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M Gowen

Publications and source records attributed to M Gowen.

At least 73 records · Page 4Linked to original sources
PubMed1997

Peptide aldehyde inhibitors of cathepsin K inhibit bone resorption both in vitro and in vivo.

We have shown previously that cathepsin K, a recently identified member of the papain superfamily of cysteine proteases, is expressed selectively in osteoclasts and is the predominant cysteine protease in these cells. Based upon its abundant cell type-selective expression, potent endoprotease activity at low pH and cellular localization at the bone interface, cathepsin K has been proposed to play a specialized role in osteoclast-mediated bone resorption. In this study, we evaluated a series of peptide aldehydes and demonstrated that they are potent cathepsin K inhibitors. These compounds inhibited osteoclast-mediated bone resorption in fetal rat long bone (FRLB) organ cultures in vitro in a concentration-dependent manner. Selected compounds were also shown to inhibit bone resorption in a human osteoclast-mediated assay in vitro. Chz-Leu-Leu-Leu-H (in vitro enzyme inhibition Ki,app = 1.4 nM) inhibited parathyroid hormone (PTH)-stimulated resorption in the FRLB assay with an IC-50 of 20 nM and inhibited resorption by isolated human osteoclasts cultured on bovine cortical bone slices with an IC-50 of 100 nM. In the adjuvant-arthritic (AA) rat model, in situ hybridization studies demonstrated high levels of cathepsin K expression in osteoclasts at sites of extensive bone loss in the distal tibia. Cbz-Leu-Leu-Leu-H (30 mg/kg, intraperitoneally) significantly reduced this bone loss, as well as the associated hind paw edema. In the thyroparathyriodectomized rat model, Cbz-Leu-Leu-Leu-H inhibited the increase in blood ionized calcium induced by a 6 h infusion of PTH. These data indicate that inhibitors of cathepsin K are effective at reducing osteoclast-mediated bone resorption and may have therapeutic potential in diseases of excessive bone resorption such as rheumatoid arthritis or osteoporosis.

Aldehydes↗
PubMed1997

Inhibition of cathepsin K--a novel approach to antiresorptive therapy.

Cathepsin K is a cysteine protease enzyme with high and specific expression in osteoclasts, the cells responsible for resorbing bone. Its discovery and the numerous lines of evidence that support a specialised role for cathepsin K in bone resorption are discussed. Inhibition of this enzyme would be anticipated to prevent bone resorption, and be of potential utility in diseases involving excess bone loss, such as osteoporosis. The pharmaceutical companies known to be involved in developing inhibitors of cathepsin K, and their progress to date, are described.

Journal Article↗
PubMed1996

Cathepsin K, but not cathepsins B, L, or S, is abundantly expressed in human osteoclasts.

Random high throughput sequencing of a human osteoclast cDNA library was employed to identify novel osteoclast-expressed genes. Of the 5475 ESTs obtained, approximately 4% encoded cathepsin K, a novel cysteine protease homologous to cathepsins S and L; ESTs for other cathepsins were rare. In addition, ESTs for cathepsin K were absent or at low frequency in cDNA libraries from numerous other tissues and cells. In situ hybridization in osteoclastoma and osteophyte confirmed that cathepsin K mRNA was highly expressed selecively in osteoclasts; cathepsins S, L, and B were not detectable. Cathepsin K was not detected by in situ hybridization in a panel of other tissues. Western blot of human osteoclastoma or fetal rat humerus demonstrated bands of 38 and 27 kDa, consistent with sizes predicted for pro- and mature cathepsin K. Immunolocalization in osteoclastoma and osteophyte showed intense punctate staining of cathepsin K exclusively in osteoclasts, with a polar distribution that was more intense at the bone surface. The abundant expression of cathepsin K selectively in osteoclasts strongly suggests that it plays a specialized role in bone resorption. Furthermore, the data suggest that random sequencing of ESTs from cDNA libraries is a valuable approach for identifying novel cell-selective genes.

Amino Acid Sequence↗
PubMed1996

Proteolytic activity of human osteoclast cathepsin K. Expression, purification, activation, and substrate identification.

