Regulation of the Insulin-like Growth Factor System by Retinoic Acid in Human Osteoblast Cells

<p>Retinoic acid (RA) is an important regulator of growth and differentiation in many cell types, including bone. However, its effects on human osteoblast cell growth and differentiation have not been well studied. Therefore, I investigated the effect of RA on proliferation and differentiation of normal human bone cells (HBCs) and human osteosarcoma, SaOS-2 cells. RA decreased baseline as well as serum-stimulated proliferation in normal HBCs. To determine the effect of RA on differentiation, expression of several osteoblastic differentiation markers were studied. RA decreased type I procollagen mRNA levels and l,25(OH)₂D₃-stimulated osteocalcin and alkaline phosphatase (ALP) mRNA levels as well as baseline and l,25(OH)₂D₃-stimulated ALP activity. These results indicate that RA inhibits human osteoblast cell proliferation and differentiation.</p> <p>The insulin-like growth factor (IGF) system is an important local regulator of bone cell proliferation and differentiation. The actions of IGF are mediated through IGF receptors and are modulated by six IGF binding proteins (IGFBPs) which either inhibit or enhance IGF action. The effect of RA on the IGF system in HBCs has not yet been determined. Therefore, because RA inhibits and IGFs stimulate proliferation and differentiation, I tested the hypothesis that RA inhibits HBC growth and differentiation by down-regulating the IGF system. Adthough the expression of IGF-II (the predominant IGF in HBCs) was not decreased by RA, cell surface binding of IGF-I and IGF-II as well as IGF-I receptor expression were decreased by RA. Furthermore RA increased expression of inhibitory IGFBP-3, -4 and -6 and decreased the expression of stimulatory IGFBP-5. These results indicate that RA markedly down-regulated the IGF system.</p> <p>To investigate the mechanism(s) by which RA regulated IGFBP expression, the effect of RA on mRNA stability of each IGFBP was studied. RA moderately increased the mRNA stabilities of IGFBP-3, -4, -5 and -6, which could explain the small increase in IGFBP-3 and -4 steady-state mRNA levels, but could not explain the dramatic decrease in IGFBP-5 mRNA levels and increase in IGFBP-6 mRNA levels. By using quantitative RT-PCR analysis, I found that RA increased nascent IGFBP-6 transcripts by a mechanism dependent on protein synthesis. These results suggest that RA regulates the expression of IGFBP-3 and -4 primarily by delayed post-transcriptional and IGFBP-5 and -6 primarily by transcriptional or early post-transcriptional mechanisms.</p>

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Microbiology, Molecular Biology and Biochemistry

Year

1995

Author dc:creator

• Zhou, Yuehua

Contributors dc:contributor

• Donna D. Strong
• Thomas A. Linkhart
• Subburaman Mohan
• John F. Sands
• Barry L. Taylor

• Cells
• Microbiology
• Molecular Genetics
• Tretinoin; Insulin-Like Growth Factor I; Insulin-Like Growth Factor II; Receptors, Insulin-Like Growth Factor I; Receptors, Insulin-Like Growth Factor II; Cells

Statement dc: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.

Language dc:language

English

Repository record dc:identifier

https://scholarsrepository.llu.edu/etd/821

OAI identifier oai:identifier

oai:scholarsrepository.llu.edu:etd-1940