Tissue and urokinase plasminogen activators in bone tissue and their regulation by parathyroid hormone

Mechanical induction of osteoanabolic Wnt1 promotes osteoblast differentiation via Plat

Physical activity-induced mechanical stimuli play a crucial role in preserving bone mass and structure by promoting bone formation. While the Wnt pathway is pivotal for mediating the osteoblast response to loading, the exact mechanisms are not fully understood. Here, we found that mechanical stimulation induces osteoblastic Wnt1 expression, resulting in an upregulation of key osteogenic marker genes, including Runx2 and Sp7, while Wnt1 knockdown using siRNA prevented these effects. RNAseq analysis identified Plat as a major target through which Wnt1 exerts its osteogenic influence. This was corroborated by Plat depletion using siRNA, confirming its positive role in osteogenic differentiation. Moreover, we demonstrated that mechanical stimulation enhances Plat expression, which, in turn leads to increased expression of osteogenic markers like Runx2 and Sp7. Notably, Plat depletion by siRNA prevented this effect. We have established that Wnt1 regulates Plat expression by activating β-Catenin. Silencing Wnt1 impairs mechanically induced β-Catenin activation, subsequently reducing Plat expression. Furthermore, our findings showed that Wnt1 is essential for osteoblasts to respond to mechanical stimulation and induce Runx2 and Sp7 expression, in part through the Wnt1/β-Catenin/Plat signaling pathway. Additionally, we observed significantly reduced Wnt1 and Plat expression in bones from ovariectomy (OVX)-induced and age-related osteoporotic mouse models compared with non-OVX and young mice, respectively. Overall, our data suggested that Wnt1 and Plat play significant roles in mechanically induced osteogenesis. Their decreased expression in bones from OVX and aged mice highlights their potential involvement in post-menopausal and age-related osteoporosis, respectively.

Interleukin-6 and cyclic AMP stimulate release of cathepsin B in human osteoblasts

Previous studies have suggested that cathepsin B participates in the joint destruction associated with rheumatoid arthritis (RA). This study examined the activity of cathepsin B (a lysosomal cysteine protease) in human osteoblasts along with its regulation by cyclic AMP and Interleukin-6 (IL-6). Cyclic AMP elevating agents activate cathepsin B and stimulate the secretion of cathepsin B via the secretion of IL-6, a potent mediator of RA. This study investigated the induction of cathepsin B using the proinflammatory cytokine in human osteoblasts (MG-63) in relation to p38 mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-kappa B transcription factor. When added to MG-63 cells, IL-6 stimulated the production of cathepsin B, which was reduced significantly by the addition of SB203580, a specific p38 MAPK inhibitor. In addition, the release of IL-6 was also inhibited by either pyrrolidine dithiocarbamate (PDTC) or NF-kappaB SN50, which are potent NF-kappaB inhibitors. Both NF-kappaB inhibitors had a larger inhibitory effect on the activity of cathepsin B in the presence of SB203580. IL-6 stimulated the NF-kappaB binding affinity as well as the activation of p38 MAP kinase, leading to the release of cathepsin B. However, SB203580 had no effect on the IL-6-induced activation of NF-kappaB, and neither of the NF-kappaB inhibitors decreased the level of p38 MAPK activation in the IL-6-stimulated osteoblasts. Moreover, IL-6 increased the activity of urokinase type plasminogen activator (uPA) in MG-63 cells, which was inhibited by SB203580, PDTC and NF-kappaB SN50. This strongly suggests that p38 MAPK and NF-kappaB are essential to the IL-6-induced activation of cathepsin B or uPA and that these two IL-6-activated pathways can act independently.

Stromal cell-derived factor-1 (SDF-1) recruits osteoclast precursors by inducing chemotaxis, matrix metalloproteinase-9 (MMP-9) activity, and collagen transmigration

Signals targeting OCs to bone and resorption sites are not well characterized. A chemoattractant receptor (CXCR4), highly expressed in murine OC precursors, mediated their chemokine (SDF-1)-induced chemoattraction, collagen transmigration, and MMP-9 expression. Thus, bone vascular and stromal SDF-1 may direct OC precursors into bone and marrow sites for development and bone resorption.

INTRODUCTION

Although chemokines are essential for trafficking and homing of circulating hematopoietic cells under normal and pathological conditions, their potential roles in osteoclast (OC) recruitment or function are generally unknown. CXCR4 and its unique ligand, stromal cell-derived factor-1 (SDF-1), critically control the matrix metalloproteinase (MMP)-dependent targeting of hematopoietic cells into bone and within the marrow microenvironment. Therefore, SDF-1/CXCR4 may regulate OC precursor recruitment to sites for development and activation.

