The role of mammalian circadian proteins in normal physiology and genotoxic stress responses

Circadian Clocks, Redox Homeostasis, and Exercise: Time to Connect the Dots?

Compelling research has documented how the circadian system is essential for the maintenance of several key biological processes including homeostasis, cardiovascular control, and glucose metabolism. Circadian clock disruptions, or losses of rhythmicity, have been implicated in the development of several diseases, premature ageing, and are regarded as health risks. Redox reactions involving reactive oxygen and nitrogen species (RONS) regulate several physiological functions such as cell signalling and the immune response. However, oxidative stress is associated with the pathological effects of RONS, resulting in a loss of cell signalling and damaging modifications to important molecules such as DNA. Direct connections have been established between circadian rhythms and oxidative stress on the basis that disruptions to circadian rhythms can affect redox biology, and vice versa, in a bi-directional relationship. For instance, the expression and activity of several key antioxidant enzymes (SOD, GPx, and CAT) appear to follow circadian patterns. Consequently, the ability to unravel these interactions has opened an exciting area of redox biology. Exercise exerts numerous benefits to health and, as a potent environmental cue, has the capacity to adjust disrupted circadian systems. In fact, the response to a given exercise stimulus may also exhibit circadian variation. At the same time, the relationship between exercise, RONS, and oxidative stress has also been scrutinised, whereby it is clear that exercise-induced RONS can elicit both helpful and potentially harmful health effects that are dependent on the type, intensity, and duration of exercise. To date, it appears that the emerging interface between circadian rhythmicity and oxidative stress/redox metabolism has not been explored in relation to exercise. This review aims to summarise the evidence supporting the conceptual link between the circadian clock, oxidative stress/redox homeostasis, and exercise stimuli. We believe carefully designed investigations of this nexus are required, which could be harnessed to tackle theories concerned with, for example, the existence of an optimal time to exercise to accrue physiological benefits.

PER3 polymorphisms and their association with prostate cancer risk in Japanese men

Introduction

Prostate cancer (PCa) is one of the most common cancers affecting men globally. Although PER3 has been suggested as a risk factor for cancer development, there are few reports elucidating the relationship between PER3 and PCa. We investigated the association between PER3 polymorphisms (rs2640908 and VNTR) and susceptibility to PCa in the Japanese population.

Methods

Eighty three patients with PCa and 122 controls participated in this study. We analyzed rs2640908 and VNTR polymorphisms by using PCR-Restriction Fragment Length Polymorphism (PCR-RFLP).

Results

Compared to the C/C genotype with the rs2640908 polymorphism, the T/T (OR: 0.35, 95% CI: 0.15-0.81, P = 0.02) and C/T + T/T (OR: 0.46, 95% CI: 0.24-0.88, P = 0.02) genotypes had a significantly lower risk of PCa. TT (OR: 0.29, 95% CI: 0.10-0.77, P = 0.02) and CT + TT (OR: 0.47, 95% CI: 0.23-0.97, P = 0.04) also had significant protection against PCa in the smoker group. Significantly, we observed an association between smoking and rs2640908 polymorphism in this study. However, no association between the VNTR polymorphisms and PCa was detected.

Conclusions

Our results suggest that PER3 rs2640908 polymorphisms influence an individual's susceptibility to PCa.

The role of circadian rhythm in choroid plexus functions

For several years, a great effort has been devoted to understand how circadian oscillations in physiological processes are determined by the circadian clock system. This system is composed by the master clock at the suprachiasmatic nucleus which sets the pace and tunes peripheral clocks in several organs. It was recently demonstrated that the choroid plexus epithelial cells that compose the blood-cerebrospinal fluid barrier hold a circadian clock which might control their multiple functions with implications for the maintenance of brain homeostasis. However, the choroid plexus activities regulated by its inner clock are still largely unknown. In this review, we propose that several choroid plexus functions might be regulated by the circadian clock, alike in other tissues. We provide evidences that the timing of cerebrospinal fluid secretion, clearance of amyloid-beta peptides and xenobiotics, and the barrier function of the blood-cerebrospinal fluid barrier are regulated by the circadian clock. These data, highlight that the circadian regulation of the blood-cerebrospinal fluid barrier must be taken into consideration for enhancing drug delivery to central nervous system disorders.

Post-translational Modifications are Required for Circadian Clock Regulation in Vertebrates

Circadian clocks are intrinsic, time-tracking systems that bestow upon organisms a survival advantage. Under natural conditions, organisms are trained to follow a 24-h cycle under environmental time cues such as light to maximize their physiological efficiency. The exact timing of this rhythm is established via cell-autonomous oscillators called cellular clocks, which are controlled by transcription/translation-based negative feedback loops. Studies using cell-based systems and genetic techniques have identified the molecular mechanisms that establish and maintain cellular clocks. One such mechanism, known as post-translational modification, regulates several aspects of these cellular clock components, including their stability, subcellular localization, transcriptional activity, and interaction with other proteins and signaling pathways. In addition, these mechanisms contribute to the integration of external signals into the cellular clock machinery. Here, we describe the post-translational modifications of cellular clock regulators that regulate circadian clocks in vertebrates.

Night workers have lower levels of antioxidant defenses and higher levels of oxidative stress damage when compared to day workers

The effects of circadian misalignment and work shift on oxidative stress profile of shift workers have not been explored in the literature. The present study aimed to evaluate the role of shift work (day and night) and social jetlag - a measure of circadian misalignment - with oxidative stress markers. A cross-sectional study was performed with 79 men (21–65 years old, 27.56 ± 4.0 kg/m²) who worked the night shift (n = 37) or daytime (n = 42). The analyzed variables included anthropometric measures and determination of systemic levels of markers of oxidative damage and antioxidant defense. Social jetlag was calculated by the absolute difference between the mean sleep point on working and rest days. The night group presented higher systemic values of thiobarbituric acid reactive substances and hydrogen peroxide, and lower levels of nitrite, total antioxidant capacity, and catalase and superoxide dismutase activities in relation to the day group. However, social jetlag was not associated with oxidative stress-related biomarkers analyzed in the night group. These results suggest that the night worker has higher levels of oxidative stress damage and lower levels of antioxidant defenses, while social jetlag was not a possible responsible factor for this condition.

Circadian genes and risk of prostate cancer: Findings from the EPICAP study

Circadian rhythms regulate several physiological functions and genes controlling the circadian rhythm were found to regulate cell proliferation, cell cycle and apoptosis. Few studies have investigated the role of those circadian genes in prostate cancer occurrence. We aim to investigate the relationship between circadian genes polymorphisms and prostate cancer risk based on data from the EPICAP study, a population-based case-control study including 1,515 men (732 cases / 783 controls) with genotyped data. Odds Ratios (ORs) for association between prostate cancer and circadian gene variants were estimated for each of the 872 single nucleotide polymorphisms (SNPs) in 31 circadian clock genes. We also used a gene-based and pathway-based approach with a focus on the pathway including 9 core circadian genes. Separate analyses were conducted by prostate cancer aggressiveness. The core-circadian pathway (p = 0.0006) was significantly associated to prostate cancer, for either low (p = 0.002) or high (p = 0.01) grade tumor. At the gene level, we observed significant associations between all prostate cancer and NPAS2 and PER1 after correcting for multiple testing, while only RORA was significant for aggressive tumors. At the SNP-level, no significant association was observed. Our findings provide additional evidence of a potential link between genetic variants in circadian genes and prostate cancer risk. Further investigation is warranted to confirm these findings and to better understand the biological pathways involved.

