Daily rhythms of glycogen synthetase and phosphorylase activities in rat liver: influence of food and light

Metabolic reconstitution of germ-free mice by a gnotobiotic microbiota varies over the circadian cycle

The capacity of the intestinal microbiota to degrade otherwise indigestible diet components is known to greatly improve the recovery of energy from food. This has led to the hypothesis that increased digestive efficiency may underlie the contribution of the microbiota to obesity. OligoMM12-colonized gnotobiotic mice have a consistently higher fat mass than germ-free (GF) or fully colonized counterparts. We therefore investigated their food intake, digestion efficiency, energy expenditure, and respiratory quotient using a novel isolator-housed metabolic cage system, which allows long-term measurements without contamination risk. This demonstrated that microbiota-released calories are perfectly balanced by decreased food intake in fully colonized versus gnotobiotic OligoMM12 and GF mice fed a standard chow diet, i.e., microbiota-released calories can in fact be well integrated into appetite control. We also observed no significant difference in energy expenditure after normalization by lean mass between the different microbiota groups, suggesting that cumulative small differences in energy balance, or altered energy storage, must underlie fat accumulation in OligoMM12 mice. Consistent with altered energy storage, major differences were observed in the type of respiratory substrates used in metabolism over the circadian cycle: In GF mice, the respiratory exchange ratio (RER) was consistently lower than that of fully colonized mice at all times of day, indicative of more reliance on fat and less on glucose metabolism. Intriguingly, the RER of OligoMM12-colonized gnotobiotic mice phenocopied fully colonized mice during the dark (active/eating) phase but phenocopied GF mice during the light (fasting/resting) phase. Further, OligoMM12-colonized mice showed a GF-like drop in liver glycogen storage during the light phase and both liver and plasma metabolomes of OligoMM12 mice clustered closely with GF mice. This implies the existence of microbiota functions that are required to maintain normal host metabolism during the resting/fasting phase of circadian cycle and which are absent in the OligoMM12 consortium.

Why meals during resting time cause fat accumulation in mammals? Mathematical modeling of circadian regulation on glucose metabolism

Synchronization of metabolic rhythms regulated by circadian clock and meal timing is essential for maintaining nutrient homeostasis in response to fluctuating food intake in animals. Despite numerous experimental findings on the involvement of circadian regulation of glucose and lipid metabolism, the optimal regulatory strategy for the maintenance of energy homeostasis remains poorly defined. A mathematical framework is useful to assess the circadian regulation of glycogen production/breakdown and de novo lipogenesis/lipolysis by evaluating the contribution of time of the day-dependent activation or the repression of each metabolic process in the maintenance of energy homeostasis. Here, we present a mathematical model that describes the dynamics of glycogen and triglyceride contents, two major forms of energy storage in the body that provide the fuel needed during different phases of food deprivation. By changing peak phases of glycogenesis and fat synthesis, we searched for the optimal phase set that minimizes the risks of two types of possible metabolic dysfunctions: (1) high blood glucose and (2) energy exhaustion. Based on the optimal phase set, we compared the level of fat accumulation between meal timing in the active and resting periods. Our results showed that an increased fat accumulation by food intake in the resting period can be the byproduct of minimizing energy homeostasis risks in the synchronized feeding schedule that animals adopt in nature. Our finding will be useful to schedule an optimal meal timing to prevent metabolic diseases caused by misalignment of biological and social time in modern society.

Circadian clock regulation of the glycogen synthase (gsn) gene by WCC is critical for rhythmic glycogen metabolism in Neurospora crassa

Circadian rhythms enable organisms to anticipate daily environmental cycles and control the timing of numerous biological processes, including metabolism, to optimize the health and survival of organisms. Glycogen metabolism is a conserved glucose homeostatic process; however, the molecular mechanisms linking the circadian clock and glycogen metabolism remain largely unknown. In this report, we demonstrate that circadian clock-dependent transcriptional regulation of glycogen synthase, gsn, regulates circadian oscillations of GSN protein and glycogen accumulation in the model filamentous fungus, Neurospora crassa.

Circadian clocks generate rhythms in cellular functions, including metabolism, to align biological processes with the 24-hour environment. Disruption of this alignment by shift work alters glucose homeostasis. Glucose homeostasis depends on signaling and allosteric control; however, the molecular mechanisms linking the clock to glucose homeostasis remain largely unknown. We investigated the molecular links between the clock and glycogen metabolism, a conserved glucose homeostatic process, in Neurospora crassa. We find that glycogen synthase (gsn) mRNA, glycogen phosphorylase (gpn) mRNA, and glycogen levels, accumulate with a daily rhythm controlled by the circadian clock. Because the synthase and phosphorylase are critical to homeostasis, their roles in generating glycogen rhythms were investigated. We demonstrate that while gsn was necessary for glycogen production, constitutive gsn expression resulted in high and arrhythmic glycogen levels, and deletion of gpn abolished gsn mRNA rhythms and rhythmic glycogen accumulation. Furthermore, we show that gsn promoter activity is rhythmic and is directly controlled by core clock component white collar complex (WCC). We also discovered that WCC-regulated transcription factors, VOS-1 and CSP-1, modulate the phase and amplitude of rhythmic gsn mRNA, and these changes are similarly reflected in glycogen oscillations. Together, these data indicate the importance of clock-regulated gsn transcription over signaling or allosteric control of glycogen rhythms, a mechanism that is potentially conserved in mammals and critical to metabolic homeostasis.

