Lipid-Induced Insulin Resistance in the Liver

Scutellarin, a modulator of mTOR, attenuates hepatic insulin resistance by regulating hepatocyte lipid metabolism via SREBP-1c suppression

High levels of consumption of saturated lipids have been largely associated with the increasing prevalence of metabolic diseases. In particular, saturated fatty acids such as palmitic acid (PA) have been implicated in the development of insulin resistance (IR). Scutellarin (Scu) is one of the effective traditional Chinese medicines considered beneficial for liver diseases and diabetes. In this study, we investigated the effect of Scu on IR and lipid metabolism disorders in vitro and in high fat diet (HFD)-fed mice. In vitro, we found that Scu decreased insulin-dependent lipid accumulation and the mRNA expression of CD36, Fasn, and ACC in PA-treated HepG2 cells. Additionally, Scu upregulated Akt phosphorylation and improved the insulin signalling pathway. Moreover, Scu downregulated mammalian target of rapamycin (mTOR) phosphorylation and the n-SREBP-1c protein level and also reduced lipid accumulation via the mTOR-dependent pathway, as confirmed by the molecular docking of Scu to mTOR. In HFD-fed C57BL/6 mice, Scu improved oral glucose tolerance, pyruvate tolerance and the IR index and also increased the Akt phosphorylation level. Moreover, Scu reduced hepatocyte steatosis, decreased lipid accumulation and triglyceride levels, inhibited mTOR phosphorylation, and decreased the SREBP-1c level in the liver. Taken together, these findings suggest that Scu ameliorates hepatic IR by regulating hepatocyte lipid metabolism via the mTOR-dependent pathway through SREBP-1c suppression.

Hypoglycemic and Hypolipidemic Effects of Phellinus Linteus Mycelial Extract from Solid-State Culture in A Rat Model of Type 2 Diabetes

Hypoglycemic and hypolipidemic effects of P. linteus have been observed in numerous studies, but the underlying molecular mechanisms are unclear. In this study, we prepared P. linteus extract (PLE) from mycelia of solid-state culture, and evaluated its hypoglycemic and hypolipidemic effects in rat models of high-fat diet (HFD)-induced and low-dose streptozotocin (STZ)-induced type 2 diabetes. PLE treatment effectively reduced blood glucose levels, and improved insulin resistance and lipid and lipoprotein profiles. The hypoglycemic effect of PLE was based on inhibition of key hepatic gluconeogenesis enzymes (FBPase, G6Pase) expression and hepatic glycogen degradation, and consequent reduction of hepatic glucose production. PLE also: (i) enhanced expression of CPT1A and ACOX1 (key proteins involved in fatty acid β-oxidation) and low-density lipoprotein receptor (LDLR) in liver, thus promoting clearance of triglycerides and LDL-C; (ii) inhibited expression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) in liver, thus reducing cholesterol production; (iii) displayed strong hepatoprotective and renal protective effects. Our findings indicate that PLE has strong potential functional food application in adjuvant treatment of type 2 diabetes with dyslipidemia.

The Proposal of Molecular Mechanisms of Weak Organic Acids Intake-Induced Improvement of Insulin Resistance in Diabetes Mellitus via Elevation of Interstitial Fluid pH

Blood contains powerful pH-buffering molecules such as hemoglobin (Hb) and albumin, while interstitial fluids have little pH-buffering molecules. Thus, even under metabolic disorder conditions except severe cases, arterial blood pH is kept constant within the normal range (7.35~7.45), but the interstitial fluid pH under metabolic disorder conditions becomes lower than the normal level. Insulin resistance is one of the most important key factors in pathogenesis of diabetes mellitus, nevertheless the molecular mechanism of insulin resistance occurrence is still unclear. Our studies indicate that lowered interstitial fluid pH occurs in diabetes mellitus, causing insulin resistance via reduction of the binding affinity of insulin to its receptor. Therefore, the key point for improvement of insulin resistance occurring in diabetes mellitus is development of methods or techniques elevating the lowered interstitial fluid pH. Intake of weak organic acids is found to improve the insulin resistance by elevating the lowered interstitial fluid pH in diabetes mellitus. One of the molecular mechanisms of the pH elevation is that: (1) the carboxyl group (R-COO⁻) but not H⁺ composing weak organic acids in foods is absorbed into the body, and (2) the absorbed the carboxyl group (R-COO⁻) behaves as a pH buffer material, elevating the interstitial fluid pH. On the other hand, high salt intake has been suggested to cause diabetes mellitus; however, the molecular mechanism is unclear. A possible mechanism of high salt intake-caused diabetes mellitus is proposed from a viewpoint of regulation of the interstitial fluid pH: high salt intake lowers the interstitial fluid pH via high production of H⁺ associated with ATP synthesis required for the Na⁺,K⁺-ATPase to extrude the high leveled intracellular Na⁺ caused by high salt intake. This review article introduces the molecular mechanism causing the lowered interstitial fluid pH and insulin resistance in diabetes mellitus, the improvement of insulin resistance via intake of weak organic acid-containing foods, and a proposal mechanism of high salt intake-caused diabetes mellitus.

