Bim, Bak, and Bax regulate osteoblast survival

Molecular mechanism of a novel root-end filling material containing zirconium oxide on the osteogenic/odontogenic differentiation of human osteosarcoma MG-63 cells

Although the novel root-end filling material containing zirconium oxide (NRFM-Zr) which is hydroxyapatite-based may promote osteoblast differentiation, the molecular mechanism remains unclear. The aim of this study is to investigate it underlying the osteogenic/odontogenic differentiation of human osteosarcoma MG-63 cells induced by NRFM-Zr, compared with calcium silicate-based mineral trioxide aggregate (MTA), and glass ionomer cement (GIC). Firstly, three different types of root filling materials were co-cultured with MG-63 cells, and their cell toxicity, alkaline phosphatase (ALP) activity, and calcium ion concentration were evaluated. Next, gene expression profiling microarray was employed to analyze the impact of the materials on the gene expression profile of MG-63 cells. The results of cell viability revealed that NRFM-Zr group had no significant difference compared to the negative control group. After 5 and 7 days of cultivation, both the NRFM-Zr and MTA groups exhibited significantly higher ALP activity compared to the negative control (p < 0.05). Moreover, the NRFM-Zr group had the highest calcium ion concentration, while the GIC group was the lowest (p < 0.05). Gene expression profiling microarray analysis identified 2915 (NRFM-Zr), 2254 (MTA) and 392 (GIC) differentially expressed genes, respectively. GO functional and KEGG pathway analysis revealed that differentially expressed genes of NRFM-Zr, MTA and GIC participated in 8, 6 and 0 differentiation-related pathways, respectively. Comparing the molecular mechanisms of osteogenic/odontogenic differentiation induced by hydroxyapatite-based NRFM-Zr and calcium silicate-based MTA, it was found that they shared similarities in their molecular mechanisms of promoting osteogenic differentiation. NRFM-Zr primarily promotes differentiation and inhibits cell apoptosis, thereby enhancing osteogenic/odontogenic differentiation of MG-63 cells. Furthermore, the inducing efficacy of NRFM-Zr was found to be superior to MTA.

The role of E3 ubiquitin ligases in bone homeostasis and related diseases

The ubiquitin-proteasome system (UPS) dedicates to degrade intracellular proteins to modulate demic homeostasis and functions of organisms. These enzymatic cascades mark and modifies target proteins diversly through covalently binding ubiquitin molecules. In the UPS, E3 ubiquitin ligases are the crucial constituents by the advantage of recognizing and presenting proteins to proteasomes for proteolysis. As the major regulators of protein homeostasis, E3 ligases are indispensable to proper cell manners in diverse systems, and they are well described in physiological bone growth and bone metabolism. Pathologically, classic bone-related diseases such as metabolic bone diseases, arthritis, bone neoplasms and bone metastasis of the tumor, etc., were also depicted in a UPS-dependent manner. Therefore, skeletal system is versatilely regulated by UPS and it is worthy to summarize the underlying mechanism. Furthermore, based on the current status of treatment, normal or pathological osteogenesis and tumorigenesis elaborated in this review highlight the clinical significance of UPS research. As a strategy possibly remedies the limitations of UPS treatment, emerging PROTAC was described comprehensively to illustrate its potential in clinical application. Altogether, the purpose of this review aims to provide more evidence for exploiting novel therapeutic strategies based on UPS for bone associated diseases.

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.

Integrative Analysis of Regulatory Module Reveals Associations of Microgravity with Dysfunctions of Multi-body Systems and Tumorigenesis

Previous studies have demonstrated that microgravity could lead to health risks. The investigation of the molecular mechanisms from the aspect of systems biology has not been performed yet. Here, we integratively analyzed transcriptional and post-transcriptional regulations based on gene and miRNA expression profiles in human peripheral blood lymphocytes cultured in modeled microgravity. Two hundred and thirty dysregulated TF-miRNA (transcription factor and microRNA) feed-forward loops (FFLs) were identified in microgravity. The immune, cardiovascular, endocrine, nervous and skeletal system subnetworks were constructed according to the functions of dysregulated FFLs. Taking the skeletal system as an example, most of genes and miRNAs in the subnetwork were involved in bone loss. In addition, several drugs have been predicted to have potential to reduce bone loss, such as traditional Chinese medicines Emodin and Ginsenoside Rh2. Furthermore, we investigated the relationships between microgravity and 20 cancer types, and found that most of cancers might be promoted by microgravity. For example, rectum adenocarcinoma (READ) might be induced by microgravity through reducing antigen presentation and suppressing IgA-antibody-secreting cells' migration. Collectively, TF-miRNA FFL might provide a novel mechanism to elucidate the changes induced by microgravity, serve as drug targets to relieve microgravity effects, and give new insights to explore the relationships between microgravity and cancers.

