The diurnal rhythms of cholesterol metabolism and plasma clearance of model chylomicrons: comparison of normal and genetically hypercholesterolemic rats (RICO)

Circadian Influence on Metabolism and Inflammation in Atherosclerosis

Many aspects of human health and disease display daily rhythmicity. The brain's suprachiasmic nucleus, which interprets recurring external stimuli, and autonomous molecular networks in peripheral cells together, set our biological circadian clock. Disrupted or misaligned circadian rhythms promote multiple pathologies including chronic inflammatory and metabolic diseases such as atherosclerosis. Here, we discuss studies suggesting that circadian fluctuations in the vessel wall and in the circulation contribute to atherogenesis. Data from humans and mice indicate that an impaired molecular clock, disturbed sleep, and shifting light-dark patterns influence leukocyte and lipid supply in the circulation and alter cellular behavior in atherosclerotic lesions. We propose that a better understanding of both local and systemic circadian rhythms in atherosclerosis will enhance clinical management, treatment, and public health policy.

Circadian rhythms in glucose and lipid metabolism in nocturnal and diurnal mammals

Most aspects of energy metabolism display clear variations during day and night. This daily rhythmicity of metabolic functions, including hormone release, is governed by a circadian system that consists of the master clock in the suprachiasmatic nuclei of the hypothalamus (SCN) and many secondary clocks in the brain and peripheral organs. The SCN control peripheral timing via the autonomic and neuroendocrine system, as well as via behavioral outputs. The sleep-wake cycle, the feeding/fasting rhythm and most hormonal rhythms, including that of leptin, ghrelin and glucocorticoids, usually show an opposite phase (relative to the light-dark cycle) in diurnal and nocturnal species. By contrast, the SCN clock is most active at the same astronomical times in these two categories of mammals. Moreover, in both species, pineal melatonin is secreted only at night. In this review we describe the current knowledge on the regulation of glucose and lipid metabolism by central and peripheral clock mechanisms. Most experimental knowledge comes from studies in nocturnal laboratory rodents. Nevertheless, we will also mention some relevant findings in diurnal mammals, including humans. It will become clear that as a consequence of the tight connections between the circadian clock system and energy metabolism, circadian clock impairments (e.g., mutations or knock-out of clock genes) and circadian clock misalignments (such as during shift work and chronic jet-lag) have an adverse effect on energy metabolism, that may trigger or enhancing obese and diabetic symptoms.

Lipids around the Clock: Focus on Circadian Rhythms and Lipid Metabolism

Disorders of lipid and lipoprotein metabolism and transport are responsible for the development of a large spectrum of pathologies, ranging from cardiovascular diseases, to metabolic syndrome, even to tumour development. Recently, a deeper knowledge of the molecular mechanisms that control our biological clock and circadian rhythms has been achieved. From these studies it has clearly emerged how the molecular clock tightly regulates every aspect of our lives, including our metabolism. This review analyses the organisation and functioning of the circadian clock and its relevance in the regulation of physiological processes. We also describe metabolism and transport of lipids and lipoproteins as an essential aspect for our health, and we will focus on how the circadian clock and lipid metabolism are greatly interconnected. Finally, we discuss how a deeper knowledge of this relationship might be useful to improve the recent spread of metabolic diseases.

Postprandial metabolic heterogeneity in men with primary dyslipidaemia

INTRODUCTION

Familial combined hyperlipidaemia (FCH) and familial hypercholesterolaemia (FH) have been strongly linked to premature coronary artery disease. Postprandial hypertriglyceridaemia is also associated with atherosclerotic disease. We evaluated the postprandial lipaemia in men with FCH and FH and compared them to a group of healthy men.

MATERIAL AND METHODS

The study population consisted of 83 men: 34 FCH, 29 FH and 20 healthy. The FCH and FH groups were further divided into five subgroups, according to their lipid phenotype: FCH-IIA (n = 13), FCH-IIB (n = 10), FCH-IV (n = 11), FH-IIA (n = 21) and FH-IIB (n = 8). Postprandial lipaemia was evaluated by the areas under the curve for triglyceride (TG) concentrations (TG-AUC).

