Rosiglitazone selectively inhibits K(ATP) channels by acting on the K(IR) 6 subunit

Identification and characterisation of functional Kir6.1-containing ATP-sensitive potassium channels in the cardiac ventricular sarcolemmal membrane

BACKGROUND AND PURPOSE

The canonical Kir6.2/SUR2A ventricular KATP channel is highly ATP-sensitive and remains closed under normal physiological conditions. These channels activate only when prolonged metabolic compromise causes significant ATP depletion and then shortens the action potential to reduce contractile activity. Pharmacological activation of KATP channels is cardioprotective, but physiologically, it is difficult to understand how these channels protect the heart if they only open under extreme metabolic stress. The presence of a second KATP channel population could help explain this. Here, we characterise the biophysical and pharmacological behaviours of a constitutively active Kir6.1-containing KATP channel in ventricular cardiomyocytes.

EXPERIMENTAL APPROACH

Patch-clamp recordings from rat ventricular myocytes in combination with well-defined pharmacological modulators was used to characterise these newly identified K+ channels. Action potential recording, calcium (Fluo-4) fluorescence measurements and video edge detection of contractile function were used to assess functional consequences of channel modulation.

KEY RESULTS

Our data show a ventricular K+ conductance whose biophysical characteristics and response to pharmacological modulation were consistent with Kir6.1-containing channels. These Kir6.1-containing channels lack the ATP-sensitivity of the canonical channels and are constitutively active.

CONCLUSION AND IMPLICATIONS

We conclude there are two functionally distinct populations of ventricular KATP channels: constitutively active Kir6.1-containing channels that play an important role in fine-tuning the action potential and Kir6.2/SUR2A channels that activate with prolonged ischaemia to impart late-stage protection against catastrophic ATP depletion. Further research is required to determine whether Kir6.1 is an overlooked target in Comprehensive in vitro Proarrhythmia Assay (CiPA) cardiac safety screens.

Electrophysiology, molecular modelling, and functional analysis of the effects of dietary quercetin and flavonoid analogues on Kir6.1 channels in rat stomach fundus smooth muscle

Flavonoids, ubiquitously distributed in the plant world, are regularly ingested with diets rich in fruit, vegetables, wine, and tea. During digestion, they are partially absorbed in the stomach. The present work aimed to assess the in vitro effects of quercetin and ten structurally related flavonoids on the rat gastric fundus smooth muscle, focussing on ATP-dependent K+ (Kir6.1) channels, which play a central role in the regulation of resting membrane potential, membrane excitability and, consequently, of gastric motility. Whole-cell currents through Kir6.1 channels (IKir6.1) were recorded with the patch-clamp technique and the mechanical activity of gastric fundus smooth muscle strips was studied under isometric conditions. Galangin ≈ tamarixetin > quercetin > kaempferol > isorhamnetin ≈ luteolin ≈ fisetin > (±)-taxifolin inhibited pinacidil-evoked, glibenclamide-sensitive IKir6.1 in a concentration-dependent manner. Morin, rutin, and myricetin were ineffective. The steric hindrance of the molecule and the number and position of hydroxyl groups on the B ring played an important role in the activity of the molecule. Molecular docking simulations revealed a possible binding site for flavonoids in the C-terminal domain of the Kir6.1 channel subunit SUR2B, in a flexible loop formed by residues 251 to 254 of chains C and D. Galangin and tamarixetin, but not rutin relaxed both high K+- and carbachol-induced contraction of fundus strips in a concentration-dependent manner. Furthermore, both flavonoids shifted to the right the concentration-relaxation curves to either pinacidil or L-cysteine constructed in strips pre-contracted by high K+, rutin being ineffective. In conclusion, IKir6.1 inhibition exerted by dietary flavonoids might counterbalance their myorelaxant activity, affect gastric accommodation or, at least, some stages of digestion.

Pharmacological Profiling of KATP Channel Modulators: An Outlook for New Treatment Opportunities for Migraine

