Glucose filtered in the glomerulus is actively reabsorbed by sodium-glucose co-transporter 2 (SGLT2) in proximal tubular epithelial cells (PTEC) and passively returned to the blood via glucose transporter 2 (GLUT2). Healthy PTEC rely primarily on fatty acid beta-oxidation (FAO) for energy. In phase III trials, SGLT2 inhibitors improved outcomes in diabetic kidney disease (DKD). Tubulointerstitial renal fibrosis due to altered metabolic reprogramming of PTEC might be at the root of the pathogenesis of DKD. Here, we investigated the molecular mechanism of SGLT2 inhibitors' renoprotective effect by examining transcriptional activity of Spp1, which encodes osteopontin, a key mediator of tubulointerstitial renal fibrosis. With primary cultured PTEC from Spp1-enhanced green fluorescent protein knock-in mice, we proved that in high-glucose conditions, increased SGLT2- and GLUT-mediated glucose uptake is causatively involved in aberrant activation of the glycolytic pathway in PTEC, thereby increasing mitochondrial reactive oxygen species (ROS) formation and transcriptional activation of Spp1. FAO activation did not play a direct role in these processes, but elevated expression of a tubular-specific enzyme, myo-inositol oxygenase, was at least partly involved. Notably, canagliflozin blocked overexpression of myo-inositol oxygenase. In conclusion, SGLT2 inhibitors exerted renoprotective effects by inhibiting aberrant glycolytic metabolism and mitochondrial ROS formation in PTEC in high-glucose conditions.
Keywords: 2-deoxy-D-glucose; GLUT2; SGLT2; myo-inositol oxygenase; osteopontin; proximal tubular epithelial cells.