Bacterial DNA induces myocardial inflammation and reduces cardiomyocyte contractility: role of toll-like receptor 9

Down-regulation of ATP-binding cassette transporter G1 expression by unmethylated CpG oligodeoxynucleotides in RAW 264.7 macrophages

We have investigated the effect of various forms of phosphodiester cytidine-phosphate-guanosine oligodeoxynucleotides (CpG ODNs) on the production of pro-inflammatory cytokines and related genes in RAW 264.7 macrophages. Treatment with the CpG ODNs increased the expression of tumor necrosis factor α (TNF-α), IL-6, and inducible nitric oxide synthase but not interleukin-1β (IL-1β). We also investigated the effect of CpG ODNs on the expression of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) genes which are known to facilitate cholesterol efflux from macrophages for anti-atherosclerosis. CpG 2006 significantly reduced the levels of ABCG1 mRNA as determined by real-time polymerase chain reaction, whereas ABCA1 mRNA level was not changed. Western blot analysis further confirmed the reduction of ABCG1 protein expression by CpG 2006. In addition, we also determined the protein level of peroxisome proliferator activated receptor γ (PPARγ), which is recognized as a transcriptional activator of ABC transporters, was also reduced by CpG 2006. Thus, these results suggest that ABCG1 is specifically down-regulated by CpG 2006 in a PPARγ-dependent manner in macrophages.

Toll-like receptors and myocardial inflammation

Toll-like receptors (TLRs) are a member of the innate immune system. TLRs detect invading pathogens through the pathogen-associated molecular patterns (PAMPs) recognition and play an essential role in the host defense. TLRs can also sense a large number of endogenous molecules with the damage-associated molecular patterns (DAMPs) that are produced under various injurious conditions. Animal studies of the last decade have demonstrated that TLR signaling contributes to the pathogenesis of the critical cardiac conditions, where myocardial inflammation plays a prominent role, such as ischemic myocardial injury, myocarditis, and septic cardiomyopathy. This paper reviews the animal data on (1) TLRs, TLR ligands, and the signal transduction system and (2) the important role of TLR signaling in these critical cardiac conditions.

Myocardial ischemia activates an injurious innate immune signaling via cardiac heat shock protein 60 and Toll-like receptor 4

Innate immune response after transient ischemia is the most common cause of myocardial inflammation and may contribute to injury, yet the detailed signaling mechanisms leading to such a response are not well understood. Herein we tested the hypothesis that myocardial ischemia activates interleukin receptor-associated kinase-1 (IRAK-1), a kinase critical for the innate immune signaling such as that of Toll-like receptors (TLRs), via a mechanism that involves heat shock proteins (HSPs) and TLRs. Coronary artery occlusion induced a rapid myocardial IRAK-1 activation within 30 min in wild-type (WT), TLR2(-/-), or Trif(-/-) mice, but not in TLR4(def) or MyD88(-/-) mice. HSP60 protein was markedly increased in serum or in perfusate of isolated heart following ischemia/reperfusion (I/R). In vitro, recombinant HSP60 induced IRAK-1 activation in cells derived from WT, TLR2(-/-), or Trif(-/-) mice, but not from TLR4(def) or MyD88(-/-) mice. Both myocardial ischemia- and HSP60-induced IRAK-1 activation was abolished by anti-HSP60 antibody. Moreover, HSP60 treatment of cardiomyocytes (CMs) led to marked activation of caspase-8 and -3, but not -9. Expression of dominant-negative mutant of Fas-associated death domain protein or a caspase-8 inhibitor completely blocked HSP60-induced caspase-8 activation, suggesting that HSP60 likely activates an apoptotic program via the death-receptor pathway. In vivo, I/R-induced myocardial apoptosis and cytokine expression were significantly attenuated in TLR4(def) mice or in WT mice treated with anti-HSP60 antibody compared with WT controls. Taken together, the current study demonstrates that myocardial ischemia activates an innate immune signaling via HSP60 and TLR4, which plays an important role in mediating apoptosis and inflammation during I/R.

