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 - RELEVANCE TO CALCIUM PYROPHOSPHATE DIHYDRATE DEPOSITION DISEASE

Objective. In subjects with idiopathic calcium pyrophosphate dihydrate (CPPD) deposition disease, cartilage chondrocytes elaborate increased amounts of PPi. The mechanism of the intracellular PPi elevation is n ot known. Plasma membrane 5'-nucleotide phosphodiesterase I/nucleotide pyrophosphohydrolase (NTPPPH) activity also is elevated in chondrocyt es and dermal fibroblasts of patients with idiopathic CPPD deposition disease. NTPPPH, as an ecto enzyme, could act within certain intracell ular compartments. Thus, we hypothesized a potential causal link betwe en increased NTPPPH activity and increased intracellular PPi. Methods. Transformed simian fibroblasts (COS cells) and human osteoblasts (U20 S cells) were transfected with the 5'-nucleotide phosphodiesterase I e ctoenzyme plasma cell membrane glycoprotein-1 (PC-1), recently shown t o be expressed in cartilage, osteoblasts, and fibroblasts. Results. Tr ansfection with PC-1 markedly upregulated 5'-nucleotode phosphodiester ase I activity and increased intracellular PPi concentrations by incre asing the capacity of cells to generate PPi. Importantly, this did not require supplementation with exogenous nucleotides. Conclusion. Cellu lar overexpression of PC-1 produces NTPPPH overactivity and increased intracellular PPi generation in vitro. These findings support the pote ntial importance of NTPPPH overactivity in PPi generation, both inside and outside the cell, in some subjects with CPPD deposition disease.