title: Stimulation of GSH synthesis to prevent oxidative stress-induced apoptosis by hydroxytyrosol in human retinal pigment epithelial cells: activation of Nrf2 and JNK-p62/SQSTM1 pathways.
abstract: The Nrf2-Keap1 pathway is believed to be a critical regulator of the phase II defense system against oxidative stress. By activation of Nrf2, cytoprotective genes such as heme oxygenase-1 (HO-1), NAD(P)H:quinone oxidoreductase (NQO-1) and γ-glutamyl-cysteine ligase (GCL) are induced. GCL-induced glutathione (GSH) production is believed to affect redox signaling, cell proliferation and death. We here report that tert-butyl hydroperoxide (t-BHP)-induced GSH reduction led to mitochondrial membrane potential loss and apoptosis in cultured human retinal pigment epithelial cells from the ARPE-19 cell line. Hydroxytyrosol (HT), a natural phytochemical from olive leaves and oil, was found to induce phase II enzymes and GSH, thus protect t-BHP-induced mitochondrial dysfunction and apoptosis. Depletion of GSH by buthionine-[S,R]-sulfoximine enhanced t-BHP toxicity and abolished HT protection. Overexpression of Nrf2 increased GSH content and efficiently protected t-BHP-induced mitochondrial membrane potential loss. Meanwhile, HT-induced GSH enhancement and induction of Nrf2 target gene (GCLc, GCLm, HO-1, NQO-1) messenger RNA (mRNA) were inhibited by Nrf2 knockdown, suggesting that HT increases GSH through Nrf2 activation. In addition, we found that HT was able to activate the PI3/Akt and mTOR/p70S6-kinase pathways, both of which contribute to survival signaling in stressed cells. However, the effect of HT was not inhibited by the PI3K inhibitor LY294002. Rather, c-Jun N-terminal kinase (JNK) activation was found to induce p62/SQSTM1 expression, which is involved in Nrf2 activation. Our study demonstrates that Nrf2 activation induced by the JNK pathway plays an essential role in the mechanism behind HT's strengthening of the antiapoptotic actions of the endogenous antioxidant system.
authors.affiliation: College of Animal Sciences, Zhejiang University, Hangzhou, China.
authors.forename: Xuan
authors.initials: X
authors.lastname: Zou
authors.forename: Zhihui
authors.initials: Z
authors.lastname: Feng
authors.forename: Yuan
authors.initials: Y
authors.lastname: Li
authors.forename: Ying
authors.initials: Y
authors.lastname: Wang
authors.forename: Karin
authors.initials: K
authors.lastname: Wertz
authors.forename: Peter
authors.initials: P
authors.lastname: Weber
authors.forename: Yan
authors.initials: Y
authors.lastname: Fu
authors.forename: Jiankang
authors.initials: J
authors.lastname: Liu
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism*; D000975:Antioxidants / Q000494:pharmacology; D017209:Apoptosis; D002460:Cell Line; D005978:Glutathione / Q000096:biosynthesis*; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism*; D051267:NF-E2-Related Factor 2 / Q000378:metabolism*; D018384:Oxidative Stress; D010626:Phenylethyl Alcohol / Q000031:analogs & derivatives* / Q000494:pharmacology; D055213:Retinal Pigment Epithelium / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D000071456:Sequestosome-1 Protein; D015398:Signal Transduction
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000975:Antioxidants; D051267:NF-E2-Related Factor 2; C495635:NFE2L2 protein, human; C099718:SQSTM1 protein, human; D000071456:Sequestosome-1 Protein; C005975:3,4-dihydroxyphenylethanol; D048031:JNK Mitogen-Activated Protein Kinases; D005978:Glutathione; D010626:Phenylethyl Alcohol