The combination of cholesterol and particles from diesel exhaust can harden arteries and lead to heart attacks and strokes, according to a new study. The two factors together appear to be much more harmful than the effects of soot containing diesel particles or cholesterol alone, the researchers found.
The relation between diesel fumes--which contain particles less than 2.5 micrometers in diameter--and cardiovascular disease is still unclear. Epidemiological evidence has shown a link, but researchers are unsure about the mechanism. Studies have suggested, however, that like the "bad" form of cholesterol known as low density lipoprotein (LDL), diesel particles cause the release into blood vessels of free radicals, a type of oxygen molecule that is damaging to human tissue.
In the new study, immunologist Andre Nel of the University of California, Los Angeles, and his colleagues exposed samples of human vascular tissue to soot, LDL cholesterol, or both at the same time. The researchers found that the combination was especially adept at setting off genes known to cause inflammation of the blood vessels and at promoting artery hardening, or atherosclerosis.
To test whether the same was true in live animals, researchers took mice genetically engineered to have high cholesterol levels and placed them in one of three environments. The first group was exposed to filtered air for 2 months, the second to the finest particles in diesel fumes, and the third group to both fine and intermediate-sized particles. Examination of the animals' lungs showed that the two diesel groups had the same amount of damage as did the human tissue samples and similar gene activation patterns. The mice exposed to only the finest particles had the most severe damage. The study is published in the 25 July issue of Genome Biology.
"This the first study to go so far into the biology behind the effect air pollutants have on the cardiovascular system," says Stanton Glantz, a toxicologist at the University of California, San Francisco. "Most people figured there was something going on, but this gives us substantial evidence."
In future studies, Nel says he hopes to discover whether antioxidants--compounds that are found in some fruits and vegetables and that can prevent harm from free radicals--can prevent damage from diesel fumes. His team also plans on identifying genes that make people more susceptible to the effects of air pollutants, so that high-risk people may be identified with a DNA test.