Osteoblastic cells from rat long bone. II: Adhesion to substrata and integrin expression in primary and propagated cultures

microRNA-25-3p suppresses osteogenic differentiation of BMSCs in patients with osteoporosis by targeting ITGB3

This study was conducted to investigate the impact of the microRNA (miR)-25-3p/ITGB3 axis on the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) from patients with osteoporosis (OP). BMSCs isolated from the bone marrow of healthy controls and OP patients were identified by flow cytometry, in which ITGB3 mRNA and miR-25-3p expression was detected by RT-qPCR and ITGB3, Runx2, OPN, ALP, and OSX protein expression by western blot. The binding between ITGB3 and miR-25-3p was assessed by dual-luciferase reporter gene and Ago2-RIP assays. BMSC osteogenic differentiation was observed by alizarin red staining and ALP activity. The differentiation of BMSCs to adipocytes and chondrocytes was measured by oil red O staining and alcian blue staining, respectively. BMSCs were successfully isolated from the bone marrow of healthy controls (normal-BMSCs) and OP patients (OP-BMSCs). ITGB3, Runx2, OPN, ALP, and OSX expression was poorer and miR-25-3p expression was higher in OP-BMSCs than in normal-BMSCs. Mechanistically, ITGB3 was negatively targeted by miR-25-3p. After osteogenic, adipogenic, and chondrogenic differentiation of BMSCs were successfully induced, adipogenic differentiation increased and osteogenic and chondrogenic differentiation decreased in OP-BMSCs compared with normal-BMSCs. Overexpression of ITGB3 facilitated mineralized nodule formation and elevated ALP activity and Runx2, OPN, and ALP expression in OP-BMSCs. miR-25-3p upregulation diminished mineralized nodule formation, ALP activity, and Runx2, OPN, and ALP expression in OP-BMSCs and normal-BMSCs, which was annulled by additional ITGB3 overexpression. miR-25-3p targets ITGB3, thereby suppressing osteogenic differentiation of BMSCs from OP patients.

Review of Integrin-Targeting Biomaterials in Tissue Engineering

The ability to direct cell behavior has been central to the success of numerous therapeutics to regenerate tissue or facilitate device integration. Biomaterial scientists are challenged to understand and modulate the interactions of biomaterials with biological systems in order to achieve effective tissue repair. One key area of research investigates the use of extracellular matrix-derived ligands to target specific integrin interactions and induce cellular responses, such as increased cell migration, proliferation, and differentiation of mesenchymal stem cells. These integrin-targeting proteins and peptides have been implemented in a variety of different polymeric scaffolds and devices to enhance tissue regeneration and integration. This review first presents an overview of integrin-mediated cellular processes that have been identified in angiogenesis, wound healing, and bone regeneration. Then, research utilizing biomaterials are highlighted with integrin-targeting motifs as a means to direct these cellular processes to enhance tissue regeneration. In addition to providing improved materials for tissue repair and device integration, these innovative biomaterials provide new tools to probe the complex processes of tissue remodeling in order to enhance the rational design of biomaterial scaffolds and guide tissue regeneration strategies.

Detachment strength of human osteoblasts cultured on hydroxyapatite with various surface roughness. Contribution of integrin subunits

Hydroxyapatite (HA) has been widely used as a bone substitute in dental, maxillofacial and orthopaedic surgery and as osteoconductive bone substitute or precoating of pedicle screws and cages in spine surgery. The aim of the present study was to investigate the osteoblastic adhesion strength on HA substrata with different surface topography and biochemistry (pre-adsorption of fibronectin) after blocking of specific integrin subunits with monoclonal antibodies. Stoichiometric HA was prepared by precipitation followed by ageing and characterized by SEM, EDX, powder XRD, Raman spectroscopy, TGA, and specific surface area analysis. Human bone marrow derived osteoblasts were cultured on HA disc-shaped substrata which were sintered and polished resulting in two surface roughness grades. For attachment evaluation, cells were incubated with monoclonal antibodies and seeded for 2 h on the substrata. Cell detachment strength was determined using a rotating disc device. Cell detachment strength was surface roughness, fibronectin preadsorption and intergin subunit sensitive.

