Prevention of dexamethasone-induced insulin resistance by metformin

Animal Models in Diabetic Research-History, Presence, and Future Perspectives

Diabetes mellitus (DM) is a very serious disease, the incidence of which has been increasing worldwide. The beginning of diabetic research can be traced back to the 17th century. Since then, animals have been experimented on for diabetic research. However, the greatest development of diabetes research occurred in the second half of the last century, along with the development of laboratory techniques. Information obtained by monitoring patients and animal models led to the finding that there are several types of DM that differ significantly from each other in the causes of the onset and course of the disease. Through different types of animal models, researchers have studied the pathophysiology of all types of diabetic conditions and discovered suitable methods for therapy. Interestingly, despite the unquestionable success in understanding DM through animal models, we did not fully succeed in transferring the data obtained from animal models to human clinical research. On the contrary, we have observed that the chances of drug failure in human clinical trials are very high. In this review, we will summarize the history and presence of animal models in the research of DM over the last hundred years. Furthermore, we have summarized the new methodological approaches, such as "organ-on-chip," that have the potential to screen the newly discovered drugs for human clinical trials and advance the level of knowledge about diabetes, as well as its therapy, towards a personalized approach.

Highly bioavailable Berberine formulation improves Glucocorticoid Receptor-mediated Insulin Resistance via reduction in association of the Glucocorticoid Receptor with phosphatidylinositol-3-kinase

Excess glucocorticoid (GC) production is known to induce obesity and insulin resistance through increased activation of the glucocorticoid receptor (GR). The molecular mechanism for the non-genomic effects of excessive circulating GC on the insulin-signalling pathway in skeletal muscle is unknown. The plant alkaloid berberine has been shown to attenuate insulin resistance and inhibit gluconeogenesis in type 2 diabetic animals. A highly bioavailable berberine formulation termed Huang-Gui solid dispersion (HGSD), is a preparation of berberine coupled to sodium caprate and this markedly improving berberines bioavailability. Here we examined how HGSD treatment attenuated GR-mediated alteration in PI3K signalling and insulin resistance in diabetic rats, dexamethasone-treated mice and in insulin resistant C2C12 skeletal muscle cells. Blood glucose and skeletal muscle GC levels were increased and insulin signalling impaired in skeletal muscle of type 2 diabetic rats compared to controls. Treatment of these animals with HGSD restored blood glucose and skeletal muscle GC levels to that of controls. Insulin resistant C2C12 skeletal muscle cells exhibited impaired insulin signalling compared to controls and treatment of HGSD and RU486, an antagonist of GR, restored insulin signalling to that of control cells. Administration of dexamethasone to mice increased GR/GRα-associated PI3K and reduced IRS1-associated PI3K, phosphorylated-AKT, and membrane GLUT4 translocation resulting in a higher blood glucose concentration compared to controls. HGSD treatment of these mice improved insulin resistance by reducing the association of GR/GRα with PI3K. Excess GC-induced insulin resistance is mediated by increased association of GR with PI3K and treatment with HGSD attenuates these effects. We hypothesize that HGSD may be a promising candidate drug for the treatment of type 2 diabetes by reducing the association of GR with PI3K in skeletal muscle.

Coumarins ameliorate diabetogenic action of dexamethasone via Akt activation and AMPK signaling in skeletal muscle

Glucocorticoids are widely prescribed for lots of pathological conditions, however, can produce 'Cushingoid' side effects including central obesity, glucose intolerance, insulin resistance and so forth. Our study is intended to investigate the improving effects of coumarins on diabetogenic action of dexamethasone in vivo and in vitro and elucidate potential mechanisms. ICR mice treated with dexamethasone for 21 days exhibited decreased body weight, increased blood glucose and impaired glucose tolerance, which were prevented by fraxetin (40 mg/kg/day), esculin (40 mg/kg/day) and osthole (20 mg/kg/day), respectively. Esculin, fraxetin and osthole also could promote glucose uptake in normal C2C12 myotubes, and improve insulin resistance in myotubes induced by dexamethasone. Western blotting results indicated that esculin, fraxetin and osthole could boost Akt activation, stimulate GLUT4 translocation, thus alleviate insulin resistance. Esculin and osthole also could activate AMPK, thereby phosphorylate TBC1D1 at Ser237, and consequently ameliorate diabetogenic action of dexamethasone. Our study indicates coumarins as potential anti-diabetic candidates or leading compounds for drug development.

Dapagliflozin for prednisone-induced hyperglycaemia in acute exacerbation of chronic obstructive pulmonary disease

The aim of the present study was to compare the effectiveness and safety of add-on treatment with dapagliflozin to placebo in patients with prednisone-induced hyperglycaemia during treatment for acute exacerbation of chronic obstructive pulmonary disease (AECOPD). We enrolled 46 patients hospitalized for an AECOPD in a multicentre double-blind randomized controlled study in which add-on treatment with dapagliflozin 10 mg was compared with placebo. Glycaemic control and incidence of hypoglycaemia were measured through a blinded subcutaneous continuous glucose monitoring device. Participants in the dapagliflozin group spent 54 ± 27.7% of the time in target range (3.9-10 mmol/L) and participants in the placebo group spent 53.6 ± 23.4% of the time in target range (P = .96). The mean glucose concentration was 10.1 mmol/L in the dapagliflozin group and 10.4 mmol/L in the placebo group (P = .66). One participant using dapagliflozin and 2 participants using placebo experienced symptomatic hypoglycaemia. Treatment with dapagliflozin was safe and there was no difference in risk of hypoglycaemia compared with placebo. Dapagliflozin did not result in better glycaemic control compared with placebo in participants with prednisone-induced hyperglycaemia during AECOPD.

Metformin prevents metabolic side effects during systemic glucocorticoid treatment

OBJECTIVES

Patients receiving glucocorticoid treatment are prone to develop metabolic complications. In preclinical studies, metformin prevented the development of the metabolic syndrome during glucocorticoid excess. We herein investigated the metabolic effect of metformin during glucocorticoid treatment in non-diabetic patients.

METHODS

In a double-blind, placebo-controlled trial, patients starting glucocorticoid treatment (prednisone, prednisolone or methylprednisolone) for four weeks were randomised to concomitantly receive metformin (850 mg once daily for one week followed by 850 mg twice daily for three weeks) or placebo. All patients underwent a standardised oral glucose tolerance test at baseline and after four weeks. The primary endpoint was change in the 2-h area under the curve (AUC) of glucose during the oral glucose tolerance test between baseline and four weeks.

RESULTS

29 of 34 randomised non-diabetic patients completed the trial (17 metformin and 12 placebo). In patients allocated to placebo, median glucose 2-h AUC increased from baseline to four weeks (836 (IQR 770-966) to 1202 (1009-1271) mmol/L per min; P = 0.01). In contrast, glucose levels remained similar to baseline in the metformin group (936 (869-1003) to 912 (825-1011) mmol/L per min; P = 0.83). This change within four weeks was different between both groups (P = 0.005). Glucocorticoid equivalent doses were similar in both groups (placebo: 980.0 (560.0-3259.8) mg/28 days; metformin: 683.0 (437.5-1970.5) mg/28 days; P = 0.26).

