Bone marrow (BM) is a clinically relevant site of micrometastatic disease in patients with solid epithelial
tumors. It is, therefore, important to establish suitable models that allow the in-depth characterization of disseminated
tumor cells present at low frequencies of 10(-5)-10(-6) nucleated BM cells. The aim of this study was to assess common phenotypic features of nine tumor cell lines established from BM of patients with
cancer of the prostate (four cell lines), breast (two cell lines), lung (two cell lines), and colon (one cell line) using immunocytochemistry, flow cytometry, and reverse transcription-PCR. All cell lines stained positive for both cytokeratins, the epithelial intermediate filaments, and the
epithelial cell adhesion molecule E-cadherin, and they lacked markers of BM-derived cells. The
tumor origin of the cell lines was supported by the expression of the ErbB2 oncogene (seven of nine) and MAGE
mRNA (eight of eight). All cell lines coexpressed
cytokeratin and
vimentin, the mesenchymal intermediate filament, indicating an epithelial-mesenchymal transition of micrometastatic cells. The invasive phenotype of the immortalized cells was also reflected by the consistent expression of several
metastasis-associated adhesion molecules, including alpha5 (eight of nine), alpha6 (five of nine), alphaV (nine of nine), beta1 (nine of nine), and beta3 (nine of nine)
integrin subunits and the Mr 67,000
laminin receptor (seven of nine). Contrary to our expectations,
metastasis-promoting CD44 variant
isoforms were only detected on two lines, whereas all cell lines expressed MUC18/
melanoma cell adhesion molecule and
intercellular adhesion molecule-1, two members of the
immunoglobulin superfamily of adhesion molecules that are not frequently found on primary
carcinoma cells. The consistent expression of various epithelial and
tumor-associated
antigens provides evidence that the established cell lines are derived from disseminated
cancer cells present in the BM. The invasive phenotype of the immortalized cells was mirrored by their epithelial-mesenchymal transition and the expression of several
metastasis-associated molecules, which might be potential candidates for novel therapeutic targets.