New Horizons: Circadian Control of Metabolism Offers Novel Insight Into the Cause and Treatment of Metabolic Diseases Free

Abstract

Metabolic homeostasis is achieved by endocrine factors, signaling cascades, and metabolic pathways that sense and respond to metabolic demands in different organs. However, the recent discovery that almost every component of this regulatory system is also modulated by circadian rhythm highlights novel etiology and prognosis of metabolic diseases. First, chronic circadian rhythm disruption, as in shiftwork or shiftwork-like lifestyle, can increase the risk for metabolic diseases. Second, by understanding factors that affect circadian rhythm, we can implement new behavioral or pharmacological interventions for the prevention and management of metabolic disorders. One of these novel circadian-based interventions is time-restricted eating (TRE) in which all daily caloric intake is restricted to a consistent window of 8 to 12 hours. In preclinical animal models, TRE can prevent or reverse many metabolic diseases. Circadian research has also catalyzed attempts to optimally time the dosing of existing drugs to treat metabolic diseases or develop new drugs that target the circadian clock to treat metabolic disorders.

All life forms on Earth evolved under predictable 24-hour cycles of light and darkness, and associated rhythms in temperature, food availability, and predation. To adapt to such predictable daily changes in survival factors, most animals have developed an internal timing mechanism to anticipate these daily changes and accordingly tune endocrine signals and metabolism to produce overt rhythms in sleep, activity, and food-seeking behavior. Such circadian (circa= approximately, dian=a day) timekeeping system in animals is based on a cell-autonomous molecular mechanism. Although the molecular clock is present in nearly all cells of the brain and other tissues, the hypothalamic brain region densely packed with ~20 000 neurons that is located right above the optic chiasma—hence the name suprachiasmatic nucleus (SCN)—plays a dominant role in the circadian control of behavior and physiology. The cell-autonomous circadian clock is based on interlocked feedback loops consisting of transcriptional activators (CLOCK, BMAL1, RORs), repressors (PERs, CRYs, REV-ERBs), and modulators (eg, CK1) (1). Concerted action of these transcriptional regulators or clock components produces ~24-hour rhythms in the mRNA and protein levels of several clock components and of hundreds of additional genes in different tissues in a tissue-specific manner. Coordination of these molecular rhythms among organs and the daily rhythms of the day-night cycle gives rise to adaptive behavioral and physiological rhythms.

The SCN plays an important role in synchronizing circadian rhythms with the light-dark cycle and orchestrating rhythms in different brain regions and peripheral organs. The SCN neurons receive monosynaptic inputs from intrinsically photosensitive ganglion cells of the retina (ipRGCs). This retina-SCN interaction ensures the SCN clock is in sync with the ambient light-dark cycle. Neurons of the SCN use synaptic mechanisms and secreted molecules to other hypothalamic brain regions, which produce daily rhythms in sleep, activity, and feeding. Some of these regions relay the timing information from the SCN to peripheral tissues via multiple mechanisms, including circadian variation in the autonomous nervous system (ANS), body temperature, and various endocrine molecules of the gonadotropic and somatotropic axes. These synaptic or systemic signals act on specific molecular circadian clock components in peripheral organs and thereby ensure the timing information from the SCN (and from the ambient light-dark cycle) is relayed to all cellular clocks throughout the body. However, these signaling molecules and mechanisms that synchronize clocks are not dedicated to transmitting the timing information between clocks. Instead, they also respond to—and hence mediate—the effects of nutrition, light, activity, and stress. Furthermore, they affect the expression and function of genes, both dependent and independent of the circadian clock. Hence, daily patterns of activity, feeding, and light exposure exert a significant impact on the synchrony among circadian clocks, the number of genes that show daily rhythms, and the magnitude or amplitude of these rhythms.

In summary, the cell-autonomous circadian clock and the dominant role of the SCN in orchestrating rhythms throughout the body offers a mechanism to sustain timing information even in the complete absence of any external timing cue (as under constant darkness). However, under normal conditions of light:dark and associated rhythms in sleep-wake and fasting-feeding, the molecular and physiological rhythms are products of both internal clocks and the external stimuli.