Migraine is a highly disabling pain disorder with huge socioeconomic and personal costs. It is genetically heterogenous leading to variability in response to current treatments and frequent lack of response. Thus, new treatment strategies are needed. A combination of preclinical and clinical data indicate that ATP-sensitive potassium (KATP) channel inhibitors could be novel and highly effective drugs in the treatment of migraine. The subtype Kir6.1/SUR2B is of particular interest and inhibitors specific for this cranio-vascular KATP channel subtype may qualify as future migraine drugs. Historically, different technologies and methods have been undertaken to characterize KATP channel modulators and, therefore, a head-to-head comparison of potency and selectivity between the different KATP subtypes is difficult to assess. Here, we characterize available KATP channel activators and inhibitors in fluorescence-based thallium-flux assays using HEK293 cells stably expressing human Kir6.1/SUR2B, Kir6.2/SUR1, and Kir6.2/SUR2A KATP channels. Among the openers tested, levcromakalim, Y-26763, pinacidil, P-1075, ZM226600, ZD0947, and A-278637 showed preference for the KATP channel subtype Kir6.1/SUR2B, whereas BMS-191095, NN414, and VU0071306 demonstrated preferred activation of the Kir6.2/SUR1 subtype. In the group of KATP channel blockers, only Rosiglitazone and PNU-37783A showed selective inhibition of the Kir6.1/SUR2B subtype. PNU-37783A was stopped in clinical development and Rosiglitazone has a low potency for the vascular KATP channel subtype. Therefore, development of novel selective KATP channel blockers, having a benign side effect profile, are needed to clinically prove inhibition of Kir6.1/SUR2B as an effective migraine treatment.

ATP-Sensitive Potassium Channels in Migraine: Translational Findings and Therapeutic Potential

Globally, migraine is a leading cause of disability with a huge impact on both the work and private life of affected persons. To overcome the societal migraine burden, better treatment options are needed. Increasing evidence suggests that ATP-sensitive potassium (KATP) channels are involved in migraine pathophysiology. These channels are essential both in blood glucose regulation and cardiovascular homeostasis. Experimental infusion of the KATP channel opener levcromakalim to healthy volunteers and migraine patients induced headache and migraine attacks in 82-100% of participants. Thus, this is the most potent trigger of headache and migraine identified to date. Levcromakalim likely induces migraine via dilation of cranial arteries. However, other neuronal mechanisms are also proposed. Here, basic KATP channel distribution, physiology, and pharmacology are reviewed followed by thorough review of clinical and preclinical research on KATP channel involvement in migraine. KATP channel opening and blocking have been studied in a range of preclinical migraine models and, within recent years, strong evidence on the importance of their opening in migraine has been provided from human studies. Despite major advances, translational difficulties exist regarding the possible anti-migraine efficacy of KATP channel blockage. These are due to significant species differences in the potency and specificity of pharmacological tools targeting the various KATP channel subtypes.

Development of IKATP Ion Channel Blockers Targeting Sulfonylurea Resistant Mutant KIR6.2 Based Channels for Treating DEND Syndrome

Introduction: DEND syndrome is a rare channelopathy characterized by a combination of developmental delay, epilepsy and severe neonatal diabetes. Gain of function mutations in the KCNJ11 gene, encoding the K_IR6.2 subunit of the I_KATP potassium channel, stand at the basis of most forms of DEND syndrome. In a previous search for existing drugs with the potential of targeting Cantú Syndrome, also resulting from increased I_KATP, we found a set of candidate drugs that may also possess the potential to target DEND syndrome. In the current work, we combined Molecular Modelling including Molecular Dynamics simulations, with single cell patch clamp electrophysiology, in order to test the effect of selected drug candidates on the K_IR6.2 WT and DEND mutant channels. Methods: Molecular dynamics simulations were performed to investigate potential drug binding sites. To conduct in vitro studies, K_IR6.2 Q52R and L164P mutants were constructed. Inside/out patch clamp electrophysiology on transiently transfected HEK293T cells was performed for establishing drug-channel inhibition relationships. Results: Molecular Dynamics simulations provided insight in potential channel interaction and shed light on possible mechanisms of action of the tested drug candidates. Effective I_KIR6.2/SUR2a inhibition was obtained with the pore-blocker betaxolol (IC₅₀ values 27-37 μM). Levobetaxolol effectively inhibited WT and L164P (IC₅₀ values 22 μM) and Q52R (IC₅₀ 55 μM) channels. Of the SUR binding prostaglandin series, travoprost was found to be the best blocker of WT and L164P channels (IC₅₀ 2-3 μM), while Q52R inhibition was 15-20% at 10 μM. Conclusion: Our combination of MD and inside-out electrophysiology provides the rationale for drug mediated I_KATP inhibition, and will be the basis for 1) screening of additional existing drugs for repurposing to address DEND syndrome, and 2) rationalized medicinal chemistry to improve I_KATP inhibitor efficacy and specificity.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Computational Identification of Novel Kir6 Channel Inhibitors

