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Abstract: PO0532

Renal FGF-23 Resistance by Phosphate Leads to NaPi-2a Internalization via Activated PiT-2/ERK1/2 Signaling in Proximal Tubule

Session Information

Category: Bone and Mineral Metabolism

  • 401 Bone and Mineral Metabolism: Basic

Authors

  • Richter, Beatrice, Hannover Medical School Department of Pediatric Kidney, Liver and Metabolic Diseases, Hannover, Germany
  • Walter, Stefanie, Hannover Medical School Department of Pediatric Kidney, Liver and Metabolic Diseases, Hannover, Germany
  • Vogt, Isabel, Hannover Medical School Department of Pediatric Kidney, Liver and Metabolic Diseases, Hannover, Germany
  • Schmitt, Roland, Hannover Medical School Department of Nephrology and Hypertension, Hannover, Germany
  • Haffner, Dieter, Hannover Medical School Department of Pediatric Kidney, Liver and Metabolic Diseases, Hannover, Germany
  • Leifheit-Nestler, Maren, Hannover Medical School Department of Pediatric Kidney, Liver and Metabolic Diseases, Hannover, Germany
Background

The bone-derived hormone fibroblast growth factor (FGF) 23 targets the kidney to promote urinary phosphate (Pi) excretion by activating the FGFR1/Klotho/ERK1/2 signaling in renal proximal tubule (PT) cells. This reduces type II sodium phosphate cotransporters NaPi-2a and NaPi-2c in the apical brush border membrane (BBM) lowering serum Pi levels. In vitro data show that under high extracellular Pi, the type III sodium-dependent phosphate transporters PiT-1 and PiT-2 activate ERK1/2 in bone cells. Moreover, Pi-regulated osseous FGF23 secretion is facilitated via PiT-2. Here we aim to analyze Pi versus FGF23 regulated Pi transport in the setting of high phosphate load in renal PT cells.

Methods

We subject C57BL/6N male mice to increased dietary Pi load (0.8% vs. 2%) for 6 months to determine phosphate homeostasis and analysing kidneys by qPCR, immunoblot and histology. In addition, we study cultured renal PT cells treated with phosphate or FGF23 to examine the activation of downstream signaling events and expression levels of specific target genes. Furthermore, we determine if co-treatment with the phosphate transporter inhibitor Foscarnet blocks the observed effects.

Results

A 2% Pi diet promotes the elevation of plasma FGF23 levels in mice. Despite reduced TRP, increased FEPi and urine Pi levels, the serum Pi levels are still enhanced. In animals fed a 2% Pi diet, we observed reduced renal NaPi-2a mRNA expression. Immunofluorescence staining revealed internalization of NaPi-2a from the apical BBM due to the high dietary Pi load. This is confirmed by analysing BBM vesicles. Interestingly, mice on 2% Pi diet have a diminished renal Klotho expression, but unaltered Fgfr1 expression. PiT-2 expression is increased and accumulated in the basolateral membrane of PT. In cultured PT cells, Pi enhanced PiT-2 expression. The Pi-mediated increase in ERK1/2 phosphorylation was blocked by Foscarnet co-treatment.

Conclusion

Hyperphosphaturia might be a result of PiT-2/ERK1/2-mediated downregulation of NaPi-2a stimulated by Pi itself. Our study indicates these Pi-mediated effects may be independent of FGF23. We postulate that high dietary Pi load causes a resistance of renal FGF23/Klotho signaling.