Circadian clock pathway genes associated with colorectal cancer risk and prognosis

Circadian clock genes influence biological processes and may be involved in tumorigenesis. We systematically evaluated genetic variants in the circadian clock pathway genes associated with colorectal cancer risk and survival. We evaluated the association of 119 single nucleotide polymorphisms (SNPs) in 27 circadian clock pathway genes with the risk of colorectal cancer in a case-control study (1150 cases and 1342 controls). The false discovery rate (FDR) method was applied to correct for multiple comparisons. Gene-based analysis was performed by the sequence kernel association test (SKAT). Cox proportional hazards regression was used to calculate the effects of SNPs on the overall survival of patients. We identified that compared to those with the G allele, individuals with the rs76436997 A allele in RORA had a significant 1.33-fold increased risk of colorectal cancer (P = 3.83 × 10^- 4). Specifically, the GA/AA genotypes were related to an enhanced risk of colorectal cancer compared with that associated with the GG genotype, which was more common in patients with well and moderately differentiated tumors and Dukes A/B stages. The SNP rs76436997 significantly increased the overall survival time of colorectal cancer patients (P = 0.044). Furthermore, RNA-seq data showed that the mRNA levels of RORA were significantly lower in colorectal tumors than the paired normal tissues. Gene-based analysis revealed a significant association between RORA and colorectal cancer risk. These findings highlight the important roles of genetic variations in circadian clock pathway genes play in colorectal cancer risk and suggest that RORA is potentially related to colorectal carcinogenesis.

Inherited variation in circadian rhythm genes and risks of prostate cancer and three other cancer sites in combined cancer consortia

Circadian disruption has been linked to carcinogenesis in animal models, but the evidence in humans is inconclusive. Genetic variation in circadian rhythm genes provides a tool to investigate such associations. We examined associations of genetic variation in nine core circadian rhythm genes and six melatonin pathway genes with risk of colorectal, lung, ovarian and prostate cancers using data from the Genetic Associations and Mechanisms in Oncology (GAME-ON) network. The major results for prostate cancer were replicated in the Prostate, Lung, Colorectal and Ovarian (PLCO) cancer screening trial, and for colorectal cancer in the Genetics and Epidemiology of Colorectal Cancer Consortium (GECCO). The total number of cancer cases and controls was 15,838/18,159 for colorectal, 14,818/14,227 for prostate, 12,537/17,285 for lung and 4,369/9,123 for ovary. For each cancer site, we conducted gene-based and pathway-based analyses by applying the summary-based Adaptive Rank Truncated Product method (sARTP) on the summary association statistics for each SNP within the candidate gene regions. Aggregate genetic variation in circadian rhythm and melatonin pathways were significantly associated with the risk of prostate cancer in data combining GAME-ON and PLCO, after Bonferroni correction (ppathway  < 0.00625). The two most significant genes were NPAS2 (pgene  = 0.0062) and AANAT (pgene  = 0.00078); the latter being significant after Bonferroni correction. For colorectal cancer, we observed a suggestive association with the circadian rhythm pathway in GAME-ON (ppathway  = 0.021); this association was not confirmed in GECCO (ppathway  = 0.76) or the combined data (ppathway  = 0.17). No significant association was observed for ovarian and lung cancer. These findings support a potential role for circadian rhythm and melatonin pathways in prostate carcinogenesis. Further functional studies are needed to better understand the underlying biologic mechanisms.

Circadian regulation of auditory function

The circadian system integrates environmental cues to regulate physiological functions in a temporal fashion. The suprachiasmatic nucleus, located in the hypothalamus, is the master clock that synchronizes central and peripheral organ clocks to orchestrate physiological functions. Recently, molecular clock machinery has been identified in the cochlea unravelling the potential involvement in the circadian regulation of auditory functions. Here, we present background information on the circadian system and review the recent findings that introduce circadian rhythms to the auditory field. Understanding the mechanisms by which circadian rhythms regulate auditory function will provide fundamental knowledge on the signalling networks that control vulnerability and resilience to auditory insults.

Differential localization of PER1 and PER2 in the brain master circadian clock

The hypothalamic suprachiasmatic nucleus (SCN), locus of the master circadian clock, bears many neuronal types. At the cellular-molecular level, the clock is comprised of feedback loops involving 'clock' genes including Period1 and Period2, and their protein products, PERIOD1 and PERIOD2 (PER1/2). In the canonical model of circadian oscillation, the PER1/2 proteins oscillate together. While their rhythmic expression in the SCN as a whole has been described, the possibility of regional differences remains unknown. To explore these clock proteins in distinct SCN regions, we assessed their expression through the rostro-caudal extent of the SCN in sagittal sections. We developed an automated method for tracking three fluorophores in digital images of sections triply labeled for PER1, PER2, and gastrin-releasing peptide (used to locate the core). In the SCN as a whole, neurons expressing high levels of PER2 were concentrated in the rostral, rostrodorsal, and caudal portions of the nucleus, and those expressing high levels of PER1 lay in a broad central area. Within these overall patterns, adjacent cells differed in expression levels of the two proteins. The results demonstrate spatially distinct localization of high PER1 vs. PER2 expression, raising the possibility that their distribution is functionally significant in encoding and communicating temporal information. The findings provoke the question of whether there are fundamental differences in PER1/2 levels among SCN neurons and/or whether topographical differences in protein expression are a product of SCN network organization rather than intrinsic differences among neurons.

CLOCK regulates mammary epithelial cell growth and differentiation

Circadian clocks influence virtually all physiological processes, including lactation. Here, we investigate the role of the CLOCK gene in regulation of mammary epithelial cell growth and differentiation. Comparison of mammary morphology in late-pregnant wild-type and ClockΔ19 mice, showed that gland development was negatively impacted by genetic loss of a functional timing system. To understand whether these effects were due, in part, to loss of CLOCK function in the gland, the mouse mammary epithelial cell line, HC11, was transfected with short hairpin RNA that targeted Clock (shClock). Cells transfected with shClock expressed 70% less Clock mRNA than wild-type (WT) HC11 cultures, which resulted in significantly depressed levels of CLOCK protein (P < 0.05). HC11 lines carrying shClock had four-fold higher growth rates (P < 0.05), and the percentage of cells in G1 phase was significantly higher (90.1 ± 1.1% of shClock vs. 71.3 ± 3.6% of WT-HC11) following serum starvation. Quantitative-PCR (qPCR) analysis showed shClock had significant effects (P < 0.0001) on relative expression levels of Ccnd1, Wee1, and Tp63 qPCR analysis of the effect of shClock on Fasn and Cdh1 expression in undifferentiated cultures and cultures treated 96 h with dexamethasone, insulin, and prolactin (differentiated) found levels were reduced by twofold and threefold, respectively (P < 0.05), in shClock line relative to WT cultures. Abundance of CDH1 and TP63 proteins were significantly reduced in cultures transfected with shClock These data support how CLOCK plays a role in regulation of epithelial cell growth and differentiation in the mammary gland.

Impaired light detection of the circadian clock in a zebrafish melanoma model

The circadian clock controls the timing of the cell cycle in healthy tissues and clock disruption is known to increase tumourigenesis. Melanoma is one of the most rapidly increasing forms of cancer and the precise molecular circadian changes that occur in a melanoma tumor are unknown. Using a melanoma zebrafish model, we have explored the molecular changes that occur to the circadian clock within tumors. We have found disruptions in melanoma clock gene expression due to a major impairment to the light input pathway, with a parallel loss of light-dependent activation of DNA repair genes. Furthermore, the timing of mitosis in tumors is perturbed, as well as the regulation of certain key cell cycle regulators, such that cells divide arhythmically. The inability to co-ordinate DNA damage repair and cell division is likely to promote further tumourigenesis and accelerate melanoma development.