KATP channels consist of four Kir6.x pore-forming subunits and four regulatory sulfonylurea receptor (SUR) subunits. These channels couple the metabolic state of the cell to membrane excitability and play a key role in physiological processes such as insulin secretion in the pancreas, protection of cardiac muscle during ischemia and hypoxic vasodilation of arterial smooth muscle cells. Abnormal channel function resulting from inherited gain or loss-of-function mutations in either the Kir6.x and/or SUR subunits are associated with severe diseases such as neonatal diabetes, congenital hyperinsulinism, or Cantú syndrome (CS). CS is an ultra-rare genetic autosomal dominant disorder, caused by dominant gain-of-function mutations in SUR2A or Kir6.1 subunits. No specific pharmacotherapeutic treatment options are currently available for CS. Kir6 specific inhibitors could be beneficial for the development of novel drug therapies for CS, particular for mutations, which lack high affinity for sulfonylurea inhibitor glibenclamide. By applying a combination of computational methods including atomistic MD simulations, free energy calculations and pharmacophore modeling, we identified several novel Kir6.1 inhibitors, which might be possible candidates for drug repurposing. The in silico predictions were confirmed using inside/out patch-clamp analysis. Importantly, Cantú mutation C166S in Kir6.2 (equivalent to C176S in Kir6.1) and S1020P in SUR2A, retained high affinity toward the novel inhibitors. Summarizing, the inhibitors identified in this study might provide a starting point toward developing novel therapies for Cantú disease.

Investigating Potassium Channels in Budding Yeast: A Genetic Sandbox

Like all species, the model eukaryote Saccharomyces cerevisiae, or Bakers' yeast, concentrates potassium in the cytosol as an electrogenic osmolyte and enzyme cofactor. Yeast are capable of robust growth on a wide variety of potassium concentrations, ranging from 10 µM to 2.5 M, due to the presence of a high-affinity potassium uptake system and a battery of cation exchange transporters. Genetic perturbation of either of these systems retards yeast growth on low or high potassium, respectively. However, these potassium-sensitized yeast are a powerful genetic tool, which has been leveraged for diverse studies. Notably, the potassium-sensitive cells can be transformed with plasmids encoding potassium channels from bacteria, plants, or mammals, and subsequent changes in growth rate have been found to correlate with the activity of the introduced potassium channel. Discoveries arising from the use of this assay over the past three decades have increased our understanding of the structure-function relationships of various potassium channels, the mechanisms underlying the regulation of potassium channel function and trafficking, and the chemical basis of potassium channel modulation. In this article, we provide an overview of the major genetic tools used to study potassium channels in S. cerevisiae, a survey of seminal studies utilizing these tools, and a prospective for the future use of this elegant genetic approach.

Lipophilicity predicts the ability of nonsulphonylurea drugs to block pancreatic beta-cell KATP channels and stimulate insulin secretion; statins as a test case

AIMS

KATP ion channels play a key role in glucose-stimulated insulin secretion. However, many drugs block KATP as "off targets" leading to hyperinsulinaemia and hypoglycaemia. As such drugs are often lipophilic, the aim was to examine the relationship between drug lipophilicity (P) and IC 50 for KATP block and explore if the IC 50's of statins could be predicted from their lipophilicity and whether this would allow one to forecast their acute action on insulin secretion.

MATERIALS AND METHODS

A meta-analysis of 26 lipophilic, nonsulphonylurea, blockers of KATP was performed. From this, the IC 50's for pravastatin and simvastatin were predicted and then tested experimentally by exploring their effects on KATP channel activity via patch-clamp measurement, calcium imaging and insulin secretion in murine beta cells and islets.

RESULTS

Nonsulphonylurea drugs inhibited KATP channels with a Log IC 50 linearly related to their logP. Simvastatin blocked KATP with an IC 50 of 25 nmol/L, a value independent of cytosolic factors, and within the range predicted by its lipophilicity (21-690 nmol/L). 10 μmol/L pravastatin, predicted IC 50 0.2-12 mmol/L, was without effect on the KATP channel. At 10-fold therapeutic levels, 100 nmol/L simvastatin depolarized the beta-cell membrane potential and stimulated Ca2+ influx but did not affect insulin secretion; the latter could be explained by serum binding.

CONCLUSIONS

The logP of a drug can aid prediction for its ability to block beta-cell KATP ion channels. However, although the IC 50 for the block of KATP by simvastatin was predicted, the difference between this and therapeutic levels, as well as serum sequestration, explains why hypoglycaemia is unlikely to be observed with acute use of this statin.

