Figure 4.

Regulation of browning of white adipocytes. Abbreviations: IL (Interleukin), UCP-1 (Uncoupling Protein-1). White adipocytes are a cell-type specialized in lipid storage but also characterized by an important endocrine function. These cells are characterized by an unilocular lipid droplet and a reduced mitochondrial mass. During the last decades, it was demonstrated that these cells could transdifferentiate into brown adipocytes under certain circumstances such as cold exposure, change in gut microbiota (decrease in Firmicutes level) or exposition to cytokine (such as IL-18), myokine (such as irisin) or chemical compounds (such as retinoic acid). This process can be inhibited by several conditions including exposition to proinflammatory cytokines such as IL-6 or IL-1β or to oxidative stress. A more detailed list of pro/anti-browning conditions can be found in the main text. The adipocyte browning is characterized by an increase in lipogenesis and changes in terms of lipid droplet number and composition. The hallmark of adipocyte browning is the induction of UCP-1 expression. Of note, if it is considered that adipocyte browning is accompanied by UCP-1 activation, it was recently suggested that UCP-1 could also play a key role in lipid metabolism. Activation of UCP-1 is often accompanied by a decrease in oxidative stress in adipose tissues. Brown adipocytes can also differentiate into white adipocytes. This process, named adipose tissue whitening, is accompanied an increase in mitophagy in order to clear cells from UCP-1 expressing mitochondria.