Proteasome inhibition-enhanced fracture repair is associated with increased mesenchymal progenitor cells in mice

The ubiquitin/proteasome system controls the stability of Runx2 and JunB, proteins essential for differentiation of mesenchymal progenitor/stem cells (MPCs) to osteoblasts. Local administration of proteasome inhibitor enhances bone fracture healing by accelerating endochondral ossification. However, if a short-term administration of proteasome inhibitor enhances fracture repair and potential mechanisms involved have yet to be exploited. We hypothesize that injury activates the ubiquitin/proteasome system in callus, leading to elevated protein ubiquitination and degradation, decreased MPCs, and impaired fracture healing, which can be prevented by a short-term of proteasome inhibition. We used a tibial fracture model in Nestin-GFP reporter mice, in which a subgroup of MPCs are labeled by Nestin-GFP, to test our hypothesis. We found increased expression of ubiquitin E3 ligases and ubiquitinated proteins in callus tissues at the early phase of fracture repair. Proteasome inhibitor Bortezomib, given soon after fracture, enhanced fracture repair, which is accompanied by increased callus Nestin-GFP+ cells and their proliferation, and the expression of osteoblast-associated genes and Runx2 and JunB proteins. Thus, early treatment of fractures with Bortezomib could enhance the fracture repair by increasing the number and proliferation of MPCs.

Incretins Enhance PGF2α-Induced Synthesis of IL-6 and Osteoprotegerin in Osteoblasts

Incretins including glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), which are secreted from the small intestine after oral food ingestion, are currently well-known to stimulate insulin secretion from pancreatic β-cells and used for the treatment of type 2 diabetes mellitus. We have previously reported that prostaglandin F_2α (PGF_2α) stimulates the synthesis of interleukin-6 (IL-6) and osteoprotegerin in osteoblast-like MC3T3-E1 cells, and that IL-6 and osteoprotegerin release are mediated through the p44/p42 mitogen-activated protein (MAP) kinase, p38 MAP kinase or stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) pathways. In the present study, we investigated the effects of incretins including GLP-1 and GIP, on the PGF_2α-induced synthesis of IL-6 and osteoprotegerin and examined the detailed mechanism in osteoblast-like MC3T3-E1 cells. We found that GIP and GLP-1 significantly stimulated the PGF_2α-induced synthesis of IL-6 in osteoblast-like MC3T3-E1 cells. In addition, GIP and GLP-1 significantly enhanced the PGF_2α-induced mRNA expression levels of IL-6. On the other hand, GIP and GLP-1 markedly stimulated the PGF_2α-induced synthesis of osteoprotegerin. However, the phosphorylation of p44/p42 MAP kinase, p38 MAP kinase, or JNK induced by PGF_2α was not affected by GIP or GLP-1. Therefore, these results strongly suggest that incretins enhance the PGF_2α-induced synthesis of IL-6 and osteoprotegerin in osteoblast-like MC3T3-E1 cells. However, these syntheses are not mediated through p44/p42 MAP kinase, p38 MAP kinase, or JNK pathways.

Overexpression of PIK3R1 Promotes Bone Formation by Regulating Osteoblast Differentiation and Osteoclast Formation

In an effort to bolster our understanding of regulation of bone formation in the context of osteoporosis, we screened out differentially expressed genes in osteoporosis patients with high and low bone mineral density by bioinformatics analysis. PIK3R1 is increasingly being nominated as a pivotal mediator in the differentiation of osteoblasts and osteoclasts that is closely related to bone formation. However, the specific mechanisms underlying the way that PIK3R1 affects bone metabolism are not fully elucidated. We intended to examine the potential mechanism by which PIK3R1 regulates osteoblast differentiation. Enrichment analysis was therefore carried out for differentially expressed genes. We noted that the estrogen signaling pathway, TNF signaling pathway, and osteoclast differentiation were markedly associated with ossification, and they displayed enrichment in PIK3R1. Based on western blot, qRT-PCR, and differentiation analysis in vitro, we found that upregulation of PIK3R1 enhanced osteoblastic differentiation, as evidenced by increased levels of investigated osteoblast-related genes as well as activities of ALP and ARS, while it notably decreased levels of investigated osteoclast-related genes. On the contrary, downregulation of PIK3R1 decreased levels of osteoblast-related genes and increased levels of osteoclast-related genes. Besides, in vitro experiments revealed that PIK3R1 facilitated proliferation and repressed apoptosis of osteoblasts but had an opposite impact on osteoclasts. In summary, PIK3R1 exhibits an osteoprotective effect via regulating osteoblast differentiation, which can be represented as a promising therapeutic target for osteoporosis.

Osteoblast-specific deficiency of ectonucleotide pyrophosphatase or phosphodiesterase-1 engenders insulin resistance in high-fat diet fed mice

Supraphysiological levels of the osteoblast-enriched mineralization regulator ectonucleotide pyrophosphatase or phosphodiesterase-1 (NPP1) is associated with type 2 diabetes mellitus. We determined the impact of osteoblast-specific Enpp1 ablation on skeletal structure and metabolic phenotype in mice. Female, but not male, 6-week-old mice lacking osteoblast NPP1 expression (osteoblast-specific knockout [KO]) exhibited increased femoral bone volume or total volume (17.50% vs. 11.67%; p < .01), and reduced trabecular spacing (0.187 vs. 0.157 mm; p < .01) compared with floxed (control) mice. Furthermore, an enhanced ability of isolated osteoblasts from the osteoblast-specific KO to calcify their matrix in vitro compared to fl/fl osteoblasts was observed (p < .05). Male osteoblast-specific KO and fl/fl mice showed comparable glucose and insulin tolerance despite increased levels of insulin-sensitizing under-carboxylated osteocalcin (195% increase; p < .05). However, following high-fat-diet challenge, osteoblast-specific KO mice showed impaired glucose and insulin tolerance compared with fl/fl mice. These data highlight a crucial local role for osteoblast NPP1 in skeletal development and a secondary metabolic impact that predominantly maintains insulin sensitivity.

PARP Activity Fine-tunes the DNA Replication Choreography of Chk1-depleted Cells

Checkpoint Kinase 1 (Chk1) prevents DNA damage by adjusting the replication choreography in the face of replication stress. Chk1 depletion provokes slow and asymmetrical fork movement, yet the signals governing such changes remain unclear. We sought to investigate whether poly(ADP-ribose) polymerases (PARPs), key players of the DNA damage response, intervene in the DNA replication of Chk1-depleted cells. We demonstrate that PARP inhibition selectively alleviates the reduced fork elongation rates, without relieving fork asymmetry in Chk1-depleted cells. While the contribution of PARPs to fork elongation is not unprecedented, we found that their role in Chk1-depleted cells extends beyond fork movement. PARP-dependent fork deceleration induced mild dormant origin firing upon Chk1 depletion, augmenting the global rates of DNA synthesis. Thus, we have identified PARPs as novel regulators of replication fork dynamics in Chk1-depleted cells.

The virulence factor GroEL promotes gelatinase secretion from cells in the osteoblast lineage: Implication for direct crosstalk between bacteria and adult cells

OBJECTIVE

The aim of this study was to demonstrate the influence of the virulence factor GroEL on osteoblast behavior by characterizing the changes of secreted gelatinases.

DESIGN

ELISA was performed to detect GroEL from samples from patients with or without apical periodontitis. An apical periodontitis model was established in rats and the expression of MMP-2, MMP-9 and NF-κB was evaluated by immunofluorescence staining. The primary osteoblasts and osteoblast-like MC3T3 cells were stimulated with recombinant GroEL, and gelatin zymography was used to determine the activity and expression of MMP-2 and MMP-9. Western blot was used to screen signaling pathways, and immunofluorescence staining was performed to confirm the activated signaling.

RESULTS

First, we found expression of GroEL to be higher in oral saliva, gingival crevicular fluid and periradicular granulation tissue of patients with apical periodontitis than it was in healthy control patients. We next found that recombinant GroEL could increase the activity of the gelatinases, MMP-2 and MMP-9, which were secreted by both primary osteoblasts and MC3T3 cells. In a rat apical periodontitis model, strong expression of gelatinases was confirmed. Then, we found that GroEL-enhanced gelatinase activity was mediated through activation of NF-κB signaling. Acetylated NF-κB accumulated in the cell nucleus and bound to the promoter of MMP-2 and MMP-9 genes, thus initiating their high expression.

CONCLUSION

This study reveals a direct interaction between oral bacteria and adult cells by demonstrating that gelatinase secretion is induced by GroEL, which partially explains bone resorption through gelatinase activation.

