Correlation between low levels of estrogen receptors and estrogen responsiveness in two rat osteoblast-like cell lines

Gonadal Hormones and Bone

In both sexes, estrogen is one of the most essential hormones for maintaining bone integrity. Also, especially in men, androgen has beneficial effects on bone independent of estrogen. However, estrogen replacement therapy for postmenopausal women increases the risk of developing breast cancer and endometrial cancer, and androgen replacement therapy for partial androgen deficiency of the aging male increases the risk of developing prostate cancer. Various mechanisms have been proposed on the effects of gonadal hormones on bone, such as effects through cytokines including IL-6 and effects on the OPG/RANKL ratio. In addition, large amounts of new information deriving from high-throughput gene expression analysis raise the possibility of multiple other effects on bone cells. Both estrogen and androgen exert their effects via the estrogen receptor (ER) or the androgen receptor (AR), which belongs to the nuclear receptor superfamily. Compounds such as selective estrogen receptor modulators (SERMs) and selective androgen receptor modulators (SARMs) also bind ER and AR, respectively. However, SERMs and SARMs alter the ER or AR structure differently from estrogen or androgen, resulting in other downstream gene responses. As a result they can exert favorable effects on bone while suppressing the undesirable actions of estrogen and androgen. Elucidation of ER and AR ligand-specific and tissue-specific gene regulation mechanisms will also provide information on the signal transduction mechanisms of other nuclear receptors and will be valuable for the development of new therapeutic agents.

Calycosin, a Phytoestrogen Isoflavone, Induces Apoptosis of Estrogen Receptor-Positive MG-63 Osteosarcoma Cells via the Phosphatidylinositol 3-Kinase (PI3K)/AKT/Mammalian Target of Rapamycin (mTOR) Pathway

Background

Osteosarcoma is the most common primary bone malignancy and often presents at an early age. Calycosin is a phytoestrogen isoflavone, which has previously been reported to inhibit tumor cell growth. The aim of this study was to investigate the effects of calycosin on apoptosis of estrogen receptor (ER)-positive and ER-negative human osteosarcoma cell lines and tumor xenografts in mice.

Material/Methods

Cultured ER-positive MG-63 human osteosarcoma cells and ER-negative U2-OS human osteosarcoma cells were treated with increasing doses of calycosin (0, 25, 50, and 100 μm). Cell viability and apoptosis were studied by an MTT assay and flow cytometry. Western blot measured the expression levels of the apoptosis-related protein p-PI3K, p-Akt, and p-mTOR in MG-63 cells, with and without pretreatment with the PI3K inhibitor, LY294002, the AKT inhibitor, MK-2206, or the mTOR inhibitor, rapamycin. MG-63 tumor-bearing nude mice were used to evaluate the effects of treatment with calycosin.

Results

Calycosin treatment inhibited proliferation and induced apoptosis in MG-63 cells, but had no effect on U2-0S cells. In MG-63 cells, calycosin treatment increased the expression of the PI3K/AKT/mTOR pathway proteins; inhibitor assays showed that expression of the PI3K protein was most strongly associated with the antitumor effects of calycosin. In the nude mouse MG-63 tumor xenografts, calycosin inhibited tumor growth and regulated the expression levels of apoptosis-related PI3K/AKT/mTOR pathway proteins.

Conclusions

The phytoestrogen, calycosin, induced apoptosis of cells of the ER-positive osteosarcoma cell line, MG-63, via the PI3K/AKT/mTOR pathway, with these effects being mainly due to PI3K.

Puerarin stimulates proliferation and differentiation and protects against cell death in human osteoblastic MG-63 cells via ER-dependent MEK/ERK and PI3K/Akt activation

Puerarin, the main isoflavone glycoside found in the Chinese herb radix of Pueraria lobata (Willd.) Ohwi, has received increasing attention because of its possible role in the prevention of osteoporosis. Previously, we showed that puerarin could inhibit the bone absorption of osteoclasts and promote long bone growth in fetal mouse in vitro. Further study confirmed that puerarin stimulated proliferation and differentiation of osteoblasts in rat. However, the mechanisms underlying its actions on human bone cells have not been well defined. Here we show that puerarin increases proliferation and differentiation and opposes cisplatin-induced apoptosis in human osteoblastic MG-63 cells containing two estrogen receptor (ER) isoforms. Puerarin promotes proliferation by altering cell cycle distribution whereas puerarin-mediated survival may be associated with up-regulation of Bcl-xL expression. Treatment with the ER antagonist ICI 182,780 abolishes the above actions of puerarin on osteoblast-derived cells. Using small interfering double-stranded RNA technology, we further demonstrate that the effects of puerarin on proliferation, differentiation and survival are mediated by both ERα and ERβ. Moreover, we also demonstrate that puerarin functions at least partially through activation of MEK/ERK and PI3K/Akt signaling. This agent also shows much weaker effect on breast epithelial cell growth than that of estrogen. Therefore, puerarin will be a promising agent that prevents or retards osteoporosis.

