Estrogen promotes apoptosis of murine osteoclasts mediated by TGF-beta

The estrogen receptor-alpha in osteoclasts mediates the protective effects of estrogens on cancellous but not cortical bone

Estrogens attenuate osteoclastogenesis and stimulate osteoclast apoptosis, but the molecular mechanism and contribution of these effects to the overall antiosteoporotic efficacy of estrogens remain controversial. We selectively deleted the estrogen receptor (ER)alpha from the monocyte/macrophage cell lineage in mice (ERalpha(LysM)(-/-)) and found a 2-fold increase in osteoclast progenitors in the marrow and the number of osteoclasts in cancellous bone, along with a decrease in cancellous bone mass. After loss of estrogens these mice failed to exhibit the expected increase in osteoclast progenitors, the number of osteoclasts in bone, and further loss of cancellous bone. However, they lost cortical bone indistinguishably from their littermate controls. Mature osteoclasts from ERalpha(LysM)(-/-) were resistant to the proapoptotic effect of 17beta-estradiol. Nonetheless, the effects of estrogens on osteoclasts were unhindered in mice bearing an ERalpha knock-in mutation that prevented binding to DNA. Moreover, a polymeric form of estrogen that is not capable of stimulating the nuclear-initiated actions of ERalpha was as effective as 17beta-estradiol in inducing osteoclast apoptosis in cells with the wild-type ERalpha. We conclude that estrogens attenuate osteoclast generation and life span via cell autonomous effects mediated by DNA-binding-independent actions of ERalpha. Elimination of these effects is sufficient for loss of bone in the cancellous compartment in which complete perforation of trabeculae by osteoclastic resorption precludes subsequent refilling of the cavities by the bone-forming osteoblasts. However, additional effects of estrogens on osteoblasts, osteocytes, and perhaps other cell types are required for their protective effects on the cortical compartment, which constitutes 80% of the skeleton.

Bone mineral density and serum levels of soluble tumor necrosis factors, estradiol, and osteoprotegerin in postmenopausal women with cirrhosis after viral hepatitis

CONTEXT

Cirrhosis after viral hepatitis has been identified as a risk factor for osteoporosis in men. However, in postmenopausal women, most studies have evaluated the effect of primary biliary cirrhosis, but little is known about the effect of viral cirrhosis on bone mass [bone mineral density (BMD)] and bone metabolism.

OBJECTIVE

Our objective was to assess the effect of viral cirrhosis on BMD and bone metabolism in postmenopausal women.

DESIGN

We conducted a cross-sectional descriptive study.

SETTING AND PATIENTS

We studied 84 postmenopausal female outpatients with viral cirrhosis and 96 healthy postmenopausal women from the general community. BMD was measured by dual-energy x-ray absorptiometry at lumbar spine (LS) and femoral neck (FN).

RESULTS

The percentage with osteoporosis did not significantly differ between patients (LS, 43.1%; FN, 32.2%) and controls (LS, 41.2%; FN, 29.4%), and there was no difference in BMD (z-score) between groups. Serum concentrations of soluble TNF receptors, estradiol, and osteoprotegerin (OPG) were significantly higher in patients vs. controls (P < 0.001, P < 0.05, and P < 0.05, respectively). No significant difference was observed in urinary deoxypyridinoline. Serum OPG levels were positively correlated with soluble TNF receptors (r = 0.35; P < 0.02) and deoxypyridinoline (r = 0.37; P < 0.05).

CONCLUSIONS

This study shows that bone mass and bone resorption rates do not differ between postmenopausal women with viral cirrhosis and healthy postmenopausal controls and suggests that viral cirrhosis does not appear to increase the risk of osteoporosis in these women. High serum estradiol and OPG concentrations may contribute to preventing the bone loss associated with viral cirrhosis in postmenopausal women.

Minireview: osteoprotective action of estrogens is mediated by osteoclastic estrogen receptor-alpha

The osteoprotective action of estrogen in women has drawn considerable attention because estrogen deficiency-induced osteoporosis became one of the most widely spread diseases in developed countries. In men, the significance of estrogen action for bone health maintenance is also apparent from the osteoporotic phenotype seen in male patients with genetically impaired estrogen signaling. Severe bone loss and high bone turnover, including typical osteofeatures seen in postmenopausal women, can also be recapitulated in rodents after ovariectomy. However, the expected osteoporotic phenotype is not observed in female mice deficient in estrogen receptor (ER)-alpha or -beta or both, even though the degenerative defects are clearly seen in other estrogen target tissues together with up-regulated levels of circulating testosterone. It has also been reported that estrogens may attenuate bone remodeling by cell autonomous suppressive effects on osteoblastogenesis and osteoclastogenesis. Hence, the effects of estrogens in bone appear to be complex, and the molecular role of bone estrogen receptors in osteoprotective estrogen action remains unclear. Instead, it has been proposed that estrogens indirectly control bone remodeling. For example, the enhanced production of cytokines under estrogen deficiency induces bone resorption through stimulation of osteoclastogenesis. However, the osteoporotic phenotype without systemic defects has been recapitulated in female (but not in male) mice by osteoclast-specific ablation of the ERalpha, proving that bone cells represent direct targets for estrogen action. An aberrant accumulation of mature osteoclasts in these female mutants indicates that in females, the inhibitory action of estrogens on bone resorption is mediated by the osteoclastic ERalpha through the shortened lifespan of osteoclasts.

The effects of corticosteroids on bone growth and bone density

OBJECTIVE

To define the effects of both parenteral and inhaled corticosteroids on bone growth in children and the development of osteopenia and osteoporosis in children and adults.

DATA RESOURCES

Articles in PubMed and MEDLINE published from 1983 to 2008 were searched. Keywords used included corticosteroids and bone growth, osteopenia, osteoporosis, and bisphosphonates.

STUDY SELECTION

Publications reviewed include randomized, placebo-controlled studies of both children and adults.

RESULTS

Because systemic and high-dose inhaled corticosteroids affect bone growth of children taking these medications, stadiometry should be used to measure the growth of children. Osteoporosis due to repetitive courses of oral or parenteral corticosteroids and inhaled corticosteroids can develop gradually in the aging adult. Prophylaxis against osteoporosis requires an index of suspicion, assessment of bone density, supplemental calcium and vitamin D, and use of bisphosphonates to prevent bone fractures that could compromise the patient's quality of life.

CONCLUSION

Preventing corticosteroid-induced effects on bone metabolism can allow effective treatment of allergic disease without long-term adverse effects.

