Mature Human White Adipocytes Cultured under Membranes Maintain Identity, Function, and Can Transdifferentiate into Brown-like Adipocytes.

White adipose tissue (WAT) is a central factor in the development of type 2 diabetes, but there is a paucity of translational models to study mature adipocytes. We describe a method for the culture of mature white adipocytes under a permeable membrane. Compared to existing culture methods, MAAC (membrane mature adipocyte aggregate cultures) better maintain adipogenic gene expression, do not dedifferentiate, display reduced hypoxia, and remain functional after long-term culture. Subcutaneous and visceral adipocytes cultured as MAAC retain depot-specific gene expression, and adipocytes from both lean and obese patients can be cultured. Importantly, we show that rosiglitazone treatment or PGC1α overexpression in mature white adipocytes induces a brown fat transcriptional program, providing direct evidence that human adipocytes can transdifferentiate into brown-like adipocytes. Together, these data show that MAAC are a versatile tool for studying phenotypic changes of mature adipocytes and provide an improved translational model for drug development.

Interaction between hormone-sensitive lipase and ChREBP in fat cells controls insulin sensitivity.

Impaired adipose tissue insulin signalling is a critical feature of insulin resistance. Here we identify a pathway linking the lipolytic enzyme hormone-sensitive lipase (HSL) to insulin action via the glucose-responsive transcription factor ChREBP and its target, the fatty acid elongase ELOVL6. Genetic inhibition of HSL in human adipocytes and mouse adipose tissue results in enhanced insulin sensitivity and induction of ELOVL6. ELOVL6 promotes an increase in phospholipid oleic acid, which modifies plasma membrane fluidity and enhances insulin signalling. HSL deficiency-mediated effects are suppressed by gene silencing of ChREBP and ELOVL6. Mechanistically, physical interaction between HSL, independent of lipase activity, and the isoform activated by glucose metabolism ChREBPα impairs ChREBPα translocation into the nucleus and induction of ChREBPß, the isoform with high transcriptional activity that is strongly associated with whole-body insulin sensitivity. Targeting the HSL-ChREBP interaction may allow therapeutic strategies for the restoration of insulin sensitivity.

Target Deconvolution Efforts on Wnt Pathway Screen Reveal Dual Modulation of Oxidative Phosphorylation and SERCA2.

Wnt signaling is critical for development, cell proliferation and differentiation, and mutations in this pathway resulting in constitutive signaling have been implicated in various cancers. A pathway screen using a Wnt-dependent reporter identified a chemical series based on a 1,2,3-thiadiazole-5-carboxamide (TDZ) core with sub-micromolar potency. Herein we report a comprehensive mechanism-of-action deconvolution study toward identifying the efficacy target(s) and biological implication of this chemical series involving bottom-up quantitative chemoproteomics, cell biology, and biochemical methods. Through observing the effects of our probes on metabolism and performing confirmatory cellular and biochemical assays, we found that this chemical series inhibits ATP synthesis by uncoupling the mitochondrial potential. Affinity chemoproteomics experiments identified sarco(endo)plasmic reticulum Ca2+ -dependent ATPase (SERCA2) as a binding partner of the TDZ series, and subsequent validation studies suggest that the TDZ series can act as ionophores through SERCA2 toward Wnt pathway inhibition.

Thermogenic activity of UCP1 in human white fat-derived beige adipocytes.

Heat-producing beige/brite (brown-in-white) adipocytes in white adipose tissue have the potential to suppress metabolic disease in mice and hold great promise for the treatment of obesity and type 2 diabetes in humans. Here, we demonstrate that human adipose-derived stromal/progenitor cells (hASCs) from subcutaneous white adipose tissue can be efficiently converted into beige adipocytes. Upon pharmacological activation of peroxisome proliferator-activated receptor-γ, hASC-derived adipocytes activated beige fat-selective genes and a brown/beige fat-selective electron transport chain gene program. Importantly, hASC-derived beige fat cells displayed the bioenergetic characteristics of genuine brown fat cells, including a capacity for increased respiratory uncoupling in response to ß-adrenergic agonists. Furthermore, knock-down experiments reveal that the thermogenic capacity of human beige fat cells was entirely dependent on the presence of Uncoupling protein 1. In summary, this study reveals that hASCs can be readily differentiated into beige adipocytes that, upon activation, undergo uncoupling protein 1-dependent thermogenesis.

Effects of pharmacological AMP deaminase inhibition and Ampd1 deletion on nucleotide levels and AMPK activation in contracting skeletal muscle.

