Effects of metformin on insulin receptor tyrosine kinase activity in rat adipocytes

Regulatory Effects of Metformin, an Antidiabetic Biguanide Drug, on the Metabolism of Primary Rat Adipocytes

Metformin is a biguanide compound commonly applied in humans with type 2 diabetes. The drug affects different tissues, including fat tissue. The direct influence of metformin on cells of fat tissue, i.e., adipocytes, is poorly elucidated. In the present study, the short-term (4-h) effects of metformin on lipogenesis, glucose transport, lipolysis, and lactate release in primary rat adipocytes were explored. It was demonstrated that metformin reduced insulin-induced lipogenesis and increased glucose transport into adipocytes. The tested compound also decreased lactate release from fat cells. It was shown that metformin substantially limited lipolysis stimulated by epinephrine (adrenergic receptor agonist) and dibutyryl-cAMP (direct activator of protein kinase A). Moreover, metformin decreased the lipolytic process triggered by DPCPX (adenosine A1 receptor antagonist). In the case of each lipolytic stimulator, the drug evoked a similar inhibitory effect in the presence of 3 and 12 mM glucose. The lipolytic response of adipocytes to epinephrine was also found to be reduced by metformin when glucose was replaced by alanine. It was demonstrated that the tested compound limits the release of both glycerol and fatty acids from fat cells. The results of the present study provided evidence that metformin significantly affects the metabolism of primary rat adipocytes. Its action covers processes related to lipid accumulation and release and occurs after relatively short-term exposure.

Impact of Molecular Symmetry/Asymmetry on Insulin-Sensitizing Treatments for Type 2 Diabetes

Although the advantages and disadvantages of asymmetrical thiazolidinediones as insulin-sensitizers have been well-studied, the relevance of symmetry and asymmetry for thiazolidinediones and biguanides has scarcely been explored. Regarding symmetrical molecules, only one thiazolidinedione and no biguanides have been evaluated and proposed as an antihyperglycemic agent for treating type 2 diabetes. Since molecular structure defines physicochemical, pharmacological, and toxicological properties, it is important to gain greater insights into poorly investigated patterns. For example, compounds with intrinsic antioxidant properties commonly have low toxicity. Additionally, the molecular symmetry and asymmetry of ligands are each associated with affinity for certain types of receptors. An advantageous response obtained in one therapeutic application may imply a poor or even adverse effect in another. Within the context of general patterns, each compound must be assessed individually. The current review aimed to summarize the available evidence for the advantages and disadvantages of utilizing symmetrical and asymmetrical thiazolidinediones and biguanides as insulin sensitizers in patients with type 2 diabetes. Other applications of these same compounds are also examined as well as the various uses of additional symmetrical molecules. More research is needed to exploit the potential of symmetrical molecules as insulin sensitizers.

Effect of metformin on estrogen and progesterone receptor-positive (MCF-7) and triple-negative (MDA-MB-231) breast cancer cells

This work aimed to investigate the effect of metformin on cellular glucose uptake and metabolism by breast cancer cells, as a mechanism contributing to its anticancer properties. Estrogen and progesterone receptor-positive (MCF-7) and triple-negative (MDA-MB-231) breast cancer cell lines were used as in vitro models of breast cancer. Short-term (26 min) exposure of MCF-7 and MDA-MB-231 cells to metformin inhibited uptake of ³H-deoxy-D-glucose (³H-DG). In contrast, long-term (24 h) exposure to metformin (5 μM-1 mM) concentration-dependently increased ³H-DG uptake in both cell lines. This effect was associated with an increase in lactate production but was not associated with changes in GLUT1 mRNA expression. Long-term exposure of MCF-7 and MDA-MB-231 cells to metformin (5 μM-1 mM) concentration-dependently reduced cell viability and culture mass and slightly increased cell proliferation rates. Combination of metformin (1 mM) with the facilitative glucose transporter (GLUT) inhibitor kaempferol (30 μM) did not change the effect of metformin on culture growth. In conclusion, short-term exposure to metformin reduces cellular glucose uptake, probably by direct inhibition of GLUT1. However, after long-term exposure to metformin, cellular uptake of glucose is significantly increased, not associated to changes in GLUT1 transcription rates. We suggest that, in the long-term, metformin induces a compensatory increase in glucose uptake in response to cellular energy depletion resulting from its inhibitory effect on mitochondrial oxidative phosphorylation machinery. Metformin-induced dependence of breast cancer cells on glycolytic pathway, associated with an anticarcinogenic effect of the drug, provides a biochemical basis for the design of new therapeutic strategies.

