Effect of ovarian steroid deficiency on oestrogen receptor alpha expression in bone

Sex Hormones and Tendon

The risk of overuse and traumatic tendon and ligament injuries differ between women and men. Part of this gender difference in injury risk is probably explained by sex hormonal differences which are specifically distinct during the sexual maturation in the teenage years and during young adulthood. The effects of the separate sex hormones are not fully elucidated. However, in women, the presence of estrogen in contrast to very low estrogen levels may be beneficial during regular loading of the tissue or during recovering after an injury, as estrogen can enhance tendon collagen synthesis rate. Yet, in active young female athletes, physiological high concentration of estrogen may enhance the risk of injuries due to reduced fibrillar crosslinking and enhanced joint laxity. In men, testosterone can enhance tendon stiffness due to an enhanced tendon collagen turnover and collagen content, but testosterone has also been linked to a reduced responsiveness to relaxin. The present chapter will focus on sex difference in tendon injury risk, tendon morphology and tendon collagen turnover, but also on the specific effects of estrogen and androgens.

Single and combined effect of high-frequency loading and bisphosphonate treatment on the bone micro-architecture of ovariectomized rats

Mechanical loading at high frequency affects bone. Whether this also applies to osteoporotic bone, combined or not with bisphosphonate therapy, was investigated in this animal study through imaging. An anabolic effect of high-frequency loading on osteoporotic bone, however non-synergistic with bisphosphonates, was found, thereby revealing its potential for treatment of osteoporosis.

INTRODUCTION

In an effort to elucidate the effect of high-frequency (HF) loading on bone and to optimize its potential for treatment osteoporosis, this study aimed to investigate the effect of HF loading via whole body vibration (WBV), alone or in association with bisphosphonate treatment (alendronate--ALN), on the micro-architecture of ovariectomy (OVX)-induced compromised bone.

METHODS

Eighty-four female Wistar rats were ovariectomized (OVX) or sham-operated (shOVX). OVX animals were treated either with ALN (3 days/week at a dose of 2 mg/kg) or with saline solution. Each group (shOVX, OVX, ALN) was further divided into subgroups relative to the loading status (sham-WBV versus WBV) and the duration of experimental period (4 days versus 14 days). (Sham)WBV loading was applied for 10 min/day using 10 consecutive steps of HF loading (130, 135, 140, 145, 150, 130, 135, 140, 145, 150 Hz). Tibial bone structural responses to WBV and/or ALN treatment were analyzed using ex vivo micro-computed tomography.

RESULTS

The animal's hormonal status displayed a major impact on the trabecular and cortical bone structural parameters. Furthermore, mechanical treatment with HF WBV increased the cortical thickness and reduced the medullar area in OVX rats. However, OVX trabecular bone was not affected by HF stimuli. Finally, ALN prevented OVX-associated bone loss, but the association of ALN with WBV did not lead to a synergistic bone response in OVX bone.

CONCLUSIONS

HF WBV mechanical stimulation displayed an anabolic effect on osteoporotic cortical bone, confirming its therapeutic properties for enhancing compromised bone. Additionally, its association with bisphosphonates' administration did not produce any additive effect on the bone micro-architecture in the present study.

Callus formation is related to the expression ratios of estrogen receptors-alpha and -beta in ovariectomy-induced osteoporotic fracture healing

INTRODUCTION

This study characterizes ovariectomized (OVX)-induced osteoporotic fracture healing with focus on estrogen receptors (ERs). Callus formation plays a critical role in fracture healing, and ERs are well-known mechanosensors in osteogenic pathways. It was hypothesized that callus formation was related to and partially determined by the difference in expression patterns of ERs in both normal and OVX-induced osteoporotic fractures.

METHODS

Closed femoral fracture in SHAM and ovariectomized rats were used in this study. Weekly callus width (CW) and area (CA), endpoint mechanical properties, gene expressions of Col-1, BMP-2, ER-α, ER-β and ER-α:ER-β ratios (ER-ratios), and correlations were assessed at 2, 4 and 8 weeks post-fracture.

