Circadian rhythm transcription factor CLOCK regulates the transcriptional activity of the glucocorticoid receptor by acetylating its hinge region lysine cluster: potential physiological implications

Molecular and biochemical modifications of liver glutamine synthetase elicited by daytime restricted feeding

BACKGROUND & AIMS

The circadian clock system in the liver plays important roles in regulating metabolism and energy homeostasis. Restricted feeding schedules (RFS) become an entraining stimulus that promotes adaptations that form part of an alternative circadian clock known as the food entrained oscillator (FEO). The aim of this study was to evaluate the daily variations of glutamine synthetase (GS) in liver under a daytime RFS.

METHODS

Hepatic GS properties were analysed at 3-h intervals over a 24-h period in adult male Wistar rats maintained in a 12:12 h light–dark cycle. RFS group: food access for 2-h in light phase, during 3 weeks. AL group: feeding ad libitum. Fa group: acute fast (21 h). Fa–Re group: acute fast followed by refed 2 h.mRNA expression was measured by RT-qPCR, protein presence by Western-blot and immunohistochemistry, enzyme activity by a spectrophotometric assay, and glutamine by high pressure liquid chromatography.

RESULTS AND CONCLUSIONS

Restricted feeding schedule induced circadian rhythmicity inmRNA levels of GS and the loss of the rhythmic pattern in mitochondrial GS activity. GS activity in liver homogenates displayed a robust rhythmic pattern in AL that was not modified by RFS. The presence of GS and its zonal distribution did not show rhythmic pattern in both groups. However, acute Fa and Fa–Re diminished GS protein and activity in liver homogenates. Hepatic glutamine concentrations showed a 24-h rhythmic pattern in both groups, in an antiphasic pattern. In conclusion, daytime RFS influences the liver GS system at different levels, that could be part of rheostatic adaptations associated to the FEO, and highlight the plasticity of this system.

Mechanisms of glucocorticoid action and insensitivity in airways disease

Glucocorticoids are the mainstay for the treatment of chronic inflammatory diseases including asthma and chronic obstructive pulmonary disease (COPD). However, it has been recognized that glucocorticoids do not work well in certain patient populations suggesting reduced sensitivity. The ultimate biologic responses to glucocorticoids are determined by not only the concentration of glucocorticoids but also the differences between individuals in glucocorticoid sensitivity, which is influenced by multiple factors. Studies are emerging to understand these mechanisms in detail, which would help in increasing glucocorticoid sensitivity in patients with chronic airways disease. This review aims to highlight both classical and emerging concepts of the anti-inflammatory mechanisms of glucocorticoids and also review some novel strategies to overcome steroid insensitivity in airways disease.

Recent advances in the molecular mechanisms determining tissue sensitivity to glucocorticoids: novel mutations, circadian rhythm and ligand-induced repression of the human glucocorticoid receptor

Glucocorticoids are pleiotropic hormones, which are involved in almost every cellular, molecular and physiologic network of the organism, and regulate a broad spectrum of physiologic functions essential for life. The cellular response to glucocorticoids displays profound variability both in magnitude and in specificity of action. Tissue sensitivity to glucocorticoids differs among individuals, within tissues of the same individual and within the same cell. The actions of glucocorticoids are mediated by the glucocorticoid receptor, a ubiquitously expressed intracellular, ligand-dependent transcription factor. Multiple mechanisms, such as pre-receptor ligand metabolism, receptor isoform expression, and receptor-, tissue-, and cell type-specific factors, exist to generate diversity as well as specificity in the response to glucocorticoids. Alterations in the molecular mechanisms of glucocorticoid receptor action impair glucocorticoid signal transduction and alter tissue sensitivity to glucocorticoids. This review summarizes the recent advances in our understanding of the molecular mechanisms determining tissue sensitivity to glucocorticoids with particular emphasis on novel mutations and new information on the circadian rhythm and ligand-induced repression of the glucocorticoid receptor.

Comprehensive overview of the structure and regulation of the glucocorticoid receptor

Glucocorticoids are among the most prescribed drugs worldwide for the treatment of numerous immune and inflammatory disorders. They exert their actions by binding to the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily. There are several GR isoforms resulting from alternative RNA splicing and translation initiation of the GR transcript. Additionally, these isoforms are all subject to several transcriptional, post-transcriptional, and post-translational modifications, all of which affect the protein's stability and/or function. In this review, we summarize recent knowledge on the distinct GR isoforms and the processes that generate them. We also review the importance of all known transcriptional, post-transcriptional, and post-translational modifications, including the regulation of GR by microRNAs. Moreover, we discuss the crucial role of the putative GR-bound DNA sequence as an allosteric ligand influencing GR structure and activity. Finally, we describe how the differential composition and distinct regulation at multiple levels of different GR species could account for the wide and diverse effects of glucocorticoids.

An epithelial circadian clock controls pulmonary inflammation and glucocorticoid action

The circadian system is an important regulator of immune function. Human inflammatory lung diseases frequently show time-of-day variation in symptom severity and lung function, but the mechanisms and cell types underlying these effects remain unclear. We show that pulmonary antibacterial responses are modulated by a circadian clock within epithelial club (Clara) cells. These drive circadian neutrophil recruitment to the lung via the chemokine CXCL5. Genetic ablation of the clock gene Bmal1 (also called Arntl or MOP3) in bronchiolar cells disrupts rhythmic Cxcl5 expression, resulting in exaggerated inflammatory responses to lipopolysaccharide and an impaired host response to Streptococcus pneumoniae infection. Adrenalectomy blocks rhythmic inflammatory responses and the circadian regulation of CXCL5, suggesting a key role for the adrenal axis in driving CXCL5 expression and pulmonary neutrophil recruitment. Glucocorticoid receptor occupancy at the Cxcl5 locus shows circadian oscillations, but this is disrupted in mice with bronchiole-specific ablation of Bmal1, leading to enhanced CXCL5 expression despite normal corticosteroid secretion. The therapeutic effects of the synthetic glucocorticoid dexamethasone depend on intact clock function in the airway. We now define a regulatory mechanism that links the circadian clock and glucocorticoid hormones to control both time-of-day variation and the magnitude of pulmonary inflammation and responses to bacterial infection.

Quantification of interactions among circadian clock proteins via surface plasmon resonance

Circadian clock is an internal time keeping system recurring 24 h daily rhythm in physiology and behavior of organisms. Circadian clock contains transcription and translation feedback loop involving CLOCK/NPAS2, BMAL1, Cry1/2, and Per1/2. In common, heterodimer of CLOCK/NPAS2 and BMAL1 binds to EBOX element in the promoter of Per and Cry genes in order to activate their transcription. CRY and PER making heterodimeric complexes enter the nucleus in order to inhibit their own BMAL1-CLOCK-activated transcription. The aim of this study was to investigate and quantify real-time binding affinities of clock proteins among each other on and off DNA modes using surface plasmon resonance. The pairwise interaction coefficients among clock proteins, as well as interaction of PER2, CRY2, and PER2 : CRY2 proteins with BMAL1 : CLOCK complex in the presence and absence of EBOX motif have been investigated via analysis of surface plasmon resonance data with pseudo first-order reaction kinetics approximation and via nonlinear regression curve fitting. The results indicated that CRY2 and PER2, BMAL1, and CLOCK proteins form complexes in vitro and that PER2, CRY2 and PER2 : CRY2 complex have similar affinities toward BMAL1 : CLOCK complex. CRY2 protein had the highest affinity toward EBOX complex, whereas PER2 and CRY2 : PER2 complexes displayed low affinity toward EBOX complex. The quantification of the interaction between clock proteins is critical to understand the operation mechanism of the biological clock and to address the behavioral and physiological disorders, and it will be useful for the design of new drugs toward clock-related diseases.

