Circadian modulation of proteasome activity and accumulation of oxidized protein in human embryonic kidney HEK 293 cells and primary dermal fibroblasts

Circadian regulation of macromolecular complex turnover and proteome renewal

Although costly to maintain, protein homeostasis is indispensable for normal cellular function and long-term health. In mammalian cells and tissues, daily variation in global protein synthesis has been observed, but its utility and consequences for proteome integrity are not fully understood. Using several different pulse-labelling strategies, here we gain direct insight into the relationship between protein synthesis and abundance proteome-wide. We show that protein degradation varies in-phase with protein synthesis, facilitating rhythms in turnover rather than abundance. This results in daily consolidation of proteome renewal whilst minimising changes in composition. Coupled rhythms in synthesis and turnover are especially salient to the assembly of macromolecular protein complexes, particularly the ribosome, the most abundant species of complex in the cell. Daily turnover and proteasomal degradation rhythms render cells and mice more sensitive to proteotoxic stress at specific times of day, potentially contributing to daily rhythms in the efficacy of proteasomal inhibitors against cancer. Our findings suggest that circadian rhythms function to minimise the bioenergetic cost of protein homeostasis through temporal consolidation of protein turnover.

Cysteine Oxidation Promotes Dimerization/Oligomerization of Circadian Protein Period 2

The molecular circadian clock is based on a transcriptional/translational feedback loop in which the stability and half-life of circadian proteins is of importance. Cysteine residues of proteins are subject to several redox reactions leading to S-thiolation and disulfide bond formation, altering protein stability and function. In this work, the ability of the circadian protein period 2 (PER2) to undergo oxidation of cysteine thiols was investigated in HEK-293T cells. PER2 includes accessible cysteines susceptible to oxidation by nitroso cysteine (CysNO), altering its stability by decreasing its monomer form and subsequently increasing PER2 homodimers and multimers. These changes were reversed by treatment with 2-mercaptoethanol and partially mimicked by hydrogen peroxide. These results suggest that cysteine oxidation can prompt PER2 homodimer and multimer formation in vitro, likely by S-nitrosation and disulphide bond formation. These kinds of post-translational modifications of PER2 could be part of the redox regulation of the molecular circadian clock.

Circadian rhythmicity of the thioredoxin system in cultured murine peritoneal macrophages

Macrophages play a pivotal role in atherosclerosis through a variety of events related to cellular oxidative stress. This process is mainly due to an excessive production of reactive oxygen species whose elimination occurs through antioxidant systems including the thioredoxin (Trx) system. In this paper, we investigated whether the Trx system would exhibit circadian rhythmicity in dexamethasone synchronized cultured macrophages and monitored the impact of the rhythmicity of Trx-1 on markers of atherosclerosis. We found that the clock-related genes BMAL-1, PER-2, CRY-1 and REV ERB α exhibited a robust circadian expression. However, the Trx genes family (Trx-1, Trx-2, TrxR1 and TXNIP) did not exhibit a circadian expression at the mRNA level in spite of the presence of E-box elements within the promoter regions of TrxR1 and TXNIP genes. Nevertheless, both Trx-1 and TXNIP exhibited a circadian expression at the protein level and proteasome inhibition abolished the rhythmicity of Trx-1. Moreover, we found a link between low Trx-1 level and elevated atherogenic markers such as 4-HNE, TNF-α and cholesterol accumulation in macrophages. Our results indicate that the Trx gene family does not exhibit the same circadian regulation and that the presence of E-box elements in the TXNIP promoter is not sufficient to ensure a circadian rhythmicity at the transcriptional level. In addition, since a link was found between a low level of Trx-1 protein during circadian rhythm and high levels of atherogenic markers, administration of Trx-1 at certain time points could be an interesting approach to protect against atherosclerosis development.

Understanding circadian regulation of mammalian cell function, protein homeostasis, and metabolism

Circadian rhythms are ∼24 h cycles of organismal and cellular activity ubiquitous to mammalian physiology. A prevailing paradigm suggests that timing information flows linearly from rhythmic transcription via protein abundance changes to drive circadian regulation of cellular function. Challenging this view, recent evidence indicates daily variation in many cellular functions arises through rhythmic post-translational regulation of protein activity. We suggest cellular circadian timing primarily functions to maintain proteome homeostasis rather than perturb it. Indeed, although relevant to timekeeping mechanism, daily rhythms of clock protein abundance may be the exception, not the rule. Informed by insights from yeast and mammalian models, we propose that optimal bioenergetic efficiency results from coupled rhythms in mammalian target of rapamycin complex activity, protein synthesis/turnover, ion transport and protein sequestration, which drive facilitatory rhythms in metabolic flux and substrate utilisation. Such daily consolidation of proteome renewal would account for many aspects of circadian cell biology whilst maintaining osmotic homeostasis.

