Cigarette smoke-inactivated SIRT1 promotes autophagy-dependent senescence of alveolar epithelial type 2 cells to induce pulmonary fibrosis

The senescence of alveolar epithelial type 2 (AT2) cells is implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF). Cigarette smoke (CS) is a strong risk factor for IPF and it is also a pro-senescent factor. Here we aimed to investigate whether and how CS induces AT2 cells senescence via a SIRT1/autophagy dependent pathway. Our results showed that CS extract (CSE) reduced autophagy and mitophagy and increased mitochondrial reactive oxygen species (mitoROS) in MLE-12 cells, an AT2 cell line. The autophagy inducer rapamycin (RAPA) and the mitochondria-targeted antioxidant mitoquinone (mitoQ) inhibited CSE-related senescence and decreased mitoROS. Next, we found that CSE promoted DNA damage, downregulated the nicotinamide adenine dinucleotide (NAD+)/nicotinamide adenine dinucleotide (NADH) ratio and suppressed SIRT1 activity. Activating SIRT1 with its activator SRT1720 attenuated senescence through an autophagy-dependent pathway. The NAD+ precursor nicotinamide mononucleotide and the poly ADP-ribose polymerase (PARP1) inhibitor olaparib also exerted anti-senescent effects by activating SIRT1. Moreover, the results showed that mitoQ and RAPA, in turn, elevated SIRT1 activity by inhibiting DNA damage. Consistent with these results, SRT1720 and mitoQ mitigated CS-induced AT2 cells senescence and lung fibrosis in vivo. Moreover, autophagy in AT2 cells was rescued by SRT1720. Taken together, our results suggested that CS-induced senescence of AT2 cells was due to decreased autophagy mediated by SIRT1 inactivation, which was attributed to competitive consumption of NAD+ caused by DNA damage-induced PARP1 activation. The reduction in autophagy, in turn, decreased SIRT1 activity by promoting mitochondrial oxidative stress-related DNA damage, thereby establishing a positive feedback loop between SIRT1 and autophagy in CS-induced AT2 cells senescence. Consequently, CS-inactivated SIRT1 promoted autophagy-dependent senescence of AT2 cells to induce pulmonary fibrosis.