Stimulation of cartilage inorganic pyrophosphate elaboration by ascorbate

Genetic factors in the pathogenesis of CPPD crystal deposition disease

Crystal deposition is a very complex process ruled by numerous factors. A small but important proportion of cases of chondrocalcinosis are monogenic, and many of the genes involved have been identified. These genetic findings strongly point to control of the level of extracellular inorganic pyrophosphate as the primary mechanism for their association with either calcium pyrophosphate dihydrate or hydroxyapatite deposition. However, effects on extracellular inorganic pyrophosphate levels do not explain the mechanism of association in all of these monogenic diseases. Further, there are likely to be several as yet unidentified genes that are important in this common condition. This review highlights what genetic studies have demonstrated about the processes involved in these diverse but related disorders.

P5L mutation in Ank results in an increase in extracellular inorganic pyrophosphate during proliferation and nonmineralizing hypertrophy in stably transduced ATDC5 cells

Ank is a multipass transmembrane protein that regulates the cellular transport of inorganic pyrophosphate. In the progressive ankylosis (ank) mouse, a premature termination mutation at glutamic acid 440 results in a phenotype characterized by inappropriate deposition of basic calcium phosphate crystals in skeletal tissues. Mutations in the amino terminus of ANKH, the human homolog of Ank, result in familial calcium pyrophosphate dihydrate deposition disease. It has been hypothesized that these mutations result in a gain-of-function with respect to the elaboration of extracellular inorganic pyrophosphate. To explore this issue in a mineralization-competent system, we stably transduced ATDC5 cells with wild-type Ank as well as with familial chondrocalcinosis-causing Ank mutations. We evaluated the elaboration of inorganic pyrophosphate, the activity of pyrophosphate-modulating enzymes, and the mineralization in the transduced cells. Expression of transduced protein was confirmed by quantitative real-time PCR and by ELISA. Levels of inorganic pyrophosphate were measured, as were the activities of nucleotide pyrophosphatase phosphodiesterase and alkaline phosphatase. We also evaluated the expression of markers of chondrocyte maturation and the nature of the mineralization phase elaborated by transduced cells. The cell line expressing the proline to leucine mutation at position 5 (P5L) consistently displayed higher levels of extracellular inorganic pyrophosphate and higher phosphodiesterase activity than the other transduced lines. During hypertrophy, however, extracellular inorganic pyrophosphate levels were modulated by alkaline phosphatase activity in this cell system, resulting in the deposition of basic calcium phosphate crystals only in all transduced cell lines. Cells overexpressing wild-type Ank displayed a higher level of expression of type X collagen than cells transduced with mutant Ank. Other markers of hypertrophy and terminal differentiation, such as alkaline phosphatase, osteopontin, and runx2, were not significantly different in cells expressing wild-type or mutant Ank in comparison with cells transduced with an empty vector or with untransduced cells. These results suggest that the P5L Ank mutant is capable of demonstrating a gain-of-function with respect to extracellular inorganic pyrophosphate elaboration, but this effect is modified by high levels of expression of alkaline phosphatase in ATDC5 cells during hypertrophy and terminal differentiation, resulting in the deposition of basic calcium phosphate crystals.

One of two chondrocyte-expressed isoforms of cartilage intermediate-layer protein functions as an insulin-like growth factor 1 antagonist

OBJECTIVE

Aging and osteoarthritic (OA) cartilage commonly demonstrate enhanced expression of the large, transforming growth factor beta (TGFbeta)-inducible glycoprotein cartilage intermediate-layer protein (CILP) as well as enhanced extracellular inorganic pyrophosphate (PPi) that promotes the deposition of calcium pyrophosphate dihydrate crystals. In normal chondrocytes, TGFbeta induces elevated chondrocyte extracellular PPi. Insulin-like growth factor 1 (IGF-1) normally blocks this response and reduces extracellular PPi. However, chondrocyte resistance to IGF-1 is observed in OA and aging. Because CILP was reported to chromatographically fractionate with PPi-generating nucleotide pyrophosphatase phosphodiesterase (NPP) activity, it has been broadly assumed that CILP itself has NPP activity. Our objective was to directly define CILP functions and their relationship to IGF-1 in chondrocytes.

