Featured ArticlesVolume 6 | No. 12 | December 2017
|Cranberry extract reverses insulin resistance and hepatic steatosis The use of polyphenol-rich fruit extracts or isolated polyphenols to alleviate obesity-linked diseases has been demonstrated in humans and in animal models, but the mechanisms of action are not yet fully elucidated. Anhê and colleagues investigated the potential of a polyphenol-rich cranberry extract (CE) to reverse an already established obesity, insulin resistance, and non-alcoholic fatty liver disease (NAFLD) in mice. They demonstrate that CE polyphenols strongly improve liver homeostasis. This was associated with improvements in glucose tolerance and full restoration of insulin sensitivity despite the maintenance of obesity. Their data also put forward potential microbial contributors to the effect of CE.|
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Objective: Previous studies have reported that polyphenol-rich extracts from various sources can prevent obesity and associated gastro-hepatic and metabolic disorders in diet-induced obese (DIO) mice. However, whether such extracts can reverse obesity-linked metabolic alterations remains unknown. In the present study, we aimed to investigate the potential of a polyphenol-rich extract from cranberry (CE) to reverse obesity and associated metabolic disorders in DIO-mice.
Methods: Mice were pre-fed either a Chow or a High Fat-High Sucrose (HFHS) diet for 13 weeks to induce obesity and then treated either with CE (200 mg/kg, Chow + CE, HFHS + CE) or vehicle (Chow, HFHS) for 8 additional weeks.
Results: CE did not reverse weight gain or fat mass accretion in Chow- or HFHS-fed mice. However, HFHS + CE fully reversed hepatic steatosis and this was linked to upregulation of genes involved in lipid catabolism (e.g., PPARα) and downregulation of several pro-inflammatory genes (eg, COX2, TNFα) in the liver. These findings were associated with improved glucose tolerance and normalization of insulin sensitivity in HFHS + CE mice. The gut microbiota of HFHS + CE mice was characterized by lower Firmicutes to Bacteroidetes ratio and a drastic expansion of Akkermansia muciniphila and, to a lesser extent, of Barnesiella spp, as compared to HFHS controls.
Conclusions: Taken together, our findings demonstrate that CE, without impacting body weight or adiposity, can fully reverse HFHS diet-induced insulin resistance and hepatic steatosis while triggering A. muciniphila blooming in the gut microbiota, thus underscoring the gut-liver axis as a primary target of cranberry polyphenols.[Hide abstract]
|Knockout of GCN5 does not enhance mitochondrial adaptationPeroxisome proliferator activated receptor-γ coactivator-1α (PGC-1α) is an important contributor to mitochondrial biogenesis and function in skeletal muscle. Combined data from several studies implicate general control of amino acid synthesis 5 (GCN5) as an important negative regulator of PGC-1α transcriptional activity in skeletal muscle and, by extension, mitochondrial biogenesis. Dent and colleagues have now directly investigated the contribution of GCN5 to skeletal muscle metabolism and mitochondrial function in vivo. Their results suggest that loss of GCN5 in muscle does not enhance in vivo basal or endurance exercise-induced metabolic adaptation.|
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Objective: Lysine acetylation is an important post-translational modification that regulates metabolic function in skeletal muscle. The acetyltransferase, general control of amino acid synthesis 5 (GCN5), has been proposed as a regulator of mitochondrial biogenesis via its inhibitory action on peroxisome proliferator activated receptor-γ coactivator-1α (PGC-1α). However, the specific contribution of GCN5 to skeletal muscle metabolism and mitochondrial adaptations to endurance exercise in vivo remain to be defined. We aimed to determine whether loss of GCN5 in skeletal muscle enhances mitochondrial density and function, and the adaptive response to endurance exercise training.
Methods: We used Cre-LoxP methodology to generate mice with muscle-specific knockout of GCN5 (mKO) and floxed, wildtype (WT) littermates. We measured whole-body energy expenditure, as well as markers of mitochondrial density, biogenesis, and function in skeletal muscle from sedentary mice, and mice that performed 20 days of voluntary endurance exercise training.
Results: Despite successful knockdown of GCN5 activity in skeletal muscle of mKO mice, whole-body energy expenditure as well as skeletal muscle mitochondrial abundance and maximal respiratory capacity were comparable between mKO and WT mice. Further, there were no genotype differences in endurance exercise-mediated mitochondrial biogenesis or increases in PGC-1α protein content.
