Effect of vitamin A on bone resorption: evidence for direct stimulation of isolated chicken osteoclasts by retinol and retinoic acid

What's new in the role of cytokines on osteoblast proliferation and differentiation?

This review assesses recent data concerning the role of cytokines produced by a variety of cells in bone on osteoblast function. The following themes are presumed: (1) osteoblasts are mesenchymal cells which act as either the major cellular agents of bone formation or as modulators of bone resorption by osteoclasts. The regulation of osteoblast proliferation and differentiation may involve a negative feedback process resulting in phenotype suppression; (2) cytokines including platelet-derived growth factors (PDGF), parathyroid hormone-related proteins (PTHrP), bone morphogenic proteins (BMP), transforming growth factor beta (TGF beta), fibroblast growth factors (FGF), insulin-like growth factors (IGF), epidermal growth factors (EGF), interleukin-1 and 6, tumour necrosis factors (TNF), interferon and haematopoietic growth factors have effects on osteoblast differentiation and proliferation but their effectiveness may not be identical in vitro and in vivo; (3) finally, therapeutic strategies for cytokine use in clinical practice are considered.

Osteoblast-osteoclast relationships in bone resorption: osteoblasts enhance osteoclast activity in a serum-free co-culture system

Osteoblast-osteoclast relationships in bone resorption are unclear. We investigated whether osteoblasts constitutively influence osteoclast activity. We employed a serum-free co-culture system in which chicken osteoclasts and chick calvaria or, alternatively, isolated chick osteoblasts were cultured in two different compartments separated by a 0.45 micron porous membrane permeable to soluble molecules. Osteoclastic bone resorption, evaluated by release of 3H-proline from prelabeled bone fragments, was significantly enhanced by bone cells resident in the calvaria, as well as by isolated osteoblasts. Stimulation was specific, since periosteal cells, or skin fibroblasts, failed to mimic osteoblast activity. Conditioned medium from osteoblast cultures stimulated osteoclast function in a similar manner, indicating that paracrine signals, capable of crossing the porous membrane separating the two compartments, are released by the bone forming cells.

Application of reflected light microscopy to identify and quantitate resorption by isolated osteoclasts

A simple technique has been developed to identify the resorption lacunae excavated by avian osteoclasts in vitro. Briefly, devitalized bovine bone wafers, with cells in situ, are fixed, stained with toluidine blue, and then examined by reflected light microscopy. Resorption pits are clearly visible using a x 10 objective, even when the resorbed areas are covered by a confluent layer of cells. The technique can be used to quantify the plan area, depth, and volume of the excavations. Depth and volume are assessed using the x 50 objective lens. This technique has been used to investigate the effects of retinyl acetate and EHDP on bone resorption.

Biphasic effect of calcitonin on tartrate-resistant acid phosphatase activity in isolated rat osteoclasts

Tartrate-resistant acid phosphatase (TRAP) has been implicated as being involved in osteoclastic bone resorption, and calcitonin (CT) is known to inhibit the resorptive process. This study investigates the kinetics of CT action on TRAP activity in isolated rat osteoclasts using both biochemical and quantitative cytochemical methods. The latter technique has been developed to detect very small changes in intracellular TRAP activity at the single-cell level. The biochemical study showed that 10(-9) M salmon CT (sCT) decreased TRAP activity in medium throughout the experimental period; TRAP activity in the cells was increased during the first 2 h but subsequently declined and was decreased to a significant level at 6 h. TRAP activity in sCT-treated osteoclasts measured by the cytochemical method showed significant increases within the first hour. This response was dose dependent between 10(-16) and 10(-11) M sCT with EC50 at 8 X 10(-14) M. After 1 h, the initial increase in intracellular TRAP activity in CT-treated osteoclasts was followed by a decline to below control levels, reaching statistical significance at 9 h. Treatment with forskolin (10(-5) M) showed a similar trend, suggesting that this response is mediated by cyclic AMP-regulated phosphorylation events. From these results, we conclude that CT has two actions on TRAP in isolated rat osteoclasts: the first to inhibit its release, the second to inhibit its synthesis and/or increase its degradation.

