) and higher HMG-CoA reductase mRNA concentrations (P < 0.05). LDL receptor mRNA concentrations were unchanged, LRP mRNA concentrations were lower (P < 0.01), and oral glucose tolerance was better (P < 0.05) with the HC diet.

CONCLUSION

The beneficial effects of the HC diet on glucose tolerance and plasma cholesterol concentrations without increases in triacylglycerol show that this diet had favorable effects on both insulin sensitivity and the plasma lipid profile.

Adipose-selective targeting of the GLUT4 gene impairs insulin action in muscle and liver

The earliest defect in developing type 2 diabetes is insulin resistance, characterized by decreased glucose transport and metabolism in muscle and adipocytes. The glucose transporter GLUT4 mediates insulin-stimulated glucose uptake in adipocytes and muscle by rapidly moving from intracellular storage sites to the plasma membrane. In insulin-resistant states such as obesity and type 2 diabetes, GLUT4 expression is decreased in adipose tissue but preserved in muscle. Because skeletal muscle is the main site of insulin-stimulated glucose uptake, the role of adipose tissue GLUT4 downregulation in the pathogenesis of insulin resistance and diabetes is unclear. To determine the role of adipose GLUT4 in glucose homeostasis, we used Cre/loxP DNA recombination to generate mice with adipose-selective reduction of GLUT4 (G4A-/-). Here we show that these mice have normal growth and adipose mass despite markedly impaired insulin-stimulated glucose uptake in adipocytes. Although GLUT4 expression is preserved in muscle, these mice develop insulin resistance in muscle and liver, manifested by decreased biological responses and impaired activation of phosphoinositide-3-OH kinase. G4A-/- mice develop glucose intolerance and hyperinsulinaemia. Thus, downregulation of GLUT4 and glucose transport selectively in adipose tissue can cause insulin resistance and thereby increase the risk of developing diabetes.

In vivo measurement of gluconeogenesis in animals and humans with deuterated water: a simplified method

The contribution of gluconeogenesis to glucose production can be measured by comparing after ingestion of deuterated water the enrichment in deuterium of the hydrogen bound to carbon 5 of glucose with that of hydrogen bound to carbon 2 or with the deuterium enrichment of plasma water. The method developed by Landau et al. for measuring deuterium enrichment on carbon 5 by gas chromatography-mass spectrometry analysis is tedious and time consuming. We developed a simpler procedure for measuring this deuterium enrichment. Deuterium enrichment on carbons 5 and 6 of glucose is measured using the 1,2-5, 6-diisopropylidene-3-O-acetyl-a-furanosyl derivative. Enrichment in position 6 is measured using the hexamethylenetetramine procedure and subtracted from the enrichment on carbons 5 and 6 to obtain the specific enrichment on carbon 5. We tested first this method in post-absorptive and fasted rats (plasma water enrichment 0.6%) infused simultaneously with [6,6-(2) H(2) ] glucose in order to obtain not only the percent contribution of gluconeogenesis, but also glucose turnover rate and absolute gluconeogenesis flux. In post-absorptive and starved rats gluconeogenesis represented respectively 46.7+/-2.0% and 94.1+/-2.0% of glucose production and a flux of 31.1+/-1.8 and 38.9+/-0.9 micromol/kg/min. The method was then used in humans. The contribution in the post-absorptive state of gluconeogenesis to glucose appearance measured in control and type 2 diabetic subjects (plasma water enrichment 0.23-0.38%) was 40. 7+/-5.0% and 65.7 +/-3.3% (p<0.05) respectively. In conclusion this simplified method appears useful for in vivo studies of gluconeogenesis.

Role of the beta(3)-adrenergic receptor and/or a putative beta(4)-adrenergic receptor on the expression of uncoupling proteins and peroxisome proliferator-activated receptor-gamma coactivator-1

