Aims/hypothesis
Insulin resistance (IR) improves with weight loss, but this response is heterogeneous. We hypothesised that metabolomic profiling would identify biomarkers predicting changes in IR ...with weight loss.
Methods
Targeted mass spectrometry-based profiling of 60 metabolites, plus biochemical assays of NEFA, β-hydroxybutyrate, ketones, insulin and glucose were performed in baseline and 6 month plasma samples from 500 participants who had lost ≥4 kg during Phase I of the Weight Loss Maintenance (WLM) trial. Homeostatic model assessment of insulin resistance (HOMA-IR) and change in HOMA-IR with weight loss (∆HOMA-IR) were calculated. Principal components analysis (PCA) and mixed models adjusted for race, sex, baseline weight, and amount of weight loss were used; findings were validated in an independent cohort of patients (
n
= 22).
Results
Mean weight loss was 8.67 ± 4.28 kg; mean ∆HOMA-IR was −0.80 ± 1.73, range −28.9 to 4.82). Baseline PCA-derived factor 3 (branched chain amino acids BCAAs and associated catabolites) correlated with baseline HOMA-IR (
r
= 0.50,
p
< 0.0001) and independently associated with ∆HOMA-IR (
p
< 0.0001). ∆HOMA-IR increased in a linear fashion with increasing baseline factor 3 quartiles. Amount of weight loss was only modestly correlated with ∆HOMA-IR (
r
= 0.24). These findings were validated in the independent cohort, with a factor composed of BCAAs and related metabolites predicting ∆HOMA-IR (
p
= 0.007).
Conclusions/interpretation
A cluster of metabolites comprising BCAAs and related analytes predicts improvement in HOMA-IR independent of the amount of weight lost. These results may help identify individuals most likely to benefit from moderate weight loss and elucidate novel mechanisms of IR in obesity.
Aims/hypothesis Insulin resistance (IR) is associated with obesity, but can also develop in individuals with normal body weight. We employed comprehensive profiling methods to identify metabolic ...events associated with IR, while controlling for obesity. Methods We selected 263 non-obese (BMI approximately 24 kg/m²) Asian-Indian and Chinese men from a large cross-sectional study carried out in Singapore. Individuals taking medication for diabetes or hyperlipidaemia were excluded. Participants were separated into lower and upper tertiles of IR based on HOMA indices of ≤1.06 or ≥1.93, respectively. MS-based metabolic profiling of acylcarnitines, amino acids and organic acids was combined with hormonal and cytokine profiling in all participants. Results After controlling for BMI, commonly accepted risk factors for IR, including circulating fatty acids and inflammatory cytokines, did not discriminate the upper and lower quartiles of insulin sensitivity in either Asian-Indian or Chinese men. Instead, IR was correlated with increased levels of alanine, proline, valine, leucine/isoleucine, phenylalanine, tyrosine, glutamate/glutamine and ornithine, and a cluster of branched-chain and related amino acids identified by principal components analysis. These changes were not due to increased protein intake by individuals in the upper quartile of IR. Increased abdominal adiposity and leptin, and decreased adiponectin and IGF-binding protein 1 were also correlated with IR. Conclusions/interpretation These findings demonstrate that perturbations in amino acid homeostasis, but not inflammatory markers or NEFAs, are associated with IR in individuals of relatively low body mass.
Hibernation is an adaptation that helps many animals to conserve energy during food shortage in winter. Brown bears double their fat depots during summer and use these stored lipids during ...hibernation. Although bears seasonally become obese, they remain metabolically healthy. We analyzed the microbiota of free-ranging brown bears during their active phase and hibernation. Compared to the active phase, hibernation microbiota had reduced diversity, reduced levels of Firmicutes and Actinobacteria, and increased levels of Bacteroidetes. Several metabolites involved in lipid metabolism, including triglycerides, cholesterol, and bile acids, were also affected by hibernation. Transplantation of the bear microbiota from summer and winter to germ-free mice transferred some of the seasonal metabolic features and demonstrated that the summer microbiota promoted adiposity without impairing glucose tolerance, suggesting that seasonal variation in the microbiota may contribute to host energy metabolism in the hibernating brown bear.
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•Bear microbiota composition differs seasonally between hibernation and active phase•Blood metabolites differ seasonally in the brown bear•The bear gut microbiota promote energy storage during summer
Sommer et al. show that the microbiota and serum metabolites in brown bears differ seasonally between hibernation and active phase. Colonization of mice with a bear microbiota promoted increased adiposity. These findings suggest that seasonal microbiota variation may contribute to metabolism of the hibernating brown bear.
