Anti-apoptotic molecule Bcl-2 regulates the differentiation, activation, and survival of both osteoblasts and osteoclasts

Bcl2l1 Deficiency in Osteoblasts Reduces the Trabecular Bone Due to Enhanced Osteoclastogenesis Likely through Osteoblast Apoptosis

Bcl2l1 (Bcl-XL) belongs to the Bcl-2 family, Bcl2 and Bcl2-XL are major anti-apoptotic proteins, and the apoptosis of osteoblasts is a key event for bone homeostasis. As the functions of Bcl2l1 in osteoblasts and bone homeostasis remain unclear, we generated osteoblast-specific Bcl2l1-deficient (Bcl2l1fl/flCre) mice using 2.3-kb Col1a1 Cre. Trabecular bone volume and the trabecular number were lower in Bcl2l1fl/flCre mice of both sexes than in Bcl2l1fl/fl mice. In bone histomorphometric analysis, osteoclast parameters were increased in Bcl2l1fl/flCre mice, whereas osteoblast parameters and the bone formation rate were similar to those in Bcl2l1fl/fl mice. TUNEL-positive osteoblastic cells and serum TRAP5b levels were increased in Bcl2l1fl/flCre mice. The deletion of Bcl2l1 in osteoblasts induced Tnfsf11 expression, whereas the overexpression of Bcl-XL had no effect. In a co-culture of Bcl2l1-deficient primary osteoblasts and wild-type bone-marrow-derived monocyte/macrophage lineage cells, the numbers of multinucleated TRAP-positive cells and resorption pits increased. Furthermore, serum deprivation or the deletion of Bcl2l1 in primary osteoblasts increased apoptosis and ATP levels in the medium. Therefore, the reduction in trabecular bone in Bcl2l1fl/flCre mice may be due to enhanced bone resorption through osteoblast apoptosis and the release of ATP from apoptotic osteoblasts, and Bcl2l1 may inhibit bone resorption by preventing osteoblast apoptosis.

Baicalin inhibits apoptosis and enhances chondrocyte proliferation in thiram-induced tibial dyschondroplasia in chickens by regulating Bcl-2/Caspase-9 and Sox-9/Collagen-II expressions

Avian tibial dyschondroplasia (TD) is a skeletal disease affecting fast growing chickens, resulting in non-mineralized avascular cartilage. This metabolic disorder is characterized by lameness and reduced growth performance causing economic losses. The aim of this study was to investigate the protective effects of baicalin against TD caused by thiram exposure. A total of two hundred and forty (n = 240) one day-old broiler chickens were uniformly and randomly allocated into three different groups (n = 80) viz. control, TD, and baicalin groups. All chickens received standard feed, however, to induce TD, the TD and baicalin groups received thiram (tetramethylthiuram disulfide) at a rate of 50 mg/kg feed from days 4-7. The thiram induction in TD and baicalin groups resulted in lameness, high mortality, and enlarged growth-plate, poor production performance, reduction in ALP, GSH-Px, SOD, and T-AOC levels, and increased AST and ALT, and MDA levels. Furthermore, histopathological results showed less vascularization, and mRNA and protein expression levels of Sox-9, Col-II, and Bcl-2 showed significant downward trend, while caspase-9 displayed significant up-regulation in TD-affected chickens. After the TD induction, the baicalin group was orally administered with baicalin at a rate of 200 mg/kg from days 8-18. Baicalin administration increased the vascularization, and chondrocytes with intact nuclei, alleviated lameness, decreased GP size, increased productive capacity, and restored the liver antioxidant enzymes and serum biochemical levels. Furthermore, baicalin significantly up-regulated the gene and protein expressions of Sox-9, Col-II, and Bcl-2, and significantly down-regulated the expression of caspase-9 (p < 0.05). Therefore, the obtained results suggest that baicalin could be a possible choice in thiram toxicity alleviation by regulating apoptosis and chondrocyte proliferation in thiram-induced tibial dyschondroplasia.

Pharmacological inhibition of BCL-2 with the FDA-approved drug venetoclax impairs longitudinal bone growth

Treatment-related skeletal complications are common in childhood cancer patients and survivors. Venetoclax is a BCL-2 inhibitor that has shown efficacy in hematological malignancies in adults and is being investigated in pediatric cancer clinical trials as a promising therapeutic modality. Venetoclax triggers cell death in cancer cells, but whether it exerts similar effects in normal bone cells, is unknown. Chondrogenic ATDC5 cells, E20 fetal rat metatarsal bones, and human growth plate biopsies were treated with different concentrations of venetoclax. Female NMRI nu/nu mice were treated with venetoclax or vehicle for 15 days. Mice were X-rayed at baseline and at the end of the experiment to assess longitudinal bone growth and body weight was monitored throughout the study. Histomorphometric and immunohistochemical analyses were performed to evaluate treatment effects on the growth plate cartilage. Venetoclax decreased the viability of chondrocytes and impaired the growth of ex vivo cultured metatarsals while reducing the height of the resting/proliferative zone and the hypertrophic cell size. When tested in vivo, venetoclax suppressed bone growth and reduced growth plate height. Our experimental data suggest that venetoclax directly targets growth plate chondrocytes suppressing bone growth and we, therefore, encourage careful monitoring of longitudinal bone growth if treating growing children with venetoclax.

TET2 regulates osteoclastogenesis by modulating autophagy in OVX-induced bone loss

Increased bone resorption by osteoclasts after estrogen deficiency is the main cause of postmenopausal osteoporosis. TET2 (tet methylcytosine dioxygenase 2) is a DNA demethylase that regulates cellular function and differentiation potential. Macroautophagy/autophagy maintains cellular homeostasis by recycling unnecessary and damaged organelles. This study revealed that TET2 promoted bone loss in oophorectomized (OVX) mice and that TET2 promoted osteoclast differentiation by regulating autophagy. Tet2 knockdown inhibited autophagy and osteoclast differentiation in vitro. Mechanistically, Tet2 knockdown increased BCL2 (B cell leukemia/lymphoma 2) expression and BCL2 exhibited increased binding to BECN1 and negatively regulated autophagy. Small interfering RNA specific to Bcl2 interfered with BCL2 expression in Tet2-knockdown bone marrow cells/precursors, partially reversing autophagy dysregulation and promoting osteoclast differentiation. Moreover, the LV-shTet2 lentivirus prevented bone loss in OVX mice. In summary, our findings provide evidence that TET2 promotes osteoclast differentiation by inhibiting BCL2 expression and positively regulating BECN1-dependent autophagy.Abbreviations: ACP5/TRAP: acid phosphatase 5, tartrate resistant; ATP6V0D2: ATPase, H+ transporting, lysosomal V0 subunit D2; BCL2: B cell leukemia/lymphoma 2; BECN1: beclin 1, autophagy related; BMs: bone marrow cells; CTSK: cathepsin K; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MMP9: matrix metallopeptidase 9; OVX: oophorectomy; RUNX1: runt related transcription factor 1; SOCS3: suppressor of cytokine signaling 3; SPI1/PU.1: Spi-1 proto-oncogene; TNFSF11/RANKL: tumor necrosis factor (ligand) superfamily, member 11; TET2: tet methylcytosine dioxygenase 2.

