Cloned endothelial cells from fetal bovine bone

Biofabrication of vasculature in microphysiological models of bone

Bone contains a dense network of blood vessels that are essential to its homoeostasis, endocrine function, mineral metabolism and regenerative functions. In addition, bone vasculature is implicated in a number of prominent skeletal diseases, and bone has high affinity for metastatic cancers. Despite vasculature being an integral part of bone physiology and pathophysiology, it is often ignored or oversimplified inin vitrobone models. However, 3D physiologically relevant vasculature can now be engineeredin vitro, with microphysiological systems (MPS) increasingly being used as platforms for engineering this physiologically relevant vasculature. In recent years, vascularised models of bone in MPSs systems have been reported in the literature, representing the beginning of a possible technological step change in how bone is modelledin vitro. Vascularised bone MPSs is a subfield of bone research in its nascency, however given the impact of MPSs has had inin vitroorgan modelling, and the crucial role of vasculature to bone physiology, these systems stand to have a substantial impact on bone research. However, engineering vasculature within the specific design restraints of the bone niche is significantly challenging given the different requirements for engineering bone and vasculature. With this in mind, this paper aims to serve as technical guidance for the biofabrication of vascularised bone tissue within MPS devices. We first discuss the key engineering and biological considerations for engineering more physiologically relevant vasculaturein vitrowithin the specific design constraints of the bone niche. We next explore emerging applications of vascularised bone MPSs, and conclude with a discussion on the current status of vascularised bone MPS biofabrication and suggest directions for development of next generation vascularised bone MPSs.

Vascularization Strategies in the Prevention of Nonunion Formation

Delayed healing and nonunion formation are major challenges in orthopedic surgery, which require the development of novel treatment strategies. Vascularization is considered one of the major prerequisites for successful bone healing, providing an adequate nutrient supply and allowing the infiltration of progenitor cells to the fracture site. Hence, during the last decade, a considerable number of studies have focused on the evaluation of vascularization strategies to prevent or to treat nonunion formation. These involve (1) biophysical applications, (2) systemic pharmacological interventions, and (3) tissue engineering, including sophisticated scaffold materials, local growth factor delivery systems, cell-based techniques, and surgical vascularization approaches. Accumulating evidence indicates that in nonunions, these strategies are indeed capable of improving the process of bone healing. The major challenge for the future will now be the translation of these strategies into clinical practice to make them accessible for the majority of patients. If this succeeds, these vascularization strategies may markedly reduce the incidence of nonunion formation. Impact statement Delayed healing and nonunion formation are a major clinical problem in orthopedic surgery. This review provides an overview of vascularization strategies for the prevention and treatment of nonunions. The successful translation of these strategies in clinical practice is of major importance to achieve adequate bone healing.

Endothelial cells produce angiocrine factors to regulate bone and cartilage via versatile mechanisms

Blood vessels are conduits distributed throughout the body, supporting tissue growth and homeostasis by the transport of cells, oxygen and nutrients. Endothelial cells (ECs) form the linings of the blood vessels, and together with pericytes, are essential for organ development and tissue homeostasis through producing paracrine signalling molecules, called angiocrine factors. In the skeletal system, ECs - derived angiocrine factors, combined with bone cells-released angiogenic factors, orchestrate intercellular crosstalk of the bone microenvironment, and the coupling of angiogenesis-to-osteogenesis. Whilst the involvement of angiogenic factors and the blood vessels of the skeleton is relatively well established, the impact of ECs -derived angiocrine factors on bone and cartilage homeostasis is gradually emerging. In this review, we survey ECs - derived angiocrine factors, which are released by endothelial cells of the local microenvironment and by distal organs, and act specifically as regulators of skeletal growth and homeostasis. These may potentially include angiocrine factors with osteogenic property, such as Hedgehog, Notch, WNT, bone morphogenetic protein (BMP), fibroblast growth factor (FGF), insulin-like growth factor (IGF), and platelet-derived growth factor (PDGF). Understanding the versatile mechanisms by which ECs-derived angiocrine factors orchestrate bone and cartilage homeostasis, and pathogenesis, is an important step towards the development of therapeutic potential for skeletal diseases.

3D-Microarchitectural patterns of Hyperostosis frontalis interna: a micro-computed tomography study in aged women

