Bone cell differentiation: a functionally coupled relationship between expression of cell-growth- and tissue-specific genes

Cell seeding via bioprinted hydrogels supports cell migration into porous apatite-wollastonite bioceramic scaffolds for bone tissue engineering

Cell seeding via cell-laden hydrogels offers a rapid way of depositing cells onto a substrate or scaffold. When appropriately formulated, hydrogels provide a dense network of fibres for cellular encapsulation and attachment, creating a protective environment that prevents cells to be washed away by media. However, when incorporating hydrogels into a cell seeding strategy the cellular capacity for migration from a hydrogel network and subsequent biofunctionality must be assessed. Here, we compare cell seeding via a bioprinted hydrogel with conventional manual cell seeding in media. To this end, we use a binder jet 3D printed bioceramic scaffold as a model system for bone tissue engineering and the reactive jet impingement (ReJI) bioprinting system to deliver high cell density cell-laden hydrogels onto the surface of the scaffolds. The bioceramic scaffolds were produced in apatite-wollastonite (AW) glass-ceramic, with a total porosity of ~50 %, with pore size predominantly around 50-200 μm. Bone marrow-derived mesenchymal stromal cells were seeded onto the porous AW substrate both in media and via ReJI bioprinting. Cell seeding in media confirmed the osteoinductive nature and the ability of the scaffold to support cell migration within the porous structure. Cell seeding via ReJI bioprinting demonstrated that the cell-laden hydrogel penetrated the porous AW structure upon hydrogel deposition. Furthermore, cells would then migrate out from the hydrogel network and interact with the bioceramic substrate. Overall, levels of cell migration and mineralisation were significant and comparable for both seeding approaches. However, cell seeding via bioprinted hydrogels may serve as an effective strategy for in situ cell seeding into implants, which is desired in clinical tissue engineering procedures, avoiding the time taken for cell attachment from media, and the requirement to maintain a specific orientation until attachment has occurred.

Identifying Biomarkers for Osteogenic Potency Assay Development

There has been extensive exploration of how cells may serve as advanced therapy medicinal products to treat skeletal pathologies. Osteoblast progenitors responsible for production of extracellular matrix that is subsequently mineralized during bone formation have been characterised as a rare bone marrow subpopulation of cell culture plastic adherent cells. Conveniently, they proliferate to form single-cell derived colonies of fibroblastoid cells, termed colony forming unit fibroblasts that can subsequently differentiate to aggregates resembling small areas of cartilage or bone. However, donor heterogeneity and loss of osteogenic differentiation capacity during extended cell culture have made the discovery of reliable potency assay biomarkers difficult. Nonetheless, functional osteoblast models derived from telomerised human bone marrow stromal cells have allowed extensive comparative analysis of gene expression, microRNA, morphological phenotypes and secreted proteins. This chapter highlights numerous insights into the molecular mechanisms underpinning osteogenic differentiation of multipotent stromal cells and bone formation, discussing aspects involved in the choice of useful biomarkers for functional attributes that can be quantitively measured in osteogenic potency assays.

Identification of special key genes for alcohol-related hepatocellular carcinoma through bioinformatic analysis

Background

Alcohol-related hepatocellular carcinoma (HCC) was reported to be diagnosed at a later stage, but the mechanism was unknown. This study aimed to identify special key genes (SKGs) during alcohol-related HCC development and progression.

Methods

The mRNA data of 369 HCC patients and the clinical information were downloaded from the Cancer Genome Atlas project (TCGA). The 310 patients with certain HCC-related risk factors were included for analysis and divided into seven groups according to the risk factors. Survival analyses were applied for the HCC patients of different groups. The patients with hepatitis B virus or hepatitis C virus infection only were combined into the HCC-V group for further analysis. The differentially expressed genes (DEGs) between the HCCs with alcohol consumption only (HCC-A) and HCC-V tumors were identified through limma package in R with cutoff criteria│log2 fold change (logFC)|>1.0 and p < 0.05. The DEGs between eight alcohol-related HCCs and their paired normal livers of GSE59259 from the Gene Expression Omnibus (GEO) were identified through GEO2R (a built-in tool in GEO database) with cutoff criteria |logFC|> 2.0 and adj.p < 0.05. The intersection of the two sets of DEGs was considered SKGs which were then investigated for their specificity through comparisons between HCC-A and other four HCC groups. The SKGs were analyzed for their correlations with HCC-A stage and grade and their prognostic power for HCC-A patients. The expressional differences of the SKGs in the HCCs in whole were also investigated through Gene Expression Profiling Interactive Analysis (GEPIA). The SKGs in HCC were validated through Oncomine database analysis.

Results

Pathological stage is an independent prognostic factor for HCC patients. HCC-A patients were diagnosed later than HCC patients with other risk factors. Ten SKGs were identified and nine of them were confirmed for their differences in paired samples of HCC-A patients. Three (SLC22A10, CD5L, and UROC1) and four (SLC22A10, UROC1, CSAG3, and CSMD1) confirmed genes were correlated with HCC-A stage and grade, respectively. SPP2 had a lower trend in HCC-A tumors and was negatively correlated with HCC-A stage and grade. The SKGs each was differentially expressed between HCC-A and at least one of other HCC groups. CD5L was identified to be favorable prognostic factor for overall survival while CSMD1 unfavorable prognostic factor for disease-free survival for HCC-A patients and HCC patients in whole. Through Oncomine database, the dysregulations of the SKGs in HCC and their clinical significance were confirmed.

Conclusion

The poor prognosis of HCC-A patients might be due to their later diagnosis. The SKGs, especially the four stage-correlated genes (CD5L, SLC22A10, UROC1, and SPP2) might play important roles in HCC development, especially alcohol-related HCC development and progression. CD5L might be useful for overall survival and CSMD1 for disease-free survival predication in HCC, especially alcohol-related HCC.

The Expanding Life and Functions of Osteogenic Cells: From Simple Bone-Making Cells to Multifunctional Cells and Beyond

During the last three decades, important progress in bone cell biology and in human and mouse genetics led to major advances in our understanding of the life and functions of cells of the osteoblast lineage. Previously unrecognized sources of osteogenic cells have been identified. Novel cellular and molecular mechanisms controlling osteoblast differentiation and senescence have been determined. New mechanisms of communications between osteogenic cells, osteocytes, osteoclasts, and chondrocytes, as well as novel links between osteogenic cells and blood vessels have been identified. Additionally, cells of the osteoblast lineage were shown to be important components of the hematopoietic niche and to be implicated in hematologic dysfunctions and malignancy. Lastly, unexpected interactions were found between osteogenic cells and several soft tissues, including the central nervous system, gut, muscle, fat, and testis through the release of paracrine factors, making osteogenic cells multifunctional regulatory cells, in addition to their bone-making function. These discoveries considerably enlarged our vision of the life and functions of osteogenic cells, which may lead to the development of novel therapeutics with immediate applications in bone disorders. © 2017 American Society for Bone and Mineral Research.

Downregulation of RANKL and RANKL/osteoprotegerin ratio in human periodontal ligament cells during their osteogenic differentiation

BACKGROUND AND OBJECTIVE

Human periodontal ligament cells (hPDLCs) are considered the promising seed cells in periodontal tissue engineering. Previous studies have discovered the ability of hPDLCs in alveolar bone formation. It remains unclear, however, how the expression of factors associated with osteoclastogenesis in hPDLCs change during their osteogenic differentiation.

OBJECTIVE

The present study aimed to observe the regulation of receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG) in hPDLCs during their osteogenic differentiation.

