Cyclic AMP-dependent and -independent effects on tissue-type plasminogen activator activity in osteogenic sarcoma cells; evidence from phosphodiesterase inhibition and parathyroid hormone antagonists

NF1 Regulatory Element Functions as a Repressor of Tissue Plasminogen Activator Expression

OBJECTIVE

Analysis of the distribution of endothelial cell tissue plasminogen activator (tPA) in the vasculature of rodents and primates demonstrated that tPA is constitutively expressed predominantly in small artery endothelial cells of brain and lung. The regulatory elements responsible for the highly selective expression of arterial endothelial cell tissue plasminogen activator were sought.

METHODS AND RESULTS

Transcription factor binding sites were defined by electrophoretic mobility-shift assay (EMSA) analysis using rat lung and brain nuclear extracts and the tPA promoter sequence from -609 to +37 bp. Protein binding to the promoter was found to be mediated by an NF1 site between -158 and -145 bp upstream from the transcriptional start site. Specific binding was confirmed through mutational analysis and competition binding studies. Infection of endothelial cells with a tPA promoter-green fluorescent protein (GFP) (-609 to +37 bp) reporter construct resulted in expression of the GFP, whereas no expression was found in smooth muscle cells. Mutation of the NF1 site increased the GFP expression indicating that the element acts as a repressor.

CONCLUSIONS

These results suggest that the 600 bp of the tPA promoter upstream of the transcription start site conveys cell specificity to tPA expression and that an NF1 site within this region acts as a repressor.

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.

Prostaglandin E2 regulates production of plasminogen activator isoenzymes, urokinase receptor, and plasminogen activator inhibitor-1 in primary cultures of rat calvarial osteoblasts

The bone resorbing agent, prostaglandin E2 (PGE2), was found to alter several components of the plasminogen activator (PA)/plasmin pathway in primary cultures of rat neonatal osteoblast-like cells. The mRNA and activities of both urokinase-type PA (uPA) and tissue-type PA (tPA) were enhanced by PGE2 treatment. The presence of mRNA for the uPA receptor (uPAR) has been demonstrated in these cells and steady-state levels shown to be greatly enhanced, the response being rapid and sustained for at least 24 hours. mRNA for plasminogen activator inhibitor 1 (PAI-1) was modulated in a biphasic manner, with inhibition of the constitutive level apparent at 4 hours of treatment and stimulation apparent at 12 hours and longer, while PAI-1 protein, measured by an ELISA assay for rat PAI-1, was diminished over this period. Neither PAI-2 mRNA nor mRNA for the broad spectrum protease inhibitor, protease nexin-1 (PN-1), was found to be modulated by PGE2. Therefore, PGE2 is likely to stimulate cell surface proteolytic activity, since uPA mRNA and cell-associated activity were elevated, as was mRNA for the cellular receptor for uPA. Although it was not possible to measure uPAR number and affinity it seems likely that elevated uPAR mRNA would translate into increased uPARs which would localize the increased uPA activity to the pericellular region. tPA mRNA and activity were also increased transiently with the activity inhibited with prolonged incubations, apparently by PAI-1. Elevation of tPA mRNA and activity may result in elevated activity within the extracellular matrix as tPA has been reported to associate with several matrix proteins. Thus the early effect of PGE2 would be to promote proteolysis, both pericellularly and in the extracellular matrix. The inhibition of PAI-1 mRNA and protein, which would contribute to the elevation of activity, is due to PGE2, but the later stimulatory effect on PAI-1 mRNA may be due to feedback regulation by transforming growth factor beta (TGF beta), secreted by osteoblasts and activated by elevated levels of PA.

The urokinase plasminogen activator (u-PA) and its inhibitor (PAI-1) in embryo-fetal bone formation in the human: an immunohistochemical study

