Calcitonin and parathyroid-hormone stimulation of acid mucopolysaccharide synthesis in cultured chondrocytes isolated from growth cartilage

Supplementation of articular cartilage-derived chondroprogenitors with bone morphogenic protein-9 enhances chondrogenesis without affecting hypertrophy

INTRODUCTION

Chondroprogenitors (CPCs) have emerged as a promising cellular therapy for cartilage-related pathologies due to their inherent primed chondrogenic potential. Studies report that the addition of growth factors such as parathyroid hormone (PTH) and Bone Morphogenic Protein (BMP) enhance the chondroinducive potential in chondrocytes and mesenchymal stem cells. This study evaluated if supplementation of the standard culture medium for cell expansion with 1-34 PTH and BMP-9 would enhance the chondrogenic potential of CPCs and reduce their hypertrophic tendency.

METHODS

Human chondrocytes were isolated from patients undergoing total knee replacement for osteoarthritis (n = 3). Following fibronectin adhesion assay, passage 1 CPCs were divided and further expanded under three culture conditions (a) control, i.e., cells continued under standard culture conditions, (b) 1-34 PTH group, additional intermittent 6 h exposure with 1-34 PTH and (c) BMP-9 group, additional BMP-9 during culture expansion. All the groups were evaluated for population-doubling, cell cycle analysis, surface marker and gene expression for chondrogenesis, hypertrophy, multilineage differentiation and GAG (glycosaminoglycan)/DNA following chondrogenic differentiation.

RESULTS

Concerning growth kinetics, the BMP-9 group exhibited a significantly lower S-phase and population-doubling when compared to the other two groups. Qualitative analysis for chondrogenic potential (Alcian blue, Safranin O staining and Toluidine blue for GAG) revealed that the BMP-9 group exhibited the highest uptake. The BMP-9 group also showed significantly higher COL2A1 expression than the control group, with no change in the hypertrophy marker expression.

CONCLUSION

BMP-9 can potentially be used as an additive for CPCs expansion, to enhance their chondrogenic potential without affecting their low hypertrophic tendency. The mitigating effects of 1-34PTH on hypertrophy would benefit further investigation when used in combination with BMP-9 to enhance chondrogenesis whilst reducing hypertrophy.

Intermittent parathyroid hormone (1-34) supplementation of bone marrow stromal cell cultures may inhibit hypertrophy, but at the expense of chondrogenesis

Background

Bone marrow stromal cells (BMSC) have promise in cartilage tissue engineering, but for their potential to be fully realised, the propensity to undergo hypertrophy must be mitigated. The literature contains diverging reports on the effect of parathyroid hormone (PTH) on BMSC differentiation. Cartilage tissue models can be heterogeneous, confounding efforts to improve media formulations.

Methods

Herein, we use a novel microwell platform (the Microwell-mesh) to manufacture hundreds of small-diameter homogeneous micro-pellets and use this high-resolution assay to quantify the influence of constant or intermittent PTH(1–34) medium supplementation on BMSC chondrogenesis and hypertrophy. Micro-pellets were manufactured from 5000 BMSC each and cultured in standard chondrogenic media supplemented with (1) no PTH, (2) intermittent PTH, or (3) constant PTH.

Results

Relative to control chondrogenic cultures, BMSC micro-pellets exposed to intermittent PTH had reduced hypertrophic gene expression following 1 week of culture, but this was accompanied by a loss in chondrogenesis by the second week of culture. Constant PTH treatment was detrimental to chondrogenic culture.

Conclusions

This study provides further clarity on the role of PTH on chondrogenic differentiation in vitro and suggests that while PTH may mitigate BMSC hypertrophy, it does so at the expense of chondrogenesis.

Role of PTHrP(1-34) Pulse Frequency Versus Pulse Duration to Enhance Mesenchymal Stromal Cell Chondrogenesis

Generation of phenotypically stable, articular chondrocytes from mesenchymal stromal cells (MSCs) is still an unaccomplished task, with formation of abundant, hyaline extracellular matrix, and avoidance of hypertrophy being prime challenges. We recently demonstrated that parathyroid hormone-related protein (PTHrP) is a promising factor to direct chondrogenesis of MSCs towards an articular phenotype, since intermittent PTHrP application stimulated cartilage matrix production and reduced undesired hypertrophy. We here investigated the role of frequency, pulse duration, total exposure time, and underlying mechanisms in order to unlock the full potential of PTHrP actions. Human MSC subjected to in vitro chondrogenesis for six weeks were exposed to 2.5 nM PTHrP(1-34) pulses from days 7 to 42. Application frequency was increased from three times weekly (3 × 6 h/week) to daily maintaining either the duration of individual pulses (6 h/day) or total exposure time (18 h/week; 2.6 h/day). Daily PTHrP treatment significantly increased extracellular matrix deposition regardless of pulse duration and suppressed alkaline-phosphatase activity by 87%. High total exposure time significantly reduced cell proliferation at day 14. Pulse duration was critically important to significantly reduce IHH expression, but irrelevant for PTHrP-induced suppression of the hypertrophic markers MEF2C and IBSP. COL10A1, RUNX2, and MMP13 expression remained unaltered. Decreased IGFBP-2, -3, and -6 expression suggested modulated IGF-I availability in PTHrP groups, while drop of SOX9 protein levels during the PTHrP-pulse may delay chondroblast formation and hypertrophy. Overall, the significantly optimized timing of PTHrP-pulses demonstrated a vast potential to enhance chondrogenesis of MSC and suppress hypertrophy possibly via superior balancing of IGF- and SOX9-related mechanisms. J. Cell. Physiol. 231: 2673-2681, 2016. © 2016 Wiley Periodicals, Inc.

Calcitonin delays the progress of early-stage mechanically induced osteoarthritis. In vivo, prospective study

BACKGROUND/RATIONALE

Introducing new or testing existing drugs in an attempt to modify the progress of osteoarthritis (OA) is of paramount importance.

QUESTIONS/PURPOSES

This study aims to determine the effect exerted by Calcitonin on the progress of early-stage osteoarthritic lesions.

