Breast cancer specifically spreads to the bone, brain, liver, and lung, and this phenomenon, tissue tropism, is not well understood. Paget’s classical seed and soil hypothesis states that specific interactions between the metastatic cell (the seed) and the recipient tissue microenvironment (the soil) mediate these non-random patterns of spread in breast cancer. We hypothesize that the ability of metastatic cells to survive dormancy, exit quiescence, and colonize a specific tissue depends both upon the ability of the seed to survive long-term growth arrest, and the ability of the soil to first sustain survival, and subsequently trigger outgrowth. To this end, we created a biomaterial platform that varies extracellular matrix (ECM) density and composition, and used cell adhesion and migration assays to study how highly metastatic cells differentiate between different tissues. In our recently published paper, we used this biomaterial system to screen the adhesion and motility of cell lines on ECM surfaces. When we compiled this data using a systems biology-like approach, we could predict in vivo metastasis with 88% accuracy using a quick screen of adhesion and motility in vitro.
With this relationship in hand, we are now using this same platform to predict long-term relapse, i.e. dormancy, in very slow growing cell lines. We have found that both integrin binding to the ECM and growth factor supplementation mediate dormancy in cell lines known to display long periods of metastatic latency in vivo. We first induce a dormant-like phenotype by culturing cell lines in serum-free medium, and surviving quiescent cells can then be stimulated to grow with serum. We uniformly observe that a laminin-coupled ECM is unable to permit survival of dormant cells, whereas binding to collagen I maintains dormancy. Increased cell density and growth factor supplementation also increase long-term dormancy, but this is a cell type-specific effect that is surprisingly uncorrelated with receptor expression. Specifically, the HCC 1428 cell line, with high epidermal growth factor receptor (EGFR) expression, was unable to maintain a dormant state beyond 7 days, even with epidermal growth factor (EGF) supplementation. In contrast, EGF maintains dormancy of a greater number of ZR-75-1 cells, despite their low EGFR expression. Upon recovery, cells that have been cultured for 1-3 months in serum-free medium proliferate more quickly, are more drug resistant, and are more homogeneously responsive to growth factors than the parental population, suggesting this selective pressure creates an aggressive subpopulation. We are currently investigating the downstream signaling responsible for long-term survival on collagen, and whether growth factor-enhanced survival results from the physical association of growth factor receptors and integrins or changes in receptor expression over time. In sum, we have used a simple biomaterial to understand the roles of integrin binding to the ECM and growth factor stimulation in mediating long-term dormancy, providing insight into combinatorial targets to prevent late breast cancer relapse.
References: 1. Barney et al., Integrative Biology, 2015.