Advantages of Circadian Rhythms

In each tissue, the circadian system generates ~24-hour rhythms in hundreds or thousands of gene products (mRNA, protein, posttranslational modifications). Results from numerous genome-wide studies in different tissues indicate that almost every gene in the mammalian genome may exhibit circadian rhythm in one or more tissues. While these studies have indicated the vast complexity of circadian regulation of gene functions, the physiological advantages of these rhythms can be simplified into 4 major functions: anticipation, separation, synchronization, and gating of acute responses to external stimuli.

The circadian system enables the organism to anticipate predictable changes in light, darkness, and food availability and prepare for such daily events, thereby offering survival advantages. The rhythms in biochemically incompatible processes with peak phases at specific times of the 24-hour day enable the cell or tissue to carry out such incompatible functions in the same cell or tissue. Along the same line, synchronized rhythms in biochemically compatible processes such as rhythms in different enzymes of a given linear anabolic or catabolic pathway can improve metabolic flux. Finally, circadian rhythm also modulates the magnitude of physiological response to stimuli such as light, food, and stress. For example, the postprandial blood glucose rise in response to a late-night meal is much higher than to an early-morning isocaloric meal.

Circadian Rhythm Disruption

As discussed above, the circadian regulatory system is built on genetic mechanisms producing molecular rhythms, which are influenced by environmental or behavioral factors. Hence genetic mutations in clock genes that weaken or mistime rhythms and abnormal light, sleep, activity, or food timing can cause circadian rhythm disruption (CRD). A common and widespread cause for CRD is shiftwork or shiftwork-like lifestyle in which light exposure, sleep, activity, and food intake occurs at an unanticipated circadian time. Such a lifestyle, when followed for several months or years, can cause chronic CRD and increase the risk for several diseases affecting different organ systems. One of the immediate and long-term adverse effects of CRD is impaired glucose homeostasis. Glucose metabolism is regulated at multiple levels—daily rhythms in hormones such as insulin, cortisol, and melatonin, the diurnal variation in tissue insulin sensitivity, rhythmic glucose metabolism rates in various tissues, and hepatic glucose production. Consequently, glucose tolerance exhibits a diurnal rhythm in healthy subjects, with lower levels observed in the evening than morning.

The circadian system can directly or indirectly modulate almost every endocrine agent, or its downstream effects. Although several hormones regulating metabolic homeostasis such as thyroid hormone, ghrelin, leptin, and adiponectin show circadian rhythm, within the scope of this article, we will present 3 different endocrine signals—melatonin, glucocorticoids, and insulin, as examples of nodes for the integration of both cell-autonomous rhythms and behavioral/environmental factors. We will discuss how disruption of normal rhythms in these hormones adversely affects overall health, and conversely, how interventions targeted at bolstering endogenous circadian rhythms impact them and improve health.

Melatonin and Sleep-Wake Cycles

Within the pineal gland, serotonin is acetylated and then methylated to produce melatonin. Melatonin is synthesized and released at night. Circadian rhythm in melatonin production is due to at least 2 mechanisms. The SCN directly controls melatonin secretion via tonic inhibition of noradrenergic stimulation of the pineal gland. Within the pinealocytes, the CLOCK-BMAL1 complex activates transcription of the rate-limiting enzyme for melatonin synthesis—arylalkylamine N-acetyltransferase (2). In anticipation of circadian sleep time, plasma or salivary melatonin levels begin to rise 1 to 3 hours before habitual bedtime and decline 1 to 3 hours after awakening. Melatonin primarily acts via the G-protein coupled receptors MT1/MTNR1A and MT2/MTNR1B, regulating sleep via a complex process. However, increased light exposure at night suppresses melatonin release, which delays sleep onset and compromises sleep quality. This sequentially affects the rhythms of several other hormones and disrupts physiological homeostasis.

