High-dose estrogen induces de novo medullary bone formation in female mice

Ovariectomy/Orchidectomy in Rodents

This chapter describes the surgical procedures for ovariectomy and orchidectomy in mice and rats. In addition to providing technical details of the surgical techniques, details of anesthesia options and pre-, peri-, and post-operative care are also included.

Resolving trabecular metaphyseal bone profiles downstream of the growth plate adds value to bone histomorphometry in mouse models

INTRODUCTION

Histomorphometry of rodent metaphyseal trabecular bone, by histology or microCT, is generally restricted to the mature secondary spongiosa, excluding the primary spongiosa nearest the growth plate by imposing an 'offset'. This analyses the bulk static properties of a defined segment of secondary spongiosa, usually regardless of proximity to the growth plate. Here we assess the value of trabecular morphometry that is spatially resolved according to the distance 'downstream' of-and thus time since formation at-the growth plate. Pursuant to this, we also investigate the validity of including mixed primary-secondary spongiosal trabecular bone, extending the analysed volume 'upstream' by reducing the offset. Both the addition of spatiotemporal resolution and the extension of the analysed volume have potential to enhance the sensitivity of detection of trabecular changes and to resolve changes occurring at different times and locations.

METHOD

Two experimental mouse studies of trabecular bone are used as examples of different factors influencing metaphyseal trabecular bone: (1) ovariectomy (OVX) and pharmacological prevention of osteopenia and (2) limb disuse induced by sciatic neurectomy (SN). In a third study into offset rescaling, we also examine the relationship between age, tibia length, and primary spongiosal thickness.

RESULTS

Bone changes induced by either OVX or SN that were early or weak and marginal were more pronounced in the mixed primary-secondary upstream spongiosal region than in the downstream secondary spongiosa. A spatially resolved evaluation of the entire trabecular region found that significant differences between experimental and control bones remained undiminished either right up to or to within 100 μm from the growth plate. Intriguingly, our data revealed a remarkably linear downstream profile for fractal dimension in trabecular bone, arguing for an underlying homogeneity of the (re)modelling process throughout the entire metaphysis and against strict anatomical categorization into primary and secondary spongiosal regions. Finally, we find that a correlation between tibia length and primary spongiosal depth is well conserved except in very early and late life.

CONCLUSIONS

These data indicate that the spatially resolved analysis of metaphyseal trabecular bone at different distances from the growth plate and/or times since formation adds a valuable dimension to histomorphometric analysis. They also question any rationale for rejecting primary spongiosal bone, in principle, from metaphyseal trabecular morphometry.

GsαR201C and estrogen reveal different subsets of bone marrow adiponectin expressing osteogenic cells

The Gsα/cAMP signaling pathway mediates the effect of a variety of hormones and factors that regulate the homeostasis of the post-natal skeleton. Hence, the dysregulated activity of Gsα due to gain-of-function mutations (R201C/R201H) results in severe architectural and functional derangements of the entire bone/bone marrow organ. While the consequences of gain-of-function mutations of Gsα have been extensively investigated in osteoblasts and in bone marrow osteoprogenitor cells at various differentiation stages, their effect in adipogenically-committed bone marrow stromal cells has remained unaddressed. We generated a mouse model with expression of GsαR201C driven by the Adiponectin (Adq) promoter. Adq-GsαR201C mice developed a complex combination of metaphyseal, diaphyseal and cortical bone changes. In the metaphysis, GsαR201C caused an early phase of bone resorption followed by bone deposition. Metaphyseal bone formation was sustained by cells that were traced by Adq-Cre and eventually resulted in a high trabecular bone mass phenotype. In the diaphysis, GsαR201C, in combination with estrogen, triggered the osteogenic activity of Adq-Cre-targeted perivascular bone marrow stromal cells leading to intramedullary bone formation. Finally, consistent with the previously unnoticed presence of Adq-Cre-marked pericytes in intraosseous blood vessels, GsαR201C caused the development of a lytic phenotype that affected both cortical (increased porosity) and trabecular (tunneling resorption) bone. These results provide the first evidence that the Adq-cell network in the skeleton not only regulates bone resorption but also contributes to bone formation, and that the Gsα/cAMP pathway is a major modulator of both functions.

Urban-Rural Differences in Bone Mineral Density and its Association with Reproductive and Menstrual Factors Among Older Women

PURPOSE

This study aimed to compare the bone mineral density (BMD) of older women living in rural and urban areas, and evaluate the potential factors affecting the risk of osteoporosis.

METHODS

We recruited 574 women aged 65 years or older from rural areas and 496 from urban areas in Shanghai, China. The BMD values of the lumbar vertebrae and total left hip were measured by a dual energy X-ray absorptiometry densitometer. We also recorded information about education level, family income, medications, reproductive and menstrual history, diet, smoking, and alcohol consumption.

RESULTS

Women in urban areas had significantly higher BMD in their lumbar spine, and there was a dramatic increase in the proportion of women with osteoporosis in rural areas. The age at menarche was significantly higher among women living in rural areas, and there were more years from menarche to menopause among urban women. Rural women had significantly higher numbers of both pregnancies and parity, and a significantly lower age at first parity. In multiple linear regression analyses, years from menarche to menopause was independently related to high lumbar spine BMD, while age at menarche and parity was independently related to low lumbar spine BMD.

CONCLUSION

More older women in rural areas had osteoporosis. Later menarche, less years from menarche to menopause and higher parity might partially contribute to decreased BMD among women in rural areas. More attention should be paid to women in rural areas to prevent bone loss and further bone and health impairment.

N-(3-methoxybenzyl)-(9Z,12Z,15Z)-octadecatrienamide promotes bone formation via the canonical Wnt/β-catenin signaling pathway

Osteoporosis is characterized by low bone mineral density and microarchitectural deterioration of bone tissue. N-(3-methoxybenzyl)-(9Z,12Z,15Z)-octadecatrienamide (MBOC) is one of the macamides isolated from Maca (Lepidium meyenii Walp.), a cruciferous plant from the Andes of Peru. In this study, C3H/10T1/2 mesenchymal stem cells were treated with MBOC in osteogenic induction medium. An ovariectomized (OVX) mouse model was used to investigate the effect of 1-month MBOC treatment on the prevention of postmenopausal osteoporosis. Remarkably, trabecular thickness, trabecular number, and bone volume/tissue volume of the distal femoral metaphysis were significantly increased in OVX + MBOC mice compared with OVX mice, as revealed by microcomputed tomography analysis. Trabecular separation was decreased in OVX + MBOC mice compared with OVX mice. Consistently, MBOC increased the levels of osteocalcin and runt-related transcription factor 2 in OVX mice, as well as the expression of runt-related transcription factor 2, osterix, and alkaline phosphatase in C3H/10T1/2 cells. Mechanistically, MBOC activates the canonical Wnt/β-catenin signaling pathway via inhibiting phosphorylation of GSK-3β at Tyr216 and maintaining β-catenin expression. Collectively, the current study demonstrates the robustness of MBOC in the induction of mesenchymal stem cells osteogenic differentiation and consequent bone formation, suggesting that MBOC may be a potentially effective drug to treat postmenopausal osteoporosis.

Ovariectomy/Orchiectomy in Rodents

This chapter describes the surgical procedures for ovariectomy and orchiectomy in mice and rats. In addition to providing technical details of the surgical techniques, details of anesthesia options and pre-, peri-, and postoperative care are also included.

Cortical bone adaptation and mineral mobilization in the subterranean mammal Bathyergus suillus (Rodentia: Bathyergidae): effects of age and sex

The patterns of bone modeling and mineral mobilization (skeletal homeostasis) among mammals other than humans and laboratory rodents are still poorly known. In this study we assessed the pattern of bone formation and bone resorption in the femur of a wild population of Cape dune molerats, Bathyergus suillus (n = 41) (Bathyergidae), a solitary subterranean mammal with a marked extended longevity among rodents, and which also lives in a naturally deficient state of vitamin D. In order to determine ontogenetic and sex effects on histomorphometric parameters of transversal undecalcified bone sections, two-way ANOVA, linear mixed-effects model and regression statistical analyses were performed. During ontogeny, B. suillus increased their cross sectional area, cortical area and cortical thickness, and most importantly, they showed scarce endosteal bone resorption which resulted in a retained medullary cavity size during ontogeny. This resulted in a positively imbalanced bone modeling, where bone formation considerably surpasses bone loss by almost 100-fold in adulthood. This differs markedly from other terrestrial mammals with relatively thin cortical walls. Regarding bone loss and remodeling, three main processes involving intracortical resorption were observed: modeling-related bone loss in early postnatal growth; secondary osteon formation occurring in both sexes; and subendosteal secondary reconstruction observed only in females. The latter is accompanied by females having six-fold more relative bone loss than males, which is evidenced by the development of enlarged resorption cavities (RCs) distributed circumferentially around the medullary cavity. Males have smaller, more circular and randomly distributed RCs. In general, our data indicate no age-related decline in mineral content in B. suillus, and provides strong support for a pattern of sexual dimorphism in skeletal homeostasis, similar to that occurring in humans and other mammals, with females losing more bone throughout aging as compared to males due to reproductive factors. Interestingly as well, despite the high mechanical loads experienced during burrow construction, bone remodeling in B. suillus is kept at very low levels throughout their lifespan, and dense Haversian tissue never forms. This study represents the first comprehensive assessment of skeletal homeostasis in a subterranean mammal, and it enables a better understanding of the complex processes governing the acquisition and maintenance of bone properties in this species with extraordinary fossorial adaptations.