RESULTS

CW and CA results confirmed that OVX-induced osteoporotic fracture was delayed at 2-4 weeks with impaired endpoint mechanical properties. Gene expressions of ER-α and ER-β were higher in the SHAM group at week 2 (p < 0.05) and later lowered at week 8; whereas the OVX group showed an opposing trend. Moderate correlation existed between ER-α and BMP-2 (0.545, p = 0.003), and ER-ratio and BMP-2 (0.601, p = 0.001), and BMP-2 to CW and CA (r = 0.709, p = 0.000 and r = 0.588, p = 0.001, respectively). ER-α and ER-β proteins expressions were confirmed by immunohistochemistry at the fracture callus in reparative progenitor cells, osteoblasts- and osteoclasts-like cells.

CONCLUSION

We conclude that the delayed healing rate and impaired callus quality in OVX-induced osteoporotic fracture is related to the delayed expression of ERs. A high ER-α:ER-β ratio favors callus formation.

Mechanical loading increases detection of estrogen receptor-alpha in osteocytes and osteoblasts despite chronic energy restriction

Estrogen receptor-α (ER-α) is an important mediator of the bone response to mechanical loading. We sought to determine whether restricting dietary energy intake by 40% limits the bone formation rate (BFR) response to mechanical loading (LOAD) by downregulating ER-α-expressing osteocytes, or osteoblasts, or both. Female rats (n = 48, 7 mo old) were randomized to ADLIB-SHAM and ADLIB-LOAD groups fed AIN-93M purified diet ad libitum or to ER40-SHAM and ER40-LOAD groups fed modified AIN-93M with 40% less energy (100% of all other nutrients). After 12 wk, LOAD rats were subjected to a muscle contraction protocol three times every third day. ER40 produced lower proximal tibia bone volume (-22%), trabecular thickness (-14%), and higher trabecular separation (+127%) in SHAM but not LOAD rats. ER40 rats exhibited reductions in mineral apposition rate, but not percent mineralizing surface or BFR. LOAD induced similar relative increases in these kinetic measures of osteoblast activity/recruitment in both diet groups., but absolute values for ER40 LOAD rats were lower vs. ADLIB-LOAD. There were fourfold and eightfold increases in proportion of estrogen receptor-α protein-positive osteoblast and osteocytes, respectively, in LOAD vs. SHAM rats, with no effect of ER40. These data suggest that a brief period of mechanical loading significantly affects estrogen receptor-α in cancellous bone osteoblasts and osteocytes. Chronic energy restriction does result in lower absolute values in indices of osteoblast activity after mechanical loading, but not by a smaller increment relative to unloaded bones; this change is not explained by an associated downregulation of ER-α in osteoblasts or osteocytes.

Is interaction between age-dependent decline in mechanical stimulation and osteocyte-estrogen receptor levels the culprit for postmenopausal-impaired bone formation?

Declining estrogen levels during menopause are widely considered to be a major cause of age-dependent bone loss, which is primarily manifested by increased bone resorption by osteoclasts. We present accumulating evidence supporting another aspect of metabolic bone loss, suggesting that the combined interaction between age-dependent factors, namely, estrogen deficiency and reduced day-by-day activity/mechanical stimulation, directly leads to a reduction in anabolic processes. Such decreased bone formation results in diminished bone strength and failure to maintain the load-bearing competence of a healthy skeleton and to postmenopausal osteoporosis disorder. Estrogen receptors (ERs), as mediators of estrogenic actions, are essential components of bone osteocyte and osteoblast mechano-adaptive responses. ER expression appears to be upregulated by adequate circulating estrogen levels. ERα signaling pathways participate in the mechanotransduction response through obligatory "non-genomic" actions that occur independently of estrogen binding to ER and by a potentially "genomic", estrogen-dependent mode. The experimental data indicate that cross talk between the ERα-"non-genomic" and Wnt/β-catenin signaling pathways constitutes the major regulatory mechanism. This interaction uses mechanically and ER-induced prostaglandin E2 as a mediator for the downregulation of osteocyte production of sclerostin. Sclerostin suppression, in turn, is a central prerequisite for load-induced formation and mineralization of the bone matrix. It is therefore plausible that future strategies for preventing and treating postmenopausal osteoporosis may use estrogenic compounds (such as selective estrogen receptor modulators or phytoestrogens) with physical activity, to complement antiresorptive therapy, aimed at stopping further bone loss and possibly even reversing it by stimulation of bone gain.