MATERIAL AND METHODS

hPDLCs were treated with (M group) or without (C group) the osteogenic induction medium. Alkaline phosphatase activity was displayed with the Gomori calcium phosphate method. Mineralized nodules were detected with von Kossa staining. Expression levels of RANKL and OPG in hPDLCs were analyzed with real-time reverse transcription-polymerase chain reaction and western blot. Tartrate-resistant acid phosphatase (TRAP) staining was used to display the TRAP activity in Raw264.7 cells co-cultured with hPDLCs in the M group and the C group.

RESULTS

We found that alkaline phosphatase staining was shown to be remarkably higher in the M group than that in the C group during the 21 d interval. Mineralized nodules could be seen in the M group but not in the C group. The expression levels of RANKL mRNA significantly decreased in the M group by 1.69-fold (p = 0.096) at day 7, by 2.04-fold (p = 0.000) at day 14 and by 1.84-fold (p = 0.023) at day 21, compared with the corresponding levels of RANKL in the C group. Similarly, the levels of RANKL protein decreased in the M group by 1.82-fold (p = 0.062) at day 7, by 5.64-fold (p = 0.000) at day 14 and by 4.84-fold (p = 0.000) at day 21. The mRNA and protein expression levels of OPG tended to increase in the M group. As a result, the RANKL/OPG mRNA and protein ratios were significantly downregulated by osteogenic induction. In addition, the number of TRAP staining-positive multinuclear cells in the M group was significantly less than in the C group (p = 0.018).

CONCLUSION

hPDLCs may help inhibit the resorption of alveolar bone during their osteogenic differentiation by reducing the RANKL expression and the RANKL/OPG ratio.

Functional roles and clinical values of insulin-like growth factor-binding protein-5 in different types of cancers

Insulin-like growth factor-binding proteins (IGFBPs) are critical regulators of the mitogenic activity of insulin-like growth factors (IGFs). IGFBP5, one of these IGFBPs, has special structural features, including a nuclear transport domain, heparin-binding motif, and IGF/extracellular matrix/acid-labile subunit-binding sites. Furthermore, IGFBP5 has several functional effects on carcinogenesis and even normal cell processes, such as cell growth, death, motility, and tissue remodeling. These biological effects are sometimes related with IGF (IGF-dependent effects) and sometimes not (IGF-independent effects). The functional role of IGFBP5 is most likely determined in a cell-type and tissue-type specific manner but also depends on cell context, especially in terms of the diversity of interacting proteins and the potential for nuclear localization. Clinical findings show that IGFBP5 has the potential to be a useful clinical biomarker for predicting response to therapy and clinical outcome of cancer patients. In this review, we summarize the functional diversity and clinical importance of IGFBP5 in different types of cancers.

Potential induction of bone regeneration by nacre: an in vitro study

PURPOSE

Because of limitations of autogenous grafts, alternative bone substitute material was investigated for its capacity in promoting bone formation. This study compared the osteogenic effects of nacre (mother of pearl) and beta-tricalcium phosphate (β-TCP).

MATERIALS AND METHODS

Human bone cells (HBCs) were obtained from the culture of bone tissues after orthognathic surgery. The HBCs were cocultured with nacre chips of the giant oyster Pinctada maxima and with β-TCP particles for 1, 2, 3, and 4 weeks. Cells of each week specimens were used to study alkaline phosphatase (ALP), bone sialoprotein (BSP), and osteocalcin (OC) gene expression by noncompetitive reverse-transcriptase polymerase chain reaction and to study BSP synthesis by means of an immunocytochemical technique in conjunction with fluorescent microscopy.

RESULTS

Reverse-transcriptase polymerase chain reaction demonstrated stronger expression levels of ALP mRNA in HBCs cocultured with the nacre chips than those with β-TCP at weeks 2 and 4. BSP gene expression levels in HBCs with nacre were more intense compared with β-TCP at weeks 3 and 4. Although the OC gene expression level in HBCs with β-TCP was higher than those with nacre at week 2, the expression was not different at weeks 3 to 4. Immunocytochemical study revealed that BSP synthesis were presented in the nacre and β-TCP from week 2 and decreased toward week 4.

CONCLUSION

This study indicated that nacre promotes ALP, BSP, and OC gene expression.

Fibrillin-1 and -2 differentially modulate endogenous TGF-β and BMP bioavailability during bone formation

Extracellular microfibrils composed of fibrillin-1 and -2 regulate bone formation through modulation of TGF-β and BMP signaling.

Extracellular regulation of signaling by transforming growth factor (TGF)–β family members is emerging as a key aspect of organ formation and tissue remodeling. In this study, we demonstrate that fibrillin-1 and -2, the structural components of extracellular microfibrils, differentially regulate TGF-β and bone morphogenetic protein (BMP) bioavailability in bone. Fibrillin-2–null (Fbn2^−/−) mice display a low bone mass phenotype that is associated with reduced bone formation in vivo and impaired osteoblast maturation in vitro. This Fbn2^−/− phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization. Cultured osteoblasts from Fbn1^−/− mice exhibit improper latent TGF-β activation as well, but mature faster because of increased availability of otherwise matrix-bound BMPs. Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition. Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

Osteo-transcriptomics of human mesenchymal stem cells: accelerated gene expression and osteoblast differentiation induced by vitamin D reveals c-MYC as an enhancer of BMP2-induced osteogenesis

Bone marrow-derived human mesenchymal stem cells (hMSCs) have the in vitro capacity to differentiate into osteoblasts, chondrocytes or adipocytes, depending on the applied stimulus. In order to identify novel regulators of osteogenesis in hMSCs, osteo-transcriptomics was performed whereby differentiation induced by dexamethasone (DEX), DEX+ bone morphogenetic protein 2 (BMP2), and DEX+ Vitamin D(3) (1,25(OH)(2)D(3)) was studied over a course of 12 days. Microarray analysis revealed that 2095 genes were significantly regulated by DEX+ 1,25(OH)(2)D(3), of which 961 showed accelerated expression kinetics compared to treatment by DEX alone. The majority of these genes were accelerated 24-48 h after onset of osteogenic treatment. Gene ontology (GO) analysis of these 1,25(OH)(2)D(3)-accelerated genes indicated their involvement in biological processes related to cellular differentiation and cell cycle regulation. When compared to cells treated with DEX or DEX+BMP2, treatment with DEX+ 1,25(OH)(2)D(3) clearly accelerated osteoprogenitor commitment and osteoblast maturation, as measured by alkaline phosphatase (ALP) activity and calcification of the matrix. Cell cycle progression, as observed after initial growth arrest, was not significantly accelerated by 1,25(OH)(2)D(3) and was not required for onset and progression of osteogenesis. However, expression of c-Myc was accelerated by 1,25(OH)(2)D(3), and binding sites for c-MYC were enriched in promoters of genes accelerated by 1,25(OH)(2)D(3). Lentiviral overexpression of c-MYC strongly promoted DEX+ BMP2-induced osteoblast differentiation and matrix maturation. In conclusion, our studies show for the first time that 1,25(OH)(2)D(3) strongly accelerates expression of genes involved in differentiation of hMSCs and, moreover, identify c-MYC as a novel regulator of osteogenesis.

Human osteocalcin: a strong promoter for nitric oxide synthase gene therapy, with specificity for hormone refractory prostate cancer

BACKGROUND

Gene therapy has been identified as a promising treatment strategy for hormone refractory prostate cancer (HRPC). We report, for the first time, the use of the human osteocalcin (hOC) promoter to control inducible nitric oxide synthase (iNOS) transgene expression in HRPC.

