The relationship between glucose disposal in response to physiological hyperinsulinemia and basal glucose and free fatty acid concentrations in healthy volunteers

This study was initiated to see if defects in the ability of physiological hyperinsulinemia (approximately 60 microU/mL) to stimulate glucose uptake in healthy, nondiabetic volunteers are associated with increases in concentrations of plasma glucose and free fatty acid (FFA) when measured at basal insulin concentrations (approximately 10 microU/mL). We recruited 22 volunteers (12 women and 10 men) for these studies, with a (mean +/- SEM) body mass index of 24.8 +/- 0.5 kg/m2. Resistance to insulin-mediated glucose disposal during physiological hyperinsulinemia was determined by suppressing endogenous insulin and determining the steady-state plasma glucose (SSPG) and steady-state plasma insulin (SSPI) concentrations at the end of a 3-h infusion, period during which glucose (267 mg/m2 x min) and insulin (32 mU/m2 x min) were infused at a constant rate. Glucose, insulin and FFA concentrations were also measured in response to infusion rates of glucose (50 mg/m2 x min) and insulin (6 mU/m2 x min). The SSPI concentration (mean +/- SEM) during physiological hyperinsulinemia was 64 +/- 3 microU/mL), in contrast to 12 +/- 0.4 microU/mL during the basal insulin study. The results demonstrated a significant relationship between SSPG concentration in response to physiological hyperinsulinemia (SSPG60) and SSPG(Basal) (r = 0.57, P < 0.01) and FFA(Basal) (r = 0.73, P < 0.001). Furthermore, FFA(Basal) and SSPG(Basal) were significantly correlated (r = 0.47, P < 0.05). Comparison of the seven most insulin-resistant and seven most insulin sensitive individuals (SSPG60 values of 209 +/- 16 vs. 64 +/- 8 mg/dL) revealed that the insulin-resistant group also had significantly higher SSPG(Basal) (105 +/- 5 vs. 78 +/- 7 mg/dL, P < 0.01) and FFA(Basal) (394 +/- 91 vs. 104 +/- 41, P < 0.02) concentrations. However, random fasting plasma glucose and FFA concentrations of the two groups were not different. The results presented demonstrate that individual differences in the ability of elevated insulin concentrations to stimulate muscle glucose disposal are significantly correlated with variations in insulin regulation of plasma glucose and FFA concentrations at basal insulin concentrations.     

Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years

Competition for respiration between substrates in animal tissues has been known for at least 80 years. The most important interaction, quantitatively is between glucose and fatty acids. The starting point in 1963 for the so called Glucose Fatty Acid Cycle was the realisation that the metabolic relationship between glucose and fatty acids is reciprocal and not dependent. Glucose provision promotes glucose oxidation and glucose and lipid storage, and inhibits fatty acid oxidation. Provision of free fatty acids promotes fatty acid oxidation and storage, inhibits glucose oxidation and may promote glucose storage if glycogen reserves are incomplete. This review is concerned predominantly with evidence in man in vivo. In the authors opinion the evidence for inhibitory effects of fatty acids on whole body glucose utilization ad oxidation (predominantly muscles) is decisive and enzyme mechanisms mediating these effects are well established. There is also much evidence that fatty acid oxidation inhibits glucose oxidation and stimulates glucose formation in liver and again enzyme mechanism are known. A permissive role for fatty acids in the insulin secretory response of islet beta-cells has now been firmly established and can be visualised as a mechanism to protect continuing provision of respiratory substrate. Longer term exposure of islet beta-cells to fatty acids impairs the insulin secretory response to glucose and mechanisms are known. There is compelling evidence that fatty acid oxidation may impair glucose oxidation in uncontrolled Type 1 and Type 2 diabetes, but no convincing evidence that fatty acids have a role in diminished glucose storage (glycogen deposition) in Type 2 diabetes. The inhibition of glucose storage which may follow prolonged elevation of plasma FFA in man and experimental animals is associated with glycogen repletion whereas the inhibition of glucose storage in Type 2 diabetes is associated with glycogen depletion. The precise role of fatty acids in disturbed carbohydrate metabolism in Type 2 diabetes is an area where future progress is confidently predicted.     

