Effects of osteogenic protein-1 (OP-1, BMP-7) on bone matrix protein expression by fetal rat calvarial cells are differentiation stage specific

The effect of autologous bone marrow stromal cells differentiated on scaffolds for canine tibial bone reconstruction

Bone marrow contains mesenchymal stem cells that form many tissues. Various scaffolds are available for bone reconstruction by tissue engineering. Osteoblastic differentiated bone marrow stromal cells (BMSC) promote osteogenesis on scaffolds and stimulate bone regeneration. We investigated the use of cultured autologous BMSC on different scaffolds for healing defects in tibias of adult male canines. BMSC were isolated from canine humerus bone marrow, differentiated into osteoblasts in culture and loaded onto porous ceramic scaffolds including hydroxyapatite 1, hydroxyapatite gel and calcium phosphate. Osteoblast differentiation was verified by osteonectine and osteocalcine immunocytochemistry. The scaffolds with stromal cells were implanted in the tibial defect. Scaffolds without stromal cells were used as controls. Sections from the defects were processed for histological, ultrastructural, immunohistochemical and histomorphometric analyses to analyze the healing of the defects. BMSC were spread, allowed to proliferate and differentiate to osteoblasts as shown by alizarin red histochemistry, and osteocalcine and osteonectine immunostaining. Scanning electron microscopy showed that BMSC on the scaffolds were more active and adhesive to the calcium phosphate scaffold compared to the others. Macroscopic bone formation was observed in all groups, but scaffolds with stromal cells produced significantly better results. Bone healing occurred earlier and faster with stromal cells on the calcium phosphate scaffold and produced more callus compared to other scaffolds. Tissue healing and osteoblastic marker expression also were better with stromal cells on the scaffolds. Increased trabecula formation, cell density and decreased fibrosis were observed in the calcium phosphate scaffold with stromal cells. Autologous cultured stromal cells on the scaffolds were useful for healing of canine tibial bone defects. The calcium phosphate scaffold was the best for both cell differentiation in vitro and bone regeneration in vivo. It may be possible to improve healing of bone defects in humans using stem cells from bone marrow.

Palmitate attenuates osteoblast differentiation of fetal rat calvarial cells

Aging is associated with the accumulation of ectopic lipid resulting in the inhibition of normal organ function, a phenomenon known as lipotoxicity. Within the bone marrow microenvironment, elevation in fatty acid levels may produce an increase in osteoclast activity and a decrease in osteoblast number and function, thus contributing to age-related osteoporosis. However, little is known about lipotoxic mechanisms in intramembraneous bone. Previously we reported that the long chain saturated fatty acid palmitate inhibited the expression of the osteogenic markers RUNX2 and osteocalcin in fetal rat calvarial cell (FRC) cultures. Moreover, the acetyl CoA carboxylase inhibitor TOFA blocked the inhibitory effect of palmitate on expression of these two markers. In the current study we have extended these observations to show that palmitate inhibits spontaneous mineralized bone formation in FRC cultures in association with reduced mRNA expression of RUNX2, alkaline phosphatase, osteocalcin, and bone sialoprotein and reduced alkaline phosphatase activity. The effects of palmitate on osteogenic marker expression were inhibited by TOFA. Palmitate also inhibited the mRNA expression of fatty acid synthase and PPARγ in FRC cultures, and as with osteogenic markers, this effect was inhibited by TOFA. Palmitate had no effect on FRC cell proliferation or apoptosis, but inhibited BMP-7-induced alkaline phosphatase activity. We conclude that palmitate accumulation may lead to lipotoxic effects on osteoblast differentiation and mineralization and that increases in fatty acid oxidation may help to prevent these lipotoxic effects.

Differentially expressed genes and signalling pathways are involved in mouse osteoblast-like MC3T3-E1 cells exposed to 17-β estradiol

Oestrogen is essential for maintaining bone mass, and it has been demonstrated to induce osteoblast proliferation and bone formation. In this study, complementary DNA (cDNA) microarrays were used to identify and study the expression of novel genes that may be involved in MC3T3-E1 cells' response to 17-β estradiol. MC3T3-E1 cells were inoculated in minimum essential media alpha (α-MEM) cell culture supplemented with 17-β estradiol at different concentrations and for different time periods. MC3T3-E1 cells treated with 10⁻⁸ mol⋅L⁻¹ 17-β estradiol for 5 days exhibited the highest proliferation and alkaline phosphatase (ALP) activity; thus, this group was chosen for microarray analysis. The harvested RNA was used for microarray hybridisation and subsequent real-time reverse transcription polymerase chain reaction (RT-PCR) to validate the expression levels for selected genes. The microarray results were analysed using both functional and pathway analysis. In this study, microarray analysis detected 5 403 differentially expressed genes, of which 1 996 genes were upregulated and 3 407 genes were downregulated, 1 553 different functional classifications were identified by gene ontology (GO) analysis and 53 different pathways were involved based on pathway analysis. Among the differentially expressed genes, a portion not previously reported to be associated with the osteoblast response to oestrogen was identified. These findings clearly demonstrate that the expression of genes related to osteoblast proliferation, cell differentiation, collagens and transforming growth factor beta (TGF-β)-related cytokines increases, while the expression of genes related to apoptosis and osteoclast differentiation decreases, following the exposure of MC3T3-E1 cells to α-MEM supplemented with 17-β estradiol. Microarray analysis with functional gene classification is critical for a complete understanding of complementary intracellular processes. This microarray analysis provides large-scale gene expression data that require further confirmatory studies.

Winner of the 2013 Young Investigator Award for the Society for Biomaterials annual meeting and exposition, April 10-13, 2013, Boston, Massachusetts. Osteogenic differentiation of mesenchymal stem cells on demineralized and devitalized biodegradable polymer and extracellular matrix hybrid constructs

Devitalization and demineralization processing of biodegradable polymer and extracellular matrix (ECM) hybrid constructs was explored for the effect on the retention of ECM components and construct osteogenicity. Hybrid constructs were generated by seeding osteogenically predifferentiated rat mesenchymal stem cells (MSCs) onto electrospun poly(ε-caprolactone) fiber meshes and culturing in osteogenic medium for 12 or 16 days within a flow perfusion bioreactor to create an ECM coating. The resulting constructs were then either devitalized (using a freeze-thaw or a detergent technique), devitalized and demineralized, or left untreated, and subsequently characterized for DNA, glycosaminoglycan, collagen, and calcium content. The osteogenicity of each construct was investigated by culturing MSCs on the hybrid constructs within a flow perfusion bioreactor for 4, 8, and 12 days in osteogenic medium. Histological staining demonstrated that devitalization via the freeze-thaw method retained the thickest coating of ECM components within the constructs. Demineralization and devitalization processing of ECM coated constructs resulted in a decrease in their osteogenicity.

The etiology of cleft palate formation in BMP7-deficient mice

Palatogenesis is a complex process implying growth, elevation and fusion of the two lateral palatal shelves during embryogenesis. This process is tightly controlled by genetic and mechanistic cues that also coordinate the growth of other orofacial structures. Failure at any of these steps can result in cleft palate, which is a frequent craniofacial malformation in humans. To understand the etiology of cleft palate linked to the BMP signaling pathway, we studied palatogenesis in Bmp7-deficient mouse embryos. Bmp7 expression was found in several orofacial structures including the edges of the palatal shelves prior and during their fusion. Bmp7 deletion resulted in a general alteration of oral cavity morphology, unpaired palatal shelf elevation, delayed shelf approximation, and subsequent lack of fusion. Cell proliferation and expression of specific genes involved in palatogenesis were not altered in Bmp7-deficient embryos. Conditional ablation of Bmp7 with Keratin14-Cre or Wnt1-Cre revealed that neither epithelial nor neural crest-specific loss of Bmp7 alone could recapitulate the cleft palate phenotype. Palatal shelves from mutant embryos were able to fuse when cultured in vitro as isolated shelves in proximity, but not when cultured as whole upper jaw explants. Thus, deformations in the oral cavity of Bmp7-deficient embryos such as the shorter and wider mandible were not solely responsible for cleft palate formation. These findings indicate a requirement for Bmp7 for the coordination of both developmental and mechanistic aspects of palatogenesis.

