Modifications of protein-DNA interactions in the proximal promoter of a cell-growth-regulated histone gene during onset and progression of osteoblast differentiation

KCa3.1 regulates cell cycle progression by modulating Ca2+ signaling in murine preosteoblasts

Osteoblasts synthesize and deposit essential components of the extracellular bone matrix and collagen scaffolds, leading to mineralized bone formation. Therefore, the proliferation of preosteoblasts (precursors of mature osteoblasts) helps in regulating skeletal homeostasis. This study demonstrated that the functional expression of KCa3.1, an intermediate-conductance Ca2+-activated K+ channel, is markedly upregulated in murine preosteoblastic MC3T3-E1 cells in the G0/G1 phase. The enhancement of KCa3.1 is involved in the establishment of more negative membrane potentials in MC3T3-E1 cells. This hyperpolarization can promote intracellular Ca2+ signaling because store-operated Ca2+ channels are activated. Treatment with TRAM-34, a specific KCa3.1 inhibitor, attenuated the cell cycle progression from the G0/G1 phase to the S/G2/M phases. In MC3T3-E1 cells, KCa3.1 significantly promoted the transition from the G1 phase to the S phase. KCa3.1 inhibition also caused G0 phase cell accumulation. Furthermore, TRAM-34 decreased the expression of alkaline phosphatase, bone sialoprotein, and osteocalcin, osteoblast differentiation markers in MC3T3-E1 cells, and inhibited the endochondral ossification of murine metatarsals. These results reveal novel ways by which KCa3.1 activity can strongly modulate osteoblast maturation during bone formation.

Oxalate induces the ossification of RTECs by activating the JAK2/STAT3 signaling pathway and participates in the formation of kidney stones

BACKGROUND

The ossification of renal tubular epithelial cells (RTECs) plays an important initial role in the formation of kidney stones, but its specific mechanism is still unclear. The JAK2/STAT3 signaling pathway is important for bone cell differentiation. Accordingly, we explored the role and mechanism of the JAK2/STAT3 signaling pathway in the ossification of RTECs.

METHODS

We used oxalate or ethylene glycol to construct kidney stone models in vitro and in vivo, and investigated the expression of osteogenic-specific genes, osteogenesis ability, and JAK2/STAT3 signaling in the kidney stone models by western blotting, qRT-PCR, immunofluorescence, and immunohistochemistry. Then, genetic engineering or drugs were used to inhibit the expression or activation of JAK2, and the expression of osteogenic-specific genes and the osteogenic ability of the RTECs were determined again.

RESULTS

In the in vitro and in vivo kidney stone models, the expression of osteogenic specific genes in the RTECs was significantly upregulated, the osteogenic capacity was significantly increased, and the expression of p-JAK2 (phospho-JAK2) and p-STAT3 (phospho-STAT3) was significantly increased. When the expression or activation of JAK2 was inhibited, the ossification of RTECs and the formation of kidney stones was reversed.

CONCLUSIONS

During the formation of kidney stones, RTECs undergo obvious ossification, and the JAK2/STAT3 signaling pathway plays a key positive regulatory role in this process.

Traditional Japanese apricot (Prunus mume) induces osteocalcin in osteoblasts

The fruit of Prunus mume (ume, also known as Japanese apricot) has been used as a functional food in Japan since ancient times. We previously reported that ume stimulates the differentiation of preosteoblastic cells. Osteocalcin (OCN) is secreted by osteoblasts, and there is known association with glucolipid metabolism and cognitive function. This study sought to clarify the relationship between ume extracts and OCN production both in vitro and in vivo. Alkaline phosphatase activity and OCN level in the ethyl acetate extracts of ume-treated extracts were significantly increased in preosteoblast MC3T3-E1 cells compared with the control group. In human study, serum OCN level was significantly higher in the high ume intake group than in the low intake group in community-dwelling participants over 60 years old. These results suggest that ume has the potential to upregulated OCN production both in vitro and in vivo.

Effects of Er:YAG laser irradiation of different titanium surfaces on osteoblast response

The aim of this in vitro study was to evaluate the effects of erbium-doped yttrium aluminum garnet (Er:YAG) laser irradiation on titanium surface topography and the proliferation and differentiation of osteoblasts using standard clinical treatment settings. Er:YAG laser irradiation at two levels ((1): 160 mJ, pulse at 20 Hz; (2): 80 mJ, pulse at 20 Hz) was applied to moderately rough and smooth titanium disks before MG-63 osteoblast-like cells were cultured on these surfaces. Titanium surface and cell morphology were observed by scanning electron microscopy. Cell proliferation/viability was measured by CCK-8 test. Gene expression of alkaline phosphatase (ALP), osteocalcin (OC), osteoprotegerin (OPG), receptor activator of nuclear factor kappa-B ligand (RANKL), and collagen type 1 was measured by qPCR, and OPG and OC protein production was determined by enzyme-linked immunosorbent assay. Treatment with Er:YAG laser at 160 mJ/20 Hz markedly caused heat-induced fusion of titanium and cell condensation on moderately rough surfaces, but not in smooth surfaces. MG-63 proliferation/viability decreased after 5 days in moderately rough surfaces. The expression of ALP, OC, OPG, and collagen type 1 was unaffected by laser treatment at 160 mJ/20. Laser irradiation at 80 mJ/20 Hz enhanced RANKL gene expression after 5 days in moderately rough surfaces. Study results suggest that Er:YAG laser irradiation at clinically relevant setting has no essential effect on osteogenic gene and protein expression of osteoblasts. However, surface structure, cell attachment, and proliferation are influenced by both treatment protocols, which implies that caution should be taken in the clinical treatment of peri-implant diseases when Er:YAG laser is used.

Effect of Multi-Phosphonate Coating of Titanium Surfaces on Osteogenic Potential

The aim of this study was to evaluate the impact of a novel multi-phosphonate (MP) coating strategy of dental implant surfaces on the expression of osteogenesis-related factors in vitro. MG-63 human osteoblast-like cells, bone marrow mesenchymal stem cells (BM-MSCs), and human periodontal ligament stem cells (hPDLSCs) were cultured separately on titanium disks with and without MP coating. Cell attachment was visualized by focal adhesion and actin cytoskeleton staining. The proliferation and gene expression of the markers related to osteogenesis and bone turnover were measured after 48 and 120 h of cell culture. Actin cytoskeleton assembly and focal adhesion were similar between test surfaces within each cell type but differed from those on tissue culture plastic (TCP). The proliferation of MG-63 cells and PDLSCs was comparable on all surfaces, while BM-MSCs showed an increase on tissue culture plastic (TCP) versus titanium. The gene expression of osteoprotegerin and receptor activator of nuclear factor-kappa B ligand was higher in MG-63 cells grown on MP-coated surfaces. At the same time, osteocalcin was decreased compared to the other surfaces. Collagen type I gene expression after 120 h was significantly lower in hPDLSCs cultivated on MP-coated surfaces. Within the limitations of this study, MP coating on titanium surfaces might have a slight beneficial effect on bone turnover in vitro.

