Transcriptional control of brown
adipocyte development and physiological
function—of mice and men
Patrick Seale, Shingo Kajimura, and Bruce M. Spiegelman1
Dana-Farber Cancer Institute and the Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA
The last several years have seen an explosion of in-
formation relating to the transcriptional control of
brown fat cell development. At the same time, new data
have emerged that clearly demonstrate that adult hu-
mans do indeed have substantial amounts of functioning
brown adipose tissue (BAT). Together, these advances
are stimulating a reassessment of the role of brown
adipose tissue in human physiology and pathophysiol-
ogy. These data have also opened up exciting new
opportunities for the development of entirely novel
classes of therapeutics for metabolic diseases like obesity
and type 2 diabetes.
Anatomists have recognized two types of adipose tissues
for at least 100 years (Cannon and Nedergaard 2004).
White fat is of course the more obvious, representing at
least 10% of the body weight of normal, healthy adult
humans. White fat tissue is specialized for the storage of
chemical energy, with white fat cells capable of both
synthesizing triglycerides de novo from glucose and
importing fatty acids from the blood. An enormous
capacity for expansion of white adipose for energy storage
was presumably a key evolutionary adaptation to cope
with periods of food shortage. However, in current times,
with ready supplies of calorie-dense food and sedentary
epidemic of obesity. This is now a formidable public
health issue, since obesity is a major risk factor for many
diseases, including type 2 diabetes, cardiovascular dis-
ease, stroke, hypertension, and many cancers (Bray and
Brown fat, a key thermogenic tissue, is a mammalian
adaptation that is most obvious in rodents and infant
humans (Cannon and Nedergaard 2004). Since smaller
animals usually have a larger surface area to volume ratio,
need for brown fat. Indeed, the existence of significant
deposits of brown fat in normal adult humans was
doubted until very recently. New data, reviewed here,
now put this question to rest, once and for all. The field
must now address the relative importance of activated
brown fat in explaining differences in susceptibility to
obesity and diabetes among human subjects. Likewise,
the ability to alter the amount and activity of brown
fat holds exciting therapeutic implications for these
Brown fat tissue is composed of brown fat cells, plus
abundant blood vessels and nerves (Fawcett 1952; Cinti
2000). Since brown fat provides heat to the body in
response to sympathetic nerve activity, and the heat is
carried via the bloodstream, this architecture makes
sense. Brown fat cells themselves have a very unique
cellular and molecular composition. They are indeed fat
cells, having all of the enzymatic machinery to synthe-
size and store triglyceride; in the case of brown fat cells,
lipid is normally stored in multiple small fat droplets
(multilocular), as opposed to white fat cells, which
usually contain one giant droplet of triglyceride (uniloc-
ular). The thermogenic business of brown fat cells is
carried out in the very dense mitochondria that these
cells possess. In fact, brown fat cells and cardiomyocytes
possess the highest levels of mitochondria in mammalian
organisms (Scheffler 1999). However, unlike heart cells,
which utilize mitochondria to generate the extremely
high amounts of ATP needed to perform cardiac contrac-
tile function, the mitochondria of brown fat cells uncouple
large amounts of fuel oxidation from ATP generation.
This uncoupling is caused by the presence of Uncoupling
Protein-1 (UCP1), which sits in the inner mitochon-
drial membrane and catalyzes a leak of protons from
the intramembrane space into the mitochondrial matrix
(Klingenberg 1999). The resulting dissipation of the mito-
chondrial membrane potential, along with extremely
high rates of mitochondrial electron transport and fuel
oxidation, results in the generation of heat and the ex-
penditure of huge amounts of chemical energy.
Brown fat has been well-established as a crucial com-
ponent in the defense against the cold in a process called
nonshivering thermogenesis (Cannon and Nedergaard
2004).Chronic coldexposure of rodents or humans causes
an expansion and activation of brown fat (Klingenspor
[Keywords: PRDM16; FoxC2; brown fat; UCP1; PGC-1a; obesity]
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788GENES & DEVELOPMENT 23:788–797 ? 2009 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/09; www.genesdev.org
2003). Cold is sensed in the central nervous system, and
the brown fat is activated via catecholamines secreted by
sympathetic nerve terminals in the brown adipose tissue
(BAT) itself (Klingenspor 2003; Cannon and Nedergaard
2004). The thermogenic function of brown fat tissue, is,
in fact, only functional in response to adrenergic input.