Mesenchymal stem cells, osteoblasts and extracellular matrix proteins: enhancing cell adhesion and differentiation for bone tissue engineering

Cell adhesion to scaffolds has remained one of the challenges in tissue engineering. Although protein surface modification has been proven to enhance cell adhesion and retention, its specificity depending on cell and biomaterial types means that the best protein and concentration must be established for each specific application. This review focuses on the improvement of cell adhesion for human mesenchymal stem cells with an osteogenesis approach. A brief outline of the cell adhesion process and extracellular matrix proteins precedes an overview of works focused on the adhesion of mesenchymal stem cells and osteoblasts to biomaterials and this effect in their differentiation into osteoblasts.

Extracellular matrix-mimetic adhesive biomaterials for bone repair

Limited osseointegration of current orthopedic biomaterials contributes to the failure of implants such as arthroplasties, bone screws, and bone grafts, which present a large socioeconomic cost within the United States. These implant failures underscore the need for biomimetic approaches that modulate host cell-implant material responses to enhance implant osseointegration and bone formation. Bioinspired strategies have included functionalizing implants with extracellular matrix (ECM) proteins or ECM-derived peptides or protein fragments, which engage integrins and direct osteoblast adhesion and differentiation. This review discusses (1) bone ECM composition and key integrins implicated in osteogenic differentiation, (2) the use of implants functionalized with ECM-mimetic peptides/protein fragments, and (3) growth factor-derived peptides to promote the mechanical fixation of implants to bone and to enhance bone healing within large defects.

Role of MT1-MMP in the osteogenic differentiation

Metalloproteinase MT1-MMP is induced and Pro-MMP-2 up modulated early in rat preosteoblasts (ROB) set to differentiate. We here show that the induction of MMPs, accompanied by activation of Pro-MMP-2, occurs by 6 h of adhesion on endogenous extracellular matrix (ECM), Fibronectin (FN) and Collagen type I (CI). These events do not occur after adhesion on Collagen III (CIII), Vitronectin (VN) or BSA. Within the first hour on inducing substrata or plastic, FAK is unchanged and ERK(1,2), is activated, but this activation is not sufficient for MT1-MMP induction. The function of p38 MAPK and PTKs is not required for the induction by substrata of MMPs. Six hours after plating preosteoblasts on MMP-inducing substrata, complexes of beta1 integrin with MT1-MMP are formed, that contain integrin dimers specifically engaged by the substratum, alpha4 and alpha5 chains for cells plated on FN, and alpha2 chain for cells plated on CI and ECM. Induction of MT1-MMP and its expression during osteogenesis pleiotropically regulate alkaline phosphatase (AP) expression. During differentiation, variant clones derived from preosteoblasts and MMPs-over-expressing osteoblasts show high MT1-MMP level associated with high AP level both persisting in time, while inhibition of MMPs is accompanied by inhibition of AP. Up or down modulation of AP, transcriptionally or by inhibition of the enzyme activity, has no effect on level or timing of expression of MT1-MMP and Pro-MMP-2. The persistence in expression of MT1-MMP during differentiation, and the associated persistence in expression of AP, as well as their inhibition, both impair the formation of nodules and mineral deposition. A transient pattern of expression of MT1-MMP is required for the establishment of nodules, and MT1-MMP decrease is permissive for nodule mineralization. The expression of AP is required for nodule formation and its level modulates the mineralization. MT1-MMP has multiple functions and is implicated in multiple steps of the differentiation process, acting to regulate homeostasis of the osteogenic differentiation.