Preventive effect of risedronate on bone loss and frailty fractures in elderly women treated with anastrozole for early breast cancer

The aim of this study was to assess the effect of adjuvant anastrozole, alone or associated with risedronate, on BMD and bone fracture risk in women more than 70 years old with hormone receptor-positive early breast cancer (EBC). In a group of 51 elderly women (aged 76.4 ± 5.0 years) considered for adjuvant aromatase inhibitors for EBC, 24 patients with T-scores ≥ -2 and no prevalent fractures received anastrozole 1 mg/day (group A), and 27 patients with T-scores < -2, or with T-scores ≥ -2 and prevalent fractures (group B), received anastrozole (1 mg/day) plus risedronate (35 mg/week). Both groups received supplementation with 1 g calcium carbonate and 800 IU vitamin D per day. Differences in BMD and frailty fractures were evaluated after 1 and 2 years. In group A, significant decreases in BMD were observed in the lumbar spine (Δ BMD, -0.030 ± 0.04 g/cm², P < 0.05), femoral neck (Δ BMD, -0.029 ± 0.05 g/cm², P < 0.05), and trochanter (Δ BMD, -0.026 ± 0.03 g/cm², P < 0.01) after 2 years. The greatest percent reduction in height (Hpr) emerged in the thoracic spine (3.6 ± 2.4%, P < 0.01), although only one incident vertebral fracture was observed. In group B, BMD increased in the lumbar spine (Δ BMD, 0.038 ± 0.04, P < 0.001), although no significant changes were seen in the hip regions. The decline in Hpr was negligible (about 1%). No incident fractures were observed at follow-up. In conclusion, anastrozole treatment for EBC in elderly women seems to have only mild negative effects on the femoral bone. Risedronate makes the use of anastrozole safer, even for osteopenic or osteoporotic elderly patients.

Estrogen, exercise, and the skeleton

Patterns of variation in bone size and shape provide crucial data for reconstructing hominin paleobiology, including ecogeographic adaptation, life history, and functional morphology. Measures of bone strength, including robusticity (diaphyseal thickness relative to length) and cross-sectional geometric properties such as moments of area, are particularly useful for inferring behavior because bone tissue adapts to its mechanical environment. Particularly during skeletal growth, exercise-induced strains can stimulate periosteal modeling so that, to some extent, bone thickness reflects individual behavior. Thus, patterns of skeletal robusticity have been used to identify gender-based activity differences, temporal shifts in mobility, and changing subsistence strategies. Although there is no doubt that mechanical loading leaves its mark on the skeleton, less is known about whether individuals differ in their skeletal responses to exercise. For example, the potential effects of hormones or growth factors on bone-strain interactions are largely unexplored. If the hormonal background can increase or decrease the effects of exercise on skeletal robusticity, then the same mechanical loads might cause different degrees of bone response in different individuals. Here I focus on the role of the hormone estrogen in modulating exercise-induced changes in human bone thickness.

Estrous Cycle‐Dependent Expression of Estrogen Receptor α in Periodontal Tissue

Estrogen receptor alpha (ERalpha) may play important roles in many estrogen physiological effects, but little is known about the fluctuation of ERalpha during the estrous cycle. In this study, the dynamic expression of ERalpha mRNA and protein in periodontal tissue during the estrous cycle were examined. Forty 12-week-old female rats were divided into four groups, based on the estrous cycle stage, and sacrificed. Immunohistochemistry and in situ hybridization were used to detect dynamic changes in ERalpha protein and mRNA in periodontal tissue during the estrous cycle, and data were analyzed by one-way ANOVA and cosinor analysis for temporal patterns. Significant differences (p<0.05) were found in the expression of ERalpha protein and mRNA among the four groups. The expression of ERalpha protein and mRNA exhibited an infradian rhythm with a period of about 120 h (five days). The phase and amplitude differences between ERalpha protein and mRNA were not significant (p>0.05). The results suggest the expression of ERalpha is dynamic during the estrous cycle and that in the future chronobiologic methods should be used to study the mechanism of estrogen effect on periodontal tissue.

