Articles

New article:  Loss of hepatic DEPTOR alters the metabolic transition to fasting

HIGHLIGHTS
  • Whole-body DEPTOR KO mice are viable and do not display abnormalities.
  • Liver-specific DEPTOR KO mice are hypoglycemic when fasted.
  • Loss of DEPTOR promotes mTORC1 and increases oxidative metabolism.
  • Rapamycin corrects hypoglycemia in liver-specific DEPTOR KO mice.
OBJECTIVE

The mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that functions into distinct protein complexes (mTORC1 and mTORC2) that regulates growth and metabolism. DEP-domain containing mTOR-interacting protein (DEPTOR) is part of these complexes and is known to reduce their activity. Whether DEPTOR loss affects metabolism and organismal growth in vivo has never been tested.

METHODS

We have generated a conditional transgenic mouse allowing the tissue-specific deletion of DEPTOR. This model was crossed with CMV-cre mice or Albumin-cre mice to generate either whole-body or liver-specific DEPTOR knockout (KO) mice.

RESULTS

Whole-body DEPTOR KO mice are viable, fertile, normal in size, and do not display any gross physical and metabolic abnormalities. To circumvent possible compensatory mechanisms linked to the early and systemic loss of DEPTOR, we have deleted DEPTOR specifically in the liver, a tissue in which DEPTOR protein is expressed and affected in response to mTOR activation. Liver-specific DEPTOR null mice showed a reduction in circulating glucose upon fasting versus control mice. This effect was not associated with change in hepatic gluconeogenesis potential but was linked to a sustained reduction in circulating glucose during insulin tolerance tests. In addition to the reduction in glycemia, liver-specific DEPTOR KO mice had reduced hepatic glycogen content when fasted. We showed that loss of DEPTOR cell-autonomously increased oxidative metabolism in hepatocytes, an effect associated with increased cytochrome c expression but independent of changes in mitochondrial content or in the expression of genes controlling oxidative metabolism. We found that liver-specific DEPTOR KO mice showed sustained mTORC1 activation upon fasting, and that acute treatment with rapamycin was sufficient to normalize glycemia in these mice.

CONCLUSION

We propose a model in which hepatic DEPTOR accelerates the inhibition of mTORC1 during the transition to fasting to adjust metabolism to the nutritional status.

Keywords:

DEPTORmTORliverglucosefasting


Article: Loss of UCP2 impairs cold-induced non-shivering thermogenesis by promoting a shift toward glucose utilization in brown adipose tissue.

Uncoupling protein 2 (UCP2) was discovered in 1997 and classified as an uncoupling protein largely based on its homology of sequence with UCP1. Since its discovery, the uncoupling function of UCP2 has been questioned and there is yet no consensus on the true function of this protein. UCP2 was first proposed to be a reactive oxygen species (ROS) regulator and an insulin secretion modulator. More recently, it was demonstrated as a regulator of the mitochondrial fatty acid oxidation, which prompted us to investigate its role in the metabolic and thermogenic functions of brown adipose tissue. We first investigated the role of UCP2 in affecting the glycolysis capacity by evaluating the extracellular flux in cells lacking UCP2. We thereafter investigated the role of UCP2 in BAT thermogenesis with positron emission tomography using the metabolic tracers [(11)C]-acetate (metabolic activity), 2-deoxy-2-[(18)F]-fluoro-d-glucose ((18)FDG, glucose uptake) and 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid [(18)FTHA, non-esterified fatty acid (NEFA) uptake]. The effect of the β3-adrenoreceptor (ADRB3) selective agonist, CL316,243 (CL), on BAT (18)FDG and (18)FTHA uptakes, as well as (11)C-acetate activity was assessed in UCP2(KO) and UCP2(WT) mice exposed at room temperature or adapted to cold. Our results suggest that despite the fact that UCP2 does not have the uncoupling potential of UCP1, its contribution to BAT thermogenesis and to the adaptation to cold exposure appears crucial. Notably, we found that the absence of UCP2 promoted a shift toward glucose utilization and increased glycolytic capacity in BAT, which conferred a better oxidative/thermogenic activity/capacity following an acute adrenergic stimulation. However, following cold exposure, a context of high-energy demand, BAT of UCP2(KO) mice failed to adapt and thermogenesis was impaired. We conclude that UCP2 regulates BAT thermogenesis by favouring the utilization of NEFA, a process required for the adaptation to cold.