The response of osteoblast-like cells towards collagen type I coating immobilized by p-nitrophenylchloroformate to titanium

The scaffold surface composition can be altered by the use of surface coatings. The use of thin coatings will give special surface properties, while the bulk properties of the scaffold are preserved. Collagen type I is known to play an important role during cell adhesion as well as osteoblast differentiation. A common way to coat surfaces is the adsorption method. An alternative way is the use of a protein immobilization method like p-nitrophenyl chloroformate. In this study, we investigated the effect of a collagen type I coating and p-nitrophenyl chloroformate as a protein immobilization method on osteoblast adhesion, proliferation, and differentiation. Titanium fiber meshes were treated with sodium hydroxide (NaOH), followed by p-nitrophenyl chloroformate, and coated with collagen type I. Osteoblast-like cells were seeded into the meshes and cultured for 24 days. The cell attachment, proliferation, and differentiation were measured by using Live and Dead assay, cell counting, DNA analysis, alkaline phosphatase activity assay, calcium content measurement, Real Time PCR (QPCR), and scanning electron microscopy (SEM). Results demonstrated that initially less cells were attached to the covalently bounded collagen meshes (NPC-Col) compared with titanium as control (Ti) and adsorbed collagen meshes (ABS-Col). Further, a decreased growth curve of cells cultured on the NPC-Col meshes was observed in comparison with Ti and ABS-Col meshes. The calcium measurements and SEM pictures revealed that all three surfaces showed differentiation of osteoblast-like cells after 8-24 days. On the basis of our results, we conclude that initially less cells were attached to the NPC-Col meshes and that they had a decreased proliferation rate. Further, we conclude that an adsorbed collagen type I coating stimulated the osteoblastic differentiation of rat bone marrow cells.

The behavior of osteoblast-like cells on various substrates with functional blocking of integrin-beta1 and integrin-beta3

This study was designed to examine the influence of integrin subunit-beta1 and subunit-beta3 on the behavior of primary osteoblast-like cells, cultured on calcium phosphate (CaP)-coated and non coated titanium (Ti). Osteoblast-like cells were incubated with specific monoclonal antibodies against integrin-beta1 and integrin-beta3 to block the integrin function. Subsequently, cells were seeded on Ti discs, either non coated or provided with a 2 microm carbonated hydroxyapatite coating using Electrostatic Spray Deposition. Results showed that on CaP coatings, cellular attachment was decreased after a pre-treatment with either anti-integrin-beta1 or anti-integrin-beta3 antibodies. On Ti, cell adhesion was only slightly affected after a pre-treatment with anti-integrin-beta3 antibodies. Scanning electron microscopy showed that on both types of substrate, cellular morphology was not changed after a pre-treatment with either antibody. With quantitative PCR, it was shown for both substrates that mRNA expression of integrin-beta1 was increased after a pre-treatment with either anti-integrin-beta1 or anti-integrin-beta3 antibodies. Furthermore, after a pre-treatment with either antibody, mRNA expression of integrin-beta3 and ALP was decreased, on both types of substrate. In conclusion, osteoblast-like cells have the ability to compensate to great extent for the blocking strategy as applied here. Still, integrin-beta1 and beta3 seem to play different roles in attachment, proliferation, and differentiation of osteoblast-like cells, and responses on CaP-coated substrates differ to non coated Ti. Furthermore, the influence on ALP expression suggests involvement of both integrin subunits in signal transduction for cellular differentiation.

Mediation of biomaterial-cell interactions by adsorbed proteins: a review

An appropriate cellular response to implanted surfaces is essential for tissue regeneration and integration. It is well described that implanted materials are immediately coated with proteins from blood and interstitial fluids, and it is through this adsorbed layer that cells sense foreign surfaces. Hence, it is the adsorbed proteins, rather than the surface itself, to which cells initially respond. Diverse studies using a range of materials have demonstrated the pivotal role of extracellular adhesion proteins--fibronectin and vitronectin in particular--in cell adhesion, morphology, and migration. These events underlie the subsequent responses required for tissue repair, with the nature of cell surface interactions contributing to survival, growth, and differentiation. The pattern in which adhesion proteins and other bioactive molecules adsorb thus elicits cellular reactions specific to the underlying physicochemical properties of the material. Accordingly, in vitro studies generally demonstrate favorable cell responses to charged, hydrophilic surfaces, corresponding to superior adsorption and bioactivity of adhesion proteins. This review illustrates the mediation of cell responses to biomaterials by adsorbed proteins, in the context of osteoblasts and selected materials used in orthopedic implants and bone tissue engineering. It is recognized, however, that the periimplant environment in vivo will differ substantially from the cell-biomaterial interface in vitro. Hence, one of the key issues yet to be resolved is that of the interface composition actually encountered by osteoblasts within the sequence of inflammation and bone regeneration.

