The Wnt antagonist secreted frizzled-related protein-1 controls osteoblast and osteocyte apoptosis

Mechanisms controlling human bone formation remain to be fully elucidated. We have used differential display-polymerase chain reaction analysis to characterize osteogenic pathways in conditionally immortalized human osteoblasts (HOBs) representing distinct stages of differentiation. We identified 82 differentially expressed messages and found that the Wnt antagonist secreted frizzled-related protein (sFRP)-1 was the most highly regulated of these. Transient transfection of HOBs with sFRP-1 suppressed canonical Wnt signaling by 70% confirming its antagonistic function in these cells. Basal sFRP-1 mRNA levels increased 24-fold during HOB differentiation from pre-osteoblasts to pre-osteocytes, and then declined in mature osteocytes. This expression pattern correlated with levels of cellular viability such that the pre-osteocytes, which had the highest levels of sFRP-1 mRNA, also had the highest rate of cell death. Basal sFRP-1 mRNA levels also increased 29-fold when primary human mesenchymal stem cells were differentiated to osteoblasts supporting the developmental regulation of the gene. Expression of sFRP-1 mRNA was induced 38-fold following prostaglandin E2 (PGE2) treatment of pre-osteoblasts and mature osteoblasts that had low basal message levels. In contrast, sFRP-1 expression was down-regulated by as much as 80% following transforming growth factor (TGF)-beta1 treatment of pre-osteocytes that had high basal mRNA levels. Consistent with this, treatment of pre-osteoblasts and mature osteoblasts with PGE(2) increased apoptosis threefold, while treatment of pre-osteocytes with TGF-beta1 decreased cell death by 50%. Likewise, over-expression of sFRP-1 in HOBs accelerated the rate of cell death threefold. These results establish sFRP-1 as an important negative regulator of human osteoblast and osteocyte survival.     

Sphingosine-1-phosphate acts as a developmental stage specific inhibitor of platelet-derived growth factor-induced chemotaxis of osteoblasts

The development and maintenance of a healthy skeleton depends on the migration of cells to areas of new bone formation. Osteoblasts, the bone forming cells of the body, mature from mesenchymal stem cells under the influence of bone morphogenetic protein. It is unclear at what developmental stage the osteoblasts start to migrate to their functional location. We have studied migration of immature pre-osteoblasts and of mature osteoblasts in response to Platelet-derived growth factor (PDGF) and sphingosine-1-phosphate (S1P). PDGF is a growth factor involved in bone remodeling and fracture healing whereas S1P is a circulating sphingolipid known to control cell trafficking. Our data indicate that PDGF acts as a chemotactic cue for pre-osteoblasts. In contrast, S1P is a chemorepellent to these cells. Upon Bone Morphogenetic Protein 2-induced conversion to the osteoblast phenotype, the chemotaxis response to PDGF is retained whereas the sensitivity to S1P is lost. By RNA interference and overexpression experiments we showed that the expression level of the S1P2 receptor is the sole determinant controlling responsiveness to S1P. The combined data indicate that migration of osteoblasts is controlled by the balance between PDGF, S1P and the differentiation state of the cells. We propose that this mechanism preserves the osteoprogenitor pool in the bone marrow, only allowing the more differentiated cell to travel to sites of bone formation.     

Tissue matrix protein expression in human osteoblasts,osteosarcoma tumors,and osteosarcoma cell lines

Treatment for osteosarcoma is problematic because there are no prognostic markers. Diagnosis is primarily limited to cytologic grading. Oncogenesis alters cell structure therefore osteoblast tissue matrix proteins (extracellular matrix, cytoskeletal, intermediate filament, and nuclear matrix proteins), components of the cell substructure, are candidates for osteosarcoma markers. Structural proteins of the extracellular matrix, e.g. the collagens, are useful for diagnosis but not for tumors that produce little osteoid. To identify principal cellular tissue matrix proteins that distinguish normal from transformed human osteoblasts, their expression in normal osteoblasts, two osteosarcoma cell lines, and three primary osteosarcoma tumors were compared. The tumors were graded as (i) intermediate, (ii) high, and (iii) high grade recurrent. The 1-D SDS/PAGE profiles of the major components of the nuclear matrix and intermediate filament fractions from normal osteoblasts did not vary with biopsy site, age, or sex of patients. These profiles included known cytoskeletal proteins and OB250, a ∼250 kD protein(s) observed in the intermediate filament fraction. A loss of protein bands, including OB250, was observed in the osteosarcoma cell lines and tumors. The intermediate and high grade tumors exhibited nearly identical protein profiles including potential tumor-specific proteins and collagen, consistent with the presence of intracellular collagen fibers in osteosarcoma. A microsequence was obtained for OT25, a novel low molecular weight protein observed in osteosarcoma cell lines. Fibrinogen γ-chain, a protein that mediates cell adhesion was recovered from the high grade recurrent tumor. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cellular and molecular alterations of osteoblasts in human disorders of bone formation

