Effect of osteoblast-targeted expression of bcl-2 in bone: differential response in male and female mice

Insights and implications of sexual dimorphism in osteoporosis

Osteoporosis, a metabolic bone disease characterized by low bone mineral density and deterioration of bone microarchitecture, has led to a high risk of fatal osteoporotic fractures worldwide. Accumulating evidence has revealed that sexual dimorphism is a notable feature of osteoporosis, with sex-specific differences in epidemiology and pathogenesis. Specifically, females are more susceptible than males to osteoporosis, while males are more prone to disability or death from the disease. To date, sex chromosome abnormalities and steroid hormones have been proven to contribute greatly to sexual dimorphism in osteoporosis by regulating the functions of bone cells. Understanding the sex-specific differences in osteoporosis and its related complications is essential for improving treatment strategies tailored to women and men. This literature review focuses on the mechanisms underlying sexual dimorphism in osteoporosis, mainly in a population of aging patients, chronic glucocorticoid administration, and diabetes. Moreover, we highlight the implications of sexual dimorphism for developing therapeutics and preventive strategies and screening approaches tailored to women and men. Additionally, the challenges in translating bench research to bedside treatments and future directions to overcome these obstacles will be discussed.

miR-126 mitigates the osteogenic differentiation of human bone marrow-derived mesenchymal stem cells by targeting the ERK1/2 and Bcl-2 pathways

Human bone marrow mesenchymal stem cells (hBMMSCs) are a promising cell source for bone engineering owing to their high potential to differentiate into osteoblasts. The objective of the present study is to assess microRNA-126 (miR-126) and examine its effects on the osteogenic differentiation of hBMMSCs. In this study, we investigate the role of miR-126 in the progression of osteogenic differentiation (OD) as well as the apoptosis and inflammation of hBMMSCs during OD induction. OD is induced in hBMMSCs, and matrix mineralization along with other OD-associated markers are evaluated by Alizarin Red S (AR) staining and quantitative PCR (qPCR). Gain- and loss-of-function studies are performed to demonstrate the role of miR-126 in the OD of hBMMSCs. Flow cytometry and qPCR-based cytokine expression studies are performed to investigate the effect of miR-126 on the apoptosis and inflammation of hBMMSCs. The results indicate that miR-126 expression is downregulated during the OD of hBMMSCs. Gain- and loss-of function assays reveal that miR-126 upregulation inhibits the differentiation of hBMMSCs into osteoblasts, whereas the downregulation of miR-126 promotes hBMMSC differentiation, as assessed by the determination of osteogenic genes and alkaline phosphatase activity. Furthermore, the miR-126 level is positively correlated with the production of inflammatory cytokines and apoptotic cell death. Additionally, our results suggest that miR-126 negatively regulates not only B-cell lymphoma 2 (Bcl-2) expression but also the phosphorylation of extracellular signal‑regulated protein kinase (ERK) 1/2. Moreover, restoring ERK1/2 activity and upregulating Bcl-2 expression counteract the miR-126-mediated suppression of OD in hBMMSCs by promoting inflammation and apoptosis, respectively. Overall, our findings suggest a novel molecular mechanism relevant to the differentiation of hBMMSCs into osteoblasts, which can potentially facilitate bone formation by counteracting miR-126-mediated suppression of ERK1/2 activity and Bcl-2 expression.

The cortical bone metabolome of C57BL / 6J mice is sexually dimorphic

Cortical bone quality, which is sexually dimorphic, depends on bone turnover and therefore on the activities of remodeling bone cells. However, sex differences in cortical bone metabolism are not yet defined. Adding to the uncertainty about cortical bone metabolism, the metabolomes of whole bone, isolated cortical bone without marrow, and bone marrow have not been compared. We hypothesized that the metabolome of isolated cortical bone would be distinct from that of bone marrow and would reveal sex differences. Metabolite profiles from liquid chromatography–mass spectrometry (LC‐MS) of whole bone, isolated cortical bone, and bone marrow were generated from humeri from 20‐week‐old female C57Bl/6J mice. The cortical bone metabolomes were then compared for 20‐week‐old female and male C57Bl/6J mice. Femurs from male and female mice were evaluated for flexural material properties and were then categorized into bone strength groups. The metabolome of isolated cortical bone was distinct from both whole bone and bone marrow. We also found sex differences in the isolated cortical bone metabolome. Based on metabolite pathway analysis, females had higher lipid metabolism, and males had higher amino acid metabolism. High‐strength bones, regardless of sex, had greater tryptophan and purine metabolism. For males, high‐strength bones had upregulated nucleotide metabolism, whereas lower‐strength bones had greater pentose phosphate pathway metabolism. Because the higher‐strength groups (females compared with males, high‐strength males compared with lower‐strength males) had higher serum type I collagen cross‐linked C‐telopeptide (CTX1)/procollagen type 1 N propeptide (P1NP), we estimate that the metabolomic signature of bone strength in our study at least partially reflects differences in bone turnover. These data provide novel insight into bone bioenergetics and the sexual dimorphic nature of bone material properties in C57Bl/6 mice. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Role of estrogen receptors in health and disease

