Daily fluctuation of hepatic P450 monooxygenase activities in male rats is controlled by the suprachiasmatic nucleus but remains unaffected by adrenal hormones

Perspectives on the relevance of the circadian time structure to workplace threshold limit values and employee biological monitoring

The circadian time structure (CTS) and its disruption by rotating and nightshift schedules relative to work performance, accident risk, and health/wellbeing have long been areas of occupational medicine research. Yet, there has been little exploration of the relevance of the CTS to setting short-term, time-weighted, and ceiling threshold limit values (TLVs); conducting employee biological monitoring (BM); and establishing normative reference biological exposure indices (BEIs). Numerous publications during the past six decades document the CTS substantially affects the disposition - absorption, distribution, metabolism, and elimination - and effects of medications. Additionally, laboratory animal and human studies verify the tolerance to chemical, biological (contagious), and physical agents can differ extensively according to the circadian time of exposure. Because of slow and usually incomplete CTS adjustment by rotating and permanent nightshift workers, occupational chemical and other contaminant encounters occur during a different circadian stage than for dayshift workers. Thus, the intended protection of some TLVs when working the nightshift compared to dayshift might be insufficient, especially in high-risk settings. The CTS is germane to employee BM in that large-amplitude predictable-in-time 24h variation can occur in the concentration of urine, blood, and saliva of monitored chemical contaminants and their metabolites plus biomarkers indicative of adverse xenobiotic exposure. The concept of biological time-qualified (for rhythms) reference values, currently of interest to clinical laboratory pathology practice, is seemingly applicable to industrial medicine as circadian time and workshift-specific BEIs to improve surveillance of night workers, in particular. Furthermore, BM as serial assessments performed frequently both during and off work, exemplified by employee self-measurement of lung function using a small portable peak expiratory flow meter, can easily identify intolerance before induction of pathology.

Sex and circadian modulatory effects on rat liver as assessed by transcriptome analyses

The present study was designed to fully uncover sex and circadian modulatory effects on rat liver. Hepatic transcriptome analyses were performed at 4 hr intervals of a day-night cycle using young adult male and female rats. Sexually dimorphic genes, which were identified by a cross-sex comparison of time series data, included representative sex-predominant genes such as male- or female-predominant cytochrome P450 subfamilies (Cyp2c11, Cyp2c12, Cyp2c13, and Cyp3a2), sulfotransferases, and glutathione S-transferase Yc2. The identified sexually dimorphic genes were over-represented in the metabolism of retinols, xenobiotics, linoleic acids, or androgen and estrogen, or bile acid biosynthesis. Furthermore, transcription factor targets modeling suggested that transcription factors SP1, hepatocyte nuclear factor 4-alpha (HNF4-alpha), and signal transducer and activator of transcription 5b (STAT5b) serve as core nodes in the regulatory networks. On the other hand, Fourier transform analyses extracted universal circadian-regulated genes in both sexes. The circadian-regulated genes included clock or clock-controlled genes such as aryl hydrocarbon receptor nuclear translocator-like (Arntl), period homolog 2 (Per2), and D site albumin promoter binding protein (Dbp). The extracted cyclic genes were over-represented in major tissue activities, e.g. the urea cycle and the metabolism of amino acids, fatty acids, or glucose, indicating that the major liver functions are under circadian control. The transcription factor targets modeling suggested that transcription factors SP1, HNF4-alpha, and c-Myc proto-oncogene protein (c-MYC) serve as major hubs in the circadian-regulatory gene networks. Interestingly, transcription factors SP1 and HNF4-alpha are likely to orchestrate not only sexually dimorphic, but also circadian-regulated genes even though each criterion was rather mutually exclusive. This suggests the cross-talk between those regulations. Sexual dimorphism is likely to interact with circadian rhythmicity via overlapping gene regulatory networks on rat liver.