METHODS

Chemokine receptor mRNA expression was analyzed during OC formation induced by RANKL in murine RAW 264.7 cells. SDF-1 versus RANKL effects on chemotaxis, transcollagen migration, MMP-9 expression and activity, OC development, and bone resorption were evaluated in RAW cells or RAW-OCs.

RESULTS

CXCR4 was highly expressed in RAW cells and downregulated during their RANKL development into bone-resorptive RAW-OCs. SDF-1, but not RANKL, elicited RAW cell chemotaxis. Conversely, RANKL, but not SDF-1, promoted RAW-OC development, TRAP activity, cathepsin K expression, and bone pit resorption, and SDF-1 did not modify these RANKL responses. Both SDF-1 and RANKL increased MMP-9, a matrix-degrading enzyme essential for OC precursor migration into developing bone marrow cavities, and increased transcollagen migration of RAW cells in a MMP-dependent manner. SDF-1 also upregulated MMP-9 in various primary murine OC precursor cells. Because RANKL induced a higher, more sustained expression of MMP-9 in RAW cells than did SDF-1, MMP-9 may have an additional role in mature OCs. Consistent with this, MMP-9 upregulation during RANKL-induced RAW-OC development was necessary for initiation of bone pit resorption.

CONCLUSIONS

SDF-1, a chemokine highly expressed by bone vascular endothelial and marrow stromal cells, may be a key signal for the selective attraction of circulating OC precursors into bone and their migration within marrow to appropriate perivascular stromal sites for RANKL differentiation into resorptive OCs. Thus, SDF-1 and RANKL likely serve complementary physiological functions, partly mediated through increases in MMP-9, to coordinate stages of OC precursor recruitment, development, and function.

Cathepsin B in osteoblasts

Active cathepsin B has been found in cell extract and medium of human osteoblast-like cells and MG-63 cells. The released form is stable at neutral and alkaline pH and, in both cell types, intracellular and extracellular cathepsin B activities are increased by interleukin-1 beta (IL-1beta) and parathyroid hormone (PTH). To evaluate the physiological role of cathepsin B in osteoblasts, we investigated the production and secretion of this enzyme in normal human synovial fibroblasts and modulation by IL-1beta and PTH. Lactate secretion concurrent with release of cathepsin B and comparable responses in osteoblasts were also examined. Our data show that synovial fibroblasts respond differently to treatment with the two agents, suggesting a cell-specific regulation of cathepsin B and possible involvement in osteoblast physiology. Cathepsin B involvement was then evaluated in the activation of plasminogen activator (PA) in MG-63 cells using two specific inhibitors of cathepsin B, CA074 and CA074-Me, in constitutive conditions and after treatment with IL-1beta. As results of PA activity obtained in the presence of IL-beta were in contrast with previous reports, we examined the activities of PA, pro-PA activated with trypsin, and plasmin in cell extract and media of MG-63 cells after 24-h treatment with IL-1beta. Results show that in normal conditions and in the presence of IL-1beta, cathepsin B is involved in the activation of PA. Moreover, IL-1beta stimulates PA, pro-PA activated by trypsin, and plasmin activity in medium, whereas in cell extract it stimulates pro-PA activated by trypsin and plasmin activity. IL-1beta has no effect on cell extract-associated PA.

Immunolocalization of urokinase and its receptor in prematurely fused cranial sutures of infants

In cranial sutural samples derived from five children with premature cranial suture fusion we have performed immunostaining for the urokinase plasminogen activator (uPA) and urokinase receptor (uPAR). We have found a strong reactivity for cell- or matrix-bound uPA and uPAR in the sutural connective tissue and associated with the osteoblasts and osteocytes lining the calvarial bone. The sutural tissue itself showed a banding with different intensity of urokinase and uPAR staining concerning connective tissue. It is proposed that the components of the plasminogen activating system are involved in tissue turnover of sutural tissue and in sutural growth.