Octaphlorethol A, a marine algae product, exhibits antidiabetic effects in type 2 diabetic mice by activating AMP-activated protein kinase and upregulating the expression of glucose transporter 4

Octaphlorethol A (OPA), a type of phlorotannin isolated from Ishige foliacea has been shown to have antidiabetic activities. However, the mechanism of action of OPA in type 2 diabetes has not been investigated extensively. Here, we investigated the antidiabetic effects and mechanism of OPA in C57BL/KsJ-db/db mice, a model of type 2 diabetes. Levels of postprandial blood glucose were significantly lower in OPAtreated db/db mice than in control db/db mice. In addition, the OPA supplements significantly improved fasting blood glucose level and impaired glucose tolerance compared to control db/db mice. OPA also significantly decreased the level of serum insulin, augmented the activation of AMP-activated protein kinase (AMPK), and increased the expression of glucose transporter 4 (GLUT4) protein in skeletal muscle. In addition, it significantly suppressed the increases in hepatic mRNA expression level of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), gluconeogenesis-related enzymes. Therefore, the mechanisms of OPA may involve suppression of gluconeogenesis by inhibiting PEPCK and G6Pase activity in the liver and affecting GLUT4-mediated glucose uptake in skeletal muscle through activation of AMPK. These findings provide a new insight into the antidiabetic clinical applications of OPA and demonstrate the potential of OPA as a new drug candidate for type 2 diabetes.

Proteomic profile of carbonylated proteins in rat liver: exercise attenuated oxidative stress may be involved in fatty liver improvement

To screen target proteins of oxidative stress which mediate the effects of exercise on preventing nonalcoholic fatty liver disease (NAFLD), the methods for selecting carbonylated proteins were modified, and carbonylated proteins were profiled. The results showed that treadmill training reduced oxidative stress and the levels of intrahepatic triglyceride (IHTG). The changes in IHTG showed a significant positive correlation with oxidative stress as indicated by malondialdehyde level. Further results from proteomics illustrated that 17 functional proteins were susceptible to oxidative modification, and exercise protected three proteins from carbonylation. The latter three proteins may serve as both direct target proteins of oxidative stress and mediators contributing to the beneficial effects of exercise. In particular, a long-chain specific acyl-CoA dehydrogenase (ACADL) which was a key enzyme in lipid metabolism was not carbonylated and with higher activities in exercise group. These findings indicate that this modified technique is practical and powerful in selecting carbonylated proteins. Long-term treadmill training is effective in ameliorating oxidative stress and preventing the accumulation of IHTG. Among the 17 target proteins of oxidative modification, three proteins contribute to the beneficial effects of exercise. Preventing ACADL from carbonylation may be involved in the physiological mechanism of exercise-induced NAFLD improvement.

Effects of brown alga, Ecklonia cava on glucose and lipid metabolism in C57BL/KsJ-db/db mice, a model of type 2 diabetes mellitus

Recently, there has been a growing interest in alternative therapies of marine algae for diabetes. Therefore, the anti-diabetic effects of brown alga, Ecklonia cava was investigated in type 2 diabetic animal. Male C57BL/KsJ-db/db (db/db) mice were divided into control, dieckol rich extract of E. cava (AG-dieckol), or rosiglitazone (RG) groups. The blood glucose, blood glycosylated hemoglobin levels, and plasma insulin levels were significantly lower in the AG-dieckol and RG groups than in the control db/db mice group, while glucose tolerance was significantly improved in the AG-dieckol group. AG-dieckol markedly lowered plasma and hepatic lipids concentration compared to the control db/db mice group. The antioxidant enzyme activities were significantly higher in the AG-dieckol group than in the control db/db mice group, yet its TBARS level was markedly lower compared to the RG group. With regard to hepatic glucose regulating enzyme activities, glucokinase activity was enhanced in the AG-dieckol group mice, while glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities in the AG-dieckol group mice were significantly lowered than those in the control db/db mice group. These results suggest that AG-dieckol exert an anti-diabetic effect in type 2 diabetic mice by improving the glucose and lipid metabolism and antioxidant enzymes.

