Mice reveal the off switch for inflammation

Working with genetically engineered mice, scientists have identified a crucial natural mechanism that rodents, and presumably people, use to shut down inflammation before it does harm.

The discovery may suggest both new types of anti-inflammatory treatments and ways to promote inflammation when it’s desired. The finding also raises the possibility that caffeine-laden drinks may interfere with this switch.

Inflammation is a double-edged sword, notes Michail Sitkovsky of the National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, Md.

Chemicals and immune cells that produce inflammation protect a body from infection. Endless or misdirected inflammation, however, can interfere with wound healing and promote heart disease and autoimmune disorders such as arthritis.

The new report centers on a nitrogen-containing molecule called adenosine, which the body uses to store energy. Buildup of adenosine in the brain may also trigger sleepiness (SN: 5/24/97, p. 316: http://www.sciencenews.org/sn_arc97/5_24_97/fob2.htm).

For more than a decade, some researchers have suggested that adenosine has anti-inflammatory powers. Consider the rheumatoid arthritis drug methotrexate. Bruce Cronstein of New York University School of Medicine and his colleagues established years ago that methotrexate thwarts inflammation by triggering the release of adenosine.

In the Dec. 20/27 Nature, Sitkovsky and his NIAID colleague Akio Ohta report determining which of the body’s several adenosine receptors–the cell-surface proteins that recognize the molecule–is essential to adenosine’s role in inflammation. By mutating the rodent gene for the so-called A2a receptor, the biologists created mice unable to limit inflammation in a variety of situations.

In one case, they gave the mutant mice and normal mice a drug that triggers inflammation in the liver. The normal mice developed little organ damage, but the mutant mice didn’t turn off the induced inflammation and suffered extensive, sometimes fatal, liver damage. Similarly, the mutant mice couldn’t dampen their inflammatory response when they were injected with pieces of the bacteria that trigger the dangerous disease sepsis.

“We proposed a long time ago that adenosine that comes out of injured, dying, or dead tissue can turn off inflammation, and [Sitkovsky] has shown the specific receptor involved in that,” says Cronstein.

There may be many ways to control inflammation, but they may all converge on this pathway, suggests Michael Yeadon of Pfizer in Sandwich, England.

Companies such as Pfizer are investigating whether drugs that activate the A2a adenosine receptor can thwart inflammation. A major concern, however, is that such drugs may have dangerous side effects because adenosine also regulates blood pressure via this receptor.

As a result, Pfizer is initially developing adenosine-receptor drugs as topical agents for direct application to sites of inflammation. People with inflammatory diseases of the skin, eyes, and lungs might benefit from such drugs, says Yeadon.

Sitkovsky notes that caffeine’s stimulant properties stem from its ability to block adenosine’s action. He suggests that people suffering from short-term inflammation may prolong their condition by drinking coffee and tea that contain caffeine.

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