Calcium phosphate particle induction of metalloproteinase and mitogenesis: effect of particle sizes

Cartilage calcification in osteoarthritis: mechanisms and clinical relevance

Pathological calcification of cartilage is a hallmark of osteoarthritis (OA). Calcification can be observed both at the cartilage surface and in its deeper layers. The formation of calcium-containing crystals, typically basic calcium phosphate (BCP) and calcium pyrophosphate dihydrate (CPP) crystals, is an active, highly regulated and complex biological process that is initiated by chondrocytes and modified by genetic factors, dysregulated mitophagy or apoptosis, inflammation and the activation of specific cellular-signalling pathways. The links between OA and BCP deposition are stronger than those observed between OA and CPP deposition. Here, we review the molecular processes involved in cartilage calcification in OA and summarize the effects of calcium crystals on chondrocytes, synovial fibroblasts, macrophages and bone cells. Finally, we highlight therapeutic pathways leading to decreased joint calcification and potential new drugs that could treat not only OA but also other diseases associated with pathological calcification.

Calcium Signaling and Tissue Calcification

Calcification is a regulated physiological process occurring in bones and teeth. However, calcification is commonly found in soft tissues in association with aging and in a variety of diseases. Over the last two decades, it has emerged that calcification occurring in diseased arteries is not simply an inevitable build-up of insoluble precipitates of calcium phosphate. In some cases, it is an active process in which transcription factors drive conversion of vascular cells to an osteoblast or chondrocyte-like phenotype, with the subsequent production of mineralizing "matrix vesicles." Early studies of bone and cartilage calcification suggested roles for cellular calcium signaling in several of the processes involved in the regulation of bone calcification. Similarly, calcium signaling has recently been highlighted as an important component in the mechanisms regulating pathological calcification. The emerging hypothesis is that ectopic/pathological calcification occurs in tissues in which there is an imbalance in the regulatory mechanisms that actively prevent calcification. This review highlights the various ways that calcium signaling regulates tissue calcification, with a particular focus on pathological vascular calcification.

Adsorption of Proteins on m-CPPD and Urate Crystals Inhibits Crystal-induced Cell Responses: Study on Albumin-crystal Interaction

The biological effects and cellular activations triggered by monosodium urate (MSU) and calcium pyrophosphate dihydrate (monoclinic: m-CPPD) crystals might be modulated by protein coating on the crystal surface. This study is aimed at: (i) Identifying proteins adsorbed on m-CPPD crystals, and the underlying mechanisms of protein adsorption, and (ii) to understand how protein coating did modulate the inflammatory properties of m-CPPD crystals. The effects of protein coating were assessed in vitro using primary macrophages and THP1 monocytes. Physico-chemical studies on the adsorption of bovine serum albumin (BSA) upon m-CPPD crystals were performed. Adsorption of serum proteins, and BSA on MSU, as well as upon m-CPPD crystals, inhibited their capacity to induce interleukin-1-β secretions, along with a decreased ATP secretion, and a disturbance of mitochondrial membrane depolarization, suggesting an alteration of NLRP3 inflammasome activation. Proteomic analysis identified numerous m-CPPD-associated proteins including hemoglobin, complement, albumin, apolipoproteins and coagulation factors. BSA adsorption on m-CPPD crystals followed a Langmuir-Freundlich isotherm, suggesting that it could modulate m-CPPD crystal-induced cell responses through crystal/cell-membrane interaction. BSA is adsorbed on m-CPPD crystals with weak interactions, confirmed by the preliminary AFM study, but strong interactions of BSA molecules with each other occurred favoring crystal agglomeration, which might contribute to a decrease in the inflammatory properties of m-CPPD crystals. These findings give new insights into the pathogenesis of crystal-related rheumatic diseases and subsequently may open the way for new therapeutic approaches.

Microcalcifications in breast cancer: From pathophysiology to diagnosis and prognosis

The implementation of mammographic screening programmes in many countries has been linked to a marked increase in early detection and improved prognosis for breast cancer patients. Breast tumours can be detected by assessing several features in mammographic images but one of the most common are the presence of small deposits of calcium known as microcalcifications, which in many cases may be the only detectable sign of a breast tumour. In addition to their efficacy in the detection of breast cancer, the presence of microcalcifications within a breast tumour may also convey useful prognostic information. Breast tumours with associated calcifications display an increased rate of HER2 overexpression as well as decreased survival, increased risk of recurrence, high tumour grade and increased likelihood of spread to the lymph nodes. Clearly, the presence of microcalcifications in a tumour is a clinically significant finding, suggesting that a detailed understanding of their formation may improve our knowledge of the early stages of breast tumourigenesis, yet there are no reports which attempt to bring together recent basic science research findings and current knowledge of the clinical significance of microcalcifications. This review will summarise the most current understanding of the formation of calcifications within breast tissue and explore their associated clinical features and prognostic value.