Paracrine and endocrine effects of adipose tissue on cancer development and progression

The past few years have provided substantial evidence for the vital role of the local tumor microenvironment for various aspects of tumor progression. With obesity and its pathophysiological sequelae still on the rise, the adipocyte is increasingly moving center stage in the context of tumor stroma-related studies. To date, we have limited insight into how the systemic metabolic changes associated with obesity and the concomitant modification of the paracrine and endocrine panel of stromal adipocyte-derived secretory products ("adipokines") influence the incidence and progression of obesity-related cancers. Here, we discuss the role of adipocyte dysfunction associated with obesity and its potential impact on cancer biology.

Evidence-based and potential benefits of metformin in the polycystic ovary syndrome: a comprehensive review

Metformin is an insulin sensitizer widely used for the treatment of patients affected by type 2 diabetes mellitus. Because many women with polycystic ovary syndrome (PCOS) are insulin resistant, metformin was introduced in clinical practice to treat these patients also. Moreover, metformin's effect has other targets beside its insulin-sensitizing action. The present review was aimed at describing all evidence-based and potential uses of metformin in PCOS patients. In particular, we will analyze the uses of metformin not only for the treatment of all PCOS-related disturbances such as menstrual disorders, anovulatory infertility, increased abortion, or complicated pregnancy risk, hyperandrogenism, endometrial, metabolic and cardiovascular abnormalities, but also for the prevention of the syndrome.

Insulin resistance and metformin in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a heterogeneous disorder with widespread systemic manifestations affecting 5-10% of women of reproductive age. The accompanying insulin resistance and hypeinsulinemia mark this syndrome as a prediabetic state, with high incidence of impaired glucose tolerance, gestational diabetes, and overt diabetes. Other metabolic and biochemical changes, such as hypertension and dyslipidemia, increase the risk of cardiovascular disease. Fertility may also be impaired due to anovulation, impaired implantation, and higher rates of spontaneous abortions. All of these effects may also be related to hyperinsulinemia. Metformin, as insulin-sensitizing drug, is being evaluated for its potential long-term disease-modifying effect, such as prevention of diabetes. Its use may also help restore spontaneous ovulation and improve menstrual cyclicity, improve the success rate of induction of ovulation with clomiphene citrate and FSH, and decrease the high rate of ovarian hyperstimulation and early pregnancy loss. Nevertheless, these new exiting potential benefits of metformin should be evaluated in large randomized controlled studies, and clinicians must counsel women appropriately before the initiation of metformin therapy.

Inhibition of leptin secretion by insulin and metformin in cultured rat adipose tissue

Leptin's role in the regulation of food intake, energy expenditure and weight control are widely recognized, especially in rodents. Likewise, the potential regulation of leptin secretion by insulin (and vice versa) has been of particular interest insofar as these nutrient signals may have meaningful, even adverse (inter)actions, in diabetes. We used a freshly isolated rat adipose tissue culture model to examine the effect of insulin, metformin and glibenclamide on basal and steroid-stimulated leptin secretion. This model was selected because of its physiologic rates of leptin formation and preservation of potentially significant cell-cell interactions compared to isolated cells. The basal rate of leptin secretion was 3. 4+/-1.2 ng/100 mg tissue/24 h. The addition of 100 nM dexamethasone or 400 nM hydrocortisone stimulated leptin secretion by 3-4 fold over basal (no steroid). Insulin inhibited both basal and steroid-activated leptin secretion by 35-50%. This inhibition was present with either 1 mM pyruvate or 5 mM glucose as a substrate suggesting that glycolysis was not required. Metformin inhibited basal and dexamethasone-stimulated leptin secretion in a dose dependent manner (50% inhibition occurred at 1 mM metformin) while glibenclamide was ineffective. The effect of insulin on isolated fat cells versus fat tissue was tested in parallel. After 24 h in culture, insulin inhibited leptin secretion similarly in both adipose preparations. The addition of 200 nM (-)N6-(2-phenylisopropyl)-adenosine did not alter the results.

