Functions of estrogen receptor b in the metabolism of rodent bone

Abstract

Estrogens are important for widespread biological functions including regulation of development and maintenance of skeletal tissue. The effects of estrogens are thought to be mediated via two intracellular proteins, estrogen receptors a and beta (ERalpha and ERbeta , respectively), which are expressed in most cell types in bone. In order to delineate the involvement of ERbeta in the development and maintenance of bone, we have analyzed mice depleted of functional ERbeta receptors following gene targeting. Both male and female homozygous ERbeta knockout (ERbetaKO) and wild-type (WT) mice were analyzed at different ages, corresponding to the phases of growth, maintenance and loss of bone.

Adolescent, female ERbetaKO mice displayed increased cortical bone mineral content, resulting from increased dimensions of bone. Analysis of both bone formation and resorption markers at the mRNA level revealed an increase in bone formation markers, correlating with the increased bone growth. Overall, these data indicate that ERbeta limits bone growth during female puberty in WT mice. In contrast, male homozygous ERbetaKO mice displayed no apparent bone phenotype alterations at any age studied. Thus, the ERbeta protein is not critically involved in development or maintenance of bone in the male mouse, and a sex specific role of ERbeta is described.

The cortical bone phenotype persisted in aged, 1 year-old, female ERbetaKO mice, which also exhibited higher trabecular bone mineral density than WT controls. Normal aging is associated with loss of trabecular bone, and this normal loss of bone was less pronounced in female ERbetaKO mice. Again, this could be correlated with an increase in specific markers related to bone formation. Furthermore, the expression of ERalpha was increased. In summary, these data indicate that, in young adults, ERbeta affects the cortical bone compartment, whereas later in adulthood ERbeta also regulates trabecular bone. In both stages ERbeta seems to act as a repressor, possibly by counteracting the stimulatory action of ERalpha on bone formation.

To assess the role of individual ERs for the estrogen response in bone, ovariectomized WT, ERalphaKO, (ERalpha knockout), ERbetaKO and ERalphabetaPKO (ERalpha and ERbeta double knockout) mice treated with 17beta-estradiol were compared to vehicle-treated mice. Estrogen-treated WT and ERbetaKO mice, but not ERalphaKO or ERalphabetaKO mice, maintained a higher trabecular bone volume, which was associated with decreased osteoclast number and activity. As expected, the ERalphabetaKO mice did not respond to estrogen treatment by modifying the bone volume, or by modifying osteoclast activity, indicating that the osteoprotective effects of estrogen in ovariectomized mice do not involve ERalpha or ERbeta-independent mechanisms. The absence of osteoprotection in ERalphaKO mice was correlated with an increase in osteoclast parameters. Deletion of ERalpha thus renders the bone susceptible to a predominantly ERbeta-regulated increase in formation and activity of osteoclasts, indicating that estrogen can exert either inhibitory or stimulatory influences on bone resorption.

In conclusion, the present study demonstrates that ERbeta affects bone in female mice, and the functional antagonism between the two ERs could provide an important means to control estrogen regulated bone metabolism.