Regulation of alphaVbeta3 and alphaVbeta5 integrins by dexamethasone in normal human osteoblastic cells

Roles of focal adhesion proteins in skeleton and diseases

The skeletal system, which contains bones, joints, tendons, ligaments and other elements, plays a wide variety of roles in body shaping, support and movement, protection of internal organs, production of blood cells and regulation of calcium and phosphate metabolism. The prevalence of skeletal diseases and disorders, such as osteoporosis and bone fracture, osteoarthritis, rheumatoid arthritis, and intervertebral disc degeneration, increases with age, causing pain and loss of mobility and creating a huge social and economic burden globally. Focal adhesions (FAs) are macromolecular assemblies that are composed of the extracellular matrix (ECM), integrins, intracellular cytoskeleton and other proteins, including kindlin, talin, vinculin, paxillin, pinch, Src, focal adhesion kinase (FAK) and integrin-linked protein kinase (ILK) and other proteins. FA acts as a mechanical linkage connecting the ECM and cytoskeleton and plays a key role in mediating cell-environment communications and modulates important processes, such as cell attachment, spreading, migration, differentiation and mechanotransduction, in different cells in skeletal system by impacting distinct outside-in and inside-out signaling pathways. This review aims to integrate the up-to-date knowledge of the roles of FA proteins in the health and disease of skeletal system and focuses on the specific molecular mechanisms and underlying therapeutic targets for skeletal diseases.

Magnesium cationic cue enriched interfacial tissue microenvironment nurtures the osseointegration of gamma-irradiated allograft bone

Regardless of the advancement of synthetic bone substitutes, allograft-derived bone substitutes still dominate in the orthopaedic circle in the treatments of bone diseases. Nevertheless, the stringent devitalization process jeopardizes their osseointegration with host bone and therefore prone to long-term failure. Hence, improving osseointegration and transplantation efficiency remains important. The alteration of bone tissue microenvironment (TME) to facilitate osseointegration has been generally recognized. However, the concept of exerting metal ionic cue in bone TME without compromising the mechanical properties of bone allograft is challenging. To address this concern, an interfacial tissue microenvironment with magnesium cationc cue was tailored onto the gamma-irradiated allograft bone using a customized magnesium-plasma surface treatment. The formation of the Mg cationic cue enriched interfacial tissue microenvironment on allograft bone was verified by the scanning ion-selective electrode technique. The cellular activities of human TERT-immortalized mesenchymal stem cells on the Mg-enriched grafts were notably upregulated. In the animal test, superior osseointegration between Mg-enriched graft and host bone was found, whereas poor integration was observed in the gamma-irradiated controls at 28 days post-operation. Furthermore, the bony in-growth appeared on magnesium-enriched allograft bone was significant higher. The mechanism possibly correlates to the up-regulation of integrin receptors in mesenchymal stem cells under modified bone TME that directly orchestrate the initial cell attachment and osteogenic differentiation of mesenchymal stem cells. Lastly, our findings demonstrate the significance of magnesium cation modified bone allograft that can potentially translate to various orthopaedic procedures requiring bone augmentation.

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A modified interfacial Mg TME was tailored onto the GI allograft bone matrix without compromising the mechanical properties.

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The SIET were applied to recognize the Mg²⁺-cue enriched interfacial TME on the surface of the Mg-treated bone allograft.

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The rodent model that is analogous to the clinical use of allograft bone were applied to charaterize the osseointegration.

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The boundary of the Mg-enriched allograft bone was already unable to be identified and become homogeneous at D28 post-op.

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The Mg²⁺-cue enriched interfacial TME is able to convince the upregulation of several integrin receptors of MSCs.

The production, detection, and origin of irisin and its effect on bone cells

Irisin is a muscle factor discovered in 2012 that plays an important role in many tissues, including bone. Eight years since its discovery, there are still many controversies regarding its molecular biology, detection, and effects on bone. This article summarizes the points raised to date, and discusses the mechanisms by which irisin regulates bone cells. The information reviewed here provides a useful foundation for future research.

Dexamethasone Reduces Cell Adhesion and Migration of T47D Breast Cancer Cell Line

BACKGROUND

Aberrant expression of cell adhesion molecules and matrix metalloproteinase (MMPs) plays a pivotal role in tumor biological processes including progression and metastasis of cancer cells. Targeting these processes and detailed understanding of their underlying molecular mechanism is an essential step in cancer treatment. Dexamethasone (Dex) is a type of synthetic corticosteroid hormone used as adjuvant therapy in combination with current cancer treatments such as chemotherapy in order to alleviate its side effects like acute nausea and vomiting. Recent evidences have suggested that Dex may have antitumor characteristics.

OBJECTIVE

Dex affects the migration and adhesion of T47D breast cancer cells as well as cell adhesion molecules e.g., cadherin and integrin, and MMPs by regulating the expression levels of associated genes.

METHODS

In this study, we evaluated the cytotoxicity of Dex on the T47D breast cancer cell line through MTT assay. Cell adhesion assay and wound healing assay were performed to determine the impact of Dex on cell adhesion and cell migration, respectively. Moreover, real-time PCR was used to measure the levels of α and β integrin, E-cadherin, N-cadherin, MMP-2, and MMP-9.

RESULTS

Dex decreased the viability of T47D cells in a time and dose-dependent manner. Cell adhesion and migration of T47D cells were reduced upon Dex treatment. The expression of α and β integrin, E-cadherin, N-cadherin, MMP-2, and MMP-9 were altered in response to the Dex treatment.

CONCLUSION

Our findings demonstrated that Dex may have a role in the prevention of metastasis in this cell line.

Actin and Actin-Associated Proteins in Extracellular Vesicles Shed by Osteoclasts

Extracellular vesicles (EVs) are shed by all eukaryotic cells and have emerged as important intercellular regulators. EVs released by osteoclasts were recently identified as important coupling factors in bone remodeling. They are shed as osteoclasts resorb bone and stimulate osteoblasts to form bone to replace the bone resorbed. We reported the proteomic content of osteoclast EVs with data from two-dimensional, high resolution liquid chromatography/mass spectrometry. In this article, we examine in detail the actin and actin-associated proteins found in osteoclast EVs. Like EVs from other cell types, actin and various actin-associated proteins were abundant. These include components of the polymerization machinery, myosin mechanoenzymes, proteins that stabilize or depolymerize microfilaments, and actin-associated proteins that are involved in regulating integrins. The selective incorporation of actin-associated proteins into osteoclast EVs suggests that they have roles in the formation of EVs and/or the regulatory signaling functions of the EVs. Regulating integrins so that they bind extracellular matrix tightly, in order to attach EVs to the extracellular matrix at specific locations in organs and tissues, is one potential active role for actin-associated proteins in EVs.

Modulation of αvβ3 Integrin via Transactivation of β3 Integrin Gene on Murine Bone Marrow Macrophages by 1,25(OH)2D3, Retinoic Acid and Interleukin-4

The interleukin (IL)-4, 1,25(OH)₂D₃ and retinoic acid, increase surface expression of functional integrin α_vβ₃ on murine osteoclast precursors. All three agonists stimulate transcription of the β₃ gene, leading to increased steady-state levels of mRNA this protein. By contrast, mRNA levels of α_v remain unchanged. In each instance, the increase in the surface expression of the integrin results in increased migration of the cells onto an α_vβ₃ substrate. Because β₃ subunit, except platelet where β₃ subunit conform a dimer with α_IIb, associates solely with α_v subunit monogamously, while promiscuous α_v subunit combines with various subunit, our present data support the idea that the β₃ subunit governs the surface-expressed functional integrin complex.

Non-invasive assessment of inter-and intrapatient variability of integrin expression in metastasized prostate cancer by PET

Purpose

Due to the high expression of the integrin αvβ3 not only on endothelial cells, but also on mature osteoclasts and prostate cancer cells, imaging of osseous metastases with αvβ3-targeted tracers seems promising. However, little is known about the patterns of αvβ3-expression in metastasized prostate cancer lesions in-vivo. Thus we evaluated the uptake of the αvβ3-specific PET tracer [¹⁸F]Galacto-RGD for assessment of bone metastases in prostate cancer patients.