Thyroid hormones also play a key role in the sympathetic
activation of nonshivering thermogenesis, in part through
the stimulation of UCP1 transcription (Bianco and Silva
1987; Ribeiro et al. 2001). Brown fat can also be activated,
and can expand quite massively, by constitutive exposure
to b-adrenergic agonists, either pharmacological or due
to catecholamine-secreting tumors. In addition to effects
gence of UCP1-expressing brown fat cells in classic de-
pots of white fat (Ghorbani and Himms-Hagen 1997;
Cinti 2005). In the genetic absence of UCP1, mice lose
their ability to produce heat by nonshivering thermo-
genesis and therefore exhibit a profound cold intolerance
(Enerback et al. 1997).
Brown fat has also been recognized for its potential and
demonstrated anti-obesity properties. Certain diets in
rodents that cause overfeeding (cafeteria diets) stimulate
the expansion and activation of brown fat, in an apparent
physiological effort to restrain weight gain and obesity,
so-called diet-induced thermogenesis (Rothwell and
Stock 1979). The genetic ablation of BAT via expression
of a targeted toxigene causes a propensity toward obesity
and metabolic disease (Lowell et al. 1993). Likewise,
deletion of UCP1 in mice causes increased weight gain,
when mice are housed at thermoneutrality (Feldmann
et al. 2009). Indeed UCP1 has now clearly been demon-
strated as a key factor in diet-induced thermogenesis. In
addition, a large number of genetic studies in mice have
shown that experimental increases in the amount and/or
function of brown fat promotes a lean and healthy pheno-
type (Kopecky et al. 1995, 1996; Cederberg et al. 2001;
Tsukiyama-Kohara et al. 2001; Xue et al. 2007). Specifi-
cally,mice withhigher amounts ofbrowntypefatgain less
weight, are more insulin-sensitive, have lower levels of
serum-free fatty acids, and are protected from diabetes and
other metabolic sequelae. Finally, stimulation of brown
fat-mediated thermogenesis in adult animals by pharma-
cological treatment with b-adrenergic agonists or thyroid
receptor agonists does reduce obesity, but thus far, these
further clinical development (Arch 2002).
Transcriptional control of BAT cell development
Extensive studies in cell culture and in vivo have revealed
the molecular pathways that control the process of
adipocyte differentiation from preadipocytes (for review,
see Farmer 2006; Rosen and MacDougald 2006; Gesta
et al. 2007). PPARg (peroxisome proliferator-activated
receptor-g), a member of the nuclear hormone receptor
superfamily, plays the central role in the differentiation of
both white and brown fat cells (Tontonoz et al. 1994;
Barak et al. 1999; Rosen et al. 1999; Nedergaard et al.
2005). Interestingly, however, mice with a particular
mutation in PPARg, P465L, display deficits in brown fat
development and thermogenic function but have appar-
ently normal white fat tissue (Gray et al. 2006). Members
of the C/EBP family of transcription factors (CCAAT/
enhancer-binding proteins: C/EBPa, C/EBPb, and C/EBPd)
also participate in activating and maintaining the expres-
sion of adipogenesis-induced genes, including PPARg.
Increased and sustained expression of C/EBPb in white
fat cells promotes the expression of brown fat cell-
selective genes (Karamanlidis et al. 2007), though it is
expressed and controls initiating adipogenic events in
both white and brown fat cells. C/EBPa is a key tran-
scriptional regulator of insulin sensitivity in mature fat
cells. PPARg and C/EBPa cooperatively regulate each
other’s expression and orchestrate a transcriptional cas-
cade that maintains the stable differentiated state of the
adipocyte (Wu et al. 1999; Lefterova et al. 2008; Nielsen
et al. 2008). However, ectopic expression of PPARg or C/
EBPa in mesenchymal cells induces only a white, not
brown, fat phenotype, suggesting that these molecules do
not control the determination of brown fat cell fate.
Several recent studies have, however, described transcrip-
tional regulators that positively or negatively influence
brown fat development (Fig. 1).
