Effects of calcitonin and parathyroid hormone on the metabolism of chondrocytes in culture

Analyses on the mechanisms that underlie the chondroprotective properties of calcitonin

INTRODUCTION

Calcitonin (CT) has recently been shown to display chondroprotective effects. Here, we investigate the putative mechanisms by which CT delivers these actions.

METHODS

Immortalized C-28/I2 cells or primary adult human articular chondrocytes (AHAC) were cultured in high-density micromasses to investigate: (i) CT anabolic effects using qPCR and immuhistochemistry analysis; (ii) CT anti-apoptotic effects using quantitation of Bax/Bcl gene products ratio, TUNEL assay and caspase-3 expression; (iii) CT effects on CREB, COL2A1 and NFAT transcription factors.

RESULTS

CT (10(-10)-10(-8)nM) induced significant up-regulation of cartilage phenotypic markers (SOX9, COL2A1 and ACAN), with down-regulation of catabolic (MMP1 and MMP13 and ADAMTS5) gene products both in resting and inflammatory conditions. This was mirrored by an augmented production of type II collagen and accumulation of glycosaminoglycan- and proteoglycan-rich extracellular matrix in vitro. Mechanistic analyses revealed only partial involvement of cyclic AMP formation in these effects of CT. Congruently, using reporter assays for specific transcription factors, there was no indication for CREB activation, whereas the COL2A1 promoter was genuinely and directly activated by cell exposure to CT. Phenotypically, these mechanisms supported the ability of CT, whilst inactive on its own, to counteract the pro-apoptotic effects of IL-1β, demonstrated by TUNEL-positive staining of chondrocytes and ratio of BAX/BCL genes products.

CONCLUSION

These data may provide a novel lead for the development of CT-based chondroprotective strategies that rely on the engagement of mechanisms that lead to augmented chondrocyte anabolism and inhibited chondrocyte apoptosis.

Evidence that human cartilage and chondrocytes do not express calcitonin receptor

OBJECTIVE

Calcitonin (CT) has been recently shown to exhibit direct protective effects on articular cartilage against joint degenerative disease. It has been proposed that CT might act via the CT receptor (CTR) to activate the cyclic AMP (cAMP) pathway and protect type II collagen degradation. In this study, we investigated the existence of CTR in human articular cartilage and chondrocytes, and examined the potential pharmacological effects and transduction pathway of salmon CT (sCT) in human chondrocytes.

METHODS

Five human articular cartilage samples were examined for the expression of the CTR by polymerase chain reaction (PCR), immunostaining and Western blot analysis. cAMP levels in human chondrocyte stimulated with sCT were assessed by ELISA. The effect of sCT on the gene expression profiles, including aggrecan, type II collagen, MMP-1, MMP-3 and MMP-13, of human chondrocytes was also examined by relative quantitative Real-time PCR.

RESULTS

We failed to detect the CTR at both the transcriptional and protein levels in human chondrocytes and cartilage tissue by PCR, immunostaining and Western blotting. cAMP levels were significantly elevated in human chondrocytes by forskolin (100muM) to more than 10-fold (P<0.001), however, were not induced by sCT (10(-7)M, 10(-8)M, 10(-9)M). Real-time PCR analysis demonstrated that sCT slightly reduced the gene expression of MMPs, although this effect was not statistically significant.

CONCLUSION

In contrary to previous reports, our data indicate that human cartilage and chondrocytes do not express CTR. Furthermore, sCT does not appear to have direct effects on human chondrocytes. We propose that the chondroprotective effect of CT observed in vivo may be indirect via its impact on subchondral bone resorptive activity of osteoclasts.

Collagen and proteoglycan production by bovine fetal and adult chondrocytes under low levels of calcium and zinc ions

The experiments described herein tested the effects of CaCl2 and ZnCl2, added at various concentrations in the culture medium, upon the synthesis of collagen and proteoglycan by adult and fetal (articular, epiphyseal and hypertrophic) bovine chondrocytes maintained in high density multilayer cultures. CaCl2 concentrations below 0.5 mM or the addition of 1-50 microM ZnCl2 to the medium selectively promoted the production of collagen by all four populations of chondrocytes but had no effect on fibroblasts. Further, these changes had no statistically significant effect on the incorporation of 35S-sulfate into macromolecules or on the synthesis of gelatinase A, measured by gelatin zymography. The addition of CaCl2 and ZnCl2 at these concentrations did not result in a change in the relative proportion of non-crosslinked 3H-collagen molecules (synthesized in the presence of beta-aminopropionitrile) partitioning in the cell layer and medium compartments, and did not appreciably alter the pattern of collagens synthesized by any of the cell populations. The hypertrophic cells synthesized high levels of collagen type X in the presence as well as absence of exogenously added cations. However, CaCl2 at 10 mM caused a marked upregulation of collagen type X synthesis by a preparation of chondrocytes derived from the entire growth plate, consistent with the view that calcium at that concentration stimulated the differentiation of some of the cells into hypertrophic chondrocytes.