METHODS

Using primary cultures of articular chondrocytes from the knee, we defined the function of the previously described CILP (CILP-1) and of a recently described 50.6% identical protein that we designated the CILP-2 isoform.

RESULTS

Both CILP isoforms were constitutively expressed by primary cultured articular chondrocytes, but only CILP-1 expression was detectable in cultured knee meniscal cartilage cells. Neither CILP isoform had intrinsic NPP activity. But CILP-1 blocked the ability of IGF-1 to decrease extracellular PPi, an activity specific for the CILP-1 N-terminal domain. The CILP-1 N-terminal domain also suppressed IGF-1-induced (but not TGFbeta-induced) proliferation and sulfated proteoglycan synthesis, and it inhibited ligand-induced IGF-1 receptor autophosphorylation.

CONCLUSION

Two CILP isoforms are differentially expressed by chondrocytes. Neither CILP isoform exhibits PPi-generating NPP activity. But, increased expression of CILP-1, via N-terminal domain-mediated inhibitory effects of CILP-1 on chondrocyte IGF-1 responsiveness, could impair chondrocyte growth and matrix repair and indirectly promote PPi supersaturation in aging and OA cartilage.

Metabolism of extracellular pyrophosphate

Accumulation of excess inorganic pyrophosphate in cartilage matrix leads to calcium pyrophosphate dihydrate crystal deposits. Recent animal and human studies now support a role for physiologic extracellular pyrophosphate levels in preventing ectopic apatite calcification in joints and extracellular tissues. Extracellular pyrophosphate is likely generated by ectoenzymes and/or is a consequence of transport of intracellular pyrophosphate to the extracellular space. Generation of pyrophosphate by chondrocytes is modulated by aging, several soluble growth factors and cytokines, and transglutaminase. The transduction mechanisms involved in regulating pyrophosphate metabolism include protein kinase C and adenylyl cyclase. It appears that regulation of extracellular pyrophosphate levels within a narrow range is complex and necessary for appropriate mineral homeostasis in articular and nonarticular tissues.

Articular cartilage calcification and matrix vesicles

There has been much research in calcium-containing crystal deposition diseases of hereditary and sporadic type. Synovial cell-induced inflammation and secondary cartilage damage are common in these diseases. In most cases of these diseases and in primary osteoarthritis, there are mineral deposits in the cartilage, mineral crystals in the synovial fluid, and aberrations of pyrophosphate metabolism.

Inorganic pyrophosphate generation and disposition in pathophysiology

Inorganic pyrophosphate (PP(i)) regulates certain intracellular functions and extracellular crystal deposition. PP(i) is produced, degraded, and transported by specialized mechanisms. Moreover, dysregulated cellular PP(i) production, degradation, and transport all have been associated with disease, and PP(i) appears to directly mediate specific disease manifestations. In addition, natural and synthetic analogs of PP(i) are in use or currently under evaluation as prophylactic agents or therapies for disease. This review summarizes recent developments in the understanding of how PP(i) is made and disposed of by cells and assesses the body of evidence for potentially significant physiological functions of intracellular PP(i) in higher organisms. Major topics addressed are recent lines of molecular evidence that directly link decreased and increased extracellular PP(i) levels with diseases in which connective tissue matrix calcification is disordered. To illustrate in depth the effects of disordered PP(i) metabolism, this review weighs the roles in matrix calcification of the transmembrane protein ANK, which regulates intracellular to extracellular movement of PP(i), and the PP(i)-generating phosphodiesterase nucleotide pyrophosphatase family isoenzyme plasma cell membrane glycoprotein-1 (PC-1).

