Cortisone is sometimes used as a drug to treat a variety of ailments. It can be administered intravenously or cutaneously.

One of cortisone's effects on the body, and a potentially harmful side effect when administered clinically, is the suppression of the immune system. This is an explanation for the apparent correlation between high stress and sickness.

U.S. scientists have learned more about why high doses of the steroid cortisone -- used to treat asthma, rheumatoid arthritis and other inflammatory and autoimmune diseases -- weaken bones and cause osteoporosis.

A team at Washington University School of Medicine in St. Louis conducted studies with mice and identified osteoclasts (cells that dismantle old bone) as the essential link between cortisone and osteoporosis.

The researchers found that when they gave cortisone to genetically modified mice whose bone-dismantling osteoclasts lacked cortisone receptors, the bone-weakening effects of cortisone were blocked.

They also found that cortisone inhibited the ability of osteoclasts to dismantle old bone in normal mice. This impairment of the regular skeletal renewal process results in dramatic age and stress-related weakening of bones.

Blocking osteoclast activity may also trigger a chain reaction that slows the activity of bone-building osteoblasts, the researchers said.

The findings were published in the August issue of the Journal of Clinical Investigation.

Learning more about this cortisone-osteoclast connection may help scientists identify targets for therapy to prevent cortisone from weakening bones, the researchers said.

"High-dose cortisone is the second most common cause of osteoporosis, and we currently have no real treatment for this serious side effect. Given how frequently these drugs are used to treat many different conditions, that's a major clinical problem," study senior author Dr. Steven L.

Teitelbaum, professor of pathology and immunology, said in a prepared statement.