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Mechanisms of thermogenesis in brown adipose tissue

Authors:
614th MEETING. OXFORD 223
Brown
Adipose Tissue
Lipid Group Colloquium organized by
P.
Trayhurn (Dunn Nutritional Laboratory, Cambridge)
and
R.
Jeffcoate (Unilever Research, Sharnbrook), and edited by
P.
Trayhurn
Mechanisms
of
thermogenesis in brown adipose tissue
DAVID NICHOLLS, SONIA CUNNINGHAM
and HERBERT WIESINGER
Neurosciences Research Group,
Ninewells Medical School, University
of
Dundee,
Dundee DDI
9SY.
Scotland,
U.K.
In 1969, Rafael and colleagues observed that the
uncoupled state of mitochondria prepared from the
brown fat of newborn guinea pigs could be reversed by
incubation in the presence of albumin and certain purine
nucleotides (Rafael
et al.,
1969). In 1974, we showed that
this uncoupled state was due to the presence in the inner
membrane of the mitochondria of a regulatable ion con-
ductance pathway, which would allow protons extruded
by the respiratory chain to re-enter the matrix without
passing through the ATP synthase, therefore allowing
the constraints of respiratory control to be bypassed
(Nicholls, 1974). We proposed that this conductance
underlay the ability of the tissue to generate heat in
response to a hormonally activated supply of free fatty
acid. Two years later we detected a high-affinity binding
site for purine nucleotides on the outer face of the inner
membrane of brown-fat mitochondria and developed an
assay to quantify the binding (Nicholls, 1976). In 1978,
the use of photoaffinity nucleotide analogues allowed the
binding site to be identified as a 32
000
M,
protein, which
we termed the ‘uncoupling protein’ (Heaton
et
al.,
1978).
The binding of noradrenaline to the plasma membrane
of the brown adipocyte is the signal for the initiation of
thermogenesis (see Nicholls
&
Locke, 1984). This activates
a lipolytic cascade whose purpose is to supply oxidizable
substrate for the mitochondria. The uncoupling protein
cannot be in an activated conformation when the tissue
is non-thermogenic and substrate supply is limited, since
this would de-energize the cell. It is clear, therefore, that
mitochondria must receive a signal synchronous with
the initiation of lipolysis so that they can activate the
uncoupling protein and start producing heat.
Fatty acids and mitochondria
Over the years a number of hypotheses have been
presented for the physiological regulation of the conduc-
tance of the uncoupling protein. These have been reviewed
elsewhere (Nicholls
&
Locke, 1984). Here we shall
present new evidence for the mechanism we favour,
namely that the free fatty acids which act as substrate are
also the messengers activating the uncoupling protein.
The origins of this proposal date back to the earliest
studies with mitochondria (Rafael
et
al.,
1969; Bulychev
et
al.,
1972) and adipocytes (Prusiner
et
al.,
1968~;
Reed
&
Fain, 1968; Williamson, 1970) and pre-date the
Abbreviation used:
FCCP,
carbonyl cyanide p-trifluoromethoxy-
phenylhydrazone.
discovery of the uncoupling protein. Since free fatty acids
can uncouple mitochondria from various sources non-
specifically, there is justifiable scepticism that such a
ubiquitous and inelegant messenger could really provide
the precise signal required to control the uncoupled
protein
in vivo,
and unusually definitive experiments are
called for.
In 1982, we designed experiments in which fatty acids
were slowly infused into plausibly physiological incu-
bations of isolated mitochondria to mimic the transition
from the non-thermogenic to the thermogenic state (Locke
et al.,
1982a,b). Membrane potential and respiration
were monitored continuously in order to detect changes
in the proton conductance. The initial (non-thermogenic)
substrate was pyruvate. Palmitate induced an immediate
increase in conductance which was reversed as soon as
the infusion was stopped, mimicking the return to non-
thermogenic conditions. When labelled palmitate was
infused and intermediates measured, the only metabolite
whose concentration correlated with the change in con-
ductance was free fatty acid.