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.

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.

Production of bone mineral material and BMP-2 in osteoblasts cultured on double acid-etched titanium

Background

The study of osteoblasts and their osteogenic functions is essential in order to understand them and their applications in implantology. In this sense, this study try to study BMP-2 production and bone matrix deposition, in addition to other biological variables, in osteoblasts cultured on a rough double acid-etched titanium surface (Osseotite®, Biomet 3i, Palm Beach Garden, Florida, USA) in comparison to a smooth titanium surface (machined) and a control Petri dish.

Material and Methods

An in vitro prospective study. NHOst human osteoblasts from the femur were cultured on three different surfaces: Control group: 25-mm methacrylate dish (n = 6); Machined group: titanium discs with machined surface (n = 6) and Experimental group: titanium discs with a double acid-etched nitric and hydrofluoric Osseotite® acid surface (n = 6). A quantification of the mitochondrial membrane potential, and studies of apoptosis, mobility and adhesion, bone productivity (BMP-2) and cellular bone synthesis were carried out after culturing the three groups for forty-eight hours.

Results

A statistically significant difference was observed in the production of BMP-2 between the experimental group and the other two groups (22.33% ± 11.06 vs. 13.10% ± 5.51 in the machined group and 3.88% ± 3.43 in the control group). Differences in cellular bone synthesis were also observed between the groups (28.34% ± 14.4% in the experimental group vs. 20.03% ± 6.79 in the machined group and 19.34% ± 15.93% in the control group).

Conclusions

In comparison with machined surfaces, Osseotite® surfaces favor BMP-2 production and bone synthesis as a result of the osteoblasts in contact with it.

Key words:BMP-2, Cytoskeleton, cell culture, bone matrix, apoptosis, cell viability.

Analysis of DNA methylation in chondrocytes in rats with knee osteoarthritis

BACKGROUND

Knee osteoarthritis (KOA) is a degenerative knee disease commonly found in the ageing population. DNA methylation works with histone acetylation to participate in aging. Alterations of DNA methylation may involve the joint chondrocyte degeneration in KOA. The aim of this study is to detect DNA methylation changes in chondrocytes of rats with KOA.

METHODS

The rat KOA model was established with the Hulth method (n = 10), while rats receiving sham operation served as the control (n = 10). At 16 weeks after modeling, the knee joint tissue was collected from half of the rats in each group for Micro-CT scanning, Haematoxylin& Eosin (HE) staining, ABH/OG staining, immunohistochemistry for Bax, Bcl-2 and Fas, and TUNNEL staining. Meanwhile, the articular cartilage was collected from the other half to detect promoter methylation in target genes with the MethylTarget approach.

RESULTS

Micro-CT scanning, HE staining, ABH/OG staining, immunohistochemistry, and TUNNEL staining all showed more severe cartilage injury in the KOA group than in the control group, indicating successful establishment of KOA model. The methylation rate in the KOA group was significantly decreased for C/ebpα-2 (within a CpG island -452 bp to the initiation codon on chromosome 1 91,363,511), Cdk2 (within a CpG island -55 bp to the initiation codon on chromosome 7 3,132,362), Bak1 (within a CpG island 6452 bp to the initiation codon on chromosome 20 5,622,277), and Fas (within a CpG island on the entire chromosome 1 gene), compared with the sham group (P = 0.005, 0.008, 0.022 and 0.027, respectively).

CONCLUSION

The chondrocyte apoptosis and significantly reduced methylation levels of C/ebpα-2, Cdk2, Bak1, and Fas may participate in the pathogenesis of KOA. However, the exact mechanisms remain to be determined.