1,25(OH)2D3 induces differentiation of osteoclast-like cells from human bone marrow cultures

Multinucleated cells were generated from human bone marrow cultured in the presence of 10(-6)M 1,25(OH)2D3 and 10(-6)M all-trans-retinoic acid for 3-4 weeks. These multinucleated cells have the phenotypic and functional characteristics of osteoclasts as judged by (a) immunostaining with osteoclast specific monoclonal antibodies 13C2 and 23C6 (b) expression of tartrate resistant acid phosphatase, an enzyme marker of osteoclast differentiation; and (c) the ability to resorb bone in vitro. The multinucleated cells appeared to form by fusion of large mononuclear cells. The monoclonal antibodies 13C2 and 23C6 stained 60-90% of the multinucleated cells, and 40-60% of the large mononuclear cells. Tartrate resistant acid phosphatase activity was expressed by 80-95% of the multinucleated cells and 60-80% of the large mononuclear cells. Scanning electron microscopy of bone wafers co-incubated with the multinucleated cells, for 7 days, revealed resorption pits. These findings suggest that in the presence of 1,25(OH)2D3 the marrow cells differentiated into multinucleated and large mononucleated cells in which a proportion of them expressed osteoclast phenotype and resorbed bone.

Osteopetrotic (grey-lethal) bone produces collagenase and TIMP in organ culture: regulation by vitamin A

Evidence has recently accumulated suggesting that osteoblasts play a direct role in bone resorption by producing collagenase. In this paper we describe studies carried out with explants of bone from osteopetrotic grey lethal (gl/gl) mice and show that despite the lack of osteoclastic activity the production of both active and latent collagenase and its specific inhibitor TIMP (tissue inhibitor of metalloproteinases) is similar to that of normal bones. Synthesis of collagenase was stimulated by the bone resorptive agent vitamin A (retinol); concomitantly, TIMP levels fell to zero and active enzyme was detected in the culture medium. This work supports the view that bone collagenase is produced by cells other than osteoclasts, since the response of the osteoblastic population to resorptive signals appears normal.

Retinol, supplemental vitamin A and bone status

Studies of hypervitaminosis A in animals and anecdotal reports of accidental vitamin A poisoning in humans suggest impairment of bone remodeling and increased numbers of fractures. Because of the widespread use of high-dose vitamin A supplements which may produce subclinical hypervitaminosis associated with decreased bone mass and increased risk of fracture, we studied the relationship between current vitamin A supplement use, serum retinol levels, radial bone mass and fracture history in a geographically-defined population of 246 postmenopausal women, 55-80 years of age. More than 36% of this population used a vitamin A supplement with 8% of these consuming an amount in excess of 2000 retinol equivalents (RE)/day. Serum retinol was measured using high-pressure liquid chromatography and radial bone mass was measured using single photon absorptiometry. After controlling for age, current estrogen replacement, and current thiazide antihypertensive use, we observed no statistically significant relationship between vitamin A supplement use or serum retinol with radial bone mass or fractures.

Fluorescence in the tandem scanning microscope

Our studies have shown that the fluorescence mode can be used to good effect in both tandem scanning microscopes (TSM: direct view confocal microscopes) as well as confocal scanning laser microscopes (CSLM). Applications are presented which show that the two great advantages of TSM are real-time viewing and real colour, which allow faster use and interpretation. CSLM are complementary, not competitive, being currently more sophisticated for low-level fluorescence work. This is equally possible with available TSM, but requires further development using CCD cameras and image-processing systems.

