David E. Moller

Role of AMP-activated protein kinase in mechanism of metformin action

Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformins beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformins inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.

Fibroblast Growth Factor 21 Corrects Obesity in Mice

Fibroblast growth factor 21 (FGF21) is a metabolic regulator that provides efficient and durable glycemic and lipid control in various animal models. However, its potential to treat obesity, a major health concern affecting over 30% of the population, has not been fully explored. Here we report that systemic administration of FGF21 for 2 wk in diet-induced obese and ob/ob mice lowered their mean body weight by 20% predominantly via a reduction in adiposity. Although no decrease in total caloric intake or effect on physical activity was observed, FGF21-treated animals exhibited increased energy expenditure, fat utilization, and lipid excretion, reduced hepatosteatosis, and ameliorated glycemia. Transcriptional and blood cytokine profiling studies revealed effects consistent with the ability of FGF21 to ameliorate insulin and leptin resistance, enhance fat oxidation and suppress de novo lipogenesis in liver as well as to activate futile cycling in adipose. Overall, these data suggest that FGF21 exhibits the therapeutic characteristics necessary for an effective treatment of obesity and fatty liver disease and provides novel insights into the metabolic determinants of these activities.

Induction of Adipocyte Complement-Related Protein of 30 Kilodaltons by PPARγ Agonists: A Potential Mechanism of Insulin Sensitization

Adipocyte complement-related protein of 30 kDa (Acrp30, adiponectin, or AdipoQ) is a fat-derived secreted protein that circulates in plasma. Adipose tissue expression of Acrp30 is lower in insulin-resistant states and it is implicated in the regulation of in vivo insulin sensitivity. Here we have characterized the ability of PPARγ agonists to modulate Acrp30 expression. After chronic treatment of obese-diabetic (db/db) mice with PPARγ agonists (11 d), mean plasma Acrp30 protein levels increased (>3×). Similar effects were noted in a nongenetic type 2 diabetes model (fat-fed and low-dose streptozotocin-treated mice). In contrast, treatment of mice (db/db or fat-fed) with metformin or a PPARα agonist did not affect plasma Acrp30 protein levels. In a cohort of normal human subjects, 14-d treatment with rosiglitazone also produced a 130% increase in circulating Acrp30 levels vs. placebo. In addition, circulating Acrp30 levels were suppressed 5-fold in patients with severe insulin resistance in association wit...

Regulation of PPAR gamma gene expression by nutrition and obesity in rodents.

The orphan nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma, is implicated in mediating expression of fat-specific genes and in activating the program of adipocyte differentiation. The potential for regulation of PPAR gamma gene expression in vivo is unknown. We cloned a partial mouse PPAR gamma cDNA and developed an RNase protection assay that permits simultaneous quantitation of mRNAs for both gamma l and gamma 2 isoforms encoded by the PPAR gamma gene. Probes for detection of adipocyte P2, the obese gene product, leptin, and 18S mRNAs were also employed. Both gamma l and gamma 2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma 1 expression was also detected at lower levels in liver, spleen, and heart; whereas, gamma l and gamma 2 mRNA were expressed at low levels in skeletal muscle. Adipose tissue levels of gamma l and gamma 2 were not altered in two murine models of obesity (gold thioglucose and ob/ob), but were modestly increased in mice with toxigene-induced brown fat ablation uncoupling protein diphtheria toxin A mice. Fasting (12-48 h) was associated with an 80% fall in PPAR gamma 2 and a 50% fall in PPAR gamma mRNA levels in adipose tissue. Western blot analysis demonstrated a marked effect of fasting to reduce PPAR gamma protein levels in adipose tissue. Similar effects of fasting on PPAR gamma mRNAs were noted in all three models of obesity. Insulin-deficient (streptozotocin) diabetes suppressed adipose tissue gamma l and gamma 2 expression by 75% in normal mice with partial restoration during insulin treatment. Levels of adipose tissue PPAR gamma 2 mRNA were increased by 50% in normal mice exposed to a high fat diet. In obese uncoupling protein diphtheria toxin A mice, high fat feeding resulted in de novo induction of PPAR gamma 2 expression in liver. We conclude (a) PPAR gamma 2 mRNA expression is most abundant in adipocytes in normal mice, but lower level expression is seen in skeletal muscle; (b) expression of adipose tissue gamma1 or gamma2 mRNAs is increased in only one of the three models of obesity; (c) PPAR gamma 1 and gamma 2 expression is downregulated by fasting and insulin-deficient diabetes; and (d) exposure of mice to a high fat diet increases adipose tissue expression of PPAR gamma (in normal mice) and induces PPAR gamma 2 mRNA expression in liver (in obese mice). These findings demonstrate in vivo modulation of PPAR gamma mRNA levels over a fourfold range and provide an additional level of regulation for the control of adipocyte development and function.