Administration of beta-adrenergic receptor (beta-AR) agonists, especially beta(3)-AR agonists, is well known to increase thermogenesis in rodents and humans. In this work we studied the role of the beta(3)-AR in regulating mRNA expression of genes involved in thermogenesis, i.e., mitochondrial uncoupling proteins UCP2 and UCP3, and peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1), in mouse skeletal muscle. For this purpose, different beta(3)-AR agonists were administered acutely to both wild type mice and mice whose beta(3)-AR gene has been disrupted (beta(3)-AR KO mice). CL 316243 increased the expression of UCP2, UCP3 and PGC-1 in wild type mice only. By contrast, BRL 37344 and CGP 12177 increased the expression of UCP2 and UCP3 in both wild type and beta(3)-AR KO mice, whereas they increased the expression of PGC-1 in wild type mice only. Finally, acute (3 h) cold exposure increased the expression of UCP2 and UCP3, but not PGC-1, in skeletal muscle of both wild type and beta(3)-AR KO mice. These results show that selective stimulation of the beta(3)-AR affects the expression of UCP2, UCP3 and PGC-1 in skeletal muscle. This effect is probably indirect, as muscle does not seem to express beta(3)-AR. In addition, our data suggest that BRL 37344 and CGP 12177 act, in part, through an as yet unidentified receptor, possibly a beta(4)-AR.

Glucose production and gluconeogenesis in postabsorptive and starved normal and streptozotocin-diabetic rats

Using a 3-hour primed-continuous infusion of [3-3H]glucose and [2-13C]glycerol, we measured glucose production, gluconeogenesis from glycerol, and total gluconeogenesis (using mass isotopomer distribution analysis [MIDA] of glucose) in postabsorptive and starved normal and streptozotocin-diabetic rats. In normal rats, 48 hours of starvation increased (P < .01) the percent contribution of both gluconeogenesis from glycerol (from 14.4% +/- 1.8% to 25.5% +/- 4.0%) and total gluconeogenesis (from 52.2% +/- 3.9% to 89.8% +/- 1.3%) to glucose production, but the absolute gluconeogenic fluxes were not modified, since glucose production decreased. Diabetic rats showed increased glucose production in the postabsorptive state; this decreased with starvation and was comparable to the of controls after 48 hours of starvation. Gluconeogenesis was increased in postabsorptive diabetic rats (69.0% +/- 1.3%, P < .05 v controls). Surprisingly, this contribution of gluconeogenesis to glucose production was not found to be increased in 24-hour starved diabetic rats (64.4% +/- 2.4%). These rats had significant liver glycogen stores, but gluconeogenesis was also low (42.8% +/- 2.1%) in 48-hour starved diabetic rats deprived of glycogen stores. Moreover, in 24-hour starved diabetic rats infused with [3-13C]lactate, gluconeogenesis was 100% when determined by comparing circulating glucose and liver pyruvate enrichment, but only 47% +/- 3% when calculated from the MIDA of glucose. Therefore, MIDA is not a valid method to measure gluconeogenesis in starved diabetic rats. This was not explained by differences in the labeling of liver and kidney triose phosphates: functional nephrectomy of starved diabetic rats decreased glucose production, but gluconeogenesis calculated by the MIDA method was only 48% +/- 3.3%. We conclude that (1) diabetic rats have increased glucose production and gluconeogenesis in the postabsorptive state; (2) starvation decreases glucose production and increases the contribution of gluconeogenesis, but MIDA is not an appropriate method in this situation; and (3) the kidneys contribute to glucose production in starved diabetic rats.

In vivo studies of intrahepatic metabolic pathways

In vivo studies of liver metabolism have long been limited to measurement by the balance technique or isotope dilution method of the amounts of substrates taken up or produced by the liver. New methods, based mainly on the use of stable isotopes, now allow important data to be obtained on intrahepatic metabolic pathways. Nuclear magnetic resonance and chemical biopsy of glucuronic acid by acetaminophen facilitate the study of glycogen metabolism. Chemical biopsies of liver glutamine by phenylacetate and of cytosolic acetylCoA by sulfamethoxazole provide important data respectively on Krebs cycle activity and gluconeogenesis and on lipogenesis and cholesterol synthesis. Mass isotopomer distribution analysis of molecules synthesised during infusion of 13C-labelled precursors allows an estimation of in vivo gluconeogenesis as well as cholesterol synthesis and lipogenesis. Finally, these metabolic pathways can be studied through the incorporation of deuterium from deuterated water in glucose, fatty acids and cholesterol. All these non-invasive techniques allow investigations to be undertaken in human beings to study the nutritional and hormonal regulation of liver metabolism in normal subjects and in pathological situations.

