Increasing evidence has confirmed that FLS play a critical role in the pathogenesis of RA, such as synovial hyperplasia and pannus formation. Therefore, inhibiting the malignant progression of FLS may be beneficial to the clinical outcome of RA. In this study, we first investigated whether overexpression of DUSP5 inhibits the proliferation, migration, and invasion of FLS, which correlated with suppression the phosphorylation of ERK signaling. Moreover, DUSP5 is a novel target gene of miR-216a-3p in FLS, and miR-216a-3p downregulates DUSP5 expression by directly binding to the 3′-UTR of DUSP5. These results indicate the critical role of the miR-216a-3p/DUSP5 axis in RA progression, which may be a new target for the treatment of RA.
DUSP5, a member of DUSP subfamily, is expressed in various cell types, and can be induced by several cytokines. Previous studies have shown that the highly conserved kinase interaction motif (KIM) of DUSP5 can specifically recognize and bind to ERK to inactivate it by dephosphorylating p-ERK [11, 33]. The ERK signaling pathway is closely associated with several chronic inflammatory diseases, such as osteoarthritis, RA, and psoriatic arthritis [17, 34]. The active form of ERK (p-ERK) can promote the expression of inflammatory genes [35]. Therefore, overexpression of DUSP5 could inactivate p-ERK and play an anti-inflammatory function. A previous study demonstrated that overexpression of DUSP5 attenuated arthritic progression, extent of histologic inflammation, and levels of osteoclast activity in an RA mouse model, which associated with expansion of Treg cells and reciprocal suppression of Th17 cells [20]. However, the exact roles and regulatory mechanisms of DUSP5 in FLS remain unclear. In this study, we observed that overexpression of DUSP5 significantly inhibited synoviocytes MH7A proliferation, migration, and invasion. On the contrary, knockdown of DUSP5 significantly promoted the malignant effect of MH7A. These findings were associated with DUSP5 modulating the phosphorylation of ERK. The above data suggested that the anti-arthritis effects of DUSP5 were associated with its suppression of proliferation, migration and invasion in FLS, which through inhibiting the ERK signaling pathway. However, the mechanism by which DUSP5 inhibits the malignant effect of FLS has not been confirmed in vivo.
Although the anti-inflammatory function of DUSP5 is well characterized, some key questions remain unanswered regarding the decreased DUSP5 expression in synovial tissues [5]. Interestingly, recent researches confirmed that decreased DUSP5 expression is related with miR dysregulation in some diseases [36, 37]. Previous study showed that DUSP5, a direct target of miR-32-5p, is involved in miR-32-5p-mediated neuropathic pain and neuroinflammation [38]. Moreover, miR-363-3p attenuates neonatal hypoxic-ischemia encephalopathy by targeting DUSP5 [39]. However, whether the expression of DUSP5 is regulated by miRs in RA remains unknown. In the present study, we identified that miR-216a-3p targeted DUSP5 to reduce its expression in FLS, and overexpression of DUSP5 partially reversed miR-216a-3p induced proliferation, migration, and invasion of FLS. These results indicated that DUSP5 is a functional target gene of miR-216a-3p in FLS, targeting miR-216a-3p to modulate DUSP5 expression may be a novel therapeutic strategy for RA.
It is well known that elevated miRs can inhibit target gene expression at the post-transcriptionally level, so the expression of target gene is negatively correlated with miRs expression. Moreover, a gene can be targeted by multiple miRs at the same time. In our study, bioinformatics analysis in TargetScan, miRDB and miRWalk databases showed that both miR-216a-3p and miR-26b-5p had potential binding sites in the 3'-UTR of DUSP5. However, a previous study confirmed that the expression of miR-26b-5p is down regulated in FLS [40]. Therefore, miR-216a-3p was selected for further investigation and we examined the effect of miR-216a-3p on DUSP5 expression in this study. Certainly, our data cannot rule out the possibility that DUSP5 may be targeted and regulated by other miRs in FLS. Therefore, the exact regulatory mechanism of DUSP5 by miRs needs to be studied further.