Autophagy-Lysosome Pathway in Renal Tubular Epithelial Cells Is Disrupted by Advanced Glycation End Products in Diabetic Nephropathy

Abstract

It has been suggested that autophagy protects renal tubular epithelial cells (TECs) from injury in diabetic nephropathy (DN). However, the manner in which the autophagy-lysosome pathway is changed in this state remains unclear. In this study of DN, we investigated the autophagic activity and lysosomal alterations in vivo and in vitro. We found that autophagic vacuoles and SQSTM1-positive proteins accumulated in TECs from patients with DN and in human renal tubular epithelial cell line (HK-2 cells) treated with advanced glycation end products (AGEs), the important factors that involved in the pathogenesis of DN. In HK-2 cells, exposure to AGEs caused a significant increase in autophagosomes but a marked decrease in autolysosomes, and the lysosomal turnover of LC3-II was not observed, although LC3-II puncta were co-localized with the irregular lysosomal-associated membrane protein1 granules after AGEs treatment. Furthermore, lysosomal membrane permeabilization was triggered by AGEs, which likely resulted in a decrease in the enzymatic activities of cathepsin B and cathepsin L, the defective acidification of lysosomes, and suppression of the lysosomal degradation of DQ-ovalbumin. Oxidative stress evoked by AGEs-receptor for AGE interaction likely played an important role in the lysosomal dysfunction. Additionally, ubiquitinated proteins were co-localized with SQSTM1-positive puncta and accumulated in HK-2 cells after exposure to AGEs, indicating blocked degradation of SQSTM1-positive and ubiquitinated aggregates. Taken together, the results show that lysosomal membrane permeabilization and lysosomal dysfunction are triggered by AGEs, which induce autophagic inactivation in TECs from patients with DN. Disruption of the autophagy-lysosome pathway should be focused when studying the mechanisms underlying DN.