Hydrogen sulfide (H₂S) has been shown previously to exert proapoptotic activity. However, the mechanism(s) by which H₂S affects cell growth and function have not been addressed adequately. In this study, cultured human lung fibroblasts were treated with the H₂S donor NaHS (10-75 µM; 12-48 h). NaHS caused a concentration-dependent increase in micronuclei formation (indicating DNA damage) and cell cycle arrest (G₁ phase). NaHS increased expression of ku 70 and ku 80 but did not affect the expression of other DNA repair proteins such as proliferating cell nuclear antigen (PCNA) or replication protein A (rNase protection assay). NaHS treatment also resulted in stabilization of p53 coupled with induction of downstream proteins such as p21, Bax, and cytochrome c, as well as translocation of Bax from the cytosol to the mitochondria and release of cytochrome c from mitonchondria. NaHS did not up-regulate cell levels of the antiapoptotic protein, Bcl-2. We propose that the genotoxic action of H₂S propels the cell toward apoptotic death triggered initially by stabilization of p53 and subsequently involving a cascade of downstream products. These results are of significance as they uncover a hitherto unknown and very fundamental role for H₂S in determining cell fate.--Baskar, R., Li, L., Moore, P. K. Hydrogen sulfide-induces DNA damage and changes in apoptotic gene expression in human lung fibroblast cells.