The effect of fluoride on the patterns of adherence of osteoclasts cultured on and resorbing dentine: a 3-D assessment of vinculin-labelled cells using confocal optical microscopy

Osteoclasts were isolated mechanically from chick long bones and cultured on dentine for three days in MEM/10% FCS with or without 1 mg/l NaF, (near to therapeutic ranges for serum fluoride levels) or 15 mg/l NaF (which has been found to reduce the volume:plan-area ratios of resorption pits in vitro). The distribution of close contacts with the substratum was determined by immunolabelling of vinculin and confocal fluorescence microscopy. Four characteristic patterns of contacts were observed: infilled discs, rings, crescents and patches: these reflect the progress of the cell in the process of pit formation. Cell-substratum adherence was more extensive than previously reported with contacts often extending down the sides of pits and sometimes across the floor. This distribution explains the curved shape of unilocular pits and the kidney-shaped extensions of multilocular pits. The peripheral creeping of the contact region was more marked in the osteoclasts of the 15 mg/l NaF cultures, and these cultures showed a decrease in pit depth and inhibition of osteoclastic movement in the presence of fluoride.

Extracellular protons acidify osteoclasts, reduce cytosolic calcium, and promote expression of cell-matrix attachment structures

Because metabolic acids stimulate bone resorption in vitro and in vivo, we focused on the cellular events produced by acidosis that might be associated with stimulation of bone remodeling. To this end, we exposed isolated chicken osteoclasts to a metabolic (butyric) acid and observed a fall in both intracellular pH and cytosolic calcium [( Ca2+]i). These phenomena were recapitulated when bone resorptive cells, alkalinized by HCO3 loading, were transferred to a bicarbonate-free environment. The acid-induced decline in osteoclast [Ca2+]i was blocked by either NaCN or Na3VO4, in a Na+-independent fashion, despite the failure of each inhibitor to alter stimulated intracellular acidification. Moreover, K+-induced membrane depolarization also reduced cytosolic calcium in a manner additive to the effect of protons. These findings suggest that osteoclasts adherent to bone lack functional voltage-operated Ca2+ channels, and they reduced [Ca2+]i in response to protons via a membrane residing Ca-ATPase. Most importantly, acidosis enhances formation of podosomes, the contact areas of the osteoclast clear zone, indicating increased adhesion to substrate, an early step in bone resorption. Thus, extracellular acidification of osteoclasts leads to decrements in intracellular pH and calcium, and appears to promote cell-matrix attachment.

Activation of the bone-derived latent TGF beta complex by isolated osteoclasts

Although TGF beta is unquestionably an important growth regulatory polypeptide with effects on many cell types, the cellular mechanisms which release it from the binding proteins which mask its biological activity are not well understood. Here we show that when isolated osteoclasts are activated, they release active TGF beta from the latent TGF beta complex produced by bone organ cultures. Since active TGF beta has powerful inhibitory effects on osteoclast formation and bone resorption and stimulates osteoblast activity, is present in abundant amounts in the bone matrix and is released during hormone-stimulated osteoclastic bone resorption, the activation of TGF beta by stimulated osteoclasts may be an important regulatory step in normal bone remodeling.

The distribution of podosomes in osteoclasts cultured on bone laminae: effect of retinol

Osteoclasts, isolated and purified from the medullary bone of calcium-deficient egg-laying hens, adhere to glass coverslips in vitro by means of specialized protrusions of the ventral membrane, denoted podosomes. These structures represent dotlike close-contact adhesion sites in which most cytoskeletal proteins involved in linking the plasma membrane to microfilaments are organized according to a specific and previously described pattern also shared by many oncogene-transformed cells. We show now that podosomes are not only a feature of osteoclasts adhering to artificial glass surfaces but are also present in the ventral membrane of osteoclasts adhering to bone laminae. Moreover, the quantity and the topography of podosomes may be modulated by retinol, which increases bone-resorbing activity of osteoclasts both in vivo and in vitro. A comparative transmission electron microscopy study of osteoclasts adhering on bone laminae in vitro or in vivo indicates that podosomes with identical features are present in the clear zone of the osteoclasts in either condition. Since podosomes are the sealing structures of the clear zone, podosome formation may represent one of the modifications involved in the reorganization process of the osteoclast that precedes bone resorption.