Small-sized granules of biphasic bone substitutes support fast implant bed vascularization

The present study investigated the influence of granule size of 2 biphasic bone substitutes (BoneCeramic® 400–700 μm and 500–1000 μm) on the induction of multinucleated giant cells (MNGCs) and implant bed vascularization in a subcutaneous implantation model in rats. Furthermore, degradation mechanisms and particle phagocytosis of both materials were examined by transmission electron microscopy (TEM). Both granule types induced tissue reactions involving primarily mononuclear cells and only small numbers of MNGCs. Higher numbers of MNGCs were detected in the group with small granules starting on day 30, while higher vascularization was observed only at day 10 in this group. TEM analysis revealed that both mono- and multinucleated cells were involved in the phagocytosis of the materials. Additionally, the results allowed recognition of the MNGCs as the foreign body giant cell phenotype. Histomorphometrical analysis of the size of phagocytosed particles showed no differences between the 2 granule types. The results indicate that granule size seems to have impact on early implant bed vascularization and also on the induction of MNGCs in the late phase of the tissue reaction. Furthermore, the results revealed that a synthetic bone substitute material can induce tissue reactions similar to those of some xenogeneic materials, thus pointing to a need to elucidate their “ideal” physical characteristics. The results also show that granule size in the range studied did not alter phagocytosis by mononuclear cells. Finally, the investigation substantiates the differentiation of material-induced MNGCs, which are of the foreign body giant cell type.

Synthesis of spherical calcium phosphate particles for dental and orthopedic applications

Calcium phosphate materials have been used increasingly in the past 40 years as bone graft substitutes in the dental and orthopedic fields. Accordingly, numerous fabrication methods have been proposed and used. However, the controlled production of spherical calcium phosphate particles remains a challenge. Since such particles are essential for the synthesis of pastes and cements delivered into the host bone by minimally-invasive approaches, the aim of the present document is to review their synthesis and applications. For that purpose, production methods were classified according to the used reagents (solutions, slurries, pastes, powders), dispersion media (gas, liquid, solid), dispersion tools (nozzle, propeller, sieve, mold), particle diameters of the end product (from 10 nm to 10 mm), and calcium phosphate phases. Low-temperature calcium phosphates such as monetite, brushite or octacalcium phosphate, as well as high-temperature calcium phosphates, such as hydroxyapatite, β-tricalcium phosphate or tetracalcium phosphate, were considered. More than a dozen production methods and over hundred scientific publications were discussed.

Inflammatory cell response to calcium phosphate biomaterial particles: an overview

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite (HA) and amorphous calcium phosphate (CaP) crystals deposited in an organic matrix. One objective of bone tissue engineering is to mimic the chemical and structural properties of this complex tissue. CaP ceramics, such as sintered HA and beta-tricalcium phosphate, are widely used as bone substitutes or prosthesis coatings because of their osteoconductive properties. These ceramic interactions with tissues induce a cell response that can be different according to the composition of the material. In this review, we discuss inflammatory cell responses to CaP materials to provide a comprehensive overview of mechanisms governing the integration or loosening of implants, which remains a major concern in tissue engineering. A focus on the effects of the functionalization of CaP biomaterials highlights potential ways to increase tissue integration and limit rejection processes.

The Therapeutic Potential of Exogenous Adenosine Triphosphate (ATP) for Cartilage Tissue Engineering

OBJECTIVE

While mechanical stimuli can be used to enhance the properties of engineered cartilage, a promising alternative may be to directly harness the underlying mechanotransduction pathways responsible. Our initial studies on the adenosine triphosphate (ATP)-purinergic receptor pathway demonstrated that stimulation by exogenous ATP improved tissue growth and properties but elicited matrix turnover under high doses (250 µM) potentially due to the accumulation of extracellular inorganic pyrophosphate (ePPi). Therefore, the purpose of this study was to identify the mechanism of ATP-mediated catabolism and determine a therapeutic dose to maximize the anabolic effect.