Early gene regulation of osteogenesis in embryonic stem cells

The early gene regulatory networks (GRNs) that mediate stem cell differentiation are complex, and the underlying regulatory associations can be difficult to map accurately. In this study, the expression profiles of the genes Dlx5, Msx2 and Runx2 in mouse embryonic stem cells were monitored over a 48 hour period after exposure to the growth factors BMP2 and TGFβ1. Candidate GRNs of early osteogenesis were constructed based on published experimental findings and simulation results of Boolean and ordinary differential equation models were compared with our experimental data in order to test the validity of these models. Three gene regulatory networks were found to be consistent with the data, one of these networks exhibited sustained oscillation, a behaviour which is consistent with the general view of embryonic stem cell plasticity. The work cycle presented in this paper illustrates how mathematical modelling can be used to elucidate from gene expression profiles GRNs that are consistent with experimental data.

Mechanical loading stimulates BMP7, but not BMP2, production by osteocytes

Bone mechanical adaptation is a cellular process that allows bones to adapt their mass and structure to mechanical loading. This process is governed by the osteocytes, which in response to mechanical loading produce signaling molecules that affect osteoblasts and osteoclasts. Bone morphogenic proteins (BMPs) are excellent candidates as signaling molecules, but it is unknown whether mechanically stimulated osteocytes affect bone adaptation through BMP production. Therefore, the aim of this study was to assess whether osteocytes produce BMPs in response to mechanical loading. In addition, since BMP7 has a vitamin D receptor (VDR) response element in the promoter region, we also investigated whether VDR is involved in the BMP7 response to mechanical loading. Human or VDR(-/-) mouse primary bone cells were submitted in vitro to 1 h pulsating fluid flow (PFF) and postincubated without PFF (PI) for 1-24 h, and gene and protein expression of BMP2 and BMP7 were quantified. In human bone cells, PFF did not change BMP2 gene expression, but it upregulated BMP7 gene expression by 4.4- to 5.6-fold at 1-3 h PI and stimulated BMP7 protein expression by 2.4-fold at 6 h PI. PFF did not stimulate BMP7 gene expression in VDR(-/-) mouse bone cells. These results show for the first time that mechanical loading upregulates BMP7, likely via the VDR, but not BMP2, gene and protein expression in osteocytes in vitro. Since BMP7 plays a major role in bone development and remodeling, these data might contribute to a better understanding of the mechanism leading to the mechanical adaptation of bone.

Culture media for the differentiation of mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) can be isolated from various tissues such as bone marrow aspirates, fat or umbilical cord blood. These cells have the ability to proliferate in vitro and differentiate into a series of mesoderm-type lineages, including osteoblasts, chondrocytes, adipocytes, myocytes and vascular cells. Due to this ability, MSCs provide an appealing source of progenitor cells which may be used in the field of tissue regeneration for both research and clinical purposes. The key factors for successful MSC proliferation and differentiation in vitro are the culture conditions. Hence, we here summarize the culture media and their compositions currently available for the differentiation of MSCs towards osteogenic, chondrogenic, adipogenic, endothelial and vascular smooth muscle phenotypes. However, optimal combination of growth factors, cytokines and serum supplements and their concentration within the media is essential for the in vitro culture and differentiation of MSCs and thereby for their application in advanced tissue engineering.

Osteogenic differentiation of mesenchymal stem cells on pregenerated extracellular matrix scaffolds in the absence of osteogenic cell culture supplements

This study utilized a full-factorial design to investigate the effect of four factors: presence of whole bone marrow cells, presence of in vitro-generated mineralized extracellular matrix (ECM), presence of dexamethasone, and variations in culture duration, on the proliferation and osteogenic differentiation of mesenchymal stem cells (MSCs) cultured on a polymer scaffold. Electrospun poly(epsilon-caprolactone) (PCL) fiber mesh scaffolds were seeded with rat MSCs and cultured in complete osteogenic medium for 12 days to generate constructs containing mineralized ECM. MSCs or MSCs and whole bone marrow cells were seeded onto decellularized ECM constructs (PCL/ECM) or plain PCL scaffolds and cultured statically for 4, 8, and 16 days in medium either with or without dexamethasone. After each culture period, the cell number was determined by DNA analysis, and the osteogenic differentiation state of the cells was determined by alkaline phosphatase activity and calcium assays. MSCs seeded onto PCL/ECM constructs and cultured in medium either with or without dexamethasone demonstrated similar amounts of calcium deposition after 16 days. A significant increase in cell number over time compared with all other groups was observed when whole bone marrow cells were cocultured with MSCs on PCL scaffolds in medium without dexamethasone. This study establishes that the osteogenic differentiation of MSCs seeded onto ECM-containing constructs is maintained even in the absence of dexamethasone and that the coculture of MSCs and whole bone marrow cells without dexamethasone and ECM enhances the proliferation of a cell population (or populations) present in the whole bone marrow.

Prostate cancer induces bone metastasis through Wnt-induced bone morphogenetic protein-dependent and independent mechanisms

Prostate cancer (PCa) is frequently accompanied by osteosclerotic (i.e., excessive bone production) bone metastases. Although bone morphogenetic proteins (BMP) and Wnts are mediators of PCa-induced osteoblastic activity, the relation between them in PCa bone metastases is unknown. The goal of this study was to define this relationship. Wnt3a and Wnt5a administration or knockdown of DKK-1, a Wnt inhibitor, induced BMP-4 and 6 expression and promoter activation in PCa cells. DKK-1 blocked Wnt activation of the BMP promoters. Transfection of C4-2B cells with axin, an inhibitor of canonical Wnt signaling, blocked Wnt3a but not Wnt5a induction of the BMP promoters. In contrast, Jnk inhibitor I blocked Wnt5a but not Wnt3a induction of the BMP promoters. Wnt3a, Wnt5a, and conditioned medium (CM) from C4-2B or LuCaP23.1 cells induced osteoblast differentiation in vitro. The addition of DKK-1 and Noggin, a BMP inhibitor, to CM diminished PCa CM-induced osteoblast differentiation in a synergistic fashion. However, pretreatment of PCa cells with DKK-1 before collecting CM blocked osteoblast differentiation, whereas pretreatment with Noggin only partially reduced osteoblast differentiation, and pretreatment with both DKK-1 and Noggin had no greater effect than pretreatment with DKK-1 alone. Additionally, knockdown of BMP expression in C4-2B cells inhibited Wnt-induced osteoblastic activity. These results show that PCa promotes osteoblast differentiation through canonical and noncanonical Wnt signaling pathways that stimulate both BMP-dependent and BMP-independent osteoblast differentiation. These results show a clear link between Wnts and BMPs in PCa-induced osteoblast differentiation and provide novel targets, including the noncanonical Wnt pathway, for therapy of PCa.

Bone morphogenetic protein (BMP) type II receptor is required for BMP-mediated growth arrest and differentiation in pulmonary artery smooth muscle cells

Bone morphogenetic protein (BMP) signals regulate the growth and differentiation of diverse lineages. The association of mutations in the BMP type II receptor (BMPRII) with idiopathic pulmonary arterial hypertension suggests an important role of this receptor in vascular remodeling. Pulmonary artery smooth muscle cells lacking BMPRII can transduce BMP signals using ActRIIa (Activin type II receptor). We investigated whether or not BMP signaling via the two receptors leads to differential effects on vascular smooth muscle cells. BMP4, but not BMP7, inhibited platelet-derived growth factor-activated proliferation in wild-type pulmonary artery smooth muscle cells, whereas neither ligand inhibited the growth of BMPRII-deficient cells. Adenoviral gene transfer of BMPRII enabled BMP4, as well as BMP7, to inhibit proliferation in BMPRII-deficient cells. BMP-mediated growth inhibition was also reconstituted by the BMPRII short isoform, lacking the C-terminal domain present in the long form. BMP4, but not BMP7, induced the expression of osteoblast markers in wild-type cells, whereas neither ligand induced these markers in BMPRII-deficient cells. Overexpression of short or long forms of BMPRII in BMPRII-deficient cells enabled BMP4 and BMP7 to induce osteogenic differentiation. Although signaling via BMPRII or ActRIIa transiently activated SMAD1/5/8, only BMPRII signaling led to persistent SMAD1/5/8 activation and sustained increases in Id1 mRNA and protein expression. Pharmacologic blockade of BMP type I receptor function within 24 h after BMP stimulation abrogated differentiation. These data suggest that sustained BMP pathway activation, such as that mediated by BMPRII, is necessary for growth and differentiation control in vascular smooth muscle.

