There is increasing evidence that dietary FFAs are a critical and independent predictor of metabolic disorders including insulin resistance, T2D and obesity (Kien et al., 2005; Park and Goldberg, 2012), and related cardiac dysfunction (Haim et al., 2010; Shao et al., 2013; O’Connell et al., 2015; Aromolaran et al., 2016; Anumonwo and Herron, 2018). This association underscores the importance of studies that provide vigorous and comprehensive molecular insights into the structural determinants of the functional properties of FFA as well as FFA-activated signaling pathways in heart. FFAs are characterized by a straight chain of carbon atoms with both a carboxylic (COOH) and a methyl (CH₃ or omega, ω) end (Rennison and Van Wagoner, 2009) and are generally classified based upon the level of saturation on the carbon atoms. Accordingly, FFAs are classified into three main groups: (1) saturated fatty acids (SFAs) that do not contain double bonds (C16:0 and C18:0), (2) monounsaturated fatty acids (MUFAs), that contain only one double bond (C18:1), and (3) polyunsaturated fatty acids (PUFAs), that contain at least two double bonds (Huang et al., 2018). PUFAs, are divide into two classes namely: ω-3 and ω-6, based on the position of the first double bond relative to the ω end.

The anti-arrhythmic or cardioprotective effects of PUFAs, especially in patients with dyslipidemia, have been studied extensively (Rimm et al., 2018; Schmocker et al., 2018; Schunck et al., 2018; Zhang et al., 2018). Billman and others have shown that the omega-3 PUFA eicosapentaenoic acid prevented ischemia-induced ventricular fibrillation in a dog model of sudden cardiac death (Billman et al., 1999), suggesting modulation by PUFAs of cardiomyocyte electrical activity (Bogdanov et al., 1998; Xiao et al., 2004; Blondeau et al., 2007; Leaf, 2007; Moreno et al., 2012). Some of the molecular mechanisms that may underlie the cardioprotective effects of PUFAs include effects on channel gating (Elinder and Liin, 2017) and membrane properties (or electrostatics) (Borjesson et al., 2010; Borjesson and Elinder, 2011; Liin et al., 2015), and have been comprehensively reviewed elsewhere (Leaf et al., 2003).

If the relative composition of FFA content in the heart can influence inflammatory responses, and affect cardiac dysfunction, then dietary PUFAs may prevent pathological levels of pro-inflammatory cytokines (Wen et al., 2011; Oikonomou et al., 2018), and cardiac dysfunction in patients with metabolic disorders. Moreover, PUFAs have been shown to decrease secretion of TNF-α, IL-1β, and IL-6 through a pathway involving M2 anti-inflammatory macrophages (Lyons et al., 2016). However, the molecular partners involved are unknown, and therefore the mechanisms are poorly understood. Future studies are needed to investigate the differences between inflammatory pathways activated by the cardioprotective PUFAs and the generally more damaging SFFAs (M1 anti-inflammatory macrophages) (Lyons et al., 2016), and whether further differences would be seen with acute versus chronic activation of these pathways.

Saturated long chain FFAs, particularly palmitic acid (PA, 16:0), which is one of the predominant FFAs in epicardial fat (Iacobellis and Bianco, 2011), are considered to be a more prominent contributor to systemic lipotoxicity compared to long chain monounsaturated FFAs such as oleic acid (OA) (van der Lee et al., 2000; Listenberger et al., 2003; Kien et al., 2005). Previously we demonstrated that exogenous application of PA conjugated with bovine serum albumin (BSA) shortened atrial action potential (AP) duration (APD), measured in guinea pig atrial myocytes, while OA prolonged atrial APD (Aromolaran et al., 2016). Similarly, Anumonwo’s group found that a short-term exposure to the saturated stearic acid caused both structural and electrical remodeling of atrial myocytes isolated from sheep (O’Connell et al., 2015), consistent with pathological cardiomyocyte remodeling. Notably, the American Heart Association (AHA) has reported a significant association between dietary FFA and the pathogenesis of a variety of cardiovascular diseases (Krauss et al., 2000). The 2013 AHA/American College of Cardiology (ACC) Guideline on Lifestyle Management to Reduce Cardiovascular Risk recommends that patients with elevated low-density lipoprotein (LDL)- cholesterol decrease the intake of dietary saturated fat to 5–6% of the total daily caloric intake (Eckel et al., 2014).