Effects of metformin and vanadium on leptin secretion from cultured rat adipocytes

OBJECTIVE

We have reported that glucose utilization regulates leptin expression and secretion from isolated rat adipocytes. In this study, we employed two antidiabetic agents that act to increase glucose uptake by peripheral tissues, metformin and vanadium, as pharmacological tools to examine the effects of altering glucose utilization on leptin secretion in primary cultures of rat adipocytes.

RESEARCH METHODS AND PROCEDURES

Isolated adipocytes (100 microL of packed cells per well) were anchored in a defined matrix of basement membrane components (Matrigel) with media containing 5.5 mM glucose and incubated for 96 hours with metformin or vanadium. Leptin secretion, glucose utilization, and lactate production were assessed.

RESULTS

Metformin (0.5 and 1.0 mM) increased glucose uptake in the presence of 0.16 nM insulin by 37 +/- 10% (p < 0.005) and 62 +/- 8% (p < 0.0001) over insulin alone, respectively. Metformin from 0.5 to 5.0 mM increased lactate production by 105 +/- 43% (p < 0.025) to 202 +/- 52% (p < 0.0025) and at 1.0 and 5.0 mM increased the proportional rate of glucose conversion to lactate by 78 +/- 18% (p < 0.005) and 166 +/- 41% (p < 0.0025), respectively. At concentrations less than 0.5 mM, metformin did not affect leptin secretion, but at 0.5 mM, the only concentration that significantly increased glucose utilization without increasing glucose conversion to lactate, leptin secretion was modestly stimulated (by 20 +/- 9%; p < 0.05). Concentrations from 1.0 to 25 mM inhibited leptin secretion by 25 +/- 8% (p < 0.005) to 89 +/- 4% (p < 0.0001). Across metformin doses, leptin secretion was inversely related to the percentage of glucose taken up and released as lactate (r = -0.74; p < 0.0001). Vanadium (5 to 20 microM) increased glucose uptake from 20 +/- 7% (p < 0.01) to 34 +/- 13% (p < 0.02) and increased lactate production at 5 microM by 17 +/- 8% (p < 0.025) and 10 microM by 61 +/- 20% (p < 0.02) but did not alter the conversion of glucose to lactate. Vanadium (5 to 50 microM) inhibited leptin secretion by 33 +/- 6% (p < 0.0025) to 61 +/- 8% (p < 0.0001).

DISCUSSION

Both metformin and vanadium increase glucose uptake and inhibit leptin secretion from cultured adipocytes. The inhibition of leptin secretion by metformin is related to an increase in the metabolism of glucose to lactate. The inhibition by vanadium most likely involves direct effects on cellular phosphatases. We hypothesize that the effect of glucose utilization to stimulate leptin production involves the metabolism of glucose to a fate other than anaerobic lactate production, possibly oxidation or lipogenesis.

Insulin sensitizers and antiandrogens in the treatment of polycystic ovary syndrome

The heterogeneous origin of polycystic ovary syndrome (PCOS) has been demonstrated by several studies. Abnormalities in steroidogenesis and metabolism are present, but the exact link between these two pathologic features remains to be clarified. In clinical practice, more than one therapeutic approach for the treatment of this syndrome has been proposed over the last few decades. Because hyperandrogenism and hyperinsulinemia contribute to a different degree to the phenotype of PCOS, therapeutic efforts have focused on agents that could treat or modify the clinical manifestations of these disorders. Antiandrogens as a sole treatment or combined with oral contraceptives are considered the treatment of choice for the manifestations of hyperandrogenemia, but there is no agreement about their efficacy on the metabolic sequelae of PCOS (insulin resistance, hyperinsulinemia, dislipidemia). Furthermore, the improvement of insulin sensitivity by insulin sensitizers may be of therapeutic value directly and/or indirectly in the management of clinical manifestations of hyperinsulinemia and hyperandrogenemia.

Metformin modulates insulin receptor signaling in normal and cholesterol-treated human hepatoma cells (HepG2)

