Proteasomal Degradation of TRAF2 Mediates Mitochondrial Dysfunction in Doxorubicin-Cardiomyopathy.

BACKGROUND: Cytokines such as tumor necrosis factor-α (TNFα) have been implicated in cardiac dysfunction and toxicity associated with doxorubicin (DOX). Although TNFα can elicit different cellular responses, including survival or death, the mechanisms underlying these divergent outcomes in the heart remain cryptic. The E3 ubiquitin ligase TRAF2 (TNF receptor associated factor 2) provides a critical signaling platform for K63-linked polyubiquitination of RIPK1 (receptor interacting protein 1), crucial for nuclear factor-κB (NF-κB) activation by TNFα and survival. Here, we investigate alterations in TNFα-TRAF2-NF-κB signaling in the pathogenesis of DOX cardiotoxicity. METHODS: Using a combination of in vivo (4 weekly injections of DOX 5 mg·kg-1·wk-1) in C57/BL6J mice and in vitro approaches (rat, mouse, and human inducible pluripotent stem cell-derived cardiac myocytes), we monitored TNFα levels, lactate dehydrogenase, cardiac ultrastructure and function, mitochondrial bioenergetics, and cardiac cell viability. RESULTS: In contrast to vehicle-treated mice, ultrastructural defects, including cytoplasmic swelling, mitochondrial perturbations, and elevated TNFα levels, were observed in the hearts of mice treated with DOX. While investigating the involvement of TNFα in DOX cardiotoxicity, we discovered that NF-κB was readily activated by TNFα. However, TNFα-mediated NF-κB activation was impaired in cardiac myocytes treated with DOX. This coincided with loss of K63- linked polyubiquitination of RIPK1 from the proteasomal degradation of TRAF2. Furthermore, TRAF2 protein abundance was markedly reduced in hearts of patients with cancer treated with DOX. We further established that the reciprocal actions of the ubiquitinating and deubiquitinating enzymes cellular inhibitors of apoptosis 1 and USP19 (ubiquitin-specific peptidase 19), respectively, regulated the proteasomal degradation of TRAF2 in DOX-treated cardiac myocytes. An E3-ligase mutant of cellular inhibitors of apoptosis 1 (H588A) or gain of function of USP19 prevented proteasomal degradation of TRAF2 and DOX-induced cell death. Furthermore, wild-type TRAF2, but not a RING finger mutant defective for K63-linked polyubiquitination of RIPK1, restored NF-κB signaling and suppressed DOX-induced cardiac cell death. Last, cardiomyocyte-restricted expression of TRAF2 (cardiac troponin T-adeno-associated virus 9-TRAF2) in vivo protected against mitochondrial defects and cardiac dysfunction induced by DOX. CONCLUSIONS: Our findings reveal a novel signaling axis that functionally connects the cardiotoxic effects of DOX to proteasomal degradation of TRAF2. Disruption of the critical TRAF2 survival pathway by DOX sensitizes cardiac myocytes to TNFα-mediated necrotic cell death and DOX cardiotoxicity.

Reductions in the Cardiac Transient Outward K+ Current Ito Caused by Chronic ß-Adrenergic Receptor Stimulation Are Partly Rescued by Inhibition of Nuclear Factor κB.

