Inhibition of basal and glucagon-induced hepatic glucose production by 991 and other pharmacological AMPK activators.

Pharmacological AMPK activation represents an attractive approach for the treatment of type 2 diabetes (T2D). AMPK activation increases skeletal muscle glucose uptake, but there is controversy as to whether AMPK activation also inhibits hepatic glucose production (HGP) and pharmacological AMPK activators can have off-target effects that contribute to their anti-diabetic properties. The main aim was to investigate the effects of 991 and other direct AMPK activators on HGP and determine whether the observed effects were AMPK-dependent. In incubated hepatocytes, 991 substantially decreased gluconeogenesis from lactate, pyruvate and glycerol, but not from other substrates. Hepatocytes from AMPKß1-/- mice had substantially reduced liver AMPK activity, yet the inhibition of glucose production by 991 persisted. Also, the glucose-lowering effect of 991 was still seen in AMPKß1-/- mice subjected to an intraperitoneal pyruvate tolerance test. The AMPK-independent mechanism by which 991 treatment decreased gluconeogenesis could be explained by inhibition of mitochondrial pyruvate uptake and inhibition of mitochondrial sn-glycerol-3-phosphate dehydrogenase-2. However, 991 and new-generation direct small-molecule AMPK activators antagonized glucagon-induced gluconeogenesis in an AMPK-dependent manner. Our studies support the notion that direct pharmacological activation of hepatic AMPK as well as inhibition of pyruvate uptake could be an option for the treatment of T2D-linked hyperglycemia.

Two isoprenylated flavonoids from Dorstenia psilurus activate AMPK, stimulate glucose uptake, inhibit glucose production and lower glycemia.

Root extracts of a Cameroon medicinal plant, Dorstenia psilurus, were purified by screening for AMP-activated protein kinase (AMPK) activation in incubated mouse embryo fibroblasts (MEFs). Two isoprenylated flavones that activated AMPK were isolated. Compound 1 was identified as artelasticin by high-resolution electrospray ionization mass spectrometry and 2D-NMR while its structural isomer, compound 2, was isolated for the first time and differed only by the position of one double bond on one isoprenyl substituent. Treatment of MEFs with purified compound 1 or compound 2 led to rapid and robust AMPK activation at low micromolar concentrations and increased the intracellular AMP:ATP ratio. In oxygen consumption experiments on isolated rat liver mitochondria, compound 1 and compound 2 inhibited complex II of the electron transport chain and in freeze-thawed mitochondria succinate dehydrogenase was inhibited. In incubated rat skeletal muscles, both compounds activated AMPK and stimulated glucose uptake. Moreover, these effects were lost in muscles pre-incubated with AMPK inhibitor SBI-0206965, suggesting AMPK dependency. Incubation of mouse hepatocytes with compound 1 or compound 2 led to AMPK activation, but glucose production was decreased in hepatocytes from both wild-type and AMPKß1-/- mice, suggesting that this effect was not AMPK-dependent. However, when administered intraperitoneally to high-fat diet-induced insulin-resistant mice, compound 1 and compound 2 had blood glucose-lowering effects. In addition, compound 1 and compound 2 reduced the viability of several human cancer cells in culture. The flavonoids we have identified could be a starting point for the development of new drugs to treat type 2 diabetes.

Genetic deletion of soluble 5'-nucleotidase II reduces body weight gain and insulin resistance induced by a high-fat diet.