Potential Role of TNF-α in the Pathogenesis of Insulin Resistance and Type 2 Diabetes

Tumor necrosis factor alpha (TNF-alpha) has well-described effects on lipid metabolism in the context of acute inflammation, as in sepsis. Recently, increased TNF-alpha production has been observed in adipose tissue derived from obese rodents or human subjects and TNF-alpha has been implicated as a causative factor in obesity-associated insulin resistance and the pathogenesis of type 2 diabetes. Thus, current evidence suggests that administration of exogenous TNF-alpha to animals can induce insulin resistance, whereas neutralization of TNF-alpha can improve insulin sensitivity. Importantly, results from knockout mice deficient in TNF-alpha or its receptors have suggested that TNF-alpha has a role in regulating in vivo insulin sensitivity. However, the absence of TNF-alpha action might only partially protect against obesity-induced insulin resistance in mice. Multiple mechanisms have been suggested to account for these metabolic effects of TNF-alpha. These include the downregulation of genes that are required for normal insulin action, direct effects on insulin signaling, induction of elevated free fatty acids via stimulation of lipolysis, and negative regulation of PPAR gamma, an important insulin-sensitizing nuclear receptor. Although current evidence suggests that neutralizing TNF-alpha in type 2 diabetic subjects is not sufficient to cause metabolic improvement, it is still probable that TNF-alpha is a contributing factor in common metabolic disturbances such as insulin resistance and dyslipidemia.

Chronic Inhibition of Dipeptidyl Peptidase-4 With a Sitagliptin Analog Preserves Pancreatic β-Cell Mass and Function in a Rodent Model of Type 2 Diabetes

Inhibitors of dipeptidyl peptidase-4 (DPP-4), a key regulator of the actions of incretin hormones, exert antihyperglycemic effects in type 2 diabetic patients. A major unanswered question concerns the potential ability of DPP-4 inhibition to have beneficial disease-modifying effects, specifically to attenuate loss of pancreatic β-cell mass and function. Here, we investigated the effects of a potent and selective DPP-4 inhibitor, an analog of sitagliptin (des-fluoro-sitagliptin), on glycemic control and pancreatic β-cell mass and function in a mouse model with defects in insulin sensitivity and secretion, namely high-fat diet (HFD) streptozotocin (STZ)-induced diabetic mice. Significant and dose-dependent correction of postprandial and fasting hyperglycemia, HbA1c, and plasma triglyceride and free fatty acid levels were observed in HFD/STZ mice following 2–3 months of chronic therapy. Treatment with des-fluoro-sitagliptin dose dependently increased the number of insulin-positive β-cells in islets, leading to the normalization of β-cell mass and β-cell–to–α-cell ratio. In addition, treatment of mice with des-fluoro-sitagliptin, but not glipizide, significantly increased islet insulin content and improved glucose-stimulated insulin secretion in isolated islets. These findings suggest that DPP-4 inhibitors may offer long-lasting efficacy in the treatment of type 2 diabetes by modifying the courses of the disease.

