Insulin Clearance Is Associated with Hepatic Lipase Activity and Lipid and Adiposity Traits in Mexican Americans

Insulin resistance and metabolic flexibility as drivers of liver and cardiac disease in T2DM

Metabolic flexibility refers to the ability of tissues to adapt their use of energy sources according to substrate availability and energy demands. This review aims to disentangle the emerging mechanisms through which altered metabolic flexibility and insulin resistance promote NAFLD and heart disease progression. Insulin resistance and metabolic inflexibility are central drivers of hepatic and cardiac diseases in individuals with type 2 diabetes. Both play a critical role in the complex interaction between glucose and lipid metabolism. Disruption of metabolic flexibility results in hyperglycemia and abnormal lipid metabolism, leading to increased accumulation of fat in the liver, contributing to the development and progression of NAFLD. Similarly, insulin resistance affects cardiac glucose metabolism, leading to altered utilization of energy substrates and impaired cardiac function, and influence cardiac lipid metabolism, further exacerbating the progression of heart failure. Regular physical activity promotes metabolic flexibility by increasing energy expenditure and enabling efficient switching between different energy substrates. On the contrary, weight loss achieved through calorie restriction ameliorates insulin sensitivity without improving flexibility. Strategies that mimic the effects of physical exercise, such as pharmacological interventions or targeted lifestyle modifications, show promise in effectively treating both diabetes and NAFLD, finally reducing the risk of advanced liver disease.

When Sugar Reaches the Liver: Phenotypes of Patients with Diabetes and NAFLD

Type 2 diabetes mellitus (T2DM) and non-alcoholic fatty liver disease (NAFLD) have been traditionally linked to one another. Recent studies suggest that NAFLD may be increasingly common in other types of diabetes such as type 1 diabetes (T1DM) and less frequently ketone-prone and Maturity-onset Diabetes of the Young (MODY) diabetes. In this review, we address the relationship between hyperglycemia and insulin resistance and the onset and progression of NAFLD. In addition, despite the high rate of patients with T2DM and other diabetes phenotypes that can alter liver metabolism and consequently develop steatosis, fibrosis, and cirrhosis, NALFD screening is not still implemented in the daily care routine. Incorporating a clinical algorithm created around a simple, non-invasive, cost-effective model would identify high-risk patients. The principle behind managing these patients is to improve insulin resistance and hyperglycemia states with lifestyle changes, weight loss, and new drug therapies.

Insulin clearance and incretin hormones following oral and "isoglycemic" intravenous glucose in type 2 diabetes patients under different antidiabetic treatments

It has not been elucidated whether incretins affect insulin clearance in type 2 diabetes (T2D). We aimed exploring possible associations between insulin clearance and endogenously secreted or exogenously administered incretins in T2D patients. Twenty T2D patients were studied (16 males/4 females, 59 ± 2 years (mean ± standard error), BMI = 31 ± 1 kg/m2, HbA1c = 7.0 ± 0.1%). Patients were treated with metformin, sitagliptin, metformin/sitagliptin combination, and placebo (randomized order). On each treatment period, oral and isoglycemic intravenous glucose infusion tests were performed (OGTT, IIGI, respectively). We also studied twelve T2D patients (9 males/3 females, 61 ± 3 years, BMI = 30 ± 1 kg/m2, HbA1c = 7.3 ± 0.4%) that underwent infusion of GLP-1(7-36)-amide, GIP, GLP-1/GIP combination, and placebo. Plasma glucose, insulin, C-peptide, and incretins were measured. Insulin clearance was assessed as insulin secretion to insulin concentration ratio. In the first study, we found OGTT/IIGI insulin clearance ratio weakly inversely related to OGTT/IIGI total GIP and intact GLP-1 (R2 = 0.13, p < 0.02). However, insulin clearance showed some differences between sitagliptin and metformin treatment (p < 0.02). In the second study we found no difference in insulin clearance following GLP-1 and/or GIP infusion (p > 0.5). Thus, our data suggest that in T2D there are no relevant incretin effects on insulin clearance. Conversely, different antidiabetic treatments may determine insulin clearance variations.

