Bone cells in culture: morphologic transformation by hormones

Historically significant events in the discovery of RANK/RANKL/OPG

After it was suggested 30 years ago that the osteoblast lineage controlled the formation of osteoclasts, methods were developed that established this to be the case, but the molecular controls were elusive. Over more than a decade much evidence was obtained for signaling mechanisms that regulated the production of a membrane - bound regulator of osteoclastogenesis, in the course of which intercellular communication in bone was revealed in its complexity. The discovery of regulation by tumor necrosis factor ligand and receptor families was made in the last few years of the twentieth century, leading since then to a new physiology of bone, and to exciting drug development.

Hyperosmotic Stimulus Down-regulates 1alpha, 25-dihydroxyvitamin D(3)-induced Osteoclastogenesis by Suppressing the RANKL Expression in a Co-culture System

The hyperosmotic stimulus is regarded as a mechanical factor for bone remodeling. However, whether the hyperosmotic stimulus affects 1alpha, 25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3))-induced osteoclastogenesis is not clear. In the present study, the effect of the hyperosmotic stimulus on 1alpha,25(OH)(2)D(3)-induced osteoclastogenesis was investigated in an osteoblast-preosteoclast co-culture system. Serial doses of sucrose were applied as a mechanical force. These hyperosmotic stimuli significantly evoked a reduced number of 1alpha,25(OH)(2)D(3)-induced tartrate-resistant acid phosphatase-positive multinucleated cells and 1alpha,25(OH)(2)D(3)-induced bone-resorbing pit area in a co-culture system. In osteoblastic cells, receptor activator of nuclear factor kappaB ligand (RANKL) and Runx2 expressions were down-regulated in response to 1alpha,25(OH)(2)D(3). Knockdown of Runx2 inhibited 1alpha,25(OH)(2)D(3)-induced RANKL expression in osteoblastic cells. Finally, the hyperosmotic stimulus induced the overexpression of TonEBP in osteoblastic cells. These results suggest that hyperosmolarity leads to the down-regulation of 1alpha,25(OH)(2)D(3)-induced osteoclastogenesis, suppressing Runx2 and RANKL expression due to the TonEBP overexpression in osteoblastic cells.

PTH-induced actin depolymerization increases mechanosensitive channel activity to enhance mechanically stimulated Ca2+ signaling in osteoblasts

Disruption of the actin cytoskeleton with cytochalasin D enhanced the mechanically induced increase in intracellular Ca(2+) ([Ca(2+)](i)) in osteoblasts in a manner similar to that of PTH. Stabilization of actin with phalloidin prevented the PTH enhanced [Ca(2+)](i) response to shear. Patch-clamp analyses show that the MSCC is directly influenced by alterations in actin integrity.

INTRODUCTION

PTH significantly enhances the fluid shear-induced increase in [Ca(2+)](i) in osteoblasts, in part, through increased activation of both the mechanosensitive, cation-selective channel (MSCC) and L-type voltage-sensitive Ca(2+) channel (L-VSCC). Both stimuli have been shown to produce dynamic changes in the organization of the actin cytoskeleton. In this study, we examined the effects of alterations in actin polymerization on [Ca(2+)](i) and MSCC activity in MC3T3-E1 and UMR-106.01 osteoblasts in response to shear +/- PTH pretreatment.

MATERIALS AND METHODS

MC3T3-E1 or UMR-106.01 cells were plated onto type I collagen-coated quartz slides, allowed to proliferate to 60% confluency, and mounted on a modified parallel plate chamber and subjected to 12 dynes/cm(2). For patch-clamp studies, cells were plated on collagen-coated glass coverslips, mounted on the patch chamber, and subjected to pipette suction. Modulators of actin cytoskeleton polymerization were added 30 minutes before the experiments, whereas channel inhibitors were added 10 minutes before mechanical stimulation. All drugs were maintained in the flow medium for the duration of the experiment.

RESULTS AND CONCLUSIONS

Depolymerization of actin with 1-5 microM cytochalasin D (cyto D) augmented the peak [Ca(2+)](i) response and increased the number of cells responding to shear, similar to the increased responses induced by pretreatment with 50 nM PTH. Stabilization of actin with phalloidin prevented the PTH enhanced [Ca(2+)](i) response to shear. Inhibition of the MSCC with Gd(3+) significantly blocked both the peak Ca(2+) response and the number of cells responding to shear in cells pretreated with either PTH or cyto D. Inhibition of the L-VSCC reduced the peak [Ca(2+)](i) response to shear in cells pretreated with PTH, but not with cyto D. Patch-clamp analyses found that addition of PTH or cyto D significantly increased the MSCC open probability in response to mechanical stimulation, whereas phalloidin significantly attenuated the PTH-enhanced MSCC activation. These data indicate that actin reorganization increases MSCC activity in a manner similar to PTH and may be one mechanism through which PTH may reduce the mechanical threshold of osteoblasts.

Novel membrane protein containing glycerophosphodiester phosphodiesterase motif is transiently expressed during osteoblast differentiation

Osteoblast maturation is a multistep series of events characterized by an integrated cascade of gene expression that are accompanied by specific phenotypic alterations. To find new osteoblast-related genes we cloned differentially expressed cDNAs characteristic of specific differentiation stages in the mouse osteoblast-like MC3T3-E1 cells by a differential display method. We identified a novel cDNA encoding a putative glycerophosphodiester phosphodiesterase, GDE3, which specifically was expressed at the stage of matrix maturation. Interestingly, the deduced amino acid sequence contains 539 amino acids including seven putative transmembrane domains and a glycerophosphodiester phosphodiesterase region in one of the extracellular loops. Northern blot analysis revealed that GDE3 was also expressed in spleen as well as primary calvarial osteoblasts and femur. We next transfected HEK293T cells with GDE3 with green fluorescent protein fused to the C terminus. The green fluorescent protein-fused protein accumulated at the cell periphery, and the transfected cells overexpressing the protein changed from a spread form to rounded form with disappearance of actin filaments. Immunofluorescence staining with GDE3 antibody and phalloidin in MC3T3-E1 cells indicated that endogenous GDE3 might be co-localized with the actin cytoskeleton. To identify a role for GDE3 in osteoblast differentiation, MC3T3-E1 cells stably expressing the full-length protein were constructed. Expression of GDE3 showed morphological changes, resulting in dramatic increases in alkaline phosphatase activity and calcium deposit. These results suggest that GDE3 might be a novel seven-transmembrane protein with a GP-PDE-like extracellular motif expressed during the osteoblast differentiation that dramatically accelerates the program of osteoblast differentiation and is involved in the morphological change of cells.

