Tamoxifen alters gating of the BK α subunit and mediates enhanced interactions with the avian β subunit

A Novel Role of Uricosuric Agent Benzbromarone in BK Channel Activation and Reduction of Airway Smooth Muscle Contraction

The uricosuric drug benzbromarone, widely used for treatment of gout, hyperpolarizes the membrane potential of airway smooth muscle cells, but how it works remains unknown. Here we show a novel role of benzbromarone in activation of large conductance calcium-activated K⁺ channels. Benzbromarone results in dose-dependent activation of macroscopic big potassium (BK) currents about 1.7- to 14.5-fold with an EC₅₀ of 111 μM and shifts the voltage-dependent channel activation to a more hyperpolarizing direction about 10 to 54 mV in whole-cell patch clamp recordings. In single-channel recordings, benzbromarone decreases single BKα channel closed dwell time and increases the channel open probability. Coexpressing β1 subunit also enhances BK activation by benzbromarone with an EC₅₀ of 67 μM and a leftward shift of conductance-voltage (G-V) curve about 44 to 138 mV. Site-directed mutagenesis reveals that a motif of three amino acids ³²⁹RKK³³¹ in the cytoplasmic linker between S6 and C-terminal regulator of potassium conductance (RCK) gating ring mediates the pharmacological activation of BK channels by benzbromarone. Further ex vivo assay shows that benzbromarone causes reduction of tracheal strip contraction. Taken together, our findings demonstrate that uricosuric benzbromarone activates BK channels through molecular mechanism of action involving the channel RKK motif of S6-RCK linker. Pharmacological activation of BK channel by benzbromarone causes reduction of tracheal strip contraction, holding a repurposing potential for asthma and pulmonary arterial hypertension or BK channelopathies. SIGNIFICANCE STATEMENT: We describe a novel role of uricosuric agent benzbromarone in big potassium (BK) channel activation and relaxation of airway smooth muscle contraction. In this study, we find that benzbromarone is an activator of the large-conductance Ca²⁺- and voltage-activated K⁺ channel (BK channel), which serves numerous cellular functions, including control of smooth muscle contraction. Pharmacological activation of BK channel by the FDA-approved drug benzbromarone may hold repurposing potential for treatment of asthma and pulmonary arterial hypertension or BK channelopathies.

Subunits of BK channels promote breast cancer development and modulate responses to endocrine treatment in preclinical models

BACKGROUND AND PURPOSE

Pore-forming α subunits of the voltage- and Ca²⁺ -activated K⁺ channel with large conductance (BKα) promote malignant phenotypes of breast tumour cells. Auxiliary subunits such as the leucine-rich repeat containing 26 (LRRC26) protein, also termed BKγ1, may be required to permit activation of BK currents at a depolarized resting membrane potential that frequently occur in non-excitable tumour cells.

EXPERIMENTAL APPROACH

Anti-tumour effects of BKα loss were investigated in breast tumour-bearing MMTV-PyMT transgenic BKα knockout (KO) mice, primary MMTV-PyMT cell cultures, and in a syngeneic transplantation model of breast cancer derived from these cells. The therapeutic relevance of BK channels in the context of endocrine treatment was assessed in human breast cancer cell lines expressing either low (MCF-7) or high (MDA-MB-453) levels of BKα and BKγ1, as well as in BKα-negative MDA-MB-157.

KEY RESULTS

BKα promoted breast cancer onset and overall survival in preclinical models. Conversely, lack of BKα and/or knockdown of BKγ1 attenuated proliferation of murine and human breast cancer cells in vitro. At low concentrations, tamoxifen and its major active metabolites stimulated proliferation of BKα/γ1-positive breast cancer cells, independent of the genomic signalling controlled by the oestrogen receptor. Finally, tamoxifen increased the relative survival time of BKα KO but not of wild-type tumour cell recipient mice.

CONCLUSION AND IMPLICATIONS

Breast cancer initiation, progression, and tamoxifen sensitivity depend on functional BK channels thereby providing a rationale for the future exploration of the oncogenic actions of BK channels in clinical outcomes with anti-oestrogen therapy.