In order to answer doubts as to the specificity of the
fatty acid effect, experiments were performed with
mitochondria from warm- and cold-adapted guinea pigs
(Locke
et al.,
1982~). Mitochondria from the warm-
adapted animals are deficient in uncoupling protein
(Rafael
&
Heldt, 1976) and show a measure of respirat-
ory control in the absence of GDP (Rafael
et
al.,
1968;
Rial
&
Nicholls, 1984). While those from cold-adapted
animals were very sensitive to fatty acids, the membrane
potential from ‘warm’ mitochondria dropped by only a
few millivolts even when the fatty acids accumulated
(Locke
et al.,
1982~).
These experiments do not prove that fatty acids act
directly on the nucleotide-sensitive pathway. However,
Rial
et al.
(1983) studied proton-dependent swelling
of non-respiring mitochondria, and found that unlike
carbonyl cyanide
p-trifluoromethoxyphenylhydrazone
(FCCP), the fatty acid-induced proton permeability can
be completely abolished by the binding of nucleotide,
showing that the two agents act at closely interacting
sites.
Recently, we have quantitatively investigated the
unbound free fatty acid concentrations needed to increase
the proton conductance of brown-fat mitochondria from
warm- and cold-adapted guinea pigs, and compared
these with the concentrations required by the fatty acyl-
CoA synthase (H. Wiesinger
&
S.
A. Cunningham,
unpublished work). Fatty acid activation has a very high
affinity, half-maximal uncontrolled respiration being
achieved respectively at 2.3n~ and 1.5n~ for the cold-
and warm-adapted states. The ability of unbound free
fatty acids to uncouple pyruvate respiration in the
presence of 3 mM-ATP differs dramatically in the two
Vol. 14
224 BIOCHEMICAL SOCIETY TRANSACTIONS
conditions. The warm mitochondria show a shallow,
linear dependency of uncoupling on unbound palmitate
concentration, indicating a non-specific action of the
fatty acid. In contrast, the cold mitochondria show an
additional saturable component (half complete by 25 nM-
unbound palmitate), proving further evidence of a specific,
high-affinity interaction with the uncoupling protein.
Instead of simply introducing an ohmic proton con-
ductance that would result in an unstable membrane
potential, fatty acids clamp the proton electrochemical
gradient at a level which is not only largely independent
of the respiratory rate but is also sufficiently high to allow
continued regulation of Ca2+, metabolite transport and
even a measure of ATP synthesis during thermogenesis
(Nicholls
&
Bernson, 1977; Rial
et
al.,
1983).
Brown adipocytes
Until recently the failure to prepare good cells from
cold-adapted animals has meant that it has not been
possible to study the adaptive response of the tissue at the
cell level, which in turn has hindered the unification of
mitochondrial and cellular studies. Indeed, in view of the
low level of uncoupling protein in the mitochondria of
warm-adapted animals, it has been less than obvious
what role the protein could play in the response of these
cells. However, we have found that the guinea pig is a
suitable source of functional brown adipocytes from
both warm- and cold-adapted animals (Locke
et
al.,
19826; Rafael
et
al.,
1985).
It is convenient to separate discussion of the response
of brown adipocytes to substrates which do not cause
activation of the uncoupling protein (non-thermogenic
substrates) and those that uncouple the mitochondria
(thermogenic substrates). The prime non-thermogenic
substrate for the brown-fat cell is pyruvate generated by
glycolysis, or added exogenously. When no substrate
other than glucose is present in the medium, cells respire
slowly and addition of protonophore causes a modest
respiratory stimulation (Locke
et
al.,
1982b; Rafael
et
al.,
1985). Respiratory control is seen much more
dramatically if pyruvate is added to the incubation (Rafael
et
al.,
1985), and respiratory control ratios exceeding 10
can be seen, indicating both that pyruvate is an excellent
substrate for intact brown adipocytes and that the
mitochondria oxidizing pyruvate within the cell are tightly
coupled.
