Publications by authors named "Alexander W Fischer"

37 Publications

TFEB deficiency attenuates mitochondrial degradation upon brown adipose tissue whitening at thermoneutrality.

Mol Metab 2021 Jan 29;47:101173. Epub 2021 Jan 29.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany; Department of Molecular Metabolism, Harvard T. H. Chan School of Public Health, Boston, MA, USA; Department of Cell Biology, Harvard Medical School, Boston, MA, USA. Electronic address:

Objective: Brown adipose tissue (BAT) thermogenesis offers the potential to improve metabolic health in mice and humans. However, humans predominantly live under thermoneutral conditions, leading to BAT whitening, a reduction in BAT mitochondrial content and metabolic activity. Recent studies have established mitophagy as a major driver of mitochondrial degradation in the whitening of thermogenic brite/beige adipocytes, yet the pathways mediating mitochondrial breakdown in whitening of classical BAT remain largely elusive. The transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy belonging to the MiT family of transcription factors, is the only member of this family that is upregulated during whitening, pointing toward a role of TFEB in whitening-associated mitochondrial breakdown.

Methods: We generated brown adipocyte-specific TFEB knockout mice, and induced BAT whitening by thermoneutral housing. We characterized gene and protein expression patterns, BAT metabolic activity, systemic metabolism, and mitochondrial localization using in vivo and in vitro approaches.

Results: Under low thermogenic activation conditions, deletion of TFEB preserves mitochondrial mass independently of mitochondriogenesis in BAT and primary brown adipocytes. However, this does not translate into elevated thermogenic capacity or protection from diet-induced obesity. Autophagosomal/lysosomal marker levels are altered in TFEB-deficient BAT and primary adipocytes, and lysosomal markers co-localize and co-purify with mitochondria in TFEB-deficient BAT, indicating trapping of mitochondria in late stages of mitophagy.

Conclusion: We identify TFEB as a driver of BAT whitening, mediating mitochondrial degradation via the autophagosomal and lysosomal machinery. This study provides proof of concept that interfering with the mitochondrial degradation machinery can increase mitochondrial mass in classical BAT under human-relevant conditions. However, it must be considered that interfering with autophagy may result in accumulation of non-functional mitochondria. Future studies targeting earlier steps of mitophagy or target recognition are therefore warranted.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.molmet.2021.101173DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7903014PMC
January 2021

Endogenous Fatty Acid Synthesis Drives Brown Adipose Tissue Involution.

Cell Rep 2021 Jan;34(2):108624

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Electronic address:

Thermoneutral conditions typical for standard human living environments result in brown adipose tissue (BAT) involution, characterized by decreased mitochondrial mass and increased lipid deposition. Low BAT activity is associated with poor metabolic health, and BAT reactivation may confer therapeutic potential. However, the molecular drivers of this BAT adaptive process in response to thermoneutrality remain enigmatic. Using metabolic and lipidomic approaches, we show that endogenous fatty acid synthesis, regulated by carbohydrate-response element-binding protein (ChREBP), is the central regulator of BAT involution. By transcriptional control of lipogenesis-related enzymes, ChREBP determines the abundance and composition of both storage and membrane lipids known to regulate organelle turnover and function. Notably, ChREBP deficiency and pharmacological inhibition of lipogenesis during thermoneutral adaptation preserved mitochondrial mass and thermogenic capacity of BAT independently of mitochondrial biogenesis. In conclusion, we establish lipogenesis as a potential therapeutic target to prevent loss of BAT thermogenic capacity as seen in adult humans.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.celrep.2020.108624DOI Listing
January 2021

Lysosomal lipoprotein processing in endothelial cells stimulates adipose tissue thermogenic adaptation.

Cell Metab 2021 Mar 22;33(3):547-564.e7. Epub 2020 Dec 22.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Electronic address:

In response to cold exposure, thermogenic adipocytes internalize large amounts of fatty acids after lipoprotein lipase-mediated hydrolysis of triglyceride-rich lipoproteins (TRL) in the capillary lumen of brown adipose tissue (BAT) and white adipose tissue (WAT). Here, we show that in cold-exposed mice, vascular endothelial cells in adipose tissues endocytose substantial amounts of entire TRL particles. These lipoproteins subsequently follow the endosomal-lysosomal pathway, where they undergo lysosomal acid lipase (LAL)-mediated processing. Endothelial cell-specific LAL deficiency results in impaired thermogenic capacity as a consequence of reduced recruitment of brown and brite/beige adipocytes. Mechanistically, TRL processing by LAL induces proliferation of endothelial cells and adipocyte precursors via beta-oxidation-dependent production of reactive oxygen species, which in turn stimulates hypoxia-inducible factor-1α-dependent proliferative responses. In conclusion, this study demonstrates a physiological role for TRL particle uptake into BAT and WAT and establishes endothelial lipoprotein processing as an important determinant of adipose tissue remodeling during thermogenic adaptation.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.cmet.2020.12.001DOI Listing
March 2021

Thermoneutrality-Induced Macrophage Accumulation in Brown Adipose Tissue Does Not Impair the Tissue's Competence for Cold-Induced Thermogenic Recruitment.

Front Endocrinol (Lausanne) 2020 30;11:568682. Epub 2020 Oct 30.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.