The fast transient outward potassium current (Ito,f) plays a critical role in the electrical and contractile properties of the myocardium. Ito,f channels are formed by the co-assembly of the pore-forming α-subunits, Kv4.2 and Kv4.3, together with the accessory ß-subunit KChIP2. Reductions of Ito,f are common in the diseased heart, which is also associated with enhanced stimulation of ß-adrenergic receptors (ß-ARs). We used cultured neonatal rat ventricular myocytes to examine how chronic ß-AR stimulation decreases Ito,f. To determine which downstream pathways mediate these Ito,f changes, adenoviral infections were used to inhibit CaMKIIδc, CaMKIIδb, calcineurin, or nuclear factor κB (NF-κB). We observed that chronic ß-AR stimulation with isoproterenol (ISO) for 48 h reduced Ito,f along with mRNA expression of all three of its subunits (Kv4.2, Kv4.3, and KChIP2). Inhibiting either CaMKIIδc nor CaMKIIδb did not prevent the ISO-mediated Ito,f reductions, even though CaMKIIδc and CaMKIIδb clearly regulated Ito,f and the mRNA expression of its subunits. Likewise, calcineurin inhibition did not prevent the Ito,f reductions induced by ß-AR stimulation despite strongly modulating Ito,f and subunit mRNA expression. In contrast, NF-κB inhibition partly rescued the ISO-mediated Ito,f reductions in association with restoration of KChIP2 mRNA expression. Consistent with these observations, KChIP2 promoter activity was reduced by p65 as well as ß-AR stimulation. In conclusion, NF-κB, and not CaMKIIδ or calcineurin, partly mediates the Ito,f reductions induced by chronic ß-AR stimulation. Both mRNA and KChIP2 promoter data suggest that the ISO-induced Ito,f reductions are, in part, mediated through reduced KChIP2 transcription caused by NF-κB activation.

IKKß stabilizes Mitofusin 2 and suppresses doxorubicin cardiomyopathy.

AIMS: The mitochondrial dynamics protein Mitofusin 2 (MFN2) coordinates critical cellular processes including mitochondrial bioenergetics, quality control, and cell viability. The NF-κB kinase IKKß suppresses mitochondrial injury in doxorubicin cardiomyopathy, but the underlying mechanism is undefined. METHODS AND RESULTS: Herein, we identify a novel signalling axis that functionally connects IKKß and doxorubicin cardiomyopathy to a mechanism that impinges upon the proteasomal stabilization of MFN2. In contrast to vehicle-treated cells, MFN2 was highly ubiquitinated and rapidly degraded by the proteasomal-regulated pathway in cardiac myocytes treated with doxorubicin. The loss of MFN2 activity resulted in mitochondrial perturbations, including increased reactive oxygen species (ROS) production, impaired respiration, and necrotic cell death. Interestingly, doxorubicin-induced degradation of MFN2 and mitochondrial-regulated cell death were contingent upon IKKß kinase activity. Notably, immunoprecipitation and proximity ligation assays revealed that IKKß interacted with MFN2 suggesting that MFN2 may be a phosphorylation target of IKKß. To explore this possibility, mass spectrometry analysis identified a novel MFN2 phospho-acceptor site at serine 53 that was phosphorylated by wild-type IKKß but not by a kinase-inactive mutant IKKßK-M. Based on these findings, we reasoned that IKKß-mediated phosphorylation of serine 53 may influence MFN2 protein stability. Consistent with this view, an IKKß-phosphomimetic MFN2 (MFN2S53D) was resistant to proteasomal degradation induced by doxorubicin whereas wild-type MFN2 and IKKß-phosphorylation defective MFN2 mutant (MFNS53A) were readily degraded in cardiac myocytes treated with doxorubicin. Concordantly, gain of function of IKKß or MFN2S53D suppressed doxorubicin-induced mitochondrial injury and cell death. CONCLUSIONS: The findings of this study reveal a novel survival pathway for IKKß that is mutually dependent upon and obligatory linked to the phosphorylation and stabilization of the mitochondrial dynamics protein MFN2.

Epigenetic regulation of canonical TNFα pathway by HDAC1 determines survival of cardiac myocytes.

