Degradation of the inducible cAMP early repressor (ICER) by the ubiquitin-proteasome pathway.

The inducible cAMP early repressor (ICER) is a powerful transcriptional inhibitor that plays an important role in the regulation of the cAMP-dependent transcriptional response in the neuroendocrine system. ICER activity is primarily determined by its intracellular concentration, rather than by post-translational modifications, such as phosphorylation. We investigated the mechanisms that regulate the levels of ICER transcript and polypeptides in cardiocytes, myogenic (C2C12) and pituitary-derived (GH3) cell lines. We show that in primary cardiocytes and GH3 cells ICER was inducible by cAMP but not by membrane depolarization. Moreover, lactacystin, a specific proteasome inhibitor, decreased the rate of ICER degradation. This effect was associated with the accumulation of ICER-ubiquitin conjugates. We conclude that the intracellular levels of ICER are controlled by the ubiquitin-proteasome pathway for protein breakdown.