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

Research progress on the regulatory mechanism of integrin-mediated mechanical stress in cells involved in bone metabolism

Mechanical stress is an internal force between various parts of an object that resists external factors and effects that cause an object to deform, and mechanical stress is essential for various tissues that are constantly subjected to mechanical loads to function normally. Integrins are a class of transmembrane heterodimeric glycoprotein receptors that are important target proteins for the action of mechanical stress stimuli on cells and can convert extracellular physical and mechanical signals into intracellular bioelectrical signals, thereby regulating osteogenesis and osteolysis. Integrins play a bidirectional regulatory role in bone metabolism. In this paper, relevant literature published in recent years is reviewed and summarized. The characteristics of integrins and mechanical stress are introduced, as well as the mechanisms underlying responses of integrin to mechanical stress stimulation. The paper focuses on integrin-mediated mechanical stress in different cells involved in bone metabolism and its associated signalling mechanisms. The purpose of this review is to provide a theoretical basis for the application of integrin-mediated mechanical stress to the field of bone tissue repair and regeneration.

Ortho-silicic Acid Plays a Protective Role in Glucocorticoid-Induced Osteoporosis via the Akt/Bad Signal Pathway In Vitro and In Vivo

Glucocorticoid-induced osteoporosis (GIOP) has been the most common form of secondary osteoporosis. Glucocorticoids (GCs) can induce osteocyte and osteoblast apoptosis. Plenty of research has verified that silicon intake would positively affect bone. However, the effects of silicon on GIOP are not investigated. In this study, we assessed the impact of ortho-silicic acid (OSA) on Dex-induced apoptosis of osteocytes by cell apoptosis assays. The apoptosis-related genes, cleaved-caspase-3, Bcl-2, and Bax, were detected by western blotting. Then, we evaluated the possible role of OSA on osteogenesis and osteoclastogenesis with Dex using Alizarin red staining and tartrate-resistant acid phosphatase (TRAP) staining. We also detected the related genes by quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and western blotting. We then established the GIOP mouse model to evaluate the potential role of OSA in vivo. We found that OSA showed no cytotoxic on osteocytes below 50 μM and prevented MLO-Y4 from Dex-induced apoptosis. We also found that OSA promoted osteogenesis and inhibited osteoclastogenesis with Dex. OSA had a protective effect on GIOP mice via the Akt signal pathway in vivo. In the end, we verified the Akt/Bad signal pathway in vitro, which showed the same results. Our finding demonstrated that OSA could protect osteocytes from apoptosis induced by GCs both in vitro and in vivo. Also, it promoted osteogenesis and inhibited osteoclastogenesis with the exitance of Dex. In conclusion, OSA has the potential value as a therapeutic agent for GIOP.

Mechanosensitive Steroid Hormone Signaling and Cell Fate

Mechanical forces collaborate across length scales to coordinate cell fate during development and the dynamic homeostasis of adult tissues. Similarly, steroid hormones interact with their nuclear and nonnuclear receptors to regulate diverse physiological processes necessary for the appropriate development and function of complex multicellular tissues. Aberrant steroid hormone action is associated with tumors originating in hormone-sensitive tissues and its disruption forms the basis of several therapeutic interventions. Prolonged perturbations to mechanical forces may further foster tumor initiation and the evolution of aggressive metastatic disease. Recent evidence suggests that steroid hormone and mechanical signaling intersect to direct cell fate during development and tumor progression. Potential mechanosensitive steroid hormone signaling pathways along with their molecular effectors will be discussed in this context.

Osteocytes and Estrogen Deficiency

PURPOSE OF REVIEW

Postmenopausal osteoporosis reduces circulating estrogen levels, which leads to osteoclast resorption, bone loss, and fracture. This review addresses emerging evidence that osteoporosis is not simply a disease of bone loss but that mechanosensitive osteocytes that regulate both osteoclasts and osteoblasts are also impacted by estrogen deficiency.

RECENT FINDINGS

At the onset of estrogen deficiency, the osteocyte mechanical environment is altered, which coincides with temporal changes in bone tissue composition. The osteocyte microenvironment is also altered, apoptosis is more prevalent, and hypermineralization occurs. The mechanobiological responses of osteocytes are impaired under estrogen deficiency, which exacerbates osteocyte paracrine regulation of osteoclasts. Recent research reveals changes in osteocytes during estrogen deficiency that may play a critical role in the etiology of the disease. A paradigm change for osteoporosis therapy requires an advanced understanding of such changes to establish the efficacy of osteocyte-targeted therapies to inhibit resorption and secondary mineralization.

Taurine is an effective therapy against thiram induced tibial dyschondroplasia via HIF-1α/VEGFA and β-catenin/ GSK-3β pathways in broilers

Thiram causes tibial dyschondroplasia in broilers, leading to a significant economic loss in the poultry industry. Our study explored the effects of taurine in thiram induced tibial dyschondroplasia (TD) through in vivo and in vitro approches. In in vivo study, thiram resulted in lameness disorder, low production parameters ALP, ACP, and a high level of NOS. While, the taurine exhibited promising effect by reducing lameness, increasing ALP, ACP levels, and significantly lowering NOS level with the restoration of the growth plate. In in vitro study, thiram caused distortion and disintegration of chondrocytes. The CCK-8 technique revealed the lower cell activity in TD as compared with the treatment group. Even, the treatment and taurine groups had higher cell activity than control group. Also, the chondrocyte morphology progressively reverted to normal after taurine treatment. It might effectively decreased the symptoms of TD in broilers and their production performance. Further research found that the taurine effectively improved chondrocytes' cell viability and recovered lameness disorder by regulation of HIF-1α, VEGFA, and Wnt/β-catenin signaling pathways. In summary, these results indicate that taurine has a protective effect on thiram-induced broilers and it can enhance the growth activity by directly affecting the development of chondrocytes and blood vessels.