Estrogen levels influence medullary bone quantity and density in female house finches and pine siskins

Medullary bone, a non-structural osseous tissue, serves as a temporary storage site for calcium that is needed for eggshell production in a number of avian species. Previous research focusing primarily on domesticated species belonging to the Anseriformes, Galliformes, and Columbiformes has indicated that rising estrogen levels are a key signal stimulating medullary bone formation; Passeriformes (which constitute over half of extant bird species and are generally small) have received little attention. In the current study, we examined the influence of estrogen on medullary bone and cortical bone in two species of Passeriformes: the Pine Siskin (Spinus pinus) and the House Finch (Haemorhous mexicanus). Females of these species received either an estradiol implant or were untreated as a control. After 4.5-5months, reproductive condition was assessed and leg (femora) and wing (humeri) bones were collected for analysis using high-resolution (10μm) micro-computed tomography scanning. We found that in both species estradiol-treated females had significantly greater medullary bone quantity in comparison to untreated females, but we found no differences in cortical bone quantity or microarchitecture. We were also able to examine medullary bone density in the pine siskins and found that estradiol treatment significantly increased medullary bone density. Furthermore, beyond the effect of the estradiol treatment, we observed a relationship between medullary bone quantity and ovarian condition that suggests that the timing of medullary bone formation may be related to the onset of yolk deposition in these species. Further research is needed to better understand the precise timing and endocrine regulation of medullary bone formation in Passerines and to determine the extent to which female Passerines rely on medullary bone calcium during the formation of calcified eggshells.

Constructing the toolbox: Patient-specific genetic factors of altered fracture healing

The multifaceted sequence of events that follow fracture repair can be further complicated when considering risk factors for impaired union, present in a large and growing percentage of the population. Risk factors such as diabetes, substance abuse, and poor nutrition affect both the young and old alike, and have been shown to dramatically impair the body's natural healing processes. To this end, biotherapeudic interventions such as ultrasound, electrical simulation, growth factor treatment (BMP-2, BMP-7, PDGF-BB, FGF-2) have been evaluated in preclinical models and in some cases are used widely for patients with established non-union or risk/indication or impaired healing (ie. ultrasound, BMP-2, etc.). Despite the promise of these interventions, they have been shown to be reliant on patient compliance and can produce adverse side-effects such as heterotopic ossification. Gene and cell therapy approaches have attempted to apply controlled regimens of these factors and have produced promising results. However, there are safety and efficacy concerns that may limit the translation of these approaches. In addition, none of the above mentioned approaches consider genetic variation between individual patients. Several clinical and preclinical studies have demonstrated a genetic component to fracture repair and that SNPs and genetic background variation play major roles in the determination of healing outcomes. Despite this, there is a need for preclinical data to dissect the mechanism underlying the influence of specific gene loci on the processes of fracture healing, which will be paramount in the future of patient-centered interventions for fracture repair.

Estrogen facilitates osteoblast differentiation by upregulating bone morphogenetic protein-4 signaling

Imbalanced functions of osteoclasts and osteoblasts are involved in various types of bone damage including postmenopausal osteoporosis. In the present study, we investigated the cellular mechanism by which estrogen interacts in the process of osteoblastic differentiation regulated by BMP-4 using mouse MC3T3-E1 cells that express estrogen receptors (ER) and BMP-4. Estradiol enhanced BMP-4-induced Runx2, osterix, ALP and osteocalcin expression in MC3T3-E1 cells. BMP-4-induced mineralization shown by Alizarin red staining was also facilitated by estrogen treatment. It was revealed that estrogen upregulated BMP-4-induced Smad1/5/8 phosphorylation, BRE-Luc activity and Id-1 mRNA expression. The expression of BMPRII was increased by estrogen in MC3T3-E1 cells, and inhibition of BMPRII or ALK-2/3 signaling impaired the effect of estrogen on BMP-4 signaling. Of note, the enhanced expression of osterix, ALP and osteocalcin mRNAs induced by BMP-4 and estrogen was reversed in the presence of an ER antagonist. Given that membrane-impermeable estrogen also upregulated BMP-4-induced expression of osteoblastic markers and Id-1 mRNA, non-genomic ER activity is involved in the mechanism by which estrogen enhances BMP-4-induced osteoblast differentiation in MC3T3-E1 cells. On the other hand, the expression of ERα and endogenous BMP-4 was suppressed by BMP-4 treatment regardless of the presence of estrogen, implying the presence of a negative feedback loop for osteoblast differentiation. Thus, estrogen is functionally involved in the process of osteoblast differentiation regulated by BMP-4 through upregulating BMP sensitivity of MC3T3-E1 cells.

Evidence for estrogen receptor expression during medullary bone formation and resorption in estrogen-treated male Japanese quails (Coturnix coturnix japonica)

The temporal expression of estrogen receptor (ER)-α and ER-β mRNA was examined in male Japanese quails. Femurs of quails receiving 17β-estradiol underwent RT-PCR and histochemical analysis 1 to 15 days after treatment. Untreated quails were used as controls (day 0). Between days 0 and 5, cells lining the bone endosteal surface differentiated into osteoblasts, which in turn formed medullary bone. Expression of ER-α was already observed on day 0 and increased slightly during bone formation whereas ER-β was hardly detected throughout this process. After osteoclasts appeared on the medullary bone surface, this type of bone disappeared from the bone marrow cavity (days 7˜15). ER-α expression simultaneously decreased slightly and ER-β levels remained very low. These results suggest that estrogen activity mediated by ER-α not only affects medullary bone formation but also bone resorption.

Sequential expression of osteoblast phenotypic genes during medullary bone formation and resorption in estrogen-treated male Japanese quails

Medullary bone is formed reticularly in the bone marrow cavity of the long bones of female birds. Although this bone matrix contains fewer collagen fibers and more acid mucopolysaccharides than cortical bone, it is not clear that the expression pattern of osteoblast phenotypic genes during bone remodeling. Therefore, 17β-estradiol (E2)-treated male Japanese quails were used to examine the temporal expression patterns of osteoblast phenotypic genes, and to simultaneously confirm the morphological changes occurring in the bone marrow cavity during medullary bone formation and resorption. After E2 treatment, bone lining cells proliferated and developed into mature osteoblasts that had intense alkaline phosphatase (ALP) activity. These cells began to form medullary bone that contained acid mucopolysaccharides and tartrate-resistantacid phosphatase. Runt-related gene 2 (Runx2) mRNA was stably expressed throughout the process. The expression of both ALP and type I collagen mRNAs increased initially, and then rapidly decreased after day 7, while osteoclasts began to resorb medullary bone at day 5. The expression of bone matrix-related genes peaked at day 5, and suddenly decreased at day 7, except for osteopontin. Taken together with these results, the expression patterns of bone matrix-related genes during the later stages might be related to osteoclast activity. Additionally, the constant expression of Runx2 during bone formation and resorption suggested that osteoprogenitor cells always exist in the bone marrow cavity. Therefore, the expression patterns of these genes and the characteristics of bone matrix might extremely be related to the quick remodeling of medullary bone.

