Brown adipose tissue (BAT) thermogenesis is an adaptive process, essential for energy expenditure and involved in the control of obesity. Obesity is associated with abnormally increased autophagy in white adipose tissue. Autophagy has been proposed as relevant for brown-vs-white adipocyte differentiation; however, its role in the response of BAT to thermogenic activation is unknown.METHODS:
The effects of thermogenic activation on autophagy in BAT were analyzed in vivo by exposing mice to 24 h cold condition. The effects of norepinephrine (NE), cAMP and modulators of lysosomal activity were determined in differentiated brown adipocytes in the primary culture. Transcript expression was quantified by real-time PCR, and specific proteins were determined by immunoblot. Transmission electron microscopy, as well as confocal microscopy analysis after incubation with specific antibodies or reagents coupled to fluorescent emission, were performed in BAT and cultured brown adipocytes, respectively.RESULTS:
Autophagy is repressed in association with cold-induced thermogenic activation of BAT in mice. This effect was mimicked by NE action in brown adipocytes, acting mainly through a cAMP-dependent protein kinase A pathway. Inhibition of autophagy in brown adipocytes leads to an increase in UCP1 protein and uncoupled respiration, suggesting a repressing role for autophagy in relation to the activity of BAT thermogenic machinery. Under basal conditions, brown adipocytes show signs of active lipophagy, which is suppressed by a cAMP-mediated thermogenic stimulus.CONCLUSIONS:
Our results show a noradrenergic-mediated inverse relationship between autophagy and thermogenic activity in BAT and point toward autophagy repression as a component of brown adipocyte adaptive mechanisms to activate thermogenesis.