FoxO transcription factors couple the urea cycle and gluconeogenesis by controlling Ass1
Amino acid catabolism during fasting requires coordinated nitrogen disposal and glucose production, but the transcriptional logic linking the urea cycle to gluconeogenesis remains unclear. Forkhead box O (FoxO) transcription factors are key regulators of fasting metabolism, yet their role in controlling urea cycle genes has not been fully defined. Here we identify FoxOs as direct regulators of hepatic argininosuccinate synthase 1 (Ass1). Because FoxOs often act through Kruppel-like factor 15 (Klf15) in amino acid metabolism, we tested whether Ass1 regulation requires Klf15. Acute hepatic FoxO1/3a knockdown in fasted mice selectively reduced Ass1 expression, lowered blood glucose, and shifted urea-cycle amino acids, with arginine decreased and ornithine increased, even in Klf15-deficient livers. Silencing Ass1 phenocopied these metabolic effects, indicating that Ass1 mediates a key FoxO-dependent branch of fasting adaptation. Mechanistically, we mapped a functional FoxO-binding element within an upstream Ass1 enhancer: FoxO1/3a activated the enhancer in reporter assays, EMSA confirmed binding, and in vivo luciferase imaging and liver ChIP demonstrated fasting-inducible enhancer activity and FoxO occupancy. Collectively, these findings establish a FoxO-Ass1 axis that couples ureagenesis to gluconeogenesis and supports metabolic flexibility during fasting. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=158 SRC="FIGDIR/small/695746v1_ufig1.gif" ALT="Figure 1"> View larger version (23K): org.highwire.dtl.DTLVardef@94ff36org.highwire.dtl.DTLVardef@1ff4f3forg.highwire.dtl.DTLVardef@abd0e5org.highwire.dtl.DTLVardef@1f8d9b_HPS_FORMAT_FIGEXP M_FIG C_FIG