CONCLUSIONS

In this first randomised controlled trial of metformin targeting metabolic complications in patients needing glucocorticoid therapy, we observed a beneficial effect of metformin on glycaemic control. Metformin thus seems to be a promising drug for preventing metabolic side effects during systemic glucocorticoid treatment.

Impaired glucose tolerance and metformin: clinical and mechanistic aspects

The Diabetes Prevention Program (DPP) showed that metformin reduced the incidence of diabetes in subjects with impaired glucose tolerance (IGT) who were at high risk of progression to type 2 diabetes. Metformin was not as efficient as intensive life style intervention, but had a clinically significant effect in obese individuals and in those with impaired fasting glucose (IFG). This review discusses the clinical implications and the mechanistic aspects of the effect of metformin in IGT and IFG. Acute actions of metformin on postprandial metabolism to improve hepatic glucose handling and improve the lipid profile could contribute to the lower incidence of diabetes. Longer term improvements in haemodynamic parameters and reduced oxidative stress are also implicated. Metformin offers a potential alternative or complement to lifestyle intervention for IGT, and deserves further evaluation in this respect.

Physician's attitudes towards diagnosing and treating glucocorticoid induced hyperglycaemia: Sliding scale regimen is still widely used despite guidelines

AIMS

Treatment with glucocorticoids for neoplasms and inflammatory disorders is frequently complicated by glucocorticoid induced hyperglycaemia (GCIH). GCIH is associated with adverse outcomes and its treatment has short term and long term benefits. Currently, treatment targets and modalities depend on local protocols and habits of individual clinicians. We explored current practice of screening and treatment of GCIH in patients receiving glucocorticoid pulse therapy.

METHODS

A factorial survey with written case vignettes. All vignette patients received glucocorticoid pulse therapy. Other characteristics (e.g., indication for glucocorticoid therapy, pre-existent diabetes) varied. The survey was held between November 2013 and May 2014 on 2 nationwide conferences and in hospitals across The Netherlands. Pulmonologists and internists expressed their level of agreement with statements on ordering capillary glucose testing and treatment initiation.

RESULTS

Respondents ordered screening for GCIH in 85% of vignette patients and initiated treatment in 56%. When initiating treatment, respondents opt for sliding scale insulin in 62% of patients. Sliding scale insulin was more frequently prescribed in patients with pre-existent insulin dependent diabetes (OR 2.4, CI 1.3-4.2) and by residents (vs. specialists, OR 2.1, CI 1.2-3.5). Sixty-nine percent of clinicians experienced a lack of guidelines for GCIH.

CONCLUSIONS

Clinicians have a strong tendency to screen for GCIH but subsequent initiation of treatment was low. Sliding scale insulin is still widely used in episodic GCIH despite evidence against its effectiveness. This may be due to lacking evidence on feasible treatment options for GCIH.

Thiazolidinediones attenuate lipolysis and ameliorate dexamethasone-induced insulin resistance

BACKGROUND

Elevated levels of circulating free fatty acids induce insulin resistance and often occur in obese and diabetic conditions. One pharmacological basis for the antidiabetic effects of thiazolidinediones (TZDs) is that TZDs reduce levels of circulating FFAs by accelerating their uptake and reesterification from plasma into adipocytes. Here, we investigated whether TZDs affect adipose lipolysis, a process controlling triglyceride hydrolysis and FFA efflux to the bloodstream.

METHODS

The effects of TZDs on lipolysis were investigated in primary rat adipocytes in vitro and in rats in vivo.

RESULTS

In rat primary adipocytes, the TZDs pioglitazone, rosiglitazone and troglitazone inhibited the lipolytic reaction dose- and time-dependently and in a post-receptor pathway by decreasing cAMP level and total lipase activity. TZDs increased the phosphorylation of Akt/protein kinase B, an action required for activating cyclic-nucleotide phosphodiesterase 3B, a major enzyme responsible for cAMP hydrolysis in adipocytes. Furthermore, rosiglitazone inhibited the lipolytic action in dexamethasone-stimulated adipocytes, thereby preventing the increased level of circulating FFAs, and ameliorated insulin resistance in vivo in dexamethasone-treated rats.

CONCLUSIONS

TZDs may attenuate lipolysis and FFA efflux by activating Akt signaling to decrease cAMP level and hence reduce lipase activity in adipocytes. Inhibiting lipolysis and FFA efflux with TZDs could be a pharmacological basis by which TZDs antagonize diabetes, particularly in patients with hypercortisolemia or glucocorticoid challenge.

Advanced glycation end-products and insulin signaling in granulosa cells

Advanced glycation end-products (AGEs) may interfere with insulin intracellular signaling and glucose transport in human granulosa cells, potentially affecting ovarian function, follicular growth, linked with diminished fertility. The potential interaction of AGEs with insulin signaling pathways and glucose transport was investigated in human granulosa KGN cells. KGN cells were cultured with variable concentrations of human glycated albumin (HGA, 50-200 µg/mL) or insulin (100 ng/mL). Combined treatments of KGN cells with insulin (100 ng/mL) and HGA (200 µg/mL) were also performed. p-AKT levels and glucose transporter type 4 (Glut-4) translocation analysis were performed by Western blot. Phosphatidylinositol-3-kinase (PI3K)-specific signaling was checked by using the PI3K-inhibitor, LY294002. p-AKT levels were significantly increased following insulin treatment compared to basal levels or HGA exposure. This insulin-mediated AKT-phosphorylation was PI3K-specific and it was inhibited after combined treatment of insulin and HGA. Furthermore, Glut-4 translocation from the cytoplasm to the membrane compartments of KGN cells was remarkably reduced after the combined treatment of insulin and HGA. The present findings support that AGEs interfere with insulin signaling in granulosa cells and prevent Glut-4 membrane translocation suggesting that intra ovarian AGEs accumulation, from endogenous or exogenous sources, may contribute to the pathophysiology of states characterized with anovulation and insulin resistance such as polycystic ovary syndrome.

Bilayer matrix tablets for prolonged actions of metformin hydrochloride and repaglinide

A combination therapy of metformin hydrochloride (MH) and repaglinide (RG) achieves a perfect glycemic control; however, the combination formulation of immediate release must be taken several times a day, compromising the therapeutic benefits and causing inconveniences to the patients. Herein, a bilayer matrix tablet that aimed at continuously releasing both MH and RG over time was developed, in which the two drugs were formulated into two separated layers. The tablets were prepared by wet granulation method, and the optimized formulation was obtained by evaluating the factors that affected the drug release. The bilayer tablets simultaneously released the two drugs over 12 h; and a better in vivo performance with a steady plasma concentration, markedly lower Cmax, prolonged Tmax, and perfect absorption was obtained. Summarily, the bilayer matrix tablets sustained both MH and RG release over time, thereby prolonging the actions for diabetic therapy and producing better health outcomes.

