TCA Cycle Analysis Service Case Study

Title: Hierarchical tricarboxylic acid cycle regulation by hepatocyte arginase 2 links the urea cycle to oxidative metabolism

Journal: Cell metabolism

Published: 2024

Background

A core challenge in liver metabolism lies in coordinating the two pathways of nitrogen metabolism (the urea cycle) and energy metabolism (the TCA cycle). Traditionally, these two pathways were considered to function independently, but recent research has revealed that metabolic intermediates (such as α-ketoglutarate and fumarate) may serve as molecular bridges connecting the two pathways. Arginase 2 (Arg2) is a mitochondrially localized urea cycle enzyme, but its regulatory role in the TCA cycle remains largely undetermined. How the liver maintains energy-nitrogen balance through Arg2-mediated metabolic reprogramming under metabolic stress (such as a high-fat diet) remains unclear. This study aimed to investigate how arginase 2 in hepatocytes influences oxidative metabolism by regulating the TCA cycle and to reveal the connection between the urea cycle and the TCA cycle.

Results

TCA cycle metabolites were quantitatively analyzed using LC-MS/MS technology to evaluate the effect of arginase 2 deficiency on metabolite levels.

Results from a [U-¹³C₆] glucose trace indicated that non-glucose carbon sources contribute to alanine accumulation in Arg2^LKO hepatocytes, and metabolite flux from non-glucose substrates is increased in Arg2^LKO hepatocytes.

Glutamine oxidative metabolism requires glutamate to enter the TCA cycle, which involves oxidative decarboxylation of glutamine-derived glutamate to form α-ketoglutarate. This is followed by desaturation to fumarate and stereospecific hydration to malate and aspartate via the TCA cycle (Figure 1K). The enhanced incorporation of glutamine into (M+5) α-ketoglutarate, (M+4) fumarate, (M+4) malate, and (M+4) aspartate suggests that Arg2^LKO hepatocytes have an increased reliance on glutamine recycling to provide energy for the TCA cycle. The authors examined the reduced carbon flux of labeled [U-¹³C₅] glutamine and found that Arg2-deficient hepatocytes significantly increased the production of [U-¹³C₅] glutamine as M+5 citrate, M+3 aspartate, M+3 malate, and M+3 fumarate compared to WT controls (Figure 1M). The ¹³C enrichment in these metabolites suggests that Arg2 LKO mouse hepatocytes metabolize glutamine through reductive catabolism to synthesize acetyl-CoA for fatty acid synthesis. In vivo and in vitro data confirmed glutamine recycling and increased transfer of glutamine to aspartate in Arg2 LKO livers.

Glutamine stable isotope tracing result Figure 1. Stable isotope tracing reveals enhanced glutaminolysis in Arg2-deficient hepatocytes

Conclusion

The research indicates that ARG2 in hepatocytes plays a key role in the regulation of the urea cycle and TCA cycle, and the losing of it can lead to mitochondrial dysfunction and metabolic disorders, thereby affecting energy metabolism and insulin sensitivity. In addition, losing of ARG2 is closely related to the occurrence of MASLD/MASH, suggesting that it is a potential target for the treatment of metabolic liver diseases.

Reference

Zhang Y.; et al. Hierarchical tricarboxylic acid cycle regulation by hepatocyte arginase 2 links the urea cycle to oxidative metabolism. Cell metabolism. 2024. 36(9):2069-2085. doi: 10.1016/j.cmet.2024.07.007

* For Research Use Only. Not for use in diagnostic procedures.

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