When researchers fed a high-fat diet to mice lacking the molecule beta3 integrin, they found the exact opposite of what they expected: The mice developed lung inflammation and clogged arteries and about two-thirds of them died within six weeks. The results suggest that long-term suppression of this molecule may exacerbate the development of heart disease, rather than prevent it.
The study will appear online the week of May 12 in the Proceedings of the National Academy of Sciences.
"We were amazed that these animals died from a high-fat diet," says the study's principal investigator Clay F. Semenkovich, M.D., professor of medicine and of cell biology and physiology and director of the Division of Endocrinology, Metabolism and Lipid Research. "This uncovers an interesting role for beta3 as a potential mediator of inflammation and may help guide new drug development strategies."
Beta3 sits on the surface of cells and interacts with other molecules in the body to help regulate functions like blood clotting and inflammation. Because one of the proteins it interacts with is critical for blood platelets to form clots, drugs that block the action of beta3 often are used to treat people who are having a heart attack. By inhibiting beta3, scientists believe these drugs prevent platelets from accumulating in blood vessels and help preserve normal blood flow.
In the past few years, many experts have hypothesized that long-term use of beta3 inhibitors may prevent arteries from clogging and thereby prevent heart attacks, which is the most common cause of death in the United States. Preventing heart disease is an exciting prospect considering that thousands of people die every year from a sudden ruptured plaque without having experienced any previous symptoms. The problem is particularly prominent, according to Semenkovich, in the current climate of high-fat diets and rising rates of obesity.
To test the role of beta3 in the development of clogged arteries (a condition called atherosclerosis), the School of Medicine team developed a strain of mice that lacked beta3 and a protein called apolipoprotein E (apoE). Mice without apoE are known to develop atherosclerosis and often are used as a model to study the disease.
The team expected that removing beta3 would prevent the development of atherosclerosis. But when they fed mice a high-fat diet akin to that of the typical American, 62 percent of the mice lacking both beta3 and apoE were dead within six weeks compared to only 4 percent of those lacking only apoE.
Autopsies of eight mice missing beta3 and apoE that had died while on the high-fat diet revealed that the cause of death was lung inflammation.
In addition, fat-fed mice lacking beta3 and apoE had significantly more fatty buildup, or "plaques," in the arteries of the heart than their solely apoE-deficient counterparts. For example, they had 3.3 times the amount of plaque buildup clogging the thoracic aorta (the artery that extends from the heart down into the diaphragm) and 5.6 times the amount of atherosclerosis in the abdominal aorta, the largest artery below the diaphragm. Even when fed a normal diet, these animals had up to three times the amount of atherosclerosis as those lacking only apoE.
"These results suggest that a complex interaction between beta3 and high-fat diets may contribute to heart disease and other inflammatory diseases," Semenkovich says.
Puzzled by this finding, the team conducted the same test in mice lacking a protein called low-density lipoprotein receptor (LDLR). After ten weeks on a high-fat diet, 48 percent of mice lacking both beta3 and LDLR had died, while none of the mice lacking only LDLR died. In addition, high fat-fed animals lacking both beta3 and LDLR had up to four times the amount of clogged arteries as those lacking only LDLR. Beyond finding physical signs of inflammation in mice lacking beta3 that had died while on the high-fat diet, the team also found significantly higher amounts of three proteins, CD36, CD40L and CD40, in the tissues of these mice. These three proteins are known to be important contributors to inflammation in several diseases.
"Although beta3 is critical for platelet function and suppression of beta3 may contribute to heart disease in mice, the relationships between platelets, inflammation and heart disease are poorly understood," Semenkovich says. "This study begins to explain the interaction, but people shouldn't use this as a reason to stop taking anti-platelet drugs as part of a strategy to prevent heart disease. Anti-platelet drugs are effective in the proper clinical context. Our findings are in mice, and we need to be cautious about extrapolating these results to people."
But Semenkovich believes the results have several potential implications. First, they may explain why people with diabetes are particularly prone to heart disease. About 80 percent of individuals with type 2 diabetes are overweight and many eat a high-fat diet. Semenkovich's team has previously observed that fatty acids, which are prevalent in high-fat diets, decrease levels of beta3. As this most recent study shows, lower levels of beta3 may account for higher amounts of atherosclerosis and heart disease.
The study also makes an important point about generalizing clinical findings, according to Semenkovich. Though targeting platelet-related molecules like beta3 may successfully treat acute heart disease, it appears to have the opposite effect when used long-term. Further research, though, may reveal alternative ways to determine individuals at high risk for atherosclerosis and ways to prevent the disease.
"We plan to extend this research into humans so we can begin to understand the interactions between these molecules and a high-fat diet," Semenkovich says. "For now, one thing is clear: High-fat diets are bad."
Weng S, Zemany L, Standley KN, Novack DV, La Regina M, Bernal-Mizrachi C, Coleman T, Semenkovich CF. Beta3 integrin deficienty promotes atherosclerosis and pulmonary inflammation in high-fat-fed, hyperlipidemic mice. Proceedings of the National Academy of Sciences Online Early Edition, May 2003.
Funding from the National Institutes of Health, Clinical Nutrition Research Unit and the Diabetes Research and Training Center supported this research.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient-care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.
Journal
Proceedings of the National Academy of Sciences