Loss of Splicing Factor SRSF3 Impairs Lipophagy Through Ubiquitination and Degradation of Syntaxin17 in Hepatocytes.

Lipid accumulation in hepatocytes is the distinctive characteristic of nonalcoholic fatty liver disease. Serine/arginine-rich splicing factor 3 (SRSF3) is highly expressed in the liver and expression decreases in high-fat conditions. However, the role of SRSF3 in hepatic lipid metabolism needs to be clarified. Here, we showed that loss of SRSF3 was associated with lipid accumulation. We determined that SRSF3 regulated lipophagy, the process of selective degradation of lipid droplets by autophagy. Mechanistically, loss of SRSF3 impaired the fusion of the autophagosome and lysosome by promoting the proteasomal degradation of syntaxin 17 (STX17), a key autophagosomal SNARE protein. We found that ubiquitination of STX17 was increased and upregulation of seven in absentia homolog 1 was responsible for the increased posttranslational modification of STX17. Taken together, our data primarily demonstrate that loss of SRSF3 weakens the clearance of fatty acids by impairing lipophagy in the progression of nonalcoholic fatty liver disease, indicating a novel potential therapeutic target for fatty liver disease treatment.

CD36 regulates diurnal glucose metabolism and hepatic clock to maintain glucose homeostasis in mice.

The mammalian circadian clock and glucose metabolism are highly interconnected, and disruption of this coupling is associated with multiple negative health outcomes. Liver is the major source of endogenous glucose production and liver clock is one of the most vital peripheral clock systems. We demonstrate that fatty acid translocase (CD36) is expressed rhythmically in mouse liver and autonomously modulates the diurnal oscillations of liver clock and glucose homeostasis. CD36 knockout in hepatocytes inhibits the relay of insulin signaling and provokes FoxO1 nuclear shuttling, consequently increasing Per1 nuclear expression. Moreover, FoxO1 can activate the central clock gene Per1 at the transcriptional level. These changes lead to a disrupted clock oscillation and behavioral rhythm. Our study first reveal that CD36 is a key regulator of the circadian oscillator and its deficiency may cause liver clock disruption, which aggravates the imbalance of glucose homeostasis and contribute to augmentation and progression of metabolic disease.

Inhibition of Fatty Acid Translocase (FAT/CD36) Palmitoylation Enhances Hepatic Fatty Acid ß-Oxidation by Increasing Its Localization to Mitochondria and Interaction with Long-Chain Acyl-CoA Synthetase 1.

Aims: Impaired fatty acid oxidation (FAO) in mitochondria of hepatocytes causes lipid accumulation and excessive production of reactive oxygen species (ROS) and oxidative damage, leading to nonalcoholic fatty liver disease (NAFLD). Fatty acid translocase (FAT/cluster of differentiation 36 [CD36]), a transmembrane protein that facilitates the uptake of long-chain fatty acids (LCFAs), is recently found to be involved in FAO. The function of FAT/CD36 is associated with its subcellular localization. Palmitoylation, one of the most common lipid modifications, is generally thought to regulate FAT/CD36 subcellular localization. Here, we aimed to investigate the role of palmitoylation in FAT/CD36 localization to mitochondria and its influence on FAO in hepatocytes. Results: We demonstrated that FAT/CD36 exists on the mitochondria of hepatocytes. Palmitoylation of FAT/CD36 was significantly upregulated in NAFLD. Inhibition of FAT/CD36 palmitoylation resulted in an obvious increase in the distribution of FAT/CD36 to mitochondria of hepatocytes. Depalmitoylated FAT/CD36 on the mitochondrial membrane continues functioning by facilitating fatty acid trafficking to mitochondria. Abundant mitochondrial FAT/CD36 interacted with long-chain acyl-CoA synthetase 1 (ACSL1), and thus, more LCFAs were transported to ACSL1. This led to an increase in the generation of long-chain acyl-CoA, contributing to the enhancement of FAO and alleviating NAFLD. Innovation and Conclusion: This work revealed that inhibiting FAT/CD36 palmitoylation alleviates NAFLD by promoting FAT/CD36 localization to the mitochondria of hepatocytes. Mitochondrial FAT/CD36 functions as a molecular bridge between LCFAs and ACSL1 to increase the production of long-chain acyl-CoA, thus promoting FAO, thereby avoiding lipid accumulation and overproduction of ROS in hepatocytes. Antioxid. Redox Signal. 36, 1081-1100.

Selenoprotein K contributes to CD36 subcellular trafficking in hepatocytes by accelerating nascent COPII vesicle formation and aggravates hepatic steatosis.

SelenoproteinK (SelK), an endoplasmic reticulum (ER) - resident protein, possesses the property of mediate oxidation resistance and ER - associated protein degradation (ERAD) in several tissues. Here, we found that increased SelK markedly promotes fatty acid translocase (CD36) subcellular trafficking and aggravates lipid accumulation in hepatocytes. We demonstrated that SelK is required for the assembly of COPII vesicles and accelerates transport of palmitoylated-CD36 from the ER to Golgi, thus facilitating CD36 plasma membrane distribution both in vivo and in vitro. The mechanism is that SelK increases the stability of Sar1B and triggers CD36-containing nascent COPII vesicle formation, consequently, promotes CD36 subcellular trafficking. Furthermore, we verified that the intervention of SelK SH3 binding domain can inhibit the vesicle formation and CD36 subcellular trafficking, significantly ameliorates NAFLD in mice. Collectively, our findings disclose an unexpected role of SelK in regulating NAFLD development, suggesting that targeting the SelK of hepatocytes may be a new therapeutic strategy for the treatment of NAFLD.