Estrogen receptors (ERs) regulate multiple complex physiological processes in humans. Abnormal ER signaling may result in various disorders, including reproductive system-related disorders (endometriosis, and breast, ovarian, and prostate cancer), bone-related abnormalities, lung cancer, cardiovascular disease, gastrointestinal disease, urogenital tract disease, neurodegenerative disorders, and cutaneous melanoma. ER alpha (ERα), ER beta (ERβ), and novel G-protein-coupled estrogen receptor 1 (GPER1) have been identified as the most prominent ERs. This review provides an overview of ERα, ERβ, and GPER1, as well as their functions in health and disease. Furthermore, the potential clinical applications and challenges are discussed.

Protective Role of DHEAS in Age-related Changes in Bone Mass and Fracture Risk

PURPOSE

Dehydroepiandrosterone sulfate (DHEAS) from the adrenal cortex substantially decreases with age, which may accelerate osteoporosis. However, the association of DHEAS with bone mineral density (BMD) and fracture is inconclusive. We conducted a Mendelian randomization (MR) analysis to investigate the role of DHEAS in age-related changes in BMD and fracture risk.

METHODS

Single nucleotide polymorphisms (SNPs) associated with serum DHEAS concentrations were used as instrumental variables (4 SNPs for main analysis; 4 SNPs for men and 5 SNPs for women in sex-related analysis). Summary statistics were obtained from relevant genome-wide association studies.

RESULTS

A log-transformed unit (µmol/L) increase in serum DHEAS concentrations was associated with an SD increase in estimated BMD at the heel (estimate, 0.120; 95% CI, 0.081-0.158; P = 9 × 10-10), and decreased fracture (odds ratio, 0.989; 95% CI, 0.981-0.996; P = 0.005), consistent with dual-energy X-ray absorptiometry-derived BMD at the femoral neck and lumbar spine. Their associations remained even after adjusting for height, body mass index, testosterone, estradiol, sex hormone-binding globulin, and insulin-like growth factor 1. The association of DHEAS with fracture remained after adjusting for falls, grip strength, and physical activity but was attenuated after adjusting for BMD. The MR-Bayesian model averaging analysis showed BMD was the top mediating factor for association of DHEAS with fracture. The association between DHEAS and BMD was observed in men but not in women.

CONCLUSION

DHEAS was associated with increased BMD and decreased fracture. DHEAS may play a protective role in decreasing fracture risk, mainly by increasing bone mass.

Role of Cell Death in Cellular Processes During Odontogenesis

The development of a tooth germ in a precise size, shape, and position in the jaw, involves meticulous regulation of cell proliferation and cell death. Apoptosis, as the most common type of programmed cell death during embryonic development, plays a number of key roles during odontogenesis, ranging from the budding of the oral epithelium during tooth initiation, to later tooth germ morphogenesis and removal of enamel knot signaling center. Here, we summarize recent knowledge about the distribution and function of apoptotic cells during odontogenesis in several vertebrate lineages, with a special focus on amniotes (mammals and reptiles). We discuss the regulatory roles that apoptosis plays on various cellular processes during odontogenesis. We also review apoptosis-associated molecular signaling during tooth development, including its relationship with the autophagic pathway. Lastly, we cover apoptotic pathway disruption, and alterations in apoptotic cell distribution in transgenic mouse models. These studies foster a deeper understanding how apoptotic cells affect cellular processes during normal odontogenesis, and how they contribute to dental disorders, which could lead to new avenues of treatment in the future.

Too much death can kill you: inhibiting intrinsic apoptosis to treat disease

Apoptotic cell death is implicated in both physiological and pathological processes. Since many types of cancerous cells intrinsically evade apoptotic elimination, induction of apoptosis has become an attractive and often necessary cancer therapeutic approach. Conversely, some cells are extremely sensitive to apoptotic stimuli leading to neurodegenerative disease and immune pathologies. However, due to several challenges, pharmacological inhibition of apoptosis is still only a recently emerging strategy to combat pathological cell loss. Here, we describe several key steps in the intrinsic (mitochondrial) apoptosis pathway that represent potential targets for inhibitors in disease contexts. We also discuss the mechanisms of action, advantages and limitations of small-molecule and peptide-based inhibitors that have been developed to date. These inhibitors serve as important research tools to dissect apoptotic signalling and may foster new treatments to reduce unwanted cell loss.

Late-onset Hypogonadism: Bone health

BACKGROUND

Bone health is underdiagnosed and undermanaged in men. Bone loss occurs in men with hypogonadism and in aging men. Thus, patients with a diagnosis of late-onset hypogonadism (LOH) are at risk of osteoporosis and osteoporotic fractures.

OBJECTIVES

To provide an update on research data and clinical implications regarding bone health in men with LOH by reviewing literature articles on this issue.

MATERIALS AND METHODS

A thorough search of listed publications in PubMed on bone health in older men with hypogonadism was performed, and other articles derived from these publications were further identified.