Although seen frequently during dissections and autopsies, Hyperostosis frontalis interna (HFI) - a morphological pattern of the frontal bone thickening - is often ignored and its nature and development are not yet understood sufficiently. Current macroscopic classification defines four grades/stages of HFI based on the morphological appearance and size of the affected area; however, it is unclear if these stages also depict the successive phases in the HFI development. Here we assessed 3D-microarchitecture of the frontal bone in women with various degrees of HFI expression and in an age- and sex-matched control group, hypothesizing that the bone microarchitecture bears imprints of the pathogenesis of HFI and may clarify the phases of its development. Frontal bone samples were collected during routine autopsies from 20 women with HFI (age: 69.9 ± 11.1 years) and 14 women without HFI (age: 74.1 ± 9.7 years). We classified the HFI samples into four groups, each group demonstrating different macroscopic type or stage of HFI. All samples were scanned by micro-computed tomography to evaluate 3D bone microarchitecture in the following regions of interest: total sample, outer table, diploe and inner table. Our results revealed that, compared to the control group, the women with HFI showed a significantly increased bone volume fraction in the region of diploe, along with significantly thicker and more plate-like shaped trabeculae and reduced trabecular separation and connectivity density. Moreover, the inner table of the frontal bone in women with HFI displayed significantly increased total porosity and mean pore diameter compared to controls. Microstructural reorganization of the frontal bone in women with HFI was also reflected in significantly higher porosity and lower bone volume fraction in the inner vs. outer table due to an increased number of pores larger than 100 μm. The individual comparisons between the control group and different macroscopic stages of HFI revealed significant differences only between the control group and the morphologically most pronounced type of HFI. Our microarchitectural findings demonstrated clear differences between the HFI and the control group in the region of diploe and the inner table. Macroscopic grades of HFI could not be distinguished at the level of bone microarchitecture and their consecutive nature cannot be supported. Rather, our study suggests that only two different types of HFI (moderate and severe HFI) have microstructural justification and should be considered further. It is essential to record HFI systematically in human postmortem subjects to provide more data on the mechanisms of its development.

Netrin-4 derived from murine vascular endothelial cells inhibits osteoclast differentiation in vitro and prevents bone loss in vivo

Bone is a highly vascularized organ, thus angiogenesis is a vital process during bone remodeling. However, the role of vascular systems in bone remodeling is not well recognized. Here we show that netrin-4 inhibits osteoclast differentiation in vitro and in vivo. Co-cultures of bone marrow macrophages with vascular endothelial cells markedly inhibited osteoclast differentiation. Adding a neutralizing antibody, or RNA interference against netrin-4, restored in vitro osteoclast differentiation. Administration of netrin-4 prevented bone loss in an osteoporosis mouse model by decreasing the osteoclast number. We propose that vascular endothelial cells interact with bone in suppressing bone through netrin-4.

Bone vascularization in normal and disease conditions

Bone vasculature is essential for many processes, such as skeletal development and growth, bone modeling and remodeling, and healing processes. Endothelium is an integral part of bone tissue, expressing a physiological paracrine function via growth factors and chemokines release, and interacting with several cellular lines. Alterations of the complex biochemical interactions between vasculature and bone cells may lead to various clinical manifestations. Two different types of pathologies result: a defect or an excess of bone vasculature or endothelium metabolism. Starting from the molecular basis of the interactions between endothelial and bone cells, the Authors present an overview of the recent acquisitions in the physiopathology of the most important clinical patterns, and the modern therapeutic strategies for their treatments.

In vitro models for the evaluation of angiogenic potential in bone engineering

Blood vessels have a fundamental role both in skeletal homeostasis and in bone repair. Angiogenesis is also important for a successful bone engineering. Therefore, scaffolds should be tested for their ability to favour endothelial cell adhesion, proliferation and functions. The type of endothelial cell to use for in vitro assays should be carefully considered, because the properties of these cells may depend on their source. Morphological and functional relationships between endothelial cells and osteoblasts are evaluated with co-cultures, but this model should still be standardized, particularly for distinguishing the two cell types. Platelet-rich plasma and recombinant growth factors may be useful for stimulating angiogenesis.

Kinetic study of early regenerative effects of RGTA11, a heparan sulfate mimetic, in rat craniotomy defects

We previously reported that RGTA, a synthetic heparan sulfate mimetic, induces almost complete closure of craniotomy defects one month after surgery in adult rats. RGTA-treated wounds showed features suggesting unusual cell and matrix interactions reminiscent of developmental events. As healing success or failure is determined shortly after wounding, we examined early events in RGTA-treated wounds. Collagen plasters soaked in a solution of RGTA11 (1.5 Microg per piece) or saline (control) were implanted in rat craniotomy defects. Seven control and seven treated rats were killed daily from days 1 to 7 after surgery. The lesions and adjacent tissues were sampled and processed for morphometry. A layer of type III collagen along the dura mater (DM) thickened up to day 5 in RGTA-treated wounds (p < 0.05 vs day 1), but became thinner in control wounds. Alkaline phosphatase-positive osteoprogenitor cells were detected on day 1 in this layer. Their number increased, and they migrated toward the mid-sagittal sinus and to connective tissue adjacent to the sinus, where they aggregated and differentiated into osteoblasts, forming bone nodules on day 6. These features were not seen in control wounds. Angiogenesis was significantly enhanced in RGTA-treated wounds, especially near the sinus. In vitro, bovine bone endothelial (BBE) cell proliferation was inhibited by RGTA11 in a concentration-dependent manner. In contrast, RGTA11 strongly enhanced the effect of fibroblast growth factor-2 on BBE cell proliferation. These results show that RGTA11, possibly by interacting with heparin-binding growth factors, elicits vascular reactions accompanying the recruitment of a large pool of committed osteoprogenitors from the DM. The DM and the sinus appear to be important centers of organization for craniotomy defect healing. RGTA probably creates an environment that starts a program of directing healing towards bone formation and defect closure.