Immunosuppressive agents: effects on glucose and lipid metabolism.

Immunosuppressive therapies are critical elements in successful organ transplantation. Although immunosuppressant drugs are essential in preventing graft rejection and graft maintenance after transplantation, their use is complicated by adverse effects, many being detrimental to graft and even patient long-term survival. Commonly used agents are associated with dysregulated glucose metabolism and dyslipidemia. This article focuses on the effects of immunosuppressive agents on glucose and lipid metabolism. Adrenal effects of these drugs, where known, also are discussed.     

Differences in free fatty acid and glucose metabolism of human blood neutrophils and lymphocytes.

Comparison of isolated human neutrophils and lymphocytes in short-term tissue culture revealed marked differences in their rates of lipid biosynthesis. Ficoll-Hypaque gradients were used to separate lymphocytes and neutrophils from the blood of normal subjects. Neutrophils incorporated more palmitate into cell lipids (151.0 +/- 16.6 nmole/hr/10(8) cells) than lymphocytes (41.6 +/- 4.1). By contrast, the lymphocytes oxidized more palmitate (8.3 +/- 0.5 nmole/hr/10(8) cells) as compared to neutrophils (1.1 +/- 0.1). The greater fatty acid uptake by the neutrophils was due to a sixfold greater rate of incoporation of palmitate into their triglyceride fraction. Triglyceride synthesis by neutrophils increased as the molar ratio of free fatty acid to albumin was raised, whereas incorporation into phospholipids remained relatively constant; there was preferential labeling of neutrophil triglycerides throughout the physiologic range. Studies using linoleate and oleate gave similar results. The distribution of radioactivity into various phospholipids determined by thin-layer chromatography was similar for the two cell types. When labeled glucose was used as a substrate to measure incorporation primarily into the glycerol backbone of the cell lipids, neutrophils incorporated more radioactivity into total lipids and triglycerides than lymphocytes. These results indicate that neutrophils take up much more fatty acid than lymphocytes primarily because they synthesize much larger quantities of triglycerides, a storage form. Since cellular triglycerides may act as a source of fatty acid for lecithin synthesis during phagocytosis, the greater rate of fatty acid incorporation in the neutrophil may reflect a metabolic pattern that permits efficient phagocytosis.     

High glucose,insulin and free fatty acid concentrations synergistically enhance perilipin 3 expression and lipid accumulation in macrophages

Objective

Perilipin (PLIN) 3, an intracellular lipid droplet (LD)-associated protein, is implicated in foam cell formation. Since metabolic derangements found in metabolic syndrome, such as high serum levels of glucose, insulin and free fatty acids (FFAs), are major risk factors promoting atherosclerosis, we investigated whether PLIN3 expression is affected by glucose, insulin and oleic acid (OA) using RAW264.7 cells.

Methods

Real-time PCR and Western blotting were performed to detect PLIN3 or PLIN2 expression. Oil-red O staining and Lipid Analysis were employed to measure cellular content of triacylglycerides (TAG) and cholesterol.

Results

PLIN3 mRNA was stimulated by high glucose or insulin concentrations individually, but not by OA. A combination of any two factors did not enhance PLIN3 expression any more than that evoked by glucose alone at 24 h. Interestingly, however, simultaneous addition of all three factors synergistically enhanced the PLIN3 expression. This synergistic effect was not apparent for PLIN2 mRNA expression. Inhibitors of Src family tyrosine kinase and/or phosphatidylinositol 3-kinase, both of which are activated by insulin and FFA signaling, partially suppressed PLIN3 expression induced by the combination of the three factors. While simultaneous addition of glucose, insulin and OA remarkably increased the cellular content of TAG and cholesterol, knocking-down of PLIN3 predominantly reduced TAG content.

