Inhibition of Rodent Protein Kinase C by the Anticarcinoma Agent Dequalinium1

Dequalinium has previously been shown to be an anticarcinoma agent (M. J. Weiss et al., Proc. Nati. Acad. Sci. USA, 84: 5444-5448, 1987). The present study demonstrates that it can inhibit protein kinase C-ß, isolated from an overproducing cell line with a 50% inhibitory concentra tion of 8-15 MM-Further examination of the inhibition by using structural analogues of dequalinium reveals that the length of the méthylà ̈ne bridge between the two quinaldinium moieties, the presence of the ring substituents, and the bipartite character of the compound each contributes to the inhibitory potency. Related studies show that the analogues display the same rank order of inhibitory potency when tested with the trypsingenerated catalytic fragment of the enzyme, indicating that dequalinium inhibits kinase activity through an interaction with the catalytic subunit. Further studies argue that the ability of a given analogue to inhibit phosphotransferase activity correlates with its ability to compete with |JH|phorbol-12,13-dibutyrate binding on the intact enzyme (50% inhibi tory concentration of 2-5 MM)-This suggests that the inhibitor is either binding directly to the regulatory subunit as well, or that due to its interaction with the catalytic subunit, dequalinium produces an indirect effect on sites defined by phorbol ester binding. Kinetic analysis revealed that inhibition is noncompetitive with respect to ATP or phosphatidylserine. Studies conducted with types I, II, and III rat brain isozymes, resolved by hydroxylapatite chromatography, demonstrate that dequa linium inhibits each of them with similar potency (50% inhibitory con centration of 11 MM)and imply that the site of contact on the enzyme is a highly conserved region. Morphology studies with dequalinium in intact cells demonstrate that the inhibitor can protect control cells against phorbol ester-induced morphology changes but cannot protect protein kinase C-overproducing cells, suggesting that an elevation in protein kinase C levels alone is sufficient to overturn the protection conferred by dequalinium. On the basis of these results, we propose that protein kinase C could be a critical in vivo target of dequalinium.