Stimulation of calcium uptake in rat calvaria by prostacyclin

Enhanced prostacyclin formation and Wnt signaling in sclerostin deficient osteocytes and bone

We show that prostacyclin production is increased in bone and osteocytes from sclerostin (Sost) knockout mice which have greatly increased bone mass. The addition of prostacyclin or a prostacyclin analog to bone forming osteoblasts enhances differentiation and matrix mineralization of osteoblasts. The increase in prostacyclin synthesis is linked to increases in β-catenin concentrations and activity as shown by enhanced binding of lymphoid enhancer factor, Lef1, to promoter elements within the prostacyclin synthase promoter. Blockade of Wnt signaling reduces prostacyclin production in osteocytes. Increased prostacyclin production by osteocytes from sclerostin deficient mice could potentially contribute to the increased bone formation seen in this condition.

Cloning and function of rabbit peroxisome proliferator-activated receptor delta/beta in mature osteoclasts

Osteoclasts modulate bone resorption under physiological and pathological conditions. Previously, we showed that both estrogens and retinoids regulated osteoclastic bone resorption and postulated that such regulation was directly mediated through their cognate receptors expressed in mature osteoclasts. In this study, we searched for expression of other members of the nuclear hormone receptor superfamily in osteoclasts. Using the low stringency homologous hybridization method, we isolated the peroxisome proliferator-activated receptor delta/beta (PPARdelta/beta) cDNA from mature rabbit osteoclasts. Northern blot analysis showed that PPARdelta/beta mRNA was highly expressed in highly enriched rabbit osteoclasts. Carbaprostacyclin, a prostacyclin analogue known to be a ligand for PPARdelta/beta, significantly induced both bone-resorbing activities of isolated mature rabbit osteoclasts and mRNA expression of the cathepsin K, carbonic anhydrase type II, and tartrate-resistant acid phosphatase genes in these cells. Moreover, the carbaprostacyclin-induced bone resorption was completely blocked by an antisense phosphothiorate oligodeoxynucleotide of PPARdelta/beta but not by the sense phosphothiorate oligodeoxynucleotide of the same DNA sequence. Our results suggest that PPARdelta/beta may be involved in direct modulation of osteoclastic bone resorption.

Differential effect of TPA on PGE2 and cicaprost-induced cAMP synthesis in UMR-106 cells

PGE2 and prostacyclin each enhance cAMP synthesis in the osteoblast-like cell line UMR-106. The amount of cAMP induced by PGE2 was 5-7-fold greater than the amount induced by cicaprost or iloprost, stable prostacyclin analogues. Both PGE2 and the two prostacyclin analogues enhanced cAMP synthesis with similar time dependence. The EC50 values of PGE2 and cicaprost were 3 X 10(-6) and 5 x 10(-8) M, respectively. Short-term incubation of the cells with 12-o-tetradecanoylphorbol 13-acetate (TPA) markedly reduced the PGE2-induced cAMP synthesis. In contrast, cells that were incubated with the same concentrations of TPA in the presence of cicaprost or iloprost showed a 1.6-fold increase in cAMP formation. The marked disparity between the cAMP response to cicaprost and PGE2 in the presence of TPA suggests that the two prostanoids induce cAMP synthesis in the UMR-106 cells by interaction with different receptors. These observations support the idea that the osteoblastic UMR-106 cells may express specific prostacyclin receptors and suggest that prostacyclin may have a unique role in osteoblasts.

Bone marrow stromal cells are load responsive in vitro

Mechanical load-related effects on bone marrow stromal cells in vitro have been investigated. A dose response of a cyclical load of 1 Hz between 350 ustrain and 2500 ustrain applied to 10-day-old cultures resulted in elevated alkaline phosphatase levels and the number of cells expressing this protein after 2 days. No significant changes in the number of cells expressing or the production of collagen type 1 was observed. A critical stage of development of the cultures must be reached before load-related elevation in alkaline phosphatase expression could be measured independent of the stage at which loading was applied. Using a prostaglandin inhibitor at concentrations previously used in vivo, the load response was abolished. We have demonstrated that bone marrow stromal cells are load responsive in culture and have made preliminary studies into determining the involvement of prostaglandins in this process.