Transforming growth factor-beta1 (TGF-beta1) induces human osteoclast apoptosis by up-regulating Bim

Transforming growth factor-beta1 (TGF-beta1) is the most abundant TGF-beta isoform detected in bone and is an important functional modulator of osteoclasts. TGF-beta1 can induce osteoclast apoptosis; however, the apoptotic pathways involved in this process are not known. We show here that human osteoclasts express both type-I and type-II TGF-beta receptors. In the absence of survival factors, TGF-beta1 (1 ng/ml) induced osteoclast apoptosis. The expression of activated caspase-9, but not that of caspase-8, was increased by TGF-beta1 stimulation, and the rate of TGF-beta1-induced apoptosis was significantly lower in the presence of a caspase-9 inhibitor. To study further the mechanisms involved in TGF-beta1-induced osteoclast apoptosis, we investigated TGF-beta1 signaling, which primarily involves the Smad pathway, but also other pathways that may interfere with intracellular modulators of apoptosis, such as mitogen-activated protein (MAP) kinases and Bcl2 family members. We show here that early events consisted of a trend toward increased expression of extracellular signal-regulated kinase (ERK), and then TGF-beta1 significantly induced the activation of p38 and Smad2 in a time-dependent manner. These signaling cascades may activate the intrinsic apoptosis pathway, which involves Bim, the expression of which was increased in the presence of TGF-beta1. Furthermore, the rate of TGF-beta1-induced osteoclast apoptosis was lower when Bim expression was suppressed, and inhibiting the Smad pathway abolished Bim up-regulation following TGF-beta stimulation. This could correspond to a regulatory mechanism involved in the inhibition of osteoclast activity by TGF-beta1.

Increased circulating heat shock protein 60 induced by menopause, stimulates apoptosis of osteoblast-lineage cells via up-regulation of toll-like receptors

Postmenopausal osteoporosis is a heterogeneous disorder characterized by accelerated bone loss after natural or surgical menopause and an increased risk of fractures. The bone loss in estrogen deficiency results from the increased bone resorption and impaired ability of osteoblastic bone formation. Previous studies have reported that the HSP60 stimulates osteoclast formation and bone resorption. Here we found that plasma HSP60 levels were significantly higher in postmenopausal (median 1152.4 ng/ml; range 724.7-2123.4 ng/ml) than in premenopausal (median 316.3 ng/ml; range 164.6-638.4 ng/ml) women. In primary human bone marrow stromal cells (hBMSC) and the HS-5 hBMSC cell line, HSP60 significantly reduced cell viability and increased caspase-dependent apoptosis. Consistent with these observations, HSP60 activated caspase-3 and -9, but not caspase-8 in HS-5 cells, and increased the release of mitochondrial cytochrome c into the cytosol. In addition, HSP60 activated p38 and NFkappaB, but not ERK or JNK; importantly, inhibitors of p38 (SB203580) and NFkappaB (PDTC) abolished HSP60-induced apoptosis. Furthermore, Western blotting showed that HSP60 up-regulated TLR-2 and TLR-4 expression, and pretreatment with blocking antibodies for TLR-2 and TLR-4 almost completely eliminated the effects of HSP60 on apoptosis, caspase-3 and -9 activation, and activation of NFkappaB and p38 MAPK. Most notably, ovariectomy-induced bone loss was attenuated in TLR-2 KO mice. In conclusion, up-regulation of TLR-2 by HSP60 may play a critical role in promoting bone loss in the estrogen-deficient state.

Apoptosis in human reproductive tissues: emerging concepts

In summary, apoptosis is an important concept in understanding many facets of human reproduction. Recent advances in the understanding of molecular mechanisms of apoptosis will allow us to understand this physiologically important process. How can the modulation of this process be applied to human reproduction? Studies to further understand the abnormalities of apoptosis, either too much or too little, may lead to a better understanding of the clinical problems in human reproduction.

We summarize future directions towards further understanding the roles of apoptotic processes in human reproduction in Table 3. The diseases listed in Table 3 are problems which could be approached from the apoptosis point of view. With further study using this concept as the lens, new diagnostic tools or therapies may be developed for these problems.

Both cell-surface and secreted CSF-1 expressed by tumor cells metastatic to bone can contribute to osteoclast activation

Tumors metastatic to the bone produce factors that cause massive bone resorption mediated by osteoclasts in the bone microenvironment. Colony stimulating factor (CSF-1) is strictly required for the formation and survival of active osteoclasts, and is frequently produced by tumor cells. Here we hypothesize that the CSF-1 made by tumor cells contributes to bone destruction in osteolytic bone metastases. We show that high level CSF-1 protected osteoclasts from suppressive effects of transforming growth factor beta (TGF-beta). r3T cells, a mouse mammary tumor cell line that forms osteolytic bone metastases, express abundant CSF-1 in vitro as both a secreted and a membrane-spanning cell-surface glycoprotein, and we show that both the secreted and the cell-surface form of CSF-1 made by r3T cells can support osteoclast formation in co-culture experiments in the presence of RankL. Mice with r3T bone metastases have elevated levels of both circulating and bone-associated CSF-1, and the majority of CSF-1 found in bone metastases is associated with the tumor cells. These results support the idea that tumor-cell produced CSF-1 contributes to osteoclast development and survival in bone metastasis.

Effects of voluntary wheel running on goserelin acetate-induced bone degeneration

A common treatment option for many breast and prostate cancer patients is the use of a luteinizing hormone-releasing hormone agonist such as goserelin acetate (GA) which reduces sex hormone levels. This treatment, however, is associated with bone degeneration, and exercise has been suggested as a means of preventing this side effect. Little is known about the effects of low intensity, low volume exercise on GA-induced bone loss. The purpose of this study, therefore, was to investigate the effects of voluntary wheel running on bone architecture in growing male (M) and female (F) rats receiving GA treatment. Rats received an 8-week GA treatment or placebo (CON) and were either housed in cages equipped with voluntary running wheels (WR) or remained sedentary (SED) in standard cages throughout the experimental period. Following treatments, tibiae were excised and analyzed for cortical bone (cross-sectional volume, cortical volume, marrow volume, cortical thickness) and cancellous bone (bone volume/total volume, trabecular number, trabecular thickness, trabecular spacing) using micro-computed tomography. Treatment with GA resulted in a significant reduction in running wheel distances in both sexes throughout the study period (P<0.05). GA treatment had no effect on cortical bone architecture in neither sex (P>0.05). Cancellous bone degeneration, however, was observed in M and F SED+GA (P<0.05). No significant differences were observed in M WR+GA animals in bone volume/total volume, trabecular number and trabecular spacing when compared to M SED+CON (P>0.05). In F WR+GA, trabecular thickness did not differ from that of F SED+CON (P>0.05), and trabecular spacing was found to be significantly lower than F SED+GA (P<0.05). The current report indicates that 8 weeks of GA treatment promotes cancellous bone degeneration, and voluntary wheel running provides no clear osteoprotection in growing male and female rats.

Prevalence of Secondary Causes of Bone Loss Among Breast Cancer Patients With Osteopenia and Osteoporosis

Purpose

To determine the prevalence of secondary causes of bone loss among patients with breast cancer with osteopenia and osteoporosis.

Patients and Methods

All women referred to a bone health clinic over a 6-year period for bone evaluation were included in this retrospective study and stratified based on presence or absence of a breast cancer history. The prevalence of secondary causes of bone loss in the two groups was compared.

Results

Of the 238 women identified, 64 women had breast cancer. The non–breast cancer group (n = 174) was significantly older (P = .015), had a lower mean weight (P = .019), lower 25 hydroxy-vitamin D level (P = .019), and greater degree of bone loss in both the spine and hip (P < .001 and 0.004, respectively). The presence of at least one secondary cause of bone loss, excluding cancer-related therapies, was seen in 78% of the breast cancer patient group and in 77% of the non–breast cancer group (P = not significant). Newly diagnosed metabolic bone disorders were seen in 58% of the breast cancer population. The most common was vitamin D deficiency, seen in 38% of patients in the breast cancer group and 51% of patients in the non–breast cancer group. Idiopathic hypercalciuria was diagnosed in 15.6%, primary hyperparathyroidism in 1.6%, and normocalcemic hyperparathyroidism in 3.1% of the breast cancer population.

