Scientists have identified a substance that appears to spur the growth of breast cancer cells, a finding announced Friday that may lead to new ways to fight the disease.

The substance, known as p75, binds to a protein on the surface of fast-spreading breast cancer cells, apparently triggering the cells to multiply, said Ruth Lupu, assistant professor of biochemistry at Georgetown University Medical Center in Washington.Patients who have more of the proteins to which p75 binds, known as receptors, tend to have a poorer prognosis, said Lupu, who presented her findings at a meeting of the American Association for Cancer Research.

While low levels of p75 stimulate rapid cell division, larger doses seem to saturate the cell and stymies growth. During saturation, the binding end of the receptor retreats inside the cell, preventing it from hooking to the activating ligand.

"By adding large amounts of the ligand, the cell's circuits overload, and the cell shuts down," Lupu said. "This hyperstimulation does not kill the cell, but it does thwart colony formation."