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Is Developmentally Regulated during Osteoblast Differentiation and Contributes to Selective Responsiveness of Gene Expression
Womens Health Research Institute, Wyeth-Ayerst (P.V.N.B., R.A.H., B.S.K.), Radnor, Pennsylvania 19087; and the Department of Cell Biology, University of Massachusetts Medical Center (J.G., M.A., T.O., G.S.S., J.B.L.), Worcester, Massachusetts 01655
Address all correspondence and requests for reprints to: Dr. Peter V. N. Bodine, Womens Health Research Institute, Wyeth-Ayerst Research, 145 King of Prussia Road, Radnor, Pennsylvania 19087. E-mail: bodinep{at}war.wyeth.com
Estrogen responsiveness of bone is a fundamental regulatory mechanism
operative in skeletal homeostasis. We examined the expression of
estrogen receptor-
(ER) messenger RNA (mRNA) in cultured rat
calvarial-derived osteoblasts during progressive development of the
osteoblast phenotype. Levels of ER message were compared with the
expression of traditional osteoblastic markers that have been mapped
throughout the differentiation process of these cells. ER transcripts,
measured using semiquantitative RT-PCR analysis, were expressed at low
levels in early stage proliferating osteoblasts and increased at
confluence upon initial expression of bone cell phenotypic genes. A
23-fold up-regulation of ER mRNA expression coincided with the
initiation of alkaline phosphatase activity (day 8). ER mRNA levels
progressively increased 70-fold, reaching a maximum level on days
2225 in fully differentiated osteoblasts when osteocalcin expression
peaked, but declined precipitously by day 32 in osteocytic cells.
Analysis of RNA isolated directly from rat calvaria confirmed these
in vitro results and demonstrated that ER message levels
become more abundant postnatally as bone becomes more mineralized. We
also examined the responsiveness of osteoblasts to 17ß-estradiol
(17ß-E2) at two periods of maturation: the nodule-forming
stage (day 14) and the late mineralization stage (day 30). Estradiol
suppressed the levels of alkaline phosphatase, osteocalcin,
osteonectin, and ER mRNAs on day 14, but up-regulated these messages on
day 30. In contrast, 17ß-E2 treatment regulated the
steady state levels of transforming growth factor-ß1 and type I
procollagen mRNAs only in the late mineralization stage, whereas
histone H4 message was unaffected by the steroid at either stage of
differentiation. Thus, the observed developmental expression of ER mRNA
correlates with progressive osteoblast differentiation and may be a
contributing factor to differential regulation of bone cell gene
expression by 17ß-E2.
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