Circadian clock disruption in the mouse ovary in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Consensus on the key characteristics of metabolism disruptors

Metabolism-disrupting agents (MDAs) are chemical, infectious or physical agents that increase the risk of metabolic disorders. Examples include pharmaceuticals, such as antidepressants, and environmental agents, such as bisphenol A. Various types of studies can provide evidence to identify MDAs, yet a systematic method is needed to integrate these data to help to identify such hazards. Inspired by work to improve hazard identification of carcinogens using key characteristics (KCs), we developed 12 KCs of MDAs based on our knowledge of processes underlying metabolic diseases and the effects of their causal agents: (1) alters function of the endocrine pancreas; (2) impairs function of adipose tissue; (3) alters nervous system control of metabolic function; (4) promotes insulin resistance; (5) disrupts metabolic signalling pathways; (6) alters development and fate of metabolic cell types; (7) alters energy homeostasis; (8) causes inappropriate nutrient handling and partitioning; (9) promotes chronic inflammation and immune dysregulation in metabolic tissues; (10) disrupts gastrointestinal tract function; (11) induces cellular stress pathways; and (12) disrupts circadian rhythms. In this Consensus Statement, we present the logic that revealed the KCs of MDAs and highlight evidence that supports the identification of KCs. We use chemical, infectious and physical agents as examples to illustrate how the KCs can be used to organize and use mechanistic data to help to identify MDAs.

Food-Borne Polycyclic Aromatic Hydrocarbons and Circadian Disruption

Circadian disruption has been found to increase the risk of metabolic diseases, brain disorders, and cancer. The aryl hydrocarbon receptor (AhR), responsible for xenobiotic metabolism, is known to be activated by certain environmental stimuli, including polycyclic aromatic hydrocarbons (PAHs). Exposure to these stimuli may lead to diseases related to circadian disruption, with AhR activation suggested as a leading cause. Both the aryl hydrocarbon receptor nuclear translocator (ARNT) and aryl hydrocarbon receptor nuclear translocator-like (BMAL1) are class II basic helix-loop-helix/Per-ARNT-SIM (bHLH-PAS) proteins. These proteins form heterodimers with stimulated class I bHLH-PAS proteins, including circadian locomotor output cycles kaput (CLOCK) and AhR. Due to their sequential similarity, the overactivation of AhR by toxicants, such as PAHs, may lead to the formation of heterodimers with BMAL1, potentially causing circadian disruption. Dysregulation of BMAL1 can affect a wide range of metabolic genes, emphasizing its crucial roles. However, this issue has not been adequately addressed. Previous studies have reported that the inhibitory effects of phytochemicals on AhR activation can ameliorate diseases induced by environmental toxicants. Additionally, some phytochemicals have shown preventive effects on circadian misalignment. Therefore, this Review aims to explore potential strategies to prevent circadian disruption induced by food-borne toxicants, such as benzo[a]pyrene; to generate new ideas for future studies; and to highlight the importance of investigating these preventive strategies.

Aryl hydrocarbon receptor impairs circadian regulation in Alzheimer's disease: Potential impact on glymphatic system dysfunction

Circadian clocks maintain diurnal rhythms of sleep-wake cycle of 24 h that regulate not only the metabolism of an organism but also many other periodical processes. There is substantial evidence that circadian regulation is impaired in Alzheimer's disease. Circadian clocks regulate many properties known to be disturbed in Alzheimer's patients, such as the integrity of the blood-brain barrier (BBB) as well as the diurnal glymphatic flow that controls waste clearance from the brain. Interestingly, an evolutionarily conserved transcription factor, that is, aryl hydrocarbon receptor (AhR), impairs the function of the core clock proteins and thus could disturb diurnal rhythmicity in the BBB. There is abundant evidence that the activation of AhR signalling inhibits the expression of the major core clock proteins, such as the brain and muscle arnt-like 1 (BMAL1), clock circadian regulator (CLOCK) and period circadian regulator 1 (PER1) in different experimental models. The expression of AhR is robustly increased in the brains of Alzheimer's patients, and protein level is enriched in astrocytes of the BBB. It seems that AhR signalling inhibits glymphatic flow since it is known that (i) activation of AhR impairs the function of the BBB, which is cooperatively interconnected with the glymphatic system in the brain, and (ii) neuroinflammation and dysbiosis of gut microbiota generate potent activators of AhR, which are able to impair glymphatic flow. I will examine current evidence indicating that activation of AhR signalling could disturb circadian functions of the BBB and impair glymphatic flow and thus be involved in the development of Alzheimer's pathology.

Effect of TCDD exposure in adult female and male mice on the expression of miRNA in the ovaries and testes and associated reproductive functions

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant found widely across the world. While animal and human studies have shown that exposure to TCDD may cause significant alterations in the reproductive tract, the effect of TCDD on the expression of miRNA in the reproductive organs has not been previously tested. In the current study, we exposed adult female or male mice to TCDD or vehicle and bred them to study the impact on reproduction. The data showed that while TCDD treatment of females caused no significant change in litter size, it did alter the survival of the pups. Also, TCDD exposure of either the male or female mice led to an increase in the gestational period. While TCDD did not alter the gross morphology of the ovaries and testes, it induced significant alterations in the miRNA expression. The ovaries showed the differential expression of 426 miRNAs, of which 315 miRNAs were upregulated and 111 miRNA that were downregulated after TCDD exposure when compared to the vehicle controls. In the testes, TCDD caused the differential expression of 433 miRNAs, with 247 miRNAs upregulated and 186 miRNAs downregulated. Pathway analysis showed that several of these dysregulated miRNAs targeted reproductive functions. The current study suggests that the reproductive toxicity of TCDD may result from alterations in the miRNA expression in the reproductive organs. Because miRNAs also represent one of the epigenetic pathways of gene expression, our studies suggest that the transgenerational toxicity of TCDD may also result from dysregulation in the miRNAs.

Aryl hydrocarbon receptor (AhR) impairs circadian regulation: impact on the aging process

Circadian clocks control the internal sleep-wake rhythmicity of 24hours which is synchronized by the solar cycle. Circadian regulation of metabolism evolved about 2.5 billion years ago, i.e., the rhythmicity has been conserved from cyanobacteria and Archaea through to mammals although the mechanisms utilized have developed with evolution. While the aryl hydrocarbon receptor (AhR) is an evolutionarily conserved defence mechanism against environmental threats, it has gained many novel functions during evolution, such as the regulation of cell cycle, proteostasis, and many immune functions. There is robust evidence that AhR signaling impairs circadian rhythmicity, e.g., by interacting with the core BMAL1/CLOCK complex and disturbing the epigenetic regulation of clock genes. The maintenance of circadian rhythms is impaired with aging, disturbing metabolism and many important functions in aged organisms. Interestingly, it is known that AhR signaling promotes an age-related tissue degeneration, e.g., it is able to inhibit autophagy, enhance cellular senescence, and disrupt extracellular matrix. These alterations are rather similar to those induced by a long-term impairment of circadian rhythms. However, it is not known whether AhR signaling enhances the aging process by impairing circadian homeostasis. I will examine the experimental evidence indicating that AhR signaling is able to promote the age-related degeneration via a disruption of circadian rhythmicity.