LC, a novel estrone-rhein hybrid compound, promotes proliferation and differentiation and protects against cell death in human osteoblastic MG-63 cells

Estrogen analogs are promising drugs for postmenopausal osteoporosis, but because of their possible side effects, estrogens which exert their estrogenic effects selectively on bone are desired. Based on our previous studies that rhein had high affinity for the bone mineral, we synthesized estrone-rhein hybrid compounds and confirmed that one of these hybrid compounds, LC, exhibited a selective profile in the bone and prevented bone loss but had no effect on endometrium growth in ovariectomized rats. However, the mechanisms underlying its actions on human bone cells have remained largely unknown. Here we show that LC increases proliferation and differentiation and opposes cisplatin-induced apoptosis in human osteoblastic MG-63 cells containing two estrogen receptor (ER) isoforms. LC promotes proliferation by altering cell cycle distribution whereas LC-mediated survival may be associated with up-regulation of X-linked inhibitor of apoptosis (XIAP) expression. Treatment with the ER antagonist ICI 182,780 abolishes the above actions of LC on osteoblast-derived cells. Using small interfering double-stranded RNAs technology, we further demonstrate that the effects of LC on proliferation and survival are mediated by both ERα and ERβ but those on differentiation primarily by ERα. Moreover, we demonstrate that LC may promote activation of the classic estrogen response element (ERE) pathway through increasing steroid receptor coactivator (SRC)-3 expression. Meanwhile, we find that regulation of osteoblastic proliferation and survival by LC involves Ras/MEK/ERK and PI3K/Akt signaling. Therefore, using rhein for conjugating compounds is a promising method of effectively targeting estrogens to the bone.

Characterization of osteoblastic properties of 7F2 and UMR-106 cultures after acclimation to reduced levels of fetal bovine serum

Estrogen plays an important role in skeletal physiology by maintaining a remodeling balance between the activity of osteoblasts and osteoclasts. In an attempt to decipher the mechanism through which estrogen elicits its action on osteoblasts, experimentation necessitated the development of a culturing environment reduced in estrogenic compounds. The selected medium (OPTI-MEM) is enriched to sustain cultures under reduced fetal bovine serum (FBS) conditions and is devoid of the pH indicator phenol red, a suspected estrogenic agent. This protocol reduced the concentration of FBS supplementation to 0% through successive 24 h incubations with diminishing amounts of total FBS (1%, 0.1%, and 0%). The protocol does not appear to alter the viability, cell morphology, or osteoblast-like phenotype of 7F2 and UMR-106 cell lines when compared with control cells grown in various concentrations of FBS. Although the rate of mitotic divisions declined, the 7F2 and UMR-106 cultures continued to express osteoblast-specific markers and exhibited estrogen responsiveness. These experimental findings demonstrate that the culture protocol developed did not alter the osteoblast nature of the cell lines and provides a model system to study estrogen's antiresorptive role on skeletal turnover.

Quantitative structure activity relationship (QSAR) study of estrogen derivatives based on descriptors of energy and softness

Quantum chemical interaction of estrogen derivatives with their receptor has been explored by using Klopman atomic softness. Four series of estrogen derivatives were taken from the literature and the structure of receptor (PDB code 1QKT) was obtained from the protein databank. It is proposed that three Lys, a His, a Tyr and a Cys residues are important for binding. The basic softness values (E(m)(double dagger)) and acidic softness values (E(n)(double dagger)) of all atoms of estrogen derivatives were evaluated. The required parameters for Klopman equation were taken from PM3 results. The highest E(n)(double dagger) values for each molecules and highest E(m)(double dagger) value for each residue were identified and Delta E(nm)(double dagger) has been derived using them. The lowest Delta E(nm)(double dagger) values were used in addition to Q(min) (highest negative charge), Delta H(f)(0) (heat of formation), E(T) (total energy), and E(E) (electronic energy). Multiple linear regression analysis was employed to correlate the variation of relative binding affinity values. The analyses show that Delta E(nm)(double dagger) values in combination with other descriptors provide significant correlation with relative binding affinity values. The result underscores that carbonyl oxygen of the receptor is important for interaction with estrogen derivatives. This model could be utilized to predict the binding affinity of a new compound of this series.

Identification of estrogen-responsive genes involved in breast cancer metastases to the bone

Bone metastasis is the most common metastasis in breast cancer patients. Clinical observations propose strong association between estrogen receptor (ER)-positive tumors and the development of bone metastases. We hypothesized of biologically diverse sets of hormone-dependent tumors predisposed to bone metastases and of possible role of ER-signaling pathways in the development and progression of bone metastases. We developed a novel in vitro estrogen (E2)-responsive model system, in which breast cancer cells and bone cells express high levels of either ERalpha or ERbeta. Using co-culture approach and gene array technology we identified E2-responsive genes involved in the interaction between cancer cells and bone cells. We detected 13 genes that were altered solely by ERalpha and 11 genes that were regulated solely by ERbeta in cancer cells. Only 5 genes were modified by both ERalpha and ERbeta. Interestingly, the majority of genes in bone cells were altered through ERbeta. Two genes, namely MacMarcks and Muc-1, whose changes in expressions in cancer cells in response to E2 were highly significant, were selected for immunohistochemical analysis using tissue microarrays of 59 infiltrating ductal carcinomas. Our results indicated that both MacMarcks and Muc-1 were expressed at high frequency in ER-positive tumors. The correlation between ERalpha- and ERbeta-status of hormone-dependent tumors with combined expression of these two markers might suggest a more aggressive tumor phenotype associated with bone metastases. Further analysis of tissues with clinicopathological characteristics and known bone metastatic disease will indicate potential prognostic values of these and other markers in the development of bone metastases in a subgroup of "bad" hormone-dependent breast cancer.