Gene transcription is regulated by post-translation modifications. Histone deacetylases (HDACs) remove acetyl groups from histone and non-histone factors inhibiting transcription. Proinflammatory cytokines such as TNFα activate the canonical nuclear factor-κB (NF-κB) pathway. Earlier we established a cytoprotective role for NF-κB in the heart. Though a causal relationship for HDAC1 and NF-κB has been established, the impact of HDAC1 on TNFα signaling is unknown. Herein, we demonstrate that HDAC1 provides a molecular switch for determining cell survival in the TNFα pathway. In contrast to vehicle-treated control cells, TNFα-treated cells displayed a marked increase in NF-κB gene transcription. Notably, cells treated with TNFα were indistinguishable from vehicle controls cells with respect to viability. Interestingly, HDAC activity was reduced in cells treated with TNFα. Conversely, in the presence of HDAC1, NF-κB gene transcription by TNFα was repressed, resulting in mitochondrial perturbations and widespread cell death. Heterologous fusion proteins comprised of yeast Gal4 DNA binding domain fused in frame to the NF-κB p65 transactivation domain were preferentially repressed by HDAC1. Moreover, transcription mediated by Gal4VP16 protein from herpes virus was unaffected by HDAC1 in cardiac myocytes. Mutations that abrogate known catalytic activities of HDAC1, small interference RNA, or pharmacological inhibition of HDAC1 restored NF-κB signaling and suppressed cell death induced by TNFα. These data provide the first evidence for an obligate link between HDAC1 and canonical TNFα pathway for cell survival of cardiac myocytes.

A novel hypoxia-inducible spliced variant of mitochondrial death gene Bnip3 promotes survival of ventricular myocytes.

RATIONALE: Alternative splicing provides a versatile mechanism by which cells generate proteins with different or even antagonistic properties. Previously, we established hypoxia-inducible death factor Bnip3 as a critical component of the intrinsic death pathway. OBJECTIVE: To investigate alternative splicing of Bnip3 pre-mRNA in postnatal ventricular myocytes during hypoxia. METHODS AND RESULTS: We identify a novel previously unrecognized spliced variant of Bnip3 (Bnip3Δex3) generated by alternative splicing of exon3 exclusively in cardiac myocytes subjected to hypoxia. Sequencing of Bnip3Δex3 revealed a frame shift mutation that terminated transcription up-stream of exon5 and exon6 ablating translation of the BH3-like domain and critical carboxyl-terminal transmembrane domain crucial for mitochondrial localization and cell death. Notably, although the 26-kDa Bnip3 protein (Bnip3FL) encoded by full-length mRNA was localized to mitochondria and provoked cell death, the 8.2-kDa Bnip3Δex3 protein encoded by the truncated spliced mRNA was defective for mitochondrial targeting but interacted with Bnip3FL resulting in less association of Bnip3FL with mitochondria and diminished apoptotic and necrotic cell death. Forced expression of Bnip3FL in cardiac myocytes or Bnip3(-/-) mouse embryonic fibroblasts triggered widespread cell death that was inhibited by coexpression of Bnip3Δex3. Conversely, RNA interference targeted against sequences encompassing the unique exon2-exon4 junction of the Bnip3Δex3 sensitized cardiac myocytes to mitochondrial perturbations and cell death induced by Bnip3FL. CONCLUSIONS: Given the otherwise lethal consequences of deregulated Bnip3FL expression in postmitotic cells, our findings reveal a novel intrinsic defense mechanism that opposes the mitochondrial defects and cell death of ventricular myocytes that is obligatorily linked and mutually dependent on alternative splicing of Bnip3FL during hypoxia or ischemic stress.

Antagonism of E2F-1 regulated Bnip3 transcription by NF-kappaB is essential for basal cell survival.