METHODS

Human prostate carcinoma cells (PC3, DU145, LNCaP), colon cancer cells (HT29) and human microvascular endothelial cells (HMEC-1) were transfected in vitro with constitutively driven CMV/iNOS or hOC/iNOS plasmid DNA by cationic lipid vector. End points of these experiments were Western blotting, NO(.) generation using the Greiss test to measure accumulated nitrite, and clonogenic assay.

RESULTS

Transfection of the hOC/iNOS plasmid increased iNOS protein and total nitrite levels in PC3 and DU145 cells, but not LNCaP or HT29. Transfection with CMV/iNOS or hOC/iNOS resulted in no additional cytotoxicity in androgen-dependent LNCaP cells or in the non-prostate cell lines. However, transfection with either construct resulted in a greatly reduced cell survival (to 10-20%) in the androgen-independent PC3 and DU145 cell lines.

CONCLUSIONS

Utilising the tumour-type specific properties of the hOC promoter in tandem with the iNOS gene, we have demonstrated target cell specificity, and transgene activation, in the androgen-independent prostate cancer cell lines (PC3 and DU145), an effect absent in normal and androgen-dependent cells. Furthermore, the levels of NO(.) generated are comparable with those seen generated with constitutively (CMV)-driven iNOS. The data obtained from this study provide a basis for future development of hOC/iNOS gene therapy.

In vitro characterization of peptide-modified p(AAm-co-EG/AAc) IPN-coated titanium implants

Interpenetrating polymer networks (IPNs) of poly(acrylamide-co-ethylene glycol/acrylic acid) [p(AAm-co-EG/AAc)] functionalized with an -Arg-Gly-Asp- containing peptide derived from rat bone sialoprotein [bsp-RGD(15)] were grafted to titanium implants in an effort to modulate osteoblast behavior in vitro. Surface characterization data were consistent with the presence of an IPN, and ligand density measurements established that the range of peptide density on the modified implants spanned three orders of magnitude (0.01-20 pmol/cm2). In vitro biological characterization of the modified implants employing the primary rat calvarial osteoblast (RCO) model resulted in the identification of a critical ligand density (0.01<Gammacrit<0.1 pmol/cm2) for maximal support of the osteoblast phenotype. After 14 and 21 days, mineralization was greater on the 0.1 and 10 pmol/cm2 bsp-RGD(15) modified implants compared to the base titanium and other control surfaces. The observed effects were attributed to specific interactions with bsp-RGD(15) and support the concept that peptide-modified implants can enhance the kinetics of differentiation of the cells they contact. These results suggest that in vivo biological performance evaluation of these biomimetic implant surfaces is merited.

Osteoblast viability and differentiation with Me2SO as cryoprotectant compared to osteoblasts from fresh human iliac cancellous bone

The aim of this study was to compare the viability of human osteoblasts cryopreserved with Me2SO to that of fresh human iliac cancellous bone using cell culture techniques. Osteoblasts were obtained by spontaneous outgrowth of human iliac cancellous bone specimens in experiment I. In experiment II, human iliac cancellous bone was frozen with 10% Me2SO at -80 degrees C for 2 weeks and osteoblasts grew spontaneously after thawing at 37 degrees C by removing Me2SO with sucrose. The cells were grown in culture flasks containing DMEM as a culture medium, supplemented with 10% fetal calf serum. They were kept at 37 degrees C in a humidified atmosphere of 95% air and 5% CO2. Cells from the second passage were plated at a density of 5 times 10(3) cells/cm2 in 24-well plates. For detection of viability and differentiation, WST-1 assay, determination of alkaline phosphatase activity, concentration of procollagen I peptide, concentration of osteocalcin, and indirect immunofluorescence for osteopontin, collagen type I, integrin beta1, and fibronectin were applied. Experiments were conducted at four stages of confluence (days 4, 7, 14, and 21 after plating the cells). Based on the results of this study, we conclude that osteoblast-like cells survived cryopreservation and synthesized a range of markers that were consistent with this cell type.

A fragment of the hypophosphatemic factor, MEPE, requires inducible cyclooxygenase-2 to exert potent anabolic effects on normal human marrow osteoblast precursors

MEPE, 56.6 kDa protein isolated from tumors associated with hypophosphatemic osteomalacia, increases renal phosphate excretion and is expressed in normal human bone cells. AC-100, a central 23-amino acid fragment of MEPE, contains motifs that are important in regulating cellular activities in the bone microenvironment. Thus, we assessed in vitro effects of AC-100 on multipotential normal human marrow stromal (hMS) cells that have the capacity to differentiate into mature osteoblasts. Proliferation was quantified by [H3]thymidine uptake and cell counting and differentiation by the levels of mRNA for the alpha2-chain of type I procollagen (COL1A2), alkaline phosphatase (AP), and osteocalcin (OC) measured using real time reverse transcriptase PCR (RT-PCR) and by the formation of mineralized nodules. AC-100 increased proliferation by 257 +/- 89% (P < 0.005), increased gene expression of COL1A2 by 339 +/- 85% (P < 0.005), AP by 1,437 +/- 40% (P < 0.001), and OC by 1,962 +/- 337% (P < 0.001). In addition, it increased mineralized nodule formation by 81 +/- 14% (P < 0.001) in a dose- and time-dependent fashion. In equimolar dosages, the parent compound, MEPE, had the full activity of the AC-100 fragment. AC-100 elicited a comparable response to both IGF-I and BMP-2 with respect to proliferation and differentiation of hMS cells. Using gene expression microarray analysis, we demonstrated that AC-100 increased (by approximately 3-fold) the mRNA for cyclooxgenase-2 (COX-2), an inducible enzyme required for prostaglandin synthesis. Moreover, NS-398, a specific inhibitor of COX-2 action completely blocked AC-100-induced increases in proliferation and differentiation. Thus, AC-100 has potent anabolic activity on osteoblast precursor cells in vitro and these effects require the induction of COX-2.

Biochemical markers of bone formation in Thai children and adolescents

The measurement of biochemical markers of bone turnover is essential in the study of skeletal metabolism in health and diseases. Due to variations in the rate of bone growth in different age groups and possible ethnic differences, age-specific reference ranges for biochemical markers should be established in a particular pediatric population. In this study, biochemical markers of bone formation, bone-specific alkaline phosphatase (BAP), and osteocalcin (OC) in healthy Thai children and adolescents aged 9 to 18 years were evaluated in relation to their ages and pubertal development. Serum BAP levels in boys increased with age and peaked at about 12 to 13 years. In contrast, there was a progressive decline of serum BAP levels with advancing age in girls older than 9 years. Serum OC also increased with age and reached a peak at ages 12 and 13 years in girls and boys, respectively. In addition, both serum BAP and OC levels also varied with pubertal stages. The BAP levels in boys increased sharply at pubertal stage 3 and decreased at pubertal stage 5. In girls, the BAP levels showed a fairly constant high level up to stage 3, followed by a remarkable decrease thereafter. The OC levels in boys increased sharply at pubertal stage 4 and decreased thereafter. In girls, OC started to increase at pubertal stage 3 with no subsequent changes. The levels of serum BAP and OC were higher in boys than in girls at pubertal stages 3 to 5 and at stages 2, 4, and 5, respectively. Moreover, only serum BAP level showed significant positive correlation with height velocity in both genders. In multiple regression analyses, gender, age, and pubertal stage were consistently correlated with both serum BAP and OC levels. In summary, male and female adolescents have different patterns of changes in biochemical markers of bone formation.

