Infectious obesity: adenovirus fattens stem cells.

Twenty-five years ago, researchers discovered that certain viruses can cause obesity in some animals. A decade ago, they extended the finding to people. Now, a team reports that one such virus works by transforming adult stem cells into fat-storing cells. The finding supports the notion that some cases of obesity may be infectious.

Magdalena Pasarica of Louisiana State University in Baton Rouge, who led the new work, stresses that obesity has many causes, including genetic factors, overeating, and a sedentary lifestyle. In some people, however, adenovirus-36 may be the culprit, she says. Adenoviruses cause colds, but adenovirus-36, apparently, does more.

In a 2005 study of 502 obese and normal-weight people, researchers reported that 30 percent of the obese group showed signs of previous adenovirus-36 infection, while only 11 percent of the lean group did.

In earlier laboratory tests, the virus made chickens, rodents, and monkeys fat, says Richard Atkinson, now president of Obetech in Richmond, Va., who led some of that work.

But how the virus might be raising obesity risk remained a mystery. To solve it, Pasarica and her colleagues collected adult stem cells from fat removed from patients during liposuction. These cells sometimes grow into adipocytes, or fat-storing cells, but can also transform into bone and cartilage.

Pasarica's team grew the stem cells in lab dishes and infected half the dishes with adenovirus-36. Most infected cells quickly transformed into adipocytes, but most of the others did not. "The difference was really big," Pasarica says. "Of the uninfected cells, just a few became fat cells. Whereas, with [the infected cells], the majority of them became fat cells." When grown on standard culture media, the adipocytes rapidly accumulated fats.

Pasarica then infected some stem cells with the virus and exposed them to a formula that usually transforms stem cells into bone. The infected cells became fat instead. "That's how we showed the virus is inducing this change at the stem cell level," says Pasarica. She presented the research this week at the American Chemical Society meeting in Boston.

Atkinson, whose company sells a test that shows whether a person has been exposed to adenovirus-36, says that the new work "closes the loops" on the earlier data. "It's the closest you can come to proving this virus makes people fat without actually infecting people," he says.

Atkinson cautions that the revelation that obesity is infectious "doesn't mean you should avoid your fat friend," because the active infectious phase lasts only a few weeks. During this time, though, the virus apparently induces long-term changes in how stem cells develop, which slowly enlarge the unlucky viral victim.

In hope of finding early treatments, Pasarica and her colleagues are now trying to track down exactly how the virus performs its cellular-fattening trick.