The clock genes period 1 and period 2 mediate diurnal rhythms in dioxin-induced Cyp1A1 expression in the mouse mammary gland and liver

Transcription factors expressing Per-Arnt-Sim (PAS) domains are key components of the mammalian circadian clockworks found in most cells and tissues. Because these transcription factors interact with other PAS genes mediating xenobiotic metabolism and because toxin responses are often marked by daily variation, we determined whether the toxin-mediated activation of the signaling pathway involving several PAS genes, the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT), fluctuates rhythmically and whether this diurnal oscillation is affected by targeted disruption of key PAS genes in the circadian clockworks, Period 1 (Per1) and Per2. Treatment with the prototypical Ahr ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), had inductive effects on a key target of AhR signaling, Cyp1A1, in both the mammary gland and liver of all animals. In wild type mice, the amplitude of this TCDD-induced Cyp1A1 expression in the mammary gland and liver was significantly greater (23-43-fold) during the night than during the daytime. However, the diurnal variation in the TCDD induction of mammary gland and liver Cyp1A1 expression was abolished in Per1(ldc), Per2(ldc) and Per1(ldc)/Per2(ldc) mutant mice, suggesting that Per1, Per2 and their timekeeping function in the circadian clockworks mediate the diurnal modulation of AhR-regulated responses to TCDD in the mammary gland and liver.

Disruption of period gene expression alters the inductive effects of dioxin on the AhR signaling pathway in the mouse liver

The aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) are transcription factors that express Per-Arnt-Sim (PAS) DNA-binding motifs and mediate the metabolism of drugs and environmental toxins in the liver. Because these transcription factors interact with other PAS genes in molecular feedback loops forming the mammalian circadian clockworks, we determined whether targeted disruption or siRNA inhibition of Per1 and Per2 expression alters toxin-mediated regulation of the AhR signaling pathway in the mouse liver and Hepa1c1c7 hepatoma cells in vitro. Treatment with the prototypical Ahr ligand, 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), had inductive effects on the primary targets of AhR signaling, Cyp1A1 and Cyp1B1, in the liver of all animals, but genotype-based differences were evident such that the toxin-mediated induction of Cyp1A1 expression was significantly greater (2-fold) in mice with targeted disruption of Per1 (Per1(ldc) and Per1(ldc)/Per2(ldc)). In vitro experiments yielded similar results demonstrating that siRNA inhibition of Per1 significantly increases the TCDD-induced expression of Cyp1A1 and Cyp1B1 in Hepa1c1c7 cells. Per2 inhibition in siRNA-infected Hepa1c1c7 cells had the opposite effect and significantly decreased both the induction of these p450 genes as well as AhR and Arnt expression in response to TCDD treatment. These findings suggest that Per1 may play a distinctive role in modulating AhR-regulated responses to TCDD in the liver.

Regulation of gene expression of hepatic drug metabolizing enzymes and transporters by the Toll-like receptor 2 ligand, lipoteichoic acid

Expression of hepatic drug metabolizing enzymes (DMEs) is altered in infection and inflammation. However, the role of Gram+ve bacterial components and their receptor, Toll-like receptor (TLR) 2 in regulation of hepatic DMEs is unknown. Gene expression of DMEs is regulated by members of the nuclear receptor superfamily (PXR, CAR and RXRalpha). The TLR2 ligand, lipoteichoic acid (LTA) reduced RNA levels of CAR and its target genes, Cyp2b10, Cyp2a4 and Sultn in mouse liver ( approximately 60-80% reduction). Hepatic genes regulated by PXR and CAR, Cyp3a11 and Mrp2 were moderately reduced by LTA, along with approximately 50% reduction of PXR RNA and nuclear protein levels of RXRalpha. The effects of LTA were significantly attenuated by pre-treatment with the Kupffer cell inhibitor, gadolinium chloride, indicating that Kupffer cells contribute to LTA-mediated down-regulation of hepatic genes. These results indicate that treatment with Gram+ve bacterial components preferentially down-regulate CAR and its target genes in the liver.

Effects of restricted feeding on daily fluctuations of hepatic functions including p450 monooxygenase activities in rats

Hepatic P450 monooxygenase activities, assessed by measurement of 7-alkoxycoumarin O-dealkylase (ACD) activities, show obvious daily fluctuations in male rats with high values during the dark period and low values during the light period. We have already confirmed that the ACD activities are controlled by the suprachiasmatic nucleus (SCN), which is well known as the oscillator of circadian rhythm. Recently, it is reported that circadian oscillators exist not only in the SCN but also in peripheral organs. To date, it is unclear which circadian oscillators predominantly drive the daily fluctuations of hepatic ACD activities. To address this question, we examined the effects of restricted feeding, which uncouples the circadian oscillators in the liver from the central pacemaker in the SCN, on the daily fluctuations in hepatic ACD activities in male rats. Here we show that restricted feeding inverts the oscillation phase of the daily fluctuations in hepatic ACD activities. Regarding the hepatic P450 content, there were no fluctuations between the light and dark periods under ad libitum and restricted feeding conditions. Therefore, it is considered that the daily fluctuations in hepatic ACD activities are predominantly driven by the circadian factors in peripheral organs rather than by the oscillator in the SCN directly.