Despite these guidelines, diet-related diseases such as obesity, T2D and insulin resistance and associated cardiac dysfunction (Kien et al., 2005; Ashrafi et al., 2017; Valli et al., 2017; Sanchez et al., 2018) are still widespread. This suggests that other mechanisms or pathways are involved, such as inflammation and cytokine release, which are activated by SFFAs. Accordingly, the anti-inflammatory effects of current therapeutic interventions (statins or APOA1) (Goonasekara et al., 2010; Shapiro and Fazio, 2016) are promising and are likely to inform future studies.

The mechanisms of FFA toxicity are complex, involving multiple combinations of distinct signaling pathways (Rennison and Van Wagoner, 2009; Park and Goldberg, 2012), suggesting the need to expand our understanding of how the interplay of these pathways lead to a diseased state. In this review we provide insights on the relatively unexplored interplay between cardiac lipotoxicity (mediated by SFFAs) and inflammatory pathways (macrophages, toll-like receptors, proinflammatory cytokines) that impair ion channel function, leading to cardiac electrical activity and conduction abnormalities. Our expectation is that understanding the cardiac-specific inflammatory pathways may facilitate the design and rational development of effective therapeutic interventions. These mechanisms are discussed below.

Over the past 40 years we have been able to establish a role for SFFAs in inflammation (Ajuwon and Spurlock, 2005; Bradley et al., 2008; Bunn et al., 2010; Guzzardi and Iozzo, 2011; Huang et al., 2012; Wang et al., 2013; Schilling et al., 2013; Haffar et al., 2015). Cardiac or systemic inflammation (or dysregulation of the innate immune system) is thought to be a function of the body’s non-specific response to injury (Pohl and Benseler, 2013). In obese and diabetic patients, the increased rates of infection and poor wound healing have been associated with immune cell dysfunction (Joshi et al., 1999; Ferrucci and Fabbri, 2018; Frydrych et al., 2018; Jin et al., 2018; van Niekerk and Engelbrecht, 2018). Importantly, macrophages are associated with heightened immune responses to infectious pathogens and tissue damage in diabetes (Mirza et al., 2009; Mirza and Koh, 2011; Das et al., 2018; Liu et al., 2018) and obesity (Lopez-Pascual et al., 2018; Ramos Muniz et al., 2018; Ding et al., 2018).

Given that FFAs are important adipocyte-derived mediators of macrophage related inflammation (Suganami et al., 2005) and macrophages can infiltrate and/or directly couple with cardiomyocytes (Figure 1; Hulsmans et al., 2016; Sager et al., 2016), then macrophages may be important mediators of SFFA effects on cardiac electrical remodeling (Wang et al., 2017, 2018). Distinct voltage-dependent K channels (VGKC) including ether-á-go-go related gene 1 (ERG1), (Dong et al., 2013), inward rectifier (Vicente et al., 2003; Moreno et al., 2013), and shaker-related or K_v1.3 (Vicente et al., 2003; Park et al., 2006; Villalonga et al., 2010) are found in macrophages. In addition to their role in controlling cardiac repolarization and resting membrane potential (Grandi et al., 2017; Jeevaratnam et al., 2018), VGKC are also involved in macrophage functions (activation, migration, proliferation) (Vicente et al., 2003). Because altered functions of macrophages could have important implications for proinflammatory cytokine release and arrhythmias, future studies will have to characterize the biophysical effects of VGKC localized to macrophages more precisely and determine whether impaired cardiac electrical activity includes altered macrophage ion channel function. For example, does the modulatory signaling pathways (including protein kinases, protein phosphatases, trafficking, anchoring proteins, and posttranslational modifications) that regulate cardiac VGKC subunit expression also regulate these channels in macrophages? These studies are a prerequisite for development of new therapeutics which target inflammatory pathways in lipotoxic-related disorders.