In addition to a major impact on sleep, melatonin affects several biological processes such as insulin secretion and blood pressure maintenance. A surprising link between melatonin and metabolism arose in the last decade. MTNR1B is expressed in human pancreatic islets, and melatonin binding can directly inhibit glucose-induced insulin secretion. This discovery has raised new questions about the potential impact of habit, pharmacology, and gene mutations in MTNR1B on glucose intolerance. For example, late dinner close to bedtime, melatonin supplementation immediately after a meal, and certain single nucleotide polymorphisms in the MTNR1B locus affecting its binding affinity to melatonin may contribute to type 2 diabetes risk (3).

Glucocorticoids and Stress Response

Glucocorticoids (GCs) are produced and secreted from the cortical region of the adrenal gland. They play an important role in the maintenance of basal and stress-related homeostasis by regulating various physiological functions such as glucose homeostasis, immunity, inflammation, and stress response. GCs bind to their cognate intracellular steroid receptor—glucocorticoid receptor and alter transcription by binding to the regulatory elements of GC-responsive genes. While GCs are released under stress conditions, they show a distinct circadian rhythm under nonstress conditions, peaking within an hour after awakening in humans. This daily rhythm is generated through the coordinated action of the SCN clock and the endogenous adrenal gland clock. Daily GC rhythms are mainly dependent on rhythmic SCN activation, which further connects to the corticotropin releasing hormone neurons in the paraventricular hypothalamic nucleus (PVN) and drives rhythmic adrenocorticotropic hormone (ACTH) release. SCN-PVN connections also regulate GC rhythms by controlling the adrenal sensitivity to ACTH via the ANS. Additionally, the adrenal clock gates response to ACTH and controls GC biosynthesis through transcriptional regulation of steroidogenic genes, eg, CLOCK-BMAL1 activates transcription of the rate-limiting enzyme in steroidogenesis—Steroidogenic acute regulatory protein (StAR) (4).

Interestingly, GCs modulate the majority of cyclic gene expression in liver, muscle, and adipose tissue, in coordination with various core clock components such as REV-ERBα, CRY1/2, and CHRONO. In the liver, GC promotes gluconeogenesis, and in muscle, it reduces glucose intake to maintain blood glucose level. In many tissues, GCs promote the expression of anti-inflammatory genes while suppressing proinflammatory genes.

GCs induce the expression of Per1 and Per2 and alter the circadian phase of responsive tissue. However, SCN cells do not express glucocorticoid receptors and are nonresponsive to changes in GC levels. Thus, GCs play an important role in relaying the timing signals from SCN to peripheral clocks. In chronic CRDs caused by shiftwork, chronic jet lag, mistimed eating, as well as during aging, GC levels increase during the trough phase and dampen the rhythm. This alteration in GC rhythm has been associated with autoimmune/proinflammatory conditions and metabolic disorders such as obesity, type 2 diabetes, dyslipidemia, and atherosclerosis (5).

Insulin and Glucose Metabolism

Insulin, produced from the pancreatic beta cells, is the dominant hormone of glucose homeostasis. It activates the insulin receptor, which is a tyrosine kinase receptor and leads to a plethora of signal transduction events, ultimately activating the downstream nodal kinase AKT. AKT upregulates several anabolic processes such as glycogen synthesis, lipogenesis, and nucleotide biosynthesis. AKT inhibits catabolic processes such as fatty-acid oxidation, gluconeogenesis, and autophagy. AKT signaling also stimulates glucose uptake in muscles and adipose tissue via membrane translocation of glucose transporter GLUT4. Several steps from insulin production from the pancreas to its action in various tissues reciprocally interact with the circadian clock.