METHODS

We used 18, skeletally mature, white, female, New Zealand rabbits. OA was operatively induced in the right knee of each animal by the complete dissection of the anterior cruciate ligament, complete medial meniscectomy and partial dissection of the medial collateral ligament. Postoperatively, animals were divided into two groups. Starting on the ninth postoperative day and daily thereafter, group A animals (n = 9) received 10 IU oculus dexter (o.d.) of synthetic Calcitonin IntraMuscularly (I.M.); group B animals (n = 9) received equal volume of saline o.d. Three animals from each group were sacrificed at 1, 2 and 3 months following treatment's initiation. The extent and the grade of OA were assessed macroscopically, histologically and by radiographs, Computed Tomography (CT) and Magnetic Resonance Imaging (MRI)-scans. The Osteoarthritis Research Society International (OARSI) score, incorporating histological and macroscopic information, was calculated for each knee.

RESULTS

Osteoarthritic changes in group A animals were less severe and progressed less rapidly when compared with those of group B animals (sham). This difference was statistically significant in the first and second month (P = 0.05), but not in the third month (P = 0.513).

CONCLUSIONS

I.M. administration of Calcitonin seems to delay the progress of early-stage osteoarthritic lesions induced by mechanical instability in a rabbit experimental model.

Human parathyroid hormone fragment stimulates the de novo synthesis of prostaglandin endoperoxide synthase in chick calvaria

The human parathyroid hormone N-terminal fragment [hPTH-(1–34)] increases the conversion of exogenous unsaturated fatty acids to prostaglandins (PGs) in calvarial homogenates. Enzyme activities were completely blocked by indomethacin (5 × 10⁻⁷ M), a PG synthase inhibitor, and actinomycin D (5 μM), an inhibitor of transcription, by binding to DNA. In addition, a potent inhibitor of protein synthesis, cycloheximide (10 μM), totally inhibited the stimulating effect of hPTH-(1–34) on prostaglandin endoperoxide synthase (PG synthase, EC 1.14.99.1). The stimulatory effect of hPTH-(1–34) on PG synthase was also reduced by the addition of stannous chloride. However, epidermal growth factor (EGF), platelet-derived activating factor (PDGF), and ionophore A23187 did not show the same stimulating effect as hPTH-(1–34) on PG synthase in calvaria. The results further demonstrated that PG synthase is a membrane-bound enzyme in chick calvaria. In this communication, evidence is presented that hPTH-(1–34) stimulates the de novo synthesis of PG synthase as demonstrated by the increased activity in calvarial homogenates and microsomes.

Identification of the calcitonin receptor in osteoarthritic chondrocytes

Background

Preclinical and clinical studies have shown that salmon calcitonin has cartilage protective effects in joint degenerative diseases, such as osteoarthritis (OA). However, the presence of the calcitonin receptor (CTR) in articular cartilage chondrocytes is yet to be identified. In this study, we sought to further investigate the expression of the CTR in naïve human OA articular chondrocytes to gain further confirmation of the existents of the CTR in articular cartilage.

Methods

Total RNA was purified from primary chondrocytes from articular cartilage biopsies from four OA patients undergoing total knee replacement. High quality cDNA was produced using a dedicated reverse transcription polymerase chain reaction (RT-PCR) protocol. From this a nested PCR assay amplifying the full coding region of the CTR mRNA was completed. Western blotting and immunohistochemistry were used to characterize CTR protein on protein level in chondrocytes.

Results

The full coding transcript of the CTR isoform 2 was identified in all four individuals. DNA sequencing revealed a number of allelic variants of the gene including two potentially novel polymorphisms: a frame shift mutation, +473del, producing a shorter form of the receptor protein, and a single nucleotide polymorphism in the 3' non coding region of the transcript, +1443 C>T. A 53 kDa protein band, consistent with non-glycosylated CTR isoform 2, was detected in chondrocytes with a similar size to that expressed in osteoclasts. Moreover the CTR was identified in the plasma membrane and the chondrocyte lacuna of both primary chondrocytes and OA cartilage section.

Conclusions

Human OA articular cartilage chondrocytes do indeed express the CTR, which makes the articular a pharmacological target of salmon calcitonin. In addition, the results support previous findings suggesting that calcitonin has a direct anabolic effect on articular cartilage.

Investigation of the direct effects of salmon calcitonin on human osteoarthritic chondrocytes

BACKGROUND

Calcitonin has been demonstrated to have chondroprotective effects under pre-clinical settings. It is debated whether this effect is mediated through subchondral-bone, directly on cartilage or both in combination. We investigated possible direct effects of salmon calcitonin on proteoglycans and collagen-type-II synthesis in osteoarthritic (OA) cartilage.

METHODS

Human OA cartilage explants were cultured with salmon calcitonin [100 pM-100 nM]. Direct effects of calcitonin on articular cartilage were evaluated by 1) measurement of proteoglycan synthesis by incorporation of radioactive labeled 35SO4 [5 microCi] 2) quantification of collagen-type-II formation by pro-peptides of collagen type II (PIINP) ELISA, 3) QPCR expression of the calcitonin receptor in OA chondrocytes using four individual primer pairs, 4) activation of the cAMP signaling pathway by EIA and, 5) investigations of metabolic activity by AlamarBlue.

RESULTS

QPCR analysis and subsequent sequencing confirmed expression of the calcitonin receptor in human chondrocytes. All doses of salmon calcitonin significantly elevated cAMP levels (P < 0.01 and P < 0.001). Calcitonin significantly and concentration-dependently [100 pM-100 nM] induced proteoglycan synthesis measured by radioactive 35SO4 incorporation, with a 96% maximal induction at 10 nM (P < 0.001) corresponding to an 80% induction of 100 ng/ml IGF, (P < 0.05). In alignment with calcitonin treatments [100 pM-100 nM] resulted in 35% (P < 0.01) increased PIINP levels.

CONCLUSION

Calcitonin treatment increased proteoglycan and collagen synthesis in human OA cartilage. In addition to its well-established effect on subchondral bone, calcitonin may prove beneficial to the management of joint diseases through direct effects on chondrocytes.

Effects of leptin to cultured growth plate chondrocytes

OBJECTIVE

This study aimed to evaluate whether leptin has any effect on growth plate chondrocytes.

