Effect of estrogen on longitudinal bone growth

Abstract

Longitudinal bone growth occurs at the growth plate, a thin layer of cartilage between the epiphysis and the metaphysis of long bones. In the growth plate resting chondrocytes proliferate, differentiate into a hypertrophic form, and finally become terminal hypertrophic chondrocytes before giving rise to bone. Estrogen is essential for skeIetal growth. Extreme early puberty often leads to short stature, whereas significantly delayed puberty leads to increased final stature. Moreover, high doses of estrogen were used to prevent extreme tall stature in girls, but the treatment was associated with severe side effects. The effect of estrogen is mediated via two known estrogen receptors, ERalpha and ERbeta that are both expressed in growth plate chondrocytes. Although the effect of estrogen on bone growth is well known, the mechanism of action on longitudinal bone growth is poorly understood.

We first studied longitudinal bone growth in female mice lacking ERalpha, ERbeta or both receptors (Paper I). Analysis of bone length and growth plate morphology revealed that ERbeta inhibits longitudinal bone growth. Moreover, in the presence of high serum concentrations of estrogen, stimulation of ERbeta induces growth plate fusion.

Expression of both androgen and estrogen receptors were demonstrated within the growth plate. To study any co-interaction between estrogen and androgen receptors, we treated female ovariectomized rats with either estrogen, dihydrotestosterone (DHT) or the combination (Paper II). We found that estrogen inhibits longitudinal bone growth and growth plate height whereas DHT is able to counteract this effect. The effect is likely to be associated with systemic actions of these hormones since we found that decreased IGF-I levels in estrogen treated rats were restored by DHT.

Estrogen regulates longitudinal bone growth either indirectly, via the modulation of the GH/IGF-I axis, or directly, via binding to estrogen receptors. Direct effects of estrogen were studied in a human chondrocytic cell line and in cultured fetal rat metatarsal bones (Paper III). We show that chondrocytes can synthesize estrogen by themselves and thereby mimic the effect of exogenous estrogen treatment. We also show that locally produced estrogen stimulates chondrocyte proliferation, inhibits apoptosis and maintains bone growth.

Selective estrogen receptor modulators (SERMs) are promising drugs to modulate longitudinal growth offering fewer side-effects. We found that tamoxifen, a well known SERM, diminishes bone growth potential in cultured fetal rat metatarsal bones through selective apoptotic elimination of stem-like chondrocytes (Paper IV). Tamoxifen triggers the Fas/FasL apoptotic pathway and activates the caspases-8, -9 and 3.

This thesis increases our understanding of how estrogen exerts its effect on longitudinal bone growth. This knowledge may be valuable for the development of new treatment strategies in children with growth disturbances, e.g. using SERMs to modulate the timing of growth plate fusion and thereby inhibit or stimulate the remaining growth potential.