Suppression of chaperone-mediated autophagy in the renal cortex during acute diabetes mellitus

Abstract

In the renal hypertrophy that occurs in diabetes mellitus, decreased proteolysis may lead to protein accumulation, but it is unclear which proteins are affected. Because the lysosomal proteolytic pathway of chaperone-mediated autophagy is suppressed by growth factors in cultured cells, we investigated whether the abundance of substrates of this pathway increase in diabetic hypertrophy. Rats with streptozotocin (STZ)-induced diabetes were pair-fed with vehicle-injected control rats. Proteolysis was measured as lysine release in renal cortical suspensions and protein synthesis as phenylalanine incorporation. Target proteins of chaperone-mediated autophagy were measured in cortical lysates and nuclear extracts by immunoblot analysis. Proteins that regulate chaperone-mediated autophagy [the lysosomal-associated membrane protein 2a (LAMP2a) or the heat shock cognate protein of 73 kD (hsc-73)] were measured in lysosomes isolated by density gradient centrifugation. Proteolysis decreased by 41% in diabetic rats; protein synthesis increased at 3 days, but returned to baseline by 7 days. The abundance of proteins containing that chaperone-mediated autophagy KFERQ signal motif increased 38% and individual KFERQ containing proteins [e.g., M2 pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and pax2] were more abundant. LAMP2a and hsc73 decreased by 25% and 81%, respectively, in cortical lysosomes from diabetic vs. control rats. The decline in proteolysis in acute diabetes mellitus is associated with an increase in proteins degraded by chaperone-mediated autophagy and a decrease in proteins which regulate this pathway. This study provides the first evidence that reduced chaperone-mediated autophagy contributes to accumulation of specific proteins in diabetic-induced renal hypertrophy.