Since cold-adapted guinea pigs have a maximal com-
plement of uncoupling protein (Rafael
&
Heldt, 1976;
Heaton
et
al.,
1978; Rial
&
Nicholls, 1984), the uncoupler-
releasable respiratory control clearly indicates that the
uncoupling protein does not conduct protons in the non-
thermogenic state. The high degree of respiratory control
also shows that only a small proportion of the respirat-
ory capacity of the mitochondria is required to sustain
the basal metabolism of these cells.
The primary interest in brown-fat cells has been as a
near-physiological system in which to study the acute
thermogenic response to noradrenaline. The respiratory
stimulation is associated with an uncoupling of respir-
ation since the maximal respiratory rate is an order of
magnitude greater than that predicted if the mitochondria
remained coupled. Additionally, oligomycin does not
cause an abrupt inhibition of
noradrenaline-stimulated
respiration, and such inhibition as eventually develops is
not reversed by protonophores and is most likely due to
ATP depletion in the cytosol inhibiting the rate of fatty
acid activation (Williamson, 1970). Finally, in the
presence of oligomycin, noradrenaline can still induce the
oxidation of the
b
cytochromes of the mitochondria
in
situ
(Prusiner, 1970). This indicates that the proton elec-
Molar ratio
of
palmitate to albumin
01
2
3
-
I
Unbound
palmitate (nM)
Fig.
1.
Fatty acid uncoupling
of
guinea-pig brown-fat
mitochondria
Brown-fat mitochondria from cold-
(0)
and warm-
(0)
adapted
guinea pigs were incubated (0.3 mg of protein/ml of incubation)
in
50
mM-KCI,
10
mM-TES,
(potassium salt),
64
pM-aIbumin
(essentially fatty acid-free), ~~M-ATP (sodium-salt)
10
mM-
pyruvate (sodium salt),
3
mM-malate (sodium salt),
1
mM-
MgSO, and lpg of oligomycin/mg. Respiratory rates were
recorded after the addition of palmitate sufficient to achieve the
molar ratios to albumin shown. Unbound palmitate was esti-
mated in separate experiments by the method of Goodman
(1958).
trochemical potential across the mitochondria1 membrane
is decreased. This could only occur through an enhanced
proton permeability.
The transition to the uncoupled thermogenic state is
freely reversible, since addition
of
the fi-antagonist
propranolol inhibits respiration within 1-2 min (Prusiner
et
al.,
19686; Bukowiecki
et
al.,
1981; Rafael
et
al.,
1985).
Propranolol restores a high degree of respiratory control
(Rafael
et
al.,
1985), clearly indicating that the respirat-
ory inhibition is not due to a limitation of substrate
supply, but to a switch-off of the uncoupling mechanism.
In the presence of exogenous pyruvate, both warrn- and
cold-adapted guinea pig brown adipocytes show a high
degree of respiratory control, releasable by added FCCP,
attesting to the integrity of the preparations. Addition of
noradrenaline, however, evokes a very different response
in the two types of cell. In the warm-adapted preparation
only a slight stimulation of respiration occurs. This is not
due to a limitation of substrate supply, since the sub-
sequent addition of FCCP causes a large stimulation,
but to a failure of the mitochondria
in
situ
to become
uncoupled. The cold-adapted cells in contrast show a
much greater respiratory stimulation with the hormone.
The lack of a subsequent effect of added uncoupler indi-
cates that the full, uncontrolled rate of respiration can be
expressed in these cells (Rafael
et
al.,
1985).
By day 4 of cold adaptation, all respiratory control can
be lost after hormone addition to the cell incubation.
When related to the induction of the uncoupled protein
(Rial
&
Nicholls, 1984), a high degree of correlation is
found consistent with an obligatory role of the uncoupling
protein in the cellular thermogenic response.
