Role of hepatic AMPK activation in glucose metabolism and dexamethasone-induced regulation of AMPK expression

To elucidate the role of AMPK in hepatic glucose metabolism, dominant negative (DN), constitutively active (CA) forms of the AMPKalpha1 subunit and control vector LacZ were overexpressed by means of adenovirus-mediated gene transfer. Five days after virus injection, hepatic AMPK activity was five-fold higher in CA mice than in DN mice. DN mice were apparently glucose intolerant with a higher fasting plasma glucose level (DN 82.3+/-0.7mg/dl, CA 42.5+/-4.8mg/dl and LacZ 54.3+/-2.4mg/dl). PEPCK, a gluconeogenic key enzyme, mRNA was increased 131.54% and 48.92% in DN mice compared to that of CA and LacZ, respectively. Thus, hepatic AMPK activation plays a role in the suppression of gluconeogenesis and this might be the cause of decreased fasting plasma glucose level in CA mice. We also investigated the effects of dexamethasone on hepatic AMPK expression and activity in rat liver, mice liver, as well as primary cultured hepatocytes. Subcutaneously injecting mice with dexamethasone (1mg/day) for 5 days significantly upregulated hepatic AMPKalpha1 and alpha2 expressions. Similarly, the treatment of primary cultured rat hepatocytes with dexamethasone (1microM) increased expression of the AMPKalpha1 subunit, AICAR-induced AMPK phosphorylation and kinase activity. Although increased AMPK expression cannot be attributed to dexamethasone-induced glucose intolerance, taken together our results raise the possibility that AMPK control liver glucose output and its expression in liver might be modulated by various hormones and growth factors.

Resistin-like Molecule β Activates MAPKs, Suppresses Insulin Signaling in Hepatocytes, and Induces Diabetes, Hyperlipidemia, and Fatty Liver in Transgenic Mice on a High Fat Diet

Resistin and resistin-like molecules (RELMs) are a family of proteins reportedly related to insulin resistance and inflammation. Because the serum concentration and intestinal expression level of RELMbeta were elevated in insulin-resistant rodent models, in this study we investigated the effect of RELMbeta on insulin signaling and metabolism using transgenic mice and primary cultured hepatocytes. First, transgenic mice with hepatic RELMbeta overexpression were shown to exhibit significant hyperglycemia, hyperlipidemia, fatty liver, and pancreatic islet enlargement when fed a high fat diet. Hyperinsulinemic glucose clamp showed a decreased glucose infusion rate due to increased hepatic glucose production. In addition, the expression levels of IRS-1 and IRS-2 proteins as well as the degrees of insulin-induced phosphatidylinositol 3-kinase and Akt activations were attenuated in RELMbeta transgenic mice. Similar down-regulations of IRS-1 and IRS-2 proteins were observed in primary cultured hepatocytes chronically treated (for 24 h) with RELMbeta, suggesting the insulin resistance-inducing effect of RELMbeta to be direct. Furthermore, it was shown that RELMbeta acutely and markedly activates ERK and p38, while weakly activating JNK, in primary cultured hepatocytes. This increased basal p38 phosphorylation level was also observed in the livers of RELMbeta transgenic mice. In conclusion, RELMbeta, a gut-derived hormone, impairs insulin signaling probably via the activations of classic MAPKs, and increased expression of RELMbeta may be involved in the pathogenesis of glucose intolerance and hyperlipidemia in some insulin-resistant models. Thus, RELMbeta is a potentially useful marker for assessing insulin resistance and may also be a target for future novel anti-diabetic agents.