The molecular nature of the 17β-Estradiol binding site in the voltage- and Ca2+-activated K+ (BK) channel β1 subunit

The accessory β1 subunit modulates the Ca2+- and voltage-activated K+ (BK) channel gating properties mainly by increasing its apparent Ca2+ sensitivity. β1 plays an important role in the modulation of arterial tone and blood pressure by vascular smooth muscle cells (SMCs). 17β-estradiol (E2) increases the BK channel open probability (Po) in SMCs, through a β1 subunit-dependent modulatory effect. Here, using molecular modeling, bioinformatics, mutagenesis, and electrophysiology, we identify a cluster of hydrophobic residues in the second transmembrane domain of the β1 subunit, including the residues W163 and F166, as the binding site for E2. We further show that the increase in Po induced by E2 is associated with a stabilization of the voltage sensor in its active configuration and an increase in the coupling between the voltage sensor activation and pore opening. Since β1 is a key molecular player in vasoregulation, the findings reported here are of importance in the design of novel drugs able to modulate BK channels.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

The large-conductance, Ca(2+)- and voltage-activated K(+) (BK) channel is ubiquitously expressed in mammalian tissues and displays diverse biophysical or pharmacological characteristics. This diversity is in part conferred by channel modulation with different regulatory auxiliary subunits. To date, two distinct classes of BK channel auxiliary subunits have been identified: β subunits and γ subunits. Modulation of BK channels by the four auxiliary β (β1-β4) subunits has been well established and intensively investigated over the past two decades. The auxiliary γ subunits, however, were identified only very recently, which adds a new dimension to BK channel regulation and improves our understanding of the physiological functions of BK channels in various tissues and cell types. This chapter will review the current understanding of BK channel modulation by auxiliary β and γ subunits, especially the latest findings.

Expression of BK-type calcium-activated potassium channel splice variants during chick cochlear development

The appearance of large-conductance, calcium-activated potassium (BK) current is a hallmark of functional maturation in auditory hair cells. Acquisition of this fast-activating current enables high-frequency, graded receptor potentials in all vertebrates and an electrical tuning mechanism in nonmammals. The gene encoding BK alpha subunits is highly alternatively spliced, and the resulting variations in channel isoforms may contribute to functional diversity at the onset of hearing. We examined the tissue specificity of nine BK alpha alternative exons and investigated changes in expression during chick cochlear development using quantitative polymerase chain reaction (qPCR). Each alternative was widely expressed in several tissues except for an insert near the C-terminus Ca(2+) sensing domain, which appeared brain-specific. The only alternative form in the membrane-bound core of the channel was expressed in brain and muscle but was undetected in cochlea. Of the remaining variants, three increased in expression prior to the onset of hearing and acquisition of BK currents. These three variants cause decreased Ca(2+) sensitivity or increased intracellular retention, traits that would not easily explain the advent of calcium-sensitive currents at embryonic day (E)18-19. Expression levels of other variants were mature and stable by E15, days before currents were acquired. Surface expression of C-terminal isoforms was examined using patch-clamp electrophysiology and immunocytochemistry. C-terminal variants that exhibit robust surface expression appeared in the membrane at E18, even though transcripts were unchanged during development starting from E12. These results indicate that delays in protein synthesis and trafficking/scaffolding of channel subunits underlie the late acquisition of BK currents in cochlear hair cells.

Bisphenol A activates Maxi-K (K(Ca)1.1) channels in coronary smooth muscle

BACKGROUND AND PURPOSE

Bisphenol A (BPA) is used to manufacture plastics, including containers for food into which it may leach. High levels of exposure to this oestrogenic endocrine disruptor are associated with diabetes and heart disease. Oestrogen and oestrogen receptor modulators increase the activity of large conductance Ca(2+)/voltage-sensitive K(+) (Maxi-K; K(Ca)1.1) channels, but the effects of BPA on Maxi-K channels are unknown. We tested the hypothesis that BPA activates Maxi-K channels through a mechanism that depends upon the regulatory beta1 subunit.

