Ubiquitin System-Dependent Regulation of Growth Hormone Receptor Signal Transduction and Effects of Oxidative Stress

The ubiquitin-proteasome system is known to be involved in GHR endocytosis, where an active ubiquitin system is necessary for receptor ubiquitination and its consequent internalization. Endocytosis of the GHR has also been shown to be regulated by the proteasome. This thesis focuses, first of all, on the effect of the ubiquitin system on the GHR signal transduction and downregulation. In chapter 2, we studied the role of the ubiquitin system in the signal transduction of the GHR. By using specific proteasome inhibitors, we determined their effect on GH-induced activity of both GHR and JAK2 in both the wild-type GHR and in the internalization- deficient GHR mutant F327A. We suggest possible models for the role of the ubiquitin-proteasome system in GHR signal transduction. Furthermore, we asked the question involving the continuation of signal transduction of the GHR after its 37 Chapter 1 internalization, where binding of JAK2 as well as other activated proteins to the GHR, not only at the cell-surface but also intracellularly, was determined. Degradation products of the GHR were determined in both cell-surface and in endosomes, indicating that part of the receptor is down-regulated by the proteasome. In chapter 3, we focused on the role of JAK2-mediated signal transduction in GHR internalization and downregulation. Using a box-1 mutant of the GHR, activation of both GHR and JAK2 was determined, as well as receptor-bound GH internalization and GHR turnover and transport to the plasma membrane. We determined the role of GHR signal transduction, via JAK2, on GHR ubiquitination, endocytosis and degradation, and the role of the ubiquitin system on the regulation of both GHR internalization and signal transduction. Finally, chapter 4 discusses the role of stress in regulating GHR levels in the cell. We determined the effect of H2O2 on receptor's turnover and on receptor's downregulation from the cell-surface. We further determined the effect of oxidative stress on the amount of ligand binding sites at the cell-surface and related these results with the enhancement of shedding activity directed to the GHR