Discussion
Brucea javanica (Linn.) Merr. (Simaroubaceae) is a traditional
Chinese medicine commonly used for the treatment of inflammatory
diseases and malaria (Huang et al., 2017; Yang et al., 2013). YDZG, a
quassinoid glucoside, is firstly purified and identified by Zhang et al
in 1983 (Zhang et al., 1983). However, as it stands now, quite few
papers were found to report its bioactivity. Our previous study
indicated that YDZG exhibited anti-complementary activity(Zhan et al.,
2018). In the present study, evidence for the direct and covalent
inhibition of NLRP3 expression by YDZG is provided. Also, the remarkable
anti-inflammatory activity of YDZG was demonstrated through the
attenuation of NLRP3 inflammasome activation both in vitro andin vivo . Taken together, these results suggest that YDZG can be
used as a lead compound to design new therapeutics against NLRP3-driven
diseases.
Previous studies have identified compounds that can directly target
NLRP3 to suppress NLRP3 activation (Jiang et al., 2017; Marchetti et
al., 2018). In contrast, our results showed that YDZG did not directly
bind to NLRP3. However, YDZG could enhance TRIM31 expression, an NLRP3
protein inhibitor, to block NLRP3 inflammasome activation. In addition,
activation of the NLRP3 inflammasome is thought be regulated at
transcriptional and post-translational levels (Jo et al., 2015). The
first signal in inflammasome activation is a priming signal, induced by
the toll-like receptor (TLR)/nuclear factor (NF)-kB pathway, to
upregulate NLRP3 expression (Swanson et al., 2019). NF-κB plays an
important role in regulating immune responses (Liang et al., 2004; Zhang
& Ghosh, 2001). However, aberrant activation of NF-κB is linked to an
up-regulation of pro-inflammatory mediators in ALI (Everhart et al.,
2006). NF-κB binding sequences have also been identified in
pro-inflammatory genes such as inducible IL-6 and TNF-α (Cho et al.,
2007; Xie et al., 2012). In our study, results suggested that YDZG
inhibited LPS-induced NF-κB activation to block the expression of NLRP3.
ROS production can stimulate NLRP3 inflammasome activation and
inflammatory cascade reactions (Heid et al., 2013; Shirasuna et al.; Zhou
et al.). When stimulated by oxidative stress, Nrf2 is dissociated from
Keap1, transferred to the nucleus, and combined with antioxidant
response elements (ARE), where it activates the transcription of the
HO-1 gene to achieve antioxidant effects (Bao et al.; Sawle et al.; Yang
et al., 2015). A recent study found that Nrf2 negatively regulates NLRP3
inflammasome activity by inhibiting ROS (Hu et al.). Our results
indicated that YDZG promoted Nrf2 translocation from the cytoplasm into
the nucleus and inhibited ROS generation.
As a primary organ of respiration, the lung encounters constant exposure
to foreign particles and infectious agents. Chronic respiratory diseases
and acute lung injuries are common and widespread problems affecting
millions of people worldwide, especially in developing countries
(Bellani et al., 2016; Guerin et al., 2015; Lorente et al., 2015). In
spite of considerable efforts, there are still no Food and Drug
Administration-approved treatments for ALI (Standiford & Ward, 2016).
In this study, the protective effect of YDZG on an LPS-induced ALI model
was evaluated. Results revealed that YDZG could significantly protect
mice from lung damage and respiratory dysfunction. Recent data has shown
that NLRP3 inflammasome is involved in the development of ALI (Dolinay
et al., 2012). IL-1β is a central pro-inflammatory cytokine in the
initiation of inflammation and transient expression of IL-1β alone can
induce acute lung injury (Kolb et al., 2001). Additionally, application
of the neutralizing antibodies for IL-18, IL-1β, and IL-1R antagonists
has reduced lung injury on ALI mouse and rat models (Kuipers et al.,
2012; Mulligan & Ward, 1992; Wu et al., 2013). In this study, YDZG
decreased IL-1β in the BALF, serum, and lung tissue of mice and reduced
NLRP3 and caspase-1 p20 in the lung.
In summary, our results indicated that YDZG effectively inhibited NLRP3
inflammasome activation in macrophages and mouse model of ALI. We
discovered that YDZG suppressed NLRP3 inflammasome via the
reduction of NF-κB and Nrf2 activation and the promotion of TRIM31
expression, suggesting that YDZG could be a potential agent of treatment
with NLRP3-driven ALI.