Prostaglandins change cell shape and increase intercellular gap junctions in osteoblasts cultured from rat fetal calvaria

Prostaglandin E2 Enhances Gap Junctional Intercellular Communication in Clonal Epithelial Cells

Prostaglandins are a group of lipids that produce diverse physiological and pathological effects. Among them, prostaglandin E2 (PGE2) stands out for the wide variety of functions in which it participates. To date, there is little information about the influence of PGE2 on gap junctional intercellular communication (GJIC) in any type of tissue, including epithelia. In this work, we set out to determine whether PGE2 influences GJIC in epithelial cells (MDCK cells). To this end, we performed dye (Lucifer yellow) transfer assays to compare GJIC of MDCK cells treated with PGE2 and untreated cells. Our results indicated that (1) PGE2 induces a statistically significant increase in GJIC from 100 nM and from 15 min after its addition to the medium, (2) such effect does not require the synthesis of new mRNA or proteins subunits but rather trafficking of subunits already synthesized, and (3) such effect is mediated by the E2 receptor, which, in turn, triggers a signaling pathway that includes activation of adenylyl cyclase and protein kinase A (PKA). These results widen the knowledge regarding modulation of gap junctional intercellular communication by prostaglandins.

Evaluation of Distraction Osteogenesis by Scanning Electron Microscopy

A model of bifocal distraction osteogenesis in the canine model was used to assess and quantitate the mineral content of the newly forming bone within the canine mandible. A 2-cm defect was created in the body of the mandible, and after a posterior osteotomy, the transport disk was advanced at 0.25 mm per 8 hours for 21 days and then held in rigid fixation for an additional week. As a control for this study, three additional dogs underwent the same procedure with the exception that the transport disk was not advanced. Electron dispersive spectroscopy analysis was performed on the newly formed regenerate bone and compared with areas of existing cortical bone of both the transport disk and the mandible. In the control model, special note was made of the pericortical callus at the osteotomy site as well as of the regenerative bone that filled the 2-cm defect in the body of the mandible. Calcium/phosphorous ratios were used to assess the composition of the mineralized regions of the mandible. The regenerate bone that filled the defect and the mineralized callus surrounding the site of osteoclasis in the control mandible were significantly different in composition when compared with the regenerate bone that formed during distraction osteogenesis. This suggests that distraction osteogenesis may effect an initial matrix production that is more similar in composition to the mature cortical bone from which it was derived than does periosteal regeneration and filling of an osseous defect.

Understanding the advances in biology of orthodontic tooth movement for improved ortho-perio interdisciplinary approach

This article provides an insight on detailed current advances in molecular understandings of periodontal ligament cells and the influence of orthodontic force on them in the light of recent advances in molecular and genetic sciences. It sequentially unfolds the cellular events beginning from the mechanical force initiated events of cellular responses to bone remodeling. It also highlights the risks and limitations of orthodontic treatment in certain periodontal conditions, the important areas of team work, orthodontic expectations from periodontal treatment and the possibility of much more future combined research to improve the best possible periodontal health and esthetic outcome of the patient.

Actin cytoskeleton controls activation of Wnt/β-catenin signaling in mesenchymal cells on implant surfaces with different topographies

Surface topography affects cell function and differentiation. It has been previously shown that rough surfaces can enhance the activation of canonical Wnt signaling, an important pathway for osteoblast differentiation and bone maintenance, but the underlying mechanisms are still poorly understood. The present paper investigates whether cytoskeletal organization contributes to regulating this pathway. Rho-associated protein kinase (ROCK), an important controller of actin microfilaments, was inhibited with 2mM specific antagonist Y-27632 in mesenchymal and osteoblastic cells growing on titanium discs with a polished or acid-etched, sand-blasted (SLA) surface. Y-27632 subverted the morphology of the cytoskeleton on polished and, to a lesser extent, on SLA surfaces, as evidenced by fluorescence microscopy. Although ROCK inhibition did not affect cell viability, it increased activation of Wnt signaling in uncommitted C2C12 mesenchymal cells on polished surfaces but not on SLA discs upon reporter assay. Consistently with this, real-time polymerase chain reaction analysis showed that MC3T3 cells on polished surfaces expressed higher mRNA levels for β-catenin and alkaline phosphatase, a known Wnt target gene, and for the osteoblastic differentiation marker osteocalcin after ROCK inhibition. Taken together, these data demonstrate that cytoskeletal organization mediates activation of Wnt canonical signaling in cells on titanium surfaces with different topographies.

Hormonal, pH, and calcium regulation of connexin 43-mediated dye transfer in osteocytes in chick calvaria

Gap junctional intercellular communication among osteocytes in chick calvaria, their natural 3D environment, was examined using FRAP analysis. Cell-cell communication among osteocytes in chick calvaria was mediated by Cx43 and was regulated by extracellular pH, extracellular calcium ion concentration, and PTH.

INTRODUCTION

The intercellular network of communication among osteocytes is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is thought to play an important role in integration and synchronization of bone remodeling. We hypothesized that extracellular pH (pH(o)) and extracellular calcium ion concentration ([Ca2+](e)), both of which are dynamically altered by osteoclasts during bone remodeling, affect GJIC among osteocytes. Using fluorescence replacement after photobleaching (FRAP) analysis, we examined the effect of changes in pH(o) and [Ca2+](e) and addition of PTH on GJIC in osteocytes in chick calvaria. Additionally, we examined the role of intracellular calcium on the regulation of GJIC among osteocytes.

MATERIALS AND METHODS

Anti-Connexin43 (Cx43) immunolabeling was used to localize gap junctions in chick calvaria. GJIC among osteocytes in chick calvariae was assessed using FRAP.

RESULTS

Cx43 immunoreactivity was detected in most of the osteocyte processes. FRAP analysis showed dye-coupling among osteocytes in chick calvariae. In untreated osteocytes, fluorescence intensity recovered 43.7 +/- 2.2% within 5 min after photobleaching. Pretreatment of osteocytes with 18 alpha-GA, a reversible inhibitor of GJIC, significantly decreased fluorescence recovery to 10.7 +/- 2.2%. When pH(o) was decreased from 7.4 to 6.9, fluorescence recovery significantly decreased from 43.3 +/- 2.9% to 19.7 +/- 2.3%. Conversely, when pH(o) was increased from 7.4 to 8.0, fluorescence recovery was significantly increased to 61.9 +/- 4.5%. When [Ca2+](e) was increased from 1 to 25 mM, fluorescence recovery was significantly decreased from 47.0 +/- 6.1% to 16.1 +/- 2.1%. In bone fragments exposed to 1.0-10 nM rPTH for 3 h, replacement of fluorescence was significantly increased to 60.7 +/- 7.2%. Chelating intracellular calcium ions affected GJIC regulation by [Ca2+](e) and PTH.

CONCLUSIONS

Our study of cell-cell communication between osteocytes in chick calvaria showed for the first time that GJIC among osteocytes is regulated by the extracellular environment and by hormonal stimulation during bone remodeling. This method may be more biologically relevant to living bone than current methods.

Gap junctions in skeletal development and function

Gap junctions play a critical role in the coordinated function and activity of nearly all of the skeletal cells. This is not surprising, given the elaborate orchestration of skeletal patterning, bone modeling and subsequent remodeling, as well as the mechanical stresses, strains and adaptive responses that the skeleton must accommodate. Much remains to be learned regarding the role of gap junctions and hemichannels in these processes. A common theme is that without connexins none of the cells of bone function properly. Thus, connexins play an important role in skeletal form and function.

Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules

The role of cytoskeletal elements in gap junction (GJ) assembly has been studied using Novikoff hepatoma cells treated with cytochalasin B (CB) to disrupt actin filaments or with colchicine or nocodazole to disrupt microtubules. After 60 min of cell reaggregation, freeze-fracture was used to evaluate quantitatively the "initiation," "maturation," and "growth" phases of GJ assembly. The development of junctional permeability to fluorescent dyes was also analyzed. The only effects of CB on the structure or permeability of the developing junctions involved an elongation of GJ aggregates and a small decrease in formation plaque areas. Colchicine (but not the inactive form, lumicolchicine) prevented the enhancement of GJ growth by cholesterol, but its effect on basal growth was equivocal. Nocodazole inhibited the growth of GJ, even under basal conditions, without an effect on initiation. Nocodazole also blocked the forskolin-enhanced increase in the growth of GJs and, in living MDCK cells, reduced the movement of transport intermediates containing green fluorescent protein-tagged connexin43. Thus, neither actin filaments nor microtubules appear to restrict GJ assembly by anchoring intramembrane GJ proteins, nor are they absolutely required for functional GJs to form. However, microtubules are necessary for enhanced GJ growth and likely for facilitating connexin trafficking under basal conditions.

Ultrastructural changes in calvarial osteoblast cytoskeleton after prostaglandin E2 administration in rats

Recent studies on the cytoskeleton of osteoblasts have been made mainly using cultured cells. However, the morphology of cultured cells may be altered during subculture. Therefore, cytoskeletal changes of calvarial osteoblasts were investigated in situ by electron microscopy using the detergent perfusion method to preserve cell morphology as well as selectively observing the cytoskeleton in the presence of a high concentration of prostaglandin E2 (PGE2) in the calvarial periphery in rats. Rats were perfused with a mixture of Triton X-100 and glutaraldehyde, then the cytoskeleton was observed by transmission electron microscopy. In osteoblasts of the control group, thick bundles of microfilaments ran parallel to the long axis of the cells immediately below the cell membrane adjacent to the osteoid layer. In contrast, in the osteoblasts of the PGE2-administered group, the external morphology was changed to an asteroid or cubic shape, and thick bundles of microfilaments immediately below the cell membrane adjacent to the osteoid were not observed, although microfilament fibres, with a diameter of 5-6 nm, were observed in the cytoplasm.

Antisense oligonucleotide blockade of connexin expression during embryonic bone formation: evidence of functional compensation within a multigene family

Prior studies in our laboratory demonstrated the presence of gap junction proteins (connexins) throughout intramembranous bone formation [Minkoff et al. (1994) Anat Embryol 190:231-241]. In addition, two members of the connexin family of gap junction proteins, connexin 43 (Cx43; Gj alpha 1) and connexin 45 (Cx45; Gj alpha 6), were found by Civitelli et al. [1993; J Clin Invest 91:1888-1896] to be associated, specifically, with osteogenesis. Recently, however, a null mutation in the gene encoding Gj alpha 1 in mice has been produced by Reaume et al. [1995; Science 267:1831-1834]. Gj alpha 1 null homozygotes survived to term but died at birth of heart abnormalities. Examination of the null homozygous embryos, surprisingly, did not reveal overt histological or anatomical abnormalities in any organ system other than the heart. In view of this, the present investigation was initiated in order to evaluate bone formation under conditions in which the expression of Gj alpha 1 and Gj alpha 6, the connexins specifically associated with osteogenesis, had been perturbed, individually as well as in combination. An in vitro system employing organ cultures of dissociated embryonic chick mandibular mesenchyme was employed. Mesenchyme was cultured in the presence and absence of sense and antisense oligodeoxynucleotides (ODN), ranging in length from 15 to 24 mer and containing sequences that included the initiation codon of Gj alpha 1 and of Gj alpha 6. In cultures of mesenchyme, grown for 6 to 13 days in the presence of the combined antisense ODNs to Gj alpha 1 and Gj alpha 6, bone formation was markedly reduced or absent. By contrast, in cultures grown in medium containing the combination of corresponding sense ODNs to both Gj alpha 1 and Gj alpha 6, bone formation was evident. In addition, when cultures were grown in the presence of antisense or sense ODNs to either Gj alpha 1 or Gj alpha 6, individually, bone formation was seen. Immunohistochemical analysis of connexin expression revealed intense immunoreactive signal to Gj alpha 1 and Gj alpha 6 in bone of the control explants, in which no ODNs were present; in those cultures in which either Gj alpha 1 and Gj alpha 6 antisense ODNs were present, however, the expression of the respective connexin protein was either significantly reduced or absent. Further, in those explants in which Gj alpha 1 expression was blocked, immunoreactive signal to Gj alpha 6 appeared to have been amplified in regions of developing bone. These results suggest that, in avian osteogenic tissue, when Gj alpha 1 protein expression has been impeded another related connexin protein (Gj alpha 6) may subserve the functions of the missing connexin. The findings of this study, therefore, support the hypothesis that, within the connexin gene family, functional compensation can occur.

Association of cell and substrate adhesion molecules with connexin43 during intramembranous bone formation

Prior studies in our laboratory have demonstrated an association of specific gap junction proteins with intramembranous bone formation in the avian mandible. The purpose of the present study was to extend these observations by determining if there was a relationship between the expression of one of the gap junction proteins examined previously (connexin43) and the expression of specific cell adhesion (CAM) and/or substrate adhesion (SAM) molecules [i.e. NCAM, A-CAM (N-cadherin) and tenascin (tenascin-C)] that have previously been shown to be associated with bone formation. Immunohistochemical localization of connexin43, tenascin, NCAM and N-cadherin was performed on serial sections of mandibles of chick embryos from 6 to 12 days of incubation. Analysis of adjacent serial sections revealed that the NCAM and tenascin immunostaining that appeared initially on the lateral aspect of Meckel's cartilage preceded the overt expression of trabecular bone. At subsequent stages, NCAM and tenascin staining gradually overlapped the region of connexin43 expression. In contrast, the expression of N-cadherin was found to colocalize with that of connexin43 from the first appearance of connexin43 expression. Most significantly, although the domains of NCAM and tenascin expression were initially separate from that of connexin43, bone formation originated only in the region where these domains intersected. These findings suggest that, of the CAMs and SAMs examined, N-cadherin appears to be associated with the establishment of cell contacts responsible for the presence and/or maintenance of connexin43-mediated gap junctional communication, while tenascin and NCAM appear to be associated, in a more specific manner, with processes that accompany the overt expression of the osteogenic phenotype.

