In the present study, the differences in gene expression between IVD patients and healthy controls were examined, and it was found that the changes in immune and metabolic functions are associated with these genes. AKR1C3, CKB, KRT19, MT1G and MUC1 were identified to have a therapeutic and preventive role in IVD, which may be a marker for early diagnosis and prognosis.
The heterogeneity of immune microenvironment in IVD is very high, and the type and number of infiltrating immune cells vary greatly in different locations. In this study, ssGSEA algorithms results revealed a higher percentage of Th2 cells and a lower percentage of Th1 cells in IVD groups. According to this outcome, it was found that the Th1/Th2 ratio of IVD patients were lower than that of the healthy control. In normal circumstances, Th1 and Th2 cytokines maintain a relative balance, which is essential for maintaining an immune equilibrium. An imbalance of Th1/Th2 cell may interfere with the cytokine network and lead to the occurrence and development of many diseases [14; 15].
IFN II response is significantly higher in the IVD group in our study, which is consistent with previous studies [10; 16; 17]. IFN-γ is a critical regulatory protein for both innate and adaptive immunity and plays an important role in immunological cell signaling [18]. Macrophages induce a large number of inflammatory responses during autoimmune and autoinflammatory diseases. TNF, IL-1α, IL-1β, IL-6, IL-8, IL-17, IL-2 and IL-10 are associated proinflammation mediators in IVD degeneration [10; 19; 20; 21]. Almost all of these inflammatory responses are exacerbated by interferon-gamma (IFN-γ), which increases the macrophage production of inflammatory mediators [22]. It has been found that the immune microenvironment containing macrophages or macrophage phenotypes involved in incomplete healing of annulus fibrosus and accelerated the progression of IVD [23]. Gabr, M. A. [24] et al. reported that IL-17 and IFN-γ contribute to the release of inflammatory mediators, and an increase in intercellular adhesion molecule 1 (ICAM − 1) expression. Meanwhile, it was discovered that the DEGs was correlated to multiple pathways, including cell adhesion, innate immune response, ECM-receptor interaction and adherens junction.
Through the verification of GSE56081 dataset, five key genes were finally identified: AKR1C3, CKB, KRT19, MT1G and MUC1. As a member of aldo-keto reductase family, AKR1C3 has a capacity to regulate the cell proliferation and differentiation in a hormone-independent manner [25]. A dual function for AKR1C3 in the metabolism of exogenous compounds was found. The aberrant expression of AKR1C3 is associated with the progression and aggressiveness of various diseases, including polycystic ovary syndrome, obesity, hyperandrogenemia, and tumors [26; 27; 28; 29]. In prostate cancer, it was confirmed that AKR1C2 and AKR1C3 mediates prostate cell proliferation via prostaglandin conversion through PI3K/Akt signaling pathways [30]. Previous studies revealed that PI3K/AKT signaling pathway can regulate the major extracellular component, aggrecan in NP cells and increase the activity of intervertebral disc cell matrix synthesis [31; 32; 33]. These findings indicated that AKR1C2 may contribute to the development of IVD via PI3K /AKT pathway. Additionally, our results displayed an inverse association between the expression of AKR1C3 and COL9A3. COL9A3 gene encodes one of the three alpha chains of type IX collagen that plays a connective role in creating cross-links between annulus fibrosis region and nucleus pulposus [34; 35; 36]. Paassilta et al. reported that at least one Trp3 allele was at three-fold greater risk of developing IVD [37]. Toktas et al. [38] found that patients with COL9A3 Trp3 were more likely to obtain a higher Pfirrmann score. Further studies should be conducted to investigate the potential pathway in AKR1C3 regulating IVD.
Creatine kinase B (CKB), a brain-type creatine kinase, reversibly catalyzes the transfer of phosphate from phosphocreatine to ADP to generate creatine and ATP [39]. CKB is crucial for the tissue with high fluctuating energy demands and plays an important part in distribution and local supply of ATP [40]. Downregulated expression of CKB severely affects cell energy homeostasis, thus leading to different cancers, obesity, and other metabolic diseases [41; 42; 43; 44]. In the model of CKB-knockout mice, Chang et al. observed markedly reduced bone resorption activity in osteoclasts [45]. The finding indicated the bone loss effect of CKB in osteoclasts [45]. What’s more, CKB was positively associated with Vitamin D receptor (VDR) gene that is a member of the nuclear steroid nuclear hormone receptor superfamily of ligand-inducible transcription factors [46]. The vitamin D endocrine system is essential to the calcium resorption, normal bone mineralization and remodeling [46]. The disorder of vitamin D metabolism is associated with pathological conditions of articular cartilage and intervertebral disc tissue, in particular osteoarthritis (OA) [47; 48] and IVD [49; 50; 51]. Furthermore, VDR is one of the most widely studied candidate genes that are supposed to involve IVD. Polymorphisms in VDR gene has been demonstrated to be associated with IVD in Japanese and Finnish populations. Sansoni et al. illustrated that FokI polymorphisms of the VDR gene, particularly the FF genotype and F allele, presented approximately 2-fold risk factors in the process of disc herniation [52]. Besides, L. Chen et al. performed a meta-analysis and found that VDR FokI polymorphism of VDR may be correlated to IDD among Caucasians [53]. These findings may provide an experimental basis for the treatment of IVD.
