The I1-imidazoline receptor in PC12 pheochromocytoma cells reverses NGF-induced ERK activation and induces MKP-2 phosphatase

We sought to further elucidate signal transduction pathways for the I1-imidazoline receptor in PC12 cells and their interaction with the well-characterized signaling events triggered by nerve growth factor (NGF) in these cells. Stimulation of the I1-imidazoline receptor with moxonidine, a centrally acting antihypertensive, increased by greater than two-fold the proportion of ERK-1 and ERK-2 in the phosphorylated active form. Similarly, NGF elicited a five-fold increase in activated ERKs. Surprisingly, treatment of NGF-treated cells with moxonidine completely reversed activation of ERK. Moxonidine-induced inhibition of ERK activation in NGF-treated cells was dose-dependent, followed a limited time course and could be blocked by the I1-antagonist efaroxan. These data suggested possible deactivation of ERK by specific phosphatases. Therefore, we assayed levels of MKP-2, a dual specificity phosphatase whose substrates include ERK. Moxonidine and NGF both increased levels of MKP-2 by three-fold. These effects were additive, as both agents together increased MKP-2 by a total of six-fold. Moxonidine-induced induction of MKP-2 was time- and dose-dependent and could be blocked by the I1-antagonist efaroxan or by D609, an inhibitor of phosphatidylcholine-selective phospholipase C known to block downstream signaling events coupled to I1-receptors. Thus, I1-receptors can abrogate the primary signaling cascade activated by NGF, most likely by increasing levels of a specific phosphatase to return dually phosphorylated ERK to its unphosphorylated state.