METHODS

We studied the effects of exogenous leptin on cultured rabbit growth plate chondrocytes. This involved assessing [3H]thymidine incorporation, alkaline phosphatase (ALP) activity, proteoglycan production, leptin receptor (Ob-R) activity, and detection of Ob-R using Western blot analysis.

RESULTS

The existence of Ob-R in growth plate chondrocytes was revealed by Western blot and Ob-R activity. Prior to semiconfluence, leptin increased [3H]thymidine incorporation while at the semiconfluent and early confluent stages, leptin promoted ALP activity and tended to promote proteoglycan production.

CONCLUSION

Growth plate chondrocytes possess Ob-Rs, and leptin enhance chondrocyte proliferation and subsequent cell differentiation.

Rapid and long-term effects of PTH(1-34) on growth plate chondrocytes are mediated through two different pathways in a cell-maturation-dependent manner

The aims of this study were to clarify the role of cell maturation stage on chondrocyte response to parathyroid hormone (PTH) by examining the effect of PTH(1-34) on alkaline-phosphatase-specific activity (ALPase) of chondrocyte cultures at two distinct stages of maturation, and to determine the signaling pathways used by the cells to mediate this effect. Confluent, fourth passage rat costochondral resting zone (RC) and growth zone (GC) chondrocytes were used. ALPase was measured in the cell layer, as well as in matrix vesicles (MV) and plasma membranes (PM), after the addition of 10(-7) 10(-11) mol/L bovine PTH(1-34), the active peptide, or bovine PTH(3-34), the inactive peptide, to the cultures. PTH(1-34) increased ALPase in the GC cultures at two separate times: between 5 and 180 min, with maximal stimulation at 10 min, and 36 to 48 h. In contrast, PTH(3-34) had no effect. At 10 min and 48 h, PTH(1-34) produced a dose-dependent increase in ALPase of both MV and PM isolated from GC cultures. Addition of forskolin and IBMX to increase cAMP increased ALPase in GC cultures to a level similar to that seen after addition of PTH(1-34). In contrast, the addition of PTH(1-34) to RC cells only increased ALPase between 5 and 60 min, with peak activity at 10 min. As with GC, PTH increased ALPase in both MV and PM. Moreover, the addition of PTH(3-34) or forskolin and IBMX had no effect on ALPase in RC. PTH(1-34) had no effect on GC protein kinase C (PKC) activity; however, the addition of PTH(1-34) to RC caused a dose-dependent increase in PKC activity. H8, an inhibitor of PKA, had no effect on PTH-stimulated ALPase in RC cells, but inhibited the PTH-dependent response in GC cells. In contrast, chelerythrine, an inhibitor of PKC activity, inhibited PTH-stimulated ALPase in RC cells, but had no effect on PTH-stimulated ALPase in GC cells. This study shows that the effect of PTH(1-34) on RC and GC cells is maturation dependent in terms of time course and mechanism. Whereas both cell types exhibit a rapid response to PTH, only GC cells show a long-term response. In GC, the effects of PTH are associated with changes in cAMP and may also involve at least one other pathway, whereas, in RC, the PTH effects appear to be associated with changes in PKC.

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.

Primary cultured chondrocytes of different origins respond differently to bFGF and TGF-beta

Responses of rib and ear chondrocytes to basic fibroblast growth factor (bFGF) and transforming growth factor-beta (TGF-beta) were investigated using high density primary culture isolated from the same rabbit. Degrees of tritiated thymidine, leucine, and proline incorporation were used as indicators of DNA, protein and collagen syntheses, respectively. 10 ng/ml bFGF increased thymidine, proline, and leucine incorporation into rib, but not ear, chondrocytes. 1 ng/ml TGF-beta enhanced thymidine incorporation into both chondrocytes but did not affect proline or leucine incorporation into the ear cells. When both growth factors were added simultaneously, both cells showed rises in syntheses of DNA, protein and collagen. Incorporation of [35S]sulfate, used as indicator of proteoglycan synthesis, was elevated by TGF-beta but was reduced by bFGF especially in the rib cells. This inhibitory effect of bFGF was not reversed by cotreatment with TGF-beta in both cell types. Thus, the origin and cellular differentiation states of chondrocytes seem to cause different responses to these growth factors.

Surface adhesion-mediated regulation of chondrocyte-specific gene expression in the nontransformed RCJ 3.1C5.18 rat chondrocyte cell line

Recent evidence suggests that decreased chondrocyte function in osteoarthritis and other articular disorders may be due to chondrocyte dedifferentiation produced by altered regulatory signals from the cartilage extracellular matrix (ECM). However, there are currently no mammalian chondrocytic cell line systems adapted to the study of this process. We therefore examined the effects of ECM growth conditions on markers of differentiated chondrocytic phenotype expression in the nontransformed rat RCJ 3.1C5.18 (RCJ) chondrocyte cell line, including type II collagen expression, aggrecan production, link protein gene expression, and parathyroid hormone (PTH) receptor number. RCJ cells grown in monolayer on plastic exhibited a dedifferentiated phenotype characterized by flattened cell morphology, with > 80% type I collagen and < 5% type II collagen production, as determined by two-dimensional gel mapping electrophoresis of collagen cyanogen bromide peptides. In addition, aggrecan production was low, and link protein mRNA was not expressed at detectable levels. After transfer to growth under minimal attachment conditions on the surface of a composite type I collagen/agarose (0.15%-0.8%) gel (CAG) for 7 days, RCJ cells developed a rounded, chondrocytic morphology and a pattern of differentiated, chondrocytic gene expression, with 79% type II and 8% type I collagen production. Steady-state type I and type II procollagen mRNA levels were altered in parallel with collagen protein expression. In cells grown on CAG, aggrecan production increased 6-fold, and there was a marked increase in both aggrecan core protein and link protein mRNA levels. In addition, maximal PTH-stimulated cAMP generation increased 15-fold in association with an increased PTH receptor number. Therefore, the RCJ chondrocyte cell line is highly sensitive to ECM regulation of chondrocyte-specific gene expression.

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.

