Knee OA is a common degenerative joint disease, which pathologically characterized by the progressive degeneration of articular cartilage. Mechanical stress may lead to alterations in the composition, structure, and metabolism of articular cartilage, therefore it was well known to be a significant risk factor for the onset and progression of OA [20]. In the current study, to explore the action mechanism of mechanical stress on the osteoarthritic development, a three-step approach was followed to perform bioinformatics analysis. Firstly, in total 213 DEGs were identified in cultured chondrocytes from knee and finger joints, of which 101 genes were upregulated and 112 were downregulated and then enrichment analysis was performed. Subsequently, 122 overlapped genes between the DEGs in the chondrocytes from knee and finger joints and those in the cartilage from healthy and osteoarthritic joints were identified, and then an enrichment analysis was performed for further understanding the regulating roles of the intersection genes in OA. Finally, analysis of miRNA regulatory network and miRNA associated pathway in the overlapped hub genes were also performed.
Compared to the proximal interphalangeal joint, the knee joint, a foremost weight-bearing joint, is continuously subjected to mechanical loading. During standing statically, the mechanical stress applied to human knee joints is about 3.5 MPa [21], while the compressive stress during activities such as stair climbing will rise up to 20 MPa [22]. The mechanical loading generated during daily activity is a fundamental stimulus for the activity of chondrocytes, physiologically necessary for maintaining cell homeostasis, while pathologically caused significant damage to the articular cartilage [23]. In this study, enrichment analysis of DEGs in cultured chondrocytes from knee and finger joints also revealed the dual character of biological process: positive or negative regulation of cell proliferation, positive regulation of cell division, positive regulation of apoptotic process and embryonic limb morphogenesis. A number of key transduction mechanisms have been identified that facilitate the mechanically-driven enhancement of chondrocyte homeostasis, including ion channel [24,25], integrins signaling [26] and primary cilia [27]. Correspondingly, the current study also revealed that DEGs exert molecular function through regulating heparin binding, growth factor activity, calcium ion binding and integrin binding. Meanwhile, the KEGG pathways analyzed by DEGs included ‘PI3K-Akt signaling pathway’, ‘FoxO signaling pathway’, ‘Ras signaling pathway’, ‘Cell cycle’ and ‘Oocyte meiosis’, which showed the consistence with the findings of previous studies [28,29,30,31].
PPI networks with the DEGs were constructed as well, and two modules included hub genes were also identified. Module 1 included Kinesin-like protein (KIF18A), KIF15, DNA topoisomerase (TOP2A), budding uninhibited by benzimidazoles (BUB1) and Cyclin-dependent kinase (CDK1), mainly involved in such biological processes as chromosome segregation, cell cycle and mitotic cell division, which agree with the results shown in the enrichment analysis. Module 2 including hub gene insulin-like growth factor 1 (IGF1) and vascular endothelial growth factor (VEGF) was associated with “platelet degranulation”, “protein processing”, “regulation of protein secretion” and “apoptotic signaling pathway”. These results are consistent with the previous studies indicating that mechanical loading directly induce weight-bearing area hypoxia followed by upregulated expression of VEGF [32], Shear stress induced the activation of IGF1 pathway which involved in the regulation of morphology and proliferation in the chondrocytes [33], and even IGF-1 has been shown to protect chondrocytes from apoptosis [34]. Collectively, mechanical stress has multiple influences on joint and cartilage, such as good ones including promoting cartilage development and maintaining chondrocyte homeostasis, or bad ones including inhibiting chondrocyte proliferation and accelerating cell apoptosis.
