TRANSFORMING GROWTH-FACTOR-BETA INCREASES ECTO-NUCLEOSIDE TRIPHOSPHATE PYROPHOSPHATASE ACTIVITY OF HUMAN BONE-DERIVED CELLS

Inorganic pyrophosphate (PPi) may be involved in the regulation of min eralization. The cell surface enzyme, ecto-NTP pyrophosphatase, could be a major source of extracellular PPi in bone, and agents that influe nce its activity in osteoblasts may modulate bone mineralization. We s tudied the effects of serum on the ecto-NTP pyrophosphatase activity o f cultured human osteoblast-like cells. Enzyme activity was lowered wh en the concentration of fetal calf serum (FGS) was reduced from 10 to 2.5% (vol/vol) for 48 h, and a further decrease in activity was observ ed after 96 h. Relative to enzyme activity in cells cultured in serum- free medium for 96 h, adult human platelet-poor plasma (HPPP; 2.5-10% vol/vol) induced a small increase, similar concentrations of adult hum an serum (HS) induced much larger increases, and charcoal-depleted FCS was ineffective. In an attempt to identify the factor(s) present in s erum that influence ecto-NTP pyrophosphatase activity, we examined tra nsforming growth factor-beta (TGF-beta) and platelet-derived growth fa ctor (PDGF). PDGFs AA, AB, and BB (0.1-10 ng/ml) were ineffective, but both TGF-beta(1) and TGF-beta(2) increased enzyme activity. The incre ase was dose dependent between 0.001 and 10 ng/ml, was enhanced in the presence of 2% vol/vol FCS, and was not potentiated by PDGF or by 1,2 5-(OH)(2)D-3. Furthermore, the increase was independent of cell densit y and was blocked by inhibitors of protein and RNA synthesis. Ecto-NTP pyrophosphatase of subject-matched human dermal fibroblasts was unaff ected by TGF-beta (10 ng/ml), suggesting that modulation of activity b y the growth factor may be tissue specific. Alkaline phosphatase (ALP) probably serves to hydrolyze extracellular PPi in bone. In contrast t o effects on NTP pyrophosphatase activity in osteoblast-like cells, TG F-beta(1) and TGF-beta(2) (0.001-10 ng/ml) decreased ALP activity dose dependently after 72 h. By inducing opposing changes in ecto-NTP pyro phosphatase and ALP activities, TGF-beta may increase extracellular PP i concentrations in osseous tissues and consequently modulate bone min eral properties in vivo.