FTO-mediated m6A demethylation of ULK1 mRNA promotes autophagy and activation of hepatic stellate cells in liver fibrosis

Abstract

The activation of hepatic stellate cells (HSCs) is central to the occurrence and development of liver fibrosis. Our previous studies showed that autophagy promotes HSC activation and ultimately accelerates liver fibrosis. Unc-51-like autophagy activating kinase 1 (ULK1) is an autophagic initiator in mammals, and N6-methyladenosine (m6A) modification is closely related to autophagy. In this study, we find that the m6A demethylase fat mass and obesity-associated protein (FTO), which is the m6A methylase with the most significant difference in expression, is upregulated during HSC activation and bile duct ligation (BDL)-induced hepatic fibrosis. Importantly, we identify that FTO overexpression aggravates HSC activation and hepatic fibrosis via autophagy. Mechanistically, compared with other autophagy-related genes, ULK1 is a target of FTO because FTO mainly mediates the m6A demethylation of ULK1 and upregulates its expression, thereby enhancing autophagy and the activation of HSCs. Notably, the m6A reader YTH domain-containing protein 2 (YTHDC2) decreases ULK1 mRNA level by recognizing the m6A binding site and ultimately inhibiting autophagy and HSC activation. Taken together, our findings highlight m6A-dependent ULK1 as an essential regulator of HSC autophagy and reveal that ULK1 is a novel potential therapeutic target for hepatic fibrosis treatment.

1. Introduction

Liver fibrosis is a complex physiological and pathophysiological condition following long-term chronic liver injury and is a necessary stage for various types of liver disease to develop into cirrhosis, liver failure and even liver cancer [1]. Cholestasis is an important factor in liver fibrosis, affecting approximately 200‒500 individuals per million inhabitants [2]. Currently, in addition to improving the etiology and reducing cholestasis, there are still some limitations in the treatment of hepatic fibrosis. Cholestatic liver disease is characterized by damage to the small intrahepatic biliary ducts, followed by proliferation and an inflammatory response, which leads to the activation of hepatic stellate cells (HSCs). Hence, the elimination of activated HSCs is recognized as the main strategy for preventing hepatic fibrosis.

Autophagy is a specific type of cell death that starts with the formation of double-membrane autophagosomes and ultimately fuses with lysosomal compartments to degrade cellular organelles and proteins [3]. Autophagy plays dual roles in hepatic fibrosis. A report showed that impairing autophagy could promote nonalcoholic liver fibrosis progression [4]. Additional studies demonstrated that autophagy in HSCs attenuated CCL4-induced liver fibrosis [5]. We previously reported that autophagy was upregulated in bile duct ligation (BDL)-induced hepatic fibrosis, causing the activation of HSCs, which ultimately accelerated liver fibrosis. Interestingly, a large number of autophagy-related genes (ATGs) are involved in autophagy; however, the specific ATGs regulating autophagy in cholestatic liver fibrosis remain unclear.