Adenosine triphosphate pyrophosphohydrolase and neutral inorganic pyrophosphatase in pathologic joint fluids. Elevated pyrophosphohydrolase in calcium pyrophosphate dihydrate crystal deposition disease

Histone Deacetylase Inhibitors Downregulate Calcium Pyrophosphate Crystal Formation in Human Articular Chondrocytes

Calcium pyrophosphate (CPP) deposition disease (CPPD) is a form of CPP crystal-induced arthritis. A high concentration of extracellular pyrophosphate (ePPi) in synovial fluid is positively correlated with the formation of CPP crystals, and ePPi can be upregulated by ankylosis human (ANKH) and ectonucleotide pyrophosphatase 1 (ENPP1) and downregulated by tissue non-specific alkaline phosphatase (TNAP). However, there is currently no drug that eliminates CPP crystals. We explored the effects of the histone deacetylase (HDAC) inhibitors (HDACis) trichostatin A (TSA) and vorinostat (SAHA) on CPP formation. Transforming growth factor (TGF)-β1-treated human primary cultured articular chondrocytes (HC-a cells) were used to increase ePPi and CPP formation, which were determined by pyrophosphate assay and CPP crystal staining assay, respectively. Artificial substrates thymidine 5′-monophosphate p-nitrophenyl ester (p-NpTMP) and p-nitrophenyl phosphate (p-NPP) were used to estimate ENPP1 and TNAP activities, respectively. The HDACis TSA and SAHA significantly reduced mRNA and protein expressions of ANKH and ENPP1 but increased TNAP expression in a dose-dependent manner in HC-a cells. Further results demonstrated that TSA and SAHA decreased ENPP1 activity, increased TNAP activity, and limited levels of ePPi and CPP. As expected, both TSA and SAHA significantly increased the acetylation of histones 3 and 4 but failed to block Smad-2 phosphorylation induced by TGF-β1. These results suggest that HDACis prevented the formation of CPP by regulating ANKH, ENPP1, and TNAP expressions and can possibly be developed as a potential drug to treat or prevent CPPD.

Synthesis of Inorganic Pyrophosphatase-Nanodiamond Conjugates Resistant to Calcium and Fluoride

Pyrophosphate arthropathy is the mineralization defect in humans caused by the deposition of microcrystals of calcium pyrophosphate dihydrate in joint tissues. As a potential therapeutic strategy for the treatment of pyrophosphate arthropathy, delivery of exogenous pyrophosphate-hydrolyzing enzymes, inorganic pyrophosphatases (PPases), to the synovial fluid has been suggested. Previously, we synthesized the conjugates of Escherichia coli PPase (Ec-PPase) with detonation synthesis nanodiamonds (NDs) as a delivery platform, obtaining the hybrid biomaterial retaining high pyrophosphate-hydrolyzing activity in vitro. However, most known PPases including Ec-PPase in the soluble form are strongly inhibited by Ca²⁺ ions. Because synovial fluid contains up to millimolar concentrations of soluble calcium, this inhibition might limit the in vivo application of Ec-PPase-based material in joint tissues. In this work, we proposed other bacterial PPases from Mycobacterium tuberculosis (Mt-PPase), which are resistant to the inhibition by Ca²⁺ ions, as an active PP_i-hydrolyzing agent. We synthesized conjugates of Mt-PPase with NDs and tested their activity under various conditions. Unexpectedly, conjugates of both Ec-PPase and Mt-PPase with aminated NDs retained significant hydrolytic activity in the presence of well-known mechanism-based PPase inhibitors, fluoride or calcium. The incomplete inhibition of PPases by fluoride or calcium was found for the first time.