FoxC2 (Forkhead box C2)
FoxC2, a member of the forkhead/winged helix transcrip-
tion factor family, promotes brown fat development. In
particular, transgenic expression of FoxC2 in adipose
tissue induces a remarkable brown fat-like phenotype in
white adipose tissue (WAT), with increased expression of
thermogenic (such as UCP1) and mitochondrial genes.
through PRDM16. PRDM16 expression in myoblasts or preadi-
pocytes induces PGC-1a gene expression. PRDM16 coactivates
the transcriptional activity of PGC-1a and PGC-1b as well as
PPARa and PPARg through direct interaction. The cAMP-
dependent thermogenic program is potentiated by FoxC2 and
PRDM16. RIP140, Rb, and p107 antagonize the expression or
function of PGC-1a. On the other hand, PRDM16 represses the
expression of white fat cell or skeletal muscle-specific genes,
mediated through its regulated association with the corepressors
CtBP1 and CtBP2.
Transcriptional cascades in brown fat development
Development and function of brown adipocytes
GENES & DEVELOPMENT789
Importantly, FoxC2 transgenic mice are lean, insulin-
sensitive, and resistant to diet-induced obesity (Cederberg
et al. 2001; Kim et al. 2005). FoxC2 achieves this
‘‘browning’’ effect, at least in part, by sensitizing cells to
the b-adrenergic cAMP–PKA pathway (Fig. 1). This occurs
by directly inducing the expression of the RIa subunit of
PKA (Cederberg et al. 2001; Dahle et al. 2002). Interest-
ingly, FoxC2 also activates expression of angiopoietin-2
to stimulate vascularization in adipose tissue, which
is also a prominent feature of BAT (Xue et al. 2008).
Although FoxC2 is abundantly expressed in adipose tis-
sue, it is equally expressed both in BATand WAT, at least
at the mRNA level. It also remains to be determined
whether brown fat differentiation and function displays
a genetic requirement for FoxC2.
PGC-1a (PPARg coactivator-1a)
PGC-1a was first identified as a PPARg-interacting pro-
tein from brown fat cells (Puigserver et al. 1998). PGC-1a
expression is highly induced and activated in response
to cold exposure, mediated by the PKA–CREB pathway
(Herzig et al. 2001; Cao et al. 2004). Over the past 10
years, PGC-1a has emerged as a dominant regulator of
mitochondrial biogenesis and oxidative metabolic path-
ways in many cell types via its coactivation of various
transcription factors (Lin et al. 2005; Finck and Kelly
2006; Handschin and Spiegelman 2006; Rosen and Mac-
Dougald 2006). Indeed, ectopic expression of PGC-1a in
white fat cells induces a number of mitochondrial genes
and thermogenic genes, including UCP1 (Puigserver et al.
1998; Tiraby et al. 2003).
Notably, several molecules have been shown to in-
fluence brown fat development and function, at least in
part, through regulating the expression or transcriptional
activity of PGC-1a. For example, RIP140, a corepressor of
many nuclear receptors, directly binds PGC-1a and
antagonizes its transcriptional function on several target
gene promoters shared by PGC-1a and RIP140 (Hallberg
et al. 2008). Expression of RIP140 in adipose and skeletal
muscle represses mitochondrial biogenesis and oxida-
tive metabolism (Leonardsson et al. 2004; Christian et al.
2005; Powelka et al. 2006; Seth et al. 2007). Genetic
ablation of RIP140 causes theemergence of brown fat-like
cells in WAT (Leonardsson et al. 2004). Although RIP140
expression is slightly enriched in WAT compared with
BAT, quantitative increases in RIP140 expression has not
been demonstrated to preferentially favor the differenti-
ation of white adipocytes.
The SRC (steroid receptor coactivator) family mem-
bers, including SRC-1/NcoA1, SRC-2/TIF2/GRIP1, and
SRC-3/p/CIP, have distinct and overlapping functions in
controlling energy metabolism and brown fat develop-
ment (Louet and O’Malley 2007). SRC-1 knockout mice
exhibit impaired adaptive thermogenesis with reduced
UCP1 expression in BAT (Picard et al. 2002). In contrast,
SRC-2 knockout mice display improved energy expendi-
ture and higher adaptive thermogenesis. SRC-2 inhibits
the interaction of PPARg with PGC-1a, whereas SRC-1
reinforces the coactivation of PGC-1a on PPARg tran-
scriptional activity (Picard et al. 2002). SRC-3-deficient
BAT has smaller lipid droplets with higher mitochondrial
content. Interestingly, SRC-3 induces GCN5, the major
acetyltransferase of PGC-1a (Lerin et al. 2006), to repress
the transcriptional activity of PGC-1a. Ablation of SRC-3
reduces acetylation of PGC-1a, leading to an increase in
mitochondrial biogenesis (Louet et al. 2006; Coste et al.