Effects of X-ray irradiation on terminal differentiation and cartilage matrix calcification of rabbit growth plate chondrocytes in culture

The retardation of long bone growth caused by irradiation is thought to be closely related to the impairment of growth plate function, but its mechanism remains unclear. In this study, we examined the effects of irradiation on the terminal differentiation of growth plate chondrocytes and on calcification. Chondrocytes were isolated from the growth plate of the ribs of four-week-old rabbits and inoculated at a high density on a type-I collagen-coated dish. Following logarithmic proliferation, they reached confluence (premature chondrocytes), then matured (mature chondrocytes), and became hypertrophied (hypertrophic chondrocytes). 10 Gy or less irradiation of the premature chondrocytes potently inhibited the terminal differentiation and matrix mineralization. Irradiation inhibited chondrocyte hypertrophy and suppressed alkaline phosphatase induction and the expression of type-X collagen without changing the protein composition profile of any other cell layer. Premature cells had the highest radiosensitivity. The sensitivity was decreased as the cells differentiated; the effects of irradiation on hypertrophic chondrocytes with terminal differentiation-related phenotypes were reduced. This study showed that 10 Gy or less irradiation of growth plate chondrocytes impaired terminal differentiation and mineralization. Since chondroclasts and bone marrow cells invade only to the mineralized cartilage, the induction of calcification in cartilage matrices is one of the most important steps in endochondral ossification. Therefore, it is conceivable that the damage in the growth plate induced by irradiation could account for the subsequent abnormal bone and skeletal growth.

Roles of cartilage matrix proteins, chondromodulin-I and -II, in endochondral bone formation: a review

Insulin induces early chondrogenesis in cultures of a clonal cell-line, ATDC5, which was derived from mouse embryonal carcinoma line AT805. Cartilage-generated matrix components chondromodulin-I (ChM-I) synergistically stimulates growth and differentiation of chondrocytes in the presence or absence of FGF-2. In contrast, ChM-I inhibits the proliferation of vascular endothelial cells and tube formation, thereby further stimulating cartilage growth and inhibiting replacing cartilage by bone in an early stage. Another cartilage-derived chondromodulin-II (ChM-II) also stimulates cartilage growth. However, ChM-II does not inhibit vascularization but stimulates osteoclast differentiation. Therefore, endochondral bone formation is regulated sequentially by cartilage-derived multiple autocrine factors. This opens a new mechanism of regulation of endochondral bone formation.

Regulation of expression of the chondrocytic phenotype in a skeletal cell line (CFK2) in vitro

We have examined in vitro the spontaneous and regulated expression of phenotypic characteristics associated with differentiated chondrocytes in an established skeletal cell line (CFK2) derived from fetal rat calvariae. Extended culture of CFK2 cells resulted in the appearance of glycosaminoglycans and type II collagen in the cell layer in association with the formation of focal nodes of cells. In addition, induction of mRNA-encoding link protein, cartilage-specific proteoglycan core protein, and thrombospondin was observed in the differentiated population (dCFK2 cells). The expression of these mRNAs was present for at least two passages after subculturing the dCFK2 cells. The dCFK2 cells also demonstrated enhanced parathyroid hormone (PTH)-stimulated adenylate cyclase activity. Proliferation of CFK2 cells was stimulated by the peptide regulatory factors EGF and PTH and inhibited by the steroidal agents dexamethasone and retinoic acid. EGF and retinoic acid inhibited the formation of cell foci and glycosaminoglycan deposition and the expression of mRNA-encoding link protein. In contrast, PTH and dexamethasone enhanced the formation of focal cellular nodes and augmented matrix deposition and link protein mRNA expression. These studies therefore show that the CFK2 cell line can serve as a nontransformed model of rat chondrocytic cells in which both induction and regulation of the expression of cartilaginous matrix components can be observed. This line thereby provides a unique renewable source of chondrocytic precursor cells and an excellent in vitro model for evaluating temporal and environmental control of chondrocyte differentiation and cartilage matrix production.

Comparison of plasma and synovial concentrations of synthetic salmon calcitonin after a single intravenous dose

The plasma and synovial fluid concentrations of synthetic salmon calcitonin in 10 patients with knee joint effusions have been compared after a single i.v. dose of 200 I.U. calcitonin. Plasma and synovial fluid concentrations of calcitonin were measured using a specific RIA before and 30 and 60 min after administration. Calcitonin was not detectable at zero time in plasma or in synovial fluid. Plasma calcitonin concentrations 60 min after administration were significantly lower than at 30 min, while the synovial fluid concentration remained relatively constant. The results show that synthetic salmon calcitonin penetrates into the articular cavity after a single i.v. dose of 200 I.U. and that a steady concentration persists there over 60 min.

Mitogenic and dentin-inductive effects of crude bone morphogenetic protein from bone and dentin in primary adult pulp cell culture

The effects of crude bone morphogenetic protein (BMP) derived from bone and dentin matrix on proliferation, production of extracellular matrix, and biologic function of the pulp cell were examined in the primary cell culture from permanent dental pulp. BMP from bone and from dentin matrix stimulated iodine 125-deoxyuridine incorporation in the absence of 10% calf serum. They increased sulfur 35-sulfate incorporation in proliferating stage and had no effects in stationary stage of culture. Alkaline phosphatase activities were inhibited in proliferating, stationary, and multilayered stages of culture. Osteocalcin synthesis was increased in culture treated with BMP from day 2 to day 10. These findings suggest that crude BMP might have mitogenic activity and some role in regulation of differentiation of pulp cells into odontoblasts.