The effects of the biguanide anti-hyperglycemic agent, metformin (N,N'-dimethyl-biguanide), on insulin signaling was studied in a human hepatoma cell line (HepG2). Cells were cultured in the absence (control cells) or in the presence of 100 microM of a cholesterol derivative, hemisuccinate of cholesterol. Cholesterol hemisuccinate-treatment alters cholesterol and lipid content of HepG2 and modulates membrane fluidity. Cholesterol hemisuccinate-treatment induces a decrease in insulin responsiveness and creates an 'insulin-resistant' state in these cells. Exposure to 100 microM of metformin resulted in a significant enhancement of insulin-stimulated lipogenesis in control and cholesterol hemisuccinate-treated cells. In control cells, metformin altered glycogenesis in a biphasic manner. In cholesterol hemisuccinate-treated cells, metformin inhibited basal glycogenesis but restored insulin-stimulated glycogenesis. Hence, to understand the mechanism of metformin action, we analyzed early steps in the insulin signaling pathway, including insulin receptor autophosphorylation, mitogen-activated-protein kinase and phosphatidylinositol 3-kinase activities, in both control and cholesterol hemisuccinate-treated cells. Overall, the results suggest that metformin may interact with the insulin receptor and/or a component involved in the early steps of insulin signal transduction.

Metformin-associated lactic acidosis: a rare or very rare clinical entity?

AIMS

Lactic acidosis is a well recognized complication of biguanide therapy which is potentially serious. Although the prevalence of metformin-associated lactic acidosis (MALA) is much lower than that associated with phenformin, it is still being reported sporadically which raises concerns for the practising clinicians. We review the currently available world-wide data of the prevalence of MALA, the risk factors for its development and the current practical guidelines on the use of metformin to minimize the risk of this potential hazard.

METHODS

An extensive literature search was conducted from both Medline and Ovid (1965-98) using the following keywords: 'Type 2 diabetes mellitus', 'oral hypoglycaemic drugs', 'biguanides', 'metformin-associated lactic acidosis' and 'renal impairment'.

RESULTS

MALA was found to be a very rare clinical entity, being 20 times less common than phenformin-associated lactic acidosis. Amongst all the risk factors, renal impairment appears to be the major precipitating factor for the development of MALA in metformin-treated patients. We also found cases of MALA where no precipitating factors were identified and the underlying mechanism in these cases remains unclear. Practical recommendations of metformin use to minimize the risk of MALA have been listed based on previous reports.

CONCLUSIONS

The low prevalence of MALA is comparable to the prevalence of sulphonylurea-induced hypoglycaemia. Metformin has many beneficial metabolic effects in the management of Type 2 diabetes mellitus. Provided that the recommended guidelines for metformin use are strictly adhered to, its widespread use would be safe and the incidence of MALA will be further reduced.

Stimulation of the intracellular portion of the human insulin receptor by the antidiabetic drug metformin

Our prior work suggested that the antidiabetic metformin must enter the cell to act and that the drug stimulates tyrosine kinase activity. We now report that therapeutic concentrations (approximately 1 microg/mL) of metformin stimulated the tyrosine kinase activity of the intracellular portion of the beta-subunit of the human insulin receptor (IPbetaIRK), the intracellular portion of the epidermal growth factor receptor and pp60-src, but not cAMP-dependent protein kinase. A derivative of metformin unable to lower glucose was ineffective in stimulating IPbetaIRK. Two derivatives more effective than metformin in patients were also more effective than metformin in stimulating IPbetaIRK. Higher levels (10-100 microg/mL) of metformin or methylglyoxyl bis(guanylhydrazone) inhibited the tyrosine kinases, and this inhibition may be responsible for the ability of these two drugs to block cell proliferation.

Metformin: an antihyperglycemic agent for treatment of type II diabetes

OBJECTIVE

To review the comparative efficacy of metformin, sulfonylureas, and insulin in the treatment of patients with type II diabetes.

DATA SOURCES

Articles were identified by a MEDLINE search of articles from 1966 to 1994, using the terms metformin, sulfonylurea, chlorpropamide, glipizide, glyburide, tolazamide, tolbutamide, and insulin, published in English, French, or German. Articles also were identified from bibliographies of pertinent articles.

STUDY SELECTION

With the exception of articles dealing with the pharmacology of metformin, only randomized, active, controlled studies were selected for review.

DATA EXTRACTION

Effects of metformin therapy on metabolic and cardiovascular risk factors were abstracted: weight, blood pressure, total and low-density lipoprotein cholesterol, triglycerides, fasting and postprandial glucose, and glycosylated hemoglobin.