Adipocytes from retinoblastoma protein (pRb)-deficient
fibroblasts or embryonic stem cells exhibit a brown-fat
phenotype with high mitochondrial content, and ele-
vated expression of UCP1, PGC-1a, and mitochondrial
genes (Hansen et al. 2004). Since most pRB knockout
mice are embryonic lethal due to abnormalities in neu-
ral and hematopoietic development (Clarke et al. 1992;
Jacks et al. 1992; Lee et al. 1992), whether BAT develop-
ment in vivo requires pRb is unknown. WAT from mice
lacking p107, another member of Rb pocket protein
family, contains abundant brown fat-like cells with
multilocular oil droplets (Scime et al. 2005). These
adipose tissues are enriched in mitochondria with high
levels of UCP1 and PGC-1a gene expression. Notably,
pRb levels are dramatically reduced in p107-deficient
Sca-1+CD31?Lin?adipogenic precursors, suggesting that
the p107 action is mediated through regulation of pRb.
Interestingly, pRb has been shown to directly bind the
PGC-1a promoter to repress its transcription (Scime et al.
Taken together, these data had suggested a dominant
role for PGC-1a in brown fat development and function.
Indeed, genetic ablation of PGC-1a results in greatly
reduced capacity for cold-induced adaptive thermogene-
sis (Lin et al. 2004). This is indeed a cell-autonomous
requirement for PGC-1a in BATcells, since immortalized
brown fat cells lacking PGC-1a display a blunted in-
duction of thermogenic genes in responseto cAMP (Uldry
et al. 2006). However, many non-cAMP-dependent brown
fat cell-selective genes are still expressed appropriately,
and the fat differentiation program itself is unaltered in
the absence of PGC-1a. These results suggest that while
PGC-1a is a crucial regulator of adaptive thermogenesis,
it is not an essential determinant of brown fat identity.
PRDM16 (PRD1-BF-1-RIZ1 homologous
In a genome-wide survey of all known and putative
transcriptional regulators in the mouse genome, PRDM16
was identified as one of only three genes whose expression
correlated strongly with the brown fat phenotype in vivo
and in cultured cell models (Seale et al. 2007). PRDM16
was originally identified at a chromosomal breakpoint of
t(1;3)(p36;q21)-positive human acute myeloid leukemia
cells (Mochizuki et al.2000;Nishikataetal.2003).PRDM16
contains seven repeats of C2H2 zinc-finger domain at its
N terminus (ZF1 domain) and three similar repeats at its
C terminus (ZF2 domain). PRDM16 also contains a puta-
tive SET domain, a conserved region among histone
lysine methyltransferases (Rea et al. 2000; Schultz et al.
2002; Trievel et al. 2002). An aberrant form of PRDM16
Seale et al.
790 GENES & DEVELOPMENT
lacking the SET domain is expressed selectively in adult
T-cell leukemia cells (Yoshida et al. 2004). This short
form of PRDM16 has been suggested to transform cells by
interfering with the function of the full-length protein
(Du et al. 2005; Shing et al. 2007). Whether multiple
spliced forms of PRDM16 exist in brown adipocytes and
what their functional role in this cell type is remain
When ectopically expressed in cultured mesenchymal
cells including white fat preadipocytes, PRDM16 induces
a complete program of brown fat differentiation, includ-
ing activation of thermogenic genes (UCP1, PGC-1a, and
Deiodinase-d2), mitochondrial genes, and other BAT-
selective genes (cidea and elovl3) (Fig. 1; Seale et al.