Differential regulation of PGC-1alpha expression in rat liver and skeletal muscle in response to voluntary running

BACKGROUND

The beneficial actions of exercise training on lipid, glucose and energy metabolism and insulin sensitivity appear to be in part mediated by PGC-1alpha. Previous studies have shown that spontaneously exercised rats show at rest enhanced responsiveness to exogenous insulin, lower plasma insulin levels and increased skeletal muscle insulin sensitivity. This study was initiated to examine the functional interaction between exercise-induced modulation of skeletal muscle and liver PGC-1alpha protein expression, whole body insulin sensitivity, and circulating FFA levels as a measure of whole body fatty acid (lipid) metabolism.

METHODS

Two groups of male Wistar rats (2 Mo of age, 188.82 +/- 2.77 g BW) were used in this study. One group consisted of control rats placed in standard laboratory cages. Exercising rats were housed individually in cages equipped with running wheels and allowed to run at their own pace for 5 weeks. At the end of exercise training, insulin sensitivity was evaluated by comparing steady-state plasma glucose (SSPG) concentrations at constant plasma insulin levels attained during the continuous infusion of glucose and insulin to each experimental group. Subsequently, soleus and plantaris muscle and liver samples were collected and quantified for PGC-1alpha protein expression by Western blotting. Collected blood samples were analyzed for glucose, insulin and FFA concentrations.

RESULTS

Rats housed in the exercise wheel cages demonstrated almost linear increases in running activity with advancing time reaching to maximum value around 4 weeks. On an average, the rats ran a mean (Mean +/- SE) of 4.102 +/- 0.747 km/day and consumed significantly more food as compared to sedentary controls (P < 0.001) in order to meet their increased caloric requirement. Mean plasma insulin (P < 0.001) and FFA (P < 0.006) concentrations were lower in the exercise-trained rats as compared to sedentary controls. Mean steady state plasma insulin (SSPI) and glucose (SSPG) concentrations were not significantly different in sedentary control rats as compared to exercise-trained animals. Plantaris PGC-1alpha protein expression increased significantly from a 1.11 +/- 0.12 in the sedentary rats to 1.74 +/- 0.09 in exercising rats (P < 0.001). However, exercise had no effect on PGC-1alpha protein content in either soleus muscle or liver tissue. These results indicate that exercise training selectively up regulates the PGC-1alpha protein expression in high-oxidative fast skeletal muscle type such as plantaris muscle.

CONCLUSION

These data suggest that PGC-1alpha most likely plays a restricted role in exercise-mediated improvements in insulin resistance (sensitivity) and lowering of circulating FFA levels.

Exercise training ameliorates the effects of rosiglitazone on traditional and novel cardiovascular risk factors in patients with type 2 diabetes mellitus

