RANKL-dependent osteoclast differentiation and gene expression in bone marrow-derived cells from adult mice is sexually dimorphic

Sex-specific differences in bone integrity and properties are associated with age as well as the number and activity of cells involved in bone remodeling. The aim of this study was to investigate sex-specific differences in adhesion, proliferation, and differentiation of mouse bone marrow derived cells into osteoclasts. The adherent fraction of bone marrow- derived cells from 12-week-old male and female C57BL/6J mice were assessed for their adhesion, proliferation, and receptor activator of nuclear factor κB (RANKL)-induced differentiation into osteoclasts. Female bone marrow derived macrophages (BMDMs) displayed higher adhesion and proliferation ratio upon macrophage colony stimulating factor (M-CSF) (day 0) and M-CSF + RANKL (day 4) treatment, respectively. On the contrary, male BMDMs differentiated more efficiently into osteoclasts upon RANKL-treatment compared to females (day 5). To further understand these sex-specific differences at the gene expression level, BMDMs treated with M-CSF (day 0) and M-CSF + RANKL (day 4), were assessed for their differential expression of genes through RNA sequencing. M-CSF treatment resulted in 1106 differentially expressed genes, while RANKL-treatment gave 473 differentially expressed genes. Integrin, adhesion, and proliferation-associated genes were elevated in the M-CSF-treated female BMDMs. RANKL-treatment further enhanced the expression of the proliferation- associated genes, and of genes associated with inhibition of osteoclast differentiation in the females, while RANK-signaling-associated genes were upregulated in males. In conclusion, BMDM adhesion, proliferation and differentiation into osteoclasts are sex-specific and may be directed by the PI3K-Akt signaling pathway for proliferation, and the colony stimulating factor 1-receptor and the RANKLsignaling pathway for the differentiation.

A COX-2 Inhibitor Does Not Interfere With the Bone-Protective Effects of Loading in Male Mice With Arthritis

Mechanical loading enhances bone strength and counteracts arthritis-induced inflammation-mediated bone loss in female mice. It is unknown whether nonsteroidal anti-inflammatory drugs (NSAIDs; eg, COX-2 inhibitors) can reduce inflammation without affecting the loading-associated bone formation in male mice. The aim of this study was to investigate if loading combined with a COX-2 inhibitor (NS-398) could prevent arthritis-induced bone loss and inflammation in male mice. Four-month-old male C57BL/6J mice were subjected to axial tibial mechanical loading three times/week for 2 weeks. Local mono-arthritis was induced with a systemic injection of methylated bovine serum albumin on the first day of loading, followed by a local injection in one knee 1 week later. The arthritis induction, knee swelling, bone architecture, and osteoclast number were evaluated in the hind limbs. C-terminal cross-links as a marker for osteoclast activity was measured in serum. Compared with loading and arthritis alone, loading of the arthritic joint enhanced swelling that was partly counteracted by NS-398. Loading of the arthritic joint enhanced synovitis and articular cartilage damage compared with loading alone. Loading increased cortical bone and counteracted the arthritis-induced decrease in epiphyseal bone. NS-398 did not alter the bone-protective effects of loading. C-terminal cross-links, a bone resorption marker, was increased by arthritis but not loading. In conclusion, loading prevented arthritis-induced epiphyseal and metaphyseal bone loss, and NS-398 reduced knee swelling without affecting the bone-protective effects of loading. If our results can be extrapolated to the human situation, specific COX-2 inhibitors could be used in combination with loading exercise to prevent pain and swelling of the joint without influencing the bone-protective effects of loading. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Dehydroepiandrosterone Supplementation Results in Varying Tissue-specific Levels of Dihydrotestosterone in Male Mice

Dehydroepiandrosterone (DHEA), an adrenal androgen precursor, can be metabolized in target tissues into active sex steroids. It has been proposed that DHEA supplementation might result in restoration of physiological local sex steroid levels, but knowledge on the effect of DHEA treatment on local sex steroid levels in multiple tissues is lacking. To determine the effects of DHEA on tissue-specific levels of sex steroids, we treated orchiectomized (ORX) male mice with DHEA for 3 weeks and compared them with vehicle-treated ORX mice and gonadal intact mice. Intra-tissue levels of sex steroids were analyzed in reproductive organs (seminal vesicles, prostate, m. levator ani), major body compartments (white adipose tissue, skeletal muscle, and brain), adrenals, liver, and serum using a sensitive and validated gas chromatography-mass spectrometry method. DHEA treatment restored levels of both testosterone (T) and dihydrotestosterone (DHT) to approximately physiological levels in male reproductive organs. In contrast, this treatment did not increase DHT levels in skeletal muscle or brain. In the liver, DHEA treatment substantially increased levels of T (at least 4-fold) and DHT (+536%, P < 0.01) compared with vehicle-treated ORX mice. In conclusion, we provide a comprehensive map of the effect of DHEA treatment on intra-tissue sex steroid levels in ORX mice with a restoration of physiological levels of androgens in male reproductive organs while DHT levels were not restored in the skeletal muscle or brain. This, and the unexpected supraphysiological androgen levels in the liver, may be a cause for concern considering the uncontrolled use of DHEA.

Comprehensive Sex Steroid Profiling in Multiple Tissues Reveals Novel Insights in Sex Steroid Distribution in Male Mice

A comprehensive atlas of sex steroid distribution in multiple tissues is currently lacking, and how circulating and tissue sex steroid levels correlate remains unknown. Here, we adapted and validated a gas chromatography tandem mass spectrometry method for simultaneous measurement of testosterone (T), dihydrotestosterone (DHT), androstenedione, progesterone (Prog), estradiol, and estrone in mouse tissues. We then mapped the sex steroid pattern in 10 different endocrine, reproductive, and major body compartment tissues and serum of gonadal intact and orchiectomized (ORX) male mice. In gonadal intact males, high levels of DHT were observed in reproductive tissues, but also in white adipose tissue (WAT). A major part of the total body reservoir of androgens (T and DHT) and Prog was found in WAT. Serum levels of androgens and Prog were strongly correlated with corresponding levels in the brain while only modestly correlated with corresponding levels in WAT. After orchiectomy, the levels of the active androgens T and DHT decreased markedly while Prog levels in male reproductive tissues increased slightly. In ORX mice, Prog was by far the most abundant sex steroid, and, again, WAT constituted the major reservoir of Prog in the body. In conclusion, we present a comprehensive atlas of tissue and serum concentrations of sex hormones in male mice, revealing novel insights in sex steroid distribution. Brain sex steroid levels are well reflected by serum levels and WAT constitutes a large reservoir of sex steroids in male mice. In addition, Prog is the most abundant sex hormone in ORX mice.

STAT3 Hyperactivation Due to SOCS3 Deletion in Murine Osteocytes Accentuates Responses to Exercise- and Load-Induced Bone Formation

Cortical bone develops and changes in response to mechanical load, which is sensed by bone-embedded osteocytes. The bone formation response to load depends on STAT3 intracellular signals, which are upregulated after loading and are subject to negative feedback from Suppressor of Cytokine Signaling 3 (Socs3). Mice with Dmp1Cre-targeted knockout of Socs3 have elevated STAT3 signaling in osteocytes and display delayed cortical bone maturation characterized by impaired accrual of high-density lamellar bone. This study aimed to determine whether these mice exhibit an altered response to mechanical load. The approach used was to test both treadmill running and tibial compression in female Dmp1Cre.Socs3^f/f mice. Treadmill running for 5 days per week from 6 to 11 weeks of age did not change cortical bone mass in control mice, but further delayed cortical bone maturation in Dmp1Cre.Socs3^f/f mice; accrual of high-density bone was suppressed, and cortical thickness was less than in genetically-matched sedentary controls. When strain-matched anabolic tibial loading was tested, both control and Dmp1Cre.Socs3^f/f mice exhibited a significantly greater cortical thickness and periosteal perimeter in loaded tibia compared with the contralateral non-loaded bone. At the site of greatest compressive strain, the loaded Dmp1Cre.Socs3^f/f tibias showed a significantly greater response than controls, indicated by a greater increase in cortical thickness. This was due to a greater bone formation response on both periosteal and endocortical surfaces, including formation of abundant woven bone on the periosteum. This suggests a greater sensitivity to mechanical load in Dmp1Cre.Socs3^f/f bone. In summary, mice with targeted SOCS3 deletion and immature cortical bone have an exaggerated response to both physiological and experimental mechanical loads. We conclude that there is an optimal level of osteocytic response to mechanical load required for cortical bone maturation and that load-induced bone formation may be increased by augmenting STAT3 signaling within osteocytes. © 2021 American Society for Bone and Mineral Research (ASBMR).