Alterations in Brca1 expression in mouse ovarian granulosa cells have short-term and long-term consequences on estrogen-responsive organs

Incessant menstrual cycle activity, uninterrupted by either pregnancy or oral contraceptive use, is the most important risk factor for sporadic ovarian cancer. Menstrual cycle progression is partly controlled by steroid hormones such as estrogens and others that are secreted by the ovarian granulosa cells. We showed earlier that mice carrying a homozygous granulosa cell-specific knockout of Brca1, the homolog of BRCA1 that is associated with familial ovarian cancer predisposition in humans, develop benign epithelial tumors in their reproductive tract. These tumors are driven, at least in part, by a prolongation of the proestrus phase of the estrus cycle (equivalent to the follicular phase of the menstrual cycle) in Brca1 mutant mice, resulting in prolonged unopposed estrogen stimulation. Mutant mice synchronized in proestrus also showed increased circulating estradiol levels, but the possibility that this change also has a role in tumor predisposition was not investigated. We sought to determine whether these changes in hormonal stimulation result in measurable changes in tissues targeted by estrogen outside the ovary. Here we show that mice carrying a Brca1 mutation in their ovarian granulosa cells show increased endometrial proliferation during proestrus, implying that the effects of Brca1 inactivation on estrogen stimulation have short-term consequences, at least on this target organ. We further show that mutant mice develop increased femoral trabecular thickness and femoral length, which are well-known consequences of chronic estrogen stimulation. Estrogen biosynthesis by granulosa cells was increased not only in mice carrying a homozygous Brca1 mutation, but also in heterozygous mutants mimicking the mutational status in granulosa cells of human BRCA1 mutation carriers. The results suggest that human germline BRCA1 mutations, although associated with increased cancer risk, may also have beneficial consequences, such as increased bone strength, that may have contributed to the maintenance of mutated BRCA1 alleles in the human gene pool.

Estradiol increases hematopoietic stem and progenitor cells independent of its actions on bone

Hematopoietic stem and progenitor cells reside in vascular and endosteal niches in the bone marrow. Factors affecting bone remodeling were reported to influence numbers and mobilization of hematopoietic stem cells. We therefore analyzed the effects of estradiol acting anabolic on bone integrity. Here we observe that estradiol increases progenitor cell numbers in the vascular but not in the endosteal compartment independent of its estrogen receptor α-dependent anabolic bone effects. Hematopoietic progenitors capable of reconstituting lethally irradiated mice are increased by enhanced cell cycle entry, leading to a diminished long-term reconstitution potential after serial transplantation. We demonstrate that estradiol action on stromal cells potently favors hematopoietic progenitor/stem cell frequency accompanied by enhanced expression of cell adhesion molecules. Finally, estradiol treatment enhances retention of hematopoietic stem cells in the vascular niche of the bone marrow. We describe for the first time the mechanism of estrogen action on hematopoietic stem and progenitor cells.

B-lymphopoiesis gains sensitivity to subsequent inhibition by estrogens during final phase of fetal development

Adult B-lymphopoiesis is suppressed by the inhibitory effects of elevated estrogens during pregnancy. At the same time, hematopoietic cells in the fetal liver are resistant to this suppression by estrogens and ensure active production of B-cells. We investigated whether this unresponsiveness to estrogens of fetal cells also applies to cells obtained from a newborn liver and projects into the adult hematopoiesis when fetal liver cells are transplanted to adult mice. Mixtures of fetal liver (E14.5), neonatal liver (P0.5) and adult bone marrow (BM) cells were co-transplanted into adult primary and secondary recipients treated with high doses of estrogen in the Ly5.1/Ly5.2 congenic mouse model. Total chimerism as a proportion of all nucleated blood cells, chimerism as a proportion of B220+ B-cells, and of other blood cell lineages as well, were determined by flow cytometry. B-lymphopoiesis derived from fetal liver (E14.5) stem cells remained resistant to estrogen after transplantation into both primary and secondary adult recipients, for up to 280 days. In contrast, B-lymphopoiesis derived from neonatal liver (P0.5) stem cells was resistant to estrogen only for approximately 50 days after the primary transplantation to the adult BM microenvironment. These results provide further evidence for a critical developmental period of B-lymphopoiesis during its fetal liver stage. In the mouse, critical developmental events that allow for the subsequent expressed sensitivity of B-lymphopoiesis for suppression by estrogens after sexual maturation appear to occur during the period of late-stage fetal liver hematopoiesis before its migration to the bone marrow.

Ovariectomy/orchidectomy in rodents

This chapter describes the surgical procedures for ovariectomy and orchidectomy in mice and rats. In -addition to providing technical details of the surgical techniques, details of anaesthesia and perioperative care are also included.

Estrogen and glucocorticoid regulate osteoblast differentiation through the interaction of bone morphogenetic protein-2 and tumor necrosis factor-alpha in C2C12 cells

Imbalanced functions between osteoclasts and osteoblasts are involved in inflammatory bone damage. The clinical effectiveness of blocking TNF-alpha in treatment of active rheumatoid arthritis established the significance of TNF-alpha in the pathogenesis. In the present study, we investigated the cellular mechanism by which estrogen and glucocorticoid interact in osteoblastic differentiation regulated by BMP and TNF-alpha using mouse myoblastic C2C12 cells. The expression of estrogen receptors, (ER)alpha and ERbeta, and glucocorticoid receptor (GCR) was significantly increased by BMP-2 treatment regardless of the presence of estradiol and dexamethasone. Estradiol, but not dexamethasone, enhanced BMP-induced Runx2 and osteocalcin expression in C2C12 cells. In addition, TNF-alpha suppressed BMP-2-induced Runx2 and osteocalcin expression, and estradiol and dexamethasone reversed the TNF-alpha effects on BMP-2-induced Runx2 expression. Dexamethasone also abolished osteocalcin expression induced by BMP-2. Interestingly, BMP-2-induced Smad1/5/8 phosphorylation and Id-1 promoter activity were enhanced by estradiol pretreatment. On the other hand, dexamethasone suppressed BMP-2-induced Smad1/5/8 activation. TNF-alpha-induced SAPK/JNK activity was suppressed by estradiol, while NFkappaB phosphorylation was inhibited by dexamethasone. Of note, the inhibitory effects of TNF- on BMP-2-induced Runx2 and osteocalcin expression were reversed by SAPK/JNK inhibition regardless of the presence of estradiol. The estradiol effects that enhance BMP-2-induced Runx2 and osteocalcin mRNA expression were restored by antagonizing ER, and moreover, membrane-impermeable estradiol-BSA failed to enhance the BMP-2-induced osteoblastic differentiation. Thus, estrogen and glucocorticoid are functionally involved in the process of osteoblast differentiation regulated by BMPs and TNF-alpha. BMP-2 increases the sensitivities of ERs and GCR, whereas estrogen and glucocorticoid differentially regulate BMP-Smad and TNF-alpha signaling.

Bone phenotypes in response to gonadotropin misexpression: the role for gonadotropins in postmenopausal osteoporosis

Scant attention has been paid to the potential role of gonadotropins in bone tissue homeostasis. The focus on estrogen and estrogen replacement therapy for osteoporosis as far back as the 1940's may account for the paucity of gonadotropin studies in bone biology. It is conceivable that prevailing dogma may have subconsciously steered us away from addressing whether gonadotropins have a place in skeletal physiology. However an examination of bone tissue catabolism in ovariectomized (OVX) and luteinizing hormone-releasing hormone (LHRH) agonist (Zoladex((R)))-treated rats generated some interesting and conflicting data; Zoladex-treated rats, unlike the OVX group, failed to exhibit increased bone collagen catabolism despite clear evidence for estrogen deficiency. The findings, although controversial, supported the possibility that elevated gonadotropins in the OVX model were in some way accountable for increased bone catabolism. In response to these initial findings further studies were performed to determine if altered LH status may in some way impact on the skeleton To this end an investigation of bone mass and histomorphometry were conducted in LH receptor nullizygous mice and human chorionic gonadotropin (hCG) overexpressing mice. There were clear phenotypic differences; the LH receptor knockout mice displayed reduced bone mass whereas the hCG overexpressing animals had stark increases in bone mass. Much more recently the team of the Mount Sinai Bone Program have made a significant discovery that bone-resorbing osteoclasts express receptors for follicle-stimulating hormone (FSH) and that mice nullizygous for FSH receptor are resistant to bone loss despite severe estrogen deficiency. Details of these fascinating models will be presented together with additional findings that give credence for exploring gonadotropin action on the skeleton as we enter the twilight of this Decade of the Bone and Joint.