EXPERIMENTAL APPROACH

Patch-clamp recordings of Maxi-K channels were made in human and canine coronary smooth muscle cells as well as in AD-293 cells expressing pore-forming alpha or alpha plus beta1 subunits.

KEY RESULTS

BPA (10 microM) activated an outward current in smooth muscle cells that was inhibited by penitrem A (1 microM), a Maxi-K blocker. BPA increased Maxi-K activity in inside-out patches from coronary smooth muscle, but had no effect on single channel conductance. In AD-293 cells with Maxi-K channels composed of alpha subunits alone, 10 microM BPA did not affect channel activity. When channels in AD-293 cells contained beta1 subunits, 10 microM BPA increased channel activity. Effects of BPA were rapid (<1 min) and reversible. A higher concentration of BPA (100 microM) increased Maxi-K current independent of the beta1 subunit.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that BPA increased the activity of Maxi-K channels and may represent a basis for some potential toxicological effects.

Channel beta2-4 subunits fail to substitute for beta1 in sensitizing BK channels to lithocholate

Large conductance, calcium- and voltage-gated potassium (BK) channels regulate numerous physiological processes. While most basic functional characteristics of native BK channels are reproduced by BK alpha (slo1) subunit homotetramers, key biophysical and pharmacological properties are drastically modified by the presence of auxiliary beta subunits (encoded by KCNMB1-4). Numerous physiological steroids, including sex hormones, gluco- and mineralocorticoids, activate beta subunit-containing BK channels, yet these steroids appear to be sensed by different types of beta subunits, with some steroids being sensed by homomeric slo1 channels as well. We recently showed that beta1 sensitizes the BK channel to microM concentrations of lithocholate (LC). Following expression of rat cerebral artery myocyte slo1 subunits ("cbv1") with beta1, beta2, beta3 or beta4 in Xenopus laevis oocytes we now demonstrate that BK beta2, beta3 and beta4 subunits fail to substitute for beta1 in providing LC-sensitivity (150 microM) to the BK channel. These findings document for the first time a rather selective steroid activation of BK channels via a particular channel accessory subunit. In addition, LC routinely activated native BK channels in myocytes freshly isolated from rat cerebral artery smooth muscle, where BK beta1 is highly expressed, while failing to do so in skeletal (flexor digitorum brevis) muscle, where BK beta1 expression is negligible. This indicates that the native environment of the BK channel sustains the LC-sensitivity distinctly provided to the BK channel by beta1 subunits. Our study indicates that LC represents a unique tool to probe the presence of functional beta1-subunits and selectively activate BK channels in tissues that highly express KCNMB1.

The large-conductance Ca(2+)-activated K(+) channel interacts with the apolipoprotein ApoA1

Owing to the multifaceted functions of the large conductance Ca(2+)-activated K(+) channel (BK), identification of protein-protein interactions is essential in determining BK regulation. A yeast two-hybrid screening of a cochlear cDNA library revealed a BK-ApoA1 interaction. Patch clamp recordings of excised membrane patches from transfected HEK293 cells showed that ApoA1 inhibits the BK alpha-subunit by significantly increasing activation and deactivation times, and shifting half-activation voltage to more positive potentials. Reciprocal coimmunoprecipitations verified the BK-ApoA1 interaction using excised sensory epithelium and ganglia. Additionally, immunocolocalization studies revealed BK and ApoA1 expression in both receptor cells and auditory neurons. These data suggest new avenues of investigation, given the importance of apolipoproteins in neurological diseases.

