| Research Abstract |
Inorganic phosphate (phosphate) is an essential nutrient in the processes of glycolysis, gluconeogenesis, energy metabolism and skeletal mineralization. Type II sodium-dependent phosphate transporter (NPT2) expressed on renal brush border membrane (BBM) serves to physiologically and pathophysiologically regulate phosphate homeostasis. In hereditary X-linked hypophosphatemia (XLH), the NPT2 protein content and the related mRNA content is reduced showing also reduced BBM phosphate transport activity. The gene causing XLH was identified as PHEX (phosphate regulating gene with homologies to endopeptidase on the X-chromosome) and a humoral factor (phosphatonin) inhibiting phosphate transport may be responsible for the renal phosphate loss observed in XLH.To test this hypothesis directly, we prepared OK-B2400 cells expressing the luciferase gene containing human NPT2 gene promoter. The findings suggest that the hypophosphatemic factor is a normal regulator of phosphate reabsorption in the ki
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dney, and hence normally present in serum, but which becomes aberrant in XLH.
Stanniocalcin (STC) is a calcium- and phosphate-regulating hormone produced by the corpuscles of Stannius in bony fishes. The mammalian homologue of STC has recently been reported (STC 1), which stimulates the phosphate uptake of the kidney. The cloning of a second mammalian stanniocalcin (STC2) from the human osteosarcoma cDNA library was identified in our laboratory. The effect of STC2 on the promoter activity of renal NPT2 was first examined, using the culture medium of STC2-transfected CHO cells. The finding suggested that STC2 would inhibit the expression of a renal NPT2 at the transcriptional level. Furthermore, we established the assay method of STC2 after preparing anti-STC2 antibody. STC2 expression is widely distributed an many organs. STC2 is constitutively excreted from chinese hamster ovary (CHO-K1) cells, but not from opposum kidney cell line (OK-B).However excretion of STC2 from OK-B cell was dramatically stimulated by 1,25 (OH) 2D3 mediated by calcium influx, indicating regulatory excretion of SCT2 from renal tubular cells. Therefore, it is concluded the STC2 will provide another dimension of the regulation of bone and mineral metabolism and a good candidate for the putative phosphatonin. Less
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