Results

[¹⁸F]Galacto-RGD PET identified 58/74 bone-lesions (detection rate of 78.4%) and lymph node metastases in 2/5 patients. The SUV_mean was 2.12+/−0.94 (range 0.70–4.38; tumor/blood 1.36+/−0.53; tumor/muscle 2.82+/−1.31) in bone-lesions and 2.21+/−1.18 (range 0.75–3.56) in lymph node metastases. Good visualization and detection of bone metastases was feasible due to a low background activity of the surrounding normal bone tissue.

Methods

12 patients with known metastasized prostate cancer according to conventional staging (including bone-scintigraphy and contrast-enhanced CT; median PSA 68.63 ng/ml, range 3.72-1935) were examined with PET after i.v.-injection of [¹⁸F]Galacto-RGD. Two blinded nuclear-medicine physicians evaluated the PET-scans in consensus concerning lesion detectability. Volumes-of-interest were drawn in the PET-scans over all metastases defined by conventional staging (maximum of 11 lesions/patient), over the left ventricle, liver and muscle and standardized-uptake-values (SUVs) were calculated.

Conclusions

Our data show generally elevated uptake of [¹⁸F]Galacto-RGD in bone metastases from prostate cancer with a marked inter- and intrapatient variability. While [¹⁸F]Galacto-RGD PET is inferior to bone scintigraphy for detection of osseous metastases, it might be valuable in patient screening and monitoring of αvβ3-targeted therapies due to the high variability of αvβ3-expression.

Driving mesenchymal stem cell differentiation from self-assembled monolayers

The utilization of self-assembled monolayer (SAM) systems to direct Mesenchymal Stem Cell (MSC) differentiation has been covered in the literature for years, but finding a general consensus pertaining to its exact role over the differentiation of stem cells had been rather challenging. Although there are numerous reports on surface functional moieties activating and inducing differentiation, the results are often different between reports due to the varying surface conditions, such as topography or surface tension. Herein, in view of the complexity of the subject matter, we have sought to catalogue the recent developments around some of the more common functional groups on predominantly hard surfaces and how these chemical groups may influence the overall outcome of the mesenchymal stem cells (MSC) differentiation so as to better establish a clearer underlying relationship between stem cells and their base substratum interactions.

Graphical illustration showing the functional groups that drive MSC differentiation without soluble bioactive cues within the first 14 days.

Synergistic influence of collagen I and BMP 2 drives osteogenic differentiation of mesenchymal stem cells: A cell microarray analysis

Cell microarrays are a novel platform for the high throughput discovery of new biomaterials. By re-creating a multitude of cell microenvironments on a single slide, this approach can identify the optimal surface composition to drive a desired cell response. To systematically study the effects of molecular microenvironments on stem cell fate, we designed a cell microarray based on parallel exposure of mesenchymal stem cells (MSCs) to surface-immobilised collagen I (Coll I) and bone morphogenetic protein 2 (BMP 2). This was achieved by means of a reactive coating on a slide surface, enabling the covalent anchoring of Coll I and BMP 2 as microscale spots printed by a robotic contact printer. The surface between the printed protein spots was passivated using poly (ethylene glycol) bisamine 10,000Da (A-PEG). MSCs were then captured and cultured on array spots composed of binary mixtures of Coll I and BMP 2, followed by automated image acquisition and quantitative, multi-parameter analysis of cellular responses. Surface compositions that gave the highest osteogenic differentiation were determined using Runx2 expression and calcium deposition. Quantitative single cell analysis revealed subtle concentration-dependent effects of surface-immobilised proteins on the extent of osteogenic differentiation obscured using conventional analysis. In particular, the synergistic interaction of Coll I and BMP 2 in supporting osteogenic differentiation was confirmed. Our studies demonstrate the value of cell microarray platforms to decipher the combinatorial interactions at play in stem cell niche microenvironments.

Dexamethasone increases αvβ3 integrin expression and affinity through a calcineurin/NFAT pathway

The purpose of this study was to determine how dexamethasone (DEX) regulates the expression and activity of αvβ3 integrin. FACS analysis showed that DEX treatment induced expression of an activated αvβ3 integrin. Its expression remained high as long as DEX was present and continued following DEX removal. FACS analysis showed that the upregulation of αvβ3 integrin was the result of an increase in the expression of the β3 integrin subunit. By real time qPCR, DEX treatment induced a 6.2-fold increase (p<0.04) in β3 integrin mRNA by day 2 compared to control and remained elevated for 6days of treatment and then an additional 10days once the DEX was removed. The increase in β3 integrin mRNA levels required only 1day of DEX treatment to increase levels for 4days in the absence of DEX. In contrast, DEX did not alter β1 integrin mRNA or protein levels. The DEX-induced upregulation of β3 integrin mRNA was partly due to an increase in its half-life to 60.7h from 22.5h in control cultures (p<0.05) and could be inhibited by RU486 and cycloheximide, suggesting that DEX-induced de novo protein synthesis of an activation factor was needed. The calcineurin inhibitors cyclosporin A (CsA) and FK506 inhibited the DEX induced increase in β3 integrin mRNA. In summary, the DEX-induced increase in β3 integrin is a secondary glucocorticoid response that results in prolonged expression of αvβ3 integrin and the upregulation of the β3 integrin subunit through the calcineurin/NFAT pathway.

Targeting integrins to promote bone formation and repair

The ageing skeleton experiences a progressive decline in the rate of bone formation, which can eventually result in osteoporosis--a common disease characterized by reduced bone mass and altered bone microarchitecture which can result in fractures. One emerging therapy involves the identification of molecules that target bone-marrow mesenchymal stromal cells (MSCs) and promote their differentiation into osteoblasts, thereby counteracting bone loss. This Review highlights the discovery that some integrins, a family of heterodimeric transmembrane proteins that can interact with matrix proteins and generate intracellular signals, can be targeted to promote homing of MSCs to bone, osteogenic differentiation and bone formation. Specifically, priming of the α(5)β(1) integrin, which is required for osteoblastic differentiation of MSCs, leads to increased bone formation and improved bone repair in mice. Additionally, treatment with a peptidomimetic ligand of the α(4)β(1) integrin coupled to an agent with a high affinity for bone improves the homing of MSCs to bone and promotes osteoblast differentiation and bone formation, leading to increased bone mass in osteopenic mice. Strategies that target key integrins expressed by MSCs might, therefore, translate into improved therapies for age-related bone loss and possibly other disorders.

Inhibition of integrins αv/α5-dependent functions in melanoma cells by an ECD-disintegrin acurhagin-C

Acurhagin-C, a Glu-Cys-Asp (ECD)-disintegrin from Agkistrodon acutus venom, has been reported as an endothelial apoptosis inducer, previously. Here we further evaluate its potential applications in cancer therapy. In vitro assays indicated that acurhagin-C not only may influence the cell viability at higher concentration, but also can potently and dose-dependently decrease cell proliferation in murine B16-F10 melanoma. Otherwise, it also had a dose-dependent inhibition on B16-F10 cell adhesion to extracellular matrices, collagen VI, gelatin B and fibronectin, as well as disturbed transendothelial migration of B16-F10 cell. Morphological study found that acurhagin-C dramatically affected B16-F10 cell adhesion to immobilized fibronectin, leading to the formation of multicellular aggregates with rounded shape. Detected by flow cytometry, acurhagin-C was able to induce B16-F10 cell apoptosis and alter cell cycle distribution through its interactions with integrins αv/α5, and thereafter initiation the apoptotic pathways of caspase-8/-9. Furthermore, acurhagin-C could synergistically enhance the anti-proliferative activity of methotrexate in B16-F10 cells and human melanoma SK-MEL-1 cells, without diminishing the growth of human epidermal melanocytes. Taken together, acurhagin-C proved to be a potent inhibitor of integrin-based functions in melanoma cells by activating the complex apoptotic pathways.