Bone mineral density and blood flow to the lower extremities: the study of osteoporotic fractures

This study tests the hypothesis that reduced blood flow to the lower extremities may affect bone remodeling, resulting in a decrease in bone mineral density (BMD). BMD was measured in the axial and appendicular skeleton of 1292 elderly women (mean age, 71 years) enrolled in the Study of Osteoporotic Fractures. The ratio of the posterior tibial and brachial systolic blood pressures, the ankle/arm index, was used as a measure of blood flow to the legs. In the cross-sectional analysis, this index was positively correlated with BMD at the radius, calcaneus, and hip, but not at the spine. A decrease in the index of 2 standard deviations (SD) (as might occur in patients with moderate arterial disease) was associated with a decrease of 3.7% (95% CI, 1.7%, 5.8%) in hip BMD. The effect size at the hip decreased from 3.7 to 1.8% (and was not statistically significant) when adjustment was made for smoking and/or body mass index (BMI). In the prospective analysis, the rate of bone loss at the hip and calcaneus was greater (p < 0.05) among women whose annual decrease in ankle/arm index was more than 1 SD greater than the mean decrease. This increase was independent of estrogen use, smoking, BMI, pattern of fat distribution, history of diabetes, exercise, and ability to walk. The results from this prospective community-based study provide the first evidence that among relatively healthy older women decreased vascular flow in the lower extremities may be associated with an increased rate of bone loss at the hip and calcaneus.

Bone resorption and response to calcium-regulating hormones in the absence of tissue or urokinase plasminogen activator or of their type 1 inhibitor

Plasminogen activators (PA) are implicated in cell migration and tissue remodeling, two components of the bone resorption processes. Using mice with inactivated tissue PA (tPA), urokinase PA (uPA), or type 1 PA inhibitor (PAI-1) genes, we evaluated whether these processes, or their stimulation by parathyroid hormone (PTH) or 1,25-dihydroxyvitamin (1,25[OH]2D3) are dependent on these genes. Two culture models were used, one involving 19-day fetal calvariae, to evaluate the direct resorptive activity of osteoclasis, and the other involving 45Ca-labeled 17-day fetal metatarsals, in which this activity depends on preliminary (pre)osteoclast migration. PTH similarly increased (about 10-fold) PA activity in calvariae from wild-type tPA+/+ and uPA+/+ or deficient uPA-/- and PAI-/- mice; it affected only tPA, not uPA. In tPA-/- bones, the low PA levels, due to uPA, were not influenced by PTH. Calcitonin did not affect PA responses to PTH. No differences were observed between tPA+/+, tPA-/-, uPA+/+, and uPA-/- calvariae for any parameter related to bone resorption (development of lacunae, release of calcium and lysosomal enzymes, accumulation of collagenase, loss of hydroxyproline), indicating similar responses to PTH or calcitonin. The progressive 45Ca release was largely similar in cultures of tPA+/+, tPA-/-, uPA+/+, uPA-/-, PAI+/+, or PAI-/- metatarsals and it was similarly enhanced by PTH or 1,25(OH)2D3. However, uPA-/- metatarsals released 45Ca at a slower rate at the beginning of the cultures, suggesting an impaired recruitment of the (pre)osteoclasts, which migrate at that time from the periosteum into the calcified cartilage. Thus, it appears that the direct resorptive activity of the osteoclasts does not necessitate the presence of either tPA or uPA, but uPA is likely to facilitate the migration of the (pre)osteoclasts toward the mineralized surfaces. Although considerably enhanced by PTH, tPA does not mediate the actions of PTH (nor of 1,25[OH]2D3) evaluated in these models.

The plasminogen activation system is upregulated in loosening of total hip prostheses

Interface tissues and pseudocapsules from loose total hip replacements were removed during revision of 11 cases and were investigated for the plasminogen activation system and IL-1beta. Control samples of synovium were taken during knee arthroscopy (n 8), and from the hip joint during primary total hip replacement (n 5). The concentrations of urokinase plasminogen activator (uPA), tissue type plasminogen activator (tPA), plasminogen activator inhibitor-1 (PAI-1) and interleukin 1beta were all found to be significantly different in interfaces and in pseudocapsules, compared to controls. Immunohistochemistry disclosed localization in periprosthetic tissues of uPA, uPA-receptor and tPA in macrophages with phagocytosed metal, polyethylene, cement particles or accompanying pieces of necrotic bone. PAI-1 staining was present in the neighboring areas that stained for uPA or tPA, but PAI-1 staining was also found overlapping and outside these areas. These findings suggest a role for the uPA/uPA- receptor and PAI-1 in activation and focalization of extracellular matrix degradation in periprosthetic tissues. The expression of the plasminogen activation system by macrophages containing phagocytosed material suggests undegradable microdebris as a possible initiating and perpetuating stimulus for a proteolytic activation cascade, which may contribute to loosening of the prosthesis.

