All ETDs from UAB

Advisory Committee Chair

Stuart J Frank

Advisory Committee Members

Jim Collawns

Joseph L Messina

Chenbei Chang

Weei-Chin Lin

Document Type

Dissertation

Date of Award

2008

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Growth hormone (GH), a 22 kD polypeptide primarily produced in the anterior pituitary gland, is a key regulator of postnatal growth and affects carbohydrate, protein and lipid metabolism. Growth hormone receptor (GHR) is a single membrane-spanning type Ⅰ glycoprotein of the cytokine receptor family and is initially synthesized as a precursor and undergoes carbohydrate processing during transport through the Golgi. It has no intrinsic kinase activity. One molecule of GH binding to predimerized GHR on the cell surface induces the receptor conformational change activating the cytoplasmic domainassociated tyrosine kinase, Janus Kinase 2 (JAK2), which phosphorylates GHR, followed by the activation of downstream pathways, including MAPK, STATs and PI3K. Thus, GH sensitivity is largely determined by cell surface GHR abundance. Previous studies have demonstrated that JAK2, in addition to its role in GH signaling, enhanced GHR maturation, mature GHR stability and surface availability. By reconsititution of γ2AGHR or γ2A-JAK2 cells with mutant JAK2 or mutant GHR respectively, we found that the membrane proximal region of GHR called Box1 element and intact N-terminus of JAK2 are necessary and sufficient for the JAK2-dependent stabilization of the GHR in the absence of its ligand. In contrast, neither the kinase domain nor the kinase-like domain of JAK2 is required for this effect. Furthermore, we investigated the effect of JAK2 on GH-induced GHR loss. Our data suggested that in the presence of JAK2, GH treatment markedly enhances GHR degradation in a dose-dependent manner. However, in ii contrast to constitutive GHR loss, GH-induced receptor downregulation relies not only on GHR-JAK2 association but also on JAK2 kinase activity and GHR tyrosine(s). Tyrosine mutation of GHR diminished GH-induced GHR internalization. Mutation of the serine residues in DSGRTS motif of GHR did not impair GH-induced GHR degradation. Both lactacystin (a proteasome inhibitor) and chloroquine (a lysosome inhibitor) blocked GHinduced GHR loss. Further experiments also indicate that, like downregulation, JAK2 kinase activity and GHR tyrosine(s) are also critical for GH-dependent ubiquitination of GHR.

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