Requirement for cAMP-response element (CRE) binding protein CRE modulator transcription factors in thyrotropin-induced proliferation of dog thyroid cells in primary culture

In several cell types, mostly of epithelial origin, activation of the cAMP pathway triggers DNA synthesis and cell division. Regulation of gene expres sion by cAMP involves phosphorylation by protein kinase A (PKA) and activat ion of cAMP-response element binding protein (CREB)/CRE modulator (CREM) tr anscription factors which bind DNA to CRE sites. On the other hand, several CREM isoforms are transcriptional repressors, such as the inducible cAMP e arly repressor (ICER) transcription factors, which are synthesized from an intronic promoter of the CREM gene. This study investigated the potential r ole of CREB/CREM transcription factors in the cAMP mitogenic pathway, using an experimental model of epithelial cells in primary culture, i.e. dog thy roid cells stimulated by thyroid-stimulating hormone (TSH). In response to TSH, CREB/CREM transcription factors were phosphorylated on the serine resi due of the PKA consensus site. In addition, the synthesis of ICER mRNAs was strongly induced by TSH. This transient upregulation of ICER expression co rrelated with increased protein levels. It was restricted to the cAMP pathw ay, as neither epidermal growth factor nor 12-O-tetradecanoylphorbol 13-ace tate (TPA), which are potent mitogens for dog thyroid cells, induced ICER e xpression. On the other hand, increased expression of ICER mRNAs was not de tected in dog thyroids chronically stimulated by TSH in vivo. The requireme nt for CREB/CREM transcription factors in the mitogenic effect of TSH was a ssessed by transfecting expression vectors encoding CREM repressors into do g thyrocytes in order to interfere with CRE-mediated gene transcription. Th e ectopic expression of ICER I gamma or CREM alpha isoforms inhibited DNA r eplication in dog thyrocytes stimulated by TSH. This inhibitory effect was dependent on the ability of CREM repressors to form dimers but did not invo lve their DNA-binding capacity, Together these results show that CREB/CREM transcription factors are tightly regulated, at the transcriptional and pos t-translational levels, by TSH in dog thyroid cells, and provide clear evid ence that their activity is required for the cAMP-dependent proliferation o f epithelial cells in primary culture. Moreover, the transient induction of ICER transcription factors during mitogenic stimulation by TSH raises ques tions about the role of these potent repressors of CRE-dependent transcript ion as timers of cellular proliferation.