Relationship between hepatic glucose production and fasting plasma glucose concentration in patients with NIDDM

Impaired plasma glucose clearance is a key determinant of fasting hyperglycemia in people with obesity

OBJECTIVE

The objective of this study was to evaluate the relative importance of the basal rate of glucose appearance (Ra) in the circulation and the basal rate of plasma glucose clearance in determining fasting plasma glucose concentration in people with obesity and different fasting glycemic statuses.

METHODS

The authors evaluated basal glucose kinetics in 33 lean people with normal fasting glucose (<100 mg/dL; Lean < 100 group) and 206 people with obesity and normal fasting glucose (Ob < 100 group, n = 118), impaired fasting glucose (100-125 mg/dL; Ob 100-125 group, n = 66), or fasting glucose diagnostic of diabetes (≥126 mg/dL; Ob ≥ 126 group, n = 22).

RESULTS

Although there was a large (up to three-fold) range in glucose Ra within each group, the ranges in glucose concentration in the Lean < 100, Ob < 100, and Ob 100-125 groups were small because of a close relationship between glucose Ra and clearance rate. However, the glucose clearance rate at any Ra value was lower in the hyperglycemic than the normoglycemic groups. In the Ob ≥ 126 group, plasma glucose concentration was primarily determined by glucose Ra, because glucose clearance was markedly attenuated.

CONCLUSIONS

Fasting hyperglycemia in people with obesity represents a disruption of the precisely regulated integration of glucose production and clearance rates.

Efficacy and safety of cetagliptin as monotherapy in patients with type 2 diabetes: A randomized, double-blind, placebo-controlled phase 3 trial

AIM

To assess the efficacy and safety of the dipeptidyl peptidase-4 inhibitor, cetagliptin, as monotherapy in Chinese patients with type 2 diabetes (T2D) and inadequate glycaemic control.

MATERIALS AND METHODS

In total, 504 eligible patients with T2D were enrolled and randomized to cetagliptin 50 mg once daily, cetagliptin 100 mg once daily or placebo at a ratio of 2:2:1 for 24 weeks of double-blind treatment, then all patients received cetagliptin 100 mg once daily for 28 weeks of open-label treatment. The primary efficacy endpoint was the change in HbA1c level from baseline at week 24.

RESULTS

After 24 weeks, HbA1c from baseline was significantly reduced with cetagliptin 50 mg (-1.08%) and cetagliptin 100 mg (-1.07%) compared with placebo (-0.35%). The placebo-subtracted HbA1c reduction was -0.72% with cetagliptin 50 mg and 100 mg. Patients with a baseline HbA1c of 8.5% or higher had a greater HbA1c reduction with cetagliptin than those patients with a baseline HbA1c of less than 8.5%. Both doses studied led to a significantly higher proportion of patients (42.3% with 100 mg and 45.0% with 50 mg) achieving an HbA1c of less than 7.0% compared with placebo (12.9%). Cetagliptin also significantly lowered fasting plasma glucose and 2-hour postmeal plasma glucose relative to placebo. The incidence of adverse experiences was similar between cetagliptin and placebo. No drug-related hypoglycaemia was reported.

CONCLUSIONS

Cetagliptin monotherapy was effective and well tolerated in Chinese patients with T2D who had inadequate glycaemic control on exercise and diet.

ALDH2 rs671 Is Associated With Elevated FPG, Reduced Glucose Clearance and Hepatic Insulin Resistance in Japanese Men

CONTEXT

A recent meta-analysis of genome-wide association studies data from East Asians identified aldehyde dehydrogenase 2 (ALDH2) rs671 as a susceptibility variant for type 2 diabetes in males.

OBJECTIVE

To investigate the association between ALDH2 rs671 and metabolic characteristics.

METHODS

We studied 94 nonobese, nondiabetic, Japanese men. Using a 2-step hyperinsulinemic-euglycemic clamp, we evaluated insulin sensitivity in muscle and liver. Intrahepatic lipid and fat distribution were measured using 1H-magnetic resonance spectroscopy and magnetic resonance imaging, respectively. We divided participants into a risk-carrying group with ALDH2 rs671 G/G (n = 53) and a nonrisk-carrying group with ALDH2 rs671 G/A or A/A (n = 41).

RESULTS

The risk-carrying group had significantly higher levels of alcohol consumption (18.4 [interquartile range, IQR, 10.4-48.9]) vs 12.1 (IQR, 1.3-29.0) g/day; P = .003), elevated fasting plasma glucose (FPG) (97.5 ± 7.9 vs 93.5 ± 6.2 mg/dL; P = .010), lower hepatic insulin sensitivity (61.7 ± 20.5% vs 73.1 ± 15.9%; P = .003), and lower fasting glucose clearance (0.84 ± 0.8 dL·m-2·min-1 vs 0.87 ± 0.09 dL·m-2·min-1; P = .047) than the nonrisk-carrying group, while insulin resistance in muscle and body fat distribution were similar. The single linear correlation analysis revealed significant correlations between alcohol consumption and hepatic insulin sensitivity (r = -0.262, P = .011), fasting glucose clearance (r = -0.370, P < .001), or FPG (r = 0.489, P < .001). The multiple regression analysis revealed that both ALDH2 rs671 G/G genotype and alcohol consumption were significant independent correlates for hepatic insulin sensitivity, whereas only alcohol consumption was a significant independent correlate for fasting glucose clearance.

CONCLUSION

Our data suggest that high-alcohol intake-dependent and independent hepatic insulin resistance and reduced fasting glucose clearance due to high alcohol intake could be a relatively upstream metabolic abnormality in ALDH2 rs671 G/G carriers.

Basal insulin requirement in patients with type 1 diabetes depends on the age and body mass index

AIMS/INTRODUCTION

To investigate the basal insulin requirement in patients with type 1 diabetes who are on multiple daily injections (MDI) and to assess the patient characteristics that affect the percent of total daily basal insulin dose to the total daily insulin dose (%TBD/TDD).

MATERIALS AND METHODS

The subjects of this study were 67 inpatients with type 1 diabetes who were served diabetic meals of 25-30 kcal/kg standard body weight during several weeks of hospitalization. The basal insulin requirement was adjusted to keep the blood glucose level from bedtime to before breakfast within a 30 mg/dL difference. The bolus insulin dose before the meal was adjusted to keep the blood glucose level below 140 and 200 mg/dL before and 2 h after each meal, respectively. The total daily insulin dose (TDD), the percent of total daily basal insulin dose (TBD) to TDD (%TBD/TDD), and clinical characteristics were collected.

RESULTS

The median (Q1, Q3) of TDD was 33.0 (26.0, 49.0) units, and the %TBD/TDD was 24.1 ± 9.8%. The %TBD/TDD was positively correlated with the body mass index (BMI) and negatively correlated with the age at the onset and at the examination according to a univariate analysis. However, the %TBD/TDD was dependent on the BMI (β = 0.340, P = 0.004) and the age at examination (β = -0.288, P = 0.012) according to the multiple regression analysis.

CONCLUSIONS

The average %TBD/TDD in patients with type 1 diabetes on MDI was approximately 24% under inpatient conditions. The basal insulin requirement was dependent on the BMI and the age at examination.

Mechanism of Action of Inhaled Insulin on Whole Body Glucose Metabolism in Subjects with Type 2 Diabetes Mellitus

In the current study we investigate the mechanisms of action of short acting inhaled insulin Exubera®, on hepatic glucose production (HGP), plasma glucose and free fatty acid (FFA) concentrations. 11 T2D (Type 2 Diabetes) subjects (age = 53 ± 3 years) were studied at baseline (BAS) and after 16-weeks of Exubera® treatment. At BAS and after 16-weeks subjects received: measurement of HGP (3-³H-glucose); oral glucose tolerance test (OGTT); and a 24-h plasma glucose (24-h PG) profile. At end of study (EOS) we observed a significant decrease in fasting plasma glucose (FPG, 215 ± 15 to 137 ± 11 mg/dl), 2-hour plasma glucose (2-h PG, 309 ± 9 to 264 ± 11 mg/dl), glycated hemoglobin (HbA1c, 10.3 ± 0.5% to 7.5 ± 0.3%,), mean 24-h PG profile (212 ± 17 to 141 ± 8 mg/dl), FFA fasting (665 ± 106 to 479 ± 61 μM), post-OGTT (433 ± 83 to 239 ± 28 μM), and triglyceride (213 ± 39 to 120 ± 14 mg/dl), while high density cholesterol (HDL-C) increased (35 ± 3 to 47 ± 9 mg/dl). The basal HGP decreased significantly and the insulin secretion/insulin resistance (disposition) index increased significantly. There were no episodes of hypoglycemia and no change in pulmonary function at EOS. After 16-weeks of inhaled insulin Exubera® we observed a marked improvement in glycemic control by decreasing HGP and 24-h PG profile, and decreased FFA and triglyceride concentrations.

