2. The Mechanism of TLR4 on Dox-Induced Cardiotoxicity
Growing evidence has shown that Dox activates the innate immune system, which is one of the expected components of the response against tumor cells
[2][9][2,9]. However, Dox-induced innate immune activation provokes the release of inflammatory cytokines in a number of tissues, including the heart, intestine, brain, and liver, which results in inflammation in non-targeted organs
[9][10][24][25][9,10,24,25]. To date, several studies have demonstrated that Dox-induced cardiac inflammation is strongly linked to Dox-induced cardiotoxicity
[10][14][26][27][10,14,26,27]. Dox induces NF-κB activity to promote cardiac inflammation by upregulating TNF-α, IL-1β, and IL-6 expression
[15][26][15,26].
TLR4 is responsible for the innate immune response; thus, the activation of TLR4 has become one of the most attractive targets in recent years
[28][29][28,29]. Moreover, the expression of TLR4 is also found in the cardiomyocytes in addition to the immune cells
[21]. This receptor responds to exogenous and endogenous ligands: pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs)
[28]. TLR4 recognizes the bacterial LPS as a major PAMP with the help of its co-receptor MD2
[21]. In addition, the TLR4 signaling pathway can also be activated by various endogenous DAMPs, namely alarmin protein, including high-mobility group protein box 1 (HMGB1) and the heat shock protein family (Hsps)
[28][30][28,30]. Upon ligand binding, TLR4 dimerization occurs, followed by activation of the myeloid differentiation primary response protein 88 (MyD88); the downstream signaling pathways propagate NF-κB phosphorylation via reducing the inhibitory κB kinase (IKK) response, leading to upregulation of inflammatory cytokines and cardiac inflammation
[21]. In addition to NF-κB activity, components of mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38, are also activated as downstream signal transducers of MyD88 to contribute to the regulation of pro-inflammatory responses
[18][21][18,21].
To date, a growing body of evidence demonstrates that Dox induces the release of PAMP and DAMP, resulting in TLR4-mediated cardiac inflammation that contributes significantly to cardiotoxicity
[10,15,28,31]. Based on evidence from [10][15][28][31]in vitro and in vivo studies, we review and discuss how Dox causes cardiac TLR4 upregulation and inflammation in this review. A simplified overview of the mechanistic activation of TLR4 in Dox-induced cardiotoxicity is shown in
Figure 1. Therefore, inhibition of TLR4 would have a therapeutic benefit against Dox-induced cardiotoxicity.
Figure 1. A schematic presentation of oxidative stress and inflammation-related mechanisms via TLR4 in Dox-induced cardiotoxicity. Activation of TLR4 was exhibited by HMGB1, NPM, and Hsp70, leading to increased cardiac inflammation, apoptosis, and fibrosis, and impaired cardiac function in Dox treatment. Its subsequent detrimental effects were effectively attenuated by treatment with C34, Hsp22, and VA.
3. The Effects of Dox on TLR4 Expression in Cardiomyocytes: Reports from In Vitro Studies
To date, the relationship between the role of TLR4 and Dox administration is still scarce; however, increasing evidence from in vitro studies suggests that Dox-mediated cardiotoxicity is related to the upregulation of cardiomyocyte TLR4 expressions
[32][33][34][32,33,34]. Li et al. reported that Dox administration upregulated the expression of TLR4 in human cardiomyocyte cell lines
[32]. In addition, Feng et al. demonstrated that Dox treatment significantly enhanced the TLR4 signaling pathway in H9c2 cardiomyocytes, as evidenced by increased TLR4 expression and its downstream signaling pathways, including MyD88, NF-κB, IL-1 β, IL-6, and TNF-α, along with decreased IkBα expression
[27]. According to these two in vitro results. However, further research is necessary to fully understand the underlying molecular signaling of Dox-induced TLR4 signaling pathway activation in cardiomyocytes.
Traditionally, TLR4 ligands, PAMPs, and DAMPs bind to MD2 to activate the TLR4 signaling pathway in cardiomyocytes, resulting in cardiac inflammation
[18][35][18,35]. The DNA-binding nuclear protein HMGB1, which regulates gene transcription and nucleosome stability, has a variety of biological functions in clinicopathological conditions
[36]. It is considered a cytokine involved in the activation of innate immunity after being actively released from cells or passively released upon cell injury
[36]. Interestingly, TLR4 functions as the major HMGB1 receptor
[37]. Disulfide HMGB1 activates the TLR4 complex, binding to MD2, which triggers dimerization of TLR4 that can stimulate downstream signal transduction molecules (e.g., NF-κB) to produce pro-inflammatory cytokines
[37]. A previous study demonstrated that the release of HMGB1 from the cells was increased following Dox treatment
[38]. Moreover, the oxidative stress and DAMPs were considered to be major stimulators of HMGB1 release, activating inflammation through the TLR4 signaling pathway
[38]. Therefore, Dox is closely related to altered HMGB1 levels, leading to induced cardiotoxicity.