Transforming growth factor-beta-induced osteoblast elongation regulates osteoclastic bone resorption through a p38 mitogen-activated protein kinase- and matrix metalloproteinase-dependent pathway

Transforming growth factor-beta (TGF-beta) is a powerful modulator of bone metabolism, and both its anabolic and catabolic effects on bone have been described. Here we have tested the hypothesis that TGF-beta-induced changes in osteoblast shape promote bone resorption by increasing the surface area of bone that is accessible to osteoclasts. The addition of TGF-beta1 to MC3T3-E1 cells resulted in cytoskeletal reorganization, augmented expression of focal adhesion kinase, and cell elongation, accompanied by an increase in the area of cell-free substratum. TGF-beta1 also triggered activation of Erk1/2 and p38 mitogen-activated protein (MAP) kinase. The p38 MAP kinase inhibitor PD169316, but not an inhibitor of the Erk1/2 pathway, abrogated the effect of TGF-beta1 on cell shape. The matrix metalloproteinase inhibitor GM6001 also interfered with osteoblast elongation. Treatment of MC3T3-E1 cells seeded at confluence onto bone slices to mimic a bone lining cell layer with TGF-beta1 also induced cell elongation and increased pit formation by subsequently added osteoclasts. These effects were again blocked by PD169316 and GM6001. We propose that this novel pathway regulating osteoblast morphology plays an important role in the catabolic effects of TGF-beta on bone metabolism.

Cellular mechanisms underlying temperature-induced bleaching in the tropical sea anemone Aiptasia pulchella

Temperature-induced bleaching in symbiotic cnidarians is a result of the detachment and loss of host cells containing symbiotic algae. We tested the hypothesis that host cell detachment is evoked through a membrane thermotropic event causing an increase in intracellular calcium concentration, [Ca2+]i, which could then cause collapse of the cytoskeleton and perturb cell adhesion. Electron paramagnetic resonance measurements of plasma membranes from the tropical sea anemone Aiptasia pulchella and the Hawaiian coral Pocillopora damicornis labeled with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) revealed no membrane thermotropic event. In addition, intracellular imaging using Fura-2AM as well as labeling anemones with 45Ca revealed no significant change in [Ca2+]i. However, bleaching could be evoked at ambient temperature with 25 mmol l–1 caffeine without affecting [Ca2+]i. [Ca2+]i could be altered with ionomycin in isolated host cells, but ionomycin could not induce bleaching in A. pulchella. As caffeine can affect levels of intracellular protein phosphorylation, the ability of other agents that alter intracellular levels of protein phosphorylation to evoke bleaching was investigated. The protein phosphatase inhibitor vanadate could induce bleaching in A. pulchella. Two-dimensional gels of 32P-labeled proteins from cold-shocked, caffeine-treated and control anemones show that both temperature shock and caffeine alter the array of phosphorylated host soluble proteins. We conclude that cnidarian bleaching is linked to a temperature-induced alteration in protein phosphorylation.

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.

Role for parathyroid hormone in mechanical responsiveness of rat bone

We investigated the relationship between parathyroid hormone (PTH) and mechanical stimulation in mechanically induced osteogenesis. In normal rats, mechanical stimulation of the eight caudal vertebra induced an osteogenic response. This was augmented by a single injection of human PTH-(1-34) 30-45 min before loading. No osteogenic response was seen in thyroparathyroidectomized (TPTX) rats; the osteogenic response was restored by a single injection of PTH before stimulation, suggesting that physiological levels of PTH are necessary for the mechanical responsiveness of bone. c-fos expression was detected only in the osteocytes of those rats that were both mechanically stimulated and given PTH. This suggests that PTH supports mechanically induced osteogenesis by sensitizing either the strain-sensing mechanism itself or early responses of bone to strain-generated signals. The osteogenic response was not augmented by two further daily injections of PTH and was not seen in TPTX rats in which PTH administration was started 3 days after loading. These results reveal a major role for PTH in the mechanical responsiveness of rat bone.

Parathyroid hormone-induced resorption in fetal rat limb bones is associated with production of the metalloproteinases collagenase and gelatinase B

The role of matrix metalloproteinases in parathyroid hormone (PTH)-induced bone resorption was assayed using a fetal rat limb bone culture system. Cotreatment of bones with PTH and recombinant inhibitor of metalloproteinases, TIMP-1, in vitro, inhibited the PTH-stimulated 45Ca release from the limb bones without affecting beta-glucuronidase release. TIMP-1 was fully effective when added during only the final 24 h of a 72 h culture with PTH but was ineffective when added for only the first 24 h of the 72 h culture. In contrast, calcitonin (CT) was effective when added for either the first 24 or the final 24 h of the culture. Using in situ hybridization, the mRNA for collagenase was detected in mononuclear cells of cultured bone. Treatment of the bones with PTH resulted in an increase in the number of cells producing collagenase mRNA, some of which had osteoclastic morphology, PTH also caused a dramatic induction of the mRNA for the 92-kD gelatinase B metalloproteinase in both mononuclear and osteoclastic cells. There was no detectable mRNA for the metalloproteinases stromelysin-1, stromelysin-2, or matrilysin in PTH-treated or control cultures. These results suggest that PTH-induced bone resorption is mediated, at least in part, by the induction of collagenase and gelatinase B mRNA in bone cells.

Mechanotransduction and the functional response of bone to mechanical strain

Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane, G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.

