Despite the advances in breast cancer management, breast cancer remains a major killer: it is estimated that in the year 2012 there were 39,510 deaths from breast cancer in the United States alone. Metastasis is the major cause of cancer mortality, and a cure for breast cancer can only emerge if we can overcome this final frontier. Understanding the processes that lead to metastasis is therefore an urgent need. However, the key barrier to progress towards addressing this challenge has been a reductionist view of cancer. Indeed, much of our approach towards understanding metastasis has been limited to either the "seeds," i.e., metastatic cells, or the "soil" i.e., the tissue the cancer cells colonize. Interestingly, the communication between the tumor cells and endothelium, which occurs during multiple steps of metastasis, has been underexplored. In a preliminary study, we have discovered a direct, physical mode of communication between metastatic breast cancer cells and endothelial cells mediated through tubular nanostructures, which we term nanobridges. This project aims to elucidate the precise role of these nanobridges in the development of metastasis.

We will achieve this goal by studying the structure and function of the nanobridges formed between the tumor and endothelial cells using a novel 3-D coculture assay that mimics tumor cell-endothelium interactions. Our preliminary results indicate that these nanobridges are continuous and formed of cytoskeletal elements; hence, it is likely that it will involve energy, cytoskeletal remodeling, and membrane fusion. Hence, it is likely that that the exocyst-GTPase complex, which has been implicated in all these processes, will play a major role in the assembly of these nanobridges. We will therefore explore the function of exocyst-GTPase complex in nanobridge assembly. Finally, we will test the hypothesis that intercellular communication mediated by nanobridges can enhance tumor cell migration across the endothelial barrier and/or transform the endothelium to a pathological phenotype (both effects can promote metastasis).

Our preliminary results indicate that the ability of a tumor cell to form nanobridges with endothelium could correlate with its metastatic potential. Achieving the above aims will provide (1) fundamental insights into how nanobridges arise and act as conduits for communication between the metastatic cells and the endothelium; (2) better clarity on how/what message is communicated; and (3) an understanding of the contribution of this communication to metastasis. The proposed project can therefore have a significant impact not only in advancing our understanding of previously unexplored physical heterotypic intercellular communication during metastasis, but also in potentially offering novel targets for perturbing metastasis in the longer term.