Adrenal insufficiency

Adrenal insufficiency is the clinical manifestation of deficient production or action of glucocorticoids, with or without deficiency also in mineralocorticoids and adrenal androgens. It is a life-threatening disorder that can result from primary adrenal failure or secondary adrenal disease due to impairment of the hypothalamic-pituitary axis. Prompt diagnosis and management are essential. The clinical manifestations of primary adrenal insufficiency result from deficiency of all adrenocortical hormones, but they can also include signs of other concurrent autoimmune conditions. In secondary or tertiary adrenal insufficiency, the clinical picture results from glucocorticoid deficiency only, but manifestations of the primary pathological disorder can also be present. The diagnostic investigation, although well established, can be challenging, especially in patients with secondary or tertiary adrenal insufficiency. We summarise knowledge at this time on the epidemiology, causal mechanisms, pathophysiology, clinical manifestations, diagnosis, and management of this disorder.

A time to remember: the role of circadian clocks in learning and memory

The circadian system has pronounced influence on learning and memory, manifesting as marked changes in memory acquisition and recall across the day. From a mechanistic perspective, the majority of studies have investigated mammalian hippocampal-dependent learning and memory, as this system is highly tractable. The hippocampus plays a major role in learning and memory, and has the potential to integrate circadian information in many ways, including information from local, independent oscillators, and through circadian modulation of neurogenesis, synaptic remodeling, intracellular cascades, and epigenetic regulation of gene expression. These local processes are combined with input from other oscillatory systems to synergistically augment hippocampal rhythmic function. This overview presents an account of the current state of knowledge on circadian interactions with learning and memory circuitry and provides a framework for those interested in further exploring these interactions.

Circadian endocrine rhythms: the hypothalamic-pituitary-adrenal axis and its actions

The stress system effectively restores the internal balance--or homeostasis--of living organisms in the face of random external or internal changes, the stressors. This highly complex system helps organisms to provide a series of neuroendocrine responses to stressors--the stress response--through coordinated activation of the hypothalamic-pituitary-adrenal (HPA) axis and the locus coeruleus/norepinephrine autonomic nervous systems. In addition to stressors, life is influenced by daily light/dark changes due to the 24-h rotation of Earth. To adjust to these recurrent day/night cycles, the biological clock system employs the heterodimer of transcription factors circadian locomotor output cycle kaput/brain-muscle-arnt-like protein 1 (CLOCK/BMAL1), along with a set of other transcription factors, to regulate the circadian pattern of gene expression. Interestingly, the stress system, through the HPA axis, communicates with the clock system; therefore, any uncoupling or dysregulation could potentially cause several disorders, such as metabolic, autoimmune, and mood disorders. In this review, we discuss the biological function of the two systems, their interactions, and the clinical implications of their dysregulation or uncoupling.

A novel point mutation of the human glucocorticoid receptor gene causes primary generalized glucocorticoid resistance through impaired interaction with the LXXLL motif of the p160 coactivators: dissociation of the transactivating and transreppressive activities

CONTEXT

Primary generalized glucocorticoid resistance is a rare genetic disorder characterized by generalized, partial, target-tissue insensitivity to glucocorticoids. The molecular basis of the condition has been ascribed to inactivating mutations in the human glucocorticoid receptor (hGR) gene.

OBJECTIVE

The objective of the study was to present three new cases caused by a novel mutation in the hGR gene and to delineate the molecular mechanisms through which the mutant receptor impairs glucocorticoid signal transduction.

DESIGN AND RESULTS

The index case (father) and his two daughters presented with increased urinary free cortisol excretion and resistance of the hypothalamic-pituitary-adrenal axis to dexamethasone suppression in the absence of clinical manifestations suggestive of Cushing syndrome. All subjects harbored a novel, heterozygous, point mutation (T→G) at nucleotide position 1724 of the hGR gene, which resulted in substitution of valine by glycine at amino acid 575 of the receptor. Compared with the wild-type receptor, the hGRαV575G demonstrated a significant (33%) reduction in its ability to transactivate the mouse mammary tumor virus promoter in response to dexamethasone, a 50% decrease in its affinity for the ligand, and a 2.5-fold delay in nuclear translocation. Although it did not exert a dominant negative effect on the wild-type receptor and preserved its ability to bind to DNA, hGRαV575G displayed significantly enhanced (∼80%) ability to transrepress the nuclear factor-κΒ signaling pathway. Finally, the mutant receptor hGRαV575G demonstrated impaired interaction with the LXXLL motif of the glucocorticoid receptor-interacting protein 1 coactivator in vitro and in computer-based structural simulation via its defective activation function-2 (AF-2) domain.

CONCLUSIONS

The natural mutant receptor hGRαV575G causes primary generalized glucocorticoid resistance by affecting multiple steps in the glucocorticoid signaling cascade, including the affinity for the ligand, the time required for nuclear translocation, and the interaction with the glucocorticoid-interacting protein-1 coactivator.

Circadian clock proteins and immunity

Immune parameters change with time of day and disruption of circadian rhythms has been linked to inflammatory pathologies. A circadian-clock-controlled immune system might allow an organism to anticipate daily changes in activity and feeding and the associated risk of infection or tissue damage to the host. Responses to bacteria have been shown to vary depending on time of infection, with mice being more at risk of sepsis when challenged ahead of their activity phase. Studies highlight the extent to which the molecular clock, most notably the core clock proteins BMAL1, CLOCK, and REV-ERBα, control fundamental aspects of the immune response. Examples include the BMAL1:CLOCK heterodimer regulating toll-like receptor 9 (TLR9) expression and repressing expression of the inflammatory monocyte chemokine ligand (CCL2) as well as REV-ERBα suppressing the induction of interleukin-6. Understanding the daily rhythm of the immune system could have implications for vaccinations and how we manage infectious and inflammatory diseases.

Nuclear receptors rock around the clock

Circadian rhythms characterize almost every aspect of human physiology, endocrinology, xenobiotic detoxification, cell growth, and behavior. Modern lifestyles that disrupt our normal circadian rhythms are increasingly thought to contribute to various disease conditions ranging from depression and metabolic disorders to cancer. This self-sustained time-keeping system is generated and maintained by an endogenous molecular machine, the circadian clock, which is a transcriptional mechanism composed of the transcription factors CLOCK and BMAL and their co-repressors, PER and CRY. Nuclear receptors (NRs) represent a large family of hormone-sensitive transcriptional regulators involved in a myriad of biological processes such as development, energy metabolism, reproduction, inflammation, and tissue homeostasis. Recent studies point not only to NR regulation by the clock, but also to NR regulation of the clock itself. Here, we discuss recent studies that functionally and mechanistically implicate NRs as key components of both the universal and adaptive circadian clock mechanisms. As proven pharmacological targets, nuclear receptors are promising targets for therapeutic control of many pathological conditions associated with the disruption of circadian rhythm.