RESULTS

The TG levels after oral fat tolerance test were significantly higher in FCH, compared to FH and healthy groups (TG-AUC in mg/dl/h; 2678 ±1415 vs. 1503 ±1147 and 1011 ±652 respectively, p < 0.001). The postprandial response was higher in FCH-IV and FCH-IIB, compared to FCH-IIA (TG-AUC in mg/dl/h; 3220 ±824 or 3409 ±770 vs. 1863 ±577 respectively, p < 0.001, for both comparisons). The FCH-IIA group showed higher postprandial TG levels when compared to FH-IIA (TG-AUC in mg/dl/h; 1863 ±577 vs. 1374 ±428 respectively, p = 0.008). There were no significant differences between FH-IIB and FCH-IIB subgroups. There was a significant correlation (r = 0.907, p < 0.001) between the postprandial TG-AUC and fasting TG levels in all FCH subjects.

CONCLUSIONS

All phenotypes of FCH and the FH IIB phenotype demonstrate an exaggerated postprandial response that could partially contribute to the high cardiovascular risk. These patients should be identified and treated early with the appropriate hypolipidaemic agents.

Clock genes, intestinal transport and plasma lipid homeostasis

Light and food are two major environmental factors that impact daily life. Light entrainment is centrally controlled by suprachiasmatic nuclei of the hypothalamus. Food entrainment might require cooperation between the intestine and dorsomedial hypothalamus. Clock genes that are essential for light entrainment also play a part in food entrainment. Understanding the role of clock genes in the entrainment of intestinal functions, as well as in gut-brain communication during food entrainment, will enhance our understanding of gastrointestinal and metabolic disorders. This review highlights recent studies examining light- and food-entrained regulation of plasma lipids and of various intestinal activities and offers insight into the role of the intestine in food entrainment.

Diurnal regulation of microsomal triglyceride transfer protein and plasma lipid levels

Plasma lipids are maintained within a narrow physiologic range and exhibit circadian rhythmicity. Plasma triglyceride and cholesterol levels were high in the night due to changes in apolipoprotein B-lipoproteins in ad libitum fed rats and mice maintained in a 12-h photoperiod. Absorption of [(3)H]triolein or [(3)H]cholesterol was higher at 2400 h than at 1200 h, indicating that intestinal lipoprotein production shows diurnal variation. Moreover, intestinal microsomal triglyceride transfer protein (MTP) activity, protein, mRNA, and gene transcription showed diurnal variations and were high at 2400 h. Similar to the small intestine, hepatic MTP activity, protein, and mRNA levels also changed significantly within a day. MTP was induced in fasted animals soon after refeeding. When mice were subjected to restricted feeding, MTP expression was high at the expected time of food availability. In contrast, extended exposures to light and dark completely abolished rhythmicity in MTP expression and plasma lipid levels. These studies show that MTP expression and plasma lipid undergo diurnal regulation and exhibit peaks and nadirs at similar times and suggest that diurnal modulation of MTP is a major determinant of daily changes in plasma lipids. Furthermore, environmental factors, such as food and light, play an important role in MTP regulation.

The influence of natural menopause on postprandial lipemia in heterozygotes for familial hypercholesterolemia

BACKGROUND

Heterozygous familial hypercholesterolemia (hFH) is a genetic disease that leads to premature atherosclerosis. Natural menopause leads to an adverse lipid profile and an enhanced risk of coronary heart disease (CHD). Raised plasma triglyceride (TG) levels also contribute to the risk of vascular events. The aim of this study was to evaluate the postprandial TG levels (after a standardized fatty meal) in premenopausal and postmenopausal women with hFH.

METHODS

Thirty-three Greek women with hFH were divided into the premenopausal group--n = 16, mean age 34(SD = 7), mean total cholesterol = 330(30) mg/dl--and the postmenopausal group--n = 17, mean age 62(5), mean total cholesterol = 346(63) mg/dl. Plasma TG concentrations were measured before and 2, 4, 6, and 8 hours after a standardized fat load. A value of >219 mg/dl (2.5 mmol/L) was taken as an abnormal response to the fat load, according to our previous studies.

RESULTS

Postmenopausal women had higher TG levels at 2 (p = 0.001), 4 (p = 0.003), 6 (p = 0.003), and 8 hours (p = 0.005) after the fatty meal compared to premenopausal women. Forty-one percent of postmenopausal hFH women had abnormal TG response (hFH-A) after a fatty meal, and such women had higher fasting TG levels than postmenopausal hFH women with a normal response to the fatty meal (hFH-N) (p = 0.0014).

CONCLUSIONS

Women with hFH tend to have an abnormal TG response to a fatty meal after the menopause. Fasting TG levels may be able to predict the abnormal response to a fatty meal.