Mild Physiologic Hyperglycemia Induces Hepatic Insulin Resistance in Healthy Normal Glucose-Tolerant Participants

CONTEXT

Chronic hyperglycemia worsens skeletal muscle insulin resistance and β-cell function. However, the effect of sustained physiologic hyperglycemia on hepatic insulin sensitivity is not clear.

OBJECTIVE

To examine the effect of sustained physiologic hyperglycemia (similar to that observed in patients with type 2 diabetes) on endogenous (primarily reflecting hepatic) glucose production (EGP) in healthy individuals.

DESIGN

Volunteers participated in a three-step hyperinsulinemic (10, 20, 40 mU/m2 per minute) euglycemic clamp before and after a 48-hour glucose infusion to increase plasma glucose concentration by ∼40 mg/dL above baseline. EGP was measured with 3-3H-glucose before and after chronic glucose infusion.

PARTICIPANTS

Sixteen persons with normal glucose tolerance [eight with and eight without a family history (FH) of diabetes] participated in the study.

MAIN OUTCOME MEASURE

EGP.

RESULTS

Basal EGP increased following 48 hours of glucose infusion (from a mean ± SEM of 2.04 ± 0.08 to 3.06 ± 0.29 mg/kgffm⋅ min; P < 0.005). The hepatic insulin resistance index (basal EGP × fasting plasma insulin) markedly increased following glucose infusion (20.1 ± 1.8 to 51.7 ± 6.6; P < 0.005) in both FH+ and FH- subjects.

CONCLUSION

Sustained physiologic hyperglycemia for as little as 48 hours increased the rate of basal hepatic glucose production and induced hepatic insulin resistance in health persons with normal glucose tolerance, providing evidence for the role of glucotoxicity in the increase in hepatic glucose production in type 2 diabetes.

Gut ghrelin regulates hepatic glucose production and insulin signaling via a gut-brain-liver pathway

BACKGROUND

Ghrelin modulates many physiological processes. However, the effects of intestinal ghrelin on hepatic glucose production (HGP) are still unclear. The current study was to explore the roles of intestinal ghrelin on glucose homeostasis and insulin signaling in the liver.

METHODS

The system of intraduodenal infusion and intracerebral microinfusion into the nucleus of the solitary tract (NTS) in the normal chow-diet rats and pancreatic-euglycemic clamp procedure (PEC) combined with [3-³H] glucose as a tracer were used to analyze the effect of intestinal ghrelin. Intraduodenal co-infusion of ghrelin, tetracaine and Activated Protein Kinase (AMPK) activator (AICAR), or pharmacologic and molecular inhibitor of N-methyl-D-aspartate receptors within the dorsal vagal complex, or hepatic vagotomy in rats were used to explore the possible mechanism of the effect of intestinal ghrelin on HGP.

RESULTS

Our results demonstrated that gut infusion of ghrelin inhibited duodenal AMP-dependent protein kinase (AMPK) signal pathways, increased HGP and expression of gluconeogenic enzymes, and decreased insulin signaling in the liver of the rat. Intraduodenal co-infusion of ghrelin receptor antagonist [D-Lys³]-GHRP-6 and AMPK agonist with ghrelin diminished gut ghrelin-induced increase in HGP and decrease in glucose infusion rate (GIR) and hepatic insulin signaling. The effects of gut ghrelin were also negated by co-infusion with tetracaine, or MK801, an N-methyl-D-aspartate (NMDA) receptor inhibitor, and adenovirus expressing the shRNA of NR1 subunit of NMDA receptors (Ad-shNR1) within the dorsal vagal complex, and hepatic vagotomy in rats. When ghrelin and lipids were co-infused into the duodenum, the roles of gut lipids in increasing the rate of glucose infusion (GIR) and lowering HGP were reversed.

CONCLUSIONS

The current study provided evidence that intestinal ghrelin has an effect on HGP and identified a neural glucoregulatory function of gut ghrelin signaling.

Glucose-Mediated Glucose Disposal at Baseline Insulin Is Impaired in IFG

OBJECTIVE

To quantify glucose-mediated glucose disposal with and without basal insulin replacement and insulin-mediated glucose disposal in subjects with impaired fasting glucose (IFG).

RESEARCH DESIGN AND METHODS

We used the hyperglycemic/pancreatic clamp and stepped euglycemic clamp techniques to quantify glucose disposal and suppression of endogenous glucose production (EGP) in those with normal glucose tolerance (NGT; n = 14) and those with IFG (n = 14).

RESULTS

Total body glucose-mediated glucose uptake, measured with the hyperglycemic/pancreatic clamp, was not significantly affected by the basal plasma insulin levels in subjects with IFG and those with NGT. Compared with subjects with NGT, those with IFG had significantly lower glucose-mediated glucose uptake (by 15%) during the hyperglycemic clamp performed with and without basal insulin replacement. In contrast, insulin-mediated glucose disposal was comparable in both groups. The suppression of EGP by hyperglycemia was similar in both groups. However, the suppression of EGP by insulin was attenuated in those with IFG compared with those with NGT.

CONCLUSIONS

The results of the present study have demonstrated that (i) glucose-mediated glucose disposal is impaired in those with IFG; (ii) insulin-mediated glucose uptake in IFG is normal; and (iii) insulin action to suppress EGP is impaired.

Predictive Accuracy of Surrogate Indices for Hepatic and Skeletal Muscle Insulin Sensitivity

Context

Surrogate indices of muscle and hepatic insulin sensitivity derived from an oral glucose tolerance test (OGTT) are frequently used in clinical studies. However, the predictive accuracy of these indices has not been validated.

Design

In this cross-sectional study, hyperinsulinemic-euglycemic glucose clamp with tritiated glucose infusion and a 75-g OGTT were performed in individuals (n = 659, aged 18 to 49 years, body mass index of 16 to 64 kg/m²) with varying degrees of glucose tolerance. A calibration model was used to assess the ability of OGTT-derived, tissue-specific surrogate indices [hepatic insulin resistance index (HIRI) and muscle insulin sensitivity index (MISI)] to predict insulin sensitivity/resistance indices derived from the reference glucose clamp [Hepatic-IR_basal, a product of fasting plasma insulin and hepatic glucose production (HGP), Hepatic-IR_clamp, reciprocal of the percent suppression of HGP during the insulin clamp corrected for plasma insulin concentration, and Muscle-IS_clamp, a measure of peripheral glucose disposal]. Predictive accuracy was assessed by root mean squared error of prediction and leave-one-out, cross-validation-type square root of the mean squared error of prediction.

Results

HIRI and MISI were correlated with their respective clamp-derived indices. HIRI was negatively related to Muscle-IS_clamp (r = -0.62, P < 0.0001) and MISI correlated with Hepatic-IR derived from the clamp (Hepatic-IR_basal: r = -0.48, P < 0.0001 and Hepatic-IR_clamp: r = -0.41, P < 0.0001). However, the accuracy of HIRI and MISI to predict Hepatic-IR (basal or during clamp) was not significantly different. Likewise, the ability of HIRI and MISI to predict Muscle-IS_clamp was also similar.

Conclusion

Our findings indicate that the surrogate indices derived from an OGTT are accurate in predicting insulin sensitivity but are not tissue specific.

Comparative effectiveness of metformin monotherapy in extended release and immediate release formulations for the treatment of type 2 diabetes in treatment-naïve Chinese patients: Analysis of results from the CONSENT trial

AIMS

Metformin treatment for type 2 diabetes mellitus (T2DM) can be limited by gastrointestinal (GI) adverse events (AEs), resulting in treatment discontinuation. We investigated whether once-daily metformin extended release (XR) is superior in terms of GI tolerability, with non-inferior efficacy, compared with thrice-daily metformin immediate release (IR) in treatment-naïve Chinese patients with T2DM.

MATERIALS AND METHODS

This prospective, open-label, randomized, multicentre, phase IV interventional study enrolled Chinese T2DM patients to receive either metformin XR or metformin IR with a 2-week screening period, a 16-week treatment period and a 2-week follow-up period without treatment. Co-primary endpoints were a non-inferiority assessment of metformin XR vs metformin IR in glycated haemoglobin (HbA1c) least squares mean (LSM) change from baseline to week 16 and the superiority of GI tolerability for metformin XR vs metformin IR.

RESULTS

Overall, 532 patients were randomized to metformin IR (n = 267) or metformin XR (n = 265). The HbA1c LSM change was -1.61% and -1.58% in each group, respectively (LSM difference, 0.03; 95% confidence interval [CI], -0.10, 0.17). Incidences of drug-related AEs were 26.5% (n = 66) in the metformin IR-only group and 32.2% (n = 85) in the metformin XR-only group, and GI AEs were 23.8% and 22.3% in each group, respectively (difference, -1.52; 95% CI, -8.60, 5.56). The treatment difference met the predefined non-inferiority upper CI margin of 0.4% in HbA1c.