DATA SYNTHESIS

Metformin is an antihyperglycemic agent with a mean bioavailability of 50-60%. It is eliminated primarily by renal filtration and secretion and has a half-life of approximately 6 hours in patients with type II diabetes. Although the half-life of metformin is prolonged in patients with renal impairment, no specific dosage adjustments have been recommended. This agent has no effect in the absence of insulin. Metformin is as effective as the sulfonylureas in treating patients with type II diabetes and has a more prominent postprandial effect than the sulfonylureas or insulin. When combined with a sulfonylurea, metformin has been shown to exert antihyperglycemic effects in addition to the sulfonylurea with which it is combined. Metformin decreases absorption of vitamin B12 and folic acid, although reported cases of megaloblastic anemia are rare. Cimetidine decreases the elimination of metformin; therefore, the manufacturer reccommends a reduced metformin dosage when these agents are combined. The most frequently reported adverse effects of metformin are gastrointestinal in nature (diarrhea, nausea, abdominal pain, and metallic taste, in decreasing order). Metformin has been used in Canada, Great Britain, and the rest of Europe for more than 30 years and was approved for use in the US in December 1994.

CONCLUSIONS

Three trials comprise the Food and Drug Administration approval database (one foreign). Metformin will be most useful in managing patients with poorly controlled postprandial hyperglycemia, as its postprandial effect is much greater than that of the sulfonylureas. In contrast, sulfonylureas or insulin are more effective for managing patients with poorly controlled fasting hyperglycemia. Metformin should be considered a first-line agent, particularly in obese or hyperlipidemic patients.

Effects of metformin on glucose and glucagon regulated gluconeogenesis in cultured normal and diabetic hepatocytes

The effects of glucose and glucagon on the anti-gluconeogenic action of metformin were investigated in normal and diabetic hepatocytes. Glucose production from lactate was elevated by 88% in hepatocytes from fasted normal rats compared with hepatocytes from fed animals. Diabetes caused 3.5- and 2.1-fold increases in hepatic gluconeogenesis under fasting and fed conditions, respectively. Metformin (250 microM) suppressed glucose production by 37% in normal and by 30% in diabetic hepatocytes from fed rats. This drug was more effective (up to 67%) with increasing concentrations of glucose in the medium. Potentiation by metformin of insulin action on gluconeogenesis was elevated significantly (P < 0.01 to 0.001) by glucose in vitro. Metformin (75-250 microM) also counteracted the effects of glucagon at optimal concentrations in normal (32-68%) as well as diabetic (8-46%) hepatocytes. The findings of this study indicate that (i) the anti-gluconeogenic effect of metformin is enhanced by glucose in vivo and in vitro; and (ii) the suppression of glucagon-induced gluconeogenesis by metformin could play a role in its glucose-lowering effects in diabetic conditions.

Treatment of non-insulin-dependent diabetes mellitus and its complications. A state of the art review

Non-insulin-dependent diabetes mellitus (NIDDM) is a major health problem which occurs predominantly in the older population; 16.8% of persons over age 65 years have NIDDM. The total health costs of NIDDM are in excess of $US20 billion annually. The primary objective in the treatment of NIDDM is to achieve normoglycaemia, without aggravating coexisting abnormalities. Common abnormalities include obesity, hypertension, retinopathy, nephropathy and neuropathies. Diet, and consequent bodyweight reduction, is the cornerstone of therapy for NIDDM. Total calorie intake should be limited, while the percentage of calories from carbohydrates should be increased and that from fats and cholesterol should be decreased. Exercise may also help to reduce bodyweight. Sulphonylurea drugs stimulate insulin secretion from beta-cells, and may be a useful adjunct to nonpharmacological therapy. Failure to respond to sulphonylurea drugs may be primary (25 to 30% of initially treated patients) or secondary (5 to 10% per year). It is not clear which is the most effective pharmacological intervention in such cases. Options include switching to or combining therapy with insulin, a biguanide, or other insulin-sparing antihyperglycaemic agents, e.g. alpha-glucosidase inhibitors, thiazolidinediones, chloroquine or hydroxychloroquine, or fibric acid derivatives such as clofibrate. Other experimental agents include the fatty acid oxidation inhibitors and dichloroacetate. Specific agents, such as antihypertensives, lipid lowering agents and sorbitol inhibitors, may be needed to prevent the complications arising from the spectrum of clinical and metabolic abnormalities which arise from insulin resistance.

