Parathyroid hormone and dietary phosphate provoke a lysosomal routing of the proximal tubular Na/Pi-cotransporter type II.
(73/75)
BACKGROUND: A decrease of proximal tubular reabsorption of phosphate (Pi), which can be provoked by parathyroid hormone (PTH) or by a high Pi-diet, has been shown to correlate with a decrease of the number of type II Na/Pi-cotransporters residing in the brush border membrane. While both PTH and a high Pi-diet lead to an internalization of type II cotransporters, the further cellular routing of internalized cotransporters has not been established unequivocally. METHODS: To prevent lysosomal degradation, rats were treated with leupeptin prior to the injection of PTH or feeding acutely with a high Pi-diet. Kidney cortex were recovered and used for immunohistochemistry. In parallel, brush border membranes and lysosomes were isolated and analyzed by Western blotting. RESULTS: Under both conditions (PTH and high Pi-diet), a strong overlap of internalized type II cotransporters with the late endosomes/lysosomes was observed by immunohistochemistry. In agreement, the content of type II Na/Pi-cotransporters was increased in lysosomes isolated from the corresponding tissues. CONCLUSIONS: These results suggest that in proximal tubular cells type II Na/Pi-cotransporters internalized due to the action of PTH and acute high Pi-diet are routed to the lysosomes, and likely do not enter a recycling compartment. (+info)
Characterization of a murine type II sodium-phosphate cotransporter expressed in mammalian small intestine.
(74/75)
An isoform of the mammalian renal type II Na/Pi-cotransporter is described. Homology of this isoform to described mammalian and nonmammalian type II cotransporters is between 57 and 75%. Based on major diversities at the C terminus, the new isoform is designed as type IIb Na/Pi-cotransporter. Na/Pi-cotransport mediated by the type IIb cotransporter was studied in oocytes of Xenopus laevis. The results indicate that type IIb Na/Pi-cotransport is electrogenic and in contrast to the renal type II isoform of opposite pH dependence. Expression of type IIb mRNA was detected in various tissues, including small intestine. The type IIb protein was detected as a 108-kDa protein by Western blots using isolated small intestinal brush border membranes and by immunohistochemistry was localized at the luminal membrane of mouse enterocytes. Expression of the type IIb protein in the brush borders of enterocytes and transport characteristics suggest that the described type IIb Na/Pi-cotransporter represents a candidate for small intestinal apical Na/Pi-cotransport. (+info)
Type II Na-Pi cotransport is regulated transcriptionally by ambient bicarbonate/carbon dioxide tension in OK cells.
(75/75)
The purpose of the present study was to determine whether isohydric changes in HCO3 concentration and PCO2 directly affect apical Na-dependent Pi (Na-Pi) cotransport in OK cells (opossum kidney cell line). Cells were kept at either 44 mM NaHCO3/10% CO2, pH 7.4 (high-HCO3/CO2 condition), or 22 mM NaHCO3/5% CO2, pH 7.4 (low-HCO3/CO2 condition) (for 14-24 h). Incubation in lower HCO3/CO2 concentrations increased Na-Pi cotransport 1.5-fold. The increased Na-Pi cotransport was paralleled by a two- to threefold increased expression of the NaPi-4 transporter protein and a two- to threefold increase in NaPi-4 mRNA abundance. The increase in NaPi-4 mRNA could be completely prevented by incubation in the presence of a transcriptional inhibitor, suggesting that the increase in NaPi-4 mRNA results from an increased NaPi-4 mRNA transcription. In agreement, the NaPi-4 promoter activity was stimulated by 50% at lower HCO3/CO2 concentrations. In conclusion, our data demonstrate that isohydric changes in HCO3 concentration and PCO2 exert a significant, direct cellular effect on Na-Pi cotransport and NaPi-4 protein expression in OK cells by affecting NaPi-4 mRNA transcription. (+info)