Simultaneous measurement of bone resorption and collagen synthesis in neonatal mouse calvaria

Intracellular measurement of prostaglandin E2: effect of anti-inflammatory drugs on cyclooxygenase activity and prostanoid expression

Cyclooxygenase (COX) converts arachidonic acid to prostaglandin (PG) H2, which is further metabolized to various prostaglandins, prostacyclin and thromboxane A2. COX exists in at least two different isoforms. COX-1 is constitutively expressed, whereas COX-2 is induced by proinflammatory stimuli. Prostaglandin E2 is a major metabolite of COX activation. In order to compare the activity of target ligands and COX inhibitors on PGE2 synthesis and release, the responsiveness of several cell lines to the calcium ionophore A23187, bacterial lipopolysaccharide (LPS), nonsteroidal anti-inflammatory drugs (NSAIDs), and the glucocorticoid, dexamethasone, were investigated. For intracellular measurements, the culture supernatant was aspirated, and the cells were thoroughly washed and lysed with dodecyltrimethylammonium bromide. Intracellular and secreted PGE2 were measured with an enzyme immunoassay. A23187 and LPS increased intracellular PGE2 in a dose-dependent manner. Kinetic experiments with A23187-stimulated mouse 3T3 fibroblast cells revealed a distinct biphasic response in COX activity. In the presence of NSAIDs or dexamethasone, there was a dose-dependent inhibition in intracellular PGE2 with A23187-stimulated 3T3 cells. Inhibitory studies demonstrated an apparent increased sensitivity of COX activity to the action of inhibitors when measuring intracellular PGE2 compared with using cell culture supernatants. Indeed, intracellular PGE2 levels were comprehensively reduced in the presence of low concentrations of inhibitor. The utilization of cell culture lysates and, in particular, measurement of intracellular PGE2 should prove useful for identifying new COX inhibitors.

Additive effects of acidosis and parathyroid hormone on mouse osteoblastic and osteoclastic function

Patients with end-stage renal disease are acidotic and often develop secondary hyperparathyroidism. Whether acidosis contributes to the bone disease observed in these patients is not clear. To determine whether acidosis and parathyroid hormone (PTH) have additive effects on net calcium efflux (JCa+) from bone and on bone cell function, we measured JCa+, osteoblastic collagen synthesis, and osteoclastic beta-glucuronidase release from neonatal mouse calvariae cultured in control (Ctl, pH approximately 7.4) or acidified (Met, pH approximately 7.1) medium with or without a submaximal concentration of PTH (10(-10) M) for 48 h. Compared with Ctl, from 24 to 48 h JCa+ was increased with Met and with PTH, and the combination of Met + PTH increased JCa+ further. Compared with Ctl, collagen synthesis was decreased with Met and with PTH and decreased further with Met + PTH. There was an inverse correlation between percent collagen synthesis and JCa+. Compared with Ctl, beta-glucuronidase release into the medium was increased with Met and with PTH and increased further with Met + PTH. There was a direct correlation between medium beta-glucuronidase activity and JCa+. Osteoclastic beta-glucuronidase activity correlated inversely with osteoblastic collagen synthesis. During cultures to 96 h, there continued to be greater JCa+ from calvariae incubated with Met + PTH than from those with either treatment alone. Thus acidosis and PTH independently stimulated JCa+ from bone, inhibited osteoblastic collagen synthesis, and stimulated osteoclastic beta-glucuronidase secretion, whereas the combination had a greater effect on each of these parameters than either treatment alone. These findings indicate that acidosis and PTH can have an additive effect on bone cell function and suggest that uremic osteodystrophy may result from a combination of a low pH and an elevated PTH.