SCN connections to the PVN and the perifornical area regulate hepatic glucose production capacity from the liver and modulate glucose-stimulated insulin secretion response via the ANS. The endogenous molecular clock in the pancreas regulates insulin secretion at multiple steps, and whole-body or pancreas-specific knockout of Bmal1 dampens glucose-stimulated insulin secretion and causes glucose intolerance. The BMAL1-CLOCK complex, along with transcription factor PDX1, binds to the distal regulatory elements of several genes involved in glucose sensing, signaling, and insulin secretion. Additionally, BMAL1 directly affects glucose sensing and insulin release by regulating mitochondrial uncoupling efficiency and adenosine triphosphate production via Ucp2 expression. BMAL1 also regulates insulin sensitivity in muscles and adipose tissue by promoting glucose uptake and utilization. Reciprocally, insulin-TORC1-S6K signaling axis in the liver regulates circadian coordination of global protein translation and can entrain peripheral clocks by stimulating protein synthesis of PER2 and inhibiting autophagy-dependent degradation of CRY1.

Circadian Resynchronization

Recent advances in circadian rhythm research have highlighted the behavioral and pharmacological approaches that can sustain or improve circadian regulatory system and thereby prevent or better manage several chronic diseases.

Light Therapy

Light is the major entraining signal for the SCN master clock. Light, specifically blue light, is sensed by melanopsin expressing ipRGCs in the retina. In addition to the SCN, the ipRGCS also send direct projections to several brain regions involved in the regulation of sleep and mood, and indirectly regulate pineal melatonin. Consequently, bright light at night suppresses melatonin release and delays sleep induction, leading to CRD. Conversely, lack of daylight exposure dampens SCN rhythms and can lead to depression and mood disorders. Several studies have shown beneficial effects of daytime blue-light stimulation and artificial-light therapy in patients suffering from insomnia and circadian rhythm sleep disorders, which may subsequently improve metabolic homeostasis. Further research will be required to understand the direct and indirect effects of light therapy on metabolism.

Timed Hormone Therapy

Among the 3 hormones discussed above, only melatonin is available without a prescription to the consumers in the United States and Canada. Melatonin or drugs that activate melatonin receptors have been extensively used to treat patients diagnosed with delayed sleep phase syndrome, undergoing shiftwork, or experiencing chronic jet lag, due to its role in sleep induction and maintenance. However, due to the short half-life of melatonin, timed melatonin supplementation or slow-release formulations of melatonin are used at target bedtime.

Physical Activity

Exercise is known to alter clock gene expression in muscles and several peripheral tissues, and timed exercise combined with light therapy or food entrainment can be a potent circadian synchronizer. Moreover, exercise improves mood, sleep quality, and fitness, promotes nutrient utilization, and reduces the risk for several metabolic and cardiovascular diseases. These effects can be brought about by improving blood circulation, signaling via secreted muscle proteins (myokines), body temperature alterations, or resetting of hormone secretion rhythms. Together, these exercise benefits can directly and indirectly entrain peripheral clocks to the master clock in both healthy conditions and under CRDs. Recent studies have shown that exercise capacity is higher in the evening than morning, possibly due to circadian variation in glucose and fatty-acid metabolism pathways in muscles (6).

Timed Feeding

Several reports have indicated that consuming a major portion of daily food intake later in the day is associated with metabolic disorders such as obesity, type 2 diabetes, inflammation, cardiovascular diseases, and gut disorders. Hence, interventions aimed at consuming the majority of calories in a short time window separated by several hours from the sleep interval are gaining interest, especially for situations where exercise or other pharmacological interventions are not possible or feasible. Studies performed in model organisms and humans have shown that time-restricted feeding/eating (TRF/TRE), wherein daily food intake is restricted to a consistent 8- to 10-hour interval during the active phase, synchronizes clock gene expression in several peripheral tissues. TRF can directly affect nutrient-sensing and metabolic pathways in several tissues and can trigger integrated stress response. These ever-expanding impacts of TRF on metabolic and signaling pathways improve glucose and lipid metabolism, reduce inflammation, preserve muscle fitness and heart functions, strengthen motor coordination, and improve sleep quality. Interestingly, TRF acts both as a preventive and therapeutic intervention against obesity, and it even drives, albeit modestly, rhythmic gene expression in mice lacking core clock components (7). Although TRE has shown beneficial effects in rodent models and small-scale human clinical trials, large-scale efforts are required to test if TRE alone or in combination with other circadian synchronization methods, can act as a combination therapy along with the standard medical interventions for treating metabolic syndrome.

