Estrogen receptor α mediates proliferation of osteoblastic cells stimulated by estrogen and mechanical strain, but their acute down-regulation of the Wnt antagonist Sost is mediated by estrogen receptor β

High circulating concentrations of estradiol are anabolic for bone mass and strength in an adult male to female transgender mouse model

The effects of gender affirming hormone therapy (GAHT) on bone microarchitecture and fracture risk in adult transgender women is unclear. To investigate the concept that skeletal integrity and strength in trans women may be improved by treatment with a higher dose of GAHT than commonly prescribed, we treated adult male mice with a sustained, high dose of estradiol. Adult male mice at 16 weeks of age were administered ~1.3 mg estradiol by silastic implant, implanted intraperitoneally, for 12 weeks. Controls included vehicle treated intact females and males. High-dose estradiol treatment in males stimulated the endocortical deposition of bone at the femoral mid-diaphysis, increasing cortical thickness and bone area. This led to higher stiffness, maximum force, and the work required to fracture the bone compared to male controls, while post-yield displacement was unaffected. Assessment of the material properties of the bone showed an increase in both elastic modulus and ultimate stress in the estradiol treated males. Treatment of male mice with high dose estradiol was also anabolic for trabecular bone, markedly increasing trabecular bone volume, number and thickness in the distal metaphysis which was accompanied by an increase in the histomorphometric markers of bone remodelling, mineralizing surface/bone surface, bone formation rate and osteoclast number. In conclusion, a high dose of estradiol is anabolic for cortical and trabecular bone in a male to female transgender mouse model, increasing both stiffness and strength. These findings suggest that increasing the current dose of GAHT administered to trans women, while considering other potential adverse effects, may be beneficial to preserving their bone microstructure and strength.

Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and disease

Wnts are secreted, lipid-modified proteins that bind to different receptors on the cell surface to activate canonical or non-canonical Wnt signaling pathways, which control various biological processes throughout embryonic development and adult life. Aberrant Wnt signaling pathway underlies a wide range of human disease pathogeneses. In this review, we provide an update of Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and diseases. The Wnt proteins, receptors, activators, inhibitors, and the crosstalk of Wnt signaling pathways with other signaling pathways are summarized and discussed. We mainly review Wnt signaling functions in bone formation, homeostasis, and related diseases, and summarize mouse models carrying genetic modifications of Wnt signaling components. Moreover, the therapeutic strategies for treating bone diseases by targeting Wnt signaling, including the extracellular molecules, cytosol components, and nuclear components of Wnt signaling are reviewed. In summary, this paper reviews our current understanding of the mechanisms by which Wnt signaling regulates bone formation, homeostasis, and the efforts targeting Wnt signaling for treating bone diseases. Finally, the paper evaluates the important questions in Wnt signaling to be further explored based on the progress of new biological analytical technologies.

Occlusal force orchestrates alveolar bone homeostasis via Piezo1 in female mice

Healthy alveolar bone is the cornerstone of oral function and oral treatment. Alveolar bone is highly dynamic during the entire lifespan and is affected by both systemic and local factors. Importantly, alveolar bone is subjected to unique occlusal force in daily life, and mechanical force is a powerful trigger of bone remodeling, but the effect of occlusal force in maintaining alveolar bone mass remains ambiguous. In this study, the Piezo1 channel is identified as an occlusal force sensor. Activation of Piezo1 rescues alveolar bone loss caused by a loss of occlusal force. Moreover, we identify Piezo1 as the mediator of occlusal force in osteoblasts, maintaining alveolar bone homeostasis by directly promoting osteogenesis and by sequentially regulating catabolic metabolism through Fas ligand (FasL)-induced osteoclastic apoptosis. Interestingly, Piezo1 activation also exhibits remarkable efficacy in the treatment of alveolar bone osteoporosis caused by estrogen deficiency, which is highly prevalent among middle-aged and elderly women. Promisingly, Piezo1 may serve not only as a treatment target for occlusal force loss-induced alveolar bone loss but also as a potential target for metabolic bone loss, especially in older patients.

El diálogo oculto entre el hueso y los tejidos a través del remodelado óseo

El hueso es mucho más que un reservorio de calcio y fósforo. Su disposición lacuno-canalicular ofrece una importante vía de intercambio con la circulación y actualmente, el esqueleto se considera un gran órgano endocrino, con acciones que van más allá del control del balance fosfocálcico mediado por el factor fibroblástico 23 (FGF23). Paralelamente al efecto modulador de las adipoquinas sobre el remodelado óseo, diversas proteínas óseas, como la osteocalcina y la esclerostina, ejercen cierta acción contra-reguladora sobre el metabolismo energético, posiblemente en un intento de asegurar los enormes requerimientos energéticos del remodelado. En esta interacción del hueso con otros tejidos, especialmente el adiposo, participa la señalización canónica Wnt/β-catenina y por ello la esclerostina, una proteína osteocítica que inhibe esta señalización, emerge como un potencial biomarcador. Es más, su participación en diversas patologías le posiciona como diana terapéutica, existiendo un anticuerpo anti-esclerostina, recientemente aprobado en nuestro país para el tratamiento de la osteoporosis. Esta revisión aborda el carácter endocrino del hueso, el papel de la osteocalcina y, especialmente, el papel regulador y modulador de la esclerostina sobre remodelado óseo y la homeóstasis energética a través de su interacción con la señalización canónica Wnt/β-catenina, así como su potencial utilidad como biomarcador.

The hidden cross talk between bone and tissues through bone turnover

Bone is more than a reservoir of calcium and phosphorus. Its lacuno-canalicular arrangement provides an important pathway for exchange with circulation and currently, the skeleton is considered a large endocrine organ with actions that go beyond the control of calcium-phosphorus balance mediated by fibroblastic growth factor 23 (FGF23). Parallel to the modulating effect of adipokines on bone turnover, certain bone proteins, such as osteocalcin and sclerostin, play a counter-regulatory role on energy metabolism, probably in an attempt to ensure its high energy requirement for bone turnover. In this crosstalk between bone and other tissues, especially with adipose tissue, canonical Wnt/β-catenin signaling is involved and therefore, sclerostin, an osteocyte derived protein that inhibits this signalling, emerges as a potential biomarker. Furthermore, its involvement in diverse pathologic conditions supports sclerostin as a therapeutic target, with an anti-sclerostin antibody recently approved in our country for the treatment of osteoporosis. This review addresses the endocrine nature of bone, the role of osteocalcin, and specially, the regulatory and modulatory role of sclerostin on bone turnover and energy homeostasis through its inhibitory effect on canonical Wnt/β-catenin signaling, as well as its potential utility as a biomarker.

Evaluation of Antioxidant Effects of Pumpkin (Cucurbita pepo L.) Seed Extract on Aging- and Menopause-Related Diseases Using Saos-2 Cells and Ovariectomized Rats

Aging and menopause are associated with oxidative stress and inflammation. Here, we evaluated the antioxidant properties of pumpkin (Cucurbita pepo L.) seed extract and assessed its ameliorative effects on aging- and menopause-related diseases using Saos-2 cells and ovariectomized rats. The seed extract had bioactive components that exhibited antioxidant activity. The extract increased the alkaline phosphatase (ALP) activity of Saos-2 cells. The oral administration of the extract to ovariectomized rats for 12 weeks decreased their body weight, fat weight, and cardiac risk indices. It also contributed to reductions in the levels of reactive oxygen species, oxidative stress, and inflammation, as assessed by measuring the serum levels of malondialdehyde and analyzing gene expression in rats. Furthermore, the administration of the extract also promoted an enhancement of the transcription of nuclear factor erythroid 2-related factor (Nrf2), heme oxygenase-1 (Ho-1), and catalase (Cat), involved in antioxidant activity; endothelial nitric oxide synthase (eNos), involved in vasculoprotective activity; and PR/SET domain 16 (Prdm16) and peroxisome proliferator-activated receptor-gamma coactivator (Pgc1α), involved in brown adipogenesis and thermogenesis. Our results using ovariectomized rats show that pumpkin seed extract may have ameliorative effects on menopause-related diseases by increasing ALP activity, evaluating the antioxidant system, ameliorating oxidative stress and thermogenesis, and enhancing lipid profiles.

Estradiol increases cortical and trabecular bone accrual and bone strength in an adolescent male-to-female mouse model of gender-affirming hormone therapy

The effects of gender-affirming hormone therapy on the skeletal integrity and fracture risk in transitioning adolescent trans girls are unknown. To address this knowledge gap, we developed a mouse model to simulate male-to-female transition in human adolescents in whom puberty is first arrested by using gonadotrophin-releasing hormone analogs with subsequent estradiol treatment. Puberty was suppressed by orchidectomy in male mice at 5 weeks of age. At 3 weeks post-surgery, male-to-female mice were treated with a high dose of estradiol (~0.85 mg) by intraperitoneal silastic implantation for 12 weeks. Controls included intact and orchidectomized males at 3 weeks post-surgery, vehicle-treated intact males, intact females and orchidectomized males at 12 weeks post-treatment. Compared to male controls, orchidectomized males exhibited decreased peak bone mass accrual and a decreased maximal force the bone could withstand prior to fracture. Estradiol treatment in orchidectomized male-to-female mice compared to mice in all control groups was associated with an increased cortical thickness in the mid-diaphysis, while the periosteal circumference increased to a level that was intermediate between intact male and female controls, resulting in increased maximal force and stiffness. In trabecular bone, estradiol treatment increased newly formed trabeculae arising from the growth plate as well as mineralizing surface/bone surface and bone formation rate, consistent with the anabolic action of estradiol on osteoblast proliferation. These data support the concept that skeletal integrity can be preserved and that long-term fractures may be prevented in trans girls treated with GnRHa and a sufficiently high dose of GAHT. Further study is needed to identify an optimal dose of estradiol that protects the bone without adverse side effects.

