Our identification of common hub genes for atherosclerosis and AAA provides new insights into the shared biological mechanisms of these two diseases. Our findings of an association between atherosclerosis and AAA is consistent with previous studies [21, 22]. Identification of the common DEGs for atherosclerosis and AAA will help to explore their common pathogenesis, identify new therapeutic targets, and predict the therapeutic effect of biological agents.
Our study identified 133 overlapping DEGs between atherosclerosis and AAA, including 10 hub genes (TYROBP, PTPRC, ITGB2, ITGAM, PLEK, CTSS, LY86, ITGAX, CCL4, and FCER1G). GO and KEGG pathway enrichment analyses showed that these genes were involved in integrin-mediated signaling pathways, integrin-mediated cell adhesion, neutrophil chemotaxis, regulation of the actin cytoskeleton, chemokine signaling pathways, and antigen processing and presentation. These results demonstrate the important role of integrins, chemokines, and immune and inflammatory responses in both diseases. The GO analysis identified that integrin-mediated signaling pathways play an important role in both diseases. Furthermore, that leukocyte integrin αxβ2 was upregulated under hypercholesterolemic conditions with reduced atherogenesis after its deletion suggests that αxβ2 may be particularly important in atherogenicity [23]. Deposition of matrix proteins in atherosclerotic plaques creates a permissive environment for cell proliferation, migration, differentiation, and inflammatory responses, primarily via integrin α5β1 and αvβ3 signaling [24]. Fibroblast growth factor 18 and integrin β1 can improve the repair of AAA by increasing elastin expression, enhancing the migration and proliferation of smooth muscle cells, and improving aortic remodeling [25]. Therefore, integrins may be the link between atherosclerosis and AAA.
In our study, we further identified nine TFs in the TRRSUT database and four TFs in the ChEA3 database which may regulate the expression of the identified hub genes. By combining these results, the high reliability of expression of one TF (SPI1) in atherosclerosis and AAA was confirmed. SPI1 is involved in the regulation of eight hub genes (PTPRC, ITGAM, ITGB2, ITGAX, PLEK, CCL4, LY86, and CTSS). Of these, after gene expression verification, only ITGB2, CTSS, LY86, and ITGAX were found to be highly expressed in both atherosclerosis and AAA.
Integrin subunit beta 2 (ITGB2) encodes the integrin beta chain. The protein encoded by this gene plays an important role in immune responses, with a defect of this gene leading to defective leukocyte adhesion. ICAM1 and endothelial cells recruit circulating ITGB2, also known as CD18, and immune cells contribute to atherosclerosis; therefore, inhibition of ITGB2 can alleviate or even prevent the development of atherosclerosis [26]. Animal experiments have shown that treatment of mice with AAA using an anti-CD18 monoclonal antibody alleviates AAA expansion and reduces the inflammatory response [27], indicative of the potential benefit of ITGB2 downregulation in patients with AAA.
Cathepsin S (CTSS) is a lysosomal cysteine proteinase that participates in the degradation of antigenic proteins into peptides for presentation on MHC class II molecules. CTSS is involved in the pathogenesis of cardiovascular diseases via its effect on extracellular matrix protein degradation, protein transport, and cell signaling [28]. CTSS can be secreted into the extracellular matrix via lysosomes, increasing collagen and elastin degradation, promoting vascular smooth muscle migration, and ultimately causing atherosclerosis [29]. Apoptosis of the medial smooth muscle cells of the arterial wall is an important marker of AAA, with an increase in apoptosis during aneurysm formation. Reduction of CTSS has been shown to attenuate smooth muscle cell apoptosis in the aorta, in vitro. and reduce smooth muscle cell loss in AAA lesions [30].
Lymphocyte antigen 86 (LY86), also known as MD-1, can form a complex with radioprotective 105 (PR105) to block the TLR4/MD-2 complex and, thus, attenuate inflammation via the NF-KB signaling pathway [31]. Therefore, an RP105 deficiency can lead to a slower progression of early atherosclerotic plaques [32]. Divanovic et al. showed that RP105 can suppress TLR4 signaling only when MD-1 is fully present [33]. Therefore, the detailed mechanism by which the specific RP105/MD-1 complex leads to atherosclerosis needs to be further elucidated. The expression of LY86 was not limited to immune cells but was also highly expressed in cardiovascular tissues. LY86 plays an important role in cardiac remodeling, myocardial hypertrophy, fibrosis, arrhythmia, and heart failure [34]. Although the effect of LY86 on AAA is still unclear, we believe that LY86 also plays an important role in the pathogenesis of AAA.
Integrin subunit alpha X (ITGAX), known as CD11C, encodes an integrin X-chain protein that binds to ITGB2 to form a leukocyte-specific integrin called inactivated-C3b (iC3b) receptor 4 (CR4). ITGAX is a fibrinogen receptor that is important for monocyte adhesion and chemotaxis, which mediates cell-to-cell interactions during inflammatory responses. Monocytes are among the main cells involved in atherosclerosis. ITGAX can mediate the adhesion of monocytes to endothelial cells and then infiltrate the arterial wall through endothelial cells [35], which is an important link in the formation of atherosclerosis [36]. In addition, CD11C expression in macrophages is regulated by interferon regulatory factor-5, promoting the presence of CD11C-expressing macrophages within atheromatous plaques [37]. Previous studies have shown that CD4 T cells and CD8 + T cells decrease significantly after CD11C deletion, which can further down-regulate activity of neutrophil elastase, thus decreasing elastase degradation and increasing collagen content and, overall, inhibiting degradation of the abdominal aortic matrix [38]. However, few studies have explored the relationship between atherosclerosis and AAA.
Our study focused on the common hub genes and related transcription factors in atherosclerosis and AAA. Hub genes were identified using a complex network of interactions and key nodes. This bioinformatics approach has proven to be reliable for other diseases [39–41]. Moreover, we verified the expression levels of the hub genes and transcription factors, which made our results more credible. We believe that our results provide a new research direction for the molecular mechanism of atherosclerosis complicated by AAA.
The limitations of our study need to be acknowledged. The datasets we selected were from different platforms and, therefore, the detection methods and algorithms for the platforms are bound to be different. In the future, we plan to use a microarray from the same platform to test our patients to eliminate this difference. The function of hub genes also needs to be further verified in cell and animal models, which will be the focus of our future studies.