Expression of alpha 6 beta 4 integrin increases during malignant conversion of mouse epidermal keratinocytes: association of beta 4 subunit to the cytokeratin fraction.

The expression of alpha 6 beta 4 integrin has been analyzed in several keratinocyte cell lines representative of various stages of mouse epidermal carcinogenesis. The immunological analyses carried out show that alpha 6 beta 4 is expressed at the cell surface of the cell lines which exhibit an epithelial or epithelioid morphology. The relative levels of alpha 6 beta 4 expressed at the cell surface increase noticeably from premalignant to malignant cells, as detected by fluorescence flow cytometry. This increase also correlates with the abundance of soluble beta 4 subunit detected by Western immunoblotting in the different cell lines. However, complete absence of alpha 6 beta 4 has been found in spindle carcinoma cells showing a fibroblast-like phenotype in culture. The integrin remains associated to detergent- and high salt-insoluble cytoskeletal components, organized in stable anchoring contacts, as in human keratinocytes (Carter et al., J. Cell Biol., 111, 3141, 1990). In addition, a significant fraction of the beta 4 subunit is detected associated to highly purified cytokeratin fractions. These results, together with those regarding the organization of both cellular components in drug-treated cells, support the existence of a close association between alpha 6 beta 4 and the intermediate filaments of the cytoskeleton.