RHD has a long history, and many patients have been killed by this disease. RHD caused 319,400 deaths in 2015 , 314,600 deaths in 2016  and 285,500 deaths in 2017 , but the pathogenesis of this disease is still unknown. In recent years, research progress has mostly focused on the signaling pathways that are related to pathogenesis. Through the efforts of researchers, some signaling pathways related to this disease have been discovered. Recently, researchers have discovered that the S1PR1/STAT3 signaling pathway is involved in vavular damage due to RHD from the study on RHD model rats . However, it is unknown how intervening in the S1PR1/STAT3 pathway affects valvular damage caused by RHD. More importantly, which method can be used to intervene in the S1PR1/STAT3 pathway and effectively attenuate valvular damage due to RHD?
S1PR1 has been extensively studied, and it is an important factor in heart diseases. The heart diseases it participates in include not only RHD [34, 35], but also myocardial infarction , Cardiac Remodelling, etc. . In these heart diseases S1PR1 participates in, it mostly plays a role in protecting the heart [9–11], and the high expression of S1PR1 can usually protect the heart during the pathogenesis of heart disease [9, 12]. However, there is also a report of high expression of S1PR1 exacerbating heart damage , and the role of mediating the pathogenesis of disease through down-regulation of S1PR1 expression has also been reported in other diseases, such as multiple sclerosis . So it can be concluded that the expression level of S1PR1 in different heart diseases is not static. Although the expression level of S1PR1 in each heart disease is variable, in most cases S1PR1 is up-regulated in heart disease and plays a cardioprotective role. In this study, after up-regulating the expression of S1PR1, the valvular damage caused by RHD is reduced, which may also be related to the cardioprotective effect of S1PR1. There is a close relationship between S1PR1 and STAT3, and there have been many reports on the role of S1PR1 in regulating STAT3 in various diseases [22–25]. In this study, the effects of S1PR1 and STAT3 are closely related to the pathogenesis of valvular damage in RHD. In previous studies on the process of valvular damage in RHD, it was reported that the down-regulation of S1PR1 and increased degree of the phosphorylation of STAT3 were involved in this process [34, 35]. This phenomenon of down-regulated S1PR1 expression and increased degree of STAT3 phosphorylation has also been reported in previous studies, such as Garris et al. found that the deficiency of S1PR1 expression would increase the phosphorylation level of STAT3 and then promote Th17 cell differentiation by studying the S1PR1 gene mutation mouse autoimmune encephalitis model . The results of our current study also showed that the RHD group also had down-regulated S1PR1 expression and increased STAT3 phosphorylation level compared to the control group. However, some studies have reported that up-regulation of S1PR1 expression increases the phosphorylation of STAT3 [23, 42], but these studies are not about RHD. Combined with the uncertainty of the expression of S1PR1 in different heart diseases mentioned above, it can be concluded that the expression level of S1PR1 and its role of regulating the phosphorylation of STAT3 in different diseases and different physiological or pathological processes may not be static according to the current research progress in the world. Two studies that previously studied the role of the S1PR1/STAT3 signaling pathway in RHD and the experimental results in this paper indicate that down-regulation of S1PR1 may increase the phosphorylation of STAT3 during RHD-induced valvular damage, and then mediates the differentiation of Th17 cells [34, 35]. Therefore, the mechanism of S1PR1/STAT3 signaling pathway in the process of valvular damage caused by RHD may be very similar to the mechanism described in the study of Garris. It may be because autoimmune encephalitis and RHD are both the autoimmune diseases, however, this is just speculation. Since S1PR1 is down-regulated in this pathway, we wondered whether overexpress its expression can attenuate valvular damage due to rheumatic heart disease.