Review: Unmet Needs and the Path Forward in Joint Disease Associated With Calcium Pyrophosphate Crystal Deposition

Calcium pyrophosphate (CPP) crystal deposition (CPPD) is prevalent and can be associated with synovitis and joint damage. The population of elderly persons predominantly affected by CPPD is growing rapidly. Since shortfalls exist in many aspects of CPPD, we conducted an anonymous survey of CPPD unmet needs, prioritized by experts from the Gout, Hyperuricemia and Crystal-Associated Disease Network. We provide our perspectives on the survey results, and we propose several CPPD basic and clinical translational research pathways. Chondrocyte and cartilage culture systems for generating CPP crystals in vitro and transgenic small animal CPPD models are needed to better define CPPD mechanism paradigms and help guide new therapies. CPPD recognition, clinical research, and care would be improved by international consensus on CPPD nomenclature and disease phenotype classification, better exploitation of advanced imaging, and pragmatic new point-of-care crystal analytic approaches for detecting CPP crystals. Clinical impacts of CPP crystals in osteoarthritis and in asymptomatic joints in elderly persons remain major unanswered questions that are rendered more difficult by current inability to therapeutically limit or dissolve the crystal deposits and assess the consequent clinical outcome. Going forward, CPPD clinical research studies should define clinical settings in which articular CPPD does substantial harm and should include analyses of diverse clinical phenotypes and populations. Clinical trials should identify the best therapeutic targets to limit CPP crystal deposition and associated inflammation and should include assessment of intraarticular agents. Our perspective is that such advances in basic and clinical science in CPPD are now within reach and can lead to better treatments for this disorder.

Mechanism of action of colchicine in the treatment of gout

PURPOSE

The aims of this article were to systematically review the literature about the mechanism of action of colchicine in the multimodal pathology of acute inflammation associated with gout and to consider the clinical utility of colchicine in other chronic inflammatory diseases.

METHODS

The English-language literature on PubMed was searched for articles published between 1990 and October 2013, with a cross-reference to citations across all years. Relevant articles pertaining to the mechanism of action of colchicine and the clinical applications of colchicine in gout and other inflammatory conditions were identified and reviewed.

FINDINGS

The molecular pathology of acute inflammation associated with gouty arthritis involves several concurrent pathways triggered by a variety of interactions between monosodium urate crystals and the surface of cells. Colchicine modulates multiple pro- and antiinflammatory pathways associated with gouty arthritis. Colchicine prevents microtubule assembly and thereby disrupts inflammasome activation, microtubule-based inflammatory cell chemotaxis, generation of leukotrienes and cytokines, and phagocytosis. Many of these cellular processes can be found in other diseases involving chronic inflammation. The multimodal mechanism of action of colchicine suggests potential efficacy of colchicine in other comorbid conditions associated with gout, such as osteoarthritis and cardiovascular disease.

IMPLICATIONS

Colchicine has multiple mechanisms of action that affect inflammatory processes and result in its utility for treating and preventing acute gout flare. Other chronic inflammatory diseases that invoke these molecular pathways may represent new therapeutic applications for colchicine.

Epidemiology of calcium pyrophosphate crystal arthritis and basic calcium phosphate crystal arthropathy

Calcium pyrophosphate crystal deposition (CPPD) is common and mainly associates with increasing age and osteoarthritis (OA). Recent studies suggest that CPPD occurs as the result of a generalized articular predisposition and may also associate with low cortical bone mineral density. The epidemiology of basic calcium phosphate (BCP) crystal deposition is poorly understood. Although periarticular BCP crystal deposits occurs at all ages and in both sexes, intra-articular BCP crystal deposition tends to associate with increasing age and OA. Calcium pyrophosphate and BCP crystals frequently coexist in joints with OA.

Treatment and management of pseudogout: insights for the clinician

Pseudogout and the associated calcium pyrophosphate dihydrate (CPPD)- crystal-related arthropathies are common conditions that present particular management problems in clinical practice as they often affect older patients with multiple medical comorbidities. The epidemiology, metabolic and endocrine disease associations, and routine investigations used in the diagnostic workup are briefly reviewed. Current treatment approaches that are mainly directed at relieving the symptoms of joint inflammation are outlined. Unlike gout, there are no agents available that have been shown to decrease crystal load in CPPD-related joint disease. Recent novel insights into the pathogenesis of crystal-induced joint inflammation and subsequent joint degeneration are also discussed. The potential of colchicine as a prophylactic agent in managing recurrent attacks and the likely mechanisms of its effects on the NACHT, LRR and PYD domains-containing protein 3 (NALP-3) inflammasome of the innate immune system are highlighted. The use of agents that directly target the inflammasome, in particular drugs which inhibit the interleukin 1 pathway, in the treatment of severe, refractory pseudogout is also discussed. Finally, there is particular emphasis on the likely pathogenic role of CPPD crystal deposition in degenerative joint disease and the use of targeted anticrystal therapies as potential disease-modifying drugs.

