Abrogation of the antifibrotic effects of natural killer cells/interferon-gamma contributes to alcohol acceleration of liver fibrosis

Abstract

Background & aims: Chronic alcohol drinking accelerates liver fibrosis in patients with viral hepatitis that cannot be fully explained by ethanol-enhanced liver damage. Here, we identified a novel mechanism by which alcohol accelerates liver fibrosis: inhibition of the antifibrotic effects of natural killer (NK) cells and interferon-gamma (IFN-gamma).

Methods: Alcohol administration was achieved by feeding mice with a liquid diet containing 5% ethanol for 8 weeks. Liver fibrosis was induced by administration of carbon tetrachloride (CCl(4)) for 2 weeks. Hepatic stellate cells (HSCs) were also isolated and cultured for in vitro studies.

Results: CCl(4) treatment induced greater fibrosis and less apoptosis of HSCs in ethanol-fed mice compared with pair-fed mice. Polyinosinic-polycytidylic acid (Poly I:C) or IFN-gamma treatment inhibited liver fibrosis in pair-fed but not in ethanol-fed mice. Poly I:C activation of NK cell cytotoxicity against HSCs was attenuated in ethanol-fed mice compared with pair-fed mice, which was due to reduced natural killer group 2 member D (NKG2D), tumor necrosis factor-related apoptosis-inducing ligand, and IFN-gamma expression on NK cells from ethanol-fed mice. In vitro, HSCs from ethanol-fed mice were resistant to IFN-gamma-induced cell cycle arrest and apoptosis compared with pair-fed mice. Such resistance was due to diminished IFN-gamma activation of signal transducer and activator of transcription 1 (STAT1) in HSCs from ethanol-fed mice caused by the induction of suppressors of cytokine signaling proteins and the production of oxidative stress. Finally, HSCs from ethanol-fed mice were resistant to NK cell killing, which can be reversed by transforming growth factor-beta1 (TGF-beta1) neutralizing antibody.

Conclusions: Chronic ethanol consumption attenuates the antifibrotic effects of NK/IFN-gamma/STAT1 in the liver, representing new and different therapeutic targets with which to treat alcoholic liver fibrosis.

Figure 1

Ethanol feeding decreases HSC apoptosis and accelerates liver fibrosis during CCl₄-induced liver fibrogenesis. Mice were fed either an ethanol diet or pair-fed a control diet for 8 weeks and then injected with CCl₄ for an additional 2 weeks while continuing the same diet. Killed at week 10, the sera were collected for measuring ALT (A) and the livers were collected for Western blotting (B) or for measuring hepatic hydroxyproline (C) or immunohistochemistry analyses (D). (D) Trichrome staining for collagen fibers, α-SMA staining for activated HSCs, and TUNEL staining for apoptosis. Arrows indicate α-SMA⁺ TUNEL⁺ cells, which were counted per field of magnification (×200) and shown in panel E. Values in panels A, C, and E are means ± SEMs (n = 7). **P < .01, ***P < .001 vs pair-fed + 0.25 mL/kg CCl₄.

Figure 2

Ethanol feeding abolishes the antifibrotic effects of poly I:C. Eight-week ethanol- and pair-fed mice were maintained on the same diet while receiving injections of CCl₄ (0.1 mL/kg) plus poly I:C or saline for an additional 2 weeks. Liver tissues were stained with anti–α-SMA antibody to detect activated HSCs (A) or subject to Western blotting (B) or measuring for hepatic hydroxyproline (C). Values in panel C are means ± SEMs (n = 6). *P < .05 vs corresponding saline group.

Figure 3

Ethanol feeding inhibits hepatic NK cell killing of HSCs and down-regulates NKG2D and TRAIL expression on NK cells. Eight-week ethanol- and pair-fed mice were treated with poly I:C (5 μg/g, intraperitoneal). After 16 hours, liver MNCs or NK cells were isolated and used in cytotoxicity assays against 4-day cultured HSCs (A). Liver MNCs were also subject to RT-PCR analyses (B). The densities of bands in panel B were quantified and shown in panel C. Liver MNCs were also analyzed by fluorescence-activated cell sorting (D). Values in panels A, C, and D are means ± SEMs (n = 6). *P < .05, **P < .01 vs corresponding pair-fed group.

Figure 4

Ethanol feeding abolishes the antifibrotic effects of IFN-γ. Eight-week ethanol- and pair-fed mice were maintained on the same diet and injected with CCl₄ (0.1 mL/kg) plus IFN-γ or saline for an additional 2 weeks. Liver tissues were stained with anti–α-SMA antibody to detect activated HSCs (A) or subject to Western blotting (B and C) or measuring hepatic hydroxyproline (D). The densities of bands in panel B were quantified and shown in panel C. Values in panels C and D are means ± SEMs (n = 4). *P < .05, **P < .01 vs corresponding IFN-γ nontreated groups.