2007, 2008; Kajimura et al. 2008). Furthermore, trans-
genic expression of PRDM16 in adipose tissue induces
the formation of copious clusters of UCP1-expressing
brown adipocytes in WAT depots. Importantly, PRDM16-
expressing cells display high levels of uncoupled cellular
respiration in response to cAMP, as do authentic brown
Although PRDM16 has been shown to bind directly to
a specific DNA sequence via two sets of zinc-fingers
(ZF1 and ZF2) in vitro (Nishikata et al. 2003), abrogation
of DNA binding by introducing a point mutation in ZF2
does not alter the ability of PRDM16 to induce a brown
fat phenotype (Seale et al. 2007). This result suggests
that the action of PRDM16 is largely mediated by pro-
tein–protein interactions, rather than by direct DNA
binding. In fact, PRDM16 powerfully coactivates the
transcriptional activity of PGC-1a and PGC-1b, as
well as PPARa and PPARg, through direct physical
interaction (Fig. 1; Seale et al. 2007, 2008). Presumably,
PRDM16 also interacts with other transcription factors
yet to be defined.
Intriguingly, coincident with the induction of brown fat
cell-selective genes, PRDM16 expression is also associ-
ated with the suppression of several markers selective for
white fat cells (such as resistin and angiotensinogen) or
muscle (myoD, myogenin, and myosin heavy chain). The
repressive action of PRDM16 on white fat cell-selective
genes is mediated by its regulated association with the
corepressors, CtBP1 and 2 (Kajimura et al. 2008). The
functions to suppress transcription of these genes remains
to be identified.
Finally, PRDM16 is required for the identity and
function of brown fat cells both in vitro and in vivo.
Depletion of PRDM16 from cultured brown fat cells
causes a near total loss of the brown fat characteristics
(Seale et al. 2007). Surprisingly, the complete loss of
PRDM16 from cultures of primary brown fat cell precur-
sors promotes overt skeletal muscle differentiation. Con-
sistent with this, BAT from PRDM16-deficient mice at
embryonic day 17 exhibits an abnormal morphology with
dramatically reduced expression of thermogenic genes
and elevated expression of muscle-specific genes (Seale
et al. 2008). Taken together, these results establish
PRDM16 as a critical determinant of the brown fat
Embryonic origin of brown fat
White and brown adipocytes have long been assumed to
share a common developmental origin because they
express a common array of genes involved in triglyceride
metabolism. They also undergo a very similar program of
morphological differentiation controlled by PPARg and
members of the C/EBP family of transcription factors
(Rosen and Spiegelman 2000; Hansen and Kristiansen
2006; Gesta et al. 2007). Recent studies, however, have
surprisingly indicated that brown and white fat cells, in
fact, arise from distinct cellular lineages. Elegant lineage
tracing experiments by Atit et al. (2006) showed that BAT,
dermis, and some skeletal muscles are derived from
a population of Engrailed-1 (En1)-expressing cells in the
dermamyotome (Fig. 2). Moreover, microarray experi-
ments revealed that brown but not white fat precursors
express many genes and microRNAs that are character-
istic of muscle precursors (Timmons et al. 2007; Walden
et al. 2009). In a completely independent line of investi-
gation, as noted above, we found that depletion of
PRDM16 from cultures of primary brown fat precursors
induces skeletal muscle differentiation (Seale et al. 2008).
Moreover, ectopic expression of PRDM16 in committed
myoblasts, both immortalized and primary, caused the
cells to adopt a brown fat phenotype when exposed to
proadipogenic stimuli (Seale et al. 2008). PRDM16 can
versus skeletal muscle cell fate. Lineage tracing experiments
suggest a model in which tripotent, Engrailed-1-expressing cells
in the central dermomyotome give rise to dermis, epaxial
muscle, and brown fat. Wnt signals appear to direct these
precursor cells to the dermal fate (Atit et al. 2006). Brown fat
and skeletal muscle arise from precursor cells that have
expressed Myf5, a gene that has been assumed to selectively
mark skeletal myogenic progenitors. Gain-of-function and loss-
of-function experiments suggest that PRDM16 specifies brown
fat cell identity from ‘‘myoblast-like’’ precursors by activating
brown adipogenesis and suppressing skeletal myogenesis. The
cues that control PRDM16 expression in the presumptive brown
fat/skeletal muscle precursors are unknown. By contrast, white
fat cells belong to a completely independent cell lineage. A
recent report suggests that at least some precursors for white fat
cells are derived from mural cells associated with blood vessels
(Tang et al. 2008).
Model for PRDM16 function in specifying brown fat
Development and function of brown adipocytes
GENES & DEVELOPMENT791
thus apparently control a bidirectional cell fate switch
between muscle and brown fat.