Oxysterols enhance osteoblast differentiation in vitro and bone healing in vivo

Oxysterols, naturally occurring cholesterol oxidation products, can induce osteoblast differentiation. Here, we investigated short-term 22(S)-hydroxycholesterol + 20(S)-hydroxycholesterol (SS) exposure on osteoblastic differentiation of marrow stromal cells. We further explored oxysterol ability to promote bone healing in vivo. Osteogenic differentiation was assessed by alkaline phosphatase (ALP) activity, osteocalcin (OCN) mRNA expression, mineralization, and Runx2 DNA binding activity. To explore the effects of osteogenic oxysterols in vivo, we utilized the critical-sized rat calvarial defect model. Poly(lactic-co-glycolic acid) (PLGA) scaffolds alone or coated with 140 ng (low dose) or 1400 ng (high dose) oxysterol cocktail were implanted into the defects. Rats were sacrificed at 6 weeks and examined by three-dimensional (3D) microcomputed tomography (microCT). Bone volume (BV), total volume (TV), and BV/TV ratio were measured. Culture exposure to SS for 10 min significantly increased ALP activity after 4 days, while 2 h exposure significantly increased mineralization after 14 days. Four-hour SS treatment increased OCN mRNA measured after 8 days and nuclear protein binding to an OSE2 site measured after 4 days. The calvarial defects showed slight bone healing in the control group. However, scaffolds adsorbed with low or high-dose oxysterol cocktail significantly enhanced bone formation. Histologic examination confirmed bone formation in the defect sites grafted with oxysterol-adsorbed scaffolds, compared to mostly fibrous tissue in control sites. Our results suggest that brief exposure to osteogenic oxysterols triggered events leading to osteoblastic cell differentiation and function in vitro and bone formation in vivo. These results identify oxysterols as potential agents in local and systemic enhancement of bone formation.

BMP-7 and CBFA1 in allograft bone in vivo bone formation and the influence of gamma-irradiation

An initial study showed that morselized human bone grafts were osteoconductive and osteoinductive when implanted in nude rat tibial window defects, and 25 kGy of gamma-irradiation significantly reduced those properties. The mechanism of the osteoinductivity and the influence of gamma-irradiation required further investigation. In this study we assessed the paraffin sections of seven morselized human bone grafts implanted into rat tibial defects for 3 weeks after being treated with 0, 15, or 25 kGy gamma-radiation respectively. Osteoclast-like cell counting and protein expressions of bone morphogenetic protein-7 (BMP-7), core binding factor alpha1 (CBFA1), and proliferating cell nuclear antigen (PCNA) were investigated and the positive signals were quantitatively analyzed. More new bone formation was observed in the 0 and 15 kGy groups compared with 25 kGy groups. The newly formed bones were found mainly from the intact cortex into the defects bridged by the implanted grafts. A dense staining of BMP-7 and CBFA1 was noted in the osteoblast-like cells in those areas. The BMP-7 and CBFA1 staining was also seen in the cells surrounding the implanted grafts in the centre areas of the defects in distance from the intact cortex. Quantitative analysis of immunohistochemical staining of the centre areas of the defects showed that gamma-irradiation (15 and 25 kGy) significantly reduced the expression of CBFA1 and BMP-7. In conclusion, morselized human bone grafts may contain some factors, which induced osteoblast lineage differentiation and bone formation and gamma-irradiation damages those bone inducing factors.

Osteogenic differentiation of human mesenchymal stem cells is regulated by bone morphogenetic protein-6

Bone marrow-derived mesenchymal stem cells (MSC) are multipotent, self-renewing, mesodermal-origin stem cells that are sequestered in the endosteal compartment. MSC are maintained in a relative state of quiescence in vivo but in response to a variety of physiological and pathological stimuli, proliferate and differentiate into osteoblasts, chondrocytes, adipocytes, or hematopoiesis-supporting stromal cells. Little is understood regarding the cellular or molecular events underlying MSC fate decisions. We report that human MSC (hMSC) cultured in defined, serum-free conditions respond to a narrow spectrum of growth factors with osteogenic commitment, differentiation, and hydroxyapatite deposition. Of the osteogenic factors we examined, only treatment with bone morphogenetic protein (BMP) results in osteoinduction under defined serum-free conditions. Among BMP-2, 4, 6, and 7, BMP-6 is the most consistent and potent regulator of osteoblast differentiation and, of these BMPs, only BMP-6 gene expression is detected prior to hMSC osteoblast differentiation. Addition of exogenous BMP-6 to hMSC induces the expression or upregulation of a repertoire of osteoblast-related genes including type I collagen, osteocalcin, bone sialoprotein, and their regulatory transcription factors Cbfa1/Runx2, and Osterix. This translates into increased production of osteogenic extracellular matrix (ECM) with subsequent hydroxyapatite deposition.

Characterization of osteoblasts derived from bone marrow stromal cells in a modified cell culture system

Bone marrow is a complex tissue composed of hematopoietic and stromal stem cells with the potential to differentiate into adipogenic, fibroblastic, reticular, osteogenic and chondrogenic lineages. Identification of differentiation markers during transformation of stromal cells into osteoblasts in a time-dependent manner may be informative for cell-based tissue engineering. Therefore, we investigated the effects of osteogenic medium (OM) on the proliferation and differentiation of rat bone marrow stromal cells (BMSCs). BMSCs from adult male rat tibia and femur were collected and cultured in alpha-MEM medium with 10% fetal bovine serum, penicillin, streptomycin and gentamycin. After three days of culture, the medium covering the adherent cells in culture was changed to OM containing dexamethasone, Na-beta-glycerophosphate and ascorbic acid. As a control, cell culture was also continued in the original medium for the same time period. Differentiated osteoblast cells were collected after 7, 10, 14, 21 and 30 days of culture, fixed with 4% paraformaldehyde and their immunolabelling for osteoblast markers osteonectin (ON) and osteocalcin (OC) was assessed using an indirect immunoperoxidase technique. Immunolabelling of ON and OC was detectable from day 10 of culture, began to increase on day 14, and increased steadily through to day 21. Labelling was highest on day 30 and was more intense in cells cultured with OM compared to the culture without OM. The control cells cultured in the absence of OM produced negligible levels of both markers. In conclusion, our culture system facilitated differentiation of BMSCs into osteoblasts featuring osteoblast markers, and these cells may be useful in autologous bone implant for the treatment of bone wound healing.

Co-transfection with the osteogenic protein (OP)-1 gene and the insulin-like growth factor (IGF)-I gene enhanced osteoblastic cell differentiation

Previous studies from this laboratory showed that the action of Osteogenic Protein-1 (OP-1, BMP-7) on osteoblastic cell differentiation could be enhanced by other protein factors, such as Insulin-like Growth Factor (IGF)-I. In the present study, we examined the effects of co-transfection with a combination of the OP-1 and the IGF-I gene on osteoblastic cell differentiation. The results first showed that fetal rat calvaria (FRC) cells transfected with the OP-1 gene under the control of the cytomegalovirus (CMV) promoter showed substantial production of the OP-1 protein. Transfected FRC cells also showed a DNA concentration-dependent increase in alkaline phosphatase (AP) activity, an osteoblastic cell differentiation marker. Von Kossa-positive nodules, a hallmark of bone formation in long-term cultures of bone-derived cells, were also observed in the transfected cells after 26 days in culture, whereas none were observed in control cells. Co-transfection of FRC cells with the combination of the OP-1 and the IGF-I gene resulted in a synergistic stimulation of AP activity. The increase was DNA dose-dependent. The current data show that transfection of OP-1 gene into osteoblastic cells stimulates osteoblastic cell differentiation in vitro. The study further demonstrates the feasibility of employing gene transfer of a second gene in combination with an OP-1 vector to synergistically enhance OP-1 activity.