Glycogen debranching enzyme association with beta-subunit regulates AMP-activated protein kinase activity

AMP-activated protein kinase (AMPK) regulates both glycogen and lipid metabolism functioning as an intracellular energy sensor. In this study, we identified a 160-kDa protein in mouse skeletal muscle lysate by using a glutathione-S-transferase (GST)-AMPK fusion protein pull-down assay. Mass spectrometry and a Mascot search revealed this protein to be a glycogen debranching enzyme (GDE). The association between AMPK and GDE was observed not only in the overexpression system but also endogenously. Next, we showed the beta1-subunit of AMPK to be responsible for the association with GDE. Furthermore, experiments using deletion mutants of the beta1-subunit of AMPK revealed amino acids 68-123 of the beta1-subunit to be sufficient for GDE binding. W100G and K128Q, both beta1-subunit mutants, are reportedly incapable of binding to glycogen, but both bound GDE, indicating that the association between AMPK and GDE does not involve glycogen. Rather, the AMPK-GDE association is likely to be direct. Overexpression of amino acids 68-123 of the beta1-subunit inhibited the association between endogenous AMPK and GDE. Although GDE activity was unaffected, basal phosphorylation and kinase activity of AMPK, as well as phosphorylation of acetyl-CoA carboxylase, were significantly increased. Thus it is likely that the AMPK-GDE association is a novel mechanism regulating AMPK activity and the resultant fatty acid oxidation and glucose uptake.

Serum concentrations of resistin-like molecules beta and gamma are elevated in high-fat-fed and obese db/db mice, with increased production in the intestinal tract and bone marrow

AIMS/HYPOTHESIS

Resistin and the resistin-like molecules (RELMs) comprise a novel class of cysteine-rich proteins. Among the RELMs, RELMbeta and RELMgamma are produced in non-adipocyte tissues, but the regulation of their expression and their physiological roles are largely unknown. We investigated in mice the tissue distribution and dimer formation of RELMbeta and RELMgamma and then examined whether their serum concentrations and tissue expression levels are related to insulin resistance.

METHODS

Specific antibodies against RELMbeta and RELMgamma were generated. Dimer formation was examined using COS cells and the colon. RELMbeta and RELMgamma tissue localisation and expression levels were analysed by an RNase protection assay, immunoblotting and immunohistochemical study. Serum concentrations in high-fat-fed and db/db mice were also measured using the specific antibodies.

RESULTS

The intestinal tract produces RELMbeta and RELMgamma, and colonic epithelial cells in particular express both RELMbeta and RELMgamma. In addition, RELMbeta and RELMgamma were shown to form a homodimer and a heterodimer with each other, in an overexpression system using cultured cells, and in mouse colon and serum. Serum RELMbeta and RELMgamma levels in high-fat-fed mice were markedly higher than those in mice fed normal chow. Serum RELMbeta and RELMgamma concentrations were also clearly higher in db/db mice than in lean littermates. Tissue expression levels revealed that elevated serum concentrations of RELMbeta and RELMgamma are attributable to increased production in the colon and bone marrow.

CONCLUSIONS/INTERPRETATION

RELMbeta and RELMgamma form homo/heterodimers, which are secreted into the circulation. Serum concentrations of RELMbeta and RELMgamma may be a novel intestinal-tract-mediating regulator of insulin sensitivity, possibly involved in insulin resistance induced by obesity and a high-fat diet.

Insulin-nonspecific reduction in skeletal muscle glucose transport in high-fat-fed rats

High-fat feeding diminishes insulin-stimulated glucose transport in skeletal muscle. However, conflicting results are reported regarding whether phosphatidylinositol (PI)-3 kinase-independent glucose transport is also impaired in insulin-resistant high-fat-fed rodents. The aim of the present study was to study whether non-insulin-dependent mechanisms for stimulation of glucose transport are defective in skeletal muscle from high-fat-fed rats. Rats were fed normal chow diet or high-fat diet for 4 weeks and isolated epitrochlearis muscles were used for measuring glucose transport. Insulin-stimulated glucose transport was significantly lower in rats fed the high-fat diet compared with chow-fed rats (P < .05). Hypoxia-stimulated glucose transport was also reduced in high-fat-fed rats (P < .05). Nevertheless, hypoxia-stimulated adenosine monophosphate-activated protein kinase (AMPK) phosphorylation (Thr172) level was not affected by high-fat feeding. Glucose transport by sodium nitroprusside stimulation was reduced in high-fat-fed rats (P < .05). Protein content of glucose transporter (GLUT)-4 and AMPK-alpha, and glycogen content were comparable between both groups. Our findings provide evidence that high-fat feeding can affect not only insulin but also non-insulin-stimulated glucose transport. A putative defect in common steps in glucose transport may play a role to account for impaired insulin-stimulated glucose transport in rats fed a high-fat diet.

