Toll-Like Receptor 9 Promotes Steatohepatitis by Induction of Interleukin-1β in Mice

Abstract

BACKGROUND & AIMS Development of nonalcoholic steatohepatitis (NASH) involves the innate immune system and is mediated by Kupffer cells and hepatic stellate cells (HSCs). Toll-like receptor 9 (TLR9) is a pattern recognition receptor that recognizes bacteria-derived cytosine phosphate guanine (CpG)–containing DNA and activates innate immunity. We investigated the role of TLR9 signaling and the inflammatory cytokine interleukin-1β (IL-1β) in steatohepatitis, fibrosis, and insulin resistance. METHODS Wild-type (WT), TLR9−/−, IL-1 receptor (IL-1R)−/−, and MyD88−/− mice were fed a choline-deficient amino acid-defined (CDAA) diet for 22 weeks and then assessed for steatohepatitis, fibrosis, and insulin resistance. Lipid accumulation and cell death were assessed in isolated hepatocytes. Kupffer cells and HSCs were isolated to assess inflammatory and fibrogenic responses, respectively. RESULTS The CDAA diet induced NASH in WT mice, characterized by steatosis, inflammation, fibrosis, and insulin resistance. TLR9−/− mice showed less steatohepatitis and liver fibrosis than WT mice. Among inflammatory cytokines, IL-1β production was suppressed in TLR9−/− mice. Kupffer cells produced IL-1β in response to CpG oligodeoxynucleotide. IL-1β but not CpG-oligodeoxynucleotides, increased lipid accumulation in hepatocytes. Lipid accumulation in hepatocytes led to nuclear factor-κB inactivation, resulting in cell death in response to IL-1β. IL-1β induced fibrogenic responses in HSCs, including secretion of tissue inhibitor of metalloproteinase-1. IL-1R−/− mice had reduced steatohepatitis and fibrosis, compared with WT mice. Mice deficient in MyD88, an adaptor molecule for TLR9 and IL-1R signaling, also had reduced steatohepatitis and fibrosis. TLR9−/−, IL-1R−/−, and MyD88−/− mice had less insulin resistance than WT mice on the CDAA diet. CONCLUSIONS In a mouse model of NASH, TLR9 signaling induces production of IL-1β by Kupffer cells, leading to steatosis, inflammation, and fibrosis.

Cellular and molecular mechanisms in liver fibrogenesis

“…However, similar to what reported for ASH, also in experimental NASH and in human NASH patients a clear increase in LPS levels has been detected, which is likely to be due to intestinal bypass [122]. Although the role of fatty acids in activating TLRs is still controversial, although described, interesting studies on transgenic mice have shown that the absence of either TLR4 or TLR9 results in a significant reduction of NASH related parameters like histological score, hepatocyte death (apoptosis and necrosis) and markers of fibrogenesis [123][124][125]. These studies pointed out that TLR9 seems to be relevant for significant IL-1β production from Kupffer cells in experimental NASH not only in relation to steatosis but also in relation to HSC activation and hepatocyte death.…”

“…Of interest, IL-1β induced fibrogenic responses in HSCs, including secretion of tissue inhibitor of metalloproteinase-1 whereas IL-1R(-/-) mice had reduced steatohepatitis and fibrosis, compared with WT mice. Mice deficient in MyD88, an adaptor molecule for TLR9 and IL-1R signaling, also had reduced steatohepatitis and fibrosis [123].…”

“…In particular, IL-1β (which is not able to induced cell death by itself) may lead to liver injury and hepatocyte death indirectly by sensitizing hepatocytes to more conventional death signals like TNF-α. [123,126].…”

PARK2-mediated mitophagy is involved in regulation of HBEC senescence in COPD pathogenesis

“…35,36 Although a direct contribution of TLR9 and inflammasome activation to COPD pathogenesis is still unclear, 37,38 IL1B/interleukin 1b, a representative senescence-associated secretory phenotype (SASP) factor involved in COPD development, 39,40 is produced and activated through TLR9 signaling and inflammasome machinery. 41,42 Accordingly, insufficient mitophagy-mediated cell autonomous activation of TLR9 and the inflammasome may partly explain persistent inflammation after smoking cessation, which may also be associated with inflammation-mediated acceleration of cell senescence during COPD progression. 43 However, we understand the potential limitations of our in vitro experimental models using short-term CSE exposure to elucidate the mechanisms for COPD development.…”

Activation of N-methyl-d-aspartate receptor downregulates inflammasome activity and liver inflammation via a β-arrestin-2 pathway

“…Activation of inflammasomes in immune cells is required in a range of tissue injury and repair, including alcoholic and nonalcoholic hepatitis and liver fibrosis (7,11,16,23,34). The minimum activation requirements for the inflammasome machinery have been identified and consist of two signals.…”