The effects of estradiol are modulated in a tissue-specific manner in mice with inducible inactivation of ERα after sexual maturation

Mouse models with lifelong inactivation of estrogen receptor-α (ERα) show that ERα is the main mediator of estrogenic effects in bone, thymus, uterus, and fat. However, ERα inactivation early in life may cause developmental effects that confound the adult phenotypes. To address the specific role of adult ERα expression for estrogenic effects in bone and other nonskeletal tissues, we established a tamoxifen-inducible ERα-inactivated model by crossing CAGG-Cre-ER and ERα^flox/flox mice. Tamoxifen-induced ERα inactivation after sexual maturation substantially reduced ERα mRNA levels in cortical bone, trabecular bone, thymus, uterus, gonadal fat, and hypothalamus, in CAGG-Cre-ERα^flox/flox (inducible ERα^KO) compared with ERα^flox/flox (control) mice. 17β-estradiol (E2) treatment increased trabecular bone volume fraction (BV/TV), cortical bone area, and uterine weight, while it reduced thymus weight and fat mass in ovariectomized control mice. The estrogenic responses were substantially reduced in inducible ERα^KO mice compared with control mice on BV/TV (-67%), uterine weight (-94%), thymus weight (-70%), and gonadal fat mass (-94%). In contrast, the estrogenic response on cortical bone area was unaffected in inducible ERα^KO compared with control mice. In conclusion, using an inducible ERα^KO model, not confounded by lack of ERα during development, we demonstrate that ERα expression in sexually mature female mice is required for normal E2 responses in most, but not all, tissues. The finding that cortical, but not trabecular bone, responds normally to E2 treatment in inducible ERα^KO mice strengthens the idea of cortical and trabecular bone being regulated by estrogen via different mechanisms.

Bone gain following loading is site-specifically enhanced by prior and concurrent disuse in aged male mice

The primary aim of osteoanabolic therapies is to strategically increase bone mass in skeletal regions likely to experience high strains. In the young healthy skeleton, this is primarily achieved by bone's adaptation to loading. This adaptation appears to fail with age, resulting in osteoporosis and fractures. We previously demonstrated that prior and concurrent disuse enhances bone gain following loading in old female mice. Here, we applied site specificity micro-computed tomography analysis to map regional differences in bone anabolic responses to axial loading of the tibia between young (19-week-old) and aged (19-month-old), male and female mice. Loading increased bone mass specifically in the proximal tibia in both sexes and ages. Young female mice gained more cortical bone than young males in specific regions of the tibia. However, these site-specific sex differences were lost with age such that bone gain following loading was not significantly different between old males and females. To test whether disuse enhances functional adaption in old male mice as it does in females, old males were subjected to sciatic neurectomy or sham surgery, and loading was initiated four days after surgery. Disuse augmented tibial cortical bone gain in response to loading in old males, but only in regions which were load-responsive in the young. Prior and concurrent disuse also increased loading-induced trabecular thickening in the proximal tibia of old males. Understanding how diminished background loading rejuvenates the osteogenic loading response in the old may improve osteogenic exercise regimes and lead to novel osteoanabolic therapies.

Changes in mechanical loading affect arthritis-induced bone loss in mice

Arthritis induces bone loss by inflammation-mediated disturbance of bone homeostasis. On the other hand, pain and impaired locomotion are highly prevalent in arthritis and result in reduced general physical activity and less pronounced mechanical loading. Bone is affected by mechanical loading, directly through impact with the ground during movement and indirectly through muscular activity. Mechanical loading in its physiological range is essential for maintaining bone mass, whereas disuse leads to bone loss. The aim of this study was to investigate the impact of mechanical loading on periarticular bone as well as inflammation during arthritis. Mechanical loading was either blocked by botulinum neurotoxin A (Botox) injections before induction of arthritis, or enhanced by cyclic compressive loading, three times per week during arthritis induction. Arthritis was verified and evaluated histologically. Trabecular and cortical bone mass were investigated using micro-computed tomography (μCT), subchondral osteoclastogenesis and bone turnover was assessed by standard methods. Inhibition of mechanical loading enhanced arthritis-induced bone loss while it did not affect inflammation. In contrast, enhanced mechanical loading mitigated arthritis-induced bone loss. Furthermore, the increase in bone resorption markers by arthritis was partly blocked by mechanical loading. In conclusion, enhanced arthritic bone loss after abrogation of mechanical loading suggests that muscle forces play an essential role in preventing arthritic bone loss. In accordance, mechanical loading of the arthritic joints inhibited bone loss, emphasizing that weight bearing activities may have the potential to counteract arthritis-mediated bone loss.

αSMA Osteoprogenitor Cells Contribute to the Increase in Osteoblast Numbers in Response to Mechanical Loading

Bone is a dynamic tissue that site-specifically adapts to the load that it experiences. In response to increasing load, the cortical bone area is increased, mainly through enhanced periosteal bone formation. This increase in area is associated with an increase in the number of bone-forming osteoblasts; however, the origin of the cells involved remains unclear. Alpha-smooth muscle actin (αSMA) is a marker of early osteoprogenitor cells in the periosteum, and we hypothesized that the new osteoblasts that are activated by loading could originate from αSMA-expressing cells. Therefore, we used an in vivo fate-mapping approach in an established axial loading model to investigate the role of αSMA-expressing cells in the load-induced increase in osteoblasts. Histomorphometric analysis was applied to measure the number of cells of different origin on the periosteal surface in the most load-responsive region of the mouse tibia. A single loading session failed to increase the number of periosteal αSMA-expressing cells and osteoblasts. However, in response to multiple episodes of loading, the caudal, but not the cranial, periosteal surface was lined with an increased number of osteoblasts originating from αSMA-expressing cells 5 days after the initial loading session. The proportion of osteoblasts derived from αSMA-labeled progenitors increased by 70% (p < 0.05), and the proportion of αSMA-labeled cells that had differentiated into osteoblasts was doubled. We conclude that αSMA-expressing osteoprogenitors can differentiate and contribute to the increase in periosteal osteoblasts induced by mechanical loading in a site-specific manner.

Interactions Between the Gravitostat and the Fibroblast Growth Factor System for the Regulation of Body Weight

Both fibroblast growth factors (FGFs), by binding to FGF receptors (FGFRs), and activation of the gravitostat, by artificial loading, decrease the body weight (BW). Previous studies demonstrate that both the FGF system and loading have the capacity to regulate BW independently of leptin. The aim of the current study was to determine the possible interactions between the effect of increased loading and the FGF system for the regulation of BW. We observed that the BW-reducing effect of increased loading was abolished in mice treated with a monoclonal antibody directed against FGFR1c, suggesting interactions between the two systems. As serum levels of endocrine FGF21 and hepatic FGF21 mRNA were increased in the loaded mice compared with the control mice, we first evaluated the loading response in FGF21 over expressing mice with constant high FGF21 levels. Leptin treatment, but not increased loading, decreased the BW in the FGF21-overexpressing mice, demonstrating that specifically the loading effect is attenuated in the presence of high activity in the FGF system. However, as FGF21 knockout mice displayed a normal loading response on BW, FGF21 is neither mediating nor essential for the loading response. In conclusion, the BW-reducing effect of increased loading but not of leptin treatment is blocked by high activity in the FGF system. We propose that both the gravitostat and the FGF system regulate BW independently of leptin and that pharmacologically enhanced activity in the FGF system reduces the sensitivity of the gravitostat.

Vitamin A decreases the anabolic bone response to mechanical loading by suppressing bone formation

Increased vitamin A consumption is associated with decreased cortical bone mass and increased fracture risk in humans. Rodent studies have demonstrated that hypervitaminosis A increases cortical bone resorption, whereas the importance of the effects on bone formation is less well defined. We used an experimental model of increased bone formation by loading of the tibiae to investigate the effect of vitamin A on bone formation. Control [retinol activity equivalents (RAE) 4.5 µg/g chow] or vitamin A (RAE 60 µg/g chow) diets were given to female C57BL/6N mice for 4 wk, after which the tibiae were subjected to axial loading on alternate days for 2 wk, while the diets were continued. Vitamin A inhibited the loading-induced increase in trabecular and cortical bone volume. This was attributed to inhibition of loading-induced increase in osteoblast number and activity, and expression of osteoblastic genes Sp7, Alpl, and Col1a1 in cortical bone. Vitamin A, loading, and combination thereof also resulted in site-specific effects on bone composition measured by Raman spectroscopy. In summary, a clinically relevant dose of vitamin A suppresses the loading-induced gain of bone mass by decreasing bone formation. These observations may have implications for regulation of bone mass caused by physical activity and the risk of osteoporosis in humans.-Lionikaite, V., Henning, P., Drevinge, C., Shah, F. A., Palmquist, A., Wikström, P., Windahl, S. H., Lerner, U. H. Vitamin A decreases the anabolic bone response to mechanical loading by suppressing bone formation.

