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      Effect of Epidermal Growth Factor on Phosphate Uptake in Renal Proximal Tubule Cells: Involvement of PKC, MAPK, and cPLA 2

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          Objective: The present study was conducted to examine the effect of epidermal growth factor (EGF) on Pi uptake and its related signal pathways in the primary cultured renal proximal tubule cells (PTCs). Results: EGF (50 ng/ml) inhibited Pi uptake, a typical marker of Na<sup>+</sup>/phosphate cotransporter, in a time- and dose-dependent manner. EGF-induced inhibition of Pi uptake was blocked by AG1478 (an EGF receptor antagonist), genistein or herbimycin A (tyrosine kinase inhibitors) and also blocked by mepacrine (a phospholipase A<sub>2</sub> (PLA<sub>2</sub>) inhibitor) and AACOCF<sub>3 </sub>(a cPLA<sub>2</sub> inhibitor). EGF increased [<sup>3</sup>H]-arachidonic acid (AA) release, which was also blocked by AG1478, genistein or herbimycin. Furthermore, EGF-induced inhibition of Pi uptake was blocked by indomethacin (a cyclooxygenase inhibitor) and econazole (a cytochrome P-450 epoxygenase inhibitor), but not by NDGA (a lipoxygenase inhibitor). On the other hand, EGF-induced inhibition of Pi uptake was blocked by staurosporine, H-7, or bisindolylmaleimide I (PKC inhibitors), PD 98059 (a p44/42 MAPK inhibitor), but not by SB 203580 (a p38 MAPK inhibitor). EGF-induced increase of [<sup>3</sup>H]-AA release was blocked by PKC inhibitors and a p44/42 mitogen-activated protein kinase (MAPK) inhibitor, but not by a p38 MAPK inhibitor. In addition, a PKC inhibitor blocked EGF-induced phosphorylation of p44/42 MAPK. Conclusion: EGF inhibits Pi uptake via PKC-p44/42 MAPK-cPLA<sub>2</sub> pathway in the PTCs.

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          Most cited references 14

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          Parathyroid hormone leads to the lysosomal degradation of the renal type II Na/Pi cotransporter.

          We have studied the involvement of proteolytic pathways in the regulation of the Na/Pi cotransporter type II by parathyroid hormone (PTH) in opossum kidney cells. Inhibition of lysosomal degradation (by leupeptin, ammonium chloride, methylamine, chloroquine, L-methionine methyl ester) prevented the PTH-mediated degradation of the transporter, whereas inhibition of the proteasomal pathway (by lactacystin) did not. Moreover it was found (i) that whereas lysosomal inhibitors prevented the PTH-mediated degradation of the transporter they did not prevent the PTH-mediated inhibition of the Na/Pi cotransport and (ii) that treating opossum kidney cells with lysosomal inhibitors led to an increased expression of the transporter without any concomitant increase in the Na/Pi cotransport. Further analysis by subcellular fractionation and morphological techniques showed (i) that the Na/Pi cotransporter is constitutively transported to and degraded within late endosomes/lysosomes and (ii) that PTH leads to the increased degradation of the transporter in late endosomes/lysosomes.
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            Characterization of primary rabbit kidney cultures that express proximal tubule functions in a hormonally defined medium

            Primary cultures of rabbit-kidney epithelial cells derived from purified proximal tubules were maintained without fibroblast overgrowth in a hormone-supplemented serum-free medium (Medium RK-1). A hormone- deletion study indicated that the primary cultures derived from purified rabbit proximal tubules required all of the three supplements in Medium RK-1 (insulin, transferrin, and hydrocortisone) for optimal growth but did not grow in response to EGF and T3. In contrast, the epithelial cells in primary cultures derived from an unpurified preparation of rabbit kidney tubules and glomeruli grew in response to EGF and T3, as well as insulin, transferrin, and hydrocortisone. These observations suggest that kidney epithelial cells derived from different segments of the nephron grow differently in response to hormones and growth factors. Differentiated functions of the primary cultures derived from proximal tubules were examined. Multicellular domes were observed, indicative of transepithelial solute transport by the monolayers. The proximal tubule cultures also accumulated alpha- methylglucoside (alpha-MG) against a concentration gradient. However, little or no alpha-MG accumulation was observed in the absence of Na+. Metabolic inhibitor studies also indicated that alpha-MG uptake by the primaries is an energy-dependent process, and depends upon the activity of the Na+/K+ ATPase. Phlorizin at 0.1 mM significantly inhibited 1 mM alpha-MG uptake whereas 0.1 mM phloretin did not have a significant inhibitory effect. Similar observations have been made concerning the Na+-dependent sugar-transport system located on the lumenal side of the proximal tubule, whereas the Na+-independent sugar transporter on the peritubular side is more sensitive to inhibition by phloretin than phlorizin. The cultures also exhibited PTH-sensitive cyclic AMP synthesis and brush-border enzymes typical of proximal cells. However, the activities of the enzymes leucine aminopeptidase, alkaline phosphatase, and gamma-glutamyl-transpeptidase were lower in the cultures than in purified proximal-tubule preparations from which they are derived.
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              Regulation of Na/Pi transporter in the proximal tubule.

              The physiological tuning and pathophysiological alterations of renal proximal reabsorption of inorganic phosphate can be ascribed to the net amount of the Na/Pi-cotransporter NaPi-IIa localized in the brush border membrane. The net amount of NaPi-IIa appears to be the result of an endocytotic rate regulated by a complex network of different protein kinases. New approaches demonstrated that NaPi-IIa is part of heteromeric protein complexes, organized by PDZ (postsynaptic protein PSD95, Drosophila junction protein Disc-large, tight junction protein ZO-1) proteins. Such complexes are thought to play important roles in the apical positioning and regulated endocytosis of NaPi-IIa and therefore such interactions have to be considered when explaining proximal phosphate ion reabsorption.

                Author and article information

                Kidney Blood Press Res
                Kidney and Blood Pressure Research
                S. Karger AG
                19 November 2003
                : 26
                : 5-6
                : 315-324
                aDepartment of Veterinary Physiology, College of Veterinary Medicine, Hormone Research Center, Chonnam National University, Gwangju, and bCollege of Veterinary Medicine, Chonbuk National University, Jeonju,Korea
                73937 Kidney Blood Press Res 2003;26:315–324
                © 2003 S. Karger AG, Basel

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                Page count
                Figures: 10, Tables: 1, References: 53, Pages: 10
                Self URI (application/pdf): https://www.karger.com/Article/Pdf/73937
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