Protective effect of Agrimonia pilosa polysaccharides on dexamethasone-treated MC3T3-E1 cells via Wnt/β-Catenin pathway

A water‐soluble polysaccharide (APP‐AW) was isolated from Agrimonia pilosa and prepared to three sulphated derivatives (S1, S2 and S3). The results showed that pre‐treatment with APP‐AW, S1, S2 and S3 each at the concentration of 50 μg/mL for 48 hours was able to prevent cytotoxicity induced by 1 μmol/L dexamethasone (Dex) in MC3T3‐E1 cells via inhibition of apoptosis, which is in line with the findings in flow cytometry analysis. Meanwhile, the decreased ALP activity, collagen content, mineralization, BMP2, Runx2, OSX and OCN protein expression in DEX‐treated MC3T3‐E1 cells were reversed by the addition of APP‐AW, S1, S2 and S3. Moreover, APP‐AW, S1, S2 and S3 rescued DEX‐induced increase of Bax, cytochrome c and caspase‐3 and decrease of Bcl‐2, Wnt3, β‐catenin and c‐Myc protein expression in MC3T3‐E1 cells. Our findings suggest that pre‐treatment with APP‐AW, S1, S2 and S3 could significantly protect MC3T3‐E1 cells against Dex‐induced cell injury via inhibiting apoptosis and activating Wnt/β‐Catenin signalling pathway, thus application of these polysaccharides may be a promising alternative strategy for steroid‐induced avascular necrosis of the femoral head (SANFH) therapy.

Promoting Effect of Pinostrobin on the Proliferation, Differentiation, and Mineralization of Murine Pre-osteoblastic MC3T3-E1 Cells

Pinostrobin (PI), a natural flavonoid found in a variety of plants, is well known for its rich pharmacological activities. However, its osteogenic function remains unclear. The aim of this study is to evaluate the effect of PI on the proliferation, differentiation, and mineralization of murine pre-osteoblastic MC3T3-E1 cells in vitro using MTT, alkaline phosphatase (ALP) activity, the synthesis of collagen I (Col I) assay, and Von-Kossa staining, respectively. The expression of osteocalcin (OCN) mRNA in cells was detected by real-time PCR. The effect of PI on the differentiation of dexamethasone (DEX)-suppressed cells was also investigated. The results showed that PI greatly promoted the proliferation of MC3T3-E1 cells at 5–80 μg/mL (p < 0.05 or p < 0.01), and caused a significant elevation of ALP activity, Col I content, and mineralization of osteoblasts at 10–40 μg/mL (p < 0.05 or p < 0.01), and the expression levels of OCN gene were greatly upregulated after PI treatment (p < 0.01). Furthermore, PI could rescue the inhibition effect of cell differentiation induced by DEX. Taken together, these results indicated that PI could directly promote proliferation, differentiation, and mineralization of MC3T3-E1 cells and has potential for use as a natural treatment for osteoporosis.

5,6-Dehydrokawain from Alpinia zerumbet promotes osteoblastic MC3T3-E1 cell differentiation

Bone homeostasis is maintained by balancing bone formation and bone resorption, but an imbalance between them is associated with various bone-related diseases such as osteoporosis and rheumatoid arthritis. We found that 5,6-dehydrokawain (DK) and dihydro-5,6-dehydrokawain (DDK), which were isolated as promising compounds from Alpinia zerumbet rhizomes, promote differentiation of osteoblastic MC3T3-E1 cells. DK and DDK increased the alkaline phosphatase activity and matrix mineralization of MC3T3-E1 cells. DK exerts larger effects than DDK. The gene expression of runt-related transcription factor 2 and osterix, which are essential transcription factors in the early period of osteoblast differentiation, was significantly increased by DK treatment. The mRNA level of distal-less homeobox 5 was also enhanced by DK treatment, and DK activated the p38 mitogen-activated protein kinase pathway. Therefore, DK may have clinical potential for preventing osteoporosis, and could be considered as a potential anabolic therapeutic agent.

Dose-dependent inhibitory effects of zoledronic acid on osteoblast viability and function in vitro

Zoledronic acid (ZA), which is one of the most potent and efficacious bisphosphonates, has been commonly used in clinical practice for the treatment of various bone disorders. The extensive use of ZA has been associated with increasing occurrence of jaw complications, now known as bisphosphonate-associated osteonecrosis of the jaw (BRONJ). However, the mechanism underlying BRONJ remains to be fully elucidated. The aim of the present study was to investigate the effects of different concentrations of ZA on the MC3T3-E1 murine preosteoblast cell line cells and examine the possible pathogenesis of BRONJ. In the present study, the effect of ZA on the viability, apoptosis, differentiation and maturation of MC3T3-E1 cells, as well as its relevant molecular mechanism, were examined The results of a Cell Counting Kit 8 assay, a flow cytometric Annexin-V/propidium iodide assay and western blot analysis demonstrated that ZA exhibited a significant inhibition of cell viability and induction of apoptosis at concentrations >10 µM. Subsequently, the effect of ZA on cell differentiation at concentrations <1 µM were investigated. In this condition, ZA inhibited bone nodule formation and decreased the activity of alkaline phosphatase. The results of reverse transcription-quantitative polymerase chain reaction and western blot analyses indicated that ZA downregulated the expression levels of the marker genes and proteins associated with osteogenic differentiation. Further investigation revealed that the suppression of differentiation by ZA was associated with decreased expression of bone morphogenetic protein-2 (BMP-2) and downregulation of the phosphorylation levels in the downstream extracellular signal-regulated kinase 1/2 and p38 pathways. These adverse effects of ZA were observed to be concentration-dependent. The results from the present study suggested that ZA at higher concentrations induces cytotoxicity towards osteoblasts, and ZA at lower concentrations suppresses osteoblast differentiation by downregulation of BMP-2. These results assist in further understanding the mechanisms of BRONJ.

The impact of osteoblastic differentiation on osteosarcomagenesis in the mouse

Osteosarcomas remain an enigmatic group of malignancies that share in common the presence of transformed cells producing osteoid matrix, even if these cells comprise a minority of the tumor volume. The differentiation state of osteosarcomas has therefore become a topic of interest and challenge to those who study this disease. In order to test how the cell of origin contributes to the final state of differentiation in the transformed cells, we compared the relative tumorigenicity of Cre-LoxP conditional disruption of the cell cycle checkpoint tumor-suppressor genes Trp53 and Rb1 using Prx1-Cre, Collagen-1α1-Cre and Osteocalcin-Cre to transform undifferentiated mesenchyme, preosteoblasts and mature osteoblasts, respectively. The Prx1 and Col1α1 lineages developed tumors with nearly complete penetrance, as anticipated. Osteosarcomas also developed in 44% of Oc-Cre;Rb1(fl/fl);Trp53(fl/fl) mice. We confirmed using 5-ethynyl-2'-deoxyuridine click chemistry that the Oc-Cre lineage includes very few actively cycling cells. By assessing radiographic mineralization and histological osteoid production, the differentiation state of tumors did not correlate with the differentiation state of the lineage of origin. Some of the osteocalcin-lineage-derived osteosarcomas were among the least osteoblastic. Osteocalcin immunohistochemistry in tumors correlated well with the expression of DNA methyl transferases, suggesting that silencing of these epigenetic regulators may influence the final differentiation state of an osteosarcoma. Transformation of differentiated, minimally proliferative osteoblasts is possible but may require such an epigenetic reprogramming that the tumors no longer resemble their differentiated origins.

