Specificity of TRH receptor coupling to G-proteins for regulation of ERG K+ channels in GH3 rat anterior pituitary cells

Role of Thyrotropin-Releasing Hormone in Regulating Fibroblast Growth Factor 21 in Mouse Pancreatic β Cells

Background: Thyrotropin-releasing hormone (TRH) is primarily produced in the hypothalamus and regulates the thyrotropin secretion from the pituitary. TRH is distributed ubiquitously in the extrahypothalamic region, especially in pancreatic islets, while its physiological role remains nebulous. We have previously established a TRH-deficient mouse model, and showed impaired glucose tolerance and downregulated expression of fibroblast growth factor 21 (FGF21) in islets. Recent studies have demonstrated the physiological roles of pancreatic FGF21. Therefore, in this study, we elucidate the direct functions of TRH in pancreatic islets via the regulation of FGF21. Methods: To explore the functions of TRH in pancreatic islets, a microarray analysis using isolated islets from TRH-knockout mice was conducted. The regulatory mechanism of TRH in pancreatic FGF21 was investigated using islet cell lines; reverse transcription-quantitative polymerase chain reaction and Western blotting were used to determine the mRNA and protein expression levels of FGF21 in pancreatic islets and islet cell lines. Induction of FGF21 expression by TRH treatment was examined in vitro. To identify the transcription factors binding to the region responsible for TRH-induced stimulation of the FGF21 promoter, electromobility shift assays were conducted. Results: Among the detected and considerably changed genes in microarray, FGF21 was the most consistently downregulated in TRH-deficient mice islets. FGF21 was strongly co-expressed with insulin in mouse islets, and TRH stimulated endogenous Fgf21 mRNA expression in the islet cell line βHC9. The E-box site in the FGF21 promoter was responsible for TRH-induced stimulation via the extracellular signal-regulated kinase (ERK)1/2 signaling pathway. The transcription factor upstream stimulatory factor 1 (USF1) could specifically bind to the E-box site. Overexpression of USF1 increased FGF21 promoter activity. Conclusion: FGF21 was transcriptionally upregulated by TRH through the ERK1/2 and USF1 pathways in pancreatic β cells.

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

The Effectiveness of Isoplumbagin and Plumbagin in Regulating Amplitude, Gating Kinetics, and Voltage-Dependent Hysteresis of erg-mediated K+ Currents

Isoplumbagin (isoPLB, 5-hydroxy-3-methyl-1,4-naphthoquinone), a naturally occurring quinone, has been observed to exercise anti-inflammatory, antimicrobial, and antineoplastic activities. Notably, whether and how isoPLB, plumbagin (PLB), or other related compounds impact transmembrane ionic currents is not entirely clear. In this study, during GH3-cell exposure to isoPLB, the peak and sustained components of an erg (ether-à-go-go related gene)-mediated K+ current (IK(erg)) evoked with long-lasting-step hyperpolarization were concentration-dependently decreased, with a concomitant increase in the decaying time constant of the deactivating current. The presence of isoPLB led to a differential reduction in the peak and sustained components of deactivating IK(erg) with effective IC50 values of 18.3 and 2.4 μM, respectively, while the KD value according to the minimum binding scheme was estimated to be 2.58 μM. Inhibition by isoPLB of IK(erg) was not reversed by diazoxide; however, further addition of isoPLB, during the continued exposure to 4,4′-dithiopyridine, did not suppress IK(erg) further. The recovery of IK(erg) by a two-step voltage pulse with a geometric progression was slowed in the presence of isoPLB, and the decaying rate of IK(erg) activated by the envelope-of-tail method was increased in its presence. The strength of the IK(erg) hysteresis in response to an inverted isosceles-triangular ramp pulse was diminished by adding isoPLB. A mild inhibition of the delayed-rectifier K+ current (IK(DR)) produced by the presence of isoPLB was seen in GH3 cells, while minimal changes in the magnitude of the voltage-gated Na+ current were demonstrated in its presence. Moreover, the IK(erg) identified in MA-10 Leydig tumor cells was blocked by adding isoPLB. Therefore, the effects of isoPLB or PLB on ionic currents (e.g., IK(erg) and IK(DR)) demonstrated herein would be upstream of our previously reported perturbations on mitochondrial morphogenesis or respiration. Taken together, the perturbations of ionic currents by isoPLB or PLB demonstrated herein are likely to contribute to the underlying mechanism through which they, or other structurally similar compounds, result in adjustments in the functional activities of different neoplastic cells (e.g., GH3 and MA-10 cells), presuming that similar in vivo observations occur.