Characterization of a volume-sensitive chloride current in rat osteoblast-like (ROS 17/2.8) cells

1. During osmotic swelling, cultured osteoblastic cells (ROS 17/2.8) exhibited activation of large amplitude Cl- currents in the whole-cell configuration of the patch-clamp technique. Effects of hypotonic shock on cell volume and membrane conductance were rapidly reversed on return to isotonic conditions. 2. Voltage command pulses in the range -80 to +50 mV produce instantaneous activation of Cl- currents. At potentials more positive than +50 mV the current exhibited time-dependent inactivation. The instantaneous current-voltage relationship was outwardly rectifying. 3. The anion permeability sequence of the induced current was SCN- (2.2) > i- (1.9) > Br- (1.5) > Cl- (1.0) > F- (0.8) > gluconate- (0.2). This corresponds to Eisenman's sequence I. 4. The volume-sensitive Cl- current was effectively inhibited by the Cl- channel blockers 4,4'- diisothiocyanatostilbene-2,2-disulphonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). Outward currents were more effectively suppressed by DIDS than inward currents. The concentrations for 50% inhibition (IC50) of outward and inward currents were 81 and 298 microM, respectively. NPPB was equally effective at inhibiting outward and inward currents (IC50 of 64 microM). The current was relatively insensitive to diphenylamine-2-carboxylate (DPC), 500 microM producing only 22.5 +/- 4.0% inhibition. 5. Inhibitors of protein kinase A (H-89, 1 microM) and tyrosine kinase (tyrphostin A25, 200 microM) were without effect upon activation of Cl- currents in response to hypotonic shock. Under isotonic conditions, elevation of intracellular Ca2+ by ionomycin (1 microM) or activation of protein kinase C by 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.1 microM) failed to evoke increases in basal Cl- conductance levels. 6. It is concluded that an outwardly rectifying Cl- conductance is activated upon osmotic swelling and may be involved in cell volume regulation of ROS 17/2.8 cells.

Signal transduction pathways mediating parathyroid hormone regulation of osteoblastic gene expression

Parathyroid hormone (PTH) plays a central role in regulation of calcium metabolism. For example, excessive or inappropriate production of PTH or the related hormone, parathyroid hormone related protein (PTHrP), accounts for the majority of the causes of hypercalcemia. Both hormones act through the same receptor on the osteoblast to elicit enhanced bone resorption by the osteoclast. Thus, the osteoblast mediates the effect of PTH in the resorption process. In this process, PTH causes a change in the function and phenotype of the osteoblast from a cell involved in bone formation to one directing the process of bone resorption. In response to PTH, the osteoblast decreases collagen, alkaline phosphatase, and osteopontin expression and increases production of osteocalcin, cytokines, and neutral proteases. Many of these changes have been shown to be due to effects on mRNA abundance through either transcriptional or post-transcriptional mechanisms. However, the signal transduction pathway for the hormone to cause these changes is not completely elucidated in any case. Binding of PTH and PTHrP to their common receptor has been shown to result in activation of protein kinases A and C and increases in intracellular calcium. The latter has not been implicated in any changes in mRNA of osteoblastic genes. On the other hand activation of PKA can mimic all the effects of PTH; protein kinase C may be involved in some responses. We will discuss possible mechanisms linking PKA and PKC activation to changes in gene expression, particularly at the nuclear level.

E-cadherins identified in osteoblastic cells: effects of parathyroid hormone and extracellular calcium on localization

The presence and regulation of cadherin localization in osteoblastic cells were examined. Monoclonal antibody (ECCD-1) that interferes with E-cadherin function prevented cell adhesion in UMR 106-H5 rat osteosarcoma cells and non-tumorigenic mouse calvarial MC3T3-E1 cells, whereas CCL39 fibroblast adhesion was not affected. Immunofluorescent antibodies (ECCD-2 and polyclonal L-CAMP P1) revealed cadherins are localized along the osteoblastic cell-cell boundaries. Exposure of UMR 106-H5 cells to bovine parathyroid hormone (1-84) (PTH; 10 ng/ml x 1 hr) or low calcium medium (1.0-0.025 mM) produced cellular retraction accompanied by intense immunofluorescence for cadherins throughout cells with a corresponding loss of punctate localization at remaining cell-cell adhesion points. Western immunoblot analysis indicated 108 kd and 115 kd cadherins are present, with a smaller 29.5 kd band that became predominantly associated with the cytosolic fraction of cells treated with parathyroid hormone or lowered calcium. The results demonstrate E-like cadherins are present in osteoblastic cells and implicate a regulatory role for parathyroid hormone and calcium in cadherin function and localization.

Regulation of integrin-mediated adhesion at focal contacts by cyclic AMP

Cyclic AMP (cAMP) elevation causes diverse types of cultured cells to round partially and develop arborized cell processes. Renal glomerular mesangial cells are smooth, muscle-like cells and in culture contain abundant actin microfilament cables that insert into substratum focal contacts. cAMP elevation causes adhesion loss, microfilament cable fragmentation, and shape change in cultured mesangial cells. We investigated the roles of the classical vitronectin (alpha V beta 3 integrin) and fibronectin (alpha 5 beta 1 integrin) receptors in these changes. Mesangial cells on vitronectin-rich substrata contained microfilament cables that terminated in focal contacts that stained with antibodies to vitronectin receptor. cAMP elevation caused loss of focal contact and associated vitronectin receptor. Both fibronectin and its receptor stained in a fibrillary pattern at the cell surface under control conditions but appeared aggregated along the cell processes after cAMP elevation. This suggested that cAMP elevation caused loss of adhesion mediated by vitronectin receptor but not by fibronectin receptor. We plated cells onto fibronectin-coated slides to test the effect of ligand immobilization on the cellular response to cAMP. On fibronectin-coated slides fibronectin receptor was observed in peripheral focal contacts where actin filaments terminated, as seen with vitronectin receptor on vitronectin-coated substrata, and in abundant linear arrays distributed along microfilaments as well. Substratum contacts mediated by fibronectin receptor along the length of actin filaments have been termed fibronexus contacts. After cAMP elevation, microfilaments fragmented and fibronectin receptor disappeared from peripheral focal contacts, but the more central contacts along residual microfilament fragments appeared intact. Also, substratum adhesion was maintained after cAMP elevation on fibronectin--but not on vitronectin-coated surfaces. Although other types of extracellular matrix receptors may also be involved, our observations suggest that cAMP regulates adhesion at focal contacts but not at fibronexus-type extracellular matrix contacts.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