Conclusion

A high prevalence of secondary causes of bone loss among patients with breast cancer supports a comprehensive evaluation in these patients, particularly those considering therapy with an aromatase inhibitor.

Sex hormones, their receptors and bone health

Sex steroids regulate skeletal maturation and preservation in both men and women, as already recognized in the 1940s by Albright and Reifenstein. The impact of gonadal insufficiency on skeletal integrity has been widely recognized in adult men and women ever since. In the context of their skeletal actions, androgens and estrogens are no longer considered as just male and female hormones, respectively. Androgens can be converted into estrogens within the gonads and peripheral tissues and both are present in men and women, albeit in different concentrations. In the late 1980s, sex steroid receptors were discovered in bone cells. However, the understanding of sex steroid receptor activation and translation into biological skeletal actions is still incomplete. Due to the complex metabolism, sex steroids may have not only endocrine but also paracrine and/or autocrine actions. Also, circulating sex steroid concentrations do not necessarily reflect their biological activity due to strong binding to sex hormone binding globulin (SHBG). Finally, sex steroid signaling may include genomic and non-genomic effects in bone and non-bone cells. This review will focus on our current understanding of gonadal steroid metabolism, receptor activation, and their most relevant cellular and biological actions on bone.

HCO3-/Cl- anion exchanger SLC4A2 is required for proper osteoclast differentiation and function

As the only cell capable of bone resorption, the osteoclast is a central mediator of skeletal homeostasis and disease. To efficiently degrade mineralized tissue, these multinucleated giant cells secrete acid into a resorption lacuna formed between their apical membrane and the bone surface. For each proton pumped into this extracellular compartment, one bicarbonate ion remains in the cytoplasm. To prevent alkalinization of the cytoplasm, a basolateral bicarbonate/chloride exchanger provides egress for intracellular bicarbonate. However, the identity of this exchanger is unknown. Here, we report that the bicarbonate/chloride exchanger, solute carrier family 4, anion exchanger, member 2 (SLC4A2), is up-regulated during osteoclast differentiation. Suppression of Slc4a2 expression by RNA interference inhibits the ability of RAW cells, a mouse macrophage cell line, to differentiate into osteoclasts and resorb mineralized matrix in vitro. Accordingly, Slc4a2-deficient mice fail to remodel the primary, cartilaginous skeletal anlagen. Abnormal multinucleated giant cells are present in the bone marrow of Slc4a2-deficient mice. Though these cells express the osteoclast markers CD68, cathepsin K, and NFATc1, compared with their wild-type (WT) counterparts they are larger, fail to express tartrate-resistant acid phosphatase (TRAP) activity, and display a propensity to undergo apoptosis. In vitro Slc4a2-deficient osteoclasts are unable to resorb mineralized tissue and cannot form an acidified, extracellular resorption compartment. These data highlight SLC4A2 as a critical mediator of osteoclast differentiation and function in vitro and in vivo.

Loss of trabeculae by mechano-biological means may explain rapid bone loss in osteoporosis

Osteoporosis is characterized by rapid and irreversible loss of trabecular bone tissue leading to increased bone fragility. In this study, we hypothesize two causes for rapid loss of bone trabeculae; firstly, the perforation of trabeculae is caused by osteoclasts resorbing a cavity so deep that it cannot be refilled and, secondly, the increases in bone tissue elastic modulus lead to increased propensity for trabecular perforation. These hypotheses were tested using an algorithm that was based on two premises: (i) bone remodelling is a turnover process that repairs damaged bone tissue by resorbing and returning it to a homeostatic strain level and (ii) osteoblast attachment is under biochemical control. It was found that a mechano-biological algorithm based on these premises can simulate the remodelling cycle in a trabecular strut where damaged bone is resorbed to form a pit that is subsequently refilled with new bone. Furthermore, the simulation predicts that there is a depth of resorption cavity deeper than which refilling of the resorption pits is impossible and perforation inevitably occurs. However, perforation does not occur by a single fracture event but by continual removal of microdamage after it forms beneath the resorption pit. The simulation also predicts that perforations would occur more easily in trabeculae that are more highly mineralized (stiffer). Since both increased osteoclast activation rates and increased mineralization have been measured in osteoporotic bone, either or both may contribute to the rapid loss of trabecular bone mass observed in osteoporotic patients.

Analysis of the association of polymorphism in the osteoprotegerin gene with susceptibility to chronic kidney disease and periodontitis

BACKGROUND AND OBJECTIVE

Chronic kidney disease (CKD) is a complex disorder, which results in several complications involving disturbance of mineral metabolism. Periodontal disease is an infectious disease that appears to be an important cause of systemic inflammation in CKD patients. Periodontal disease is characterized by clinical attachment loss (CAL) caused by alveolar bone resorption around teeth, which may lead to tooth loss. Osteoprotegerin (OPG) is a key regulator of osteoclastogenesis. Polymorphisms are the main source of genetic variation, and single nucleotide polymorphisms (SNPs) have been reported as major modulators of disease susceptibility. The aim of this study was to investigate the association of a polymorphism located at position -223 in the untranslated region of the OPG gene, previously known as -950, with susceptibility to CKD and periodontal disease.

MATERIAL AND METHODS

A sample of 224 subjects without and with CKD (in hemodialysis) was divided into groups with and without periodontal disease. The OPG polymorphism was analyzed by polymerase chain reaction and restriction fragment length polymorphism.

RESULTS

No association was found between the studied OPG polymorphism and susceptibility to CKD or periodontal disease.

CONCLUSION

It was concluded that polymorphism OPG-223 (C/T) was not associated with CKD and periodontal disease in a Brazilian population. Studies on other polymorphisms in this and other genes of the host response could help to clarify the involvement of bone metabolism mediators in the susceptibility to CKD and periodontal disease.

TGF-beta coordinately activates TAK1/MEK/AKT/NFkB and SMAD pathways to promote osteoclast survival

To better understand the roles of TGF-beta in bone metabolism, we investigated osteoclast survival in response TGF-beta and found that TGF-beta inhibited apoptosis. We examined the receptors involved in promotion of osteoclast survival and found that the canonical TGF-beta receptor complex is involved in the survival response. The upstream MEK kinase TAK1 was rapidly activated following TGF-beta treatment. Since osteoclast survival involves MEK, AKT, and NFkappaB activation, we examined TGF-beta effects on activation of these pathways and observed rapid phosphorylation of MEK, AKT, IKK, IkappaB, and NFkappaB. The timing of activation coincided with SMAD activation and dominant negative SMAD expression did not inhibit NFkappaB activation, indicating that kinase pathway activation is independent of SMAD signaling. Inhibition of TAK1, MEK, AKT, NIK, IKK, or NFkappaB repressed TGF-beta-mediated osteoclast survival. Adenoviral-mediated TAK1 or MEK inhibition eliminated TGF-beta-mediated kinase pathway activation and constitutively active AKT expression overcame apoptosis induction following MEK inhibition. TAK1/MEK activation induces pro-survival BclX(L) expression and TAK1/MEK and SMAD pathway activation induces pro-survival Mcl-1 expression. These data show that TGF-beta-induced NFkappaB activation is through TAK1/MEK-mediated AKT activation, which is essential for TGF-beta to support of osteoclast survival.