SFFAs have been shown to stimulate an inflammatory response by acting on TLRs present on macrophages (Figure 1). TLRs are pattern recognition receptors that play an important role in the body’s innate immune response (Takeda et al., 2003; Schilling et al., 2013). Thus, if inflammation is a hallmark of lipotoxicity (Ertunc and Hotamisligil, 2016; Ralston et al., 2017), then the role of SFFAs as an activator of TLRs must be understood because they initiate inflammation.

Structurally, TLRs are characterized by three distinct domains, namely: (1) an extracellular leucine-rich repeat (LRR) domain, which is required and necessary for ligand binding and the recognition of pathogen-associated molecular patterns (PAMPs); (2) a transmembrane domain important for receptor localization to the surface (TLR1, TLR2, TLR4, TLR5, TLR6) and intracellular membranes (TLR3, TLR7, TLR8, TLR9); 3) a cytoplasmic conserved toll/interleukin-1 receptor (TIR) domain, which plays a role in the activation of NF-κB, and cytokine secretion (Figure 2; Fuentes-Antras et al., 2014). TLR2 and TLR4 have been widely studied and have both been shown to play a role in the pathogenesis of lipid disorders like atherosclerotic cardiovascular disease (Curtiss and Tobias, 2009), insulin resistance (Devaraj et al., 2008; Wong and Wen, 2008; Kim et al., 2010; Dong et al., 2012), and obesity (Kim et al., 2007; Ghanim et al., 2017).

SFFAs have been shown to promote both TLR4-dependent and TLR2-dependent signaling in multiple cell models. For example, Lee et al. (2001, 2004), using RAW 264.7 macrophages, demonstrated that the saturated lauric acid (C12:0), signaled via: (1) TLR4-myeloid differentiation primary response 88 (MyD88) to activate NF-κB; (2) TLR4-Toll-IL-1 receptor (TIR)-domain-containing adapter-inducing interferon-β (TRIF), leading to the activation of interferon-stimulated regulatory element. Furthermore, endogenous SFFAs released from adipocytes have also been shown to activate co-cultured macrophages via TLR4 (Suganami et al., 2007), demonstrating an important crosstalk in adipose tissue, with implications for lipotoxicity and cardiac electrical remodeling. Similarly, TLR2 has been shown to mediate palmitate-induced insulin resistance in C2C12 myoblasts (Senn, 2006). Endothelial overexpression of TLR2 led to early development of atherosclerotic processes in the aorta of LDLr^-/- mice (Mullick et al., 2008), while the downregulation of TLR2 protected against atherosclerosis in LDLr^-/- mice (Mullick et al., 2005). Others have also shown similar effects of TLR2-deficiency in apoE^-/- mice (Liu et al., 2008; Madan and Amar, 2008). Reduced TLR4 expression and function protected against insulin resistance in a mouse model of systemic lipid infusion (Shi et al., 2006), demonstrating a role for TLRs in lipotoxic disorders (Bashir et al., 2016; Shen et al., 2018). Flier and colleagues also found that female C57BL/6 mice lacking TLR4 and fed a high fat diet developed increased obesity, but are partially protected from insulin resistance through a mechanism involving reduced inflammatory gene expression (including IL-6) (Shi et al., 2006).

T2 diabetic mice with mutated TLR4 prevented endothelial cell dysfunction, hyperglycemia and hypertension when compared with wild-type. These effects were largely due to suppression of oxidative stress signaling molecules nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 and 4 (Liang et al., 2013). Furthermore, mice lacking either TLR4 or its downstream adapter protein MyD88 are protected against atherosclerosis (Bjorkbacka et al., 2004; Michelsen et al., 2004; Ding et al., 2012). In agreement, humans with TLR4 mutations, which lead to reduced receptor signaling and depressed inflammatory response, are also less susceptible to atherosclerosis (Arbour et al., 2000; Kiechl et al., 2002; Lin et al., 2012). Schwartz’s group (Kim et al., 2007) also found that thoracic aortic samples from TLR4 knockout mice (TLR4^-/-) fed a high-fat diet did not develop vascular inflammation or insulin resistance.