Pathogenesis of calcium pyrophosphate crystal deposition disease

Calcium pyrophosphate dihydrate deposition (CPPDD) disease is an increasingly common form of arthritis affecting the elderly. It is characterized by the formation of CPPD crystals in articular cartilage and usually results in severe cartilage destruction with loss of joint function. This article discusses our understanding of how and why these crystals form, highlighting recent developments in the field.

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.

The tetrabasic KKKK(147-150) motif determines intracrine regulatory effects of PthrP 1-173 on chondrocyte PPi metabolism and matrix synthesis

Expression of PTHrP is a major regulator of growth cartilage development and also becomes robust in osteoarthritic cartilage. We further defined how PTHrP 1-173, which we observed to be the preferentially expressed PTHrP isoform in normal and osteoarthritic cartilage, functions in chondrocytes. We transfected both immortalized human juvenile costal chondrocytes (TC28 cells) and rabbit articular chondrocytes with wild-type PTHrP 1-173 and mutants of putative PTHrP 1-173 endoproteolytic processing sites. Wild-type PTHrP 1-173 inhibited collagen synthesis and decreased extracellular PPi (which critically regulates hydroxyapatite deposition) by 50-80% in both chondrocytic cell types. In contrast, PTHrP 1-173 mutated at the PTHrP 147-150 motif KKKK (but not the other site-directed mutants) and increased both extracellular PPi and collagen synthesis by >50%. Synthetic PTHrP 140-173 mutated at amino acids 147-150 and also increased extracellular PPi, and wild-type 140-173 decreased extracellular PPi in permeabilized cells. The 147-nuclear localization of PTHrP. We conclude that the tetrabasic 147-150 motif functions to determine how PTHrP 1-173 regulates collagen synthesis and levels of extracellular PPi by an intracrine mechanism in chondrocytes, and it may prove useful as a therapeutic target for regulation of mineralization.

Formation of calcium pyrophosphate crystals: biologic implications

The formation of calcium pyrophosphate dihydrate (CPPD) crystals in articular cartilage marks the earliest known phase of CPPD deposition disease. Although the exact mechanisms through which these crystals form remains unknown, work over the last year has added useful details to our current paradigms of crystal nucleation and growth. Key advances include (1) progress in understanding pyrophosphate elaboration and its modifiers, (2) further characterization of the enzymes responsible for pyrophosphate elaboration, and (3) the discovery of an association between two seemingly unrelated metabolic risk factors for CPPD deposition disease.

Differential mechanisms of inorganic pyrophosphate production by plasma cell membrane glycoprotein-1 and B10 in chondrocytes

OBJECTIVE

Increased nucleoside triphosphate pyrophosphohydrolase (NTPPPH) activity in chondrocytes is associated with cartilage matrix inorganic pyrophosphate (PPi) supersaturation in chondrocalcinosis. This study compared the roles of the transforming growth factor beta (TGFbeta)-inducible plasma cell membrane glycoprotein-1 (PC-1) and the closely related B10 NTPPPH activities in chondrocyte PPi metabolism.

METHODS

NTPPPH expression was studied using reverse transcriptase-polymerase chain reaction and Western blotting. Transmembrane PC-1 (tmPC-1), water-soluble secretory PC-1 (secPC-1), and transmembrane B10 were expressed by adenoviral gene transfer or plasmid transfection, and expression of PPi was assessed in cultured articular chondrocytes and immortalized NTPPPH-deficient costal chondrocytes (TC28 cells).

RESULTS

PC-1 and B10 messenger RNA were demonstrated in articular cartilages in situ, in untreated cultured normal articular chondrocytes, and in TC28 cells. Expression of tmPC-1 and secPC-1, but not B10, rendered the NTPPPH-deficient TC28 cells able to increase expression of extracellular PPi, with or without addition of TGFbeta (10 ng/ml) to the media. More plasma membrane NTPPPH activity was detected in cells transfected with tmPC-1 than in cells transfected with B10. Furthermore, confocal microscopy with immunofluorescent staining of articular chondrocytes confirmed preferential plasma membrane localization of PC-1, relative to B10. Finally, both PC-1 and B10 increased the levels of intracellular PPi, but PC-1 and B10 appeared to act principally in different intracellular compartments (Golgi and post-Golgi versus pre-Golgi, respectively).

