Overexpression of MARCKS, but not protein kinase C-α, increases phorbol ester-stimulated synthesis of phosphatidylcholine in human SK-N-MC neuroblastoma cells

To investigate the regulation of phorbol ester-stimulated synthesis of phosphatidylcholine (PtdCho), myristoylated alanine-rich protein kinase C substrate (MARCKS) and the α-isoform of protein kinase C (PKC-α) were overexpressed in a human neuroblastoma (SK-N-MC) cell line that does not increase PtdCho synthesis in response to 4β-12-O-tetradecanoylphorbol 13- acetate (TPA). In five clones with a less than fivefold increase in MARCKS protein level, the synthesis of PtdCho from [methyl-³H]choline was stimulated 1.88-2.34-fold in the presence of 100-200 nM TPA. In clones overexpressing PKC-α (30-40-fold increased level of protein) or in mock- transfected vector controls, TPA had much less of a stimulatory effect (1.04- 1.43-fold) on PtdCho synthesis. TPA caused translocation of PKC-α and increased phosphorylation of MARCKS, indicating that both overexpressed proteins responded to stimulation. Thus, in SK-N-MC cells, MARCKS is required for TPA-stimulated synthesis of PtdCho, and PKC-α alone is insufficient for supporting enhanced synthesis.