The aim of the study was to investigate the effects of rosiglitazone and/or exercise training on novel cardiovascular risk factors in patients with type 2 diabetes mellitus. One hundred overweight/obese type 2 diabetes mellitus patients, with inadequate glycemic control (hemoglobin A(1c) >7%) despite combined treatment with gliclazide plus metformin, were randomized using a 2 x 2 factorial design to 4 equivalent (n = 25) groups, as follows: (1) CO: maintenance of habitual activities, (2) RSG: add-on therapy with rosiglitazone (8 mg/d), (3) EX: adjunctive exercise training, and (4) RSG + EX: supplementary administration of rosiglitazone (8 mg/d) plus exercise training. No participant had diabetic vascular complications or was receiving lipid-lowering therapy. Anthropometric parameters, cardiorespiratory capacity, glycemic and lipid profile, apolipoprotein (apo) A-I, apo B, interleukin (IL)-10, IL-18, insulin resistance, and blood pressure were measured before and after 12 months of intervention (P < .05). Both RSG and EX groups significantly reduced glycemic indexes, insulin resistance, blood pressure, and IL-18, whereas they significantly increased high-density lipoprotein, cardiorespiratory capacity, and IL-10, compared with CO group (P < .05). Besides this, exercise-treated patients conferred a remarkable down-regulation in the rest of lipid parameters (total cholesterol, low-density lipoprotein cholesterol, triglycerides, apo B) and body fat content (P < .05) in comparison with CO group. On the other hand, RSG group rather than CO group considerably increased apo A-I levels and body mass index (P < .05). Notably, the combined treatment group yielded pronounced beneficial changes in glycemic indexes, lipid profile, insulin resistance, blood pressure, IL-10, IL-18, apo A-I, and apo B (vs CO group, P < .05). Furthermore, the addition of exercise to rosiglitazone treatment counteracted the drug-related negative effects on body weight, low-density lipoprotein, and total cholesterol. Rosiglitazone plus exercise training elicited additive effects on body composition, glycemic control, and traditional and novel cardiovascular risk factors in type 2 diabetes mellitus patients, indicating complementary effects.

Independent effects of physical activity in patients with nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease, characterized by elevated liver enzymes, central obesity, and insulin resistance, is becoming increasingly prevalent. The effects of changes in physical activity on the metabolic profile of this group have not been reported. We assessed at 3 months the impact of a behavior change-based lifestyle intervention on physical activity and the effects of this change on the metabolic profile of people with fatty liver disease. In all, 141 participants with nonalcoholic fatty liver disease were prospectively enrolled into either a low- or moderate-intensity lifestyle intervention or to a control group. Physical activity was assessed using a validated reporting tool and physical fitness was measured using the YMCA protocol on a cycle ergometer. Individualized counseling to increase physical activity was provided. Overall, 96% of participants attended the 3-month follow-up assessment. Participants in the moderate- and low-intensity intervention groups were 9 times more likely to increase physical activity by an hour or more per week compared to controls. Patients increasing or maintaining their reported physical activity to > or =150 minutes/week, and those who increased their objective levels of fitness, had the greatest improvements in liver enzymes and other metabolic indices compared to those who were least active. This effect was independent of weight loss and was corroborated by an objective measure of fitness. There was no dose-response effect on liver enzymes with incremental increases in physical activity above 60 minutes/week.

CONCLUSION

Lifestyle counseling interventions are effective in improving physical activity behavior. Maintaining or increasing physical activity provides health benefits for patients with fatty liver, independent of changes in weight.

Genistein and daidzein modulate hepatic glucose and lipid regulating enzyme activities in C57BL/KsJ-db/db mice

This study examines whether anti-diabetic effects of genistein and daidzein are mediated by hepatic glucose and lipid regulating enzyme activities in type 2 diabetic animals. Male C57BL/KsJ-lepr(db)/lepr(db) (db/db) mice and age-matched non-diabetic littermates (db/+) were used in this study. The db/db mice were divided into control, genistein (0.02%, w/w) and daidzein (0.02%, w/w) groups. The blood glucose and HbA(1c) levels were significantly lower in the genistein and daidzein groups than in the control group, while glucose tolerance only was significantly improved in the genistein-supplemented group. The plasma insulin and C-peptide levels did not differ significantly between groups, yet the glucagon level was lower in the genistein and daidzein groups compared to that in the control db/db or db/+ group. The genistein and daidzein supplements increased the insulin/glucagon ratio in the type 2 diabetic animals. While the hepatic glucokinase activity was significantly lower in the db/db control group, the glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities were significantly higher in the control group compared to the db/+ group. Interestingly, these hepatic glucose metabolizing enzyme activities were reversed by the genistein and daidzein supplementation in db/db mice compared to the control group. The hepatic fatty acid synthase, beta-oxidation and carnitine palmitoyltransferase activities were all significantly lower in the genistein and daidzein groups than in the control group. The genistein and daidzein supplements also improved the plasma total cholesterol, triglyceride, HDL-cholesterol/total cholesterol, free fatty acid and hepatic triglyceride concentrations in the db/db mice. These results suggest that genistein and daidzein exert anti-diabetic effect in type 2 diabetic conditions by enhancing the glucose and lipid metabolism.