Staying lean may help body reject cancer cells

WASHINGTON — Fatty tissue may decrease the body's ability to kill off cancer, says a study that found making mice leaner — through exercise or surgery — seemed to help them fight skin tumors.

Scientists have long known that people who are overweight are at increased risk of certain types of cancer. The question is why, and whether slimming down will lower that risk or do any good after a tumor forms.

Rutgers University scientists took a closer look at that question using mice engineered to get skin cancer, and reported Monday that fat cells may secrete substances that short-circuit one of the body's main anti-tumor defenses.

When cells become genetically damaged — such as the DNA damage caused by the sun's ultraviolet rays — they're supposed to self-destruct. It's a process called apoptosis, which helps clear out bad cells before they can grow into tumors.

The Rutgers team put running wheels in the cages of some laboratory mice. They apparently loved the new toy, logging an estimated 2 to 3 miles a day, estimated professor Allan Conney, director of cancer research in Rutgers' pharmacy school.

After two weeks, the mice hadn't lost weight but had lost a significant amount of fat in favor of muscle when compared to sedentary mice.

More importantly, the exercising mice experienced higher levels of apoptosis — the self-destruction of bad cells — in both sun-damaged skin and in already-formed skin tumors, Conney found.

The researchers then tried the experiment again, this time surgically removing pads of fat from chubby mice instead of having them exercise. Getting rid of fat stimulated tumor-cell death, they report in the journal Proceedings of the National Academy of Sciences.

That suggests something in fat lowers normal levels of cancer-fighting apoptosis. Now Conney is trying to figure out what.

What happens in mice doesn't always happen in people, cautioned Dr. Len Lichtenfeld, the American Cancer Society's deputy chief medical officer.

"We have a long way to go to prove" if cutting fat really cuts cancer, he said. But, "this particular study certainly provides a biologic rationale or explanation about why weight loss may be helpful."

In people, studies to date are conflicting. There's some evidence that a low-fat diet may reduce the risk of breast cancer returning, for instance, while other studies found no benefit — possibly because losing weight and keeping it off is difficult, Lichtenfeld said.