Glucocorticoid treatment and endocrine pancreas function: implications for glucose homeostasis, insulin resistance and diabetes

Glucocorticoids (GCs) are broadly prescribed for numerous pathological conditions because of their anti-inflammatory, antiallergic and immunosuppressive effects, among other actions. Nevertheless, GCs can produce undesired diabetogenic side effects through interactions with the regulation of glucose homeostasis. Under conditions of excess and/or long-term treatment, GCs can induce peripheral insulin resistance (IR) by impairing insulin signalling, which results in reduced glucose disposal and augmented endogenous glucose production. In addition, GCs can promote abdominal obesity, elevate plasma fatty acids and triglycerides, and suppress osteocalcin synthesis in bone tissue. In response to GC-induced peripheral IR and in an attempt to maintain normoglycaemia, pancreatic β-cells undergo several morphofunctional adaptations that result in hyperinsulinaemia. Failure of β-cells to compensate for this situation favours glucose homeostasis disruption, which can result in hyperglycaemia, particularly in susceptible individuals. GC treatment does not only alter pancreatic β-cell function but also affect them by their actions that can lead to hyperglucagonaemia, further contributing to glucose homeostasis imbalance and hyperglycaemia. In addition, the release of other islet hormones, such as somatostatin, amylin and ghrelin, is also affected by GC administration. These undesired GC actions merit further consideration for the design of improved GC therapies without diabetogenic effects. In summary, in this review, we consider the implication of GC treatment on peripheral IR, islet function and glucose homeostasis.

Metformin and male reproduction: effects on Sertoli cell metabolism

BACKGROUND AND PURPOSE

Metformin is commonly used to treat type 2 diabetes (T2D). While new clinical applications have been ascribed to metformin, including treatment of anovulatory infertility, its effects on male reproduction have not been investigated. The Sertoli cell (SC) is crucial for germ cell development, exerting metabolic control of spermatogenesis, therefore, we investigated the effects of metformin on SC metabolism.

EXPERIMENTAL APPROACH

Rat SCs were cultured in the absence and presence of metformin (5, 50 and 500 μM). mRNA and protein levels of glucose transporters (GLUT1 and GLUT3), phosphofructokinase 1 (PFK 1), lactate dehydrogenase (LDH) and monocarboxylate transporter 4 (MCT4) were determined by quantitative PCR and Western blot respectively. LDH activity was assessed and metabolite production/consumption determined by (1) H-NMR.

KEY RESULTS

Metformin (50 μM) decreased mRNA and protein levels of GLUT1, GLUT3, MCT4 and PFK 1 but did not affect LDH mRNA or protein levels. However, although glucose consumption was maintained in metformin-treated cells, LDH activity, lactate and alanine production were increased, indicating an enhanced glycolytic flux. No metabolic cytotoxicity was detected in SCs exposed to supra-pharmacological concentration of metformin.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that metformin: (i) decreases mRNA and protein levels of glycolysis-related transporters in SCs but increases their activity; and (ii) stimulates alanine production, which induces antioxidant activity and maintains the NADH/NAD(+) equilibrium. The increased lactate in metformin-treated SCs provides nutritional support and has an anti-apoptotic effect in developing germ cells. Thus, metformin can be considered as a suitable antidiabetic drug for male patients of reproductive age with T2D.

Glucose kinetics in the collagen-induced arthritis model: an all-in-one model to assess both efficacy and metabolic side effects of glucocorticoids

Prednisolone and other glucocorticoids (GCs) are potent anti-inflammatory drugs, but chronic use is hampered by metabolic side effects. Therefore, there is an urgent medical need for improved GCs that are as effective as classical GCs but have a better safety profile. A well-established model to assess anti-inflammatory efficacy is the chronic collagen-induced arthritis (CIA) model in mice, a model with features resembling rheumatoid arthritis. Models to quantify undesired effects of glucocorticoids on glucose kinetics are less well-established. Recently, we have described a model to quantify basal blood glucose kinetics using stably-labeled glucose. In the present study, we have integrated this blood glucose kinetic model in the CIA model to enable quantification of both efficacy and adverse effects in one animal model. Arthritis scores were decreased after treatment with prednisolone, confirming the anti-inflammatory properties of GCs. Both inflammation and prednisolone induced insulin resistance as insulin secretion was strongly increased whereas blood glucose concentrations and hepatic glucose production were only slightly decreased. This insulin resistance did not directly resulted in hyperglycemia, indicating a highly adaptive compensatory mechanism in these mice. In conclusion, this 'all-in-one' model allows for studying effects of (novel) GC compounds on the development of arthritis and glucose kinetics in a single animal. This integrative model provides a valuable tool for investigating (drug-induced) metabolic dysregulation in an inflammatory setting.

Murraya koenigii (L.) Spreng. ameliorates insulin resistance in dexamethasone-treated mice by enhancing peripheral insulin sensitivity

BACKGROUND

Murraya koenigii (L.) Spreng. is an important medicinal plant used traditionally as an antiemetic, antidiarrhoeal agent and blood purifier and as a medicine for a variety of ailments. This study investigated the effects of ethanolic extract of M. koenigii (MK) on diabetes-associated insulin resistance induced in mice by chronic low-dose injection of dexamethasone.

RESULTS

Mice treated with dexamethasone exhibited hyperglycaemia and impaired glucose tolerance. Treatment with MK reduced the extent of dexamethasone-induced hyperglycaemia and decreased insulin resistance as indicated by improved glucose tolerance and increased insulin-stimulated AKT phosphorylation in skeletal muscle tissue. Further evaluation in clonal skeletal muscle cell lines suggested that MK increased glucose uptake in L6 skeletal muscle cells by increasing cell surface GLUT4 density via an AKT-mediated pathway.

CONCLUSION

MK can ameliorate dexamethasone-induced hyperglycaemia and insulin resistance in part by increasing glucose disposal into skeletal muscle.

Two weeks of metformin treatment induces AMPK-dependent enhancement of insulin-stimulated glucose uptake in mouse soleus muscle

Metformin-induced activation of the 5'-AMP-activated protein kinase (AMPK) has been associated with enhanced glucose uptake in skeletal muscle, but so far no direct causality has been examined. We hypothesized that an effect of in vivo metformin treatment on glucose uptake in mouse skeletal muscles is dependent on AMPK signaling. Oral doses of metformin or saline treatment were given to muscle-specific kinase dead (KD) AMPKα2 mice and wild-type (WT) littermates either once or chronically for 2 wk. Soleus and extensor digitorum longus muscles were used for measurements of glucose transport and Western blot analyses. Chronic treatment with metformin enhanced insulin-stimulated glucose uptake in soleus muscles of WT (∼45%, P < 0.01) but not of AMPK KD mice. Insulin signaling at the level of Akt protein expression or Thr(308) and Ser(473) phosphorylation was not changed by metformin treatment. Insulin signaling at the level of Akt and TBC1D4 protein expression as well as Akt Thr(308)/Ser(473) and TBC1D4 Thr(642)/Ser(711) phosphorylation were not changed by metformin treatment. Also, protein expressions of Rab4, GLUT4, and hexokinase II were unaltered after treatment. The acute metformin treatment did not affect glucose uptake in muscle of either of the genotypes. In conclusion, we provide novel evidence for a role of AMPK in potentiating the effect of insulin on glucose uptake in soleus muscle in response to chronic metformin treatment.

