Fig. 9. Schematic representation of the proposed mechanism of
GP on ameliorating intestinal inflammation.
Autophagy, a highly conserved process that evolved in eukaryotes, is
involved in maintaining organism homeostasis via lysosome-mediated
self-digestion and recycling of organelles and
proteins. Although the pathogenesis of IBD is ambiguous, studies have
pointed out that autophagic dysfunction is a key factor in the
persistence of intestinal inflammation (M.
Zhou et al., 2018). Pioneer evidence from genome-wide association
studies suggested that defective autophagy is associated with an
increased risk of IBD (Retnakumar &
Muller, 2019; Voinchet et al., 2010).
Several roles of autophagy in gut inflammation have been summarized:
elimination of pathogens, regulation of antigen presentation, governance
of the secretion of cytokines, and maintenance of lymphocyte
homeostasis. Notably, the accumulation of p62 in DSS-induced rats
confirms the dysfunction of autophagy in the
pathologicalprocess of intestinal inflammation.
After treatment with GP, both groups of rats with intestinal
inflammation presented diminished levels of
phosphorylated mTOR, LC3B, and Beclin-1, which
paralleled with a decrease of p62.
On the basis above, we further discussed the interaction between gut
microbiome, TLR4 and autophagy, aiming to reveal the potential mechanism
of the impact of GP on intestinal inflammation. In this study, 16S rRNA
sequencing was used to evaluate the changes in intestinal flora before
and after GP administration. The Spearman correlation coefficient was
used to evaluate the correlation between high abundances of bacteria at
the genus level and important characteristics of intestinal inflammation
in order to explore the correlation between intestinal flora and
autophagy, inflammation and oxidative stress. The results showed that
the expression of TLR4 was positively correlated with four of the
bacteria with high abundance in intestinal flora.
p__ Verrucomicrobia−g__Akkermansia showed significant
negative correlation with serum endotoxin, while displaying significant
positive correlation with LC3BII/I. It can be seen that changes in
intestinal flora, TLR4 pathway activation, and autophagy disorder are
connected together in the occurrence and development of intestinal
inflammation.
In recent years, the focus of research on intestinal flora has changed
from association to modulation (Schmidt,
Raes, & Bork, 2018). The most convincing experimental evidence of the
role of intestinal microbiota in human diseases can be obtained from
relevant experiments of intestinal flora transplantation (FMT)
(Liu et al., 2021). However, there are
few studies on the treatment of inflammatory bowel disease by FMT with
the addition of GP. Therefore, indirect experimental animal models are
needed to establish causal relationships between altered microbiomes and
disease pathogenesis. In this study, FMT-related experiments showed that
GP could improve the symptoms of DSS-induced intestinal inflammation by
inhibiting the TLR4-NF-κB pathway and activating mTOR-dependent
autophagy.
Recently, emerging research has focused on the connection between
inflammation and autophagy (Deretic et al.,
2013). Accumulating evidence indicates that inflammation and autophagy
are linked by reciprocal regulation, and they are orchestrated by the
upstream TLR4-mTOR pathway. It has been reported that inhibiting
autophagy is a novel role of the TLR4-MyD88 in intestinal epithelial
dysfunction. Subsequently, the release of NF-κB-induced cytokines was
orchestrated by autophagy. In addition to previous research, our data
also demonstrated that TLR4-MyD88 is involved in the regulation of mTOR
phosphorylation, and that mTOR is a newfound intersection between
inflammation and autophagy. Blocking the TLR4 pathway not only promotes
the repair of autophagy, but also inhibits the
disaggregation of IκBα from NF-κB. Consequently,
cytokine release and oxidative stress were diminished. Importantly,
autophagy also participates in
reducing the degradation of the IκBα-NF-κB complex
(R. Zhou, Yazdi, Menu, & Tschopp, 2011).
Treatment with 3-MA deteriorated the LPS-induced
inflammatory response in HT-29 cells. Meanwhile, rapamycin, an autophagy
activator, ameliorated inflammation to a great extent, which further
indicates that autophagy is involved in suppressing NF-κB activation.
Therefore, for the first time, this
study elucidated that polysaccharide purified from P. ginseng
ameliorates intestinal inflammation via the TLR4-autophagy axis through
regulating gut microbiota by experiments with FMT. Our research revealed
the mechanism concerning the intestinal anti-inflammatory effect of GP
and provided a promising therapy for IBD.
Conclusion
The present results highlighted that the purified polysaccharides from
P. ginseng showed intestinal anti-inflammatory effects in DSS-induced
rats by regulating gut microbiota and mTOR-dependent autophagy. It was
suggested that GP-treated intestinal flora transplantation elevated
autophagy and suppressed TLR4-MAPK was associated with the
downregulation of LPS-producing bacteria. Consequently, NF-κB-induced
inflammation was attenuated via the activation of mTOR-dependent
autophagy. The results suggested the view that GP may have potential in
intestinal inflammation disease therapies, providing a foundation for
the potential utilization of polysaccharide from P. ginseng for
functional foods and complementary medicines.