Clinically relevant doses of vitamin A decrease cortical bone mass in mice

Excess vitamin A has been associated with decreased cortical bone thickness and increased fracture risk. While most studies in rodents have employed high dosages of vitamin A for short periods of time, we investigated the bone phenotype in mice after longer exposure to more clinically relevant doses. For 1, 4 and 10 weeks, mice were fed a control diet (4.5 µg retinyl acetate/g chow), a diet modeled from the human upper tolerable limit (UTL; 20 µg retinyl acetate/g chow) and a diet three times UTL (supplemented; 60 µg retinyl acetate/g chow). Time-dependent decreases in periosteal circumference and bone mineral content were noted with the supplemented dose. These reductions in cortical bone resulted in a significant time-dependent decrease of predicted strength and a non-significant trend toward reduced bone strength as analyzed by three-point bending. Trabecular bone in tibiae and vertebrae remained unaffected when vitamin A was increased in the diet. Dynamic histomorphometry demonstrated that bone formation was substantially decreased after 1 week of treatment at the periosteal site with the supplemental dose. Increasing amount of vitamin A decreased endocortical circumference, resulting in decreased marrow area, a response associated with enhanced endocortical bone formation. In the presence of bisphosphonate, vitamin A had no effect on cortical bone, suggesting that osteoclasts are important, even if effects on bone resorption were not detected by osteoclast counting, genes in cortical bone or analysis of serum TRAP5b and CTX. In conclusion, our results indicate that even clinically relevant doses of vitamin A have a negative impact on the amount of cortical bone.

Correction to: Compressive loading of the murine tibia reveals site-specific micro-scale differences in adaptation and maturation rates of bone

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ERα expression in T lymphocytes is dispensable for estrogenic effects in bone

Estrogen treatment has positive effects on the skeleton, and we have shown that estrogen receptor alpha (ERα) expression in cells of hematopoietic origin contributes to a normal estrogen treatment response in bone tissue. T lymphocytes are implicated in the estrogenic regulation of bone mass, but it is not known whether T lymphocytes are direct estrogen target cells. Therefore, the aim of this study was to determine the importance of ERα expression in T lymphocytes for the estrogenic regulation of the skeleton using female mice lacking ERα expression specifically in T lymphocytes (Lck-ERα^-/-) and ERα^flox/flox littermate (control) mice. Deletion of ERα expression in T lymphocytes did not affect bone mineral density (BMD) in sham-operated Lck-ERα^-/- compared to control mice, and ovariectomy (ovx) resulted in a similar decrease in BMD in control and Lck-ERα^-/- mice compared to sham-operated mice. Furthermore, estrogen treatment of ovx Lck-ERα^-/- led to an increased BMD that was indistinguishable from the increase seen after estrogen treatment of ovx control mice. Detailed analysis of both the appendicular (femur) and axial (vertebrae) skeleton showed that both trabecular and cortical bone parameters responded to a similar extent regardless of the presence of ERα in T lymphocytes. In conclusion, ERα expression in T lymphocytes is dispensable for normal estrogenic regulation of bone mass in female mice.

Prednisolone treatment reduces the osteogenic effects of loading in mice

Glucocorticoid treatment, a major cause of drug-induced osteoporosis and fractures, is widely used to treat inflammatory conditions and diseases. By contrast, mechanical loading increases bone mass and decreases fracture risk. With these relationships in mind, we investigated whether mechanical loading interacts with GC treatment in bone. Three-month-old female C57BL/6 mice were treated with high-dose prednisolone (15 mg/60 day pellets/mouse) or vehicle for two weeks. During the treatment, right tibiae were subjected to short periods of cyclic compressive loading three times weekly, while left tibiae were used as physiologically loaded controls. The bones were analyzed using peripheral quantitative computed tomography, histomorphometry, real-time PCR, three-point bending and Fourier transform infrared micro-spectroscopy. Loading alone increased trabecular volumetric bone mineral density (vBMD), cortical thickness, cortical area, osteoblast-associated gene expression, osteocyte- and osteoclast number, and bone strength. Prednisolone alone decreased cortical area and thickness and osteoblast-associated gene expression. Importantly, prednisolone treatment decreased the load-induced increase in trabecular vBMD by 57% (p < 0.001) and expression of osteoblast-associated genes, while completely abolishing the load-induced increase in cortical area, cortical thickness, number of osteocytes and osteoclasts, and bone strength. When combined, loading and prednisolone decreased the collagen content. In conclusion, high-dose prednisolone treatment strongly inhibits the loading-induced increase in trabecular BMD, and abolishes the loading-induced increase in cortical bone mass. This phenomenon could be due to prednisolone inhibition of osteoblast differentiation and function.

The Gravitostat Regulates Fat Mass in Obese Male Mice While Leptin Regulates Fat Mass in Lean Male Mice

Leptin has been the only known homeostatic regulator of fat mass, but we recently found evidence for a second one, named the gravitostat. In the current study, we compared the effects of leptin and increased loading (gravitostat stimulation) on fat mass in mice with different levels of body weight (lean, overweight, and obese). Leptin infusion suppressed body weight and fat mass in lean mice given normal chow but not in overweight or obese mice given a high-fat diet for 4 and 8 weeks, respectively. The maximum effect of leptin on body weight and fat mass was obtained already at <44 ng/mL of serum leptin. Increased loading using intraperitoneal capsules with different weights decreased body weight in overweight and obese mice. Although the implantation of an empty capsule reduced the body weight in lean mice, only a nonsignificant tendency of a specific effect of increased loading was observed in the lean mice. These findings demonstrate that the gravitostat regulates fat mass in obese mice, whereas leptin regulates fat mass only in lean mice with low endogenous serum leptin levels. We propose that activation of the gravitostat primarily protects against obesity, whereas low levels of leptin protect against undernutrition.

Roles of activating functions 1 and 2 of estrogen receptor α in lymphopoiesis

Apart from the role of sex steroids in reproduction, sex steroids are also important regulators of the immune system. 17β-estradiol (E2) represses T and B cell development, but augments B cell function, possibly explaining the different nature of immune responses in men and women. Both E2 and selective estrogen receptors modulators (SERM) act via estrogen receptors (ER). Activating functions (AF)-1 and 2 of the ER bind to coregulators and thus influence target gene transcription and subsequent cellular response to ER activation. The importance of ERαAF-1 and AF-2 in the immunomodulatory effects of E2/SERM has previously not been reported. Thus, detailed studies of T and B lymphopoiesis were performed in ovariectomized E2-, lasofoxifene- or raloxifene-treated mice lacking either AF-1 or AF-2 domains of ERα, and their wild-type littermate controls. Immune cell phenotypes were analyzed with flow cytometry. All E2 and SERM-mediated inhibitory effects on thymus cellularity and thymic T cell development were clearly dependent on both ERαAFs. Interestingly, divergent roles of ERαAF-1 and ERαAF-2 in E2 and SERM-mediated modulation of bone marrow B lymphopoiesis were found. In contrast to E2, effects of lasofoxifene on early B cells did not require functional ERαAF-2, while ERαAF-1 was indispensable. Raloxifene reduced early B cells partly independent of both ERαAF-1 and ERαAF-2. Results from this study increase the understanding of the impact of ER modulation on the immune system, which can be useful in the clarification of the molecular actions of SERMs and in the development of new SERM.

Increased adipose tissue aromatase activity improves insulin sensitivity and reduces adipose tissue inflammation in male mice

Females are, in general, more insulin sensitive than males. To investigate whether this is a direct effect of sex-steroids (SS) in white adipose tissue (WAT), we developed a male mouse model overexpressing the aromatase enzyme, converting testosterone (T) to estradiol (E₂), specifically in WAT (Ap2-arom mice). Adipose tissue E₂ levels were increased while circulating SS levels were unaffected in male Ap2-arom mice. Importantly, male Ap2-arom mice were more insulin sensitive compared with WT mice and exhibited increased serum adiponectin levels and upregulated expression of Glut4 and Irs1 in WAT. The expression of markers of macrophages and immune cell infiltration was markedly decreased in WAT of male Ap2-arom mice. The adipogenesis was enhanced in male Ap2-arom mice, supported by elevated Pparg expression in WAT and enhanced differentiation of preadipocyte into mature adipocytes. In summary, increased adipose tissue aromatase activity reduces adipose tissue inflammation and improves insulin sensitivity in male mice. We propose that estrogen increases insulin sensitivity via a local effect in WAT on adiponectin expression, adipose tissue inflammation, and adipogenesis.

