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.