Genetic association between PER3 genetic polymorphisms and cancer susceptibility: a meta-analysis

The genes along the circadian pathways control and modulate circadian rhythms essential for the maintenance of physiological homeostasis through self-sustained transcription-translation feedback loops. PER3 (period 3) is a circadian pathway gene and its variants (rs1012477, 4/5-repeat) have frequently been associated with human cancer. The mixed findings, however, make the role of the 2 variants in cancer susceptibility elusive. We aimed in this article to clarify the association of PER3 variants with cancer. We collected genetic data from 8 studies, providing 6149 individuals for rs1012477 and 5241 individuals for 4/5-repeat. Based on the genotype and allele frequency, we chose the fixed-effects model to estimate risk of cancer. Overall analysis did not suggest a global role of rs1012477 in cancer susceptibility. For PER3 4/5-repeat variant, we found a moderate increase in risk of cancer among individuals with the 5-allele compared to individuals with the 4-allele, although this association was not statistically significant (homozygous model: odds ratio [OR] 1.17, 95% confidence interval [CI] 0.81-1.67; recessive model: OR 1.17, 95% CI 0.82-1.67). No substantial heterogeneity was revealed in this analysis. Our meta-analysis provides no evidence supporting a global association of PER3 genetic variants with the incidence of cancer.

The circadian gene CRY2 is associated with breast cancer aggressiveness possibly via epigenomic modifications

Although the role of core circadian gene cryptochrome 2 (CRY2) in breast tumorigenesis has been demonstrated, the correlations of CRY2 with clinical parameters in breast cancer patients and its involvement in epigenetic processes such as DNA methylation remain relatively unexplored. In the current study, we first queried the Oncomine database and the Gene Expression-Based Outcome for Breast Cancer Online (GOBO) database to identify associations between CRY2 expression levels and clinical parameters in breast cancer patients. We then silenced CRY2 in vitro and performed a genome-wide methylation array to determine the epigenetic impact of CRY2 silencing. The Ingenuity Pathway Analysis software was used to further explore the genes exhibiting altered methylation identified using the array. We found that CRY2 was frequently down-regulated in breast cancer tissue compared to adjacent normal tissue or breast tissue from healthy controls. Lower CRY2 expression was associated with estrogen receptor (ER)-negativity (P < 0.0001), higher tumor grade (P < 0.0001), and shorter overall survival time in breast cancer patients (HR = 1.44, 95 % confidence interval (CI) 1.09-1.91). Genome-wide methylation analysis showed that a total of 515 CpG sites were hypermethylated following CRY2 knockdown, while 730 sites were hypomethylated. The pathway analysis revealed several cancer-relevant networks with genes exhibiting significantly altered methylation following CRY2 silencing. These findings suggest that the core circadian gene CRY2 is associated with breast cancer progression and prognosis, and that knockdown of CRY2 causes the epigenetic dysregulation of genes involved in cancer-relevant pathways, which provide further evidence supporting a role of the circadian system in breast tumorigenesis.

Circadian clock genes and risk of fatal prostate cancer

PURPOSE

Circadian genes may be involved in regulating cancer-related pathways, including cell proliferation, DNA damage response, and apoptosis. We aimed to assess the role of genetic variation in core circadian rhythm genes with the risk of fatal prostate cancer and first morning void urinary 6-sulfatoxymelatonin levels.

METHODS

We used unconditional logistic regression to evaluate the association of 96 single-nucleotide polymorphisms (SNPs) across 12 circadian-related genes with fatal prostate cancer in the AGES-Reykjavik cohort (n = 24 cases), the Health Professionals Follow-Up Study (HPFS) (n = 40 cases), and the Physicians' Health Study (PHS) (n = 105 cases). We used linear regression to evaluate the association between SNPs and first morning void urinary 6-sulfatoxymelatonin levels in AGES-Reykjavik. We used a kernel machine test to evaluate whether multimarker SNP sets in the pathway (gene based) were associated with our outcomes.

RESULTS

None of the individual SNPs were consistently associated with fatal prostate cancer across the three cohorts. In each cohort, gene-based analyses showed that variation in the CRY1 gene was nominally associated with fatal prostate cancer (p values = 0.01, 0.01, and 0.05 for AGES-Reykjavik, HPFS, and PHS, respectively). In AGES-Reykjavik, SNPs in TIMELESS (four SNPs), NPAS2 (six SNPs), PER3 (two SNPs) and CSNK1E (one SNP) were nominally associated with 6-sulfatoxymelatonin levels.

CONCLUSION

We did not find a strong and consistent association between variation in core circadian clock genes and fatal prostate cancer risk, but observed nominally significant gene-based associations with fatal prostate cancer and 6-sulfatoxymelatonin levels.

Functional polymorphisms of circadian negative feedback regulation genes are associated with clinical outcome in hepatocellular carcinoma patients receiving radical resection

Previous studies have demonstrated that circadian negative feedback loop genes play an important role in the development and progression of many cancers. However, the associations between single-nucleotide polymorphisms (SNPs) in these genes and the clinical outcomes of hepatocellular carcinoma (HCC) after surgical resection have not been studied so far. Thirteen functional SNPs in circadian genes were genotyped using the Sequenom iPLEX genotyping system in a cohort of 489 Chinese HCC patients who received radical resection. Multivariate Cox proportional hazards model and Kaplan-Meier curve were used for the prognosis analysis. Cumulative effect analysis and survival tree analysis were used for the multiple SNPs analysis. Four individual SNPs, including rs3027178 in PER1, rs228669 and rs2640908 in PER3 and rs3809236 in CRY1, were significantly associated with overall survival (OS) of HCC patients, and three SNPs, including rs3027178 in PER1, rs228729 in PER3 and rs3809236 in CRY1, were significantly associated with recurrence-free survival (RFS). Moreover, we observed a cumulative effect of significant SNPs on OS and RFS (P for trend < 0.001 for both). Survival tree analysis indicated that wild genotype of rs228729 in PER3 was the primary risk factor contributing to HCC patients' RFS. Our study suggests that the polymorphisms in circadian negative feedback loop genes may serve as independent prognostic biomarkers in predicting clinical outcomes for HCC patients who received radical resection. Further studies with different ethnicities are needed to validate our findings and generalize its clinical utility.

Functional polymorphisms in the NPAS2 gene are associated with overall survival in transcatheter arterial chemoembolization-treated hepatocellular carcinoma patients

The functional abnormality of circadian regulation genes is involved in the development and progression of hepatocellular carcinoma (HCC). However, the association between functional single nucleotide polymorphisms (SNPs) in circadian gene NPAS2 and the overall survival of HCC patients treated with transcatheter arterial chemoembolization (TACE) has never been investigated. Six functional SNPs in the NPAS2 gene were genotyped using the Sequenom iPLEX genotyping system in a cohort of 448 unresectable Chinese patients with HCC treated with TACE. Multivariate Cox proportional hazards model and Kaplan–Meier curves were used for the prognosis analysis. We found that two SNPs, rs1053096 and rs2305160, in the NPAS2 gene showed significant associations with overall death risk in HCC patients in the recessive model (hazard ratio [HR] = 1.48; 95% confidence interval [CI], 1.13–1.94; P = 0.004) and in the dominant model (HR = 1.63; 95% CI, 1.29–2.07; P < 0.001), respectively. Moreover, we observed a cumulative effect of these two SNPs on HCC overall survival, indicating a significant trend of increasing death risk with increasing number of unfavorable genotypes (P for trend < 0.001). Compared with the patients without any unfavorable genotypes, the HRs for patients with one and two unfavorable genotypes were 1.41 (95% CI, 1.10–1.82; P = 0.007) and 2.09 (95% CI, 1.46–2.97, P < 0.001), respectively. The haplotype and diplotype analyses further characterized the association between NPAS2 genotype and survival of HCC patients. Our results for the first time suggest that NPAS2 gene polymorphisms may serve as an independent prognostic marker for HCC patients treated with TACE.

Understanding circadian gene function: animal models of tissue-specific circadian disruption

Circadian rhythms are the daily patterns that occur within an organism, from gene expression to behavior. These rhythms are governed not only externally by environmental cues but also internally, with cell-autonomous molecular clock mechanisms present nearly ubiquitously throughout the cells of organisms. In more complex organisms, it has been suggested that the clock mechanisms serve varied functions depending on the tissue in which they are found. By disrupting core circadian gene function in specific tissues of animal models, the various roles of the circadian clock in differing tissues can begin to be defined. This review provides an overview of the model organisms used to elucidate tissue-specific functions of the molecular circadian clock.