Heterogeneous dental follicle cells and the regeneration of complex periodontal tissues

Dental follicle cells (DFCs) are a heterogeneous population that exhibit a variety of phenotypes. However, it remains unclear whether DFCs can maintain stem cell characteristics, or mediate tissue-regeneration to form single or complex tissues in the periodontium, after long-term culturing. Therefore, DFCs were isolated from human impacted molars (HIM-DFCs), passaged >30 times, and then evaluated for their heterogeneity and multipotential differentiation. Morphology, proliferation, epitope profile, and mineralization characteristics of clones derived from single HIM-DFCs in vitro were also assayed. HIM-DFCs (passage #30) were found to be positive for the heterogeneous markers, Notch-1, stro-1, alkaline phosphomonoesterase (ALP), type I collagen (COL-I), type III collagen (COL-III), and osteocalcine. Moreover, passage #30 of the HDF1, 2, and 3 subclone classes identified in this study were found to express high levels of the mesenchymal stem cells markers, CD146 and Stro1. HDF3 subclones were also associated with the strongest ALP staining detected, and strongly expressed osteoblast and cementoblast markers, including COL-I, COL-III, bone sialoprotein (BSP), and Runx2. In contrast, HDF1 subclone analyzed strongly expressed COL-I and COL-III, yet weakly expressed BSP and Runx2. The HDF2 subclone was associated with the strongest proliferative capacity. To evaluate differentiation characteristics in vivo, these various cell populations were combined with ceramic bovine bone and implanted into subcutaneous pockets of nude mice. The 30th passage of subclone HDF1 and 3 were observed to contribute to fiber collagens and the mineralized matrix present, respectively, whereas HDF2 subclones were found to have a minimal role in these formations. The formation of a cementum-periodontal ligament (PDL) complex was observed 6 weeks after HIM-DFCs (passage #30) were implanted in vivo, thus suggesting that these cells maintain stem cell characteristics. Therefore, subclone HDF1-3 may be related to the differentiation of fibroblasts in the PDL, undifferentiated cells, and osteoblasts and cementoblasts, respectively. Overall, this study is the first to amplify HIM-DFCs and associated subclones with the goal of reconstructing complex or single periodontium. Moreover, our results demonstrate the potential for this treatment approach to address periodontal defects that result from periodontitis, or for the regeneration of teeth.

Dental follicle cells and treated dentin matrix scaffold for tissue engineering the tooth root

Tissue engineering strategies to reconstruct tooth roots are an effective therapy for the treatment of tooth loss. However, strategies to successfully regenerate tooth roots have not been developed and optimized. In the present study, rat dental follicle stem cells (DFCs) were characterized, followed by a thorough investigation of tooth roots regeneration for a combination of DFCs seeding cells, treated dentin matrix (TDM) scaffolds, and an inductive alveolar fossa microenvironment. Eighteen clones derived from single DFCs were harvested; however, only three clones were amplified successfully more than five passages and 90-95 days in culture. Following 270 days or 30 passages, the heterogeneous DFCs showed suitable characteristics for seeding cells to regenerate tooth roots. However, various features, such as variable proliferation rates, differentiation characteristics, apoptosis rates, and total lifespan were observed in DFCs and the three clones. Importantly, upon transplantation of DFCs combined with TDM for four weeks, root-like tissues stained positive for markers of dental pulp and periodontal tissues were regenerated in the alveolar fossa, but not in the skull and omental pockets. These results indicate that tooth roots were successfully regenerated and suggest that the combination of DFCs with TDM in the alveolar fossa is a feasible strategy for tooth roots regeneration. This strategy could be a promising approach for the treatment of clinical tooth loss and provides a perspective with potential applications to regeneration of other tissues and organs.

The Ric-8B gene is highly expressed in proliferating preosteoblastic cells and downregulated during osteoblast differentiation in a SWI/SNF- and C/EBPbeta-mediated manner

The Ric-8 gene encodes a guanine exchange factor (GEF) that modulates G protein-mediated signaling, exhibiting a relevant role during regulation of cell division. In mammals, two Ric-8 homologues have been reported (Ric-8A and Ric-8B), and recent studies indicate equivalent roles for each protein. Here, we show that the Ric-8B gene is negatively regulated during osteoblast differentiation by the transcription factor C/EBPβ. Only the larger C/EBPβ isoform (C/EBPβ-LAP*) downregulates Ric-8B gene promoter activity in osteoblastic cells. Accordingly, knockdown of C/EBPβ expression by small intefering RNA in osteoblastic cells results in a significant increase of Ric-8B gene expression. Transient overexpression of Brg1 or Brm, the catalytic subunits of the SWI/SNF chromatin-remodeling complex, inhibits Ric-8B promoter activity. Also, the presence of inactive SWI/SNF complexes in osteoblastic cells results in increased endogenous Ric-8B transcription, indicating that SWI/SNF activity negatively regulates Ric-8B expression. During osteoblast differentiation, Ric-8B gene repression is accompanied by changes in nucleosome placement at the proximal Ric-8B gene promoter and reduced accessibility to regulatory sequences.

Polyester copolymer scaffolds enhance expression of bone markers in osteoblast-like cells

In tissue engineering, the resorbable aliphatic polyester poly(L-lactide) (PLLA) is used as scaffolds in bone regeneration. Copolymers of poly(L-lactide)-co-(epsilon-caprolactone) [poly(LLA-co-CL)] and poly(L-lactide)-co-(1,5-dioxepan-2-one) [poly(LLA-co-DXO)], with superior mechanical properties to PLLA, have been developed to be used as scaffolds, but the influence on the osteogenic potential is unclear. This in vitro study of test scaffolds of poly(LLA-co-CL) and poly(LLA-co-DXO) using PLLA scaffolds as a control demonstrates the attachment and proliferation of human osteoblast-like cells (HOB) as measured by SEM and a methylthiazol tetrazolium (MTT) colorimetric assay, and the progression of HOB osteogenesis for up to 3 weeks; expressed as synthesis of the osteoblast differentiation markers: collagen type 1 (Col 1), alkaline phosphatase, bone sialoprotein, osteocalcin (OC), osteopontin and runt related gene 2 (Runx2). Surface analysis disclosed excellent surface attachment, spread and penetration of the cells into the pores of the test scaffolds compared to the PLLA. MTT results indicated that test scaffolds enhanced the proliferation of HOBs. Cells grown on the test scaffolds demonstrated higher synthesis of Col 1 and OC and also increased bone markers mRNA expression. Compared to scaffolds of PLLA, the poly(LLA-co-CL) and poly(LLA-co-DXO) scaffolds enhanced attachment, proliferation, and expression of osteogenic markers by HOBs in vitro. Therefore, these scaffolds might be appropriate carriers for bone engineering.