Conclusions: These results demonstrate that loss of GCN5 in vivo does not promote metabolic remodeling in mouse skeletal muscle.[Hide abstract]
|Discovery of agonists for the succinate receptor GPR91 Succinate functions not only as an energy source and building block but also as an extracellular messenger, signaling via the G-protein coupled receptor (GPCR) GPR91. Metabolic stress conditions cause the levels of succinate to rise, enabling activation of GPR91. The physiological role of GPR91 on whole body metabolism, however, is still unclear. Trauelsen and colleagues developed drug-like non-metabolite GPR91 agonists as potential pharmacological tools by using a receptor structure-based approach. The compounds they identified should make it possible to study effects of selective GPR91 activation in an in vivo setting after oral administration.|
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Objective: Besides functioning as an intracellular metabolite, succinate acts as a stress-induced extracellular signal through activation of GPR91 (SUCNR1) for which we lack suitable pharmacological tools.
Methods and results: Here we first determined that the cis conformation of the succinate backbone is preferred and that certain backbone modifications are allowed for GPR91 activation. Through receptor modeling over the X-ray structure of the closely related P2Y1 receptor, we discovered that the binding pocket is partly occupied by a segment of an extracellular loop and that succinate therefore binds in a very different mode than generally believed. Importantly, an empty side-pocket is identified next to the succinate binding site. All this information formed the basis for a substructure-based search query, which, combined with molecular docking, was used in virtual screening of the ZINC database to pick two serial mini-libraries of a total of only 245 compounds from which sub-micromolar, selective GPR91 agonists of unique structures were identified. The best compounds were backbone-modified succinate analogs in which an amide-linked hydrophobic moiety docked into the side-pocket next to succinate as shown by both loss- and gain-of-function mutagenesis. These compounds displayed GPR91-dependent activity in altering cytokine expression in human M2 macrophages similar to succinate, and importantly were devoid of any effect on the major intracellular target, succinate dehydrogenase.
Conclusions: These novel, synthetic non-metabolite GPR91 agonists will be valuable both as pharmacological tools to delineate the GPR91-mediated functions of succinate and as leads for the development of GPR91-targeted drugs to potentially treat low grade metabolic inflammation and diabetic complications such as retinopathy and nephropathy.[Hide abstract]
|Muscle autophagy in endurance-trained runners before and after a high-fat meal Metabolic inflexibility has been implicated in the pathogenesis of obesity and the development of insulin resistance. Tarpey et al. observed elevated markers of mitophagy activity in skeletal muscle of endurance-trained runners (ET) compared with sedentary (SED) males, without a discernible difference in markers of autophagy. However, the greater content of mitophagy markers in skeletal muscle of ET individuals was not associated with a corresponding higher level of metabolic flexibility. Still, skeletal muscle metabolic flexibility increased following a high-fat meal in the ET but not SED individuals.|
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Objective: We tested the hypothesis that skeletal muscle of endurance-trained male runners would exhibit elevated autophagy and mitophagy markers, which would be associated with greater metabolic flexibility following a high-fat meal (HFM).
Methods: Muscle biopsies were collected to determine differences in autophagy and mitophagy protein markers and metabolic flexibility under fasting conditions and 4 h following a HFM between endurance-trained male runners (n = 10) and sedentary, non-obese controls (n = 9).
Results: Maximal oxygen consumption (ml·kg·min−1) was approximately 50% higher (p < 0.05) in endurance-trained runners compared with sedentary controls (65.8 ± 2.3 and 43.1 ± 3.4, respectively). Autophagy markers were similar between groups. Mitophagy and mitochondrial dynamics protein markers were significantly higher in skeletal muscle of endurance-trained runners compared with sedentary controls in the fasted state, although unaffected by the HFM. Skeletal muscle metabolic flexibility was similar between groups when fasted (p > 0.05), but increased in response to the HFM in endurance-trained athletes only (p < 0.005). Key mitophagy markers, phospho-Pink1Thr257 and phospho-ParkinS65 (r = 0.64, p < 0.005), and phospo-ParkinSer65 and phospho-Drp1Ser616 (r = 0.70, p < 0.05) were correlated only within the endurance-trained group. Autophagy and mitophagy markers were not correlated with metabolic flexibility.
Conclusions: In summary, mitophagy may be enhanced in endurance-trained runners based on elevated markers of mitophagy and mitochondrial dynamics. The HFM did not alter autophagy or mitophagy in either group. The absence of a relationship between mitophagy markers and metabolic flexibility suggests that mitophagy is not a key determinant of metabolic flexibility in a healthy population, but further investigation is warranted.[Hide abstract]
|Modulation of cognition and anxiety-like behavior by bone remodelingOsteocalcin is a bone-derived hormone that regulates a growing number of physiological functions. Osteocalcin-/- mice have increased anxiety-like behavior and depression, decreased exploratory behavior, and impaired learning and memory. Given that osteocalcin is produced only by osteoblasts, Khrimian et al. asked whether an impairment in osteoblast differentiation and function, as may occur in various skeletal dysplasias or with aging, could affect cognition or anxiety. Their results suggest that a decrease in osteocalcin is indeed the cause for these cognitive impairments.|
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Objective: That the bone-derived hormone osteocalcin is necessary to promote normal brain development and function, along with its recently described sufficiency in reversing cognitive manifestations of aging, raises novel questions. One of these is to assess whether bone health, which deteriorates rapidly with aging, is a significant determinant of cognition and anxiety-like behavior.