The transcription factor E2F-1 drives proliferation and death, but the mechanisms that differentially regulate these divergent actions are poorly understood. The hypoxia-inducible death factor Bnip3 is an E2F-1 target gene and integral component of the intrinsic mitochondrial death pathway. The mechanisms that govern Bnip3 gene activity remain cryptic. Herein we show that the transcription factor NF-kappaB provides a molecular switch that determines whether E2F-1 signals proliferation or death under physiological conditions. We show under basal nonapoptotic conditions that NF-kappaB constitutively occupies and transcriptionally silences Bnip3 gene transcription by competing with E2F-1 for Bnip3 promoter binding. Conversely, in the absence of NF-kappaB, or during hypoxia when NF-kappaB abundance is reduced, basal Bnip3 gene transcription is activated by the unrestricted binding of E2F-1 to the Bnip3 promoter. Genetic knock-down of E2F-1 or retinoblastoma gene product over-expression in cardiac and human pancreatic cancer cells deficient for NF-kappaB signaling abrogated basal and hypoxia-inducible Bnip3 transcription. The survival kinase PI3K/Akt inhibited Bnip3 expression levels in cells in a manner dependent upon NF-kappaB activation. Hence, by way of example, we show that the transcriptional inhibition of E2F-1-dependent Bnip3 expression by NF-kappaB highlights a survival pathway that overrides the E2F-1 tumor suppressor program. Our data may explain more fundamentally how cells, by selectively inhibiting E2F-1-dependent death gene transcription, avert apoptosis down-stream of the retinoblastoma/E2F-1 cell cycle pathway.

Epigenetic regulation of E2F-1-dependent Bnip3 transcription and cell death by nuclear factor-κB and histone deacetylase-1.

A delicate balance exists between cell growth and cell death. In the context of the adult myocardium, inappropriate or inordinate cell loss through an apoptotic process may profoundly influence cardiac structure, function, or both given the limited and meager ability of the heart for repair after injury. Earlier work by the authors' laboratory identified a close relation between cell cycle factor E2F-1 and hypoxia-inducible factor Bnip3 as the key regulator of apoptosis and autophagy in ventricular myocytes. Epigenetic changes by histone-modifying proteins, namely, histone deacetylases (HDACs) influence cell survival by altering the activity of histone core proteins, transcription factors, or both. This report highlights the intricate nature between the cellular factors E2F-1 and nuclear factor-κB (NF-κB) and the epigenetic regulation of Bnip3 gene transcription by HDAC1 for cell survival of ventricular myocytes.

Impaired NF-κB signalling underlies cyclophilin D-mediated mitochondrial permeability transition pore opening in doxorubicin cardiomyopathy.

AIMS: The chemotherapy drug doxorubicin (Dox) is commonly used for treating a variety of human cancers; however, it is highly cardiotoxic and induces heart failure. We previously reported that the Bcl-2 mitochondrial death protein Bcl-2/19kDa interaction protein 3 (Bnip3), is critical for provoking mitochondrial perturbations and necrotic cell death in response to Dox; however, the underlying mechanisms had not been elucidated. Herein, we investigated mechanism that drives Bnip3 gene activation and downstream effectors of Bnip3-mediated mitochondrial perturbations and cell death in cardiac myocytes treated with Dox. METHODS AND RESULTS: Nuclear factor-κB (NF-κB) signalling, which transcriptionally silences Bnip3 activation under basal states in cardiac myocytes was dramatically reduced following Dox treatment. This was accompanied by Bnip3 gene activation, mitochondrial injury including calcium influx, permeability transition pore (mPTP) opening, loss of nuclear high mobility group protein 1, reactive oxygen species production, and cell death. Interestingly, impaired NF-κB signalling in cells treated with Dox was accompanied by protein complexes between Bnip3 and cyclophilin D (CypD). Notably, Bnip3-mediated mPTP opening was suppressed by inhibition of CypD-demonstrating that CypD functionally operates downstream of Bnip3. Moreover, restoring IKKß-NF-κB activity in cardiac myocytes treated with Dox suppressed Bnip3 expression, mitochondrial perturbations, and necrotic cell death. CONCLUSIONS: The findings of the present study reveal a novel signalling pathway that functionally couples NF-κB and Dox cardiomyopathy to a mechanism that is mutually dependent upon and obligatorily linked to the transcriptional control of Bnip3. Our findings further demonstrate that mitochondrial injury and necrotic cell death induced by Bnip3 is contingent upon CypD. Hence, maintaining NF-κB signalling may prove beneficial in reducing mitochondrial dysfunction and heart failure in cancer patients undergoing Dox chemotherapy.

