Olle Vidal

Increased cortical bone mineral content but unchanged trabecular bone mineral density in female ERbeta(-/-) mice.

Ovariectomy in young, growing rodents results in decreased trabecular bone mineral density (BMD) and increased radial growth of the cortical bone. Both of these effects are reversed by treatment with estrogen. The aim of the present study was to determine the physiological role of estrogen receptor-beta (ERbeta) on bone structure and bone mineral content (BMC). The BMC was increased in adult (11 weeks old), but not prepubertal (4 weeks old), female ERbeta(-/-) mice compared with wild-type (WT) mice. This increase in BMC in females was not due to increased trabecular BMD, but to an increased cross-sectional cortical bone area associated with a radial bone growth. Male ERbeta(-/-) mice displayed no bone abnormalities compared with WT mice. Ovariectomy decreased the trabecular BMD to the same extent in adult female ERbeta(-/-) mice as in WT mice. The expression levels of osteoblast-associated genes - alpha1(I) collagen, alkaline phosphatase, and osteocalcin mRNAs - were elevated in bone from adult ERbeta(-/-) females compared with WT mice. These observations provide a possible explanation for the increased radial bone growth seen in female mutants, suggesting a repressive function for ERbeta in the regulation of bone growth during female adolescence. In summary, ERbeta is essential for the pubertal feminization of the cortical bone in female mice but is not required for the protective effect of estrogens on trabecular BMD.

Expression and Localization of Estrogen Receptor‐β in Murine and Human Bone

Estrogens have profound effects on bone metabolism. Cellular responses to estrogens are mediated by estrogen receptors (ERs) which belong to the nuclear receptor superfamily. Two estrogen receptors, ERα and ERβ, have been cloned. Previously expression of ERα has been shown in osteoblasts. Here we demonstrate that the transcript for ERβ can be detected in the human osteosarcoma cell lines (MG‐63 and SaOS‐2) and in cultured human osteoblast‐like cells. We also show that ERβ protein is present in nuclear extracts from these cells. Furthermore, ERβ immunoreactivity is found in sections of murine and human bone. Murine and human osteoblast and osteocyte nuclei are immunoreactive for ERβ. Osteoclasts are also ERβ immunoreactive but the staining is mainly cytoplasmic. The present study demonstrates that ERβ is present in all the cellular compartments involved in bone formation and bone resorption, both in human and in murine bone tissue.

Female Estrogen Receptor β−/− Mice Are Partially Protected Against Age-Related Trabecular Bone Loss

Recently, it has been shown that inactivation of estrogen receptor β (ER‐β) by gene targeting results in increased cortical bone formation in adolescent female mice. To study the possible involvement of ER‐β in the regulation of the mature skeleton, we have extended the analyses to include 1‐year‐old ER‐β knockout mice (ER‐β−/−). Male ER‐β−/− mice did not express any significant bone phenotypic alterations at this developmental stage. However, the increase in cortical bone parameters seen already in the adolescent female ER‐β−/− mice was maintained in the older females. The aged female ER‐β−/− mice further exhibited a significantly higher trabecular bone mineral density (BMD) as well as increased bone volume/total volume (BV/TV) compared with wild‐type (wt) mice. This was caused by a less pronounced loss of trabecular bone during adulthood in female ER‐β−/− mice. The growth plate width was unaltered in the female ER‐β−/− mice. Judged by the expression of the osteoclast marker tartrate‐resistant acid phosphatase (TRAP) and cathepsin K (cat K; reverse‐transcription‐polymerase chain reaction [RT‐PCR]) as well as the serum levels of C‐terminal type I collagen cross‐linked peptide, bone resorption appeared unaffected. However, an increase in the messenger RNA (mRNA) expression levels of the osteoblast marker core‐binding factor α1 (Cbfa1) suggested an anabolic effect in bones of old female ER‐β−/− mice. In addition, the mRNA expression of ER‐α was augmented, indicating a role for ER‐α in the development of this phenotype. Taken together, the results show that ER‐β is involved in the regulation of trabecular bone during adulthood in female mice and suggest that ER‐β acts in a repressive manner, possibly by counteracting the stimulatory action of ER‐α on bone formation.