To confirm the effect of mechanical stress in the osteoarthritis development, the overlap between the DEGs in the chondrocytes from knee and finger joints and those in the cartilage from healthy and osteoarthritic joints was also performed. In total, we screened out 122 jointly expressive intersection genes, brought a main enrichment in DNA replication, cell division, negative regulation of stress-activated MAPK cascade, calcium ion binding and positive regulation of apoptotic process. In general, the loss of cartilage homeostasis and the dysfunction of chondrocytes phenotypes which includes cell apoptosis, cell division and proliferation, are the important pathological process of OA [35,36]. This study also suggested that the overlapped DEGs may involve in such biological process as DNA replication, cell division and positive regulation of apoptotic process. Meanwhile, the MAPK signaling pathway has been found to receive, transduce and convey mechanical information to the chondrocyte interior [37], which also evidenced by the enrichment of negative regulation of stress-activated MAPK cascade in the overlapped genes. Intracellular calcium signaling is among the earliest responses of chondrocytes to physical stimuli and plays an important role in the mechanobiology of chondrocytes in a natural residing environment [38]. Here, calcium ion binding was also found to be the significant enrichment for the DEGs, which suggested the role of calcium ion channel and downstream pathway in the mechanical stress-induced osteoarthritic development.
Additionally, many overlapped hub genes were identified in the PPI analysis, and submodule including hub genes CDK1 and Forkhead box M1 (FOXM1) was also selected, which enriched in cell cycle, regulation of cell cycle process and DNA replication. Cell cycle regulation is critical for chondrocyte differentiation and hypertrophy. Due to terminal differentiation is usually accompanied with cell cycle exit and the non-proliferative maintenance, cell cycle arrest is required for chondrocyte transition from proliferation to hypertrophic differentiation [39,40], which played a vital role in early and late stage OA [41,42]. In the present study, significantly, the hub genes involved in cell cycle were entirely down-regulated, which suggested the abnormal regulation of cell cycle process, implicating for chondrocyte differentiation and hypertrophy. CDK1, a member of the Ser/Thr protein kinase family, functions as a molecular switch from proliferation to hypertrophic differentiation of chondrocytes. Therefore, CDK1 is usually highly expressed in columnar proliferative chondrocytes and greatly downregulated upon differentiation into hypertrophic chondrocytes [43]. FOXM1 is a transcription factor that acts as a regulator of the cell cycle [44], which may involve in such biological process as mitotic cell cycle, Wnt signaling and apoptosis [45]. In addition, the role of overlapped hub genes including KIF11, BUB1 and MAD2L1 in the pathogenesis of OA was also found in the previous studies [46,47]. Accordingly, under mechanical stress, the cell cycle-related genes were downregulated, as a result, chondrocyte cell proliferation would be blocked and transformed to differentiation and hypertrophy, which may be important in the pathogenesis of OA.
Finally, the miRNA-regulatory network in the overlapped hub genes was also identified in this study. 120 predicted miRNAs were identified, including miR-146a-5p, miR-19b-3p, miR-122-5p and MiR-15a-5p, which were mainly enriched in TGF-β signaling pathway, Wnt signaling pathway and MAPK signaling pathway. Actually, these predicted miRNAs and pathways played important roles in OA has been verified in previous studies. Zhang et al have suggested that mechanical stress contributes to osteoarthritis development through the activation of TGF-β signaling pathway [5]. In addition, some professores suggested that circulating miRNAs, for example, miR-19b-3p and miR-122-5p, are promising biomarkers to diagnose knee OA [48, 49]. Furthermore, Chen et.al also suggested that MiR-15a-5p regulated the viability of human osteoarthritis chondrocytes via targeting VEGF [50]. Thus, the critical miRNAs and signaling pathways can be used as new targets for OA diagnosis and therapy, and can make some contributions for further experimental studies on the part of potential hallmarks.
In conclusion, mechanical stress has shown the multiple influences on chondrocytes and cartilage, physiologically necessary for maintaining chondrocyte homeostasis and promoting cartilage development. While, pathologically, mechanical stress could induce cartilage destruction and advance osteoarthritic development via boosting chondrocyte differentiation and hypertrophy, and accelerating cell apoptosis. Additionally, TGF-β signaling pathway, Wnt signaling pathway and MAPK signaling pathway is likely related to mechanical stress-induced OA progression and can be the potential biomarkers and therapeutic targets, which needs further experiment to verify and provide useful evidence for future researches in OA.