Distinct effects of RGD-glycoproteins on Integrin-mediated adhesion and osteogenic differentiation of human mesenchymal stem cells

The detailed interactions of mesenchymal stem cells (MSCs) with their extracellular matrix (ECM) and the resulting effects on MSC differentiation are still largely unknown. Integrins are the main mediators of cell-ECM interaction. In this study, we investigated the adhesion of human MSCs to fibronectin, vitronectin and osteopontin, three ECM glycoproteins which contain an integrin-binding sequence, the RGD motif. We then assayed MSCs for their osteogenic commitment in the presence of the different ECM proteins. As early as 2 hours after seeding, human MSCs displayed increased adhesion when plated on fibronectin, whereas no significant difference was observed when adhering either to vitronectin or osteopontin. Over a 10-day observation period, cell proliferation was increased when cells were cultured on fibronectin and osteopontin, albeit after 5 days in culture. The adhesive role of fibronectin was further confirmed by measurements of cell area, which was significantly increased on this type of substrate. However, integrin-mediated clusters, namely focal adhesions, were larger and more mature in MSCs adhering to vitronectin and osteopontin. Adhesion to fibronectin induced elevated expression of α₅-integrin, which was further upregulated under osteogenic conditions also for vitronectin and osteopontin. In contrast, during osteogenic differentiation the expression level of β₃-integrin was decreased in MSCs adhering to the different ECM proteins. When MSCs were cultured under osteogenic conditions, their commitment to the osteoblast lineage and their ability to form a mineralized matrix in vitro was increased in presence of fibronectin and osteopontin. Taken together these results indicate a distinct role of ECM proteins in regulating cell adhesion, lineage commitment and phenotype of MSCs, which is due to the modulation of the expression of specific integrin subunits during growth or osteogenic differentiation.

Optimal intensity shock wave promotes the adhesion and migration of rat osteoblasts via integrin β1-mediated expression of phosphorylated focal adhesion kinase

To search for factors promoting bone fracture repair, we investigated the effects of extracorporeal shock wave (ESW) on the adhesion, spreading, and migration of osteoblasts and its specific underlying cellular mechanisms. After a single period of stimulation by 10 kV (500 impulses) of shock wave (SW), the adhesion rate was increased as compared with the vehicle control. The data from both wound healing and transwell tests confirmed an acceleration in the migration of osteoblasts by SW treatment. RT-PCR, flow cytometry, and Western blotting showed that SW rapidly increased the surface expression of α5 and β1 subunit integrins, indicating that integrin β1 acted as an early signal for ESW-induced osteoblast adhesion and migration. It has also been found that a significant elevation occurred in the expression of phosphorylated β-catenin and focal adhesion kinase (FAK) at the site of tyrosine 397 in response to SW stimulation after the increasing expression of the integrin β1 molecule. When siRNAs of integrin α5 and β1 subunit were added, the level of FAK phosphorylation elevated by SW declined. Interestingly, the adhesion and migration of osteoblasts were decreased when these siRNA reagents as well as the ERK1/2 signaling pathway inhibitors, U0126 and PD98059, were present. Further studies demonstrated that U0126 could inhibit the downstream integrin-dependent signaling pathways, such as the FAK signaling pathway, whereas it had no influence on the synthesis of integrin β1 molecule. In conclusion, these data suggest that ESW promotes the adhesion and migration of osteoblasts via integrin β1-mediated expression of phosphorylated FAK at the Tyr-397 site; in addition, ERK1/2 are also important for osteoblast adhesion, spreading, migration, and integrin expression.

Tailored integrin-extracellular matrix interactions to direct human mesenchymal stem cell differentiation

Integrins provide the primary link between mesenchymal stem cells (MSCs) and their surrounding extracellular matrix (ECM), with different integrin pairs having specificity for different ECM molecules or peptide sequences contained within them. It is widely acknowledged that the type of ECM present can influence MSC differentiation; however, it is yet to be determined how specific integrin-ECM interactions may alter this or how they change during differentiation. We determined that human bone marrow-derived mesenchymal stem cells (hMSCs) express a broad range of integrins in their undifferentiated state and show a dramatic, but transient, increase in the level of α5 integrin on day 7 of osteogenesis and an increase in α6 integrin expression throughout adipogenesis. We used a nonfouling polystyrene-block-poly(ethylene oxide)-copolymer (PS-PEO) surface to present short peptides with defined integrin-binding capabilities (RGD, IKVAV, YIGSR, and RETTAWA) to hMSCs and investigate the effects of such specific integrin-ECM contacts on differentiation. hMSCs cultured on these peptides displayed different morphologies and had varying abilities to differentiate along the osteogenic and adipogenic lineages. The peptide sequences most conducive to differentiation (IKVAV for osteogenesis and RETTAWA and IKVAV for adipogenesis) were not necessarily those that were bound by those integrin subunits seen to increase during differentiation. Additionally, we also determined that presentation of RGD, which is bound by multiple integrins, was required to support long-term viability of hMSCs. Overall we confirm that integrin-ECM contacts change throughout hMSC differentiation and show that surfaces presenting defined peptide sequences can be used to target specific integrins and ultimately influence hMSC differentiation. This platform also provides information for the development of biomaterials capable of directing hMSC differentiation for use in tissue engineering therapies.

Effect of blasting treatment and Fn coating on MG63 adhesion and differentiation on titanium: a gene expression study using real-time RT-PCR

Biomaterial surface properties, via alterations in the adsorbed protein layer, and the presence of specific functional groups can influence integrin binding specificity, thereby modulating cell adhesion and differentiation processes. The adsorption of fibronectin, a protein directly involved in osteoblast adhesion to the extracellular matrix, has been related to different physical and chemical properties of biomaterial surfaces. This study used blasting particles of different sizes and chemical compositions to evaluate the response of MG63 osteoblast-like cells on smooth and blasted titanium surfaces, with and without fibronectin coatings, by means of real-time reverse transcription-polymerase chain reaction (qRT-PCR) assays. This response included (a) expression of the α(5), α(v) and α(3) integrin subunits, which can bind to fibronectin through the RGD binding site, and (b) expression of alkaline phosphatase (ALP) and osteocalcin (OC) as cell-differentiation markers. ALP activity and synthesis of OC were also tested. Cells on SiC-blasted Ti surfaces expressed higher amounts of the α(5) mRNA gene than cells on Al(2)O(3)-blasted Ti surfaces. This may be related to the fact that SiC-blasted surfaces adsorbed higher amounts of fibronectin due to their higher surface free energy and therefore provided a higher number of specific cell-binding sites. Fn-coated Ti surfaces decreased α(5) mRNA gene expression, by favoring the formation of other integrins involved in adhesion over α(5)β(1). The changes in α(5) mRNA expression induced by the presence of fibronectin coatings may moreover influence the osteoblast differentiation pathway, as fibronectin coatings on Ti surfaces also decreased both ALP mRNA expression and ALP activity after 14 and 21 days of cell culture.

Phase II study of cilengitide (EMD 121974, NSC 707544) in patients with non-metastatic castration resistant prostate cancer, NCI-6735. A study by the DOD/PCF prostate cancer clinical trials consortium

BACKGROUND

Integrins mediate invasion and angiogenesis in prostate cancer bone metastases. We conducted a phase II study of cilengitide, a selective antagonist of α(v)β(3) and α(v)β(5) integrins, in non-metastatic castration resistant prostate cancer with rising PSA.

METHODS

Patients were observed for 4 weeks with PSA monitoring, and then treated with 2,000 mg IV of cilengitide twice weekly until toxicity/progression. PSA, circulating tumor cells (CTCs) and circulating endothelial cells (CECs) were monitored each cycle with imaging performed every three cycles. Primary end point was PSA decline by ≥ 50%. Secondary endpoints were safety, PSA slope, time to progression (TTP), overall survival (OS), CTCs, CECs and gene expression.

RESULTS

16 pts were enrolled; 13 were eligible with median age 65.5 years, baseline PSA 8.4 ng/mL and median Gleason sum 7. Median of three cycles was administered. Treatment was well tolerated with two grade three toxicities and no grade four toxicities. There were no PSA responses; 11 patients progressed by PSA after three cycles. Median TTP was 1.8 months and median OS has not been reached. Median pre- and on-treatment PSA slopes were 1.1 and 1.8 ng/mL/month. Baseline CTCs were detected in 1/9 patients. CTC increased (0 to 1; 2 pts), remained at 0 (2 pts) or decreased (23 to 0; 1 patient) at progression. Baseline median CEC was 26 (0-61) and at progression, 47 (15-148). Low cell counts precluded gene expression studies.

CONCLUSIONS

Cilengitide was well tolerated but had no detectable clinical activity. CTCs are of questionable utility in non-metastatic prostate cancer.