PTH-induced osteoblast contraction is mediated by cysteine proteases

E-64d, a membrane-permeable cysteine protease inhibitor, was tested for its ability to inhibit PTH-induced contraction in intact mouse MC-3T3-E1 osteoblastic cells. Incubation of MC-3T3-E1 cells with vehicle (DMSO) or E-64c, a nonpermeant cysteine protease inhibitor, in the presence or in the absence of PTH had no effects on cAMP production or on morphology from 0 to 90 minutes after addition. In contrast, treatment with E-64d markedly attenuated PTH-induced contraction in these cells. These findings suggest that cysteine proteases, such as the calcium-activated neutral proteases (calpains), are involved in PTH-induced osteoblastic contraction. The observation that cysteine protease activity mediates PTH-induced osteoblastic contraction also suggests that endogenous inhibitors, such as calpastatin, may also be present in the osteoblast and play a role in the regulation of stimulus-response coupling in bone. This mechanism may provide another regulatory point at which bone cells may be pharmacologically manipulated in clinical situations characterized by excessive bone resorption.

Adhesion, growth, and matrix production by osteoblasts on collagen substrata

A number of studies have demonstrated the pivotal role of collagen molecules in modulating cell growth and differentiation. In order to analyze the direct effects of collagen type I on the osteoblastic phenotype, we have devised an in vitro culture system for studying the interactions between bovine collagen type I and Saos-2 cells, a human osteoblastic cell line. Saos-2 cells were cultured both on top of collagen-coated culture dishes as well as inside a three-dimensional collagen network. Plating on dishes treated with collagen induced maximal adhesion of Saos-2 cells after 24-hour incubation. Cells cultured on collagen gel matrix expressed about 2.5-fold more alkaline phosphatase when compared with untreated plastic dishes. On collagen-coated dishes the responsiveness of Saos-2 cells to parathyroid hormone was decreased, whereas no modifications were observed in the effect of vasoactive intestinal peptide on these cells. Using a microfluorimetric measurement of DNA, an increase of proliferation was observed in Saos-2 cells cultured on collagen gel. Saos-2 cells were also able to colonize collagen sponges and in this three-dimensional network they were able to synthesize osteocalcin, as assessed both by immunocytochemistry and radioimmunoassay. In this study we have demonstrated that bovine collagen type I exhibits favorable effects on attachment and functional and growth activities of a human osteoblastic cell line, encouraging its use as a bone graft material.

Endosteal surfaces in hyperparathyroidism: an enzyme cytochemical study on low-temperature-processed, glycol-methacrylate-embedded bone biopsies

Alkaline phosphatase (AlP) and tartrate-resistant acid phosphatase (TRAP) activities have been studied comparatively in needle biopsies of the iliac crest of four cases of secondary hyperparathyroidism (renal osteodystrophy). AlP activity was associated with the plasma membrane of osteoblasts and their processes, of reticular cells of bone marrow and of young osteocytes of osteoid borders and woven bone. Moreover, it was detected in the fibroblast-like cells of the endosteal "fibrosis". These cells were orderly in arrangement and were parallel to the endosteal surfaces near zones of bone formation. They were disorderly near zones of bone resorption. A strong TRAP-positive reaction was present in osteoclasts and mononuclear cells of endosteal "fibrosis" and in osteocytes located near active osteoclasts and in woven bone. These results suggest that the so-called fibrosis of hyperparathyroidism, rather than representing reparative, inert tissue, consists of osteoblast-like cells, probably precursors of osteoblasts derived by parathormone-stimulated proliferation of AlP-positive stromal cells of bone marrow, and of TRAP-positive, mononuclear cells, probably preosteoclasts. Moreover, they show that TRAP activity can be present in osteocytes, probably under stimulation by the same factors which stimulate osteoclast activity. The histochemical demonstration of AlP and TRAP facilitates the morphological diagnosis of metabolic bone disease and may improve knowledge of bone physiopathology.

Stimulation of cell proliferation in skeletal tissues of the rat by defined parathyroid hormone fragments

We have found, in previous studies in vitro using skeletal derived cell cultures, that mid-region fragments of human parathyroid hormone (hPTH) stimulate [3H]thymidine incorporation into DNA and increase the specific activity of the brain-type isoenzyme of creatine kinase (CK). These changes occurred without an increase in cyclic AMP formation which is linked to bone resorption. In this study, we found that the mid-region fragment hPTH-(28-48) stimulated CK activity in diaphysis, epiphysis and kidney in a time- and dose-dependent manner, parallel to the effects of the whole molecule bovine (b)PTH-(1-84) and the fully active fragment hPTH-(1-34). The increase caused by hPTH-(28-48) at a dose of 1.25 micrograms/rat was not less than the 2-fold increase caused by a roughly equimolar concentration bPTH-(1-84). A significant increase was reached at 1 h after intraperitoneal injection in all cases. All three sequences of PTH caused an increase in [3H]thymidine incorporation into DNA in diaphysis and epiphysis, but not in kidney, 24 h after injection. A fragment further towards the C-terminal, hPTH-(34-47), was inactive compared with an equimolar concentration of the fragment hPTH-(25-39), which stimulated both CK activity and DNA synthesis. These results in vivo are in line with previous findings in vitro; they provide further support for the suggestion that mid-region fragments of the PTH molecule could be used to induce bone formation without incurring the deleterious effect of bone resorption.