Parallel regulation of extracellular ATP and inorganic pyrophosphate: roles of growth factors, transduction modulators, and ANK

OBJECTIVE

Extracellular inorganic pyrophosphate (ePPi) is a key regulator of pathologic mineralization in articular cartilage. Articular chondrocytes generate ePPi by the transportation of intracellular PPi (iPPi) through transport mechanisms such as ANK or by the degradation of extracellular adenosine triphosphate (eATP) by ectoenzymes. Although numerous modulators of ePPi have been characterized, little is known about eATP elaboration in cartilage. We sought to determine (1) whether eATP is coordinately regulated with ePPi and (2) whether ANK transports ATP.

METHODS

Primary articular chondrocytes were treated with factors known to modulate ePPi levels including growth factors (TGFβ1 and IGF-1), anion channel inhibitors, and chemicals that alter adenylyl cyclase and protein kinase C activities. Additional chondrocyte monolayers were infected with adenovirus containing functional (Ad-ANK) or mutated (Ad-ANK mutant) ANK sequences. eATP levels were measured with a bioluminescent assay.

RESULTS

TGFβ1 enhanced eATP accumulation by 33%, whereas IGF-1 decreased eATP accumulation by 63% and attenuated TGFβ1-induced eATP release by 72%. Forskolin and probenecid diminished eATP accumulation by 55% and 89%. Phorbol-12-myristate-13-acetate increased eATP by 29%. Transfection of chondrocytes with Ad-ANK caused a 10-fold increase in eATP compared with control values.

CONCLUSION

Modulation of eATP by various factors paralleled their effects on ePPi production, suggesting a shared pathway of ePPi and eATP production and implicating ANK in eATP transport. As eATP directly contributes to pathologic mineralization in articular cartilage, understanding eATP regulation may lead to effective therapies for crystal-associated arthritis.

Up-regulated expression of cartilage intermediate-layer protein and ANK in articular hyaline cartilage from patients with calcium pyrophosphate dihydrate crystal deposition disease

OBJECTIVE

Excess accumulation of extracellular inorganic pyrophosphate (ePPi) in aged human cartilage is crucial in calcium pyrophosphate dihydrate (CPPD) crystal formation in cartilage matrix. Two sources of ePPi are ePPi-generating ectoenzymes (NTPPPH) and extracellular transport of intracellular PPi by ANK. This study was undertaken to evaluate the role of NTPPPH and ANK in ePPi elaboration, by investigating expression of NTPPPH enzymes (cartilage intermediate-layer protein [CILP] and plasma cell membrane glycoprotein 1 [PC-1]) and ANK in human chondrocytes from osteoarthritic (OA) articular cartilage containing CPPD crystals and without crystals.

METHODS

Chondrocytes were harvested from knee cartilage at the time of arthroplasty (OA with CPPD crystals [CPPD], n = 8; OA without crystals [OA], n = 10). Normal adult human chondrocytes (n = 1) were used as a control. Chondrocytes were cultured with transforming growth factor beta1 (TGFbeta1), which stimulates ePPi elaboration, and/or insulin-like growth factor 1 (IGF-1), which inhibits ePPi elaboration. NTPPPH and ePPi were measured in the media at 48 hours. Media CILP, PC-1, and ANK were determined by dot-immunoblot analysis. Chondrocyte messenger RNA (mRNA) was extracted for reverse transcriptase-polymerase chain reaction to study expression of mRNA for CILP, PC-1, and ANK. NTPPPH and ANK mRNA and protein were also studied in fresh frozen cartilage.