Chondrogenic potential of chick embryonic calvaria: II. Matrix calcium may repress cartilage differentiation

Chick embryos cultured in the absence of their eggshell are rendered severely calcium-deficient, and develop a cartilage-like phenotype in the calvarium, a normally osteogenic tissue. In the preceding paper (Jacenko and Tuan [1995] Dev. Dyn. 202:13-26), experiments using organ cultured calvaria from day-12 normal and shell-less embryos showed that depletion of calcium alone may be responsible in promoting chondrogenic differentiation in calvaria. Here these findings were extended using an in vivo calvarial grafting technique, such that the extent of calvarial matrix calcification was a function of the calcium status of both the graft and the host. In these calvarial grafts, undermineralized regions again were shown to support chondrogenesis. To identify possible mechanisms which promote chondrogenesis in the calvaria, cells were enzymatically dissociated from the calvaria and cultured in media with varied levels of soluble calcium, under conditions which should modulate cell-to-cell interactions, including monolayer, micromass, agarose gels, and suspension cultures. Soluble calcium had no effect on calvarial cell differentiation, whereas conditions which enhanced cell-cell interactions, e.g., suspension culture, elicited cartilage expression. Based on these findings, we propose that the calcified matrix of the calvarium is repressive to chondrogenesis during normal development, but that the lack of mineral in a calcium-deficient calvarium creates a microenvironment permissive for cell-to-cell interactions which lead to chondrogenic differentiation.

Effects of thapsigargin, an intracellular calcium-mobilizing agent, on synthesis and secretion of cartilage collagen and proteoglycan

The calcium-mobilizing agents thapsigargin and 2,5-di-(tert-butyl)-1,4- benzohydroquinone were shown to markedly elevate the intracellular calcium concentration of chick embryo chondrocytes in a dose-dependent manner. Under these conditions, the metabolism of macromolecules was variably affected. The synthesis and secretion of protein in general, and of collagen in particular, were significantly inhibited; in contrast, proteoglycan synthesis (but not glycosaminoglycan synthesis) was inhibited, whereas secretion was unaffected. Flunarizine, which prevented the thapsigargin-induced intracellular calcium elevation, and EGTA, which caused only a transient thapsigargin-induced intracellular calcium elevation, did not reverse these alterations. It was concluded, therefore, that the observed effects of thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone on chondrocyte macromolecule metabolism were not related to the ability of these drugs to increase the cytosolic free calcium concentration but may have been due to the specific depletion of the calcium sequestered in the endoplasmic reticulum. The differential effect of these drugs on protein and proteoglycan secretion suggests that the intracellular trafficking of these two classes of macromolecules may be controlled independently.

Differential effects of parathyroid hormone on chick growth plate and articular chondrocytes

Parathyroid hormone (PTH) binds specifically to the hypertrophic region of growth plate cartilage [16]. This specific binding suggests a role for this hormone in chondrocyte maturation. Enzymatically isolated chick articular and growth plate chondrocytes grown in monolayer culture were used to assay the direct effects of PTH on chondrocytes. The articular chondrocytes were unresponsive to PTH. The growth plate chondrocytes, however, demonstrated a marked mitogenic response to PTH, with a 39-fold increase of [3H]-thymidine incorporation into DNA. PTH also affected matrix production by the growth plate chondrocytes causing a twofold stimulation of proteoglycan synthesis as determined by the rate of 35SO4 incorporated into matrix macromolecules. Furthermore, PTH depressed collagen synthesis as measured by [3H]-proline incorporation. PTH caused a 12-fold increase in intracellular cAMP in growth plate chondrocytes but no increase in the articular cells. This specificity of PTH for growth plate chondrocytes suggests a possible regulatory role in enchondral ossification.