Metformin--an update

Metformin (dimethylbiguanide) is an antihyperglycaemic drug used to treat non-insulin dependent diabetes mellitus. It acts in the presence of insulin to increase glucose utilization and reduce glucose production, thereby countering insulin resistance. The effects of metformin include increased glucose uptake, oxidation and glycogenesis by muscle, increased glucose metabolism to lactate by the intestine, reduced hepatic gluconeogenesis and possibly a reduced rate of intestinal glucose absorption. Metformin appears to facilitate steps in the postreceptor pathways of insulin action, and may exert effects that are independent of insulin. In muscle, metformin increases translocation into the plasma membrane of certain isoforms of the glucose transporter. The effects of metformin are generally moderate, and do not cause clinical hypoglycaemia or increased weight gain. Metformin has an antihypertriglyceridaemic effect and exerts various potentially useful effects on haemostasis. A risk of lactic acidosis is negligible provided that the contraindications, particularly renal incompetence are respected.

Functional characterization of insulin and IGF-I receptors in chicken lens epithelial and fiber cells

Insulin and insulin-like growth factor I (IGF-I) play a role in lens cell growth and development. The binding of these hormones to their respective receptors with its concomitant signal transduction is an important step in these cellular processes. Hormone binding to adult chicken lens insulin and IGF-I receptors, partially purified from epithelial and fiber cells, was studied to examine this activity in lens. The associated stimulation of receptor-mediated tyrosine kinase by the hormones was also studied. At an insulin concentration of 0.02 nM, specific binding was similar for epithelial and fiber receptor preparations (Epi = 0.23 +/- 0.03 fmol, Fib = 0.19 +/- 0.02 fmol). Displacement studies revealed that there was also no difference between epithelial and fiber receptor preparations in the concentration of insulin necessary for half maximal displacement of specific [125I]-insulin binding (IC50: Epi = 0.32 nM +/- 0.07 nM, Fib = 0.31 nM +/- 0.05 nM). Comparison of IGF-I (0.02 nM) binding to receptor preparations from epithelial and fiber cells demonstrated that specific binding was similar in the two preparations (Epi = 0.50 +/- 0.05 fmol, Fib = 0.42 +/- 0.05 fmol). Also, there was no difference in the concentration of IGF-I necessary for half maximal displacement of specific [125I]-IGF-I binding (IC50 = Epi: 0.27 +/- 0.05 nM, Fib: 0.28 +/- 0.04 nM). The ability of IGF-I to displace bound [125I]-insulin was also examined. The IC50 for IGF-I binding to the insulin receptors isolated from epithelial and fiber cells was 37.4 +/- 2.4 nM, and 35.4 +/- 2.8 nM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of oral hypoglycaemic drugs with insulin on steroid metabolism in hepatocytes isolated from control and diabetic male rats

The effects of the oral hypoglycaemic drugs, phenformin and tolbutamide, and insulin, alone and in combination, on steroid metabolism in hepatocytes isolated from control and streptozotocin-diabetic male rats has been studied. Both phenformin and tolbutamide mimic the action of insulin in stimulating hepatic steroid metabolism in a dose-dependent manner in control cells. Unlike insulin, however, both drugs give a similar effect in cells derived from diabetic animals although to a lesser extent. Both drugs can partially restore the effect of insulin in cells derived from diabetic animals. Biguanides and sulphonylureas, therefore, have a direct effect on liver cells to mimic insulin action and can still have an effect under conditions where insulin is inactive. Both types of oral hypoglycaemics can also affect insulin-insensitive cells isolated from diabetic rat liver to restore to a certain extent their response to insulin.

Dual effect of metformin in cultured rat hepatocytes: potentiation of insulin action and prevention of insulin-induced resistance

The ability of the biguanide hypoglycemic agent metformin to improve the acute effects of insulin on glucose and/or lipid metabolism was investigated in both insulin-responsive and insulin-resistant cultured rat hepatocytes: (1) metformin (20 micrograms/mL, 16 hours) increased the insulin-dependent stimulation of glycogen and lipid synthesis through an exclusive enhancement of the responsiveness without modification of the cell sensitivity to the hormone; (2) metformin neither altered basal glycogenesis from [U-14C]glucose and basal lipogenesis from [1-14C]acetate nor insulin binding. These results indicate the ability of this drug to selectively potentiate the acute action of insulin at a postreceptor step in normal liver cells. A prolonged incubation with insulin (16 hours, 5 x 10(-7) mol/L) led the hepatocytes to a state of resistance evidenced by a 50% decrease in their maximal responsiveness and sensitivity to a subsequent acute stimulation by the hormone, as assessed on lipogenesis. Addition of metformin (20 micrograms/mL) during the overnight incubation of hepatocytes with insulin prevented the decrease in cell responsiveness and sensitivity to the hormone for the stimulation of lipogenesis, thus showing that metformin was able to hamper the development of the resistant state to the hormone in this pathway. These results strongly suggest that metformin improves type 2 diabetes through an effect at the hepatic level on both insulin action and insulin-induced resistance.

