Exacerbated tissue destruction in DSS-induced acute colitis of OPN-null mice is associated with downregulation of TNF-alpha expression and non-programmed cell death

Osteopontin (OPN), a pro-inflammatory mediator, is constitutively expressed in normal gut and is upregulated in inflammatory colitis. To determine the significance of OPN in inflammatory bowel disease, we studied the development of acute, experimental colitis induced by dextran sulfate sodium (DSS) in OPN-null and wild-type (WT) mice. OPN expression was markedly increased in WT diseased colons, while a higher disease activity index, including spleen enlargement, bowel shortening, and mucosal destruction, was observed in OPN-null mice. Although peripheral blood neutrophil numbers were lower in DSS-treated OPN-null mice, tissue myeloperoxidase levels, reflecting enhanced neutrophil activity, were increased in the diseased colons. In comparison, lymphocyte numbers in peripheral blood were increased earlier than in DSS-treated WT mice. Despite a significantly greater spleen enlargement, flow cytometric analysis of splenocytes from the DSS-treated OPN-null mice revealed lower numbers of differentiated macrophages and (CD4+ and CD8alpha+) lymphocytes. Whereas pro-inflammatory cytokines, including G-CSF, RANTES, MIP1alpha, and TNF-alpha, were increased < 10-fold in DSS-treated WT splenocytes, expression of these cytokines was dramatically suppressed in the DSS-treated OPN-null splenocytes as well as gut tissues. The suppressed TNF-alpha response in OPN-null mice was reflected in a marked increase in non-apoptotic cell death in diseased colons. Collectively, these studies demonstrate that OPN is required for mucosal protection in acute inflammatory colitis.

Osteopontin is associated with nuclear factor κB gene expression during tail-suspension-induced bone loss

Osteoporosis due to unloading-induced bone loss is a critical issue in the modern aging society. Although the mechanisms underlying this phenomenon are largely unknown, osteopontin (OPN) is one of the critical mediators required for unloading-induced bone loss [M. Ishijima, S.R. Rittling, T. Yamashita, K. Tsuji, H. Kurosawa, A. Nifuji, D.T. Denhardt, and M. Noda, Enhancement of osteoclastic bone resorption and suppression of osteoblastic bone formation in response to reduced mechanical stress do not occur in the absence of osteopontin, J Exp Med, 193 (2001) 399-404]. To clarify the molecular bases for OPN actions, we carried out microarray analyses on the genes expressed in the femoral bone marrow cells in wild type and OPN-/- mice. The removal of the mechanical load induced bone loss in wild type, but not in OPN-/- mice, as previously reported. Expression analysis of 9586 cDNAs on a microarray system revealed that OPN deficiency blocked tail-suspension-induced expression of ten genes (group A). This observation was confirmed based on semi-quantitative RT-PCR analyses. On the other hand, expression of four genes (group B) was not altered by tail suspension in wild type but was enhanced in OPN-deficient mice. NF-kappaB p105 subunit gene (Nfkb1) was found in group A and Bax in group B. p53 gene expression was upregulated by tail suspension in wild type mice, but it was no longer observed in OPN-/- mice. These data indicate that OPN acts to mediate mechanical stress signaling upstream to the genes encoding apoptosis-related molecules, and its action is associated with alteration of the genes.

Osteopontin deficiency suppresses high phosphate load-induced bone loss via specific modulation of osteoclasts

Phosphate (Pi) plays a critical role in the maintenance of mineralized tissues and signaling in the intracellular environment. Although extracellular phosphate concentration is maintained at fixed levels, physiological machineries involved in phosphate homeostasis in bone, which is the largest phosphate storage site, have not yet been fully elucidated. Here we examined the role of osteopontin (OPN) in a high-Pi diet load-induced bone loss. A high-Pi diet significantly reduced bone mineral density as well as bone mass in wild type. In contrast, OPN deficiency totally prevented reduction in bone mineral density and bone mass. Analyses of bone turnover-related components revealed that bone formation parameters (bone formation rate and mineral apposition rate) were enhanced by high-Pi diet load similarly in wild-type and OPN-deficient mice. In sharp contrast, bone resorption parameters (osteoclast number and osteoclast surface) were enhanced by high-Pi diet load in wild type but not at all in OPN-deficient mice. Bone marrow cell cultures revealed no major effects of OPN deficiency on high-Pi diet modulation of mineralized nodule formation in culture. On the other hand, tartrate-resistant acid phosphatase-positive multinucleated cell development in cultures were enhanced by high-Pi diet load in wild-type cells, but such effects of high Pi-diet were totally abolished in the absence of OPN. These data indicated that OPN is needed for osteoclastic activity to resorb bone on high phosphate loading.