1986
614th MEETING, OXFORD 225
In the guinea pig, cold-adaptation is associated with a
doubling of cell number, a doubling of mitochondria per
cell, and a seven-fold increase in uncoupling protein per
mitochondrion (Rial
&
Nicholls, 1984; Rafael
et al.,
1985), the last correlating with the six-fold increase in
noradrenaline-stimulated respiration per mitochondrion
(Rafael
et al.,
1985).
While isolated mitochondria may be used to model
plausible uncoupling mechanisms as discussed above,
evidence which can be obtained using the more physio-
logical intact adipocyte preparation is likely to be much
less ambiguous. ‘Fatty acid uncoupling’ of brown adipo-
cytes after addition of exogenous fatty acids was first
observed by Fain
et al.
(1967) and by Prusiner
et al.
(1968a). The fatty acids could mimic the respiratory
stimulation seen on noradrenaline addition, and the
proposal was therefore made that fatty acids could act as
both substrates and uncouplers, although these early
studies could provide no mechanistic basis for the tissue
selectivity of the process. The uncoupling by added fatty
acids has been more recently explored by Bukowiecki
et al.,
(198 1). Although the concentrations of added fatty
acids seem extremely high (typically 0.41 mM), it must be
born in mind that the fatty acids are equilibrating with
the albumin present in all incubation media. We have
recently determined the unbound fatty acid in equilibrium
with the albumin during the uncoupling of cold-adapted
brown adipocytes from the guinea pig
(H.
Wiesinger
&
S.
A. Cunningham, unpublished work) and have con-
cluded that the cells are stimulated by palmitate over the
range 20-200 nM-unbound fatty acid. Cells from warm-
adapted guinea pigs are substantially less sensitive to
fatty acid uncoupling compared with the cold-adapted
adipocytes, consistent with an interaction between the
permeating fatty acids and the uncoupling protein in the
mitochondria
in
situ.
This result suggests that the dif-
ferential ability of noradrenaline to uncouple the two
types of guinea-pig cell can be explained simply by the
sensitivity of the mitochondria
in
situ
to fatty acid
uncoupling.
Work from
our
laboratory is supported by grants from the Medical
Research Council and the Scottish Home and Health Department.
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Fuel supply
to
brown
adipose tissue
DERMOT
H.
WILLIAMSON
Metabolic Research Laboratory,
Nufield Department of Clinical Medicine,
Radclife Infirmary, Woohtock Road,
Oxford
OX2
6HE,
U.K.
Thermogenesis in brown adipose tissue requires a con-
tinual supply of oxidizable substrate (Nicholls
&
Locke,
1984), which must either be provided directly by the
circulation or be mobilized from triacylglycerols stored
within the tissue. When the latter is the source of substrate
then there are mechanisms to maintain or replenish the
intracellular pool
of
triacylglycerol, either by exogenous
supply of triacylglycerol and non-esterified fatty acids, or
by synthesis
de novo
of fatty acids from lipogenic pre-
cursors, e.g. glucose (McCormack, 1982). The aim of this
contribution is to discuss the regulation of substrate util-
ization by brown adipose tissue of the rat, and the changes
which occur during lactation (a situation associated with
physiological hyperphagia) and in the postnatal period.
The substrates to be considered are triacylglycerols,
non-esterified fatty acids, glucose and ketone bodies
(acetoacetate and 3-hydroxybutyrate). Other fuels may
be used by brown adipose tissue, e.g. acetate, lactate and
certain amino acids, but it is likely that their contribution
to the overall substrate supply to the tissue is low. It
is not intended to deal with the factors which control
the final fate of the substrates (oxidation versus con-
version to lipid) because this area will be discussed by
other contributors.