Brown adipose tissue from mice living under conditions approaching human thermal and nutritional conditions (prolonged exposure to thermoneutral temperature and to an energy-rich (high-fat, high-sugar) diet) - referred to as "physiologically humanized" mice, displays morphological and molecular characteristics significantly different from those observed in young, chow-fed mice maintained at room temperature - referred to as "standard" mice. Here, we further examined brown fat from physiologically humanized and standard mice, as well as from mice exposed to thermoneutrality for a long time but not to an energy-rich diet - referred to here as "long-term thermoneutral" mice. Global transcriptome analysis of brown fat revealed that genes that were the most upregulated in brown fat of thermoneutral mice (both physiologically humanized and long-term thermoneutral) were those related to inflammatory processes, including genes expressed selectively in macrophages. Cellular and molecular analyses confirmed that brown fat from thermoneutral mice was heavily infiltrated by macrophages, predominantly organized into crown-like structures. However, despite this, the brown fat of thermoneutral mice retained full competence to attain the greatest possible recruitment state and became macrophage-depleted during the process of cold acclimation. Thus, profound macrophage accumulation does not influence the thermogenic recruitment competence of brown fat.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3389/fendo.2020.568682DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7662517PMC
October 2020

Lipid Droplets in Brown Adipose Tissue Are Dispensable for Cold-Induced Thermogenesis.

Cell Rep 2020 11;33(5):108348

Department of Molecular Metabolism, Harvard T.H. Chan School of Public Health, Boston, MA 02115, USA; Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA; Broad Institute of Harvard and MIT, Cambridge, MA 02142, USA; Howard Hughes Medical Institute, Boston, MA 02115, USA. Electronic address:

Brown adipocytes store metabolic energy as triglycerides (TGs) in lipid droplets (LDs). Fatty acids released from brown adipocyte LDs by lipolysis are thought to activate and fuel UCP1-mediated thermogenesis. Here, we test this hypothesis by preventing fatty acid storage in murine brown adipocytes through brown adipose tissue (BAT)-specific deletions of the TG synthesis enzymes DGAT1 and DGAT2 (BA-DGAT KO). Despite the absence of TGs in brown adipocytes, BAT is functional, and BA-DGAT-KO mice maintain euthermia during acute or chronic cold exposure. As apparent adaptations to the lack of TG, brown adipocytes of BA-DGAT-KO mice appear to use circulating glucose and fatty acids, and stored glycogen, to fuel thermogenesis. Moreover, BA-DGAT-KO mice are resistant to diet-induced glucose intolerance, likely because of increased glucose disposal by BAT. We conclude that TGs in BAT are dispensable for its contribution to cold-induced thermogenesis, at least when other fuel sources are available.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.celrep.2020.108348DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7696656PMC
November 2020

CD38 downregulation modulates NAD and NADP(H) levels in thermogenic adipose tissues.

Biochim Biophys Acta Mol Cell Biol Lipids 2021 01 30;1866(1):158819. Epub 2020 Sep 30.

DIMES-Section of Biochemistry, University of Genova, Italy. Electronic address:

Different strategies to boost NAD levels are considered promising means to promote healthy aging and ameliorate dysfunctional metabolism. CD38 is a NAD-dependent enzyme involved in the regulation of different cell functions. In the context of systemic energy metabolism, it has been demonstrated that brown adipocytes, the parenchymal cells of brown adipose tissue (BAT) as well as beige adipocytes that emerge in white adipose tissue (WAT) depots in response to catabolic conditions, are important to maintain metabolic homeostasis. In this study we aim to understand the functional relevance of CD38 for NAD and energy metabolism in BAT and WAT, also using a CD38 mouse model. During cold exposure, an increase in NAD levels occurred in BAT of wild type mice, together with a marked downregulation of CD38, as detected at the mRNA and protein level. CD38 downregulation was observed also in WAT of cold-exposed mice, where it was accompanied by a strong increase in NADP(H) levels. Accordingly, NAD kinase and glucose-6-phosphate dehydrogenase activities were enhanced in WAT (but not in BAT). Increased NAD levels were observed in BAT/WAT from CD38 compared with wild type mice, in line with CD38 being a major NAD-consumer in AT. CD38 mice kept at 6 °C had higher levels of Ucp1 and Pgc-1α in BAT and WAT, and increased levels of phosphorylated hormone-sensitive lipase in BAT, compared with wild type mice. These results demonstrate that CD38, by modulating cellular NAD(P) levels, is involved in the regulation of thermogenic responses in cold-activated BAT and WAT.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.bbalip.2020.158819DOI Listing
January 2021

Brown adipose tissue lipoprotein and glucose disposal is not determined by thermogenesis in uncoupling protein 1-deficient mice.