Bone morphogenetic protein-6 promotes osteoblastic prostate cancer bone metastases through a dual mechanism

Prostate cancer frequently metastasizes to bone where it forms osteoblastic lesions through unknown mechanisms. Bone morphogenetic proteins (BMP) are mediators of skeletal formation. Prostate cancer produces a variety of BMPs, including BMP-6. We tested the hypothesis that BMP-6 contributes to prostate cancer-induced osteosclerosis at bone metastatic sites. Prostate cancer cells and clinical tissues produced BMP-6 that increased with aggressiveness of the tumor. Prostate cancer-conditioned medium induced SMAD phosphorylation in the preosteoblast MC3T3 cells, and phosphorylation was diminished by anti-BMP-6 antibody. Prostate cancer-conditioned medium induced mineralization of MC3T3 cells, which was blocked by both the BMP inhibitor noggin and anti-BMP-6. Human fetal bones were implanted in severe combined immunodeficient mice and after 4 weeks, LuCaP 23.1 prostate cancer cells were injected both s.c. and into the bone implants. Anti-BMP-6 or isotype antibody administration was then initiated. Anti-BMP-6 reduced LuCaP 23.1-induced osteoblastic activity, but had no effect on its osteolytic activity. This was associated with increased osteoblast numbers and osteoblast activity based on bone histomorphometric evaluation. As endothelin-1 has been implicated in bone metastases, we measured serum endothelin-1 levels but found they were not different among the treatment groups. In addition to decreased bone production, anti-BMP-6 reduced intraosseous, but not s.c., tumor size. We found that BMP-2, BMP-4, BMP-6, and BMP-7 had no direct effect on prostate cancer cell growth, but BMP-2 and BMP-6 increased the in vitro invasive ability of prostate cancer cell. These data show that prostate cancer promotes osteoblastic activity through BMP-6 and that, in addition to its bone effects, suggest that BMPs promote the ability of the prostate cancer cells to invade the bone microenvironment.

Effects of bone morphogenetic protein-2 and transforming growth factor beta1 on gene expression of transcription factors, AJ18 and Runx2 in cultured osteoblastic cells

Osteoblast differentiation is controlled by multiple transcription factors, Runx2, AJ18, Osterix, Dlx5 and Msx2. The mechanisms of regulation of AJ18 mRNA expression by the transforming growth factor beta (TGF-beta) superfamily remain poorly understood. However, it is known that BMP-2 induces differentiation of C26 cells into more mature osteoblastic cells. The present study, using Northern blot and real-time reverse transcription polymerase chain reaction analyses, investigated the effects of bone morphogenetic protein-2 (BMP-2) and TGF-beta1 on mRNA expression of AJ18 and Runx2 in a clonal osteoblast precursor cell line ROB-C26 (C26) cultured for 3, 6 or 9 days in the presence or absence of BMP-2. Although mRNA expression of Osterix and bone sialoprotein (BSP) was undetectable in the C26 culture, BMP-2 induced Osterix expression on days 3-9, but not BSP expression. BMP-2 also stimulated significantly Dlx5 expression on days 3-9, Msx2 and matrix Gla protein expressions on days 3 and 6, Runx2, alkaline phosphatase and osteocalcin expressions on days 6 and 9 in the culture. Furthermore, BMP-2 increased significantly Smad5 mRNA in the culture on day 3, indicating BMP-2 involvement in the regulation of Smad5 mRNA expression. In contrast, the inhibitory effects of BMP-2 on AJ18 mRNA expression were significant on days 3-9, indicating that a decrease in AJ18 mRNA expression is essential for the increased osteoblastic differentiation. Furthermore, TGF-beta1 (0, 0.1, 1.0 and 5.0 ng/ml) treatment of C26 cells cultured for 6 days in the presence or absence of BMP-2 for 24h stimulated mRNA levels of AJ18 and Runx2, maximal stimulation occurring principally at 1.0 ng/ml. These observations indicate that the expression of AJ18 and Runx2 mRNAs in C26 cells is under the control of BMP-2 and TGF-beta1, which exert different effects on AJ18 mRNA expression, but are potent stimulators of Runx2 mRNA expression during osteoblast differentiation.

Prostate cancer bone metastases promote both osteolytic and osteoblastic activity

Advanced prostate cancer is frequently accompanied by the development of metastasis to bone. In the past, prostate cancer bone metastases were characterized as being osteoblastic (i.e., increasing bone density) based on radiographs. However, emerging evidence suggests that development of prostate cancer bone metastases requires osteoclastic activity in addition to osteoblastic activity. The complexities of how prostate tumor cells influence bone remodeling are just beginning to be elucidated. Prostate cancer cells produce a variety of pro-osteoblastic factors that promote bone mineralization. For example, both bone morphogenetic proteins and endothelin-1 have well recognized pro-osteoblastic activities and are produced by prostate cancer cells. In addition to factors that enhance bone mineralization prostate cancer cells produced factors that promote osteoclast activity. Perhaps the most critical pro-osteoclastogenic factor produced by prostate cancer cells is receptor activator of NFkappaB ligand (RANKL), which has been shown to be required for the development of osteoclasts. Blocking RANKL results in inhibiting prostate cancer-induced osteoclastogenesis and inhibits development and progression of prostate tumor growth in bone. These findings suggest that targeting osteoclast activity may be of therapeutic benefit. However, it remains to be defined how prostate cancer cells synchronize the combination of osteoclastic and osteoblastic activity. We propose that as the bone microenvironment is changed by the developing cancer, this in turn influences the prostate cancer cells' balance between pro-osteoclastic and pro-osteoblastic activity. Accordingly, the determination of how the prostate cancer cells and bone microenvironment crosstalk are important to elucidate how prostate cancer cells modulate bone remodeling.

Bone morphogenetic protein-7 selectively enhances mechanically induced bone formation

The responses of bone cells to skeletal loading are clearly an important factor in bone biology, but much remains to be learned about the role of these responses in skeletal development, maintenance, and tissue repair. Bone morphogenetic proteins (BMPs) are key regulators of bone formation. We examined the effect of BMP-7 on periosteal and endosteal bone formation in response to increased mechanical loading using the rat tibial bending model. Female Sprague-Dawley rats were divided into four groups of six rats each. Three groups received four point bending loading at 60 N force; the fourth group received sham loading at the same force. The right tibia received 36 cycles of loading on Monday, Wednesday, and Friday for 2 weeks; the left tibia served as a nonloaded control. Just prior to loading, the three loaded groups were injected intraperitoneally with vehicle only or 10 microg/kg or 100 microg/kg of recombinant human BMP-7. Half the sham group received vehicle, and half were given 100 microg/kg of BMP-7. Bone forming surfaces were labeled twice in vivo with calcein, and histomorphometry was performed to quantify periosteal and endosteal bone formation in the loaded and control tibiae. BMP-7 had no effect on periosteal or endosteal bone formation in control or sham-loaded tibiae. Loading produced significantly more woven bone on the periosteal surface than sham loading, but BMP-7 treatment had no effect on this response. Endosteal bone formation was entirely lamellar, and loading (but not sham loading) increased the endosteal mineral apposition and bone formation rates. The higher BMP-7 dose more than doubled the load-induced increase in endosteal lamellar bone formation rate, primarily by increasing the amount of bone forming surface.

