Maintaining body temperature in a cold environment is key
for the survival of euthermic animals. In mammals, it is
achieved by both shivering and nonshivering thermogenesis.
Nonshivering thermogenesis is mainly mediated by brown adipose
tissue, in which chemical energy is dissipated in the form
of heat through the actions of uncoupling protein 1 (UCP1) in
mitochondria (1). In addition to its function to prevent hypothermia,
brown fat has also been of interest as a defense against
obesity and diabetes. Indeed, ablation of brown fat through
the use of a UCP1 promoter-driven toxin has shown that loss
of brown adipose tissue (BAT) causes an increased propensity to
weight gain and diabetes in experimental animals (2). Similarly,
ablation of UCP1 itself causes increased fat accumulation and
insulin resistance in mice, at least at thermoneutrality (3). Because
of the recent realization that adult humans have active brown fat
(4–6), UCP1-mediated thermogenesis has becoming a compelling
subject for research in obesity and other metabolic disorders.
In addition to the “classic” brown fat that is formed during
embryonic development, certain white fat depots contain cells
that can activate UCP1 and a broad thermogenic program (7–
11). This also occurs in response to the β-adrenergic stimulation
that is downstream of cold exposure. These cells have been
called beige (12) or brite adipocytes (13). Recently, it has been
shown that these beige cells have a developmental origin (14, 15)
and molecular signature (12) that is distinct from that of classic
brown adipocytes. Although classic brown cells come from a
muscle-like myogenic factor 5 (myf5)-positive lineage, the
beige cells come from a myf5-negative lineage. Interestingly,
the UCP1-positive adipose tissue of adult humans seems closer
in its gene expression pattern to the beige cells of mice than to
the classic brown cells (12).
Cold is a powerful environmental signal to activate thermogenesis
in vivo. According to the best studied pathway, cold is
first sensed by the sensory nerves in peripheral tissues. This information
is then received and processed in the hypothalamus,
which controls the activity of the sympathetic nervous system
(SNS), leading to release of norepinephrine (NE) onto brown and
beige fat cells. NE works through G protein-coupled β-adrenergic
receptors (β-ARs) on adipocytes to activate the cAMP/Protein
Kinase A/cAMP response element-binding protein signaling
cascade that controls the transcription of the thermogenic
gene program.
Thus, although cold is the natural stimulus for the activation of
thermogenesis, the prevailing model is that it exerts its effect on
adipocytes indirectly (i.e., via the central nervous system and SNS).
Here, we report that cold can directly act on beige and white adipocytes
in a cell-autonomous manner to activate the expression
of a thermogenic gene program. This provides a unique view to
the understanding and control of adaptive thermogenesis.