Calcitonin-like effects of forskolin and choleratoxin on surface area and motility of isolated rabbit osteoclasts

Vasoactive intestinal peptide regulates osteoclast activity via specific binding sites on both osteoclasts and osteoblasts

Clinical and experimental observations, together with immunohistochemical findings, suggest that neuro-osteogenic interactions may occur in the skeleton. In this study, we have examined the effect of vasoactive intestinal peptide (VIP), one of the neuropeptides present in bone, on the activity of the bone-resorbing osteoclast. Effects on bone resorption were assessed by counting the number of pits formed by rat osteoclasts incubated on devitalized slices of bovine cortical bone. Under conditions with an initially sparse density of stromal cells/osteoblasts, VIP caused a rapid cytoplasmic contraction and decreased motility of osteoclasts. This was coupled with a decrease in the number of resorption lacunae and a decrease in the total area resorbed by the osteoclasts in 48-h cultures. Time-course experiments revealed that the inhibitory effects on contraction and motility were transient and that the cells gradually regained their activity, such that, when culture time was prolonged to 120 h, a stimulatory effect by VIP on bone resorption was observed. When osteoclasts were incubated on bone slices, in the presence of an initially large number of stromal cells/osteoblasts, VIP treatment increased the number of resorption pits and total bone area resorbed in 48-h cultures. Using atomic force microscopy, we provide direct evidence that both osteoclasts and stromal cells/osteoblasts bind VIP. Also, VIP was shown to cause a rapid rise of intracellular calcium in osteoclasts and in a proportion (20%) of stromal cells/osteoblasts. Taken together, these data suggest that differentiated osteoclasts are equipped with receptors for VIP that are linked to a transient inhibition of osteoclast activity and, in addition, that stromal cells/osteoblasts have VIP receptors coupled to a delayed stimulation of osteoclastic resorption.

Role of phosphodiesterase 4 isoenzyme in alkaline phosphatase activation by calcitonin in porcine kidney LLC-PK1 cells

To confirm the intracellular signal transduction in regulation of alkaline phosphatase (ALP) activity by calcitonin in kidney tubular cells, effects of several inhibitors of cyclic nucleotide phosphodiesterase (PDE) isoenzymes and cyclic AMP-dependent protein kinase (PKA) on the action of salmon calcitonin in porcine kidney tubular epithelial cells LLC-PK1 were examined. A confluent culture of LLC-PK1 cells was treated with calcitonin and inhibitors in Dulbecco's modified Eagle's medium supplemented with 0.1% bovine serum albumin, and intracellular cyclic AMP content and ALP activity were measured after incubation for 30 min and 48 hr, respectively. Calcitonin and PDE 4 inhibitors increased cyclic AMP level and ALP activity in the cells, and PDE 4 inhibitors synergistically potentiated the effects of calcitonin. Calcitonin induced ALP activation by treatment for the first 1 hr, as well as continuous treatment for 48 hr, while it never increased the enzyme activity just after 1-hr exposure. Rolipram, an inhibitor of PDE 4 isoenzyme, induced ALP activation by itself and in combination with calcitonin by only a long term treatment (48 hr). The activation of ALP by calcitonin and rolipram each alone and in combination was completely abolished by a PKA inhibitor, H-89. These results confirm that calcitonin induces ALP activation through the cyclic AMP-PKA pathway and that PDE 4 isoenzyme is closely associated with the calcitonin-receptor system and plays a major role in hydrolysis of cyclic AMP produced in the kidney tubular cells.

Effects of continuous calcitonin treatment on osteoclast-like cell development and calcitonin receptor expression in mouse bone marrow cultures

Continuous treatment with calcitonin (CT) to inhibit osteoclastic bone resorption results in acquired resistance. The mechanisms of this "escape" phenomenon are not yet established. The aim of this study was to examine the effects of continuous treatment with CT on the generation of osteoclasts and calcitonin receptor (CTR) expression in mouse bone marrow cultures. This was done by daily CT treatment of mouse bone marrow cultures from day 0, when only undifferentiated mononuclear precursors of osteoclast-like cells were present, or commencing from day 6, when differentiated osteoclast-like cells were abundant. The response to CT treatment was determined by quantitation of cells positive for tartrate-resistant acid phosphatase (TRAP) and binding of 125I-salmon CT. Calcitonin receptor and TRAP mRNA levels were determined using semi-quantitative reverse transcription/polymerase chain reaction. When cultures were treated with CT from day 0, TRAP-positive multinucleated cells appeared. These cells expressed only very low levels of CTR or CTR mRNA and were morphologically indistinguishable from osteoclast-like cells formed in control cultures. They also displayed the ability to resorb bone. Continuous CT treatment of cultures from day 6 rapidly reduced the CTR mRNA levels, with a t1/2 of 6 to 12 h, and these levels remained low thereafter. 125I-salmon CT binding capacity, as determined by autoradiography, was lost in parallel. These effects were specific for the CTR since there was no consistent effect on TRAP mRNA levels. Based on these data, we suggest that the "escape" phenomenon may result from a prolonged CT-induced loss of CT responsiveness due, at least in part, both to reduced synthesis of CTR, and to the appearance in bone of CTR-deficient osteoclasts.

