The Role of Matrix Metalloproteinase 13 in Breast Cancer

Institution: University of California, San Francisco
Investigator(s): Mikala Egeblad, Ph.D. -
Award Cycle: 2002 (Cycle VIII) Grant #: 8FB-0180 Award: $80,000
Award Type: Postdoctoral Fellowship
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2002)
The normal breast contains many cell types, including milk-producing (epithelial) cells, fat cells and fibroblasts (stromal or supporting cells). Most research on breast cancer focuses on the genetic changes in the epithelial cells. However, the other cells in the breast tumor, although they are not cancer cells, also contribute to disease initiation, permissiveness, and progression. These non-epithelial cells respond to changes in the breast tissue microenvironment induced by the cancer cells. One recently identified difference between supporting, stromal cells in the normal breast and in breast cancer is the secretion of matrix metalloproteinase 13 (MMP-13), an extracellular matrix degrading enzyme. It is thought that cancer cells stimulate the stromal cells to make MMP-13, but it is not known how they do so and what the presence of MMP-13 contributes to breast cancer.

Previous studies have demonstrated that MMP-13 is expressed by the non-cancerous cells in breast tumors that have migrated into the surrounding tissue. MMP-13 is not found in tumors that have not spread. Therefore, we hypothesize that MMP-13 is necessary for the events that make a breast tumor progress from a localized tumor (i.e., having a good clinical prognosis) to a tumor that has infiltrated the surrounding breast tissue (i.e., a less favorable clinical prognosis). Furthermore, we believe that cancer cells in early stage cancer stimulate supporting cells to make MMP-13.

The only way to directly test if MMP-13 has a role in breast cancer is to perform animal experiments. We start with a mouse model in which the animals are genetically deficient in MMP-13. Then, we cross these MMP-13-deficient animals with mouse strains that express the rodent form of either the HER-2 oncogene, which is highly expressed in 30% of human breast cancers, or the WNT1 gene. These mice develop forms of breast cancer that resemble the human disease. If the hybrid crosses between the MMP-13-deficient and tumor producing strains develop either fewer breast tumors or fewer tumors that have spread to other organs, then we will better understand the role of MMP-13 in breast cancer. Finally, in other experiments we will demonstrate how MMP-13 is distributed in stromal cells adjacent to tumor cells, how MMP-13 is induced in stromal cells, and what proteins are degraded by MMP-13.

Collectively, these animal studies will clarify the function of MMP-13 in breast cancer.

Final Report (2004)
The normal breast contains many cell types, including epithelial (milk-producing) cells and stromal (supporting) cells. The stromal cells include, for example, fat cells and fibroblasts. Breast cancer cells are epithelial in origin; however the stromal cells also undergo changes as a breast cancer evolves. One difference between stromal cells in the normal breast and in breast cancer is the secretion of matrix metalloproteinase 13 (MMP-13), an extracellular matrix degrading enzyme. Type I collagen is one of the best characterized substrates of MMP-13 and of particularly interest because increased levels of collagen has been correlated with increased risk of developing breast cancer and distant metastasis.

The aim of our project was to determine the importance of MMP-13 and its substrates in the normal breast and in breast cancer. We found that MMP-13 was upregulated in breast tumors in several different mouse models of breast cancer, similarly to what is reported in humans. We initiated animal studies to test if our mouse strain that lacks MMP-13 develops breast cancer with the same time course as normal mice. However, we initially had technical problems with this study. These problems have been solved and the studies are still ongoing.

We decided to focus on the role of cleavage of MMP-13 substrates in the normal breast and in cancer. One of MMP-13's substrates, namely type I collagen. Type I collagen was chosen because its expression already had been linked to breast cancer. In order to understand its role in cancer it is important to understand its normal functions in the breast. We observed that mice treated with an MMP inhibitor had a delay in the normal invasion of the epithelium into the mammary fat pad. This was associated with a dramatic increase in fibrillar collagen expression in the stroma. This suggests that cleavage of collagen by MMPs is necessary for the invasion of the normal epithelium into the fat pad. Therefore, we studied mice that had type I collagen genes that were mutated so that collagen could not be cleaved by MMPs. The mammary gland development was indeed also delayed in these mice. We had expected that MMP-13 would be the MMP responsible for cleavage of collagen but instead found that MMP-14 was responsible. Thus, we have determined that type I collagen remodeling by MMPs is required for the normal development of the mammary gland.

We plan to continue these studies, because type I collagen has been implicated as a predisposing factor for breast cancer and its expression is associated with metastasis. Type I collagen is a substrate for several matrix metalloproteinases (MMPs), a family of proteolytic enzymes identified as modifiers of mammary carcinogenesis in humans and in experimental mice predisposed to breast cancer development. We have now shown that cleavage of collagen by MMPs is important for the normal development of the breast. Future studies will concentrate on determining the functional significance of collagen metabolism as it relates to the etiology and progression of mammary carcinogenesis, and will set the stage for evaluation of this important molecule as a prognostic and/or diagnostic marker during human mammary carcinoma progression.

Symposium Abstract (2003)
During tumor progression, changes occur in both the cancer cells and in the stromal compartment of the tumor, i.e. the non-epithelial cells and the extracel-lular matrix. The matrix metalloproteinases (MMPs) may participate in the regulation of the tumor microenvironment and, and they are expressed in human breast cancer. We used a genetic approach and the mouse mammary tumor virus (MMTV)-Neu mouse model of breast cancer to determine if and how MMP-9 contributes to mammary carcinogenesis. In the carcinomas of the MMTV-Neu mice, MMP-9 was not expressed by the cancer cells, but by stromal cells - including macrophages, neutrophils and fibroblasts - at the invasive border of the tumors and within adjacent cells in the tumor microvasculature. Surprisingly, when the MMTV-Neu mouse strain was crossed with MMP-9 null mice, the MMTV-Neu/MMP-9 null mice developed mammary tumors without any delay in tumor onset compared to MMTV-Neu mice. Furthermore, the percentage of mice with lung metastases was the same, approximately 50%, in MMTV-Neu/MMP-9 null and MMTV-Neu mice. Counter-intuitively, the MMTV-Neu/MMP-9 null mice tend to developed more tumors per mouse than MMTV-Neu mice. Our data contrasts with previous studies using the RIP-Tag mouse model of pancreatic cancer and the K14-HPV16 mouse model of skin cancer where tumor onset is delayed in the absence of MMP-9. This suggests that MMP-9 plays a different role in breast cancer than in other cancers and may explain the lack of success in the clinical trials that used MMP inhibitors for the treatment of breast cancer.

Ets2-dependent stromal regulation of mouse mammary tumors.
Periodical:Molecular and Cellular Biology
Index Medicus: Mol Cell Biol
Authors: Man AK, Young LJ, Tynan JA, Lesperance J, Egeblad M, Werb Z, Hauser CA, Muller WJ, Cardiff
Yr: 2003 Vol: 23 Nbr: Abs: Pg:8614-25