Impairment of the Vascular KATP Channel Imposes Fatal Susceptibility to Experimental Diabetes Due to Multi-Organ Injuries

The vascular isoform of ATP-sensitive K(+) (KATP ) channels regulates blood flow to all organs. The KATP channel is strongly inhibited by reactive oxygen and carbonyl species produced in diabetic tissue inflammation. To address how such channel inhibition impacts vascular regulation as well as tissue viability, we performed studies in experimental diabetic mice. Strikingly, we found that knockout of the Kcnj8 encoding Kir6.1 subunit (Kcnj8-KO) caused mice to be fatally susceptible to diabetes. Organ perfusion studies suggested that the lack of this vascular K(+) channel handicapped activity-dependent vasodilation, leading to hypoperfusion, tissue hypoxia, and multi-organ failure. Morphologically, Kcnj8-KO mice showed greater inflammatory cell infiltration, higher levels of expression of inflammation indicator proteins, more severe cell apoptosis, and worse tissue disruptions. These were observed in the kidney, liver, and heart under diabetic condition in parallel comparison to tissues from WT mice. Patch clamping and molecular studies showed that the KATP channel was S-glutathionylated in experimental diabetes contributing to the inhibition of channel activity as well as the reduced arterial responses to vasodilators. These results suggest that the vascular KATP channel is organ protective in diabetic condition, and since the channel is suppressed by diabetic oxidative stress, therapeutical interventions to the maintenance of functional KATP channels may help to lower or prevent diabetic organ dysfunction.

TRPs: modulation by drug-like compounds

Drug-like compounds that exert biological activity towards TRP channels are either being used as cell biological tools or further developed into pharmacological lead structures aiming at therapeutic use in diseased states. Although drug-likeliness is not easy to predict, common rules include a relatively low molecular weight, physicochemical constraints, and the absence of known reactive or otherwise toxic groups. Small molecules that exert a biological activity to block, activate, or modulate TRP channels are intensely sought. Such tool compounds may be useful to assign native currents to a certain TRP channel and to validate the channel as a candidate target for future pharmacological intervention. Depending on the TRP channel isotype, these activities have reached different levels, with only few TRP channels modulators already being clinically tested in humans, whereas other compounds only underwent a preliminary validation. For some TRP channels, reliable low molecular weight inhibitors are not yet available. Hence, further efforts need to be undertaken in order to explore the physiological impact and possible therapeutic potential of TRP channel targeting with drug-like compounds.

Differential sensitivities of the vascular K(ATP) channel to various PPAR activators

Several agonists of the peroxisome proliferator-activated receptors (PPARs) are currently used for the treatment of metabolic disorders including diabetes. We have recently shown that one of them, Rosiglitazone, inhibits the vascular ATP-sensitive K⁺ (K(ATP)) channel and compromises the coronary vasodilation by the β-adrenoceptor agonist. Here, we show evidence for the channel inhibition by various PPAR agonists, information that may be useful for finding new therapeutical agents with less cardiovascular side-effects and more selective K(ATP) channel blockers targeting at the K(ir)6.1 subunit. Structural comparison of these PPAR agonists may shed insight into the critical chemical groups for the channel inhibition. K(ir)6.1/SUR2B channel was expressed in HEK293 cells and studied in whole-cell voltage clamp. The K(ir)6.1/SUR2B channel was strongly inhibited by several PPAR(γ) agonists with potencies similar to, or higher than, that of Rosiglitazone, while other PPAR(γ) agonists barely inhibited the channel. The K(ir)6.1/SUR2B channel was also inhibited by PPAR(α) and PPAR(β/δ) agonists with intermediate potencies. The structure necessary for the channel inhibition appears to include the thiazole linked to an aromatic or furan ring. Additions of side groups such as small aliphatic chain increased the potency for channel inhibition, while additions of aromatic rings reduced it. These results indicate that the PPAR(γ) agonists with weak K(ATP) channel inhibition may be potential candidates as therapeutical agents, and those with strong channel inhibition may be used as selective K(ATP) channel blockers. The structural information of the PPAR agonists may be useful for the development of new therapeutical modalities with less cardiovascular side-effects.

Selective block of K(ATP) channels: why the anti-diabetic sulphonylureas and rosiglitazone have more in common than we thought

Rosiglitazone, the thiazolidinedione class anti-diabetic withdrawn from Europe in 2010 amid reports of adverse cardiovascular effects, is revealed by Yu et al. in this issue of the British Journal of Pharmacology to be a selective blocker of ATP-sensitive potassium (K(ATP) ) channels. This seems little cause for excitement given that the closure of pancreatic K(ATP) channels is integral to insulin secretion; and sulphonylureas, which inhibit K(ATP) channels, are widely used to treat type II diabetes. However, rosiglitazone, whose primary targets are nuclear transcription factors that regulate genes involved in lipid metabolism, blocks K(ATP) channels by a novel mechanism different to that of the sulphonylureas and has a worrying preference for blood flow-regulating vascular K(ATP) channels. Identification of a new molecule that modulates K(ATP) channel gating will not only tell us more about how these complex metabolic sensors work but also raises questions as to whether rosiglitazone suppresses the cardiovascular system's ability to cope with metabolic stress - a claim that has dogged the sulphonylureas for many years.