J Lipid Res 2020 11 7;61(11):1377-1389. Epub 2020 Aug 7.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany

Adaptive thermogenesis is highly dependent on uncoupling protein 1 (UCP1), a protein expressed by thermogenic adipocytes present in brown adipose tissue (BAT) and white adipose tissue (WAT). Thermogenic capacity of human and mouse BAT can be measured by positron emission tomography-computed tomography quantifying the uptake of F-fluodeoxyglucose or lipid tracers. BAT activation is typically studied in response to cold exposure or treatment with β-3-adrenergic receptor agonists such as CL316,243 (CL). Currently, it is unknown whether cold-stimulated uptake of glucose or lipid tracers is a good surrogate marker of UCP1-mediated thermogenesis. In metabolic studies using radiolabeled tracers, we found that glucose uptake is increased in mildly cold-activated BAT of versus WT mice kept at subthermoneutral temperature. Conversely, lower glucose disposal was detected after full thermogenic activation achieved by sustained cold exposure or CL treatment. In contrast, uptake of lipoprotein-derived fatty acids into chronically activated thermogenic adipose tissues was substantially increased in UCP1-deficient mice. This effect is linked to higher sympathetic tone in adipose tissues of mice, as indicated by elevated levels of thermogenic genes in BAT and WAT. Thus, glucose and lipoprotein handling does not necessarily reflect UCP1-dependent thermogenic activity, but especially lipid uptake rather mirrors sympathetic activation of adipose tissues.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1194/jlr.RA119000455DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7604721PMC
November 2020

Human brown adipose tissue is phenocopied by classical brown adipose tissue in physiologically humanized mice.

Nat Metab 2019 08 19;1(8):830-843. Epub 2019 Aug 19.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.

Human and rodent brown adipose tissues (BAT) appear morphologically and molecularly different. Here we compare human BAT with both classical brown and brite/beige adipose tissues of 'physiologically humanized' mice: middle-aged mice living under conditions approaching human thermal and nutritional conditions, that is, prolonged exposure to thermoneutral temperature (approximately 30 °C) and to an energy-rich (high-fat, high-sugar) diet. We find that the morphological, cellular and molecular characteristics (both marker and adipose-selective gene expression) of classical brown fat, but not of brite/beige fat, of these physiologically humanized mice are notably similar to human BAT. We also demonstrate, both in silico and experimentally, that in physiologically humanized mice only classical BAT possesses a high thermogenic potential. These observations suggest that classical rodent BAT is the tissue of choice for translational studies aimed at recruiting human BAT to counteract the development of obesity and its comorbidities.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s42255-019-0101-4DOI Listing
August 2019

Human brown adipose tissue: Classical brown rather than brite/beige?

Exp Physiol 2020 08 30;105(8):1191-1200. Epub 2020 May 30.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.

New Findings: What is the topic of this review? It has been suggested that human brown adipose tissue (BAT) is more similar to the brite/beige adipose tissue of mice than to classical BAT of mice. The basis of this is discussed in relationship to the physiological conditions of standard experimental mice. What advances does it highlight? We highlight that, provided mouse adipose tissues are examined under physiological conditions closer to those prevalent for most humans, the gene expression profile of mouse classical BAT is more similar to that of human BAT than is the profile of mouse brite/beige adipose tissue. Human BAT is therefore not different in nature from classical mouse BAT.

Abstract: Since the presence of brown adipose tissue (BAT) was established in adult humans some 13 years ago, its physiological significance and molecular characteristics have been discussed. In particular, it has been proposed that the mouse adipose tissue depot most closely resembling and molecularly parallel to human BAT is not classical mouse BAT. Instead, so-called brite or beige adipose tissue, which is characteristically observed in the inguinal 'white' adipose tissue depot of mice, has been proposed to be the closest mouse equivalent of human BAT. We summarize here the published evidence examining this question. We emphasize the differences in tissue appearance and tissue transcriptomes from 'standard' mice [young, chow fed and, in effect semi-cold exposed (20°C)] versus 'physiologically humanized' mice [middle-aged, high-fat diet-fed mice living at thermoneutrality (30°C)]. We find that in the physiologically humanized mice, classical BAT displays molecular and cellular characteristics that are more akin to human BAT than are those of brite/beige adipose tissues from either standard or physiologically humanized mice. We suggest, therefore, that mouse BAT is the more relevant tissue for translational studies. This is an invited summary of a presentation given at Physiology 2019 (Aberdeen).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1113/EP087875DOI Listing
August 2020

Leptin: Is It Thermogenic?

Endocr Rev 2020 04;41(2)

Department of Molecular Biosciences, The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, Stockholm, Sweden.

Animals that lack the hormone leptin become grossly obese, purportedly for 2 reasons: increased food intake and decreased energy expenditure (thermogenesis). This review examines the experimental evidence for the thermogenesis component. Analysis of the data available led us to conclude that the reports indicating hypometabolism in the leptin-deficient ob/ob mice (as well as in the leptin-receptor-deficient db/db mice and fa/fa rats) derive from a misleading calculation artefact resulting from expression of energy expenditure per gram of body weight and not per intact organism. Correspondingly, the body weight-reducing effects of leptin are not augmented by enhanced thermogenesis. Congruent with this, there is no evidence that the ob/ob mouse demonstrates atrophied brown adipose tissue or diminished levels of total UCP1 mRNA or protein when the ob mutation is studied on the inbred C57BL/6 mouse background, but a reduced sympathetic nerve activity is observed. On the outbred "Aston" mouse background, brown adipose tissue atrophy is seen, but whether this is of quantitative significance for the development of obesity has not been demonstrated. We conclude that leptin is not a thermogenic hormone. Rather, leptin has effects on body temperature regulation, by opposing torpor bouts and by shifting thermoregulatory thresholds. The central pathways behind these effects are largely unexplored.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1210/endrev/bnz016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021639PMC
April 2020

No insulating effect of obesity, neither in mice nor in humans.