Aryl hydrocarbon receptor affects circadian-regulated lipolysis through an E-Box-dependent mechanism

An internal circadian clock regulates timing of systemic energy homeostasis. The central clock in the hypothalamic suprachiasmatic nucleus (SCN) directs local clocks in peripheral tissues such as liver, muscle, and adipose tissue to synchronize metabolism with food intake and rest/activity cycles. Aryl hydrocarbon receptor (AhR) interacts with the molecular circadian clockworks. Activation of AhR dampens rhythmic expression of core clock genes, which may lead to metabolic dysfunction. Given the importance of appropriately-timed adipose tissue function to regulation of energy homeostasis, this study focused on mechanisms by which AhR may influence clock-controlled adipose tissue activity. We hypothesized that AhR activation in adipose tissue would impair lipolysis by dampening adipose rhythms, leading to a decreased lipolysis rate during fasting, and subsequently, altered serum glucose concentrations. Levels of clock gene and lipolysis gene transcripts in mouse mesenchymal stem cells (BMSCs) differentiated into mature adipocytes were suppressed by the AhR agonist β-napthoflavone (BNF), in an AhR dependent manner. BNF altered rhythms of core clock gene and lipolysis gene transcripts in C57bl6/J mice. BNF reduced serum free fatty acids, glycerol and liver glycogen. Chromatin immunoprecipitation indicated that BNF increased binding of AhR to E-Box elements in clock gene and lipolysis gene promoters. These data establish a link between AhR activation and impaired lipolysis, specifically by altering adipose tissue rhythmicity. In response to the decreased available energy from impaired lipolysis, the body increases glycogenolysis, thereby degrading more glycogen to provide necessary energy.

Control of Arterial Hypertension by the AhR Blocker CH-223191: A Chronopharmacological Study in Chronic Intermittent Hypoxia Conditions

Chronic intermittent hypoxia (CIH) is a major contributor to the development of hypertension (HTN) in obstructive sleep apnea (OSA). OSA subjects frequently display a non-dipping pattern of blood pressure (BP) and resistant HTN. After discovering that AHR-CYP1A1 axis is a druggable target in CIH-HTN, we hypothesized that CH-223191 could control BP in both active and inactive periods of the animals, recovering the BP dipping profile in CIH conditions.We evaluated the chronopharmacology of the antihypertensive efficacy of the AhR blocker CH-223191 in CIH conditions (21% to 5% of O₂, 5.6 cycles/h, 10.5 h/day, in inactive period of Wistar rats). BP was measured by radiotelemetry, at 8 am (active phase) and at 6 pm (inactive phase) of the animals. The circadian variation of AhR activation in the kidney in normoxia was also assessed, measuring the CYP1A1 (hallmark of AhR activation) protein levels.Despite drug administration before starting the inactive period of the animals, CH-223191 was not able to decrease BP during the inactive phase, in CIH conditions, therefore not reverting the non-dipping profile. These results suggest that a higher dose or different time of administration of CH-223191 might be needed for an antihypertensive effect throughout the 24-h cycle.

From Nucleus to Organs: Insights of Aryl Hydrocarbon Receptor Molecular Mechanisms

The aryl hydrocarbon receptor (AHR) is a markedly established regulator of a plethora of cellular and molecular processes. Its initial role in the detoxification of xenobiotic compounds has been partially overshadowed by its involvement in homeostatic and organ physiology processes. In fact, the discovery of its ability to bind specific target regulatory sequences has allowed for the understanding of how AHR modulates such processes. Thereby, AHR presents functions in transcriptional regulation, chromatin architecture modifications and participation in different key signaling pathways. Interestingly, such fields of influence end up affecting organ and tissue homeostasis, including regenerative response both to endogenous and exogenous stimuli. Therefore, from classical spheres such as canonical transcriptional regulation in embryonic development, cell migration, differentiation or tumor progression to modern approaches in epigenetics, senescence, immune system or microbiome, this review covers all aspects derived from the balance between regulation/deregulation of AHR and its physio-pathological consequences.

Environmental disruption of reproductive rhythms

Reproduction is a key biological function requiring a precise synchronization with annual and daily cues to cope with environmental fluctuations. Therefore, humans and animals have developed well-conserved photoneuroendocrine pathways to integrate and process daily and seasonal light signals within the hypothalamic-pituitary-gonadal axis. However, in the past century, industrialization and the modern 24/7 human lifestyle have imposed detrimental changes in natural habitats and rhythms of life. Indeed, exposure to an excessive amount of artificial light at inappropriate timing because of shift work and nocturnal urban lighting, as well as the ubiquitous environmental contamination by endocrine-disrupting chemicals, threaten the integrity of the daily and seasonal timing of biological functions. Here, we review recent epidemiological, field and experimental studies to discuss how light and chemical pollution of the environment can disrupt reproductive rhythms by interfering with the photoneuroendocrine timing system.

Derailed peripheral circadian genes in polycystic ovary syndrome patients alters peripheral conversion of androgens synthesis

STUDY QUESTION

Do circadian genes exhibit an altered profile in peripheral blood mononuclear cells (PBMCs) of polycystic ovary syndrome (PCOS) patients and do they have a potential role in androgen excess?

SUMMARY ANSWER

Our findings revealed that an impaired circadian clock could hamper the regulation of peripheral steroid metabolism in PCOS women.

WHAT IS KNOWN ALREADY

PCOS patients exhibit features of metabolic syndrome. Circadian rhythm disruption is involved in the development of metabolic diseases and subfertility. An association between shift work and the incidence of PCOS in females was recently reported.

STUDY DESIGN, SIZE, DURATION

This is a retrospective case-referent study in which peripheral blood samples were obtained from 101 control and 101 PCOS subjects. PCOS diagnoses were based on Rotterdam Consensus criteria.