TLR4 are also expressed in normal myocardium (Vaez et al., 2016), suggesting that it may play a role in cardiac function. Moreover, increased TLR4 expression has been reported in failing myocardium (Frantz et al., 1999), and isolated cardiomyocytes from humans and animal models of different cardiomyopathies (Dange et al., 2014; Avlas et al., 2015; Liu et al., 2015), including myocarditis (Timmers et al., 2008). These findings provide strong hints that blocking TLR4 may be a promising cardioprotective target in patients. Whether and how effective modulation of the TLR4 pathway is as a therapeutic target is yet to be determined.

Activated TLR4 and myeloid differentiation protein-2 (MD-2) signals through both the MyD88-dependent and MyD88-independent pathways (Figure 2; Lee et al., 2003; Gao et al., 2017). The MyD88-dependent pathway is initially triggered by the recruitment of the TIR domain containing adaptor protein (or TIRAP), an adaptor between the TIR domain of TLR4 and MyD88 (Sakaguchi et al., 2011). This is followed by activation of the interleukin-1 receptor-associated kinase (IRAK) 4 family of protein kinases which induces IRAK1/2 phosphorylation and the subsequent phosphorylation of TNF Receptor Associated Factor 6 (TRAF6) (Vilahur and Badimon, 2014).

However, with the MyD88-independent pathway, the TLR4 pathway is activated through a TIR-containing adapter molecule 1 (TRIF)-related adaptor molecule TRAM. This allows the recruitment of TRIF and subsequent activation of TRAF6 (Figure 2). TRAF6, either directly, or via thylakoid arabidopsis kinase (TAK) 1, stimulates the inhibitor of nuclear factor-κB kinase (IKK) complex, which promotes phosphorylation of the inhibitor of NF-κB (IkB). In the resting state of a cell, IkB exists in a complex with NF-κB, which limits its spatial localization to cytosol. Phosphorylated IkB is ubiquitinated, dissociates from NF-κB, and is then subjected to proteasomal degradation. Free NF-κB (p50/p65 heterodimer) translocate to the nucleus and binds to κB sites in the promoter regions of genes involved in the secretion of inflammatory cytokines which includes TNF-α and IL-6 (Youssef-Elabd et al., 2012). Furthermore, the p50/p65 heterodimer may undergo a series of cellular protein modifications including phosphorylation, acetylation, and methylation with implications for a finely-tuned regulation of transcriptional activity of pro-inflammatory cytokines. Moreover, NF-κB transcriptional activity may also be regulated by secreted pro-inflammatory cytokines through a positive and/or negative feedback mechanism depending on the pathological state of the cell.

NF-κB-mediated cytokine release and inflammation may also be augmented through adenosine triphosphate (ATP) via purinergic signaling (Figure 1; Dosch et al., 2018; Lee et al., 2018). In pathological conditions, ATP from cells pass through hemichannels to activate purinergic receptors, leading to an amplification of inflammation (Mezzaroma et al., 2011). Moreover inhibition of the ATP receptor P2X7 prevented cardiac dysfunction in a mouse model of acute myocardial infarction (Mezzaroma et al., 2011) and LPS-primed naive rats (Yin et al., 2017). P2X7 deficiency inhibited inflammasome activation and reduced atherosclerosis in P2X7^-/- mice (Stachon et al., 2017). Therefore, activation of connexins and P2X7 receptors (P2X7R) signaling pathways represent emerging targets that can be explored in lipotoxic cardiomyopathies.