JNK/c-Jun signaling mediates an anti-apoptotic effect of RANKL in osteoclasts

INTRODUCTION

RANKL is known to be important not only for differentiation and activation of osteoclasts but also for their survival. Experimentally, apoptosis of osteoclasts is rapidly induced by the deprivation of RANKL. RANKL activates Elk-related tyrosine kinase (ERK), p38, c-Jun N-terminal kinase (JNK), and NF-kappaB pathways through TRAF6 in osteoclasts and the precursor cells. It has been shown that ERK is critical for regulation of osteoclast survival. However, an involvement of other RANKL signaling pathways such as JNK signaling in survival of osteoclasts has not been fully understood yet.

MATERIALS AND METHODS

Osteoclasts derived from primary mouse bone marrow cells by soluble RANKL (sRANKL) were treated with a JNK inhibitor, SP600125, or infected with adenovirus carrying dominant-negative (DN)-c-jun, DN-c-fos, mitogen-activated protein kinase kinase 1 (MEKK1), I-kappaBalpha mutant, or NF-kappaB components, p50 and p65. Osteoclasts were cultured with or without sRANKL, and apoptotic phenotype was determined by TUNEL assay, DAPI staining, and expression of cleaved caspase 3 followed by TRACP staining.

RESULTS

Overexpression of TRAF6 activated JNK and NF-kappaB signaling pathways and clearly prevented osteoclasts from apoptosis caused by abrogation of sRANKL. An anti-apoptotic effect of RANKL/RANK/TRAF6 signaling on osteoclast was inhibited by JNK-specific inhibitor SP600125 and by overexpression of dominant-negative JNK1, c-jun, and c-fos. Also, overexpression of MEKK1 inhibited apoptosis of osteoclasts even in the absence of sRANKL along with activation of JNK/c-jun signaling. On the other hand, blockade of NF-kappaB signaling by I-kappaBalpha mutant or overexpression of NF-kappaB components showed a marginal effect on apoptosis of osteoclasts.

CONCLUSIONS

An important role of RANKL-induced activation of MEKK1/JNK/c-jun signaling in the regulation of apoptosis in osteoclasts was shown. Our study suggests that c-fos plays a role as a partner of activator protein-1 factor, c-jun, during the regulation of apoptosis in osteoclasts.

Estrogen-TGFbeta cross-talk in bone and other cell types: role of TIEG, Runx2, and other transcription factors

It is well established that E(2) and TGFbeta have major biological effects in multiple tissues, including bone. The signaling pathways through which these two factors elicit their effects are well documented. However, the interaction between these two pathways and the potential consequences of cross-talk between E(2) and TGFbeta continue to be elucidated. In this prospectus, we present known and potential roles of TIEG, Runx2, and other transcription factors as important mediators of signaling between these two pathways.

Materials in particulate form for tissue engineering. 2. Applications in bone

Materials in particulate form have been the subjects of intensive research in view of their use as drug delivery systems. While within this application there are still issues to be addressed, these systems are now being regarded as having a great potential for tissue engineering applications. Bone repair is a very demanding task, due to the specific characteristics of skeletal tissues, and the design of scaffolds for bone tissue engineering presents several difficulties. Materials in particulate form are now seen as a means of achieving higher control over parameters such as porosity, pore size, surface area and the mechanical properties of the scaffold. These materials also have the potential to incorporate biologically active molecules for release and to serve as carriers for cells. It is believed that the combination of these features would create a more efficient approach towards regeneration. This review focuses on the application of materials in particulate form for bone tissue engineering. A brief overview of bone biology and the healing process is also provided in order to place the application in its broader context. An original compilation of molecules with a documented role in bone tissue biology is listed, as they have the potential to be used in bone tissue engineering strategies. To sum up this review, examples of works addressing the above aspects are presented.

Inhibitory effect of Zn2+ in zinc-containing beta-tricalcium phosphate on resorbing activity of mature osteoclasts

Long term effect of the growing instability of the bone-implant interface due to bone resorption at the interface is a problem for the implants, including bioactive ceramics. Zn2+ -containing tricalcium phosphate (ZnTCP) is a material which may overcome this problem. The present study aims to clarify whether Zn2+ -containing tricalcium phosphate (ZnTCP) ceramics with a Zn2+ content of 0.316 (ZnTCP316) and 0.633 (ZnTCP633) wt % suppress resorption by mature osteoclasts in vitro. Suppression would be due to an increase in the number of apoptotic osteoclasts and the inhibition of the resorbing activity of osteoclasts, the latter being the major mechanism of the suppression. The number of apoptotic osteoclasts was significantly 6.3 times higher with ZnTCP633 than with tricalcium phosphate ceramic (TCP) after 24-h culture. The net contribution to resorption of this change in apoptotic cell numbers is much smaller than that of the change in resorbing activity. The osteoclasts cultured on ZnTCP formed fewer actin rings than those cultured on the TCP. The mRNA expression of CAII and cathepsin K/OC2 in the osteoclasts on ZnTCP633 was downregulated 0.5-fold and 0.6-fold, respectively, compared with that on the TCP. The volume of resorption pits was downregulated 0.4-fold in the ZnTCP633 than that in TCP. These results suggest that ZnTCPs suppressed the resorbing activity of mature osteoclasts probably through a local increase in the level of Zn2+. Bone substitutes or coating layers containing ZnTCP would be promising biomaterials from the viewpoint of counteracting osteoclastic bone resorption at the bone-implant interface.

Enamel matrix derivative induces connective tissue growth factor expression in human osteoblastic cells

BACKGROUND

Enamel matrix derivative (EMD) stimulates the production of transforming growth factor-beta (TGF-beta), which has been suggested to play a role in mediating the effects of EMD in periodontal tissue regeneration. Connective tissue growth factor (CTGF) is a mediator of TGF-beta and promotes cell development. The interaction between EMD and CTGF is unknown. This study explored the effects of EMD on CTGF expression in human osteoblastic cells and whether the interaction is modulated by the TGF-beta signaling pathway. Also, the roles of CTGF in cell proliferation, cell cycle progression, and mineralized nodule formation of EMD-induced osteoblastic cultures were examined.

METHODS

Human osteoblastic cells (Saos-2) were treated with 25 to 100 microg/ml EMD with or without the addition of TGF-beta inhibitor. CTGF mRNA expression was detected by reverse transcription-polymerase chain reaction (RT-PCR), and CTGF protein levels were assayed by Western blot analysis. In addition, cell cycle progression and DNA synthesis were determined by flow cytometry and 5-bromo-2'-deoxyuridine (BrdU) incorporation following EMD treatment with or without CTGF antibody. Mineralization was examined by alizarin red staining and quantified by elution with cetylpyridinium chloride.