Parathyroid hormone promotes the disassembly of cytoskeletal actin and myosin in cultured osteoblastic cells: mediation by cyclic AMP

Parathyroid hormone (PTH) alters the shape of osteoblastic cells both in vivo and in vitro. In this study, we examined the effect of PTH on cytoskeletal actin and myosin, estimated by polyacrylamide gel electrophoresis of Triton X-100 (1%) nonextractable proteins. After 2-5 minutes, PTH caused a rapid and transient decrease of 50-60% in polymerized actin and myosin associated with the Triton X-100 nonextractable cytoskeleton. Polymerized actin returned to control levels by 30 min. The PTH effect was dose-dependent with an IC50 of about 1 nM, and was partially inhibited by the (3-34) PTH antagonist. PTH caused a rapid transient rise in cyclic AMP (cAMP) in these cells that peaked at 4 min, while the nadir in cytoskeletal actin and myosin was recorded around 5 min. The intracellular calcium chelator Quin-2/AM (10 microM) also decreased cytoskeletal actin and myosin, to the same extent as did PTH (100 nM). To distinguish between cAMP elevation and Ca++ reduction as mediators of PTH action, we measured the phosphorylation of the 20 kD (PI 4.9) myosin light chain in cells preincubated with [32P]-orthophosphate. The phosphorylation of this protein decreased within 2-3 min after PTH addition and returned to control levels after 5 min. The calcium ionophore A-23187 did not antagonize this PTH effect. Visualization of microfilaments with rhodamine-conjugated phalloidin showed that PTH altered the cytoskeleton by decreasing the number of stress fibers. These changes in the cytoskeleton paralleled changes in the shape of the cells from a spread configuration to a stellate form with retracting processes. The above findings indicate that the alteration in osteoblast shape produced by PTH involve relatively rapid and transient changes in cytoskeletal organization that appear to be mediated by cAMP.

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)

Cyclic AMP-induced shape changes of astrocytes are accompanied by rapid depolymerization of actin

Agents that increase intracellular cyclic AMP produce a process-bearing morphology in astrocytes. We have examined short-term re-arrangements of the astrocyte cytoskeleton during this shape conversion. Primary cultures of astrocytes from neonatal rat forebrain were grown at low density as polygonal shaped cells. Treatment with 1 mM dibutyryl cAMP in the absence of serum produced rapid changes in cell shape (100% of cells as flat to 90% showing cytoplasmic contraction and processes in 60 min). In the presence of serum, similar changes took place, but more slowly. No changes in total cellular levels of GFAP, vimentin, tubulin or actin were observed over a 2-h period of treatment. There was a shift in actin from a Triton X-100-insoluble pool to a soluble pool, with a 40% reduction in insoluble actin. The kinetics of this shift paralleled kinetics of shape change. The shift also corresponded to a loss of stress fibers, visualized with rhodamine-phalloidin. Intermediate stages of stress fiber loss were observed as short, wavy or small ring profiles. Colchicine prevented the dBcAMP-induced changes in shape. If cells were first treated with taxol, however, subsequent exposure to colchicine did not inhibit contraction. Thus, dBcAMP, presumably through a cAMP-dependent kinase, depolymerizes actin in stress fiber form as cells contract. In addition, an intact microtubule system may be required for the changes in shape. Treatment with dBcAMP also caused the disappearance of vinculin-containing attachment sites, indicating that adhesion plaques, or at least the association of vinculin with them, are lost during the time of microfilament bundle dissociation.

Thrombin reverts the beta-adrenergic agonist-induced morphological response in rat glioma C6 cells

Treatment of rat glioma C6 cells with a beta-adrenergic agonist leads to a rise in cAMP level and a subsequent change in cell morphology from an epithelial to an astrocyte type of appearance. This morphological change is reverted by the addition of thrombin. In 10-15 min the cells acquire their normal epithelial morphology. The reversion by thrombin is inhibited by hirudin, but not by antithrombin III (an inhibitor of the proteolytic action of thrombin). Using the intracellular Ca2(+)-indicator fura-2, we observed that the addition of thrombin to the glioma cells generated a Ca2(+)-signal which was inhibited by pretreatment of the cells with hirudin or with 1 mM neomycin. These results suggest that thrombin uses the phospholipid-inositol pathway to counteract the morphological response, which was induced by activation of the cAMP pathway.

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.

Distinct effects of calcium- and cyclic AMP-enhancing factors on cytoskeletal synthesis and assembly in mouse osteoblastic cells

Previous studies have indicated that the effects of parathyroid hormone (PTH) on osteoblastic function involve alteration of cytoskeletal assembly. We have reported that after a transitory cell retraction, PTH induces respreading with stimulation of actin, vimentin and tubulins synthesis in mouse bone cells and that this effect is not mediated by cAMP. In order to further elucidate the role of intracellular cAMP and calcium on PTH action on bone cell shape and cytoskeleton we have compared the effects of calcium- and cAMP-enhancing factors on actin, tubulin and vimentin synthesis in relation with mouse bone cell morphology, DNA synthesis and alkaline phosphatase activity as a marker of differentiation. Confluent mouse osteoblastic cells were treated with 0.1 mM isobutylmethylxanthine (IBMX) for 24 h. This treatment caused an increase in the levels of cytoskeletal subunits associated with an elevation of cAMP. Under these conditions, PTH (20 nM) and forskolin (0.1 microM) produced persistent cytoplasmic retraction. PTH and forskolin treatment in presence of IBMX (24 h) induced inhibitory effects on actin and tubulin synthesis evaluated by [35S]methionine incorporation into cytoskeletal proteins identified on two-dimensional gel electrophoresis. Under these culture conditions PTH and forskolin also caused disassembly of microfilament and microtubules as shown by the marked reduction in Triton X soluble-actin and alpha- and beta-tubulins. In contrast, incubation of mouse bone cells with 1 microM calcium ionophore A23187 (24 h) resulted in increased monomeric and polymeric forms of actin and tubulin while not affecting intracellular cAMP. Alkaline phosphatase activity was increased in all conditions while DNA synthesis evaluated by [3H]thymidine incorporation into DNA was stimulated by PTH combined with forskolin and inhibited by the calcium ionophore. These data indicate that persistent elevation of cAMP levels induced by PTH and forskolin with IBMX cause cell retraction with actin and tubulin disassembly whereas rising cell calcium induces cytoskeletal protein assembly and synthesis in mouse osteoblasts. The results point to a distinct involvement of calcium and cAMP in both cytoskeletal assembly and DNA synthesis in mouse bone 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.