Parathyroid hormone's enhancement of bones' osteogenic response to loading is affected by ageing in a dose- and time-dependent manner

Decreased effectiveness of bones' adaptive response to mechanical loading contributes to age-related bone loss. In young mice, intermittent administration of parathyroid hormone (iPTH) at 20-80μg/kg/day interacts synergistically with artificially applied loading to increase bone mass. Here we report investigations on the effect of different doses and duration of iPTH treatment on mice whose osteogenic response to artificial loading is impaired by age. One group of aged, 19-month-old female C57BL/6 mice was given 0, 25, 50 or 100μg/kg/day iPTH for 4weeks. Histological and μCT analysis of their tibiae revealed potent iPTH dose-related increases in periosteally-enclosed area, cortical area and porosity with decreased cortical thickness. There was practically no effect on trabecular bone. Another group was given a submaximal dose of 50μg/kg/day iPTH or vehicle for 2 or 6weeks with loading of their right tibia three times per week for the final 2weeks. In the trabecular bone of these mice the loading-related increase in BV/TV was abrogated by iPTH primarily by reduction of the increase in trabecular number. In their cortical bone, iPTH treatment time-dependently increased cortical porosity. Loading partially reduced this effect. The osteogenic effects of iPTH and loading on periosteally-enclosed area and cortical area were additive but not synergistic. Thus in aged, unlike young mice, iPTH and loading appear to have separate effects. iPTH alone causes a marked increase in cortical porosity which loading reduces. Both iPTH and loading have positive effects on cortical periosteal bone formation but these are additive rather than synergistic.

Compressive loading of the murine tibia reveals site-specific micro-scale differences in adaptation and maturation rates of bone

Loading increases bone mass and strength in a site-specific manner; however, possible effects of loading on bone matrix composition have not been evaluated. Site-specific structural and material properties of mouse bone were analyzed on the macro- and micro/molecular scale in the presence and absence of axial loading. The response of bone to load is heterogeneous, adapting at molecular, micro-, and macro-levels.

INTRODUCTION

Osteoporosis is a degenerative disease resulting in reduced bone mineral density, structure, and strength. The overall aim was to explore the hypothesis that changes in loading environment result in site-specific adaptations at molecular/micro- and macro-scale in mouse bone.

METHODS

Right tibiae of adult mice were subjected to well-defined cyclic axial loading for 2 weeks; left tibiae were used as physiologically loaded controls. The bones were analyzed with μCT (structure), reference point indentation (material properties), Raman spectroscopy (chemical), and small-angle X-ray scattering (mineral crystallization and structure).

RESULTS

The cranial and caudal sites of tibiae are structurally and biochemically different within control bones. In response to loading, cranial and caudal sites increase in cortical thickness with reduced mineralization (-14 and -3%, p < 0.01, respectively) and crystallinity (-1.4 and -0.3%, p < 0.05, respectively). Along the length of the loaded bones, collagen content becomes more heterogeneous on the caudal site and the mineral/collagen increases distally at both sites.

CONCLUSION

Bone structure and composition are heterogeneous, finely tuned, adaptive, and site-specifically responsive at the micro-scale to maintain optimal function. Manipulation of this heterogeneity may affect bone strength, relative to specific applied loads.

Extra-nuclear effects of estrogen on cortical bone in males require ERαAF-1

Estradiol (E2) signaling via estrogen receptor alpha (ERα) is important for the male skeleton as demonstrated by ERα inactivation in both mice and man. ERα mediates estrogenic effects not only by translocating to the nucleus and affecting gene transcription but also by extra-nuclear actions e.g., triggering cytoplasmic signaling cascades. ERα contains various domains, and the role of activation function 1 (ERαAF-1) is known to be tissue specific. The aim of this study was to determine the importance of extra-nuclear estrogen effects for the skeleton in males and to determine the role of ERαAF-1 for mediating these effects. Five-month-old male wild-type (WT) and ERαAF-1-inactivated (ERαAF-1⁰) mice were orchidectomized and treated with equimolar doses of 17β-estradiol (E2) or an estrogen dendrimer conjugate (EDC), which is incapable of entering the nucleus and thereby only initiates extra-nuclear ER actions or their corresponding vehicles for 3.5 weeks. As expected, E2 treatment increased cortical thickness and trabecular bone volume per total volume (BV/TV) in WT males. EDC treatment increased cortical thickness in WT males, whereas no effect was detected in trabecular bone. In ERαAF-1⁰ males, E2 treatment increased cortical thickness, but did not affect trabecular bone. Interestingly, the effect of EDC on cortical bone was abolished in ERαAF-1⁰ mice. In conclusion, extra-nuclear estrogen signaling affects cortical bone mass in males, and this effect is dependent on a functional ERαAF-1. Increased knowledge regarding estrogen signaling mechanisms in the regulation of the male skeleton may aid the development of new treatment options for male osteoporosis.

Female Mice Lacking Estrogen Receptor-α in Hypothalamic Proopiomelanocortin (POMC) Neurons Display Enhanced Estrogenic Response on Cortical Bone Mass

Estrogens are important regulators of bone mass and their effects are mainly mediated via estrogen receptor (ER)α. Central ERα exerts an inhibitory role on bone mass. ERα is highly expressed in the arcuate (ARC) and the ventromedial (VMN) nuclei in the hypothalamus. To test whether ERα in proopiomelanocortin (POMC) neurons, located in ARC, is involved in the regulation of bone mass, we used mice lacking ERα expression specifically in POMC neurons (POMC-ERα(-/-)). Female POMC-ERα(-/-) and control mice were ovariectomized (OVX) and treated with vehicle or estradiol (0.5 μg/d) for 6 weeks. As expected, estradiol treatment increased the cortical bone thickness in femur, the cortical bone mechanical strength in tibia and the trabecular bone volume fraction in both femur and vertebrae in OVX control mice. Importantly, the estrogenic responses were substantially increased in OVX POMC-ERα(-/-) mice compared with the estrogenic responses in OVX control mice for cortical bone thickness (+126 ± 34%, P < .01) and mechanical strength (+193 ± 38%, P < .01). To test whether ERα in VMN is involved in the regulation of bone mass, ERα was silenced using an adeno-associated viral vector. Silencing of ERα in hypothalamic VMN resulted in unchanged bone mass. In conclusion, mice lacking ERα in POMC neurons display enhanced estrogenic response on cortical bone mass and mechanical strength. We propose that the balance between inhibitory effects of central ERα activity in hypothalamic POMC neurons in ARC and stimulatory peripheral ERα-mediated effects in bone determines cortical bone mass in female mice.

Liver-derived IGF-I regulates cortical bone mass but is dispensable for the osteogenic response to mechanical loading in female mice

Low circulating IGF-I is associated with increased fracture risk. Conditional depletion of IGF-I produced in osteoblasts or osteocytes inhibits the bone anabolic effect of mechanical loading. Here, we determined the role of endocrine IGF-I for the osteogenic response to mechanical loading in young adult and old female mice with adult, liver-specific IGF-I inactivation (LI-IGF-I(-/-) mice, serum IGF-I reduced by ≈70%) and control mice. The right tibia was subjected to short periods of axial cyclic compressive loading three times/wk for 2 wk, and measurements were performed using microcomputed tomography and mechanical testing by three-point bending. In the nonloaded left tibia, the LI-IGF-I(-/-) mice had lower cortical bone area and increased cortical porosity, resulting in reduced bone mechanical strength compared with the controls. Mechanical loading induced a similar response in LI-IGF-I(-/-) and control mice in terms of cortical bone area and trabecular bone volume fraction. In fact, mechanical loading produced a more marked increase in cortical bone mechanical strength, which was associated with a less marked increase in cortical porosity, in the LI-IGF-I(-/-) mice compared with the control mice. In conclusion, liver-derived IGF-I regulates cortical bone mass, cortical porosity, and mechanical strength under normal (nonloaded) conditions. However, despite an ∼70% reduction in circulating IGF-I, the osteogenic response to mechanical loading was not attenuated in the LI-IGF-I(-/-) mice.

