Ethanol-induced sensitization to endotoxin in Kupffer cells is dependent upon oxidative stress

Activation of Kupffer cells by gut-derived endotoxin plays a pivotal role in alcoholic liver injury. On the other hand, it was reported that acute ethanol administration reduced activation of Kupffer cells. We found that Kupffer cells isolated from rat treated only once with ethanol were sensitized to endotoxin 24 hrs later correlatively with CD14 expression. Moreover, it was shown that Kupffer cell activation by endotoxin via Toll-like receptor (TLR)-4 is involved in alcohol-induced liver injury and ethanol-induced oxidative stress is important in the regulation of transcription factor NFkappaB activation and cytokine production by Kupffer cells. Here, we show that IRAK, one of signaling molecules of TLR-4, regulates tolerance and sensitization to LPS and acute ethanol increases in IRAK expression through a mechanism dependent upon oxidant production.

Female C57BL/6 mice were given ethanol (5 g/kg) intragastrically, and LPS was injected 1 or 21 hrs later. Serum transaminase levels were measured. Moreover, some mice were treated with NADPH oxidase inhibitor diphenyleneiodonium sulfate (DPI, 1 mg/kg/day) or infected with adenovirus (1 x 10 plaque-forming units, intravenously) containing IkappaB superrepressor gene, which prevent NFkappaB activation of Kupffer cells, for three days. Kupffer cells were isolated from mice 1 hr and 21 hrs after ethanol treatment. After the addition of LPS, TNF-alpha in the media was measured using ELISA, Electrophoretic mobility shift assay (EMSA) was performed to analyze DNA binding activity of NFkappaB. Further, expression of Interleukin-1 receptor-associated kinase (IRAK) was evaluated by Western blotting.

LPS-induced increases in transaminases were blunted in mice treated with ethanol before 1 hr. However, ethanol treatment 21 hrs earlier augmented LPS-increased transaminases three-fold over controls. Pretreatment with nonabsorbable antibiotics blocked these effects of ethanol. LPS-induced TNF-alpha production by Kupffer cells isolated from mice 1 hr after ethanol was reduced to about 60% of values from control Kupffer cells, while LPS-induced TNF-alpha production by Kupffer cells isolated from mice treated with ethanol 21 hrs earlier increased 1.5-fold over control Kupffer cells. In Kupffer cells from mice 1 hr after ethanol treatment, expression of IRAK was decreased, and LPS-induced activation of NFkappaB was decreased correlatively. In contrast, ethanol treatment to mice increased expression of IRAK in Kupffer cells 21hrs later and LPS-induced activation of NFkappaB was elevated significantly. On the other hands, DPI treatment for three days prior to ethanol did not prevent decreases in IRAK expression due to ethanol treatment for 1 hr. However, DPI treatment blunted ethanol-induced increases in IRAK expression. Additionally, inhibition of NKkappaB activation with dominant-negative IkappaBalpha blunted ethanol-induced increase in IRAK expression. Contrary, inhibition of NKkappaB did not affect decrease of IRAK expression due to ethanol treatment for 1 hr.

Ethanol causes tolerance in the early phase after ethanol consumption, while sensitization was observed later. Both tolerance and sensitization were induced by gut-derived endotoxin. These findings indicate that ethanol-induced both tolerance and sensitization of Kupffer cells to endotoxin involve IRAK expression. Further, NADPH oxidase plays a pivotal role in the increase in IRAK expression due to ethanol via activation of NFkappaB signaling pathway. In conclusion, these data indicate that acute ethanol causes sensitization to endotoxin through mechanisms dependent upon oxidative stress.