The attachment and growth behavior of osteoblast-like cells on microtextured surfaces

In previous studies, we showed that the application of microgrooves on a surface can direct cellular morphology and the deposition of mineralized matrix of osteoblast-like cells (Biomaterials 20 (1999) 1293; Clin. Oral Impl Res. 11 (2000) 325). In this study, we evaluated the attachment and growth behavior of these cells, using scanning- and transmission electron microscopy (SEM/TEM). Smooth and microgrooved polystyrene substrates were made (groove depth 0.5-1.5 microm, groove- and ridge width 1-10 microm). On these substrates, osteoblast-like cells were cultured for periods up to 16 days. SEM showed that the cells, and their extensions, closely followed the surface on smooth and wider grooved (>5 microm) substrates. In contrast, narrow grooves (<2 microm) were bridged. After 16 days of incubation, the matrix showed extensive deposition of collagen fibrils, and the formation of calcified nodules. With TEM it was shown that on the smooth and wider grooved substrates, focal adhesions were spread throughout the surface. However, on narrow grooves focal adhesions were always positioned on the edges of surface ridges only. Apparently, most extracellular matrix (ECM) was produced by the cells that directly adhered to the substrate. Deposition of ECM was seen in the surface grooves, as well as in between the cell layers. On basis of the current study and previous experiments, we conclude that microgrooves are able to influence bone cell behavior by (1) determining the alignment of cells and cellular extensions, (2) altering the formation and placement of cell focal adhesions, and (3) altering ECM production. Therefore, microgrooved surfaces seem interesting to be applied on bone-anchored implants.

Effect of fibronectin- and collagen I-coated titanium fiber mesh on proliferation and differentiation of osteogenic cells

The objective of this study was to evaluate the effects of fibronectin and collagen I coatings on titanium fiber mesh on the proliferation and osteogenic differentiation of rat bone marrow cells. Three main treatment groups were investigated in addition to uncoated titanium fiber meshes: meshes coated with fibronectin, meshes coated with collagen I, and meshes coated first with collagen I and then subsequently with fibronectin. Rat bone marrow cells were cultured for 1, 4, 8, and 16 days in plain and coated titanium fiber meshes. In addition, a portion of each of these coating treatment groups was cultured in the presence of antibodies against fibronectin and collagen I integrins. To evaluate cellular proliferation and differentiation, constructs were examined for DNA, osteocalcin, and calcium content and alkaline phosphatase activity. There were no significant effects of the coatings on cellular proliferation as indicated by the DNA quantification analysis. When antibodies against fibronectin and collagen I integrins were used, a significant reduction (p < 0.05) in cell proliferation was observed for the uncoated titanium meshes, meshes coated with collagen, and meshes coated with collagen and fibronectin. The different coatings also did not affect the alkaline phosphatase activity of the cells seeded on the coated meshes. However, the presence of antibodies against fibronectin or collagen I integrins resulted in significantly delayed expression of alkaline phosphatase activity for uncoated titanium meshes, meshes coated with collagen, and meshes coated with collagen and fibronectin. Calcium measurements did not reveal a significant effect of fibronectin or collagen I coating on calcium deposition in the meshes. Also, no difference in calcium content was observed in the uncoated titanium meshes and meshes coated with fibronectin when antibodies against fibronectin or collagen I integrins were present. Meshes coated with both collagen I and fibronectin showed significantly higher calcium content when cultured in the presence of antibodies to collagen and fibronectin integrins. A similar phenomenon was also observed for collagen-coated meshes cultured in the presence of antibodies to fibronectin integrins. No significant differences in osteocalcin content were observed between the treatment groups. However, all groups exposed to antibodies against fibronectin integrins showed a significant decrease in osteocalcin content on day 16. These results show that a fibronectin or collagen I coating does not stimulate the differentiation of rat bone marrow cells seeded in a titanium fiber mesh.