We previously investigated whether inhibition of AMP-metabolizing enzymes could enhance AMP-activated protein kinase (AMPK) activation in skeletal muscle for the treatment of type 2 diabetes. Soluble 5'-nucleotidase II (NT5C2) hydrolyzes IMP and its inhibition could potentially lead to a rise in AMP to activate AMPK. In the present study, we investigated effects of NT5C2 deletion in mice fed a normal-chow diet (NCD) or a high-fat diet (HFD). On a NCD, NT5C2 deletion did not result in any striking metabolic phenotype. On a HFD however, NT5C2 knockout (NT5C2-/-) mice displayed reduced body/fat weight gain, improved glucose tolerance, reduced plasma insulin, triglyceride and uric acid levels compared with wild-type (WT) mice. There was a tendency towards smaller and fewer adipocytes in epididymal fat from NT5C2-/- mice compared to WT mice, consistent with a reduction in triglyceride content. Differences in fat mass under HFD could not be explained by changes in mRNA expression profiles of epididymal fat from WT versus NT5C2-/- mice. However, rates of lipolysis tended to increase in epididymal fat pads from NT5C2-/- versus WT mice, which might explain reduced fat mass. In incubated skeletal muscles, insulin-stimulated glucose uptake and associated signalling were enhanced in NT5C2-/- versus WT mice on HFD, which might contribute towards improved glycemic control. In summary, NT5C2 deletion in mice protects against HFD-induced weight gain, adiposity, insulin resistance and associated hyperglycemia.

Structural Determinants for Small-Molecule Activation of Skeletal Muscle AMPK α2ß2γ1 by the Glucose Importagog SC4.

The AMP-activated protein kinase (AMPK) αßγ heterotrimer regulates cellular energy homeostasis with tissue-specific isoform distribution. Small-molecule activation of skeletal muscle α2ß2 AMPK complexes may prove a valuable treatment strategy for type 2 diabetes and insulin resistance. Herein, we report the small-molecule SC4 is a potent, direct AMPK activator that preferentially activates α2 complexes and stimulates skeletal muscle glucose uptake. In parallel with the term secretagog, we propose "importagog" to define a substance that induces or augments cellular uptake of another substance. Three-dimensional structures of the glucose importagog SC4 bound to activated α2ß2γ1 and α2ß1γ1 complexes reveal binding determinants, in particular a key interaction between the SC4 imidazopyridine 4'-nitrogen and ß2-Asp111, which provide a design paradigm for ß2-AMPK therapeutics. The α2ß2γ1/SC4 structure reveals an interaction between a ß2 N-terminal α helix and the α2 autoinhibitory domain. Our results provide a structure-function guide to accelerate development of potent, but importantly tissue-specific, ß2-AMPK therapeutics.

A novel PKB/Akt inhibitor, MK-2206, effectively inhibits insulin-stimulated glucose metabolism and protein synthesis in isolated rat skeletal muscle.

PKB (protein kinase B), also known as Akt, is a key component of insulin signalling. Defects in PKB activation lead to insulin resistance and metabolic disorders, whereas PKB overactivation has been linked to tumour growth. Small-molecule PKB inhibitors have thus been developed for cancer treatment, but also represent useful tools to probe the roles of PKB in insulin action. In the present study, we examined the acute effects of two allosteric PKB inhibitors, MK-2206 and Akti 1/2 (Akti) on PKB signalling in incubated rat soleus muscles. We also assessed the effects of the compounds on insulin-stimulated glucose uptake, glycogen and protein synthesis. MK-2206 dose-dependently inhibited insulin-stimulated PKB phosphorylation, PKBß activity and phosphorylation of PKB downstream targets (including glycogen synthase kinase-3α/ß, proline-rich Akt substrate of 40 kDa and Akt substrate of 160 kDa). Insulin-stimulated glucose uptake, glycogen synthesis and glycogen synthase activity were also decreased by MK-2206 in a dose-dependent manner. Incubation with high doses of MK-2206 (10 µM) inhibited insulin-induced p70 ribosomal protein S6 kinase and 4E-BP1 (eukaryotic initiation factor 4E-binding protein-1) phosphorylation associated with increased eEF2 (eukaryotic elongation factor 2) phosphorylation. In contrast, Akti only modestly inhibited insulin-induced PKB and mTOR (mammalian target of rapamycin) signalling, with little or no effect on glucose uptake and protein synthesis. MK-2206, rather than Akti, would thus be the tool of choice for studying the role of PKB in insulin action in skeletal muscle. The results point to a key role for PKB in mediating insulin-stimulated glucose uptake, glycogen synthesis and protein synthesis in skeletal muscle.