Modulation of proteases and their inhibitors in immortal human osteoblast-like cells by tumor necrosis factor-alpha in vitro

Inflammatory cytokines like interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) are linked to abnormal cartilage and bone loss in a variety of pathological conditions. We have investigated the effect of TNF-alpha on the synthesis and/or steady-state mRNA levels of collagen, alkaline phosphatase (ALP), plasminogen activators (PAs) and their inhibitor PAI-1, and collagenases (MMPs) and their inhibitor TIMP-1 by human osteoblastic, HOS TE85, cells in monolayer cultures. HOS TE85 cells possess approximately 2000 TNF-alpha receptors per cell with a Kd value of 0.67 nM and receptor of approximately 60 kDa. TNF-alpha enhances urokinase-plasminogen activator (u-PA) activity and steady-state mRNA levels twofold without affecting tissue-plasminogen activator (t-PA) or PAI-1. The increase in u-PA mRNA is due to enhanced transcription of this gene. mRNA levels or activities of collagenase 1 (MMP-1), 72- and 92-kDa gelatinases (MMP-2 and MMP-9) are also nearly doubled with little change in the level of expression of TIMP-1. TNF-alpha does not significantly affect the activity or mRNA levels of ALP. TNF-alpha decreases collagen as well as general protein synthesis. However, the steady-state mRNA for the alpha 2 chain of collagen type I is increased three- to fourfold. These results show that TNF-alpha may increase pathological bone turnover by enhancing the rate of transcription of proteases capable of degrading the nonmineralized osteoid layer and decelerating the maturation of the extracellular matrix formed by osteoblasts.

Relationship of the plasminogen activator/plasmin cascade to osteoclast invasion and mineral resorption in explanted fetal metatarsal bones

An attempt was made to establish whether the activation of plasminogen into plasmin is necessary either for the preparatory phases to bone resorption, involving the recruitment of osteoclast precursors, their migration toward mineralized surfaces, and their final differentiation, or for the subsequent osteoclastic resorption phase. 45Ca-labeled fetal (17 day) mouse metatarsals were cultured under conditions in which they pursue their modeling for a few days. In this model, the resorption phase, monitored by the release of 45Ca into the medium, is entirely dependent on the preparatory phases affecting osteoclast precursors. It was, as expected, stimulated by parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 and inhibited by calcitonin. PTH also enhanced the activity of tissue-type plasminogen activator (PA) in extracts of metatarsals but not that of urokinase (which is, however, the main PA present in the mouse fetal metatarsal culture model). The resorption processes were not dependent on the presence of plasminogen in the media, even when the rudiments were precultured with tranexamic acid to remove their endogenous plasminogen. Moreover, they were not influenced by inhibitors of plasmin, either the plasma inhibitors alpha 2-antiplasmin, alpha 2-macroglobulin, and alpha 1-antitrypsin, or aprotinin, which was tested under a variety of conditions. Aprotinin also did not influence the resorption (loss of calcium and hydroxyproline) of 19 day fetal mouse calvariae cultured with PTH in a medium devoid of plasminogen. It is concluded that the various steps implicated in the bone resorption processes that occur in the metatarsals and in the calvariae culture models are not dependent on the activity of plasmin. The function of PAs in bone, however, could be exerted through direct proteolysis of extracellular proteins other than plasminogen or be mediated by a molecular structural domain distinct from their catalytic domain.