SERMs have substance-specific effects on bone, and these effects are mediated via ERαAF-1 in female mice

The bone-sparing effect of estrogens is mediated primarily via estrogen receptor (ER)α, which stimulates gene transcription through activation function (AF)-1 and AF-2. The role of ERαAF-1 for the estradiol (E2) effects is tissue specific. The selective ER modulators (SERMs) raloxifene (Ral), lasofoxifene (Las), and bazedoxifene (Bza) can be used to treat postmenopausal osteoporosis. They all reduce the risk for vertebral fractures, whereas Las and partly Bza, but not Ral, reduce the risk for nonvertebral fractures. Here, we have compared the tissue specificity of Ral, Las, and Bza and evaluated the role of ERαAF-1 for the effects of these SERMs, with an emphasis on bone parameters. We treated ovariectomized (OVX) wild-type (WT) mice and OVX mice lacking ERαAF-1 (ERαAF-1(0)) with E2, Ral, Las, or Bza. All three SERMs increased trabecular bone mass in the axial skeleton. In the appendicular skeleton, only Las increased the trabecular bone volume/tissue volume and trabecular number, whereas both Ral and Las increased the cortical bone thickness and strength. However, Ral also increased cortical porosity. The three SERMs had only a minor effect on uterine weight. Notably, all evaluated effects of these SERMs were absent in ovx ERαAF-1(0) mice. In conclusion, all SERMs had similar effects on axial bone mass. However, the SERMs had slightly different effects on the appendicular skeleton since only Las increased the trabecular bone mass and only Ral increased the cortical porosity. Importantly, all SERM effects require a functional ERαAF-1 in female mice. These results could lead to development of more specific treatments for osteoporosis.

Enzalutamide Reduces the Bone Mass in the Axial But Not the Appendicular Skeleton in Male Mice

Testosterone is a crucial regulator of the skeleton, but the role of the androgen receptor (AR) for the maintenance of the adult male skeleton is unclear. In the present study, the role of the AR for bone metabolism and skeletal growth after sexual maturation was evaluated by means of the drug enzalutamide, which is a new AR antagonist used in the treatment of prostate cancer patients. Nine-week-old male mice were treated with 10, 30, or 100 mg/kg·d of enzalutamide for 21 days or were surgically castrated and were compared with vehicle-treated gonadal intact mice. Although orchidectomy reduced the cortical bone thickness and trabecular bone volume fraction in the appendicular skeleton, these parameters were unaffected by enzalutamide. In contrast, both enzalutamide and orchidectomy reduced the bone mass in the axial skeleton as demonstrated by a reduced lumbar spine areal bone mineral density (P < .001) and trabecular bone volume fraction in L5 vertebrae (P < .001) compared with vehicle-treated gonadal intact mice. A compression test of the L5 vertebrae revealed that the mechanical strength in the axial skeleton was significantly reduced by enzalutamide (maximal load at failure -15.3% ± 3.5%; P < .01). The effects of enzalutamide in the axial skeleton were associated with a high bone turnover. In conclusion, enzalutamide reduces the bone mass in the axial but not the appendicular skeleton in male mice after sexual maturation. We propose that the effect of testosterone on the axial skeleton in male mice is mainly mediated via the AR.

Identification of proteins highly expressed in uterine fluid from mice with hydrometra

Estrogen receptor alpha (ERα) is an important regulator of the estrous cycle and mice with global ERα deletion, as well as some conditional knockout mouse lines, have an interruption in the estrous cycle. In this study we observed that conditional ERα knockout mice where the Cre gene is regulated by the rat insulin promoter (RIP), RIP-Cre/ERα(KO) mice, have a 3.7-fold increase in serum 17β-estradiol levels, blocked estrous cycle, and develop a fluid-filled uterus (hydrometra). Using a proteomics approach, we identified three proteins, lactoferrin, complement C3 and chitinase 3-like protein 1 (CHI3L1), as highly expressed proteins in hydrometra fluid. The mRNA levels of the corresponding genes were more than 50-fold higher in RIP-Cre/ERα(KO) uterus compared to controls. High expression of CHI3L1 in the uterine fluid was not reflected as elevated levels in the serum. The high expression of lactoferrin, complement C3 and CHI3L1 in the uterine fluid, in association with elevated estrogen levels, prompted us to address if the expression of these genes is related to reproduction. However, gonadotropin treatment of mice reduced the uterine expression of these genes in a model of in vitro fertilization. Our findings identify lactoferrin, complement C3 and CHI3L1 as highly expressed proteins in hydrometra fluid in association with chronically elevated serum estradiol levels.

Measurement of a Comprehensive Sex Steroid Profile in Rodent Serum by High-Sensitive Gas Chromatography-Tandem Mass Spectrometry

Accurate measurement of sex steroid concentrations in rodent serum is essential to evaluate mouse and rat models for sex steroid-related disorders. The aim of the present study was to develop a sensitive and specific gas chromatography-tandem mass spectrometry (GC-MS/MS) method to assess a comprehensive sex steroid profile in rodent serum. A major effort was invested in reaching an exceptionally high sensitivity for measuring serum estradiol concentrations. We established a GC-MS/MS assay with a lower limit of detection for estradiol, estrone, T, DHT, progesterone, androstenedione, and dehydroepiandrosterone of 0.3, 0.5, 4.0, 1.6, 8, 4.0, and 50 pg/mL, respectively, whereas the corresponding values for the lower limit of quantification were 0.5, 0.5, 8, 2.5, 74, 12, and 400 pg/mL, respectively. Calibration curves were linear, intra- and interassay coefficients of variation were low, and accuracy was excellent for all analytes. The established assay was used to accurately measure a comprehensive sex steroid profile in female rats and mice according to estrous cycle phase. In addition, we characterized the impact of age, sex, gonadectomy, and estradiol treatment on serum concentrations of these sex hormones in mice. In conclusion, we have established a highly sensitive and specific GC-MS/MS method to assess a comprehensive sex steroid profile in rodent serum in a single run. This GC-MS/MS assay has, to the best of our knowledge, the best detectability reported for estradiol. Our method therefore represents an ideal tool to characterize sex steroid metabolism in a variety of sex steroid-related rodent models and in human samples with low estradiol levels.

The effect of estrogen on bone requires ERα in nonhematopoietic cells but is enhanced by ERα in hematopoietic cells

The effects of estrogen on bone are mediated mainly via estrogen receptor (ER)α. ERα in osteoclasts (hematopoietic origin) is involved in the trabecular bone-sparing effects of estrogen, but conflicting data are reported on the role of ERα in osteoblast lineage cells (nonhematopoietic origin) for bone metabolism. Because Cre-mediated cell-specific gene inactivation used in previous studies might be confounded by nonspecific and/or incomplete cell-specific ERα deletion, we herein used an alternative approach to determine the relative importance of ERα in hematopoietic (HC) and nonhematopoietic cells (NHC) for bone mass. Chimeric mice with selective inactivation of ERα in HC or NHC were created by bone marrow transplantations of wild-type (WT) and ERα-knockout (ERα(-/-)) mice. Estradiol treatment increased both trabecular and cortical bone mass in ovariectomized WT/WT (defined as recipient/donor) and WT/ERα(-/-) mice but not in ERα(-/-)/WT or ERα(-/-)/ERα(-/-) mice. However, estradiol effects on both bone compartments were reduced (∼50%) in WT/ERα(-/-) mice compared with WT/WT mice. The effects of estradiol on fat mass and B lymphopoiesis required ERα specifically in NHC and HC, respectively. In conclusion, ERα in NHC is required for the effects of estrogen on both trabecular and cortical bone, but these effects are enhanced by ERα in HC.

