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The role of the clock in lipid metabolism
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
In mammals, the circadian clock coordinates multiple behavioural and physical processes, including energy homeostasis. At the centre of these rhythms lies the circadian clock machinery, a precisely coordinated transcription-translation feedback system required to maintain the correct time. Metabolic homeostasis requires accurate and coordinated circadian timing within individual cells and tissues of the body. Moreover, recent evidence has shown that the coupling of circadian and metabolic circuits involves reciprocal regulatory feedback. In line with this, mounting evidence suggests that disruption of the clock contributes to the development of obesity and its comorbidities. This is particularly concerning given that modern lifestyles often undermine our bodies’ clock. However, the casual mechanisms which link circadian disruption to metabolic disease are not well defined. This work aims to gain a further understanding of clock control of metabolic homeostasis and especially regulation of lipid metabolism.This work uses dietary challenge to determine which peripheral clocks and downstream metabolic pathways are particularly susceptible to diet induced obesity (DIO). We demonstrate that although behavioural rhythmicity was maintained in DIO, gene expression profiling revealed tissue-specific alteration to the phase and amplitude of the molecular clockwork. Clock function was most significantly attenuated in visceral white adipose tissue (WAT) of DIO mice, and was coincident with elevated tissue inflammation, and dysregulation of clock-coupled metabolic regulators PPARα/γ.The rhythmic expression of Rev-erbα, a nuclear receptor involved in the circadian clock, was particularly affected in DIO mice. This study uses the Rev-erbα-/- mouse to explore clock-metabolic coupling, specifically lipid metabolism. In line with published work, Rev-erbα-/- mice exhibit an obese phenotype with associated upregulation in gWAT of lipogenic (Dgat2, Fasn) and fatty acid liberation (Lpl) genes. Differences in fat mobilization are observed as Rev-erbα-/- mice show a heightened insulin stimulated lipogenic drive and an attenuation of the lipolytic drive in the fasted state, suggesting an increased propensity for fat accumulation. The role of the clock was further investigated in adipose tissue by deletion of Bmal1 (clock ablation) or Rev-erbα (clock manipulation) specifically in adipocytes using Cre-Lox methodology. AdipoCREBmal1flox/flox mice showed attenuated feeding rhythms, indicating a direct effect of the adipocyte circadian clock on hypothalamic feeding centres and severe dysregulation of metabolic genes. However, AdipoCRERev-erbαflox/flox displayed very little phenotypic difference compared to control littermates, suggesting that global loss of Rev-erbα may have reinforcing metabolic consequences.This work suggests a key role of the clock in lipid handling and the pathogenesis of obesity. Insights into this link may lead to novel targets for treating both obesity and metabolic complications.