Urinary and Serum Amino Acids May Be Associated with Podocyte, Proximal Tubule, and Renal Endothelial Injury in Early Diabetic Kidney Disease in Type 2 Diabetes Mellitus Patients

Background/Objectives: The pathogenesis of diabetic kidney disease (DKD) is complex and multifactorial. Because of its complications and reduced number of diagnostic biomarkers, it is important to explore new biomarkers with possible roles in the early diagnosis of DKD. Our study aims to investigate the pattern of previously identified metabolites and their association with biomarkers of endothelial dysfunction, proximal tubule (PT) dysfunction, and podocyte injury. Methods: A total of 110 participants, comprising 20 healthy individuals and 90 patients divided in three groups were enrolled in the study: normoalbuminuria, microalbuminuria, and macroalbuminuria. Untargeted and targeted metabolomic methods were employed to assess urinary and serum biomarkers, as well as indicators of endothelial dysfunction, podocyte damage, and PT dysfunction through ELISA techniques. Results: Our research uncovered specific metabolites that exhibit varying levels across different sub-groups. Notably, glycine serves as a distinguishing factor between group C and the normoalbuminuric group. Furthermore, glycine is correlated with endothelial markers, especially VCAM. We observed a gradual decrease in kynurenic acid levels from group C to group P3; this biomarker also demonstrates an inverse relationship with both p-selectin and VCAM. Additionally, tryptophan levels decline progressively from group C to group P3, accompanied by a negative correlation with p-selectin and VCAM. Urinary tiglylglycine also differentiates among the patient groups, with concentrations decreasing as the condition worsens. It shows a strong positive correlation with nephrin, podocalyxin, KIM1, and NAG. Conclusions: In conclusion, glycine, tiglylglycine, kynurenic acid and tryptophan may be considered putative biomarkers for early diagnosis of DKD and T2DM progression.