Estrogen Blocks Immune System from Attacking Breast Cancer

There's more evidence pointing to a connection between estrogen and breast cancer. New research indicates that the female hormone appears to shield breast cancer cells from attack by the body's immune cells.

It was already known that estrogen enhances the growth and migration of breast cancer cells. But this study, by researchers at the University of Illinois at Urbana-Champaign, finds that estrogen also stimulates a certain chemical-called protease inhibitor 9 (PI-9)—that hinders the ability of immune cells to kill tumor cells.

This cause-and-effect mechanism may lead to new breast cancer therapies.

"The amounts of estrogen required to do this are quite small," notes researcher David J. Shapiro, professor of biochemistry at the University of Illinois at Urbana-Champaign.

PI-9 has already been implicated in other kinds of cancers. For example, high levels of PI-9 in some lymphomas are associated with poor patient prognoses.

But this is the first time that estrogen's role in shielding breast cancer cells from immune cells has been identified.