The role of NF-κB as a key transcription factor critical for regulation of cardiac inflammatory signaling pathways strongly suggests its involvement in cardiac remodeling leading to pathogenesis of heart failure (Zhou et al., 2009; Santos et al., 2010; Schreiber et al., 2011). Previous reports have shown that NF-κB is activated in the failing human heart (Wong et al., 1998; Gupta and Sen, 2005; Kawamura et al., 2005). Further, in vitro studies have also demonstrated that activation of NF-κB plays a role in hypertrophic growth of primary rat neonatal ventricular cardiomyocytes in response to angiotensin II, phenylephrine, and endothelin-1 (Purcell et al., 2001). Recent studies have also demonstrated that NF-κB plays a partial role in the depression of the fast transient outward K current (I_to,f) caused by chronic β-adrenergic receptor stimulation in cultured neonatal rat ventricular myocytes (Panama et al., 2011, 2016). In these studies NF-κB effects were attributed to a downregulation of the pore-forming (K_v4.3) and regulatory/auxiliary (KChIP2) subunits of I_to,f (Panama et al., 2011). The functional consequence of NF-κB modulation may be due to transmural changes in I_to,f in the ventricles and possibly the atria. These effects further highlight an emerging role for NF-κB as a substrate for cardiac dysfunction in lipotoxicity.

In a study by Kawamura and others, the blockade of NF-κB did not ameliorate myocardial inflammation, but significantly improved cardiac function and survival in a transgenic mice model with cardiac overexpression of TNF-α (Kawamura et al., 2005). Recently, a similar mechanism has been proposed for TRAF1, an inhibitory adapter of TLRs (Anto Michel et al., 2018). It was found that downregulation of macrophage-resident TRAF1 induced increased expression of inflammatory genes and protected against metabolic dysfunction in mice. Therefore, it is possible that activation of the non-canonical NF-κB/TRAF1 pathway (Choudhary et al., 2013), may at least, in part, explain the effects of TNF-α described by Sunagawa’ group (Kawamura et al., 2005).

The studies of Wolf and others also showed a convincing correlation between higher TRAF1 expression, body mass index, and fasting plasma lipid in patients with severe metabolic syndrome (Anto Michel et al., 2018). Albeit interesting, the implications of these findings on cardiac function were not investigated. To reconcile existing knowledge of the relationship between metabolic disorders, inflammation and heart failure, we will need additional studies related to the spatial and temporal effects of TRAF1, inflammatory responses and effects on cardiac remodeling.

Nonetheless, we speculate that NF-κB activation may contribute to cardiac dysfunction independent of macrophage-related inflammation. Therefore, blockade of NF-κB leading to cardiac electrical remodeling may be a novel therapeutic strategy for cardiac diseases worth investigating. However, little is known about the effects of NF-κB inhibition in patients with metabolic disorders or any animal models of cardiac lipotoxicity and metabolic disorders including T2D, insulin resistance, and cardiac inflammation.

A major signaling pathway for SFFA-mediated inflammatory response and cardiac electrical dysfunction may involve the following steps: (1) infiltration of cardiac macrophages into cardiomyocytes (2) increased functional expression and activation of TLR; (3) activation of NF-κB; (4) altered secretion of pro-inflammatory cytokines; and (4) ion channel remodeling (Figure 3). Recently the significance of this pathway was demonstrated in a LIPGENE cohort study (Cruz-Teno et al., 2012), wherein NF-κB was found to regulate TNF-α release during postprandial period (Lefebvre and Scheen, 1998). Because the activation of this pathway may also depend on the amount and type of dietary fat, the potential of this pathway as an intervention strategy in overweight and obese patients with cardiac dysfunction deserves further investigation.

The TNF-α signaling pathway and its role in cardiac ion channel remodeling leading to cardiac dysfunction in animal models has been extensively studied (London et al., 2003; Wang et al., 2004; Hatada et al., 2006; Kawada et al., 2006; Petkova-Kirova et al., 2006; Fernandez-Velasco et al., 2007; Grandy and Fiset, 2009). However, the functional consequence of the relative contribution of each one of the steps (SFFAs/TLR4/NF-κB) involved in TNF-α production and its subsequent regulation of cardiac function in relevant animal models of cardiac lipotoxicity is poorly understood. In contrast, it is known that the stimulatory effects of distinct SFFAs (lauric, palmitic and stearic acids) cause increased IL-6 functional expression in macrophages via TLR4 activation (Shi et al., 2006). Despite the role of IL-6 as a cardiovascular risk indicator (Lazzerini et al., 2017a), its function as modulator of cardiac electrical remodeling with implications for the pathogenesis of arrythmias is only beginning to be understood. In this review, we focus on the pathophysiology of the IL-6 signaling pathway and how its modulation may reveal crucial insights that will inform future lipid studies (Table 1).