Effects of vitamin D3, 17beta-estradiol, vasoactive intestinal peptide, and glutamate on electric coupling between rat osteoblast-like cells in vitro

Osteoblast-like cells express receptors for various hormones and neurotransmitters that induce widespread actions in the bone to which intercellular communication and its modulation may contribute. Therefore, we examined the effects of the osteotropic hormones vitamin D3 (vitD3) and 17beta-estradiol (17beta-E2) as well as the neurotransmitter vasoactive intestinal peptide (VIP) and the excitatory amino acid glutamate (Glu) on gap junctions between rat osteoblast-like (ROB) cells in vitro. Electric coupling was measured by simultaneous intracellular recordings from neighboring cells. The coupling factor (cf) was calculated from membrane potential changes induced by alternate current injections into both cells. In ROB cells cf was increased by 5 x 10(-8) mol/L vitD3 to 130 +/- 13% (mean +/- SD; n = 6) of the initial value within 5-20 min. This effect was not reversible after washing with control saline for 10-15 min. In six cell pairs, cf was not affected by vitD3 (94 +/- 5%). In three cell pairs superfusion of 10(-8) mol/L E2 reduced cf to 80 +/- 6% within 10 min, whereas, in two cell pairs, this hormone improved cf to 140% within 20 min. Exposure of VIP (3 x 10(-8) mol/L) did not alter cf in the majority of cells (99 +/- 3%; n = 11). In five cell pairs, cf was improved within 5-15 min to 133 +/- 12%, whereas, in one cell pair, cf was reduced to 22% by VIP. In contrast, brief application of Glu (5 x 10(-3) mol/L) decreased cf to 75 +/- 5% (n = 5), whereas, in nine other cell pairs, cf was not affected (96 +/- 5%). The findings indicate that cell-cell coupling of gap junctions between bone cells can be altered by actions of hormones and transmitters in a cell-pair-specific way, which may depend on their functional state.

Morphological changes induced by prostaglandin E in cultured rat osteoblasts

Prostaglandin E (PGE)-induced morphological changes of osteoblasts and its possible mechanisms were investigated in cultured calvaria and isolated osteoblasts from long bone fragments of neonatal rats. The control osteoblasts, either on the calvaria or isolated from the long bone fragments, were flat, polygonal in shape, and arranged in a monolayer under scanning electron microscopy (SEM) or phase contrast microscopy. Treatment with 1 mumol/L of prostaglandin E2 (PGE2, 2 h) caused these bone cells to contract a soma, whereas 10 and 100 mumol/L PGE2 (2 h) caused 18%-30% of the bone cells to elongate and expose the undersurface. Incubation of the cultured osteoblasts with PGE2 at different time periods showed a bell-shaped pattern with the optimal response at 2 h of incubation. A similar reaction can be induced by treatment with prostaglandin E1 (PGE1) or dibutyryl cyclic adenosine monophosphate (DBcAMP) in combination with 3-isobutyl-1-methylxanthine (IBMX). Furthermore, we assessed the percentage of responsive isolated bone cells to investigate interactions with other agents. The morphological changes induced by PGEs were inhibited by H-8, a protein kinase inhibitor. On the other hand, elevated intracellular calcium enhanced the PGE-induced morphological changes. Fluorescence labeling showed that PGEs caused the breakdown of the actin microfilaments, but spared the microtubules and vimentin filaments in the isolated osteoblast-like cells. These results suggest that the morphological changes of osteoblasts induced by PGEs may be related to the intracellular cAMP and calcium levels.

Estrogen modulation of osteoblastic cell-to-cell communication

Two osteoblastic cell populations, calvarial and marrow stromal cells, were exposed to estrogen derivatives in vitro. The hormonal effect was monitored by following intracellular Ca+2 levels [Ca+2]i and gap-junction communication. We measured fast changes in intracellular Ca+2 levels in response, of these cells, to the steroid hormones. The changes were dose dependent revealing maximal activity at 100 pM by 17-beta-Estradiol and 1 nM by estradiol-CMO. Additionally, the effect of estrogen, on functional coupling of the cells, was measured using fluorescence dye migration and counting the number of neighboring cells coupled by gap junctions. An uncoupling effect was demonstrated in response of these cells to estrogen treatment. The quick stereospecific effect was achieved in the presence of 17-beta-estradiol but not in the presence of 17-alpha-estradiol. These results suggest the involvement of plasma membrane receptors in addition to the already known nuclear receptors in transducing the hormone effects in the osteoblastic cells.

Parathyroid hormone effect on cell-to-cell communication in stromal and osteoblastic cells

We characterized the formation and regulation of the gap junction in calvarial osteoblasts and in a series of subtypes from marrow stromal cells. The stromal cells included osteogenic, chondro-osteogenic, and endothelial cells. The cell coupling was measured by using fluorescence dye injected into single cells, and its migration to neighboring cells was measured. The functional coupling of cells was highly expressed by the osteoblastic cells. This process is mediated through fast changes in intracellular Ca+2 levels. Calcium ionophore (A 23,187) demonstrated an uncoupling effect on the cells. In addition, the exposure of the cells to the parathyroid hormone increased the formation of the gap junction complex; the highest level was demonstrated in the osteoblastic cells.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

Parathyroid hormone up-regulation of connexin 43 gene expression in osteoblasts depends on cell phenotype

Accumulating evidence indicates that gap junctions, primarily composed of connexin 43 (Cx43), are distributed extensively throughout bone. We have previously reported that in osteoblastic cells parathyroid hormone (PTH) increases both the steady-state levels of transcripts for Cx43 and gap-junctional intercellular communication in a process involving cyclic adenosine monophosphate (cAMP). We now present data showing that the mechanism of stimulation of Cx43 gene expression by PTH involves an increased rate of Cx43 gene transcription without affecting Cx43 transcript stability in UMR 106 osteoblastic cells. Activation of the protein kinase C pathway is not involved in this process. Inhibiting translation consistently decreases the PTH-mediated stimulation of Cx43 gene expression at all the times we tested (1-3 h). However, this effect is only partial, demonstrating that de novo protein synthesis is required for full stimulation. PTH increases the steady-state levels of Cx43 mRNA in several osteoblastic cell lines, albeit to different levels. We were unable to detect PTH stimulation in ROS 17/2.8 osteoblastic cells, suggesting that the effect of PTH on Cx43 gene expression may depend on the developmental state of the cell along the osteoblastic differentiation pathway. In the MC3T3-E1 preosteoblastic cell line, we find that PTH increases Cx43 gene expression in proliferating and maturing osteoblastic cells, but not in nondividing, differentiated osteoblasts, where the basal level of Cx43 gene expression is elevated. Unlike PTH, the osteotropic hormones 1,25-dihydroxyvitamin D3 and 17beta-estradiol do not appear to affect Cx43 gene expression in UMR 106 osteoblastic cells.

Down-regulation of gap junctional intercellular communication between osteoblastic MC3T3-E1 cells by basic fibroblast growth factor and a phorbol ester (12-O-tetradecanoylphorbol-13-acetate)

To address the relation between osteoblast growth and cell-to-cell communication, we examined the effects of basic fibroblast growth factor (bFGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA), both potent stimulators of osteoblastic proliferation, on gap junctional intercellular communication between osteoblastic MC3T3-E1 cells. The level of intercellular communication was estimated by a photobleaching method. TPA inhibited the degree of intercellular communication in two different time-dependent manners. The early (< 1 h) inhibition by TPA was consistent with an increase in the phosphorylation of connexin 43 (Cx43). The later inhibition was caused by reduction in the total amount of Cx43 on the plasma membrane, due to the decrease in the level of Cx43 transcripts. These qualitative and quantitative modulations by TPA were inhibited by a selective inhibitor of protein kinase C, GF109203X. bFGF also attenuated the gap junctional intercellular communication. However, short exposure (< 5 h) to bFGF did not affect the communication. The fact that the growth factor immediately stimulated the phosphorylation of Cx43 indicates that the phosphorylation site(s) affected by bFGF was not involved in the inhibition of communication. The decrease in the intercellular communication level was detected by the longer exposure (> 8 h) to bFGF and paralleled the decline in the Cx-mRNA level. This inhibitory effect of bFGF was abolished by the addition of a tyrosine kinase inhibitor, herbimycin A. Thus, gap junctional intercellular communication between osteoblasts was down-regulated by osteoblastic mitogens through different mechanisms of the modulation of Cx43.

