FN3K deficiency drives tubular injury in diabetic kidney disease through impaired mitochondrial respiratory chain function.

AIMS: Despite growing evidence implicating FN3K in the pathogenesis of cancer and diabetes, its precise role in modulating renal tubular injury within the context of diabetic nephropathy has yet to be fully elucidated. MATERIALS AND METHODS: FN3K expression was examined in DKD model mice, human kidney biopsy samples, and high glucose-treated HK-2 cells via immunohistochemistry (IHC) and Western blotting. Associations between serum FN3K levels and DKD risk were analyzed in the Kidney Precision Medicine Project cohort via multivariable logistic regression, restricted cubic spline modelling, and receiver operating characteristic (ROC) analysis. Regional proteomics, single-cell RNA sequencing, untargeted metabolomics, and joint pathway analyses were integrated to delineate FN3K-associated pathways and cell type-specific metabolic alterations. The effect of FN3K on mitochondrial function was evaluated using Mitochondrial respiratory chain activity assays. RESULTS: FN3K expression was significantly reduced in the renal tubular compartments of DKD mice and patients and was downregulated by high glucose in HK-2 cells. Low circulating FN3K levels were independently associated with increased DKD risk and improved diagnostic performance when combined with conventional biomarkers. Proteomic and single-cell analyses revealed the activation of PI3K/AKT signaling and the suppression of mitochondrial metabolic pathways in DKD tubules, whereas FN3K-high proximal tubule cells presented increased oxidative phosphorylation and reduced inflammatory and apoptotic signaling. Metabolomic profiling demonstrated that FN3K overexpression restored the levels of mitochondrial- and nitrogen-related metabolites and attenuated injury-associated lipid remodeling. Joint pathway analysis highlighted nitrogen and glutathione metabolism as convergent FN3K-related signatures. Over-expression of FN3K would improves the function of mitochondrial respiratory chain I and III activity. CONCLUSIONS: FN3K deficiency is associated with tubular injury and DKD progression, potentially through increased glycation stress, mitochondrial metabolic dysfunction.