A jet impingement investigation of osteoblastic cell adhesion

When designing dental and orthopedic implants, it is important to consider phenomena occurring at the microscopic level, particularly at the bone-implant interface. The presence of hard tissue at this interface is essential to implant viability. The integrity of this tissue-biomaterial interface is dependent on appropriate osteoblast functions (adhesion, matrix deposition, etc.) in the immediate area. Researchers have modified various materials with cell-adhesive peptides with the ultimate goal of controlling osteoblast functions. This study used microjet impingement to compare the strength of adhesion of osteoblastic cells (at varying populations) and fibroblasts to peptide-modified substrates in the presence and absence of fetal bovine serum. In the presence of the serum, there was no significant difference in cellular adhesion strength between substrates. In the absence of serum, all cells tested adhered more strongly to underlying substrates, and the strength of cellular adhesion was greater on modified surfaces than on plain glass surfaces. In the absence of serum, second-passage osteoblastic cells generally adhered to substrates more strongly than first-passage osteoblastic cells; fibroblasts adhered similarly to second-passage osteoblastic cells. Fundamental studies such as the present increase the understanding of cell adhesion to various substrates--knowledge that may be ultimately useful in creating an optimal bone-implant interface.

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.

Function of linear and cyclic RGD-containing peptides in osteoprogenitor cells adhesion process

Cell adhesion directly influences cell growth, differentiation and migration as well as morphogenesis, integrity and repair. The extracellular matrix (ECM) elaborated by osteoblast cells constitutes a regulator of the cell adhesion process and then of the related phenomenon. These regulatory effects of ECM are mediated through integrins and some of them are able to bind RGD sequences. The aim of this study was to determine the role of the sequence and the structure of RGD-containing peptides (linear and cyclic) as well as their role in the cell adhesion process. Cell adhesion assays onto ECM proteins coated surfaces were performed using a range of linear and cyclic RGD-containing peptides. We showed a different human osteoprogenitor cell adhesion according to the coating for ECM proteins and for RGD-peptides. Inhibition assays using peptides showed different responses depending on the coated protein. Depending on the amino-acid sequence and the structure of the peptides (cyclic linear), we observed 100% inhibition of cell adhesion onto vitronectin. These results suggest the importance of sequence, structure and conformation of the peptide, which may play a crucial function in the ligand/receptor interaction and/or in the stability of the interaction.

The expression of metalloproteinase-2, -9, and -14 and of tissue inhibitors-1 and -2 is developmentally modulated during osteogenesis in vitro, the mature osteoblastic phenotype expressing metalloproteinase-14

During osteogenesis, in vitro, of tibial-derived rat osteoblasts (ROB) and derived clones, changes occur in the interactions of mature osteoblasts with the endogenous extracellular matrix (ECM) and these culminate in the formation of tridimensional nodules, which become sites of mineral deposition. We investigated if these changes might be mediated by remodeling of ECM, and we focused our study on the neutral metalloproteinases (MMPs), known agents of matrix remodeling, and on their tissue inhibitors (TIMPs). We report that during in vitro differentiation, osteoblasts express the secreted MMP-2 and -9 and the membrane gelatinase MMP-14. These, along with the tissue inhibitors TIMP-1 and -2, are developmentally regulated according to the maturation stage of osteoblasts. Their levels change in a similar association with osteoblast phenotypic maturation in different populations of ROB, which take different times to complete osteogenesis in vitro. MMP-14 expression coincides in both cell populations with the mature osteoblastic phenotype and is localized in the cells forming nodules. MMP-2 and -9 are expressed diffusely in the osteoblast population. Developmentally associated changes in the activation of MMP-2 are detected, associated in their timing with the expression of MMP-14 in both populations of ROB, and MMP-14 activates pro-MMP-2 in vitro. Expression of messenger RNAs (mRNAs) for the three MMPs increases up to the time of nodule formation. At this stage, TIMP-1 mRNA levels are lowest. TIMP-2 mRNA decreases throughout osteogenesis. In situ hybridization in 7-day-old rat tibias shows the strongest expression of MMP-14 among osteogenic cells, in lining osteoblasts on the newly formed trabeculae under the growth plate, and on the endosteal surface of cortical bone. Our data support the concept that the developmentally regulated expression of MMP-14 triggers localized proteolysis within the osteogenic population, concomitant in vitro to nodule formation.

