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Hye Jung Cho1
, Kyu Youn Ahn1
, Kyu Youn Ahn1
Abstract
The Nuclear factor-erythroid-2-related factor 2 (Nrf2) plays a key role in the cellular defense against oxidative stress. Low K+ increased the reactive oxygen species and it stimulate Nrf2 activation. Previous our study demonstrated that low potassium promoted expression of H/K-ATPase and kNBC1 by Nrf2 transcription factor in cultured models. In addition, phosphorylation of ERK, JNK, p38 and PI3K was involved in the activation of Nrf2 expression. This study aims to elucidate the mechanism which low potassium regulates Nrf2 expression through various in vitro and in vivo models. Using various kinase inhibitors, promotion of Nrf2 expression in low potassium condition was inhibited by LY294002 and SP600125 while PD98059 and SB203580 did not affect Nrf2, suggesting that phosphorylation of Akt and JNK is specifically involved in Nrf2 expression in low potassium condition. Kidney tissues from low potassium diet rats showed increased phospho-ERK1/2 and phospho-Akt in diet time dependent manner but no effect to JNK and p38 phosphorylation. Specifically, Phospho-Nrf2 was also increased in nuclear compartment by low potassium diet. In order to demonstrate direct evidence that low potassium regulates ionic transporters by Nrf2, Nrf2 knockout mice were employed. Mouse embryonic fibroblasts (MEF) were harvested for the study. As expected, low potassium promotes expression of Nrf2 and level of phospho-ERK1/2 and phospho-Akt in MEF-Nrf2 (+/+). Low potassium promoted expression of kNBC1 and H/K-ATPase in MEF-Nrf2 (+/+), but unchanged or even decreased in MEF-Nrf2 (+/−) and MEF-Nrf2 (−/−). Taken together, these results show that Nrf2 was activated by ERK1/2 and AKT in low potassium condition and further regulates expression of kNBC1 and colonic H/K-ATPase.
Figures and Tables
Fig. 1
Akt and JNK specific inhibitors block low-potassium stimulated Nrf2 activation. (a) 293T cells were treated with indicated pharmacological inhibitors in the normal or low potassium media for 12 h (LY294002, 10 µM; PD98059, 20 µM; SB203580, 20 µM; SP600125, 20 µM). Cells were lysed, and Western blotting was performed using anti-Nrf2 antibodies. (b), (c) Cells were treated with indicated concentration of LY294002 or SP600125 for 12 h. Cells were lysed, and Western blotting was performed using anti-Nrf2 antibodies.
Fig. 2
P-Nrf2 accumulates in the nucleus following low K+ stimulation. (a) Western blot analysis reveal that the accumulation of p-Nrf2 and Nrf2 protein from nuclear extract increased in the rat kidney according to potassium depleted diet. (b) In cytoplasm extract, the abundance of Nrf2 protein increased but p-Nrf2 protein was undetected.
Fig. 3
Effect of low K+ on phosphorylation of ERK1/2 and Akt in rat kidney, CV-1 cell and MEF cell. (a) Expression levels of both phosphorylated and total ERK1/2, Akt, JNK and p38 were measured by western blot analysis in the kidney of potassium depleted rats. A transient increase in the phosphorylation of ERK1/2 and Akt was observed in potassium depleted rat kidney, whereas the phosphorylation of JNK and p38 did not changed. (b) CV-1 cells exposed to normal or low potassium media for indicated time periods and harvested in protein lysis buffer. Western blotting was performed using specific antibodies. In low potassium condition, stimulated phosphorylation of ERK1/2 as early as 5 min, and this increase persisted through 30min. The activation of Akt was observed at 30min, and this activation persisted through 24 h. (c) MEF cells stimulated phosphorylation of ERK1/2 as early as 3 min, and this increase persisted through 30min. The activation of Akt was observed at 10min, and this activation persisted through 3 h.
Fig. 4
Low-potassium Enhances Nrf2 gene in MEF cells. (a, b) MEF cells were treated with low potassium media, total RNA was extracted, and Nrf2 mRNA expression was analyzed by RT-PCR. Nrf2 expression was measured with various concentrations of low potassium at 12 h (control 5.4 mM; low potassium 2.7 mM, 2 mM, 1.5 mM, 1 mM). The expression levels of Nrf2 are up-regulated during low potassuim at 2.7 mM. (c) Expression of Nrf2, kNBC1 and colonic H/K at various time periods in 2.7mM low potassium conditions. (d~f) Densitometric analysis of mRNA expression levels of Nrf2, kNBC1 and colonic H/K.
Fig. 5
Effect of low K+ on Nrf2 expression in Nrf2 (−/−) MEF cell. (a) MEF cells of each genotype were incubated in low potassium (2.7 mM) media for 24 and 48 hr. (b) Densitometric analysis of mRNA expression levels of Nrf2, kNBC1 and colonic H/K in MEF cells during pretreatment with low potassium. Low potassium promoted expression of Nrf2, kNBC1 and colonic H/K-ATPasein MEF-Nrf2 (+/+), but unchanged or even decreased in MEF-Nrf2 (+/−) and MEF-Nrf2 (−/−).
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