Evidence that protein kinase-A, calcium-calmodulin kinase and cytoskeletal proteins are involved in osteoclast retraction induced by calcitonin

Calcitonin promotes outgrowth of trophoblast cells on endometrial epithelial cells: involvement of calcium mobilization and protein kinase C activation

Embryo implantation is a complex process that requires coordinated trophoblast-endometrial interactions. During implantation, trophoblast cells of the attached blastocyst penetrate the luminal epithelium of the endometrium before invasion into the endometrial stroma. Previous studies demonstrated that calcitonin was actively secreted by rat and human endometrial epithelial cells (EEC) during the implantation window and targeted disruption of endometrial calcitonin expression dramatically decreased embryo implantation rates; however, the role and signal transduction of calcitonin in trophoblast-endometrial interactions remained unclear and are therefore examined in this study. BeWo trophoblast and RL95-2 EEC lines were used because they preserve many properties of their respective normal tissues. We co-cultured BeWo trophoblast spheroids with RL95-2 EEC monolayers to mimic the blastocyst-endometrial interaction, and found that most spheroids quickly attached to EEC monolayers and then progressively expanded, with marked displacement of EEC adjacent to the outgrowing trophoblast cells. Interestingly, pretreatment of EEC monolayers with calcitonin before the addition of spheroids significantly enhanced trophoblast expansion on EEC monolayers. Cytosolic calcium (Ca(2+)) levels in EEC increased rapidly upon exposure to calcitonin, and blockade of Ca(2+) release by BAPTA-AM effectively prevented the promoting effect of calcitonin on trophoblast expansion on EEC. The Ca(2+)-dependent protein kinase C (PKC) was also activated in EEC after calcitonin treatment, and the PKC inhibitors staurosporine and calphostin C could completely abolish calcitonin-induced augmentation of trophoblast expansion on EEC. Our results suggest that calcitonin promotes trophoblastic displacement of EEC through calcium mobilization and PKC activation, thereby facilitating embryo implantation.

In vitro characterization of a human calcitonin receptor gene polymorphism

Calcitonin is a 32 amino acid peptide hormone that inhibits bone resorption by stimulating calcitonin receptors (CTR) located on the surfaces of osteoclasts. A polymorphism at nucleotide 1340 of the human calcitonin receptor gene (CALCR) lies within the coding region and has the potential to change the amino acid at codon 447 from leucine to proline. In the present study, we scanned the coding region, portions of the 5'-flanking and 3'-flanking sequences, and the intron-exon boundaries of the human CALCR gene for additional polymorphisms, and determined the frequency of the codon 447 polymorphism in several ethnic groups. Because a leucine to proline change has the potential for significant structural alteration, receptor genes encoding either leucine or proline at residue 447 were transiently expressed in COS-7 cells to determine the binding and functional consequences of this polymorphism. Our complete polymorphism scan of the CALCR gene identified 11 polymorphic sites in the gene and confirmed the presence of the previously identified nucleotide T1340C (codon 447) polymorphism. Ten of the 11 polymorphisms were single nucleotide polymorphisms (SNPs). For the codon 447 polymorphism, the prevalence of the TT genotype (leucine) was 59% in Caucasians, 27% in African-Americans, 0% in Asians, and 20% in Hispanics. The presence of this SNP appears to have no statistically significant difference with the receptor's ability to bind calcitonin or signal when activated with the hormone.

Organization of cytoskeletal F-actin, G-actin, and gelsolin in the adhesion structures in cultured osteoclast

Immunofluorescence using Gc protein (group-specific component or vitamin D binding protein [DBP]) as a marker of G-actin showed that nonfilamentous, monomeric G-actin is a component of the podosomes of osteoclasts cultured on glass plates or bone slices. Typical individual podosomes of the well-spread cells on glass plates were rosette in form. When viewed from the basolateral surface, the core portion of the dotlike podosomes was associated with packed F-actin filaments surrounded by G-actin organized in a ringlike structure. The podosomes, when viewed perpendicular to the substrate, showed a conical shape as a bundle of short F-actin core and a ring of G-actin. With cell spreading on glass plates, the clustering of the podosomes formed a continuous belt of tightly packed podosomes as an adhesion structure at the paramarginal area. In addition, these structures were seen on the ventral cell surface. Similar changes in cell shape were seen in the osteoclasts when they were plated on bone slices. With the loss of dotlike podosomes, a continuous band of F-actin was formed around the resorption lacunae. It became evident then that F- and G-actin dissociated from each other in the podosomes. The staining patterns of G-actin varied from a discrete dot to a diffuse one. Toward the nonresorption phase, the osteoclasts lost their continuous F-actin band but dotlike podosomes appeared in the leading and the trailing edges. In such a cell undergoing translational movements, G-actin was located diffusely in the cytoplasm behind the lamellipodia and along some segments of the leading edge. Cytochalasin B treatment caused cells to disorganize the actin cytoskeletal architecture, which indicated the disassembling of F-actin into G-actin in podosomes and disappearance of actin-ring of cultured osteoclasts. Staining with polyclonal actin antibody or monoclonal beta-actin was overlapped with the distribution pattern of G- and F-actin. Gelsolin was detected in the region of the adhesion area corresponding to the podosome. The observation that F-actin, G-actin, and gelsolin were detected in the osteoclastic adhesion structures suggests that the podosomes may represent sites where a rapid polymerization/depolymerization of actin occurs. These dynamic changes in cytoskeletal organization and reorganization of G-actin may reflect changes in the functional polarization of the osteoclast during the bone resorption cycle and suggest the important role of G-actin in the regulation of osteoclast adhesion.