Analysis of the role of chemokines in angiogenesis

Chemokines, a large family of inflammatory cytokines, have been shown to play a critical role in the regulation of angiogenesis during several pathophysiologic processes, such as tumor growth, wound healing and ischemia. Semiquantitative or quantitative angiogenesis assays are commonly utilized to screen the angiogenic or angiostatic activity of chemokines. These include in vitro endothelial cell activation assays and ex vivo or in vivo models of neovascularization. Chemokines may exert their regulatory activity on angiogenesis directly or as a consequence of leukocyte infiltration and/or the induction of growth factor expression. The effect of chemokines on endothelium can be assessed by performing in vitro assays on purified endothelial cell populations or by in vivo assays. Nevertheless, each model used to evaluate the angiogenic or angiostatic activity of a discrete factor has advantages and limitations. Thus, in order to avoid under- or overestimating the regulatory effect of chemokines on angiogenesis and to evaluate all aspects of the angiogenic process, multiple assays are usually performed. This review summarizes past and recent studies on chemokines as modulators of angiogenesis with particular emphasis on the methods currently used for the assessment of chemokine-mediated angiogenic or angiostatic responses.

Oxidation-reduction (redox) controls fetal hypoplastic lung growth

INTRODUCTION

The persistent morbidity and mortality of congenital diaphragmatic hernia are largely due to associated pulmonary hypoplasia. We have shown previously that three antioxidants (vitamin C, glutathione, and vitamin E) could accelerate the growth of fetal hypoplastic lungs grown in culture. We hypothesize that this occurs via a reductant mechanism.

METHODS

Timed-pregnant rats were gavage-fed nitrofen (100 mg) on day 9.5 of gestation (term = day 22). Fetal lungs were harvested on day 13.5 and placed in organ culture containing serum-free BGJb medium with antibiotics. After randomization, the lung organ cultures were divided into a control group (n = 31) and an experimental group that received the antioxidant N-acetylcysteine (NAC, 100 microM, n = 31). The fetal lung organ cultures were grown for 4 days at 37 degrees C with 5% CO(2). Computer-assisted digital tracings of the airways were performed daily on live, unstained specimens, and lung bud count, perimeter, and area were measured. After 4 days, lungs were pooled, homogenized, and assayed for reduced and oxidized glutathione, normalized to protein, as an estimate of the tissue redox potential. Data were expressed as means +/- SEM, and statistical comparisons were performed using Student's unpaired t test, with P < 0.05 considered significant.

RESULTS

Area, perimeter, lung bud count, and complexity (as measured by the perimeter/square root of area) were all significantly increased with NAC treatment from day 2 onward. Reduced glutathione levels were significantly increased following NAC administration (67.1 +/- 5.8 versus 37.5 +/- 4.2 micromol/mg, P = 0.0004). The ratio of reduced to oxidized glutathione was 2.23.

CONCLUSIONS

N-Acetylcysteine stimulates nitrofen-induced hypoplastic fetal lung growth in organ culture and increases the ratio of reduced to oxidized glutathione. These data support the concept that oxidation-reduction (redox) may be an important control mechanism for fetal lung growth.

Transforming growth factor-beta induces expression of receptor activator of NF-kappa B ligand in vascular endothelial cells derived from bone

Vascular endothelial cells in bone are thought to have significant roles on pathological bone resorption such as bone metastasis and hypercalcemia because this resorption is often seen where blood vessels are abundant. However, the detailed mechanisms have not yet been elucidated. Here, we focused on transforming growth factor-beta (TGF-beta) and studied its effects on vascular endothelial cells because TGF-beta is abundantly stored in bone matrix and is released and activated during bone resorption. We found that TGF-beta up-regulated the expression of receptor activator of NF-kappa B ligand (RANKL) mRNA and protein in bone marrow-derived endothelial cells and in primary vascular endothelial cells but not in osteoblasts. Further analysis revealed that TGF-beta promoted phosphorylation of cAMP response element-binding protein and p38. Protein kinase A inhibitor KT5720 and p38 inhibitor SB203580 significantly reduced the TGF-beta-induced RANKL expression. Moreover, we found two CRE-like domains in murine RANKL promoter region that were critical for TGF-beta-dependent RANKL expression. Therefore, protein kinase A and p38 signaling pathways are involved in TGF-beta-induced RANKL expression by stimulating transcription factors that bind to the CRE-like domains. Our findings indicate that TGF-beta stimulates osteoclastogenesis by promoting RANKL expression in endothelial cells under pathological conditions.