An osteopontin-NADPH oxidase signaling cascade promotes pro-matrix metalloproteinase 9 activation in aortic mesenchymal cells

Osteopontin (OPN) is a cytokine upregulated in diabetic vascular disease. To better understand its role in vascular remodeling, we assessed how OPN controls metalloproteinase (MMP) activation in aortic adventitial myofibroblasts (AMFs) and A7r5 vascular smooth muscle cells (VSMCs). By zymography, OPN and tumor necrosis factor (TNF)-alpha preferentially upregulate pro-matrix metalloproteinase 9 (pro-MMP9) activity. TNF-alpha upregulated pro-MMP9 in AMFs isolated from wild-type (OPN(+/+)) mice, but pro-MMP9 induction was abrogated in AMFs from OPN(-/-) mice. OPN treatment of VSMCs enhanced pro-MMP9 activity, and TNF-alpha induction of pro-MMP9 was inhibited by anti-OPN antibody and apocynin. Superoxide and the oxylipid product 8-isoprostaglandin F(2) alpha-isoprostane (8-IsoP) were increased by OPN treatment, and anti-OPN antibody suppressed 8-IsoP production. Like OPN and TNF-alpha, 8-IsoP preferentially activated pro-MMP9. Superoxide, 8-IsoP, and NADPH oxidase 2 (Nox2) subunits were reduced in OPN(-/-) AMFs. Treatment of A7r5 VSMCs with OPN upregulated NADPH oxidase subunit accumulation. OPN structure/function studies mapped these activities to the SVVYGLR heptapeptide motif in the thrombin-liberated human OPN N-terminal domain (SLAYGLR in mouse OPN). Treatment of aortic VSMCs with SVVYGLR upregulated pro-MMP9 activity and restored TNF-alpha activation of pro-MMP9 in OPN(-/-) AMFs. Injection of OPN-deficient OPN(+/-) mice with SVVYGLR peptide upregulated pro-MMP9 activity, 8-IsoP levels, and Nox2 protein levels in aorta and increased panmural superoxide production (dihydroethidium staining). At equivalent hyperglycemia and dyslipidemia, 8-IsoP levels and aortic pro-MMP9 were reduced with complete OPN deficiency in a model of diet-induced diabetes, achieved by comparing OPN(-/-)/LDLR(-/-) versus OPN(+/-)/LDLR(-/-) siblings. Thus, OPN provides a paracrine signal that augments vascular pro-MMP9 activity, mediated in part via superoxide generation and oxylipid formation.

A Mouse Model of Breast Cancer Metastasis to the Choroid of the Eye

Transformed mouse mammary epithelial cells, r3T, injected into the arterial circulation form bone metastases with high frequency. Here we report that metastases to the choroid of the eye also occur in these mice with a penetrance of at least 50%. The tumors can occupy as much as half the volume of the eye, and pigmented cells become incorporated into and distributed throughout the tumors. Pigmentation is also observed in the brains and optic nerves of mice with choroidal tumors, suggesting that the tumor cells stimulate migration of pigmented cells along the optic nerve into the brain. To our knowledge, this is the first mouse model of breast cancer choroidal metastasis, and should be useful in the study of this disease.

Osteopontin participates in Th1-mediated host resistance against nonlethal malaria parasite Plasmodium chabaudi chabaudi infection in mice

Osteopontin (OPN) knockout mice (OPN-KO mice) died of Plasmodium chabaudi chabaudi infection, although wild-type (WT) mice had self-limiting infections. OPN was detected in the WT mice at 2 days postinfection. OPN-KO mice produced significantly smaller amounts of interleukin-12 and gamma interferon than WT mice produced. These results suggested that OPN is involved in Th1-mediated immunity against malaria infection.

Override of the osteoclast defect in osteopontin-deficient mice by metastatic tumor growth in the bone

Osteopontin (OPN) is a major noncollagenous protein of bone that is frequently up-regulated in tumors, where it enhances tumor growth. OPN-deficient mice are resistant to stimulated bone resorption, including that occurring after ovariectomy. Using a new syngeneic model of bone metastasis (r3T), we examined whether OPN-deficient mice are similarly resistant to bone loss resulting from osteolytic tumor growth. Transformed mammary epithelial cells, r3T, which express parathyroid hormone-related protein but not receptor activator of nuclear factor-kappaB ligand, were injected via the intracardiac route into both wild-type and OPN-/- mice. We measured tumor burden in the bone by quantitative polymerase chain reaction assay and evaluated bone loss by X-ray and microCT. Unexpectedly, bone loss was similar in OPN-/- and wild-type mice bearing similar-sized tumors. Osteoclast number was comparable in both genotypes, and the expression of bone sialoprotein was similar in tumor-bearing bones of both genotypes, excluding two potential mechanisms of overriding the defect. Taken together, these results indicate that in the absence of OPN, the bone loss associated with tumor growth at the bone site proceeds rapidly despite the osteoclast defects documented in OPN-/- mice, suggesting that the mechanism of bone loss due to tumor growth differs from that occurring in other pathologies.