Human cathepsin K is a recently identified protein with high primary sequence homology to members of the papain cysteine protease superfamily including cathepsins S, L, and B and is selectively expressed in osteoclasts (Drake, F.H., Dodds, R., James I., Connor J., Debouck, C., Richardson, S., Lee, E., Rieman, D., Barthlow, R., Hastings, G., and Gowen, M. (1996) J. Biol., Chem. 271, 12511-12516). To characterize its catalytic properties, cathepsin K has been expressed in baculovirus-infected SF21 cells and the soluble recombinant protein isolated from growth media was purified. Purified protein includes an inhibitory pro-leader sequence common to this family of protease. Conditions for enzyme activation upon removal of the pro-sequence have been identified. Fluorogenic peptides have been identified as substrates for mature cathepsin K. In addition, two protein components of bone matrix, collagen and osteonectin, have been shown to be substrates of the activated protease. Cathepsin K is inhibited by E-64 and leupeptin, but not for by pepstatin, EDTA, phenylmethylsulfonyl fluoride, or phenanthroline, consistent with its classification within the cysteine protease class. Leupeptin has been characterized as a slow binding inhibitor of cathepsin K (kobs/[I] = 273,000 m(-1).s(-1)). Cathepsin K may represent the elusive protease implicated in degradation of protein matrix during bone resorption and represents a novel molecular target in treatment of disease states associated with excessive bone loss such as osteoporosis.

Amino Acid Sequence↗
PubMed1996

Human osteoclastoma-derived stromal cells: correlation of the ability to form mineralized nodules in vitro with formation of bone in vivo.

It has been suggested that the stromal element of human osteoclastomas contains osteoblastic cells. In this study, we demonstrate that osteoclast-depleted, passaged stromal cells express alkaline phosphatase and osteocalcin in vitro and form mineralized nodules under appropriate culture conditions. In addition, we describe a model in which severe combined immunodeficient (SCID) mice were used to support the differentiation of these putative human osteoblast progenitors in vivo. Lesions formed from human stromal cells were identified using the OKa blood group antigen and human procollagen type I antibodies. By 21 days, the lesion was a complete bone unit: a fully mineralized cortex, remodeling trabeculae, and a highly cellular marrow space. Stromal cells derived from six out of seven osteoclastomas produced identical lesions. Further studies have demonstrated that the capacity of the osteoclastoma-derived stromal cells to form bone in vivo and in vitro is passage dependent; early passages were osteogenic in both model systems, while later passages were not. In conclusion, we have developed a model in which the osteogenic nature of cells can be confirmed in vivo. Furthermore, human osteoclastoma-derived stromal cells provide a source of these osteogenic cells to study human osteoblast differentiation, both in vivo and in vitro.

Alkaline Phosphatase↗
PubMed1996

Purification and characterization of fully functional human osteoclast precursors.

The identification and purification of human osteoclast precursors is essential to further our understanding of the mechanisms that control human osteoclast differentiation. Osteoclastoma tissue potentially provides a rich source of human osteoclast precursors, and in previous studies we have demonstrated the existence of a population of mononuclear cells within this tissue that is reactive with osteoclast-selective vitronectin receptor monoclonal antibodies. In this study, mononuclear cells expressing the vitronectin receptor, as defined by their ability to react with a murine monoclonal antibody to the beta 3 chain of the vitronectin receptor (87MEM1), were isolated from collagenase digests of osteoclastoma tissue using a fluorescence activated cell sorter. Based on their fluorescence signal and size, approximately 2-3% of the viable cells (typically 2 x 10(5)) were obtained and prepared for further phenotyping. The isolated cells demonstrated a number of phenotypic characteristics of osteoclasts: positive tartrate-resistant acid phosphatase (TRAP) activity, reactivity with human osteoclast-selective antibodies, expression of calcitonin receptors, cathepsin K (a novel osteoclast-selective cysteine proteinase) mRNA, and osteopontin mRNA and protein. These phenotypic characteristics were also detected in mononuclear cells within cryostat sections of the native osteoclastoma tissue as well as in resorption lacunae of sections of human bone. In contrast, isolated peripheral blood monocytes were negative for TRAP activity and osteopontin expression and, unlike the osteoclastoma-derived cells, demonstrated strong nonspecific esterase activity. Significantly, when the osteoclastoma-derived 87MEM1 positive cells were cocultured on whale dentine for 1-3 weeks with stromal cells, extensive resorption of the dentine surface was observed. This is the first demonstration of the purification of human osteoclast precursors. These cells provide an homogeneous cell population for studying cellular events that occur during human osteoclast differentiation.

