MO551: Targeted Ablation of PTEN in Osteocytes Induces Severe Anemia and Increases FGF23 Production in Renal Insufficiency

Abstract BACKGROUND AND AIMS Renal disease patients present bone alterations due to an imbalance of mineral metabolism. Previous studies have demonstrated that mice with conditional ablation of the phosphatase and tensin homolog (PTEN) in osteoblasts show increases in bone volume. The aim of this study was to investigate whether PTEN ablation in osteocytes has potential benefits on bone and mineral metabolism in the context of renal insufficiency. METHOD A few 2-month-old mice with constitutive ablation of PTEN in osteocytes (Ocy-PTEN-cKO) underwent 5/6 nephrectomy (5/6Nx) and were fed a 0.9% phosphate diet. After 2 weeks, 24-h urine samples were collected and animals were sacrificed to obtain blood samples for biochemical studies and organs for histological and biochemical analyses. Bone histomorphometry and microcomputed tomography analyses were performed in femurs. Fibroblast growth factor 23 (FGF23) expression was quantified in bone lysates and intact and c-terminal FGF23 concentrations were measured in plasma samples. Animals with normal renal function and Cre-negative littermates were used as controls. In addition, the effects of PTEN silencing were assessed in vitro in osteocyte-like bone marrow mesenchymal stem cells and in the osteosarcoma cell line SAOS-2. RESULTS Ocy-PTEN-cKO mice showed increased bone volume, mainly in the cortical compartment which showed higher cortical thickness and lower porosity. Of interest, we observed lower haematocrit in the Ocy-PTEN-cKO mice that was much lower in those with 5/6Nx. Anaemia was not associated with a lower renal EPO expression. Bone mRNA and protein expression of FGF23 were increased in Ocy-PTEN-cKO mice with normal and reduced renal function. 5/6Nx resulted in higher plasma intact FGF23, which remained similar between both, Ocy-PTEN-cKO and control groups, due to increased FGF23 cleavage as indicated by the higher plasma c-terminal FGF23 levels in the Ocy-PTEN-cKO groups, resulting in increased plasma phosphate. In vitro, PTEN silencing in bone marrow mesenchymal stem cells differentiated into osteoblasts or in the osteoblastic cell line SAOS-2 did not show significant changes regarding FGF23 levels, indicating that the effects must be indirect. CONCLUSION PTEN ablation in osteocytes increases cortical bone volume and exacerbates anemia in mice with reduced renal function. Moreover, bone FGF23 production was increased but subsequent proteolysis resulted in higher plasma phosphate. These results demonst