DESIGN

Isolated bovine articular chondrocytes were seeded in high-density, 3-dimensional culture supplemented with varying doses of ATP for 4 weeks. The effects on biosynthesis, matrix metalloproteinase 13 (MMP-13) protein activity, and PPi accumulation were determined. Separate monolayer experiments were conducted to determine the effect of ePPi on MMP-13 activity.

RESULTS

High doses of ATP resulted in an increase in ePPi accumulation (by 54%) and MMP-13 activity (by 39%). Monolayer experiments confirmed a link between increased ePPi accumulation and MMP-13 activity, which appeared to require calcium and was inhibited by the MEK1/2 inhibitor U0126. Cultures supplemented with 62.5 to 125 µM ATP favored an anabolic response, which represented the therapeutic dose range.

CONCLUSIONS

A therapeutic dose range of exogenous ATP to improve the properties of engineered cartilage has been identified, and a possible catabolic mechanism involving excess PPi was determined. Future research into PPi signal transduction and pathological crystal formation is necessary to maximize the beneficial effect of exogenous ATP on chondrocyte cultures.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

Serum-derived nanoparticles: de novo generation and growth in vitro, and internalization by mammalian cells in culture

AIM

While nanoparticles (NPs) have been shown to form spontaneously in body fluids such as serum, the possible implications of these NPs for cell cultures that use supporting media containing serum remain unclear. To understand the de novo formation of NPs, we delineated their growth characteristics, chemical composition and interaction with cells in culture.

MATERIALS & METHODS

Serum-derived particles were analyzed using a combination of dynamic light scattering, turbidity measurements, spectroscopic techniques and optical/electron microscopies.

RESULTS

NPs were found in serum and in serum-containing medium and they increased in size and number during incubation. The mineral particles, consisting mainly of calcium carbonate phosphate bound to organics such as proteins, underwent an amorphous-to-crystalline transformation with time. Serum-derived particles were internalized by the cells tested, eventually reaching lysosomal compartments.

CONCLUSION

The spontaneous formation of serum-derived NPs and their internalization by cells may have overlooked effects on cultured cells in vitro as well as potential pathophysiological consequences in vivo.

Polymorphonuclear neutrophil response to hydroxyapatite particles, implication in acute inflammatory reaction

Hydroxyapatite (HA) is widely used as a bone substitute or coating biomaterial in bone diseases or prosthesis metal parts. The release of HA particles induces an inflammatory response and, if uncontrolled, could result in implant loss. Among the hallmarks of such inflammatory response is early recruitment of the polymorphonuclear cells (PMNs). The purpose of this work is to investigate the response of PMNs following exposure to HA in terms of secreted mediators. Our study shows that HA particles increase the release of pro-inflammatory mediators such as interleukin-1alpha, as well as chemotactic factors such as interleukin-8, macrophage inflammatory protein-1alpha and macrophage inflammatory protein-1beta. HA also induces an increase in matrix metalloproteinase 9 expression. Taken together, our data demonstrate for the first time that HA is capable of activating PMNs, a phenomenon that could potentially contribute to the onset of implant-associated inflammation.

Effect of particle size on hydroxyapatite crystal-induced tumor necrosis factor alpha secretion by macrophages

Macrophages may promote a vicious cycle of inflammation and calcification in the vessel wall by ingesting neointimal calcific deposits (predominantly hydroxyapatite) and secreting tumor necrosis factor (TNF)alpha, itself a vascular calcifying agent. Here we have investigated whether particle size affects the proinflammatory potential of hydroxyapatite crystals in vitro and whether the nuclear factor (NF)-kappaB pathway plays a role in the macrophage TNFalpha response. The particle size and nano-topography of nine different crystal preparations was analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and gas sorbtion analysis. Macrophage TNFalpha secretion was inversely related to hydroxyapatite particle size (P=0.011, Spearman rank correlation test) and surface pore size (P=0.014). A necessary role for the NF-kappaB pathway was demonstrated by time-dependent I kappaB alpha degradation and sensitivity to inhibitors of I kappaB alpha degradation. To test whether smaller particles were intrinsically more bioactive, their mitogenic activity on fibroblast proliferation was examined. This showed close correlation between TNFalpha secretion and crystal-induced fibroblast proliferation (P=0.007). In conclusion, the ability of hydroxyapatite crystals to stimulate macrophage TNFalpha secretion depends on NF-kappaB activation and is inversely related to particle and pore size, with crystals of 1-2 microm diameter and pore size of 10-50 A the most bioactive. Microscopic calcific deposits in early stages of atherosclerosis may therefore pose a greater inflammatory risk to the plaque than macroscopically or radiologically visible deposits in more advanced lesions.

