Estrogen receptor specificity in the regulation of skeletal growth and maturation in male mice

Androgens and bone

Loss of estrogens or androgens increases the rate of bone remodeling by removing restraining effects on osteoblastogenesis and osteoclastogenesis, and also causes a focal imbalance between resorption and formation by prolonging the lifespan of osteoclasts and shortening the lifespan of osteoblasts. Conversely, androgens, as well as estrogens, maintain cancellous bone mass and integrity, regardless of age or sex. Although androgens, via the androgen receptor (AR), and estrogens, via the estrogen receptors (ERs), can exert these effects, their relative contribution remains uncertain. Recent studies suggest that androgen action on cancellous bone depends on (local) aromatization of androgens into estrogens. However, at least in rodents, androgen action on cancellous bone can be directly mediated via AR activation, even in the absence of ERs. Androgens also increase cortical bone size via stimulation of both longitudinal and radial growth. First, androgens, like estrogens, have a biphasic effect on endochondral bone formation: at the start of puberty, sex steroids stimulate endochondral bone formation, whereas they induce epiphyseal closure at the end of puberty. Androgen action on the growth plate is, however, clearly mediated via aromatization in estrogens and interaction with ERalpha. Androgens increase radial growth, whereas estrogens decrease periosteal bone formation. This effect of androgens may be important because bone strength in males seems to be determined by relatively higher periosteal bone formation and, therefore, greater bone dimensions, relative to muscle mass at older age. Experiments in mice again suggest that both the AR and ERalpha pathways are involved in androgen action on radial bone growth. ERbeta may mediate growth-limiting effects of estrogens in the female but does not seem to be involved in the regulation of bone size in males. In conclusion, androgens may protect men against osteoporosis via maintenance of cancellous bone mass and expansion of cortical bone. Such androgen action on bone is mediated by the AR and ERalpha.

Osteoblast-like cells from estrogen receptor alpha knockout mice have deficient responses to mechanical strain

INTRODUCTION

In vivo, bones' osteogenic response to mechanical loading involves proliferation of surface osteoblasts. This response is replicated in vitro and involves ERK-mediated activation of the estrogen receptor (ER) alpha and upregulation of estrogen response element activity. This proliferative response can be blocked by selective estrogen receptor modulators and increased by transfection of additional ERalpha.

MATERIALS AND METHODS

We have now investigated the mechanisms of ER involvement in osteoblast-like cells' early responses to strain by comparing the responses of primary cultures of these cells derived from homozygous ERalpha knockout (ERKO) mice (ERalpha-/-) with those from their wildtype (ERalpha+/+) and heterozygous (ERalpha+/-) littermates and from ER/beta knockout (BERKO) mice (ERbeta+/+, ERbeta+/-, and ERbeta-/-).

RESULTS

Whereas ERalpha+/+, ERalpha+/-, ERbeta+/+, and ERbeta-/- cells proliferate in response to a single 10-minute period of cyclic strain, ERalpha-/- cells do not. Transfection of fully functional, but not mutant, ERalpha rescues the proliferative response to strain in these cells. The strain-related response of ERalpha-/- cells is also deficient in that they show no increased activity of an AP-I driven reporter vector and no strain-related increases in NO production. Their strain-related increase in prostacyclin production is retained. They proliferate in response to fibroblast growth factor-2 but not insulin-like growth factor (IGF)-I or IGF-II, showing the importance of ERalpha in the IGF axis and the ability of ERalpha-/- cells to proliferate normally in response to a mitogenic stimulus that does not require functional ERalpha.

CONCLUSIONS

These data indicate ERalpha's obligatory involvement in a number of early responses to mechanical strain in osteoblast-like cells, including those that result in proliferation. They support the hypothesis that reduction in ERalpha expression or activity after estrogen withdrawal results in a less osteogenic response to loading. This could be important in the etiology of postmenopausal osteoporosis.

Estrogen receptor beta polymorphisms are associated with bone mass in women and men: the Framingham Study

ESR2 is expressed in bone cells, yet few studies have tested its variation for association with BMD, an important determinant of osteoporotic fractures. This was investigated in 723 men and 795 women from the Framingham study. Results show association of variation in this gene with BMD in both women and men.

INTRODUCTION

Osteoporotic fracture risk is highly dependent on bone density, a quantitative multifactorial trait with a substantial genetic component. In contrast to the growing body of evidence that estrogen receptor alpha (ESR1) plays a role in bone metabolism, few studies have examined the estrogen receptor beta (ESR2) gene for association with BMD. An ESR2 CA repeat polymorphism, D14S1026, was associated with BMD in two small studies, each with <200 women.

MATERIALS AND METHODS

The objective of this investigation was to assess whether D14S1026 or four other intronic polymorphisms were associated with BMD in 723 men and 795 women (mean age, 60 years) from the offspring cohort of the population-based Framingham Study. BMD was measured at the femur (neck, trochanter, and Ward's area) and the lumbar spine (L(2)-L(4)).

RESULTS

In both women and men, there was significant association of D14S1026 genotype with measures of femoral but not spinal BMD. In addition, genotypes of two common single nucleotide polymorphisms, rs1256031 and rs1256059, in strong linkage disequilibrium with one another but not with D14S1026, were associated with measures of femoral BMD in men. The rs1256031 genotypes had up to a 4.0% difference in mean femoral BMD. An inferred rs1256031-D14S1026-rs1256059 haplotype C-23CA-T was significantly associated with reduced femoral BMD in women (p = 0.03, 0.003, and 0.01 for neck, trochanter, and Ward's area, respectively). Haplotype-based BMD differences ranged from 3.0% to 4.3%.

CONCLUSIONS

We have observed significant association of common ESR2 variants with measures of femoral BMD in both men and women.

Prenatal exposure to IL-1beta results in disturbed skeletal growth in adult rat offspring

Events occurring early in life or prenatally are able to play important roles in the pathogenesis of diseases in adult life. Different sorts of stress or hormonal influences, during particular periods of pregnancy, may result in persistent or transient changes in physiology. IL-1 is a multifunctional cytokine that is involved in bone metabolism. The aim of the present study was to investigate whether exposure to IL-1beta during fetal life has any effect on skeletal growth or bone mineral density in adult rat offspring. Pregnant rats were given intraperitoneal injections of IL-1beta, 1 microg/rat, or saline on days 8, 10, and 12 of gestation. Male IL-1-exposed offspring showed reduced height, areal bone mineral density, and bone mineral content at vertebra L5. Tibial length was reduced in both male and female offspring. Peripheral quantitative computed tomography analyses revealed reduced cortical bone mineral content caused by a decreased cortical cross-sectional area as a result of a decreased cortical thickness, whereas there was no reduction in the amount of trabecular bone in the tibia of male offspring. Our results demonstrate that prenatal exposure to IL-1 can induce specific programming of skeletal tissue. In conclusion, prenatal IL-1 exposure results in decreased skeletal growth and a reduced amount of cortical bone but unchanged trabecular bone mineral density in adult rat offspring.

Estrogen receptor-beta inhibits skeletal growth and has the capacity to mediate growth plate fusion in female mice

To determine the long-term role of ER beta in the regulation of longitudinal bone growth, appendicular and axial skeletal growth was followed and compared in female ER beta-/-, ER alpha-/-, and ER alpha-/- beta-/- mice. Our results show that ER beta inhibits appendicular and axial skeletal growth and has the capacity to induce fusion of the growth plates.

