Date of Award
Doctor of Philosophy
The circadian clock system is essential for mammals to adapt to environmental conditions such as light-dark cycles and to manage the optimal timing for cyclical physiological processes, including sleep-wakefulness, fasting-feeding and multiple aspects of metabolism. The circadian timing system is arranged in hierarchical fashion, with the master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus, acting as the pace-maker and maintaining synchrony among clocks found in every organ system throughout the body. The core molecular clock consists of two interconnected transcriptional-translational feedback loops comprising core clock components: Brain and Muscle Arnt-Like protein1 (BMAL1), Circadian Locomoter Output Cycles Kaput (CLOCK), Period (PER), Cryptochrome (CRY), Nuclear Receptor family1 D1 (REV-ERB) and Retinoic acid-related Orphan Receptor (ROR). Circadian clock disruptions, through environmental changes to light-dark cycles or through genetic modification of core clock genes cause metabolic disturbances. Aryl hydrocarbon receptor (AhR), also known as the dioxin receptor, mediates systemic metabolism and toxicity of a range of environmental contaminants. Epidemiological studies have established a positive correlation between exposure to dioxins and other synthetic organic chemicals and metabolic diseases such as diabetes and dyslipidemia. Animal research have supported these findings by showing that AhR activation has detrimental effects on glucose and lipid homeostasis. Mechanisms for AhR-mediated metabolic dysfunction remain unknown. Coincidently, both AhR and many core clock components, for example BMAL1 and CLOCK, belong to the basic helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) domain family. Previous studies have linked AhR signaling to circadian rhythm. Importantly, activation of the AhR can impair transcriptional activity of the CLOCK: BMAL1 heterodimer in cultured cells. However, because the AhR is differentially expressed among the body’s tissues, its activation may have distinctive, tissue-specific effects on the hierarchical circadian clock oscillators in vivo, which have not been investigated. Therefore, this dissertation is designed to examine the short-term and long-term effects of AhR activation on circadian clocks and downstream clock-regulated metabolic pathways. Specifically, this dissertation is aimed to explore how acute and chronic activation of AhR affects rhythmic aspects of behavior, as well as clock-controlled glucose and lipid metabolism. In the acute AhR activation model, a single dose of the synthetic AhR agonist, β-Naphthoflavone (BNF), was administered to C57Bl/6J wild type mice. Circadian behavior was monitored before and after acute AhR activation. Circadian expression of core clock genes, as well as key metabolic genes in the liver, skeletal muscle and adipose tissue were examined. Compared to the vehicle group, BNF-treated mice displayed a transient loss of behavioral rhythmicity and delayed activity onset, which suggest that acute activation of AhR acts directly on the central clock, the suprachiasmatic nucleus of the hypothalamus. In contrast, circadian oscillations of core clock genes were not eliminated in the peripheral tissues (liver, skeletal muscle and adipose tissue), but changes were observed in their rhythmic amplitude or phase. Rhythms of key enzymes related to glucose and lipid metabolic pathways in the liver and adipose were decreased while those in the skeletal muscle were increased. These results indicate that acute AhR activation affects the central clock and peripheral clock differently. Moreover, acute AhR activation significantly dampened the rhythm of genes involved in lipogenesis, lipolysis and lipid storage. In the chronic AhR activation model, C57Bl6/J mice were exposed to BNF for a month to explore whether long-term AhR activation can cause bigger disruption of circadian clocks and lead to metabolic dysfunction in vivo. Unexpectedly, general circadian behavior was maintained although after each dose of BNF there was a consistent, transient loss of behavioral rhythmicity and significant phase delay (about 30 minutes) in BNF-treated mice. Liver and skeletal muscle clocks were not significantly altered after 4 doses of BNF, and the in-phase oscillations of core clock genes in liver and skeletal muscle suggested a functional SCN as well as the two peripheral clocks. However, the adipose clock was significantly disrupted. Altered clock-regulated rhythms in lipid metabolism genes are associated with impaired lipid storage functions in white adipose tissues and deregulated plasma lipids in BNF-treated mice. The results of acute and chronic AhR activation support a significant interaction of AhR with the circadian clock system. Although future studies are needed to elucidate how AhR signaling specifically interacts with the clock in different cell types, the current research establishes a model for studying the crosstalk between AhR and circadian rhythm and provides new perspectives into the mechanisms of metabolic diseases correlated with exposure to synthetic organic chemicals.
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