Running for time: circadian rhythms and melanoma

Circadian timing system includes an input pathway transmitting environmental signals to a core oscillator that generates circadian signals responsible for the peripheral physiological or behavioural events. Circadian 24-h rhythms regulate diverse physiologic processes. Deregulation of these rhythms is associated with a number of pathogenic conditions including depression, diabetes, metabolic syndrome and cancer. Melanoma is a less common type of skin cancer yet more aggressive often with a lethal ending. However, little is known about circadian control in melanoma and exact functional associations between core clock genes and development of melanoma skin cancer. This paper, therefore, comprehensively analyses current literature data on the involvement of circadian clock components in melanoma development. In particular, the role of circadian rhythm deregulation is discussed in the context of DNA repair mechanisms and influence of UV radiation and artificial light exposure on cancer development. The role of arylalkylamine N-acetyltransferase (AANAT) enzyme and impact of melatonin, as a major output factor of circadian rhythm, and its protective role in melanoma are discussed in details. We hypothesise that further understanding of clock genes' involvement and circadian regulation might foster discoveries in the field of melanoma diagnostics and treatment.

Minireview: The versatile roles of lysine deacetylases in steroid receptor signaling

Lysine deacetylases have been known to regulate nuclear receptor function for many years. In the unliganded state, nuclear receptors that form heterodimers with retinoid X receptors, such as the retinoic acid and thyroid hormone receptors, associate with deacetylases to repress target genes. In the case of steroid receptors, binding of an antagonist ligand was initially reported to induce association of deacetylases to prevent activation of target genes. Since then, deacetylases have been shown to have diverse functions in steroid receptor signaling, from regulating interactions with molecular chaperones to facilitating their ability to activate transcription. The purpose of this review is to summarize recent studies on the role of deacetylases in steroid receptor signaling, which show deacetylases to be highly versatile regulators of steroid receptor function.

Modulation of glucocorticoid receptor induction properties by core circadian clock proteins

Glucocorticoid (GC) plays important roles in diverse physiological processes including metabolism and immune functions. While circadian control of GC synthesis and secretion is relatively well appreciated, circadian control of GC action within target tissues remains poorly understood. Here, we demonstrate that CLOCK/BMAL1, the core circadian clock components, reduces maximal GR transactivation (A(max)) as well as efficacy (EC₅₀) by a novel mechanism that requires binding to DNA and transactivation of target genes. Accordingly, we observe that PER1 and CRY1, the primary targets of CLOCK/BMAL1 action, reduce maximal GR transactivation while not affecting the efficacy. Moreover, we observe hyper-activations of GRE-dependent transcription in BMAL1- or PERs-deficient MEFs. In addition, endogenous GC target genes expression negatively correlates with the CLOCK/BMAL1 activity. Considering that GC sensitivity is widely implicated in human health and diseases, these results provide valuable insights into plethora of GC-related physiology and pathology.

Circadian rhythms in leukocyte trafficking

A broad range of immunological processes oscillates over the course of a day. Recent findings have identified a molecular basis for the circadian clock in the regulation of the immune system. These rhythms manifest themselves in oscillatory behavior of immune cells and proinflammatory mediators, which causes a time-dependent sensitivity in the reaction to pathogens. This rhythmicity impacts disease manifestations and severity and provides an option for therapy that incorporates chronopharmacological considerations. This review will focus on the current knowledge and relevance of rhythmic immune cell trafficking. It will provide an overview of the molecular clock machinery and its interrelations with leukocyte migration and the immune response.

Dosing schedule-dependent attenuation of dexamethasone-induced muscle atrophy in mice

Many inflammatory and autoimmune diseases are treated using synthetic glucocorticoids. However, excessive glucocorticoid can often cause unpredictable effects including muscle atrophy. Endogenous glucocorticoid levels robustly fluctuate in a circadian manner and peak just before the onset of the active phase in both humans and nocturnal rodents. The present study determines whether muscle atrophy induced by exogenous glucocorticoid can be avoided by optimizing dosing times. We administered single daily doses of the glucocorticoid analog dexamethasone (Dex) to mice for 10 days at the times of day corresponding to peak (early night) or trough (early morning) endogenous glucocorticoid levels. Administration at the acrophase of endogenous glucocorticoids significantly attenuated Dex-induced wasting of the gastrocnemius (Ga) and tibialis anterior (TA) muscles that comprise mostly fast-twitch muscle fibers. Real-time RT-PCR revealed that the Dex-induced mRNA expression of genes encoding the atrophy-related ubiquitin ligases Muscle Atrophy F-box (Fbxo32, also known as MAFbx/Atrogin-1) and Muscle RING finger 1 (Trim63, also known as MuRF1) in the Ga and TA muscles was significantly attenuated by Dex when administered during the early night. Dex negligibly affected the weight of the soleus (So) muscle that mostly comprises slow-twitch muscle fibers, but significantly and similarly decreased the weight of the spleen at both dosing times. These results suggest that glucocorticoid-induced muscle atrophy can be attenuated by optimizing the dosing schedule.

Interaction of circadian and stress systems in the regulation of adipose physiology

Endogenous circadian clocks facilitate the adaptation of physiology and behavior to recurring environmental changes brought about by the Earth’s rotation around its axis. Adipose tissues harbor intrinsic circadian oscillators based on interlocked transcriptional-translational feedback loops built from a set of clock genes that regulate important aspects of lipid metabolism and adipose endocrine function. These adipocyte clocks are reset via neuronal and endocrine pathways originating from a master circadian pacemaker residing in the hypothalamic suprachiasmatic nucleus. One important mediator of circadian output is the stress hormone cortisol, which, at the same time, is one of the major regulators of adipose physiology. In this review we summarize recent findings on the interaction between circadian and stress systems in the regulation of adipose physiology and discuss the implications of this crosstalk for the development of metabolic disorders associated with circadian disruption and/or chronic stress, for example in shift workers.

Dominance of the strongest: inflammatory cytokines versus glucocorticoids

Pro-inflammatory cytokines are involved in the pathogenesis of many inflammatory diseases, and the excessive expression of many of them is normally counteracted by glucocorticoids (GCs), which are steroids that bind to the glucocorticoid receptor (GR). Hence, GCs are potent inhibitors of inflammation, and they are widely used to treat inflammatory diseases, such as asthma, rheumatoid arthritis and inflammatory bowel disease. However, despite the success of GC therapy, many patients show some degree of GC unresponsiveness, called GC resistance (GCR). This is a serious problem because it limits the full therapeutic exploitation of the anti-inflammatory power of GCs. Patients with reduced GC responses often have higher cytokine levels, and there is a complex interplay between GCs and cytokines: GCs downregulate pro-inflammatory cytokines while cytokines limit GC action. Treatment of inflammatory diseases with GCs is successful when GCs dominate. But when cytokines overrule the anti-inflammatory actions of GCs, patients become GC insensitive. New insights into the molecular mechanisms of GR-mediated actions and GCR are needed for the design of more effective GC-based therapies.

