Zinc oxide nanoparticles induce apoptosis by enhancement of autophagy via PI3K/Akt/mTOR inhibition

- ZNPs induced ROS generation whereas depleted antioxidant enzymes.
- ZNPs were found concentrated within autophagosomes and lysosomes.
- ZNPs followed extrinsic and intrinsic pathways of apoptosis in macrophages.
- Autophagy involved PI3K/pAKTpMTOR cascade in ZNPs induced macrophages.
- NAC pre-treatment and LC3B-siRNA suppressed apoptosis and autophagy.

Zinc oxide nanoparticles (ZnO NPs) induced macrophage cell death and its mechanism remains to be solved. Herein, we report that ZnO NPs induced ROS generation by depleting antioxidant enzymes, increasing lipid peroxidation and protein carbonyl contents in macrophages. The oxidative stress was induced by the inhibition of Nrf2 transcription factor release. ZnO NPs also activated the cleavage of apoptosis markers like caspases 3, 8 and 9. γH2Ax activation and cleavage of poly (ADP-ribose) polymerase (PARP) that are known indicators of genotoxicity were found to be activated by following p53, p21/waf1 signaling. ZnO NPs increased the number of autophagosomes and autophagy marker proteins such as microtubule-associated protein 1 light chain 3-isoform II (MAP-LC3-II) and Beclin 1 after 0.5-24 h of treatment. Phosphorylated Akt, PI3K and mTOR were significantly decreased on ZnO NPs exposure. Moreover, the apoptotic and autophagic cell death could be inhibited on blocking of ROS generation by N-acetylcysteine (NAC) which demonstrated the critical role of ROS in both types of cell death. In addition, inhibition of LC3-II by siRNA-dependent knockdown attenuated the cleavage of caspase 3. This study demonstrates autophagy supports apoptosis on ZnO NPs exposure.

Schematic representation of the possible way of internalization of ZnO NPs into the macrophages and its after-effects.