Type I collagen degradation by mouse calvarial osteoblasts stimulated with 1,25-dihydroxyvitamin D-3: evidence for a plasminogen-plasmin-metalloproteinase activation cascade

To understand the mechanisms regulating osteoid removal by osteoblasts, mouse calvarial osteoblasts were grown on 14C-labelled type I collagen films and stimulated with 1,25-dihydroxyvitamin D-3 (2.5.10(-8) M) for 48-72 h. In the presence of 5% non-inhibitory rabbit serum this resulted in a 2-3-fold increase in collagen degradation and a dramatic change in osteoblast morphology, when compared with untreated osteoblasts. Collagenolysis was accompanied by increased synthesis and release of latent collagenase, gelatinase and stromelysin and a concomitant decrease in their specific inhibitor, TIMP (tissue inhibitor of metalloproteinases). In serum-free medium, osteoblasts failed to degrade collagen, but their ability to lyse collagen could be restored by adding plasminogen (5 micrograms/ml) to the cultures. Plasminogen-dependent collagenolysis was inhibited by human recombinant TIMP (5 units/ml), demonstrating that plasmin, derived from plasminogen, activated latent collagenase and did not itself degrade collagen. Plasminogen activator production was confirmed by culturing osteoblasts on 125I-labelled fibrin plates. Comparison with urokinase-type and tissue-type plasminogen activator standards suggested that osteoblast plasminogen activator was predominantly cell-associated and likely to be of the urokinase type. Immunocytochemistry indicated that osteoblasts also constitutively produce plasminogen activator inhibitor-1. These findings provide evidence for the involvement of a plasminogen-plasmin-latent metalloproteinase activation cascade in type I collagen degradation by osteoblasts, and for its regulation by TIMP and plasminogen activator inhibitor-1.

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.

The metabolism and immunology of bone

Many cells and their cytokines produce a significant effect on bone metabolism. Bone matrix synthesis is a function of the osteoblast (Fig 1), influenced directly by numerous local and systemic factors (Tables 1 and 2). Locally synthesized factors such as SGF, BMP, and BDGF may be particularly important in stimulating new bone formation at sites of bone resorption or following bony injury. Of the systemic factors, GH; somatomedin C (IGF-1); high concentrations of insulin, testosterone, PDGF and TGF beta; and low concentrations of PGE2 and IL-1 appear to stimulate bone formation in vitro. These latter factors may be more important in maintaining skeletal growth and bone mass. Bone resorption by osteoclasts (Figs 2 and 3) is also controlled by the osteoblast, as this cell produces a leukotriene-dependent polypeptide that stimulates osteoclastic bone resorption. Osteoblasts cover the periosteal and endosteal bone-surfaces and limit exposure of the underlying bone to osteoclasts. PTH, vitamin D, PGE2, and other systemic factors interact directly with the osteoblast, not the osteoclast. Surface receptor binding of PTH increases intracellular cAMP and calcium and results in release of the factor that stimulates osteoclastic bone resorption. PGE2 induces osteoblasts to activate osteoclasts and is a major controlling factor in bone metabolism; the osteoblast produces PGE2, which can then modify osteoblastic function by positive feedback. Although low concentrations of PGE2 stimulate bone formation, higher concentrations promote osteoblast-mediated bone resorption. Furthermore, many of the systemic factors stimulate bone resorption via a PGE2-associated mechanism. Immune cytokines also appear to exert a profound influence on bone metabolism. INF-gamma inhibits osteoclastic resorption, whereas IL-1, TNF, and LT strongly stimulate bone resorption. However, low concentrations of IL-1 paradoxically result in stimulation of bone formation. These cytokines, particularly in various combinations, may prove extremely important in understanding and treating the bone loss associated with malignancies, osteoporosis, and rheumatoid arthritis.

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.

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.

Phorbol 12-myristate 13-acetate prevents isoproterenol-induced morphological change in cultured vascular smooth muscle cells

The effect of phorbol 12-myristate 13-acetate (PMA) on isoproterenol (ISO)- and dibutyryl cAMP (dBcAMP)-induced morphological change and cytoskeletal reorganization was studied in cultured vascular smooth muscle cells (VSMC) using the fluorescence staining of actin and microtubules. The treatment of VSMC with 1.0 microM of ISO or with 1.0 mM of dBcAMP for 90 min induced the disruption of actin-containing stress fibers followed by cytoplasmic arborization. The addition of 100 or 10 nM of PMA prevented both the destruction of actin fibers and cell arborization induced either by ISO or by dBcAMP. However, PMA rather enhanced cAMP production stimulated by ISO. 1-Oleoyl-2-acetyl-sn-glycerol (100 micrograms/ml) mimicked this inhibitory effect of PMA whereas 4 alpha-phorbol 12,13-didecanoate (100 nM) failed to block the arborization. These results indicated that the inhibition of arborization by PMA was mediated through the activation of protein kinase C. Colchicine at 5.0 microM also had an inhibitory effect on ISO- and dBcAMP-induced cell arborization. However, immunofluorescence studies revealed that colchicine but not PMA elicited the reorganization of microtubules, suggesting that the effect of PMA was mediated through a mechanism different from that of colchicine. These observations indicated that the morphology of VSMC was regulated through the alteration of cytoskeletal organization induced by cAMP-mediated and by protein kinase C-dependent systems.

