Etoposide-induced protein 2.4 homolog promotes argininosuccinate synthase 1 and cancer cell survival upon arginine deprivation.

BACKGROUND: Arginine auxotrophy has been reported in a subset of cancers with inherently defective de novo arginine synthesis. However, the use of arginine deprivation therapy seems to be unequally effective, partially owing to the resistance acquired by cancer cells. Study of underlying factors involved in this response thus becomes of utmost importance. Meanwhile, the function of etoposide-induced 2.4 homolog (EI24) in cancer metabolism, and specifically in arginine metabolism, remains unknown. METHODS: EI24 was overexpressed in cancer cells using a doxycycline-inducible system or adenovirus transduction, while siRNA was used to knockdown EI24. Amino acid(s) deprivation medium was exploited with a cell viability assay to check the reliance of cancer cell survival on arginine. Protein expression and activation were examined through western blot and co-immunoprecipitation blot. Furthermore, global and specific protein translation were assessed through the SUnSET assay and polysome fractionation analysis. Gene expression and arginine level were downloaded from public cancer datasets for in silico validation including gene set enrichment and survival analysis to objectively evaluate the association between EI24 and arginine metabolism. RESULTS: EI24 promoted cancer survival under arginine starvation. Mechanistically, EI24 replenished translation of argininosuccinate synthase 1 (ASS1) by inducing the inactive S-nitrosylated form of phosphatase and tensin homolog (PTEN), leading to release of the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) axis. This tumor-promoting action of EI24 could be found in multiple ASS1-deficient cancer cells regardless of p53 status. Furthermore, expression of EI24 was linked to enrichment of arginine metabolism pathway as well as poor survival of patients with cancer across various cancer types, suggesting its role in cancer resistance to arginine deprivation. CONCLUSIONS: This study is the first to report the role of EI24 in promoting cancer survival via translational regulation of the metabolic enzyme ASS1, thus paving a route for further investigation into the link between EI24 and cancer metabolism.