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

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 microfilaments (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 substratum 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.

Biochemical localization of the transformation-sensitive 52 kDa (p52) protein to the substratum contact regions of cultured rat fibroblasts. Butyrate induction, characterization, and quantification of p52 in v-ras transformed cells

“…Total profile and individual peak areas were determined by scanning densitometry. The identification of a-actinin, vimentin and actin in each electrophoretic profile utilized criteria described in detail previously (Ryan & Higgins, 1988a;Higgins et al, 1987 The present and previous investigations into the mechanism of action of NaB on KNRK fibroblasts have clearly indicated an association between p52 expression and induced cell spreading. Induction of p52 occurs before NaB-initiated cell spreading (Ryan & Higgins, 1988b); it is the only major protein obviously deficient in the SAM compartment of virus-transformed fibroblasts (e.g.…”

“…6). The p52 content of NRK and K/NaB SAM fractions closely approximated that of actin [which was identified using criteria described previously in detail (Ryan & Higgins, 1988a,b;Higgins et al, 1987)]. …”