Cellular Senescence and Lung Function during Aging Yin and Yang

Cellular senescence is a cell fate decision and stress response that entails a permanent arrest of cell proliferation coupled to a complex secretory phenotype. Senescent cells increase in number with age in most, if not all, mammalian tissues, including the airways and lungs. They also increase at greater than expected numbers, compared with age-matched controls, at sites of age-related pathologies such as chronic obstructive pulmonary disorder and emphysema. The senescence response is a double-edged sword. The proliferative arrest suppresses the development of cancer by preventing the propagation of stressed or damaged cells that are at risk for neoplastic transformation. However, this arrest can also curtail the proliferation of stem or progenitor cells and thus hamper tissue repair and regeneration. Similarly, the secretory phenotype can promote wound healing by transiently providing growth factors and the initial inflammatory stimulus that is required for tissue repair. However, when chronically present, the secretory phenotype of senescent cells can drive pathological inflammation, which contributes to a host of age-related pathologies, including cancer. There are now transgenes and prototype small molecules that can clear senescent cells, at least in mouse models, and thus improve health span and median life span. The next challenge will be to develop interventions and supplements that can abrogate the deleterious effects of senescent cells while preserving their beneficial effects.