Col3.6-HSD2 transgenic mice: a glucocorticoid loss-of-function model spanning early and late osteoblast differentiation

The goal of this study was to characterize the bone phenotype and molecular alterations in Col3.6-HSD2 mice in which a 3.6-kb Col1a1 promoter fragment drives 11beta-HSD2 expression broadly in the osteoblast lineage to reduce glucocorticoid signaling. Serum corticosterone was unchanged in transgenic females excluding a systemic effect of the transgene. Adult transgenic mice showed reduced vertebral trabecular bone volume and reduced femoral and tibial sub-periosteal and sub-endosteal areas as assessed by microCT. In adult female transgenic mice, histomorphometry showed that vertebral bone mass and trabecular number were reduced but that osteoblast and osteoclast numbers and the mineral apposition and bone formation rates were not changed, suggesting a possible developmental defect in the formation of trabeculae. In a small sample of male mice, osteoblast number and percent osteoid surface were increased but the mineral apposition bone formation rates were not changed, indicating subtle sex-specific phenotypic differences in Col3.6-HSD2 bone. Serum from transgenic mice had decreased levels of the C-terminal telopeptide of alpha1(I) collagen but increased levels of osteocalcin. Transgenic calvarial osteoblast and bone marrow stromal cultures showed decreased alkaline phosphatase and mineral staining, reduced levels of Col1a1, bone sialoprotein and osteocalcin mRNA expression, and decreased cell growth and proliferation. Transgenic bone marrow cultures treated with RANKL and M-CSF showed greater osteoclast formation; however, osteoclast activity as assessed by resorption of a calcium phosphate substrate was decreased in transgenic cultures. Gene profiling of cultured calvarial osteoblasts enriched in the Col3.6-HSD2 transgene showed modest but significant changes in gene expression, particularly in cell cycle and integrin genes. In summary, Col3.6-HSD2 mice showed a low bone mass phenotype, with decreased ex vivo osteogenesis. These data further strengthen the concept that endogenous glucocorticoid signaling is required for optimal bone mass acquisition and highlight the complexities of glucocorticoid signaling in bone cell lineages.

Involvement of osteopontin upregulation on mesangial cells growth and collagen synthesis induced by intermittent high glucose

Glucose fluctuations are strong predictor of diabetic vascular complications. We explored the effects of constant and intermittent high glucose on the proliferation and collagen synthesis of cultured rat mesangial cells. Furthermore, the possible involvement of osteopontin (OPN) was assessed. In rat mesangial cells cultured in 5, 25, or 5 mmol/L alternating with 25 mmol/L glucose in the absence or presence of neutralizing antibodies to OPN, beta3 integrin receptor and beta5 integrin receptor, the cell proliferation, collagen synthesis, and the expression of OPN and type IV collagen were assessed. In cultured mesangial cells, treatment with constant or intermittent high glucose significantly increased [(3)H]thymidine incorporation in a time-dependent manner. A modest increase was observed at 12 h, and further deteriorated afterwards, and reached the maximum incorporation at 48 h. Treatment with constant high glucose for 48 h resulted in significant increases in [(3)H]thymidine incorporation, cell number, [(3)H]proline incorporation, mRNA, and protein levels of type IV collagen and OPN compared with mesangial cells treated with the normal glucose, which were markedly enhanced in cells exposed to intermittent high glucose medium. In addition, neutralizing antibodies to either OPN or its receptor beta3 integrin but not neutralizing antibodies to beta5 integrin can effectively prevented proliferation and collagen synthesis of mesangial cells induced by constant or intermittent high glucose. Intermittent high glucose exacerbates mesangial cells growth and collagen synthesis by upregulation of OPN expression, indicating that glycemic variability have important pathological effects on the development of diabetic nephropathy, which is mediated by the stimulation of OPN expression and synthesis.

Dentin matrix protein-1 isoforms promote differential cell attachment and migration

Dentin matrix protein-1 (DMP1), bone sialoprotein (BSP), and osteopontin (OPN) are three SIBLINGs (small integrin-binding ligand, N-linked glycoproteins) co-expressed/secreted by skeletal and active ductal epithelial cells. Although etiological mechanisms remain unclear, DMP1 is the only one of these three genes currently known to have mutations resulting in human disease, and yet it remains the least studied. All three contain the highly conserved integrin-binding tripeptide, RGD, and experiments comparing the cell attachment and haptotactic migration-enhancing properties of DMP1 to BSP and OPN were performed using human skeletal (MG63 and primary dental pulp cells) and salivary gland (HSG) cells. Mutation of any SIBLING's RGD destroyed all attachment and migration activity. Using its alphaVbeta5 integrin, HSG cells attached to BSP but not to DMP1 or OPN. However, HSG cells could not migrate onto BSP in a modified Boyden chamber assay. Expression of alphaVbeta3 integrin enhanced HSG attachment to DMP1 and OPN and promoted haptotactic migration onto all three proteins. Interchanging the first four coding exons or the conserved amino acids adjacent to the RGD of DMP1 with corresponding sequences of BSP did not enhance the ability of DMP1 to bind alphaVbeta5. For alphaVbeta3-expressing cells, intact DMP1, its BMP1-cleaved C-terminal fragment, and exon six lacking all post-translational modifications worked equally well but the proteoglycan isoform of DMP1 had greatly reduced ability for cell attachment and migration. The sequence specificity of the proposed BMP1-cleavage site of DMP1 was verified by mutation analysis. Direct comparison of the three proteins showed that cells discriminate among these SIBLINGs and among DMP1 isoforms.

Simple application of fibronectin-mimetic coating enhances osseointegration of titanium implants

Integrin-mediated cell adhesion to biomolecules adsorbed onto biomedical devices regulates device integration and performance. Because of the central role of integrin-fibronectin (FN) interactions in osteoblastic function and bone formation, we evaluated the ability of FN-inspired biomolecular coatings to promote osteoblastic differentiation and implant osseointegration. Notably, these biomolecular coatings relied on physical adsorption of FN-based ligands onto biomedical-grade titanium as a simple, clinically translatable strategy to functionalize medical implants. Surfaces coated with a recombinant fragment of FN spanning the central cell binding domain enhanced osteoblastic differentiation and mineralization in bone marrow stromal cell cultures and increased implant osseointegration in a rat cortical bone model compared to passively adsorbed arginine-glycine-aspartic acid peptides, serum proteins and full-length FN. Differences in biological responses correlated with integrin binding specificity and signalling among surface coatings. This work validates a simple, clinically translatable, surface biofunctionalization strategy to enhance biomedical device integration.

The effect of integrin-specific bioactive coatings on tissue healing and implant osseointegration

Implant osseointegration, defined as bone apposition and functional fixation, is a requisite for clinical success in orthopaedic and dental applications, many of which are restricted by implant loosening. Modification of implants to present bioactive motifs such as the RGD cell-adhesive sequence from fibronectin (FN) represents a promising approach in regenerative medicine. However, these biomimetic strategies have yielded only marginal enhancements in tissue healing in vivo. In this study, clinical-grade titanium implants were grafted with a non-fouling oligo(ethylene glycol)-substituted polymer coating functionalized with controlled densities of ligands of varying specificity for target integrin receptors. Biomaterials presenting the alpha5beta1-integrin-specific FN fragment FNIII 7-10 enhanced osteoblastic differentiation in bone marrow stromal cells compared to unmodified titanium and RGD-presenting surfaces. Importantly, FNIII 7-10-functionalized titanium significantly improved functional implant osseointegration compared to RGD-functionalized and unmodified titanium in vivo. This study demonstrates that bioactive coatings that promote integrin binding specificity regulate marrow-derived progenitor osteoblastic differentiation and enhance healing responses and functional integration of biomedical implants. This work identifies an innovative strategy for the rational design of biomaterials for regenerative medicine.

