Bringing the Cellular Clock into Understanding Lung Disease: It's Time, Period!

Chronic respiratory diseases continue to be a worldwide burden with asthma affecting ~350 million people and chronic obstructive pulmonary disease (COPD) affecting almost 400 million (1, 19). Asthma prevalence has increased 3.6% since 2006 (19) and COPD is predicted to be the third leading cause of death by 2030 (1). Although chronologic age increases risk for developing chronic respiratory diseases, environmental exposures (pollutants, allergens, infectious agents) during early life can increase susceptibility to developing airway diseases in childhood and through adulthood. Indeed, the incidence of asthma in children and the elderly is increasing. These stark facts have led to substantial efforts towards understanding the mechanisms underlying asthma or COPD. An increasing portfolio of pharmacological strategies is being implemented including well-established drugs such as corticosteroids and -adrenoceptor agonists, and emerging therapeutics such as biologics targeting the inflammatory system. Even with such advancements, significant gaps remain in treating or preventing chronic respiratory diseases, highlighting the need for better understanding of the underlying pathophysiology and identification of novel therapeutic targets. One potential missing link in lung disease research endeavors has attracted a lot of attention over the past few years and has gained momentum following the Nobel Prize in Physiology or Medicine in 2017: circadian rhythms and peripheral clock biology. The notion that the respiratory system and its diseases exhibit circadian rhythms was first noted in the 5th century by Roman physician Caelius Aurelianus, who described worsening nocturnal asthma symptoms compared with the daytime (49). Today, experimental studies have shown that lung function does exhibit circadian fluctuations in healthy individuals, including diurnal variation in airway caliber [forced expiratory volume in one second (FEV1), peak expiratory flow (PEF), and airway resistance] (5). Normal lungs reach maximum functionality (FEV1 and FEV1/forced vital capacity, FVC) around noon with lowest lung function around 4:00 AM (5, 46). Initial studies in the field hypothesized that disrupted circadian rhythms and subsequently altered fluctuations in lung function inherently affected bronchial reactivity, immune-inflammatory response, adrenergic responsiveness, airway tone, and circulating cortisol and catecholamines (5). Epidemiological and experimental studies of asthmatics revealed elevated asthma exacerbations around 4:00 AM (50) and 50% greater change in nocturnal PEF compared with nonasthmatic individuals (24). Additionally, asthmatics exhibit heightened bronchial response to methacholine and histamine when challenged at 4:00 AM compared with later in the day (8) and have higher eosinophil, macrophage, neutrophil, lymphocyte, and CD4 T cells at 4:00 AM compared with 4:00 PM (27, 29, 30). Considering the dynamic nature of this so-called “nocturnal asthma” it is unsurprising that 70% of asthma-related deaths in the hospital occur during the early morning hours (10). Daily oscillations in lung function (decreased FEV1, FVC, PEF, cortisol, epinephrine, 2-receptor, vagal activity, and body temperature during the night) can explain heightened nocturnal asthma symptoms (49). Asthmatics consequently demonstrate a time-of-day responsiveness to treatment that is highly dependent on classification of medication and route of administration [we direct the readers to two excellent reviews on this topic (31, 49)]. For example, 3:00 PM administration of oral or intravenous steroids increased early morning FEV1, decreased inflammatory cell infiltration (7), and increased early morning PEF (42) compared with administration at other times of day. Inhaled steroid triamcinolone proved more efficacious when administered at 3:00 PM by providing as much benefit as administration 4/day (40, 41). Additionally, a long-acting 2-agonist had increased efficacy when administered in the evening, reducing dosing by half (17) with a leukotriene receptor antagonist administered in the evening resulting in increased early morning FEV1 compared with morning administration (34). Overall, these data in humans should excite the lung research community in a joint effort to understand the mechanisms underlying the links between circadian rhythms and asthma. With the discovery of peripheral circadian clocks, the time is ripe for such endeavors. The importance of peripheral circadian clocks in health and disease is now well-recognized and key to unlocking mechanisms underlying lung (patho)physiology and future therapeutic targets (3, 43). Peripheral circadian clocks are intracellular networks of transcription-translation feedback loops of the core clock components [BMAL1 (aryl hydrocarbon receptor nuclear translocatorlike protein 1; gene name ARNTL), CLOCK (circadian locomoter output cycles protein kaput), PER1/2/3 (period 1/2/3), CRY1/2 (cryptochrome 1/2)] that function in a highly cell type-specific manner (2, 13). BMAL1 is a key element for oscillations and in combination with CLOCK, transcriptionally regulates CRY1/2 and PER1/2. CRYs and PERs heterodimerize in the cytoplasm before translocating back to the nucleus where they inhibit the BMAL1:CLOCK complex, thereby inhibiting their own expression (2, 13). Nuclear receptors REV-ERB / (nuclear receptor subfamily 1, group D, member 1/2; gene names NR1D1/2) and ROR / / (retinoic acid receptor-related orphan receptors) provide stability for the clock, regulating its oscillation by inhibiting or driving BMAL1 expression, respectively (REV-ERBs competing with RORs at binding sites) (20). Furthermore, BMAL1: CLOCK regulate REV-ERB gene expression at promoter E-box domains, creating a feedback loop (11). Another stabilizing comCorrespondence: Y. S. Prakash (prakash.ys@mayo.edu). Am J Physiol Lung Cell Mol Physiol 319: L273–L276, 2020; First published July 8, 2020; doi:10.1152/ajplung.00320. 2020.