Employing a transgenic animal model to obtain cementoblasts in vitro

BACKGROUND

Proper formation of cementum, a mineralized tissue lining the tooth root surface, is required for development of a functional periodontal ligament. Further, the presence of healthy cementum is considered to be an important criterion for predictable restoration of periodontal tissues lost as a consequence of disease. Despite the significance of cementum to general oral health, the mechanisms controlling development and regeneration of this tissue are not well understood and research has been hampered by the lack of adequate in vitro experimental models.

METHODS

In an effort to establish cementoblast cell populations, without the trappings of a heterogeneous population containing periodontal ligament (PDL) cells, cells were obtained from the root surface of first mandibular molars of OC-TAg transgenic mice. These mice contain the SV40 large T-antigen (TAg) under control of the osteocalcin (OC) promoter. Therefore, only cells that express OC also express TAg and are immortalized in vitro. Based on results of prior in situ studies, OC is expressed by cementoblasts during root development, but not by cells within the PDL. Consequently, when populations are isolated from developing molars using collagenase/trypsin digestion, only cementoblasts, not PDL cells, are immortalized and thus, will survive in culture.

RESULTS

The resulting immortalized cementoblast population (OC/CM) expressed bone sialoprotein (BSP), osteopontin (OPN), and OC, markers selective to cells lining the root surface. These cells also expressed type I and XII collagen and type I PTH/PTHrP receptor (PTH1R). In addition to expression of genes associated with cementoblasts, OC/CM cells promoted mineral nodule formation and exhibited a PTHrP mediated cAMP response.

CONCLUSIONS

This approach for establishing cementoblasts in vitro provides a model to study cementogenesis as required to enhance our knowledge of the mechanisms controlling development, maintenance, and regeneration of periodontal tissues.

High-resolution morphometric analysis of human osteoblastic cell adhesion on clinically relevant orthopedic alloys

Understanding the cellular basis of osteoblastic cell-biomaterial interaction is crucial to the analysis of the mechanism of osseointegration, a requirement of long-term orthopedic implant stability. Clinically, the amount of bone ingrowth is variable, and cellular parameters that influence ingrowth have yet to be clearly determined. In this study, two clinically relevant orthopedic alloys, titanium Ti6A14V (Ti) and cobalt-chrome-molybdenum (CC), were used for a comparative analysis of primary human osteoblastic cell adhesion and spreading, where cell adhesion represents the initial interaction between cellular elements and the biomaterial surface. The kinetic profile of adhesion revealed enhanced cell attachment upon rough Ti surfaces relative to rough CC. Using confocal laser scanning microscopy (CLSM), we observed that, during the first 12 h of contact with the substratum, osteoblastic cells were relatively less spread on rough Ti, whereas cells appeared elongated with multiple cellular extensions on rough CC. Focal adhesion contacts, as indicated by vinculin immunostaining, were distributed throughout the cells adhering to Ti, but were relatively sparse and localized to cellular processes on CC. Furthermore, three-dimensional CLSM reconstruction analysis indicated the presence of vinculin at all membrane-to-surface contact points on both Ti and CC. On Ti, these contact points closely followed the surface contour, whereas, on CC, they were restricted to relative topographic peaks only. Actin cytoskeletal reorganization was prominent in cells cultured on Ti, with stress fibers arranged throughout the cell body, whereas, on CC, actin filaments were sparse and localized primarily to cellular extensions. Because cell attachment mechanisms are likely to influence signal transduction and regulation of gene expression, these early differential responses of osteoblastic cells on Ti and CC may have functional implications on subsequent extracellular matrix mineralization and bone ingrowth at the cell-biomaterial interface.