Figure 5

HSCs from ethanol-fed mice are resistant to IFN-γ stimulation. (A) HSCs isolated from 8-week pair- or ethanol-fed mice were cultured for 4 days (D4 HSCs) and treated with IFN-γ for 24 hours. Cell proliferation was then measured by [³H] thymidine incorporation assay. (B) D4 HSCs were stimulated with IFN-γ (5 ng/mL) for 6 or 12 hours. Caspase-3 activity was measured. (C–E) D4 HSCs were stimulated with IFN-γ for 30 minutes, and STAT1 activation was detected by immunostaining with anti-pSTAT1 antibody (C) or Western blotting (D). The densities of bands in panel D were quantified and shown in panel E. In panel F, HSCs isolated from 8-week pair- and ethanol-fed mice were cultured for 4 or 7 days. The SOCS1 protein was then examined by Western blotting. Values in panels A, B, and E are means ± SEMs from 3 to 4 independent experiments. *P < .05, **P < .01 vs corresponding pair-fed groups.

Figure 6

Coculture of HSCs with hepatocytes from ethanol-fed mice attenuates IFN-γ/STAT1 signaling in HSCs. (A) D4 HSCs were cocultured with hepatocytes from pair- or ethanol-fed mice for 24 hours and subsequent treatment with IFN-γ (5 ng/mL) for 30 minutes, followed by immunostaining with anti-pSTAT1 antibody. (B) D4 HSCs and hepatocytes were cocultured with or without antioxidants for 24 hours, then treated with IFN-γ (5 ng/mL) for 30 minutes, followed by Western blot analyses. The densities of bands were quantified and shown in the right panel. Values represent means ± SEMs from 4 independent experiments. ***P < .001 vs ethanol group without antioxidants. (C and D) D4 HSCs were incubated with or without 0.5 mmol/L H₂O₂ for 5 minutes, then stimulated with IFN-γ (5 ng/mL) for 30 minutes, followed by immunostaining with anti-pSTAT1 antibody (C) or RT-PCR analyses of IRF-1 expression (D). (E and F) D4 HSCs were cocultured with hepatocytes from pair- or ethanol-fed mice for 24 hours, then treated with TGF-β1 (2.5 ng/mL for 45 minutes) or IFN-γ + TGF-β1 (pretreatment with 50 ng/mL IFN-γ for 45 minutes first then 2.5 ng/mL TGF-β1 for an additional 45 minutes), followed by immunostaining with anti-pSmad3 antibody (E) or RT-PCR analyses of COL1A2 expression (F). The densities of bands in panel F were quantified and shown in the upper panel. Values represent means ± SEMs from 5 independent experiments. ***P < .001 vs the corresponding IFN-γ(−)TGF-β(+) group. (A, C, and E, black arrow shows positive reaction and blank arrow shows very weak reaction). (Pair-Hep: pair-fed hepatocytes; EtOH-Hep: ethanol-fed hepatocytes).

Figure 7

HSCs from ethanol-fed mice are resistant to NK cell killing by a TGF-β1– dependent mechanism. (A) D4 HSCs from pair- or ethanol-fed mice were used as target cells, and liver MNCs from poly I:C-treated normal mice were used as effector cells. The cytotoxicity of liver MNCs against D4 HSCs was measured. Values are means ± SEMs from 3 independent experiments. **P < .01 vs corresponding pair-fed groups. (B) HSCs from pair- or ethanol-fed mice were cultured for various days and then subject to RT-PCR analyses. (C) D4 HSCs from pair- or ethanol-fed mice were incubated with murine recombinant TGF-β1 protein or TGF-β1 antibody for 20 minutes, then cytotoxicity assays similar to panel A were performed. Values are means ± SEMs from 6 independent experiments. *P < .05; **P < .01 vs corresponding 0 groups. (D) Poly I:C treated MNCs were incubated with or without TGF-β1 for 1 hour, then expression of NKG2D, TRAIL, or IFN-γ were measured by RT-PCR analyses. (P, pair-fed; Et, EtOH-fed).

Figure 8

A model for chronic alcohol acceleration of liver fibrosis by inhibition of NK cell killing of HSCs and suppression of IFN-γ signaling in HSCs. (1) Ethanol directly inhibits NK cytotoxicity against HSCs by down-regulating expression of NKG2D (NK cell activating receptor), TRAIL, and IFN-γ. (2) Ethanol stimulates HSCs to produce TGF-β, followed by inhibiting NK cell killing of HSCs; (3) ethanol induces expression of SOCS1 protein, followed by inhibiting IFN-γ signaling in HSCs; (4) ethanol stimulates hepatocytes to produce oxidative stress, which subsequently inhibits IFN-γ signaling in HSCs.