INTRODUCTION

Estrogen affects skeletal growth and promotes growth plate fusion in humans. In rodents, the growth plates do not fuse after sexual maturation, but prolonged treatment with supraphysiological levels of estradiol has the capacity to fuse the growth plates. It should be emphasized that the estrogen receptor (ER) alpha-/- and the ER alpha-/- beta-/-, but not the ER beta-/-, mouse models have clearly increased serum levels of estradiol.

MATERIALS AND METHODS

The skeletal growth was monitored by X-ray and dynamic histomorphometry, and the growth plates were analyzed by quantitative histology, calcein double labeling, bromodeoxyuridine (BrdU) incorporation, and TUNEL assay in 4- and 18-month-old female ER beta-/-, ER alpha-/-, and ER alpha-/- beta-/- mice.

RESULTS

Young adult (4-month-old) ER beta-/- mice demonstrated an increased axial- and appendicular-skeletal growth, supporting the notion that ER beta inhibits skeletal growth in young adult female mice. Interestingly, the growth plates were consistently fused in the appendicular skeleton of 18-month-old female ER alpha-/- mice. This fusion of growth plates, caused by a prolonged exposure to supraphysiological levels of estradiol in female ER alpha-/- mice, must be mediated through ER beta because old ER alpah-/- beta-/- mice displayed unchanged, unfused growth plates.

CONCLUSIONS

Our results confirm that ER beta is a physiological inhibitor of appendicular- and axial-skeletal growth in young adult female mice. Furthermore, we made the novel observation that ER beta, after prolonged supraphysiological estradiol exposure, has the capacity to mediate growth plate fusion in old female mice.

Estrogen Receptor Knockout Mice as a Model for Endocrine Research

The biological effects of estrogen in mammalian target tissues are important for multiple organ systems including the male and female reproductive tract and the neuroendocrine, skeletal, and cardiovascular systems. Numerous physiological effects of estradiol are modulated by the estrogen receptor (ER), a Class I member of the nuclear receptor superfamily. However, more recent studies have also implicated nongenomic effects of estrogen, which may involve a membrane-binding site. The two forms of the ER are the classical estrogen receptor-alpha (ERalpha) and the more recently discovered estrogen receptor-beta (ERbeta). Gene-targeting techniques were used to generate mice lacking either functional ERalpha (alphaERKO), ERbeta (betaERKO), or both ERs (alphabetaERKO) to provide a model for evaluating estrogen receptor action. These knockout models provide a unique tool to study the effects of estrogen in the context of the whole animal and to discern the role of each ER in various tissues. The reproductive phenotypes as well as some of the nonreproductive phenotypes of the different ERKO models are summarized.

Systemic and local regulation of the growth plate

The growth plate is the final target organ for longitudinal growth and results from chondrocyte proliferation and differentiation. During the first year of life, longitudinal growth rates are high, followed by a decade of modest longitudinal growth. The age at onset of puberty and the growth rate during the pubertal growth spurt (which occurs under the influence of estrogens and GH) contribute to sex difference in final height between boys and girls. At the end of puberty, growth plates fuse, thereby ceasing longitudinal growth. It has been recognized that receptors for many hormones such as estrogen, GH, and glucocorticoids are present in or on growth plate chondrocytes, suggesting that these hormones may influence processes in the growth plate directly. Moreover, many growth factors, i.e., IGF-I, Indian hedgehog, PTHrP, fibroblast growth factors, bone morphogenetic proteins, and vascular endothelial growth factor, are now considered as crucial regulators of chondrocyte proliferation and differentiation. In this review, we present an update on the present perception of growth plate function and the regulation of chondrocyte proliferation and differentiation by systemic and local regulators of which most are now related to human growth disorders.

Estren Is a Selective Estrogen Receptor Modulator with Transcriptional Activity

It was recently reported that the synthetic compound estren increases bone mass without affecting reproductive organs or classic transcription. The aim of the present study was to further characterize the in vivo and in vitro effects of estren. We demonstrate that estren is a selective estrogen receptor modulator (SERM) with a strong effect on thymus, a moderate effect on uterus and trabecular bone, but no major effect on fat or cortical bone in 11-month-old ovariectomized mice. The effect of estren on trabecular bone and uterus is mediated via estrogen receptors (ERs) because no effect is seen in ER double-inactivated mice. Furthermore, with the use of ERalpha- and ERbeta-expressing reporter cell lines, we demonstrate that estren displays an agonistic effect on transcriptional activity of an estrogen-responsive element-driven reporter gene with a degree of agonism similar to that of 17beta-estradiol for both ERalpha and ERbeta. Thus, estren has the capacity to exert genomic effects via both ERalpha and ERbeta. We conclude, in contrast to what was previously reported by others, that estren is a SERM with transcriptional activity.

Longitudinal association between sex hormone levels, bone loss, and bone turnover in elderly men

Male osteoporosis is an increasingly important health problem. It is known that sex steroid hormones play an important role in regulating bone turnover and bone mass in males as well as in females. However, the exact mechanism of bone loss in men remains unknown. In the present study, 200 elderly men (age range, 55-85 yr) were followed for 4 yr to evaluate the relationships between hormone levels, bone turnover markers, bone mineral density, and rates of bone loss. Femoral and lumbar bone mineral density, bone ultrasound parameters at the os calcis, serum testosterone (T), serum estradiol (E(2)), SHBG levels, and bone turnover markers (urinary crosslaps and bone alkaline phosphatase) were evaluated for each man at enrollment and 4 yr afterward. The free androgen index (FAI) and free estrogen index (FEI) as well as measures of the bioavailable sex hormones [calculated bioavailable E(2) (c-bioE(2)) and T (c-bioT)] were calculated from total hormone levels and SHBG. In the total population, T, c-bioT, c-bioE(2), FAI, and FEI, but not E(2), decreased significantly with age, whereas SHBG increased significantly. Subjects with FEI, c-bioE(2), and E(2) levels below the median showed higher rates of bone loss at the lumbar spine and the femoral neck as well as higher speed-of-sounds decrease at the calcaneus with respect to men with FEI, c-bioE(2), and E(2) levels above the median. Serum bone alkaline phosphatase and urinary crosslaps were significantly higher in men with FEI, c-bioE(2), and E(2) in the lower quartile than in men with FEI, c-bioE(2), and E(2) levels in the higher quartile. No statistically significant differences were observed in relation to T, c-bioT, or FAI levels. Finally, the ratio between E(2) and T, an indirect measure for aromatase activity, increased significantly with age and was higher in normal than in osteoporotic subjects. In conclusion, results from the present study indicate an important role of estrogens, and particularly of the ability to aromatize T to E(2), in the regulation of bone loss and bone metabolism in elderly men.

Differential effects on bone of estrogen receptor alpha and androgen receptor activation in orchidectomized adult male mice

Androgens may regulate the male skeleton either directly by stimulation of the androgen receptor (AR) or indirectly by aromatization of androgens into estrogens and, thereafter, by stimulation of the estrogen receptors (ERs). To directly compare the effect of ER activation on bone in vivo with the effect of AR activation, 9-month-old orchidectomized wild-type and ER-inactivated mice were treated with the nonaromatizable androgen 5alpha-dihydrotestosterone, 17beta-estradiol, or vehicle. Both ERalpha and AR but not ERbeta activation preserved the amount of trabecular bone. ERalpha activation resulted both in a preserved thickness and number of trabeculae. In contrast, AR activation exclusively preserved the number of trabeculae, whereas the thickness of the trabeculae was unaffected. Furthermore, the effects of 17beta-estradiol could not be mediated by the AR, and the effects of 5alpha-dihydrotestosterone were increased rather than decreased in ER-inactivated mice. ERalpha, but not AR or ERbeta, activation resulted in preserved thickness, volumetric density, and mechanical strength of the cortical bone. ERalpha activation increased serum levels of insulin-like growth factor I, which were positively correlated with all the cortical and trabecular bone parameters that were specifically preserved by ERalpha activation but not by AR activation, suggesting that insulin-like growth factor I might mediate these effects of ERalpha activation. Thus, the in vivo bone-sparing effect of ERalpha activation is distinct from the bone-sparing effect of AR activation in adult male mice. Because these two pathways are clearly distinct from each other, one may speculate that a combined treatment of selective ER modulators and selective AR modulators might be beneficial in the treatment of osteoporosis.

