New insights into cell suicide could provide new avenues for cancer therapies

When it comes to the complex life of the multicellular variety, cell death can be as important as survival. It allows organisms to clean the house and prevent the proliferation of damaged cells that could compromise tissue function.

Several years ago, biologist Denise Montell, a distinguished professor at UC Santa Barbara, discovered that cells sometimes survive after what was considered the key step in cell suicide. Now, she and her lab have identified two key factors involved in this extraordinary recovery.

The results, published in Nature Communications, indicate that this survival mechanism may be critical for normal tissue recovery from extreme stress rather than a fluke. Understanding the nuances could also provide new strategies for cancer treatment.

Apoptosis is the most common way in which cells commit suicide and this process is fundamental to maintaining the well-being of an organism. Living things need a way to terminate cells when they are badly injured or their DNA is damaged. Apoptosis is also part of natural turnover, especially in blood cells, skin cells and the lining of the intestine.

“Before our work, people really thought apoptosis was an all-or-nothing decision,” said Montell, Duggan Professor in the Department of Molecular, Cellular and Developmental Biology. “Either you committed suicide and went through with it, or you didn’t.”

Scientists have considered the activation of an enzyme aptly called “executioner’s caspase” as the point of no return. This enzyme essentially slices and cubes many of the cell’s proteins. But it turns out that apoptosis is more nuanced than previously known, and sometimes cells survive the hangman’s caspase through another process – anastasis.

Back from the abyss

This phenomenon first came to Montell’s attention around 2010. Typically, scientists studying apoptosis use extreme conditions that cause all cells in their sample to die. A doctoral student in her lab at the time was curious about whether cells could survive caspase activation if she removed the substance that induced apoptosis. To everyone’s surprise, many of them did.

Scientists have since observed anastasis in the cells of many different organisms, including humans, mice and fruit flies. Montell and his team set out to look for genes that could enhance or inhibit the ability of cells to undergo this process.

To this end, the researchers applied a technique developed in 2016. By breeding transgenic fruit flies that express a specific protein cut from the executioner’s caspase, they initiated a series of events that eventually make the cells fluorescent green. This permanently identifies any cell that survived this phase of apoptosis.

With this tool at hand, the team, led by former postdoctoral fellow Gongping Sun, set out to identify the genes involved in the anastasis. Because they couldn’t investigate all 13,000 genes in the fruit fly genome, the researchers sifted through their data and literature to identify candidate genes, eventually settling on around 200 to investigate further.

Sun and his lab mates took hundreds of fruit flies and eliminated the expression of a different gene in half of each animal’s cells. This allowed them to control other factors that could affect the results.

In the paper published in 2016, the team found that some cells undergo anastasis during normal fruit fly development. In the new paper, therefore, they looked for changes in the percentage of cells that went through this process during development. They also tested genes for their ability to influence anastasis in response to stresses such as radiation and heat.

Distinguishing between the genes involved in anastasis and those that are simply necessary for basic survival has been a challenge. “Because if it’s needed for survival, period, then it will also be needed for recovery from the brink of death,” Montell said.

Then, the team looked at not only how many cells in a sample turned fluorescent green after the experiment, but the ratio of green cells to non-green cells. If the gene in question was needed for baseline survival but not involved in anastasis, it affects all cells equally. This would impact the overall number of fluorescent cells, but leave the ratio unchanged.

The researchers found two proteins, and the genes that encode them, were instrumental in the anastasis. The first, AKT1, is a well-researched and renowned survival protein that is activated in response to growth factors, essentially telling the cell to grow and divide. Scientists were aware that it can block executioner caspase activation, but the team found it can also make the difference between survival and death after caspase is activated.

The other protein, CIZ1, is not as well studied and appears in a number of unrelated articles throughout the literature. In almost all of these cases it appears that CIZ1 also promotes survival from stress. For example, a reduced amount of CIZ1 is associated with increased age-dependent neurodegeneration in mice.

The involvement of these two proteins in the anastasis indicates that it is probably a very ancient process. “Not only the phenomenon of cells recovering from the brink of death, but also the mechanism – the molecules involved – are so deeply conserved in evolution that flies and mice are using the same molecules,” Montell said.

Apoptosis and the fight against cancer

These findings are a huge step forward in understanding apoptosis at a fundamental level. They also suggest possible applications, especially in efforts to fight cancer.

Apoptosis plays an important role in maintaining a stable balance within complex organisms. Under normal circumstances, such as UV damage to a skin cell, the body wants the injured cell to die so that it doesn’t develop into a condition like melanoma.

“However, if you’ve been under extreme stress you may not want every cell to commit apoptosis,” Montell said. “This could cause permanent tissue damage from which it would be very difficult to recover.”

In response to severe but temporary trauma, it may be beneficial for some cells to be able to recover. Montell suspects this is the main reason organisms have developed a way to circumvent apoptosis.

The temporary nature of stress appears to be the critical factor both in the role the anastasis plays in promoting healing and in the mechanism itself. When a cell is subjected to extreme stress, such as exposure to radiation or chemicals, two things happen simultaneously: the cell activates the apoptosis response – including the hangman’s caspase – while it also activates favorable survival responses.

“It’s like putting on the accelerator and the brake at the same time,” Montell said.

Apoptotic factors are strengthened, so if the stressful conditions persist, the process crosses a threshold and the cell dies. But if the stress is only transient, the pro-survival pathway is ready to kick in and help the cell recover. Researchers don’t fully understand how the cell turns off the apoptotic pathway, but proteins such as AKT1 and CIZ1 are likely involved.

There is, however, a dark side to this survival mechanism. “Anastasis might be a good thing if you’re trying to repair damaged tissue, but it could be a bad thing as it could promote tumor growth,” Montell pointed out, “especially in response to chemotherapy and radiation treatments. which are extreme temporary stresses. “

This matches the experience of many doctors, Montell explained. Many cancer patients initially respond well to treatments; their tumors shrink and their condition improves. But unfortunately, the tumors often grow back. And scientists aren’t sure why this is.

Some think the rebirth could be the result of drug-resistant cells existing in the tumor, which then sow relapse. This paper provides another hypothesis: “the idea that the treatment itself could induce cancer cells to undergo this stress-dependent survival process,” Montell said.

This notion could fundamentally change the way doctors think about relapse prevention. There’s not much you can do about drug-resistant cells, Montell said, but if the relapse is due to this survival mechanism, these findings could inform new therapies.

Drugs that inhibit AKT1 are currently in clinical trials. These could be combined with other therapies to increase their effectiveness, potentially allowing doctors and researchers to inhibit anastasis in cancer cells while promoting it to normal cells.

Furthermore, successful cancer cells can actually induce apoptosis in T cells that the immune system sends to attack them, according to Montell. This represents another goal for anastasis therapies.

“There’s this war going on between the immune system and cancer,” Montell said, “and if you can tip the scales even a little, you can start winning.”