The effects of growth factors on DNA synthesis, proteoglycan synthesis and alkaline phosphatase activity in bovine dental pulp cells

Platelet-derived growth factor (PDGF), insulin-like growth factor-I and -II (IGF-I and -II), acidic fibroblast growth factor (aFGF), basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) stimulated [125I]-deoxyuridine incorporation about 13, 6.2-, 4.6-, 3.8-, 3.1- and 1.2-fold, respectively, above control values at a concentration of 50 ng/ml. Transforming growth factor-beta (TGF-beta) decreased incorporation about 30% at the same dose. aFGF, IGF-I, IGF-II, bFGF and TGF-beta increased [35S]-sulphate incorporation 231, 71, 64, 42 and 39%, respectively, in proliferating cells, while EGF, IGF-I, TGF-beta and PDGF decreased incorporation about 30%, and aFGF increased incorporation 80% in stationary-stage culture. TGF-beta, PDGF, aFGF and bFGF caused 65-40% inhibition of alkaline phosphatase activity in proliferating and stationary cultures. These findings suggest that the proliferation of pulp cells may be stimulated mainly by PDGF and IGF-I, and the production of extracellular matrix proteoglycan may be enhanced by aFGF, IGF-I and IGF-II. Furthermore, TGF-beta, PDGF, aFGF and bFGF may regulate the differentiation of pulp cells into odontoblasts.

Effects of various growth factors on a chondrocyte differentiation model

We developed a new 4-dimensional differentiation model of growth-cartilage cells (temporally and spatially regulated). composed of a high-density suspension culture of rabbit costal chondrocytes. In this system, fibroblast growth factor (FGF) or transforming growth factor-beta (TGF beta) stimulated proliferation and matrix synthesis, but reversibly inhibited terminal differentiation-induction of alkaline phosphatase (Alpase) activity, type X collagen synthesis and subsequent calcification. When parathyroid hormone (PTH) was added to the differentiation model, both the induction of Alpase and calcium uptake were reversibly suppressed. In contrast, calcitonin (CT) stimulated Alpase activity and calcium uptake dose-dependently. Bone morphogenetic protein (BMP) strongly stimulated DNA synthesis of chondrocytes in the presence of FGF and induced rapid maturation of chondrocytes at a growing stage. Moreover, BMP stimulated Alpase activity in multilayered chondrocytes at the calcifying stage.

Bone morphogenetic proteins (BMP-2 and BMP-3) promote growth and expression of the differentiated phenotype of rabbit chondrocytes and osteoblastic MC3T3-E1 cells in vitro

We studied the effects of highly purified bone morphogenetic protein 2 and 3 (BMP-2 and -3) on growth plate chondrocytes and osteoblastic cells in vitro and compared to TGF-beta. A mixture of BMP-2 and 3 (BMPs) strongly stimulated DNA synthesis of chondrocytes in the presence of fibroblast growth factor (FGF). BMPs induced rapid maturation of chondrocytes at a growing stage: BMPs transformed the cells into rounded cells and induced marked accumulation of cartilage matrix; TGF-beta slightly reduced matrix accumulation and changed cell morphology into spindle-like in the presence of FGF. Moreover, exposure of chondrocytes to BMPs resulted in a dramatic increase of the putative approximately 80 kD PTH receptors expressed on the cell surface. In multilayered chondrocytes at the calcifying stage, BMPs stimulated alkaline phosphatase (ALPase) activity but TGF-beta inhibited it. In osteoblastic MC3T3-E1 cells, BMPs were found to be the most potent stimulator of ALPase activity thus far described: ALPase in the cells treated with approximately 100 ng/ml of BMPs reached 5- to 20-fold over the basal, whereas TGF-beta inhibited expression of ALPase activity in these cells. The stimulatory action of BMPs overrode the inhibition of ALPase activity by TGF-beta when the cells were incubated with TGF-beta and BMPs. BMPs also upregulated expression of the approximately 80 kD PTH receptor on the cells. These results suggest that BMPs have unique biologic activities in vitro that lead to growth and phenotypic expression of cells playing a critical role in endochondral bone formation.

Chondroprotective action of salmon calcitonin in experimental arthropathies

To assess whether calcitonin exerts an influence on cartilage, three models of arthropathies in rabbits--representing three different modes of cartilage destruction--were used: (1) corticosteroid administration (endocrinological disturbances model); (2) meniscectomy (mechanical stress model); and (3) immobilization of the hind leg (nutritional disorder model). After 12 weeks of methylprednisolone (MP) administration, the rabbit femur heads displayed cartilage erosions, marked decrease of glycosaminoglycans (GAG) content, and narrowing of joint spaces. Elevation of serum uronic acid, activity of alkaline phosphatase, and calmodulin content was evident. All these changes were minimal--close to normal--in the group treated for 12 weeks with MP + salmon calcitonin (sCT). Partial meniscectomy and hind leg immobilization caused statistically significant loss of GAG from the cartilage and narrowing of the knee joint space during the same experimental period, 12 weeks. In both these models the groups of rabbits treated simultaneously with sCT showed only insignificantly smaller joint spaces and GAG content. These results support our hypothesis of a chondroprotective property of calcitonin. However, the mechanism through which calcitonin influences joint cartilage remains unknown. A direct effect of calcitonin on cultivated chondrocytes, as well as the role of calmodulin, beta-endorphins, calcium, and interleukin-1 in the process are discussed.

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Differential modulation of growth and phenotypic expression of chondrocytes in sparse and confluent cultures by growth factors in cartilage

The growth-promoting actions of cartilage extracts (CE) on rabbit cultured chondrocytes were studied to assess the role of local acting growth factors in the generation and expansion of highly differentiated cells. In the present study, DNA synthesis and proteoglycan synthesis in the cultured chondrocytes were monitored by flow cytofluorometry and double-isotope autoradiography by using [3H]thymidine and [35S]sulfate. We report here that actions of the same set of growth factors extracted from cartilage evokes differential cellular responses depending upon cell density. Growth factors in the optimal dose of CE (2 micrograms/ml) or epidermal growth factor (EGF, 40 ng/ml) did not reveal such a cell density-dependent effect on cellular proliferation. However, growth factors in CE induced proteoglycan synthesis selectively in nonproliferating and expressing cells in confluent culture.

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.