The role of urokinase plasminogen activator and plasminogen activator inhibitor-1 in human embryo-fetal bone formation between the 9th and the 20th week of gestation has been studied immunohistochemically. While mature osteocytes of the secondary spongiosa and resting chondrocytes of the bone epiphyses were negative for both antigens in each developmental stage, metabolically active parts of the osseocartilaginous system showed a strong immunoreactivity. Until the end of the 10th week of gestation urokinase plasminogen activator and plasminogen activator inhibitor-1 could not be demonstrated in the shaft of the preexisting cartilaginous models of bones, which correlates with the morphological developmental stage of the embryos. Later, osteoblasts and chondrocytes in the areas of enchondral ossification, and the perivascular chondrocytes of the epiphyseal secondary ossification centres, showed similarly high concentrations of urokinase plasminogen activator and plasminogen activator inhibitor-1. Moreover, the individual ossification stages of the different bones in embryo-fetal development could be demonstrated immunohistochemically. While humeri and femora showed diaphyseal immunoreactivities at an early stage, positive reactions in the phalanges were found only much later. Thus, the enzymes of the fibrinolytic system studied are clearly involved in the desmal and enchondral ossification process in the osseocartilaginous compartment.

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.

Plasminogen-dependent activation of latent transforming growth factor beta (TGF beta) by growing cultures of osteoblast-like cells

Osteoblasts secrete transforming growth factor beta (TGF beta) as a biologically inert, latent complex that must be dissociated before the growth factor can exert its effects. We have examined the production and proteolytic activation of latent TGF beta (LTGF beta) by clonal UMR 106-01 rat osteosarcoma cells and neonatal mouse calvarial (MC) osteoblast-like cells in vitro. Synthetic bPTH-(1-34) increased the activity of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PA) in cell lysates (CL) of UMR 106-01 cells. The concentration of active TGF beta in serum-free CM from cultures treated with bPTH-(1-34) and plasminogen was significantly greater than in CM from untreated controls and cultures treated with either bPTH-(1-34) or plasminogen alone. This effect occurred at concentrations of PTH-(1-34) that increased PA activity and was prevented by aprotinin, an inhibitor of plasmin activity. Treatment with bPTH-(1-34) had no effect on the concentration of TGF beta in acid-activated samples of CM. Functional consequences of proteolytically activated TGF beta was examined in primary cultures of neonatal MC osteoblast-like cells. Human platelet TGF beta 1 caused a dose-dependent increase in the migration of these cells in an in vitro wound healing assay. Cell migration was also stimulated in cultures treated with bPTH-(1-34) and plasminogen together. This effect was blocked by an anti-TGF beta 1 antibody. The results of these studies demonstrate that (1) LTGF beta secreted by osteoblasts in vitro is activated under conditions where the plasmin activity in the cultures is increased, and (2) the TGF beta generated by plasmin-mediated proteolysis is biologically active. We suggest that the local concentration of TGF beta in bone may be controlled by the osteoblast-associated plasminogen activator/plasmin system. Furthermore, since several calciotropic factors influence osteoblast PA activity, this system may have an important role in mediating their anabolic and/or catabolic effects.

The effect of transforming growth factor beta on the plasminogen activator activity of normal human osteoblast-like cells and a human osteosarcoma cell line MG-63

Transforming growth beta (TGF-beta) has been proposed to have a role in bone remodeling by affecting the differentiation and activity of osteoblasts and osteoclasts and by inhibiting the production of proteinases, such as plasminogen activators (PAs). Studies on PAs have largely been based on data from nonhuman and fetal cell lines, however. The purpose of this study was to investigate the effect of TGF-beta on the PA activity of normal human osteoblast-like cells and to compare this with its action on the human osteosarcoma cell line MG-63. The action of interleukin-1 beta (IL-1 beta) was also assessed because it has been shown to increase PA activity in other connective tissue cell types. Normal osteoblast-like cells had low to undetectable basal urokinase (uPA) and tissue plasminogen activator (tPA) activity, which was significantly stimulated by TGF-beta 1. This action was shown to be dependent on transcription and new protein synthesis. TGF-beta 2 had a similar action. IL-1 beta did not stimulate PA activity. In contrast, the MG-63 cell line had high basal tPA and uPA activities. TGF-beta 1 decreased basal PA activity, the effect being most marked for uPA activity. IL-1 beta stimulated uPA and tPA activity. TGF-beta 1 inhibited IL-1 beta-stimulated uPA activity, but the effect on tPA was more variable. This study has shown that TGF-beta has opposite effects on the PA activity of the two osteoblast-like cell types studied. Care must therefore be used before extrapolating data from one cell type to another.(ABSTRACT TRUNCATED AT 250 WORDS)

What's new in the role of cytokines on osteoblast proliferation and differentiation?

