Mitogen-activated Protein Kinase Phosphorylates and Targets Inducible cAMP Early Repressor to Ubiquitin-mediated Destruction

Abstract

Inducible cAMP early repressor (ICER) is an important mediator of cAMP antiproliferative activity that acts as a putative tumor suppressor gene product. In this study, we examined the regulation of ICER protein by phosphorylation and ubiquitination in human choriocarcinoma JEG-3 and mouse pituitary AtT20 cells. We found that cAMP stabilized ICER protein by inhibiting the mitogen-activated protein kinase (MAPK) cascade. Activation of the MAPK pathway increased ICER phosphorylation. ICER phosphorylation was abrogated by inhibition of the MAPK pathway either by cAMP or directly by the MAPK inhibitor PD098059. The MAPKs extracellular signal-regulated kinases 1 and 2 physically interact with ICER and mediated the phosphorylation of ICER on a critical serine residue (Ser-41). A mutant form of ICER in which Ser-41 was substituted by alanine had a half-life 4 -5 h longer than its wild-type counterpart. This alteration in stability was due to the inability of the Ser-41-mutant ICER to be efficiently ubiquitinated and degraded via the ubiquitin-proteasome pathway. These results present a novel cell signaling crosstalk mechanism at the cell nucleus between the MAPK and cAMP pathways, whereby MAPK targets a repressor of the cAMP-dependent gene expression for ubiquitination and proteasomal degradation.The MAPK 1 pathway controls cell growth by regulating gene expression, protein synthesis, nucleotide synthesis, the interaction between cyclins and cyclin-dependent kinases, and protein stability by ubiquitin-mediated proteasomal degradation (Refs. 1 and 2 and references therein). In the classic pathway, activation of MAPK occurs by peptide growth factors that bind to a transmembrane tyrosine kinase receptor. This results in the activation of the small GTP-binding protein Ras that recruits a member of the Raf kinase family (Raf-1, A-Raf, or B-Raf) to the plasma membrane. Raf phosphorylates and activates the MEKs (MEK1 and MEK2), which in turn activates the MAPKs ERK1 and ERK2 by phosphorylation on threonine and tyrosine.

ICER/CREM‐mediated transcriptional attenuation of IL‐2 and its role in suppression by regulatory T cells

“…6) [5]. Moreover, previous reports indicated that CTLA-4/B7 engagement could inhibit extracellular signal-regulated kinases (ERK1 and ERK2) [45] thus protecting ICER from ERK-mediated phosphorylation and subsequent degradation by ubiquitination [46]. Therefore, contactdependent 'infectious' tolerance acting through ICER/ CREM-mediated inhibition of IL-2 synthesis could explain (i) how a relatively small population of T R cells effectively suppresses a much larger population of Foxp3 -responders, and (ii) how inducible CTLA-4 and B7 expression contributes to the spread of this infectious suppression conveyed by induction of ICER/CREM in antigen-nonspecific fashion (Fig.…”

Regulatory T cells facilitate the nuclear accumulation of inducible cAMP early repressor (ICER) and suppress nuclear factor of activated T cell c1 (NFATc1)

“…nTregs harbor constitutively high levels of cAMP and are able to confer cAMP through gap junctions into conventional CD4 + T cells (3). The increase in cellular cAMP levels enhances the expression and nuclear localization of ICER (19). This results in the binding of ICER to the inducible P1 promoter of the Nfatc1 gene and the repression of NFATc1/αA induction.…”

MAPK signal pathways in the regulation of cell proliferation in mammalian cells

“…The small G proteins such as Rap1, Rac and Cdc42 play a key role in this decision. cAMP inhibits the growth of fibroblasts cells, smooth muscle cells, and adipocytes at least in part, by blocking the binding of Raf-1 to Ras, thus blocking the MAPK pathway [35]. On the contrary, in PC12 cells, cAMP induces activation of MAPK through PKA-induced activation Rap1.…”