Potent mitogenic effects of parathyroid hormone (PTH) on embryonic chick and rabbit chondrocytes. Differential effects of age on growth, proteoglycan, and cyclic AMP responses of chondrocytes to PTH

The effect of PTH on chondrocyte proliferation as a function of cartilage age was examined. PTH[1-34] induced a 12- to 15-fold increase in the efficiency of colony formation in soft agar by chondrocytes from embryonic 13- to 19-d-old chickens and fetal 25-d-old rabbits with a 10-fold increase in their DNA content. It also caused a 2.5-fold increase in [3H]thymidine incorporation into DNA in fetal 25-d-old rabbit chondrocytes. No mitogenic responses to PTH were observed, however, in postnatal 7- to 21-d-old chick chondrocytes or postnatal 21-d-old rabbit chondrocytes. This age dependency was observed only with PTH: fibroblast growth factor, epidermal growth factor, and insulin stimulated chondrocyte proliferation irrespective of cartilage age. The absence of a mitogenic effect in postnatal chondrocytes was not due to a decrease in number or a reduction in affinity of receptors for PTH. PTH also increased [35S]sulfate incorporation into proteoglycans and the cyclic AMP level in fetal and postnatal chondrocytes, but at 100-fold higher concentrations (10(-8)-10(-7) M) than those (10(-10)-10(-9) M) required for the stimulation of cell division. These results suggest that PTH is a potent mitogen for embryonic chondrocytes, and that its mitogenic effect disappears selectively after birth.

Mechanism of action of parathyroid hormone-induced proteoglycan synthesis in the growth plate chondrocyte

In the growth plate chondrocyte, parathyroid hormone (PTH) stimulates phosphoinositol 4,5 bisphosphate (PIP2) degradation, which results in the rapid production of inositol (1,4,5) triphosphate (IP3). IP3 induced the release of calcium from an intracellular store, which caused a rapid increase in the cytosolic ionized calcium concentration. Parathyroid hormone also induced a 30-50% increase in proteoglycan synthesis. Phorbol esters, which pharmacologically activate protein kinase C, resulted in a 70-80% increase in proteoglycan synthesis. Treatment of the chondrocytes with retinoic acid (0.2 microM) inhibited the parathyroid hormone and phorbol ester-induced increase in intracellular ionized calcium and the increase in proteoglycan synthesis. From this data we postulate that the stimulation of proteoglycan synthesis in growth plate chondrocytes by PTH is mediated by the breakdown of membrane phosphoinositides, which results in the production of IP3 and an increase in ionized intracellular calcium. It is suggested that the degradation of membrane phosphoinositides also results in production of diacylglycerol and, thereby, an activation of protein kinase C, which has a large stimulatory effect on proteoglycan synthesis. The increase in cytosolic calcium most likely acts synergetically with diacylglycerol to activate protein kinase C. Retinoic acid blocks the effect of PTH and phorbol ester-induced proteoglycan synthesis and may act through the inhibition of protein kinase C. The overall effect of PTH on the growth plate chondrocyte appears to be a stimulation of proteoglycan synthesis that is mediated by the degradation products of membrane phosphoinositides.

Demonstration of receptors for parathyroid hormone on cultured rabbit costal chondrocytes

Parathyroid hormone (PTH) receptors on cultured rabbit costal chondrocytes were demonstrated using HPLC-purified, radioiodinated [Nle8,-Nle18, Tyr34] bovine PTH-(1-34)amide. PTH binding was found to be specific for PTH agonists and antagonists and dependent on the time and temperature of incubation. Both growth cartilage (GC) cells and resting cartilage (RC) cells were shown to have a single class of saturable, high affinity PTH binding sites with a dissociation constant of 0.6-0.7 nM. However, the numbers of receptors per cell were approximately 49,000 on GC cells and 19,000 on RC cells. After crosslinking the receptors on these cells with the radioligand, one, major 125I-labeled band of 76 kDa was separated by SDS-PAGE.

Rat epiphyseal cells in culture: responsiveness to bone-seeking hormones

Cell cultures derived from young rat epiphyseal cartilage were grown for approximately 2 wk in BGJb medium supplemented with 10% fetal bovine serum to reach confluence. These cells were identified as chondrocytes as checked by morphology, the presence of alkaline phosphatase, and a positive type II collagen antibody reaction. The cells also responded to different hormonal treatment. Parathyroid hormone (PTH) increased cyclic AMP production by 50% within 15 min of treatment, whereas prostaglandin E2 (PGE2) caused an increase of 160%. Calcitonin (CT) did not affect cAMP production in these cells. DNA synthesis 24 h after hormonal treatment was increased by PTH (2.5-fold) and PGE2 (2-fold), but not by CT. Among the vitamin D metabolites, 24,25(OH)2D3 increased significantly the [3H]thymidine incorporation into DNA, whereas 1,25(OH)2D3 effect was minimal. These results provide evidence for the use of these cell cultures as a model for cartilage in vitro when studying biological and hormonal responsiveness.

Effect of transforming growth factor beta on cell proliferation and glycosaminoglycan synthesis by rabbit growth-plate chondrocytes in culture

The effects of the transforming growth factor beta (TGF-beta) on the growth and glycosaminoglycan synthesis of rabbit growth plate-chondrocytes in culture were studied. In serum-free medium, TGF-beta caused dose-dependent inhibition of DNA synthesis by chondrocytes, measured as [3H]thymidine incorporation (ED50 = 0.1-0.3 ng/ml). The inhibitory effect was maximal at a dose of 1 ng/ml, and extended for a duration of 16-42 h. In contrast, TGF-beta potentiated the synthesis of DNA stimulated by fetal calf serum (FCS). Addition of TGF-beta (1 ng/ml) to cultures containing 10% FCS increased [3H]thymidine incorporation to 1.6-times that in cultures with 10% FCS alone. Consistent with this finding, TGF-beta potentiated DNA synthesis stimulated by the purified growth factors such as platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factor (FGF). The maximal stimulation of DNA synthesis by FGF (0.4 ng/ml) was further potentiated dose dependently by TGF-beta (ED50 = 0.1 ng/ml, maximum at 1 ng/ml). When the cultures were treated with the optimal concentrations of TGF-beta (1 ng/ml) and FGF (0.4 ng/ml), [3H]thymidine incorporation was 3-times higher than that of cultures treated with FGF alone. This TGF-beta-induced potentiation of DNA synthesis was associated with replication of chondrocytes, as shown by a marked increase in the amount of DNA during treatment of sparse cultures of the cells with the growth factors for 5 days. In contrast, TGF-beta caused dose-dependent stimulation of glycosaminoglycan synthesis in confluent cultures of growth-plate chondrocytes (ED50 = 0.3 ng/ml, maximum at 1 ng/ml). This stimulatory effect of TGF-beta was greater than that of insulin-like growth factor I (IGF-I) or PDGF. Furthermore, TGF-beta stimulated glycosaminoglycan synthesis additively with IGF-I or PDGF. Recently, it has been suggested that bone and articular cartilage are rich sources of TGF-beta, whereas epiphyseal growth cartilage is not. Thus, the present data indicate that TGF-beta may be important in bone formation by modulating growth and phenotypic expression of chondrocytes in the growth plate, possibly via a paracrine mechanism.