Bone morphogenetic protein and retinoic acid signaling cooperate to induce osteoblast differentiation of preadipocytes

Mesenchymal cells can differentiate into osteoblasts, adipocytes, myoblasts, or chondroblasts. Whether mesenchymal cells that have initiated differentiation along one lineage can transdifferentiate into another is largely unknown. Using 3T3-F442A preadipocytes, we explored whether extracellular signals could redirect their differentiation from adipocyte into osteoblast. 3T3-F442A cells expressed receptors and Smads required for bone morphogenetic protein (BMP) signaling. BMP-2 increased proliferation and induced the early osteoblast differentiation marker alkaline phosphatase, yet only mildly affected adipogenic differentiation. Retinoic acid inhibited adipose conversion and cooperated with BMP-2 to enhance proliferation, inhibit adipogenesis, and promote early osteoblastic differentiation. Expression of BMP-RII together with BMP-RIA or BMP-RIB suppressed adipogenesis of 3T3-F442A cells and promoted full osteoblastic differentiation in response to retinoic acid. Osteoblastic differentiation was characterized by induction of cbfa1, osteocalcin, and collagen I expression, and extracellular matrix calcification. These results indicate that 3T3-F442A preadipocytes can be converted into fully differentiated osteoblasts in response to extracellular signaling cues. Furthermore, BMP and retinoic acid signaling cooperate to stimulate cell proliferation, repress adipogenesis, and promote osteoblast differentiation. Finally, BMP-RIA and BMP-RIB induced osteoblast differentiation and repressed adipocytic differentiation to a similar extent.

Characterization of a novel KRAB/C2H2 zinc finger transcription factor involved in bone development

Osteogenic differentiation involves a cascade of coordinated gene expression that regulates cell proliferation and matrix protein formation in a defined temporo-spatial manner. Here we have used differential display to identify a novel zinc finger transcription factor (AJ18) that is induced during differentiation of bone cells in vitro and in vivo. The 64-kDa protein, encoded by a 7- kilobase mRNA, contains a Krüppel-associated box (KRAB) domain followed by 11 successive C(2)H(2) zinc finger motifs. AJ18 mRNA, which is also expressed in kidney and brain, is developmentally regulated in embryonic tibiae and calvariae, with little expression in neonate and adult animals. During osteogenic differentiation in vitro AJ18 mRNA is expressed as cells approach confluence and declines as bone formation occurs. Using bacterially expressed, His-tagged AJ18 in a target detection assay, we identified a consensus binding sequence of 5'-CCACA-3', which forms part of the consensus element for Runx2, a master gene for osteogenic differentiation. Overexpression of AJ18 suppressed Runx2-mediated transactivation of an osteocalcin promoter construct in transient transfection assays and reduced alkaline phosphatase activity in bone morphogenetic protein-induced C3H10T1/2 cells. These studies, therefore, have identified a novel zinc finger transcription factor in bone that can modulate Runx2 activity and osteogenic differentiation.

Functional analysis of bone sialoprotein: identification of the hydroxyapatite-nucleating and cell-binding domains by recombinant peptide expression and site-directed mutagenesis

Mammalian bone sialoprotein (BSP) is a mineralized tissue-specific protein containing an RGD (arginine-glycine-aspartic acid) cell-attachment sequence and two distinct glutamic acid (glu)-rich regions, with each containing one contiguous glu sequence. These regions have been proposed to contribute to the attachment of bone cells to the extracellular matrix and to the nucleation of hydroxyapatite (HA), respectively. To further delineate the domains responsible for these activities, porcine BSP cDNA was used to construct expression vectors coding for two partial-length recombinant BSP peptides: P2S (residues 42-87), containing the first glutamic acid-rich domain; and P1L (residues 69-300), containing the second glutamic acid-rich region and the RGD sequence. These peptides were expressed in Escherichia coli as his-tag fusion proteins and purified by nickel affinity columns and FPLC chromatography. Digestion with trypsin released the his-tag fusion peptide, which generated P2S-TY (residues 42-87) and P1L-TY (residues 132-239). Using a steady-state agarose gel system, P2S-TY promoted HA nucleation, whereas P2S, P1L, and P1L-TY did not. This implies that the minimum requirement for nucleation of HA resides within the amino acid sequence of the first glutamic acid-rich domain, whereas the second glutamic acid-rich domain may require posttranslational modifications for activity. P1L, but not P2S, promoted RGD-mediated attachment of human gingival fibroblasts in a manner similar to that of native BSP. Deletion of the RGD domain or conversion of it to RGE (arginine-glycine-glutamic acid) abolished the cell-attachment activity of P1L. This suggests that, at least for human gingival fibroblasts, the major cell-attachment activity in the recombinant BSP peptides studied (residues 42-87 and 69-300) requires the RGD sequence located at the C-terminal domain.

Osteogenic protein-1 differentially regulates the mRNA expression of bone morphogenetic proteins and their receptors in primary cultures of osteoblasts

The mRNA expression patterns of several bone morphogenetic proteins (BMPs) and their receptors (BMPRs) in long-term primary cultures of fetal rat calvaria (FRC) cells were examined by Northern analysis. Their temporal orders of expression were correlated with those of several biochemical markers characteristic of osteoblastic cell differentiation. Distinct temporal patterns of expression of BMPs and BMPRs during osteoblastic cell differentiation were observed. BMP-2 and BMP-7 mRNA levels did not change significantly. BMP-4 mRNA expression increased and reached a peak prior to matrix formation. BMP-5 mRNA expression increased during the mineralization phase and BMP-6 mRNA expression increased throughout all phases of cell differentiation. Effects of BMP-7 (Osteogenic Protein-1; OP-1) on the expression patterns of several other members of the BMP family and the receptors were also studied. OP-1 downregulated the BMP-4, -5, and -6 mRNA levels by a maximal of 2-fold, 1.5-fold, and 6-fold, respectively. OP-1 did not change significantly the OP-1 and BMP-2 mRNA expression. Of the three type I BMPR examined, OP-1 upregulated ActR-I and BMPR-IA mRNA expression slightly but with statistical significance. OP-1 downregulated BMPR-IB mRNA expression slightly. OP-1 upregulated BMPR-II mRNA expression by a maximum of 2-fold. Our findings demonstrate that OP-1 differentially regulates the mRNA expression of several related members of the BMP family and their receptors in osteoblasts. The observations suggest that OP-1 action on osteoblastic cells involves a complex regulation of gene expression of related members of the BMP family and their receptors in a cell differentiation stage dependent manner.

Regulation of osteoblast differentiation mediated by bone morphogenetic proteins, hedgehogs, and Cbfa1

Osteoblasts arise from common progenitors with chondrocytes, muscle and adipocytes, and various hormones and local factors regulate their differentiation. We review here regulation of osteoblast differentiation mediated by the local factors such as bone morphogenetic proteins (BMPs) and hedgehogs and the transcription factor, core-binding factor alpha-1 (Cbfa1). BMPs are the most potent regulators of osteoblast differentiation among the local factors. Sonic and Indian hedgehogs are involved in osteoblast differentiation by interacting with BMPs. Cbfa1, a member of the runt domain gene family, plays a major role in the processes of a determination of osteoblast cell lineage and maturation of osteoblasts. Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation, because Cbfa1-deficient mice completely lacked bone formation due to maturation arrest ofosteoblasts. Although the regulatory mechanism of Cbfa1 expression has not been fully clarified, BMPs are an important local factor that up-regulates Cbfa1 expression. Thus, the intimate interaction between local factors such as BMPs and hedgehogs and the transcription factor, Cbfa1, is important to osteoblast differentiation and bone formation.