This review assesses recent data concerning the role of cytokines produced by a variety of cells in bone on osteoblast function. The following themes are presumed: (1) osteoblasts are mesenchymal cells which act as either the major cellular agents of bone formation or as modulators of bone resorption by osteoclasts. The regulation of osteoblast proliferation and differentiation may involve a negative feedback process resulting in phenotype suppression; (2) cytokines including platelet-derived growth factors (PDGF), parathyroid hormone-related proteins (PTHrP), bone morphogenic proteins (BMP), transforming growth factor beta (TGF beta), fibroblast growth factors (FGF), insulin-like growth factors (IGF), epidermal growth factors (EGF), interleukin-1 and 6, tumour necrosis factors (TNF), interferon and haematopoietic growth factors have effects on osteoblast differentiation and proliferation but their effectiveness may not be identical in vitro and in vivo; (3) finally, therapeutic strategies for cytokine use in clinical practice are considered.

Plasminogen activator regulation in osteoblasts: parathyroid hormone inhibition of type-1 plasminogen activator inhibitor and its mRNA

In order to determine the mechanism by which parathyroid hormone (PTH) stimulates plasminogen activator (PA) activity in rat osteoblasts, we investigated the effect of human PTH(1-34) [hPTH(1-34)] on the synthesis of mRNAs for tissue-type PA (tPA), urokinase-type PA (uPA), and PA inhibitor-1 (PAI-1), and on release of PA activity and PAI-1 protein in both normal rat calvarial osteoblasts and UMR 106-01 osteogenic sarcoma cells. hPTH(1-34) (0.25-25 nM) decreased PAI-1 mRNA and protein, and increased PA activity in both cell types in a dose-dependent manner with ED50 of about 1 nM for both responses. Forskolin and isobutylmethylxanthine also stimulated PA activity and decreased PAI-1 protein and mRNA in both cell types. hPTH(1-34) did not show any consistent effect on tPA and uPA mRNA in calvarial osteoblasts, but a modest (two-fold) increase of both mRNAs was observed in UMR 106-01 cells treated with 25 nM hPTH(1-34). However, when protein synthesis was inhibited with 100 microM cycloheximide, the increase of tPA and uPA mRNA by hPTH(1-34) was enhanced in UMR 106-01 cells and became evident in calvarial osteoblasts. Fibrin autography also revealed that hPTH(1-34) increases tPA and uPA activity, especially after cycloheximide treatment in UMR 106-01 cells. These results strongly suggest that PTH increases PA activity predominantly by decreasing PAI-1 protein production through a cyclic adenosine monophosphate (cAMP)-dependent mechanism in rat osteoblasts. The reduction of PAI-1 protein by PTH results in enhanced action of both tPA and uPA, and would contribute to the specific roles of these PAs in bone.

The effect of tissue type plasminogen activator (tPA) on osteoclastic resorption in embryonic mouse long bone explants: a possible role for the growth factor domain of tPA

Osteoblasts produce proteolytic enzymes and their production is regulated by osteotropic agents. It has been suggested that these proteases play a role in bone resorption by removing the superficial collagenous layer from the bone matrix and indirectly inducing migration of osteoclast precursors towards the bone matrix. We examined the effect of the plasminogen activator tPA on osteoclastic resorption using 17-day-old mouse embryonic long bone explants representing different stages of osteoclast development, that is, radii containing already mature osteoclasts and metacarpals containing no mature osteoclasts but only osteoclast precursors/progenitors which are still confined to the periosteum. Tissue type PA stimulated osteoclastic resorption (measured as 45Ca-release) in 17-day-old fetal metacarpals but not in radii of the same animal. Blocking the enzymatic activity of tPA did not inhibit its effect on osteoclastic resorption. Plasmin, the direct product of PA enzymatic activity, did not induce osteoclastic resorption. However, a tPA-mutant missing the growth-factor-like domain of the molecule, failed to stimulate 45Ca-release from the metacarpals. In addition, in both systems tPA and transforming growth factor alpha had similar effects on osteoclastic resorption. The finding that tPA stimulated 45Ca-release only in the metacarpals suggests that tPA has an effect on osteoclast formation rather than on the activity of already mature osteoclasts. Under the experimental conditions used this effect seems to be mediated by the growth factor domain of tPA rather than by the enzymatic activity of the molecule.