2mit, an intronic gene of Drosophila melanogaster timeless2, is involved in behavioral plasticity

Background

Intronic genes represent ~6% of the total gene complement in Drosophila melanogaster and ~85% of them encode for proteins. We recently characterized the D. melanogastertimeless2 (tim2) gene, showing its active involvement in chromosomal stability and light synchronization of the adult circadian clock. The protein coding gene named 2mit maps on the 11^thtim2 intron in the opposite transcriptional orientation.

Methodology/Principal Findings

Here we report the molecular and functional characterization of 2mit. The 2mit gene is expressed throughout Drosophila development, localizing mainly in the nervous system during embryogenesis and mostly in the mushroom bodies and ellipsoid body of the central complex in the adult brain. Insilico analyses revealed that 2mit encodes a putative leucine-Rich Repeat transmembrane receptor with intrinsically disordered regions, harboring several fully conserved functional interaction motifs in the cytosolic side. Using insertional mutations, tissue-specific over-expression, and down-regulation approaches, it was found that 2mit is implicated in adult short-term memory, assessed by a courtship conditioning assay. In D. melanogaster, tim2 and 2mit do not seem to be functionally related. Bioinformatic analyses identified 2MIT orthologs in 21 Drosophilidae, 4 Lepidoptera and in Apis mellifera. In addition, the tim2-2mit host-nested gene organization was shown to be present in A. mellifera and maintained among Drosophila species. Within the Drosophilidae 2mit-hosting tim2 intron, insilico approaches detected a neuronal specific transcriptional binding site which might have contributed to preserve the specific host-nested gene association across Drosophila species.

Conclusions/Significance

Taken together, these results indicate that 2mit, a gene mainly expressed in the nervous system, has a role in the behavioral plasticity of the adult Drosophila. The presence of a putative 2mit regulatory enhancer within the 2mit-hosting tim2 intron could be considered an evolutionary constraint potentially involved in maintaining the tim2-2mit host-nested chromosomal architecture during the evolution of Drosophila species.

Pharmacological modulators of the circadian clock as potential therapeutic drugs: focus on genotoxic/anticancer therapy

The circadian clock is an evolutionary conserved intrinsic timekeeping mechanism that controls daily variations in multiple biological processes. One important process that is modulated by the circadian clock is an organism's response to genotoxic stress, such as that induced by anticancer drug and radiation treatments. Numerous observations made in animal models have convincingly demonstrated that drug-induced toxicity displays prominent daily variations; therefore, undesirable side effects could be significantly reduced by administration of drugs at specific times when they are better tolerated. In some cases, these critical times of the day coincide with increased sensitivity of tumor cells allowing for a greater therapeutic index. Despite encouraging results of chronomodulated therapies, our knowledge of molecular mechanisms underlying these observations remains sketchy. Here we review recent progress in deciphering mechanistic links between circadian and stress response pathways with a focus on how these findings could be applied to anticancer clinical practice. We discuss the potential for using high-throughput screens to identify small molecules that can modulate basic parameters of the entire circadian machinery as well as functional activity of its individual components. We also describe the discovery of several small molecules that can pharmacologically modulate clock and that have a potential to be developed into therapeutic drugs. We believe that translational applications of clock-targeting pharmaceuticals are twofold: they may be developed into drugs to treat circadian-related disorders or used in combination with existing therapeutic strategies to improve therapeutic index of a given genotoxic treatment via the intrinsic clock mechanism.

Rhythmic profiles of cell cycle and circadian clock gene transcripts in mice: a possible association between two periodic systems

The circadian system shapes the rhythms of most biological functions. The regulation of the cell cycle by a circadian clock was suggested to operate via stages S, G2 and G2/M. This study investigated a possible time link at stages G1 and G1/S as well. The daily expression profiles of cell cycle markers (Ccnd1, Ccne1 and Pcna) and circadian clock genes (Per2 and Clock) were monitored in liver and esophagus (low and high proliferation index, respectively) of BALB/c mice. Locomotor activity displayed a 24 h rhythm, establishing the circadian organization of the suprachiasmatic nucleus. In the liver, the mRNA level of Per2 and Clock fitted the circadian rhythm with a 7.5 h shift. This temporal pattern suggests that the liver harbors a functional circadian clock. The rhythm of the analyzed cell cycle genes, however, was of low significance fitness and showed an opposite peak time between Pcna and Clock. These results indicate a weak regulatory role of the circadian clock. In the esophagus, the rhythms of Clock and Per2 mRNA had a similar peak time and non-circadian periods. These results suggest either that the esophagus does not harbor a functional circadian apparatus or that the phenotypes stem from differences in phase and amplitude of the rhythms of its various cell types. The similarity in the rhythm parameters of Clock, Ccne1 and Pcna transcripts questions the control of the circadian clock on the cell cycle along the G1 and G1/S stages. Yet the G1/S transition may play a role in modulating the local clock of proliferating tissues.

Association of CLOCK gene variants with semen quality in idiopathic infertile Han-Chinese males

Recent experimental animal studies suggested that the circadian locomotor output cycles kaput protein gene (CLOCK) may play an important role in male reproduction. So far, such data for humans are not available. This study used single-nucleotide polymorphisms (SNP) to examine the association between CLOCK and semen quality in a human population with idiopathic infertility. Three-variant genotyping of CLOCK and semen analysis were performed in 478 men with idiopathic infertility by SNP genotyping assays and computer-aided sperm analysis. Subjects carrying a C allele at rs3749474 (CC and TC) presented significantly lower semen volume (P=<0.001 and 0.001, respectively) compared with the TT genotype. Subjects carrying the rs3749474 CC genotype had significantly lower sperm number per ejaculate (P=0.026) and sperm motility (P=0.021) than TT genotype carriers. rs1801260 TC genotype carriers had significantly lower sperm motility compared with the TT genotype (P=0.028). For the rs3817444 genotypes, CA and AA genotype carriers presented significantly lower semen volume compared with the CC genotype (P=0.022 and 0.001, respectively). The findings suggest, as far as is known for the first time, an association between CLOCK genetic variants and altered semen quality in a human population with idiopathic infertility. The gene encoding the circadian locomotor output cycles kaput protein (CLOCK) functions as an important positive enhancer of the circadian system. The observations reported in recent experimental animal studies suggested that CLOCK may play an important role in male reproduction. So far, such data for humans are not available. In this study, single-nucleotide polymorphisms (SNP) were used to examine the association between CLOCK and semen quality in human population with idiopathic infertility. Three-variant genotyping of CLOCK and semen analysis were performed in 478 males with idiopathic infertility by SNP genotyping assays and computer-assisted semen analysis. The results showed that the subjects carrying a C allele at rs3749474 (CC and TC) presented significantly lower semen volume compared with the TT genotype. For subjects carrying the CC genotype, sperm number per ejaculate and sperm motility were significantly lower compared with TT genotype carriers. The rs1801260 TC genotype carriers also had significantly lower sperm motility compared with the TT genotype. For the rs3817444 genotypes, the CA and AA genotype carriers presented significantly lower semen volume compared with the CC genotype. The findings suggested, as far as is known for the first time, an association between CLOCK genetic variants and altered semen quality in a human population with idiopathic infertility.

A functional polymorphism in PER3 gene is associated with prognosis in hepatocellular carcinoma

BACKGROUND

Previous studies have revealed that circadian genes play important roles in cell proliferation, apoptosis, cell cycle control, DNA damage response and treatment response of chemotherapy agents in cancers.