Tuning of Titanium Microfiber Scaffold with UV-Photofunctionalization for Enhanced Osteoblast Affinity and Function

Titanium (Ti) is an osteoconductive material that is routinely used as a bulk implant to fix and restore bones and teeth. This study explored the effective use of Ti as a bone engineering scaffold. Challenges to overcome were: (1) difficult liquid/cell infiltration into Ti microfiber scaffolds due to the hydrophobic nature of Ti; and (2) difficult cell attachment on thin and curved Ti microfibers. A recent discovery of UV-photofunctionalization of Ti prompted us to examine its effect on Ti microfiber scaffolds. Scaffolds in disk form were made by weaving grade 4 pure Ti microfibers (125 µm diameter) and half of them were acid-etched to roughen the surface. Some of the scaffolds with original or acid-etched surfaces were further treated by UV light before cell culture. Ti microfiber scaffolds, regardless of the surface type, were hydrophobic and did not allow glycerol/water liquid to infiltrate, whereas, after UV treatment, the scaffolds became hydrophilic and immediately absorbed the liquid. Osteogenic cells from two different origins, derived from the femoral and mandibular bone marrow of rats, were cultured on the scaffolds. The number of cells attached to scaffolds during the early stage of culture within 24 h was 3–10 times greater when the scaffolds were treated with UV. The development of cytoplasmic projections and cytoskeletal, as well as the expression of focal adhesion protein, were exclusively observed on UV-treated scaffolds. Osteoblastic functional phenotypes, such as alkaline phosphatase activity and calcium mineralization, were 2–15 times greater on UV-treated scaffolds, with more pronounced enhancement on acid-etched scaffolds compared to that on the original scaffolds. These effects of UV treatment were associated with a significant reduction in atomic carbon on the Ti microfiber surfaces. In conclusion, UV treatment of Ti microfiber scaffolds tunes their physicochemical properties and effectively enhances the attachment and function of osteoblasts, proposing a new strategy for bone engineering.

DKK1 Induced by 1,25D3 Is Required for the Mineralization of Osteoblasts

1α,25-dihydroxyvitamin D3 (1,25D3), the most popular drug for osteoporosis treatment, drives osteoblast differentiation and bone mineralization. Wnt/β-catenin signaling is involved in commitment and differentiation of osteoblasts, but the role of the Dickkopf-related protein 1 (DKK1), a Wnt antagonist, in osteoblasts remains unknown. Here, we demonstrate the molecular mechanism of DKK1 induction by 1,25D3 and its physiological role during osteoblast differentiation. 1,25D3 markedly promoted the expression of both CCAAT/enhancer binding protein beta (C/EBPβ) and DKK1 at day 7 during osteoblast differentiation. Interestingly, mRNA and protein levels of C/EBPβ and DKK1 in osteoblasts were elevated by 1,25D3. We also found that C/EBPβ, in response to 1,25D3, directly binds to the human DKK1 promoter. Knockdown of C/EBPβ downregulated the expression of DKK1 in osteoblasts, which was partially reversed by 1,25D3. In contrast, overexpression of C/EBPβ upregulated DKK1 expression in osteoblasts, which was enhanced by 1,25D3. Furthermore, 1,25D3 treatment in osteoblasts stimulated secretion of DKK1 protein within the endoplasmic reticulum to extracellular. Intriguingly, blocking DKK1 attenuated calcified nodule formation in mineralized osteoblasts, but not ALP activity or collagen synthesis. Taken together, these observations suggest that 1,25D3 promotes the mineralization of osteoblasts through activation of DKK1 followed by an increase of C/EBPβ.

Carbon Nanotube Reinforced Hydroxyapatite Nanocomposites As Bone Implants: Nanostructure, Mechanical Strength And Biocompatibility

PURPOSE

Hydroxyapatite (HA) is a biologically active ceramic which promotes bone growth, but it suffers from relatively weak mechanical properties. Multi-walled carbon nanotubes (MWCNTs) have high tensile strength and a degree of stiffness that can be used to strengthen HA; potentially improving the clinical utility of the bone implant.

METHODS

HA was precipitated by the wet precipitation method in the presence of pristine (p) or functionalised (f) MWCNTs, and polyvinyl alcohol (PVA) or hexadecyl trimethyl ammonium bromide (HTAB) as the surfactant. The resulting composites were characterised and the diametral tensile strength and compressive strength of the composites were measured. To determine the biocompatibility of the composites, human osteoblast cells (HOB) were proliferated in the presence of the composites for 7 days.

RESULTS

The study revealed that both the MWCNTs and surfactants play a crucial role in the nucleation and growth of the HA. Composites made with f-MWCNTs were found to have better dispersion and better interaction with the HA particles compared to composites with p-MWCNTs. The mechanical strength was improved in all the composites compared to pure HA composites. The biocompatibility study showed minimal LDH activity in the media confirming that the composites were biocompatible. Similarly, the ALP activity confirmed that the cells grown on the composites containing HTAB were comparable to the control whereas the composites containing PVA surfactant showed significantly reduced ALP activity.

CONCLUSIONS

The study shows that the composites made of f-MWCNTs HTAB are stronger than pure HA composites and biocompatible making it a suitable material to study further.

Effects of glucocorticoids on vitamin D3-metabolizing 24-hydroxylase (CYP24A1) in Saos-2 cells and primary human osteoblasts

Vitamin D is essential for bone function and deficiency in active vitamin D hormone can lead to bone disorders. Long-term treatment with glucocorticoids results in osteoporosis and increased risk of fractures. Much remains unclear regarding the effects of these compounds in bone cells. In the current study, human osteosarcoma Saos-2 cells and primary human osteoblasts were found to express mRNA for the vitamin D receptor as well as activating and deactivating enzymes in vitamin D₃ metabolism. These bone cells exhibited CYP24A1-mediated 24-hydroxylation which is essential for deactivation of the active vitamin form. However, bioactivating vitamin D₃ hydroxylase activities could not be detected in either of these cells. Several glucocorticoids, including prednisolone, down regulated CYP24A1 mRNA and CYP24A1-mediated 24-hydroxylase activity in both Saos-2 and primary human osteoblasts. Also, prednisolone significantly suppressed a human CYP24A1 promoter-luciferase reporter gene in Saos-2 cells co-transfected with the glucocorticoid receptor. Thus, the results of the present study show suppression by glucocorticoids on CYP24A1 mRNA, CYP24A1-mediated metabolism and CYP24A1 promoter activity in human osteoblast-like cells. As part of this study we examined if glucocorticoids are formed locally in Saos-2 cells. The experiments indicate formation of 11-deoxycortisol, a steroid with glucocorticoid activity, which can bind the glucocorticoid receptor. Our data showing suppression by glucocorticoids on CYP24A1 expression in human osteoblasts suggest a previously unknown mechanism for effects of glucocorticoids in human bone, where these compounds may interfere with regulation of active vitamin D levels.

Deficiency of sphingosine-1-phosphate receptor 3 does not affect the skeletal phenotype of mice lacking sphingosine-1-phosphate lyase

Albeit osteoporosis is one of the most prevalent disorders in the aged population, treatment options stimulating the activity of bone-forming osteoblasts are still limited. We and others have previously identified sphingosine-1-phosphate (S1P) as a bone remodeling coupling factor, which is released by bone-resorbing osteoclasts to stimulate bone formation. Moreover, S1pr3, encoding one of the five known S1P receptors (S1P3), was found differentially expressed in osteoblasts, and S1P3 deficiency corrected the moderate high bone mass phenotype of a mouse model (deficient for the calcitonin receptor) with increased S1P release from osteoclasts. In the present study we addressed the question, if S1P3 deficiency would also influence the skeletal phenotype of mice lacking S1P-lyase (encoded by Sgpl1), which display markedly increased S1P levels due to insufficient degradation. Consistent with previous reports, the majority of Sgpl1-deficient mice died before or shortly after weaning, and this lethality was not influenced by additional S1P3 deficiency. At 3 weeks of age, Sgpl1-deficient mice displayed increased trabecular bone mass, which was associated with enhanced osteoclastogenesis and bone resorption, but also with increased bone formation. Most importantly however, none of the skeletal parameters assessed by μCT, histomorphometry and serum analyses were significantly influenced by additional S1P3 deficiency. Taken together, our findings fully support the concept that S1P is a potent osteoanabolic molecule, although S1P3 is not the sole receptor mediating this influence. Since S1P receptors are considered excellent drug targets, it is now required to screen for the impact of other family members on bone formation.

Pyk2 deficiency potentiates osteoblast differentiation and mineralizing activity in response to estrogen or raloxifene

Bone remodeling is controlled by the actions of bone-degrading osteoclasts and bone-forming osteoblasts (OBs). Aging and loss of estrogen after menopause affects bone mass and quality. Estrogen therapy, including selective estrogen receptor modulators (SERMs), can prevent bone loss and increase bone mineral density in post-menopausal women. Although investigations of the effects of estrogen on osteoclast activity are well advanced, the mechanism of action of estrogen on OBs is still unclear. The proline-rich tyrosine kinase 2 (Pyk2) is important for bone formation and female mice lacking Pyk2 (Pyk2-KO) exhibit elevated bone mass, increased bone formation rate and reduced osteoclast activity. Therefore, in the current study, we examined the role of estrogen signaling on the mechanism of action of Pyk2 in OBs. As expected, Pyk2-KO OBs showed significantly higher proliferation, matrix formation, and mineralization than WT OBs. In addition we found that Pyk2-KO OBs cultured in the presence of either 17β-estradiol (E2) or raloxifene, a SERM used for the treatment of post-menopausal osteoporosis, showed a further robust increase in alkaline phosphatase (ALP) activity and mineralization. We examined the possible mechanism of action and found that Pyk2 deletion promotes the proteasome-mediated degradation of estrogen receptor α (ERα), but not estrogen receptor β (ERβ). As a consequence, E2 signaling via ERβ was enhanced in Pyk2-KO OBs. In addition, we found that Pyk2 deletion and E2 stimulation had an additive effect on ERK phosphorylation, which is known to stimulate cell differentiation and survival. Our findings suggest that in the absence of Pyk2, estrogen exerts an osteogenic effect on OBs through altered ERα and ERβ signaling. Thus, targeting Pyk2, in combination with estrogen or raloxifene, may be a novel strategy for the prevention and/or treatment of bone loss diseases.