IL-6 is a pleiotropic cytokine that is involved in a variety of biological effects that occur in cells of the immune system and also cardiomyocytes in response to injury (Ancey et al., 2002; Yang et al., 2004). IL-6 mediates its effects either through its membrane-bound receptor, IL-6R alpha (α) subunit (classical signaling) or the soluble receptor (sIL-6R) (Taga and Kishimoto, 1997; Fontes et al., 2015), in complex with the signal transduction protein glycoprotein 130 (gp130) leading to the activation of the janus kinase (JAK)-related signaling pathways (Akira et al., 1990, 1994; Naka et al., 1997; Taga and Kishimoto, 1997; Figure 3).

Cardiac arrhythmias are irregular variations from the normal cardiac sinus rhythm due to conduction abnormalities, or electrical impulses in the heart. It is well established that the abnormal electrical activity that predispose to arrhythmias is commonly due to dysfunction and/or structural disruption of the electrical conduction system of the heart and can be classified based on their pathogenesis. In particular, the underlying molecular mechanisms of IL-6 in the pathogenesis of ventricular and supraventricular arrhythmias are poorly understood. In this review we highlight current reports of IL-6 involvement in the pathogenesis of cardiac dysfunctions associated with increased vulnerability to fatal ventricular arrhythmias (long QT syndrome or LQTS) (Schwartz et al., 2009; Beitland et al., 2014) or increased morbidity (atrial fibrillation or AF) and mortality (Table 1). Our hope is that studies that distinguish between the molecular mechanisms of IL-6 contribution to LQTS and AF may reveal important mechanistic insights that will inform targeted therapeutic interventions that will be beneficial to all patients and help improve quality of life.

The electrical activity of the human heart is controlled by the coordinated action of ion channels localized to distinct compartments (surface sarcolemma, t-tubular system, intercalated disk), within the myocardium. The temporal and biophysical properties of individual ionic channels allow the exchange of ions between distinct compartments and are responsible for generation of an AP in individual cardiomyocytes. The normal cardiac AP is defined by 5 distinct phases, namely: phase 0 or phase of rapid depolarization, due to a large inward Na current, I_Na followed by currents due to voltage-gated L-type Ca (I_Ca,L) and the sodium-calcium exchanger (I_NCX) channels (Bers and Despa, 2009); phase 1 or phase of early repolarization controlled by the transient outward K current, I_to; phase 2 or plateau phase accomplished by a balance between the depolarizing Ca current (I_Ca,L) and the repolarizing rapidly (I_Kr) and slowly (I_Ks) activating component of the delayed rectifier K currents. The atria specific ultra-rapidly (I_Kur) activating delayed rectifier K current controls repolarization (Tian et al., 2006) and underlies the triangular signature of the atrial AP (Ford et al., 2013); phase 3 (or phase of final repolarization) represents the inactivation of I_Ca,L and is therefore predominantly regulated by the delayed rectifier K currents and the inwardly rectifying K current (I_K1) (Varro et al., 1993). While extensive reviews about the pathophysiology of major cardiac ionic channels have recently been published (Aromolaran and Boutjdir, 2017; Dan and Dobrev, 2018; Heijman et al., 2018; Rahm et al., 2018), the importance of ion channel function to normal cardiac rhythm is exemplified in a variety of inherited and acquired pathological conditions. For example, in LQTS decreases in outward currents (Aromolaran et al., 2014; Puckerin et al., 2016) or increases in depolarizing mechanisms (Wehrens et al., 2003; Fredj et al., 2006; Cheng et al., 2011; Hsiao et al., 2013), predispose to fatal ventricular arrhythmias (such as TdP) (El-Sherif et al., 2017; Muser et al., 2018; Wit, 2018) and sudden cardiac death (Anderson et al., 2019; Liu et al., 2018).