Intracellular calcium puffs in osteoclasts

We studied intracellular calcium ([Ca(2+)](i)) in acid-secreting bone-attached osteoclasts, which produce a high-calcium acidic extracellular compartment. Acid secretion and [Ca(2+)](i) were followed using H(+)-restricted dyes and fura-2 or fluo-3. Whole cell calcium of acid-secreting osteoclasts was approximately 100 nM, similar to cells on inert substrate that do not secrete acid. However, measurements in restricted areas of the cell showed [Ca(2+)](i) transients to 500-1000 nM consistent with calcium puffs, transient (millisecond) localized calcium elevations reported in other cells. Spot measurements at 50-ms intervals indicated that puffs were typically less than 400 ms. Transients did not propagate in waves across the cell in scanning confocal measurements. Calcium puffs occurred mainly over regions of acid secretion as determined using lysotracker red DND99 and occurred at irregular periods averaging 5-15 s in acid secreting cells, but were rare in lysotracker-negative nonsecretory cells. The calmodulin antagonist trifluoperazine, cell-surface calcium transport inhibitors lanthanum or barium, and the endoplasmic reticulum ATPase inhibitor thapsigargin had variable acute effects on the mean [Ca(2+)](i) and puff frequency. However, none of these agents prevented calcium puff activity, suggesting that the mechanism producing the puffs is independent of these processes. We conclude that [Ca(2+)](i) transients in osteoclasts are increased in acid-secreting osteoclasts, and that the puffs occur mainly near the acid-transporting membrane. Cell membrane acid transport requires calcium, suggesting that calcium puffs function to maintain acid secretion. However, membrane H(+)-ATPase activity was insensitive to calcium in the 100 nM-1 microM range. Thus, any effects of calcium puffs on osteoclastic acid transport must be indirect.

Novel mechanisms of calcium handling by the osteoclast: A review-hypothesis

The osteoclast is a cell that is unique in its ability to resorb bone and, in doing so, becomes exposed to unusually high millimolar Ca2+ concentrations. It is generally accepted that, during resorption, osteoclasts can "sense" changes in their ambient Ca2+ concentration. This triggers a sharp cytosolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The change in cytosolic Ca2+ is transduced finally into inhibition of bone resorption. It has been shown that a type 2 ryanodine receptor isoform, expressed uniquely in the plasma membrane, functions as a Ca2+ influx channel and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily Ca2+ release channels that have a microsomal membrane location in a wide variety of eukaryotic cells, including the osteoclasts. However, only recently has it become obvious that ryanodine receptors are also expressed in osteoclast nuclear membranes, at which site they probably gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+ in turn regulates key nuclear processes, including gene expression and apoptosis. Here, we review the potential mechanisms underlying the recognition, movement, and effects of Ca2+ in the osteoclast. We will also speculate on the general biological significance of the unique processes used by the osteoclast to handle high Ca2+ loads during bone resorption.

Calcitonin inhibits phospholipase A2 and collagenase activity of human osteoarthritic chondrocytes

Calcitonin (CT) is a known potent inhibitor of bone resorption but its effect on cartilage enzymatic degradation has been incompletely studied. Salmon CT, at a concentration of 0, 0.1, 0.25, 0.5, 2.5 and 50 ng/ml, was added at 24 or 72 h to the culture medium of chondrocytes from human osteoarthritic hips and knees. The spontaneous collagenolytic activity, measured using a radiolabeled type II collagen, was inhibited by CT in a dose-dependent manner. However, CT had no effect on the total collagenolytic activity assayed after APMA activation. Stromelysin and plasmin activity, measured by degradation of casein and a synthetic substrate, were also unaffected by CT. Chondrocyte phospholipase A2 activity, assayed using a labeled specific substrate, was decreased by CT. Chondrocyte pre-incubation with CT significantly decreased the cell binding of labeled TNF alpha, but did not affect IL-1 beta cell binding. Attachment of chondrocytes on fibronectin was markedly stimulated by CT, while attachment to type II collagen was not. Significant effects were obtained using at least 2 or 5 ng/ml of CT. CT appears to decrease collagenolytic activity by decreasing its activation and/or increasing its inhibition by tissue inhibitors of metalloproteinases (TIMP). CT might act on osteoarthritic chondrocyte activation via mechanisms such as phospholipase A2 activity, human necrosis factor-alpha or fibronectin receptor expression.