Carbonated apatite-induced arthropathy: a consideration in cases of polyarthritis

BACKGROUND

A 79-year-old woman was referred for evaluation of her painful and swollen joints. She had a medical history of congestive heart failure, renal insufficiency and peptic ulcer disease. For the past 3 years she had experienced recurrent bouts of debilitating arthritis, lasting approximately 3-4 weeks at a time. The symptoms were most severe in the hands and knees, where the joints were warm, swollen and tender. During each flare-up, the patient was housebound and required therapeutic dosing of nonsteroidal anti-inflammatory drugs and codeine to control joint pain.

INVESTIGATIONS

Physical examination, fine-detailed radiographs of the hands, standing radiographs of the knees, arthrocentesis including cell count and gram stain, compensated polarized light microscopy, alizarin-red staining, X-ray diffraction, scanning and transmission electron microscopy with energy dispersive spectrometry, electron microprobe analysis with energy dispersive spectrometry, Fourier transform infrared spectroscopy, and atomic force microscopy.

DIAGNOSIS

Carbonated-substituted apatite arthropathy.

MANAGEMENT

Both knees were aspirated and large volumes of a straw-colored synovial fluid was removed. The knees were injected with corticosteroid, resulting in excellent symptomatic response.

Monosodium urate and calcium pyrophosphate dihydrate (CPPD) crystals, inflammation, and cellular signaling

Monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD) crystals are responsible for acute synovial inflammation but also contribute to cartilage degradation and bone lesions within the joint. They activate multiple signal transduction pathways leading to cell activation and recruitment. Some signalling pathways are activated by both types of crystals, and other pathways may only be activated by one type depending on cell type, namely neutrophils, monocytes, macrophages, synovial fibroblasts, endothelial cells and chondrocytes. Cascades of activated proteins involve cytoplasmic membrane related proteins (FAK complex, Src family tyrosine kinases), but also MAPK and NF-kB pathways, leading to NO, prostanoid and cytokine production, and protease activation. This review will also focus on potential therapeutic targets related to cellular signalling in MSU and CPPD crystal-induced inflammation.

Importance of the surface area ratio on cytokines production by human monocytes in vitro induced by various hydroxyapatite particles

A possible complication associated with the implantation of hydroxyapatite (HA)-based prosthesis is the release of particles. Those particles can be phagocyted by monocytes that are among the first cells to colonize the inflammatory site. The activated monocytes produce inflammatory mediators, such as cytokines, which cause osteoclasts activation. It has previously been demonstrated using a surface area ratio (ratio of the total surface of the given particles to the surface area of cells) of 1 to 1 that there was a correlation between the expression and production of cytokines induced by HA. The present work studies the effect of physical characteristics of HA particles on the production of various inflammatory cytokines (tumour necrosis factor-alpha, interleukin (IL)-6, and IL-8) and anti-inflammatory cytokine (IL-10). However, the experiments were performed using a surface area ratio of 10 to 1. Our data demonstrate that all the particles, whatever their characteristics, induced a high expression of cytokines but the production was different, meaning that there was a post-transcriptional regulation. The size and sintering temperature seemed to be a characteristics that were less important compared to the shape; the needle particles appeared to induce the most important production of all the cytokines studied.

MMP-2, MMP-9 and their inhibitors TIMP-2 and TIMP-1 production by human monocytes in vitro in the presence of different forms of hydroxyapatite particles

After calcium-phosphates biomaterials based implantation like hydroxyapatite (HA) coating, particles are released in the periprosthetic tissues. Wear-debris induced fibrous membranes contain macrophage subsets that can produce metalloproteinases (MMPs), which are considered to be key enzymes in extra-cellular matrix turnover. Tissue inhibitors of metalloproteinases (TIMPs) are important regulator of MMPs activity. Interleukin-1 mainly produced by monocytes can also regulate MMPs production. In the present work, we have evaluated the effect of HA particles characteristics (size, shape and sintering temperature) on the MMP-2, -9 and their respective inhibitors TIMP-2, -1 production. Our results demonstrate that sintering temperature (that modify crystal size and surface area) have little effect on MMPs and TIMPs production. Non-phagocytable particles induced more MMP-9, although phagocytable particles induced more IL-1beta release. The shape of the particles was the most important factor since needle-shaped particles induced the most significant up-regulated expression of MMPs and IL-1beta.

