BACKGROUND: Lipopolysaccharide (LPS) and ammonia act synergistically in mediating the severity of hepatic encephalopathy (HE) in cirrhosis. Although LPS results in neuroinflammation, it is not clear whether it induces hyperammonemia (HA) contributing to HE. This study addressed the following questions: 1) Does LPS worsen HA in cirrhosis? 2) Does treatment with an antagonist (TAK242) of the LPS receptor, toll-like receptor 4 (TLR4), prevent HA? 3) Is a TLR4 knock-out (TLR4-/-) animal protected from HA? 4) If so, what is the underlying mechanism? Do they have a more effective urea cycle? METHODS: Study 1: Sprague Dawley rats were treated with LPS (0.025 mg/kg, ip.) 4 weeks after bile duct ligation (BDL). 4 groups of rats were studied: sham (n = 4), BDL (n = 4), BDL+LPS (n = 6) and BDL+TAK242 (10 mg/kg ip.) 3 hours before LPS injection (n = 7). Study 2: 4 groups of mice were studied: wild type control (WTC, n = 7), WT with HA (WTH, n = 10), TLR4-/- control (TLR4-/-C, n = 10) and TLR4-/- with HA (TLR4-/-H, n = 10). HA was induced by adding 0.28M ammonium chloride to drinking water for 3 days. For both studies, plasma ammonia and liver gene expression (qPCR, data shown as 2^-ΔΔCT compared to sham/WT) of the 5 urea cycle enzymes (UCEs) were assessed. For study 2, protein expression of the key, rate-limiting enzyme carbamoyl phosphate synthetase 1 (CPS1) was also assessed (Western Blot, immunohistochemistry). RESULTS: Study 1: There was a stepwise increase in plasma ammonia throughout sham, BDL and BDL+LPS groups (P < 0.001). Pre-treatment with TAK242 prior to LPS injection in BDL rats was associated with a reduction in plasma ammonia (P < 0.01, Figure 1a) and a higher coma-free survival rate (100% vs. 15%). Gene expression of all UCEs showed a stepwise decrease throughout sham, BDL and BDL+LPS (all P < 0.05), which was prevented by TAK242 (all P < 0.05). This was most pronounced for CPS1, for which expression levels in the TAK242-treated group were restored to that of the sham animals (Figure 1b). Study 2: In TLR4-/- mice, the increase in plasma ammonia was less compared to WT mice (P < 0.001). Although no significant changes were found for gene expression of UCEs between groups, protein expression of CPS1 was significantly higher in TLR4-/- mice as compared to WT mice, as shown by both Western Blot and immunohistochemistry. CONCLUSIONS: These data suggest that TLR4 signaling contributes to the development of hyperammonemia by modulating the urea cycle function. Inhibition of TLR4 with TAK242 offers a potential novel therapy for HA and HE in cirrhosis.
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