In the atria, remodeled biophysical properties of ionic channels (Aromolaran et al., 2016) serve as substrates for AF, including accelerated repolarization, spatial and temporal AP instabilities, atrial refractoriness, early and delayed afterdepolarizations, ectopic firing, and single/multiple wave re-entrant mechanisms (Nattel and Dobrev, 2017; Dan and Dobrev, 2018; Heijman et al., 2018). Collectively these observations reveal potential targets for modulation by SFFA pathways.

Our recently published data suggest that IL-6 may be a critical inflammatory signaling molecule contributing to TdP in patients with rheumatoid arthritis (Lazzerini et al., 2017b). This finding identifies IL-6 as a potential therapeutic target in arrhythmias. In the following sections we highlight the modulation by IL-6 of I_Na, I_Ca,L, and I_K currents critically involved in cardiac instabilities, that ultimately predisposes to lipotoxic cardiomyopathies.

The modulation of I_Na by IL-6 in cardiomyocytes is poorly understood; however, there is evidence for modulation of I_Na by other pro-inflammatory cytokines in other cell systems. The pro-inflammatory cytokine IL-2, which is also associated with arrhythmias, (Rizos et al., 2007) has been shown to increase the transcriptional levels of SCN3B leading to increased peak I_Na density in Hela and HL-1 cells (Zhao et al., 2016b) and suggests a role for cytokine modulation of I_Na functional expression in lipotoxicity. From the perspective of cardiac electrical remodeling, cytokine-mediated increases in I_Na density will be expected to delay cardiac repolarization leading to prolongation of the QT interval. Future IL-6 studies in native atrial and ventricular cardiomyocytes and animal models of lipotoxicity are critical for fundamental insights into the functional consequence of IL-6 modulation of Na current in metabolic disease-related arrhythmias.

Unlike I_Na, IL-6 has been shown to regulate I_Ca,L, density. Hagiwara and others (Hagiwara et al., 2007) demonstrated that acute (30 min) exposure to IL-6 and sIL-6R significantly increased I_Ca,L density in mouse ventricular myocytes in line with a role for IL-6 in LQTS. In guinea pig ventricular myocytes acute (5 min) exposure to IL-6 had no effect on I_Ca,L, but reversed the increased I_Ca,L due to sympathetic stimulation with isoproterenol (Sugishita et al., 1999). By contrast, chronic (2 h) exposure to IL-6 alone had no effect on I_Ca,L in adult rat ventricular myocytes (Yu et al., 2005) and therefore suggests temporal differences in IL-6 effects on channel function.

The Ca_v1.2 channel subunit mediates I_Ca,L current in both the atria and ventricles (Mancarella et al., 2008). Targeted deletion in mice has also demonstrated a functional role of the Ca_v1.3 isoform in the pathogenesis of AF (Zhang et al., 2005; Mancarella et al., 2008; Lu et al., 2015; Sun et al., 2017). Therefore, it will be interesting to investigate the differential regulation of Ca_v1.2/Ca_v1.3 by IL-6. With expression of Ca_v1.3 only in the atria we may be able to identify new pathways that could be targeted for arrhythmias in patients with metabolic disorders, without off-target ventricular effects.

In agreement with this notion, we have recently found that Ca_v1.3 expression is significantly downregulated in the atria of high-fat diet induced obese guinea pigs, while Ca_v1.2 expression remained essentially unchanged (Ademuyiwa S Aromolaran, personal communication, Biophysical meeting 2018, San Diego, CA, United States). Although the role of Ca_v1.3 in inflammation remains to be defined, the data suggests that Ca_v1.3 expression and regulation could be participating in arrhythmogenic responses to lipotoxicity.