RESULTS

Basal ePPi elaboration and NTPPPH activity in conditioned media from CPPD chondrocytes were elevated compared with normal chondrocytes, and tended to be higher compared with OA chondrocytes. Basal expression of mRNA for CILP (chondrocytes) and ANK (cartilage) was higher in both CPPD chondrocytes and CPPD cartilage extract than in OA or normal samples. PC-1 mRNA was less abundant in CPPD chondrocytes and cartilage extract than in OA chondrocytes and extract, although the difference was not significant. CILP, PC-1, and ANK protein levels were similar in CPPD, OA, and normal chondrocytes or cartilage extracts. Both CILP and ANK mRNA expression and ePPi elaboration were stimulated by TGFbeta1 and inhibited by IGF-1 in chondrocytes from all sources.

CONCLUSION

CILP and ANK mRNA expression correlates with chondrocyte ePPi accumulation around CPPD and OA chondrocytes, and all respond similarly to growth factor stimulation. These findings suggest that up-regulated CILP and ANK expression contributes to higher ePPi accumulation from CPPD crystal-forming cartilage.

Rheumatic manifestations of primary hyperparathyroidism and parathyroid hormone therapy

A frequent manifestation of severe primary hyperparathyroidism was the bone disease osteitis fibrosa cystica. Rarely, excess parathyroid hormone (PTH) was associated with gout and calcium pyrophosphate crystal deposition disease. Surgical cure of primary hyperparathyroidism was occasionally associated with pseudogout. Today, primary hyperparathyroidism is generally asymptomatic. Clinically overt rheumatologic and skeletal effects are mainly of historical interest. Skeletal disease is still detectable by more sensitive techniques. In certain circumstances, PTH may be protective and anabolic for the skeleton.

Variations in site and levels of expression of chondrocyte nucleotide pyrophosphohydrolase with aging

The aim of this study was to identify changes in cartilage intermediate layer protein/nucleotide pyrophosphohydrolase (CILP/NTPPH) expression in articular cartilage during aging. Adult (3-4 years old) and young (7-10 days old) porcine articular hyaline cartilage and fibrocartilage were studied by Northern blot analysis, in situ hybridization, and immunohistochemistry using a complementary DNA (cDNA) probe encoding porcine CILP/NTPPH and antibody to a synthetic peptide corresponding to a CILP/NTPPH sequence. Northern blot analysis of chondrocytes showed lower expression of CILP/NTPPH messenger RNA (mRNA) in young cartilage than in adult cartilage. In adult cartilage, extracellular matrix from the surface to the middeep zone was immunoreactive for CILP/NTPPH, especially in the pericellular matrix surrounding the middeep zone chondrocytes. In young cartilage, chondrocytes were moderately immunoreactive for CILP/NTPPH throughout all zones except the calcified zone. The matrix of young cartilage was negative except in the superficial zone. In young cartilage, CILP/NTPPH mRNA expression was undetectable. In adult cartilage, chondrocytes showed strong mRNA expression for CILP/NTPPH throughout middeep zones. Protein and mRNA signals were not detectable below the tidemark. CILP/NTPPH secretion into matrix around chondrocytes increases with aging. In this extracellular site it may generate inorganic pyrophosphate and contribute to age-related calcium pyrophosphate dihydrate crystal deposition disease.

Transforming growth factor beta-1 stimulates articular chondrocyte elaboration of matrix vesicles capable of greater calcium pyrophosphate precipitation

Objective To determine the role of transforming growth factor beta1 (TGFbeta) in early calcium pyrophosphate formation by measuring its effects on articular chondrocyte matrix vesicle (MV) formation, specific activity of the inorganic pyrophosphate(PPi)-generating enzyme nucleoside triphosphate pyrophospho-hydrolase (NTPPPH) and biomineralization capacity. Methods MV elaborated from mature porcine chondrocyte monolayers+/-TGFbeta were compared for protein content, NTPPPH activity, and ATP-dependent biomineralization. Precipitation of calcium pyrophosphate mineral phases by MV was determined by a radiometric assay and by Fourier transform infrared spectroscopy (FTIR). Results MV from monolayers exposed to TGFbeta were enriched in NTPPPH activity compared to MV from control monolayers (P< 0.01) and precipitated more calcium/mg MV protein than controls (P</= 0.01). FTIR spectra of mineral generated by monolayer-elaborated MV were consistent with poorly crystalline CPPD. Conclusions TGFbeta is capable of increasing the capacity of articular chondrocyte-derived MV to generate PPi via NTPPPH and precipitate calcium in the form of CPPD mineral. These data support the concept that this growth factor plays a key role in cartilage matrix CPPD deposition.

