Abstract:

ERBB receptors in humans (EGFR, ERBB2, ERBB3, and ERBB4) are involved in a broad range of cellular responses including proliferation, growth inhibition, differentiation, and apoptosis. Because of its potential to cause cellular transformation, the ERBB signaling network is tightly regulated. Receptor degradation serves as an important mechanism for ERBB signal attenuation. While a large pool of data has been accumulated for the mechanisms of EGFR down-regulation, the regulation of signal attenuation and steady state levels for ERBB2, and particularly for ERBB3, is much less understood. Here we describe post-translational control of both ligand-induced attenuation of ERBB2/ERBB3 signaling as well as the constitutive turnover of ERBB3. We compared the signal attenuation properties of the EGF-like domain of neuregulin-?1 with the full-length ligand, and demonstrated that the presence of the N-terminal domains enhanced signal attenuation and receptor degradation to levels comparable to those of EGF activated EGFR. In addition, we addressed whether the differences in outcome of signaling between the EGF-like domain of neuregulin-?1 and the full-length ligand are based on timing of the response, or if stimulation with the two ligands results in qualitatively different signaling. Based on ongoing difference gel electrophoresis (DIGE) experiments, we found that the differences in outcome are likely a result of changes in the duration of signaling. Furthermore, we evaluated the mechanisms that regulate the ubiquitination of ERBB3 under ligand-independent turnover and ligand-induced down-regulation. We demonstrated that tyrosine phosphorylation of ERBB3 by ERBB2 or EGFR is required for ligand-induced ubiquitination, but is not necessary for ligand-independent ubiquitination. Taken together, our results demonstrate that different levels of post-translational control are used to regulate ERBB3 signaling.