Effects of interleukin-1, tumor necrosis factor -beta, and forskolin on tissue plasminogen activator activity in human osteoblastic osteosarcoma cells

The effects of interleukin-1 (IL-1), forskolin, and tumor necrosis factor beta (TNF-beta) on tissue plasminogen activator (t-PA) activity were studied in the human osteoblastic osteosarcoma cell line, G292. t-PA activity was measured in the cell media using the chromogenic substrate, S-2251. After a 24 hour incubation period, IL-1 increased t-PA in a dose-dependent manner. The effect of IL-1 at 10.0 U/ml was partially inhibited in the presence of indomethacin. Forskolin (1.0 microM) increased t-PA activity after 24 hours with the effects of combined treatment of IL-1 (1.0 U/ml, 10.0 U/ml) and forskolin being apparently additive in nature. TNF-beta (10(-8)-10(-7)M) also produced increased t-PA activity in the cell media after a 24 hour incubation period. These results suggest that the cytokines, IL-1 and TNF-beta, can increase t-PA activity in G292 cells and that there is both a cAMP-dependent as well as a cAMP-independent pathway involved in the regulation of this osteoblastic cell function.

Specific down-regulation of parathyroid hormone (PTH) receptors and responses to PTH by tumour necrosis factor alpha and retinoic acid in UMR 106-06 osteoblast-like osteosarcoma cells

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) act via PTH receptors in bone to stimulate bone resorption. Bone resorption is also stimulated by certain cytokines, which are produced in bone and bone marrow. The effects of such cytokines on the PTH-receptor system were studied in the osteoblast-like osteosarcoma cell line UMR 106-06. 125I-labelled PTHrP-(1-84)-peptide bound specifically to the cells, and PTHrP-(1-34) and -(1-84) competed with equimolar affinity for binding to UMR 106-06 cells. The specific binding of 125I-PTHrP-(1-84) could be completely blocked by PTH. Therefore 125I-PTHrP-(1-84) bound to a classical receptor in UMR 106-06 cells. Preincubation for 3 days with either tumour necrosis factor alpha (TNF alpha) or retinoic acid (RA) both decreased the specific binding of 125I-PTHrP-(1-84) to about 40% of control levels. These effects were specific for PTH binding, since there was little effect on 125I-salmon-calcitonin binding. Both TNF alpha and RA required 24 h exposure to cells to produce a measurable effect. The decrease in 125I-PTHrP-(1-84) binding was due to a reduced number of binding sites, with little apparent change in affinity. Half-maximal effects were seen with 1 ng of TNF alpha/ml, whereas 1 microM-RA was needed to observe the loss of PTH receptors. Combinations of RA and TNF alpha produced a greater effect than that of either agonist alone. The loss of PTH receptors was accompanied by a specific loss of PTH-stimulated cyclic AMP production. Preincubation with TNF alpha increased the basal plasminogen activator (PA) activity in the cells and decreased the amplitude of the response of PA activity to PTH compared with control cells. Furthermore TNF alpha decreased sensitivity to PTH (50% stimulation of PA activity with 0.1 nM-PTH in control cells versus 50% stimulation with 0.3 nM-PTH in TNF alpha-treated cells). In contrast, TNF alpha pretreatment increased the amplitude of the response of PA activity to calcitonin, whereas sensitivity to calcitonin was not altered. These data are consistent with a specific down-regulation of PTH receptors in osteoblast-like UMR 106-06 cells after exposure to TNF alpha or RA. The loss of PTH receptors is accompanied by a decreased responsiveness to PTH, as measured with the PA system in these cells. A loss of PTH receptors could modulate PTH responses in osteoblasts, either in the local control of bone formation and resorption, or in pathological conditions such as humoral hypercalcaemia of malignancy.

