Identification of ecto-nucleoside triphosphate pyrophosphatase in human articular chondrocytes in monolayer culture

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

The Effect of Rusa unicolor Antler Deer Extracts from East Kalimantan in Bone Turnover Cell Models

Objectives

Osteoporosis is a condition characterized by skeletal degradation of osseous tissue resulting in an escalated chance of broken bones. Traditionally, Rusa unicolor horn from East Kalimantan is used to treat many diseases, including conditions associated with bone turnover. The aim of the present research was to analyze the effects of 70% ethanol and aqueous extracts of R. unicolor antler's horn from East Kalimantan on nitric oxide inhibition, osteoblast differentiation, and mineralization related to bone turnover.

Materials and Methods

Nitric oxide inhibition of the extracts in lipopolysaccharide-stimulated RAW 264.7 macrophages was evaluated by Griess reagent, while the effects of extracts on osteoblast differentiations were measured by alkaline phosphatase in p-nitrophenyl phosphate. Their effects on mineralization was determined using alizarin red staining.

Results

The 70% ethanol and aqueous extracts inhibited cell inflammation (40% and 80%, respectively) and stimulated osteoblast differentiation (65% and 52%, respectively). In the mineralization test, the aqueous extract showed an effect two times higher than that of 70% ethanol extract.

Conclusion

The extracts can effectively degrade inflammatory marker expression and preserve osteoblast functions.

Extracellular Nucleotides Selectively Induce Migration of Chondrocytes and Expression of Type II Collagen

The migration of chondrocytes from healthy to injured tissues is one of the most important challenges during cartilage repair. Additionally, maintenance of the chondrogenic phenotype remains another limitation, especially during monolayer culture in vitro. Using both the differentiated and undifferentiated chondrogenic ATDC5 cell line, we showed that extracellular nucleotides are able to increase the migration rate of chondrocytes without affecting their chondrogenic phenotype. We checked the potency of natural nucleotides (ATP, ADP, UTP, and UDP) as well as their stable phosphorothioate analogs, containing a sulfur atom in the place of one nonbridging oxygen atom in a phosphate group. We also detected P2y1, P2y2, P2y4, P2y6, P2y12, P2y13, and P2y14 mRNA transcripts for nucleotide receptors, demonstrating that P2y1 and P2y13 are highly upregulated in differentiated ATDC5 cells. We showed that ADPβS, UDPβS, and ADP are the best stimulators of migration of differentiated chondrocytes. Additionally, ADP and ADPβS positively affected the expression of type II collagen, a structural component of the cartilage matrix.

Articular chondrocytes express connexin 43 hemichannels and P2 receptors - a putative mechanoreceptor complex involving the primary cilium?

Mechanical loading is essential for the health and homeostasis of articular cartilage, although the fundamental mechanotransduction pathways are unclear. Previous studies have demonstrated that cyclic compression up-regulates proteoglycan synthesis via an intracellular Ca(2+) signalling pathway, mediated by the release of ATP. However, the mechanism(s) of ATP release has not been elucidated. The present study examines expression of the putative mechanosensitive ATP-release channel, connexin 43 and whether it is expressed on the chondrocyte primary cilium, which acts as a mechanosensor in a variety of other cell types. In addition the study characterized the expression of a range of purine receptors through which ATP may activate downstream signalling events controlling cell function. Bovine articular chondrocytes were isolated by sequential enzyme digestion and seeded in agarose constructs. To verify the presence of functional hemichannels, Lucifer yellow (LY) uptake into viable cells was quantified following treatment with a hemichannel agonist (EGTA) and antagonist (flufenamic acid). LY uptake was observed in 45% of chondrocytes, increasing to 83% following EGTA treatment (P < 0.001). Treatment with the hemichannel blocker, flufenamic acid, significantly decreased LY uptake to less than 5% with and without EGTA. Immunofluorescence and confocal microscopy confirmed the presence of primary cilia and the expression of connexin 43. Approximately 50% of bovine chondrocyte primary cilia were decorated with connexin 43. Human chondrocytes in situ within cartilage explants also expressed connexin 43 hemichannels. However, expression was confined to the upper 200 microm of the tissue closest to the articular surface. Immunofluorescence revealed the expression of a range of P2X and P2Y receptor subtypes within human articular cartilage. In conclusion, the expression of functional hemichannels by articular chondrocytes may represent the mechanism through which mechanical loading activates ATP release as part of a purinergic mechanotransduction pathway. Furthermore, the expression of connexin 43 on the chondrocyte primary cilium suggests the possible involvement of the cilium in this pathway.