Defining circadian disruption in neurodegenerative disorders

Neurodegenerative diseases encompass a large group of conditions that are clinically and pathologically diverse yet are linked by a shared pathology of misfolded proteins. The accumulation of insoluble aggregates is accompanied by a progressive loss of vulnerable neurons. For some patients, the symptoms are motor focused (ataxias), while others experience cognitive and psychiatric symptoms (dementias). Among the shared symptoms of neurodegenerative diseases is a disruption of the sleep/wake cycle that occurs early in the trajectory of the disease and may be a risk factor for disease development. In many cases, the disruption in the timing of sleep and other rhythmic physiological markers immediately raises the possibility of neurodegeneration-driven disruption of the circadian timing system. The aim of this Review is to summarize the evidence supporting the hypothesis that circadian disruption is a core symptom within neurodegenerative diseases, including Alzheimer's disease, Huntington's disease, and Parkinson's disease, and to discuss the latest progress in this field. The Review discusses evidence that neurodegenerative processes may disrupt the structure and function of the circadian system and describes circadian-based interventions as well as timed drug treatments that may improve a wide range of symptoms associated with neurodegenerative disorders. It also identifies key gaps in our knowledge.

Expanding the link between circadian rhythms and redox metabolism of epigenetic control

Circadian rhythms play a central role in physiological and metabolic processes. This is mostly achieved through rhythmic regulation of myriad genes via dynamic epigenome changes. Accumulating evidence indicates that oxidative stress and redox balance are under circadian control and feedback on the clock system. Circadian perturbations induce oxidative stress accumulation and disturb redox balance. Along with these changes, epigenomic landscape changes are a remarkable hallmark of clock disruption. This review aims to summarize evidence supporting the link between the circadian clock and redox metabolism, focusing on possible connections through epigenetic mechanisms.

The Circadian Clock Regulates the Expression of the Nuclear Factor Erythroid 2-Related Factor 2 in Acute Kidney Injury following Myocardial Ischemia-Reperfusion in Diabetic Rat

Cardiac surgery-associated acute kidney injury (AKI) is a serious and frequent complication with poor prognosis, and disruption in circadian rhythm shall adversely influence cardiovascular and renal functions via oxidative stress mechanisms. However, the role of circadian clock genes (circadian locomotor output cycle kaput (CLOCK) and brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein-1 (BMAL1)) and its interaction with nuclear factor erythroid 2-related factor 2 (Nrf2) in AKI following myocardial ischemia-reperfusion (MIR) in the diabetic rat has not yet been explored. In this study, rats were divided into the sham (S) group, MIR (M) group, diabetic (D) group, and diabetic+MIR (DM) group. At light (zeitgeber time (ZT) 0) and dark time points (ZT12), rat MIR model was established by occlusion of the left anterior descending coronary artery for 30 min followed by 2 -hour reperfusion, and then renal injury was evaluated. The renal histological changes in the DM group were significantly high compared to other groups; serum creatinine, blood urea nitrogen, and neutrophil gelatinase-associated lipocalin levels, as well as malondialdehyde and 8-iso-prostaglandin-F2α levels in renal tissues of M ZT12 and DM ZT12 subgroups, were significantly higher than those of M ZT0 and DM ZT0 subgroups, individually indicating increased oxidative stress at a dark cycle. Further, Nrf2 protein accumulated in a circadian manner with decreasing levels at night in the DM and M groups. In conclusion, renal injury following MIR was exacerbated in the diabetic rat at night through molecular mechanisms involving transcriptional control of the circadian clock on light-dark activation of Nrf2.

Mechanism of circadian regulation of the NRF2/ARE pathway in renal ischemia-reperfusion

