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

Abstract

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.

Figure 1

DXA measurements of bone parameters of excised bones. The area BMD (a and d), bone area (b and e), and BMC (c and f) of excised tibiae (a–c) and vertebrae L₅ (d–f) were measured using DXA as described in Methods. Four-week-old mice are indicated as prepubertal mice (female WT and ERβ^–/– mice [n = 6]; male WT and ERβ^–/– mice [n = 7]). Eleven-week-old mice are indicated as adult mice (female WT and ERβ^–/– mice [n =7]; male WT and ERβ^–/– mice [n = 5]). Values are given as mean ± SEM. *P < 0.05, **P < 0.01 vs. WT.

Figure 2

DXA scan of femur (top), tibia (middle), and vertebrae (bottom) from ERβ^–/– and WT mice. The relative areal BMD is indicated: H = high density, L = low density.

Figure 3

Expression of osteoblast- and osteoclast-associated genes in long bones of prepubertal (4 weeks old) and adult (11 weeks old) male (M) and female (F) wild-type (W) mice and ERβ^–/– knockout (K) mice by RT-PCR. Osteoblast markers: (a) Col. I, (b) ALP, and (c) OC. Osteoclast markers: (d) TRAP, (e) Cath. K, and (f) GAPDH (the control).