PARTICIPANTS/MATERIALS, SETTING, METHODS

This study comprised 101 women with PCOS and 101 control volunteers, as well as Swiss albino mice treated with dehydroepiandrosterone (DHEA) to induce PCOS development. Gene expression analyses of circadian and steroidogenesis genes in human PBMC and mice ovaries and blood were executed by quantitative real-time PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

We observed aberrant expression of peripheral circadian clock genes in PCOS, with a significant reduction in the core clock genes, circadian locomotor output cycles kaput (CLOCK) (P ≤ 0.00001), brain and muscle ARNT-like 1 (BMAL1) (P ≤ 0.00001) and NPAS2 (P ≤ 0.001), and upregulation of their negative feedback loop genes, CRY1 (P ≤ 0.00003), CRY2 (P ≤ 0.00006), PER1 (P ≤ 0.003), PER2 (P ≤ 0.002), DEC1 (P ≤ 0.0001) and DEC2 (P ≤ 0.00005). Transcript levels of an additional feedback loop regulating BMAL1 showed varied expression, with reduced RORA (P ≤ 0.008) and increased NR1D1 (P ≤ 0.02) in PCOS patients in comparison with the control group. We also demonstrated the expression pattern of clock genes in PBMCs of PCOS women at three different time points. PCOS patients also exhibited increased mRNA levels of steroidogenic enzymes like StAR (P ≤ 0.0005), CYP17A1 (P ≤ 0.005), SRD5A1 (P ≤ 0.00006) and SRD5A2 (P ≤ 0.009). Knockdown of CLOCK/BMAL1 in PBMCs resulted in a significant reduction in estradiol production, by reducing CYP19A1 and a significant increase in dihydrotestosterone production, by upregulating SRD5A1 and SRD5A2 in PBMCs. Our data also showed that CYP17A1 as a direct CLOCK-BMAL1 target in PBMCs. Phenotypic classification of PCOS subgroups showed a higher variation in expression of clock genes and steroidogenesis genes with phenotype A of PCOS. In alignment with the above results, altered expression of ovarian core clock genes (Clock, Bmal1 and Per2) was found in DHEA-treated PCOS mice. The expression of peripheral blood core clock genes in DHEA-induced PCOS mice was less robust and showed a loss of periodicity in comparison with that of control mice.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

We could not evaluate the circadian oscillation of clock genes and clock-controlled genes over a 24-h period in the peripheral blood of control versus PCOS subjects. Additionally, circadian genes in the ovaries of PCOS women could not be evaluated due to limitations in sample availability, hence we employed the androgen excess mouse model of PCOS for ovarian circadian assessment. Clock genes were assessed in the whole ovary of the androgen excess mouse model of PCOS rather than in granulosa cells, which is another limitation of the present work.

WIDER IMPLICATIONS OF THE FINDINGS

Our observations suggest that the biological clock is one of the contributing factors in androgen excess in PCOS, owing to its potential role in modulating peripheral androgen metabolism. Considering the increasing prevalence of PCOS and the rising frequency of delayed circadian rhythms and insufficient sleep among women, our study emphasizes the potential in modulating circadian rhythm as an important strategy in PCOS management, and further research on this aspect is highly warranted.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the RGCB-DBT Core Funds and a grant (#BT/PR29996/MED/97/472/2020) from the Department of Biotechnology (DBT), India, to M.L. B.S.J. was supported by a DST/INSPIRE Fellowship/2015/IF150361 and M.B.K. was supported by the Research Fellowship from Council of Scientific & Industrial Research (CSIR) (10.2(5)/2007(ii).E.U.II). The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Pharmacological Actions of Carbamate Insecticides at Mammalian Melatonin Receptors

Integrated in silico chemical clustering and melatonin receptor molecular modeling combined with in vitro 2-[125I]-iodomelatonin competition binding were used to identify carbamate insecticides with affinity for human melatonin receptor 1 (hMT1) and human melatonin receptor 2 (hMT2). Saturation and kinetic binding studies with 2-[125I]-iodomelatonin revealed lead carbamates (carbaryl, fenobucarb, bendiocarb, carbofuran) to be orthosteric ligands with antagonist apparent efficacy at hMT1 and agonist apparent efficacy at hMT2 Furthermore, using quantitative receptor autoradiography in coronal brain slices from C3H/HeN mice, carbaryl, fenobucarb, and bendiocarb competed for 2-[125I]-iodomelatonin binding in the suprachiasmatic nucleus (SCN), paraventricular nucleus of the thalamus (PVT), and pars tuberalis (PT) with affinities similar to those determined for the hMT1 receptor. Carbaryl (10 mg/kg i.p.) administered in vivo also competed ex vivo for 2-[125I]-iodomelatonin binding to the SCN, PVT, and PT, demonstrating the ability to reach brain melatonin receptors in C3H/HeN mice. Furthermore, the same dose of carbaryl given to C3H/HeN mice in constant dark for three consecutive days at subjective dusk (circadian time 10) phase-advanced circadian activity rhythms (mean = 0.91 hours) similar to melatonin (mean = 1.12 hours) when compared with vehicle (mean = 0.04 hours). Carbaryl-mediated phase shift of overt circadian activity rhythm onset is likely mediated via interactions with SCN melatonin receptors. Based on the pharmacological actions of carbaryl and other carbamate insecticides at melatonin receptors, exposure may modulate time-of-day information conveyed to the master biologic clock relevant to adverse health outcomes. SIGNIFICANCE STATEMENT: In silico chemical clustering and molecular modeling in conjunction with in vitro bioassays identified several carbamate insecticides (i.e., carbaryl, carbofuran, fenobucarb, bendiocarb) as pharmacologically active orthosteric melatonin receptor 1 and 2 ligands. This work further demonstrated that carbaryl competes for melatonin receptor binding in the master biological clock (suprachiasmatic nucleus) and phase-advances overt circadian activity rhythms in C3H/HeN mice, supporting the relevance of circadian effects when interpreting toxicological findings related to carbamate insecticide exposure.

Cross-species physiological interactions of endocrine disrupting chemicals with the circadian clock

Endocrine disrupting chemicals (EDCs) are endocrine-active chemical pollutants that disrupt reproductive, neuroendocrine, cardiovascular and metabolic health across species. The circadian clock is a transcriptional oscillator responsible for entraining 24-hour rhythms of physiology, behavior and metabolism. Extensive bidirectional cross talk exists between circadian and endocrine systems and circadian rhythmicity is present at all levels of endocrine control, from synthesis and release of hormones, to sensitivity of target tissues to hormone action. In mammals, a range of hormones directly alter clock gene expression and circadian physiology via nuclear receptor (NR) binding and subsequent genomic action, modulating physiological processes such as nutrient and energy metabolism, stress response, reproductive physiology and circadian behavioral rhythms. The potential for EDCs to perturb circadian clocks or circadian-driven physiology is not well characterized. For this reason, we explore evidence for parallel endocrine and circadian disruption following EDC exposure across species. In the reviewed studies, EDCs dysregulated core clock and circadian rhythm network gene expression in brain and peripheral organs, and altered circadian reproductive, behavioral and metabolic rhythms. Circadian impacts occurred in parallel to endocrine and metabolic alterations such as impaired fertility and dysregulated metabolic and energetic homeostasis. Further research is warranted to understand the nature of interaction between circadian and endocrine systems in mediating physiological effects of EDC exposure at environmental levels.

