Claudia P. Coomans

Detrimental effects of constant light exposure and high-fat diet on circadian energy metabolism and insulin sensitivity

Circadian rhythm disturbances are observed in, e.g., aging and neurodegenerative diseases and are associated with an increased incidence of obesity and diabetes. We subjected male C57Bl/6J mice to constant light [12‐h light‐light (LL) cycle] to examine the effects of a disturbed circadian rhythm on energy metabolism and insulin sensitivity. In vivo electrophysiological recordings in the central pacemaker of the suprachiasmatic nuclei (SCN) revealed an immediate reduction in rhythm amplitude, stabilizing at 44% of normal amplitude values after 4 d LL. Food intake was increased (+26%) and energy expenditure decreased (–13%), and we observed immediate body weight gain (d 4: +2.4%, d 14: +5.0%). Mixed model analysis revealed that weight gain developed more rapidly in response to LL as compared to high fat. After 4 wk in LL, the circadian pattern in feeding and energy expenditure was completely lost, despite continuing low‐amplitude rhythms in the SCN and in behavior, whereas weight gain had stabilized. Hyperinsulinemic‐euglycemic clamp analysis revealed complete abolishment of normal circadian variation in insulin sensitivity in LL. In conclusion, a reduction in amplitude of the SCN, to values previously observed in aged mice, is sufficient to induce a complete loss of circadian rhythms in energy metabolism and insulin sensitivity.—Coomans, C. P., van den Berg, S. A. A., Houben, T., van Klinken, J.‐B., van den Berg, R., Pronk, A. C. M., Havekes, L. M., Romijn, J. A., Willems van Dijk, K., Biermasz, N. R., Meijer, J. H. Detrimental effects of constant light exposure and high‐fat diet on circadian energy metabolism and insulin sensitivity. FASEB J. 27, 1721–1732 (2013). www.fasebj.org

The Suprachiasmatic Nucleus Controls Circadian Energy Metabolism and Hepatic Insulin Sensitivity

Disturbances in the circadian system are associated with the development of type 2 diabetes mellitus. Here, we studied the direct contribution of the suprachiasmatic nucleus (SCN), the central pacemaker in the circadian system, in the development of insulin resistance. Exclusive bilateral SCN lesions in male C57Bl/6J mice, as verified by immunochemistry, showed a small but significant increase in body weight (+17%), which was accounted for by an increase in fat mass. In contrast, mice with collateral damage to the ventromedial hypothalamus and paraventricular nucleus showed severe obesity and insulin resistance. Mice with exclusive SCN ablation revealed a loss of circadian rhythm in activity, oxygen consumption, and food intake. Hyperinsulinemic–euglycemic clamp analysis 8 weeks after lesioning showed that the glucose infusion rate was significantly lower in SCN lesioned mice compared with sham-operated mice (−63%). Although insulin potently inhibited endogenous glucose production (−84%), this was greatly reduced in SCN lesioned mice (−7%), indicating severe hepatic insulin resistance. Our data show that SCN malfunctioning plays an important role in the disturbance of energy balance and suggest that an absence of central clock activity, in a genetically intact animal, may lead to the development of insulin resistance.

Thyroid hormone effects on whole-body energy homeostasis and tissue-specific fatty acid uptake in vivo.

The effects of thyroid hormone (TH) status on energy metabolism and tissue-specific substrate supply in vivo are incompletely understood. To study the effects of TH status on energy metabolism and tissue-specific fatty acid (FA) fluxes, we used metabolic cages as well as (14)C-labeled FA and (3)H-labeled triglyceride (TG) infusion in rats treated with methimazole and either 0 (hypothyroidism), 1.5 (euthyroidism), or 16.0 (thyrotoxicosis) microg per 100 g/d T(4) for 11 d. Thyrotoxicosis increased total energy expenditure by 38% (P = 0.02), resting energy expenditure by 61% (P = 0.002), and food intake by 18% (P = 0.004). Hypothyroidism tended to decrease total energy expenditure (10%; P = 0.064) and resting energy expenditure (12%; P = 0.025) but did not affect food intake. TH status did not affect spontaneous physical activity. Thyrotoxicosis increased fat oxidation (P = 0.006), whereas hypothyroidism decreased glucose oxidation (P = 0.035). Plasma FA concentration was increased in thyrotoxic but not hypothyroid rats. Thyrotoxicosis increased albumin-bound FA uptake in muscle and white adipose tissue (WAT), whereas hypothyroidism had no effect in any tissue studied, suggesting mass-driven albumin-bound FA uptake. During thyrotoxicosis, TG-derived FA uptake was increased in muscle and heart, unaffected in WAT, and decreased in brown adipose tissue. Conversely, during hypothyroidism TG-derived FA uptake was increased in WAT in association with increased lipoprotein lipase activity but unaffected in oxidative tissues and decreased in liver. In conclusion, TH status determines energy expenditure independently of spontaneous physical activity. The changes in whole-body lipid metabolism are accompanied by tissue-specific changes in TG-derived FA uptake in accordance with hyper- and hypometabolic states induced by thyrotoxicosis and hypothyroidism, respectively.

