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Free Radic Biol Med. 2014 Jul;72:296-307. doi: 10.1016/j.freeradbiomed.2014.04.004. Epub 2014 Apr 13.

AKT/mTOR and c-Jun N-terminal kinase signaling pathways are required for chrysotile asbestos-induced autophagy.

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  • 1Clinical Research Center, Affiliated Hospital of Guangdong Medical College, Zhanjiang 524001, China.
  • 2Department of Hematology, The Second Affiliated Hospital, Medical School of Xi׳an Jiaotong University, Xi׳an 710004, Shanxi, China.
  • 3Department of Medicine, Northwestern University Feinberg School of Medicine, and Jesse Brown VA Medical Center, Chicago, IL 60611, USA. Electronic address: d-kamp@northwestern.edu.
  • 4Clinical Research Center, Affiliated Hospital of Guangdong Medical College, Zhanjiang 524001, China. Electronic address: gangliu11@gdmc.edu.cn.

Abstract

Chrysotile asbestos is closely associated with excess mortality from pulmonary diseases such as lung cancer, mesothelioma, and asbestosis. Although multiple mechanisms in which chrysotile asbestos fibers induce pulmonary disease have been identified, the role of autophagy in human lung epithelial cells has not been examined. In this study, we evaluated whether chrysotile asbestos induces autophagy in A549 human lung epithelial cells and then analyzed the possible underlying molecular mechanism. Chrysotile asbestos induced autophagy in A549 cells based on a series of biochemical and microscopic autophagy markers. We observed that asbestos increased expression of A549 cell microtubule-associated protein 1 light chain 3 (LC3-II), an autophagy marker, in conjunction with dephosphorylation of phospho-AKT, phospho-mTOR, and phospho-p70S6K. Notably, AKT1/AKT2 double-knockout murine embryonic fibroblasts (MEFs) had negligible asbestos-induced LC3-II expression, supporting a crucial role for AKT signaling. Chrysotile asbestos also led to the phosphorylation/activation of Jun N-terminal kinase (JNK) and p38 MAPK. Pharmacologic inhibition of JNK, but not p38 MAPK, dramatically inhibited the protein expression of LC3-II. Moreover, JNK2(-/-) MEFs but not JNK1(-/-) MEFs blocked LC3-II levels induced by chrysotile asbestos. In addition, N-acetylcysteine, an antioxidant, attenuated chrysotile asbestos-induced dephosphorylation of P-AKT and completely abolished phosphorylation/activation of JNK. Finally, we demonstrated that chrysotile asbestos-induced apoptosis was not affected by the presence of the autophagy inhibitor 3-methyladenine or autophagy-related gene 5 siRNA, indicating that the chrysotile asbestos-induced autophagy may be adaptive rather than prosurvival. Our findings demonstrate that AKT/mTOR and JNK2 signaling pathways are required for chrysotile asbestos-induced autophagy. These data provide a mechanistic basis for possible future clinical applications targeting these signaling pathways in the management of asbestos-induced lung disease.

KEYWORDS:

A549 cells; Autophagy; Chrysotile asbestos; Free radicals; LC3-II; Pulmonary disease

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