TOR (target of rapamycin) integrates nutrient and energy signals in eukaryotic cells to regulate growth and cell size, and has been linked to aging in various model systems, including yeast.
Yeast is an important model system for biogerontology in general and for the study of telomere biology in particular: yeast cells grow quickly, and its genome can be manipulated with relative ease, alllowing the study of knock in/out strains. The yeast protein Cdc13p (homologous to the human protein Pot1) binds single-stranded telomeric repeats, maintaining genomic integrity and suppressing the activation of Mec1p (the yeast homologue of human ATR, which is involved in the DNA-damage response pathway). Qi et al. have previously demonstrated that inactivation of Cd13p using a temperature-sensitive mutant results in deprotection of telomeres, activation of Mec1p and either senescence or apoptosis (depending on the cellular context). These same authors have continued their studies and now report that TOR plays a pivotal role in the apoptosis that results from deprotected yeast telomeres:
TOR Regulates Cell Death Induced by Telomere Dysfunction in Budding Yeast
Telomere dysfunction is known to induce growth arrest (senescence) and cell death. However, the regulation of the senescence-death process is poorly understood. Here using a yeast dysfunctional telomere model cdc13-1, which carries a temperature sensitive-mutant telomere binding protein Cdc13p, we demonstrate that inhibition of TOR (Target of Rapamycin), a central regulator of nutrient pathways for cell growth, prevents cell death, but not growth arrest, induced by inactivation of Cdc13-1p. This function of TOR is novel and separable from its G1 inhibition function, and not associated with alterations in the telomere length, the amount of G-tails, and the telomere position effect (TPE) in cdc13-1 cells. Furthermore, antioxidants were also shown to prevent cell death initiated by inactivation of cdc13-1. Moreover, inhibition of TOR was also shown to prevent cell death induced by inactivation of telomerase in an est1 mutant. Interestingly, rapamycin did not prevent cell death induced by DNA damaging agents such as etoposide and UV. In the aggregate, our results suggest that the TOR signaling pathway is specifically involved in the regulation of cell death initiated by telomere dysfunction.
The authors began by inactivating Cdc13p to leave the single stranded telomeric overhang permanently unprotected. This resulted in growth arrest and cell death characterized by increased ROS production, increased phosphatidylserine flipping, and caspase activation. However, treatment with rapamycin (which inhibits TOR, and which has been much discussed lately as a possible anti-aging therapeutic) significantly attenuated these effects; cell survival was increased 100-fold and the aforementioned markers of cell death were effectively inhibited.
As Cdc13p binds to telomeres in S-phase, and TOR inhibition delays the G1/S transition, the authors reasoned that the observed protective effects of rapamycin may simply reflect its effect on the cell cycle — i.e., cells do not reach S-phase, and so do not require Cdc13p. As rapamycin did not result in G1 accumulation, however, the authors concluded that the protective effect of rapamycin is unrelated to the delay in the cell cycle.
They next asked whether rapamycin affected the state of the telomeres, by examining the number of telomeric G-tails, telomere length, and the telomere-position effect (TPE; the phenomenon whereby telomere status influences gene expression in sub-telomeric regions). All three measures remained unchanged upon treatment with rapamycin, suggesting that the effect is mediated downstream of telomeric status.
Cell death can also be triggered by mechanisms that are independent of telomere dysfunction. Exposure to either UV or etoposide (a topoisomerase-II poison) induced an apoptotic response that is not rescued by rapamycin treatment, supporting the authors’ hypothesis that the drug specifically prevents telomere-initiated cell death. Inactivation of Cdc13p resulted in elevated ROS, which is abolished by rapamycin (and also by the antioxidants vitamin C and NAC), raising the possibility that rapamycin prevents telomere-initiated cell death via an antioxidant mechanism was raised.
Inhibition of TOR is known to up-regulate Sod2p, but deletion of Sod2p did not affect the protective effect of rapamycin. However, rapamycin treatment was shown to reduce mitochondiral ROS production and increase mitochondiral mass, pointing to regulation of mitochondrial function as the mechanism through which rapamycin prevents cell death. The implied model is that treatment with rapamycin improves mitochondrial bioenergetics, thus reducing the generation of ROS, which are known to impact of telomere dynamics. To provide additional support for this model, the final experiment involved inactivation of Est1p (an essential subunit of telomerase). As with Cdc13p inactivation, rapamycin treatment suppressed the elevated PS flipping and ROS production.
Taken together, the findings support the hypothesis that “the TOR signaling pathway is specifically involved in the regulation of cell death initiated by telomere dysfunction”.
I have often wondered just how telomere biology in yeast relates to telomere dynamics in higher organisms. In particular, S. cerevisiae (used in the present study) is quite different from other organisms in terms of telomere sequences and telomerase activity. A minor point perhaps, but important to note.