Influenza A Virus Infection Induces White Adipose Tissue Browning: A Metabolic Adaptation to Infection?

Mammals have two main types of adipose tissue (commonly defined as “fat”): the white adipose tissue (WAT), which represents the main energy reservoir of the body and provides lipids as substrates for other tissues, and the brown adipose tissue (BAT), which uses lipids for heat production (a process called “thermogenesis”). WAT and BAT are mainly composed of white and brown adipocytes, respectively. Whereas white adipocytes contain few mitochondria and a single large lipid droplet (unilocular), brown adipocytes contain many mitochondria and multiple small lipid droplets (multilocular). More recently, another type of adipocytes has been uncovered: the beige/brite (“BRown-in-whITE”) adipocytes, which locate in WAT and share some morphological and functional features with brown adipocytes. Indeed, alike brown adipocytes, beige/ brite cells contain many mitochondria that express high levels of the thermogenic uncoupling protein 1 (UCP1) in their inner membrane [1-4] (Figure 1a).