Obesity and type 2 diabetes are associated with mitochondrial dysfunction in adipose tissue, but the role for adipose tissue mitochondria in the development of these disorders is currently unknown. ...To understand the impact of adipose tissue mitochondria on whole-body metabolism, we have generated a mouse model with disruption of the mitochondrial transcription factor A (TFAM) specifically in fat. F-TFKO adipose tissue exhibit decreased mtDNA copy number, altered levels of proteins of the electron transport chain, and perturbed mitochondrial function with decreased complex I activity and greater oxygen consumption and uncoupling. As a result, F-TFKO mice exhibit higher energy expenditure and are protected from age- and diet-induced obesity, insulin resistance, and hepatosteatosis, despite a greater food intake. Thus, TFAM deletion in the adipose tissue increases mitochondrial oxidation that has positive metabolic effects, suggesting that regulation of adipose tissue mitochondria may be a potential therapeutic target for the treatment of obesity.
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► Adipose tissue-specific TFAM KO protects from obesity and insulin resistance ► Adipose tissue-specific TFAM KO increases energy expenditure ► TFAM KO increases mitochondrial uncoupling and decreases complex I activity ► Increased adipose tissue mitochondrial oxidation has positive metabolic effect
Blood glucose levels are maintained by the balance between glucose uptake by peripheral tissues and glucose secretion by the liver. Gluconeogenesis is strongly stimulated during fasting and is ...aberrantly activated in diabetes mellitus. Here we show that the transcriptional coactivator PGC-1 is strongly induced in liver in fasting mice and in three mouse models of insulin action deficiency: streptozotocin-induced diabetes, ob/ob genotype and liver insulin-receptor knockout. PGC-1 is induced synergistically in primary liver cultures by cyclic AMP and glucocorticoids. Adenoviral-mediated expression of PGC-1 in hepatocytes in culture or in vivo strongly activates an entire programme of key gluconeogenic enzymes, including phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, leading to increased glucose output. Full transcriptional activation of the PEPCK promoter requires coactivation of the glucocorticoid receptor and the liver-enriched transcription factor HNF-4alpha (hepatic nuclear factor-4alpha) by PGC-1. These results implicate PGC-1 as a key modulator of hepatic gluconeogenesis and as a central target of the insulin-cAMP axis in liver.
Isolation of INS-1-derived cell lines with robust ATP-sensitive K+ channel-dependent and -independent glucose-stimulated insulin
secretion.
H E Hohmeier ,
H Mulder ,
G Chen ,
R Henkel-Rieger ,
M ...Prentki and
C B Newgard
BetaGene, Inc., Dallas, Texas, USA.
Abstract
The biochemical mechanisms involved in regulation of insulin secretion are not completely understood. The rat INS-1 cell line
has been used to gain insight in this area because it secretes insulin in response to glucose concentrations in the physiological
range. However, the magnitude of the response is far less than that seen in freshly isolated rat islets. In the current study,
we have stably transfected INS-1 cells with a plasmid containing the human proinsulin gene. After antibiotic selection and
clonal expansion, 67% of the resultant clones were found to be poorly responsive to glucose in terms of insulin secretion
(< or =2-fold stimulation by 15 mmol/l compared with 3 mmol/l glucose), 17% of the clones were moderately responsive (2- to
5-fold stimulation), and 16% were strongly responsive (5- to 13-fold stimulation). The differences in responsiveness could
not be ascribed to differences in insulin content. Detailed analysis of one of the strongly responsive lines (832/13) revealed
that its potent response to glucose (average of 10-fold) was stable over 66 population doublings (approximately 7.5 months
of tissue culture) with half-maximal stimulation at 6 mmol/l glucose. Furthermore, in the presence of 15 mmol/l glucose, insulin
secretion was potentiated significantly by 100 pmol/l isobutylmethylxanthine (320%), 1 mmol/l oleate/palmitate (77%), and
50 nmol/l glucagon-like peptide 1 (60%), whereas carbachol had no effect. Glucose-stimulated insulin secretion was also potentiated
by the sulfonylurea tolbutamide (threefold at 3 mmol/l glucose and 50% at 15 mmol/l glucose) and was abolished by diazoxide,
which demonstrates the operation of the ATP-sensitive K+ channel (K(ATP)) in 832/13 cells. Moreover, when the K(ATP) channel
was bypassed by incubation of cells in depolarizing K+ (35 mmol/l), insulin secretion was more effectively stimulated by glucose
in 832/13 cells than in parental INS-1 cells, which demonstrates the presence of a K(ATP) channel-independent pathway of glucose
sensing. We conclude that clonal selection of INS-1 cells allows isolation of cell lines that exhibit markedly enhanced and
stable responsiveness to glucose and several of its known potentiators. These lines may be attractive new vehicles for studies
of beta-cell function.
