Discussion
In this study, we confirmed that TAF alleviated NASH in two different
NASH mouse models by decreasing the activity of recruited and
intrahepatic MPs by blocking the AKT pathway. Our results demonstrated
that liver damage and
IAIE+/PD-L1+/MerTK+intrahepatic MP number were lower in TAF-treated NASH mice. Moreover,
LPS-stimulated TAF-treated CD14+ human monocytes
expressed lower PD-L1+HLA-DR+ levels
than LPS-stimulated mock-treated ones. These results suggest that TAF
decreased the numbers of recruited and intrahepatic MPs in the NASH
liver. Moreover, pAKT levels were decreased in TAF-treated NASH mice
livers and isolated peripheral human CD14+ monocytes,
suggesting that TAF blocks AKT phosphorylation in MPs.
During NASH, recruited and intrahepatic MPs differentiate into
pro-inflammatory MPs (Sung, 2021). The single-cell transcriptome
analysis of NASH mouse livers showed that genes most specific to MPs
were primarily NASH-induced genes (Xiong et al., 2019). Another recent
single cell analysis also demonstrated that the recruited and
intrahepatic MPs exhibit distinct inflammatory phenotypes during NAFLD
progression (Krenkel et al., 2020). Other studies also demonstrated that
in NASH/ALD livers, CD68+ MP numbers increased with
the progression of fibrosis (Pil Soo Sung et al., 2022), and
Trem2+CD9+ MPs derived from the
recruited MPs display a profibrogenic phenotype (Ramachandran et al.,
2019). Therefore, it is critical to investigate the mechanisms that
target the activated MPs to treat NASH.
The NASH mouse models show pathological features similar to that of
human NASH (Saito et al., 2015). Therefore, we established a NASH mouse
model using a combination of chemical (STZ) and dietary interventions
with HFHC diets. A recent study on human NAFLD livers suggested that
gut-derived LPS induces liver damage by activating MPs via the TLR-4
pathway (Carpino et al., 2020). Our IHC results showed that LPS and
TLR-4 expression was significantly reduced in the TAF-treated HFHC
diet-fed mice compared to the mock-treated ones. A previous study
revealed that TAF mitigates ALT levels in chronic hepatitis B virus
infection maybe due to metabolic factors associated with NAFLD
(Sripongpun et al., 2022). The NASH mouse model showed elevated serum
ALT, AST, and TRIG levels, indicating severe liver damage. After TAF
administration in the NASH mouse model, serum ALT, AST, and TRIG levels
were remarkably restored to normal ranges. CD11bhighF4/80low +, used as a recruited MP marker, exhibited a
higher percentage in the mock-treated HFHC diet group and was markedly
decreased in the TAF-treated group. Previous studies have revealed that
TDF, an antiviral agent used to treat chronic hepatitis B, has
additional immune-modulatory effects (S. W. Lee et al., 2021; Zhao et
al., 2020). To investigate anti-inflammatory effects of TAF on inflamed
intrahepatic MPs, MPs were isolated from a TAA-induced liver injury
mouse model and treated with TAF ex vivo . The activation status
of the isolated MPs was significantly reduced after TAF treatment,
suggesting that TAF has anti-inflammatory effects.
Akt is known as a key player in signal transduction and regulation of
cellular processes. The PI3K/AKT pathway regulates MP survival,
migration, proliferation, and inflammatory
signals and promotes the
development of inflammation (Linton, Moslehi, & Babaev, 2019). The
PI3K/Akt pathway is activated by TLR4 or other cytokines, chemokines,
and Fc receptors. TLR activation ultimately affects the activation state
of macrophages (Lopez-Pelaez, Soria-Castro, Bosca, Fernandez, &
Alemany, 2011). Moreover, LPS increases the phosphorylation of Akt (S.
J. Lee, Seo, & Kim, 2015). Our data demonstrated that AKT
phosphorylation was inhibited by TAF administration in a NASH mouse
model. Moreover, pAKT levels were decreased in TAF-treated activated
peripheral human CD14+ monocytes, suggesting that TAF
blocks AKT phosphorylation in MPs. Further human studies are necessary
to discover the safe and effective anti-inflammatory dose of TAF in
patients with NASH.