Estrogen depletion alters osteogenic differentiation and matrix production by osteoblasts in vitro

Recent studies have revealed that the effects of estrogen deficiency are not restricted to osteoclasts and bone resorption, but that bone matrix composition is altered and osteoblasts exhibit an impaired response to mechanical stimulation. In this study, we test the hypothesis that estrogen depletion alters osteogenic differentiation and matrix production by mechanically stimulated osteoblasts in vitro. MC3T3-E1 cells were pre-treated with estrogen for 14 days, after which estrogen was withdrawn or inhibited with Fulvestrant up to 14 days. Fluid shear stress (FSS) was applied using an orbital shaker. Under estrogen depletion in static culture, osteogenic marker (ALP) and gene expression (Runx2) were decreased at 2 and after 7 days of estrogen depletion, respectively. In addition, up to 7 day the inhibition of the estrogen receptor significantly decreased fibronectin expression (FN1) under static conditions. Under estrogen depletion and daily mechanical stimulation, changes in expression of Runx2 occurred earlier (4 days) and by 14 days, changes in matrix production (Col1a1) were reported. We propose that changes in osteoblast differentiation and impaired matrix production during estrogen depletion may contribute to the altered quality of the bone and act as a contributing factor to increased bone fragility in postmenopausal osteoporosis.

Metabolomics strategy reveals the osteogenic mechanism of yak (Bos grunniens) bone collagen peptides on ovariectomy-induced osteoporosis in rats

Our previous work demonstrated that yak bone collagen peptides (YBP) possessed excellent osteogenic activity in vitro. However, associations between YBP and osteoporosis were less established, and the positive effect and underlying mechanism of YBP in the treatment of osteoporotic rats in vivo remained unclear. Herein, ovariectomized rats were intragastrically administered with YBP or 17β-estradiol for 12 weeks. Bone turnover markers, bone biomechanical parameters and bone microarchitecture were investigated to identify the specific changes of potential antagonistic effects of YBP on ovariectomized rats. Then, serum samples were analyzed by UPLC/Q-TOF-MS to identify metabolites. The results showed that YBP treatment remarkably altered the content of serum bone turnover markers and prevented the ovariectomy-induced deterioration of bone mechanical and microarchitecture characteristics. A total of forty-one biomarkers for which levels changed markedly upon treatment have been identified based on non-targeted metabolomics. Among them, twenty-one metabolites displayed a downward expression level, while twenty metabolites showed an upward expression level in the YBP group and finally were selected as potential biomarkers. The levels of these biomarkers displayed significant alterations and a tendency to be restored to normal values in YBP treated osteoporotic rats. A systematic network analysis of their corresponding pathways delineated that the protective or recovery effect of YBP on osteoporosis occurred primarily by regulating the amino acid metabolism and lipid metabolism (especially unsaturated fatty acid). Collectively, these findings highlight that such peptides hold promise in further advancement as a natural alternative for functional and health-promoting foods, which could be potentially used in mediated treatment of osteoporosis.

ROCK-II inhibition suppresses impaired mechanobiological responses in early estrogen deficient osteoblasts

Mechanobiological responses by osteoblasts are governed by downstream Rho-ROCK signalling through actin cytoskeleton re-arrangements but whether these responses are influenced by estrogen deficiency during osteoporosis remains unknown. The objective of this study was to determine alterations in the mechanobiological responses of estrogen-deficient osteoblasts and investigate whether an inhibitor of the Rho-ROCK signalling can revert these changes. MC3T3-E1 cells were pre-treated with 10 nM 17-β estradiol for 7 days and further cultured with or without estradiol for next 2 days. These cells were treated with or without ROCK-II inhibitor, Y-27632, and oscillatory fluid flow (OFF, 1Pa, 0.5 Hz, 1 h) was applied. Here, we report that Prostaglandin E₂ release, Runt-related transcription factor 2 and Osteopontin gene expression were significantly enhanced in response to OFF in estrogen-deficient cells than in cells with estrogen (3.73 vs 1.63 pg/ng DNA; 13.5 vs 2.6 fold, 2.1 vs 0.4 fold respectively). Upon ROCK-II inhibition, these enhanced effects of estrogen deficiency were downregulated. OFF increased the fibril anisotropy in cells pre-treated with estrogen and this increase was suppressed upon ROCK-II inhibition. This study is the first to demonstrate altered mechanobiological responses by osteoblasts during early estrogen deficiency and that these responses to OFF can be suppressed upon ROCK inhibition.

Estrogen effects on orthodontic tooth movement and orthodontically-induced root resorption

Estrogen is an essential regulator of the bone tissue. The remodeling of the alveolar bone and periodontal ligament is the basis of orthodontic tooth movement (OTM). There is a negative coregulation between physiological estrogen levels and the rate of OTM. As a possible inhibitory factor of OTM, estrogen suppresses bone resorption by inhibiting osteoclastic differentiation and restraining osteoclast lifespan though multiple pathways and cytokines, leading to the suppression of the initiation step of bone remodeling. On the other hand, estrogen stimulates osteoblastic differentiation and function. Estrogen receptor-α (ERα) involves in the osteogenic responses to mechanical stimulation, and the ERα expression is regulated positively by the levels of circulatory estrogen. Orthodontically induced root resorption (OIRR) is a common side-effect of orthodontic treatment. Estrogen may have some inhibitory effects on OIRR, but more studies are needed to get an effective conclusion.

A Novel 3D Osteoblast and Osteocyte Model Revealing Changes in Mineralization and Pro-osteoclastogenic Paracrine Signaling During Estrogen Deficiency

Recent in vitro studies have revealed that the mechanobiological responses of osteoblasts and osteocytes are fundamentally impaired during estrogen deficiency. However, these two-dimensional (2D) cell culture studies do not account for in vivo biophysical cues. Thus, the objectives of this study are to (1) develop a three-dimensional (3D) osteoblast and osteocyte model integrated into a bioreactor and (2) apply this model to investigate whether estrogen deficiency leads to changes in osteoblast to osteocyte transition, mechanosensation, mineralization, and paracrine signaling associated with bone resorption by osteoclasts. MC3T3-E1s were expanded in media supplemented with estrogen (17β-estradiol). These cells were encapsulated in gelatin-mtgase before culture in (1) continued estrogen (E) or (2) no further estrogen supplementation. Constructs were placed in gas permeable and water impermeable cell culture bags and maintained at 5% CO2 and 37°C. These bags were either mechanically stimulated in a custom hydrostatic pressure (HP) bioreactor or maintained under static conditions (control). We report that osteocyte differentiation, characterized by the presence of dendrites and staining for osteocyte marker dentin matrix acidic phosphoprotein 1 (DMP1), was significantly greater under estrogen withdrawal (EW) compared to under continuous estrogen treatment (day 21). Mineralization [bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), calcium] and gene expression associated with paracrine signaling for osteoclastogenesis [receptor activator of nuclear factor kappa-β ligand (RANKL)/osteoprotegerin OPG ratio] were significantly increased in estrogen deficient and mechanically stimulated cells. Interestingly, BSP and DMP-1 were also increased at day 1 and day 21, respectively, which play a role in regulation of biomineralization. Furthermore, the increase in pro-osteoclastogenic signaling may be explained by altered mechanoresponsiveness of osteoblasts or osteocytes during EW. These findings highlight the impact of estrogen deficiency on bone cell function and provide a novel in vitro model to investigate the mechanisms underpinning changes in bone cells after estrogen deficiency.