Expression of cartilage intermediate layer protein/nucleotide pyrophosphohydrolase parallels the production of extracellular inorganic pyrophosphate in response to growth factors and with aging

OBJECTIVE

To evaluate the role of the extracellular inorganic pyrophosphate (ePPi)-generating ectoenzyme cartilage intermediate layer protein/nucleotide pyrophosphohydrolase (CILP/NTPPH) in chondrocyte PPi elaboration, we studied CILP/NTPPH expression in response to growth factors during aging.

METHODS

Porcine chondrocytes from adult (3-4-year-old) and young (2-week-old) animals were stimulated with transforming growth factor beta1 (TGFbeta1), which enhances ePPi elaboration, and/or insulin-like growth factor 1 (IGF-1), which diminishes ePPi elaboration. Measurements of ePPi, NTPPH enzyme activity, Western blot analysis, reverse transcriptase-polymerase chain reaction (RT-PCR), and Northern blot analysis were performed.

RESULTS

Elaboration of ePPi into conditioned media from adult chondrocytes was significantly increased by TGFbeta1 and significantly inhibited by IGF-1, but no significant differences were observed in young chondrocytes. The protein levels of CILP/NTPPH by Western analysis in the media from adult and young porcine chondrocytes were increased by TGFbeta1. RT-PCR and Northern analysis showed that CILP/NTPPH messenger RNA (mRNA) expression in both adult and young chondrocytes was increased by TGFbeta1 and decreased by IGF-1, but these changes were less significant in the young chondrocytes. Basal and TGFbeta1-up-regulated levels of CILP/NTPPH expression were higher in adult chondrocytes than in young chondrocytes.

CONCLUSION

These results provide evidence that CILP/NTPPH expression and ePPi elaboration are concomitantly stimulated by TGFbeta1 and down-regulated by IGF-1, especially in adult chondrocytes, implicating CILP/NTPPH as a functional participant in ePPi elaboration. Increased CILP/NTPPH mRNA expression in chondrocytes derived from aged animals compared with young animals might promote the formation of calcium pyrophosphate dihydrate crystals in aged cartilage.

Transduction mechanisms of porcine chondrocyte inorganic pyrophosphate elaboration

OBJECTIVE

To investigate cellular signaling mechanisms that influence chondrocyte production of inorganic pyrophosphate (PPi), which promotes calcium pyrophosphate dihydrate (CPPD) crystal deposition.

METHODS

Articular chondrocyte and cartilage cultures were stimulated with protein kinase C (PKC) activator and adenyl cyclase activator. Generation of extracellular PPi was measured.

RESULTS

Adenyl cyclase activation resulted in diminished pyrophosphate generation. PKC activation stimulated pyrophosphate elaboration.

CONCLUSION

Two signaling pathways, cAMP and PKC, modulate generation of extracellular pyrophosphate by cartilage and chondrocytes. They are novel targets for potentially diminishing extracellular pyrophosphate elaboration that leads to CPPD crystal deposition.

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.

Differential effects of aging on human chondrocyte responses to transforming growth factor beta: increased pyrophosphate production and decreased cell proliferation

OBJECTIVE

To address the influence of age on inorganic pyrophosphate (PPi) accumulation in human articular chondrocytes.

METHODS

Articular cartilage was obtained from men and women in 2 different age groups: ages 15-55 and 56-91. The effects of transforming growth factor beta1 (TGFbeta1) on PPi levels in the media and cell lysates of chondrocytes were investigated. In addition, the effects of TGFbeta on PPi accumulation were compared with chondrocyte proliferation.

