New findings offer key to understanding breast cancer

June 23, 1995

By Lynn Yarris,

Two more critical pieces have been added to the puzzle of how breakdowns in the extracellular matrix (ECM)--a mass of protein that serves as support "scaffolding" for cells--can lead to breast cancer. The pieces represent the findings of two separate but related studies conducted through a collaboration led by Mina Bissell, director of the Laboratory's Life Sciences Division (LSD), and Zena Werb, a professor with UC San Francisco's Laboratory of Radiobiology and Environmental Health.

In the first study, Bissell, Werb and colleagues demonstrated that uncontrolled production of metalloproteinase (MMP), an enzyme known to break down the ECM, can initiate the development of breast cancer. In the second study, the collaboration demonstrated that a breakdown of the ECM results in an abnormally high rate of apoptosis--programmed cell death--another significant factor in the development of breast cancer.

Breast cancer is the most common malignancy in women of the western hemisphere and accounts for the greatest number of deaths among western women between the ages of 40 to 45. It is estimated about five percent of all women in the United States will develop breast cancer.

About 15 years ago, working with tissue cultures, Bissell demonstrated that the ECM surrounding mammary epithelial cells in the breast plays a vital role in cell growth and development. This discovery led her to propose a direct link between the ECM and breast cancer, which occurs when cell growth and development runs amok. Since then, Bissell, working with other scientists at LBNL and elsewhere, has been steadily building up a picture of the interaction between breast cells and the ECM.

"If my hypothesis was correct, then a loss of ECM should destroy functional and morphological differentiation in cells," says Bissell. "We found this to be the case in tissue cultures, but we needed to show it was true in living organisms."

In addition to Bissell and Werb, the other participants in these studies were LSD's Nancy Boudreau, and Carolyn Sympson, a post-doc who was splitting her time between LBNL and UCSF. The research has been reported to the American Society for Cell Biology, and in the journal Science.

Since the ECM is so crucial to the overall development of an organism, its gene cannot simply be knocked out as is done with most other genes. Instead, the ECM must be studied through an indirect genetic route. In their collaborations, the Bissell-Werb team worked with mice that had been genetically engineered to produce an abundance of an MMP called stromelysin-1. Normally activated during pregnancy, this enzyme is an important regulator of milk production in the mammary gland. The research teams focused on the interactions between stromelysin-1 and a special type of ECM called "basement membrane" that specifically supports epithelial cells.

In the first study, the increased activity of stromelysin-1 destroyed the ECM and caused the mammary glands to lose function as if the mice were already involuting (returning to a pre-pregnancy state). The Bissell-Werb team found that 12 percent of these mice developed tumors similar to those formed in human breast cancer, whereas no tumors were found in the normal mice.

"We destroyed the ECM inappropriately and got inappropriate results," says Bissell. "This is what we would expect if the hypothesis is correct and the ECM is crucial to cell development."

The researchers suspected that the breakdown of ECM by stromelysin-1 triggers the development of genetic abnormalities in associated cells. As a followup, working first with tissue culture then with the same strain of stromelysin-1 producing mice, they investigated the effect that a breakdown of basement membrane ECM has on apoptosis in mammary epithelial cells.

Controlled death is as much a part of normal cell development and functioning as controlled growth and differentiation. If a cell does not die when it is supposed to, Bissell says, problems occur throughout the rest of the cycle that can lead to the development and spread of cancer.

In both the tissue culture and the mouse experiments, the Bissell-Werb collaboration found that the destruction of basement membrane ECM accelerated the rate of apoptosis in epithelial cells. Absent the ECM, apoptosis set in as much as 10 days earlier in the transgenic mice than in the normal mice.

"A cell can either differentiate, grow or die, depending on the signal sent to it by the ECM," says Bissell. "The ECM suppresses death by telling the cell to grow or differentiate instead. If we remove the ECM signals, then normal cells die without first doing what they are supposed to."

Investigating the mechanisms behind the ECM's regulation of apoptosis, the Bissell-Werb collaboration learned that the process revolves around the expression of a protein called the interleukin-1-beta converting enzyme (ICE). Apoptosis was induced by the activation of ICE which correlated with the loss of ECM. When ECM was present, the activity of ICE was inhibited and apoptosis did not occur. In normal breasts, ICE increases at the end of lactation. This leads to cell apoptosis and involution.