In mice genetically manipulated to develop atherosclerosis, scientists at the University of Pennsylvania Medical Center have for the first time conclusively demonstrated that vitamin E confers potent protection from the disease. Atherosclerotic damage to cardiovascular tissues was limited by about 40 percent in at-risk mice receiving doses of the vitamin for only 16 weeks.
The researchers were also able to show that the benefit was due to the antioxidant action of vitamin E, not to other possible effects such as a reduction in blood cholesterol levels. This fact strongly suggests that free radicals do, in fact, play a central role in heart disease, a theory long proposed but never proven. A novel method recently developed by the same group for directly measuring oxidant stress in the body provided the data in support of the observations.
A report on the new findings appears in the October issue of Nature Medicine.
"This study offers powerful evidence for the efficacy of vitamin E as an antioxidant in atherosclerosis," says Garret A FitzGerald, MD, chairman of the department pharmacology and senior author on the report. "Significantly, it also shows that free-radical injury is functionally important in the development of cardiovascular disease."
The innovative measurement technique used in the study was developed in FitzGerald's laboratory in collaboration with Joshua Rokach, PhD, a chemist at the Florida Institute of Technology. It assesses the levels in blood or urine of certain biochemicals called isoprostanes. These isoprostanes are stable byproducts of free-radical catalyzed damage to lipids, or fatty molecules, found in tissues throughout the body, and they serve as quantitative markers for that damage.
The strain of mice used in the experiments has been genetically engineered to lack the apoE gene, resulting in extremely high levels of cholesterol and triglycerides in their blood. Atherosclerotic lesions develop in the cardiovasculature of these mice that are similar to those seen in humans with the disease.
Using the isoprostane assay, the investigators were able to determine not only the degree of oxidant stress at work in the mice but also the precise effects of the vitamin E given as an experimental intervention to counter the corrosive process. The results showed that lesions were reduced from 31.7 percent of the aortic arch surface area in the untreated mice to 19.3 percent in the mice receiving vitamin E for 16 weeks, an improvement of approximately 40 percent.
According to FitzGerald, the measure of lipid oxidation used in these experiments could form the basis for clinical trials to test effectively the use of antioxidants against a range of diseases in humans. In recently published studies, his laboratory has demonstrated that levels of the isoprostanes are also increased in human atherosclerosis.
He notes that trials to date of antioxidants have resulted in conflicted findings, perhaps because it could not be known whether the volunteers were under high oxidant stress to begin and, thus, able to benefit from the antioxidant vitamins and other compounds being tested. Also, because antioxidants like vitamins C and E have been shown in the test tube to act as prooxidants when used at high levels, the assay could be used to determine the doses of an antioxidant that would be beneficial rather than damaging.
"One's responsiveness to an antioxidant is a function of the degree to which the oxidation system is activated," FitzGerald says. "So, in clinical trials where there's no biochemical basis for either selecting a dose or identifying patients susceptible to respond, the results can be skewed and erroneous conclusions can be drawn from them."
He adds: "We need to know which individuals should take an antioxidant like vitamin E, because it's not entirely innocuous, and we need to know what dose would be beneficial for them -- as is the case with any drug."
The lead author on the study was Domenico Pratico, MD, an assistant professor of pharmacology, and Rajendra K. Tangirala, MD, was a coauthor. Daniel J. Rader, MD, an assistant professor of medicine, collaborated on the study. Funding was provided by the National Institutes of Health, the National Science Foundation, the American Heart Association, and the W.W. Smith Charitable Trust.
The University of Pennsylvania Medical Center's sponsored research and training ranks third in the United States based on grant support from the National Institutes of Health, the primary funder of biomedical research and training in the nation -- $175 million in federal fiscal year 1997. In addition, for the third consecutive year, the institution posted the highest annual growth in these areas -- 17.6 percent -- of the top ten U.S. academic medical centers. News releases from the University of Pennsylvania Medical Center are available to reporters by direct e-mail, fax, or U.S. mail, upon request. They are also posted electronically to the medical center's home page (http://www.med.upenn.edu), to EurekAlert! (http://www.eurekalert.org), an Internet resource sponsored by the American Association for the Advancement of Science, and to the electronic news services SciNews-MedNews and Quadnet.
Journal
Nature Medicine