Direct effect of parathyroid hormone on the proliferation of osteoblast-like cells; a possible involvement of cyclic AMP

Parathyroid hormone depresses cytosolic pH and DNA synthesis in osteoblast-like cells

It has recently become apparent that a number of hormones and growth factors modulate cytosolic pH (pHi), and there is some evidence that this in turn may influence cell growth. We have examined the effects of parathyroid hormone (PTH) on both these parameters in an osteoblast-like cell line, UMR 106. Preliminary studies, using the pH-sensitive fluorescent probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein indicated that these cells regulate pHi by means of an amiloride-inhibitable Na+-H+ exchanger. Rat PTH-(1-34) (rPTH) caused a progressive dose-related decrease in pHi with a half-maximal effect at 10(-11) M. At 1 h, the maximal depression of pHi was 0.1 +/- 0.01 U. This effect was reproduced by forskolin, but neither agent influenced pHi in the presence of amiloride. Incorporation of [3H]thymidine was reduced by rPTH (half-maximal dose approximately 10(-11) M), forskolin, and N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate. The diacylglycerol analogue, phorbol 12-myristate 13-acetate, increased both pHi and [3H]thymidine incorporation, and amiloride reduced both indexes. However, rPTH remained a potent inhibitor of [3H]thymidine incorporation in the presence of amiloride, even though it did not affect pHi in these circumstances. It is concluded that PTH decreases pHi and growth in UMR 106 cells but that these changes can be dissociated. Depression of pHi may have other important effects on bone metabolism, such as reducing cell-cell communication, and may be associated with alkalinization of the bone fluid compartment.

Parathyroid hormone and prostaglandin E2 stimulate both inositol phosphates and cyclic AMP accumulation in mouse osteoblast cultures

Parathyroid hormone (PTH) and prostaglandin E2 (PGE2) are physiological agonists which stimulate bone cells to resorb bone, a process by which the mineralized extracellular bone matrix is dissolved. Bone resorption has a key role in the maintenance of plasma calcium levels. It has been established that both PTH and PGE2 activate adenylate cyclase in osteoblasts, but it is apparent that (1) the two agents have qualitatively different effects on osteoblasts, and (2) the generation of cyclic AMP cannot account for all the effects of PTH on bone cell metabolism. Others have demonstrated that PTH and PGE2 may also elevate intracellular calcium levels, but the mechanism by which this is achieved has not been fully defined. Here we have investigated the effects of PTH on neonatal mouse osteoblasts in culture and shown that physiological concentrations of the hormone (50 nM) caused a small increase (22%) in total inositol phosphates accumulation, with a larger increase (40%) in inositol trisphosphate. We found that this activation occurred at lower concentration than was necessary to activate adenylate cyclase. PGE2 was a more effective activator of inositol phosphates accumulation than PTH, causing up to 300% increase in the total inositol phosphates after 30 min. Both PTH and PGE2 stimulated cyclic AMP accumulation, but the activation of adenylate cyclase by forskolin did not enhance inositol phosphates production. We conclude that both PTH and PGE2 stimulate phosphoinositide turnover in mouse osteoblasts and suggest that this mechanism may contribute to their elevation of intracellular calcium in bone cells.

Inhibition of bone collagen synthesis by the tumor promoter phorbol 12-myristate 13-acetate

We characterized the effect of the tumor promoter phorbol 12-myristate 13-acetate (PMA) on osteoblast function and DNA synthesis in 21-day-old fetal rat calvaria maintained in organ culture. Protein synthesis was determined by measuring the incorporation of [3H]proline into collagenase-digestible (CDP) and noncollagen protein (NCP), respectively. Alkaline phosphatase activity was assessed as the release of p-nitrophenol from p-nitrophenol phosphate. DNA synthesis was determined by the incorporation of [3H]thymidine into acid-insoluble bone and total DNA content. PMA at 3-100 ng/ml (4-133 nM) caused a dose-related inhibition of collagen synthesis that was observed 6 hours after adding PMA to calvaria. PMA inhibited collagen synthesis in the osteoblast-rich central bone of calvaria but did not alter collagen synthesis in the periosteum. There was little effect of PMA on noncollagen protein synthesis in the central bone or periosteum. Phorbol esters that do not promote tumor formation in vivo did not alter collagen synthesis in calvaria. PMA stimulated prostaglandin E2 (PGE2) production in calvaria, but indomethacin did not alter the inhibitory effect of PMA on bone collagen synthesis. PMA decreased alkaline phosphatase activity measured after 48 hr of culture and increased the incorporation of [3H]thymidine into bone and DNA content after 96 hr of culture. These data indicate that PMA inhibits collagen synthesis and alkaline phosphatase activity, while stimulating DNA synthesis, suggesting that activation of protein kinase C might regulate osteoblast function and bone cell replication.