Regulation of substrate utilization
The factors involved in the regulation of substrate util-
ization by mammalian tissues include: (a) the availablility
of the substrate in the circulation, (b) the blood flow to
the tissue, (c) the transport of the substrate into the tissue
compartment where it is utilized, (d) the activity of the
enzymes which initiate the metabolism of the substrate,
(e) other regulatory sites in the metabolic pathway, and
(f)
intracellular and inter-tissue metabolic integration of
pathways (Williamson, 1984). Of special significance for
substrate utilization by brown adipose tissue is the range
of blood flow, which can increase up to an order of
magnitude (Portet
et al.,
1974; Foster
&
Frydman, 1979),
and the noradrenergic stimulation of thermogenesis (see
Nicholls
&
Locke, 1984).
Measurement of substrate utilization
The most direct method of assessment of substrate
utilization is by measurement of arteriovenous differences
together with information on blood flow. In the case
of brown adipose tissue, blood can be collected from
Sulzer’s vein and from the carotid artery (Portet
et al.,
1974), but because of the surgical expertise involved this
Vol. 14
... Lors d'une exposition au froid, le cerveau libèrera la NE qui agira sur le récepteur β-adrénergique (β-AR), principalement l'isoforme 3 dans le BAT. La signalisation de celui-ci entraînera l'induction des gènes impliqués dans la thermogénèse, et ce, à court ou long terme (Lowell et Flier, 1997;Nicholls et al., 1986). On retrouve la présence du BAT chez des patients souffrant de tumeurs bénignes du tissu adipeux brun (hibernome) ou de phéochromocytomes, des tumeurs de la glande surrénale sécrétant de fortes quantités de catécholamines (Houstĕk et al., 1993;Lean et al., 1986;Ricquier et al., 1982). ...
... La perte de PRDM16 dans des précurseurs adipocytaires entraîne la différenciation en cellules musculaires. À l'inverse, l'expression ectopique de PDRM16 dans des précurseurs de muscles entraîne la différenciation en adipocytes bruns (Farmer, 2008;Lepper et Fan, 2010;Sanchez-Gurmaches et al., 2012;Seale et al., 2008;Wang et Seale, 2016 Gesta et al., 2007;Nicholls et al., 1986;Robidoux et al., 2005). La voie de l'AMPc permet aussi d'induire l'expression d'enzymes capables d'augmenter la quantité de T3 amplifiant la transcription d'UCP1 (Bianco et Silva, 1988). ...
Thesis
Il existe deux types de tissus adipeux (TA). Le tissu adipeux blanc stocke les lipides sous forme de triglycérides. Le tissu adipeux brun possède une signature thermogénique via la protéine UCP1 utilisant les lipides pour former de la chaleur. Il existe aussi des adipocytes qui ont des caractéristiques similaires aux adipocytes bruns (adipocytes beiges) au sein du TA blanc. Le TA sécrète également des hormones lui conférant une fonction endocrinienne. Il maintient l’homéostasie énergétique et peut être altéré de différentes façons, ce qui conduit à des dysfonctionnements métaboliques : Une perte importante du TA dans les lipoatrophies est observée lors d’un traitement antirétroviral hautement actif contre le VIH (thérapie HAART). Ceci amène à des modifications métaboliques graves, dues à des niveaux élevés de lipides circulants et à une résistance à l’insuline systémique. Cette thérapie HAART est composée d’inhibiteurs de la protéase du VIH (IPs) ou de la transcriptase inverse (INTI). Les effets inhibiteurs des IPs sur le processus de différenciation adipocytaire blanche sont bien connus. Cependant, les mécanismes spécifiques qui affectent les différents dépôts adipeux humains distinctement ainsi que la différenciation adipocytaire brune le sont moins. Le cancer est une pathologie caractérisée par la prolifération dérégulée de cellules capables de former des métastases. Les cellules tumorales interagissent activement avec leur microenvironnement, notamment avec le TA qui est présent autour de nombreux organes et qui peut favoriser la progression tumorale (tissu adipeux associé au cancer). Le TA promeut la prolifération des cellules cancéreuses par la sécrétion d’adipocytokines. De plus, les cellules tumorales modifient le TA pour tirer leur énergie des lipides ce qui favorise leur expansion et leur dissémination. Nous avons étudié les interactions entre adipocytes et cellules tumorales de sein puisque le TA fait partie intégrante de la glande mammaire. Mon travail de thèse a consisté à identifier de nouveaux mécanismes moléculaires importants pour le développement physiopathologique et/ou l’altération du TA. Nous avons d’abord