Variation in estradiol level affects cortical bone growth in response to mechanical loading in sheep

Although mechanical loading can stimulate cortical bone growth, little is known about how individual physiology affects this response. This study demonstrates that in vivo variation in estradiol (E2) level alters osteoblast sensitivity to exercise-induced strains, affecting cortical bone responses to mechanical loading. Subadult sheep were divided into treatment groups that varied in terms of circulating E2 levels and loading (exercised and sedentary). After 45 days, periosteal cortical bone growth rates and cross-sectional properties were measured at the midshafts of hindlimb bones and compared with strain data. The results indicate significant interactions between E2 and strain. Cortical bone growth in exercised animals with elevated E2 levels was 27% greater in the femur, 6% greater in the tibia, and 14% greater in the metatarsal than in exercised animals with lower E2 levels, or sedentary animals regardless of E2 dose (P<0.05). There was also a trend toward greater resistance to deformation in the tibia, but not the metatarsal, in the exercised, high-E2 group compared to the other treatment groups. These results demonstrate that E2 plays a role in mediating skeletal responses to strain, such that physiological variation in E2 levels among individuals may lead to differential growth responses to similar mechanical loading regimes. Efforts to model the relationship between environmental strain and bone morphology should include the effects of physiological variation in hormone levels.

Wnt/beta-catenin signaling is a component of osteoblastic bone cell early responses to load-bearing and requires estrogen receptor alpha

The Wnt/beta-catenin pathway has been implicated in bone cell response to their mechanical environment. This response is the origin of the mechanism by which bone cells adjust bone architecture to maintain bone strength. Osteoporosis is the most widespread failure of this mechanism. The degree of osteoporotic bone loss in men and women is related to bio-available estrogen. Here we report that in osteoblastic ROS 17/2.8 cells and primary osteoblast cultures, a single short period of dynamic mechanical strain, as well as the glycogen synthase kinase-3beta (GSK-3beta) inhibitor LiCl, increased nuclear accumulation of activated beta-catenin and stimulated TCF/LEF reporter activity. This effect was blocked by the estrogen receptor (ER) modulators ICI 182,780 and tamoxifen and was absent in primary osteoblast cultures from mice lacking ERalpha. Microarray expression data for 25,000 genes from total RNA extracted from tibiae of wild-type mice within 24 h of being loaded in vivo showed differential gene regulation between loaded and contralateral non-loaded bones of 10 genes established to be involved in the Wnt pathway. Only 2 genes were involved in loaded tibiae from mice lacking ERalpha (ERalpha(-/-)). Together these data suggest that Wnt/beta-catenin signaling contributes to bone cell early responses to mechanical strain and that its effectiveness requires ERalpha. Reduced effectiveness of bone cell responses to bone loading, associated with estrogen-related decline in ERalpha, may contribute to the failure to maintain structurally appropriate bone mass in osteoporosis in both men and women.

Soy isoflavones and bone health: a double-edged sword?

Numerous publications and research studies on isoflavones have prompted a nationwide increase in the consumption of soy-based foods and supplements in the United States. Isoflavones are natural endocrine active compounds generally considered to promote health and prevent or slow the onset of certain chronic diseases such as osteoporosis. The beneficial effects of soy isoflavones on bone may, however, be life-stage specific and dependent on the estrogen receptor number and endogenous hormone milieu. Perimenopausal and early menopausal women may therefore be more receptive to the therapeutic effects of isoflavones on bone loss prior to the diminution of estrogen receptors that occurs in the postmenopausal years, whereas laboratory studies in developmental age range animals have demonstrated the potential for adverse effects following exposure to high levels of soy isoflavones. Clinical studies in developing humans that either support or refute findings in animal studies are lacking. The effects of chronic consumption of high levels of soy isoflavones at each life stage to assess risk-benefit ratios should be a high priority of research.