Expression of Estrogen Receptors Alfa and Beta mRNA and Alkaline Phosphatase in the Differentiation of Osteoblasts from Elderly Postmenopausal Women: Comparison with Osteoblasts from Osteosarcoma Cell Lines

OBJECTIVE

To evaluate the expression of estrogen receptors (ER) alpha and beta, and activity of alkaline phosphatase during differentiation of primary osteoblast cells (hOB) from aged postmenopausal women and human osteosarcoma cell lines (HOS, MG63).

MATERIALS AND METHODS

Osteoblast cultures were prepared from the upper femur of postmenopausal patients (age, 60-74 years) and HOS. At the indicated times (days 5, 10, 15, 20, and 25), alkaline phosphatase activity and expression of ERalpha and ERbeta mRNA were evaluated.

RESULTS

In both cultures of primary hOB and HOS, alkaline phosphatase activity decreased at the osteoblast proliferation stage, whereas it subsequently increased at the matrix maturation stage. ER beta mRNA was strongly expressed in HOS on day 15 and remained at high levels of transcription through to day 25 (matrix maturation phase), whereas ERalpha mRNA was barely detectable during osteoblast differentiation. In hOB, transcription of ERalpha mRNA was much stronger than that of ERbeta mRNA.

CONCLUSION

The presence of ERalpha and ERbeta mRNA in osteoblasts supports the involvement of estrogen in human bone formation. The developmental expression of alkaline phosphatase was not correlated to ER mRNA expression during osteoblast differentiation. ER isoforms may have different functions or interact with each other during osteoblast differentiation. Since the expression of ER isoforms is different between postmenopausal women and osteosarcoma cell lines, characteristics of osteosarcoma cell lines may not be suitable as a model for the evaluation of estrogen effects on postmenopausal osteoporosis.

Identification of a PTH regulated gene selectively induced in vivo during PTH-mediated bone formation

The biological activities of parathyroid hormone (PTH) on bone are quite complex as demonstrated by its catabolic and anabolic activities on the skeleton. Although there have been many reports describing genes that are regulated by PTH in osteoblast cells, the goal of this study was to utilize a well-established in vivo PTH anabolic treatment regimen to identify genes that mediate bone anabolic effects of PTH. We identified a gene we named PTH anabolic induced gene in bone (PAIGB) that has been reported as brain and acute leukemia cytoplasmic (BAALC). Therefore, using the latter nomenclature, we have discovered that BAALC is a PTH-regulated gene whose mRNA expression was selectively induced in rat tibiae nearly 100-fold (maximal) by a PTH 1-34 anabolic treatment regimen in a time-dependent manner. Although BAALC is broadly expressed, PTH did not regulate BAALC expression in other PTH receptor expressing tissues and we find that the regulation of BAALC protein by PTH in vivo is confined to mature osteoblasts. Further in vitro studies using rat UMR-106 osteoblastic cells show that PTH 1-34 rapidly induces BAALC mRNA expression maximally by 4 h while the protein was induced by 8 h. In addition to being regulated by PTH 1-34, BAALC expression can also be induced by other bone forming factors including PGE(2) and 1,25 dihydroxy vitamin D(3). We determined that BAALC is regulated by PTH predominantly through the cAMP/PKA pathway. Finally, we demonstrate in MC3T3-E1 osteoblastic cells that BAALC overexpression regulates markers of osteoblast differentiation, including downregulating alkaline phosphatase and osteocalcin expression while inducing osteopontin expression. We also demonstrate that these transcriptional responses mediated by BAALC are similar to the responses elicited by PTH 1-34. These data, showing BAALC overexpression can mimic the effect of PTH on markers of osteoblast differentiation, along with the observations that BAALC is induced selectively with a bone anabolic treatment regimen of PTH (not a catabolic treatment regimen), suggest that BAALC may be an important mediator of the PTH anabolic action on bone cell function.

Long-term culture in dexamethasone unmasks an abnormal phenotype in osteoblasts isolated from osteoporotic subjects

We have shown that osteoblastic cells derived from trabecular bone explants of osteoporotic subjects (OP cells) exhibited an altered alkaline phosphatase (ALP) response to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] compared to control (CON) cells. Our hypothesis that OP cells have other intrinsic abnormalities was investigated using our cell models representing two different stages of differentiation. OP and CON cells were cultured in the absence (-DEX) or presence (+DEX) of 10 nM dexamethasone (DEX) in 10% fetal calf serum (FCS) prior to exposure to serum-free medium containing 1 nM of PTH and/or 17-beta estradiol (E2). Both OP and CON cells responded to DEX with a two-fold increase in basal ALP activity. While E2 or PTH+E2 had no effect on OP cells, both treatments inhibited ALP activity in CON cells (p<0.05). OP and CON cells grown in DEX also expressed PTH-stimulated adenylate cyclase (AC) activities higher than those of (-DEX) cells. OP+DEX cells, however, exhibited activities which were 8-fold higher than those of CON+DEX cells (p<0.001). In OP+DEX cells, E2 stimulated basal AC activity (p<0.05) but did not affect PTH-stimulated activity. In contrast, in CON+DEX cells, E2 had no effect on basal activity but inhibited PTH-stimulated AC activity (p<0.001). Osteocalcin production was 4-fold lower in OP+DEX cells compared to OP-DEX and CON cells (p<0.05) while osteocalcin mRNA levels were significantly lower in OP+DEX and CON+/-DEX cells compared to OP-DEX cells (p<0.05). E2 did not affect osteocalcin protein or mRNA levels in either OP or CON cells. No differences in mRNA levels were found for estrogen receptor-alpha (ER-a) in OP+/-DEX cells whereas these levels were significantly higher in CON+DEX compared to CON-DEX cells (p<0.05). These results indicate that DEX amplified the differences between OP and CON cells and confirm the presence of intrinsic osteoblastic abnormalities in patients with osteoporosis that persist in culture.

