The stress system consists of the locus caeruleus/norepinephrine autonomic nervous systems and the hypothalamic–pituitary–adrenal (HPA) axis. These two components interact with each other, as well as with other brain subsystems, such as the mesocortical and the mesolimbic dopaminergic system, which is involved in reward and motivation, the central nucleus of the amygdalae, which generate fear and/or anger, and the arcuate nucleus of the hypothalamus participating in stress system control (1–4). The activity of the stress system is influenced by several neurochemical modulators (e.g., serotonin, acetylcholine, γ-aminobutyric acid, glutamate and endogenous cannabinoids, and benzodiazepines) (1–4). When homeostasis is threatened or perceived by the individual as threatened by stressors, the locus caeruleus/norepinephrine/autonomic nervous systems release norepinephrine in the brain and the systemic circulation, while epinephrine is secreted by the adrenal medulla. On the other hand, the HPA axis is associated with the production and secretion of glucocorticoids by the zona fasciculata of the adrenal cortex (1–4). Glucocorticoids play a fundamental role in the maintenance of basal and stress-related homeostasis, regulating many physiologic functions through genomic actions mediated by their cognate intracellular receptor, the glucocorticoid receptor (GR); the latter belongs to the steroid receptor family of the nuclear receptor superfamily of transcription factors (4–8).

The human glucocorticoid receptor (hGR) is encoded by the NR3C1 gene, which is located in the long arm of chromosome 5 and is composed of 10 exons. The alternative splicing of exon 9 generates the two main protein isoforms of the receptor, the hGRα and the hGRβ. Expressed in every tissue except the suprachiasmatic nucleus (SCN) of the hypothalamus, the hGRα is activated following binding of natural or synthetic glucocorticoids to its ligand-binding domain and binds to the regulatory regions of glucocorticoid-responsive genes through its DNA-binding domain and/or interacts with other transcription factors altering their transcriptional activities (vide infra) (6–10). On the other hand, the hGRβ isoform is an enigma in endocrine physiology. Exclusively localized in the nucleus of certain cell types, such as endothelial cells, the hGRβ acts as a dominant-negative inhibitor of hGRα-induced transcriptional activity through well-delineated molecular mechanisms (11–13). Interestingly, this receptor isoform can influence the transcription rate of several genes independently of hGRα (14, 15). Recent studies have demonstrated that hGRβ may be involved in insulin signaling and implicated in gluconeogenesis and inflammation in mouse liver (16, 17). New evidence suggests a pivotal role of the GRβ isoform in the molecular cascades of glioma formation and bladder cancer cells migration (18–20). Further to the alternative splicing of exon 9, Lu and Cidlowski showed that the initiation of the hGRα mRNA translation might occur through eight different sites giving rise to receptor isoforms with variable N-terminal domains: hGRα-A (classic GRα), hGRα-B, hGRα-C1, hGRα-C2, hGRα-C3, hGRα-D1, hGRα-D2, and hGRα-D3, which have distinct properties in terms of intracellular localization and transcriptional activity (10, 21). We assume similar translation processing of the GRβ isoform.

At the cellular level, the glucocorticoid signaling pathway is initiated by ligand-induced activation of the primarily cytoplasmic hGRα, which dissociates from chaperon heat shock proteins and immunophillins, and translocates into the nucleus, where it binds, as homo- or heterodimer, to specific DNA sequences, the glucocorticoid response elements, within the regulatory regions of target genes, thereby influencing their transcription in a positive or negative fashion (4, 6–8, 10). In addition to direct hGRα binding to glucocorticoid-responsive genes, glucocorticoids can influence the transcription of several other genes independently of DNA binding. Indeed, the activated hGRα isoform can interact, possibly as a monomer, with other transcription factors, such as the nuclear factor-κB, the activator protein-1, and the signal transducers and activators of transcription, suppressing or inducing their transcriptional activity (4, 6–8, 10). In addition to the well-described genomic actions, glucocorticoids can induce some cellular effects in a very short-time frame. These effects are referred to as “non-genomic glucocorticoid actions” and are likely to be mediated by membrane-bound GRs, which may trigger the activation of kinase signaling pathways (22–24).

To adjust their daily activities to light/dark changes, organisms have developed a highly conserved timekeeping system, the circadian clock system (from the Latin “circa diem” meaning “approximately a day”), which creates internal rhythmicity under the influence of day/night cycles. This regulatory system is composed of a central “master” clock located in the SCN of hypothalamus, and peripheral “slave” clocks, which are ubiquitously expressed in all tissues (25–27). Importantly, peripheral clocks are tightly synchronized to the central clock through as yet unknown possibly neural or neuroendocrine mechanisms. The central clock influences virtually all physiologic functions, such as sleep/wakefulness, feeding, thermoregulation, energy expenditure, glucose homeostasis, and the activity of the HPA axis. Similarly, peripheral clocks regulate several functions of their residing tissues, ultimately contributing to the homeostasis of living organisms (25–28).