Effect of metformin treatment on insulin action in diabetic rats: in vivo and in vitro correlations

The mechanism (both at the whole body and cellular level) by which metformin improves insulin sensitivity has yet to be defined. In the present study, we examined in vivo insulin-mediated whole-body glucose disposal, glycogen synthesis, hepatic glucose production, and insulin secretion, as well as in vitro muscle insulin receptor tyrosine kinase activity in eight control, eight neonatal streptozotocin diabetic rats, and eight diabetic rats before and after treatment with metformin. Ten weeks after birth diabetic rats had higher fasting (132 + 5 v 101 + 2 mg/dL) and postmeal (231 + 10 v 133 + 3) plasma glucose levels compared with controls (P less than .001). Metformin treatment was followed by a significant decrease in the growth rate and normalized glucose tolerance without enhancing the deficient insulin response. Insulin-mediated glucose uptake in diabetic versus control rats was reduced (P less than .01) during the high-dose (15.4 + 0.6 v 18.3 + 1.0 mg/kg.min) insulin clamp study and was increased to values greater (P less than .05) than controls following metformin treatment. Muscle glycogen synthetic rate in vivo, measured by incorporation of 3H-3-glucose radioactivity, was diminished by 25% (P less than .01) in diabetic rats, restored to normal values with metformin, and correlated closely (r = .82, P less than .002) with total-body glucose uptake during the insulin clamp in all three groups. Insulin receptor tyrosine kinase activity, measured in partially purified insulin receptors, was reduced in diabetic rats and increased to supernormal levels after metformin. The decrease in muscle tyrosine kinase activity in diabetic versus control animals was entirely accounted for by a reduction in maximal velocity (Vmax) (32 v 45 pmol/mg.min, P less than .01) and increased to supernormal levels following metformin (91 pmol/mg.min, P less than .001) without any change in affinity (Km). Muscle tyrosine kinase activity was closely correlated with both the muscle glycogen synthetic rate (r = .82, P less than .002) and total-body insulin-mediated glucose disposal (r = .64, P less than .01) in vivo. The close correlation between in vivo insulin action, muscle glycogen synthesis, and muscle insulin receptor tyrosine kinase activity is consistent with an important role of the enzyme in the insulin resistance of diabetes and its improvement following metformin treatment.

[Physiopathology of non-insulin-dependent diabetes: current data and therapeutic consequences]

Non insulin-dependent diabetes mellitus results from the combination in varying proportions of low plasma insulin levels (insulinopenia), peripheral resistance to insulin and increased hepatic glucose production. Abnormalities of insulin secretion can be demonstrated without and after stimulation. Insulin resistance mainly occurs in skeletal muscle and is primarily due to a "postreceptor" defect. A pancreatic peptide, amylin, may participate in insulin resistance. Hepatic glucose production correlates with high fasting plasma glucose concentrations. Whatever its initial mechanism, hyperglycaemia maintains low insulin secretion and insulin resistance by its toxicity. In the light of these data, the effects of weight loss in obese non insulin-dependent diabetics have become clearer. The action of biguanides on insulin sensitivity is confirmed. Sulphonylureas have a pancreatic and an extrapancreatic action. The normoglycaemia obtained by intermittent insulin therapy can break the vicious circle of glucose toxicity. The use of prolonged insulin therapy is discussed. Finally, new compounds with an original mode of action offer hopes for the future.

