Tissue factor and cell morphology variations in cell lines subcloned from U87-MG

Tissue factor is heterogeneously distributed within and among cells in cultures of U87-MG, a glioblastoma-derived line. The heterogeneity among cells may reflect the presence of distinct populations within the U87-MG cultures. This hypothesis has been confirmed by cloning of five distinct sublines from the parent population. These subpopulations have remained distinct through 4 months of growth in culture and one cycle of cryogenic preservation and thawing. The cultures differ in growth rates, amounts of tissue factor activity expressed, tissue factor antigen measured by flow cytometry, and patterns of tissue factor distribution studied by immunofluorescence microscopy. Characterization of these sublines allowed us to recognize that the tissue factor distribution on polarized cells (e.g. spindle-shaped) differed from that on cells with less polar morphologies. Finely speckled tissue factor staining tended to be localized to polarized aspects of the cell body where actin stress fibers are commonly present, whereas larger distinct foci of tissue factor were present in regions of membrane spreading. These results show that tissue factor is distributed differently in distinct regions of plasma membrane differentiation. Furthermore, the isolation of distinct stable subpopulations by dilutional cloning of U87-MG cultures serves as a reminder that cell culture heterogeneity can complicate experiments using molecular genetic manipulation of cultured cells which require clonal isolation of genetically altered lines.