title: Acidosis is a key regulator of osteoblast ecto-nucleotidase pyrophosphatase/phosphodiesterase 1 (NPP1) expression and activity.
abstract: Previous work has shown that acidosis prevents bone nodule formation by osteoblasts in vitro by inhibiting mineralisation of the collagenous matrix. The ratio of phosphate (Pi ) to pyrophosphate (PPi ) in the bone microenvironment is a fundamental regulator of bone mineralisation. Both Pi and PPi , a potent inhibitor of mineralisation, are generated from extracellular nucleotides by the actions of ecto-nucleotidases. This study investigated the expression and activity of ecto-nucleotidases by osteoblasts under normal and acid conditions. We found that osteoblasts express mRNA for a number of ecto-nucleotidases including NTPdase 1-6 (ecto-nucleoside triphosphate diphosphohydrolase) and NPP1-3 (ecto-nucleotide pyrophosphatase/phosphodiesterase). The rank order of mRNA expression in differentiating rat osteoblasts (day 7) was Enpp1 > NTPdase 4 > NTPdase 6 > NTPdase 5 >  alkaline phosphatase > ecto-5-nucleotidase > Enpp3 > NTPdase 1 > NTPdase 3 > Enpp2 > NTPdase 2. Acidosis (pH 6.9) upregulated NPP1 mRNA (2.8-fold) and protein expression at all stages of osteoblast differentiation compared to physiological pH (pH 7.4); expression of other ecto-nucleotidases was unaffected. Furthermore, total NPP activity was increased up to 53% in osteoblasts cultured in acid conditions (P < 0.001). Release of ATP, one of the key substrates for NPP1, from osteoblasts, was unaffected by acidosis. Further studies showed that mineralised bone formation by osteoblasts cultured from NPP1 knockout mice was increased compared with wildtypes (2.5-fold, P < 0.001) and was partially resistant to the inhibitory effect of acidosis. These results indicate that increased NPP1 expression and activity might contribute to the decreased mineralisation observed when osteoblasts are exposed to acid conditions.
authors.affiliation: Department of Comparative Biomedical Sciences, Royal Veterinary College, London, UK.|Department of Cell and Developmental Biology, University College London, London, UK.
authors.forename: Isabel R
authors.initials: IR
authors.lastname: Orriss
authors.affiliation: Department of Cell and Developmental Biology, University College London, London, UK.
authors.forename: Michelle L
authors.initials: ML
authors.lastname: Key
authors.affiliation: Department of Cell and Developmental Biology, University College London, London, UK.
authors.forename: Mark O R
authors.initials: MO
authors.lastname: Hajjawi
authors.affiliation: Sanford-Burnham Medical Research Institute, La Jolla, California.
authors.forename: José L
authors.initials: JL
authors.lastname: Millán
authors.affiliation: Department of Cell and Developmental Biology, University College London, London, UK.
authors.forename: Timothy R
authors.initials: TR
authors.lastname: Arnett
mesh_terms: D000138:Acidosis / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D015519:Bone Density; D002478:Cells, Cultured; D015971:Gene Expression Regulation, Enzymologic; D006863:Hydrogen-Ion Concentration; D057507:Mice, 129 Strain; D018345:Mice, Knockout; D010006:Osteoblasts / Q000201:enzymology* / Q000473:pathology; D010012:Osteogenesis; D010727:Phosphoric Diester Hydrolases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D011755:Pyrophosphatases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D013997:Time Factors; D015854:Up-Regulation
chemical_list: D012333:RNA, Messenger; D000255:Adenosine Triphosphate; D010727:Phosphoric Diester Hydrolases; C065962:ectonucleotide pyrophosphatase phosphodiesterase 1; D011755:Pyrophosphatases