RESULTS

TGFbeta1 increased PPi levels to a greater extent in chondrocytes from subjects in the older age group compared with those obtained from younger subjects. Treatment of chondrocytes with TGFbeta1 led to a similar increase in total intracellular protein in both age groups. Although TGFbeta increased nucleoside triphosphate pyrophosphohydrolase activity and decreased alkaline phosphatase activity, these effects did not differ between the 2 age groups. Analysis of the same cell preparations showed an age-related decrease in TGFbeta-induced chondrocyte proliferation, whereas these same cells showed an increased response with respect to PPi elaboration.

CONCLUSION

These results show that aging differentially affected TGFbeta-induced PPi accumulation versus proliferation in human articular chondrocytes. These differences in TGFbeta response are likely to contribute to the development of age-associated cartilage diseases such as osteoarthritis.

Identification of a nucleotide pyrophosphohydrolase from articular tissues in human serum

OBJECTIVE

To characterize the nucleotide pyrophosphohydrolase (NTPPHase) in human serum.

METHODS

NTPPHase activity and kinetic analysis were performed using thymidine monophosphate paranitrophenyl ester (TMPNP) or 32Pgamma-labeled ATP as substrate. Sera were chromatographed (dye column), and peak fractions were analyzed kinetically and by immunoblot using antibodies to 127-kd articular cartilage vesicle (ACV) NTPPHase as well as to PC-1 and to 58 kd, two plasma membrane ecto-NTPPHases. Enzyme activity was measured before and after sample ultracentrifugation.

RESULTS

NTPPHase activity was found in all sera tested (2 normal subjects, 9 arthritis patients). Specific activity was increased 9-32-fold after chromatography; 60-80% of total activity was recovered in a single peak containing an approximately 100-kd soluble peptide related to the 127-kd ACV enzyme. The apparent Km of this peptide (TMPNP) was virtually identical to that of the porcine ACV 127-kd enzyme. No immunoreactivity against PC-1 or 58-kd NTPPHase was found.

CONCLUSION

Human serum NTPPHase is derived from 127-kd ACV-related enzyme.

Specificity of a porcine 127-kd nucleotide pyrophosphohydrolase for articular tissues

OBJECTIVE

To determine the tissue specificity of a porcine 127-kd nucleotide pyrophosphohydrolase (NTPPHase) found in vesicles derived from hyaline articular cartilage (ACV).

METHODS

Homogenates of porcine brain, lung, liver, kidney, urinary bladder, pancreas, spleen, skin, vena cava, marrow, bone (cells), tendon (Achilles), ligament (anterior cruciate), elastic cartilage, meniscus, and hyaline cartilage were analyzed for NTTPHase activity (thymidine monophosphate paranitrophenyl ester substrate) and by Western blot using polyclonal antibodies against 127-kd NTPPHase and against recombinant PC-1, another ecto-NTPPHase.

RESULTS

All tissues contained NTPPHase activity; the highest specific activity was found in hyaline articular cartilage, the lowest in brain. ACV-associated 127-kd NTTPHase was expressed in cartilage, ligament, and tendon. PC-1 was also expressed in those tissues and in skin, kidney, bone cells, and (probably) in liver and muscle.

CONCLUSION

The 127-kd NTPPHase appears to be highly specific for articular tissues.

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.

ATP-induced chondrocalcinosis

OBJECTIVE

To determine whether adult articular cartilage mineralizes in the presence of ATP.

METHODS

Intact adult porcine articular cartilage and monolayers of chondrocytes were cultured in physiologic media containing ATP, and mineralization was measured as retention of 45Ca. Cartilage was analyzed by electron microscopy.

RESULTS

Articular cartilage sequestered 45Ca when incubated with 100 microM ATP: Use of the ATP analog alpha, beta-methylene ATP did not promote mineralization and addition of pyrophosphatase inhibited mineralization, indicating that hydrolysis of ATP to AMP and inorganic pyrophosphate is necessary for the process to occur. Mineral was concentrated in articular cartilage vesicles in the perichondral area.

CONCLUSION

Adult articular cartilage mineralizes in the presence of ATP, in a manner similar to that found with isolated matrix or articular cartilage vesicles. This supports the notion that these structures have a role in chondrocalcinosis.