The circadian clock and glucocorticoids--interactions across many time scales

Glucocorticoids are steroid hormones of the adrenal gland that are an integral component of the stress response and regulate many physiological processes, including metabolism and immune response. Their release into the blood is highly dynamic and occurs in about hourly pulses, the amplitude of which is modulated in a daytime dependent fashion. In addition, in many species seasonal changes in basal glucocorticoid levels have been reported. In their target tissues, glucocorticoids bind to cytoplasmic receptors of the nuclear receptor superfamily. Upon binding, these receptors regulate transcription in a highly dynamic fashion, which involves stochastic binding to regulatory DNA elements on a time scale of seconds and heat shock protein mediated receptor-ligand complex recycling within minutes. The glucocorticoid hormone system interacts with another highly dynamic system, the circadian clock. The circadian clock is an endogenous biological timing mechanism that allows organisms to anticipate regular daily changes in their environment. It regulates daily rhythms of glucocorticoid release by a variety of mechanisms, modulates glucocorticoid signaling and is itself influenced by glucocorticoids. Here, we discuss mechanisms, functions and interactions of the circadian and glucocorticoid systems across time scales ranging from seconds (DNA binding by transcriptional regulators) to years (seasonal rhythms).

Metabolic control through glucocorticoid hormones: an update

In the past decades, glucocorticoid (GC) hormones and their cognate, intracellular receptor, the glucocorticoid receptor (GR), have been well established as critical checkpoints in mammalian energy homeostasis. Whereas many aspects in healthy nutrient metabolism require physiological levels and/or action of GC, aberrant GC/GR signalling has been linked to severe metabolic dysfunction, including obesity, insulin resistance and type 2 diabetes. Consequently, studies of the molecular mechanisms within the GC signalling axis have become a major focus in biomedical research, up-to-date particularly focusing on systemic glucose and lipid handling. However, with the availability of novel high throughput technologies and more sophisticated metabolic phenotyping capabilities, as-yet non-appreciated, metabolic functions of GC have been recently discovered, including regulatory roles of the GC/GR axis in protein and bile acid homeostasis as well as metabolic inter-organ communication. Therefore, this review summarises recent advances in GC/GR biology, and summarises findings relevant for basic and translational metabolic research.

Glucocorticoid sensitivity in health and disease

Glucocorticoids regulate many physiological processes and have an essential role in the systemic response to stress. For example, gene transcription is modulated by the glucocorticoid-glucocorticoid receptor complex via several mechanisms. The ultimate biologic responses to glucocorticoids are determined by not only the concentration of glucocorticoids but also the differences between individuals in glucocorticoid sensitivity, which is influenced by multiple factors. Differences in sensitivity to glucocorticoids in healthy individuals are partly genetically determined by functional polymorphisms of the gene that encodes the glucocorticoid receptor. Hereditary syndromes have also been identified that are associated with increased and decreased sensitivity to glucocorticoids. As a result of their anti-inflammatory properties, glucocorticoids are widely used in the treatment of allergic, inflammatory and haematological disorders. The variety in clinical responses to treatment with glucocorticoids reflects the considerable variation in glucocorticoid sensitivity between individuals. In immune-mediated disorders, proinflammatory cytokines can induce localized resistance to glucocorticoids via several mechanisms. Individual differences in how tissues respond to glucocorticoids might also be involved in the predisposition for and pathogenesis of the metabolic syndrome and mood disorders. In this Review, we summarize the mechanisms that influence glucocorticoid sensitivity in health and disease and discuss possible strategies to modulate glucocorticoid responsiveness.

Effects of chronic jet lag on the central and peripheral circadian clocks in CBA/N mice

The disruption of the circadian clock by frequent shifts in the light-dark cycle, such as shift-work or frequent jet lag, increases the risk of many diseases, including cancer. Experimental disruption of the circadian clock also increases tumor development in mice, although most studies used the strains that are genetically impaired in melatonin synthesis and secretion. Here, we examined the effects of experimental chronic jet lag with 8 h advances of the light-dark cycle every 2 days for 10 days on the central and peripheral clocks of CBA/N mice, the strain with normal profiles of melatonin synthesis and secretion. Mice were exposed to constant darkness after the 10 days of chronic jet lag. In the suprachiasmatic nucleus (SCN), chronic jet lag shifted the temporal expression of most clock genes examined without causing total disturbance of circadian oscillations. In the liver, the temporal patterns of Per1, Bmal1, and Dbp expression were phase-shifted, and Per2 expression was significantly upregulated by chronic jet lag. Further, the expression of cell cycle-related genes, c-Myc and p53 in the liver was significantly activated by the chronic jet lag schedule with a significant positive correlation between Per2 and p53 expression. We determined the plasma concentrations of melatonin and corticosterone as candidate hormonal messengers of chronic jet lag, but their overall levels were not affected by chronic jet lag. Moreover, the expression of the MT1 melatonin and glucocorticoid receptors in the liver was suppressed by chronic jet lag. These data suggest that in CBA/N mice, frequent advances of light-dark cycles modify the phases of central clock in the SCN and disturb the peripheral clock in the liver and apoptotic functions, which may be associated with the suppression of hormone receptors.

Class I lysine deacetylases facilitate glucocorticoid-induced transcription

Nuclear receptors use lysine acetyltransferases and lysine deacetylases (KDACs) in regulating transcription through histone acetylation. Lysine acetyltransferases interact with steroid receptors upon binding of an agonist and are recruited to target genes. KDACs have been shown to interact with steroid receptors upon binding to an antagonist. We have shown previously that KDAC inhibitors (KDACis) potently repress the mouse mammary tumor virus promoter through transcriptional mechanisms and impair the ability of the glucocorticoid receptor (GR) to activate it, suggesting that KDACs can play a positive role in GR transactivation. In the current study, we extended this analysis to the entire GR transcriptome and found that the KDACi valproic acid impairs the ability of agonist-bound GR to activate about 50% of its target genes. This inhibition is largely due to impaired transcription rather than defective GR processing and was also observed using a structurally distinct KDACi. Depletion of KDAC1 expression mimicked the effects of KDACi in over half of the genes found to be impaired in GR transactivation. Simultaneous depletion of KDACs 1 and 2 caused full or partial impairment of several more GR target genes. Altogether we found that Class I KDAC activity facilitates GR-mediated activation at a sizable fraction of GR-activated target genes and that KDAC1 alone or in coordination with KDAC2 is required for efficient GR transactivation at many of these target genes. Finally, our work demonstrates that KDACi exposure has a significant impact on GR signaling and thus has ramifications for the clinical use of these drugs.

Molecular clocks in pharmacology

Circadian rhythms regulate a vast array of biological processes and play a fundamental role in mammalian physiology. As a result, considerable diurnal variation in the pharmacokinetics, efficacy, and side effect profiles of many therapeutics has been described. This variation has subsequently been tied to diurnal rhythms in absorption, distribution, metabolism, and excretion, as well as in pharmacodynamic variables, such as target expression. More recently, the molecular basis of circadian rhythmicity has been elucidated with the identification of clock genes, which oscillate in a circadian manner in most cells and tissues and regulate transcription of large sets of genes. Ongoing research efforts are beginning to reveal the critical role of circadian clock genes in the regulation of pharmacologic parameters, as well as the reciprocal impact of drugs on circadian clock function. This chapter will review the role of circadian clocks in the pharmacokinetics and pharmacodynamics of drug response and provide several examples of the complex regulation of pharmacologic systems by components of the molecular circadian clock.

