Expression of clock genes in human subcutaneous and visceral adipose tissues

Visceral adiposity and expression of clock genes in peripheral blood mononuclear cells: A pilot study

Increasing evidence suggests a close interrelationship between disrupted circadian rhythms and obesity and metabolic disturbances. In particular, abdominal obesity, which contributes to the pathogenesis of metabolic disease, is associated with disrupted clock gene expression. However, little is known about the relationship between clock gene expression and accurate computed tomography (CT)-based measurements of visceral adiposity. Therefore, we examined the relationship between expression of clock genes in peripheral blood mononuclear cells (PBMCs) with visceral and subcutaneous adiposity in 75 healthy overweight or obese individuals. PBMCs were obtained from blood samples collected at 8 AM, and gene expression was analyzed by real-time reverse transcription polymerase chain reaction. Visceral and subcutaneous adiposity were measured by CT. Our results showed that visceral fat area was significantly positively correlated with BMAL1 and CRY1 mRNA levels and significantly negatively correlated with CLOCK, PER2, PER3 and CRY2 mRNA levels. In contrast, subcutaneous fat area was not correlated with the expression of any of the clock genes analyzed. After adjusting for multiple variables, visceral fat area was significantly associated with the expression of BMAL1, PER2 and CRY1. Taken together, our results indicate that visceral adiposity, but not subcutaneous adiposity, correlates with expression of clock genes in PBMCs.

Synchronized human skeletal myotubes of lean, obese and type 2 diabetic patients maintain circadian oscillation of clock genes

Cell and animal studies have demonstrated that circadian rhythm is governed by autonomous rhythmicity of clock genes. Although disturbances in circadian rhythm have been implicated in metabolic disease development, it remains unknown whether muscle circadian rhythm is altered in human models of type 2 diabetes. Here we used human primary myotubes (HPM) to investigate if rhythmicity of clock- and metabolic gene expression is altered in donors with obesity or type 2 diabetes compared to metabolically healthy donors. HPM were obtained from skeletal muscle biopsies of four groups: type 2 diabetic patients and their BMI- and age-matched obese controls and from lean, healthy and young endurance trained athletes and their age-matched sedentary controls. HPM were differentiated for 7 days before synchronization by serum shock followed by gene expression profiling over the next 72 hours. HPM display robust circadian rhythms in clock genes, but REVERBA displayed dampened rhythmicity in type 2 diabetes. Furthermore, rhythmicity in NAMPT and SIRT1 expression was only observed in HPM from trained athletes. Rhythmicity in expression of key-regulators of carbohydrate and lipid metabolism was modest. We demonstrate that in human skeletal muscle REVERBA/B, NAMPT and SIRT1 circadian rhythms are affected in donors of sedentary life style and poor health status.

The effects of light at night on circadian clocks and metabolism

Most organisms display endogenously produced ∼ 24-hour fluctuations in physiology and behavior, termed circadian rhythms. Circadian rhythms are driven by a transcriptional-translational feedback loop that is hierarchically expressed throughout the brain and body, with the suprachiasmatic nucleus of the hypothalamus serving as the master circadian oscillator at the top of the hierarchy. Appropriate circadian regulation is important for many homeostatic functions including energy regulation. Multiple genes involved in nutrient metabolism display rhythmic oscillations, and metabolically related hormones such as glucagon, insulin, ghrelin, leptin, and corticosterone are released in a circadian fashion. Mice harboring mutations in circadian clock genes alter feeding behavior, endocrine signaling, and dietary fat absorption. Moreover, misalignment between behavioral and molecular circadian clocks can result in obesity in both rodents and humans. Importantly, circadian rhythms are most potently synchronized to the external environment by light information and exposure to light at night potentially disrupts circadian system function. Since the advent of electric lights around the turn of the 20th century, exposure to artificial and irregular light schedules has become commonplace. The increase in exposure to light at night parallels the global increase in the prevalence of obesity and metabolic disorders. In this review, we propose that exposure to light at night alters metabolic function through disruption of the circadian system. We first provide an introduction to the circadian system, with a specific emphasis on the effects of light on circadian rhythms. Next we address interactions between the circadian system and metabolism. Finally, we review current experimental and epidemiological work directly associating exposure to light at night and metabolism.