Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P00750 (PLA)
 16,800 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
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PMID:Plasminogen activator regulation in osteoblasts: parathyroid hormone inhibition of type-1 plasminogen activator inhibitor and its mRNA. 132 17

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.
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PMID:Tissue and urokinase plasminogen activators in bone tissue and their regulation by parathyroid hormone. 179 56

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.
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PMID:Parathyroid hormone-related protein of malignancy: active synthetic fragments. 368 95

Hormonal control of plasminogen activator (PA) was studied in clonal rat osteogenic sarcoma cells which are phenotypically osteoblast, and in osteoblast-rich rat bone cell cultures. The bone-resorbing hormones (parathyroid hormone, prostaglandin E2, epidermal growth factor and 1,25-dihydroxyvitamin D3) stimulated PA activity in both cell types. The relative efficacies of vitamin D metabolites and of prostanoids reflect their relative potencies as stimulators of bone resorption.
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PMID:Stimulation of plasminogen activator in osteoblast-like cells by bone-resorbing hormones. 661 Nov 56

In rat calvarial osteoblast-like cells and in clonal osteogenic sarcoma cells (UMR 106-01), 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3) enhanced plasminogen activator (PA) activity and decreased PA inhibitor-1 (PAI-1) production over the same concentration range. Steady-state levels of mRNA for PAI-1 were also decreased by 1,25(OH)2D3 in a dose-dependent manner, without significant effects on mRNA for either tissue-type PA (tPA) or urokinase-type PA (uPA). When protein synthesis was inhibited by cycloheximide treatment in UMR 106-01 cells, the action of 1,25(OH)2D3 on PAI-1 mRNA was abolished, as was observed previously with parathyroid hormone (PTH) treatment. In osteoblast-like cells however, 1,25(OH)2D3 and PTH actions differed, in that 1,25(OH)2D3 had no effect on either PAI-1 or uPA mRNA levels under conditions of protein synthesis inhibition, whereas PTH decreased PAI-1, and increased uPA mRNA. Identification of proteins involved in these actions may help to explain differences in molecular regulation by PTH and 1,25(OH)2D3, two agents which have similar actions on osteoblasts, but employ different signal transduction pathways.
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PMID:Regulation of plasminogen activator inhibitor-1 (PAI-1) expression by 1,25-dihydroxyvitamin D-3 in normal and malignant rat osteoblasts. 794 35

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.
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PMID:Relationship of the plasminogen activator/plasmin cascade to osteoclast invasion and mineral resorption in explanted fetal metatarsal bones. 807 64

Recent in vivo and in vitro studies show that high phosphate directly stimulates parathyroid hormone (PTH) secretion. However, little is known about the intracellular signaling system involved in the regulation of PTH secretion by extracellular phosphate. High extracellular calcium is coupled to the activation of phospholipase A(2) (PLA(2)) and the formation of arachidonic acid (AA), a potent inhibitor of PTH release. The present study was designed to evaluate whether a high phosphate concentration has an effect on the PLA(2)-AA pathway in parathyroid cells. In vitro experiments were performed in parathyroid tissue obtained from normal rats and dogs. AA production was measured in parathyroid tissue in response to 1- and 4-mM phosphate concentration and after addition of PLA(2) to the medium. To determine whether the effect of phosphate on AA production in parathyroid cells was tissue specific, separate experiments were performed to test the effect of phosphate in rat adrenal glomerulosa cells, which are known to increase AA production in response to angiotensin II. The effect of sulfate, an ion with chemical characteristics similar to phosphate, on PTH secretion was also evaluated. In parathyroid tissue, a high phosphate concentration decreased the high calcium-induced AA production. This effect of phosphate was associated with an increase in PTH secretion. The addition of AA reversed the stimulatory effect of phosphate on PTH secretion. In another type of AA-responsive tissue, the adrenal glomerulosa, a high phosphate concentration did not affect the production of AA when stimulated by angiotensin II. In a normal phosphate concentration, the addition of PLA(2) stimulated AA production and decreased the PTH secretion. However, in a 4-mM phosphate concentration, the addition of PLA(2) did not reduce PTH secretion and did not stimulate AA production. Finally, sulfate did not affect PTH secretion. In conclusion, a high phosphate concentration affects the production of AA by parathyroid tissue. This effect of phosphate may be the mechanism by which a high phosphate concentration stimulates PTH secretion.
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PMID:Effect of high extracellular phosphate concentration on arachidonic acid production by parathyroid tissue in vitro. 1096 96