Dexamethasone induces human spinal ligament derived cells toward osteogenic differentiation

Ossification of spinal ligament is characterized by heterotopic bone formation in the spinal ligaments that are normally composed of fibrous tissues. The pathogenesis of ossification of spinal ligament has been suggested to be associated with osteogenic differentiation of the spinal ligament cells. In order to address this hypothesis, cells derived from human spinal ligament were investigated for their osteogenic potential by the treatment of dexamethasone in vitro. Yellow ligaments were obtained from patients with spinal disorders except ossification of spinal ligament during surgery, and the adhering tissues were removed completely. Most of the ligament cells treated with vehicle exhibited a fibroblast-like spindle shape, while the dexamethasone-treated cells acquired a polygonal morphology. Growth of the ligament cells was suppressed by dexamethasone at a high concentration. Some of the vehicle treated-cells were alkaline phosphatase-positive, and dexamethasone increased the alkaline phosphatase-positive cells and alkaline phosphatase activity in the cells. Northern blot analysis demonstrated that mRNAs expression of pro-alpha1(I) collagen and alkaline phosphatase were promoted by dexamethasone. Analysis by reverse transcription-polymerase chain reaction showed that expression of osteocalcin mRNA was detected in the dexamethasone-treated cells but not in the vehicle-treated cells, and dexamethasone-induced osteocalcin mRNA expression was promoted by 1,25-dihydroxyvitamin D(3). Finally, mineralization of extracellular matrix in the cells was induced by the presence of dexamethasone and 1,25-dihydroxyvitamin D(3). These results suggest for the first time that dexamethasone has a possible involvement in the osteoblastic differentiation of human spinal ligament cells.

Peptide-modified p(AAm-co-EG/AAc) IPNs grafted to bulk titanium modulate osteoblast behavior in vitro

Interpenetrating polymer networks (IPNs) of poly(acrylamide-co-ethylene glycol/acrylic acid) (p(AAm-co-EG/AAc) applied to model surfaces prevent protein adsorption and cell adhesion. Subsequently, IPN surfaces functionalized with the RGD cell-binding domain from rat bone sialoprotein (BSP) modulated bone cell adhesion, proliferation, and matrix mineralization. The objective of this study was to utilize the same biomimetic modification strategy to produce functionally similar p(AAm-co-EG/AAc) IPNs on clinically relevant titanium surfaces. Contact angle goniometry and X-ray photoelectron spectroscopy (XPS) data were consistent with the presence of the intended surface modifications. Cellular response was gauged by challenging the surfaces with primary rat calvarial osteoblast (RCO) surfaces in serum-containing media. IPN modified titanium and negative control (RGE-IPN) surfaces inhibit cell adhesion and proliferation, while RGD-modified IPNs on titanium supported osteoblast attachment and spreading. Furthermore, the latter surfaces supported significant mineralization despite exhibiting lower levels of proliferation than positive control surfaces. These results suggest that with the appropriate optimization, this approach may be practical for surface engineering of osseous implants.

Effect of mechanical loading on periodontal cells

Mechanical loading is an important regulatory factor in alveolar bone homeostasis, and plays an essential role in maintaining the structure and mass of the alveolar processes throughout lifetime. A better understanding of the cellular and molecular responses of periodontal cells is a prerequisite for further improvements of therapeutic approaches in orthodontics, periodontal and alveolar bone repair and regeneration, implantology, and post-surgical wound healing. The purpose of this review is to provide an insight into some cell culture and animal models used for studying the effects of mechanical loading on periodontal cells, and into the recent developments and utilization of new in vivo animal models. There has been an increased awareness about the need for improvement and development of in vivo models to supplement the widely used cell culture models, and for biological validation of in vitro results, especially in the light of evidence that developmental models may not always reflect bone homeostasis in an adult organism. Due to the limitations of in vivo models, previous studies on mechanical regulation of alveolar bone osteoblasts and cementoblasts mostly focused on proliferative responses, rather than on the stimulation of cell differentiation. To address this problem, we have recently characterized and implemented a mouse osteoinductive tooth movement model for studying mechanically induced regulation of osteoblast- and cementoblast-associated genes. In this model, a defined and reproducible mechanical osteogenic loading is applied during a time course of up to two weeks. Regulation of gene expression in either wild-type or transgenic animals is assessed by a relative quantitative measurement of the level of target mRNAs directly within the subpopulations of periodontal cells. To date, results demonstrate a defined temporal pattern of cell-specific gene regulation in periodontal osteoblasts mechanically stimulated to differentiate and deposit bone matrix. The responses of osteoblast-associated genes to mechanical loading were 10- to 20-fold greater than the increase in the numbers of these cells, indicating that the induction of differentiation and an increase of cell function are the primary responses to osteogenic loading. The progression of the osteoblast phenotype in the intact mouse periodontium was several-fold faster compared with that in cultured cells, suggesting that the mechanical signal may be targeting osteoblast precursors in the state of readiness to respond to an environmental challenge, without the initial proliferative response. An early response of alkaline phosphatase and bone sialoprotein genes was detected after 24 hrs of treatment, followed by a concomitant stimulation of osteocalcin and collagen I between 24 and 48 hrs, and deposition of osteoid after 72 hrs. Although cementoblasts constitutively express biochemical markers similar to those of osteoblasts, distinct responses of osteocalcin, collagen I, and bone sialoprotein genes to mechanical loading were observed in the two cell phenotypes. This finding indicates that differential genetic responses to mechanical loading provide functional markers for distinction of the cementoblast and osteoblast phenotypes.

Effects of pH on human bone marrow stromal cells in vitro: implications for tissue engineering of bone

The objective of this study was to address the hypothesis that changes in extracellular pH alter collagen gene expression, collagen synthesis, and alkaline phosphatase activity in bone marrow stromal cells (BMSCs). Potential effects of pH on cell function are of particular importance for tissue engineering because considerable effort is being placed on engineering biodegradable polymers that may generate a local acidic microenvironment on degradation. Human and murine single-cell marrow suspensions were plated at a density of 2 x 10(4) cells/cm(2). After 7 days in culture, the pH of the culture medium was adjusted to one of six ranges: > or = 7.8, 7.5.-7.7, 7.2-7.4, 6.9-7.1, 6.6-6.8, or < or = 6.5. After 48 h of exposure to an altered pH, alkaline phosphatase activity and collagen synthesis decreased significantly with decreasing pH. This decrease was two-to threefold as pH decreased from 7.5 to 6.6. In contrast, alpha1(I) procollagen mRNA levels increased two- to threefold as pH was decreased. The trend in osteocalcin mRNA expression was opposite to that of collagen. Small shifts in extracellular pH led to significant changes in the ability of BMSCs to express markers of the osteoblast phenotype. These pH effects potentially relate to the microenvironment supplied by a tissue-engineering scaffold and suggest that degrading polymer scaffolds may influence the biologic activity of the cells in the immediate environment.

Bone morphogenetic protein-2 stimulates cell recruitment and cementogenesis during early wound healing

BACKGROUND

The unique action of bone morphogenetic proteins (BMPs) on mineralised tissue formation indicates that BMPs are good candidates for use in stimulating periodontal regeneration. Relatively little is known about the mechanisms of actions of BMPs during periodontal regeneration, although recent evidence from our laboratory suggests that the effects of BMPs may be profoundly influenced by various factors including root surface conditioning, delivery systems and masticatory forces.

AIM

The aim of this study was to investigate the effect of rhBMP-2 on cell recruitment during periodontal regeneration using a pulse-chase technique where cells are labelled with a thymidine analogue (BrdU) (pulse) and the migration of their progeny is followed (chase) during early wound healing. The relationship between the rhBMP-2 influence on cell recruitment from the periodontal ligament (PDL) and its ability to stimulate cementogenesis was also evaluated.

