Regulation of phosphate (Pi) transport and NaPi-III transporter (Pit-1) mRNA in rat osteoblasts.
In osteoblasts only the type III Na(+)-dependent phosphate (NaPi) transporter isoforms Pit-1 and Pit-2 have been identified. We tested the effects of extracellular Pi, Ca(2+) and IGF-I on Na(d)Pi transport and Pit-1 or Pit-2 mRNA expression in rat osteoblastic (PyMS) cells. The v(max) of Na(d)Pi transport was higher in cells kept in Pi-free, serum-free medium for 24 h than in controls at 1 mM Pi (2.47+/-0.20 vs 1.83+/-0.17 nmol/mg protein x 10 min). The apparent affinity constant (K(M)) for Pi remained unchanged. Pi withdrawal for 24 h did not impair cell viability whereas increasing the extracellular Pi to 5 mM resulted in cell death. Pit-1 (but not Pit-2) mRNA was upregulated following Pi deprivation, Ca(2+) treatment or after treatment with 1 nM IGF-I, known to stimulate Na(d)Pi transport and cell proliferation. IGF-I also stimulated Na(d)Pi transport and Pit-1 mRNA in primary rat calvarial osteoblasts. Expression of Pit-1 mRNA in vivo and the coordinate regulation of Pit-1 mRNA and Pi transport in osteoblastic cells suggest that Pit-1 is a candidate transporter of physiological relevance in bone.
Zoidis
E
E
Department of Internal Medicine, University Hospital of Zurich, CH-8091 Zurich, Switzerland.
Ghirlanda-Keller
C
C
Gosteli-Peter
M
M
Zapf
J
J
Schmid
C
C
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D001692:Biological Transport; D015152:Blotting, Northern / Q000379:methods; D002118:Calcium / Q000494:pharmacology; D002460:Cell Line; D015870:Gene Expression / Q000187:drug effects; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D008297:Male; D010006:Osteoblasts / Q000187:drug effects / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D027981:Symporters / Q000235:genetics*
D010710:Phosphates; D012333:RNA, Messenger; C493588:Slc20a1 protein, rat; D050608:Sodium-Phosphate Cotransporter Proteins; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D027981:Symporters; D007334:Insulin-Like Growth Factor I; D000469:Alkaline Phosphatase; D002118:Calcium