Comparison of skeletal effects of ovariectomy versus chemically induced ovarian failure in mice

Bone loss associated with menopause leads to an increase in skeletal fragility and fracture risk. Relevant animal models can be useful for evaluating the impact of ovarian failure on bone loss. A chemically induced model of menopause in which mice gradually undergo ovarian failure yet retain residual ovarian tissue has been developed using the chemical 4-vinylcyclohexene diepoxide (VCD). This study was designed to compare skeletal effects of VCD-induced ovarian failure to those associated with ovariectomy (OVX). Young (28 day) C57Bl/6Hsd female mice were dosed daily with vehicle or VCD (160 mg/kg/d, IP) for 15 days (n = 6-7/group) and monitored by vaginal cytology for ovarian failure. At the mean age of VCD-induced ovarian failure (approximately 6 wk after onset of dosing), a different group of mice was ovariectomized (OVX, n = 8). Spine BMD (SpBMD) was measured by DXA for 3 mo after ovarian failure and OVX. Mice were killed approximately 5 mo after ovarian failure or OVX, and bone architecture was evaluated by microCT ex vivo. In OVX mice, SpBMD was lower than controls 1 mo after OVX, whereas in VCD-treated mice, SpBMD was not lower than controls until 2.9 mo after ovarian failure (p < 0.05). Both VCD-induced ovarian failure and OVX led to pronounced deterioration of trabecular bone architecture, with slightly greater effects in OVX mice. At the femoral diaphysis, cortical bone area and thickness did not differ between VCD mice and controls but were decreased in OVX compared with both groups (p < 0.05). Circulating androstenedione levels were preserved in VCD-treated mice but reduced in OVX mice relative to controls (p < 0.001). These findings support that (1) VCD-induced ovarian failure leads to trabecular bone deterioration, (2) bone loss is attenuated by residual ovarian tissue, particularly in diaphyseal cortical bone, and (3) the VCD mouse model can be a relevant model for natural menopause in the study of associated bone disorders.

Hormone therapy for breast cancer, with an emphasis on the pure antiestrogen fulvestrant: mode of action, antitumor efficacy and effects on bone health

Breast cancer is a major health problem in women of developed Western countries. Whereas estrogen receptor (ER) may be involved in many cases in breast carcinogenesis, its expression in breast tumors may predict a favorable response to hormone therapy. In this review, we report the role played by ER in breast cancer and compare the effects and mechanisms of action of partial (tamoxifen) and pure (fulvestrant) antiestrogens, as well as of aromatase inhibitors. Moreover, as ER also has a critical role in bone metabolism, we review the beneficial and adverse effects of breast cancer hormone therapy on bone health, with a particular emphasis on fulvestrant, the only pure antiestrogen recently approved by the FDA for Phase III clinical trials. We conclude that, because of its therapeutic efficacy and its seemingly minimal effect on bone integrity, fulvestrant represents a new option for the hormonal treatment of breast cancer that deserves further clinical evaluation.

Inhibin and the regulation of bone mass

Inhibins A and B are gonadal peptide members of the transforming growth factor-beta superfamily that serve as negative feedback regulators of pituitary follicle-stimulating hormone (FSH). Accumulating evidence suggests that bone turnover and bone loss increase in women before menopause and the decrease in serum estradiol levels. Increased FSH levels have been correlated with some of these perimenopausal changes, whereas decreased inhibins strongly correlate with increases in bone formation and resorption across the menopause transition, and predict lumbar bone mass in perimenopausal women, likely resulting from the direct inhibin suppression of osteoblast and osteoclast development. Interestingly, continuous exposure of mice to inhibin A in vivo is anabolic and protective against gonadectomy-induced bone loss. Together, these data suggest inhibins contribute to the endocrine regulation of bone metabolism via a bimodal mechanism of action such that cycling inhibin exposure suppresses bone turnover, and continuous exposure to inhibins is anabolic.

An ENU-induced mutation in the Ankrd11 gene results in an osteopenia-like phenotype in the mouse mutant Yoda

The mechanisms that regulate bone mass are important in a variety of complex diseases such as osteopenia and osteoporosis. Regulation of bone mass is a polygenic trait and is also influenced by various environmental and lifestyle factors, making analysis of the genetic basis difficult. As an effort toward identifying novel genes involved in regulation of bone mass, N-ethyl-N-nitrosourea (ENU) mutagenesis in mice has been utilized. Here we describe a mouse mutant termed Yoda that was identified in an ENU mutagenesis screen for dominantly acting mutations. Mice heterozygous for the Yoda mutation exhibit craniofacial abnormalities: shortened snouts, wider skulls, and deformed nasal bones, underlined by altered morphology of frontonasal sutures and failure of interfrontal suture to close. A major feature of the mutant is reduced bone mineral density. Homozygosity for the mutation results in embryonic lethality. Positional cloning of the locus identified a missense mutation in a highly conserved region of the ankyrin repeat domain 11 gene (Ankrd11). This gene has not been previously associated with bone metabolism and, thus, identifies a novel genetic regulator of bone homeostasis.

Impaired growth plate function in bmp-6 null mice

Bone morphogenetic protein 6 (BMP-6) is expressed by different skeletal cells including osteoblasts and growth plate chondrocytes, suggesting roles in bone formation and growth regulation. To address these possibilities, we examined whether cancellous and cortical bone parameters, or indices of growth plate function, are altered in bmp-6 null mice as assessed under basal conditions, and following stimulation of bone formation and suppression of growth by estrogen treatment. Ten-week-old female littermate bmp-6 null and wild-type (WT) mice were administered vehicle or E(2) 4, 40, 400 or 4,000 microg/kg/day by daily sc injection for 28 days (6-8 per group). Tibias were removed, and detailed histomorphometric analysis of the proximal metaphysis and growth plates, and tibial diaphysis were performed on longitudinal and transverse sections respectively. Long bone area as measured by DXA was reduced in vehicle-treated bmp-6 null mice compared with WT littermate controls. In addition, vehicle-treated bmp-6 null mice had a reduced cross-sectional area at the tibial mid-diaphysis as assessed by histomorphometry, whereas cancellous bone indices were unaffected. Histomorphometry of the proximal tibial metaphysis demonstrated a defect in bone formation immediately adjacent to the growth plate in bmp-6 null mice compared to WT mice following E(2) treatment. E(2) administration was also associated with a dose-responsive decrease in longitudinal growth rate, and proliferative and hypertrophic zone parameters of the growth plate (p<0.0001). Significantly greater reductions following E(2) treatment were observed in longitudinal growth rate (p<0.01), proliferating and hypertorphic zone widths (p<0.001), and proliferating (p<0.0002) and hypertrophic (p<0.002) cells per column of bmp-6 null mice compared to WT mice. Our observation that long bones are reduced in size compared to wild-type mice primarily through a decrease in cortical cross-sectional area, whilst cancellous bone mass is unaltered, suggests a non-redundant role for BMP-6 in periosteal but not trabecular bone formation. Moreover, growth plate function was reduced in bmp-6 null mice receiving estrogen, leading to an impaired cancellous bone response to estrogen at the highest dose, suggesting that BMP-6 also plays a physiological role in maintaining growth plate function.

Mice rendered severely deficient in megakaryocytes through targeted gene deletion of the thrombopoietin receptor c-Mpl have a normal skeletal phenotype

To explore whether a functional relationship exists between megakaryocytes and the cellular processes responsible for bone formation, we examined if Mpl ( -/- ) mice, which are severely megakaryocyte-deficient through c-Mpl gene deletion, have an abnormal skeletal phenotype compared to Mpl ( +/- ) and wild-type littermates. We also analyzed whether the osteogenic response to high-dose estrogen treatment is altered in Mpl ( -/- ) mice. Megakaryocyte numbers and skeletal indices were compared between Mpl ( -/- ) mice and littermate Mpl ( +/- ) and wild-type 12-week-old mice (six per group). Dual-energy X-ray absorbtiometry of whole body, excised tibias, and femurs was performed. Histomorphometric analyses of the proximal metaphysis and mid-diaphysis were carried out on longitudinal and transverse sections, respectively. Histomorphometry was performed on the proximal tibial metaphysis of four Mpl ( -/- ) and four wild-type mice following high-dose estrogen treatment (0.5 mg/animal/week) for 4 weeks. Mpl ( -/- ) mice had 10% the megakaryocyte number of Mpl ( +/- ) and wild-type littermates. Bone mineral density values in Mpl ( -/- ) mice were identical to those in Mpl ( +/- ) and wild-type mice for whole body, femur, and tibia. Histomorphometric analysis demonstrated that cancellous and cortical tibial bone parameters were similar across all genotypes. The osteogenic response to estrogen treatment was indistinguishable between Mpl ( -/- )and wild-type mice. We found that mice severely deficient in megakaryocytes have a normal skeletal phenotype. Additionally, the deficiency did not diminish the osteogenic marrow response to high-dose estrogen treatment. These results represent the first in vivo evidence that severe megakaryocyte deficiency does not affect bone formation, suggesting that this process is not dependent on normal megakaryocyte number.