Estradiol shapes auditory processing in the adult brain by regulating inhibitory transmission and plasticity-associated gene expression

Estradiol impacts a wide variety of brain processes, including sex differentiation, mood, and learning. Here we show that estradiol regulates auditory processing of acoustic signals in the vertebrate brain, more specifically in the caudomedial nidopallium (NCM), the songbird analog of the mammalian auditory association cortex. Multielectrode recordings coupled with local pharmacological manipulations in awake animals reveal that both exogenous and locally generated estradiol increase auditory-evoked activity in NCM. This enhancement in neuronal responses is mediated by suppression of local inhibitory transmission. Surprisingly, we also found that estradiol is both necessary and sufficient for the induction of multiple mitogen-activated protein kinase (MAPK)-dependent genes thought to be required for synaptic plasticity and memorization of birdsong. Specifically, we show that local blockade of estrogen receptors or aromatase activity in awake birds decrease song-induced MAPK-dependent gene expression. Infusions of estradiol in acoustically isolated birds induce transcriptional activation of these genes to levels comparable with song-stimulated animals. Our results reveal acute and rapid nongenomic functions for estradiol in central auditory physiology and suggest that such roles may be ubiquitously expressed across sensory systems.

Tamoxifen inhibits BK channels in chick cochlea without alterations in voltage-dependent activation

Large-conductance, Ca(2+)-activated, and voltage-gated potassium channels (BK, BK(Ca), or Maxi-K) play an important role in electrical tuning in nonmammalian vertebrate hair cells. Systematic changes in tuning frequency along the tonotopic axis largely result from variations in BK channel kinetics, but the molecular changes underpinning these functional variations remain unknown. Auxiliary beta(1) have been implicated in low-frequency tuning at the cochlear apex because these subunits dramatically slow channel kinetics. Tamoxifen (Tx), a (xeno)estrogen compound known to activate BK channels through the beta-subunit, was used to test for the functional presence of beta(1). The hypotheses were that Tx would activate the majority of BK channels in hair cells from the cochlear apex due to the presence of beta(1) and that the level of activation would exhibit a tonotopic gradient following the expression profile of beta(1). Outside-out patches of BK channels were excised from tall hair cells along the apical half of the chicken basilar papilla. In low-density patches, single-channel conductance was reduced and the averaged open probability was unaffected by Tx. In high-density patches, the amplitude of ensemble-averaged BK current was inhibited, whereas half-activation potential and activation kinetics were unaffected by Tx. In both cases, no tonotopic Tx-dependent activation of channel activity was observed. Therefore, contrary to the hypotheses, electrophysiological assessment suggests that molecular mechanisms other than auxiliary beta-subunits are involved in generating a tonotopic distribution of BK channel kinetics and electric tuning in chick basilar papilla.

Structural determinants of monohydroxylated bile acids to activate beta 1 subunit-containing BK channels

Lithocholate (LC) (10-300 microM) in physiological solution is sensed by vascular myocyte large conductance, calcium- and voltage-gated potassium (BK) channel beta(1) accessory subunits, leading to channel activation and arterial dilation. However, the structural features in steroid and target that determine LC action are unknown. We tested LC and close analogs on BK channel (pore-forming cbv1+beta(1) subunits) activity using the product of the number of functional ion channels in the membrane patch (N) and the open channel probability (Po). LC (5beta-cholanic acid-3alpha-ol), 5alpha-cholanic acid-3alpha-ol, and 5beta-cholanic acid-3beta-ol increased NPo (EC(50) approximately 45 microM). At maximal increase in NPo, LC increased NPo by 180%, whereas 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol raised NPo by 40%. Thus, the alpha-hydroxyl and the cis A-B ring junction are both required for robust channel potentiation. Lacking both features, 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol were inactive. Three-dimensional structures show that only LC displays a bean shape with clear-cut convex and concave hemispheres; 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol partially matched LC shape, and 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol did not. Increasing polarity in steroid rings (5beta-cholanic acid-3alpha-sulfate) or reducing polarity in lateral chain (5beta-cholanic acid 3alpha-ol methyl ester) rendered poorly active compounds, consistent with steroid insertion between beta(1) and bilayer lipids, with the steroid-charged tail near the aqueous phase. Molecular dynamics identified two regions in beta(1) transmembrane domain 2 that meet unique requirements for bonding with the LC concave hemisphere, where the steroid functional groups are located.