Receptor activator of NF-kappa B and osteoprotegerin expression by human microvascular endothelial cells, regulation by inflammatory cytokines, and role in human osteoclastogenesis

The receptor activator of NF-kappaB (RANKL) is the essential signal required for full osteoclast (OC) development, activation, and survival. RANKL is highly expressed in areas of trabecular bone remodeling and inflammatory bone loss, is increased on marrow stromal cells or osteoblasts by osteotropic hormones or cytokines, and is neutralized by osteoprotegerin (OPG), a soluble decoy receptor also crucial for preventing arterial calcification. Vascular endothelial cells (VEC) are critically involved in bone development and remodeling and influence OC recruitment, formation, and activity. Although OCs develop and function in close association with bone VEC and sinusoids, signals mediating their interactions are not well known. Here, we show for the first time that human microvascular endothelial cells (HMVEC) express transcripts for both RANKL and OPG; inflammatory cytokines tumor necrosis factor-alpha and interleukin-1alpha elevate RANKL and OPG expression 5-40-fold in HMVEC (with an early OPG peak that declines as RANKL rises), and RANKL protein increases on the surface of tumor necrosis factor-alpha-activated HMVEC. Cytokine-activated HMVEC promoted the formation, fusion, and bone resorption of OCs formed in co-cultures with circulating human monocytic precursors via a RANKL-mediated mechanism fully antagonized by exogenous OPG. Furthermore, paraffin sections of human osteoporotic fractured bone exhibited increased RANKL immunostaining in vivo on VEC located near resorbing OCs in regions undergoing active bone turnover. Therefore, cytokine-activated VEC may contribute to inflammatory-mediated bone loss via regulated production of RANKL and OPG. VEC-derived OPG may also serve as an autocrine signal to inhibit blood vessel calcification.

In vitro expression of proalpha1(I) collagen mRNA by human pre-osteoclastic cells

In vitro studies have demonstrated that the extracellular matrix modulates the cell phenotype. In the present study we have investigated in vitro proalpha1(I) collagen mRNA expression in a human pre-osteoclastic cell line (FLG 29.1 cells) in basal condition and after various stimuli. In addition, in order to evaluate the effect of cell-cell interactions on collagen type I mRNA expression, we have cultured the human pre-osteoclastic cells FLG 29.1 with either the human osteoblast-like cell line Saos-2 or the bovine bone endothelial cell line BBE. We showed that the FLG 29.1 cells express proal (I) collagen mRNA, whose expression is modulated by phorbol esters (TPA). Co-culturing FLG 29.1 cells with either Saos-2 or BBE cells induced decrease of proalpha1(I) collagen mRNA expression.

Apoptosis induced by sulindac sulfide in epithelial and mesenchymal cells from human abdominal neoplasms

We investigated whether the therapeutic action of sulindac, used for the treatment of familial adenomatous polyposis, desmoid tumors, and against colon cancer, could be mediated by its active metabolite, sulindac sulfide, in cell growth and apoptosis on cell lines derived from abdominal neoplasms. Sulindac sulfide actions on cell growth and apoptosis were evaluated in epithelial human colon tumor 8 (HCT8) cell line and mesenchymal cell lines (bovine bone endothelial (BBE) cell line, desmoid tumor-derived cells, human colorectal cancer-derived fibroblasts). Sulindac sulfide (0.1-60 microg/ml) induced a dose-dependent inhibition of cell proliferation of all cell lines tested. Apoptosis was induced at doses of 20 and 40 microg/ml, respectively, in BBE and HCT8 cells with no effect on desmoid tumor cells and colorectal cancer-derived fibroblasts. Since mesenchymal cells respond to clinically effective concentrations of the compound, its preferential action on the stromal compartment of intestinal polyps, desmoid tumors and colon cancer can be proposed, with consequent regression of the tumor.

Effects of angiotensin II on bone cells in vitro

Other than its known effects on the cardiovascular system, angiotensin II (Ang II) stimulates cell growth in several cell types. In this study, we examined whether it also might affect bone cell metabolism. Ang II stimulated DNA and collagen synthesis and decreased alkaline phosphatase (AP) activity in bone cell populations derived from the periosteum of fetal rat calvariae. Similar effects of Ang II were observed on human adult bone cells obtained by collagenase digestion from trabecular bone. Clonal cell analysis, autoradiographic studies, and receptor subtype analysis suggested the presence of specific Ang II receptor subtype 1 (AT1) binding sites on AP+ osteoblastic precursor cells. Ang II had no direct effects on osteoblastic cells with a mature phenotype, but paracrine effects of Ang II on mature osteoblasts could be observed upon coculture with Ang II-responsive bone cell populations. Because Ang II is known to be locally generated by endothelial cells, Ang II might play an important role in coordinating capillary cell growth and osteoblastic bone formation during bone remodeling.