High-dose estrogen-induced osteogenesis is decreased in aged RUNX2(+/-) mice

Runx2 is a transcription factor that is not only critical in embryonic skeletal development but also important in regulating osteoblast function in the adult. Heterozygosity of RUNX2 (RUNX2(+/-)) leads to haploinsufficiency and manifests as a condition with distinctive skeletal features in humans and mice. Aged but not young RUNX2(+/-) adult mice may also display reduced intramembranous bone formation. To clarify the role of Runx2 in intramembranous bone formation in adult mice a histomorphometric study was performed to compare the osteogenic response to high-dose estrogen in RUNX2(+/-) and wild-type mice. Young (10 weeks) and aged (26 weeks) RUNX2(+/-) and wild-type littermate mice were treated with vehicle or high-dose estrogen (0.5 mg/animal/week) by subcutaneous injection for 4 weeks. Mice were divided into 8 groups according to age, genotype and treatment with 6 animals per group. Following sacrifice, longitudinal tibial sections were prepared and examined by static and dynamic histomorphometry. Estrogen treatment induced formation of new cancellous bone in both wild-type and RUNX2(+/-) mice. This occurred to the same extent in young mice of both genotypes. However, in the aged RUNX2(+/-) mice this response as assessed by bone volume (BV/TV%) was decreased by over 70% (p<0.001) when compared to aged wild-type mice. Furthermore, significant reductions in cancellous double-labelled surfaces (dls/TV, 1.7+/-0.2 vs 1.0+/-0.4 mm(2)/mm(3), p<0.05) and mineral apposition rate (1.8+/-0.1 vs 1.4+/-0.1 microm/day, p<0.01) were observed in aged RUNX2(+/-) mice compared to wild-types. Aged RUNX2(+/-) mice display an abrogated osteogenic response to high-dose estrogen. This may have occurred through combined reductions in recruitment of osteoprogenitor cells, osteoblast activity and mineralization. Since the characteristic histological changes in the marrow cavity which precede the formation of cancellous bone following estrogen treatment was seen in the aged RUNX2(+/-) mice we suggest that they may eventually be capable of a full osteogenic response but haploinsufficiency leads to delayed bone formation.

Inhibin A is an endocrine stimulator of bone mass and strength

Gonadal function plays a major role in bone homeostasis. It is widely held that the skeletal consequences of hypogonadism are solely due to a loss of sex steroids; however, increases in bone turnover begin during perimenopause before decreases in serum estradiol levels. These data and our demonstration that inhibins acutely regulate bone cell differentiation in vitro led us to test whether inhibin A (InhA) regulates bone mass in vivo. Using a transgenic model of inducible human InhA expression, InhA increased total body bone mineral density, increased bone volume, and improved biomechanical properties at the proximal tibia in intact mice and also prevented the loss of BMD and bone volume and strength associated with gonadectomy at both the spine and proximal tibia. In addition, InhA increased mineral apposition rate, double-labeled surface, and serum osteocalcin levels in vivo and osteoblastogenesis ex vivo without affecting osteoclast number or activity. Together these results demonstrate novel stimulatory effects of InhA on the skeleton in vivo. These studies provide in vivo evidence demonstrating that gonadal factors other than sex steroids play an important role in regulating bone mass and strength and, combined with our previous clinical data, suggest that gonadal InhA may be a component of the normal endocrine repertoire that regulates bone quality in both the axial and appendicular skeleton.

Strain-dependent variations in the response of cancellous bone to ovariectomy in mice

The goal of this study was to characterize the skeletal response to ovariectomy in mice (129P3, C57BL/6, and B6129PF2) commonly used in gene manipulation studies to evaluate their potential as preclinical models of postmenopausal osteoporosis. The magnitude of cancellous bone loss and cellular indices of increased bone turnover in response to ovariectomy varied with mouse type and skeletal site, but in general, were less pronounced and less consistent than in Sprague-Dawley rats, the established preclinical model for postmenopausal bone loss.

INTRODUCTION

The ovariectomized (OVX) rat is the most widely used preclinical rodent model for postmenopausal osteoporosis. However, the underlying mechanisms of bone disorders, including osteoporosis, have been explored predominantly in the mouse. The purpose of this study was to evaluate mice (129P3 and C57BL/6 inbred strains and their F2 hybrid offspring, B6129PF2), commonly used for gene knockout and overexpression studies, for their potential as preclinical models of postmenopausal bone loss.

MATERIALS AND METHODS

The mice were OVX or sham-operated at 4 months of age and killed at 1 or 3 months after surgery. Lumbar vertebrae and distal femora were subjected to histomorphometric assessment.

RESULTS

Mice in the two strains and the F2 hybrids (will be referred to as strain for the remainder of the abstract) lost vertebral cancellous bone after OVX; bone volume (BV/TV) was 20% and 27% lower at 1 and 3 months after surgery, respectively. The decreased cancellous BV/TV was associated with an increase in osteoclast surface at 1 month after OVX in the 129P3 strain only. Osteoblast surface was increased by 20% with OVX at both 1 and 3 months after surgery, irrespective of mouse strain. However, bone formation rate was not altered by OVX in any of the mouse strains. In contrast to the lumbar vertebrae, cancellous bone loss in response to OVX differed in the distal femur among the three mouse strains. OVX had no significant effect on distal femur BV/TV in the B6129PF2 mouse strain. In the C57BL/6 strain, cancellous BV/TV was reduced by OVX at 1 month after surgery but not at 3 months after surgery, whereas distal femur BV/TV in 129P3 mice was reduced at 3 months after surgery. Osteoclast surface was not affected by OVX at either time-point in the C57BL/6 strain, but was increased by 116% at 1 month after surgery in the 129P3 strain. Osteoblast surface was increased with OVX at 1 month after surgery, irrespective of strain, whereas bone formation rate was not altered by OVX at either time-point in any of the strains.

CONCLUSIONS

The magnitude of cancellous bone loss and cellular indices of increased bone turnover in response to OVX varied with mouse strain and skeletal site, but in general, were less pronounced and less consistent than in the Sprague-Dawley rat. Although mouse models will continue to provide insights into genetic influences on bone mass and turnover, caution should be exercised when using 129P3 and C57BL/6 mice, and their F2 hybrids, as models for postmenopausal bone loss and preclinical testing of potential therapies for osteoporosis.

The response to sex steroid hormones and vitamin D of cultured osteoblasts derived from ovariectomized mice with and without 17beta-estradiol pretreatment

This study investigated whether 17beta-estradiol (E2) may have different effects on osteoblasts derived from estrogen-deficient ovariectomized (OVX) mice compared to sham-operated normal animals. We studied the specific effects of 17beta-estradiol on the differentiation and function of cultured osteoblasts derived from these groups of animals, with or without estrogen replacement treatment. One-month-old mice were ovariectomized or sham-operated, and treated (every second day) for 4 weeks with 0.5 mg/kg 17beta-estradiol or with vehicle alone. At the end of the experiment, bones were removed for primary osteoblast cultures or for morphological and chemical evaluation. In cells from untreated OVX animals, alkaline phosphatase (ALP) specific activity was reduced, while collagen production and mineralization were unchanged when compared to cells from controls. In vivo estrogen pretreatment of the OVX mice elevated ALP activity and mineralization of the cells, while collagen production was reduced. The addition of 17beta-estradiol to the culture medium increased ALP activity, collagen production, and mineralization by all cultured osteoblasts, except in those derived from sham-operated, estrogen-pretreated mice, where these features remained unchanged. Osteocalcin production was unchanged. Addition of testosterone or 1,25(OH)2D3 to the culture medium induced changes that differed among the groups depending on the source of the cultured cells. It seems that ovariectomy in mice prior to culture affected the phenotype of the cultured osteoblasts and their response to estradiol, testosterone, and 1,25(OH)2D3, depending on whether animals were pretreated with estradiol or not. These results imply that the animal's estrogen status prior to culture can influence the response to estrogens; this finding may have important implications for hormone replacement therapy (HRT) in postmenopausal women.

Identification of estrogen receptor ligands leading to activation of non-genomic signaling pathways while exhibiting only weak transcriptional activity

Estrogen receptors (ERs) stimulate genomic effects by acting as nuclear transcription factors as well as non-genomic effects by activating distinct cytoplasmic protein kinase cascades. Non-genomic effects have been implicated in numerous cellular processes, such as proliferation, differentiation, apoptosis and vasorelaxation. To exploit non-genomic effects mediated by ERalpha for novel hormone replacement regimens, we screened a focused library of steroid receptor ligands to identify compounds exhibiting properties different from estradiol, i.e. substances that selectively stimulate non-genomic signal transduction pathways while exhibiting low genomic activities. Treatment of breast cancer cells and osteosarcoma cells with estradiol, estren, substance A and substance B led to non-genomic activation of Akt (protein kinase B) and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling cascades mediated by Src (Rous Sarcoma Virus, non-receptor tyrosine kinase) and phosphatidylinositol-3-kinase (PI3K) stimulation. Such compounds leading to prominent Akt/ERK activation but exhibiting only weak genomic properties were applied in vasorelaxation assays, modeling physiological non-genomic ER responses. As expected from PI3K and Src activation data, substances were as effective as estradiol in mediating vasorelaxation. We assume that these pathway-selective estrogen receptor ligands may serve as potent lead structures for novel hormone replacement strategies exhibiting lesser side effects than the existing treatment paradigms.