To more closely examine whether the BATand skeletal
muscle lineages could be developmentally related in an in
vivo context, we traced the ontogeny of Myf5-expressing
cells in mice. Myf5 is a crucial early myogenic transcrip-
tion factor whose expression had been thought to be
highly specific to committed skeletal myoblastic cells
(Pownall et al. 2002). In these studies, we observed that
only skeletal muscle and BAT are derived from cells that
have previously activated Myf5 (Fig. 2). Importantly,
white fat from several depots and all other tissues
examined had not been formed from a Myf5-expressing
cell lineage. These experiments thus reveal that white
and brown fat cell lineages have disparate developmental
origins. Consistent with this notion, a new report by Graff
and colleagues (Tang et al. 2008) indicates that at least
some white adipose precursors derive from mural cells
(pericytes) associated with blood vessels in adipose (Fig. 2).
The Myf5-lineage tracing studies suggest that brown fat
is derived from a precursor cell type that is equipped with
the potential to give rise to brown fat and muscle.
Notably, myogenin-deficient mice that are devoid of
differentiated skeletal muscle fibers have prominent de-
position of brown fat tissue especially in the dorsal
cervival region (Hasty et al. 1993), suggesting that cells
incapable of completing terminal muscle differentiation
give rise to brown fat cells. However, the identity of this
‘‘putative’’ brown fat/skeletal muscle progenitor cell
population remains to be localized and characterized in
mouse embryogenesis. The lineage tracing studies do not
dismiss the possibility that distinct pools of Myf5-
expressing precursors exist that are committed to either
muscle or brown fat
Interestingly, the pockets of brown fat cells that emerge
in white fat depots in response to chronic b-adrenergic
stimulation were not derived from a Myf5-expressing cell
lineage (Seale et al. 2008). Notably, Kozak’s group (Xue
et al. 2007) had shown previously that BAT cells in the
interscapular depot and those localized in WAT depots
are genetically distinct.We speculate that the brown fat-
like cells in WAT tissues possess a more plastic pheno-
type, enabling them to adapt to environmental cues,
whereas the distinct depots of brown fat cells that are
present beforebirthare stably committed to thebrown fat
fate. It remains to be determined which cell type(s) in
white fat tissues are able to give rise to UCP1-expressing
brown type adipocytes. It is possible that certain white fat
preadipocytes or mature white fat cells can be stimulated
to undergo brown adipogenesis. Alternatively, a distinct
population of committed brown fat precursors or other
more primitive stem cells may reside in WATs. Although
we hypothesize that PRDM16 is involved in the forma-
tion of these brown-typefat cells invivo, this hasyet to be
examined. Interestingly, brown fat cells and their precur-
sors have been localized in mouse and human skeletal
muscle tissue and may be protective against metabolic
disease (Almind et al. 2007; Crisan et al. 2008). Whether
these cells arise from Myf5+ myoblastic cells has not yet
It will now be important to carefully map the embry-
onic expression pattern of PRDM16 in an effort to
pinpoint the putative brown fat–skeletal muscle precur-
sors. In addition, signaling molecules that control the
timing and specificity of PRDM16 expression during
development are unknown. Certain growth factors have
been shown to influence both brown and white adipo-
genesis. Of particular note, bone morphogenetic proteins
(BMPs) have emerged as powerful regulators of both
white and brown adipocyte differentiation (Sottile and
Seuwen 2000; Hata et al. 2003; Tang et al. 2004; Taha
et al. 2006; Tseng et al. 2008). Consistent with a crucial
role for BMP signaling in adipogenesis, mice lacking
Schnurri-2, an important mediator of the BMP pathway,
have profound deficits in adipose tissue development (Jin
et al. 2006). Notably, BMP2 and BMP4 have been shown
to promote the differentiation of white adipose cells,
whereas BMP7 selectively stimulates the process of
brown adipogenesis in committed precursors. In particu-
lar, exposure of brown preadipocytes to BMP7 induces
a full program of brown fat differentiation, including
induction of PRDM16 and UCP1 expression (Tseng
et al. 2008). Moreover, BMP7-deficient mice display a pro-
found deficit in the development of brown fat tissue
(Tseng et al. 2008). It is not known, however, if BMP7
plays a direct role in inducing or activating PRDM16. In
addition, the transcriptional components in preadipo-
cytes that control the differential adipogenic response to
various BMPs remain to be found. Interestingly, BMPs
have also been shown to negatively regulate myogenesis
in certain contexts (Murray et al. 1993; Katagiri et al.