Rhythmic Food Intake Drives Rhythmic Gene Expression More Potently than the Hepatic Circadian Clock in Mice

Every mammalian tissue exhibits daily rhythms in gene expression to control the activation of tissue-specific processes at the most appropriate time of the day. Much of this rhythmic expression is thought to be driven cell autonomously by molecular circadian clocks present throughout the body. By manipulating the daily rhythm of food intake in the mouse, we here show that more than 70% of the cycling mouse liver transcriptome loses rhythmicity under arrhythmic feeding. Remarkably, core clock genes are not among the 70% of genes losing rhythmic expression, and their expression continues to exhibit normal oscillations in arrhythmically fed mice. Manipulation of rhythmic food intake also alters the timing of key signaling and metabolic pathways without altering the hepatic clock oscillations. Our findings thus demonstrate that systemic signals driven by rhythmic food intake significantly contribute to driving rhythms in liver gene expression and metabolic functions independently of the cell-autonomous hepatic clock.

Temporal partitioning of cardiac metabolism by the cardiomyocyte circadian clock

NEW FINDINGS

What is the topic of this review? This review highlights temporal partitioning of cardiac metabolism by the cardiomyocyte circadian clock. What advances does it highlight? Advances include: 1) cardiac glucose utilization peaks during the active period to meet increased energetic demands at this time; 2) synthesis of glycogen and triglyceride peak in the heart during the latter half of the active period, likely in anticipation of the upcoming sleep/fasting period; and 3) protein turnover increases in the heart at the beginning of the sleep phase, probably to promote growth and repair at this time. Cell-autonomous circadian clocks have emerged as crucial mediators of 24 h rhythms in cellular processes. In doing so, these molecular timekeepers confer the selective advantage of anticipation, allowing cells and organs to prepare for stimuli and stresses before their onset. The heart is subjected to dramatic fluctuations in energetic demand and nutrient supply in association with sleep-wake and fasting-feeding cycles. Recent studies suggest that the cardiomyocyte circadian clock orchestrates daily rhythms in both oxidative and non-oxidative glucose and fatty acid metabolism, as well as protein turnover. Here, I review this evidence and discuss whether disruption of these rhythms can contribute to cardiovascular disease.

Pregnancy-induced changes in the circadian expression of hepatic clock genes: implications for maternal glucose homeostasis

Adaptations in maternal carbohydrate metabolism are particularly important in pregnancy because glucose is the principal energy substrate used by the fetus. As metabolic homeostasis is intricately linked to the circadian system via the rhythmic expression of clock genes, it is likely that metabolic adaptations during pregnancy also involve shifts in maternal circadian function. We hypothesized that maternal adaptation in pregnancy involves changes in the hepatic expression of clock genes, which drive downstream shifts in circadian expression of glucoregulatory genes. Maternal liver and plasma (n = 6-8/group) were collected across 24-h periods (0800, 1200, 1600, 2000, 0000, 0400) from C57Bl/6J mice under isoflurane-nitrous oxide anesthesia prior to and on days 6, 10, 14 and 18 of pregnancy (term = day 19). Hepatic expression of clock genes and glucoregulatory genes was determined by RT-qPCR. Hepatic clock gene expression was substantially altered across pregnancy, most notably in late gestation when the circadian rhythmicity of several clock genes was attenuated (≤64% reduced amplitude on day 18). These changes were associated with a similar decline in rhythmicity of the key glucoregulatory genes Pck1, G6Pase, and Gk, and by day 18, Pck1 was no longer rhythmic. Overall, our data show marked adaptations in the liver clock during mouse pregnancy, changes that may contribute to the altered circadian variation in glucoregulatory genes near term. We propose that the observed reduction of daily oscillations in glucose metabolism ensure a sustained supply of glucose to meet the high demands of fetal growth.

Circadian regulation of metabolism

In association with sleep-wake and fasting-feeding cycles, organisms experience dramatic oscillations in energetic demands and nutrient supply. It is therefore not surprising that various metabolic parameters, ranging from the activity status of molecular energy sensors to circulating nutrient levels, oscillate in time-of-day-dependent manners. It has become increasingly clear that rhythms in metabolic processes are not simply in response to daily environmental/behavioral influences, but are driven in part by cell autonomous circadian clocks. By synchronizing the cell with its environment, clocks modulate a host of metabolic processes in a temporally appropriate manner. The purpose of this article is to review current understanding of the interplay between circadian clocks and metabolism, in addition to the pathophysiologic consequences of disruption of this molecular mechanism, in terms of cardiometabolic disease development.

Changes in glycogen structure over feeding cycle sheds new light on blood-glucose control

Liver glycogen, a highly branched polymer of glucose, is important for maintaining blood-glucose homeostasis. It was recently shown that db/db mice, a model for Type 2 diabetes, are unable to form the large composite glycogen α particles present in normal, healthy mice. In this study, the structure of healthy mouse-liver glycogen over the diurnal cycle was characterized using size exclusion chromatography and transmission electron microscopy. Glycogen was found to be formed as smaller β particles, and then only assembled into large α particles, with a broad size distribution, significantly after the time when glycogen content had reached a maximum. This pathway, missing in diabetic animals, is likely to give optimal blood-glucose control during the daily feeding cycle. Lack of this control may contribute to, or result from, diabetes. This discovery suggests novel approaches to diabetes management.