CONCLUSION

PC-1 and B10 NTPPPH activities were not redundant in chondrocytes. Although increased PC-1 and B10 expression caused elevations in intracellular PPi, the major effects of PC-1 and B10 were exerted in distinct subcellular compartments. Moreover, PC-1 (transmembrane and secreted), but not B10, increased the levels of extracellular PPi. Differential expression of PC-1 and B10 could modulate cartilage mineralization in degenerative joint diseases.

Matrix vesicle plasma cell membrane glycoprotein-1 regulates mineralization by murine osteoblastic MC3T3 cells

A naturally occurring nonsense truncation mutation of the inorganic pyrophosphate (PPi)-generating nucleoside triphosphate pyrophosphohydrolase (NTPPPH) PC-1 is associated with spinal and periarticular ligament hyperostosis and cartilage calcification in "tiptoe walking" (ttw) mice. Thus, we tested the hypothesis that PC-1 acts directly in the extracellular matrix to restrain mineralization. Cultured osteoblastic MC3T3 cells expressed PC-1 mRNA and produced hydroxyapatite deposits at 12-14 days. NTPPPH activity increased steadily over 14 days. Transforming growth factor-beta and 1,25-dihydroxyvitamin D3 increased PC-1 and NTPPPH in matrix vesicles (MVs). Because PC-1/NTPPPH was regulated in mineralizing MC3T3 cells, we stably transfected or infected cells with recombinant adenovirus, in order to express 2- to 6-fold more PC-1. PC-1/NTPPPH and PPi content increased severalfold in MVs derived from cells transfected with PC-1. Furthermore, MC3T3 cells transfected with PC-1 deposited approximately 80-90% less hydroxyapatite (by weight) than cells transfected with empty plasmid or enzymatically inactive PC-1. ATP-dependent 45Ca precipitation by MVs from cells overexpressing active PC-1 was comparably diminished. Thus, regulation of PC-1 controls the PPi content and function of osteoblast-derived MVs and matrix hydroxyapatite deposition. PC-1 may provide a novel therapeutic target in certain disorders of bone mineralization.

Parathyroid hormone-related proteins is abundant in osteoarthritic cartilage, and the parathyroid hormone-related protein 1-173 isoform is selectively induced by transforming growth factor beta in articular chondrocytes and suppresses generation of extracellular inorganic pyrophosphate

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a major, locally expressed regulator of growth cartilage chondrocyte proliferation, differentiation, synthetic function, and mineralization. Because mechanisms that limit cartilage chondrocytes from maturing and mineralizing are diminished in osteoarthritis (OA), we studied PTHrP expression by articular chondrocytes.

METHODS

PTHrP was studied in normal knee cartilage samples and cultured articular chondrocytes, and in cartilage specimens from knees with advanced OA, obtained at the time of joint replacement.

RESULTS

PTHrP was more abundant in OA than in normal human knee articular cartilage. Both demonstrated PTH/PTHrP receptor expression. PTHrP 1-173, one of three alternatively spliced PTHrP isoforms, was exclusively expressed and induced by transforming growth factor beta in cultured chondrocytes. Chondrocytes mainly used the GC-rich P2 alternative promoter to express PTHrP messenger RNA. Inhibition by PTHrP 1-173, but not by PTHrP 1-146 or PTHrP 1-87, of inorganic pyrophosphate (PPi) elaboration suggested selective functional properties of the 1-173 isoform. Exposure to a neutralizing antibody to PTHrP increased PPi elaboration by articular chondrocytes.

CONCLUSION

Increased expression of PTHrP, including the 1-173 isoform, has the potential to contribute to the pathologic differentiated functions of chondrocytes, including mineralization, in OA.

