c-Fms and the alphavbeta3 integrin collaborate during osteoclast differentiation

beta(3) integrin-null osteoclasts are dysfunctional, but their numbers are increased in vivo. In vitro, however, the number of beta(3)(-/-) osteoclasts is reduced because of arrested differentiation. This paradox suggests cytokine regulation of beta(3)(-/-) osteoclastogenesis differs in vitro and in vivo. In vitro, additional MCSF, but not receptor activator of NF-kappaB ligand (RANKL), completely rescues beta(3)(-/-) osteoclastogenesis. Similarly, activation of extracellular signal-regulated kinases (ERKs) and expression of c-Fos, both essential for osteoclastogenesis, are attenuated in beta(3)(-/-) preosteoclasts, but completely restored by additional MCSF. In fact, circulating and bone marrow cell membrane-bound MCSFs are enhanced in beta(3)(-/-) mice, correlating with the increase in the osteoclast number. To identify components of the MCSF receptor that is critical for osteoclastogenesis in beta(3)(-/-) cells, we retrovirally transduced authentic osteoclast precursors with chimeric c-Fms constructs containing various cytoplasmic domain mutations. Normalization of osteoclastogenesis and ERK activation, in beta(3)(-/-) cells, uniquely requires c-Fms tyrosine 697. Finally, like high-dose MCSF, overexpression of c-Fos normalizes the number of beta(3)(-/-) osteoclasts in vitro, but not their ability to resorb dentin. Thus, while c-Fms and alpha(v)beta(3) collaborate in the osteoclastogenic process via shared activation of the ERK/c-Fos signaling pathway, the integrin is essential for matrix degradation.

Tyrosines 559 and 807 in the cytoplasmic tail of the macrophage colony-stimulating factor receptor play distinct roles in osteoclast differentiation and function

Osteoclast (OC) differentiation requires that precursors, such as macrophage colony-stimulating factor (M-CSF)-dependent bone marrow macrophages, receive signals transduced by receptor activator of nuclear factor kappaB (RANK) and c-Fms, receptors for RANK ligand (RANKL) and M-CSF, respectively. Activated c-Fms autophosphorylates cytoplasmic tail tyrosine residues, which, by recruiting adaptor molecules, initiate specific signaling pathways. To identify which tyrosine residues are involved in c-Fms signaling in primary cells, we retrovirally transduced M-CSF-dependent bone marrow macrophages with a chimera comprising the external domain of the erythropoietin (Epo) receptor linked to the transmembrane and cytoplasmic domains of c-Fms. Transduced cells differentiate into bone-resorbing osteoclasts when treated with RANKL and either M-CSF or Epo, confirming that both endogenous and chimeric receptors transmit osteoclastogenic signals. Cells expressing chimeric receptors with Y(697)F, Y(706)F, Y(721)F, and Y(921)F single point mutations generate normal numbers of bone-resorbing OCs, with normal bone-resorbing activity when treated with RANKL and Epo. In contrast, those expressing Y(559)F generate fewer OCs, whereas theY807F mutant is incapable of osteoclastogenesis. Finally, although mature OCs expressing Y(559)F exhibit impaired bone resorption, those bearing Y807F do not. Thus, we have identified specific tyrosine residues in the cytoplasmic tail of c-Fms that are critical for transmitting M-CSF-initiated signals individually required for OC formation or function, respectively.

SHIP-deficient mice are severely osteoporotic due to increased numbers of hyper-resorptive osteoclasts

The hematopoietic-restricted protein Src homology 2-containing inositol-5-phosphatase (SHIP) blunts phosphatidylinositol-3-kinase-initiated signaling by dephosphorylating its major substrate, phosphatidylinositol-3,4,5-trisphosphate. As SHIP(-/-) mice contain increased numbers of osteoclast precursors, that is, macrophages, we examined bones from these animals and found that osteoclast number is increased two-fold. This increased number is due to the prolonged life span of these cells and to hypersensitivity of precursors to macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappa B ligand (RANKL). Similar to pagetic osteoclasts, SHIP(-/-) osteoclasts are enlarged, containing upwards of 100 nuclei, and exhibit enhanced resorptive activity. Moreover, as in Paget disease, serum levels of interleukin-6 are markedly increased in SHIP(-/-) mice. Consistent with accelerated resorptive activity, 3D trabecular volume fraction, trabecular thickness, number and connectivity density of SHIP(-/-) long bones are reduced, resulting in a 22% loss of bone-mineral density and a 49% decrease in fracture energy. Thus, SHIP negatively regulates osteoclast formation and function and the absence of this enzyme results in severe osteoporosis.

