SV40, Asbestos Cause Mesothelioma
A possible cancer-causing link has been found between asbestos fibers and a monkey virus, SV40, that infects people in the U.S., as reported in a study in the Aug. 31 issue of the Proceedings of the National Academy of Sciences. The research, conducted by Michele Carbone at the Cardinal Bernardin Cancer Center of Loyola University Medical Center (Maywood, IL), offers preliminary evidence that the virus, Simian virus 40 (SV40), and asbestos fibers work together to transform healthy cells into malignant ones, resulting in mesothelioma, a rare and fatal form of cancer.
About 2,500 deaths due to mesothelioma occur annually in the U.S., and the number of cases is growing, says Carbone. Approximately 80% of patients with this form of cancer have had some occupational exposure to asbestos.
Infection with SV40 apparently became widespread during the late 1950s and early 1960s when the U.S. population was inoculated with polio vaccine made from monkey cell cultures contaminated by the virus. The contaminated vaccine was no longer used after 1963. Even so, the virus can be found today in both adults and children. The exact means by which the virus is transmitted from person to person is unknown, although the virus is apparently present in blood and other bodily fluids, including breast milk.
Normally, SV40 kills most human cells it enters because it keeps making copies of itself inside the cell until the cell dies. Because the infected cell dies, no cancer can occur.
However, when mesothelial cells (found, for instance, in the lining of the lungs) are infected with the virus, they do not die. Because mesothelial cells contain uniquely high amounts of the tumor-fighting p53 protein, which attaches itself to the cancer- promoting protein of the virus, viral replication is slowed and the virus lives inside the cell without killing it, the study showed. The combination of these two proteins–the p53 and the viral protein–disrupts the anti-tumor effect of p53, increasing the possibility that the cell becomes malignant.
“Mesothelial cells infected with this virus are at a 1,000-times greater risk of becoming malignant than other normal healthy human cells,” Carbone says. “Asbestos fibers alone are unable to transform the cell, but they seem to promote the cancer-causing work of SV40, thereby increasing the cell’s potential for becoming malignant.”
Although SV40 can make mesothelial cells in tissue culture malignant, infected mesothelial cells will not automatically become cancerous. However, the risk of malignancy increases if cells are exposed to both SV40 and asbestos fibers. Exactly how asbestos and the virus work together will be the focus of future studies.
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