Aprt/Opn double knockout mice: osteopontin is a modifier of kidney stone disease severity

BACKGROUND

Osteopontin (OPN) is reported to have two distinct functions in kidney disease: Promotion of inflammation at sites of tissue injury, and inhibition of calcium oxalate monohydrate stone formation. However, many of the studies supporting these functions were carried out in animal models of acute renal injury or in cultured cells; thus, the role of OPN in chronic renal disease is not well defined. We examined the role of OPN in adenine phosphoribosyltransferase (Aprt) knockout mice, in which inflammation and formation of 2,8-dihydroxyadenine (DHA) kidney stones are prominent features, by generating Aprt/Opn double knockout mice.

METHODS

We characterized the phenotypes of six- and 12-week-old Aprt-/- Opn-/-, Aprt-/- Opn+/+, Aprt+/+ Opn-/-, and Aprt+/+ Opn+/+ male and female mice using biochemical, histologic, immunohistochemical, and in situ hybridization techniques.

RESULTS

At 6 weeks of age, there was no difference in phenotype between double knockout and Aprt knockout mice. At 12 weeks, there was increased adenine and DHA excretion, renal crystal deposition, and inflammation in double knockout versus Aprt knockout male mice. Double knockout and Aprt knockout female mice at 12 weeks had less pathology than their male counterparts, but kidneys from double knockout females showed more inflammation compared with Aprt knockout females; both genotypes had similar levels of DHA crystal deposition.

CONCLUSION

We conclude that (1) OPN is a major inhibitor of DHA crystal deposition and inflammation in male mice; and (2) OPN is a major modifier of the inflammatory response but not of crystal deposition in female mice. Thus, separate mechanisms appear responsible for the tissue changes seen in DKO males versus females.

Apatite plaque particles in inner medulla of kidneys of calcium oxalate stone formers: osteopontin localization

BACKGROUND

We have previously shown that interstitial plaque particles appear first in the basement membranes of thin loops of Henle and then in the interstitial space. However, it is not known if the plaque in the basement membrane of thin loops of Henle is of the same or different form than the interstitial plaque. Thus our purpose here is to detail the structure of the interstitial and membrane-bound plaque and explore the relationship of plaque apatite to osteopontin, a well-known crystal-associated urine protein.

METHODS

Deep papillary biopsy tissue was studied from all 15 calcium oxalate stone formers and four nonforming subjects that we previously reported on [Evan et al, J Clin Ivest, 2003]. Routine light and transmission electron microscopy (TEM) as well as light microscopy and TEM immunohistochemical localization of osteopontin antibody were performed on all 19 subjects.

RESULTS

In the basement membrane, plaque particles are individual and appear laminated with alternating light regions of crystal and electron-dense organic layers. In the interstitium, individual particles are not abundant but are instead aggregated to form regions of attached particles and in some regions what appears to be a fusion or syncytium in which crystal islands float in an organic sea. By light microscopy immunohistochemistry, osteopontin was localized to cells of the loops of Henle and collecting ducts as well as on sites of plaque. By immunoelectron microscopy, osteopontin immunogold label was found mainly on the surfaces of apatite crystal phase, at the junction of the crystal/organic layers. A similar immunogold labeling pattern was seen in the particles forming the syncytial islands of interstitial plaque.

CONCLUSION

If indeed we accept the hypothesis that apatite plaque may be an anchored site on which calcium oxalate stones form and grow, the present work makes clear that it is unlikely that the surface of plaque presented to the final urine will be apatite crystal per se. However, our findings clearly show osteopontin is one of the crystal-associated urine proteins involved in the formation of the organic layers of the plaque particles.

