The pathophysiology of RA is complex, perpetuated by an array of cells whose functions have been altered and converted to autoimmune cells, or made to follow an inflammatory pathway [10]. As an autoimmune disease, the reactivity of immune cells towards articular cells is central to RA. Neutrophils, lymphocytes, and platelets have been reported to play a role in the control of inflammation and are associated with alterations in secondary to inflammation [11]. Meanwhile, pro-inflammatory molecules are usually present in the circulation of patients with RA and contribute to the immunopathogenesis. These uncontrolled molecules are also closely linked to abnormal blood clotting via increased oxidative stress that leads to changes in cells of haematological system (including platelets) [12]. Research has showed that platelets and platelet-derived microparticles (MPs) are involved in the inflammatory processes and are found in arthritic joints [13]. Their activation is regulated by platelet signaling and secreted proteins, miRNAs, and molecular pathways. Then, they release cytokines, chemokines and growth factor stored in α-granules and dense granules to mediate their main functions, support the recruitment of inflammatory cells to tissue sites and trigger inflammation [14, 15].
The platelets (PLTs) arise in the bone marrow from megakaryocytes, some studies found that platelets have a significant role in skeletal homeostasis, modulating bone formation and resorption [16]. Many studies indicated the supportive effect of PLTs on bone formation result from platelet-derived growth factors (PDGFs) which favor bone formation by affecting cell proliferation, chemotaxis differentiation, and extracellular matrix synthesis [17]. However, the exact function of PLTs and its MPs on bone resorption is still complex to understand. There are several preclinical and clinical researches showing the role of PLTs in osteoclastogenesis and bone resorption. PLTs have vitamin D receptors which are less expressed in osteoporosis patients and related to the variation of BMD [18]. Eroglu, S. et al. found that the PLT/lymphocyte ratio also correlated with low BMD [19]. The mediators released from PLTs may be involved in bone remodeling. EGF cooperating with RNAKL with osteoclasts and TGF-Β stimulating OPG synthesis are more likely to enhance the bone resorption. Moreover, TXA2 induces osteoclastogenesis and enhances bone resorption [17, 20]. Chronic inflammation may play a pivotal role in osteoporosis and activated platelet [21]. Pro-inflammatory cytokines enhance oxidative stress which contributes to platelet activation. On one hand, those cytokines promote osteoclast formation and simulate bone resorption. On the other hand, activated PLTs affect osteoclastogenesis through prostaglandin and RANKL signaling [22].
In the current study, we identified 385 up-regulated and 264 down-regulated DEmRNAs between RA and control samples using bioinformatics analysis. Similarly, a total of 535 DEmRNAs including 227 up- and 308 down-regulated were identified between osteoporosis and control samples. As cumulative evidence has shown that co-expressed genes normally represent those with similar expression profiles that also frequently participate in similar biological processes [23], we further performed GO and pathway enrichment analyses. These analyses suggested that the DEmRNAs in RA and osteoporosis were commonly involved in “platelet degranulation”,” platelet alpha granule”,” platelet activation”,” tight junction” and “leukocyte transendothelial migration”. Most of them were related to PLTs, and the results were consistent with what has been discussed above. Furthermore, to identify common genes both in RA and osteoporosis, each DEmRNAs in them were intersected. A total of 28 common DEmRNAs including 23 up-regulated and 5 down-regulated were identified. Heatmap provided sufficient evidence indicating the presence of differentially expressed mRNAs. Chromosome mapping showed the distribution of genes in chromosome. Previously studies confirmed that X-chromosome consisted of many genes which were associated with RA and osteoporosis. Like CD99, IRAK-1, LAMP-2, CD40L,TLR7,DDX3X, XIAP and USP27X,they are involved in autoimmunity [24]. Van Dijk FS.et al. found that mutation in PLS3 might result in X-linked osteoporosis [25]. Our results indicated that the dysregulation of SH3BGRL and TMSB4X on X chromosome may be associated with RA and osteoporosis. Meantime, some studies indicated that the genes in chromosomes 1 and 13 may be related to RA and osteoporosis [26, 27, 28]. Our results showed chromosome 1 containing the greatest number of dysregulated genes.
The subsequent construction of the PPI network using the common DEmRNAs identified 17 genes as potential key genes involved in RA and osteoporosis. PTGS1 was identified as one of the hub genes and has been connected with multiple pathological disorders including inflammation, arthritis and cancer [29]. Upregulation of the PTGS1 and PTGS2 pathways of arachidonic acid is thought to be involved in the development of rheumatic diseases [30]. And another study in both ex vivo and in vivo showed that PTGS1 which controlled osteogenesis of adipose-derived stem cells was involved in the osteogenic differentiation [31]. Cho HW et al. reported that PTGS1 was associated with osteoporosis [29]. In our study, PTGS1 was over-expressed and also enriched in platelet activation pathway both in RA and osteoporosis, indicating its potential role in pathogenesis. In turn, MT-ATP6, another hub gene, derives from mitochondrial DNA. Previous studies have confirmed that each mitochondrion has multiple copies of mitochondrial DNA (mtDNA) which encode 13 protein subunits of the electron transport chain and 22 tRNAs and 2rRNAs [32]. In the plasma and synovial fluid of RA patients, mtDNA levels are higher than in control subjects. Moreover, they observed that mtDNA induces TNF-α expression through NF-κB activation, participating in inflammation and tissue injury [33]. Juping Du et al. found that enrichment of variants in MT-ATP6 was detected in RA patients [34]. In our study, MT-ATP6 was enriched both in RA and osteoporosis. Interestingly, another mitochondrial DNA A3243G mutation in blood leukocytes were found to be significantly associated with lower bone mineral density [35]. Further evidence for mitochondrial dysfunction as a potential contributor to osteoporosis is seen in mice with a mitochondrial transcription factor A(TFAM) knockout specific to osteoclasts [36]. So the accumulating mtDNA mutations may play a significant role in RA and osteoporosis.
In recent years, lncRNA related research has attracted the attention of various fields. However, lncRNAs have just begun to be understood, and the majority of them have not yet been researched. To explore the biological functions of lncRNAs, we undertook a comprehensive analysis and employed an co-expression network to identify interactions based on the 2 common differentially expressed lncRNAs and the 21 common differentially expressed mRNAs. Importantly, we identify two potential key pairs of lncRNAs and target mRNAs, including RP11 − 815J21.2 - MT − ATP6 and RP11 − 815J21.2- PTGS1. Prior study had showed that MT − ATP6 and PTGS1 were related to RA and osteoporosis, and we found the two pairs were significantly higher than those of control. But up to now, there is no report on this novel lncRNA in any disease, and our work filled the gap of lncRNA RP11 − 815J21.2. The result suggests that lncRNA RP11 − 815J21.2 could be a potential candidate as biomarker and need to be further evaluated and investigated using advanced diagnostic methods and more samples.