Effects of eccentric exercise training on cortical bone and muscle strength in the estrogen-deficient mouse

The purpose of this study was to determine whether eccentrically biased exercise training could attenuate changes in muscle and bone function associated with estrogen deficiency in the mouse model. Four groups of ICR mice were used: control (Con), sham ovariectomized (Sham), ovariectomized (OVX), and ovariectomized + high-force resistance training (OVX+Train). All groups except Con were implanted with a nerve cuff surrounding the peroneal nerve to stimulate the left ankle dorsiflexors. Training consisted of 30 stimulated eccentric contractions of the left ankle dorsiflexors at approximately 150% of peak isometric torque every third day for 8 wk. After the training period, groups were not significantly different with regard to peak torque or muscle size. However, the tibial midshaft of the trained leg in the OVX+Train mice exhibited greater stiffness (+15%) than that in the untrained OVX mice, which could not be explained by changes in cross-sectional geometry of the tibia. Scaling of bone mechanical properties to muscle strength were not altered by ovariectomy or training. These data indicate that eccentric exercise training in adult mice can significantly increase bone stiffness, despite the absence of ovarian hormones.

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.

Annual Changes of Bone Density over 12 Years in an Amenorrheic Athlete

PURPOSE

To link annual changes of bone mineral density (BMD) over 12 consecutive years to pharmacological intervention and to fluctuations of body mass and body composition in an amenorrheic athlete.

METHODS

BMD of the lumbar spine (LS) and total proximal femur (PF) were measured using dual energy x-ray absorptiometry (DXA), every 11-13 months between ages 24.8 and 36.9 yr. Body composition was assessed every 3-4 yr from a whole body DXA scan. Body mass was recorded every 3 months. For the first 5 yr of study, the subject used oral contraceptives (OC). For the subsequent 7 yr, she used estradiol skin patches (EP) with oral norethisterone.

RESULTS

The first DXA scan (age 24.8 yr) revealed a low BMD at both LS and PF, with T-scores of -1.4 and -2.8, respectively. During the next 5 yr, while adhering to OC, the BMD of her LS and PF declined by 9.8% and 12.1%, respectively. Concomitantly, her body mass fell from 45.1 to 41.4 kg, her body mass index (BMI) from 16.4 to 15.0 kg.m-2, and her percent body fat from 8.3 to <4.0%. While treated with EP and norethisterone (age 29.8-33.5 yr), her LS BMD gradually increased by 9.4%, despite a further 0.8 kg decline of body mass. From age 33.8 to 36.9 yr, voluntary weight gain (2-3 kg.yr-1; total: 8.1 kg) was accompanied by an increase of her PF BMD (16.9%), with no further increase at the LS.

CONCLUSION

Changes of BMD at the total proximal femur reflected changes of body mass in this subject. At the lumbar spine, BMD declined with weight loss but increased in association with transdermal estradiol treatment in the absence of weight gain.

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.

The effect of in vivo mechanical loading on estrogen receptor alpha expression in rat ulnar osteocytes

The presence of estrogen receptor alpha (ER alpha) in osteocytes was identified immunocytochemically in transverse sections from 560 to 860 microm distal to the midshaft of normal neonatal and adult male and female rat ulnas (n = 3 of each) and from adult male rat ulnas that had been exposed to 10 days of in vivo daily 10-minute periods of cyclic loading producing peak strains of either -3000 (n = 3) or -4000 microstrain (n = 5). Each animal ambulated normally between loading periods, and its contralateral ulna was used as a control. In animals in which limbs were subject to normal locomotor loading alone, 14 +/- 1.2% SEM of all osteocytes in each bone section were ER alpha positive. There was no influence of either gender (p = 0.725) or age (p = 0.577) and no interaction between them (p = 0.658). In bones in which normal locomotion was supplemented by short periods of artificial loading, fewer osteocytes expressed ER alpha (7.5 +/- 0.91% SEM) than in contralateral control limbs, which received locomotor loading alone (14 +/- 1.68% SEM; p = 0.01; median difference, 6.43; 95% CI, 2.60, 10.25). The distribution of osteocytes expressing ER alpha was uniform across all sections and thus did not reflect local peak strain magnitude. This suggests that osteocytes respond to strain as a population, rather than as individual strain-responsive cells. These data are consistent with the hypothesis that ER alpha is involved in bone cells' responses to mechanical strain. High strains appear to decrease ER alpha expression. In osteoporotic bone, the high strains assumed to accompany postmenopausal bone loss may reduce ER alpha levels and therefore impair the capacity for appropriate adaptive remodeling.