Purendan alleviates non-alcoholic fatty liver disease in aged type 2 diabetic rats via regulating mTOR/S6K1/SREBP-1c signaling pathway

Older people are more likely to develop insulin resistance and lipid metabolism disorders. Purendan (PRD) is a clinically verified traditional Chinese medicine compound, which plays an obvious role in regulating lipid metabolism disorder and improving insulin sensitivity. Our study aimed to investigate the efficacy and mechanism of PRD on aged type 2 diabetes mellitus (T2DM) complicated with non-alcoholic fatty liver disease (NAFLD) rats. Sprague-Dawley rats (13 months) were fed with high-fat diet (HFD) and injected with low-dose STZ to replicate T2DM model. PRD was treated at three concentrations with metformin as a positive control. After administration, blood and liver tissue samples were collected to measure glucose metabolism indexes such as serum glucose and insulin, as well as lipid metabolism indexes such as TC, TG, LDL, HDL and FFA. Liver fat accumulation was observed by HE staining and oil red O staining. And protein expression levels of mTOR, p-mTOR, S6K1, p-S6K1 and SREBP-1c were detected by western blot. After PRD treatment, not only the insulin sensitivity and insulin resistance were significantly improved, but also the TC, TG, LDL, FFA, AST and ALT in serum and the lipid accumulation in liver tissue were significantly decreased. Moreover, PRD significantly down-regulated the expression of p-mTOR, p-S6K1 and SREBP-1c in liver tissues. In conclusion, PRD can alleviate NAFLD in aged T2DM rats by inhibiting the mTOR /S6K1/ SREBP-1c pathway.

Exercise Training Rapidly Increases Hepatic Insulin Extraction in NAFLD

PURPOSE

We aimed to determine the immediacy of exercise intervention on liver-specific metabolic processes in nonalcoholic fatty liver disease.

METHODS

We undertook a short-term (7-d) exercise training study (60 min·d treadmill walking at 80%-85% of maximal heart rate) in obese adults (N = 13, 58 ± 3 yr, 34.3 ± 1.1 kg·m, >5% hepatic lipid by H-magnetic resonance spectroscopy). Insulin sensitivity index was estimated by oral glucose tolerance test using the Soonthorpun model. Hepatic insulin extraction (HIE) was calculated as the molar difference in area under the curve (AUC) for insulin and C-peptide (HIE = 1 - (AUCInsulin/AUCC-Pep)).

RESULTS

The increases in HIE, V˙O2max, and insulin sensitivity index after the intervention were 9.8%, 9.8%, and 34%, respectively (all, P < 0.05). Basal fat oxidation increased (pre: 47 ± 6 mg·min vs post: 65 ± 6 mg·min, P < 0.05) and carbohydrate oxidation decreased (pre: 160 ± 20 mg·min vs post: 112 ± 15 mg·min, P < 0.05) with exercise training. After the intervention, HIE correlated positively with adiponectin (r = 0.56, P < 0.05) and negatively with TNF-α (r = -0.78, P < 0.001).

CONCLUSIONS

By increasing HIE along with peripheral insulin sensitivity, aerobic exercise training rapidly reverses some of the underlying physiological mechanisms associated with nonalcoholic fatty liver disease, in a weight loss-independent manner. This reversal could potentially act through adipokine-related pathways.

The Relation of Omentin-1 Levels and Some Trace Elements as a Potential Markers for Diagnosis of Prediabetic Obese Patients

The obesity is one of the most common physiological disorders, also it is linked with a variety of circumstances like hypertension, dyslipidemia, T2DM, non-alcoholic fatty liver disease, and cardiovascular diseases. The aim of this study was to check the changes and compare serum omentin-1, lipid profile (TC, TG, VLDL-C, LDL-C, and HDL-C), trace elements (Mg, Zn, Cu, Fe) and insulin resistance between the prediabetic obese patients, healthy obese, and control subjects and assess the correlation between omentin-1, lipid profile, trace elements (Mg, Zn, Cu, Fe) levels and insulin resistance in prediabetic obese patients. Correlation analysis between omentin-1 levels with the biochemical parameters in the prediabetic obese patients with WC, body mass index, FBG, Insulin, HOMA-IR, HbA1c, TG, TC, LDL-C, VLDL-C, and copper was found to be negative; while significant positive correlation of omentin-1 with HDL-C, Mg, Zn, and Fe was noticed. Omentin-1 serum concentration level decrease and insulin resistance increased in prediabetic obese patients compared to healthy obese and control individuals. Omentin-1 inversely associated with obesity and insulin resistance, therefore can be used as a biomarker for obesity related metabolic disorders.

Hepatic Insulin Extraction in NAFLD Is Related to Insulin Resistance Rather Than Liver Fat Content

CONTEXT

Total insulin clearance is decreased in nonalcoholic fatty liver disease (NAFLD), but the relationship between liver fat and hepatic insulin extraction (HIE) is unknown.