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.

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.

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.

Ovarian hormone depletion affects cortical bone quality differently on different skeletal envelopes

The physical properties of bone tissue are determined by the organic and mineral matrix, and are one aspect of bone quality. As such, the properties of mineral and matrix are a major contributor to bone strength, independent of bone mass. Cortical bone quality may differ regionally on the three skeletal envelopes that compose it. Each of these envelopes may be affected differently by ovarian hormone depletion. Identifying how these regions vary in their tissue adaptive response to ovarian hormones can inform our understanding of how tissue quality contributes to overall bone strength in postmenopausal women. We analyzed humeri from monkeys that were either SHAM-operated or ovariectomized. Raman microspectroscopic analysis was performed as a function of tissue age based on the presence of multiple fluorescent double labels, to determine whether bone compositional properties (mineral/matrix ratio, tissue water, glycosaminoglycan, lipid, and pyridinoline contents, and mineral maturity/crystallinity) are similar between periosteal, osteonal, and endosteal surfaces, as well as to determine the effects of ovarian hormone depletion on them. The results indicate that mineral and organic matrix characteristics, and kinetics of mineral and organic matrix modifications as a function of tissue age are different at periosteal vs. osteonal and endosteal surfaces. Ovarian hormone depletion affects the three cortical surfaces (periosteal, osteonal, endosteal) differently. While ovarian hormone depletion does not significantly affect the quality of either the osteoid or the most recently mineralized tissue, it significantly affects the rate of subsequent mineral accumulation, as well as the kinetics of organic matrix modifications, culminating in significant differences within interstitial bone. These results highlight the complexity of the cortical bone compartments, add to existing knowledge on the effects of ovarian hormone depletion on local cortical bone properties, and may contribute to a better understanding of the location specific action of drugs used in the management of postmenopausal osteoporosis.

Mechanical loading reduces inflammation-induced human osteocyte-to-osteoclast communication

Multiple factors contribute to bone loss in inflammatory diseases such as rheumatoid arthritis (RA), but circulating inflammatory factors and immobilization play a crucial role. Mechanical loading prevents bone loss in the general population, but the effects of mechanical loading in patients with RA are less clear. Therefore, we aimed to investigate whether mechanical stimuli reverse the stimulatory effect of RA serum on osteocyte-to-osteoclast communication. Human primary osteocytes were pretreated with 10 % RA serum or healthy control serum for 7 days, followed by 1 h ± mechanical loading by pulsating fluid flow (PFF). Nitric oxide (NO) and prostaglandin E2 were measured in the medium. Receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG), interleukin-6 (IL-6), cyclooxygenase-2 (COX2), matrix-extracellular phosphoglycoprotein (MEPE), cysteine-rich protein 61 (CYR61), and SOST gene expression was quantified by qPCR. Osteoclast precursors were cultured with PFF-conditioned medium (PFF-CM) or static-conditioned medium (stat-CM), and osteoclast formation was assessed. RA serum alone did not affect IL-6, CYR61, COX2, MEPE, or SOST gene expression in osteocytes. However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF nullified this effect. PFF enhanced NO production to the same extent in control serum (2.6-3.5-fold) and RA serum-pretreated (2.7-3.6-fold) osteocytes. Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF nullified this effect. In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication. Mechanical loading nullified this upregulation, suggesting that mechanical stimuli could contribute to the prevention of osteoporosis in inflammatory disease.

CXCL8 and CCL20 Enhance Osteoclastogenesis via Modulation of Cytokine Production by Human Primary Osteoblasts

Generalized osteoporosis is common in patients with inflammatory diseases, possibly because of circulating inflammatory factors that affect osteoblast and osteoclast formation and activity. Serum levels of the inflammatory factors CXCL8 and CCL20 are elevated in rheumatoid arthritis, but whether these factors affect bone metabolism is unknown. We hypothesized that CXCL8 and CCL20 decrease osteoblast proliferation and differentiation, and enhance osteoblast-mediated osteoclast formation and activity. Human primary osteoblasts were cultured with or without CXCL8 (2–200 pg/ml) or CCL20 (5–500 pg/ml) for 14 days. Osteoblast proliferation and gene expression of matrix proteins and cytokines were analyzed. Osteoclast precursors were cultured with CXCL8 (200 pg/ml) and CCL20 (500 pg/ml), or with conditioned medium (CM) from CXCL8 and CCL20-treated osteoblasts with or without IL-6 inhibitor. After 3 weeks osteoclast formation and activity were determined. CXCL8 (200 pg/ml) and CCL20 (500 pg/ml) enhanced mRNA expression of KI67 (2.5–2.7-fold), ALP (1.6–1.7-fold), and IL-6 protein production (1.3–1.6-fold) by osteoblasts. CXCL8-CM enhanced the number of osteoclasts with 3–5 nuclei (1.7-fold), and with >5 nuclei (3-fold). CCL20-CM enhanced the number of osteoclasts with 3–5 nuclei (1.3-fold), and with >5 nuclei (2.8-fold). IL-6 inhibition reduced the stimulatory effect of CXCL8-CM and CCL20-CM on formation of osteoclasts. In conclusion, CXCL8 and CCL20 did not decrease osteoblast proliferation or gene expression of matrix proteins. CXCL8 and CCL20 did not directly affect osteoclastogenesis. However, CXCL8 and CCL20 enhanced osteoblast-mediated osteoclastogenesis, partly via IL-6 production, suggesting that CXCL8 and CCL20 may contribute to osteoporosis in rheumatoid arthritis by affecting bone cell communication.