Semiempirical QSAR study and ligand receptor interaction of estrogens

Softness values E(n)+/+ of estrogen derivatives and softness values E(m)+/+ of receptor lysine, histidine, tyrosine and cysteine have been evaluated by Klopman equation. The required parameters for the solution of Klopman equation have been calculated with the help of PM3 method. The difference deltaE(nm)+/+ between E(n)+/+ and E(m)+/+ has been derived for QSAR study. The estrogen derivatives have been divided into four different sets on the basis of their structural similarities, and their biological activity taken from literature in terms of relative binding affinity (RBA). The QSAR study shows that, deltaE(nm)+/+ values provide good relationship with biological activity.

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.

Abnormal bone composition in female juvenile American alligators from a pesticide-polluted lake (Lake Apopka, Florida)

Reproductive disorders have been found in pesticide-exposed alligators living in Lake Apopka, Florida (USA). These disorders have been hypothesized to be caused by exposure to endocrine- disruptive estrogen-like contaminants. The aim of this study was to expand our analysis beyond previous studies by investigating whether bone tissue, known to be affected by sex steroid hormones, is a potential target of endocrine disruptors. Long bones from 16 juvenile female alligators from Lake Apopka (pesticide-contaminated lake) and Lake Woodruff (control lake) were evaluated by peripheral quantitative computed tomography. We observed significant differences in bone composition, with female alligators from the contaminated lake having greater trabecular bone mineral density (BMD), total BMD, and trabecular mineral content compared with females from the control lake (p < 0.05). Increased trabecular and total BMD measurements suggest that juvenile female alligators from Lake Apopka were exposed to contaminants that created an internal environment more estrogenic than that normally observed. This estrogenic environment could be caused by both natural and anthropogenic compounds. Effects on BMD indicate interference with bone homeostasis. We hypothesize that contaminants present in the lake inhibit the natural and continuous resorption of bone tissue, resulting in increased bone mass. Although this is the only study performed to date examining effects of environmental estrogenic compounds on alligator bones, it supports previous laboratory-based studies in rodents. Further, this study is important in demonstrating that the alterations in morphology and physiology induced in free-ranging individuals living in environments contaminated with endocrine-active compounds are not limited to a few systems or tissues; rather, effects can be observed in many tissues affected by these hormones.

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.

Mechanical strain activates estrogen response elements in bone cells

The involvement of the estrogen receptor in the early responses of bone cells to mechanical strain was investigated by subjecting subconfluent monolayer cultures of ROS.SMER #14 cells (ROS 17/2.8 cells stably transfected with additional ER alpha) to 17 beta-estradiol or a single short period of dynamic mechanical strain (600 cycles, 1 Hz). The basal proliferation rate of ROS.SMER #14 cells was similar to ROS 17/2.8 cells, whose proliferative responsiveness to strain and estrogen is similar to that of primary cultures of rat long bone-derived osteoblasts. At peak strains of 3400 mu epsilon, strain-related proliferation in ROS.SMER #14 cells was 1.4 times that of ROS 17/2.8 cells. At 10(-8) mol/L, 17 beta-estradiol-related proliferation was nearly twice greater. The ROS.SMER #14 cells were transiently transfected with an estrogen-responsive reporter, 2ERE-pS2-CAT, containing two consensus estrogen response elements (ERE) linked to a chloroamphenicol acetyl transferase gene. Strain increased normalized ERE-CAT activity threefold and estradiol (10(-8) mol/L) sixfold. Both strain-related and estradiol-related increases in proliferation and ERE-CAT activity were blocked by the estrogen antagonist ICI 182,780 (10(-6) mol/L). These data show that strain as well as estrogen stimulates increased proliferation in ROS 17/2.8 cells and increased ER alpha-related ERE activity in ROS cells transfected with ER alpha. Proliferation is greater in the cells with more estrogen receptors. Both strain- and estrogen-related proliferation and ERE activity are blocked by the estrogen antagonist ICI 182,780. This indicates that ROS cells' early responses to mechanical strain involve ER alpha and estrogen-responsive genes.