The emerging role of innate immunity in the heart and vascular system: for whom the cell tolls

Recent studies suggest that the heart possesses an innate immune system that is intended to delimit tissue injury, as well as orchestrate homoeostatic responses, within the heart. The extant literature suggests that this intrinsic stress response system is mediated, at least in part, by a family of pattern recognition receptors, most notably the Toll-like receptors. Although the innate immune system provides a short-term adaptive response to tissue injury, the beneficial effects of this phylogenetically ancient system may be lost if innate immune signaling becomes sustained and/or excessive; in which case, the salutary effects of activation of these pathways are contravened by the known deleterious effects of inflammatory signaling. Herein, the biology of innate immune signaling in the heart is reviewed, as well as the literature suggesting that the innate immune system is involved in the pathogenesis of atherosclerosis, acute coronary syndromes, stroke, viral myocarditis, sepsis, ischemia/reperfusion injury, and heart failure. The review concludes by discussing new therapies that are being developed to modulate the innate immune system.

Role of toll-like receptors in cardiovascular diseases

The discovery and characterization of the TLR (Toll-like receptor) family has led to a better understanding of the innate immune system. The strategy of innate immune recognition is based on the detection of constitutive and conserved products of micro-organisms. However, host molecules that are released during injury can also activate TLRs. Engagement of TLRs by microbial or host-derived molecules induces the expression of pro-inflammatory cytokines, which may have both beneficial and detrimental effects on the host. In addition to being expressed in immune cells, TLRs are expressed in other tissues such as those of the cardiovascular system. In the present review, the role of TLRs in septic cardiomyopathy, viral myocarditis, atherosclerosis, ischaemia/reperfusion injury and cardiac remodelling after myocardial infarction are outlined, with attention paid to genetically modified murine models. Although much has been learned about stress-induced TLR activation in the tissues of the cardiovascular system, the role of individual TLRs in initiating and integrating homoeostatic responses within the heart remains to be defined. Accumulating evidence indicates that TLRs may play an important role in the pathogenesis of atherosclerosis, viral myocarditis, dilated cardiomyopathy, cardiac allograft rejection and sepsis-induced left ventricular dysfunction. Moreover, heart failure of diverse aetiology is also now recognized to have an important immune component, with TLR signalling influencing the process of cardiac remodelling and prognosis. In the present review, we outline the biology of TLRs as well as the current experimental and clinical evidence for the role of TLRs in cardiovascular diseases.

Innate immune signaling in cardiac ischemia

Despite advances in treatment of patients who suffer from ischemic heart disease, morbidity related to myocardial infarction is increasing in Western societies. Acute and chronic immune responses elicited by myocardial ischemia have an important role in the functional deterioration of the heart. Research on modulation of the inflammatory responses was focused on effector mediators such as leukocytes. However, increasing evidence indicates that various endogenous ligands that act as 'danger signals', also called danger-associated molecular patterns (DAMPs), are released upon injury and modulate inflammation. Originally described as part of the first-line defense against invading microorganisms, several Toll-like receptors (TLRs) on leukocytes and parenchymal cells have now been shown to respond to such signals and to have a pivotal role in noninfectious pathological cardiovascular conditions, such as ischemia-reperfusion injury and heart failure. From a therapeutic perspective, DAMPs are attractive targets owing to their specific induction after injury. In this Review, we will discuss innate immune activation through TLRs in cardiac ischemia mediated by DAMPs.

Systemically administered ligands of Toll-like receptor 2, -4, and -9 induce distinct inflammatory responses in the murine lung

Objective. To determine whether systemically administered TLR ligands differentially modulate pulmonary inflammation. Methods. Equipotent doses of LPS (20 mg/kg), CpG-ODN (1668-thioat 1 nmol/g), or LTA (15 mg/kg) were determined via TNF activity assay. C57BL/6 mice were challenged intraperitoneally. Pulmonary NFκB activation (2 h) and gene expression/activity of key inflammatory mediators (4 h) were monitored. Results. All TLR ligands induced NFκB. LPS increased the expression of TLR2, 6, and the cytokines IL-1αβ, TNF-α, IL-6, and IL-12p35/p40, CpG-ODN raised TLR6, TNF-α, and IL12p40. LTA had no effect. Additionally, LPS increased the chemokines MIP-1α/β, MIP-2, TCA-3, eotaxin, and IP-10, while CpG-ODN and LTA did not. Myeloperoxidase activity was highest after LPS stimulation. MMP1, 3, 8, and 9 were upregulated by LPS, MMP2, 8 by CpG-ODN and MMP2 and 9 by LTA. TIMPs were induced only by LPS. MMP-2/-9 induction correlated with their zymographic activities. Conclusion. Pulmonary susceptibility to systemic inflammation was highest after LPS, intermediate after CpG-ODN, and lowest after LTA challenge.

