Although several studies investigated the correlation between hResistin and the pro-inflammatory signaling pathways, the main cause of the promoting effect of hResistin on the promotion of IL-1β-associated pro-inflammatory cytokine cascades is still not fully understood. However, it is suspected that the activation of toll-like receptor (TLR)4, which is reported to serve as a receptor for the pro-inflammatory effects of resistin in human cells, could trigger the upregulation of not only the IL-1 family of cytokines but also of TNF-α 11–14. As shown in Fig. 3, hResistin stimulation of both hAF and hNP cells induced the phosphorylation of p38 MAPK and p65 NF-κB, which promote inflammatory cytokine activation cascades in hIVD tissues. MAPK is activated by various exogenous factors and is involved in various inflammatory diseases, such as osteoarthritis, rheumatoid arthritis, and neurodegenerative disease 15,16. MAPKs are divided into three subunit types: p38, ERK, and JNK. Regarding the inflammatory cytokines of the hIVD, p38 and ERK are associated with a decrease in ECM synthesis, and p38 and JNK are involved in the activation of ECM catabolic enzymes, such as MMPs 17,18. Another important pathway involved in the inflammatory response involves NF-κB, a central component of the hIVD cellular response to damage, stress, and inflammation. NF-κB is composed of the most common heterodimer, the p50/p65 subunit complex, which regulates several pro-inflammatory mediators, such as TNF-α, IL-1β, IL-6, and MMPs 19,20. Thus, p38 MAPK and p65 NF-κB may play important roles in inflammatory IVD degeneration. The phosphorylation of p38 MAPK and p65 NF-κB is associated with TLR4-related inflammasome upregulation 21–23. The interaction between hResistin and hIVD cells results in the mRNA expression of NLRP3, Caspase-1, and pro-IL-1β. The inflammasome components involved in the inflammatory response are shown in Fig. 4. The gene expression of hResistin-induced inflammasome components exerts a greater influence in hNP cells, and the role of hResistin in the inflammatory response is expected to be more dominant in hNP cells than in hAF cells. Nevertheless, the gene expression induced by hResistin stimulation was significantly different from that of the unstimulated control groups, but the fold-change value was not significantly different; thus, hResistin mono-stimulation did not seem to significantly affect inflammatory cytokine expression in hIVD cells.
Based on the significant increase in secretion of IL-1β, IL-6, IL-8, MMP-1, MMP-3, and MMP-13 induced by combined-hResistin and IL-1β stimulation of both hAF and hNP cells, compared with that induced by IL-1β mono-stimulation (Figs. 5 and 6), the promoting effect of hResistin on the activation of the IL-1β-associated pro-inflammatory cytokine cascade was observed. This cascade is associated with the progression of obesity-related inflammatory disc degeneration; this further supports the low-grade pro-inflammatory cytokine cascade activation nature of hResistin, which is widely reported to be associated with the development of insulin resistance in obesity 2,6,24−27. hResistin is primarily expressed in immune cells, such as monocytic cells, which are key players in the inflammatory response, in contrast to rodent resistin, which releases rodent adipocytes in white adipose tissues 13,14,28. Thus, hResistin is expected to have a greater influence on the inflammatory response in adipokine-related diseases than rodent resistin. The promoting effect of hResistin on the IL-1β-associated pro-inflammatory cytokine cascade might be influenced by the correlation between TLR4 and hResistin-induced expression of IL-1β and the phosphorylation of p38 MAPK and p65 NF-κB. The treatment with each specific inhibitor of IL-1β, p38 MAPK, and p65 NF-κB effectively inhibited the promoting effect of hResistin-related inflammatory mediator protein expression (Figs. 7 and 8). The IL-1 signaling inhibition of the combined groups reduced both the increased inflammatory cytokine and ECM catabolic enzyme expression levels, which may be due to a promoting effect of hResistin-induced low-grade inflammatory cytokine activation in the IL-1β-associated inflammatory response. Nevertheless, the decrease