Conclusions

These results indicate that PLIN3 expression is synergistically stimulated by high glucose, insulin and FFA concentrations, in parallel with TAG accumulation in macrophages. This finding raises new evidence of PLIN3 involvement in conversion of macrophages into foam cells     

Insulin sensitivity indices of glucose and free fatty acid metabolism in obese children and adolescents in relation to serum lipids

OBJECTIVE: Most studies concerning the association between insulin resistance and the features of metabolic syndrome in obese children are based on measurement of insulin sensitivity indices (ISI) of glucose metabolism and not of fat metabolism. METHODS: We studied fasting low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglycerides, insulin, free fatty acids (FFA), blood glucose, ISI of glucose (homeostasis model assessment [HOMA] %S), and FFA metabolism (ISI-FFA) in 124 obese children aged 6 to 16 years. ISI-FFA was calculated based on the formula 2/(insulin x FFA + 1). Stepwise forward regression analyses were performed with triglycerides, HDL-C and LDL-C as dependent variables and age, sex, stage of puberty, body mass index, insulin, FFA, and blood glucose as independent variables. Direct multiple regression analyses were conducted with the dependent variables triglycerides, HDL-C, and LDL-C including age, sex, stage of puberty, body mass index, HOMA %S, and ISI-FFA as independent variables. Furthermore, ISI-FFA was measured in 13 normal-weight children aged 6 to 16 years. RESULTS: ISI-FFA (median 0.30) was significantly (P < .05) reduced in obese children compared with normal-weight children (median ISI-FFA 0.64). In stepwise regression analyses, triglycerides were significantly correlated with insulin and FFA (P < .05), LDL-C levels were significantly correlated with FFA (P < .05), and HDL-C was related to stage of puberty (P < .05), whereas all other variables demonstrated no significant associations with triglycerides, LDL-C, and HDL-C levels. In contrast to HOMA %S, ISI-FFA was significantly (P < .05) related to triglycerides and LDL-C in direct multiple regression analysis. CONCLUSIONS: Insulin resistance in respect to FFA metabolism is already detectable in childhood. Insulin sensitivity index of FFA metabolism seems to be a better tool for describing insulin resistance in lipid metabolism than ISI of glucose metabolism, because FFA and partially insulin but not glucose were related to triglycerides and LDL-C.     

Effect of differences in glucose tolerance on insulin's ability to regulate carbohydrate and free fatty acid metabolism in obese individuals

The effect of variations in glucose tolerance on insulin's ability to regulate glucose uptake and plasma glucose and FFA concentrations was assessed in 22 obese individuals [8 with normal glucose tolerance, 7 with impaired glucose tolerance (IGT), and 7 with noninsulin-dependent diabetes mellitus (NIDDM)]. Patients with IGT had ambient insulin levels that were higher than normal, associated with elevated postprandial glucose levels and a marked reduction in insulin-stimulated glucose uptake. On the other hand, plasma FFA levels were relatively normal in IGT, possibly because of the hyperinsulinemia. Patients with NIDDM were also hyperinsulinemic, with insulin levels throughout the day that were approximately twice normal. Hyperinsulinemia in patients with NIDDM was associated with a significant decline in insulin-stimulated glucose uptake as well as with significant increases in both ambient plasma glucose and FFA concentrations. Thus, and in contrast to patients with IGT, plasma FFA metabolism in NIDDM was grossly abnormal, despite the concomitant hyperinsulinemia. These data indicate that insulin resistance in obese individuals varies as a function of degree of glucose tolerance, and insulin resistance in patients with NIDDM involves defects in the regulation of both plasma glucose and FFA metabolism.     

Regulation of glucose metabolism by free fatty acid availability in septic and nonseptic rats.