Chromium enhances insulin responsiveness via AMPK

Trivalent chromium (Cr(3+)) is known to improve glucose homeostasis. Cr(3+) has been shown to improve plasma membrane-based aspects of glucose transporter GLUT4 regulation and increase activity of the cellular energy sensor 5' AMP-activated protein kinase (AMPK). However, the mechanism(s) by which Cr(3+) improves insulin responsiveness and whether AMPK mediates this action is not known. In this study we tested if Cr(3+) protected against physiological hyperinsulinemia-induced plasma membrane cholesterol accumulation, cortical filamentous actin (F-actin) loss and insulin resistance in L6 skeletal muscle myotubes. In addition, we performed mechanistic studies to test our hypothesis that AMPK mediates the effects of Cr(3+) on GLUT4 and glucose transport regulation. Hyperinsulinemia-induced insulin-resistant L6 myotubes displayed excess membrane cholesterol and diminished cortical F-actin essential for effective glucose transport regulation. These membrane and cytoskeletal abnormalities were associated with defects in insulin-stimulated GLUT4 translocation and glucose transport. Supplementing the culture medium with pharmacologically relevant doses of Cr(3+) in the picolinate form (CrPic) protected against membrane cholesterol accumulation, F-actin loss, GLUT4 dysregulation and glucose transport dysfunction. Insulin signaling was neither impaired by hyperinsulinemic conditions nor enhanced by CrPic, whereas CrPic increased AMPK signaling. Mechanistically, siRNA-mediated depletion of AMPK abolished the protective effects of CrPic against GLUT4 and glucose transport dysregulation. Together these findings suggest that the micronutrient Cr(3+), via increasing AMPK activity, positively impacts skeletal muscle cell insulin sensitivity and glucose transport regulation.

Chronic prednisolone treatment aggravates hyperglycemia in mice fed a high-fat diet but does not worsen dietary fat-induced insulin resistance

Synthetic glucocorticoids such as prednisolone have potent antiinflammatory actions. Unfortunately, these drugs induce severe adverse effects in patients, many of which resemble features of the metabolic syndrome, such as insulin resistance. In this study, we investigated whether adverse effects of prednisolone on glucose homeostasis are aggravated in mice with compromised insulin sensitivity due to a high-fat diet by applying various methods to analyze changes in insulin sensitivity in mice. C57BL/6J mice were fed a high-fat diet for 6 wk and treated with either prednisolone (10 mg/kg · d) or vehicle for the last 7 d. Insulin sensitivity and blood glucose kinetics were analyzed with state-of-the-art stable isotope procedures in different experimental conditions. Prednisolone treatment aggravated fasting hyperglycemia and hyperinsulinemia caused by high-fat feeding, resulting in a higher homeostatic assessment model of insulin resistance. In addition, prednisolone-treated high-fat diet-fed mice appeared less insulin sensitive by detailed analysis of basal glucose kinetics. Remarkably, using hyperinsulinemic-euglycemic or hyperglycemic clamp techniques, neither hepatic nor peripheral insulin resistance was worsened in the group that was treated with prednisolone. Yet analysis of hepatic glucose metabolism revealed that prednisolone did alter glycogen balance by reducing glycogen synthase flux under hyperinsulinemic as well as hyperglycemic conditions. In addition to elevated insulin levels, prednisolone-treated mice showed a major rise in plasma leptin and fibroblast growth factor 21 levels. Our data indicate that prednisolone-induced adverse effects on glucose metabolism in high-fat diet-fed mice do not reflect impaired insulin sensitivity but may be caused by other changes in the hormonal regulatory network controlling glucose metabolism such as fibroblast growth factor 21 and leptin.

Defining the contribution of AMP-activated protein kinase (AMPK) and protein kinase C (PKC) in regulation of glucose uptake by metformin in skeletal muscle cells

The importance of AMP-activated protein kinase (AMPK) and protein kinase C (PKC) as effectors of metformin (Met) action on glucose uptake (GU) in skeletal muscle cells was investigated. GU in L6 myotubes was stimulated 2-fold following 16 h of Met treatment and acutely enhanced by insulin in an additive fashion. Insulin-stimulated GU was sensitive to PI3K inhibition, whereas that induced by Met was not. Met and its related biguanide, phenformin, stimulated AMPK activation/phosphorylation to a level comparable with that induced by the AMPK activator, 5-amino-1-β-d-ribofuranosyl-imidazole-4-carboxamide (AICAR). However, the increase in GU elicited by AICAR was significantly lower than that induced by either biguanide. Expression of a constitutively active AMPK mimicked the effects of AICAR on GU, whereas a dominant interfering AMPK or shRNA silencing of AMPK prevented AICAR-stimulated GU and Met-induced AMPK signaling but only repressed biguanide-stimulated GU by ∼20%. Consistent with this, analysis of GU in muscle cells from α1(-/-)/α2(-/-) AMPK-deficient mice revealed a significant retention of Met-stimulated GU, being reduced by ∼35% compared with that of wild type cells. Atypical PKCs (aPKCs) have been implicated in Met-stimulated GU, and in line with this, Met and phenformin induced activation/phosphorylation of aPKC in L6 myotubes. However, although cellular depletion of aPKC (>90%) led to loss in biguanide-induced aPKC phosphorylation, it had no effect on Met-stimulated GU, whereas inhibitors targeting novel/conventional PKCs caused a significant reduction in biguanide-induced GU. Our findings indicate that although Met activates AMPK, a significant component of Met-stimulated GU in muscle cells is mediated via an AMPK-independent mechanism that involves novel/conventional PKCs.

Action of metformin on the insulin-signaling pathway and on glucose transport in human granulosa cells

CONTEXT

Hyperinsulinemia in polycystic ovary syndrome is widely treated with the insulin sensitizer metformin, which, in addition to its systemic effects, directly affects the ovarian insulin-stimulated steroidogenesis pathway.

OBJECTIVE

Our aim was to investigate the interaction of metformin with the other insulin-stimulated ovarian pathway, namely that leading to glucose uptake.