Na+ permeation and block of hERG potassium channels.

The inactivation gating of hERG channels is important for the channel function and drug-channel interaction. Whereas hERG channels are highly selective for K+, we have found that inactivated hERG channels allow Na+ to permeate in the absence of K+. This provides a new way to directly monitor and investigate hERG inactivation. By using whole cell patch clamp method with an internal solution containing 135 mM Na+ and an external solution containing 135 mM NMG+, we recorded a robust Na+ current through hERG channels expressed in HEK 293 cells. Kinetic analyses of the hERG Na+ and K+ currents indicate that the channel experiences at least two states during the inactivation process, an initial fast, less stable state followed by a slow, more stable state. The Na+ current reflects Na+ ions permeating through the fast inactivated state but not through the slow inactivated state or open state. Thus the hERG Na+ current displayed a slow inactivation as the channels travel from the less stable, fast inactivated state into the more stable, slow inactivated state. Removal of fast inactivation by the S631A mutation abolished the Na+ current. Moreover, acceleration of fast inactivation by mutations T623A, F627Y, and S641A did not affect the hERG Na+ current, but greatly diminished the hERG K+ current. We also found that external Na+ potently blocked the hERG outward Na+ current with an IC50 of 3.5 mM. Mutations in the channel pore and S6 regions, such as S624A, F627Y, and S641A, abolished the inhibitory effects of external Na+ on the hERG Na+ current. Na+ permeation and blockade of hERG channels provide novel ways to extend our understanding of the hERG gating mechanisms.

PDK2-mediated alternative splicing switches Bnip3 from cell death to cell survival.

Herein we describe a novel survival pathway that operationally links alternative pre-mRNA splicing of the hypoxia-inducible death protein Bcl-2 19-kD interacting protein 3 (Bnip3) to the unique glycolytic phenotype in cancer cells. While a full-length Bnip3 protein (Bnip3FL) encoded by exons 1-6 was expressed as an isoform in normal cells and promoted cell death, a truncated spliced variant of Bnip3 mRNA deleted for exon 3 (Bnip3Δex3) was preferentially expressed in several human adenocarcinomas and promoted survival. Reciprocal inhibition of the Bnip3Δex3/Bnip3FL isoform ratio by inhibiting pyruvate dehydrogenase kinase isoform 2 (PDK2) in Panc-1 cells rapidly induced mitochondrial perturbations and cell death. The findings of the present study reveal a novel survival pathway that functionally couples the unique glycolytic phenotype in cancer cells to hypoxia resistance via a PDK2-dependent mechanism that switches Bnip3 from cell death to survival. Discovery of the survival Bnip3Δex3 isoform may fundamentally explain how certain cells resist Bnip3 and avert death during hypoxia.

Production in Pichia pastoris and characterization of genetic engineered chimeric HBV/HEV virus-like particles.

OBJECTIVE: To investigate the presentation of a neutralization epitope-containing peptide antigen of hepatitis E virus (HEV) on chimeric virus-like particles (VLPs) of hepatitis B surface antigen (HBsAg). METHODS: The gene fragment corresponding to amino acids (aa) 551-607 (HEnAg) of HEV capsid protein, which contains the only neutralization epitope identified to date, was fused via a synthetic glycine linker in frame with the gene of HBsAg. The resulted fusion gene was then integrated through transformation into the genome of Pichia pastoris under the control of a methanol-induced alcohol oxidase 1 (AOX1) promoter and expressed intracellularly. The expression products in the soluble cell extracts were characterized by Western blot, ELISA, CsCl density gradient analysis, and electron microscopic visualization. RESULTS: The novel fusion protein incorporating HBsAg and the neutralization epitope-containing HEnAg was expressed successfully in Pichia pastoris with an expected molecular weight of approximately 32 kD. It was found to possess the ability to assemble into chimeric HBV/HEV VLPs with immunological physical and morphological characteristics akin to HBsAg particles. Not only did the chimeric VLPs show high activity levels in a HBsAg particle-specific ELISA but they were also strongly immunoreactive with hepatitis E (HE) positive human serum in a HEV specific ELISA, indicating that HEnAg peptide fragments were exposed on VLP surfaces and would be expected to be readily accessible by cells and molecules of the immune system. Similarity between chimeric VLPs to highly immunogenic HBsAg particles may confer good immunogenicity on surface-displayed HEnAg. CONCLUSION: The chimeric HBV/HEV VLPs produced in this study may have potential to be a recombinant HBV/HEV bivalent vaccine candidate.

