Combined exposure to PM_2.5 and high-fat diet facilitates the hepatic lipid metabolism disorders via ROS/miR-155/PPARγ pathway

Highlights：

• Combined exposure to PM_2.5 and high-fat diet (HFD) facilitated the occurrence and development of MAFLD in C57BL/6J mice.

• There was a synergistic effect between PM_2.5 and HFD on hepatic lipotoxicity.

• The ROS/miR-155/PPARγ pathway played a crucial role in promoting hepatic metabolism disorders by PM_2.5 and HFD exposure.

Abstract

Environmental fine particulate matter (PM_2.5), which has attracted worldwide attention, is associated with the progression of metabolic-associated fatty liver disease (MAFLD). However, it is unclear whether dietary habit exacerbate liver damage caused by PM_2.5. The current study aimed to investigate the combined negative effects of PM_2.5 and high-fat diet (HFD) on liver lipid metabolism in C57BL/6J mice. Histopathological and Oil-Red O staining analysis illustrated that PM_2.5 exposure resulted in increased liver fat content in HFD-fed C57BL/6J mice, but not in standard chow diet (STD)-fed mice. And there was a synergistic effect between PM_2.5 and HFD on hepatic lipotoxicity. The increased ROS levels and augmented oxidative damage were evaluated in liver tissue of mice treated with PM_2.5 and HFD together. In addition, excessive ROS production could activate the miR-155/peroxisome proliferator-activated receptor gamma (PPARγ) pathway, including up-regulation of lipid accumulation-related protein expressions of recombinant liver X receptor alpha (LXRα), sterol regulatory element binding protein-1 (SREBP-1), stearoyl-CoA desaturase-1 (SCD1), fatty acid synthase (FAS) and acetyl-CoA carboxylase 1 (ACC1).The use of miR-155 inhibitors demonstrated the indispensable role of miR-155 in the activation of lipid-regulated proteins by PM_2.5 and palmitic acid (PA). Collectively, altering high-fat dietary habits could protect against MAFLD motivated by air pollution, and miR-155 might be an effective preventive and therapeutic target for this process.

Introduction

Environmental fine particulate matter (PM_2.5), the fifth largest risk factor for death globally, has contributed to the global burden of disease in recent years [1]. Epidemiological and clinical studies have linked PM_2.5 exposure to respiratory, cardiovascular, metabolic disorders and reproductive diseases [2,3]. This means that PM_2.5 is not only deposited in the lungs, but also travels through the bloodstream to other organs in the body, including the liver [4]. Recently, a precise imaging technique was developed to visualize the deposition of PM_2.5 particles in the liver through inhalation, providing solid evidence that the PM_2.5 particles could enter the extrapulmonary organs [5]. Therefore, as the main responsibility for the metabolism and absorption of drugs and poisons, liver is also a considerable target organ of PM_2.5.

Non-alcoholic fatty liver disease (NAFLD) is characterized by intrahepatic triglyceride accumulation exceeds 5% of liver wet weight [6]. It encompasses nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH), which can progress to fibrosis, cirrhosis and even liver cancer [7]. It is estimated that one quarter of the global population has NAFLD, and its incidence and prevalence continue to rise rapidly [8]. Between 2016 and 2030, NASH prevalence is projected to double in some rapidly advancing countries (e.g., China, France, Germany, and so on) [9]. Environment plays a catalytic role in the progression of hepatic lipid accumulation. Evidence from an epidemiological study showed that for each 10 μg/m³ increase in PM_2.5 exposure, the odds ratios for NAFLD was 1.29 [10]. A similar study found that the incidence of NAFLD increased by 34% among people living in environments with high levels of PM_2.5 [11]. With the progress of science and the improvement of people's understanding of existing diseases, NAFLD has now been renamed metabolic-associated fatty liver disease (MAFLD) [12].

The prevalence of MAFLD evolved with increased obesity burden. Recently, the urbanization in many countries has led to overnutrition and unhealthy lifestyles, setting the stage for obesity and MAFLD epidemic [13]. Epidemiological studies have shown that people with MAFLD typically have diets higher in saturated fat and cholesterol [14]. In a clinical study of overweight subjects, triglyceride levels in the liver increased by 55% after three weeks of high fat diet (HFD) [15]. It was reported that HFD led to obesity, liver lipid metabolism disorders and liver damage in C57BL/6J mice [16]. Many studies have used HFD-fed mice as MAFLD models. Lu et al. manipulated the expression levels of liver target genes in HFD-induced obese mice by a gene delivery system to investigate their role in hepatic steatosis [17]. Stephania et al. found that compensatory mechanisms for triglyceride synthesis were inhibited in MAFLD patients and HFD-fed mice, thereby exacerbating liver steatosis [18]. However, there are few studies on the effects of combined intervention of PM_2.5 and HFD on liver lipotoxicity, which requires further exploration.

The progression of MAFLD involves the occurrence of " multiple-hit” injury, in which oxidative stress is the central mechanism [19]. The imbalance of oxygen species (ROS) generation causes excessive fatty acids and lipid peroxidation in liver, resulting in hepatocyte damage [20]. Previous studies have confirmed that oxidative stress is also a key mechanism by which PM_2.5 exerts its harmful effects. Organic compounds attached to the surface of PM_2.5 particles can be metabolized into electrophilic metabolites, inducing an increase in intracellular ROS [21]. It has been confirmed that PM_2.5 exposure promotes oxidative stress and hepatic inflammation, thereby disrupting normal hepatic lipid metabolism in mice [22,23]. However, the specific mechanism by which the combined exposure of PM_2.5 and HFD promotes the development of MAFLD by inducing oxidative stress is still not completely clear.

MiRNA is a regulator of gene expression and translation efficiency. It is involved in the physiological and pathological process of liver function such as maladjustment of liver metabolism, liver injury and liver fibrosis [24]. MiR-155, one of the differentially expressed miRNA in obese individuals, can directly affect fatty acid uptake, oxidation and output by targeting peroxisome proliferator-activated receptor gamma (PPARγ) [25]. In rats with fatty liver, high-throughput sequencing technology detected a significant up-regulation of miR-155 in liver tissue [26]. Moreover, it has been reported that fatty hepatitis mice with miR-155 KO showed reduced liver steatosis and decreased expression of proteins involved in fatty acid metabolism [27]. However, whether PM_2.5-mediated oxidative stress can activate miR-155 remains unknown. In consideration of the above evidences, we hypothesized that combined treatment to PM_2.5 and HFD may expedite liver lipid metabolism disorder via triggering ROS/miR-155/PPARγ signaling pathway, and PM_2.5 and HFD have a synergistic effect on the occurrence and development of MAFLD.