A miRNA-101-3p/Bim axis as a determinant of serum deprivation-induced endothelial cell apoptosis

Serum deprivation or withdrawal induces apoptosis in endothelial cells, resulting in endothelial cell dysfunction that is associated with cardiovascular disease. However, there is still limited information on the role of miRNA in serum deprivation-induced apoptosis. Here we found that serum deprivation increased caspase-dependent apoptosis through miRNA-101-3p downregulation, without altering expression of its host gene RNA 3′-terminal phosphate cyclase-like 1, which was highly correlated with suppressed expression levels of Dicer and Argonaute 2 (Ago2), indicating that miR-101-3p is post-transcriptionally elevated in serum-deprived conditions. The decreased miR-101-3p caused elevated Bim expression by targeting its 3′-untranslated region (3′-UTR). This resulted in activation of the intrinsic pathway of apoptosis via interaction with Bcl-2, decreased mitochondrial membrane potential, cytochrome c release, mitochondrial reactive oxygen species (ROS) production, and caspase activation. These events were abrogated by miR-101-3p mimic and the proapoptotic Bim siRNA, which suggest a determinant role of the miR-101-3p/Bim axis in serum deprivation-induced apoptosis. The apoptosis induced by miR-101-3p-mediated Bim expression is mediated by both caspase-3 and -1, which are activated by two distinct intrinsic mechanisms, cytochrome c release and ROS-induced inflammasome activation, respectively. In other words, the antioxidant inhibited endothelial cell death mediated by caspase-1 that activated caspase-7, but not caspase-3. These findings provide mechanistic insight into a novel function of miR-101-3p in serum withdrawal-induced apoptosis triggered by activating two different intrinsic or mitochondrial apoptosis pathways, implicating miR-101-3p as a therapeutic target that limits endothelial cell death associated with vascular disorders.

25-Hydroxy- and 1α,25-Dihydroxycholecalciferol Have Greater Potencies than 25-Hydroxy- and 1α,25-Dihydroxyergocalciferol in Modulating Cultured Human and Mouse Osteoblast Activities

Despite differences in the phamacokinetics of 25-hydroxycholecalciferol (25(OH)D3) and 25-hydroxyergocalciferol (25(OH)D2) in man, the effects of these and their 1α-hydroxylated forms (1,25(OH)2D3 and 1,25(OH)2D2) on cellular activity of vitamin D-responsive cells have hardly been compared. We studied differences in the effects of these metabolites on cell number, gene transcription, protein expression and mineralisation of cultured human bone marrow-derived stromal cells (hBMSC) and rapidly mineralising mouse 2T3 osteoblasts. 50-1000 nM 25(OH) and 0.05-10 nM 1,25(OH)2 metabolites were used. At high concentrations, 25(OH)D2/D3 and 1,25(OH)2D2/D3 suppressed cell number in both human and mouse cells. The suppression was greater with cholecalciferol (D3) metabolites than with those of ergocalciferol (D2). In both cell types, 25(OH)D2 and 25(OH)D3 increased the expression of osteopontin, osteocalcin, collagen-1, receptor activator of nuclear factor kappa-B ligand, vitamin D receptor, CYP24A1 and CYP27B1 genes. Whereas there was little or no difference between the effects of 25(OH)D2 and 25(OH)D3 in hBMSCs, differences were observed in the magnitude of the effects of these metabolites on the expression of most studied genes in 2T3 cells. Alkaline phosphatase (ALP) activity was increased by 25(OH)D2/D3 and 1,25(OH)2D2/D3 in hBMSC and 2T3 cells, and the increase was greater with the D3 metabolites at high concentrations. In hBMSCs, mineralisation was also increased by 25(OH)D2/D3 and 1,25(OH)2D2/D3 at high concentrations, with D3 metabolites exerting a greater influence. In 2T3 cells, the effects of these compounds on mineralisation were stimulatory at low concentrations and inhibitory when high concentrations were used. The suppression at high concentrations was greater with the D3 metabolites. These findings suggest that there are differences in the effects of 25-hydroxy and 1α,25(OH)2 metabolites of D3 and D2 on human preosteoblasts and mouse osteoblasts, with the D3 metabolites being more potent in suppressing cell number, increasing ALP activity and influencing mineralisation.

