title: The I(1)-imidazoline receptor in PC12 pheochromocytoma cells reverses NGF-induced ERK activation and induces MKP-2 phosphatase.
abstract: We sought to further elucidate signal transduction pathways for the I(1)-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 I(1)-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 I(1)-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 I(1)-antagonist efaroxan or by D609, an inhibitor of phosphatidylcholine-selective phospholipase C known to block downstream signaling events coupled to I(1)-receptors. Thus, I(1)-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.
authors.affiliation: Department of Nutrition, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4906, USA.
authors.forename: Lincoln
authors.initials: L
authors.lastname: Edwards
authors.forename: Paul
authors.initials: P
authors.lastname: Ernsberger
mesh_terms: D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology; D015153:Blotting, Western; D004305:Dose-Response Relationship, Drug; D007093:Imidazoles / Q000494:pharmacology; D054755:Imidazoline Receptors; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D020932:Nerve Growth Factor / Q000378:metabolism*; D016716:PC12 Cells / Q000187:drug effects* / Q000201:enzymology*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D051381:Rats; D011955:Receptors, Drug / Q000819:agonists / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D000959:Antihypertensive Agents; D007093:Imidazoles; D054755:Imidazoline Receptors; D011955:Receptors, Drug; C410887:imidazoline I1 receptors; D020932:Nerve Growth Factor; C043482:moxonidine; D020928:Mitogen-Activated Protein Kinases; D010744:Phosphoric Monoester Hydrolases