Mechanical strain stimulates ROS cell proliferation through IGF-II and estrogen through IGF-I

The mechanism by which mechanical strain stimulates bone cell proliferation was investigated and compared with that of estrogen in ROS 17/2.8 cells. Similarity of strain-related responses between ROS cells and osteoblasts was established by demonstrating that ROS cells respond to a short single period of strain in their substrate (1000-3500 microepsilon, 600 cycles, 1 Hz) by a similar strain magnitude-related increase in glucose 6-phosphate dehydrogenase activity as rat osteoblasts and osteocytes in explants in situ. ROS17/2.8 cells also showed similar proliferative responses to strain and 17beta-estradiol, as assessed by [3H]thymidine incorporation and cell counting, as primary cultures of long bone-derived osteoblast-like cells. Strain-related increase in proliferation in ROS cells was accompanied by a 4-fold increase in levels of insulin-like growth factor-II (IGF-II) in conditioned medium. Neither strain nor estrogen had an effect on the conditioned medium levels of IGF-I. Exogenous truncated IGFs tIGF-I and tIGF-II both increased proliferation in a dose-dependent manner. The neutralizing monoclonal antibody (nMAb) to IGF-I blocked proliferation stimulated by tIGF-I but not that due to tIGF-II and vice versa. IGF-I receptor blocking antibody (IGF-IRBAb) blocked the proliferative effect of tIGF-I but not that to tIGF-II. The proliferative effect of estrogen was abolished by IGF-I nMAb and IGF-IRBAb, but these antibodies had no effect on the proliferative response to strain. In contrast IGF-II nMAb abolished the proliferative effect of strain but had no effect on that of estrogen. These data show that ROS17/2.8 cells have similar responses to strain and estrogen qualitatively and quantitatively as rat osteoblasts in situ and rat long bone-derived osteoblast-like cells in primary culture. Estrogen-related proliferation in ROS17/2.8 cells appears to be mediated by IGF-I acting through the IGF-I receptor and does not involve IGF-II. In contrast, strain-related proliferation appears to be mediated by IGF-II and does not involve either IGF-I or the IGF-I receptor.

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

The mechanism by which oestrogen and hormone replacement therapy (HRT) maintain bone mass in women is still unclear. It has previously been shown that cells of osteoblast lineage in vivo, particularly osteocytes, express oestrogen receptor alpha (ERalpha). Nevertheless, it is still debatable whether oestrogen and the ovarian steroids have a direct affect on osteocytes. If they could regulate osteocyte ERalpha expression, this would be strong evidence for the involvement of these cells in the hormonal regulation of bone mass. This study therefore aimed to compare bone biopsies from women who were replete with ovarian steroids (pre-ovariectomy or post-HRT) with those from the same women when hormone-deficient (post-ovariectomy or pre-HRT) for cellular localization of ERalpha protein or mRNA expression by indirect immunofluorescence, or by in situ hybridization combined with reverse transcriptase-polymerase chain reaction (IS-RT-PCR) respectively. Image analysis showed that proportions of osteocytes positive for immunodetectable ERalpha were higher in hormone-replete than in hormone-deficient women (25+/-SEM 3 per cent, 12+/-SEM 4 per cent, respectively; n=5), with similar but non-statistically significant changes in osteoblasts. This was observed even when HRT was commenced 18 years after menopause. In contrast, grain volume/unit cell area of osteoblast mRNA signal was markedly higher when hormone-deficient (0.055+/-0.01) than when hormone-replete (0.016+/-0.004), with similar but non-significant differences in osteocytes. This preliminary study indicates up-regulation of osteocyte ERalpha protein by ovarian steroids in these patients, which is accompanied by decreased osteoblast ERalpha mRNA expression, providing further evidence for the involvement of osteocytes in the regulation of skeletal structure by ovarian steroids.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]

Expression and localization of estrogen receptor-beta in murine and human bone

Estrogens have profound effects on bone metabolism. Cellular responses to estrogens are mediated by estrogen receptors (ERs) which belong to the nuclear receptor superfamily. Two estrogen receptors, ERalpha and ERbeta, have been cloned. Previously expression of ERalpha has been shown in osteoblasts. Here we demonstrate that the transcript for ERbeta can be detected in the human osteosarcoma cell lines (MG-63 and SaOS-2) and in cultured human osteoblast-like cells. We also show that ERbeta protein is present in nuclear extracts from these cells. Furthermore, ERbeta immunoreactivity is found in sections of murine and human bone. Murine and human osteoblast and osteocyte nuclei are immunoreactive for ERbeta. Osteoclasts are also ERbeta immunoreactive but the staining is mainly cytoplasmic. The present study demonstrates that ERbeta is present in all the cellular compartments involved in bone formation and bone resorption, both in human and in murine bone tissue.

Characterization of aromatase and 17 beta-hydroxysteroid dehydrogenase expression in rat osteoblastic cells

Postmenopausal loss of 17 beta-estradiol (E2) in women is associated with decreased bone mineral density and increased susceptibility to osteoporotic bone fracture. These changes in bone status are assumed to be due to circulating levels of the hormone; therapeutic replacement of E2 can alleviate the bone disease. However, recent reports have shown that human osteoblastic (OB) cells are able to synthesize estrogens locally, via expression of the enzyme aromatase. In this study, we have characterized the expression and activity of aromatase and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) in rat OB cell lines. Aromatase activity in ROS 17/2.8, ROS 25/1, and UMR 106 cells was similar to that shown in human OB cells, with the highest levels of activity observed in the more differentiated ROS 17/2.8 cells (Vmax = 45 pmol/h/mg of protein). The rat OB cells also showed 17 beta-HSD activity, with the predominant metabolism in all three cell lines being estrone (E1) to E2. As with aromatase, the highest activity was observed in ROS 17/2.8 cells (Vmax = 800 pmol/h/mg of protein). Northern analyses indicated the variable presence of transcripts corresponding to the type 1, 2, 3, and 4 isoforms of 17 beta-HSD. Further analysis of androstenedione metabolism indicated that the net effect of aromatase and 17 beta-HSD activity varied with cell type and culture treatment. All three OB cell lines were able to synthesize E1, E2, and testosterone from androstenedione, although activity varied between OB cell types. Regulatory effects were observed with 1,25-dihydroxyvitamin D3 (positive) and dexamethasone (negative). These data suggest that local synthesis of sex hormones is an important function of OB cells and may play a key role in the modulation of bone turnover independent of circulating hormone concentrations.

