Balance between senescence and apoptosis is regulated by telomere damage-induced association between p16 and caspase-3.

Telomerase activation protects cells from telomere damage by delaying senescence and inducing cell immortalization, whereas telomerase inhibition mediates rapid senescence or apoptosis. However, the cellular mechanisms that determine telomere damage-dependent senescence versus apoptosis induction are largely unknown. Here, we demonstrate that telomerase instability mediated by silencing of sphingosine kinase 2 (SPHK2) and sphingosine 1-phosphate (S1P), which binds and stabilizes telomerase, induces telomere damage-dependent caspase-3 activation and apoptosis, but not senescence, in p16-deficient lung cancer cells or tumors. These outcomes were prevented by knockdown of a tumor-suppressor protein, transcription factor 21 (TCF21), or by ectopic expression of WT human telomerase reverse transcriptase (hTERT) but not mutant hTERT with altered S1P binding. Interestingly, SphK2-deficient mice exhibited accelerated aging and telomerase instability that increased telomere damage and senescence via p16 activation especially in testes tissues, but not in apoptosis. Moreover, p16 silencing in SphK2-/- mouse embryonic fibroblasts activated caspase-3 and apoptosis without inducing senescence. Furthermore, ectopic WT p16 expression in p16-deficient A549 lung cancer cells prevented TCF21 and caspase-3 activation and resulted in senescence in response to SphK2/S1P inhibition and telomere damage. Mechanistically, a p16 mutant with impaired caspase-3 association did not prevent telomere damage-induced apoptosis, indicating that an association between p16 and caspase-3 proteins forces senescence induction by inhibiting caspase-3 activation and apoptosis. These results suggest that p16 plays a direct role in telomere damage-dependent senescence by limiting apoptosis via binding to caspase-3, revealing a direct link between telomere damage-dependent senescence and apoptosis with regards to aging and cancer.