Leukemia/lymphoma-related factor (LRF) or osteoclast zinc finger protein (OCZF) overexpression promotes osteoclast survival by increasing Bcl-xl mRNA: A novel regulatory mechanism mediated by the RNA binding protein SAM68

RANKL induces NFATc1, a key transcriptional factor to induce osteoclast-specific genes such as cathepsin K, whereas transcriptional control of osteoclast survival is not fully understood. Leukemia/lymphoma-related factor (LRF) in mouse and osteoclast zinc finger protein (OCZF) in rat are zinc finger and BTB domain-containing protein (zBTB) family of transcriptional regulators, and are critical regulators of hematopoiesis. We have previously shown that differentiation and survival were enhanced in osteoclasts from OCZF-Transgenic (Tg) mice. In the present study, we show a possible mechanism of osteoclast survival regulated by LRF/OCZF and the role of OCZF overexpression in pathological bone loss. In the in vitro cultures, LRF was highly colocalized with NFATc1 in cells of early stage in osteoclastogenesis, but only LRF expression persisted after differentiation into mature osteoclasts. LRF expression was further enhanced in resorbing osteoclasts formed on dentin slices. Osteoclast survival inhibitor such as alendronate, a bisphosphonate reduced LRF expression. Micro CT evaluation revealed that femurs of OCZF-Tg mice showed significantly lower bone volume compared to that of WT mice. Furthermore, OCZF overexpression markedly promoted bone loss in ovariectomy-induced osteolytic mouse model. The expression of anti-apoptotic Bcl-xl mRNA, which is formed by alternative splicing, was enhanced in the cultures in which osteoclasts are formed from OCZF-Tg mice. In contrast, the expression of pro-apoptotic Bcl-xs mRNA was lost in the culture derived from OCZF-Tg mice. We found that the expression levels of RNA binding splicing regulator, Src substrate associated in mitosis of 68 kDa (Sam68) protein were markedly decreased in OCZF-Tg mice-derived osteoclasts. In addition, shRNA-mediated knockdown of Sam68 expression increased the expression of Bcl-xl mRNA, suggesting that SAM68 regulates the expression of Bcl-xl. These results indicate that OCZF overexpression reduces protein levels of Sam68, thereby promotes osteoclast survival, and suggest that LRF/OCZF is a promising target for regulating pathological bone loss.

Proteus mirabilis Vesicles Induce Mitochondrial Apoptosis by Regulating miR96-5p/Abca1 to Inhibit Osteoclastogenesis and Bone Loss

Bone loss due to an increased osteoclast activity is common in osteoporosis and rheumatoid arthritis. For the first time, we observed an inhibition of osteoclast formation and bone resorption by outer-membrane vesicles (OMVs) from a Gram-negative, pathogenic bacterium, Proteus mirabilis (P.M). Gene ontogeny and KEGG enrichment analyses of miRNA and mRNA sequencing data demonstrated a significant effect of P.M OMVs on mitochondrial functions and apoptotic pathways. OMVs induced mitochondrial dysfunction through an increased level of intracellular ROS, collapse of mitochondrial membrane potential (ΔΨm), and modulation of Bax, Bcl-2, caspase-3, and cytochrome c expression. In addition, P.M OMVs strongly inhibited miR-96-5p expression, which caused an upregulation of ATP binding cassette subfamily A member 1 (Abca1) in osteoclasts leading to an increased level of mitochondria-dependent apoptosis. Moreover, treatment with P.M but not Escherichia coli OMVs attenuated bone loss in experimental osteoporosis and collagen-induced arthritis. Collectively, we demonstrated osteoprotective functions of OMVs from Proteus mirabilis, which downregulated miR-96-5p causing an increased Abca1 expression and mitochondria-dependent apoptosis.

The Role of miR-34c-5p in Osteogenic Differentiation of Bone Marrow Mesenchymal Stem Cells

Background and Objectives

Osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) plays a critical role in the success of lumbar spinal fusion with autogenous bone graft. This study aims to explore the role and specific mechanism of miR-34c-5p in osteogenic differentiation of BMSCs.

Methods and Results

Rabbit model of lumbar fusion was established by surgery. The osteogenic differentiation dataset of mesenchymal stem cells was obtained from the Gene Expression Omnibus (GEO) database, and differentially expressed miRNAs were analyzed using R language (limma package). The expressions of miR-34c-5p, miR-199a-5p, miR-324-5p, miR-361-5p, RUNX2, OCN and Bcl-2 were determined by qRT-PCR and Western blot. ELISA, Alizarin red staining and CCK-8 were used to detect the ALP content, calcium deposition and proliferation of BMSCs. The targeted binding sites between miR-34c-5p and Bcl-2 were predicted by the Target database and verified using dual-luciferase reporter assay. MiR-34c-5p expression was higher in rabbit lumbar fusion model and differentiated BMSCs than normal rabbit or BMSCs. The content of ALP and the deposition of calcium increased with the osteogenic differentiation of BMSCs. Upregulation of miR-34c-5p reduced cell proliferation and promoted ALP content, calcium deposition, RUNX2 and OCN expression compared with the control group. The effects of miR-34c-5p inhibitor were the opposite. In addition, miR-34c-5p negatively correlated with Bcl-2. Upregulation of Bcl-2 reversed the effects of miR-34c-5p on ALP content, calcium deposition, and the expressions of RUNX2 and OCN.

Conclusions

miR-34c-5p could promote osteogenic differentiation and suppress proliferation of BMSCs by inhibiting Bcl-2.

Mitophagy and apoptosis mediated by ROS participate in AlCl3-induced MC3T3-E1 cell dysfunction

Aluminum (Al), as a common environmental pollutant, causes osteoblast (OB) dysfunction and then leads to Al-related bone diseases (ARBD). One of the mechanisms of ARBD is oxidative stress, which leads to an increase in the production of reactive oxygen species (ROS). ROS can induce mitochondrial damage, thereby inducing mitophagy and apoptosis. But whether mitophagy and apoptosis mediated by ROS, and the role of ROS in AlCl₃-induced MC3T3-E1 cell dysfunction is still unclear. In this study, MC3T3-E1 cells used 0 mM Al (control group), 2 mM Al (Al group), 5 mM N-acetyl cysteine (NAC) (NAC group), 2 mM Al and 5 mM NAC (Al + NAC group) for 24 h. We found AlCl₃-induced MC3T3-E1 cell dysfunction accompanied by oxidative stress, apoptosis, and mitophagy. While NAC, a ROS scavenger treatment, restored cell function and alleviated the mitophagy and apoptosis. These results suggested that mitophagy and apoptosis mediated by ROS participate in AlCl₃-induced MC3T3-E1 cell dysfunction.

Phenanthrenoid Coelogin Isolated from Coelogyne cristata Exerts Osteoprotective Effect Through MAPK-Mitogen-Activated Protein Kinase Signaling Pathway

Osteoporosis is a major health problem in postmenopausal women globally. This study determined the mechanism through which coelogin stimulates osteoblastogenesis and its osteoprotective and bone regenerating potential. Coelogin effect on primary calvarial osteoblast cells was determined by measuring alkaline phosphatase activity, mineralization, osteoblast survival, and apoptosis and protein expression studies. The osteoprotective effect of coelogin was also evaluated on osteopenic adult female Swiss mice. At autopsy, bones were collected for dynamic and histomorphometry studies. Serum samples were also collected for assessment of serum parameters. Coelogin treatment led to increased osteoblast proliferation, survival, differentiation, and mineralization in osteoblast cells. Coelogin supplementation to Ovx mice promoted new bone formation, prevented Ovx-induced deterioration of bone microarchitecture, and enhanced bone regeneration. In addition, signaling studies revealed that coelogin treatment activates the ER-Erk and Akt-dependent signaling pathways which stimulate the osteoblastogenesis in osteoblast cells.