Toll-like receptors as potential therapeutic targets in cardiac dysfunction

INTRODUCTION

The innate immune system can detect the highly conserved, relatively invariant structural motifs of pathogens. The most important innate immune receptors, Toll-like receptors (TLRs), represent a first line of defense against infectious pathogens, and play a pivotal role in initiating and shaping innate and adaptive immune responses. TLRs are not only expressed in immune cells, but also in cardiovascular cells. In addition to their role in response to microbial infections, evidence suggests that TLRs can also recognize endogenous ligands and may play a role in mediating cardiomyocyte cell death and survival after non-infectious injury.

AREAS COVERED

TLRs could be a link between cardiovascular diseases and the immune system. Experimentally, there is good evidence that TLR activation contributes to development and progression of both acute cardiac injury and chronic heart failure. The role of TLRs in myocardial ischemia-reperfusion, remodeling, septic cardiomyoparthy, autoimmune- and viral myocarditis, anthracycline-induced cardiomyopathy and cardiac hypertrophy, in basic as well as clinical science are discussed.

EXPERT OPINION

Evidence, mainly from animal experiments, indicates that TLRs contribute to all of the myocardial disease states reviewed in this paper. However, the relevance of TLRs as therapeutic targets remains to be defined as clinical data is sparse.

TRIF is a critical survival factor in viral cardiomyopathy

TRIF is a member of the innate immune system known to be involved in viral recognition and type I IFN activation. Because IFNs are thought to play an important role in viral myocarditis, we investigated the role of TRIF in induced myocarditis in mice. Whereas C57BL/6 (wild-type) mice showed only mild myocarditis, including normal survival postinfection with coxsackievirus group B serotype 3 (CVB3), infection of TRIF(-/-) mice led to the induction of cardiac remodeling, severe heart failure, and 100% mortality (p < 0.0001). These mice showed markedly reduced virus control in cardiac tissues and cardiomyocytes. This was accompained with dynamic cardiac cytokine activation in the heart, including a suppression of the antiviral cytokine IFN-β in the early viremic phase. TRIF(-/-) myocytes displayed a TLR4-dependent suppression of IFN-β, and pharmacological treatment of CVB3-infected TRIF(-/-) mice with murine IFN-β led to improved virus control and reduced cardiac inflammation. Additionally, this treatment within the viremic phase of myocarditis showed a significant long-term outcome indexed by reduced mortality (20 versus 100%; p < 0.001). TRIF is essential toward a cardioprotection against CVB3 infection.

CpG oligodeoxynucleotides as TLR9 agonists: therapeutic application in allergy and asthma

Unmethylated cytosine-phosphate-guanine (CpG) dinucleotides in microbial DNA sequences activate Toll-like receptor (TLR) 9, and previous studies have shown that oligodeoxynucleotides (ODNs) containing CpG in specific base sequence motifs (CpG ODNs) can reiterate the majority of the immunomodulatory effects produced by bacterial DNA. Many of the manifestations in allergic diseases are primarily due to T helper (T(h))-2 cell-type responses. CpG ODNs can induce T(h)1 and T-regulatory (T(reg)) cell-type cytokines that can suppress the T(h)2 response. The therapeutic application of TLR9 has been explored extensively in recent years, and many studies are being conducted to assess the safety and efficacy of TLR9 agonists in various diseases, including atopic and infectious diseases, and cancer. Studies in murine models have shown that the development of atopic airway disease can be prevented by treatment with CpG ODNs. Various clinical trials are currently ongoing to determine the efficacy of CpG ODNs as a therapeutic tool for atopic diseases. In this review, we discuss the therapeutic application of CpG ODNs in allergy and asthma. CpG ODNs may be used alone or as an adjuvant to immunotherapy to treat these disorders.

