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  • Title: NaPO(4) cotransport type III (PiT1) expression in human embryonic kidney cells and regulation by PTH.
    Author: Fernandes I, Béliveau R, Friedlander G, Silve C.
    Journal: Am J Physiol; 1999 Oct; 277(4):F543-51. PubMed ID: 10516278.
    Abstract:
    The aim of the present study was to characterize the type(s) of NaPO(4) cotransporter expressed in the human renal cell line HEK-293 and its regulation by parathyroid hormone (PTH) in wild-type cells and in cells transfected by the PTH/PTH-related protein (PTHrP) receptor. The results showed that human embryonic kidney HEK-293 cells expressed NaPO(4) cotransporter type III (PiT1) mRNA and protein. In contrast, type I (NPT1) or II (NPT2) cotransporter mRNA were not expressed. Na(+)-dependent phosphate uptake followed a Michaelis-Menten model (apparent maximal transport rate and affinity constant: 23.32 +/- 0.69 nmol PO(4). mg protein(-1). 10 min(-1) and 0.147 +/- 0.014 mM KH(2)PO(4), respectively), was stimulated by phosphate deprivation (maximal increase 24.5 +/- 0.8%, P < 0.001, after 15 h of phosphate deprivation), and was inhibited by increasing pH (3.6 +/- 0.2-fold decrease at pH 8.5, P < 0.0001). It was inhibited in a time- and concentration-dependent fashion by PTH in HEK-293 cells stably transfected by PTH/PTHrP receptors but not in parental HEK-293 cells. Maximal inhibition of Na(+)-dependent phosphate transport was observed at 30 min after the addition of 72 nM PTH-(1-34) (31.5 +/- 2.4% inhibition, P < 0.01). PTH inhibition of phosphate transport was maintained in phosphate-deprived cells and reversed by both GF109203X (10(-6) M) or staurosporine (5.5 nM), two protein kinase C inhibitors. Na(+)-dependent phosphate uptake was also significantly inhibited by phorbol 12-myristate 13-acetate (20.9 +/- 3.9% inhibition, P < 0.001) but not by dibutyril-cAMP (10(-4) M) or forskolin (50 microM). The physiological role played by type III NaPO(4) cotransport expression in the overall renal regulation of phosphate homeostasis remains to be established.
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