Even though aspirin's pain-killing capacity was well-known to Hippocrates in the fifth century B.C., exactly what it does remains somewhat of a mystery. Now, Johns Hopkins researchers have shown that aspirin inhibits interleukin-4, a protein involved in allergic reactions and inflammation.

"The finding appears to explain some of aspirin's less obvious beneficial effects, such as how the drug might help prevent heart disease or the ravages of rheumatoid arthritis," says Vincenzo Casolaro, assistant professor of clinical immunology, reporting in the March issue of Blood.

Aspirin, the world's most used non-steroidal anti-inflammatory drug, provides the mainstay of therapy for inflammatory musculoskeletal disorders and has been shown to be effective in the management and prevention of a wide variety of noninflammatory conditions, including coronary and cerebral ischemia and possibly gastrointestinal cancer.

Since the 1970s, researchers have known that aspirin works, in part, by inhibiting prostaglandin, a hormonelike substance present in a variety of tissues and body fluids that has many roles, including causing contraction of smooth muscle and promoting inflammation. Specifically, aspirin inhibits an enzyme, cyclo-oxygenase, or COX, that catalyzes the generation of prostaglandin from cell membrane fatty acid precursors. But the suppression of prostaglandin production could not fully account for aspirin's effectiveness because, after 15 minutes, aspirin is broken down in the body and becomes salicylic acid, a completely ineffective inhibitor of COX.

By 1994, aspirin was found to actually repress the activation of NF-KB, a molecular activator of cytokines, chemicals that trigger inflammation. Casolaro, an allergy researcher, wondered whether IL-4, a cytokine, was in this category. His tests showed that IL-4 was not, however, and, in fact, that NF-KB repressed IL-4 expression in immune system cells.

Putting the pieces of the puzzle together, Casolaro and his team embarked on a study to test the notion that aspirin might actually increase IL-4 production using T cells from healthy donor blood and culturing them in the presence of aspirin. To his surprise, aspirin significantly reduced IL-4.

"The bottom line was a paradox," he says. "We found that aspirin had the opposite effect of what we might have expected and was clearly acting in a completely novel way." In mapping the gene responsible for the production of IL-4, they found that aspirin targets part of a complex of DNA binding proteins that form on the IL-4 promoter, the region that regulates the quantity of protein manufactured.

"Before this study, we knew that, fundamentally, aspirin had at least two pathways of action, on COX and on NF-KB. This study is the first evidence that aspirin influences IL-4, and does so via a third, novel pathway," Casolaro says.

The researchers speculate that because IL-4 favors recruitment of inflammatory cells from the blood stream, a process implicated in rheumatoid arthritis and heart disease, suppressing IL-4 with aspirin may be one reason the drug protects against both conditions.

Other authors include Antonella Cianferoni, John Schroeder, Jean Kim, John Schmidt, Lawrence Lichtenstein and Steve Georas. The research was funded by grants from the National Institutes of Health.