p53 mediates autophagy and cell death by a mechanism contingent on Bnip3.

Myocardial ischemia and angiotensin II activate the tumor suppressor p53 protein, which promotes cell death. Previously, we showed that the Bcl-2 death gene Bnip3 is highly induced during ischemia, where it triggers mitochondrial perturbations resulting in autophagy and cell death. However, whether p53 regulates Bnip3 and autophagy is unknown. Herein, we provide new compelling evidence for a novel signaling axis that commonly links p53 and Bnip3 for autophagy and cell death. p53 overexpression increased endogenous Bnip3 mRNA and protein levels resulting in mitochondrial defects leading to loss of mitochondrial ΔΨ(m). This was accompanied by an increase in autophagic flux and cell death. Notably, genetic loss of function studies, such as Atg7 knock-down or pharmacological inhibition of autophagy with 3-methyl adenine, suppressed cell death induced by p53--indicating that p53 induces maladaptive autophagy. Our previous work demonstrated that Bnip3 induces mitochondrial defects and autophagic cell death. Conversely, loss of function of Bnip3 in cardiac myocytes or Bnip3(-/-) mouse embryonic fibroblasts prevented mitochondrial targeting of p53, autophagy, and cell death. To our knowledge, these data provide the first evidence for the dual regulation of autophagy and cell death of cardiac myocytes by p53 that is mutually dependent on and obligatorily linked to Bnip3 gene activation. Hence, our findings may explain more fundamentally, how, autophagy and cell death are dually regulated during cardiac stress conditions where p53 is activated.

Bidirectional regulation of nuclear factor-κB and mammalian target of rapamycin signaling functionally links Bnip3 gene repression and cell survival of ventricular myocytes.

BACKGROUND: Tumor necrosis factor-α and other proinflammatory cytokines activate the canonical nuclear factor (NF)-κB pathway through the kinase IKKß. Previously, we established that IKKß is also critical for Akt-mediated NF-κB activation in ventricular myocytes. Akt activates the kinase mammalian target of rapamycin (mTOR), which mediates important processes such as cardiac hypertrophy. However, whether mTOR regulates cardiac myocyte cell survival is unknown. METHODS AND RESULTS: Herein, we demonstrate bidirectional regulation between NF-κB signaling and mTOR, the balance which determines ventricular myocyte survival. Overexpression of IKKß resulted in mTOR activation and conversely overexpression of mTOR lead to NF-κB activation. Loss of function approaches demonstrated that endogenous levels of IKKß and mTOR also signal through this pathway. NF-κB activation by mTOR was mediated by phosphorylation of the NF-κB p65 subunit increasing p65 nuclear translocation and activation of gene transcription. This circuit was also important for NF-κB activation by the canonical tumor necrosis factor-α pathway. Our previous work has shown that NF-κB signaling suppresses transcription of the death gene Bnip3 resulting in ventricular myocyte survival. Inhibition of mTOR with rapamycin decreased NF-κB activation resulting in increased Bnip3 expression and cell death. Conversely, mTOR overexpression suppressed Bnip3 levels and cell death of ventricular myocytes in response to hypoxia. CONCLUSIONS: To our knowledge, these data provide the first evidence for a bidirectional link between NF-κB signaling and mTOR that is critical in the regulation of Bnip3 expression and cardiac myocyte death. Hence, modulation of this axis may be cardioprotective during ischemia.