ER-β accelerates the process of primary osteoporosis by promoting VEGFA-mediated apoptosis of osteoblasts

Primary osteoporosis (POP) is a widespread and severe disorder of bone metabolism characterized by reduced bone mass and destruction of bone structure, frequently inducing fracture risk and imposing a heavy economic burden on public life. The development of POP partially revolves around the estrogen receptor β (ER-β), one of the major mediator receptors of estrogen that influences apoptosis in a range of cells. We performed KEGG and GO analysis by mining the transcriptomic dataset of POP samples showing significant enrichment of differentially expressed genes (DEGs) in multiple apoptosis-related pathways. The results of the Spearman correlation analysis and Protein-Protein Interaction (PPI) Networks screening of hub genes indicated that vascular endothelial growth factor A (VEGFA) may be a key target of ER-β in controlling osteoblast apoptosis. Further, we carried out high-throughput sequencing of ESR2-silenced MC3T3-E1 cells and noticed a substantial suppression in VEGFA expression and all apoptosis-related pathways. In addition, we determined the cell cycle and apoptosis by constructing a VEGFA-silenced cell model utilizing flow cytometry (FCM), and the results showed that ER-β could regulate the osteoblast cycle and thus promote osteoblast apoptosis by promoting VEGFA expression. And Western blot results showed that apoptosis was most likely realized through the regulation of downstream apoptosis markers c-JUN (c-Jun N-terminal kinase, JNK) and GADD45G (Growth Arrest and DNA Damage-Inducible Protein 45 gamma). The effects of ESR2 and VEGFA on the proliferation of osteoblasts were lastly assessed using the cell counting kit- 8 (CCK-8) assay. In conclusion, this study identifies that the roles of ER-β in the regulation of osteoblast apoptosis are closely related to VEGFA and provides a new target for POP treatment.

Saos-2 cells cultured under hypoxia rapidly differentiate to an osteocyte-like stage and support intracellular infection by Staphylococcus aureus

The intracellular infection of osteocytes represents a clinically important aspect of osteomyelitis. However, few human osteocyte in vitro models exist and the differentiation of immature osteoblasts to an osteocyte stage typically takes at least 4-weeks of culture, making the study of this process challenging and time consuming. The osteosarcoma cell line Saos-2 has proved to be a useful model of human osteoblast to mature osteocyte differentiation. Culture under osteogenic conditions in a standard normoxic (21% O2 ) atmosphere results in reproducible mineralization and acquisition of mature osteocyte markers over the expected 28-35 day culture period. In order to expedite experimental assays, we tested whether reducing available oxygen to mimic concentrations experienced by osteocytes in vivo would increase the rate of differentiation. Cells cultured under 1% O2 exhibited maximal mineral deposition by 14 days. Early (COLA1, MEPE) and mature (PHEX, DMP1, GJA1, SOST) osteocyte markers were upregulated earlier under hypoxia compared to normoxia. Cells differentiated under 1% O2 for 14 days displayed a similar ability to internalize Staphylococcus aureus as day 28 cells grown under normoxic conditions. Thus, low oxygen accelerates Saos-2 osteocyte differentiation, resulting in a useful human osteocyte-like cell model within 14 days.

YAP/TEAD1 and β-catenin/LEF1 synergistically induce estrogen receptor α to promote osteogenic differentiation of bone marrow stromal cells

Bone remodeling is vital to the maintenance of bone homeostasis and may lead to destructive skeletal diseases once the balance is disrupted. Crosstalk between Wnt and estrogen receptor (ER) signaling has been proposed in bone remodeling, but the underlying mechanism remains unclear. This study was designed to explore the effect of Wnt-ER signaling during the osteogenic differentiation of bone marrow stromal cells (BMSCs). Rat BMSCs were isolated and identified using flow cytometry and stimulated with Wnt3a. Wnt3a treatment promoted osteogenic differentiation and mineralization of the BMSCs. Meanwhile, Wnt3a enhanced the expression of ERα as well as the canonical Wnt signaling mediator β-catenin and the alternative Wnt signaling effector Yes-associated protein 1 (YAP1). Interestingly, DNA pulldown assay revealed direct binding of transcriptional enhanced associate domain 1 (TEAD1) and lymphoid enhancer binding factor 1 (LEF1), transcriptional partners of YAP1 and β-catenin, respectively, to the promoter region of ERα. In addition, inhibition of TEAD1 and LEF1 suppressed Wnt3-promoted BMSC osteogenic differentiation and blocked Wnt3a-induced ERα expression. Furthermore, an in vivo model of femoral bone defect also supported that Wnt3a facilitated bone healing in an ERα-dependent way. Together, we suggest that Wnt3a promotes the osteogenic activity of BMSCs through YAP1 and β-catenin-dependent activation of ERα, via direct binding of TEAD1 and LEF1 to the ERα promoter.

New Emerging Biomarkers for Bone Disease: Sclerostin and Dickkopf-1 (DKK1)

A healthy skeleton depends on a continuous renewal and maintenance of the bone tissue. The process of bone remodeling is highly controlled and consists of a fine-tuned balance between bone formation and bone resorption. Biochemical markers of bone turnover are already in use for monitoring diseases and treatment involving the skeletal system, but novel biomarkers reflecting specific biological processes in bone and interacting tissues may prove useful for diagnostic, prognostic, and monitoring purposes. The Wnt-signaling pathway is one of the most important pathways controlling bone metabolism and consequently the action of inhibitors of the pathway such as sclerostin and Dickkopf-related protein 1 (DKK1) have crucial roles in controlling bone formation and resorption. Thus, they might be potential markers for clinical use as they reflect a number of physiological and pathophysiological events in bone and in the cross-talk with other tissues in the human body. This review focuses on the clinical utility of measurements of circulating sclerostin and DKK1 levels based on preanalytical and analytical considerations and on evidence obtained from published clinical studies. While accumulating evidence points to clear associations with a number of disease states for the two markers, and thus, the potential for especially sclerostin as a biochemical marker that may be used clinically, the lack of standardization or harmonization of the assays still hampers the clinical utility of the markers.

Synchronisation of apical constriction and cell cycle progression is a conserved behaviour of pseudostratified neuroepithelia informed by their tissue geometry

Neuroepithelial cells balance tissue growth requirement with the morphogenetic imperative of closing the neural tube. They apically constrict to generate mechanical forces which elevate the neural folds, but are thought to apically dilate during mitosis. However, we previously reported that mitotic neuroepithelial cells in the mouse posterior neuropore have smaller apical surfaces than non-mitotic cells. Here, we document progressive apical enrichment of non-muscle myosin-II in mitotic, but not non-mitotic, neuroepithelial cells with smaller apical areas. Live-imaging of the chick posterior neuropore confirms apical constriction synchronised with mitosis, reaching maximal constriction by anaphase, before division and re-dilation. Mitotic apical constriction amplitude is significantly greater than interphase constrictions. To investigate conservation in humans, we characterised early stages of iPSC differentiation through dual SMAD-inhibition to robustly produce pseudostratified neuroepithelia with apically enriched actomyosin. These cultured neuroepithelial cells achieve an equivalent apical area to those in mouse embryos. iPSC-derived neuroepithelial cells have large apical areas in G2 which constrict in M phase and retain this constriction in G1/S. Given that this differentiation method produces anterior neural identities, we studied the anterior neuroepithelium of the elevating mouse mid-brain neural tube. Instead of constricting, mid-brain mitotic neuroepithelial cells have larger apical areas than interphase cells. Tissue geometry differs between the apically convex early midbrain and flat posterior neuropore. Culturing human neuroepithelia on equivalently convex surfaces prevents mitotic apical constriction. Thus, neuroepithelial cells undergo high-amplitude apical constriction synchronised with cell cycle progression but the timing of their constriction if influenced by tissue geometry.

Osteocyte Estrogen Receptor β (Ot-ERβ) Regulates Bone Turnover and Skeletal Adaptive Response to Mechanical Loading Differently in Male and Female Growing and Adult Mice

Age-related bone loss is a failure of balanced bone turnover and diminished skeletal mechanoadaptation. Estrogen receptors, ERα and ERβ, play critical roles in osteoprotective regulation activated by estrogen and mechanical signals. Previous studies mainly focused on ERα and showed that osteocyte-ERα (Ot-ERα) regulated trabecular, but not cortical bone, and played a minor role in load-induced cortical adaptation. However, the role of Ot-ERβ in bone mass regulation remains unrevealed. To address this issue, we characterized bone (re)modeling and gene expression in male and female mice with Ot-ERβ deletion (ERβ-dOT) and littermate control (LC) at 10 weeks (young) or 28 weeks (adult) of age, as well as their responses to in vivo tibial compressive loading. Increased cancellous bone mass appeared in the L4 vertebral body of young male ERβ-dOT mice. At the same time, femoral cortical bone gene expression showed signs consistent with elevated osteoblast and osteoclast activities (type-I collagen, Cat K, RANKL). Upregulated androgen receptor (AR) expression was observed in young male ERβ-dOT mice relative to LC, suggesting a compensatory effect of testosterone on male bone protection. In contrast, bone mass in L4 decreased in adult male ERβ-dOT mice, attributed to potentially increased bone resorption activity (Cat K) with no change in bone formation. There was no effect of ERβ-dOT on bone mass or gene expression in female mice. Sex-dependent regulation of Ot-ERβ also appeared in load-induced cortical responsiveness. Young female ERβ-dOT mice showed an enhanced tibial cortical anabolic adaptation compared with LC. In contrast, an attenuated cortical anabolic response presented at the proximal tibia in male ERβ-dOT mice at both ages. For the first time, our findings suggest that Ot-ERβ regulates bone (re)modeling and the response to mechanical signals through different mechanisms in males and females. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Deletion of osteopontin or bone sialoprotein induces opposite bone responses to mechanical stimulation in mice

