Effect of chronic metformin treatment of hepatic and muscle glycogen metabolism in KK mice

Crataegus aronia prevents high-fat diet-induced hepatic steatosis in rats by activating AMPK-induced suppression of SREBP1 and activation of PPARα

This study examined if the aqueous extract of Crataegus aronia (C. aronia) can prevent high-fat diet (HFD)-induced hepatic steatosis in rats by activating AMPK. Adult male Wistar rats were fed either a control diet or HFD for 12 weeks and treated either with vehicle (normal saline) or C. aronia extract (200 mg/kg/orally), daily. Also, hepatocytes were treated with increasing concentrations of the extract in the presence or absence of compound C (CC), an AMPK inhibitor. C. aronia prevented the increase in serum and hepatic lipids, reduced hepatic levels of reactive oxygen species, and increased hepatic glutathione and superoxide dismutase levels. It also downregulated the hepatic expression of SREBP1/2, fatty acid synthase, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase but stimulated the activity of AMPK and levels of peroxisome proliferator-activated receptor-alpha. Similar effects were reported in the cultured cells, in a dose-dependent manner but were prevented by CC. In conclusion, C. aronia ameliorates HFD-induced hepatic steatosis and oxidative stress by activating AMPK. PRACTICAL APPLICATIONS: The use of the aqueous extract of Crataegus aronia has been extensively used during the last years in traditional medicine to treat chronic disorders including nonalcoholic fatty liver disease. The findings of this study support these findings and suggest that oral administration of C. aronia aqueous extract has potent hypoglycemic effect and demonstrate the mechanism of action mimics such drugs such as metformin and involves activation of AMPK and peroxisome proliferator-activated receptor-alpha. These findings are very encouraging for further biochemical analysis and isolation of active ingredients responsible for these effects to be used in more clinical trials.

Solid Lipid Nanoparticles of Myricitrin Have Antioxidant and Antidiabetic Effects on Streptozotocin-Nicotinamide-Induced Diabetic Model and Myotube Cell of Male Mouse

Type 2 diabetes mellitus (T2DM) may occur via oxidative stress. Myricitrin is a plant-derived antioxidant, and its solid lipid nanoparticle (SLN) may be more potent. Hence, the present study was conducted to evaluate the effects of myricitrin SLN on streptozotocin-nicotinamide- (STZ-NA-) induced T2DM of the mouse and hyperglycemic myotube. In this experimental study, cold homogenization method was used to prepare SLN. Then, 120 adult male NMRI mice were divided into 7 groups: control, vehicle, diabetes (received STZ 65 mg/kg 15 min after injected NA 120 mg/kg), diabetes + SLN containing myricitrin 1, 3, and 10 mg/kg, and diabetes + metformin. For in vitro study, myoblast (C2C12) cell line was cultured and divided into 6 groups (n = 3): control, hyperglycemia, hyperglycemia + SLN containing myricitrin 1, 3, and, 10 μM, and hyperglycemia + metformin. After the last nanoparticle treatment, plasma samples, pancreas and muscle tissues, and myotubes were taken for experimental assessments. Diabetes increased lipid peroxidation and reduced antioxidant defense along with the hyperglycemia, insulin resistance, and pancreas apoptosis. Hyperglycemia induced oxidative stress, antioxidant impairment, and cellular apoptosis. Myricitrin SLN improved diabetes and hyperglycemia complications in the in vivo and in vitro studies. Therefore, SLN of myricitrin showed antioxidant, antidiabetic, and antiapoptotic effects in the mouse and myotube cells.

A novel peptide nanomedicine for treatment of pancreatogenic diabetes

Pancreatogenic diabetes (PD) is a potentially fatal disease that occurs secondary to pancreatic disorders. The current anti-diabetic therapy for PD is fraught with adverse effects that can increase morbidity. Here we investigated the efficacy of novel peptide nanomedicine: pancreatic polypeptide (PP) in sterically stabilized micelles (SSM) for management of PD. PP exhibits significant anti-diabetic efficacy but its short plasma half-life curtails its therapeutic application. To prolong and improve activity of PP in vivo, we evaluated the delivery of PP in SSM. PP-SSM administered to rats with PD, significantly improved glucose tolerance, insulin sensitivity and hepatic glycogen content compared to peptide in buffer. The studies established the importance of micellar nanocarriers in protecting enzyme-labile peptides in vivo and delivering them to target site, thereby enhancing their therapeutic efficacy. In summary, this study demonstrated that PP-SSM is a promising novel anti-diabetic nanomedicine and therefore should be further developed for management of PD.

FROM THE CLINICAL EDITOR

Pancreatic peptide was earlier demonstrated to address pancreatogenic diabetes, but its short half-life represented major difficulties in further development for therapeutic use. PP-SSM (pancreatic polypeptide in sterically stabilized micelles) is a promising novel anti-diabetic nanomedicine that enables prolonged half-life and increased bioactivity of PP, as shown in this novel study, paving the way toward clinical studies in the near future.