Caron, A., Labbé, S. M., Carter, S., Roy, M.-C., Lecomte, R., Ricquier, D., et al. (2017). Loss of UCP2 impairs cold-induced non-shivering thermogenesis by promoting a shift toward glucose utilization in brown adipose tissue. Biochimie. http://doi.org/10.1016/j.biochi.2017.01.006


Deficiency of Interleukin-15 Confers Resistance to Obesity by Diminishing Inflammation and Enhancing the Thermogenic Function of Adipose Tissues. PLOS One. September 29, 2016 http://dx.doi.org/10.1371/journal.pone.0162995
Gregory Lacraz, Volatiana Rakotoarivelo, Sebastien M Labbé, Mathieu Vernier, Christophe Noll, Marian Mayhue, Jana Stankova, Adel Schwertani, Guillaume Grenier, André Carpentier, Denis Richard, Gerardo Ferbeyre, Julie Fradette, Marek Rola-Pleszczynski, Alfredo Menendez, Marie-France Langlois, Subburaj Ilangumaran, Sheela Ramanathan.
Abstract : Objective – IL-15 is an inflammatory cytokine secreted by many cell types. IL-15 is also produced during physical exercise by skeletal muscle and has been reported to reduce weight gain in mice. Contrarily, our findings on IL-15 knockout (KO) mice indicate that IL-15 promotes obesity. The aim of this study is to investigate the mechanisms underlying the pro-obesity role of IL-15 in adipose tissues. Methods – Control and IL-15 KO mice were maintained on high fat diet (HFD) or normal control diet. After 16 weeks, body weight, adipose tissue and skeletal mass, serum lipid levels and gene/protein expression in the adipose tissues were evaluated. The effect of IL-15 on thermogenesis and oxygen consumption was also studied in primary cultures of adipocytes differentiated from mouse preadipocyte and human stem cells. Results – Our results show that IL-15 deficiency prevents diet-induced weight gain and accumulation of lipids in visceral and subcutaneous white and brown adipose tissues. Gene expression analysis also revealed elevated expression of genes associated with adaptive thermogenesis in the brown and subcutaneous adipose tissues of IL-15 KO mice. Accordingly, oxygen consumption was increased in the brown adipocytes from IL-15 KO mice. In addition, IL-15 KO mice showed decreased expression of pro-inflammatory mediators in their adipose tissues. Conclusions – Absence of IL-15 results in decreased accumulation of fat in the white adipose tissues and increased lipid utilization via adaptive thermogenesis. IL-15 also promotes inflammation in adipose tissues that could sustain chronic inflammation leading to obesity-associated metabolic syndrome.

Contributions of white and brown adipose tissues and skeletal muscles to acute cold‐induced metabolic responses in healthy men. The Journal of Physiology, 15 December 2014, DOI: 10.1113/jphysiol.2014.283598

Denis P Blondin, Sébastien M Labbé, Serge Phoenix, Brigitte Guérin, Eric E Turcotte, Denis Richard, André C Carpentier, François Haman.

Abstract: Cold exposure stimulates the sympathetic nervous system (SNS), triggering the activation of cold-defence responses and mobilizing substrates to fuel the thermogenic processes. Although these processes have been investigated independently, the physiological interaction and coordinated contribution of the tissues involved in producing heat or mobilizing substrates has never been investigated in humans. Using [U-13C]-palmitate and [3-3H]-glucose tracer methodologies coupled with positron emission tomography using 11C-acetate and 18F-fluorodeoxyglucose, we examined the relationship between whole body sympathetically induced white adipose tissue (WAT) lipolysis and brown adipose tissue (BAT) metabolism and mapped the skeletal muscle shivering and metabolic activation pattern during a mild, acute cold exposure designed to minimize shivering response in 12 lean healthy men. Cold-induced increase in whole-body oxygen consumption was not independently associated with BAT volume of activity, BAT oxidative metabolism, or muscle metabolism or shivering intensity, but depended on the sum of responses of these two metabolic tissues. Cold-induced increase in non-esterified fatty acid (NEFA) appearance rate was strongly associated with the volume of metabolically active BAT (r = 0.80, P = 0.005), total BAT oxidative metabolism (r = 0.70, P = 0.004) and BAT glucose uptake (r = 0.80, P = 0.005), but not muscle glucose metabolism. The total glucose uptake was more than one order of magnitude greater in skeletal muscles compared to BAT during cold exposure (674 ± 124 vs. 12 ± 8 μmol min−1, respectively, P < 0.001). Glucose uptake demonstrated that deeper, centrally located muscles of the neck, back and inner thigh were the greatest contributors of muscle glucose uptake during cold exposure due to their more important shivering response. In summary, these results demonstrate for the first time that the increase in plasma NEFA appearance from WAT lipolysis is closely associated with BAT metabolic activation upon acute cold exposure in healthy men. In humans, muscle glucose utilization during shivering contributes to a much greater extent than BAT to systemic glucose utilization during acute cold exposure.