AIMS

We hypothesized that the polymorphisms in circadian genes may be associated with prognosis of hepatocellular carcinoma (HCC) patients treated with transcatheter arterial chemoembolization (TACE).

METHODS

Twelve functional single nucleotide polymorphisms (SNPs) in circadian negative feedback regulation genes (including CRY1, CRY2, PER1, PER2 and PER3) were genotyped using Sequenom iPLEX genotyping method in 337 HCC patients treated with TACE and analysed for associations with overall survival.

RESULTS

Our data showed that one SNP rs2640908 in PER3 gene was significantly associated with overall survival of HCC patients (P = 0.027). Patients carrying at least one variant allele of rs2640908 (WV + VV) had a significantly decreased risk of death (hazard ratio, 0.71; 95% confidence interval, 0.53-0.90), when compared with those carrying homozygous wild-type alleles (WW). Kaplan-Meier analyses showed a significantly longer median survival time in patients with WV + VV genotypes of SNP rs2640908 than those with WW genotype (11.6 months vs. 8.1 months; log rank P = 0.030). In addition, we also observed a significant difference on the genotype distribution of SNP rs2640908 in patients with and without portal vein thrombus (P = 0.041).

CONCLUSIONS

Our study provides the first evidence that a single functional polymorphism of PER3 gene is significantly associated with overall survival in HCC patients treated with TACE.

Functional polymorphisms of circadian positive feedback regulation genes and clinical outcome of Chinese patients with resected colorectal cancer

BACKGROUND

Previous studies have demonstrated that circadian genes play a role in the development and progression of many cancers. This study aims to assess the effects of single nucleotide polymorphisms (SNPs) in circadian genes on recurrence and survival of colorectal cancer (CRC) patients.

METHODS

Nine functional SNPs in 3 genes (CLOCK, NPAS2, and BMAL1) on the circadian positive feedback loop were selected and genotyped using the Sequenom iPLEX genotyping system in a cohort of 411 resected Chinese CRC patients. Multivariate Cox proportional hazards model and Kaplan-Meier curve were used for the prognosis analysis.

RESULTS

The authors identified 2 SNPs in the CLOCK gene to be significantly associated with CRC overall survival. SNP rs3749474 exhibited a significant association with survival of CRC patients in the additive model (hazard ratio [HR], 0.55; 95% confidence interval [CI], 0.37-0.81; P = .003). In addition, patients carrying the heterozygous variant of rs1801260 had significantly increased overall survival compared with those carrying homozygous wild-type genotype (HR, 0.31; 95% CI, 0.11-0.88; P = .03). Findings from functional assay provided further biological support for these significant associations. Stratified analysis found no modifying effect of chemotherapy on the prognostic significance of both SNPs. Moreover, we observed cumulative effects of these 2 SNPs on CRC overall survival (P for trend = .01). Compared with patients carrying no unfavorable genotypes, those carrying 2 unfavorable genotypes had a 2.92-fold increased risk of death (P = .03).

CONCLUSIONS

The results suggest for the first time that CLOCK gene polymorphisms may serve as an independent prognostic marker for CRC patients.

The de-ubiquitinylating enzyme, USP2, is associated with the circadian clockwork and regulates its sensitivity to light

We have identified a novel component of the circadian clock that regulates its sensitivity to light at the evening light to dark transition. USP2 (Ubiquitin Specific Protease 2), which de-ubiquitinylates and stabilizes target proteins, is rhythmically expressed in multiple tissues including the SCN. We have developed a knockout model of USP2 and found that exposure to low irradiance light at ZT12 increases phase delays of USP2(-/-) mice compared to wildtype. We additionally show that USP2b is in a complex with several clock components and regulates the stability and turnover of BMAL1, which in turn alters the expression of several CLOCK/BMAL1 controlled genes. Rhythmic expression of USP2 in the SCN and other tissues offers a new level of control of the clock machinery through de-ubiqutinylation and suggests a role for USP2 during circadian adaptation to environmental day length changes.

Deficiency of circadian protein CLOCK reduces lifespan and increases age-related cataract development in mice

Circadian clock is implicated in the regulation of aging. The transcription factor CLOCK, a core component of the circadian system, operates in complex with another circadian clock protein BMAL1. Recently it was demonstrated that BMAL1 deficiency results in premature aging in mice. Here we investigate the aging of mice deficient for CLOCK protein. Deficiency of the CLOCK protein significantly affects longevity: the average lifespan of Clock-/- mice is reduced by 15% compared with wild type mice, while maximum lifespan is reduced by more than 20%. CLOCK deficiency also results in the development of two age-specific pathologies in these mice, cataracts and dermatitis, at a much higher rate than in wild type mice. In contrast to BMAL1 deficient animals, Clock-/- mice do not develop a premature aging phenotype and do not develop the multiple age-associated pathologies characteristic of BMAL1 deficiency. Thus, although CLOCK and BMAL1 form a transcriptional complex, the physiological result of their deficiency is different. Our results suggest that CLOCK plays an important role in aging, specifically; CLOCK activity is critical for the regulation of normal physiology and aging of the lens and skin.

A common origin: signaling similarities in the regulation of the circadian clock and DNA damage responses

Circadian clocks are intrinsic, time-tracking systems that endow organisms with a survival advantage. Studies of animal models and human tumor samples have revealed that the disruption of circadian rhythms is an important endogenous factor that can contribute to mammalian cancer development. The core of the circadian clock mechanism is a cell-autonomous and self-sustained oscillator system mediated by a transcription/translation-based negative feedback loop that relies on positive and negative elements. Recent studies have implicated these core circadian components in the regulation of both the cell cycle and DNA damage responses (DDR). Indeed, the circadian feedback loop controls the timing of cell proliferation by regulating the expression of key cell cycle genes. Conversely, several intracellular signaling cascades and post-translational modifications that play important roles in the cell cycle and DDR are also essential for circadian clock regulation. Importantly, alteration of a cell's reduction-oxidation (redox) state triggers the transduction of photic signals that regulate circadian clock gene transcription, suggesting that cellular responses to photo-oxidative stress may have been the evolutionary origin of the circadian clock. This review describes selected regulatory aspects of circadian machinery that are evidence of a molecular link between the circadian clock and DDR, focusing particularly on the signaling cascades involved in the light entrainment of the zebrafish circadian clock.

Circadian clock proteins control adaptation to novel environment and memory formation

Deficiency of the transcription factor BMAL1, a core component of the circadian clock, results in an accelerated aging phenotype in mice. The circadian clock regulates many physiological processes and was recently implicated in control of brain-based activities, such as memory formation and the regulation of emotions. Aging is accompanied by the decline in brain physiology, particularly decline in the response and adaptation to novelty. We investigated the role of the circadian clock in exploratory behavior and habituation to novelty using the open field paradigm. We found that mice with a deficiency of the circadian transcription factor BMAL1 display hyperactivity in novel environments and impaired intra- and intersession habituation, indicative of defects in short- and long-term memory formation. In contrast, mice double-deficient for the circadian proteins CRY1 and CRY2 (repressors of the BMAL1-mediated transcription) demonstrate reduced activity and accelerated habituation when compared to wild type mice. Mice with mutation in theClock gene (encoding the BMAL1 transcription partner) show normal locomotion, but increased rearing activity and impaired intersession habituation. BMAL1 is highly expressed in the neurons of the hippocampus - a brain region associated with spatial memory formation; BMAL1 deficiency disrupts circadian oscillation in gene expression and reactive oxygen species homeostasis in the brain, which may be among the possible mechanisms involved. Thus, we suggest that the BMAL1:CLOCK activity is critical for the proper exploratory and habituation behavior, and that the circadian clock prepares organism for a new round of everyday activities through optimization of behavioral learning.