An inhibitory effect of fatty acid oxidation on glucose uptake and oxidation has been demonstrated in heart and skeletal muscle under certain experimental conditions. This reciprocal relationship has been termed the glucose-fatty acid cycle. The purpose of the present study was to determine under in vivo conditions whether this interaction was operational in various nonmuscle tissues, and whether infection altered the activity of this cycle. Oral administration of alpha-methylpalmoxirate (MPA; 75 mg/kg), a known inhibitor of long-chain fatty acid oxidation, suppressed hepatic glucose production by 54% and increased whole body glucose disappearance by 15% in nonseptic rats. In contrast, MPA produced a larger reduction of glucose output in septic rats, but did not enhance glucose disposal. In vivo glucose uptake (Rg) by individual tissues was determined using the tracer 2-deoxyglucose technique. In nonseptic animals, MPA increased Rg in "working" muscles (heart and diaphragm; 12-fold and two-fold respectively), but not in "resting" skeletal muscles. MPA increased the Rg of heart and diaphragm to the same level in septic animals. Inhibition of fatty acid oxidation in nonseptic rats also enhanced Rg in liver (174%), spleen (158%), lung (153%), ileum (52%), skin (28%), kidney (115%), and epididymal fat (135%). In septic rats, MPA only increased Rg in the ileum (23%) and kidney (50%). This increased glucose uptake was independent of increases in plasma glucose and insulin concentrations. The infusion of heparin and intralipid, which increased circulating levels of fatty acids, failed to produce consistent changes in either the whole body glucose turnover or glucose uptake by individual tissues. We conclude that under basal in vivo conditions the inhibition of fatty acid oxidation suppresses glucose production and increases peripheral glucose disposal in nonseptic animals. These data support the presence of the glucose-fatty acid cycle in nonmuscle tissues and emphasizes its importance in whole body glucose homeostasis in situations where fatty acid oxidation is impaired. Infection increases glucose uptake by nonmuscle tissues which, for the most part, cannot be further enhanced by the inhibition of lipid oxidation.     

Effects of lowering circulating free fatty acid levels on protein metabolism in adult growth hormone deficient patients.

Our study was conducted to define the roles of lowering circulating free fatty acids (FFA) and of growth hormone (GH) replacement on protein metabolism in GH deficient patients.To isolate the specific effects of FFA and GH we studied seven adult subjects with GH deficiency four times: (A) with administration of GH and Acipimox (an inhibitor of lipolysis), (B) with GH, without Acipimox, (C) without GH, with Acipimox and (D) without either. Each study included a 3 h basal period and a 3 h euglycemic clamp. Amino acid metabolism was assessed by stable isotope dilution technique at the whole body level and across the forearm. Overall, we saw no intervention effect on protein metabolism, but when the two situations in which Acipimox was given were combined, Acipimox decreased basal plasma FFA concentrations by 75% and increased serum urea concentrations by 20%, whole body appearance rates (reflecting protein degradation) of phenylalanine (by 7%) and tyrosine (by 11%) and protein synthesis rates for phenylalanine (by 7%), whereas phenylalanine-to-tyrosine conversion was unaffected. Acipimox more than doubled net forearm phenylalanine release during the clamp and increased basal forearm phenylalanine disappearance (reflecting muscle protein synthesis). During the clamp whole body amino acid fluxes and phenylalanine-to-tyrosine conversion decreased together with a decrease in forearm protein breakdown. GH replacement did not affect any of these metabolic parameters. Although we failed to show any role for GH, the results show that lowering of FFA concentrations with Acipimox has pronounced effects on protein metabolism, including increased whole body and forearm protein breakdown, together with increased protein synthesis systemically and locally in the forearm. The increase in serum urea and a doubling of net forearm phenylalanine release after lowering of FFA strongly indicate that the overall effect is catabolic and supports a pivotal protein conserving role of lipids.     

Effect of heparin on plasma free fatty acid concentrations after acute myocardial infarction.