Regulation and Role of Transcription Factors in Osteogenesis

Bone is a dynamic tissue constantly responding to environmental changes such as nutritional and mechanical stress. Bone homeostasis in adult life is maintained through bone remodeling, a controlled and balanced process between bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoblasts secrete matrix, with some being buried within the newly formed bone, and differentiate to osteocytes. During embryogenesis, bones are formed through intramembraneous or endochondral ossification. The former involves a direct differentiation of mesenchymal progenitor to osteoblasts, and the latter is through a cartilage template that is subsequently converted to bone. Advances in lineage tracing, cell sorting, and single-cell transcriptome studies have enabled new discoveries of gene regulation, and new populations of skeletal stem cells in multiple niches, including the cartilage growth plate, chondro-osseous junction, bone, and bone marrow, in embryonic development and postnatal life. Osteoblast differentiation is regulated by a master transcription factor RUNX2 and other factors such as OSX/SP7 and ATF4. Developmental and environmental cues affect the transcriptional activities of osteoblasts from lineage commitment to differentiation at multiple levels, fine-tuned with the involvement of co-factors, microRNAs, epigenetics, systemic factors, circadian rhythm, and the microenvironments. In this review, we will discuss these topics in relation to transcriptional controls in osteogenesis.

Exosomal lncRNAs NONMMUT000375.2 and NONMMUT071578.2 derived from titanium particle treated RAW264.7 cells regulate osteogenic differentiation of MC3T3-E1 cells

Periprosthetic osteolysis and the subsequent aseptic loosening can lead to the failure of joint replacement. Wear particles are well known to be the initiative cause inducing osteolysis through enhancing osteoclast-mediated bone resorption and reducing osteogenic differentiation. The purpose of this study was to investigate the effects of osteoclast-secreted exosomal long noncoding RNAs (lncRNAs) on osteogenesis in the process of particle-induced osteolysis. RAW264.7 cells were treated by titanium particles (TI). The inflammatory cytokines were increased, and expression of Receptor Activator of Nuclear Factor-κB and Nuclear factor of activated T cells c1 were also increased, indicating osteoclast differentiation occurred. The purified exosomes from RAW264.7 cells induced with TI inhibited osteogenic differentiation of MC3T3-E1 cells. RNA sequencing generated lncRNAs expression profiles (458 up-regulated and 1641 down-regulated) of the exosomes derived from RAW264.7 cells treated with TI. Based on the results of gene ontology/Kyoto Encyclopedia of Genes and Genomes analysis and quantitative real time polymerase chain reaction validation, we confirmed two candidate lncRNAs, NONMMUT000375.2 and NONMMUT071578.2. The regulation network presented that some vital genes involved in osteoclast differentiation, such as Bcl2, Wnt11, TGF-β, and Pdk1, were under the regulation of NONMMUT000375.2 and NONMMUT071578.2. Taken together, exosomes derived from TI treated RAW264.7 cells inhibit the osteogenic activity of MC3T3-E1 cells. Exosomal lncRNAs, NONMMUT000375.2 and NONMMUT071578.2 may potentially play their roles in promoting osteoclast differentiation and suppressing osteogenesis, which aggravates the osteoclastogenesis/osteogenesis imbalance.

Novel gene Merlot inhibits differentiation and promotes apoptosis of osteoclasts

Extended osteoclast longevity is deeply involved in the pathogenesis of bone diseases such as osteoporosis and rheumatoid arthritis, though the mechanisms that determine osteoclast lifespan are not fully understood. Here we present findings indicating that the newly characterized gene Merlot, which encodes a highly conserved yet uncharacterized protein in vertebrates, is an important regulator for termination of osteoclastogenesis via induction of apoptosis. Mice lacking Merlot exhibited low bone mass due to increased osteoclast and bone resorption. Furthermore, osteoclast precursors overexpressing Merlot failed to differentiate into mature osteoclasts, while Merlot deficiency led to hyper-nucleation and prolonged survival of osteoclasts, accompanied by sustained nuclear localization of nuclear factor of activated T cell c1 (NFATc1) and derepression of glycogen synthase kinase-3β (GSK3β) activity, known to regulate NFATc1 activity and induce apoptosis. Merlot-deficient osteoclasts were found to represent suppression of caspase-3-mediated apoptosis and Merlot deficiency caused transcriptional downregulation of a proapoptotic cascade, including Bax, Bak, Noxa, and Bim, as well as the executor caspase members Casp-3, -6, and -7, and upregulation of anti-apoptotic Bcl2, resulting in a low apoptotic threshold. Thus, Merlot regulates osteoclast lifespan by inhibition of differentiation and simultaneous induction of apoptosis via regulation of the NFATc1-GSK3β axis.

Implication of the p53-Related miR-34c, -125b, and -203 in the Osteoblastic Differentiation and the Malignant Transformation of Bone Sarcomas

The formation of the skeleton occurs throughout the lives of vertebrates and is achieved through the balanced activities of two kinds of specialized bone cells: the bone-forming osteoblasts and the bone-resorbing osteoclasts. Impairment in the remodeling processes dramatically hampers the proper healing of fractures and can also result in malignant bone diseases such as osteosarcoma. MicroRNAs (miRNAs) are a class of small non-coding single-strand RNAs implicated in the control of various cellular activities such as proliferation, differentiation, and apoptosis. Their post-transcriptional regulatory role confers on them inhibitory functions toward specific target mRNAs. As miRNAs are involved in the differentiation program of precursor cells, it is now well established that this class of molecules also influences bone formation by affecting osteoblastic differentiation and the fate of osteoblasts. In response to various cell signals, the tumor-suppressor protein p53 activates a huge range of genes, whose miRNAs promote genomic-integrity maintenance, cell-cycle arrest, cell senescence, and apoptosis. Here, we review the role of three p53-related miRNAs, miR-34c, -125b, and -203, in the bone-remodeling context and, in particular, in osteoblastic differentiation. The second aim of this study is to deal with the potential implication of these miRNAs in osteosarcoma development and progression.

Puerarin enhance vascular proliferation and halt apoptosis in thiram-induced avian tibial dyschondroplasia by regulating HIF-1α, TIMP-3 and BCL-2 expressions

Tetramethyl thiuram disulfide (thiram) is a dithiocarbamate pesticide used for crop protection and storage. But, it's widespread utilization is associated with deleterious growth plate cartilage disorder in broilers termed as avian tibial dyschondroplasia (TD). TD results in non-mineralized and less vascularized proximal tibial growth plate cartilage causing lameness and poor growth performance. This study investigated the therapeutic potential of puerarin against thiram toxicity in TD affected chickens. One-day-old broiler chickens (n = 240) were alienated into three equal groups i.e. control, TD and puerarin (n = 80) and were offered standard feed. Additionally, TD and puerarin groups were offered thiram at 50 mg/kg of feed from 4 to 7 days for TD induction followed by puerarin therapy at 120 mg/kg to puerarin group only from 8 to 18 days for TD treatment. Thiram feeding to TD and puerarin group chickens caused lameness, mortality, and increased the aspartate aminotransferase (AST), alanine aminotransferase (ALT), malondialdehyde (MDA) levels and growth plate (GP) size and upregulated HIF-1α expression. Besides, the production parameters, alkaline phosphatase (ALP), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) levels and the expressions of TIMP-3 and BCL-2 were decreased (p < 0.05). Puerarin alleviated lameness, enhanced angiogenesis and growth performance and serum and antioxidant enzymes, decreased apoptosis and recuperated GP width by significantly downregulating HIF-1α and upregulating the TIMP-3 and BCL-2 mRNA and protein expressions in puerarin group chickens (p < 0.05). In conclusion, the toxic effects associated with thiram can be mitigated using puerarin.

Positive and negative regulators of osteoclast apoptosis

Survival and apoptosis are of major importance in the osteoclast life cycle. As osteoclasts have short lifespan, any alteration that prolongs their viability may cause enhanced osteoclast activity. Hence, the regulation of OC apoptosis has been recognized as a critical factor in bone remodeling. An imbalance in bone remodeling due to increased osteoclast activity leads to most adult bone diseases such as osteoporosis, rheumatoid arthritis and multiple myeloma. Therefore, manipulating osteoclast death would be a viable therapeutic approach in ameliorating bone diseases, with accelerated resorption. Over the last few decades we have witnessed the unraveling of many of the intracellular mechanisms responsible for osteoclast apoptosis. Thus, an understanding of the underlying mechanisms by which osteoclasts undergo programmed cell death and the regulators that modulate that activity will undoubtedly provide an insight into the development of pharmacological agents to treat such pathological bone diseases.