ATP release by mechanically loaded porcine chondrons in pellet culture

OBJECTIVE

To determine whether ATP is released from chondrocytes during mechanical stimulation and whether degradation of ATP generates inorganic pyrophosphate in chondron pellet cultures.

METHODS

Chondron pellets were formed from 1.6 x 10(6) cells that had been enzymatically isolated from porcine articular cartilage. ATP was measured in media from cultures at rest and during fluid movement and cyclic compression. ATP hydrolysis was examined by high-performance liquid chromatography following the addition of gamma32P-ATP to resting cultures.

RESULTS

Pellet cultures at rest maintained a steady-state concentration of 2-4 nM ATP in 2 ml of medium. The ATP concentration increased 5-12-fold with cyclic compression (7.5 and 15 kPa at 0.5 Hz), then decreased to preloading levels within 60 minutes despite continued loading. A subsequent increase in pressure stimulated a further increase in ATP release, suggesting that chondrocytes desensitize to load. Cell viability was similar for pellets at rest and up to 24 hours after compression. ATP released in response to mechanical stimulation was inhibited 50% by 0.5 mM octanol, suggesting a regulated mechanism for ATP release. Exogenous ATP was rapidly hydrolyzed to pyrophosphate in resting cultures.

CONCLUSION

The occurrence of basal levels of extracellular ATP in the presence of pyrophosphohydrolase activity indicates that ATP was continuously released by chondrocytes at rest. Considering that chondrocytes express purinoceptors that respond to ATP, we suggest a role for ATP in extracellular signaling by chondrocytes in response to mechanical load. ATP released by chondrocytes in response to mechanical load is a likely source of pyrophosphate in calcium pyrophosphate dihydrate crystal deposition diseases.

Exclusion of the gene for human cartilage intermediate layer protein in currently mapped calcium pyrophosphate dihydrate deposition syndromes

OBJECTIVE

To map the gene for human cartilage intermediate layer protein (CILP) in order to assess its involvement in some familial forms of calcium pyrophosphate dihydrate (CPPD) deposition disease.

METHODS

A radiation hybrid panel was analyzed for chromosomal assignment of the CILP gene within a 1-cM limit of resolution. The location of the gene for CILP was confirmed to reside at the observed radiation hybrid locus by fluorescence in situ hybridization.

RESULTS

The human CILP gene resides at chromosome 15q21.

CONCLUSION

This map location definitively excludes mutations in the CILP gene as the cause of certain familial forms of CPPD deposition disease that have been genetically mapped to chromosomes 8q and 5p.

Proteoglycan breakdown from bovine nasal cartilage is increased, and from articular cartilage is decreased, by extracellular ATP

The addition of ATP, but not ADP or AMP, to the culture media of bovine nasal cartilage explants caused an acceleration in the rate of proteoglycan loss from the tissue. The ATP-stimulated loss of proteoglycan was not inhibited by the IL1-receptor antagonist protein, but was partially inhibited by the presence of ADP or AMP. The proteolytic events resulting from the presence of ATP were found to be similar to those following treatment with IL1, in that inhibitors of the cysteine-peptidase cathepsin B, serine-proteinases with trypsin-like specificity, and of some of the matrixins, could all prevent proteoglycan loss, which was mediated, at least in part, by the action of 'aggrecanase'. In contrast to its effects on nasal cartilage, ATP inhibited basal and stimulated proteoglycan release from articular cartilage. Both ADP and AMP had no effect on proteoglycan release in articular cartilage but enhanced the response to ATP when added concurrently. We conclude that extracellular ATP, probably acting via P2-purinoceptors, stimulates proteoglycan breakdown from bovine nasal cartilage and thus, may have a role in diseases which primarily involve destruction of non-articular cartilage. Extracellular ATP has, in contrast, a chondroprotective effect on bovine articular cartilage.

