Introduction
Fatty acids (FAs) are indispensable sources of energy in cells, and also important bioactive mediators involved in many homeostasis processes, including metabolism and regulating inflammatory immune responses (Masoodi et al., 2015). However, unlike adipocytes, which possess a strong capacity to store excessive free fatty acids (FFAs) in lipid droplets in the form of triglycerides, lipid overload in non-adipose tissues (such as heart, liver, kidney, skeletal muscle, and pancreatic β-cells) causes lipotoxicity, leading to cell dysfunction or death. Lipotoxicity has been reported to be mainly caused by long-chain saturated fatty acids (SFAs), especially palmitic acid (PA, C16: 0) (Leamy et al., 2013). Accumulating data suggest that abnormal FAs metabolism and its induced lipotoxicity are closely related to the risk of developing non-alcoholic fatty liver disease (NAFLD), diabetes, atherosclerosis, heart failure, and even multiple cancers (Afonso et al., 2016; Blucher and Stadler, 2017; Cansancao et al., 2018; Jiang et al., 2019; Zhou et al., 2019). Therefore, inhibition of FAs-related lipotoxicity represents a potential therapeutic strategy, which is of great interest.
The liver is the main metabolic organ of FAs, and the imbalance in its synthesis, uptake and disposal (mainly including mitochondrial oxidation and endoplasmic reticulum re-esterification) will induce lipotoxicity, further leading to the body dysfunction. Mitochondrial β-oxidation, the most important metabolic pathway of FAs, is mainly regulated by rate-limiting enzymes such as carnitine palmitoyl-transferase 1α (CPT1α), which serves as a gatekeeper for FAs to enter mitochondria. In addition, it has been demonstrated that in starved mitochondrial fusion protein 1 knockout (Mitofusin1KO) mouse embryonic fibroblasts (MEFs), mitochondria were fragmented, reducing the β-oxidation rate and leading to lipid accumulation in lipid droplets (Rambold et al., 2015; Wrighton, 2015). However, there are no published data on the relationship between liver FAs metabolism and Mitofusin1 expression during nutrient oversupply.
Hydrogen sulfide (H2S), a well-known novel gaseous signaling molecule, is increasingly recognized as a crucial regulator of cardiovascular diseases. H2S in mammalian tissues is synthesized endogenously by three enzymes: cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MPST). Emerging data indicate that endogenous H2S level is significantly negatively correlated with abnormal lipid metabolism, including hyperlipidemia, NAFLD, and atherosclerosis(Mani et al., 2013; Sun et al., 2015; Li et al., 2017). Exogenous H2S donors (NaHS, GYY4137, etc.) can significantly improve these metabolic diseases through multiple properties, such as anti-inflammatory, antioxidant, inhibiting foam cell formation, improving endothelial function, and activating liver autophagy (Liu et al., 2013; Sun et al., 2015; Durante, 2016; Li et al., 2017). And more notably, a recent study found that FFAs up-regulate the liver expression of 3-MPST, and subsequently inhibit the CSE/H2S pathway, impairing the endogenous synthesis of H2S, and leading to NAFLD (Li et al., 2017). However, the effect of H2S on FAs metabolism in the liver remains unclear.
At present, H2S-releasing “drugs” used in research have been largely limited to simple sulfide salts, mainly NaHS. However, NaHS, the so-called immediate-release H2S donor, releases excessive H2S instantaneously and therefore does not mimic the production of endogenous H2S (Li et al., 2008). Anethole dithiolethione (ADT; 5-(4-methoxyphenyl)-3H-1,2-dithiole-3-thione), clinically used as a hepatoprotective and choleretic drug, is also a prodrug of H2S (Wang et al., 2014; Szabo and Papapetropoulos, 2017). In vivo, ADT is rapidly metabolized into the active metabolite ADT-OH, with a half-life of about 3.1 and 4.4 h, respectively(Li W. et al., 2008; Wang et al., 2014). Compared with NaHS, ADT releases H2S and significantly increases serum H2S levels over a long period of time. In 2019, Dulac et al. proposed the first detailed mechanism for the production of H2S by ADT and ADT-OH in the presence of rat liver microsomes, NADPH and O2 (Figure 1) (Dulac et al., 2019). Therefore, this article aims to investigates how clinically available sustained-release H2S donor ADT affects hepatic fatty acid metabolism under high-fat diet (HFD) and explores its possible mechanisms.