Alendronate enhances osteogenic differentiation of bone marrow stromal cells: a preliminary study

Alendronate inhibits osteoclastic activity. However, some studies suggest alendronate also has effects on osteoblast activity. We hypothesized alendronate would enhance osteoblastic differentiation without causing cytotoxicity of the osteoblasts. We evaluated the effect of alendronate on the osteogenic differentiation of mouse mesenchymal stem cells. D1 cells (multipotent mouse mesenchymal stem cells) were cultured in osteogenic differentiation medium for 7 days and then treated with alendronate for 2 days before being subjected to various tests using MTT assays, Alizarin Red, enzyme-linked immunosorbent assay, energy-dispersive xray spectrophotometry, reverse transcriptase-polymerase chain reaction, confocal microscopy, and flow cytometric analysis. D1 cells differentiated into osteoblasts in the presence of osteogenic differentiation medium as confirmed by positive Alizarin Red S staining, increased alkaline phosphatase activity and osteocalcin mRNA expression, a calcium peak by energy-dispersive xray spectrophotometry, and by positive immunofluorescence staining against CD44. Osteogenic differentiation was enhanced after treatment with alendronate as confirmed by Alizarin Red S staining, elevated alkaline phosphatase activity and osteocalcin mRNA expression, a greater calcium peak by energy-dispersive xray spectrophotometry, and by immunofluorescence staining against CD44 by flow cytometric analysis. These data suggest alendronate enhances osteogenic differentiation when treated with mouse mesenchymal stem cells in osteogenic differentiation medium.

In Vitro Evaluation of the Efficacy of a New Laser Surface Implant: Cellular Adhesion and Alkaline Phosphatase Production Tests

Bone tissue reacts in different ways to implant surfaces with different patterns. The aim of this study is to understand which laser generated surface pattern produces the best cell adhesion in vitro, evaluating both the activity of the alkaline phosphatase and the cells adhering to titanium samples. Tests were carried out on titanium samples with sandblasted surfaces with laser-produced holes with diameters of 5, 10, and 20 μm, and on sandblasted titanium cylinders without holes as controls. The samples were inserted into culture medium containing SaOS-2 cells for 3, 7 and 10 days. The results showed that at days 3 and 7 the laser surfaces stimulated a higher production of alkaline phosphatase (ALP) compared to the data from the control group. At day 10 there were no significant differences between the test group and the control group.

Differentiation and cytokine synthesis of human alveolar osteoblasts compared to osteoblast-like cells (MG63) in response to titanium surfaces

OBJECTIVES

The aim of this study was to investigate the influence of different implant surface topographies and chemistries on the expression of differentiation/proliferation markers on MG63 cells and primary human alveolar osteoblasts.

METHODS

Hydrophobic acid-etched (A) and hydrophobic coarse-grit-blasted, acid-etched (SLA) surfaces and hydrophilic acid-etched (modA) and hydrophilic coarse-grit-blasted (modSLA) surfaces were produced. Thereby, modA and modSLA surfaces were rinsed under nitrogen protection and stored in a sealed glass tube containing isotonic NaCl solution at pH 4-6. Tissue culture plates without specimens served as controls. The behavior of MG63 cells and primary human alveolar osteoblasts (AOB) grown on all surfaces was compared through determination of alkaline phosphatase (ALP) activity, cell proliferation ((3)H-thymidin incorporation, MTT colorimetric assay) and expression of osteocalcin (OC), osteoprotegerin (OPG), transforming growth factor-beta1 (TGF-beta(1)) and vascular endothelial growth factor (VEGF), detected with commercial available test kits.

RESULTS

Proliferation of MG63 and primary cells was highest on controls, followed by A surfaces, modA and SLA surfaces being almost on the same level and lowest on modSLA surfaces. modSLA surfaces exhibited highest ALP and OC production, followed by SLA, modA and A surfaces. Proliferation and OC production were comparable for MG63 cells and AOB. OPG, TGF-beta(1) and VEGF produced on primary cells showed a slightly different rank order on different surfaces compared to MG63 cells. modSLA still showed the highest production of OPG, TGF-beta(1) and VEGF, but was followed by modA, SLA and A. Statistical significance was checked by ANOVA (p<0.0035).

SIGNIFICANCE

MG63 cells and primary human alveolar osteoblasts showed similar proliferation and differentiation characteristics on different titanium surfaces. Only modA surfaces showed enhanced expression of OPG, TGF-beta(1) and VEGF on MG63 cells compared to primary human alveolar osteoblasts. Overall, the lowest proliferation rates and the highest expressions of differentiation markers and growth factor productions were observed on modSLA.

Osteoblast differentiation and skeletal development are regulated by Mdm2-p53 signaling

Mdm2 is required to negatively regulate p53 activity at the peri-implantation stage of early mouse development. However, the absolute requirement for Mdm2 throughout embryogenesis and in organogenesis is unknown. To explore Mdm2–p53 signaling in osteogenesis, Mdm2-conditional mice were bred with Col3.6-Cre–transgenic mice that express Cre recombinase in osteoblast lineage cells. Mdm2-conditional Col3.6-Cre mice die at birth and display multiple skeletal defects. Osteoblast progenitor cells deleted for Mdm2 have elevated p53 activity, reduced proliferation, reduced levels of the master osteoblast transcriptional regulator Runx2, and reduced differentiation. In contrast, p53-null osteoprogenitor cells have increased proliferation, increased expression of Runx2, increased osteoblast maturation, and increased tumorigenic potential, as mice specifically deleted for p53 in osteoblasts develop osteosarcomas. These results demonstrate that p53 plays a critical role in bone organogenesis and homeostasis by negatively regulating bone development and growth and by suppressing bone neoplasia and that Mdm2-mediated inhibition of p53 function is a prerequisite for Runx2 activation, osteoblast differentiation, and proper skeletal formation.

The Vitamin D Response Element in the Distal Osteocalcin Promoter Contributes to Chromatin Organization of the Proximal Regulatory Domain

Vitamin D receptor (VDR) and Runx2 are key regulators of tissue-specific gene transcription. Using the bone-related osteocalcin (OC) gene, we have previously shown that Runx2 is required for the extensive chromatin remodeling that accompanies gene activation. Here, we have addressed the direct contribution of the VDR to chromatin remodeling events necessary for regulation of OC transcription using mutational analysis. Our studies demonstrate that both the distal and proximal DNase I-hypersensitive sites characteristic of the transcriptionally active OC promoter are not enhanced in the absence of a functional vitamin D response element (VDRE). Furthermore, restriction enzyme accessibility studies reveal that nucleosomal reorganization of the proximal promoter occurs in response to vitamin D and this reorganization is abrogated by mutation of the VDRE. These findings indicate that binding of liganded VDR in the distal promoter directly impacts the chromatin structure of the proximal promoter. We find that, in the absence of functional Runx sites, the VDR cannot be recruited to the OC promoter and, therefore, the VDRE is not competent to mediate vitamin D responsiveness. On the other hand, chromatin immunoprecipitation assays show that Runx2 association with the OC promoter is not significantly impaired when the VDRE is mutated. Chromatin immunoprecipitation assays also demonstrate that basal levels of histone acetylation occur in the absence of Runx2 binding but that the VDRE and vitamin D are required for enhanced acetylation of histones H3 and H4 downstream of the VDRE. Together our results support a stepwise model for chromatin remodeling of the OC promoter and show that binding of the liganded VDR.retinoid X receptor directly impacts both the distal and proximal regulatory domains.

