Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
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398 COUSE AND KORACH Vol. 20, No. 3<br />
bisphosphates, and estrogens (435). The obvious beneficial<br />
effects of estrogen replacement therapy have evoked an intense<br />
research effort for bone-specific estrogen agonists that<br />
lack the potentially harmful side effects in the breast and<br />
reproductive tissues. Ironically, the currently available “selective<br />
ER modulators” or SERMs, as these drugs have come<br />
to be termed, have been selected from the pool of nonsteroidal<br />
estrogen antagonists (reviewed in Refs. 8, 434, and<br />
436).<br />
For several years it was believed that the effects of estrogens<br />
on bone physiology were indirect, inferred from the<br />
inability to detect ER within bone and bone cell cultures.<br />
However, in 1988, Komm et al. (437) and Eriksen et al. (438)<br />
simultaneously reported the detection of high-affinity, competable<br />
estradiol binding, ER� mRNA, and the induction of<br />
estrogen-responsive genes in cultured rat and human osteoblast-like<br />
cells, the bone-forming cell. We have reported similar<br />
findings, including the inhibition of estradiol transactivational<br />
activity with antiestrogens, in two separate<br />
osteoblast-like cell lines from the rat (439). Oursler et al. (440)<br />
have since demonstrated ER� and estrogenic activity in cultured<br />
avian osteoclasts, the bone-resorbing cell type, including<br />
estrogen induction of the genes for c-fos and c-jun. Using<br />
in situ RT-PCR analysis, Hoyland et al. (441) demonstrated<br />
the presence of ER� mRNA in both osteoblasts and osteoclasts<br />
in bone grafts from human females. Immunocytochemical<br />
methods have also been used to demonstrate the<br />
presence of ER� in multiple bone cell lines (442). Recently,<br />
Bodine et al. (443) reported significant increases in the levels<br />
of ER� transcripts during dexamethasone-induced differentiation<br />
of rat osteoblasts in vitro. Therefore, there is adequate<br />
experimental evidence to support the presence of a direct<br />
ER�-mediated estrogen-signaling pathway in bone.<br />
The discovery of the ER� introduced renewed vigor in the<br />
search for SERMs, allowing for the greater possibility of<br />
finding a receptor-selective agonist. The distinct expression<br />
pattern of the two ERs among various tissues has further<br />
enhanced the possibility of finding tissue-specific SERMs.<br />
Several recent studies have reported the detection of ER� in<br />
bone cells. Onoe et al. (444) employed RT-PCR to demonstrate<br />
the presence of both ER� and ER� mRNA in immortalized<br />
as well as primary osteoblast cell cultures from the rat.<br />
Similar to reports of ER�, both Onoe et al. (444) and Arts et<br />
al. (95) report significant increases in ER� mRNA levels during<br />
in vitro dexamethasone-induced differentiation of osteoblasts<br />
derived from the rat and human, respectively. Therefore,<br />
the generation of mice lacking ER� or ER� will once<br />
again prove invaluable in delineating the roles of the two<br />
receptors in bone physiology.<br />
The majority of animal studies concerning the role of estrogens<br />
in bone morphology and metabolism have been carried<br />
out in the rat (reviewed in Ref. 445). Ovariectomy in the<br />
rodent results in increased bone turnover similar to that seen<br />
in postmenopausal women; however, the mechanisms of<br />
action may differ between the species (445). The effects of<br />
ovariectomy in the rat include decreases in bone mineral<br />
density, cancellous bone area, and bone strength, whereas<br />
increases are observed in radial and longitudinal growth,<br />
osteoblast and osteoclast activity, and overall rates of bone<br />
turnover (445). <strong>Estrogen</strong> replacement, including those com-<br />
pounds with mixed agonist/antagonist activity, has been<br />
shown to reverse several of the effects induced by ovariectomy<br />
(445). However, the extent and direction of the changes<br />
induced by ovariectomy, as well as the protection provided<br />
by estrogen replacement, vary depending on the bone parameter,<br />
sex, and type of bone being evaluated, e.g., femur,<br />
tibia, calvaria, or vertebrae (445). Interestingly, a study of the<br />
androgen-resistant Tfm rat describes a bone phenotype similar<br />
to a wild-type female, i.e., shorter and thinner femurs,<br />
indicating that androgen action may also be critical to longitudinal<br />
and radial bone growth in the male rat (446). However,<br />
endogenous gonadal estrogens were able to maintain<br />
a normal cancellous bone mass in the Tfm rat (446). It is<br />
noteworthy that the first description of a human case of<br />
estrogen insensitivity due to a spontaneous mutation of the<br />
ER� gene exhibits severe osteoporosis as well as significant<br />
increases in longitudinal growth of bones (see Section VIII.C)<br />
(116).<br />
Unfortunately, few studies of the effects of steroids on<br />
bone physiology have been carried out in the mouse. Analysis<br />
of femoral bone length in �ERKO mice indicates a significant<br />
decrease in length and diameter in females and a<br />
slight decrease in males, when compared with age- and sexmatched<br />
wild-type controls (447). However, measurements<br />
of bone density and mineral content indicate the opposite<br />
effect, i.e., �ERKO males exhibited significant decreases<br />
throughout the femur (448), whereas the �ERKO females<br />
demonstrate just slight and localized decreases (447). In<br />
agreement with the ovariectomized rat model, �ERKO female<br />
mice exhibit increased bone resorption-remodeling<br />
rates (448). However, the decreased femur length observed<br />
in the �ERKO is in contrast to that reported in the ovariectomized<br />
rat and the ER�-deficient human male. Interestingly,<br />
a series of studies by Migliaccio et al. (449, 450) illustrated<br />
that prenatal and neonatal exposure to the synthetic<br />
estrogen, DES, also results in significantly shorter femur<br />
lengths as well as increased cortical bone thickness and increased<br />
trabecular bone at the epiphysis of the femur during<br />
adulthood in female mice. Therefore, it appears that in the<br />
mouse, aberrant estrogen exposure during development or<br />
a hereditary loss of ER� action leads to decreased longitudinal<br />
bone growth, contrasting experimental schemes resulting<br />
in a similar phenotype.<br />
These data indicate that pathways other than ER� may<br />
mediate the negative regulatory effects of estradiol on bone<br />
growth, suggesting a possible role for ER�. Longitudinal<br />
bone growth is a poorly understood process that depends on<br />
chondrocyte activity, including proliferation, hypertrophy,<br />
and the secretion of extracellular matrix at the growth plate<br />
(445). <strong>Estrogen</strong> is thought to slow this process by reducing<br />
the recruitment, proliferation, and synthetic activity of chondrocytes,<br />
thereby resulting in a maturation of the epiphyseal<br />
plate and inhibition of further longitudinal growth (445). The<br />
detection of both ER� (451) and ER� (452) in human epiphyseal<br />
chondrocytes has recently been described. Therefore, it<br />
is possible that ER�, in the context of significantly elevated<br />
levels of estradiol, results in an inhibition of long bone<br />
growth in the �ERKO mouse. It is also possible that the<br />
significantly elevated levels of serum androgens in the<br />
�ERKO female may be playing an influential role. This may