The Molecular Size Distribution of Glycogen and its Relevance to Diabetes

Glycogen is a highly branched polymer of glucose, functioning as a blood-glucose buffer. It comprises relatively small β-particles, which may be joined as larger aggregate α-particles. The size distributions from size-exclusion chromatography (SEC, also known as GPC) of liver glycogen from non-diabetic and diabetic mice show that diabetic mice have impaired α-particle formation, shedding new light on diabetes. SEC data also suggest the type of bonding holding β-particles together in α-particles. SEC characterisation of liver glycogen at various time points in a day/night cycle indicates that liver glycogen is initially synthesised as β-particles, and then joined by an unknown process to form α-particles. These α-particles are more resistant to degradation, presumably because of their lower surface area-to-volume ratio. These findings have important implications for new drug targets for diabetes management.

PER2 promotes glucose storage to liver glycogen during feeding and acute fasting by inducing Gys2 PTG and G L expression

The interplay between hepatic glycogen metabolism and blood glucose levels is a paradigm of the rhythmic nature of metabolic homeostasis. Here we show that mice lacking a functional PER2 protein (Per2 (Brdm1) ) display reduced fasting glycemia, altered rhythms of hepatic glycogen accumulation, and altered rhythms of food intake. Per2 (Brdm1) mice show reduced hepatic glycogen content and altered circadian expression during controlled fasting and refeeding. Livers from Per2 (Brdm1) mice display reduced glycogen synthase protein levels during refeeding, and increased glycogen phosphorylase activity during fasting. The latter is explained by PER2 action on the expression of the adapter proteins PTG and GL, which target the protein phosphatase-1 to glycogen to decrease glycogen phosphorylase activity. Finally, PER2 interacts with genomic regions of Gys2, PTG, and G L . These results indicate an important role for PER2 in the hepatic transcriptional response to feeding and acute fasting that promotes glucose storage to liver glycogen.

Metabolism as an integral cog in the mammalian circadian clockwork

Circadian rhythms are an integral part of life. These rhythms are apparent in virtually all biological processes studies to date, ranging from the individual cell (e.g. DNA synthesis) to the whole organism (e.g. behaviors such as physical activity). Oscillations in metabolism have been characterized extensively in various organisms, including mammals. These metabolic rhythms often parallel behaviors such as sleep/wake and fasting/feeding cycles that occur on a daily basis. What has become increasingly clear over the past several decades is that many metabolic oscillations are driven by cell-autonomous circadian clocks, which orchestrate metabolic processes in a temporally appropriate manner. During the process of identifying the mechanisms by which clocks influence metabolism, molecular-based studies have revealed that metabolism should be considered an integral circadian clock component. The implications of such an interrelationship include the establishment of a vicious cycle during cardiometabolic disease states, wherein metabolism-induced perturbations in the circadian clock exacerbate metabolic dysfunction. The purpose of this review is therefore to highlight recent insights gained regarding links between cell-autonomous circadian clocks and metabolism and the implications of clock dysfunction in the pathogenesis of cardiometabolic diseases.

Melatonin and pancreatic islets: interrelationships between melatonin, insulin and glucagon

The pineal hormone melatonin exerts its influence in the periphery through activation of two specific trans-membrane receptors: MT1 and MT2. Both isoforms are expressed in the islet of Langerhans and are involved in the modulation of insulin secretion from β-cells and in glucagon secretion from α-cells. De-synchrony of receptor signaling may lead to the development of type 2 diabetes. This notion has recently been supported by genome-wide association studies identifying particularly the MT2 as a risk factor for this rapidly spreading metabolic disturbance. Since melatonin is secreted in a clearly diurnal fashion, it is safe to assume that it also has a diurnal impact on the blood-glucose-regulating function of the islet. This factor has hitherto been underestimated; the disruption of diurnal signaling within the islet may be one of the most important mechanisms leading to metabolic disturbances. The study of melatonin–insulin interactions in diabetic rat models has revealed an inverse relationship: an increase in melatonin levels leads to a down-regulation of insulin secretion and vice versa. Elucidation of the possible inverse interrelationship in man may open new avenues in the therapy of diabetes.

Regulation of myocardial metabolism by the cardiomyocyte circadian clock

On a daily basis, the heart is subjected to dramatic fluctuations in energetic demand and neurohumoral influences, many of which occur in a temporally predictable manner. In order to preserve cardiac performance, the heart must therefore maintain metabolic flexibility, even within the confines of a single day. Recent studies have established mechanistic links between time-of-day-dependent oscillations in myocardial metabolism and the cardiomyocyte circadian clock. More specifically, evidence suggests that this cell autonomous molecular mechanism regulates myocardial glucose uptake, flux through both glycolysis and the hexosamine biosynthetic pathway, and pyruvate oxidation, as well as glycogen, triglyceride, and protein turnover. These observations have led to the hypothesis that the cardiomyocyte circadian clock confers the selective advantage of anticipation of increased energetic demand during the awake period. Here, we review the accumulative evidence in support of this hypothesis thus far, and discuss the possibility that attenuation of these metabolic rhythms, through disruption of the cardiomyocyte circadian clock, contributes towards the etiology of cardiac dysfunction in various disease states. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Melatonin stimulates glucagon secretion in vitro and in vivo

