Rexford S. Ahima

Rapamycin-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity

Dissecting Rapamycin Responses Long-term treatment of mice and other organisms with the drug rapamycin extends life span. But, at the same time, the drug disrupts metabolic regulation and the action of the hormone insulin. Lamming et al. (p. 1638; see the Perspective by Hughes and Kennedy) dissected the action of rapamycin in genetically modified mice and found, encouragingly, that these two actions of rapamycin can be separated. Rapamycin inhibits a protein kinase complex known as mTORC1, and this appears to provide most of the life-lengthening effects of the drug. However, rapamycin also acts on a related complex known as mTORC2, and it is the disruption of mTORC2 action that produces the diabetic-like symptoms of decreased glucose tolerance and insensitivity to insulin. The effect of the drug rapamycin on life span can be separated from its effects on metabolism. Rapamycin, an inhibitor of mechanistic target of rapamycin complex 1 (mTORC1), extends the life spans of yeast, flies, and mice. Calorie restriction, which increases life span and insulin sensitivity, is proposed to function by inhibition of mTORC1, yet paradoxically, chronic administration of rapamycin substantially impairs glucose tolerance and insulin action. We demonstrate that rapamycin disrupted a second mTOR complex, mTORC2, in vivo and that mTORC2 was required for the insulin-mediated suppression of hepatic gluconeogenesis. Further, decreased mTORC1 signaling was sufficient to extend life span independently from changes in glucose homeostasis, as female mice heterozygous for both mTOR and mLST8 exhibited decreased mTORC1 activity and extended life span but had normal glucose tolerance and insulin sensitivity. Thus, mTORC2 disruption is an important mediator of the effects of rapamycin in vivo.

Hepatic Hdac3 promotes gluconeogenesis by repressing lipid synthesis and sequestration

Fatty liver disease is associated with obesity and type 2 diabetes, and hepatic lipid accumulation may contribute to insulin resistance. Histone deacetylase 3 (Hdac3) controls the circadian rhythm of hepatic lipogenesis. Here we show that, despite severe hepatosteatosis, mice with liver-specific depletion of Hdac3 have higher insulin sensitivity without any changes in insulin signaling or body weight compared to wild-type mice. Hdac3 depletion reroutes metabolic precursors towards lipid synthesis and storage within lipid droplets and away from hepatic glucose production. Perilipin 2, which coats lipid droplets, is markedly induced upon Hdac3 depletion and contributes to the development of both steatosis and improved tolerance to glucose. These findings suggest that the sequestration of hepatic lipids in perilipin 2–coated droplets ameliorates insulin resistance and establish Hdac3 as a pivotal epigenomic modifier that integrates signals from the circadian clock in the regulation of hepatic intermediary metabolism.

Insulin regulates liver metabolism in vivo in the absence of hepatic Akt and Foxo1

Considerable data support the idea that forkhead box O1 (Foxo1) drives the liver transcriptional program during fasting and is then inhibited by thymoma viral proto-oncogene 1 (Akt) after feeding. Here we show that mice with hepatic deletion of Akt1 and Akt2 were glucose intolerant, insulin resistant and defective in their transcriptional response to feeding in the liver. These defects were normalized with concomitant liver-specific deletion of Foxo1. Notably, in the absence of both Akt and Foxo1, mice adapted appropriately to both the fasted and fed state, and insulin suppressed hepatic glucose production normally. A gene expression analysis revealed that deletion of Akt in liver led to the constitutive activation of Foxo1-dependent gene expression, but again, concomitant ablation of Foxo1 restored postprandial regulation, preventing the inhibition of the metabolic response to nutrient intake caused by deletion of Akt. These results are inconsistent with the canonical model of hepatic metabolism in which Akt is an obligate intermediate for proper insulin signaling. Rather, they show that a major role of hepatic Akt is to restrain the activity of Foxo1 and that in the absence of Foxo1, Akt is largely dispensable for insulin- and nutrient-mediated hepatic metabolic regulation in vivo.