[Estradiol significantly increases the expression of antioxidant enzymes in osteoporotic rats and osteoblasts in vitro]

OBJECTIVE

To investigate the effect of estradiol on the expression of antioxidant enzymes in osteoblasts and its role in postmenopausal osteoporosis.

METHODS

Rat models of osteoporosis established by ovariectomy were treated with estradiol for 3 months, and the changes in serum levels of reactive oxygen species (H₂O₂) and antioxidant enzymes (γ -GCS, GSH-ST and GSH-px) were detected. The effects of estradiol on the expression of γ -GCS mRNA and protein in osteoblast-like cells MC3T3-E1, MG63 and OB were examined with PCR and Western blotting. Using a mRNA microarray, we analyzed the changes in the expressions of 84 antioxidant enzymes in the osteoblast cell line MC3T3-E1 following estradiol treatment, and the enzymes with significant changes were verified by PCR. CCK-8 kit was used to evaluate the effect of estradiol and antioxidant NAC on the proliferation of MC3T3-E1 cells.

RESULTS

Rat models of osteoporosis were successfully established with ovariectomy. The osteoporotic rats showed significantly increased serum level of reactive oxygen species (H2O2) and decreased levels of antioxidant enzymes. Estrogen treatment of the osteoporotic rats obviously reversed the phenotype of osteoporosis, lowered serum level of reactive oxygen species, and increased the level of γ -GCS. In MC3T3-E1, MG63 and OB cells, estradiol treatment significantly upregulated the expression levels of γ -GCS mRNA and protein. In MC3T3-E1 cells treated with estrogen, the mRNA chip identified 6 upregulated antioxidant enzymes (Gpx6, Gstk1, Nos2, Prdx2, Ngb and Ccs), and the results of PCR verified that estradiol upregulated Ccs and Ngb mRNAs in MC3T3-E1, MG63 and OB cells. Estradiol and antioxidant NAC obviously promoted the proliferation of MC3T3-E1 cells.

CONCLUSION

Estradiol significantly increases the expression of antioxidase γ -Gcs, Ccs and Ngb in osteoblasts in vitro. Postmenopausal osteoporosis is closely related with the increase of reactive oxygen species and the decrease of antioxidant levels. In osteoblasts, estrogen deficiency may increase the level of reactive oxygen species, decrease the level of antioxidant enzymes, activate the oxidative stress cascade, and consequently inhibit the proliferation of osteoblasts to aggravate the condition of osteoporosis.

Osteoblast AMP-Activated Protein Kinase Regulates Postnatal Skeletal Development in Male Mice

Studies have shown that AMP-activated protein kinase (AMPK), a crucial regulator of energy homeostasis, plays important roles in osteoblast differentiation and mineralization. However, little is known about in vivo roles of osteoblastic AMPK in bone development. Thus, to investigate in vivo roles of osteoblast AMPK, we conditionally inactivated Ampk in osterix (Osx)-expressing cells by crossing Osx-Cre mice with floxed AMPKα1 to generate mice lacking AMPKα1 in osteoblasts (Ampk-/- mice). Compared with wild-type and Ampk+/- mice, Ampk-/- mice displayed retardation of postnatal bone development, although bone deformity was not observed at birth. Microcomputed tomography showed significant reductions in trabecular bone volume, cortical bone length, and density, as well as increased cortical porosity in femur as well as development defects of skull in 8-week-old Ampk-/- mice. Surprisingly, histomorphometric analysis demonstrated that the number of osteoclasts was significantly increased, although bone formation rate was not altered. Loss of trabecular network connections and mass, as well as shortened growth plates and reduced thickness of cartilage adjacent to the growth plate, was observed in Ampk-/- mice. In primary cultured osteoblasts from calvaria, the expressions of alkaline phosphatase, type 1 collagen, osteocalcin, bone morphogenetic protein 2, Runx2, and osterix were significantly inhibited in Ampk-/- osteoblasts, whereas the expression of receptor activator of nuclear κB ligand (RANKL) and the RANKL/osteoprotegerin ratio were significantly increased. These findings indicate that osteoblastic AMPK plays important roles in bone development in vivo and that deletion of AMPK in osteoblasts decreases osteoblastic differentiation and enhances bone turnover by increasing RANKL expression.

Differentially activated Src kinase in chemo-naïve human primary osteosarcoma cells and effects of a Src kinase inhibitor

The therapeutic treatment of osteosarcoma (OS), a rare malignant teenage cancer of the skeletal system, still represents a great challenge as patient survival after conventional protocol chemotherapy treatment has not improved in the last four decades leaving poor patient prognoses. Therefore, many efforts have been done to find increasingly reliable OS cell models and to identify "druggable" targets in OS, in order to identify novel effective therapeutic approaches and treatment strategies. In this contest, the more successful use of patient-derived cell cultures in respect to human commercial lines and findings of Src kinase deregulation in cancer, prompted us to study for the first time the activation state of Src and the potential activity of our Src inhibitor SI-83 in a number of chemo-naïve patient-derived primary OS cells. We here demonstrate that Src is hyperactivated in OS cells in respect to the nonmalignant counterpart and that SI-83 is able to strongly decrease cell viability, proliferation, Src₄₁₆ phosphorylation, and cell migration. © 2017 BioFactors, 43(6):801-811, 2017.

Cold atmospheric plasma enhances osteoblast differentiation

This study was designed to assess the effects of cold atmospheric plasma on osteoblastic differentiation in pre-osteoblastic MC3T3-E1 cells. Plasma was irradiated directly to a culture medium containing plated cells for 5 s or 10 s. Alkaline phosphatase (ALP) activity assay and alizarin red staining were applied to assess osteoblastic differentiation. The plasma-generated radicals were detected directly using an electron spin resonance-spin trapping technique. Results show that plasma irradiation under specific conditions increased ALP activity and enhanced mineralization, and demonstrated that the yield of radicals was increased in an irradiation-time-dependent manner. Appropriate plasma irradiation stimulated the osteoblastic differentiation of the cells. This process offers the potential of promoting bone regeneration.

Dried Plum Ingestion Increases the Osteoblastogenic Capacity of Human Serum

In cell culture studies, dried plum (Prunus domestica L.) polyphenols increased osteoblast alkaline phosphatase (ALP) activity, mineralized nodule formation, and the expression of the bone marker genes runt-related transcription factor 2 (RUNX2) and osterix. The purpose of this study was to determine whether human serum collected 1 and 2 h after dried plum ingestion influenced osteoblast cell activity and gene expression. Five healthy women ingested 100 g of dried plum, and serum samples were collected at baseline (before dried plum ingestion) and 1 and 2 h postingestion of dried plum. MC3T3-E1 osteoblast cells were treated (2% of medium) with these serum samples for 3 or 9 days. Intracellular and extracellular ALP activities were significantly increased after 3 or 9 days of treatment with serum both postingestion time points, with no effect seen in baseline samples. Also, serum obtained 1 and 2 h postingestion significantly increased the mRNA expression of bone markers RUNX2 and connexin43 (CX43) after both 3 and 9 days of incubation periods. Finally, serum obtained 1 and 2 h postingestion increased the mRNA expression of β-catenin after 9 days of incubation. We conclude that osteoblast activity and function are increased by dried plum ingestion, which may, in part, explain its beneficial effects on bone health.

Study of GSK3b inhibitors SB415286 and SB216763 to improve osteoblastic differentiation on microstructured titanium

Rough titanium surfaces enhance cell response to activation of Wnt canonical signalling, a pathway required for osteoblast differentiation. The present study investigated the effects of GSK3β-inhibitors SB216763 and SB415286 on osteoblastic differentiation on titanium surfaces with different topography and wettability. Osteoblastic MC3T3 cells were plated on smooth (Pickled), sand-blasted/acid-etched (SLA) or hyper hydrophilic SLA (modSLA) titanium discs and transfected with a reporter vector sys-tem for Wnt canonical signalling. Cells were also seeded in the presence or in the absence of GSK3b-inhibitors SB216763 or SB415286 and their viability, morphology and the expression of Wnt target and osteoblast specific genes was assessed by Real Time PCR. Inhibitors altered cell morphology and mostly reduced cell viability at high concentration. SB415286 markedly increased the expression of ALP in MC3T3 cells on rough surfaces at the concentration of 100 nM before decreasing its expression at higher concentrations. OCN expression was unaffected. Increasing concentrations of SB216763 increased the expression of ALP in MC3T3 cells on rough surfaces but OCN expression was not changed at any con-centration. SB216763 and SB415286 inhibitors should be further investigated as potential tools to improve cell differentiation on titanium surfaces for endosseous implants.

Enhancement of in Vitro Osteoblastic Potential after Selective Sorting of Osteoblasts with High Alkaline Phosphatase Activity from Human Osteoblast-Like Cells

In this article we describe the expansion of in vitro osteogenic capability of human osteoblasts (HOS cells) after sorting by fluorescence-activated cell sorting (FACS) with the osteoblastic marker of human bone alkaline phosphatase (AP) monoclonal antibody. After culturing for 7 days, the HOS cells were incubated with fluorescein isothiocyanate (FITC)-labeled AP monoclonal antibody. The antibody recognized the cells with high AP activity (high AP cells), which were about 76% of the total cells. After the HOS cells were sorted, the high AP cells could be recovered, and almost all of them reacted strongly with the AP antibody. Therefore, we were able to condense the high AP cells about 1.3 times. We further cultured the sorted cells as well as the unsorted control cells. After the initial seeding, the culturing periods for both groups of cells were 20 days. At the end of this period, we measured AP activity per DNA and osteocalcin contents. In contrast to the low condensation ratio of the high AP cells in the sorted fraction, the AP activity and osteocalcin contents were about nine times and four times greater than those of the unsorted cells, respectively. These results demonstrated that using the sorting technique to isolate the high AP cells might be a useful method for applications in bone tissue engineering.