Interleukin-4 reversibly inhibits osteoclastogenesis via inhibition of NF-kappa B and mitogen-activated protein kinase signaling

To define the molecular mechanism(s) by which interleukin (IL)-4 reversibly inhibits formation of osteoclasts (OCs) from bone marrow macrophages (BMMs), we examined the capacity of this T cell-derived cytokine to impact signals known to modulate osteoclastogenesis, which include those initiated by macrophage colony-stimulating factor (M-CSF), receptor for activation of NF-kappa B ligand (RANKL), tumor necrosis factor (TNF), and IL-1. We find that although pretreatment of BMMs with IL-4 does not alter M-CSF signaling, it reversibly blocks RANKL-dependent activation of the NF-kappa B, JNK, p38, and ERK signals. IL-4 also selectively inhibits TNF signaling, while enhancing that of IL-1. Contrary to previous reports, we find that MEK inhibitors dose-dependently inhibit OC differentiation. To identify more proximal signals mediating inhibition of OC formation by IL-4, we used mice lacking STAT6 or SHIP1, two adapter proteins that bind the IL-4 receptor. IL-4 fails to inhibit RANKL/M-CSF-induced osteoclastogenesis by BMMs derived from STAT6-, but not SHIP1-, knockout mice. Consistent with this observation, the inhibitory effects of IL-4 on RANKL-induced NF-kappa B and mitogen-activated protein kinase activation are STAT6-dependent. We conclude that IL-4 reversibly arrests osteoclastogenesis in a STAT6-dependent manner by 1) preventing I kappa B phosphorylation and thus NF-kappa B activation, and 2) blockade of the JNK, p38, and ERK mitogen-activated protein kinase pathways.

Abnormal osteoclast morphology and bone remodeling in a murine model of a lysosomal storage disease

Mucopolysaccharidosis type VII (MPS VII) is a heritable lysosomal storage disease caused by a deficiency in beta-glucuronidase (GUSB) activity, leading to progressive accumulation of undegraded glycosaminoglycans in many tissues. Clinical features include growth and mental retardation, hearing and visual defects, shortened lifespan, and skeletal deformities. A murine model of MPS VII has been described that shares many of the manifestations of the human disease, including the skeletal dysplasia. In this study we describe abnormalities in the cellular morphology and function of osteoclasts and a localized defect in bone formation rate in the MPS VII mouse. Ultrastructural analysis revealed that MPS VII osteoclasts fail to form ruffled border membranes and many appeared to be detached from the bone surface. Following bone marrow transplantation, osteoclasts derived from wild-type donors showed normal morphology and were closely associated with the bone surface in MPS VII recipients. In vitro bone resorption assays demonstrated that MPS VII osteoclasts formed significantly smaller and fewer pits than those formed by osteoclasts derived from normal mice of the same strain. Although osteoclast morphology and function appeared to be abnormal in the MPS VII mouse, interleukin-1 (IL-1)-induced osteoclastogenesis in vivo was not affected. In addition to the osteoclast defects, MPS VII mice demonstrated a slower rate of bone matrix deposition in the epiphysis by in vivo calcein labeling experiments. These data suggest that abnormal morphology and function of MPS VII osteoclasts, combined with deficient matrix deposition, may contribute to the skeletal defects observed in this lysosomal storage disease.

