Protein kinase C is necessary for recovery from the thyrotropin-releasing hormone-induced r-ERG current reduction in GH3 rat anterior pituitary cells

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.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

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.

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

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.

G(alpha)q-mediated regulation of TASK3 two-pore domain potassium channels: the role of protein kinase C

The TASK subfamily of two pore domain potassium channels (K2P) gives rise to leak potassium currents, which contribute to the resting membrane potential of many neurons and regulate their excitability. K2P channels are highly regulated by phosphorylation and by G protein-mediated pathways. In this study, we show that protein kinase C (PKC) inhibits recombinant TASK3 channels. Inhibition by PKC is blocked by the PKC inhibitors bisindolylmaleimide 1 hydrochloride (BIM) and 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (Gö6976). Gene-silencing experiments with a validated small interfering RNA sequence against PKCalpha ablates the effect of PKC. PKC acts directly on hTASK3 channels to phosphorylate an identified amino acid in the C terminus region (Thr341), thereby reducing channel current. PKC also inhibits mTASK3 channels despite their having a quite different C-terminal structure to hTASK3 channels. Activation of M(3) muscarinic receptors inhibits both hTASK3 channels expressed in tsA-201 cells and standing outward potassium current (IK(SO)) in mouse cerebellar granule neurons through the activation of the G protein Galpha(q), because both effects are abolished by the selective Galpha(q) antagonist YM-254890 (J Biol Chem 279:47438-47445, 2004). This inhibition is not directly transduced through activation of PKC because inhibition persists in mutated PKC-insensitive hTASK3 channels. Instead, inhibition seems to occur through a direct action of Galpha(q) on the channel. Nevertheless, preactivation of PKC blocks muscarinic inhibition of both TASK3 channels and IK(SO). Our results suggest that activation of PKC (via phospholipase C) has a role in opposing inhibition after M(3) receptor activation rather than transducing it and may act as a negative regulator of G protein modulation to prevent prolonged current inhibition.

Modulation of hERG potassium currents in HEK-293 cells by protein kinase C. Evidence for direct phosphorylation of pore forming subunits

The human ether-a-go-go related gene (hERG) potassium channel is expressed in a variety of tissues including the heart, neurons and some cancer cells. hERG channels are modulated by several intracellular signalling pathways and these provide important mechanisms for regulating cellular excitability. In this study, we investigated muscarinic modulation of hERG currents and direct phosphorylation of channel subunits expressed in HEK-293 cells at physiologically relevant temperatures by protein kinase C (PKC). Activation of G(alpha q/11)-coupled M(3)-muscarinic receptors with methacholine, reduced current amplitudes at all potentials with minor effects on the voltage dependence of activation and inactivation. The response to methacholine was insensitive to intracellular BAPTA, but was attenuated by either acute inhibition of PKC with 300 nm bisindolylmaleimide-1 (bis-1) or chronic down-regulation of PKC isoforms by 24 h pretreatment of cells with phorbol 12-myristate 13-acetate (PMA). Stimulation of PKC with 1-oleoyl 2-acetylglycerol (OAG), an analogue of diacylglycerol (DAG), mimicked the actions of muscarinic receptor stimulation. Direct phosphorylation of hERG was measured by [(32)P]orthophosphate labelling of immunoprecipitated protein with an anti-hERG antibody. Basal phosphorylation was high in unstimulated cells and further increased by OAG. The OAG dependent increase was abolished by bis-1 and down-regulation of PKC, but basal levels of phosphorylation were unchanged. Deletion of the amino-terminus of hERG prevented both the modulation of channel activity and the increase of phosphorylation by OAG. Our results are consistent with calcium and/or DAG sensitive isotypes of PKC modulating hERG currents through a mechanism that involves direct phosphorylation of sites on the amino terminus of hERG.

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.