Specific regulation of lipocalin-type prostaglandin D synthase in mouse heart by estrogen receptor beta

Estrogens have important physiological roles in the cardiovascular system. We use DNA microarray technology to study the molecular mechanism of estrogen action in the heart and to identify novel estrogen-regulated genes. In this investigation we identify genes that are regulated by chronic estrogen treatment of mouse heart. We present our detailed characterization of one of these genes, lipocalin-type prostaglandin D synthase (L-PGDS). Northern and Western blot analysis revealed that L-PGDS was induced both by acute and chronic estrogen treatment. Northern blot analysis, using estrogen receptor (ER)-disrupted mice, suggests that L-PGDS is specifically induced by ERbeta in vivo. In further support of ERbeta-selective regulation, we identify a functional estrogen-responsive element in the L-PGDS promoter, the activity of which is up-regulated by ERbeta, but not by ERalpha. We demonstrate that a one-nucleotide change (A to C) in the L-PGDS estrogen-responsive element affects receptor selectivity.

Influence of oestrogen receptor alpha and beta on the immune system in aged female mice

Oestrogen has a dichotomous effect on the immune system. T and B lymphopoiesis in thymus and bone marrow is suppressed, whereas antibody production is stimulated by oestrogen. In this study the importance of the oestrogen receptors (ER) ER-alpha and ER-beta in the aged immune system was investigated in 18 months old-wild type (WT), ER-alpha (ERKO), ER-beta (BERKO) and double ER-alpha and ER-beta (DERKO) knock-out mice, and compared with 4 months old WT mice. Cell phenotypes in bone marrow, spleen and thymus, and the frequency of immunoglobulin (Ig) spot forming cells (SFC) were determined. We show here that the 17-beta-oestradiol (E2)-induced downregulation of B lymphopoietic cells in bone marrow of young ovariectomized mice can be mediated through both ER-alpha and ER-beta. However, only ER-alpha is required for the age-related increased frequency of immunoglobulin M (IgM) SFC in the bone marrow, as well as for the increased production of interleukin-10 (IL-10) from cultured splenocytes in aged mice. Furthermore, increased age in WT mice resulted in lower levels of both pro- and pre-B cells but increased frequency of IgM SFC in the bone marrow, as well as increased frequency of both IgM and IgA SFC in the spleen. Results from this study provide valuable information regarding the specific functions of ER-alpha and ER-beta in the aged immune system.

Suppressive function of androgen receptor in bone resorption

As locally converted estrogen from testicular testosterone contributes to apparent androgen activity, the physiological significance of androgen receptor (AR) function in the beneficial effects of androgens on skeletal tissues has remained unclear. We show here that inactivation of AR in mice using a Cre-loxP system-mediated gene-targeting technique caused bone loss in males but not in females. Histomorphometric analyses of 8-week-old male AR knockout (ARKO) mice showed high bone turnover with increased bone resorption that resulted in reduced trabecular and cortical bone mass without affecting bone shape. Bone loss in orchidectomized male ARKO mice was only partially prevented by treatment with aromatizable testosterone. Analysis of primary osteoblasts and osteoclasts from ARKO mice revealed that AR function was required for the suppressive effects of androgens on osteoclastogenesis supporting activity of osteoblasts but not on osteoclasts. Furthermore, expression of the receptor activator of NF-kappaB ligand (RANKL) gene, which encodes a major osteoclastogenesis inducer, was found to be up-regulated in osteoblasts from AR-deficient mice. Our results indicate that AR function is indispensable for male-type bone formation and remodeling.

Molecular mechanisms underlying osteoclast formation and activation

Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a progressive loss of total bone mass and bone density. Bone loss in osteoporosis is due to the persistent excess of osteoclastic bone resorption over osteoblastic bone formation. Receptor activator of NFkappaB ligand (RANKL) critically regulates both osteoclast differentiation and activation. TRAFs appear to be central coupling molecules in the signal transduction pathways that regulate osteoclastogenesis, cathepsin K is the major mediator of osteoclastic bone resorption, and sex steroids and aging also affect osteoclastogenesis and osteoclast activity. However, bone homeostasis depends upon the intimate coupling of bone formation and bone resorption, wherein both osteoclasts and osteoblasts exert vital stimulatory and inhibitory effects upon each other via molecules such as RANKL, TGFbeta, PDGF, BMP2, and Mim-1. This review will highlight some of the major features of the complex circuit of cytokines, growth factors, and hormones that underlies the formation and function of osteoclasts and the dynamic equilibrium that marks the interaction between osteoclasts and osteoblasts.

Effects of SP500263, a novel selective estrogen receptor modulator, on bone, uterus, and serum cholesterol in the ovariectomized rat

We describe here the activity of a novel selective estrogen receptor modulator, SP500263. When given to adult ovariectomized (OVX) rats for 28 days at doses of 0.3, 1, or 3 mg/kg/day, we found that SP500263 partially protected against OVX-induced loss of bone mineral content in the distal ends of femurs and in the whole bone. SP500263 also antagonized the OVX-induced increase in body weight. However, unlike 17beta-estradiol, SP500263 at efficacious doses did not prevent the OVX-induced loss in uterine wet weight. A small but significant effect on uterine wet weight was noted with raloxifene dosed at 1 mg/kg. As expected, SP500263 but not raloxifene acted as an estrogen antagonist on the uterus in adult rats when administered for 7 days at 30 mg/kg/day. Finally, SP500263 had no statistically significant effects on total serum cholesterol and serum triglycerides in OVX rats treated for 28 days. Raloxifene had no significant effects on body weight, bone mineral content, and serum cholesterol or triglycerides in the OVX-rat model. In summary, SP500263 is a new orally active SERM that acts in rats as an estrogen agonist on bone without causing uterine stimulatory effects.

Estrogen receptor-alpha dependency of estrogen's stimulatory action on cancellous bone formation in male mice

We examined whether estrogen receptor (ER)alpha is required for estrogen to stimulate cancellous bone formation in long bones of male mice. 17 beta-Estradiol (E(2)) was administered to ER alpha(-/-) male mice or wild-type (WT) littermate controls at 40, 400, or 4000 microg/kg by daily sc injection for 28 d and histomorphometric analysis performed at the distal femoral metaphysis. In WT mice, treatment with E(2) (40 microg/kg per d) increased the proportion of cancellous bone surfaces undergoing mineralization and stimulated mineral apposition rate. In addition, higher doses of E(2) induced the formation of new cancellous bone formation surfaces in WT mice. In contrast, E(2) had little effect on any of these parameters in ER alpha(-/-) mice. Immunohistochemistry was subsequently performed using an ER alpha-specific C-terminal polyclonal antibody. In WT mice, ER alpha was expressed both by cancellous osteoblasts and a significant proportion of mononuclear bone marrow cells. Immunoreactivity was also observed in cancellous osteoblasts of ER alpha(-/-) mice, resulting from expression of the activation function-1-deficient 46-kDa ER alpha isoform previously reported to be expressed in normal osteoblasts and bones of ER alpha(-/-) mice. Taken together, our results suggest that estrogen stimulates bone formation in mouse long bones via a mechanism that requires the presence of full-length ER alpha possessing activation function-1.