Flowtaxis of osteoblast migration under fluid shear and the effect of RhoA kinase silencing

Despite the important role of mechanical signals in bone remodeling, relatively little is known about how fluid shear affects osteoblastic cell migration behavior. Here we demonstrated that MC3T3-E1 osteoblast migration could be activated by physiologically-relevant levels of fluid shear in a shear stress-dependent manner. Interestingly, shear-sensitive osteoblast migration behavior was prominent only during the initial period after the onset of the steady flow (for about 30 min), exhibiting shear stress-dependent migration speed, displacement, arrest coefficient, and motility coefficient. For example, cell speed at 1 min was 0.28, 0.47, 0.51, and 0.84 μm min-1 for static, 2, 15, and 25 dyne cm-2 shear stress, respectively. Arrest coefficient (measuring how often cells are paused during migration) assessed for the first 30 min was 0.40, 0.26, 0.24, and 0.12 respectively for static, 2, 15, and 25 dyne cm-2. After this initial period, osteoblasts under steady flow showed decreased migration capacity and diminished shear stress dependency. Molecular interference of RhoA kinase (ROCK), a regulator of cytoskeletal tension signaling, was found to increase the shear-sensitive window beyond the initial period. Cells with ROCK-shRNA had increased migration in the flow direction and continued shear sensitivity, resulting in greater root mean square displacement at the end of 120 min of measurement. It is notable that the transient osteoblast migration behavior was in sharp contrast to mesenchymal stem cells that exhibited sustained shear sensitivity (as we recently reported, J. R. Soc. Interface. 2015; 12:20141351). The study of fluid shear as a driving force for cell migration, i.e., "flowtaxis", and investigation of molecular mechanosensors governing such behavior (e.g., ROCK as tested in this study) may provide new and improved insights into the fundamental understanding of cell migration-based homeostasis.

Protein tyrosine phosphatase SHP-1 modulates osteoblast differentiation through direct association with and dephosphorylation of GSK3β

SHP-1, the Src homology-2 (SH2) domain-containing phosphatase 1, is a cytosolic protein-tyrosine phosphatase (PTP) predominantly expressed in hematopoietic-derived cells. Previous studies have focused on the involvement of SHP-1 in osteoclastogenesis. Using primary cultured mouse fetal calvaria-derived osteoblasts as a model, this study aims to investigate the effects of SHP-1 on differentiation and mineralization of osteoblasts and elucidate the signaling pathways responsible for these effects. We found that osteoblasts treated by osteogenic media showed significant increase in SHP-1 expression, which contributed to osteoblastic differentiation and mineralization. Using immunoprecipitation assay, we found that a direct association between SHP-1 and glycogen synthase kinase (GSK)-3β could be detected in differentiated osteoblasts and was significantly inhibited by SHP-1 inhibitor NSC87877. Inhibition of SHP-1 activated GSK3β, thereby leading to suppression of osteoblast differentiation and mineralization, which could be rescued by the inhibitor of GSK3β. In addition, we found that rosiglitazone (RSG) treatment led to significant decrease in SHP-1 expression. Overexpression of SHP-1 reversed RSG-induced GSK3β activation, thus rescuing the inhibitory effect of RSG on osteoblast differentiation and mineralization. These findings suggest that protein tyrosine phosphatase SHP-1 may act as a positive regulator of osteoblast differentiation through direct association with and dephosphorylation of GSK3β. Downregulation of SHP-1 may contribute to RSG-induced inhibition of mouse calvaria osteoblast differentiation by activating GSK3β-dependent pathway.

Effects of a Hydroxyapatite-based Biomaterial on Gene Expression in Osteoblast-like Cells

Biostite® is a hydroxyapatite-derived biomaterial that is used in periodontal and bone reconstructive procedures due to its osteoconductive properties. Since the molecular effects of this biomaterial on osteoblasts are still unknown, we decided to assess whether it may specifically modulate osteoblast functions in vitro. We found that a brief exposure to Biostite® significantly reduced the proliferation of MG-63 and SaOS-2 osteoblast-like cells to ~ 50% of the plateau value. Furthermore, gene array analysis of MG-63 cells showed that Biostite® caused a differential expression of 37 genes which are involved in cell proliferation and interaction, and related to osteoblast differentiation and tissue regeneration. Results were confirmed by RT-PCR, Western blot, and by an increase in alkaline phosphatase (ALP) specific activity. Biostite® also increased levels of polycystin-2, a mechano-sensitive Ca2+ channel, a promising new marker of bone cell differentiation. Biostite®, therefore, may directly affect osteoblasts by enhancing chondro/osteogenic gene expression and cytoskeleton-related signaling pathways, which may contribute to its clinical efficacy.

Compressive Force Induces Osteoblast Apoptosis via Caspase-8

Periodontal remodeling during orthodontic tooth movement is a result of mechanical stresses. The application of excessive orthodontic force induces cell death. However, the nature of compressive force-induced cell death is unclear. We examined whether the in vitro application of continuous compressive force would induce apoptosis in human osteoblast-like cells (MG-63 cells), and investigated the mechanism by which apoptosis was initiated. The cells became aligned irregularly, and cell viability decreased, indicating that the compressive force caused cell death. According to the TUNEL analysis, the number of apoptotic cells increased significantly in a time-and force-dependent manner. Caspase-3 activity increased with the magnitude of the compressive force, and this effect was reduced significantly by a caspase-8 inhibitor, whereas a caspase-9 inhibitor had no such effect. We conclude that the in vitro application of compressive force can induce apoptosis in MG-63 cells through the activation of caspase-3 via the caspase-8 signaling cascade.

Alkaline phosphatase determines polyphosphate-induced mineralization in a cell-type independent manner

Polyphosphate [Poly(P)] has positive effects on osteoblast mineralization; however, the underlying mechanism remains unclear. In addition, it is unknown whether Poly(P) promotes mineralization in soft tissues. We investigated this by using various cells. Poly(P) concentrations of 1 and 0.5 mg/mL yielded high levels of mineralization in ROS17/2.8 osteoblast cells. Similarly, Poly(P) induced mineralization in cell types expressing alkaline phosphatase (ALP), namely, ATDC5 and MC3T3-E1, but not in CHO, C3H10T1/2, C2C12, and 3T3-L1 cells. Furthermore, forced expression of ALP caused Poly(P)-induced mineralization in CHO cells. These results suggest that ALP determines Poly(P)-induced mineralization in a cell-type independent manner.

Effect of NSAIDs on the aminopeptidase activity of cultured human osteoblasts

Aminopeptidases (APs) are involved in various physiological and pathological processes. In tumor tissues the expression of APs, cyclooxygenase-2 and its metabolites are increased. The objective was to determine the effect of certain NSAIDs on the AP activity of osteoblasts. Primary cultures of osteoblast were treated with different concentrations of indomethacin, meloxicam, naproxen, nimesulide, and piroxicam. The AP activity was fluorimetrically determined using aminoacyl-β-naphthylamides (aa-βNAs) as substrates: Ala-βNA, Arg-βNA, Gly-βNA, Leu-βNA, Lys-βNA, Met-βNA, and Phe-βNA. The five NSAIDs showed an inhibitory effect of AP activity against the study substrates depending on the dose tested. Meloxicam and piroxicam had the highest inhibitory effect on enzymatic activity, with an IC50 of around 70 μM. Our results suggest that the physiological alteration of osteoblasts in the presence of NSAIDs may be a consequence of AP inhibition, suggesting a potential clinical role for these drugs against cancer in combination with chemotherapeutic agents.

Peroxidase Enzymes Regulate Collagen Biosynthesis and Matrix Mineralization by Cultured Human Osteoblasts

The early recruitment of inflammatory cells to sites of bone fracture and trauma is a critical determinant in successful fracture healing. Released by infiltrating inflammatory cells, myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are heme-containing enzymes, whose functional involvement in bone repair has mainly been studied in the context of providing a mechanism for oxidative defense against invading microorganisms. We report here novel findings that show peroxidase enzymes have the capacity to stimulate osteoblastic cells to secrete collagen I protein and generate a mineralized extracellular matrix in vitro. Mechanistic studies conducted using cultured osteoblasts show that peroxidase enzymes stimulate collagen biosynthesis at a post-translational level in a prolyl hydroxylase-dependent manner, which does not require ascorbic acid. Our studies demonstrate that osteoblasts rapidly bind and internalize both MPO and EPO, and the catalytic activity of these peroxidase enzymes is essential to support collagen I biosynthesis and subsequent release of collagen by osteoblasts. We show that EPO is capable of regulating osteogenic gene expression and matrix mineralization in culture, suggesting that peroxidase enzymes may play an important role not only in normal bone repair, but also in the progression of pathological states where infiltrating inflammatory cells are known to deposit peroxidases.