Cell-cell interactions in the osteogenic compartment of bone

The interactions between two different cell populations within the osteogenic compartment have been examined. The proliferation of periosteal fibroblasts (PF) in the presence or absence of osteoblast-like cells (OB), whose proliferative capacity was inhibited by irradiation, was measured. OB stimulated [3H]thymidine incorporation in PF and parathyroid hormone (PTH) enhanced the stimulation. In the reverse situation, however, PF inhibited OB. Irradiated OB also stimulated 3H-thymidine incorporation in OB, and irradiated PF in PF, but both to a lesser extent. Co-culture experiments showed that direct cell-cell contact was a prerequisite for stimulation of PF. Medium mediated contact between physically separated OB and PF did not stimulate, but rather inhibited PF proliferation. These results demonstrate that OB regulate the proliferation of cells in the PF population and can transmit the proliferation stimulating message of PTH to PF. This implies that in vivo the mature osteoblast may play a pivotal role in the (hormonal) regulation of osteoprogenitor cell proliferation and therefore bone formation.

In vitro evidence that local and systemic skeletal effectors can regulate 3[H]-thymidine incorporation in chick calvarial cell cultures and modulate the stimulatory actions(s) of embryonic chick bone extract

These investigations were intended to determine whether local and systemic skeletal effectors--3'5'-cyclic adenosine monophosphate (cAMP), prostaglandin E2 (PGE2), parathyroid hormone (PTH), 1,25-dihydroxyvitamin D (1,25(OH)2D), calcitonin, and NaF--could regulate 3[H]-thymidine incorporation (i.e., into DNA) in serum-free, monolayer cultures of embryonic chick calvarial cells, and/or modulate the activity of embryonic chick bone extracts to increase 3[H]-thymidine incorporation. In the absence of added bone extract, we found that calcitonin (0.1 U/ml), NaF (100 microM) and low-dose PTH (0.1 nM) stimulated 3[H]-thymidine incorporation, P less than .05 for each; isobutylmethylxanthine (IBMX--1 mM), 1,25OHD (10 nM), and high-dose PTH (10 nM) decreased 3[H]-thymidine incorporation; and PGE2 (1 microM) had no effect. The stimulatory actions of calcitonin, fluoride, and low-dose PTH were inductive, and the inhibitory actions of IBMX and 1,25(OH)2D were acute. PTH had complex time-dependent actions on 3[H]-thymidine incorporation, being inhibitory after 4-8 hours of exposure and stimulatory after 20-24 hours (P less than .001 for each). The effects of calcitonin, fluoride, and low-dose PTH to increase 3[H]-thymidine incorporation were greater in calvarial cell cultures enriched for undifferentiated osteoprogenitor cells than in cultures enriched for differentiated osteoblastlike cells. PTH inhibited 3[H]-thymidine incorporation in the latter (i.e., osteoblastlike) cultures (P less than .005). The inhibitory actions of IBMX and 1,25(OH)2D were independent of cell differentiation. Additional studies further revealed that these local and systemic skeletal effectors could also modulate the activity of embryonic chick bone extracts to increase 3[H]-thymidine incorporation in calvarial cell cultures. We found that calcitonin, fluoride, and low-dose PTH enhanced the effect of the extracts to increase 3[H]-thymidine incorporation (P less than .001 for each). These activations were noncompetitive, indicating (1) mechanistic differences between the stimulatory actions of the effectors and the chick bone extract (i.e., different rate-limiting steps for the effects of each on 3[H]-thymidine incorporation); and (2) that neither calcitonin, fluoride, nor 0.1 nM PTH altered the apparent affinity of the cells for stimulatory activity(s) in the extract. High-dose PTH was a noncompetitive inhibitor with respect to bone extract activity, indicating that the effect of 10 nM PTH to decrease 3[H]-thymidine incorporation was mechanistically distinct from the effect of the bone extract to increase 3[H]-thymidine incorporation.(ABSTRACT TRUNCATED AT 400 WORDS)

Parathyroid hormone induction of creatine kinase activity and DNA synthesis is mimicked by phospholipase C, diacylglycerol and phorbol ester