RESULTS

Western blot and RT-PCR analysis demonstrated a dose-dependent increase of CTGF expression by EMD. EMD-induced CTGF expression was reduced significantly in the presence of TGF-beta inhibitor. Cell cycle and BrdU analysis revealed an increase in cell proliferation following EMD treatment, which was due to an increase in the percentage of cells in the G2/M phase of the cell cycle. No significant effect was found when anti-CTGF antibody was added. Conversely, mineralization was inhibited significantly in EMD-treated cultures in the presence of anti-CTGF antibody.

CONCLUSIONS

EMD stimulates CTGF expression, and the interaction is modulated via TGF-beta in osteoblastic cells. Also, CTGF affects EMD-induced osteoblastic mineralization but not cell proliferation. To our knowledge, these results provide novel insight into EMD-CTGF interaction, two biomodifiers that have therapeutic relevance to tissue engineering and regeneration.

Osteoclast receptors and signaling

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. Besides the well known Receptor Activator of Nuclear factor-kappaB (RANK), RANK ligand and osteoprotegerin axis, a variety of factors tightly regulate osteoclast formation, adhesion, polarization, motility, resorbing activity and life span, maintaining bone resorption within physiological ranges. Receptor-mediated osteoclast regulation is rather complex. Nuclear receptors, cell surface receptors, integrin receptors and cell death receptors work together to control osteoclast activity and prevent both reduced or increased bone resorption. Here we will discuss the signal transduction pathways activated by the main osteoclast receptors, integrating their function and mechanisms of action.

Estrogen deficiency, T cells and bone loss

Estrogen plays a fundamental role in the maintenance of skeletal homeostasis. Although estrogen is established to have direct effects on bone cells, animal studies have identified additional regulatory effects of estrogen centered at the level of the adaptive immune response. Furthermore, a potential role for reactive oxygen species has now been identified in both humans and animals. One of the major challenges has been to integrate a multitude of redundant pathways and cytokines, that all appear capable of playing a relevant role, into a global model of postmenopausal osteoporosis. This review presents our current understanding of the process of estrogen deficiency mediated bone destruction and explores some of the most recent findings and hypotheses to explain estrogen action in bone.

Detection of apoptosis of bone cells in vitro

Studies during the last decade demonstrated that apoptosis is as important as mitosis for the growth and maintenance of the skeleton and provided information on the significance and molecular regulation of apoptosis of bone cells. It is now known that: (1) all osteoclasts die by apoptosis after completing a bone resorption cycle; (2) the majority of osteoblasts also die, whereas the remainder become lining cells or osteocytes; and (3) osteocytes, although long-living cells, also can die prematurely. Furthermore, mounting evidence indicates that systemic hormones, local growth factors, cytokines, and pharmacological agents, as well as mechanical forces regulate the rate of bone cell apoptosis. This chapter summarizes the methods developed in the last few years to examine apoptosis of cultured bone cells and identify the signaling pathways and molecules involved in apoptosis regulation by diverse skeletal stimuli.

The osteoclast: friend or foe?

Bone is a dynamic organ constantly remodeled to support calcium homeostasis and structural needs. The osteoclast is the cell responsible for removing both the organic and inorganic components of bone. It is derived from hematopoietic progenitors in the macrophage lineage and differentiates in response to the tumor necrosis factor family cytokine receptor activator of NF kappa B ligand. alpha v beta 3 integrin mediates cell adhesion necessary for polarization and formation of an isolated, acidified resorptive microenvironment. Defects in osteoclast function, whether genetic or iatrogenic, may increase bone mass but lead to poor bone quality and a high fracture risk. Pathological stimulation of osteoclast formation and resorption occurs in postmenopausal osteoporosis, inflammatory arthritis, and metastasis of tumors to bone. In these diseases, osteoclast activity causes bone loss that leads to pain, deformity, and fracture. Thus, osteoclasts are critical for normal bone function, but their activity must be controlled.

T cells and post menopausal osteoporosis in murine models

Estrogen deficiency is one of the most frequent causes of osteoporosis in women and a possible cause of bone loss in men. But the mechanism involved remains largely unknown. Estrogen deficiency leads to an increase in the immune function, which culminates in an increased production of tumor necrosis factor (TNF) by activated T cells. TNF increases osteoclast formation and bone resorption both directly and by augmenting the sensitivity of maturing osteoclasts to the essential osteoclastogenic factor RANKL (the RANK ligand). Increased T cell production of TNF is induced by estrogen deficiency via a complex mechanism mediated by antigen presenting cells and the cytokines IFNγ, IL-7 and transforming growth factor-β. The experimental evidence that suggests that estrogen prevents bone loss by regulating T cell function and the interactions between immune cells and bone is reviewed here.

Antiresorptive agents and osteoclast apoptosis

Antiresorptive agents have proven to be effective therapies for the treatment of bone diseases associated with excessive osteoclast activity. Decreased osteoclast formation, inhibition of osteoclast actions, and reduced osteoclast survival represent mechanisms by which antiresorptive agents could act. The goals of this article are to present the evidence that antiresorptive agents can decrease osteoclast survival through apoptosis, to review the mechanisms by which they are thought to activate the apoptotic process, and to consider whether the actions on apoptosis fully account for the antiresorptive effects. As background, the apoptotic process will be briefly summarized together with the evidence that factors that promote osteoclast survival affect steps in the process. Following this, therapeutic agents that are both antiresorptive and can stimulate osteoclast apoptosis will be discussed. Other bone therapeutic agents that are either antiresorptive or apoptotic, but not both, will be described. Finally, newer antiresorptive compounds that elicit apoptosis and could represent potential therapeutic agents will be noted.

Quantifying osteoblast and osteocyte apoptosis: challenges and rewards

Since the initial demonstration of the phenomenon in murine and human bone sections approximately 10 yr ago, appreciation of the biologic significance of osteoblast apoptosis has contributed greatly not only to understanding the regulation of osteoblast number during physiologic bone remodeling, but also the pathogenesis of metabolic bone diseases and the pharmacology of some of the drugs used for their treatment. It is now appreciated that all major regulators of bone metabolism including bone morphogenetic proteins (BMPs), Wnts, other growth factors and cytokines, integrins, estrogens, androgens, glucocorticoids, PTH and PTH-related protein (PTHrP), immobilization, and the oxidative stress associated with aging contribute to the regulation of osteoblast and osteocyte life span by modulating apoptosis. Moreover, osteocyte apoptosis has emerged as an important regulator of remodeling on the bone surface and a critical determinant of bone strength, independently of bone mass. The detection of apoptotic osteoblasts in bone sections remains challenging because apoptosis represents only a tiny fraction of the life span of osteoblasts, not unlike a 6-mo-long terminal illness in the life of a 75-yr-old human. Importantly, the phenomenon is 50 times less common in human bone biopsies because human osteoblasts live longer and are fewer in number. Be that as it may, well-controlled assays of apoptosis can yield accurate and reproducible estimates of the prevalence of the event, particularly in rodents where there is an abundance of osteoblasts for inspection. In this perspective, we focus on the biological significance of the phenomenon for understanding basic bone biology and the pathogenesis and treatment of metabolic bone diseases and discuss limitations of existing techniques for quantifying osteoblast apoptosis in human biopsies and their methodologic pitfalls.