Glucose-Dependent Insulinotropic Peptide Prevents Serum Deprivation-Induced Apoptosis in Human Bone Marrow-Derived Mesenchymal Stem Cells and Osteoblastic Cells

Human bone marrow-derived mesenchymal stem cells (hBMSC) are able to differentiate into cells of connective tissue lineages, including bone and cartilage. They are therefore considered as a promising tool for the treatment of bone degenerative diseases. One of the major issues in regenerative cell therapy is the biosafety of fetal bovine serum used for cell culture. Therefore, the development of a culture medium devoid of serum but preserving hBMSC viability will be of clinical value. The glucose-dependent insulinotropic peptide (GIP) has an anti-apoptotic action in insulin-producing cells. Interestingly, GIP also exerts beneficial effects on bone turnover by acting on osteoblasts and osteoclasts. We therefore evaluated the ability of GIP to prevent cell death in osteoblastic cells cultured in serum-free conditions. In hBMSC and SaOS-2 cells, activation of the GIP receptor increased intracellular cAMP levels. Serum deprivation induced apoptosis in SaOS-2 and hBMSC that was reduced by 30 and 50 %, respectively, in the presence of GIP. The protective effect of GIP involves activation of the adenylate cyclase pathway and inhibition of caspases 3/7 activation. These findings demonstrate that GIP exerts a protective action against apoptosis in hBMSC and suggest a novel approach to preserve viability of hBMSC cultured in the absence of serum.

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.

Lovastatin-Mediated Changes in Human Tendon Cells

Statins are among the most widely prescribed drugs worldwide. Numerous studies have shown their beneficial effects in prevention of cardiovascular disease through cholesterol-lowering and anti-atherosclerotic properties. Although some statin patients may experience muscle-related symptoms, severe side effects of statin therapy are rare, primarily due to extensive first-pass metabolism in the liver. Skeletal muscles appear to be the main site of side effects; however, recently some statin-related adverse effects have been described in tendon. The mechanism behind these side effects remains unknown. This is the first study that explores tendon-specific effects of statins in human primary tenocytes. The cells were cultured with different concentrations of lovastatin for up to 1 week. No changes in cell viability or morphology were observed in tenocytes incubated with therapeutic doses. Short-term exposure to lovastatin concentrations outside the therapeutic range had no effect on tenocyte viability; however, cell migration was reduced. Simvastatin and atorvastatin, two other drug family members, also reduced the migratory properties of the cells. Prolonged exposure to high concentrations of lovastatin induced changes in cytoskeleton leading to cell rounding and decreased levels of mRNA for matrix proteins, but increased BMP-2 expression. Gap junctional communication was impaired but due to cell shape change and separation rather than direct gap junction inhibition. These effects were accompanied by inhibition of prenylation of Rap1a small GTPase. Collectively, we showed that statins in a dose-dependent manner decrease migration of human tendon cells, alter their expression profile and impair the functional network, but do not inhibit gap junction function.

Glucocorticoid induced osteoblast apoptosis by increasing E4BP4 expression via up-regulation of Bim

It is well known that glucocorticoid (GC)-induced bone loss is caused primarily by hypofunction and apoptosis of osteoblasts. However, the precise molecular events underlying the effect of GC on osteoblast apoptosis are not fully understood. Recent studies implicated an important role of E4BP4 in the regulation of osteoblast apoptosis and differentiation. Furthermore, E4BP4 is a GC-regulated gene required for GC-induced apoptosis in many cells. Therefore, we hypothesize that E4BP4 may be implicated in the process of GC-induced osteoblast apoptosis. Western blot, reverse-transcription-PCR, flow cytometry, and Hoechst 33258 staining were employed to investigate the role of E4BP4 in dexamethasone (DEX)-induced osteoblast apoptosis. We found that the expression of E4BP4 is significantly up-regulated in osteoblasts exposed to DEX. Furthermore, the depletion of E4BP4 significantly decreased DEX-induced osteoblast apoptosis. In addition, E4BP4 plays a crucial role in GC-evoked apoptosis of osteoblasts by enabling induction of Bim. On the basis of these results above, we can draw the conclusion that E4BP4 may contribute to the process of DEX-induced osteoblast apoptosis.

