An extraordinary array of behavioural and physiological systems is regulated by endogenous circadian timekeepers within mammals. This circadian machinery is comprised of intricate autoregulatory transcription feedback loops, which interact to generate a time delay of approximately 24 hours in the cyclical transcription of components. In addition to the master clock in the suprachiasmatic nucleus it is clear that peripheral tissues also express clock components. The central clock entrains these in part through the HPA axis and changes in serum glucocorticoid levels.
Several inflammatory diseases show distinct diurnal profiles of activity eg asthma and rheumatoid arthritis. Serum concentrations of pro-inflammatory cytokines, IL-6 and TNFα, and also glucocorticoids oscillate in circadian manner suggesting a direct causative effect. It is uncertain if the IL-6 and TNFα cycles are regulated by changes in the HPA axis or whether their expression is driven by peripheral clock mechanisms.
We now show that IL-6 promoter activity is directly regulated by expression of core clock gene CLOCK and BMAL1, but with a phase delay compared to the index clock controlled gene RevErbα. We then generated stable cell lines expressing IL6-luc and TNFα-luc. The cells were synchronised with serum shock and continuous, broad-field, integrated luciferase activity was measured at 60 s intervals for up to 10 days. The data showed unexpectedly strong, robust oscillations in TNF promoter activity with a period of approximately 24 h. Less marked oscillations were seen in IL6-luc, but again with a 24 h period. A key upstream mediator of cytokine expression is C/EBPβ, or NF-IL6. Strikingly, there was also a robust circadian oscillation in C/EBPβ activity, measured with reporter gene. This is the first demonstration of endogenous clock control of proinflammatory cytokine gene expression. This finding implies that circadian variation in disease severity has a significant, and previously unexpected peripheral clock component.
01 - 05 Apr 2006
European Society of Endocrinology