At the cellular level, the circadian clock system is composed of transcriptional/translational loops, which create an intrinsic, self-oscillating circadian rhythm in both the central and peripheral clocks (Figure 1). At the molecular level, these feedback loops are mediated by the circadian locomotor output cycle kaput/brain–muscle–arnt-like protein 1 (CLOCK/BMAL1) heterodimer and other negative transcription factors, such as the Periods (PER1, PER2, and PER3) and Cryptochromes (CRY1 and CRY2). In the principal or core transcription loop, the activated CLOCK/BMAL1 heterodimer binds to the E-box response elements and induces the expression of Pers and Crys. These proteins associate with casein kinase (Csnk) 1ε and δ and undergo phosphorylation (29–31). The phosphorylated isoforms then translocate to the nucleus and suppress the transcriptional activity of the CLOCK/BMAL1 heterodimer. In addition, several other clock-related genes, such as retinoic acid receptor-related orphan receptor α (RORα) and reverse viral erythroblastosis oncogene product (REV-ERBα), are upregulated by the CLOCK/BMAL1 heterodimer, forming an auxiliary loop, which stabilizes the transcriptional activity of the core loop. The transcription factors of both principal and auxiliary loops can modulate the expression of many clock-responsive genes in various tissues (27, 29–31).

In addition to light/dark signals, the circadian clock system is strongly influenced by several metabolic inputs, particularly those associated with ingestion of food (Figure 1) (33–39). Indeed, the adenosine monophosphate-activated protein kinase (AMPK), a tissue sensor and master regulator of energy balance, seems to influence the activity of the clock system through energy-dependent signals. AMPK does so by phosphorylating CRYs and PERs leading to their degradation. AMPK can also cause destabilization of PERs indirectly by increasing the activity of Csnk1ε through phosphorylation, resulting in Csnk1ε-mediated degradation of PERs (Figure 1) (33–36). In addition to AMPK, the longevity and metabolism-associated sirtuin 1 (SIRT1) was demonstrated to deacetylate BMAL1, PER2, and histone 3 depending on the NAD+ cellular levels, possibly functioning as a counter-regulatory mechanism for the histone acetyltransferase activity of the CLOCK (Figure 1) (35–39).

Alterations in circadian release of glucocorticoids have been found in several pathologic conditions, such as mood, metabolic and inflammatory disorders, as well as in cancers (60). On the other hand, chronically stressed (CS) humans, rotating shift workers and subjects frequently exposed to jet lag because of trans-timezone traveling have been demonstrated to be at increased risk for cardiometabolic disorders and their sequelae myocardial infarction and stroke (69). Compared to normal subjects, CS individuals might display an uncoupling between the circadian clock and the HPA axis, a decreased variance between evening nadir and morning zenith cortisol concentrations, as well as an inadequate response to a low-dose dexamethasone suppression test as a result of chronic hyperactivation of PVN CRH and vasopressin secretion (90) (Figure 3). We hypothesize that shift workers or trans-timezone travelers might show a phase-altered curve of cortisol concentrations (Figure 3). Individuals with alterations in circadian secretion of glucocorticoids might develop psychiatric diseases (e.g., anxiety and depression) and autoimmune/inflammatory conditions, with rheumatoid arthritis and asthma as representative examples (91). Furthermore, chronic stress with loss of a proper cortisol circadian rhythm result in glucocorticoid excess-related increased appetite, splachnic obesity, and metabolic disturbances, such as hyperglycemia, insulin resistance, dyslipidemia, osteopenia/osteoporosis, and hypertension (27). All the above pieces of evidence strongly suggest that any dysfunction of the stress system may cause dysregulation of the circadian clock system and vice versa.

Several studies in animals and humans have shown that diurnal fluctuations of cortisol concentrations are flattened in obesity. Indeed, genetically obese rats, db/db mice, and obese adults display dampened glucocorticoid circadian rhythms (92, 93). In addition, childhood overweight and obesity are associated with a reduction in the amplitude of ultradian glucocorticoid secretion (94). Moreover, subjects carrying specific Clock polymorphisms are more susceptible to develop obesity and metabolic syndrome (95, 96). Not only in obesity but also in diabetes mellitus there is a flattened 24-h pattern of cortisol concentrations. In diabetic adults, salivary cortisol was low in the morning and high in the afternoon and evening (97). Finally, mice deficient in the Cry gene had a defective suppression of HPA axis, ultimately leading to glucose intolerance and metabolic syndrome (98).

Disruption of circadian rhythms, as often observed in shift workers, may cause mood disorders (99). Mice exposed to a short 7-h light/dark change have higher concentrations of corticosterone and exhibit depressive symptoms (100). On the other hand, subjects suffering from major depression have flattened diurnal glucocorticoid rhythms, probably due to altered secretion of CRH and AVP in the brain (101). They also display defective clock-related gene expression in the peripheral blood cells, as well as in brain tissues (102, 103). Indeed, the rhythmic expression of core clock transcription factor genes, such as Bmal1, Per1-3, Dec1/2, and Rev-erbα is attenuated in brain regions influencing mood, compared to healthy controls. In addition to depression, several other psychiatric conditions, including bipolar disorder, posttraumatic stress disorder, attention deficit hyperactivity disorder, schizophrenia, and chronic alcoholism, are examples in which the interconnection between the stress system and the circadian clock system is dysregulated (104).