Exogenous prostacyclin, but not prostaglandin E2, produces similar responses in both G6PD activity and RNA production as mechanical loading, and increases IGF-II release, in adult cancellous bone in culture

Cyclic mechanical loading in vivo that leads to new bone formation is also associated in osteocytes and surface bone cells with almost immediate increases in G6PD activity, and later increases in RNA production. Both these early, loading-related, responses can be reproduced in organ culture of adult cancellous bone, and both are abolished by the presence of indomethacin in the culture medium at the time of loading. The implication that prostaglandins (PGs) are involved in the control of loading-related osteogenesis is supported by increases in prostacyclin (PGI2) and PGE2 release from cores of cancellous bone during loading. In the experiments reported here, PGE2 and PGI2 were added exogenously (10(-6) M) to perfusable cores of adult canine cancellous bone to determine whether they would simulate the loading-related responses in G6PD activity and RNA synthesis. PGE2 increased G6PD activity in surface cells and osteocytes within 8 minutes but had no effect on [3H]-uridine incorporation at 6 hours. PGI2 stimulated both G6PD activity and [3H]-uridine incorporation equally in osteocytes and surface cells. Neither PG produced any significant change in medium concentrations of IGF-I, and PGE2 had no effect on IGF-II. In contrast PGI2 elevated the medium concentration of IGF-II threefold. IGF-I and IGF-II were localized immunocytochemically to osteocytes and surface cells in both treated and untreated cores. Prostacyclin, but not PGE2, appears to imitate the early loading-related increases in G6PD activity and RNA synthesis in bone cells in situ. Prostacyclin, but not PGE2, also stimulates the early release of IGF-II.

Loading-related increases in prostaglandin production in cores of adult canine cancellous bone in vitro: a role for prostacyclin in adaptive bone remodeling?

Cyclic mechanical loading sufficient to engender strains of physiologic magnitude applied to recently excised canine cancellous bone cores in vitro increased the release of prostaglandin E (PGE) and prostacyclin (PGI2, measured as its breakdown product 6-keto-PGF1 alpha), during a 15 minute loading period in which PG levels were measured in perfusing medium at 5 minute intervals. Peak production occurred in the 0-5 minute sample. Mean levels preload compared to during load were PGE, 2.66 and 3.67 ng/ml (p less than 0.002); and 6-keto-PGF1 alpha, 543 and 868 pg/ml (p less than 0.007). The elevated levels then declined to preload levels during the loading period. However, the 5-10 minute but not the 10-15 minute samples still contained levels greater than preload values. A second 15 minute period of load, 1 h following the end of the first, produced smaller increases in the levels of release that were statistically significant only for the first 0-5 minute sample during load (preload compared to load mean values, PGE, 1.09-1.66 ng/ml, p less than 0.02; 6-keto-PGF1 alpha, 401-558 pg/ml, p less than 0.04). Immunolocalization revealed PGE and 6-keto-PGF1 alpha in lining cells and 6-keto-PGF1 alpha but not PGE in osteocytes. Addition to the medium of 1 microM PGE2, approximating the concentration produced by loading, had no significant effect on the specific activity of the extractable RNA fraction labeled with [3H]uridine, whereas 1 microM PGI2 produced an increase similar to that seen previously with loading.(ABSTRACT TRUNCATED AT 250 WORDS)

A biochemical analysis of the effects of arachidonic acid on sarcoplasmic reticulum function

Discontinuous sucrose gradients were used to determine the degree of association between arachidonic acid and sarcoplasmic reticulum vesicle membranes. Fraction analyses showed that arachidonic acid migrated to a different region of the sucrose gradient in the presence of sarcoplasmic reticulum membranes. This could suggest that arachidonic acid was complexed into the membranes. Arrhenius curves representing the temperature dependency of Ca2+-Mg2+-ATPase activity and calcium uptake in the presence and absence of arachidonic acid were constructed. The activation energy for ATPase did not change significantly due to the presence of arachidonic acid. The curve representing control calcium uptake did not show a discontinuity. However, the curve representing calcium uptake in the presence of arachidonic acid showed discontinuities at 18 degrees C and 21 degrees C. Activation energy increased sharply between these temperatures. The results suggest that arachidonic acid reached the critical micellar concentration between these temperatures. Enthalpy decreased in the presence of arachidonic acid. This observation could suggest a transition of the protein-phospholipid complex to a less rigid state since decreased order in the membrane would decrease the energy barrier for activation of ATPase.