Mechanical strain enhances extracellular matrix-induced gene focusing and promotes osteogenic differentiation of human mesenchymal stem cells through an extracellular-related kinase-dependent pathway

Human mesenchymal stem cells (hMSCs) are a population of multipotent bone marrow cells capable of differentiating along multiple lineages, including bone. Our recently published proteomics studies suggest that focusing of gene expression is the basis of hMSC osteogenic transdifferentiation, and that extracellular matrix proteins play an important role in controlling this focusing. Here, we show that application of a 3-5% tensile strain to a collagen I substrate stimulates osteogenesis in the attached hMSCs through gene focusing via a MAP kinase signaling pathway. Mechanical strain increases expression levels of well-established osteogenic marker genes while simultaneously reducing expression levels of marker genes from three alternate lineages (chondrogenic, adipogenic, and neurogenic). Mechanical strain also increases matrix mineralization (a hallmark of osteogenic differentiation) and activation of extracellular signal-related kinase 1/2 (ERK). Addition of the MEK inhibitor PD98059 to reduce ERK activation decreases osteogenic gene expression and matrix mineralization while also blocking strain-induced down-regulation of nonosteogenic lineage marker genes. These results demonstrate that mechanical strain enhances collagen I-induced gene focusing and osteogenic differentiation in hMSCs through the ERK MAP kinase signal transduction pathway.

Type I collagen is a genetic modifier of matrix metalloproteinase 2 in murine skeletal development

Recessive inactivating mutations in human matrix metalloproteinase 2 (MMP2, gelatinase A) are associated with syndromes that include abnormal facial appearance, short stature, and severe bone loss. Mmp2(-/-) mice have only mild aspects of these abnormalities, suggesting that MMP2 function is redundant during skeletal development in the mouse. Here, we report that Mmp2(-/-) mice with additional mutations that render type I collagen resistant to collagenase-mediated cleavage to TC(A) and TC(B) fragments (Col1a1(r/r) mice) have severe developmental defects resembling those observed in MMP2-null humans. Composite Mmp2(-/-);Col1a1(r/r) mice were born in expected Mendelian ratios but were half the size of wild-type, Mmp2(-/-), and Col1a1(r/r) mice and failed to thrive. Furthermore, composite Mmp2(-/-);Col1a1(r/r) animals had very abnormal craniofacial features with shorter snouts, bulging skulls, incompletely developed calvarial bones and unclosed cranial sutures. In addition, trabecular bone mass was reduced concomitant with increased numbers of bone-resorbing osteoclasts and osteopenia. In vitro, MMP2 had a unique ability among the collagenolytic MMPs to degrade mutant collagen, offering a possible explanation for the genetic interaction between Mmp2 and Col1a1(r). Thus, because mutations in the type I collagen gene alter the phenotype of mice with null mutations in Mmp2, we conclude that type I collagen is an important modifier gene for Mmp2. Developmental Dynamics 236:1683-1693, 2007. (c) 2007 Wiley-Liss, Inc.

Serumfreie Kultivierung von Osteoprogenitorzellen und Osteoblasten zur Testung von Biomaterialien

The aim of the study is to describe a model for testing biocompatibility of implant materials. Usually cells do not bind the biomaterial surface itself via integrins but adsorbed proteins of blood or interstitial fluids. To eliminate the influence of serum proteins on cell adhesion to the test materials we cultivated osteoprogenitor cells and osteoblasts with a serum replacement or with fetal calf serum, but seeded them likewise without serum or serum replacement on cell culture polystyrene, sandblasted titanium and titanium coated with the peptide c(RGDfK) or hydroxyapatite (Bonemaster) and determined cell adhesion. In addition, the surfaces were preincubated with the serum proteins albumin, fetuin, fibronectin and vitronectin to examine specifically their influence on cell adhesion. Clearly cell adhesion depended on cell culture conditions and state of differentiation, especially with prominent differences in adhesion to c(RGDfK). Precoating with serum proteins demonstrated that besides fibronectin and vitronectin fetuin can function as an adhesion protein, whereas albumin demonstrated an antiadhesive effect. Depending on the material they affected cell adhesion differently. Although osteoprogenitor cells and osteoblasts could bind to tissue culture polystyrene, titanium and especially hydroxyapatite without mediation of proteins, it has to be taken into consideration that cell spreading and proliferation of cells on a scaffold are more important than adhesion alone and may not be ensured in the absence of adhesion proteins.

Four and half lim protein 2 (FHL2) stimulates osteoblast differentiation

FHL2, a molecule that interacts with many integrins and transcription factors, was found to play an important role in osteoblast differentiation. Overexpression of FHL2 increases the accumulation of osteoblast differentiation markers and matrix mineralization, whereas FHL2 deficiency results in inhibition of osteoblast differentiation and decreased bone formation.

INTRODUCTION

Integrin-matrix interaction plays a critical role in osteoblast function. It has been shown that the cytoplasmic domains of integrin beta subunits mediate signal transduction induced by integrin-matrix interaction. We reasoned that the identification of proteins interacting with beta-cytoplasmic tails followed by analysis of the function of these proteins would enhance our understanding on integrin signaling and the roles of these proteins in osteoblast activities.

MATERIALS AND METHODS

Yeast two hybrid assay was used to identify proteins interacting with the cytoplasmic domain of integrin beta5 subunit. The association of these proteins with integrin alphavbeta5 was confirmed by confocal analysis and co-immunoprecipitation. A stable MC3T3-E1 cells line overexpressing Four and Half Lim Protein 2 (FHL2) and mouse osteoblasts deficient in FHL2 were used to study the roles of FHL2 in osteoblast differentiation and bone formation. Matrix protein expression was determined by mRNA analysis and Western blotting. Matrix mineralization was detected by Alizarin red staining. Alkaline phosphatase activity was also measured. muCT was used to determine bone histomorphometry.

RESULTS AND CONCLUSIONS

FHL2 and actin-binding proteins, palladin and filamin A, were identified as proteins interacting with beta5 cytoplasmic domain. FHL2 co-localized with alphavbeta5 at the focal adhesion sites in association with palladin and filamin A. FHL2 was also present in nuclei. Osteoblasts overexpressing FHL2 exhibited increased adhesion to and migration on matrix proteins. Conversely, FHL2 stimulation of CREB activity was dependent on integrin function because it was inhibited by Gly-Arg-Gly-Asp-Ser (GRGDS) peptide. The expression of osteoblast differentiation markers and Msx2 was upregulated, and bone matrix mineralization was increased in FHL2 overexpressing cells. In contrast, FHL2-deficient bone marrow cells and osteoblasts displayed decreased osteoblast colony formation and differentiation, respectively, compared with wildtype cells. Moreover, FHL2-deficient female mice exhibited greater bone loss than the wildtype littermates after ovariectomy. Thus, FHL2 plays an important role in osteoblast differentiation and bone formation.

Ectopic bone formation in rat marrow stromal cell/titanium fiber mesh scaffold constructs: Effect of initial cell phenotype

Titanium fiber mesh scaffolds have been shown to be a suitable material for culture of primary marrow stromal cells in an effort to create tissue engineered constructs for bone tissue replacement. In native bone tissue, these cells are known to attach to extracellular matrix molecules via integrin receptors for specific peptide sequences, and these attachments can be a source of cell signaling, affecting cell behaviors such as differentiation. In this study, we examined the ability of primary rat marrow stromal cells at two different stages of osteoblastic differentiation to further differentiate into osteoblasts both in vitro and in vivo when seeded on titanium fiber mesh scaffolds either with or without RGD peptide tethered to the surface. In vitro, the tethered RGD peptide resulted in reduced initial cell proliferation. In vivo, there was no effect of tethered RGD peptide on ectopic bone formation in a rat subcutaneous implant model. Scaffold/cell constructs exposed to dexamethasone for 4 days prior to implantation (+dex constructs) resulted in significant bone formation whereas no bone formation was observed in--dex constructs. These results show that the osteoblastic differentiation of marrow stromal cells was not dependent on surface tethered RGD peptide, and that the initial differentiation stage of implanted cells plays an important role in bone formation in titanium fiber mesh bone tissue engineering constructs.

