Transcriptional and Epigenetic Regulation of Thermogenesis in Brown Adipose Tissue

Adipose tissue has a central role in controlling mammalian metabolism. Whilewhite adipose tissue (WAT) is to store excess in the form of triglycerides, brown adipose tissue (BAT) is specialized in burning calories and generating heat. Brown adipose tissue is a key thermogenic organ and functions as a mechanism to combat hypothermia in small mammals such as rodents as well as in humans. Classic brown adipocytes contain a high density of mitochondria that constitutively express a protein called Uncoupling protein 1 (UCP1), which dissipates chemical energy by combusting energy and generating heat instead. Mice housed in the cold environment undergo a marked remodeling of their white fat and form beige fat, a third class of inducible cells that appears within white adipose depot that expresses UCP1 in response to cold stimulus. Importantly, there is emerging evidence of thermogenic tissues in human adults after chronic cold, and it is inversely correlated with body mass index and visceral fat. The aim of this dissertation work was to identify and characterize critical thermogenic factors that activate the UCP1 promoter and increase thermogenesis which may serve as promising avenues to combat obesity and associated metabolic diseases. Chapter 1 reviews advances made in understanding the molecular mechanism underlying the thermogenic gene program. A number of key transcriptional regulators critical for the thermogenic gene program centering on activating the UCP1 promoter, have been discovered. Thermogenic gene expression in brown adipocytes relies on coordinated actions of a multitude of transcription factors, including EBF2, PPARγ, Zfp516 and Zc3h10. Moreover, these transcription factors recruit epigenetic factors, such as LSD1 and Dot1l, for specific histone signatures to establish the favorable chromatin landscape. Additionally, this chapter discusses environmental signals that affect gene expression via various signaling pathways and by changes in intracellular metabolites, hence altering epigenome. Chapter 2 demonstrates my effort to characterize a key thermogenic transcription factor, Zc3h10 and discusses its molecular significance in thermogenesis both in vitro and in vivo. As a member of CCCH zinc finger proteins, Zc3h10 directly binds to the distal UCP1 promoter and increases the UCP1 gene expression. Upon sympathetic 2 stimulation, Zc3h10 is phosphorylated at S126 by p38 mitogen-activated protein kinase to increase binding to the distal region of the UCP1 promoter. Zc3h10 overexpression in mice increases thermogenic gene expression and energy expenditure, resulting in a lean phenotype. Conversely, Zc3h10 ablation in mice impairs thermogenic capacity, leading to weight gain. Chapter 3 focuses on the function of the epigenetic factor, Dot1l, the only known H3K79 methyltransferase, in the Zc3h10-mediated activation of the thermogenic program. Through a direct interaction, Dot1l is recruited by Zc3h10 to the promoter regions of thermogenic genes to function as a coactivator by methylating H3K79, and Dot1l and its H3K79 methyltransferase activity are required for activating the thermogenic gene program. Dot1l ablation in Ucp1+ cells in mice prevents activation of Ucp1 and other target genes to reduce thermogenic capacity and energy expenditure, promoting adiposity. Chapter 4 concludes my work describing the importance of key factors identified for thermogenesis and presents future directions and remaining questions.

Advisor: Sul, Hei Sook