DESIGN

Human granulosa-luteal cells were cultured with metformin (10(-7) M), insulin (10 ng/ml) or metformin and insulin (met + ins) combined. Insulin receptor (IR) involvement was assessed by culture with an (anti)-insulin receptor (IR) antibody.

MAIN OUTCOME MEASURES

The effect of metformin on insulin-receptor substrate proteins 1 and 2 (IRS-1 and -2) mRNA and protein expression was determined. The KGN granulosa-cell line was used to investigate the effect of insulin and metformin on Akt activation and glucose transporter-4 (Glut-4) expression. Glut-4 translocation from the cytosol to the membrane was determined in cytoplasmic and membrane-enriched fractions of protein lysates.

RESULTS

IRS-1 mRNA and protein increased with all treatments. In contrast, basal IRS-2 mRNA levels were barely detectable, but transcription was up-regulated by metformin. The anti-IR antibody reduced treatment-stimulated IRS-1 to basal levels and IRS-2 expression to an even greater extent than IRS-1, showing greater dependence on the IR than IRS-1. Metformin in the presence of insulin activated Akt and this was dependent on phosphoinositide-3 kinase, as was translocation of Glut-4 to the membrane. Metformin was able to substantially enhance the insulin-stimulated translocation of Glut-4 transporters from the cytosol to the membrane.

CONCLUSION

This net increase in Glut-4 transporters in the plasma membrane has the potential to increase glucose uptake and metabolism by granulosa cells of the insulin-resistant polycystic ovary, thereby facilitating follicle growth.

Chronic prednisolone treatment reduces hepatic insulin sensitivity while perturbing the fed-to-fasting transition in mice

Chronic glucocorticoid use for treatment of inflammatory diseases is accompanied by severe side effects in humans (e.g. hyperglycemia and insulin resistance). The present studies were conducted to characterize consequences of chronic treatment with the synthetic glucocorticoid prednisolone on insulin sensitivity and blood glucose kinetics in mice. Prednisolone treatment increased fasting blood glucose and plasma insulin concentrations, but this apparently reduced insulin sensitivity could not be confirmed in hyperinsulinemic euglycemic clamp studies. Therefore, a novel method to study whole body glucose kinetics was used. This method revealed that prednisolone-treated mice show an increased hepatic glucose production (HGP). The increased HGP was accompanied by elevated plasma insulin concentrations, indicating reduced insulin sensitivity of hepatic glucose metabolism in prednisolone-treated mice. Compared with vehicle, prednisolone-treated mice had lower blood glucose concentrations, higher plasma free fatty acids, and higher plasma fibroblast growth factor-21 concentrations in the fed condition, i.e. mimicking a fasting situation. Next, the effects of 24-h fasting on energy metabolism were studied. Compared with controls, fasted prednisolone-treated mice had higher blood glucose concentrations and lower plasma beta-hydroxybutyrate concentrations. In conclusion, these results indicate that chronic prednisolone treatment reduces insulin sensitivity of HGP, induces a fasting-like phenotype in fed mice, and perturbs the fed-to-fasting transition.

Early toxicology signal generation in the mouse

The rat has been the preferred rodent toxicology species since before regulatory requirements have been in place, and there exists in the pharmaceutical industry and the regulatory agencies a significant amount of historical data for the rat. The resulting experience base with the rat makes the possibility of replacing it with the mouse for regulated toxicology studies untenable for all but the most extreme circumstances. However, toxicologists are very familiar with the mouse as a model for chronic carcinogenicity studies, and there exist multiple preclinical mouse models of disease. The authors evaluated the use of the mouse for early in vivo toxicology signal generation and prioritization of small molecule lead compounds prior to nomination of a development candidate. In five-day oral gavage studies with three test agents in the mouse, the authors were able to identify the same dose-limiting toxicities as those identified in the rat, including examples of compound-mediated hemolysis as well as microscopic lesions in the alimentary canal, kidney, and pancreas. Performing early signal generation studies in the mouse allows for earlier assessment of the safety liabilities of small molecules, requires significantly less compound, and allows evaluation of more compounds earlier in the project's life cycle.

Attenuation of hepatic expression and secretion of selenoprotein P by metformin

High serum selenium levels have been associated epidemiologically with increased incidence of type 2 diabetes. The major fraction of total selenium in serum is represented by liver-derived selenoprotein P (SeP). This study was undertaken to test for a hypothesized effect of hyperglycemia and the antihyperglycemic drug metformin on hepatic selenoprotein P biosynthesis. Cultivation of rat hepatocytes in the presence of high glucose concentrations (25 mmol/l) resulted in increased selenoprotein P mRNA expression and secretion. Treatment with metformin dose-dependently downregulated SeP mRNA expression and secretion, and suppressed glucocorticoid-stimulated production of SeP. Moreover, metformin strongly decreased mRNA levels of selenophosphate synthetase 2 (SPS-2), an enzyme essential for selenoprotein biosynthesis. Taken together, these results indicate an influence of metformin on selenium metabolism in hepatocytes. As selenoprotein P is the major transport form of selenium, metformin treatment may thereby diminish selenium supply to extrahepatic tissues.

Pharmacodynamics of glucose regulation by methylprednisolone. I. Adrenalectomized rats

Mechanisms related to the adverse effects of corticosteroids on glucose homeostasis were studied. Five groups of adrenalectomized (ADX) rats were given methylprednisolone (MPL) intravenously at 10 and 50 mg/kg, or a continuous 7 day infusion at rates of 0, 0.1, 0.3 mg/kg/h via subcutaneously implanted Alzet mini-pumps. Plasma concentrations of MPL, glucose and insulin were determined at various time points up to 72 h after injection or 336 h after infusion. The pharmacokinetics of MPL was captured with a two-compartment model. The Adapt II software was used in modeling. Injection of MPL caused a temporary glucose increase over 6 h by stimulating gluconeogenesis. The glucose changes stimulated pancreatic beta-cell secretion yielding a later insulin peak at around 10 h. In turn, insulin can stimulate glucose disposition. However, long-term MPL treatment caused continuous hyperglycemia during and after infusion. Insulin was increased during infusion, and immediately returned to baseline after the infusion was terminated, despite the almost doubled glucose concentration. A disease progression model incorporating the reduced endogenous glucose disposition was included to capture glucose homeostasis under different treatments. The results exemplify the importance of the steroid dosing regimen in mediating pharmacological and adverse metabolic effects. This mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model quantitatively describes the induction of hyperglycemia and provides additional insights into metabolic disorders such as diabetes.