Cytoskeletal changes in osteoclasts during the resorption cycle

Osteoclasts are large, multinucleated cells which change their shape and polarity according to their resorptive activity. At least in vitro, nonresorbing osteoclasts move on the bone surface and do not show clear evidence of apical-basolateral polarity. When stimulated for resorption, osteoclasts undergo a rapid reorganization of the cytoskeleton and appear clearly polarized. The detailed nature of different membrane domains in polarized osteoclasts is still far from clear, but a remarkable feature is the formation of a tight sealing zone between the ruffled border and the rest of the cell membrane. Characteristic organization of F-actin into a belt or ring-like structure with a double circle of vinculin around it is needed for the formation of the sealing zone. This type of microfilament organization is typical only for resorbing osteoclasts and can thus be used as a marker for resorbing cells. These characteristic changes in the molecular organization of the cytoskeleton in osteoclasts during the resorption cycle offer several potential targets to inhibit bone resorption, perhaps cell-specific.

Volumes of chick and rat osteoclasts cultured on glass

We have examined the relationship between the number of nuclei of an osteoclast and its volume. Chick and rat cells were released from long bones by chopping the shafts and flushing the fragments in Eagle's Minimum Essential Medium with added 10% fetal calf serum. The bone cell suspension was seeded onto glass coverslips. In Experiment 1, rat and chick cells were allowed to settle for 15 minutes, more medium was then added, and the cells were cultured in 5% CO2 at 37 degrees C for 4 hours. In Experiment 2, only rat cells were used, and the cells were cultured in the presence or absence of 10(-6) M 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (APD) in the medium for 4 or 6 hours. The coverslips were washed in 37 degrees C phosphate-buffered saline and fixed for 24 hours in 2.5% glutaraldehyde in isotonic cacodylate buffer (initially 37 degrees C). The chick cells were critical point dried (CPD) or freeze dried (FD); all rat cells were FD. After drying, cells were coated with gold by vacuum evaporation. The volumes and areas of osteoclasts were measured using a video-rate, line-confocal reflection laser scanning microscope and the number of nuclei in each cell was counted. The volumes and volumes per nucleus of the FD cells were larger than those of the CPD cells but there was no significant difference in plan-areas. Rat osteoclasts were larger than chick cells in all the measured parameters except the mean number of nuclei/cell. The correlation coefficients for the areas, volumes, and the numbers of nuclei for rat and chick cells were all high (r > 0.725). The volumes and volumes per nucleus, but not the areas or areas per nucleus, of the osteoclasts cultured with APD were significantly smaller than control cells. We conclude that FD causes less shrinkage than CPD; chick osteoclasts are about two-thirds the size of rat osteoclasts; and 10(-6) M APD caused a reduction of rat osteoclast volume and volume per nucleus of 21%.

Cholera toxin-stimulated bone resorption in cultured mouse calvarial bones not inhibited by calcitonin: a possible interaction at the stimulatory G protein

We examined the effect of calcitonin in cultured mouse calvarial bones after prestimulation with different activators of adenylyl cyclase. Calcitonin (100 ng/ml), added after 48 h of culture, inhibited bone resorption (assessed as release of 45Ca from prelabeled bones cultured for 96-144 h) stimulated with parathyroid hormone (PTH, 10 nM; 0-144 h) or the adenylyl cyclase stimulator forskolin (2 microM; 0-144 h). However, no effect of calcitonin was demonstrated when bone resorption was prestimulated with the adenylyl cyclase stimulator cholera toxin, at and above 1 ng/ml, at any time point studied. In contrast, two other types of inhibitors of bone resorption in vitro, the carbonic anhydrase inhibitor acetazolamide (10 microM) and the aminobisphosphonate AHPrBP (10 microM), significantly inhibited cholera toxin-stimulated bone resorption. No cyclic AMP response to calcitonin was seen after preculture for 48 h with cholera toxin (0.1-100 ng/ml), although bones precultured in basic medium, in the absence or presence of forskolin, were still able to respond to calcitonin with elevation of cyclic AMP. Binding studies with [125I]calcitonin demonstrated that the preculture with cholera toxin did not affect the binding of calcitonin to the receptor. In summary, our data show that cholera toxin pretreatment makes calvarial bones insensitive to calcitonin-induced inhibition of bone resorption as a result of an interaction with cholera toxin at the level of calcitonin receptor-linked signal transduction. We suggest that the interaction, distal to the calcitonin receptor, is caused by the irreversible activation of Gs produced by cholera toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies against the putative divalent cation-receptor that is located on parathyroid cells do not stain isolated rat osteoclasts

