Experimental co-expression of vimentin and keratin intermediate filaments in human breast cancer cells results in phenotypic interconversion and increased invasive behavior

The expression of intermediate filament proteins is remarkably tissue specific, which suggests that the intermediate filament type(s) present in cells is somehow related to their biological function. However, in some cancers, particularly malignant breast carcinoma, there is a strong indication that vimentin is co-expressed with keratins, thus presenting as a dedifferentiated or interconverted (between epithelial and mesenchymal) phenotype. In the present study, we recapitulated the interconverted phenotype by developing stable transfectants of MCF-7 human breast cancer cells, termed MoVi clones, to express both vimentin and keratins. Over expression of vimentin in these cells led to augmentation of motility and invasiveness in vitro. These activities could be transiently down-regulated by vimentin antisense oligonucleotides in MoVi clones and MDA-MB-231 cells (which constitutively coexpress keratins and vimentin). Furthermore, in the MoVi experimental transfectants expressing the highest percentage of vimentin-positive cells, their proliferative capacity, clonogenic potential, and tumorigenicity increased. However, the metastatic ability of the MoVi transfectants remained unchanged compared with MCF-7neo controls. The MDA- MB-231 cells metastasized to axillary lymph nodes in a SCID mouse model. Finally, we explored the possibility that potential changes could occur with respect to cell surface integrins. These studies revealed a decrease in the α₂- and α₃-containing promiscuous integrins, in addition to β₁ containing integrins, concomirant with an increase in the α₆-containing laminin receptor integrin. Further functional analysis of the α₆ observation showed an increase in the haptotactic migration of MoVi transfectants toward a laminin substrate. From these data, it is postulated that the ability to co-express vimentin and keratins confers a selective advantage to breast cancer cells in their interpretation of signaling cues from the extracellular matrix; however the addition of vimentin intermediate filaments alone is not sufficient to confer the metastatic phenotype.