Introduction
Obstructive sleep apnea (OSA), which affects more than 5% of the adult population, is an increasing prevalent disease [
1]. It is characterized by repetitive episodes of complete or partial upper airway obstruction when asleep, resulting in subsequent arousals [
2]. The published literatures show that OSA patients have an increased risk of cardiovascular events, such as hypertension, myocardial infarction, heart failure, nocturnal dysrhythmias, and pulmonary hypertension [
3]. It has been reported that exaggerated negative pressure in thorax, hypercapnia, intermittent hypoxia, and surges of sympathetic activity might contribute to these cardiovascular diseases mediated through the endothelial dysfunction [
4].
Among these cardiovascular diseases, coronary heart disease is the result of the accumulation of atheromatous plaques within the coronary artery walls [
5]. Some studies have showed the high prevalence of OSA among patient with coronary heart disease and high prevalence of coronary heart disease among patient with OSA [
6,
7]. Intermittent hypoxia could activate the inflammatory reaction which is critically involved in OSA as demonstrated by both in vitro and in vivo studies [
8]. The activation of leukocyte and endothelial cell and the adhesion of leukocyte to endothelium are known to result in the inflammation and the development of atherosclerosis [
9]. The transmigration of circulating monocytes into vascular intimal space initially attracted by various chemokines secreted by vascular endothelial cells is an important tread in the progression of atherosclerosis [
10].
Regulated upon activation normal T cell expressed and secreted (RANTES), an 8-kDa polypeptide of the C-C chemokine family, was a mighty chemotactic factor for monocytes and T lymphocytes [
11]. RANTES was expressed highly in atheroma, and higher plasma RANTES levels were related to the extent of carotid atherosclerosis and high-risk plaques [
12]. C-C chemokine receptor type 5 (CCR5), one of the receptors of RANTES, is a G protein–coupled receptor that belongs to the beta chemokine receptor family of integral membrane proteins [
13]. Blocking RANTES/CCR5 signaling with antagonist in vivo influences the development of atherosclerotic lesions [
14].
Although one study revealed that inhibition of RANTES attenuated intermittent hypoxia (IH)–evoked inflammatory preatherosclerotic remodeling and some literatures demonstrated that hypoxia can induce CCR5 expression [
15‐
17], there are still puzzles in understanding whether “intermittent hypoxia” can activate monocyte to express more CCR5, which causes subsequent atherosclerosis development. Also, no currently published literature has mentioned about the changes of CCR5 expression in monocyte from OSA patients. In the present study, we therefore investigated how intermittent hypoxia affects the regulation of CCR5 expression and related signal transduction pathways in monocytic THP-1 cells. The CCR5 expression was also examined in monocytes isolated from OSA patients.
Discussion
We demonstrated in the present study that intermittent hypoxia can stimulate the monocytes to actively express CCR5 at both the membrane protein levels and mRNA, which subsequently increased the migratory ability of monocytes toward RANTES and adhesion to endothelial cell. Besides, the p44/42 MAPK pathway was demonstrated to contribute to the activation of monocytes by intermittent hypoxia. Furthermore, increased monocytic CCR5 expression was found in severe OSA patients.
It has been demonstrated that some chemokines and their receptors are in charge of the adhesion, transendothelial migration, and chemotaxis of monocytes which is important in the initiation of atherosclerosis [
22]. Studies using ApoE-null mice combined with the deficient chemokine or its receptor have further confirmed their roles in the pathogenesis of atherosclerosis [
23]. C-C chemokine receptor type 5 (CCR5), which belongs to the beta chemokine receptors family of integral membrane proteins, is expressed in peripheral blood leukocytes, including monocytes, macrophages, and T cells [
13,
24]. It regulates leukocyte chemotaxis in inflammation and serves as a functional receptor for various inflammatory CC-chemokines [
25]. Among these chemokines, RANTES immobilized on activated endothelium can trigger leukocyte transmigration, which is mediated by specialized roles of CCR5 [
26]. Also, in an animal study, CCR5 plays a role in the recruitment and activation of leukocytes as well as of vascular cells, and the blockade of CCR5 by RANTES antagonist would prevent leukocyte migration into lesion [
14]. In OSA patients, serum RANTES level was found to be independently associated with AHI after an acute cardiovascular event [
27]. In this study, the upregulation of CCR5 expression in monocytes of severe OSA patient was confirmed. The earlier published data showed that the level of RANTES markedly higher in OSA patients might further augment the consequence of intermittent hypoxia on the adhesion and chemotaxis of monocytes toward endothelial cells.