Genetic ablation of the CDP/Cux protein C terminus results in hair cycle defects and reduced male fertility

Murine CDP/Cux, a homologue of the Drosophila Cut homeoprotein, modulates the promoter activity of cell cycle-related and cell-type-specific genes. CDP/Cux interacts with histone gene promoters as the DNA binding subunit of a large nuclear complex (HiNF-D). CDP/Cux is a ubiquitous protein containing four conserved DNA binding domains: three Cut repeats and a homeodomain. In this study, we analyzed genetically targeted mice (Cutl1(tm2Ejn), referred to as Delta C) that express a mutant CDP/Cux protein with a deletion of the C terminus, including the homeodomain. In comparison to the wild-type protein, indirect immunofluorescence showed that the mutant protein exhibited significantly reduced nuclear localization. Consistent with these data, DNA binding activity of HiNF-D was lost in nuclear extracts derived from mouse embryonic fibroblasts (MEFs) or adult tissues of homozygous mutant (Delta C(-/-)) mice, indicating the functional loss of CDP/Cux protein in the nucleus. No significant difference in growth characteristics or total histone H4 mRNA levels was observed between wild-type and Delta C(-/-) MEFs in culture. However, specific histone genes (H4.1 and H1) containing CDP/Cux binding sites have reduced expression levels in homozygous mutant MEFs. Stringent control of growth and differentiation appears to be compromised in vivo. Homozygous mutant mice have stunted growth (20 to 50% weight reduction), a high postnatal death rate of 60 to 70%, sparse abnormal coat hair, and severely reduced fertility. The deregulated hair cycle and severely diminished fertility in Cutl1(tm2Ejn/tm2Ejn) mice suggest that CDP/Cux is required for the developmental control of dermal and reproductive functions.

The role of vascular endothelial growth factor in human dental pulp cells: induction of chemotaxis, proliferation, and differentiation and activation of the AP-1-dependent signaling pathway

Vascular endothelial growth factor (VEGF) is a potent mitogen in endothelial cells, but little is known about its activity in other cell types. To clarify the role of VEGF in human dental pulp cells and pulp tissue, we investigated the effects of VEGF on the chemotaxis, proliferation, and differentiation of human dental pulp cells. VEGF induced a strong chemotactic response in human dental pulp cells in a dose-dependent manner. VEGF also marginally enhanced the proliferation of human dental pulp cells and induced an increase in alkaline phosphatase in human dental pulp cells. However, these effects of VEGF were not observed in reference to human skin fibroblasts. Analyses by the reverse-transcription/polymerase-chain-reaction method and flow cytometry showed that the mRNAs of two VEGF receptors, fins-like tyrosine kinase and kinase insert domain-containing receptor, were expressed in human dental pulp cells, whereas only fms-like tyrosine kinase mRNA was expressed in human skin fibroblasts. VEGF induced the activation of activator protein 1 (AP-1) and c-fos mRNA expression in human dental pulp cells. The AP-1 inhibitor curcumin strongly inhibited VEGF-induced alkaline phosphatase production in human dental pulp cells. In addition, VEGF antisense oligonucleotide suppressed the production of VEGF and alkaline phosphatase in human dental pulp cells. These results suggest that VEGF produced by human dental pulp cells acts directly upon human dental pulp cells in an autocrine manner, and may promote the chemotaxis, proliferation, and/or differentiation of human dental pulp cells via the utilization of kinase insert domain-containing receptor and in part through AP-1 by increasing c-fos.

Communication between the cell membrane and the nucleus: role of protein compartmentalization

Understanding how the information is conveyed from outside to inside the cell is a critical challenge for all biologists involved in signal transduction. The flow of information initiated by cell-cell and cell-extracellular matrix contacts is mediated by the formation of adhesion complexes involving multiple proteins. Inside adhesion complexes, connective membrane skeleton (CMS) proteins are signal transducers that bind to adhesion molecules, organize the cytoskeleton, and initiate biochemical cascades. Adhesion complex-mediated signal transduction ultimately directs the formation of supramolecular structures in the cell nucleus, as illustrated by the establishment of multi complexes of DNA-bound transcription factors, and the redistribution of nuclear structural proteins to form nuclear subdomains. Recently, several CMS proteins have been observed to travel to the cell nucleus, suggesting a distinctive role for these proteins in signal transduction. This review focuses on the nuclear translocation of structural signal transducers of the membrane skeleton and also extends our analysis to possible translocation of resident nuclear proteins to the membrane skeleton. This leads us to envision the communication between spatially distant cellular compartments (i.e., membrane skeleton and cell nucleus) as a bidirectional flow of information (a dynamic reciprocity) based on subtle multilevel structural and biochemical equilibria. At one level, it is mediated by the interaction between structural signal transducers and their binding partners, at another level it may be mediated by the balance and integration of signal transducers in different cellular compartments.

Soluble factors produced by PC-3 prostate cells decrease collagen content and mineralisation rate in fetal rat osteoblasts in culture

Approximately 70% of patients with prostate cancer develop bone metastases in the advanced state of the disease. In the present study, we sought to test the hypothesis that prostatic cancer cells produce factors that inhibit the mineralisation process in vitro, decreasing the content of type I collagen in rat fetal calvaria osteoblasts. We investigated the capacity of conditioned media (CM) from the human prostatic tumour cell line PC-3 to inhibit the expression of the differentiation programme on osteoblasts in culture, with a primary focus on type I collagen synthesis and degradation. Our results show that PC-3 CM inhibits collagen synthesis and stimulates the production of interstitial collagenase from osteoblasts. A consequential decrease in the content of immunoreactive type I collagen was observed. We have previously demonstrated that PC-3 CM blocks osteoblast differentiation in culture. We propose that under the effect of factors present in PC-3 CM, osteoblastic cells retain the undifferentiated phenotype.

Bipartite structure of the proximal promoter of a human H4 histone gene

The proximal promoter of the human H4 histone gene FO108 contains two regions of in vivo protein-DNA interaction, Sites I and II. Electrophoretic mobility shift assays using a radiolabeled DNA probe revealed that several proteins present in HeLa cell nuclear extracts bound specifically to Site I (nt-125 to nt-86). The most prominent complex, designated HiNF-C, and a complex of greater mobility, HiNF-C', were specifically compatable by an Sp1 consensus oligonucleotide. Fractionation of HiNF-C using wheat germ agglutinin affinity chromatography suggested that, like Sp1, HiNF-C contains N-acetylglucosamine moieties. Two minor complexes of even greater mobility, designated HiNF-E and F, were compatable by ATF consensus oligonucleotides. A DNA probe carrying a site-specific mutation in the distal portion of Site I failed to bind HiNF-E, indicating that this protein associated specifically to this region. UV cross-linking analysis showed that several proteins of different molecular weights interact specifically with Site I. These data indicate that Site I possesses a bipartite structure and that multiple proteins present in HeLa cell nuclear extracts interact specifically with Site I sequences.