The Landscape of AhR Regulators and Coregulators to Fine-Tune AhR Functions

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates numerous cellular responses. Originally investigated in toxicology because of its ability to bind environmental contaminants, AhR has attracted enormous attention in the field of immunology in the last 20 years. In addition, the discovery of endogenous and plant-derived ligands points to AhR also having a crucial role in normal cell physiology. Thus, AhR is emerging as a promiscuous receptor that can mediate either toxic or physiologic effects upon sensing multiple exogenous and endogenous molecules. Within this scenario, several factors appear to contribute to the outcome of gene transcriptional regulation by AhR, including the nature of the ligand as such and its further metabolism by AhR-induced enzymes, the local tissue microenvironment, and the presence of coregulators or specific transcription factors in the cell. Here, we review the current knowledge on the array of transcription factors and coregulators that, by interacting with AhR, tune its transcriptional activity in response to endogenous and exogenous ligands.

Metabolic rivalry: circadian homeostasis and tumorigenesis

Circadian rhythms govern a large array of physiological and metabolic functions. Perturbations of the daily cycle have been linked to elevated risk of developing cancer as well as poor prognosis in patients with cancer. Also, expression of core clock genes or proteins is remarkably attenuated particularly in tumours of a higher stage or that are more aggressive, possibly linking the circadian clock to cellular differentiation. Emerging evidence indicates that metabolic control by the circadian clock underpins specific hallmarks of cancer metabolism. Indeed, to support cell proliferation and biomass production, the clock may direct metabolic processes of cancer cells in concert with non-clock transcription factors to control how nutrients and metabolites are utilized in a time-specific manner. We hypothesize that the metabolic switch between differentiation or stemness of cancer may be coupled to the molecular clockwork. Moreover, circadian rhythms of host organisms appear to dictate tumour growth and proliferation. This Review outlines recent discoveries of the interplay between circadian rhythms, proliferative metabolism and cancer, highlighting potential opportunities in the development of future therapeutic strategies.

2,3,7,8-Tetrachlorodibenzo-p-dioxin abolishes circadian regulation of hepatic metabolic activity in mice

Aryl hydrocarbon receptor (AhR) activation is reported to alter the hepatic expression of circadian clock regulators, however the impact on clock-controlled metabolism has not been thoroughly investigated. This study examines the effects of AhR activation on hepatic transcriptome and metabolome rhythmicity in male C57BL/6 mice orally gavaged with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) every 4 days for 28 days. TCDD diminished the rhythmicity of several core clock regulators (e.g. Arntl, Clock, Nr1d1, Per1, Cry1, Nfil3) in a dose-dependent manner, involving either a ≥ 3.3-fold suppression in amplitude or complete loss of oscillation. Accordingly, protein levels (ARNTL, REV-ERBα, NFIL3) and genomic binding (ARNTL) of select regulators were reduced and arrhythmic following treatment. As a result, the oscillating expression of 99.6% of 5,636 clock-controlled hepatic genes was abolished including genes associated with the metabolism of lipids, glucose/glycogen, and heme. For example, TCDD flattened expression of the rate-limiting enzymes in both gluconeogenesis (Pck1) and glycogenesis (Gys2), consistent with the depletion and loss of rhythmicity in hepatic glycogen levels. Examination of polar hepatic extracts by untargeted mass spectrometry revealed that virtually all oscillating metabolites lost rhythmicity following treatment. Collectively, these results suggest TCDD disrupted circadian regulation of hepatic metabolism, altering metabolic efficiency and energy storage.

Circadian rhythms and environmental disturbances – underexplored interactions

Biological rhythms control the life of virtually all organisms, impacting numerous aspects ranging from subcellular processes to behaviour. Many studies have shown that changes in abiotic environmental conditions can disturb or entrain circadian (∼24 h) rhythms. These expected changes are so large that they could impose risks to the long-term viability of populations. Climate change is a major global stressor affecting the fitness of animals, partially because it challenges the adaptive associations between endogenous clocks and temperature – consequently, one can posit that a large-scale natural experiment on the plasticity of rhythm–temperature interactions is underway. Further risks are posed by chemical pollution and the depletion of oxygen levels in aquatic environments. Here, we focused our attention on fish, which are at heightened risk of being affected by human influence and are adapted to diverse environments showing predictable changes in light conditions, oxygen saturation and temperature. The examined literature to date suggests an abundance of mechanisms that can lead to interactions between responses to hypoxia, pollutants or pathogens and regulation of endogenous rhythms, but also reveals gaps in our understanding of the plasticity of endogenous rhythms in fish and in how these interactions may be disturbed by human influence and affect natural populations. Here, we summarize research on the molecular mechanisms behind environment–clock interactions as they relate to oxygen variability, temperature and responses to pollutants, and propose ways to address these interactions more conclusively in future studies.

Regulation of Innate Lymphoid Cells by Aryl Hydrocarbon Receptor

With striking similarity to their adaptive T helper cell counterparts, innate lymphoid cells (ILCs) represent an emerging family of cell types that express signature transcription factors, including T-bet+ Eomes+ natural killer cells, T-bet+ Eomes- group 1 ILCs, GATA3+ group 2 ILCs, RORγt+ group 3 ILCs, and newly identified Id3+ regulatory ILC. ILCs are abundantly present in barrier tissues of the host (e.g., the lung, gut, and skin) at the interface of host-environment interactions. Active research has been conducted to elucidate molecular mechanisms underlying the development and function of ILCs. The aryl hydrocarbon receptor (Ahr) is a ligand-dependent transcription factor, best known to mediate the effects of xenobiotic environmental toxins and endogenous microbial and dietary metabolites. Here, we review recent progresses regarding Ahr function in ILCs. We focus on the Ahr-mediated cross talk between ILCs and other immune/non-immune cells in host tissues especially in the gut. We discuss the molecular mechanisms of the action of Ahr expression and activity in regulation of ILCs in immunity and inflammation, and the interaction between Ahr and other pathways/transcription factors in ILC development and function with their implication in disease.

Effect of lipopolysaccharide on circadian clock genes Per2 and Bmal1 in mouse ovary

In mammals, circadian rhythms are associated with multiple physiological events. The aim of the present study was to examine the effect of lipopolysaccharide (LPS) on circadian systems in the ovary. Immature female mice were received an intra-peritoneal injection of equine chorionic gonadotropin (eCG) and LPS. Total RNA was collected from the ovary at 6-h intervals throughout a 48 h of experimental period. The expression of the circadian genes period 2 (Per2) and brain and muscle ARNT-like 1 (Bmal1) such as circadian genes was measured by quantitative PCR. Although expression of Per2 and Bmal1 in the ovary did not display clear diurnal oscillation, LPS suppressed the amplitude of Per2 expression. Additionally, LPS inhibited the expression of cytochrome P450 aromatase (CYP19) and luteinizing hormone receptor (LHr) genes in the ovary of eCG-treated mice. Our data suggest that Per2 may be associated with the inhibition of CYP19 and LHr expression by LPS in the ovaries of immature mice.