Transforming growth factor beta inhibits plasminogen activator (PA) activity and stimulates production of urokinase-type PA, PA inhibitor-1 mRNA, and protein in rat osteoblast-like cells

Transforming growth factor beta (TGF beta) treatment of rat osteoblast-rich calvarial cells or of the clonal osteogenic sarcoma cells, UMR 106-01, resulted in dose-dependent inhibition of plasminogen activator (PA) activity, and increased production of 3.2 kb mRNA and protein for PA inhibitor -1 (PAI-1). Although tissue-type PA (tPA) protein was not measured, TGF beta did not influence production of mRNA for tPA. Production of 2.3 kb mRNA for urokinase-type PA (uPA) was also increased by TGF beta in a dose-dependent manner. The effects of TGF beta on synthesis of mRNA for PAI-1 and uPA were maintained when protein synthesis was inhibited, and were abolished by inhibition of RNA synthesis. Although uPA had not been detected previously as a product of rat osteoblasts, treatment of lysates of osteoblast-like cells with plasmin yielded a band of PA activity on reverse fibrin autography, corresponding to a low Mr form of uPA. Untreated conditioned media from normal osteoblasts or UMR 106-01 cells contained no significant TGF beta activity, but activity could be detected in acidified medium. Treatment of conditioned media with plasmin resulted in activation of approximately 50% of the TGF beta detectable in acidified media. The results identify several effects of TGF beta on the PA-PA inhibitor system in osteoblasts. Net regulation of tPA activity through the stimulatory actions of several calciotropic hormones and the promotion of PAI-1 formation by TGF beta could determine the amount of osteoblast-derived TGF beta activated locally in bone. Stimulation of osteoblast production of mRNA for uPA could reflect effects on the synthesis of sc-uPA, a precursor for the active form of the enzyme.

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

The identification of the plasminogen activator (PA) types present in bone and the regulation of their activity by parathyroid hormone (PTH) were investigated in cultures of fetal mouse calvariae with the use of either a chromogenic substrate or a zymographic assay. PA was detected essentially in the tissue extracts of the explanted bones, with only 1-2% of the total activity released in the surrounding culture media. From their electrophoretic behavior compared to PAs of other mouse tissues and from their response to a specific antibody raised against the tissue type PA (tPA), two major molecular species, of 70 and 48 kD were identified as tPA and urokinase (uPA), respectively, a third minor species of 105 kD being likely to correspond to complexes between tPA and an inhibitor; the culture fluids, moreover, contained enzymatically active degradation products of uPA of 42 and 29 kD. The PA activity of the bone extracts was only minimally affected by the addition of fibrinogen fragments to the chromogenic assays. PTH induced bone resorption and stimulated in parallel the accumulation of PA in the tissue; other bone-resorbing agents, 1,25-dihydroxyvitamin D3 and prostaglandin E2, had similar effects. Densitometric scanning of the zymograms of the bone extracts indicated that PTH stimulated only the production of tPA and had no effect on that of uPA. However, PTH also enhanced the release of uPA (both the 48 kD and the 29 kD forms) from the bones into the media. Although inhibiting bone resorption, calcitonin had no effect on the PTH-induced accumulation of PA in bone or on the release of tPA, but it prevented the PTH-induced accumulation of 29 kD uPA in the culture fluids. Thus these studies support the view that tPA and possibly also uPA may have a role in the physiology of bone; the nature of this role remains to be elucidated, however.

Regulation of the production of plasminogen activators by bone resorption enhancing and inhibiting factors in three types of osteoblast-like cells

Production of proteolytic enzymes by osteoblasts is considered to be important for the initiation of osteoclastic bone resorption. We examined the production of tissue-type (tPA) and urokinase-type plasminogen activator (uPA) activity by three types of osteoblast-like cells (normal rat osteoblasts, rat and human osteosarcoma cells) using a quantitative spectrophotometric assay and a qualitative gel overlay technique. All 3 types of cells released both types of PA-activity into the medium, but normal rat osteoblasts released uPA probably in an inactive form. Treatment with different concentrations of the bone resorbing factors bovine Parathyroid Hormone [1-84], synthetic human Parathyroid Hormone-Like Protein [1-34]. Prostaglandin E2, Interleukin-1 beta, Tumor Necrosis Factor alpha and 1,25-dihydroxyvitamin D3 increased in general the production of both PA's by all three cell types. However, there were differences in the relative potencies of these factors. In contrast, Transforming Growth Factor beta, which inhibits bone resorption, decreased PA-activity in osteoblast-like cells. In all three types of cells, under control as well as under stimulated conditions, a high molecular weight form of PA was demonstrated by the gel overlay technique, most likely a complex of tPA with the PA-inhibitor PAI-1. The uniform increase in production of PA's by osteoblast-like cells in response to bone resorbing factors and its decrease by TGF beta supports the notion that PA's are involved in bone resorption. The exact mechanism however, remains to be elucidated.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteoblasts display receptors for and responses to leukemia-inhibitory factor