Estrogen receptor-related receptor alpha impinges on the estrogen axis in bone: potential function in osteoporosis

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha) is expressed by osteoblastic cells and plays a functional role in osteoprogenitor proliferation and differentiation. To dissect further the role of ERRalpha in bone, we investigated the effects of estrogen (E2) on ERRalpha both in vitro and in vivo. Chronic treatment of fetal rat calvaria cells with E2-stimulated bone nodule formation and up-regulated ERRalpha mRNA expression at early (10 h and d 8) but not later times in culture, suggesting a link between ERRalpha and E2 during osteoprogenitor proliferation. ERRalpha mRNA levels were significantly lower in ovariectomized adult rat bones vs. those of sham-operated rats early (1 d and 1 wk) post surgery, but levels returned to control levels thereafter. ERRalpha is also expressed in osteoclasts (tartrate-resistant acid phosphatase + multinucleated cells) in vivo and in vitro (RAW 264.7 cells) and ovariectomization lowered the OPG/receptor activator of nuclear factor kappaB ligand expression ratio. Down-regulation of ERRalpha expression via antisense treatment of rat calvaria cells not only inhibited osteogenesis but also increased adipocyte colony formation and changed the OPG/receptor activator of nuclear factor kappaB ligand ratio. These data suggest that ERRalpha is regulated by estrogen in bone in which it may play a functional role at several levels (osteoblasts, adipocytes, and osteoclasts) in E2 deficiency diseases such as osteoporosis.

Differential expression of estrogen receptor-related receptor alpha and estrogen receptors alpha and beta in osteoblasts in vivo and in vitro

The orphan nuclear estrogen receptor-related receptor (ERR) alpha is expressed by osteoblastic cells, is known to transactivate at least one osteoblast-associated gene osteopontin (OPN) and plays a functional role in osteoprogenitor cell proliferation and differentiation. To dissect further the role of ERR-alpha in bone formation, we compared its expression to that of the estrogen receptor (ER) alpha and ER-beta in rat calvaria (RC) and fetal tibia in vivo and in RC and rat bone marrow (RBM) cells in vitro. We found that ERR-alpha is highly and widely expressed in most, if not all, cells in RC cell cultures from early proliferation stages through mineralized nodule formation; ER-alpha was localized similarly but at lower levels and ER-beta, although present, was barely detectable. These patterns of expression in vitro correlated with what we observed in vivo in sections of 21-day fetal RC, in which ERR-alpha appeared to be more highly expressed than either of the ERs. Interestingly, ERR-a also is highly expressed in RBM cells, while ER-alpha and ER-beta mRNA is expressed, but at lower levels. Moreover, we found that ERR-alpha, ER-alpha, and ER-beta were all expressed in osteoblasts in fetal and adult tibia whereas they were expressed differentially in calvaria in vivo in subsets of osteoblasts, supporting the hypothesis that ERR-alpha may interact with one or both of the ERs in those osteoblasts in which they are coexpressed and that all three receptors may be required for bone formation but at different times and for different functions.

