Autophagic flux blockage in alveolar epithelial cells is essential in silica nanoparticle-induced pulmonary fibrosis

Silica nanoparticles (SiNPs) have been reported to induce pulmonary fibrosis (PF) with an unknown mechanism. Recently, the activation of autophagy, a lysosome-dependent cell degradation pathway, by SiNPs has been identified in alveolar epithelial cells (AECs). However, the underlying mechanism and the relevance of SiNPs-induced autophagy to the development of PF remain elusive. Here, we report that autophagy dysfunction and subsequent apoptosis in AECs are involved in SiNPs-induced PF. SiNPs engulfed by AECs enhance autophagosome accumulation and apoptosis both in vivo and in vitro. Mechanically, SiNPs block autophagy flux through impairing lysosomal degradation via acidification inhibition. Lysosomal reacidification by cyclic-3′,5′-adenosine monophosphate (cAMP) significantly enhances autophagic degradation and attenuate apoptosis. Importantly, enhancement of autophagic degradation by rapamycin protects AECs from apoptosis and attenuates SiNPs-induced PF in the mouse model. Altogether, our data demonstrate a repressive effect of SiNPs on lysosomal acidification, contributing to the decreased autophagic degradation in AECs, thus leading to apoptosis and subsequent PF. These findings may provide an improved understanding of SiNPs-induced PF and molecular targets to antagonize it.