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 (IKr) 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/IKr 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.
Footnotes
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This work was supported by National Institute of Health Grant HL71789 (to E.F.) and CA106028 (to B.A.W.). B.A.W. is an employee of ChanTest Inc., a company that provides hERG trafficking assays as part of their services.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.106.113043.
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ABBREVIATIONS: hERG, human ether a-go-go-related gene, IKr, rapidly activating delayed rectifier K+ current, IKs, slowly activating delayed rectifier K+ current; LQTS, long QT syndrome; acLQTS, acquired long QT syndrome; HEK, human embryonic kidney; WT, wild-type; HA, hemagglutinin; DMEM, Dulbecco's modified Eagle's medium; [K+]ex, extracellular K+ concentration; MEM, minimum Eagle's medium; K-sf, serum-free potassium; CHO, Chinese hamster ovary; IK1, inward rectifier current; ER, endoplasmic reticulum; fg, fully glycosylated; cg, core glycosylated; APD, action potential duration; E4031, 1-[2-(6-methyl-2-pyridyl)ethyl]-4-(4-methylsulfonyl aminobenzoyl)piperidine.
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↵ The online version of this article (available at http://jpet.aspetjournals.org) contains supplemental material.
- Received August 26, 2006.
- Accepted November 8, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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