UF Researcher Unravels Estrogen’s Antioxidant Mechanism

September 24, 2003

GAINESVILLE, Fla. — New insight into how estrogen works may lead to better treatments for stroke patients, say researchers with the University of Florida’s Evelyn F. and William L. McKnight Brain Institute.

The hormone estrogen commonly is accepted as an antioxidant that can minimize brain damage from stroke, but it’s not widely used therapeutically because of potential side effects, including uterine cancer and breast cancer in women and feminizing effects in men.

Now a medicinal chemist with UF’s College of Pharmacy has discovered how the body naturally regenerates estrogen after a stroke, pinpointing a compound he believes also may provide estrogen’s benefits without its risks.

Researchers at UF and the University of North Texas Health Science Center collaborated to make the discovery, which is reported online this week in the Proceedings of the National Academy of Sciences. More investigation is required, but the findings may lead to therapies for postmenopausal women who no longer benefit from estrogen’s neuroprotective qualities, as well as for men who are at risk for stroke. Stroke is the nation’s third-leading killer, according to the American Stroke Association. About 700,000 Americans will have a stroke this year.

“During a stroke, free radicals damage important cells in the body, most notably nerve cells,” said Laszlo Prokai, of UF’s McKnight Brain Institute. “That’s where much of the debilitating effect of the stroke comes from, not from the primary incident – the clogging of the arteries and the shortage of blood – but after the surgeon opens an artery, for example, and the blood flows through the previously blocked territory. Then the reactive oxygen species (free radicals) just start going rampant.”

Free radicals are unbalanced molecules that have lost an electron and try to stabilize themselves by stealing an electron from a nearby molecule, which creates a jumble of high-energy particles that ricochet wildly and damage cells. In the case of stroke, the hydroxyl radical – a chemical compound consisting of one atom of hydrogen and one of oxygen – does a large part of the damage.

Estrogen comes to the rescue by capturing the hydroxyl radical.

“In layman’s terms, a spill occurs when the blood starts flowing into the blocked territory, and the overflow hydroxyl radical is the spill,” Prokai said. “The estrogen is the mop, soaking up the hydroxyl radicals before they do damage. But when the mop is saturated, you have to squeeze it to continue mopping. This mechanism has never been fully understood before.”

When the estrogen and hydroxyl radicals combine, an unusual molecule called a quinol is produced. In this form, the hydroxyl radicals are harmless, but the estrogen is no longer useful as an antioxidant. Prokai suspected that chemicals in the body were transforming the quinol back to estrogen, effectively wringing out the mop and making it useful again. He confirmed his hypothesis in experiments using laboratory cultures of rat brain tissue, and in live rats. The quinol molecules eventually all would convert to estrogen.

“It has been talked about for years that estrogen somehow participates in an (antioxidant) cycle and until this work, no one really knew what the cycle was,” said Bruce McEwen, a professor of neuroendocrinology at Rockefeller University in New York. “The cycle implies that estrogen doesn’t have to be used up, that it can be rejuvenated to produce a chemical shield that gets rid of free radicals. Further, you can use tiny amounts of the estrogen derivative to set off this cycle. Showing a mechanism in which these effects can take place is an important step forward.”

In terms of therapies, scientists believe administering the quinol – the saturated mop – may deliver the protective benefits of estrogen, because the body will naturally wring it out and convert it to estrogen, while side effects associated with estrogen drug therapy, such as feminization in men, may remain in check.

This could be a viable alternative to hormone replacement therapy, which is a combination of estrogen plus progestin, a synthetic substance that mimics the hormone progesterone to oppose estrogen’s undesirable effects on the uterus. While the natural loss of estrogen as women age may contribute to higher risks for stroke and heart disease, the American Heart Association does not advise women to take hormone replacement therapy. Furthermore, scientists at the National Institutes of Health in 2002 stopped a large study of such therapy after research showed it increased the risk of invasive breast cancer and blood clots in the legs and lungs and failed to protect women from stroke.

“Many women are interested in hormone replacement therapy, but they are afraid of the side effects,” said James Simpkins, a professor of pharmacology and neuroscience at the University of North Texas Health Science Center and a co-author of the paper. “But if a compound can retain estrogen’s protective benefits and reduce the side effects because the estrogen is produced in a controlled fashion in the body, it could change the way one treats acute events, such as stroke and heart attack, and chronic conditions such as Alzheimer’s disease.”

Production of such a drug will require additional experimentation and clinical testing and is at least five years away, Simpkins said.

Prokai’s analyses of about 30 female rats in which experimental stroke was induced showed both estrogen and quinol equally reduced brain cell death by about 50 percent, compared with the control animals. However, scientists found the weight of the uteruses in rats treated with estrogen doubled, while the uterine weight of quinol-treated rats did not change. That suggests estrogen, when administered into the body, stimulates sexual characteristics in addition to acting as an antioxidant. Prolonged and unopposed stimulation of the uterus by estrogen is believed to increase the incidence of endometrial cancer in women.

Technically, the quinol compound is referred to as a prodrug, which is a substance that avoids the undesirable properties of its parent by undergoing a biotransformation before delivering its pharmacological effects. An example is aspirin, which is the inactive precursor of salicylic acid – a substance that can be toxic when consumed in large amounts.