Autophagy is a cellular recycling process induced in response to many types of stress. However, little is known of the signaling pathways that regulate autophagy during acute kidney injury (AKI). Bcl-2/adenovirus E1B 19 kDa-interacting protein (BNIP)3 and sestrin-2 are the target proteins of hypoxia-inducible factor (HIF)-1␣ and p53, respectively. The aim of this study was to investigate the roles of BNIP3 and sestrin-2 in oxidative stressinduced autophagy during AKI. We used rat ischemia-reperfusion injury and cultured renal tubular (NRK-52E) cells as in vivo and in vitro models of AKI, respectively. Renal ischemia-reperfusion injury upregulated the expression of BNIP3 and sestrin-2 in the proximal tubules, as measured by immunohistochemical staining and Western blot analysis. In vitro, NRK-52E cells exposed to hypoxia showed increased expression of BNIP3 mRNA and protein in a HIF-1␣-dependent manner. In contrast, sestrin-2 mRNA and protein expression were upregulated in a p53-dependent manner after exposure to oxidative stress (exogenous H2O2). NRK-52E cells stably transfected with a fusion protein between green fluorescent protein and light chain 3 were used to investigate autophagy. Overexpression of BNIP3 or sestrin-2 in these cells induced light chain 3 expression and formation of autophagosomes. Interestingly, BNIP3-induced autophagosomes were mainly localized to the mitochondria, suggesting that this protein selectively induces mitophagy. These observations demonstrate that autophagy is induced in renal tubules by at least two independent pathways involving p53-sestrin-2 and HIF-1␣-BNIP3, which may be activated by different types of stress to protect the renal tubules during AKI.Bcl-2/adenovirus E1B 19 kDa-interacting protein-3; acute kidney injury; autophagy; mitophagy; sestrin-2 ISCHEMIA is the leading cause of acute kidney injury (AKI) in the adult population. Prominent morphological features of ischemic AKI include effacement and loss of the proximal tubule brush border, patchy loss of tubular cells, focal areas of proximal tubular dilation, and increased apoptosis (9). The mechanisms that dictate the survival or death of renal cells under oxidative stress must be more completely understood before novel therapeutic strategies for the treatment of ischemic AKI can be explored. Proximal renal tubular cells have high rates of ATP consumption and are very sensitive to hypoxia; thus, mitochondrial damage is one of the most important factors in determining the survival of these cells (1,43).Autophagy is one of the cellular processes that protect cells from genotoxic stress, oxidative stress, accumulation of misfolded proteins, and nutrient deprivation. We (20) have previously reported results from a study of autophagy in a mouse model of AKI. Autophagy plays roles in the pathogenesis of many diseases, and, in kidney disease, both beneficial and detrimental effects of autophagy have been reported (19 -22). Our understanding of autophagy has expanded greatly in recent years, largely due to the identification...

BackgroundHeat shock protein beta-1 (HSPB1, also known as HSP27) is a small heat shock protein involved in many cellular processes and reportedly protects cells against oxidative stress. Autophagy protects cells from many types of stress and is thought to play a key role in preventing stress in acute kidney injury (AKI). However, little is known about the role of HSPB1 in autophagy and apoptosis in the pathogenesis of AKI.MethodsWe used a rat ischemia/reperfusion AKI model and cultured renal tubular cells as an in vitro model. To elucidate the regulation of HSPB1, we evaluated the promoter activity and expression of HSPB1 in normal rat kidney (NRK)-52E cells in the presence of H2O2. To examine the regulation of autophagy by HSPB1, we established NRK-light chain 3 (NRK-LC3) cells that were stably transfected with a fusion protein of green fluorescent protein and LC3.ResultsThe results of immunohistological examination showed that HSPB1 was expressed in proximal tubule cells after AKI. Real-time quantitative reverse transcription-polymerase chain reaction and western blot analysis showed that HSPB1 messenger RNA and protein expression were upregulated 6–72 h and 12–72 h, respectively, after ischemia/reperfusion injury. HSPB1 promoter activity as well as messenger RNA and protein expression indicated dose-dependent induction by H2O2. HSPB1 overexpression-induced autophagy in NRK-LC3 cells under normoxic conditions was confirmed with confocal microscopy, which revealed the presence of LC3-positive granules. Furthermore, H2O2-induced autophagy was inhibited by the transfection of small interfering RNAs for HSPB1. Overexpression of HSPB1 reduced BAX activation and H2O2-induced apoptosis, as measured by caspase 3 activity and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling assay.ConclusionsWe showed that HSPB1 expression increased during oxidative stress in AKI. Incremental HSPB1 expression increased autophagic flux and inhibited apoptosis in renal tubular cells. These results indicate that HSPB1 upregulation plays a role in the pathophysiology of AKI.

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