étudié les effets des IPs sur la perte de l’auto-renouvellement des progéniteurs adipeux (PAs) (1) et sur les modifications métaboliques des adipocytes (2). Nous étudions aussi les interactions entre les cellules de cancer du sein et le microenvironnement adipeux (3). Tout d’abord, les IPs inhibent l’auto-renouvellement des PAs en diminuant IER3 ce qui déstabilise en aval la boucle autocrine de l’Activine A. Les IPs bloquent la différenciation des PAs en adipocytes. La perte de ces deux processus indique que les IPs induisent des lipoatrophies retrouvées au cours de la thérapie HAART. Par la suite, nous observons que les IPs réduisent l’expression des marqueurs thermogéniques dans les adipocytes beiges et bruns par l’inhibition de la transcription d’UCP1. Ils altèrent aussi l’expression des sirtuines, enzymes antivieillissement. L’utilisation d’un activateur de la sirtuine 1 permet de renverser partiellement les effets des IPs sur l’expression d’UCP1. Enfin, nos résultats démontrent que des mammosphères de cancer de sein induisent la protéine UCP1 dans les adipocytes adjacents. L’adrénomedulline produite par les mammosphères participe à ce processus et nous avons pu caractériser son mécanisme d’action. En conclusion, les travaux réalisés pendant ma thèse ont permis de mieux comprendre les mécanismes par lesquels les IPs inhibent l’auto-renouvellement des progéniteurs adipeux ainsi que l’altération de la signature thermogénique via la perte d’UCP1 dans les adipocytes bruns. Les cellules tumorales, quant à elles, induisent l’expression d’UCP1 résultant en une conversion métabolique des adipocytes blancs en adipocytes bruns.
... recruitment of protein complexes to specific DNA sequences 16 . They are preferentially expressed in tissues with high OXPHOS needs, and the first member, PGC1α was identified in brown adipose tissue where it was shown to regulate mitochondrial biogenesis 17,18 . The PGC1α homologs, PGC1β and PGC-1-related coactivator (PRC), exert similar downstream effects 19,20 . ...
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... Under aerobic conditions, mitochondrial metabolism is a central provider of cellular energy in the form of ATP. While the overall mitochondrial architecture and organization is generally conserved among organisms, there can be enormous differences in mitochondrial content and capacity among different tissues and in response to various physiological constraints (Hood et al., 2006;Kraft et al., 2006;Lyons et al., 2006;Moyes et al., 1997;Nicholls et al., 1986). This inherent plasticity of the organelle provides metabolic flexibility that certainly plays an integral part in the resilience of species to environmental and physiological change (Seebacher et al., 2010). ...
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The peroxisome proliferator activated receptor γ coactivator-1 (PGC-1) family is composed of three coactivators whose role in regulating mammalian bioenergetics regulation is clear, but is much less certain in other vertebrates. Current evidence suggests that in fish, PGC-1α and PGC-1β may exhibit much less redundancy in the control of fatty acid oxidation and mitochondrial biogenesis compared to mammals. To assess these roles directly, we knocked down PGC-1α and PGC-1β expression with morpholinos in zebrafish embryos, and we investigated the resulting molecular and physiological phenotypes. First, we found no effects of either morpholinos on larval hatching, heart rates and oxygen consumption over the first few days of development. Next, at 3 days post fertilization (dpf), we confirmed by real time PCR a specific knock down of both coactivators, that resulted in a significant reduction in the transcript levels of citrate synthase (CS), 3-hydroxyacyl-CoA dehydrogenase (HOAD), and medium-chain acyl-coenzyme A dehydrogenase (MCAD) in both morphant groups. However, there was no effect on transcription factors’ gene expression except for a marked reduction in estrogen related receptor α (ERRα) transcripts in PGC-1α morphants. Finally, we assessed whole embryonic enzyme activity for CS, cytochrome oxidase (COX), HOAD and carnitine palmitoyltransferase I (CPT-1) at 4 dpf. The only significant effect of the knockdown was a reduced CS activity in PGC-1α morphants and a counterintuitive increase of cytochrome oxidase activity in PGC-1β morphants. Overall, our results indicate that in larval zebrafish, PGC-1α and PGC-1β both play a role in regulating expression of important mitochondrial genes potentially through ERRα.