Effects of imidapril, an angiotensin-converting enzyme inhibitor, on insulin sensitivity and responsiveness in streptozotocin-induced diabetic rats

We have studied the effect of imidapril, an angiotensin-converting enzyme inhibitor, on streptozotocin-induced diabetic rats. A sequential euglycemic hyperinsulinemic clamp procedure was used (insulin infusion rates: 3 and 30 mU/kg BW/min) in 30 diabetic rats. The rats were divided in 6 groups: a control group, a control group with N-monomethyl-L-arginine (L-NMMA, 1 mg/kg/min, a nitric oxide synthase inhibitor) infusion, a streptozotocin-induced diabetic group, a diabetic group with L-NMMA infusion, a diabetic group involving imidapril infusion (5 microg/kg/min), and a diabetic group involving simultaneous imidapril and L-NMMA infusion. Glucose concentrations were maintained around 140 mg/dl during the clamp studies. Plasma insulin levels during the 3 and 30 mU/kg BW/min insulin infusions were 30 and 400 microU/ml, respectively. Glucose infusion rates (GIR) in STZ-induced diabetic rats showed a significant decrease compared to controls. At both insulin infusion rates, imidapril-infused diabetic rats showed an increased GIR, compared with the saline infused ones. There was no significant difference in GIR between L-NMMA and saline infusion in diabetic rats. Simultaneous infusion of imidapril and L-NMMA did not significantly decrease GIR with low-dose insulin infusion, but the increase in GIR induced by imidapril with high-dose insulin infusion was impaired by 100 % by L-NMMA infusion in diabetic rats. These results suggest that imidapril may improve insulin action, in part, via nitric oxide.

Hepatic Akt activation induces marked hypoglycemia, hepatomegaly, and hypertriglyceridemia with sterol regulatory element binding protein involvement

Akt is critical in insulin-induced metabolism of glucose and lipids. To investigate functions induced by hepatic Akt activation, a constitutively active Akt, NH(2)-terminally myristoylation signal-attached Akt (myr-Akt), was overexpressed in the liver by injecting its adenovirus into mice. Hepatic myr-Akt overexpression resulted in a markedly hypoglycemic, hypoinsulinemic, and hypertriglyceridemic phenotype with fatty liver and hepatomegaly. To elucidate the sterol regulatory element binding protein (SREBP)-1c contribution to these phenotypic features, myr-Akt adenovirus was injected into SREBP-1 knockout mice. myr-Akt overexpression induced hypoglycemia and hepatomegaly with triglyceride accumulation in SREBP-1 knockout mice to a degree similar to that in normal mice, whereas myr-Akt-induced hypertriglyceridemia in knockout mice was milder than that in normal mice. The myr-Akt-induced changes in glucokinase, phosphofructokinase, glucose-6-phosphatase, and PEPCK expressions were not affected by knocking out SREBP-1, whereas stearoyl-CoA desaturase 1 induction was completely inhibited in knockout mice. Constitutively active SREBP-1-overexpressing mice had fatty livers without hepatomegaly, hypoglycemia, or hypertriglyceridemia. Hepatic acetyl-CoA carboxylase, fatty acid synthase, stearoyl-CoA desaturase 1, and glucose-6-phosphate dehydrogenase expressions were significantly increased by overexpressing SREBP-1, whereas glucokinase, phospho-fructokinase, glucose-6-phosphatase, and PEPCK expressions were not or only slightly affected. Thus, SREBP-1 is not absolutely necessary for the hepatic Akt-mediated hypoglycemic effect. In contrast, myr-Akt-induced hypertriglyceridemia and hepatic triglyceride accumulation are mediated by both Akt-induced SREBP-1 expression and a mechanism involving fatty acid synthesis independent of SREBP-1.