The Emerging Role of Mesenchymal Stem Cells in Vascular Calcification

Vascular calcification (VC), characterized by hydroxyapatite crystal depositing in the vessel wall, is a common pathological condition shared by many chronic diseases and an independent risk factor for cardiovascular events. Recently, VC is regarded as an active, dynamic cell-mediated process, during which calcifying cell transition is critical. Mesenchymal stem cells (MSCs), with a multidirectional differentiation ability and great potential for clinical application, play a duplex role in the VC process. MSCs facilitate VC mainly through osteogenic transformation and apoptosis. Meanwhile, several studies have reported the protective role of MSCs. Anti-inflammation, blockade of the BMP2 signal, downregulation of the Wnt signal, and antiapoptosis through paracrine signaling are possible mechanisms. This review displays the evidence both on the facilitating role and on the protective role of MSCs, then discusses the key factors determining this divergence.

Autophagy Protects MC3T3-E1 Cells upon Aluminum-Induced Apoptosis

Aluminum (Al) exposure has adverse effects on osteoblasts, and the effect might be through autophagy-associated apoptosis. In this study, we showed that aluminum trichloride (AlCl₃) could induce autophagy in MC3T3-E1 cells, as demonstrated by monodansylcadaverine (MDC) staining and the expressions of the ATG3, ATG5, and ATG9 genes. We found AlCl₃ inhibited MC3T3-E1 cell survival rate and caused apoptosis, as evidenced by CCK-8 assay, Annexin V/PI double staining, and increased expressions of Bcl-2, Bax, and Caspase-3 genes. In addition, increased autophagy induced by rapamycin further attenuated the MC3T3-E1 cell apoptosis rate after AlCl₃ exposure. These results support the hypothesis that autophagy plays a protective role in impeding apoptosis caused by AlCl₃. Activating autophagy may be a strategy for treatment of Al-induced bone disease.

Nanostructured Ti surfaces and retinoic acid/dexamethasone present a spatial framework for the maturation and amelogenesis of LS-8 cells

PURPOSE

To investigate the amelogenesis-inductive effects of surface structures at the nanoscale. For this purpose, variable nanostructured titanium dioxide (TiO2) surfaces were used as a framework to regulate the amelogenic behaviors of ameloblasts with the administration of retinoic acid (RA)/dexamethasone (DEX).

MATERIALS AND METHODS

TiO2 nanotubular (NT) surfaces were fabricated via anodization. Mouse ameloblast-like LS-8 cells were seeded and cultured on NT surfaces in the presence or absence of RA/DEX for 48 h. The amelogenic behaviors and extracellular matrix (ECM) mineralization of LS-8 cells on nanostructured Ti surfaces were characterized using field emission scanning electron microscope, laser scanning confocal microscope, quantitative polymerase chain reaction, MTT assay, and flow cytometry.

RESULTS

TiO2 NT surfaces (tube size ~30 and ~80 nm) were constructed via anodization at 5 or 20 V and denoted as NT5 and NT20, respectively. LS-8 cells exhibited significantly increased spread and proliferation, and lower rates of apoptosis and necrosis on NT surfaces. The amelogenic gene expression and ECM mineralization differed significantly on the NT20 and the NT5 and polished Ti sample surfaces in standard medium. The amelogenic behaviors of LS-8 cells were further changed by RA/DEX pretreatment, which directly drove maturation of LS-8 cells.

CONCLUSION

Controlling the amelogenic behaviors of ameloblast-like LS-8 cells by manipulating the nanostructure of biomaterials surfaces represents an effective tool for the establishment of a systemic framework for supporting enamel regeneration. The administration of RA/DEX is an effective approach for driving the amelogenesis and maturation of ameloblasts.

Fas- and Mitochondria-Mediated Signaling Pathway Involved in Osteoblast Apoptosis Induced by AlCl3

Aluminum (Al) is known to induce apoptosis of osteoblasts (OBs). However, the mechanism is not yet established. To investigate the apoptotic mechanism of OBs induced by aluminum trichloride (AlCl₃), the primary OBs from the craniums of fetal Wistar rats were exposed to 0 mg/mL (control group, CG), 0.06 mg/mL (low-dose group, LG), 0.12 mg/mL (mid-dose group, MG), and 0.24 mg/mL (high-dose group, HG) AlCl₃ for 24 h, respectively. We observed that AlCl₃ induced OB apoptosis with the appearance of apoptotic morphology and increase of apoptosis rate. Additionally, AlCl₃ treatment activated mitochondrial-mediated signaling pathway, accompanied by mitochondrial membrane potential (ΔΨm) depolarization, release of cytochrome c from the mitochondria to the cytoplasm, as well as survival signal-related factor caspase-9 and caspase-3 activation. AlCl₃ exposure also activated Fas/Fas ligand signaling pathway, presented as Fas, Fas ligand, and Fas-associated death domain expression enhancement and caspase-8 activation, as well as the hydrolysis of Bid to truncated Bid, suggesting that the Fas-mediated signaling pathway might aggravate mitochondria-mediated OB apoptosis through hydrolyzing Bid. Furthermore, AlCl₃ exposure inhibited Bcl-2 protein expression and increased the expressions of Bax, Bak, and Bim in varying degrees. These results indicated that AlCl₃ exposure induced OB apoptosis through activating Fas- and mitochondria-mediated signaling pathway and disrupted B-cell lymphoma-2 family proteins.

The emerging role of Hippo signaling pathway in regulating osteoclast formation

A delicate balance between osteoblastic bone formation and osteoclastic bone resorption is crucial for bone homeostasis. This process is regulated by the Hippo signaling pathway including key regulatory molecules RASSF2, NF2, MST1/2, SAV1, LATS1/2, MOB1, YAP, and TAZ. It is well established that the Hippo signaling pathway plays an important part in regulating osteoblast differentiation, but its role in osteoclast formation and activation remains poorly understood. In this review, we discuss the emerging role of Hippo-signaling pathway in osteoclast formation and bone homeostasis. It is revealed that specific molecules of the Hippo-signaling pathway take part in a stage specific regulation in pre-osteoclast proliferation, osteoclast differentiation and osteoclast apoptosis and survival. Upon activation, MST and LAST, transcriptional co-activators YAP and TAZ bind to the members of the TEA domain (TEAD) family transcription factors, and influence osteoclast differentiation via regulating the expression of downstream target genes such as connective tissue growth factor (CTGF/CCN2) and cysteine-rich protein 61 (CYR61/CCN1). In addition, through interacting or cross talking with RANKL-mediated signaling cascades including NF-κB, MAPKs, AP1, and NFATc1, Hippo-signaling molecules such as YAP/TAZ/TEAD complex, RASSF2, MST2, and Ajuba could also potentially modulate osteoclast differentiation and function. Elucidating the roles of the Hippo-signaling pathway in osteoclast development and specific molecules involved is important for understanding the mechanism of bone homeostasis and diseases.

Magnetic Nanoparticle-Based Upregulation of B-Cell Lymphoma 2 Enhances Bone Regeneration

Clinical translation of cell-based strategies for tissue regeneration remains challenging because survival of implanted cells within hostile, hypoxic wound environments is uncertain. Overexpression of B-cell lymphoma 2 (Bcl-2) has been shown to inhibit apoptosis in implanted cells. The present study describes an "off the shelf" prefabricated scaffold integrated with magnetic nanoparticles (MNPs) used to upregulate Bcl-2 expression in implanted adipose-derived stromal cells for bone regeneration. Iron oxide cores were sequentially coated with branched polyethyleneimine, minicircle plasmid encoding green fluorescent protein and Bcl-2, and poly-β-amino ester. Through in vitro assays, increased osteogenic potential and biological resilience were demonstrated in the magnetofected group over control and nucleofected groups. Similarly, our in vivo calvarial defect study showed that magnetofection had an efficiency rate of 30%, which in turn resulted in significantly more healing compared with control group and nucleofected group. Our novel, prefabricated MNP-integrated scaffold allows for in situ postimplant temporospatial control of cell transfection to augment bone regeneration. Stem Cells Translational Medicine 2017;6:151-160.

