In the study in the November Nature Medicine, the team from the MGH and the Howard Hughes Medical Institute at Washington University School of Medicine in St. Louis describes its finding that treatment with the anti-cancer drug doxorubicin (DXR) causes mouse oocytes or egg cells to undergo apoptosis or programmed cell death. Apoptosis is a natural process that usually involves the elimination of unneeded or worn-out cells from the body. The researchers also confirmed the involvement of several participants in a known cell-death pathway in the DXR-associated death of oocytes and discovered that another common pathway for the destruction of tumor cells is not involved in oocyte death.
"It's been known for years that women treated with chemotherapy become infertile, but we've never known what the mechanisms were," says Jonathan Tilly, PhD, associate director of the MGH Vincent Center for Reproductive Biology, the paper's senior author. "While this research is a long way from clinical application, we hope it will lead us to ways of selectively short-circuiting the cell-death pathway in ovarian cells."
In the first stage of the project the researchers exposed mouse oocytes to doses of DXR equivalent to levels seen in the blood of patients treated with this drug. They observed changes in the cells characteristic of apoptosis, such as shrinkage and fragmentation. In contrast, fertilized mouse oocytes exposed to DXR reacted differently, by halting the process involved in the growth and reproduction of cells.
The process of cell death is known to involve the sequential activity of several genes and their protein products in a cascade eventually leading to the cell's destruction. The researchers looked at several of these known molecular signals for apoptosis and found several indications of their activity in DXR-induced death of oocytes.
One of the signals initiating the cell-death process involves the molecule ceramide, and the team found that use of a substance known to inhibit ceramide signaling prevented the death of oocytes exposed to DXR. They also found that eggs from mice with a mutation in a cell-death-promoting gene called bax (part of a key group called the bcl-2 gene family) were almost completely resistant to DXR-induced apoptosis. Inhibition of the activity of proteins called caspases, which function as cellular executioners by snipping apart other proteins, also inhibited apoptosis in DXR-treated oocytes.
All three of these signals -- ceramide, Bax and caspases -- are involved in the same pathway leading to cell death. "I often describe it as though the cell is traveling along a network of roads that can lead either to cell death or to cell survival, which in the case of oocytes would mean continued fertility," says Tilly. "The signals early in the process, such as ceramide, are like neighborhood streets along which the cell travels slowly and has a lot of ways to get off the cell-death route. When it reaches the 'Bax boulevard,' it's traveling faster, and by the time a cell reaches the 'ICE interstate' [ICE is a specific caspase protein] there are very few 'off ramps' that would prevent it from reaching the point of apoptosis. While we want to speed cancer cells along this route, we'd like to provide the oocytes with a detour to survival."
The cell-death process also can be initiated by the activity of p53, a key tumor-suppressor protein. The researchers found, however, that oocytes from mice with a disruption in the gene for p53 responded normally to DXR treatment by undergoing apoptosis, indicating that p53 is not part of the pathway leading to DXR-induced apoptosis in these cells. This finding suggests that anti-cancer drugs that depend on the activity of p53 might successfully destroy tumor cells without damaging oocytes.
Improved understanding of chemotherapy-induced oocyte death also could lead to new knowledge about the normal death of these cells that occurs throughout a woman's life -- starting before birth and eventually leading to menopause when the supply of eggs is finally depleted.
Tilly's coauthors include Gloria Perez, DVM, PhD, first author, and Lucy Leykin, PhD, of the MGH, and Michael Knudson, MD, PhD, and Stanley Korsmeyer, MD, at Howard Hughes/Washington University.