Over 60% of breast cancer patients are diagnosed with "luminal" breast cancer, but the mechanisms that contribute to tumor progression and more importantly to metastasis formation, are poorly understood. Acquisition of a metastatic phenotype by breast tumor cells might be mediated by Epithelial-to-Mesenchymal Transition (EMT) and by up-regulation of ECM-receptors.
Our recent findings indicated that the inflammatory cytokine TNFa induced EMT and invasion in breast tumor cells. To better recapitulate the tumor microenvironment of breast tumors, we determined the joint effects of TNFa, hormones and GF (Growth Factors) on EMT and metastasis formation in luminal breast tumor cells.
Our findings indicate that following exposure to TNFa+hormones+GF (="combined stimulation"), breast tumor cells expressed characteristics associated with a more metastatic phenotype: tumor cells exhibited substantial morphological changes, up-regulation of ECM-receptors, and enhanced EMT phenotype. The effect of the "combined stimulation" was more prominent than each of the stimuli alone, indicating that the pro-metastatic effects of each of the factors were intensified when it cooperated with other factors.
The implications of these in-vitro results were manifested in-vivo, when 45% of mice inoculated with "combine stimulated" cells developed metastases, while no metastases were evident in mice inoculated with unstimulated cells. Our data suggest that the metastatic phenotype might be potentiated by "combined stimulation"-induced retention of tumor cells at CXCL12/CCL21-enriched sites. This possibility is implicated by the fact that the "combined stimulation" induced selection of tumor cells expressing high levels of CXCR4/CCR7, which exhibited low motility upon exposure to CXCL12/CCL21, and therefore might arrest at specific sites, resulting in metastasis formation.
Overall, our findings point to instrumental interactions occurring at breast cancer microenvironment, which may take place in patients diagnosed with "luminal" breast tumors, and potentiate metastasis formation. Such interactions emphasize the critical importance of studying the combined effects of tumor-promoting factors in cancer microenvironment.