1997; Reshef et al. 1998), identifying them as potential
candidates for controlling brown adipogenic versus skel-
etal muscle cell fate (Fig. 2).
Activation of the canonical Wnt (wingless) signaling
pathway has been shown to inhibit both brown and white
adipogenesis by blocking the induction of PPARg and C/
EBPa (Ross et al. 2000; Bennett et al. 2002; Longo et al.
2004; Kang et al. 2005; Li et al. 2008). Wnt10a and
Wnt10b are expressed in brown fat tissue and are down-
regulated during in vitro differentiation of brown fat
preadipocytes. In mature brown fat cells, ectopic Wnt10b
expression inhibits the expression of PGC-1a and thereby
promotes a white fat phenotype (Longo et al. 2004; Kang
et al. 2005). Wnt signaling is also an important promyo-
genic cue during embryogenesis and adult muscle re-
generation (Cossu and Borello 1999; Polesskaya et al.
2003; Otto et al. 2008). As with the BMPs, the reciprocal
effects of Wnt signaling in adipogenesis and myogenesis
suggest that it could also play an instructive role in
presumptive precursors for brown fat and skeletal muscle.
Members of the fibroblast growth factors (FGFs) have
also been suggested to play developmental and functional
roles in brown adipogenesis. FGF16 is highly expressed in
growing BAT tissue during late stages of fetal develop-
ment and decreases postnatally (Konishi et al. 2000). In
cell culture assays, recombinant FGF16 stimulates the
proliferation of BAT preadipocytes. Transgenic expres-
sion of FGF19 (FGF15 in mouse) in skeletal muscle causes
an expansion of BAT and elevated whole-body energy
Seale et al.
792GENES & DEVELOPMENT
expenditure; however, whether this effect is a direct
endocrine action of FGF19 is not known (Tomlinson
et al. 2002).
Brown fat tissue and thermogenesis in adult humans
The prevailing dogma that BAT is not present to signif-
icant levels in adult humans has now been refuted.
Positron emission tomography (PET) is used diagnosti-
cally to detect cancer tissues/cells based on their capacity
to take up large amounts of18fluoro-labeled 2-deoxy-
glucose (FDG). During the course of these imaging stud-
ies, symmetrically distributed hot spots for FDG uptake
were noted that had the characteristic density of adi-
pose tissue (Hany et al. 2002; Yeung et al. 2003; Tatsumi
et al. 2004; Weber 2004; Alkhawaldeh and Alavi 2008).
Moreover, the incorporation of FDG in these tissues
disappeared when patients were kept warm. Altogether,
a large number of such reports had strongly suggested
that these tissues were in fact BAT (Nedergaard et al.
2007). These reports, however, were largely overlooked
by the metabolism/obesity community because they
were published in very specialized medical journals. Very
recently, the nature of these putative brown fat depots
in adult humans has been carefully examined by dedi-
cated FDG-PETanalyses, combined with tissue biopsies.
Most importantly, by all criteria, these tissues are
now unequivocally identified as thermogenic brown fat
(S. Enerback and C.R Kahn, pers. comm.). Most impor-
tantly, the results of Enerback and colleagues (S. Enerback,
pers. comm.) clearly demonstrate in the same subjects
that human BAT is dramatically activated by cold expo-
sure. The human brown fat depots are localized in the
supraclavicular, cervical, and paravertebral regions and
express high levels of PRDM16, PGC-1a, and UCP1.
Analysis of a large number of historical FDG-PET scans
by Kahn and colleagues (C.R. Kahn, pers. comm.) suggests
mass index). Based on the amount of active BAT detected
by these studies, it seems very likely that these tissues
may make an important contribution to whole-body
energy balance. However, because the detection of BAT
by FDG-PETis so temperature-sensitive, dedicated studies
to examine the relationship between obesity and BAT
activity under standardized conditions are needed. Taken
together, these seminal studies have revitalized interest in
BAT as an important metabolic tissue in humans.