Gap junctional intercellular communication of bovine luteal cells from several stages of the estrous cycle: effects of prostaglandin F2 alpha, protein kinase C and calcium

Cellular interactions mediated by both contact-dependent and contact-independent mechanisms are probably important to maintain luteal function. The present studies were performed to evaluate the effects of luteotropic and luteolytic hormones, and also intracellular regulators, on contact-dependent gap junctional intercellular communication (GJIC) of bovine luteal cells from several stages of luteal development. Bovine corpora lutea (CL) from the early, mid and late luteal phases of the estrous cycle were dispersed with collagenase and incubated with no treatment, LH, PGF or LH + PGF (Experiment 1), or with no treatment, or agonists or antagonists of protein kinase C (TPA or H-7) or calcium (A23187 or EGTA; Experiment 2). After incubation, media were collected for determination of progester-one concentrations. Then the rate of GJIC was evaluated for small luteal cells in contact with small luteal cells, and large luteal cells in contact with small luteal cells by using the fluorescence recovery after photobleaching technique and laser cytometry. Luteal cells from each stage of the estrous cycle exhibited GJIC, but the rate of GJIC was least (P < 0.05) for luteal cells from the late luteal phase. LH increased (P < 0.05) GJIC between small luteal cells from the mid and late but not the early luteal phase. PGF increased (P < 0.05) GJIC between small luteal cells from the mid luteal phase and diminished (P < 0.05) LH-stimulatory effects on GJIC between small luteal cells from the late luteal phase. Throughout the estrous cycle, TPA decreased (P < 0.05) the rate of GJIC between large and small, and between small luteal cells, and A23187 decreased (P < 0.05) the rate of GJIC between large and small luteal cells. LH and LH + PGF, but not PGF alone increased (P < 0.05) progesterone secretion by luteal cells from the mid and late luteal phases. Agonists or antagonists of PKC or calcium did not affect progesterone secretion by luteal cells. These data demonstrate that both luteal cell types communicate with small luteal cells, and the rate of communication depends on the stage of luteal development. LH and PGF affect GJIC between small luteal cells during the fully differentiated (mid-luteal) and regressing (late luteal) stages of the estrous cycle. In contrast, at all stages of luteal development, activation of PKC decreases GJIC between small and between large and small luteal cells, whereas calcium ionophore decreases GJIC only between large and small luteal cells. Luteotropic and luteolytic hormones, and intracellular regulators, may be involved in regulation of cellular interactions within bovine CL which likely is an important mechanism for coordination of luteal function.

Effects of luteinizing hormone and prostaglandin F(2α) on gap junctional intercellular communication of ovine luteal cells throughout the estrous cycle

Cellular interactions mediated by contact-dependent pathways may be important to maintain luteal function. The objective of the present experiment was to evaluate the role of LH and prostaglandin F(2α) (PGF) in regulation of contact-dependent, gap junctional intercellular communication (GJIC) of ovine luteal cells from several stages of luteal development. Corpora lutea (CL) obtained from superovulated ewes on days 5 (n=7), 10 (n=8), and 15 (n=9) after estrus were dispersed with collagenase and cell types were separated by elutriation. Cells were plated as a mixed population (nonelutriated), or as small or large luteal cell fractions, and incubated in serum-free media containing no hormone, LH (100 ng/mL), PGF (100 ng/mL), LH+PGF, or dibutyryl cAMP (dbcAMP; 2 mM) for 18-24 h. Media were collected for evaluation of progesterone (P4) concentrations and replaced with media containing fluorescent dye. Then the rate of GJIC was evaluated by using the fluorescence recovery after photobleaching technique and laser cytometry. The rate of GJIC was determined for selected cells: small luteal cells in contact only with small luteal (S-S) cells; large luteal cells in contact only with small luteal (L-S) cells; and large luteal cells in contact only with large luteal (L-L) cells. LH increased (p<0.01) GJIC for S-S on d 5 and 10 and for L-S cells across the estrous cycle, but did not affect GJIC for L-L cells. PGF increased (p<0.05) GJIC for L-L cells on d 10 and 15, and decreased (p<0.05) GJIC for S-S cells from d 5 and 10 of the estrous cycle. LH+PGF increased (p<0.05) GJIC for S-S cells on d 5 and 10, and for L-S and L-L cells on d 10 and 15 of the estrous cycle. In addition, PGF diminished (p<0.05) LH-stimulatory effects on GJIC for S-S cells from d 5 and 10, and for L-S cells from d 5 of the estrous cycle. Dibutyryl cAMP stimulated (p<0.05) GJIC between all evaluated cell types across the estrous cycle. LH and dbcAMP stimulated (p<0.05) P4 secretion by mixed and small luteal cell fractions, PGF alone did not affect basal P4 secretion, but LH+PGF stimulated (p<0.05) P4 production by small luteal cells across the estrous cycle. PGF diminished (p<0.05) LH-stimulatory effects on P4 production in mixed populations of luteal cells across the estrous cycle.These data demonstrate that both luteal cell types communicate with each other, and the rate of communication was affected by LH, PGF, and dbcAMP. Modulation of gap junctional contact-dependent intercellular communication may be an important mechanism by which regulatory signals are transduced during luteal growth, differentiation, and regression in sheep.

Transfected connexin45 alters gap junction permeability in cells expressing endogenous connexin43

Many cells express multiple connexins, the gap junction proteins that interconnect the cytosol of adjacent cells. Connexin43 (Cx43) channels allow intercellular transfer of Lucifer Yellow (LY, MW = 443 D), while connexin45 (Cx45) channels do not. We transfected full-length or truncated chicken Cx45 into a rat osteosarcoma cell line ROS-17/2.8, which expresses endogenous Cx43. Both forms of Cx45 were expressed at high levels and colocalized with Cx43 at plasma membrane junctions. Cells transfected with full-length Cx45 (ROS/Cx45) and cells transfected with Cx45 missing the 37 carboxyl-terminal amino acids (ROS/Cx45tr) showed 30-60% of the gap junctional conductance exhibited by ROS cells. Intercellular transfer of three negatively charged fluorescent reporter molecules was examined. In ROS cells, microinjected LY was transferred to an average of 11.2 cells/injected cell, while dye transfer between ROS/Cx45 cells was reduced to 3.9 transfer between ROS/Cx45 cells was reduced to 3.9 cells. In contrast, ROS/Cx45tr cells transferred LY to > 20 cells. Transfer of calcein (MW = 623 D) was also reduced by approximately 50% in ROS/Cx45 cells, but passage of hydroxycoumarin carboxylic acid (HCCA; MW = 206 D) was only reduced by 35% as compared to ROS cells. Thus, introduction of Cx45 altered intercellular coupling between cells expressing Cx43, most likely the result of direct interaction between Cx43 and Cx45. Transfection of Cx45tr and Cx45 had different effects in ROS cells, consistent with a role of the carboxyl-terminal domain of Cx45 in determining gap junction permeability or interactions between connexins. These data suggest that coexpression of multiple connexins may enable cells to achieve forms of intercellular communication that cannot be attained by expression of a single connexin.