Basic calcium phosphate crystals stimulate the endocytotic activity of cells—inhibition by anti-calcification agents

Pathological calcifications are associated with many medical conditions including diabetes, breast cancer, and crystals-associated osteoarthritis. The deposition of calcium-containing crystals on cells induces detrimental cellular effects and speeds up the progression of associated diseases. We carried out the present study to test the hypotheses that calcium-containing crystals may stimulate the influx of other molecules existing in the extracellular fluid disturbing normal molecular signaling and that anti-calcification agent will inhibit such endocytotic process. We found that basic calcium phosphate (BCP) crystals greatly stimulated the endocytotic activity of cells by rendering the cells more permeable and that the anti-calcification agent phosphocitrate and several others inhibited the crystals-mediated endocytosis. This is the first study reporting that the endocytotic activity of cells is affected by BCP crystals and that such endocytotic activity can be inhibited by anti-calcification agents. Since calcium-containing crystals are associated with many human diseases and in many circumstances are associated with apoptotic bodies, extracellular and matrix vesicles where DNA fragments, small peptides, and minerals are released into extracellular space, the findings reported here are important for our understanding of the complex biological effects and the potential pathological role of calcium-containing crystals in crystals-associated diseases, and for the development of disease modifying agents as well.

The biodegradation mechanism of calcium phosphate biomaterials in bone

This study was undertaken to understand the biodegradation mechanisms of calcium phosphate (Ca-P) biomaterials with different crystallization. Two types of sintered Ca-P porous ceramic (HA and beta-TCP) and a Ca-P bone cement (CPC) were implanted into cavities drilled in rabbit femoral and tibiae condyles. The results have shown that a material biodegradation was rapid in the beta-TCP and the CPC, but very weak in the HA. This biodegradation presented a decrease of material volume from the periphery to the center as well as a particle formation causing phagocytosis by numerous macrophages and multinucleated giant cells in the CPC. In the beta-TCP, there was a peripheral and central decrease of material volume as well as an absence of particle formation or visible phagocytosis. The process of biodegradation is considered to be directly influenced by the type of material crystallization. The sintered bioceramics processed at a high temperature exhibit good crystallization and are primarily degraded by a process dependent on interstitial liquids. However, the bone cement is formed by physicochemical crystallization and is degraded through a dissolution process associated with a cellular process.

Importance of hydroxyapatite particles characteristics on cytokines production by human monocytes in vitro

Calcium phosphate bioceramics have been applied as bone substitutes for several decades. Aseptic loosening after total joint arthroplasty is a major problem in orthopaedic surgery. Hydroxyapatite particles from materials wear have been reported as the main cause of implant failure. For this reason, an investigation into possible wear particles from materials used in the implant may lead to longevity after arthroplasty. Monocytes are among the first cells to colonize the inflammatory site. In the present study, we have evaluated the inflammatory response after exposition to particles with different characteristics (size, sintering temperature and shape). Our data demonstrate that the most important characteristic was the shape and the size of the particles. The needle shaped particles induced the larger production of TNF-alpha, IL-6 and IL-10 by cells. To a less manner, the smallest particles induced an increase of the expression and production of the cytokines studied (TNF-alpha, IL-6 and IL-10). The sintering temperature appeared to be a less important characteristic even though it was involved in the dissolution/precipitation process.

Calcium-deficient apatite: a first in vivo study concerning bone ingrowth

Biphasic calcium phosphate (BCP) materials are increasingly used to restore bone loss in surgery. Calcium-deficient apatites (CDA), the precursors of BCP, are closer in structure to biological apatites and can be associated with therapeutic agents to form drug-delivery systems. The purpose of this first in vivo study of CDA was to evaluate the osteoconductive properties of two composites, consisting of 40-80 microm granules carried by a cellulose-derived polymer, used to fill critical size bone defects in rabbit femoral ends. Animals were sacrificed 2 or 3 weeks after implantation. Histomorphometric analysis of scanning electron microscopy implant surface files was performed using gray level threshold that distinguish between bone or materials (white) and noncalcified tissue (black). Quantitative results for new bone formation showed no significant differences between the composites or the implantation periods. However, nearly all of the CDA disappeared early while supporting more extensive bone colonization than biphasic calcium phosphates implanted in the same conditions.

