Abstract
The role of albumin reabsorption in proximal tubule (PT) cells has emerged as an important factor in the genesis of albuminuria observed in the early stages of diabetes. Evidence has shown that a decrease in megalin expression could be the key mechanism in this process. In the present work, we investigated the molecular mechanism underlying the modulation of albumin endocytosis in LLC-PK1 cells, a model of PT cells. High glucose concentrations (HG) inhibited megalin expression and albumin endocytosis after 48 h of incubation. This inhibitory effect involves the entrance of glucose into PT cells through SGLT located at the luminal membrane. Once inside PT cells, glucose is diverted to the hexosamine biosynthetic pathway (HBP) increasing O-GlcNAcylation of several intracellular proteins, including PKB. This process promotes the inhibition of PKB activity measured by its phosphorylation at Thr-308 and Ser-473 and phosphorylation of specific substrates, glycogen synthase kinase 3β (GSK3β) and tuberous sclerosis complex 2. The decrease in PKB activity led to a decrease in megalin expression and, consequently, reducing albumin endocytosis in LLC-PK1 cells. HG did not change mammalian target of rapamycin (mTOR) C2 activity, responsible for phosphorylated PKB at Ser-473. In addition, HG activated the mTORC1/S6K pathway, but this effect was not correlated to the decrease in megalin expression or albumin endocytosis. Taken together, our data help to clarify the current understanding underlying the genesis of tubular albuminuria induced by hyperglycemia in the early stage of diabetes pathogenesis.
Highlights
The role of albumin reabsorption in proximal tubule (PT) cells has emerged as an important factor in the genesis of albuminuria observed in the early stages of diabetes
High glucose concentrations (HG) inhibited albumin endocytosis after 48 h of incubation (Fig. 1, A and B). To further characterize this effect, the cells were incubated with HG for different periods of incubation, and albumin endocytosis was measured by cell-associated fluorescence using albumin–FITC as a tracer
The role of albumin reabsorption in PT cells has been highlighted as an important mechanism in this process and a possible therapeutic target to halt the progression of renal disease [5, 16, 37,38,39]
Summary
Once inside PT cells, glucose is diverted to the hexosamine biosynthetic pathway (HBP) increasing O-GlcNAcylation of several intracellular proteins, including PKB This process promotes the inhibition of PKB activity measured by its phosphorylation at Thr-308 and Ser-473 and phosphorylation of specific substrates, glycogen synthase kinase 3 (GSK3) and tuberous sclerosis complex 2. Has emerged as an important factor in the genesis of albuminuria observed in the early stages of diabetes [3,4,5] Albumin reabsorption in these cells involves receptor-mediated endocytosis, with megalin as a central receptor [6, 7]. The molecular mechanism underlying this effect involves the inhibition of PKB activity as a result of its O-GlcNAcylation, leading to a decrease in megalin expression and, to the decrease in megalin-mediated albumin endocytosis
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