Osteogenic capacity of cultured human periosteal cells

We developed a culture system of cells isolated from juvenile human periosteum. The culture consisted of epithelial-like and fibroblast-like cells. Both types of cells had intense alkaline phosphatase activity maintained in subculture. When these cells, loaded into diffusion chambers, were implanted subcutaneously in rats, cartilage tissue was mainly formed and bone was seen scantily. 1 alpha-OH-D3 and 1,25-(OH)2-D3 increased the alkaline phosphatase activity and proteoglycan synthesis in the periosteal cells. Calcitonin also stimulated the proteoglycan synthesis, but parathyroid hormone had no effect.

Hydrocortisone stimulation of proliferation and glycosaminoglycan synthesis in rabbit craniofacial chondrocytes in vitro

Hydrocortisone stimulated glycosaminoglycan (GAG) synthesis, a cartilage phenotype, in chondrocytes from mandibular condylar cartilage (MCC), nasal septal cartilage (NSC) and sphenooccipital synchondrosis (SOS). These stimulations were dose- and time-dependent, being maximal 27 h after addition of 10(-7) M hydrocortisone. The maximal induced increase of GAG synthesis was about 100%, 50% and 20% that of non-stimulated MCC, SOS and NSC chondrocytes, respectively. When stained with toluidine blue, all three types of cortisone-treated chondrocytes showed stronger metachromasia than non-treated controls. DNA synthesis was also increased by hydrocortisone, reaching a maximum 20 h after the addition; stimulation was also dose-dependent and maximal at a concentration of 10(-6) M. The maximal increase in DNA synthesis was 200% in NSC chondrocytes, 90% in SOS chondrocytes, and slight in MCC chondrocytes. However, there was no stimulation of DNA synthesis in serum-free medium, in contrast to that of GAG synthesis. These observations suggest that hydrocortisone regulates craniofacial growth by controlling the differentiation of these chondrocytes directly and their proliferation indirectly, and that the difference in their responses to hydrocortisone may reflect different responses in vivo.

The effect of mechanical stress on cultured growth cartilage cells

Mechanical stress seems to influence the growth of bone, but little is known about the transduction of mechanical forces into biochemical signals. The present study was done to resolve the important question of how a mechanical stimulus is transduced into a metabolic response by chondrocytes. Cultured growth cartilage cells isolated from rat ribs and subjected to mechanical stress (tensile force) showed a significant increase in intracellular cyclic AMP levels, but no increase in prostaglandin E2 levels. In glycosaminoglycan synthesis measured by radioactive sulfate incorporation, long-term exposure of chondrocytes to mechanical loading caused increased synthesis. Moreover, long-term exposure of chondrocytes exerted an alteration of responsiveness to parathyroid hormone and calcitonin measured as intracellular cyclic AMP content. These results suggest that mechanical stress can alter bone growth by modulating the metabolism of growth cartilage cells.

Enhanced responsiveness to parathyroid hormone and induction of functional differentiation of cultured rabbit costal chondrocytes by a pulsed electromagnetic field

Pulsed electromagnetic fields promote healing of delayed united and ununited fractures by triggering a series of events in fibrocartilage. We examined the effects of a pulsed electromagnetic field (recurrent bursts, 15.4 Hz, of shorter pulses of an average of 2 gauss) on rabbit costal chondrocytes in culture. A pulsed electromagnetic field slightly reduced the intracellular cyclic adenosine 3',5'-monophosphate (cAMP) level in the culture. However, it significantly enhanced cAMP accumulation in response to parathyroid hormone (PTH) to 140% of that induced by PTH in its absence, while it did not affect cAMP accumulation in response to prostaglandin E1 or prostaglandin I2. The effect on cAMP accumulation in response to PTH became evident after exposure of the cultures to the pulsed electromagnetic field for 48 h, and was dependent upon the field strength. cAMP accumulation in response to PTH is followed by induction of ornithine decarboxylase, a good marker of differentiated chondrocytes, after PTH treatment for 4 h. Consistent with the enhanced cAMP accumulation, ornithine decarboxylase activity induced by PTH was also increased by the pulsed electromagnetic field to 170% of that in cells not exposed to a pulsed electromagnetic field. Furthermore, stimulation of glycosaminoglycan synthesis, a differentiated phenotype, in response to PTH was significantly enhanced by a pulsed electromagnetic field. Thus, a pulsed electromagnetic field enhanced a series of events in rabbit costal chondrocytes in response to PTH. These findings show that exposure of chondrocytes to a pulsed electromagnetic field resulted in functional differentiation of the cells.

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.

Stimulation of DNA synthesis in quiescent rabbit chondrocytes in culture by limited exposure to somatomedin-like growth factors

Cartilage-derived factor (CDF), extracted from fetal bovine cartilage, and multiplication-stimulating activity (MSA) stimulated DNA synthesis in quiescent rabbit costal chondrocytes in culture under serum-free conditions. As described previously, when added in the presence of fibroblast growth factor (FGF) or epidermal growth factor (EGF) a somatomedin-like growth factor, CDF or MSA, synergistically stimulated DNA synthesis in the cultured chondrocytes. The present study showed that exposure of the cells to MSA or CDF for only the initial 5 h was sufficient for transmission of their full stimulatory effect. Furthermore, the limited exposure did not alter the time course of stimulation of DNA synthesis: [3H]thymidine incorporation into DNA began to increase after 16 h and reached a maximum after 24 h. In contrast to the somatomedin-like growth factors, FGF and EGF were required continuously in the culture medium during traverse of the entire G1 phase for stimulation of DNA synthesis, and the mitogenic effects of FGF and EGF in cultured chondrocytes were stronger than those of CDF and MSA. Synergistic stimulation of DNA synthesis by CDF or MSA in the presence of FGF or EGF could be observed as long as FGF or EGF was continuously present, even when CDF or MSA was withdrawn after the first 5 h of culture. These findings suggest that, in contrast to FGF and EGF, somatomedin-like growth factors affect an early distinct stage in the G1 phase of chondrocytes.