METHOD

The buccal aspect of the distal root of the first molar was denuded of its PDL, cementum and superficial dentine through a bony window created in the mandible of 64 Wistar rats under general anaesthesia. Test animals were treated with 10 microL of 500 microg/ml rhBMP-2 in a collagen membrane sponge (n=32) and control defects received 10 microl of saline in a collagen sponge (n=32). All animals received an intraperitoneal single pulse injection of 40 mg/kg BrdU label 2 days postoperatively. Groups of test and control animals (n=8) were killed 2 hours later on day 2 and at 4, 7 and 10 days postoperatively. Mandibles were processed for histological examination.

RESULTS

The results show that rhBMP-2 had a profound effect on proliferation and migration of cells in the adjacent and deeper aspects of the PDL at 7 and 10 days post periodontal wounding (p<0.05). Significantly greater new cementum formation occurred in the test group at 10 days (p=0.03).

CONCLUSION

This study shows that following periodontal wounding rhBMP-2 stimulates cell recruitment by increasing proliferation and migration of cells from the adjacent unwounded PDL into the wounded area, thus promoting periodontal regeneration by increasing new cementum formation.

Osteogenesis from cultured chick periostea has a specific requirement for chloride

Bone development, like embryonic development in general, depends on a particular internal electrical milieu. Ions are the carriers of currents that maintain this internal environment. In embryonic bone, chloride is a major carrier of such current. To explore the role chloride plays in embryonic bone development we performed several ion-removal experiments, using the chick periosteal osteogenesis (CPO) system as our model. We found that if chloride is reduced in the medium and replaced with a nontoxic anion, alkaline phosphatase (ALP) activity does not rise, nor does osteogenic development occur. However, acid phosphatase (AP) activity is not affected by level of chloride. Experiments using metabolic inhibitors showed that explants cultured in low chloride medium remain viable. Dose-response studies revealed that the response of ALP activity to chloride concentration is sigmoidal, with a [Cl-]0.5 of 45.9 mM. Reciprocal transfers of explants between complete and low chloride medium show that the rise in ALP activity depends on the length of time explants are cultured with chloride. In contrast, such transfer experiments show that osteogenesis requires chloride only during days 2-3 of culture.

A Cbfa1-dependent genetic pathway controls bone formation beyond embryonic development

The molecular mechanisms controlling bone extracellular matrix (ECM) deposition by differentiated osteoblasts in postnatal life, called hereafter bone formation, are unknown. This contrasts with the growing knowledge about the genetic control of osteoblast differentiation during embryonic development. Cbfa1, a transcriptional activator of osteoblast differentiation during embryonic development, is also expressed in differentiated osteoblasts postnatally. The perinatal lethality occurring in Cbfa1-deficient mice has prevented so far the study of its function after birth. To determine if Cbfa1 plays a role during bone formation we generated transgenic mice overexpressing Cbfa1 DNA-binding domain (DeltaCbfa1) in differentiated osteoblasts only postnatally. DeltaCbfa1 has a higher affinity for DNA than Cbfa1 itself, has no transcriptional activity on its own, and can act in a dominant-negative manner in DNA cotransfection assays. DeltaCbfa1-expressing mice have a normal skeleton at birth but develop an osteopenic phenotype thereafter. Dynamic histomorphometric studies show that this phenotype is caused by a major decrease in the bone formation rate in the face of a normal number of osteoblasts thus indicating that once osteoblasts are differentiated Cbfa1 regulates their function. Molecular analyses reveal that the expression of the genes expressed in osteoblasts and encoding bone ECM proteins is nearly abolished in transgenic mice, and ex vivo assays demonstrated that DeltaCbfa1-expressing osteoblasts were less active than wild-type osteoblasts. We also show that Cbfa1 regulates positively the activity of its own promoter, which has the highest affinity Cbfa1-binding sites characterized. This study demonstrates that beyond its differentiation function Cbfa1 is the first transcriptional activator of bone formation identified to date and illustrates that developmentally important genes control physiological processes postnatally.

Restricted and coordinated expression of beta3-integrin and bone sialoprotein during cultured osteoblast differentiation

In this study, the expression of beta3-integrin was examined in relationship to the restricted expression of bone sialoprotein (BSP). Immunohistochemical analysis indicated that the alpha(v)beta3-integrin was coincident and proximal to BSP expression in the fetal mandible bovine osteoblast culture model. Alpha(v)beta3-integrin expression was expressed predominantly in a region proximal to, but not including, the substrate adherent cells. In comparison, the alpha5beta1-integrin was expressed in a generalized pattern throughout the culture layers in a coordinated fashion to fibronectin. The temporal expression of beta1- and beta3-integrin was evaluated using RT-PCR and southern blot analysis. Unlike the generalized expression of beta1-integrin, beta3-integrin was restricted to days 3 and 5 of the culture period. The previous demonstration of similar restriction of BSP expression and the present colocalization of BSP suggests the potential coordinated expression of a specific extracellular matrix ligand with a select integrin. Beta3-integrin/BSP adhesion-mediated signaling may play a significant role in the process of osteoblast morphodifferentiation.

Advances in the osteoblast lineage

Osteoblasts are the skeletal cells responsible for synthesis, deposition and mineralization of the extracellular matrix of bone. By mechanisms that are only beginning to be understood, stem and primitive osteoprogenitors and related mesenchymal precursors arise in the embryo and at least some appear to persist in the adult organism, where they contribute to replacement of osteoblasts in bone turnover and in fracture healing. In this review, we describe the morphological, molecular, and biochemical criteria by which osteoblasts are defined and cell culture approaches that have helped to clarify transitional stages in osteoblast differentiation. Current understanding of differential expression of osteoblast-associated genes during osteoprogenitor proliferation and differentiation to mature matrix synthesizing osteoblasts is summarized. Evidence is provided to support the hypothesis that the mature osteoblast phenotype is heterogeneous with subpopulations of osteoblasts expressing only subsets of the known osteoblast markers. Throughout this paper, outstanding uncertainties and areas for future investigation are also identified.Key words: skeletal development, differential gene expression, heterogeneity.

Developmental and hormonal regulation of keratinocyte growth factor expression and action in the ovarian follicle

The developing ovarian follicle is one of the most rapidly proliferating normal tissues in vivo. Mesenchymal-epithelial cell interactions between theca cells and granulosa cells are essential for this follicular expansion. Ovarian hormones (i.e. estrogen and LH) may promote follicular development by regulating the local production of mesenchymal inducer proteins that mediate theca cell-granulosa cell interactions. Recently, theca cells were shown to produce keratinocyte growth factor (KGF) that can act in a paracrine manner to stimulate granulosa cell growth. In this study, the developmental and hormonal regulation of KGF was examined during follicular development in the bovine ovary. Expression of KGF in theca cells and the KGF receptor (KGFR, or splice variant of the fibroblast growth factor family receptor family, FGFR-2) in granulosa cells was examined using RT-PCR. Both KGF and KGFR were detected throughout follicular development in small (<5 mm), medium (5-10 mm), and large (>10 mm) follicles. Quantitative RT-PCR assays were used to determine steady-state levels of KGF and KGFR messenger RNAs. Developmental regulation of KGF and KGFR was analyzed in freshly isolated theca cells and granulosa cells from small, medium, and large follicles. Observations demonstrated that expression of KGF (in theca cells) and KGFR (in granulosa cells) was highest in large follicles. These results suggest that KGF actions are important for the rapid proliferation of granulosa cells in large follicles. Estrogen and LH are the primary endocrine hormones that regulate theca cell function in vivo. Therefore, hormonal regulation of KGF was analyzed by treating serum-free theca cell cultures with estrogen and human CG (hCG, an LH agonist). Results showed that both estrogen and hCG stimulated KGF gene expression in theca cells. These results suggest that estrogen and LH may promote follicular growth (i.e. granulosa cell proliferation), in part, by stimulating the local production of KGF. Effects of KGF on granulosa cell differentiated functions were examined. Treatment with KGF reduced basal levels and FSH-stimulated levels of aromatase activity in bovine and rat granulosa cells. In addition, KGF inhibited the ability of hCG to stimulate progesterone production by granulosa cells. The inhibition of granulosa cell steroid production by KGF was likely the indirect effect of promoting cellular proliferation. Therefore, KGF directly stimulates granulosa cell proliferation and indirectly inhibits granulosa cell differentiated functions. Combined results suggest that theca cell production of KGF may be important for ovarian folliculogenesis. This is the first report of the regulation of KGF expression in the ovary. The developmental and hormonal regulation of KGF and KGFR during folliculogenesis provides evidence that KGF may be important for hormone-induced granulosa cell proliferation. As a result, KGF may be essential for establishing the microenvironment required for oocyte maturation in the ovary.