Histone gene transcription: a model for responsiveness to an integrated series of regulatory signals mediating cell cycle control and proliferation/differentiation interrelationships

Histone gene expression is restricted to the S-phase of the cell cycle. Control is at multiple levels and is mediated by the integration of regulatory signals in response to cell cycle progression and the onset of differentiation. The H4 gene promoter is organized into a series of independent and overlapping regulatory elements which exhibit selective, phosphorylation-dependent interactions with multiple transactivation factors. The three-dimensional organization of the promoter and, in particular, its chromatin structure, nucleosome organization, and interactions with the nuclear matrix may contribute to interrelationships of activities at multiple promoter elements. Molecular mechanisms are discussed that may participate in the coordinate expression of S-phase-specific core and H1 histone genes, together with other genes functionally coupled with DNA replication.

Repression of histone gene transcription in quiescent 3T6 fibroblasts

Maintaining murine 3T6 fibroblasts in serum-depleted medium for a period of three days results in a resting cell population that does not synthesize DNA. Histone mRNA levels, closely tied to the cell-proliferation rate, are low due to a reduced rate of synthesis. A comparison of histone gene transcription in vitro by nuclear extracts of quiescent or proliferative 3T6 cells showed that a 200-bp segment of the promoter was responsible for repressing gene activity when cells were in a G0 state. In the absence of the distal promoter region (-200 to -400), gene transcription remained high in quiescent cells, indicating the proximal promoter region (+1 to -200) was responsible for basal gene activity. Alterations in protein binding to the distal promoter region correlated with histone H4 gene activity, suggesting that repression of histone gene transcription is linked to the attachment of a specific nuclear protein. During G1, the histone H4 gene was efficiently transcribed in vitro, but an inability to process the histone pre-mRNA limited the cellular content of mature histone mRNA. This distinction between transcriptional (in G0) and post-transcriptional (in G1) mechanisms for modulating histone mRNA levels suggests that gene-regulatory factors are specifically activated in quiescent cells to reduce expression of non-essential genes.

Differential expression of the chromosomal high mobility group proteins 14 and 17 during the onset of differentiation in mammalian osteoblasts and promyelocytic leukemia cells

The expression of chromosomal proteins HMG 14 and HMG 17 during proliferation and differentiation into the osteoblast and monocyte phenotypes was studied. Cellular levels of HMG 14 and HMG 17 mRNA were assayed in primary cultures of calvarial-derived rat osteoblasts under conditions that: (1) support complete expression of the mature osteocytic phenotype and development of a bone tissue-like organization; and (2) where development of osteocytic phenotypic properties are both delayed and reduced in extent of expression. HMG 14 and HMG 17 are preferentially expressed in proliferating osteoblasts and decline to basal levels post-proliferatively at the onset of extracellular matrix mineralization. In contrast, under conditions that are not conducive to extracellular matrix mineralization, HMG 14 is maximally expressed following the downregulation of proliferation. Consistent with previous reports by Bustin and co-workers [Crippa et al., 1990], HMG 14 and HMG 17 are expressed in proliferating HL-60 promyelocytic leukemia cells and downregulated post-proliferatively following phorbol ester-induced monocytic differentiation. However, differentiation into the monocyte phenotype is accompanied by reinitiation of HMG 17 gene expression. The results indicate that the levels of HMG 14 and HMG 17 mRNA are selectively down-regulated during differentiation.

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.

Pattern of gene expression following rat tibial marrow ablation

Following injury to bone marrow there is a phase of osteogenesis in which bone trabeculae replace the initial blood clot and fill the marrow cavity. The newly formed bone is subsequently fully resorbed by osteoclasts and normal bone marrow is restored. In this study we correlated the morphologic events with the pattern of gene expression that defines this sequence. Following marrow ablation, the trabecular bone volume in the affected section of the marrow cavity increased from control to 27% at day 6, declined to 18% at day 8, and eventually returned to control levels at day 14. Osteoblast number increased up to day 6 and declined substantially by day 8, but the number of osteoclasts peaked between days 8 and 10. Histologic analysis of alkaline phosphatase (AP) and tartrate-resistant acid phosphatase (TRAP) activity correlated with the observed cellular changes. Northern blot analysis of the levels of AP, osteocalcin (OC), and osteopontin (OP) mRNA shows a specific pattern of regulated gene expression, with AP mRNA maximal at day 6, OC mRNA very low until days 6-8, and OP mRNA expressed at very high levels throughout. In addition, procollagen alpha 1(I) and alpha 1(III) mRNAs show a regulated pattern of expression, with procollagen alpha 1(I) maximally expressed between days 4 and 10 and procollagen alpha 1(III) expressed at lower levels between days 4 and 6. The mRNA encoding insulin-like growth factor I (IGF-I) was found to be highly expressed between days 5 and 12; however, transforming growth factor beta 1 (TGF-beta 1) and TGF-beta 3 mRNA were only weakly expressed between days 4 and 10. These data demonstrate a temporal pattern of gene expression consistent with the observed morphologic profile, identify changes in growth factor mRNA that may be related to this repair process, and suggest that this is a suitable model for studying in vivo a synchronized sequence of bone formation and resorption at a well-defined anatomic site.

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.

Overlapping and CpG methylation-sensitive protein-DNA interactions at the histone H4 transcriptional cell cycle domain: distinctions between two human H4 gene promoters

Transcriptional regulation of vertebrate histone genes during the cell cycle is mediated by several factors interacting with a series of cis-acting elements located in the 5' regions of these genes. The arrangement of these promoter elements is different for each gene. However, most histone H4 gene promoters contain a highly conserved sequence immediately upstream of the TATA box (H4 subtype consensus sequence), and this region in the human H4 gene FO108 is involved in cell cycle control. The sequence-specific interaction of nuclear factor HiNF-D with this key proximal promoter element of the H4-FO108 gene is cell cycle regulated in normal diploid cells (J. Holthuis, T.A. Owen, A.J. van Wijnen, K.L. Wright, A. Ramsey-Ewing, M.B. Kennedy, R. Carter, S.C. Cosenza, K.J. Soprano, J.B. Lian, J.L. Stein, and G.S. Stein, Science, 247:1454-1457, 1990). Here, we show that this region of the H4-FO108 gene represents a composite protein-DNA interaction domain for several distinct sequence-specific DNA-binding activities, including HiNF-D, HiNF-M, and HiNF-P. Factor HiNF-P is similar to H4TF-2, a DNA-binding activity that is not cell cycle regulated and that interacts with the analogous region of the H4 gene H4.A (F. LaBella and N. Heintz, Mol. Cell. Biol. 11:5825-5831, 1991). The H4.A gene fails to interact with factors HiNF-M and HiNF-D owing to two independent sets of specific nucleotide variants, indicating differences in protein-DNA interactions between these H4 genes. Cytosine methylation of a highly conserved CpG dinucleotide interferes with binding of HiNF-P/H4TF-2 to both the H4-FO108 and H4.A promoters, but no effect is observed for either HiNF-M or HiNF-D binding to the H4-FO108 gene. Thus, strong evolutionary conservation of the H4 consensus sequence may be related to combinatorial interactions involving overlapping and interdigitated recognition nucleotides for several proteins, whose activities are regulated independently. Our results also suggest molecular complexity in the transcriptional regulation of distinct human H4 genes.

