Phenotypic properties of cultured tumor cells: Integrin αIIbβ3 expression, tumor‐cell‐induced platelet aggregation, and tumor‐cell adhesion to endothelium as important parameters of experimental metastasis

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Abstract
The present study was undertaken to investigate the factors involved in determining the metastatic potential of cultured cells derived from solid tumors. We first investigated the effects of cell source and culture conditions on lung colony formation by i.v. injected B16a (B16 amelanotic melanoma) cells and inhibition of tumor colony formation by the thromboxane A2 syn‐thase inhibitor, CGS14854. Prolonged culture resulted in a 10‐fold decrease in the incidence of B16a lung colonies, whereas passage in vivo for 150 days did not affect lung colony formation by tumor cells isolated from enzymatic dispersates by centrifugal elutriation. Cultured BI6a cells maintained at low density (LD) and harvested at low passage (LP) formed significantly more lung colonies than B16a cells harvested at high densities (HD) or high passage (HP). Over‐confluent tumor cells produced even lower number of lung colonies. Lung colony formation by elutriated BI6a cells (i.e., cells freshly isolated from tumor tissue) was consistently inhibited by CGS14854, whereas inhibition of lung colony formation by cultured B16a cells was dependent upon culture conditions. CGS 14854 was ineffective or less effective against HD/HP B16a cells. The differences in lung colony formation between LD, HD and elutriated B16a cells were not due to differential cell‐cycle distribution. Mechanistic studies indicated that LD/LP tumor cells induced aggregation of homologous platelets, whereas HD/HP BI6a cells failed to induce significant platelet aggregation. Aggregation of homologous platelets correlated positively with lung‐colonizing ability. Additionally, LD/LP cells demonstrated higher adhesion to endothelium than HD/HP BI6a cells. Finally, LD/LP BI6a cells expressed higher levels of αIIbβ3 integrins than HD/HP tumor cells, as determined by flow cytometry and immunofluorescence.