Activating AMP-activated protein kinase by an α1 selective activator compound 13 attenuates dexamethasone-induced osteoblast cell death

Excessive glucocorticoid (GC) usage may lead to non-traumatic femoral head osteonecrosis. Dexamethasone (Dex) exerts cytotoxic effect to cultured osteoblasts. Here, we investigated the potential activity of Compound 13 (C13), a novel α1 selective AMP-activated protein kinase (AMPK) activator, against the process. Our data revealed that C13 pretreatment significantly attenuated Dex-induced apoptosis and necrosis in both osteoblastic-like MC3T3-E1 cells and primary murine osteoblasts. AMPK activation mediated C13' cytoprotective effect in osteoblasts. The AMPK inhibitor Compound C, shRNA-mediated knockdown of AMPKα1, or dominant negative mutation of AMPKα1 (T172A) almost abolished C13-induced AMPK activation and its pro-survival effect in osteoblasts. On the other hand, forced AMPK activation by adding AMPK activator A-769662 or exogenous expression a constitutively-active (ca) AMPKα1 (T172D) mimicked C13's actions and inhibited Dex-induced osteoblast cell death. Meanwhile, A-769662 or ca-AMPKα1 almost nullified C13's activity in osteoblast. Further studies showed that C13 activated AMPK-dependent nicotinamide adenine dinucleotide phosphate (NADPH) pathway to inhibit Dex-induced reactive oxygen species (ROS) production in MC3T3-E1 cells and primary murine osteoblasts. Such effects by C13 were almost reversed by Compound C or AMPKα1 depletion/mutation. Together, these results suggest that C13 alleviates Dex-induced osteoblast cell death via activating AMPK signaling pathway.

Aluminum Chloride Induces Osteoblasts Apoptosis via Disrupting Calcium Homeostasis and Activating Ca(2+)/CaMKII Signal Pathway

Aluminum promotes osteoblast (OB) apoptosis. Apoptosis is induced by the disordered calcium homeostasis. Therefore, to investigate the relationship between Al-induced OB apoptosis and calcium homeostasis, calvarium OBs from neonatal rats (3-4 days) were cultured and exposed to 0.048-mg/mL Al(3+) or 0.048-mg/mL Al(3+) combined with 5 μM BAPTA-AM (OBs were pretreated with 5 μM BAPTA-AM for 1 h, then added 0.048 mg/mL Al(3+)), respectively. Then OB apoptosis rate, intracellular calcium ions concentration ([Ca(2+)]i), mRNA expression level of calmodulin (CaM), and protein expression levels of CaM and p-CaMKII in OBs were examined. The result showed that AlCl3 increased OB apoptosis rate, and [Ca(2+)]i and p-CaMKII expression levels and decreased CaM expression levels, whereas BAPTA-AM relieved the effects. These results proved that AlCl3 induced OB apoptosis by disrupting the intracellular Ca(2+) homeostasis and activating the Ca(2+)/CaMKII signal pathway. Our findings can provide new insights for revealing the apoptosis mechanism of OBs exposed to AlCl3.

Elevated marrow inflammatory cells and osteoclasts in subchondral osteosclerosis in human knee osteoarthritis

Subchondral osteosclerosis, characterized by an increase of hypomineralized bone material, is a pathological hallmark of osteoarthritis. The cellular components in the subchondral marrow compartment that participate in this aberrant bone remodeling process remain to be elucidated. This study assessed the presence of marrow inflammatory cells and their relative abundance between nonsclerotic and sclerotic tissues in knee osteoarthritis. Bone samples from osteoarthritic knee tibial plateaus were stratified for histological analyses using computed tomography osteoabsorptiometry. Immunohistological analysis revealed the presence of CD20 (B-lymphocyte) and CD68 (macrophage), but not CD3 (T-lymphocyte) immunoreactive mononuclear cells in subchondral marrow tissues and their relative abundance was significantly increased in sclerotic compared with nonsclerotic bone samples. Multinucleated osteoclasts that stained positive for CD68 and tartrate-resistant acid phosphatase, predominantly associated with CD34-positive blood vessels and their abundance was strongly increased in sclerotic samples. Bone-specific alkaline phosphatase activity in outgrowth osteoblasts was induced by conditioned medium from nonsclerotic, but not sclerotic, bone pieces. These results suggest that an interaction between bone-resident cells and marrow inflammatory cells might play a role in aberrant bone remodeling leading to subchondral osteosclerosis. Elevated osteoclast activity in sclerotic bone suggests that bone formation and resorption activities are increased, yet uncoupled, in human knee osteoarthritis.

Glabridin Alleviates the Toxic Effects of Methylglyoxal on Osteoblastic MC3T3-E1 Cells by Increasing Expression of the Glyoxalase System and Nrf2/HO-1 Signaling and Protecting Mitochondrial Function

Methylglyoxal (MG) contributes to the pathogenesis of age- and diabetes-associated complications. The present study investigated the effects of glabridin on MG-induced cytotoxicity in MC3T3-E1 osteoblastic cells. MC3T3-E1 cells were treated with glabridin in the presence of MG, and markers of mitochondrial function and oxidative damage were examined. Pretreatment of MC3T3-E1 osteoblastic cells with glabridin prevented MG-induced cell death, the production of intracellular reactive oxygen species and mitochondrial superoxides, cardiolipin peroxidation, and the production of inflammatory cytokines. The soluble form of receptor for advanced glycation end products (sRAGEs)/RAGE ratio increased upon MG treatment, but less so after pretreatment with glabridin, which also increased the level of reduced glutathione and the activities of glyoxalase I and heme oxygenase-1, all of which were reduced by MG. In addition, glabridin elevated the level of nuclear factor erythroid 2-related factor 2. These findings suggest that glabridin protects against MG-induced cell damage by inhibiting oxidative stress and increasing MG detoxification. Pretreatment of MC3T3-E1 osteoblastic cells with glabridin reduced MG-induced mitochondrial dysfunction. Additionally, the nitric oxide level significantly increased upon glabridin pretreatment. Together, these data show that glabridin may potentially serve to prevent the development of diabetic bone disease associated with MG-induced oxidative stress.

Activation of Protein Kinase A in Mature Osteoblasts Promotes a Major Bone Anabolic Response

Protein kinase A (PKA) regulates osteoblast cell function in vitro and is activated by important bone mass modulating agents. We determined whether PKA activation in osteoblasts is sufficient to mediate a bone anabolic response. Thus, a mouse model conditionally expressing a constitutively active PKA (CA-PKA) in osteoblasts (CA-PKA-OB mouse) was developed by crossing a 2.3-kb α1 (I)-collagen promoter-Cre mouse with a floxed-CA-PKA mouse. Primary osteoblasts from the CA-PKA-OB mice exhibited higher basal PKA activity than those from control mice. Microcomputed tomographic analysis revealed that CA-PKA-OB female mice had an 8.6-fold increase in femoral but only 1.16-fold increase in lumbar 5 vertebral bone volume/total volume. Femur cortical thickness and volume were also higher in the CA-PKA-OB mice. In contrast, alterations in many femoral microcomputed tomographic parameters in male CA-PKA-OB mice were modest. Interestingly, the 3-dimensional structure model index was substantially lower both in femur and lumbar 5 of male and female CA-PKA-OB mice, reflecting an increase in the plate to rod-like structure ratio. In agreement, femurs from female CA-PKA-OB mice had greater load to failure and were stiffer compared with those of control mice. Furthermore, the CA-PKA-OB mice had higher levels of serum bone turnover markers and increased osteoblast and osteoclast numbers per total tissue area compared with control animals. In summary, constitutive activation of PKA in osteoblasts is sufficient to increase bone mass and favorably modify bone architecture and improve mechanical properties. PKA activation in mature osteoblasts is, therefore, an important target for designing anabolic drugs for treating diseases with bone loss.

Involvement of Prolyl Hydroxylase Domain Protein in the Rosiglitazone-Induced Suppression of Osteoblast Differentiation

Rosiglitazone is a well-known anti-diabetic drug that increases insulin sensitivity via peroxisome proliferator-activated receptor γ (PPARγ) activation, but unfortunately it causes bone loss in animals and humans. A previous study showed that prolyl hydroxylase domain protein (PHD) plays a role in rosiglitazone-induced adipocyte differentiation. Based on the inverse relationship between adipocyte and osteoblast differentiation, we investigated whether PHD is involved in the effects of rosiglitazone on osteoblast differentiation. Rosiglitazone inhibited osteoblast differentiation in a concentration-dependent manner, and in parallel induced three PHD isoforms (PHD1, 2, and 3). PHD inhibitors and knockdown of each isoform prevented the inhibitory effects of rosiglitazone on osteoblast differentiation and increased the expression of Runx2, a transcription factor essential for osteoblastogenesis. MG-132, a proteasomal inhibitor also prevented the rosiglitazone-induced degradation of Runx2. Furthermore, both increased PHD isoform expressions and reduced osteoblast differentiation by rosiglitazone were prevented by PPARγ antagonists, indicating these effects were mediated via PPARγ activation. In vivo oral administration of rosiglitazone to female ICR mice for 8 weeks reduced bone mineral densities and plasma alkaline phosphatase (ALP) activity, and increased PHD expression in femoral primary bone marrow cells and the ubiquitination of Runx2. Together, this suggests that the rosiglitazone-induced suppression of osteoblast differentiation is at least partly induced via PPARγ-mediated PHD induction and subsequent promotion of the ubiquitination and degradation of Runx2.