Overlapping and CpG methylation-sensitive protein-DNA interactions at the histone H4 transcriptional cell cycle domain: distinctions between two human H4 gene promoters

Transcriptional regulation of vertebrate histone genes during the cell cycle is mediated by several factors interacting with a series of cis-acting elements located in the 5' regions of these genes. The arrangement of these promoter elements is different for each gene. However, most histone H4 gene promoters contain a highly conserved sequence immediately upstream of the TATA box (H4 subtype consensus sequence), and this region in the human H4 gene FO108 is involved in cell cycle control. The sequence-specific interaction of nuclear factor HiNF-D with this key proximal promoter element of the H4-FO108 gene is cell cycle regulated in normal diploid cells (J. Holthuis, T.A. Owen, A.J. van Wijnen, K.L. Wright, A. Ramsey-Ewing, M.B. Kennedy, R. Carter, S.C. Cosenza, K.J. Soprano, J.B. Lian, J.L. Stein, and G.S. Stein, Science, 247:1454-1457, 1990). Here, we show that this region of the H4-FO108 gene represents a composite protein-DNA interaction domain for several distinct sequence-specific DNA-binding activities, including HiNF-D, HiNF-M, and HiNF-P. Factor HiNF-P is similar to H4TF-2, a DNA-binding activity that is not cell cycle regulated and that interacts with the analogous region of the H4 gene H4.A (F. LaBella and N. Heintz, Mol. Cell. Biol. 11:5825-5831, 1991). The H4.A gene fails to interact with factors HiNF-M and HiNF-D owing to two independent sets of specific nucleotide variants, indicating differences in protein-DNA interactions between these H4 genes. Cytosine methylation of a highly conserved CpG dinucleotide interferes with binding of HiNF-P/H4TF-2 to both the H4-FO108 and H4.A promoters, but no effect is observed for either HiNF-M or HiNF-D binding to the H4-FO108 gene. Thus, strong evolutionary conservation of the H4 consensus sequence may be related to combinatorial interactions involving overlapping and interdigitated recognition nucleotides for several proteins, whose activities are regulated independently. Our results also suggest molecular complexity in the transcriptional regulation of distinct human H4 genes.

Sequence-specific DNA-binding proteins are components of a nuclear matrix-attachment site

We have identified a nuclear matrix-attachment region within an upstream element of a human H4 histone gene promoter. Nuclear matrix proteins, isolated and solubilized from HeLa S3 cells, were found to interact with sequence specificity at this matrix-attachment region. Several types of assays for protein-DNA interaction showed that the minimal sequence for the nuclear matrix protein-DNA interaction was 5'-TGACGTCCATG-3'; the underlined region corresponds to the core consensus sequence for ATF transcription factor binding. Two proteins with molecular masses of 43 and 54 kDa were identified by UV-crosslinking analysis as integral components of this protein-DNA complex. The molecular masses of these proteins and the ATF-binding site consensus sequence suggest that these proteins are members of the ATF family. Our results provide direct evidence for nuclear matrix localization of sequence-specific DNA-binding factors for an actively transcribed gene. The proximity of a strong positive transcriptional regulatory element to the matrix-attachment region of this gene suggests that the nuclear matrix may serve to localize and concentrate trans-acting factors that facilitate regulation of gene expression.

Protein-DNA interactions at the H4-site III upstream transcriptional element of a cell cycle regulated histone H4 gene: differences in normal versus tumor cells

Upstream sequences of the H4 histone gene FO108 located between nt -418 to -213 are stimulatory for in vivo transcription. This domain contains one protein/DNA interaction site (H4-Site III) that binds factor H4UA-1. Based on methylation interference, copper-phenanthroline protection, and competition assays, we show that H4UA-1 interacts with sequences between nt -345 to -332 containing an element displaying sequence-similarity with the thyroid hormone response element (TRE). Using gel retardation assays, we also demonstrate that H4UA-1 binding activity is abolished at low concentrations of Zn2+ (0.75 mM), a characteristic shared with the thyroid hormone (TH) receptor DNA binding protein. Interestingly, phosphatase-treatment of nuclear proteins inhibits formation of the H4UA-1 protein/DNA complex, although a complex with higher mobility (H4UA-1b) can be detected; both complexes share identical protein-DNA contacts and competition behaviors. These findings suggest that phosphorylation may be involved in the regulation of H4-Site III protein/DNA interactions by directly altering protein/protein associations. H4-Site III interactions were examined in several cell culture systems during cell growth and differentiation. We find that H4UA-1 binding activity is present during the cell cycle of both normal diploid and transformed cells. However, during differentiation of normal diploid rat calvarial osteoblasts, we observe a selective loss of the H4UA-1/H4-Site III interaction, concomitant with an increase of the H4UA-1b/H4-Site III complex, indicating modifications in the heteromeric nature of protein/DNA interactions during downregulation of transcription at the cessation of proliferation. Transformed cells have elevated levels of H4UA-1, whereas H4UA-1b is predominantly present in normal diploid cells; this alteration in the ratio of H4UA-1 and H4UA-1b binding activities may reflect deregulation of H4-Site III interactions in transformed cells. We propose that H4-Site III interactions may contribute, together with protein/DNA interactions at proximal regulatory sequences, in determining the level of H4-FO108 histone gene transcription.

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.

Multiple mechanisms regulate the proliferation-specific histone gene transcription factor HiNF-D in normal human diploid fibroblasts

The proliferation-specific transcription factor complex HiNF-D interacts with sequence specificity in a proximal promoter element of the human H4 histone gene FO108, designated Site II. The occupancy of Site II by HiNF-D has been implicated in proper transcription initiation and as a component of the cell cycle regulation of this gene. In the present study we have investigated the role of the HiNF-D/Site II interaction in controlling the level of H4 histone gene transcription during modifications of normal cellular growth. HiNF-D binding activity is present at high levels in rapidly proliferating cultures of human diploid fibroblasts and is reduced to less than 2% upon the cessation of proliferation induced by serum deprivation of sparsely population fibroblast cultures. Density-dependent quiescence also abolishes HiNF-D binding activity. Downregulation of transcription from the H4 gene occurs concomitant with the loss of the HiNF-D/Site II interaction, further suggesting a functional relationship between Site II occupancy and the capacity for transcription. Serum stimulation of quiescent preconfluent cells results in an increase in HiNF-D binding activity as the cells are resuming DNA synthesis and H4 histone gene transcription. Density-inhibited quiescent cells respond to serum stimulation with only a minimal increase in the HiNF-D binding activity, 30% of maximal levels. However, H4 histone gene transcription is stimulated to a level equal to that detected in extracts of the sparsely populated serum-stimulated cultures. These results suggest that there is a threshold level of HiNF-D binding activity necessary for the activation of H4 histone gene transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of cell cycle and growth control

The potential biological effects of electric and/or magnetic fields on cells and tissues must be addressed systematically within a context of perturbations in cell cycle control. Such studies should not be pursued in an isolated manner but as a component of the fundamental relationship between proliferation and differentiation, the multi-step process by which structural and functional properties of specialized cells, tissues, and organs progressively develop. It is necessary to quantitatively establish the influence of electric and magnetic fields on the integrated signalling mechanisms which transduce regulatory information for 1) control of the proliferative process and 2) down-regulation of proliferation associated with the initiation of gene expression that mediates the development and maintenance of phenotypic properties characteristic of differentiated cells. We will present an overview of our current understanding of regulatory mechanisms that control proliferation and cell specialization in normal diploid cells with emphasis on rate limiting steps that may be the basis for biological perturbations by electric and magnetic fields. Addressing such questions in normal diploid cells is essential since the loss of growth control in transformed and tumor cells is accompanied by an abrogation of developmental regulatory mechanisms that are functionally coupled to proliferation.

