It is known that vascular endothelial cells (ECs) first sense ESS changes, a known activator of ECs that contributes to atherosclerotic plaque formation mainly in bifurcated vessels such as carotid arteries [14,19,20]. In fact, vascular injury and endothelial dysfunction induced by abnormal ESS are often regarded as a hallmark for AS. Continuous low-grade injury to ECs, induced by disturbed flow at arterial branch points and curvatures, could lead to apoptosis and inflammation, causing endothelial cell dysfunction which was considered critical initiating step in the pathogenesis of AS . Therefore, further studies are necessary to fully understand the potential mechanisms. In the present study, we focused on alterations in ECs gene expression that result from abnormal blood flow shear forces, which are capable of inducing atherosclerotic plaque progression.
ECs express a unique transcriptional profile under very high ESS, which is known to induce extracellular matrix synthesis and expansive arterial remodeling [15,21]. In addition, excessively low blood flow shear is able to induce the change of the gene expression pattern of ECs, leading to the progression of atherosclerotic lesions [6,16]. Studies have reported that shear stress may regulate the growth characteristics of vascular smooth muscle cells by altering the EC and inflammatory regulation, and then contribute the formation of atherosclerotic lesions [15,17]. Based on the above researches, we further studied the specific molecular mechanism of atherosclerotic plaque progression induced by abnormal blood flow shear stress using bioinformatics technology.
Previous studies have shown that physiological shear stress is in the ranges of 1-50 dyne/cm2 [22,23]. In this study, we analyzed the changes of gene expression profiles of ECs exposed to low, physiological and high (0,10 or 15,75 dyne/cm2) shear stress (GSE45225 and GSE23289). Using bioinformatics techniques, we identified 1141 LESS-related DEGs from GSE45225 and 1844 HESS-related DEGs from GSE23289. From GSE43292, we got 1481AS-related DEGs between AS tissues and control samples.
To search for preventive and therapeutic targets for AS induced by ESS, three co-hub genes were screened out: FBXO32, ICAM1 and TNFRSF1B. In addition, the expression of these genes in advanced atherosclerotic plaques was also studied. Compared with their expression levels in control tissues, ICAM1 and TNFRSF1B expression increased in atherosclerotic plaques while FBXO32 expression decreased. ROC analysis demonstrates that the differential expression of these three genes had reliable value in differentiating plaques and even identifying advanced stages. The three promising mRNAs, proposed by this study, that could provide some clues to reveal the potential molecular mechanism of abnormal ESS and AS. These data will also help to predict the clinical deterioration of patients with advanced and ruptured AS plaque, and may also provide potential targets for treatment.
ICAM1, a member of the immunoglobulin family, is expressed on cell surface and mediates the adhesion of the cell to other cells or to the extracellular matrix[24,25]. Our analysis demonstrates that excessively low blood flow shear forces produce a significant increase in ICAM1 expression in ECs. In atherosclerotic plaques, increased ICAM1 expression was detected. Ishibazawa et al. also proved that LSS could promote the expression of proinflammatory genes ICAM1[26]. Earlier studies showed that ICAM1 was barely expressed on the cell surface in a normal endothelium or without proinflammatory stimuli [27]. Under pathological conditions, increased expression of ICAM1 can mediate endothelial cell activation, causes enhanced interaction of ECs with leukocytes. This is followed by a stepwise recruitment of leukocytes, rolling, activation, migration and adhesion on the vascular basement membrane, which in turn induce atherosclerotic plaque formation [28,29]. The AS risk in communities (ARIC) study indicate that plasma levels of ICAM1 may serve as molecular markers for AS and the development of coronary heart disease (CHD) [30]. In patients with coronary slow flow (CSF), ICAM1 K allele plays an important role in the pathogenesis of AS, which is related to the decrease of coronary blood flow. Both above studies and results of our analysis suggest that ICAM1 plays a central role in endothelial dysfunction. Combined with our research, it is reasonable to suggest that ICAM1 plays an important role in AS progression resulting from abnormal ESS and might be a possible target for AS prevention and treatment.