Irradiation inhibits the maturation and mineralization of osteoblasts via the activation of Nrf2/HO-1 pathway

Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) regulates the induction of antioxidant gene expression and protects cells against oxidative injury. However, there are controversial findings regarding the roles of Nrf2 on bone metabolism under oxidative stress. The role of Nrf2 on the differentiation of radiation-exposed osteoblasts is also unclear. We investigated whether Nrf2 negatively or positively affects osteoblast differentiation in response to irradiation. Irradiation inhibited osteoblast differentiation of MC3T3-E1 cells in a dose-dependent manner. This inhibition was evidenced by the irradiation-mediated decreases in bone-like nodule formation, alkaline phosphatase (ALP) activity, calcium accumulation, and expression of osteoblast markers, such as ALP, osteocalcin, osteopontin, bone sialoprotein, osterix, and Runx2. These reductions were accompanied by increased induction of Nrf2 and heme oxygenase-1 (HO-1), accumulation of cellular oxidants, and depletion of antioxidant defense enzymes. siRNA-mediated silencing of Nrf2 markedly reversed the negative effect of irradiation on osteoblast differentiation of the cells, leading to a decrease in HO-1 and an increase in Runx2 levels. Irradiation-mediated decreases in the levels of Runx2 and osteocalcin mRNA, but not of Nrf2 protein, were also significantly inhibited by HO-1 inhibitor, zinc protoporphyrin IX. Furthermore, N-acetyl cysteine restored all of the changes induced by irradiation to near-normal levels in the cells. These results demonstrate that irradiation inhibits osteoblast differentiation and mineralization of MC3T3-E1 cells through the oxidative stress-mediated activation of Nrf2/HO-1 pathway.

Effects on MC3T3-E1 Cells and In silico Toxicological Study of Two 6-(Propan-2-yl)-4-methyl-morpholine-2,5-diones

Recently, we found that two cyclodidepsipeptides, 3,6-di-(propan-2-yl)-4-methyl-morpholine-2,5-dione (1) and 3-(2-methylpropyl)-6-(propan-2-yl)-4-methyl- morpholine-2,5-dione (2), are excellent inhibitors of xanthine oxidase. In order to obtain more information about the toxicological potential of compounds 1 and 2 on bone cells, the current study was designed to evaluate the effect of these compounds on viability and proliferation of MC3T3-E1 cells. Compound 1 showed neither cytotoxic nor stimulatory effect on cell viability, while compound 2 showed a slight stimulatory effect on cell viability. Both studied compounds showed slight stimulatory effects on proliferation of MC3T3-E1 cells, in a dose dependent manner. Additionally, an in silico toxicological study of compounds 1 and 2 was performed, and the results indicate that they have a good probability of safe biological intake.

[Establishment of osteoblast primary cilia model removed by chloral hyrate]

OBJECTIVE

To establish osteoblast model, primary cilla model was removed by chloral hyrate, observe effects of osteoblast primary cilla moved on enhancing ALP staining and calcified nodules staining in electromagnetic field.

METHODS

Three 3-day-old male SD rats weighed between 6 and 9 g were killed, cranial osteoblast was drawed and adherencing cultured respectively. Cells were subcultured and randomly divided into 4 groups until reach to fusion states. The four groups included chloral hydrate non-involved group (control group), 2 mM, 4 mM and 8 mM chloral hydrate group, and cultured in 37 °C, 5% CO2 incubator for 72 h. Morphology of primary cilla was observed by laser confocal scanning microscope, and incidence of osteoblast primary cilia was analyzed by Image-Pro Plus 6.0 software. Cells in the correct concentration group which can removed cillia most effectively were selected and divided into 3 groups, including control group (C), Electromagnetic fields group (EMFs), and EMFs with 4 mM chloral hydrate group. DMEM nutrient solution contained 10%FBS were added into three groups and cultured for 9 days and formation of ALP were observed by histochemical staining of alkaline phosphatase. After 12 days' cultivation, formation of mineralization nodes was observed by alizarin red staining.

RESULTS

Compared with control group and 2mM chloral hydrate group,4 mM chloral hydrate group could effectively remove osteoblast primary cilla (P<0.01). Removal of osteoblast primary cilla could weaken the formation of ALP and mineralization nodes in osteoblast in EMFS. Compared with EMFs group, the area of ALP and mineralization nodes in EMFs with 4 mM chloral hydrate group were decreased obviously (P<0.01).

CONCLUSION

4mM chloral hydrate could effectively remove osteoblast primary cilia. Primary cilla participate in EMFs promoting formation of ALP and mineralization nodes in osteoblast and provide new ideas for exploring mechanism of EMFs promoting osteoblast maturation and mineralization.

ERK1 and ERK2 regulate chondrocyte terminal differentiation during endochondral bone formation

Chondrocytes in the epiphyseal cartilage undergo terminal differentiation prior to their removal through apoptosis. To examine the role of ERK1 and ERK2 in chondrocyte terminal differentiation, we generated Osterix (Osx)-Cre; ERK1(-/-) ; ERK2(flox/flox) mice (conditional knockout Osx [cKOosx]), in which ERK1 and ERK2 were deleted in hypertrophic chondrocytes. These cKOosx mice were grossly normal in size at birth, but by 3 weeks of age exhibited shorter long bones. Histological analysis in these mice revealed that the zone of hypertrophic chondrocytes in the growth plate was markedly expanded. In situ hybridization and quantitative real-time PCR analyses demonstrated that Matrix metalloproteinase-13 (Mmp13) and Osteopontin expression was significantly decreased, indicating impaired chondrocyte terminal differentiation. Moreover, Egr1 and Egr2, transcription factors whose expression is restricted to the last layers of hypertrophic chondrocytes in wild-type mice, were also strongly downregulated in these cKOosx mice. In transient transfection experiments in the RCS rat chondrosarcoma cell line, the expression of Egr1, Egr2, or a constitutively active mutant of MEK1 increased the activity of an Osteopontin promoter, whereas the MEK1-induced activation of the Osteopontin promoter was inhibited by the coexpression of Nab2, an Egr1 and Egr2 co-repressor. These results suggest that MEK1-ERK signaling activates the Osteopontin promoter in part through Egr1 and Egr2. Finally, our histological analysis of cKOosx mice demonstrated enchondroma-like lesions in the bone marrow that are reminiscent of human metachondromatosis, a skeletal disorder caused by mutations in PTPN11. Our observations suggest that the development of enchondromas in metachondromatosis may be caused by reduced extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK MAPK) signaling.

Hypoxia promotes AMP-activated protein kinase (AMPK) and induces apoptosis in mouse osteoblasts

The hypoxic environment around the fracture site develops post the blood flow disruption and leads to osteoblast cell death and further impairs fracture healing. Hypoxia usually leads to the mitochondrial dysfunction and then results in apoptotic cell death. AMPK is ubiquitously expressed and functions as an intracellular fuel sensor by maintaining energy balance, as is potentially activated by hypoxia, ischemia, and ROS, however, the regulatory role of AMPK in hypoxia-induced apoptosis in osteoblasts and in the fracture healing has not been identified. In present study, we firstly determined the apoptosis induction by hypoxia in mouse osteoblastic MC3T3-E1 cells via examining the apoptotic cells and the activation of apoptosis-related molecules, then investigated the activation of AMPK signaling by hypoxia via analyzing the phosphorylation of AMPKα and ACC1, finally we explored the association of the AMPK activation with the hypoxia-induced apoptosis using loss-of-function strategy. Results demonstrated that hypoxia induced apoptosis in MC3T3-E1 cells and activated the AMPK signaling. And the knockdown of AMPK via chemical treatment or RNA interfering significantly decreased the hypoxia-induced apoptosis in MC3T3-E1 cells. Taken together, present study unveiled the regulatory role of AMPK signaling in the hypoxia-induced osteoblast apoptosis.

Inhibition of FSS-induced actin cytoskeleton reorganization by silencing LIMK2 gene increases the mechanosensitivity of primary osteoblasts

Mechanical stimulation plays an important role in bone cell metabolic activity. However, bone cells lose their mechanosensitivity upon continuous mechanical stimulation (desensitization) and they can recover the sensitivity with insertion of appropriate rest period into the mechanical loading profiles. The concrete molecular mechanism behind the regulation of cell mechanosensitivity still remains unclear. As one kind of mechanosensitive cell to react to the mechanical stimulation, osteoblasts respond to fluid shear stress (FSS) with actin cytoskeleton reorganization, and the remodeling of actin cytoskeleton is closely associated with the alteration of cell mechanosensitivity. In order to find out whether inhibiting the actin cytoskeleton reorganization by silencing LIM-kinase 2 (LIMK2) gene would increase the mechanosensitivity of primary osteoblasts, we attenuated the formation of actin stress fiber under FSS in a more specific way: inhibiting the LIMK2 expression by RNA interference. We found that inhibition of LIMK2 expression by RNA interference attenuated the formation of FSS-induced actin stress fiber, and simultaneously maintained the integrity of actin cytoskeleton in primary osteoblasts. We confirmed that the decreased actin cytoskeleton reorganization in response to LIMK2 inhibition during FSS increased the mechanosensitivity of the osteoblasts, based on the increased c-Fos and COX-2 expression as well as the enhanced proliferative activity in response to FSS. These data suggest that osteoblasts can increase their mechanosensitivity under continuous mechanical stimulation by reducing the actin stress fiber formation through inhibiting the LIMK2 expression. This study provides us with a new and more specific method to regulate the osteoblast mechanosensitivity, and also a new therapeutic target to cure bone related diseases, which is of importance in maintaining bone mass and promoting osteogenesis.

