Cytoskeletal reorganizations responsible for the phorbol ester-induced formation of cytoplasmic processes: possible involvement of intermediate filaments.

The tumor promoter phorbol 12-myristate 13-acetate (PMA) induces characteristic reversible changes of cell shape in certain fibroblastic lines: motile lamellas are transformed into noncontractile narrow processes; simultaneously, the actin microfilament network of lamellas is locally disorganized. This reaction to PMA may be regarded as a prototype of reorganizations involving formation of stable cytoplasmic processes. Specific drugs, Taxol and Colcemid, were used to study the role of microtubules and vimentin-containing intermediate filaments (IF) in the development of PMA-induced reorganizations. PMA readily induced formation of noncontractile processes in Taxol-treated fibroblasts; these cells had a profoundly altered microtubular system but noncollapsed IF. A short (1 hr) exposure to PMA induced formation of processes in control cells but not in the Colcemid-treated cells, which had depolymerized microtubules and IF that collapsed around the nucleus. Longer (3-4 hr) exposure of the Colcemid-treated cells to PMA induced partial reversal of the IF collapse; those parts of the peripheral lamellas that contained IF were transformed into narrow noncontractile processes. It is suggested that the local interaction of IF with the actin system is an essential step in the formation of processes from lamellas. The microtubular system controls distribution of IF in the cytoplasm and thus plays an indirect role in the reorganization of the actin cortex.