The synergistic activity of alphavbeta3 integrin and PDGF receptor increases cell migration

Integrins and growth factor receptors act synergistically to modulate cellular functions. The alphavbeta3 integrin and the platelet-derived growth factor receptor have both been shown to play a positive role in cell migration. We show here that a platelet derived growth factor-BB gradient stimulated migration of rat microvascular endothelial cells on vitronectin (9.2-fold increase compared to resting cells) in a alphavbeta3 and RGD-dependent manner. In contrast, this response was not observed on a beta1 integrin ligand, laminin; background levels of migration, in response to a platelet derived growth factor-BB gradient, were observed on this substrate or on bovine serum albumin (2.4- or 2.0-fold, respectively). Comparable results were obtained using NIH-3T3 cells. Platelet derived growth factor-BB did not change the cells' ability to adhere to vitronectin, nor did it stimulate a further increase in proliferation on vitronectin versus laminin. In addition, platelet derived growth factor-BB stimulation of NIH-3T3 cells did not alter the ability of alphavbeta3 to bind RGD immobilized on Sepharose. The alphavbeta3 integrin and the platelet derived growth factor receptor-beta associate in both microvascular endothelial cells and NIH-3T3 cells, since they coprecipitated using two different antibodies to either alphavbeta3 or to the platelet derived growth factor receptor-beta. In contrast, beta1 integrins did not coprecipitate with the platelet derived growth factor receptor-beta. These results point to a novel pathway, mediated by the synergistic activity of alphavbeta3 and the platelet derived growth factor receptor-beta, that regulates cell migration and, therefore, might play a role during neovessel formation and tissue infiltration.

HPE cells: a clonal endothelial cell line established from human parathyroid tissue (human parathyroid cell line)

We report the culture and cloning of human endothelial cells derived from parathyroid tissue surgically removed from a patient affected by Multiple Endocrine Neoplasia Type 1 syndrome. These cells, known as HPE, have been isolated and maintained in culture by serial passages for more than 15 months. The clonal cell line grows in a medium containing serum substitutes which favour endothelial cell growth. HPE cells replicate with a mean doubling time of 120 h, showing typical functional and morphological features of endothelial cells, such as uptake of acetylated low density lipoprotein and positive reaction for Factor VIII-Related Antigen. Basic fibroblast growth factor, vascular endothelial growth factor, insulin-like growth factor type I and ascorbic acid stimulate cell proliferation, whereas transforming growth factor beta and heparin act as inhibitory factors. Prostaglandin E2, secretin and epinephrine increased cAMP production, while human parathyroid hormone, histamine and glucagon were inert. Cells were found to express pro-collagen alpha 1 (type I) mRNA. In HPE cells Restriction Fragments Length Polymorphism and PCR analysis did not show allelic loss at chromosome 11q12-13, known to be a typical feature of MEN 1 parathyroid tumors. These cells are the first example of an established normal human clonal cell line with an endothelial phenotype.

Histamine receptors and bioeffects on clonal parathyroid endothelial cells

Using a clonal line of bovine parathyroid endothelial cells (BPE-1) we defined the presence on these cells of a histamine H2 receptor and characterized its pharmacological properties. Interaction of histamine with its receptor induced an increase of cAMP accumulation in a dose- and time-dependent fashion. This effect appears unique for parathyroid endothelial cells, in fact, clonal parathyroid epithelial cells did not exhibit a similar response. No effect of histamine was observed on BPE-1 cell proliferation.

Increase of vascular endothelial growth factor mRNA expression by 1,25-dihydroxyvitamin D3 in human osteoblast-like cells

Vascular endothelial growth factor (VEGF), a secreted endothelial cell-specific mitogen, is produced in endocrine organs and regulated by trophic hormones. Because angiogenesis and osteogenesis are closely regulated, we studied whether human osteoblast-like cells produce VEGF, and if so, what factors regulate VEGF mRNA expression. Human osteoblast-like cells (HObLC) derived from trabecular bone explants were cultured in alpha-MEM supplemented with 10% fetal calf serum. Northern blot analysis revealed that HObLC expressed VEGF mRNA, as did several human osteosarcoma cells. 1,25-(OH)2D3 increased the steady-state levels of VEGF mRNA in a time- and concentration-dependent manner in HObLC and one of the osteosarcoma cell lines, SaOS-2, accompanied by an increase in the concentration of immunoreactive VEGF in the conditioned medium. PTH and IGF-I also increased the level of VEGF mRNA in HObLC and SaOS-2 cells. Furthermore, 12-O-tetradecanoylphorbol ester stimulated VEGF mRNA in a time-and concentration-dependent manner. The VEGF mRNA expression induced by 1,25-(OH)2D3 was completely inhibited by H-7, but only partially by staurosporine. We have demonstrated that PTH, IGF-I, and most potently 1,25-(OH)2D3 stimulate the mRNA expression and secretion of VEGF in human osteoblast-like cells, suggesting that one of the anabolic effects of 1,25-(OH)2D3 on skeletal tissue may be mediated by VEGF produced by osteoblasts.

