Activation of the STING-IRF3 pathway promotes hepatocyte inflammation, apoptosis and induces metabolic disorders in nonalcoholic fatty liver disease

Abstract

Background

Nonalcoholic fatty liver disease (NAFLD) is a common result of obesity and metabolic syndrome. Hepatocyte injury and metabolic disorders are hallmarks of NAFLD. Stimulator of interferon genes (STING) and its downstream factor interferon regulatory factor 3 (IRF3) trigger inflammatory reaction in response to the presence of cytosolic DNA. STING has recently been shown to play an important role in early alcoholic liver disease. However, little is known about the role of STING-IRF3 pathway in hepatocyte injury. Here, we aimed to examine the effect of STING-IRF3 pathway on hepatocyte metabolism, inflammation and apoptosis.

Methods

We examined the activation of the STING-IRF3 pathway, a high-fat diet (HFD)-induced obese mouse model, and determined the role of this pathway in a free fatty acid (FFA)-induced hepatocyte inflammatory response, injury, and dysfunction in L-O2 human liver cells.

Results

STING and IRF3 were upregulated in livers of HFD-fed mice and in FFA-induced L-O2 cells. Knocking down either STING or IRF3 led to a significant reduction in FFA-induced hepatic inflammation and apoptosis, as evidenced by modulation of the nuclear factor κB (NF-κB) signaling pathway, inflammatory cytokines, and apoptotic signaling. Additionally, STING/IRF3 knockdown enhanced glycogen storage and alleviated lipid accumulation, which were found to be associated with increased expression of hepatic enzymes in glycolysis and lipid catabolism, and attenuated expression of hepatic enzymes in gluconeogenesis and lipid synthesis.

Conclusions

Our results suggest that the STING-IRF3 pathway promotes hepatocyte injury and dysfunction by inducing inflammation and apoptosis and by disturbing glucose and lipid metabolism. This pathway may be a novel therapeutic target for preventing NAFLD development and progression.

Introduction

In recent decades, the rapid increase in the prevalence of obesity has become a serious global health problem [1]. Nonalcoholic fatty liver disease (NAFLD) is common in obesity and metabolic syndrome [2], and has become a main cause of liver disease in industrialized countries, including China [3], [4], [5]. Hepatic insulin resistance and type 2 diabetes are considered sequelae of NAFLD [6]. In brief, NAFLD increases the risk of insulin resistance and leads to obesity-related glucose metabolic disorders [1], yet the pathogenesis of NAFLD is complex and not well understood. The two-hit hypothesis [7] has been proposed to explain NAFLD pathogenesis. In this hypothesis, steatosis represents the “first hit”, increasing the vulnerability of the liver to various “second hits” including immune-related reactions, oxidative stress, lipid peroxidation damage, etc. [8], [9], [10], finally leading to hepatic inflammation and cellular death. These mechanisms are not well understood, and effective measures for preventing and treating NAFLD are lacking.

Stimulator of interferon genes (STING) is an important protein in the innate immune signaling pathway, newly discovered in 2008 [11]. The majority of previous studies have focused on the effects of activated STING signaling upon viral infections in immune cells. After infection, the pathogenic DNA is always present in the cytoplasm, and is sensed by cytosolic DNA sensors to initial innate immune responses [12], [13], [14], [15], [16]. However, under certain pathological conditions, self DNA also can be recognized by innate immune sensors causing several autoimmune and auto-inflammatory diseases [17]. STING, as a mediator of the innate immune signaling pathway, plays a role in linking upstream DNA sensors to downstream factors [11]. A recent study suggests that STING and interferon (IFN) regulatory factor 3 (IRF3), a downstream factor of STING, play an important role in early alcoholic liver disease [18]. Furthermore, chronic activation of STING can lead to cell metabolic disorders in an autoimmune/auto-inflammatory disease mouse model [19]. However, the specific molecular mechanism of STING in NAFLD remains poorly understood.

Given the importance of STING in inflammatory response, we explored the role of STING in NAFLD and its possible regulatory mechanism in hepatic inflammation, apoptosis, and glucose and lipid metabolic disorders using an obese mouse model with a high-fat diet (HFD) in vivo and the L-O2 human liver cell line in vitro.