Prolonged daily light exposure increases body fat mass through attenuation of brown adipose tissue activity

Significance Increased light exposure has been associated with obesity in both humans and mice. In this article, we elucidate a mechanistic basis of this association by performing studies in mice. We report that prolonging daily light exposure increases adiposity by decreasing energy expenditure rather than increasing food intake or locomotor activity. This was caused by a light-exposure period-dependent attenuation of the noradrenergic activation of brown adipose tissue that has recently been shown to contribute substantially to energy expenditure by converting fatty acids and glucose into heat. Therefore, we conclude that impaired brown adipose tissue activity may mediate the relationship between increased light exposure and adiposity. Disruption of circadian rhythmicity is associated with obesity and related disorders, including type 2 diabetes and cardiovascular disease. Specifically, prolonged artificial light exposure associates with obesity in humans, although the underlying mechanism is unclear. Here, we report that increasing the daily hours of light exposure increases body adiposity through attenuation of brown adipose tissue (BAT) activity, a major contributor of energy expenditure. Mice exposed to a prolonged day length of 16- and 24-h light, compared with regular 12-h light, showed increased adiposity without affecting food intake or locomotor activity. Mechanistically, we demonstrated that prolonged day length decreases sympathetic input into BAT and reduces β3-adrenergic intracellular signaling. Concomitantly, prolonging day length decreased the uptake of fatty acids from triglyceride-rich lipoproteins, as well as of glucose from plasma selectively by BAT. We conclude that impaired BAT activity is an important mediator in the association between disturbed circadian rhythm and adiposity, and anticipate that activation of BAT may overcome the adverse metabolic consequences of disturbed circadian rhythmicity.

The suprachiasmatic nuclei as a seasonal clock.

In mammals, the suprachiasmatic nucleus (SCN) contains a central clock that synchronizes daily (i.e., 24-h) rhythms in physiology and behavior. SCN neurons are cell-autonomous oscillators that act synchronously to produce a coherent circadian rhythm. In addition, the SCN helps regulate seasonal rhythmicity. Photic information is perceived by the SCN and transmitted to the pineal gland, where it regulates melatonin production. Within the SCN, adaptations to changing photoperiod are reflected in changes in neurotransmitters and clock gene expression, resulting in waveform changes in rhythmic electrical activity, a major output of the SCN. Efferent pathways regulate the seasonal timing of breeding and hibernation. In humans, seasonal physiology and behavioral rhythms are also present, and the human SCN has seasonally rhythmic neurotransmitter levels and morphology. In summary, the SCN perceives and encodes changes in day length and drives seasonal changes in downstream pathways and structures in order to adapt to the changing seasons.

Circulating insulin stimulates fatty acid retention in white adipose tissue via KATP channel activation in the central nervous System only in insulin-sensitive mice