Comparing the Protein Expression Profiles of Human Mesenchymal Stem Cells and Human Osteoblasts Using Gene Ontologies

One of the hallmark events regulating the process of osteogenesis is the transition of undifferentiated human mesenchymal stem cells (hMSCs) found in the bone marrow into mineralized-matrix producing osteoblasts (hOSTs) through mechanisms that are not entirely understood. With recent developments in mass spectrometry and its potential application to the systematic definition of the stem cell proteome, proteins that govern cell fate decisions can be identified and tracked during this differentiation process. We hypothesize that protein profiling of hMSCs and hOSTs will identify potential osteogenic marker proteins associated with hMSC commitment and hOST differentiation. To identify markers for each cell population, we analyzed the expression of hMSC proteins and compared them to that of hOST by two-dimensional gel electrophoresis and two-dimensional liquid chromatography tandem mass spectrometry (2D LC-MS/MS). The 2D LC-MS/MS data sets were analyzed using the Database for Annotation, Visualization and Integrated Discovery (DAVID). Only 34% of the spots in 2D gels were found in both cell populations; of those that differed between populations, 65% were unique to hOST cells. Of the 755 different proteins identified by 2D LCMS/ MS in both cell populations, two sets of 247 and 158 proteins were found only in hMSCs and hOST cells, respectively. Differential expression of some of the identified proteins was further confirmed by Western blot analyses. Substantial differences in clusters of proteins responsible for calcium- based signaling and cell adhesion were found between the two cell types. Osteogenic differentiation is accompanied by a substantial change in the overall protein expression profile of hMSCs. This study, using gene ontology analysis, reveals that these changes occur in clusters of functionally related proteins. These proteins may serve as markers for identifying stem cell differentiation into osteogenic fates because they promote differentiation by mechanisms that remain to be defined.

Enamel matrix derivative is a potent inhibitor of breast cancer cell attachment to bone

This study examined whether enamel matrix derivative (EMD) inhibits the adhesion of cancer cells to bone. A typical breast cancer cell line, MCF-7, was used. Conditioned human osteosarcoma cell (Saos-2) medium was used as extracellular bone matrix (ECBM) to measure cell attachment. MCF-7 cells were incubated on ECBM-coated culture plates with or without soluble EMD, Arg-Gly-Asp (RGD) sequence blocking peptides, recombinant bone sialoprotein (rBSP), or specific integrin antibodies, and the attached cells were quantified using toluidine blue staining. EMD markedly reduced the attachment of MCF-7 cells to ECBM in a dose-dependent manner. An RGD peptide (GRGDSP) and recombinant BSP inhibited cell attachment to the same degree as EMD. Similarly, anti-alphavbeta3 integrin antibody strongly reduced cell attachment, whereas anti-alphavbeta5 and anti-beta1 integrin antibodies had less marked effects on cell attachment. These results show that EMD inhibits MCF-7 cell attachment to a bone matrix and that it might be useful as an anti-adhesive agent for breast cancer cells to bone in vivo.

Steroid hormones regulate gene expression posttranscriptionally by altering the stabilities of messenger RNAs

Hormones exert powerful effects on reproductive physiology by regulating gene expression. Recent discoveries in hormone action emphasize that regulation of gene expression is not restricted to their alterations of the rate of gene transcription. On the contrary, hormonal effects on the stability of a specific mRNA can profoundly alter its steady-state concentration. The mRNAs encoding hormone receptors are commonly regulated by their own hormones to create autoregulatory feedback loops. Negative and positive autoregulatory feedback loops serve to limit or augment hormonal responses, respectively. After introducing the topics of mRNA degradation and regulated stability, this review focuses on steroid hormone effects on mRNA stabilities. Autoregulation of the mRNAs encoding estrogen, progesterone, androgen, and glucocorticoid receptors by the steroid hormones in reproductive tissues is discussed. In addition, steroid hormone effects on the stabilities of many other mRNAs that are important to reproductive biology are reviewed. These include mRNAs that encode gonadotropin hormones, integrins, growth factors, and inflammatory response proteins. Through these posttranscriptional effects, steroid hormones impact the expression of a large population of genes. Studies of the molecular mechanisms of hormonally regulated mRNA stabilities continue to identify critical mRNA sequence elements and their interactions with proteins. Increased understanding of how hormones affect mRNA stability may yield novel approaches to the therapeutic control of hormone effects, including those essential to reproductive physiology in animals.

Beta ig-h3 mediates osteoblast adhesion and inhibits differentiation

betaig-h3 is an extracellular matrix (ECM) protein induced by TGF-beta, and it has motifs interacting with the alpha3beta1, alphavbeta5, and alphavbeta3 integrins. Our previous study shows the role of betaig-h3 in osteoblast differentiation and its involvement in melorheostosis, a rare bone disease. Here we demonstrate that betaig-h3 expression is down-regulated during the early stage of differentiation of the murine preosteoblastic cell line, KS483. The recombinant betaig-h3 and its FAS1 domain significantly inhibited in vitro osteoblast differentiation as evaluated by matrix mineralization/bone nodule formation. Furthermore, inhibition of expression of osteoblast differentiation marker genes [such as type I collagen, alkaline phosphatase, and osteocalcin (OC)] was accompanied by suppression of osteoblast-specific transcription factors, Cbfa1/Runx2 and osterix. Flow cytometric analyses, cell adhesion, and inhibition assays disclosed alphavbeta3 and alphavbeta5 as the principal integrins mediating the adhesion of osteoblastic cells to betaig-h3. The disruption of interactions between betaig-h3 and osteoblasts by a function-blocking antibody specific for alphavbeta3 but not for alphavbeta5 abolished the inhibitory effect of betaig-h3 on osteoblast differentiation. We suggest that these interacting integrins may play an important role in betaig-h3-mediated inhibition of osteoblast differentiation.

Alphavbeta integrins play an essential role in BMP-2 induction of osteoblast differentiation

Both integrins and BMP-2 exert similar effects on osteoblasts. We examined the relationship between the alphav-containing integrins (alphavbeta) and BMP-2 in osteoblast function. BMP-2 stimulates alphavbeta expression. BMP-2 receptors co-localize/overlap with alphavbeta integrins, and the intact function of alphavbeta is essential in BMP-2 activity.

INTRODUCTION

Bone morphogenetic protein (BMP)-2 not only induces osteoblast differentiation and bone matrix mineralization, but also stimulates osteoblast migration on and adhesion to bone matrix proteins. The alphavbeta- and beta1- (alphabeta1) containing integrins mediate osteoblast interaction with many bone matrix proteins and play important roles in osteoblast adhesion, migration, and differentiation. Because alphavbeta integrins and BMP-2 share common effects on osteoblasts, we analyzed their relationship in osteoblast function.

MATERIALS AND METHODS

The effects of BMP-2 on integrin expression were determined by surface labeling/immunoprecipitation and cell adhesion to matrix proteins. Confocal analysis of the immunostained cells and co-immunoprecipitation of cell extracts were used to study the spatial relationship between integrins and BMP-2 receptors. A function-blocking anti-alphavbeta integrin antibody (L230) was employed to investigate the roles of alphavbeta integrins in BMP-2 function.

RESULTS

Human osteoblasts (HOBs) express alphabeta1, alphavbeta3, alphavbeta5, alphavbeta6, and alphavbeta8 integrins at focal adhesion sites. BMP-2 increases the levels of these integrins on osteoblast surface and enhances HOB adhesion to osteopontin and vitronectin. Immunoprecipitation and immunostaining analyses show that BMP-2 receptors co-localize or overlap with alphavbeta and alphabeta1 integrins. Incubation of HOBs with L230 abolishes the antiproliferative effect of BMP-2 and reduces the capacity of BMP-2 to stimulate alkaline phosphatase activity and the expression of osteocalcin, osteopontin, and bone sialoprotein. Furthermore, L230 prevents BMP-2 induction of matrix mineralization. Although BMP-2 retains its receptor-binding capability in the presence of L230, BMP-2 stimulation of Smad signaling is abolished by L230.

CONCLUSION

BMP-2 upregulates the expression of alphavbeta integrins, and these integrins, in turn, play a critical role in BMP-2 function in osteoblasts.

Integrins as linker proteins between osteoblasts and bone replacing materials. A critical review

The adhesion of osteoblasts to substrates is mediated through proteins that have adsorbed to the substrate, providing integrins on the cell membrane with ligands to connect to. The integrins regulate cell behavior through bi-directional signaling pathways. This critical review has the purpose to consider the research that has been performed with osteoblasts, integrins, and bone replacing materials. Until now, most research has been done to investigate the integrin expression of osteoblasts in culture during cellular adhesion. However, it remains difficult to draw general conclusions from this research. Nevertheless, it can be concluded that the used substrates and protein or peptide coatings can influence the integrin expression and cellular behavior. Additional research has to be done to fully understand all the parameters involved in integrin expression, the adhesion of cells to substrates, and the subsequent cellular behavior. For this purpose, model substrates are under development. The signaling pathway is receiving more and more attention, but for biomaterial purposes, too little consideration is paid to the translation of the in vitro results to the in vivo situation, and to practical applications.