Stimulated osteoclastic and suppressed osteoblastic activity in metabolic but not respiratory acidosis

When bone is cultured in acidic medium produced by a reduced bicarbonate concentration ([HCO(3-)]), a model of metabolic acidosis, there is greater net calcium efflux than when the same decrement in pH is produced by an increased partial pressure of carbon dioxide (PCO2), a model of respiratory acidosis. To determine the effects of metabolic and respiratory acidosis on bone cell function we cultured neonatal mouse calvariae for 48 h under control conditions (pH approximately 7.40, PCO2 approximately 41 mmHg, [HCO(3-)] approximately 25 meq/l) or under isohydric acidic conditions simulating metabolic (pH approximately 7.09, [HCO(3-)] approximately 12) or respiratory (pH approximately 7.10, PCO2 approximately 86) acidosis and measured osteoblastic collagen synthesis and alkaline phosphatase activity and osteoclastic beta-glucuronidase activity. Collagen synthesis was inhibited by metabolic (23.2 +/- 1.3 vs. 30.3 +/- 1.0% in control) but was not altered by respiratory (32.3 +/- 0.6) acidosis. Alkaline phosphatase activity was inhibited by metabolic (402 +/- 16 vs. 471 +/- 15 nmol P.min-1.mg protein-1 in control) but not altered by respiratory (437 +/- 25) acidosis. beta-Glucuronidase activity was stimulated by metabolic (1.02 +/- 0.06 vs. 0.78 +/- 0.05 micrograms phenolphthalein released.bone-1.h-1 in control) but not altered by respiratory (0.73 +/- 0.06) acidosis. Net calcium efflux in control was increased by metabolic (783 +/- 57 vs. 20 +/- 57 nmol.bone-1.48 h-1 in control) and by respiratory (213 +/- 45) acidosis; however, calcium efflux with metabolic was greater than with respiratory acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of sodium/calcium exchange in rodent osteoblasts

Based on the inhibition of stimulated Ca release from cultured bone by several different agents that alter Na transport, we proposed that hormonally stimulated bone resorption requires Na/Ca exchange. Calcemic hormones appear to interact primarily directly with the osteoblast, which then mediates the activation of osteoclast activity. In organ culture it is not possible to determine whether Na/Ca exchange is involved in this initiating step in the osteoblast or directly in osteoclast-mediated Ca release, and there have been no prior direct measurements of Na/Ca exchange in bone or bone cells. The purpose of this study was to demonstrate the presence of Na/Ca exchange transport in the osteoblast. Thus, we characterized Na-dependent Ca transport in osteoblast-like rat osteosarcoma cells (UMR-106) and primary bone cells isolated from neonatal mouse calvaria. Cells were loaded with the Ca-sensitive dye fura-2 in the presence of physiologic NaCl and the absence of Ca with or without 0.3 mM ouabain. Changes in free cytosolic Ca after the extracellular addition of 1.5 mM Ca were measured spectrofluorimetrically. An outward Na gradient was generated by decreasing extracellular Na while maintaining isotonicity. UMR-106 cells that were Na loaded by ouabain inhibition of Na,K-ATPase activity exhibited 30% greater Ca uptake than control cells. Similar results were obtained with primary bone cells. This uptake required extracellular Ca, was not inhibited by 200 microM verapamil, and was reversible upon reversal of the Na gradient. These data demonstrate the presence of a Na/Ca exchange transport system in osteoblasts.

Gallium nitrate inhibits bone resorption and collagen synthesis in neonatal mouse calvariae