The CCR5 gene expression has been reported to be induced by hypoxia in dendritic cells [
28], and hypoxia-ischemic injury can enhance CCR5 gene expression localized to endothelium in rat brain [
29]. Many studies have demonstrated that the expression of CCR5 is absolutely critical in human immunodeficiency virus (HIV)–positive patients and various CCR5 inhibitors have been developed to treat HIV disease [
30]. In recent years, obstructive sleep apnea has been increasingly reported in HIV patients [
31,
32]. This study was the first to show that in vitro intermittent hypoxia can upregulate the monocytic CCR5 expression at the protein levels and mRNA. Besides, we further examined the correlation between human monocytic CCR5 expression with oxygen parameters in PSG and revealed that only ODI and mean SpO
2 have positive correlation with CCR5 gene expression but not lowest SpO
2 or time with SpO
2 < 85%. These results supported our point of view that the increased CCR5 expression correlates more with the frequency of hypoxemic episodes rather than the duration of the hypoxemic episodes, or the severity of hypoxemia.
The upregulation of monocytic CCR5 mRNA expression has been found to be mediated by the activation of some signal pathways such as p44/42 or p38 MAPK [
33,
34]. The inhibition of p44/42 and p38 MAPK signal pathway in monocytes by PD98095 and MSB202190 respectively diminished the CCR5 gene expression enhanced by different stimulator [
35,
36]. However, there is no reported investigation on the CCR5 gene expression pathway under hypoxia or intermittent hypoxia. In our study, the inhibition of p44/42 by PD98095 decreased the monocytic CCR5 gene expression stimulated by intermittent hypoxia. On the other hand, there was no inhibitory effect on the intermittent hypoxia-induced monocytic CCR5 expression by pretreatment with p38 MAPK inhibitor. Together, results revealed the activation of p44/42 was needed for the upregulated monocytic CCR5 expression triggered by intermittent hypoxia.
Some studies have revealed the underlying mechanism and possible pathway in intermittent hypoxia and atherosclerosis by in vivo and in vitro studies. The underlying mechanisms of intermittent hypoxia related to the atherosclerosis formation include inflammation, oxidative stress, platelet activation, cell apoptosis, vascular endothelial injury, insulin resistance, and neuroendocrine disorders [
37]. Endothelial cell injury is an important mechanism of atherosclerosis, and chemotaxis plays the initial crucial role in that process [
22]. We have found in our previous reports that the upregulated expression of MCP-1 and CCR2 in monocytes of patients with severe OSA and monocytic MCP-1 and CCR2 gene expression can be activated under intermittent hypoxia which subsequently promotes the adhesion and chemotaxis of monocytes [
18,
20]. Combined together, this present study proved that intermittent hypoxia can induce CCR5 expression in monocytes, directly supporting the interpretation that intermittent hypoxia can enhance the chemotaxic ability of monocytes, which consequently results in more cardiovascular events in severe OSA patients. This is truly another new possible molecular mechanism that differs from other studies. In addition to atherosclerosis, pulmonary hypertension is frequently seen in patients with OSA, and the coexistence of pulmonary hypertension and OSA predicts a worse prognosis and higher morbidity [
38]. It is interesting to find out that the marked CCR5 expression in the macrophages of the lungs from patients with pulmonary hypertension might represent a new therapeutic target to modulate the cell growth and arterial remodeling [
39].
The novelty of our study is to prove the upregulated gene expression of monocytic CCR5 in OSA patient; however, there are some limits that can be discussed. We have excluded the potential confounders in this study that might interfere with the CCR5 expression such as ischemic heart disease. Although we tried to exclude the potential cause of systemic inflammation as much as we could, there are still myriad of possible causes of inflammation beyond we thought. It is a better design to test the inflammatory effect of intermittent hypoxia in the same patient group to eliminate the possible confounders. In addition, body weight is proved to be an important factor affecting OSA. It is difficult to enroll patients with high BMI and without any respiratory event during sleep. We separated recruited patients into four groups in accordance with the OSA diagnostic criteria; the ODI and AHI in 72 patients were noticeably different among these groups. Although the BMI was also significantly different among groups, the possible effect of BMI was excluded because only ODI and AHI have positive correlation with monocytic CCR5 expression after multiple regression analyses. Although OSA is obviously a chronic disorder, the night time events, such as intermittent hypoxia, may have acute effects contributing to this disease. According to the published paper by Dr. Tamaki, they found that just one-night hypoxic stress can activate the invasive function of monocytes in patients with OSA [
40]. Also, our previous published literature comparing the serum MMP-9 expression in patients with OSA revealed the same phenomenon that just one-night events can increase MMP-9 expression after sleep [
21]. It seems that some parts of OSA-related injury occur on a nightly basis. On the other hand, during event-free daytime, some injuries may have been recovered in somewhat degree, depending on the severity of injury itself and host repair ability [
41]. Thus, different measuring time point, such as before sleep or after sleep, may yield different results in OSA patients.
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