Selective inhibition of protein kinase C isozymes by the indolocarbazole Gö 6976

Indolocarbazoles have been identified as novel inhibitors of protein kinase C (PKC), with Go 6976 as one of its most potent and selective representatives. Recombinant PKC isozymes alpha, beta 1, delta, epsilon, and zeta were used in in vitro kinase assays to investigate Go 6976 with respect to isozyme-specific PKC inhibition. Go 6850, identical with GF 109203X, another PKC-specific kinase inhibitor, was included in this study as a reference compound. Nanomolar concentrations of the indolocarbazole Go 6976 inhibited the Ca(2+)-dependent isozymes alpha and beta 1, whereas even micromolar concentration of Go 6976 had no effect on the kinase activity of the Ca(2+)-independent PKC subtypes delta, epsilon, and zeta. In contrast, the bisindolymaleimide Go 6850 inhibited all PKC isozymes, however, with a ranked order of potency (alpha > beta 1 > epsilon > delta > zeta). Kinetic analysis revealed that PKC inhibition by Go 6976 was competitive with respect to ATP, non-competitive with respect to the protein substrate, and mixed type with respect to phosphatidylserine. Further experiments in the presence of different amounts of free Ca2+ indicated that interference with Ca2+ or its binding site is not responsible for the differential inhibition of PKC isozymes by Go 6976.