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  • The following are the supplementary data related to

    2019-07-30

    The following are the supplementary data related to this article.
    Conflicts of interest
    Transparency document
    Abbreviations
    Introduction The incidence of overweight and obesity in the Western world has risen dramatically. In the United States, more than one-third of the adult population is obese [1]. Obesity, defined as a body mass index of ≥30kg/m2, is associated with an increased risk of some leading causes of preventable death, including heart diseases, metabolic syndrome, type 2 diabetes and certain types of cancer [2], [3], [4]. Among cardiovascular diseases, obesity has a great impact on the cardiac remodeling and function in terms of hemodynamic load, myocardial fibrosis and impaired ventricular contractility, which leads to the progression of heart failure [5], [6], [7]. Current pharmacological therapies used for the treatment of heart failure cannot potentially reduce the risk of obese-induced health outcomes; therefore, new therapeutic strategies are needed to reliably prevent development of obesity-related heart diseases. One major mechanism by which obesity induces cardiovascular diseases is inflammatory perturbations [8]. Toll-like receptor 4 (TLR4), an immune receptor that enhances inflammation, has been shown to be involved in several obesity-induced health conditions [9]. Obesity and overweight are associated with significant increased (±)-Baclofen of TLR4 and its adaptor protein MyD88 [10]. Under conditions of hyperlipidemia, TLR4 serves as a receptor for free fatty acids, and the activation of TLR4 leads to increased inflammation by stimulating the production of inflammatory cytokines such as TNF-α, IL-1b, and IL-6 [10], [11], contributing to increased risk of cardiovascular events [12]. On the other hand, TLR4 deficiency or polymorphism ameliorates obesity-associated hypertension in patients [13], implicating that TLR4 plays an important role in the progression of cardiovascular diseases. Therefore, inactivating TLR4-dependent inflammatory pathway may be beneficial to prevent obesity-related heart diseases. In endotoxin lipopolysacchride (LPS) stimulation, myeloid differentiation 2 (MD2) has been identified as a co-receptor of TLR4 [14]. It interacts with TLR4 on the cell membrane and enables TLR4 to response to LPS [15]. Upon LPS binding, the TLR4/MD2 complex triggers two major downstream signaling pathways, the MyD88-dependent and TRIF-dependent (TIR domain-containing adaptor inducing IFN-β) pathways. The MyD88-dependent pathway further stimulates the expression of pro-inflammatory cytokines, while TRIF-dependent pathway is activated after endocytosis of LPS/MD2/TLR4 complex and leads to the expression of interferon (IFN)-inducible genes [16]. Small molecule inhibitors of MD2 suppress LPS-stimulated MAPK phosphorylation, NF-κB activation and cytokine expression in macrophages [17]. Moreover, mice with MD2 gene knockout have improved survival rate, less lung injury and reduced serum and hepatic cytokine levels in the LPS-induced septic shock model [17]. Deficiency in MD2 gene expression also attenuates liver steatosis and fibrosis in a non-alcoholic steatohepatitis (NASH) model in mice [18]. These studies demonstrate the critical role of MD2 in LPS/TLR4-mediated inflammation and tissue injury, and suggest that MD2 could serve as a therapeutic target for inflammatory disorders. The present study aims to investigate the role of MD2 in obesity-related cardiomyopathy. We used MD2 inhibitor L6H21 [17] as well as MD2 knockout mice in a model of high-fat-diet (HFD)-induced obesity, as tools to evaluate mechanisms of high lipid-mediated inflammation and cardiac injury in vitro and in vivo. Our results identify the mechanistic role for MD2 and TLR4 receptor complex in the pathogenesis of cardiomyopathy, including inflammation, hypertrophy and fibrosis.
    Materials and methods
    Results
    Discussion It is noteworthy that the activation of TLR4 signaling pathway correlates with heart dysfunction in human obesity and a high expression of TLR4 occurs in the obesity model [10]. Significantly, we also found increased MD2 in the left ventricle of obesity hearts, which is consistent with MD2/TLR4 signaling activation. Increased MD2 activity has been reported in serum and urine of patients with sepsis, and this increased MD2 activity further promotes LPS responsiveness to TLR4 signaling pathway activation [24]. Inhibition of MD2-induced inflammation has been suggested as a therapeutic strategy for sepsis and inflammation, and antagonists of MD2 have been studied in clinical trials for the treatment of sepsis [25], [26]. However, it is unknown if antagonizing MD2 is able to prevent obesity-related inflammation and tissue injuries. Our data suggest a major role for MD2 in obesity-associated cardiac injury, as indicated by profound attenuation of cardiac hypertrophy, fibrosis and dysfunction in MD2 genetic deletion or pharmacological inhibition models. These data shed additional light for future MD2-based therapeutic interventions in obesity-related cardiomyopathy.