AMP-activated protein kinase (AMPK) plays a central role in regulating metabolism and energy homeostasis. It achieves its function by sensing fluctuations in the AMP:ATP ratio. AMP deaminase (AMPD) converts AMP into IMP, and the AMPD1 isoenzyme is expressed in skeletal muscles. Here, effects of pharmacological inhibition and genetic deletion of AMPD were examined in contracting skeletal muscles. Pharmacological AMPD inhibition potentiated rises in AMP, AMP:ATP ratio, AMPK Thr172, and acetyl-CoA carboxylase (ACC) Ser218 phosphorylation induced by electrical stimulation, without affecting glucose transport. In incubated extensor digitorum longus and soleus muscles from Ampd1 knockout mice, increases in AMP levels and AMP:ATP ratio by electrical stimulation were potentiated considerably compared with muscles from wild-type mice, whereas enhanced AMPK activation was moderate and only observed in soleus, suggesting control by factors other than changes in adenine nucleotides. AMPD inhibitors could be useful tools for enhancing AMPK activation in cells and tissues during ATP-depletion.

Inhibition of AMP deaminase activity does not improve glucose control in rodent models of insulin resistance or diabetes.

Inhibition of AMP deaminase (AMPD) holds the potential to elevate intracellular adenosine and AMP levels and, therefore, to augment adenosine signaling and activation of AMP-activated protein kinase (AMPK). To test the latter hypothesis, novel AMPD pan inhibitors were synthesized and explored using a panel of in vitro, ex vivo, and in vivo models focusing on confirming AMPD inhibitory potency and the potential of AMPD inhibition to improve glucose control in vivo. Repeated dosing of selected inhibitors did not improve glucose control in insulin-resistant or diabetic rodent disease models. Mice with genetic deletion of the muscle-specific isoform Ampd1 did not showany favorable metabolic phenotype despite being challenged with high-fat diet feeding. Therefore, these results do not support the development of AMPD inhibitors for the treatment of type 2 diabetes.

A small-molecule benzimidazole derivative that potently activates AMPK to increase glucose transport in skeletal muscle: comparison with effects of contraction and other AMPK activators.

AMPK (AMP-activated protein kinase) is an attractive therapeutic drug target for treating metabolic disorders. We studied the effects of an AMPK activator developed by Merck (ex229 from patent application WO2010036613), comparing chemical activation with contraction in intact incubated skeletal muscles. We also compared effects of ex229 with those of the Abbott A769662 compound and AICAR (5-amino-4-imidazolecarboxamide riboside). In rat epitrochlearis muscle, ex229 dose-dependently increased AMPK activity of α1-, α2-, ß1- and ß2-containing complexes with significant increases in AMPK activity seen at a concentration of 50 µM. At a concentration of 100 µM, AMPK activation was similar to that observed after contraction and importantly led to an ~2-fold increase in glucose uptake. In AMPK α1-/α2-catalytic subunit double-knockout myotubes incubated with ex229, the increases in glucose uptake and ACC (acetyl-CoA carboxylase) phosphorylation seen in control cells were completely abolished, suggesting that the effects of the compound were AMPK-dependent. When muscle glycogen levels were reduced by ~50% after starvation, ex229-induced AMPK activation and glucose uptake were amplified in a wortmannin-independent manner. In L6 myotubes incubated with ex229, fatty acid oxidation was increased. Furthermore, in mouse EDL (extensor digitorum longus) and soleus muscles, ex229 increased both AMPK activity and glucose uptake at least 2-fold. In summary, ex229 efficiently activated skeletal muscle AMPK and elicited metabolic effects in muscle appropriate for treating Type 2 diabetes by stimulating glucose uptake and increasing fatty acid oxidation.

Structural basis of AMPK regulation by small molecule activators.

AMP-activated protein kinase (AMPK) plays a major role in regulating cellular energy balance by sensing and responding to increases in AMP/ADP concentration relative to ATP. Binding of AMP causes allosteric activation of the enzyme and binding of either AMP or ADP promotes and maintains the phosphorylation of threonine 172 within the activation loop of the kinase. AMPK has attracted widespread interest as a potential therapeutic target for metabolic diseases including type 2 diabetes and, more recently, cancer. A number of direct AMPK activators have been reported as having beneficial effects in treating metabolic diseases, but there has been no structural basis for activator binding to AMPK. Here we present the crystal structure of human AMPK in complex with a small molecule activator that binds at a site between the kinase domain and the carbohydrate-binding module, stabilising the interaction between these two components. The nature of the activator-binding pocket suggests the involvement of an additional, as yet unidentified, metabolite in the physiological regulation of AMPK. Importantly, the structure offers new opportunities for the design of small molecule activators of AMPK for treatment of metabolic disorders.

Design of small molecule inhibitors of acetyl-CoA carboxylase 1 and 2 showing reduction of hepatic malonyl-CoA levels in vivo in obese Zucker rats.

