Figure 7. Opposing activities of p53 and mTOR/AKT upon telomere shortening promote a switch from apoptosis to senescence.

Early telomere shortening triggers p53-dependent apoptosis and inhibition of cell proliferation. At early age, apoptosis is the predominant cell fate and it mostly counterbalanced by compensatory proliferation of neighboring cells. However, inhibition of cell proliferation results in a progressive loss of tissue cellularity, eventually leading to tissue damage. As age progresses, loss of tissue homeostasis triggers the pro-proliferative mTOR/AKT pathway. Akt phosphorylates FoxO, inducing its translocation from the nucleus to the cytoplasm. Loss of FoxO transcriptional activity reduces mitochondrial SOD2 expression generating mitochondria oxidative stress through increased ROS levels. Mitochondrial dysfunction eventually triggers p15/16 expression and senescence becomes the predominant cell fate.