Activation of protein kinase C subtypes α, γ, δ, ϵ, ζ, and η by tumor-promoting and nontumor-promoting agents

Abstract Protein kinase C (PKC) subtypes α, γ, δ, ϵ, ζ, and η have been expressed using the baculovirus expression system. The partially purified PKC subtypes have been studied for their substrate specificities and phospholipid-independent activation by various chemically different nontumor- and tumor-promoting agents, as well as their inhibition of kinase activity by staurosporine and two related compounds. An endogenous PKC-like kinase activity of Sf9 cells was detected and analyzed for cofactor requirements and inhibition. Protamine sulfate was most efficiently phosphorylated by all of the PKC subtypes tested, although this phosphorylation was independent of phosphatidylserine (PS) and diacylglycerol (DAG) or 12-O-tetradecanoylphorbol 13-acetate (TPA). Except for PKC-ζ, all subtypes tested phosphorylated myelin basic protein (MBP), histone, or a peptide derived from the pseudosubstrate region of PKC-α in a PS/DAG -dependent manner but to varying extents. Among the various agents tested, TPA most efficiently stimulated the kinase activities of the PKC subtypes in a phospholipid-dependent manner. Phorbol 12,13-dibutyrate (PDBu) was less effective than TPA but displayed no major difference among the subtypes. Activation of PKC-α by bryostatin-1 reached only half of the TPA response whereas the other subtypes were activated more effectively. The weak tumor promoter resiniferonol 9,13,14-orthophenyl acetate (ROPA) mainly stimulated PKC-α and PKC-γ at 1 μM concentration, whereas PKC-ϵ and PKC-η were much less activated. Sapintoxin D, mezerein, indolactam V, and resiniferatoxin at concentrations of 1–100 nM preferentially activated PKC-α in a DAG-like manner, whereas at 1 μM other subtypes were activated as well. Preferential activation of PKC-α was also noted for tinyatoxin and thapsigargin, but their mode of activation is unclear because these two compounds did not compete for the phorbol ester binding of the PKC subtypes as the other agents did. Of the three PKC inhibitors tested, staurosporine most efficiently inhibited kinase activity of the PKC subtypes, whereas K252a and CGP 41251 were at least 10 times less effective. However, K252a showed certain specificity for inhibition of PKC-α, and CGP 41251 failed to inhibit PKC-ϵ and PKC-ζ. Given the different substrate specificities and modes of activation by various tumor-promoting and nontumor-promoting agents, as well as the different sensitivities towards different inhibitors, our results indicate a divergence of individual PKC subtypes in signal transduction.