The nuclear erythroid 2-related factor 2 (NRF2)/antioxidant response element (ARE) pathway has been shown to provide strong protection against oxidative stress injury induced by renal ischemia-reperfusion (IR). However, the endogenous regulatory mechanism of the NRF2/ARE pathway in renal IR injury is incompletely understood. A rat model of renal IR was established by occlusion of the bilateral renal pedicle for 45 min, followed by reperfusion for 24 h. Renal injury was assessed by light microscopy and levels of serum creatinine, blood urea nitrogen and neutrophil gelatinase-associated lipocalin was measured using enzyme-linked immunosorbent assay. Renal oxidative stress was also evaluated by measuring superoxide dismutase and malondialdehyde in renal tissues. Protein expression levels of brain and muscle ARNT-like 1 (BMAL1), nuclear factor erythroid 2-related factor 2 (NRF2), NAD(P)H dehydrogenase [quinone] 1 (NQO1), glutamate-cysteine ligase modifier (GCLM) and heme oxygenase 1 (HO1) in the kidney were determined by western blotting and immunohistochemistry. Reverse transcription-quantitative PCR was used to evaluate rhythmic transcription of the core clock genes (CLOCK and BMAL1) and the NRF2 gene. The nature of the binding of BMAL1 to the promoter regions in the NRF2 gene was assessed by chromatin immunoprecipitation assays in rat kidneys. BMAL1 was found to bind to the promoter of the NRF2 gene through an E-BOX element associated with strongly rhythmic activation of NRF2 in both the normal kidney and those exposed to IR. The ARE-regulated anti-oxidative stress protein was affected by the circadian rhythm of the NRF2 gene. As the NRF2 level was at a circadian nadir, the expression of the proteins NQO1, GCLM and HO1 was weakened, resulting in more serious renal oxidative stress injury and pathological and functional impairment induced by IR. It can be concluded that the circadian rhythm of the NRF2/ARE pathway controlled by the circadian clock is essential for regulating antioxidant stress in renal IR injury, which might prompt new therapeutic strategies associated with the diurnal variability of human kidney disease, including renal transplantation.

Reduced Proteasome Activity and Enhanced Autophagy in Blood Cells of Psoriatic Patients

Psoriasis is a skin disease that is accompanied by oxidative stress resulting in modification of cell components, including proteins. Therefore, we investigated the relationship between the intensity of oxidative stress and the expression and activity of the proteasomal system as well as autophagy, responsible for the degradation of oxidatively modified proteins in the blood cells of patients with psoriasis. Our results showed that the caspase-like, trypsin-like, and chymotrypsin-like activity of the 20S proteasome in lymphocytes, erythrocytes, and granulocytes was lower, while the expression of constitutive proteasome and immunoproteasome subunits in lymphocytes was increased cells of psoriatic patients compared to healthy subjects. Conversely, the expression of constitutive subunits in erythrocytes, and both constitutive and immunoproteasomal subunits in granulocytes were reduced. However, a significant increase in the autophagy flux (assessed using LC3BII/LC3BI ratio) independent of the AKT pathway was observed. The levels of 4-HNE, 4-HNE-protein adducts, and proteins carbonyl groups were significantly higher in the blood cells of psoriatic patients. The decreased activity of the 20S proteasome together with the increased autophagy and the significantly increased level of proteins carbonyl groups and 4-HNE-protein adducts indicate a proteostatic imbalance in the blood cells of patients with psoriasis.

Age-dependent altered redox homeostasis in the chronodisrupted rat model and moderation by melatonin administration

Circadian disruption or chronodisruption (CD) occurs when day-night cycles and other internal rhythms are not adjusted to environmental light-dark regimens and are unable to synchronize among each other. Artificial light-induced oxidative stress is a major concern as the circadian physiology of the cell is chronically altered due to suppression of the time-keeping hormone, melatonin. The relationship between age-related impaired redox status and disrupted circadian rhythms is still not fully understood. The present study evaluated the effect of artificial light at night (ALAN) with respect to aging and role of melatonin supplementation. This study was conducted on young (3 months) and old (24 months) male Wistar rats subdivided into four groups control (C), melatonin treated (MLT), artificial light at night (ALAN), and ALAN+MLT group. Pronounced changes were observed in the old compared to the young rats. Reactive oxygen species (ROS), malondialdehyde (MDA), plasma membrane redox system (PMRS), protein carbonyl (PCO), and sialic acid (SA) were significantly (p ≤ 0.05) increased, while ferric reducing ability of plasma (FRAP) and reduced glutathione (GSH) were significantly (p ≤ 0.05) suppressed in light-exposed young and old animals compared to their age-matched controls. Advanced oxidation protein products (AOPP) increased non-significantly in young rats of the ALAN group; however, significant (p ≤ 0.05) changes were observed in the old rats of the ALAN group compared to their respective controls. Advanced glycation end products (AGEs) increased and acetylcholinesterase (AChE) activity decreased, significantly (p ≤ 0.05) in young animals of the ALAN group, while nonsignificant changes of both parameters were recorded in the old animals of the ALAN groups compared with their age-matched controls. Melatonin supplementation resulted in maintenance of the normal redox homeostasis in both young and old animal groups. Our study suggests that aged rats are more susceptible to altered photoperiod as their circadian redox homeostasis is under stress subsequent to ALAN. Melatonin supplementation could be a promising means of alleviating age-related circadian disturbances, especially in light-polluted areas.