New therapeutic opportunities
The expanding epidemic of obesity and diabetes linked to
obesity serves clear notice that medical treatments for
obesity have not been particularly effective. In addition,
a growing understanding that obesity is also driving large
increases in the rates of certain cancers, arthritic con-
ditions, and cardiovascular disorders makes the develop-
ment of new therapeutic approaches to obesity an
Any effective treatment for obesity must fundamen-
tally impact whole-body energy balance, either by re-
ducing energy intake from food or increasing energy
expenditure. Understanding the complex genetic, behav-
ioral, and environmental networks that regulate food
intake and satiety is an enormous and important area of
study that has been reviewed elsewhere. Conceptually, at
least, the possibility of raising energy expenditure could
be achievedby increasing theamount of brown fat. Brown
fat evolved for the sole purpose of safely dissipating large
amounts of chemical energy. Hence, enhancing the in-
herent metabolic inefficiency of brown fat cells would be
an effective strategy to burn off excess calories. Given the
fact that most obese people are not that far out of energy
balance, the amount of activated BAT necessary to
achieve long-term weight control should not require
excessive or unacceptable heat generation.
Given the new knowledge that PRDM16 can act as
a dominant regulator of brown fat cell determination,
how canthis factor and related pathways beused to create
new therapies for obesity and linked metabolic diseases?
Development of drugs that elevate PRDM16 expression
in myoblasts or white preadipocytes in vivo may be
possible. The PRDM16 gene is dynamically regulated,
showing an increase in determined brown fat precursor
cells, relative to white fat precursors or myoblastic cells.
PRDM16 is further increased during the process of brown
fat cell differentiation in culture. Hence, it is probably
worthwhile to screen chemical libraries, as well as col-
lections of known drugs and drug-like compounds, to
determine if synthetic chemicals can be found that can
activate PRDM16 expression. The target cells for such
a therapy would presumably be the myoblastic precursors
in postnatal skeletal muscle, as well as the preadipoctyes
resident in the white fat tissues.
Furthermore, it is highly likely that PRDM16 is in-
duced in the dermomyotome during brown fat cell de-
termination in the embryo, in response to biological cues
that have not yet been identified. If a growth/differenti-
ation factor is involved in this transformation, the same
factor could have therapeutic potential postnatally. These
studies may not only find new chemical matter with
therapeutic potential, but they should also identify bio-
chemical pathways that control PRDM16 expression.
Having precise molecular targets that can alter PRDM16
expression within cells could open the door to more
precisely targeted drugs. BMP7, which can promote
PRDM16 expression and brown adipogenesis in culture
and in vivo, may be of direct therapeutic benefit for
obesity and type 2 diabetes (Tseng et al. 2008). The full
potential of therapies using BMP7 remains to be fully
explored. It should also be noted that the thiazolidine-
diones, which are highly selective PPARg agonists that
act as insulin sensitizing agents, are thought to function,
in part, by enhancing a brown adipose phenotype in WAT
(Wilson-Fritch et al. 2004; Hondares et al. 2006).
One caveat to these approaches is that we know very
little about the function of PRDM16 in nonadipose
tissues. Certain human leukemias have a chromosomal
translocation in lymphocytes that results in overexpres-
sion of PRDM16 (Mochizuki et al. 2000; Nishikata et al.
2003; Shing et al. 2007; Modlich et al. 2008). Obviously,
Development and function of brown adipocytes
GENES & DEVELOPMENT793
synthetic drugs or natural compounds that elevate PRDM16
globally will have to be carefully analyzed for both ben-
efits and toxicities.
A different approach would be to utilize cellular trans-
plantation of modified cells. Adipose tissues are removed
from many patients during liposuction; other patients
have adipose tissues injected in cosmetic surgery. Thus, it
should be feasible to grow either adipose of muscle
precursor cells from a given obese patient in culture and
stably express PRDM16 in these cells to make them
essentially brown fat precursors. Similar approaches
could also be attempted by exposing cells to BMP7 or
by overexpressing FoxC2. These engineered cells can
be injected back into the same patients and, at least
in principle, increase whole-body energy expenditure.
Among the serious questions we must ask, however, is
where such autologous transplants should be placed
anatomically in order to optimize their establishment
and proper vascularization and innervation. Whether
a therapeutic effect via such engineered autologous trans-
plantations would require a thimbleful or a bucketful of
cells remains to be determined.
P.S. is supported by an NIH grant (DK081605). S.K. is supported
by a fellowship from the Japan Society for the Promotion of
Science. This work is funded by the Picower foundation and an
NIH grant to B.M.S (DK081605).
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Development and function of brown adipocytes
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