Density-dependent expression of keratins in transformed rat liver cell lines

Immunomorphological examination of the distribution of three keratins in cultured rat liver-derived epithelial cell lines of the IAR series was performed in order to find out the effects of neoplastic evolution on the expression of these epithelium-specific markers. Specific monoclonal antibodies were used to reveal various intermediate filament proteins: keratins with molecular masses of 55, 49 or 40 kD (K55, K49 or K40), and vimentin. The expression of keratins was negligible in sparse and dense cultures of nontransformed lines, which had typical epithelial morphology. The examined keratins were also absent in the sparse cultures of transformed lines, which have lost partially or completely the morphological features of epithelia. However, cells in dense cultures of most transformed lines contained numerous keratin filaments. It is suggested that the paradoxical increase of keratin expression after transformation is due to increased saturation density of transformed cultures; this high density favours the expression. As shown by the experiments with culture wounding, the effects of density are local and reversible. While K55 was present in all the cells of dense cultures, the expression of the other two keratins was dependent on the cell position within these cultures. It is suggested that the expression of the latter two keratins, besides high cell density, also requires the presence (K40) or the absence (K49) of cell-substratum contacts. Possible mechanisms of the effects of cell density on the expression of keratins are discussed.