in levels of ECM catabolic enzymes in hAF cells was lower than that in hNP cells, which is expected to be more associated with hNP cells than with hAF cells in the correlation between hResistin and IL-1β-associated inflammatory response. When comparing p38 MAPK and p65 NF-κB, which are involved in the expression of inflammatory mediators in hIVDD, the NF-κB signaling inhibition showed a greater reduction in levels of inflammatory mediators than p38 MAPK signaling inhibition, indicating that NF-κB signaling plays a more important role than p38 MAPK signaling. It is also thought that pathways other than the p38 MAPK pathway may be involved in ECM catabolic enzyme expression 29,30. In 2014, Eltom et al. reported the release of IL-1β, induced by TLR4, which was activated via direct exposure to endotoxin lipopolysaccharide (LPS) in living mouse lung tissues in vivo 31. Similarly, in the following year, Lin et al. also reported an increased expression of IL-1β and TNF-α induced by the activation of TLR4, which was induced by in vitro stimulation of synovial fibroblast cells with LPS 32. It is widely accepted that IVDD is evident by the increased secretion of TNF, IL‑1α, IL‑1β, IL‑6, IL‑17, IL‑8, and IL‑2, which leads to an imbalance in catabolic and anabolic responses, leading to the degeneration of IVD tissues, as well as disc herniation and radicular pain via the promotion of extracellular matrix degradation, chemokine secretion, and IVD cell phenotype changes 33–35. However, increases in the expression of MMP‑1, MMP‑3, MMP‑7, MMP‑9, and MMP‑13 are associated with the promotion of a degenerative response 30,36−38. However, the specific inflammatory disc degeneration-associated inflammatory pathways that are responsible for the promoting effect of hResistin on the promotion of IL-1β-associated inflammatory cytokine cascade activation remain to be identified and should be investigated in future studies.
Additionally, it was interesting to observe that the changes in secretion levels of all inflammatory mediators in hNP cells induced by the combined-hResistin and IL-1β stimulation were determined to be marginally greater than those induced in hAF cells. This trend may be due to the accelerated dehydration and desiccation of NP cells, induced by the promotion of apoptosis of NP cells activated by the IL family of inflammatory cytokines. Wang et al. reported that IL-2, which is associated with the onset of thinning and tearing of hAF cells at the early stage of inflammatory IVDD, promotes the apoptosis of NP cells by upregulating destructive enzymes, such as MMPs or ADAMTS, downregulating the aggrecan expression levels, and altering the levels of collagen type I–II via the death receptor pathway activated by an increased activity of caspase-3 and caspase-8, which is associated with the upregulation of Fas protein expression 39. The NP is encapsulated by endplates and the AF has a significant influence on the overall function and homeostasis of the intervertebral discs by acting as a pump to regulate the flow of liquids and gases in the discs via proteoglycan‑rich ECM, maintaining the hydration levels 40.
In conclusion, based on the significant upregulation of target inflammatory IVDD-associated inflammatory mediators, induced by the combined-hResistin and IL-1β stimulation of hIVD cells, compared to that induced by mono-IL-1β stimulation, the promoting effect of hResistin, which is associated with the development of insulin resistance in obesity, on the progression of inflammatory IVDD via upregulation of inflammatory mediators associated with the activation of IL-1β pro-inflammatory cytokine cascade has been observed. These results further support the low-grade pro-inflammatory cytokine cascade activation nature of hResistin. Moreover, the significance of NP in the progression of inflammatory IVDD has been observed via a marginally greater secretion of inflammatory mediators from NP cells than from AF cells, after stimulation with hResistin and IL-1β. The results of our study improve our understanding of the pro-inflammatory mechanism of action of hResistin, which is associated not only with the progression of inflammatory IVDD but also with other obesity- and diabetes-related inflammatory diseases.