Direct block of hERG potassium channels by the protein kinase C inhibitor bisindolylmaleimide I (GF109203X).

OBJECTIVE: The human ether-a-go-go-related gene (hERG) encodes the rapid component of the cardiac repolarizing delayed rectifier potassium current, I(Kr). The direct interaction of the commonly used protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM I) with hERG, KvLQT1/minK, and I(Kr) currents was investigated in this study. METHODS: hERG and KvLQT1/minK channels were heterologously expressed in Xenopus laevis oocytes, and currents were measured using the two-microelectrode voltage clamp technique. In addition, hERG currents in stably transfected human embryonic kidney (HEK 293) cells, native I(Kr) currents and action potentials in isolated guinea pig ventricular cardiomyocytes were recorded using whole-cell patch clamp electrophysiology. RESULTS: Bisindolylmaleimide I blocked hERG currents in HEK 293 cells and Xenopus oocytes in a concentration-dependent manner with IC(50) values of 1.0 and 13.2 muM, respectively. hERG channels were primarily blocked in the open state in a frequency-independent manner. Analysis of the voltage-dependence of block revealed a reduction of inhibition at positive membrane potentials. BIM I caused a shift of -20.3 mV in the voltage-dependence of inactivation. The point mutations tyrosine 652 alanine (Y652A) and phenylalanine 656 alanine (F656A) attenuated hERG current blockade, indicating that BIM I binds to a common drug receptor within the pore region. KvLQT1/minK currents were not significantly altered by BIM I. Finally, 1 muM BIM I reduced native I(Kr) currents by 69.2% and lead to action potential prolongation. CONCLUSION: In summary, PKC-independent effects have to be carefully considered when using BIM I as PKC inhibitor in experimental models involving hERG channels and I(Kr) currents.

Activation of cardiac human ether-a-go-go related gene potassium currents is regulated by alpha(1A)-adrenoceptors.

Patients with cardiac disease typically develop life-threatening ventricular arrhythmias during physical or emotional stress, suggesting a link between adrenergic stimulation and regulation of the cardiac action potential. Human ether-a-go-go related gene (hERG) potassium channels conduct the rapid component of the repolarizing delayed rectifier potassium current, I(Kr). Previous studies have revealed that hERG channel activation is modulated by activation of the beta-adrenergic system. In contrast, the influence of the alpha-adrenergic signal transduction cascade on hERG currents is less well understood. The present study examined the regulation of hERG currents by alpha(1A)-adrenoceptors. hERG channels and human alpha(1A)-adrenoceptors were heterologously coexpressed in Xenopus laevis oocytes, and currents were measured using the two-microelectrode voltage clamp technique. Stimulation of alpha(1A)-receptors by applying 20 microM phenylephrine caused hERG current reduction due to a 9.6-mV shift of the activation curve towards more positive potentials. Simultaneous application of the alpha(1)-adrenoceptor antagonist prazosin (20 microM) prevented the activation shift. Inhibition of PKC (3 microM Ro-32-0432) or PKA (2.5 microM KT 5720) abolished the alpha-adrenergic activation shift, suggesting that PKC and PKA are required within the regulatory mechanism. The effect was still present when the PKA- and PKC-dependent phosphorylation sites in hERG were deleted by mutagenesis. In summary, cardiac repolarizing hERG/I(Kr) potassium currents are modulated by alpha(1A)-adrenoceptors via PKC and PKA independently of direct channel phosphorylation. This novel regulatory pathway of alpha1-adrenergic hERG current regulation provides a link between stress and ventricular arrhythmias, in particular in patients with heart disease.

Inhibition of cardiac HERG currents by the DNA topoisomerase II inhibitor amsacrine: mode of action.

1 The topoisomerase II inhibitor amsacrine is used in the treatment of acute myelogenous leukemia. Although most anticancer drugs are believed not to cause acquired long QT syndrome (LQTS), concerns have been raised by reports of QT interval prolongation, ventricular fibrillation and death associated with amsacrine treatment. Since blockade of cardiac human ether-a-go-go-related gene (HERG) potassium currents is an important cause of acquired LQTS, we investigated the acute effects of amsacrine on cloned HERG channels to determine the electrophysiological basis for its proarrhythmic potential. 2 HERG channels were heterologously expressed in human HEK 293 cells and Xenopus laevis oocytes, and the respective potassium currents were recorded using patch-clamp and two-microelectrode voltage-clamp electrophysiology. 3 Amsacrine blocked HERG currents in HEK 293 cells and Xenopus oocytes in a concentration-dependent manner, with IC50 values of 209.4 nm and 2.0 microm, respectively. 4 HERG channels were primarily blocked in the open and inactivated states, and no additional voltage dependence was observed. Amsacrine caused a negative shift in the voltage dependence of both activation (-7.6 mV) and inactivation (-7.6 mV). HERG current block by amsacrine was not frequency dependent. 5 The S6 domain mutations Y652A and F656A attenuated (Y652A) or abolished (F656A, Y652A/F656A) HERG current blockade, indicating that amsacrine binding requires a common drug receptor within the pore-S6 region. 6 In conclusion, these data demonstrate that the anticancer drug amsacrine is an antagonist of cloned HERG potassium channels, providing a molecular mechanism for the previously reported QTc interval prolongation during clinical administration of amsacrine.

Inhibition of human ether-a-go-go-related gene potassium channels by alpha 1-adrenoceptor antagonists prazosin, doxazosin, and terazosin.

Human ether-a-go-go-related gene (HERG) potassium channels are expressed in multiple tissues including the heart and adenocarcinomas. In cardiomyocytes, HERG encodes the alpha-subunit underlying the rapid component of the delayed rectifier potassium current, I(Kr), and pharmacological reduction of HERG currents may cause acquired long QT syndrome. In addition, HERG currents have been shown to be involved in the regulation of cell proliferation and apoptosis. Selective alpha 1-adrenoceptor antagonists are commonly used in the treatment of hypertension and benign prostatic hyperplasia. Recently, doxazosin has been associated with an increased risk of heart failure. Moreover, quinazoline-derived alpha 1-inhibitors induce apoptosis in cardiomyocytes and prostate tumor cells independently of alpha1-adrenoceptor blockade. To assess the action of the effects of prazosin, doxazosin, and terazosin on HERG currents, we investigated their acute electrophysiological effects on cloned HERG potassium channels heterologously expressed in Xenopus oocytes and HEK 293 cells.Prazosin, doxazosin, and terazosin blocked HERG currents in Xenopus oocytes with IC(50) values of 10.1, 18.2, and 113.2 microM respectively, whereas the IC(50) values for HERG channel inhibition in human HEK 293 cells were 1.57 microM, 585.1 nM, and 17.7 microM. Detailed biophysical studies revealed that inhibition by the prototype alpha 1-blocker prazosin occurred in closed, open, and inactivated channels. Analysis of the voltage-dependence of block displayed a reduction of inhibition at positive membrane potentials. Frequency-dependence was not observed. Prazosin caused a negative shift in the voltage-dependence of both activation (-3.8 mV) and inactivation (-9.4 mV). The S6 mutations Y652A and F656A partially attenuated (Y652A) or abolished (F656A) HERG current blockade, indicating that prazosin binds to a common drug receptor within the pore-S6 region. In conclusion, this study demonstrates that HERG potassium channels are blocked by prazosin, doxazosin, and terazosin. These data may provide a hypothetical molecular explanation for the apoptotic effect of quinazoline-derived alpha1-adrenoceptor antagonists.