Additive effects of estrogen and mechanical stress on nitric oxide and prostaglandin E2 production by bone cells from osteoporotic donors

Mechanical loading is thought to provoke a cellular response via loading-induced flow of interstitial fluid through the lacuno-canalicular network of osteocytes. This response supposedly leads to an adaptation of local bone mass and architecture. It has been suggested that loss of estrogen during menopause alters the sensitivity of bone tissue to mechanical load, thereby contributing to the rapid loss of bone. The present study aimed to determine whether estrogen modulates the mechanoresponsiveness of bone cells from osteoporotic women. Bone cell cultures from nine osteoporotic women (aged 62-90 years) were pre-cultured for 24 h with 10(-11) mol/l 17beta-estradiol (E2) or vehicle, and subjected to 1 h of pulsating fluid flow (PFF) or static culture. E2 alone enhanced prostaglandin E(2) (PGE(2)) and nitric oxide (NO) production by 2.8-fold and 2.0-fold, respectively, and stimulated endothelial nitric oxide synthase protein expression by 2.5-fold. PFF, in the absence of E2, stimulated PGE(2) production by 3.1-fold and NO production by 3.9-fold. Combined treatment with E2 and PFF increased PGE(2) and NO production in an additive manner. When expressed as PFF-treatment-over-control ratio, the response to fluid shear stress was similar in the absence or presence of E2. These results suggest that E2 does not affect the early response to stress in bone cells. Rather, E2 and shear stress both promote the production of paracrine factors such as NO and PGE(2) in an additive manner.

Tamoxifen stimulates cancellous bone formation in long bones of female mice

Selective estrogen receptor modulators (SERMs) have been developed as a means of targeting estrogen's protective effect on the skeleton in the treatment of postmenopausal osteoporosis. Although it is well established that SERMs such as tamoxifen inhibit bone resorption in a similar manner to estrogen, whether this agent shares estrogen's stimulatory action on bone formation is currently unclear. To address this question, we compared the effect of treatment for 28 d with 17beta-estradiol (E2; 0.1, 1.0 mg/kg x d) and tamoxifen (0.1, 1.0, or 10 mg/kg x d) on cancellous bone formation at the proximal tibial metaphysis of intact female mice. E2 stimulated the formation of new cancellous bone throughout the metaphysis. A similar response was observed after administration of tamoxifen, the magnitude of which was approximately 50% of that seen after E2. As expected, E2 was found to suppress longitudinal bone growth, but in contrast, this parameter was stimulated by tamoxifen. We conclude that tamoxifen acts as an agonist with respect to estrogen's stimulatory action on bone formation but as an antagonist in terms of estrogen's inhibition of longitudinal growth, suggesting that the protective effect of SERMs on the skeleton is partly mediated by stimulation of osteoblast activity.

Estrogen receptor beta: the antimechanostat?

We have known for sometime that sex hormones influence the growth, preservation, and loss of bone tissue in the skeleton. However, we are only beginning to recognize how estrogen influences the responsiveness of the skeleton to exercise. Frost's mechanostat theory proposes that estrogen reduces the mechanical strain required to initiate an osteogenic response, but this may only occur at the endocortical and trabecular bone surfaces. The discovery of estrogen receptors alpha and beta may help us to understand the bone surface-specific effects of exercise. Findings from estrogen receptor knockout mice suggest that the activity of ERalpha may explain the positive interaction between estrogen and exercise on bone formation near marrow, that is, endocortical and trabecular bone surfaces. Estrogen inhibits the anabolic exercise response at the periosteal surface, and this we propose is due to the activation of ERbeta. Signaling through this receptor retards periosteal bone formation and suppresses gains in bone size and bone strength, and for these reasons it behaves as an antimechanostat.

Skeletal changes in transgenic male mice expressing human cytochrome p450 aromatase

In this study, we showed that overexpressing P450 aromatase in male mice can increase bone mass and strengthen the tibia. Probably as a result of the action of products of local estrogen biosynthesis at different stages of life, the increased bone mass in young mice was induced by decreased bone turnover, but in aged animals, it was induced by increased bone formation.

INTRODUCTION

To understand the skeletal responses to the testosterone/estrogen balance, especially to excess estrogen produced by extragonadal biosynthesis, we investigated the bone changes in transgenic mice overexpressing human aromatase.

MATERIALS AND METHODS

Sixty-one young (40 days) and 25 aged (9 months) transgenic and wildtype (WT) mice were used. Bone samples were analyzed using pQCT, histomorphometry, and mechanical testing. Concentrations of testosterone (T) and estradiol (E2) were measured in serum and testicular interstitial fluid.

RESULTS AND CONCLUSIONS

Young P450 aromatase-positive (AROM+) mice had much higher trabecular BMD in the proximal tibia than WT mice, and the tissue area was significantly smaller in the former. Histomorphometric data further showed that the longitudinal growth rate of the tibia was decreased in AROM+ mice, and the bone formation rate (BFR) was decreased in trabecular bone and periosteum. All the changes were more striking in males than in females. Aged male AROM+ mice showed similar changes in trabecular bone as young animals, but their BFR was obviously increased. Another dramatic change was in the tibias of aged AROM+ mice: length was shorter (-23.2%), whereas ash weight was much heavier (+24.0%), and bending strength was markedly higher (+21.2%) compared with WT mice. The concentration of T was decreased in both serum and testicular interstitial fluid in young AROM+ mice versus WT animals; E2 levels were increased only in the testes of young AROM+ mice. However, in aged AROM+ mice, the levels of T and E2 were highly increased in both serum and testis versus WT animals. These results are in agreement with the suggestion that enhanced production of estrogen from testosterone in the peripheral tissues as a result of aromatase action can affect skeletal growth and strengthen bone in males. The results also suggest a marked difference in response between femur and tibia.

Differential transcriptional effects of PTH and estrogen during anabolic bone formation

The aim of this study was to compare transcriptional regulation in vivo during anabolic bone formation induced by either estradiol (E2) treatment or intermittent parathyroid hormone[1-34] (PTH) therapy. We utilized an ovariectomized (OVX) mouse model of osteoporosis and transcriptional profiling to identify genes upregulated by either high-dose E2 or PTH. Five weeks post-OVX, the mice were administered either E2 and/or PTH, or vehicle for 4 weeks. Femoral bones were analyzed by microCT and histomorphometry to confirm the anabolic effect of each treatment. OVX vehicle-treated control mice lost metaphyseal trabecular bone, with significant decrease in trabecular number, thickness, and connectivity. Both E2 and PTH treatments increased trabecular and cortical bone indices above the level of the sham operated controls, fully restoring both bone volume and bone mineral density (BMD). Moreover, PTH/E2 combination treatment led to significantly greater increase in cancellous bone and BMD than would be expected from the additive effects of the separate treatments. To determine whether PTH and E2 treatments were stimulating similar bone anabolic mechanisms, or were activating distinct signaling pathways, we compared patterns of gene expression using transcriptional profiling after either E2 or PTH treatment. After 4, 11, and 24 days of treatment, total RNA was collected from both the distal femoral metaphysis and diaphysis. Transcriptional profiling was performed using Affymetrix GeneChip probe arrays, comprised of approximately 36,000 full-length mouse genes and EST clusters from the UniGene database. Several markers of osteoblast activity, including c-fos, RANKL, PHEX, and PTHR1, were consistently upregulated by PTH in both skeletal sites. PTH treatment also increased expression of Cathespin K, consistent with the predicted increase in osteoclast activity. E2 treatment upregulated a largely distinct set of genes, including TGFbeta3, and BMP1, as well as several genes critical for cell cycle control, including Cyclin D1 and CDK inhibitor 1A. Overall, comparison of transcriptional profiles suggest that anabolic responses in bone to PTH and high-dose E2 treatment after OVX-induced osteoporosis involve largely distinct patterns of gene regulation, each resulting in restoration of bone mass.