Osteogenesis is a complex process characterized sequentially by the commitment of precursor cells, the proliferation of osteoprogenitor cells, the differentiation of pre-osteoblasts into mature osteoblasts and the apposition of a calcified bone matrix. Recent advances in cell and molecular biology have improved our knowledge of the cellular and molecular mechanisms controlling the different steps of bone formation in humans. Using ex vivo/in vitro studies of disorders of bone formation, we showed that the recruitment of osteoprogenitor cells is the most important step controlling the rate of bone formation in both rodents and humans. Accordingly, treatments stimulating osteoblast recruitment were found to increase bone formation in experimental models of osteopenic disorders. Using models of human osteoblastic cells, we identified the profile of phenotypic markers expressed during osteoblast differentiation, and found that hormones and growth factors control osteoblastic cell proliferation and differentiation in a sequential and coordinate manner during osteogenesis in vitro. Our recent evaluation of the phenotypic osteoblast abnormalities induced by genetic mutations in the Gs alpha and FGFR-2 genes led to the characterization of the role of these genes in the alterations of osteoblast proliferation and differentiation in humans. These studies at the histological, cellular and molecular levels provided new insight into the mechanisms that are involved in pathological bone formation in humans. It is expected that further determination of the pathogenic pathways in metabolic and genetic abnormalities in human osteoblasts will help to identify novel target genes and to conceive new therapeutic tools to stimulate bone formation in osteopenic disorders.     

Advanced glycation end-products (AGEs) induce concerted changes in the osteoblastic expression of their receptor RAGE and in the activation of extracellular signal-regulated kinases (ERK)

An increase in the interaction between advanced glycation end-products (AGEs) and their receptor RAGE is believed to contribute to the pathogenesis of chronic complications of Diabetes mellitus, which can include bone alterations such as osteopenia. We have recently found that extracellular AGEs can directly regulate the growth and development of rat osteosarcoma UMR106 cells, and of mouse calvaria-derived MC3T3E1 osteoblasts throughout their successive developmental stages (proliferation, differentiation and mineralisation), possibly by the recognition of AGEs moieties by specific osteoblastic receptors which are present in both cell lines. In the present study we examined the possible expression of RAGE by UMR106 and MC3T3E1 osteoblastic cells, by immunoblot analysis. We also investigated whether short-, medium- or long-term exposure of osteoblasts to extracellular AGEs, could modify their affinity constant and maximal binding for AGEs (by 125I-AGE-BSA binding experiments), their expression of RAGE (by immunoblot analysis) and the activation status of the osteoblastic ERK 1/2 signal transduction mechanism (by immunoblot analysis for ERK and P-ERK). Our results show that both osteoblastic cell lines express readily detectable levels of RAGE. Short-term exposure of phenotypically mature osteoblastic UMR106 cells to AGEs decrease the cellular density of AGE-binding sites while increasing the affinity of these sites for AGEs. This culture condition also dose-dependently increased the expression of RAGE and the activation of ERK. In proliferating MC3T3E1 pre-osteoblasts, 24–72 h exposure to AGEs did not modify expression of RAGE, ERK activation or the cellular density of AGE-binding sites. However, it did change the affinity of these binding sites for AGEs, with both higher- and lower-affinity sites now being apparent. Medium-term (1 week) incubation of differentiated MC3T3E1 osteoblasts with AGEs, induced a simultaneous increase in RAGE expression and in the relative amount of P-ERK. Mineralising MC3T3E1 cultures grown for 3 weeks in the presence of extracellular AGEs showed a decrease both in RAGE and P-ERK expression. These results indicate that, in phenotypically mature osteoblastic cells, changes in ERK activation closely follow the AGEs-induced regulation of RAGE expression. Thus, the AGEs-induced biological effects that we have observed previously in osteoblasts, could be mediated by RAGE in the later stages of development, and mediated by other AGE receptors in the earlier pre-osteoblastic stage.     

Bone regenerates via dedifferentiation of osteoblasts in the zebrafish fin

While mammals have a limited capacity to repair bone defects, zebrafish can completely regenerate amputated bony structures of their fins. Fin regeneration is dependent on formation of a blastema, a progenitor cell pool accumulating at the amputation plane. It is unclear which cells the blastema is derived from, whether it forms by dedifferentiation of mature cells, and whether blastema cells are multipotent. We show that mature osteoblasts dedifferentiate and form part of the blastema. Osteoblasts downregulate expression of intermediate and late bone differentiation markers and induce genes expressed by bone progenitors. Dedifferentiated osteoblasts proliferate in a FGF-dependent manner and migrate to form part of the blastema. Genetic fate mapping shows that osteoblasts only give rise to osteoblasts in the regenerate, indicating that dedifferentiation is not associated with the attainment of multipotency. Thus, bone can regenerate from mature osteoblasts via dedifferentiation, a finding with potential implications for human bone repair.     