Cell-to-cell communication in osteoblastic networks: cell line-dependent hormonal regulation of gap junction function

We have characterized the distribution, expression, and hormonal regulation of gap junctions in primary cultures of rat osteoblast-like cells (ROBs), and three osteosarcoma cell lines, ROS 17/2.8, UMR-106, and SAOS-2, and a continuous osteoblastic cell line, MC3T3-E1. All cell lines we examined were functionally coupled. ROS 17/2.8 were the more strongly coupled, while ROB and MC3T3-E1 were moderately coupled and UMR-106 and SAOS-2 were weakly coupled. Exposure to parathyroid hormone (PTH) for 1 h increased functional coupling in ROB cells in a concentration-dependent manner. Furthermore, PTH(3-34), an analog of PTH with binds to the PTH receptor and thus attenuates PTH-stimulated cAMP accumulation, also attenuated PTH-stimulated functional coupling in ROB. This suggests that PTH increases functional coupling partly through a cAMP-dependent mechanism. A 1 h exposure to PTH did not affect coupling in ROS 17/2.8, UMR-106, MC3T3-E1, or SAOS-2. To examine whether connexin43 (Cx43), a specific gap junction protein, is present in functionally coupled osteoblastic cells, we characterized Cx43 distribution and expression. Indirect immunofluorescence with antibodies to Cx43 revealed that ROS 17/2.8, ROB, and to a lesser extent MC3T3-E1 and UMR-106, expressed Cx43 immunoreactivity. SAOS-2 showed little if any Cx43 immunoreactivity. Cx43 mRNA and Cx43 protein were detected by Northern blot analysis and immunoblot analysis, respectively, in all cell lines examined, including SAOS-2. Our findings suggest that acute exposure to PTH regulates gap junction coupling, in a cell-line dependent manner, in osteoblastic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of gap junctions in the osteoblastic MC3T3-E1 cell line

Gap junctions are channels connecting cells that function in cell-to-cell communication. Gap junctions are abundant in osteoblastic cells. Membranes enriched for gap junction plaques were obtained by differential centrifugation, followed by treatment of the membranes with potassium iodide and sarkosyl before sucrose density gradient centrifugation. Electron microscopy showed that the preparation was enriched for electron-dense membranes consistent with gap junctions. Coomassie Blue staining of SDS-PAGE preparations revealed a prominent band at approximately 41 kD. Western analysis with a site-directed antibody, CT-360 (D. Laird, California Institute of Technology, Pasadena, CA), to the C-terminal portion of the rat heart connexin 43 molecule was positive in the MC3T3-E1 cell line, a phenotypic osteoblastic cell line derived from normal neonatal mouse calvariae. Western analysis using a monoclonal antibody, R5.21C, to rat liver connexin 32 was negative. Additionally, a prominent band at 59 kD was detected by CT-360 in both gap junction-enriched preparations and cell lysates. Treatment of diluted samples of gap junction-enriched preparations with sulfhydryl reducing agents in combination with detergents resulted in the enhancement and diminution of the 41 and 59 kD bands, respectively. Immunoprecipitation following [35S]methionine/[35S]cysteine labeling revealed a significant band detected at 122 kD in addition to the 41 kD band. To demonstrate functional gap junctions, transfer of lucifer yellow dye to surrounding cells was monitored after microinjection of a target cell. Between passages 10 and 25 in culture, functional cell coupling was found in approximately 70% of injected cells. Coupling was detected within 1-2 minutes after injection. Simultaneous microinjection of the CT-360 antibody with lucifer yellow resulted in the decoupling of cells. In conclusion, (1) MC3T3-E1 cells possess a 41 kD protein that is recognized by connexin 43 antibody to rat heart gap junction; (2) multimers of the MC3T3-E1 gap junctions occur in the preparation; and (3) functional coupling demonstrated by dye transfer may be regulated by region(s) in the C terminus of the connexin molecule.

Connexin43 mediates direct intercellular communication in human osteoblastic cell networks

We have examined cell coupling and expression of gap junction proteins in monolayer cultures of cells derived from human bone marrow stromal cells (BMC) and trabecular bone osteoblasts (HOB), and in the human osteogenic sarcoma cell line, SaOS-2. Both HOB and BMC cells were functionally coupled, since microinjection of Lucifer yellow resulted in dye transfer to neighboring cells, with averages of 3.4 +/- 2.8 (n = 131) and 8.1 +/- 9.3 (n = 51) coupled cells per injection, respectively. In contrast, little diffusion of Lucifer yellow was observed in SaOS-2 monolayers (1.4 +/- 1.8 coupled cells per injection, n = 100). Dye diffusion was inhibited by octanol (3.8 mM), an inhibitor of gap junctional communication. All of the osteoblastic cells expressed mRNA for connexin43 and connexin45, but not for connexins 26, 32, 37, 40, or 46. Whereas all of the osteoblastic cells expressed similar quantities of mRNA for connexin43, the poorly coupled SaOS-2 cells produced significantly less Cx43 protein than either HOB or BMC, as assessed by immunofluorescence and immunoprecipitation. Conversely, more Cx45 mRNA was expressed by SaOS-2 cells than by HOB or BMC. Thus, intercellular coupling in normal and transformed human osteoblastic cells correlates with the level of expression of Cx43, which appears to mediate intercellular communication in these cells. Gap junctional communication may serve as a means by which osteoblasts can work in synchrony and propagate locally generated signals throughout the skeletal tissue.

The incidence and size of gap junctions between the bone cells in rat calvaria

Polyclonal antisera to synthetic peptides matching sequences on the cytoplasmic regions of connexin-43, a gap junction protein first identified in rat heart, have been used to immunolabel gap junctions in the calvarial bone, maintained intact as in vivo, of 1- to 2-week-old rats. The specimens were examined in reflection and fluorescence modes by scanning laser confocal microscopy, and the numbers of gap junctions and their sizes estimated. The mean number of connexin-43 immunolabelled junctions per osteoblast (n = 65) was 15.3 (SD +/- 4.5). The mean length of 227 junctions, selected for the sharpness of the image of the fluorescent spot, was 0.67 micron (SD +/- 0.18; range 0.37-1.29 microns) and their mean area 0.26 micron2 (SD +/- 0.145; range 0.075-0.93 micron2); these probably fell within the upper half of the total size range. Gap junctions were detected between preosteoblasts, osteoblasts, osteocytes and chondrocytes, and between these juxtaposed cell types. In addition, connexin-43 immunolabelled junctions were found between some osteoclasts and overlying mononuclear cells at active sites of resorption.