Crystal structure of the TRANCE/RANKL cytokine reveals determinants of receptor-ligand specificity

RANK, the receptor activator of NF-kappaB, and its ligand RANKL (initially termed TRANCE, also termed ODF and OPGL), are a TNF superfamily receptor-ligand pair that govern the development and function of osteoclasts, lymphoid tissue, and mammary epithelium. While TNF family cytokines share a common structural scaffold, individual receptor-ligand pairs associate with high specificity. Given the low level of amino acid conservation among members of the TNF superfamily, the means by which these molecules achieve specificity cannot be completely understood without knowledge of their three-dimensional structures. To determine the elements of RANKL that mediate RANK activation, we have crystallized the ectodomain of murine RANKL and solved its structure to a resolution of 2.6 A. RANKL self-associates as a homotrimer with four unique surface loops that distinguish it from other TNF family cytokines. Mutagenesis of selected residues in these loops significantly modulates RANK activation, as evidenced by in vitro osteoclastogenesis, thereby establishing their necessity in mediating the biological activities of RANKL. Such structural determinants of RANKL-RANK specificity may be of relevance in the pharmacologic design of compounds to ameliorate osteopenic disorders of bone.

TAT fusion proteins containing tyrosine 42-deleted IkappaBalpha arrest osteoclastogenesis

In most circumstances, NF-kappaB, which is essential for osteoclastogenesis, is activated following serine 32/36 phosphorylation of its cytosolic inhibitory protein, IkappaBalpha. In contrast to other cell types, IkappaBalpha, in bone marrow macrophages (BMMs), which are osteoclast precursors, is tyrosine-phosphorylated by c-Src kinase. To address the role of IkappaBalpha phosphorylation in osteoclastogenesis, we generated TAT fusion proteins containing wild-type IkappaBalpha (TAT-WT-IkappaB), IkappaBalpha lacking its NH(2)-terminal 45 amino acids (TAT-IkappaB(46-317)), and IkappaBalpha in which tyrosine residue 42, the c-Src target, is mutated into phenylalanine (TAT-IkappaB(Y42F)). TAT-IkappaB efficiently enters BMMs, and the NF-kappaB-inhibitory protein, once intracellular, is functional. While TAT-WT-IkappaB only slightly inhibits osteoclastogenesis, osteoclast recruitment is diminished >80% by TAT-IkappaB(46-317), an event mirrored by dentin resorption. The fact that TAT alone does not impact osteoclastogenesis, which also resumes following withdrawal of TAT-IkappaB(46-317), establishes that the mutant's anti-osteoclastogenic properties do not reflect toxicity. Affirming a functional role for IkappaB(Tyr(42)) in osteoclastogenesis, TAT-IkappaB(Y42F) is as efficient as TAT-IkappaB(46-317) in blocking osteoclast differentiation. Thus, dominant-negative IkappaBalpha constructs block osteoclastogenesis, and Tyr(42) is essential to the process, increasing the possibility that nonphosphorylatable forms of IkappaBalpha may be a means of preventing pathological bone loss.

Integrin αvβ3 mediates K1735 murine melanoma cell motility in vivo and in vitro

The integrin αvβ3 has been shown to be tightly linked to progression of human melanoma. In this study, using two clones from the K1735 murine melanoma system, we investigated the role of αvβ3 in metastasis. The highly metastatic K1735M2 cells express the αvβ3 integrin, whereas the poorly metastatic K1735C23 cells do not. When transduced with the β3 integrin subunit cDNA, the K1735C23 cells produced lung lesions and, in two animals, cardiac metastases, whereas the parental C23 cells did not. By contrast, transduction of the full-length β3 integrin antisense DNA into the K1735M2 cells suppressed metastatic colonization. To specifically investigate the activation of β3 integrin-mediated pathways, the β3-positive and the β3-negative K1735 cells were plated onto vitronectin, a major matrix molecule of both primary and metastatic melanomas. Tyr397 of FAK was phosphorylated several times higher in β3-expressing K1735 melanoma cells than in β3-negative cells. To determine whether phosphorylation of FAK was associated with K1735 melanoma motility, we expressed the FAK-related non-kinase (FRNK) in the highly metastatic K1735M2 cells. Exogenous expression of FRNK suppressed phosphorylation of FAK at Tyr397 and decreased the invasive ability of these cells. In addition, expression of a constitutively active mutant Src in poorly metastatic K1735C23 cells increased invasion in vitro; whereas expression of a kinase-inactive Src mutant suppressed invasion. Our results suggest that signals initiated by αvβ3 promote metastasis in K1735 melanoma cells through the phosphorylation of FAK and activation of Src.