Early osteoblastic differentiation induced by dexamethasone enhances adenoviral gene delivery to marrow stromal cells

We investigated the implications of induced osteogenic differentiation on gene delivery in multipotent rat marrow stromal cells (MSCs). Prior to genetic manipulation cells were cultured with or without osteogenic supplements (5x10(-8) M dexamethasone, 160 microM l-ascorbic acid 2-phosphate, and 10 mM beta-glycerophosphate). Comparison of liposome, retroviral, and adenoviral vectors demonstrated that all three vectors could mediate gene delivery to primary rat MSCs. When these vectors were applied in the absence or presence of osteogenic supplements, we found that MSCs differentiated prior to transduction with adenovirus type 5 vectors produced a 300% increase in transgene expression compared to MSCs that were not exposed to osteogenic supplements. This differentiation effect appeared specific to adenoviral mediated gene delivery, since there was minimal increase in retroviral gene delivery and no increase in liposome gene delivery when MSCs were treated with osteogenic supplements. In addition, we also determined this increase in transgene production to occur at a higher concentration of dexamethasone (5x10(-8) M) in the culture medium of MSCs prior to adenoviral transduction. We found that this increased transgene production could be extended to the osteogenic protein, human bone morphogenetic protein 2 (hBMP-2). When delivered by an adenoviral vector, hBMP-2 transgene production could be increased from 1.4 ng/10(5) cells/3 days to 4.3 ng/10(5) cells/3 days by culture of MSCs with osteogenic supplements prior to transduction. These results indicate that the utility of MSCs as a therapeutic protein delivery mechanism through genetic manipulation can be enhanced by pre-culture of these cells with dexamethasone.

Adhesion to Vitronectin and Collagen I Promotes Osteogenic Differentiation of Human Mesenchymal Stem Cells

The mechanisms controlling human mesenchymal stem cells (hMSC) differentiation are not entirely understood. We hypothesized that the contact with extracellular matrix (ECM) proteins normally found in bone marrow would promote osteogenic differentiation of hMSC in vitro. To test this hypothesis, we cultured hMSC on purified ECM proteins in the presence or absence of soluble osteogenic supplements, and assayed for the presence of well-established differentiation markers (production of mineralized matrix, osteopontin, osteocalcin, collagen I, and alkaline phosphatase expression) over a 16-day time course. We found that hMSC adhere to ECM proteins with varying affinity (fibronectin>collagen I≥collagen IV≥vitronectin>laminin-1) and through distinct integrin receptors. Importantly, the greatest osteogenic differentiation occurred in cells plated on vitronectin and collagen I and almost no differentiation took place on fibronectin or uncoated plates. We conclude that the contact with vitronectin and collagen I promotes the osteogenic differentiation of hMSC, and that ECM contact alone may be sufficient to induce differentiation in these cells.

Association of sustained ERK activity with integrin beta3 induction during receptor activator of nuclear factor kappaB ligand (RANKL)-directed osteoclast differentiation

Osteoclast differentiation is a multi-step process that involves cell proliferation, commitment, and fusion. Some adhesion molecules, including integrin alphavbeta3, have been shown to have roles in osteoclast fusion. In the course of studying with pharmacologic agents known to inhibit protein tyrosine kinases of the Src family, we found that radicicol increased cell fusion during receptor activator of nuclear factor kappaB ligand (RANKL)-driven differentiation of osteoclasts at concentrations far below the ones shown to inhibit its targets in previous studies. Treatments of low doses of radicicol to RAW 264.7 cells that undergo osteoclastic differentiation in the presence of RANKL enhanced the RANKL-induced gene expression of integrin beta3 without any effect on the expression of integrin alphav, which was constitutively high. The cell surface level of integrin alphavbeta3 complexes was consequently augmented by radicicol. In addition, sustained ERK and MEK activation was observed in cells treated with both radicicol and RANKL. More importantly, modulation of ERK activity by the MEK inhibitor U0126 or the gene transduction of a constitutively active form of MEK resulted in a suppression and increment, respectively, of integrin beta3 induction by RANKL. Our data indicate that sustained ERK activity is associated with integrin beta3 induction and subsequent cell surface expression of the alphavbeta3 integrin complex, which may contribute to cell fusion during RANKL-directed osteoclastogenesis.

Effect of dexamethasone withdrawal on osteoblastic differentiation of bone marrow stromal cells

Dexamethasone is capable of directing osteoblastic differentiation of bone marrow stromal cells (BMSCs) in vitro, but its effects are not lineage-specific, and sustained exposure has been shown to down-regulate collagen synthesis and induce maturation of an adipocyte subpopulation within BMSC cultures. Such side effects might be reduced if dexamethasone is applied in a regimented manner, but the discrete steps in osteoblastic maturation that are stimulated by dexamethasone are not known. To examine this, dexamethasone was added to medium to initiate differentiation of rat BMSCs cultures and then removed after a varying number of days. Cell layers were analyzed for cell number, rate of collagen synthesis, expression of osteocalcin (OC), bone sialoprotein (BSP) and lipoprotein lipase (LpL), and matrix mineralization. Withdrawal of dexamethasone at 3 and 10 days was found to enhance cell number relative to continuous exposure, but did not affect to decrease collagen synthesis slightly. Late markers of osteoblastic differentiation, BSP expression and matrix mineralization, were also sensitive to dexamethasone and increased systematically with exposure while LpL systematically decreased. These results indicate that dexamethasone acts at both early and late stages to direct proliferative osteoprogenitor cells toward terminal maturation.

Integrin-mediated adhesion of human articular chondrocytes to cartilage

OBJECTIVE

To determine 1) the kinetics and strength of adhesion of human articular chondrocytes to a cut cartilage surface, and 2) the role of specific integrins in mediating such adhesion, using an in vitro model.

METHODS

Human articular chondrocytes isolated from cadaveric donors (mean +/- SD age 38 +/- 13 years) were cultured in high-density or low-density monolayer. Following release from culture with trypsin and a 2-2.5-hour recovery period, chondrocytes were analyzed either for adhesion to cartilage or for integrin expression by flow cytometry.

RESULTS

Following culture in monolayer, adhesion of chondrocytes to cartilage increased with time, from 6-16% at 10 minutes to a maximum of 59-82% at 80-320 minutes. After 80 minutes of adhesion, the resistance of cells to flow-induced shear stress (50% detachment) was approximately 21 Pa. Chondrocyte adhesion to cartilage decreased with pretreatment of cells with monoclonal antibodies that bound to and blocked certain integrins. After an 80-minute incubation time, adhesion of chondrocytes cultured in high-density monolayer decreased from the value of IgG1-treated controls (55%) with blocking of the beta1 integrin subunit (to 23%) or with blocking of alpha 5 beta 1 (to 36%). Following expansion of chondrocytes in low-density monolayer, the mechanisms of adhesion to cartilage were generally similar. After an 80-minute incubation time, adhesion of chondrocytes cultured in low-density monolayer decreased from the value of IgG1-treated controls (62%) with blocking of the beta1 integrin subunit (to 30%) or with blocking of alpha 5 beta 1 (to 44%). Additionally, adhesion of these cells decreased to 46% by blocking of alpha v beta 5, with a similar trend in effect for chondrocytes cultured in high-density monolayer. Blocking of the alpha 1 or alpha 3 integrin subunits or alpha v beta 3 had no detectable effect on adhesion, even though these receptors were detected by flow cytometry.

CONCLUSION

Under the culture and seeding conditions studied, beta1, alpha 5 beta 1, and alpha v beta 5 integrins mediate human chondrocyte adhesion to cartilage. These chondrocyte integrins have a potential role in the initial adhesion and retention of chondrocytes at a cartilage defect site following clinical procedures of chondrocyte transplantation.