Bone sialoprotein

The search for a protein nucleator of hydroxyapatite crystal formation has been a focus for the isolation and characterization of the major non-collagenous proteins in bone. Of the proteins characterized to date, bone sialoprotein (BSP) has emerged as the only bona fide candidate for nucleation. BSP is a highly glycosylated and sulphated phosphoprotein that is found almost exclusively in mineralized connective tissues. Characteristically, polyglutamic acid and arginine-glycine-aspartate (RGD) motifs with the ability to bind hydroxyapatite and cell-surface integrins, respectively, have been conserved in the protein sequence. Expression of the BSP gene, which is induced in newly formed osteoblasts, is up-regulated by hormones and cytokines that promote bone formation and down-regulated by factors that suppress bone formation. Thus, BSP has the biophysical and chemical properties of a nucleator, and its temporo-spatial expression coincides with de novo mineralization in bone and cementum. Moreover, BSP has been associated with mineral crystal formation in several pathologies, including breast carcinomas. However, the ability of BSP to mediate cell attachment and to signal through the RGD motif points to alternate functions for BSP which need further investigation. In combination, the hydroxyapatite-binding polyglutamic acid sequences and the RGD provide bi-functional entities through which BSP may mediate the targeting and attachment of normal and metastasizing cells to the bone surface.

The effect of bone morphogenetic protein-7 on the expression of type I inositol 1,4,5-trisphosphate receptor in G-292 osteosarcoma cells and primary osteoblast cultures

Bone morphogenetic protein-7 (BMP-7) affects differentiation of preosteoblasts enabling the resultant cells to respond optimally to acutely acting regulators. As the phosphoinositide cascade and, particularly, the calcium-mobilizing inositol 1,4,5-trisphosphate (InsP3) receptor are integral to stimulus-secretion coupling in osteoblasts, the hypothesis that BMP-7 affects InsP3 receptor expression was examined in the G-292 human osteosarcoma cell line and in primary cultures of human osteoblasts. G-292 osteosarcoma cells were found to be a valid experimental model for primary human osteoblasts, expressing osteoblastic mRNAs encoding osteocalcin, bone sialoprotein, alkaline phosphatase, alpha1-collagen, epidermal growth-factor receptor, and BMP type II receptor. When cultured long term in the presence of ascorbic acid and beta-glycerophosphate, G-292 cells underwent further osteoblastic differentiation, forming nodules and exhibiting restricted mineralization. G-292 cells responded to BMP-7 with an increase in InsP3 receptor density. Ligand-binding studies established that BMP-7 (50 ng/ml) treatment of G-292 cells increased InsP3 receptor density 2.4-fold with no apparent change in affinity. Immunoblot analysis with antibodies specific for type I, type II, and type III InsP3 receptors revealed that BMP-7 (50 ng/ml) treatment resulted in a specific increase (206+/-8%) in the type I receptor. Reverse transcription-polymerase chain reaction and Northern blot analyses of G-292 and primary human osteoblasts confirmed an increase in type I InsP3 receptor mRNA upon BMP-7 treatment. These results demonstrate that G-292 cells respond to BMP-7 with an increase InsP3 receptor density, consistent with the enhanced capacity of these cells to respond to Ca2+-mobilizing secretory hormones during osteoblast differentiation.

Correlation between ALK-6 (BMPR-IB) distribution and responsiveness to osteogenic protein-1 (BMP-7) in embryonic mouse bone rudiments

Osteogenic protein-1 (OP-1) or bone morphogenetic protein-7 (BMP-7) stimulates cartilage formation in mouse bone rudiments in vitro but arrests terminal differentiation of prehypertrophic chondrocytes into hypertrophic chondrocytes. In this study we report that these effects of OP-1 depend on the developmental stage of the bone rudiment, early stages (E14 and E15 metatarsals) being most responsive. E17 metatarsals that already contained a hypertrophic area that had initiated mineralization were no longer affected by OP-1. We then investigated whether the sensitivity of the early long bone rudiments to OP-1 correlated with high expression of the OP-1 binding type I serine/threonine kinase receptors (activin receptor-like kinase: ALK-2/ActR-I, ALK-3/BMPR-IA or ALK-6/BMPR-IB) at this early stage. We did not find any significant difference in overall mRNA levels of these ALKs between stages E14 through E17 as assessed by RNase protection assays. However, by immunohistochemistry we found that ALK-6 staining was strong in E14 early cartilage primordium and its future perichondrium but dropped sharply to low levels in these cell types until onset of chondrocyte (pre)hypertrophy at E16. By contrast, ALK-2 and ALK-3 immunostainings in E14 were barely detectable. We also examined by immunohistochemistry the local synthesis of OP-1. OP-1 was present in E14 early chondrocytes and forming perichondrium but in low amounts; however, production of OP-1 increased in these cell types with age. All three receptor types as well as OP-1 were present in significant amounts in prehypertrophic chondrocytes and late hypertrophic chondrocytes including those undergoing mineralization. The temporary high immunostaining for ALK-6 in the early proliferating chondrocytes and future perichondrium of E14 bone rudiments, and its absence in older bones correlated with the sensitivity of chondrocytes and perichondrium to (exogenous) OP-1. We therefore propose that the effects of OP-1 on these cells in vitro are mediated by ALK-6/BMPR-IB. We furthermore conclude that locally produced OP-1 is a potential autocrine/paracrine growth factor. Increased local production of OP-1 may be partially responsible for the age-related decrease in responsiveness to exogenous OP-1 with respect to hypertrophy and mineralization of cartilage.

Melatonin promotes osteoblast differentiation and bone formation

Prior studies have demonstrated that the pineal hormone, melatonin, can stimulate chloramphenicol acetyltransferase activity in Drosophila SL-3 cells transfected with a chloramphenicol acetyltransferase reporter construct containing the response element of rat bone sialoprotein (BSP). Based on these findings, studies were performed to determine whether melatonin could similarly modulate the expression of BSP in two cell lines, the MC3T3-E1(MC3T3) pre-osteoblast and rat osteoblast-like osteosarcoma 17/2.8 cell. Initial studies demonstrated that MC3T3 cells grown in the presence of 50 nM melatonin underwent cell differentiation and mineralization by day 12 instead of the 21-day period normally required for cells grown in untreated media. Melatonin increased gene expression of BSP and the other bone marker proteins, including alkaline phosphatase (ALP); osteopontin; secreted protein, acidic and rich in cysteine; and osteocalcin in MC3T3 cells in a concentration-dependent manner. Levels of melatonin as low as 10 nM were capable of stimulating transcription of these genes when cells were grown in the presence of beta-glycerophosphate and ascorbic acid. Under these conditions, melatonin induced gene expression of the bone marker proteins; however, this does not occur until the 5th day after seeding the culture dishes. Thereafter, MC3T3 cells responded to melatonin within 2 h of treatment. The fully differentiated rat osteoblast-like osteosarcoma 17/2.8 cells responded rapidly to melatonin and displayed an increase in the expression of BSP, ALP, and osteocalcin genes within 1 h of exposure to the hormone. To determine whether melatonin-induced osteoblast differentiation and bone formation are mediated via the transmembrane receptor, MC3T3 cells were treated in the presence and absence of melatonin with either luzindole, a competitive inhibitor of the binding of melatonin to the transmembrane receptors, or pertussis toxin, an uncoupler of G(i) from adenylate cyclase. Both luzindole and pertussis toxin were shown to reduce melatonin-induced expression of BSP and ALP. These results demonstrate, for the first time, that the pineal hormone, melatonin, is capable of promoting osteoblast differentiation and mineralization of matrix in culture and suggest that this hormone may play an essential role in regulating bone growth.

Osteogenic protein-1 increases gene expression of vascular endothelial growth factor in primary cultures of fetal rat calvaria cells

Osteogenic protein-1 (OP-1 or BMP-7) stimulates new bone formation in vivo and induces cell proliferation and differentiation of osteoblasts in vitro. In the present study, we examined effects of OP-1 on the expression of vascular endothelial growth factor (VEGF) in primary cultures of fetal rat calvaria (FRC) cells. OP-1 increased the steady-state level of VEGF mRNA by about 3-fold in an OP-1 concentration- and time-dependent manner. The increase in VEGF mRNA level depended on transcription and was sensitive to cell replication. The VEGF mRNA stability was unaffected. The mRNA levels for both types of VEGF receptors, Flk-1 and Flt-1 were low but detectable in FRC cells by RT-PCR and were not changed by OP-1. Inhibition of VEGF synthesis and function by antisense oligonucleotide and by suramin, respectively arrested the OP-1-induced alkaline phosphatase activity and mineralized bone nodule formation. Together with published studies of VEGF on vascular endothelial cells which are usually found in close proximity to osteoblastic cells in vivo, these results suggest that VEGF participates in the OP-1-induced osteogenesis by taking part in bone cell differentiation and by promoting angiogenesis at the site of bone formation.

