LM2I leads to CAD ubiquitination and liver cancer suppression through activation of ASS1

The urea cycle occurs mainly in the liver and undergoes changes during hepatocarcinogenesis. Argininosuccinate synthase 1 (ASS1) is a key enzyme in the urea cycle and is expressed at low levels in certain cancers. LM2I, a specific activator of ASS1, exhibits significant antitumor activity. However, the antitumor mechanism of LM2I in liver cancer remains unclear. In this study, we find that LM2I is more effective for liver cancer cells with low ASS1 expression. The results of the IP-LC/MS experiments reveal that ASS1 interacts with CAD. The expressions of ASS1 and CAD in liver cancer tissues and cells are negatively correlated. LM2I promotes the ubiquitination of CAD protein through ASS1. LM2I inhibits the proliferation of liver cancer cells in vivo and in vitro. However, its efficacy is weak in liver cancer cells stably overexpressing CAD. The H&E staining results reveal that LM2I has no toxicity in mice. In terms of metabolism, LM2I increases the urea content and decreases the pyrimidine content in liver cancer cells. Overexpression of CAD can reduce the inhibitory effect of LM2I on pyrimidine. Pyrimidine supplementation facilitates the proliferation of liver cancer cells, particularly when they are treated with LM2I. In summary, ASS1 interacts with CAD, and LM2I enhances CAD degradation through the activation of ASS1, consequently inhibiting pyrimidine synthesis and the progression of liver cancer.