MO044HIGH DIETARY PHOSPHATE INTAKE ENHANCES INTACT FGF23 AND CAUSES ACUTE TUBULAR INJURY WITH SEVERE INFLAMMATION AND FIBROSIS IN UNTREATED MICE
Authors: Vogt Isabel , Walter Stefanie , Veenstra Anna Catharina , Kapanadze Tamar , Schmitz Jessica , Bräsen Jan Hinrich , Schmitt Roland , Limbourg Florian , Haffner Dieter , Leifheit-Nestler Maren 
Published: June 2020

Abstract

Background and Aims
In early stages of chronic kidney disease (CKD) rising levels of the phosphaturic hormone fibroblast growth factor (FGF) 23 maintain the phosphate homeostasis, whereas in end-stage kidney disease hyperphosphatemia manifests. Both elevated FGF23 and phosphate levels contribute to CKD progression and are associated with an increased mortality in CKD patients. So far, it is unknown whether high inorganic phosphate uptake causes renal injury as well in the absence of CKD. Therefore, we analyzed the effects of a long-term high phosphate diet (HPD) on the renal function in healthy mice.

Method
We studied the effects of a 2% HPD compared to a 0.8% normal phosphate diet (NPD) fed to healthy male C57BL/6 mice. After six month, blood pressure and metabolic serum and urine parameters were measured. The renal function was characterized by histology, flow cytometry and gene expression analysis. Additionally, we investigate the effects of elevated FGF23 and phosphate on mouse proximal tubular cells (mPT) in vitro.

Results
Feeding of a HPD for 6 months significantly increased systolic blood pressure (95 vs 79 mmHg; p=0.003), plasma creatinine (0.33±0.01 vs 0.29±0.02 mg/dL; p=0.004), C-term FGF23 (1991±394 vs 309±56 pg/mL; p&lt;0.0001), intact FGF23 (1701±429 vs 191±30 pg/mL; p&lt;0.0001) and urinary phosphate excretion (1732±968 vs 520±123 mg/dL; p=0.0005) compared to NPD controls, whereas serum phosphate (14.3±5.3 vs 13.4±2.9 mg/dL; p=0.68) and parathyroid hormone levels (417±104 vs 544±187 pg/mL; p=0.51) remained unchanged. The HPD caused acute tubular injury in the renal cortex with loss of polarity of proximal tubular epithelial cells and enhanced renal fibrosis and inflammation demonstrated by PAS and Picrosirius red staining. These histological pathologies were associated with a 100-fold increase of KIM-1 and a 7-fold increase of NGAL mRNA expression in the kidneys of HPD mice compared to the NPD group. In HPD mice, flow cytometry analysis showed a reduced storage of Ly6Chi monocytes in the spleen, whereas Ly6Chi monocytes, granulocytes, macrophages, B-cells, and CD4+ T-cells (each p&lt;0.05) were significantly induced in the kidney compared to NPD-fed mice. Histological staining’s displayed an accumulation of B- and T-cells in regions of tubular injury in HPD mice. Furthermore, the HPD significantly enhanced the mRNA expression of the chemotactic markers for monocytes Il34 and Ccl2 and inflammatory cytokines Tnfα and Il1β. Whether enhanced phosphaturia directly or HPD-mediated increase of intact FGF23 cause the renal toxicity is currently investigated in mPTs.

Conclusion
Chronic high phosphate load induces kidney injury in the proximal tubules with severe inflammation and fibrosis in healthy mice. Our results suggest that a controlled dietary intake of inorganic phosphate is not only important for CKD patients but also for the general population.