Binding and activation of plasminogen on the surface of osteosarcoma cells

Plasmin (Pm) is a broad action serine protease implicated in numerous physiological functions. In bone, Pm may play a role in growth, resorption, metastasis, and the activation of growth factors. The various components of the Pm system are known to bind and function on the cell surface of various cell types, but no pertinent data are available describing membrane-bound Pm or its zymogen, plasminogen (Pg), in either normal or neoplastic bone cells. We report here that Pg binds to the surface of the human osteosarcoma cell line MG-63 and is activated to Pm by endogenous urokinase plasminogen activator (uPA). These conclusions are based on experiments utilizing radiolabeled compounds and a cell surface proteolytic assay measuring amidolytic activity of Pm. 125I-Pg binding to cells was time dependent, saturable, reversible, and specific. Binding was characterized by a relatively low affinity (Kd approximately 0.9 microM) and a high capacity (approximately 7.5 x 10(6) sites/cell). The binding of 125I-Pg was associated with lysine binding sites of the plasminogen molecule. Activation of 125I-Pg to 125I-Pm occurred on the cell surface and was dependent upon cell bound uPA, as determined by inhibitory antibodies. Binding of Pg to MG-63 monolayers represented approximately 80% bound specifically to the cell surface and the remainder to the surrounding extra-cellular matrix. Either co-incubation with uPA or pre-incubation with Pm resulted in increased 125I-Pg binding to osteosarcoma cells. Cell surface Pm proteolytic activity was confirmed by an amidolytic chromogenic assay. Both Pm and Pg bound to cells with Pg being activated by endogenous uPA. Plasmin activated on the cell surface was partially protected from inhibition by alpha 2-antiPm (requiring Pm lysine binding site interaction) but inhibited by aprotinin, (interacting directly with the Pm catalytic site). Resistance of cell bound Pm to alpha 2-antiPm inhibition suggests that cell surface proteolysis can occur in the presence of a soluble Pm inhibitor known to exist in the extracellular space. Based on these results, we speculate that the various bone physiological processes implicating Pm may occur at or near the bone cell surface.

Bone cell biology: new approaches and unanswered questions

In an effort to define the major unanswered questions in bone cell biology and suggest new approaches to answering these questions, I have outlined the bone remodeling cycle and briefly described the major local and systemic factors that regulate bone cell function. These factors include calcium-regulating and systemic hormones as well as locally produced prostaglandins, cytokines, and growth factors. To understand the individual roles of this large number of regulators, it will be necessary to develop new approaches to measure their production and activity in bone under physiologic and pathologic conditions. Quantitative methods in molecular and cellular biology have been developed that should make this identification possible.

Association between low bone density and stroke in elderly women. The study of osteoporotic fractures

BACKGROUND AND PURPOSE

To determine whether women with low bone mineral density are at increased risk of stroke, the present study was conducted.

METHODS

We studied 4024 ambulatory women aged 65 years or older participating in the prospective Study of Osteoporotic Fractures. Bone mineral density was measured at baseline using single photon absorptiometry; strokes were ascertained using a computerized Medicare data base and death certificates.

RESULTS

During a mean of 1.98 years of follow-up, 83 women suffered first strokes (five fatal). Osteopenia was associated with an increased stroke risk: Each SD decrease in bone mineral density at the calcaneus (0.09 g/cm2) was associated with a 1.31-fold increase in stroke (95% confidence interval, 1.03-1.65), adjusted for age, follow-up time, and several potential confounders, including diabetes, systolic blood pressure, use of alcohol, cigarettes or postmenopausal estrogens, cognitive ability, grip strength, and functional ability. The observed relation between bone density and stroke was strongest for intracerebral hemorrhages and occlusions.

CONCLUSIONS

Most likely, low bone density does not cause stroke; some other process probably results in both osteopenia and cerebrovascular disease.

Specific binding of parathyroid hormone to living osteoclasts

We show that osteoclasts bind parathyroid hormone (PTH) in a manner that displays the properties of receptor-dependent hormone binding, that is, saturability, time dependence, temperature dependence, and hormone specificity. Osteoclasts were isolated from the endosteum of 2 to 3 week chick tibiae and maintained in culture for 4-6 days. Bovine PTH-(1-84) was biotinylated with N-hydroxysuccinimidobiotin. Biotinyl-PTH (btPTH, 10(-5)-10(-11) M) was added to the cultured osteoclasts for 2-20 minutes. After rinsing away unbound btPTH, fluorescein isothiocyanate-labeled avidin (FITC-avidin) at a concentration of 66 micrograms/ml was applied. Receptor binding characteristics were assessed: (1) saturation occurred at around 10(-6) M btPTH; (2) competition of excess unlabeled PTH was found, namely, a 10-fold excess abolished fluorescence; (3) specificity was shown by adding other polypeptide hormones (insulin, glucagon, and calcitonin) in 10- to 100-fold excess--no effect on PTH binding was observed; and (4) affinity of btPTH for its binding site was indicated by half-maximal binding approximately equal to 10(-7) M for both osteoclasts and osteoblasts. Biotin (10(-5) M) or FITC-avidin (66 micrograms/ml) alone did not cause fluorescence. The time course of btPTH on the cell exterior was short: at 2 and 5 minutes dots of fluorescence were randomly dispersed over the cell surface, by 10 minutes most of the fluorescence was clustered in one region of the membrane, and by 20 minutes most of the hormone was no longer present on the surface of the cells.(ABSTRACT TRUNCATED AT 250 WORDS)