Unloading induces osteoblastic cell suppression and osteoclastic cell activation to lead to bone loss via sympathetic nervous system

Osteoporosis is one of the major health problems in our modern world. Especially, disuse (unloading) osteoporosis occurs commonly in bedridden patients, a population that is rapidly increasing due to aging-associated diseases. However, the mechanisms underlying such unloading-induced pathological bone loss have not yet been fully understood. Since sympathetic nervous system could control bone mass, we examined whether unloading-induced bone loss is controlled by sympathetic nervous tone. Treatment with beta-blocker, propranolol, suppressed the unloading-induced reduction in bone mass. Conversely, beta-agonist, isoproterenol, reduced bone mass in loaded mice, and under such conditions, unloading no longer further reduced bone mass. Analyses on the cellular bases indicated that unloading-induced reduction in the levels of osteoblastic cell activities, including mineral apposition rate, mineralizing surface, and bone formation rate, was suppressed by propranolol treatment and that isoproterenol-induced reduction in these levels of bone formation parameters was no longer suppressed by unloading. Unloading-induced reduction in the levels of mineralized nodule formation in bone marrow cell cultures was suppressed by propranolol treatment in vivo. In addition, loss of a half-dosage in the dopamine beta-hydroxylase gene suppressed the unloading-induced bone loss and reduction in mineralized nodule formation. Unloading-induced increase in the levels of osteoclastic activities such as osteoclast number and surface as well as urinary deoxypyridinoline was all suppressed by the treatment with propranolol. These observations indicated that sympathetic nervous tone mediates unloading-induced bone loss through suppression of bone formation by osteoblasts and enhancement of resorption by osteoclasts.

Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size

Stem cells reside in a specialized niche that regulates their abundance and fate. Components of the niche have generally been defined in terms of cells and signaling pathways. We define a role for a matrix glycoprotein, osteopontin (OPN), as a constraining factor on hematopoietic stem cells within the bone marrow microenvironment. Osteoblasts that participate in the niche produce varying amounts of OPN in response to stimulation. Using studies that combine OPN-deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell number and function in a stem cell–nonautonomous manner. The OPN-null microenvironment was sufficient to increase the number of stem cells associated with increased stromal Jagged1 and Angiopoietin-1 expression and reduced primitive hematopoietic cell apoptosis. The activation of the stem cell microenvironment with parathyroid hormone induced a superphysiologic increase in stem cells in the absence of OPN. Therefore, OPN is a negative regulatory element of the stem cell niche that limits the size of the stem cell pool and may provide a mechanism for restricting excess stem cell expansion under conditions of niche stimulation.

The cytokine osteopontin modulates the severity of rotavirus diarrhea

Osteopontin (OPN) is a sialated phosphoprotein found in tissues and secreted into body fluids. It is an integrin ligand with pleiotropic functions as an extracellular matrix protein in mineralized tissues and a cytokine that is active in cell signaling (A. B. Tuck, C. Hota, S. M. Wilson, and A. F. Chambers, Oncogene 22:1198-1205, 2003). To determine whether OPN may be important in mucosal defense against viral pathogens, we evaluated the OPN response to rotavirus infection and the extent of diarrhea manifested by infected opn null mutant (opn-/-) mice. Reverse transcription-PCR, Northern and Western blots, and immunohistochemical studies of the HT-29 intestinal epithelial cell line and murine intestine were used to evaluate OPN mRNA and product. Intestinal closed loops and diarrheal observations determined disease severity and duration. OPN mRNA levels increased after infection of HT-29 cells, peaking in 4 to 6 h. Infected cultures contained 925 microg of OPN/ml, while for controls the levels were below detection (50 microg/ml). Infection increased OPN mRNA levels in intestinal tissue between 2 and 24 h postinoculation and increased OPN protein in intestinal fluid. The cellular localization of OPN was supranuclear and apical, and responding cells were diffusely distributed on the villus surface. Three days after infection, closed intestinal loops from opn-/- mice contained more fluid than loops from controls, although secretion levels at the onset of illness were similar. Null mutant mice experienced more intense and prolonged diarrhea than controls. Rotavirus infection of intestinal epithelial cells and murine intestine caused marked increases in OPN mRNA levels and secreted OPN protein. OPN-deficient mice suffered prolonged disease.

Osteoclasts resorb protein-free mineral (Osteologic discs) efficiently in the absence of osteopontin