Cell Death in Chondrocytes, Osteoblasts, and Osteocytes

Cell death in skeletal component cells, including chondrocytes, osteoblasts, and osteocytes, plays roles in skeletal development, maintenance, and repair as well as in the pathogenesis of osteoarthritis and osteoporosis. Chondrocyte proliferation, differentiation, and apoptosis are important steps for endochondral ossification. Although the inactivation of P53 and RB is involved in the pathogenesis of osteosarcomas, the deletion of p53 and inactivation of Rb are insufficient to enhance chondrocyte proliferation, indicating the presence of multiple inhibitory mechanisms against sarcomagenesis in chondrocytes. The inflammatory processes induced by mechanical injury and chondrocyte death through the release of danger-associated molecular patterns (DAMPs) are involved in the pathogenesis of posttraumatic osteoarthritis. The overexpression of BCLXL increases bone volume with a normal structure and maintains bone during aging by inhibiting osteoblast apoptosis. p53 inhibits osteoblast proliferation and enhances osteoblast apoptosis, thereby reducing bone formation, but also exerts positive effects on osteoblast differentiation through the Akt–FoxOs pathway. Apoptotic osteocytes release ATP, which induces the receptor activator of nuclear factor κ-B ligand (Rankl) expression and osteoclastogenesis, from pannexin 1 channels. Osteocyte death ultimately results in necrosis; DAMPs are released to the bone surface and promote the production of proinflammatory cytokines, which induce Rankl expression, and osteoclastogenesis is further enhanced.

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.

Histone demethylase Jmjd3 regulates osteoblast apoptosis through targeting anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bim

Posttranslational modifications including histone methylation regulate gene transcription through directly affecting the structure of chromatin. Trimethylation of histone H3K27 (H3K27me3) contributes to gene silencing and the histone demethylase Jumonji domain-containing 3 (Jmjd3) specifically removes the methylation of H3K27me3, followed by the activation of gene expression. In the present study, we explored the roles of Jmjd3 in regulating osteoblast apoptosis. Knockdown of Jmjd3 promoted osteoblast apoptosis induced by serum deprivation with decreased mitochondrial membrane potential and increased levels of caspase-3 activation, PARP cleavage, and DNA fragmentation. B cell lymphoma-2 (Bcl-2), an anti-apoptotic protein, was down-regulated by knockdown of Jmjd3 through retaining H3K27me3 on its promoter region. Knockdown of Jmjd3 increased the pro-apoptotic activity of Bim through inhibiting ERK-dependent phosphorylation of Bim. Protein kinase D1 (PKD1), which stimulates ERK phosphorylation, decreased in the Jmjd3-knockdown cells and introduction of PKD1 relieved osteoblast apoptosis in the Jmjd3-knockdown cells through increasing ERK-regulated Bim phosphorylation. These results suggest that Jmjd3 regulates osteoblast apoptosis through targeting Bcl-2 expression and Bim phosphorylation.

Predictive genetic markers of coagulation, inflammation and apoptosis in Perthes disease—Serbian experience

Perthes disease is one of the most common forms of pediatric femoral head osteonecrosis with an unknown etiology. Coagulation factors were the first genetic factors suspected to have a role in the pathogenesis of this disease, but studies showed inconsistent results. It is described that inflammation is present during early stages of Perthes disease, but its genetic aspect has not been studied extensively. Little is known regarding the status of apoptotic factors during the repair process that leads to the occurrence of hip deformity in patients. Therefore, the aim of this study was to analyze major mediators involved in coagulation, inflammation, and apoptotic processes as possible causative factors of Perthes disease. The study cohort consisted of 37 patients. Gene variants of TNF-α, FV, FII, and MTHFR genes were determined by PCR-RFLP, while IL-3 and PAI-1 were genotyped by direct sequencing. The expression level of Bax, Bcl-2, Bcl2L12, Fas and FasL was analyzed by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) technique. Our results showed a significantly increased level of expression of pro-apoptotic factor Bax along with significantly higher Bax/Bcl-2 ratio in the patient group.

CONCLUSION

The results presented indicate that apoptosis could be one of the factors contributing to the lack of balanced bone remodeling process in Perthes patients.

EphrinB2 signaling in osteoblasts promotes bone mineralization by preventing apoptosis

Cells that form bone (osteoblasts) express both ephrinB2 and EphB4, and previous work has shown that pharmacological inhibition of the ephrinB2/EphB4 interaction impairs osteoblast differentiation in vitro and in vivo. The purpose of this study was to determine the role of ephrinB2 signaling in the osteoblast lineage in the process of bone formation. Cultured osteoblasts from mice with osteoblast-specific ablation of ephrinB2 showed delayed expression of osteoblast differentiation markers, a finding that was reproduced by ephrinB2, but not EphB4, RNA interference. Microcomputed tomography, histomorphometry, and mechanical testing of the mice lacking ephrinB2 in osteoblasts revealed a 2-fold delay in bone mineralization, a significant reduction in bone stiffness, and a 50% reduction in osteoblast differentiation induced by anabolic parathyroid hormone (PTH) treatment, compared to littermate sex- and age-matched controls. These defects were associated with significantly lower mRNA levels of late osteoblast differentiation markers and greater levels of osteoblast and osteocyte apoptosis, indicated by TUNEL staining and transmission electron microscopy of bone samples, and a 2-fold increase in annexin V staining and 7-fold increase in caspase 8 activation in cultured ephrinB2 deficient osteoblasts. We conclude that osteoblast differentiation and bone strength are maintained by antiapoptotic actions of ephrinB2 signaling within the osteoblast lineage.

Age-related marrow adipogenesis is linked to increased expression of RANKL

With advancing age bone marrow is progressively replaced with adipose tissue, accompanied by a concomitant decline in bone mass and strength. The mechanism underlying the increase in marrow fat and bone destruction remains elusive. We found that on the way of adipogenic differentiation of marrow stromal cells, receptor activator for NF-κB ligand (Rankl) expression was induced, concomitantly with a down-regulation of osteoprotegerin, which prompted us to hypothesize that cells at a preadipocyte stage express RANKL. This concept was supported by the findings that the early adipogenic transcription factors C/EBPβ and C/EBPδ, but not the late factor peroxisome proliferator-activated receptor γ, bind to the Rankl promoter and stimulate Rankl gene transcription. In fact, when cells isolated from the bone marrow of aging mice were analyzed by flow cytometry, we found that cells expressing the pre-adipocyte marker Pref-1 were RANKL-positive, and the number of these cells was increased with aging, with concomitant down-regulation of osteoprotegerin, and most importantly, that these RANKL(+)/Pref-1(+) marrow cells were capable of generating osteoclasts from bone marrow macrophages. Thus, the capacity of cells at a pre-adipocyte stage to express RANKL via C/EBPβ and C/EBPδ and to support osteoclastogenesis may account partly for the co-progression of fatty marrow and bone destruction with aging.