Gallium nitrate (GN) is an agent used in the treatment of hypercalcemia. To more fully characterize the direct actions of GN on bone, we examined its effects on medium calcium, medium beta-glucuronidase (beta-GLU), and collagen synthesis in control and hormone-stimulated neonatal (4-6 days) mouse calvariae in vitro. GN (10 micrograms/ml) inhibited parathyroid hormone-stimulated (PTH; 1 nM) calcium release. A 24 h preincubation with 10 micrograms/ml of GN was required for complete inhibition; partial inhibition was seen with 12 h preincubation; 1, 3, or 6 h was inadequate. A dose-response study showed that with 24 h preincubation, 5, 3, and 1 microgram/ml of GN inhibited 81, 62, and 0% of PTH-induced calcium release. The effects of GN on the release of beta-GLU generally paralleled those on the release of calcium except that 10 micrograms/ml of GN stimulated beta-GLU release. Collagen synthesis was inhibited 50% by 3 micrograms/ml of GN, whereas noncollagen protein synthesis was unaffected. With PTH + GN no further decrease was observed. When GN was withdrawn from the medium after 24 h of preincubation, the inhibitory effect on calcium release and beta-GLU activity, but not on collagen synthesis, persisted through the 72 h of culture. GN also inhibited the resorption elicited by thyroxine (1 microM) and interleukin-1 beta (10 nM) but not by 1,25-dihydroxyvitamin D3 (30 pM). Our results indicate that GN is a powerful inhibitor of bone resorption in neonatal mouse calvariae even at low doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclooxygenase inhibitors enhance cell growth in an osteoblastic cell line, MC3T3-E1

To elucidate the significance of endogenous prostaglandin E2 (PGE2) in osteoblastic cell function, we studied the effects of cyclooxygenase inhibitors on cell growth and alkaline phosphatase (ALP) activity in MC3T3-E1 cells. UMR-106 cells were also used as references in our experiments. MC3T3-E1 cells, cultured in alpha-minimal essential medium containing 10% fetal bovine serum, were shown to produce PGE2, which was markedly suppressed in the presence of indomethacin. Addition of indomethacin resulted in an increase in DNA content and [3H]thymidine incorporation. A similar growth stimulatory effect was observed when structurally different cyclooxygenase inhibitors, that is, acetyl salicylic acid (ASA), flurbiprofen, and piroxicam, were added. These cyclooxygenase inhibitors, however, differed in their effects on ALP activity. Indomethacin and ASA enhanced ALP activity, whereas flurbiprofen and piroxicam suppressed it. We then examined the effects of exogenous addition of PGE2. Although exogenous PGE2 at 6 x 10(-6) M slightly stimulated cell growth, it inhibited cell growth at 6 x 10(-8) M and 6 x 10(-7) M. ALP activity was reduced in a dose-dependent fashion by exogenous PGE2. These results suggest that PGE2 produced by MC3T3-E1 may be suppressing cell proliferation and that cyclooxygenase inhibitors, per se, may stimulate cell growth by inhibiting endogenous PGE2 production in MC3T3-E1 cells. UMR-106 cells also produced PGE2, although less than MC3T3-E1 cells. In UMR-106 cells, the cyclooxygenase inhibitors did not influence DNA content or ALP activity as distinctly as in MC3T3-E1 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of specific thyroid hormone receptors in bone

Thyroid hormones stimulate bone turnover in vivo and increase Ca release from bone in vitro. To investigate further the effects of thyroid hormones in bone, we have characterized specific nuclear receptors for [125I]tri-iodothyronine (T3) in neonatal mouse calvaria. Maximal specific binding of [125I]T3 to isolated nuclei occurred within 60 min at 22 degrees C. [125I]T3 binding was completely and rapidly displaced by the addition of 10(-6) M unlabeled T3; the dissociation appears to be first order with t1/2 = 36 min. The IC50 for competition by unlabeled T3 was approximately 10(-8) M. The relative affinity of thyroxine (T4) for the receptor was approximately 10 X lower than T3, consistent with its lower biological potency in most target tissues for thyroid hormones. Only weak competition was observed with diiodotyrosine at concentrations up to 10(-4) M. We have previously shown that the cardiotonic agent milrinone stimulates bone resorption in vitro with characteristics similar to those of T4. Structural homology between milrinone and T4 was recently reported. Milrinone, like diiodotyrosine, was only a weak competitor for binding at concentrations up to 10(-4) M. Milrinone inhibited collagen synthesis in the calvaria. The results suggest that the effects of milrinone on bone turnover in calvaria in vitro are probably not mediated through a thyroid hormone receptor.