Metabolic Regulation in Diabetic Kidney Disease

Abstract

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease in the US and developed countries. Metabolomics studies have demonstrated that significant renal metabolic reprogramming occurs in DKD and potentially contributes to disease pathogenesis and progression. In this thesis, I explored the regulatory mechanisms of nutrient metabolism in DKD and how this altered metabolism may serve as a biomarker of disease severity and progression. We found that malonylation, a non-enzymatically acylated post-translation modification (PTM), is reduced in the kidney cortex of the type 2 diabetic T2D db/db mice and that this process is regulated by Sirtuin 5 (SIRT5), which removes negatively charged lysine modifications such as malonylation. Proteomic analysis of db/db and db/+ kidney cortex found that targets with significant reduction in malonylation are enriched in non-mitochondrial metabolic pathways such as glycolysis/gluconeogenesis and peroxisomal fatty acid oxidation (FAO). We generated diabetic SIRT5 knockout (KO) and overexpression (OE) mouse models to understand the role of SIRT5 in DKD with high-fat diet and streptozotocin (HFD-STZ) treatment. We found that increased SIRT5 levels are protective against DKD; SIRT5 OE mice were relatively protected against DKD. These studies highlight an important adaptive role for SIRT5 in DKD. However, we also observed a large effect of mouse genetic background strains on the development of diabetes and kidney disease, especially for SIRT5 KO mice, confounding our observations. We then investigated potential biomarkers of DKD. We found that higher levels of L and D 2-hydroxyglutarate (2-HG) are found in the urine of db/db mice and that this increase likely driven by increased kidney tricarboxylic acid (TCA) cycle levels and hypoxia and acidosis that occurs in DKD. Elevations in 2-HG may lead to altered epigenetic programming, in particular by inhibiting α-ketoglutarate-dependent dioxygenases and may contribute to DKD. We also conducted a study to assess whether increased glucose flux, which has been demonstrated to occur in the db/db kidney cortex, also occurs in diabetic DKD patients. We conducted euglycemic and hyperglycemic clamp studies with 13C6 - glucose in healthy subjects, patients with type 1 diabetes (T1D) and no microvascular complications and patients with T1D and DKD. We analyzed urine collected during the clamp studies for relative glucose incorporation and found decreased incorporation of glucose into urinary metabolites, especially TCA cycle metabolites, in the T1D subjects. Our findings suggest that glucose incorporation into TCA cycle is blunted in T1D and potentially reflects reduced mitochondrial activity in the diabetic kidney. However, as urine metabolites are a culmination of systemic and renal metabolism, further investigation is necessary to understand exactly how glucose metabolism is altered between the three groups.