RESULTS

Late-onset Hypogonadism may be associated with reduced bone mineral density (BMD). In a pathophysiological perspective, the detrimental effects of testosterone (T) deficiency on BMD are partly ascribed to relative estrogen deficiency and both serum T and serum estradiol (E2) need to be above 200 ng/dL and 20 pg/mL to prevent bone loss. The effects of exogenous T on BMD are controversial, but most of the studies confirm that testosterone replacement therapy (TRT) increases BMD and prevents further bone loss in men with hypogonadism. No data are available on TRT and the prevention of fractures.

DISCUSSION AND CONCLUSION

In men with documented LOH, a specific clinical workup should be addressed to the diagnosis of osteoporosis in order to program subsequent follow-up and consider specific bone active therapy. TRT should be started according to guidelines of male hypogonadism while keeping in mind that it may also have positive effects also on bone health in men with LOH.

Overexpression of BCLXL in Osteoblasts Inhibits Osteoblast Apoptosis and Increases Bone Volume and Strength

The Bcl2 family proteins, Bcl2 and BclXL, suppress apoptosis by preventing the release of caspase activators from mitochondria through the inhibition of Bax subfamily proteins. We reported that BCL2 overexpression in osteoblasts using the 2.3 kb Col1a1 promoter increased osteoblast proliferation, failed to reduce osteoblast apoptosis, inhibited osteoblast maturation, and reduced the number of osteocyte processes, leading to massive osteocyte death. We generated BCLXL (BCL2L1) transgenic mice using the same promoter to investigate BCLXL functions in bone development and maintenance. Bone mineral density in the trabecular bone of femurs was increased, whereas that in the cortical bone was similar to that in wild-type mice. Osteocyte process formation was unaffected and bone structures were similar to those in wild-type mice. A micro-CT analysis showed that trabecular bone volume in femurs and vertebrae and the cortical thickness of femurs were increased. A dynamic bone histomorphometric analysis revealed that the mineralizing surface was larger in trabecular bone, and the bone-formation rate was increased in cortical bone. Serum osteocalcin but not TRAP5b was increased, BrdU-positive osteoblastic cell numbers were increased, TUNEL-positive osteoblastic cell numbers were reduced, and osteoblast marker gene expression was enhanced in BCLXL transgenic mice. The three-point bending test indicated that femurs were stronger in BCLXL transgenic mice than in wild-type mice. The frequency of TUNEL-positive primary osteoblasts was lower in BCLXL transgenic mice than in wild-type mice during cultivation, and osteoblast differentiation was enhanced but depended on cell density, indicating that enhanced differentiation was mainly owing to reduced apoptosis. Increased trabecular and cortical bone volumes were maintained during aging in male and female mice. These results indicate that BCLXL overexpression in osteoblasts increased the trabecular and cortical bone volumes with normal structures and maintained them majorly by preventing osteoblast apoptosis, implicating BCLXL as a therapeutic target of osteoporosis. © 2016 American Society for Bone and Mineral Research.

Estrogen receptors alpha and beta in bone

Estrogens are important for bone metabolism via a variety of mechanisms in osteoblasts, osteocytes, osteoclasts, immune cells and other cells to maintain bone mineral density. Estrogens bind to estrogen receptor alpha (ERα) and ERβ, and the roles of each of these receptors are beginning to be elucidated through whole body and tissue-specific knockouts of the receptors. In vitro and in vivo experiments have shown that ERα and ERβ antagonize each other in bone and in other tissues. This review will highlight the role of these receptors in bone, with particular emphasis on their antagonism.

The effects of α-zearalanol on the proliferation of bone-marrow-derived mesenchymal stem cells and their differentiation into osteoblasts

The aim of this study was to explore the effects of α-zearalanol (α-ZAL) on the proliferation of mouse bone-marrow-derived mesenchymal stem cells (BMSCs) and their differentiation into osteoblasts. Six- to eight-week-old BALB/C mice were used either as recipients or as bone marrow donors. BMSCs were isolated and collected using a differential adhesion method, with use of 10 % fetal bovine serum and Iscove's modified Dulbecco's medium. After the third generation, the BMSCs were randomly placed into the following subgroups: a control group, an osteogenic medium (OM) group, a 17β-estradiol group, an α-ZAL 10(-7) mol/L group, an α-ZAL 10(-6) mol/L group, and an α-ZAL 10(-5) mol/L group. Flow cytometry was used to identify the BMSCs collected from the bone marrow. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test was performed, and markers of the osteoblasts were measured in the different subgroups. In addition, expression of osteoprotegerin and expression of receptor activator of nuclear factor κB ligand were examined using Western blot. In contrast to the control and OM groups, BMSCs in the α-ZAL groups exhibited long fusiform shapes, and contact inhibition was observed when the cells were closely packed. After induction, the BMSCs grew well and exhibited triangular, star, polygonal, or irregular shapes. Clumps and multiple cells were evident. The trends of the proliferation and differentiation for the control, OM, 17β-estradiol, and α-ZAL groups were similar. Compared with the control and OM groups, in the α-ZAL groups the expression levels of alkaline phosphatase, procollagen type I N-terminal propeptide, bone morphogenetic protein 2, and osteocalcin were significantly increased (p < 0.05). In addition, α-ZAL inhibited osteoclastogenesis by increasing the expression of osteoprotegerin and decreasing the expression of nuclear factor κB ligand. In conclusion, α-ZAL can increase the proliferation of BMSCs and their differentiation into osteoblasts and can effectively suppress osteoclastogenesis.