CONCLUSIONS

Metformin XR was non-inferior to metformin IR for the LSM change in HbA1c from baseline to week 16 and not superior to metformin IR for overall GI AE incidence during treatment of Chinese T2DM patients.

Decreased basal hepatic glucose uptake in impaired fasting glucose

AIMS/HYPOTHESIS

This research aimed to define the pathophysiological defects responsible for the elevated fasting plasma glucose (FPG) concentration and excessive rise in post-load plasma glucose observed in individuals with impaired fasting glucose (IFG).

METHODS

We used tracer techniques to quantify basal splanchnic (primarily hepatic) glucose uptake and glucose fluxes following glucose ingestion in individuals with normal glucose tolerance (NGT; n = 10) and IFG (n = 10).

RESULTS

Individuals with IFG had a comparable basal rate of hepatic glucose production to those with NGT (15.2 ± 0.2 vs 18.0 ± 0.8 μmol min^-1 [kg lean body mass (LBM)]^-1; p = 0.09). However, they had a significantly reduced glucose clearance rate during the fasting state compared with NGT (2.64 ± 0.11 vs 3.62 ± 0.20 ml min^-1 [kg LBM]^-1; p < 0.01). The difference between the basal rate of glucose appearance measured with [3-³H]glucose and [1-¹⁴C]glucose, which represent basal splanchnic glucose uptake, was significantly reduced in IFG compared with NGT (1.39 ± 0.28 vs 3.16 ± 0.44 μmol min^-1 [kg LBM]^-1; p = 0.02). Following glucose ingestion, the total amount of exogenous glucose that appeared in the systemic circulation was not significantly different between groups. However, suppression of endogenous glucose production (EGP) was markedly impaired in individuals with IFG.

CONCLUSIONS/INTERPRETATION

These results demonstrate that decreased tissue (liver) glucose uptake, not enhanced EGP, is the cause for elevated FPG concentration in individuals with IFG, while the excessive rise in plasma glucose concentration following a glucose load in these individuals is the result of impaired suppression of hepatic glucose production.

Duodenal GLP-1 signaling regulates hepatic glucose production through a PKC-δ-dependent neurocircuitry

Intestinal glucagon-like peptide-1 (GLP-1) is a hormone that stimulates insulin secretion and acts as a neuropeptide to control glucose homeostasis, but little is known whether intestinal GLP-1 has any effect in the control of hepatic glucose production (HGP). Here we found that intraduodenal infusion of GLP-1 activated duodenal PKC-δ, lowered HGP and was accompanied by a decrease in hepatic expression of gluconeogenic enzymes and an increase in hepatic insulin signaling in rats. However, gut co-infusion of either the GLP-1 receptor antagonist Ex-9, or the PKC-δ inhibitor rottlerin with GLP-1, negated the ability of gut GLP-1 to lower HGP and to increase hepatic insulin signaling during clamps. The metabolic and molecular signal effects of duodenal GLP-1 were also negated by co-infusion with tetracaine, pharmacologic inhibition of N-methyl-d-aspartate receptors within the dorsalvagal complex, or hepatic vagotomy in rats. In summary, we identified a neural glucoregulatory function of gut GLP-1 signaling.

Effect of Peripheral Electrical Stimulation (PES) on Nocturnal Blood Glucose in Type 2 Diabetes: A Randomized Crossover Pilot Study

Background

Regulation of hepatic glucose production has been a target for antidiabetic drug development, due to its major contribution to glucose homeostasis. Previous pre-clinical study demonstrated that peripheral electrical stimulation (PES) may stimulate glucose utilization and improve hepatic insulin sensitivity. The aim of the present study was to evaluate safety, tolerability, and the glucose-lowering effect of this approach in patients with type 2 diabetes (T2DM).

Methods

Twelve patients with T2DM were recruited for an open label, interventional, randomized trial. Eleven patients underwent, in a crossover design, an active, and a no-intervention control periods, separated with a two-week washout phase. During the active period, the patients received a daily lower extremity PES treatment (1.33Hz/16Hz burst mode), for 14 days. Study endpoints included changes in glucose levels, number of hypoglycemic episodes, and other potential side effects. Endpoints were analyzed based on continuous glucose meter readings, and laboratory evaluation.

Results

We found that during the active period, the most significant effect was on nocturnal glucose control (P < 0.0004), as well as on pre-meal mean glucose levels (P < 0.02). The mean daily glucose levels were also decreased although it did not reach clinical significance (P = 0.07). A reduction in serum cortisol (P < 0.01) but not in insulin was also detected after 2 weeks of treatment. No adverse events were recorded.

Conclusions

These results indicate that repeated PES treatment, even for a very short duration, can improve blood glucose control, possibly by suppressing hepatic glucose production. This effect may be mediated via hypothalamic-pituitary-adrenal axis modulation.

Trial registration

ClinicalTrials.gov NCT02727790

Verapamil decreases the glucose-lowering effect of metformin in healthy volunteers

AIM

The organic cation transporter 1 (OCT1) plays a key role in the cellular transport of metformin and its subsequent glucose-lowering effect. A recent non-clinical study reported that metformin uptake into hepatocytes is regulated via OCT1, and that uptake was strongly inhibited by verapamil. Therefore, we investigated the effects of verapamil co-administration on the pharmacokinetics and pharmacodynamics of metformin in humans.

METHODS

We evaluated the pharmacokinetics and the anti-hyperglycaemic effects of metformin using an oral glucose tolerance test (OGTT) in 12 healthy participants, before (day 1) and after metformin treatment (day 2), and again on days 15 and 16 after co-administration with verapamil.

RESULTS

Verapamil inhibited the ability of metformin to reduce maximum blood glucose concentrations (ΔGmax ) by 62.5% (P = 0.008) and decreased the area under the glucose concentration-time curve (ΔAUCgluc ) by 238% (P = 0.015). However, verapamil did not significantly alter the Cmax and the AUC of metformin, nor its renal clearance.

CONCLUSIONS

Our results suggest that verapamil remarkably decreases the glucose-lowering effect of metformin, possibly by acting as a competitive inhibitor of OCT1.

Discovery of novel indole derivatives as allosteric inhibitors of fructose-1,6-bisphosphatase

A series of novel indole derivatives was designed and synthesized as inhibitors of fructose-1,6-bisphosphatase (FBPase). The most potent compound 14c was identified with an IC50 value of 0.10 μM by testing the inhibitory activity against recombinant human FBPase. The structure-activity relationships were investigated on the substitution at 4- and 5-position of the indole scaffold. The binding interactions of the title compounds at AMP binding site of FBPase were predicted using CDOCKER algorithm.

Metformin Suppresses MHC-Restricted Antigen Presentation by Inhibiting Co-Stimulatory Factors and MHC Molecules in APCs

Metformin is widely used for T2D therapy but its cellular mechanism of action is undefined. Recent studies on the mechanism of metformin in T2D have demonstrated involvement of the immune system. Current immunotherapies focus on the potential of immunomodulatory strategies for the treatment of T2D. In this study, we examined the effects of metformin on the antigen-presenting function of antigen-presenting cells (APCs). Metformin decreased both MHC class I and class II-restricted presentation of OVA and suppressed the expression of both MHC molecules and co-stimulatory factors such as CD54, CD80, and CD86 in DCs, but did not affect the phagocytic activity toward exogenous OVA. The class II-restricted OVA presentation-regulating activity of metformin was also confirmed using mice that had been injected with metformin followed by soluble OVA. These results provide an understanding of the mechanisms of the T cell response-regulating activity of metformin through the inhibition of MHC-restricted antigen presentation in relation to its actions on APCs.

Exendin-4 reduces glycemia by increasing liver glucokinase activity: an insulin independent effect

Exendin-4 is a stable peptide agonist of GLP-1 receptor that exhibits insulinotropic actions. Some in vivo studies indicated insulin-independent glucoregulatory actions of exendin-4. That finding prompted us to evaluate effects of exendin-4 on liver glucose metabolism. Acute and chronic treatment of exendin-4 resulted in increased hepatic glucokinase activity in db/db mice but not in lean C57 mice. The stimulatory effect of exendin-4 on glucokinase activity was abrogated by exendin 9-39, a GLP-1 antagonist. Exposure of hepatocytes isolated from db/db mice to exendin-4 elicited a rapid increase in cAMP, which was synergized by IBMX, an inhibitor of cAMP degradation. The GLP-1 antagonist, exendin 9-39, has abolished the cAMP generating effects of exendin-4 as well. Furthermore, chronic treatment of exendin-4 in streptozotocin-treated C57 mice resulted in restoration of hepatic glycogen, an indicator of improved glucose metabolism, without apparent changes in serum insulin levels. In conclusion, exendin-4 increased glucokinase enzyme protein and activity in liver via a mechanism parallel to and independent of insulin. Exendin-4-induced increase in hepatic glucokinase activity is more pronounced in the presence of hepatic insulin resistance. This beneficial effect of exendin-4 on liver glucokinase activity may be mediated by GLP-1 receptor.

