Brown Adipose Tissue: Research Milestones of a Potential Player in Human Energy Balance and Obesity

Abstract

Obesity and diabetes mellitus are worldwide epidemics driven by the disruption in energy balance. In recent years, it was discovered that functional brown adipose tissue (BAT), once thought to exist mainly in infants, is present in adults, and can be detected during cold stimulation, and is associated with decreased adiposity. Brown fat pads were shown to be highly vascularized and metabolically active and on stimulation, they caused enhanced energy expenditure and increased glucose and fatty acid uptake. These observations drew attention to the possibility that nonshivering thermogenesis mediated by activation of BAT might be important in human energy balance and a potential tool to counter obesity. Recent investigations have revealed significant advances in the understanding of the role of BAT-mediated thermogenesis, uncovering essential knowledge on the origin, differentiation, activation, and regulation of BAT in both murine models and humans. In addition to classic BAT depots, transformation of white adipocytes into brown-like adipocytes, and the development of "beige" cells from distinct precursors, were demonstrated in different animal models and resulted in increased thermogenic activity. Several transcription factors, activating proteins, and hormones are increasingly identified as regulating the development and function of both brown-like adipocytes and classic brown fat pads. This review will summarize the evolution of research on BAT in humans, in light of the renewed scientific interest and growing body of evidence showing that recruitment and activation of BAT and browning of white adipose tissue can affect energy expenditure and may be a future feasible target in the treatment of metabolic diseases.