Conformation-sensitive antibody reveals an altered cytosolic PAS/CNBh assembly during hERG channel gating

The human ERG (hERG) K⁺ channel has a crucial function in cardiac repolarization, and mutations or channel block can give rise to long QT syndrome and catastrophic ventricular arrhythmias. The cytosolic assembly formed by the Per-Arnt-Sim (PAS) and cyclic nucleotide binding homology (CNBh) domains is the defining structural feature of hERG and related KCNH channels. However, the molecular role of these two domains in channel gating remains unclear. We have previously shown that single-chain variable fragment (scFv) antibodies can modulate hERG function by binding to the PAS domain. Here, we mapped the scFv2.12 epitope to a site overlapping with the PAS/CNBh domain interface using NMR spectroscopy and mutagenesis and show that scFv binding in vitro and in the cell is incompatible with the PAS interaction with CNBh. By generating a fluorescently labeled scFv2.12, we demonstrate that association with the full-length hERG channel is state dependent. We detect Förster resonance energy transfer (FRET) with scFv2.12 when the channel gate is open but not when it is closed. In addition, state dependence of scFv2.12 FRET signal disappears when the R56Q mutation, known to destabilize the PAS-CNBh interaction, is introduced in the channel. Altogether, these data are consistent with an extensive structural alteration of the PAS/CNBh assembly when the cytosolic gate opens, likely favoring PAS domain dissociation from the CNBh domain.

Influence of Kv11.1 (hERG1) K+ channel expression on DNA damage induced by the genotoxic agent methyl methanesulfonate

Besides their crucial role in cell electrogenesis and maintenance of basal membrane potential, the voltage-dependent K⁺ channel Kv11.1/hERG1 shows an essential impact in cell proliferation and other processes linked to the maintenance of tumour phenotype. To check the possible influence of channel expression on DNA damage responses, HEK293 cells, treated with the genotoxic agent methyl methanesulfonate (MMS), were compared with those of a HEK-derived cell line (H36), permanently transfected with the Kv11.1-encoding gene, and with a third cell line (T2) obtained under identical conditions as H36, by permanent transfection of another unrelated plasma membrane protein encoding gene. In addition, to gain some insights about the canonical/conduction-dependent channel mechanisms that might be involved, the specific erg channel inhibitor E4031 was used as a tool. Our results indicate that the expression of Kv11.1 does not influence MMS-induced changes in cell cycle progression, because no differences were found between H36 and T2 cells. However, the canonical ion conduction function of the channel appeared to be associated with decreased cell viability at low/medium MMS concentrations. Moreover, direct DNA damage measurements, using the comet assay, demonstrated for the first time that Kv11.1 conduction activity was able to modify MMS-induced DNA damage, decreasing it particularly at high MMS concentration, in a way related to PARP1 gene expression. Finally, our data suggest that the canonical Kv11.1 effects may be relevant for tumour cell responses to anti-tumour therapies.