Deletion of CaMKK2 from the liver lowers blood glucose and improves whole-body glucose tolerance in the mouse

Ca(2+)/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a member of the Ca(2+)/CaM-dependent protein kinase family that is expressed abundantly in brain. Previous work has revealed that CaMKK2 knockout (CaMKK2 KO) mice eat less due to a central nervous system -signaling defect and are protected from diet-induced obesity, glucose intolerance, and insulin resistance. However, here we show that pair feeding of wild-type mice to match food consumption of CAMKK2 mice slows weight gain but fails to protect from diet-induced glucose intolerance, suggesting that other alterations in CaMKK2 KO mice are responsible for their improved glucose metabolism. CaMKK2 is shown to be expressed in liver and acute, specific reduction of the kinase in the liver of high-fat diet-fed CaMKK2(floxed) mice results in lowered blood glucose and improved glucose tolerance. Primary hepatocytes isolated from CaMKK2 KO mice produce less glucose and have decreased mRNA encoding peroxisome proliferator-activated receptor γ coactivator 1-α and the gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, and these mRNA fail to respond specifically to the stimulatory effect of catecholamine in a cell-autonomous manner. The mechanism responsible for suppressed gene induction in CaMKK2 KO hepatocytes may involve diminished phosphorylation of histone deacetylase 5, an event necessary in some contexts for derepression of the peroxisome proliferator-activated receptor γ coactivator 1-α promoter. Hepatocytes from CaMKK2 KO mice also show increased rates of de novo lipogenesis and fat oxidation. The changes in fat metabolism observed correlate with steatotic liver and altered acyl carnitine metabolomic profiles in CaMKK2 KO mice. Collectively, these results are consistent with suppressed catecholamine-induced induction of gluconeogenic gene expression in CaMKK2 KO mice that leads to improved whole-body glucose homeostasis despite the presence of increased hepatic fat content.

Fructose-1, 6-bisphosphatase inhibitors for reducing excessive endogenous glucose production in type 2 diabetes

Fructose-1,6-bisphosphatase (FBPase), a rate-controlling enzyme of gluconeogenesis, has emerged as an important target for the treatment of type 2 diabetes due to the well-recognized role of excessive endogenous glucose production (EGP) in the hyperglycemia characteristic of the disease. Inhibitors of FBPase are expected to fulfill an unmet medical need because the majority of current antidiabetic medications act primarily on insulin resistance or insulin insufficiency and do not reduce gluconeogenesis effectively or in a direct manner. Despite significant challenges, potent and selective inhibitors of FBPase targeting the allosteric site of the enzyme were identified by means of a structure-guided design strategy that used the natural inhibitor, adenosine monophosphate (AMP), as the starting point. Oral delivery of these anionic FBPase inhibitors was enabled by a novel diamide prodrug class. Treatment of diabetic rodents with CS-917, the best characterized of these prodrugs, resulted in a reduced rate of gluconeogenesis and EGP. Of note, inhibition of gluconeogenesis by CS-917 led to the amelioration of both fasting and postprandial hyperglycemia without weight gain, incidence of hypoglycemia, or major perturbation of lactate or lipid homeostasis. Furthermore, the combination of CS-917 with representatives of the insulin sensitizer or insulin secretagogue drug classes provided enhanced glycemic control. Subsequent clinical evaluations of CS-917 revealed a favorable safety profile as well as clinically meaningful reductions in fasting glucose levels in patients with T2DM. Future trials of MB07803, a second generation FBPase inhibitor with improved pharmacokinetics, will address whether this novel class of antidiabetic agents can provide safe and long-term glycemic control.

Hepatic dysfunction and insulin insensitivity in type 2 diabetes mellitus: a critical target for insulin-sensitizing agents

The liver plays an essential role in maintaining glucose homeostasis, which includes insulin-mediated processes such as hepatic glucose output (HGO) and uptake, as well as in clearance of insulin itself. In type 2 diabetes, the onset of hyperglycaemia [itself a potent inhibitor of hepatic glucose output (HGO)], alongside hyperinsulinaemia, indicates the presence of hepatic insulin insensitivity. Increased HGO is central to the onset of hyperglycaemia and highlights the need to target hepatic insulin insensitivity as a central component of glucose-lowering therapy. The mechanisms underlying the development of hepatic insulin insensitivity are not well understood, but may be influenced by factors such as fatty acid oversupply and altered adipocytokine release from dysfunctional adipose tissue and increased liver fat content. Furthermore, although the impact of insulin insensitivity as a marker of cardiovascular disease is well known, the specific role of hepatic insulin insensitivity is less clear. The pharmacological tools available to improve insulin sensitivity include the biguanides (metformin) and thiazolidinediones (rosiglitazone and pioglitazone). Data from a number of sources indicate that thiazolidinediones, in particular, can improve multiple aspects of hepatic dysfunction, including reducing HGO, insulin insensitivity and liver fat content, as well as improving other markers of liver function and the levels of mediators with potential involvement in hepatic function, including fatty acids and adipocytokines. The current review addresses this topic from the perspective of the role of the liver in maintaining glucose homeostasis, its key involvement in the pathogenesis of type 2 diabetes and the tools currently available to reduce hepatic insulin insensitivity.

Differences in insulin sensitivity and secretory capacity based on OGTT in subjects with impaired glucose regulation

BACKGROUND

This study examined whether defects in insulin secretion contribute to the development and progression of type 2 diabetes mellitus (T2DM).

METHODS

Plasma insulin and glucose were measured after a glucose tolerance test to calculate the insulinogenic index (IGI) and the HOMA-IR Homeostasis model assessment of insulin resistance in subjects with normal glucose tolerance (NGT), prediabetes (preDM, n = 49), and T2DM patients with disease duration < 1 year (n = 84), 1 approximately 5 years (n = 45), or > 5 years (n = 37). Plasma proinsulin and adiponectin levels were also measured as a parameter of insulin secretion and resistance.

RESULTS

The mean HOMA-IR increased and the adiponectin levels decreased relative to the deterioration of glucose tolerance in NGT and preDM subjects. However, differences in the HOMA-IR were not related to disease duration in T2DM subjects. The mean IGI was similar in NGT and preDM subjects, but there were significant deteriorations in IGI relative to the duration of diabetes.

CONCLUSIONS

Defects in both insulin sensitivity and insulin secretion contribute to T2DM, but decreased insulin secretion may be more important in the development and progression of T2DM.

An assessment of the in vivo efficacy of the glycogen phosphorylase inhibitor GPi688 in rat models of hyperglycaemia

BACKGROUND AND PURPOSE

Studies in cultured hepatocytes demonstrate glycogen synthase (GS) activation with glycogen phosphorylase (GP) inhibitors. The current study investigated whether these phenomena occurred in vivo using a novel GP inhibitor.

EXPERIMENTAL APPROACH

An allosteric GP inhibitor, GPi688, was evaluated against both glucagon-mediated hyperglycaemia and oral glucose challenge-mediated hyperglycaemia to determine the relative effects against GP and GS in vivo.

KEY RESULTS

In rat primary hepatocytes, GPi688 inhibited glucagons-mediated glucose output in a concentration dependent manner. Additionally GP activity was reduced and GS activity increased seven-fold. GPi688 inhibited glucagon-mediated hyperglycaemia in both Wistar (65%) & obese Zucker (100%) rats and demonstrated a long duration of action in the Zucker rat. The in vivo efficacy in the glucagon challenge model could be predicted by the equation; % glucagon inhibition=56.9+34.3[log ([free plasma]/rat IC50)], r=0.921). GPi688 also reduced the blood glucose of obese Zucker rats after a 7 h fast by 23%. In an oral glucose tolerance test in Zucker rats, however, GPi688 was less efficacious (7% reduction) than a glycogen synthase kinase-3 (GSK-3) inhibitor (22% reduction), despite also observing activation (by 45%) of GS in vivo.

CONCLUSIONS AND IMPLICATIONS

Although GP inhibition can inhibit hyperglycaemia mediated by increased glucose production, the degree of GS activation induced by allosteric GP inhibitors in vivo, although discernible, is insufficient to increase glucose disposal. The data suggests that GP inhibitors might be more effective clinically against fasting rather than prandial hyperglycaemic control.

Relationship between hepatic/visceral fat and hepatic insulin resistance in nondiabetic and type 2 diabetic subjects

BACKGROUND AND AIMS

Abdominal fat accumulation (visceral/hepatic) has been associated with hepatic insulin resistance (IR) in obesity and type 2 diabetes (T2DM). We examined the relationship between visceral/hepatic fat accumulation and hepatic IR/accelerated gluconeogenesis (GNG).

METHODS

In 14 normal glucose tolerant (NGT) (body mass index [BMI] = 25 +/- 1 kg/m(2)) and 43 T2DM (24 nonobese, BMI = 26 +/- 1; 19 obese, BMI = 32 +/- 1 kg/m(2)) subjects, we measured endogenous (hepatic) glucose production (3-(3)H-glucose) and GNG ((2)H(2)O) in the basal state and during 240 pmol/m(2)/min euglycemic-hyperinsulinemic clamp, and liver (LF) subcutaneous (SAT)/visceral (VAT) fat content by magnetic resonance spectroscopy/magnetic resonance imaging.