Aryl Hydrocarbon Receptor Deficiency Alters Circadian and Metabolic Rhythmicity

PAS domain-containing proteins can act as environmental sensors that capture external stimuli to allow coordination of organismal physiology with the outside world. These proteins permit diverse ligand binding and heterodimeric partnership, allowing for varied combinations of PAS-dependent protein-protein interactions and promoting crosstalk among signaling pathways. Previous studies report crosstalk between circadian clock proteins and the aryl hydrocarbon receptor (AhR). Activated AhR forms a heterodimer with the circadian clock protein Bmal1 and thereby functionally inhibits CLOCK/Bmal1 activity. If physiological activation of AhR through naturally occurring, endogenous ligands inhibits clock function, it seems plausible to hypothesize that decreased AhR expression releases AhR-induced inhibition of circadian rhythms. Because both AhR and the clock are important regulators of glucose metabolism, it follows that decreased AhR will also alter metabolic function. To test this hypothesis, rhythms of behavior, metabolic outputs, and circadian and metabolic gene expression were measured in AhR-deficient mice. Genetic depletion of AhR enhanced behavioral responses to changes in the light-dark cycle, increased rhythmic amplitude of circadian clock genes in the liver, and altered rhythms of glucose and insulin. This study provides evidence of AhR-induced inhibition that influences circadian rhythm amplitude.

Circadian Regulation of Benzo[a]Pyrene Metabolism and DNA Adduct Formation in Breast Cells and the Mouse Mammary Gland

The circadian clock plays a role in many biologic processes, yet very little is known about its role in metabolism of drugs and carcinogens. The purpose of this study was to define the impact of circadian rhythms on benzo-a-pyrene (BaP) metabolism in the mouse mammary gland and develop a circadian in vitro model for investigating changes in BaP metabolism resulting from cross-talk between the molecular clock and aryl hydrocarbon receptor. Female 129sv mice (12 weeks old) received a single gavage dose of 50 mg/kg BaP at either noon or midnight, and mammary tissues were isolated 4 or 24 hours later. BaP-induced Cyp1a1 and Cyp1b1 mRNA levels were higher 4 hours after dosing at noon than at 4 hours after dosing at midnight, and this corresponded with parallel changes in Per gene expression. In our in vitro model, we dosed MCF10A mammary cells at different times after serum shock to study how time of day shifts drug metabolism in cells. Analysis of CYP1A1 and CYP1B1 gene expression showed the maximum enzyme-induced metabolism response 12 and 20 hours after shock, as determined by ethoxyresorufin-O-deethylase activity, metabolism of BaP, and formation of DNA-BaP adducts. The pattern of PER-, BMAL-, and aryl hydrocarbon receptor-induced P450 gene expression and BaP metabolism was similar to BaP-induced Cyp1A1 and Cyp1B1 and molecular clock gene expression in mouse mammary glands. These studies indicate time-of-day exposure influences BaP metabolism in mouse mammary glands and describe an in vitro model that can be used to investigate the circadian influence on the metabolism of carcinogens.

Mechanisms and therapeutic prospects of polyphenols as modulators of the aryl hydrocarbon receptor

Polyphenolic AhR modulators displayed concentration-, XRE-, gene-, species- and cell-specific agonistic/antagonistic activity.

The Circadian Timing System and Environmental Circadian Disruption: From Follicles to Fertility

The internal or circadian timing system is deeply integrated in female reproductive physiology. Considerable details of rheostatic timing function in the neuroendocrine control of pituitary hormone secretion, adenohypophyseal hormone gene expression and secretion, gonadal steroid hormone biosynthesis and secretion, ovulation, implantation, and parturition have been reported. The molecular clock, an autonomous feedback loop oscillator of interacting transcriptional regulators, dictates the timing and amplitude of gene expression in each tissue of the female hypothalamic-pituitary-gonadal (HPG) axis. Although multiple targets of the molecular clock have been identified, many associated with critical physiological functions in the HPG axis, the full extent of clock-driven gene expression and physiology in this critical system remains unknown. Environmental circadian disruption (ECD), the disturbance of temporal relationships within and between internal clocks (brain and periphery), and external timing cues (eg, light, nutrients, social cues) due to rotating/night shift work or transmeridian travel have been linked to reproductive dysfunction and subfertility. Moreover, ECD resulting from exposure to endocrine disrupting chemicals, environmental toxins, and/or irregular hormone levels during sexual development can also reduce fertility. Thus, perturbations that disturb clock function at the molecular, cellular or systemic level correlate with significant declines in female reproductive function. Here we briefly review the evidence for molecular clock function in each tissue of the female HPG axis (GnRH neuron, pituitary, uterus, oviduct, and ovary), describe the human epidemiological and animal data supporting the negative effects of ECD on fertility, and explore the potential for novel chronotherapeutics in women's health and fertility.

Role of Aryl Hydrocarbon Receptor in Circadian Clock Disruption and Metabolic Dysfunction

The prevalence of metabolic syndrome, a clustering of three or more risk factors that include abdominal obesity, increased blood pressure, and high levels of glucose, triglycerides, and high-density lipoproteins, has reached dangerous and costly levels worldwide. Increases in morbidity and mortality result from a combination of factors that promote altered glucose metabolism, insulin resistance, and metabolic dysfunction. Although diet and exercise are commonly touted as important determinants in the development of metabolic dysfunction, other environmental factors, including circadian clock disruption and activation of the aryl hydrocarbon receptor (AhR) by dietary or other environmental sources, must also be considered. AhR binds a range of ligands, which prompts protein-protein interactions with other Per-Arnt-Sim (PAS)-domain-containing proteins and subsequent transcriptional activity. This review focuses on the reciprocal crosstalk between the activated AhR and the molecular circadian clock. AhR exhibits a rhythmic expression and time-dependent sensitivity to activation by AhR agonists. Conversely, AhR activation influences the amplitude and phase of expression of circadian clock genes, hormones, and the behavioral responses of the clock system to changes in environmental illumination. Both the clock and AhR status and activation play significant and underappreciated roles in metabolic homeostasis. This review highlights the state of knowledge regarding how AhR may act together with the circadian clock to influence energy metabolism. Understanding the variety of AhR-dependent mechanisms, including its interactions with the circadian timing system that promote metabolic dysfunction, reveals new targets of interest for maintenance of healthy metabolism.

