Contact‐inhibitory factor induces alterations in the distribution and content of specific cytoskeletal elements in an established line of rat hepatic tumor cells

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Abstract
Established 72/22 rat hepatic epithelial tumor cells, which possess intracellular aggregates of intermediate‐sized filaments resembling Mallory‐body‐like inclusions, were used to assess changes in tumor cell growth and morphology associated with exposure to contact‐inhibitory factor (CIF). CIF reduced 72/22 proliferative rate, increased mean population doubling time by 42%, lowered culture saturation densities to 34–50% of control values and inhibited formation of dense foci. These proliferative changes were due to an apparent prolongation of the G1 phase of the cell cycle during the period of CIF exposure. CIF concomitantly induced a marked increase (by 70%) in cell spreading and loss of both the usual tight (epithelioid) cell juxtaposition and typical ordered colony structure characteristic of untreated populations. However, CIF exposure failed to achieve complete cytoarchitectural “normalization” in 72/22 cells (i.e., dispersal of the Mallory‐body‐like aggregate of intermediate filaments and restoration of a more typical hepatocytic phenotype). Most obvious was a reduction In the integrity of the peripheral band of mlcrofilaments (a structure involved in the maintenance of epithelial cell shape) and a decrease in the content of desmoplakin (a protein component of desmosomal plaques). Changes in these major structural elements appear to be critical events in development of the pleomorphic phenotype and reduced sub‐stratum adhesiveness observed during treatment. CIF‐related fragmentation of peripheral band structures was not reflected in changes in either the total cellular or cytoskeletal‐associated actin contents. The morphologic changes observed under conditions of CIF exposure closely paralleled induced decreases in the cellular content of the actin‐associated membrane skeleton protein p35. These data collectively suggest that CIF may act to alter the composition of the cortical skeleton in cultured liver tumor cells.