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

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

Effect of bone proteins on human prostate cancer cell lines in vitro

BACKGROUND

Despite the high incidence and serious consequences of skeletal metastasis in prostate cancer patients, the mechanisms involved in establishing secondary lesions in bone are not well-understood. In this study, the role of the mineralized bone matrix in the process of skeletal metastasis was evaluated.

METHODS

Attachment, migration, and proliferation responses of human prostate cancer cells to a crude bone protein extract (CBE) were studied. LNCaP and DU145 cells were utilized in 24-hr attachment assays. Boyden chamber chemotactic assays and cell proliferation assays utilized DU145 cells.

RESULTS

CBE and fibronectin (FN) promoted attachment of DU145 cells, whereas only FN facilitated attachment of LNCaP cells. CBE-mediated adhesion of DU145 cells was reduced by 94% with cycloheximide, by 98% with RGD peptides, and by 94% with an antibody to alphavbeta3. Although DU145 cells migrated toward FN, CBE did not promote migration of DU145 cells. DU145 cells grown in the presence of CBE-containing media demonstrated a significant reduction in cell number by day 4. The antiproliferative effect of CBE was not due to cell toxicity.

CONCLUSIONS

In conclusion, results from this study indicate that mineralized bone proteins promote the attachment of DU145 cells in vitro and suggest that bone proteins may play a key role in vivo during the development of metastatic prostate lesions in bone.

Rat tibial osteoblasts III: propagation in vitro is accompanied by enhancement of osteoblast phenotype

Postproliferative confluent cultures of primary rat tibial osteoblasts (ROB), cultured in medium supplemented with ascorbic acid and beta-glycerophosphate (AS-bGP, differentiation medium) express, in sequence, specific bone markers which identify a succession of maturation stages, and eventually form mineralized noduli. We report an investigation on the effect of extensive proliferation in vitro in unsupplemented medium on the osteogenic potential of mass cultures of ROB. The growth rates of the populations, derived from two independent primary cultures, was constant throughout 110 cumulative population doublings (CPD) in culture. Propagated cells maintained features similar to osteoblasts in primary cultures with respect to serum and anchorage dependence for growth and to the chemokinetic effect on endothelial cells exerted by their conditioned media (CM). Propagated populations, set at confluence in differentiation medium, were tested for the expression of early [alkaline phosphatase (AP)] and late [osteocalcin (OC); bone sialoprotein (BSP); 45Ca incorporation and mineralization] osteogenic markers. We observed an increase, parallel to the increase in CPD, in both the level of maximal expression of AP (enzyme/microgram cellular DNA) and in the frequency of nodules, reaching five- to sixfold (at 78 CPD) and eightfold (at 60 CPD), respectively, the levels of primary cultures. AP expression (enzyme and mRNA) persisted during mineralization and 45Ca incorporation. The time required by propagated cultures for the formation of nodules decreased with increase of CPD, and was reduced to less than one third at 87 CDP. Nodules became mineralized over a similar lapse of time as in primary cultures and were positive by histochemistry for BSP and OC. We also obtained osteogenic clones from two independent cultures after 72 CPD. 90% of these showed an osteoblast phenotype, expressing AP and forming nodules positive for OC and BSP, which mineralized. Timing of formation and frequency of nodules/plated cells in clones was similar to that found in propagated cultures of equivalent CPD. In summary, propagated ROB populations and derived clones showed enhanced osteoblast phenotype, possibly due to an increase in osteogenic cells and enrichment of proliferating mature osteoblasts, consequent to extended propagation in culture.