Dissemination of cancer breast cancer cells from the primary tumor is an essential step in disease mortality. Dissemination requires invasion into the surrounding stroma, intravasation into the local blood or lymph vasculature, convective transport via circulation to distant tissues, extravasation out of the vasculature at the secondary tissue site, and, finally, growth in said secondary tissue. Many have noted that only a small fraction of cells (less than 1%) that are capable of invading into the surrounding stroma (the first step of metastasis) actually contribute to secondary tumors. Therefore, colonization and growth at a distant tissue site, such as the brain, bone, lung, and liver, is a rate-limiting step in the overall metastatic cascade.
We hypothesize that the ability of breast cancer cells to attach to their surroundings (via integrins) and respond to growth factors (via growth factor receptors) at these distant tissues dictates the ability of these cells to survive and grow. In this in vitro study, we created biomaterials that covalently and permanently present different extracellular matrix (ECM) proteins, such as collagen, fibronectin, and laminin, to cells, so that we may control integrin binding. We then studied the response of different breast cancer cell lines that span the disease spectrum to growth factors (epidermal growth factor (EGF), fibroblast growth factor 1 (FGF-1) and the insulin-like growth factor 1 (IGF-1)) on these ECM-controlled biomaterials. We quantified membrane extension (protrusion), morphological polarization, and spreading rate of SkBr3s (representing the HER2-overexpessing subtype of breast cancer), MCF-7s (representing luminal A), and MDA-MB-231s (triple negative). We found that SkBr3s were extremely sensitive to EGF stimulation on all three integrin-binding conditions, while FGF-1 stimulated membrane protrusion only on the fibronectin-treated surface, and SkBr3s were insensitive to IGF-1. MCF-7 cells did not show increases in membrane protrusion or spreading rate under any growth factor or ECM conditions. The MDA-MB-231s showed increases in membrane protrusion with EGF, FGF-1 and IGF-1 on collagen, and only with EGF, and FGF-1 on laminin. Thus far, our findings show that breast cancer subtype has some relationship to growth factor sensitivity and integrin-binding, and we are currently investigating whether this applies to other cell lines within these same disease subtype categories. We are now quantifying if these phenomenological observations correlate with growth factor receptor phosphorylation on the different integrin-binding regimes using a high-throughput bead-based ELISA platform. Coupled with our ongoing studies in tumor dormancy, we suspect that the ability of cells to respond to growth factors in different integrin-binding conditions is related to their ability to grow aggressively at certain tissues and not others. We propose this type of in vitro screening approach as a way to predict for tumor dormancy or relapse, and in the long-term, as a theranostic platform for patient prognosis.
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