Glucocorticoid-mediated Period2 induction delays the phase of circadian rhythm

Glucocorticoid (GC) signaling synchronizes the circadian rhythm of individual peripheral cells and induces the expression of circadian genes, including Period1 (Per1) and Period2 (Per2). However, no GC response element (GRE) has been reported in the Per2 promoter region. Here we report the molecular mechanisms of Per2 induction by GC signaling and its relevance to the regulation of circadian timing. We found that GC prominently induced Per2 expression and delayed the circadian phase. The overlapping GRE and E-box (GE2) region in the proximal Per2 promoter was responsible for GC-mediated Per2 induction. The GRE in the Per2 promoter was unique in that brain and muscle ARNT-like protein-1 (BMAL1) was essential for GC-induced Per2 expression, whereas other GRE-containing promoters, such as Per1 and mouse mammary tumor virus, responded to dexamethasone in the absence of BMAL1. This specialized regulatory mechanism was mediated by BMAL1-dependent binding of the GC receptor to GRE in Per2 promoter. When Per2 induction was abrogated by the mutation of the GRE or E-box, the circadian oscillation phase failed to be delayed compared with that of the wild-type. Therefore, the current study demonstrates that the rapid Per2 induction mediated by GC is crucial for delaying the circadian rhythm.

Circulating cortisol-associated signature of glucocorticoid-related gene expression in subcutaneous fat of obese subjects

OBJECTIVE

Serum cortisol concentrations fluctuate in a circadian fashion, and glucocorticoids exert strong effects on adipose tissue and induce obesity through the glucocorticoid receptor.

DESIGN AND METHODS

To examine the impact of physiologic levels of circulating cortisol on subcutaneous adipose tissue, 25 overweight and obese subjects were employed, and their serum levels of morning (AM) and evening (PM) cortisol, AM/PM cortisol ratios, and 24-h urinary-free cortisol (UFC) were compared with their clinical parameters, serum cytokine levels, and mRNA expression of 93 receptor action-regulating and 93 glucocorticoid-responsive genes in abdominal subcutaneous fat.

RESULTS AND CONCLUSIONS

AM cortisol levels did not correlate with mRNA expression of the all genes examined, whereas PM cortisol levels, AM/PM cortisol ratios, and 24-h UFC were associated with distinct sets of these genes. Body mass index did not significantly correlate with the four cortisol parameters employed. These results suggest that physiologic levels of AM serum cortisol do not solely represent biological effects of circulating cortisol on the expression of glucocorticoid-related genes in subcutaneous adipose tissue, whereas PM levels, amplitude, and net amounts of the diurnally fluctuating serum cortisol have distinct effects. Through the genes identified in this study, glucocorticoids appear to influence intermediary metabolism, energy balance, inflammation, and local circadian rythmicity in subcutaneous fat. Our results may also explain in part the development of metabolic abnormality and obesity in subjects under stress or patients with melancholic/atypical depression who demonstrate elevated levels of PM serum cortisol.

Relative metabolic stability, but disrupted circadian cortisol secretion during the fasting month of Ramadan

BACKGROUND

Chronic feeding and sleep schedule disturbances are stressors that exert damaging effects on the organism. Practicing Muslims in Saudi Arabia go through strict Ramadan fasting from dawn till sunset for one month yearly. Modern era Ramadan practices in Saudi Arabia are associated with disturbed feeding and sleep patterns, namely abstaining from food and water and increasing daytime sleep, and staying awake and receiving food and water till dawn.

HYPOTHESIS

Strict Ramadan practices in Saudi Arabia may influence metabolism, sleep and circadian cortisol secretion.

PROTOCOL

Young, male Ramadan practitioners were evaluated before and two weeks into the Ramadan. Blood samples were collected at 9.00 am and 9.00 pm for measurements of metabolic parameters and cortisol. Saliva was collected serially during the day for cortisol determinations.

RESULTS

Ramadan practitioners had relative metabolic stability or changes expected by the pattern of feeding. However, the cortisol circadian rhythm was abolished and circulating insulin levels and HOMA index were increased during this period.

DISCUSSION

The flattening of the cortisol rhythm is typical of conditions associated with chronic stress or endogenous hypercortisolism and associated with insulin resistance.

CONCLUSIONS

Modern Ramadan practices in Saudi Arabia are associated with evening hypercortisolism and increased insulin resistance. These changes might contribute to the high prevalence of chronic stress-related conditions, such as central obesity, hypertension, metabolic syndrome and diabetes mellitus type 2, and their cardiovascular sequelae observed in the Kingdom.

A novel point mutation in the DNA-binding domain (DBD) of the human glucocorticoid receptor causes primary generalized glucocorticoid resistance by disrupting the hydrophobic structure of its DBD

CONTEXT

Primary generalized glucocorticoid resistance is a rare genetic condition characterized by partial end-organ insensitivity to glucocorticoids. Most affected subjects present with clinical manifestations of mineralocorticoid and androgen excess. The condition has been associated with inactivating mutations in the human glucocorticoid receptor (hGR) gene, which impair the molecular mechanisms of hGRα action, thereby reducing tissue sensitivity to glucocorticoids.

OBJECTIVE

ΤHE aim of our study was to investigate the molecular mechanisms through which one previously described natural heterozygous V423A mutation, the second mutation detected in the DNA-binding domain (DBD) of the hGRα, affects glucocorticoid signal transduction.

DESIGN AND RESULTS

Compared with the wild-type receptor, hGRαV423A demonstrated a 72% reduction in its ability to transactivate the glucocorticoid-inducible mouse mammary tumor virus promoter in response to dexamethasone. The hGRαV423A receptor showed a significant reduction in its ability to bind to glucocorticoid-response elements of glucocorticoid-responsive genes, owing to structural alterations of the DBD confirmed by computer-based structural analysis. In addition, hGRαV423A demonstrated a 2.6-fold delay in nuclear translocation following exposure to the ligand, although it did not exert a dominant negative effect on the wild-type hGRα, had a similar affinity to the ligand with the wild-type receptor, and displayed a normal interaction with the GRIP1 coactivator in vitro.

CONCLUSIONS

The natural mutant receptor hGRαV423A causes primary generalized glucocorticoid resistance by affecting multiple steps in the cascade of glucocorticoid receptor action, which primarily involve decreased ability to bind to target glucocorticoid response elements and delayed translocation into the nucleus.

The mammalian clock and chronopharmacology

Increases in our understanding of the molecular control of circadian rhythms and subsequent signaling pathways has allowed for new therapeutic drug targets to be identified as well as for a better understanding of how to more efficaciously and safely utilize current drugs. Here, we review recent advances in targeting components of the molecular clock in mammals for the development of novel therapeutics as well as describe the impact of the circadian rhythm on drug efficacy and toxicity.

Histone Deacetylase Inhibition Attenuates Transcriptional Activity of Mineralocorticoid Receptor Through Its Acetylation and Prevents Development of Hypertension

Rationale:

Inhibition of histone deacetylases (HDACs) results in attenuated development of hypertension in deoxycorticosterone acetate–induced hypertensive rats and spontaneously hypertensive rats. However, the molecular mechanism remains elusive.

