Gene expression profiling of tumor xenografts: In vivo analysis of organ‐specific metastasis

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Abstract
Mammary carcinoma frequently metastasizes to specific organs, including the regional lymph nodes, the lung and bone marrow. The mechanisms that guide organ‐specific metastasis and the molecular players that are involved remain to be established. To gain insight into this problem, we used an orthotopic model of breast cancer in which the MDA‐MB‐435 cells are implanted into the mouse mammary fat pad. Sublines that preferentially metastasized to specific sites were isolated by excising metastatic tumors and growing explants of these tumors in culture. Cells lines that preferentially metastasize to the lymph node and thoracic cavity were obtained. The gene expression profiles of primary tumors from these sublines were then compared with cDNA arrays containing 5,800 known genes. In tumors that preferentially metastasize to the lymph node, several genes encoding adhesion and matrix proteins were upregulated. Genes encoding proteins involved in metabolism were downregulated. One of the upregulated genes in lymph‐homing tumors was CD73. Immunohistochemistry showed that the CD73 protein is also upregulated in primary tumors of this cell line and that its expression is elevated in the lymph node metastases. CD73 is a transmembrane protein that has previously been implicated in the homing of normal lymphocytes to the nodes. This raises the hypothesis that tumors preferentially metastasize to lymph nodes by using CD73 to mimic part of the lymphocyte homing process.