Molecular cloning and expression of a porcine chondrocyte nucleotide pyrophosphohydrolase

The porcine 127-kDa nucleotide pyrophosphohydrolase (NTPPHase) had been previously purified from the conditioned culture media of porcine articular cartilage. Protein sequencing of an internal 61-kDa proteolytic fragment of NTPPHase (61-kDa NTPPHase) determined the 26 N-terminal amino acids. This sequence was used to amplify a DNA fragment, which was used as a probe to clone the gene encoding the 61-kDa NTPPHase from a porcine chondrocyte cDNA library. DNA sequence analysis showed the cDNA insert to be 2509 bp, corresponding to a predicted open reading frame (ORF) encoding 599 amino acids. The 26 N-terminal amino acids of the 61-kDa NTPPHase were located within the ORF immediately downstream of a putative protease recognition region, RRKRR. This is consistent with this cDNA insert representing an internal proteolytic fragment of the full length 127-kDa NTPPHase. BLAST and FASTA analysis confirmed that the deduced amino acid sequence of 61-kDa NTPPHase was unique and did not possess a high degree of homology to sequence in the non-redundant protein and nucleotide databases. Proteins that possess limited homology (< 17%) with the 61-kDa NTTPPHase include several prokaryotic and eukaryotic ATP pyrophosphate-lyases (adenylate cyclase). Northern blot analysis of porcine chondrocyte RNA showed that the DNA encoding the 61-kDa NTPPHase hybridized to a single 4.0-kb RNA transcript. This DNA probe also hybridized to a single species of human chondrocyte RNA. Expression of a 61-kDa protein was detected by coupled in-vitro transcription/translation. Western blot analysis of this in-vitro transcription/translation reaction detected a 61-kDa protein, using an antibody raised against the peptide sequence that was originally used to clone the 61-kDa NTPPHase. These data indicate the successful in-vitro cloning and expression of the porcine chondrocyte 61-kDa NTPPHase. Future studies that utilize the gene encoding the 61-kDa NTPPHase may allow the characterization of the role of NTPPHase in calcium pyrophosphate dihydrate (CPPD) crystal deposition disease.

Calcium-mobilizing purine receptors on the surface of mammalian articular chondrocytes

If we are to fully understand mechanisms of cartilage homeostasis, it is essential that we know the full catalogue of receptors present on the surface of a chondrocyte and the pathways regulated by ligands that bind to these receptors. In this study, we describe chondrocyte responses to adenosine 5'-triphosphate and related molecules. Adenosine 5'-triphosphate stimulated a statistically significant, dose-dependent, transient rise in the concentration of calcium ions in Fura 2-loaded, differentiated, primary chondrocytes. The increase occurred in the absence of extracellular calcium, indicating a mobilization from intracellular stores. The increase in concentration of cytoplasmic calcium ions induced by adenosine 5'-triphosphate was mimicked by uridine 5'-triphosphate but not by 2-methylthioadenosine 5'-triphosphate, cytidine 5'-triphosphate, or adenosine. Heterologous desensitization experiments demonstrated that chondrocytes showed no subsequent response to uridine 5'-triphosphate after initial stimulation with adenosine 5'-triphosphate nor did they respond to adenosine 5'-triphosphate in inverse conditions, thereby indicating competition for the same receptor site. Together, these results are consistent with the presence of a P2U receptor on the cell surface of chondrocytes. Purine-induced calcium mobilization in passaged chondrocytes showed the same pharmacological profile with respect to agonist sensitivity, but responses were of greater magnitude than responses in primary differentiated chondrocytes, suggesting upregulation of the receptor with time in culture. Adenosine 5'-triphosphate and uridine 5'-triphosphate (1-100 microM) did not alter cartilage matrix synthesis as measured by rate of incorporation of [35S]sulfate into glycosaminoglycan by cartilage explants or primary chondrocytes. Matrix degradation, measured by release of glycosaminoglycan from cartilage explants, was also unaltered by adenosine 5'-triphosphate or uridine 5'-triphosphate (1-100 microM). Production of prostaglandin E2 was upregulated by incubation with either adenosine 5'-triphosphate or uridine 5'-triphosphate. These data demonstrate the presence of a functional P2U-like purine receptor on the surface of primary articular chondrocytes and support the hypothesis that altered concentrations of extracellular purines may influence chondrocyte metabolism.

