The spread of cancer cells from a primary tumor to form metastasis at distant sites is a complex multistep process. The cancer cell proteins and plasma membrane proteins in particular involved in this process are poorly defined, and a study of the very early events of the metastatic process using clinical samples or in vitro assays is not feasible. We have used a unique model system consisting of two isogenic human breast cancer cell lines that are equally tumorigenic in mice; but although one gives rise to metastasis, the other disseminates single cells that remain dormant at distant organs. Membrane purification and comparative quantitative LC-MS/MS proteomics identified 13 membrane proteins that were expressed at higher levels and three that were underexpressed in the metastatic compared with the non-metastatic cell line from a total of 1919 identified protein entries. Among the proteins were ecto-5-nucleotidase (CD73), NDRG1, integrin ␤1, CD44, CD74, and major histocompatibility complex class II proteins. The altered expression levels of proteins identified by LC-MS/MS were validated using flow cytometry, Western blotting, and immunocyto-and immunohistochemistry. Analysis of clinical breast cancer biopsies demonstrated a significant correlation between high ecto-5-nucleotidase and integrin ␤1 expression and poor outcome, measured as tumor spread or distant recurrence within a 10-year follow-up. Further the tissue analysis suggested that NDRG1, HLA-DR␣, HLA-DR␤, and CD74 were associated with the ER

No abstract