Ether-à-go-go K+ channels: effective modulators of neuronal excitability

Mammalian ether-à-go-go (EAG) channels are voltage-gated K⁺ channels. They are encoded by the KCNH gene family and divided into three subfamilies, eag (Kv10), erg (eag-related gene; Kv11) and elk (eag-like; Kv12). All EAG channel subtypes are expressed in the brain where they effectively modulate neuronal excitability. This Topical Review describes the biophysical properties of each of the EAG channel subtypes, their function in neurons and the neurological diseases induced by EAG channel mutations. In contrast to the function of erg currents in the heart, where they contribute to repolarization of the cardiac action potential, erg currents in neurons are involved in the maintenance of the resting potential, setting of action potential threshold and frequency accommodation. They can even support high frequency firing by preventing a depolarization-induced Na⁺ channel block. EAG channels are modulated differentially, e.g. eag channels by intracellular Ca²⁺ , erg channels by extracellular K⁺ and GPCRs, and elk channels by changes in pH. So far, only currents mediated by erg channels have been recorded in neurons with the help of selective blockers. Neuronal eag and elk currents have not been isolated due to the lack of suitable channel blockers. However, findings in KO mice indicate a physiological role of eag1 currents in synaptic transmission and an involvement of elk2 currents in cognitive performance. Human eag1 and eag2 gain-of-function mutations underlie syndromes associated with epileptic seizures.

ERK and RSK are necessary for TRH-induced inhibition of r-ERG potassium currents in rat pituitary GH3 cells

The transduction pathway mediating the inhibitory effect that TRH exerts on r-ERG channels has been thoroughly studied in GH3 rat pituitary cells but some elements have yet to be discovered, including those involved in a phosphorylation event(s). Using a quantitative phosphoproteomic approach we studied the changes in phosphorylation caused by treatment with 1μM TRH for 5min in GH3 cells. The activating residues of Erk2 and Erk1 undergo phosphorylation increases of 5.26 and 4.87 fold, respectively, in agreement with previous reports of ERK activation by TRH in GH3 cells. Thus, we studied the possible involvement of ERK pathway in the signal transduction from TRH receptor to r-ERG channels. The MEK inhibitor U0126 at 0.5μM caused no major blockade of the basal r-ERG current, but impaired the TRH inhibitory effect on r-ERG. Indeed, the TRH effect on r-ERG was also reduced when GH3 cells were transfected with siRNAs against either Erk1 or Erk2. Using antibodies, we found that TRH treatment also causes activating phosphorylation of Rsk. The TRH effect on r-ERG current was also impaired when cells were transfected with any of two different siRNAs mixtures against Rsk1. However, treatment of GH3 cells with 20nM EGF for 5min, which causes ERK and RSK activation, had no effect on the r-ERG currents. Therefore, we conclude that in the native GH3 cell system, ERK and RSK are involved in the pathway linking TRH receptor to r-ERG channel inhibition, but additional components must participate to cause such inhibition.

A comprehensive metabolic modeling of thyroid pathway in relation to thyroid pathophysiology and therapeutics

The thyroid pathway represents a complex interaction of different glands for thyroid hormone synthesis. Thyrotropin releasing hormone is synthesized in the hypothalamus and regulates thyrotropin stimulating hormone gene expression in the pituitary gland. In order to understand the complexity of the thyroid pathways, and using experimental data retrieved from the biomedical literature (e.g., NCBI, HuGE Navigator, Protein Data Bank, and KEGG), we constructed a metabolic map of the thyroid hormone pathway at a molecular level and analyzed it topologically. A total of five hub nodes were predicted in regards to the transcription thyroid receptor (TR), cAMP response element-binding protein (CREB), signal transducer and activator of transcription 3 (STAT 3), nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB), and activator protein 1 (AP-1) as being potentially important in study of thyroid disorders and as novel putative therapeutic drug targets. Notably, the thyroid receptor is a highly connected hub node and currently used as a therapeutic target in hypothyroidism. Our analysis represents the first comprehensive description of the thyroid pathway, which pertains to understanding the function of the protein and gene interaction networks. The findings from this study are therefore informative for pathophysiology and rational therapeutics of thyroid disorders.