Protein-tyrosine phosphatase 1B expression is induced by inflammation in vivo

Protein-tyrosine phosphatase 1B (PTP1B) is a major negative regulator of insulin and leptin sensitivity. PTP1B overexpression in adipose tissue and skeletal muscle of humans and rodents may contribute to insulin resistance and obesity. The mechanisms mediating PTP1B overexpression in obese and diabetic states have been unclear. We find that adipose tissue inflammation and the pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha) regulate PTP1B expression in vivo. High fat feeding of mice increased PTP1B expression 1.5- to 7-fold in adipose tissue, liver, skeletal muscle, and arcuate nucleus of hypothalamus. PTP1B overexpression in high fat-fed mice coincided with increased adipose tissue expression of the macrophage marker CD68 and TNFalpha, which is implicated in causing obesity-induced insulin resistance. TNFalpha increased PTP1B mRNA and protein levels by 2- to 5-fold in a dose- and time-dependent manner in adipocyte and hepatocyte cell lines. TNFalpha administration in mice increased PTP1B mRNA 1.4- to 4-fold in adipose tissue, liver, skeletal muscle, and hypothalamic arcuate nucleus and PTP1B protein 2-fold in liver. Actinomycin D treatment blocked, and high dose salicylate treatment inhibited by 80%, TNFalpha-induced PTP1B expression in adipocyte cell lines, suggesting TNFalpha may induce PTP1B transcription via nuclear factor kappaB (NFkappaB) activation. Chromatin immunoprecipitation from adipocyte cell lines and liver of mice demonstrated TNFalpha-induced recruitment of NFkappaB subunit p65 to the PTP1B promoter in vitro and in vivo. In mice with diet-induced obesity, TNFalpha deficiency also partly blocked PTP1B overexpression in adipose tissue. Our data suggest that PTP1B overexpression in multiple tissues in obesity is regulated by inflammation and that PTP1B may be a target of anti-inflammatory therapies.

Antidiabetogenic effects of chromium mitigate hyperinsulinemia-induced cellular insulin resistance via correction of plasma membrane cholesterol imbalance

Previously, we found that a loss of plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated filamentous actin (F-actin) structure contributes to insulin-induced insulin resistance. Interestingly, we also demonstrated that chromium picolinate (CrPic), a dietary supplement thought to improve glycemic status in insulin-resistant individuals, augments insulin-regulated glucose transport in insulin-sensitive 3T3-L1 adipocytes by lowering PM cholesterol. Here, to gain mechanistic understanding of these separate observations, we tested the prediction that CrPic would protect against insulin-induced insulin resistance by improving PM features important in cytoskeletal structure and insulin sensitivity. We found that insulin-induced insulin-resistant adipocytes display elevated PM cholesterol with a reciprocal decrease in PM PIP2. This lipid imbalance and insulin resistance was corrected by the cholesterol-lowering action of CrPic. The PM lipid imbalance did not impair insulin signaling, nor did CrPic amplify insulin signal transduction. In contrast, PM analyses corroborated cholesterol and PIP2 interactions influencing cytoskeletal structure. Because extensive in vitro study documents an essential role for cytoskeletal capacity in insulin-regulated glucose transport, we next evaluated intact skeletal muscle from obese, insulin-resistant Zucker (fa/fa) rats. Because insulin resistance in these animals likely involves multiple mechanisms, findings that cholesterol-lowering restored F-actin cytoskeletal structure and insulin sensitivity to that witnessed in lean control muscle were striking. Also, experiments using methyl-beta-cyclodextrin to shuttle cholesterol into or out of membranes respectively recapitulated the insulin-induced insulin-resistance and protective effects of CrPic on membrane/cytoskeletal interactions and insulin sensitivity. These data predict a PM cholesterol basis for hyperinsulinemia-associated insulin resistance and importantly highlight the reversible nature of this abnormality.

Glucocorticoids produce whole body insulin resistance with changes in cardiac metabolism

Insulin resistance is viewed as an insufficiency in insulin action, with glucocorticoids being recognized to play a key role in its pathogenesis. With insulin resistance, metabolism in multiple organ systems such as skeletal muscle, liver, and adipose tissue is altered. These metabolic alterations are widely believed to be important factors in the morbidity and mortality of cardiovascular disease. More importantly, clinical and experimental studies have established that metabolic abnormalities in the heart per se also play a crucial role in the development of heart failure. Following glucocorticoids, glucose utilization is compromised in the heart. This attenuated glucose metabolism is associated with altered fatty acid supply, composition, and utilization. In the heart, elevated fatty acid use has been implicated in a number of metabolic, morphological, and mechanical changes and, more recently, in "lipotoxicity". In the present article, we review the action of glucocorticoids, their role in insulin resistance, and their influence in modulating peripheral and cardiac metabolism and heart disease.

Metformin increases the PGC-1α protein and oxidative enzyme activities possibly via AMPK phosphorylation in skeletal muscle in vivo

AMP-activated protein kinase (AMPK), which was activated by an antihyperglycemic drug metformin, has been hypothesized to mediate metabolic adaptations. The purposes of the present study were 1) to confirm whether acute metformin administration induced AMPK phosphorylation and 2) to determine whether chronic metformin treatment increased the peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) protein expression, glycolytic and oxidative enzyme activities, and cytochrome c and glucose transporter-4 (GLUT4) protein expressions in the rat soleus and red and white gastrocnemius muscles. The single oral administration of metformin (300 mg/kg body wt) enhanced the AMPK phosphorylation at 5 and/or 6 h after treatment. In the chronic study, rats were fed either normal chow or chow containing 1% metformin for 14 days. Metformin treatment resulted in a mean daily metformin intake of 631 mg.kg body wt(-1).day(-1). Metformin increased the PGC-1alpha content in all three muscles. Metformin increased the hexokinase activity in the white gastrocnemius, the citrate synthase activity in all three muscles, and the beta-hydroxyacyl-CoA dehydrogenase activity in the soleus. The cytochrome c protein content in the soleus muscle also increased. The GLUT4 content was unchanged by metformin. These results suggest that metformin enhances the PGC-1alpha expression and mitochondrial biogenesis possibly at least in part via AMPK phosphorylation in the skeletal muscle. Metformin has thus been proposed to possibly ameliorate insulin resistance, at least partially, by means of such metabolic effects.

Determination of metformin based on amplification of its voltammetric response by a combination of molecular wire and carbon nanotubes

Molecular wires containing copper(II) (CuMW), in the form of the coordination polymer (Cu(II)4(bpp)4(maa)8(H2O)2).2H2O (bpp=1,3-bis(4-pyridyl)propane, maa=2-methylacrylic acid), and multiwalled carbon nanotubes (CNT) have been combined to prepare a paste electrode (CuMW/CNT/PE). The voltammetric response of the CuMW/CNT/PE to metformin (MET) was significantly greater than that of electrodes prepared from other materials, because of both the surface effect of CuMW and CNT and coordination of MET with the Cu(II) ion in the CuMW. A novel voltammetric method for determination of MET is proposed. In pH 7.2 Britton-Robinson buffer, using single sweep voltammetry, the second-order derivative peak current for oxidation of MET at 0.97 V (relative to SCE) increased linearly with MET concentration in the range 9.0 x 10(-7)-5.0 x 10(-5) mol L(-1) and the detection limit was 6.5 x 10(-7) mol L(-1).

