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

The circadian clock controls many physiological parameters including immune response to infectious agents, which is mediated by activation of the transcription factor NF-kappa B. 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-kappa B response to a variety of immunomodulators are mediated by core circadian protein CLOCK, which can up-regulate NF-kappa B-mediated transcription in the absence of BMAL1; moreover, BMAL1 counteracts the CLOCK-dependent increase in the activation of NF-kappa B-responsive genes. Consistent with its regulatory function, CLOCK is found in protein complexes with the p65 subunit of NF-kappa B, and its overexpression correlates with an increase in specific phosphorylated and acetylated transcriptionally active forms of p65. In addition, activation of NF-kappa B 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-Delta 19 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-kappa B-responsive promoters. These findings establish a molecular link between two essential determinants of the circadian and immune mechanisms, the transcription factors CLOCK and NF-kappa B, respectively.