Starting with an extract derived from the stem of Macleaya cordata (Papaveraceae) that was active in the process of inhibiting
phorbol 12,13-dibutyrate binding to partially purified
protein kinase C (PKC), the benzophenanthridine
alkaloid angoline was isolated and identified. This discovery appeared in context, as a related benzophenanthridine
alkaloid,
chelerythrine, has been reported to mediate a variety of
biological activities, including potent and selective inhibition of
protein kinase C (PKC). However, in our studies,
angoline was not observed to function as a potent inhibitor of PKC. Moreover, we were unable to confirm the reported inhibitory activity of
chelerythrine. In a comprehensive series of studies performed with various PKC
isozymes derived from a variety of mammalian species, neither
chelerythrine nor
angoline inhibited activity with high potency. To the contrary,
chelerythrine stimulated PKC activity in the cytosolic fractions of rat and mouse brain in concentrations up to 100 microM. In addition,
chelerythrine and
angoline did not inhibit [3H]
phorbol 12,13-dibutyrate binding to the regulatory domain of PKC at concentrations up to 40 microg/ml, and no significant alteration of PKC-alpha, -beta, or -gamma translocation was observed with human
leukemia (HL-60) cells in culture. Further,
chelerythrine did not inhibit 12-O-tetradecanoylphorbol 13-acetate-induced
ornithine decarboxylase activity with cultured mouse 308 cells, but
angoline was active in this capacity with an IC50 value of 1.0 microg/ml. A relatively large number of
biological responses have been reported in studies conducted with
chelerythrine, and alteration of PKC activity has been considered as a potential mechanism of action. In light of the current report, mechanisms independent of PKC inhibition should be considered as responsible for these effects.