Chromium picolinate positively influences the glucose transporter system via affecting cholesterol homeostasis in adipocytes cultured under hyperglycemic diabetic conditions

Since trivalent chromium (Cr(3+)) enhances glucose metabolism, interest in the use of Cr(3+)as a therapy for type 2 diabetes has grown in the mainstream medical community. Moreover, accumulating evidence suggests that Cr(3+) may also benefit cardiovascular disease (CVD) and atypical depression. We have found that cholesterol, a lipid implicated in both CVD and neurodegenerative disorders, also influences cellular glucose uptake. A recent study in our laboratory shows that exposure of 3T3-L1 adipocytes to chromium picolinate (CrPic, 10 nM) induces a loss of plasma membrane cholesterol. Concomitantly, accumulation of intracellularly sequestered glucose transporter GLUT4 at the plasma membrane was dependent on the CrPic-induced cholesterol loss. Since CrPic supplementation has the greatest benefit on glucose metabolism in hyperglycemic insulin-resistant individuals, we asked here if the CrPic effect on cells was glucose-dependent. We found that GLUT4 redistribution in cells treated with CrPic occurs only in cells cultured under high glucose (25 mM) conditions that resemble the diabetic-state, and not in cells cultured under non-diabetic (5.5 mM glucose) conditions. Examination of the effect of CrPic on proteins involved in cholesterol homeostasis revealed that the activity of sterol regulatory element-binding protein (SREBP), a membrane-bound transcription factor ultimately responsible for controlling cellular cholesterol balance, was upregulated by CrPic. In addition, ABCA1, a major player in mediating cholesterol efflux was decreased, consistent with SREBP transcriptional repression of the ABCA1 gene. Although the exact mechanism of Cr(3+)-induced cholesterol loss remains to be determined, these cellular responses highlight a novel and significant effect of chromium on cholesterol homeostasis. Furthermore, these findings provide an important clue to our understanding of how chromium supplementation might benefit hypercholesterolemia-associated disorders.

The phosphatidylinositol-3-kinase inhibitor PX-866 overcomes resistance to the epidermal growth factor receptor inhibitor gefitinib in A-549 human non-small cell lung cancer xenografts

Epidermal growth factor receptor (EGFR) inhibitors such as gefitinib show antitumor activity in a subset of non-small cell lung cancer (NSCLC) patients having mutated EGFR. Recent work shows that phosphatidylinositol-3-kinase (PI3-K) is coupled to the EGFR only in NSCLC cell lines expressing ErbB-3 and that EGFR inhibitors do not inhibit PI3-K signaling in these cells. The central role PI3-K plays in cell survival suggests that a PI3-K inhibitor offers a strategy to increase the antitumor activity of EGFR inhibitors in resistant NSCL tumors that do not express ErbB-3. We show that PX-866, a PI3-K inhibitor with selectivity for p110alpha, potentiates the antitumor activity of gefitinib against even large A-549 NSCL xenografts giving complete tumor growth control in the early stages of treatment. A-549 xenograft phospho-Akt was inhibited by PX-866 but not by gefitinib. A major toxicity of PX-866 administration was hyperglycemia with decreased glucose tolerance, which was reversed upon cessation of treatment. The decreased glucose tolerance caused by PX-866 was insensitive to the AMP-activated protein kinase inhibitor metformin but reversed by insulin and by the peroxisome proliferator-activated receptor-gamma activator pioglitazone. Prolonged PX-866 administration also caused increased neutrophil counts. Thus, PX-866, by inhibiting PI3-K signaling, may have clinical use in increasing the response to EGFR inhibitors such as gefitinib in patients with NSCLC and possibly in other cancers who do not respond to EGFR inhibition.

Is non-insulin dependent glucose uptake a therapeutic alternative? Part 1: physiology, mechanisms and role of non insulin-dependent glucose uptake in type 2 diabetes

Several decades of research for treating type 2 diabetes have yielded new drugs but the actual experience with the available oral antidiabetic compounds clearly shows that therapeutic needs are not matched. This highlights the urgent need for exploring other pathways. All cell types have the capacity to take up glucose independently of insulin, whereby basal but also hyperglycaemia-promoted glucose supply is ensured. Although poorly explored, insulin-independent glucose uptake might nevertheless represent a therapeutic target, as an alternative to the clear limits of actual drug treatments. This review not only critically examines some major pathways not requiring insulin (although they may be influenced by the hormone) but importantly, this analysis extends to the clinical applicability of these potential therapeutic principles by also considering their predictable tolerability for long-term therapy. In particular vascular safety (the ultimate problem linked with diabetes) will be envisaged because of the ubiquitous distribution of glucose transporters and some linked mechanisms. Several mechanisms can be identified which do not require insulin for their functioning. The first part of this review deals with the description, the regulation and the limits of some mechanisms representing potential pharmacological targets capable of having a highly significant impact on glucose uptake. These selected topics are: a) unmasking and/or activation of glucose transporters in cell plasma membranes, b) insulin mimetics acting at postreceptor level, c) activation of AMPK, d) increasing nitric oxide and e) increasing glucose-6P and glycogen stores.

Dexamethasone treatment causes resistance to insulin-stimulated cellular potassium uptake in the rat

Patients treated with glucocorticoids have elevated skeletal muscle ouabain binding sites. The major Na(+)-K(+)-ATPase (NKA) isoform proteins found in muscle, alpha2 and beta1, are increased by 50% in rats treated for 14 days with the synthetic glucocorticoid dexamethasone (DEX). This study addressed whether the DEX-induced increase in the muscle NKA pool leads to increased insulin-stimulated cellular K+ uptake that could precipitate hypokalemia. Rats were treated with DEX or vehicle via osmotic minipumps at one of two doses: 0.02 mg.kg(-1).day(-1) for 14 days (low DEX; n = 5 pairs) or 0.1 mg.kg(-1).day(-1) for 7 days (high DEX; n = 6 pairs). Insulin was infused at a rate of 5 mU.kg(-1).min(-1) over 2.5 h in conscious rats. Insulin-stimulated cellular K+ and glucose uptake rates were assessed in vivo by measuring the exogenous K+ infusion (K+(inf)) and glucose infusion (Ginf) rates needed to maintain constant plasma K+ and glucose concentrations during insulin infusion. DEX at both doses decreased insulin-stimulated glucose uptake as previously reported. Ginf (in mmol.kg(-1).h(-1)) was 10.2 +/- 0.6 in vehicle-treated rats, 5.8 +/- 0.8 in low-DEX-treated rats, and 5.2 +/- 0.6 in high-DEX-treated rats. High DEX treatment also reduced insulin-stimulated K+) uptake. K+(inf) (in mmol.kg(-1).h(-1)) was 0.53 +/- 0.08 in vehicle-treated rats, 0.49 +/- 0.14 in low-DEX-treated rats, and 0.27 +/- 0.08 in high-DEX-treated rats. DEX treatment did not alter urinary K+ excretion. NKA alpha2-isoform levels in the low-DEX-treated group, measured by immunoblotting, were unchanged, but they increased by 38 +/- 15% (soleus) and by 67 +/- 3% (gastrocnemius) in the high-DEX treatment group. The NKA alpha1-isoform level was unchanged. These results provide novel evidence for the insulin resistance of K+ clearance during chronic DEX treatment. Insulin-stimulated cellular K+ uptake was significantly depressed despite increased muscle sodium pump pool size.

