Cartilage inorganic pyrophosphate elaboration is independent of sulfated glycosaminoglycan synthesis

Abstract

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 PP, was measured. When stimulated by 0.8 mM xyloside to increase 35S0, incorporation (mean f SEM % of control 183 f 16, n = 5), PP, accumulation changed little (from 54 2 6 pmoleshg to 63 -+ 8 pmoleshg of cartilage wet weight). Inhibition of sulfation with monensin or diethylcarbamazine disproportionately lowered 35S0, incorporation compared with PP, elaboration. Using 60 mM diethylcarbamazine, PP, production was preserved (105 -+ 8% mean -t SEM) compared with control cultures, while sulfation was markedly inhibited (7 f 1 %). This dissociation of sulfate incorporation and PP, secretion indicates that it is not likely that glycosaminoglycan sulfation is the source of the PP, that escapes from chondrocytes to participate in the formation of extracellular crystals.

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

“…Because PPi does not freely diffuse across membranes, it was hypothesized that the PPi might be released through an anion channel (23) or cosecreted with basic proteins such as collagen (24). Chondrocyte PPi elaboration is not dependent on de novo synthesis of glycosaminoglycans (25,26) but appears to be partially mediated by de novo protein synthesis (24). In this regard, TGF3 treatment of porcine chondrocytes up-regulated membrane and extracellular NT-PPPH activity by up to 36% (22).…”

Causal link between nucleotide pyrophosphohydrolase overactivity and increased intracellular inorganic pyrophosphate generation demonstrated by transfection of cultured fibroblasts and osteoblasts with plasma cell membrane glycoprotein–1

“…The source of the intracellular PP, that is secreted is not clear (20). Studies analyzing intracellular PP, in vivo from different groups of CPPD deposition disease patients have been limited because of difficulties in standardization needed to compare PPi in cell populations (8).…”

“…In cartilage, extracellular PP, production mediated by NTPPPH is complemented by the relatively unique ability of chondrocytes to secrete substantial amounts of intracellular PPi (18,19). The source of the intracellular PP, that is secreted is not clear (20). Studies analyzing intracellular PP, in vivo from different groups of CPPD deposition disease patients have been limited because of difficulties in standardization needed to compare PPi in cell populations (8).…”