The I1-imidazoline receptor in PC12 pheochromocytoma cells activates protein kinases C, extracellular signal-regulated kinase (ERK) and c-jun N-terminal kinase (JNK)

We sought to further elucidate signal transduction pathways for the I-1-imidazoline receptor in PC12 cells by testing involvement of protein kinase C (PKC) isoforms (beta (II), epsilon, xi), and the mitogen-activated protein kinases (MAPK) ERK and JNK. Stimulation of I-1-imidazoline receptor with moxonidine increased enzymatic activity of the classical beta (II) isoform in membranes by about 75% and redistributed the atypical isoform into membranes (40% increase in membrane-bound activity), but the novel isoform of PKC was unaffected. Moxonidine and clonidine also increased by greater than two-fold the proportion of ERK-1 and ERK-2 in the phosphorylated active form. In addition, JNK enzymatic activity was increased by exposure to moxonidine. Activation of ERK and JNK followed similar time courses with peaks at 90 min. The action of moxonidine on ERK activation was blocked by the I-1-receptor antagonist efaroxan and by D609, an inhibitor of phosphatidylcholine-selective phospholipase C (PC-PLC), previously implicated as the initial event in I-1-receptor signaling. Inhibition or depletion of PKC blocked activation of ERK by moxonidine. Two-day treatment of PC12 cells with the I-1/alpha (2)-agonist clonidine increased cell number by up to 50% in a dose related manner. These data suggest that ERK and JNK, along with PKC, are signaling components of the I-1-receptor pathway, and that this receptor may play a role in cell growth.