Second messenger signaling in the regulation of cytosolic pH and DNA synthesis by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: role of Na+/H+ exchange

The present study was performed to investigate the regulation of cytosolic pH (pHi) and DNA synthesis by parathyroid hormone(PTH) and PTH-related peptide (PTHrP) in osteoblasts, using osteoblastic osteosarcoma cells, UMR-106 which possessed PTH-responsive dual signal transduction systems (cAMP-dependent protein kinase (PKA) and calcium/protein kinase C [Ca/PKC]) and amiloride-inhibitable Na+/H+ exchange system. Both human (h)PTH-(1-34) and hPTHrP-(1-34) caused a progressive decrease in pHi and the inhibition of [3H]thymidine incorporation (TdR) to the same degree in a dose-dependent manner with a minimal effective dose of 10(-10) M. Dibutyryl cAMP (10(-4) M and Sp-cAMPS (10(-4) M), a direct stimulator of PKA also caused a progressive decrease in pHi, and calcium ionophores (A23187 and ionomycin, 10(-6) M) caused a transient decrease in pHi. Pretreatment with amiloride (0.3 mM) mostly blocked dbcAMP- and Sp-cAMPS-induced decrease in pHi but did not affect calcium ionophore-induced decrease in pHi. In the presence of amiloride, PTH and PTHrP caused a transient decrease in pHi, which was similar to the pattern of calcium ionophore-induced change in pHi. Amiloride did not affect the inhibition of TdR by PTH or PTHrP as well as that by cAMP analogues or calcium ionophores. The present study indicated that PTH and PTHrP caused cytosolic acidification through PKA-inhibited Na+/H+ exchange and increased cytosolic calcium-induced pathway and that the regulation of DNA synthesis by PTH and PTHrP was not via Na+/H+ exchange system.

Isolation and purification of osteocytes

An isolation method for osteocytes is described. After removal of the periostea, bone cells were isolated from calvariae of 18-day-old chicken embryos by alternating treatments with collagenase and EDTA. Osteocytes were purified from the heterogeneous bone cell population with the help of the osteocyte-specific MAb OB 7.3 bound to protein G-conjugated magnetic beads. The purity of the osteocyte population ultimately obtained was more than 95%. Osteocytes were found to adhere rapidly to glass or plastic substrates. They showed numerous processes of various types. These processes could branch and make contact with those of other osteocytes. After 1-2 days of culture, the isolated osteocytes formed a network of apparently interconnected cell processes very similar to the osteocyte network in bone.

Tooth movement

This article reviews the evolution of concepts regarding the biological foundation of force-induced tooth movement. Nineteenth century hypotheses proposed two mechanisms: application of pressure and tension to the periodontal ligament (PDL), and bending of the alveolar bone. Histologic investigations in the early and middle years of the 20th century revealed that both phenomena actually occur concomitantly, and that cells, as well as extracellular components of the PDL and alveolar bone, participate in the response to applied mechanical forces, which ultimately results in remodeling activities. Experiments with isolated cells in culture demonstrated that shape distortion might lead to cellular activation, either by opening plasma membrane ion channels, or by crystallizing cytoskeletal filaments. Mechanical distortion of collagenous matrices, mineralized or non-mineralized, may, on the other hand, evoke the development of bioelectric phenomena (stress-generated potentials and streaming potentials) that are capable of stimulating cells by altering the electric charge on their membrane or their fluid envelope. In intact animals, mechanical perturbations on the order of about 1 min/d are apparently sufficient to cause profound osteogenic responses, perhaps due to matrix proteoglycan-related "strain memory". Enzymatically isolated human PDL cells respond biochemically to mechanical and chemical signals. The latter include endocrines, autocrines, and paracrines. Histochemical and immunohistochemical studies showed that during the early places of tooth movement, PDL fluids are shifted, and cells and matrix are distorted. Vasoactive neurotransmitters are released from periodontal nerve terminals, causing leukocytes to migrate out of adjacent capillaries. Cytokines and growth factors are secreted by these cells, stimulating PDL cells and alveolar bone lining cells to remodel their related matrices. This remodeling activity facilitates movement of teeth into areas in which bone had been resorbed. This emerging information suggests that in the living mammal, many cell types are involved in the biological response to applied mechanical stress to teeth, and thereby to bone. Essentially, cells of the nervous, immune, and endocrine systems become involved in the activation and response of PDL and alveolar bone cells to applied stresses. This fact implies that research in the area of the biological response to force application to teeth should be sufficiently broad to include explorations of possible associations between physical, cellular, and molecular phenomena. The goals of this investigative field should continue to expound on fundamental principles, particularly on extrapolating new findings to the clinical environment, where millions of patients are subjected annually to applications of mechanical forces to their teeth for long periods of time in an effort to improve their position in the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

Homeostatic control of bone structure: an application of feedback theory

The regulation of bone mass and structure in the weight-bearing skeleton is governed to a great extent by the mechanical demands placed upon the bone tissue. The apparent biological goal is the maintenance of a minimum adequate structure, in which the margin of safety between normal mechanical demands and fracture is balanced by the cost of excessive bone mass on mobility. Frost has developed two powerful postulates concerning bone adaptation: (a) there exist threshold levels of mechanical strain, above or below which bone adaptation is turned on, and (b) the set point for normal bone structure can be modulated by hormones. A model was developed, using Frost's postulates and simple feedback theory, that describes the interaction between biochemical influences and mechanical influences on bone structure. The model predicts that biochemical agents that influence bone structure independently of the mechanical feedback system (e.g., calcitonin) are capable of only limited anabolic effects on bone mass because their influences conflict with mechanical influences. However, biochemical agents that influence bone structure by changing the set point of the mechanical feedback system (e.g., estrogen) will provide lasting changes in bone structure. Age-related changes occur within the effector and transduction components of the mechanical feedback system that tend to make it sluggish. These changes may lead to increased bone fragility because the system is no longer capable of maintaining adequate bone structure.

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)

Changes in cytoskeletal proteins in response to parathyroid hormone and 1,25-dihydroxyvitamin D in human osteoblastic cells

We have examined the influence of parathyroid hormone (PTH) and 1,25(OH)2 vitamin D (1,25(OH)2D) on cytoskeletal assembly and biosynthesis in relation with cAMP production and parameters of cell growth and differentiation in normal human osteoblastic cells. Untreated human bone cells showed elongated morphology associated with high levels of actin, vimentin, alpha- and beta-tubulins and alpha-actinin as determined by 2-dimensional-gel electrophoresis and [35S]methionine labelling of cytoskeletal proteins. PTH (20 nM, 24 h) decreased the de novo biosynthesis of vimentin and alpha-actinin in human bone cells, an effect associated with a rise in intracellular cyclic AMP. In addition, PTH induced cytoskeletal disassembly as shown by a 52-70% decrease in the Triton-insoluble fractions of actin, alpha-tubulins and alpha-actinin. 1,25(OH)2D (10 nM, 24 h) also induced a 40-64% decrease in the polymerized fractions of actin, alpha-tubulins and alpha-actinin. These changes were associated with an 83% increase in osteocalcin production. Under these conditions, neither PTH nor 1,25(OH)2D at the doses tested affected alkaline phosphatase activity or cell growth as assessed by [3H]thymidine incorporation into DNA. The results show that PTH and 1,25(OH)2D induce similar inhibition of cytoskeletal proteins assembly involving microfilaments and microtubules in human osteoblastic cells. These alterations of cytoskeletal arrangement in response to PTH and 1,25(OH)2D may contribute to the functional response of human osteoblastic cells to these bone-resorbing hormones.