Ghrelin uses the GHS-R1a/Gi/cAMP pathway and induces differentiation only in mature osteoblasts. This ghrelin pathway is impaired in AIS patients

We have examined the Acylated Ghrelin (AG)/Gi pathway in different human osteoblastic cell lines. We have found that: 1) AG induces differentiation/mineralization only in mature osteoblasts; 2) the expression of GHS-R1a increases up to the mature cell stage, 3) the action is mediated via the GHS-R/Gi/cAMP pathway only in mature osteoblasts, and 4) osteoblastic cells from adolescent idiopathic scoliosis (AIS) are resistant to the AG/Gi/cAMP pathway. Altogether, these results suggested that AG uses the GHS-R1a/Gi/cAMP pathway to induce differentiation in mature osteoblasts only. This pathway is impaired in AIS osteoblasts. Understanding AG-specific pathways involved in normal and pathological osteoblasts may be useful for developing new treatments for pathologies such as AIS or osteoporosis.     

Osteogenic differentiation of immature osteoblasts: Interplay of cell culture media and supplements

Differentiation of immature osteoblasts to mature osteoblasts in vitro initially was induced by supplementing the medium with β-gylcerophosphate and dexamethasone. Later, ascorbic acid, vitamin D3, vitamin K3 and TGFβ1 were used in varying concentrations as supplements to generate a mature osteoblast phenotype. We tested the effects of several combinations of cell culture media, seeding protocols and osteogenic supplements on osteogenic differentiation of human primary osteoblasts. Osteogenic differentiation was analyzed by staining alkaline phosphatase (ALP) with 5-bromo-4-chloro-3-indolyl-phosphate/nitro blue tetrazolium (BCIP/NBT) and by von Kossa staining of deposited calcium phosphate. The combinations of culture media and supplements significantly influenced osteogenic differentiation, but the seeding protocol did not. Staining of ALP and calcium phosphate could be achieved only if our own mix of osteogenic supplements was used in combination with Dulbecco's modified Eagle medium or if a commercial mix of osteogenic supplements was used in combination with osteoblast growth medium. Especially for von Kossa, we observed great variations in the staining intensity. Because osteogenic differentiation is a complex process, the origin of the osteoblasts, cell culture media and osteogenic supplements should be established by preliminary experiments to achieve optimal differentiation. Staining of ALP or deposited calcium phosphate should be supplemented with qRT-PCR studies to learn more about the influence of specific supplements on osteogenic markers.     

Coexpression of osteogenic and adipogenic differentiation markers in selected subpopulations of primary human mesenchymal progenitor cells

Knowledge of the basic mechanisms controlling osteogenesis and adipogenesis might provide new insights into the prevention of osteoporosis and age-related osteopenia. With the help of magnetic cell sorting and fluorescence activated cell sorting (FACS), osteoblastic subpopulations of mesenchymal progenitor cells were characterized. Alkaline phosphatase (AP) negative cells expressed low levels of osteoblastic and adipocytic markers. AP positive cells expressed adipocytic markers more strongly than the AP negative cell populations, thus suggesting that committed osteoblasts exhibit a greater adipogenic potential. AP negative cells differentiated to the mature osteoblastic phenotype, as demonstrated by increased AP-activity and osteocalcin secretion under standard osteogenic culture conditions. Surprisingly, this was accompanied by increased expression of adipocytic gene markers such as peroxisome proliferator-activated receptor-gamma2, lipoprotein lipase and fatty acid binding protein. The induction of adipogenic markers was suppressed by transforming growth factor-beta1 (TGF-beta1) and promoted by bone morphogenetic protein 2 (BMP-2). Osteogenic culture conditions including BMP-2 induced both the formation of mineralized nodules and cytoplasmic lipid vacuoles. Upon immunogold electron microscopic analysis, osteoblastic and adipogenic marker proteins were detectable in the same cell. Our results suggest that osteogenic and adipogenic differentiation in human mesenchymal progenitor cells might not be exclusively reciprocal, but rather, a parallel event until late during osteoblast development.     