Cytosolic free calcium concentrations in avian growth plate chondrocytes

Isolated avian growth plate chondrocytes convert the acetoxymethyl ester (AM) form of Fura-2 quickly and efficiently to the Ca2(+)-sensitive pentacarboxylic acid (FA) form. Control experiments indicate that the Kd for intracellular Fura-2/FA is very close to that of extracellular Fura-2/FA at the same ionic strength and pH and that the Fura-2/FA fluorescence from indicator converted by intracellular organelles is quite small. Correcting for the effects of extracellular Fura-2/FA and partial hydrolysis products has improved the accuracy of determination of intracellular [Ca2+] over earlier measurements in chondrocytes. Cytosolic [Ca2+] in isolated growth plate chondrocytes (containing cells from each maturational stage) is found to require approximately 9 hours to recover from the isolation process. After this recovery period, cytosolic [Ca2+] in these cells converges to approximately 70 nM regardless of the [Ca2+] of the recovery medium, suggesting regulation of cytosolic [Ca2+] to a set point. Chondrocytes that are separated into maturationally distinct fractions using countercurrent centrifugal elutriation show an increase in cytosolic [Ca2+] with cellular maturation. The least mature resting cells have a [Ca2+] near 57 nM, while the most mature hypertrophic cells are around 95 nM.

Stimulation of adult chondrocyte metabolism by a thyroid-derived factor

This paper reports the effects of adding partially purified bovine thyroid calcitonin, thyrocalcitonin, to adult bovine articular cartilage cells. Thyrocalcitonin stimulated chondrocyte proliferation fourfold under low serum (0.5%) culture conditions. In serum-free medium, thyrocalcitonin stimulated cell proliferation more than twofold. With high-density cultures in serum-free medium, chondrocyte glycosaminoglycan (GAG) synthesis was stimulated 60% by thyrocalcitonin. Cell-associated radioactivity was increased twofold. In contrast to thyrocalcitonin, addition of human and salmon calcitonin peptides as well as the thyroid hormones T3 and T4 had no effect on adult cartilage cell proliferation or GAG synthesis. The data reported here suggest the existence of a thyroid-derived factor, independent of calcitonin peptides or thyroid hormones, which stimulates adult articular chondrocyte metabolism.

Effects of parathyroid hormone on odontogenesis of the mouse embryonic molar tooth in vitro

Mandibular first molars of 17-day-old mouse embryos were cultured in vitro to examine the histological effects of various concentrations of parathyroid hormone (PTH) on odontogenesis of the molars. PTH did not affect the cytodifferentiation of mesenchymal cells into preodontoblasts but inhibited that of preodontoblasts into odontoblasts. Consequently, the odontoblasts failed to undergo dentinogenesis. On the other hand, inner enamel epithelium achieved terminal cytodifferentiation into secretory ameloblasts and these cells partially formed enamel in spite of the absence of dentin. All treated molars showed the same histological disturbances and these effects were independent of PTH dose. The present study indicated that PTH had an influence on mesenchyme-derived cells, inhibiting both the differentiation of odontoblasts and the formation of predentin and dentin.

Hormone-responsive cells derived from human dental papilla: characterization in vitro and in vivo in diffusion chambers

Cells of the dental papilla are capable of odontoblastic, fibroblastic, and endothelial differentiation and formation of dentin and the dental pulp. In the present study dental papilla cells, obtained from human tooth buds (HDP cells), were cultured in vitro through 3 to 7 passages. After exposure to prostaglandin E2 there was a marked decrease in intracellular cyclic AMP (cAMP) levels as compared to hormone-free controls. Parathyroid hormone and calcitonin had stimulatory effects with 1 and 2 log increases in cAMP, respectively. The HDP cells showed moderate activity of alkaline phosphatase, 1 log higher than that of hamster kidney fibroblasts (BHK 13) and 1 log lower than that of osteoblastic osteosarcoma cells (ROS 17/2). When cultured for 4 or 8 wk in diffusion chambers (DC) implanted in athymic mice, many of the HDP cells underwent odontoblastic morphodifferentiation with very long, single processes extending into the matrix. This matrix contained banded and unbanded collagen fibers. Neither light nor electron microscopy of the DC content revealed mineral deposits. These results suggest that HDP cells have an intrinsic potential for partial odontoblastic differentiation; inductive signals like those originating from odontogenic epithelium are probably essential for the completion of hard tissue formation.

The morphology and hormonal responsiveness of developing skeletal elements in chick limb buds