Gap junctions mediate intercellular calcium signalling in cultured articular chondrocytes

Gap junction-mediated intercellular communication has been implicated in a variety of cellular functions. Among these, signal transduction can be coordinated among several cells due to gap junctional permeability to intracellular second messengers. Chondrocytes from articular cartilage in primary culture respond to extracellular ATP by rhythmically increasing their cytosolic Ca2+ concentration. Digital imaging fluorescence microscopy of Fura-2 loaded cells was used to monitor Ca2+ in confluent and semi-confluent cell layers. Under these conditions, Ca2+ spikes propagate from cell to cell giving rise to intercellular Ca2+ waves. The functional expression of gap junctions was assessed, in confluent chondrocyte cultures, by the intercellular transfer of Lucifer yellow dye in scrape-loading experiments. Intercellular dye transfer was blocked by the gap junction inhibitor 18 alpha-glycyrrhetinic acid. In imaging experiments, the inhibitor caused the loss of synchrony of ATP-induced Ca2+ oscillations, and blocked the intercellular Ca2+ propagation induced by mechanical stimulation of a single cell in a monolayer. It is concluded that gap junctions mediate intercellular signal transduction in cartilage cells and may provide a mechanism for co-ordinating their metabolic activity.

A unique ectonucleotide pyrophosphohydrolase associated with porcine chondrocyte-derived vesicles

Previous studies have shown increased nucleotide pyrophosphohydrolase (EC 3.6.1.8) (NTPPHase) activity in detergent extracts of degenerated human cartilage containing calcium pyrophosphate dihydrate (CPPD) crystals relative to those from osteoarthritis or normal cartilage. NTPPHase was later shown to be an ectoenzyme and its activity was increased in synovial fluid from patients with CPPD crystal deposits relative to fluids from other types of arthritis. We have purified a soluble 61-kD NTPPHase from conditioned media of organ-cultured porcine articular cartilage to electrophoretic homogeneity. Its NH2-terminal sequence through 26 cycles showed < 30% homology to any previously reported protein sequence. An antibody raised to a synthetic peptide corresponding to this sequence reacted with denatured but not native enzyme. This antibody reacted against a sedimentable vesicle-associated 127-kD protein in conditioned media from cultured articular cartilage or from chondrocytes in primary monolayer culture and against a series of soluble proteins in conditioned media supernatant, including a 61-kD protein representing our original isolate. No reactivity was found in 1% SDS extracts of washed cultured chondrocytes, although these contained greater NTPPHase activity than the conditioned media. Antibody to PC-1, another ectoNTPPHase, reacted with 1% SDS extracts of whole chondrocytes but not against those chromatographic fractions containing the major portion of NTPPHase activity. Release of the vesicle-associated 127-kD enzyme into conditioned medium was stimulated three- to sevenfold by TGF beta 1. The antibody also reacted with a series of soluble proteins and with 127-kD sedimentable protein in human synovial fluid. Kinetic studies supported the existence of a unique vesicle-associated NTPPHase; apparent Km (mM) of chondrocyte membrane NTPPHase was 1.5 and 3.0 at pH 7.3 and 9.88, respectively; apparent Km (mM) of vesicle associated NTPPHase was 0.83 and 1.28 at pH 7.3 and 9.88. The data suggest the existence of a unique ecto-NTPPHase associated with vesicles derived from normal articular cartilage.