Targeting the Activin Type IIB Receptor to Improve Muscle Mass and Function in the mdx Mouse Model of Duchenne Muscular Dystrophy

The activin receptor type IIB (ActRIIB) is a transmembrane receptor for transforming growth factor-β superfamily members, including myostatin, that are involved in the negative regulation of skeletal muscle mass. We tested the translational hypothesis that blocking ligand binding to ActRIIB for 12 weeks would stimulate skeletal muscle growth and improve muscle function in the mdx mouse. ActRIIB was targeted using a novel inhibitor comprised of the extracellular portion of the ActRIIB fused to the Fc portion of murine IgG (sActRIIB), at concentrations of 1.0 and 10.0 mg/kg(-1) body weight. After 12 weeks of treatment, the 10.0 mg/kg(-1) dose caused a 27% increase in body weight with a concomitant 33% increase in lean muscle mass. Absolute force production of the extensor digitorum longus muscle ex vivo was higher in mice after treatment with either dose of sActRIIB, and the specific force was significantly higher after the lower dose (1.0 mg/kg(-1)), indicating functional improvement in the muscle. Circulating creatine kinase levels were significantly lower in mice treated with sActRIIB, compared with control mice. These data show that targeting the ActRIIB improves skeletal muscle mass and functional strength in the mdx mouse model of DMD, providing a therapeutic rationale for use of this molecule in treating skeletal myopathies.

Inflammatory Induction of Human Resistin Causes Insulin Resistance in Endotoxemic Mice

OBJECTIVE Although adipocyte-derived murine resistin links insulin resistance to obesity, the role of human resistin, predominantly expressed in mononuclear cells and induced by inflammatory signals, remains unclear. Given the mounting evidence that obesity and type 2 diabetes are inflammatory diseases, we sought to determine the relationship between inflammatory increases in human resistin and insulin resistance. RESEARCH DESIGN AND METHODS To investigate the role of human resistin on glucose homeostasis in inflammatory states, we generated mice lacking murine resistin but transgenic for a bacterial artificial chromosome containing human resistin (BAC-Retn), whose expression was similar to that in humans. The metabolic and molecular phenotypes of BAC-Retn mice were assessed after acute and chronic endotoxemia (i.e., exposure to inflammatory lipopolysaccharide). RESULTS We found that BAC-Retn mice have circulating resistin levels within the normal human range, and similar to humans, lipopolysaccharide markedly increased serum resistin levels. Acute endotoxemia caused hypoglycemia in mice lacking murine resistin, and this was attenuated in BAC-Retn mice. In addition, BAC-Retn mice developed severe hepatic insulin resistance under chronic endotoxemia, accompanied by increased inflammatory responses in liver and skeletal muscle. CONCLUSIONS These results strongly support the role of human resistin in the development of insulin resistance in inflammation. Thus, human resistin may link insulin resistance to inflammatory diseases such as obesity, type 2 diabetes, and atherosclerosis.

Loss of IL-15 receptor α alters the endurance, fatigability, and metabolic characteristics of mouse fast skeletal muscles

IL-15 receptor α (IL-15Rα) is a component of the heterotrimeric plasma membrane receptor for the pleiotropic cytokine IL-15. However, IL-15Rα is not merely an IL-15 receptor subunit, as mice lacking either IL-15 or IL-15Rα have unique phenotypes. IL-15 and IL-15Rα have been implicated in muscle phenotypes, but a role in muscle physiology has not been defined. Here, we have shown that loss of IL-15Rα induces a functional oxidative shift in fast muscles, substantially increasing fatigue resistance and exercise capacity. IL-15Rα-knockout (IL-15Rα-KO) mice ran greater distances and had greater ambulatory activity than controls. Fast muscles displayed fatigue resistance and a slower contractile phenotype. The molecular signature of these muscles included altered markers of mitochondrial biogenesis and calcium homeostasis. Morphologically, fast muscles had a greater number of muscle fibers, smaller fiber areas, and a greater ratio of nuclei to fiber area. The alterations of physiological properties and increased resistance to fatigue in fast muscles are consistent with a shift toward a slower, more oxidative phenotype. Consistent with a conserved functional role in humans, a genetic association was found between a SNP in the IL15RA gene and endurance in athletes stratified by sport. Therefore, we propose that IL-15Rα has a role in defining the phenotype of fast skeletal muscles in vivo.

Maternal obesity associated with inflammation in their children.

BackgroundThis study explored the association between maternal obesity during pregnancy and the inflammatory markers, tumor necrosis factor-α, interleukin-6 and high sensitivity C-reactive protein (hs-CRP), and the cytokine, adiponectin, in the offspring.MethodsWeight, height, Tanner stage and biomarkers were measured in thirty-four 12-year-old children, from the Infant Growth Study, who were divided into high risk (HR) and low risk (LR) groups based on maternal pre-pregnancy body mass index (BMI).ResultsThe two groups differed markedly in their hs-CRP levels, but no group difference was found for the other three biomarkers. The odds ratio (OR) of HR children having detectable hs-CRP levels was 16 times greater than that of LR children after adjusting for confounding variables, including BMI z-score, Tanner stages and gender (OR: 16; 95% CI: 2–123).ConclusionsThese results suggest that maternal obesity during pregnancy is associated with later development of elevated hs-CRP in the offspring, even after controlling for weight.

