Xiaoying Wang

Abstract

Glucocorticoids (GCs) inhibit bone formation in vivo and inhibit osteoblast proliferation and collagen synthesis in vitro. These effects may be mediated by alterations in the insulin-like growth factor (IGF) system. In the present study of normal human osteoblast-like (HOB) cells, we tested the hypothesis that dexamethasone (Dex) inhibits the expression of IGF binding protein-5 (IGFBP-5). Dex decreased IGFBP-5 mRNA levels to 54% of control after 4 hr. Dex did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested osteoblast cells. Dex decreased IGFBP-5 hnRNA levels to 67% of control after 2 hr, and the activity of the human IGFBP- 5 promoter by 35% in transient transfection experiments. Analysis of 5' deletion constructs indicates that Dex inhibition of IGFBP-5 promoter activity does not require the GR response element half sites in the 5' flanking region but does require the region from base pairs -70 to -35. Transcription factor AP-2 may be involved in Dex regulation. Mutation of the AP-2 binding site and NF-1 binding site at this region eliminated Dex inhibition. Gel mobility shift assays using -85 to -35 DNA fragment, U2 cell nuclear extracts, or purified AP-2 protein indicate that nuclear factors bind to a AP-2 binding site at the region -70 to -35 in the IGFBP-5 promoter. GR interacts with AP-2 in vitro and in vivo as shown by GST-pulldown assay, coimmunopricipitation assay and mammalian two-hybrid assay.

In conclusion, Dex inhibits IGFBP-5 expression at the transcription level through a mechanism involving interaction of GR and transcription factor AP-2.

LLU Discipline

Biochemistry

Department

Biochemistry

School

Graduate School

First Advisor

Thomas A. Linkhart

Second Advisor

Alan P. Escher

Third Advisor

William H.R. Langridge

Fourth Advisor

Subburaman Mohan

Fifth Advisor

Donna D. Strong

Degree Name

Doctor of Philosophy (PhD)

Degree Level

Ph.D.

Year Degree Awarded

2000

Date (Title Page)

12-2000

Language

English

Library of Congress/MESH Subject Headings

Transcription Factors -- genetics; Somatodins; Glucocorticoids -- metabolism; Osteoblasts; Bone Development; Insulin-Like Growth Factor-Binding Protein 5

Type

Thesis

Page Count

xiv; 169

Digital Format

PDF

Digital Publisher

Loma Linda University Libraries

Usage Rights

This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.

Collection

Loma Linda University Electronic Theses and Dissertations

Collection Website

http://scholarsrepository.llu.edu/etd/

Repository

Loma Linda University. Del E. Webb Memorial Library. University Archives

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