Osteopontin (OPN) and Bone Sialoprotein (BSP) are co-expressed in bone and display overlapping and complementary physiological properties. Both genes show a rapid expression response to mechanical stimulation. We used mice with single and double deletions (DKO) of BSP and OPN to assess the specificity of their roles in skeletal adaptation to loading. Two-month-old Wild-Type (WT), BSP knockout (BSP^−/−), OPN^−/− and DKO male mice were submitted to two mechanical stimulation regimen (n = 10 mice/group) respectively impacting trabecular bone (Hypergravity, HG) and cortical bone (Whole Body Vibration, WBV). HG increased trabecular bone volume (BV/TV) in WT femur through reduced resorption, and in BSP^−/− mice femur and vertebra through increased bone formation. In contrast, HG increased the turnover of OPN^−/− bone, resulting in reduced femur and vertebra BV/TV. HG did not affect DKO bones. Similarly, WBV increased cortical thickness in BSP^−/− mice and decreased it in OPN^−/−, without affecting structurally WT and DKO bone. Vibrated BSP^−/− mice displayed increased endocortical bone formation with a drop in Sclerostin expression, and reduced periosteal osteoclasts with lower Rankl and Cathepsin K expression. In contrast, vibrated OPN^−/− endocortical bone displayed decreased formation and increased osteoclast coverage. Therefore, under two regimen (HG and WBV) targeting distinct bone compartments, absence of OPN resulted in bone loss while lack of BSP induced bone gain, reflecting divergent structural adaptations. Strikingly, absence of both proteins led to a relative insensitivity to either mechanical challenge. Interplay between OPN and BSP thus appears as a key element of skeletal response to mechanical stimulation.

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Osteopontin gene knockout induces bone loss under mechanical stimulation

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Bone Sialoprotein gene knockout potentiates bone gain under mechanical stimulation

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Knockout of both genes leads to bone insensitivity

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Their interplay is crucial for bone response to mechanical challenges

Bisphenol A inhibits osteogenic activity and causes bone resorption via the activation of retinoic acid-related orphan receptor α

Bisphenol A (BPA) has deleterious effects on bone metabolism; however, its underlying mechanism has not yet been comprehensively understood. Here, we investigated whether RORα plays an important role in BPA-induced bone resorption both in vitro and in vivo. We found that BPA (0.1-1 μM) inhibited osteogenic activity (including ALP activity and mineralization), decreased the expression levels of osteoblast markers (such as RUNX2, OSX, and ALP) in human MG-63 osteoblast-like osteosarcoma cells, and inhibited spontaneous vertebral formation in zebrafish larvae. Additionally, BPA diminished β-glycerophosphate-induced osteoblast differentiation and vertebral formation, while simultaneously downregulating the expression levels of RUNX2a, OSX, and ALP. Furthermore, molecular docking data showed that a hydroxyl group of BPA dominantly binds to the H3 (ALA70) and/or H5 (ARG107) of RORα-ligand binding domain with hydrogen bonding (ALA330 and/or ARG367 in the full length of RORα, respectively), which another hydroxyl group of BPA fits into H3, H6, and H7 elements with non-covalent interactions, resulting in the activation of RORα. However, an RORα inverse agonist potently inhibited BPA-induced anti-osteogenic activity and vertebral formation in zebrafish larvae, concomitant with inhibition of osteogenic gene expression. Overall, our findings reveal that BPA inhibits osteoblast differentiation and bone formation by activating RORα. These results suggest that BPA exposure (0.1-1 μM) can cause various bone-resorptive diseases, such as osteoporosis.

Activation of canonical Wnt signaling accelerates intramembranous bone regeneration in male mice

Canonical Wnt signaling plays an important role in skeletal development, homeostasis, and both endochondral and intramembranous repair. While studies have demonstrated that the inhibition of Wnt signaling impairs intramembranous bone regeneration, how its activation affects intramembranous bone regeneration has been underexplored. Therefore, we sought to determine the effects of activation of canonical Wnt signaling on intramembranous bone regeneration by using the well-established marrow ablation model. We hypothesized that mice with a mutation in the Wnt ligand coreceptor gene Lrp5 would have accelerated intramembranous bone regeneration. Male and female wild-type and Lrp5-mutant mice underwent unilateral femoral bone marrow ablation surgery in the right femur at 4 weeks of age. Both the left intact and right operated femurs were assessed at Days 3, 5, 7, 10, and 14. The intact femur of Lrp5 mutant mice of both sexes had higher bone mass than wild-type littermates, although to a greater degree in males than females. Overall, the regenerated bone volume in Lrp5 mutant male mice was 1.8-fold higher than that of littermate controls, whereas no changes were observed between female Lrp5 mutant and littermate control mice. In addition, the rate of intramembranous bone regeneration (from Day 3 to Day 7) was higher in Lrp5 mutant male mice compared to their same-sex littermate controls with no difference in the females. Thus, activation of canonical Wnt signaling increases bone mass in intact bones of both sexes, but accelerates intramembranous bone regeneration following an injury challenge only in male mice.

Identification of Di/Tripeptide(s) With Osteoblasts Proliferation Stimulation Abilities of Yak Bone Collagen by in silico Screening and Molecular Docking

Endothelial protein C receptor (EPCR), cannabinoid receptor 2 (CBR2), and estrogen receptor α (ERα) play vital roles in osteoblasts proliferation. Also, collagen peptides have osteoblasts proliferation stimulation abilities, and di/tri-peptides could be absorbed by the intestine more easily. This study obtained three di/tripeptides with potential osteoblasts proliferation stimulation abilities of yak bone collagen, namely, MGF, CF, and MF, by in silico screening. Results suggested that these three peptides exhibited good absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties. They also had strong affinities with EPCR, CBR2, and ERα, and the total -CDOCKER energy (-CE) values were 150.9469, 113.1835, and 115.3714 kcal/mol, respectively. However, further Cell Counting Kit-8 (CCK-8) assays indicated that only MGF could significantly (P < 0.05) stimulate osteoblasts proliferation at 0.3 mg/ml. At the same time, the proliferating index (PI) of the osteoblasts treated with MGF increased significantly (P < 0.05), and the alkaline phosphatase (ALP) activity decreased highly significantly (P < 0.01). In summary, MGF exhibited the potential to be an effective treatment for osteoporosis.

GATA4 and estrogen receptor alpha bind at SNPs rs9921222 and rs10794639 to regulate AXIN1 expression in osteoblasts

Osteoporosis is a serious public health problem that affects 200 million people worldwide. Genome-wide association studies have revealed the association between several single nucleotide polymorphisms (SNPs) near WNT/β-catenin signaling genes and bone mineral density (BMD). The activation of β-catenin by WNT ligands is required for osteoblast differentiation. SNP rs9921222 is an intronic variant of AXIN1 (a scaffold protein in the destruction complex that regulates β-catenin signaling) that correlates with BMD. However, the biological mechanism of SNP rs9921222 has never been reported. Here, we show that the genotype of SNP rs9921222 correlates with the expression of AXIN1 in human osteoblasts. RNA and genomic DNA were analyzed from primary osteoblasts from 111 different individuals. Homozygous TT at rs9921222 correlates with a higher expression of AXIN1 than homozygous CC. Regional association analysis showed that rs9921222 is in high linkage disequilibrium (LD) with SNP rs10794639. In silico transcription factor analysis predicted that rs9921222 is within a GATA4 motif and rs10794639 is adjacent to an estrogen receptor alpha (ERα) motif. Mechanistically, GATA4 and ERα bind at SNPs rs9921222 and rs10794639 as detected by ChIP-qPCR. Luciferase assays demonstrate that rs9921222 is the causal SNP to alter ERα and GATA4 binding. GATA4 promoted the expression, and in contrast, ERα suppressed the expression of AXIN1 via the histone deacetylase complex member SIN3A. Functionally, the level of AXIN1 negatively correlates with the level of transcriptionally active β-catenin. In summary, we have discovered a molecular mechanism of the SNP rs9921222 to regulate AXIN1 through GATA4 and ERα binding in human osteoblasts.

Sclerostin: From Molecule to Clinical Biomarker

Sclerostin, a glycoprotein encoded by the SOST gene, is mainly produced by mature osteocytes and is a critical regulator of bone formation through its inhibitory effect on Wnt signaling. Osteocytes are differentiated osteoblasts that form a vast and highly complex communication network and orchestrate osteogenesis in response to both mechanical and hormonal cues. The three most commonly described pathways of SOST gene regulation are mechanotransduction, Wnt/β-catenin, and steroid signaling. Downregulation of SOST and thereby upregulation of local Wnt signaling is required for the osteogenic response to mechanical loading. This review covers recent findings concerning the identification of SOST, in vitro regulation of SOST gene expression, structural and functional properties of sclerostin, pathophysiology, biological variability, and recent assay developments for measuring circulating sclerostin. The three-dimensional structure of human sclerostin was generated with the AlphaFold Protein Structure Database applying a novel deep learning algorithm based on the amino acid sequence. The functional properties of the 3-loop conformation within the tertiary structure of sclerostin and molecular interaction with low-density lipoprotein receptor-related protein 6 (LRP6) are also reviewed. Second-generation immunoassays for intact/biointact sclerostin have recently been developed, which might overcome some of the reported methodological obstacles. Sclerostin assay standardization would be a long-term objective to overcome some of the problems with assay discrepancies. Besides the use of age- and sex-specific reference intervals for sclerostin, it is also pivotal to use assay-specific reference intervals since available immunoassays vary widely in their methodological characteristics.