Specific binding of leukemia-inhibitory factor (LIF) to osteoblasts, but not multinucleated osteoclasts, was demonstrated by receptor autoradiography by using cells isolated from newborn rat long bones. The clonal rat osteogenic sarcoma cells, UMR 106-06, which have several phenotypic properties of osteoblasts, expressed 300 LIF receptors per cell, with an apparent KD of 60 pM. Treatment of calvarial osteoblasts or UMR 106-01 cells with LIF resulted in a dose-dependent inhibition of plasminogen activator (PA) activity. Both calvarial osteoblasts and osteogenic sarcoma cells were shown by Western blotting and reverse fibrin autography to produce plasminogen activator inhibitor-1 (PAI-1), the production of which was increased by LIF treatment. Northern blot analysis revealed that LIF treatment resulted in a rapid (peak 1 hour), dose-dependent increase in mRNA for PAI-1. LIF treatment of the preosteoblast cell line, UMR 201, enhanced the alkaline phosphatase response of these cells to retinoic acid. Each of the osteoblast-like cell types (calvarial osteoblasts, UMR 106-06, and UMR 201) was shown to produce LIF by bioassay and, by using the polymerase chain reaction (PCR), was shown to express low levels of mRNA for LIF. These data establish that cells of the osteoblast lineage are targets for LIF action. The reported anabolic effects of this cytokine on bone formation in vivo could be related to inhibition of protease activity. LIF may be an important paracrine modulator in bone, or perhaps an autocrine one, based on the evidence for its production by osteoblasts and osteoblast-like cells.

Effects of helodermin on fetal rat bone metabolism in vitro

Helodermin belongs to the VIP family of polypeptides. Recent in vivo data suggest that helodermin-like peptides might be involved in the regulation of calcium metabolism. We show that helodermin specifically binds to a secretin-type receptor in osteoblast-like cells from fetal rat calvaria and increases the basal and PTH-stimulated cAMP concentration of these cells. In organ cultures of fetal rat calvaria, helodermin strongly inhibits bone matrix apposition and augments PTH-induced bone resorption. Helodermin-like peptides may thus be capable of enhancing the direct effects of PTH on bone metabolism.

Identification of plasminogen activator in osteoclasts

Plasminogen activator (PA) was located in newborn rat osteoclasts using a single-cell assay. Immunohistochemistry using biotin-streptavidin-peroxidase indicated the presence of both tissue-type plasminogen activator (tPA) and urokinase (uPA) within the cytoplasm of osteoclasts isolated from newborn rat long bones. Electron microscopic immunohistochemistry using the biotin-streptavidin-colloidal gold system on L.R. Gold thin resin sections of undecalcified, newborn rat tibial metaphyseal trabecular bone identified these proteases in the lysosomal network of osteoclasts. uPA was also localized in marrow macrophage lysosomes, but tPA was not detected in these cells. The localization of these enzymes within osteoclasts may imply their involvement in bone resorption.

Adenylate cyclase blockers dissociate PTH-stimulated bone resorption from cAMP production

It is uncertain whether adenosine 3',5'-cyclic monophosphate (cAMP) or the inositol-calcium pathway mediates the stimulation of bone resorption by parathyroid hormone (PTH). Incubation of bone organ cultures with cAMP analogues and forskolin has not resolved this question because of the cellular inhomogeneity of bone and the consequent presence of adenylate cyclase-linked receptors for both PTH and calcitonin, hormones with opposite effects on bone resorption. We have used two new inhibitors of adenylate cyclase, 9-(tetrahydro-2-furyl)adenine (SQ 22536) and 2',5'-dideoxyadenosine (DDA), to directly reassess the role of cAMP in PTH-stimulated osteolysis. SQ 22536 (0.01-1.0 mM) and DDA (0.01-1.0 mM) completely blocked PTH stimulation of cAMP production measured in the absence of a phosphodiesterase blocker. In the presence of 1 mM 3-isobutyl-1-methylxanthine, half-maximal inhibition of PTH-induced cAMP production occurred with 0.2 mM SQ and 0.1 mM DDA, respectively. These concentrations of SQ and DDA had no effect on PTH-stimulated 45Ca release from calvaria, although both agents inhibited bone resorption when present at concentrations of 1-2 mM. At these levels, SQ and DDA caused equivalent inhibition of 45Ca release stimulated by 1,25-dihydroxyvitamin D3 but did not affect basal 45Ca release or [3H]-phenylalanine incorporation. It is concluded that substantial blockade of PTH-induced cAMP production does not affect this hormone's stimulation of bone resorption, which is therefore likely to be mediated by another intracellular messenger system, possibly calcium. In millimolar concentrations, SQ and DDA appear to be nonspecific blockers of osteoclastic bone resorption.