Chloroform extract of deer antler inhibits osteoclast differentiation and bone resorption

It has been reported that deer antler extract has anti-bone resorptive activity in vivo. However, little is known about the cellular and molecular mechanism of this effect. In this study, we investigated the effects of deer antler extracts on osteoclast differentiation and bone-resorption in vitro. Chloroform extract (CE-C) of deer antler inhibited osteoclast differentiation in mouse bone marrow cultures stimulated by receptor activator of NF-kappaB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF). CE-C suppressed the activation of extracellular signal-regulated kinase (ERK), protein kinase B (PKB/Akt) and inhibitor of kappa B (I-kappaB) by RANKL in osteoclast precursor cells. It also inhibited the bone resorptive activity of differentiated osteoclasts that was accompanied by disruption of actin rings and induction of the apoptosis. These results demonstrate deer antler extract may be a useful remedy for the treatment of bone-resorption diseases such as osteoporosis.

Decrease in the number and apoptosis of alveolar bone osteoclasts in estrogen-treated rats

BACKGROUND AND OBJECTIVE

Bone is a mineralized tissue that is under the influence of several systemic, local and environmental factors. Among systemic factors, estrogen is a hormone well known for its inhibitory function on bone resorption. As alveolar bone of young rats undergoes continuous and intense remodeling to accommodate the growing and erupting tooth, it is a suitable in vivo model for using to study the possible action of estrogen on bone. Thus, in an attempt to investigate the possibility that estrogen may induce the death of osteoclasts, we examined the alveolar bone of estrogen-treated rats.

MATERIAL AND METHODS

Fifteen, 22-d-old female rats were divided into estrogen, sham and control groups. The estrogen group received estrogen and the sham group received corn oil used as the dilution vehicle. After 8 d, fragments containing alveolar bone were removed and processed for light microscopy and transmission electron microscopy. Sections were stained with hematoxylin and eosin and tartrate-resistant acid phosphatase (TRAP)-an osteoclast marker. Quantitative analysis of the number of TRAP-positive osteoclasts per mm of bone surface was carried out. For detecting apoptosis, sections were analyzed by the Terminal deoxynucleotidyl transferase-mediated dUTP Nick-End Labeling (TUNEL) method; TUNEL/TRAP combined methods were also used.

RESULTS

The number of TRAP-positive osteoclasts per mm of bone surface was significantly reduced in the estrogen group compared with the sham and control groups. TRAP-positive osteoclasts exhibiting TUNEL-positive nuclei were observed only in the estrogen group. In addition, in the estrogen group the ultrastructural images revealed shrunken osteoclasts exhibiting nuclei with conspicuous and tortuous masses of condensed chromatin, typical of apoptosis.

CONCLUSION

Our results reinforce the idea that estrogen inhibits bone resorption by promoting a reduction in the number of osteoclasts, thus indicating that this reduction may be, at least in part, a consequence of osteoclast apoptosis.

Weaning triggers a decrease in receptor activator of nuclear factor-kappaB ligand expression, widespread osteoclast apoptosis, and rapid recovery of bone mass after lactation in mice

A significant portion of milk calcium comes from the mother's skeleton, and lactation is characterized by rapid bone loss. The most remarkable aspect of this bone loss is its complete reversibility, and the time after weaning is the most rapid period of skeletal anabolism in adults. Despite this, little is known of the mechanisms by which the skeleton repairs itself after lactation. We examined changes in bone and calcium metabolism defining the transition from bone loss to bone recovery at weaning in mice. Bone mass decreases during lactation and recovers rapidly after weaning. Lactation causes changes in bone microarchitecture, including thinning and perforation of trabecular plates that are quickly repaired after weaning. Weaning causes a rapid decline in urinary C-telopeptide levels and stimulates an increase in circulating levels of osteocalcin. Bone histomorphometry documented a significant reduction in the numbers of osteoclasts on d 3 after weaning caused by a coordinated wave of osteoclast apoptosis beginning 48 h after pup removal. In contrast, osteoblast numbers and bone formation rates, which are elevated during lactation, remain so 3 d after weaning. The cessation of lactation stimulates an increase in circulating calcium levels and a reciprocal decrease in PTH levels. Finally, weaning is associated with a decrease in levels of receptor activator of nuclear factor-kappaB ligand mRNA in bone. In conclusion, during lactation, bone turnover is elevated, and bone loss is rapid. Weaning causes selective apoptosis of osteoclasts halting bone resorption. The sudden shift in bone turnover favoring bone formation subsequently contributes to the rapid recovery of bone mass.

Ovariectomy-induced bone loss occurs independently of B cells

Estrogen withdrawal is associated with a significant expansion in B cell precursor and mature B cell populations. However, despite significant circumstantial evidence the role of B lineage cells in ovariectomy-induced bone loss in vivo is unclear. In vitro studies have demonstrated that mature B cells have the potential to both positively and negatively impact osteoclastogenesis by virtue of their capacity to secrete pro-osteoclastogenic cytokines including receptor activator of NFkappaB ligand (RANKL), as well as anti-osteoclastogenic cytokines such as osteoprotegerin (OPG) and transforming growth factor beta (TGFbeta). Although several studies have suggested that expansion of the B lineage following ovariectomy may play a key role in the etiology of ovariectomy-induced bone loss, in vivo studies to directly test this notion have yet to be conducted. In this study, we performed ovariectomy on microMT(-/-) mice which are specifically deficient in mature B cells. Analysis of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) and micro-computed tomography (CT) demonstrate that mature B cell-deficient mice undergo an identical loss of bone mass relative to wild-type (WT) control mice. Our data demonstrate that mature B cells are not central mediators of ovariectomy-induced bone loss in vivo.

Strategies for the Prevention of Treatment-Related Bone Loss in Women Receiving Adjuvant Hormonal Therapy

More than 220,000 women will be diagnosed with breast cancer this year, and approximately 75% of these women will be long-term survivors of this disease. Survival has improved largely because of advances in adjuvant hormone therapy and chemotherapy, as well as early detection strategies. Because most women will receive adjuvant treatment, and the majority will survive cancer, it is increasingly important to understand the resultant toxicities and to devise monitoring and treatment strategies to avoid adverse long-term effects. Loss of bone mineral density leading to osteoporosis and increased risk of fracture as well as other morbidities is a well known complication of estrogen suppression associated with use of aromatase inhibitors (AIs) in postmenopausal women, and ovarian suppression with GnRH agonists or chemotherapy in premenopausal women. Hormone receptor positivity is increasingly frequent with increasing patient age, so that a large number of women already at risk for osteopenia associated with menopause are at risk for further bone loss caused by adjuvant hormone therapy with AIs. This article will review data on bone mineral density loss and risk of fracture in the large, randomized phase III trials comparing tamoxifen to AIs using the upfront, switching or extended hormone therapy approach. Data from prophylactic bisphosphonate intervention trials in both post- and premenopausal women will be discussed. Ongoing trials are described.

