Implications of cytokeratin 8/18 filament formation in stratified epithelial cells: induction of transformed phenotype.

The cytokeratin (CK) pair 8 and 18 is normally expressed in all simple epithelia. This pair is not normally seen in stratified epithelial cells. Squamous cell carcinomas derived from stratified epithelia show anomalous expression of this CK pair. It is not known whether CKs 8 and 18 in any way contribute to the malignant phenotype of these cells. We used an immortalised, nontransformed human foetal buccal mucosa (FBM) cell line that expresses significantly higher amounts of CK18 compared to CK8. FBM cells were transfected with the full-length CK8 gene to study the role of CKs 8 and 18 in malignant transformation. Clones with higher expression of CK8 compared to untransfected FBM cells were studied for changes in their phenotypic characteristics. Immunofluorescence studies using antibodies to CKs 8 and 18 revealed well-decorated filaments throughout the cytoplasm in CK8 gene-transfected cells vs. untransfected FBM cells. Transmission images showed that FBM cells were isolated while transfected cells were in groups of well-spread cells with cellular projections. Transfected cells were independent of growth supplement requirements and showed anchorage-independent growth in soft agar assay and significantly reduced doubling time compared to nontransfected FBM cells. DNA flow-cytometric studies revealed increased DNA content and prolonged S phase in transfected clones vs. FBM cells. Injection of cells s.c. obtained from soft agar colonies developed from 2 of the clones resulted in tumour formation at the site of injection. In both cases, lung metastasis was also seen. Thus, in conclusion, it appears that increased expression of CK8 in some way changes the phenotypic characteristics of stratified epithelial cells, resulting in malignant transformation.