Parathyroid hormone (PTH), which increases cAMP levels, also induces an increase in the activity of the brain isozyme of creatine kinase and in DNA synthesis in osteoblast-enriched bone cell cultures by a cAMP-independent mechanism. The following results lead us to the conclusion that PTH induction of brain isozyme of creatine kinase activity and DNA synthesis occurs by activation of membranal phospholipid metabolism leading to increased protein kinase C activity and Ca2+ mobilization, a mechanism demonstrated for several growth factors and other hormones. (1) Binding of membranal phospholipids by agents such as gentamycin or antiphospholipid antibodies abolishes the stimulation by PTH of creatine kinase activity and DNA synthesis but not of cAMP production. (2) Treatment of cell cultures with exogenous phospholipase C increases brain isozyme of creatine kinase activity and DNA synthesis, but not cAMP production; these stimulations are also blocked by serum containing anti-phospholipid antibodies. PTH has no additional effect on stimulation of creatine kinase activity by phospholipase C (and only a slight effect on DNA synthesis). (3) A synthetic diacylglycerol (1-oleyl-2-acetyl glycerol) or phorbol ester (phorbol 12-myristate 13-acetate) or Ca2+ ionophore, A23187 induces creatine kinase activity and DNA synthesis in the cultures. However, this effect is not blocked by antiphospholipid sera and PTH has no additional effect. (4) Inhibition of protein kinase C activity by drugs reported to inhibit the enzyme (retinoic acid, quercetin) abolishes the stimulation of brain isozyme of creatine kinase activity and of DNA synthesis by PTH.

Isolation and characterization of highly serum-dependent cells released early from collagenase digested calvaria

A subclass of highly serum-dependent bone cells has been identified among the cells released early from calvaria following digestion in collagenase. Partial purification for these cells has been carried out based on the observation that they require serum for attachment to polystyrene culture flasks. This subclass of bone cells differs from adherent cells and late released osteoblasts, in that they express almost no cAMP response to PTH, require high levels of serum (10%) for initial growth and proliferation, and do not increase DNA synthesis in response to PTH. In common with adherent cells and late released osteoblasts, their proliferation is decreased by 1,25(OH)2D3 at doses above 0.2 ng/ml and they respond to PGE2 with increased DNA synthesis. These similarities suggest an ontogenic relationship with osteoblasts. Based on their differences, however, provisional identification of these cells as relatively undifferentiated mesenchymal cells is suggested.

Parathyroid hormone stimulation of alkaline phosphatase activity in cultured neonatal mouse calvarial bone cells: involvement of cyclic AMP and calcium

The involvement of cAMP and calcium in the rise in alkaline phosphatase (AP) activity observed when confluent, serum-free primary cultures of neonatal mouse calvarial cells are treated with parathyroid hormone (PTH) has been studied. Synthetic bovine PTH [bPTH-(1-34)] increased cellular cAMP at concentrations (10(-9) to 10(-7) M) previously found to elevate AP activity. Other substances that increase cAMP in these cells (forskolin, prostaglandin E2, 8-bromoadenosine cAMP and 3-isobutyl-1-methylxanthine) also increased enzyme activity. By comparison, increasing the concentration of calcium in the culture medium from 1.8 to 3.8 or 5.8 mM lowered the magnitude of the maximal AP response. In addition, treatment of cultures with the divalent cation ionophore A23187 caused a significant decrease in AP activity. These results suggest that: 1) cAMP mediates the rise in the specific activity of AP in cultured neonatal mouse calvarial cells treated with bPTH-(1-34) and 2) the concentration of calcium in the environment significantly influences the responsivity of bone cells to the hormone.

Effects of exogenous prostanoids on the proliferation of osteoblast-like cells in vitro

The effects of several prostaglandins on the proliferation of secondary cultures of osteoblast-like cells, as measured by the incorporation of [3H]-thymidine into DNA and total DNA content of the cultures, were studied. PGE2 in the concentration range of 10(-8) to 10(-5) M caused a direct, dose-related stimulation of proliferation, while PGF2 alpha and PGD2 were less effective. PGA2 and 6-keto-PGF1 alpha were inactive in the osteoblasts in concentrations of 10(-7) to 10(-6) M. A similar stimulation profile was observed for the induction of ornithine decarboxylase (ODC, L-ornithine decarboxy-lyase, EC 4.1.1.17): the order of potency of the different prostaglandins in the induction of the ODC activity was PGE2 greater than PGF2 alpha = PGD2; again, PGA2 and 6-keto-PGF1 alpha were without effect in concentrations up to 10(-6) M. These results show that the primary prostaglandins, in order of potency PGE2 greater than PGF2 alpha = PGD2, can have a direct, stimulatory effect on the proliferation of osteoblasts, which is closely related to the induction of ODC activity.