Attenuation of ATM signaling by ROS delays replicative senescence at physiological oxygen.

Replicative senescence is a powerful tumor suppressor pathway that curbs proliferation of human cells when a few critically-short telomeres activate the DNA damage response (DDR). We show that ATM is the sole DDR kinase responsible for the induction and maintenance of replicative senescence and that ATM inhibition can induce normal cell divisions in senescent cells. Compared to non-physiological atmospheric (∼20%) oxygen, primary fibroblast cells grown at physiological (3%) oxygen were more tolerant to critically short telomeres, explaining their extended replicative lifespan. We show that this tolerance is due to attenuation of the ATM response to double-strand breaks (DSBs) and unprotected telomeres. Our data indicate that the reduced ATM response to DSBs at 3% oxygen is due to increased ROS, which induces disulfide crosslinked ATM dimers that do not respond to DSBs. This regulation of cellular lifespan through attenuation of ATM at physiological oxygen has implications for tumor suppression through telomere shortening.