Molecular Mechanisms of Binge Drinking-Induced Atrial Fibrillation

Alcohol is an established risk factor for atrial fibrillation (AF).¹ Moderate, habitual consumption increases the incidence of AF in a dose-dependent manner.² Holiday heart syndrome, characterized by paroxysmal AF following excessive alcohol intake, occurs in regular drinkers, but also in infrequent and nondrinkers after a binge.² Many of such patients develop AF at the time of intoxication, but others present 12–36 h later.² Although human and animal studies have reported that alcohol acutely causes shortening of the atrial refractory period and slow conduction that favor reentry,² the mechanisms and underlying molecular bases of binge drinking-induced AF, particularly in the subacute phase, are largely unknown.

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The T-type Ca-channel Ca_V3.1/3.2 expressed greatly in the cardiac conduction system of adult hearts, including the sinoatrial node and cardiac Purkinje fibers, contributes to the genesis of pacemaker activity. In this issue of the Journal, Wang et al³ create a rat model of binge drinking-induced AF and demonstrate that ethanol caused an increase in the T-type Ca²⁺-current (I_Ca-T) through upregulation of Ca_V3.1/3.2 mRNA, the changes being greater in the pulmonary veins (PVs) than in the atria. At 8 h after ethanol injection in rats, AF inducibility was increased and the isolated PV cardiomyocytes had enhanced automaticity and abbreviated action potential duration (APD). Moreover, using neonatal rat cardiomyocytes cultured in a medium including ethanol for 24 h, Wang et al³ found that ethanol caused an increase in Cav3.1/3.2 transcription by activating protein kinase cascades related to cardiac hypertrophy. Ethanol stimulated protein kinase C (PKC), which phosphorylates and inactivates glycogen synthase kinase-3β (GSK3β), a counter-regulator of calcineurin/nuclear factor of activated T-cell (NFAT) signaling. The cultured cardiomyocytes showed GSK3β inhibition and subsequent NFAT translocation into the nucleus, leading to release of Cav3.1/3.2 gene repression. These findings, together with the result showing that AF was not inducible immediately after ethanol injection, suggest involvement of electrical and structural remodeling in association with activated PKC/GSK3β/NFAT signaling and I_Ca-T channel upregulation.³

Another paper by Ai’s group published in 2018 reported that binge drinking activates c-Jun N-terminal kinase (JNK) and subsequently phosphorylates Ca²⁺/calmodulin-dependent protein kinase-II (CaMKII), enhancing sarcoplasmic reticulum (SR) Ca²⁺-mishandling and thereby delayed afterdepolarization (DAD) facilitation in a set of experiments using human atria, alcohol-exposed rabbits and mice (including wild-type mice treated with a JNK1/2 inhibitor and transgenic mice overexpressing inactive cardiac-specific JNK1/2 proteins).⁴ JNK is a well-characterized stress response kinase that is activated in response to various cellular stresses (including ultraviolet light, ischemia, inflammatory cytokines and aging) and in cardiovascular diseases (ischemic heart disease, cardiac hypertrophy and heart failure).⁴ CaMKII is an established proarrhythmic signaling molecule that is activated by intracellular Ca²⁺-overlord, oxidative stress and inflammatory cytokines.^1,5 CaMKII-mediated hyperphosphorylation of SR Ca²⁺-release channel ryanodine receptor type-2 makes it leaky. The resulting diastolic Ca²⁺-elevation coupled with Na⁺/Ca²⁺-exchanger causes DADs, leading to atrial ectopy.^1,5

There is literature on the acute effects of alcohol. Alcohol shortens the effective refractory period and slows intra- and interatrial conduction, causing a shortened wavelength that facilitates reentry in humans.² Ethanol exposure causes ion-channel current changes in animals: reduced L-type Ca²⁺ (I_Ca-L)- and Na⁺-current densities in rabbit atrial myocytes,² increased transient outward K⁺-current in rabbit PV cardiomyocytes,⁶ and increased atrial-selective acetylcholine-sensitive inward-rectifier K⁺ current (I_K,ACh) in rat and guinea-pig atrial myocytes.⁷ However, electrophysiological changes persisting during the washout or hangover period have not been characterized. The current work by Wang et al³ and the article by Ai’s group⁴ have respectively found that the signaling molecules PKC/GSKβ/NFAT and JNK/CaMKII connect from alcohol bingeing to AF-related mechanisms.