Identification of Potentially Pathogenic Variants Associated with Recurrence in Medication-Related Osteonecrosis of the Jaw (MRONJ) Patients Using Whole-Exome Sequencing

Background: Bisphosphonates are antiresorptive and antiangiogenic drugs that prevent and treat bone loss and mineralization in women with postmenopausal osteoporosis and cancer patients. Medication-related osteonecrosis of the jaw (MRONJ) is commonly caused by tooth extraction and dental trauma. Although genetic and pathological studies about MRONJ have been conducted, the pathogenesis of MRONJ still remains unclear. Methods: We aimed to identify genetic variants associated with MRONJ, using whole-exome sequencing (WES). Ten MRONJ patients prescribed bisphosphonates were recruited for WES, and jawbone tissue and blood samples were collected from the patients. Results: The analysis of the WES data found a total of 1866 SNP and 40 InDel variants which are specific to MRONJ. The functional classification assay using Gene Ontology and pathway analysis discovered that genes bearing the MRONJ variants are significantly enriched for keratinization and calcium ion transport. Some of the variants are potential pathogenic variants (24 missense mutations and seven frameshift mutations) with MAF < 0.01. Conclusions: The variants are located in eight different genes (KRT18, MUC5AC, NBPF9, PABPC3, MST1L, ASPN, ATN1, and SLAIN1). Nine deleterious SNPs significantly associated with MRONJ were found in the KRT18 and PABPC3 genes. It suggests that KRT18 and PABPC3 could be MRONJ-related key genes.

Transplantation of bone marrow mesenchymal stem cells and fibrin glue into extraction socket in maxilla promoted bone regeneration in osteoporosis rat

AIMS

Bone defect repair in osteoporosis remains a tremendous challenge for clinicians due to increased bone metabolism resulted from estrogen deficiency. This study aims to investigate the effect of bone marrow mesenchymal stem cells (BMSCs) combined with fibrin glue (FG) in the extraction socket healing process of osteoporosis rats, as well as estimate the role of estrogen receptors (ERs) played in BMSCs differentiation in vitro and in the alveolar bone reconstruction process in vivo.

MAIN METHODS

Forty rats were randomly divided into four groups, under general anesthesia, three groups underwent bilateral ovariectomy(OVX) and one group with the sham operation. Three months later, the osteogenic ability of BMSCs, isolated from healthy and osteoporosis rats, respectively, was tested. The ERα and ERβ mRNA expression in BMSCs was also evaluated by RT-PCR analysis. In vivo experiment, Micro-CT detection, histological and immunofluorescent analysis, tissue PCR was conducted up to 2, 4 and 6 weeks after transplantation of BMSCs/FG to assess the newly formed bone in the extraction socket.

KEY FINDINGS

The BMSCs from osteoporosis rats displayed weaker osteogenic potential and lower ERs expression compared with the BMSCs from healthy rats. Newly formed bone tissue filled the socket defect in BMSCs/FG treated VOX rats after six weeks, which was comparable to the sham group, while reduced ERs expression was found in the regenerated bone of the OVX group.

SIGNIFICANCE

The BMSCs seeded within FG might provide an alternative therapeutic method for repairing the extraction socket defect in osteoporosis condition.

Osteocyte Wnt/β-catenin pathway activation upon mechanical loading is altered in ovariectomized mice

Estrogen levels decline in both sexes with age, but more dramatically in females. Activation of the Wnt/β-catenin signaling pathway is central to the regulation of bone mass accrual and maintenance and in response to mechanical loading. Using the ovariectomized mouse model we examined the effect of estrogen loss on the osteocyte's ability to activate the Wnt/β-catenin pathway following mechanical loading. Female TOPGAL mice underwent ovariectomy (OVX) (n = 10) or sham surgery (n = 10) at 16 weeks of age. Four weeks post-surgery, a single loading session (global strain of 2200 με for 100 cycles at 2 Hz) was performed on the right forearm with the left as a non-loaded control. Mice (n = 5) were sacrificed at 1 or 24 hr post-load. Ulnae were stained for β-catenin activation, femurs were used for μCT and 3-pt bending/biomechanical testing, and tibiae were used for histology analysis and to determine osteocyte lacunar size using SEM and high resolution micro-XCT. A 2.2-fold increase in β-catenin signaling activation was observed 24 hr post-load in the Sham group but did not occur in the OVX group. The OVX group versus control had significant losses (p < 0.05) in trabecular BMD (−8%), BV/TV (−35%) and thickness (−23%), along with cortical thickness (−6%) and periosteal perimeter (−4%). The OVX group had significantly higher trabecular bone osteoclast numbers (63%), OCS/BS (77%) and N.OC/BPm (94%) and a significant decrease in osteoblast number (53%), OBS/BS (37%) and N.OB/BPm (40%) compared to the sham group (p < 0.05). Cortical bone lacunar number/lacunar volume and bone biomechanical properties did not change between groups. Given that the ulna is a cortical bone loading model and the lack of changes in osteocyte lacunar number/volume in cortical bone, which would alter strains experienced by osteocytes, these data suggest the absence of estrogen resulted in intrinsic changes in the ability of the osteocyte to respond to mechanical load, rather than changes in the biomechanical and architectural properties of bone.

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In vivo mechanical loading activates β-catenin signaling in osteocytes.

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Ovariectomy induced estrogen loss attenuates in vivo loading induced β-catenin signaling in osteocytes.

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Changes in bone material and architectural properties do not appear to explain attenuated pathway activation.

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Our data suggests estrogen loss alters the intrinsic ability of the osteocyte to respond to mechanical load.

Effects of Extracellular Osteoanabolic Agents on the Endogenous Response of Osteoblastic Cells

The complex multidimensional skeletal organization can adapt its structure in accordance with external contexts, demonstrating excellent self-renewal capacity. Thus, optimal extracellular environmental properties are critical for bone regeneration and inextricably linked to the mechanical and biological states of bone. It is interesting to note that the microstructure of bone depends not only on genetic determinants (which control the bone remodeling loop through autocrine and paracrine signals) but also, more importantly, on the continuous response of cells to external mechanical cues. In particular, bone cells sense mechanical signals such as shear, tensile, loading and vibration, and once activated, they react by regulating bone anabolism. Although several specific surrounding conditions needed for osteoblast cells to specifically augment bone formation have been empirically discovered, most of the underlying biomechanical cellular processes underneath remain largely unknown. Nevertheless, exogenous stimuli of endogenous osteogenesis can be applied to promote the mineral apposition rate, bone formation, bone mass and bone strength, as well as expediting fracture repair and bone regeneration. The following review summarizes the latest studies related to the proliferation and differentiation of osteoblastic cells, enhanced by mechanical forces or supplemental signaling factors (such as trace metals, nutraceuticals, vitamins and exosomes), providing a thorough overview of the exogenous osteogenic agents which can be exploited to modulate and influence the mechanically induced anabolism of bone. Furthermore, this review aims to discuss the emerging role of extracellular stimuli in skeletal metabolism as well as their potential roles and provide new perspectives for the treatment of bone disorders.

TGFB-induced factor homeobox 1 (TGIF) expression in breast cancer

BACKGROUND

Breast cancer (BC) is the most frequent female cancer and preferentially metastasizes to bone. The transcription factor TGFB-induced factor homeobox 1 (TGIF) is involved in bone metabolism. However, it is not yet known whether TGIF is associated with BC bone metastasis or patient outcome and thus of potential interest.

METHODS

TGIF expression was analyzed by immunohistochemistry in 1197 formalin-fixed, paraffin-embedded tissue samples from BC patients treated in the GAIN (German Adjuvant Intergroup Node-Positive) study with two adjuvant dose-dense schedules of chemotherapy with or without bisphosphonate ibandronate. TGIF expression was categorized into negative/low and moderate/strong staining. Endpoints were disease-free survival (DFS), overall survival (OS) and time to primary bone metastasis as first site of relapse (TTPBM).

RESULTS

We found associations of higher TGIF protein expression with smaller tumor size (p = 0.015), well differentiated phenotype (p < 0.001) and estrogen receptor (ER)-positive BC (p < 0.001). Patients with higher TGIF expression levels showed a significantly longer disease-free (DFS: HR 0.75 [95%CI 0.59-0.95], log-rank p = 0.019) and overall survival (OS: HR 0.69 [95%CI 0.50-0.94], log-rank p = 0.019), but no association with TTPBM (HR 0.77 [95%CI 0.51-1.16]; p = 0.213). Univariate analysis in molecular subgroups emphasized that elevated TGIF expression was prognostic for both DFS and OS in ER-positive BC patients (DFS: HR 0.68 [95%CI 0.51-0.91]; log-rank p = 0.009, interaction p = 0.130; OS: HR 0.60 [95%CI 0.41-0.88], log-rank p = 0.008, interaction p = 0.107) and in the HER2-negative subgroup (DFS:HR 0.67 [95%CI 0.50-0.88], log-rank p = 0.004, interaction p = 0.034; OS: HR 0.57 [95%CI 0.40-0.81], log-rank p = 0.002, interaction p = 0.015).