... It is also well known that the major components of the adaptive response to cold are increased in the number of mitochondria and in the activity of their electron transport system [5]. ...
Article
Continuous exposure of homeothermic animals to low environmental temperatures elicits physiological adaptations necessary for animal survival, which are associated to higher generation of pro-oxidants in thermogenic tissues. It is not known whether intermittent cold exposure (cold training) is able to affect tissue responses to continuous cold exposure. Therefore, we investigated whether rat liver responses to continuous cold exposure of 2 days are modified by cold training (1 hour daily for 5 day per week for 3 consecutive weeks). Continuous cold increased liver oxidative metabolism by increasing tissue content of mitochondrial proteins and mitochondrial aerobic capacity. Cold training did not affect such parameters, but attenuated or prevented the changes elicited by continuous cold exposure. Two-day cold exposure increased lipid hydroperoxide and protein-bound carbonyl levels in homogenates and mitochondria, whereas cold training decreased such effects although it decreased only homogenate protein damage in control rats. The activities of the antioxidant enzymes GPX and GR and H2O2 production were increased by continuous cold exposure. Despite the increase in GPX and GR activities, livers from cold-exposed rats showed increased susceptibility to in vitro oxidative challenge. Such cold effects were decreased by cold training, which in control rats reduced only H2O2 production and susceptibility to stress. The changes of PGC-1, NRF-1, and NRF-2 expression levels were consistent with those induced by cold exposure and cold training in mitochondrial protein content and antioxidant enzyme activities. However, the mechanisms by which cold training attenuates the effects of the continuous cold exposure remain to be elucidated.
... The general concept in this hypothesis is simply that the free fatty acids themselves, liberated from the triglycerides within the tissue, interact with thermogenin to activate it. The hypothesis is summarised, e.g. in ref. [85]. ...
Article
This chapter discusses the uncoupling protein thermogenin and mitochondrial thermogenesis. The activity of thermogenin has traditionally been studied in isolated brown-fat mitochondria, however it has become possible to isolate thermogenin itself, and this has opened new avenues for thermogenesis research. Thermogenin has successfully been reconstituted into lipid vesicles, and in these, it displays most of the characteristics expected from earlier studies. Thermogenin functions as a transporter over the mitochondrial membrane. The transported species concentrate on the molecular bioenergetics of thermogenin and on developments in understanding. A thermogenin molecule which is always active or always inactive would not make much physiological sense. Thus, thermogenin activity is under physiological control. Despite much effort and despite the unravelling of the primary structure, a molecular understanding of this control has not been achieved. A physiological activation of thermogenin via this mechanism has as a prerequisite that a physiological mechanism for cellular alkalinization exists and can be evoked upon adrenergic stimulation of the cells.
... La UCP1 tiene la capacidad de disipar el gradiente protónico generado por la cadena respiratoria, desacoplando la fosforilación oxidativa. De esta forma la UCP1 aumenta la conductividad protónica de la membrana interna, reactivando la oxidación de sustratos y generando calor de forma inherente al proceso (Cannon and Nedergaard, 2004;Himms-Hagen, 1990;Nicholls et al., 1986). La actividad de la UCP1 es sensible a variaciones del metabolismo celular, ya que es inhibida por nucleótidos de purina di-o trifosfato (ADP, ATP, GDP y GTP) y activada en presencia de ácidos grasos (Kozak and Harper, 2000;Nicholls, 1974), siendo éstos últimos los principales combustibles encargados de mantener la capacidad termogénica del tejido (Rousset et al., 2004). ...