Inhibition of acetyl-CoA carboxylases has the potential for modulating long chain fatty acid biosynthesis and mitochondrial fatty acid oxidation. Hybridization of weak inhibitors of ACC2 provided a novel, moderately potent but lipophilic series. Optimization led to compounds 33 and 37, which exhibit potent inhibition of human ACC2, 10-fold selectivity over inhibition of human ACC1, good physical and in vitro ADME properties and good bioavailability. X-ray crystallography has shown this series binding in the CT-domain of ACC2 and revealed two key hydrogen bonding interactions. Both 33 and 37 lower levels of hepatic malonyl-CoA in vivo in obese Zucker rats.

Chronic hyperglycemia reduces substrate oxidation and impairs metabolic switching of human myotubes.

Skeletal muscle of insulin resistant individuals is characterized by lower fasting lipid oxidation and reduced ability to switch between lipid and glucose oxidation. The purpose of the present study was to examine if chronic hyperglycemia would impair metabolic switching of myotubes. Human myotubes were treated with or without chronic hyperglycemia (20mmol/l glucose for 4 days), and metabolism of [(14)C]oleic acid (OA) and [(14)C]glucose was studied. Myotubes exposed to chronic hyperglycemia showed a significantly reduced OA uptake and oxidation to CO(2), whereas acid-soluble metabolites were increased compared to normoglycemic cells (5.5mmol/l glucose). Glucose suppressibility, the ability of acute glucose (5mmol/l) to suppress lipid oxidation, was 50% in normoglycemic cells and reduced to 21% by hyperglycemia. Adaptability, the capacity to increase lipid oxidation with increasing fatty acid availability, was not affected by hyperglycemia. Glucose uptake and oxidation were reduced by about 40% after hyperglycemia, and oxidation of glucose in presence of mitochondrial uncouplers showed that net and maximal oxidative capacities were significantly reduced. Hyperglycemia also abolished insulin-stimulated glucose uptake. Moreover, ATP concentration was reduced by 25% after hyperglycemia. However, none of the measured mitochondrial genes were downregulated nor was mitochondrial DNA content. Microarray and real-time RT-PCR showed that no genes were significantly regulated by chronic hyperglycemia. Addition of chronic lactate reduced both glucose and OA oxidation to the same extent as hyperglycemia. In conclusion, chronic hyperglycemia reduced substrate oxidation in skeletal muscle cells and impaired metabolic switching. The effect is most likely due to an induced mitochondrial dysfunction.

Cell based in vitro and ex vivo models in metabolic disease drug discovery: nice to have or critical path?

BACKGROUND: The use of cellular models as tools in drug discovery is almost universal. However, in disease areas such as metabolic diseases, are they relevant to the process and do they add value? OBJECTIVE: In this article, we explore the variety of cellular models now used in drug discovery in metabolic diseases as revealed by publication. We have tried to make some connections between drug phenotypes in these models with clinical parallels. We also ask the question as to whether such models add value in the drug discovery process. This overview is not about recombinant cell systems used in target-based screening; rather, we focus on in vitro, including ex vivo, models as physiological systems in drug discovery in obesity and diabetes. CONCLUSION: In terms of building target confidence, in vitro models are often the only mechanistic link to human systems early in a projects life. Many of the current targets in metabolic diseases in the early discovery phase are not yet clinically supported, let alone validated. In this respect, therefore, in vitro models warrant a place in the critical path in early discovery. In terms of any predictive role for decision-making today, this is much more difficult and is more likely pushed to a supporting role as part of a wider package. However, there is a rapid rate of advancement in this field and future developments hold much promise.

Topography of the membrane domain of the liver Na+-dependent bile acid transporter.

The ileal apical and liver basolateral bile acid transporters catalyze the Na+-dependent uptake of these amphipathic molecules in the intestine and liver. They contain nine predicted helical hydrophobic sequences (H1-H9) between the exoplasmic N-glycosylated N terminus and the cytoplasmic C terminus. Previous in vitro translation and in vivo alanine insertion scanning studies gave evidence for either nine or seven transmembrane segments, with H3 and H8 noninserted in the latter model. N-terminal GFP constructs containing either successive predicted segments or only the last two domains of the liver transporter following a membrane anchor signal were expressed in HEK-293 cells, and a C-terminal glycosylation flag allowed detection of membrane insertion. Western blot analysis with anti-GFP antibody after alkali and PNGase treatment showed that H1, H2, H3 behaved as competent transmembrane (TM) sequences. Results from longer constructs were difficult to interpret. H9, however, but not H8 was membrane-inserted. To analyze the intact transporter, a C-terminal YFP fusion protein was expressed as a functionally active protein in the plasma membrane of HEK-293 cells as seen by confocal microscopy. After limited tryptic digestion to ensure the accessibility of only exoplasmic lysine or arginine residues, molecular weight (MW) analysis of the five cleavage products on SDS-PAGE predicted the presence of seven transmembrane segments, H1, H2, H3, H4, H5, H6, and H9, with H7 and H8 exoplasmic. This new method provided evidence for seven membrane segments giving a new model of the membrane domain of this protein and probably the homologous ileal transporter, with H7/H8 as the transport region.