Diseases associated with calcium pyrophosphate deposition disease

Although many metabolic and endocrine diseases have been reported to predispose to calcium pyrophosphate dihydrate crystal deposition, the validity of many of these associations remains unclear. A critical review of the literature relating to these associations, with illustrative cases and data derived from the authors' own experience, is presented. It is concluded that there is good evidence to associate hypophosphatasia, hypomagnesemia, and hyperparathyroidism with chondrocalcinosis and acute attacks of "pseudogout." Meta-analysis also suggests a small but significant association between hypothyroidism and chondrocalcinosis. Hemochromatosis stands alone in clearly associating not only with chondrocalcinosis but also with structural change and chronic arthropathy. The biochemical mechanisms that may produce these various associations are discussed. Recommendations are made concerning appropriate screening for metabolic and endocrine disease in patients with chondrocalcinosis.

Synovial fluid 5'-nucleotidase activity. Relationship to other purine catabolic enzymes and to arthropathies associated with calcium crystal deposition

We measured 5'-nucleotidase (5NT) activity in synovial fluid from 159 patients with various diagnoses. The activity of 5NT was compared with activities of nucleotide pyrophosphohydrolase, alkaline and neutral phosphatases, and adenosine deaminase, in the same samples. Higher levels of 5NT activity occurred in synovial fluid from osteoarthritic joints than from joints of patients with gout, pseudogout, or rheumatoid arthritis. The highest levels of 5NT activity were found in synovial fluid from patients with Milwaukee shoulder syndrome and from osteoarthritis patients in whom deposition of calcium-containing crystals was also present.

Transforming growth factor beta 1 stimulates inorganic pyrophosphate elaboration by porcine cartilage

The overproduction of inorganic pyrophosphate (PPi) by cartilage is thought to be a key element in the formation of calcium pyrophosphate dihydrate (CPPD) crystals in joints, and the subsequent development of pseudogout or chondrocalcinosis. We report herein that transforming growth factor beta 1 (TGF beta 1), alone and in synergy with epidermal growth factor (EGF) or TGF alpha, markedly stimulates PPi elaboration by porcine articular cartilage in organ culture and monolayer culture. This effect is not seen with platelet-derived growth factor, basic fibroblast growth factor, or insulin-like growth factor types 1 and 2, substances which also affect chondrocyte metabolism or are mitogenic. TGF beta 1 produces only a modest increase in nucleoside triphosphate pyrophosphohydrolase (NTPPPH), a chondrocyte ectoenzyme that produces PPi; this implies the existence of other pathways for PPi elaboration. TGF beta 1 is present in joint fluid and cartilage. TGF beta 1, TGF alpha, and EGF are the first known physiologic modifiers of cartilage PPi production. They provide a novel model for the study of CPPD crystal formation in cartilage, as well as new insights into the pathogenesis of this common affliction of aging.

Cartilage inorganic pyrophosphate elaboration is independent of sulfated glycosaminoglycan synthesis

Inorganic pyrophosphate (PPi), a product of glycosaminoglycan synthesis, may be cosecreted with matrix proteoglycan to reach the extracellular site where calcium pyrophosphate dihydrate crystals form. To test this hypothesis, sulfated glycosaminoglycan synthesis by articular cartilage in culture was stimulated or inhibited while the effect on extracellular PPi was measured. When stimulated by 0.8 mM xyloside to increase 35SO4 incorporation (mean +/- SEM % of control 183 +/- 16, n = 5), PPi accumulation changed little (from 54 +/- 6 pmoles/mg to 63 +/- 8 pmoles/mg of cartilage wet weight). Inhibition of sulfation with monensin or diethylcarbamazine disproportionately lowered 35SO4 incorporation compared with PPi elaboration. Using 60 mM diethylcarbamazine, PPi production was preserved (105 +/- 8% mean +/- SEM) compared with control cultures, while sulfation was markedly inhibited (7 +/- 1%). This dissociation of sulfate incorporation and PPi secretion indicates that it is not likely that glycosaminoglycan sulfation is the source of the PPi that escapes from chondrocytes to participate in the formation of extracellular crystals.