Osteocytes inhibit osteoclastic bone resorption through transforming growth factor-beta: enhancement by estrogen

Osteocytes are the most abundant cells in bone and distributed throughout the bone matrix. They are connected to the each other and to the cells on the bone surface. Thus, they may also secrete some regulatory factors controlling bone remodeling. Using a newly established osteocyte-like cell line MLO-Y4, we have studied the interactions between osteocytes and osteoclasts. We collected the conditioned medium (CM) from MLO-Y4 cells, and added it into the rat osteoclast cultures. The conditioned medium had no effect on osteoclast number in 24-h cultures, but it dramatically inhibited resorption. With 5, 10, and 20% CM, there was 25, 39, and 42% inhibition of resorption, respectively. Interestingly, the inhibitory effect was even more pronounced, when MLO-Y4 cells were pretreated with 10(-8) M 17-beta-estradiol. With 5, 10, and 20% CM, there was 46, 51, and 58% of inhibition. When the conditioned medium was treated with neutralizing antibody against transforming growth factor-beta (TGF-beta), the inhibitory effect was abolished. This suggests that osteocytes secrete significant amounts of TGF-beta, which inhibits bone resorption and is modulated by estrogen. RT-PCR and Western blot analysis show that in MLO-Y4 cells, the prevalent TGF-beta isoform is TGF-beta3. We conclude that osteocytes have an active, inhibitory role in the regulation of bone resorption. Our results further suggest a novel role for TGF-beta in the regulation of communication between different bone cells and suggest that at least part of the antiresorptive effect of estrogen in bone could be mediated via osteocytes.

Postmenopausal osteoporosis as a failure of bone's adaptation to functional loading: a hypothesis

There is substantial evidence that bones' ability to withstand functional loading without damage depends on the processes of bone modeling and remodeling, which are responsible for establishing and maintaining bone architecture, being influenced by a feedback mechanism related to the control of functional strains. It is probably useful to consider the diminished ability to maintain bone strength in postmenopausal osteoporosis as a failure of this mechanism. Acceptance of this approach would not only increase understanding of the etiology of postmenopausal osteoporosis but also significantly influence the ways in which it is investigated and treated. This would not mean that the many other factors affecting bone mass and bone cell activity will be ignored, but rather these factors will be put in perspective. Research to prevent or treat osteoporosis could be directed usefully to understanding how osteoblasts, lining cells, and osteocytes respond to mechanically derived information and how these responses are converted into stimuli controlling structurally appropriate modeling and remodeling. Evidence suggesting that early strain-related responses of bone cells in males and females involve the estrogen receptor (ER) could explain decreased effectiveness of this pathway when ER levels are low.

Mechanical strain and estrogen activate estrogen receptor alpha in bone cells

Bone cells' early responses to estrogen and mechanical strain were investigated in the ROS 17/2.8 cell line. Immunoblotting with antiphosphorylated estrogen receptor a (ER-alpha) antibody showed that when these cells were exposed for 10 minutes to estrogen (10(-8) M) or a single period of cyclic dynamic strain (peak 3400 microepsilon, 1 Hz, 600 cycles), there was an increase in the intensity of a 66-kDa band, indicating phosphorylation of ser122 in the amino terminus of ER-alpha. Increased phosphorylation was detected within 5 minutes of exposure to estrogen and 5 minutes after the end of the period of strain. Estrogen and strain also activated the mitogen-activated protein kinase (MAPK) family member extracellular regulated kinase-1 (ERK-1). Increases in ERK activation coincided with increased ER-alpha phosphorylation. Activation of ERK-1 and the phosphorylation of ER-alpha, by both estrogen and strain, were prevented by the MAP kinase kinase (MEK) inhibitor U0126 and the protein kinase A (PKA) inhibitor (PKI). These data support previous suggestions that resident bone cells' early responses to strain and estrogen share a common pathway, which involves ER-alpha. This pathway also appears to involve PKA and ERK-mediated phosphorylation of ser122 within the amino terminus of ER-alpha. Reduced availability of this pathway when estrogen levels are reduced could explain diminished effectiveness of mechanically related control of bone architecture after the menopause.