RESULTS

LF was increased in lean T2DM compared with lean NGT (18% +/- 3% vs 9% +/- 2%, P < .03), but was similar in lean T2DM and obese T2DM (18% +/- 3% vs 22% +/- 3%; P = NS). Both VAT and SAT increased progressively from lean NGT to lean T2DM to obese T2DM. T2DM had increased basal endogenous glucose production (EGP) (NGT, 15.1 +/- 0.5; lean T2DM, 16.3 +/- 0.4; obese T2DM, 17.2 +/- 0.6 micromol/min/kg(ffm); P = .02) and basal GNG flux (NGT, 8.6 +/- 0.4; lean T2DM, 9.6 +/- 0.4; obese T2DM, 11.1 +/- 0.6 micromol/min/kg(ffm); P = .02). Basal hepatic IR index (EGP x fasting plasma insulin) was increased in T2DM (NGT, 816 +/- 54; lean T2DM, 1252 +/- 164; obese T2DM, 1810 +/- 210; P = .007). In T2DM, after accounting for age, sex, and BMI, both LF and VAT, but not SAT, were correlated significantly (P < .05) with basal hepatic IR and residual EGP during insulin clamp. Basal percentage of GNG and GNG flux were correlated positively with VAT (P < .05), but not with LF. LF, but not VAT, was correlated with fasting insulin, insulin-stimulated glucose disposal, and impaired FFA suppression by insulin (all P < .05).

CONCLUSIONS

Abdominal adiposity significantly affects both lipid (FFA) and glucose metabolism. Excess VAT primarily increases GNG flux. Both VAT and LF are associated with hepatic IR.

Diurnal rhythm in endogenous glucose production is a major contributor to fasting hyperglycaemia in type 2 diabetes. Suprachiasmatic deficit or limit cycle behaviour?

AIMS/HYPOTHESIS

An increase in endogenous glucose production (EGP) is a major contributor to fasting morning hyperglycaemia in type 2 diabetes. This increase is dissipated with fasting, later in the day. To understand its origin, EGP, gluconeogenesis and hormones that regulate metabolism were measured over 24 h. We hypothesised that EGP, and therefore glycaemia, would demonstrate a centrally mediated circadian rhythm in type 2 diabetes.

SUBJECTS AND METHODS

Seven subjects with type 2 diabetes and six age- and BMI-matched control subjects, fasting after breakfast (08.00 h), underwent a further 24-h fast, with the infusion of [U-(13)C]glucose and [3-(14)C]lactate, starting at 14.00 h. The MCR and production of total and gluconeogenic glucose were determined from the tracer concentrations using compartmental analysis.

RESULTS

MCR was near constant: 1.73+/-0.10 in control and 1.40+/-0.14 ml kg(-1) min(-1) in diabetic subjects (p=0.04). EGP in diabetes rose gradually overnight from 8.2+/-0.7 to 11.3+/-0.5 micromol kg(-1) min(-1) at 06.00 h (p<0.05). Glucose utilisation lagged EGP, rising from 8.5+/-0.6 to 10.5+/-0.4 micromol kg(-1) min(-1) (p<0.05), inducing a fall in glycaemia from a peak of 8.0+/-0.5 mmol/l to 6.3+/-0.4 mmol/l (p<0.05). Cortisol and melatonin showed diurnal variations, whereas insulin, glucagon and leptin did not. Melatonin was most closely related to EGP, but its secretion was attenuated in diabetes (p<0.05).

CONCLUSIONS/INTERPRETATION

In type 2 diabetes, EGP and gluconeogenesis display diurnal rhythms that drive the fasting hyperglycaemia and are absent in healthy control subjects. The rise in EGP may be related to a deficit in suprachiasmatic nucleus activity in diabetes, or result from non-linear behaviour plus a transition from a normal steady state to a limit cycle pattern in diabetes, or both.

The relationship between peripheral glucose utilisation and insulin sensitivity in the regulation of hepatic glucose production: studies in normal and alloxan-diabetic dogs

BACKGROUND

Hepatic glucose overproduction (HGP) of diabetes could be primary or could occur in response to the metabolic needs of peripheral (skeletal muscle (SkM)) tissues. This question was tested in normal and diabetic dogs.

METHODS

HGP, SkM glucose uptake (Rd(tissue)), metabolic clearance of glucose (MCRg) and glycolytic flux (GF(exog)), and SkM biopsies were measured in the same dogs before and after alloxan-induced diabetes. Normal dogs were exposed to (1) an extended 20-h fast, (2) low- and high-dose glucose infusions (GINF) at basal insulinaemia, and chronic diabetic dogs were exposed to (3) hyperglycaemia, (4) phlorizin-induced normoglycaemia, and (5) poor and good diabetic control.

RESULTS

(1) Prolonged fast: HGP, Rd(tissue), and GF(exog) fell in parallel (p < 0.05). (2) Low-dose GINF: plasma glucose, insulin, Rd(tissue), MCRg, and GF(exog) were unchanged, but HGP fell by approximately 40%, paralleling the supplemental GINF. (3) High-dose GINF at basal insulin: plasma glucose doubled and synchronous changes in HGP, Rd(tissue), MCRg, and GF(exog) occurred; IC(glucose), G6P, and glycogen were unchanged. (4) Hyperglycaemic diabetes: HGP was raised (p < 0.05), matching urinary glucose loss (UGL) and decreased MCR(g), and maintaining normal basal Rd(tissue) and GF(exog). SkM IC(glucose) was increased and glycogen decreased (both p < 0.05). (5) Phlorizin-induced normoglycaemia in diabetic dogs: HGP rose, matching the increased UGL, while maintaining normal Rd(tissue) and GF(exog). Intramuscular substrates normalised. (6) Whole body and SkM metabolism normalised with correction of the insulin resistance and good diabetic control.

CONCLUSION

HGP reflects whether SkM is in a state of relative glucose 'excess' or absolute/relative glucose 'deprivation'.

Mitochondrial dysfunction and type 2 diabetes

Insulin resistance plays a major role in the pathogenesis of the metabolic syndrome and type 2 diabetes, and yet the mechanisms responsible for it remain poorly understood. Magnetic resonance spectroscopy studies in humans suggest that a defect in insulin-stimulated glucose transport in skeletal muscle is the primary metabolic abnormality in insulin-resistant patients with type 2 diabetes. Fatty acids appear to cause this defect in glucose transport by inhibiting insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and IRS-1-associated phosphatidylinositol 3-kinase activity. A number of different metabolic abnormalities may increase intramyocellular and intrahepatic fatty acid metabolites; these include increased fat delivery to muscle and liver as a consequence of either excess energy intake or defects in adipocyte fat metabolism, and acquired or inherited defects in mitochondrial fatty acid oxidation. Understanding the molecular and biochemical defects responsible for insulin resistance is beginning to unveil novel therapeutic targets for the treatment of the metabolic syndrome and type 2 diabetes.

MB06322 (CS-917): A potent and selective inhibitor of fructose 1,6-bisphosphatase for controlling gluconeogenesis in type 2 diabetes

In type 2 diabetes, the liver produces excessive amounts of glucose through the gluconeogenesis (GNG) pathway and consequently is partly responsible for the elevated glucose levels characteristic of the disease. In an effort to find safe and efficacious GNG inhibitors, we targeted the AMP binding site of fructose 1,6-bisphosphatase (FBPase). The hydrophilic nature of AMP binding sites and their widespread use for allosteric regulation of enzymes in metabolic pathways has historically made discovery of AMP mimetics suitable for drug development difficult. By using a structure-based drug design strategy, we discovered a series of compounds that mimic AMP but bear little structural resemblance. The lead compound, MB05032, exhibited high potency and specificity for human FBPase. Oral delivery of MB05032 was achieved by using the bisamidate prodrug MB06322 (CS-917), which is converted to MB05032 in two steps through the action of an esterase and a phosphoramidase. MB06322 inhibited glucose production from a variety of GNG substrates in rat hepatocytes and from bicarbonate in male Zucker diabetic fatty rats. Analysis of liver GNG pathway intermediates confirmed FBPase as the site of action. Oral administration of MB06322 to Zucker diabetic fatty rats led to a dose-dependent decrease in plasma glucose levels independent of insulin levels and nutritional status. Glucose lowering occurred without signs of hypoglycemia or significant elevations in plasma lactate or triglyceride levels. The findings suggest that potent and specific FBPase inhibitors represent a drug class with potential to treat type 2 diabetes through inhibition of GNG.