Antibodies, Monoclonal↗
PubMed1995

Human osteoclasts, not osteoblasts, deposit osteopontin onto resorption surfaces: an in vitro and ex vivo study of remodeling bone.

Osteopontin is a phosphorylated glycoprotein believed to be secreted by osteoblasts and deposited into the bone matrix to facilitate osteoclasts adhesion or to initiate osteoid mineralization. Previously we have presented contradictory evidence that osteoclasts express osteopontin mRNA in human remodeling bone. The aim of this study was to ascertain whether osteoclasts synthesize and deposit osteopontin in resorption lucunae. We characterized expression of osteopontin mRNA and protein expression in both intramembranous and endochondral ossification, as well as remodeling bone, in the human osteophyte. Osteopontin mRNA was expressed in osteoclast with tartrate-resistant acid phosphatase (TRAP) positivity within resorption lacunae. The osteoclasts and immediate resorption surfaces also expressed osteopontin. However, osteopontin mRNA and protein were weak (transient) or undetectable in osteoblasts at adjacent bone formation sites; no osteopontin expression was observed in the osteoid, although occasional reactivity was observed in osteocytes and the mineral-osteoid interface. In contrast, osteopontin was highly expressed in the osteoblasts and matrix of woven bone during intramembranous and endochondral ossification. The matrix expression correlated with mineralization; however, in some instances osteopontin deposition was observed prior to mineralization. Similarly, osteopontin expression was evident in cartilage matrix, solely at foci of mineralization. Chondroclasts expressed osteopontin mRNA and protein: the surfaces of resorbed calcified cartilage also expressed osteopontin. Abnormal, unmineralized matrices apparently lacked deposited osteopontin, but were nevertheless resorbed by osteoclasts; the osteoclasts and resorbed surfaces expressed no osteopontin protein. That osteoclasts are responsible for the deposition of osteopontin was confirmed in vitro, whereby resorption pits in whale dentine and bovine bone slices, produced by isolated human osteoclasts, contained deposited osteopontin. Osteopontin may facilitate the adhesion (or detachment) of the osteoclast to the bone surface. Alternatively, the possibility that osteopontin may act as a postresorptive signal to recruit osteoblasts, or to polarize and direct the mineralization of the formed osteoid, is discussed.

Acid Phosphatase↗
PubMed1995

Human osteoclast and giant cell differentiation: the apparent switch from nonspecific esterase to tartrate resistant acid phosphatase activity coincides with the in situ expression of osteopontin mRNA.

Animal model and in vitro cultures suggest that osteoclasts and cells of the mononuclear phagocyte system share a common precursor. However, the human osteoclast precursor has not been positively identified. We attempted to identify the precursor in situ by using a number of osteoclast- and macrophage-selective markers, together with the expression of osteopontin mRNA, previously shown to be abundant in human osteoclasts. Sections of osteophytic bone and a panel of inflammatory connective tissues were processed for in situ hybridization; serial sections were analyzed for tartrate-resistant acid phosphatase (TRAP) and nonspecific esterase (NSE) activity, selective cytochemical markers for the osteoclast and cells of the macrophage/monocyte lineage, respectively. The murine anti-human osteoclast monoclonal antibodies 23C6 (vitronectin receptor) and C35 (osteoclast-selective) were used to further identify the osteoclast phenotype. We compared osteoclasts, giant cells, and their respective putative mononuclear precursors. At resorption sites within osteophytic bone, osteopontin mRNA was expressed in osteoclasts and a distinct population of TRAP+, NSE- mononuclear cells. Adjacent clusters of mononuclear cells were TRAP- and NSE+ or were active for both enzymes; these cells demonstrated variable expression of osteopontin mRNA. In the inflammatory connective tissues, abundant macrophage-like cells (NSE+/TRAP-) did not express osteopontin mRNA. However, TRAP+ mononuclear cells observed among clusters of NSE+ cells did express osteopontin mRNA. At these sites, clusters of putative macrophage polykaryons removing fragments of bone debris were observed. These giant cells and associated mononuclear cells were NSE- and distinctly TRAP+, and expressed osteopontin mRNA, C35, and 23C6 (human osteoclast) reactivity. Therefore, cells involved in the remodeling (resorption) of bone or the removal of bone debris, together with their immediate precursors, switch from being NSE+/TRAP- to NSE-/TRAP+ cells that express osteopontin mRNA. We propose that the clusters of NSE+/TRAP- mononuclear cells represent the immature osteoclast precursor. In support of this, TRAP+/NSE+ cells were occasionally observed in both tissues, representing an intermediate stage in differentiation. These results further suggest that cells of the mononuclear phagocyte lineage within bone and inflammatory connective tissue have the potential to differentiate into osteoclasts.