Bisphosphonates and connexin 43: a critical review of evidence

Bisphosphonates (BPs) are drugs commonly used in the treatment of various disease arising or affecting bone tissue. There is a standard use in bone neoplasia and metastasis, hormonal and developmental disorders as well as for compensation of adverse effects in several medical therapies. Many in-vivo and in-vitro studies have assessed the efficacy of this drug and its function in cellular scale. In this concern, BPs are described to inhibit the resorptive function of osteoclasts and to prevent apoptosis of osteoblasts and osteocytes. They can preserve the osteocytic network, reduce fracture rate, and increase the bone mineral content, which is therapeutically used. Connexin 43 (Cx43) is a crucial molecule for basal regulation of bone homeostasis, development, and differentiation. It is described for signal transduction in many physiological and pathological stimuli and recently to be involved in BP action.

Dysapoptosis of osteoblasts and osteocytes increases cancellous bone formation but exaggerates cortical porosity with age

Skeletal aging is accompanied by decreased cancellous bone mass and increased formation of pores within cortical bone. The latter accounts for a large portion of the increase in nonvertebral fractures after age 65 years in humans. We selectively deleted Bak and Bax, two genes essential for apoptosis, in two types of terminally differentiated bone cells: the short-lived osteoblasts that elaborate the bone matrix, and the long-lived osteocytes that are immured within the mineralized matrix and choreograph the regeneration of bone. Attenuation of apoptosis in osteoblasts increased their working lifespan and thereby cancellous bone mass in the femur. In long-lived osteocytes, however, it caused dysfunction with advancing age and greatly magnified intracortical femoral porosity associated with increased production of receptor activator of nuclear factor-κB ligand and vascular endothelial growth factor. Increasing bone mass by artificial prolongation of the inherent lifespan of short-lived osteoblasts, while exaggerating the adverse effects of aging on long-lived osteocytes, highlights the seminal role of cell age in bone homeostasis. In addition, our findings suggest that distress signals produced by old and/or dysfunctional osteocytes are the culprits of the increased intracortical porosity in old age.

Identification of estrogen target genes during zebrafish embryonic development through transcriptomic analysis

Estrogen signaling is important for vertebrate embryonic development. Here we have used zebrafish (Danio rerio) as a vertebrate model to analyze estrogen signaling during development. Zebrafish embryos were exposed to 1 µM 17β-estradiol (E2) or vehicle from 3 hours to 4 days post fertilization (dpf), harvested at 1, 2, 3 and 4 dpf, and subjected to RNA extraction for transcriptome analysis using microarrays. Differentially expressed genes by E2-treatment were analyzed with hierarchical clustering followed by biological process and tissue enrichment analysis. Markedly distinct sets of genes were up and down-regulated by E2 at the four different time points. Among these genes, only the well-known estrogenic marker vtg1 was co-regulated at all time points. Despite this, the biological functional categories targeted by E2 were relatively similar throughout zebrafish development. According to knowledge-based tissue enrichment, estrogen responsive genes were clustered mainly in the liver, pancreas and brain. This was in line with the developmental dynamics of estrogen-target tissues that were visualized using transgenic zebrafish containing estrogen responsive elements driving the expression of GFP (Tg(5xERE:GFP)). Finally, the identified embryonic estrogen-responsive genes were compared to already published estrogen-responsive genes identified in male adult zebrafish (Gene Expression Omnibus database). The expressions of a few genes were co-regulated by E2 in both embryonic and adult zebrafish. These could potentially be used as estrogenic biomarkers for exposure to estrogens or estrogenic endocrine disruptors in zebrafish. In conclusion, our data suggests that estrogen effects on early embryonic zebrafish development are stage- and tissue- specific.

Glucocorticoid-induced osteoporosis in children with 21-hydroxylase deficiency

21-Hydroxylase deficiency (21-OHD) is the most common cause of congenital adrenal hyperplasia (CAH), resulting from deletions or mutations of the P450 21-hydroxylase gene (CYP21A2). Children with 21-OHD need chronic glucocorticoid (cGC) therapy, both to replace congenital deficit in cortisol synthesis and to reduce androgen secretion by adrenal cortex. GC-induced osteoporosis (GIO) is the most common form of secondary osteoporosis that results in an early, transient increase in bone resorption accompanied by a decrease in bone formation, maintained for the duration of GC therapy. Despite the conflicting results in the literature about the bone status on GC-treated patients with 21-OHD, many reports consider these subjects to be at risk for osteoporosis and fractures. In bone cells, at the molecular level, GCs regulate various functions including osteoblastogenesis, osteoclastogenesis, and the apoptosis of osteoblasts and osteocytes. In this paper, we focus on the physiology and biosynthesis of endogenous steroid hormones as well as on the effects of GCs on bone cells, highlighting the pathogenetic mechanism of GIO in children with 21-OHD.