Insulin-like growth factor I stimulates proliferation, migration, and plasminogen activator release by human retinal pigment epithelial cells

The migration of retinal pigment epithelial (RPE) cells from their normal anatomic position to a new position in the vitreous cavity is a critical feature of proliferative vitreous retinopathy. To determine if insulin-like growth factor I (IGF I), which is present in the vitreous fluid of diabetics, stimulates RPE cells, we examined the effects of IGF I on the proliferation, chemotaxis, and release of plasminogen activator by these cells. At the concentrations of IGF I tested, significant proliferation of RPE cells is seen. Significant chemotaxis of the RPE cells also is seen at all the concentrations of IGF I tested. The mean number of migrating cells per high-powered field in control studies was 43 +/- 13 (x +/- SEM), and for IGF I at 2.5 ng and 50 ng/ml the mean numbers of migrating cells were 96 +/- 17 and 483 +/- 62, respectively (P less than 0.001 for each comparison). The IGF I response was noted to be dose-dependent. The chemotactic response noted at 50 ng/ml of IGF I was greater than the positive chemotactic control of 10% fetal calf serum. Addition of alpha IR-3, an IGF I receptor antibody, eliminated the IGF I chemotactic response. The effect of IGF I on the secretion of plasminogen activators was assessed using an immunological assay for tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI). Media conditioned by RPE cells have measureable levels of PAI and t-PA antigen. IGF I supplementation resulted in an increase of t-PA secretion and PAI secretion over basal levels. These studies support a role for IGF I in modulating RPE cell function.

Cloning and sequencing of cDNA for the rat plasminogen activator inhibitor-1

A cDNA encoding rat plasminogen activator-inhibitor (PAI-1) has been isolated from an HTC rat hepatoma cell cDNA library constructed in phage lambda gt10. The cDNA contains 118 bp of 5'-untranslated sequence, 1206 bp encoding a 402-amino acid (aa) protein and 1747 bp of 3'-untranslated sequence. The protein-coding sequence and the derived amino acid sequence share 82% and 81% identity, respectively, with human PAI-1 cDNA and protein. The rat cDNA encodes a preprotein with a 23-aa leader peptide and a predicted N-terminal serine for the mature protein. Three of four potential N-glycosylation acceptor sites as well as the active site of rat PAI-1 are identical to the human protein. The 3'-untranslated region contains a number of unusual regions, including 80 bp of tandemly repeated GpA dinucleotides, a 115-bp stretch which shares greater than 90% sequence identity with a region within the 3'-untranslated cDNA of human PAI-1, and two 70-bp stretches of highly T-rich sequence located close to the 3'-terminus of the cDNA.

Parathyroid hormone-related protein of malignancy: active synthetic fragments

Peptides corresponding to the amino-terminal region of the parathyroid hormone-related protein (PTHrP) of humoral hypercalcemia of malignancy were synthesized. A 34-amino acid peptide, PTHrP(1-34), was two to four times more potent than bovine or human PTH(1-34) in bioassays promoting the formation of adenosine 3',5'-monophosphate (cAMP) and plasminogen activator activity in osteogenic sarcoma cells and adenylate cyclase activity in chick kidney membranes. Like parathyroid hormone itself, in which the activity resides in the first 34 residues, PTHrP peptides of less than 30 residues from the amino terminus showed substantially reduced activity. PTHrP(1-34) had only 6% of the potency of bovine PTH(1-34) in promoting bone resorption in vitro. PTHrP(1-34) strongly promoted the excretion of cAMP and phosphorus and reduced the excretion of calcium in the isolated, perfused rat kidney consistent with the symptoms seen in malignant hypercalcemia.