Osteopontin (OPN) is both a matrix protein in mineralized tissues and a cytokine, and it has a pivotal role in osteoclast-mediated bone resorption. Here, using a proprietary hydroxyapatite substitute for bone mineral (Osteologic discs), we investigated the requirement for OPN in mineral resorption. Resorption pits formed in the Osteologic discs, revealed by staining with silver nitrite (Von Kossa stain), were analyzed using the NIH Image J program, which can determine the number of pits formed per unit area, their average size, and the fractional area resorbed. After a preincubation of bone marrow cells from OPN -/- and OPN +/+ mice with M-CSF to allow the multiplication of osteoclast precursors on cell culture plastic, osteoclast formation on both Osteologic discs and standard cell culture plates was induced with soluble receptor activator of NFkappaB ligand, sRANKL. We did not detect a dramatic difference in osteoclast formation between OPN +/+ and OPN -/- cells, as judged by staining for tartrate-resistant acid phosphatase in osteoclasts formed on cell culture plastic, nor was there a significant difference in the ability of the osteoclasts to form resorption pits in the Osteologic discs. Additionally, none of six different anti-OPN monoclonal antibodies had a significant and reproducible effect on the formation or subsequent functioning of the OPN+/+ osteoclasts. These studies suggest that, in contrast to what has been found for normal bone, the efficiency of dissolution of a ceramic, protein-free (excepting protein adsorbed from the culture medium) hydroxyapatite/tri-calcium phosphate substrate by osteoclasts is not substantially enhanced by endogenous or exogenous OPN.

Role of osteopontin in tumour progression

Since its first identification as a transformation-associated protein, osteopontin (OPN) has been recognised as important in the processes of tumorigenicity and metastasis. Here, we review the evidence that OPN might be considered as a candidate prognostic marker in human cancer. In animal systems, evidence from cell injection experiments and genetically manipulated mice suggest an important but complex role for the protein in tumour progression. Moreover, studies in a variety of human cancers associate high levels of OPN expression in tumours or in blood with more advanced cancers. The mechanism of action of OPN in promoting cancer is still unclear, and we consider aspects of OPN biology that can complicate interpretation of human studies. Nevertheless, growing evidence supports a role for OPN as a potential prognostic factor for various human cancers.

Role of osteopontin in cardiac fibrosis and remodeling in angiotensin II-induced cardiac hypertrophy

Osteopontin (OPN) is upregulated in several experimental models of cardiac fibrosis and remodeling. However, its direct effects remain unclear. We examined the hypothesis that OPN is important for the development of cardiac fibrosis and remodeling. Moreover, we examined whether the inhibitory effect of eplerenone (Ep), a novel aldosterone receptor antagonist, was mediated through the inhibition of OPN expression against cardiac fibrosis and remodeling. Wild-type (WT) and OPN-deficient mice were treated with angiotensin II (Ang II) for 4 weeks. WT mice receiving Ang II were divided into 2 groups: a control group and an Ep treatment group. Ang II treatment significantly elevated blood pressure and caused cardiac hypertrophy and fibrosis in WT mice. Ep treatment and OPN deficiency could reduce the Ang II-induced elevation of blood pressure and ameliorate the development of cardiac fibrosis, whereas Ep-only treatment abolished the development of cardiac hypertrophy. Most compelling, the reduction of cardiac fibrosis led to an impairment of cardiac systolic function and subsequent left ventricular dilatation in Ang II-treated OPN-deficient mice. These results suggest that OPN has a pivotal role in the development of Ang II-induced cardiac fibrosis and remodeling. Moreover, the effect of Ep on the prevention of cardiac fibrosis, but not cardiac hypertrophy, might be partially mediated through the inhibition of OPN expression.

Impaired anti-tumor cytotoxicity of macrophages from osteopontin-deficient mice

Osteopontin (OPN) expression in tumors is associated with more aggressive tumor growth; however, several studies have suggested that OPN as a host protein can regulate tumor growth as well. OPN is produced by macrophages and T cells, and reportedly modifies macrophage function. Here, we have investigated the effect of OPN on macrophage function, and its role in host defense against tumor growth. OPN deficient (-/-) and wild-type (WT) peritoneal macrophages were assessed for their ability to mediate cytotoxicity of tumor cells. Thioglycollate-elicited peritoneal exudate cells (PEC) were stimulated in vitro with interferon-gamma and lipopolysaccharide. [(3)H]Thymidine-labeled ras-transformed tumor cells were then added and (3)H release and nitrite accumulation were measured. OPN -/- PEC exhibited as much as a 70% reduction in cytotoxicity as compared to WT PEC. Tumor cell OPN status, on the other hand, had little effect on the extent of cytotoxicity. Production of nitrite by the PEC correlated with their capacity to kill tumor cells. L-929 cells, which are relatively resistant to nitric oxide-induced cytotoxicity and sensitive to that effected by TNF-alpha, were killed equally well by wild-type and OPN-deficient PEC, suggesting that the effect of OPN is not mediated through TNF-alpha. No difference was seen in the cytotoxicity of resident macrophages from mice of different genotypes, indicating that the defect in the OPN-deficient macrophages may result from altered differentiation in vivo. In support of this idea, we show that the expression of the macrophage markers F4/80 in peritoneal cells and of Mac-2 in spleen cells is altered in OPN -/- mice as compared to WT. These data support the hypothesis that host-derived osteopontin may inhibit tumor growth and provide a mechanism for this effect.