EDC-2: The Endocrine Society's Second Scientific Statement on Endocrine-Disrupting Chemicals

The Endocrine Society's first Scientific Statement in 2009 provided a wake-up call to the scientific community about how environmental endocrine-disrupting chemicals (EDCs) affect health and disease. Five years later, a substantially larger body of literature has solidified our understanding of plausible mechanisms underlying EDC actions and how exposures in animals and humans-especially during development-may lay the foundations for disease later in life. At this point in history, we have much stronger knowledge about how EDCs alter gene-environment interactions via physiological, cellular, molecular, and epigenetic changes, thereby producing effects in exposed individuals as well as their descendants. Causal links between exposure and manifestation of disease are substantiated by experimental animal models and are consistent with correlative epidemiological data in humans. There are several caveats because differences in how experimental animal work is conducted can lead to difficulties in drawing broad conclusions, and we must continue to be cautious about inferring causality in humans. In this second Scientific Statement, we reviewed the literature on a subset of topics for which the translational evidence is strongest: 1) obesity and diabetes; 2) female reproduction; 3) male reproduction; 4) hormone-sensitive cancers in females; 5) prostate; 6) thyroid; and 7) neurodevelopment and neuroendocrine systems. Our inclusion criteria for studies were those conducted predominantly in the past 5 years deemed to be of high quality based on appropriate negative and positive control groups or populations, adequate sample size and experimental design, and mammalian animal studies with exposure levels in a range that was relevant to humans. We also focused on studies using the developmental origins of health and disease model. No report was excluded based on a positive or negative effect of the EDC exposure. The bulk of the results across the board strengthen the evidence for endocrine health-related actions of EDCs. Based on this much more complete understanding of the endocrine principles by which EDCs act, including nonmonotonic dose-responses, low-dose effects, and developmental vulnerability, these findings can be much better translated to human health. Armed with this information, researchers, physicians, and other healthcare providers can guide regulators and policymakers as they make responsible decisions.

Back to the future: transgenerational transmission of xenobiotic-induced epigenetic remodeling

Epigenetics, or regulation of gene expression independent of DNA sequence, is the missing link between genotype and phenotype. Epigenetic memory, mediated by histone and DNA modifications, is controlled by a set of specialized enzymes, metabolite availability, and signaling pathways. A mostly unstudied subject is how sub-toxic exposure to several xenobiotics during specific developmental stages can alter the epigenome and contribute to the development of disease phenotypes later in life. Furthermore, it has been shown that exposure to low-dose xenobiotics can also result in further epigenetic remodeling in the germ line and contribute to increase disease risk in the next generation (multigenerational and transgenerational effects). We here offer a perspective on current but still incomplete knowledge of xenobiotic-induced epigenetic alterations, and their possible transgenerational transmission. We also propose several molecular mechanisms by which the epigenetic landscape may be altered by environmental xenobiotics and hypothesize how diet and physical activity may counteract epigenetic alterations.

Hypoxia and the pharmaceutical diclofenac influence the circadian responses of three-spined stickleback

Pollution with low concentrations of pharmaceuticals, especially when combined with low-oxygen conditions (hypoxia), is a threat to aquatic ecosystems worldwide. The non-steroidal anti-inflammatory drug diclofenac is commonly detected in wastewater effluents, and has potential to accumulate in the bile of fish. Diclofenac has been shown to activate aryl hydrocarbon receptor (AHR), which induces transcription in the metabolic enzyme cytochrome P450 1a (cyp1a). Previously, crosstalk has been shown to occur between AHR and hypoxia inducible factor 1 (HIF-1). In addition, both of these transcription factors interact with the proteins regulating circadian (24-h) rhythms in vertebrates. Yet little is known about the significance of these interactions during simultaneous exposure to chemicals and hypoxia in fish in vivo. We exposed wild-caught three-spined sticklebacks (Gasterosteus aculeatus) to diclofenac (1 μg/L, 14 days), hypoxia (2.0 mg/L, up to 24h) and the combination of both. We then analyzed markers of chemical biotransformation (EROD activity, cyp1a and ahr mRNA levels), glycolysis (lactate dehydrogenase (LDH) enzyme activity, ldh and enolase 1a mRNA levels), and the transcription of core circadian clock genes clock and period 1 in liver tissue. Samples were taken at three time points during the light period in order to address disturbances in the circadian variation of metabolic processes. The results show that mRNA levels and LDH activity tended to be lowest before the dark period, but this pattern was disturbed by hypoxia and diclofenac. Diclofenac and hypoxia co-exposure induced EROD activity more strongly than diclofenac exposure alone, while cyp1a mRNA level was increased also by hypoxia and diclofenac alone. LDH activity and mRNA expression showed a clear time-dependent response during hypoxia, which is consistent with the previously suggested decreased accumulation of HIF-1 during the dark period. Furthermore, LDH activity and transcription was disturbed by diclofenac, indicating important effects of environmental pollutants in disturbing natural acclimation. This study demonstrates the need for more studies to understand the potential disturbances in endogenous rhythms caused by environmental pollution in natural populations.

Circadian Clock Function in the Mammalian Ovary

Rhythmic events in the female reproductive system depend on the coordinated and synchronized activity of multiple neuroendocrine and endocrine tissues. This coordination is facilitated by the timing of gene expression and cellular physiology at each level of the hypothalamo-pituitary-ovarian (HPO) axis, including the basal hypothalamus and forebrain, the pituitary gland, and the ovary. Central to this pathway is the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) that, through its myriad outputs, provides a temporal framework for gonadotropin release and ovulation. The heart of the timing system, a transcription-based oscillator, imparts SCN pacemaker cells and a company of peripheral tissues with the capacity for daily oscillations of gene expression and cellular physiology. Although the SCN sits comfortably at the helm, peripheral oscillators (such as the ovary) have undefined but potentially critical roles. Each cell type of the ovary, including theca cells, granulosa cells, and oocytes, harbor a molecular clock implicated in the processes of follicular growth, steroid hormone synthesis, and ovulation. The ovarian clock is influenced by the reproductive cycle and diseases that perturb the cycle and/or follicular growth can disrupt the timing of clock gene expression in the ovary. Chronodisruption is known to negatively affect reproductive function and fertility in both rodent models and women exposed to shiftwork schedules. Thus, influencing clock function in the HPO axis with chronobiotics may represent a novel avenue for the treatment of common fertility disorders, particularly those resulting from chronic circadian disruption.