Signaling mechanisms involved in crystal-induced tissue damage

The association of crystal deposition with osteoarthritis and joint destruction is well established. Recent advances in understanding the mechanisms whereby calcium crystals contribute to cartilage damage are highlighted in this review. In vitro studies have shown that when calcium-containing crystals come in contact with cells they cause an influx in Ca 2+ concentration and activation of p42/44 mitogen-activated protein kinases. This is followed by induction of proto-oncogenes (c- fos, c- jun ) and induction of the nuclear transcription factors activator protein-1 and nuclear factor-kappaB, which in turn lead to crystal-induced modulation of normal gene expression. Some of the downstream effects known to date include increased mitogenesis, up-regulation of members of the matrix metalloproteinase family, down-regulation of tissue inhibitor of metalloproteinase-1 and -2 in fibroblasts, induction of neutrophil chemotactic chemokines such as interleukin-8, activation and degranulation of neutrophils, and inhibition of neutrophil apoptosis. Because no known drug prevents or treats the consequences of basic calcium phosphate crystal deposition, an improved understanding of the molecular mechanisms leading to crystal-induced joint degeneration is essential to the development of a rational approach to target the consequences of crystal deposition.

Basic calcium phosphate crystals induce matrix metalloproteinase-1 through the Ras/mitogen-activated protein kinase/c-Fos/AP-1/metalloproteinase 1 pathway. Involvement of transcription factor binding sites AP-1 and PEA-3

Synovial fluid basic calcium phosphate (BCP) crystals are common in osteoarthritis and are associated with severe degenerative arthropathy. Besides stimulating synovial fibroblast-like cells to proliferate, BCP crystals are a potent inducer of human matrix metalloproteinases (hMMPs), which can speed up the articular joint tissue degeneration of osteoarthritis patients. Here, we report that transfections with hMMP1 luciferase reporter plasmids in fibroblast-like synoviocytes revealed that the induction of hMMP1 promoter by BCP crystals was mainly mediated through the -72AP-1 element. Elimination of the -72AP-1 element either by mutation or deletion abolished the induction of hMMP1 promoter activity by BCP crystals almost completely. Interestingly, a mutation at the -88PEA-3 site also abolished the induction of hMMP1 promoter. Further mutation at the -181AP-1 site resumed the induction, indicating that the -181AP-1 element had an effect opposite to the -72AP-1 element. The effect of -181AP-1 could be inactivated either by a mutation at this -181AP-1 site or by the -88PEA-3 element. In addition, dominant negative Ras, Raf, and MEK1/2 could block the induction of hMMP1, and a MEK1/2-specific inhibitor (UO126) could block the induction of hMMP1 and c-Fos by BCP crystals. Taken together, these data indicate that multiple elements, including at least AP-1 and PEA-3, are involved in the induction of hMMP1 gene expression by BCP crystals and that the induction follows the Ras/MAPK/c-Fos/AP-1/MMP1 signaling pathway.

Activation of metalloproteinase-2, loss of matrix scleroprotein content and coronary artery calcification

Plaques from the coronary arteries of explanted hearts showed massive calcification (15-fold increase) with a loss of scleroproteins (-36%), an increase in the collagen to elastin ratio (twofold) and activation (+15%) of matrix metalloproteinase-2 (MMP-2). Plaque-free portions of the coronary artery gave results similar to those obtained with the internal mammary artery. There was a significant correlation between plaque calcification and MMP-2 activation, suggesting that the two processes may be linked.