Regulation of transcription-factor activity during growth and differentiation: involvement of the nuclear matrix in concentration and localization of promoter binding proteins

Several lines of evidence are presented which support involvement of the nuclear matrix in regulating the transcription of two genes, histone and osteocalcin, that are reciprocally expressed during development of the osteoblast phenotype. In the 5' regulatory region of an H4 histone gene, which is expressed in proliferating osteoblasts early during the developmental/differentiation sequence, a dual role is proposed for the nuclear matrix binding domain designated NMP-1 (-589 to -730 upstream from the transcription start site). In addition to functioning as a nuclear matrix attachment site, the sequences contribute to the upregulation of histone gene transcription, potentially facilitated by concentration and localization of an 84kD ATF DNA binding protein. A homologous nuclear matrix binding domain was identified in the promoter of the osteocalcin gene, which is expressed in mature osteoblasts in an extracellular matrix undergoing mineralization. The NMP binding domain in the osteocalcin gene promoter resides contiguous to the vitamin D responsive element. Together with gene and transcription factor localization, a model is proposed whereby nuclear matrix-associated structural constraints on conformation of the osteocalcin gene promoter facilitates vitamin D responsiveness mediated by cooperativity at multiple regulatory elements.

Protein/DNA interactions involving ATF/AP1-, CCAAT-, and HiNF-D-related factors in the human H3-ST519 histone promoter: cross-competition with transcription regulatory sites in cell cycle controlled H4 and H1 histone genes

Protein/DNA interactions of the H3-ST519 histone gene promoter were analyzed in vitro. Using several assays for sequence specificity, we established binding sites for ATF/AP1-, CCAAT-, and HiNF-D related DNA binding proteins. These binding sites correlate with two genomic protein/DNA interaction domains previously established for this gene. We show that each of these protein/DNA interactions has a counterpart in other histone genes: H3-ST519 and H4-F0108 histone genes interact with ATF- and HiNF-D related binding activities, whereas H3-ST519 and H1-FNC16 histone genes interact with the same CCAAT-box binding activity. These factors may function in regulatory coupling of the expression of different histone gene classes. We discuss these results within the context of established and putative protein/DNA interaction sites in mammalian histone genes. This model suggests that heterogeneous permutations of protein/DNA interaction elements, which involve both general and cell cycle regulated DNA binding proteins, may govern the cellular competency to express and coordinately control multiple distinct histone genes.

Influence of dexamethasone on the vitamin D-mediated regulation of osteocalcin gene expression

The influence of dexamethasone on expression of the osteocalcin gene which encodes the most abundant non-collagenous and only reported bone-specific protein was examined in ROS 17/2.8 osteosarcoma cells which express a broad spectrum of genes related to bone formation. Consistent with previous reports, quantitation of cellular osteocalcin mRNA levels by Northern blot analysis, osteocalcin gene transcription by activity of the osteocalcin gene promoter fused to a chloramphenicol acetyl-transferase (CAT) mRNA coding sequence following transfection into ROS 17/2.8 cells, and osteocalcin biosynthesis by radioimmunoassay indicate that dexamethasone in a concentration range of 10(-6) to 10(-9) M only modestly modifies basal levels of osteocalcin gene expression. However, dexamethasone significantly inhibits these parameters of the vitamin D-induced upregulation of osteocalcin gene expression in both proliferating and in confluent ROS 17/2.8 cells. In this study, we observed that the extent to which abrogation of the vitamin D response occurs is dependent on basal levels of osteocalcin gene expression as reflected by a complete inhibition of the vitamin D-induced upregulation in a ROS 17/2.8K subline with low basal expression and only a partial reduction of the vitamin D stimulation in a ROS 17/2.8C subline with eightfold higher levels of basal expression. This effect of glucocorticoid appears to be at the transcriptional and post-transcriptional levels as demonstrated by a parallel decline in the cellular representation of osteocalcin mRNA, osteocalcin gene promoter activity, and osteocalcin biosynthesis. The complexity of the glucocorticoid effect on vitamin D-mediated transcriptional properties of the osteocalcin gene is indicated by persistence of sequence-specific protein-DNA interactions at two principal osteocalcin gene promoter regulatory elements, the osteocalcin (CCAAT) box which modulates basal level of transcription, and the vitamin D responsive element, where vitamin D-mediated enhancement of osteocalcin gene transcription is controlled.

Transcriptional element H4-site II of cell cycle regulated human H4 histone genes is a multipartite protein/DNA interaction site for factors HiNF-D, HiNF-M, and HiNF-P: involvement of phosphorylation

Cell cycle regulated gene expression was studied by analyzing protein/DNA interactions occurring at the H4-Site II transcriptional element of H4 histone genes using several approaches. We show that this key proximal promoter element interacts with at least three distinct sequence-specific DNA binding activities, designated HiNF-D, HiNF-M, and HiNF-P. HiNF-D binds to an extended series of nucleotides, whereas HiNF-M and HiNF-P recognize sequences internal to the HiNF-D binding domain. Gel retardation assays show that HiNF-D and HiNF-M each are represented by two distinct protein/DNA complexes involving the same DNA binding activity. These results suggest that these factors are subject to post-translational modifications. Dephosphorylation experiments in vitro suggest that both electrophoretic mobility and DNA binding activities of HiNF-D and HiNF-M are sensitive to phosphatase activity. We deduce that these factors may require a basal level of phosphorylation for sequence specific binding to H4-Site II and may represent phosphoproteins occurring in putative hyper- and hypo-phosphorylated forms. Based on dramatic fluctuations in the ratio of the two distinct HiNF-D species both during hepatic development and the cell cycle in normal diploid cells, we postulate that this modification of HiNF-D is related to the cell cycle. However, in several tumor-derived and transformed cell types the putative hyperphosphorylated form of HiNF-D is constitutively present. These data suggest that deregulation of a phosphatase-sensitive post-translational modification required for HiNF-D binding is a molecular event that reflects abrogation of a mechanism controlling cell proliferation. Thus, phosphorylation and dephosphorylation of histone promoter factors may provide a basis for modulation of protein/DNA interactions and H4 histone gene transcription during the cell cycle and at the onset of quiescence and differentiation.