Recent investigations have demonstrated that melatonin influences carbohydrate metabolism mediated by insulin-inhibiting effects on pancreatic β-cells. This study evaluated whether melatonin has also an effect on pancreatic α-cells and glucagon expression as well as the glucagon secretion in vitro and in vivo. Glucagon-producing pancreatic α-cell line αTC1 clone 9 (αTC1.9) was used, which was characterized as an appropriate model with glucose responsiveness and expression of the melatonin receptors MT1 and MT2. The results demonstrate that melatonin incubation significantly enhanced the expression as well as the secretion of glucagon. These effects appeared to be more pronounced under hyperglycemic conditions compared to basal glucose concentrations. Notably, in vivo studies demonstrated that long-term oral melatonin administration led to significantly elevated plasma glucagon concentrations in Wistar rats. In contrast, plasma glucagon levels were found to be slightly decreased in type 2 diabetic Goto-Kakizaki rats. Moreover, investigations measuring the relative glucagon receptor mRNA expression showed marked differences in the liver of melatonin-substituted rats as well as in melatonin receptor knockout mice. In conclusion, these findings revealed evidence that melatonin influences pancreatic glucagon expression and secretion as well as the peripheral glucagon action.

CLOCK regulates circadian rhythms of hepatic glycogen synthesis through transcriptional activation of Gys2

Hepatic glycogen content is important for glucose homeostasis and exhibits robust circadian rhythms that peak at the end of the active phase in mammals. The activities of the rate-limiting enzymes for glycogenesis and glycogenolysis also show circadian rhythms, and the balance between them forms the circadian rhythm of the hepatic glycogen content. However, no direct evidence has yet implicated the circadian clock in the regulation of glycogen metabolism at the molecular level. We show here that a Clock gene mutation damps the circadian rhythm of the hepatic glycogen content, as well as the circadian mRNA and protein expression of Gys2 (glycogen synthase 2), which is the rate-limiting enzyme of glycogenesis in the liver. Transient reporter assays revealed that CLOCK drives the transcriptional activation of Gys2 via two tandemly located E-boxes. Chromatin immunoprecipitation assays of liver tissues revealed that CLOCK binds to these E-box elements in vivo, and real time reporter assays showed that these elements are sufficient for circadian Gys2 expression in vitro. Thus, CLOCK regulates the circadian rhythms of hepatic glycogen synthesis through transcriptional activation of Gys2.

Anticipating anticipation: pursuing identification of cardiomyocyte circadian clock function

Diurnal rhythms in myocardial physiology (e.g., metabolism, contractile function) and pathophyiology (e.g., sudden cardiac death) are well establish and have classically been ascribed to time-of-day-dependent alterations in the neurohumoral milieu. Existence of an intramyocellular circadian clock has recently been exposed. Circadian clocks enable the cell to anticipate environmental stimuli, facilitating a timely and appropriate response. Generation of genetically modified mice with a targeted disruption of the cardiomyocyte circadian clock has provided an initial means for deciphering the functions of this transcriptionally based mechanism and allowed predictions regarding which environmental stimuli the heart anticipates (i.e., "anticipating anticipation"). Recent studies show that the cardiomyocyte circadian clock influences myocardial gene expression, beta-adrenergic signaling, transcriptional responsiveness to fatty acids, triglyceride metabolism, heart rate, and cardiac output, as well as ischemia-reperfusion tolerance. In addition to reviewing current knowledge regarding the roles of the cardiomyocyte circadian clock, this article highlights putative frontiers in this field. The latter includes establishing molecular links between the cardiomyocyte circadian clock with identified functions, understanding the pathophysiological consequences of disruption of this mechanism, targeting resynchronization of the cardiomyocyte circadian clock for prevention/treatment of cardiovascular disease, linking the circadian clock with the cardiobeneficial effects of caloric restriction, and determining whether circadian clock genes are subject to epigenetic regulation. Information gained from studies investigating the cardiomyocyte circadian clock will likely translate to extracardiac tissues, such as skeletal muscle, liver, and adipose tissue.

Linking the cardiomyocyte circadian clock to myocardial metabolism

INTRODUCTION

The energetic demands imposed upon the heart vary dramatically over the course of the day. In the face of equally commanding oscillations in the neurohumoral mileu, the heart must respond both rapidly and appropriately to its diurnal environment, for the survival of the organism. A major response of the heart to alterations in workload, nutrients, and various neurohumoral stimuli is at the level of metabolism. Failure of the heart to achieve adequate metabolic adaptation results in contractile dysfunction.

DISCUSSION

Substantial evidence is accumulating which suggests that a transcriptionally based timekeeping mechanism known as the circadian clock plays a role in mediating myocardial metabolic rhythms. Here, we provide an overview of our current knowledge regarding the interplay between the circadian clock within the cardiomyocyte and myocardial metabolism. This includes a particular focus on circadian clock mediated regulation of endogenous energy stores, as well as those mechanisms orchestrating circadian rhythms in metabolic gene expression.

CONCLUSION

An essential need to elucidate fully the functions of this molecular mechanism in the heart remains.