Acid Phosphatase↗
PubMed1994

Monoclonal antibodies with selective reactivity against osteoblasts and osteocytes in human bone.

Monoclonal antibodies (MAb) may provide valuable tools for studying osteoblast differentiation. We therefore raised a panel of MAb reactive with cells of this phenotype using 1,25(OH)2D3-treated human trabecular osteoblast-like cells (HOBS) as the immunogen. Immunohistochemical studies on various tissues, including undecalcified cryostat sections of fetal and adult human bone, identified 11 bone cell-reactive MAb. Of these, 2 demonstrated particularly selective reactivities against osteocytes (OB/M) and osteoblasts (OB/L). These reactivities were also seen in developing bone from rat, rabbit, and marmoset. OB/L and OB/M demonstrated limited reactivity against a small number of human tissues from the extensive panel of substrates tested. Both MAb exhibited reactivity against discrete populations of cells in the large and small intestine. In addition, OB/L reacted with cells in the basal epidermis of skin and OB/M with cells in blood vessel walls. Both antibodies demonstrated reactivity against a variety of cultured osteoblast-like cell lines and other cultured cell types. These MAb may therefore provide a valuable means of studying osteoblast ontogeny.

Adult↗
PubMed1994

Microcytophotometric analysis of human osteoclast metabolism: lack of activity in certain oxidative pathways indicates inability to sustain biosynthesis during resorption.

It has been proposed that highly biosynthetic cells oxidize fatty acids to generate ATP while maintaining high levels of glucose metabolism through the glycolytic and pentose shunt systems to supply biosynthetic intermediates. We investigated the metabolic strategies and substrate for ATP production in the osteoclast. We used in situ quantitative microcytophotometric techniques to determine the maximal activity of the pentose shunt (glucose-6-phosphate dehydrogenase; G6PD), the glycolytic pathway (glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase; G3PD and LDH), fatty acid oxidation (beta-hydroxyacyl dehydrogenase; HOAD), and the Krebs cycle (succinate dehydrogenase; SDH) in human osteoclasts in situ, and related these enzyme activities to the degree of involvement of the cells in resorption. Unlike other highly biosynthetic cells, such as chondrocytes and macrophage polykaryons, osteoclasts associated with bone resorption were deficient in G3PD, LDH, and G6PD activity. However, osteoclasts did demonstrate a capacity for fatty acid oxidation which increased in cells apposed to the bone surface. The lack of significant glycolytic and pentose shunt activity in the osteoclast provides good evidence that resorbing osteoclasts, unlike phagocytosing macrophage polykaryons, have the metabolic characteristics of cells with greatly reduced capabilities of de novo mRNA synthesis but which do maintain high rates of ATP production. The possibility that the loss of glycolytic activity is a prelude to cell death is discussed.

Adenosine Triphosphate↗
PubMed1994

Expression of mRNA for IL1 beta, IL6 and TGF beta 1 in developing human bone and cartilage.

Using in situ hybridization, we investigated the expression of mRNA for interleukin-1 beta (IL1 beta), interleukin-6 (IL6), and transforming growth factor-beta-1 (TGF beta 1) in sections of developing bone in human osteophytes. The expression was related to the cellular activity of alkaline phosphatase to aid in the identification of pre-osteoblast populations. IL1 beta mRNA was localized in active osteoblasts within distinct areas of intramembranous ossification. However, the expression was sporadic and appeared to occur at a specific stage of the osteoblast life cycle. There was no IL1 beta mRNA expression in any cell types during endochondral ossification. IL6 mRNA expression was located within pre-osteoblasts and in newly differentiated and matrix-secreting osteoblasts; expression was absent or reduced in flattened, inactive osteoblasts. Weak or no IL6 expression was observed in chondroblasts and chondrocytes, respectively. However, there was a close association between IL6 mRNA expression and the differentiation of mesenchymal cells into osteoblasts. TGF beta 1 expression was localized to osteoblasts apposed to bone or cartilage matrix; the intensity of expression correlated with matrix secretion. Chondroblasts and chondrocytes expressed lower but significant levels of TGF beta 1 mRNA; the expression was lost with the progression to calcifying cartilage. The three cytokines studied were differentially expressed both temporally and spatially, suggesting different roles for each in osteoblast and chondrocyte function.

