Non-alcoholic fatty liver disease (NAFLD) remains the most threatening liver disease worldwide. The incidence rate is increasing annually and is related to obesity, dyslipidemia, insulin resistance and other factors. NAFLD is characterized by the excessive accumulation of fatty acid and metabolic dysfunction (Paul and Davis, 2018). The manifestations of NAFLD vary with disease progression. Statistically, 20–27% of patients with NAFLD progress from simple steatosis to nonalcoholic steatohepatitis (NASH), which, over time, develops into cirrhosis and hepatocellular carcinoma (Sanyal, 2019). Over the past decades, research on molecular mechanisms has extended from the initial “double hit” theory to the current “multiple hit” theory, with influencing factors including metabolism, genetics, inflammation and injury, innate immunity, and mitochondrial dysfunction (Sanyal et al., 2021). However, the pathogenesis remains unclear. Several studies have shown that hepatocyte mitochondria play an important role in the pathogenesis of NAFLD (Pirola et al., 2013). Recent studies have reported that changes in mitochondrial structure and function are characteristic of NAFLD (Ma et al., 2023). Mitochondrial dysfunction causes energy depletion in hepatocytes, accompanied by proton pump dysfunction, increased permeability of the mitochondrial inner membrane, rapid dissipation of membrane potential, mitochondrial swelling, and mutations in mitochondrial DNA (Garcia-Ruiz et al., 2013, Wang et al., 2020). Mitophagy is also involved in the selective degradation of lipid droplets in the cytoplasm (Undamatla et al., 2023). However, mitophagy has received much attention, but its role in NAFLD is unclear.

Various forms of oxidative stress, hypoxia, and pathogen infections affect autophagy by inducing the formation of free radicals and reactive oxygen species (ROS) formation, thereby promoting antioxidant responses (Albano et al., 2023). Hypoxia can upregulate hypoxia inducible factor 1 (HIF-1), activate BNIP3, BNIP3L/NIX, and FUNDC1 mediated mitophagy, reduce ROS production, maintain self survival, and induce hypoxia adaptation in the body (Gonzalez et al., 2018). Mitophagy can alleviate liver lipid metabolism disorders and mitochondrial dysfunction caused by NAFLD (Nobili et al., 2014). Hypoxia relieves liver triglyceride accumulation, necrotising inflammation, and fibrosis during NAFLD (Chen et al., 2023). In addition, PPARα under hypoxia stress is essential for fatty acid metabolism homeostasis in the liver and inhibition of the development of NAFLD. Free fatty acid-induced PPAR-α upregulates carnitine palmitoyltransferase 1 (CPT1) expression and increases mitochondrial β oxidation, as well as regulates fatty acid uptake and clearance (Golinska et al., 2022). Activation of PPARα reverses inhibited autophagy and lipid degradation, whereas PPARα induces genes for fatty acid β-oxidation and lipid droplet autophagy, slowing NAFLD development (Theys et al., 2022). The specific role of mitophagy in the pathogenesis of NAFLD is emerging, but whether obesity-related NAFLD changes with altitude remains to be fully understood. It is important to understand the role of mitophagy in NAFLD under hypoxia to understand how the risk of NAFLD in residents of high altitudes under hypoxia is significantly lower than that in plain areas and how the metabolic function of patients is enhanced. As mitophagy is involved in the selective degradation of lipid droplets in the cytoplasm, mitophagy in cells may be a self-defense mechanism to prevent NAFLD (Wang et al., 2015). BNIP3 has been demonstrated to be extensively involved in abnormalities to mitochondrial metabolic function and dynamicsand in non-alcoholic fatty liver disease. Mitophagy and lipid metabolism are also associated, however, the specific relationship of BNIP3 in NAFLD lipid metabolism under hypoxia remains unclear. Consequently, to address the prevention and treatment challenges of NAFLD, it is important to study the specific role of mitophagy in the lipid metabolism of NAFLD in high-altitude hypoxic environments.

In total, we investigate the expression and the role of BNIP3 in NAFLD under hypoxia, and explore its involvement in regulating NAFLD mitophagy, fatty acid β-oxidation both in vivo and in vitro. The study design is illustrated in Fig. 1.