Chronopharmacology

Studies have shown that the molecular targets of the majority of Food and Drug Administration–approved drugs show rhythmic expression. Moreover, various processes involved from the consumption of drug to its target action, such as absorption from the gut lumen, transport inside the target cells/tissue, detoxification by liver, and kidney to its excretion from the body, can be rhythmic. Thus it is crucial that drugs are administered taking into account all these parameters. This is especially true for drugs with shorter half-lives. Timed administration of drugs—chronopharmacology—can substantially reduce the amount of drug required, improve efficacy, and potentially lower harmful side effects. For example, the nighttime administration of antihypertensive drugs has a more significant impact on reducing BP than when they are taken in the morning. Short-acting statins such as simvastatin lower cholesterol more efficiently when administered in the evening than morning. In rats, the sulfonylurea drug tolbutamide is more effective in lowering blood glucose when administered during the start of the active phase (dark) than inactive phase (light) (8).

Small Molecule Clock Modulators

Since the molecular clock entrains to environmental inputs, much research is ongoing to identify various small molecules that can target clock components and alter its period, amplitude, or phase (8). Since clock components regulate multiple pathways, the effects of these small molecules are usually pleiotropic. Several molecules have been identified that target the negative feedback arm of the molecular clock and lengthen period, eg, CK1 inhibitors prevent PER/CRY degradation and are useful for treating mood and sleep disorders. KL001 is a CRY stabilizer, and it improves glucose tolerance in obese mice. Isoform selective stabilizers for CRY1 and CRY2 have been identified: KL101/KL201 and TH101, respectively, which affect brown adipose tissue functions. REV-ERB agonists GSK4112, SR9009, and SR9011 improve energy homeostasis, glucose tolerance, and metabolic syndrome. They also suppress inflammation and inhibit cancer cell proliferation.

On the other hand, various strategies have been used to modify clock amplitude, for example, the ROR agonist Nobiletin (NOB) improves metabolic homeostasis in obese mice and reduces inflammation. Recently, adenosine analog cordycepin was shown to alter the phase of molecular clock by binding to the chaperone RUVBL2 and disrupting the repressive super-complex of BMAL1, CLOCK, PER, CRY, and CK1. This resulted in a faster adaptation of mice to a phase-advance jet lag paradigm.

Conclusion

Hormonal rhythms are generated and maintained by the coordinated action of central and peripheral molecular clocks, which are entrained by light-dark cycles and feeding-fasting periods. Disruption of these hormonal rhythms due to stress, aging, or conditions affecting circadian rhythms such as shiftwork, jet lag, and mistimed eating alters circadian gene expression and metabolic processes and predisposes individuals to develop metabolic syndrome. It is imperative that interventions aimed at improving endogenous rhythms are extensively studied and tested for better treatment and management strategies for metabolic diseases.

Abbreviations

Abbreviations
 
• ACTH

  adrenocorticotropic hormone

 
• ANS

  autonomous nervous system

 
• CRD

  circadian rhythm disruption

 
• GC

  glucocorticoid

 
• ipRGCs

  intrinsically photosensitive ganglion cells of the retina

 
• PVN

  paraventricular nucleus

 
• SCN

  suprachiasmatic nucleus

 
• TRE

  time-restricted eating

 
• TRF

  time-restricted feeding

Acknowledgments

Research in S.P.’s lab is funded by US National Institutes of Health (grants EY016807, DK118278, DK115214), Department of Defense (grant W81XWH1810645), Department of Homeland Security (grant EMW-2016-FP-00788), the Robert Wood Johnson Foundation (grant 76014), the Paul F. Glenn Center for the Biology of Aging.

Additional Information

Disclosure: S.P. has received author royalties for the book “The Circadian Code.”

Data Availability

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

References