A Glanzmann's mutation in beta 3 integrin specifically impairs osteoclast function

Osteoclastic bone resorption requires cell-matrix contact, an event mediated by the alpha v beta 3 integrin. The structural components of the integrin that mediate osteoclast function are, however, not in hand. To address this issue, we generated mice lacking the beta 3 integrin gene, which have dysfunctional osteoclasts. Here, we show the full rescue of beta 3(-/-) osteoclast function following expression of a full-length beta 3 integrin. In contrast, truncated beta 3, lacking a cytoplasmic domain (h beta 3c), is completely ineffective in restoring function to beta 3(-/-) osteoclasts. To identify the components of the beta 3 cytoplasmic domain regulating osteoclast function, we generated six point mutants known, in other circumstances, to mediate beta integrin signaling. Of the six, only the S(752)P substitution, which also characterizes a form of the human bleeding disorder Glanzmann's thrombasthenia, fails to rescue beta 3(-/-) osteoclasts or restore ligand-activated signaling in the form of c-src activation. Interestingly, the double mutation Y(747)F/Y(759)F, which disrupts platelet function, does not affect the osteoclast. Thus similarities and distinctions exist in the mechanisms by which the beta 3 integrin regulates platelets and osteoclasts.

Superactivation of integrin (α)v(β)3 by low antagonist concentrations

Integrins are implicated in cell adhesion, migration and homeostasis. An important feature is their ability to adopt different affinity states that can be regulated by a variety of intra- and extracellular factors. To study affinity modulation of the integrin ectodomain by extracellular factors, we produced a soluble recombinant form of mouse integrin (α)v(β)3 in a mammalian expression system and isolated it to purity. We show that the two transmembrane truncated integrin subunits stably associate to form a functional receptor, soluble recombinant (α)v(β)3. The affinity of this receptor for its ligands vitronectin, fibronectin and fibrinogen can be modulated by the divalent cations magnesium, calcium and manganese. Most importantly, we found that a cyclic RGD-peptide has a biphasic effect on rs(α)v(β)3and native purified (α)v(β)3, with an antagonistic phase at high concentrations, and an agonistic phase at low concentrations. This integrin superactivation by low antagonist concentrations is shown in binding of sr(α)v(β)3 to immobilized ligands by ELISA, and in adhesion of cells that express the chimaeric integrin ligand KISS31 to immobilized rs(α)v(β)3 and native purified (α)v(β)3. Our results indicate that low concentrations of the ligand mimetic cyclo-RGD can result in superactivation of the extracellular domain of integrin (α)v(β)3 to a comparable level as activation by manganese.

Cloning and characterization of the murine beta(3) integrin gene promoter: identification of an interleukin-4 responsive element and regulation by STAT-6

Expression of the alpha(v)beta(3) integrin by murine bone marrow macrophages is regulated by cytokines such as IL-4 and GM-CSF through transcriptional activation of the beta(3) subunit gene. To characterize the molecular mechanisms by which such regulation occurs, we isolated the murine beta(3) integrin promoter. To this end, we first cloned a full length beta(3) cDNA and used the 5'UTR and leader peptide coding sequence to identify genomic clones containing the beta(3) promoter region. The transcriptional start site, identified by primer extension and S1 nuclease assay, is 34 nt upstream of the translation initiation codon. A 1.1 kb fragment of the promoter region drives IL-4 responsive transcription in transiently transfected murine bone marrow macrophages. Deletion analysis of the beta(3) promoter indicates the IL-4 responsive element lies between -465 to -678 nt relative to the transcriptional start site. This promoter fragment contains two overlapping STAT consensus recognition sites and nuclear extracts from BMMs contain an IL-4-inducible DNA binding factor, identified by super shift analysis, as STAT-6. Furthermore, an oligonucleotide which includes the two STAT recognition sites residing in the IL-4 responsive region of the beta(3) promoter, competes for STAT-6 binding. Confirming IL-4 induction of the integrin subunit is specifically mediated by STAT-6, beta(3) mRNA is not enhanced in BMMs derived from STAT-6 deleted mice, which however, retain their capacity to respond to GM-CSF.

Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-kappa B ligand (RANKL)-induced JNK activation

The differentiation of cells of the monocytic lineage into mature osteoclasts (OC) is specifically induced by the tumor necrosis factor-related factor, RANKL (receptor activator of NF-kappaB ligand; also known as OPGL, ODF, or TRANCE). Because inhibition of osteoclastogenesis is one of the main mechanisms by which estrogen (E2) prevents bone loss, it is likely that E2 may regulate either the production of, or the target cell responsiveness to RANKL. We found that E2 decreases the differentiation into OC of both murine bone marrow monocytes and RAW 264.7 cells, a monocytic line, by down-regulating the activation of Jun N-terminal kinase 1 (JNK1). Diminished JNK1 activity results in decreased nuclear levels of the key osteoclastogenic transcription factors, c-Fos and c-Jun, and lower binding of these transcriptional inducers to DNA. Thus, one novel mechanism by which E2 down-regulates osteoclastogenesis is by decreasing the responsiveness of OC precursors to RANKL.

Receptor activator of nuclear factor-kappa b ligand activates nuclear factor-kappa b in osteoclast precursors

Receptor activator of nuclear factor-kappa B ligand [RANK ligand (RANK-L)] stimulates mature osteoclasts to resorb bone, a process associated with NF-kappa B activation. RANK-L also prompts macrophages to develop the osteoclast phenotype. Although NF-kappa B is essential for osteoclast differentiation, it is not known whether RANK-L activates this transcription complex in osteoclast precursors. We report that RANK-L rapidly induces NF-kappa B activation in both authentic osteoclast precursors, namely bone marrow macrophages, and RAW 264.7 cells, a murine macrophage line also capable of RANK-L-mediated osteoclastogenesis. Supershift studies reveal the RANK-L-induced DNA binding moiety contains p50/p65, the most common NF-kappa B complex. Subcellular translocation of p50 and p65 subunits is confirmed by Western blots and immunofluorescence analysis. RANK-L activates NF-kappa B in both bone marrow macrophages and RAW 264.7 cells by serine phosphorylation of I kappa B alpha within 5 min, resulting in rapid I kappa B alpha degradation and resynthesis. Attesting to function, RANK-L treatment of RAW 264.7 cells transiently transfected with a plasmid containing NF-kappa B consensus elements linked to luciferase greatly enhances reporter activity. Our data suggest that activation of the NF-kappa B pathway is an integral component of RANK-L-induced osteoclast differentiation.

TNF-alpha induces osteoclastogenesis by direct stimulation of macrophages exposed to permissive levels of RANK ligand

While TNF-alpha is pivotal to the pathogenesis of inflammatory osteolysis, the means by which it recruits osteoclasts and promotes bone destruction are unknown. We find that a pure population of murine osteoclast precursors fails to undergo osteoclastogenesis when treated with TNF-alpha alone. In contrast, the cytokine dramatically stimulates differentiation in macrophages primed by less than one percent of the amount of RANKL (ligand for the receptor activator of NF-kappaB) required to induce osteoclast formation. Mirroring their synergistic effects on osteoclast differentiation, TNF-alpha and RANKL markedly potentiate NF-kappaB and stress-activated protein kinase/c-Jun NH(2)-terminal kinase activity, two signaling pathways essential for osteoclastogenesis. In vivo administration of TNF-alpha prompts robust osteoclast formation in chimeric animals in which ss-galactosidase positive, TNF-responsive macrophages develop within a TNF-nonresponsive stromal environment. Thus, while TNF-alpha alone does not induce osteoclastogenesis, it does so both in vitro and in vivo by directly targeting macrophages within a stromal environment that expresses permissive levels of RANKL. Given the minuscule amount of RANKL sufficient to synergize with TNF-alpha to promote osteoclastogenesis, TNF-alpha appears to be a more convenient target in arresting inflammatory osteolysis.