Modulation of ecto-nucleoside triphosphate pyrophosphatase activity of human osteoblast-like bone cells by 1 alpha,25-dihydroxyvitamin D3, 24R,25-dihydroxyvitamin D3, parathyroid hormone, and dexamethasone

Extracellular inorganic pyrophosphate (PPi) is involved in the regulation of mineralization, and there is evidence that the cell surface enzyme, NTP pyrophosphatase, is a major source of this metabolite in bone. Osteotrophic agents that influence bone turnover may exert their effects, in part, by modulating the activity of ecto-NTP pyrophosphatase in bone cells. We investigated the effect of 1, 25(OH)2 D3, 24, 25(OH)2D3, dexamethasone, and parathyroid hormone (PTH) on the activity of this enzyme in cultured human trabecular bone-derived osteoblast-like cells. 1,25(OH)2D3 at 10(-11)-10(-9) M induced a dose- and time-dependent increase in activity (at 96 h; maximum 10(-9) M, p < 0.001), whereas higher concentrations (10(-8) and 10(-7) M) had no effect. In contrast, 24,25(OH)2D3 was effective only at 10(-8) and 10(-6) M (at 96 h; p < 0.01). Dexamethasone (10(-9)-10(-7) M) caused a dose-dependent decrease in ecto-NTP pyrophosphatase activity (10(-7) M, p < 0.001); concentrations higher than 10(-7) M did not evoke greater inhibition. This effect became apparent by 48 h and was significantly enhanced after 72 h. The response to dexamethasone was attenuated by cycloheximide, indicating a requirement for de novo protein synthesis. Interestingly, the stimulatory effect of 10(-9) M 1,25(OH)2D3 on ecto-NTP pyrophosphatase activity was significantly enhanced in the presence of dexamethasone (10(-9)-10(-7) M). Human PTH(1-34) and bovine PTH(1-34) in the range 10(-10)-10(-7) M had no effect on enzyme activity over a 72 h period.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the murine plasma cell nucleotide pyrophosphohydrolase PC-1 is shared by human liver, bone, and cartilage cells. Regulation of PC-1 expression in osteosarcoma cells by transforming growth factor-beta

A bone and cartilage enzyme with both 5'-nucleotide phosphodiesterase I and nucleotide pyrophosphohydrolase (NTPPPH) activity modulates physiologic mineralization and pathologic chondrocalcinosis by generating inorganic pyrophosphate. We hypothesized that, as for alkaline phosphatase, expression of an NTPPPH gene can be shared by cells from bone, cartilage, and liver and by certain leukocytes. Recently, we demonstrated the hepatocyte and murine plasma cell membrane glycoprotein PC-1 to have both 5'-nucleotide phosphodiesterase I and NTPPPH activity. We detected polypeptides cross-reactive with PC-1 in human U20S osteosarcoma cells, articular chondrocytes, homogenized human knee cartilages, human knee synovial fluids, hepatoma cells, and murine plasmacytoma cells. Constitutive low abundance PC-1 mRNA expression was detected in U20S cells and chondrocytes by a nested RNA-PCR assay and by Northern blotting. TGF beta is known to substantially increase NTPPPH activity in primary osteoblast cultures. We demonstrated that TGF beta 1 increased NTPPPH activity and the level of PC-1 mRNA and immunoprecipitable [35S]-methionine-labeled PC-1 polypeptides in U20S cells. The identification of PC-1 as an NTPPPH expressed in cells derived from bone and cartilage may prove useful in furthering the understanding of the role of NTPPPH i n physiologic and pathologic mineralization.

Transforming growth factor beta increases ecto-nucleoside triphosphate pyrophosphatase activity of human bone-derived cells