Increased endoplasmic reticulum stress in mouse osteocytes with aging alters Cox-2 response to mechanical stimuli

Aging reduces bone mass as well as the anabolic response of bone to mechanical stimuli, resulting in osteopenia. Endoplasmic reticulum (ER) stress impairs the response of myogenic cells to anabolic stimuli, and is involved in sarcopenia, but whether ER stress also contributes to osteopenia is unknown. Therefore, we tested whether ER stress exists in bones of aged mice, and whether this impairs the osteocyte response to mechanical stimulation. Primary osteocytes were obtained from long bones of adult (8 months) and old (24-26 months) mice, treated with or without the pharmacological ER stress inducer tunicamycin, and either or not subjected to mechanical loading by pulsating fluid flow (PFF). The osteocyte response to PFF was assessed by measuring cyclooxygenase-2 (Cox-2) mRNA levels and nitric oxide (NO) production. mRNA levels of ER stress markers were higher in old versus adult osteocytes (+40% for activating transcription factor-4, +120% for C/EBP homologous protein, and +120% for spliced X-box binding protein-1, p < 0.05). The Cox-2 response to PFF was fourfold decreased in cells from old bones (p < 0.001), while tunicamycin decreased PFF-induced Cox-2 expression by threefold in cells from adult bones (p < 0.01). PFF increased NO production by 50% at 60 min in osteocytes from old versus adult bones (p < 0.01). In conclusion, our data indicate that the expression of several ER stress markers was higher in osteocytes from bones of old compared to adult mice. Since ER stress altered the response of osteocytes to mechanical loading, it could be a novel factor contributing to osteopenia.

Bone cell mechanosensitivity, estrogen deficiency, and osteoporosis

Adaptation of bone to mechanical stresses normally produces a bone architecture that combines a proper resistance against failure with a minimal use of material. This adaptive process is governed by mechanosensitive osteocytes that transduce the mechanical signals into chemical responses, i.e. the osteocytes release signaling molecules, which orchestrate the recruitment and activity of bone forming osteoblasts and/or bone resorbing osteoclasts. Computer models have shown that the maintenance of a mechanically-efficient bone architecture depends on the intensity and spatial distribution of the mechanical stimulus as well as on the osteocyte response. Osteoporosis is a condition characterized by a reduced bone mass and a compromized resistance of bone against mechanical loads, which has led us to hypothesize that mechanotransduction by osteocytes is altered in osteoporosis. One of the major causal factors for osteoporosis is the loss of estrogen, the major hormonal regulator of bone metabolism. Loss of estrogen may increase osteocyte-mediated activation of bone remodeling, resulting in impaired bone mass and architecture. In this review we highlight current insights on how osteocytes perceive mechanical stimuli placed on whole bones. Particular emphasis is placed on the role of estrogen in signaling pathway activation by mechanical stimuli, and on computer simulation in combination with cell biology to unravel biological processes contributing to bone strength.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

Icariin stimulates the osteogenic differentiation of rat bone marrow stromal cells via activating the PI3K-AKT-eNOS-NO-cGMP-PKG

Icariin, a prenylated flavonol glycoside isolated from Epimedii herba, has been found to be a potent stimulator of osteogenic differentiation and has potential application in preventing bone loss. However, the signaling pathway underlying its osteogenic effect remains unclear. We hypothesized that the osteogenic activity of icariin is related to the nitric oxide (NO) signal pathway and PI3K/AKT pathway in its upstream. Rat bone marrow stromal cells (rBMSCs) were cultured in osteogenic medium and treated with icariin or together with L-NAME, ODQ, PDE5, and/or LY294002 (the inhibitor of NOS, sGC, cGMP, and PI3K respectively), and effects were examined on the expression of signal messengers (NOS, NO, sGC, cGMP, PKG and PI3K) and the levels of osteogenic markers (alkaline phosphatase or ALP, osteocalcin and calcified nodules). It was found that icariin dose-dependently increased ALP activity, and treatment at the optimal concentration (10(-5)M) increased NOS activity, iNOS and eNOS expression, NO production, sGC and cGMP contents and PKG expression besides the phosphorylation of AKT. The addition of L-NAME, ODQ and PDE5 significantly inhibited the icariin effects on above markers respectively. The addition of LY294002 decreased the p-AKT level, NOS activity, eNOS expression and NO production significantly, but had no significant effect on iNOS expression. The addition of any of the four inhibitors also abolished the osteogenic effect of icariin on rBMSCs as indicated by ALP activity, osteocalcin synthesis, calcium deposition and the number and areas of calcified nodules. These results suggest that the osteogenic effect of icariin involves the PI3K-AKT-eNOS-NO-cGMP-PKG signal pathway. Furthermore, dosage response studies showed that icariin at 10(-6)M (a physiologically achievable concentration in vivo) also activated this signal pathway.

Physical activity improves age-related decline in the osteogenic potential of rats' bone marrow-derived mesenchymal stem cells

AIM

To examine whether physical activity increases osteogenic differentiation of bone marrow mesenchymal stem cells (BMMSCs) from adult rats compared with young rats.

METHODS

Eighteen female Wistar rats were divided into three groups and the following cells isolated: (1) differentiated BMMSCs from young donors, (2) differentiated BMMSCs from sedentary adult donors and (3) differentiated BMMSCs from active adult donors. We analysed MTT conversion, percentage of cells per field, mineralized nodule number and gene expression for telomerase reverse transcriptase (TERT), alkaline phosphatase, caspase 3, osteocalcin, bone sialoprotein and collagen I.

RESULTS

Telomerase reverse transcriptase expression and the percentage of cells per field in BMMSCs cultures from adult rats were smaller than those observed in young donors. However, levels of caspase 3 expression were higher in BMMSCs from adult donors (P < 0.05). Despite the fact that physical activity was associated with an increase in expression of caspase 3 (P < 0.05), there was no difference in the percentage of cells per field between groups of adult BMMSCs (active or sedentary). However, physical activity increased the number of mineralized nodules and osteocalcin expression after 21 days, and alkaline phosphatase expression at 7, 14 and 21 days in the BMMSCs of adult donors (P < 0.05). However, those values were smaller when compared with young donors BMMSCs (P < 0.05). Only the expression levels of alkaline phosphatase were similar to young donors BMMSCs (P ≥ 0.05).

CONCLUSION

Physical activity increases osteogenic differentiation of BMMSCs from adult donors but does not increase the differentiation to the levels observed in BMMSCs from young donor rats.