Altered expression of bone sialoproteins in vitamin D-deficient rBSP2.7Luc transgenic mice

Bone sialoprotein (BSP) and osteopontin (OPN) are two major noncollagenous matrix proteins in mineralized connective tissue that have discrete roles in bone matrix formation, mineralization, and remodeling. The osteotropic secosteroid, 1,25-dihydroxyvitamin D3, a potent regulator of bone remodeling required for normal bone development, has been shown to exert differential effects on OPN and BSP expression by bone cells in vitro. To investigate these effects in vivo, we induced vitamin D3 deficiency in a transgenic mouse line (rBSP2.7Luc) that has a 2.7 kb rat BSP promoter linked to a luciferase reporter gene in its genome. Pregnant rBSP2.7Luc mice were fed vitamin D3-deficient food and demineralized water for 6 weeks. Their offspring were weaned at 3 weeks of age and then fed vitamin D-deficient food for an additional week. The control group were fed normal rodent pellets and water during the entire experimental procedure. Bone tissues from 40, 4-week-old offspring in each group were analyzed for BSP, OPN and luciferase expression. Vitamin D3-deficient mice displayed a rachitic phenotype that included reduced size and malformation of bones. Assays of the BSP promoter transgene in calvariae, mandibles, and tibiae of the rachitic mice showed increases in luciferase activity of 3.1-, 1.9-, and 4.6-fold, respectively, when compared with control littermates. Semiquantitative reverse transcriptase polymerase chain reaction assays of BSP mRNA revealed increases of 7-, 74-, and 66-fold, respectively, in the same rachitic bones, while OPN mRNA was reduced 12.5-fold in calvariae and 2-fold in tibiae and mandibles. In situ hybridization using mouse cRNA probes revealed that the increased BSP expression and decreased OPN expression in the vitamin D3-deficient mice was primarily in osteoblastic cells on the surface of calvariae and endosteal spaces of alveolar bone, on newly formed epiphyseal bone, and in cementoblasts and in hypertrophic chondrocytes. These studies are the first to show that BSP and OPN are differentially regulated by vitamin D3 in vivo, reflecting the diverse roles of these protein in bone remodeling. Moreover, the increased expression of the BSP transgene in the rachitic mice demonstrates that vitamin D3 regulation of BSP expression is mediated, in part, by element(s) within the 2.7 kb promoter region.

Effects of bone morphogenetic protein-2 on human neonatal calvaria cell differentiation

Bone morphogenetic proteins (BMPs) are factors that promote osteoblastic cell differentiation and osteogenesis. It is unknown whether BMPs may act on human osteoblastic cells by increasing immature cell growth and/or differentiation. We investigated the short- and long-term effects of recombinant human (rh)BMP-2 on cell growth and osteoblast phenotype in a new model of human neonatal pre-osteoblastic calvaria cells (HNC). In short-term culture, rhBMP-2 (20-100 ng/ml) inhibited DNA synthesis and increased alkaline phosphatase (ALP) activity without affecting osteocalcin (OC) production. When cultured for 3 weeks in the presence of ascorbic acid and inorganic phosphate to induce cell differentiation, HNC cells initially proliferated, type 1 collagen mRNA and protein levels rose, and then decreased, whereas OC mRNA and protein levels, and calcium accumulation into the extracellular matrix increased at 2 to 3 weeks. A transient treatment with rhBMP-2 (50 ng/ml) for 1 to 7 days which affected immature HNC cells, decreased cell growth, increased ALP activity and mRNA, and induced cells to express ALP, osteopontin, and OC at 7 days, as shown by immunocytochemistry. At 2 to 3 weeks, matrix mineralization was markedly increased despite cessation of treatment, and although OC and Col 1 mRNA and protein levels were not changed. A continuous treatment with rhBMP-2 for 3 weeks which affected immature and mature cells reduced cell growth, increased ALP activity and mRNA at 1 week and increased OC mRNA and protein levels and calcium content in the matrix at 3 weeks, indicating complete osteoblast differentiation. These results indicate that the differentiating effects of BMP-2 on human neonatal calvaria are dependent on duration of exposure. Although long-term exposure led to complete differentiation of OC-synthesizing osteoblasts, the primary effect of rhBMP-2 was to promote osteoblast marker expression in immature cells, which was sufficient to induce optimal matrix mineralization independently of cell growth and type 1 collagen expression.

Inhibition of BMP receptor synthesis by antisense oligonucleotides attenuates OP-1 action in primary cultures of fetal rat calvaria cells

Osteogenic protein-1 (OP-1 or bone morphogenetic protein-7 [BMP-7]) stimulates osteoblast differentiation in vitro and induces bone formation in vivo. BMPs exert their effects through complex formation with a heterodimeric receptor composed of a type I and a type II polypeptide. In the present study, mRNAs for three BMP subtype I receptors (ActR-I, BMPR-IA, and BMPR-IB) and one BMPR-II receptor were detected by Northern analysis in two human osteosarcoma cell lines (SaOS-2 and TE85) and in the primary cultures of fetal rat calvaria (FRC) cells. OP-1 affected the steady-state mRNA levels of these receptors differently among these cell types. To study the role of each receptor type in OP-1 action in FRC cells, receptor synthesis was inhibited by antisense oligonucleotides. Inhibition of receptor synthesis was confirmed by immunoprecipitation of radiolabeled cellular proteins with specific antibodies. The osteogenic action of OP-1 was measured by alkaline phosphatase (ALP) activity and mineralized bone nodule formation in FRC cells. Results showed that inhibition of synthesis of a single subtype I receptor alone did not affect significantly the OP-1-stimulated ALP activity. Inhibition of BMPR-II synthesis reduced the OP-1-stimulated ALP activity by about 50%. Inhibition of synthesis of any one of the type I receptor plus the BMPR-II receptor did not reduce the OP-1-stimulated ALP activity significantly beyond that observed by inhibition of BMPR-II alone. Under these conditions, nodule formation was affected similarly, thus supporting the observations made with the ALP measurements. The present results suggest that the ActR-I, BMPR-IA, and BMPR-IB receptors and the BMPR-II receptor are expressed and functional for OP-1 in FRC cells and that regulation of synthesis of these receptors may be a mechanism by which a specific cell type responds to OP-1. The turnover rate of these receptor proteins might be relatively long and another type II receptor(s) for OP-1 might be functional in FRC cells.

Expression of fetal kidney growth factors in a kidney tumor line: role of FGF2 in kidney development

To identify growth factors which may play a role in kidney organogenesis, we have analyzed culture supernatants from the pediatric kidney tumor cell line G401. G401 cells were found to secrete fibroblast growth factor 2 (FGF2), a potent mitogen for mesenchymal cells, OP-1/BMP7, an epithelial cell growth inhibitor, and midkine (MK). Northern blotting confirmed expression of FGF2, OP-1/BMP7 and MK mRNA, as well as Wnt5A mRNA in G401 cells. In situ hybridization and immunocytochemistry on human fetal kidney demonstrated FGF2 expression in epithelial cells of the branching ureteric bud epithelium, nephron precursors ("S-shaped bodies"), proximal tubule epithelium and the parietal epithelium of the glomerulus. FGF2 protein in condensed "caps" of induced mesenchymal cells was also detected by immunocytochemistry. FGF2 protein was found to be concentrated in nuclei, particularly in proximal tubule epithelial cells. Recombinant FGF2 was found to act as a mitogen on primary mouse fetal kidney cell cultures. The results demonstrate G401 cells secrete a variety of fetal kidney growth factors and that FGF2 may act as a mitogen for fetal kidney cells and thus could play a role in the morphogenesis of the kidney.