Objective:

We hypothesized that HDAC inhibition attenuates transcriptional activity of mineralocorticoid receptor (MR) through its acetylation and prevents development of hypertension in deoxycorticosterone acetate–induced hypertensive rats.

Methods and Results:

Expression of MR target genes was measured by quantitative real-time polymerase chain reaction. Recruitment of MR and RNA polymerase II on promoters of target genes was analyzed by chromatin immunoprecipitation assay. Live cell imaging was performed for visualization of nuclear translocation of MR. MR acetylation was determined by Western blot with anti-acetyl-lysine antibody after immunoprecipitation with anti-MR antibody. Transcriptional activity of MR was determined by luciferase assay. For establishment of a hyperaldosteronism animal, Sprague-Dawley rats underwent uninephrectomy and received subcutaneous injection of 40 mg/kg per week of deoxycorticosterone acetate and drinking water containing 1% NaCl. Treatment with a HDAC class I inhibitor resulted in reduced expression of MR target genes in accordance with reduced recruitment of MR and RNA polymerase II on promoters of target genes. HDAC inhibition promoted MR acetylation, leading to decreased transcriptional activity of MR. Knockdown or inhibition of HDAC3 resulted in reduced expression of MR target genes induced by mineralocorticoids.

Conclusions:

These results indicate that HDAC inhibition attenuates transcriptional activity of MR through its acetylation and prevents development of hypertension in deoxycorticosterone acetate–induced hypertensive rats.

Circadian acetylome reveals regulation of mitochondrial metabolic pathways

The circadian clock is constituted by a complex molecular network that integrates a number of regulatory cues needed to maintain organismal homeostasis. To this effect, posttranslational modifications of clock proteins modulate circadian rhythms and are thought to convert physiological signals into changes in protein regulatory function. To explore reversible lysine acetylation that is dependent on the clock, we have characterized the circadian acetylome in WT and Clock-deficient (Clock(-/-)) mouse liver by quantitative mass spectrometry. Our analysis revealed that a number of mitochondrial proteins involved in metabolic pathways are heavily influenced by clock-driven acetylation. Pathways such as glycolysis/gluconeogenesis, citric acid cycle, amino acid metabolism, and fatty acid metabolism were found to be highly enriched hits. The significant number of metabolic pathways whose protein acetylation profile is altered in Clock(-/-) mice prompted us to link the acetylome to the circadian metabolome previously characterized in our laboratory. Changes in enzyme acetylation over the circadian cycle and the link to metabolite levels are discussed, revealing biological implications connecting the circadian clock to cellular metabolic state.

Characterisation of the maternal response to chronic phase shifts during gestation in the rat: implications for fetal metabolic programming

Disrupting maternal circadian rhythms through exposure to chronic phase shifts of the photoperiod has lifelong consequences for the metabolic homeostasis of the fetus, such that offspring develop increased adiposity, hyperinsulinaemia and poor glucose and insulin tolerance. In an attempt to determine the mechanisms by which these poor metabolic outcomes arise, we investigated the impact of chronic phase shifts (CPS) on maternal and fetal hormonal, metabolic and circadian rhythms. We assessed weight gain and food consumption of dams exposed to either CPS or control lighting conditions throughout gestation. At day 20, dams were assessed for plasma hormone and metabolite concentrations and glucose and insulin tolerance. Additionally, the expression of a range of circadian and metabolic genes was assessed in maternal, placental and fetal tissue. Control and CPS dams consumed the same amount of food, yet CPS dams gained 70% less weight during the first week of gestation. At day 20, CPS dams had reduced retroperitoneal fat pad weight (-15%), and time-of-day dependent decreases in liver weight, whereas fetal and placental weight was not affected. Melatonin secretion was not altered, yet the timing of corticosterone, leptin, glucose, insulin, free fatty acids, triglycerides and cholesterol concentrations were profoundly disrupted. The expression of gluconeogenic and circadian clock genes in maternal and fetal liver became either arrhythmic or were in antiphase to the controls. These results demonstrate that disruptions of the photoperiod can severely disrupt normal circadian profiles of plasma hormones and metabolites, as well as gene expression in maternal and fetal tissues. Disruptions in the timing of food consumption and the downstream metabolic processes required to utilise that food, may lead to reduced efficiency of growth such that maternal weight gain is reduced during early embryonic development. It is these perturbations that may contribute to the programming of poor metabolic homeostasis in the offspring.

Metastasis-associated protein 1 is an integral component of the circadian molecular machinery

The mammalian circadian clock regulates the daily cycles of many important physiological processes, but its mechanism is not well understood. Here we provide genetic and biochemical evidence that metastasis-associated protein 1 (MTA1), a widely upregulated gene product in human cancers, is an integral component of the circadian molecular machinery. Knockout of MTA1 in mice disrupts the free-running period of circadian rhythms under constant light and normal entrainment of behaviour to 12-h-light/12-h-dark cycles. The CLOCK-BMAL1 heterodimer activates MTA1 transcription through a conserved E-box element at its promoter. MTA1, in turn, interacts with and recruits CLOCK-BMAL1 at its own and CRY1 promoters and promotes their transcription. Moreover, MTA1 deacetylates BMAL1 at lysine 538 through regulating deacetylase SIRT1 expression, thus disturbing the CRY1-mediated negative feedback loop. These findings uncover a previously unappreciated role for MTA1 in maintenance of circadian rhythmicity through acting on the positive limb of the clock machinery.

Circadian rhythm disruption in cancer biology

Circadian rhythms show universally a 24-h oscillation pattern in metabolic, physiological and behavioral functions of almost all species. This pattern is due to a fundamental adaptation to the rotation of Earth around its own axis. Molecular mechanisms of generation of circadian rhythms organize a biochemical network in suprachiasmatic nucleus and peripheral tissues, building cell autonomous clock pacemakers. Rhythmicity is observed in transcriptional expression of a wide range of clock-controlled genes that regulate a variety of normal cell functions, such as cell division and proliferation. Desynchrony of this rhythmicity seems to be implicated in several pathologic conditions, including tumorigenesis and progression of cancer. In 2007, the International Agency for Research on Cancer (IARC) categorized "shiftwork that involves circadian disruption [as] probably carcinogenic to humans" (Group 2A in the IARC classification system of carcinogenic potency of an agentagent) (Painting, Firefighting, and Shiftwork; IARC; 2007). This review discusses the potential relation between disruptions of normal circadian rhythms with genetic driving machinery of cancer. Elucidation of the role of clockwork disruption, such as exposure to light at night and sleep disruption, in cancer biology could be important in developing new targeted anticancer therapies, optimizing individualized chronotherapy and modifying lighting environment in workplaces or homes.

