ESPN 53rd Annual Meeting

ESPN 2021


 
Renal activation of PiT-2/ERK1/2 signalling by phosphate mediates internalization of NaPi-2a in the proximal tubule independent of FGF23
BEATRICE RICHTER 1 STEFANIE WALTER 1 ISABEL VOGT 1 ROLAND SCHMITT 2 DIETER HAFFNER 1 MAREN LEIFHEIT-NESTLER 1

1- DEPARTMENT OF PAEDIATRIC KIDNEY, LIVER AND METABOLIC DISEASES, PAEDIATRIC RESEARCH CENTRE, HANNOVER MEDICAL SCHOOL, HANNOVER, GERMANY
2- DEPARTMENT OF NEPHROLOGY AND HYPERTENSION, HANNOVER MEDICAL SCHOOL, HANNOVER, GERMANY
 
Introduction:

In renal proximal tubule (PT) cells, bone derived fibroblast growth factor (FGF) 23 activates the FGFR1/Klotho/ERK1/2 signalling to reduce the type II sodium phosphate transporters NaPi-2a and NaPi-2c in the apical brush border membrane (BBM) causing lower serum phosphate levels. In the presence of high extracellular phosphate, the type III sodium-dependent phosphate transporters PiT-1 and PiT-2 activate ERK1/2 in bone cells in vitro. It has been shown that PiT-2 is responsible for the phosphate-mediated osseous FGF23 secretion. Yet, it is unknown if phosphate sensing plays a role in renal PT cells and thereby regulates its own excretion. Thus, we aimed to analyse renal actions of phosphate in the setting of chronic high phosphate loading independent of FGF23.

Material and methods:

C57BL/6N male mice were fed a 0.8% normal phosphate diet (NPD) or a 2% high phosphate diet (HPD) for 6 months, phosphate homeostasis was determined and kidneys were analysed by qPCR, immunoblot and histology. Cultured murine PT cells were treated with phosphate or FGF23 with or without the phosphate transporter inhibitor Foscarnet followed by analyses.

Results:

Elevated plasma FGF23 levels in HPD-fed mice resulted in reduced TRP and increased FEPi with still enhanced serum phosphate levels compared to NPD. HPD diminished renal Klotho protein, while Fgfr1 was unaltered. Although ERK1/2 activation was inconsistent, NaPi-2a mRNA expression was significantly reduced in HPD-fed mice compared to NPD. Analysing BBM vesicles and immunofluorescence staining confirmed NaPi-2a internalization from the apical BBM. HPD-fed mice showed increased PiT-2 expression and enhanced accumulation on the basolateral membrane of PT. In cultured murine PT cells, Foscarnet prevented the phosphate-mediated PiT-2 upregulation and ERK1/2 phosphorylation but not the FGF23-induced effects.

Conclusions:

In the setting of high phosphate-mediated renal resistance of FGF23/Klotho signalling, phosphate itself may stimulate its urinary secretion via PiT-2/ERK1/2-mediated downregulation of NaPi-2a resulting in hyperphosphaturia FGF23-independent.