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Tuesday, September 01, 2009

Cells don’t like to be alone

Detached Early Cancer Cells May Die from Lack of Nourishment
Antioxidants Could Rescue Starving Tumors-to-be
http://focus.hms.harvard.edu/2009/082809/antioxidants.shtml

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Detached Early Cancer Cells May Die from Lack of Nourishment

Antioxidants Could Rescue Starving Tumors-to-be
Cells don’t like to be alone. In the early stages of tumor formation, a cell might be pushed out of its normal environment due to excessive growth. But a cell usually responds to this homeless state by dismantling its nucleus, packing up its DNA, and offering itself to be eaten by immune cells. Simply put, the homeless cell kills itself. This process, known as apoptosis, typically stops potential cancer cells before they have a chance to proliferate.
Joan Brugge
Photo by Liza Green, HMS Media Services
Joan Brugge and collaborators have identified metabolic defects with a lethal effect on cells that stray too far from their home environment. The defects might be a way for the body to stop potential tumor cells from proliferating.


Now, researchers from the lab of Joan Brugge, the Louise Foote Pfeiffer professor of cell biology and chair of that department, have discovered another mechanism that these precancerous, homeless cells use to commit suicide. By studying two different types of human breast epithelial cells, the researchers found that when separated from their natural environment, these cells lose their ability to harvest energy from their surroundings. Eventually, they starve.
“We originally thought that in order for cells to survive outside their normal environment, they would simply need to suppress apoptosis,” said Brugge, senior author on the paper, which appeared online Aug. 19 in Nature. “But our studies indicate that this activity is not sufficient to prevent the demise of homeless cells. Even if they escape apoptosis, these cells can’t transport enough glucose to sustain an energy supply.”
Surprisingly, metabolic function is restored if antioxidant activity is increased inside the cells, allowing them to use energy pathways that do not rely on glucose.
“It raises the interesting idea that antioxidants, which are typically thought to be protective because they prevent genomic damage, might be allowing these potentially dangerous cells to survive,” said first author Zachary Schafer, assistant professor at the University of Notre Dame and a former postdoc in Brugge’s lab.
“It raises the interesting idea that antioxidants, which are typically thought to be protective because they prevent genomic damage, might be allowing these potentially dangerous cells to survive.”
—Zachary Schafer
The authors caution against extrapolating too far from their data, which were based on cell culture. They also emphasize that the experiments were not designed to mimic the effect of dietary antioxidants. The researchers used two specific antioxidant compounds—chemically distinct from those found in food and supplements—only to understand how oxidants contribute to the metabolic defects.
“We think that genes with antioxidant activity play a much bigger role than antioxidant compounds administered from outside the body,” said Brugge.
Beyond Cell Suicide
The team had previously reported that when cells were endowed with a cancer-causing gene that prevents them from committing suicide, they still died when cut off from their extracellular environment. This puzzled the researchers since they had long thought that apoptosis was the only way the cells could die.
In the recent study, Schafer and colleagues took a closer look, measuring the levels of proteins and molecules associated with metabolic activity in the displaced, but apoptosis-resistant, cells. They found that the cells had become incapable of taking up glucose, their primary energy source. Under the microscope, the cells also displayed telltale signs of oxidative stress, a harmful accumulation of oxygen-derived molecules called reactive oxygen species (ROS). The result was a halt in the production of ATP, the molecular lifeblood that transports energy in the cells. The unmoored cells were literally starving to death.
“The idea that a lack of extracellular matrix can prevent cells from accessing nutrients hasn’t been shown conclusively before,” said Schafer. “Loss of glucose transport, decreased ATP production, increased oxidative stress—all those things turn out to be interrelated.”
Tumor Metabolism
To figure out what was wrong, the researchers took a direct approach: they tried to fix it. Schafer engineered the homeless cells to express high levels of a gene, HER2, known to be hyperactive in many breast tumors. He also treated the cells with antioxidants in an attempt to relieve oxidative stress and help the cells survive.
Both strategies worked. The cells with the breast cancer gene regained glucose transport, preventing ROS accumulation, and recovered their ATP levels. The antioxidant-treated cells also survived, but by using fatty acids instead of glucose as an energy source.


10A
10A Trolox
Courtesy Zachary Schafer
In these microscope images, human mammary cells (blue) grow in clusters surrounded by a membrane of extracellular matrix (red), which usually keeps them alive. Normally (left), the cells in the middle of the cluster die due to lack of contact with the extracellular matrix, leaving an empty space. In cells treated with Trolox (right), an antioxidant derived from Vitamin E, cells separated from the extracellular matrix survive, filling up the middle of the cluster.

“Our results raise the possibility that antioxidant activity might allow early-stage tumor cells to survive where they otherwise would die from these metabolic defects,” said Schafer.
The researchers are currently planning to test the effects of antioxidant genes, some of which are abnormally regulated in human tumors, and a wider range of antioxidants in animal models. They also plan on characterizing the metabolic consequences of matrix detachment in more detail.
“Ultimately,” Brugge said, “we want to understand enough about the metabolism of tumor cells so that new types of drugs can be designed to target them.”
Students may contact Joan Brugge at joan_brugge@hms.harvard.edu for more information.
Conflict Disclosure: The authors declare no conflict of interest.
Funding Sources: The National Cancer Institute and the National Institutes of Health; the authors are solely responsible for the content of this work.

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