Effects of hydroxyapatite particulate debris on the production of cytokines and proteases in human fibroblasts

Cytokines and proteases are secreted by fibroblasts in response to particulate wear debris, and these proteins are felt to play an important role in the development of osteolysis and implant loosening. Although metallic and polyethlyene debris have been studied extensively, little is known about the cellular responses to hydroxyapatite, despite the wide clinical use of these materials. Therefore, the effects of hydroxyapatite (HA) and hydroxyapatite/beta-tricalciumphosphate (HA/TCP) on cellular proliferation, cytokine gene expression and protein secretion, protease synthesis, and gelatinolytic activity were investigated in human fibroblasts. HA and HA/TCP particles were synthesized, and their effects were compared to the responses elicited by titanium and cobalt chromium. Sample characterization by scanning electron microscopy and Coulter Counter demonstrated that the materials had a mean particle size of less than 10 microm, and all of the particles were compared using the same concentration ranges. Aliquots of particle suspensions were added to human fibroblasts maintained in tissue culture, and dose-response and time-course experiments were performed. Effects of the particles on fibroblast proliferation were assessed, and alterations in cytokine levels were determined by specific enzyme linked immunosorbent assays (ELISA). Cytokines that were evaluated included interleukin-1 (IL-1beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha), all of which have been demonstrated to enhance bone resorption and are associated with osteolysis and implant loosening. Gene expression was determined using Northern blot analysis with cytokine-specific probes, while secretion of the proteases collagenase and stromelysin was determined by Western blot analysis. Functional gelatinolytic assay was assessed using zymogram gels. The particles were evaluated in a concentration range from 0.000021 to 0.021 vol%. All of the particles produced increases in cellular proliferation up to 0.0021 vol%, with the largest increases being seen at 0.021 vol% with HA/TCP and titanium. At the highest concentration, both cobalt chromium and HA samples decreased cellular proliferation relative to lower doses, possibly representing cytotoxicity. Hydroxyapatite particles yielded a 30-fold increase in interleukin-6 secretion compared to unstimulated controls, which was also greater than three times the levels produced by cobalt chromium, titanium, or HA/TCP. HA particles also tripled the secretion of IL-1beta at 0.00021 vol%, and doubled TNF-alpha secretion at 0.021 vol%. Addition of conditioned media prepared by incubation of the particles in culture medium in the absence of cells did not alter the secretion of any of the cytokines. Northern blot analysis using IL-6 probes also demonstrated strong increases with HA compared to the other materials, suggesting that the action of the HA particles was at the level of transcription. Secretion of the protease collagenase was increased by all of the samples including HA when compared to unstimulated controls. Stromelysin secretion into the culture medium was decreased by cobalt chromium, but increased by titanium, HA, and HA/TCP. All of the particles including HA increased the gelatinolytic activity of the fibroblasts. These findings demonstrate that HA and HA/TCP particles are capable of stimulating the expression and secretion of cytokines and proteases that enhance bone resorption, and suggest that particulate debris from implants using these coatings may also increase osteolysis and loosening.

Matrix regulation of skeletal cell apoptosis. Role of calcium and phosphate ions

Previously, we noted that inorganic phosphate (P(i)), a major component of bone extracellular matrix, induced osteoblast apoptosis (Meleti, Z., Shapiro, I. M., and Adams, C. S. (2000) Bone (NY) 27, 359-366). Since Ca(2+) along with P(i) is released from bone during the resorption process, we advanced the hypothesis that Ca(2+) modulates P(i)-mediated osteoblast apoptosis. To test this hypothesis, osteoblasts were incubated with both ions, and cell death was determined. We noted that a modest increase in the medium Ca(2+) concentrations ([Ca(2+)](e)) of 0.1-1 mm caused a profound and rapid enhancement in P(i)-dependent death of cultured osteoblasts. An elevation in [Ca(2+)](e) alone had no effect on osteoblast viability, whereas Ca(2+) channel blockers failed to inhibit killing of ion pair-treated cells. These results indicated that P(i)-mediated cell death is not dependent on a sustained increase in the cytosolic Ca(2+) concentration. Terminal dUTP nick-end labeling analysis and measurement of caspase-3 activity of the ion pair-treated cells suggested that death was apoptotic. Apoptosis was confirmed using caspase-3 and endonuclease inhibitors. The mitochondrial membrane potential and cytosolic Ca(2+) status of the treated cells were evaluated. After incubation with [Ca(2+) ](e) and P(i), a decrease in mitochondrial fluorescence was noted, suggesting that the ions decreased the mitochondrial transmembrane potential. Subsequent to the fall in mitochondrial membrane potential, there was a transient elevation in the cytosolic Ca(2+) concentration. Results of the study suggest that the ion pair conspire at the level of the plasma membrane to induce intracellular changes that result in loss of mitochondrial function. The subsequent increase in the cytosolic Ca(2+) concentration may trigger downstream events that transduce osteoblast apoptosis.

