Cross-regulation of Immunometabolism and Circadian Pathways in Obesity Pathophysiology

Obesity and diabetes are increased among individuals subjected to shiftwork, reduced sleep, and social jetlag. High fat diet (HFD) misaligns intrinsic circadian cycles with the light/dark cycle and alters oscillations of metabolic genes in visceral adipose tissue, a key site in the control of energy balance, glucose regulation, and inflammatory disorders. Conversely, restricting HFD to the dark period only realigns meal time with circadian rhythms and enhances insulin sensitivity, promoting healthful obesity. How circadian disruption in visceral adipose tissue contributes to obesity pathophysiology remains unknown. In exciting new data, we show strong day/night rhythms in adipocyte mitochondrial respiration with maximal uncoupling at the onset of the active period that is dependent upon a functional clock. Further, 13C-glucose entry into the tricarboxylic acid (TCA) cycle is also highest at the beginning of the active period, indicating autonomous circadian control of WAT metabolic flux across the day/night cycle. Surprisingly, in Bmal1-/- adipocytes, we observe reprogramming of adipocyte metabolism with increased 13C labeling in succinate and reduced levels of other TCA intermediates, a signature of stress and ROS accumulation. Here we seek to test the hypothesis that the circadian clock controls energy flux within visceral adipose tissue at the level of fuel entry into the TCA cycle through a process disrupted by HFD. In Aim 1, we will test the hypothesis that circadian coordination of feeding time and adipose energy utilization cycles promote healthful adipose expansion using genetic lineage tracer animals and metabolic phenotyping. We will assess adipose tissue remodeling and 13C glucose flux into the TCA cycle in addition to lipid and organic acids across the light-dark cycle in mice fed regular chow or HFD either ad lib or time-restricted to the light (misttimed feeding) or dark (optimal time feeding) period at thermoneutrality (30oC). We will also test the requirement of the WAT clock and the effect of adipose-specific BMAL1 overexpression in HFD on adipose remodeling, inflammation, fibrosis, and glucose homeostasis. Aim 1 results will establish the interplay between the WAT clock and feeding time in energy flux, metabolic health, and capacity for healthful adipose expansion, particularly with the new addition of the light-only feeding group for comparison to the dark-only and AL cohorts. In Aim 2, we will test the hypothesis that HFD abrogates circadian energetic cycles in visceral adipose tissue and induces epigenetic remodeling towards a proinflammatory cell fate. We will perform tandem chromatin and expression profiling in adipocytes to identify the time signature and molecular drivers of visceral WAT remodeling in ad lib and light- and dark-only-fed mice on HFD. Finally, we will examine whether BMAL1 overexpression during HFD preserves the healthful chromatin landscape of regular chow fed mice. Results of Aim 2 will determine how clock control of chromatin activity and transcription contributes to healthful obesity. Collectively, these studies will define the role of time-of-day in adipogenesis and nutrient flux, uncovering novel targets to combat the immunometabolic complications of obesity and circadian disruption.