Rheumatoid arthritis (RA) is a chronic and systemic autoimmune disease with the symptom of hyperplastic synovium and destruction of joints(26). Excessive proliferation and migration of FLS directly promote cartilage erosion and articular bone destruction, which eventually results in RA development and progression(27). Currently, the main strategy for RA treatment is non-steroidal anti-inflammatory drugs (NSAIDs), glucocorticoids, disease-modifying antirheumatic drugs (DMARDs), and more(28). These drugs are mainly focused on relieving pain and improving patients' quality of life to a certain extent(29). However, the subsequent side effects of the drugs include gastrointestinal reactions, abnormal liver and kidney functions, osteoporosis, and systemic complications that cannot be ignored(30–32). The introduction of targeted biologics has been regarded as a new effective therapy(33, 34). Increasing evidence has suggested that activated fibroblast-like synoviocytes (FLSs) play a critical role in synovitis inflammation, hyperplasia, pannus formation, and joint destruction. The excessive proliferation and enhanced migration capabilities of RA-FLS are considered the major cause of tissue damage(35). Therefore, searching for novel factors or mechanisms that regulate biological behaviors and inhibit the activation of FLSs is necessary for the treatment of RA.
Interestingly, previous studies have reported that Cdc37 can influence cell cycle progression, cell proliferation, and migration(17, 36). The orderly transformation of the cell cycle among different phases is helpful to prevent uncontrolled cell proliferation. Particularly, Cdc37 knockdown in human colon cancer cells decreased the association with HSP90 and inhibited cell proliferation via the inhibition of G1/S-phase transition, as well as reduced levels of ERBB2, CRAF, CDK4, and CDK6(13). In human prostate cancer cells, the inhibition of growth arrest and multiple signaling pathways was also observed after silencing of Cdc37(37). In human HCC, overexpression of Cdc37 was associated with the downregulation of p16(38). Indeed, the growth of HepG2 cells was inhibited in vitro within Cdc37 suppression(16). In addition, Cdc37 could mediate cell growth and migration during Drosophila wound healing(36). Meanwhile, Cdc37 contributed to Schwann cell proliferation and migration after sciatic nerve crush(19). Combining the above results, we can speculate that Cdc37 cannot be neglected for cell cycle regulation, cell proliferation, and migration.
In this study, we first found a high expression of Cdc37 in human synovial tissues and RA-FLS. Next, we demonstrated that Cdc37 had a close connection with RA-FLS proliferation. Interestingly, protein levels of PCNA and CyclinD1 were elevated in the LPS-induced RA-FLS proliferation model. Besides, Cdc37 promoted RA-FLS proliferation by facilitating transformation from G0/G1 phase to the S phase. But the molecular mechanism of Cdc37-mediated cell proliferation and migration was still unclear. Various studies have shown that activation of the ERK signaling pathway is tightly correlated with cell proliferation and migration in RA(39). As we previously found that Cdc37 might participate in Schwann cell proliferation partially via the ERK pathway. This provides good foundations for our present study. Based on these findings, we sought to determine whether Cdc37 contributes to RA-FLS proliferation and migration through ERK signaling. Amazingly, we detected the up-regulation of p-ERK once Cdc37 was overexpressed in our research. Meanwhile, knockdown of Cdc37 distinctly caused down-regulation of p-ERK. Moreover, this change caused by the overexpression of Cdc37 can be reversed by PD98059. Similarly, the promotion of cell proliferation and migration resulting from Cdc37 also could be attenuated by the treatment of PD98059, further indicating that the ERK pathway is related to the influence of Cdc37 on the biological behaviors of RA-FLS. Finally, we found that synovial inflammation and cartilage erosion could be effectively alleviated, and the ERK pathway in joint tissues was inhibited by the injection of sh-Cdc37. In vivo experiments demonstrated that inhibiting Cdc37 mitigated the symptoms of RA rats.
In conclusion, our study demonstrates the interaction between Cdc37 and the ERK signaling pathway in the regulation of RA-FLS proliferation and migration. This presents new evidence that Cdc37 is a critical factor that can regulate the biological behaviors of RA-FLS and may be a prospective strategy for RA treatment. On the other side, RA-FLS secrete various pro-inflammatory cytokines and chemokines to establish an inflammatory microenvironment typically that is favorable for the occurrence of RA. However, whether there is an interrelation between Cdc37 and inflammatory microenvironment is not known clearly. Therefore, future studies are needed to investigate the anti-inflammatory effects of Cdc37 on RA.