The Banting Medal for Scientific Achievement Award is the American Diabetes Association’s highest scientific award and honors an individual who has made significant, long-term contributions to the understanding of diabetes, its treatment, and/or prevention. The award is named after Nobel Prize winner Sir Frederick Banting, who codiscovered insulin treatment for diabetes.

Dr. Eisenbarth received the American Diabetes Association’s Banting Medal for Scientific Achievement at the Association’s 69th Scientific Sessions, June 5–9, 2009, in New Orleans, Louisiana. He presented the Banting Lecture, An Unfinished Journey—Type 1 Diabetes—Molecular Pathogenesis to Prevention, on Sunday, June 7, 2009.

 

OBJECTIVE

In metazoans, target of rapamycin complex 1 (TORC1) plays the key role in nutrient- and hormone-dependent control of metabolism. However, the role of TORC1 in regulation of triglyceride storage and metabolism remains largely unknown.


RESEARCH DESIGN AND METHODS

In this study, we analyzed the effect of activation and inhibition of the mammalian TORC1 (mTORC1) signaling pathway on the expression of adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), lipolysis, lipogenesis, and lipid storage in different mammalian cells.


RESULTS

Activation of mTORC1 signaling in 3T3-L1 adipocytes by ectopic expression of Rheb inhibits expression of ATGL and HSL at the level of transcription, suppresses lipolysis, increases de novo lipogenesis, and promotes intracellular accumulation of triglycerides. Inhibition of mTORC1 signaling by rapamycin or by knockdown of raptor stimulates lipolysis primarily via activation of ATGL expression. Analogous results have been obtained in C2C12 myoblasts and mouse embryonic fibroblasts with genetic ablation of tuberous sclerosis 2 (TSC2) gene. Overexpression of ATGL in these cells antagonized the lipogenic effect of TSC2 knockout.


CONCLUSIONS

Our findings demonstrate that mTORC1 promotes fat storage in mammalian cells by suppression of lipolysis and stimulation of de novo lipogenesis.

 

OBJECTIVE

Leptin acts via its receptor (LepRb) to signal the status of body energy stores. Leptin binding to LepRb initiates signaling by activating the associated Janus kinase 2 (Jak2) tyrosine kinase, which promotes the phosphorylation of tyrosine residues on the intracellular tail of LepRb. Two previously examined LepRb phosphorylation sites mediate several, but not all, aspects of leptin action, leading us to hypothesize that Jak2 signaling might contribute to leptin action independently of LepRb phosphorylation sites. We therefore determined the potential role in leptin action for signals that are activated by Jak2 independently of LepRb phosphorylation (Jak2-autonomous signals).


RESEARCH DESIGN AND METHODS

We inserted sequences encoding a truncated LepRb mutant (LepRb65c, which activates Jak2 normally, but is devoid of other LepRb intracellular sequences) into the mouse Lepr locus. We examined the leptin-regulated physiology of the resulting / mice relative to LepRb-deficient db/db animals.


RESULTS

The / animals were similar to db/db animals in terms of energy homeostasis, neuroendocrine and immune function, and regulation of the hypothalamic arcuate nucleus, but demonstrated modest improvements in glucose homeostasis.


CONCLUSIONS

The ability of Jak2-autonomous LepRb signals to modulate glucose homeostasis in / animals suggests a role for these signals in leptin action. Because Jak2-autonomous LepRb signals fail to mediate most leptin action, however, signals from other LepRb intracellular sequences predominate.

 

OBJECTIVE

Adiponectin is one of several important metabolically active cytokines secreted from adipose tissue. Epidemiologic studies have associated low-circulating levels of this adipokine with multiple metabolic disorders including obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. To investigate adiponectin-mediated changes in metabolism in vivo, we generated transgenic mice that specifically express the gene coding for human adiponectin in mouse macrophages using the human scavenger receptor A-I gene enhancer/promoter.