IL-6 studies in mice ventricular myocytes showed that acute exposure (10 min) to IL-6 and sIL-6R significantly increased intracellular Ca transients (Hagiwara et al., 2007). This suggests a role for IL-6 in mediating arrhythmias through modulation of Ca-handling proteins. Yu et al. (2005) showed that IL-6 induced negative inotropy, decreased postrest potentiation, as well as responsiveness to the ryanodine receptor (RyR) agonist caffeine in adult rat ventricular myocytes. Similarly, acute exposure to IL-6 produced decreased peak systolic intracellular Ca concentration ([Ca²⁺]_i) and cell shortening leading to a negative ionotropic effect in guinea pig ventricular myocytes despite a lack of effect on I_Ca,L (Sugishita et al., 1999). Human recombinant IL-6 significantly decreased peak systolic [Ca²⁺]_i and the amplitude of cell contraction in cultured chick embryo ventricular myocytes (Kinugawa et al., 1994).

IL-6 may also directly regulate the activity of the sarcoplasmic reticulum Ca-ATPase (SERCA2). Previous reports showed that chronic (48 h) exposure of IL-6 caused a significant downregulation of SERCA2 gene and protein expression levels in cultured neonatal rat ventricular myocytes (Villegas et al., 2000), while a 6 h exposure to IL-6 significantly decreased SERCA2 expression in cultured rat ventricular myocytes (Tanaka et al., 2004), consistent with impaired sarcoplasmic reticular (SR) function and propensity for arrhythmias.

The regulation by IL-6 of intracellular Ca dynamics through modulation of RyRs or the inositol triphosphate receptor (IP₃R) is currently unknown. In the context of metabolic diseases, altered IL-6 mechanisms that decrease I_Ca,L density, reduce intracellular Ca transients and impair cardiac contractility are also likely to promote supraventricular arrhythmias (Mancarella et al., 2008). To our knowledge, the underlying molecular mechanisms of IL-6 modulation of Ca_v1.3 and Ca handling proteins in atrial myocytes are poorly understood and therefore warrants future studies. Nevertheless, the outcome of IL-6 studies in ventricular myocytes demonstrate that IL-6-mediated altered Ca handling proteins and subsequent inhibition of the SR function may contribute to impaired cardiac electrical activities in lipotoxicity.

Cardiac delayed rectifier K current, (or I_K) contributes prominently to normal repolarization (Sanguinetti and Jurkiewicz, 1990). The molecular partners of I_K, namely: I_Kr and I_Ks determine spatial and temporal activation and modulation of repolarization by I_K (Sanguinetti and Jurkiewicz, 1991; Aromolaran et al., 2014). Thus, inactivation and/or downregulation of I_K, either due to congenital mutations (Aromolaran et al., 2014; Puckerin et al., 2016), or secondary to pathological disease states including diabetes (Eranti et al., 2016), obesity (Papaioannou et al., 2003), or drugs (Shenthar et al., 2017; Grouthier et al., 2018), will delay repolarization leading to LQTS. Recently, we demonstrated that both gating and trafficking defects underlie pathological decreases in I_Kr and I_Ks in heart (Aromolaran et al., 2014; Puckerin et al., 2016).

A previous report by Wang et al. (2004), showed TNF-α mediated depression of the human ether-à-go-go-related gene (or hERG) current density in human embryonic kidney (HEK293) cells. Further, TNF-α significantly reduced I_Kr density and prolonged APD in canine ventricular myocytes, primarily though changes in reactive oxygen species (Wang et al., 2004). The effects of IL-6 on I_Kr and I_Ks currents as targets in its reported link to LQTS (Lazzerini et al., 2017a) is currently unknown. This is further complicated by a lack of clarity about the role of I_K in AF. We have previously reported that the SFFA PA increased the densities of I_Kr and I_Ks currents in HEK293 cells and shortened atrial APD measured in adult guinea pig myocytes, in line with a role for I_K in AF associated with metabolic disorders (Aromolaran et al., 2016). Therefore, considering the idea that SFFAs activate macrophages and promote increased expression of IL-6 (Shi et al., 2006; Figure), our expectation is that I_K may also be an important target for IL-6 modulation in AF pathogenesis in patients with metabolic disorders.