VDR dependent and independent effects of 1,25-dihydroxyvitamin D3 on nitric oxide production by osteoblasts

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) strongly mediates bone mass. Mechanical stimulation also affects bone mass, partly via enhancing nitric oxide (NO) production by osteoblasts. We aimed to determine whether 1,25(OH)(2)D(3) affects NO production by osteoblasts in the presence or absence of mechanical stimulation. We hypothesised that 1,25(OH)(2)D(3) stimulates NO production via nuclear actions of the vitamin D receptor (VDR), which requires hours of incubation with 1,25(OH)(2)D(3) to occur. MC3T3-E1 osteoblasts and long-bone osteoblasts of adult wildtype and VDR(-/-) mice were pre-incubated for 24h with or without 1,25(OH)(2)D(3) (10(-13)-10(-9)M), followed by 30 min pulsating fluid flow (PFF; 0.7±0.3 Pa, 5 Hz) or static culture with or without 1,25(OH)(2)D(3). NO production and NO synthase (NOS) expression were quantified. 10(-11)M 1,25(OH)(2)D(3) for 24h, but not 30 min, stimulated NO production by MC3T3-E1 osteoblasts (eightfold). 1,25(OH)(2)D(3) for 24h increased inducible-NOS gene-expression (twofold), suggesting that 1,25(OH)(2)D(3) stimulated NO production via activation of NOS gene transcription. PFF rapidly increased NO production by MC3T3-E1 osteoblasts, wildtype osteoblasts, and VDR(-/-) osteoblasts. This PFF effect was abolished after incubation with 1,25(OH)(2)D(3) for 24h, or during PFF only. Our results suggest that 1,25(OH)(2)D(3) stimulates inducible-NOS expression and NO production by osteoblasts in the absence of mechanical stimulation, likely via genomic VDR action. In contrast, 1,25(OH)(2)D(3) may affect mechanical loading-induced NO production independent of genomic VDR action, since 1,25(OH)(2)D(3) diminished PFF-induced NO production in VDR(-/-) bone cells. In conclusion, 1,25(OH)(2)D(3) and mechanical loading interact at the level of mechanotransduction, whereby 1,25(OH)(2)D(3) seems to act independently of VDR genomic mechanism.

Loading-related regulation of transcription factor EGR2/Krox-20 in bone cells is ERK1/2 protein-mediated and prostaglandin, Wnt signaling pathway-, and insulin-like growth factor-I axis-dependent

Of the 1,328 genes revealed by microarray to be differentially regulated by disuse, or at 8 h following a single short period of osteogenic loading of the mouse tibia, analysis by predicting associated transcription factors from annotated affinities revealed the transcription factor EGR2/Krox-20 as being more closely associated with more pathways and functions than any other. Real time quantitative PCR confirmed up-regulation of Egr2 mRNA expression by loading of the tibia in vivo. In vitro studies where strain was applied to primary cultures of mouse tibia-derived osteoblastic cells and the osteoblast UMR106 cell line also showed up-regulation of Egr2 mRNA expression. In UMR106 cells, inhibition of β1/β3 integrin function had no effect on strain-related Egr2 expression, but it was inhibited by a COX2-selective antagonist and imitated by exogenous prostaglandin E2 (PGE2). This response to PGE₂ was mediated chiefly through the EP1 receptor and involved stimulation of PKC and attenuation by cAMP/PKA. Neither activators nor inhibitors of nitric oxide, estrogen signaling, or LiCl had any effect on Egr2 mRNA expression, but it was increased by both insulin-like growth factor-1 and high, but not low, dose parathyroid hormone and exogenous Wnt-3a. The increases by strain, PGE2, Wnt-3a, and phorbol 12-myristate 13-acetate were attenuated by inhibition of MEK-1. EGR2 appears to be involved in many of the signaling pathways that constitute early responses of bone cells to strain. These pathways all have multiple functions. Converting their strain-related responses into coherent “instructions” for adaptive (re)modeling is likely to depend upon their contextual activation, suppression, and interaction probably on more than one occasion.

Is nitric oxide a mediator of the effects of low-intensity electrical stimulation on bone in ovariectomized rats?

Low-intensity electrical stimulation (LIES) may counteract the effects of ovariectomy (OVX) on nitric oxide synthase (NOS) expression, osteocyte viability, bone structure, and microarchitecture in rats (Lirani-Galvão et al., Calcif Tissue Int 84:502-509, 2009). The aim of the present study was to investigate if these effects of LIES could be mediated by NO. We analyzed the effects of NO blockage (by L-NAME) in the response to LIES on osteocyte viability, bone structure, and microarchitecture in OVX rats. Sixty rats (200-220 g) were divided into six groups: sham, sham-L-NAME (6 mg/kg/day), OVX, OVX-L-NAME, OVX-LIES, and OVX-LIES-L-NAME. After 12 weeks, rats were killed and tibiae collected for histomorphometric analysis and immunohistochemical detection of endothelial NOS (eNOS), inducible NOS (iNOS), and osteocyte apoptosis (caspase-3 and TUNEL). In the presence of L-NAME, LIES did not counteract the OVX-induced effects on bone volume and trabecular number (as on OVX-LIES). L-NAME blocked the stimulatory effects of LIES on iNOS and eNOS expression of OVX rats. Both L-NAME and LIES decreased osteocyte apoptosis. Our results showed that in OVX rats L-NAME partially blocks the effects of LIES on bone structure, turnover, and expression of iNOS and eNOS, suggesting that NO may be a mediator of some positive effects of LIES on bone.

Prostaglandin E(2) is crucial in the response of podocytes to fluid flow shear stress

Podocytes play a key role in maintaining and modulating the filtration barrier of the glomerulus. Because of their location, podocytes are exposed to mechanical strain in the form of fluid flow shear stress (FFSS). Several human diseases are characterized by glomerular hyperfiltration, such as diabetes mellitus and hypertension. The response of podocytes to FFSS at physiological or pathological levels is not known. We exposed cultured podocytes to FFSS, and studied changes in actin cytoskeleton, prostaglandin E(2) (PGE(2)) production and expression of cyclooxygenase-1 and-2 (COX-1, COX-2). FFSS caused a reduction in transversal F-actin stress filaments and the appearance of cortical actin network in the early recovery period. Cells exhibited a pattern similar to control state by 24 h following FFSS without significant loss of podocytes or apoptosis. FFSS caused increased levels of PGE(2) as early as 30 min after onset of shear stress, levels that increased over time. PGE(2) production by podocytes at post-2 h and post-24 h was also significantly increased compared to control cells (p < 0.039 and 0.012, respectively). Intracellular PGE(2) synthesis and expression of COX-2 was increased at post-2 h following FFSS. The expression of COX-1 mRNA was unchanged. We conclude that podocytes are sensitive and responsive to FFSS, exhibiting morphological and physiological changes. We believe that PGE(2) plays an important role in mechanoperception in podocytes.