In addition to HMGB1, there is also nucleophosmin (NPM), which behaves similarly to an alarmin protein released in response to cellular excess due to severe injury
[39]. Similar to HMGB1, NPM can also bind to the TLR4 signaling cascade, leading it to exerting pro-inflammatory cytokines
[39]. Interestingly, a previous study showed that Dox induced nucleolar stress, subsequently disrupting and releasing the NPM from the nucleolus. Then, the extracellular NPM induced inflammation via TLR4 signaling pathway activation
[39]. To date, there is only one study that has shown that NPM can bind to the TLR4 signaling cascade, promoting pro-inflammatory function following the Dox treatment of human cardiac mesenchymal progenitor cells (hCmPCs)
[39]. Thus, NPM is a novel ligand of TLR4 that activates inflammation in Dox-treated cardiotoxicity
[39].
Inhibition of TLR4 via genetic deletion suppressed Dox-induced cardiotoxicity
[27][31][32][27,31,32]. This evidence was provided by previous studies, showing that genetic ablation of TLR4 not only reduced inflammation but also decreased apoptosis in cardiomyocytes after exposure to Dox
[27][31][32][27,31,32]. Although molecular signaling through TLR4 has not been demonstrated in the genetic deletion of TLR4 on apoptosis in cardiomyocytes after Dox exposure, the aforementioned discussions suggest that pro-inflammatory cytokines
[27], mediated by TLR4, were suppressed, resulting in a decrease in cardiomyocyte apoptosis, as evidenced by the decrease in apoptotic proteins (Bax and cleaved caspase-3) and the increase in anti-apoptotic proteins (Bcl-2)
[31][32][31,32]. These in vitro studies demonstrate the plausible involvement of TLR4 in Dox-induced cardiotoxicity via the implication in cardiomyocyte apoptosis.
4. The TLR4 Expression in Dox-Induced in Cardiomyocytes: Reports from In Vivo Studies
Several studies have demonstrated that Dox treatment leads to impaired cardiac function in rodents, as assessed via echocardiography and invasive hemodynamic assessment
[14][23][40][14,23,40]. Cellular and molecular studies have also been carried out using the cardiomyocytes isolated from these Dox-treated animals, and the results are largely consistent with the findings from the in vitro models discussed in the previous section. Previous in vivo studies have shown that Dox enhanced the expression of TLR4 in the cardiac tissue of rodents
[14][23][40][14,23,40]. Furthermore, the levels of TLR4 ligands, such as HMGB1 and Hsp70, were also significantly increased in the cardiac tissue of Dox-treated mice
[40]. This result further confirms that the TLR4 signaling pathway was activated following Dox treatment, leading to the triggering of NF-κB activity, which leads to the generation of pro-inflammatory cytokines in cardiac tissue, as evidenced by increases in TNF-α, IL-6, IL-13, monocyte chemotactic protein (MCP-1), and transforming growth factor (TGF)-β1
[14][40][14,40]. Consistently, the expression of cardiac macrophage markers, including CD45 and CD68, was also elevated in Dox-treated mice
[14]. Taken together, elevated cardiac macrophages expressing TLR4 can lead to overwhelming pro-inflammatory cytokines in Dox-treated animals
[14][40][14,40]. In addition to cardiac inflammation, cardiac remodeling was also observed in Dox-treated mice
[40], which was attributed to the fact that TGF-β1 was increased in the hearts of these mice
[40]. TGF-β1 is involved in the cardiac remodeling process, since it is a multifunctional cytokine and a growth factor that plays multiple roles in inflammation and fibrosis
[41]. Therefore, an increase in cardiac TGF-β1 expression promoted collagen accumulation in the heart of Dox-treated mice, as indicated by increasing α-smooth muscle actin (α-SMA)
[40].
Dox induces ROS through redox reactions due to its quinone component, with the resulting production of the superoxide anion (O
2−), hydrogen peroxide (H
2O
2), and hydroxyl radical (∙OH)
[8]. This ROS can lead to lipid peroxidation, as indicated by an increase in malondialdehyde (MDA) that was observed in the hearts of Dox-treated rats
[14]. In addition, this ROS can trigger apoptosis in cardiac tissue in Dox-treated mice, as evidenced by an increase in Bax, cytochrome
c, and TUNEL
+ cells, as well as a decrease in Bcl-2 in cardiac tissue
[14]. According to in vitro studies
[31][32][31,32], an in vivo study has also shown that increasing TLR4 activation was implicated in cardiac apoptosis in Dox-treated mice
[14]. This could be due to the oxidative stress associated with the increase in TLR4 expression that further promotes inflammation
[42], which, in turn, contributes to apoptosis. Therefore, oxidative stress not only directly induces cardiac apoptosis per se but also promotes inflammation via increasing TLR4 expression, leading to cardiac apoptosis in Dox-treated mice.