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

The identity of the G-protein coupling thyrotropin-releasing hormone (TRH) receptors to rat ether-à-go-go related gene (r-ERG) K+ channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH(3) cells and dominant-negative variants (Galpha-QL/DN) of G-protein alpha subunits. Expression of dominant-negative Galpha(q/11) that minimizes the TRH-induced Ca2+ signal had no effect on r-ERG current inhibition elicited by the hormone. In contrast, the introduction of dominant-negative variants of Galpha13 and the small G-protein Rho caused a significant loss of the inhibitory effect of TRH on r-ERG. A strong reduction of this TRH effect was also obtained in cells expressing either dominant-negative Galpha(s) or transducin alpha subunits, an agent known to sequester free G-protein betagamma dimers. As a further indication of specificity of the dominant-negative effects, only the dominant-negative variants of Galpha13 and Rho (but not Galpha(s)-QL/DN or Galpha(t)) were able to reduce the TRH-induced shifts of human ERG (HERG) activation voltage dependence in HEK293 cells permanently expressing HERG channels and TRH receptors. Our results demonstrate that whereas the TRH receptor uses a G(q/11) protein for transducing the Ca2+ signal during the initial response to TRH, this G-protein is not involved in the TRH-induced inhibition of endogenous r-ERG currents in pituitary cells. They also identify G(s) (or a G(s)-like protein) and G13 as important contributors to the hormonal effect in these cells and suggest that betagamma dimers released from these proteins may participate in modulation of ERG currents triggered by TRH.

Muscarinic modulation of erg potassium current

We studied modulation of current in human embryonic kidney tsA-201 cells coexpressing rat erg1 channels with M(1) muscarinic receptors. Maximal current was inhibited 30% during muscarinic receptor stimulation, with a small positive shift of the midpoint of activation. Inhibition was attenuated by coexpression of the regulator of G-protein signalling RGS2 or of a dominant-negative protein, G(q), but not by N-ethylmaleimide or C3 toxin. Overexpression of a constitutively active form of G(q) (but not of G(13) or of G(s)) abolished the erg current. Hence it is likely that G(q/11), and not G(i/o) or G(13), mediates muscarinic inhibition. Muscarinic suppression of erg was attenuated by chelating intracellular Ca(2+) to < 1 nm free Ca(2+) with 20 mm BAPTA in the pipette, but suppression was normal if internal Ca(2+) was strongly clamped to a 129 nm free Ca(2+) level with a BAPTA buffer and this was combined with numerous other measures to prevent intracellular Ca(2+) transients (pentosan polysulphate, preincubation with thapsigargin, and removal of extracellular Ca(2+)). Hence a minimum amount of Ca(2+) was necessary for the inhibition, but a Ca(2+) elevation was not. The ATP analogue AMP-PCP did not prevent inhibition. The protein kinase C (PKC) blockers staurosporine and bisindolylmaleimide I did not prevent inhibition, and the PKC-activating phorbol ester PMA did not mimic it. Neither the tyrosine kinase inhibitor genistein nor the tyrosine phosphatase inhibitor dephostatin prevented inhibition by oxotremorine-M. Hence protein kinases are not needed. Experiments with a high concentration of wortmannin were consistent with recovery being partially dependent on PIP(2) resynthesis. Wortmannin did not prevent muscarinic inhibition. Our studies of muscarinic inhibition of erg current suggest a role for phospholipase C, but not the classical downstream messengers, such as PKC or a calcium transient.

Thyrotropin-releasing hormone increases phospholipase D activity through stimulation of protein kinase C in GH3 cells

Activation of phospholipase D was investigated after treatment of GH3 cells with thyrotropin-releasing hormone. Thyrotropin-releasing hormone treatment resulted in both time- and dose-dependent increases of phospholipase D activity, translocation of protein kinase C-alpha and -beta I isozymes from cytosol to membrane within 30 min, and approx 43-fold increase of phosphatidylinositol-specific phospholipase C activity. Intracellular calcium concentration was rapidly increased and diacyglycerol level remained high up to 3 h after the treatment. Pretreatment of the cells with U73122, a potent inhibitor of phosphatidylinositol-specific phospholipase C, inhibited thyrotropin-releasing hormone-induced phospholipase D activation. Protein kinase C activity was down-regulated by pretreatment of the GH3 cells with either protein kinase C inhibitors (RO320432, GF109203X) or preincubation of the cells with phorbol myristrate acetate (500 nM) for 24 h. This treatment largely abolished the thyrotropin-releasing hormone-induced activation of phospholipase D, thus further confirming the involvement of protein kinase C in the activation. These results suggest that thyrotropin-releasing hormone-induced phospholipase D activation may be due to phosphatidylinositol-specific phospholipase C, and activation of protein kinase C isozymes is responsible for this stimulation.