Functional and structural interactions between osteoblastic and preosteoclastic cells in vitro

Osteoblasts are involved in the bone resorption process by regulating osteoclast maturation and activity. In order to elucidate the mechanisms underlying osteoblast/preosteoclast cell interactions, we developed an in vitro model of co-cultured human clonal cell lines of osteoclast precursors (FLG 29.1) and osteoblastic cells (Saos-2), and evaluated the migratory, adhesive, cytochemical, morphological, and biochemical properties of the co-cultured cells. In Boyden chemotactic chambers, FLG 29.1 cells exhibited a marked migratory response toward the Saos-2 cells. Moreover, they preferentially adhered to the osteoblastic monolayer. Direct co-culture of the two cell types induced: (1) positive staining for tartrate-resistant acid phosphatase in FLG 29.1 cells; (2) a decrease of the alkaline phosphatase activity expressed by Saos-2 cells; (3) the appearance of typical ultrastructural features of mature osteoclasts in FLG 29.1 cells; (4) the release into the culture medium of granulocyte-macrophage colony stimulating factor. The addition of parathyroid hormone to the co-culture further potentiated the differentiation of the preosteoclasts, the cells tending to fuse into large multinucleated elements. These in vitro interactions between osteoblasts and osteoclast precursors offer a new model for studying the mechanisms that control osteoclastogenesis in bone tissue.

In vitro structural and functional relationships between preosteoclastic and bone endothelial cells: a juxtacrine model for migration and adhesion of osteoclast precursors

The role of vascularization in the process of bone resorption has not been clarified. The interactions between vascular endothelium and osteoclast progenitors were analyzed using clonal cell lines of bone-derived endothelial and preosteoclastic cells. Insulin-like growth factor I is a major chemotactic stimulator of preosteoclastic cell migration mediated by bone endothelial cells. Osteoclast precursors rapidly adhered to bone endothelial monolayers. This phenomenon appeared to be cell-specific and mediated through the binding of vitronectin and fibronectin receptors to fibronectin. In addition, direct contact with bone endothelial cells induced osteoclast progenitors to differentiate into more mature elements, with the tendency to cluster together to form large multinucleated cells. These findings demonstrated specific in vitro interactions between bone endothelial cells and osteoclast progenitors, offering a new model for understanding the molecular mechanisms which direct the processes of osteoclast recruitment and ontogeny.

Prostaglandin-stimulated second messenger signaling in bone-derived endothelial cells is dependent on confluency in culture

New bone formation is associated with an increase in blood flow by the invasion of capillaries. Endothelial cells that line the capillaries can produce paracrine factors that affect bone growth and development, and in turn, could be affected by products produced by bone cells, in particular the osteoblasts. Since osteoblasts produce prostaglandins E2 and F2 alpha (PGE2, PGF2 alpha), it was investigated if these PGs were agonists to bone-derived endothelial cells (BBE) by assessing changes in cAMP and free cytosolic calcium concentration ([Ca2+]i) second messenger generation. We found that confluent cultures of BBE cells, a clonal endothelial cell line derived from bovine sternal bone, responded to 1 microM PGE2 by an increase in cAMP. PGF2 alpha at the same concentration was less potent in stimulating an increase in cAMP production in confluent BBE cells. Subconfluent cells with a morphology similar to that of fibroblastic cells were not as sensitive to PGE2-stimulated cAMP generation. PGF2 alpha failed to elicit any cAMP production in subconfluent cultures. PGE2 and PGF2 alpha both stimulated an increase in [Ca2+]i concentration in a dose-dependent manner. The potency of PGE2 was similar to that of PGF2 alpha in stimulating an increase in [Ca2+]i. The Ca2+ response was mostly independent of extracellular Ca+, was unchanged even with prior indomethacin treatment, was unaffected by caffeine pretreatment, but was abolished subsequent to thapsigargin pretreatment. The PG-induced increase in [Ca2+]i was also dependent on the confluency of the cells. In a subconfluent state, the responses to PGE2, or PGF2 alpha were either negligible, or only small increases in [Ca2+]i were noted with high concentrations of these two PGs. Consistent, dose-dependent increases in [Ca2+]i were stimulated by these PGs only when the cells were confluent and had a cobblestoned appearance. Since it was previously demonstrated that BBE cells respond to parathyroid hormone (PTH) by the production of cAMP, we tested if bovine PTH(1-34) amide ]bPTH(1-34) also increased [Ca2+]i in these cells. No change in [Ca2+]i was found in response to bPTH (1-34), although bPTH (1-34) stimulated a nine to tenfold increase in cAMP. We conclude that BBE cells respond to PGE2 and PGF2 alpha but not to bPTH(1-34) by an increase in [Ca2+]i probably secondary to stimulation of phospholipase C and that the cAMP and [Ca2+]i second messenger responses in BBE cells are dependent on the state of confluency of the cells.