Stress response and child health

Stress is defined as a state of real or perceived threat to homeostasis. The principal effectors of the stress system are corticotropin-releasing hormone (CRH), arginine vasopressin, the proopiomelanocortin-derived peptides α-melanocyte-stimulating hormone and β-endorphin, the glucocorticoids, and the catecholamines norepinephrine and epinephrine. Appropriate responsiveness of the stress system to stressors is a crucial prerequisite for a sense of well-being, adequate performance of tasks, and positive social interactions. By contrast, inappropriate responsiveness of the stress system may impair growth and development and may account for various endocrine, metabolic, autoimmune, and psychiatric disorders. The development and severity of these conditions primarily depend on the genetic vulnerability of the individual, the exposure to adverse environmental factors, and the timing of stressful event(s). Prenatal life, infancy, childhood, and adolescence are critical periods characterized by increased vulnerability to stressors. This review summarizes the topics presented at the fifth New Inroads to Child Health (NICHe) Conference "Stress Response and Child Health" held at Heraklion, Crete, Greece, in May 2012.

Circadian rhythms of glucocorticoid hormone actions in target tissues: potential clinical implications

Organisms face unforeseen short- and long-term changes in the environment (stressors). To defend against these changes, organisms have developed a stress system that includes the hypothalamic-pituitary-adrenal (HPA) axis, which employs glucocorticoids and the glucocorticoid receptor (GR) for signal transduction. In addition, organisms live under the strong influence of day-night cycles and, hence, have also developed a highly conserved circadian clock system for adjusting their activities to recurring environmental changes. This regulatory system creates and maintains internal circadian rhythmicity by employing a self-oscillating molecular pacemaker composed of the Clock-Bmal1 heterodimer and other transcription factors. The circadian clock consists of a central master clock in the suprachiasmatic nucleus of the brain hypothalamus and peripheral slave clocks in virtually all organs and tissues. The HPA axis and the circadian clock system communicate with each other at multiple levels. The central clock controls the HPA axis, creating the diurnal oscillation of circulating adrenocorticotropic hormone and cortisol, and the HPA axis adjusts the circadian rhythmicity of the peripheral clocks in response to various stressors through the GR. Further, Clock-Bmal1 regulates the response to glucocorticoids in peripheral tissues through acetylation of the GR, possibly antagonizing the biologic actions of diurnally fluctuating circulating cortisol. Importantly, dysregulation in the clock system and the HPA axis may cause similar pathologic manifestations--including obesity, metabolic syndrome, and cardiovascular disease--by uncoupling circulating cortisol concentrations from tissue sensitivity to glucocorticoids.

Glucose can promote a glucocorticoid resistance state

It has been shown that ingestion of glucose, amino acids, protein or mixed meals tends to increase serum and salivary cortisol concentrations in healthy adults. Recently, it has been demonstrated that morning glucose ingestion stimulates pulsatile cortisol and adrenocorticotropic hormone (ACTH) secretion, thus elevating their mean concentrations. In light of the above, a question arises: could the frequent food – and specifically glucose – consumption lead to hypercortisolism with possible clinical implications? And can the human body, under normal conditions raise defence mechanisms against the transient hypercortisolism caused by the frequent glucose consumption? Studies have revealed novel mechanisms, which are implicated in the glucocorticoid receptor (GR)-mediated action, providing a kind of glucocorticoid resistance. This glucocorticoid resistance could be mediated through both enhancing acetylation (via, among others, regulation of essential clock genes such as Per) and inhibiting deacetylation of GR (via possible regulation of sirtuin activity). Interestingly, the acetylation/deacetylation processes seem to be regulated by glucose. Thus, glucose apart from causing increased cortisol secretion can, simultaneously, counter-regulate this hypercortisolism, by promoting directly and/or indirectly a glucocorticoid resistance state. Undoubtedly, before extracting conclusions regarding the clinical significance of the increased cortisol secretion following glucose ingestion, we should first thoroughly investigate the ‘defence’ mechanisms provided by ‘nature’ to handle this hypercortisolism.

Core circadian protein CLOCK is a positive regulator of NF-κB-mediated transcription

The circadian clock controls many physiological parameters including immune response to infectious agents, which is mediated by activation of the transcription factor NF-κB. It is widely accepted that circadian regulation is based on periodic changes in gene expression that are triggered by transcriptional activity of the CLOCK/BMAL1 complex. Through the use of a mouse model system we show that daily variations in the intensity of the NF-κB response to a variety of immunomodulators are mediated by core circadian protein CLOCK, which can up-regulate NF-κB-mediated transcription in the absence of BMAL1; moreover, BMAL1 counteracts the CLOCK-dependent increase in the activation of NF-κB-responsive genes. Consistent with its regulatory function, CLOCK is found in protein complexes with the p65 subunit of NF-κB, and its overexpression correlates with an increase in specific phosphorylated and acetylated transcriptionally active forms of p65. In addition, activation of NF-κB in response to immunostimuli in mouse embryonic fibroblasts and primary hepatocytes isolated from Clock-deficient mice is significantly reduced compared with WT cells, whereas Clock-Δ19 mutation, which reduces the transactivation capacity of CLOCK on E-box-containing circadian promoters, has no effect on the ability of CLOCK to up-regulate NF-κB-responsive promoters. These findings establish a molecular link between two essential determinants of the circadian and immune mechanisms, the transcription factors CLOCK and NF-κB, respectively.

[Physiological and pathophysiological role of the circadian clock system]

It has been well known for ages that in living organisms the rhythmicity of biological processes is linked to the ~ 24-hour light-dark cycle. However, the exact function of the circadian clock system has been explored only in the past decades. It came to light that the photosensitive primary "master clock" is situated in the suprachiasmatic photosensitive nuclei of the special hypothalamic region, and that it is working according to ~24-hour changes of light and darkness. The master clock sends its messages to the peripheral "slave clocks". In many organs, like pancreatic β-cells, the slave clocks have autonomic functions as well. Two essential components of the clock system are proteins encoded by the CLOCK and BMAL1 genes. CLOCK genes are in interaction with endonuclear receptors such as peroxisoma-proliferator activated receptors and Rev-erb-α, as well as with the hypothalamic-pituitary-adrenal axis, regulating the adaptation to stressors, energy supply, metabolic processes and cardiovascular system. Melatonin, the product of corpus pineale has a significant role in the functions of the clock system. The detailed discovery of the clock system has changed our previous knowledge about the development of many diseases. The most explored fields are hypertension, cardiovascular diseases, metabolic processes, mental disorders, cancers, sleep apnoe and joint disorders. CLOCK genes influence ageing as well. The recognition of the periodicity of biological processes makes the optimal dosing of certain drugs feasible. The more detailed discovery of the interaction of the clock system might further improve treatment and prevention of many disorders.

Current concepts in glucocorticoid resistance

Glucocorticoids (GCs) are the most potent anti-inflammatory agents known. A major factor limiting their clinical use is the wide variation in responsiveness to therapy. The high doses of GC required for less responsive patients means a high risk of developing very serious side effects. Variation in sensitivity between individuals can be due to a number of factors. Congenital, generalized GC resistance is very rare, and is due to mutations in the glucocorticoid receptor (GR) gene, the receptor that mediates the cellular effects of GC. A more common problem is acquired GC resistance. This localized, disease-associated GC resistance is a serious therapeutic concern and limits therapeutic response in patients with chronic inflammatory disease. It is now believed that localized resistance can be attributed to changes in the cellular microenvironment, as a consequence of chronic inflammation. Multiple factors have been identified, including alterations in both GR-dependent and -independent signaling downstream of cytokine action, oxidative stress, hypoxia and serum derived factors. The underlying mechanisms are now being elucidated, and are discussed here. Attempts to augment tissue GC sensitivity are predicted to permit safe and effective use of low-dose GC therapy in inflammatory disease.