Osteopontin expression in osteoblasts and osteocytes during bone formation under mechanical stress in the calvarial suture in vivo

To clarify the role of OPN in bone formation under mechanical stress, we examined the expression and the function of OPN in bone using an expansion force-induced osteogenesis model. Our results indicated that OPN expression was enhanced during the bone formation and that OPN would be one of the positive factors for the bone formation under mechanical stress.

INTRODUCTION

Bone formation is known to be stimulated by mechanical stress; however, molecules involved in stress-dependent regulation of bone formation have not yet been fully characterized. Extracellular matrix proteins such as osteopontin (OPN) could play a role in mediation of the mechanical stress signal to osteoblasts. However, the function of OPN in bone formation under mechanical force is not known. Therefore, we examined the expression and the role of OPN in bone formation in vivo under tensile mechanical stress.

MATERIALS AND METHODS

Sagittal sutures of mice were subjected to expansion mechanical stress by setting orthodontic spring wires, and OPN expression during bone formation within the suture gap was examined.

RESULTS

Expansion of the sutures resulted in bone formation at the edges of the parietal bones within the sagittal suture. Immunohistochemical analysis revealed abundant accumulation of OPN protein in the matrix of newly formed bone on the inner edge of the parietal bone within the mechanically expanded sutures. Osteoblasts forming bone within the suture subjected to tensile stress also exhibited high levels of OPN protein expression. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicated that OPN mRNA expression was enhanced in wild-type calvariae subjected to expansion force compared with the control calvariae where dead spring wires were set without expansion stress. In addition, type I collagen mRNA was also expressed in the calvariae under the mechanical stimuli. To understand the function of OPN, sagittal sutures in OPN-deficient mice were subjected the expansion stress, and bone formation within the suture to fill the expanded gap was compared with that observed in wild-type mice. OPN deficiency reduced bone formation at the edge of the parietal bone in contact with the expanded suture gap.

CONCLUSIONS

These observations revealed that OPN plays a pivotal role in bone formation under tensile mechanical stress.

Rho-dependent Rho kinase activation increases CD44 surface expression and bone resorption in osteoclasts

Osteoclasts from osteopontin-deficient mice exhibit decreased CD44 surface expression [corrected]. Osteopontin (OPN)/alphavbeta3 generated Rho signaling pathway is required for the surface expression of CD44. In this work we show the Rho effector, Rho kinase (ROK-alpha), to be a potent activator of CD44 surface expression. ROK-alpha activation was associated with autophosphorylation, leading to its translocation to the plasma membrane, as well as its association with CD44. ROK-alpha promoted CD44 surface expression through phosphorylation of CD44 and ezrin-radixin-moesin (ERM) proteins and CD44.ERM.actin complex formation. Osteoclasts from OPN-/- mice exhibited an approximately 55-60% decrease in basal level ROK-alpha phosphorylation as compared with wild type osteoclasts. Furthermore, RhoVal-14 transduction was only partially effective in stimulating ROK-alpha/CD44 phosphorylation, as well as CD44 surface expression, in these osteoclasts. Studies on the inhibition of Rho by C3 transferase or ROK-alpha by the specific inhibitor, Y-27632, showed a decrease in the phosphorylation mediated by ROK-alpha and CD44 surface expression. Neutralizing antibodies to alphav, beta3, or CD44 inhibited the migration and bone resorption of wild type osteoclasts. However, only anti-alphav or -beta3 antibodies blocked OPN-induced phosphorylation of ROK-alpha, CD44, and the ERM proteins. Our results strongly suggest a role for ROK-alpha in alphavbeta3-mediated Rho signaling, which is required for the phosphorylation events and CD44 surface expression. The functional deficiencies in the Rho effector(s) because of the lack of OPN were associated with decreased CD44 surface expression and hypomotility in the OPN-/- osteoclasts. Finally, we find that cooperativity exists between alphavbeta3 and CD44 for osteoclast motility and bone resorption.

Osteopontin deficiency attenuates atherosclerosis in female apolipoprotein E-deficient mice

OBJECTIVE

Osteopontin (OPN), a noncollagenous adhesive protein, is implicated in atherosclerosis, in which macrophages within atherosclerotic plaques express OPN. However, it is not known whether the elevated OPN expression is a cause or result of atherosclerosis.