CMF608-a novel mechanical strain-induced bone-specific protein expressed in early osteochondroprogenitor cells

Microarray gene expression analysis was utilized to identify genes upregulated in primary rat calvaria cultures in response to mechanical force. One of the identified genes designated CMF608 appeared to be novel. The corresponding full-length cDNA was cloned and characterized in more details. It encodes a putative 2597 amino acid protein containing N-terminal signal peptide, six leucine-rich repeats (LRRs), and 12 immunoglobulin-like repeats, 10 of which are clustered within the C-terminus. Expression of CMF608 is bone-specific and the main type of CMF608-positive cells is mesenchymal osteochondroprogenitors with fibroblast-like morphology. These cells reside in the perichondral fibrous ring of La Croix, periosteum, endosteum of normal bone as well as in the activated periosteum and early fibrous callus generated postfracture. Expression of CMF608 is notably absent from the regions of endochondral ossification. Mature bone cell types do not produce CMF608 with the exception of chondrocytes of the tangential layer of the articular cartilage, which are thought to be under constant mechanical loading. Ectopic expression of CMF608 in HEK293T cells shows that the protein is subjected to post-translational processing and its N-terminal approximately 90 kDa polypeptide can be found in the conditioned medium. Ectopic expression of either the full-length cDNA of CMF608 or of its N-terminal region in CMF608-negative ROS17/2.8 rat osteosarcoma cells results in transfected clones displaying increased proliferation rate and the characteristics of less-differentiated osteoblasts compared to the control cells. Our data indicate that CMF608 is a unique marker of early osteochondroprogenitor cells. We propose that it could be functionally involved in maintenance of the osteochondroprogenitor cells pool and its down-regulation precedes terminal differentiation.

Estrogen added intermittently, but not continuously, stimulates differentiation and bone formation in SaOS-2 cells

Although it is well established that estrogen inhibits bone resorption, its effects on bone formation remain controversial. We studied the effects of intermittent and continuous treatment with estrogen on bone formation in vitro using long term cultures of SaOS-2 cells under conditions that permit mineralization. SaOS-2 cells cultured in dexamethasone, ascorbic acid and beta-glycerophosphate for up to 17 d formed mineralized bone nodules as visualized by von Kossa staining. Electron microscopic analysis of ultrathin sections of representative mineralized nodules showed the presence of mineral deposits, collagen fibrils and osteocytes. Both the mineralized nodule numbers and areas increased exponentially with time of culture after addition of beta-glycerophophate at day 8. Intermittent addition of 17beta-estradiol (E(2)) for 6 h or 24 h of every 48 h starting at day 3 or day 8 to the end of culture period resulted in a specific time- and dose-dependent stimulation of mineralized bone nodule number and area, and alkaline phosphatase activity which were accompanied with increase in cell numbers. On the other hand, continuous treatment with E(2) added every 48 h had no effect. The estrogen receptor alpha (ERalpha) mRNA expression was stimulated after 6 or 24-h (intermittent), but not after 48-h (continuous) treatment with E(2). The stimulatory effect of E(2), when added intermittently, but not continuously, on differentiation and bone formation in human osteoblasts in culture may be relevant to previous reports of stimulatory effects of E(2) on bone formation in vivo.

Selective inhibitors of the osteoblast proteasome stimulate bone formation in vivo and in vitro

We have found that the ubiquitin-proteasome pathway exerts exquisite control of osteoblast differentiation and bone formation in vitro and in vivo in rodents. Structurally different inhibitors that bind to specific catalytic beta subunits of the 20S proteasome stimulated bone formation in bone organ cultures in concentrations as low as 10 nM. When administered systemically to mice, the proteasome inhibitors epoxomicin and proteasome inhibitor-1 increased bone volume and bone formation rates over 70% after only 5 days of treatment. Since the ubiquitin-proteasome pathway has been shown to modulate expression of the Drosophila homologue of the bone morphogenetic protein-2 and -4 (BMP-2 and BMP-4) genes, we examined the effects of noggin, an endogenous inhibitor of BMP-2 and BMP-4 on bone formation stimulated by these compounds and found that it was abrogated. These compounds increased BMP-2 but not BMP-4 or BMP-6 mRNA expression in osteoblastic cells, suggesting that BMP-2 was responsible for the observed bone formation that was inhibited by noggin. We show proteasome inhibitors regulate BMP-2 gene expression at least in part through inhibiting the proteolytic processing of Gli3 protein. Our results suggest that the ubiquitin-proteasome machinery regulates osteoblast differentiation and bone formation and that inhibition of specific components of this system may be useful therapeutically in common diseases of bone loss.

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

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

Increased metabolism of bone collagen in post-menopausal female osteoporotic femoral heads

Our previous studies demonstrated that the residual collagen in osteoporotic bone was not normal but possessed higher levels of lysine hydroxylation and modified cross-linking. However, the mechanism for these changes was not clear. In the current investigation, an assessment of bone collagen metabolism in osteoporosis (OP) revealed an increase in the overall metabolism of collagen relative to age-matched controls. The increased metabolism accounts for the observed post-translational modifications of collagen which lead to a more fragile bone matrix. The rate of bone metabolism is therefore an important aspect of the pathogenesis of osteoporosis, the greater the turnover the greater the propensity of a more fragile tissue. Clearly, the quality of bone tissue does not depend solely on adequate bone density but also on the state of the collagenous matrix.

Estrogens activate bone morphogenetic protein-2 gene transcription in mouse mesenchymal stem cells

Estrogens exert their physiological effects on target tissues by interacting with the estrogen receptors, ERalpha and ERbeta. Estrogen replacement is one the most common and effective strategies used to prevent osteoporosis in postmenopausal women. Whereas it was thought that estrogens work exclusively by inhibiting bone resorption, our previous results show that 17beta-estradiol (E2) increases mouse bone morphogenetic protein (BMP)-2 mRNA, suggesting that estrogens may also enhance bone formation. In this study, we used quantitative real-time RT-PCR analysis to demonstrate that estrogens increase BMP-2 mRNA in mouse mesenchymal stem cells. The selective ER modulators, tamoxifen, raloxifene, and ICI-182,780 (ICI), failed to enhance BMP-2 mRNA, whereas ICI inhibited E2 stimulation of expression. To investigate if estrogens increase BMP-2 expression by transcriptional mechanisms and if the response is mediated by ERalpha and/or ERbeta, we studied the effects of estrogens on BMP-2 promoter activity in transient transfected C3H10T1/2 cells. E2 produced a dose-dependent induction of the mouse -2712 BMP-2 promoter activity in cells cotransfected with ERalpha and ERbeta. At a dose of 10 nM E2, ERalpha induced mouse BMP-2 promoter activity 9-fold, whereas a 3-fold increase was observed in cells cotransfected with ERbeta. Tamoxifen and raloxifene were weak activators of the mouse BMP-2 promoter via ERalpha, but not via ERbeta. ICI blocked the activation of BMP-2 promoter activity by E2 acting via both ERalpha and ERbeta, indicating that mouse BMP-2 promoter activation is ER dependent. In contrast to E2 and selective ER modulators, the phytoestrogen, genistein was more effective at activating the mouse BMP-2 promoter with ERbeta, compared with ERalpha. Using a deletion series of the BMP-2 promoter, we determined that AP-1 or Sp1 sites are not required for E2 activation. A mutation in a sequence at -415 to -402 (5'-GGGCCActcTGACCC-3') that resembles the classical estrogen-responsive element abolished the activation of the BMP-2 promoter in response to E2. Our studies demonstrate that E2 activation of mouse BMP-2 gene transcription requires ERalpha or ERbeta acting via a variant estrogen-responsive element binding site in the promoter, with ERalpha being the more efficacious regulator. Estrogenic compounds may enhance bone formation by increasing the transcription of the BMP-2 gene.

Reversal of bone loss in mice by nongenotropic signaling of sex steroids

We show that sex steroids protect the adult murine skeleton through a mechanism that is distinct from that used to preserve the mass and function of reproductive organs. The classical genotropic actions of sex steroid receptors are dispensable for their bone protective effects, but essential for their effects on reproductive tissues. A synthetic ligand (4-estren-3alpha,17beta-diol) that reproduces the nongenotropic effects of sex steroids, without affecting classical transcription, increases bone mass and strength in ovariectomized females above the level of the estrogen-replete state and is at least as effective as dihydrotestosterone in orchidectomized males, without affecting reproductive organs. Such ligands merit investigation as potential therapeutic alternatives to hormone replacement for osteoporosis in both women and men [corrected].

