This picture of how cancer cells shift between two alternating states -- travelers and nesters -- represents a new understanding of how cancer metastasizes, or spreads to other parts of the body, said the Duke Comprehensive Cancer Center researchers who conducted the study.
"Understanding this toggle switch might ultimately enable scientists to find ways to stop cells from metastasizing, which is the most deadly trait of cancer," said the study's lead investigator, Mariano Garcia-Blanco, M.D., Ph.D., professor of molecular genetics and microbiology.
The researchers will publish their findings in the Sept. 19, 2006, issue of the journal Proceedings of the National Academy of Sciences, now available on line. The research was funded by the National Cancer Institute.
Until now, scientists have believed that cancer cells must transform permanently from stationary epithelial cells into migratory mesenchymal cells in order to metastasize.
The Duke team discovered that highly malignant cells are equal parts epithelial and mesenchymal, transitioning between the two as their surroundings necessitate. The proteins that the cell produces dictate which way the cell shifts.
In a classic example of survival of the fittest, a cancer cell's ability to toggle between epithelial and mesenchymal enables the most malignant cells to aggressively invade and then peacefully adapt in unfamiliar territory, the scientists said.
"The prevailing notion has been that the more mesenchymal the cancer cells, the more mobile and metastatic they would be," Garcia-Blanco said. "In reality, aggressive cancer cells are not homogenous, but are extremely versatile in their ability to adapt as their survival needs shift."
The researchers discovered this transition in cancer cells when they observed an error in "alternative splicing," a key element of the genetic copying program inside cells. Alternative splicing determines how the DNA is chopped into pieces and then reassembled. The order in which DNA is reassembled determines which proteins the gene produces.
In cancer cells, the splicing machinery goes awry -- as do myriad functions within the cells. When the splicing process proceeds one way, the cells become mesenchymal. Spliced another way, the cells turn epithelial.
To determine which way a cancer cell would turn, the scientists constructed a fluorescent "reporter" -- a protein that illuminates if the cell turns epithelial but lies dormant if the cell reverts to mesenchymal state.
By following the reporter's illumination within cancer cells in rats, the team viewed the very process of alternative splicing as it occurred in the tumors. The researchers were able to visualize specific portions of DNA, called exons, to see if they were included or excluded in the splicing process as the cell transformed.
"We found that the regulation of alternative splicing is different in mesenchymal versus epithelial cells," Garcia-Blanco said. "A particular exon, FGFR2 IIIc, is silenced in mesenchymal cells but is active in epithelial cells.
"We can visualize the genes as they are dynamically changing," he said. "We can define the cell types by observing their splicing patterns."
According to Garcia-Blanco, the cellular switch that is believed to guide the regulation of splicing is a protein called Fox. Both mesenchymal and epithelial cells produce Fox, but the protein is active only in epithelial cells, Garcia-Blanco said.
Fox also may have an accomplice or "co-factor" in or around epithelial cells that prompts it to activate, the researchers said. They speculate that this co-factor could be activated by contact with stroma --the suppor ting structural cells of a tumor -- because the stroma is where the majority of epithelial-type cancer cells were observed. Their heavy presence implies that the stroma may have induced the cancer cells to revert to epithelial when they reached a new destination, so they could stabilize to populate a new tumor site.
"Our findings validate that tumors are highly complex in their behavior and don't necessarily need a gene mutation to alter their behavior," said Sebastian Oltean, M.D., Ph.D., research associate and first author of the journal article.
"Alterations in gene splicing can be much more subtle in nature but still have a major impact on the cancer cell and can be targets of therapy."
The team's next step is to determine precisely what controls the toggle mechanism in cancer cells, Garcia-Blanco said. Identifying the various steps that occur during the natural progression of tumors could lead to therapies for blocking metastasis, he said.