Am J Physiol Endocrinol Metab 2019 11;317(5):E952-E953

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden.

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1152/ajpendo.00333.2019DOI Listing
November 2019

Thyroid-Hormone-Induced Browning of White Adipose Tissue Does Not Contribute to Thermogenesis and Glucose Consumption.

Cell Rep 2019 06;27(11):3385-3400.e3

Internal Medicine I, Molecular Endocrinology, Center of Brain, Behavior and Metabolism, University of Lübeck, 23562 Lübeck, Germany. Electronic address:

Regulation of body temperature critically depends on thyroid hormone (TH). Recent studies revealed that TH induces browning of white adipose tissue, possibly contributing to the observed hyperthermia in hyperthyroid patients and potentially providing metabolic benefits. Here, we show that browning by TH requires TH-receptor β and occurs independently of the sympathetic nervous system. The beige fat, however, lacks sufficient adrenergic stimulation and is not metabolically activated despite high levels of uncoupling protein 1 (UCP1). Studies at different environmental temperatures reveal that TH instead causes hyperthermia by actions in skeletal muscle combined with a central body temperature set-point elevation. Consequently, the metabolic and thermogenic effects of systemic hyperthyroidism were maintained in UCP1 knockout mice, demonstrating that neither beige nor brown fat contributes to the TH-induced hyperthermia and elevated glucose consumption, and underlining that the mere presence of UCP1 is insufficient to draw conclusions on the therapeutic potential of browning agents.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.celrep.2019.05.054DOI Listing
June 2019

The answer to the question "What is the best housing temperature to translate mouse experiments to humans?" is: thermoneutrality.

Mol Metab 2019 08 18;26:1-3. Epub 2019 May 18.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden. Electronic address:

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.molmet.2019.05.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6667698PMC
August 2019

Glucocorticoid-Induced Obesity Develops Independently of UCP1.

Cell Rep 2019 05;27(6):1686-1698.e5

Department of Molecular Biosciences, the Wenner-Gren Institute, Stockholm University, 106 91 Stockholm, Sweden. Electronic address:

An excess of glucocorticoids leads to the development of obesity in both mice and humans, but the mechanism for this is unknown. Here, we determine the extent to which decreased BAT thermogenic capacity (as a result of glucocorticoid treatment) contributes to the development of obesity. Contrary to previous suggestions, we show that only in mice housed at thermoneutrality (30°C) does corticosterone treatment reduce total BAT UCP1 protein. This reduction is reflected in reduced brown adipocyte cellular and mitochondrial UCP1-dependent respiration. However, glucocorticoid-induced obesity develops to the same extent in animals housed at 21°C and 30°C, whereas total BAT UCP1 protein levels differ 100-fold between the two groups. In corticosterone-treated wild-type and UCP1 knockout mice housed at 30°C, obesity also develops to the same extent. Thus, our results demonstrate that the development of glucocorticoid-induced obesity is not caused by a decreased UCP1-dependent thermogenic capacity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.celrep.2019.04.041DOI Listing
May 2019

Synergistic Effects of DHA and Sucrose on Body Weight Gain in PUFA-Deficient Elovl2 -/- Mice.

Nutrients 2019 Apr 15;11(4). Epub 2019 Apr 15.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University,SE-10691, Sweden.

The omega-3 polyunsaturated fatty acid docosahexaenoic acid (DHA) is implicated in theregulation of both lipid and carbohydrate metabolism. Thus, we questioned whether dietary DHAand low or high content of sucrose impact on metabolism in mice deficient for elongation of verylong-chain fatty acids 2 (ELOVL2), an enzyme involved in the endogenous DHA synthesis. Wefound that Elovl2 -/- mice fed a high-sucrose DHA-enriched diet followed by the high sucrose, highfat challenge significantly increased body weight. This diet affected the triglyceride rich lipoproteinfraction of plasma lipoproteins and changed the expression of several genes involved in lipidmetabolism in a white adipose tissue. Our findings suggest that lipogenesis in mammals issynergistically influenced by DHA dietary and sucrose content.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.3390/nu11040852DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6520711PMC
April 2019

Dissecting hiPSC-CM pacemaker function in a cardiac organoid model.

Biomaterials 2019 06 21;206:133-145. Epub 2019 Mar 21.

University Medical Center Hamburg Eppendorf, Institute of Experimental Pharmacology and Toxicology, 20246, Hamburg, Germany; German Centre for Cardiovascular Research (DZHK), partner site Hamburg/Kiel/Lübeck, Hamburg, Germany. Electronic address:

Biological pacemakers could be a promising alternative to electronic pacemakers and human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) may represent a suitable source for implantable cells. To further unravel this potential a thorough understanding of pacemaker function with regard to coupling processes both in the physiological and in the graft-host context is required. Here we developed a 2-component cardiac organoid model with a hiPSC-CM embryoid body (EB) as trigger casted into a rat engineered heart tissue (EHT) as arrhythmic beating substrate. Contractility recordings revealed that the EB controlled the beating activity of the EHT, leading to a regular hiPSC-CM-like beating pattern instead of the irregular beating typically seen in rat EHT. Connectivity was observed with action potential (AP) measurements and calcium transients transmitting from the EB directly into the rat EHT. Immunohistochemistry and genetically labeled hiPSC-CMs demonstrated that EB-derived and rat cells intermingled and formed a transitional zone. Connexin 43 expression followed the same pattern as histological and computer models have indicated for the human sinoatrial node. In conclusion, hiPSC-CM EBs function as a biological pacemaker in a 2-component cardiac organoid model, which provides the possibility to study electrophysiological and structural coupling mechanisms underlying propagation of pacemaker activity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.biomaterials.2019.03.023DOI Listing
June 2019

PID1 regulates insulin-dependent glucose uptake by controlling intracellular sorting of GLUT4-storage vesicles.