Probiotics protect mice from ovariectomy-induced cortical bone loss

The gut microbiota (GM) modulates the hosts metabolism and immune system. Probiotic bacteria are defined as live microorganisms which when administered in adequate amounts confer a health benefit on the host and can alter the composition of the GM. Germ-free mice have increased bone mass associated with reduced bone resorption indicating that the GM also regulates bone mass. Ovariectomy (ovx) results in bone loss associated with altered immune status. The purpose of this study was to determine if probiotic treatment protects mice from ovx-induced bone loss. Mice were treated with either a single Lactobacillus (L) strain, L. paracasei DSM13434 (L. para) or a mixture of three strains, L. paracasei DSM13434, L. plantarum DSM 15312 and DSM 15313 (L. mix) given in the drinking water during 6 weeks, starting two weeks before ovx. Both the L. para and the L. mix treatment protected mice from ovx-induced cortical bone loss and bone resorption. Cortical bone mineral content was higher in both L. para and L. mix treated ovx mice compared to vehicle (veh) treated ovx mice. Serum levels of the resorption marker C-terminal telopeptides and the urinary fractional excretion of calcium were increased by ovx in the veh treated but not in the L. para or the L. mix treated mice. Probiotic treatment reduced the expression of the two inflammatory cytokines, TNFα and IL-1β, and increased the expression of OPG, a potent inhibitor of osteoclastogenesis, in cortical bone of ovx mice. In addition, ovx decreased the frequency of regulatory T cells in bone marrow of veh treated but not probiotic treated mice. In conclusion, treatment with L. para or the L. mix prevents ovx-induced cortical bone loss. Our findings indicate that these probiotic treatments alter the immune status in bone resulting in attenuated bone resorption in ovx mice.

aP2-Cre-mediated inactivation of estrogen receptor alpha causes hydrometra

In this study we describe the reproductive phenotypes of a novel mouse model in which Cre-mediated deletion of ERα is regulated by the aP2 (fatty acid binding protein 4) promoter. ERα-floxed mice were crossed with transgenic mice expressing Cre-recombinase under the control of the aP2 promoter to generate aP2-Cre/ERα(flox/flox) mice. As expected, ERα mRNA levels were reduced in adipose tissue, but in addition we also detected an 80% reduction of ERα levels in the hypothalamus of aP2-Cre/ERα(flox/flox) mice. Phenotypic analysis revealed that aP2-Cre/ERα(flox/flox) female mice were infertile. In line with this, aP2-Cre/ERα(flox/flox) female mice did not cycle and presented 3.8-fold elevated estrogen levels. That elevated estrogen levels were associated with increased estrogen signaling was evidenced by increased mRNA levels of the estrogen-regulated genes lactoferrin and aquaporin 5 in the uterus. Furthermore, aP2-Cre/ERα(flox/flox) female mice showed an accumulation of intra-uterine fluid, hydrometra, without overt indications for causative anatomical anomalies. However, the vagina and cervix displayed advanced keratosis with abnormal quantities of accumulating squamous epithelial cells suggesting functional obstruction by keratin plugs. Importantly, treatment of aP2-Cre/ERα(flox/flox) mice with the aromatase inhibitor Letrozole caused regression of the hydrometra phenotype linking increased estrogen levels to the observed phenotype. We propose that in aP2-Cre/ERα(flox/flox) mice, increased serum estrogen levels cause over-stimulation in the uterus and genital tracts resulting in hydrometra and vaginal obstruction.

The role of activation functions 1 and 2 of estrogen receptor-α for the effects of estradiol and selective estrogen receptor modulators in male mice

Estradiol (E2) is important for male skeletal health and the effect of E2 is mediated via estrogen receptor (ER)-α. This was demonstrated by the findings that men with an inactivating mutation in aromatase or a nonfunctional ERα had osteopenia and continued longitudinal growth after sexual maturation. The aim of the present study was to evaluate the role of different domains of ERα for the effects of E2 and selective estrogen receptor modulators (SERMs) on bone mass in males. Three mouse models lacking either ERαAF-1 (ERαAF-1(0)), ERαAF-2 (ERαAF-2(0)), or the total ERα (ERα(-/-)) were orchidectomized (orx) and treated with E2 or placebo. E2 treatment increased the trabecular and cortical bone mass and bone strength, whereas it reduced the thymus weight and bone marrow cellularity in orx wild type (WT) mice. These parameters did not respond to E2 treatment in orx ERα(-/-) or ERαAF-2(0). However, the effects of E2 in orx ERαAF-1(0) [corrected] were tissue-dependent, with a clear response in cortical bone parameters and bone marrow cellularity, but no response in trabecular bone. To determine the role of ERαAF-1 for the effects of SERMs, we treated orx WT and ERαAF-1(0) mice with raloxifene (Ral), lasofoxifene (Las), bazedoxifene (Bza), or vehicle. These SERMs increased total body areal bone mineral density (BMD) and trabecular volumetric BMD to a similar extent in orx WT mice. Furthermore, only Las increased cortical thickness significantly and only Bza increased bone strength significantly. However, all SERMs showed a tendency toward increased cortical bone parameters. Importantly, all SERM effects were absent in the orx ERαAF-1(0) mice. In conclusion, ERαAF-2 is required for the estrogenic effects on all evaluated parameters, whereas the role of ERαAF-1 is tissue-specific. All evaluated effects of Ral, Las and Bza are dependent on a functional ERαAF-1. Our findings might contribute to the development of bone-specific SERMs in males.

Estrogen receptor-α is required for the osteogenic response to mechanical loading in a ligand-independent manner involving its activation function 1 but not 2

Estrogen receptor-α (ERα) is crucial for the adaptive response of bone to loading but the role of endogenous estradiol (E2) for this response is unclear. To determine in vivo the ligand dependency and relative roles of different ERα domains for the osteogenic response to mechanical loading, gene-targeted mouse models with (1) a complete ERα inactivation (ERα(-/-) ), (2) specific inactivation of activation function 1 (AF-1) in ERα (ERαAF-1(0) ), or (3) specific inactivation of ERαAF-2 (ERαAF-2(0) ) were subjected to axial loading of tibia, in the presence or absence (ovariectomy [ovx]) of endogenous E2. Loading increased the cortical bone area in the tibia mainly as a result of an increased periosteal bone formation rate (BFR) and this osteogenic response was similar in gonadal intact and ovx mice, demonstrating that E2 (ligand) is not required for this response. Female ERα(-/-) mice displayed a severely reduced osteogenic response to loading with changes in cortical area (-78% ± 15%, p < 0.01) and periosteal BFR (-81% ± 9%, p < 0.01) being significantly lower than in wild-type (WT) mice. ERαAF-1(0) mice also displayed a reduced response to mechanical loading compared with WT mice (cortical area -40% ± 11%, p < 0.05 and periosteal BFR -41% ± 8%, p < 0.01), whereas the periosteal osteogenic response to loading was unaffected in ERαAF-2(0) mice. Mechanical loading of transgenic estrogen response element (ERE)-luciferase reporter mice did not increase luciferase expression in cortical bone, suggesting that the loading response does not involve classical genomic ERE-mediated pathways. In conclusion, ERα is required for the osteogenic response to mechanical loading in a ligand-independent manner involving AF-1 but not AF-2.

Reduced bone mass and muscle strength in male 5α-reductase type 1 inactivated mice

Androgens are important regulators of bone mass but the relative importance of testosterone (T) versus dihydrotestosterone (DHT) for the activation of the androgen receptor (AR) in bone is unknown. 5α-reductase is responsible for the irreversible conversion of T to the more potent AR activator DHT. There are two well established isoenzymes of 5α-reductase (type 1 and type 2), encoded by separate genes (Srd5a1 and Srd5a2). 5α-reductase type 2 is predominantly expressed in male reproductive tissues whereas 5α-reductase type 1 is highly expressed in liver and moderately expressed in several other tissues including bone. The aim of the present study was to investigate the role of 5α-reductase type 1 for bone mass using Srd5a1^−/− mice. Four-month-old male Srd5a1^−/− mice had reduced trabecular bone mineral density (−36%, p<0.05) and cortical bone mineral content (−15%, p<0.05) but unchanged serum androgen levels compared with wild type (WT) mice. The cortical bone dimensions were reduced in the male Srd5a1^−/− mice as a result of a reduced cortical periosteal circumference compared with WT mice. T treatment increased the cortical periosteal circumference (p<0.05) in orchidectomized WT mice but not in orchidectomized Srd5a1^−/− mice. Male Srd5a1^−/− mice demonstrated a reduced forelimb muscle grip strength compared with WT mice (p<0.05). Female Srd5a1^−/− mice had slightly increased cortical bone mass associated with elevated circulating levels of androgens. In conclusion, 5α-reductase type 1 inactivated male mice have reduced bone mass and forelimb muscle grip strength and we propose that these effects are due to lack of 5α-reductase type 1 expression in bone and muscle. In contrast, the increased cortical bone mass in female Srd5a1^−/− mice, is an indirect effect mediated by elevated circulating androgen levels.

The role of estrogen receptor α in growth plate cartilage for longitudinal bone growth

Estrogens enhance skeletal growth during early sexual maturation, whereas high estradiol levels during late puberty result in growth plate fusion in humans. Although the growth plates do not fuse directly after sexual maturation in rodents, a reduction in growth plate height is seen by treatment with a high dose of estradiol. It is unknown whether the effects of estrogens on skeletal growth are mediated directly via estrogen receptors (ERs) in growth plate cartilage and/or indirectly via other mechanisms such as the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis. To determine the role of ERα in growth plate cartilage for skeletal growth, we developed a mouse model with cartilage-specific inactivation of ERα. Although mice with total ERα inactivation displayed affected longitudinal bone growth associated with alterations in the GH/IGF-1 axis, the skeletal growth was normal during sexual maturation in mice with cartilage-specific ERα inactivation. High-dose estradiol treatment of adult mice reduced the growth plate height as a consequence of attenuated proliferation of growth plate chondrocytes in control mice but not in cartilage-specific ERα(-/-) mice. Adult cartilage-specific ERα(-/-) mice continued to grow after 4 months of age, whereas growth was limited in control mice, resulting in increased femur length in 1-year-old cartilage-specific ERα(-/-) mice compared with control mice. We conclude that during early sexual maturation, ERα in growth plate cartilage is not important for skeletal growth. In contrast, it is essential for high-dose estradiol to reduce the growth plate height in adult mice and for reduction of longitudinal bone growth in elderly mice.

