Insulin-like growth factor binding proteins (IGFBPs) are a family of six secreted proteins that bind to and modulate the functions of insulin-like growth factors (IGF-I and IGF-II). As a relatively new member of the IGF system, IGFBP-6 was found to be produced by human osteoblasts and exert significant biological effects on human osteoblasts.
Our previous studies indicated that all-trans retinoic acid (ATRA) caused more than a10 fold increase in IGFBP-6 production and a50-60% reduction of ALP activity in human osteoblasts. Based on these findings, we proposed the hypothesis that ATRA induced human osteoblast differentiation was mediated at least in part by IGFBP-6. To test this hypothesis, we undertook studies to specifically block the actions of IGFBP-6 by antisense overexpression and evaluate the consequences of this blockage on ATRA-induced hOB differentiation. Cells overexpressing IGFBP-6 were also prepared. Three phenotypically appropriate IGFBP-6 antisense clones were identified which showed significant reduction of IGFBP-6 mRNA (50-60%) and protein production (40-50%) while other IGFBP (IGFBP-3 and IGFBP-4) production profiles were unchanged. Treatment of vector control clones or parental SaOS-2 cells with ATRA caused a 50-60% reduction of ALP (alkaline phosphatase activity), but this reduction was significantly diminished (to10-20%) in the three phenotypically appropriate clones. These results indicate that the ATRA induced reduction of ALP activity in human osteoblasts is mediated at least in part by IGFBP-6.
Previous studies provided evidence that the IGF system modulated 1,25(OH)2D induced osteoblast differentiation. A second hypothesis was proposed that IGFBP-6 could affect 1,25(OH)2D stimulated differentiation of human osteoblasts. Cytochemical staining for ALP showed that 1,25(OH)2D treatment increased the ALP activity to a greater extent in IGFBP-6 antisense clones compared to vector control clones. Cells overexpressing IGFBP-6 produced only low levels of ALP activity with or without treatment. Taken together, these results indicate that IGFBP-6 plays an important role in mediating the 1,25(OH)2D induced increase of ALP activity in human osteoblasts.
Microbiology and Molecular Genetics
Donna D. Strong
Barry L. Taylor
Hansel M. Fletcher
Thomas A. Linkhart
Doctor of Philosophy (PhD)
Year Degree Awarded
Date (Title Page)
Library of Congress/MESH Subject Headings
Insulin-Like Growth Factor-Binding Proteins -- analysis; Insulin-Like-Growth-Factor-Binding Protein-6 -- analysis; Growth Substances; Osteoblasts -- analysis; Gene Expression Regulation; Receptors, Growth Factor.
xv; 233 p.
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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.
Yan, Tao, "Functional Analysis of Human IGFBP-6 in Human Osteoblasts" (1999). Loma Linda University Electronic Theses, Dissertations & Projects. 1644.
Loma Linda University Electronic Theses and Dissertations
Loma Linda University. Del E. Webb Memorial Library. University Archives