Osteoblastic phenotype of rat marrow stromal cells cultured in the presence of dexamethasone, beta-glycerolphosphate, and L-ascorbic acid

We investigated the effects of the time course of addition of osteogenic supplements dexamethasone, beta-glycerolphosphate, and L-ascorbic acid to rat marrow stromal cells, and the exposure time on the proliferation and differentiation of the cells. It was the goal of these experiments to determine the time point for supplement addition to optimize marrow stromal cell proliferation and osteoblastic differentiation. To determine this, two studies were performed; one study was based on the age of the cells from harvest, and the other study was based on the duration of exposure to supplemented medium. Cells were seen to proliferate rapidly at early time points in the presence and absence of osteogenic supplements as determined by 3H-thymidine incorporation into the DNA of replicating cells. These results were supported by cell counts ascertained through total DNA analysis. Alkaline phosphatase (ALP) activity and osteocalcin production at 21 days were highest for both experimental designs when the cells were exposed to supplemented medium immediately upon harvest. The ALP levels at 21 days were six times greater for cells maintained in supplements throughout than for control cells cultured in the absence of supplements for both studies, reaching an absolute value of 75 x 10(-7) micromole/min/cell. Osteocalcin production reached 20 x 10(-6) ng/cell at 21 days in both studies for cells maintained in supplemented medium throughout the study, whereas the control cells produced an insignificant amount of osteocalcin. These results suggest that the addition of osteogenic supplements to marrow-derived cells early in the culture period did not inhibit proliferation and greatly enhanced the osteoblastic phenotype of cells in a rat model.

Osteogenic protein-1 regulates insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding protein-5 (IGFBP-5) gene expression in fetal rat calvaria cells by different mechanisms

Osteogenic protein-1 (OP-1 or BMP-7) stimulates new bone formation in vivo and induces cell proliferation and differentiation of osteoblasts in vitro. Previous studies from our laboratory revealed that OP-1 led to a two- to threefold increase in steady-state insulin-like growth factor-I (IGF-I) and IGF-II mRNA levels and a fivefold decrease in IGF-binding protein-5 (IGFBP-5) mRNA levels in primary cultures of fetal rat calvaria (FRC) cells. In the present study, we determined whether the effects of OP-1 were at the transcriptional or posttranscriptional level. OP-1 increased the half-life of the IGF-I mRNA from 6 to 17 h without changing the level of IGF-I nuclear pre-mRNA. In transiently transfected FRC cells, the luciferase activity driven by the -1122/+362 or the -133/+362 IGF-I exon 1 promoter fragment was not changed by OP-1. Similar results were observed using the -1500/+44 or -362/+44 IGF-I exon 2 promoter constructs. Effects of OP-1 on IGF-I mRNA were independent of cell division, as they remained elevated in the presence of hydroxyurea. Cycloheximide inhibited moderately the OP-1-induced increase in IGF-I mRNA, suggesting partial dependency on protein synthesis. On the other hand, the IGF-II nuclear pre-mRNA levels were increased by OP-1 but the half-life of the mature IGF-II mRNA was not affected. Effects of OP-1 on IGF-II mRNA were also independent of cell division, but were dependent on protein synthesis. OP-1 caused a 43-50% reduction in the level of IGFBP-5 nuclear pre-mRNA transcripts and a 40% decrease in the IGFBP-5 promoter activity in FRC cells transfected with the -1278/+1 IGFBP-5 promoter fragment. The half-life of the mature IGFBP-5 mRNA was not affected by OP-1. Hydroxyurea did not prevent the OP-1-induced reduction in IGFBP-5 mRNA. The level of IGFBP-5 mRNA was barely detectable in the presence of cycloheximide, and further suppressive effect of OP-1 on IGFBP-5 mRNA could not be determined. In conclusion, OP-1 regulates IGF-I gene expression at the posttranscriptional level, but regulates IGF-II and IGFBP-5 gene expression at the transcriptional level.

Flow cytometric analysis of recombinant human osteogenic protein-1 (BMP-7) responsive subpopulations from fetal rat calvaria based on intracellular osteopontin content

The bone morphogenetic proteins (BMPs) are characterized by their ability to induce both chondrogenic and osteogenic differentiation of mesenchymal cells in vivo and in vitro. Primary cultures of fetal rat calvarial cells contain a broad spectrum of osteogenic cells at various stages of differentiation, but the responsive subpopulations are incompletely characterized. To identify responsive cells in osteogenic cell differentiation, we have treated fetal rat calvarial cells with recombinant osteogenic protein-1 and used flow cytometric analyses of intracellular osteopontin, and of cartilage and bone nodule formation, to evaluate the effects. When administered as a single dose at confluence, osteogenic protein-1 stimulated bone nodule formation in fetal rat calvarial cultures in dose-dependently way. To determine the response of osteogenic subpopulations at two discrete stages of differentiation, fetal rat calvaria cells were cultured for 2 days (proliferative stage) or 12 days (early mineralization stage) and treated with 100 ng/ml recombinant osteogenic protein-1 for 12 h before analysis by flow cytometry. Flow cytometry analyses of cell suspensions revealed that osteogenic protein-1 increased the total protein content of cells, and selectively increased the mean expression of osteopontin and the size and granularity of osteopontin expressing cells, particularly at day 12, consistent with a stimulation of osteogenic differentiation and matrix formation. Pulse administration of 100 ng/ml osteogenic protein-1 to sorted, osteopontin-negative subpopulations enriched for stem cells reduced by more than four-fold the number and size of bone nodules while promoting chondrogenesis and adipogenesis. In contrast, a pulse administration of osteogenic protein-1 to more differentiated, large osteopontin-positive cells increased bone nodule formation two-fold. Continuous administration of 100 ng/ml osteogenic protein-1 to the large osteopontin-positive and small osteopontin-negative cell populations obliterated bone nodule formation and promoted chondrogenesis. We conclude that pulse administration of osteogenic protein-1 promotes osteogenic differentiation of cells committed to the osteogenic lineage, whereas undifferentiated periosteal cells are induced to differentiate along the chondrogenic pathway. In contrast, continuous exposure to osteogenic protein-1 promotes chondrogenesis in populations of committed osteogenic cells and in undifferentiated periosteal cells.

Influence of osteogenic protein-1 (OP-1;BMP-7) and transforming growth factor-beta 1 on bone formation in vitro

The bone morphogenetic proteins (BMPs) and transforming growth factor-beta s (TGF-beta s), are a group of structurally related proteins which have been shown to stimulate bone formation in vivo. Since these proteins are concentrated in the organic matrix of bone and would be released during bone resorption, they are likely to have a profound effect on the remodeling bone and may provide a link between bone resorption and bone formation. We are using primary cultures of fetal rat calvarial cells (FRCC) to study the independent and combined effects of OP-1/BMP-7 and TGF-beta 1 on bone cells at different stages of differentiation in order to identify responding cell populations and target genes. We have confirmed prior reports that OP-1 stimulates, while TGF-beta 1 inhibits, osteogenic differentiation in this system. The increase in both number and size of the mineralized nodules induced by OP-1 was accompanied by increased expression of alkaline phosphatase and type I collagen with an induction of bone sialoprotein (BSP) suggesting that OP-1 stimulates both differentiation and clonal expansion of osteoblastic cells. Interestingly, TGF-beta 1 abrogated OP-1 induced nodule formation. Despite these opposing effects on osteogenic differentiation, TGF-beta 1 (Wrana et al, 1991) and OP-1 both stimulated a rapid induction of osteopontin (OPN) mRNA in confluent FRCC cultures enriched in pre-osteoblastic cells. In contrast, when OP-1 was added to nodule-forming cultures which are enriched in osteoblastic cells, there was only a weak induction of OPN. Moreover, while the expression of one marker for mature osteoblasts (BSP) was refractory to OP-1, another (osteocalcin) was markedly stimulated. Thus OP-1 has selective effects on bone matrix protein expression that are dependent on the differentiated state of the cells.