Regulation of hypoxia-induced cell death in human tenocytes

Degenerate shoulder tendons display evidence of hypoxia. However tendons are relatively avascular and not considered to have high oxygen requirements and the vulnerability of tendon cells to hypoxia is unclear. Cultured human tenocytes were exposed to hypoxia and the cellular response detected using QPCR, Western blotting, viability, and ELISA assays. We find that tenocytes respond to hypoxia in vitro by activating classical HIF-1α-driven pathways. Total hypoxia caused significant tenocyte apoptosis. Transcription factors typically involved in hypoxic response, HIF-1α and FOXO3A, were upregulated. Hypoxia caused sustained upregulation of several proapoptotic proteins known to mediate hypoxia-induced apoptosis, such as Bnip3 and Nix, but others were unchanged although they were reportedly hypoxia-sensitive in other cell types. Antiapoptotic proteins Bcl2 and Bcl-xL were unchanged by hypoxia. Normal human tenocytes expressed all isoforms of the hypoxia-induced vascular growth factor VEGF except VEGF-D. Hypoxia markedly upregulated VEGF-A mRNA, followed by increased VEGF protein secretion. However treatment with VEGF did not improve tenocyte survival. As a protective strategy for tenocytes at risk of hypoxic death we added prosurvival growth factors insulin or platelet rich plasma (PRP). Both agents strongly protected tenocytes from hypoxia-induced death over 48 h, suggesting possible efficacy in the acute postrupture tendon or integrating graft.

Ablation of the pro-apoptotic protein Bax protects mice from glucocorticoid-induced bone growth impairment

Dexamethasone (Dexa) is a widely used glucocorticoid to treat inflammatory diseases; however, a multitude of undesired effects have been reported to arise from this treatment including osteoporosis, obesity, and in children decreased longitudinal bone growth. We and others have previously shown that glucocorticoids induce apoptosis in growth plate chondrocytes. Here, we hypothesized that Bax, a pro-apoptotic member of the Bcl-2 family, plays a key role in Dexa-induced chondrocyte apoptosis and bone growth impairment. Indeed, experiments in the human HCS-2/8 chondrocytic cell line demonstrated that silencing of Bax expression using small-interfering (si) RNA efficiently blocked Dexa-induced apoptosis. Furthermore, ablation of Bax in female mice protected against Dexa-induced bone growth impairment. Finally, Bax activation by Dexa was confirmed in human growth plate cartilage specimens cultured ex vivo. Our findings could therefore open the door for new therapeutic approaches to prevent glucocorticoid-induced bone growth impairment through specific targeting of Bax.

Protection against glucocorticoid-induced damage in human tenocytes by modulation of ERK, Akt, and forkhead signaling

Antiinflammatory glucocorticoid (GC) injections are extensively used to treat painful tendons. However, GC cause severe tissue wasting in other collagen-producing tissues such as skin and bone. The objective of this study was to determine the effects of GC on tenocytes and to explore strategies to protect against unwanted side effects of GC treatment. Cell survival, collagen production, and the induction of signaling pathways in primary human tenocytes treated with dexamethasone (Dex) were assessed. Antioxidant and growth factor approaches to protection were tested. Dex treatment resulted in reduced viable cell number, cell proliferation, and collagen production. Dex induced reactive oxygen species generation in tenocytes and strongly up-regulated the stress-response transcription factors FOXO1 and FOXO3A. Phosphorylation of ERK and protein kinase B/Akt, which regulate cell proliferation and also inhibit forkhead activity, was decreased. Chemical inhibition of ERK or Akt activity significantly reduced tenocyte cell number. Ameliorating the Dex-induced reduction in ERK or Akt activity by cotreatment with vitamin C or insulin protected against the Dex-induced reduction in cell number. Silencing FOXO1 prevented the Dex-induced reduction in collagen 1α1 expression. Cotreatment with vitamin C or insulin protected against the Dex-induced increase in FOXO and the Dex-induced inhibition of collagen 1α1 expression. Reduced ERK and Akt activation and increased forkhead signaling contribute to the negative effects of GC on tenocytes. Cotreatment therapies that target these signaling pathways are protective. Vitamin C in particular may be a clinically useable co-therapy to reduce connective tissue side effects associated with GC therapy.