Parathyroid hormone stimulates proliferation of chondroprogenitor cells in vitro

Mandibular condylar explants of newborn ICR mice were maintained as serum-free organ culture systems and were used to study the effects of 0.1-10.0U/ml parathyroid hormone (PTH) on the morphology of the organ and the ultrastructure of the chondroprogenitor cells. Parameters of proliferation such as 3H-thymidine autoradiography and incorporation into the explants were also studied. The chondroprogenitoric zone gradually increased with increasing dosages of the hormone up to a maximum of 5-fold of the control with 5.0 U/ml PTH. Autoradiographic studies showed a 3-fold increase in the number of 3H-thymidine-labeled cells in the chondroprogenitoric zone of PTH-treated explants. This was matched by a dose-dependent stimulation of 3H-thymidine incorporation, reaching maximal values at 5.0 U/ml PHT. At this concentration, the stimulated incorporation of 3H-thymidine was found to be dependent on the Ca2+ concentration of the medium. Chondroprogenitor cells located adjacent to the chondroblastic zone tended to pile up and aggregate in "syncytium"-like clusters, establishing intercellular gap junctions. All PTH-treated chondroprogenitor cells demonstrated large deposits of glycogen and highly elaborated stacks of their Golgi systems; the latter were associated with large numbers of vesicular elements. On the other hand, the chondroblastic zone was significantly reduced in size. Hence, it seems that PTH possesses a rather intense mitogenic effect upon chondroprogenitor cells and might possibly interfere with their normal pattern of differentiation into mature cartilage cells.

Study of the localization of [3H]bovine parathyroid hormone in bone by light microscope autoradiography

Bovine parathyroid hormone labeled with tritium on the methionine residues by [3H]methyl exchange ([3H]bPTH) was administered intravenously to 35-day-old mice and localized in tissues by light microscope autoradiography. After 10 min most of the [3H]bPTH had been taken up from plasma. In the kidney and liver, radioactivity was not displaced by simultaneous administration of unlabeled bPTH, and was as intense after giving oxidized [3H]bPTH ([3H]ox bPTH) as [3H]bPTH, suggesting that binding was largely associated with nonspecific processes. In long bones, [3H]bPTH was bound to osteoblasts and preosteoblasts lining the endosteal and periosteal surfaces of compact bone. In the area of the growth-plate there was intense labeling on new endochondrial bone, and on hypertrophic chondrocytes in the region of calcification. There was little labeling in marrow and, in contrast to other tissues, much reduced binding was seen when a large excess of unlabeled parathyroid hormone was administered together with [3H]bPTH, or when [3H]ox bPTH was given. It is concluded that binding of parathyroid hormone to cells in bone is largely specific, and that the hormone has a function in cartilage which relates to the differentiation and calcification of chondrocytes of the growth-plate that occurs prior to its replacement by new bone tissue.

Differential effects of parathyroid hormone and somatomedin-like growth factors on the sizes of proteoglycan monomers and their synthesis in rabbit costal chondrocytes in culture

In the proteoglycans extracted from rabbit costal chondrocytes in culture, two populations of proteoglycans were distinguished by density gradient centrifugation under dissociative conditions. The major component was the faster sedimenting population (proteoglycan I), the putative 'cartilage-specific' proteoglycans, and the minor component was the slower sedimenting population (proteoglycan II). The monomeric size of proteoglycan I was closely related to the differentiation-state of chondrocytes and was a good marker of the differentiated chondrocytes. Treatment of the cultures with parathyroid hormone (PTH) induced an increase in the monomeric size of proteoglycan I. This increase was ascribed to an increase in the molecular size of the glycosaminoglycan chain in proteoglycan I. On the other hand, somatomedin-like growth factors, such as multiplication-stimulating activity (MSA) and cartilage-derived factor (CDF), did not affect the size of proteoglycan I, while they markedly stimulated the synthesis of proteoglycan I. In contrast, treatment with nonsomatomedin growth factors, such as fibroblast growth factor (FGF) and epidermal growth factor (EGF), resulted in not only a decrease in glycosaminoglycan synthesis but also a slight decrease in size of proteoglycan I. However, synthesis and size of proteoglycan II were little affected by these agents. Thus, the present study clearly shows that PTH and somatomedin-like growth factors have differential functions in bringing about the expression of the differentiated phenotype of chondrocytes: PTH influences chain elongation and termination of glycosaminoglycans in proteoglycan I, while somatomedin-like growth factors affect primarily the synthesis and secretion of proteoglycan I.

Cytoskeleton and differentiation: effects of cytochalasin B and colchicine on expression of the differentiated phenotype of rabbit costal chondrocytes in culture

Cytochalasin B changed the shape of cultured rabbit costal chondrocytes from polygonal to nearly spherical and stimulated glycosaminoglycan synthesis, which is a differentiated phenotype of chondrocytes, whereas colchicine changed them from polygonal to flattened and inhibited glycosaminoglycan synthesis. These morphological changes occurred parallel with the changes in glycosaminoglycan synthesis. Induction of ornithine decarboxylase by parathyroid hormone, which is a good marker of differentiated chondrocytes, was markedly potentiated in the spherical cells which had been pretreated with cytochalasin B, whereas pretreatment with colchicine inhibited the induction of the enzyme. Both cytochalasin B and colchicine inhibited DNA synthesis. The inhibitions were observed after the appearance of changes in the morphology of the cells and glycosaminoglycan synthesis. These findings suggest that intactness of microtubules and disruption of microfilaments are involved in regulating the expression of the differentiated phenotype of chondrocytes in culture.

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.