Up-regulated expression of the phosphodiesterase nucleotide pyrophosphatase family member PC-1 is a marker and pathogenic factor for knee meniscal cartilage matrix calcification

Objective. Elevated cartilage inorganic pyrophosphate (PPi) production and PPi-generating nucleoside triphosphate pyrophosphohydrolase (NTPPPH) activi ty are strongly linked with aging-related cartilage calcification in menisc al and articular cartilages. We hypothesized that there were divergent rela tionships of 3 NTPPPH isozymes with cartilage matrix calcification and soug ht to identify them. Methods. We studied knee medial meniscal expression in situ of 3 NTPPPH iso zymes of the phosphodiesterase nucleotide pyrophosphatase (PDNP) family: pl asma cell membrane glycoprotein 1 (PC-1, or PDNP1), autotaxin (ATX, or PDNP 2), and BIO/PDNP3. We also used complementary DNA transfection to assess di fferential functions in matrix calcification of each NTPPPH isozyme in vitr o in meniscal cells. Results. We observed diffuse cell-associated ATX and BIO/PDNP3 expression i n central (chondrocytic) and, to a lesser degree, peripheral (fibroblastic) regions of normal, degenerative uncalcified, and degenerative calcified me nisci. In contrast, PC-1 expression was only robust at sites of apoptotic c ells and calcification in central regions of degenerative menisci. Only PC- 1 was abundant at the perimeter of meniscal cells and in association with m eniscal cell-derived matrix vesicles (MVs). Because each PDNP-family isozym e was expressed by cells near calcifications, we transfected the isozymes i n nonadherent knee meniscal cells cultured with ascorbic acid, beta -glycer ophosphate, and dexamethasone supplementation to stimulate them to calcify the matrix. PC-1, but not ATX or BIO/PDNP3, consistently promoted increased MV NTPPPH, NW-associated PPi, and extracellular PPi. PC-1 also increased m atrix calcification (with hydroxyapatite crystals) by meniscal cells.,A,TX uniquely induced alkaline phosphatase activity, but promoted only moderatel y increased matrix calcification. Conclusion. We identified divergent effects of 3 PDNP-family NTPPPH isozyme s on meniscal cell matrix calcification. Increased expression of PC-1 is bo th a marker and a potential pathogenic factor for knee meniscal cartilage m atrix calcification.