Biol. Pharm. Bull. 30(4) 621—626 (2007)

logical functions, including metabolism, cell growth, etc. The master pacemaker of circadian rhythm resides in the suprachiasmatic nucleus (SCN) of the hypothalamus and coordinates autonomous peripheral clocks located in organs such as the liver and kidney. The molecular clock is composed of transcriptional feedback loops in organisms ranging from cyanobacteria to humans. Two transcription factors, the brain and muscle Arnt-like protein-1 (BMAL1; also referred to as MOP3 or Arnt3) and CLOCK, play central roles in the regulation of circadian rhythms. BMAL1 and CLOCK form a heterodimer and drive transcription from E-box elements found in the promoter of circadian-responsive genes, including period (Per)1 and cryptochrome (Cry). After translation of the Per and Cry proteins, the Per/Cry complex translocates to the nucleus, where it inhibits gene expression driven by BMAL1 and CLOCK. There is a growing body of evidence that circadian rhythms also govern immunoreactions such as antigen presentation, lymphocyte proliferation, and cytokine expression. More recently, diurnal expression of granzyme B and perforin in natural killer (NK) cells was observed in vivo. These circadian variations in the immune system are likely to be regulated by molecular clocks for the following reasons. First, RNAi-mediated Per2 knockdown caused a significant decrease of granzyme B and perforin levels in the rat-derived NK cell line RNK16. Second, Per2-deficient mice were more resistant to lipopolysaccharide (LPS)-induced endotoxic shock than wild-type mice. Also, the levels of the proinflammatory cytokines gamma interferon (IFN) and interleukin (IL)-1beta were dramatically decreased in Per2 / mice following LPS challenge, while the productions of tumor necrosis factor alpha (TNFa), IL-6, and IL-10 were approximately equal to those in wild-type mice. Furthermore, studies using BMAL1-deficient mice revealed that BMAL1 is required for B cell development. Although several studies have shown that the immunoreactions and onset of the inflammatory diseases exhibit circadian variation, the molecular details of the circadian rhythms in macrophages are still poorly understood. Therefore, in this study, we attempted to characterize circadian gene expression in mice peritoneal macrophages. We found that the expression of several clock genes such as BMAL1 exhibited daily oscillations in resident peritoneal macrophages. The expression of inflammatory factors such as monocyte chemoattractant protein-1 (MCP-1/JE) exhibited robust circadian rhythms. Suppression of BMAL1 expression by an RNAi technique lowered the nuclear factor-kappa B (NF-kB) activity followed by down-regulation of MCP-1/JE mRNA expression in RAW264.7 macrophage cells. Macrophages are known to play essential roles in immunoreactions and pathogenesis of atherosclerosis and arthropathy. Consequently, these results may provide further insight into the immunoreactions and the inflammatory diseases mediated by macrophages.