Alkaline Phosphatase↗
PubMed1993

Proliferative responses to estradiol, IL-1 alpha and TGF beta by cells expressing alkaline phosphatase in human osteoblast-like cell cultures.

The use of primary (nontransformed) bone cell cultures is hampered by their cellular heterogeneity. Primary cultures of osteoblast-like cells have been shown to proliferate in response to several osteotropic agents, but because mixed cell populations are present it is uncertain whether a true osteoblastic response was observed. By combining (1) localization of [3H]-thymidine incorporation into the nuclei of actively dividing cells by autoradiography with (2) subsequent induction of osteoblast differentiation by 1,25(OH)2D3 to optimize the number of cells expressing high alkaline phosphatase activity and (3) its localization by histochemical staining, it is possible to measure the proliferation of cells that are capable of expressing a more mature osteoblastic phenotype in heterogeneous human trabecular bone cell cultures. Over a 72-hour incubation period, rhIL-1 alpha (0.2-2 ng/ml) exerted a dose-dependent stimulation of proliferation of cells expressing alkaline phosphatase. Purified human TGF beta 1 produced a biphasic increase in the proliferation of these cells (0.01-1 ng/ml) but 17 beta and 17 alpha-estradiol (10(-12)-10(-8) M) failed to consistently regulate cell growth. Furthermore, 17 beta-estradiol did not reproducibly modulate proliferation induced by IL-1 alpha or TGF beta when added together in cultures. This procedure represents a more accurate method for the assessment of osteoblast proliferation in primary bone cell cultures and demonstrates that estrogen is not mitogenic for human osteoblasts and does not potentiate the actions of putative local stimulators of osteoblast replication.

Adult↗
PubMed1993

The cell biology of bone.

Bone remodelling and repair are accomplished by the co-ordinated activity of cells of the osteoclast and osteoblast lineages. Small changes in the balance between formation and resorption will, when magnified by repeated cycles, lead to significant reduction in bone mass and strength, ultimately resulting in fracture. This review focuses on the cellular features of bone remodelling and the known regulators of bone cell function. These include systemic and local factors, both soluble and contained within the complex extracellular matrix of bone.

Animals↗
PubMed1993

Expression of osteopontin mRNA by osteoclasts and osteoblasts in modelling adult human bone.

Over recent years several non-collagenous matrix proteins of bone have been isolated and characterized. One of these proteins, osteopontin, has been shown to be synthesized by osteoblasts and deposited in the bone matrix where it is thought to bind to hydroxyapatite. However much of the functional evidence is circumstantial, and the precise function of osteopontin has not been fully elucidated. We have used in situ hybridization techniques to investigate the expression of osteopontin mRNA in a variety of human bone tissues. Cryostat sections of human osteophyte and osteoclastoma tissue were hybridized with an antisense RNA probe for osteopontin. Sense transcripts were used as a negative control to assess non-specific binding. There was a very distinct pattern of osteopontin mRNA expression in these tissues. Plump osteoblasts adjacent to the osteoid matrix expressed high levels of osteopontin mRNA, whilst flattened osteoblasts demonstrated weaker expression. The most striking feature of osteopontin mRNA expression was the high levels detected in osteoclasts. Osteoclasts in resorption lacunae and those distant from resorption sites both expressed osteopontin mRNA, suggesting that attachment was not a prerequisite for osteopontin expression. A population of mononuclear cells in resorption lacunae was also observed to express high levels of osteopontin mRNA. The whole population of osteoclasts in the osteoclastoma tissue expressed high levels of osteopontin mRNA, indicating that expression is not restricted to osteoclasts involved in bone resorption. This study confirms that human osteoblasts are capable of synthesizing osteopontin.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Oestradiol inhibits the release of tumour necrosis factor but not interleukin 6 from adult human osteoblasts in vitro.

