Abstract
ERK, an extracellular signal-regulated protein kinase, is involved in various biological responses, such as cell proliferation and differentiation, cell morphology maintenance, cytoskeletal construction, apoptosis, and canceration of cells. In this study, we focused on ERK pathway on cellular injury and autophagy-associated adaptive response in urinary protein-irritated renal tubular epithelial cells and explored the potential mechanisms underlying it. By using antioxidants N-acetylcysteine and catalase, we found that ERK pathway was activated by a reactive oxygen species- (ROS-) dependent mechanism after exposure to urinary proteins. What is more, ERK inhibitor U0126 could decrease the release of neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), and the number of apoptotic cells induced by urinary proteins, indicating the damaging effects of ERK pathway in mediating cellular injury and apoptosis in HK-2 cells. Interestingly, we also found that the increased expression of microtubule-associated protein 1 light chain 3 (LC3)-II (a key marker of autophagy) and the decreased expression of p62 (autophagic substrate) induced by urinary proteins were reversed by U0126, suggesting autophagy was activated by ERK pathway. Furthermore, rapamycin reduced urinary protein-induced NGAL and KIM-1 secretion and cell growth inhibition, while chloroquine played the opposite effect, indicating that autophagy activation by ERK pathway was an adaptive response in the exposure to urinary proteins. Taken together, our results indicate that activated ROS-ERK pathway can induce cellular injury and in the meantime provide an autophagy-associated adaptive response in urinary protein-irritated renal tubular epithelial cells.
Highlights
Renal tubular epithelial cell injury is central to the pathophysiology of tubulointerstitial fibrosis, which strongly correlates with the decline of renal function [1,2,3]
We first evaluated the activity of Extracellular signal-regulated protein kinases kidney injury molecule-1 (KIM-1) (ERK) pathway by examining the expression of phosphorylated ERK (p-ERK), a key marker of ERK pathway activation
U0126 reduced the level of tubular injury markers neutrophil gelatinase-associated lipocalin (NGAL) and KIM-1, which have been increased with urinary protein stimulation (Figures 2(e) and 2(f)). These results indicated that ERK pathway activation caused by urinary protein led to cellular injury and apoptosis in HK-2 cells
Summary
Renal tubular epithelial cell injury is central to the pathophysiology of tubulointerstitial fibrosis, which strongly correlates with the decline of renal function [1,2,3]. As a pathological product of glomerular diseases, is an important factor leading to renal tubular epithelial cell injury. It is proved to cause cellular apoptosis, resulting in the detachment of renal tubular epithelial cells from the basement membrane and losing their functions to the renal tubules [4, 5]. It is of great importance to further explore the mechanism of urinary protein-induced cellular injury. One of the important ways that urinary protein causes cellular injury is by elevating the production of reactive oxygen species (ROS) [6]. With the phosphorylation of the Ras/Raf/MEK/ERK cascade, the ERK pathway is activated and participates in the regulation of a variety of cellular processes, which includes
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