Biochim Biophys Acta Mol Basis Dis 2019 06 21;1865(6):1592-1603. Epub 2019 Mar 21.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg Eppendorf, 20246 Hamburg, Germany. Electronic address:

The phosphotyrosine interacting domain-containing protein 1 (PID1) serves as a cytosolic adaptor protein of the LDL receptor-related protein 1 (LRP1). By regulating its intracellular trafficking, PID1 controls the hepatic, LRP1-dependent clearance of pro-atherogenic lipoproteins. In adipose and muscle tissues, LRP1 is present in endosomal storage vesicles containing the insulin-responsive glucose transporter 4 (GLUT4). This prompted us to investigate whether PID1 modulates GLUT4 translocation and function via its interaction with the LRP1 cytosolic domain. We initially evaluated this in primary brown adipocytes as we observed an inverse correlation between brown adipose tissue glucose uptake and expression of LRP1 and PID1. Insulin stimulation in wild type brown adipocytes induced LRP1 and GLUT4 translocation from endosomal storage vesicles to the cell surface. Loss of PID1 expression in brown adipocytes prompted LRP1 and GLUT4 sorting to the plasma membrane independent of insulin signaling. When placed on a diabetogenic high fat diet, systemic and adipocyte-specific PID1-deficient mice presented with improved hyperglycemia and glucose tolerance as well as reduced basal plasma insulin levels compared to wild type control mice. Moreover, the improvements in glucose parameters associated with increased glucose uptake in adipose and muscle tissues from PID1-deficient mice. The data provide evidence that PID1 serves as an insulin-regulated retention adaptor protein controlling translocation of LRP1 in conjunction with GLUT4 to the plasma membrane of adipocytes. Notably, loss of PID1 corrects for insulin resistance-associated hyperglycemia emphasizing its pivotal role and therapeutic potential in the regulation of glucose homeostasis.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.bbadis.2019.03.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6624118PMC
June 2019

Intact innervation is essential for diet-induced recruitment of brown adipose tissue.

Am J Physiol Endocrinol Metab 2019 03 21;316(3):E487-E503. Epub 2018 Dec 21.

Department of Molecular Biosciences, Wenner-Gren Institute, Stockholm University , Stockholm , Sweden.

The possibility that recruitment and activation of brown adipose tissue (BAT) thermogenesis could be beneficial for curtailing obesity development in humans prompts a need for a better understanding of the control of these processes [that are often referred to collectively as diet-induced thermogenesis (DIT)]. Dietary conditions are associated with large changes in blood-borne factors that could be responsible for BAT recruitment, but BAT is also innervated by the sympathetic nervous system. To examine the significance of the innervation for DIT recruitment, we surgically denervated the largest BAT depot, i.e., the interscapular BAT depot in mice and exposed the mice at thermoneutrality to a high-fat diet versus a chow diet. Denervation led to an alteration in feeding pattern but did not lead to enhanced obesity, but obesity was achieved with a lower food intake, as denervation increased metabolic efficiency. Conclusively, denervation totally abolished the diet-induced increase in total UCP1 protein levels observed in the intact mice, whereas basal UCP1 expression was not dependent on innervation. The denervation of interscapular BAT did not discernably hyper-recruit other BAT depots, and no UCP1 protein could be detected in the principally browning-competent inguinal white adipose tissue depot under any of the examined conditions. We conclude that intact innervation is essential for diet-induced thermogenesis and that circulating factors cannot by themselves initiate recruitment of brown adipose tissue under obesogenic conditions. Therefore, the processes that link food intake and energy storage to activation of the nervous system are those of significance for the further understanding of diet-induced thermogenesis.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1152/ajpendo.00443.2018DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6459298PMC
March 2019

Abscisic acid stimulates the release of insulin and of GLP-1 in the rat perfused pancreas and intestine.

Diabetes Metab Res Rev 2019 02 13;35(2):e3102. Epub 2018 Dec 13.

Department of Experimental Medicine, Section of Biochemistry, and CEBR, University of Genova, Genoa, Italy.

Aims: Previous results indicate that nanomolar concentrations of abscisic acid (ABA) stimulate insulin release from β-pancreatic cells in vitro and that oral ABA at 50 mg/kg increases plasma GLP-1 in the fasted rat. The aim of this study was to test the effect of ABA on the perfused rat pancreas and intestine, to verify the insulin- and incretin-releasing actions of ABA in controlled physiological models.

Materials And Methods: Rat pancreas and small intestine were perfused with solutions containing ABA at high-micromolar concentrations, or control secretagogues. Insulin and GLP-1 concentrations in the venous effluent were analysed by radioimmunoassay, and ABA levels were determined by ELISA.

Results: High micromolar concentrations of ABA induced GLP-1 secretion from the proximal half of the small intestine and insulin secretion from pancreas. GLP-1 stimulated ABA secretion from pancreas in a biphasic manner. Notably, a positive correlation was found between the ABA area under the curve (AUC) and the insulin AUC upon GLP-1 administration.