Transforming growth factor-beta up-regulates the beta 5 integrin subunit expression via Sp1 and Smad signaling

Integrin-mediated cell-matrix interactions play important roles in regulating cell function. Since transforming growth factor-beta (TGF-beta) modulates many osteoblast activities, we hypothesized that the growth factor acts in part by modulating integrin expression. TGF-beta increased cell adhesion to vitronectin and up-regulated the surface level of alpha(v)beta(5) via increasing beta(5) protein synthesis by a transcriptional mechanism. Promoter activity analysis demonstrated that a TGF-beta-responsive element resides between nucleotides -63 and -44. Electrophoretic mobility shift assay and immunoprecipitation/Western studies indicated that the nuclear complex formed using the -66/-42 oligonucleotide contained both Sp1/Sp3 and Smad proteins. Since nuclear Sp1/Sp3 levels were not altered, whereas Smad levels were increased by TGF-beta, we investigated the roles of Smad proteins in the up-regulation of beta(5) gene activation. Co-transfection of cells with beta(5) promoter reporter construct and expression vectors for Smad3, Smad4, and Sp1 increased the stimulatory effect of TGF-beta. Furthermore, expression of dominant negative Smad3 or Smad4 in cells decreased or abolished the stimulation of beta(5) promoter activity by TGF-beta. Smad4 mutant also inhibited the up-regulation of surface beta(5) level by TGF-beta. Thus, TGF-beta increases expression of the integrin beta(5) gene by mechanisms involving Sp1/Sp3 and Smad transcription factors.

Tumor necrosis factor receptors types 1 and 2 differentially regulate osteoclastogenesis

The potent osteoclastogenic agent, tumor necrosis factor-alpha (TNF), exerts its biological effects via two receptors, namely TNF receptors 1 (p55r) and 2 (p75r), each present on osteoclast precursors. Thus, we asked if p55r and p75r differentially impact the osteoclastogenic process. Marrow derived from mice expressing only p55r generates substantially more osteoclasts, in the basal state, than does wild type, while marrow expressing only p75r, produces substantially fewer. Reflecting its preferential activation of p55r, exogenous TNF stimulates osteoclast formation by p55r(+/+)p75r(-/-), but not p55r(-/-)p75r(+/+), marrow. Consistent with the fact that NF-kappaB is essential for osteoclastogenesis, this transcription complex is activated, relative to wild type, in p55r(+/+)p75r(-/-) osteoclast precursors and suppressed in those expressing only p75r. Because p55r enhances, and p75r suppresses, osteoclastogenesis, we asked if their principal ligands, namely soluble and membrane-residing TNF, respectively, differentially impact basal osteoclast recruitment. We find, in contrast to the significant level of osteoclast formation in wild type marrow, osteoclastogenesis by that derived from mice expressing membrane, but not soluble, TNF, is negligible. Thus, optimal therapeutic inhibition of bone resorption may entail selective TNF receptor modulation and/or arrested cleavage of membrane TNF to its soluble form.