Osteoclasts prefer aged bone

We investigated whether the age of the bones endogenously exerts control over the bone resorption ability of the osteoclasts, and found that osteoclasts preferentially develop and resorb bone on aged bone. These findings indicate that the bone matrix itself plays a role in targeted remodeling of aged bones.

INTRODUCTION

Osteoclasts resorb aging bone in order to repair damage and maintain the quality of bone. The mechanism behind the targeting of aged bone for remodeling is not clear. We investigated whether bones endogenously possess the ability to control osteoclastic resorption.

METHODS

To biochemically distinguish aged and young bones; we measured the ratio between the age-isomerized betaCTX fragment and the non-isomerized alphaCTX fragment. By measurement of TRACP activity, CTX release, number of TRACP positive cells and pit area/pit number, we evaluated osteoclastogenesis as well as osteoclast resorption on aged and young bones.

RESULTS

We found that the alphaCTX/betaCTX ratio is 3:1 in young compared to aged bones, and we found that both alpha and betaCTX are released by osteoclasts during resorption. Osteoclastogenesis was augmented on aged compared to young bones, and the difference was enhanced under low serum conditions. We found that mature osteoclasts resorb more on aged than on young bone, despite unchanged adhesion and morphology.

CONCLUSIONS

These data indicate that the age of the bone plays an important role in controlling osteoclast-mediated resorption, with significantly higher levels of osteoclast differentiation and resorption on aged bones when compared to young bones.

NF-kappaB p50 and p52 regulate receptor activator of NF-kappaB ligand (RANKL) and tumor necrosis factor-induced osteoclast precursor differentiation by activating c-Fos and NFATc1

Postmenopausal osteoporosis and rheumatoid joint destruction result from increased osteoclast formation and bone resorption induced by receptor activator of NF-kappaB ligand (RANKL) and tumor necrosis factor (TNF). Osteoclast formation induced by these cytokines requires NF-kappaB p50 and p52, c-Fos, and NFATc1 expression in osteoclast precursors. c-Fos induces NFATc1, but the relationship between NF-kappaB and these other transcription factors in osteoclastogenesis remains poorly understood. We report that RANKL and TNF can induce osteoclast formation directly from NF-kappaB p50/p52 double knockout (dKO) osteoclast precursors when either c-Fos or NFATc1 is expressed. RANKL- or TNF-induced c-Fos up-regulation and activation are abolished in dKO cells and in wild-type cells treated with an NF-kappaB inhibitor. c-Fos expression requires concomitant RANKL or TNF treatment to induce NFATc1 activation in the dKO cells. Furthermore, c-Fos expression increases the number and resorptive capacity of wild-type osteoclasts induced by TNF in vitro. We conclude that NF-kappaB controls early osteoclast differentiation from precursors induced directly by RANKL and TNF, leading to activation of c-Fos followed by NFATc1. Inhibition of NF-kappaB should prevent RANKL- and TNF-induced bone resorption.

Low transforming growth factor-beta1 serum levels in idiopathic male osteoporosis

INTRODUCTION

Although the etiology of osteoporosis is different between men and women, the underlying pathophysiological mechanism is similar, namely an absolute or relative increase in bone resorption, leading to progressive bone loss. Transforming growth factor (TGF)-beta1 is a growth factor in human bone, which is produced by osteoblasts, and which has various effects on osteoclasts and osteoblasts. The aim of our study was to determine serum TGF-beta1 levels in male patients with idiopathic osteoporosis.

METHODS

Twenty five males with idiopathic osteoporosis and 25 age-matched controls were studied. Osteoporosis was defined by a T score of <-2.5 in the lumbar spine or at the femoral neck. We measured levels of TGF-beta1, estradiol, total and bioactive testosterone. Various markers of bone remodeling were also measured.

RESULTS

TGF-beta1 was significantly lower in osteoporotic patients than in controls (3.706 ng/dl, 25-75 percentiles: 2.81-5.33 vs 8.659 ng/dl, 25-75 percentiles: 4.837-11.835; p=0.000). Moreover, TGF-beta1 levels were positively correlated with bone mineral density (BMD) at the femoral neck (r=0.439, p=0.028), and at the lumbar spine (r=0.41, p=0.042). No correlation was found between serum estradiol, testosterone and TGF-beta1 levels.

DISCUSSION

Serum TGF-beta1 levels are depressed in osteoporotic men and are positively correlated with hip and spine BMD. The results of our study suggest that TGF-beta1 may play a role in the pathogenesis of idiopathic male osteoporosis.

Determination of oxidative stress status and concentration of TGF-beta 1 in the blood and saliva of osteoporotic subjects

Preliminary reports indicate the influence of oxidative stress and interleukins, particularly TGF-beta1, in maintenance of bone mass. This study was designed to determine any possible variations of cellular lipid peroxidation, the total antioxidant power, and concentration of TGF-beta1 in blood and saliva of osteoporotic subjects in comparison to healthy people. Blood and saliva samples of 22 osteoporotic women and 22 age-matched healthy women were collected. Samples were analyzed for thiobarbituric acid-reactive substances (TBARS) as a marker of lipid peroxidation, ferric reducing ability (total antioxidant power, TAP), and concentration of TGF-beta1. The blood and saliva TAP (mean +/- SD) of osteoporotic subjects was significantly lower than that of healthy controls (606.65 +/- 119.13 vs. 665.64 +/- 63.73 mmol/L and 560.43 +/- 84.70 vs. 612.05 +/- 81.5, respectively). Blood and saliva TBARS (mean +/- SD) of osteoporotic subjects were significantly higher than those of healthy controls (0.30 +/- 0.04 vs. 0.26 +/- 0.04 and 0.23 +/- 0.03 vs. 0.16 +/- 0.04 micromol/L, respectively). Concentrations of TGF-beta1 (mean +/- SD) in plasma and saliva of osteoporotic subjects were not different in comparison to healthy subjects. Results indicate that persons with osteoporosis have an increased oxidative stress that is not accompanied by changes in TGF-beta1 levels. Use of supplementary antioxidants in osteoporotic patients may be helpful.

CYP19A1 polymorphisms are associated with bone mineral density in Chinese men

Aromatase-dependent biosynthesis of estrogen plays an important role in maintenance of the male skeleton, and Cytochrome p450 aromatase is the key enzyme to catalyze the conversion of androgen precursors to estrogens. We investigated the association of polymorphisms in the CYP19A1 gene and bone mineral density in a Chinese cohort. 2392 extreme low femoral neck BMD cases or extreme high femoral neck BMD controls were selected from a population-based cohort and genotyped for eight SNPs in the CYP19A1 gene. Significant associations for rs17703883, rs12594287 and rs16964201 SNPs with BMD were found in men only. Men with TC/CC genotypes in the rs17703883 SNP had a 1.5 times higher risk of having extreme low femoral neck BMD (P = 0.003, empirical P value = 0.05), and decreased BMDs at total body (P = 0.004, empirical P value = 0.07) and total hip (P = 0.003, empirical P value = 0.05). Men carrying AA/AG genotypes in the rs12594287 SNP had a 30% reduced risk of having extreme low femoral neck BMD (P = 0.007, empirical P value = 0.12), and increased BMDs at total body (P = 0.0009, empirical P value = 0.018) and total hip (P = 0.001, empirical P value = 0.02). Men carrying TT/TC genotypes in the rs16964201 SNP had a 40% reduced risk of having extreme low femoral neck BMD (P = 0.005, empirical P value = 0.087), and increased BMDs at total body (P = 0.0001, empirical P value = 0.002) and total hip (P = 0.0006, empirical P value = 0.012). Haplotype analysis showed that the G-C-T-A-T haplotype was significantly related to higher BMD. Our finding suggests that genetic variations in the CYP19A1 gene are significantly associated with BMD at different skeletal sites in adult men, but not in women.

