Cancer cells are known to hijack the process of autophagy, helping them survive stress induced by anti-cancer drugs. As a workaround, autophagy inhibitors are now being tested in clinical trials in combination with other chemotherapeutics. Such treatment appears to effectively drive cancer cells to death by apoptosis, although the mechanism remains unknown. A new study published in Developmental Cell now identifies the key component that links autophagy and apoptosis—the transcription factor FOXO3a. The work comes from a team at the University of Colorado.
"The problem is this: many anti-cancer treatments push cancer cells to the brink of death. But the cells use autophagy to go into a kind of suspended animation, pausing but not dying,” says study senior author Andrew Thorburn. “We show that FOXO3a may make the difference between these two outcomes.”
The team found that FOXO3a is regulated in a feedback loop by basal autophagy. When autophagy goes up, FOXO3a goes down, which then dials back autophagy, and vice versa. Thus when autophagy is inhibited, FOXO3a is activated and, the team finds, subsequently drives transcription of another key player—the pro-apoptotic gene PUMA. Activation of PUMA is known to trigger the apoptotic signaling cascade that leads to cell death.
The team used CRISPR/Cas9 gene editing to delete a FOXO3a-binding region downstream of PUMA’s transcriptional start site. This was enough to keep PUMA from being upregulated when autophagy was inhibited with drugs. Autophagy regulation of PUMA was thus shown to happen through FOXO3a.
The team then looked at another transcription factor that drives PUMA expression, p53. Previous studies have shown that a class of drugs called Nutlins stimulate p53, but this mainly results in growth inhibition rather than tumor cell death.
The team sought to mobilize these two transcription factor pathways. "What we wanted to see is whether these two things together—Nutlin along with autophagy inhibition—would increase PUMA past the point of growth inhibition and into actual cell death," said first author Brent Fitzwalter.
This was indeed the case, as shown by experiments in human cell lines and xenografted mouse models. The combination was much more effective against cancer cells than either strategy alone. The team’s paper concludes that via FOXO3a, autophagy inhibitors can change the mode of action of a cancer drug. "The punchline was that we turned a drug that could slow down tumor growth but couldn't kill cancer cells into one that now kills the cells," Thorburn says.