title: Cardiac glycosides as novel inhibitors of human ether-a-go-go-related gene channel trafficking.
abstract: Direct block of the cardiac potassium channel human ether-a-go-go-related gene (hERG) by a large, structurally diverse group of therapeutic compounds causes drug-induced QT prolongation and torsades de pointes arrhythmias. In addition, several therapeutic compounds have been identified more recently that prolong the QT interval by inhibition of hERG trafficking to the cell surface. We used a surface expression assay to identify novel compounds that interfere with hERG trafficking and found that cardiac glycosides are potent inhibitors of hERG expression at the cell surface. Further investigation of digitoxin, ouabain, and digoxin revealed that all three cardiac glycosides reduced expression of the fully glycosylated cell surface form of hERG on Western blots, indicating that channel exit from the endoplasmic reticulum is blocked. Likewise, hERG currents were reduced with nanomolar affinity on long-term exposure. hERG trafficking inhibition was initiated by cardiac glycosides through direct block of Na(+)/K(+) pumps and not via off-target interactions with hERG or another closely associated protein in its processing or export pathway. In isolated guinea pig myocytes, long-term exposure to 30 nM of the clinically used drugs digoxin or digitoxin reduced hERG/rapidly activating delayed rectifier K(+) current (I(Kr)) currents by approximately 50%, whereas three other cardiac membrane currents--inward rectifier current, slowly activating delayed rectifier K(+) current, and calcium current--were not affected. Importantly, 100 nM digitoxin prolonged action potential duration on long-term exposure consistent with a reduction in hERG/I(Kr) channel number. Thus, cardiac glycosides are able to delay cardiac repolarization at nanomolar concentrations via hERG trafficking inhibition, and this may contribute to the complex electrocardiographic changes seen with compounds such as digitoxin.
authors.affiliation: Rammelkamp Center for Education and Research, Case Western Reserve University, Cleveland, OH 44109, USA.
authors.forename: Lu
authors.initials: L
authors.lastname: Wang
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Wible
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wan
authors.forename: Eckhard
authors.initials: E
authors.lastname: Ficker
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D015153:Blotting, Western; D002301:Cardiac Glycosides / Q000494:pharmacology*; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism; D051638:Ether-A-Go-Go Potassium Channels / Q000037:antagonists & inhibitors* / Q000378:metabolism; D006168:Guinea Pigs; D006801:Humans; D051657:KCNQ1 Potassium Channel / Q000187:drug effects / Q000378:metabolism; D051666:Kv1.5 Potassium Channel / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology; D024642:Potassium Channels, Voltage-Gated / Q000187:drug effects / Q000378:metabolism; D021381:Protein Transport / Q000187:drug effects; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
chemical_list: D002301:Cardiac Glycosides; D051638:Ether-A-Go-Go Potassium Channels; C489622:KCNE1 protein, human; D051657:KCNQ1 Potassium Channel; D051666:Kv1.5 Potassium Channel; D024642:Potassium Channels, Voltage-Gated; D000254:Sodium-Potassium-Exchanging ATPase