B cells and T cells are critical for the preservation of bone homeostasis and attainment of peak bone mass in vivo

Bone homeostasis is regulated by a delicate balance between osteoblastic bone formation and osteoclastic bone resorption. Osteoclastogenesis is controlled by the ratio of receptor activator of NF-kappaB ligand (RANKL) relative to its decoy receptor, osteoprotegerin (OPG). The source of OPG has historically been attributed to osteoblasts (OBs). While activated lymphocytes play established roles in pathological bone destruction, no role for lymphocytes in basal bone homeostasis in vivo has been described. Using immunomagnetic isolation of bone marrow (BM) B cells and B-cell precursor populations and quantitation of their OPG production by enzyme-linked immunosorbent assay (ELISA) and real-time reverse transcriptase-polymerase chain reaction (RT-PCR), cells of the B lineage were found to be responsible for 64% of total BM OPG production, with 45% derived from mature B cells. Consistently B-cell knockout (KO) mice were found to be osteoporotic and deficient in BM OPG, phenomena rescued by B-cell reconstitution. Furthermore, T cells, through CD40 ligand (CD40L) to CD40 costimulation, promote OPG production by B cells in vivo. Consequently, T-cell-deficient nude mice, CD40 KO mice, and CD40L KO mice display osteoporosis and diminished BM OPG production. Our data suggest that lymphocytes are essential stabilizers of basal bone turnover and critical regulators of peak bone mass in vivo.

The differential effects of bisphosphonates, SERMS (selective estrogen receptor modulators), and parathyroid hormone on bone remodeling in osteoporosis

Osteoporosis is a skeletal metabolic disease characterized by a compromised bone fragility, leading to an increased risk of developing spontaneous and traumatic fractures. Osteoporosis is considered a multifactorial disease and fractures are the results of several different risk factors both extra- and intraskeletal. Thus bone fragility can be the end point of several different causes: a) failure to reach an optimal peak bone mass during growth; b) excessive bone resorption resulting in decreased bone mass and microarchitectural deterioration; c) inadequate formation upon an increased resorption during the process of bone remodeling. The pharmacological therapeutical options, available to date, are directed on prevention of fractures. The aim of this paper is to describe the activities and the mechanisms of action, as known at present, of the most used therapies for osteoporosis and their clinical implications. Improvement of knowledge in this field will allow us to further improve therapeutical choices and pharmacological interventions.

Osteoinductivity of demineralized bone matrix in immunocompromised mice and rats is decreased by ovariectomy and restored by estrogen replacement

The osteoinduction potential of human demineralized bone matrix (DBM) in females with low estrogen (E2) is unknown. Moreover, the osteoinductivity of commercial human DBM is tested in male athymic rats and mice, but DBM performance in these animals may not reflect performance in female animals or provide information on E2's role in the process. To gain insight, human DBM was implanted bilaterally in the gastrocnemius of twenty-four athymic female mice (10 mg/implant) and twenty-four athymic female rats (15 mg/implant). Eight animals in each group were sham-operated (SHAM), ovariectomized (OVX), or ovariectomized with E2-replacement (OVX+E2) via subcutaneous slow release capsules of 17beta-estradiol. OVX and OVX+E2 animals were pair-fed to SHAM animals. Four animals from each group were euthanized at 35 days and four at 56 days. Animal weight, uterine weight, and blood estrogen levels confirmed that pair feeding, ovariectomy, and E2 replacement were successful. Histological sections of implanted tissues were evaluated qualitatively for absence or presence of DBM, ossicle formation, and new bone or cartilage using a previously developed qualitative scoring system (QS) and by histomorphometry to obtain a quantitative assessment of osteoinduction. OVX mice had a small but significant QS decrease at 35 days compared to SHAM mice, confirmed by quantitative measurement of ossicle, marrow space, and new bone areas. The QS in rats was not affected by OVX but histomorphometry showed decreased new bone in OVX rats, which was restored by E2. The QS indicated that the number of new bone sites was not reduced by OVX in rats or mice at 56 days, but the relative amount of new bone v. marrow space was affected and differed with animal species. Residual DBM was less in OVX animals, indicating that DBM resorption was affected. Cartilage was present in rats but not in mice, suggesting that endochondral ossification was slower and indicating that bone graft studies in these species are not necessarily comparable. These results show the importance of E2 in human DBM-induced bone formation and suggest that E2 may be needed for clinical effectiveness in post-menopausal women.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

RANKL expression in rat periodontal ligament subjected to a continuous orthodontic force

OBJECTIVES

This study investigated longitudinal changes in receptor activator NF kappa B ligand (RANKL) expression in periodontal ligament (PDL) cells subjected to a continuous orthodontic force.

DESIGN

Fifty-five-day-old male Wistar rats were divided into experimental and control groups. The experimental group had the first molars laterally expanded by a continuous orthodontic force. In each group, the horizontal section specimens were embedded in OTC compound and frozen at 0, 1, 3 and 7 days after the expansion. Sections were observed by immunostaining with anti-RANKL and the tartrate-resistant acid phosphatase (TRAP) staining.

RESULT

Immunoreaction of RANKL and TRAP-positive cells were observed in the distal region of the controls and on the compressed side of the expansion group in the 3 and 7 days. Immunoreaction of RANKL was also observed after 1 day on the compression side of the expansion group, but here TRAP-positive cells were few.

CONCLUSIONS

The experiments have showed that PDL cells are continuously producing RANKL on the PDL pressure side of rats subjected to mechanical stress with a continuous orthodontic force, there was no noticeable the excessive appearance of osteoclasts however. Considering this, it is expected that not only RANKL production but also other cytokines play an important role in the balancing adjustment in the alveolar bone remodeling.

Skeletal complications of breast cancer therapies

Nonsurgical treatment options, such as hormonal therapy, chemotherapy, radiation, and bisphosphonate therapy, are undoubtedly improving outcomes for women with breast cancer; however, these therapies also carry significant skeletal side effects. For example, adjuvant hormonal treatments, such as aromatase inhibitors that disrupt the estrogen-skeleton axis, have the potential to cause decreased bone mineral density. Similarly, chemotherapy often induces primary ovarian failure in premenopausal women, resulting in decreased levels of circulating estrogen and subsequent osteopenia. In both cases, women receiving these therapies are at an increased risk for the development of osteoporosis and skeletal fracture. Furthermore, women undergoing radiation therapy to the upper body may have an increased incidence of rib fracture, and those receiving bisphosphonates may be vulnerable to the development of osteonecrosis of the jaw. Therefore, women with breast cancer who are undergoing any of these therapies should be closely monitored for bone mineral loss and advised of skeletal health maintenance strategies.