Conclusion: Our results indicate the existence of a cross talk between GLP-1 and ABA, whereby ABA stimulates GLP-1 secretion, and vice versa. Release of ABA could be considered as a new promising molecule in the strategy of type 2 diabetes treatment and as a new endogenous hormone in the regulation of glycaemia.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/dmrr.3102DOI Listing
February 2019

Altered hepatic glucose homeostasis in AnxA6-KO mice fed a high-fat diet.

PLoS One 2018 15;13(8):e0201310. Epub 2018 Aug 15.

School of Pharmacy, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia.

Annexin A6 (AnxA6) controls cholesterol and membrane transport in endo- and exocytosis, and modulates triglyceride accumulation and storage. In addition, AnxA6 acts as a scaffolding protein for negative regulators of growth factor receptors and their effector pathways in many different cell types. Here we investigated the role of AnxA6 in the regulation of whole body lipid metabolism and insulin-regulated glucose homeostasis. Therefore, wildtype (WT) and AnxA6-knockout (KO) mice were fed a high-fat diet (HFD) for 17 weeks. During the course of HFD feeding, AnxA6-KO mice gained less weight compared to controls, which correlated with reduced adiposity. Systemic triglyceride and cholesterol levels of HFD-fed control and AnxA6-KO mice were comparable, with slightly elevated high density lipoprotein (HDL) and reduced triglyceride-rich lipoprotein (TRL) levels in AnxA6-KO mice. AnxA6-KO mice displayed a trend towards improved insulin sensitivity in oral glucose and insulin tolerance tests (OGTT, ITT), which correlated with increased insulin-inducible phosphorylation of protein kinase B (Akt) and ribosomal protein S6 kinase (S6) in liver extracts. However, HFD-fed AnxA6-KO mice failed to downregulate hepatic gluconeogenesis, despite similar insulin levels and insulin signaling activity, as well as expression profiles of insulin-sensitive transcription factors to controls. In addition, increased glycogen storage in livers of HFD- and chow-fed AnxA6-KO animals was observed. Together with an inability to reduce glucose production upon insulin exposure in AnxA6-depleted HuH7 hepatocytes, this implicates AnxA6 contributing to the fine-tuning of hepatic glucose metabolism with potential consequences for the systemic control of glucose in health and disease.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0201310PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6093612PMC
January 2019

The adaptor protein PID1 regulates receptor-dependent endocytosis of postprandial triglyceride-rich lipoproteins.

Mol Metab 2018 10 30;16:88-99. Epub 2018 Jul 30.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, 20246, Hamburg, Germany. Electronic address:

Objective: Insulin resistance is associated with impaired receptor dependent hepatic uptake of triglyceride-rich lipoproteins (TRL), promoting hypertriglyceridemia and atherosclerosis. Next to low-density lipoprotein (LDL) receptor (LDLR) and syndecan-1, the LDLR-related protein 1 (LRP1) stimulated by insulin action contributes to the rapid clearance of TRL in the postprandial state. Here, we investigated the hypothesis that the adaptor protein phosphotyrosine interacting domain-containing protein 1 (PID1) regulates LRP1 function, thereby controlling hepatic endocytosis of postprandial lipoproteins.

Methods: Localization and interaction of PID1 and LRP1 in cultured hepatocytes was studied by confocal microscopy of fluorescent tagged proteins, by indirect immunohistochemistry of endogenous proteins, by GST-based pull down and by immunoprecipitation experiments. The in vivo relevance of PID1 was assessed using whole body as well as liver-specific Pid1-deficient mice on a wild type or Ldlr-deficient (Ldlr) background. Intravital microscopy was used to study LRP1 translocation in the liver. Lipoprotein metabolism was investigated by lipoprotein profiling, gene and protein expression as well as organ-specific uptake of radiolabelled TRL.

Results: PID1 co-localized in perinuclear endosomes and was found associated with LRP1 under fasting conditions. We identified the distal NPxY motif of the intracellular C-terminal domain (ICD) of LRP1 as the site critical for the interaction with PID1. Insulin-mediated NPxY-phosphorylation caused the dissociation of PID1 from the ICD, causing LRP1 translocation to the plasma membrane. PID1 deletion resulted in higher LRP1 abundance at the cell surface, higher LDLR protein levels and, paradoxically, reduced total LRP1. The latter can be explained by higher receptor shedding, which we observed in cultured Pid1-deficient hepatocytes. Consistently, PID1 deficiency alone led to increased LDLR-dependent endocytosis of postprandial lipoproteins and lower plasma triglycerides. In contrast, hepatic PID1 deletion on an Ldlr background reduced lipoprotein uptake into liver and caused plasma TRL accumulation.

Conclusions: By acting as an insulin-dependent retention adaptor, PID1 serves as a regulator of LRP1 function controlling the disposal of postprandial lipoproteins. PID1 inhibition provides a novel approach to lower plasma levels of pro-atherogenic TRL remnants by stimulating endocytic function of both LRP1 and LDLR in the liver.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.molmet.2018.07.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6158030PMC
October 2018

Lipolysis Triggers a Systemic Insulin Response Essential for Efficient Energy Replenishment of Activated Brown Adipose Tissue in Mice.

