Chemical switch determines if healthy cells are killed by chemotherapy

Investigators at Washington University School of Medicine in St. Louis have discovered a mechanism that helps explain why healthy cells are not killed by DNA-damaging cancer chemotherapy drugs. The findings are published in the Oct. 4 issue of the journal Cell.

DNA-damaging agents are the most common kind of drugs used to treat cancer. Like most chemotherapy drugs, these are carried in the blood and travel throughout the body. They work by irreparably gumming up DNA in rapidly dividing tumor cells. That damage then triggers the cells to self-destruct through a natural process known as apoptosis, or active cell death.

The drugs also can harm rapidly dividing healthy cells, such as those in the hair follicles, but most healthy cells are unaffected. It is not known why these drugs do not trigger apoptosis in healthy cells.

“The standard answer is that tumor cells are dividing and normal cells are not,” says Steve J. Weintraub, M.D., assistant professor of surgery, division of urologic surgery, of medicine and of cell biology and physiology. “But that’s an observation, not an explanation.”

The study led by Weintraub found that healthy, nondividing cells have a biochemical switch that when triggered allows apoptosis. The switch is found in a protein that blocks apoptosis known as Bcl-xL.

“Our findings show that if Bcl-xL is inactivated through a chemical process known as deamidation, DNA-damaging chemotherapy will kill even healthy cells,” says Weintraub, who is a researcher with the Cellular Proliferation research program at the Alvin J. Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine.

The study focuses on a family of proteins known as Bcl-2, which play a central role in both promoting and inhibiting apoptosis. The investigators first exposed cancer cells from bone, ovarian and other tumors to the anti-cancer drug cisplatin. When they looked at the Bcl-2 proteins from the cells that had died by apoptosis, they found that in each case one member of the Bcl-2 family, the protein Bcl-xL, had been modified by deamidation.

Deamidation makes slight changes in two amino acids in the Bcl-xL protein. As if someone had thrown a switch, those changes alter the shape of Bcl-xL and thereby inactivate it. In its active state, Bcl-xL is tightly joined with another Bcl-2 protein that when free triggers apoptosis. When Bcl-xL is switched off through deamidation, it releases the second protein, and apoptosis can proceed.

The researchers also exposed a line of healthy, nondividing human fibroblasts and several lines of mouse fibroblasts to cisplatin. In some of the cells, the investigators had artificially inactivated the Bcl-xL protein. They found that cells with normal Bcl-xL were not affected by the drug, while those with the inactive Bcl-xL protein died by apoptosis, indicating they were now susceptible to cisplatin.

“Our findings show that normal cells somehow suppress the signal that throws the switch and avoid self-destructing,” says Weintraub. They also suggest that tumor cells that suppress the same signal also might be resistant to chemotherapy drugs, he says.

Weintraub is now studying the nature and regulation of the signal that targets Bcl-xL.