Glucocorticoid receptor signaling in bone cells

Glucocorticoids are used for treating a wide range of diseases including inflammation and autoimmune disorders. However, there are drawbacks, primarily due to adverse effects on bone cells resulting in osteoporosis. Evidence indicates that the ratio of benefits to adverse effects depends greatly on glucocorticoid receptor (GR)-mediated mechanisms. Delineating GR-mediated signaling in bone cells will allow development of selective GR ligands/agonists (SEGRAs), which would dissociate the positive therapeutic (anti-inflammatory) effects from the negative effects on the skeleton. The present review provides an in-depth account of the current knowledge of GR-mediated transcriptional regulation of specific genes and proteins engaged in the proliferation, differentiation, and apoptosis of bone cells (osteoblasts, osteocytes, osteoclasts). We hope this knowledge will advance research in the development of SEGRAs with improved benefit/risk ratios.

Tissue-Specific Effects of Loss of Estrogen during Menopause and Aging

The roles of estrogens have been best studied in the breast, breast cancers, and in the female reproductive tract. However, estrogens have important functions in almost every tissue in the body. Recent clinical trials such as the Women's Health Initiative have highlighted both the importance of estrogens and how little we know about the molecular mechanism of estrogens in these other tissues. In this review, we illustrate the diverse functions of estrogens in the bone, adipose tissue, skin, hair, brain, skeletal muscle and cardiovascular system, and how the loss of estrogens during aging affects these tissues. Early transcriptional targets of estrogen are reviewed in each tissue. We also describe the tissue-specific effects of selective estrogen receptor modulators (SERMs) used for the treatment of breast cancers and postmenopausal symptoms.

Overexpression of Bcl2 in osteoblasts inhibits osteoblast differentiation and induces osteocyte apoptosis

Bcl2 subfamily proteins, including Bcl2 and Bcl-X_L, inhibit apoptosis. As osteoblast apoptosis is in part responsible for osteoporosis in sex steroid deficiency, glucocorticoid excess, and aging, bone loss might be inhibited by the upregulation of Bcl2; however, the effects of Bcl2 overexpression on osteoblast differentiation and bone development and maintenance have not been fully investigated. To investigate these issues, we established two lines of osteoblast-specific BCL2 transgenic mice. In BCL2 transgenic mice, bone volume was increased at 6 weeks of age but not at 10 weeks of age compared with wild-type mice. The numbers of osteoblasts and osteocytes increased, but osteoid thickness and the bone formation rate were reduced in BCL2 transgenic mice with high expression at 10 weeks of age. The number of BrdU-positive cells was increased but that of TUNEL-positive cells was unaltered at 2 and 6 weeks of age. Osteoblast differentiation was inhibited, as shown by reduced Col1a1 and osteocalcin expression. Osteoblast differentiation of calvarial cells from BCL2 transgenic mice also fell in vitro. Overexpression of BCL2 in primary osteoblasts had no effect on osteoclastogenesis in co-culture with bone marrow cells. Unexpectedly, overexpression of BCL2 in osteoblasts eventually caused osteocyte apoptosis. Osteocytes, which had a reduced number of processes, gradually died with apoptotic structural alterations and the expression of apoptosis-related molecules, and dead osteocytes accumulated in cortical bone. These findings indicate that overexpression of BCL2 in osteoblasts inhibits osteoblast differentiation, reduces osteocyte processes, and causes osteocyte apoptosis.

Odontoblast-targeted Bcl-2 overexpression promotes dentine damage repair

OBJECTIVE

Bcl-2 is widely expressed in a developing tooth organ and regulates tooth morphogenesis. However, whether Bcl-2 is related to tooth damage repair is unknown yet. Using an odontoblast-targeted Bcl-2 overexpression transgenic mouse (Col2.3Bcl-2) and artificial cavity preparation as a model system, the relationship between Bcl-2 and reparative dentinogenesis is investigated in this study.

METHODS

The odontoblastic-like cell cultures derived from mouse molar pulps were established. The expression of transgenic human Bcl-2 (hBcl-2) and endogenous mouse Bcl-2 (mBcl-2) and mouse Bax (mBax, a Bcl-2 antagonist) was detected in vivo and in vitro by Western blot and immunocytochemistry, respectively. Basal level and artificial cavity-induced odontoblast apoptosis was detected by the Deoxynucleotidyl Transferase (TdT) dUTP Nick End labelling (TUNEL) technique. Reparative dentine formation induced by artificial cavity drilled to a half dentine thickness on mesial cervical region of mandibular first molars 2, 4, and 6 weeks post-op was evaluated histologically and via micro-CT.