METHODS AND RESULTS

Using this transgenic mouse model, we found that adiponectin expression was associated with reduced whole-animal body and fat-pad weight and an improved lipid accumulation in macrophages when these transgenic mice were fed with a high-fat diet. Moreover, these macrophage Ad-TG mice exhibit enhanced whole-body glucose tolerance and insulin sensitivity with reduced proinflammatory cytokines, MCP-1 and TNF-a (both in the serum and in the metabolic active macrophage), adipose tissue, and skeletal muscle under the high-fat diet condition. Additional studies demonstrated that these macrophage adiponectin transgenic animals exhibit reduced macrophage foam cell formation in the arterial wall when these transgenic mice were crossed with an LDL receptor–deficient mouse model and were fed a high-fat diet.


CONCLUSIONS

These results suggest that adiponectin expressed in macrophages can physiologically modulate metabolic activities in vivo by improving metabolism in distal tissues. The use of macrophages as carriers for adiponectin, a molecule with antidiabetes, anti-inflammatory, and antiatherogenic properties, provides a novel and unique strategy for studying the mechanisms of adiponectin-mediated alterations in body metabolism in vivo.

 

OBJECTIVE

Insulin-mediated glucose uptake is highly sensitive to the levels of the facilitative GLUT protein GLUT4. Transcription of the GLUT4 gene is repressed in states of insulin deficiency and insulin resistance and can be induced by states of enhanced energy output, such as exercise. The cellular signals that regulate GLUT4 transcription are not well understood. We hypothesized that changes in energy substrate flux regulate GLUT4 transcription.


RESEARCH DESIGN AND METHODS

To test this hypothesis, we used transgenic mice in which expression of the chloramphenicol acetyltransferase (CAT) gene is driven by a functional 895-bp fragment of the human GLUT4 promoter, thereby acting as a reporter for transcriptional activity. Mice were treated with a single dose of etomoxir, which inhibits the transport of long-chain fatty acids into mitochondria and increases basal, but not insulin-mediated, glucose flux. GLUT4 and transgenic CAT mRNA were measured.


RESULTS

Etomoxir treatment significantly reduced CAT and GLUT4 mRNA transcription in adipose tissue, but did not change transcription in heart and skeletal muscle. Downregulation of GLUT4 transcription was cell autonomous, since etomoxir treatment of 3T3-L1 adipocytes resulted in a similar downregulation of GLUT4 mRNA. GLUT4 transcriptional downregulation required the putative liver X receptor (LXR) binding site in the human GLUT4 gene promoter in adipose tissue and 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with the LXR agonist, TO901317, partially restored GLUT4 expression in etomoxir-treated cells.


CONCLUSIONS

Our data suggest that long-chain fatty acid import into mitochondria in adipose tissue may produce ligands that regulate expression of metabolic genes.

 

OBJECTIVE

Vascular endothelial cells (VECs) downregulate their rate of glucose uptake in response to hyperglycemia by decreasing the expression of their typical glucose transporter GLUT-1. Hitherto, we discovered critical roles for the protein calreticulin and the arachidonic acid–metabolizing enzyme 12-lipoxygenase in this autoregulatory process. The hypothesis that 4-hydroxydodeca-(2E,6Z)-dienal (4-HDDE), the peroxidation product of 12-lipoxygenase, mediates this downregulatory mechanism by activating peroxisome proliferator–activated receptor (PPAR) was investigated.


RESEARCH DESIGN AND METHODS

Effects of 4-HDDE and PPAR on the glucose transport system and calreticulin expression in primary bovine aortic endothelial cells were evaluated by pharmacological and molecular interventions.


RESULTS

Using GW501516 (PPAR agonist) and GSK0660 (PPAR antagonist), we discovered that high-glucose–induced downregulation of the glucose transport system in VECs is mediated by PPAR. A PPAR-sensitive luciferase reporter assay in VECs revealed that high glucose markedly increased luciferase activity, while GSK0660 abolished it. High-performance liquid chromatography analysis showed that high-glucose incubation substantially elevated the generation of 4-HDDE in VECs. Treatment of VECs, exposed to normal glucose, with 4-HDDE mimicked high glucose and downregulated the glucose transport system and increased calreticulin expression. Like high glucose, 4-HDDE significantly activated PPAR in cells overexpressing human PPAR (hPPAR) but not hPPAR, -1, or -2. Moreover, silencing of PPAR prevented high-glucose–dependent alterations in GLUT-1 and calreticulin expression. Finally, specific binding of PPAR to a PPAR response element in the promoter region of the calreticulin gene was identified by utilizing a specific chromatin immunoprecipitation assay.