Insulin signaling in the central nervous system (CNS) is required for the inhibitory effect of insulin on glucose production. Our aim was to determine whether the CNS is also involved in the stimulatory effect of circulating insulin on the tissue-specific retention of fatty acid (FA) from plasma. In wild-type mice, hyperinsulinemic-euglycemic clamp conditions stimulated the retention of both plasma triglyceride-derived FA and plasma albumin-bound FA in the various white adipose tissues (WAT) but not in other tissues, including brown adipose tissue (BAT). Intracerebroventricular (ICV) administration of insulin induced a similar pattern of tissue-specific FA partitioning. This effect of ICV insulin administration was not associated with activation of the insulin signaling pathway in adipose tissue. ICV administration of tolbutamide, a KATP channel blocker, considerably reduced (during hyperinsulinemic-euglycemic clamp conditions) and even completely blocked (during ICV administration of insulin) WAT-specific retention of FA from plasma. This central effect of insulin was absent in CD36-deficient mice, indicating that CD36 is the predominant FA transporter in insulin-stimulated FA retention by WAT. In diet-induced insulin-resistant mice, these stimulating effects of insulin (circulating or ICV administered) on FA retention in WAT were lost. In conclusion, in insulin-sensitive mice, circulating insulin stimulates tissue-specific partitioning of plasma-derived FA in WAT in part through activation of KATP channels in the CNS. Apparently, circulating insulin stimulates fatty acid uptake in WAT but not in BAT, directly and indirectly through the CNS.

Stimulatory Effect of Insulin on Glucose Uptake by Muscle Involves the Central Nervous System in Insulin-Sensitive Mice

OBJECTIVE Insulin inhibits endogenous glucose production (EGP) and stimulates glucose uptake in peripheral tissues. Hypothalamic insulin signaling is required for the inhibitory effects of insulin on EGP. We examined the contribution of central insulin signaling on circulating insulin–stimulated tissue-specific glucose uptake. RESEARCH DESIGN AND METHODS Tolbutamide, an inhibitor of ATP-sensitive K+ channels (KATP channels), or vehicle was infused into the lateral ventricle in the basal state and during hyperinsulinemic-euglycemic conditions in postabsorptive, chow-fed C57Bl/6J mice and in postabsorptive C57Bl/6J mice with diet-induced obesity. Whole-body glucose uptake was measured by d-[14C]glucose kinetics and tissue-specific glucose uptake by 2-deoxy-d-[3H]glucose uptake. RESULTS During clamp conditions, intracerebroventricular administration of tolbutamide impaired the ability of insulin to inhibit EGP by ∼20%. In addition, intracerebroventricular tolbutamide diminished insulin-stimulated glucose uptake in muscle (by ∼59%) but not in heart or adipose tissue. In contrast, in insulin-resistant mice with diet-induced obesity, intracerebroventricular tolbutamide did not alter the effects of insulin during clamp conditions on EGP or glucose uptake by muscle. CONCLUSIONS Insulin stimulates glucose uptake in muscle in part through effects via KATP channels in the central nervous system, in analogy with the inhibitory effects of insulin on EGP. High-fat diet–induced obesity abolished the central effects of insulin on liver and muscle. These observations stress the role of central insulin resistance in the pathophysiology of diet-induced insulin resistance.

Lack of exercise leads to significant and reversible loss of scale invariance in both aged and young mice

Significance Healthy systems are characterized by scale invariance across multiple timescales. We investigated whether loss of scale invariance that occurs with aging and disease can be counteracted by exercise, in four age groups of mice. Surprisingly, we observed that lack of exercise was detrimental not only in old but also in young mice, raising the possibility of an unforeseen role of behavioral activity for health in aged and young subjects alike. Moreover, we show that scale invariance could be restored by high levels of exercise, even in old animals. The World Health Organization has pinpointed lack of exercise and a sedentary lifestyle as a major risk factor for various diseases. Our measures may guide health programs. In healthy humans and other animals, behavioral activity exhibits scale invariance over multiple timescales from minutes to 24 h, whereas in aging or diseased conditions, scale invariance is usually reduced significantly. Accordingly, scale invariance can be a potential marker for health. Given compelling indications that exercise is beneficial for mental and physical health, we tested to what extent a lack of exercise affects scale invariance in young and aged animals. We studied six or more mice in each of four age groups (0.5, 1, 1.5, and 2 y) and observed an age-related deterioration of scale invariance in activity fluctuations. We found that limiting the amount of exercise, by removing the running wheels, leads to loss of scale-invariant properties in all age groups. Remarkably, in both young and old animals a lack of exercise reduced the scale invariance in activity fluctuations to the same level. We next showed that scale invariance can be restored by returning the running wheels. Exercise during the active period also improved scale invariance during the resting period, suggesting that activity during the active phase may also be beneficial for the resting phase. Finally, our data showed that exercise had a stronger influence on scale invariance than the effect of age. The data suggest that exercise is beneficial as revealed by scale-invariant parameters and that, even in young animals, a lack of exercise leads to strong deterioration in these parameters.