Functional characterization of the adenosine receptor contributing to glycogenolysis and gluconeogenesis in rat hepatocytes

The adenosine receptor subtype mediating glucose production by glycogenolysis and gluconeogenesis was studied in primary cultured rat hepatocytes. Adenosine and adenosine agonists caused cyclic AMP accumulation in rat hepatocytes. The order of potency was 5'-N-ethylcarboxamidoadenosine (NECA)>R(-)-N(6)-(2-phenylisopropyl)adenosine (RPIA)>adenosine>2-[p-(carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). Furthermore, adenosine agonists stimulated glycogenolysis and gluconeogenesis. The order of potency was NECA>RPIA>CGS21680. The rank order of potency is typical for adenosine A(2B) receptors. Glycogenolysis stimulated by NECA was fully inhibited by nonselective adenosine antagonists, 9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS15943). However, the adenosine A(2A) receptor-selective antagonist, 8-(3-chlorostyryl)caffeine (CSC), and the adenosine A(1) receptor-selective antagonist, (+)-(R)-[(E)-3-(2-phenylpyrazolo[1,5-alpha]pyridin-3-yl)acryloyl]-2-piperidine ethanol (FK453), had a low inhibitory potency. A strong correlation was found between the inhibitory effect of adenosine antagonists on NECA-induced glucose production and that on intracellular cyclic AMP generation in rat hepatocytes. Our results suggest that adenosine stimulates cyclic AMP formation and regulates glycogenolysis and gluconeogenesis, most likely through the adenosine A(2B) receptor subtype in rat hepatocytes.

Effects of combined pioglitazone and metformin on diabetes and obesity in Wistar fatty rats

1. The effects of combined treatment with pioglitazone.HCl and metformin on diabetes and obesity were investigated in Wistar fatty rats, which are hyperglycaemic and hypertriglyceridaemic and have higher plasma levels of total ketone bodies than lean rats. 2. Plasma glucose was significantly decreased when pioglitazone.HCl or metformin was administered alone and combined treatment accentuated this decrease. The administration of pioglitazone.HCl, but not metformin, also decreased plasma levels of triglyceride and total ketone bodies. 3. The glycogen content of skeletal muscle was not increased by pioglitazone.HCl or metformin alone, but was increased by combined treatment (P=0.003, ANOVA). 4. Pioglitazone.HCl produced increased food intake and bodyweight in hyperphagic Wistar fatty rats; however, concurrent administration of metformin significantly ameliorated these pioglitazone.HCl-induced increases. 5. These results indicate that combined treatment with pioglitazone.HCl and metformin induces a marked hypoglycaemic effect accompanied by a reduction in plasma levels of total ketone bodies and prevention of excessive bodyweight gain in Wistar fatty rats. These favourable effects suggest that the combination would be beneficial in treating patients with type 2 diabetes.

Obligatory role of membrane events in the regulatory effect of metformin on the respiratory chain function

From recent findings about the indirect effect of metformin (MET) targeted on the respiratory chain complex I, we reconsidered this question and tried to determine the causality of any alteration at this enzymatic level using Xenopus laevis oocytes. Addition of MET (50 microM) reduced by 40% the rotenone-sensitive activity of complex I only in incubating intact oocytes but not in mitochondria isolated by differential centrifugation. The drug prior injected inside these cells had also no measurable effect. In spite of this and the weak binding of MET to the mitochondrial fraction, there was a fairly good correlation between the marked inhibitory action of MET on complex I and its progressive appearance within the oocyte cytoplasm. The intriguing observation that MET as a liposomal form was again able to exert its role when added directly to isolated mitochondria is in accordance with a membrane-mediated uptake and vesicular routing of MET. Furthermore, a temperature-dependent effect was clearly shown. At 4 degrees, oocytes failed to take up efficiently MET and accordingly its subsequent action on respiration was therefore lost. Likewise, MET transport was hindered and inhibition of complex I totally disappeared when a structural analog, asymmetrical dimethylarginine (ADMA), was placed together with MET either at an identical concentration or in excess. These data strongly support the view that MET may recognise some specific membranous sites, likely belonging to effector systems, before penetrating the cell in a bound state via an obscure endocytotic event which still has to be identified.

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.

Importance of the intestine as a site of metformin-stimulated glucose utilization

1. The intestine has been implicated as a site of increased glucose utilization by the antihyperglycaemic drug, metformin. This study makes a quantitative assessment of this effect. 2. Glucose utilization by the intestine and hind limb region was determined by arterial-venous glucose difference adjusted for blood flow rate in fasted rats receiving a hyperglycaemic hyperinsulinaemic infusion. 3. Intrajejunal administration of metformin, 250 mg kg-1, increased glucose disposal during the infusion procedure, associated with increased glucose utilization in the intestine by 69% and in the hind limb region by 40%. 4. Metformin, 250 mg kg-1, increased glucose disappearance during an intravenous glucose tolerance test. This was accompanied by increased uptake of tritiated 2-deoxy-D-glucose into the intestinal mucosa to a greater extent than into skeletal muscles (per unit wet weight of tissue). 5. The results demonstrate that the intestinal mucosa is a quantitatively important site of increased glucose utilization during the blood glucose-lowering effect of metformin.

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.