Core circadian protein CLOCK is a positive regulator of NF-κB–mediated transcription

Abstract

The circadian clock controls many physiological parameters including immune response to infectious agents, which is mediated by activation of the transcription factor NF-κB. It is widely accepted that circadian regulation is based on periodic changes in gene expression that are triggered by transcriptional activity of the CLOCK/BMAL1 complex. Through the use of a mouse model system we show that daily variations in the intensity of the NF-κB response to a variety of immunomodulators are mediated by core circadian protein CLOCK, which can up-regulate NF-κB-mediated transcription in the absence of BMAL1; moreover, BMAL1 counteracts the CLOCK-dependent increase in the activation of NF-κB-responsive genes. Consistent with its regulatory function, CLOCK is found in protein complexes with the p65 subunit of NF-κB, and its overexpression correlates with an increase in specific phosphorylated and acetylated transcriptionally active forms of p65. In addition, activation of NF-κB in response to immunostimuli in mouse embryonic fibroblasts and primary hepatocytes isolated from Clock-deficient mice is significantly reduced compared with WT cells, whereas Clock-Δ19 mutation, which reduces the transactivation capacity of CLOCK on E-box-containing circadian promoters, has no effect on the ability of CLOCK to up-regulate NF-κB-responsive promoters. These findings establish a molecular link between two essential determinants of the circadian and immune mechanisms, the transcription factors CLOCK and NF-κB, respectively.

A circadian based inflammatory response – implications for respiratory disease and treatment

“…A reduction of cytokine production (in response to the LPS challenge or Salmonella Typhimurium infection) is observed in macrophages from Clock mutant mice (Bellet et al 2013). This is in agreement with the finding that CLOCK protein activates the NF-κB pathway leading to upregulation of cytokines (Spengler et al 2012). …”

“…Similarly, bacterial infection has been shown to lead to higher mortality when initiated during the rest period (Shackelford and Feigin 1973). More recently, these results were confirmed and extended showing that exposing mice to LPS at the end of their rest period or beginning of the active period resulted in a stronger cytokine response and NF-κB activation compared with LPS exposure starting during the active period or beginning of the rest period (Marpegan et al 2009;Gibbs et al 2012;Nguyen et al 2013;Spengler et al 2012). Similar results have been obtained in humans using the LPS challenge both in vivo injecting LPS to healthy volunteers (Alamili et al 2014) and in vitro exposing blood samples obtained at different times of the day from volunteers to LPS (Petrovsky et al 1998;Rahman et al 2015).…”

Immunometabolism: Is it under the eye of the clock?

“…Activation of the subunit of nuclear factor kappa-light-chainenhancer of activated B-cells (NF-B) correlates positively with the circadian variation in lethality to LPS [115]. CLOCK complexes with p65 causing increased acetylation and phosphorylation of p65 and enhancing the activity of the NF-B complex [115].…”

“…CLOCK complexes with p65 causing increased acetylation and phosphorylation of p65 and enhancing the activity of the NF-B complex [115]. Both CLOCK mutant MEFs [116] and bone marrow derived macrophages (BMDMs) lacking CLOCK, are less responsive to LPS, and TNF␣ induced NF-B activation [65].…”

Mechanism of the circadian clock in physiology

“…One recent study showed that CLOCK positively modulates NF-B in a possible "circadian clock-independent fashion" that does not involve the transcriptional activity of CLOCK-BMAL1. This is suggested due to the effects still being seen in CLOCK (⌬19) mutant mice (129). This is one of the few studies that identify CLOCK having effects that do not involve the classical interaction with the E-box; if more of these interactions exist will be of great interest.…”