RESULTS

The transgenic hBcl-2 was stably expressed in odontoblasts of the transgenic animals without interference with the expression of mBcl-2 and mBax. Basal level as well as artificial cavity- induced odontoblast apoptosis was prevented by the transgene. Compared to the wild type, the transgenic animals produced reparative dentine with significantly higher mineral density 6 weeks after the operation.

CONCLUSIONS

Bcl-2 overexpression prevents odontoblast apoptosis and promotes dentine damage repair, indicating that genetic manipulation of Bcl-2 may be a novel strategy to maintain the vitality and function of dentine-pulp complex under detrimental mechanical stimuli.

Novel actions of bisphosphonates in bone: preservation of osteoblast and osteocyte viability

Bisphosphonates stop bone loss by inhibiting the activity of bone-resorbing osteoclasts. However, the effect of bisphosphonates on bone mass cannot completely explain the reduction in fracture incidence observed in patients treated with these agents. Recent research efforts provided an explanation to this dichotomy by demonstrating that part of the beneficial effect of bisphosphonates on the skeleton is due to prevention of osteoblast and osteocyte apoptosis. Work of our group, independently confirmed by other investigators, demonstrated that bisphosphonates are able to prevent osteoblast and osteocyte apoptosis in vitro and in vivo. This prosurvival effect is strictly dependent on the expression of connexin (Cx) 43, as demonstrated in vitro using cells lacking Cx43 or expressing dominant-negative mutants of the protein as well as in vivo using Cx43 osteoblast/osteocyte-specific conditional knock-out mice. Remarkably, this Cx43-dependent survival effect of bisphosphonates is independent of gap junctions and results from opening of Cx43 hemichannels. Hemichannel opening leads to activation of the kinases Src and extracellular signal-regulated kinases (ERKs), followed by phosphorylation of the ERK cytoplasmic target p90(RSK) kinase and its substrates BAD and C/EBPβ, resulting in inhibition of apoptosis. The antiapoptotic effect of bisphosphonates is separate from the effect of the drugs on osteoclasts, as analogs that lack antiresorptive activity are still able to inhibit osteoblast and osteocyte apoptosis in vitro. Furthermore, a bisphosphonate analog that does not inhibit osteoclast activity prevented osteoblast and osteocyte apoptosis and the loss of bone mass and strength induced by glucocorticoids in mice. Preservation of the bone-forming function of mature osteoblasts and maintenance of the osteocytic network, in combination with lack anticatabolic actions, open new therapeutic possibilities for bisphosphonates in the treatment of osteopenic conditions in which decreased bone resorption is not desired.

Direct transcriptional targets of sex steroid hormones in bone

The sex steroid hormones, androgens and estrogens, via their respective nuclear receptors, regulate bone mineral density in humans and mice. Very little is known about the direct targets of the androgen and estrogen receptors in bone cells. First, models of hormone and receptor deficiency in mouse and human bone are discussed. This review then focuses on the direct targets of the receptors in osteoblasts and osteoclasts. A direct target of a NR is defined here as a gene that is regulated by NR binding to the DNA (either through DNA binding or association with a DNA binding protein) at an enhancer or promoter of that gene. The experimental evidence that illustrates androgen and estrogen gene regulation in osteoblasts and osteoclasts will be summarized and compared with the phenotype of the hormones in vivo.

Odontoblast-targeted Bcl-2 overexpression impairs dentin formation

Apoptosis has been described extensively in tooth development, which is under tight control of multiple apoptosis regulators, including anti-apoptotic protein Bcl-2. However, it is totally unclear how Bcl-2 is related to odontogenesis, especially dentinogenesis. Using a transgenic mouse Col2.3Bcl-2 in which human Bcl-2 was overexpressed in odontoblasts, the effect of Bcl-2 on dentinogenesis was investigated. Overexpression of Bcl-2 was detected by immunohistochemistry and Western blot. Odontoblast apoptosis was evaluated by TUNEL and Western blot detection of cleaved caspase-3. Odontoblast differentiation was assessed by real-time PCR detection of dentin matrix expression. Dentin mineralization was evaluated by micro-CT in vivo, and alizarin red S staining and calcium content analysis in vitro. Bcl-2 was found to be overexpressed in odontoblasts and prevent their apoptosis. Odontoblast differentiation and mineralization was inhibited by Bcl-2, as evidenced by lower expressions of DMP-1, OC, and DSPP, and decreased odontoblast mineralization in vitro, as well as decreased dentin thickness and mineral density in vivo when compared to the wild-type animals. Inhibition of odontoblast differentiation by Bcl-2 occurs, at least partially, via a suppression of MEK-ERK1/2 signaling pathway. In conclusion, Bcl-2 overexpression prevents odontoblast apoptosis and impairs dentin formation, partially via an inhibition of odontoblast differentiation. This study revealed some novel functions of Bcl-2 in dentinogenesis in addition to its anti-apoptotic effect, which shed some light on the genetic complexity of tooth development.