Chronic inflammatory disorders, such as rheumatoid arthritis and asthma, are characterized by worsening symptoms in early morning hours (60). This phenomenon has been attributed to the circadian fluctuation of circulating cytokines, such as interleukin (IL)-1α, IL-6, and tumor necrosis factor-α, which strongly participate in the pathogenesis of these diseases (105, 106). These inflammatory cytokines reach their peak concentrations earlier than cortisol; however, the target tissue glucocorticoid sensitivity is low during that time because of CLOCK-mediated suppressed hGRα-transcriptional and transrepressive activities (48). Furthermore, night-shift workers have increased risk for common infections and multiple sclerosis, indicating that dysregulation of the circadian clock system contributes undoubtedly to the development of inflammatory diseases (107, 108).

Subjects with prolonged night-shift work are also more susceptible to develop several cancers (109). Previous studies have shown that SCN damage, chronic jet lag, as well as Per2 deficiency contribute to cancer initiation and progression (110–112). On the other hand, patients with breast, ovarian, lung, and kidney cancers have flattened or antiphasic diurnal oscillations of cortisol, compared to normal subjects (60). Future studies are still needed to clarify the molecular mechanisms underlying the association between stress, circadian rhythms, and carcinogenesis.

In addition to metabolic, psychiatric, autoimmune/inflammatory, and malignant disorders, sleep disturbances and disorders have been associated with alterations in circadian secretion of glucocorticoids (69). Indeed, a prolonged or excessive activation of the HPA axis results in insufficient sleep, which, in turn, may cause an elevation of glucocorticoid concentrations and cytokines in the early evening, forming a vicious cycle (69). Moreover, sleep deprivation has been associated with an increase in HPA axis activity in the evening hours, thereby altering the ability of the latter to properly autoregulate itself (113–115). Idiopathic chronic insomnia has been associated with evening hypercortisolism and hypercytokinemia influencing substantially the transcription rate of numerous genes in the brain (116, 117). On the other hand, a dysfunctional HPA axis could promote insomnia (69) or be affected by obstructive sleep apnea (118, 119). Not only chronic insomnia but also chronic fatigue syndrome, fibromyalgia and posttraumatic stress disorder have been linked to consistent alterations in HPA axis activity (120, 121).

Synthetic glucocorticoids have been widely used in the treatment of several inflammatory disorders and hematologic malignancies (122). Since target tissue glucocorticoid sensitivity is lower in the morning and higher in the evening, glucocorticoid analogs should be administered in a time-of-day dependent fashion to achieve a beneficial therapeutic outcome and to avoid their detrimental side effects, such as osteoporosis, weight gain, glucose intolerance, and psychiatric symptoms. Therefore, patients with autoimmune disorders are treated with prednisolone or other synthetic glucocorticoids in the evening, given that the hGRα is less acetylated during that time (106). In addition to inflammatory disorders, chronic administration of glucocorticoids is frequently used as substitution treatment of hypocortisolemic disorders, such as adrenal insufficiency, regardless of its etiology. Approximately two-thirds of hydrocortisone dose is usually given in the morning, while the remainder one-third is administered in two doses (in the mid-day and in the early evening) (123). Recent advances in the therapeutic manipulation of adrenal insufficiency have shown that a dual-release hydrocortisone formulation, which resembles circadian cortisol secretion, results in improved quality of life, decreased body weight and blood pressure, and improved glucose tolerance (124, 125).

Although we have gained important insight in the molecular communication between the stress system and the circadian clock system, there are many physiologic and pathophysiologic aspects of their interrelation that still elude us. The molecular mechanisms underlying resetting of peripheral clocks by glucocorticoids are under intense investigation both in normal and pathologic conditions. Moreover, the tight synchronization of peripheral clocks with the central clock remains poorly understood. Furthermore, our understanding on the function and significance of local adrenal clocks is still increasing. Future in vitro and in vivo studies will shed light on the functional significance of the cross talk between the stress system and the circadian clock system of living organisms to increase survival chance. Importantly, in parallel with the tremendous progress of molecular, cellular, and structural biology, significant advances in the field of mathematical and computer biosciences will undoubtedly help us have a deeper understanding of system interrelations. Interestingly, accumulating evidence suggests that stress-related “static” signaling pathways can be effectively transformed into functionally predictive computerized kinetic models (126–129). Such efforts will be useful for accurate predictions of a system response to acute or chronic stress, as well as to pharmacotherapy with novel medications.

All authors contributed equally to the conception of the work, drafted the manuscript or revisited it critically for important intellectual content, finally approved the version to be published, and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.