Diurnal variations in myocardial metabolism

The heart is challenged by a plethora of extracellular stimuli over the course of a normal day, each of which distinctly influences myocardial contractile function. It is therefore not surprising that myocardial metabolism also oscillates in a time-of-day dependent manner. What is becoming increasingly apparent is that the heart exhibits diurnal variations in its intrinsic properties, including responsiveness to extracellular stimuli. This article summarizes our current knowledge regarding the mechanism(s) mediating diurnal variations in myocardial metabolism. Particular attention is focused towards the intramyocardial circadian clock, a cell autonomous molecular mechanism that appears to regulate myocardial metabolism both directly (e.g. triglyceride and glycogen metabolism) and indirectly (through modulation of the responsiveness of the myocardium to workload, insulin, and fatty acids). In doing so, the circadian clock within the cardiomyocyte allows the heart to anticipate environmental stimuli (such as changes in workload, feeding status) prior to their onset. This synchronization between the myocardium and its environment is enhanced by regular feeding schedules. Conversely, loss of synchronization may occur through disruption of the circadian clock and/or diurnal variations in neurohumoral factors (as observed during diabetes mellitus). Here, we discuss the possibility that loss of synchronization between the heart and its environment predisposes the heart to metabolic maladaptation and subsequent myocardial contractile dysfunction.

The daily timing of gene expression and physiology in mammals

Mammalian behavior and physiology undergo daily rhythms that are coordinated by an endogenous circadian timing system. This system has a hierarchical structure, in that a master pacemaker, residing in the suprachiasmatic nucleus of the ventral hypothalamus, synchronizes peripheral oscillators in virtually all body cells. While the basic molecular mechanisms generating the daily rhythms are similar in all cells, most clock outputs are cell-specific. This conclusion is based on genome-wide transcriptome profiling studies in several tissues that have revealed hundreds of rhythmically expressed genes. Cyclic gene expression in the various organs governs overt rhythms in behavior and physiology, encompassing sleep-wake cycles, metabolism, xenobiotic detoxification, and cellular proliferation. As a consequence, chronic perturbation of this temporal organization may lead to increased morbidity and reduced lifespan.

Diurnal rhythm in endogenous glucose production is a major contributor to fasting hyperglycaemia in type 2 diabetes. Suprachiasmatic deficit or limit cycle behaviour?

AIMS/HYPOTHESIS

An increase in endogenous glucose production (EGP) is a major contributor to fasting morning hyperglycaemia in type 2 diabetes. This increase is dissipated with fasting, later in the day. To understand its origin, EGP, gluconeogenesis and hormones that regulate metabolism were measured over 24 h. We hypothesised that EGP, and therefore glycaemia, would demonstrate a centrally mediated circadian rhythm in type 2 diabetes.

SUBJECTS AND METHODS

Seven subjects with type 2 diabetes and six age- and BMI-matched control subjects, fasting after breakfast (08.00 h), underwent a further 24-h fast, with the infusion of [U-(13)C]glucose and [3-(14)C]lactate, starting at 14.00 h. The MCR and production of total and gluconeogenic glucose were determined from the tracer concentrations using compartmental analysis.

RESULTS

MCR was near constant: 1.73+/-0.10 in control and 1.40+/-0.14 ml kg(-1) min(-1) in diabetic subjects (p=0.04). EGP in diabetes rose gradually overnight from 8.2+/-0.7 to 11.3+/-0.5 micromol kg(-1) min(-1) at 06.00 h (p<0.05). Glucose utilisation lagged EGP, rising from 8.5+/-0.6 to 10.5+/-0.4 micromol kg(-1) min(-1) (p<0.05), inducing a fall in glycaemia from a peak of 8.0+/-0.5 mmol/l to 6.3+/-0.4 mmol/l (p<0.05). Cortisol and melatonin showed diurnal variations, whereas insulin, glucagon and leptin did not. Melatonin was most closely related to EGP, but its secretion was attenuated in diabetes (p<0.05).

CONCLUSIONS/INTERPRETATION

In type 2 diabetes, EGP and gluconeogenesis display diurnal rhythms that drive the fasting hyperglycaemia and are absent in healthy control subjects. The rise in EGP may be related to a deficit in suprachiasmatic nucleus activity in diabetes, or result from non-linear behaviour plus a transition from a normal steady state to a limit cycle pattern in diabetes, or both.

Suprachiasmatic GABAergic inputs to the paraventricular nucleus control plasma glucose concentrations in the rat via sympathetic innervation of the liver

Daily peak plasma glucose concentrations are attained shortly before awakening. Previous experiments indicated an important role for the biological clock, located in the suprachiasmatic nuclei (SCN), in the genesis of this anticipatory rise in plasma glucose concentrations by controlling hepatic glucose production. Here, we show that stimulation of NMDA receptors, or blockade of GABA receptors in the paraventricular nucleus of the hypothalamus (PVN) of conscious rats, caused a pronounced increase in plasma glucose concentrations. The local administration of TTX in brain areas afferent to the PVN revealed that an important part of the inhibitory inputs to the PVN was derived from the SCN. Using a transneuronal viral-tracing technique, we showed that the SCN is connected to the liver via both branches of the autonomic nervous system (ANS). The combination of a blockade of GABA receptors in the PVN with selective removal of either the sympathetic or parasympathetic branch of the hepatic ANS innervation showed that hyperglycemia produced by PVN stimulation was primarily attributable to an activation of the sympathetic input to the liver. We propose that the daily rise in plasma glucose concentrations is caused by an SCN-mediated withdrawal of GABAergic inputs to sympathetic preautonomic neurons in the PVN, resulting in an increased hepatic glucose production. The remarkable resemblance of the presently proposed control mechanism to that described previously for the control of daily melatonin rhythm suggests that the GABAergic control of sympathetic preautonomic neurons in the PVN is an important pathway for the SCN to control peripheral physiology.