Human dental pulp cells exhibit bone cell-like responsiveness to fluid shear stress

BACKGROUND AIMS

For engineering bone tissue to restore, for example, maxillofacial defects, mechanosensitive cells are needed that are able to conduct bone cell-specific functions, such as bone remodelling. Mechanical loading affects local bone mass and architecture in vivo by initiating a cellular response via loading-induced flow of interstitial fluid. After surgical removal of ectopically impacted third molars, human dental pulp tissue is an easily accessible and interesting source of cells for mineralized tissue engineering. The aim of this study was to determine whether human dental pulp-derived cells (DPC) are responsive to mechanical loading by pulsating fluid flow (PFF) upon stimulation of mineralization in vitro.

METHODS

Human DPC were incubated with or without mineralization medium containing differentiation factors for 3 weeks. Cells were subjected to 1-h PFF (0.7 ± 0.3 Pa, 5 Hz) and the response was quantified by measuring nitric oxide (NO) and prostaglandin E₂ (PGE₂) production, and gene expression of cyclooxygenase (COX)-1 and COX-2.

RESULTS

We found that DPC are intrinsically mechanosensitive and, like osteogenic cells, respond to PFF-induced fluid shear stress. PFF stimulated NO and PGE₂ production, and up-regulated COX-2 but not COX-1 gene expression. In DPC cultured under mineralizing conditions, the PFF-induced NO, but not PGE₂, production was significantly enhanced.

CONCLUSIONS

These data suggest that human DPC, like osteogenic cells, acquire responsiveness to pulsating fluid shear stress in mineralizing conditions. Thus DPC might be able to perform bone-like functions during mineralized tissue remodeling in vivo, and therefore provide a promising new tool for mineralized tissue engineering to restore, for example, maxillofacial defects.

Mechanosensitivity of dental pulp stem cells is related to their osteogenic maturity

For engineering bone tissue, mechanosensitive cells are needed for bone (re)modelling. Local bone mass and architecture are affected by mechanical loading, which provokes a cellular response via loading-induced interstitial fluid flow. We studied whether human dental pulp-derived mesenchymal stem cells (PDSCs) portraying mature (PDSC-mature) or immature (PDSC-immature) bone cell characteristics are responsive to pulsating fluid flow (PFF) in vitro. We also assessed bone formation by PDSCs on hydroxyapatite-tricalcium phosphate granules after subcutaneous implantation in mice. Cultured PDSC-mature exhibited higher osteocalcin and alkaline phosphatase gene expression and activity than PDSC-immature. Pulsating fluid flow (PFF) stimulated nitric oxide production within 5 min by PDSC-mature but not by PDSC-immature. In PDSC-mature, PFF induced prostaglandin E(2) production, and cyclooxygenase 2 gene expression was higher than in PDSC-immature. Implantation of PDSC-mature resulted in more osteoid deposition and lamellar bone formation than PDSC-immature. We conclude that PDSCs with a mature osteogenic phenotype are more responsive to pulsating fluid shear stress than osteogenically immature PDSCs and produce more bone in vivo. These data suggest that PDSCs with a mature osteogenic phenotype might be preferable for bone tissue engineering to restore, for example, maxillofacial defects, because they might be able to perform mature bone cell-specific functions during bone adaptation to mechanical loading in vivo.

Estrogen augments shear stress-induced signaling and gene expression in osteoblast-like cells via estrogen receptor-mediated expression of beta1-integrin

Estrogen and mechanical forces are positive regulators for osteoblast proliferation and bone formation. We investigated the synergistic effect of estrogen and flow-induced shear stress on signal transduction and gene expression in human osetoblast-like MG63 cells and primary osteoblasts (HOBs) using activations of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) and expressions of c-fos and cyclooxygenase-2 (I) as readouts. Estrogen (17beta-estradiol, 10 nM) and shear stress (12 dyn/cm(2)) alone induced transient phosphorylations of ERK and p38 MAPK in MG63 cells. Pretreating MG63 cells with 17beta-estradiol for 6 hours before shearing augmented these shear-induced MAPK phosphorylations. Western blot and flow cytometric analyses showed that treating MG63 cells with 17beta-estradiol for 6 hrs induced their beta(1)-integrin expression. This estrogen-induction of beta(1)-integrin was inhibited by pretreating the cells with a specific antagonist of estrogen receptor ICI 182,780. Both 17beta-estradiol and shear stress alone induced c-fos and Cox-2 gene expressions in MG63 cells. Pretreating MG63 cells with 17beta-estradiol for 6 hrs augmented the shear-induced c-fos and Cox-2 expressions. The augmented effects of 17beta-estradiol on shear-induced MAPK phosphorylations and c-fos and Cox-2 expressions were inhibited by pretreating the cells with ICI 182,780 or transfecting the cells with beta(1)-specific small interfering RNA. Similar results on the augmented effect of estrogen on shear-induced signaling and gene expression were obtained with HOBs. Our findings provide insights into the mechanism by which estrogen augments shear stress responsiveness of signal transduction and gene expression in bone cells via estrogen receptor-mediated increases in beta(1)-integrin expression.

Epimedium-derived flavonoids promote osteoblastogenesis and suppress adipogenesis in bone marrow stromal cells while exerting an anabolic effect on osteoporotic bone

BACKGROUND

Epimedium-derived flavonoids (EFs) have been reported to prevent bone loss in ovariectomized (OVX) rats and late postmenopausal women but the underlying mechanism of the anabolic effect is unknown.

OBJECTIVE

This study aimed to investigate the effect of EFs on osteoporotic bone using histomorphometry and on osteoblastogenesis/adipogenesis of bone marrow stromal cells (BMSCs).

METHODS

11-month-old female Wistar rats were divided into Sham, OVX, Sham+soluble vehicle (Sham+SV), OVX+SV and OVX+EFs (10 mg/kg/day) groups. 3 months after surgery, rats from the first two groups were euthanized to verify the establishment of OVX-induced osteoporosis. Other groups were orally treated with either daily SV or EFs for 4 months. At sacrifice, serum was analyzed for the levels of osteocalcin and TRACP 5b, BMD in the proximal femur was measured by pQCT. Static and dynamic bone histomorphometry was performed in proximal tibiae with microCT and undecalcified sections, respectively. The effect of EF treatment on differentiation of rat BMSCs was assessed by colony formation assays and gene expression analysis, respectively. Gene expression, ALP activity and adipocyte numbers were determined in differentiating human BMSCs after exposure to conditioned serum from SV- or EFs-treated OVX rats.

