CLOCK is a positive component of a transcription/translation-based negative feedback loop of the central
circadian oscillator in the
suprachiasmatic nucleus in mammals. To examine CLOCK-regulated
circadian transcription in peripheral tissues, we performed
microarray analyses using
liver RNA isolated from Clock
mutant mice. We also compared expression profiles with those of
Cryptochromes (Cry1 and Cry2) double
knockout mice. We identified more than 100 genes that fluctuated from day to night and of which expression levels were decreased in Clock
mutant mice. In Cry-deficient
mice, the expression levels of most CLOCK-regulated genes were elevated to the upper range of normal oscillation. Most of the screened genes had a
CLOCK/
BMAL1 binding site (
E box) in the 5'-flanking region. We found that
CLOCK was absolutely concerned with the
circadian transcription of one type of
liver genes (such as DBP, TEF, and Usp2) and partially with another (such as mPer1, mPer2, mDec1, Nocturnin,
P450 oxidoreductase, and FKBP51) because the latter were damped but remained rhythmic in the
mutant mice. Our results showed that
CLOCK and CRY proteins are involved in the
transcriptional regulation of many
circadian output genes in
the mouse liver. In addition to being a core component of the negative feedback loop that drives the
circadian oscillator,
CLOCK also appears to be involved in various physiological functions such as
cell cycle,
lipid metabolism,
immune functions, and
proteolysis in peripheral tissues.
