The unexpected role of Htra2/Omi, an apoptotic protein, in cellular senescence

Cellular senescence is considered a contributor to radiation resistance in cancer. The molecular mechanisms underlying radiation-induced senescence are poorly understood. We hypothesized that proteins from apoptotic, autophagic and programmed necrotic pathways that control life and death decisions of a cell, may have a non-canonical function in irradiation (IR)-induced cellular senescence. Hence, modulation of these pathways may influence the fate of cancer cells following irradiation. To this end, we developed a novel experimental model system of IR-induced senescence in NCI-H460 (NSCLC cell line). Subsequently, we performed an siRNA screen of programmed cell death genes, using IR-induced senescence as an endpoint, based upon the CellTiter glo assay. A reduction in the extent of senescence response to IR indicates the corresponding gene functions as a positive mediator of senescence. The top hit was Htra2/Omi, a mitochondrial serine protease with a well-known pro-apoptotic activity, yet not previously linked to cellular senescence. We demonstrated that Omi knock down mitigated the IR induced reduction in cell number and metabolic state, the increase in cell size characteristic of cellular senescence and the increase in SA-β-gal staining. Omi’s critical role in IR-induced senescence was further validated using a pharmacological inhibitor. Vimentin, a cytoskeletal protein, was previously identified as a potential substrate of Omi. We found that cleavage and filamentous organization following IR was mediated by the proteolytic activity of Omi. Altogether, we report that Omi functions as a positive mediator of IR-induced senescence, and by Vimentin cleavage Omi has an important role in the cellular mechanism and cytoskeletal morphological changes.