The dynamics of the transcriptional response to cyclic adenosine 3',5'-monophosphate: recurrent inducibility and refractory phase.

Abstract

Activation of the cAMP signal transduction pathway results in the transcriptional induction of many genes. Several of them are induced with kinetics characteristic of the early response. One of these, the cAMP response element modulator (CREM) gene, is cAMP-inducible by virtue of an intronic promoter that directs the synthesis of the dominant negative inducible cAMP early repressor (ICER). ICER is involved in the down-regulation of its own promoter via an autoregulatory loop. Thus, while phosphorylation of cAMP response element binding protein (CREB) by the cAMP-dependent protein kinase A is the prerequisite for induction, it has been proposed that the following attenuation involves both CREB dephosphorylation and repression by the inducible repressor ICER. Here we show that ectopic expression of sense or antisense ICER in corticotroph AtT20 cells dramatically modifies the normal CREM inducibility profile. We have investigated the kinetics of CREM inducibility by recurrent stimulation of the cAMP-signaling pathway. We define the presence of a refractory phase that follows the first induction cycle. Accumulation of cAMP, protein kinase A activity, CREB/CREM phosphorylation, and ICER levels contribute to the refractory period. Strikingly, the length of the refractory period is determined by the length of the stimulation by cAMP responsible for the first cycle of induction.