It has been reported that osteoclastic function is regulated by calcium-induced alterations in cytoplasmic free calcium ([Ca2+]i), possibly through a specific receptor. We have investigated whether osteoclasts, isolated from neonatal rat long bones, possess the divalent cation-receptor that has been demonstrated on parathyroid cells. Studies with fura-2 loaded adherent single cells showed that an increase in extracellular Ca2+ ([Ca2+]e) from 0.5 mM to 10 mM resulted in an increase in [Ca2+]i in isolated rat osteoclasts, from a basal value of 94.7 +/- 16.2 to 150.6 +/- 22.4 nM (means +/- SEM; n = 14). The shape and time course of the [Ca2+]i increase varied considerably from cell to cell. Less than half of the cells responded with a rapid transient increase whereas the rest responded with a slow increase that reached a plateau within 1-2 minutes. When [Ca2+]e was changed back to 0.5 mM, a slow decrease in [Ca2+]i was monitored. Immunohistochemical staining with two different monoclonal antibodies, recognizing the putative Ca2+ receptor on parathyroid cells, did not indicate any staining on freshly isolated rat osteoclasts. Thus, our data demonstrate that an increase in [Ca2+]e causes an elevation of [Ca2+]i in osteoclasts. This increase is not mediated via the putative cation-receptor found on parathyroid cells.

Effects of phorbol myristate acetate on rat and chick osteoclasts

The role of protein kinase C in the regulation of osteoclast function is not known. We therefore compared the effect of phorbol myristate acetate (PMA), which activates protein kinase C, on the resorptive function, motility, and morphology of osteoclasts from rat and chick. PMA caused a significant reduction in resorption pit number in both species; rat osteoclasts were more sensitive, being significantly inhibited at doses of 10(-9)-10(-6) M compared with 10(-7)-10(-6) M for chick osteoclasts. The inactive analog PMA-alpha was without significant effect, and inhibition was not blocked by 10(-6) M indomethacin. In time course experiments, inhibition at 24 h was similar to or greater than inhibition at 6 h, indicating a persistent or progressive effect on bone resorption. Removal of PMA after 6 h prompted partial recovery of bone-resorptive ability in chick osteoclasts but not rat, at least over a 48 h incubation. In time-lapse video studies of rat osteoclasts, 10(-6) M PMA produced an immediate but transient cessation of motility and retraction of the cell margin into prominent filopodia. Motility resumed within 2.5 h after addition, but the osteoclasts remained partially contracted. Chick osteoclasts behaved similarly but showed no formation of filopodia at the cell periphery and a more rapid recovery of motility than rat osteoclasts; chick osteoclasts also underwent a transient vacuolation following PMA exposure, whereas rat osteoclasts did not. Despite differences in the sensitivity of rat and chick osteoclasts to PMA, these results suggest a fundamental role for protein kinase C in the inhibition of osteoclasts from both species.

Effects of phorbol esters and pertussis toxin on calcitonin-stimulated accumulation of cyclic AMP in neonatal mouse calvarial bones

Calcitonin (CT) is a well-known inhibitor of osteoclastic bone resorption both in vivo and in vitro. The effect is mediated by activation of adenylate cyclase and subsequent increased levels of cyclic AMP (cAMP). We report here that CT-induced (30 nmol/liter) accumulation of cAMP in cultured neonatal mouse calvaria is enhanced two-fold by 12-O-tetradecanoylphorbol-13-acetate (TPA; 100 nmol/liter) and phorbol 12,13-dibutyrate (PDBU; 100 nmol/liter), two protein kinase C (PKC)-activating phorbol esters, whereas phorbol 13-monoacetate (phorb-13; 100 nmol/liter), a related compound that does not activate PKC, has no effect. The ability of TPA and PDBU to enhance CT-stimulated cAMP accumulation was obtained also in the presence of indomethacin (1 mumol/liter). Kinetic studies revealed that TPA enhanced the cAMP response to CT at all the time points at which CT had a significant effect per se and that TPA did not alter the time-course of the cAMP response to CT. Treatment with pertussis toxin (100 ng/ml) enhanced cAMP response to parathyroid hormone (10 nmol/liter) and prostaglandin E2, but not to CT. From these data it is concluded that PKC, but not pertussis toxin-sensitive guanyl nucleotide-binding proteins (G-proteins), can interact with and modify the signal transducing system for CT in osteoclasts.