Free fatty acid concentrations in plasma measured after in vivo heparinisation are often overestimated because of ex vivo lipolysis of variable degrees. A new method has been developed using immediate extraction of blood which obviates this and shows that the true rise in plasma free fatty acid concentration after heparin in ambulant ward patients and in patients with acute myocardial infarction is less than previously reported. The small rise in plasma free fatty acid concentration after heparin is unlikely to have adverse metabolic effects in most patients during acute myocardial infarction.     

The Farnesoid X receptor: a molecular link between bile acid and lipid and glucose metabolism

Bile acids are the end products of cholesterol metabolism. They are synthesized in the liver and secreted via bile into the intestine, where they aid in the absorption of fat-soluble vitamins and dietary fat. Subsequently, bile acids return to the liver to complete their enterohepatic circulation. The Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and has emerged as a key player in the control of multiple metabolic pathways. On its activation by bile acids, FXR regulates bile acid synthesis, conjugation, and transport, as well as various aspects of lipid and glucose metabolism. This review summarizes recent advances in deciphering the role of FXR in the context of hepatic lipid and glucose homeostasis and discusses the potential of FXR as a pharmacological target for therapeutic applications.     

葡萄糖转运子9与尿酸代谢间关系的研究进展

生物体内尿酸代谢异常可导致多种疾病的发生.新近发现,葡萄糖转运子家族中的葡萄糖转运子9除了转运葡萄糖外,还在尿酸的转运中起重要作用,其转运过程还受到pH、葡萄糖和雌激素等的影响.而葡萄糖转运子9的变异也会造成代谢病的发生.因此,针对葡萄糖转运子9的尿酸转运机制和相关药物的研究,将为临床上控制高尿酸血症和相关心血管病的发生提供新的思路.     

Resistance to insulin mediated glucose disposal in obese subjects: respective effect of lipid metabolism and glycemia.

The present study was designed to assess the respective effect of altered lipid metabolism and hyperglycemia on glucose metabolism in vivo in obese subjects. Six young obese non-diabetic volunteers were studied on four occasions during hyperinsulinemic clamp, twice during euglycemia and twice during hyperglycemia, with or without the infusion of beta-pyridylcarbinol, an inhibitor of lipid metabolism. Glucose oxidation was calculated from continuous respiratory exchange measurements, and glucose storage was obtained as the difference between total glucose disposal and glucose oxidation. Two-way analysis of variance (with interaction term) demonstrated (i) a significant increase for total glucose disposal with beta-pyridylcarbinol but no significant effect of hyperglycemia and no interaction between the two treatments, and (ii) an important increase of beta-pyridylcarbinol to enhance glucose storage but no significant effect of hyperglycemia and no interaction between the two treatments. These results show that obese people, at physiological insulinemia, enhance their glucose disposal and glucose storage when lipid oxidation is artificially lowered. This suggests that enhanced lipid oxidation is related to insulin resistance in these patients. However, hyperglycemia in these patients failed to compensate for defective glucose disposal or storage.     

Comparison of the pattern of postprandial carbohydrate metabolism after ingestion of a glucose drink or a mixed meal

To determine whether the postprandial pattern of carbohydrate metabolism differs after ingestion of an identical amount of glucose as either a drink or as a part of a mixed meal, normal subjects were studied on two occasions. On both occasions, hepatic and extrahepatic glucose metabolism were assessed using the dual isotope and forearm catheterization techniques as well as indirect calorimetry. Plasma glucose, insulin, and C-peptide concentrations and rate of systemic entry of ingested glucose all were lower (P less than 0.05) during the first 15 min after the mixed meal than after the glucose drink. The integrated C-peptide response was greater (P less than 0.05) after the mixed meal, whereas the integrated suppression of glucagon was greater (P less than 0.05) after the glucose drink. Despite these differences in circulating hormone concentrations, after the first 15 min, the rates of systemic entry of ingested glucose, endogenous glucose release, incorporation of carbon dioxide into glucose, and glucose and lipid oxidation as well as nonoxidative glucose storage were virtually the same after the mixed meal and the glucose drink. We conclude that the pattern of postprandial carbohydrate metabolism after ingestion of a glucose meal is remarkably similar to that after ingestion of a more traditional mixed meal. These data suggest that insights regarding the pattern of postprandial carbohydrate metabolism derived from previous studies employing only a glucose drink are likely to pertain to those observed when healthy individuals ingest a meal that contains protein and fat.     