Liver-enriched transcription factors in liver function and development. Part II: the C/EBPs and D site-binding protein in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation

In the first part of our review (see Pharmacol Rev 2002;54:129-158), we discussed the basic principles of gene transcription and the complex interactions within the network of hepatocyte nuclear factors, coactivators, ligands, and corepressors in targeted liver-specific gene expression. Now we summarize the role of basic region/leucine zipper protein family members and particularly the albumin D site-binding protein (DBP) and the CAAT/enhancer-binding proteins (C/EBPs) for their importance in liver-specific gene expression and their role in liver function and development. Specifically, regulatory networks and molecular interactions were examined in detail, and the experimental findings summarized in this review point to pivotal roles of DBP and C/EBPs in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation. These regulatory proteins are therefore of great importance in liver physiology, liver disease, and liver development. Furthermore, interpretation of the vast data generated by novel genomic platform technologies requires a thorough understanding of regulatory networks and particularly the hierarchies that govern transcription and translation of proteins as well as intracellular protein modifications. Thus, this review aims to stimulate discussions on directions of future research and particularly the identification of molecular targets for pharmacological intervention of liver disease.

The mammalian circadian timing system: from gene expression to physiology

Many physiological processes in organisms from bacteria to man are rhythmic, and some of these are controlled by self-sustained oscillators that persist in the absence of external time cues. Circadian clocks are perhaps the best characterized biological oscillators and they exist in virtually all light-sensitive organisms. In mammals, they influence nearly all aspects of physiology and behavior, including sleep-wake cycles, cardiovascular activity, endocrinology, body temperature, renal activity, physiology of the gastro-intestinal tract, and hepatic metabolism. The master pacemaker is located in the suprachiasmatic nuclei, two small groups of neurons in the ventral part of the hypothalamus. However, most peripheral body cells contain self-sustained circadian oscillators with a molecular makeup similar to that of SCN (suprachiasmatic nucleus) neurons. This organization implies that the SCN must synchronize countless subsidiary oscillators in peripheral tissues, in order to coordinate cyclic physiology. In this review, we will discuss some recent studies on the structure and putative functions of the mammalian circadian timing system, but we will also point out some apparent inconsistencies in the currently publicized model for rhythm generation.

Restricted feeding uncouples circadian oscillators in peripheral tissues from the central pacemaker in the suprachiasmatic nucleus

In mammals, circadian oscillators exist not only in the suprachiasmatic nucleus, which harbors the central pacemaker, but also in most peripheral tissues. It is believed that the SCN clock entrains the phase of peripheral clocks via chemical cues, such as rhythmically secreted hormones. Here we show that temporal feeding restriction under light-dark or dark-dark conditions can change the phase of circadian gene expression in peripheral cell types by up to 12 h while leaving the phase of cyclic gene expression in the SCN unaffected. Hence, changes in metabolism can lead to an uncoupling of peripheral oscillators from the central pacemaker. Sudden large changes in feeding time, similar to abrupt changes in the photoperiod, reset the phase of rhythmic gene expression gradually and are thus likely to act through a clock-dependent mechanism. Food-induced phase resetting proceeds faster in liver than in kidney, heart, or pancreas, but after 1 wk of daytime feeding, the phases of circadian gene expression are similar in all examined peripheral tissues.

The circadian rhythm in activity of the periodic acid Schiff reaction and of the Mg and Ca content in rat liver in relation to age and season of the year

The experiments were carried out on male Wistar rats aged 6, 12 and 24 months, over the four seasons of the year. Analysis of the results obtained in all age groups disclosed that changes in activity of the p.a.S (periodic acid Schiff) reaction and in concentrations of Mg and Ca in the liver showed rhythmic oscillations with a period of 12 hours. The maximal p.a.S reaction activity and of Ca and Mg levels were generally found to coincide throughout all seasons and in all age groups. The rhythms of change in these parameters in 12- and 24-month-old rats showed a phase shift as compared to the 6-month-old animals.

Rhythms in morphology and function of hepatocytes

In standard textbooks of histology, hepatocyte structure is described as being, for the most part, static. However, more than 50 years ago, it was reported that the glycogen content of rabbit hepatocytes varies significantly over 24 h. Since then, numerous investigators have confirmed this phenomenon in a variety of animals by use of morphological, histochemical and biochemical techniques, and it has been shown that the variation in glycogen content of hepatocytes corresponds with variations in enzymes related to glycogen metabolism. Moreover, there are also variations in most subcellular organelles and in many other proteins over 24 h. It is clear that circadian rhythms need to be taken into account in studies of morphology and function of hepatocytes.