Peroxisome proliferator-activated receptor γ coactivator (PGC)-1α is a coactivator of nuclear receptors and other transcription factors that regulates several components of energy metabolism, ...particularly certain aspects of adaptive thermogenesis in brown fat and skeletal muscle, hepatic gluconeogenesis, and fiber type switching in skeletal muscle. PGC-1α has been shown to induce mitochondrial biogenesis when expressed in muscle cells, and preliminary analysis has suggested that this molecule may specifically increase the fraction of uncoupled versus coupled respiration. In this paper, we have performed detailed bioenergetic analyses of the function of PGC-1α and its homolog PGC-1β in muscle cells by monitoring simultaneously oxygen consumption and membrane potential. Cells expressing PGC-1α or PGC-1β display higher proton leak rates at any given membrane potential than control cells. However, cells expressing PGC-1α have a higher proportion of their mitochondrial respiration linked to proton leak than cells expressing PGC-1β. Although these two proteins cause a similar increase in the expression of many mitochondrial genes, PGC-1β preferentially induces certain genes involved in the removal of reactive oxygen species, recently recognized as activators of uncoupling proteins. Together, these data indicate that PGC-1α and PGC-1β profoundly alter mitochondrial metabolism and suggest that these proteins are likely to play different physiological functions.
Isoprenylcysteine carboxylmethyltransferase (Icmt) catalyzes the last of the three-step posttranslational protein prenylation process for the so-called CaaX proteins, which includes many signaling ...proteins, such as most small GTPases. Despite extensive studies on Icmt and its regulation of cell functions, the mechanisms of much of the impact of Icmt on cellular functions remain unclear. Our recent studies demonstrated that suppression of Icmt results in induction of autophagy, inhibition of cell growth and inhibition of proliferation in various cancer cell types, prompting this investigation of potential metabolic regulation by Icmt. We report here the findings that Icmt inhibition reduces the function of mitochondrial oxidative phosphorylation in multiple cancer cell lines. In-depth oximetry analysis demonstrated that functions of mitochondrial complex I, II and III are subject to Icmt regulation. Consistently, Icmt inhibition decreased cellular ATP and depleted critical tricarboxylic acid cycle metabolites, leading to suppression of cell anabolism and growth, and marked autophagy. Several different approaches demonstrated that the impact of Icmt inhibition on cell proliferation and viability was largely mediated by its effect on mitochondrial respiration. This previously unappreciated function of Icmt, which can be therapeutically exploited, likely has a significant role in the impact of Icmt on tumorigenic processes.
The balance between hepatic glucose uptake and production is perturbed in both major forms of diabetes. It has been suggested that pharmacologic or genetic methods for enhancing glucokinase (GK) ...enzymatic activity in liver might be a means of increasing glucose disposal and lowering blood glucose in diabetic patients. To better evaluate this possibility, we used a recombinant adenovirus containing the cDNA encoding GK (AdCMV-GKL) to achieve overexpression of the enzyme at different levels in liver of normal rats. In a first set of experiments, in rats fasted for 18 h, AdCMV-GKL infusion caused a 211% increase in hepatic GK activity relative to animals infused with a control virus (AdCMV-betaGAL). AdCMV-GKL-treated fasted rats exhibited no significant changes in circulating glucose, free fatty acids (FFAs), lactate, beta-hydroxybutyrate, or insulin levels relative to controls, whereas triglyceride (TG) levels were slightly increased (53%). In a second set of studies, in rats fed ad libitum, GK was overexpressed in liver by 3- and 6.4-fold. Animals with the lower degree of GK overexpression exhibited no significant changes in circulating glucose, FFAs, insulin, TG, or lactate levels relative to controls that received a virus encoding a catalytically inactive mutant GK (AdCMV-GK203), but did show a modest increase in lactate (58%) relative to AdCMV-betaGAL-infused controls. In contrast, the higher level of GK overexpression caused a 38% decrease in blood glucose levels and a 67% decrease in circulating insulin levels relative to AdCMV-GK203-infused animals. The decline in glucose levels was accompanied by a 190% increase in circulating TG and a 310% increase in circulating FFAs; total plasma cholesterol was unaffected. Finally, fasted animals treated with AdCMV-GKL had 5.4 times as much liver glycogen as AdCMV-betaGAL-treated controls; no significant increases in liver glycogen were observed at either level of GK overexpression in ad libitum-fed rats relative to fed controls. In sum, levels of hepatic GK overexpression associated with a decline in blood glucose are accompanied by equally dramatic increases in FFAs and TG, raising concerns about manipulation of liver GK activity as a viable strategy for treatment of diabetes.
Single-gene mutations that disrupt mitochondrial respiratory chain function in Caenorhabditis elegans change patterns of protein expression and metabolites. Our goal was to develop useful molecular ...fingerprints employing adaptable techniques to recognize mitochondrial defects in the electron transport chain. We analyzed mutations affecting complex I, complex II, or ubiquinone synthesis and discovered overarching patterns in the response of C. elegans to mitochondrial dysfunction across all of the mutations studied. These patterns are in KEGG pathways conserved from C. elegans to mammals, verifying that the nematode can serve as a model for mammalian disease. In addition, specific differences exist between mutants that may be useful in diagnosing specific mitochondrial diseases in patients.
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