Age-Associated Activation of the cGAS-STING Pathway and Impairment of DNA Damage Repair in Human Primary Alveolar Type II Cells

Homeostatic imbalance and lung function decline are central physiological characteristics of aging and susceptibility to respiratory diseases. Senescence contributes to tissue damage and alveolar epithelial cell injury and decreases reparative capacity. Alveolar type II (ATII) cells have stem cell potential and self-renew to regenerate the alveoli after damage. They were isolated from younger and older non-smoker and smoker organ donors to define their function in the lung. Smoking and older age increased ATII cell senescence as detected by high β-galactosidase activity and P21 levels by Western blotting and RT-PCR. Also, the number of ATII cells was the lowest in lung tissue in older smokers. This was associated with increased stress signaling, as shown by elevated 4-HNE and G3BP1 expression in ATII cells, and inflammation indicated by high IL-8 levels in BAL fluid. In addition, DNA damage and decreased repair were observed using the comet assay, especially in ATII cells isolated from older smokers. This was accompanied by the highest levels of cytosolic double-strand DNA in this group and correlated with the activated cGAS-STING pathway and increased IRF3 expression. Moreover, telomere shortening, accumulation of TERRA molecules, and increased ZBP1 protein expression in ATII cells were associated with smoking and older age. Reduced NRF2 and DJ-1 expression in ATII cells was detected by Western blotting, especially in older smokers, which suggests an antioxidant defense system dysfunction. Our study provides insights into the impaired interconnected signaling network, which can contribute to ATII cell senescence.