Differential expression of alphav integrins in K1735 melanoma cells

Tumor cell adherence to and migration on the extracellular matrix is an important aspect of cancer progression. This interaction with the extracellular matrix is mediated primarily through the integrin class of cell adhesion molecules. We identified a restricted expression of alphavbeta3 in highly metastatic K1735M2 and of alphavbeta5 in poorly metastatic K1735C23 murine melanoma cells. The highly metastatic cells were ten times more motile on vitronectin and fibronectin and approximately three times more invasive through a reconstituted basement membrane than the poorly metastatic cells. This motility was inhibited by addition of anti-beta3 antibodies. Injection of the alphavbeta3-negative K1735C23 cells into syngeneic mice resulted in the generation of a metastatic variant (K1735C23PM) that neo expressed the alphavbeta3 complex, indicating that expression of alphavbeta3 is required for K1735 melanoma metastasis. Injection of highly metastatic K1735M2 cells in the presence of blocking antibody to beta3 reduced tumor size by approximately 80%. Treatment of the K1735M2 cells with a retroviral antisense beta3 construct significantly reduced their expression of alphavbeta3 and also reduced their motility on extracellular matrix ligands and their invasion through a reconstituted basement membrane. In contrast, when the K1735C23 cells were treated with a construct containing the full-length beta3 cDNA, their motility on extracellular matrix proteins and invasion of a reconstituted basement membrane were significantly increased. These results indicate that alphavbeta3 is required for migration and invasion of K1735 melanoma cells in vitro and primary tumor growth and metastasis in vivo.

Sp1/Sp3 and PU.1 differentially regulate beta(5) integrin gene expression in macrophages and osteoblasts

Murine osteoclast precursors and osteoblasts express the integrin alpha(v)beta(5), the appearance of which on the cell surface is controlled by the beta(5), and not the alpha(v), subunit. Here, we show that a 173-base pair proximal region of the beta(5) promoter mediates beta(5) basal transcription in macrophage (osteoclast precursor)-like and osteoblast-like cells. DNase I footprinting reveal four regions (FP1-FP4) within the 173-base pair region, protected by macrophage nuclear extracts. In contrast, osteoblast nuclear extracts protect only FP1, FP2, and FP3. FP1, FP2, and FP3 bind Sp1 and Sp3 from both macrophage and osteoblast nuclear extracts. FP4 does not bind osteoblast proteins but binds PU.1 from macrophages. Transfection studies show that FP1 and FP2 Sp1/Sp3 sites act as enhancers in both MC3T3-E1 (osteoblast-like) and J774 (macrophage-like) cell lines, whereas the FP3 Sp1/Sp3 site serves as a silencer. Mutation of the FP2 Sp1/Sp3 site totally abolishes promoter activity in J774 cells, with only partial reduction in MC3T3-E1 cells. Finally, we demonstrate that PU.1 acts as a beta(5) silencer in J774 cells but plays no role in MC3T3-E1 cells. Thus, three Sp1/Sp3 sites regulate beta(5) gene expression in macrophages and osteoblast-like cells, with each element exhibiting cell-type and/or activation-suppression specificity.

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

Long-term administration of pharmacological doses of glucocorticoids inhibits bone formation and results in osteoporosis. Since integrin-mediated cell-matrix interactions are essential for osteoblast function, we hypothesized that the detrimental effect of glucocorticoids on bone derived, at least in part, from decreased integrin-matrix interactions. Because alphavbeta3 and alphavbeta5 integrins can interact with several bone matrix proteins, we analyzed the effects of dexamethasone (Dex) on the expression of these integrins in normal human osteoblastic cells. We found adhesion of these cells to osteopontin and vitronectin to be dependent on alphavbeta3 and alphavbeta5, respectively; this ligand specificity was not altered by Dex. The effects of Dex on the adhesion of human osteoblastic cells to osteopontin and vitronectin were biphasic with an increase after 2 days, followed by a decrease after 8 days of treatment. Consistently, surface alphavbeta3 and alphavbeta5 integrins, which were increased after 2 days of Dex treatment, were decreased after 8 days. Similarly, total cellular alphav, beta3, and beta5 proteins, which were increased by Dex early in the culture, were diminished after 8 days. Metabolic labeling studies indicated that Dex exhibited biphasic regulation on the biosynthesis of alphavbeta5, with stimulation observed during the second day of treatment, followed by inhibition during the 8th day of exposure. By contrast, the biosynthesis of alphavbeta3 was inhibited by Dex on day 1 and remained inhibited on day 8. Analysis of the mRNA indicated that alphav and beta5 levels were increased by Dex during early exposure (1-3 days), followed by inhibition after prolonged exposure (>/=7 days). By contrast, Dex decreased beta3 mRNA level at all the time points analyzed. Consistently, Dex decreased beta3 promoter activity after 1 day and persisted over 8-day period. By contrast, Dex stimulated beta5 promoter activity after 1 or 2 days but had no effect after 8 days. To further evaluate mechanism(s) leading to the decreased integrin expression after prolonged Dex treatment, mRNA stability was analyzed. Dex was found to accelerate the degradation of alphav, beta3 and beta5 mRNA after an 8-day treatment. Thus, the regulation of alphavbeta3 was dependent on transcription and posttranscriptional events whereas the expression of alphavbeta5 was dependent mainly on posttranscriptional events after prolonged Dex treatment. In conclusion, Dex exhibited time-dependent regulation on the expression of alphavbeta3 and alphavbeta5 integrins in normal human osteoblastic cells. Short-term exposure to Dex increased the levels of alphavbeta3 and alphavbeta5 on the surface and cell adhesion to osteopontin and vitronectin whereas long-term exposure to Dex decreased the expression of both integrins and inhibited the cell adhesion to matrix proteins.