Bacterial flagellin triggers cardiac innate immune responses and acute contractile dysfunction

Background

Myocardial contractile failure in septic shock may develop following direct interactions, within the heart itself, between molecular motifs released by pathogens and their specific receptors, notably those belonging to the toll-like receptor (TLR) family. Here, we determined the ability of bacterial flagellin, the ligand of mammalian TLR5, to trigger myocardial inflammation and contractile dysfunction.

Methodology/Principal Findings

TLR5 expression was determined in H9c2 cardiac myoblasts, in primary rat cardiomyocytes, and in whole heart extracts from rodents and humans. The ability of flagellin to activate pro-inflammatory signaling pathways (NF-kappaB and MAP kinases) and the expression of inflammatory cytokines was investigated in H9c2 cells, and, in part, in primary cardiomyocytes, as well as in the mouse myocardium in vivo. The influence of flagellin on left ventricular function was evaluated in mice by a conductance pressure-volume catheter. Cardiomyoyctes and intact myocardium disclosed significant TLR5 expression. In vitro, flagellin activated NF-kappaB, MAP kinases, and the transcription of inflammatory genes. In vivo, flagellin induced cardiac activation of NF-kappaB, expression of inflammatory cytokines (TNF alpha, IL-1 beta, IL-6, MIP-2 and MCP-1), and provoked a state of reversible myocardial dysfunction, characterized by cardiac dilation, reduced ejection fraction, and decreased end-systolic elastance.

Conclusion/Significance

These results are the first to indicate that flagellin has the ability to trigger cardiac innate immune responses and to acutely depress myocardial contractility.

Myeloid differentiation factor-88 contributes to TLR9-mediated modulation of acute coxsackievirus B3-induced myocarditis in vivo

Toll-like receptor 9 (TLR9) is a member of the innate immune system and has been shown to influence myocardial function, but its role in myocarditis is hitherto unknown. We therefore investigated whether or not TLR9 plays a role in this disease in coxsackievirus B3 (CVB3)-induced myocarditis in mice. Left ventricular (LV) function, cardiac immune cell infiltration, virus mRNA, and components of the TLR9 downstream pathway were investigated in TLR9-deficient [knockout (KO)] and wild-type (WT) mice after infection with CVB3. Murine cardiac TLR9 expression was significantly increased in WT mice with acute CVB3 infection but not in WT mice with chronic myocarditis. Furthermore, in the acute phase of CVB3-induced myocarditis, CVB3-infected KO mice displayed improved LV function associated with reduced cardiac inflammation indexed by reduced amounts of immune cells compared with CVB3-infected WT mice. In contrast, in the chronic phase, LV function and inflammation were not seen to differ among the infected groups. The cardioprotective effects due to TLR9 deficiency were associated with suppression of the TLR9 downstream pathway as indexed by reduced cardiac levels of the adapter protein myeloid differentiation factor (MyD)-88 and the proinflammatory cytokine TNF-alpha. In addition, TLR9 deficiency led to an activation of the antiviral cytokine interferon-beta in the heart as a result from viral infection. In conclusion, the MyD88/TNF-alpha axis due to TLR9 activation in the heart contributes the development of acute myocarditis but not of chronic myocarditis.

Activation of cardiac hypertrophic signaling pathways in a transgenic mouse with the human PRKAG2 Thr400Asn mutation