Methods: To begin addressing this question, we used mice haploinsufficient for Runx2, the master gene of osteoblast differentiation and the main regulator of Osteocalcin expression. Control and Runx2+/− mice were evaluated for the expression of osteocalcin's target genes in the brain and for behavioral parameters, using two assays each for cognition and anxiety-like behavior.
Results: We found that adult Runx2+/− mice had defects in bone resorption, reduced circulating levels of bioactive osteocalcin, and reduced expression of osteocalcin's target genes in the brain. Consequently, they had significant impairment in cognitive function and increased anxiety-like behavior.
Conclusions: These results indicate that bone remodeling is a determinant of brain function.[Hide abstract]
|Induction of FGF21 does not require activation of the hepatic XBP1Fibroblast growth factor 21 (FGF21) has emerged as a key regulator of the metabolic response to fasting. Endoplasmic reticulum (ER) stress is a well-established inducer of hepatic FGF21 expression. X-box binding protein 1 (XBP1) has been strongly implicated in regulating hepatic lipid and glucose metabolism, making it an intriguing candidate for mediating the effect of ER stress on FGF21 expression. To directly determine whether hepatic Xbp1 is required for induction of hepatic Fgf21 in vivo, Olivares and Henkel subjected mice bearing a hepatocyte-specific deletion of Xbp1 to fasting, a ketogenic diet, or pharmacologic ER stress, all potent stimuli of Fgf21 expression. By this, they provide definitive evidence that hepatic Xbp1 is not required for induction of hepatic Fgf21.
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Objective: Fibroblast growth factor 21 (FGF21), a key regulator of the metabolic response to fasting, is highly induced by endoplasmic reticulum (ER) stress. The X-box binding protein 1 (Xbp1) is one of several ER stress proteins that has been shown to directly activate the FGF21 promoter. We aimed to determine whether hepatic Xbp1 is required for induction of hepatic FGF21 in vivo.
Methods: Mice bearing a hepatocyte-specific deletion of Xbp1 (Xbp1LKO) were subjected to fasting, pharmacologic ER stress, or a ketogenic diet, all potent stimuli of Fgf21 expression.
Results: Hepatocyte-specific Xbp1 knockout mice demonstrated normal induction of FGF21 in response to fasting or pharmacologic ER stress and enhanced induction of FGF21 in response to a ketogenic diet. Consistent with preserved induction of FGF21, Xbp1LKO mice exhibited normal induction of FGF21 target genes and normal ketogenesis in response to fasting or a ketogenic diet.
Conclusions: Hepatic Xbp1 is not required for induction of FGF21 under physiologic or pathophysiologic conditions in vivo.[Hide abstract]
|AMPK activation protects against hepatic steatosis in MCT1 haploinsufficient mice Hepatic steatosis can evolve into non-alcoholic fatty liver disease (NAFLD). Monocarboxylate transporter isoform 1 (MCT1) haploinsufficient mice (MCT1+/-) exhibit resistance to high-fat diet-induced obesity and associated NAFLD. Carneiro et al. reinforce the concept that AMP-activated protein kinase (AMPK) activation mediates this effect and promotes a protection against hepatic steatosis by inhibiting Sterol Regulatory Element Binding Protein 1 (SREBP1). They also reveal an unexpected regulatory role of circulating lactate in the regulation of hepatic lipid metabolism.|
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Objective: Hepatic steatosis is the first step leading to non-alcoholic fatty liver disease, which represents a major complication of obesity. Here, we show that MCT1 haploinsufficient mice resist to hepatic steatosis development when fed a high fat diet. They exhibit a reduced hepatic capacity to metabolize monocarboxylates such as lactate compared to wildtype mice.
Methods: To understand how this resistance to steatosis develops, we used HFD fed wildtype mice with hepatic steatosis and MCT1 haploinsufficient mice to study hepatic metabolism.
Results: AMPK is constitutively activated in the liver of MCT1 haploinsufficient mice, leading to an inactivation of SREBP1. Therefore, expression of key transcription factors for lipid metabolism, such as PPARα and γ, CHREB, or SREBP1 itself, as well as several enzymes including FAS and CPT1, was not upregulated in these mice when fed a high fat diet. It is proposed that reduced hepatic lactate metabolism is responsible for the protection against hepatic steatosis in MCT1 haploinsufficient mice via a constitutive activation of AMPK and repression of several major elements involved in hepatic lipid metabolism.
Conclusions: Our results support a role of increased lactate uptake in hepatocytes during HFD that, in turn, induce a metabolic shift stimulating SREBP1 activity and lipid accumulation.[Hide abstract]