Mice lacking beta3 integrins are osteosclerotic because of dysfunctional osteoclasts

Osteoclasts express the alphavbeta3 integrin, an adhesion receptor that has been implicated in bone resorption and that is therefore a potential therapeutic target. To assess the role of this heterodimer in skeletal development in vivo, we engineered mice in which the gene for the beta3 integrin subunit was deleted. Bone marrow macrophages derived from these mutants differentiate in vitro into numerous osteoclasts, thus establishing that alphavbeta3 is not necessary for osteoclast recruitment. Furthermore, the closely related integrin, alphavbeta5, does not substitute for alphavbeta3 during cytokine stimulation or authentic osteoclastogenesis. beta3 knockout mice, but not their heterozygous littermates, develop histologically and radiographically evident osteosclerosis with age. Despite their increased bone mass, beta3-null mice contain 3.5-fold more osteoclasts than do heterozygotes. These mutant osteoclasts are, however, dysfunctional, as evidenced by their reduced ability to resorb whale dentin in vitro and the significant hypocalcemia seen in the knockout mice. The resorptive defect in beta3-deficient osteoclasts may reflect absence of matrix-derived intracellular signals, since their cytoskeleton is distinctly abnormal and they fail to spread in vitro, to form actin rings ex vivo, or to form normal ruffled membranes in vivo. Thus, although it is not required for osteoclastogenesis, the integrin alphavbeta3 is essential for normal osteoclast function.

Tumor necrosis factor alpha regulates alpha(v)beta5 integrin expression by osteoclast precursors in vitro and in vivo

Early osteoclast precursors, in the form of murine bone marrow macrophages (BMMs), while expressing no detectable alpha(v)beta3 integrin, contain abundant alpha(v)beta5 and attach to matrix in an alpha(v) integrin-dependent manner. Furthermore, alpha(v)beta5 expression by osteoclast precursors progressively falls as they assume the resorptive phenotype. We find the osteoclastogenic agent, tumor necrosis factor-alpha, (TNF) down-regulates alpha(v)beta5 expression by BMMS via attenuation of beta5 messenger RNA (mRNA) t1/2. Using BMMs from TNF receptor knockout mice we establish the p55 receptor transmits the beta5 suppressive effect. The functional implications of TNF-mediated alpha(v)beta5 down-regulation are underscored by the capacity of an alpha(v) inhibitory peptide mimetic to prevent spreading by BMMs expressing abundant alpha(v)beta5 while failing to impact those in which the integrin has been diminished by TNF. Finally, beta5 mRNA in BMMs of wild-type mice administered lipopolysaccharide (LPS) progressively falls with time of in vivo treatment. Alternatively, beta5 mRNA does not decline in BMMs of LPS-treated mice lacking both TNF receptors, documenting down-regulation of the beta5 integrin subunit, in vivo, is mediated by TNF. Thus, matrix attachment of osteoclast precursors and mature osteoclasts are governed by distinct alpha(v) integrins which are differentially regulated by specific cytokines.