Elsevier

Atherosclerosis

Volume 221, Issue 1, March 2012, Pages 249-253
Atherosclerosis

Differential expression of Toll-like receptor 4 and human monocyte subsets in acute myocardial infarction

https://doi.org/10.1016/j.atherosclerosis.2011.12.030Get rights and content

Abstract

Objective

To investigate the involvement of Toll-like receptor 4 (TLR4) expression on two monocyte subsets in the pathologic processes related to acute coronary syndrome. How monocytes, which have recently been shown to comprise two distinct subsets, mediate the process of coronary plaque rupture remains to be fully elucidated. Recent studies have shown that TLR4 is involved in monocyte activation of patients with accelerated forms of atherosclerosis.

Methods

We enrolled 65 patients with acute myocardial infarction (AMI, n = 22), unstable angina pectoris (UAP, n = 16), and stable angina pectoris (SAP, n = 27) who underwent coronary angiography and 15 healthy controls. The expression of TLR4 on two monocyte subsets (CD14+CD16 and CD14+CD16+) was measured by flow cytometry.

Results

In patients with AMI, TLR4 was more expressed on circulating CD14+CD16+ monocytes than on CD14+CD16 monocytes (p < 0.001). The expression levels of TLR4 on CD14+CD16+ monocytes were significantly elevated in patients with AMI compared with other 3 groups. TLR4 expression levels on CD14+CD16+ monocytes were significantly elevated at the culprit site compared with the systemic level (p = 0.044). The up-regulation of TLR4 on admission was remarkably decreased 12 days after AMI (p < 0.001). In addition, plasma levels of tumor necrosis factor-α were positively correlated with TLR4 expression levels on monocytes in patients with AMI (r = 0.47, p = 0.027).

Conclusion

TLR overexpression on CD14+CD16+ monocytes in AMI, as demonstrated both in the circulation and at the coronary culprit site, might be associated with the pathogenesis of AMI.

Highlights

► The expression levels of TLR4 on CD14+ monocytes were significantly elevated in patients with AMI. ► TLR4 expression levels on CD14+CD16+ monocytes were significantly elevated at culprit site compared with the systemic levels. ► Plasma levels of tumor necrosis factor-α were positively correlated with TLR4 expression levels on monocytes in patients with AMI.

Introduction

Monocytes appear to be actively involved in all stages of acute coronary syndrome (ACS). Although the mechanisms of plaque destabilization can be attributed mainly to the processes taking place inside the plaque, circulating monocytes are able to generate and secrete mediators of all the major factors involved in plaque destabilization, including inflammation, and matrix degradation. Monocytes in human peripheral blood are heterogeneous [1], [2], [3]. Differential expression of CD14 and CD16 enables monocytes to be divided into two subsets: CD14+CD16 monocytes expressing C–C motif chemokine receptor 2 (CCR2); and CD14+CD16+ monocytes expressing C-X3-C motif chemokine receptor 1 (CX3CR1). The discovery that monocytes comprise distinct subsets in humans suggests a specialization of function and has stimulated an interest in approaches that discriminate between “harmful” and “beneficial” subsets [4], [5], [6], [7], [8]. In terms of the relationship between monocyte subsets and coronary plaque destabilization, we previously showed by using optical coherence tomography that up-regulation of CD14+CD16+ monocytes is associated with plaque rupture in patients with unstable angina pectoris (UAP) [9]. These findings highlight the importance of characterizing different monocyte subsets, which differentially contribute to plaque infiltration and vulnerability.

Toll-like receptor 4 (TLR4) is the receptor for exogenous lipopolysaccharide (LPS) and endogenous heat shock protein [10], [11]. TLR4 plays an important role in the innate immune and inflammatory responses. The expression of TLR4 has recently been described in macrophages and endothelial cells in lipid-rich atherosclerotic plaques [12], [13]. TLR4 is also involved in monocyte activation in patients with accelerated forms of atherosclerosis. In particular, acute myocardial infarction (AMI) is associated with enhanced expression of TLR4 in circulating monocytes [14]. Moreover, TLR4 plays a key role in the activation of inflammatory pathways of ACS [15], [16].

The assessment of monocyte subsets and TLR4 is important from the perspective of prevention. To the best of our knowledge, the relationship between expression of TLR4 and human monocyte subsets has not been examined in patients with ACS. In the present study, we investigated the association between human monocyte subsets and the expression levels of TLR4 in patients with AMI. Moreover, we examined whether expression levels of TLR4 on specific monocyte subsets were elevated at the culprit site compared with the systemic level in patients with AMI.

Section snippets

Patient population

We enrolled 65 patients; 22 with AMI, 16 with UAP, and 27 with stable angina pectoris (SAP) who underwent coronary angiography. Patients were diagnosed with AMI according to the following criteria: (1) chest pain within 24 h before admission that lasted for 30 min and was not relieved by sublingual nitroglycerin; (2) ST-segment elevation and/or abnormal Q-wave on an electrocardiogram; and (3) elevated serum creatine kinase (CK) levels. Exclusion criteria included: (1) AMI for 24 h from onset; (2)

Patient characteristics

The characteristics of this study population are summarized in Table 1. Current smoking and drug medication of β-blocker, statin, and hypoglycemic agent on admission were more frequently observed in patients with SAP. In AMI patients, there were 16 men and 6 women with a mean age of 67 ± 10 years (range 50–80 years). The mean value of the maximum CK level was 2970 ± 2318 IU/L. The mean interval from AMI onset to reperfusion was 7.2 ± 6.5 h.

Heterogeneous of monocyte subsets

The CD14+CD16+ were less observed in patients with AMI than

Discussion

During the pathogenesis of ACS, monocytes undergo phenotypic transformation, leading to their activation. TLR4 appears to be particularly important for monocyte activation in ACS. To date, the association between TLR4 and monocyte subsets in patients with ACS has not been investigated. In the present study, we showed for the first time the different expression levels of TLR4 between monocyte subsets in patients with AMI. In patients with AMI, TLR4 was more frequently expressed on CD14+CD16+

Study limitations

A number of limitations may be associated with this study. First, the study population was relatively small. Second, we did not investigate the differences in mRNA expression or intracellular signaling differences between the two monocyte subsets. Therefore, our results might be limited and did not show enough information about mechanistic link between monocyte and acute coronary syndrome. Third, we did not examine TLR2 expression in this study. Wyss et al. [15] have shown the over-expressions

Conclusions

The expression levels of TLR4 on CD14+CD16+ monocytes in patients with AMI were significantly elevated compared with those with UAP or SAP. TLR4 expression levels on CD14+CD16+ monocytes at the culprit site in AMI patients were significantly higher than at the peripheral site, suggesting the involvement of these specific monocyte subsets in the process of coronary plaque rupture. Further studies are needed to clarify the precise role of monocyte subsets in ACS as well as any potential novel

Conflict of interest and financial disclosure

None.

Acknowledgment

This work was supported in part by Grant-in-Aid for Scientific Research (C), Japan (No. 23591058).

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