IL-6, recognized as one of major pro-inflammatory cytokines, like IL-1 β and TNF-α, belongs to the family of IL-6-type cytokines which includes IL-6, leukemia inhibitory factor 9 (LIF), IL-11, ciliary neurotrophic factor (CNTF), oncostatin M (OSM), cardiotrophin-like cytokine (CLC) and cardiotrophin-1 (CT-1). These aforementioned cytokines are mainly involved in activating target genes responsible for cell differentiation, survival, proliferation, and apoptosis (19). After IL-6 is bound to IL-6R (IL-6 receptor α on the cell membrane, CD126) or sIL-6 (the soluble form of IL-6R), the ligand-receptor complex will associate with glycoprotein 130 (gp130), promoting dimerization and the subsequent initiation of intracellular signaling. The three major downstream signaling pathways include the MEK/ERK, PI3-K/AKT, and JAK/STAT3 pathways (20). When considering the role of inflammatory cytokines in inducing NPC degeneration, IL-6 receives less attention than IL-1β and TNF-α, though they all belongs to the family of proinflammatory cytokines. It is known that some classical inflammatory cytokines, like TNF-α, IL-1β, and IL-6, are found to be overexpressed in degenerated or herniated discs, but IL-6 is not so ‘classical’ than the other two because the expression levels of IL-6 are demonstrated to be affected by IL-1β or TNF-α, especially when the other two are applied to induce NPC degeneration. Additionally, researchers have become more willing to view this as a ‘sign’ of inflammatory response or it being a ‘participant’, just like metalloproteases (MMPs) but not an ‘inducer’ like IL-1β or TNF-α. Rebecca et al. found that after addition of exogenous IL-6 and IL-6 soluble receptor, the effects of which had been induced by IL-1β or TNF-α on NPCs were significantly enlarged (21). Therefore, what exactly the role of a dose of IL-6 play in IDD remains a question. Actually, IL-6 itself and its downstream JAK/STAT3 pathway has already been shown to be involved in the pathogenesis of IDD in recent years (22). In that study, the activation of the JAK/STAT3 pathway can directly increase the expression levels of cyclooxygenase-2 and matrix metalloprotease-13, thus causing IDD without the involvement of ‘classical’ IL-1β or TNF-α. Thus, with further unraveling of the mechanism of IL-6 and its downstream pathways in recent years, we potentially must look more closely at them for their involvement in the initiation or progression of IDD in the past. So, in terms of RSV, there is no need to doubt its therapeutic impact on IDD because there have been plenty of studies demonstrating that in the past. However, nearly all of them have described the mechanism from the view of inhibiting the downstream pathways that have already been activated by IL-1β or TNF-α (8, 9, 16). We definitely do not deny that, but we want to explain the therapeutic mechanism from a novel angle, which is IL-6 and the JAK/STAT3 pathway, the reason why we have concentrated on them and established meaningful findings.
IL-6 itself can be induced by various stimuli that mostly exert their functions through the activation of NF-κB, CREB, C/EBP, and AP-1, which are transcription factors shown to bind to IL-6 promoter (23–25). As such, IL-6 exhibits a special characteristic whereby it can be auto-regulated (26, 27). Studies have found that certain downstream pathways triggered by IL-6 can sometimes act upstream, thus regulating the expression of IL-6. For example, MEK/ERK kinase does this by activating NF-κB and PI3-K/AKT pursues this purpose by activating IKK-α, which in turn activates AP-1 and NF-κB (28–30). However, the JAK/STAT3 pathway, as the most well-known IL-6 downstream pathway, remains controversial until Huang et al. claimed that JAK/STAT3, in combination with other downstream IL-6 pathways, frequently and significantly promoted IL-6 autocrine through a positive feedback loop in a wide range of cancer cell lines and clinical samples (31). The phenomenon of feedback regulation of IL-6 in cancer cells has received more attention than that in NPCs for many reasons. There is no such official statement that type of phenomenon exists in NPCs, not to mention its role in IDD. However, it is possible to still find certain clues to indicate the existence of the phenomenon in NPCs. For example, the expression levels of IL-6 and the JAK/STAT3 pathway are elevated in degenerative lumber disc tissues, which have often been observed in various cancer tissues (6). Interestingly, in the study of Rebecca et al., the IL-6 expression levels of NPCs were also elevated after exogenous IL-6 was added (21). Yet, it did not occur to them that this could be a clue to demonstrate the existence of positive feedback regulation of IL-6 in NPCs at that time.
Our study has shown that the phosphorylation levels of JAK1/STAT3 and the expression levels of IL-6 are both decreased after treatment with different concentrations of RSV, and that could be explained by the blockage of a positive feedback loop of IL-6/JAK/STAT3 by RSV. Though this supposedly makes sense to some degree, we still do not know if RSV may also inactivate the upstream pathways of IL-6, even when IL-1β or TNF-α is not added to NPCs. To our knowledge, we have been the first to investigate the biological function of RSV regarding NPCs under relatively normal physiological conditions. We hope our findings could be meaningful in terms of further investigation surrounding the mechanistic study of RSV, especially as that pertains to NPCs.