Mechanisms of Binge Drinking-Induced AF

The development of AF involves ectopic activity and a reentrant substrate.^1,5 The Figure illustrates potential mechanisms of binge drinking-induced AF. Excessive alcohol causes I_Ca-T channel upregulation via PKC/GSKβ/NFAT signaling³ and aberrant SR Ca²⁺-handling by activating JNK/CaMKII,⁴ both of which promote focal activity. Theses signaling molecules also activate targets participating in a substrate for AF-promoting reentry. NFAT nuclear translocation leads to downregulation of the I_Ca-L channel Ca_V 1.2 in canine atrial myocytes.^5,8 In parallel, NFAT suppresses the production of microRNA (miR)-26 that negatively regulate the transcription of KCNJ2 encoding the inward-rectifier K⁺ (I_K1)-channel Kir2.1.^5,8 The reduction of miR26 leads to removal of miR-26-induced destabilization of the KCNJ2 mRNA and thereby increases its translation, resulting in Kir2.1 upregulation. Reduced I_Ca-L and enhanced I_K1, together with an increase in constitutively active acetylcholine-sensitive K⁺-current (I_K,ACh), are hallmarks of the electrical remodeling that is often observed in AF patients and atrial tachypaced dogs,^5,8 leading to acceleration of repolarization (effective refractory period abbreviation) and resting membrane potential hyperpolarization, both of which stabilize AF-maintaining rotors.

Figure.

Potential mechanisms of binge alcohol-induced atrial fibrillation (AF). The determinants of AF-related mechanisms with the related cell signaling molecules (in boxes). CaMKII, Ca²⁺/calmodulin-dependent protein kinase-II; Cx40/43, connexin-40/43; GSK3β, glycogen synthase kinase-3β; I_Ca-L, L-type Ca²⁺-current; I_Ca-T, T-type Ca²⁺-current; I_K1, inward rectifier K⁺-current; I_K,ACh, acetylcholine-sensitive K⁺-current; IL-6, interleukin-6; JNK, c-Jun N-terminal kinase; NFAT, nuclear factor of activated T-cells; NLRP3, NACHT, LRR, and PYD domain containing protein 3; PKC, protein kinase C; SR, sarcoplasmic reticulum.

The gap junction protein connexin (Cx) is an important determinant of cardiac conduction velocity and the principal isoforms expressed in atria are Cx43 and Cx40. Ai and colleagues recently reported that JNK is activated in aged rabbit left atria and that JNK activation causes a reduction in Cx43 mRNA expression by phosphorylating (activating) a transcription factor c-Jun.⁹ Connexin channel function and membrane expression and localization are regulated by phosphorylation. Because Cx43 and Cx40 have multiple phosphorylation sites for a variety of kinases, including PKC and CaMKII, altered phosphorylation of connexins is presumably involved in alcohol-induced conduction abnormalities in the atria and PVs.

Dephosphorylated GSKβ promotes NFAT nuclear translocation, and both NFAT and CaMKII are established mediators of cardiac hypertrophy.^10,11 Although the 2 studies did not provide any data on cell size or the dimensions of the left atrium in their animal models, cell hypertrophy may be an intermediate phenotype that links alcohol bingeing to AF.

Excessive alcohol intake induces myocardial inflammation. Cardiac magnetic resonance imaging 1 day after binge drinking in healthy volunteers demonstrates transient increases in ventricular T2-signal intensity representative of myocardial edema, and global relative enhancement consistent with hyperemia.¹² A recent study in cultured HL-1 atrial cells showed that interleukin-6 (IL-6) rapidly causes reversible downregulation of Cx43 and Cx40 and conduction slowing.¹³ These findings, together with the fact that there is a close association between inflammation and AF development,¹⁴ suggest that binge drinking has the potential to cause electrical remodeling by activating proinflammatory cytokines in normal atria. The notion is strongly supported by emerging evidence that the cardiomyocyte NLRP3 (NACHT, LRR, and PYD domain containing protein 3) inflammasome plays a causal role in the etiology of AF. The recent work by Li’s group¹⁵ has demonstrated that transgenic mice with constitutively active NLRP inflammasome have atrial ectopy associated with aberrant Ca²⁺-handling, as well as the substrate resulting from APD abbreviation due to increased atrial-selective ultra-rapid delayed rectifier K⁺-current (I_Kur) and I_K,ACh. The transcription of RYR2, KCN5 (encoding I_Kur channel Kv1.5) and GIRK1/4 (encoding I_K,ACh channel Kir3.1/3.4) is upregulated in the transgenic mice,¹⁵ although the mechanisms remain to be established.

The articles by Wang et al³ and Ai’s group⁴ advance our understanding of the mechanisms of binge drinking-induced AF. Wang et al made great efforts to elucidate the signaling pathway related to I_Ca-T channel upregulation, but it remains unclear whether and how I_Ca-T channel upregulation affects the substrate for AF maintenance. Future studies to shed light on the issue might strengthen the signature of the I_Ca-T channel in AF-related mechanisms.

Acknowledgment

The author’s work is supported by funds from Japan Society for the Promotion of Science (20K08450 and 18H02802).

IRB Information / Disclosures

None.

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