Staff | United Press International

CHAMPAIGN, Ill. -- U.S. scientists have found estrogen not only enhances the growth and migration of breast cancer cells, but also shields the cells from immune cells.

The University of Illinois medical researchers, in what's described as the first study of its kind, discovered the hormone estrogen induces the expression of an inhibitor that blocks immune cells' ability to kill tumor cells.

The scientists analyzed estrogen's role in the cascade of events that occurs when immune cells, called natural killer cells, encounter a tumor cell. Normally, natural killer cells release granules that contain enzymes called granzymes, which enter and kill the tumor cell.

But the researchers found when estrogen binds with an estrogen receptor the complex promotes production of a granzyme inhibitor, proteinase inhibitor 9 (PI-9). That inhibitor binds the granzyme, preventing it from initiating the molecular cascade that kills tumor cells.

"It wasn't known estrogen could do this in breast cancer cells," said principal investigator David Shapiro, a UI professor of biochemistry. "The amounts of estrogen required to do this are quite small."

The study conducted by Shapiro, graduate student Xinguo Jiang and collaborators from the University of Wisconsin, is detailed online in the journal Oncogene.