Both fasting glucose production and disappearance are abnormal in people with "mild" and "severe" type 2 diabetes

To determine whether regulation of fasting endogenous glucose production (EGP) and glucose disappearance (R(d)) are both abnormal in people with type 2 diabetes, EGP and R(d) were measured in 7 "severe" (SD), 9 "mild" (MD), and 12 nondiabetic (ND) subjects (12.7 +/- 0.6 vs. 8.1 +/- 0.4 vs. 5.1 +/- 0.4 mmol/l) after an overnight fast and during a hyperglycemic pancreatic clamp. Fasting insulin was higher in both the SD and MD than ND subjects, whereas fasting glucagon only was increased (P < 0.05) in SD. Fasting EGP, glycogenolysis, gluconeogenesis, and R(d) all were increased (P < 0.05) in SD but did not differ in MD or ND. On the other hand, when glucose ( approximately 11 mmol/l), insulin ( approximately 72 pmol/l), and glucagon ( approximately 140 pg/ml) concentrations were raised to values similar to those observed in the severe diabetic subjects, EGP was higher (P < 0.001) and R(d) lower (P < 0.01) in both SD and MD than in ND. The higher EGP in the SD and MD than ND during the clamp was the result of increased (P < 0.05) rates of glycogenolysis (4.2 +/- 1.7 vs. 3.5 +/- 1.0 vs. 0.0 +/- 0.8 micromol.kg(-1).min(-1)), since gluconeogenesis did not differ among groups. We conclude that neither glucose production nor disappearance is appropriate for the prevailing glucose and insulin concentrations in people with mild or severe diabetes. Both increased rates of gluconeogenesis (likely because of higher glucagon concentrations) and lack of suppression of glycogenolysis contribute to excessive glucose production in type 2 diabetics.

Parallel manifestation of insulin resistance and beta cell decompensation is compatible with a common defect in Type 2 diabetes

AIMS/HYPOTHESIS

The aim of the study was to evaluate the relationship between insulin sensitivity, beta cell function and glucose tolerance, and its dependence on variants in the newly identified Type 2 diabetes susceptibility gene, calpain-10 ( CAPN10).

METHODS

We studied 203 men of the same age but with varying degrees of glucose tolerance. These men participated in (i) an oral glucose tolerance test, (ii) a euglycaemic clamp combined with indirect calorimetry and infusion of [3-(3)H]-glucose and (iii) a stepwise assessment of acute insulin response to arginine (AIR) at three different glucose concentrations (fasting, 14 and 28 mmol/l).

RESULTS

There was a linear increase in NEFA levels ( p<0.0005) and WHR ( p<0.0005) and decrease in glucose uptake due to a reduction in glucose storage over the entire range of glucose tolerance ( r=-0.404; p<0.005). No increase in endogenous glucose production (EGP) was seen until patients had manifest diabetes. However, when EGP was expressed relative to fasting insulin concentrations, there was a linear deterioration of basal hepatic insulin sensitivity ( r=-0.514; p<0.005). The AIR followed a bell-shaped curve with an initial rise and subsequent decrease. However, AIR adjusted for insulin sensitivity (disposition index) showed a linear decrease with increasing glucose concentrations ( r=-0.563; p<0.001) starting already in subjects with normal glucose tolerance. There was an inverse correlation between increase in WHR and NEFA and peripheral as well as hepatic insulin sensitivity. Subjects with the genotype combination of CAPN10 consisting of SNP44 TT and SNP43 GG genotypes had significantly lower insulin-stimulated glucose uptake than carriers of the other genotype combinations (5.3+/-0.4 vs 7.2+/-0.4 mg.ffm kg(-1).min(-1).mU.l(-1); p<0.005).

CONCLUSIONS/INTERPRETATION

We conclude that the pre-diabetic state is characterised by a similar linear deterioration of peripheral and hepatic insulin sensitivity as beta cell function and that variants in the CAPN10 gene modify this relationship. These findings are compatible with a common defect in muscle, liver and beta cells in the pathogenesis of Type 2 diabetes.

Clinical pharmacokinetics of nateglinide: a rapidly-absorbed, short-acting insulinotropic agent

The prevalence and medical and economic impact of type 2 diabetes mellitus is increasing in Western societies. New agents have been developed that act primarily to reduce postprandial glucose excursions, which may be of particular significance now that postprandial glucose excursions are known to be correlated with cardiovascular morbidity and mortality. Nateglinide is a phenylalanine derivative that blocks K+ channels in pancreatic beta-cells, facilitating insulin secretion. Nateglinide sensitises beta-cells to ambient glucose, reducing the glucose concentration needed to stimulate insulin secretion. The pharmacokinetics of nateglinide are characterised by rapid absorption and elimination, with good (73%) bioavailability. Nateglinide is more rapidly absorbed when given 0-30 minutes prior to meal ingestion than if given during the meal. Nateglinide is extensively metabolised, primarily by cytochrome P450 2C9, and eliminated primarily by the kidney. Nateglinide pharmacokinetics are linear over the dose range 60-240 mg. No significant pharmacokinetic alterations occur in renally impaired patients, in the elderly, or in mildly hepatically impaired patients. Nateglinide administered prior to meals stimulates rapid, short-lived insulin secretion in a dose-dependent manner, thus decreasing mealtime plasma glucose excursions. Its effects on insulin secretion are synergistic with those of a meal. With increasing nateglinide doses, the risk of hypoglycaemia also increases, but its incidence is low. Even if a meal is missed, and the patient skips the dose of nateglinide (as recommended in the event of a missed meal), the incidence of subsequent hypoglycaemia remains low compared with long-acting agents. The postprandial insulinotropic effects of nateglinide are more rapid than those of repaglinide and more rapid and greater than those of glibenclamide (glyburide), while producing less prolonged insulin exposure and less risk of delayed hypoglycaemia. Further investigation is required to determine if nateglinide inhibition of postprandial glucose excursions will help to prevent diabetic complications or preserve pancreatic beta-cell function.

Metformin: new understandings, new uses

Metformin, a biguanide, has been available in the US for the treatment of type 2 diabetes mellitus for nearly 8 years. Over this period of time, it has become the most widely prescribed antihyperglycaemic agent. Its mechanism of action involves the suppression of endogenous glucose production, primarily by the liver. Whether the drug actually has an insulin sensitising effect in peripheral tissues, such as muscle and fat, remains somewhat controversial. Nonetheless, because insulin levels decline with metformin use, it has been termed an 'insulin sensitiser'. Metformin has also been shown to have several beneficial effects on cardiovascular risk factors and it is the only oral antihyperglycaemic agent thus far associated with decreased macrovascular outcomes in patients with diabetes. Cardiovascular disease, impaired glucose tolerance and the polycystic ovary syndrome are now recognised as complications of the insulin resistance syndrome, and there is growing interest in the management of this extraordinarily common metabolic disorder. While diet and exercise remain the cornerstone of therapy for insulin resistance, pharmacological intervention is becoming an increasingly viable option. We review the role of metformin in the treatment of patients with type 2 diabetes and describe the additional benefits it provides over and above its effect on glucose levels alone. We also discuss its potential role for a variety of insulin resistant and prediabetic states, including impaired glucose tolerance, obesity, polycystic ovary syndrome and the metabolic abnormalities associated with HIV disease.

Does overnight normalization of plasma glucose by insulin infusion affect assessment of glucose metabolism in Type 2 diabetes?

AIMS

In order to perform euglycaemic clamp studies in Type 2 diabetic patients, plasma glucose must be reduced to normal levels. This can be done either (i) acutely during the clamp study using high-dose insulin infusion, or (ii) slowly overnight preceding the clamp study using a low-dose insulin infusion. We assessed whether the choice of either of these methods to obtain euglycaemia biases subsequent assessment of glucose metabolism and insulin action.

METHODS

We studied seven obese Type 2 diabetic patients twice: once with (+ ON) and once without (- ON) prior overnight insulin infusion. Glucose turnover rates were quantified by adjusted primed-constant 3-3H-glucose infusions, and insulin action was assessed in 4-h euglycaemic, hyperinsulinaemic (40 mU m-2 min-1) clamp studies using labelled glucose infusates (Hot-GINF).

RESULTS

Basal plasma glucose levels (mean +/- sd) were 5.5 +/- 0.5 and 10.7 +/- 2.9 mmol/l in the + ON and - ON studies, respectively, and were clamped at -5.5 mmol/l. Basal rates of glucose production (GP) were similar in the + ON and - ON studies, 83 +/- 13 vs. 85 +/- 14 mg m-2 min-1 (NS), whereas basal rates of glucose disappearance (Rd) were lower in the + ON than in the - ON study, 84 +/- 8 vs. 91 +/- 11 mg m-2 min-1 (P = 0.02). During insulin infusion in the clamp period, rates of GP, 23 +/- 11 vs. 25 +/- 10 mg m-2 min-1, as well as rates of Rd, 133 +/- 32 vs. 139 +/- 37 mg m-2 min-1, were similar in the + ON and - ON studies, respectively (NS).