RESULTS

The serum level of osteocalcin was higher and TRACP 5b was lower in EFs versus SV-treated OVX rats. BMD, BV/TV, Tb.N and Conn.D in EFs-treated OVX rats were significantly greater than those of SV-treated OVX rats. Bone histomorphometric parameters OS/BS, MAR, and BFR/BS were significantly higher in EFs versus SV-treated OVX rats. EFs significantly increased osteogenesis and decreased adipogenesis of BMSCs, as evidenced by CFU-ALP and CFU-Adipo assays, respectively. The mRNA level of Runx2 and bone sailoprotein was significantly higher while PPARgamma2 was significantly lower in BMSCs from EFs-treated versus SV-treated OVX rats. ALP activity and Runx2 mRNA was significantly higher while adipocyte number and PPARgamma2 mRNA was significantly lower in human BMSCs after exposure to conditioned serum from EFs versus SV-treated OVX rats.

CONCLUSION

EFs exerted anabolic effect on osteoporotic bone by concomitantly promoting osteogenic and suppressing adipogenic differentiation of BMSCs.

Low-intensity electrical stimulation counteracts the effects of ovariectomy on bone tissue of rats: effects on bone microarchitecture, viability of osteocytes, and nitric oxide expression

Low Intensity Electrical Stimulation (LIES) has been used for bone repair, but little is known about its effects on bone after menopause. Osteocytes probably play a role in mediating this physical stimulus and they could act as transducers through the release of biochemical signals, such as nitric oxide (NO). The aim of the present study was to investigate the effects of LIES on bone structure and remodeling, NOS expression and osteocyte viability in ovariectomized (OVX) rats. Thirty rats (200-220 g) were divided into 3 groups: SHAM, OVX, and OVX subjected to LIES (OVX + LIES) for 12 weeks. Following the protocol, rats were sacrificed and tibias were collected for histomorphometric analysis and immunohistochemical detection of endothelial NO synthase (eNOS), inducible NOS (iNOS), and osteocyte apoptosis (caspase-3 and TUNEL). OVX rats showed significant (p < 0.05 vs. SHAM) decreased bone volume (10% vs. 25%) and trabecular number (1.7 vs. 3.9), and increased eroded surfaces (4.7% vs. 3.2%) and mineralization surfaces (15.9% vs. 7.7%). In contrast, after LIES, all these parameters were significantly different from OVX but not different from SHAM. eNOS and iNOS were similarly expressed in subperiosteal regions of tibiae cortices of SHAM, not expressed in OVX, and similarly expressed in OVX + LIES when compared to SHAM. In OVX, the percentage of apoptotic osteocytes (24%) was significantly increased when compared to SHAM (11%) and OVX + LIES (8%). Our results suggest that LIES counteracts some effects of OVX on bone tissue preserving bone structure and microarchitecture, iNOS and eNOS expression, and osteocyte viability.

The Src inhibitor AZD0530 reversibly inhibits the formation and activity of human osteoclasts

Tumor cells in the bone microenvironment are able to initiate a vicious cycle of bone degradation by mobilizing osteoclasts, multinucleated cells specialized in bone degradation. c-Src is highly expressed both in tumors and in osteoclasts. Therefore, drugs like AZD0530, designed to inhibit Src activity, could selectively interfere with both tumor and osteoclast activity. Here we explored the effects of AZD0530 on human osteoclast differentiation and activity. The effect on osteoclasts formed in vivo was assessed in mouse fetal calvarial explants and in isolated rabbit osteoclasts, where it dose-dependently inhibited osteoclast activity. Its effect on formation and activity of human osteoclasts in vitro was determined in cocultures of human osteoblasts and peripheral blood mononuclear cells. AZD0530 was most effective in inhibiting osteoclast-like cell formation when present at the onset of osteoclastogenesis, suggesting that Src activity is important during the initial phase of osteoclast formation. Formation of active phosphorylated c-Src, which was highly present in osteoclast-like cells in cocultures and in peripheral blood mononuclear cell monocultures, was significantly reduced by AZD0530. Furthermore, it reversibly prevented osteoclast precursor migration from the osteoblast layer to the bone surface and subsequent formation of actin rings and resorption pits. These data suggest that Src is pivotal for the formation and activity of human osteoclasts, probably through its effect on the distribution of the actin microfilament system. The reversible effect of AZD0530 on osteoclast formation and activity makes it a promising candidate to temper osteoclastic bone degradation in bone diseases with enhanced osteoclast activity such as osteolytic metastatic bone disease.

Hormone therapy improves femur geometry among ethnically diverse postmenopausal participants in the Women's Health Initiative hormone intervention trials

Loss of bone strength underlies osteoporotic fragility fractures. We hypothesized that hormone interventions significantly improve the structural geometry of proximal femur cross-sections. Study participants were from the Women's Health Initiative hormone intervention trials: either the conjugated equine estrogen (CEE) only (N(placebo) = 447, N(CEE) = 422) trial or the estrogen (E) plus progestin (P) (N(placebo) = 441, N(E+P) = 503) trial, who were 50-79 yr old at baseline and were followed up to 6 yr. BMD scans by DXA were conducted at baseline, year 1, year 3, and year 6. Femur geometry was derived from hip DXA scans using the hip structural analysis (HSA) method. Mixed effects models with the intent-to-treat analysis approach were used. There were no significant differences in treatment effects between the E-alone and the E + P trial, so the analyses were conducted with participants combined from both trials. Treatment benefits (p < 0.05) on femur geometry were observed as early as 1 yr after the intervention. From baseline to year 6, section modulus (a measure of maximum bending stress) was preserved, and buckling ratio (an index of cortical instability under compression) was reduced by hormone interventions (p < 0.05); the differences in the percent changes from baseline to year 6 between women on hormone intervention versus women on placebo were 2.3-3.6% for section modulus and -5.3% to - 4.3% for buckling ratio. Hormone interventions led to favorable changes in femur geometry, which may help explain the reduced fracture risk observed in hormone interventions.