Effect of novel glucose lowering agents on non-alcoholic fatty liver disease: A systematic review and meta-analysis

BackgroundThe efficacy of novel glucose-lowering drugs in treating non-alcoholic fatty liver disease (NAFLD) in patients with and without type-2 diabetic patients (T2DM) remains unclear.AimTo conduct a meta-analysis to evaluate the efficacy of 3 novel glucose-lowering drug classes, namely glucagon-like peptide-1 receptor agonists (GLP-1RA), sodium-glucose co-transporter 2 (SGLT2) inhibitors, and dipeptidyl-peptidase-4 (DPP4) inhibitors on hepatic parameters: Aspartate Aminotransferase (AST), Alanine Aminotransferase (ALT), Gamma-Glutamyl Transferase (GGT), Bilirubin, and FIB-4 (Fibrosis).MethodsMEDLINE was searched from inception through October 2021 for randomized placebo or active glucose-lowering drug-controlled trials. A random-effects model was used to pool the results. A p-value of less than or equal to 0.05 was considered significant. Results were presented as weighted mean differences (WMD) and corresponding 95% confidence intervals (CIs).ResultsOur pooled analysis consisted of 40 studies. A significant reduction was seen in AST with SGLT2 inhibitors (WMD = -2.31 IU/L, 95%CI: -3.16 to -1.47 IU/L, P < 0.00001) and GLP-1RA (WMD = -3.29 IU/L, 95%CI: -5.98 to -0.61 IU/L, P = 0.02). Similarly, significant reduction was seen in ALT with SGLT2 inhibitors (WMD = -5.93 IU/L, 95%CI: -7.70 to -4.16 IU/L, P < 0.00001) and GLP-1RAs (WMD = -9.92 IU/L, 95%CI: -19.89 to 0.05 IU/L, P = 0.05). In contrast, DPP-4 inhibitors showed no significant reduction in AST (WMD = -3.20 IU/L, 95%CI: -11.13 to 4.73 IU/L, P = 0.43) or ALT (WMD = -4.81 IU/L, 95%CI: -15.83 to 6.21 IU/L, P = 0.39). A significant reduction in GGT was seen with SGLT2 inhibitors (WMD = -6.49 IU/L, 95%CI: -11.09 to -1.89 IU/L, P = 0.006) and GLP-1RAs (WMD = -12.38 IU/L, 95%CI: -15.69 to -9.07 IU/L, P < 0.00001). However, significant results were not observed with DPP-4 inhibitors (WMD = -0.92 IU/L, 95%CI: -5.80 to 3.96 IU/L, P = 0.71). There was a statistically significant reduction in FIB-4 index with SGLT2 inhibitors (WMD = -0.21, 95%CI: -0.40 to -0.03, P = 0.02) and GLP-1 RA (WMD = -0.15, 95%CI: -0.29 to 0.00, P = 0.05). Lastly, SGLT2 inhibitors led to a significant change in bilirubin levels (WMD = 2.03, 95%CI: 0.76 to 3.30, P = 0.002) while the change in bilirubin was not significant with GLP-1 agonists (WMD = -0.21, 95%CI: -1.09 to 0.66, P = 0.63) and DPP-4 inhibitors (WMD = 0.14, 95%CI: -1.55 to 1.83, P = 0.87).ConclusionSGLT2 inhibitors and GLP-1 agonists have a beneficial effect on hepatic parameters in patients with NAFLD. However, further research is needed to evaluate the effect of DPP-4 inhibitors on hepatic function properly.