The first part of the thesis reports the role of Ten-eleven translocation methylcytosinedioxygenase 1 (TET1) in beige adipocyte thermogenesis. It has been suggested that beige
fat thermogenesis is tightly controlled by epigenetic regulators that sense environmental
cues such as temperature. We report that subcutaneous adipose expression of the DNA
demethylase TET1 is suppressed by cold and other stimulators of beige adipocyte
thermogenesis. TET1 acts as an autonomous repressor of key thermogenic genes,
including Ucp1 and Ppargc1a, in beige adipocytes. Adipose-selective Tet1 knockout mice
generated by using Fabp4-Cre improves cold tolerance and increases energy expenditure
and protects against diet-induced obesity and insulin resistance. Moreover, the suppressive
role of TET1 in the thermogenic gene regulation of beige adipocytes is largely DNA
demethylase–independent. Rather, TET1 coordinates with HDAC1 to mediate the
epigenetic changes to suppress thermogenic gene transcription. Taken together, TET1 is
a potent beige-selective epigenetic breaker of the thermogenic gene program. Our findings
from this chapter may lead to a therapeutic strategy to increase energy expenditure in
obesity and related metabolic disorders.
The second part of the thesis demonstrates the role of DNA (cytosine-5)-methyltransferase
3A (DNMT3A) in exercise regulation. Exercise can alter the skeletal muscle DNA
methylome, yet little is known about the role of the DNA methylation machinery in exercise
capacity. Here, we show that DNMT3A expression in oxidative red muscle increases greatly
following a bout of endurance exercise. Muscle-specific Dnmt3a knockout mice have
reduced tolerance to endurance exercise, accompanied by reduction in oxidative capacity
and mitochondrial respiration. Moreover, Dnmt3a deficient muscle overproduces reactive
oxygen species (ROS), the major contributors to muscle dysfunction. Mechanistically, we
show that DNMT3A suppresses the Aldh1l1 transcription by binding to its promoter region,
altering its epigenetic profile. Forced expression of ALDH1L1 elevates NADPH levels,
which results in overproduction of ROS by the action of NADPH oxidase complex, ultimately
resulting in mitochondrial defects in myotubes. Thus, inhibition of ALDH1L1 pathway can
rescue oxidative stress and mitochondrial dysfunction from Dnmt3a deficiency in myotubes.
Finally, we show that in vivo knockdown of Aldh1l1 largely rescues exercise intolerance in
Dnmt3a deficient mice. Together, we establish that DNMT3A in skeletal muscle plays a
pivotal role in endurance exercise by controlling intracellular oxidative stress.
The aim of this dissertation work was to identify and characterize the role of adipose Tet1
in the epigenetic regulation of thermogenesis and the role of skeletal muscle Dnmt3a in
exercise metabolism and oxidative stress. This may lead to a therapeutic strategy in
obesity, related metabolic disorders and exercise intolerance.
Chapter 1 provides a detailed understanding of the mechanisms of DNMT3A and TET2,
which may lead to identifying novel targets for the treatment of IR and relevant human
diseases. Even though a plethora of studies have found that changes in DNA methylation
are associated with metabolic dysregulation, the functional role is poorly understood. Here,
I will review the currently available literature and point out the remaining questions to be
answered in order to gain a better understanding of the mechanisms of DNMT3a and TET2.
Chapter 2 exhibits my efforts in characterizing the epigenetic role of DNA demethylase
TET1 in suppressing key thermogenic genes. TET1 suppresses the thermogenic activation
of beige adipocytes. Moreover, adipose-specific TET1 loss-of function in vivo led to
increased energy expenditure and protection from diet-induced obesity, insulin resistance,
and glucose tolerance. TET1 coordinates with HDAC1 to suppress thermogenic gene
transcription in a DNA demethylase-independent manner. These findings will open new
avenues for developing therapeutic strategies to increase energy expenditure in obesity
and related metabolic disorders.
Chapter 3 shows the previously unknown role of DNMT3A in endurance exercise skeletal
muscle mitochondrial biology. DNMT3A expression in oxidative red muscle increases
greatly following a bout of endurance exercise. Mechanistically, we reveal that ALDH1L1
serves as a novel molecular link that contributes to oxidative stress and mitochondrial
dysfunction following the loss of Dnmt3a in red muscle. This is of great importance from
the standpoint of exercise physiology, as physical activity is strongly encouraged as a key
strategy for preventing and treating a wide range of human diseases.
Chapter 4 concludes my work on the role of epigenetic enzymes TET1 and DNMT3A in
metabolism and related disorders. Further, it also presents future directions and additional
questions to get a further understanding.
