Fumarate hydratase orchestrates hepatic steatosis via ULK1 mediated lipophagy

Hepatic lipid homeostasis is coordinated by various metabolic pathways, such as lipogenesis, lipid uptake, storage, and oxidation. However, the underlying mechanism that orchestrates metabolic crosstalk remains unclarified. Herein, we demonstrated that fumarate hydratase (FH) functioned as an indispensable adaptor governing mitochondria metabolism and lipid droplets (LDs) degradation. Hepatic FH overexpression prevented hepatic steatosis in normal mice and ob/ob mice without interfering mitochondrial lipid oxidation and lipogenesis. Strikingly, we found that FH selectively activated lipophagy-mediated LDs lipidolysis. Mechanistically, FH interacted with ULK1 and stabilized ULK1 through preventing its ubiquitination and degradation, resulting in lipophagy activation and the elimination of hepatic lipid accumulation. Meanwhile, hepatic ULK1 deficiency abrogated the protective effects in FH-overexpressing mice. Also, we clarified that the interaction between FH and ULK1 occurred in cytoplasm, but not in mitochondria. Cytoplasmic FH was sensitive to lipids and downregulated without affecting mitochondrial FH in vivo. Moreover, the FH-ULK1 axis was identified closely associated with hepatic steatosis in human patients. Taken together, our research demonstrates a “two-side” insight of FH on hepatic lipid metabolism and provides a promising strategy for fatty liver treatment. Overall design: To investigate the role of FH in the protection of fatty liver, we used liver-specific adenovirus AAV8 to administer tail vein injections to 6-8 week old C57BL/6 mice, which were fasted for 24h after one month. And for another model we used liver-specific adenovirus AAV8 to inject 6-8 week old ob/ob mice into the tail vein and then continued to feed them a high-fat diet for one month.