METHODS AND RESULTS

We generated mice that lacked OPN and crossed them with apolipoprotein (apo) E-deficient mice and analyzed these mice with a mixed C57BL/6x129 background after 36 weeks on a normal chow diet. In female mice, OP+/-E-/- and OP-/-E-/- mice had significantly smaller atherosclerotic and inflammatory lesions compared with OP+/+E-/- mice, and that was reflected by smaller area of MOMA-2-positive staining. In male mice, however, there was no significant difference in the atherosclerosis lesion areas among 3 genotypes. In both OP-/-E-/- and OP+/+E-/- mice, typical atherosclerotic lesions were detected, which include necrotic core, foamy cell collections, and cholesterol clefts. However, we found that vascular mineral-deposited areas in 60-week-old male OP-/-E-/- mice were significantly increased compared with those in OP+/+E-/- male mice.

CONCLUSIONS

These results suggest that OPN plays a promoting effect in atherosclerosis and inhibitory effect in vascular calcification. The suppression of OPN expression in females should be considered a therapeutic possibility in atherosclerosis.

Novel murine mammary epithelial cell lines that form osteolytic bone metastases: effect of strain background on tumor homing

We have developed a series of novel mammary epithelial cell lines from tumors arising in strain 129 mice, with the ultimate goal of evaluating the role of host factors in the development of bone metastases. Mammary tumors were induced in mice with subcutaneously implanted medroxyprogesterone acetate (MPA) pellets followed by administration of DMBA by oral gavage. Mammary tumor development was efficient in the 129 strain and was independent of osteopontin (OPN) expression. Epithelial cell lines were isolated from these tumors; surprisingly, these cells did not form tumors upon inoculation into the mammary fat pad of syngeneic mice, even when MPA was present. One OPN-deficient cell line was selected for further study; full transformation of these cells required expression of both polyoma middle T and activated ras. These doubly transfected cells, 1029 GP+Er3, grew in soft agar, and formed hormone-independent tumors efficiently in the mammary fat pad that spontaneously metastasized to several soft tissue sites but not to the bone. Derivatives of these cells were isolated from tumors arising in the fat pad and from a lung metastasis (r3T and r3L, respectively): these cells formed tumors more rapidly in the fat pad than the parental GP+Er3 cells. Upon left ventricle injection, the r3T and r3L cells formed osteolytic bone metastases in 129 mice, with few metastases seen in other organs. These tumors filled the marrow cavity, and caused extensive destruction of both cortical and trabecular bone. Intriguingly, in an alternative syngeneic host, (129xC57B1/6) F1, osteolytic bone metastases were not seen on x-ray; instead extensive liver metastasis was present in these mice, indicating that genetic factors in these two strains regulate tumor cell homing and distribution during metastasis. These cell lines provide an important new tool in the study of bone metastasis, particularly in elucidating the role of host factors in the development of these lesions, as the 129 mouse strain is frequently used for genetic manipulations in the mouse.

Osteopontin deficiency induces parathyroid hormone enhancement of cortical bone formation

Intermittent PTH treatment increases cancellous bone mass in osteoporosis patients; however, it reveals diverse effects on cortical bone mass. Underlying molecular mechanisms for anabolic PTH actions are largely unknown. Because PTH regulates expression of osteopontin (OPN) in osteoblasts, OPN could be one of the targets of PTH in bone. Therefore, we examined the role of OPN in the PTH actions in bone. Intermittent PTH treatment neither altered whole long-bone bone mineral density nor changed cortical bone mass in wild-type 129 mice, although it enhanced cancellous bone volume as reported previously. In contrast, OPN deficiency induced PTH enhancement of whole-bone bone mineral density as well as cortical bone mass. Strikingly, although PTH suppressed periosteal bone formation rate (BFR) and mineral apposition rate (MAR) in cortical bone in wild type, OPN deficiency induced PTH activation of periosteal BFR and MAR. In cancellous bone, OPN deficiency further enhanced PTH increase in BFR and MAR. Analysis on the cellular bases for these phenomena indicated that OPN deficiency augmented PTH enhancement in the increase in mineralized nodule formation in vitro. OPN deficiency did not alter the levels of PTH enhancement of the excretion of deoxypyridinoline in urine, the osteoclast number in vivo, and tartrate-resistant acid phosphatase-positive cell development in vitro. These observations indicated that OPN deficiency specifically induces PTH activation of periosteal bone formation in the cortical bone envelope.