Biochemical evaluation of rat hepatocyte primary cultures as a model for carbon tetrachloride hepatotoxicity: comparative studies in vivo and in vitro

In order to evaluate how well the development of CCl4 hepatotoxicity in vivo can be modeled in primary cultures of rat hepatocytes, biochemical alterations were determined in liver samples from rats given CCl4 and in liver cells cultured for 18 hr then exposed to CCl4. Soluble thiol levels matched closely between tissue and hepatocytes (11 vs 12 micrograms-SH/mg protein) prior to exposure. Comparable concentrations of CCl4 were measured in blood (0.30 mM at 30 min) and in culture medium (0.49 mM at 5 min). Simultaneous inhibition of the endoplasmic reticulum calcium pump and stimulation of phosphorylase a activity occurred at early times in vivo (30 min) and in vitro (5 min). Glucose-6-phosphatase was inhibited next in liver (120 min) and in cells (20 min). 5'-Nucleotidase was not affected at any time points examined in either system. Leakage of glutamic-pyruvic transaminase and depletion of glycogen were maximal at later times in vivo (greater than or equal to 8 hr) and in cells (30 min). Total calcium content was increased severalfold in liver tissue (24 hr), but was not elevated in hepatocytes. This lack of calcium accumulation in cells appeared to result from impaired mitochondrial calcium uptake. Thus CCl4-induced biochemical changes followed nearly the same continuum in both models, although the progression was much more rapid in vitro than in vivo.

Liver phosphorylase activation during early phase of feeding behavior: a neurohumoral regulation

The mechanisms of liver glycogen depletion during the early phase of spontaneous feeding were examined in adult rats. A 40% increase in the activity of phosphorylase a, the active form of a key glycogenolytic enzyme, was detected 5 min after the onset of feeding, with a concomitant decrease of glycogen in the liver after 5 and 15 min. Within 60 min after the onset of feeding, the enzyme activity returned to the basal level and the glycogen content was restored. These metabolic responses were impaired by hepatic sympathetic denervation or by bilateral adrenodemedullation. It was concluded that the onset of meals may evoke an arousal of the sympathoadrenal system including the hepatic innervation, and that activation of this system is important in stimulating liver phosphorylase activity and glycogenolysis during the early phase of spontaneous feeding.

Diurnal rhythms in liver carbohydrate metabolism. Comparative aspects and critical review

Literature data on the diurnal rhythms of blood glucose, liver glycogen levels and key hepatic enzyme activities of glycolysis, gluconeogenesis, glycogen metabolism and lipogenesis in animals are reviewed. Materials on the diurnal rhythms of the activities of other enzymes involved in carbohydrate metabolism and related pathways such as the equilibrium glycolytic enzymes are also given. Interspecies comparison and analysis of the results and their interpretation are given.

The diurnal rhythm of liver glycogen phosphorylase: correlating changes in enzyme activity and enzymic protein

The diurnal rhythm of phosphorylase activity in mouse liver extracts was correlated with the 24 h fluctuations in phosphorylase protein. This last measurement was made using rocket immunoelectrophoresis. The peak activity of phosphorylase appeared coincident (at 18:00 h) with the greatest amount of phosphorylase protein detected. Conversely, the lowest activity measured and lowest enzymic protein content both occurred at 02:00 h. Regression analysis revealed a significant positive correlation between enzyme activity and protein. Thus changes in the cellular concentration of this enzyme are implicated in the diurnal rhythm of liver glycogen.

Diurnal variations in the activities of the glycogen metabolizing enzymes in mouse liver

The glycogen level in mouse liver was maximal during the night and decreased to the lowest level during the light period. The peak activity of phosphorylase alpha was observed during the light hours and thus paralleled the decline of hepatic glycogen concentrations. The period of rapid glycogen synthesis (1800-2200 hr) was immediately preceded by maximum glycogen synthase alpha activity. Significant diurnal rhythms for phosphorylase kinase and phosphorylase phosphatase activities were also observed and appear to play a role in regulating the diurnal rhythm of phosphorylase alpha activity.

Effect of starvation-refeeding and an exogenous glucocorticoid on carbohydrate metabolism in chick liver

Broiler chicks, 4 weeks of age, were subjected to a regimen of 48-hr starvation and 24-hr refeeding as a means of inducing hepatic glycogen supercompensation. A synthetic glucocorticoid (prednisolone) and transcription inhibitor (actinomycin D) treatment were superimposed on the starvation-refeeding regimen to examine the effect of an exogenous glucocorticoid and the necessity for de novo protein synthesis during glycogen supercompensation. Starvation decreased plasma glucose, immunoreactive insulin and liver glycogen. These parameters returned to, or overshot prefasting levels after a 48-hr refeeding period. Prednisolone magnified the overshoot response but some de novo protein synthesis was required. Glycogen synthase a activity was opposite that of liver glycogen content. A possible nonhormone stimulated glycogen synthetic mechanism in the starvation-refeeding response of the chick was noted.

Biological time-related changes in tolerance of male rats to hypoxia--I. Survival rate and carbohydrate metabolism

Investigations were carried out on male Wistar rats, synchronized in standard conditions to a light-dark regiment (LD 12:12 with L from 0600 to 1800). Rats exposed to hypoxia equivalent to 10,500m at a clock-hour of 1000 had a survival time twice as long as that of animals exposed at 2200. Data from this study indicate the ability to mobilize energy stores through the conversion of liver glycogen to glucose along with circadian differences in hormonal response (e.g. corticosterone and insulin) contributes to the tolerance to hypoxia being greater during diurnal rest than nocturnal activity in rats.