Enzymes in mineralizing systems: state of the art

The hallmark of biological mineralization is the precise regulation of mineral deposition in space and time. The cells which produce mineralized tissues are themselves controlled by developmental programs and hormonal signals which result in regulation of gene expression and modulation of protein function. These signals are transduced into changes in enzyme levels and/or activity. Upon activation, cellular enzymes then act to synthesize the organic matrix and process it extracellularly, utilize metabolic energy to transport ions from the blood to the matrix, and to initiate the mineralization cascade. The first enzyme activity described in mineralizing tissues was alkaline phosphatase and it is still the best characterized enzyme in the mineralization process. Yet, important questions about the role of this protein remain unanswered, and it continues to occupy a central focus in mineralized tissue investigation. Other phosphatases, including protein tyrosine phosphatases are important in regulating tyrosine kinase mediated signals. Investigators have now begun to look closely at several groups of kinases which are also important for proper mineralization. As peptide hormones are important modulators of mineralized tissues, protein kinase A has always been presumed to play a key role in phosphorylating intracellular proteins. There is also considerable interest in protein kinase C, as well as tyrosine kinases in mineralized tissue signal transduction. Another group of kinases important in mineralized tissues are the enzymes which phosphorylate the matrix phosphoproteins. Of these, casein kinase II appears to be involved in intracellular and extracellular protein phosphorylation. Several enzymes present in the premineralized matrix are thought to be significant in triggering mineralization. Alkaline phosphatase may act at this level, but new data also suggests that metalloproteases and gelatinases, by modifying or digesting matrix components, may be important in the initiation of calcification.

Carbonic anhydrase II gene transcript in cultured osteoclasts from neonatal rats: effect of calcitonin

Carbonic anhydrase II (CA II), an enzyme catalyzing the interconversion of CO2 and water to HCO3- and protons, has a key role in osteoclastic bone resorption, but little is known of the regulation of CA II gene expression by calcitonin. Analysis of mRNA in osteoclasts has been difficult because of the problems of obtaining sufficient number of purified osteoclasts from bone. In this study, however, we have investigated the regulation of CA II mRNA in rat osteoclasts and their putative mononuclear precursors by using in situ hybridization. We have found that the CA II gene is expressed at high levels in osteoclasts and what are probably their maturing mononuclear precursors. Measurement of CA II mRNA in cultured osteoclasts and their putative mononuclear precursor cells by cytophotometry provided evidence that calcitonin, a direct inhibitor of mammalian osteoclast activity, reduces the levels of CA II mRNA in a dose dependent manner; maximum reduction was observed at a concentration of 100 pM of calcitonin. In addition, calcitonin reduced the number of CA II mRNA-positive mononuclear precursor cells. The results also suggest that expression of the CA II gene is a feature of cells committed to the osteoclast lineage.

Tartrate resistant acid phosphatase activity in rat cultured osteoclasts is inhibited by a carboxyl terminal peptide (osteostatin) from parathyroid hormone-related protein

A carboxyl-terminal peptide sequence ("osteostatin") from parathyroid hormone related protein has been shown to have an inhibitory effect on osteoclastic bone resorption--an action opposite to its amino-terminal sequence. In this study, we proposed that inhibition of osteoclastic bone resorption by osteostatin was associated with reduction of tartrate resistant acid phosphatase (TRAcP) activity in osteoclasts. Our results have indicated that osteostatin reduced TRAcP activity in a dose dependent manner. This effect of osteostatin was both sensitive (half maximal effect approximately 5 x 10(-13) M) and potent (maximum inhibition approximately 50% of control). In the first 90 min of treatment, however, reduction of TRAcP activity was erratic but became persistent and progressive when the time course was extended. Moreover, throughout the experimental period the levels of TRAcP activity in the culture medium had fallen significantly. It appears that osteostatin has a biphasic effect on TRAcP activity, inhibiting its secretion and either suppressing its synthesis or increasing its degradation. In addition, osteostatin induced rapid cellular retraction of both human and rat cultured osteoclasts, which was morphologically distinct from that produced by calcitonin.

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.