UC Berkeley

Obesity, an excess accumulation of white adipose tissue, has become a global epidemic. White adipose tissue (WAT) plays a critical role by serving as the major energy storage site and by secreting adipokines that control various biological processes such as appetite, metabolism and insulin sensitivity. In contrast, brown adipose tissue (BAT) or BAT-like tissue dissipate energy via non-shivering thermogenesis to maintain body temperature and is known to inversely correlate with adiposity. BAT not only has high capacity of glucose and fatty acid (FA) uptake, but also secretes adipokines, all potentially contributing to insulin sensitivity. Understanding how adipocyte differentiation (adipogenesis) and thermogenesis is regulated and supported can further effort of developing therapeutic strategies against obesity/lipodystrophy. The aims of this dissertation work were to identify the receptor of Pref-1, an adipose tissue-specific secreted factor that inhibits adipogenesis and to characterize Aifm2 function during thermogenesis in BAT.

Chapter 1 reviews the developmental of various adipose depots. While subcutaneous WAT develops perinatally, visceral WAT develops after birth. They are derived from adipose progenitors expressing distinctive markers depending on the anatomical location. Adipogenesis is controlled by transcription factors which is tightly regulated by secreted factors such as Wnt, and Pref-1. In contrast, BAT forms in early embryogenesis from Myf5+, Pax7+ precursors. Brown fat-like cells can be found in WAT upon cold exposure or -adrenergic stimulation. In this chapter, transcriptional and epigenetic control of brown and beige adipocyte differentiation and thermogenic program is discussed. Finally, how metabolites can regulate this transcriptional machinery and support thermogenesis in BAT and BAT-like tissues is also highlighted.

Chapter 2 profiles my effort to identify the receptor of Pref-1 by employing a new improved and more sensitive method of crosslinking in live cells. I found a high level of plasma membrane ST2 in 3T3-L1 preadipocytes and in preadipocytes of SVF from WAT. Like Pref-1, ST2 is downregulated during adipocyte differentiation. In contrast, IL-33, the ST2 known ligands, is upregulated during adipocyte differentiation. I also found that Pref-1 binds tightly to ST2 at the plasma membrane of adipose precursor cells to inhibit adipogenesis. ST2 KO prevented Pref-1 activation of phospho-ERK and its inhibitory effect on adipogenesis. Moreover, our ST2 loss-of function approaches in mice showed in vivo evidence of ST2 requirement for Pref-1 to exert its effects on adipogenesis.

Chapter 3 profiles my discovery of Aifm2 as a lipid droplet (LD) associated NADH oxidase that is expressed at a high level only in BAT, but not in other tissues, and is cold inducible. I found Aifm2 to increase cytosolic NAD/NADH, correlating with higher glycolytic rate leading to higher oxygen consumption/uncoupled respiration and heat production in BAT cells. The Aifm2 BAT specific knockout mice were cold-sensitive due to impaired thermogenesis and, with decreased energy expenditure, they exhibited a higher adiposity. Conversely, our transgenic mice overexpressing Aifm2 in UCP1+ cells had a higher thermogenic capacity to better maintain body temperature upon cold exposure, increasing energy expenditure with decreasing adiposity.