Expression of Eag1 K+ channel and ErbBs in human pituitary adenomas: cytoskeleton arrangement patterns in cultured cells

Pituitary adenomas can invade surrounded tissue, but the mechanism remains elusive. Ether à go-go-1 (Eag1) potassium channel and epidermal growth factor receptors (ErbB1 and ErbB2) have been associated to invasive phenotypes or poor prognosis in cancer patients. However, cells arrange their cytoskeleton in order to acquire a successful migration pattern. We have studied ErbBs and Eag1 expression, and cytoskeleton arrangements in 11 human pituitary adenomas. Eag1, ErbB1 and ErbB2 expression were studied by immunochemistry in tissue and cultured cells. The cytoskeleton arrangement was analyzed in cultured cells by immunofluorescence. Normal pituitary tissue showed ErbB2 expression and Eag1 only in few cells. However, Eag1 and ErbB2 were expressed in all the tumors analyzed. ErbB1 expression was observed variable and did not show specificity for a tumor characteristic. Cultured cells from micro- and macro-adenomas clinically functional organize their cytoskeleton suggesting a mesenchymal pattern, and a round leucocyte/amoeboid pattern from invasive clinically silent adenoma. Pituitary tumors over-express EGF receptors and the ErbB2 repeated expression suggests is a characteristic of adenomas. Eag 1 was express, in different extent, and could be a therapeutic target. The cytoskeleton arrangements observed suggest that pituitary tumor cells acquire different patterns: mesenchymal, and leucocyte/amoeboid, the last observed in the invasive adenomas. Amoeboid migration pattern has been associated with high invasion capacity.

Cytoplasmic domains and voltage-dependent potassium channel gating

The basic architecture of the voltage-dependent K⁺ channels (Kv channels) corresponds to a transmembrane protein core in which the permeation pore, the voltage-sensing components and the gating machinery (cytoplasmic facing gate and sensor–gate coupler) reside. Usually, large protein tails are attached to this core, hanging toward the inside of the cell. These cytoplasmic regions are essential for normal channel function and, due to their accessibility to the cytoplasmic environment, constitute obvious targets for cell-physiological control of channel behavior. Here we review the present knowledge about the molecular organization of these intracellular channel regions and their role in both setting and controlling Kv voltage-dependent gating properties. This includes the influence that they exert on Kv rapid/N-type inactivation and on activation/deactivation gating of Shaker-like and eag-type Kv channels. Some illustrative examples about the relevance of these cytoplasmic domains determining the possibilities for modulation of Kv channel gating by cellular components are also considered.

Cell type influences the molecular mechanisms involved in hormonal regulation of ERG K+ channels

While the thyrotropin-releasing hormone (TRH) effect of raising intracellular Ca(2+) levels has been shown to rely on G(q/11) and PLC activation, the molecular mechanisms involved in the regulation of ERG K(+) channels by TRH are still partially unknown. We have analysed the effects of βγ scavengers, Akt/PKB inactivation, and TRH receptor (TRH-R) overexpression on such regulation in native and heterologous expression cell systems. In native rat pituitary GH(3) cells β-ARK/CT, Gα(t), and phosducin significantly reduced TRH inhibition of rERG currents, whereas in HEK-H36/T1 cells permanently expressing TRH-R and hERG, neither of the βγ scavengers affected the TRH-induced shift in V (1/2). Use of specific siRNAs to knock Akt/PKB expression down abolished the TRH effect on HEK-H36/T1 cell hERG, but not on rERG from GH(3) cells. Indeed, wortmannin or long insulin pretreatment also blocked TRH regulation of ERG currents in HEK-H36/T1 but not in GH(3) cells. To determine whether these differences could be related to the amount of TRH-Rs in the cell, we studied the TRH concentration dependence of the Ca(2+) and ERG responses in GH(3) cells overexpressing the receptors. The data indicated that independent of the receptor number additional cellular factor(s) contribute differently to couple the TRH-R to hERG channel modulation in HEK-H36/T1 cells. We conclude that regulation of ERG currents by TRH and its receptor is transduced in GH(3) and HEK-H36/T1 cell systems through common and different elements, and hence that the cell type influences the signalling pathways involved in the TRH-evoked responses.