NOVEL PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR (PPAR) GAMMA AND PPARDELTA LIGANDS PRODUCE DISTINCT BIOLOGICAL EFFECTS

The peroxisome proliferator-activated receptors (PPARs) include three receptor subtypes encoded by separate genes: PPARα, PPARδ, and PPARγ. PPARγ has been implicated as a mediator of adipocyte differentiation and the mechanism by which thiazolidinedione drugs exert in vivo insulin sensitization. Here we characterized novel, non-thiazolidinedione agonists for PPARγ and PPARδ that were identified by radioligand binding assays. In transient transactivation assays these ligands were agonists of the receptors to which they bind. Protease protection studies showed that ligand binding produced specific alterations in receptor conformation. Both PPARγ and PPARδ directly interacted with a nuclear receptor co-activator (CREB-binding protein) in an agonist-dependent manner. Only the PPARγ agonists were able to promote differentiation of 3T3-L1 preadipocytes. In diabeticdb/db mice all PPARγ agonists were orally active insulin-sensitizing agents producing reductions of elevated plasma glucose and triglyceride concentrations. In contrast, selectivein vivo activation of PPARδ did not significantly affect these parameters. In vivo PPARα activation with WY-14653 resulted in reductions in elevated triglyceride levels with minimal effect on hyperglycemia. We conclude that: 1) synthetic non-thiazolidinediones can serve as ligands of PPARγ and PPARδ; 2) ligand-dependent activation of PPARδ involves an apparent conformational change and association of the receptor ligand binding domain with CREB-binding protein; 3) PPARγ activation (but not PPARδ or PPARα activation) is sufficient to potentiate preadipocyte differentiation; 4) non-thiazolidinedione PPARγ agonists improve hyperglycemia and hypertriglyceridemia in vivo; 5) although PPARα activation is sufficient to affect triglyceride metabolism, PPARδ activation does not appear to modulate glucose or triglyceride levels.

The Effects of LY2405319, an FGF21 Analog, in Obese Human Subjects with Type 2 Diabetes

Fibroblast growth factor 21 (FGF21) is a recently discovered metabolic regulator. Exogenous FGF21 produces beneficial metabolic effects in animal models; however, the translation of these observations to humans has not been tested. Here, we studied the effects of LY2405319 (LY), a variant of FGF21, in a randomized, placebo-controlled, double-blind proof-of-concept trial in patients with obesity and type 2 diabetes. Patients received placebo or 3, 10, or 20 mg of LY daily for 28 days. LY treatment produced significant improvements in dyslipidemia, including decreases in low-density lipoprotein cholesterol and triglycerides and increases in high-density lipoprotein cholesterol and a shift to a potentially less atherogenic apolipoprotein concentration profile. Favorable effects on body weight, fasting insulin, and adiponectin were also detected. However, only a trend toward glucose lowering was observed. These results indicate that FGF21 is bioactive in humans and suggest that FGF21-based therapies may be effective for the treatment of selected metabolic disorders.

Mice lacking dipeptidyl peptidase IV are protected against obesity and insulin resistance

Dipeptidyl peptidase IV (DP-IV), a member of the prolyl oligopeptidase family of peptidases, is involved in the metabolic inactivation of a glucose-dependent insulinotropic hormone, glucagon-like peptide 1 (GLP-1), and other incretin hormones. Here, we investigated the impact of DP-IV deficiency on body weight control and insulin sensitivity in mice. Whereas WT mice displayed accelerated weight gain and hyperinsulinemia when fed a high-fat diet (HFD), mice lacking the gene encoding DP-IV (DP-IV-/-) are refractory to the development of obesity and hyperinsulinemia. Pair-feeding and indirect calorimetry studies indicate that reduced food intake and increased energy expenditure accounted for the resistance to HFD-induced obesity in the DP-IV-/- mice. Ablation of DP-IV also is associated with elevated GLP-1 levels and improved metabolic control in these animals, resulting in improved insulin sensitivity, reduced pancreatic islet hypertrophy, and protection against streptozotocin-induced loss of β cell mass and hyperglycemia. Together, these observations suggest that chronic deletion of DP-IV gene has significant impact on body weight control and energy homeostasis, providing validation of DP-IV inhibition as a viable therapeutic option for the treatment of metabolic disorders related to diabetes and obesity.

Activation of PPARδ alters lipid metabolism in db/db mice

Peroxisome proliferator‐activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARα and PPARγ, relatively little is known about the most widely expressed PPAR subtype, PPARδ. Here we show that treatment of insulin resistant db/db mice with the PPARδ agonist L‐165 041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L‐165 041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARδ ligand, but was increased by a PPARγ agonist. These data suggest both that PPARδ is involved in the regulation of cholesterol metabolism in db/db mice and that PPARδ ligands could potentially have therapeutic value.