After repeated cell divisions, human cells eventually stop dividing due to the shortening of telomeres, the protective ends of chromosomes. This process, known as replicative senescence, is a key mechanism in preventing early-stage cancers from developing further. A new study published in Molecular Cell has identified ATM kinase as the central factor driving this cellular aging process.
The research clarifies why cells cultured in laboratories under high oxygen conditions reach senescence more quickly than those grown at oxygen levels similar to those found in the human body. According to the findings, ATM becomes hyperactive when exposed to higher oxygen concentrations, which lowers a cell's ability to tolerate short telomeres and accelerates entry into senescence.
"Our results have illuminated the mechanism underlying the aging of human cells through replicative senescence," said Titia de Lange, head of the Laboratory of Cell Biology and Genetics. "These insights are critical for understanding how this tumor suppression pathway prevents cancer."
Replicative senescence acts by halting the growth of cells with damaged or critically short telomeres. When telomeres can no longer attract enough TRF2—a protein that protects them—they are mistaken for DNA breaks, triggering a cellular response that stops division.
"Replicative senescence is a remarkably effective tumor suppressor pathway," de Lange said. "We know this from patients with long telomeres in which this system does not work properly. These patients can get as many as five different cancers before the age of 70, indicating that in people with normal length telomeres, the telomere tumor suppressor pathway prevents many cancers."
The study also addresses longstanding questions about whether ATM or ATR kinases were responsible for enforcing senescence and how oxygen levels influence cellular aging. Researchers observed that standard laboratory conditions (about 20 percent oxygen) lead to earlier onset of senescence compared to physiological conditions (1-8 percent oxygen). Previous theories suggested high oxygen might speed up telomere loss; however, recent evidence contradicts this explanation.
To investigate further, de Lange’s team tracked primary human fibroblasts at both low (3 percent) and high (20 percent) oxygen concentrations. Alexander Stuart, a former graduate student now at Harvard University, described working with low-oxygen environments as challenging because even brief exposure to atmospheric oxygen could alter experimental outcomes.
"Any time the cells or the reagents are outside of the special low oxygen incubator, they are exposed to 20 percent oxygen which can change the molecular environment within minutes," Stuart explained. "That means you're often in a race to do all the standard protocol steps extremely quickly so you can keep the samples at low oxygen as much as possible."
Stuart demonstrated that only ATM—not ATR—enforces senescence under both conditions. Inhibiting ATM allowed cells to continue dividing beyond their usual limit and even reversed arrest in already-senescent cells.
Further experiments revealed that high oxygen creates a hyperactive form of ATM. The difference in lifespan between cells grown at low versus high oxygen was linked to their ability to withstand very short telomeres: at lower oxygen levels, cells could continue dividing despite shortened telomeres but would stop if moved back into higher-oxygen settings where ATM activity increased.
"I don't think of it as low oxygen extending the lifespan of human cells—that's the physiological state of our bodies. Rather, the question was: why do high oxygen conditions shorten cellular lifespan? One could then extend that question: why aren't high oxygen conditions accurate systems for studying senescence?" Stuart said. "We've now shown that high oxygen represents a hyperactive ATM setting, which leads to fewer divisions than cells would naturally undergo."
The researchers traced these effects back to reactive oxygen species (ROS), molecules present even under low-oxygen conditions. ROS promote formation of disulfide bonds within ATM molecules, causing them to dimerize and lose responsiveness to DNA damage signals like short telomeres.
With support from Ekaterina V. Vinogradova at Rockefeller's Laboratory of Chemical Immunology and Proteomics, they mapped these disulfide bonds within ATM and identified one bond necessary for regulating its activity based on environmental oxygen levels.
Their results suggest important considerations for laboratory research using cultured human cells: “Studying that in human cells cultured at 20 percent oxygen means you're basically studying the ATM kinase under hyperactive conditions,” de Lange said. “We're not saying that everybody should switch to working at low oxygen because it's very hard to do, but it may be a good idea to verify that what is observed at 20 percent also holds at 3 percent oxygen.”
The findings may also have implications for cancer therapy since most tumors experience lower-than-atmospheric levels of oxygen—conditions under which ATM activity is suppressed and cancerous cells tolerate shorter telomeres than normal tissues allow. Restoring proper ATM function could force malignant cells into growth arrest by making them less tolerant of critically short telomeres.
"Telomere shortening represents a very important cancer prevention program." de Lange said. "Questions about how this system works have been at the heart of the work in my lab for years now, and we'll continue to dig deeper into this pathway."