Kinetics of the osteoclast cytoskeleton during the resorption cycle in vitro

Resorption and migration phases alternate in the life of the osteoclast. We have previously described a specific microfilament structure at the attachment sites in resorbing osteoclasts. In the present study we have examined microfilaments and microtubules in both resorbing and migrating rat osteoclasts cultured on bone slices. In migrating osteoclasts microfilaments form so-called podosome structures containing vinculin, talin, and F-actin at the paramarginal area of the cell. When the osteoclast prepares itself for resorption, the podosomes gather to a certain area and form a broad ring around the area, which is then resorbed. In the resorbing osteoclast, vinculin and talin form a continuous double circle, which may be partially formed by podosomes, and between these double circles a broad zone is formed by F-actin. Narrow vinculin and F-actin rings were found in osteoclasts at the end of the resorption phase. The different configurations of microfilaments in 1 and 2 day cultures were correlated in terms of their relationship to the resorption lacunae. The vitamin A derivative isotretinoin significantly stimulated resorption and increased the number of microfilament configurations associated with the resorption pits. On the other hand, Bt2cAMP abolished resorption and prevented the formation of a specific ring structure of microfilaments. Based on these data, a kinetic model of the whole migration-resorption cycle of the osteoclast cultured on the bone slice is presented. With alpha-tubulin stainings of microtubules two different cytoskeletal organizations were observed. In migrating osteoclasts, microtubules were evenly distributed over the whole cell. In the resorbing osteoclast, there was a noticeable concentration of these cytoskeletal structures at cytoplasmic sites closest to the resorption lacuna. This orientation of microtubules may reflect the active secretory function of the resorbing osteoclast.

The effects of stimulators of intracellular cyclic AMP on rat and chick osteoclasts in vitro: validation of a simplified light microscope assay of bone resorption

The aim of this study was to investigate whether a cyclic AMP-mediated inhibitory mechanism is present in embryonic chick osteoclasts and to extend data implicating cyclic AMP in the inhibition of neonatal rat osteoclasts. Dibutyryl cyclic AMP ((Bu)2cAMP) (5 x 10(-4) M and above) and isobutylmethylxanthine (IBMX) (10(-4) M and above) reduced the number of pits made in slices of devitalized bovine cortical bone by chick osteoclasts over 24 h. The effect of forskolin (FSK) on chick osteoclasts was biphasic, 10(-5) M producing a weak and variable reduction in pit number while 10(-6) M and 10(-7) M stimulated resorption. Doses of FSK (10(-5) M) and (Bu)2cAMP (3 x 10(-4) M), which individually produced no consistent significant effect, produced a synergistic and highly significant reduction in pit number when used in combination, implying that these agents were acting through a common mechanism, presumably cyclic AMP. Stimulatory doses of FSK were associated with increased osteoclast numbers, implicating cyclic AMP in the formation of osteoclasts. In comparative experiments using neonatal rat osteoclasts, (Bu)2cAMP (10(-4) M and above), IBMX (10(-3) M) and FSK (10(-7) M and above) all reduced the number of pits excavated. Strongly inhibitory doses of these agents caused contraction of chick osteoclasts into a hemispherical shape; contraction of rat osteoclasts into a stellate shape occurred with (Bu)2cAMP and FSK, but not with IBMX. Our results implicate cyclic AMP in the inhibition of both rat and chick osteoclasts, and show that pit counting in the light microscope is a valid method of analyzing the disaggregated osteoclast resorption assay.

Comparison between the effects of forskolin and calcitonin on bone resorption and osteoclast morphology in vitro

The adenylate cyclase activator forskolin (1-10 mumol/L) inhibited 45Ca release from parathyroid hormone (PTH; 10 nmol/L) stimulated prelabeled neonatal mouse calvaria in short term culture (24 h). This effect of forskolin was potentiated by rolipram, Ro 20-1724, and isobutyl-methylxanthine, three structurally different inhibitors of cyclic AMP phosphodiesterase. Forskolin (10 mumol/L) and calcitonin (30 mU/mL) inhibited the mobilization of stable calcium and inorganic phosphate as well as the release of the lysomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase from PTH-stimulated unlabeled bones. Osteoclasts in PTH-stimulated calvaria showed active ruffled borders with numerous membrane infoldings. Treatment of PTH-stimulated bones with forskolin and calcitonin resulted in a rapid (2 h) loss of the active ruffled border. In addition, forskolin and calcitonin induced similar changes with respect to the number and size distribution of cytoplasmic vesicles in PTH-activated osteoclasts. After 24 h, all signs of osteoclast inactivation were still prominent, whereas after 48 h of treatment with forskolin or calcitonin, the reappearance of a ruffled border on a number of osteoclasts signaled an escape from the inhibitory action of both calcitonin or forskolin. These data indicate that forskolin inhibits bone resorption by a cyclic AMP dependent mechanism and that the effect of forskolin and calcitonin on bone resorption and osteoclast morphology are comparable. These observations lend further support to the view that cyclic AMP may be an intracellular mediator of the inhibitory action of calcitonin on multinucleated osteoclasts.