Probucol and the cholesterol synthesis inhibitors simvastatin and triparanol regulate I ks channel function differently

Probucol is anti-hyperalgesic in a mouse peripheral nerve injury model of neuropathic pain

2,6-di-tert-butylphenol (2,6-DTBP) ameliorates mechanical allodynia and thermal hyperalgesia produced by partial sciatic nerve ligation in mice, and selectively inhibits HCN1 channel gating. We hypothesized that the clinically utilized non-anesthetic dimerized congener of 2,6-DTBP, probucol (2,6-di-tert-butyl-4-[2-(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanylpropan-2-ylsulfanyl]phenol), would relieve the neuropathic phenotype that results from peripheral nerve damage, and that the anti-hyperalgesic efficacy in vivo would correlate with HCN1 channel inhibition in vitro. A single oral dose of probucol (800 mg/kg) relieved mechanical allodynia and thermal hyperalgesia in a mouse spared-nerve injury neuropathic pain model. While the low aqueous solubility of probucol precluded assessment of its possible interaction with HCN1 channels, our results, in conjunction with recent data demonstrating that probucol reduces lipopolysaccharide-induced mechanical allodynia and thermal hyperalgesia, support the testing/development of probucol as a non-opioid, oral antihyperalgesic albeit one of unknown mechanistic action.

In-Situ imaging detection of cell membrane and intracellular cholesterol via cascade reactions

Herein, an effective membrane-to-intracellular cholesterol detection strategy was designed based on cascade reactions. A biochip array was firstly fabricated by consecutively immobilizing luminol modified gold nanoparticles (Au@luminol), soybean peroxidase (SBP) and cholesterol oxidase (ChoX) on the cellulose acetate (CA) membrane functionalized home-made micropore array. When cholesterol existed, it was oxidized by ChoX generating H₂O₂, which further triggered the CL reaction under the SBP catalysis, the CL signals were collected by a charge-coupled device (CCD). The proposed strategy exhibited a wide linear range from 0.12 μM to 1000 μM and relatively low detection limit (LOD) of 0.08 μM. Furthermore，it could be used to in-situ detect membrane cholesterol and intracelluar esterified cholesterol in HepG2 cells. After activated HepG2 cells were added to the modified biochip, membrane cholesterol was detected directly. Intracelluar esterified cholesterol was detected through the introduction of triton X-100 and cholesteryl esterase (ChoE). Additionally, the cholesterol content in cells was changed after stimulated by drugs, such as apolipoprotein A-I (ApoA-I), pitavastatin or probucol. The correlation of the CL signal with the amount of cholesterol confirmed that our strategy was feasible to simultaneously detect membrane and intracellular cholesterol at different cellular states. The proposed strategy exhibited excellent sensitivity, selectivity, stability, and reproducibility in a simple, cheap way, which opened a new door for studying clinic treatment of the cholesterol-related diseases.

Regulation of KCNQ/Kv7 family voltage-gated K+ channels by lipids

Many years of studies have established that lipids can impact membrane protein structure and function through bulk membrane effects, by direct but transient annular interactions with the bilayer-exposed surface of protein transmembrane domains, and by specific binding to protein sites. Here, we focus on how phosphatidylinositol 4,5-bisphosphate (PIP₂) and polyunsaturated fatty acids (PUFAs) impact ion channel function and how the structural details of the interactions of these lipids with ion channels are beginning to emerge. We focus on the Kv7 (KCNQ) subfamily of voltage-gated K⁺ channels, which are regulated by both PIP₂ and PUFAs and play a variety of important roles in human health and disease. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Triggering of Erythrocyte Death by Triparanol

The cholesterol synthesis inhibitor Triparanol has been shown to trigger apoptosis in several malignancies. Similar to the apoptosis of nucleated cells, erythrocytes may enter eryptosis, the suicidal death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Triggers of eryptosis include oxidative stress which may activate erythrocytic Ca²⁺ permeable unselective cation channels with subsequent Ca²⁺ entry and increase of cytosolic Ca²⁺ activity ([Ca²⁺]_i). The present study explored whether and how Triparanol induces eryptosis. To this end, phosphatidylserine exposure at the cell surface was estimated from annexin-V-binding, cell volume from forward scatter, hemolysis from hemoglobin release, [Ca²⁺]_i from Fluo3-fluorescence, and ROS formation from 2’,7’-dichlorodihydrofluorescein diacetate (DCFDA) dependent fluorescence. As a result, a 48 h exposure of human erythrocytes to Triparanol (20 µM) significantly increased DCFDA fluorescence and significantly increased Fluo3-fluorescence. Triparanol (15 µM) significantly increased the percentage of annexin-V-binding cells, and significantly decreased the forward scatter. The effect of Triparanol on annexin-V-binding was significantly blunted, but not abolished by removal of extracellular Ca²⁺. In conclusion, Triparanol leads to eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and phospholipid scrambling of the erythrocyte cell membrane. Triparanol is at least in part effective by stimulating ROS formation and Ca²⁺ entry.

A long QT mutation substitutes cholesterol for phosphatidylinositol-4,5-bisphosphate in KCNQ1 channel regulation

Introduction

Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP₂. A previous study suggested that R539W is paradoxically less sensitive to intracellular magnesium inhibition than the WT channel, despite a decreased interaction with PIP₂. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it.

Methods and Results

COS-7 cells were transfected with WT or mutated KCNE1-KCNQ1 channel, and patch-clamp recordings were performed in giant-patch, permeabilized-patch or ruptured-patch configuration. Similar to other channels with a decreased PIP₂ affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when membrane PIP₂ levels are decreasing. As opposed to R243H and R555C mutants, R539W is not more but rather less sensitive to PIP₂ decrease than the WT channel. A molecular model of a fragment of the KCNQ1 C-terminus and the membrane bilayer suggested that a potential novel interaction of R539W with cholesterol stabilizes the channel opening and hence prevents rundown upon PIP₂ depletion. We then carried out the same rundown experiments under cholesterol depletion and observed an accelerated R539W rundown that is consistent with this model.

Conclusions

We show for the first time that a mutation may shift the channel interaction with PIP₂ to a preference for cholesterol. This de novo interaction wanes the sensitivity to PIP₂ variations, showing that a mutated channel with a decreased affinity to PIP₂ could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP₂ sensitivity.