Circadian Effects of Timed Meals (and Other Rewards)

Mammals organize many of their activities around rhythmic events in their environments. Primary among these events is the daily light-dark cycle. However, for many animals, food availability is rhythmic or quasi-rhythmic and is therefore a potential synchronizing cue. While circadian rhythms in both behavior and physiological activity can be entrained in animals via meal-feeding schedules, the mechanism by which this occurs remains poorly understood. Similarities between the circadian effects of restricted feeding and the effects of chronic methamphetamine treatment may be indicative of a common mechanism. This article argues that reward (or the arousal that accompanies it) may be the final common pathway for such nonphotic circadian inputs.

Diurnal difference in CAR mRNA expression

Background

The constitutive androstane receptor (CAR, NR1I3) plays a key role in the transcriptional activation of genes that encode xenobiotic/steroid and drug metabolizing enzymes.

Results

The expression of CAR mRNA throughout the circadian rhythm is reported for the first time in phase with the clock gene Bmal1 and in antiphase with the clock-controlled gene Rev-erbα mRNAs, with a peak at Zeitgeber time (ZT) 20 and a trough at ZT8, and a peak/trough ratio of 2.0. The diurnal difference in CAR mRNA expression might underlie the 1.7-fold difference in the magnitude of the PB-dependent induction of CYP2B1/2 mRNA.

Conclusion

The circadian oscillation of xenosensor gene CAR mRNA expression is partially responsible for chronopharmacokinetics and chronopharmacology in disease.

The mammalian circadian timing system: from gene expression to physiology

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

Circadian rhythms of body temperature and liver function in fed and food-deprived goats

Daily rhythms of body core temperature and liver function were recorded in goats maintained under various schedules of lighting and feeding. Concentration of urea in the blood was used as an index of digestion-driven hepatic activity, whereas concentration of cholesterol served as an index of autonomous hepatic activity. Body temperature exhibited robust circadian rhythmicity in the presence and absence of a light-dark cycle and/or a feeding regime. The rhythm was more responsive to shifts in feeding time than to shifts in the light-dark cycle. Urea concentration in the blood exhibited daily rhythmicity only in the presence of a daily feeding regime and, therefore, was driven by ingestive and digestive processes. The rhythm of cholesterol concentration persisted in the presence or absence of a light-dark cycle and/or a feeding regime, except when the feeding time was shifted under constant light. However, the cholesterol rhythm did not respond either to shifts in the light-dark cycle or, more importantly, to shifts in feeding time. Thus, based on this index of hepatic function, the liver cannot be identified as the site of the putative food-entrainable pacemaker.

Sex difference in the daily rhythm of hepatic P450 monooxygenase activities in rats is regulated by growth hormone release

Daily fluctuations have been observed in the hepatic cytochrome P450 monooxygenase activities of rodents. It was confirmed in our previous study that the P450 activities assessed by measurement of 7-alkoxycoumarin O-dealkylase (ACD) showed obvious daily fluctuations in male rats with high values during the dark period. However, there is still very little information about the daily fluctuation of P450 activities in female rats. In the first experiment of the present study, the daily fluctuation of ACD activities in female rats was examined. The results indicated the absence of any apparent daily fluctuation in the hepatic ACD activities of the females, so the study confirmed a sex difference in the daily rhythm of the ACD activities. In the second experiment using hypophysectomized (Hpx) rats, it was investigated whether this sex difference in daily rhythm was attributable to growth hormone (GH) release, an activity which is known to affect the sex difference in the expression of P450 isoforms. It was found that the Hpx rats, given subcutaneous injections of GH twice during the light period to mimic a male pattern of GH secretion, showed obvious light-dark fluctuation in ACD activities with high values in the dark period, and similar results were obtained in sham-operated males. Conversely, the Hpx rats given continuous infusion of GH to mimic a female pattern of GH secretion did not show the light-dark fluctuation in ACD activities, and similar results were obtained in sham-operated females. In conclusion, there is a sex difference in the daily rhythm of the hepatic P450 activities in rats and this difference is apparently influenced by the difference in the patterns of GH secretions.