Estrogen Protects Male Rats From Aortic Aneurysms

During 2000, about 11,000 people in the United States died from a ruptured abdominal aortic aneurysm. Eighty percent of these aneurysms, which doctors call AAAs for short, occur in men. Scientists know very little about why this often-undetected condition, for which there is no medical treatment, strikes men more often than women. But vascular surgeons at the University of Michigan Medical School have found some intriguing clues.

At this week's American College of Surgeons meeting in New Orleans, Derek T. Woodrum, M.D., a U-M resident in general surgery, will present new research results showing that smooth muscle cells from aortas of male rats contain 2.5 times more destructive MMP-9 protein and 10 times the level of MMP-9 gene expression compared to the same cells from female rat aortas. Known to be involved in AAA formation, MMP-9 is a cell-digesting enzyme that eats away at the wall of the aorta, leaving it vulnerable to expansion and rupture.

However, when Woodrum treated male rats with estradiol, a form of the female hormone estrogen, and then tested their aortas, he found that MMP-9 activity was substantially decreased. At this year's meeting, Woodrum will receive an American College of Surgeons "Excellence in Research Award" for his study.

Woodrum conducted his research in the laboratory of Gilbert Upchurch, M.D., an associate professor of surgery in the U-M Medical School, who studies factors responsible for gender differences in abdominal aortic aneurysms.

"Earlier studies have demonstrated that increased estrogen systemically inhibits the development of AAAs," Upchurch says. "Dr. Woodrum's study extends earlier research and suggests that there also is something inherent in males that increases MMP-9 and may lead to greater AAA formation."

"Estrogen affects production of MMP-9 by white blood cells called macrophages," Upchurch adds. "MMPs degrade collagen and elastin, two major proteins in the aortic wall. Typically production of MMP's are part of the body's natural healing response to injury, but left unchecked, MMPs – in particular MMP-9 – can cause extensive tissue damage and lead to the formation of an aneurysm."

In a recent paper published in Atheriosclerosis, Thrombosis and Vascular Biology, Upchurch described a series of U-M experiments focused on gender differences in AAA formation and estrogen's protective effect. This study was one of the first to include both female and male experimental animals and to compare results between sexes.

The ATVB paper describes how scientists in the Upchurch laboratory first perfused the aortas of male and female rats with elastase, an enzyme that triggers an uncontrolled inflammatory reaction and creates an aneurysm in the aortic wall. Fourteen days later, 82 percent of the male rats, but only 29 percent of the female rats, showed evidence of AAA formation. Macrophages from male rats were shown to be responsible for the higher levels of the MMP-9 enzyme.

In a second group of experiments, U-M scientists surgically transplanted aortas from male to male rats, from female to male, and from female to female. After the rats were treated with elastase to stimulate aneurysm formation, scientists compared the effects on the transplanted aortas.

All male aortas transplanted into male rats developed an aneurysm, while only 17 percent of female aortas transplanted into female rats did so. But when female aortas were transplanted into male rats, the protective effect disappeared. Every male rat that received a female aorta developed an aneurysm.

"The fact that all the male and female aortas developed aneurysms when transplanted into male rats, compared to only 17 percent of the female aortas in female rats, suggests that host environment is the critical factor in aneurysm formation," Upchurch says.

In the third part of the study, Upchurch and his research team implanted a slow-release estrogen pellet in half the male rats, while the other half went through the same implantation procedure, but did not receive the pellet. After being exposed to elastase perfusion, the scientists compared the effects on aortas from each group. All the male rats developed aneurysms, but the estradiol-treated rats had significantly smaller aneurysms and less damage to the aortic wall than the control rats.

Even though his results provide strong evidence for an estrogen-mediated protective effect, Upchurch cautions that it does not mean everyone should start taking estrogen to protect against AAAs.

"There is no medical therapy for abdominal aortic aneurysms," he says. "But understanding exactly what triggers them and how they develop could lead to an effective therapy someday. It also could help us identify people with a high risk of AAA formation who need regular diagnostic testing."

Scientists in Upchurch's lab are studying other factors involved in AAAs, including how tamoxifen, a drug used to treat breast cancer, affects the process of AAA formation.

The research was supported by the National Institutes of Health, the Jobst Vascular Research Fund, the Lifeline Foundation and the University of Michigan.

Gorav Ailawadi, M.D., a U-M resident in surgery, was first author of the ATVB paper. Additional U-M collaborators included Jonathan L. Eliason, M.D., Karen J. Roelofs, D.V.M., Indranil Sinha, Kevin K. Hannawa, Eric P. Kaldjian, M.D., Guanyi Lu, M.D., Ph.D., Peter K. Henke, M.D., James C. Stanley, M.D., Stephen J. Weiss, M.D., and Robert W. Thompson.