Even though the LPS-induced periodontitis model has been established for years and is widely used in pre-clinical studies, the precise mechanisms remain unclear. It is reported that HGFs can be activated by LPS and release significant inflammatory factors, which contribute to periodontal tissue destruction [25, 26]. What’s more, there are still exit oxidative stress injuries because of the accumulation of oxidation products[27]. It has been reported that PRDX6 is multifunctional to scavenge peroxides and generate ROS via PLA2 activity[28]. Therefore, PRDX6 is a regulator of the cellular redox of biological systems. In this study, we mainly discussed the role of PRDX6 in LPS-induce inflammation and oxidative stress in HGFs.
PRDX6 holds an essential position in inflammatory diseases and hasn’t been reported in periodontitis. We found that the expression level of PRDX6 was decreased in inflamed gingival tissues due to severe oxidative damage in chronic periodontitis. HGFs are the most abundant cells in periodontal tissue and the critical barrier against bacterial infection. In HGFs, the expression of PRDX6 was increased considerably to defend against the LPS-induced oxidative damage at first, while it eventually decreased and lost the protective effect.
Interestingly, PRDX6 has been recently identified as the regulator of redox balance. We then identified the inflammatory cytokines and ROS in PRDX6-silenced cells. PRDX6 knockdown elevated the levels of inflammatory cytokines released by HGFs, which indicated that PRDX6 plays a protective role in LPS-induced inflammation. Furthermore, more ROS production could be observed in silencing HGFs treated with LPS and participates in the onset and amplification of inflammatory response. PRDX6 could play peroxidase activity to act as an antioxidant reducing short-chain hydroperoxide. Thus, PRDX6 limits oxidative stress in LPS-induced HGFs. It has been reported that PRDX6 plays a paradoxical role in inflammatory and immune responses. For example, in the LPS-induced acute kidney injury model, PRDX6-overexpressed mice showed less renal apoptosis and leukocyte infiltration than wild mice [29]. Conversely, PRDX6 lost its antioxidant activity in aortic lesions or aneurysms, characterized by high inflammation and oxidative stress [30]. Our results showed that PRDX6 could alleviate ROS and has a protective effect on periodontitis. However, we only demonstrated this mechanism in vitro experiments, and the vivo model was necessary to analyze the essential role of PRDX6 in the future.
Notably, PRDX6 is a peroxiredoxin that owns multiple enzyme activities, peroxidase, PLA2, and LPCAT. It has been reported that blocking the PLA2 activity of PRDX6 by MJ33 enhanced ROS [31]. In our study, the inhibition of PRDX6-PLA2 activity down-regulated the production of LPS-induced ROS and inflammatory cytokines. The potential mechanism for MJ33 to alleviate the inflammation and oxidative stress through the inhibition of the PLA2 activity to block NOX2 activation. NOX2 is the enzyme responsible for ROS generation associated with inflammation [32]. It has been reported that enhanced NOX2 activity is most likely one of the ROS sources in periodontal tissues [33]. PRDX6-PLA2 could play a protective role in reducing phospholipid hydroperoxides and repairing peroxidized cell membranes. However, it is less than the protective role of PHGPx activity and LPCAT activity. This result was in line with the study that blocking PRDX6-PLA2 activity could attenuate lungs damage, presumably by inhibiting oxidative stress and preventing the amplification of lungs inflammation [32]. Therefore, inhibiting PRDX6-PLA2 activity alleviated the LPS-induced inflammation and oxidative injury in HGFs.
Furthermore, the NRF2 is a vital regulator of the cellular antioxidant response, as many of its downstream genes prevent oxidative stress [34]. The activated NRF2 can be transported to the nucleus and subsequently bind to ARE to improve the transcription of some antioxidant genes, such as HO-1 and NQO1[35, 36]. In our study, the downregulation of NRF2 was observed in inflammatory gingival tissues compared to the normal gingival tissues. What’s more, NRF2 was intensely activated to promote HGFs at first and eventually was decreased with long-term LPS treatment, which is in line with the result of PRDX6. With the treatment of NRF2 inhibitor ML385, the expression of PRDX6 was down-regulated. In addition, the levels of inflammatory cytokines and ROS were increased as ML385 eliminated the protective role of NRF2. Therefore, the NRF2 could play a protective role in LPS induced-HGFs. However, this study only explored the primary mechanism between PRDX6 and NRF2 (Fig.6), and the deep mechanism still needs to be further explored.