KRT19 belongs to a family of keratins, which is responsible for structural rigidity and multipurpose scaffolds [54]. KRT19 is often used as a positive marker for the identification of nucleus pulposus cells [55]. Moreover, some studies have shown that KRT19 expression in degenerated NP tissue is lower than that in normal NP tissue [55; 56; 57], which is consistent with our outcome that the expression of KRT19 is lower in the IVD tissues. Lv et al. [56] found that KRT19 presented a high NP:AF ratio and a high NP:AC ratio in degenerated NP. However, little is known about the potential mechanism about the downregulation of KRT 19 in aging IVD tissues or cartilage tissue.
Metallothioneins (MTs) are a family of low-molecular weight and cysteine-rich intracellular proteins that are expressed in response to oxidative stress and metal-induced toxicity [58]. MTs might also regulate the function of zinc-dependent proteins, such as p53, transcription factor, and chemo-sensitivity, by donating or taking away of zinc ions. MT1G is a widely studied isoform of MTs in humans, and it has been reported to play an important part in repressing carcinogenesis and inhibiting tumor metastasis and cancer cells differentiation because of excessive oxidative damage. The expression of MT1G is closely associated with aggressive phenotype, drug resistance, and poor prognosis for tumor progression of different types [59; 60; 61; 62].
It has been well established that MUC1, a membrane-bound mucin expressed on the surface of airway epithelial cells [63; 64], also serves an anti-inflammatory role in the course of various infections [65; 66; 67]. Some studies have demonstrated that the suppression of MUC1 expression can induce a recruitment of macrophages and trigger a hallmark of overstimulated immune responses [67; 68; 69]. McAuley et al. [68] found that the MUC1-knockout (KO) mice exhibited more susceptible to influenza A virus. They observed that MUC1-KO mice have a higher number of macrophages and a high level of pro-inflammatory cytokines, particularly IL-6 and MCP-1. Moreover, under the repetitive airway Pseudomonas aeruginosa (Pa) infection, MUC1-KO mice presented significantly elevated macrophages and airspace enlargement compared with Muc1 wild-type mice [70]. Based on our results, the course of IVD is closely associated with immune response and MUC1 may have vital mechanism behind IVD. Gruber et al. [71] ever reported that the expression of MUC1 increased in high-grade IVD. Interestingly, in vitro studies showed a significant downregulation of MUC1 in human annulus cells while they were exposed to IL-1 and TNF, which indicated that MUC1 may participate important immune response and result in the consumption of it. On the other hand, it was speculated that degenerated intervertebral may have different stages and we have reasons to believe that the activation of immune response may exist in some specific stages of IVD. Promoting the expression of MUC1 in IVD may serve as a new direction in the treatment of IVD.
Since there were no effective drugs for IVD, the online database was widely used to aid our understanding of human diseases and the prediction of new drugs. CMap is an easily accessible and effective tool in exploring new drugs, and it has been confirmed by many studies. From the CMap database, four compounds, including 1-Phenylbiguanide, LY2183240, Flubendazole and penciclovir, were identified and may have significant potential therapeutic effects on IVD. Penciclovir is a novel acyclic nucleoside analogue that has been shown to be effective against HSV1 and HSV2 [72]. In addition, penciclovir can alleviate neuroinflammation in neurodegenerative diseases [73], and inflammation can promote the degradation of extracellular matrix of intervertebral disc, thereby leading to intervertebral disc dysfunction and structural damage [74]. The role of penciclovir in IVD degeneration deserves further discussion. Flubendazole was approved in 1980 for the treatment of gastrointestinal nematode infection [75]. Recent studies have reported that flubendazole has significant antiproliferative activity in vitro and in vivo [76]. Inducing autophagy cell death seems to be the key to flubendazole mediated TNBC cell growth inhibition [76], and the aging and degeneration of IVDs are related to autophagy signal pathway transduction[77].
Some limitations of our study should be mentioned. First, the performance of our five key genes should be evaluated in more IVD datasets. Second, the main results of our study were based on public datasets, so they require further validation by actual experiments. Third, the clinical information of the IVD sample is unknown, which may influence the results of our study.
In conclusion, the five immune-related genes signature were identified to be related to IVD, with potentially substantial clinical significance. Most importantly, two prospective drugs for the treatment of IVD were identified. More studies are needed to confirm the therapeutic targets of these genes. Furthermore, bioinformatic analyses will provide new approaches for studying the pathogenesis and treatment of IVD degeneration.