Inorganic pyrophosphate (PPi) may be involved in the regulation of mineralization. The cell surface enzyme, ecto-NTP pyrophosphatase, could be a major source of extracellular PPi in bone, and agents that influence its activity in osteoblasts may modulate bone mineralization. We studied the effects of serum on the ecto-NTP pyrophosphatase activity of cultured human osteoblast-like cells. Enzyme activity was lowered when the concentration of fetal calf serum (FCS) was reduced from 10 to 2.5% (vol/vol) for 48 h, and a further decrease in activity was observed after 96 h. Relative to enzyme activity in cells cultured in serum-free medium for 96 h, adult human platelet-poor plasma (HPPP; 2.5-10% vol/vol) induced a small increase, similar concentrations of adult human serum (HS) induced much larger increases, and charcoal-depleted FCS was ineffective. In an attempt to identify the factor(s) present in serum that influence ecto-NTP pyrophosphatase activity, we examined transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF). PDGFs AA, AB, and BB (0.1-10 ng/ml) were ineffective, but both TGF-beta 1 and TGF-beta 2 increased enzyme activity. The increase was dose dependent between 0.001 and 10 ng/ml, was enhanced in the presence of 2% vol/vol FCS, and was not potentiated by PDGF or by 1,25-(OH)2D3. Furthermore, the increase was independent of cell density and was blocked by inhibitors of protein and RNA synthesis. Ecto-NTP pyrophosphatase of subject-matched human dermal fibroblasts was unaffected by TGF-beta (10 ng/ml), suggesting that modulation of activity by the growth factor may be tissue specific. Alkaline phosphatase (ALP) probably serves to hydrolyze extracellular PPi in bone. In contrast to effects on NTP pyrophosphatase activity is osteoblast-like cells, TGF-beta 1 and TGF-beta 2 (0.001-10 ng/ml) decreased ALP activity dose dependently after 72 h. By inducing opposing changes in ecto-NTP pyrophosphatase and ALP activities, TGF-beta may increase extracellular PPi concentrations in osseous tissues and consequently modulate bone mineral properties in vivo.

Interleukin-1 beta enhances the response of human articular chondrocytes to extracellular ATP

It has been observed that both interleukin-1 (IL-1) and extracellular ATP stimulate the production of prostaglandin E (PGE) by human articular chondrocytes in monolayer culture. The combined effects of recombinant human IL-1 beta and ATP were therefore studied using these cells. IL-1 beta rapidly enhanced the response to a maximally effective concentration of ATP (100 microM). On continuous exposure of the cells to the cytokine, its effect was greatest after approx. 24 h and tended to decline thereafter. The enhancement of the response to 100 microM ATP by IL-1 beta was dose-dependent. Removal of IL-1 beta prior to treating the cells with 100 microM ATP did not affect the degree of enhancement of the response. The effect of the cytokine on the response to suboptimal concentrations of extracellular ATP was also tested. IL-1 beta lowered the minimum concentration of ATP required to elicit an increase in the production of PGE by human articular chondrocytes. These findings are of interest, since they indicate a synergistic interaction between a cytokine and a purinergic agonist. Moreover, since both the sensitivity of the cells to extracellular ATP and the maximum response to this agent were enhanced, it is possible that IL-1 modulates more than one step in the process of P2-purinoceptor-mediated stimulation of PGE production. These observations may be relevant to the pathogenesis of some forms of arthritis.

Evidence for the presence of P2-purinoceptors at the surface of human articular chondrocytes in monolayer culture

Extracellular purines can act at purinoceptors to influence metabolic processes. Nucleotide-metabolizing ectoenzymes may modulate such purinergic effects, and their occurrence in a tissue may suggest the presence of purinoceptors. Thus, following the identification of ecto-nucleoside triphosphate pyrophosphatase in cultured human articular chondrocytes, we have studied whether these cells express P2-type purinoceptors. Release of prostaglandin E (PGE) was monitored, since articular chondrocytes synthesize and secrete PGE, and activation of P2-purinoceptors frequently results in enhanced prostaglandin production. Extracellular ATP and ADP stimulated PGE production, whereas AMP and adenosine had only limited effects. ATP concentrations as low as 5 microM were effective, and maximal responses were achieved at 50-100 microM ATP. GTP, UTP and ITP also elicited responses, but tended to be less effective than ATP at equivalent concentrations. Of the analogues of ATP that were tested, only adenosine 5'-(beta,gamma-methylene)triphosphate stimulated PGE production. The response to extracellular ATP was virtually abolished by indomethacin. Treatment of the cells with the P1-purinoceptor antagonist, 8-phenyltheophylline, or with pertussis toxin reduced both basal and ATP-stimulated PGE production, but did not substantially decrease the ratio of ATP-stimulated to basal PGE production. These results indicate the presence of P2-purinoceptors in cultured human articular chondrocytes, and suggest that extracellular ATP may have physiological and pathological effects in human articular cartilage.