Augmentation of inorganic pyrophosphate elaboration in cartilage by serum factors

The disordered production of inorganic pyrophosphate (PPi) by articular cartilage is thought to have an important role in the pathogenesis of calcium pyrophosphate dihydrate deposition disease and perhaps osteoarthritis. We have previously shown that fetal calf serum added to the culture media of porcine articular cartilage explants increases the elaboration of PPi into the ambient media. We have examined this PPi stimulatory activity by studying the effects of adult human serum (HS), serum derived from adult human plasma (HP), and an acid-alcohol extract of human platelets (PE) on PPi production in cartilage organ culture. Ten percent HS produces a 1.4-fold increase in PPi production after 48 h of culture, while cartilage incubated in media containing 10% HP produces no more PPi than that incubated in media alone. PE stimulates a mean 2-fold increase in PPi production at 48 h in the presence of low concentrations of HP, and has no effect alone. It does not appear to up-regulate the activity of the ectoenzyme nucleoside triphosphate pyrophosphohydrolase (NTPPPH), nor does it promote the release of enzyme substrate into the extracellular space. Cartilage exposed to 0.5% HP and PE has 1.51 +/- 0.36 units of NTPPPH activity whereas cartilage exposed to 0.5% HP alone has 1.52 +/- 0.41 units of enzyme activity. PE does not increase the release of [14C]adenine-labeled compounds into the media. Approximately 13% of soluble 14C counts was found in the media of chondrocytes treated with PE while 18% of counts was released in the presence of HP alone. We have demonstrated a factor or factors present in FCS, HS, and an acid-ethanol extract of human platelets which represent(s) the first known physiologic modulators of PPi production in articular cartilage and may increase PPi production without affecting NTPPPH activity.

Stimulation of inorganic pyrophosphate elaboration by cultured cartilage and chondrocytes

Inorganic pyrophosphate elaboration by articular cartilage may favor calcium pyrophosphate dihydrate crystal deposition. Frequently crystal deposits form in persons affected with metabolic diseases. The cartilage organ culture system was used to model these metabolic conditions while measuring the influence on extracellular pyrophosphate elaboration. Alterations of ambient pH, thyroid stimulating hormone levels, and parathyroid hormone levels did not change pyrophosphate accumulation in the media. However, subphysiologic ambient calcium concentrations (25, 100, 500 microM) increased pyrophosphate accumulation about chondrocytes 3- to 10-fold. Low calcium also induced release of [14C]adenine-labeled nucleotides from chondrocytes, potential substrates for generation of extracellular pyrophosphate by ectoenzymes. Exposing cartilage to 10% fetal bovine serum also enhanced by 50% the egress of inorganic pyrophosphate from the tissue.

Synovial fluid inorganic pyrophosphate concentration and nucleotide pyrophosphohydrolase activity in basic calcium phosphate deposition arthropathy and Milwaukee shoulder syndrome

Synovial fluid (SF) inorganic pyrophosphate (PPi) concentration is elevated in calcium pyrophosphate dihydrate (CPPD) crystal deposition arthropathy. Since CPPD and basic calcium phosphate (BCP) crystals often are present in the same joints, we determined [PPi] and activity of the PPi-generating enzyme, nucleotide pyrophosphohydrolase (NPPH), in SF from the joints of patients with various arthropathies, including those with BCP crystals. We found elevated SF [PPi] in joints with BCP crystals, as well as in joints with CPPD crystals. The presence of BCP crystals in synovial fluids was also predictive of elevated NPPH activity.

Crystals, inflammation, and osteoarthritis

Calcium pyrophosphate and apatite crystals are common in osteoarthritic knee effusions. One or the other crystal was found in 60 percent or more of cases. These crystals offer the potential for mechanical effects in cartilage but are also often phagocytized by synovial cells. A low-grade inflammation with release of proteases and other mediators of inflammation may be an important factor in the pain and joint damage of osteoarthritis. Crystal-associated changes may merit specific attention in any future approach to therapy.