Metastatic tumor formation via vessel route begins with cancer cell extravasation from vessel lumen, migration into the connective tissue surrounding vessels, and invasion into target organ parenchyma. Epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET) have been recognized to play an important role in metastatic process, however, how and where these biological changes take place in the early phase of metastatic tumor development has never been clarified. We morphologically evaluated 34 small intrapulmonary metastases formed after cancer cell extravasation from lymphatics (lymphogenic metastasis) and 40 formed in the absence of extravasation (aerogenous metastasis) in human specimens and found that isolated or small clusters of invasive cancer cells (tumor budding) were frequently observed in lymphogenic metastasis (24/34; 71%), but were never observed within aerogenous metastasis. We immunostained 34 lymphogenic metastases for 13 molecular markers of EMT and MET and scored the immunostaining intensity of cancer cells floating in lymphatic vessels (LVs), migrating into the connective tissue surrounding vessels [bronchovascular bundle (BVB)], and growing in lung parenchyma (LP). Cancer cells within BVBs stained more weakly for E-cadherin (p < 0.001), β-catenin (p < 0.001), and Geminin (p < 0.001) and more strongly for MMP-7 (p = 0.046) and Laminin-5 γ2 (p = 0.037) than tumor cells in LVs. However, cancer cells in LP exhibited resurgent E-cadherin (p = 0.011), β-catenin (p < 0.001), and Geminin (p = 0.037) expression and reduced MMP-7 (p = 0.038) and Laminin-5 γ2 (p = 0.001) expression in comparison with cancer cells in BVBs. Our results suggested that in the early phase of metastatic tumor formation cancer cells undergo dynamic phenotypic change associated with EMT and subsequent MET.
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