Articular chondrocytes and synoviocytes in culture: influence of antioxidants on lipid peroxidation and proliferation

Chondrocytes and synoviocytes are the main cell types in articular joints. Articular cartilage is fed by synoviocytes via synovial fluid and has a low partial oxygen pressure. Thus, chondrocytes show oxygen radical protective mechanisms in vivo and are unprotected against these factors under common culture conditions. We investigated the influence of ascorbic acid, Fe2+, glutathione and alpha-tocopherol on lipid peroxidation and proliferation of rat articular chondrocytes and rabbit synoviocytes (HIG-82) in vitro. A combination of ascorbic acid and Fe2+ induced the production of thiobarbituric acid-reactive material as a marker of radical-mediated lipid peroxidation in homogenates and/or supernatants of cultured chondrocytes and synoviocytes. The amount of lipid peroxidation of chondrocytes was about 3-fold higher than that of synoviocytes. Ascorbic acid or Fe2+ alone had no significant influence on the production of thiobarbituric acid-reactive material. Lipid peroxidation could be abolished by addition of the radical scavenger alpha-tocopherol, whereas glutathione had no effect. 25-50 microM alpha-tocopherol decreased the ascorbic acid-(100 micrograms/ml) and Fe(2+)-(3 microM) induced lipid peroxidation to a basal level. Moreover, ascorbic acid inhibited the proliferation of rat chondrocytes and rabbit synoviocytes measured by [3H]-thymidine incorporation. Alpha-tocopherol and glutathione had no influence on the proliferation of chondrocytes but alpha-tocopherol decreased the growth of synoviocytes and increased the anti-proliferative effect of ascorbic acid on these cells. The importance of these findings for the use of ascorbic acid, glutathione and alpha-tocopherol in chondrocyte and synoviocyte cultures, or the influence of these molecules on the etiology and treatment of articular diseases will be discussed.

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.

Retinoic acid stimulates pyrophosphate elaboration by cartilage and chondrocytes

Abnormal metabolism of extracellular inorganic pyrophosphate (PPi) by articular cartilage contributes to calcium pyrophosphate dihydrate (CPPD) crystal formation and the resultant arthritis known as CPPD deposition disease. The factors causing excess PPi elaboration in affected cartilage remain poorly defined. Retinoic acid (RA), a naturally occurring vitamin A metabolite, promotes cartilage degeneration and mineralization, two correlates of CPPD crystal deposition. RA was examined as a potential modifier of cartilage PPi elaboration. All-trans RA (200-1000 nM) increased PPi levels in culture medium of normal porcine cartilage and chondrocytes 2-3-fold over control values at 96 hours of incubation (P < 0.01). IGF1 and anti-EGF antibody diminished the effects of RA on PPi elaboration. RA modestly increased activity of the PPi-generating ectoenzyme NTPPPH in culture medium (P < 0.01). As some RA effects are mediated through increased activity of TGFbeta, a known PPi stimulant, we examined the effect of anti-TGFbeta antibody on RA-induced PPi elaboration. PPi levels in medium were reduced from 30 +/- 7 microM in cartilage cultures with 500 nM RA to 14 +/- 4 microM PPi in cartilage cultures with RA and anti-TGFbeta. Anti-TGFbeta antibody, however, had no significant effect on RA-induced PPi elaboration in chondrocyte cultures. Thus, RA, along with TGFbeta and ascorbate, can now be included in the list of known PPi stimulants. All three of these factors promote mineralization in growth plate cartilage. These data support a central role for TGFbeta in CPPD disease, and provide further evidence linking processes of normal and pathologic calcification in cartilage.

