Protein Kinase C-α But Not Protein Kinase C-ϵ Is Differentially Down-Regulated by Bryostatin 1 and Tetradecanoyl Phorbol 13-Acetate in SH-SY5Y Human Neuroblastoma Cells

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SH-SY5Y human neuroblastoma cells can be induced to differentiate by phorbol esters but not by bryostatins although both agents increase protein kinase C (PKC) activity in these cells to a similar extent. We examined whether this difference could be explained by differences in the responses of specific PKC isoenzymes. Both TPA and bryostatin 1 at 10 nM induced a rapid increase in membrane-associated PKC-α immunoreactivity which was sustained for 72 hours in TPA-treated cells, but was down-regulated within 24 hours in bryostatin-treated cells. TDA likewise induced a sustained phosphorylation of an 80 kDa PKC substrate whereas in bryostatin-treated cells the 80 kDa substrate was rapidly phosphorylated reaching a maximum at 6 hours followed by a decline to basal level within 48 hours. A higher concentration of TPA (300 nM), which results in a less differentiated phenotype, induced down-regulation of PKC-α within 24 hours. In contrast, both TPA and bryostatin 1 stimulated translocation and a partial down-regulation of PKC-ϵ with similar kinetics. These results suggest that the divergent actions of bryostatin I and TPA in SH-SY5Y cells are at least partially due to differential modulation of PKC-α but not PKC-ϵ by these two agents.