Tumor necrosis factor (TNF) is a pleiotropic cytokine that involved in the pathogenesis of inflammatory diseases such as endothelial injury and AS. TNF exerts its biological functions by binding to its two different receptors, the TNF receptor 1 (TNFR1, TNFRSF1A) and the TNF receptor 2 (TNFR2, TNFRSF1B), which differ in structure, expression patterns and signaling pathways that they induce [31,32].TNFRSF1A is expressed in most cell types, while TNFRSF1B appears to function exclusively in immune and ECs [33]. Some studies have shown that TNFRSF1B mainly triggers tissue repair and regeneration, whereas TNFRSF1A promotes apoptosis or inflammation [34]. In the study by Luo, Y et al. have shown that overexpression of TNFRSF1B in ECs can reduce cell death induced by ischemia-reperfusion and promote the proliferation of ECs, vasoformation and vessel maturation after injury [35]. Our analysis showed that the abnormal increase or decrease of ESS could inhibit the expression of TNFRSF1B in ECs. These results have shown that TNFRSF1B plays a critical role in mediating endothelial dysfunction induced by abnormal ESS.
It is interesting to note that the expression of TNFRSF1B was increased in atherosclerotic plaque tissue. We suspected that in the early stages of endothelium dysfunction, abnormal ESS promotes endothelial cell death, inhibits endothelial cell proliferation and neovascularization after injury, and finally leads to endothelial dysfunction by inhibiting the expression of TNFRSF1B in ECs [35]. Moreover, the high expression of TNFRSF1B in atherosclerotic plaque tissue contributes to the progression, rupture or erosion of atherosclerotic plaques by stimulating leucocyte adhesion and inflammatory cell recruitment [36]. In addition, ROC analysis indicates that the expression level of TNFRSF1B represented highly efficient in discriminate AS (AUC = 0.801). Previous studies have also revealed the association of the TNFRSF1B allele with increased risk of development of myocardial infarction [37,38]. We can see through our studying and analyzing, that TNFRSF1B might be a potential new atherosclerosis prevention and therapeutic target.
FBXO32 was originally identified as a muscle-specific ubiquitin-E3 ligase, and further study indicated that it plays a key role in heart development and muscle homeostasis [39]. Serum starvation and hypoxia can also induce the expression of this gene, which can regulate apoptosis [40,41]. Another study showed that the deficiency of FBXO32 can lead to cardiomyopathy due to impaired autophagy [42]. Autophagy and apoptosis may be the potential mechanisms by which FBXO32 promotes the progression of AS. Nevertheless, the above researches on FBXO32 were mainly focused on cardiomyocytes and cardiac function. Currently, the role of FBXO32 in endothelial injury and AS are still poorly investigated, and the underlying mechanism is poorly understood. Through the analysis of datasets GSE23289 and GSE45225, we found that abnormal ESS can lead to the overexpression of FBXO32 in ECs. Further studies showed that, compared with the control group, the expression of FBXO32 in atherosclerotic plaque was significantly decreased and the ROC curves confirmed that FBXO32 could distinguish the AS state. Subsequent cellular experiments also demonstrated a significant decrease in FBXO32 expression in foam cells. Although the exact mechanisms still need to be identified, FBXO32 may represent an important target for prevention and treatment of AS.
We think that a series of genome-wide, unbiased screens for identifying hub genes would be of considerable value for revealing mechanisms and potential therapeutic targets. Our present findings have demonstrated that FBXO32, ICAM1 and TNFRSF1B play vital roles in abnormal ESS and endothelial dysfunction. These genes may warrant as valuable targets for prevention and treatment of AS induced by abnormal ESS.
This study has some limitations. First, due to the small sample size of datasets GSE45225 and GSE23289, larger studies are required to confirm our findings. Second, different sample sources and detection microarray platforms may contribute to some of the differences in gene expression. In addition, we verified the expression of these genes only in vitro assays, and no clinical specimens were obtained for further validation. In the future, to verify our hypothesis, more research that includes larger samples, randomized trial designs, further mechanistic studies, and even more clinical trials is needed.