Effects of global or targeted deletion of the EP4 receptor on the response of osteoblasts to prostaglandin in vitro and on bone histomorphometry in aged mice

Because global deletion of the prostaglandin EP4 receptor results in neonatal lethality, we generated a mouse with targeted EP4 receptor deletion using Cre-LoxP methodology and a 2.3 kb collagen I a1 promoter driving Cre recombinase that is selective for osteoblastic cells. We compared wild type (WT), global heterozygote (G-HET), targeted heterozygote (T-HET) and knockout (KO) mice. KO mice had one targeted and one global deletion of the EP4 receptor. All mice were in a mixed background of C57BL/6 and CD-1. Although there were one third fewer G-HET or KO mice at weaning compared to WT and T-HET mice, G-HET and KO mice appeared healthy. In cultures of calvarial osteoblasts, prostaglandin E(2) (PGE(2)) increased alkaline phosphatase (ALP) activity in cells from WT mice, and this effect was significantly decreased in cells from either G-HET or T-HET mice and further decreased in cells from KO mice. A selective agonist for EP4 receptor increased ALP activity and osteocalcin mRNA levels in cells from WT but not KO mice. A selective COX-2 inhibitor, NS-398, decreased osteoblast differentiation in WT but not KO cells. At 15 to 18 months of age there were no differences in serum creatinine, calcium, PTH, body weight or bone mineral density among the different genotypes. Static and dynamic histomorphometry showed no consistent changes in bone volume or bone formation. We conclude that expression of the EP4 receptor in osteoblasts is critical for anabolic responses to PGE(2) in cell culture but may not be essential for maintenance of bone remodeling in vivo.

Lef1 haploinsufficient mice display a low turnover and low bone mass phenotype in a gender- and age-specific manner

We investigated the role of Lef1, one of the four transcription factors that transmit Wnt signaling to the genome, in the regulation of bone mass. Microcomputed tomographic analysis of 13- and 17-week-old mice revealed significantly reduced trabecular bone mass in Lef1(+/-) females compared to littermate wild-type females. This was attributable to decreased osteoblast activity and bone formation as indicated by histomorphometric analysis of bone remodeling. In contrast to females, bone mass was unaffected by Lef1 haploinsufficiency in males. Similarly, females were substantially more responsive than males to haploinsufficiency in Gsk3beta, a negative regulator of the Wnt pathway, displaying in this case a high bone mass phenotype. Lef1 haploinsufficiency also led to low bone mass in males lacking functional androgen receptor (AR) (tfm mutants). The protective skeletal effect of AR against Wnt-related low bone mass is not necessarily a result of direct interaction between the AR and Wnt signaling pathways, because Lef1(+/-) female mice had normal bone mass at the age of 34 weeks. Thus, our results indicate an age- and gender-dependent role for Lef1 in regulating bone formation and bone mass in vivo. The resistance to Lef1 haploinsufficiency in males with active AR and in old females could be due to the reduced bone turnover in these mice.

Role of Bcl2 in osteoclastogenesis and PTH anabolic actions in bone

INTRODUCTION

B-cell leukemia/lymphoma 2 (Bcl2) is a proto-oncogene best known for its ability to suppress cell death. However, the role of Bcl2 in the skeletal system is unknown. Bcl2 has been hypothesized to play an important anti-apoptotic role in osteoblasts during anabolic actions of PTH. Although rational, this has not been validated in vivo; hence, the impact of Bcl2 in bone remains unknown.

MATERIALS AND METHODS

The bone phenotype of Bcl2 homozygous mutant (Bcl2(-/-)) mice was analyzed with histomorphometry and muCT. Calvarial osteoblasts were isolated and evaluated for their cellular activity. Osteoclastogenesis was induced from bone marrow cells using RANKL and macrophage-colony stimulating factor (M-CSF), and their differentiation was analyzed. PTH(1-34) (50 microg/kg) or vehicle was administered daily to Bcl2(+/+) and Bcl2(-/-) mice (4 days old) for 9 days to clarify the influence of Bcl2 ablation on PTH anabolic actions. Western blotting and real-time PCR were performed to detect Bcl2 expression in calvarial osteoblasts in response to PTH ex vivo.

RESULTS

There were reduced numbers of osteoclasts in Bcl2(-/-) mice, with a resultant increase in bone mass. Bcl2(-/-) bone marrow-derived osteoclasts ex vivo were significantly larger in size and short-lived compared with wildtype, suggesting a pro-apoptotic nature of Bcl2(-/-) osteoclasts. In contrast, osteoblasts were entirely normal in their proliferation, differentiation, and mineralization. Intermittent administration of PTH increased bone mass similarly in Bcl2(+/+) and Bcl2(-/-) mice. Finally, Western blotting and real-time PCR showed that Bcl2 levels were not induced in response to PTH in calvarial osteoblasts.

