The phenomenon of white fat “browning,” in which certain white adipose tissue depots significantly increase gene expression for the uncoupling protein UCP1 and thus supposedly acquire thermogenic, ...fat-burning properties, has attracted considerable attention. Because the mRNA increases are from very low initial levels, the metabolic relevance of the change is unclear: is the UCP1 protein thermogenically competent in these brite/beige-fat mitochondria? We found that, in mitochondria isolated from the inguinal “white” adipose depot of cold-acclimated mice, UCP1 protein levels almost reached those in brown-fat mitochondria. The UCP1 was thermogenically functional, in that these mitochondria exhibited UCP1-dependent thermogenesis with lipid or carbohydrate substrates with canonical guanosine diphosphate (GDP) sensitivity and loss of thermogenesis in UCP1 knockout (KO) mice. Obesogenic mouse strains had a lower thermogenic potential than obesity-resistant strains. The thermogenic density (UCP1-dependent oxygen consumption per g tissue) of inguinal white adipose tissue was maximally one-fifth of interscapular brown adipose tissue, and the total quantitative contribution of all inguinal mitochondria was maximally one-third of all interscapular brown-fat mitochondria, indicating that the classical brown adipose tissue depots would still predominate in thermogenesis.
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•In recruited brite/beige-fat mitochondria, UCP1 protein is present in functional amount•UCP1 is canonically regulated and thermogenically competent with regular substrates•UCP1-mediated thermogenic capacity is lower in obesogenic C57Bl/6 than in 129Sv mice•Total brite adipose tissue UCP1-mediated thermogenic capacity is low at a systemic level
There is great interest in the phenomenon of white fat “browning”; i.e., the appearance of the uncoupling protein UCP1 in white fat depots, supposedly bestowing the white fat with thermogenic, fat-burning properties. However, there has been little evidence that the UCP1 in the white fat is functional. Here, Nedergaard and colleagues clearly show that the white-fat UCP1 as such is truly thermogenic. However, the combined functional effect of the UCP1 in the browned white fat is still only a fraction of that in the brown fat.
Perfluorooctanoic acid (PFOA) is a synthetic organofluoride surfactant associated with several toxic effects in humans and animals. Particularly, it has been observed that PFOA treatment of mice ...results in weight loss associated with recruited brown adipose tissue (BAT), including an increased amount of uncoupling protein 1 (UCP1). The molecular mechanism behind this BAT recruitment is presently unknown. To investigate the existence of possible cell-autonomous effects of PFOA, we treated primary cultures of brown and white (inguinal) adipocytes with PFOA, or with the non-fluorinated equivalent octanoate, or with vehicle, for 48 h (from day 5 to day 7 of differentiation). PFOA in itself increased the gene expression (mRNA levels) of UCP1 and carnitine palmitoyltransferase 1A (CPT1α) (thermogenesis-related genes) in both brown and white adipocytes. In addition, PFOA increased the expression of fatty acid binding protein 4 (FABP4) and peroxisome proliferator-activated receptor α (PPARα) (adipogenesis-related genes). Also the protein levels of UCP1 were increased in brown adipocytes exposed to PFOA. This increase was more due to an increase in the fraction of cells that expressed UCP1 than to an increase in UCP1 levels per cell. The PFOA-induced changes were even more pronounced under simultaneous adrenergic stimulation. Octanoate induced less pronounced effects on adipocytes than did PFOA. Thus, PFOA in itself increased the levels of thermogenic markers in brown and white adipocytes. This could enhance the energy metabolism of animals (and humans) exposed to the compound, resulting in a negative energy balance, leading to diminished fitness.
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•How the persistent environmental pollutant PFOA affects exposed organisms is examined.•Cell-autonomous effects of PFOA are seen in primary brown and white adipocyte cultures.•PFOA in itself augments gene expression and protein levels of UCP1 and FABP4.•PFOA augments effects of adrenergic stimulation on gene expression.•PFOA may promote a negative energy balance in exposed animals and humans.