Cell Metab 2018 10 19;28(4):644-655.e4. Epub 2018 Jul 19.

Department of Biochemistry and Molecular Cell Biology, University Medical Center Hamburg-Eppendorf, Martinistr. 52, 20246 Hamburg, Germany. Electronic address:

The coordination of the organ-specific responses regulating systemic energy distribution to replenish lipid stores in acutely activated brown adipose tissue (BAT) remains elusive. Here, we show that short-term cold exposure or acute β3-adrenergic receptor (β3AR) stimulation results in secretion of the anabolic hormone insulin. This process is diminished in adipocyte-specific Atgl mice, indicating that lipolysis in white adipose tissue (WAT) promotes insulin secretion. Inhibition of pancreatic β cells abolished uptake of lipids delivered by triglyceride-rich lipoproteins into activated BAT. Both increased lipid uptake into BAT and whole-body energy expenditure in response to β3AR stimulation were blunted in mice treated with the insulin receptor antagonist S961 or lacking the insulin receptor in brown adipocytes. In conclusion, we introduce the concept that acute cold and β3AR stimulation trigger a systemic response involving WAT, β cells, and BAT, which is essential for insulin-dependent fuel uptake and adaptive thermogenesis.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.cmet.2018.06.020DOI Listing
October 2018

Rab18 is not necessary for lipid droplet biogenesis or turnover in human mammary carcinoma cells.

Mol Biol Cell 2018 08 27;29(17):2045-2054. Epub 2018 Jun 27.

Department of Genetics and Complex Diseases, Harvard T. H. Chan School of Public Health, Boston, MA 02115.

Rab GTPases recruit peripheral membrane proteins and can define organelle identity. Rab18 localizes to the endoplasmic reticulum (ER) but also to lipid droplets (LDs), where it has been implicated in effector protein recruitment and in defining LD identity. Here, we studied Rab18 localization and function in a human mammary carcinoma cell line. Rab18 localized to the ER and to LD membranes on LD induction, with the latter depending on the Rab18 activation state. In cells lacking Rab18, LDs were modestly reduced in size and numbers, but we found little evidence for Rab18 function in LD formation, LD turnover on cell starvation, or the targeting of several proteins to LDs. We conclude that Rab18 is not a general, necessary component of the protein machinery involved in LD biogenesis or turnover.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1091/mbc.E18-05-0282DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6232964PMC
August 2018

FoxO transcription factors are required for hepatic HDL cholesterol clearance.

J Clin Invest 2018 04 19;128(4):1615-1626. Epub 2018 Mar 19.

Naomi Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York, New York, USA.

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of the effect of insulin on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of the HDL-C clearance factors scavenger receptor class B type I (SR-BI) and hepatic lipase and defective selective uptake of HDL cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1172/JCI94230DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5873864PMC
April 2018

Brown adipose tissue thermogenic adaptation requires Nrf1-mediated proteasomal activity.

Nat Med 2018 03 5;24(3):292-303. Epub 2018 Feb 5.

Department of Genetics and Complex Diseases and Sabri Ülker Center, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, USA.

Adipocytes possess remarkable adaptive capacity to respond to nutrient excess, fasting or cold exposure, and they are thus an important cell type for the maintenance of proper metabolic health. Although the endoplasmic reticulum (ER) is a critical organelle for cellular homeostasis, the mechanisms that mediate adaptation of the ER to metabolic challenges in adipocytes are unclear. Here we show that brown adipose tissue (BAT) thermogenic function requires an adaptive increase in proteasomal activity to secure cellular protein quality control, and we identify the ER-localized transcription factor nuclear factor erythroid 2-like 1 (Nfe2l1, also known as Nrf1) as a critical driver of this process. We show that cold adaptation induces Nrf1 in BAT to increase proteasomal activity and that this is crucial for maintaining ER homeostasis and cellular integrity, specifically when the cells are in a state of high thermogenic activity. In mice, under thermogenic conditions, brown-adipocyte-specific deletion of Nfe2l1 (Nrf1) resulted in ER stress, tissue inflammation, markedly diminished mitochondrial function and whitening of the BAT. In mouse models of both genetic and dietary obesity, stimulation of proteasomal activity by exogenously expressing Nrf1 or by treatment with the proteasome activator PA28α in BAT resulted in improved insulin sensitivity. In conclusion, Nrf1 emerges as a novel guardian of brown adipocyte function, providing increased proteometabolic quality control for adapting to cold or to obesity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/nm.4481DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5839993PMC
March 2018

Optimal housing temperatures for mice to mimic the thermal environment of humans: An experimental study.

Mol Metab 2018 01 31;7:161-170. Epub 2017 Oct 31.

Department of Molecular Biosciences, The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, SE-106 91 Stockholm, Sweden. Electronic address:

Objectives: The laboratory mouse is presently the most common model for examining mechanisms of human physiology and disease. Housing temperatures can have a large impact on the outcome of such experiments and on their translatability to the human situation. Humans usually create for themselves a thermoneutral environment without cold stress, while laboratory mice under standard conditions (≈20° C) are under constant cold stress. In a well-cited, theoretical paper by Speakman and Keijer in Molecular Metabolism, it was argued that housing mice under close to standard conditions is the optimal way of modeling the human metabolic situation. This tenet was mainly based on the observation that humans usually display average metabolic rates of about 1.6 times basal metabolic rate. The extra heat thereby produced would also be expected to lead to a shift in the 'lower critical temperature' towards lower temperatures.