Direct actions of kit-ligand on theca cell growth and differentiation during follicle development

The direct actions of kit-ligand/stem cell factor (KL) in developing ovarian follicles were investigated. Previous studies have shown that granulosa cells express KL that can support oocyte development. The current study demonstrates that KL can also act directly on theca cells to promote cellular growth and differentiation. Through RT-PCR analysis it was shown that bovine granulosa cells express KL, and theca cells express the receptor c-kit. Bovine theca interna cells were isolated and cultured in serum-free conditions to study KL actions. KL stimulated theca cell growth in a dose-dependent manner as measured by [3H]thymidine incorporation into DNA when cells were cultured under subconfluent conditions. KL had no effect on theca cell androstenedione or progesterone production under these growth-permissive conditions. In contrast, KL stimulated theca cell androstenedione production but had no effect on progesterone production when theca cells were cultured under confluent (non-growth-permissive) conditions. Estradiol (10(-7) M) and human CG (100 ng/ml) were used as controls and regulated theca cell steroid production at any cell density. These results demonstrate that KL can directly stimulate theca cell growth and steroid production during follicular development. The observation that KL stimulated androstenedione production but not progesterone production suggests that KL promotes a follicular phase differentiated state in theca cells. The potential regulation of KL and c-kit expression during follicular development was studied using a specific quantitative RT-PCR procedure. Total RNA from granulosa cells (for KL) and theca cells (for c-kit) was examined from small (<5 mm), medium (5-10 mm), and large (>10 mm) size follicles. Steady state levels of KL messenger RNA were highest in granulosa cells from large size follicles and lowest in small and medium size follicles. No differences were observed in the steady state levels of c-kit messenger RNA in theca cells from small, medium, or large size follicles. The observation that KL expression is highest in large size follicles suggests that KL may be important for increased growth and steroid production in large and dominant follicles. Observations demonstrate that KL can dramatically alter theca cell function and support the hypothesis that local granulosa-theca cell interactions play an important role in regulating cellular function within ovarian follicles. This study identifies KL as the first granulosa cell-derived growth factor that can directly stimulate theca cell growth and androstenedione production in the absence of gonadotropins.

Expression of bone matrix proteins during dexamethasone-induced mineralization of human bone marrow stromal cells

Glucocorticoids have been shown to induce the differentiation of bone marrow stromal osteoprogenitor cells into osteoblasts and the mineralization of the matrix. Since the expression of bone matrix proteins is closely related to the differentiation status of osteoblasts and because matrix proteins may play important roles in the mineralization process, we investigated the effects of dexamethasone (Dex) on the expression of bone matrix proteins in cultured normal human bone marrow stromal cells (HBMSC). Treatment of HBMSC with Dex for 23 days resulted in a significant increase in alkaline phosphatase activity with maximum values attained on day 20 at which time the cell matrix was mineralized. Northern blot analysis revealed an increase in the steady-state mRNA level of alkaline phosphatase over 4 weeks of Dex exposure period. The observed increase in the alkaline phosphatase mRNA was effective at a Dex concentration as low as 10(-10) M with maximum values achieved at 10(-8)M. In contrast, Dex decreased the steady-state mRNA levels of both bone sialoprotein (BSP) and osteopontin (OPN) over a 4 week observation period when compared to the corresponding control values. The relative BSP and OPN mRNA levels among the Dex treated cultures, however, showed a steady increase after more than 1 week exposure. The expression of osteocalcin mRNA which was decreased after 1 day Dex exposure was undetectable 4 days later. Neither control nor Dex-treated HBMSC secreted osteocalcin into the conditioned media in the absence of 1 ,25(OH)(2)D(3) during a 25-day observation period. The accumulated data indicate that Dex has profound and varied effects on the expression of matrix proteins produced by human bone marrow stromal cells. With the induced increment in alkaline phosphatase correlating with the mineralization effects of Dex, the observed concomitant decrease in osteopontin and bone sialoprotein mRNA levels and the associated decline of osteocalcin are consistent with the hypothesis that the regulation of the expression of these highly negatively charged proteins is essential in order to maximize the Dex-induced mineralization process conditioned by normal human bone marrow stromal osteoprogenitor cells.

Stromelysin-3 in the biology of the normal and neoplastic mammary gland

Stromelysin-3 (ST3) is an extracellular proteinase predominantly expressed in fibroblasts. The particular structural features and in vitro functions of this molecule suggest it could be the first member of a new subgroup of the matrix metalloproteinase family. ST3 is transiently expressed during mammary gland post-weaning involution, embryonic implantation, various organogeneses, and during amphibian metamorphosis. Moreover, ST3 is expressed in a panel of human invasive carcinomas including breast, colon, and head and neck carcinomas. Almost all ST3-expressing tissues show intense extracellular matrix remodeling activities including the loss of basement membrane integrity. Thus, either directly, or indirectly in association with other proteinases, ST3 might be involved in tissue remodeling processes occurring in both physiological and pathological processes. In vitro and in vivo studies using malignant cells stably transfected in such a way as to modulate their ST3 expression levels indicate that ST3 modifies neither cell proliferation nor invasive properties, but rather favors tumor cell survival in host tissues. This hypothesis is consistent with clinical data showing that ST3 expression could be predictive of tumor progression leading to metastases.

The architectural organization of nuclear metabolism

Nucleic acid metabolism is structurally organized in the nucleus. DNA replication and transcription have been localized to particular nuclear domains. Additional domains have been identified by their morphology or by their composition; for example, by their high concentration of factors involved in RNA splicing. The domain organization of the nucleus is maintained by the nuclear matrix, a nonchromatin nuclear scaffolding that holds most nuclear RNA and organizes chromatin into loops. The nuclear matrix is built on a network of highly branched core filaments that have an average diameter of 10 nm. Many of the intermediates and the regulatory and catalytic factors of nucleic acid metabolism are retained in nuclear matrix preparations, suggesting that nucleic acid synthesis and processing are structure-bound processes in cells. Tissue-specific and malignancy-induced variations in nuclear structure and metabolism may result from altered matrix architecture and composition.

Ontogeny of the 1,25-dihydroxyvitamin D3 receptor in fetal rat bone

To gain insights into 1,25-dihydroxyvitamin D3 receptor (VDR) function during fetal bone development, we examined fetal rat tissues from gestational days 13-21 for the presence and distribution of VDR using immunohistochemistry. Prior to ossification, VDR epitopes were observed in the mesenchyme condensing to form skeletal tissues, on day 13 in the developing vertebral column and limbs, and on day 17 of gestation in developing calvaria. Immunostaining for VDR was seen in proliferating and hypertrophic chondrocytes and in osteoblasts of limb buds and the vertebral column by day 17 of gestation. In calvaria, VDR epitopes were observed in osteoblasts by gestational day 19. VDR immunostaining was also evident in the skin of fetal limbs at all gestational ages examined. We show for the first time that the VDR appears very early in the developing fetal rat skeleton, suggesting that the VDR, in concert with its ligand, 1,25-dihydroxyvitamin D3, may play a role in the differentiation of mesenchymal precursors into bone tissue.