While parathyroid hormone (PTH), calcitonin (CT), and certain prostaglandins (PGs) are known to regulate the metabolism of both osteogenic and osteolytic cells of the adult skeleton through an adenosine 3', 5'-monophosphate-dependent mechanism, little is known about the development of this hormonally mediated response in embryonic skeletal tissues. In the present study, the responsiveness of embryonic skeletal elements to PTH and PGE2 was examined during various stages of development utilizing cAMP concentrations as an indicator of hormone-receptor interaction. The cytology of the limb skeletal system was examined also at each stage tested in order to compare the differentiated cellular phenotypes with their hormonal responsiveness. Prior to differentiation of cartilaginous elements in developing limb buds (stage 20-21), cells were responsive to PGE2 and epinephrine (EPI) but not to PTH. The first consistent response to PTH occurred coincident with the initial differentiation of the cartilage phenotype in limb buds (stage 24-25). A responsiveness to both PTH and PGE2 was progressively increased as maturation of cartilaginous and osteogenic elements occurred (stage 26-35). The initial response to CT was detected within cartilage rods in which osteogenic cells had differentiated (stage 33-35). The results of this study indicate that PGE2-sensitive cells exist within the developing limb prior to cytodifferentiation. The development of PTH responsiveness within embryonic chick limb buds is correlated with the onset of both chondrogenesis and osteogenesis in vivo.

Ultrastructure of the effects of calcitonin on the development of mouse tooth germs in vitro

Mandibular first molars, from 17-day-old embryos, were cultivated in control medium or medium containing 0.1, 0.01 or 0.001 unit/ml of calcitonin (CT) for periods up to 10 days. In untreated tooth germs, cells of the dental papilla differentiated into pre-odontoblasts up to 4 days and predentine was seen on day 6. Cells treated with 0.1 unit/ml of CT differentiated into pre-odontoblasts up to 4 days, but no predentine was formed even after 10 days in culture. With 0.01 unit/ml, cells differentiated into odontoblasts, and had already secreted predentine a few days earlier than the untreated group. With 0.001 unit/ml, the developing germs were similar to the control explants during the entire 10-day cultivation period. The proportional area of rough endoplasmic reticulum to cytoplasm of the odontoblasts was low at 0.1 unit/ml of CT and high at 0.01 unit/ml compared to the untreated explants.

Effects of parathyroid hormone and cyclic AMP analogues on the activity of ornithine decarboxylase and expression of the differentiated phenotype of chondrocytes in culture

Parathyroid hormone (PTH) greatly increased the level of adenosine 3', 5' cyclic monophosphate (cAMP) in rabbit costal chondrocytes in culture 2 minutes after its addition. PTH, as well as N6 O2' dibutyryl adenosine 3', 5' cyclic monophosphate (DBcAMP) and 8 Bromo adenosine 3', 5' cyclic monophosphate (8 Br-cAMP) induced ornithine decarboxylase (ODC; L-ornithine carboxylyase; EC 4.1.1.17), which reached a maximum 4 hours after their addition. Neither cAMP, N6 O2' dibutyryl guanosine 3', 5' cyclic monophosphate (DBcGMP), nor sodium butyrate increased the activity of the enzyme. PTH had no effect on DNA synthesis, while DBcAMP and 8 Br-cAMP decreased DNA synthesis. Expression of the differentiated phenotype of chondrocytes in culture was also induced by PTH, DBcAMP, and 8 Br-cAMP, but not by cAMP, DBcGMP, or sodium butyrate, as judged by morphological change. Glycosaminoglycan synthesis, a characteristic of the cartilage phenotype, began to increase 8 hours after addition of PTH or DBcAMP, reaching a plateau 32 hours after their addition. These findings suggest that PTH induces increase of ODC activity and expression of the differentiated phenotype of chondrocytes through increase of cAMP and that induction of OCD is closely related to expression of the differentiated phenotype of chondrocytes.

Polyamine and differentiation: induction of ornithine decarboxylase by parathyroid hormone is a good marker of differentiated chondrocytes

The activity of ornithine decarboxylase (OD-Case:L-ornithine carboxy-lyase, EC 4.1.1.17) in rabbit costal chondrocytes in culture increased markedly after addition of parathyroid hormone (PTH), reaching a maximum 4 to 5 hr after PTH addition. The increase in ODCase activity was followed by increase in the intracellular concentrations of polyamines, especially putrescine, which increased in 6 hr to about 3-fold that of untreated cultures. The induction of ODCase by PTH was not observed in L, 3T3, HeLa, buffalo rat liver, or BHK cells. Retinyl acetate and retinoic acid both inhibited expression of the differentiated phenotype of chondrocytes by rabbit costal chondrocytes in culture within 3 days after their addition, as judged by morphological change and decrease in sulfate incorporation into glycosaminoglycans but did not inhibit cell proliferation. PTH could not induce an increase in ODCase in de-differentiated cells that had been pretreated with retinyl acetate or retinoic acid for 3 days. but 4 days after removal of the retinoids, these de-differentiated cells regained the ability to synthesize ODCase in response to PTH. These facts suggest that the induction of ODCase and the formation of putrescine by PTH are good markers of the differentiated phenotype of cultured chondrocytes.