Amelioration of collagen-induced arthritis and immune-associated bone loss through signaling via estrogen receptor alpha, and not estrogen receptor beta or G protein-coupled receptor 30

OBJECTIVE

The effects of estrogen may be exerted via the nuclear estrogen receptors (ERs) ERalpha or ERbeta or via the recently proposed transmembrane estrogen receptor G protein-coupled receptor 30 (GPR-30). The purpose of this study was to elucidate the ER specificity for the ameliorating effects of estrogen on arthritis and bone loss in a model of postmenopausal rheumatoid arthritis (RA).

METHODS

Female DBA/1 mice underwent ovariectomy or sham operation, and type II collagen-induced arthritis was induced. Mice were treated subcutaneously 5 days/week with the specific agonists propylpyrazoletriol (PPT; for ERalpha), diarylpropionitrile (DPN; for ERbeta), G1 (for GPR-30), or with a physiologic dose of estradiol. Clinical arthritis scores were determined continuously. At termination of the study, bone mineral density (BMD) was analyzed, paws were collected for histologic assessment, serum was analyzed for cytokines and markers of bone and cartilage turnover, and bone marrow was subjected to fluorescence-activated cell sorting.

RESULTS

Treatment with PPT as well as estradiol dramatically decreased the frequency and severity of arthritis. Furthermore, estradiol and PPT treatment resulted in preservation of bone and cartilage, as demonstrated by increased BMD and decreased serum levels of bone resorption markers and cartilage degradation markers, whereas no effect was seen after DPN or G1 treatment.

CONCLUSION

In a well-established model of postmenopausal RA, ERalpha, but not ERbeta or GPR-30 signaling, was shown to ameliorate the disease and the associated development of osteoporosis. Since long-term treatment with estrogen has been associated with significant side effects, increased knowledge about the mechanisms behind the beneficial effects of estrogen is useful in the search for novel treatments of postmenopausal RA.

The role of the G protein-coupled receptor GPR30 in the effects of estrogen in ovariectomized mice

In vitro studies suggest that the membrane G protein-coupled receptor GPR30 is a functional estrogen receptor (ER). The aim of the present study was to determine the possible in vivo role of GPR30 as a functional ER primarily for the regulation of skeletal parameters, including bone mass and longitudinal bone growth, but also for some other well-known estrogen-regulated parameters, including uterine weight, thymus weight, and fat mass. Three-month-old ovariectomized (OVX) GPR30-deficient mice (GPR30(-/-)) and wild-type (WT) mice were treated with either vehicle or increasing doses of estradiol (E(2); 0, 30, 70, 160, or 830 ng.mouse(-1).day(-1)). Body composition [bone mineral density (BMD), fat mass, and lean mass] was analyzed by dual-energy-X ray absorptiometry, while the cortical and trabecular bone compartments were analyzed by peripheral quantitative computerized tomography. Quantitative histological analyses were performed in the distal femur growth plate. Bone marrow cellularity and distribution were analyzed using a fluorescence-activated cell sorter. The estrogenic responses on most of the investigated parameters, including increase in bone mass (total body BMD, spine BMD, trabecular BMD, and cortical bone thickness), increase in uterine weight, thymic atrophy, fat mass reduction, and increase in bone marrow cellularity, were similar for all of the investigated E(2) doses in WT and GPR30(-/-) mice. On the other hand, E(2) treatment reduced longitudinal bone growth, reflected by decreased femur length and distal femur growth plate height, in the WT mice but not in the GPR30(-/-) mice compared with vehicle-treated mice. These in vivo findings demonstrate that GPR30 is not required for normal estrogenic responses on several major well-known estrogen-regulated parameters. In contrast, GPR30 is required for a normal estrogenic response in the growth plate.

Identification of target cells for the genomic effects of estrogens in bone

Estrogen has bone protective effects, but the exact mechanism behind these effects remains unclear. The aim of the present study was to identify the primary target cells in bone for the classical genomic effects of estrogens in vivo. For this purpose we have used reporter mice with a luciferase gene under the control of three estrogen-responsive elements (EREs), enabling detection of in vivo activation of gene transcription. Three-month-old ovariectomized mice were treated with a single dose (50 mug/kg) 17beta-estradiol (E2). Luciferase activity was analyzed in several tissues and in different bone marrow-derived lymphocyte enriched/depleted preparations using MacsMouse CD19 (for B lymphocytes) or CD90 (for T lymphocytes) MicroBeads (Miltenyi Biotec GmbH, Bergisch Gladbach, Germany). Histological characterization of cells with high luciferase content was performed using immunohistochemistry. Both cortical bone and bone marrow displayed a rapid (within 1 h) and pronounced E2-induced increase in luciferase activity. The luciferase activity in total bone marrow and in bone marrow depleted of lymphocytes was increased six to eight times more than in either B-lymphocyte or T-lymphocyte enriched cell fractions 4 h after the E2 injection, demonstrating that mature lymphocytes are not major direct targets for the genomic effect of estrogens in bone. Immunohistochemistry identified clear luciferase staining in hypertrophic growth plate chondrocytes, megakaryocytes, osteoblasts, and lining cells, whereas no staining was seen in proliferative chondrocyte. Although most of the osteocytes did not display any detectable luciferase staining, a subpopulation of osteocytes both in cortical and trabecular bone stained positive for luciferase. In conclusion, hypertrophic growth plate chondrocytes, megakaryocytes, osteoblasts, lining cells, and a subpopulation of osteocytes were identified to respond to estrogen via the classical ERE-mediated genomic pathway in bone. Furthermore, our findings indicate that possible direct estrogenic effects on the majority of osteocytes, not staining positive for luciferase, on proliferative chondrocytes and on mature lymphocytes are mediated by non-ERE actions.

SHBG gene promoter polymorphisms in men are associated with serum sex hormone-binding globulin, androgen and androgen metabolite levels, and hip bone mineral density

CONTEXT

SHBG regulates free sex steroid levels, which in turn regulate skeletal homeostasis. Twin studies have demonstrated that genetic factors largely account for interindividual variation in SHBG levels. Glucuronidated androgen metabolites have been proposed as markers of androgenic activity.

OBJECTIVE

Our objective was to investigate whether polymorphisms in the SHBG gene promoter [(TAAAA)(n) microsatellite and rs1799941 single-nucleotide polymorphism] are associated with serum levels of SHBG, sex steroids, or bone mineral density (BMD) in men.

DESIGN AND STUDY SUBJECTS

We conducted a population-based study of two cohorts of Swedish men: elderly men (MrOS Sweden; n congruent with 3000; average age, 75.4 yr) and young adult men (GOOD study; n = 1068; average age, 18.9 yr).

MAIN OUTCOME MEASURES

We measured serum levels of SHBG, testosterone, estradiol, dihydrotestosterone, 5alpha-androstane-3alpha,17beta-diol glucuronides, androsterone glucuronide, and BMD determined by dual-energy x-ray absorptiometry.

RESULTS

In both cohorts, (TAAAA)(n) and rs1799941 genotypes were associated with serum levels of SHBG (P < 0.001), dihydrotestosterone (P < 0.05), and 5alpha-androstane-3alpha,17beta-diol glucuronides (P < 0.05). In the elderly men, they were also associated with testosterone and BMD at all hip bone sites. The genotype associated with high levels of SHBG was also associated with high BMD. Interestingly, male mice overexpressing human SHBG had increased cortical bone mineral content in the femur, suggesting that elevated SHBG levels may cause increased bone mass.

CONCLUSIONS

Our findings demonstrate that polymorphisms in the SHBG promoter predict serum levels of SHBG, androgens, and glucuronidated androgen metabolites, and hip BMD in men.

