Circulating Tumor Cells as a Window on Metastasis Biology in Lung Cancer

Abstract

Circulating tumor cell (CTC) number in metastatic cancer patients yields prognostic information consistent with enhanced cell migration and invasion via loss of adhesion, a feature of epithelial-to-mesenchymal transition (EMT). Tumor cells also invade via collective migration with maintained cell-cell contacts and consistent with this is the circulating tumor microemboli (CTM; contiguous groups of tumor cells) that are observed in metastatic cancer patients. Using a blood filtration approach, we examined markers of EMT (cytokeratins, E-cadherin, vimentin, neural cadherin) and prevalence of apoptosis in CTCs and CTM to explore likely mechanism(s) of invasion in lung cancer patients and address the hypothesis that cells within CTM have a survival advantage. Intra-patient and inter-patient heterogeneity was observed for EMT markers in CTCs and CTM. Vimentin was only expressed in some CTCs, but in the majority of cells within CTM; E-cadherin expression was lost, cytoplasmic or nuclear, and rarely expressed at the surface of the cells within CTM. A subpopulation of CTCs was apoptotic, but apoptosis was absent within CTM. This pilot study suggests that EMT is not prosecuted homo- Metastasis usually portends a dismal prognosis for cancer patients and effective therapeutic intervention in the metastatic process remains elusive. This is the case despite decades of research after Paget's "seed and soil" hypothesis in 1889 to explain why primary tumors within one particular organ give rise to secondary tumors at nonrandom sites 1 and Ewing's suggestion in 1929 that mechanical factors associated with the anatomy of human vasculature also determine the final destination of metastasizing tumor cells. 2 It is now apparent that tumor cell invasion and formation of distant metastasis can progress via three major routes: i) via the bloodstream, ii) via lymphatic vessels, and iii) via transcoelomic spread into the pleural, pericardial, and abdominal cavities. 3 The hematogenous system is thought to be the primary and most common route for the formation of distant metastases. Disseminating tumor cells can also circulate to and lie dormant in the bone marrow, potentially for a number of years, and then re-enter the bloodstream en route to secondary metastatic sites. 4 According to the widely espoused epithelial-to-mesenchymal transition (EMT) paradigm, suggested by some as essential for metastasis, 5,6 invading mesenchymal tumor cells lose cell-cell adhesion. Consistent with this concept, there are increasing reports enumerating individual circulating tumor cells (CTCs) in cancer patients' blood samples. Moreover, using the Food and Drug Administration's approved CellSearch platform, the CTC number is a prognostic biomarker in metastatic breast,

Circulating tumor cell interactions with macrophages: implications for biology and treatment

“…These data suggest that SCLC CTCs seem to recruit and "educate" a specific type of macrophages operative in invasion, immune protection, extravasation and possibly cachexia (Figure 2). Thus, the well-established role of CTCs has to be extended to specific effects on monocyte-macrophage differentiation and specific priming (133,134). Intravasation is a significant rate limiting step in the metastatic process (69).…”

Small cell lung cancer: Recruitment of macrophages by circulating tumor cells

“…Thus, the wellestablished role of CTCs has to be extended to specific effects on monocyte-macrophage differentiation and specific priming, possibly overlapping with the cancer stem cell characteristics. [74][75][76] …”

Towards elucidating the connection between epithelial–mesenchymal transitions and stemness

“…Cells belonging to this phenotype have combined epithelial (cell-cell adhesion) and mesenchymal (motility) traits that enable them to migrate collectively, as seen during wound healing and branching morphogenesis [6]. In cancer, such cohort migration is manifested as the migration of clusters of CTCs in the bloodstream, as observed with lung, prostate and breast cancer patients [7][8][9][10]. Such collective migration obviates the need for all cells to detect external signals for migration and facilitates subsequent rounds of dissemination [11].…”