Comparative secretome analysis of human bone marrow‐derived mesenchymal stem cells during osteogenesis

Osteogenesis is a tightly regulated process that involves coordinated extracellular signals from autocrine and paracrine loops. Secretory proteins during osteogenesis can inhibit cell proliferation and activate cell differentiation toward mature osteoblasts, which are characterized by mineralization. In this study, we attempted to identify these secretory proteins during osteogenesis using LC–MS/MS analysis. We compared the secretome between undifferentiated human bone marrow‐derived mesenchymal stem cells (hBMSCs) and differentiated osteoblasts. Among 315 proteins that were identified, 177 proteins were present at increased levels in osteoblasts, whereas 88 proteins were present at decreased levels. Among the identified proteins, several were validated by quantitative RT‐PCR and immunoblot analysis. Of particular interest, calcium homeostasis‐related proteins were upregulated, whereas stem cell proliferation‐related proteins and other lineage‐related proteins were downregulated during osteogenesis. These findings provide information about the dynamic changes in the expression and secretion of proteins during osteogenesis and suggest the putative role of secretory proteins in osteogenesis. J. Cell. Physiol. 228: 216–224, 2013. © 2012 Wiley Periodicals, Inc.     

Primary human osteoblasts and bone cancer cells as models to study glycodynamics in bone

Bone cells produce many glycoproteins potentially involved in the maintenance of healthy bone tissues. Two cytokines produced in inflamed joints, tumor necrosis factor (TNF)alpha and transforming growth factor (TGF)beta, have previously been shown to alter cellular glycosylation which may potentially affect the expression and function of glycoproteins. In order to evaluate models to study the glycodynamics of bone cells, we examined primary human osteoblastic cells from osteoarthritis patients, and compared these to human osteosarcoma cells MG63 and SJSA-1. We showed here for the first time that all of the human osteoblastic cells actively synthesize complex N- and O-glycan chains of bone cell glycoproteins, with quantitative differences between cell types. TNFalpha-induced apoptosis or TGFbeta-induced cell differentiation and proliferation had significant effects on both cell surface carbohydrates and glycosyltransferase activities of osteoblasts and osteosarcoma cells. The results indicate that cultured human bone-derived osteoblastic cells are good models to examine the glycodynamics of osteoblasts under conditions of cell growth and cell death. The changes induced by cytokines can result in altered cell surface functions which may be of importance in osteoarthritis, osteoporosis and other bone diseases.     

Interleukin-1 regulates FGF-2 mRNA and localization of FGF-2 protein in human osteoblasts

Interleukin-1 (IL-1) and basic fibroblast growth factor (FGF-2) are potent stimulators of osteoclast formation. However, the role of FGF-2 in the responses to IL-1 in bone has not been reported. We examined the effect of IL-1 on FGF-2 mRNA and protein expression in human osteosarcoma MG-63 osteoblasts, normal human osteoblasts (NHOB), and osteoblasts from osteoarthritic patients (F2 and F13). IL-1 increased FGF-2 mRNA expression in osteoblasts within 1.5 to 3 h. Multiple FGF-2 protein isoforms were expressed in human osteoblasts. Twenty-four hours of treatment of MG-63 and NHOB cells with IL-1 increased the high-molecular-weight(HMW, 22/24 kDa) and low-molecular-weight (LMW, 18 kDa) FGF-2 proteins intracellularly. In contrast, IL-1 preferentially increased the LMW protein signal intracellularly as well as on the cell surface of F2 and F13 osteoblasts. We conclude that IL-1 is a major stimulator of FGF-2 expression in human osteoblasts. Furthermore, selective increases in the exportable LMW protein in osteoblasts from osteoarthritic patients may be of clinical relevance.     

Thrombospondin is synthesized and secreted by human osteoblasts and osteosarcoma cells. A model to study the different effects of thrombospondin in cell adhesion

In this study we have shown by both immunofluorescence and immunoprecipitation techniques that human osteoblasts and osteosarcoma cells synthesize and secrete thrombospondin, a 450-kDa glycoprotein initially found in platelets. Immunofluorescence with a mouse monoclonal antibody to human platelet thrombospondin yielded specific granular staining within the cytoplasm of human osteoblasts. SDS/polyacrylamide gel electrophoresis analysis of immunoprecipitates obtained with polyclonal and monoclonal anti-thrombospondin antibodies allows the identification of thrombospondin in the cellular lysates and the culture media of biosynthetically labelled osteoblasts and osteosarcoma cells. Kinetic and dose/response studies of osteoblasts and of two osteosarcoma cell lines (MG-63, SaOs-2) were performed to assess the ability of these cells to adhere to thrombospondin and type-I collagen. Thrombospondin promoted the attachment of human osteoblasts whereas it inhibited the adhesion of MG-63 and SaOs-2 cells, both when it was directly adsorbed to plastic and when it was bound to type-I collagen. Therefore osteoblasts and osteosarcoma cells may be valuable tools to study the role of thrombospondin in cell adhesion.