Despite the widespread use of vaccines and antiviral drugs, approximately 350‐400 million patients with chronic hepatitis B (CHB) remain worldwide, who carry high risk of cirrhosis and liver carcinoma. Moreover, owing to improvements in global living standards and lifestyle changes, non‐alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease. Coexistence of NAFLD and CHB is commonly observed, especially in Asian CHB populations; however, little is known regarding the relationship between these two diseases as comorbidities. In this review, we summarize recent advances in clinical and basic researches related to the underlying mutual interactions, as well as potential animal models to facilitate further investigation.

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.

The gut-liver axis is increasingly recognized as being involved in the pathogenesis and progression of non-alcoholic fatty liver disease (NAFLD). Prolyl endopeptidase (PREP) plays a role in gut metabolic homeostasis and neurodegenerative diseases. We investigated the role of PREP disruption in the crosstalk between gut flora and hepatic steatosis or inflammation in mice with NAFLD. Wild-type mice (WT) and PREP gene knocked mice (PREP^gt) were fed a low-fat diet (LFD) or high-fat diet (HFD) for 16 or 24 weeks. Murine gut microbiota profiles were generated at 16 or 24 weeks. Liver lipogenesis-associated molecules and their upstream mediators, AMP-activated protein kinase (AMPK) and sirtuin1 (SIRT1), were detected using RT-PCR or western blot in all mice. Inflammatory triggers and mediators from the gut or infiltrated inflammatory cells and signal mediators, such as p-ERK and p-p65, were determined. We found that PREP disruption modulated microbiota composition and altered the abundance of several beneficial bacteria such as the butyrate-producing bacteria in mice fed a HFD for 16 or 24 weeks. The level of butyrate in HFD-PREP^gt mice significantly increased compared with that of the HFD-WT mice at 16 weeks. Interestingly, PREP disruption inhibited p-ERK and p-p65 and reduced the levels of proinflammatory cytokines in response to endotoxin and proline-glycine-proline, which guided macrophage/neutrophil infiltration in mice fed a HFD for 24 weeks. However, at 16 weeks, PREP disruption, other than regulating hepatic inflammation, displayed improved liver lipogenesis and AMPK/SIRT1 signaling. PREP disruption may target multiple hepatic mechanisms related to the liver, gut, and microbiota, displaying a dynamic role in hepatic steatosis and inflammation during NAFLD. PREP might serve as a therapeutic target for NAFLD.