Role of vascular endothelial cells in bone biology

Bone development and remodeling depend on complex interactions between bone-forming osteoblasts, bone-degrading osteoclasts, and other cells present within the bone microenvironment. Balanced control of bone formative and degradative processes is normally carefully maintained in the adult skeleton but becomes uncoupled in the course of aging or in various pathological disease states. Systemic regulators of bone metabolism and local mediators, including matrix molecules, cytokines, prostaglandins, leukotrienes, and other autocrine or paracrine factors, regulate the recruitment, differentiation, and function of cells participating in bone formation and turnover. Although some of these interactions are now understood, many yet remain to be elucidated. Recent studies have begun exploring in detail how vascular endothelial cells and their products function in bone physiology. The findings are revealing that bone vascular endothelial cells may be members of a complex communication network in bone which operates between endothelial cells, osteoblasts, osteoclasts, macrophages, stromal cells, and perhaps other cell types found in bone as well. Therefore, multiple systemic and locally produced signals may be received, transduced, and integrated by individual cells and then propagated by the release from these cells of further signals targeted to other members of the bone cell network. In this manner, bone cell activities may be continuously coordinated to afford concerted actions and rapid responses to physiological changes. The bone microvasculature may play a pivotal role in these processes, both in linking circulatory and local signals with cells of the bone microenvironment and in actively contributing itself to the regulation of bone cell physiology.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization, regulation, and function of specific cell membrane receptors for insulin-like growth factor I on bone endothelial cells

It is now widely accepted that insulin-like growth factor-I (IGF-I) has a local regulatory role in bone remodeling. IGF-I has also been demonstrated to regulate proliferation of bone-derived endothelial cells. Such studies suggest a role of IGF-I in skeletal angiogenesis. Using BBE cells, a bovine bone endothelial cell line, we characterized the kinetics and chemical properties of IGF-I receptors and examined the effect of IGF-I on bone endothelium migration. Two classes of binding sites with high affinity for IGF-I were detected by binding experiments on bone endothelial cells. Both competition analyses and cross-linking studies revealed the presence of type I IGF receptor in bone endothelial cells. Moreover, these cells produced and released authentic IGF-I into the medium, as evidenced by radioimmunoassay analyses of gel-filtered conditioned media. Both IGF-I binding capacity and release decreased either with increases in cell number or after treatment with 17 beta-estradiol (17 beta E2) and parathyroid hormone (PTH). Both hormones also inhibited chemotactic responses of bone endothelial cells to IGF-I. Taken together, these results strongly suggest that IGF-I, a growth factor that promotes the proliferation of various bone cell types, also induces growth and chemotactic responses in bone endothelium acting through the type I IGF receptor. This may be part of a generalized response of bone cells to IGF-I that facilitates cell migration.

Bone endothelial cells as estrogen targets

In the present study, we investigated the effects of estrogens on bone endothelial cell metabolism and the presence of estrogen binding sites in the same cells. For these studies, we have used a continuous cell line of clonal bovine bone endothelial cells for evidence of a direct response to estrogens in vitro. Receptor analysis to intact viable cells was steroid specific and saturable, with an apparent dissociation constant of 17.2 nM and a Bmax of 3.2 x 10(4) sites/cell. Northern blot analysis revealed a 6.5-kilobase mRNA that hybridized with a cDNA to human estrogen receptor. The 6.5-kilobase size is in close agreement with the reported size of the human estrogen receptor mRNA. In vitro estrogen responses of bone endothelial cells included a stimulation of cell proliferation as well as an inhibition of parathyroid hormone responsiveness. These findings clearly demonstrate the presence of functional estrogen receptors in bone endothelial cells in vitro, suggesting a role of estrogens in bone angiogenesis and in the entire process of bone remodeling.

Decreased cultured endothelial cell proliferation in high glucose medium is reversed by antioxidants: new insights on the pathophysiological mechanisms of diabetic vascular complications

Exposure to hyperglycemia slows the rate of proliferation of cultured human endothelial cells. Recently, it has been reported that glucose may autoxidize generating free radicals which have been hypothesized to delay cell replication time. To test whether oxidative stress has an effect on delaying cell replication time in hyperglycemic conditions, human endothelial cells cultured from umbilical veins were incubated in 5 or 20 mM glucose, either alone or in the presence of one of three different antioxidants: superoxide dismutase (SOD), catalase and glutathione (GSH). Cells grown in medium with 5 mM glucose, with or without antioxidants, yielded similar population doubling times and cell cycle phase distributions. Significantly lower growth parameters were observed in cells grown in medium with 20 mM glucose, without antioxidants. The presence of the antioxidant reverted them to almost normal growth. These data show that high glucose levels may delay endothelial cells replication time through the generation of free radicals, suggesting a possible pathophysiological linkage between the high levels of glucose and the development of microvascular complications of diabetes, possibly suggesting a new therapeutic approach to prevent such complications.

