All ETDs from UAB

Advisory Committee Chair

Stuart Frank

Advisory Committee Members

Rosa Serra

Joseph Messina

Chenbei Chang

Xu Feng

Document Type

Dissertation

Date of Award

2012

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

GH receptor (GHR) binds GH in its extracellular domain (ECD) to activate the GHR-associated cytoplasmic tyrosine kinase, JAK2. IGF-1 binds IGF-1R, a disulfide-linked heterotetramer with tyrosine kinase activity in its intracellular domain (ICD). Classically, IGF-1 is a GH effector in a “linear” GH→GHR→IGF-1→IGF-1R pathway. Our recent studies suggest IGF-1R also subserves GH signaling in several novel ways: 1) GH induces a GHR-JAK2-IGF-1R complex, whose formation is independent of tyrosine phosphorylation of any of the partners; 2) Cotreatment with IGF-1 augments acute GH signaling; 3) deletion of IGF-1R in primary osteoblasts or human prostate cancer cells blunts acute GH signaling. In IGF-1R-deficient primary osteoblasts, adenovirally-driven re-expression of wild-type IGF-1R normalizes GH-induced STAT5 phosphorylation, but re-expression of a truncated IGF-1R that lacks most of its ICD (including its kinase domain) partially rescues GH-induced STAT5 activation and IGF-1 gene expression. Thus, IGF-1R ICD and kinase are dispensable for some of the IGF-1R functional collaboration with acute GH signaling. We now examine how protein tyrosine phosphatase (PTP) activity influences GHR-IGF-1R collaboration. In primary osteoblasts, pre-incubation with orthovanadate, a general phosphatase inhibitor, is capable of rescuing diminished STAT5 activation induced by GH upon the deletion of IGF-1R, suggesting the involvement of phosphatase(s) in the functional collaboration between IGF-1R and GH signaling pathway. Furthermore, specific PTP-1B inhibitor recapitulates the rescuing effect of orthovanadate, but specific SHP1/2 inhibitor does not. Analogous to orthovanadate and specific PTP-1B inhibitor, adenovirus driven expression of a catalytically-inactive PTP1B also rescues diminished STAT5 activation induced by GH upon the deletion of IGF-1R. Similar re-sults are obtained as well in LnCap cells wherein the IGF-1R is markedly deleted via Sh-RNA mediated strategy. Taken together, these data suggest that the activity of PTP-1B or association of PTP-1B with some component(s) within GH signaling pathway is en-hanced upon the deletion of IGF-1R, hence the observed result of diminished STAT5 activation.

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