CONCLUSIONS

Apart from basal rates of Rd, assessment of glucose turnover rates in euglycaemic clamp studies of Type 2 diabetic patients is not dependent on the method by which plasma glucose levels are lowered.

S(G), S(I), and EGP of exercise-trained middle-aged men estimated by a two-compartment labeled minimal model

To examine the effects of physical training on glucose effectiveness (S(G)), insulin sensitivity (S(I)), and endogenous glucose production (EGP) in middle-aged men, stable-labeled frequently sampled intravenous glucose tolerance tests (FSIGTT) were performed on 11 exercise-trained middle-aged men and 12 age-matched sedentary men. The time course of EGP during the FSIGTT was estimated by nonparametric stochastic deconvolution. Glucose uptake-specific indexes of glucose effectiveness (S(2*)(G) x 10(2): 0.81 +/- 0.08 vs. 0.60 +/- 0.05 dl. min(-1). kg(-1), P < 0.05) and insulin sensitivity [S(2*)(I) x 10(4): 24.59 +/- 2.98 vs. 11.89 +/- 2.36 dl. min(-1). (microU/ml)(-1). kg(-1), P < 0.01], which were analyzed using the two-compartment minimal model, were significantly greater in the trained group than in the sedentary group. Plasma clearance rate (PCR) of glucose was consistently greater in the trained men than in sedentary men throughout FSIGTT. Compared with sedentary controls, EGP of trained middle-aged men was higher before glucose load. The EGP of the two groups was similarly suppressed by approximately 70% within 10 min, followed by an additional suppression after insulin infusion. EGP returned to basal level at approximately 60 min in the trained men and at 100 min in the controls, followed by its overshoot, which was significantly greater in the trained men than in the controls. In addition, basal EGP was positively correlated with S(2*)(G) . The higher basal EGP and greater EGP overshoot in trained middle-aged men appear to compensate for the increased insulin-independent (S(2*)(G)) and -dependent (S(2*)(I)) glucose uptake to maintain glucose homeostasis.

Effect of pioglitazone on abdominal fat distribution and insulin sensitivity in type 2 diabetic patients

We examined the effect of pioglitazone on abdominal fat distribution to elucidate the mechanisms via which pioglitazone improves insulin resistance in patients with type 2 diabetes mellitus. Thirteen type 2 diabetic patients (nine men and four women; age, 52 +/- 3 yr; body mass index, 29.0 +/- 1.1 kg/m(2)), who were being treated with a stable dose of sulfonylurea (n = 7) or with diet alone (n = 6), received pioglitazone (45 mg/d) for 16 wk. Before and after pioglitazone treatment, subjects underwent a 75-g oral glucose tolerance test (OGTT) and two-step euglycemic insulin clamp (insulin infusion rates, 40 and 160 mU/m(2).min) with [(3)H]glucose. Abdominal fat distribution was evaluated using magnetic resonance imaging at L4-5. After 16 wk of pioglitazone treatment, fasting plasma glucose (179 +/- 10 to 140 +/- 10 mg/dl; P < 0.01), mean plasma glucose during OGTT (295 +/- 13 to 233 +/- 14 mg/dl; P < 0.01), and hemoglobin A(1c) (8.6 +/- 0.4% to 7.2 +/- 0.5%; P < 0.01) decreased without a change in fasting or post-OGTT insulin levels. Fasting plasma FFA (674 +/- 38 to 569 +/- 31 microEq/liter; P < 0.05) and mean plasma FFA (539 +/- 20 to 396 +/- 29 microEq/liter; P < 0.01) during OGTT decreased after pioglitazone. In the postabsorptive state, hepatic insulin resistance [basal endogenous glucose production (EGP) x basal plasma insulin concentration] decreased from 41 +/- 7 to 25 +/- 3 mg/kg fat-free mass (FFM).min x microU/ml; P < 0.05) and suppression of EGP during the first insulin clamp step (1.1 +/- 0.1 to 0.6 +/- 0.2 mg/kg FFM.min; P < 0.05) improved after pioglitazone treatment. The total body glucose MCR during the first and second insulin clamp steps increased after pioglitazone treatment [first MCR, 3.5 +/- 0.5 to 4.4 +/- 0.4 ml/kg FFM.min (P < 0.05); second MCR, 8.7 +/- 1.0 to 11.3 +/- 1.1 ml/kg FFM(.)min (P < 0.01)]. The improvement in hepatic and peripheral tissue insulin sensitivity occurred despite increases in body weight (82 +/- 4 to 85 +/- 4 kg; P < 0.05) and fat mass (27 +/- 2 to 30 +/- 3 kg; P < 0.05). After pioglitazone treatment, sc fat area at L4-5 (301 +/- 44 to 342 +/- 44 cm(2); P < 0.01) increased, whereas visceral fat area at L4-5 (144 +/- 13 to 131 +/- 16 cm(2); P < 0.05) and the ratio of visceral to sc fat (0.59 +/- 0.08 to 0.44 +/- 0.06; P < 0.01) decreased. In the postabsorptive state hepatic insulin resistance (basal EGP x basal immunoreactive insulin) correlated positively with visceral fat area (r = 0.55; P < 0.01). The glucose MCRs during the first (r = -0.45; P < 0.05) and second (r = -0.44; P < 0.05) insulin clamp steps were negatively correlated with the visceral fat area. These results demonstrate that a shift of fat distribution from visceral to sc adipose depots after pioglitazone treatment is associated with improvements in hepatic and peripheral tissue sensitivity to insulin.

Free fatty acids in obesity and type 2 diabetes: defining their role in the development of insulin resistance and beta-cell dysfunction

Plasma free fatty acids (FFA) play important physiological roles in skeletal muscle, heart, liver and pancreas. However, chronically elevated plasma FFA appear to have pathophysiological consequences. Elevated FFA concentrations are linked with the onset of peripheral and hepatic insulin resistance and, while the precise action in the liver remains unclear, a model to explain the role of raised FFA in the development of skeletal muscle insulin resistance has recently been put forward. Over 30 years ago, Randle proposed that FFA compete with glucose as the major energy substrate in cardiac muscle, leading to decreased glucose oxidation when FFA are elevated. Recent data indicate that high plasma FFA also have a significant role in contributing to insulin resistance. Elevated FFA and intracellular lipid appear to inhibit insulin signalling, leading to a reduction in insulin-stimulated muscle glucose transport that may be mediated by a decrease in GLUT-4 translocation. The resulting suppression of muscle glucose transport leads to reduced muscle glycogen synthesis and glycolysis. In the liver, elevated FFA may contribute to hyperglycaemia by antagonizing the effects of insulin on endogenous glucose production. FFA also affect insulin secretion, although the nature of this relationship remains a subject for debate. Finally, evidence is discussed that FFA represent a crucial link between insulin resistance and beta-cell dysfunction and, as such, a reduction in elevated plasma FFA should be an important therapeutic target in obesity and type 2 diabetes.

Is hepatic glucose production increased in type 2 diabetes mellitus?

Based on recent studies, including our own, using what we consider to be an appropriate technique to estimate rates of hepatic glucose production (HGP), this article can be summarized as follows: 1) HGP in the overnight fasted state is near normal in obese type 2 diabetes (T2D) subjects, i.e., it may be increased by a mean of 12% compared to matched control subjects. 2) Suppression of HGP by insulin shows a rightward shift of the dose response curve (reduced insulin sensitivity) but normal maximal suppression (no maximum velocity defect). 3) In the overnight fasted state, gluconeogenesis is responsible for two thirds of HGP in T2D subjects and is about 5% to 10% increased compared to healthy subjects. 4) Suppression of HGP during a meal is close to normal. 5) The slightly increased HGP values throughout the 24-hour period together with reduced metabolic clearance rate (peripheral insulin resistance) and increased carbohydrate intake is responsible for the increase in fasting plasma glucose values. 6) Hypothetically, the role of the liver in nondiabetic and T2D subjects may be to produce the amount of glucose needed for metabolism in peripheral tissues. If insulin-mediated glucose uptake in skeletal muscle is reduced, plasma glucose will increase due to the "nonsuppressed" HGP values. Plasma glucose continues to rise until glucose-mediated glucose uptake compensates completely for the reduction in insulin-mediated glucose uptake.

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

Comparisons between white ginseng radix and rootlet for antidiabetic activity and mechanism in KKAy mice

The mechanisms responsible for the antidiabetic activity of both the white ginseng radix (Ginseng Radix Alba, GRA) and the rootlet (Ginseng Radix Palva, GRP) were investigated. After a four week oral administration, the fasting blood glucose levels in the GRA- and GRP-treated groups were lower when compared to the control group. To elucidate the hypoglycemic mechanism(s) of the ginseng radices, glucose absorption from the small intestine, hepatic hexokinase and glucose-6-phosphatase activities, in addition to PPAR-gamma expression in adipose tissue were examined. The results strongly suggest that GRA can improve hyperglycemia in KKAy mice, possibly by blocking intestinal glucose absorption and inhibiting hepatic glucose-6-phosphatase, and GRP through the upregulation of adipocytic PPAR-y protein expression as well as inhibiting intestinal glucose absorption.