Platelet-derived growth factor is angiogenic in vivo

PDGF receptors have recently been found to be expressed in microvascular endothelium in vivo under circumstances of endothelial cell activation and angiogenesis suggesting that PDGF may have a direct effect on endothelial cells. We have tested the angiogenic activity of PDGF-AA and -BB homodimers in the chick chorioallantoic membrane in vivo. PDGF-BB was found to consistently induce an angiogenic response whereas PDGF-AA was less active. Morphological analyses revealed that there was little inflammation associated with this response but an increase in vessel density suggested a direct effect of PDGF on embryonic chorioallantoic endothelial cells. In vitro, PDGF-BB was found to be more potent than PDGF-AA in stimulating the chemotaxis of rat brain capillary endothelial cells. This is consistent with a direct effect of PDGF on endothelial cells. Thus, this novel angiogenic activity of PDGF has implications for several developmental and pathological events in which PDGF, particularly the B-chain, is expressed.

Partial characterization of rat marrow stromal cells

Fibroblast-like rat marrow stromal cell (CFU-F) cultures have been characterized in terms of their responsiveness to calciotropic hormones, metal ions, the nonsteroidal antiinflammatory drug, and by their putative paracrine role in the maintenance of active populations of osteoblasts at the marrow-bone interface. These studies indicate that CFU-Fs lack a complete osteoblast signature. Subconfluent CFU-Fs grown in the presence or absence of 10(-7) M dexamethasone lack receptors for PTH and calcitonin, and fail to show enhanced cAMP or cGMP responses to 10(-7) M 1-34 PTH (rat), or any evidence of osteocalcin production [+/- 10(-9) M 1,25-(OH)2D3]. Low concentrations of fluoride [10(-12) and 10(-9) M] stimulated CFU-F grown in vitro in serum-free media, though higher levels (10(-7) and 10(-6) M), inhibited growth in vivo and in vitro. Aluminum (10(-12)-10(-7) M) and ibuprofen (10(-7) M) did not alter normal growth patterns, indicating an action on bone cells more differentiated than CFU-Fs. Serum-free conditioned medium (CM) from control and ovariectomized (OVX)/OVX+ dihydrotachysterol-Rx rat CFU-F cultures was mitogenic for neonatal rat calvarial osteoblasts in vitro, but not for ROS 17/2.8 cells. The studies affirm the mesenchymal-like character of CFU-Fs and project their significant role in sustaining functional endosteal osteogenic cell populations.

Human microvascular endothelial cells express receptors for platelet-derived growth factor

Endothelial cells have been widely thought to be unresponsive to platelet-derived growth factor (PDGF, a major growth factor released from stimulated platelets at the sites of vascular insults) and devoid of PDGF receptors. Nevertheless, in examining the growth-factor responses of microvascular endothelial cells isolated from human omental adipose tissue, we were surprised to detect PDGF-induced tyrosine phosphorylation of a 180-kDa glycoprotein, subsequently identified as the cellular receptor for PDGF by specific immunoprecipitation. Scatchard analysis of 125I-labeled PDGF binding to human microvascular endothelial cells revealed 30,000 PDGF receptors per cell with a Kd of 0.14 nM. PDGF stimulated tyrosine phosphorylation of PDGF receptors and other cellular proteins in a dose- and time-dependent manner, with half-maximal receptor phosphorylation occurring at 0.3 nM recombinant human PDGF (B chain) and a less than or equal to 1-min exposure to PDGF. Normal cellular consequences of receptor activation were also observed, including tyrosine phosphorylation of a 42-kDa protein and serine phosphorylation of ribosomal protein S6. Furthermore, PDGF was mitogenic for these cells. Microvascular endothelial cells play a central role in neovascularization required for wound healing and solid tumor growth. Thus, the discovery of functional PDGF receptors on human microvascular endothelial cells suggests a direct role for PDGF in this process.

In vitro development of hypertrophic chondrocytes starting from selected clones of dedifferentiated cells

Single cells from enzymatically dissociated chick embryo tibiae have been cloned and expanded in fresh or conditioned culture media. A cloning efficiency of approximately 13% was obtained using medium conditioned by dedifferentiated chondrocytes. A cloning efficiency of only 1.4% was obtained when conditioned medium from hypertrophic chondrocytes was used, and efficiencies of essentially 0 were found with fresh medium or medium conditioned by J2-3T3 mouse fibroblasts. Cell clones were selected by morphological criteria and clones showing a dedifferentiated phenotype (fibroblast-like) were further characterized. Out of 38 clones analyzed, 17 were able to differentiate to the hypertrophic chondrocyte stage and reconstitute hypertrophic cartilage when placed in the appropriate culture conditions. Cells from these clones expressed the typical markers of chondrocyte differentiation, i.e., type II and type X collagens. Clones not undergoing differentiation continued to express only type I collagen. Hypertrophic chondrocytes from differentiating clones were analyzed at the single cell level by immunofluorescence; all the cells were positive for type X collagen, while approximately 50% of them showed positivity for type II collagen.