In vivo bone formation by human bone marrow stromal cells: effect of carrier particle size and shape

Successful closure of bone defects in patients remains an active area of basic and clinical research. A novel and promising approach is the transplantation of human bone marrow stromal cells (BMSCs), which have been shown to possess a significant osteogenic potential. The extent and quality of bone formation by transplanted human BMSCs strongly depends on the carrier matrix with which cells are transplanted; to date, hydroxyapatite/tricalcium phosphate (HA/TCP) supports far more osteogenesis than any other matrix tested. In order to further improve the technique of BMSC transplantation, we studied whether commercially available HA/TCP particles, clinically approved as an osteoconductive material and commercially available as particles measuring 0.5-1.0 mm diameter, is an optimum matrix for promoting bone development by BMSCs. HA/TCP and HA particles of varying size were sieved into a variety of size ranges, from <0.044 mm to 1.0-2.0 mm. Transplants were formed by mixing 40 mg aliquots of particles with cultured passaged human BMSCs. They were placed in subcutaneous pockets in immunocompromised Bg-Nu-XID mice and harvested 4 or 10 weeks later. The transplants were examined histologically; the presence of bone within each transplant was evaluated using histomorphometry or blindly scored on a semiquantitative scale. Transplant morphology and the amount of new bone varied in a consistent fashion based on particle size and shape. Transplants incorporating HA/TCP particles of 0.1-0.25 mm size demonstrated the greatest bone formation at both 4 and 10 weeks; larger or smaller particles were associated with less extensive bone formation, while a size of 0.044 mm represented a threshold below which no bone formation could be observed. Flat-sided HA particles measuring 0.1-0.25 mm formed no bone. The differences in bone formation were not attributable to the differences in cell attachment among the groups. Instead, the size and spatial and structural organization of the particles within BMSC transplants appear to determine the extent of bone formation. These findings provide necessary information for the successful clinical application of BMSC transplantation techniques.

Calcium crystal effects on the cells of the joint: implications for pathogenesis of disease

In the past three years, there has been considerable progress in delineating the mechanism of calcium-containing crystal-induced cell activation: (1) the identification of Ca2+ influx and p42/44 mitogen-activated protein kinase activation as the signal transduction pathways; (2) induction of nuclear transcription factors of cyclic adenosine monophosphate (cAMP) response element binding protein, activator protein-1, and nuclear factor kappaB; (3) the differential role of crystal endocytosis and dissolution in crystal-induced metalloproteinase synthesis and mitogenesis; (4) crystal upregulation of matrix metalloproteinases, including MMP-13 but downregulation of tissue inhibitor of metalloproteinase-1 and -2, thus magnifying the degenerative effect of crystals. Phosphocitrate, a specific inhibitor of biologic effect of calcium crystals, reverses the degenerative effects of crystals.

Short-term effects of mineral particle sizes on cellular degradation activity after implantation of injectable calcium phosphate biomaterials and the consequences for bone substitution

This in vivo study investigated the influence of two calcium phosphate particle sizes (40-80 microm and 200-500 microm) on the cellular degradation activity associated with the bone substitution process of two injectable bone substitutes (IBS). The tested biomaterials were obtained by associating a biphasic calcium phosphate (BCP) ceramic mineral phase and a 3% aqueous solution of a cellulosic polymer (hydroxypropylmethylcellulose). Both were injected into osseous defects at the distal end of rabbit femurs for 2- and 3-week periods. Quantitative results for tartrate-resistant acid phosphatase (TRAP) cellular activity, new bone formation, and ceramic resorption were studied for statistical purposes. Positive TRAP-stained degradation cells were significantly more numerous for IBS 40-80 than IBS 200-500, regardless of implantation time. BCP degradation was quite marked during the first 2 weeks for IBS 40-80, and bone colonization occurred more extensively for IBS 40-80 than for IBS 200-500. The resorption-bone substitution process occurred earlier and faster for IBS 40-80 than IBS 200-500. Both tested IBS displayed similar biological efficiency, with conserved in vivo bioactivity and bone-filling ability. Differences in calcium phosphate particle sizes influenced cellular degradation activity and ceramic resorption but were compatible with efficient bone substitution.