Improved redox anti-cancer treatment efficacy through reactive species rhythm manipulation

Rhythms in the pseudo-steady state (PSS) levels of reactive species (RS), particularly superoxide and hydroxyl radicals, exist in cancer cells. The RS rhythm characteristics, particularly frequency and amplitude, are entrained (reset) by the anticancer compounds/drugs. In this work, we show for the first time that the phase of the RS rhythm at which the drug is added is significantly important in determining the cytotoxicity of anticancer compounds/drugs such as menadione and curcumin, in two different cancer cell lines. Curcumin, the more effective of the two drugs (IC₅₀ = 15 µM, SiHa; 6 µM, HCT116) induced reset of superoxide and hydroxyl rhythms from 15.4 h to 9 h, and 25 h to 11 h respectively, as well as caused increases in these radical levels. However, menadione (IC₅₀ = 20 µM, SiHa; 17 µM, HCT116) affected only the superoxide levels. Drug treatment at different time points/phase of the RS rhythm resulted in a maximum of 27% increase in cytotoxicity, which is significant. Further, we report for the first time, an unexpected absence of a correlation between the intracellular PSS RS and antioxidant levels; thus, the practice of using antioxidant enzyme levels as surrogate markers of intracellular oxidative stress levels may need a re-consideration. Therefore, the RS rhythm could be a fundamental/generic target to manipulate for improved cancer therapy.

Diurnal Rhythms Spatially and Temporally Organize Autophagy

Circadian rhythms are a hallmark of physiology, but how such daily rhythms organize cellular catabolism is poorly understood. Here, we used proteomics to map daily oscillations in autophagic flux in mouse liver and related these rhythms to proteasome activity. We also explored how systemic inflammation affects the temporal structure of autophagy. Our data identified a globally harmonized rhythm for basal macroautophagy, chaperone-mediated autophagy, and proteasomal activity, which concentrates liver proteolysis during the daytime. Basal autophagy rhythms could be resolved into two antiphase clusters that were distinguished by the subcellular location of targeted proteins. Inflammation induced by lipopolysaccharide reprogrammed autophagic flux away from a temporal pattern that favors cytosolic targets and toward the turnover of mitochondrial targets. Our data detail how daily biological rhythms connect the temporal, spatial, and metabolic aspects of protein catabolism.

Adaptive homeostasis and the free radical theory of ageing

The Free Radical Theory of Ageing, was first proposed by Denham Harman in the mid-1950's, based largely on work conducted by Rebeca Gerschman and Daniel Gilbert. At its core, the Free Radical Theory of Ageing posits that free radical and related oxidants, from the environment and internal metabolism, cause damage to cellular constituents that, over time, result in an accumulation of structural and functional problems. Several variations on the original concept have been advanced over the past six decades, including the suggestion of a central role for mitochondria-derived reactive species, and the proposal of an age-related decline in the effectiveness of protein, lipid, and DNA repair systems. Such innovations have helped the Free Radical Theory of Aging to achieve widespread popularity. Nevertheless, an ever-growing number of apparent 'exceptions' to the Theory have seriously undermined its acceptance. In part, we suggest, this has resulted from a rather simplistic experimental approach of knocking-out, knocking-down, knocking-in, or overexpressing antioxidant-related genes to determine effects on lifespan. In some cases such experiments have yielded results that appear to support the Free Radical Theory of Aging, but there are just as many published papers that appear to contradict the Theory. We suggest that free radicals and related oxidants are but one subset of stressors with which all life forms must cope over their lifespans. Adaptive Homeostasis is the mechanism by which organisms dynamically expand or contract the homeostatic range of stress defense and repair systems, employing a veritable armory of signal transduction pathways (such as the Keap1-Nrf2 system) to generate a complex profile of inducible and enzymatic protection that best fits the particular need. Viewed as a component of Adaptive Homeostasis, the Free Radical Theory of Aging appears both viable and robust.

Redox biology and the interface between bioenergetics, autophagy and circadian control of metabolism

Understanding molecular mechanisms that underlie the recent emergence of metabolic diseases such as diabetes and heart failure has revealed the need for a multi-disciplinary research integrating the key metabolic pathways which change the susceptibility to environmental or pathologic stress. At the physiological level these include the circadian control of metabolism which aligns metabolism with temporal demand. The mitochondria play an important role in integrating the redox signals and metabolic flux in response to the changing activities associated with chronobiology, exercise and diet. At the molecular level this involves dynamic post-translational modifications regulating transcription, metabolism and autophagy. In this review we will discuss different examples of mechanisms which link these processes together. An important pathway capable of linking signaling to metabolism is the post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc). This is a nutrient regulated protein modification that plays an important role in impaired cellular stress responses. Circadian clocks have also emerged as critical regulators of numerous cardiometabolic processes, including glucose/lipid homeostasis, hormone secretion, redox status and cardiovascular function. Central to these pathways are the response of autophagy, bioenergetics to oxidative stress, regulated by Keap1/Nrf2 and mechanisms of metabolic control. The extension of these ideas to the emerging concept of bioenergetic health will be discussed.