Advanced Glycation End Products and Bone Metabolism in Patients with Chronic Kidney Disease

**Background:** Mineral and bone disorder (MBD) is a major complication of chronic kidney disease (CKD), leading to cardiovascular disease, fractures, and mortality. Advanced glycation end products (AGEs) are uremic toxins that may contribute to bone dysfunction, but their effects in CKD patients are poorly understood. This study aimed to comprehensively evaluate AGEs accumulation in blood, skin, and bone and its associations with bone histomorphometry, protein expression, gene expression, and serum biomarkers in CKD patients.

**Methods:** Eighty-six patients with CKD stages 3–5D (mean age 51±13 years; 56% male; 19% with type 2 diabetes; 70% on dialysis) were enrolled from two centers. Serum AGEs were measured as pentosidine, N-carboxymethyl lysine (CML), and glycated hemoglobin (A1c). Skin autofluorescence (SAF) was assessed using an AGE-Reader. Bone biopsies from the iliac crest were analyzed for AGEs and RAGE expression by immunohistochemistry, and for histomorphometry (static, dynamic, structural indices). Bone protein levels (sclerostin, DKK1, FGF-23, osteoprotegerin, osteocalcin) were quantified by multiplex assay, and gene expression (SOST, RANKL, OPG, β-catenin, FGF-23, p53, DKK, osterix, ALP-1, collagen 1, BGLAP) by qPCR. Serum biomarkers included PTH, TRACP-5b, RANKL, FGF-23, and sclerostin. Statistical comparisons were made using median splits for AGEs parameters, with Spearman correlations and Mann-Whitney tests.

**Key Results:** Median serum pentosidine was 71.6 pmol/mL, CML 15.2 ng/mL, A1c 5.4%, and SAF 3.05 AU. Bone AGEs covered 3.92% of trabecular and 5.42% of cortical bone surface; RAGE expression covered 0.7% and 0.83%, respectively. AGEs accumulation in bone was inversely correlated with serum RANKL/PTH ratio (R=-0.25, p=0.03), and RAGE expression was negatively correlated with TRACP-5b/PTH ratio (R=-0.31, p=0.01). Patients with high trabecular bone AGEs (≥3.92%) had significantly lower bone protein levels: sclerostin (1.96 vs. 89.3 ng/mg, p=0.004), DKK1 (0.064 vs. 1.36 ng/mg, p=0.0001), FGF-23 (1.07 vs. 44.1 ng/mg, p=0.01), and osteoprotegerin (0.16 vs. 6.5 ng/mg, p=0.001). Gene expression analysis showed upregulation of p53 and downregulation of DKK1 in the high AGEs group. High serum A1c was associated with greater cortical porosity (1.9% vs. 1.18%, p=0.02), longer mineralization lag time (24.8 vs. 19.1 days, p=0.03), and reduced osteoblast surface/bone surface (1.7% vs. 3.6%, p=0.04), eroded surface/bone surface (2.4% vs. 4.9%, p=0.0001), osteoclast surface/bone surface (0.1% vs. 0.29%, p=0.0001), mineral apposition rate (0.59 vs. 0.78 μm/d, p=0.02), and adjusted apposition rate (0.26 vs. 0.38 μm/d, p=0.009). Cortical thickness was negatively correlated with A1c (R=-0.28, p=0.02) and pentosidine (R=-0.27, p=0.02). No significant differences in bone parameters were observed based on median CML or SAF levels.

**Clinical Implications:** This study provides novel evidence that AGEs accumulate in bone of CKD patients and are associated with reduced expression of key bone proteins (sclerostin, DKK1, FGF-23, osteoprotegerin), altered gene expression (p53 upregulation, DKK1 downregulation), and increased skeletal resistance to PTH. Serum A1c and pentosidine levels were linked to cortical bone deterioration (reduced thickness, increased porosity, prolonged mineralization lag time). These findings suggest that AGEs may contribute to the pathogenesis of CKD-MBD and that reducing AGEs accumulation could be a therapeutic target. However, the observational design limits causal inference, and further interventional studies are needed to confirm these mechanisms and explore potential treatments (e.g., pharmacological or dietary approaches) to mitigate skeletal complications in CKD.