Differential effects of estrogen and progesterone on potassium channels expressed in Xenopus oocytes

HYPOTHESIS

Potassium (K(+)) channel activation contributes in part to estrogen-mediated vasorelaxation. However, the underlying mechanism is still unclear. We hypothesize that estrogen increases K(+) currents via membrane-associated, non-genomic interaction and that steroid hormones have differential effects on different types of K(+) channels.

EXPERIMENTAL

Human large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) and human voltage-gated K(+) channels (K(V1.5)) were expressed in Xenopus oocytes, and K(+) currents elicited by voltage clamp were measured.

RESULTS

Both 17beta-estradiol and BSA-conjugated 17beta-estradiol increased the BK(Ca) current in a concentration-dependent manner and this effect was abolished by tetraethylammonium ions and iberiotoxin (putative BK(Ca) channel blockers). 17beta-estradiol-stimulated increase in the BK(Ca) current was unaffected by treatment with ICI 182,780 (classic estrogen receptor antagonist), tamoxifen (estrogen receptor agonist/antagonist), actinomycin D (RNA synthesis inhibitor), or cycloheximide (protein synthesis inhibitor). In contrast, progesterone reduced the BK(Ca) current in the absence or presence of NS 1619 (BK(Ca) channel activator). Progesterone also inhibited 17beta-estradiol-stimulated increase in the BK(Ca) current. Finally, progesterone but not 17beta-estradiol reduced the K(V1.5) current.

CONCLUSIONS

The present results show that 17beta-estradiol stimulates BK(Ca) channels without affecting K(V1.5) channels. This effect is ICI 182,780-insensitive and is likely mediated via a membrane-bound binding site. Progesterone inhibits both BK(Ca)- and K(V1.5)-encoded currents. The present results suggest that inhibition of K(+) channels may contribute in part to its reported antagonism against 17beta-estradiol-mediated vascular relaxation via BK(Ca) channels.

The second transmembrane domain of the large conductance, voltage- and calcium-gated potassium channel beta(1) subunit is a lithocholate sensor

Bile acids and other steroids modify large conductance, calcium- and voltage-gated potassium (BK) channel activity contributing to non-genomic modulation of myogenic tone. Accessory BK beta(1) subunits are necessary for lithocholate (LC) to activate BK channels and vasodilate. The protein regions that sense steroid action, however, remain unknown. Using recombinant channels in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylserine bilayers we now demonstrate that complex proteolipid domains and cytoarchitecture are unnecessary for beta(1) to mediate LC action; beta(1) and a simple phospholipid microenvironment suffice. Since beta(1) senses LC but beta(4) does not, we made chimeras swapping regions between these subunits and, following channel heterologous expression, demonstrate that beta(1) TM2 is a bile acid-recognizing sensor.

Beta1 (KCNMB1) subunits mediate lithocholate activation of large-conductance Ca2+-activated K+ channels and dilation in small, resistance-size arteries

Among the nongenomic effects of steroids, control of vasomotion has received increasing attention. Lithocholate (LC) and other physiologically relevant cholane-derived steroids cause vasodilation, yet the molecular targets and mechanisms underlying this action remain largely unknown. We demonstrate that LC (45 microM) reversibly increases the diameter of pressurized resistance cerebral arteries by approximately 10%, which would result in approximately 30% increase in cerebral blood flow. LC action is independent of endothelial integrity, prevented by 55 nM iberiotoxin, and unmodified by 0.8 mM 4-aminopyridine, indicating that LC causes vasodilation via myocyte BK channels. Indeed, LC activates BK channels in isolated myocytes through a destabilization of channel long-closed states without modifying unitary conductance. LC channel activation occurs within a wide voltage range and at Ca2+ concentrations reached in the myocyte at rest and during contraction. Channel accessory beta1 subunits, which are predominant in smooth muscle, are necessary for LC to modify channel activity. In contrast, beta4 subunits, which are predominant in neuronal tissues, fail to evoke LC sensitivity. LC activation of cbv1+beta1 and native BK channels display identical characteristics, including EC50 (46 microM) and Emax (approximately 300 microM) values, strongly suggesting that the cbv1+beta1 complex is necessary and sufficient to evoke LC action. Finally, intact arteries from beta1 subunit knockout mice fail to relax in response to LC, although they are able to respond to other vasodilators. This study pinpoints the BK beta1 subunit as the molecule that senses LC, which results in myocyte BK channel activation and, thus, endothelial-independent relaxation of small, resistance-size arteries.