Interplay between Dioxin-mediated signaling and circadian clock: a possible determinant in metabolic homeostasis

The rotation of the earth on its axis creates the environment of a 24 h solar day, which organisms on earth have used to their evolutionary advantage by integrating this timing information into their genetic make-up in the form of a circadian clock. This intrinsic molecular clock is pivotal for maintenance of synchronized homeostasis between the individual organism and the external environment to allow coordinated rhythmic physiological and behavioral function. Aryl hydrocarbon receptor (AhR) is a master regulator of dioxin-mediated toxic effects, and is, therefore, critical in maintaining adaptive responses through regulating the expression of phase I/II drug metabolism enzymes. AhR expression is robustly rhythmic, and physiological cross-talk between AhR signaling and circadian rhythms has been established. Increasing evidence raises a compelling argument that disruption of endogenous circadian rhythms contributes to the development of disease, including sleep disorders, metabolic disorders and cancers. Similarly, exposure to environmental pollutants through air, water and food, is increasingly cited as contributory to these same problems. Thus, a better understanding of interactions between AhR signaling and the circadian clock regulatory network can provide critical new insights into environmentally regulated disease processes. This review highlights recent advances in the understanding of the reciprocal interactions between dioxin-mediated AhR signaling and the circadian clock including how these pathways relate to health and disease, with emphasis on the control of metabolic function.

In vitro neuroendocrine effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the AhR-expressing hypothalamic rat GnV-3 cell line

The aryl hydrocarbon receptor (AhR) is involved in a wide variety of biological and toxicological responses, including neuroendocrine signaling. Due to the complexity of neuroendocrine pathways in e.g. the hypothalamus and pituitary, there are limited in vitro models available despite the strong demand for such systems to study and predict neuroendocrine effects of chemicals. In this study, the applicability of the AhR-expressing rat hypothalamic GnV-3 cell line was investigated as a novel model to screen for neuroendocrine effects of AhR ligands using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as reference compound. The qRT-PCR analyses demonstrated the presence of several sets of neurotransmitter receptors in the GnV-3 cells. TCDD (10nM) altered neurotransmitter signaling by up-regulation of glutamate (Grik2), gamma-amino butyric acid (Gabra2) and serotonin (Ht2C) receptor mRNA levels. However, no significant changes in basal and serotonin-evoked intracellular Ca(2+) concentration ([Ca(2+)]i) or serotonin release were observed. On the other hand, TCDD de-regulated period circadian protein homolog 1 (Per1) and gonadotropin releasing hormone (Gnrh) mRNA levels within a 24-h time period. Both Per1 and Gnrh genes displayed a similar mRNA expression pattern in GnV-3 cells. Moreover, the involvement of AhR in TCDD-induced alteration of Neuropeptide Y (Npy) gene expression was found and confirmed by using siRNA targeted against Ahr in GnV-3 cells. Overall, the combined results demonstrate that GnV-3 cells may be a suitable model to predict some mechanisms of action and effects of AhR ligands in the hypothalamus.

Long-term dietary-exposure to non-coplanar PCBs induces behavioral disruptions in adult zebrafish and their offspring

The use of polychlorinated biphenyls (PCBs) has been banned for several decades. PCBs have a long biological half-life and high liposolubility which leads to their bioaccumulation and biomagnification through food chains over a wide range of trophic levels. Exposure can lead to changes in animal physiology and behavior and has been demonstrated in both experimental and field analyses. There are also potential risks to high trophic level predators, including humans. A maternal transfer has been demonstrated in fish as PCBs bind to lipids in eggs. In this study, behavioral traits (exploration and free swimming, with or without challenges) of contaminated zebrafish (Danio rerio) adults and their offspring (both as five-day-old larvae and as two-month-old fish reared under standard conditions) were measured using video-tracking. Long-term dietary exposure to a mixture of non-coplanar PCBs was used to mimic known environmental contamination levels and congener composition. Eight-week-old fish were exposed for eight months at 26-28 °C. Those exposed to an intermediate dose (equivalent to that found in the Loire Estuary, ∑(CB)=515 ng g⁻¹ dry weight in food) displayed behavioral disruption in exploration capacities. Fish exposed to the highest dose (equivalent to that found in the Seine Estuary, ∑(CB)=2302 ng g⁻¹ dry weight in food) displayed an increased swimming activity at the end of the night. In offspring, larval activity was increased and two-month-old fish occupied the bottom section of the tank less often. These findings call for more long-term experiments using the zebrafish model; the mechanisms underlying behavioral disruptions need to be understood due to their implications for both human health and their ecological relevance in terms of individual fitness and survival.

Genetic polymorphisms in the aryl hydrocarbon receptor-signaling pathway and sleep disturbances in middle-aged women

OBJECTIVE

We aimed to determine if selected genetic polymorphisms in the aryl hydrocarbon receptor (AHR)-signaling pathway and circadian locomotor output cycles kaput (CLOCK) are associated with insomnia and early awakening in middle-aged women.

METHODS

Women aged 45 to 54years (n=639) were recruited into a middle-aged health study and agreed to complete questionnaires and donate blood samples. Questionnaires were used to assess sleep outcomes. Blood samples were processed for genotyping for the selected polymorphisms: AHR (rs2066853), AHR repressor (AHRR) (rs2292596), aryl hydrocarbon nuclear translocator (ARNT) (rs2228099), and CLOCK (rs1801260). Data were analyzed using multivariable logistic regression.

RESULTS

Women heterozygous for the AHRR alleles (GC) had decreased odds of insomnia compared to women homozygous for the AHRR_C allele (adjusted odds ratio [aOR], 0.69; 95% confidence interval [CI], 0.49-0.96). Women with at least one of the AHRR_G or CLOCK_C alleles had significantly decreased odds of insomnia compared to women homozygous for the AHRR_C and CLOCK_T alleles (aOR, 0.64; 95% CI, 0.43-0.96). Additionally, women homozygous for the AHRR_G and CLOCK_C alleles had significantly decreased odds of insomnia compared to women homozygous for the AHRR_C and CLOCK_T alleles (aOR, 0.56; 95% CI, 0.35-0.89). None of the selected single nucleotide polymorphisms (SNPs) or combinations of SNPs were significantly associated with early awakening.

CONCLUSIONS

Selected genetic polymorphisms in the AHR-signaling pathway (i.e., AHRR) and CLOCK may play a role in decreasing the risk for experiencing insomnia during the menopausal transition.