The action of extracellular calcium on the calcium receptor in parathyroid cells results in activation of phospholipase C (PLC), PLD, and PLA(2). The PLA(2)-arachidonic acid (AA) intracellular signaling pathway mediates inhibition of parathyroid hormone (PTH) secretion. In addition, stimulation of the calcium receptor produces increases in intracellular calcium levels. It was demonstrated that high extracellular phosphate levels reduce the production of AA, a mechanism by which phosphate may stimulate PTH secretion. The objective was to determine, in parathyroid tissue, whether AA production is stimulated by increases in intracellular calcium levels and to investigate whether the decreased AA production induced by high extracellular phosphate levels could be modified by increases in intracellular calcium levels. Experiments were performed in vitro using parathyroid tissue. The intracellular calcium level was increased by incubation with an ionophore (A23187), which increases calcium influx across the cell membrane, or thapsigargin, which releases calcium from intracellular stores. The phosphate concentration in the medium was normal (1 mM) or high (4 mM). The response to calcium was evaluated by incubation with 0.6 or 1.35 mM calcium concentrations. AA production by parathyroid tissue was measured by gas chromatography. In parathyroid tissue incubated with either a calcium ionophore or thapsigargin, there was an increase in AA production, together with inhibition of PTH secretion, suggesting that PLA(2) is activated by the elevation in intracellular calcium levels. Therefore, the effect of intracellular calcium level elevation on AA production in the presence of high extracellular phosphate levels was evaluated. The results demonstrate that, despite high phosphate levels in the medium, both the ionophore and thapsigargin were capable of inducing a marked increase in AA production, which was associated with a decrease in PTH secretion. In conclusion, in parathyroid tissue, AA levels can be regulated by an ionophore and thapsigargin, both of which increase cytosolic calcium concentrations. The stimulation of PTH secretion by high phosphate levels can be prevented by increases in intracellular calcium levels.
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PMID:Regulation of arachidonic acid production by intracellular calcium in parathyroid cells: effect of extracellular phosphate. 1185 73

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.
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PMID:Cathepsin B in osteoblasts. 1272 91

Current therapy for secondary hyperparathyroidism in uremia has relatively poor success in achieving the target levels of parathyroid hormone (PTH), calcium and phosphate established by the NKF-K/DOQI guidelines. The discovery and characterization of a new membrane receptor able to sense minimal Ca changes (CaSR) started intensive research in the attempt to characterize better its functions and its finding compounds, which could modulate its activity. CaSR is expressed not only in the cells that secrete calcium-regulating hormones (parathyroid cells and thyroid C-cells) and in cells involved in calcium transport mechanisms (ie intestinal cells, bone-forming osteoblasts, and cells of different nephron segments), but also in other tissues with, as yet, a not completely defined role. CaSR stimulation by the agonists is followed by the activation of a great number of G-proteins mediated intracellular signalling pathways (PLC, PLA, PLD, PKC, PKA, etc). At the level of parathyroid cells, the main effect is the increase in IP3, followed by a mobilization of intracellular Ca stores, which inhibit PTH secretion in a few seconds or minutes. Long-term CaSR stimulation is also able to induce a reduction in both PTH synthesis and parathyroid cell proliferation. More than 100 mutations of the gene coding for CaSR have been described. Some of these mutations are matched by a gain or reduction/loss of function. Notwithstanding, CaSR is widely represented on different tissue cells, the main clinical manifestations of the above genetic changes mainly involve PTH and calcium metabolism. A great number of inorganic and organic cations can interact with the Ca-sensitive N-terminus domain of CaSR, mimicking Ca effects (type I calcimimetics), but these substances have substantial limitations for use in clinical practice. A second class of compounds was produced (NPS R-467, S-467, R-568, S-568, AMG 073), for use in the clinical setting, type II calcimimetics. These compounds, after having interacted with the membrane-spanning domains of the CaSR, induce conformational changes in the N-terminus domain, increasing its affinity for Ca. The preclinical experiences with calcimimetics demonstrated that they were effective in reducing circulating PTH, preventing the progression of secondary hyperparathyroidism, suppressing parathyroid cell proliferation, and reversing osteitis fibrosa at least in animal models. Clinical studies were performed mainly using AMG 073, due to its greater bioavailability and more consistent pharmacokinetic profile. Clinical studies performed in primary hyperparathyroidism proved AMG 073 to be effective in reducing both PTH and Ca serum levels, with a good safety profile. Further studies, mainly focused on the efficacy of AMG 073 in the control of secondary hyperparathyroidism in uremia, confirmed the efficacy of this compound in reducing PTH levels >30% in about 50% of patients. Furthermore, the fall in PTH was matched by a reduction in both calcium and phosphate serum levels of about 5-7%, with a significant reduction in calcium x phosphate product (about 15%). The latter aspect represents a unique pharmacological profile, as compared to all the other available therapeutic means to control secondary hyperparathyroidism in uremia. In addition to their effectiveness, calcimimetics present a relatively safe profile, the only adverse events referred to consist of transient and easily remediable hypocalcemic episodes and some gastrointestinal discomfort symptoms. However, although calcimimetics represent a real advancement in the field of treating secondary hyperparathyroidism in uremic patients, their use should be matched by the awareness that previously the success of a high number of new drugs proposed have been flawed by negative consequences in the long term. Therefore, strict clinical control is necessary in the next few years when the use of these new compounds will widen.
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PMID:[Calcimimetics]. 1652 Oct 71


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