Bcl2 deficiency activates FoxO through Akt inactivation and accelerates osteoblast differentiation

Osteoblast apoptosis plays an important role in bone development and maintenance, and is in part responsible for osteoporosis in sex steroid deficiency, glucocorticoid excess, and aging. Although Bcl2 subfamily proteins, including Bcl2 and Bcl-XL, inhibit apoptosis, the physiological significance of Bcl2 in osteoblast differentiation has not been fully elucidated. To investigate this, we examined Bcl2-deficient (Bcl2(-/-)) mice. In Bcl2(-/-) mice, bromodeoxyuridine (BrdU)-positive osteoblasts were reduced in number, while terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL)-positive osteoblasts were increased. Unexpectedly, osteoblast differentiation was accelerated in Bcl2(-/-) mice as shown by the early appearance of osteocalcin-positive osteoblasts. Osteoblast differentiation was also accelerated in vitro when primary osteoblasts were seeded at a high concentration to minimize the reduction of the cell density by apoptosis during culture. FoxO transcription factors, whose activities are negatively regulated through the phosphorylation by Akt, play important roles in multiple cell events, including proliferation, death, differentiation, longevity, and stress response. Expressions of FasL, Gadd45a, and Bim, which are regulated by FoxOs, were upregulated; the expression and activity of FoxOs were enhanced; and the phosphorylation of Akt and that of FoxO1 and FoxO3a by Akt were reduced in Bcl2(-/-) calvariae. Further, the levels of p53 mRNA and protein were increased, and the expression of p53-target genes, Pten and Igfbp3 whose proteins inhibit Akt activation, was upregulated in Bcl2(-/-) calvariae. However, Pten but not Igfbp3 was upregulated in Bcl2(-/-) primary osteoblasts, and p53 induced Pten but not Igfbp3 in vitro. Silencing of either FoxO1 or FoxO3a inhibited and constitutively-active FoxO3a enhanced osteoblast differentiation. These findings suggest that Bcl2 deficiency induces and activates FoxOs through Akt inactivation, at least in part, by upregulating Pten expression through p53 in osteoblasts, and that the enhanced expression and activities of FoxOs may be one of the causes of accelerated osteoblast differentiation in Bcl2(-/-) mice.

Signaling cascades of Pasteurella multocida toxin in immune evasion

Pasteurella multocida toxin (PMT) is a protein toxin found in toxigenic strains of Pasteurella multocida. PMT is the causative agent for atrophic rhinitis in pigs, a disease characterized by loss of nasal turbinate bones due to an inhibition of osteoblast function and an increase in osteoclast activity and numbers. Apart from this, PMT acts as a strong mitogen, protects from apoptosis and has an impact on the differentiation and function of immune cells. Many signaling pathways have been elucidated, however, the effect of these signaling cascades as a means to subvert the host’s immune system are just beginning to unravel.

Simvastatin enhances human osteoblast proliferation involved in mitochondrial energy generation

Simvastatin has been shown to stimulate osteogenic cell differentiation. Our previous study showed osteoblasts on trabecular surface are increased by simvastatin treatment in animal study. However, whether simvastatin stimulates osteoblast proliferation and by what molecular mechanism have not been adequately investigated. Because the mitochondrial function is crucial for cell survival and proliferation, we hypothesize that simvastatin may promote human osteoblast (hOBs) proliferation and it may be related to mitochondrial function. Our results showed that simvastatin significantly enhanced proliferation and increased both mRNA and protein levels of cyclin D2, Bcl-2 and the ratio of Bcl-2 to Bax (Bcl-2/Bax). Furthermore, simvastatin increased mitochondrial activity and ATP content of hOBs. Most importantly, treatment with ATP synthase blocker, oligomycin, significantly decreased both simvastatin-stimulated ATP content and cell proliferation, and completely reversed the simvastatin-induced up-regulation of cyclin D2 and Bcl-2 expression in hOBs. On the other hand, rotenone, the complex I blocker, also partially blocked simvastatin-stimulated ATP content and cell proliferation, but the blocker did not suppress the effect of simvastatin on cyclin D2 and Bcl-2 expression. These results indicate that the up-regulation of cyclin D2 and Bcl-2/Bax by simvastatin depends on the intact function of ATP synthase in the mitochondria of hOBs. It suggests that simvastatin may promote hOB proliferation, at least partly, via up-regulating mitochondrial function and subsequently cyclin D2 and Bcl-2/Bax expression. The findings provide new information for the basic medical science in bone physiology and for new therapy strategy of simvastatin on bone formation in future.

Necrotic and apoptotic cells serve as nuclei for calcification on osteoblastic differentiation of human mesenchymal stem cells in vitro

A close relationship between cell death and pathological calcification has recently been reported, such as vascular calcification in atherosclerosis. However, the roles of cell death in calcification by osteoblast lineage have not been elucidated in detail. In this study, we investigated whether cell death is involved in the calcification on osteoblastic differentiation of human bone marrow mesenchymal stem cells (hMSC) under osteogenic culture in vitro. Apoptosis and necrosis occurred in an osteogenic culture of hMSC, and cell death preceded calcification. The generation of intracellular reactive oxygen species, chromatin condensation and fragmentation, and caspase-3 activation increased in this culture. A pan-caspase inhibitor (Z-VAD-FMK) and anti-oxidants (Tiron and n-acetylcysteine) inhibited osteogenic culture-induced cell death and calcification. Furthermore, calcification was significantly promoted by the addition of necrotic dead cells or its membrane fraction. Spontaneously dead cells by osteogenic culture and exogenously added necrotic cells were surrounded by calcium deposits. Induction of localized cell death by photodynamic treatment in the osteogenic culture resulted in co-localized calcification. These findings show that necrotic and apoptotic cell deaths were induced in an osteogenic culture of hMSC and indicated that both necrotic and apoptotic cells of osteoblast lineage served as nuclei for calcification on osteoblastic differentiation of hMSC in vitro.

The deep-sea natural products, biogenic polyphosphate (Bio-PolyP) and biogenic silica (Bio-Silica), as biomimetic scaffolds for bone tissue engineering: fabrication of a morphogenetically-active polymer

Bone defects in human, caused by fractures/nonunions or trauma, gain increasing impact and have become a medical challenge in the present-day aging population. Frequently, those fractures require surgical intervention which ideally relies on autografts or suboptimally on allografts. Therefore, it is pressing and likewise challenging to develop bone substitution materials to heal bone defects. During the differentiation of osteoblasts from their mesenchymal progenitor/stem cells and of osteoclasts from their hemopoietic precursor cells, a lineage-specific release of growth factors and a trans-lineage homeostatic cross-talk via signaling molecules take place. Hence, the major hurdle is to fabricate a template that is functioning in a way mimicking the morphogenetic, inductive role(s) of the native extracellular matrix. In the last few years, two naturally occurring polymers that are produced by deep-sea sponges, the biogenic polyphosphate (bio-polyP) and biogenic silica (bio-silica) have also been identified as promoting morphogenetic on both osteoblasts and osteoclasts. These polymers elicit cytokines that affect bone mineralization (hydroxyapatite formation). In this manner, bio-silica and bio-polyP cause an increased release of BMP-2, the key mediator activating the anabolic arm of the hydroxyapatite forming cells, and of RANKL. In addition, bio-polyP inhibits the progression of the pre-osteoclasts to functionally active osteoclasts. Based on these findings, new bioinspired strategies for the fabrication of bone biomimetic templates have been developed applying 3D-printing techniques. Finally, a strategy is outlined by which these two morphogenetically active polymers might be used to develop a novel functionally active polymer.