OBJECTIVE

This cross-sectional study addresses the hypothesis that HIE is reduced in NAFLD and investigates metabolic and/or anthropometric characteristics most closely associated with insulin clearance.

PARTICIPANTS

Nondiabetic subjects with NAFLD (n = 13) and age- and body mass index (BMI)-matched controls with normal liver enzymes (n = 15) underwent abdominal CT, dual-energy X-ray absorptiometry, oral glucose tolerance test (OGTT), and labeled two-step hyperinsulinemic-euglycemic clamps.

OUTCOME MEASUREMENTS

Liver fat was estimated by the CT liver/spleen ratio. Hepatic and extrahepatic insulin clearances were modeled using clamp and OGTT data.

RESULTS

Extrahepatic insulin clearance and HIE were not different between NAFLD and controls and did not correlate with liver fat. HIE was positively correlated with insulin sensitivity [rate of glucose disposal (Rd; low r = +0.7, P < 0.001; high r = +0.6, P = 0.001), adiponectin (r = +0.55, P = 0.004), and insulin-mediated suppression of clamp nonesterified free fatty acid (NEFA; r = +0.67, P < 0.001)] but was not associated with fasting NEFA, insulin-mediated suppression of glucose production, or measures of adiposity. Extrahepatic insulin clearance was positively associated with percent body fat (r = +0.44, P = 0.02) and subcutaneous fat (r = +0.42, P = 0.03) but not BMI, intra-abdominal fat, liver fat, Rd, adiponectin, or NEFA.

CONCLUSIONS

HIE is not directly associated with hepatic steatosis but is associated with muscle and adipose tissue insulin resistance. The data suggest differential regulation of insulin clearance with extrahepatic insulin clearance being associated with body fat and not insulin sensitivity.

Hepatitis C virus infection is associated with hepatic and adipose tissue insulin resistance that improves after viral cure

BACKGROUND

Chronic hepatitis C (CHC) is associated with systemic insulin resistance, yet there are limited data on the tissue-specific contribution in vivo to this adverse metabolic phenotype, and the effect of HCV cure.

METHODS

We examined tissue-specific insulin sensitivity in a cohort study involving 13 patients with CHC compared to 12 BMI-matched healthy control subjects. All subjects underwent a two-step clamp incorporating the use of stable isotopes to measure carbohydrate and lipid flux (hepatic and global insulin sensitivity) with concomitant subcutaneous adipose tissue microdialysis and biopsy (subcutaneous adipose tissue insulin sensitivity). Investigations were repeated in seven patients with CHC following antiviral therapy with a documented sustained virological response.

RESULTS

Adipose tissue was more insulin resistant in patients with CHC compared to healthy controls, as evidence by elevated glycerol production rate and impaired insulin-mediated suppression of both circulating nonesterified fatty acids (NEFA) and adipose interstitial fluid glycerol release during the hyperinsulinaemic euglycaemic clamp. Hepatic and muscle insulin sensitivity were similar between patients with CHC and controls. Following viral eradication, hepatic insulin sensitivity improved as demonstrated by a reduction in endogenous glucose production rate. In addition, circulating NEFA decreased with sustained virological response (SVR) and insulin was more effective at suppressing adipose tissue interstitial glycerol release with a parallel increase in the expression of insulin signalling cascade genes in adipose tissue consistent with enhanced adipose tissue insulin sensitivity.

CONCLUSION

Chronic hepatitis C patients have profound subcutaneous adipose tissue insulin resistance in comparison with BMI-matched controls. For the first time, we have demonstrated that viral eradication improves global, hepatic and adipose tissue insulin sensitivity.

Adiposity-Independent Effects of Aging on Insulin Sensitivity and Clearance in Mice and Humans

OBJECTIVE

Aging is associated with impaired insulin sensitivity and increased prevalence of type 2 diabetes. However, it remains unclear whether aging-associated insulin resistance is due to increased adiposity or other age-related factors. To address this question, the impact of aging on insulin sensitivity was investigated independently of changes in body composition.

METHODS

Cohorts of mice aged 4 to 8 months ("young") and 18 to 27 months ("aged") exhibiting similar body composition were characterized for glucose metabolism on chow and high-fat diets. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp analyses. The relationship between aging and insulin resistance in humans was investigated in 1,250 nondiabetic Mexican Americans who underwent hyperinsulinemic-euglycemic clamps.