Evidence for cell-specific changes with age in expression of oestrogen receptor (ER) alpha and beta in bone fractures from men and women

Oestrogen is recognized as important for maintaining bone mass in men and women. Oestrogen receptor (ER) alpha and the recently described ER-beta are both expressed in bone cells, but have different affinities for oestrogen agonists and plant oestrogens, which could be important in developing treatments for bone loss in both men and women. It is unclear, however, which isoform predominates in bone; cell type and age may influence their relative expression. The present study has compared ER-alpha and ER-beta expression in serial sections of human fracture callus from males (n = 19, age range 5-72 years) and females (n = 15, age range 3-86 years) by indirect immunoperoxidase. Fracture callus was used as it can be readily obtained from individuals over a wide age range and contains a variety of bone cells. Antibody specificity was confirmed by western blotting and comparison of immunoreactivity in sections of breast tumour and benign prostate hyperplasia. No gender difference in ER expression was found in bone from individuals less than 40 years old. Proliferative chondrocytes were positive for both isoforms, but few larger hypertrophic cells were immunoreactive. ER-alpha and ER-beta were co-expressed in osteoclasts, suggesting that oestrogen may act directly on these cells. Osteoblasts, osteocytes, and mesenchymal cells also expressed both isoforms. In women over 40 years of age, however, relatively fewer biopsies contained osteocytes positive for ER-alpha and ER-beta. Likewise, the proportions of osteoblasts and mesenchymal cells expressing ER-beta were reduced but ER-alpha remained unaffected. In contrast, in men over 40 years, only the proportion of biopsies containing ER-beta-positive mesenchymal cells was lower. In these older men and women, ER-alpha and ER-beta expression was retained by the small proliferative chondrocytes. These results demonstrate that gender, age, and cell type are important determinants of ER isoform expression in skeletal cells.

A functional androgen receptor is not sufficient to allow estradiol to protect bone after gonadectomy in estradiol receptor-deficient mice

Although the role of estradiol in maintaining bone mass is well established, the relative contributions of the estradiol receptors ERalpha and ERbeta and of the androgen receptor (AR) remain controversial. To determine the role of ERalpha-mediated, ERbeta-mediated, and non-ER-mediated mechanisms in maintaining bone mass, gonadectomy and estradiol treatment were studied in ER-knockout mice. Estradiol treatment of ovariectomized ERalphabeta(-/-) mice failed to prevent bone loss, precluding significant effects of estradiol on bone through non-ER-signaling pathways. In contrast, estradiol prevented ovariectomy-induced bone loss in ERbeta(-/-) mice, as in WT males and females, indicating that ERalpha is the major mediator of estradiol effects in bone. No response of bone to estradiol was detected in orchidectomized ERalpha(-/-) mice, suggesting estradiol cannot protect bone mass via the AR in vivo. In contrast to female ERalphabeta(-/-) and male ERalpha(-/-) mice, female ERalpha(-/-) mice were partially protected against ovariectomy-induced bone loss by estradiol, confirming that ERbeta mediates estradiol effects in bone, but only in females and with a lower efficacy than ERalpha. We conclude that ERalpha is the main effector of estradiol's protective function in bone in both male and female mice, and that, in its absence, AR is not sufficient to mediate this response.

Men, bone and estrogen: unresolved issues

Gonadal function has long been known to be important for skeletal health in men. Prepubertal hypogonadism is clearly associated with impairment in peak bone mass development and adult-onset hypogonadism with accelerated bone loss. Gonadal failure results in deficits in both androgen and estrogen action, but traditionally androgens were assumed to have the most important skeletal effect in men. Recently that model has been reconsidered as a variety of kinds of evidence have appeared to document a critical role for estrogen in bone physiology. As a result of this fresh perspective a host of interesting new dilemmas and hypotheses are being examined, including those related to the mechanisms of sex steroid action in bone, the origins of gender differences in skeletal morphology and physiology, and the role of estrogen in diagnostic and therapeutic strategies in men with metabolic bone disorders.

Oestrogen receptor beta is the predominant oestrogen receptor in human scalp skin

Oestrogens play a major role in non-classic target tissues in both sexes, yet there have been few studies on estrogens and skin. Recently a second oestrogen receptor (ERbeta) has been discovered. Therefore, we have compared the expression of oestrogen receptor alpha (ERalpha), beta (ERbeta), the androgen receptor (AR) and a cell proliferation marker in male and female non-balding scalp skin. ERbeta was the major steroid receptor expressed in human skin. It was highly expressed in epidermis, blood vessels and dermal fibroblasts, in contrast to ERalpha and AR. In the hair follicle, ERbeta expression was localized to nuclei of outer root sheath, epithelial matrix and dermal papilla cells, in contrast to ERalpha, and the AR, which was only expressed in dermal papilla cells. Serial sections also showed strong nuclear expression of ERbeta in the cells of the bulge, while neither ERalpha nor AR was expressed. In the sebaceous gland, ERbeta was expressed in both basal and partially differentiated sebocytes. ERalpha exhibited a similar pattern of expression, while the AR was expressed in the basal and very early differentiated sebocytes. There was no obvious difference in the expression of either oestrogen receptor in male or female skin. The wide distribution of ERbeta in human skin suggests that oestrogens may play an important role in the maintenance of skin and in the regulation of the pilosebaceous unit, and provides further evidence for oestrogen action in non-classic target tissues. The differential expression of ERalpha, ERbeta and AR in human skin suggests that the mechanisms by which steroid hormones mediate their effects may be more complex than previously thought.

The role of estrogens in men and androgens in women

The past years several have witnessed a significant transformation in our understanding of sex steroid action in the male and female skeleton. Data from animal and human studies indicate that sex steroids have important skeletal effects in both genders. It seems from the in vivo human data that estrogen is likely more potent than testosterone in inhibiting bone resorption. Estrogen and testosterone appear to be important for maintaining bone formation. In addition, androgens clearly enhance bone size, likely through effects on periosteal bone formation. How much of this gender cross-talk at the physiological level is caused by "promiscuous" actions of sex steroids at the molecular level, with estrogen acting by way of the androgen receptor (and androgens via the estrogen receptor) is an interesting and important question, the answer to which may well provide additional surprises.

Oestrogen receptor specificity in oestradiol-mediated effects on B lymphopoiesis and immunoglobulin production in male mice

Oestrogen treatment down-regulates B lymphopoiesis in the bone marrow of mice. Meanwhile it up-regulates immunoglobulin production. To understand better the oestrogen action on bone marrow male mice lacking oestrogen receptor alpha (ERalpha; ERKO mice), lacking ERbeta (BERKO mice), lacking both receptors (DERKO mice) or wild-type (wt) littermates were castrated and treated for 2.5 weeks with 30 microg/kg 17beta-oestradiol (E2) or vehicle oil as controls. The B lymphopoiesis in the bone marrow was examined by flow cytometry and mature B-cell function was studied using an ELISPOT assay enumerating the B cells in bone marrow and spleen that were actively producing immunoglobulins. In wt mice the frequency of B-lymphopoietic (B220+) cells in the bone marrow decreased from 15% to 5% upon E2 treatment. In ERKO and BERKO mice significant reduction was seen but not of the same magnitude. In DERKO mice no reduction of B lymphopoiesis was seen. In addition, our results show that E2 mediated reduction of different steps in B lymphopoiesis require only ERalpha or both receptors. In wt and BERKO mice E2 treatment resulted in significantly increased levels of B cells actively producing immunoglobulin, while in ERKO and DERKO mice no such change was seen. Similar results were found in both bone marrow and spleen. In conclusion our results clearly show that both ERalpha and ERbeta are required for complete down-regulation of B lymphopoiesis while only ERalpha is needed to up-regulate immunoglobulin production in both bone marrow and spleen.