Regulation of autophagy in the heart: "you only live twice".

Autophagy is a highly orchestrated cellular process by which proteins and organelles are degraded via an elaborate lysosomal pathway to generate free amino acids and sugars for ATP during metabolic stress. At present, the exact role of autophagy in the heart is highly debated but suggested to play a key role in regulating cell turnover in cardiomyopathies and heart failure. The signaling pathways and molecular effectors that govern autophagy are incomplete, as are the mechanisms that determine whether autophagy promotes or prevents cell death. The mitochondrion has been identified as a key organelle centrally involved in regulating autophagy. Certain members of the Bcl-2 gene family, including Beclin-1, Bcl-2 nineteen kilodaltons interacting protein (Bnip3), and Nix/Bnip3L, provoke mitochondrial perturbations leading to permeability transition pore opening, resulting in apoptosis, autophagy, or both. These and other aspects of autophagy processes have been discussed.

Molecular determinants of cocaine block of human ether-á-go-go-related gene potassium channels.

The use of cocaine causes cardiac arrhythmias and sudden death. Blockade of the cardiac potassium channel human ether-á-go-go-related gene (hERG) has been implicated as a mechanism for the proarrhythmic action of cocaine. hERG encodes the pore-forming subunits of the rapidly activating delayed rectifier K(+) channel (I(Kr)), which is important for cardiac repolarization. Blockade of I(Kr)/hERG represents a common mechanism for drug-induced long QT syndrome. The mechanisms for many common drugs to block the hERG channel are not well understood. We investigated the molecular determinants of hERG channels in cocaine-hERG interactions using site-targeted mutations and patch-clamp method. Wild-type and mutant hERG channels were heterologously expressed in human embryonic kidney 293 cells. We found that there was no correlation between inactivation gating and cocaine block of hERG channels. We also found that consistent with Thr-623, Tyr-652, and Phe-656 being critical for drug binding to hERG channels, mutations in these residues significantly reduced cocaine-induced block, and the hydrophobicity of the residues at position 656 dictated the cocaine sensitivity of the channel. Although the S620T mutation, which removed hERG inactivation, reduced cocaine block by 21-fold, the S620C mutation, which also completely removed hERG inactivation, did not affect the blocking potency of cocaine. Thus, Ser-620 is another pore helix residue whose mutation can interfere with cocaine binding independently of its effect on inactivation.

Intracellular K+ is required for the inactivation-induced high-affinity binding of cisapride to HERG channels.

Many commonly used medications can cause long QT syndrome and thus increase the risk of life-threatening arrhythmias. High-affinity human Ether-à-go-go-related gene (HERG) potassium channel blockade by structurally diverse compounds is almost exclusively responsible for this side effect. Understanding drug-HERG channel interactions is an important step in avoiding drug-induced long QT syndromes. Previous studies have found that disrupting HERG inactivation reduces the degree of drug block and have suggested that the inactivated state is the preferential state for drug binding to HERG channels. However, recent studies have also shown that inactivation does not dictate drug sensitivity of HERG channels. In the present study, we examined the effect of inactivation gating on cisapride block of HERG. Modulation of HERG inactivation was achieved by either changing extracellular K+ or Cs+ concentrations or by mutations of the channel. We found that although inactivation facilitated cisapride block of the HERG K+ current, it was not coupled with cisapride block of HERG when the Cs+ current was recorded. Furthermore, cisapride block of the HERG K+ current was not linked with inactivation in the mutant HERG channels F656V and F656M. Our results suggest that inactivation facilitates cisapride block of HERG channels through affecting the positioning of Phe-656.