Intercellular communication in bronchial epithelial cells: review of evidence for a possible role in lung carcinogenesis

A challenging aspect of lung carcinogenesis is the elucidation of the mechanisms which permit initiated bronchial epithelial cells to attain a growth advantage over normal bronchial epithelial cells, and subsequently evolve into a malignant phenotype. In this review, the effects of interactions between normal and transformed cells, and the potential role of representative extrinsic factors on cell-cell communication are discussed. Evidence is presented to show how cell injury and the effects of serum and calcium may affect morphology and communication, and tumor development. A large number of autocrine-paracrine factors (e.g., TGF beta, TGF alpha) are released by bronchial epithelial cells. These factors may inhibit or promote the proliferation of normal and transformed bronchial epithelial cells, respectively. The ability of certain injurious and tumor promoting agents (e.g., formaldehyde, TPA) to select for the transformed phenotype may involve selective cell injury, the induction of terminal differentiation and an inhibition of gap junction communication among normal BE cells.

Parathyroid hormone, but not prostaglandin E2, changes the shape of osteoblasts maintained on bone in vitro

Parietal bones from 2-week-old rats were dissected free from the sutural regions, dura mater, and periosteum, leaving the surface covered with osteoblasts and some osteoclasts. Prostaglandin (PG) production by these "stripped" bones under basal conditions and after exposure to parathyroid hormone (PTH) was measured by radioimmunoassay of the culture medium (minimum essential medium with or without added 10% heat-inactivated fetal calf serum). Cultured specimens were examined by scanning electron microscopy for changes in osteoblast length, orientation, ruffling, and overlap. As demonstrated previously, PTH caused the osteoblasts to elongate, align, and show fewer ruffles compared to controls. PTH increased PG synthesis by the stripped bones. Indomethacin inhibited PG formation but did not affect the osteoblast shape change. PGE2, indomethacin, or both drugs together had no discernible effect on any morphologic features. These findings indicate that PGE2 does not change osteoblast shape and that the cell shape change with PTH is not mediated by endogenous prostanoids.

Forskolin has biphasic effects on osteoprogenitor cell differentiation in vitro

Cells isolated from fetal rat calvaria (RC) and maintained in vitro in medium containing ascorbic acid and B-glycerophosphate form three-dimensional, mineralized nodules having the histological, immunohistological, and ultrastructural characteristics of woven bone. We have studied the effects of forskolin (FSK), a diterpene that activates adenylate cyclase, in this system. While 10(-7)-10(-5) M FSK significantly stimulated cAMP levels in RC cells, lower concentrations did not. cAMP levels with 10(-5) M FSK reached a maximum by 30 min at 37 degrees C and returned to basal level in 2-3 hr. Changes in cAMP levels correlated with changes in cellular shape: cells treated with 10(-5) M FSK assumed a stellate morphology, lost microfilament bundles, and reduced their substrate adhesiveness, while cells treated with 10(-9) M were not affected. Exponential growth and saturation densities of FSK-treated cultures were similar to untreated cultures, indicating that FSK was neither toxic nor stimulatory to the population. The effect on bone nodule formation of FSK present continuously depended on concentration: 10(-5) M FSK significantly inhibited the number of nodules formed, while 10(-9) M FSK significantly stimulated bone nodule formation. Single short treatments with either 10(-5) M or 10(-9) M FSK had no effect on nodule formation, but repeated short duration treatments (1 hr every 2 days for 21 days) gave results similar to continuous exposure. These results indicate that intermittent elevations in intracellular cAMP have an inhibitory effect on bone formation. In addition, our work indicates that low concentrations of FSK stimulate differentiation of osteoprogenitor cells possibly through a non-cAMP-dependent process.

Neurotransmitter regulation of cytosolic calcium in osteoblast-like bone cells

The nervous system may play a role in regulation of bone metabolism. The effects of norepinephrine(NE), vasoactive intestinal peptide(VIP), and ATP on cytosolic Ca2+ were assessed in a rat osteoblast-like osteosarcoma cell line (UMR-106) responsive to PTH. All three transmitters transiently increased Ca2+, with ATP much greater than PTH greater than NE = VIP, and then caused sustained increases in Ca2+. The ATP-induced transient resulted from mobilization of intracellular Ca2+ store, while NE and VIP-induced transients also involved influx of Ca2+. Later sustained increases by all agonists were dependent upon extracellular Ca2+. Release of intracellular Ca2+ by ATP was associated with a marked increase in IP3 but without a significant change in cAMP. NE, VIP, and ATP, through regulation of Ca2+ metabolism, may be involved in various osteoporotic conditions.

Human osteoblasts in vitro secrete tissue inhibitor of metalloproteinases and gelatinase but not interstitial collagenase as major cellular products

Human osteoblast cultures (hOB) were examined for the production of interstitial collagenase, tissue inhibitor of metalloproteinases (TIMP), and gelatinolytic enzymes. Cells were isolated by bacterial collagenase digestion of trabecular bone (vertebra, rib, tibia, and femur) from 11 subjects (neonatal to adult). Confluent cultures were exposed to phorbol 12-myristate 13-acetate, PTH, PGE2, epidermal growth factor, 1,25(OH)2 vitamin D3, recombinant human IL-1 beta, and dexamethasone. Collagenase and TIMP were assayed immunologically and also by measurements of functional activity. Collagenase was not secreted in significant quantities by human bone cells under any tested condition. Furthermore, collagenase mRNA could not be detected in hOB. However, hOB spontaneously secreted large amounts of TIMP for at least 72 h in culture. hOB TIMP was found to be identical to human fibroblast TIMP by double immunodiffusion, metabolic labeling and immunoprecipitation, Northern blot analysis, and stoichiometry of collagenase inhibition. SDS-substrate gel electrophoresis of hOB-conditioned media revealed a prominent band of gelatinolytic activity at 68 kD, and specific polyclonal antisera established its identity with the major gelatinolytic protease of human fibroblasts. Abundant secretion of gelatinolytic, but not collagenolytic, enzymes by hOB may indicate that human osteoblasts do not initiate and direct the cleavage of osteoid collagen on the bone surface, but may participate in the preparation of the bone surface for osteoclast attachment by removal of denatured collagen peptides. The constitutive secretion of TIMP may function to regulate metalloproteinase activity.

Indomethacin modulation of load-related stimulation of new bone formation in vivo

The capacity of bone to organize and reorganize its structure in response to changing mechanical demands is well recognized. However, the mechanism by which the changing mechanical environment is detected, and the means by which this information is translated into a stimulus for structural modification, are not understood. A group of substances suggested to be involved in the initial transduction of strain information are the prostaglandins. In this experiment we used a single period of dynamic loading to stimulate an adaptive osteogenic response in vivo. Loading was performed in the presence and absence of indomethacin. Measurements of the periosteum 5 days after loading showed that the presence of indomethacin at the time of loading reduced the osteogenic response. Though consistent with the hypothesis that prostaglandins are involved in the initial transduction of tissue strain into a biochemical response, this result is not sufficient to demonstrate this conclusively because reduced prostaglandin levels during the 24 hours immediately after the period of loading may affect many other points in the cascade of events between strain transduction and adaptive new bone formation. Furthermore, indomethacin at the relatively high levels we used (40 mg/kg) may have effects other than those on prostaglandin synthesis.