Localization of estrogen receptor beta protein expression in adult human bone

Evidence suggests that the newly described estrogen receptor beta (ER-beta) may be important for estrogen (17beta-estradiol) action on the skeleton, but its cellular localization in adult human bone requires clarification. We addressed this by using indirect immunoperoxidase with a novel affinity purified polyclonal antibody to human ER-beta, raised to hinge domain (D) sequences from the human receptor. Bone was demineralized in 20% EDTA and all biopsy specimens were formalin-fixed and wax-embedded. Vigorous retrieval was essential for ER-beta detection. In sections (5 microm) of benign prostate hyperplasia, used as positive control, clear nuclear immunoreactivity was seen in glandular epithelial cells, with a 1:500 dilution of ER-beta40. For bone sections, optimal antibody dilutions were 1:100-1:250. We found that in normal bone (from graft operations), in fracture callus from both men and women (>25 years old), pagetic bone, osteophytes, and secondary hyperparathyroid bone, all from older patients, ER-beta was expressed clearly in osteoclast nuclei, with little cytoplasmic immunoreactivity. Nuclear immunoreactivity was still prominent in osteoclasts, with antibody diluted 1:500, although it faded in other cells. Osteoblasts, in areas of active bone formation or bone remodeling, also expressed ER-beta, as did some osteocytes. However, hypertrophic chondrocytes were negative, unlike mesenchymal cells, adjacent to the osteogenesis. Megakaryocytes and some capillary blood vessels cells were receptor positive. All ER-beta expression was blocked totally by preincubation of antibody with antigen. We conclude that ER-beta is expressed in cells of osteoblast lineage and in osteoclasts. The latter appear relatively abundant in this receptor and this might provide a means for direct action of estrogen on osteoclasts.

Preliminary report of impaired oestrogen receptor-alpha expression in bone, but no involvement of androgen receptor, in male idiopathic osteoporosis

In western countries, osteoporosis affects at least 1 in 12 of all adult males and a third of osteoporotic men have idiopathic disease (MIO). Both oestrogen and testosterone are now known to be important to the male skeleton. As normal oestrogen levels have been found in younger MIO cases, it is hypothesized that, in bone, their responses to gonadal steroids may be defective, through impaired receptor expression. This study therefore compared oestrogen receptor (ER)-alpha and androgen receptor (AR) expression, by indirect immunofluorescence and semi-quantitative image analysis, in undecalcified fresh frozen bone sections from MIO patients (33-56 years), age-matched control men (n=7), and, for reference, ovarian steroid-replete (n=7) and -deficient women (n=6). In normal men, 23%+/-SEM 6% osteoblasts and 14%+/-SEM 2% osteocytes expressed ERalpha protein, similar to hormone-replete women. Although receptor expression decreased in hormone-deficient women, loss of ERalpha protein in MIO patients was more severe (1%+/-SEM 0.5% osteocytes, 2%+/-SEM 1% osteoblasts expressed receptor). In all four groups, there was little osteocyte AR expression, but in the women, a proportion of osteoblasts were receptor-positive. Deficient osteoblast and osteocyte ERalpha protein expression could explain the bone loss in these MIO patients.

Preliminary evidence for impaired estrogen receptor-alpha protein expression in osteoblasts and osteocytes from men with idiopathic osteoporosis

Although osteoporosis is usually associated with women, 1 in 12 men in the UK have the disease, and a third of these cases are idiopathic. Estrogen is now known to be associated with bone loss in older men, but we found, previously, that levels of this hormone were normal in younger cases of male idiopathic osteoporosis (MIO) in the age range 33-61 years. We therefore hypothesized that their estrogen responses in bone might be defective, through impaired estrogen receptor-alpha (ER)-alpha expression. Consequently, in the present study, we compared expression of ER-alpha by indirect immunofluorescence, semiquantitative image analysis, and in situ reverse transcription-polymerase chain reaction in bone sections from MIO patients (33-56 years) (N = 7); age-matched control men (N = 7); and, for reference, ovarian steroid (OS)-replete (N = 7) and OS-deficient women (N = 6). In the control men, 23 +/- 6% (mean +/- SEM) of osteoblasts and 14 +/- 2% of osteocytes expressed ER-alpha protein, similar to OS-replete women. Although receptor expression decreased in OS-deficient women, the loss of ER-alpha protein in MIO patients was more severe (1 +/- 0.5% osteocytes, 2 +/- 1% osteoblasts expressed receptor); however, ER-alpha messenger RNA (mRNA) was still expressed in controls and MIO patients. Bone loss in these patients may be due to deficient ER-alpha protein expression.