Osteocyte Wnt/beta-catenin signaling is required for normal bone homeostasis

Beta-Catenin-dependent canonical Wnt signaling plays an important role in bone metabolism by controlling differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. To investigate its function in osteocytes, the cell type constituting the majority of bone cells, we generated osteocyte-specific beta-catenin-deficient mice (Ctnnb1(loxP/loxP); Dmp1-Cre). Homozygous mutants were born at normal Mendelian frequency with no obvious morphological abnormalities or detectable differences in size or body weight, but bone mass accrual was strongly impaired due to early-onset, progressive bone loss in the appendicular and axial skeleton with mild growth retardation and premature lethality. Cancellous bone mass was almost completely absent, and cortical bone thickness was dramatically reduced. The low-bone-mass phenotype was associated with increased osteoclast number and activity, whereas osteoblast function and osteocyte density were normal. Cortical bone Wnt/beta-catenin target gene expression was reduced, and of the known regulators of osteoclast differentiation, osteoprotegerin (OPG) expression was significantly downregulated in osteocyte bone fractions of mutant mice. Moreover, the OPG levels expressed by osteocytes were higher than or comparable to the levels expressed by osteoblasts during skeletal growth and at maturity, suggesting that the reduction in osteocytic OPG and the concomitant increase in osteocytic RANKL/OPG ratio contribute to the increased number of osteoclasts and resorption in osteocyte-specific beta-catenin mutants. Together, these results reveal a crucial novel function for osteocyte beta-catenin signaling in controlling bone homeostasis.

Osteoblastic molecular scaffold Gab1 is required for maintaining bone homeostasis

The Grb2-associated binder 1 (Gab1), which serves as a scaffolding adaptor protein, plays a crucial role in transmitting key signals that control cell growth, differentiation and function from multiple receptors. However, its biological role in osteoblast activity and postnatal bone metabolism remains unclear. To elucidate the in vivo function of Gab1 in postnatal bone remodeling, we generated osteoblast-specific Gab1 knockout mice. Disruption of Gab1 expression in osteoblasts led to decreased trabecular bone mass with a reduced bone formation rate and a decreased bone resorption. Bones from Gab1 mutants also exhibited inferior mechanical properties. Moreover, primary osteoblasts from Gab1 mutant mice demonstrated markedly suppressed osteoblast mineralization, increased susceptibility to apoptosis and decreased expression of receptor activator of NF-kappaB ligand (RANKL). Activation of serine-threonine Akt kinase and extracellular signal-regulated kinase in response to insulin and insulin-like growth factor 1 was attenuated in Gab1 mutant osteoblasts. Our results show that Gab1-mediated signals in osteoblasts are crucial for normal postnatal bone homeostasis.

Differentiation-related gene-1 decreases Bim stability by proteasome-mediated degradation

Drg1 was identified as a differentiation-related, putative metastatic suppressor gene in human colon and prostate cancer. Its expression is associated with resistance to irinotecan (CPT-11) therapy in preclinical colorectal cancer models both in vitro and in vivo. However, the functional significance of Drg1 in these processes is unknown. We have shown for the first time that Drg1 directly binds to the BH3-only proapoptotic protein Bim. Depletion of Drg1 by small interfering RNA induced up-regulation of Bim and its accumulation in the mitochondria, which correlated with loss of mitochondrial membrane potential and induction of apoptosis in cells exposed to SN-38. Further analyses revealed that Drg1 promotes degradation of Bim through the Cullin2/ElonginB-CIS ubiquitin-protein ligase complex. Conversely, in the absence of Drg1, Bim was stabilized and bound more abundantly to Hsp70. These results show that Drg1 renders cancer cells more resistant to chemotherapy through enhanced proteasome-mediated Bim degradation.

Loss of the pro-apoptotic BH3-only Bcl-2 family member Bim sustains B lymphopoiesis in the absence of IL-7

IL-7 is pivotal for B cell development. Proteins of the Bcl-2 family are essential regulators of lymphocyte survival. Particularly, the pro-apoptotic BH3-only members Bim and Puma mediate lymphocyte apoptosis provoked by cytokine deprivation. Herein, we addressed whether the absence of Bim or Puma within the hematopoietic compartment could bypass the requirement for IL-7-driven B cell development in adult mice. We found that deficiency of Bim, but not Puma, partially rescued B cell development in the absence of IL-7. The numbers of both sIgM(-) and sIgM(+) B cells were markedly increased in the bone marrow of recipients lacking IL-7 upon reconstitution with Bim-deficient hematopoietic progenitors, compared with their control or Puma-deficient counterparts. The augmentation of B cell lymphopoiesis in the absence of Bim was reflected in the mature peripheral compartment by an increase in both the number of immature and mature B cells in the spleen and in the circulating IgM levels. Bim-deficient B cells were also increased in IL-7-sufficient recipients suggesting that peripheral B cells homeostasis is governed by a Bim-dependent and IL-7-independent mechanism. Our data highlight the role of Bim as a key regulator of cell survival during B lymphocyte development in vivo.