The effects of metformin on adipocyte insulin action and metabolic control in obese subjects with type 2 diabetes

To investigate the mechanisms of action of metformin, insulin receptor binding and the activity of several insulin-controlled metabolic pathways were measured in adipocytes taken from 10 obese Type 2 diabetic patients treated for 4 weeks with either metformin (0.5 g x 3 daily) or matching placebo using a double-blind crossover design. Metformin therapy was associated with a significant fall in serum fructosamine levels (3.1 +/- 0.4 vs 2.8 +/- 0.4 mmol l-1, p less than 0.02) as well as fasting (10.8 +/- 2.4 vs 9.4 +/- 2.1 mmol l-1) and daytime (11.5 +/- 2.4 vs 10.0 +/- 2.2 mmol l-1) plasma glucose concentrations (p less than 0.05). Fasting and postprandial plasma levels of C-peptide and insulin were unchanged. While fasting plasma lactate concentrations remained unaltered after metformin, a rise was noted in response to meals (from 1.4 +/- 0.1 to 1.8 +/- 0.2 mmol l-1, p less than 0.05). Adipocyte insulin receptor binding was unaffected by drug treatment. Moreover, no insulin-like effects or post-binding potentiation of insulin action could be found on adipocyte glucose transport, glucose oxidation, lipogenesis, glycolysis or antilipolysis. A complementary in vitro study using adipocytes from non-obese healthy volunteers failed to show any direct effect of metformin on adipocyte insulin binding or glucose transport and metabolism, at media drug concentrations corresponding to therapeutic plasma levels.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

Metformin enhances certain insulin actions in cultured rat hepatoma cells

The effect of the oral antidiabetic agent metformin on insulin regulation of glycogen metabolism, tyrosine-aminotransferase activity, and [1-14C]aminoisobutyric acid uptake was studied in H4IIE cultured rat hepatoma cells. Metformin enhanced both basal (from 0.213 +/- 0.016 to 0.262 +/- 0.024 nmol/mg protein, p less than 0.01) and insulin stimulated [3H] glucose incorporation into glycogen in a time-dependent and dose-dependent manner. A small effect of metformin was seen at 1 mumol/l, and its greatest effects were obtained at 10 mumol/l. At the same concentrations, metformin did not influence basal tyrosine-aminotransferase activity but it potentiated insulin stimulated tyrosine-aminotransferase activity (+29.2 +/- 1.4%, p less than 0.01) and prevented the loss of tyrosine-aminotransferase responsiveness to insulin in H4IIE cells desensitised by a previous exposure to insulin. In contrast, metformin had no effect on basal or insulin-stimulated [1-14C]aminoisobutyric acid uptake. Over the concentrations of metformin that enhanced insulin action in H4IIE cells, the drug had no significant effect on insulin binding to its receptor. These studies suggest, therefore, that metformin may influence cellular metabolism by potentiating certain insulin actions through mechanisms that may be beyond insulin receptor binding.

Treatment--metformin

The hyperglycaemia of NIDDM is associated with insulin resistance due, in part, to reduced insulin receptor binding and more especially postreceptor defects. Metformin is an antihyperglycaemic agent which can be used to ameliorate insulin resistance. It appears to act directly on insulin target cells to enhance insulin action. Although metformin may increase insulin-receptor binding, its main effect appears to be directed at the postreceptor level of insulin action. Accordingly the drug potentiates insulin-suppression of hepatic gluconeogenesis and increases insulin-mediated peripheral glucose uptake and metabolism. It does not stimulate insulin release, does not cause weight gain and does not cause clinical hypoglycaemia. The risk of lactate accumulation should be appreciated in patients with renal insufficiency, liver dysfunction and following acute illness with hypoxia, when therapy should be stopped. Although metformin is often bracketed with phenformin in the context of lactic acidosis, different pharmacodynamics and adherence to prescribing guidelines render such a comparison unwarranted.

The effect of metformin on glycaemic control, intermediary metabolism and blood pressure in non-insulin-dependent diabetes mellitus

Fourteen non-insulin-dependent diabetics (9 female, 5 male), aged 46 to 64 years, uncontrolled by diet (fasting plasma glucose greater than or equal to 8 mmol/l), were treated with metformin, 1-3 g daily, and followed prospectively at 1 week, then at 2-weekly intervals for 6 months. The fasting plasma glucose fell significantly (p less than 0.01) after 1 week of therapy and HbA1 showed a significant reduction, 13.4 +/- 2.5% (mean +/- SD) to 10.7 +/- 1.8% (normal range 5.1-9.3%) at 8 weeks (p less than 0.001). There was no significant change in body weight, plasma insulin, serum cholesterol or triglycerides, blood lactate, pyruvate, glycerol, alanine, 3-hydroxybutyrate or acetoacetate concentration. Systolic, diastolic, and mean arterial blood pressures, along with resting heart rate remained unchanged. This study shows that metformin effectively lowers plasma glucose without hyperinsulinism or significant rise in fasting blood lactate and other gluconeogenic precursors.