Neutrophil-Derived Myeloperoxidase Aggravates Non-Alcoholic Steatohepatitis in Low-Density Lipoprotein Receptor-Deficient Mice

Abstract

BackgroundChronic inflammation and oxidative stress play fundamental roles in the pathogenesis of non-alcoholic steatohepatitis (NASH). Previously, we reported that myeloperoxidase (MPO), an aggressive oxidant-generating neutrophil enzyme, is associated with NASH severity in man. We now investigated the hypothesis that MPO contributes to the development and progression of NASH.MethodologyLow-density lipoprotein receptor-deficient mice with an MPO-deficient hematopoietic system (LDLR−/−/MPO−/−tp mice) were generated and compared with LDLR−/−/MPO+/+tp mice after induction of NASH by high-fat feeding.ResultsHigh-fat feeding caused a ∼4-fold induction of liver MPO in LDLR−/−/MPO+/+ mice which was associated with hepatic sequestration of MPO-positive neutrophils and high levels of nitrotyrosine, a marker of MPO activity. Importantly, LDLR−/−/MPO−/−tp mice displayed markedly reduced hepatic neutrophil and T-lymphocyte infiltration (p<0.05), and strong down regulation of pro-inflammatory genes such as TNF-α and IL-6 (p<0.05, p<0.01) in comparison with LDLR−/−/MPO+/+tp mice. Next to the generalized reduction of inflammation, liver cholesterol accumulation was significantly diminished in LDLR−/−/MPO−/−tp mice (p = 0.01). Moreover, MPO deficiency appeared to attenuate the development of hepatic fibrosis as evident from reduced hydroxyproline levels (p<0.01). Interestingly, visceral adipose tissue inflammation was markedly reduced in LDLR−/−/MPO−/−tp mice, with a complete lack of macrophage crown-like structures. In conclusion, MPO deficiency attenuates the development of NASH and diminishes adipose tissue inflammation in response to a high fat diet, supporting an important role for neutrophils in the pathogenesis of metabolic disease.

Adipose tissue macrophages induce hepatic neutrophil recruitment and macrophage accumulation in mice

“…A potential role of neutrophils in NASH has been previously suggested as MPO-deficient mice have reduced hepatic inflammation and attenuated NASH development 24. However, the increase in hepatic neutrophil accumulation we observed 12 days after AT transplantation did not coincide with macrophage accumulation or elevated liver enzyme levels.…”

The role of neutrophils in the development of liver diseases

“…Neutrophil-derived MPO can enhance macrophage cytotoxicity and induce neutrophil activation in a NASH mouse model. 27 In the foz/foz NASH metabolic syndrome model, a reduction of hepatic cholesterol stores was associated with ameliorated liver injury, and apoptosis and macrophage and neutrophil accumulation. 28 Additionally, dietary reversion suppressed uncoupling protein 2 expression and increased hepatic ATP levels, conditions that might favor apoptosis rather than reactive oxygen speciesmediated hepatocyte necrosis.…”

Non-alcoholic steatohepatitis pathogenesis: sublethal hepatocyte injury as a driver of liver inflammation

“…Mice lacking key neutrophilic enzymes, such as myeloperoxidase or elastase, displayed attenuated hepatic inflammation or improved insulin resistance in murine models of NAFLD. 6970 Recently, neutrophil extracellular trap formation has been implicated in liver sterile injury. 71 However, whether this phenomenon occurs in the lipotoxic liver is not known.…”