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Previous studies indicated that mouse transformed keratinocytes undergo an epithelial-fibroblastic conversion when cultured in the presence of TGF- β1. This conversion is associated in vivo with a squamous-spindle carcinoma transition. We derived epithelioid (A6, FPA6) and spindle (B5) clonal cell variants from a squamous carcinoma cell line (PDV) after treatment with TGF- β1. FPA6 cells were isolated from the ascites fluid of an A6-tumor-bearing mouse. FPA6 and A6 cell lines produced in nude mice mixed carcinomas with a squamous and poorly differentiated component. Both cell lines coexpressed keratins and vimentin and synthesized E-cadherin protein, although FPA6 cells cultured at early passages (FPA6-ep) had reduced levels of E-cadherin mRNA and increased synthesis of keratin K8, a marker of malignant progression. Immunofluorescence analysis revealed that FPA6-ep cells exhibited a disorganized cytoskeleton with keratins forming focal juxtanuclear aggregates and loss of F-actin stress fibers and cortical bundles, and E-cadherin was localized in the cytoplasm out of cell-cell contact areas. Sporadic cells in A6 and PDV cultures also presented those anomalous keratin structures, suggesting that FPA6 cells originated from a subpopulation of A6 tumor cells that metastasized into the peritoneal cavity. The analysis of the spontaneous and experimental metastatic potentials of the cell lines showed that epithelioid and fibroblastic cell variants had acquired metastatic abilities compared to PDV which was nonmetastatic. The FPA6-ep cell line exhibited a highly aggressive behavior, killing the animals at about 17 days after intravenous injection of the cells into athymic mice. The phenotype of FPA6- ep cells was unstable and reverted at later passages in which the normal organization of keratin and F-actin in filaments and the localization of E- cadherin at cell-cell contacts were restored. This phenotypic reversion occurred concomitantly with a reduction of the experimental metastatic potential of FPA6 cells., This work was supported by Grants SAF95-1547 from the Comisión Interministerial de Ciencia y Tecnología (CICYT) and 7/118/96, 8.1/22/97 from the Comunidad Autónoma de Madrid (CAM).