Altered body mass regulation in male mPeriod mutant mice on high-fat diet

The circadian clock orchestrates most physiological processes in mammals. Disruption of circadian rhythms appears to contribute to the development of obesity and metabolic syndrome. The Period genes mPer1 and mPer2, but not mPer3, are essential for core clock function in mice. To assess the impact of mPer genes on body mass regulation, mPer mutant and control mice were fed a high-fat diet. Here the authors report that male mPer1/2/3 triple-deficient mice gain significantly more body mass than wild-type controls on high-fat diet. Surprisingly, mPer3 single-deficient animals mimicked this phenotype, suggesting a previously unrecognized role for mPer3 in body mass regulation.

The core circadian gene Cryptochrome 2 influences breast cancer risk, possibly by mediating hormone signaling

As transcriptional regulators, circadian genes have the potential to influence a variety of biological pathways, including many cancer-related processes. Cryptochrome 2 (CRY2) is essential for proper circadian timing and is a key component of the circadian regulatory feedback loop. Here, we report findings from genetic, epigenetic, loss-of-function, and transcriptional profiling analyses of CRY2 in breast cancer. Six single-nucleotide polymorphisms in CRY2 were identified for genotyping in a case-control population (n = 441 cases and n = 479 controls), and three single-nucleotide polymorphisms (rs11038689, rs7123390, and rs1401417) were significantly associated with postmenopausal breast cancer risk, with significant effect modification by menopausal status [dominant model for rs11038689: odds ratio (OR), 0.71; 95% confidence interval (95% CI), 0.51-0.99; P for trend = 0.028; homozygous variants for rs7123390: OR, 0.44; 95% CI, 0.22-0.86; P for trend = 0.028; and rs1401417: OR, 0.44; 95% CI, 0.21-0.92; P for trend = 0.017]. Interestingly, this association was only evident in women with estrogen and progesterone receptor (ER/PR)-negative breast tumors but not with ER/PR-positive tumors. Breast cancer patients also had significantly higher levels of CRY2 promoter methylation relative to controls, which is consistent with tissue array data showing lower levels of CRY2 expression in tumor tissue relative to adjacent normal tissue. Furthermore, in vitro analyses identified several breast cancer-relevant genes that displayed altered expression following CRY2 knockdown. These findings suggest a role for CRY2 in breast tumorigenesis and provide further evidence that the circadian system may be an important modulator of hormone-related cancer susceptibility.

Circadian rhythm and its role in malignancy

Circadian rhythms are daily oscillations of multiple biological processes directed by endogenous clocks. The circadian timing system comprises peripheral oscillators located in most tissues of the body and a central pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Circadian genes and the proteins produced by these genes constitute the molecular components of the circadian oscillator which form positive/negative feedback loops and generate circadian rhythms. The circadian regulation extends beyond clock genes to involve various clock-controlled genes (CCGs) including various cell cycle genes. Aberrant expression of circadian clock genes could have important consequences on the transactivation of downstream targets that control the cell cycle and on the ability of cells to undergo apoptosis. This may lead to genomic instability and accelerated cellular proliferation potentially promoting carcinogenesis. Different lines of evidence in mice and humans suggest that cancer may be a circadian-related disorder. The genetic or functional disruption of the molecular circadian clock has been found in various cancers including breast, ovarian, endometrial, prostate and hematological cancers. The acquisition of current data in circadian clock mechanism may help chronotherapy, which takes into consideration the biological time to improve treatments by devising new therapeutic approaches for treating circadian-related disorders, especially cancer.

Phenotypic effects of the circadian gene Cryptochrome 2 on cancer-related pathways

BACKGROUND

Circadian genes continue to gain attention as important transcriptional regulators with the potential to influence a variety of biological pathways, including many cancer-related processes. The core circadian gene cryptochrome 2 (CRY2) is essential for proper circadian timing, and is a key component of the negative arm of the circadian feedback loop. As such, aberrant expression of CRY2 may influence carcinogenic processes and thereby impact cancer susceptibility.

METHODS

We silenced CRY2 in breast cancer cell lines (MCF-7) using small-interfering oligos (siRNA) and measured the impact of CRY2 knockdown on a number of cancer-relevant parameters. Cell cycle distribution, cell viability, and apoptotic response were measured in CRY2 knockdown (CRY2-) and normal (CRY2+) cell populations using flow cytometry in cells with and without exposure to a mutagen challenge. DNA damage accumulation was measured using the single cell gel electrophoresis (comet) assay, and damage was quantified using the Olive tail moment, which considers the amount and distance of DNA migration away from the nucleus, indicative of DNA strand breaks. Expression changes in cancer-relevant transcripts were measured by whole genome microarray. The Student's t-test was used for statistical comparisons, and P-values obtained from the microarray were adjusted for multiple comparisons using the false discovery rate correction, in order to obtain an adjusted Q-value for each observation.

RESULTS

The comet assay results indicated that upon exposure to the same dose of chemical mutagen, CRY2- cells accumulate significantly more unrepaired DNA damage than CRY2+ cells (P = 0.040), suggesting that CRY2 may be important for DNA repair. In addition, a number of transcripts with relevance for DNA damage repair displayed altered expression following CRY2 silencing. These included BCCIP (Q = 0.002), BCL2 (Q = 0.049), CCND1 (Q = 0.009), CDKN1A (Q < 0.001), GADD45A (Q = 0.002), HERC5 (Q < 0.001), MCM5 (Q = 0.042), PPP1R15A (Q < 0.001), SUMO1 (Q < 0.001), and UBA1 (Q = 0.023). However, no significant influence of CRY2 knockdown on cell cycle distributions, cell cycle checkpoints in response to mutagen challenge, or apoptotic response was detected.

CONCLUSIONS

In total, these data suggest a limited, but potentially important role for CRY2 in the regulation of DNA damage repair and the maintenance of genomic stability. Future investigations may focus on identifying the mechanisms by which CRY2 may regulate the expression of transcripts with known relevance for carcinogenesis.

Circadian rhythms and metabolic syndrome: from experimental genetics to human disease

The incidence of the metabolic syndrome represents a spectrum of disorders that continue to increase across the industrialized world. Both genetic and environmental factors contribute to metabolic syndrome and recent evidence has emerged to suggest that alterations in circadian systems and sleep participate in the pathogenesis of the disease. In this review, we highlight studies at the intersection of clinical medicine and experimental genetics that pinpoint how perturbations of the internal clock system, and sleep, constitute risk factors for disorders including obesity, diabetes mellitus, cardiovascular disease, thrombosis and even inflammation. An exciting aspect of the field has been the integration of behavioral and physiological approaches, and the emerging insight into both neural and peripheral tissues in disease pathogenesis. Consideration of the cell and molecular links between disorders of circadian rhythms and sleep with metabolic syndrome has begun to open new opportunities for mechanism-based therapeutics.

Circadian proteins and genotoxic stress response

The circadian clock is an evolutionarily conserved time-keeping system that coordinates the physiology of the organism with daily changes in the environment. A growing body of evidence gradually leads to the conception that virtually all aspects of the biochemical, physiological, and behavioral functions of the animal are linked to circadian regulation. Moreover, proper synchronization of various processes through the activity of circadian components is important for the well-being of many organisms, including humans. The focus of this review is the circadian control of an organism's response to genotoxic stress, which is a major contributor to life-threatening human pathologies such as cancer and cardiovascular disease.

