Hepatic ureagenesis plays a predominant role in maintaining nitrogen and ammonia homeostasis in mammals. Disruption of ureagenesis leads to hyperammonemia, accompanied by hepatic encephalopathy and liver fibrosis. BAF60a is a subunit of the SWItch/Sucrose NonFermentable (SWI/SNF) complexes that regulates a series of metabolic pathways as a transcription factor. Zhang, Dong, et al. have elucidated a BAF60a-orchestrated upstream regulatory mechanism in the urea cycle and provide a mechanism through which steatotic hepatocytes produce ammonia via formation of a BAF60a-YB-1 complex.
SWI/SNF complex subunit BAF60a represses hepatic ureagenesis through a crosstalk between YB-1 and PGC-1α
Objective: Ureagenesis predominantly occurs in the liver and functions to remove ammonia, and the dysregulation of ureagenesis leads to the development of hyperammonemia. Recent studies have shown that ureagenesis is under the control of nutrient signals, but the mechanism remains elusive. Therefore, intensive investigation of the molecular mechanism underlying ureagenesis will shed some light on the pathology of metabolic diseases related to ammonia imbalance.
Methods: Mice were fasted for 24 h or fed a high-fat diet (HFD) for 16 weeks. For human evaluation, we obtained a public data set including 41 obese patients with and without hepatic steatosis. We analyzed the expression levels of hepatic BAF60a under different nutrient status. The impact of BAF60a on ureagenesis and hyperammonemia was assessed by using gain- and loss-of-function strategies. The molecular chaperons mediating the effects of BAF60a on ureagenesis were validated by molecular biological strategies.
Results: BAF60a was induced in the liver of both fasted and HFD-fed mice and was positively correlated with body mass index in obese patients. Liver-specific overexpression of BAF60a inhibited hepatic ureagenesis, leading to the increase of serum ammonia levels. Mechanistically, BAF60a repressed the transcription of Cps1, a rate-limiting enzyme, through interaction with Y-box protein 1 (YB-1) and by switching the chromatin structure of Cps1 promoter into an inhibitory state. More importantly, in response to different nutrient status, PGC-1α (as a transcriptional coactivator) and YB-1 competitively bound to BAF60a, thus selectively regulating hepatic fatty acid β-oxidation and ureagenesis.
Conclusions: The BAF60a-YB-1 axis represses hepatic ureagenesis, thereby contributing to hyperammonemia under overnutrient status. Therefore, hepatic BAF60a may be a novel therapeutic target for the treatment of overnutrient-induced urea cycle disorders and their associated diseases.