CONCLUSIONS

Bcl2 is critical in osteoclasts but not osteoblasts. Osteoclast suppression is at least in part responsible for increased bone mass of Bcl2(-/-) mice, and Bcl2 is dispensable in PTH anabolic actions during bone growth.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Continuous Activation of Gαq in Osteoblasts Results in Osteopenia through Impaired Osteoblast Differentiation

We explored the role of G alpha(q)-mediated signaling on skeletal homeostasis by selectively expressing a constitutively active G alpha(q) (mutation of Q209L) in osteoblasts. Continuous signaling via G alpha(q) in mouse osteoblastic MC3T3-E1 cells impaired differentiation. Mice that expressed the constitutively active G alpha(q) transgene in cells of the osteoblast lineage exhibited severe osteopenia in cortical and trabecular bones. Osteoblast number, bone volume, and trabecular thickness were reduced in transgenic mice, but the osteoclasts were unaffected. Osteoblasts from transgenic mice showed impaired differentiation and matrix formation. In the presence of a protein kinase C inhibitor GF109203X, this impairment was not seen, indicating mediation by the protein kinase C pathway. We propose that continuous activation of the G alpha(q) signal in osteoblasts plays a crucial, previously unrecognized role in bone formation.

Bone-targeted overexpression of Bcl-2 increases osteoblast adhesion and differentiation and inhibits mineralization in vitro

Apoptosis is a process important for the development and homeostasis of self-renewing tissues, including bone. However, little is known about the function of Bcl-2, a key player of apoptosis, in the regulation of osteoblast activity. Ex vivo cultures of osteoblasts from Col2.3Bcl-2 mice, in which human Bcl-2 was targeted to bone by the 2.3 kb fragment of the type I collagen promoter, were used to study the effect of Bcl-2 in osteoblasts. During 35 days of culture, hBcl-2 expression increased without any effect on endogenous mouse Bcl-2 and Bax expression. Adhesion of transgenic (TG) osteoblasts was twofold more than that of wild-type (WT) cells, with significantly higher expression of integrins alpha(1), alpha(2), and alpha(5) but similar levels of alpha(v) and beta(1) relative to WT cells. Proliferation of osteoblasts was not affected. Overexpression of hBcl-2 promoted the differentiation of osteoblasts, as shown by increased message levels of alkaline phosphatase, type I collagen, bone sialoprotein, and osteocalcin in the TG compared to WT cells throughout the culture period. The two transcription factors essential for osteoblast differentiation, core binding factor alpha 1 (Cbfa-1) and osterix, had significantly higher expression in TG than WT cells during the early culture period. ss-Catenin, a central player in the canonical Wnt pathway, also had higher expression in TG than WT cultures. Mineralization was significantly decreased in TG cultures, with less osteoblast apoptosis, compared to WT. Thus, Bcl-2 seems to have multiple roles in modulating osteoblast activities.

Osteoblast and osteocyte apoptosis associated with androgen action in bone: requirement of increased Bax/Bcl-2 ratio

Both the number and the activity of osteoblasts are critical for normal bone growth and maintenance. Although a potential role for estrogen in protection of bone mass through inhibition of osteoblast apoptosis has been proposed, a function for androgen is much less clear. The aim of this study was to establish a direct role for androgen to influence osteoblast apoptosis both in vitro and in vivo. AR-MC3T3-E1 cells, with androgen receptor (AR) overexpression controlled by the type I collagen promoter, were treated with the non-aromatizable androgen 5alpha-dihydrotestosterone (DHT). Apoptosis was assessed by three different techniques including DNA fragmentation, caspase-3 activation, and changes in mitochondrial membrane potential. Transactivation of AR by DHT enhanced apoptosis while 17beta-estradiol (E(2)) treatment reduced apoptosis in both proliferating preosteoblasts and mature osteocyte-like cells. To explore mechanism, the apoptosis regulators Bcl-2 (antiapoptotic) and Bax (proapoptotic) were evaluated. Western analysis revealed that DHT decreased Bcl-2 resulting in a significantly increased Bax/Bcl-2 ratio. Regulation of Bcl-2 was post-transcriptional since bcl-2 mRNA levels were unaffected by DHT treatment. Furthermore, ubiquitination of Bcl-2 was increased and serine phosphorylation was reduced, consistent with inhibition of MAP kinase signaling by DHT. Increased Bax/Bcl-2 ratio was essential since either Bcl-2 overexpression or Bax downregulation by RNA interference (RNAi) partially abrogated or reversed DHT-enhanced osteoblastic apoptosis. In order to establish physiologic significance in vivo, AR-transgenic mice with AR overexpression in the osteoblast lineage and thus enhanced androgen sensitivity were characterized. In male AR-transgenic mice, increased osteoblast apoptosis was observed in vivo even in association with new bone formation. Thus, although estrogen can be antiapoptotic, androgen stimulates osteoblast and osteocyte apoptosis through an increased Bax/Bcl-2 ratio even in anabolic settings. These results identify a new mechanism for androgen regulation of osteoblast activity distinct from estrogen, and suggest that enhanced apoptosis can be associated with anabolic stimulation of new bone growth. Androgens thus play a distinct role in skeletal homeostasis.