RESULTS

In mice with similar body composition, age had no detrimental effect on plasma glucose and insulin levels. While aging did not diminish glucose tolerance, hyperinsulinemic-euglycemic clamps demonstrated impaired insulin sensitivity and reduced insulin clearance in aged mice on chow and high-fat diets. Consistent with results in the mouse, age remained an independent determinant of insulin resistance after adjustment for body composition in Mexican American males.

CONCLUSIONS

This study demonstrates that in addition to altered body composition, adiposity-independent mechanisms also contribute to aging-associated insulin resistance in mice and humans.

Species differences in lipoprotein lipase and hepatic lipase activities: comparative studies of animal models of lifestyle-related diseases

Lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) have an important role in lifestyle-related diseases. To evaluate species differences, we compared LPL and HTGL activities in different animal models of lifestyle-related diseases using the same assay kit. Normal animals (JW rabbits, ICR mice, and SD rats), a hypercholesterolemic animal model (WHHLMI rabbits), and obese animal models (KK-A^y mice and Zucker fatty rats) fed standard chow were used in this study. Plasma was prepared before and after an intravenous injection of heparin sodium under fasting and feeding. LPL and HTGL activities were measured with the LPL/HTGL activity assay kit (Immuno-Biological Laboratories) using an auto-analyzer. Only in mice, high HTGL activity was observed in pre-heparin plasma. In normal animals, LPL and HTGL activities were high in ICR mice and SD rats but low in JW rabbits. Compared to normal animals, LPL activity was high in Zucker fatty rats and WHHLMI rabbits at both fasting and feeding, while LPL activity after feeding was low in KK-A^y mice. HTGL activity was higher in fasted and fed WHHLMI rabbits and fasted Zucker fatty rats, but was lower in fed KK-A^y mice. Gender difference was observed in HTGL activity in SD rats and LPL activity in WHHLMI rabbits but not in ICR mice. In conclusion, this simple assay method was effective for measuring LPL and HTGL activities of experimental animals, and the activities are highly regulated depending on animal species, animal models, feeding/fasting conditions and genders.

Determinants of longitudinal change in insulin clearance: the Prospective Metabolism and Islet Cell Evaluation cohort

OBJECTIVE

To evaluate multiple determinants of the longitudinal change in insulin clearance (IC) in subjects at high risk for type 2 diabetes (T2D).

RESEARCH DESIGN AND METHODS

Adults (n=492) at risk for T2D in the Prospective Metabolism and Islet Cell Evaluation cohort, a longitudinal observational cohort, had four visits over 9 years. Values from oral glucose tolerance tests collected at each assessment were used to calculate the ratios of both fasting C peptide-to-insulin (IC_FASTING) and areas under the curve of C peptide-to-insulin (IC_AUC). Generalized estimating equations (GEE) evaluated multiple determinants of longitudinal changes in IC.

RESULTS

IC declined by 20% over the 9-year follow-up period (p<0.05). Primary GEE results indicated that non-European ethnicity, as well as increases in baseline measures of waist circumference, white cell count, and alanine aminotransferase, was associated with declines in IC_FASTING and IC_AUC over time (all p<0.05). There were no significant associations of IC with sex, age, physical activity, smoking, or family history of T2D. Both baseline and longitudinal IC were associated with incident dysglycemia.

CONCLUSIONS

Our findings suggest that non-European ethnicity and components of the metabolic syndrome, including central obesity, non-alcoholic fatty liver disease, and subclinical inflammation, may be related to longitudinal declines in IC.

Pathogenesis of Lipid Disorders in Insulin Resistance: a Brief Review

PURPOSE OF REVIEW

Insulin resistance (IR) is recognized to play an important role in the pathogenesis of dyslipidemia. This review summarizes the complex interplay between IR and dyslipidemia in people with and without diabetes.

RECENT FINDINGS

IR impacts the metabolism of triglycerides, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), and very low-density lipoprotein cholesterol (VLDL-C) by several mechanisms. Trials with insulin sensitizing therapies, including biguanides and thiazolidinediones, have provided inconsistent results on lipid lowering in people with and without diabetes. In this review, we focus on the pathophysiological interplay between IR and dyslipidemia and recapitulate lipid and lipoprotein data from insulin-sensitizing trials. Further research elucidating the reciprocal relationship between IR and dyslipidemia is needed to better target these important risk factors for cardiovascular disease.