Researchers at Duke University Medical Center report animal studies that may explain, for the first time, why oral contraceptives offer protection against ovarian cancer. Their findings suggest that the progestin in birth control pills induces damaged ovarian cells to die before they turn malignant.
The discovery offers hope that periodically administering progestin to wipe out precancerous ovarian cells could be a highly effective preventive treatment for ovarian cancer, which kills 17,000 women in the United States annually. The researchers plan to begin human trials of the therapy this fall.
"Progestin could be one of the most significant cancer preventive agents ever developed, and its one that millions of are already exposed to through oral contraceptives and hormone replacement therapy," said Dr. Gus Rodriguez, lead author of the study published in the September 9th Journal of the Society for Gynecologic Investigation. "Theoretically, we could wipe out precancerous cells in the ovarian epithelium by delivering bursts of progestin throughout the lifespan," he said.
Studies in Macaque Monkeys
Rodriguez and colleagues studied how hormones affect the ovaries of macaque monkeys, whose reproductive biology closely mimics that of humans. They found that progestin activated the critical process of apoptosis in the ovarian lining. Apoptosis is a genetic suicide program triggered when cells have sustained irreparable genetic damage. Such mutated cells in the ovarian lining, if allowed to survive, could proliferate and turn malignant.
The researchers said theirs is the first study to directly implicate this action of progestin as the mechanism by which birth control pills protect against ovarian cancer--a phenomenon that has been observed widely. Well-documented epidemiologic studies spanning 3 decades have shown that taking birth control pills for just 3 years can reduce a womans lifetime risk of ovarian cancer by 40%.
Rodriguez said progestin treatment would target precancerous cells in the ovarian lining that should have been slated for apoptosis but somehow escaped the process. Once such cells have been destroyed, it takes years or decades for the ovarian lining to accumulate the kind of damage that would lead to ovarian cancer, at which point progestin could be delivered again. Pregnancy may also confer protection against ovarian cancer because progesterone(Drug information on progesterone) (the natural form of progestin) peaks during gestation.
Until now, scientists had presumed that oral contraceptives reduced the risk of ovarian cancer by suppressing monthly ovulation, in which constant cell division can spontaneously damage DNA in ovarian cells. By halting ovulation for a period of time, a woman would theoretically have less chance of sustaining enough genetic damage to cause ovarian cancer.
But the Duke researchers questioned how only 3 years of oral contraceptive use--or a mere 10% fewer lifetime ovulations--could reduce the lifetime risk of ovarian cancer by 40%. So they decided to study the cellular and molecular effects of various contraceptive hormones on the ovaries.For 35 months, they gave three groups of monkeys one of four different combinations of hormones--either an oral contraceptive progestin, an oral contraceptive estrogen, or a combination of both. A fourth control group received no hormones.
The monkeys receiving progestin alone had a six times greater level of apoptosis (24%) in their ovarian epithelium than did the control group or the group receiving estrogen alone. The combination group also showed a high level of apoptosis, but less than that of the monkeys receiving progestin alone. Monkeys receiving estrogen alone and those in the control group had the lowest rate of apoptosis, about 3.9%.
While cells routinely self-destruct when they sustain irreparable DNA damage, the suicide process does not always work as it should. Thats especially the case in women who sustain serious damage to critical regulatory genes that normally activate the suicide genetic switch. When such genes are damaged, the apoptosis switch is never activated.
Progestin May Tip the Scales Toward Apoptosis
Any compound that can help cells overcome the cells resistance to apoptosis can prevent them from becoming malignant, Rodriguez said. "Progestin may lower the threshold of resistance by tipping the scales toward apoptosis," he said.
Although the researchers dont know precisely how progestin activates apoptosis, knowing how progestin works is not critical to its success. As long as it works and is safe in humans, which the researchers plan to test this fall, the hormone could be used to protect millions of women against ovarian cancer, Rodriguez said.
"If you want to make an impact on ovarian cancer from a public health standpoint, theoretically you should give progestin to the entire population of women," he said. "Most ovarian cancer occurs sporadically, not in response to an inherited genetic defect, so there is no standard for judging definitively who is at risk and who isnt."
Multiple Benefits of Progestin Cited
The benefits of progestin are many, he says:
Thirty years of evidence show the safety of progestins among women taking oral contraceptives and hormone replacement therapy.
Progestin selectively targets those cells that have progestin receptors, such as cells in the breast, uterus and ovary, so there is little risk of inadvertently causing harm to other cells in the body.
Hormone therapy with progestin can be implemented in all age groups.
Women are already exposed to progestins through oral contraceptives and hormone replacement therapy, and post-menopausal women who arent taking it can be encouraged to do so.
A French study has suggested that the type of progestin used in oral contraceptives may decrease the risk of subsequent breast cancer.
Upcoming studies will focus on progestins effects in women. Specifically, the researchers will develop a risk-assessment profile for women by analyzing their genetic susceptibility and pertinent aspects of their history, relative to risk of ovarian cancer. Researchers will use the profile to determine who is at greatest risk for developing ovarian cancer, with the goal of delivering preventive agents more aggressively to this population.
The researchers also plan to study the molecular pathways through which progestins work, with the hope of developing additional agents that stimulate apoptosis in the ovarian epithelium.