PPARgamma inhibits osteogenesis via the down-regulation of the expression of COX-2 and iNOS in rats

Peroxisome proliferator-activated receptor gamma (PPARgamma), a ligand-activated transcription factor, is considered as an anti-osteoblastic factor associated with adiposity and the elderly osteoporosis due to a defect in osteoblastogenesis. We have found that oral administration of PPARgamma activator rosiglitazone decreased tibia BMD and serum ALP but left serum calcium and osteoclast marker C-terminal telopeptide unaffected. In addition, we examined the inhibitory mechanisms of PPARgamma on the bone formation by using PPARgamma activators ciglitazone and 15-deoxy-Delta(12,14)-prostaglandin-J2 (15d-PGJ2). Our data indicated that PPARgamma ligands decreased both mineralized bone nodules and alkaline phosphatase (ALP) activities in cultured primary osteoblasts. Reverse transcription polymerase chain reaction (RT-PCR) showed that the expression of bone morphogenetic protein-2 (BMP-2) and osteocalcin (OCN) was inhibited by ciglitizone and 15d-PGJ2. Furthermore, PPARgamma ligands inhibited NF-kappaB associated downstream COX-2 and iNOS osteogenic signaling. The ultrasound (US)-induced elevation of COX-2 and iNOS expression and nitric oxide (NO) production were attenuated in the presence of PPARgamma ligands. Furthermore, local administration of PPARgamma ligands into the metaphysis of rat tibia decreased the bone volume in secondary spongiosa. These results suggest that the activation of PPARgamma inhibits osteoblastic differentiation and the expression of several anabolic mediators involved in bone formation. These data may reflect osteoporosis and less bone formation in the aging people and patients treated with thiazolidinediones.

Influence of hormones on osteogenic differentiation processes of mesenchymal stem cells

Bone development, regeneration and maintenance are governed by osteogenic differentiation processes from mesenchymal stem cells through to mature bone cells, which are directed by local growth and differentiation factors and modulated strongly by hormones. Mesenchymal stem cells develop from both mesoderm and neural crest and can give rise to development, regeneration and maintenance of mesenchymal tissues, such as bone, cartilage, muscle, tendons and discs. There are only limited data regarding the effects of hormones on early events, such as regulation of stemness and maintenance of the mesenchymal stem cell pool. Hormones, such as estrogens, vitamin D-hormone and parathyroid hormone, besides others, are important modulators of osteogenic differentiation processes and bone formation, starting off with fate decision and the development of osteogenic offspring from mesenchymal stem cells, which end up in osteoblasts and osteocytes. Hormones are involved in fetal bone development and regeneration and, in childhood, adolescence and adulthood, they control adaptive needs for growth and reproduction, nutrition, physical power and crisis adaptation. As in other tissues, aging in mesenchymal stem cells and their osteogenic offspring is accompanied by the accumulation of genomic and proteomic damage caused by oxidative burden and insufficient repair. Failsafe programs, such as apoptosis and cellular senescence avoid tumorigenesis. Hormones can influence the pace of such events, thus supporting the quality of tissue regeneration in aging organisms in vivo; for example, by delaying osteoporosis development. The potential for hormones in systemic therapeutic strategies is well appreciated and some concepts are approved for clinical use already. Their potential for cell-based therapeutic strategies for tissue regeneration is probably underestimated and could enhance the quality of tissue-engineering constructs for transplantation and the concept of in situ-guided tissue regeneration.

Signal transduction pathways involved in mechanotransduction in bone cells

Several in vivo and in vitro studies with different loading regimens showed that mechanical stimuli have an influence on proliferation and differentiation of bone cells. Prerequisite for this influence is the transduction of mechanical signals into the cell, a phenomenon that is termed mechanotransduction, which is essential for the maintenance of skeletal homeostasis in adults. Mechanoreceptors, such as the integrins, cadherins, and stretch-activated Ca2+ channels, together with various signal transduction pathways, are involved in the mechanotransduction process that ultimately regulates gene expression in the nucleus. Mechanotransduction itself is considered to be regulated by hormones, the extracellular matrix of the osteoblastic cells and the mode of the mechanical stimulus.

Different responsiveness to mechanical stress of bone cells from osteoporotic versus osteoarthritic donors

INTRODUCTION

Osteoporosis (OP) and osteoarthritis (OA) are both common diseases in the elderly, but remarkably seldom coexist. The bone defects that are related to both diseases develop with increasing age, which suggests that they are related to some form of imperfect bone remodeling. Current opinion holds that the bone remodeling process is supervised by bone cells that respond to mechanical stimuli. An imperfect response of bone cells to mechanical stimuli might thus relate to imperfect bone remodeling, which could eventually lead to a lack bone mass and strength, such as in OP patients.

MATERIALS

To investigate whether the cellular response to mechanical stress differs between OP and OA patients, we compared the response of bone cells from both groups to fluid shear stress of increasing magnitude. Bone cells from 9 female OP donors (age 60-90 year) and 9 female age-matched OA donors were subjected to pulsating fluid flow (PFF) of low (0.4+/-0.1 Pa at 3 Hz), medium (0.6+/-0.3 Pa at 5 Hz), or high shear stress (1.2+/-0.4 at 9Hz), or were kept under static culture conditions.

RESULTS

We found subtle differences in the shear-stress response of the two groups, measured as nitric oxide (NO) and prostaglandin E2 (PGE2) production. The NO-response to shear stress was higher in the OP than the OA cells, while the PGE2-response was higher in the OA cells.

CONCLUSIONS

Assuming that NO and PGE2 play a role in cell-cell communication during remodeling, these results suggest that slight differences in mechanotransduction might relate to the opposite bone defects in osteoporosis and osteoarthritis.

Pathogenesis of osteoporosis: concepts, conflicts, and prospects

Osteoporosis is a disorder in which loss of bone strength leads to fragility fractures. This review examines the fundamental pathogenetic mechanisms underlying this disorder, which include: (a) failure to achieve a skeleton of optimal strength during growth and development; (b) excessive bone resorption resulting in loss of bone mass and disruption of architecture; and (c) failure to replace lost bone due to defects in bone formation. Estrogen deficiency is known to play a critical role in the development of osteoporosis, while calcium and vitamin D deficiencies and secondary hyperparathyroidism also contribute. There are multiple mechanisms underlying the regulation of bone remodeling, and these involve not only the osteoblastic and osteoclastic cell lineages but also other marrow cells, in addition to the interaction of systemic hormones, local cytokines, growth factors, and transcription factors. Polymorphisms of a large number of genes have been associated with differences in bone mass and fragility. It is now possible to diagnose osteoporosis, assess fracture risk, and reduce that risk with antiresorptive or other available therapies. However, new and more effective approaches are likely to emerge from a better understanding of the regulators of bone cell function.