Bone protection by estrens occurs through non-tissue-selective activation of the androgen receptor

The use of estrogens and androgens to prevent bone loss is limited by their unwanted side effects, especially in reproductive organs and breast. Selective estrogen receptor modulators (SERMs) partially avoid such unwanted effects, but their efficacy on bone is only moderate compared with that of estradiol or androgens. Estrens have been suggested to not only prevent bone loss but also exert anabolic effects on bone while avoiding unwanted effects on reproductive organs. In this study, we compared the effects of a SERM (PSK3471) and 2 estrens (estren-alpha and estren-beta) on bone and reproductive organs to determine whether estrens are safe and act via the estrogen receptors and/or the androgen receptor (AR). Estrens and PSK3471 prevented gonadectomy-induced bone loss in male and female mice, but none showed true anabolic effects. Unlike SERMs, the estrens induced reproductive organ hypertrophy in both male and female mice and enhanced MCF-7 cell proliferation in vitro. Estrens directly activated transcription in several cell lines, albeit at much higher concentrations than estradiol or the SERM, and acted for the most part through the AR. We conclude that the estrens act mostly through the AR and, in mice, do not fulfill the preclinical efficacy or safety criteria required for the treatment or prevention of osteoporosis.

Cholesterol-sensing receptors, liver X receptor alpha and beta, have novel and distinct roles in osteoclast differentiation and activation

The liver X receptor (alpha,beta) is responsible for regulating cholesterol homeostasis in cells. However, our studies using the LXRalpha-/-, LXRbeta-/-, and LXRalpha-/-beta-/- mice show that both LXRalpha and beta are also important for bone turnover, mainly by regulating osteoclast differentiation/activity.

INTRODUCTION

The liver X receptors (alpha,beta) are primarily responsible for regulating cholesterol homeostasis within cells and the whole body. However, as recent studies show that the role for this receptor is expanding, we studied whether the LXRs could be implicated in bone homeostasis and development.

MATERIALS AND METHODS

pQCT was performed on both male and female LXRalpha-/-, LXRbeta-/-, LXRalpha-/-beta-/-, and WT mice at 4 months and 1 year of age. Four-month-old female mice were additionally analyzed with reference to qPCR, immunohistochemistry, histomorphometry, transmission electron microscopy, and serum bone turnover markers.

RESULTS

At the mRNA level, LXRbeta was more highly expressed than LXRalpha in both whole long bones and differentiating osteoblast-like MC3T3-E1 and osteoclast-like RAW 264.7 cells. Four-month-old female LXRalpha-/- mice had a significant increase in BMD because of an increase in all cortical parameters. No difference was seen regarding trabecular BMD. Quantitative histomorphometry showed that these mice had significantly more endosteal osteoclasts in the cortical bone; however, these cells appeared less active than normal cells as suggested by a significant reduction in serum levels of cross-linked carboxyterminal telopeptides of type I collagen (CTX) and a reduction in bone TRACP activity. Conversely, the female LXRbeta-/- mice exhibited no change in BMD, presumably because a significant decline in the number of the trabecular osteoclasts was compensated for by an increase in the expression of the osteoclast markers cathepsin K and TRACP. These mice also had a significant decrease in serum CTX, suggesting decreased bone resorption; however, in addition presented with an increase in the expression of osteoblast associated genes, bone formation markers, and serum leptin levels.

CONCLUSIONS

Our findings show that both LXRs influence cellular function within the bone, with LXRalpha having an impact on osteoclast activity, primarily in cortical bone, whereas LXRbeta modulates trabecular bone turnover.

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

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

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

Recently, it has been shown that inactivation of estrogen receptor beta (ER-beta) by gene targeting results in increased cortical bone formation in adolescent female mice. To study the possible involvement of ER-beta in the regulation of the mature skeleton, we have extended the analyses to include 1-year-old ER-beta knockout mice (ER-beta-/-). Male ER-beta-/- mice did not express any significant bone phenotypic alterations at this developmental stage. However, the increase in cortical bone parameters seen already in the adolescent female ER-beta-/- mice was maintained in the older females. The aged female ER-beta-/- mice further exhibited a significantly higher trabecular bone mineral density (BMD) as well as increased bone volume/total volume (BV/TV) compared with wild-type (wt) mice. This was caused by a less pronounced loss of trabecular bone during adulthood in female ER-beta-/- mice. The growth plate width was unaltered in the female ER-beta-/- mice. Judged by the expression of the osteoclast marker tartrate-resistant acid phosphatase (TRAP) and cathepsin K (cat K; reverse-transcription-polymerase chain reaction [RT-PCR]) as well as the serum levels of C-terminal type I collagen cross-linked peptide, bone resorption appeared unaffected. However, an increase in the messenger RNA (mRNA) expression levels of the osteoblast marker core-binding factor alpha1 (Cbfa1) suggested an anabolic effect in bones of old female ER-beta-/- mice. In addition, the mRNA expression of ER-alpha was augmented, indicating a role for ER-alpha in the development of this phenotype. Taken together, the results show that ER-beta is involved in the regulation of trabecular bone during adulthood in female mice and suggest that ER-beta acts in a repressive manner, possibly by counteracting the stimulatory action of ER-alpha on bone formation.

Cellular distribution of estrogen receptor beta in neonatal rat bone

Estrogens affect bone metabolism, and ovariectomy of rats results in marked bone loss caused by stimulation of osteoclastic bone resorption. Estrogen receptors (ER) have been demonstrated in osteoblasts and bone marrow stromal cells, but their presence in osteoclasts is controversial. Until recently, only one type of ER (now renamed ERalpha) had been identified. After the discovery of a novel ER subtype (ERbeta), it became necessary to re-investigate the ER expression in human and rodent bone. In the present study we examined the expression of ER mRNA in neonatal rat bone. Expression of ER alpha and beta mRNA (RT-PCR) was evident in femurs of 3-week-old male and female rats. In situ hybridization histochemistry of femural bones with digoxigenin labelled riboprobes, as well as radioactively labeled riboprobes, revealed that ERbeta mRNA was predominantly expressed in osteoblasts covering the metaphyseal bone trabecular surface. The presence of ERbeta mRNA in osteoblasts of rat bone suggests that ERbeta is involved in the mechanism of action of estrogens in bone.

The nuclear-receptor interacting protein (RIP) 140 binds to the human glucocorticoid receptor and modulates hormone-dependent transactivation

The glucocorticoid receptor (GR) regulates target gene expression in response to corticosteroid hormones. We have investigated the mechanism of transcriptional activation by the GR by studying the role of the receptor interacting protein RIP140. Both in vivo and in vitro protein-protein interaction assays revealed a ligand-dependent interaction between the GR and RIP140. The ligand binding domain of the GR was sufficient for this interaction, while both the N- and C-terminal regions of RIP140 bound to the receptor. In a yeast transactivation assay RIP140 and SRC-1, a member of the steroid receptor coactivator family of proteins, both enhanced the transactivation activity of a GR protein (GRA-1) in which the potent N-terminal tau1 transactivation domain has been deleted. In contrast, in COS-7 cells increasing amounts of RIP140 significantly inhibited GRdeltatau1 function. In cotransfection studies in COS-7 cells, RIP140 also inhibited receptor activity in presence of both SRC-1 and the coactivator protein CBP together. Thus, in yeast cells a stimulation of receptor activity was observed, while in mammalian cells RIP140 repressed GR function. Taken together, these data suggest that, (1) RIP140 is a target protein for the GR and (2) RIP140 can modulate the transactivation activity of the receptor.

Increased cortical bone mineral content but unchanged trabecular bone mineral density in female ERbeta(-/-) mice

Ovariectomy in young, growing rodents results in decreased trabecular bone mineral density (BMD) and increased radial growth of the cortical bone. Both of these effects are reversed by treatment with estrogen. The aim of the present study was to determine the physiological role of estrogen receptor-beta (ERbeta) on bone structure and bone mineral content (BMC). The BMC was increased in adult (11 weeks old), but not prepubertal (4 weeks old), female ERbeta(-/-) mice compared with wild-type (WT) mice. This increase in BMC in females was not due to increased trabecular BMD, but to an increased cross-sectional cortical bone area associated with a radial bone growth. Male ERbeta(-/-) mice displayed no bone abnormalities compared with WT mice. Ovariectomy decreased the trabecular BMD to the same extent in adult female ERbeta(-/-) mice as in WT mice. The expression levels of osteoblast-associated genes - alpha1(I) collagen, alkaline phosphatase, and osteocalcin mRNAs - were elevated in bone from adult ERbeta(-/-) females compared with WT mice. These observations provide a possible explanation for the increased radial bone growth seen in female mutants, suggesting a repressive function for ERbeta in the regulation of bone growth during female adolescence. In summary, ERbeta is essential for the pubertal feminization of the cortical bone in female mice but is not required for the protective effect of estrogens on trabecular BMD.