The Fires WithinJuly 2004
By Christine Gorman and Alice Park
|© 2004 TIME, Inc. Reprinted with permission.|
IS YOUR HEART ON FIRE?
There's just one problem with that explanation: sometimes it's dead wrong. Indeed, half of all heart attacks occur in people with normal cholesterol levels. Not only that, as imaging techniques improved, doctors found, much to their surprise, that the most dangerous plaques weren't necessarily all that large. Something that hadn't yet been identified was causing those deposits to burst, triggering massive clots that cut off the coronary blood supply. In the 1990s, Ridker became convinced that some sort of inflammatory reaction was responsible for the bursting plaques, and he set about trying to prove it.
To test his hunch, Ridker needed a simple blood test that could serve as a marker for chronic inflammation. He settled on C-reactive protein (CRP), a molecule produced by the liver in response to an inflammatory signal. During an acute illness, like a severe bacterial infection, levels of CRP quickly shoot from less than 10 mg/L to 1,000 mg/L or more. But Ridker was more interested in the low levels of CRP—less than 10 mg/L—that he found in otherwise healthy people and that indicated only a slightly elevated inflammation level. Indeed, the difference between normal and elevated is so small that it must be measured by a specially designed assay called a high-sensitivity CRP test.
By 1997, Ridker and his colleagues at Brigham and Women's had shown that healthy middle-aged men with the highest CRP levels were three times as likely to suffer a heart attack in the next six years as were those with the lowest CRP levels. Eventually, inflammation experts determined that having a CRP reading of 3.0 mg/L or higher can triple your risk of heart disease. The danger seems even greater in women than in men. By contrast, folks with extremely low levels of CRP, less than 0.5 mg/L, rarely have heart attacks.
Physicians still don't know for sure how inflammation might cause a plaque to burst. But they have a theory. As the level of LDL cholesterol increases in the blood, they speculate, some of it seeps into the lining of the coronary arteries and gets stuck there. Macrophages, alerted to the presence of something that doesn't belong, come in and try to clean out the cholesterol. If, for whatever reason, the cytokine signals begin ramping up the inflammatory process instead of notching it down, the plaque becomes unstable. "This is not about replacing cholesterol as a risk factor," Ridker says. "Cholesterol deposits, high blood pressure, smoking—all contribute to the development of underlying plaques. What inflammation seems to contribute is the propensity of those plaques to rupture and cause a heart attack. If there is only inflammation but no underlying heart disease, then there is no problem."
At this point, cardiologists are still not ready to recommend that the general population be screened for inflammation levels. But there's a growing consensus that CRP should be measured in those with a moderately elevated risk of developing cardiovascular disease. At the very least, a high CRP level might tip the balance in favor of more aggressive therapy with treatments—such as aspirin and statins—that are already known to work.
A NEW VIEW OF DIABETES
What they have discovered is a complex interplay between inflammation, insulin and fat—either in the diet or in large folds under the skin. (Indeed, fat cells behave a lot like immune cells, spewing out inflammatory cytokines, particularly as you gain weight.) Where inflammation fits into this scenario—as either a cause or an effect—remains unclear. But the case for a central role is getting stronger. Dr. Steve Shoelson, a senior investigator at the Joslin Diabetes Center in Boston, has bred a strain of mice whose fat cells are supercharged inflammation factories. The mice become less efficient at using insulin and go on to develop diabetes. "We can reproduce the whole syndrome just by inciting inflammation," Shoelson says.
That suggests that a well-timed intervention in the inflammatory process might reverse some of the effects of diabetes. Some of the drugs that are already used to treat the disorder, like metformin, may work because they also dampen the inflammation response. In addition, preliminary research suggests that high CRP levels may indicate a greater risk of diabetes. But it's too early to say whether reducing CRP levels will actually keep diabetes at bay.
CANCER: THE WOUND THAT NEVER HEALS
How might that happen? One of the most potent weapons produced by macrophages and other inflammatory cells are the so-called oxygen free radicals. These highly reactive molecules destroy just about anything that crosses their path—particularly DNA. A glancing blow that damages but doesn't destroy a cell could lead to a genetic mutation that allows it to keep on growing and dividing. The abnormal growth is still not a tumor, says Lisa Coussens, a cancer biologist at the Comprehensive Cancer Center at the University of California, San Francisco. But to the immune system, it looks very much like a wound that needs to be fixed. "When immune cells get called in, they bring growth factors and a whole slew of proteins that call other inflammatory cells," Coussens explains. "Those things come in and go 'heal, heal, heal.' But instead of healing, you're 'feeding, feeding, feeding.'" Sometimes the reason for the initial inflammatory cycle is obvious—as with chronic heartburn, which continually bathes the lining of the esophagus with stomach acid, predisposing a person to esophageal cancer. Other times, it's less clear. Scientists are exploring the role of an enzyme called cyclo-oxygenase 2 (COX-2) in the development of colon cancer. COX-2 is yet another protein produced by the body during inflammation.
Over the past few years, researchers have shown that folks who take daily doses of aspirin—which is known to block COX-2—are less likely to develop precancerous growths called polyps. The problem with aspirin, however, is that it can also cause internal bleeding. Then in 2000, researchers showed that Celebrex®, another COX-2 inhibitor that is less likely than aspirin to cause bleeding, also reduces the number of polyps in the large intestine.
So, should you be taking Celebrex® to prevent colon cancer? It's still too early to say. Clearly COX-2 is one of the factors in colon cancer. "But I don't think it's the exclusive answer," says Ray DuBois, director of cancer prevention at the Vanderbilt-Ingram Cancer Center in Nashville, Tenn. "There are a lot of other components that need to be explored."