Estrogen Receptor-α Is Developmentally Regulated during Osteoblast Differentiation and Contributes to Selective Responsiveness of Gene Expression

Estrogen responsiveness of bone is a fundamental regulatory mechanism operative in skeletal homeostasis. We examined the expression of estrogen receptor-alpha (ER) messenger RNA (mRNA) in cultured rat calvarial-derived osteoblasts during progressive development of the osteoblast phenotype. Levels of ER message were compared with the expression of traditional osteoblastic markers that have been mapped throughout the differentiation process of these cells. ER transcripts, measured using semiquantitative RT-PCR analysis, were expressed at low levels in early stage proliferating osteoblasts and increased at confluence upon initial expression of bone cell phenotypic genes. A 23-fold up-regulation of ER mRNA expression coincided with the initiation of alkaline phosphatase activity (day 8). ER mRNA levels progressively increased 70-fold, reaching a maximum level on days 22-25 in fully differentiated osteoblasts when osteocalcin expression peaked, but declined precipitously by day 32 in osteocytic cells. Analysis of RNA isolated directly from rat calvaria confirmed these in vitro results and demonstrated that ER message levels become more abundant postnatally as bone becomes more mineralized. We also examined the responsiveness of osteoblasts to 17beta-estradiol (17beta-E2) at two periods of maturation: the nodule-forming stage (day 14) and the late mineralization stage (day 30). Estradiol suppressed the levels of alkaline phosphatase, osteocalcin, osteonectin, and ER mRNAs on day 14, but up-regulated these messages on day 30. In contrast, 17beta-E2 treatment regulated the steady state levels of transforming growth factor-beta1 and type I procollagen mRNAs only in the late mineralization stage, whereas histone H4 message was unaffected by the steroid at either stage of differentiation. Thus, the observed developmental expression of ER mRNA correlates with progressive osteoblast differentiation and may be a contributing factor to differential regulation of bone cell gene expression by 17beta-E2.

Estrogen regulation of a transforming growth factor-beta inducible early gene that inhibits deoxyribonucleic acid synthesis in human osteoblasts

This laboratory reported the identification and characterization of a unique three zinc finger, transcription factor-like, transforming growth factor-beta inducible early gene (TIEG) (see Ref. 35). TIEG expression has been shown to be tissue- and cell type specific, enhanced by specific growth factors, and to decrease with advancing stages of breast cancer. Recent studies involving TIEG overexpression in pancreatic carcinoma cells indicate that TIEG expression inhibits DNA synthesis, similar to a tumor suppressor-like gene, and plays a role in apoptosis (see Ref. 37). This paper describes the rapid, but transient, induction of TIEG steady-state messenger RNA (mRNA) levels by 17beta-estradiol (E2) in estrogen receptor (ER)-positive, human fetal osteoblastic (hFOB/ER) cells. This rapid induction is shown to be ER- and steroid dose-dependent but protein synthesis independent. An antagonism between E2 and PTH, which occurs in skeletal metabolism, is shown to concur rapidly with TIEG mRNA expression. Scanning confocal microscopy (using polarized, laser-based immunofluorescence) shows that TIEG protein is localized in the nucleus of hFOB/ER cells, with the levels rapidly increasing after E2 treatment. The rapid E2-induced increase in TIEG expression is followed by an E2-induced inhibition of DNA synthesis in the hFOB/ER cells. Antiestrogens block not only the induction of TIEG mRNA levels but also the inhibition of cell proliferation. Lastly, hFOB cells, stably transfected with a TIEG expression vector, display markedly reduced DNA synthesis/cell proliferation, compared with nontransfected cells. These results support the finding that TIEG is an early responding regulatory gene for E2 in human osteoblast cells that inhibits DNA synthesis. It is speculated that TIEG may play a role in the signaling pathway for E2 in inhibiting cell proliferation.

Estrogen receptor expression and growth-promoting function in human choriocarcinoma cells

Tamoxifen (1.0 microM) was found to inhibit the expression of a thymidine kinase (TK) promoter-reporter gene, lacking an estrogen response element (ERE), in transiently transfected BeWo cells, suggesting that inhibition of TK promoter activity was linked to secondary estrogen-dependent effects on BeWo cell function. Estradiol (0.05-0.45 microM) stimulated BeWo cell proliferation and increased the percentage of S-phase cells. Tamoxifen (1.35-4.05 microM) inhibited BeWo cell growth and antagonized the stimulatory actions of 0.15 microM estradiol. Reverse transcription-polymerase chain reaction and Western analyses confirmed the presence of estrogen receptor (ER) transcripts and the 67-kD ER in BeWo cells. The BeWo cell ER binds to an ERE consensus sequence and the ER-ERE complex is supershifted by antibodies directed against the ER. We conclude that BeWo cells express a functional ER that is important for the control of BeWo cell proliferation, suggesting a potential role for estrogens in mediating placental trophoblast growth and development.

