Incubation of HT-29 M6 cells with the phorbol ester phorbol 12-myristate 13-acetate (PMA) induces cell scattering, loss of cellular contacts and inactivation of E-cadherin. We have investigated the involvement of different protein kinase C (PK-C) isoforms in these processes using specific activators. Thymeleatoxin, a derivative of mezerein that activates conventional PK-Cs (cPK-Cs) but not novel PK-Cs (nPK-Cs), promoted effects that were similar to those of PMA, i.e. at concentrations of 200 nM it induced scattering of HT-29 M6 colonies, loss of homotypic contacts and dissociation of E-cadherin from the cytoskeleton. Among the isoforms activated by this compound, only cPK-Cα was detected in HT-29 M6 cells by Western blot. The specificity of this compound with respect to the rest of the PK-C isoforms present in these cells was determined; thymeleatoxin induced, as did PMA, the translocation of cPK-Cα from the cytosol to the membrane and the cytoskeleton, and its partial down-regulation. On the other hand, thymeleatoxin did not modify the cellular levels or localization of nPK-Cε or atypical PK-Cζ. ‘In vitro’ assays also showed that thymeleatoxin did not activate nPK-Cε at the concentrations added to the cell cultures. These results indicate that thymeleatoxin is selective for cPK-Cα over nPK-Cε and show a role for the former enzyme in the regulation of cell–cell contacts and the inactivation of E-cadherin in HT-29 M6 cells.

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