Testing the circadian gene hypothesis in prostate cancer: a population-based case-control study

Circadian genes are responsible for maintaining the ancient adaptation of a 24-hour circadian rhythm and influence a variety of cancer-related biological pathways, including the regulation of sex hormone levels. However, few studies have been undertaken to investigate the role of circadian genes in the development of prostate cancer, the most common cancer type among men (excluding nonmelanoma skin cancer). The current genetic association study tested the circadian gene hypothesis in relation to prostate cancer by genotyping a total of 41 tagging and amino acid-altering single nucleotide polymorphisms (SNP) in 10 circadian-related genes in a population-based case-control study of Caucasian men (n = 1,308 cases and 1,266 controls). Our results showed that at least one SNP in nine core circadian genes (rs885747 and rs2289591 in PER1; rs7602358 in PER2; rs1012477 in PER3; rs1534891 in CSNK1E; rs12315175 in CRY1; rs2292912 in CRY2; rs7950226 in ARNTL; rs11133373 in CLOCK; and rs1369481, rs895521, and rs17024926 in NPAS2) was significantly associated with susceptibility to prostate cancer (either overall risk or risk of aggressive disease), and the risk estimate for four SNPs in three genes (rs885747 and rs2289591 in PER1, rs1012477 in PER3, and rs11133373 in CLOCK) varied by disease aggressiveness. Further analyses of haplotypes were consistent with these genotyping results. Findings from this candidate gene association study support the hypothesis of a link between genetic variants in circadian genes and prostate cancer risk, warranting further confirmation and mechanistic investigation of circadian biomarkers in prostate tumorigenesis.

Circadian disruption, Per3, and human cytokine secretion

Circadian disruption has been linked with inflammation, an established cancer risk factor. Per3 clock gene polymorphisms have also been associated with circadian disruption and with increased cancer risk. Patients completed a questionnaire and provided a blood sample prior to undergoing a colonoscopy (n = 70). Adjusted mean serum cytokine concentrations (IL-6, TNF-alpha, gamma-INF, IL-1ra, IL-1-beta, VEGF) were compared among patients with high and low scores for fatigue (Multidimensional Fatigue Inventory), depressive symptoms (Beck Depression Inventory II), or sleep disruption (Pittsburgh Sleep Quality Index), or among patients with different Per3 clock gene variants. Poor sleep was associated with elevated VEGF, and fatigue-related reduced activity was associated with elevated TNF-alpha concentrations. Participants with the 4/5 or 5/5 Per3 variable tandem repeat sequence had elevated IL-6 concentrations compared to those with the 4/4 genotype. Biological processes linking circadian disruption with cancer remain to be elucidated. Increased inflammatory cytokine secretion may play a role.

Pharmacological modulators of the circadian clock as potential therapeutic drugs

Circadian clocks are molecular time-keeping systems that underlie daily fluctuations in multiple physiological and biochemical processes. It is well recognized now that dysfunctions of the circadian system (both genetically and environmentally induced) are associated with the development of various pathological conditions. Here we describe the application of high throughput screening approach designed to search for small molecules capable of pharmacological modulation of the molecular clock. We provide evidence for the feasibility and value of this approach for both scientific and therapeutic purposes.

Developmental profiles of PERIOD and DOUBLETIME in Drosophila melanogaster ovary

The clock protein PERIOD (PER) displays circadian cycles of accumulation, phosphorylation, nuclear translocation and degradation in Drosophila melanogaster clock cells. One exception to this pattern is in follicular cells enclosing previtellogenic ovarian egg chambers. In these cells, PER remains high and cytoplasmic at all times of day. Genetic evidence suggest that PER and its clock partner TIMELESS (TIM) interact in these cells, yet, they do not translocate to the nucleus. Here, we investigated the levels and subcellular localization of PER in older vitellogenic follicles. Cytoplasmic PER levels decreased in the follicular cells at the onset of vitellogenesis (stage 9). Interestingly, PER was observed in the nuclei of some follicular cells at this stage. PER signal disappeared in more advanced (stage 10) vitellogenic follicles. Since the phosphorylation state of PER is critical for the progression of circadian cycle, we investigated the status of PER phosphorylation in the ovary and the expression patterns of DOUBLETIME (DBT), a kinase known to affect PER in the clock cells. DBT was absent in previtellogenic follicular cells, but present in the cytoplasm of some stage 9 follicular cells. DBT was not distributed uniformly but was present in patches of adjacent cells, in a pattern resembling PER distribution at the same stage. Our data suggest that the absence of dbt expression in the follicular cells of previtellogenic egg chambers may be related to stable and cytoplasmic expression of PER in these cells. Onset of dbt expression in vitellogenic follicles coincides with nuclear localization of PER protein.

Clock-cancer connection in non-Hodgkin's lymphoma: a genetic association study and pathway analysis of the circadian gene cryptochrome 2

Circadian genes have the potential to influence a variety of cancer-related biological pathways, including immunoregulation, which may influence susceptibility to non-Hodgkin's lymphoma (NHL). However, few studies have examined the role of circadian genes in lymphomagenesis. The current study examined Cryptochrome 2 (CRY2), a core circadian gene and transcriptional repressor, as a potential circadian biomarker for NHL. We first performed genetic association analyses of tagging single nucleotide polymorphisms (SNP) in CRY2 and NHL risk using DNA samples from a population-based case-control study (n = 455 cases and 527 controls). Three SNPs were found to be significantly associated with risk of NHL when combining all subtypes [dbSNP IDs, odds ratios (ORs), and 95% confidence intervals: rs11038689, OR, 2.34 (1.28-4.27), P = 0.006; rs7123390, OR, 2.40 (1.39-4.13), P = 0.002; and rs1401417, OR, 2.97 (1.57-5.63, P = 0.001)]. Each of these associations remained significant when restricting the analysis to B-cell cases and when further restricting to follicular lymphomas. An analysis of CRY2 diplotypes confirmed these significant findings. To further determine the functional effect of CRY2, we silenced the gene in vitro and performed a whole genome expression microarray. A pathway-based analysis showed that genes significantly altered by CRY2 knockdown formed networks associated with immune response and hematologic system development. In addition, these genes were predicted to have significant effects on several disease processes, including cancer (B-H P = 3.75E(-9)) and hematologic disease (B-H P = 8.01E(-8)). In conclusion, both genetic association and functional analyses suggest that the circadian gene CRY2 may play an important role in NHL development.

Expression and functional analyses of circadian genes in mouse oocytes and preimplantation embryos: Cry1 is involved in the meiotic process independently of circadian clock regulation

In mammals, circadian genes, Clock, Arntl (also known as Bmal1), Cry1, Cry2, Per1, Per2, and Per3, are rhythmically transcribed every 24 h in almost all organs and tissues to tick the circadian clock. However, their expression and function in oocytes and preimplantation embryos have not been investigated. In this study we found that the circadian clock may stop in mouse oocytes and preimplantation embryos. Real-time PCR analysis revealed the presence of transcripts of these genes in both oocytes and preimplantation embryos; however, their amounts did not oscillate every 24 h in one- to four-cell and blastocyst-stage embryos. Moreover, immunofluorescence analyses revealed that CLOCK, ARNTL, and CRY1 were localized similarly in the nuclei of germinal vesicle (GV) oocytes and one-cell- to four-cell-stage embryos. Because CRY1 is known to interact with the CLOCK-ARNTL complex to suppress transcription-promoting activity of the complex for genes such as Wee1, Cry2, Per1, Per2, and Per3 in cells having the ticking circadian clock, we hypothesized that if the circadian clock functions in GV oocytes and one-cell- to four-cell-stage embryos, CLOCK, ARNTL, and CRY1 might suppress the transcription of these genes in GV oocytes and one-cell- to 4-cell-stage embryos as well. As a result, knockdown of CRY1 in GV oocytes by RNA interference did not affect the transcription levels of Wee1, Cry2, Per1, Per2, and Per3, but it reduced maturation ability. Thus, it seems that circadian genes are not involved in circadian clock regulation in mouse oocytes and preimplantation embryos but are involved in physiologies, such as meiosis.

The meter of metabolism

The circadian system orchestrates the temporal organization of many aspects of physiology, including metabolism, in synchrony with the 24 hr rotation of the Earth. Like the metabolic system, the circadian system is a complex feedback network that involves interactions between the central nervous system and peripheral tissues. Emerging evidence suggests that circadian regulation is intimately linked to metabolic homeostasis and that dysregulation of circadian rhythms can contribute to disease. Conversely, metabolic signals also feed back into the circadian system, modulating circadian gene expression and behavior. Here, we review the relationship between the circadian and metabolic systems and the implications for cardiovascular disease, obesity, and diabetes.