Adrenergic stimulation of brown adipocytes (BA) induces mitochondrial uncoupling, thereby increasing energy expenditure by shifting nutrient oxidation towards thermogenesis. Here we describe that ...mitochondrial dynamics is a physiological regulator of adrenergically‐induced changes in energy expenditure. The sympathetic neurotransmitter Norepinephrine (NE) induced complete and rapid mitochondrial fragmentation in BA, characterized by Drp1 phosphorylation and Opa1 cleavage. Mechanistically, NE‐mediated Drp1 phosphorylation was dependent on Protein Kinase‐A (PKA) activity, whereas Opa1 cleavage required mitochondrial depolarization mediated by FFAs released as a result of lipolysis. This change in mitochondrial architecture was observed both in primary cultures and brown adipose tissue from cold‐exposed mice. Mitochondrial uncoupling induced by NE in brown adipocytes was reduced by inhibition of mitochondrial fission through transient Drp1 DN overexpression. Furthermore, forced mitochondrial fragmentation in BA through Mfn2 knock down increased the capacity of exogenous FFAs to increase energy expenditure. These results suggest that, in addition to its ability to stimulate lipolysis, NE induces energy expenditure in BA by promoting mitochondrial fragmentation. Together these data reveal that adrenergically‐induced changes to mitochondrial dynamics are required for BA thermogenic activation and for the control of energy expenditure.
Synopsis
In this study, norepinephrine‐mediated mitochondrial dynamics are shown to contribute to uncoupled respiration during thermogenesis in brown adipocytes.
Norepinephrine induced complete and rapid mitochondrial fragmentation in brown adipocytes through PKA‐dependent Drp1 phosphorylation and Opa1 cleavage.
Norepinephrine‐induced mitochondrial uncoupling was reduced by inhibition of mitochondrial fission and enhanced by Mfn2 knock down.
Knockdown of Mfn2 enhanced the capacity of exogenous free fatty acids to increase energy expenditure.
We conclude that in addition to its ability to stimulate lipolysis, norepinephrine induces energy expenditure in brown adipocytes by promoting mitochondrial fragmentation.
In this study, norepinephrine‐mediated mitochondrial dynamics are shown to contribute to uncoupled respiration during thermogenesis in brown adipocytes.
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.
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).
Attainment of a brown adipocyte cell phenotype in white adipocytes, with their abundant mitochondria and increased energy expenditure potential, is a legitimate strategy for combating obesity. The ...unique transcriptional regulators of the primary brown adipocyte phenotype are unknown, limiting our ability to promote brown adipogenesis over white. In the present work, we used microarray analysis strategies to study primary preadipocytes, and we made the striking discovery that brown preadipocytes demonstrate a myogenic transcriptional signature, whereas both brown and white primary preadipocytes demonstrate signatures distinct from those found in immortalized adipogenic models. We found a plausible SIRT1-related transcriptional signature during brown adipocyte differentiation that may contribute to silencing the myogenic signature. In contrast to brown preadipocytes or skeletal muscle cells, white preadipocytes express Tcf21, a transcription factor that has been shown to suppress myogenesis and nuclear receptor activity. In addition, we identified a number of developmental genes that are differentially expressed between brown and white preadipocytes and that have recently been implicated in human obesity. The interlinkage between the myocyte and the brown preadipocyte confirms the distinct origin for brown versus white adipose tissue and also represents a plausible explanation as to why brown adipocytes ultimately specialize in lipid catabolism rather than storage, much like oxidative skeletal muscle tissue.
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.
In humans, Cidea (cell death-inducing DNA fragmentation factor alpha-like effector A) is highly but variably expressed in white fat, and expression correlates with metabolic health. Here we generate ...transgenic mice expressing human Cidea in adipose tissues (aP2-hCidea mice) and show that Cidea is mechanistically associated with a robust increase in adipose tissue expandability. Under humanized conditions (thermoneutrality, mature age and prolonged exposure to high-fat diet), aP2-hCidea mice develop a much more pronounced obesity than their wild-type littermates. Remarkably, the malfunctioning of visceral fat normally caused by massive obesity is fully overcome-perilipin 1 and Akt expression are preserved, tissue degradation is prevented, macrophage accumulation is decreased and adiponectin expression remains high. Importantly, the aP2-hCidea mice display enhanced insulin sensitivity. Our data establish a functional role for Cidea and suggest that, in humans, the association between Cidea levels in white fat and metabolic health is not only correlative but also causative.
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.
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