CONCLUSIONS

Our results suggest that moderate to high TGIF expression is a common feature of breast cancer cells and that this is not associated with bone metastases as first site of relapse. However, a reduced expression is linked to tumor progression, especially in HER2-negative breast cancer.

TRIAL REGISTRATION

This clinical trial has been registered with ClinicalTrials.gov ; registration number: NCT00196872 .

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Bone Sclerostin and Dickkopf-related protein-1 are positively correlated with bone mineral density, bone microarchitecture, and bone strength in postmenopausal osteoporosis

BACKGROUND

Wnt-catenin signaling antagonists sclerostin and dickkopf-related protein-1 (Dkk-1) inhibit bone formation and are involved in the pathogenesis of postmenopausal osteoporosis (PO). However, the association between sclerostin and Dkk-1 and bone mineral density (BMD) in women with PO remains unclear.

OBJECTIVE

This study aimed to determine the association between sclerostin and Dkk-1 and BMD, bone microarchitecture, and bone strength in PO.

METHODS

Trabecular bone specimens were obtained from the femoral heads of 76 Chinese women with PO who underwent hip arthroplasty for femoral neck fractures. Micro-computed tomography (Micro-CT) was used to assess the BMD and bone microarchitecture of the trabecular bone. Subsequently, a mechanical test was performed. Finally, sclerostin and Dkk-1 in the bone were measured by enzyme-linked immunosorbent (Elisa) assay. Serum ionized serum ionised calcium, propeptide of type 1 collagen, C-terminal β-telopeptide of type-1 collagen, sclerostin, and Dkk-1 were also detected.

RESULTS

Bone sclerostin was positively correlated with serum ionised calcium, serum sclerostin, BMD, bone volume/tissue volume (BV/TV), trabecular number (Tb.N), maximum compressive force, and yield strength (r = 0.32, 0.906, 0.355, 0.401, 0.329, 0.355, and 0.293, respectively, P < 0.05) and negatively correlated with age and trabecular separation (Tb.Sp) (r = - 0.755 and - 0.503, respectively, P < 0.05). Bone Dkk-1 was positively correlated with serum ionised calcium, serum Dkk-1, BMD, BV/TV, trabecular thickness, Tb.N, maximum compressive force, yield strength, and Young's modulus (r = 0.38, 0.809, 0.293, 0.293, 0.228, 0.318, 0.352, 0.315, and 0.266, respectively, P < 0.05) and negatively correlated with age and Tb.Sp (r = - 0.56 and - 0.38, respectively, P < 0.05). Serum levels of sclerostin and Dkk-1 reflected the levels of sclerostin and Dkk-1 in the bone.

CONCLUSION

Bone sclerostin and Dkk-1 were positively correlated with BMD in women with PO, and higher levels of bone sclerostin and Dkk-1 might predict better BMD, bone microarchitecture, and bone strength. The potential molecular mechanisms still require further study.

Long-term exposure to fine particulate matter and osteoporotic fracture: A case-control study in Taiwan

Few studies have explored the relationship between long-term exposure to particulate matter with an aerodynamic diameter of ≤2.5 μm (PM_2.5) and osteoporotic fracture, particularly in high PM_2.5 level areas. The aim of this study was to assess the association between long-term exposure to PM_2.5 and osteoporotic fracture. We performed a matched case-control study of 16,175 participants obtained from a hospital registry during 2005-2014 in Taiwan. A major osteoporotic fracture was defined as a fracture of the spine, hip, proximal humerus, and forearm. We applied satellite-based spatiotemporal models with 1-km resolution to individually calculate the 1-year average PM_2.5 concentration before the index date which was defined as the first visit date for the osteoporotic fracture. Logistic regression models with and without potential confounding factors were used to estimate odds ratios (OR) and 95% confidence intervals (CI) between PM_2.5 and osteoporotic fracture, whereas a restricted cubic spline model was used to estimate the dose-response relationship. The sample's median age was 44.7 years (interquartile range: 30.7, 63.1 years). We observed that long-term PM_2.5 exposure was associated with osteoporotic fracture, the OR was 1.12 (95% CI: 1.03, 1.22) per 10-μg/m³ increase in PM_2.5 in women. In the dose-response association, the OR of osteoporotic fracture was significantly increased for PM_2.5 exposures more than 41 μg/m³. We did not find a significant association between PM_2.5 (per 10-μg/m³ increase) and osteoporotic fracture among overall population (adjusted OR, 1.02 [95% CI, 0.97 to 1.08]) and men (adjusted OR, 0.94 [95% CI, 0.86 to 1.02]). The results of the stratified analysis showed that women were more sensitive to the adverse impact of PM2.5 that were men, and evidence was obtained of sex-based effect modification (P for interaction = 0.002). Our findings suggest that long-term exposure to PM_2.5 is associated with osteoporotic fracture, particularly among women.

[Estradiol activates ERK phosphorylation by binding to ERβ to inhibit proliferation and promote apoptosis of human chondrocytes]

OBJECTIVE

To explore the effect of estradiol (E2) binding to its receptor ERβ on the proliferation and apoptosis of C28I2 cells.

OBJECTIVE

We cloned the sequence of ESR2 into a recombinant adenovirus plasmid (pAd-ESR2) and packaged the plasmid in HEK293 cells. Normal human chondrocyte C28I2 cells were transfected with Ad-ESR2 or small interfering RNA targeting ESR2-siRNA (ESR2-siRNA), and the effects of treatment with DMSO or E2 on the expression of the proteins associated with endoplasmic reticulum (ER) stress and cell apoptosis were determined using Western blotting. qRT-PCR was used to detect the expressions of proliferation-related marker genes, and an EdU kit and flow cytometry were used to assess cell proliferation and apoptosis. We also tested the effects of U0126 (an ERK pathway inhibitor) and E2, alone or in combination, on ER stress, apoptosis and the ERK signaling pathway in C28I2 cells infected with Ad-ESR2 using Western blotting.

OBJECTIVE

Overexpression of Ad-ESR2 in C28I2 cells significantly promoted the expressions of IRE1α, PERK, XBP1s, and cleaved caspase-12, inhibited proliferation related marker genes PCNA, cyclin B1, cyclin D1, and decreased the level of ERK phosphorylation following E2 treatment (all P < 0.05). Interference of ESR2 caused significant reduction in the expressions of ER stress-related proteins and apoptosis-related proteins, up-regulated the genes related to cell proliferation, and increased intracellular pERK/ERK ratio in C28I2 cells. The effect of E2 binding to ERβ, which promoted the expressions of ER stress associated proteins and apoptosis related proteins, was obviously antagonized by treatment of the cells with U0126.

OBJECTIVE

The binding of E2 to ERβ promotes ER stress and apoptosis in human chondrocytes by activating ERK pathway phosphorylation inhibit cell proliferation.

Voluntary Wheel Running Partially Compensates for the Effects of Global Estrogen Receptor-α Knockout on Cortical Bone in Young Male Mice

Estrogen receptor-α knockout (ERKO) in female, but not male, mice results in an impaired osteogenic response to exercise, but the mechanisms behind this ability in males are unknown. We explored the main and interactive effects of ERKO and exercise on cortical geometry, trabecular microarchitecture, biomechanical strength, and sclerostin expression in male mice. At 12 weeks of age, male C57BL/6J ERKO and WT animals were randomized into two groups: exercise treatment (EX) and sedentary (SED) controls, until 22 weeks of age. Cortical geometry and trabecular microarchitecture were measured via μCT; biomechanical strength was assessed via three-point bending; sclerostin expression was measured via immunohistochemistry. Two-way ANOVA was used to assess sclerostin expression and trabecular microarchitecture; two-way ANCOVA with body weight was used to assess cortical geometry and biomechanical strength. ERKO positively impacted trabecular microarchitecture, and exercise had little effect on these outcomes. ERKO significantly impaired cortical geometry, but exercise was able to partially reverse these negative alterations. EX increased cortical thickness regardless of genotype. There were no effects of genotype or exercise on sclerostin expression. In conclusion, male ERKO mice retain the ability to build bone in response to exercise, but altering sclerostin expression is not one of the mechanisms involved.

Critical Role of Estrogens on Bone Homeostasis in Both Male and Female: From Physiology to Medical Implications

Bone is a multi-skilled tissue, protecting major organs, regulating calcium phosphate balance and producing hormones. Its development during childhood determines height and stature as well as resistance against fracture in advanced age. Estrogens are key regulators of bone turnover in both females and males. These hormones play a major role in longitudinal and width growth throughout puberty as well as in the regulation of bone turnover. In women, estrogen deficiency is one of the major causes of postmenopausal osteoporosis. In this review, we will summarize the main clinical and experimental studies reporting the effects of estrogens not only in females but also in males, during different life stages. Effects of estrogens on bone involve either Estrogen Receptor (ER)α or ERβ depending on the type of bone (femur, vertebrae, tibia, mandible), the compartment (trabecular or cortical), cell types involved (osteoclasts, osteoblasts and osteocytes) and sex. Finally, we will discuss new ongoing strategies to increase the benefit/risk ratio of the hormonal treatment of menopause.

Differential Effects of Estrogen Receptor Alpha and Beta on Endogenous Ligands of Peroxisome Proliferator-Activated Receptor Gamma in Papillary Thyroid Cancer

PURPOSE

The inhibition of estrogen receptor alpha (ERα) or the activation of ERβ can inhibit papillary thyroid cancer (PTC), but the precise mechanism is not known. We aimed to explore the role of ERα and ERβ on the production of endogenous peroxisome proliferator-activated receptor gamma (PPARγ) ligands in PTC.