Peripheral quantitative computed tomography for the detection of diabetic osteopathy: a study in the Goto-Kakizaki rat

RATIONALE AND OBJECTIVE

To assess the utility of dual energy x-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT) in detecting trabecular and cortical bone changes in diabetes as a model of osteopenia.

MATERIALS AND METHODS

The tibia from 10 type-2 diabetic Goto-Kakizaki (GK) rats and 10 control Wistar rats were analyzed by DEXA, pQCT, and ash weight determination.

RESULTS

DEXA of GK rats showed a significant reduction in mineral content (32%) and density (24%) of the metaphysis, but not of the diaphysis. PQCT disclosed that the reduction of density predominantly pertained to the trabecular bone (reduced by 62%). Periosteal and endosteal circumferences of the diaphyses were increased and cortical thickness was unchanged leading to increased moment of inertia.

CONCLUSIONS

This study suggests that in osteopathic conditions, cortical and trabecular bone should be separately examined within specific subregions to obtain relevant information. Loss of metaphyseal trabecular bone seems to be a predominant feature in diabetic rats. Moreover, there is increased moment of inertia in the diaphysis implying increased strength. These diagnostic features of diabetic osteopathy can only be assessed by pQCT. It may prove that similar changes occur in human type-2 diabetes, which could explain the susceptibility to periarticular fracture and Charcot arthropathy.

Estrogen receptor-alpha and estrogen receptor-beta are present in the human growth plate in childhood and adolescence, in identical distribution

OBJECTIVE

To localize estrogen receptor-alpha (ER-alpha) and estrogen receptor-beta (ER-beta) within the growth plate and adjacent bony tissue of children in the prepubertal and pubertal age period.

METHODS

Tissue was taken during orthopedic surgery (epiphysiodesis) for correction of congenital or traumatic leg length difference in 2 prepubertal females and 2 adolescent males. Immunohistochemistry was performed on paraffin-embedded or cryostat sections by using commercially available rabbit polyclonal antibodies for ER-alpha and ER-beta.

RESULTS

Both ER-alpha and ER-beta were detected within the growth plate in all sections investigated. Immunostaining was restricted to hypertrophic chondrocytes. In the bony tissue adjacent to the growth plate, osteoblasts stained positive for both ER-alpha and ER-beta, whereas osteocytes and osteoclasts were negative. Staining with ER-alpha was mainly nuclear but some cells also showed cytoplasmic signals, while ER-beta staining was predominantly cytoplasmic, only few nuclei stained positive. There was no difference in the local distribution of both ERs between tissue from prepubertal and pubertal patients.

CONCLUSION

Our findings indicate that the hypertrophic chondrocyte is the main target cell for estrogen action within the growth plate. The presence of ER in prepubertal children suggests that estrogens play a role in skeletal maturation under physiological conditions also in this age-group.

Estrogen receptor (ER)-beta reduces ERalpha-regulated gene transcription, supporting a "ying yang" relationship between ERalpha and ERbeta in mice

Estrogen is of importance for the regulation of adult bone metabolism. The aim of the present study was to determine the role of estrogen receptor-beta (ERbeta) in vivo on global estrogen-regulated transcriptional activity in bone. The effect of estrogen in bone of ovariectomized mice was determined using microarray analysis including 9400 genes. Most of the genes (95% = 240 genes) that were increased by estrogen in wild-type (WT) mice were also increased by estrogen in ERbeta-inactivated mice. Interestingly, the average stimulatory effect of estrogen on the mRNA levels of these genes was 85% higher in ERbeta-inactivated than in WT mice, demonstrating that ERbeta reduces estrogen receptor-alpha (ERalpha)-regulated gene transcription in bone. The average stimulatory effect of estrogen on estrogen-regulated bone genes in ERalpha-inactivated mice was intermediate between that seen in WT and ERalphabeta double-inactivated mice. Thus, ERbeta inhibits ERalpha-mediated gene transcription in the presence of ERalpha, whereas, in the absence of ERalpha, it can partially replace ERalpha. In conclusion, our in vivo data indicate that an important physiological role of ERbeta is to modulate ERalpha-mediated gene transcription supporting a "Ying Yang" relationship between ERalpha and ERbeta in mice.

Knockouts model the 100 best-selling drugs--will they model the next 100?

The biopharmaceutical industry is currently faced with a tremendous number of potential drug targets identified through the sequencing of the human genome. The challenge ahead is to delineate those targets with the greatest value for therapeutic intervention. Here, we critically evaluate mouse-knockout technology for target discovery and validation. A retrospective evaluation of the knockout phenotypes for the targets of the 100 best-selling drugs indicates that these phenotypes correlate well with known drug efficacy, illuminating a productive path forward for discovering future drug targets. Prospective mining of the druggable genome is being catalysed by large-scale mouse knockout programs combined with phenotypic screens focused on identifying targets that modulate mammalian physiology in a therapeutically relevant manner.

Identification of estrogen-regulated genes of potential importance for the regulation of trabecular bone mineral density

Estrogen is of importance for the regulation of trabecular bone mineral density (BMD). The aim of this study was to search for possible mechanisms of action of estrogen on bone. Ovariectomized (OVX) mice were treated with 17beta-estradiol. Possible effects of estrogen on the expression of 125 different bone-related genes in humerus were analyzed using the microarray technique. Estrogen regulated 12 of these genes, namely, two growth factor-related genes, 8 cytokines, and 2 bone matrix-related genes. Five of the 12 genes are known to be estrogen-regulated, and the remaining 7 genes are novel estrogen-regulated genes. Seven genes, including interleukin-1 receptor antagonist (IL-1ra), IL-1receptor type II (IL-1RII), insulin-like growth factor-binding protein 4 (IGFBP-4), transforming growth factor beta (TGF-beta), granulocyte colony-stimulating factor receptor (G-CSFR), leukemia inhibitory factor receptor (LIFR), and soluble IL-4 receptor (sIL-4R) were selected as probable candidate genes for the trabecular bone-sparing effect of estrogen, as the mRNA levels of these genes were highly correlated (r2 > 0.65) to the trabecular BMD. The regulation of most of these seven genes was predominantly estrogen receptor alpha (ER-alpha)-mediated (5/7) while some genes (2/7) were regulated both via ER-alpha and ER-beta. In conclusion, by using the microarray technique, we have identified four previously known and three novel estrogen-regulated genes of potential importance for the trabecular bone-sparing effect of estrogen.