Silicate modulates the cross-talk between osteoblasts (SaOS-2) and osteoclasts (RAW 264.7 cells): inhibition of osteoclast growth and differentiation

It has been shown that inorganic monomeric and polymeric silica/silicate, in the presence of the biomineralization cocktail, increases the expression of osteoprotegerin (OPG) in osteogenic SaOS-2 sarcoma cells in vitro. In contrast, silicate does not affect the steady-state gene expression level of the osteoclastogenic ligand receptor activator of NF-κB ligand (RANKL). In turn it can be expected that the concentration ratio of the mediators OPG/RANKL increases in the presence of silicate. In addition, silicate enhances the growth potential of SaOS-2 cells in vitro, while it causes no effect on RAW 264.7 cells within a concentration range of 10-100 µM. Applying a co-cultivation assay system, using SaOS-2 cells and RAW 264.7 cells, it is shown that in the presence of 10 µM silicate the number of RAW 264.7 cells in general, and the number of TRAP(+) RAW 264.7 cells in particular markedly decreases. The SaOS-2 cells retain their capacity of differential gene expression of OPG and RANKL in favor of OPG after exposure to silicate. It is concluded that after exposure of the cells to silicate a factor(s) is released from SaOS-2 cells that causes a significant inhibition of osteoclastogenesis of RAW 264.7 cells. It is assumed that it is an increased secretion of the cytokine OPG that is primarily involved in the reduction of the osteoclastogenesis of the RAW 264.7 cells. It is proposed that silicate might have the potential to stimulate osteogenesis in vivo and perhaps to ameliorate osteoporotic disorders.

Intracellular and extracellular ATP coordinately regulate the inverse correlation between osteoclast survival and bone resorption

Osteoclasts, highly differentiated bone-resorbing cells of hematopoietic origin, have two conflicting tendencies: a lower capacity to survive and a higher capacity to execute energy-consuming activities such as bone resorption. Here, we report that when compared with their precursors, mature mitochondria-rich osteoclasts have lower levels of intracellular ATP, which is associated with receptor activator of nuclear factor κ-B ligand (RANKL)-induced Bcl-x(L) down-regulation. Severe ATP depletion, caused by disrupting mitochondrial transcription factor A (Tfam) gene, leads to increased bone-resorbing activity despite accelerated apoptosis. Although AMP-activated protein kinase (AMPK) activation by ATP depletion is not involved in the regulation of osteoclast function, the release of ATP from intracellular stores negatively regulates bone-resorbing activity through an autocrine/paracrine feedback loop by altering cytoskeletal structures. Furthermore, osteoclasts derived from aged mice exhibit reduced mitochondrial DNA (mtDNA) and intracellular ATP levels with increased bone-resorbing activity, implicating the possible involvement of age-related mitochondrial dysfunction in osteoporosis. Thus, our study provides evidence for a mechanism underlying the control of cellular functions by reciprocal changes in intracellular and extracellular ATP, which regulate the negative correlation between osteoclast survival and bone resorption.

There are many potential medical therapies for atraumatic osteonecrosis

Atraumatic osteonecrosis is a common complication of SLE and is seen in other connective tissue diseases, in patients treated with high doses of CSs, in HIV-infected patients and in alcoholic patients. Standard care is confined to analgesia, core decompression if the condition is early and affects the femoral head and joint replacement. However, consideration of the underlying biological mechanisms leads to the recognition of many potential therapies that might either prevent progression or, even, reverse the process if it is not yet too far advanced. These potential therapies merit detailed consideration. Critical points are that (i) histopathological evidence shows that the initial cellular event is apoptosis of osteocytes; and (ii) another requisite, as homeostasis requires that death and rebirth of osteocytes be balanced, is an accompanying inadequate proliferative capacity of osteoblasts. Thus, a logical approach to treatment includes measures that (i) reduce apoptosis of osteocytes and (ii) enhance proliferation of osteoblasts/pre-osteoblasts. Measures to reduce the ongoing apoptosis of osteocytes require reinforcing the effects of members of the Bcl-2 family (Bcl-2 itself and Mcl-1), the Wnt/catenin pathways (using an available sclerostin antibody) and HSPs (by application of local heat using US, deep wave diathermy or infrared), as well as administration of bisphosphonates and nitrates. Measures to enhance proliferation of osteoblasts/pre-osteoblasts include the use of stem cells, extracorporeal shock wave therapy, aspirin, the proteosome inhibitor bortezomib, melatonin and application of local heat. Use of VEGF would encourage proliferation of blood vessels and osteogenesis. Certain drugs that inhibit osteoblast proliferation should be avoided, including NSAIDs, serotonin reuptake inhibitors and thiazolidinediones.

Anti-apoptotic gene Bcl2 is required for stapes development and hearing

In this paper we describe novel and specific roles for the apoptotic regulators Bcl2 and Bim in hearing and stapes development. Bcl2 is anti-apoptotic while Bim is pro-apoptotic. Characterization of the auditory systems of mice deficient for these molecules revealed that Bcl2^−/− mice suffered severe hearing loss. This was conductive in nature and did not affect sensory cells of the inner ear, with cochlear hair cells and neurons present and functional. Bcl2^−/− mice were found to have a malformed, often monocrural, porous stapes (the small stirrup-shaped bone of the middle ear), but a normally shaped malleus and incus. The deformed stapes was discontinuous with the incus and sometimes fused to the temporal bones. The defect was completely rescued in Bcl2^−/−Bim^−/− mice and partially rescued in Bcl2^−/−Bim^+/− mice, which displayed high-frequency hearing loss and thickening of the stapes anterior crus. The Bcl2^−/− defect arose in utero before or during the cartilage stage of stapes development. These results implicate Bcl2 and Bim in regulating survival of second pharyngeal arch or neural crest cells that give rise to the stapes during embryonic development.

Celecoxib and Bcl-2: emerging possibilities for anticancer drug design

Celecoxib is a multifaceted drug with promising anticancer properties. A number of studies have been conducted that implicate the compound in modulating the expression of Bcl-2 family members and mitochondria-mediated apoptosis. The growing data surrounding the role of celecoxib in the regulation of the mitochondrial death pathway provides a platform for ongoing debate. Studies that describe celecoxib's properties as a BH3 mimic or as a direct inhibitor of Bcl-2 are not available. The motivations for this review are: to provide the basis for the development of novel compounds that modulate Bcl-2 expression using celecoxib as a structural starting point and to encourage additional biological studies (such as binding and enzymatic assays) that would provide information regarding celecoxib's role as a Bcl-2 antagonist. The current review summarizes work that identifies the role of celecoxib in blocking the activity of Bcl-2.

Distinguishing the proapoptotic and antiresorptive functions of risedronate in murine osteoclasts: role of the Akt pathway and the ERK/Bim axis

OBJECTIVE

Nitrogen-containing bisphosphonates are one of the most successful therapeutics for osteoporosis. The aim of this study was to elucidate the functional mechanism of one of the typical nitrogen-containing bisphosphonates, risedronate.

METHODS

Osteoclasts generated from murine bone marrow macrophages were treated with risedronate in vitro, and its effects on apoptosis and bone-resorbing activity were examined. The mechanism of action of risedronate was examined by gene induction of constitutively active Akt-1 and constitutively active MEK-1, and by gene deletion of Bim. Bim(-/-) mice, in which osteoclasts were resistant to apoptosis, were treated with risedronate and analyzed radiographically, biochemically, and histologically.

RESULTS

Risedronate induced osteoclast apoptosis through the mitochondria-dependent pathway with an increased expression of Bim, and the proapoptotic effect of risedronate was suppressed by Bim deletion and constitutively active MEK-1 introduction. In contrast, the risedronate-induced suppression of bone resorption was completely reversed by inducing constitutively active Akt-1, but not by Bim deletion or constitutively active MEK-1 introduction. These results suggested that apoptosis and bone-resorbing activity of osteoclasts were regulated through the ERK/Bim axis and the Akt pathway, respectively, both of which were suppressed by risedronate. Although osteoclast apoptosis in response to risedronate administration was suppressed in the Bim(-/-) mice, risedronate treatment increased bone mineral density in Bim(-/-) mice at a level equivalent to that in wild-type mice.

CONCLUSION

Our findings indicate that the antiresorptive effect of risedronate in vivo is mainly mediated by the suppression of the bone-resorbing activity of osteoclasts and not by the induction of osteoclast apoptosis.