Cellular and molecular effects of unoprostone as a BK channel activator

Effects of unoprostone isopropyl (unoprostone), a prostaglandin metabolite analog; latanoprost, a PGF(2alpha) analog; and PGF(2alpha) were examined in HCN-1A cells, a model system for studies of large conductance Ca(2+) activated K(+)(BK) channel activator-based neuroprotective agents. Unoprostone and latanoprost, both used as anti-glaucoma agents, have been suggested to act through FP receptors and have neuroprotective effects. Ion channel activation, plasma membrane polarization, [Ca(2+)](i) changes and protection against long-term irreversible glutamate-induced [Ca(2+)](i) increases were studied. Unoprostone activated iberiotoxin (IbTX)-sensitive BK channels in HCN-1A cells with an EC(50) of 0.6+/-0.2 nM and had no effect on Cl(-) currents. Unoprostone caused IbTX-sensitive plasma membrane hyperpolarization that was insensitive to AL8810, an FP receptor antagonist. In contrast, latanoprost and PGF(2alpha) activated a Cl(-) current sensitive to [Ca(2+)](i) chelation, tamoxifen and AL8810, and caused IbTX-insensitive, AL8810-sensitive membrane depolarization consistent with FP receptor-mediated Ca(2+) signaling Cl(-) current activation. Latanoprost and PGF(2alpha), but not unoprostone, increased [Ca(2+)](i). Unoprostone, PGF(2alpha) only partially, but not latanoprost protected HCN-1A cells against glutamate-induced Ca(2+) deregulation. These findings show that unoprostone has a distinctly different mechanism of action from latanoprost and PGF(2alpha). Whether unoprostone affects the BK channel directly or an unidentified signaling mechanism has not been determined.

The Antiestrogen Tamoxifen Activates BK Channels and Stimulates Proliferation of MCF-7 Breast Cancer Cells

In the present study, we investigated the effect of the antiestrogen compound tamoxifen on BK channels by the use of the patch-clamp technique. The perfusion of 10 nM tamoxifen significantly increased the magnitude of a voltage-dependent K+ current by 22.6 +/- 10.6% (n = 23). The effect of tamoxifen was always obtained in the first minute, peaked at 5.9 +/- 2.2 min (n = 23), and was abolished by the perfusion of tetraethylammonium (0.5 mM), charybdotoxin (50 nM), or iberiotoxin (100 nM). The stimulatory effect of 10 nM tamoxifen was the same at low (50 nM) and high (700 nM) internal calcium concentration and was not additive to that of 17-beta-estradiol (E2) or its membrane-impermeant form, beta-estradiol 6-(O-carboxymethyl)oxime:bovine serum albumin. Furthermore, the effect of tamoxifen was still recorded in the presence of the selective estrogen receptor antagonist faslodex (ICI-182,780; 1 microM). At the single-channel level, tamoxifen significantly increased the open probability of the BK channel by 46.2 +/- 10.1% (n = 4) without changing its unitary conductance. Moreover, we show here that the stimulation of BK channel activity by tamoxifen is involved in MCF-7 cell proliferation. Taken together, these results permitted us to identify the BK channel as the molecular target of tamoxifen that probably acts at the same extracellular molecular level as E2. The site of action of tamoxifen is probably the channel itself or the auxiliary beta subunits.

Compounds Structurally Related to Tamoxifen as Openers of Large-Conductance Calcium-Activated K+ Channel

We found that a variety of compounds containing partial structures of tamoxifen showed activity as chemical modulators of large-conductance calcium-activated K+ channels (BK channels).