Reduction of TIP47 improves hepatic steatosis and glucose homeostasis in mice

Lipid droplets in the liver are coated with the perilipin family of proteins, notably adipocyte differentiation-related protein (ADRP) and tail-interacting protein of 47 kDa (TIP47). ADRP is increased in hepatic steatosis and is associated with hyperlipidemia, insulin resistance, and glucose intolerance. We have shown that reducing ADRP in the liver via antisense oligonucleotide (ASO) treatment attenuates steatosis and improves insulin sensitivity and glucose tolerance. We hypothesized that TIP47 has similar effects on hepatic lipid and glucose metabolism. We found that TIP47 mRNA and protein levels were increased in response to a high-fat diet (HFD) in C57BL/6J mice. TIP47 ASO treatment decreased liver TIP47 mRNA and protein levels without altering ADRP levels. Low-dose TIP47 ASO (15 mg/kg) and high-dose TIP47 ASO (50 mg/kg) decreased triglyceride content in the liver by 35% and 52%, respectively. Liver histology showed a drastic reduction in hepatic steatosis following TIP47 ASO treatment. The high dose of TIP47 ASO significantly blunted hepatic triglyceride secretion, improved glucose tolerance, and increased insulin sensitivity in liver, adipose tissue, and muscle. These findings show that TIP47 affects hepatic lipid and glucose metabolism and may be a target for the treatment of nonalcoholic fatty liver and related metabolic disorders.

Effects of perilipin 2 antisense oligonucleotide treatment on hepatic lipid metabolism and gene expression

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. We previously showed that Perilipin 2 (Plin2), a member of lipid droplet protein family, is markedly increased in fatty liver, and its reduction in the liver of diet-induced obese mice by antisense oligonucleotide (ASO) decreased steatosis and enhanced insulin sensitivity. Plin2-ASO treatment markedly suppressed lipogenic gene expression. To gain a better understanding of the biological role of Plin2 in liver, we performed microarray analysis to determine genes differentially regulated by Plin2-ASO compared with a control (scrambled) oligonucleotide (Cont). Male C57BL/6J mice on a high-fat diet were treated with Plin2- or Cont-ASO for 4 wk. Plin2-ASO decreased hepatic triglycerides, and this was associated with changes in expression of 1,363 genes. We analyzed the data for functional clustering and validated the expression of representative genes using real-time PCR. On the high-fat diet, Plin2-ASO decreased the expression of enzymes involved in fatty acid metabolism (acsl1, lipe) and steroid metabolism (hmgcr, hsd3b5, hsd17b2), suggesting that Plin2 affects hepatic lipid metabolism at the transcriptional level. Plin2-ASO also increased the expression of genes involved in regulation of hepatocyte proliferation (afp, H19), mitosis (ccna2, incenp, sgol1), and extracellular matrix (col1a1, col3a1, mmp8). Plin2-ASO had similar effects on gene expression in chow-fed mice. Together, these results indicate that Plin2 has diverse metabolic and structural roles in the liver, and its downregulation promotes hepatic fibrosis and proliferation.

Metabolic dysfunction associated with adiponectin deficiency enhances kainic acid-induced seizure severity

Metabolic syndrome has deleterious effects on the CNS, and recent evidence suggests that obesity rates are higher at presentation in children who develop epilepsy. Adiponectin is secreted by adipose tissue and acts in the brain and peripheral organs to regulate glucose and lipid metabolism. Adiponectin deficiency predisposes toward metabolic syndrome, characterized by obesity, insulin resistance, impaired glucose tolerance, hyperlipidemia, and cardiovascular morbidity. To investigate the relationship between metabolic syndrome and seizures, wild-type C57BL/6J and adiponectin knock-out mice were fed a high-fat diet, followed by treatment with low doses of kainic acid to induce seizures. Adiponectin deficiency in mice fed a high-fat diet resulted in greater fat accumulation, impaired glucose tolerance, hyperlipidemia, increased seizure severity, and increased hippocampal pathology. In contrast, there were no adverse effects of adiponectin deficiency on metabolic phenotype or seizure activity in mice fed a normal (low-fat) chow diet. These findings demonstrate that metabolic syndrome modulates the outcome of seizures and brain injury.