β-AR–Independent Thermogenic Pathway in Subcutaneous Fat. Mice
that lack all three β-adrenergic receptors (β-less mice) have been
shown to have severely impaired thermogenesis in their interscapular
BAT in response to cold (16). Recently, it has become
clear that certain white adipose depots can activate a thermogenic
program once exposed to prolonged cold or β-adrenergic
stimulation. Increasing evidence suggests that these cells play
a significant role in thermogenesis and energy homeostasis (17);
however, the extent to which the thermogenic capacity of these
cells is dependent on β-adrenergic signaling is unclear.
Prolonged exposure to 4 °C can be fatal to β-less mice (16);
therefore, a milder cold challenge was given by exposing these
animals to 10 °C for 20 h. As expected, the thermogenic gene
expression [including peroxisome proliferator-activated receptor
gamma coactivator 1-alpha (Pgc1a), Ucp1, and type II iodothyronine
deiodinase (Dio2)] in the classic (interscapular) BAT
was severely diminished in the β-less mice compared with the
wild-type controls (Fig. 1A). Interestingly, and in contrast, the
induction of thermogenic genes in the s.c. (inguinal) fat in the
β-less mice was largely preserved on cold exposure (Fig. 1B),
suggesting there is a β-AR–independent pathway that controls
thermogenic gene expression in the s.c. fat. In the visceral (epididymal)
fat, although a β3 agonist caused a β-AR-dependent
thermogenic response, cold exposure did not lead to significant
induction of thermogenic gene expression, as it did in the s.c. fat,
suggesting the β-AR–independent thermogenic response is limited
to the s.c. fat (Fig. 1C).