| Summary: | Abstract Forkhead‐Box Class O 4 (FOXO4) is involved in critical biological functions, but its response to EGF‐PKB/Akt signal regulation is not well characterized. Here, it is reported that FOXO4 levels are downregulated in response to EGF treatment, with concurrent elevation of COP9 Signalosome subunit 6 (CSN6) and E3 ubiquitin ligase constitutive photomorphogenic 1 (COP1) levels. Mechanistic studies show that CSN6 binds and regulates FOXO4 stability through enhancing the E3 ligase activity of COP1, and that COP1 directly interacts with FOXO4 through a VP motif on FOXO4 and accelerates the ubiquitin‐mediated degradation of FOXO4. Metabolomic studies demonstrate that CSN6 expression leads to serine and glycine production. It is shown that FOXO4 directly binds and suppresses the promoters of serine‐glycine‐one‐carbon (SGOC) pathway genes, thereby diminishing SGOC metabolism. Evidence shows that CSN6 can regulate FOXO4‐mediated SGOC gene expression. Thus, these data suggest a link of CSN6‐FOXO4 axis and ser/gly metabolism. Further, it is shown that CSN6‐COP1‐FOXO4 axis is deregulated in cancer and that the protein expression levels of CSN6 and FOXO4 can serve as prognostic markers for cancers. The results illustrate a pathway regulation of FOXO4‐mediated serine/glycine metabolism through the function of CSN6‐COP1 axis. Insights into this pathway may be strategically designed for therapeutic intervention in cancers.
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