The transcription factor FBI-1/OCZF/LRF is expressed in osteoclasts and regulates RANKL-induced osteoclast formation in vitro and in vivo

OBJECTIVE

Since transcription factors expressed in osteoclasts are possible targets for regulation of bone destruction in bone disorders, we investigated the expression of the transcription factor FBI-1/OCZF/LRF (in humans, factor that binds to inducer of short transcripts of human immunodeficiency virus type 1; in rats, osteoclast-derived zinc finger; in mice, leukemia/lymphoma-related factor) in patients with rheumatoid arthritis (RA), and assessed its role in osteoclastogenesis in vivo.

METHODS

Expression of FBI-1/OCZF was investigated in subchondral osteoclasts in human RA and in rat adjuvant-induced arthritis (AIA) using immunostaining and in situ hybridization, respectively. Transgenic mice overexpressing OCZF (OCZF-Tg) under the control of the cathepsin K promoter were generated, and bone mineral density and bone histomorphometric features were determined by peripheral quantitative computed tomography, calcein double-labeling, and specific staining for osteoclasts and osteoblasts. LRF/OCZF expression and the consequence of LRF inhibition were assessed in vitro with RANKL-induced osteoclast differentiation.

RESULTS

FBI-1/OCZF was detected in the nuclei of osteoclasts in rat AIA and human RA. RANKL increased the levels of LRF messenger RNA and nuclear-localized LRF protein in primary macrophages. In OCZF-Tg mice, bone volume was significantly decreased, the number of osteoclasts, but not osteoblasts, was increased in long bones, and osteoclast survival was promoted. Conversely, inhibition of LRF expression suppressed the formation of osteoclasts from macrophages in vitro.

CONCLUSION

FBI-1/OCZF/LRF regulates osteoclast formation and apoptosis in vivo, and may become a useful marker and target in treating disorders leading to reduced bone density, including chronic arthritis.

MLK3 regulates bone development downstream of the faciogenital dysplasia protein FGD1 in mice

Mutations in human FYVE, RhoGEF, and PH domain-containing 1 (FGD1) cause faciogenital dysplasia (FGDY; also known as Aarskog syndrome), an X-linked disorder that affects multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase CDC42. However, the mechanisms by which mutations in FGD1 affect skeletal development are unknown. Here, we describe what we believe to be a novel signaling pathway in osteoblasts initiated by FGD1 that involves the MAP3K mixed-lineage kinase 3 (MLK3). We observed that MLK3 functions downstream of FGD1 to regulate ERK and p38 MAPK, which in turn phosphorylate and activate the master regulator of osteoblast differentiation, Runx2. Mutations in FGD1 found in individuals with FGDY ablated its ability to activate MLK3. Consistent with our description of this pathway and the phenotype of patients with FGD1 mutations, mice with a targeted deletion of Mlk3 displayed multiple skeletal defects, including dental abnormalities, deficient calvarial mineralization, and reduced bone mass. Furthermore, mice with knockin of a mutant Mlk3 allele that is resistant to activation by FGD1/CDC42 displayed similar skeletal defects, demonstrating that activation of MLK3 specifically by FGD1/CDC42 is important for skeletal mineralization. Thus, our results provide a putative biochemical mechanism for the skeletal defects in human FGDY and suggest that modulating MAPK signaling may benefit these patients.

The role of the BH3-only protein Noxa in bone homeostasis

Bone homeostasis is maintained by a dynamic balance between bone resorption by osteoclasts and bone formation by osteoblasts. Since excessive osteoclast activity is implicated in pathological bone resorption, understanding the mechanism underlying osteoclast differentiation, function and survival is of both scientific and clinical importance. Osteoclasts are monocyte/macrophage lineage cells with a short life span that undergo rapid apoptosis, the rate of which critically determines the level of bone resorption in vivo. However, the molecular basis of rapid osteoclast apoptosis remains obscure. Here we report the role of a BH3-only protein, Noxa (encoded by the Pmaip1 gene), in bone homeostasis using Noxa-deficient mice. Among the Bcl-2 family members, Noxa was selectively induced during osteoclastogenesis. Mice lacking Noxa exhibit a severe osteoporotic phenotype due to an increased number of osteoclasts. Noxa deficiency did not have any effect on the number of osteoclast precursor cells or the expression of osteoclast-specific genes, but led to a prolonged survival of osteoclasts. Furthermore, adenovirus-mediated Noxa overexpression remarkably reduced bone loss in a model of inflammation-induced bone destruction. This study reveals Noxa to be a crucial regulator of osteoclast apoptosis, and may provide a molecular basis for a new therapeutic approach to bone diseases.

Bim: guardian of tissue homeostasis and critical regulator of the immune system, tumorigenesis and bone biology

One of the most important roles of apoptosis is the maintenance of tissue homeostasis. Impairment of apoptosis leads to a number of pathological conditions. In response to apoptotic signals, various proteins are activated in a pathway and signal-specific manner. Recently, the pro-apoptotic molecule Bim has attracted increasing attention as a pivotal regulator of tissue homeostasis. The Bim expression level is strictly controlled in both transcriptional and post-transcriptional levels. This control is dependent on cell, tissue and apoptotic stimuli. The phenotype of Bim-deficient mice is a systemic lupus erythematosus-like autoimmune disease with an abnormal accumulation of hematopoietic cells. Bim is thus a critical regulator of hematopoietic cells and immune system. Further studies have revealed the critical roles of Bim in various normal and pathological conditions, including bone homeostasis and tumorigenesis. The current understanding of Bim signaling and roles in the maintenance of tissue homeostasis is reviewed in this paper, focusing on the immune system, bone biology and tumorigenesis to illustrate the diversified role of Bim.

New insight into the antifibrotic effects of praziquantel on mice in infection with Schistosoma japonicum

BACKGROUND

Schistosomiasis is a parasitic disease infecting more than 200 million people in the world. Although chemotherapy targeting on killing schistosomes is one of the main strategies in the disease control, there are few effective ways of dealing with liver fibrosis caused by the parasite infection in the chronic and advanced stages of schistosomiasis. For this reason, new strategies and prospective drugs, which exert antifibrotic effects, are urgently required.

METHODS AND FINDINGS

The antifibrotic effects of praziquantel were assessed in the murine models of schistosomiasis japonica. Murine fibrosis models were established by cutaneous infection with 14 ± 2 Schistosoma japonicum cercariae. Then, the mice of both chronic (8 weeks post-infection) and advanced (15 weeks post-infection) schistosomiasis were treated by gavage of praziquantel (250 mg/kg, once daily for 3 days) to eliminate worms, and followed by praziquantel anti-fibrosis treatment (300 mg/kg, twice daily for 30 days). The fibrosis-related parameters assessed were areas of collagen deposition, content of hydroxyproline and mRNA expressions of Col1α1, Col3α1, α-SMA, TGF-β, MMP9, TIMP1, IL-4, IL-10, IL-13 and IFN-γ of liver. Spleen weight index, alanine aminotransferase activity and liver portal venous pressure were also measured. The results showed that anti-fibrosis treatment improved liver fibrosis, splenomegaly, hepatic function, as well as liver portal hypertension. In order to confirm the anti-fibrotic properties of praziquantel, we established a CCL(4)-induced model and revealed that CCL(4)-induced liver fibrosis was inhibited by PZQ treatment for 30 days. Furthermore, we analyzed the effects of praziquantel on mouse primary hepatic stellate cells (HSCs). It is indicated that mRNA expressions of Col1α1, Col3α1, α-SMA, TGF-β, MMP9 and TIMP1 of HSCs were all inhibited after praziquantel anti-parasite treatments.

CONCLUSIONS

The significant amelioration of hepatic fibrosis by praziquantel treatment validates it as a promising drug of anti-fibrosis and offers potential of a new chemotherapy for hepatic fibrosis resulting from schistosomiasis.