Structure and function of animal cryptochromes

Cryptochrome (CRY) is a photolyase-like flavoprotein with no DNA-repair activity but with known or presumed blue-light receptor function. Animal CRYs have DNA-binding and autokinase activities, and their flavin cofactor is reduced by photoinduced electron transfer. In Drosophila, CRY is a major circadian photoreceptor, and in mammals, the two CRY proteins are core components of the molecular clock and potential circadian photoreceptors. In mammals, CRYs participate in cell cycle regulation and the cellular response to DNA damage by controlling the expression of some cell cycle genes and by directly interacting with checkpoint proteins.

Disruption of the circadian clock due to the Clock mutation has discrete effects on aging and carcinogenesis

The mammalian circadian system has been implicated in the regulation of various biological processes including those involved in genotoxic stress responses and tumor suppression. Here we report that mice with the functional deficiency in circadian transcription factor CLOCK (Clock/Clock mutant mice) do not display predisposition to tumor formation both during their normal lifespan or when challenged by gamma- radiation. This phenotype is consistent with high apoptotic and low proliferation rate in lymphoid tissues of Clock mutant mice and is supported by the gene expression profiling of a number of apoptosis and cell cycle-related genes, as well as by growth inhibition of cells with CLOCK downregulation. At the same time, Clock mutant mice respond to low-dose irradiation by accelerating their aging program, and develop phenotypes that are reminiscent of those in Bmal1-deficient mice. Taken together, our results demonstrate the dichotomy in biological consequences of the disruption of the circadian clock with respect to ageing and cancer. They also highlight the existence of a complex interconnection between ageing, carcinogenesis and individual components of the circadian clock machinery.

Disruption of clock gene expression alters responses of the aryl hydrocarbon receptor signaling pathway in the mouse mammary gland

The biological effects of many environmental toxins are mediated by genes containing Per-Arnt-Sim (PAS) domains, the aryl hydrocarbon receptor (AhR), and AhR nuclear translocator. Because these transcription factors interact with other PAS genes that form the circadian clockworks in mammals, we determined whether targeted disruption of the clock genes, Per1 and/or Per2, alters toxin-induced expression of known biological markers in the AhR signaling pathway. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a prototypical Ahr agonist, had an inductive effect on mammary gland expression of cytochrome P450, subfamily I, polypeptide 1 (Cyp1A1) mRNA regardless of genotype. However, TCDD-mediated Cyp1A1 induction in the mammary glands of Per1(ldc) and Per1(ldc)/Per2(ldc) mice was significantly (17.9- and 5.9-fold) greater than that in wild-type (WT) animals. In addition, TCDD-induced Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mammary glands was significantly increased relative to that in WT mice. Similar to in vivo observations, experiments using primary cultures of mammary gland tissue demonstrated that TCDD-induced Cyp1A1 and Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mutant cells was significantly greater than that in WT cultures. AhR mRNA levels were distinctively elevated in cells derived from all mutant genotypes, but they were commonly decreased in WT and mutant cultures after TCDD treatment. In WT mice, an interesting corollary is that the inductive effects of TCDD on mammary gland expression of Cyp1A1 and Cyp1B1 vary over time and are significantly greater during the night. These findings suggest that clock genes, especially Per1, may be involved in TCDD activation of AhR signaling pathways.

24-Hour rhythm in gene expression of nitric oxide synthase and heme-peroxidase in anterior pituitary of ethanol-fed rats

Chronic exposure of rats to ethanol results in significant changes in pituitary hormone secretion. However, identification of the site(s) and mechanism of action of ethanol to induce these effects remains elusive. Free radical damage at the adenohypophyseal level may play a role in the decline in serum gonadotropin levels in ethanol-fed rats. Since 24-h changes in redox state occurred, we analyzed the 24-h changes in pituitary gene expression of the prooxidant enzymes nitric oxide synthase (NOS) 1 and 2, and of heme oxygenase-1 (HO-1) enzyme, and in plasma NO(2)(-) and NO(3)(-) (NO(x)) levels, in ethanol and control rats. Male rats, 35-day-old, received a liquid diet for 4 weeks. The ethanol-fed group received a similar diet to controls except for that maltose was isocalorically replaced by ethanol. Animals were killed at six time intervals during a 24-h cycle. Anterior pituitary mRNA levels encoding NOS1, NOS2 and HO-1 were measured by real-time PCR analysis. Plasma NO(x) concentration was determined by the Griess reaction. Ethanol feeding of prepubertal rats changed significantly the 24-h pattern of expression of NOS1, NOS2 and HO-1 in the adenohypophysis and augmented NOS2 and HO-1 mRNA levels. Peak values for the three enzymes in ethanol-fed rats occurred at the beginning of the scotophase (i.e., at 21:00 h). Ethanol feeding augmented mean values plasma NO(x) levels with a maximum at 13:00 h while in controls a biphasic pattern was observed, with peaks at 09:00 h and 17:00-21:00 h. One of the mechanisms by which ethanol augments oxidative damage in the adenohypophysis may include overproduction of nitric oxide and carbon monoxide.

Neuroendocrine-immune correlates of circadian physiology: studies in experimental models of arthritis, ethanol feeding, aging, social isolation, and calorie restriction

Virtually all neuroendocrine and immunological variables investigated in animals and humans display biological periodicity. Circadian rhythmicity is revealed for every hormone in circulation as well as for circulating immune cells, lymphocyte metabolism and transformability, cytokines, receptors, and adhesion molecules. Clock genes, notably the three Period (Per1/Per2/Per3) genes and two Cryptochrome (Cry1/Cry2) genes, are present in immune and endocrine cells and are expressed in a circadian manner in human cells. This review discusses the circadian disruption of hormone release and immune-related mechanisms in several animal models in which circulating cytokines are modified including rat adjuvant arthritis, social isolation in rats and rabbits and alcoholism, the aging process and calorie restriction in rats. In every case the experimental manipulation used perturbed the temporal organization by affecting the shape and amplitude of a rhythm or by modifying the intrinsic oscillatory mechanism itself.

Circadian proteins in the regulation of cell cycle and genotoxic stress responses

The mammalian circadian system has been implicated in the regulation of the genotoxic stress response of an organism; however, the underlying molecular mechanisms are not well understood. Recent data suggest that, in addition to circadian variations in the expression of genes involved in genotoxic stress responses, core circadian proteins PERIOD1 (PER1) and TIMELESS (TIM) interact with components of the cell cycle checkpoint system, such as ataxia telangiectasia mutated (ATM)-checkpoint kinase 2 (Chk2) and ataxia telangiectasia and Rad3-related (ATR)-Chk1, and are necessary for activation of Chk1 and Chk2 by DNA damage. Moreover, in complex with its recently identified partner, TIM-interacting protein (TIPIN), TIM interacts with components of the DNA replication system to regulate DNA replication processes under both normal and stress conditions. These discoveries shed new light on the role of core circadian proteins in various cellular and physiological processes.

The clock proteins, aging, and tumorigenesis

Many aspects of mammalian physiology and behavior are driven by an intrinsic timekeeping system that has an important role in synchronizing various biological processes within an organism and coordinating them with the environment. It is believed that deregulation of this coordination may cause the development of various pathologies. However, recent studies using mice deficient in individual components of the circadian system clearly demonstrated more complex interaction of the circadian system with various biological processes. The growing amount of evidence suggests that in addition to their roles in the core clock mechanism, some of the components of the molecular oscillator are involved in modulation of such diverse physiological processes as response to genotoxic stress, regulation of the cell cycle, aging, and carcinogenesis. These new data provide a mechanistic link between deregulation of the circadian system and/or some of its core components and the development of various pathologies, suggesting novel strategies for the disease treatment and prevention.