USF1 deficiency activates brown adipose tissue and improves cardiometabolic health

Deficiency of USF1 protects against obesity, insulin resistance, and cardiovascular disease in mice and humans, and induces brown adipose tissue to burn triglycerides and glucose. Boosting metabolism with low USF1 Laurila et al. show that a deficiency of the transcription factor USF1 protects against obesity, insulin resistance, and cardiovascular disease. Even when fed a high-fat diet, USF1-deficient mice stayed lean and maintained a beneficial lipid profile with low triglycerides and high high-density lipoprotein (HDL) cholesterol. The mice had elevated energy expenditure because their brown adipose tissue was more active. In USF1-deficient mice, glucose and lipids were rapidly cleared from the circulation to be burned by brown fat. In humans, individuals with reduced USF1 expression also had higher plasma HDL cholesterol and lower triglycerides, and were more insulin-sensitive and less prone to hardening of the arteries. These findings identify USF1 as a potential therapeutic target for treating metabolic and cardiac diseases. USF1 (upstream stimulatory factor 1) is a transcription factor associated with familial combined hyperlipidemia and coronary artery disease in humans. However, whether USF1 is beneficial or detrimental to cardiometabolic health has not been addressed. By inactivating USF1 in mice, we demonstrate protection against diet-induced dyslipidemia, obesity, insulin resistance, hepatic steatosis, and atherosclerosis. The favorable plasma lipid profile, including increased high-density lipoprotein cholesterol and decreased triglycerides, was coupled with increased energy expenditure due to activation of brown adipose tissue (BAT). Usf1 inactivation directs triglycerides from the circulation to BAT for combustion via a lipoprotein lipase–dependent mechanism, thus enhancing plasma triglyceride clearance. Mice lacking Usf1 displayed increased BAT-facilitated, diet-induced thermogenesis with up-regulation of mitochondrial respiratory chain complexes, as well as increased BAT activity even at thermoneutrality and after BAT sympathectomy. A direct effect of USF1 on BAT activation was demonstrated by an amplified adrenergic response in brown adipocytes after Usf1 silencing, and by augmented norepinephrine-induced thermogenesis in mice lacking Usf1. In humans, individuals carrying SNP (single-nucleotide polymorphism) alleles that reduced USF1 mRNA expression also displayed a beneficial cardiometabolic profile, featuring improved insulin sensitivity, a favorable lipid profile, and reduced atherosclerosis. Our findings identify a new molecular link between lipid metabolism and energy expenditure, and point to the potential of USF1 as a therapeutic target for cardiometabolic disease.

Apolipoprotein CI enhances the biological response to LPS via the CD14/TLR4 pathway by LPS-binding elements in both its N- and C-terminal helix

Timely sensing of lipopolysaccharide (LPS) is critical for the host to fight invading Gram-negative bacteria. We recently showed that apolipoprotein CI (apoCI) (apoCI1–57) avidly binds to LPS, involving an LPS-binding motif (apoCI48–54), and thereby enhances the LPS-induced inflammatory response. Our current aim was to further elucidate the structure and function relationship of apoCI with respect to its LPS-modulating characteristics and to unravel the mechanism by which apoCI enhances the biological activity of LPS. We designed and generated N- and C-terminal apoCI-derived peptides containing varying numbers of alternating cationic/hydrophobic motifs. ApoCI1–38, apoCI1–30, and apoCI35–57 were able to bind LPS, whereas apoCI1–23 and apoCI46–57 did not bind LPS. In line with their LPS-binding characteristics, apoCI1–38, apoCI1–30, and apoCI35–57 prolonged the serum residence of 125I-LPS by reducing its association with the liver. Accordingly, both apoCI1–30 and apoCI35–57 enhanced the LPS-induced TNFα response in vitro (RAW 264.7 macrophages) and in vivo (C57Bl/6 mice). Additional in vitro studies showed that the stimulating effect of apoCI on the LPS response resembles that of LPS-binding protein (LBP) and depends on CD14/ Toll-like receptor 4 signaling. We conclude that apoCI contains structural elements in both its N-terminal and C-terminal helix to bind LPS and to enhance the proinflammatory response toward LPS via a mechanism similar to LBP.