Senescence as an anticancer mechanism.

Senescence was originally described as a terminal nondividing state of normal human cells reached after many cell divisions in culture. The cause was shown to be shortening of telomeres, leading to telomere dysfunction and cell cycle arrest. Subsequently, a more rapid, nontelomere-dependent form of senescence, often termed stress-induced premature senescence, was described. Mostly importantly, it occurs in response to activated oncogene products. Oncogene-induced senescence has been shown to play a role in tumor suppression in vivo; it does not seem to involve changes in telomeres. A second phenomenon that plays a role in tumor suppression, which does involve progressive telomere shortening, is crisis, the state that cells reach when cell cycle checkpoints are impaired and cells can no longer respond to telomere shortening or oncogene activation by entering senescence. These two processes, oncogene-induced senescence and telomere-based crisis, exert powerful anticancer effects.