Oestrogens may control bone remodelling by directly regulating the synthesis of cytokines in osteoblasts. We have investigated the effects of oestradiol on the release of two cytokines, IL-6 and TNF, known to be produced by normal human osteoblast-like cells. The effect of oestradiol on basal and stimulated IL-6 and TNF release was investigated. The concentration of IL-6 and TNF in 24 h bone cell-conditioned medium was determined using bio- and immunoradiometric assays. The results showed that 17 beta-oestradiol (10(-10) and 10(-8) M) inhibited IL-1-stimulated TNF release in a dose dependent manner in 7 out of 9 patients. Maximal inhibition was observed with 10(-8) M 17 beta-oestradiol, producing an average 30% reduction in TNF release. In contrast 17 beta-oestradiol (10(-12)-10(-8) M) failed to consistently regulate basal or stimulated IL-6 release. IL-6 mRNA levels were also shown not to be modulated by 17 beta-oestradiol (10(-9) M) under stimulatory conditions. rhIL-1 alpha (10 U/ml) was a consistent and potent stimulator of IL-6 and TNF release, and the glucocorticoid hydrocortisone was found to be a powerful suppressor of both IL-6 and TNF release under basal or stimulatory conditions. In conclusion direct regulation of bone remodelling by oestradiol does not appear to be effected via the control of IL-6 production in osteoblasts. However, a suppression of osteoblastic TNF release could represent one facet of the control of bone formation and resorption by oestrogens.

Cells, Cultured↗
PubMed1992

The transcriptional control of TGF-beta in human osteoblast-like cells is distinct from that of IL-1 beta.

Transforming growth factor beta (TGF-beta) and interleukin 1 (IL-1) are among the most potent osteotropic cytokines. The expression of mRNA for both TGF-beta and IL-1 beta was studied in human osteoblast-like cells in vitro. These cells constitutively expressed TGF-beta but not IL-1 beta mRNA. Treatment of the cells with the systemic hormones 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] (10(-8) M) and parathyroid hormone (10(-7) M) induced an increase in TGF-beta mRNA but failed to stimulate the production of IL-1-beta mRNA. Retinoic acid (10(-8) M) had no effect on either mRNA species. The cytokines IL-1 alpha (200 pg/ml), tumour necrosis factor alpha (TNF-alpha) (17 ng/ml) and bacterial lipopolysaccharide (LPS) (500 ng/ml) stimulated the production of IL-1 beta mRNA after 6-8 hours. This was followed by an increase in protein production after 24 hours. In contrast, the production of TGF-beta mRNA remained constant after treatment with these agents. Treatment of the cells with hydrocortisone (10(-8) M) resulted in the suppression of both TGF-beta and IL-1 beta mRNA. However, when the stimulating agent 1,25-(OH)2D3 was added in conjunction with hydrocortisone the mRNA expression of TGF-beta mRNA returned to 70% of the stimulated level. In contrast, the addition of the stimulatory agent IL-1 alpha to hydrocortisone-treated cells resulted in no increase in IL-1 beta mRNA. In-situ hybridization demonstrated both TGF-beta and IL-1 beta mRNA at the cellular level.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone Remodeling↗
PubMed1992

Integrin subunit expression by human osteoblasts and osteoclasts in situ and in culture.

The extracellular matrix may be considered as an insoluble local mediator which plays an important role in regulating cell function. Communication between the cell and its matrix occurs via the integrins, a family of transmembrane proteins composed of non-covalently linked alpha and beta subunits. The aim of this study was to establish which integrins are present on human bone cells in situ and in culture, using cryostat sections of undecalcified human bone, osteoclastoma tissue and cultured human osteoblasts. Integrin subunit expression was identified indirectly using alkaline phosphatase anti-alkaline phosphatase conjugates and FITC-labelled secondary antibodies. Subunits expressed by cultured human osteoblast-like cells were then quantified by FACS analysis. Staining patterns observed in situ show that osteoblasts and osteoclasts possess different integrin subunits. Osteoblasts primarily express alpha 1, alpha 3 and beta 1 and weakly express alpha 2. Osteoclasts express alpha 2, alpha V, beta 1 and beta 3. Subunits alpha 4, alpha 5, alpha 6, alpha L, alpha M and beta 2 were not expressed by either of these cell types. Expression of beta 1 by all cells of the osteoblastic lineage was constitutive, but alpha 1 and alpha 3 subunits were expressed by osteoblasts actively synthesizing bone and some of the osteoblast lining cells. All integrin subunits identified on osteoblasts in situ were maintained on culture but there was an increased expression of alpha 2 and alpha V subunits were weakly positive. Expression of alpha 2, alpha 3, alpha V and beta 1 subunits was independent of cell density but expression of alpha 1 was much greater in confluent cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Bone Matrix↗