Estrogen-induced osteogenesis in intact female mice lacking ERbeta

We recently found that estrogen receptor (ER) antagonists prevent high-dose estrogen from inducing the formation of new cancellous bone within the medullary cavity of mouse long bones. In the present investigation, we studied the role of specific ER subtypes in this response by examining whether this is impaired in female ERbeta(-/-) mice previously generated by targeted gene deletion. Vehicle or 17beta-estradiol (E(2)) (range 4-4,000 microg. kg(-1). day(-1)) was administered to intact female ERbeta(-/-) mice and wild-type littermates by subcutaneous injection for 28 days. The osteogenic response was subsequently assessed by histomorphometry performed on longitudinal and cross sections of the tibia. E(2) was found to cause an equivalent increase in cancellous bone formation in ERbeta(-/-) mice and littermate controls, as assessed at the proximal and distal regions of the proximal tibial metaphysis. E(2) also resulted in a similar increase in endosteal mineral apposition rate in these two genotypes, as assessed at the tibial diaphysis. In contrast, cortical area in ERbeta(-/-) mice was found to be greater than that in wild types irrespective of E(2) treatment, as was tibial bone mineral density as measured by dual-energy X-ray absorptiometry, consistent with previous reports of increased cortical bone mass in these animals. We conclude that, although ERbeta acts as a negative modulator of cortical modeling, this isoform does not appear to contribute to high-dose estrogen's ability to induce new cancellous bone formation in mouse long bones.

Increased bone morphogenetic protein-6 expression in mouse long bones after estrogen administration

High-dose estrogen administration is known to induce new bone formation in mouse long bones. To study the role of regulatory proteins in this response, we examined associated changes in femoral messenger RNA (mRNA) for candidate factors. 17beta-estradiol (E2) 0.5 mg was administered to intact female mice by weekly injection, and Northern blot analysis was performed 1, 2, 4, 8, 12, and 16 days after the first injection. In contrast to other factors, an increase was observed in mRNA for bone morphogenetic protein-6 (BMP-6), which reached significance at day 8 and subsequent time-points. Estrogen-induced changes in BMP-6 protein expression were assessed by immunocytochemistry in longitudinal femoral sections. In untreated animals, BMP-6 was expressed by a significant proportion of growth plate chondrocytes and a subpopulation of bone marrow cells. In contrast, osteoblasts were consistently BMP-6 negative. From as early as 4 days after starting estrogen, clusters of slightly elongated BMP-6-positive cells were observed within the marrow cavity; the majority were close to active bone formation surfaces. Double immunolabeling studies revealed that only approximately 10% of BMP-6-positive bone marrow cells co-expressed the osteoblast transcription factor Cbfa1 suggesting that they are largely distinct from the osteoblast precursor population generated concurrently. BMP-6-positive cells expressed neither leukocyte nor erythroid markers (CD45 and TER-119, respectively), consistent with a stromal origin. We conclude that estrogen-induced osteogenesis in female mice is associated with increased levels of BMP-6 mRNA in mouse femurs, which seems to reflect the emergence of clusters of BMP-6 positive stromal cells adjacent to active bone formation surfaces. These findings raise the possibility that BMP-6 serves as a paracrine mediator of estrogen's osteogenic action in mice.

Estrogen-induced osteogenesis in mice is associated with the appearance of Cbfa1-expressing bone marrow cells

Cbfa1 is a transcription factor recognised as being involved in early osteoblast differentiation during embryonic skeletogenesis. To determine whether Cbfa1 plays a similar role in bone formation in the adult, we analysed whether its expression is altered during estrogen-induced osteogenesis, following our recent studies which suggest that this response involves the generation of early osteoblast precursors within bone marrow. To facilitate identification of Cbfa1-expressing cells, these studies were performed in mice heterozygous for a cbfa1 gene deletion (cbfa1(+/-)) using beta-galactosidase (lacZ) as a genetic marker. Cbfa1-expressing cells were identified by lacZ staining of longitudinal sections of the proximal tibial metaphysis. Treatment of cbfa1(+/-) mice with 17beta-estradiol 0.5 mg/week for 24 days led to the appearance of new cancellous bone surfaces. This response was associated with a marked increase in number of Cbfa1-expressing cells within the metaphysis, consisting not only of osteoblasts on bone surfaces but also of cells within the adjacent bone marrow. We subsequently enumerated Cbfa1-expressing cells at earlier time-points following estrogen, in sections co-stained for ALP activity. After 4 days of estrogen treatment, a population of cells appeared within the marrow cavity which expressed Cbfa1, but were negative for ALP. At later time-points, large numbers of Cbfa1 + bone marrow cells were still present, but the majority of these were close to new trabecular bone surfaces at sites which showed high levels of ALP activity. An equivalent distribution of Cbfa1-expressing cells was observed in further studies where Cbfa1 expression was analysed in wild-type mice by immunohistochemistry. We conclude that estrogen-induced osteogenesis is associated with the appearance of a population of Cbfa1-expressing cells within bone marrow, which we hypothesize to represent the osteoblast precursor population responsible for subsequent new bone formation.

Characterisation of the temporal sequence of osteoblast gene expression during estrogen-induced osteogenesis in female mice

Osteoblast differentiation under in vitro conditions is associated with increased expression of non-collagenous bone proteins including osteocalcin, osteopontin, and osteonectin, the exact function of which remain poorly understood. To determine whether these proteins play an important role in the formation of mineralised bone matrix by osteoblasts in vivo, we analysed the time-course of their expression during estrogen-induced osteogenesis in female mice, and compared this with the formation of new cancellous bone. Female mice were sacrificed prior to or following treatment with 17beta-estradiol for up to 32 days (500 microg/animal/week). Total RNA was extracted from femurs, and changes in expression of genes for a range of osteoblast-derived proteins assessed by Northern blot analysis. In parallel experiments, the time course of cancellous bone formation was determined by measuring bone mineral density (BMD) of the distal femur. Estrogen led to a rapid increase in BMD, which reached significance by Day 16. This was preceded by three-fold increases in expression of alkaline phosphatase (ALP) and type I collagen (COL I) at Days 8 and 12 respectively. In contrast, osteocalcin, osteopontin, and osteonectin expression showed no change during this initial period, although modest increases were observed at later times (i.e., Days 20 and 24). Our results suggest that osteocalcin, osteopontin, and osteonectin are not involved in the initial phase of the osteogenic response to estrogen, suggesting that these non-collagenous bone proteins do not play a direct role in the formation of mineralised bone matrix by osteoblasts in vivo.

Estrogen modulates estrogen receptor alpha and beta expression, osteogenic activity, and apoptosis in mesenchymal stem cells (MSCs) of osteoporotic mice

In the mouse, ovariectomy (OVX) leads to significant reductions in cancellous bone volume while estrogen (17beta-estradiol, E2) replacement not only prevents bone loss but can increase bone formation. As the E2-dependent increase in bone formation would require the proliferation and differentiation of osteoblast precursors, we hypothesized that E2 regulates mesenchymal stem cells (MSCs) activity in mouse bone marrow. We therefore investigated proliferation, differentiation, apoptosis, and estrogen receptor (ER) alpha and beta expression of primary culture MSCs isolated from OVX and sham-operated mice. MSCs, treated in vitro with 10(-7) M E2, displayed a significant increase in ERalpha mRNA and protein expression as well as alkaline phosphatase (ALP) activity and proliferation rate. In contrast, E2 treatment resulted in a decrease in ERbeta mRNA and protein expression as well as apoptosis in both OVX and sham mice. E2 up-regulated the mRNA expression of osteogenic genes for ALP, collagen I, TGF-beta1, BMP-2, and cbfa1 in MSCs. In a comparison of the relative mRNA expression and protein levels for two ER isoforms, ERalpha was the predominant form expressed in MSCs obtained from both OVX and sham-operated mice. Cumulatively, these results indicate that estrogen in vitro directly augments the proliferation and differentiation, ERalpha expression, osteogenic gene expression and, inhibits apoptosis and ERbeta expression in MSCs obtained from OVX and sham-operated mice. Co-expression of ERalpha, but not ERbeta, and osteogenic differentiation markers might indicate that ERalpha function as an activator and ERbeta function as a repressor in the osteogenic differentiation in MSCs. These results suggest that mouse MSCs are anabolic targets of estrogen action, via ERalpha activation. J. Cell. Biochem. Suppl. 36: 144-155, 2001.

Growth-hormone-stimulated dentinogenesis in Lewis dwarf rat molars

In dentinogenesis, certain growth factors, matrix proteoglycans, and proteins are directly or indirectly dependent on growth hormone. The hypothesis that growth hormone up-regulates the expression of enzymes, sialoproteins, and other extracellular matrix proteins implicated in the formation and mineralization of tooth and bone matrices was tested by the treatment of Lewis dwarf rats with growth hormone over 5 days. The molar teeth were processed for immunohistochemical demonstration of bone-alkaline phosphatase, bone morphogenetic proteins-2 and -4, osteocalcin, osteopontin, bone sialoprotein, and E11 protein. Odontoblasts responded to growth hormone by more cells expressing bone morphogenetic protein, alkaline phosphatase, osteocalcin, and osteopontin. No changes were found in bone sialoprotein or E11 protein expression. Thus, growth hormone may stimulate odontoblasts to express several growth factors and matrix proteins associated with dentin matrix biosynthesis in mature rat molars.