Effect of parathyroid hormone and forskolin on cytoskeletal protein synthesis in cultured mouse osteoblastic cells

Parathyroid hormone (PTH) has been shown to cause transient cell shape changes in bone cells. We have examined the effects of parathyroid hormone and forskolin on the organization and expression of cytoskeletal proteins in cultured mouse endosteal osteoblastic cells. Analysis of [35S]methionine-labeled cytoskeletal proteins isolated on two-dimensional gel electrophoresis showed that PTH treatment (24 h) stimulated the de novo biosynthesis of actin, vimentin and tubulins in confluent cells, whereas forskolin had a minor effect despite a huge stimulation of cAMP production. This PTH-induced stimulation was associated with cell respreading following a mild and transitory cell retraction. PTH increased the synthesis of monomeric subunits of actin and beta-tubulins in subconfluent bone cells, whereas both monomeric and polymeric levels of beta-tubulins were increased in confluent osteoblasts. Under conditions reducing cell spreading, osteoblastic cells had initially high levels of unpolymerized subunits. In these poorly spread cells, parathyroid hormone or forskolin had no effect on the de novo synthesis of cytoskeletal proteins despite a marked elevation in intracellular cAMP levels. It is concluded that PTH affects the biosynthesis of cytoskeletal proteins in osteoblastic cells and that cAMP production does not seem to be directly involved. In addition, the effect of PTH is modulated by cell spreading and by the initial pool of cytoskeletal subunits.

Microinjection of catalytic subunit of cyclic AMP-dependent protein kinase triggers acute morphological changes in thyroid epithelial cells

In dog thyroid epithelial cells in primary culture, thyrotropin acting through cyclic AMP induced rapid morphological changes associated with complete disruption of actin containing stress fibers. This modification preceded cell retraction and rounding up. These morphological effects were also induced by glass capillary microinjection of purified catalytic subunit of cAMP-dependent protein kinase. This provides the first direct evidence in intact cells that catalytic subunit, which is released upon activation of cAMP-dependent protein kinases, is responsible for cAMP-dependent morphological transformation.

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.

Study on normal and otosclerotic bone cell cultures: an advance in understanding the pathogenesis of otosclerosis

The authors first reviewed the main theories concerning the pathogenesis of otosclerosis and studied the morphologic and functional characteristics of cell cultures derived from normal and otosclerotic bones. Light transmission and scanning electron microscopy did not permit definite identification of the cultured cells as predominantly osteoblasts, nor did these techniques show significant differences between cultured cells derived from normal and pathologic bone. Functional tests of the cell cultures proved more interesting. First, the bony nature of the cultured cells was demonstrated by studying the intracellular 45Ca++ uptake after stimulation with calcitonin and dybutryl-cAMP. Second, cell cultures derived from otosclerotic bone behaved differently from those derived from normal bone. Their peak uptake of calcium appeared later, and post-stimulatory values were higher, suggesting that cells derived from otosclerotic bone store a greater quantity of 45Ca++. Furthermore, after stimulation with calcitonin and propranolol, we observed an inhibition of the calcium uptake and decreased intracellular cAMP levels in normal bone cell cultures. In contrast, the cell cultures derived from otosclerotic bone exhibited an initial inhibition of calcium absorption followed by massive calcium penetration. The response of adenylate cyclase to the action of Mg++, Ca++, and F- ions was evaluated in cultures derived from normal bone, otosclerotic bone, and normal skin fibroblasts. The resulting data show that activation due to Mg++ is much lower in cultured cells derived from otosclerotic bone than in those from either normal bone or skin fibroblasts. No significant differences were found after Ca++ inhibition in any of the cell cultures. Moreover, in cell cultures derived from normal bone, F- ions induced a strong activation that was lower than the levels observed in cultures of otosclerotic bone or in normal fibroblasts. We hypothesize that an alteration at the calcitonin receptor site is responsible for the difference in calcium uptake and cAMP levels observed in the cells derived from otosclerotic bone as compared to those cultured from normal cells.

New concepts in bone remodeling: an expanding role for the osteoblast

Until recently, the prevailing view regarding the function of osteoblasts and osteoclasts was to attribute bone formation to the former and bone resorption to the latter. While the capacity of the osteoclast to degrade bone matrix remains unquestioned, there is now provocative evidence indicating that the osteoblast plays a critical role in regulating osteoclast resorptive activity as well as in contributing directly to matrix dissolution. The first of these points follows from observations indicating that the osteoblast (but not the osteoclast) 1) exhibits receptors and/or responses to resorption-promoting agents (including parathyroid hormone and vitamin D), and 2) releases agents capable of stimulating bone resorption. The second point is derived from studies demonstrating that the osteoblast produces neutral collagenase (an enzyme specialized to degrade type I collagen, the principal organic constituent of bone matrix) and an inhibitor capable of blocking collagenase activity. The synthesis of both of these proteins is, in part, regulated by parathyroid hormone and other resorption-stimulating agents and appears to involve control at the transcriptional, translational, and secretory levels. Thus, in both physiologic bone remodeling and modeling, as well as the altered bone turnover associated with some disease states, it is the osteoblast rather than the osteoclast that may hold the key to understanding the mechanism of tissue form and function.

Comparative effects of parathyroid hormone on osteoblasts and cementoblasts

Although bone, dentin and dental cementum are mesenchymal mineralized tissues composed mainly of collagen and hydroxy apatite, they differ markedly in their suceptibility to resorption. Bone undergoes physiological resorption to which the dental tissues appear to be resistant. Recently, the cells covering bone surfaces have been attributed a regulatory role in osteoclastic bone resorption by exposing the bone surface following stimulation with hormones and inflammatory mediators, thereby allowing osteoclasts to colonize the bone surface. In the present study, it was shown that reparative cementum-forming cells covering an experimental cavity in the root surface of replanted monkey incisors were unaffected by parathyroid hormone, a potent mediator of bone resorption. In parallel experiments, endocranial osteoblasts exposed bone surface by widening their inercellular spaces. It was concluded that the layer of cells covering the root surface forms a protective barrier against resorption which serves to preserve the integrity of the dental root.