Gluconeogenesis in moderately and severely hyperglycemic patients with type 2 diabetes mellitus

We tested the generally accepted concept that increased gluconeogenesis (GNG) and endogenous glucose production (EGP) are the main reasons for postabsorptive hyperglycemia in patients with type 2 diabetes mellitus (T2DM). GNG was measured with the (2)H(2)O method by use of both the C5-to-C2 ratio (C5/C2, with gas chromatography-mass spectrometry) and the C5-to-(2)H(2)O ratio (C5/(2)H(2)O, with isotope ratio mass spectrometry), and EGP was measured with 3-[(3)H]glucose in 27 patients with T2DM [13 with fasting plasma glucose (FPG) >10 mM and 14 with FPG <10 mM] and in 7 weight- and age-matched nondiabetic controls. The results showed 1) that GNG could be determined accurately with (2)H(2)O by using either C5/C2 or C5/(2)H(2)O; 2) that whereas after an overnight fast of 16 h, GNG was higher in the entire group of patients with T2DM than in controls (6.4 vs. 5.0 micromol. kg(-1). min(-1) or 60.4 vs. 51.4% of EGP, P < 0.02), GNG was within normal limits (less than the mean +/- 2 SD of controls or <65.3%) in 11/14 (79%) patients with mild to moderate hyperglycemia (FPG <10 mM) and in 5/13 (38%) of patients with severe hyperglycemia (FPG 10-20 mM); 3) that elevated GNG in T2DM was associated with a 43% decrease in prehepatic insulin secretion, i.e., with hepatic insulin deficiency; and 4) that FPG correlated significantly with glucose clearance (insulin resistance) (r = 0.70) and with GNG (r = 0.50) or EGP (r = 0.45). We conclude 1) that peripheral insulin resistance is at least as important as GNG (and EGP) as a cause of postabsorptive hyperglycemia in T2DM and 2) that GNG and EGP in T2DM are increased under conditions of significant hepatic insulin deficiency and thus probably represent a late event in the course of T2DM.

Mechanism by which metformin reduces glucose production in type 2 diabetes

To examine the mechanism by which metformin lowers endogenous glucose production in type 2 diabetic patients, we studied seven type 2 diabetic subjects, with fasting hyperglycemia (15.5 +/- 1.3 mmol/l), before and after 3 months of metformin treatment. Seven healthy subjects, matched for sex, age, and BMI, served as control subjects. Rates of net hepatic glycogenolysis, estimated by 13C nuclear magnetic resonance spectroscopy, were combined with estimates of contributions to glucose production of gluconeogenesis and glycogenolysis, measured by labeling of blood glucose by 2H from ingested 2H2O. Glucose production was measured using [6,6-2H2]glucose. The rate of glucose production was twice as high in the diabetic subjects as in control subjects (0.70 +/- 0.05 vs. 0.36 +/- 0.03 mmol x m(-2) min(-1), P < 0.0001). Metformin reduced that rate by 24% (to 0.53 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0009) and fasting plasma glucose concentration by 30% (to 10.8 +/- 0.9 mmol/l, P = 0.0002). The rate of gluconeogenesis was three times higher in the diabetic subjects than in the control subjects (0.59 +/- 0.03 vs. 0.18 +/- 0.03 mmol x m(-2) min(-1) and metformin reduced that rate by 36% (to 0.38 +/- 0.03 mmol x m(-2) x min(-1), P = 0.01). By the 2H2O method, there was a twofold increase in rates of gluconeogenesis in diabetic subjects (0.42 +/- 0.04 mmol m(-2) x min(-1), which decreased by 33% after metformin treatment (0.28 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0002). There was no glycogen cycling in the control subjects, but in the diabetic subjects, glycogen cycling contributed to 25% of glucose production and explains the differences between the two methods used. In conclusion, patients with poorly controlled type 2 diabetes have increased rates of endogenous glucose production, which can be attributed to increased rates of gluconeogenesis. Metformin lowered the rate of glucose production in these patients through a reduction in gluconeogenesis.

Metabolic complications of obesity

The rising prevalence of obesity is accompanied by an increasing number of patients with the metabolic complications of obesity. The major complications come under the heading of the metabolic syndrome. This syndrome is characterized by plasma lipid disorders (atherogenic dyslipidemia), raised blood pressure, elevated plasma glucose, and a prothrombotic state. The clinical consequences of the metabolic syndrome are coronary heart disease and stroke, type 2 diabetes and its complications, fatty liver, cholesterol gallstones, and possibly some forms of cancer. At the heart of the metabolic syndrome is insulin resistance, which represents a generalized derangement in metabolic processes. Obesity is the predominant factor leading to insulin resistance, although other factors play a role. The mechanistic link between insulin resistance and the metabolic syndrome is complex. The relationship is modulated by yet other factors, such as physical activity, body fat distribution, hormones, and a person's genetic polymorphic architecture. A better understanding of the molecular basis of this relationship is needed to suggest new targets for prevention and treatment of the complications of obesity. In addition, understanding at the clinical level will lead to improved management of these complications.

Insulin resistance and antidiabetic drugs

Insulin resistance describes an impaired biological response to insulin, which underpins the development of type 2 (non-insulin-dependent) diabetes mellitus (T2DM). Initially, insulin resistance causes a compensatory hyperinsulinaemia, which gives way to pancreatic beta-cell failure. Insulin resistance and hyperinsulinaemia conspire together in the development of a diverse collection of risk factors for coronary heart disease, namely obesity, T2DM, dyslipidaemia, hypertension, atherosclerosis, and a pro-coagulant state. This collection of factors is commonly found in T2DM patients, and is recognised as the Insulin Resistance Syndrome or Syndrome X. By targeting insulin resistance as a treatment strategy for T2DM, it should be possible to broaden the potential benefits, so that improved glycaemic control is complemented with improvements to other components of Syndrome X. At present, metformin and thiazolidinediones are the only therapies for T2DM that directly address aspects of insulin resistance. Increasing awareness of the clinical implications of insulin resistance, and increasing knowledge of the cellular basis of insulin resistance, provide the rationale and a means for developing an anti-insulin resistance approach to the treatment of T2DM.

Ethnicity affects the postprandial regulation of glycogenolysis

We investigated the effect of nutrient intake on glucose metabolism in normal Mexican-Americans (n = 6) and European-Americans (n = 6). Subjects were studied after an 18-h fast and after 5-6 h of ingestion of hourly meals that supplied 6.35 or 12.75 micromol glucose. kg(-1). min(-1). Endogenous glucose production (EGP), gluconeogenesis (GNG), and glycogenolysis (GLY) were estimated by mass isotopomer analysis with [U-(13)C]glucose infusions. Fasting EGP, GNG, and GLY did not differ between the groups. Food ingestion lowered the molar rate of GNG by only 31%. However, while consuming the lower quantity of nutrients, Mexican-Americans had higher plasma glucose (P < 0.05), a 39% higher rate of EGP (P < 0.05), and a 68% (P < 0.025) higher rate of GLY than the European-Americans. At the higher intake, EGP and GLY were suppressed completely in both groups. There was a linear relationship between insulin concentrations, EGP, and GLY in both groups, but the slope of the line was significantly (P < 0.05) greater in the European-Americans. We conclude that the sensitivity of GLY to nutrient intake differs between ethnic groups and that this may play a role in the increased predisposition of Mexican-Americans to type II diabetes.

Quantifying the contribution of gluconeogenesis to glucose production in fasted human subjects using stable isotopes

The contribution of gluconeogenesis to glucose production is estimated from the enrichment of the H bound to C-5 of glucose relative to either that bound to C-2 of glucose or the enrichment in body water on ingesting 2H2O in the fasted state. Contributions of all gluconeogenic substrates are included in the estimate and the limitation of an uncertain precursor enrichment removed. The half-life of 2H2O in body water precludes a repeat study for many weeks. Glycogen cycling could result in underestimation, but there is evidence that glycogen cycling does not occur in liver in the fasted state. Gluconeogenesis has been estimated by mass-isotopomer-distribution analyses, usually by administering 13C-labelled glycerol. Underestimates emphasize the major limitation of the method, i.e. the need to assume a single enrichment of the precursor pool. Estimates of gluconeogenesis from isotopomer distribution in arterial-blood glucose and lactate on infusing [U-13C6] glucose are unreliable, as a proportion of the glucose is formed from glycerol and from amino acids not converted to glucose via pyruvate. Loss of label in the Krebs cycle and relying on enrichment of arterial-blood lactate as a measure of hepatic pyruvate further add to the uncertainty. Estimates of the rate of gluconeogenesis by NMR are obtained by subtraction of the rate of glycogenolysis determined by NMR from the rate of glucose production. Estimates are then the mean rate for the period over which glycogen contents are measured. Technical considerations can limit the accuracy of analyses and result in overestimates.