Differential N termini in epithelial Na+ channel δ-subunit isoforms modulate channel trafficking to the membrane

The epithelial Na(+) channel (ENaC) is a heteromultimeric ion channel that plays a key role in Na(+) reabsorption across tight epithelia. The canonical ENaC is formed by three analogous subunits, α, β, and γ. A fourth ENaC subunit, named δ, is expressed in the nervous system of primates, where its role is unknown. The human δ-ENaC gene generates at least two splice isoforms, δ(1) and δ(2) , differing in the N-terminal sequence. Neurons in diverse areas of the human and monkey brain differentially express either δ(1) or δ(2) , with few cells coexpressing both isoforms, which suggests that they may play specific physiological roles. Here we show that heterologous expression of δ(1) in Xenopus oocytes and HEK293 cells produces higher current levels than δ(2) . Patch-clamp experiments showed no differences in single channel current magnitude and open probability between isoforms. Steady-state plasma membrane abundance accounts for the dissimilarity in macroscopic current levels. Differential trafficking between isoforms is independent of β- and γ-subunits, PY-motif-mediated endocytosis, or the presence of additional lysine residues in δ(2)-N terminus. Analysis of δ(2)-N terminus identified two sequences that independently reduce channel abundance in the plasma membrane. The δ(1) higher abundance is consistent with an increased insertion rate into the membrane, since endocytosis rates of both isoforms are indistinguishable. Finally, we conclude that δ-ENaC undergoes dynamin-independent endocytosis as opposed to αβγ-channels.

Molecular determinants of interactions between the N-terminal domain and the transmembrane core that modulate hERG K+ channel gating

A conserved eag domain in the cytoplasmic amino terminus of the human ether-a-go-go-related gene (hERG) potassium channel is critical for its slow deactivation gating. Introduction of gene fragments encoding the eag domain are able to restore normal deactivation properties of channels from which most of the amino terminus has been deleted, and also those lacking exclusively the eag domain or carrying a single point mutation in the initial residues of the N-terminus. Deactivation slowing in the presence of the recombinant domain is not observed with channels carrying a specific Y542C point mutation in the S4–S5 linker. On the other hand, mutations in some initial positions of the recombinant fragment also impair its ability to restore normal deactivation. Fluorescence resonance energy transfer (FRET) analysis of fluorophore-tagged proteins under total internal reflection fluorescence (TIRF) conditions revealed a substantial level of FRET between the introduced N-terminal eag fragments and the eag domain-deleted channels expressed at the membrane, but not between the recombinant eag domain and full-length channels with an intact amino terminus. The FRET signals were also minimized when the recombinant eag fragments carried single point mutations in the initial portion of their amino end, and when Y542C mutated channels were used. These data suggest that the restoration of normal deactivation gating by the N-terminal recombinant eag fragment is an intrinsic effect of this domain directed by the interaction of its N-terminal segment with the gating machinery, likely at the level of the S4–S5 linker.

FRET with multiply labeled HERG K(+) channels as a reporter of the in vivo coarse architecture of the cytoplasmic domains

The intracellular N-terminus of human ether-a-go-go-related gene (HERG) potassium channels constitutes a key determinant of activation and deactivation characteristics and is necessary for hormone-induced modifications of gating properties. However, the general organization of the long amino and carboxy HERG terminals remains unknown. In this study we performed fluorescence resonance energy transfer (FRET) microscopy with a library of fluorescent HERG fusion proteins obtained combining site-directed and transposon-based random insertion of GFP variants into multiple sites of HERG. Determinations of FRET efficiencies with functional HERG channels labeled in different combinations localize the fluorophores, introduced in the amino and carboxy ends, in two quadratic planes of 7.8 and 8.6 nm lateral size, showing a vertical separation of nearly 8 nm without major angular torsion between the planes. Similar analysis using labels at positions 345 and 905 of the amino and carboxy terminals, located them slightly above the planes delimited by the amino and carboxy end labels, respectively. Our data also indicate an almost vertical arrangement of the fluorophores introduced in the NH(2) and COOH ends and at position 905, but a near 45 degrees angular rotation between the planes delimited by these labels and the 345-located fluorophores. Systematic triangulation using interfluorophore distances coming from multiply labeled channels provides an initial constraint on the overall in vivo arrangement of the HERG cytoplasmic domains, suggesting that the C-linker/CNBD region of HERG hangs centrally below the transmembrane core, with the initial portion of the amino terminus around its top and side surfaces directed towards the gating machinery.

