Whether breast tumors develop and metastasize and how well they respond to therapy depend upon how well they are able to resist death. Despite concerted effort towards understanding what factors drive death resistance in primary and metastatic tumors, clinically translatable advancements in this important area have been frustratingly slow. For the past few decades studies have focused predominantly on identifying whether death resistant tumors acquire changes in specific genes and proteins that improve their survival. However, clinical data clearly indicate that the extracellular protein microenvironment that the tumor resides within can and does influence its ability to survive and resist treatment. How the extracellular tissue environment might protect tumors from death is not well understood.
We have been studying whether extracellular matrix (ECM, the protein from the extracellular environment that surrounds cells in tissues) receptors called integrins could improve the survival of mammary tumor cells. We found that the levels and function of integrins change when cells become malignant, and that altering integrins in normal cells dramatically modifies their behavior and enhances their survival. If we correct the expression of integrins in transformed cells we can restore their normal behavior and increase their sensitivity to death. Interestingly, we found that the ability of integrins such as a6b4 to protect cells from death is influenced by the composition of the ECM protein in the environment as well as how the ECM protein is organized and modified. What factors drive changes in integrins in tumors, how integrins protect cells from death, and why the organization of the ECM can modulate the efficiency of integrin-dependent survival and the clinical relevance of these findings require clarification. As tumors develop in the breast they evoke a response in the surrounding ECM microenvironment clinically termed the desmoplastic response. Desmoplasia is characterized by alterations in ECM expression, processing and organization. We found that these desmoplastic changes in the ECM are associated with an incremental and significant increase in stiffness or rigidity (termed tension) of the gland. An increase in tension can regulate the behavior of many cell types including endothelial, fibroblasts, neurons, and mammary epithelial cells (MECs). Although the mammary gland is not traditionally viewed as a mechanically challenged tissue, MECs within the tissue are nevertheless subject to tension during normal development and pregnancy. Given that, we found that MECs are extremely sensitive to changes in tension. It is highly likely that the behavior of MECs within the mammary gland will also be influenced by tension. If true, this could have strong implications for breast cancer because once tumor cells have metastasized to different tissues they frequently reside within ECMs with compositions and tensions different from the original breast. For example, a common site for breast tumor metastasis is the bone, which is very stiff (high vitronectin & fibronectin), the lung, which is very malleable (surrounded by fibrin), and the circulation where they encounter high tension from blood pressure (high fibronectin). Because tension can influence cell behavior so strongly, we believe that differences in ECM tension may constitute a previously unrecognized but critical regulator of mammary tumor behavior and treatment responsiveness. Using three-dimensional (3D) natural and synthetic ECMs with defined tensions (similar to what we could measure in normal, premalignant and breast tumors in vivo), we conducted a pilot study to explore how ECM tension could regulate mammary tissue behavior and survival. We found that ECM tension alters integrins, changes key enzymes that regulate critical cell processes, and modifies the activity of genes that regulate stress and cell death in MECs. When we varied ECM tension in 3D we found that matrix tension drives MECs to grow, alters their ability to differentiate, and regulates their ability to survive in response to frequently used chemo and immune breast cancer therapies (unpublished data). We predict that the biophysical properties of the ECM constitute a critical regulator of mammary tumor behavior and survival. To test this we will: (1) Engineer appropriate 3D organo-typic models that recapitulate the composition and tensional properties of primary and metastatic breast tumor tissues to identify candidate tension-dependent molecular survival regulators. (2) Develop xenograft and transgenic mouse models to test whether ECM tension regulates behavior and apoptotic responsiveness of mammary epithelia in vivo and define how. (3) Build a computational model that can predict how changes in ECM tension could influence integrin-dependent function such as survival in MECs, and query this model with neoadjuvant therapy clinical trial data. (4) Develop non-invasive imaging tools that could be used to monitor changes in ECM tension or tension-induced modifications in normal and transformed mammary tissues in vivo using phage display and quantum dots.
Defining how matrix tension could regulate tumor behavior and treatment resistance will have important consequences for tumor diagnosis and therapy.