Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. Pathways responsible for the activation of IL-1 family cytokines are key in the development of NAFLD but underlying mechanisms are not fully understood. Many studies have focused on the inflammasome-caspase-1 pathway and have shown that this pathway is an important inducer of inflammation in NAFLD. However, this pathway is not solely responsible for the activation of proinflammatory cytokines. Also, neutrophil serine proteases (NSPs) are capable of activating cytokines and recent studies reported that these proteases also contribute to NAFLD. These studies provided, for the first time, evidence that this inflammasome-independent pathway is involved in NAFLD. In our opinion, these new insights open up new approaches for therapeutic intervention.

Introduction Non-alcoholic fatty liver disease (NAFLD) is becoming a major health problem worldwide. Inflammation plays an important role in disease pathogenesis and recent studies have shown a potential role for the neutrophil serine proteases (NSPs) proteinase-3 (PR3) and neutrophil elastase (NE) in NAFLD as well as an imbalance between NSPs and their natural inhibitor alpha-1 antitrypsin (AAT). The aim of this study was to investigate whether PR3 and NE plasma concentrations are associated with NAFLD and/or type 2 diabetes. Methods To explore this hypothesis we used several cohorts: a cohort of 271 obese individuals with liver steatosis, a cohort of 41 patients with biopsy-proven NAFLD, a cohort of 401 obese type 2 diabetes patients and a cohort of 205 lean healthy controls; and measured PR3 and NE plasma concentrations. In addition, we measured AAT plasma concentrations in order to investigate if the ratios between NSPs and their natural inhibitor were altered in NAFLD and type 2 diabetes when compared to healthy controls. Results Our data shows an increase in PR3 and NE concentrations and a decrease in AAT concentrations in obese patients when compared to controls. Moreover, PR3 plasma concentrations are increased in patients with liver steatosis. Furthermore, PR3 and NE concentrations in the liver are associated with the advanced stages of NAFLD characterized by NASH and/ or liver fibrosis. Additionally, PR3 and NE concentrations were up-regulated in patients with type 2 diabetes when compared to lean and obese controls. Conclusion We conclude that circulating levels of NSPs associate with obesity-related metabolic disorders. Further research is needed to clearly establish the role of these proteases and investigate whether they could be used as non-invasive markers for NAFLD and/or type 2 diabetes. Electronic supplementary material The online version of this article (10.1186/s10020-019-0084-3) contains supplementary material, which is available to authorized users.

Activation of inflammatory pathways is known to accompany development of obesity-induced nonalcoholic fatty liver disease (NAFLD), insulin resistance and type 2 diabetes. In addition to caspase-1, the neutrophil serine proteases proteinase 3, neutrophil elastase and cathepsin G are able to process the inactive proinflammatory mediators interleukin (IL)-1β and IL-18 to their bioactive forms, thereby regulating inflammatory responses. In this study, we investigated whether proteinase 3 is involved in obesity-induced development of insulin resistance and NAFLD. We investigated the development of NAFLD and insulin resistance in mice deficient for neutrophil elastase/proteinase 3 and neutrophil elastase/cathepsin G and in wild-type mice treated with the neutrophil serine proteinase inhibitor human α-1 antitrypsin. Expression profiling of metabolically relevant tissues obtained from insulin-resistant mice showed that expression of proteinase 3 was specifically upregulated in the liver, whereas neutrophil elastase, cathepsin G and caspase-1 were not. Neutrophil elastase/proteinase 3-deficient mice showed strongly reduced levels of lipids in the liver after being fed a high-fat diet. Moreover, these mice were resistant to high-fat-diet-induced weight gain, inflammation and insulin resistance. Injection of proteinase 3 exacerbated insulin resistance in caspase-1 -/-mice, indicating that proteinase 3 acts independently of caspase-1. Treatment with α-1 antitrypsin during the last 10 d of a 16-wk high-fat diet reduced hepatic lipid content and decreased fasting glucose levels. We conclude that proteinase 3 is involved in NAFLD and insulin resistance and that inhibition of proteinase 3 may have therapeutic potential. online address: http://www.molmed.org

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease. Inflammatory pathways contribute to disease pathogenesis; however, regulation of the underlying mechanism is not completely understood. IL-1β, a proinflammatory cytokine, participates in the development and progression of NAFLD. To become bioactive, IL-1β requires enzymatic processing. Mechanisms that activate IL-1β include the classical NLRP3 inflammasome-caspase-1 and the neutrophil serine proteases, neutrophil elastase, and proteinase-3. Several studies have shown that both caspase-1 and the neutrophil serine proteases are important for NAFLD development. However, it is unknown whether these pathways interact and if they have a synergistic effect in promoting NAFLD. In the present study, we developed a novel and unique mouse model by intercrossing caspase-1/ 11 knockout mice with neutrophil elastase/proteinase-3 double knockout mice. Subsequently, these mice were examined regarding the development of high-fat diet-induced NAFLD. Our results show that mice deficient in caspase-1, neutrophil elastase, and proteinase-3 were protected from developing diet-induced weigh gain, liver steatosis, and adipose tissue inflammation when compared with controls. We conclude that pathways that process pro-IL-1β to bioactive IL-1β play an important role in promoting the development of NAFLD and obesity-induced inflammation. Targeting these pathways could have a therapeutic potential in patients with NAFLD.