Developmental expression of mouse stromelysin-3 mRNA

We have used northern blot analysis and in situ hybridization to study the spatial distribution of stromelysin-3 (ST3) expression during mouse embryogenesis. ST3 mRNA was observed in trophoblastic cells at the site of embryonic implantation (7.5-8.5 days) and in a variety of developing embryonic tissues. In these tissues, the highest ST3 expression levels were observed during the development of the external features of limb, tail and snout, and during bone and spinal cord morphogenesis. In limb, tail and snout, ST3 expression was specifically detected in mesenchymal cells lining the basement membrane at the junction of primitive dermis and epidermis, and adjacent to epithelial cells undergoing proliferation and/or apoptosis. In bone, ST3 was expressed in invasive mesenchymal cells and, in the spinal cord in neuroepithelial cells of the floor plate, at the time that this structure is crossed by commissural axons. Altogether, these observations suggest a role for ST3 during embryonic morphogenesis, in tissue remodeling processes associated with cell proliferation, death and/or invasion. Moreover, when compared to urokinase and tissue plasminogen activators, the spatiotemporal pattern of ST3 expression shows some similarities, but was not completely superimposable, suggesting that these genes may cooperate in some developing tissues and have specific functions in others.

Monoclonal antibodies with selective reactivity against osteoblasts and osteocytes in human bone

Monoclonal antibodies (MAb) may provide valuable tools for studying osteoblast differentiation. We therefore raised a panel of MAb reactive with cells of this phenotype using 1,25(OH)2D3-treated human trabecular osteoblast-like cells (HOBS) as the immunogen. Immunohistochemical studies on various tissues, including undecalcified cryostat sections of fetal and adult human bone, identified 11 bone cell-reactive MAb. Of these, 2 demonstrated particularly selective reactivities against osteocytes (OB/M) and osteoblasts (OB/L). These reactivities were also seen in developing bone from rat, rabbit, and marmoset. OB/L and OB/M demonstrated limited reactivity against a small number of human tissues from the extensive panel of substrates tested. Both MAb exhibited reactivity against discrete populations of cells in the large and small intestine. In addition, OB/L reacted with cells in the basal epidermis of skin and OB/M with cells in blood vessel walls. Both antibodies demonstrated reactivity against a variety of cultured osteoblast-like cell lines and other cultured cell types. These MAb may therefore provide a valuable means of studying osteoblast ontogeny.

Expression of cell growth and bone phenotypic genes during the cell cycle of normal diploid osteoblasts and osteosarcoma cells

Establishing regulatory mechanisms that mediate proliferation of osteoblasts while restricting expression of genes associated with mature bone cell phenotypic properties to post-proliferative cells is fundamental to understanding skeletal development. To gain insight into relationships between growth control and the developmental expression of genes during osteoblast differentiation, we have examined expression of three classes of genes during the cell cycle of normal diploid rat calvarial-derived osteoblasts and rat osteosarcoma cells (ROS 17/2.8): cell cycle and growth-related genes (e.g., histone), genes that encode major structural proteins (e.g., actin and vimentin), and genes related to the biosynthesis, organization, and mineralization of the bone extracellular matrix (e.g., alkaline phosphatase, collagen I, osteocalcin, and osteopontin). In normal diploid osteoblasts as well as in osteosarcoma cells we found that histone genes, required for cell progression, are selectively expressed during S phase. All other genes studied were constitutively expressed both at the transcriptional and posttranscriptional levels. Alkaline phosphatase, an integral membrane protein in both osteoblasts and osteosarcoma cells, exhibited only minimal changes in activity during the osteoblast and osteosarcoma cell cycles. Our findings clearly indicate that despite the loss of normal proliferation-differentiation interrelationships in osteosarcoma cells, cell cycle regulation or constitutive expression of growth and phenotypic genes is maintained.

Variations in vitamin D receptor transcription factor complexes associated with the osteocalcin gene vitamin D responsive element in osteoblasts and osteosarcoma cells

Vitamin D responsive transcription of the bone-specific osteocalcin gene differs markedly in osteosarcoma cells and normal diploid osteoblasts. In osteoblasts the osteocalcin gene is transcribed, and upregulated by Vitamin D, only in post-proliferative cells, but in osteosarcoma cells expression is constitutive. This distinction in transcriptional regulation of the osteocalcin gene correlates with striking differences in the relative representation of two principal Vitamin D-dependent protein/DNA complexes designated V1 and V2 at the Vitamin D responsive element in the osteocalcin promoter. Formation of both complexes is Vitamin D dependent and they contain the Vitamin D receptor as well as an RXR related protein. Pore size exclusion and sedimentation velocity analyses suggest that the V1 and V2 complexes represent oligomeric protein assemblies (respectively, tetramers and trimers), and reflect primarily DNA-directed association of the monomeric protein components at the osteocalcin Vitamin D responsive element. UV crosslinking and methylation interference analyses of the V1 and V2 complexes at the osteocalcin Vitamin D responsive element indicate differences in protein/DNA recognition. For example, the V1 complex interacts with both steroid half-elements, whereas the V2 complex appears to recognize the proximal half-element. Our findings suggest variations in protein/protein and protein/DNA interactions of the VDR and RXR related complexes V1 and V2 at the osteocalcin Vitamin D responsive element that reflect unique properties of the osteosarcoma and normal diploid osteoblast phenotype.

Direct modulation of osteoblastic activity with estrogen

Estrogens play an important but poorly understood role in the maintenance of skeletal mass. Whereas the mechanisms of estrogen action on bone may be complex, the finding that osteoblasts express estrogen receptors suggests that this class of hormones exerts direct effects on bone cells. To understand how estrogens regulate osteoblastic function, the physiologically active estrogen metabolite 17 beta-estradiol was tested to determine its effects on the well characterized murine osteoblastic cell-line MC3T3-E1. Experiments were designed to identify the effects of estrogen on osteoblastic activities associated with both the formation and the resorption of bone. Estrogen treatment coordinately increased DNA content and alkaline phosphatase activity in MC3T3-E1 cells as much as twofold. The stimulatory effect on alkaline phosphatase was stereospecific, dose-dependent between 0.1 and ten nanomolar, and dependent on the time in culture when the hormone was administered. The effect was also persistent, since alkaline phosphatase activity remained elevated for several days after withdrawal of the hormone. Estrogen increased the levels of messenger RNA for alkaline phosphatase and type-I collagen as well, and these effects also persisted after removal of the hormone. The levels of messenger RNA for osteopontin, another bone-matrix protein, were only slightly affected by estrogen. Finally, estrogen inhibited the activation of adenylate cyclase by three osteotropic agents known to stimulate the resorption of bone: parathyroid hormone, prostaglandin E2, and the beta-adrenergic agonist isoproterenol. Thus, estrogen promoted the expression of traits associated with the formation of bone while reducing cellular responsiveness to hormones that may trigger the resorption of bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Upstream regulatory elements necessary for expression of the rat COL1A1 promoter in transgenic mice

The activity of fusion genes containing fragments of the COL1A1 promoter was measured in tissues from 6- to 8-day-old transgenic mice. ColCAT3.6 contains approximately 3.6 kb (-3521 to 115 bp) of the rat COL1A1 gene, the chloramphenicol acetyltransferase (CAT) reporter gene, and the SV40 splice and polyadenylation sequences. ColCAT2.3 and ColCAT1.7 are deletion constructs that contain 2296 and 1667 bp of COL1A1 upstream from the RNA start site, respectively. For each transgene, up to six lines of mice were characterized. Both ColCAT3.6 and ColCAT2.3 had similar activity in bone and tooth; ColCAT1.7 was inactive. In transgenic calvariae, levels of transgene mRNA paralleled levels of CAT activity. In tendon, the activity of ColCAT2.3 was 3- to 4-fold lower than that of ColCAT3.6, and the activity ColCAT1.7 was 16-fold lower than that of ColCAT2.3. There was little activity of the ColCAT constructs in liver and brain. These data show that DNA sequences between -2.3 and -1.7 kb are required for COL1A1 promoter expression in bone and tooth; sequences that control expression in tendon are distributed between -3.5 and -1.7 kb of the promoter, with sequences downstream of -1.7 kb still capable of directing expression to this tissue. The cis elements that govern basal expression of COL1A1 in transgenic calvariae appear to be different from those required for optimal expression of the COL1A1 promoter in stably transfected osteoblastic cells.