STAT3 plays a key pathogenic role in many inflammatory conditions. Researchers have found that STAT3 mediates immune myocarditis due to IL-6 production-induced liver complement component C3 and Th17 cell differentiation , and the differentiation of Th17 cells plays an important role in the occurrence and development of myocarditis . Furthermore, tissue signaling cytokines such as IL-17 and IL-22 have been proposed to have actions on the heart that involve STAT3 . Th17 cells and related inflammatory factors (such as IL17) play an important role in the process of inflammation and autoimmune response [18, 46, 47].Th17 cell-related factors are increased in peripheral blood and serum of RHD patients , and the level of Th17-related cytokines in the mitral valve is also significantly increased . Therefore, Th17 cells are likely to promote the development of RHD disease. Previous studies have shown high expression of p-STAT3 in rheumatoid arthritis , and so we consider STAT3 to be the key part of this signaling pathway, we wondered whether suppressing its expression can attenuate valvular damage due to rheumatic heart disease.
Based on the research results of the relationship between the S1PR1/STAT3 signaling pathway and valvular damage caused by RHD, we speculated that the phosphorylation level of STAT3 is increased via S1PR1/STAT3 signaling pathway, thereby promoting the differentiation of CD4 + T cells into Th17 cells, and then released Th17 cells-related cytokines and participated in the process of valvular damage caused by RHD. So we designed experiments to overexpress S1PR1 and inhibit STAT3 to interfere with the S1PR1/STAT3 signaling pathway. The results of Group Ⅰ showed that after the overexpression treatment of S1PR1, the originally down-regulated expression of S1PR1 rosed, the phosphorylation level of STAT3 dropped, Th17 cell-related cytokines expressed in the valve and serum also decreased, and eventually the level of inflammation and fibrosis of the valve caused by RHD was attenuated. The results of Group Ⅱ showed that after the inhibition treatment of STAT3, the total amount of both STAT3 and p-STAT3 was reduced, the expression of Th17-related transcription factor and cytokines was then reduced, and the level of inflammation and fibrosis of the valve caused by RHD was also attenuated. These results showed that the S1PR1/STAT3 signaling pathway is involved in regulating the Th17 cell-related cytokines during the vavular damage due to RHD, and altering the S1PR1/STAT3 signaling pathway could affect the expression of Th17 cell-related cytokines then attenuate the vavular damage due to RHD.
At present, there are few studies on the signaling pathways related to the pathogenesis of RHD, and there are only 6 signaling pathways with significant research progress: RhoA/Rho-dependent kinase (RhoA/ROCK) signaling pathway,Mitogen‑activated protein kinase (MAPK) signaling pathway༌Protein kinase B/S6 kinase (AKT/S6K) signaling pathway༌TGF-β1/Smad signaling pathway༌Wnt signaling pathway and S1PR1/STAT3 signaling pathway [34, 51–55]. And there are only three potential intervention targets found in these signaling pathway studies: Intervene in the expression of interferon (IFN)-γ and tumour necrosis factor (TNF)-α to regulate extracellular matrix remodeling to reduce heart damage caused by RHD; Intervene in AKT/S6K signaling pathway to inhibit TGF-β1-induced fibroblasts; Intervene in the S1PR1/STAT3 signaling pathway to reduce RHD-induced valvular damage. And whether these intervention targets are effective for the prevention and treatment of RHD requires the efforts of researchers to further study. This shows that there is a lack of research on the pathogenesis of RHD worldwide, so that the pathogenesis of RHD is still unclear. The threat of RHD to patients' lives is huge, the damage to patients' property and quality of life is devastating. RHD should be taken seriously, and it is worthy of researchers' efforts to explore its pathogenesis. The object of our study, S1PR1/STAT3 signaling pathway, may become a potential intervention target for RHD. We hope that this study will enrich the research on the signaling pathways related to the pathogenesis of RHD, and we hope that this research will contribute to revealing the pathogenesis of RHD, and it can provide a little help for the development of effective and inexpensive control methods for RHD in the future.
There are some limitations in our research. This was a rat study, and further research on humans is required. S1PR1 is already down-regulated during RHD-induced valvular damage, and what will happen if S1PR1 expression is completely blocked, higher technical requirements and further experiments are needed. How will up-regulating the expression of STAT3 affect RHD is unknown. Cell experiments may provide cell-level evidence for our research, but we have not conducted it. In addition, we did not detect the expression of FSP1 and Col3a1 at the protein level.