Transcriptional regulation of osteopontin and the metastatic phenotype: evidence for a Ras-activated enhancer in the human OPN promoter

Elevated osteopontin (OPN) transcription often correlates with increased metastatic potential of transformed cells, and in several model systems OPN--whether produced by the tumor cells or by stromal cells - has been shown to enhance metastatic ability. Sequence elements in the OPN promoter have been identified on the basis of their ability to interact with protein factors associated with the tumorigenic process in one or more cell lineages. One of these is a Ras-activated enhancer (RAE) that binds a protein, the Ras-response factor (RRF), whose ability to form a complex with the RAE is stimulated by Ras signaling in fibroblasts and epithelial cells. Another is the T cell factor-4 binding site, which in the OPN promoter can retard OPN transcription when bound by the Tcf-4 protein. In Rama 37 rat mammary epithelial cells Tcf-4 suppresses OPN transcription and the metastatic phenotype. A third promoter segment consists of two sequences in the -94 to -24 region of the human OPN promoter able to bind several known transcription factors, including Sp1, Myc and Oct-1, which may act synergistically to stimulate OPN transcription in malignant astrocytic cells. Although expression of other genes may also be regulated by these transcription factors, evidence suggests that often OPN alone can stimulate metastasis. In this communication we address two issues: (1) How does OPN facilitate the metastatic phenotype? (2) What mechanisms are responsible for the increase in OPN transcription in metastatic cells?

Osteopontin modulates CD44-dependent chemotaxis of peritoneal macrophages through G-protein-coupled receptors: Evidence of a role for an intracellular form of osteopontin

Expression of osteopontin (OPN) by activated T-cells and macrophages is required for the development of cell-mediated inflammatory responses. Acting through integrin alpha(v)beta(3) and CD44 receptors, OPN can promote chemoattraction and pro-inflammatory cytokine expression by macrophages. In this study, we have used peritoneal macrophages from OPN-/, CD44-/-, and WT mice to study the relationship between OPN and CD44 in macrophage migration. Using confocal microscopy, we show that OPN co-distributes with CD44 inside macrophages at cell edges and in cell processes in a mutually dependent manner. The existence of an intracellular form of OPN is supported by pulse-chase studies in which a thrombin-sensitive, phosphorylated protein immunoprecipitated with OPN antibodies is retained inside macrophages. In OPN-/- and CD44-/- macrophages, the absence of CD44 and OPN, respectively, is associated with the formation of fewer cell processes, reduced cell fusion required to form functional multinucleated osteoclasts in the presence of CSF-1 and RANKL, and impaired chemotaxis. Whereas the chemotaxis of CD44-/- cells to various chemoattractants is almost completely abrogated, a differential effect is seen with the OPN-/- cells. Thus, OPN-/- cells migrate normally towards CSF-1 but not towards fMLP and MCP-1, which signal through G-protein coupled receptors (GPCRs). That the GPCR-mediated migration is dependent upon the level of cell-surface CD44 is indicated by the reduced cell-surface expression of CD44 in OPN-/- cells and a comparable impairment in the chemotaxis of CD44+/- cells. Although chemotaxis of OPN-/- cells could be rescued by an OPN substratum, or by addition of high levels of OPN in solution, no response is evident with physiological levels of OPN, indicating a requirement for the CD44-associated intracellular OPN in CD44 cell-surface expression. These studies indicate, therefore, that the level of cell surface CD44 is critical for GPCR-mediated chemotaxis by peritoneal macrophages and suggest that a novel intracellular form of OPN may modulate CD44 activities involved in these processes.

Osteopontin deficiency produces osteoclast dysfunction due to reduced CD44 surface expression

Osteopontin (OPN) was expressed in murine wild-type osteoclasts, localized to the basolateral, clear zone, and ruffled border membranes, and deposited in the resorption pits during bone resorption. The lack of OPN secretion into the resorption bay of avian osteoclasts may be a component of their functional resorption deficiency in vitro. Osteoclasts deficient in OPN were hypomotile and exhibited decreased capacity for bone resorption in vitro. OPN stimulated CD44 expression on the osteoclast surface, and CD44 was shown to be required for osteoclast motility and bone resorption. Exogenous addition of OPN to OPN-/- osteoclasts increased the surface expression of CD44, and it rescued osteoclast motility due to activation of the alpha(v)beta(3) integrin. Exogenous OPN only partially restored bone resorption because addition of OPN failed to produce OPN secretion into resorption bays as seen in wild-type osteoclasts. As expected with these in vitro findings of osteoclast dysfunction, a bone phenotype, heretofore unappreciated, was characterized in OPN-deficient mice. Delayed bone resorption in metaphyseal trabeculae and diminished eroded perimeters despite an increase in osteoclast number were observed in histomorphometric measurements of tibiae isolated from OPN-deficient mice. The histomorphometric findings correlated with an increase in bone rigidity and moment of inertia revealed by load-to-failure testing of femurs. These findings demonstrate the role of OPN in osteoclast function and the requirement for OPN as an osteoclast autocrine factor during bone remodeling.