Measuring glycerol turnover, gluconeogenesis from glycerol, and total gluconeogenesis with [2-13C] glycerol: role of the infusion-sampling mode

Mass isotopomer distribution analysis (MIDA) of glucose during infusion of [2-13C]glycerol is a new method for measuring total gluconeogenesis (GNG). Since this method relies on calculation of the isotopic enrichment (IE) of hepatic triose phosphates (TP), the results should be independent of the sites of tracer infusion and blood sampling. Postabsorptive and starved rats were infused with [2-13C]glycerol and sampled either in the arterial-venous (A-V) or venous-arterial (V-A) modes. Blood was also sampled from the portal vein. In both postabsorptive and starved rats, glycerol turnover rate (Rt) and the percent contribution of glycerol to total glucose production were higher in the A-V mode than in the V-A mode (P < .05). Glycerol IE in portal venous blood was intermediate between IE values observed in peripheral arterial and venous blood. Its use for calculating the contribution of glycerol to glucose production reconciled the results obtained with the two infusion-sampling modes in both postabsorptive and starved rats; this contribution was increased by starvation (P < .01). In postabsorptive rats, total GNG calculated from MIDA of glucose accounted for approximately 50% of glucose production whatever the infusion-sampling mode (A-V, 48.8% +/- 4.7%; V-A, 52.2% +/- 3.9%). This contribution increased to 90% in starved rats, again, with no difference between A-V (95.2% +/- 1.8%) and V-A (89.2% +/- 1.3%) modes. In conclusion, during infusion of [2-13C]glycerol, total GNG measured from MIDA of glucose is independent of the infusion-sampling mode, contrary to calculations of Rt and GNG from glycerol. Measurement of glycerol IE in portal venous blood reconciles the results obtained with the two modes with respect to the contribution of glycerol to GNG.

New methods for in vivo studies of hepatic metabolism

In vivo studies of liver metabolism have been limited for a long time to measurements, by the balance technique or the isotope dilution method, of the amounts of substrates taken up or produced by liver. New methods have been developed that now permit us to obtain important information on intrahepatic metabolic pathways. Nuclear magnetic resonance permits noninvasive studies of liver glycogen synthesis and breakdown. Chemical biopsy of glucuronic acid by acetaminophen also permits the study of glycogen synthesis whereas chemical biopsies of liver glutamine by phenylacetate and of cytosolic acetyl-CoA by sulfamethoxazole give important information concerning, respectively, Krebs cycle activity and glconeogenesis and on lipogenesis and cholesterol synthesis. Mass isotopomer distribution analysis of molecules synthesized during the infusion of a deuterium of 13C-labeled precursor permits the estimation of in vivo gluconeogenesis as well as cholesterol synthesis and lipogenesis. Finally, these metabolic pathways can be studied through the incorporation of deuterium from deuterated water in glucose, fatty acids and cholesterol. All these noninvasive techniques will allow investigations to be undertaken in humans, addressing the nutritional and hormonal regulation of liver metabolism in normal subjects and in pathological situations.

Measuring gluconeogenesis with [2-13C]glycerol and mass isotopomer distribution analysis of glucose

We tested the validity of the use of [2-13C]glycerol and of the mass isotopomer distribution analysis of glucose for measuring gluconeogenesis in vitro and in vivo. When isolated rat livers (starved for 48 h) were infused with labeled glycerol without or with lactate+pyruvate, gluconeogenesis accounted for > 90% of glucose production. When glucose was added to the infusate so that glucose produced by the liver represented only 80 or 45% of total glucose output, this dilution could be calculated from the mass isotopomer distribution of glucose. When postabsorptive and starved rats were infused with [2-13C]glycerol, gluconeogenesis accounted for 54 +/- 2 and 89 +/- 1%, respectively, of glucose production. However, accurate measures could be obtained, particularly in postabsorptive rats, only with high tracer infusion rates (representing > or = 50% of endogenous glycerol production rate). In both groups of rats, these infusion rates resulted in an increase in total glycerol turnover rate and gluconeogenesis from glycerol. In addition, hepatic concentration of glycerol 3-phosphate was increased. In conclusion, [2-13C]glycerol infusion and mass isotopomer distribution analysis of glucose appear to be useful methods for studies of gluconeogenesis in vitro and in vivo; however, accurate measurements in vivo can be obtained only at the expense of some perturbation of the metabolic pathway studied.