Thermodynamic and kinetic properties of amino-terminal and S4-S5 loop HERG channel mutants under steady-state conditions

Gating kinetics and underlying thermodynamic properties of human ether-a-go-go-related gene (HERG) K(+) channels expressed in Xenopus oocytes were studied using protocols able to yield true steady-state kinetic parameters. Channel mutants lacking the initial 16 residues of the amino terminus before the conserved eag/PAS region showed significant positive shifts in activation voltage dependence associated with a reduction of z(g) values and a less negative DeltaG(o), indicating a deletion-induced displacement of the equilibrium toward the closed state. Conversely, a negative shift and an increased DeltaG(o), indicative of closed-state destabilization, were observed in channels lacking the amino-terminal proximal domain. Furthermore, accelerated activation and deactivation kinetics were observed in these constructs when differences in driving force were considered, suggesting that the presence of distal and proximal amino-terminal segments contributes in wild-type channels to specific chemical interactions that raise the energy barrier for activation. Steady-state characteristics of some single point mutants in the intracellular loop linking S4 and S5 helices revealed a striking parallelism between the effects of these mutations and those of the amino-terminal modifications. Our data indicate that in addition to the recognized influence of the initial amino-terminus region on HERG deactivation, this cytoplasmic region also affects activation behavior. The data also suggest that not only a slow movement of the voltage sensor itself but also delaying its functional coupling to the activation gate by some cytoplasmic structures possibly acting on the S4-S5 loop may contribute to the atypically slow gating of HERG.

Phosphatidylinositol 4,5-bisphosphate interactions with the HERG K(+) channel

Regulation of ion channel activity plays a central role in controlling heart rate, rhythm, and contractility responses to cardiovascular demands. Dynamic beat-to-beat regulation of ion channels is precisely adjusted by autonomic stimulation of cardiac G protein-coupled receptors. The rapidly activating delayed rectifier K(+) current (I (Kr)) is produced by the channel that is encoded by human ether-a-gogo-related gene (HERG) and is essential for the proper repolarization of the cardiac myocyte at the end of each action potential. Reduction of I (Kr) via HERG mutations or drug block can lead to lethal cardiac tachyarrhythmias. Autonomic regulation of HERG channels is an area of active investigation with the emerging picture of a complex interplay of signal transduction events, including kinases, second messengers, and protein-protein interactions. A recently described pathway for regulation of HERG is through channel interaction with the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). Changes in cellular PIP2 concentrations may occur with Gq-coupled receptor activation. Here, we review the evidence for PIP2-HERG interactions, its potential biological significance, and unfilled gaps in our understanding of this regulatory mechanism.

Retracted: Overexpression HERG K+ channel gene mediates cell-growth signals on activation of oncoproteins SP1 and NF-κB and inactivation of tumor suppressor Nkx3.1