Methods: To examine these tenets experimentally, we performed high time-resolution indirect calorimetry at different environmental temperatures on mice acclimated to different housing temperatures.

Results: Based on the high time-resolution calorimetry analysis, we found that mice already under thermoneutral conditions display mean diurnal energy expenditure rates 1.8 times higher than basal metabolism, remarkably closely resembling the human situation. At any temperature below thermoneutrality, mice metabolism therefore exceeds the human equivalent: Mice under standard conditions display energy expenditure 3.1 times basal metabolism. The discrepancy to previous conclusions is probably attributable to earlier limitations in establishing true mouse basal metabolic rate, due to low time resolution. We also found that the fact that mean energy expenditure exceeds resting metabolic rate does not move the apparent thermoneutral zone (the lower critical temperature) downwards.

Conclusions: We show that housing mice at thermoneutrality is an advantageous step towards aligning mouse energy metabolism to human energy metabolism.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.molmet.2017.10.009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5784327PMC
January 2018

Impairment of systemic DHA synthesis affects macrophage plasticity and polarization: implications for DHA supplementation during inflammation.

Cell Mol Life Sci 2017 08 15;74(15):2815-2826. Epub 2017 Mar 15.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, SE10691, Stockholm, Sweden.

Docosahexaenoic acid (DHA) is an omega-3 fatty acid obtained from the diet or synthesized from alpha-linolenic acid through the action of fatty acid elongases (ELOVL) and desaturases. DHA plays important roles in the central nervous system as well as in peripheral organs and is the precursor of several molecules that regulate resolution of inflammation. In the present study, we questioned whether impaired synthesis of DHA affected macrophage plasticity and polarization both in vitro and in vivo models. For this we investigated the activation status and inflammatory response of bone marrow-derived M1 and M2 macrophages obtained from mice deficient of Elovl2 (Elovl2), a key enzyme for DHA synthesis in mammals. Although both wild type and Elovl2 mice were able to generate efficient M1 and M2 macrophages, M1 cells derived from Elovl2 mice showed an increased expression of key markers (iNOS, CD86 and MARCO) and cytokines (IL-6, IL-12 and IL-23). However, M2 macrophages exhibited upregulated M1-like markers like CD80, CD86 and IL-6, concomitantly with a downregulation of their signature marker CD206. These effects were counteracted in cells obtained from DHA-supplemented animals. Finally, white adipose tissue of Elovl2 mice presented an M1-like pro-inflammatory phenotype. Hence, impairment of systemic DHA synthesis delineates an alteration of M1/M2 macrophages both in vitro and in vivo, with M1 being hyperactive and more pro-inflammatory while M2 less protective, supporting the view that DHA has a key role in controlling the balance between pro- and anti-inflammatory processes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s00018-017-2498-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5491590PMC
August 2017

Leptin-deficient mice are not hypothermic, they are anapyrexic.

Mol Metab 2017 02 9;6(2):173. Epub 2016 Nov 9.

Department of Molecular Biosciences, The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, SE-106 91 Stockholm, Sweden. Electronic address:

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.molmet.2016.10.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5279901PMC
February 2017

UCP1 inhibition in Cidea-overexpressing mice is physiologically counteracted by brown adipose tissue hyperrecruitment.

Am J Physiol Endocrinol Metab 2017 01 6;312(1):E72-E87. Epub 2016 Dec 6.

Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm, Sweden;

Cidea is a gene highly expressed in thermogenesis-competent (UCP1-containing) adipose cells, both brown and brite/beige. Here, we initially demonstrate a remarkable adipose-depot specific regulation of Cidea expression. In classical brown fat, Cidea mRNA is expressed continuously and invariably, irrespective of tissue recruitment. However, Cidea protein levels are regulated posttranscriptionally, being conspicuously induced in the thermogenically recruited state. In contrast, in brite fat, Cidea protein levels are regulated at the transcriptional level, and Cidea mRNA and protein levels are proportional to tissue "briteness." Although routinely followed as a thermogenic molecular marker, Cidea function is not clarified. Here, we employed a gain-of-function approach to examine a possible role of Cidea in the regulation of thermogenesis. We utilized transgenic aP2-hCidea mice that overexpress human Cidea in all adipose tissues. We demonstrate that UCP1 activity is markedly suppressed in brown-fat mitochondria isolated from aP2-hCidea mice. However, mitochondrial UCP1 protein levels were identical in wild-type and transgenic mice. This implies a regulatory effect of Cidea on UCP1 activity, but as we demonstrate that Cidea itself is not localized to mitochondria, we propose an indirect inhibitory effect. The Cidea-induced inhibition of UCP1 activity (observed in isolated mitochondria) is physiologically relevant since the mice, through an appropriate homeostatic compensatory mechanism, increased the total amount of UCP1 in the tissue to exactly match the diminished thermogenic capacity of the UCP1 protein and retain unaltered nonshivering thermogenic capacity. Thus, we verified Cidea as being a marker of thermogenesis-competent adipose tissues, but we conclude that Cidea, unexpectedly, functions molecularly as an indirect inhibitor of thermogenesis.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1152/ajpendo.00284.2016DOI Listing
January 2017