Monokines produced by macrophages stimulate the growth of osteoblasts

We have previously reported that the J774A.1 macrophage-like tumor cell line produces two potent monokines which stimulate the growth of osteoblasts and chondrocytes. These growth factors, which have an affinity for heparin-agarose, have been termed HEP I (a 30 Kd PDGF-like molecule) and HEP II (an approximately 20 Kd molecule), respectively, based on their elution profile. Unlike HEP I, HEP II does not stimulate the growth of fibroblasts. Extensive biological and chromatographic studies disclosed that HEP II appears to be a unique bone cell mitogen unlike any known growth factor, including the FGFs, IL-1s, and TNFs, EGF, IGF-I and -II, TGF-beta, beta 2 microglobulin, G-CSF, CSF-1 and GM-CSF. To characterize more fully the effects of the macrophage-derived monokines on osteoblast growth and function, clones were derived from calvaria explant cultures. Two clones, SDFRC-2.05 and SDFRC-3, were developed and found to exhibit osteoblastic characteristics, including high levels of alkaline phosphatase, synthesis of type I but not type III collagen, and an increased intracellular cAMP production in response to PTH. The SDFRC-3 cells exhibited a polygonal morphology like that of the explant-derived cells while SDFRC-2.05 cells exhibited a more fibroblastic morphology. When tested on the explant cultures and clones, HEP I and HEP II were found to stimulate DNA synthesis and increase protein per culture, but decreased alkaline phosphatase activity. Clone SDFRC-3 was found to be more responsive to HEP II than clone SDFRC-2.05. Both monokines were found to be more potent mitogens for bone cells than TGF-beta. HEP II, but not HEP I or TGF-beta, induced a transformation of bone cells from a polygonal to a fibroblastic morphology, suggesting the induction of migration prior to proliferation. Thus, macrophages may be responsible not only for bone repair but also for ensuring the linkage of bone formation to resorption during physiological remodeling.

Bone-resorbing agents promote and interferon-gamma inhibits bone cell collagenase production

Parathyroid hormone, prostaglandin E2, 1 alpha,25-dihydroxyvitamin D3, interleukin-1, tumor necrosis factor alpha, and epidermal growth factor, all known stimulators of bone resorption, markedly enhanced collagenase secretion by rat fetus osteoblastlike cells in primary culture as judged by enzyme-linked immunosorbent assay. Untreated cells contained no immunostainable or extractable collagenase. Collagenase was detected in the treated cells and media only after 1-3 h of treatment, and there was no increment in collagenase activity when cells were treated in the presence of actinomycin D or cycloheximide. Cells secreted collagenase in a latent form and also elaborated collagenase inhibitor; chromatographic separation of collagenase from collagenase inhibitor and subsequent activation of the collagenase with trypsin yielded the active species in stimulated but not in unstimulated cells. The ability of individual prostanoids, among seven tested, to promote collagenase production correlated positively with their reported capacity to promote bone resorption. Interferon-gamma (IFN-gamma), a known resorption inhibitor, blocked the increment in collagenase production caused by all agents tested. These results indicate a close linkage between stimulation of bone resorption and collagenase production by osteoblastlike cells. Various resorption stimulators, including some not previously tested for effects on collagenase, augment the de novo synthesis and secretion of collagenase and act by an IFN-gamma-inhibitable mechanism.

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.

Systemic effects of prostaglandin E2 on vertebral trabecular remodeling in beagles used in a healing study

Prostaglandin E2 (PGE2) at a dose of 10 mg/kg was administered orally to beagles used in a study of rib fracture and drill hole defect healing. Double fluochrome labels were given prior to surgical manipulation and before necropsy at 30 days. Bone remodeling was evaluated in trabecular bone of the fourth lumbar vertebra. There was a decrease in the number and extent of posttreatment labels (P less than 0.05) in the controls, with decreased mineral apposition rate (P less than 0.05) and decreased active bone formation rate (P less than 0.01). In dogs given PGE2 for 30 days following surgery, the extent of posttreatment labels (P less than 0.05) and bone formation rate (P less than 0.01) were increased. There was no difference found, however, in static morphometric parameters, including osteoid and osteoblast-covered surface, indicating that the stimulation of bone formation may have been transitory and matrix synthesis had declined. In another group of dogs given PGE2 for 5 days prior to surgical manipulation and between the first and second pretreatment labels, the extent of the double-labeled surface was increased (P less than 0.05) indicating an acute PG effect to sustain formation at remodeling sites. These studies show that PGE2 given orally has a systemic effect on bone remodeling in vertebral trabeculae that involves the stimulation of formation activity.

Transforming growth factor-beta modulates the expression of osteoblast and chondroblast phenotypes in vitro

Transforming growth factor beta (TGF-beta) has been shown to induce chondrogenesis by embryonic rat mesenchymal cells (Seyedin et al., J. Biol. Chem., 261: 5693, 1986). Here we report the effects of bovine TGF-beta on the phenotypic expression of differentiated primary rat osteoblastic and chondroblastic cells. Culture of rat calvarial osteoblasts with TGF-beta resulted in a dose and time-dependent decrease in alkaline phosphatase activity. Levels of alkaline phosphatase were reduced to less than 10% of control values by 0.4 nM TGF-beta. The decrease became apparent after 24 hours and reached a maximum by 72 hours. Similarly, treatment of chondroblasts with 0.4 nM TGF-beta resulted in decreased production of cartilage-specific macromolecules: type II collagen and cartilage proteoglycan. Both cell types exhibited dramatic changes in cell shape after treatment with TGF-beta. Modulation of these differentiated markers by TGF-beta could be mimicked, in part, by addition of fibronectin. Addition of dihydrocytochalasin B blocked the inhibition of phenotypic expression by TGF-beta. These results indicate that TGF-beta inhibits phenotypic expression by osteoblasts and chondroblasts in vitro and suggest that this activity of TGF-beta may be mediated through interactions between the extracellular matrix and cytoskeletal elements.

Role of prostaglandins in bone resorption in a synchronized remodeling sequence in the rat

The role of prostaglandins (PGs) in physiological remodeling has not yet been defined. The present study was undertaken to determine whether they intervene during the activation phase in a highly reproducible and synchronized model of bone remodeling. Indomethacin was employed to inhibit PG synthesis. This treatment throughout the entire activation period (4 days in this model) inhibited osteoclastic resorption completely. By modifying the treatment procedure, it appeared that PGs were operative mainly between the second and third day of activation. PGs did not seem to act on precursor recruitment, since off-bone osteoclasts (putatively inactive cells) were numerous in the treated groups. PGs might also be involved in osteoclast activity as the mean interface between osteoclasts and bone surface was reduced in the treated groups. However, indomethacin was unable to inhibit the remodeling sequence durably since a 6-day treatment resulted in a high profile of resorption. This suggests that factors other than PGs were responsible for activating resorption.

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.

The bone lining cell: a distinct phenotype?

It is argued that the flat cells that line nonremodeling endosteal bone surfaces are a distinct phenotype. These cells have a distinct morphology and they most likely have important functional roles in skeletal physiology, metabolism, and remodeling. For these reasons this cell seems deserving of a proper name. The name bone lining cell seems to have gained some acceptance but there will be continued confusion as long as skeletologists use this same term to generically describe cells that line bone surfaces, regardless of their actual identity.