Evidence from the aged orchidectomized male rat model that 17beta-estradiol is a more effective bone-sparing and anabolic agent than 5alpha-dihydrotestosterone

This study was designed to evaluate the impact of estrogen versus androgen action on orchidectomy (ORX)-induced bone loss and associated changes in body composition. During an experimental period of 4 months, aged (12-month-old) ORX rats were treated with 17beta-estradiol (E2; 0.75 microg/day) or different doses of the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT; 45, 75, and 150 microg/day, respectively), via subcutaneous (sc) silastic implants. Low doses of DHT and E2 inhibited the ORX-induced rise of bone turnover markers (serum osteocalcin and urinary deoxypyridinoline [DPD]) to a similar extent. High-dose DHT prevented the ORX-induced decrease of trabecular bone density but had no significant effect on cortical thinning as assessed by peripheral quantitative computed tomography (pQCT). This bone-sparing action of DHT occurred at the expense of hypertrophy of the ventral prostate and seminal vesicles. On the other hand, E2 restored both trabecular bone density and cortical thickness in ORX rats and even prevented age-related bone loss. In contrast to DHT, E2 increased lean body mass and inhibited the ORX-associated increase of fat mass, as measured by DXA. Administration of E2 was associated with increased serum concentrations of insulin-like growth factor (IGF) I and decreased circulating levels of leptin. We conclude that, in the aged ORX rat model, E2 is more effective in preventing ORX-induced bone loss than DHT. Additionally, E2 has anabolic effects on muscle tissue and prevents the ORX-related increase of fat mass. Overall, these data suggest that androgen action on bone and body composition is dependent on stimulation of both androgen receptors (ARs) and estrogen receptors (ERs).

Effects of liver-derived insulin-like growth factor I on bone metabolism in mice

Insulin-like growth factor (IGF) I is an important regulator of both skeletal growth and adult bone metabolism. To better understand the relative importance of systemic IGF-I versus locally expressed IGF-I we have developed a transgenic mouse model with inducible specific IGF-I gene inactivation in the liver (LI-IGF-I-/-). These mice are growing normally up to 12 weeks of age but have a disturbed carbohydrate and lipid metabolism. In this study, the long-term effects of liver-specific IGF-I inactivation on skeletal growth and adult bone metabolism were investigated. The adult (week 8-55) axial skeletal growth was decreased by 24% in the LI-IGF-I-/- mice whereas no major reduction of the adult appendicular skeletal growth was seen. The cortical cross-sectional bone area, as measured in the middiaphyseal region of the long bones, was decreased in old LI-IGF-I-/- mice. This reduction in the amount of cortical bone was caused mainly by decreased periosteal circumference and was associated with a weaker bone determined by a decrease in ultimate load. In contrast, the amount of trabecular bone was not decreased in the LI-IGF-I-/- mice. DNA microarray analysis of 30-week-old LI-IGF-I-/- and control mice indicated that only four genes were regulated in bone whereas approximately 40 genes were regulated in the liver, supporting the hypothesis that liver-derived IGF-I is of minor importance for adult bone metabolism. In summary, liver-derived IGF-I exerts a small but significant effect on cortical periosteal bone growth and on adult axial skeletal growth while it is not required for the maintenance of the trabecular bone in adult mice.

Estrogen receptor alpha, but not estrogen receptor beta, is involved in the regulation of the hair follicle cycling as well as the thickness of epidermis in male mice

Estrogen is of importance for the regulation of hair growth and epidermal thickness. The effects of estrogen have predominantly been studied in females; however, recent studies demonstrate that estrogen also is critical for males. The aim of this study was to investigate the relative functional importance of estrogen receptor alpha and estrogen receptor beta in the regulation of the hair follicle cycling and epidermal thickness in male mice. Seven month old transgenic male mice, lacking estrogen receptor alpha (ERKO), estrogen receptor beta (BERKO), or both receptors (DERKO), were orchidectomized and treated for 3 week with 17beta-estradiol or vehicle. Orchidectomy induced a synchronized anagen phase of the hair follicles, which was inhibited by 17beta-estradiol treatment in wild-type and BERKO mice, but not in ERKO and DERKO mice. Furthermore, 17beta-estradiol treatment increased the thickness of epidermis in wild-type and BERKO mice, but not in ERKO and DERKO. This study demonstrates that estrogen is of importance for the regulation of hair follicle cycling and epidermal thickness in male mice. The effect on hair follicle cycling is caused by an estrogen receptor alpha mediated inhibition of telogen-anagen transition and the effect of estrogen to increase epidermal thickness is associated with an estrogen receptor alpha mediated increase in the proliferative rate of the keratinocytes in the basal cell layer of the epidermis.

Effect of estrogen deficiency in the male: the ArKO mouse model

Aromatase, the enzyme responsible for the conversion of androgens to estrogens, is present in the mouse gonads, brain, adipose tissue and bone. Depletion of endogenous estrogens in the aromatase deficient mouse (ArKO) caused by the targeted disruption of the Cyp19 gene resulted in an impairment of sexual behaviour and an age-dependent disruption of spermatogenesis. This disruption occurred during early spermiogenesis, due possibly to increased number of apoptotic round spermatids. Development of obesity was associated with ageing, decrease in lean mass, hypercholesterolemia, hyperleptinemia, and insulin resistance and hepatic steatosis. However, it was not correlated with hyperphagia but to decreased physically-active behaviour. ArKO mice also developed osteoporosis. Thus, studies using the ArKO mice model has led to several insights into the multiple roles played by estrogens in the development and maintenance of fertility, sexual behaviour, lipid metabolism and bone remodelling.

The molecular genetics of bone formation: implications for therapeutic interventions in bone disorders

Skeletal biology is a complex process involving the developmental commitment and differentiation of chondrocytes and osteoblasts which produce and mineralize cartilage and bone matrix during growth and postnatal life. Several genes are involved in controlling osteogenesis by acting on target cells in a very complex manner. Manipulation of genes in mice and studies of genetic mutations affecting the skeleton in humans have enabled the assessment of the role of transcription factors, bone matrix proteins and regulatory factors involved in the control of chondrocyte and osteoblast differentiation, and have considerably improved our understanding of the bone formation process. Clinical studies and gene polymorphism analyses suggest that the variable expression of particular genes may be linked to clinical osteoporosis. A major challenge in the future will be to develop molecularly targeted approaches to stimulating bone formation and increasing bone mass. The use of mouse strain models and transgenic animals with variable bone density may be useful to identify genetic determinants of bone mass which may serve as a basis for drug discovery and development. On the other hand, the availability of gene microarrays and other emerging genomic techniques are promising tools to identify genes that are distinctly expressed in health and disease. These technologies may also serve to test the mechanisms of action of drugs aimed at increasing bone formation. Genetic studies of the molecular signaling pathways involved in normal and pathological osteogenesis may also help to identify genes that could be targeted for therapeutic intervention. Candidate approaches include selective gene transfection in target cells and the use of drugs acting on gene promoters to selectively enhance gene expression in osteoblasts. The development of these strategies is expected not only to bring new insight into the molecular mechanisms that govern bone formation in normal and pathological situations but, in the long term, may also result in the identification of novel molecular targets for therapeutic interventions for bone formation disorders.

Elucidation of estrogen receptor function in bone with the use of mouse models

Since the discovery that estrogen receptors (ERs) are present in bone cells, there has been intense research into the action of estrogen in bone. During the past decade, humans with disturbed estrogen signaling, either as a result of ER alpha or aromatase deficiency, have been reported. Furthermore, mouse models have been established with a deficiency of ER alpha, ER beta or both, in addition to deficiency of aromatase. This review focuses on data accumulated during the past three years from studies of knockout mice with impaired estrogen signaling resulting from ER or aromatase deficiency.

Sex steroids and the construction and conservation of the adult skeleton

Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4-8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.