Contraction and relaxation of cultured mesangial cells on a silicone rubber surface

Glomerular mesangial cells (MC) in culture are believed to contract or relax in response to agents such as angiotensin II and cyclic AMP. However, cells grown on glass or plastic surfaces are limited in their response to vasoactive agents because of the rigid surfaces to which they adhere; thus, interpretation of a change in shape as contraction, relaxation, or detachment is difficult. We have grown MC on a flexible silicone rubber (dimethylpolysiloxane) substrate (DMPS), and studied with sequential photographs several models of cell contraction, relaxation, and detachment. When the cells contracted, the DMPS became wrinkled; when the cells relaxed, the DMPS lost wrinkles. In contrast, if the cells detached, the sheet lost wrinkles as the cells became smaller and rounder. Angiotensin II (5 X 10(-7) M), and calcium ionophore A23187 (2 X 10(-6) M) increased wrinkles in more than 30% of cells at 22 degrees C and more than 40% of the cells at 36 degrees C. The earliest effect was visible within five to 10 minutes at 22 degrees C and within one minute at 36 degrees C and increased until 40 minutes; thereafter, the cells relaxed and wrinkles were reduced. 10(-1) M Na azide prevented the increase in wrinkles produced by angiotensin II. Seventy-two percent of the angiotensin II-treated cells whose margins could be seen in their entirety, and 78% of the calcium ionophore-treated cells showed a reduction in surface area at a time when new wrinkles were appearing or wrinkles were increasing in size. Dibutyryl cyclic AMP, a known smooth muscle relaxant, produced a decrease or loss of wrinkles in 90% of the cells, and an accompanying increase in surface area. Untreated control cells, observed in conjunction with the above series, showed little change in wrinkles. Ten percent DMSO, an actin-translocating agent, produced a reversible disappearance of wrinkles. These models of contraction and relaxation could be distinguished from cell detachment; EDTA, for example, in the presence of zero calcium, diminished both cell size and wrinkles, with an accompanying lifting of cells from the surface. Similar results were obtained with cytochalasin B and chlorpromazine. Thus, the silicone rubber system accurately reflects the contraction, relaxation and detachment of cultured mesangial cells in response to a variety of agents.

The effect of 1,25-dihydroxyvitamin D3 on the cytoskeleton of rat calvaria and rat osteosarcoma (ROS 17/2.8) osteoblastic cells

1,25-dihydroxyvitamin D3 produces pronounced shape changes in fetal rat calvaria and osteosarcoma-derived (ROS 17/2.8) osteoblastic cells, characterized by retracting processes and cell rounding followed by aggregation of cells. The 1,25(OH)2D3 effect on ROS 17/2.8 morphology was determined morphometrically on scanning electron micrographs. The hormone effect was found to be dose dependent between 10(-12) and 10(-9) M. The shape changes appeared 12 h after hormone (10(-10) M) addition and were present in 80% of the ROS 17/2.8 cells and in 50% of the calvaria cells at 72 h. Cycloheximide at 1 microM, inhibited the hormone-dependent change in morphology. The 1,25(OH)2D3 effects were partially mimicked by 10(-8) M 25(OH)D3 but not by 10(-10) M 25(OH)D3 or 10(-11)-10(-8) M 24,25(OH)2D3. 1,25-dihydroxyvitamin D3 also increased cell proliferation twofold at 14 days in serum-free medium. 1,25(OH)2D3 treatment produced changes in microfilament organization, visualized with rhodamine-conjugated phalloidin. Microfilaments were localized at the terminal attachment points and in the perinuclear region, and few if any, were seen in the retracting processes themselves. Estimation of cytoskeletal actin and myosin by gel electrophoresis of Triton X-100 nonextractable proteins showed a 30% reduction in these proteins in the hormone-treated cells. Microtubules visualized by indirect immunofluorescence showed no major changes in organization. Both colchicine and cytochalasin D altered the hormone-induced shape change, suggesting that both microfilaments and microtubules were required for this process. Thus, 1,25(OH)2D3 had pronounced effects on cell shape in osteoblastic cells, probably via de novo protein synthesis. These changes lead to rearrangement of the cytoskeleton, primarily the microfilaments.

Inhibition of bone resorption in culture by (+)-catechin

A pretreatment with (+)-catechin renders embryonic mouse calvaria in culture resistant to the action of bone resorbing agents, either parathyroid hormone (PTH), prostaglandin E2 or retinoic acid, and inhibits in a parallel way the enhanced excretion of N-acetyl-beta-glucosaminidase, a reference lysosomal enzyme, induced by these agents; it has, however, no effect on the small spontaneous leakage of lactate dehydrogenase from the explants. Moreover, the resorption induced in calvaria by a pretreatment with PTH or retinoic acid is inhibited by a further culture with catechin. This inhibition of bone resorption is discussed in relation with the collagen-stabilizing properties of (+)-catechin.

Effect of parathyroid hormone on phospholipid metabolism in osteoblast-like rat osteogenic sarcoma cells

Previous results have shown that 1,25-dihydroxycholecalciferol [1,25(OH)2D3] enhances the synthesis of phosphatidylserine (PS) and suppresses the synthesis of phosphatidylethanolamine (PE) in osteoblast-like rat osteogenic sarcoma UMR 106 cells [Matsumoto, Kawanobe, Morita & Ogata (1985) J. Biol. Chem. 260, 13704-13709]. In the present study, the effect of parathyroid hormone (PTH) on phospholipid metabolism is examined by using these cells. Treatment of UMR 106 cells with human PTH-(1-34)-peptide suppresses the synthesis of phosphatidylethanolamine in a dose- and time-dependent manner without affecting the synthesis of PS. The maximal effect on PE synthesis is obtained with 2.4 nM-human PTH-(1-34)-peptide when the cells are treated for 48 h or longer. In addition, when human PTH-(1-34)-peptide is added together with the maximal dose of 1,25(OH)2D3, there is a further decline in PE synthesis, whereas the stimulation of PS synthesis by 1,25(OH)2D3 is not altered. Because methylation of PE is suggested to affect hormone receptor-adenylate cyclase coupling, the observed change in PE metabolism by PTH and 1,25(OH)2D3 may be, at least in part, involved in the development of desensitization phenomenon to PTH in these cells.