Protein kinase C regulates dopamine D4 receptor-mediated phospholipid methylation

Dopamine D4 receptors (D4 receptors) mediate dopamine-stimulated, folate-de pendent phospholipid methylation. To investigate possible regulation of thi s multi-step D4 receptor-mediated phospholipid methylation cycle by protein kinases, specific kinase activators and inhibitors were studied in SK-N-MC human neuroblastoma cells, using [C-14] formate to label folate-derived si ngle-carbon groups. Phorbol dibutyrate (PDB), an activator of protein kinas e C, stimulated basal phospholipid methylation and also shifted the dose-re sponse curve for dopamine-stimulated phospholipid methylation to the right by more than an order of magnitude, Calphostin C, an inhibitor of protein k inase C, had little effect on basal phospholipid methylation but significan tly inhibited dopamine-stimulated phospholipid methylation and also blocked the stimulatory response to PDB. Chelerythrine, which inhibits protein kin ase C and other kinases. strongly inhibited both basal and dopamine-stimula ted phospholipid methylation. Forskolin, an activator of protein kinase A, inhibited basal and dopamine-stimulated phospholipid methylation, but only at high concentrations while Rp-cAMP, an inhibitor of protein kinase A. did not block this effect. Inhibition of protein kinase G produced a modest de crease in dopamine-stimulated phospholipid methylation. but neither sodium nitroprusside, which increases nitric oxide (NO) production and activates p rotein kinase G, nor the NO synthase inhibitor N-nitro-L-arginine had any e ffect on basal or dopamine-stimulated phospholipid methylation. These obser vations indicate that protein kinase C is an important regulator of basal a nd D4 receptor-mediated folate-dependent phospholipid methylation, whereas protein kinase A and protein kinase G have a lesser or minimal role. (C) 20 01 Elsevier Science B.V. All rights reserved.