Peptide Orders Tumor Cell Suicide

Most cancer treatments come with a serious downside: They also harm normal cells. Now researchers have found a way to kill tumor cells in test tubes without inflicting any collateral damage on healthy cells. The finding, reported in today's Proceedings of the National Academy of Sciences, suggests a new molecular target for treating all kinds of cancer.

The growth of cancer cells is spurred by a family of transcription factors, called E2Fs. But E2Fs can turn traitor if they become too abundant, ordering cells to commit suicide. "The cancer cell walks a tightrope," says William Kaelin, a molecular oncologist at the Dana-Farber Cancer Institute in Boston, Massachusetts. "It wants E2F, but it has to be careful to sidestep E2F's ability to cause cell death."

Normal cells keep E2F levels in check with a protein called pRB and an enzyme called cyclin A-cdk2. For some reason, pRB is turned off in all types of cancer cells, leaving them with only cyclin A-cdk2 to regulate E2F. Kaelin's group reasoned that if they could disable this remaining safety check in cancer cells, then E2F would become abundant enough to provoke a spiral of programmed cell death. To test this theory, they identified a short peptide that inhibits the binding of cyclin A-cdk2 to E2F and used it to treat cultured cells. The peptide quickly killed the cancer cells but left normal cells unaffected.

The finding will trigger a new wave of rational drug design, predicts Robert Weinberg, a cancer biologist at the Massachusetts Institute of Technology. But he warns, "We are still a long way from converting this cleverness into drugs that are effective in the oncology clinic."

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