CONCLUSIONS

Collectively, our data show that 4-HDDE plays a central role in the downregulation of glucose uptake in VECs by activating PPAR.

 

OBJECTIVE

We examined in insulin-resistant muscle if, in contrast to long-standing dogma, mitochondrial fatty acid oxidation is increased and whether this is attributed to an increased nuclear content of peroxisome proliferator–activated receptor (PPAR) coactivator (PGC) 1 and the adaptations of specific mitochondrial subpopulations.


RESEARCH DESIGN AND METHODS

Skeletal muscles from male control and Zucker diabetic fatty (ZDF) rats were used to determine 1) intramuscular lipid distribution, 2) subsarcolemmal and intermyofibrillar mitochondrial morphology, 3) rates of palmitate oxidation in subsarcolemmal and intermyofibrillar mitochondria, and 4) the subcellular localization of PGC1. Electotransfection of PGC1 cDNA into lean animals tested the notion that increased nuclear PGC1 preferentially targeted subsarcolemmal mitochondria.


RESULTS

Transmission electron microscope analysis revealed that in ZDF animals the number (+50%), width (+69%), and density (+57%) of subsarcolemmal mitochondria were increased (P < 0.05). In contrast, intermyofibrillar mitochondria remained largely unchanged. Rates of palmitate oxidation were ~40% higher (P < 0.05) in ZDF subsarcolemmal and intermyofibrillar mitochondria, potentially as a result of the increased PPAR-targeted proteins, carnitine palmitoyltransferase-I, and fatty acid translocase (FAT)/CD36. PGC1 mRNA and total protein were not altered in ZDF animals; however, a greater (~70%; P < 0.05) amount of PGC1 was located in nuclei. Overexpression of PGC1 only increased subsarcolemmal mitochondrial oxidation rates.


CONCLUSIONS

In ZDF animals, intramuscular lipids accumulate in the intermyofibrillar region (increased size and number), and this is primarily associated with increased oxidative capacity in subsarcolemmal mitochondria (number, size, density, and oxidation rates). These changes may result from an increased nuclear content of PGC1, as under basal conditions, overexpression of PGC1 appears to target subsarcolemmal mitochondria.

 

OBJECTIVE

Sirtuin 1 (SIRT1) is implicated in the regulation of mitochondrial function, energy metabolism, and insulin sensitivity in rodents. No studies are available in humans to demonstrate that SIRT1 expression in insulin-sensitive tissues is associated with energy expenditure and insulin sensitivity.


RESEARCH DESIGN AND METHODS

Energy expenditure (EE), insulin sensitivity, and SIRT1 mRNA adipose tissue expression (n = 81) were measured by indirect calorimetry, hyperinsulinemic-euglycemic clamp, and quantitative RT-PCR in 247 nondiabetic offspring of type 2 diabetic patients.


RESULTS

High EE during the clamp (r = 0.375, P = 2.8 x 10–9) and high EE (EE during the clamp – EE in the fasting state) (r = 0.602, P = 2.5 x 10–24) were associated with high insulin sensitivity. Adipose tissue SIRT1 mRNA expression was significantly associated with EE (r = 0.289, P = 0.010) and with insulin sensitivity (r = 0.334, P = 0.002) during hyperinsulinemic-euglycemic clamp. Furthermore, SIRT1 mRNA expression correlated significantly with the expression of several genes regulating mitochondrial function and energy metabolism (e.g., peroxisome proliferator–activated receptor coactivator-1β, estrogen-related receptor , nuclear respiratory factor-1, and mitochondrial transcription factor A), and with several genes of the respiratory chain (e.g., including NADH dehydrogenase [ubiquinone] 1 subcomplex 2, cytochrome c, cytochrome c oxidase subunit IV, and ATP synthase).


CONCLUSIONS

Impaired stimulation of EE by insulin and low SIRT1 expression in insulin-sensitive tissues is likely to reflect impaired regulation of mitochondrial function associated with insulin resistance in humans.

 
« Older PostsNewer Posts »