| Abstract | | Light, restricted feeding, and hormonal inputs may operate as time-givers (zeitgebers) for the circadian clock within peripheral organs through the activation of tissue-specific signaling cascades. The cAMP signaling through cAMP Responsive Element Modulator (CREM) and its variant Inducible cAMP Early Repressor (ICER) is linked to the circadian regulation of the pineal melatonin synthesis, although little is known about its influence in other organs. We have performed experiments in the absence of light and feeding-time cues to test which core clock genes are controlled by CREM / ICER in the liver and the adrenal gland. In vivo, Crem loss-of-function mutation results in fine-tuning of all measured adrenal clock genes (Per 1,2,3, Cry 1,2, Bmal1, Rev-erb?), while only Per1 and Cry1 were affected in the liver. Icer expression is circadian in the adrenal gland with peak gene expression at ZT 12 and highest protein levels around ZT 20. The expression of both Icer and Per1 genes responds to cAMP stimuli in an immediate early fashion. In immortal cells forskolin induces expression of Per1 after 2 hours, and de novo protein synthesis leads to Per1 attenuation. We show that the de novo synthetized protein responsible for Per1 attenuation is ICER. Indeed, Per1 expression is up-regulated in cells ectopically expressing antisense-Icer, and mobility-shift experiments identify ICER binding to CRE elements of the Per1 promoter. We propose that ICER acts as a noise filter for different signals that could affect transcription in the adrenal gland. Since ICER is an immediate early repressor, the circadian nature of adrenal Icer expression could serve a role in a time- dependent gating mechanism.
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