Urokinase induces survival or pro-apoptotic signals in human mesangial cells depending on the apoptotic stimulus

Deregulated apoptosis of MCs (mesangial cells) is associated with a number of kidney diseases including end-stage diabetic nephropathy. Cell death by apoptosis is a tightly orchestrated event, whose mechanisms are not completely defined. In the present study we show that the uPA (urokinase-type plasminogen activator)/uPAR (uPA receptor) system can initiate both cell survival and pro-apoptotic signals in human MCs in response to different apoptotic stimuli. uPA abrogated MC apoptosis induced by serum withdrawal conditions and enhanced apoptosis initiated in MCs by high glucose. Effects of uPA were independent of its proteolytic activity and required uPAR for both pro- and anti-apoptotic effects. Studies on the uPAR interactome provide evidence that the opposing effects of uPA were directed via different uPAR-interacting transmembrane partners. Exposure of MCs to RGD (Arg-Gly-Asp) peptide led to abrogation of the anti-apoptotic effect of uPA, which implies involvement of integrins in this process. A pro-apoptotic effect of uPA under high-glucose conditions was mediated via association of uPAR and the cation-independent M6P (mannose-6-phosphate)/IGF2R (insulin-like growth factor 2 receptor). Both receptors were co-precipitated and co-localized in MCs. Studies on the underlying signalling indicate that the ERK1/2 (extracellular-signal-regulated kinase 1/2), Akt and BAD (Bcl-2/Bcl-X(L)-antagonist, causing cell death) protein were involved in regulation of apoptosis by uPA in MCs. M6P/IGF2R mediated BAD perinuclear localization during apoptosis initiated by uPA and high glucose. In conclusion, we provide evidence that, in MCs, the uPA/uPAR system regulates survival/apoptosis processes in a stimulus-specific fashion via a mitochondria-dependent mechanism and that BAD protein serves as a downstream molecule.

Bax deficiency in mice increases cartilage production during fracture repair through a mechanism involving increased chondrocyte proliferation without changes in apoptosis

This study sought to determine the role of the pro-apoptotic gene, Bax, in fracture healing by comparing femoral fracture healing in Bax knockout (KO) and wild-type C57BL/6J (background strain) mice. Bax KO fractures were larger, had more bone mineral content, had approximately 2-fold larger cartilage area per callus area in the first and second weeks of fracture healing, and showed an increased osteoclast surface area in the third and fourth weeks of fracture healing compared to C57BL/6J fractures. The increased cartilage area in the Bax KO fracture callus was due to increases in number of both pre-hypertropic and hypertropic chondrocytes. TUNEL analysis showed no significant differences in the number of either chondrocyte or non-chondrocyte apoptotic cells between Bax KO and C57BL/6J fractures at 7 or 14 days post-fracture, indicating that the increased number of chondrocytes in Bax KO fractures was not due to reduced apoptosis. Analysis of expression of apoptotic genes revealed that although the expression levels of Bcl-2 and Bcl-xL were not different between the Bax KO and C57BL/6J mice at 7 or 14 days post-fracture, the expression of BH3-domain only Bak and "Bik-like" pro-apoptotic gene increased approximately 1.5-fold and approximately 2-fold, respectively, in Bax KO fractures at 7 and 14 days post-fracture, compared to C57BL/6J fractures, suggesting that up-regulation of the Bak and Bik-like pro-apoptotic genes in Bax KO mice might compensate for the lack of Bax functions in the context of apoptosis. Analysis by in vivo incorporation of bromodeoxyuridine into chondrocytes within the fracture tissues indicated a highly significant increase in chondrocyte proliferation in Bax KO fractures compared to C57BL/6J fractures at day 7. The increased expression of collagen 2alpha1 and 9alpha1 gene in Bax KO fractures during early healing was consistent with an increased chondrocyte proliferation. In conclusion, this study demonstrates for the first time that Bax has an important role in the early stage of fracture healing, and that the increased callus size and cartilage area in Bax KO fractures was due to increased chondrocyte proliferation and not to reduced apoptosis or increased chondrocyte hypertrophy. The unexpected effect of Bax deficiency on chondrocyte proliferation implicates a novel regulatory function for Bax on chondrocyte proliferation during fracture repair.