Background/Aims: Fatty acid-binding proteins (FABPs) are a family of intracellular lipid chaperones. Among FABPs, FABP1 (liver FABP) is expressed in proximal tubular epithelial cells in the kidney, and urinary FABP1 has been reported to reflect damage of proximal tubular epithelial cells. However, roles of other FABP isoforms in renal pathologies have not been reported. Recently, FABP4 (adipocyte FABP/aP2) was reported to be expressed in peritubular capillaries (PTCs), but not in glomerular capillaries in the normal kidney. We examined the hypothesis that pathological conditions alter the level and localization of FABP4 expression in the kidney, which mediates renal dysfunction. Methods: A total of 112 consecutive patients who underwent renal biopsy were retrospectively enrolled. Expression of FABP4 protein and mRNA in the kidney was examined by immunohistochemistry and in situ hybridization, respectively. The ratio of FABP4-positive area to total area within glomeruli (G-FABP4-Area), urinary protein level (U-Protein), and change in estimated glomerular filtration rate (eGFR) 1 year after biopsy were examined. Results: FABP4 protein and mRNA were expressed not only in PTCs, but also in endothelial cells and macrophages in the glomerulus. G-FABP4-Area was correlated with U-Protein (r = 0.497, p < 0.001). As a subanalysis, in patients with IgA nephropathy (n = 34), G-FABP4-Area was significantly larger in cases with an endocapillary proliferative lesion, and change in eGFR was negatively correlated with G-FABP4-Area at baseline (r = -0.537, p = 0.008). Conclusion: Ectopic FABP4 expression in the glomerulus is induced by renal diseases and is closely associated with proteinuria and renal dysfunction.

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