Synovial fluid pyrophosphate and nucleoside triphosphate pyrophosphatase: comparison between normal and diseased and between inflamed and non-inflamed joints

Deposition of intra-articular calcium pyrophosphate is associated with both aging and arthropathy; increased concentrations of free pyrophosphate (PPi) may contribute to such deposition. Free pyrophosphate and nucleoside triphosphate pyrophosphatase (NTPase) were estimated in synovial fluids from 50 subjects with normal knees and from 44 patients with rheumatoid arthritis, 61 with pyrophosphate arthropathy, and 59 with osteoarthritis. For arthropathic knees clinically assessed inflammation was classified as active or inactive using a summated score of six clinical features. The order of PPi (mumol/l) and NTPase (mumol PPi/30 min/mg protein) was pyrophosphate arthropathy greater than osteoarthritis greater than rheumatoid arthritis (median PPi, NTPase respectively: for pyrophosphate arthropathy 15.9, 0.45; for osteoarthritis 9.3, 0.25; for rheumatoid arthritis 4.4, 0.18), with significant differences between all groups. In pyrophosphate arthropathy both PPi (mumol/l) and NTPase (mumol PPi/30 min/mg protein) were higher than normal (15.9, 0.45 v 8.6, 0.2 respectively), but findings in osteoarthritis did not differ from normal. The inflammatory state of the knee had a distinct but variable effect on synovial fluid findings in rheumatoid arthritis and pyrophosphate arthropathy, but not in osteoarthritis. There was no correlation of either PPi or NTPase with age, or between PPi and NTPase in any group. This study provides in vivo data for synovial fluid PPi and NTPase. It suggests that factors other than PPi need to be considered in a study of crystal associated arthropathy. Clinical inflammation, as well as diagnosis, is important in synovial fluid studies.

Pyrophosphate-loaded hepatocytes show increased iron uptake from transferrin

Exogenously added pyrophosphate was found associated with hepatocytes in suspension, particularly in the presence of calcium ions. The cell-associated pyrophosphate was recovered with the stroma fraction. Pyrophosphate thus bound did not affect cellular ATP, leakage of lactate dehydrogenase or trypan blue exclusion. Compared to control cells, pyrophosphate-loaded hepatocytes showed an approximately 70% increase in iron accumulation from transferrin. The excess iron was handled by the pyrophosphate-loaded cells as control cells. The relevance of pyrophosphate to hepatocyte iron uptake is discussed.

Evidence that ecto-nucleoside-triphosphate pyrophosphatase serves in the generation of extracellular inorganic pyrophosphate in human bone and articular cartilage

Extracellular inorganic pyrophosphate (PPi) is important in the regulation of mineralisation of bone, and in the pathogenesis of chondrocalcinosis, an arthritic disease in which calcium pyrophosphate dihydrate crystals form in articular cartilage. Nucleoside-triphosphate pyrophosphatase, which catalyses the formation of PPi, was previously observed at the surface of human articular chondrocytes in culture. A similar enzyme has been identified in osteoblast-like human bone cells in culture, and is active towards purine and pyrimidine nucleoside triphosphates. The enzyme has high affinity for ATP and is located on the cell surface, and thus could serve in the generation of extracellular PPi. Moreover, no other mechanism for the catabolism of small amounts of exogenous ATP is present in human bone cells. Further evidence for ecto-nucleoside-triphosphate pyrophosphatase serving in the generation of extracellular PPi in articular cartilage and bone was obtained by studying the ability of alternative substrates (which do not yield PPi) to inhibit generation of PPi from ATP. In both articular chondrocytes and bone cells, the enzyme exhibited an apparent preference for ATP over dinucleotide and phosphodiester substrates. Some potential inhibitors of the enzyme activity were also studied in both cell types. ADP moderately inhibited the activity but two bisphosphonate drugs were only slightly inhibitory.