The long QT syndrome gene human ether-a-go-go related gene (HERG) encodes a K(+) channel critical to cardiac repolarization. It peculiarly overexpresses in cancer cells of different histogenesis and promotes tumorigenesis. To decipher the molecular mechanisms for HERG overexpression, we identified and characterized the promoter region of the HERG gene, which contains cis-elements for multiple oncoproteins and tumor suppressors. Oncoprotein Sp1 was found to be essential to driving the HERG promoter thereby transcription. Another oncoprotein NF-kappaB transactivated, while tumor suppressor Nkx3.1 repressed HERG promoter activity and endogenous HERG transcription. Loss-of-function mutations in the corresponding cis-elements rendered a loss of the ability of the oncoproteins Sp1 and NF-kappaB to transactivate, and of the tumor repressor Nkx3.1 to repress, HERG transcription. Either activation of Sp1 and NF-kappaB or silencing of Nkx3.1 promoted tumor cell growth, and the effects were abrogated by HERG inhibition or knockdown, but facilitated by overexpression of HERG, indicating that HERG mediates the cell growth signals generated by activation of oncoproteins or inactivation of tumor suppressors. Binding of Sp1, NF-kappaB, and Nkx3.1 to their respective cis-elements in the HERG promoter in vitro and their presence on the HERG promoter in vivo were confirmed. Therefore, the HERG promoter region is characterized by multiple Sp1 binding sites that are responsible for transcription initiation of the HERG gene and by binding sites for multiple other oncogenes and tumor suppressor genes being important for regulating HERG expression. The HERG K(+) channel is likely a mediator of growth-promoting processes induced by oncoproteins and/or by silencing of tumor suppressors.

Merg1a K+ channel induces skeletal muscle atrophy by activating the ubiquitin proteasome pathway

Skeletal muscle atrophy results from an imbalance in protein degradation and protein synthesis and occurs in response to injury, various disease states, disuse, and normal aging. Current treatments for this debilitating condition are inadequate. More information about mechanisms involved in the onset and progression of muscle atrophy is necessary for development of more effective therapies. Here we show that expression of the mouse ether-a-go-go related gene (Merg1a) K+ channel is up-regulated in skeletal muscle of mice experiencing atrophy as a result of both malignant tumor expression and disuse. Further, ectopic expression of Merg1a in vivo induces atrophy in healthy wt-bearing mice, while expression of a dysfunctional Merg1a mutant suppresses atrophy in hindlimb-suspended mice. Treatment of hindlimb-suspended mice with astemizole, a known Merg1a channel blocker, inhibits atrophy in these animals. Importantly, in vivo expression of Merg1a in mouse skeletal muscle activates the ubiquitin proteasome pathway that is responsible for the majority of protein degradation that causes muscle atrophy, yet expression of a dysfunctional Merg1a mutant decreases levels of ubiquitin-proteasome proteolysis. Thus, expression of Merg1a likely initiates atrophy by activating ubiquitin-proteasome proteolysis. This gene and its product are potential targets for prevention and treatment of muscle atrophy.

Effects of TRH on heteromeric rat erg1a/1b K+ channels are dominated by the rerg1b subunit

The erg1a (HERG) K+ channel subunit and its N-terminal splice variant erg1b are coexpressed in several tissues and both isoforms have been shown to form heteromultimeric erg channels in heart and brain. The reduction of erg1a current by thyrotropin-releasing hormone (TRH) is well studied, but no comparable data exist for erg1b. Since TRH and TRH receptors are widely expressed in the brain, we have now studied the different TRH effects on the biophysical properties of homomeric rat erg1b as well as heteromeric rat erg1a/1b channels. The erg channels were overexpressed in the clonal somatomammotroph pituitary cell line GH3/B6, which contains TRH receptors and endogenous erg channels. Compared to rerg1a, homomeric rerg1b channels exhibited not only faster deactivation kinetics, but also considerably less steady-state inactivation, and half-maximal activation occurred at about 10 mV more positive potentials. Coexpression of both isoforms resulted in erg currents with intermediate properties concerning the deactivation kinetics, whereas rerg1a dominated the voltage dependence of activation and rerg1b strongly influenced steady-state inactivation. Application of TRH induced a reduction of maximal erg conductance for all tested erg1 currents without effects on the voltage dependence of steady-state inactivation. Nevertheless, homomeric rerg1b channels significantly differed in their response to TRH from rerg1a channels. The TRH-induced shift in the activation curve to more positive potentials, the dramatic slowing of activation and the acceleration of deactivation typical for rerg1a modulation were absent in rerg1b channels. Surprisingly, most effects of TRH on heteromeric rerg1 channels were dominated by the rerg1b subunit.