Lack of prolidase causes a bone phenotype both in human and in mouse

The degradation of the main fibrillar collagens, collagens I and II, is a crucial process for skeletal development. The most abundant dipeptides generated from the catabolism of collagens contain proline and hydroxyproline. In humans, prolidase is the only enzyme able to hydrolyze dipeptides containing these amino acids at their C-terminal end, thus being a key player in collagen synthesis and turnover. Mutations in the prolidase gene cause prolidase deficiency (PD), a rare recessive disorder. Here we describe 12 PD patients, 9 of whom were molecularly characterized in this study. Following a retrospective analysis of all of them a skeletal phenotype associated with short stature, hypertelorism, nose abnormalities, microcephaly, osteopenia and genu valgum, independent of both the type of mutation and the presence of the mutant protein was identified. In order to understand the molecular basis of the bone phenotype associated with PD, we analyzed a recently identified mouse model for the disease, the dark-like (dal) mutant. The dal/dal mice showed a short snout, they were smaller than controls, their femurs were significantly shorter and pQCT and μCT analyses of long bones revealed compromised bone properties at the cortical and at the trabecular level in both male and female animals. The differences were more pronounce at 1 month being the most parameters normalized by 2 months of age. A delay in the formation of the second ossification center was evident at postnatal day 10. Our work reveals that reduced bone growth was due to impaired chondrocyte proliferation and increased apoptosis rate in the proliferative zone associated with reduced hyperthrophic zone height. These data suggest that lack of prolidase, a cytosolic enzyme involved in the final stage of protein catabolism, is required for normal skeletogenesis especially at early age when the requirement for collagen synthesis and degradation is the highest.

Alkaline phosphatase activity-guided isolation of active compounds and new dammarane-type triterpenes from Cissus quadrangularis hexane extract

ETHNOPHARMACOLOGICAL RELEVANCE

The stem of Cissus quadrangularis L. (CQ) is used in traditional medicine to treat bone fractures and swelling. Anti-osteoporotic activity of CQ hexane extract has been reported, but the active compounds in this extract remain unknown. Thus, we aimed to identify the active compounds in CQ hexane extract using bioassay-guided isolation.

MATERIALS AND METHODS

The CQ hexane extract was fractionated sequentially with benzene, dichloromethane, ethyl acetate, and methanol. The examination of CQ extract and its fractions was guided by bioassays for alkaline phosphatase (ALP) activity during the differentiation of MC3T3-E1 osteoblastic cells. The cells were treated with or without the CQ extract and its fractions for a period of time, and then the stimulatory effect of the alkaline phosphatase enzyme, a bone differentiation marker, was investigated. The compounds obtained were structurally elucidated using spectroscopic techniques and re-evaluated for activity during bone differentiation.

RESULTS

A total of 29 compounds were isolated, viz., triterpenes, fatty acid methyl esters, glycerolipids, steroids, phytols, and cerebrosides. Four new dammarane-type triterpenes were isolated for the first time from nature, and this report is the first to identify this group of compounds from the Vitaceae family. Seven compounds, viz., glycerolipids and squalene, stimulated ALP activity at a dose of 10μg/mL. Moreover, the synergistic effect of these compounds on bone formation was demonstrated.

CONCLUSION

This report describes, for the first time, the isolation of active compounds from CQ hexane extract; these active compounds will be useful for the quality control of extracts from this plant used to treat osteoporosis.

STAT-6 mediates TRAIL induced RANK ligand expression in stromal/preosteoblast cells

Receptor activator of nuclear factor kappa-B ligand (RANKL) is a critical osteoclastogenic factor expressed in bone marrow stromal/osteoblast lineage cells. Tumor necrosis factor (TNF) related apoptosis-inducing ligand (TRAIL) levels are elevated in pathologic conditions such as multiple myeloma and inflammatory arthritis, and have been positively correlated with osteolytic markers. Osteoprotegerin (OPG) which inhibits osteoclastogenesis is a decoy receptor for RANKL and also known to interact with TRAIL. Herein, we show that TRAIL increases DR5 and DcR1 receptors but no change in the levels of DR4 and DcR2 expression in human bone marrow derived stromal/preosteoblast (SAKA-T) cell line. We further demonstrated that TRAIL treatment significantly decreased OPG mRNA expression. Interestingly, TRAIL treatment induced RANKL mRNA expression in these cells. In addition, TRAIL significantly increased NF-kB and c-Jun N-terminal kinase (JNK) activity. Human transcription factor array screening by real-time RT-PCR identified TRAIL up-regulation of the signal transducers and activators of the transcription (STAT)-6 expression in SAKA-T cells. TRAIL stimulation induced p-STAT-6 expression in human bone marrow derived primary stromal/preosteoblast cells. Confocal microscopy analysis further revealed p-STAT-6 nuclear localization in SAKA-T cells. Chromatin immunoprecipitation (ChIP) assay confirmed p-STAT-6 binding to the hRANKL gene distal promoter region. In addition, siRNA suppression of STAT-6 expression inhibits TRAIL increased hRANKL gene promoter activity. Thus, our results suggest that TRAIL induces RANKL expression through a STAT-6 dependent transcriptional regulatory mechanism in bone marrow stromal/preosteoblast cells.

Nanostructured Zr-Pd metallic glass thin film for biochemical applications

Zr-Pd metallic glassy thin films with a hierarchical nano-scale structure, produced by magnetron sputtering of the Zr and Pd powder mixture, demonstrate a unique combination of physical and biochemical properties. Thermal stability of the nano-structured glassy samples, their resistance to oxidation in dry air and phase transformation behavior are discussed in the present work. These binary alloy samples also show exceptionally high corrosion resistance and spontaneous passivation in a simulated body fluid. Experiments on the catalytic activity and biocompatibility of this nanostructured metallic glass indicate that this is a very suitable material for biochemical applications. Compared to the multicomponent alloys studied earlier this binary alloy has much simpler chemical composition, which makes preparation of the sample with defined stoichiometry easier, especially when the elements have different sputtering rates.

Expression of poly(ADP-ribose) polymerase in bone regeneration

Poly(ADP-ribose) polymerase (PARP) is a 116kDa enzyme catalysing the synthesis of ADP-ribose polymers from NAD+. PARP is activated in response to DNA strand breaks and plays a critical role in the maintenance of genomic integrity. However, considering its role also in transcription, proliferation as well as apoptosis in biological process, in the present study the role of PARP in bone regeneration was evaluated, in particular in bone cell proliferation and differentiation processes. Thus, formalin fixed paraffin embedded specimens of 10 human bone samples after sinus lift were collected and investigated by immunohistochemistry using a mouse monoclonal anti-human PARP antibody. PARP was expressed in cells with morphological features of osteoblasts in the areas of new bone formation at the junction between mineralized and unmineralized tissue, between osteoid tissue and bone. Few osteoclasts were observed and showed only focal nuclear expression of PARP, while osteocytes showed no positivity for PARP. Our data showed an overall involvement of PARP enzyme in human bone tissues, in particular during bone regeneration process.

MiR-378 overexpression attenuates high glucose-suppressed osteogenic differentiation through targeting CASP3 and activating PI3K/Akt signaling pathway

Hyperglycemia is one of the possible causes for osteoporosis and bone fracture in diabetes mellitus. Here we modeled diabetes-induced osteoporosis in vitro using preosteoblastic cell line MC3T3-E1 and a diabetic mice model for in vivo studies. We found that in addition to reducing osteoblast viability and differentiation (mineralization), culture in elevated glucose down regulated microRNA-378 (miR-378) expression but ectopic miR-378 expression reversed the effects of high glucose. We identified caspase-3 (CASP3) as a target of miR-378 and showed that miR-378 repressed CASP3 mRNA and protein expression under high glucose condition. We further showed that both miR-378 expression and CASP3 silencing independently restored alkaline phosphatase (ALP) activity and the expression of osteoblastic differentiation markers Runt-related transcription factor 2 (Runx2), osteorix (Osx), collagen I (Col I), osteocalcin (OCN), and osteonectin (ON). We also found that under high glucose conditions miR-378 activated the PI3K/Akt signaling pathway and down regulated pro-apoptotic CytC, Apaf-1 and Bax proteins via the PI3K/Akt pathway. Collectively, these results suggest that miR-378 overexpression attenuates high glucose-suppressed osteogenic differentiation through targeting CASP3 and activating the PI3K/Akt pathway.

Enhancement of cell ingrowth, proliferation, and early differentiation in a three-dimensional silicon carbide scaffold using low-intensity pulsed ultrasound

Concerns over the use of autografts or allografts have necessitated the development of biomaterials for bone regeneration. Various studies have been performed to optimize the cultivation of osteogenic cells using osteoconductive porous scaffolds. The aim of this study was to evaluate the osteogenic efficiency of bone cell ingrowth, proliferation, and early differentiation in a silicon carbide (SiC) porous ceramic scaffold promoted with low-intensity pulsed ultrasound. MC3T3-E1 mouse preosteoblasts were seeded onto scaffolds and cultured for 4 and 7 days with daily of 20-min ultrasound treatment. The cells were evaluated for cell attachment, morphology, viability, ingrowth depth, volumetric proliferation, and early differentiation. After 4 and 7 days of culture and ultrasound exposure, the cell density was higher in the ultrasound-treated group compared with the sham-treated group on SiC scaffolds. The cell ingrowth depths inside the SiC scaffolds were 149.2±27.3 μm at 1 day, 310.1±12.6 μm for the ultrasound-treated group and 248.0±19.7 μm for the sham control at 4 days, and 359.6±18.5 μm for the ultrasound-treated group and 280.0±17.7 μm for the sham control at 7 days. They were significantly increased, that is, 25% (p=0.0029) and 28% (p=0.0008) increase, respectively, with ultrasound radiation force as compared with those in sham control at 4 and 7 days postseeding. The dsDNA contents were 583.5±19.1 ng/scaffold at 1 day, 2749.9±99.9 ng/scaffold for the ultrasound-treated group and 2514.9±114.7 ng/scaffold for the sham control at 4 days, and 3582.3±325.3 ng/scaffold for the ultrasound-treated group and 2825.7±134.3 ng/scaffold for the sham control at 7 days. There was a significant difference in the dsDNA content between the ultrasound- and sham-treated groups at 4 and 7 days. The ultrasound-treated group with the SiC construct showed a 9% (p=0.00029) and 27% (p=0.00017) increase in the average dsDNA content at 4 and 7 days over the sham control group, respectively. Alkaline phosphatase activity was significantly increased by the treatment of ultrasound at 4 (p=0.012) and 7 days (p=0.035). These results suggested that ultrasound treatment with low-intensity acoustic energy facilitated the cellular ingrowth and enhanced the proliferation and early differentiation of osteoblasts in SiC scaffolds.