Genetics of osteoporosis: role of steroid hormone receptor gene polymorphisms

Osteoporosis is a common skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture. In the past years, twin and family study have shown that this disease recognizes a strong genetic component and that genetic factors play an important role in regulating bone mineral density (BMD). While in few isolate conditions osteoporosis can be inherited in a simple Mendelian pattern, due to single gene mutations, in the majority of cases has to be considered a multifactorial polygenic disease in which genetic determinants are modulated by hormonal, environmental and nutritional factors. Given the important role that steroid hormones play in bone cell development and in the maintenance of normal bone architecture, polymorphisms at receptor of the steroid/thyroid hormone receptor superfamily, such as estrogen receptor alpha (ERalpha) and Vitamin D receptor (VDR) have been thoroughly investigated in the last years and appeared to represent important candidate genes. The individual contribution of these genetic polymorphisms to the pathogenesis of osteoporosis remains to be universally confirmed and an important aim in future work will be to define their functional molecular consequences and how these polymorphisms interact with each other and with the environment to cause the osteoporotic phenotype. A further promising application of genetic studies in osteoporosis comes from their pharmacogenomic implications, with the possibility to give a better guidance for therapeutic agents commonly used to treat this invalidating disorder or to identify target molecules for new therapeutic agents.

Sex steroids in serum of prepubertal male and female horses and correlation with bone characteristics

We used radioimmunoassay (RIA) to measure monthly serum levels of unconjugated and conjugated sex steroids (testosterone T, androstenedione A, estradiol E(2), and estrone E(1)) in 4 male and 4 female foals during their first year of life. Maximal production of sex steroids was detected from April to August with hormonal peaks, corresponding to the natural breeding season in adults. In males, only A levels were more steady. Total estrogens (unconjugated plus conjugated E(2) and E(1)) were the major steroids in immature males in contrast to adults. Estrogens generally peaked in young females before males; the major estrogen was E(1), and total estrogens overtook total androgens (unconjugated and conjugated T and unconjugated A). We also sampled 3 male and 3 female foals with bone alterations in adulthood. For all animals, serum levels of four bone formation markers were obtained: osteocalcin (O), hydroxyproline (HP), and alkaline phosphatase (AP), and a radiographic score was determined. Only male foals with normal skeletal frame (good radiographic score GRS) in adulthood showed a correlation (P < 0.01) between the distribution frequency of each bone formation marker and unconjugated E(2) or E(1) levels; this finding highlighted the role of unconjugated estrogens in bone maturation in horses, since this was not found in the groups with bone alterations. In females, the threshold of estrogen synthesis and sensitivity was probably sufficient to be a nonlimiting factor at this stage of development. Our results strongly suggest a differential regulation of the estrogen/androgen balance in horses according to sex, sexual maturation, and photoperiod. Moreover, estrogens appear to be crucial for skeletal development in male colts, and these steroids are good modulators of skeletal frame characteristics in adulthood.

Clinical review 144: Estrogen and the male skeleton

Because estrogen (E) and T are the major sex steroids in women and men, respectively, the traditional view had been that E primarily regulated bone turnover in women and T played the analogous role in men. The description of ER- deficient and aromatase-deficient males, however, initiated a major shift in our thinking on the relative roles of T and E in regulating the male skeleton, because these individuals all had unfused epiphyses, high bone turnover, and osteopenia. Similar, albeit less striking, findings were noted in mouse models with knock-out of either the ER-alpha or the aromatase genes. Although these human experiments of nature and mouse knock-out models clearly demonstrated an important role for E in the growth and maturation of the male skeleton, they did not define the role of E vs. T in regulating the adult male skeleton. The past several years have witnessed an accumulation of evidence from observational as well as direct interventional studies that now clearly indicates that E plays a major, and likely dominant, role in bone metabolism in men. These data also suggest that a threshold level of bioavailable (or non-SHBG bound) E is needed for skeletal E sufficiency in the male, and that with aging, an increasing percentage of elderly men begin to fall below this level. It is this subset of men who may be at greatest risk for the development of age-related bone loss and osteoporosis. Moreover, these men may also be the ones most likely to respond favorably to treatment with selective E receptor modulators, or perhaps even to T replacement, because the skeletal effects of the latter may be mediated largely via aromatization to E.

Two different pathways for the maintenance of trabecular bone in adult male mice

Androgens may regulate the male skeleton either directly via activation of the androgen receptor (AR) or indirectly via aromatization of androgens into estrogen and, thereafter, via activation of estrogen receptors (ERs). There are two known estrogen receptors, ER-alpha and ER-beta. The aim of this study was to investigate the relative roles of ER-alpha, ER-beta, and AR in the maintenance of trabecular bone in male mice. Seven-month-old male mice, lacking ER-alpha (ERKO), ER-beta (BERKO), or both receptors (DERKO), were orchidectomized (orx) and treated for 3 weeks with 0.7 microg/mouse per day of 17beta-estradiol or vehicle. No reduction in trabecular bone mineral density (BMD) was seen in ERKO, BERKO, or DERKO mice before orx, showing that neither ER-a nor ER-beta is required for the maintenance of a normal trabecular BMD in male mice. After orx, there was a pronounced decrease in trabecular BMD, similar for all groups, resulting in equal levels of trabecular BMD in all genotypes. This reduction was reversed completely in wild-type (WT) and BERKO mice treated with estrogen, and no significant effect of estrogen was found in ERKO or DERKO mice. In summary, the trabecular bone is preserved both by a testicular factor, presumably testosterone acting via AR and by an estrogen-induced activation of ER-alpha. These results indicate that AR and ER-alpha are redundant in the maintenance of the trabecular bone in male mice. In contrast, ER-beta is of no importance for the regulation of trabecular bone in male mice.

Deletion of estrogen receptors reveals a regulatory role for estrogen receptors-beta in bone remodeling in females but not in males

To determine the contributions of estrogen receptor (ER)alpha and ERbeta in bone growth and remodeling in male and female mice, we generated and analyzed full knockouts for each receptor, and a double ER knockout. Although suppression of the ligand to the ERs (i.e., estradiol) after menopause or gonadectomy in females led to a catastrophic increase in bone turnover and concomitant bone loss, deletion of one or both ERs failed to show such an effect. Complete deletion of ERalpha led to a decrease, not an increase, in bone turnover and an increase, not a decrease, in trabecular bone volume in both male and female animals. Deletion of ERbeta led to different responses in males, where bone was unaffected, and in females, where bone resorption was decreased and trabecular bone volume increased. In contrast, deletion of both ERs led to a profound decrease in trabecular bone volume in females, which was associated with a decrease, not an increase, in bone turnover. Finally, deletion of ERalpha, but not ERbeta, led to major changes in circulating levels of estradiol and/or testosterone, indirectly affecting bone remodeling and bone mass. Thus, only ERalpha was shown to regulate bone remodeling in males, whereas in females both receptor subtypes influenced this process and could, at least under basal knockout conditions, compensate for each other.

ERalpha gene expression in human primary osteoblasts: evidence for the expression of two receptor proteins

The beneficial influence of E2 in the maintenance of healthy bone is well recognized. However, the way in which the actions of this hormone are mediated is less clearly understood. Western blot analysis of ERalpha in osteoblasts clearly demonstrated that the well characterized 66-kDa ERalpha was only one of the ERalpha isoforms present. Here we describe a 46-kDa isoform of ERalpha, expressed at a level similar to the 66-kDa isoform, that is also present in human primary osteoblasts. This shorter isoform is generated by alternative splicing of an ERalpha gene product, which results in exon 1 being skipped with a start codon in exon 2 used to initiate translation of the protein. Consequently, the transactivation domain AF-1 of this ERalpha isoform is absent. Functional analysis revealed that human (h)ERalpha46 is able to heterodimerize with the full-length ERalpha and also with ERbeta. Further, a DNA-binding complex that corresponds to hERalpha46 is detectable in human osteoblasts. We have shown that hERalpha46 is a strong inhibitor of hERalpha66 when they are coexpressed in the human osteosarcoma cell line SaOs. As a functional consequence, proliferation of the transfected cells is inhibited when increasing amounts of hERalpha46 are cotransfected with hERalpha66. In addition to human bone, the expression of the alternatively spliced ERalpha mRNA variant is also detectable in bone of ERalpha knockout mice. These data suggest that, in osteoblasts, E2 can act in part through an ERalpha isoform that is markedly different from the 66-kDa receptor. The expression of two ERalpha protein isoforms may account, in part, for the differential action that estrogens and estrogen analogs have in different tissues. In particular, the current models of the action of estrogens should be reevaluated to take account of the presence of at least two ERalpha protein isoforms in bone and perhaps in other tissues.