Prostaglandin E2 causes a transient inhibition of mineral mobilization, matrix degradation, and lysosomal enzyme release from mouse calvarial bones in vitro

The effect of prostaglandin E2 (PGE2) on the kinetic of bone resorption in vitro was assessed by following the release of minerals and degradation of matrix in cultured mouse calvarial bones. PGE2 (1 and 3 mumol/liter) caused an initial inhibition of the release of 45Ca, stable calcium, and inorganic phosphate from unstimulated calvarial bones. The effect was transient and after 24 and 48 hours the release of 45Ca, stable calcium, and inorganic phosphate from PGE2-treated bones was enhanced. 0.3 mumol/liter of PGE2 stimulated the release of 45Ca after 24 hours, but at this concentration no initial inhibition was observed. The initial inhibitory effect of PGE2 (1 mumol/liter) could be further increased by three structurally different inhibitors of cyclic AMP breakdown. PGE2 (1 mumol/liter) caused not only an initial inhibition of mineral release but also an initial inhibition of matrix degradation, as assessed by the release of 3H from [3H]-proline labeled bones. In addition, PGE2 (3 mumol/liter), in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine, caused a rapid (6 hours) inhibition of the release of the lysosomal enzymes beta-glucuronidase and beta-N-acetyl-glucosaminidase, without affecting the release of the cytosolic enzyme lactate dehydrogenase. Similar specific initial inhibition of lysosomal enzyme release was also seen in the presence of calcitonin and dibutyryl cyclic AMP, but not in the presence of parathyroid hormone (PTH). Neither PGE2 nor the phosphodiesterase inhibitors rolipram and Ro 20.1724, could inhibit the initial stages of PTH-induced 45Ca release. Nor did PGE2 inhibit the stimulation of radioactive calcium mobilization induced by 1 alpha (OH)-vitamin D3.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prostacyclin and prostaglandin E1 (PGE1) on bone resorption in the presence and absence of parathyroid hormone

Prostaglandins have been shown to stimulate osteoclastic bone resorption in organ culture but morphologic studies of isolated osteoclasts have shown a transient calcitonin-like inhibiting effect of these agents. We looked for a dual effect on bone resorption by comparing the early and late effects of prostaglandin E1 (PGE1), prostacyclin (PGI2), 6 alpha-carbaprostaglandin I2 (C-PGI2), a carbon substituted analog of PGI2, and salmon calcitonin (CT) on the release of previously incorporated 45Ca from fetal rat long bones cultured in the presence of an inhibitor of cyclooxygenase, RO-20-5720. Experiments were performed in both the presence and absence of PTH (400 ng/ml), which was administered 24 hours before addition of prostaglandins or CT. In control cultures not stimulated by PTH, CT (100 mU/ml) produced significant decreases in 45Ca release at 48, 72, and 96 hours while PGE1 (10(-6) M), PGI2 (10(-5)), and C-PGI2 (10(-6) M) each produced significant increases in resorption at 24 through 96 hours. PGE1 at 10(-5) M, but not 10(-6) M, caused a significant decrease in medium 45Ca of 21% at 1 and 2 hours. Medium calcium measurements suggest that the change in 45Ca was due to inhibition of release and not to increased uptake. PGI2 (10(-5) M) and C-PGI2 (10(-6) M) caused no significant inhibitory effect. In cultures stimulated by PTH, CT produced significant inhibition of bone resorption of 6 through 96 hours, but no inhibition of bone resorption was noted at either early or late time points with PGE1, PGI2, or C-PGI2.(ABSTRACT TRUNCATED AT 250 WORDS)