METHODS

2 PTC cell lines, 32 pairs of PTC tissues and matched normal thyroid tissues were used in this study. The levels of endogenous PPARγ ligands 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), 13-S-hydroxyoctadecadienoic acid (13(S)-HODE), and15-deoxy-Δ12,14-prostaglandin J2 (PGJ2) were measured by ELISA.

RESULTS

The levels of PGJ2 and 15(S)-HETE were significantly reduced in PTC, but 13(S)-HODE was not changed. Activation of ERα or inhibition of ERβ significantly downregulated the production of PGJ2, 15(S)-HETE and 13(S)-HODE, whereas inhibition of ERα or activation of ERβ markedly upregulated the production of these three ligands. Application of endogenous PPARγ ligands inhibited growth, induced apoptosis of cancer cells, and promoted the efficacy of chemotherapy.

CONCLUSION

The levels of endogenous PPARγ ligands PGJ2 and 15(S)-HETE are significantly decreased in PTC. The inhibition of ERα or activation of ERβ can inhibit PTC by stimulating the production of endogenous PPARγ ligands to induce apoptosis in cancer cells.

17α-Estradiol prevents ovariectomy-mediated obesity and bone loss

Menopause is a natural physiological process in older women that is associated with reduced estrogen production and results in increased risk for obesity, diabetes, and osteoporosis. 17α-estradiol (17α-E2) treatment in males, but not females, reverses several metabolic conditions associated with advancing age, highlighting sexually dimorphic actions on age-related pathologies. In this study we sought to determine if 17α-E2 could prevent ovariectomy (OVX)-mediated detriments on adiposity and bone parameters in females. Eight-week-old female C57BL/6J mice were subjected to SHAM or OVX surgery and received dietary 17α-E2 during a six-week intervention period. We observed that 17α-E2 prevented OVX-induced increases in body weight and adiposity. Similarly, uterine weight and luminal cell thickness were decreased by OVX and prevented by 17α-E2 treatment. Interestingly, 17α-E2 prevented OVX-induced declines in tibial metaphysis cancellous bone. And similarly, 17α-E2 improved bone density parameters in both tibia and femur cancellous bone, primarily in OVX mice. In contrast, to the effects on cancellous bone, cortical bone parameters were largely unaffected by OVX or 17α-E2. In the non-weight bearing lumbar vertebrae, OVX reduced trabecular thickness but not spacing, while 17α-E2 increased trabecular thickness and reduced spacing. Despite this, 17α-E2 did improve bone volume/tissue volume in lumbar vertebrae. Overall, we found that 17α-E2 prevented OVX-induced increases in adiposity and changes in bone mass and architecture, with minimal effects in SHAM-operated mice. We also observed that 17α-E2 rescued uterine tissue mass and lining morphology to control levels without inducing hypertrophy, suggesting that 17α-E2 could be considered as an adjunct to traditional hormone replacement therapies.

Bone's Response to Mechanical Loading in Aging and Osteoporosis: Molecular Mechanisms

Mechanotransduction is pivotal in the maintenance of homeostasis in different tissues and involves multiple cell signaling pathways. In bone, mechanical stimuli regulate the balance between bone formation and resorption; osteocytes play a central role in this regulation. Dysfunctions in mechanotransduction signaling or in osteocytes response lead to an imbalance in bone homeostasis. This alteration is very relevant in some conditions such as osteoporosis and aging. Both are characterized by increased bone weakness due to different causes, for example, the increase of osteocyte apoptosis that cause an alteration of fluid space, or the alteration of molecular pathways. There are intertwined yet very different mechanisms involved among the cell-intrinsic effects of aging on bone, the cell-intrinsic and tissue-level effects of estrogen/androgen withdrawal on bone, and the effects of reduced mechanical loading on bone, which are all involved to some degree in how aged bone fails to respond properly to stress/strain compared to younger bone. This review aims at clarifying how the cellular and molecular pathways regulated and induced in bone by mechanical stimulation are altered with aging and in osteoporosis, to highlight new possible targets for antiresorptive or anabolic bone therapeutic approaches.

Molecular and cellular mechanisms linking air pollution and bone damage

Air pollution is the second most important risk factor associated with noncommunicable diseases after smoking. The effects of pollution on health are commonly attributable to particulate matter (PM), a complex mixture of particles suspended in the air. PM can penetrate the lower respiratory tract and has harmful direct and indirect effects on different organs and tissues. Direct effects are caused by the ability of PM components to cross the respiratory membrane and enter the bloodstream; indirect effects are systemic consequences of the local airway response. Recent work suggests that PM is an independent risk factor for low bone mineral density and osteoporosis-related fractures. Osteoporosis is a common age-related disease closely linked to bone fractures, with severe clinical consequences affecting quality of life, morbidity, and mortality. In this review, we discuss potential mechanisms behind the association between outdoor air pollution, especially PM, and bone damage. The discussion features four main mechanisms: 1) several different atmospheric pollutants can induce low-grade systemic inflammation, which affects bone metabolism through a specific effect of cytokines such as TNFα, IL-1β, IL-6, and IL-17 on osteoblast and osteoclast differentiation and function; 2) some pollutants, particularly certain gas and metal compounds, can cause oxidative damage in the airway and bone cells; 3) different groups of pollutants can act as endocrine disruptors when binding to the receptors in bone cells, changing their functioning; and 4) air pollution can directly and indirectly cause vitamin D deficiency. Characterizing these mechanisms will better define the physiopathology of bone damage, and recognizing air pollution as a modifiable risk factor for osteoporosis will inform environmental policies. Such knowledge will also guide the prevention of fractures due to fragility and help reduce health-related costs.

Alterations in osteocyte mediated osteoclastogenesis during estrogen deficiency and under ROCK-II inhibition: An in vitro study using a novel postmenopausal multicellular niche model

This study sought to derive an enhanced understanding of the complex intracellular interactions that drive bone loss in postmenopausal osteoporosis. We applied an in-vitro multicellular niche to recapitulate cell-cell signalling between osteocytes, osteoblasts and osteoclasts to investigate (1) how estrogen-deficient and mechanically loaded osteocytes regulate osteoclastogenesis and (2) whether ROCK-II inhibition affects these mechanobiological responses. We report that mechanically stimulated and estrogen-deficient osteocytes upregulated RANKL/OPG and M-CSF gene expression, when compared to those treated with 10 nM estradiol. Osteoclast precursors (RAW 264.7) cultured within this niche underwent significant reduction in osteoclastogenic gene expression (CTSK), and there was an increasing trend in the area covered by TRAP⁺ osteoclasts (24% vs. 19.4%, p = 0.06). Most interestingly, upon treatment with the ROCK-II inhibitor, RANKL/OPG and M-CSF gene expression by estrogen-deficient osteocytes were downregulated. Yet, this inhibition of the pro-osteoclastogenic factors by osteocytes did not ultimately reduce the differentiation of osteoclast precursors. Indeed, TRAP and CTSK gene expressions in osteoclast precursors were upregulated, and there was an increased trend for osteoclast area (30.4% vs. 24%, p = 0.07), which may have been influenced by static osteoblasts (MC3T3-E1) that were included in the niche. We conclude that ROCK-II inhibition can attenuate bone loss driven by osteocytes during estrogen deficiency.

Modulation of bone turnover aberration: A target for management of primary osteoporosis in experimental rat model

Osteoporosis is a skeletal degenerative disease characterised by abnormal bone turnover with scant bone formation and overabundant bone resorption. The present approach was intended to address the potency of nanohydroxyapatite (nHA), chitosan/hydroxyapatite nanocomposites (nCh/HA) and silver/hydroxyapatite nanoparticles (nAg/HA) to modulate bone turnover deviation in primary osteoporosis induced in the experimental model. Characterisation techniques such as TEM, zeta-potential, FT-IR and XRD were used to assess the morphology, the physical as well as the chemical features of the prepared nanostructures. The in vivo experiment was conducted on forty-eight adult female rats, randomised into 6 groups (8 rats/group), (1) gonad-intact, (2) osteoporotic group, (3) osteoporotic + nHA, (4) osteoporotic + nCh/HA, (5) osteoporotic + nAg/HA and (6) osteoporotic + alendronate (ALN). After three months of treatment, serum sclerostin (SOST), bone alkaline phosphatase (BALP) and bone sialoprotein (BSP) levels were quantified using ELISA. Femur bone receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL) and cathepsin K (CtsK) mRNA levels were evaluated by quantitative RT-PCR. Moreover, alizarin red S staining was applied to determine the mineralisation intensity of femur bone. Findings in the present study indicated that treatment with nHA, nCh/HA or nAg/HA leads to significant repression of serum SOST, BALP and BSP levels parallel to a significant down-regulation of RANKL and CtsK gene expression levels. On the other side, significant enhancement in the calcification intensity of femur bone has been noticed. The outcomes of this experimental setting ascertained the potentiality of nHA, nCh/HA and nAg/HA as promising nanomaterials in attenuating the excessive bone turnover in the primary osteoporotic rat model. The mechanisms behind the efficacy of the investigated nanostructures involved the obstacle of serum and tissue indices of bone resorption besides the strengthening of bone mineralisation.