Food availability and daily biological rhythms

Restricted daily feeding schedules result in the partial or complete synchronization of a wide range of rhythmic biological functions in rodents. In some cases, exemplified by drinking behavior and liver tyrosine transaminase activity, this represents primarily a direct, exogenous influence of food intake. In others, synchronization is achieved by entrainment of a circadian time-keeping mechanism distinct from that which underlies free-running rhythms in these functions. This food-entrained mechanism is responsible for the timing of anticipatory increases in locomotor and lever-pressing activity immediatley prior to food delivery, and may also underlie similar anticipatory increases in body temperature, corticosterone secretion, and in the activities of some intestinal enzymes. It is suggested that such a mechanism may enable an animal to recognize and take advantage of the periodic recurrence of significant events in its biotic environment.

Feeding schedules and the circadian organization of behavior in the rat

Feeding and drinking behavior of rats maintained in constant light were recorded before, during and after feeding schedules with periods lying within or outside the range of circadian entrainment. Regardless of period, all schedules immediately resulted in the partial or complete synchronization of drinking behavior, but failed to entrain the free-running circadian feeding and drinking rhythms. This indicated that drinking can be passively driven by periodic access to food. However, other results suggested that a separate circadian system was entrained by feeding schedules: First, the 24-h periodicity induced by 24-h feeding schedules often continued for several days after termination of the schedules. Second, the rats showed anticipatory activity under schedules with periods within, but not outside, the circadian range of entrainment. Third, lesions of the suprachiasmatic nuclei (SCN), which resulted in the immediate elimination of free-running rhythms, did not alter the rhythmic influences of the feeding schedules. These results provide evidence for the participation of two distinct circadian systems in the control of behavior in the rat. The two systems appear to have different entrainment characteristics and separate physiological substrates.

Scanning microdensitometry of glycogen zonation in the livers of rats adapted to a controlled feeding schedule and to 30, 60, or 90% casein diets

The effect of diet composition on diurnal changes in glycogen zonation patterns in rat liver was investigated in individually-caged male Sprague-Dawley rats adapted to the 2 + 22 controlled feeding and lighting schedule and to diets containing 30% casein/55% carbohydrates, 60% casein/25% carbohydrates, or 9.0% casein (30 rats/dietary group). Three rats from each dietary group were killed at the following times relative to the onset of feeding (0 min):--60, --30, 0, 15, 30, 45, 60, 90, 120, and 180 min. Glycogen in cryostat sections from the median and right lateral lobes of the liver was fixed and stained by standard techniques. The optical density of glycogen at points along the path between the central and portal veins of a given lobule was determined, and lobular glycogen gradients of replicate animals were integrated to form a composite lobular glycogen distribution profile. In the period from--60 to 0 min, liver glycogen levels were similar for rats on any of the diets, and the glycogen concentration was similar in periportal (P), midlobular (M), and centrilobular (C) hepatocytes. During the 0- to 45-min period, diet-related glycogen depletion occurred (90 > 60 > 30% casein) by asymmetrical glycogen loss (P > M > C hepatocytes) from the liver lobules. Similar food intake curves occurred for all diets. During the 45- to 180-min period, asymmetrical glycogen accumulation began in lobular parenchymal cells (P > M > C hepatocytes), and rate of accumulation was related to dietary to dietary composition (30 > 60 > 90% casein). The differential responses of parenchymal cells within liver lobules to physiological stimuli resulted in glycogen distribution changes that were rapid and of large magnitude. Our results are consistent with the hypothesis that periportal and midlobular hepatocytes are more metabolically responsive and active than centrilobular hepatocytes

Circadian rhythm of liver glycogen metabolism in rats: effects of hypothalamic lesions

The effects of electrolytic lesions of the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMH) of the hypothalamus on the circadian rhythm of liver glycogen metabolism were studied in rats fed during the dark period (2000-0800 h). Bilateral lesions of the SCN did not affect appreciably the circadian rhythmicity in three parameters of liver glycogen metabolism, i.e., glycogen content and glycogen synthetase I and phosphorylase a activities. In contrast, bilateral lesions of the VMH resulted in almost complete loss of circadian rhythmicity in phosphorylase a activity by lowering the high activity normally observed in late evening. VMH lesions also reduced the amplitudes of the rhythms of synthetase I activity and glycogen content, but did not abolish their rhythmicities. These results suggest that the integrity of the VMH, rather than the SCN, is important for regulation or expression of the food-triggered circadian rhythm of liver phosphorylase a activity. The implication of the lateral hypothalamic nucleus in regulation of the circadian rhythm of liver synthetase I is also discussed.

On the activities of glycogen phosphorylase and glycogen synthase in the liver of the rat

A procedure was developed for determination of glycogen synthase and phosphorylase activities in liver after various in vivo physiological treatments. Liver samples were obtained from anaesthetised rats by freeze-clamping in situ. Other procedures were shown to stimulate the activity of phosphorylase and depress the activity of glycogen in the liver. The direction of glycogen metabolism appears to be regulated by the relative proportions of the two enzymes, as shown by a strong positive correlation between total activities and active forms of phosphorylase and synthase. The enzyme activities responded as expected to stimuli such as insulin and glucose, which depressed phosphorylase and increased synthase activity, and glucagon, which increased phosphorylase and decreased synthase activity. In fasted animals approximately 50% of each enzyme was in the active form, which suggests the existence of a potential futile cycle for glycogen metabolism. The role for such a cycle in the regulation of glycogen synthesis and degradation is discussed.