Clocks, metabolism, and the epigenome

Many behaviors and physiological activities in living organisms display circadian rhythms, allowing the organisms to anticipate and prepare for the diurnal changes in the living environment. In this way, metabolic processes are aligned with the periodic environmental changes and behavioral cycles, such as the sleep/wake and fasting/feeding cycles. Disturbances of this alignment significantly increase the risk of metabolic diseases. Meanwhile, the circadian clock receives signals from the environment and feedback from metabolic pathways, and adjusts its activity and function. Growing evidence connects the circadian clock with epigenomic regulators. Here we review the recent advances in understanding the crosstalk between the circadian clock and energy metabolism through epigenomic programming and transcriptional regulation.

Complex dynamics of transcription regulation

Transcription is a tightly regulated cellular function which can be triggered by endogenous (intrinsic) or exogenous (extrinsic) signals. The development of novel techniques to examine the dynamic behavior of transcription factors and the analysis of transcriptional activity at the single cell level with increased temporal resolution has revealed unexpected elements of stochasticity and dynamics of this process. Emerging research reveals a complex picture, wherein a wide range of time scales and temporal transcription patterns overlap to generate transcriptional programs. The challenge now is to develop a perspective that can guide us to common underlying mechanisms, and consolidate these findings. Here we review the recent literature on temporal dynamics and stochastic gene regulation patterns governed by intrinsic or extrinsic signals, utilizing the glucocorticoid receptor (GR)-mediated transcriptional model to illustrate commonality of these emerging concepts. This article is part of a Special Issue entitled: Chromatin in time and space.

The nuclear receptor REV-ERBα is required for the daily balance of carbohydrate and lipid metabolism

Mutations of clock genes can lead to diabetes and obesity. REV-ERBα, a nuclear receptor involved in the circadian clockwork, has been shown to control lipid metabolism. To gain insight into the role of REV-ERBα in energy homeostasis in vivo, we explored daily metabolism of carbohydrates and lipids in chow-fed, unfed, or high-fat-fed Rev-erbα(-/-) mice and their wild-type littermates. Chow-fed Rev-erbα(-/-) mice displayed increased adiposity (2.5-fold) and mild hyperglycemia (∼10%) without insulin resistance. Indirect calorimetry indicates that chow-fed Rev-erbα(-/-) mice utilize more fatty acids during daytime. A 24-h nonfeeding period in Rev-erbα(-/-) animals favors further fatty acid mobilization at the expense of glycogen utilization and gluconeogenesis, without triggering hypoglycemia and hypothermia. High-fat feeding in Rev-erbα(-/-) mice amplified metabolic disturbances, including expression of lipogenic factors. Lipoprotein lipase (Lpl) gene, critical in lipid utilization/storage, is triggered in liver at night and constitutively up-regulated (∼2-fold) in muscle and adipose tissue of Rev-erbα(-/-) mice. We show that CLOCK, up-regulated (2-fold) at night in Rev-erbα(-/-) mice, can transactivate Lpl. Thus, overexpression of Lpl facilitates muscle fatty acid utilization and contributes to fat overload. This study demonstrates the importance of clock-driven Lpl expression in energy balance and highlights circadian disruption as a potential cause for the metabolic syndrome.

Metabolic consequences of stress during childhood and adolescence

Stress, that is, the state of threatened or perceived as threatened homeostasis, is associated with activation of the stress system, mainly comprised by the hypothalamic-pituitary-adrenal axis and the arousal/sympathetic nervous systems. The stress system normally functions in a circadian manner and interacts with other systems to regulate a variety of behavioral, endocrine, metabolic, immune, and cardiovascular functions. However, the experience of acute intense physical or emotional stress, as well as of chronic stress, may lead to the development of or may exacerbate several psychologic and somatic conditions, including anxiety disorders, depression, obesity, and the metabolic syndrome. In chronically stressed individuals, both behavioral and neuroendocrine mechanisms promote obesity and metabolic abnormalities: unhealthy lifestyles in conjunction with dysregulation of the stress system and increased secretion of cortisol, catecholamines, and interleukin-6, with concurrently elevated insulin concentrations, lead to development of central obesity, insulin resistance, and the metabolic syndrome. Fetal life, childhood, and adolescence are particularly vulnerable periods of life to the effects of intense acute or chronic stress. Similarly, these life stages are crucial for the later development of behavioral, metabolic, and immune abnormalities. Developing brain structures and functions related to stress regulation, such as the amygdala, the hippocampus, and the mesocorticolimbic system, are more vulnerable to the effects of stress compared with mature structures in adults. Moreover, chronic alterations in cortisol secretion in children may affect the timing of puberty, final stature, and body composition, as well as cause early-onset obesity, metabolic syndrome, and type 2 diabetes mellitus. The understanding of stress mechanisms leading to metabolic abnormalities in early life may lead to more effective prevention and intervention strategies of obesity-related health problems.

Entrainment of peripheral clock genes by cortisol

Circadian rhythmicity in mammals is primarily driven by the suprachiasmatic nucleus (SCN), often called the central pacemaker, which converts the photic information of light and dark cycles into neuronal and hormonal signals in the periphery of the body. Cells of peripheral tissues respond to these centrally mediated cues by adjusting their molecular function to optimize organism performance. Numerous systemic cues orchestrate peripheral rhythmicity, such as feeding, body temperature, the autonomic nervous system, and hormones. We propose a semimechanistic model for the entrainment of peripheral clock genes by cortisol as a representative entrainer of peripheral cells. This model demonstrates the importance of entrainer's characteristics in terms of the synchronization and entrainment of peripheral clock genes, and predicts the loss of intercellular synchrony when cortisol moves out of its homeostatic amplitude and frequency range, as has been observed clinically in chronic stress and cancer. The model also predicts a dynamic regime of entrainment, when cortisol has a slightly decreased amplitude rhythm, where individual clock genes remain relatively synchronized among themselves but are phase shifted in relation to the entrainer. The model illustrates how the loss of communication between the SCN and peripheral tissues could result in desynchronization of peripheral clocks.

Circadian rhythms: from basic mechanisms to the intensive care unit

OBJECTIVE

: Circadian rhythms are intrinsic timekeeping mechanisms that allow for adaptation to cyclic environmental changes. Increasing evidence suggests that circadian rhythms may influence progression of a variety of diseases as well as effectiveness and toxicity of drugs commonly used in the intensive care unit. In this perspective, we provide a brief review of the molecular mechanisms of circadian rhythms and its relevance to critical care. DATA SOURCES, STUDY SELECTION, DATA EXTRACTION, AND DATA SYNTHESIS:: Articles related to circadian rhythms and organ systems in normal and disease conditions were searched through the PubMed library with the goal of providing a concise review.

CONCLUSIONS

: Critically ill patients may be highly vulnerable to disruption of circadian rhythms as a result of the severity of their underlying diseases as well as the intensive care unit environment where noise and frequent therapeutic/diagnostic interventions take place. Further basic and clinical research addressing the importance of circadian rhythms in the context of critical care is warranted to develop a better understanding of the complex pathophysiology of critically ill patients as well as to identify novel therapeutic approaches for these patients.