Mechanical strain-mediated reduction in RANKL expression is associated with RUNX2 and BRD2

Mechanical loading-related strains trigger bone formation by osteoblasts while suppressing resorption by osteoclasts, uncoupling the processes of formation and resorption. Osteocytes may orchestrate this process in part by secreting sclerostin (SOST), which inhibits osteoblasts, and expressing receptor activator of nuclear factor-κB ligand (RANKL/TNFSF11) which recruits osteoclasts. Both SOST and RANKL are targets of the master osteoblastic transcription factor RUNX2. Subjecting human osteoblastic Saos-2 cells to strain by four point bending down-regulates their expression of SOST and RANKL without altering RUNX2 expression. RUNX2 knockdown increases basal SOST expression, but does not alter SOST down-regulation following strain. Conversely, RUNX2 knockdown does not alter basal RANKL expression, but prevents its down-regulation by strain. Chromatin immunoprecipitation revealed RUNX2 occupies a region of the RANKL promoter containing a consensus RUNX2 binding site and its occupancy of this site decreases following strain. The expression of epigenetic acetyl and methyl writers and readers was quantified by RT-qPCR to investigate potential epigenetic bases for this change. Strain and RUNX2 knockdown both down-regulate expression of the bromodomain acetyl reader BRD2. BRD2 and RUNX2 co-immunoprecipitate, suggesting interaction within regulatory complexes, and BRD2 was confirmed to interact with the RUNX2 promoter. BRD2 also occupies the RANKL promoter and its occupancy was reduced following exposure to strain. Thus, RUNX2 may contribute to bone remodeling by suppressing basal SOST expression, while facilitating the acute strain-induced down-regulation of RANKL through a mechanosensitive epigenetic loop involving BRD2.

Flow-induced mechanotransduction in skeletal cells

Human body is subject to many and variegated mechanical stimuli, actuated in different ranges of force, frequency, and duration. The process through which cells "feel" forces and convert them into biochemical cascades is called mechanotransduction. In this review, the effects of fluid shear stress on bone cells will be presented. After an introduction to present the major players in bone system, we describe the mechanoreceptors in bone tissue that can feel and process fluid flow. In the second part of the review, we present an overview of the biological processes and biochemical cascades initiated by fluid shear stress in bone cells.

Integrins in Osteocyte Biology and Mechanotransduction

PURPOSE OF REVIEW

Osteocytes are the main mechanosensitive cells in bone. Integrin-based adhesions have been shown to facilitate mechanotransduction, and therefore play an important role in load-induced bone formation. This review outlines the role of integrins in osteocyte function (cell adhesion, signalling, and mechanotransduction) and possible role in disease.

RECENT FINDINGS

Both β₁ and β₃ integrins subunits have been shown to be required for osteocyte mechanotransduction. Antagonism of these integrin subunits in osteocytes resulted in impaired responses to fluid shear stress. Various disease states (osteoporosis, osteoarthritis, bone metastases) have been shown to result in altered integrin expression and function. Osteocyte integrins are required for normal cell function, with dysregulation of integrins seen in disease. Understanding the mechanism of faulty integrins in disease may aid in the creation of novel therapeutic approaches.

Methylation of bone SOST impairs SP7, RUNX2, and ERα transactivation in patients with postmenopausal osteoporosis

Sclerostin (SOST), a glycoprotein predominantly secreted by bone tissue osteocytes, is an important regulator of bone formation, and loss of SOST results in Van Buchem disease. DNA methylation regulates SOST expression in human osteocytes, although the detailed underlying mechanisms remain unknown. In this study, we compared 12 patients with bone fractures and postmenopausal osteoporosis with eight patients without postmenopausal osteoporosis to understand the mechanisms via which SOST methylation affects osteoporosis. Serum and bone SOST expression was reduced in patients with osteoporosis. Bisulfite sequencing polymerase chain reaction revealed that the methylation rate was higher in patients with osteoporosis. We identified osterix (SP7), Runt-related transcription factor 2 (RUNX2), and estrogen receptor α (ERα) as candidate transcription factors activating SOST expression. Increased SOST methylation impaired the transactivation function of SP7, RUNX2, and ERα in MG-63 cells. AzadC treatment and SOST overexpression in MG-63 cells altered cell proliferation and apoptosis. Chromatin immunoprecipitation showed that higher methylation was associated with reduced SP7, RUNX2, and ERα binding to the SOST promoter in patients with osteoporosis. Our studies provide new insight into the role of SOST methylation in osteoporosis.

Sclerostin inhibition alleviates breast cancer-induced bone metastases and muscle weakness

Breast cancer bone metastases often cause a debilitating non-curable condition with osteolytic lesions, muscle weakness and a high mortality. Current treatment comprises chemotherapy, irradiation, surgery and anti-resorptive drugs that restrict but do not revert bone destruction. In metastatic breast cancer cells, we determined the expression of sclerostin, a soluble Wnt inhibitor that represses osteoblast differentiation and bone formation. In mice with breast cancer bone metastases, pharmacological inhibition of sclerostin using an anti-sclerostin antibody (Scl-Ab) reduced metastases without tumor cell dissemination to other distant sites. Sclerostin inhibition prevented the cancer-induced bone destruction by augmenting osteoblast-mediated bone formation and reducing osteoclast-dependent bone resorption. During advanced disease, NF-κB and p38 signaling was increased in muscles in a TGF-β1-dependent manner, causing muscle fiber atrophy, muscle weakness and tissue regeneration with an increase in Pax7-positive satellite cells. Scl-Ab treatment restored NF-κB and p38 signaling, the abundance of Pax7-positive cells and ultimately muscle function. These effects improved the overall health condition and expanded the life span of cancer-bearing mice. Together, these results demonstrate that pharmacological inhibition of sclerostin reduces bone metastatic burden and muscle weakness with a prolongation of the survival time. This might provide novel options for treating musculoskeletal complications in breast cancer patients. .

Combination of estrogen deficiency and excessive mechanical stress aggravates temporomandibular joint osteoarthritis in vivo

OBJECTIVE

It has been suggested that degenerative conditions of the temporomandibular joint (TMJ), such as osteoarthritis (OA) and progressive condylar resorption, are caused by multiple etiological factors, such as hormonal imbalance and excessive mechanical stress. However, it is unclear whether these factors interrelate in the degenerative process of the condyle. The aim of this study was to observe the effects of combined hormonal imbalance and excessive mechanical stress on the condyle using a mouse model.

MATERIALS AND METHODS

Ovariectomy (OVX) was performed in 8-week-old female mice. Three weeks after OVX, a build-up resin was bonded to the right maxillary molars to create imbalanced occlusion (increased occlusal vertical dimension, iOVD). Mice were divided into four groups: control, OVX, iOVD, and OVX + iOVD.

RESULTS

Histomorphometric analysis showed the lowest cartilage thickness and the highest TMJ-OA score in the OVX + iOVD group. Bone structural analysis showed significantly lower subchondral bone mass in all experimental groups. Additionally, the OVX + iOVD group showed up-regulated osteoclastic activity and increased apoptosis in the condyle. Gene expression analysis showed significantly elevated expression of pre-inflammatory cytokines in the OVX + iOVD group. These data showed that the OVX + iOVD group exhibited the most severe inflammatory TMJ-OA. Upregulation of ERα and activation of the ERK pathway was observed in the OVX + iOVD group.

CONCLUSIONS

Additive effects of estrogen deficiency and excessive mechanical stress on the condyle exacerbate TMJ-OA. Furthermore, estrogen deficiency and excessive mechanical stress combined may exacerbate TMJ-OA though activation of the ERK pathway.

Estrogen deficiency impairs integrin αvβ3-mediated mechanosensation by osteocytes and alters osteoclastogenic paracrine signalling

The integrin α_vβ₃ has been shown to play an important role in osteocyte mechanotransduction. It has been reported that there are fewer β₃ integrin-containing cells in osteoporotic bone cells. Osteocytes cultured in vitro under estrogen deficient conditions demonstrate altered mechanotransduction. However, it is unknown whether the altered mechanotransduction in estrogen deficient osteocytes is directly associated with defective α_vβ₃ expression or signalling. The objective of this study is to investigate the role of estrogen deficiency for regulating MLO-Y4 cell morphology, α_vβ₃ expression, focal adhesion formation and mechanotransduction by osteocytes. Here, we report that estrogen withdrawal leads to a smaller focal adhesion area and reduced α_vβ₃ localisation at focal adhesion sites, resulting in an increased Rankl/Opg ratio and defective Cox-2 responses to oscillatory fluid flow. Interestingly, α_vβ₃ antagonism had a similar effect on focal adhesion assembly, Rankl/Opg ratio, and Cox-2 responses to oscillatory fluid flow. Taken together, our results provide the first evidence for a relationship between estrogen withdrawal and defective α_vβ₃-mediated signalling. Specifically, this study implicates estrogen withdrawal as a putative mechanism responsible for altered α_vβ₃ expression and resultant changes in downstream signalling in osteocytes during post-menopausal osteoporosis, which might provide an important, but previously unidentified, contribution to the bone loss cascade.

ERα and Wnt/β‑catenin signaling pathways are involved in angelicin‑dependent promotion of osteogenesis

Reports of the ameliorative effect of angelicin on sex hormone deficiency-induced osteoporosis have highlighted this compound as a candidate for the treatment of osteoporosis. However, the molecular mechanisms of action of angelicin on osteoblast differentiation have not been thoroughly researched. The aim of the present study was to evaluate the effect of angelicin on the proliferation, differentiation and mineralization of rat calvarial osteoblasts using a Cell Counting Kit-8, alkaline phosphatase activity and the expression of osteogenic genes and proteins. Treatment with angelicin promoted the proliferation, matrix mineralization and upregulation of osteogenic marker genes including collagen type I α 1 and bone γ-carboxyglutamate in fetal rat calvarial osteoblasts. Furthermore, angelicin promoted the expression of β-catenin and runt related transcription factor 2, which serve a vital role in the Wnt/β-catenin signaling pathway. Consistently, the osteogenic effect of angelicin was attenuated by the use of a Wnt inhibitor. Moreover, angelicin increased the expression of estrogen receptor α (ERα), which also serves a key role in osteoblast differentiation. Taken together, these results demonstrated that angelicin may promote osteoblast differentiation through activation of ERα and the Wnt/β-catenin signaling pathway.

