Background and objectives:
Nonalcoholic steatohepatitis (NASH), which is characterized by concurrent
existence of hepatic steatosis and predominantly lobular necroinflammation, represents
the more advanced stage in the spectrum of nonalcoholic fatty liver disease (NAFLD).
NASH exhibits dramatically increased risk of progression to end-stage liver diseases
than simple steatosis. Therefore, the progression of hepatic steatosis to steatohepatitis is
the crucial step in the development of obesity-related NASH. Toll like receptor 4
(TLR4), a master regulator of innate immunity, is the principal receptor for endotoxin,
which is a central mediator of liver inflammation associated with both alcoholic and
nonalcoholic liver disease. However, due to a lack of suitable animal models which
fully recapitulate the natural history of obesity-induced NASH, the precise
pathophysiological function of TLR4 signaling in the development of this disease
remains poorly understood.
The objective of this study is to investigate the role of TLR4 in mediating
inflammatory responses in obesity-induced NASH using both in vivo and ex vivo
approaches, and to unveil cellular and molecular mechanisms responsible for TLR4
actions.
Key findings:
1. To address the role of TLR4 in the pathogenesis of NASH, we crossed ApoEdeficient
mice (ApoE-/-) with TLR4 mutant mice (TLR4-/-) to generate ApoE-/-
/TLR4 wild type mice (ApoE-/-/TLR4-WT) and ApoE-/-/TLR4-/- mice. Noticeably,
when fed with high fat high cholesterol (HFHC) diet, ApoE-/-/TLR4-WT mice
developed the typical pathology of NASH (hepatic steatosis, lobular inflammation,
and hepatocyte ballooning) in the context of obesity and metabolic syndrome,
suggesting HFHC-fed ApoE-/- mice as a suitable animal model for NASH.
2. TLR4 inactivation protected ApoE-/- mice against HFHC diet-induced liver injury,
as indicated by a significant improvement in liver histology, a a marked reduction
in serum ALT activity, a dramatic repression of inflammatory infiltrates, as well as
an obvious decrease in hepatic production of pro-inflammatory cytokines.
3. In ApoE-/-/TLR4-WT mice, TLR4 expression was selectively elevated in Kupffer
cells in response to HFHC diet feeding.
4. The activation of XBP1, a transcription factor involved in endoplasmic reticulum
stress, was markedly elevated in liver of ApoE-/-/TLR4-WT mice fed with HFHC
diet, whereas this change was abrogated in HFHC diet-fed ApoE-/-/TLR4-/- mice.
5. In rat primary Kupffer cells, treatment with anti-oxidants blocked endotoxininduced
activation of XBP1 and NF-κB, leading to decreased cytokine production.
In addition, siRNA-mediated knockdown of XBP1 inhibited NF-κB activation and
cytokine production resulted from the treatment with the TLR4 agonist LPS.
6. In ApoE-/-/TLR4-WT mice, adenovirus-mediated expression of dominant negative
XBP1 had no obvious effect on HFHC diet-induced hepatic steatosis and ROS
production, but markedly decreased lobular inflammation, NF-κB activation,
cytokine production in the liver and significantly reduced serum levels of ALT.
Conclusions:
These findings support the role of TLR4 in Kupffer cells as a key player in
mediating the progression of simple steatosis to NASH, by inducing ROS-dependent
activation of XBP1. In light of the obligatory role of XBP1 in TLR4-induced liver
inflammation and injury, therapeutic interventions that inhibit TLR4/XBP1 activation
may represent a promising strategy for treatment of NASH. / published_or_final_version / Medicine / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/174471 |
| Date | January 2012 |
| Creators | Ye, Dewei., 叶得伟. |
| Contributors | Xu, A, Wang, Y |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B47752919 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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