Article
Rat brown preadipocytes cultured in low serum conditions increase DNA synthesis and proliferate in response to serum and a variety of growth factors and hormones. Epidermal growth factor, platelet-derived growth factor, and acidic and basic fibroblast growth factors stimulate DNA synthesis in a dose-dependent manner and induce at least a 5-fold increase in [³H]thymidine incorporation after 40 h of exposure. The physiological activator of brown adipose tissue, norepinephrine, has a low mitogenic effect per se, but increases DNA synthesis stimulation exerted by serum, epidermal growth factor, basic fibroblast growth factor, and the neuropeptide vasopressin. The addition of vasopressin plus norepinephrine greatly potentiates the mitogenic effect of growth factors to levels comparable to the effect of 10% serum. Preadipocytes cultured in the presence of these mitogen combinations (growth factor, vasopressin, and norepinephrine) express a differentiation marker, the uncoupling protein. Thus, our results show 1) that a variety of growth factors and hormones induce DNA synthesis in a synergistic fashion in brown preadipocytes in primary culture; and 2) there is evidence for a role of norepinephrine in the regulation of brown adipocyte proliferation, potentiating the action of serum and mitogens, besides its role in uncoupling protein messenger RNA expression. The acquisition of a total thermogenic capacity also involves an augmented content of UCP. In this context, we tested the ability of cultured cells to express UCP mRNA as an indication of its thermogenic capacity in the presence of the different mitogen combinations. Brown preadipocytes maintained in the continuous presence of polypeptide growth factor, vasopressin, and NE express UCP mRNA when T3 is present. The addition of NE 4 h before RNA isolation increases the amount of the UCP transcripts. Thus, it seems that the presence of NE not only potentiates the stimulatory effect of other mitogens, but also increases the ability of cells to express differentiation markers. NE stimulates both proliferation and differentiation in brown fat cells, and this effect seems to be caused by an increase in intracellular cAMP levels (6, 7, 8). Canine thyrocytes in primary culture also undergo both proliferation and differentiation when stimulated by TSH (30, 31). On the other hand, the different growth factor combinations exert different effects on UCP mRNA expression. Some growth factors, such as EGF and PDGF, could interfere with the induction of UCP mRNA expression in response to differentiating conditions. Further work is needed to elucidate the role of NE in the stimulation of proliferation and differentiation as well as the effects of the different mitogens on expression of the differentiation marker, UCP.
Article
Systemic inflammation resulting from dysfunction of white adipose tissue (WAT) accelerates the pathologies of diabetes and cardiovascular diseases. In contrast to WAT, brown adipose tissue (BAT) is abundant in mitochondria that produce heat by uncoupling respiratory chain process of ATP synthesis. Besides BAT’s role in thermogenesis, accumulating evidence has shown that it is involved in regulating systemic metabolism. Studies have analyzed the “browning” processes of WAT as a means to combat obesity, whereas few studies have focused on the impact and molecular mechanisms that contribute to obesity-linked BAT dysfunction—a process that is associated with the “whitening” of this tissue. Compared to WAT, a dense vascular network is required to support the high energy consumption of BAT. Recently, vascular rarefaction was shown to be a significant causal factor in the whitening of BAT in mouse models. Vascular insufficiency leads to mitochondrial dysfunction and loss in BAT and contributes to systemic insulin resistance. These data suggest that BAT “whitening,” resulting from vascular dysfunction, can impact obesity and obesity-linked diseases. Conversely, agents that promote BAT function could have utility in the treatment of these conditions.
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