Gonadal Hormones and Bone

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

Site-specific differences in osteoblast phenotype, mechanical loading response and estrogen receptor-related gene expression

The osteoporosis-resistant nature of skull bones implies inherent differences exist between their cellular responses and those of other osteoporosis-susceptible skeletal sites. Phenotypic differences in calvarial and femoral osteoblastic responses to induction of osteogenesis, mechanical loading, estrogen, growth factor and cytokine stimulation were investigated. Primary rat calvarial and femoral adult male osteoblasts were cultured and osteoblastic mineralisation and maturation determined using Alizarin Red staining and expression of osteogenic marker genes assessed. Expression of known mechanically-responsive genes was compared between sites following loading of scaffold-seeded cells in a bioreactor. Cell proliferation and differentiation following growth factor and estrogen stimulation were also compared. Finally expression of estrogen receptors and associated genes during osteogenic differentiation were investigated. Calvarial osteoblasts exhibited delayed maturation (45d. vs 21d.) and produced less mineralised matrix than femoral osteoblasts when osteogenically induced. PDGF-BB and FGF2 both caused a selective increase in proliferation and decrease in osteoblastic differentiation of femoral osteoblasts. Mechanical stimulation resulted in the induction of the expression of Ccl2 and Anx2a selectively in femoral osteoblasts, but remained unchanged in calvarial cells. Estrogen receptor beta expression was selectively upregulated 2-fold in calvarial osteoblasts. Most interestingly, the estrogen responsive transcriptional repressor RERG was constitutively expressed at 1000-fold greater levels in calvarial compared with femoral osteoblasts. RERG expression in calvarial osteoblasts was down regulated during osteogenic induction whereas upregulation occurred in femoral osteoblasts. Bone cells of the skull are inherently different to those of the femur, and respond differentially to a range of stimuli. These site-specific differences may have important relevance in the development of strategies to tackle metabolic bone disorders.

Estrogen-induced Tgfbr1 and Bmpr1a Expression Repressed via Estrogen Receptor Beta in MC3T3-E1 Cells

Background

Estrogen, as an important hormone in human physiological process, is closely related to bone metabolism. The aim of this study was to investigate the mechanism of estrogen on osteoblasts metabolism in MC3T3-E1 cells.

Methods

We treated the MC3T3-E1 cells with different concentrations of β-estradiol (0.01, 0.1, 1, and 10 nmol/L), observed the morphological changes of the cells, and detected the cell's proliferation and apoptosis of MC3T3-E1 cells. Two transcriptome libraries were constructed and sequenced. Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to confirm the differentially expressed genes (DEGs), and then treated the MC3T3-E1 cells with estrogen receptor (ER) inhibitors α and β, respectively, and then examined the expression of Tgfbr1 and Bmpr1a genes. The promoter of Tgfbr1 and Bmpr1a gene was analyzed, and the ER response elements were identified. Finally, ChIP was used to verify the binding of ER to Tgfbr1 and Bmpr1a promoter.

Results

In the high-concentration β-estradiol treatment group (1 nmol/L and 10 nmol/L), there was no significant difference in the morphology of the cells under the microscope, 1 nmol/L and 10 nmol/L treated group appeared statistically significant difference in cell apoptosis and proliferation (P < 0.05 and P < 0.01, respectively). We found 460 DEGs compared with the control group. Among the DEGs, there were 66 upregulated genes and 394 downregulated genes. Gene ontology classification and Kyoto Encyclopedia of Genes and Genomes pathway analysis revealed that many bone metabolism-related biological processes and cell signaling pathways were disordered. The qRT-PCR verification showed that the expressions of Tgfbr1- and Bmpr1a-related genes in bone metabolism pathway in the 10 nmol/L treatment group were significantly decreased (P < 0.05). ER β was involved in the inhibitory effect of Tgfbr1 and Bmpr1a genes. The bioinformatics of the promoter found that there were three ER response elements in the promoter of Tgfbr1, and there were two ER response elements in Bmpr1a promoter regions. ChIP experiments showed that estrogen could enhance the binding of ERs to Tgfbr1 and Bmpr1a genes.

Conclusions

Estrogen can promote the apoptosis and proliferation of osteoblasts simultaneously, and the mechanism may be the joint action of transforming growth factor-beta, Wnt, mitogen-activated protein kinase, and nuclear factor-kappaB bone metabolism-related signaling pathway. Estrogen inhibits the expression of Tgfbr1 and Bmpr1a genes through ER β and affects the metabolism of MC3T3-E1 osteoblasts.

Pyk2 deficiency potentiates osteoblast differentiation and mineralizing activity in response to estrogen or raloxifene

Bone remodeling is controlled by the actions of bone-degrading osteoclasts and bone-forming osteoblasts (OBs). Aging and loss of estrogen after menopause affects bone mass and quality. Estrogen therapy, including selective estrogen receptor modulators (SERMs), can prevent bone loss and increase bone mineral density in post-menopausal women. Although investigations of the effects of estrogen on osteoclast activity are well advanced, the mechanism of action of estrogen on OBs is still unclear. The proline-rich tyrosine kinase 2 (Pyk2) is important for bone formation and female mice lacking Pyk2 (Pyk2-KO) exhibit elevated bone mass, increased bone formation rate and reduced osteoclast activity. Therefore, in the current study, we examined the role of estrogen signaling on the mechanism of action of Pyk2 in OBs. As expected, Pyk2-KO OBs showed significantly higher proliferation, matrix formation, and mineralization than WT OBs. In addition we found that Pyk2-KO OBs cultured in the presence of either 17β-estradiol (E2) or raloxifene, a SERM used for the treatment of post-menopausal osteoporosis, showed a further robust increase in alkaline phosphatase (ALP) activity and mineralization. We examined the possible mechanism of action and found that Pyk2 deletion promotes the proteasome-mediated degradation of estrogen receptor α (ERα), but not estrogen receptor β (ERβ). As a consequence, E2 signaling via ERβ was enhanced in Pyk2-KO OBs. In addition, we found that Pyk2 deletion and E2 stimulation had an additive effect on ERK phosphorylation, which is known to stimulate cell differentiation and survival. Our findings suggest that in the absence of Pyk2, estrogen exerts an osteogenic effect on OBs through altered ERα and ERβ signaling. Thus, targeting Pyk2, in combination with estrogen or raloxifene, may be a novel strategy for the prevention and/or treatment of bone loss diseases.

Mechanical Strain Regulates Osteoblast Proliferation Through Ca2+-CaMK-CREB Signal Pathway

Objective To investigate the effects of mechanical strain on Ca²⁺-calmodulin dependent kinase (CaMK)-cAMP response element binding protein (CREB) signal pathway and proliferation of osteoblasts.Methods Using a four-point bending device, MC3T3-E1 cells were exposed to mechanical tensile strains of 2500 µs and 5000 µs at 0.5 Hz respectively. The intracellular free Ca²⁺ ([Ca²⁺]i) concentration and calmodulin activity were assayed by fluorospectrophotometry, CaMK II β, CREB, and phosphorylated (activated) CREB (p-CREB) were assessed by Western blot, and cells proliferation was assayed with MTT. Pretreatment with verapamil was carried out to block Ca²⁺ channel, and inhibitor U73122 was used to inhibit phospholipase C (PLC).Results Mechanical strains of 2500 µs and 5000 µs for 1 to 10 minutes both increased [Ca²⁺]i level of the cells. The 2500 µs strain, a periodicity of 1 h/d for 3 days, activated calmodulin, elevated protein levels of CaMK II β and p-CREB, and promoted cells proliferation, which were attenuated by pretreatment of verapamil or U73122. The effects of 5000 µs strain on calmodulin, CaMK II β, p-CREB and proliferation were contrary to 2500 µs strain.Conclusion The mechanical strain regulates osteoblasts proliferation through Ca²⁺-CaMK-CREB signal pathway via Ca²⁺ channel and PLC/IP₃ transduction cascades.

Decreased local and systemic levels of sFRP3 protein in osteosarcoma patients

Osteosarcoma is a malignant bone tumor that occurs mainly in children and adolescents. Because Wnt signaling has been implicated in the pathogenesis of osteosarcoma, we have investigated the circulating and local levels of the Wnt antagonist protein, Secreted Frizzled Related Protein (sFRP) 3, in osteosarcoma patients. Enzyme linked immunosorbent assay (ELISA) analysis of 67 osteosarcoma and age-matched non-diseased control sera showed that sFPR3 protein levels were significantly lower in osteosarcoma than in normal. Analysis of tumor and adjacent normal tissues (9 pairs) from osteosarcoma patients showed a decrease in sFRP3 expression in 5 out of 9 tumor samples compared to normal tissues. Furthermore, immunohistochemical analysis of tissue microarray revealed a significant decrease in sFRP3 levels in tumor compared to normal bone. RNA sequencing analysis in osteosarcoma cells shows suppression of sFRP3 and concomitant expression of multiple Wnt family members mediating canonical or non-canonical Wnt signaling. Taken together, our findings show that the systemic and local levels of sFRP3 protein are downregulated in osteosarcoma and sFRP3 levels could be explored further in the diagnosis and the care of osteosarcoma patients.