Interleukin 1 beta suppresses transforming growth factor-induced inorganic pyrophosphate (PPi) production and expression of the PPi-generating enzyme PC-1 in human chondrocytes

Articular cartilage chondrocytes have the unique ability to elaborate large amounts of extracellular pyrophosphate (PPi), and transforming growth factor beta (TGF beta) appears singular among cartilage regulatory factors in stimulating PPi production. TGF beta caused a time and dose-dependent increase in intracellular and extracellular PPi in human articular chondrocyte cultures. TGF beta and interleukin 1 beta (IL-1 beta) antagonistically regulate certain chondrocyte functions. IL-1 beta profoundly inhibited basal and TGF beta-induced PPi elaboration. To address mechanisms involved with the regulation of PPi synthesis by IL-1 beta and TGF beta, we analyzed the activity of the PPi-generating enzyme NTP pyrophosphohydrolase (NTPPPH) and the PPi-hydrolyzing enzyme alkaline phosphatase. Human chondrocyte NTPPPH activity was largely attributable to plasma cell membrane glycoprotein 1, PC-1. Furthermore, TGF beta induced comparable increases in the activity of extracellular PPi, intracellular PPi, and cellular NTPPPH and in the levels of PC-1 protein and mRNA in chondrocytes as well as a decrease in alkaline phosphatase. All of these TGF beta-induced responses were completely blocked by IL-1 beta. Thus, IL-1 beta may be an important regulator of mineralization in chondrocytes by inhibiting TGF beta-induced PPi production and PC-1 expression.

Ageing increases growth factor-induced inorganic pyrophosphate elaboration by articular cartilage

Advanced age is the most common risk factor for the development of calcium pyrophosphate dihydrate (CPPD) crystal-associated arthritis. However, the link between ageing and CPPD crystal formation in cartilage remains unexplained. In CPPD deposition disease, excess extracellular inorganic pyrophosphate (ePPi), generated by articular chondrocytes, accumulates in affected joints and contributes to CPPD crystallogenesis. Transforming growth factor beta 1 (TGF beta 1) is the first known physiologic stimulant of ePPi elaboration by adult porcine and human cartilage. We hypothesized that sensitivity of articular cartilage to the ePPi-stimulatory effects of TGF beta 1 may increase with ageing. Accordingly, we compared the effects of TGF beta 1 on cartilage ePPi elaboration from juvenile, young adult, and old adult pigs. Cartilage organ cultures from old animals increased ePPi elaboration in response to TGF beta 1 to a greater extent than did cartilage from juvenile and young adult animals. Similar results were seen in chondrocyte monolayers. Concurrent exposure to epidermal growth factor (EGF) augmented, but was not necessary for TGF beta 1-induced ePPi elaboration by adult cartilage. In contrast, in juvenile cartilage, concurrent exposure to EGF was required to permit TGF beta 1-induced ePPi elaboration. Thus, increased cartilage responsiveness to the ePPi-stimulatory effects of TGF beta 1 occurs with ageing, and may explain the link between advanced age and CPPD deposition disease.

Crystals and arthritis

Monosodium urate, calcium pyrophosphate dihydrate, and basic calcium phosphate (carbonate-substituted hydroxyapatite and octacalcium phosphate) crystal aggregates are associated with gout, pseudogout, and cartilage degeneration (osteoarthritis, Milwaukee Shoulder/Knee Syndrome), respectively. Hyperuricemia is a frequent but nonspecific and inconstant feature of gout just as an elevated synovial fluid inorganic pyrophosphate level is an inconstant feature of pseudogout. Monosodium urate, calcium pyrophosphate dihydrate, or basic calcium phosphate crystals can cause acute inflammation associated with phagocytosis by neutrophilic leukocytes. Each induces neutral protease synthesis and secretion and arachidonic acid metabolism by synoviocytes and macrophages in a dose-dependent fashion, postulated to produce the damage to bone, cartilage, and other joint tissues that is perceived clinically as tophaceous destruction or degenerative joint disease. Crystals containing calcium are potent mitogens. All three types of crystals are more common in older persons and will attract additional attention as the mean age of our population increases. Gout is perhaps the most treatable disease in medicine, although mistakes in diagnosis and in choice of appropriate therapy are very common. Acute pseudogout and acute calcific periarthritis are readily treated medically, but the chronic effects of crystals containing calcium are not. New approaches using drugs derived from scientific study of the biologic effects of these crystals may become useful therapeutically.