Adiponectin receptor 1 regulates bone formation and osteoblast differentiation by GSK-3β/β-catenin signaling in mice

Adiponectin and its receptors are expressed in bone marrow-derived osteoblasts. Previous studies in vivo and in vitro have produced controversial results. The purpose of this study was to use porcine adiponectin receptor 1 transgenic mice (pAdipoR1) as a model to evaluate the role of AdipoR1 on bone physiology at different ages. pAdipoR1 transgenic mice had higher bone mineral density than wild-type mice in both genders at 56 weeks of age. The bone volume and trabecular number, measured by micro-computed tomography (μCT) was significantly greater in transgenic than in wild-type female mice at both 8 and 56 weeks of age. ELISA analysis revealed that both serum osteocalcin and osteoprotegerin (OPG) were significantly increased in 8-week old pAdipoR1 transgenic mice of both genders. Furthermore, serum OPG was elevated at 32 and 56 weeks of age in female and male pAdipoR1 transgenic mice. Serum TRAP5b concentration was reduced in 8 and 56 weeks old male pAdipoR1 mice compared with wild-type male mice. Knock-down of AdipoR1 significantly decreased gene expression of osteocalcin, OPG, alkaline phosphatase and msh homeobox 2 and the mineralization in MC3T3-E1 cells and mesenchymal stem cells. In addition, pathscan analysis and real-time PCR analysis suggest AdipoR1 regulates osteoblast differentiation through GSK-3 β and β-Catenin signaling. Consequently, the lack of AdipoR1 impaired osteoblast differentiation and bone formation. We conclude that AdipoR1 is a critical factor for the osteoblast differentiation and bone homeostasis.

Estrogen withdrawal from osteoblasts and osteocytes causes increased mineralization and apoptosis

Recent studies have demonstrated increased bone mineral heterogeneity following estrogen withdrawal in vivo. Such changes likely contribute to fracture risk during post-menopausal osteoporosis since tissue mineralization is correlated with bone strength and stiffness. However, the cellular mechanisms responsible for increased mineral variability have not yet been distinguished. The objective of this study is to elucidate how alterations in mineral distribution are initiated during estrogen depletion. Specifically, we tested two separate hypotheses; (1) estrogen deficiency directly alters osteoblast mineralization and (2) estrogen deficiency increases bone cell apoptosis. Osteoblast-like cells (MC3T3-E1) and osteocyte-like cells (MLO-Y4) were pretreated with or without estrogen (17β-estradiol) for 14 days. Estrogen deficiency was subsequently induced by either withdrawing estrogen from cells or blocking estrogen receptors using an estrogen antagonist, fulvestrant (ICI 182,780). Cell number (Hoechst DNA), alkaline phosphatase activity (p-NPP), mineralization (alizarin red) and apoptosis (Caspase 3/7) were evaluated. Whether estrogen withdrawal altered apoptosis rates in the presence of an apoptosis promoting agent (etoposide) was also determined. Interestingly, estrogen withdrawal from cells accustomed to estrogen exposure caused significantly increased osteoblast mineralization and osteocyte apoptosis compared with continued estrogen treatment. In contrast, blocking estrogen receptors with fulvestrant abrogated the mineralization induced by estrogen treatment. When apoptosis was induced using etoposide, cells undergoing estrogen withdrawal increased apoptosis compared to cells with continued estrogen treatment. Recognizing the underlying mechanisms regulating bone cell mineralization and apoptosis during estrogen deficiency and their consequences is necessary to further our knowledge of osteoporosis.

Pharmacological GSK-3beta inhibition improves osteoblast differentiation on titanium surfaces

Rough titanium surfaces enhance the activation of Wnt canonical signaling, a pathway required for osteoblast differentiation. The present study investigated the effects of GSK3b-inhibitor (2'Z,3'E)- 6-Bromoindirubin-3'-oxime (BIO) on osteoblastic differentiation on titanium surfaces with different topography and wettability. C2C12 cells were plated on pickled, acid-etched/sand-blasted (SLA), modified hydrophilic SLA titanium discs (modSLA) and stimulated with increasing doses of BIO. Activation of Wnt canonical signaling was measured with a reporter system. Gene expression was measured in the same cell system by Real Time PCR. Osteoblastic MC3T3 cells were then plated on discs with or without BIO and the expression of osteoblast specific genes was assessed by Real Time PCR. One mM BIO activated Wnt canonical signaling in C2C12 cells on all surfaces, and the highest effect was on rough surfaces. BIO markedly increased the expression of Osteoprotegerin and Osteocalcin in MC3T3 cells on rough surfaces at the concentration of 100 nM, and on all surfaces at the concentration of 1 mM. BIO enhances Wnt signaling activation and the expression of osteoblastic genes on rough surfaces and could be a viable approach to improve cell response to implant surfaces.

A novel function for lysyl oxidase in pluripotent mesenchymal cell proliferation and relevance to inflammation-associated osteopenia

Lysyl oxidase is a multifunctional enzyme required for collagen biosynthesis. Various growth factors regulate lysyl oxidase during osteoblast differentiation, subject to modulation by cytokines such as TNF-α in inflammatory osteopenic disorders including diabetic bone disease. Canonical Wnt signaling promotes osteoblast development. Here we investigated the effect of Wnt3a and TNF-α on lysyl oxidase expression in pluripotent C3H10T1/2 cells, bone marrow stromal cells, and committed osteoblasts. Lysyl oxidase was up-regulated by a transcriptional mechanism 3-fold in C3H10T1/2 cells, and 2.5-fold in bone marrow stromal cells. A putative functional TCF/LEF element was identified in the lysyl oxidase promoter. Interestingly, lysyl oxidase was not up-regulated in committed primary rat calvarial- or MC3T3-E1 osteoblasts. TNF-α down-regulated lysyl oxidase both in Wnt3a-treated and in non-treated C3H10T1/2 cells by a post-transcriptional mechanism mediated by miR203. Non-differentiated cells do not produce a collagen matrix; thus, a novel biological role for lysyl oxidase in pluripotent cells was investigated. Lysyl oxidase shRNAs effectively silenced lysyl oxidase expression, and suppressed the growth of C3H10T1/2 cells by 50%, and blocked osteoblast differentiation. We propose that interference with lysyl oxidase expression under excess inflammatory conditions such as those that occur in diabetes, osteoporosis, or rheumatoid arthritis can result in a diminished pool of pluripotent cells which ultimately contributes to osteopenia.

Design and in vitro evaluation of simvastatin-hydroxyapatite coatings by an electrochemical process on titanium surfaces

The aim of this study was to deposit simvastatin-hydroxyapatite (sim-HA) coatings on titanium surfaces by an electrochemical process and evaluate osteoprogenitor cell responses to sim-HA-coated surfaces. Simvastatin was prepared onto titanium surfaces with varying concentration (10(-7), 10(-6), 10(-5) and 10(-4) mol/L). Surface characteristics were performed by FSEM, XRD and FTIR. LC-MS/MS method was used for simvastatin quantification in Sim-HA-coated surfaces. Murine calvaria-derived pre-osteoblastic cell (MC3T3-E1) proliferation, alkaline phosphatase activity (ALP) and osteocalcin release were used to measure osteoblastic activities. FSEM observation showed rod-like HA crystals covered on all surfaces. As drug concentration increased, the crystal diameter decreased. XRD and FTIR observations showed all coatings consisted of HA. LC-MS/MS test showed the simvastatin concentration in 10(-6) mol/L group was 2.77 x 10(-7) mol/L/cm2, while in 10(-7) mol/L group was 1.89 x 10(-7) mol/L/cm2. MC3T3-E1 cells grown on 10(-7) mol/L and 10(-6) mol/L Sim-HA surfaces showed increased ALP activity as compared to HA-coated surfaces on day 7 (P < 0.05) while at 10 day, all Sim-HA groups were significantly increased than HA group (P < 0.05). Two Sim-HA groups showed significant increases in osteocalcin production on 7 10 and 14 day (P < 0.05). It showed that incubation with 10(-7) mol/L simvastatin enhanced the cell proliferation to a statistically significant extent (P < 0.05) compared with the HA group at day 7. It was concluded that simvastatin was successfully deposited into HA coatings using the electrochemical process and the sim-HA coatings enhanced differentiation of osteoprogenitor cells.