Influence of DNA replication inhibition on expression of cell growth and tissue-specific genes in osteoblasts and osteosarcoma cells

Interrelationships between proliferation and expression of cell growth as well as bone cell-related genes were examined from two standpoints. First, the consequence of downregulating proliferation by DNA synthesis inhibition on expression of a cell cycle-regulated histone gene and genes associated with development of the bone cell phenotype (type I collagen, alkaline phosphatase, osteopontin, and osteocalcin) was investigated. Second, the requirement for stringent growth control to support functional relationships between expression of proliferation and differentiation-related genes was explored. Parameters of cell growth and osteoblast-related gene expression in primary cultures of normal diploid osteoblasts, that initially express proliferation-dependent genes and subsequently postproliferative genes associated with mature bone cell phenotypic properties, were compared to those operative in ROS 17/2.8 osteosarcoma cells that concomitantly express cell growth and mature osteoblast phenotypic genes. Our findings indicate that in both normal diploid osteoblasts and osteosarcoma cells, expression of the cell cycle regulated histone genes is tightly coupled with DNA synthesis and controlled predominantly at a posttranscriptional level. Inhibition of proliferation by blocking DNA synthesis with hydroxyurea upregulates a subset of developmentally expressed genes that postproliferatively support progressive establishment of mature osteoblast phenotypic properties (e.g., alkaline phosphatase, type 1 collagen, and osteopontin). However, the osteocalcin gene, which is expressed during the final stage of osteoblast differentiation when extracellular matrix mineralization occurs, is not upregulated. Variations in the extent to which inhibition of proliferation in normal diploid osteoblasts and in ROS 17/2.8 osteosarcoma cells selectively affects transcription and cellular levels of mRNA transcripts from bone cell-related genes (e.g., osteocalcin) may reflect modifications in proliferation/differentiation interrelationships when stringent growth control is abrogated.

The cell biology of bone

Bone remodelling and repair are accomplished by the co-ordinated activity of cells of the osteoclast and osteoblast lineages. Small changes in the balance between formation and resorption will, when magnified by repeated cycles, lead to significant reduction in bone mass and strength, ultimately resulting in fracture. This review focuses on the cellular features of bone remodelling and the known regulators of bone cell function. These include systemic and local factors, both soluble and contained within the complex extracellular matrix of bone.

Constitutive transcription of the osteocalcin gene in osteosarcoma cells is reflected by altered protein-DNA interactions at promoter regulatory elements

The bone-specific osteocalcin (OC) gene is transcribed only after completion of proliferation in normal diploid calvarial-derived osteoblasts during extracellular matrix mineralization. In contrast, the OC gene is expressed constitutively in both proliferating and nonproliferating ROS 17/2.8 osteosarcoma cells. To address molecular mechanisms associated with these tumor-related modifications in transcriptional control, we examined sequence-specific interactions of transactivation factors at key basal and hormone-responsive elements in the OC gene promoter. In ROS 17/2.8 cells compared to normal diploid osteoblasts, the absence of a stringent requirement for cessation of proliferation to support both induction of OC transcription and steroid hormone-mediated transcriptional modulation is reflected by modifications in transcription factor binding at (i) the two primary basal regulatory elements, the OC box (which contains a CCAAT motif as a central core) and the TATA/glucocorticoid-responsive element domain, and (ii) the vitamin D-responsive element. Particularly striking are two forms of the vitamin D receptor complex that are present in proliferating osteoblasts and osteosarcoma cells. Both forms of the complex are sensitive to vitamin D receptor antibody and retinoic X receptor antibody. After the down-regulation of proliferation, only the lower molecular weight complex is found in normal diploid osteoblasts. Both forms of the complex are present in nonproliferating ROS 17/2.8 cells with increased representation of the complex exhibiting reduced electrophoretic mobility that is phosphorylation-dependent.

Postproliferative transcription of the rat osteocalcin gene is reflected by vitamin D-responsive developmental modifications in protein-DNA interactions at basal and enhancer promoter elements

In the osteocalcin (OC) gene promoter, both independent positive and negative regulatory elements, as well as others with contiguous [TATA/glucocorticoid-responsive elements (GRE)] or overlapping [TATA/GRE, vitamin D-responsive enhancer elements (VDRE)/AP-1, and OC box/AP-1] domains, are sites for modifications in protein-DNA interactions. In the present studies, we have examined nuclear protein extracts from fetal rat calvarial cells that undergo a developmental sequence of bone cell differentiation. Our results demonstrate modifications in protein-DNA interactions that relate to the developmental stages of the osteoblast and support developmental regulation of OC gene transcription. Basal expression of the OC gene is associated with sequence-specific protein-DNA interactions at the OC box, VDRE, and TATA/GRE box. Distinct differences are observed in proliferating osteoblasts, where the OC gene is not transcribed compared to postproliferative, differentiated osteoblasts that transcribe the OC gene. Furthermore, the protein-DNA complexes that reflect hormonal control are also developmentally regulated, mediating both the transcriptionally active and repressed states of the OC gene. For example, in proliferating osteoblasts, a vitamin D receptor-antibody-sensitive complex is formed that is different from the DNA binding complex induced by vitamin D postproliferatively when the OC gene is transcribed. Mutational analysis of the steroid hormone binding domain and the overlapping AP-1 site at the VDRE supports mutually exclusive occupancy by Fos-Jun heterodimers and vitamin D receptor. Such protein-DNA interactions at the VDRE are consistent with repression of competency for vitamin D-mediated transcriptional enhancement in proliferating osteoblasts expressing high levels of Fos and Jun.

Concepts of osteoblast growth and differentiation: basis for modulation of bone cell development and tissue formation

The combined application of molecular, biochemical, histochemical, and ultrastructural approaches has defined a temporal sequence of gene expression associated with development of the bone cell phenotype in primary osteoblast cultures. The peak levels of expressed genes reflect a developmental sequence of bone cell differentiation characterized by three principal periods: proliferation, extracellular matrix maturation and mineralization, and two restriction points to which the cells can progress but cannot pass without further signals. The regulation of cell growth and bone-specific gene expression has been examined during this developmental sequence and is discussed within the context of several unique concepts. These are (1) that oncogene expression in proliferating osteoblasts contributes to the suppression of genes expressed postproliferatively, (2) that hormone modulation of a gene is dependent upon the maturational state of the osteoblast, and (3) that chromatin structure and the presence of nucleosomes contribute to three-dimensional organization of gene promoters that support synergistic and/or antagonistic activities of physiologic mediators of bone cell growth and differentiation.