IDD is a major cause of low back pain and various degenerative spinal disorders. IDD has been a global health issue, which places a heavy burden on the healthcare system and results in severe economic consequences [23]. To date, little is known about the molecular mechanisms and early diagnosis method of IDD. Thus, it is necessary to obtain a more comprehensive understanding of IDD progression.
miRNAs, a group of small non-coding RNAs, are regulators of target genes by altering mRNA translation or stability [3-5]. Emerging evidence indicates that miRNAs play a key role in the development and progression of IDD [6, 7], and may be effective therapeutic targets [9] and biomarkers[8]. Recent advances in microarray techniques enable us to determine the functional miRNAs and target genes in the progression of diseases, which can be used to identify key miRNAs for diagnosis and therapy of IDD.
In this study,3 miRNA microarray datasets were downloaded from GEO to obtain DEMs between normal and degenerative disc samples. Two DEMs, miR-557 and miR-516a-3p, were present in all the datasets. In the three microarray datasets, miR-557 was down-regulated. miR-516a-3p was down-regulated in datasets. However, it was up-regulated in GSE116726 dataset. Our qRT-PCR data showed that miR-577 was significantly downregulated,and miR-516a-3p was significantly upregulated,which consistent with the GSE116726 dataset.MiR-557 is a recently identified miRNA. Previous studies have been reported that it is related to the progression of various human cancers, such as lung cancer, gastric cancer, hepatocellular carcinoma and breast cancer [24-27]. Qiu et al. found that miR-557 can suppress the proliferation and invasion of lung cancer cells by negatively regulating the expression of LEF1 [24], which corresponded to our analysis.Yu et al. studies have shown that miR-577 could be a marker for the diagnosis andprognosis of gastric cancer[25]. Regarding miR-516a-3p, there are only two publications in PubMed. The studies indicated that miR-516a-3p can regulate the metastasis and progression of tumor,and influence chemosensitivity[28, 29].These findings might suppose that miR-577 andmiR-516a-3p may be potential key regulators in the development of IDD.
Our GO analysis showed that the target genes were mainly associated with cell development and cell differentiation functions. The current lack of early-stage diagnosis and treatment for IDD is associated with a poor understanding of the biological mechanisms that regulate intervertebral disc (IVD) development and degeneration [30]. Intervertebral discs consist of a nucleus pulposus (NP), an annulus fibrosus (AF) and cartilage endplates (CEP). NP cells play critical roles in maintaining the integrity of intervertebral discs by secreting a complex extracellular matrix (ECM)[31]. Understanding the mechanism of NP development will provide the foundation for future repair, regeneration, and tissue engineering strategies for IVD disease [32]. Moreover, KEGG pathway analysis showed that most significant pathways were classical signaling pathways, such as the p53 signaling pathway and the Wnt signaling pathway. p53 is a famous tumor suppressor and plays key roles in metabolism. Xiong et al. found that increasing p53 levels could improve the negative effects of NP cell viability and integrity under low glucose conditions[33]. The Wnt signaling pathway is a major signaling pathway that participates in IDD progression. Activating Wnt/β-catenin signaling could regulates aggrecan expression,and induces expression of matrix metalloproteinases and TGF-β in NP cells, thereby promoting cellular senescence and IVD degeneration[34]. Previous studies have reported that miRNA can modulate intervertebral disc NP cells apoptosis, senescence, proliferation, and ECM synthesis via Wnt signaling [35-37]. All of these research findings are consistent with our results.
By constructing a PPI network, we identified 13 hub genes with degrees ≥ 10, including MAPK8, BMP4, and GSK3B, which were the 3 nodes with the highest degrees. To analyze the potential role of hub genes, an miRNA-target gene GO and KEGG analysis network was created and it was found that the hub genes, MAPK8, BMP4 and GSK3B, were mainly modulated by miR-577. Further analysis indicated that the hub genes were significantly associated with cell development, cell differentiation, and the Wnt signaling pathway. A literature search showed that the interaction among IDD and hub genes has rarely been reported. MAPK8, which is also known as JNK1, is a member of the MAP kinase and JNK family. Its functions include cell proliferation, cell differentiation, and apoptosis [38, 39].Hua et al.found that MAPK8 might be candidate biomarker for apoptosis in Melanosis coli [40]. Wang et al. verified that MAPK8 regulates the differentiation of embryonic stem cells by inhibiting/activating JNK signaling [41]. Furthermore, Ren et al.conclude that enhanced expressions of miR-330 inhibits atherosclerotic plaqueformation while promoting vascular endothelial cell proliferation by targeting MAPK8 via the Wnt signaling pathway in acute coronary syndrome rats [42]. Thus, MAPK8 might regulate the process of IDD. BMP4 is a member of the bone morphogenetic protein (BMP) family which belongs to the TGF-beta superfamily. It has been reported that BMPs play key roles in development of human diseases, and in maintenance of tissue homeostasis[43]. From the network, we found that BMP4 is only involved in the regulation of cell development, cell differentiation and the TGF-beta signaling pathway. Data from El-Safadi et al. suggests the existence of reciprocal relationship between TGF-beta and BMP signaling that regulates hBMSC lineage commitment and differentiation [44]. In addition, some research has shown that TGF--beta, Wnt signaling pathway components and BMP4 are essential genes during the notochord to NP transition [45, 46]. Therefore, BMP4 maybe closely involved with development of NP cells. GSK3B is an integral kinase within the β-catenin destruction complex. GSK3B can suppress Wnt signaling by phosphorylation and proteasomal degradation of β-catenin[47]. Several studies have shown modifications of GSK3B leading to Wnt/β-catenin signaling activation/suppression,which promotes cancer progression and chemosensitivity [48, 49]. Importantly, Wang et al. found that circSEMA4Bcanacts as a miRNA sponge and competes with GSK-3β for binding to miR-431,thereby inhibiting IL-1β-induced degeneration in NPCsvia regulating Wnt signaling[50]. The results of these previous studies were in agreement with our study, which showed that GSK3B through modulation of Wnt signaling pathway activity might affect the development of IDD.
In conclusion, this study identified two novel DEMs (miR-557 and miR-516a-3p) and 13 hub genes via a comprehensive bioinformatics analysis approach. Validation experiments by qRT-PCR suggested that miR-557 and miR-516a-3p may have a certain influence on the pathological processes of IDD. Notably, the roles of miR-557 through its targeting of MAPK8, BMP4 and GSK3B may improve our understanding and provide new therapeutic targets for IDD. However, there are still some deficiencies in this study. First, the sample sizes of microarray data are small, so further studies with larger sample sizes are still needed in order to confirm the results. second, only the top three hub genes and Wnt signaling were selected for further analysis, which might overlook important functions of other hub genes and pathways.