SENP1-Sirt3 Axis Shields Lung Stem Cells from Oxidative Damage and Fibrosis

Pulmonary fibrosis is driven in part by the failure of alveolar type II (AT2) epithelial cells — the lung's resident stem cells — to properly repair injured tissue. When AT2 cells cannot proliferate and differentiate into alveolar type I cells, they can instead stall in a pathological 'transitional' state marked by Keratin 8 (KRT8+), secreting pro-fibrotic signals that activate fibroblasts and cause scarring. Understanding what tips AT2 cells into dysfunction is therefore critical for developing anti-fibrotic therapies.

This study focused on the SENP1-Sirt3 axis, a mitochondrial regulatory pathway in which the SUMO-specific protease SENP1 removes SUMO modifications from the deacetylase Sirtuin 3 (Sirt3), thereby activating it. Active Sirt3 deacetylates Superoxide Dismutase 2 (SOD2), enhancing its antioxidant activity and reducing mitochondrial ROS. The researchers showed that bleomycin (BLM) lung injury in mice and in A549 cells suppresses mitochondrial SENP1, causing Sirt3 hyper-SUMOylation, elevated mitochondrial acetylation, ROS accumulation, and structural mitochondrial damage including loss of cristae and matrix swelling.

To test whether restoring this axis is protective, the team generated Sirt3 K223R knock-in mice, where the primary SUMOylation site (lysine 223) is mutated to arginine, constitutively mimicking SENP1 activation. In BLM models, Sirt3 K223R mice showed dramatically improved survival at high doses, less weight loss, reduced bronchoalveolar lavage cell counts and protein, lower TNF-α and IL-6 levels at day 7, and significantly attenuated collagen deposition and Ashcroft fibrosis scores at day 14 compared to wild-type controls. Fibrotic markers collagen I and α-SMA were also markedly reduced.

Transcriptomic profiling of sorted AT2 cells at day 7 revealed that Sirt3 K223R cells downregulated chemokine signaling, apoptosis, oxidative damage, IL-5 signaling, and matrix metalloproteinase pathways relative to injured wild-type AT2 cells. Upregulated pathways included Wnt/β-catenin signaling, lipid and cholesterol biosynthesis (SREBP, PPAR, fatty acid synthesis), and pluripotency-associated programs — all consistent with enhanced AT2 stemness and regenerative capacity. Functionally, Sirt3 K223R AT2 cells showed greater proliferation (EdU incorporation), higher proSPC+ cell numbers, more efficient lineage tracing into AT1 cells, and a marked reduction in the pro-fibrotic KRT8+ transitional cell population. Antioxidant N-acetylcysteine (NAC) supplementation in wild-type BLM mice phenocopied the K223R protective effects, confirming that ROS clearance is the operative mechanism.

These findings establish the SENP1-Sirt3-SOD2 axis as a central node governing AT2 cell resilience under oxidative stress. The results suggest that pharmacological strategies to activate this axis — or direct antioxidant supplementation — could represent viable therapeutic approaches to preserve AT2 function and slow fibrotic progression in conditions such as idiopathic pulmonary fibrosis.