Modulation of integrin expression on rat bone marrow cells by substrates with different surface characteristics

Biomaterials have been shown to be able to influence the growth and differentiation of osteogenic cells cultured on the surface. Although the precise mechanisms by which the materials influence osteogenic cells are unclear, it is possible that the materials manipulate the expression of integrins by the cells. We therefore studied the expression of a number of integrins by rat bone marrow (RBM) cells, after culture on culture polystyrene, on machined and grit-blasted titanium, and on calcium phosphate-coated titanium. Integrin expression was studied by FACS analysis. We found a large variation in the expression of integrins by cells in replicate experiments. After culture on polystyrene for 7 days, cells expressed alpha1, alpha2, alpha3, alpha5, alpha6, beta1, and beta3, although some of the subunits were expressed only occasionally. The cells did not express the alpha4 subunit. After culture of RBM cells for 8 days on coated and noncoated titanium substrates, cells always expressed alpha3, alpha5, alpha6, and beta1. The alpha1 and beta3 subunits were only expressed in some of the experiments. Frequently, the expression of alpha5, alpha6, and beta1 was higher on the coated than on the noncoated titanium substrates. Based on our results, we conclude that the studied materials are capable of influencing the expression of integrins by RBM cells cultured on relevant implant materials.

Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human osteoblastic cells

Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-beta function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-beta suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-beta and BMP-2 function in osteoblasts.

The role of alpha(v)beta(3) in prostate cancer progression

Integrin alpha(v)beta(3) is involved in varied cell biological activities, including angiogenesis, cell adhesion, and migration on several extracellular matrix components. Although alpha(v)beta(3) is not typically expressed in epithelial cells, it is expressed in macrophages, activated leukocytes, cytokine-stimulated endothelial cells, osteoclasts, and certain invasive tumors. Interestingly, the adhesion and migration of breast cancer cells on bone matrix are mediated, in part, by alpha(v)beta(3). Similar to breast cancer cells, prostate cancer cells preferentially metastasize to the bone. The biological events that mediate this metastatic pattern of prostate cancer are not well defined. This review discusses the role alpha(v)beta(3) plays in prostate cancer progression, with specific emphasis on bone metastasis and on alpha(v)beta(3) signaling in prostate cancer cells. The data suggest that alpha(v)beta(3), in part, facilitates prostate cancer metastasis to bone by mediating prostate cancer cell adhesion to and migration on osteopontin and vitronectin, which are common proteins in the bone microenvironment. These biological events require the activation of focal adhesion kinase and the subsequent activation of PI-3 kinase/Akt signaling pathway.

The role of alpha(v)beta(3) in prostate cancer progression

Integrin alpha(v)beta(3) is involved in varied cell biological activities, including angiogenesis, cell adhesion, and migration on several extracellular matrix components. Although alpha(v)beta(3) is not typically expressed in epithelial cells, it is expressed in macrophages, activated leukocytes, cytokine-stimulated endothelial cells, osteoclasts, and certain invasive tumors. Interestingly, the adhesion and migration of breast cancer cells on bone matrix are mediated, in part, by alpha(v)beta(3). Similar to breast cancer cells, prostate cancer cells preferentially metastasize to the bone. The biological events that mediate this metastatic pattern of prostate cancer are not well defined. This review discusses the role alpha(v)beta(3) plays in prostate cancer progression, with specific emphasis on bone metastasis and on alpha(v)beta(3) signaling in prostate cancer cells. The data suggest that alpha(v)beta(3), in part, facilitates prostate cancer metastasis to bone by mediating prostate cancer cell adhesion to and migration on osteopontin and vitronectin, which are common proteins in the bone microenvironment. These biological events require the activation of focal adhesion kinase and the subsequent activation of PI-3 kinase/Akt signaling pathway.

The metastasis gene osteopontin: a candidate target for cancer therapy

Malignant tumors are characterized by dysregulated growth control, overcoming of replicative senescence, and metastasis formation. Current therapeutic regimens mostly exert their effects through inhibition of cell cycle progression, leaving two major components of transformation untouched. The cytokine osteopontin is essential for the dissemination of various cancers. Past research has implied several modes in which osteopontin and its main receptors on tumor cells can be suppressed. Osteopontin expression is inhibitable on the levels of gene transcription and the RNA message, and the osteopontin protein can be blocked with antibodies or synthetic peptides. The osteopontin receptor CD44 has been targeted by diverse therapeutic strategies, including cytotoxic and immunotherapeutic approaches. The receptor integrin alpha(V)beta(3) contributes not only to tumor cell dissemination, but also to angiogenesis and osteolysis in bone metastases. Small molecule inhibitors of this receptor are under study as drug candidates. Because receptors and cytokine ligands that mediate metastasis formation are sparsely expressed in the adult healthy organism and are more readily reached by pharmaceuticals than intracellular drug targets they may represent a particularly suitable focus for therapeutic intervention.

Attachment of human periodontal ligament cells to enamel matrix-derived protein is mediated via interaction between BSP-like molecules and integrin alpha(v)beta3

BACKGROUND

Although enamel matrix-derived protein (EMD) can stimulate attachment of human periodontal ligament (HPDL) cells to the root surface, the biological mechanism of this phenomenon is unclear. The purpose of this study was to determine which molecules in EMD are involved in the attachment of HPDL cells, and which types of integrins on the cell surface mediate the interaction between the cells and EMD.

METHODS

HPDL explants were obtained from tooth surfaces extracted from 4 individuals, and cells taken from the individual explants were separately harvested and subcultured through as many as 5 passages. Cells were incubated on EMD-coated culture plates with and without neutral antibodies for integrins or RGD-sequence blocking peptides and stained with toluidine blue. Proteins in EMD that were able to induce cell attachment were identified by incubating sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) replicas with HPDL cells; the cell-binding regions were detected by staining the cells with toluidine blue. Characteristics of the cell-binding proteins in the EMD were identified by Western blot analysis.

RESULTS

It was shown that anti-alpha(v)beta3 antibody and GRGDSP peptide markedly reduced attachment of HPDL cells to EMD. When the cells were incubated with SDS-PAGE replicas, distinct cell attachment was observed at a molecular mass of approximately 55 kDa. The cell-binding ability of this protein was completely blocked by treatment with anti-alpha(v)beta3 antibody or GRGDSP peptide. In the Western blot analysis, the 55 kDa protein was recognized by anti-bone sialoprotein (BSP) antibody as a single band.

CONCLUSIONS

Our study provides the first evidence that the attachment of HPDL cells to EMD can be mediated by interaction between a BSP-like molecule and integrin alpha(v)beta3 on the cell surface.

Hypoxia stimulates osteopontin expression and proliferation of cultured vascular smooth muscle cells: potentiation by high glucose

We examined the effect of hypoxia on proliferation and osteopontin (OPN) expression in cultured rat aortic vascular smooth muscle (VSM) cells. In addition, we determined whether hypoxia-induced increases in OPN and cell proliferation are altered under hyperglycemic conditions. Quiescent cultures of VSM cells were exposed to hypoxia (3% O(2)) or normoxia (18% O(2)) in a serum-free medium, and cell proliferation as well as the expression of OPN was assessed. Cells exposed to hypoxia for 24 h exhibited a significant increase in [(3)H]thymidine incorporation followed by a significant increase in cell number at 48 h in comparison with respective normoxic controls. Exposure to hypoxia produced significant increases in OPN protein and mRNA expression at 2 h followed by a gradual decline at 6 and 12 h, with subsequent significant increases at 24 h. Neutralizing antibodies to either OPN or its receptor beta3 integrin but not neutralizing antibodies to beta5 integrin prevented the hypoxia-induced increase in [(3)H]thymidine incorporation. Inhibitors of protein kinase C (PKC) and p38 mitogen-activated protein (MAP) kinase also reduced the hypoxia-induced stimulation of proliferation and OPN synthesis. Exposure to high-glucose (HG) (25 mmol/l) medium under normoxic conditions also resulted in significant increases in OPN protein and mRNA levels as well as the proliferation of VSM cells. Under hypoxic conditions, HG further stimulated OPN synthesis and cell proliferation in an additive fashion. In conclusion, hypoxia-induced proliferation of cultured VSM cells is mediated by the stimulation of OPN synthesis involving PKC and p38 MAP kinase. In addition, hypoxia also enhances the effect of HG conditions on both OPN and proliferation of cultured VSM cells, which may have important implications in the development of diabetic atherosclerosis associated with arterial wall hypoxia.