Human mutations in PRKAG2, the gene encoding the gamma2 subunit of AMP activated protein kinase (AMPK), cause a glycogen storage cardiomyopathy. In a transgenic mouse with cardiac specific expression of the Thr400Asn mutation in PRKAG2 (TG(T400N)), we previously reported initial cardiac hypertrophy (ages 2-8 weeks) followed by dilation and failure (ages 12-20 weeks). We sought to elucidate the molecular mechanisms of cardiac hypertrophy. TG(T400N) mice showed significantly increased cardiac mass/body mass ratios up to approximately 3-fold beginning at age 2 weeks. Cardiac expression of ANP and BNP were approximately 2- and approximately 5-fold higher, respectively, in TG(T400N) relative to wildtype (WT) mice at age 2 weeks. NF-kappaB activity and nuclear translocation of the p50 subunit were increased approximately 2- to 3-fold in TG(T400N) hearts relative to WT during the hypertrophic phase. Phosphorylated Akt and p70S6K were elevated approximately 2-fold as early as age 2 weeks. To ascertain whether these changes in TG(T400N) mice were a consequence of increased AMPK activity, we crossbred TG(T400N) with TG(alpha2DN) mice, which express a dominant negative, kinase dead mutant of the AMPK alpha2 catalytic subunit and have low myocardial AMPK activity. Genetic reversal of AMPK overactivity led to a reduction in hypertrophy, nuclear translocation of NF-kappaB, phosphorylated Akt, and p70S6K. We conclude that inappropriate activation of AMPK secondary to the T400N PRKAG2 mutation is associated with the early activation of NF-kappaB and Akt signaling pathway, which mediates cardiac hypertrophy.

Innate immunity and toll-like receptor antagonists: a potential role in the treatment of cardiovascular diseases

Toll-like receptors (TLRs) are germline-encoded receptors that recognize various pathogen-associated molecular patterns (PAMPs). They are key components of the innate immunity which are activated in response to pathogens as well as non-pathogenic components of damaged tissues. TLR agonists have been developed to treat allergies, cancers, and chronic infections by upregulating the innate immune system. TLR antagonists may be used to treat a number of inflammatory conditions, such as rheumatoid arthritis and systemic lupus erythematosus. Recent research also has shown that TLRs are involved in the pathogenesis of atherosclerosis, thrombosis, myocardial remodeling, ischemic/reperfusion injury, and valvular disease. This article reviews the current experimental and clinical evidence for the role of TLRs in the cardiovascular system, and examines the mechanisms by which TLR antagonists could potentially be used in targeted therapy.

Oxidative stress and inflammation in atrial fibrillation: role in pathogenesis and potential as a therapeutic target

Atrial fibrillation (AF) is one of the most prevalent and vexing cardiovascular conditions. Available treatments for AF based on ion channel blockade are only poorly effective. The fundamental mechanisms that underlie AF are still not clearly understood, and likely vary depending on the etiology of AF. In older individuals with senile AF, likely mechanisms include abnormal calcium cycling, oxidant stress, and deleterious inflammatory responses. Clinical and experimental evidence is provided to support the role of oxidant and inflammatory mechanisms in AF. On the basis of these studies, the prospects of manipulating oxidant and inflammatory pathways as targets for therapeutic intervention are discussed.

Toll-like receptor signaling: a critical modulator of cell survival and ischemic injury in the heart

Toll-like receptors (TLRs) represent the first line of host defense against microbial infection and play a pivotal role in both innate and adaptive immunity. TLRs recognize invading pathogens through molecular pattern recognition, transduce signals via distinct intracellular pathways involving a unique set of adaptor proteins and kinases, and ultimately lead to the activation of transcription factors and inflammatory responses. Among 10 TLRs identified in humans, at least two exist in the heart, i.e., TLR2 and TLR4. In addition to the critical role of these in mediating cardiac dysfunction in septic conditions, emerging evidence suggests that the TLRs can also recognize endogenous ligands and may play an important role in modulating cardiomyocyte survival and in ischemic myocardial injury. In animal models of ischemia-reperfusion injury or in hypoxic cardiomyocytes in vitro, the administration of a sublethal dose of lipopolysaccharide, which signals through TLR4, reduces subsequent myocardial infarction, improves cardiac functions, and attenuates cardiomyocyte apoptosis. By contrast, a systemic deficiency of TLR2, TLR4, or myeloid differentiation primary-response gene 88, an adaptor critical for all TLR signaling, except TLR3, leads to an attenuated myocardial inflammation, a smaller infarction size, a better preserved ventricular function, and a reduced ventricular remodeling after ischemic injury. These loss-of-function studies suggest that both TLRs contribute to myocardial inflammation and ischemic injury in the heart although the exact contribution of cardiac (vs. circulatory cell) TLRs remains to be defined. These recent studies demonstrate an emerging role for TLRs as a critical modulator in both cell survival and tissue injury in the heart.