Estrogen receptor mRNA in mineralized tissues of rainbow trout: calcium mobilization by estrogen

RT-PCR was undertaken on total RNA extracts from bone and scales of the rainbow trout, Oncorhynchus mykiss. The rainbow trout estrogen receptor (ER)-specific primers used amplified a single product of expected size from each tissue which, using Southern blotting, strongly hybridized with a 32P-labelled rtER probe under stringent conditions. These data provide the first in vivo evidence of ER mRNA in bone and scale tissues of rainbow trout and suggest that the effects of estrogen observed in this study (increased bone mineral and decreased scale mineral contents, respectively) may be mediated directly through ER.

Estrogen regulation of human osteoblastic cell proliferation and differentiation

Estrogen (E2) has been shown to prevent bone loss among postmenopausal women. The molecular mechanism(s) by which this is accomplished is not clear. The discovery of E2 receptor (ER) in osteoblasts and osteoclasts has implicated these cells as direct targets for E2. Previous studies on the effects of E2 on osteoblastic cells in vitro or in organ culture present conflicting results, possibly due to heterogeneity in cell types, stage of differentiation, ER levels, and/or species differences. The effects of E2 on gene expression during various stages of human osteoblast cell differentiation has not been investigated extensively. In this study we employed a newly developed human fetal osteoblastic cell line (hFOB/ER9) that contains high levels of ER to examine the effects of E2 on osteoblast proliferation and differentiation. The basal levels and E2 effects on the expression of various extracellular matrix proteins were also characterized throughout different stages of differentiation. These stages include a proliferative/relatively undifferentiated stage (day 6), a matrix maturation stage (days 10-14), and a mineralization/calcified nodule stage (day 18). During the stage of rapid cell proliferation, E2 treatment of hFOB/ER9 cells resulted in a dose-dependent decrease in [3H]thymidine incorporation to a maximum of 72% compared to the vehicle control value. Treatment of hFOB/ER9 cells with 10(-9) M E2 for 48 h resulted in an increase in alkaline phosphatase (AP) activity throughout cell differentiation. The magnitude of AP induction varied from approximately 200-500%. In contrast, E2 decreased osteocalcin protein levels to a minimum of 54% compared to the vehicle control value. The steady state messenger RNA levels for AP increased and osteocalcin decreased after E2 treatment, similar to the responses observed at the protein level. At all stages, there was little or no effect of E2 on type I collagen protein levels or osteonectin steady state messenger RNA levels. The E2 responses on hFOB/ER9 cell matrix protein expression and cell proliferation were mediated through the ER, as cultures cotreated with a 100-fold molar excess of a type II anti-E2 (ICI 182,780) abrogated these effects. These results support the hypothesis that E2 does have an effect on osteoblastic differentiation by decreasing hFOB/ER9 cell proliferation and differentially regulating extracellular matrix expression.

Functional properties of a conditionally phenotypic, estrogen-responsive, human osteoblast cell line

Osteoblasts are established targets of estrogen action in bone. We screened 66 conditionally immortalized clonal human osteoblast cell lines for estrogen receptors (ERs) using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis for ER alpha mRNA and transactivation of adenovirus-estrogen response element (ERE)-tk-luciferase by 17 beta-estradiol (17 beta-E2) for functional ER protein. One of these cell lines, termed HOB-03-CE6, was chosen for further characterization. The cells, which were conditionally immortalized with a temperature-sensitive SV40 large T antigen, proliferated at the permissive temperature (34 degrees C) but stopped dividing at the nonpermissive temperature (> or = 39 degrees C). Alkaline phosphatase activity and osteocalcin secretion were upregulated by 1 alpha, 25-dihydroxyvitamin D3 in a dose-dependent manner. The cells also expressed type I collagen and other bone matrix proteins, secreted a variety of growth factors and cytokines, formed mineralized nodules based on alizarin red-S and von Kossa histochemical staining, and responded to dexamethasone, all-trans retinoic acid, and transforming growth factor-beta 1. This cell line expressed 42-fold less ER message than MCF-7 human breast cancer cells, as determined by quantitative RT-PCR. However, adenovirus-ERE-tk-luciferase activity was upregulated three- to fivefold in these cells by 17 beta-E2 with an EC50 of 64 pM. Furthermore, this upregulation was suppressed by co-treatment with the anti-estrogen ICI-182, 780. Cytosolic extracts of these cells specifically bound [125I]-17 beta-E2 in a concentration-dependent manner with a Bmax of 2.7 fmoles/mg protein (approximately 1,200 ERs/cell) and a Kd of 0.2 nM. DNA gel-shift analysis using a [32P]-ERE demonstrated the presence of ERs in nuclear extracts of these cells. Moreover, binding of the extracts to this ERE was blocked by a monoclonal antibody to the human ER DNA-binding domain. We evaluated these cells for 14 of 20 reported endogenous responses to 17 beta-E2 in osteoblasts. Although most of these responses appeared to be unaffected by the steroid, 17 beta-E2 suppressed parathyroid hormone-induced cAMP production, as well as basal interleukin-6 mRNA expression; conversely, the steroid upregulated the steady-state expression of alkaline phosphatase message in these cells. In summary, we have identified a clonal, conditionally phenotypic, human osteoblast cell line that expresses functional ERs and exhibits endogenous responses to 17 beta-E2. This cell line will be a valuable in vitro model for exploring some of the molecular mechanisms of estrogen action in bone.