Hypothalamic control of brown adipose tissue thermogenesisFront Syst Neurosci. 2015; 9: 150. Published online 2015 Nov 3. doi:  10.3389/fnsys.2015.00150
Sebastien M Labbé, Alexandre Caron, Damien Lanfray, Boris Monge-Rofarello, Timothy J Bartness, Denis Richard (2015)
Abstract: It has long been known, in large part from animal studies, that the control of brown adipose tissue (BAT) thermogenesis is insured by the central nervous system (CNS), which integrates several stimuli in order to control BAT activation through the sympathetic nervous system (SNS). SNS-mediated BAT activity is governed by diverse neurons found in brain structures involved in homeostatic regulations and whose activity is modulated by various factors including oscillations of energy fluxes. The characterization of these neurons has always represented a challenging issue. The available literature suggests that the neuronal circuits controlling BAT thermogenesis are largely part of an autonomic circuitry involving the hypothalamus, brainstem and the SNS efferent neurons. In the present review, we recapitulate the latest progresses in regards to the hypothalamic regulation of BAT metabolism. We briefly addressed the role of the thermoregulatory pathway and its interactions with the energy balance systems in the control of thermogenesis. We also reviewed the involvement of the brain melanocortin and endocannabinoid systems as well as the emerging role of steroidogenic factor 1 (SF1) neurons in BAT thermogenesis. Finally, we examined the link existing between these systems and the homeostatic factors that modulate their activities.

 

Keywords: brown adipose tissue, melanocortin, endocannabinoid, hypothalamus, steroidogenic factor 1, non-shivering thermogenesis


Increased Postprandial Nonesterified Fatty Acid Appearance and Oxidation in Type 2 Diabetes Is Not Fully Established in Offspring of Diabetic Subjects. PLoS ONE 5(6): e10956. doi:10.1371/journal.pone.0010956
Normand-Lauziere F, Frisch F, Labbé SM, Bherer P, Gagnon R, et al. (2010)
Abstract: Background: It has been proposed that abnormal postprandial plasma nonesterified fatty acid (NEFA) metabolism may participate in the development of tissue lipotoxicity and type 2 diabetes (T2D). We previously found that non-diabetic offspring of two parents with T2D display increased plasma NEFA appearance and oxidation rates during intravenous administration of a fat emulsion. However, it is currently unknown whether plasma NEFA appearance and oxidation are abnormal during the postprandial state in these subjects at high-risk of developing T2D. Methodology: Palmitate appearance and oxidation rates and glycerol appearance rate were determined in eleven healthy offspring of two parents with T2D (positive family history, FH+), 13 healthy subjects without first-degree relatives with T2D (FH-) and 12 subjects with T2D at fasting, during normoglycemic hyperinsulinemic clamp and during continuous oral intake of a standard liquid meal to achieve steady postprandial NEFA and triacylglycerols (TG) without and with insulin infusion to maintain similar glycemia in all three groups. Principal findings: Plasma palmitate appearance and oxidation were higher at fasting and during the clamp conditions in the T2D group (all P<0.05). In the postprandial state, palmitate appearance, oxidative and non oxidative rates were all elevated in T2D (all P<0.05) but not in FH+. Both T2D and FH+ displayed elevated postprandial TG vs. FH- (P<0.001). Acute correction of hyperglycemia during the postprandial state did not affect these group differences. Increased waist circumference and BMI were positively associated with elevated postprandial plasma palmitate appearance and oxidation. Conclusions/significance: Postprandial plasma NEFA intolerance observed in subjects with T2D is not fully established in non-diabetic offspring of both parents with T2D, despite the presence of increased postprandial plasma TG in the later. Elevated postprandial plasma NEFA appearance and oxidation in T2D is observed despite acute correction of the exaggerated glycemic excursion in this group.