Cell localization, physiology, and nongenomic actions of estrogen receptors

The existence of binding proteins for the female sex steroid, 17beta-estradiol, has been known for almost 50 years. Presently, two estrogen receptors (ERs), ER-alpha and ER-beta, have been cloned in mammals, and they are expressed in many cell types of metazoans. ERs act primarily as nuclear transcription factors, and this effect is enhanced by ligand binding. Emerging data have identified a separate pool of receptors for this steroid in the plasma membrane, but the mechanisms of action and cellular functions of these proteins are just beginning to be defined. In this review, the known details of the nuclear and plasma membrane ER functions will be discussed. A particular focus will be to define the signaling pathways from the membrane that lead to important cell physiology effects of estrogen. The potential interactions of membrane ER with other local proteins will also be discussed, and the unique but often complementary roles of the receptor pools will be highlighted. These details may be of additional relevance to other steroid receptors, since there is evidence of their existence in the cell membrane.

Female estrogen receptor beta-/- mice are partially protected against age-related trabecular bone loss

Recently, it has been shown that inactivation of estrogen receptor beta (ER-beta) by gene targeting results in increased cortical bone formation in adolescent female mice. To study the possible involvement of ER-beta in the regulation of the mature skeleton, we have extended the analyses to include 1-year-old ER-beta knockout mice (ER-beta-/-). Male ER-beta-/- 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-beta-/- mice was maintained in the older females. The aged female ER-beta-/- 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-beta-/- mice. The growth plate width was unaltered in the female ER-beta-/- 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 alpha1 (Cbfa1) suggested an anabolic effect in bones of old female ER-beta-/- mice. In addition, the mRNA expression of ER-alpha was augmented, indicating a role for ER-alpha in the development of this phenotype. Taken together, the results show that ER-beta is involved in the regulation of trabecular bone during adulthood in female mice and suggest that ER-beta acts in a repressive manner, possibly by counteracting the stimulatory action of ER-alpha on bone formation.

Phytoestrogens from the roots of Polygonum cuspidatum (Polygonaceae): structure-requirement of hydroxyanthraquinones for estrogenic activity

The methanolic extract from the roots of Polygonum (P.) cuspidatum was found to enhance cell proliferation at 30 or 100 microg/mL in MCF-7, an estrogen-sensitive cell line. By bioassay-guided separation from P. cuspidatum with the most potent activity, emodin and emodin 8-O-beta-D-glucopyranoside were isolated as active principles. The methanolic extracts from Polygonum, Cassia, Aloe, and Rheum species, which were known to contain anthraquinones, also showed the MCF-7 proliferation. As a result of the evaluation of various anthraquinones from plant sources and synthetic anthraquinones, aloe-emodin, chrysophanol, chrysophanol 8-O-beta-D-glucopyranoside, and 1,8-dihydroxyanthraquinone showed weak activity. On the other hand, alizalin and 2,6-dihydroxyanthraquinone as well as emodin having the 2- and/or 6-hydroxyl groups showed potent activity. These results show that the unchelated hydroxyl group is essential for strong activity. Emodin and 2,6-dihydroxyanthraquinone also inhibited 17beta-estradiol binding to human estrogen receptors (ERs) with K(i) values of 0.77 and 0.31microM for ERalpha and 1.5 and 0.69 microM for ERbeta. These findings indicate that hydroxyanthraquinones such as emodin are phytoestrogens with an affinity to human estrogen receptors.

Testosterone prevents orchidectomy-induced bone loss in estrogen receptor-alpha knockout mice

To examine the role of the estrogen receptor-alpha (ERalpha) during male skeletal development, bone density and structure of aged ERalphaKO mice and wild-type (WT) littermates were analyzed and skeletal changes in response to sex steroid deficiency and replacement were also studied. In comparison to WT, ERalphaKO mice had smaller and thinner bones, arguing for a direct role of ERalpha to obtain full skeletal size in male mice. However, male ERalphaKO mice had significantly more trabecular bone as assessed both by pQCT and histomorphometry, indicating that ERalpha is not essential to maintain cancellous bone mass. Six weeks following orchidectomy (ORX), both WT and ERalphaKO mice showed high-turnover osteoporosis as revealed by increases in serum osteocalcin and decreases in trabecular (-38% and -58% in WT and ERalphaKO, respectively) and cortical bone density (-5% and -4% in WT and ERalphaKO, respectively). Administration of testosterone propionate (T, 5 mg/kg/day) completely prevented bone loss both in ERalphaKO and in WT mice. As expected, estradiol (E2, 60 microg/kg/day) replacement did not prevent cancellous bone loss in ORX ERalphaKO mice. However, E2 stimulated bone formation at the endocortical surface in ORX ERalphaKO, suggesting that osteoblasts may respond to nonERalpha-mediated estrogen action. In conclusion, although functional ERalpha may play a significant role during male skeletal development, this receptor does not seem essential for androgen-mediated skeletal maintenance in older male mice.

Transgenic mouse models of metabolic bone disease

The approach of gene-targeted animal models is likely the most important experimental tool contributing to recent advances in skeletal biology. Modifying the expression of a gene in vivo, and the analysis of the consequences of the mutation, are central to the understanding of gene function during development and physiology, and therefore to our understanding of the gene's role in disease states. Researchers had been limited to animal models primarily involving pharmaceutical manipulations and spontaneous mutations. With the advent of gene targeting, however, animal models that impact our understanding of metabolic bone disease have evolved dramatically. Interestingly, some genes that were expected to yield dramatic phenotypes in bone, such as estrogen receptor-alpha or osteopontin, proved to have subtle phenotypes, whereas other genes, such as interleukin-5 or osteoprotegerin, were initially identified as having a role in bone metabolism via the analysis of their phenotype after gene ablation or overexpression. Particularly important has been the advance in knowledge of osteoblast and osteoclast independent and dependent roles via the selective targeting of genes and the consequent disruption of bone formation, bone resorption, or both. Our understanding of interactions of the skeletal system with other systems, ie, the vascular system and homeostatic controls of adipogenesis, has evolved via animal models such as the matrix gla protein, knock-out, and the targeted overexpression of Delta FosB. Challenging transgenic models such as the osteopontin-deficient mice with mediators of bone remodeling like parathyroid hormone and mechanical stimuli and extending phenotype characterization to mechanistic in vitro studies of primary bone cells is providing additional insight into the mechanisms involved in pathologic states and their potentials for therapeutic strategies. This review segregates characterization of transgenic models based on the category of gene altered, eg, reproductive hormones, calcitropic hormones, growth factors and cytokines, signaling molecules, extracellular matrix molecules and "other" genes. Models are also segregated based on phenotypes that are primarily osteoclastic, osteoblastic or mixed. As the technical ability to alter gene expression negatively or positively and in a tissue-specific and temporal manner continues to evolve, there are endless possibilities for generating genetically altered animal models with which to gain insight into metabolic bone diseases.