Effect of dietary Platycodon grandiflorum on the improvement of insulin resistance in obese Zucker rats

The effect of dietary Platycodon grandiflorum on the improvement of insulin resistance and lipid profile was investigated in lean (Fa/-) and obese (fa/fa) Zucker rats, a model for noninsulin dependent diabetes mellitus. Dietary Platycodon grandiflorum feeding for 4 weeks resulted in a significant decrease in the concentration of plasma triglyceride in both lean and obese Zucker rats. Furthermore, dietary Platycodon grandiflorum markedly decreased both plasma cholesterol and fasting plasma insulin levels, and significantly decreased the postprandial glucose level at 30 min during oral glucose tolerance test in obese Zucker rats. Although there was no statistical significance, the crude glucose transporter 4 protein level of obese rats fed Platycodon grandiflorum tended to increase when compared with that of obese control rats. Therefore, the present results suggested that dietary Platycodon grandiflorum may be useful in prevention and improvement of metabolic disorders characterized by hyperinsulinemia states such as noninsulin dependent diabetes mellitus, syndrome X, and coronary artery disease.

Troglitazone prevents and reverses dexamethasone induced insulin resistance on glycogen synthesis in 3T3 adipocytes

Troglitazone lowers blood glucose levels in Type II diabetic patients. To evaluate the insulin sensitizing action of troglitazone on glycogen synthesis we have used dexamethasone-treated 3T3 adipocytes as an in vitro model. Differentiated 3T3 adipocytes were incubated with 100 nM dexamethasone for 6 days. Troglitazone (1.0 microM) or metformin (1.0 mM) with or without 200 nM insulin was added during the last 4 days. At the end, insulin (100 nM) stimulated glycogen synthesis was determined using (14)C-glucose. Dexamethasone caused a 50% reduction in glycogen synthesis. Troglitazone caused an approximately 3 fold increase in glycogen synthesis from 43.9+/-3.4 to 120+/-16.2 nmols h(-1). Under identical conditions metformin had no significant effect. When cells were incubated with troglitazone and dexamethasone simultaneously for 6 days, troglitazone but not metformin completely prevented dexamethasone-induced insulin resistance. RU 486 (1.0 microM) also completely prevented the insulin resistance. Chronic incubation with dexamethasone and insulin resulted in a 73% reduction in glycogen synthesis. In these adipocytes, troglitazone was partially active with glycogen synthesis rising from 23.1+/-3.0 to 44.4+/-4.5 nmol h(-1), P<0.01 while metformin was inactive. Troglitazone stimulated 2-deoxyglucose uptake by 2 - 3 fold in dexamethasone-treated adipocytes. Metformin also increased glucose uptake significantly. Troglitazone did not affect insulin binding while a 2 fold increase was observed in normal adipocytes where it exhibited a modest effect. Since the effect of troglitazone was greater in dexamethasone-treated adipocytes, troglitazone is likely to act by preventing dexamethasone-induced alterations which may include (i) binding to glucocorticoid receptor and (ii) effect on glucose uptake. These data demonstrate the direct insulin sensitizing action of troglitazone on glycogen synthesis and suggest a pharmacological profile different from metformin.

The antihyperglycaemic effect of metformin: therapeutic and cellular mechanisms

Metformin is regarded as an antihyperglycaemic agent because it lowers blood glucose concentrations in type 2 (non-insulin-dependent) diabetes without causing overt hypoglycaemia. Its clinical efficacy requires the presence of insulin and involves several therapeutic effects. Of these effects, some are mediated via increased insulin action, and some are not directly insulin dependent. Metformin acts on the liver to suppress gluconeogenesis mainly by potentiating the effect of insulin, reducing hepatic extraction of certain substrates (e.g. lactate) and opposing the effects of glucagon. In addition, metformin can reduce the overall rate of glycogenolysis and decrease the activity of hepatic glucose-6-phosphatase. Insulin-stimulated glucose uptake into skeletal muscle is enhanced by metformin. This has been attributed in part to increased movement of insulin-sensitive glucose transporters into the cell membrane. Metformin also appears to increase the functional properties of insulin- and glucose-sensitive transporters. The increased cellular uptake of glucose is associated with increased glycogen synthase activity and glycogen storage. Other effects involved in the blood glucose-lowering effect of metformin include an insulin-independent suppression of fatty acid oxidation and a reduction in hypertriglyceridaemia. These effects reduce the energy supply for gluconeogenesis and serve to balance the glucose-fatty acid (Randle) cycle. Increased glucose turnover, particularly in the splanchnic bed, may also contribute to the blood glucose-lowering capability of metformin. Metformin improves insulin sensitivity by increasing insulin-mediated insulin receptor tyrosine kinase activity, which activates post-receptor insulin signalling pathways. Some other effects of metformin may result from changes in membrane fluidity in hyperglycaemic states. Metformin therefore improves hepatic and peripheral sensitivity to insulin, with both direct and indirect effects on liver and muscle. It also exerts effects that are independent of insulin but cannot substitute for this hormone. These effects collectively reduce insulin resistance and glucotoxicity in type 2 diabetes.

Metformin interaction with insulin-regulated glucose uptake, using the Xenopus laevis oocyte model expressing the mammalian transporter GLUT4

The primary goal of this work was to better define, in molecular terms, the impact of metformin on hexose carriers. The methodology consisted of determining the zero-trans kinetics of 2-deoxy-D-glucose uptake for the mammalian insulin-sensitive glucose transporter (GLUT4) expressed in Xenopus laevis oocytes. These cells possessed the specialized protein and, when treated with insulin (2 microM) plus metformin (20 microM), showed a markedly enhanced hexose transport activity (2.4-fold increase over basal) as compared to that of cells incubated in the presence of insulin alone (1.8-fold increase over basal). Kinetic analysis of this process revealed that insulin induced a similar response to that observed for the native carrier, i.e., a higher Vmax. When metformin was added together with insulin, we mainly recorded a significant decrease in apparent Km for the sugar transported, Vmax being only marginally modified. Parathyroid hormone (PTH), which is known to impair the intrinsic activity of GLUT4, prevented the stimulatory effect of metformin in both kinds of oocytes whereas cytochalasin D, which interferes with the translocation of carriers, was without effect. These results suggest that metformin combined with insulin can maintain glucose homeostasis by increasing the catalytic activity of some hexose carriers or by improving the affinity of GLUT4 for glucose.