Thiamethoxam induces nonalcoholic fatty liver disease in mice via methionine metabolism disturb via nicotinamide N-methyltransferase overexpression
Graphical abstract
Introduction
Within 20 years from their introduction, neonicotinoids have replaced carbamate and organophosphate pesticides to become the most widely used class of insecticides worldwide, with registration in 120 countries (Craddock et al., 2019). They are widely applied in crop and vegetable protection, and their use has also expanded to home, lawn, and garden products (Craddock et al., 2019). It is estimated that >1.8 million kg of neonicotinoid active ingredients have been used on approximately 567–809,000 km2 of farmland annually in the United States of America (Douglas et al., 2015). The extensive application of neonicotinoids has resulted in frequent exposure of humans through multiple routes. Evidence has shown that neonicotinoids are detected in the surface water, groundwater, and soil (Huseth and Groves, 2014; Morrissey et al., 2015; Starner and Goh, 2012). Worryingly, it has also been evidenced through detection of neonicotinoids in urine that children in China and Japan are exposed to neonicotinoids (Osaka et al., 2016; Wang et al., 2015).
Thiamethoxam (TMX) is one of the major compounds of neonicotinoids. A study conducted by Lu et al. reported that TMX was one of the most commonly detected neonicotinoids in fruits and vegetables, with 51% and 53% detection rates in the Hangzhou (China) study and the U.S. Congressional Cafeteria study, respectively (Liu et al., 2018). Moreover, the small reference dose of TMX suggests a high relative toxicity of TMX (Jameel et al., 2020). Some studies indicated the hepatotoxic/hepatocarcinogenic effect in mice, and clarified the mechanism of the specific hepatotoxic/hepatocarcinogenic effect in mice by comparing the TMX metabolism in mouse, rat, and human microsomes with nicotinamide adenine dinucleotide phosphate (NADPH) (Swenson and Casida, 2013). Interestingly, neonicotinoids, such as imidacloprid, could induce lipid biosynthesis disorder in mouse liver by activating acetyl CoA carboxylase (Sun et al., 2017). However, the pathomorphological toxicology of TMX on the liver remains unclear.
Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease globally and an emerging risk factor for liver-related complications, including cirrhosis and hepatocellular carcinoma (Chalasani et al., 2018). NAFLD encompasses a histologic spectrum ranging from simple steatosis (nonalcoholic fatty liver) to nonalcoholic steatohepatitis, with variable degrees of fibrosis; the latter has the potential to progress to cirrhosis (Musso et al., 2016). Multiple risk factors of NAFLD have been documented in numerous studies. It has been shown that drugs such as rosiglitazone, tetracycline, phenobarbital, pentoxifylline, etc., aggravate pre-existing NAFLD (Fromenty, 2013). The disorder of nicotinic acid metabolism is associated with blood lipid and cholesterol profiles changes and metabolic diseases. TMX may disturb NADPH conversion (Honda et al., 2006), potentially associated with the development of metabolic diseases, such as NAFLD. However, thus far, it remains unclear whether TMX may induce or aggravate NAFLD.
Nicotinamide N-methyltransferase (NNMT) is strongly expressed in the adipose tissue and liver (Aksoy et al., 1994; Riederer et al., 2009), and plays a critical role in body energy expenditure. Increased NNMT activity was observed in patients with obesity and type 2 diabetes, suggesting a causative role of NNMT in these diseases (Salek et al., 2007). In addition, NNMT activation is also associated with human liver cirrhosis (Pumpo et al., 2001). Inhibition of NNMT activation may attenuate high fat diet-induced obesity and fatty liver disease (Kraus et al., 2014). NNMT may be a target gene to attenuate metabolic diseases, such as type 2 diabetes and obesity. However, it remains unknown whether NNMT participates in the process of TMX-induced changes.
Consequently, in this study, we determined the potential effect of TMX on the development of NAFLD and investigated the mechanism of TMX-induced NAFLD.
Section snippets
Animals and treatments
All animal experiments were reviewed and approved by the Ethics Research Committee of the School of Life Science and Technology of Harbin Institute of Technology and carried out according to guidelines for the care and use of experimental animals approved by the Heilongjiang Province People’s Congress.
ICR mouse is a strain of albino mice originating in Switzerland and selected by Dr. Hauschka to create a fertile mouse line. Healthy male ICR mice (25 ± 5 g body weight, 6–8 weeks age, n = 30)
TMX-induced dyslipidemia in mice
The concentrations of plasma NEFA and TG are commonly detected to describe lipid metabolism in the body. The concentrations of plasma NEFA in the TMX-H group were significantly increased compared with those detected in the control group (p < 0.05). Significant difference only in the concentration of TG was observed between the TMX-H and control groups (p < 0.05) (Fig. 1A and B). Additionally, the concentrations of plasma LDLC and HDLC were examined to further determined. TMX significantly
Discussion
Generally, TMX is considered a nearly non-toxic neonicotinoid insecticide to mammals. Hence, it is widely utilized in agriculture and gardening, resulting in its detection in human urine and animals. However, in this study, chronic TMX exposure induced dyslipidemia in mice in a dose-dependent manner, suggesting that TMX is not as safe as previously considered for mammals and may cause metabolic syndrome.
Liver is the critical tissue of lipid biosynthesis and metabolism in the organism. In the
Conclusion
In conclusion, TMX exposure evoked the occurrence and development of dyslipidemia and NAFLD in mice, partly through NNMT-mediated depletion of SAM and methyl donor imbalance, which finally regulate PPARα signaling pathway. Inhibition of NNMT could be a potentially novel strategy for blocking the progression to dyslipidemia and NAFLD induced by TMX in mice.
Author statement
Daqian Yang: Conceptualization, Methodology, Investigation, Writing – original draft. Xiaoting Zhang: Investigation, Methodology, Formal analysis. Lei Yue: Methodology, Histopathological diagnosis, cell morphology. Hailong Hu: Methodology, Validation, Data curation. Xiangjuan Wei: Investigation, Formal analysis. Qian Guo: Methodology, Formal analysis, Investigation. Boya Zhang: Investigation, Methodology. Xinpei Fan: Methodology, Validation. Yuan Xin: Methodology, Investigation. Yuri Oh:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by funds of the National Natural Science Foundation of China (Grant No. 21677044), the Open Project of State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology (Grant No. HCK201805).
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