Evidence of a correlation of estrogen receptor level and avian osteoclast estrogen responsiveness

Isolated osteoclasts from 5-week-old chickens respond to estradiol treatment in vitro with decreased resorption activity, increased nuclear proto-oncogene expression, and decreased lysosomal enzyme secretion. This study examines osteoclasts from embryonic chickens and egg-laying hens for evidence of estrogen responsiveness. Although osteoclasts from both of these sources express estrogen receptor mRNA and protein, estradiol treatment had no effect on resorption activity. In contrast to the lack of effect on resorption, estradiol treatment for 30 minutes resulted in steady-state mRNA levels of c-fos and c-jun increasing in osteoclasts from embryonic chickens and decreasing in osteoclasts from egg-laying hens. These data suggest that a nuclear proto-oncogene response may not be involved in estradiol-mediated decreased osteoclast resorption activity. To examine the influence of circulating estrogen on osteoclast estrogen responsiveness, 5-week-old chickens were injected with estrogen for 4 days prior to sacrifice. Estradiol treatment of osteoclasts from these chickens did not decrease resorption activity in vitro. Transfection of an estrogen receptor expression vector into osteoclasts from the estradiol-injected chickens and egg-laying hens restored estrogen responsiveness. Osteoclasts from 5-week-old chickens and estradiol treated 5-week-old chickens transfected with the estrogen receptor expression vector contained significantly higher levels of estrogen receptor protein and responded to estradiol treatment by decreasing secretion of cathepsins B and L and tartrate-resistant acid phosphatase. In contrast, osteoclasts from embryonic chickens, egg-laying hens, and estradiol-treated 5-week-old chickens either untransfected or transfected with an empty expression vector did not respond similarly. These data suggest that modulation of osteoclast estrogen responsiveness may be controlled by changes in the osteoclast estrogen receptor levels.

Immunohistochemical localization of the estrogen receptor in human osteoblastic SaOS-2 cells: association of receptor levels with alkaline phosphatase activity

We have previously shown that the combination of estrogen (E2) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] enhanced alkaline phosphatase (ALP) activity in human osteosarcoma SaOS-2 cells which had been grown in the presence of 10 nmol/L dexamethasone (SaOS + DEX cells). To determine whether this increase in ALP activity was associated with changes in receptor protein levels for E2 (ER) in individual SaOS + DEX cells, a monoclonal antibody to ER and a histochemical stain for ALP were used localize the expression of these proteins in fixed cells. Western and Northern blot analyses were used to determine whether E2 and 1,25(OH)2D3 affected immunoreactive ER protein and mRNA levels, respectively. Our results showed that immunohistochemical staining for ER was primarily nuclear, whereas histochemical staining for ALP was cytosolic. Treatment of cells with 1,25(OH)2D3, E2, or E2 + 1,25(OH)2D3 increased the levels of both ER and ALP activity, as visualized by enhanced cellular staining. Western analyses showed that 1,25(OH)2D3 and E2, separately and in combination, significantly increased ER protein levels. 1,25(OH)2D3 enhanced ER levels in a dose-dependent manner [analysis of variance (ANOVA), F = 3.91, p < 0.05]; this effect was augmented by E2 (ANOVA, F = 5.98, p < 0.005). In comparison, 17 alpha-E2 + 1,25(OH)2D3 and tamoxifen + 17 beta-E2 + 1,25(OH)2D3 did not increase ER levels compared with those obtained with 17 beta-E2 + 1,25(OH)2D3. ER mRNA levels were not significantly increased by E2, 1,25(OH)2D3, or E2 + 1,25(OH)2D3 together. In contrast, in a population of SaOS cells which had been in culture longer (approximately 40 passages more) than the previous cells, E2 + 1,25(OH)2D3 did not enhance ALP activity or ER levels above those obtained with 1,25(OH)2D3 alone. These results showed that in responsive SaOS cells, E2 enhanced both the stimulatory effects of 1,25(OH)2D3 on ALP activity and the activation of ER. Thus changes in ALP activity are associated with changes in ER levels in SaOS + DEX cells.

Estrogen inhibits interleukin-6 production and gene expression in a human osteoblastic cell line with high levels of estrogen receptors

Some studies suggest that estrogen acts on bone by decreasing the production of interleukin-6 (IL-6), a cytokine that increases bone resorption, by osteoblasts or bone marrow cells. However, other studies have not confirmed this, possibly because of a low and variable number of estrogen receptors (ER) in the model systems used. Thus, we employed a recently developed human fetal osteoblast cell line with high levels of ER. Treatment (n = 4 experiments) with 0.01 to 10 nM of 17 beta-estradiol had no effect on the constitutive production of IL-6. However, stimulated production, induced by treatment with IL-1 beta plus tumor necrosis factor-alpha (TNF-alpha), was reduced in a dose-dependent manner to 74 +/- 3% (mean +/- SEM) of control (p < 0.01). This response was blocked by cotreatment with the type II antiestrogen ICI 182,780. Treatment with hydrocortisone (1 microM), a known inhibitor of IL-6 production in many cell types, reduced IL-6 production to 17 +/- 1% of control (p < 0.001). As assessed by Northern analysis, treatment (n = 3 experiments) with 0.01-10 nM of 17 beta-estradiol decreased steady-state levels of IL-6 mRNA in a dose-dependent manner. These data support the hypothesis that at least part of the antiresorptive action of estrogen in humans is mediated by decreased production of IL-6 by osteoblastic cells.