The circadian clock circuitry and the AHR signaling pathway in physiology and pathology

Life forms populating the Earth must face environmental challenges to assure individual and species survival. The strategies predisposed to maintain organismal homeostasis and grant selective advantage rely on anticipatory phenomena facing periodic modifications, and compensatory phenomena facing unpredictable changes. Biological processes bringing about these responses are respectively driven by the circadian timing system, a complex of biological oscillators entrained to the environmental light/dark cycle, and by regulatory and metabolic networks that precisely direct the body's adjustments to variations of external conditions and internal milieu. A critical role in organismal homeostatic functions is played by the aryl hydrocarbon receptor (AHR) complex, which senses environmental and endogenous compounds, influences metabolic responses controlling phase I/II gene expression, and modulates vital phenomena such as development, inflammation and adaptive immunity. A physiological cross-talk between circadian and AHR signaling pathways has been evidenced. The alteration of AHR signaling pathway deriving from genetic damage with polymorphisms or mutations, or produced by exogenous or endogenous AHR activation, and chronodisruption caused by mismatch between the body's internal clock and geophysical time/social schedules, are capable of triggering pathological mechanisms involved in metabolic, immune-related and neoplastic diseases. On the other hand, the molecular components of the circadian clock circuitry and AHR signaling pathway may represent useful tools for preventive interventions and valuable targets of therapeutic approaches.

Dynamic postnatal developmental and sex-specific neuroendocrine effects of prenatal polychlorinated biphenyls in rats

Gestational exposures to estrogenic compounds, both endogenous hormones and exogenous endocrine-disrupting chemicals (EDCs), have long-term effects on reproductive physiology and behavior. We tested the hypothesis that prenatal treatment of rats with low doses of Aroclor 1221 (A1221), a weakly estrogenic polychlorinated biphenyl mix previously used in industry, or estradiol benzoate (EB), alters development of the hypothalamus in a sexually dimorphic manner and subsequently perturbs reproductive function. Pregnant Sprague-Dawley rats were injected on embryonic days 16 and 18 with vehicle (dimethylsulfoxide), A1221 (1 mg/kg), or EB (50 μg/kg). Developmental milestones were monitored, and on postnatal days 15, 30, 45, and 90, 1 male and 1 female per litter were euthanized. Because of their key roles in the mediation of steroid actions on reproductive function, the anteroventral periventricular nucleus (AVPV) and the arcuate nucleus (ARC) were punched for a low-density quantitative PCR array of 48 neuroendocrine genes and analysis of DNA methylation of a subset of genes. Gestational exposure to A1221 or EB delayed the timing of puberty in males and disrupted estrous cyclicity in females. In the AVPV, 28 genes were affected by treatment in a developmental stage-specific manner, mostly in females, which exhibited a masculinized expression profile. This included 2 clock genes, Per2 and Arntl, implicating circadian circuits as being vulnerable to endocrine disruption. DNA methylation analysis of 2 genes, Per2 and Ar, showed no effect of EDCs and suggested alternative mechanisms for the altered mRNA levels. In the ARC, 12 genes were affected by treatment, mostly in males, again with dynamic developmental changes. Bionetwork analysis of relationships among genes, hormones, and physiological markers showed sexually dimorphic effects of estrogenic EDC exposures, with the female AVPV and the male ARC being most vulnerable, and provided novel relationships among hypothalamic genes and postnatal reproductive maturation.

Clocks underneath: the role of peripheral clocks in the timing of female reproductive physiology

The central circadian pacemaker in the suprachiasmatic nucleus (SCN) is a critical component of the neuroendocrine circuit controlling gonadotropin secretion from the pituitary gland. The SCN conveys photic information to hypothalamic targets including the gonadotropin releasing hormone neurons. Many of these target cells are also cell autonomous clocks. It has been suggested that, rather then being singularly driven by the SCN, the timing of gonadotropin secretion depends on the activity of multiple hypothalamic oscillators. While this view provides a novel twist to an old story, it does little to diminish the central role of rhythmic hypothalamic output in this system. It is now clear that the pituitary, ovary, uterus, and oviduct have functional molecular clocks. Evidence supports the notion that the clocks in these tissues contribute to the timing of events in reproductive physiology. The aim of this review is to highlight the current evidence for molecular clock function in the peripheral components of the female hypothalamo-pituitary-gonadal axis as it relates to the timing of gonadotropin secretion, ovulation, and parturition.

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.

The mammalian circadian system is resistant to dioxin

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that is bound and activated by many toxic ubiquitous environmental contaminants, including the halogenated aromatic hydrocarbon, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The AhR belongs to a family of proteins that contain basic helix-loop-helix/Per-ARNT-SIM (bHLH/PAS) domains. The circadian clock protein, BMAL1, is also a bHLH-PAS transcription factor and has been shown to interact with the AhR. AhRs are expressed in nearly every mammalian tissue, including the suprachiasmatic nuclei (SCN), and previous studies have suggested that activation of the AhR with dioxins affects rhythmicity in circadian clocks. In this study, the authors tested the hypothesis that activation of the aryl hydrocarbon receptor with the potent dioxin, TCDD, alters the organization of the mammalian circadian system by measuring bioluminescence from tissues explanted from PER2::LUCIFERASE mice. They found that in vitro treatment of explanted tissues with TCDD did not alter the periods, amplitudes, or damping rates of the PER2::LUC rhythms compared with controls. Likewise, in vivo treatment with TCDD had no effect on the phase relationship between central and peripheral oscillators. Together, these data demonstrate that activation of the AhR with TCDD does not directly or systemically alter the mouse circadian system.

Persistent polar depletion of stratospheric ozone and emergent mechanisms of ultraviolet radiation-mediated health dysregulation

Year 2011 noted the first definable ozone "hole" in the Arctic region, serving as an indicator to the continued threat of dangerous ultraviolet radiation (UVR) exposure caused by the deterioration of stratospheric ozone in the northern hemisphere. Despite mandates of the Montreal Protocol to phase out the production of ozone-depleting chemicals (ODCs), the relative stability of ODCs validates popular notions of persistent stratospheric ozone for several decades. Moreover, increased UVR exposure through stratospheric ozone depletion is occurring within a larger context of physiologic stress and climate change across the biosphere. In this review, we provide commentaries on stratospheric ozone depletion with relative comparisons between the well-known Antarctic ozone hole and the newly defined ozone hole in the Arctic. Compared with the Antarctic region, the increased UVR exposure in the Northern Hemisphere poses a threat to denser human populations across North America, Europe, and Asia. In this context, we discuss emerging targets of UVR exposure that can potentially offset normal biologic rhythms in terms of taxonomically conserved photoperiod-dependent seasonal signaling and entrainment of circadian clocks. Consequences of seasonal shifts during critical life history stages can alter fitness and condition, whereas circadian disruption is increasingly becoming associated as a causal link to increased carcinogenesis. We further review the significance of genomic alterations via UVR-induced modulations of phase I and II transcription factors located in skin cells, the aryl hydrocarbon receptor (AhR), and the nuclear factor (erythroid-derived 2)-related factor 2 (Nrf2), with emphasis on mechanism that can lead to metabolic shifts and cancer. Although concern for adverse health consequences due to increased UVR exposure are longstanding, recent advances in biochemical research suggest that AhR and Nrf2 transcriptional regulators are likely targets for UVR-mediated dysregulations of rhythmicity and homeostasis among animals, including humans.