Prolyl endopeptidase gene disruption attenuates high fat diet-induced nonalcoholic fatty liver disease in mice by improving hepatic steatosis and inflammation

BACKGROUND: Prolyl endopeptidase (PREP) is a serine endopeptidase that regulates inflammatory responses. PREP inhibitors can reduce hepatocyte lipid accumulation and may participate in the progression of nonalcoholic fatty liver disease (NAFLD). We investigated whether disruption of PREP regulates hepatic steatosis and inflammation in mice with NAFLD. METHODS: Wild-type and PREP gene disrupted mice were randomly divided into low-fat diet wild-type (LFD-WT), high-fat diet wild-type (HFD-WT), low-fat diet PREP disruption (LFD-PREP(gt)), and high-fat diet PREP disruption (HFD-PREP(gt)) groups. Animals were euthanized at the endpoint of 32 weeks. The NAFLD activity score and number of inflammatory cells were determined by hematoxylin-eosin staining and immunohistochemical staining of liver tissue. The expression levels of inflammation- and lipid metabolism-associated genes in the liver and serum were detected by quantitative reverse transcription PCR, mass spectrometry, or enzyme-linked immunosorbent assay. RESULTS: The body weight and epididymal fat tissue index of the HFD-PREP(gt) mice were significantly decreased compared with that of the HFD-WT mice. Moreover, the NAFLD activity score and liver function were attenuated in the HFD-PREP(gt) mice. Fat accumulation and the level of expression of mRNAs associated with lipid metabolism and proinflammatory responses were improved in the HFD-PREP(gt) mice. The number of CD68-positive cells in liver tissue and the serum levels of inflammation-associated factors were significantly decreased in the HFD-PREP(gt) mice compared with those in the HFD-WT mice. Further mechanistic investigations indicated that the protective effect of PREP disruption on liver inflammation was associated with the suppressed production of matrix metalloproteinases (MMPs) and proline-glycine-proline (PGP) and the inhibition of neutrophil infiltration. CONCLUSIONS: Loss of PREP lowers the severity of hepatic steatosis and inflammatory responses in a high-fat diet-induced nonalcoholic steatohepatitis model. PREP inhibition may protect against NAFLD.